Niches provide structural and functional cues that are both biochemical and biophysical, stem cells integrate this complex array of signals with intrinsic regulatory networks to meet physiological demands. Although, some of these regulatory mechanisms remain poorly understood or difficult to harness with traditional culture systems. Biomaterial strategies are being developed that aim to recapitulate stem cell niches, by engineering microenvironments with physiological-like niche properties that aim to elucidate stem cell-regulatory mechanisms, and to harness their regenerative capacity in vitro. In the future, engineered niches will prove important tools for both regenerative medicine and therapeutic discoveries.Mesenchymal stem cells, characterized by their ability to differentiate into skeletal tissues and self-renew, hold great promise for both regenerative medicine and novel therapeutic discovery. However, their regenerative capacity is retained only when in contact with their specialized microenvironment, termed the  Unlike embryonic stem cells (ESCs), MSCs must tightly balance specialization in response to regenerative demand and retention of a stem cell pool throughout life and this balance is controlled by the niche environment e.g. the bone marrow . FurtherHowever, since their discovery in 1974  there hain vitro and in vivo with their ability to form bone and cartilage as well as through their immunomodulatory capacity; indeed, this capacity is finding use in e.g. islet transplant procedures [MSCs are easily harvested from autologous bone marrow, fat or umbilical cord and have shown promise ocedures . Autologstem cell niche. First described four decades ago by Schofield [MSCs mediate the fine balance between differentiation, proliferation, self-renewal and quiescence through cell-intrinsic regulatory networks . Howeverchofield , it refechofield , neural chofield , epidermchofield ,20 and cchofield  and intechofield . The perBox 1.Box 1. Model of the organization of stem cells and tissue-specific mesenchymal progenitors. Multipotent stem cells exist in homeostasis between quiescence and an activated state. Activation, and entry to the cell cycle, occurs upon tissue damage or other physiological stimuli. Upon activation (regenerative demand), multipotent progenitor cells with transit amplification capacity arise. These progenitors are the precursors to the tissue-specific mature cells, for example osteoblasts, adipocytes and chondrocytes in the mesenchymal stem cell compartment. The activated stem cell and its daughter cells differentiate or can return to a quiescent state once the tissue repair or other physiological process is complete. This homeostasis between quiescence and self-renewal is tightly regulated to avoid transformations and to retain a viable stem cell pool throughout the life of the organism [organism . , biofabrication  and bioreactor  techniques hold the potential to allow us to construct, deconstruct and investigate the important components of cellular microenvironments. Such approaches could evolve the development of both reductionist stem cell interfaces allowing high throughput analysis and discovery and, perhaps more importantly, non-animal technologies (NATs) that recreate tissue complexity and reduce costly/inefficient animal experimentation. The challenges, however, are great. The niche, as highlighted in l trials . This isl trials , built uHere we review recent progress in the area and give a forward look on the development of artificial niches, with particular focus on MSCs. First, we discuss how biomaterial technologies have developed our understanding of cell–substrate interactions, and consider important factors in a cells' niche that allow us to differentiate stem cell populations for potential use in regenerative medicine. We then discuss how this understanding has led to recent advances in harnessing the capacity of stem cell self-renewal for prospective use in stem cell transplants and for immunosuppression. Finally, we provide an outlook on how combinations of such techniques provide opportunities for the generation of complex artificial stem cell niches.2.Stem cell niches maintain self-renewal/quiescence . However2.1.To understand how properties of the biomaterial interface, such as stiffness, topography and chemistry can regulate cell behaviour, we must first consider how cells adhere to substrates. The architecture of a cell's microenvironment contains stimuli ranging from the micro to the nanoscale; microscale features are in the range of the size of the cell itself and result in whole-cell responses such as alignment of cells with topographical features, known as contact guidance . HoweverCell adherence to substrates is typically through integrins, transmembrane receptors that tether to the ECM, which itself forms an intermediate layer of proteins adsorbed on the surface of materials exposed to serum ,37. InteControl over adhesion size has been achieved using lithography strategies to create nanopatterned substrates with controlled size, shape, spacing and symmetry in a variety of materials; patterns have included nanopits –45, nanoSuch alterations in cell adhesion, cytoskeletal organization and mechanotransductive cell fate changes are likely to be driven by the topography–protein interface. Indeed, if fibronectin (FN), a major cell-adhesive protein of the ECM is absorbed onto nanopit patterned surfaces, FN adsorbs within the pits and it was seen that cells probed these pits with filopodia, leading to ‘nanoimprinting’ of the pits on the cell membrane, an effect that was not observed when the substrate was not coated in FN . NanoimpCell adhesion and subsequent spreading that governs size and shape influence physiological processes such as cell survival . Using mTopographical RGD coupled substrates have been used to decouple adhesion requirements for cell spreading. As integrins ligate, they are coupled to the actin cytoskeleton. Through activation of G-proteins inducing actin–myosin contraction, the integrins are gathered together leading to mature adhesion formation comprising many integrins. Using nanocolloid particles with one RGD motif covalently linked to each colloid, it has been demonstrated that when RGD is at a density of less than 70 nm apart, integrin gathering can occur; above this density integrins cannot gather together and form mature adhesions ,60. Usinb).This topographical control over how cells adhere to substrates has been used to control either self-renewal or osteogenic differentiation of MSCs, using topographies that can be remarkably similar. For example an electron beam lithographically defined pattern that permits out-of-niche self-renewal is comprised of pits with a diameter of 120 nm, depth of 100 nm and centre–centre spacing of 300 nm in a square lattice (SQ) . Adding figure 3Osteogenic nanotopographies were originally developed with orthopaedic implants in mind, where patterns alone would guide cell fate. Such implants are typically made from titanium and its alloys. For example, hip replacements are stabilized by stems into the bone marrow of the femoral canals. The bone marrow, once disrupted, can no longer function effectively as a niche and the stem cells differentiate in response to the implant, but mainly into soft tissue forming fibroblasts. Fibroblasts are the default differentiation of the MSC, with MSCs originally identified as fibroblastic colony forming units . This so2.2.The ECM and surrounding cell junctions have a major physical influence in transmitting forces between cells, which ultimately regulate intracellular signalling pathways and therefore fate ,66,76,77Intrinsic and extrinsic mechanical forces guide early embryo development, even from as early as the blastocyst stage, when remodelling of cell–cell junctions is driven by intrinsic cell forces to relieve tension as the embryo transitions through germ-band elongation. This remodelling of junctions is not determined by external forces at tissue boundaries but rather depends on myosin II-dependent spatial reorganization leading local forces at cell boundaries . Later iet al. used polyacrylamide (PAM) gels of tuneable matrix stiffness to guide stem cell fate through distinct tissue lineages; neural at 1 kPa, muscle at 12 kPa and bone at 30 kPa. This simple tuneable system has somewhat set the pace over the last decade for the unravelling and exploiting the biological mechanisms linked to mechanoregulation of MSCs [Much research on MSCs, has previously been carried out on tissue culture plastic, or on other stiff and planar substrates. However, many stem cell niches often have low stiffness and are three-dimensional. Hydrogel systems are optimal for investigating mechanobiology due to their unique properties. Natural or synthetic polymers can be physically or chemically cross-linked in a controlled manner to produce hydrogel systems with tuned degradability, hydrophilicity and stiffness. The water that fills the space between the macromolecules leads to a degree of flexibility similar to that of natural tissues, making them both biocompatible and biomimicking . Thus, h of MSCs . Recent  of MSCs , prolife of MSCs  and spre of MSCs .in vivo exhibit gradients of stiffness, such as those at tissue junctions or at pathological boundaries, e.g. neuromuscular junctions and the tumour boundary [−1. This allows for an in vitro model spanning the in vivo physiological and pathological range that can facilitate investigation of a range of mechanical signals on one surface [Physiological interfaces boundary . Isolateboundary ,88. Cancboundary ,90. Multboundary . These o surface . Other s surface .Elucidating the mechanism behind what micro- and nanoscale properties cells sense at the cell–material interface of substrates of given stiffnesses has been a matter of some debate. Differentiation of MSCs on polydimethylsiloxane (PDMS) and PAM hydrogels of varying stiffness (0.1 kPa–2.3 MPa) with a covalently attached collagen coating were used to show that cell spreading and differentiation were unaffected by differing PDMS stiffness, with MSCs committing to osteogenic lineages. PAM substrates of high elastic modulus also exhibited osteogenic differentiation of MSCs, whereas this potential was lost on softer PAM substrates, decreased ERK/MAPK signalling was observed, suggesting MSCs were unable to form stable focal