The synthesis and use of supported metal nanoparticlecatalystshave a long-standing tradition in catalysis, typically associatedwith the field of heterogeneous catalysis. More recently, the developmentand understanding of catalytic systems composed of metal nanoparticles(NPs) that are synthesized from organometallic precursors on molecularlymodified surfaces (MMSs) have opened a conceptually new approach tothe design of multifunctional catalysts (NPs@MMS). These complex yetfascinating materials bridge molecular (“homogeneous”)and material (“heterogeneous”) approaches to catalysisand provide access to catalytic systems with tailor-made reactivitythrough judicious combinations of supports, molecular modifiers, andnanoparticle precursors. A particularly promising field of applicationis the controlled activation and transfer of dihydrogen, enablinghighly selective hydrogenation and hydrogenolysis reactions as relevantfor the conversion of biogenic feedstocks and platform chemicals aswell as for novel synthetic pathways to fine chemicals and even pharmaceuticals.Consequently, the topic offers an emerging field for interdisciplinaryresearch activities involving organometallic chemists, material scientists,synthetic organic chemists, and catalysis experts.This Accountwill provide a brief overview of the historical backgroundand cover examples from the most recent developments in the field.A coherent account on the methodological and experimental basis willbe given from the long-standing experience in our laboratories. MMSsare widely accessible via chemisorption and physisorption methodsfor the generation of stable molecular environments on solid surfaces,whereby a special emphasis is given here to ionic liquid-type moleculesas modifiers  and silica assupport material. Metal nanoparticles are synthesized following anorganometallic approach, allowing the controlled formation of smalland uniformly dispersed monometallic or multimetallic NPs in definedcomposition. A combination of techniques from molecular and materialcharacterization provides a detailed insight into the structure ofthe resulting materials across various scales .2 is predominant.The incorporation of 3d metals in noble metal NPs to give bimetallic monofunctional NPs@MMSs favors a more polarizedH2 activation and thus its transfer to the C=O bond,while at the same time preventing the arrangement of noble metal atomsnecessary for ring hydrogenation. The incorporation of reactive functionalities,such as, for example, a −SO3H moiety on NPs@MMSs,results in bifunctional catalysts enabling the heterolytic cleavagecorresponding to a formal H–/H+ transfer.Consequently, such catalysts possess excellent deoxygenation activitywith strong synergistic effects arising from an intimate contact betweenthe nanoparticles and the molecular functionality.The molecular functionalitiesgrafted on the silica surface havea pronounced influence on the formation, stabilization, and reactivityof the NPs. The complementary and synergistic fine-tuning of the metaland its molecular environment in NPs@MMSs allow in particular thecontrol of the activation of hydrogen and its transfer to substrates.Monometallic  monofunctional NPs@MMSs possess excellentactivities for the hydrogenation of alkenes, alkynes, and arenes forwhich a nonpolarized (homolytic) activation of HWhile manymore efforts are still required to explore, control,and understand the chemistry of NPs@MMS catalysts fully, the currentlyavailable examples already highlight the large potential of this approachfor the rational design of multifunctional catalytic systems. Molecular Control of the Catalytic Properties of Rhodium Nanoparticlesin Supported Ionic Liquid Phase (SILP) Systems.ACS Catal.2020, 10, 13904–1391233343998Bordet, A.; Moos,G.; Welsh, C.; License, P.; Luska, K. L.; Leitner, W. .1Influence of the ionic liquid structure in SILPson the formation of Rh nanoparticles and their catalytic propertiesfor the hydrogenation of biomass-derived furfuralacetone. Comparisonwith nonmolecularly modified surfaces.xRhx100–@SILP) as Catalysts for the Selective Hydrogenationof Multifunctional Aromatic Substrates.Organometallic Synthesisof Bimetallic Cobalt–Rhodium Nanoparticles in Supported IonicLiquid Phases .19 In particular, the molecular imprinting of solid supports,14 the development of SILP (supported ionic liquid phase) and SCILL catalysts,16 and the immobilization of polymeric structures18 received great attention, leading to novel design options for solidcatalyst materials.With the depletionof fossil resources and the rise of alternativerenewable energy sources and chemical feedstock, the permanent evolutionand adaptation of catalysts are of critical importance.inter alia enhanced NPs stability,21 control over NP growth,1 tunable wettability,18 enhanced mass transport,22 and the introductionof additional functionalities.23 Most importantly, theirversatile and modular synthetic accessibility from defined molecularprecursors and materials allows countless variations in the combinationof supports, molecular modifiers, and nanoparticles and is thus highlysuitable for the development of multifunctional catalytic systemswith tailor-made reactivity.22Metal nanoparticles immobilized on MMSs(NPs@MMSs) are complexmulticomponent systems but possess several attractive features comparedto classical NPs@support catalysts comprising solely inorganic oxidesas supports. Potential advantageous properties have been reportedto include pars pro toto for the general strategy.Despite a numberof pioneering contributions and a highly dynamicrecent development, it is fair to say that the potential of NPs@MMScatalytic systems has only been tapped upon until now. We will describein this Account principal methods to synthesize and characterize NPs@MMSmaterials and highlight their promising properties in the controlledactivation of molecular hydrogen for its selective transfer to variousunsaturated functional groups. A particular focus will be placed onthe organometallic synthesis of metal nanoparticles on MMSs producedthrough the covalent attachment of organic compounds on porous solids.Some digressions toward materials obtained through physisorption techniquesand alternative NP syntheses will be made when necessary. While abroad range of molecular modifiers can be envisaged in this approach,we will lay special emphasis on ionic liquid (IL)-type structures 22The molecularmodification of porous solids (supports) has been accomplished throughthe development of physisorption and chemisorption techniques. Whenconsidering physisorption, the organic molecules are immobilized onthe support via weak interactions such as dispersion forces, hydrogenbonds, or stronger electrostatic interactions. In contrast, chemisorptionimplies a covalent attachment of the organic molecules at the surfaceof the support. While both approaches present advantages and drawbacks,chemisorption is generally preferred when possible, since the resultingmaterials are typically more stable, well-defined, and less proneto leaching.24 When considering the chemisorption of polymers on solid surfaces,the grafted from method is popular. In this case,the polymer chains grow in situ from an initiator,which has been previously anchored to the support surface.24 Another widely used and highlyversatile approach is the so-called grafted to method,which involves the reaction of a molecule possessing a reactive endat the surface of a support to form a covalent bond  with alkoxysilane-functionalized molecules. This methodproduces particularly robust MMSs and has been used to functionalizethe surface of solid supports with a wide range of organic compoundspossessing various functionalities including small organic molecules,25 ionic liquids,23 or polymers.24 Such molecular modifierscan be relatively complex and expensive to produce, hindering theirdirect use in large volumes as solvents for industrial processes.In contrast, the preparation of MMSs requires only small quantitiesof molecular modifiers and generates materials that are stable andeasy to reuse, thus reducing the cost factors and improving theireffectivity significantly. Most of the materials discussed in thisAccount have been obtained by variations of this general methodology.Many techniques have been developedin the past decade to achieve the chemisorption of various organicmodifiers on solid supports .ent bond 1.17 A ty2 adsorption experiments, electronmicroscopy, and diffraction techniques. In particular, N2 adsorption experiments can give information on the modificationof the surface area, pore diameter, and pore volume caused by thefunctionalization. Scanning electron microscopy (SEM) and transmissionelectron microscopy (TEM) give an overview of the materials’morphology and show evidence of the major changes. In the case ofcrystalline supports, X-ray diffraction (XRD) is the technique ofchoice to investigate potential structural changes.The influence of the chemisorption of organic compounds onthetextural, morphological, and structural properties of the solid supportscan be characterized by N29Si NMR. In 29Si CP-MASNMR showed the presence of trifunctionalized signals (T3: R-Si(OSi)3; T2: R-Si(OSi)2(OR′))corresponding to Si atoms of the ionic liquid bound to the SiO2 surface, thus evidencing the covalent attachment of the ILon the silica support  can be used to examine bandsthat are characteristic of the grafted molecules, thus confirmingboth their presence and their preserved structure/functionality.23 In addition, elemental mapping using SEM with energy dispersiveX-ray spectroscopy (SEM-EDX) provides valuable information concerningthe homogeneity of the surface coverage.Other NMR active nuclei such as naturally abundant bottom-up chemical method) has been demonstrated to be particularlypowerful to produce size- and shape-controlled nanoparticles possessingwell-defined physicochemical properties.28 This approach,developed in particular by Chaudret and co-workers, involves the thermaldecomposition of high energy organometallic complexes or hydrogenationof labile hydrocarbon ligands under relatively mild conditions, avoidingthe use of potential sources of pollution arising from the precursoror from the reducing agent.28 The nanoparticle formationcan be achieved directly from the solution or after impregnation ofthe precursor complex on the MMSs, producing NPs@MMS materials thatcan be used directly in catalysis without the need for additionalprereduction treatment. The simplicity and versatility of this methodcombined with its capacity to produce various types of metal nanoparticleswith practically clean surfaces makes this approach particularly usefulfor the generation of NPs on the MMSs.The developmentof physical and chemical methods allowing the preparation of metalnanoparticles has received broad attention in the past decades. Inthe context of the present Account, the organometallic approach .In our experience, theorganometallic approach has proven to be highly versatile for thegeneration of well-defined NPs in the 1–5 nm range from variousmetals and mixtures of metals, including, but not limited to, Fe,Co, Ni, Ru, Rh, FexRh100–x 3a. The ptroduced 3b.2,4,3131 the alloying extent in the case of bimetallics (EXAFS)31 and to study the electronic interactions between the NPs and theMMS (XPS).3 SEM, TEM, and scanning transmission electronmicroscopy with energy dispersive X-ray spectroscopy (STEM-EDX) provideinformation about the size of the NPs, their dispersion on the support,their composition, their interaction with the molecular modifier,and their bimetallic nature when applicable . The elemental mappingevidences that the Ru NPs are concentrated in the zones containingthe ionic liquid layer, providing direct evidence for the stabilizingeffect of the IL environment and the close contact of the metal NPand the molecular functionality.X-ray spectroscopy techniques can be used to determine the oxidationstate of the metals (XPS and XANES),A summary of the keyinformation required to characterize MMS andNPs@MMS as well as the associated techniques is provided in After catalysis, the same techniquescan be used to investigatethe potential changes in NPs@MMS materials including leaching of themolecular modifier, leaching of the metal, aggregation, and growthof the metal nanoparticles.10 as highlighted in 2 activated through homolytic dissociation(H–H), while the hydrogenation of ketones requires more polarizedhydrogen activation (Hδ+–Hδ−) resulting in formal hydride and proton delivery.10 Going one step further, the deoxygenation of alcohols involvesdehydration via carbocation intermediates or transition states, andthe presence of acidic protons from the heterolytic cleavage of hydrogen(H+–H–) is expected to facilitatetheses elementary steps.We will describein this part recent advances in the use of NPs@MMS catalysts for theactivation of molecular hydrogen and the selective hydrogenation andhydrogenolysis of unsaturated functional groups in various aromaticsubstrates. The concept takes advantage of the modularity of NPs@MMScatalytic systems to control the elementary processes of how molecularhydrogen is activated and transferred to substrates. The requirementsto hydrogenate alkenes, aromatic rings, or ketones or to hydrogenolyticallydeoxygenate alcohols are different especially in terms of hydrogenpolarization,In the following sections,we present selected recent efforts fromour laboratories and other teams to exploit the modularity of NPs@MMScatalytic systems in this context. The discussion will be organizedaccording to the metal component  and the functionalityof the molecular modifier .31 In this case, the IL grafted on silica does not take part directlyin the catalysis but contributes to the stabilization of the Ru NPs.Interestingly, biomass-derived furfuralacetone 2(cod)] or [Ru(cod)(cot)]on an imidazolium-based SILP are able to completely hydrogenate variousaromatic substrates, including benzylideneacetone 6.31 In t33 the influenceof chemisorbed molecular modifiers on the growth of NPs is far lessstudied. To investigate the influence of the structure of the molecularmodifiers on the synthesis of NPs, we synthesized Rh NPs on a rangeof imidazolium-based SILPs with systematic variations of the IL structure and well dispersed Rh NPswere observed in all cases on the SILPs, the structure of the graftedIL was found to have a significant influence on the size of the NPsand on their activity for the full hydrogenation of furfuralacetone.At similar NP sizes (1.2 nm), Rh@SILP catalysts were found to be moreactive and much more stable under continuous flow conditions thana reference Rh@SiO2 catalyst. The characterization of Rh@SILPmaterials after catalysis (6 h on stream) did not evidence any leachingof the molecular modifier or of the metal or any growth of the metalnanoparticles. In sharp contrast, Rh NPs on Rh@SiO2 werefound to aggregate, growing up to twice their original size over thecourse of the reaction. Interestingly, a decrease in the size of thespacer led to the formation of bigger NPs (2.0 nm) that were stillhighly active for C=C and furan ring hydrogenation but leftthe ketone intact.While solvents and ligands are known to control the formation ofmetal NPs in solution,34 The increase of the alkyl chains grafted on silica rendersthe environment of the Rh NPs apolar and hydrophobic, thus favoringthe hydrogenation pathway over hydrodefluorination. Excellent yieldsof fluorinated cycloalkanes could be achieved for a broad range ofsubstrates, matching or even surpassing that of molecular Rh-complexesas precatalysts and showing in addition the superior catalyst stabilityand reusability.35The modification of thesurface polarity by alkylsilanes was foundto be highly beneficial for the use of Rh nanoparticles in the challengingselective hydrogenation of fluoroaromatics to fluorocyclohexanes 7b.34 The2P@SiO2 materials through the chemisorptionof organosilanes and used these catalysts for the hydrodeoxygenationof guaiacol. Fast catalyst deactivation due to the formation of watercould be prevented by generating a nonpolar catalyst surface throughthe chemisorption of methyltriethoxysilane.36Along the sameline, Schüth and co-workers tuned the polarityof Ni6, NTf2; TOF = ca. 3 s–1) than for hydrophilicones .37 The same group investigated the potential ofconfinement effects in catalytic systems composed of Au NPs immobilizedon Al2O3-based SILPs in the hydrogenation of trans-cinnamaldehyde. From kinetic studies and the determinationof kinetic isotopic effects, the authors concluded that the dynamicchemisorbed IL layers act as nanocontainers for the Au NPs and playa key role in controlling the mode of activation of H2 (homolyticvs heterolytic).38Dupont and co-workers showed that the activity of imidazolium-basedPd@SILP catalysts for the partial hydrogenation of 1,3-cyclohexadienecan be modulated by changing the anion of the SILP, resulting in higherturnover frequencies for hydrophobic anions . In the presence of excess amine ligands,the Pd NPs were more stable and selective for the semihydrogenationof alkynes to alkenes than on bare SiOThese selected studies demonstrate thatthe molecular functionalityincorporated on the support plays a crucial role in controlling thegrowth of the NPs and enhancing their stability. In addition, evenif they are not reacting directly with the substrates, the molecularmodifiers can significantly influence the reactivity observed, forexample, through stereoelectronic interactions with the NPs, modificationof the polarity and hydrophilicity of their environment, or confinementeffects.31 In particular, we synthesizeda series of alloy-type FexRux100–@SILP catalysts using the complexes {Fe[N(Si(CH3)3)2]2}2 and [Ru(cod)(cot)]as precursors for the organometallic approach.31 The selection of organometallic precursors of a similarstability is important to ensure their simultaneous decompositionunder the conditions applied and thus avoid the formation of monometallicNPs and/or core–shell structures. In addition, the presenceof the IL layer chemisorbed on the support was found to be beneficialfor the formation and stabilization of small and uniformly dispersedbimetallic FexRux100– NPs on the SILP, which was not possible on unmodifiedSiO2 using the same conditions. While the monometallicRu@SILP catalyst led to the full hydrogenation of the substrates,the bimetallic Fe25Ru75@SILP catalyst was foundto be highly active and selective for the partial hydrogenation ofaromatic ketones to aromatic alcohols. Intriguingly, we observed thatnot only was the arene hydrogenation suppressed almost completelybut also, at the same time, the rate of C=O hydrogenation wasca. 4 times higher when using Fe25Ru75@SILPinstead of Ru@SILP. This enhancement of the C=O hydrogenationrate can be explained by a more polarized activation of hydrogen onthe FeRu surface than on the pure Ru surface as well as by an activationof the C=O bond on the more oxophilic FeRu surface. One possiblereason for the lack of ring hydrogenation could be the disruptionof the necessary arrangements of Ru atoms on the particle surfaceby the incorporation of Fe atoms. Dupont and co-workers used bimetallicFeRu NPs in IL media for the hydrogenation of CO2 to formicacid and heavy hydrocarbons. The CO2 hydrogenation pathwaywas dictated by both the basicity and hydrophilicity of the imidazoliumIL and by the composition of the NPs. In particular, the use of bimetallicFeRu NPs resulted in enhanced selectivity toward the formation ofheavy hydrocarbons as compared to monometallic Fe and Ru catalysts,presumably due to synergistic effects arising from the combinationof a reverse water gas shift-active metal (Fe) and a Fischer–Tropsch-activemetal (Ru (+Fe)).41As seenfrom the previous part, Rh, Ru, and Pd@MMS are excellent catalystsfor the hydrogenation of olefins and arenes. However, the selectivehydrogenation of C=O bonds in aromatic substrates for the productionof aromatic alcohols is particularly difficult with these metals,which typically result in full hydrogenation. In fact, in many cases,the hydrogenation of aromatics occurs either in parallel or even fasterthan that of C=O units,for example, with heterogeneous Ru and Rh catalysts. The developmentof catalytic systems able to efficiently hydrogenate C=O bondswhile leaving aromatics untouched is synthetically highly desirable,yet challenging. Following the general concept of xRhx100–@SILP catalysts with various Co/Rh ratios using [Co(cod)(cyclooctadienyl)]and [Rh3] and applied them for the hydrogenationof aromatic ketones and bicyclic heteroaromatics  and the partially saturated product furfurylalcohol at high Ag content (>50%).42 DFTcalculations indicate that the adsorption free energy of the intermediatefurfuryl alcohol on the Pd(111) surface is inversely proportionalto the Ag content in PdAg bimetallic NPs. Rana and Jonnalagadda showedthat bimetallic Ni50Cu50 NPs supported on organo-functionalizedgraphene oxide allow the selective hydrogenation of nitrophenol andcinnamaldehyde to aminophenol and cinnamyl alcohol, respectively.The organic amine groups chemisorbed on graphene oxide play the roleof stabilizers, increasing the binding capacity of the NPs.43 Han and co-workers prepared bimetallic CuxRux100– NPs on bentonite functionalized with a guanidinium IL. The IL wasfound to be crucial for the formation of the NPs and their stabilizationunder reaction conditions . In addition, bimetallic catalysts were moreactive and selective for the hydrogenolysis of glycerol to 1,2-propanediolthan the monometallic Cu and Ru versions with particularly good resultsobtained for the Cu25Ru75 catalyst .21Zhao and co-workers recently showed that the selectivityof furfuralhydrogenation using supported PdThese examples demonstrate that the selectivity of the hydrogenationreactions, and especially the catalyst’s ability to hydrogenatepolarized C=O units versus aromatic rings, can be effectivelyand finely controlled through the preparation of bimetallic NPs withtunable metal ratios on SILP-type materials. Intriguingly, the incorporationof 3d metals in nanoalloys does not modify the performance of noblemetals only via “dilution” effects but allows the generationof very significant synergistic effects leading to the accelerationof some transformations while others are completely shut down. Themodularity of the organometallic approach opens the possibility toexplore almost any combination of metals by systematic variation ofthe composition, providing a unique platform for optimization andmechanistic studies.45 We particularly used this approach to develop catalytic systemsable to perform selective hydrodeoxygenation reactions. As describedin +, and Brønsted acid functionalities can be introduced in themolecular modifiers in many different ways. Lewis acidity can alsobe provided through the modifier, whereby the anions of IL-type structuresoffer a versatile opportunity to fine-tune the acid strength.Thisthird part will cover monometallic nanoparticles immobilized on MMSswhere the molecular modifier possesses a functionality that is directlyinvolved in the catalysis. These materials fall in the class of multifunctionalcatalysts, which currently attract considerable attention not onlyfor the production of fine chemicals and pharmaceuticals but alsofor the conversion of oxygen-containing renewable feedstocks and platformchemicals into value-added chemicals and fuel components.3H) are excellent hydrodeoxygenation catalysts  and deoxygenation (−SO3H)active sites on a single-support was confirmed to generate synergisticeffects, leading to enhanced hydrodeoxygenation activities of Ru@SILP-SO3H as compared to a mixture of its single components.46 Recently, we further illustrated the versatilityof Ru@SILP-SO3H through its application in selective hydrogenolysisof various substituted diaryl ethers.48 The presence of the strong acid sites in combination with the RuNPs was found to be the key to high ether bond cleavage activity.In several of these reactions, the Ru@SILP-SO3H materialwas used for several hours under continuous flow conditions withoutleaching of the active phase or growth of the Ru NPs, highlightingits stability.48Our group has shown that bifunctional catalysts made of Ru NPs embeddedin sulfonic acid functionalized imidazolium-based SILPs  are responsiblefor the deoxygenation of fructose to 5-hydroxymethylfurfural, whilethe Pd NPs perform the hydrogenolysis and hydrogenation of 5-hydroxymethylfurfuralto 2,5-dimethylfuran. Following this one-pot strategy, excellent 2,5-dimethylfuranyields  were obtained.49 Hou and co-workers have shown that supportingRu NPs on sulfonic