adhesions at this low modulus. Trappmann and colleagues used differences in ECM tethering to explain this effect, pore size on PAM substrates, but not PDMS, decreased almost twofold on soft substrates, proposing that varying pore size alters collagen tethering and thus local stiffness i.e. lower collagen anchoring density enhanced cell spreading and therefore differentiation .On the other hand, work from the Engler group systematically modulated porosity of PAM substrates, without altering stiffness, to control ligand–substrate tethering. MSCs were cultured on these substrates and stiffness was indeed found to be the driving factor for cellular spreading and differentiation. Varying the degree of ECM–protein anchoring reaffirmed this indicating bulk stiffness of two-dimensional matrices as the main driver of cell response, irrespective of protein tethering and porosity .a). YAP/TAZ activation, until recently, was thought to be exclusive to the Hippo signalling pathway [Despite the exact rules for mechanical control remaining to be fully resolved, elucidation of cell signalling is emerging. Osteogenesis of MSCs on stiff substrates is regulated by the nuclear translocation of Yes-associated protein and its transcriptional co-activator (YAP and TAZ), which consequently activates the osteogenic transcription factor runt-related transcription factor 2 (RUNX2) . Preservfigure 3 pathway , however pathway . By vary pathway . TherefoAs substrate stiffness correlates to the expression and localization of cadherins and integrins, and therefore cell–cell and cell–matrix adhesions, this mechanosensitive mechanism can be exploited to modulate biochemical signalling –99. One Mechanical memory may have importance and implications for cell–material interactions, but also for clinical use. MicroRNAs (miRNAs) are short, single-stranded, non-coding RNAs. They bind one or more messenger RNA (mRNA) and therefore regulate protein expression; typically, miRNAs attenuate the ability of mRNAs to be translated to protein by binding the mRNA sequence . MiRNA21Recently, a more counterintuitive mechanism of stiffness related integrin–ligand interactions has emerged. The ‘molecular clutch model’ has long been proposed to explain actin cytoskeleton and cell migration dynamics and has now been employed to explain cell mechanotransduction . Using get al. linked the mechanical tension exerted on a cell through tissue specificity and ECM stiffness to changes in transcriptional programmes. They proposed a model supporting a physical link between the nucleus and mechanical properties of the extracellular environment, whereby tension from the ECM lead to biophysical changes in the cell cytoskeleton on the nucleus [et al. showed that environmental stiffness can change the transcription of the gene LMNA, and the stability of its protein lamin-A. In response to increased cellular tension, turnover of lamin-A is reduced leading to physical stiffening of the nucleus and stabilization of the nuclear lamina and chromatin, and thus accumulation of YAP [Other key stiffness-related mechanisms are also becoming elucidated with the use of biomaterial strategies. In 2013 Swift  nucleus . Cells d nucleus , and alt nucleus . Swift en of YAP .To describe how the cytoskeleton can transmit such forces, elegant, tension based, cell-tensegrity models have been proposed by Inger –108. Hisin vitro modelling systems.The last decade has focused heavily on tuning and defining matrix stiffness of biomaterial niches; it has been a key facet in controlling cellular fate through traction forces. However, what is often neglected is that tissues and organs of the body are often not purely elastic. Non-degradable hydrogel systems capture some aspects of the physiological ECM environment, however reconstituted ECMs such as collagen and fibrin, and tissues such as adipose tissues, brain, liver, fracture haematoma, and the soft callus of regenerating bone, are all temporally viscoelastic . The micBecause of this, there is now focus on the effects of three-dimensional matrices on MSC differentiation that go beyond bulk material mechanical properties. MSCs encapsulated in alginate of varied stiffness (2.5–110 kPa), presenting RGD motifs at various clustering densities, highlighted that stiff substrates that hindered cell spreading still enabled osteogenic commitment. Hence, rather than cell morphology it is the cells ability to recognize and cluster adhesion ligands, thus generating traction forces, that drives cell fate . This suAnother key example combines both three-dimensional culture with matrix degradability; a tuneable HA based system that either permits or prevents cell-mediated degradation was used to show cell fate is morphology-independent when the matrix is degradable, but is instead directed by cell-mediated traction forces. Local degradability allowed cells entrapped in gels to reach otherwise unavailable adhesive ligands, rearranging their cytoskeletal structure to generate traction forces that potentially lead to osteogenic commitment, irrespective of morphology or bulk matrix stiffness . These tin vivo-like mechanical behaviour, and is also important when considering MSC mechanical memory [Fibre architecture has also been highlighted as a mechanism for transducing matrix stiffness, where lower fibre stiffness allow cells to recruit nearby fibres and correlates with enhanced cell spreading and proliferation . Dynamicl memory . The impl memory . The inc2.3.t-butyl groups, and osteogenesis with charged phosphate groups [Chemistry-based strategies to engineering substrates have also been employed, changes in cell adhesion to substrates can be controlled by manipulation of properties of materials such as surface wettability ,123, ande groups . Howevere groups –129 and e groups ,131, it in situ. Photodegradation of the hydrogel network changes the physical three-dimensional microenvironment of the encapsulated cells and can lead to migration or lineage commitment [Commonly used PEG hydrogels can be modified to contain photodegradable cross linkers, allowing the user post-gelation control over substrate properties such as gel conformation and biochemical composition with MSCs mmitment . Incorpommitment . Additiommitment .Similarly, enzymatic activation was exploited in a 2D PEG based system. Here RGD groups were attached to a low-adhesion PEG or Fmoc (fluorenylmethyloxycarbonyl), the ‘blocking group’ via an elastase cleavable linker, with the blocking group preventing easy access to the RGD moieties for cell integrins. With the blocking group in place, adhesion and intracellular tension were lowered sufficiently to support MSC self-renewal. However, upon elastase cleavage of the dialanine (AA) linker, cells could access the RGD more easily, form larger adhesions supporting greater intracellular tension, driving osteogenic differentiation .in vitro, and improve osteointegration of titanium implants in vivo [Other chemical approaches aim to create ECM-mimetic materials, the FN RGD adhesion domain has been incorporated into many materials, however, when it is presented with its synergy domain PHSRN it enhances the affinity of integrin binding over 40-fold . This FN in vivo –138. Suc in vivo . Titaniua), meaning this molecule could also be exploited as an approach to deliver GFs. GFs have been widely employed in the clinic, due to their capacity to regulate cell growth, healing and stem cell differentiation, however soluble GF administration usually means application of supraphysiological doses which can lead to serious off-target side effects [b) [in vivo [in vivo [In addition to its integrin binding domains, FN also contains a highly promiscuous GF binding domain in its 12th to 14th type III repeats  figure a, meanin effects . One stu effects . This reects [b) . This ap[in vivo . This sy[in vivo , and fur[in vivo .Figure in vivo, immobilization also permits a more biomimetic recapitulation of stem cell niches [c). Controlled tethering of such ligands to synthetic materials has proved difficult [et al., were a poly(ethylene oxide) (PEO) based co-polymer was used to covalently tether epidermal growth factor (EGF) to the substrate, a GF associated with tissue repair. Specific tethering at the N-terminus retained EGF bioactivity but restricted it to the surface. EGF tethering led to signalling through its receptor EGFR, in a manner akin to physiological-like matrix-embedded EGF, where EGFR is stimulated but not internalized [Immobilized GF technologies not only permit lower doses of the GF to be employed (thus being more cost effective than solubilized GF delivery) but as many of these ligands act at interfaces l niches ,145  [As ll level ,167. Onell level . This apll level . More res (MPPs) .Vβ3 and αVβ5 with adsorbed vitronectin promoted hPSC self-renewal for long-term culture periods [Similar microarray platforms have been developed to screen for self-renewal promoting materials discovery. Both human embryonic and induced pluripotent stem cells (hPSC) have the ability to self-renew indefinitely in culture, and hence hold great promise for drug discovery and regenerative medicine –174. How periods . Scalabi periods , however4.in vitro to create more tissue-like systems, as there is a strong need for more humanized, NATs  for drug discovery, disease modelling and regenerative medicine.Many of the approaches discussed above are reductionist in nature and have been key to elucidating and understanding key mechanisms for how cells sense and respond to their complex microenvironment, however, while these approaches are easily scalable and robust, they lack biological functionality. Recent material strategies are now beginning to re-build the complexity of the niche For example, one study designed a synthetic hydrogel (PEG based) system to define the ECM parameters that govern ISC expansion and organoid formation. Organoid formation is through self-organization of stem cells into structures that resemble their native multicellular architecture and many of their functional features. Here, by designing a matrix that was mechanically dynamic, it allowed for optimal ISC expansion , and subsequently for differentiation and intestinal organoid formation . This dynamic substrate design could thus support the need for the changing mechanical environments throughout the native cellular evolution during organ development . AnotherFurther to organoids, organ-on-a-chip (OOC) platforms seek to recapitulate the multifaceted tissue-specific cellular microenvironment in three-dimensional settings for a given organ system. OOC aims not to build a whole living organ, but to establish a minimally functional unit that is representative of aspects of human physiology in a controlled system . SeveralOn the other hand, microwell formats that recapitulate cell–cell and cell–matrix interactions typical of stem cell niches will be valuable for analysis of heterogeneous populations of stem cells at the single cell level. Integration of material technologies and protein patterning will be crucial in fabrication of microwell arrays, these will allow high throughput screens to identify ECM molecules relevant to ‘niche-like’ regulation. By developing arrays that mimic cell–cell interactions through peptide presentation, single or combinations of molecules can be identified that induce effects on cell behaviours, such as self-renewal and lineage commitment ,170. Typin vitro and pre-clinical in vivo screens. It is envisioned that technologies such as many discussed above will solve these issues, by engineering validated NAT models that can more readily predict drug targeting and tissue dose exposure, and hence leads with high risk of failure can be dropped quickly—before expensive clinical trial stages. As discussed above, biomaterial engineered systems can easily introduce complex physiological-like parameters, removing problems of overly simple in vitro cell lines and in vivo models . Currently, regulators are encouraging the development of tissue models, such as organoids, to help provide ‘humanized’ data alongside traditional in vivo data [Technologies such as these will prove valuable for the pharmaceutical industry, as although in recent decades advances in the molecular understanding of diseases has underpinned new potential therapeutic targets there is a lack of corresponding increase in drug discovery and manufacture. This is irrespective of substantial increases in research and development investment, which led to a ‘volume research’ approach using new genomic and high-throughput technological approaches, and yet productivity has remained static . This frivo data . Furtherivo data –197.NAT development will also play a key role in the advancement of the use of stem cells for clinical applications in regenerative medicine. Current culture systems rely on non-human (xeno) serums in culture mediums or feeder layers to support SC survival and expansion. However, these xeno systems can induce as immune response upon clinical transplantation, for example, FBS is a potential source of pathogens, such as endogenous retroviruses and xenoepitopes, and batches of the feeder layer Matrigel have been found to be contaminated with lactate-dehydrogenase-elevating virus (LDV). Further to this, xeno-derived components suffer greatly from batch-to-batch variability. This has driven searches for serum-free and feeder-free culture systems, although current systems contain several recombinant proteins or inhibitors whose long-term effects are yet to be understood. Suggesting further, the need for more humanized models that remove contaminating animal factors, it is highly anticipated that this can be approached by the advancement in understanding of the stem cell niche with biomaterial and biofabricated systems.In conclusion, the future for materials as biological tools to dissect stem cell functions, as the building blocks to develop NATs to accelerate drug screening and to underpin regenerative medicine is very promising.


Cross-talk between adipose tissue and central nervous system (CNS) underlies the increased risk of obese people to develop brain diseases such as cognitive and mood disorders. Detailed mechanisms for how peripheral changes caused by adipose tissue accumulation in obesity impact the CNS to cause brain dysfunction are poorly understood. Adipokines are a large group of substances secreted by the white adipose tissue to regulate a wide range of homeostatic processes including, but not limited to, energy metabolism and immunity. Obesity is characterized by a generalized change in the levels of circulating adipokines due to abnormal accumulation and dysfunction of adipose tissue. Altered adipokine levels underlie complications of obesity as well as the increased risk for the development of obesity-related comorbidities such as type 2 diabetes, cardiovascular and neurodegenerative diseases. Here, we review the literature for the role of adipokines as key mediators of the communication between periphery and CNS in health and disease. We will focus on the actions of leptin and adiponectin, two of the most abundant and well studied adipokines, in the brain, with particular emphasis on how altered signaling of these adipokines in obesity may lead to cognitive dysfunction and augmented risk for Alzheimer’s disease. A better understanding of adipokine biology in brain disorders may prove of major relevance to diagnostic, prevention and therapy. Lifestyle, nutrition and lack or inefficient exercise to control body weight is making the global population more susceptible to obesity. Food with high sugar and carbohydrate contents are in general cheaper and more readily available for the current busy lives. These factors contribute to a widespread epidemic of overweight and obesity, metabolic disorders characterized by the accumulation of peripheral and/or visceral adipose tissue . ObesityThe obese body accumulates adipose tissues broadly classified in white (WAT) and brown adipose tissue (BAT) . InitialIn the following sections, we review the literature for studies on the role of adipokines as the possible mediators of signals from the periphery to the CNS in obesity-associated brain dysfunctions. Specifically, we will focus on two of the most abundant and well studied adipokines – leptin and adiponectin – and their interaction with cognitive processes of the CNS in health and disease.Disproportional accumulation of white adipose tissue in overweight and obesity is accompanied by a generalized change in the circulating levels of several adipokines. Adipose dysfunction and adipokine dysregulation are thought to be responsible for or, at least, be an important contributor to the increased risk of obese people to develop a number of related diseases. For instance, increased levels of proinflammatory adipokines such as interleukin (IL)-1β, IL-6, TNFα and leptin, and decreased levels of anti-inflammatory adipokines, such as adiponectin, in obesity produce a chronic state of low-grade inflammation which promotes the development of insulin resistance and type-2 diabetes, hypertension, atherosclerosis and other cardiovascular diseases, and some types of cancer . MoreoveThe CNS is not exempt from negative effects of obesity, as adipose dysfunction associated with obesity have been linked to altered brain metabolism, neuroinflammation, neuronal dysfunction, brain atrophy, impaired mood and cognitive decline . StudiesObesity has also been linked to cognitive disorders. Obese individuals are under greater risk to develop age-related cognitive decline, vascular dementia, mild cognitive impairment (MCI) and AD . FurtherThe increased risk for obese individuals to develop CNS pathology reflects the capacity of adipose tissue to communicate with the brain and impact brain function. It is not clear yet how this cross-talk occurs, but growing evidence indicate that adipokines are involved. Adipokines may impact brain physiology through different mechanisms. Some adipokines such as leptin and TNFα can cross the BBB and act directly in the brain while other adipokines would act on endothelial brain cells, regulating BBB permeability and the access of other circulating mediators into the brain. Importantly, in pathological states such as inflammation, the BBB integrity is compromised allowing the penetration of adipokines and other substances to which the brain is normally inaccessible. Finally, local expression of adipokines such as leptin and adiponectin have been reported in the mammalian brain . AdvanceIn the following sections, we will examine key aspects of leptin and adiponectin – the two most abundant and well studied adipokines – regarding their roles in brain physiology and their involvement in obesity-related cognitive dysfunction, dementia and AD Figure .The discovery of leptin in 1994 is considered the cornerstone for adipokine research . Leptin Leptin is mainly produced by adipose tissue, more specifically by visceral white adipocytes in rodent and by subcutaneous adipose tissue in human . Howeverdb gene expression result in six different LepR isoforms (LepRa-f) (Leptin receptors (LepR or ObR) were first detected in 1995 at the mouse choroid plexus . PosteriLepRa-f) . Long leLepRa-f) . LepRs aLepRa-f) .in situ hybridization (LepRb is the main isoform of leptin receptor expressed in the brain. Robust expression comprehends the arcuate nucleus (ARC), the paraventricular nucleus and the dorsomedial, lateral and ventromedial regions of the hypothalamus . Extrahydization , but thidization . The hipdization .The wide variety of leptin effects in the CNS likely reflects the peculiar distribution of leptin receptors through different brain structures as well as the particular signaling cascades activated by these receptors in distinct regions .In the following sections, we review key aspects of brain leptin signaling that are relevant for its role in cognitive decline and AD Table .Synaptic plasticity is a crucial event for learning and memory process. Long-term potentiation (LTP) and long-term depression (LTD) of synaptic transmission are pivotal mechanisms in hippocampal memory formation and consolidation . At the There is substantial evidence indicating that leptin regulates hippocampal synaptic plasticity and memory. LepRs in hippocampal neurons are closely associated with somato-dendritic and synaptic regions, indicating the potential for leptin to modulate synaptic function . Recent Leptin was shown to regulated synaptic plasticity by targeting NMDA- and AMPA-receptors trafficking, particularly in the CA1 region of the hippocampus. Schaffer collaterals (SCs) and temporoammonic (TA) projections to CA1 pyramidal neurons exhibit an age-dependent response to leptin. Interestingly, these functionally