acid-functionalized silica (Ru@SiO2–SO3H) produces a bifunctional catalyst possessing a high activityfor the hydrogenolysis of cellulose into sorbitol. Using XPS and FT-IRwith pyridine as the molecular probe, the authors evidenced strongelectronic interactions between the sulfonic groups and the Ru NPs.The bifunctional catalyst was found to be more selective toward theformation of sorbitol than a physical mixture of its individual components(Ru@SiO2 + SiO2–SO3H) with,in particular, less side reactions of the sorbitol presumably dueto the “poisoning” of the Ru NP surface by the sulfonicacid functionalities.50 Qu and co-workersprepared Ru NPs immobilized on benzenesulfonic acid-functionalizedgraphene. The resulting bifunctional catalyst was found to be highlyactive for the conversion of biomass-derived levulinic acid to γ-valerolactone. In this case, the Ru NPs are responsible for the hydrogenationof the levulinic acid to 4-hydroxyvaleric acid while the sulfonicacid-functionalized surface catalyzes the dehydration of 4-hydroxyvalericto γ-valerolactone.51Kim and co-workers usedPd NPs supported on a Zr-based MOF depositedon sulfonated graphene oxide (Pd/UiO-66@SGO) as a bifunctional catalystto convert monosaccharides into 2,5-dimethylfuran in one pot. In thiscase, the Brønsted acidic functionalities  evidenced strong interactionsbetween the chlorozincate species (mainly ZnCl42–) and the Ru NPs surface. Ru@SILP-LA was found to be highly active,selective, and stable for the partial hydrogenation of benzofuranswith catalytic properties superior to Ru/C and nonfunctionalized Ru@SILPcatalysts .atalysts 10. The o2 anion. The deoxygenation activity was attributedin this case to the formation of Lewis acidic Rh–F speciesdue to the partial decomposition of the NTf2 anion on theNPs’ surface.32 The resulting balancebetween hydrogenation and hydrogenolysis activity enabled precisetemperature control, and thus, the selectivity could be switched toproduce aromatic products with either alcohol or alkyl side chainsand materials (“heterogeneous”) catalysis. The preparationmethods are highly versatile, allowing the production of multimetaland multifunctional systems. In particular, this modularity allowsone to fine-tune the balance between homolytic and heterolytic activationof Hl groups 14. In co58 Recently demonstratedexamples that are very challenging to achieve with classical heterogeneousor homogeneous catalysts include the selective hydrogenation of bicyclicheteroaromatics, fluorinated aromatics, aryl ether cleavage, and hydrogenationand deoxygenation of side chains in aromatics. Most recently, it hasbeen demonstrated that the introduction of reactive functional groupscan open the path to adaptive catalytic systems enabling reversiblecontrol over their performance by applying external stimuli.62 The manifold opportunities arising from theemerging design criteria motivate future developments in this direction.The synthetic potentialof such controlled transformations canhelp to exploit the green chemistry principles for the existing chemicalvalue chain as well as for novel biomass-derived product streams.in operando characterization under catalytic conditions to understand the structuraland reactive dynamics beyond the static structures before and aftercatalysis. In addition, detailed kinetic studies and determinationof kinetic isotopic effects will contribute to a better understandingof hydrogen activation processes in these complex catalytic systems.While the interest of NPs@MMS materials as multifunctional catalystsfor synthesis applications is clear, significant efforts are stillrequired to fully unlock their potential. Although the importanceof intimate contact between the NPs and the modifier has been demonstratedin particular for the SILP-type supports, the fundamental understandingof the interactions between the different catalyst components andtheir implications in catalysis currently remains largely elusive.Obviously, this is of crucial importance for the rational developmentof NPs@MMS catalysts and should be investigated in detail using acombination of characterization and computational techniques. A particularchallenge in this context is the need for The methodology toolbox and the selected examples presented inthis Account evidence that NPs@MMSs are now widely accessible andthat the knowledge-based design and preparation of NPs@MMS catalystsoffer new pathways for challenging transformations. As the availableinformation is assembled from individual examples to systematic classesof metal-functionality combinations, the capacity to develop catalystswith increasing complexity and controlled reactivity by this approachwill benefit from interdisciplinary research efforts by organometallicchemists, catalysis experts, synthetic chemists, and process engineers.We therefore hope that this Account will encourage researchers toinclude NPs@MMSs in the portfolio of multifunctional catalytic systems.


Polyomaviruses are a family of non-enveloped DNA viruses with wide host ranges. Human polyomaviruses typically cause asymptomatic infection and establish persistence but can be reactivated under certain conditions and cause severe diseases. Most well studied polyomaviruses encode a viral miRNA that regulates viral replication and pathogenesis by targeting both viral early genes and host genes. In this review, we summarize the current knowledge of polyomavirus miRNAs involved in virus infection. We review in detail the regulation of polyomavirus miRNA expression, as well as the role polyomavirus miRNAs play in viral pathogenesis by controlling both host and viral gene expression. An overview of the potential application of polyomavirus miRNA as a marker for the progression of polyomaviruses associated diseases and polyomaviruses reactivation is also included. Polyomaviridae family contains more than 100 members with natural hosts of primarily mammals and birds1 , which contains the origin of DNA replication and bi-directional promoters and transcriptional enhancers for early and late region transcription from opposite strands of the genome . The earMicroRNAs (miRNAs) are a group of non-coding RNAs with an average of 22 nucleotides that post-transcriptionally regulate gene expression by binding most often to 3′untranslated regions (UTR) of target messenger RNAs (mRNAs) . TypicalA small RNA that we now know to be a miRNA encoded by a polyomavirus was first identified in simian virus 40 (SV40) . Since tAlthough most well studied polyomaviruses encode a viral miRNA, how miRNA expression is regulated is poorly understood. Since the NCCR harbors the bi-directional promoter and transcriptional enhancer for both early and late gene transcription, it is not unexpected that studies have shown that the NCCR plays an important role in regulating polyomavirus miRNA expression. Because the polyomavirus miRNA is encoded on the late strand DNA of the viral genome, studies have shown that it was expressed in the late phase of the viral infection . HoweverIt was also demonstrated, however, that the total amount of miRNA expressed by a rearranged BKPyV is 1,000 times higher than the archetype virus, raising the question of whether the miRNA truly regulates replication . In suppAlthough numerous transcription factors (TFs) have been predicted or confirmed to bind to the NCCR of various polyomaviruses to regulate viral early and late gene expression and determine the polyomavirus cell or tissue tropism, few studies have been performed to investigate how viral miRNA expression is regulated by TFs, even though the TFs that could regulate viral late gene expression probably may also control miRNA expression . Sp1 wasTypically, cellular miRNAs are expressed from introns of protein coding genes . HoweverStrong evidence exists that there are also viral sequences in the genome containing promoter activity that regulate miRNA expression independent of the NCCR . This waAnalysis of 643 publicly available JCPyV genome sequences showed that three major miRNA genotypes exist in JCPyV isolates, with some minor variant genotypes. All the mutations located in the miRNA coding region form three single nucleotide polymorphisms (SNPs), with two located in the stem region of the pre-miRNA and one in miRNA-J1-5p. The SNP site located in miRNA-J1-5p does not alter the miRNA seed sequence and no SNPs were identified in miRNA-J1-3p. Interestingly, the SNP located in miRNA-J1-5p changes the TAg amino acid, while the two SNPs located in the stem region are synonymous mutations and cause no amino acid change in TAg . The samAnother study focused on miRNA polymorphisms demonstrates that naturally occurring SV40 and JCPyV variants rarely lose miRNA expression. The few variants with insertions or deletions in the miRNA coding region are unable to express viral miRNA due to a defect in the processing of the pri-miRNA by the Drosha complex . ConsideThe functions of polyomavirus miRNAs in viral pathogenesis and latent infection have begun to be elucidated. Due to the intrinsic property of the perfect complementarity to the early gene, the polyomavirus miRNA has a universal role in inhibiting viral T antigen expression in all reported polyomaviruses that encode a miRNA . HoweverFor BKPyV, a miRNA null mutant of a rearranged variant showed no difference in progeny virion production. Of note, however, the same mutation in an archetype virus background results in significantly higher viral replication and progeny production than the wild type virus . This isA study on MCPyV showed that its miRNA negatively regulates TAg expression like that of other polyomaviruses, and a mutant lacking miRNA expression showed higher viral DNA replication than the wild type virus. More importantly, this mutant lost the ability to establish long term persistence in transfected cells in culture . HoweverIn contrast to the low expression level of MCPyV miRNA in MCC, the miRNA expressed by a racoon polyomavirus (RacPyV) is highly abundant in RacPyV-associated neuroglial brain tumors . AlthougStudies have also shown that Skp-F-box-cullin (SCF) E3 ubiquitin ligase family members including Fbw7, βTrCP, and Skp2 induce the degradation of TAg protein and maintain MCPyV persistent infection . HoweverIdentification of the host targets of polyomavirus miRNAs is a useful strategy to understand the function of the viral miRNAs in polyomavirus pathogenesis or oncogenesis. However, at present, only a few miRNA host targets have been identified or predicted. The JCPyV and BKPyV 3p-miRNAs, which are identical, have been reported to facilitate viral immune evasion by specifically downregulating the expression of ULBP3, a stress-induced ligand that is recognized by the killer receptor NKG2D, to avoid recognition by NK cells or CD8+ T cells . Of noteAs discussed above, the main difference between archetype viruses and rearranged variants is the structure of the NCCR . NCCR reIn recent years, increasing evidence has suggested that polyomaviruses take advantage of exosomes (or extracellular vesicles) for viral transmission and spread, including JCPyV and BKPyV . The virFor JCPyV, detection of viral DNA in plasma (viremia) is rare and has been shown not to be useful for predicting PML . As the Increasing evidence has suggested a connection between human polyomaviruses and infections of the respiratory tract and oral cavity. Many of human polyomaviruses including JCPyV, BKPyV, MCPyV, HPyV6, KIPyV, and WUPyV have been isolated from or detected in tonsils, salivary glands, and/or respiratory secretions . The expSince JCPyV and BKPyV miRNA have been detected in the plasma, urine, and CSF of immunocompromised patients and healthy subjects, with similar detection rates in PyV-seropositive and PyV-seronegative subjects, all these data suggest polyomavirus miRNA may be a surrogate marker for polyomavirus associated disease . HowevermiRNAs post-transcriptionally regulate gene expression and play an important role in a variety of biological processes including development, cell growth and differentiation, survival, regulation of apoptosis, oncogenesis, and viral infection . Most poWZ wrote the first draft and edited subsequent the versions. MI edited the versions. Both authors contributed to the article and approved the submitted version.The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.


Non-alcoholic fatty liver disease (NAFLD) is the liver component of the metabolic syndrome. The disease mainly but not exclusively develops in people who are overweight or obese and in patients with type 2 diabetes. In Western society, NAFLD is highly prevalent and is gaining importance among people in Latin America and Asia . NAFLD cIn search for therapeutic targets, investigators realized that bile salts may play a role in the development of NASH. Serum and intrahepatic bile salt levels in patients with NASH are increased and regulatory mechanisms to prevent bile salt-induced hepatotoxicity are disturbed , 4. BileThe traditional role of bile salts is in the digestion of dietary fats. Bile salts enable pancreatic lipase in the small intestine to hydrolyze triglycerides to mono- and diglycerides and free fatty acids, molecules that can be absorbed. For this function, bile salts need to be present in the intestine in millimolar concentration. Therefore, bile salt metabolism needs to be tightly regulated, not only to avoid bile salt overload but also bile salt depletion. Ninety-five percent of bile salts secreted into the small intestine is reabsorbed Fig. . This prUpon bile salt absorption, FGF19 synthesis in the intestinal mucosa cells is activated. FGF19 is released into the portal blood and delivered to the liver to bind to the FGFR4/βKlotho receptor complex . ThroughCYP7A1 is a tightly regulated hepatic enzyme. Its main action is the conversion of cholesterol to bile salts, thereby compensating for the 5% loss of bile salts that escape from reabsorption. Through the action of hepatic FXR and SHP, CYP7A1 transcription and CYP7A1 protein expression are down-regulated, thus preventing bile salts to reach hepatotoxic levels. In healthy persons, the FXR-mediated down-regulation of CYP7A1 serves in the feed-back regulation of bile salt synthesis. In NASH, these regulatory mechanisms are disturbed causing inappropriately high intrahepatic and serum bile salt levels .Recent studies showed that bile salts through the action of FXR, FGF19 (fibroblast growth factor 19) and TGR5  regulate energy metabolism, bile salt synthesis, lipid oxidation, lipogenesis and gluconeogenesis , 11. IncThe association between bile salts and metabolism is complex and the interplay of bile salts, lipids and glucose can be approached from different angles, some pathways appear more promising than others. We now have steroidal and non-steroidal FXR agonists, agents that sequester bile salts in the intestine and drugs that inhibit ASBT , 13. OthIn this issue of Hepatology International, Japanese investigators report on the effect of the ASBT inhibitor elobixibat in mice with methionine choline-deficient (MCD) diet-induced NAFLD/NASH . These mThe mechanism of action of ASBT inhibitors in NASH is not completely understood. ASBT inhibitors have an interesting developmental history. Initially, they were advanced as candidates for the treatment of hypercholesterolemia. Irritable bowel syndrome and chronic constipation are other indications. This action is based on the fact that ASBT inhibitors increase the bile salt concentration in the colon and so stimulate chloride and water secretion and alter the intestinal microbiota. By doing this, they soften the stools and stimulate colonic motility. For NASH, ASBT inhibitors are under investigation both in animal studies and in human phase II trials. A recently published study reported the effect of the ASBT inhibitor volixibat in patients with NASH . The stuBoth volixibat and elobixibat inhibit bile acid absorption in the terminal ileum. By doing this, these drugs lower the circulating bile salt pool but by inhibiting bile salt reabsorption they also reduce the synthesis and release of FGF19. Lowered FGF19 secretion impairs the physiological feed-back inhibition of CYP7A1 in the liver and reduce the beneficial effects of natural FGF19 on lipidogenesis and gluconeogenesis. In the volixibat study, an increase in 7alpha-hydroxy-4-cholesten-3-one  was observed as indication of increased bile salt synthesis in the liver due to a lowered activity of FGF19.Other targets, such as an increased bile salt concentration in colon and stool and changes of the microbial colonic flora, were achieved in the Japanese study . However


Background and Objectives: Dental occlusion and gingival recession have been studied over the past years especially because of the increasing incidence of occlusal interferences in young patients. The purpose of this pilot study is to investigate any association between occlusal dysfunctions and gingival recessions. Data on gingival phenotype and previous orthodontic treatment were also collected to assess any correlation with the presence of gingival recession. Materials and Methods: Forty systemically healthy subjects, without signs of periodontitis and with gingival recessions, were included in the study. The following parameters were determined: location and extent of the gingival recession, gingival phenotype and functional occlusion by means of observing and registering the occlusal contacts in maximum intercuspation position, protrusive and lateral guidance. Results: Premolars were mostly affected in cases of working-side interferences during lateral guidance (71.19% of the affected teeth during left and 75% during right mandibular movements). The chi-squared exact test applied for the analysis of contingency tables revealed statistically significant associations between excursive interferences during lateral guidance and anterior guidance and the presence of gingival recession on the involved group of teeth. Conclusions: The results suggest that most gingival recessions might be associated with working-side interferences, the highest number of gingival recessions being associated with active interferences during lateral guidance. Gingival recessions are defined as apical displacement of the gingival margin beyond the cementoenamel junction; they appear either as a response against local irritating factors  or as incapacity of the gingival mucosa to adapt to excessive occlusal forces . Often, Occlusal relationships of maxillary and mandibular arches are of paramount importance in dentistry. Whether it is a fully dentate patient or a complex situation of complete mouth rehabilitation, the clinical analysis of dental occlusal requires time and precision, for both static and dynamic occlusion . SeveralAnterior guidance is the dynamic relationship among the lower anterior teeth and the upper anterior teeth through protrusive excursions. Lateral guidance refers to the dynamic relationship among lower teeth and upper teeth in lateral excursions; canine guidance or group function are considered physiologic situations in case of lateral excursions . During Occlusal trauma refers to injuries determining tissue changes within the attachment apparatus, including periodontal ligament, supporting alveolar bone and cementum, as a result of occlusal forces ,10. OcclDental occlusion and gingival recessions have been studied over the past years, especially with an increasing incidence of occlusal interferences in young patients. Moreover, patient’s expectations regarding pink–white esthetics have increased ,11. AbnoSeveral classifications of gingival recessions are reported in the literature. Cairo’s classification (2011), based on the assessment of the clinical attachment level at both the buccal and interproximal sites, proposes three situations: (1) recession type 1 (RT1)—no loss of interproximal attachment; (2) recession type 2 (RT2)—gingival recession associated with loss of interproximal attachment, in a lower or equal amount compared to the buccal attachment; (3) recession type 3 (RT3)—gingival recession associated with loss of interproximal attachment, in a higher amount than the buccal attachment The hypothesis of the study was that gingival recession that occurred on a certain group of teeth, irrespective of the gingival phenotype, can be correlated with occlusal interferences. In this context, the following questions are addressed: (1) Is there a relationship between occlusal interferences and gingival recessions? (2) Does the gingival phenotype influence the development of gingival recession in patients with occlusal dysfunction? (3) Which group of teeth would be more prone to gingival recession in case of occlusal interferences?The present study aims to evaluate the incidence of occlusal interferences in young patients with gingival recession and no other signs of gingival inflammation or periodontitis and determine a potential relationship between occlusal interferences and gingival recession.Eighty-two healthy subjects were selected for participating in our research study; all of them were informed about the examination protocols and all signed the informed written consent. The inclusion criteria were as follows: (1) age, between 20 and 35 years old; (2) Angle’s class 1 occlusal relationships; (3) full dentition except for third molars; (4) no ongoing orthodontic therapy; (5) at least one gingival recession ≥1 mm with no interproximal attachment loss (RT1); (6) no other clinical signs of gingival inflammation or supragingival calculus; (7) intact cementoenamel junction (CEJ) for the examined teeth. The exclusion criteria were as follows: (1) gingival inflammation; (2) smoking habit; (3) ongoing orthodontic treatment; (4) incorrect direct or direct restorations in violation of the biological width; (5) non-carious cervical lesions; (6) diagnostic of temporomandibular disorders; (7) Angle’s class II or III; (8) recession types 2 or 3.