distinct circuits appear to respond differently to leptin exposure. In SC-CA1 synapses, leptin was shown to induce depression of synaptic transmission in young rodents hippocampus  whereas,Several studies demonstrated that leptin responsiveness decreases with obesity, aging and neurodegenerative diseases, a phenomenon called leptin resistance. Leptin resistance affects a range of processes such as food intake, insulin sensitivity, inflammation and cognition. In obesity, leptin resistance leads to increased production of leptin by adipocytes and hyperleptinemia, in an attempt of the organism to compensate for low leptin responsiveness. Decreased leptin signaling in the CNS may be related to defective leptin transport across BBB, LepR downregulation and/or deficient leptin signaling downstream LepRs .Triglycerides can impair BBB leptin transport causing central leptin deficiency . FurtherAt the intracellular level, leptin signaling is negatively regulated by the suppressor of SOCS3 and by the PTP1B. SOCS3 binds to LepR and JAK2 to inhibit their activities, whereas PTP1B dephosphorylates tyrosine residues deactivating LepR and JAK2. PTP1B have been linked to central leptin resistance in humans  as well Central leptin resistance can also be mediated by downregulation of LepRs expression. In this regard, LepR levels were found to be decreased in the hippocampus of AD patients, whereas leptin levels were upregulated both in CSF and locally in the hippocampus, possibly due to a compensatory mechanism for receptor dysfunction . Age-depTaken together, the results described above indicate that impaired brain leptin signaling may play a role in AD pathophysiology, and that restoring leptin signaling may constitute a valid approach to restore synaptic function and cognition in AD.Several studies indicate that over-weight and obesity during mid-age increases risk to develop AD . In contEvidence regarding how leptin levels are affected in AD are controversial. Several studies show decreased leptin levels in CSF and plasma in AD patients  whereas Leptin signaling was found to interact with several mechanisms associated with AD physiopathology. Aβ disrupt leptin signaling leading to down-regulation of hippocampal leptin and LepR expression . ConversTau phosphorylation can be suppressed by leptin modulation of GSK3β activity . ImportaAdiponectin is a 30 KDa adipokine encoded by the AdipoQ gene, mainly produced and secreted by adipocytes and highly abundant in human plasma. Adiponectin is known to increase insulin sensitivity of target organs such as liver and muscle, ultimately regulating peripheral glucose and fatty acid metabolism , 2002. BAdiponectin naturally self-associate to form different types of aggregates. In plasma, adiponectin exists as trimers, hexamers or high molecular weight (HMW) multimers. Adiponectin trimers are mainly stabilized by non-covalent interactions, whereas larger aggregates require crosslinking between subunits by disulfide bonds . In addiAdiponectin acts through binding to three different receptors: adiponectin receptor 1 (AdipoR1), adiponectin receptor 2 (AdipoR2) and T-cadherin. AdipoR1 and AdipoR2 are the most abundant sites for adiponectin binding and mediate most of adiponectin actions through the organism , 2007. AData from epidemiological studies demonstrate that diabetes, obesity, and metabolic syndrome increase the risk of developing cognitive problems and dementia . More reAdiponectin is present in human and mouse CSF, albeit in much lower concentrations than in blood . HoweverAdiponectin receptors are also expressed in different structures of the brain. The hypothalamus is the best documented site for AdipoRs expression in the mammalian brain, but there is also evidence for expression in endothelial brain cells, cerebral cortex, brain stem and hippocampus , 2007 reAdiponectin functions in the brain are highly diversified. It has been shown to act locally in the brain to control key processes of brain physiology including neuronal excitability and synaptic plasticity, neuroprotection, neurogenesis and regulation of glial cells activation . More rein vitro and in vivo -induced LTP and suppresses low-frequency stimulation (LFS)-induced LTD. Furthermore, adiponectin administration alone induced a chemical LTP, independent of presynaptic stimulus . OsmotinCollectively, these recent studies consistently established a previously ignored ability of adiponectin to regulate hippocampal synaptic function and plasticity and to improve cognitive function, learning and memory. This further support a protective role for adiponectin in AD, suggesting that diminished brain adiponectin signaling in obesity may favor AD onset and progression and that adiponectin signaling may be an interesting target in AD therapy.in vitro and in vivo and can be prevented by treatment with insulin sensitizing drugs and by insulin itself (Neuronal insulin signaling is important for synaptic plasticity and memory, mainly by regulating glutamate receptors trafficking . Impairen itself . These dn itself . In thisn itself . Conversn itself . These dAdiponectin is well known for its anti-inflammatory activity in peripheral tissues, which include suppression of macrophage activation and secretion of pro-inflammatory cytokines , 2018. FAlzheimer’s disease is also characterized by a chronic state of low-grade inflammation in the brain. This response is mediated by microglial cells activation and secretion of pro-inflammatory cytokines TNFα, IL-6, and IL-1β . Proinflin vivo and in vitro (in vitro, via PPARγ activation (Adiponectin-knockout mice develop a series of pathological features in the brain resembling AD, including insulin resistance, reduced levels of synaptic proteins and the presence of neuroinflammatory markers such as microgliosis, astrogliosis and elevated levels of the pro-inflammatory cytokines TNFα and IL-1β . Environin vitro  in a mectivation . Furthertivation . Moreovetivation . Finallytivation . These ein vitro and in vivo, and represents an important pathological mechanism in synaptic failure and neuronal death in AD (reviewed in Neurodegeneration in AD is mediated by overactivation of glutamate receptors and excessive neuronal calcium influx, a process called excitotoxicity . Excitotin vitro and in vivo. In vitro, adiponectin was shown to protect SH-SY5Y human neuroblastoma cells against oxidative stress and cytotoxicity induced by Aβ and MPP+, an inhibitor of mitochondrial complex I (In the past few years, a growing number of studies demonstrated the neuroprotective properties of adiponectin and other AdipoRs agonists against a variety of neuronal toxic insults omplex I . Adiponeomplex I . A serieomplex I . These nThe antidepressant effects of physical exercise are well known and widely used in the clinic as a non-pharmacological treatment for depression. Recently, adiponectin was shown to play a crucial role in antidepressant effects of exercise, by mediating exercise-induced hippocampal neurogenesis . The neuRecent studies indicate that adiponectin signaling deficiency is sufficient to induce an AD-like phenotype in mice. Aged adiponectin-knockout mice recapitulate several aspects of AD pathology, including increased Aβ levels and deposition, tau hyperphosporylation, neuroinflammation, synapse loss, neuronal apoptosis and impaired insulin signaling. Importantly, aged adiponectin knockout mice also performed poorly in spatial memory and fear conditioning behavioral tests . These oConversely, impairment in adiponectin function is also observed in amyloid-based AD models, whereas activating adiponectin signaling reduces AD-like pathology. In APP/PS1 mice, changes in AdipoRs expression levels were less responsive to a stress-inducing paradigm as compared to wild-type mice . In the AdipoRon, an orally bioavailable small-molecule agonist of adiponectin receptors , was shoStudies relating adiponectin levels to AD in humans are controversial. Increased baseline adiponectin levels in plasma have been associated with a higher risk of women, but not men, to develop AD and other types of dementia . FurtherAvailable data regarding a possible association between adiponectin levels in blood and CSF to MCI and AD are conflicting and inconclusive. Therefore, further studies are warranted to reveal the potential use of adiponectin measurement for diagnostic purposes and its clinical relevance for the physiopathology of these neurological conditions. Worth mentioning, some studies found that the association between adiponectin levels and dementia can be sex-specific . This maObesity is pandemic in present days. Beyond its intrinsic complications, and the obvious social and psychological impact, obesity harms extend to a wide range of associated health conditions to which it represents a major risk factor. More recently, growing attention is being given to the impact of obesity on CNS function, as accumulating evidence indicate higher incidence of neurological disorders in the obese population. The mechanisms by which fat accumulation and adipose tissue dysfunction in obesity result in CNS pathology are poorly understood. It is widely accepted that dissection of such mechanisms will greatly improve our knowledge on the cross-talk between peripheral metabolism and brain physiology, and may provide novel targets for therapeutic intervention for prevention and/or treatment of neurological dysfunctions associated with obesity.Adipokines are secreted factors which carry regulatory signals from adipose tissue through systemic circulation to control a wide range of physiological functions throughout the human body. Not surprisingly, adipokine dysregulation in obesity results in the disruption of homeostasis in a variety of organs and systems, and underlie obesity complications and risk for associated chronic conditions. In this context, adipokines emerged as strong candidates to represent the mediators of pathological signals from adipose tissue to CNS in metabolic disorders.The studies reviewed here provide evidence supporting a role of leptin and adiponectin, two highly abundant and well characterized adipokines, as key mediators of obesity-related CNS dysfunctions Figure . We founLF-G and MV wrote the manuscript. MV designed the figure. LF-G, FDF, and MV planned the scope and reviewed the final manuscript.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.