®, Chicago, IL, USA), 200 µm and 40 µm red/blue articulating paper , shimstock occlusal registration strips 12 µm thick  and Miller forceps.The examination procedures were performed by two experienced specialists and consisted of a general anamnestic evaluation, followed by a periodontal and occlusal examination, using an examination kit, a periodontal probe PCP UNC-15  and the most apical part of the free gingival margin of the teeth and recording the value in mm in the patient’s chart. The Cairo classification was used for rating gingival recession. The gingival phenotype was determined by the mucosal transparency probe technique, using the same periodontal probe. The probe was gently inserted into the gingival sulcus of the examined teeth and gingival transparency was evaluated. The gingival phenotype was classified as thin if the examiner was able to see the probe through the gingival margin and thick if the examiner was unable to see the probe trough the tissues.Occlusal evaluation consisted in recording the contact points between antagonistic arches in maximum intercuspation MIP and eccentric movements . To determine the MIP, 200 µm blue articulating paper was used and the distribution and intensity of the marks was analyzed. The articulating paper was held intraorally (using Miller forceps) while the subject tapped their teeth together firmly through the articulating paper 5 times in succession, through a single marking strip. Each subject was instructed to attempt to generate their perceived maximum occlusal force while tapping through the articulation strips. The presence or absence of contact points was additionally checked with 12 µm shimstock articulating paper. The Angle class of the patient in MIP was also determined.During lateral guidance, canine and group guidance were considered functional; excursive interferences occurred when either 1 or 2 posterior teeth on the same side were involved (in canine guidance) or when 1 posterior tooth was more strongly marked (in group function). Balancing-side interferences were considered when posterior teeth on the opposite side did not disocclude. The interferences were assessed with shimstock and marked with 40 µm red articulating paper.During anterior guidance, at least one pair of incisors (maxillary and mandibular), on each side of the midline associated with the immediate disocclusion of posterior teeth, was considered functional. Excursive interferences were considered when only one pair of teeth guided the movement, or all protrusive contacts were distributed only one side of the midline, as well as when the marks’ intensity differed between the guiding teeth. Posterior interferences were considered when disocclusion did not occur immediately between posterior teeth, right after the mandible left the centric relation.The markings resulted during MIP or excursive movements were photographed off a mirror using a digital SLR camera , using an aperture of F32 and shutter speed of 2/5 s. For each patient, a photo portfolio was obtained, where all markings were easily discernable. Afterwards, two examinators evaluated each photograph in order to identify the more intense and large marks  compared to the rest, as well as a marking on the inclines, meaning a contact point outside the dental fossa or marginal ridge, which would generate mandibular instability.® SPSS® Statistics 25.0  and the Microsoft Excel application. The data are presented as mean ± standard deviation of the mean (SD). The qualitative analysis of the variables was performed using the chi-squared test, Fisher’s exact test and the Z test. The variations in the results were considered statistically significant if p < 0.05.The statistical analyses were performed using IBMOut of the 82 subjects examined, 40  were further included in this pilot study, based on the inclusion and exclusion criteria . In totaIn total, 16 out of the 27 subjects presenting excursive interferences in anterior guidance displayed gingival recession localized on the guidance teeth. In addition, three subjects out of the six presenting posterior interferences displayed gingival recessions.A total of 20 out of the 26 subjects presenting working-side interferences  displayed gingival recession on the guidance teeth, while 17 out of the 22 subjects with working-side interferences  displayed gingival recessions. Two out of the four subjects with balancing-side interferences also had gingival recessions .Among the 16 subjects with excursive interferences in protrusion, 9 of them presented a thin gingival phenotype, while 7 had a thick phenotype. For the subjects with excursive interferences in lateral guidance, 18 had a thick gingival phenotype compared with 19 with a thin phenotype. Two out of four subjects with balancing interferences in lateral guidance had a thin gingival phenotype .The chi-squared exact test applied for the analysis of contingency tables revealed statistically significant associations between excursive interferences during lateral guidance and anterior guidance and the presence of gingival recession on the involved group of teeth. No statistically significant associations were identified between gingival recessions and balancing interferences in lateral guidance or posterior interferences in anterior guidance .The effects of occlusal trauma and excessive occlusal forces on the periodontium are still a controversial subject in periodontology. According to Glickman, trauma from occlusion is defined and diagnosed based on the histologic changes in the periodontium. In addition, Glickman clearly stated that, as long as the periodontium adapts to the occlusal relationships  without loss of tooth-supporting tissues, a diagnostic of occlusal trauma cannot be confirmed ,17. HoweThe aim of our study is to evaluate the incidence of occlusal interferences in young patients with gingival recession, in the absence of any gingival inflammation or periodontitis and determine whether there might be a connection between occlusal interferences and gingival recession.The statistical analyses revealed that a high percentage of gingival recessions (62.5%) were associated with excursive interferences, especially during right lateral guidance. For the anterior guidance, central upper and lower incisors were equally affected, as the lateral incisors tended to be less affected. During lateral guidance, the premolars involved in excursive interferences were mostly affected by gingival recessions . In lateral guidance, the incidence of gingival recessions was low (12%) in case of balancing interferences and usually localized in the posterior area .Several studies revealed similar results. Krishna et al., in their study published in 2013, a selective sample of 50 subjects with gingival recessions and 10 subjects who had gingival clefts (mean age of 22) were examined in static and dynamic occlusion. The authors concluded that the large majority of subjects showed excursive interferences, while gingival recessions were more frequently related to the group function in lateral guidance, rather than canine guidance. For cases with canine protection in lateral guidance, gingival recessions were observed on the buccal surfaces, while, for group protection, gingival recessions were distributed on the lingual surfaces of the anterior teeth. The authors also emphasized how occlusal interferences, in the case of dynamic occlusion associated with an absence of mutually protected occlusion, may contribute to gingival lesions, such as gingival recessions and fissures .In their study published in 2019, Kundapur et al. concluded that there was no association between the presence of gingival recession and occlusal interferences. While their group size was larger than that in our research study, with similar inclusion criteria, the parameters evaluated were limited to the frontal mandibular area. There is a risk a bias due to the small sample size taken in consideration for our study; however, the results based on our research work might indicate a strong association between gingival recession on the premolar area and eccentric lateral movements of the mandible [We also want to emphasize the fact that our study does not provide information to support the hypothesis that patients with a thin phenotype are more exposed to gingival recession; on the contrary, the results revealed that patients with a thick phenotype were diagnosed with gingival recession and occlusal interferences in a similar percentage compared to the subjects with thin phenotype.In our pilot study, the method used for occlusal analysis was the conventional method, consisting of the use of articulating paper. While several studies consider the method subjective, as it depends on the operator’s interpretation ,21,22, sIt is very difficult to isolate a single trigger point for the development of gingival recession and, to this matter, several articles reported that gingival recession was more likely to occur in patients who underwent orthodontic treatment ,30,31. HOur study might lay the foundation for future research focused on the correlation between occlusal interferences during lateral movements and gingival recessions in the premolar region, in young population without periodontal infectious disease.We strongly believe there is a strong association between recession and working-side (excursive) interferences, especially for the lateral movements, aspect indicated by the preliminary data of our research. The lack of literature evidence may be due to the absence of uniformity regarding case definition, as well as proper diagnostics of the occlusal interferences and gingival recessions.® systems) for occlusal analysis, as those systems are reliable and less prone to subjective interpretation [Nevertheless, the limitations of our pilot study arise also from the analog method used for occlusal examination. There are several clinical studies suggesting the importance of using digital devices (such as Tscanretation ,25,26,32retation .The findings of our pilot research study suggest that further larger clinical studies, investigating the association between occlusal trauma and gingival recession are required, with the use of digital systems in order to reduce or to eliminate the subjective variables.Within the limitations of our study, we can conclude that gingival recessions were present in a high percentage in cases of excursive interferences, especially in lateral guidance. Passive interference caused recessions less frequently, although in those cases, they were evenly distributed throughout the teeth affected. During the active interferences in the lateral guidance movements, it was observed that the premolars are the most affected teeth. In passive interference of the protrusive movement, the mandibular premolars are affected to a greater extent than the maxillary premolars. Molars are the most exposed teeth to passive interferences in laterotrusive movements.Despite the uncertainty about the true value of gingival thickness, the present results demonstrate that the gingival phenotype did not influence the occurrence of gingival recession associated with excursive/balancing-side interference in young patients.We strongly believe that, in the coming years, the number of patients with gingival recession will raise along with the need for predictable and effective treatments. Therefore, a proper diagnostic of gingival recession should include the primary and secondary causes, as the treatment should focus on all factors that contributed to the trigger and progression of such pathology. While the results of our study indicate that there might be a connection among the factors investigated, further randomized clinical trials are needed in order to identify a strong association between occlusal trauma and gingival recession.


Selenium (Se) is a micronutrient essential for life. Dietary intake of Se within the physiological range is critical for human health and reproductive functions. Selenium levels outside the recommended range have been implicated in infertility and variety of other human diseases. However, presently it is not clear how different dietary Se sources are processed in our bodies, and in which form or how much dietary Se is optimum to maintain metabolic homeostasis and boost reproductive health. This uncertainty leads to imprecision in published dietary guidelines and advice for human daily intake of Se and in some cases generating controversies and even adverse outcomes including mortality. The chief aim for this review is to describe the sources of organic and inorganic Se, the metabolic pathways of selenoproteins synthesis, and the critical role of selenprotenis in the thyroid gland homeostasis and reproductive/fertility functions. Controversies on the use of Se in clinical practice and future directions to address these challenges are also described and discussed herein. Selenium (Se) is a metalloid  trace element represented with the symbol Se and atomic number 34 in the periodic table ,2. SelenElemental Se is not biologically active per se. However, this trace element exerts its biological function(s) through its incorporation as selenocysteine and selenomethionine with subsequent integration into 25 selenoproteins encoded by the human genome ,11 and 2The trace nutrient is generally bioavailable in two forms: organic and inorganic Se. It is reported that the supplemented form of Se has an important influence on whether it is beneficial or harmful to organisms’ development and overall well-being ,16,17. A2SeO4] and selenite [Me2SeO3])  or selenocysteine [SeCys]) in comparison with inorganic forms (selenate [Mee2SeO3]) . While oe2SeO3]) .Despite the lower levels of bioavailability of inorganic types of Se, this form of the micronutrient may still be useful for synthesizing selenoproteins under stressful conditions or pathogenic insult ,22. Inte2Se), and selenium oxide (SeO2). The estimated Se concentration in the agricultural soil ranges from 0.33 to 2 mg/kg globally Sec aminoacylation. A critical RNA element of selenocysteine injection machinery is Sec-tRNA Sec, which is the only tRNA that alone regulates expression within the selenoproteomes ,48. Sec-Aminoacylation of Sec-tRNA is especially unusual in that it involves four enzymes rather than one for traditional tRNAs as shown in −) is central to both inorganic and organic Se crucial to the biosynthetic pathway. A screen of the available data indicates three metabolic pathways have indeed been documented for the generation of HSe−, which include the substrates selenocysteine, selenite, and methylselenol derived from Se-rich food  and lipid hydroperoxide, and inflammation  Selenoproteins that contain enzymatic activities and are closely linked to thyroid derived metabolism are glutathione peroxidase (GPX), iodothyronine deiodinase (DIO), and thioredoxin reductase (TXNRD) , which aGPXs: The GPXs are antioxidant enzymes with several biological roles, including hydrogen peroxide (H2O2) signaling, catalysis of hydroperoxide degradation, and maintenance of cellular redox homeostasis [2O2) involved in thyroid hormone synthesis into water  within the thyroid and all three play a role in thyroid hormone metabolism, where TH has downstream metabolic regulatory roles including growth, differentiation, thermogenesis, and metabolism that are mediated by nuclear receptors of thyroid hormones expressed in various tissues [ tissues . Both DI tissues . However tissues .TXNRDs: The TXNRDs selenoenzymes have a wide range of disulfide-containing substrates other than thioredoxin (Trx) and are responsible for the regulation of redox signalling system, transcription, immunomodulation, and cell growth factors [2)—a high-capacity biological reductant for thioredoxin, Trx-(SH)2 using NADPH as the hydrogen donor [2 then reduces other protein sulfides to regenerate the original Trx-S2, which may activate DNA synthesis, regulate transcription factors of numerous inflammatory genes or downstream signaling for apoptosis in mammals [2) to form reduced peroxiredoxin, Prx-(SH)2, which then catalyzes the chemical reduction and degradation of H2O2 and organic lipid hydroperoxides thereby playing a role in host defense against oxidative and inflammatory stress and damage, and their associated disease processes [ factors . In the en donor . The oxi mammals ,74. The rocesses .2 highlights its importance as an antioxidant that indirectly protects against oxidative stress and associated disease processes including diabetes and obesity-related metabolic disorders [As an example, Trx’s role as the electron donor in the formation of reduced Prx-(SH)isorders . In the isorders . Recentlisorders . These rInterestingly, there is a preferential synthesis of a specific selenoprotein type over the other during insufficiency of Se supply . For exaAs described earlier, thyroid hormone is widely known to regulate a range of metabolic processes throughout the body including growth, development, and notably, thermogenesis, basal metabolic rate, energy expenditure, and body weight . While h-deficient mice by regulating cholesterol metabolism and reducing oxidative stress [More recent studies have even linked Se deficiency with other comorbidities of obesity including hypertension and impaired glycemic control ,88. Impoe stress . These oe stress . Despitee stress . This cae stress . Notablye stress . Therefoe stress .As indicated above, the daily recommended Se intake in the general population varies due to geographical and racial characteristics, and dietary habits, thus a clear threshold for daily Se intake in obese population is undefined despite of the fact that Se is essential for thyroid functions, in modulating body’s response to inflammation and oxidative stress . NonetheThe previous section has highlighted the potential of Se supplementation to complement with other weight loss strategies including hypocaloric diets and exercise to maximize weight loss and enhance the general health of obese populations. However, the effects of Se diets may vary according to the different forms and doses of Se resulting in differences in absorption and metabolic pathways, along with the length of supplementation and Se status of subjects. A recent study revealed that obese individuals, when they adopted a slightly hypocaloric diet for 3 months, showed reduced body weight and fat mass, however the weight loss and changes in body composition were more evident when coupled with selenomethionine supplementation . Yet, anThese contradicting results could partly be explained by the fact that the increase in T3 levels and subsequent fat metabolism and weight loss would only manifest in Se-deficient individuals, which was consistent with the results from the first study that was based in Italy with Se-deficient soil and lower Se status. It was postulated that the high Se intake in populations with sufficient Se intake at the baseline could depress the activity of DIO1 involved in the production of active T3 hormone, leading to reduced T3 levels and thus body metabolism . This th2O2 production to activate its oxidative properties [2O2 is not adequately removed by the less active GPX, due to Se deficiency, significant host damage occurs to thyrocytes followed by necrosis and fibrosis of the thyroid [Moderate Se deficiency has been linked to decreased thyroid function and increased occurrences of thyroid disorder as summarized in operties . However thyroid . Even th thyroid . For exa thyroid .A growing interest in the beneficial role of Se in human health is now focused on disease-prevention. Despite the significant role of dietary iodine in normal thyroid functioning, patients with thyroiditis and severe iodine deficiency failed to respond to iodine supplementation treatment . HoweverIn addition, low Se status is associated to an increased risk of autoimmune thyroiditis, Grave’s disease and goiter (enlarged thyroid gland) . It is nThe optimum reproductive ability relies on several variables, including genes, external ecological criteria, and individual diet . Out of Currently there is scarce information on the significance of Se to female fertility. In female reproductive health, the production of a healthy oocyte involves a number of sequential steps. One primary step is folliculogenesis which is the developmental process of primordial ovarian follicles at birth into mature follicles after puberty. An essential step of folliculogenesis is the proliferation of granulosa cells, where Se has been shown to control the development of granulosa cells and the biosynthesis of one of the major female sex hormones, 17-estradiol (E2) in adult ovaries in vitro . Recent Thyroid hormones are essential for the optimization of mammalian reproduction and growth . InadequThis available data clearly demonstrates the important role of uterine and placental tissues in normal pregnancy and the control of fetal exposure to maternal thyroid hormones . Huang aLower Se levels are reported during different stages of pregnancy suggesting that a stage-dependent relationship is operational. For example, Se deficiency reduces selenoproteins levels involved in redox regulation, impairs placental function and fetal development leading to miscarriage or complicated preterm birth ,119. In Sodium selenite supplementation (5 ng/mL) promotes oocyte growth and increases the rate of proliferation in theca and granulosa cells . This imRecently, Se supplementation (both organic and inorganic forms) has significantly reduced the apoptotic rate in developing follicles in aging mice and improved the rate of in vitro produced pre-implantation embryos . Taken t2O2), and lipid peroxides from oxidative stress as described above. Many of these are tissue-specific and seems to be expressed in sex-specific manners [The role of Se in male reproduction is relatively well-established and reported compared to female reproduction. Among all selenoproteins, the glutathione peroxidase (GPX) family plays a critical role in many redox responses involved in male reproduction. The key function of these enzymatic isoforms is to defend and guard the cells by catalyzing the reduction of organic hydroperoxides (via glutathione), hydrogen peroxide  and thus plays a critical role in sperm motion . For natThe significance of GPX4 is illustrated by the death in GPX4 homozygous embryonic mice (KO) after a specific target disruption to GPX4. However, heterozygote mice in gestational stage were unremarkably alive and were born as healthy looking fertile pups . This in(–/–) KO (gene responsible for SelenoP protein production) showed considerable reduction in fertility, lower Se levels, and decreased GPX activity [(–/–) KO male mice, which again iterates the significance of Se in spermatogenesis [Similar with the case with GPX4, selenoprotein P (SelenoP) encoded by the SELENOP gene is thought to have pivotal role in the male reproductive functions. It acts as a Se transporting protein and is also present in vesicle structures in the basal region of Sertoli cells . Sertoliactivity . These cogenesis .GPX1 and GPX3 selenoproteins, though less abundant are also present in male reproductive tissues and secretions involved in male reproduction. These are well-expressed and defined in sperm and epididymal epithelial cells . In someSperm maturation is closely associated with sperm quality and ejaculation, as well as reproductive efficacy in males. Any anomalies in these processes may therefore lead to insufficient, lower quality ejaculates, and decreased male fertility . IncreasMost importantly, the parameters of the quality of sperm including gross count and motility in the treatment group was also improved compared to control group . These fIncreased ROS levels have been strongly associated with infertility . Thus, fThis outcome demonstrates the need to reconsider which Se form should be promoted for supplementation to the public and how much is needed to maintain homeostasis. In an Se-deficient diet, a reduced GPX level has also been observed, whereas Se-excess group showed comparatively high GPX levels . It is nFurthermore, the health benefits offered by organic vs inorganic Se also differ making decision on Se supplementation more complex. For example, a substantial improvement in the overall morphological and histomorphological indexes was observed in testes of young male goats fed with organic Se (Se yeast) of 0.3 mg/kg body weight for 2 months, in addition to the improved GPX enzymatic activity and superoxide dismutase (SOD) in their serum and testicular tissue compared to controls . AnotherSelenium deficiency leads to several thyroid gland disorders which can impact either synthesis of thyroid hormones or their function at the target tissues which can have implication on global body metabolism. Se supplementation can significantly improve sperm fertility parameters. Although, data on Se and female reproduction are scarce but do suggest that Se supplementation is beneficial in restoring ovarian function and minimizing pregnancy related complications.Almost all the available literature evidence is based on single time point investigations on either thyroid or reproductive organs studied in isolation. However, given the strong link between thyroid function and reproduction, the root cause must be addressed as targeting reproduction alone will not resolve the issue unless thyroid function is restored that will subsequently improve the downstream associated reproductive functions.Although Se supplementation has shown convincing results in improving thyroid and reproductive functions, the border between selection of dose and form of Se is very thin as slightly higher doses may lead to Se intoxication. Hence, further well-designed longitudinal studies are required to investigate a range of Se doses and forms to characterize and identify the best form that offers maximum and safe protection.