Natural killer (NK) cells are essential in the early immune response against viral infections, in particular through clearance of virus-infected cells. In return, viruses have evolved multiple mechanisms to evade NK cell-mediated viral clearance. Several unrelated viruses, including influenza virus, respiratory syncytial virus, and human immunodeficiency virus, can directly interfere with NK cell functioning through infection of these cells. Viral infection can lead to immune suppression, either by downregulation of the cytotoxic function or by triggering apoptosis, leading to depletion of NK cells. In contrast, some viruses induce proliferation or changes in the morphology of NK cells. In this review article, we provide a comprehensive overview of the viruses that have been reported to infect NK cells, we discuss their mechanisms of entry, and describe the interference with NK cell effector function and phenotype. Finally, we discuss the contribution of virus-infected NK cells to viral load. The development of specific therapeutics, such as viral entry inhibitors, could benefit from an enhanced understanding of viral infection of NK cells, opening up possibilities for the prevention of NK cell infection. Natural killer (NK) cells are innate lymphocytes that represent the first line of defense against tumor cells and viral infections ,2. The iNK cells have multiple mechanisms to kill virus-infected cells, including the engagement of extracellular death receptors and exocytosis of cytolytic granules . To mediActivation of NK cells is regulated by a balance in the engagement of its activating and inhibitory receptors in combination with the presence of certain cytokines. Together, these stimuli determine the type and strength of NK cell activity .Healthy cells inhibit NK cell activation mainly through the expression of major histocompatibility complex class I (MHC-I) molecules, which interact with inhibitory receptors present on the NK cell surface. Inhibitory NK cell receptors that ligate to MHC-I include killer cell immunoglobulin-like receptors (KIRs) and leukocyte immunoglobulin-like receptors (LILRs) . This inThe activating receptors that are expressed by NK cells facilitate activation upon detection of viral or stress-induced ligands on target cells. For example, the natural cytotoxicity receptors (NCRs), including NKp46, NKp44, and NKp30, are known to bind viral glycoproteins ,11, alloDue to the important role of NK cells in the early antiviral immune response, viruses have evolved numerous strategies to evade NK cell effector functions. One of these evasion strategies is the manipulation of NK cells through direct infection. In this review, we provide a comprehensive overview of the viruses that have been reported to infect NK cells. We discuss their mechanisms of entry, describe how they affect NK cell function, and indicate which viruses deplete NK cells through the induction of apoptosis. Moreover, we address the contribution of infected NK cells to viral load.Viruses have evolved many mechanisms to enter host cells. The best-known mechanism is entry through binding to specific receptors, which either leads to fusion directly at the plasma membrane, or to entry following clathrin- or caveolin-dependent endocytosis of the viral particle. Additionally, viruses may require direct cell–cell interactions for infection. Finally, viral entry may occur through macropinocytosis, which is the nonspecific uptake of extracellular material. Hence, non-specific binding of virus can lead to internalization, even in the absence of specific viral entry receptors on the target cell. Macropinocytosis has been described for multiple viruses , but to One of the entry pathways for viruses is through binding of cellular receptors by viral proteins, which leads to receptor-mediated entry. NK cells express multiple receptors that are known to be used for virus entry. Additionally, it has been described that NK cells acquire entry receptors from infected cells, either through direct contact via the immunological synapse or by exosome transfer (extensively reviewed in Reference ).Human influenza A virus (IAV) is an enveloped virus that has been shown to directly infect both human and mouse NK cells in vitro ,15. In vHuman immunodeficiency virus 1 (HIV-1) has also been shown to infect human NK cells both in vitro  and in vEpstein Barr virus  is a lymphotropic virus that is present in a variety of lymphocyte neoplasms including NK cell lymphoma , indicatThe receptor-mediated entry of IAV, HIV, and EBV into NK cells indicates that NK cells are permissive for infection due to expression or acquisition of virus entry receptors. However, for many other viruses known to infect NK cells, no receptor has been identified yet. Our group recently showed that RSV can also infect NK cells in vitro and that infection is enhanced in the presence of sub-neutralizing concentrations of RSV-specific antibodies . This stNK cells closely interact with infected cells, for example through the interaction between death receptors and their ligands, in order to clear the viral infection. During this interaction, they may acquire membrane fragments that contain entry receptors for viruses. This transfer of entry receptors between cells could take place via the immunological synapse, or via intercellular transfer of exosomes (reviewed in Reference ). In add+ T cells and NK cells [The human T cell leukemia virus 1  is known to require cell–cell interactions for infection, as infected cells produce virtually no cell-free infectious viral particles . In an eNK cells . EarlierNK cells .Vaccinia virus  was also proposed to spread to NK cells through cell–cell contact ,44. ThisTwo other cell-associated viruses are herpes simplex virus  and varicella zoster virus . HSV was shown to enter NK cells after co-culture with infected fibroblasts . IncubatAlthough some of these viruses are known to produce virtually no cell-free infectious viral particles, an indirect effect of other infected cell types cannot be ruled out. Infected cells may secrete multiple soluble mediators  that could prime NK cells and make them more prone to infection. Significantly higher NK cell infection rates are seen after stimulation with IL-2 for VZV, and after stimulation with IL-15 for RSV, compared to unstimulated cells ,42 [unpuThere are many more viruses found to infect NK cells, but research into their entry mechanisms is lacking. DNA of the asymptomatic Torque teno virus , also known as the transfusion-transmitted virus, has been found in NK cells of viremic individuals . RNA froBecause of the prominent role of NK cells in antiviral immunity, viruses have evolved multiple evasion strategies that affect NK cell effector functions and phenotype. Both these aspects are addressed below and are summarized in Inhibition of cytotoxicity is the most prominent effect caused by viral infection of NK cells. In vitro infection by IAV results in decreased cytotoxic function and induction of apoptosis in both adult and cord blood NK cells ,15,38. ASimilar effects were demonstrated for VV infection, which also leads to decreased cytotoxicity . HoweverDecreased ability to lyse target cells has also been reported for HSV and MV-infected NK cells ,40. The Besides their cytotoxic potential, NK cells are also prominent cytokine and chemokine secreters. Multiple viruses have been shown to affect cytokine secretion. In the presence of IAV, a downregulation was observed for the production of multiple proinflammatory cytokines and chemokines including IFN-γ, GM-CSF, MIP-1α (CCL3), MIP-1β (CCL4), and RANTES (CCL5) . On the Besides IAV, there are more viruses that increase or suppress NK cell IFN-γ production. RSV-infected NK cells have an increased IFN-γ production, compared to RSV-exposed uninfected or mock-infected NK cells . The comTaken together, it seems that viruses have evolved multiple mechanisms to inhibit NK cell cytotoxicity or induce apoptosis, which could serve as an important immune evasion strategy. Additionally, the effect of viruses on effector cytokine production by NK cells can vary per virus, leading to variable polarization of the NK cells and (ultimately) in skewed steering of other immune cells.Virus infection does not necessarily modulate NK cell effector functions directly, but often causes changes in receptor expression. As the balance in activating and inhibiting receptor signaling is crucial for NK cell activity, these phenotypical changes may indirectly influence effector functions. Additionally, the phenotype of infected NK cells can be altered by changes in morphology or even transformation into malignant cells.Downregulation of the expression of activating NK cell receptors has been described for RSV . IndepenChanges to the morphology of NK cells have been reported following EBV infection in vitro, where infection of NK cells caused cell deformation and increased size . MoreoveThese studies indicate that each virus induces a different phenotype, often skewed towards an NK cell state that promotes the establishment of a successful infection. Replication of the viral genome, transcription of viral proteins, and subsequent release of new infectious virions are necessary for virus spread. Since NK cells are infected, they may also contribute to the viral load. A significant role for production of virus particles within NK cells has not been described for the majority of viruses mentioned in this review. Nevertheless, in vitro studies have observed (limited) production of infectious viral particles by NK cells infected with HIV-1 ,46, VZV Accumulating evidence demonstrates that NK cells can serve as target cells for viral infections. Viruses have evolved multiple mechanisms to enter NK cells, change the cellular effector function and affect the subsequent immune response. Literature discussed in this review shows that viruses from nine different families can infect NK cells, indicating this is not an uncommon phenomenon. An overview of the most thoroughly studied viruses that infect NK cells and their effect on NK cell function is shown in To date, almost all viruses that have been found to infect NK cells are enveloped viruses, with the exception of TTV. This suggests that there is a bigger (evolutionary) advantage for enveloped viruses compared to naked viruses to infect NK cells and modulate anti-viral NK cell immunity. Cells infected with enveloped viruses display viral proteins on their membrane, making them prone to ADCC by NK cells . TherefoIt is apparent that viruses that cause systemic infections will encounter NK cells during an infection. However, infections with viruses like influenza and RSV are restricted to the lungs. Consistent with their important immunological role NK cells are distributed throughout the whole body, including the lungs . MoreoveGenerally, only a proportion of NK cells is infected in in vitro experiments. For viruses that enter NK cells through target cell interactions, the proximity of NK cells with infected cells seem to be the main determinant for infection. In the case of receptor-mediated entry, limited expression of specific entry receptors may result in non-homogeneous infection of NK cells, although other factors or phenotypic alterations may also be responsible. As NK cells are among the first cells to react to viral infections, inhibition or skewing of a small subset may already have a significant impact on the subsequent immune response. This is supported by increasing evidence showing that NK cells have an important role in shaping the adaptive immune response to viral infections . ThiA caveat of multiple studies that investigate NK cell infection is the lack of a clear distinction between the effect of infection as opposed to mere interaction. Incubation of NK cells with viral particles can lead to modulation of NK cell function in absence of infection. The mere interaction between viral particle and NK cell can be enough to trigger or inhibit certain effects. Whether infection is a prerequisite for NK cell modulation, can be studied using UV-inactivated viruses, as has been done for VV , or in tSeveral questions remain unresolved and need further investigation. The contribution of infected NK cells to disease severity and viral load remains unclear. Answering this question is challenging, as isolation of  cell subsets from patients is often impossible, making it difficult to demonstrate infected NK cells in vivo. The potential use of animal models is limited, as NK cell receptor repertoires differ between species. Moreover, many viruses are host-specific and infection in animals may not mimic the human situation. It is essential to expand our knowledge on possible cellular targets and reservoirs of viral infections. Intervention strategies to treat viral infections, such as viral entry inhibitors, may have different effects on particular cell types, as was demonstrated for influenza in murine NK cells . Hence,


The majority of children undergoing Hematopoietic Stem cell Transplantation (HSCT) require conditioning therapy to make space and prevent rejection of the donor stem cells. The exception is certain children with Severe Combined immune deficiency, who have limited or no ability to reject the donor graft. Transplant conditioning is associated with significant morbidity and mortality from both direct toxic effects of chemotherapy as well as opportunistic infections associated with profound immunosuppression. The ultimate goal of transplant practice is to achieve sufficient engraftment of donor cells to correct the underlying disease with minimal short- and long-term toxicity to the recipient. Traditional combinations, such as busulfan and cyclophosphamide, achieve a high rate of full donor engraftment, but are associated with significant acute transplant-related-mortality and late effects such as infertility. Less “intensive” approaches, such as combinations of treosulfan or melphalan with fludarabine, are less toxic, but may be associated with rejection or low level chimerism requiring the need for re-transplantation. The major benefit of these novel approaches, however, which we hope will be realized in the decades to come, may be the preservation of fertility. Future approaches look to replace chemotherapy with non-toxic antibody conditioning. The lessons learnt in refining conditioning for HSCT are likely to be equally applicable to gene therapy protocols for the same diseases. Conditioning is required for the majority of children with non-malignant diseases.Some children with severe combined immune deficiency require little or no conditioning—but this usually only corrects T cell function.Mixed chimerism may indicate that re-transplantation may be required.○ Less toxic.○ Associated with more mixed chimerism and may require further procedures.○ May allow long term preservation of fertility.Less intensive conditioning is:conditioning therapy plays a vital role in allowing engraftment of new Hematopoietic progenitor cells (HPC) in the patient. These new HPC can correct some, or occasionally all, of the manifestations of a non-malignant disease.With the exception of some children with severe combined immunodeficiencies , all patients who undergo allogeneic Hematopoietic Stem cell Transplantation (HSCT) require therapy prior to receipt of the graft. This myeloablative combinations (MAC) achieved prolonged aplasia, and were associated with full donor chimerism (DC). However, such therapy is associated with a significant burden of early and late toxicities, making MAC less suitable for older patients, or those with significant co-morbidities. This led to the concept of reduced intensity (RIC) regimens, which are defined as regimens containing reduced doses of myeloablative drugs (or radiotherapy), which are therefore less likely to achieve marrow ablation and more likely to produce mixed chimerism (MC). For the vast majority of HSCTs, namely older adults with malignant disease, the balance between MAC and RIC is a clear trade off: more transplant–related mortality (TRM) is seen with MAC, and more relapse with RIC; these 2 often counter-balancing each other. Multiple attempts to define RIC in terms of specific drug doses were made during 2006–2009, however, there is a spectrum of conditioning and it is preferable to define truly non-myeloablative or minimally intensive conditioning protocols (MIC) where the initial neutrophil recovery is frequently recipient, MAC protocols which mostly achieve sustained full donor chimerism, and then RIC protocols which comprise all those in-between , with a partial or complete inability to reject a graft, can achieve MC or full DC with no conditioning or RIC. The first successful allogeneic transplantation reported was performed without conditioning (or graft-versus-host disease prophylaxis) in a patient with X-linked Severe Combined Immunodeficiency (SCID) in 1968 . AlthougFor decades, a combination of myeloablative doses of busulfan (Bu) and cyclophosphamide was used for many children with a wide variety of non-malignant diseases. Its efficacy, in reliably achieving full DC, is not questioned. However, the profile of toxicity, in particular with endothelial injury, most often manifested as veno-occlusive disease (VOD) means we have continued to look for alternatives.Reducing the Bu dose and increasing the immunosuppression, e.g., by replacing cyclophosphamide with fludarabine (mirroring the RIC approach), has produced good results , particureduced toxicity” was first used to describe a Treo-based regimen (Substituting treosulfan (Treo) for Bu is undoubtedly less toxic, with a low rate of VOD. Although the term “ regimen , the com regimen . Treo ma regimen , 9. This regimen –12, for  regimen . In addi regimen  and a va regimen .The combination of Flu with Melphalan was the first widely used RIC protocol in PID . HoweverIn addition to the conditioning therapy given to allow engraftment of donor cells, it is also important to prevent allogeneic T-lymphocytes from the graft reacting against the donor (or host), which causes graft-versus-host disease (GvHD).ex-vivo Sheep red cell rosetting and soybean lectin . In general, the more myeloablative combinations are associated with gonadal failure and infertility; whereas the RIC combinations may be associated with preservation of these important long term toxicities. Currently, long term follow up data for children treated with Treo are lacking, but early data looks promising .The development of malignancy is another major long term consequence of transplant, But the majority of these appear to be secondary malignancies in patients transplanted for malignant disease, with the exception of Fanconi anemia.As mentioned above, for the sick infant with newly diagnosed SCID, an unconditioned graft may be required, to achieve some T-lymphocyte function and control of infection, and then allow an older child in better shape to tolerate a conditioned graft and achieve a better functioning (donor) immune system. With the adoption of newborn screening for SCID, younger, well infants are being diagnosed earlier, at an age where the use of conventional chemotherapy is less studied and with diagnoses (such as Artemis) where the adverse consequences of chemotherapy are well-known . To achiPreclinical studies identified CD45  and CD11A synergistic combination of two unconjugated rat anti-human CD45 antibodies has been tested in two clinical studies. A Phase 1 dose escalation study used these mAbs in patients with hematologic malignancies that received standard conditioning  and as pClinical testing of radioisotope-labeled anti-CD45 mAbs are in the advanced phase in adults with myeloid disorders, and second generation agents continue to be developed  CD45 antAntibodies that target the cell surface receptor CD117 can also effectively clear host HSC niches. CD117 is a receptor tyrosine kinase expressed on the surface of HSC and early hematopoietic progenitors. Interaction of CD117 with its ligand, stem cell factor (SCF), provides signals necessary for HSC survival, proliferation, and differentiation. Treatment of mice with anti-CD117 mAbs results in transient depletion of HSC with return of normal blood formation within 2 weeks of antibody administration . This wiA humanized anti-CD117 mAb, AMG 191 has been developed and is currently undergoing testing as sole conditioning for patients with SCID. Large animal studies support the safety and efficacy of this agent in depleting HSC . Early dPre-clinical studies have focused on increasing the niche clearing potency of the naked anti-CD117 antibodies. The combination of a second mAb directed against the macrophage checkpoint inhibitor, CD47, with an anti-CD117 mAb demonstrated marked increase in myeloablation and donor cell engraftment compared to the CD117 mAb alone . Other sAll authors listed have made a substantial, direct and intellectual contribution to the work, and approved it for publication.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.