Breastfeeding is associated with numerous health benefits, but early life nutrition has not been specifically studied in the neurodevelopmental disorder fragile X syndrome (FXS). Herein, I evaluate associations between the consumption of breast milk during infancy and the prevalence of autism, allergies, diabetes, gastrointestinal (GI) problems and seizures in FXS. The study design was a retrospective survey of families enrolled in the Fragile X Online Registry and Accessible Research Database (FORWARD). There was a 1.7-fold reduction in the prevalence of autism in FXS participants who were fed breast milk for 12 months or longer. There were strong negative correlations between increased time the infant was fed breast milk and the prevalence of autism and seizures and moderate negative correlations with the prevalence of GI problems and allergies. However, participants reporting GI problems or allergies commenced these comorbidities significantly earlier than those not fed breast milk. Parsing the data by sex indicated that males exclusively fed breast milk exhibited decreased prevalence of GI problems and allergies. These data suggest that long-term or exclusive use of breast milk is associated with reduced prevalence of key comorbidities in FXS, although breast milk is associated with the earlier development of GI problems and allergies. The American Academy of Pediatrics recommends exclusive breastfeeding for the first 6 months of life and continued breastfeeding with the introduction of complementary foods . BreastfFMR1) gene located on the X-chromosome . Thus, the earlier age of onset of GI problems and allergies in FXS infants fed breast milk was not associated with reported autoimmune disorders in the mothers.I did not observe a higher prevalence of material autoimmune disorders in the breastfeeding mothers [Q38: thyroid disorder: 16% no BM  levels in breast milk. Axons in the autistic brain have a thinner coating of myelin . IGF-1 r1KO mice ,31, and 1KO mice ,33. Brea1KO mice  and is a1KO mice  and alte1KO mice .Bifidobacteria, which are associated with reduced risk of infection in infancy, as well as a reduced risk of certain chronic illnesses in adulthood [GI problems are common in children and adults with FXS ,38. Breadulthood . Formuladulthood , which cdulthood .There are racial and economic disparities in breastfeeding with higher rates among white, college-educated, middle class women . This stAn important question raised by this study is whether breast milk is protective, or formula is detrimental, in regard to FXS comorbidities. I hypothesize that formula is detrimental in terms of autism in FXS because breast milk feeding (47% autism comorbidity) is more similar to no breast milk feeding (53%) than to exclusive breast milk (28%). Likewise, GI problems are more similar when comparing breast milk feeding (31% GI problem comorbidity) to no breast milk (33%) than to exclusive breast milk (23%) in FXS. Retrospective studies are considered a low level of evidence and prospective evaluation is required to justify specific dietary recommendations in FXS.This study also evokes the question of whether or not there are differences in mothering habits or in breast milk from FXS premutation and full-mutation carriers that could contribute to disease severity in their children with FXS. For example, further studies are needed to explain the reported earlier onset of GI problems and allergies in response to breast milk in FXS. This study did not collect data on maternal premutation and full-mutation status; however, since participant eligibility included full-mutation FXS status, the biological mothers were all pre- or full-mutation carriers. Female premutation carriers are at higher risk for obsessive-compulsive symptoms and autoimmune disorders ,44,45. IIt is unlikely that increased autoimmune antibodies in the breast milk caused the reported earlier onset of GI problems and allergies. Although higher titers of anti-neuronal antibodies have been reported in both autism and FXS ,47,48, fThe nutritional profile of premutation and full-mutation breast milk remains to be determined. The most common substance in breast milk found to cause allergies is casein protein. However, casein protein is not the cause of earlier onset of allergies in the FXS breast milk cohort, as those participants were compared to participants with FXS fed cow milk formula. The scientific literature is inconclusive as regards the effects of breastfeeding in preventing or delaying the onset of specific food allergies ,36,50. Tn-3 to n-6 fatty acids [KOFmr1 mice [KOFmr1 mice exhibit reduced triglycerides, total cholesterol, carnitine, leptin and insulin, and increased levels of free fatty acid and the ketone bodies acetone and acetoacetate [FMR1 mutation size, that could affect developmental outcomes in FXS. In addition, while breast milk from premutation carriers (n = 5) and controls (n = 25) are similar in terms of protein and lactose concentrations, the incidences of low protein and low lactose are higher in premutation carriers, while the quantity of the micronutrient zinc is lower [The number of trinucleotide cytosine-guanine-guanine (CGG) repeats in premutation carriers correlates with multiple plasma metabolite levels including lower levels of fatty acids and the ketone body 3-hydroxybutyrate, as well as the ratio of ty acids . Omega-31KO mice . Fmr1KO oacetate . Male paoacetate . In totaoacetate , future is lower . Zinc isis lower . Low zinis lower . Macro- FMR1 gene and socioeconomic status of the mothers. Recall bias, particularly the use of breast milk, was not expected to be a problem as parents typically have specific reasons for their choice of infant feeding and switch for specific reasons such as GI problems or allergies. Reporting bias could be an issue with the reported age of onset for GI problems and allergies because mothers are solely responsible for breastfeeding whereas fathers, who may be more risk tolerant, can assist with formula feeding and due to the timing between the cessation of breastfeeding and the survey. It should be noted that there are practical implications of exclusive, prolonged breastfeeding. The major reasons reported for discontinuation of breast milk feeding in this study were inadequate quantify of milk; child age and transition to baby food and sippy cup; and mother need to wean child for work, health or convenience-related issues. Also of note, the average age of an FXS diagnosis is 35–37 months in boys and 41 months in girls with an average of 10 symptom-related visits to health care professionals before a diagnostic FXS DNA test is ordered [The limitations of this study include the absence of longitudinal studies, which are not possible in this population because most participants are not diagnosed until three years of age or older after breastfeeding has been discontinued, recall and reporting biases which are potential issues with all retrospective studies, and unknown CGG repeat number in the  ordered . An earlThe strengths of this study include the survey links parent-reported data on breastfeeding to the development of comorbid conditions including autism in their children with FXS and the higher than expected rate of breastfeeding in this population allowed evaluation of the timing of GI and allergy comorbidities. Although clinical autism diagnostic test scores were not available, it is reassuring that caregiver-reported incidence of autism concurs with survey questions adapted from standardized instruments that assess cognitive ability and autistic behaviors . Of 32 questions on our survey, the caregiver-reported autism cohort exhibited statistically significant differences with Bonferroni correction on 14 questions and statistically significant differences without Bonferroni correction on 24 questions. All differences in outcomes were in the expected direction for an autism diagnosis.Collectively, the association between breastmilk and decreased comorbidities in FXS concurs with extensive literature supporting the opinion that, “Human breastmilk is therefore not only a perfectly adapted nutritional supply for the infant, but probably the most specific personalised medicine he or she is likely to receive, given at a time when gene expression is being fine-tuned for life.” . It remaFMR1 mutation status, which may affect development in their children. FXS is typically not diagnosed until 3 years of age or older; however, NBS protocols are validated and could be implemented if an early-life intervention such as a dietary supplement were available.In conclusion, this retrospective survey study provides data regarding an association between breastfeeding and the reduced prevalence of autism in participants with FXS recruited through FORWARD. Although the overall prevalence of GI problems and allergies were not statistically different with breast milk in the total study population and were higher with cow milk formula compared to breast milk in males, the age of onset of GI problems and allergies in FXS participants were significantly younger in those fed breast milk. Overall, these findings indicate that breastmilk is associated with improved outcomes in FXS compared to infant formula and that mothers should be encouraged to breastfeed. The findings also support further studies to identify potentially altered levels of breastmilk components as a function of maternal


Defences to pathogens such as SarCoV2 in mammals involves interactions between immune functions and metabolic pathways to eradicate infection while preventing hyperinflammation. Amino acid metabolic pathways represent with other antimicrobial agent potential targets for therapeutic strategies. iNOS-mediated production of NO from Arg is involved in the innate inflammatory response to pathogens and NO overproduction can induce hyperinflammation. The two Arg- and Trp-catabolising enzymes Arg1 and IDO1 reduce the hyperinflammation by an immunosuppressive effect via either Arg starvation (for Arg1) or via the immunoregulatory activity of the Trp-derived metabolites Kyn (for IDO1). In response to amino acid abundance mTOR activates the host protein translation and Coronaviruses use this machinery for their own protein synthesis and replication. In contrast GCN2, the sensor of amino acid starvation, activates pathways that restrict inflammation and viral replication. Gln depletion alters the immune response that become more suppressive, by favouring a regulatory T phenotype rather than a Th1 phenotype. Proliferating activated immune cells are highly dependent on Ser, activation and differentiation of T cells need enough Ser and dietary Ser restriction can inhibit their proliferation. Cys is strictly required for T-cell proliferation because they cannot convert Met to Cys. Restricting Met inhibits both viral RNA cap methylation and replication, and the proliferation of infected cells with an increased requirement for Met. Phe catabolism produces antimicrobial metabolites resulting in the inhibition of microbial growth and an immunosuppressive activity towards T lymphocytes. Host defences to pathogen invasion, including bacteria or virus such as SarCoV2, involves in mammals a combination of initial innate immune and protective responses to eradicate infection that proceeds by directly acting on the pathogens and with locally induced inflammation, subsequently balanced by different mechanisms of immunomodulation and disease tolerance to prevent secondary harmful hyperinflammation. These processes include for a part a series of complex