Jiao and colleagues reported a case of hemorrhagic fever with renal syndrome who developed respiratory failure and symmetrical flaccid paralysis of all extremities. Electrophysiology revealed peripheral nerve injuries mainly in axons. They reached a diagnosis of Guillain-Barré syndrome (GBS) associated with hemorrhagic fever with renal syndrome. Although the case is interesting, the diagnosis of GBS in such a patient should be with caution. Critical illness polyneuropathy (CIP) is an important and common differential diagnosis of GBS, especially in intensive care settings. Differentiating CIP from the axonal variants of GBS may be difficult on purely clinical grounds. Albumino-cytologic dissociation in CSF can help differentiate GBS from other disorders. Jiao and colleagues reported a case of hemorrhagic fever with renal syndrome who developed respiratory failure and symmetrical flaccid paralysis of all extremities . ElectroGBS is an immune-mediated disorder of the peripheral nervous system and respiratory failure requiring mechanical ventilation is a serious complication of GBS . HoweverCIP and critical illness myopathy (CIM) are frequent complications of critical illness involving both of the motor and sensory axons , 4. The We previously reported a case of critical illness polyneuropathy and myopathy with reversible tetraplegia after percutaneous nephrostolithotomy and septic shock . The axoIn summary, CIP is an important and common differential diagnosis of GBS, especially in intensive care settings. Differentiating CIP from the axonal variants of GBS may be difficult on purely clinical grounds. Albumino-cytologic dissociation in CSF can help differentiate between these two diseases. Zhang and colleagues suggested a reasonable doubt about the patient’s diagnosis that we should take critical illness polyneuropathy (CIP) or critical illness myopathy (CIM), instead of Guillain–Barré syndrome (GBS), into consideration based on the scenario of critical illness. Although it is difficult to distinguish CIP/CIM and GBS because of their similar clinical manifestations , some factors should be taken into account when we deal with this dilemma.First, prolongation of F-waves on electromyography (EMG) is the most important feature of GBS other than CIP/CIM . Nerve rSecond, the lack of evidence of albuminocytological dissociation should not be an exclusion criterion for GBS. Based on our current knowledge, only 64% of patients with GBS have the feature with albuminocytological dissociation . AccordiThird, we cannot arbitrarily exclude the diagnosis of GBS due to the absence of cranial nerve deficits, such as diplopia or facioplegia. GBS is highly diverse in terms of the presence, distribution and extent of cranial nerve deficits, sensory symptoms, weakness, ataxia, and the course of the disease . Only 50Fourth, the presence or absence of respiratory problems cannot be used in differential diagnosis of ICUAW and GBS since respiratory muscle weakness also occurs commonly in patients with CIP/CIM .Fifth, good response to intravenous immunoglobin (IVIG) therapy strongly supported the diagnosis of GBS. For GBS, IVIG or plasma exchange (PE) is the optimal management approach, alongside supportive care . CIP andLast, etiological agents might highlight the diagnosis of GBS. Wachira et al. found that virus  and Campylobacter jejuni infection were the most commonly etiological agents in GBS development in a meta-analysis . HoweverIn summary, we hold the opinion that our patient is a case of GBS (AMSAN) associated with Hantavirus infection. In intensive care settings, ICUAW should be suspected once the patients presented flaccid quadriparesis and neuromuscular respiratory failure, especially after recovery of sepsis, shock and multiorgan failure. Of note is that in patients with virus or Campylobacter jejuni infection, EMG should be performed to identify GBS. In that case, targeting therapy, like IVIG or PE, could be immediately initiated to get a better prognosis.


Hematopoietic stem cell transplantation (HSCT) has been established as an effective therapy for selected inborn errors of metabolism. The success of HSCT in metabolic disease is best exemplified through the treatment of Hurler's syndrome, a lysosomal storage disease. Through the collaborative effort of several international centers, factors that predict successful patient and transplant outcomes have been identified. In this review, we discuss the principles that underlie the use of HSCT in metabolic diseases. We consider the clinical indications, conditioning regimens, and disease-specific follow-up for HSCT in different metabolic diseases. We highlight persisting challenges in HSCT to delay progression of certain organ systems that remain refractory to HSCT and the relatively high rates of aplastic graft failure. Finally, we evaluate the variable applicability of these principles to other inherited metabolic disorders including peroxisomal, mitochondrial, and other lysosomal storage diseases. Hematopoietic Stem Cell Transplantation (HSCT) for metabolic diseases is specialized medicine. It requires multidisciplinary management and continuous collaboration with other specialties and allied health professionals before, during and beyond the period of transplantation.HSCT is better at preventing disease progression than in reversing already established disease manifestations.Early intervention with HSCT is associated with better outcomes.Transplant outcomes should be distinguished from disease outcomes in metabolic disease, and much progress has been made in recent years from careful, multi-center and collaborative analysis of the factors that separately influence each.ex-vivo T-cell depletion.Engraftment rates are improved with pharmacokinetic analysis-guided, myeloablative busulfan without Autologous HSCT using gene therapy may provide an improved treatment option for inherited metabolic diseases in the future, both by reducing allogeneic treatment-related toxicities, and by improving efficacy through augmented graft enzyme delivery.Inherited metabolic disorders comprise a large, diverse, and complex group of diseases caused by defects in genes that code for proteins involved in metabolic pathways. HSCT is an option and even standard of care for specific metabolic diseases, where other available therapies are less effective and where the benefit of HSCT outweighs the risk of a transplant. This chapter will serially discuss the use of HSCT in certain lysosomal storage and peroxisomal diseases where HSCT is standard of care. Furthermore, it will discuss its conditional role in other metabolic disease including mitochondrial disease .The Lysosomal Storage Diseases (LSDs) encompass over 70 diseases, which comprise genetic defects in specific lysosomal proteins. In the past decade, the function of lysosomes has extended beyond their involvement in degradation and recycling of extracellular and intracellular material. They play a crucial role in plasma membrane repair, lipid and metabolite exchange between organelles and have recently been found to regulate energy metabolism via calcium signaling –3. UnderIDUA gene which codes for alpha-l-iduronidase, resulting in ineffective catabolism of heparan and dermatan sulfate (Mucopolysaccharidosis Type 1 (MPS I) is an autosomal recessive LSD characterized by lysosomal accumulation of mucopolysaccharides or glycosaminoglycans (GAGs) . In MPS  sulfate . Accumul sulfate . The cli sulfate , 8.LSDs require early intervention and multi-disciplinary management to optimize treatment response, quality of life and prevent premature mortality. The principle of HSCT in LSDs is in cross-correction. HSCT provides the recipient with a continuous source of enzyme produced by donor-derived myeloid cells, which are then taken up by enzyme-deficient host cells . FurtherFull intensity myeloablative conditioning with fludarabine and pharmacokinetic-guided busulfan dosing is the current recommendation for LSDs . ParenteIn the past two decades, the proportion of MPS IH patients with graft failure has declined by more than 3-fold . Prefereex vivo stem cell gene therapy procedure and this may improve skeletal outcomes.HSCT undoubtedly improves the clinical course of patients with MPS IH. Following HSCT, early disease-related mortality is reduced and prolonged survival possible and even expected , 25. HowMultidisciplinary management beyond the period of transplantation is fundamental in the care of patients with LSD. HSCT improves the clinical course of disease but it is not curative. Regular, long-term follow up with several specialties is mandatory including a bone marrow transplant clinician, metabolic disease specialist, endocrinologist, orthopedic specialist, and spinal surgeon. The psychosocial impact of the disease should not be neglected and psychological support should be offered.MPS IH has exemplified core underlying principles of HSCT in LSDs. It has demonstrated the concept of efficacious enzyme delivery via cross-correction, facilitated optimization of a cell source hierarchy that includes UCB, and optimization of a fludarabine and pharmacokinetic-guided busulfan conditioning regimen. It has highlighted the importance of performing HSCT early and pre-symptomatically. While some of these principles are applicable to a few LSDs, it is not indicated in others as it fails to improve patient outcomes.In Alpha Mannosidosis (AM), HSCT is considered standard of care based on limited published evidence . DeficieEx-vivo stem cell gene therapy of autologous HSC improves graft enzyme delivery and has been shown to be dramatically beneficial in modifying disease progression in infantile MLD (There is no role for HSCT in Sanfilippo syndrome (MPS III) or infantile Metachromatic Leukodystrophy (MLD). MLD is caused by deficiency of arylsulfatase A and subsequent accumulation of sulfatides in the central and peripheral nervous system, resulting in widespread demyelination , 33. Whatile MLD   is caused by a genetic mutation in roxisome . Subsequroxisome , 41. Theroxisome . Childhoroxisome . The iniroxisome . Complexroxisome , 45.HSCT is only indicated in CCALD and is the single treatment modality available in this phenotype to demonstrate amelioration of disease . The effMyeloablative conditioning with busulfan and CY with or without total body irradiation (TBI) has originally been used for HSCT in X-ALD . RecentlRegular evaluation with multi-disciplinary input is necessary to provide appropriate assessment and management for the diverse complications that may arise with X-ALD. This includes follow-up by an endocrinologist, neurologist, and ongoing radiological monitoring by MRI to detect early cerebral disease in genetically affected boys, or evaluate neurological response to treatment.Mitochondrial neurogastrointestinal encephalomyopathy (MNGIE) is an autosomal recessive disease characterized by deficiency of thymidine phosphorylase (TP) . NucleosThe collaborative effort of several international studies has led to the success of HSCT in treating metabolic disease, particularly MPS IH.The principle of cross-correction does not reliably translate to all LSDs or other metabolic diseases, and persistent progression of organs that are refractory to HSCT cause significant morbidity following transplant.ex vivo, and infusing them back into the patient following appropriate conditioning.Autologous HSCT using gene therapy may facilitate supranormal enzyme production by transducing a patient's HSCs with a viral vector These gene-corrected cells can integrate into the host genome, where certain viral vectors such as lentiviruses can do so with a lower risk of insertional mutagenesis.This may help treat refractory diseases such as MLD and MPSIIIA, and better correct refractory organs in a responding disease, such as the skeleton in MPSIH, while avoiding significant HSCT-associated morbidity. Pre-clinical experiments in animal models are promising and clinical trials are ongoing.ET and RW were the main authors with contributions by JB and SJ. All authors reviewed the manuscript.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.