interactions between immune functions and metabolic pathways resulting from a convergent evolution of the two systems , 2. ThisArginine (Arg) is a non-essential amino acid for healthy humans involved in the synthesis of proteins and in the urea cycle and acting as precursor for different molecules including glutamate (Glu), citrulline (Cit), and nitric oxide (NO), an important bioactive molecule with both cardiovascular, immunological, and neurological signalling functions, and antimicrobial cytotoxic activity . In humaArg catabolism involves in mammals four enzymes including nitric oxide synthase , arginase , arginine decarboxylase (ADC), and arginine glycine amidinotransferase (AGAT) . The thrThe relative expression of the two Arg catabolizing enzymes iNOS and Arg1, and the production of associated Arg-derived metabolites, are related to the intrinsic regulation of macrophage polarisation, function, and activation state, and to the profile and time course of the immune response against viruses and bacteria pathogens  is a precursor for the synthesis of proteins and for several molecules involved in the regulation of diverse biological processes, including the amino acid kynurenine (Kyn), NAD, serotonin and melatonin . In mammThe enzyme IDO1, mainly expressed by DCs, is involved in the control of acute inflammation by an immunoregulatory immunosuppressive effects linked to its catabolic activity that induce a rapid change of Trp and Kyn levels that causes Trp deprivation in the microenvironment and the generation of immunoactive Kyn metabolites . Kyn is Identification of mechanisms that control the functions and fate of IDO1 opens new strategy for the pharmacologic control of IDO1 functioning and its immunoregulatory effect and modulating IDO1 catalytic activity is a promising therapeutic approach to modulate the immune response during the different steps of the immune response to pathogens infection. For some viral infections high IDO1 activity represents a main cause of immune unresponsiveness during the initial immune responses that aims to eradicate pathogens and downregulation of the short-term immunosuppressive regulatory effects of IDO1 should improve this short-term immune response. In contrast, during the second phase of the response, upregulating and increasing the activity of IDO1 could participate to prevent harmful hyperinflammation.At the crossroad between Arg and Trp metabolism and their regulatory effects, the two enzymes Arg1 and IDO1 are involved in the secondary control of acute hyperinflammation associated to pathogens infection by an immunoregulatory immunosuppressive effects via either amino acid depletion (as for Arg1) or via the combined effects of immunoregulatory Kyn and signalling activity (as for IDO1) , 32. HowRapidly proliferating cells, including several immune cells, require a supply of some amino acids such as Gln and upregulate metabolic pathways providing these amino acids, and these processes can be used as therapeutic targets against infection . The ami3 and aspartate and to the synthesis of the nucleotides purines and pyrimidines involved in the synthesis of DNA and RNA. Gln acts as a nitrogen donor for rapidly dividing cells, such as lymphocytes, in which it is critical for nucleotide synthesis and energy production [3 and in mammals glutaminase is highly expressed in lymphocytes and macrophages of lymphoid organs and increase in response to inflammatory stimuli [The degradation of Gln leads to the formation of NHoduction  and thisoduction . Glutami stimuli . Gln act stimuli . The TCANaive T lymphocytes are rapidly activated following pathogens invasion and Gln is essential for their proliferation and biosynthetic activities requiring high energy supply met by increased absorption and use of Gln –51. DuriGln depletion alters the balance of the immune response that become more suppressive, by favouring a regulatory T phenotype rather than a Th1 phenotype, even in a cytokine environment inducing the Th1 phenotype, associated with an inhibition of IFN‑γ secretion . Gln comAmino acid sufficiency and deficiency and amino acid and protein metabolism are controlled by the amino acid sensing pathways mTOR and GCN2 with a crosstalk between the two pathways . In respAnother example relies to serine (Ser) that is also required in large amount for rapidly growing cells –69. Ser In the same way, the two sulphur-containing amino acids, the essential amino acid methionine (Met) and the semi-essential amino cysteine (Cys), are also involved in immunoregulation. Cys is strictly required for T-cell proliferation because T cells lack the enzyme converting Met to Cys and must import Cys via their transport system . Cys avaMoreover, the interleukin 4-inducible 1 enzyme, secreted or localised in lysosomes, catalyses Phe catabolism leading Phe depletion and to the production of the a-ketoacid phenylpyruvate, and H2O2 with antibacterial toxic effects potentiated by the associated production of NH3 and basification of the medium , 80. TheAmino acid metabolism and signalling processes participate to the control of pathogen infection and to the regulation of the inflammation induced by activation of innate, adaptive, and regulatory immune responses. The crosstalk between catabolism of amino acids and the immune system represents an important process for tuning immune reactivity and reducing hyperinflammation associated to immune responses to infections. Among amino acids, Arg availability and the catabolic enzyme iNOS are involved in the inflammatory innate response through the production of NO, while Arg and Trp availability, associated to the two catabolic enzymes Arg1 and IDO1, represent catabolic pathways involved in modulating immune reactivity and exerting regulatory effect on inflammation and on the adaptive immune responses. Moreover, the amino acids Gln, Ser, Cys, Met, and Phe are also being considered as amino acids involved in pathogen development and in the balance of the immune reactivity. Overall, excess amino acid and mTOR hyperactivation appear as an environment favouring virus invasion while amino acid starvation and GCN2 activation restrict viral invasion. These different mechanisms represent potential therapeutic targets to control infection and associated hyperinflammatory response causally related to morbidity and mortality induced by pathogens, including coronavirus.


Skeletal muscle development and regeneration rely on the successive activation of specific transcription factors that engage cellular fate, promote commitment, and drive differentiation. Emerging evidence demonstrates that epigenetic regulation of gene expression is crucial for the maintenance of the cell differentiation status upon division and, therefore, to preserve a specific cellular identity. This depends in part on the regulation of chromatin structure and its level of condensation. Chromatin architecture undergoes remodeling through changes in nucleosome composition, such as alterations in histone post-translational modifications or exchange in the type of histone variants. The mechanisms that link histone post-translational modifications and transcriptional regulation have been extensively evaluated in the context of cell fate and differentiation, whereas histone variants have attracted less attention in the field. In this review, we discuss the studies that have provided insights into the role of histone variants in the regulation of myogenic gene expression, myoblast differentiation, and maintenance of muscle cell identity. Skeletal muscle tissue has a high regenerative potential that relies on tissue-specific stem cells, the satellite cells. These cells are characterized by the expression of the paired-homeobox transcription factor PAX7 and, in a subset of muscles, by the co-expression of its paralog PAX3 . Muscle Skeletal muscle development and regeneration depend on the fine regulation of transcription factor activity and subsequent gene expression. The epigenetic status of a specific gene locus constitutes additional layers of regulatory events that control cell differentiation, fate, and identity. The epigenetic regulation of gene expression depends on the chromatin architecture, the histone post-translational modifications (PTMs), the nucleosome composition, and DNA methylation. In eukaryotic cells, nucleosomes are composed of 147 bp of DNA that wraps around the histone octamer complex, which contains a central tetramer comprising the histones H3-H4 and two external dimers of histones H2A-H2B ,7. The nEzh2 deletion specifically in satellite cells, further revealed its role in controlling stem cell self-renewal and maintaining myogenic gene expression and identity . I. IMsx1 ipression . Mass sppression . During and Myog B 84]. T. TMsx1 iThe role of the histone variants in myogenesis and more generally in the regulation of somatic cell identity and differentiation has relatively recently started to be unveiled. In contrast, studies on histone variants in pluripotent or embryonic stem cells have greatly contributed to describe the function of specific histone variants in maintaining pluripotency and/or regulating developmental gene expression upon differentiation. In addition, the use of pluripotent cell lines to study histone variants further contributed to uncovering the genomic enrichment of histone variants, the histone chaperones mediating histone variant deposition, or the mechanisms by which specific histone variants interfere with transcriptional modulators to regulate gene expression. With this review and the different studies that combine histone variants and myogenesis, we can highlight the importance of the H3 histone variant H3.3 and the H2A histone variants H2A.Z and mH2A1.2 in promoting myogenic gene expression and differentiation. These histone variants are enriched at myogenic gene loci during differentiation and they either recruit transcriptional regulators or undergo specific post-translational modifications required to maintain or activate gene expression. Studies on H1 variants, such as H1b, further show that replacement histones from distinct families can function as transcriptional regulators and control myogenic differentiation. This suggests that there is a broad regulation of gene expression by distinct histone variants; however, whether the role of each histone variant is specific to muscle cells or whether similar or contrasting functions operate in other somatic cells remains to be studied.The field of somatic cell differentiation started recently to give more attention to the function of histone variants in transcriptional regulation, but has largely relied on in vitro studies. This is associated with the need to use fusion and/or tagged proteins, or the requirement of high cell numbers to accomplish specific experiments, where cell lines provide easier manipulation and virtually no limit of material. With the constant update of the technologies and protocols to perform genome-wide analysis with smaller inputs, it might be easier in the near future to combine epigenetic studies with in vivo approaches. Techniques such as single cell RNA-seq (scRNA-seq) combined with scATAC-seq start to make it possible to study gene expression and chromatin accessibility at the single cell level and can be performed on in vivo cells or tissues. Moreover, ChIP-seq improved techniques such as Cleavage Under Targets and Tagmentation (CUT&Tag) are now available for use with small numbers of cells and should lead to the identification of histone or histone mark enrichment from in vivo cells.