Reducing meat consumption may improve human health, curb environmental damage and greenhouse gas emissions, and limit the large-scale suffering of animals raised in factory farms. Previous work has begun to develop interventions to reduce individual meat consumption, often by appealing directly to individual health motivations. However, research on nutritional behavior change suggests that interventions additionally linking behavior to ethical values, identity formation, and existing social movements may be particularly effective and longer-lasting. Regarding meat consumption, preliminary evidence and psychological theory suggest that appeals related to animal welfare may have considerable potential to effectively leverage these elements of human psychology. We aim to conduct a systematic review and quantitative meta-analysis evaluating the effectiveness of animal welfare-related appeals on actual or intended meat consumption or purchasing. Our investigation will critically synthesize the current state of knowledge regarding psychological mechanisms of intervening on individual meat consumption and empirically identify the psychological characteristics underlying the most effective animal welfare-based interventions.We will systematically search eight academic databases and extensively search unpublished grey literature. We will include studies that assess interventions intended to reduce meat consumption or purchase through the mention or portrayal of animal welfare, that measure outcomes related to meat consumption or purchase, and that have a control condition. Eligible studies may recruit from any human population, be written in any language, and be published or released any time. We will meta-analyze the studies, reporting the pooled point estimate and additional metrics that describe the distribution of potentially heterogeneous effects. We will assess studies’ risk of bias and conduct sensitivity analyses for publication bias. We describe possible follow-up analyses to investigate hypothesized moderators of intervention effectiveness.The findings of the proposed systematic review and meta-analysis, including any identified methodological limitations of the existing literature, could inform the design of successful evidence-based interventions with broad potential to improve human, animal, and environmental well-being.https://osf.io/d3y56/registrations).The protocol was preregistered via the Open Science Framework ( Excessive consumption of meat and animal products may be deleterious to human health  are eligibleComparison: The study must include a control group, condition, or time period not subjected to any form of intervention intended to change meat consumption . Not exhaustively, eligible study designs therefore include randomized controlled trials, repeated-measures designs in which subjects serve as their own controls, and quasi-experiments such as interrupted time-series analyses. For example, a repeated-measures study might estimate the difference in subjects’ meat consumption after versus before all subjects are exposed to an eligible intervention. In a quasi-experiment, posters encouraging reduced meat consumption might be introduced to a cafeteria on a particular day, with meat consumption measured before and after this intervention.Outcome: Our review will consider meat consumption or purchase as outcomes. Specifically, the outcome must be actual , self-reported, or intended consumption or purchase of meat or animal products. Willingness to pay for these products is an eligible outcome, though we anticipate that most such studies will include only comparisons of different meat products without a non-meat option, hence failing the previous inclusion criterion requiring a control condition. Studies assessing these outcomes in virtual or lab environments are eligible. Studies measuring only a specific type of meat or animal product  are eligible. Outcomes such as empathy, affective responses to meat, beliefs about factory farming, etc., are ineligible, though these outcomes may be relevant for narrative discussion in the theoretical review. Table Time frame: Articles released or published at any point prior to our final search are eligible.In stage 1 searches, we will search both the peer-reviewed literature and the grey literature to identify eligible articles. Then, in stage 2 searches, we will use “snowballing” of the articles identified in stage 1 to attempt to identify additional articles, as described below. We will include articles and grey literature published or released at any time and written in any language, seeking translation for full texts if necessary.We developed a sensitive search strategy for peer-reviewed literature in collaboration with an academic reference librarian (PAB). We will search eight databases  using search terms in Additional file An author (JP) who is the director of an evidence-based animal welfare research organization will lead the search for non-academic grey literature. We will identify relevant studies conducted or referenced by evidence-based animal welfare organizations by searching, not exhaustively, the websites of Faunalytics, Animal Welfare Action Lab, The Humane League Labs, Mercy for Animals, Animal Charity Evaluators, and Better Buying Lab  will extract point estimate(s) and variance estimate(s) most closely approximating the treatment effect of the animal welfare appeal  may similarly contribute one point estimate per eligible intervention and outcome because different point estimates may be relevant to different moderator analyses described in the “We will use a table to describe study characteristics, such as the study’s randomization status, the specific intervention and outcome , the length of follow-up between the intervention and outcome measurement, subject demographics , and the analyzed sample size. We will also descriptively present a table of risk-of-bias criteria . We will discuss the state of theoretical development and open issues in each of these categories, paying particular attention to any longitudinal studies that conduct mediation analyses using measurements of proposed mechanistic variables  occurring after exposure to the intervention and before measurement of the outcome. We may also discuss articles that contribute useful theoretical insights but that are not eligible for inclusion in the quantitative meta-analyses, for example, because they measure proposed mechanistic variables without an eligible outcome measure of meat consumption or purchase.metafor. If the point estimates appear non-normal or if many studies contribute multiple point estimates, potentially introducing correlation between point estimates, we will instead  fit a robust semi-parametric meta-analysis model using the R package robumeta [p values for the mean effect size and the estimated standard deviation of the effect sizes  will conduct all statistical analyses in R . We willrobumeta , 42. RepFor the primary analysis, we will estimate the interventions’ average effect size by meta-analyzing the point estimates from all eligible studies that provided sufficient information to extract data. We have designated this analysis as primary because it represents a reasonable first step in pursuing the first quantitative meta-analysis of this nascent literature. The primary analysis will aggregate across all eligible outcomes Table  rather tdetrimental effects of the interventions [If the interventions’ effects appear heterogeneous based on the statistical estimate of their standard deviation (and 1.2) , 44. We ventions , 44. Theventions . Second,ventions , qualitaWe will conduct several sensitivity analyses regarding statistical biases and the scope of articles included in the analysis. First, we will assess publication bias using selection model methods , sensitiWe will consider using meta-regression or subset analyses to assess the role of hypothesized moderators (e.g., Table Developing effective interventions to reduce excessive meat consumption could carry broad societal benefits to human health, animal welfare, and the environment. Interventions involving appeals regarding animal welfare may have unique potential to transform widespread ethical concerns about farm animal suffering into actual behavioral change and to tap potent psychological levers, such as cognitive dissonance and the connection between physical and moral disgust. However, these types of interventions have received little academic attention to date. The present systematic review will discuss theoretical advances underlying effective interventions based on animal welfare appeals. By further conducting the first quantitative meta-analysis of animal welfare-based appeals to reduce meat consumption, we will estimate the effectiveness of the interventions that have been attempted to date, assess what characteristics of interventions may contribute to greater effectiveness, and present findings using metrics that directly summarize their societal impact on human health, animal welfare, and the environment. Last, by carefully assessing risks of bias of the existing literature, we will provide concrete methodological recommendations for future work.Additional file 1:Section 1. Examples of eligible interventions. Section 2. Search terms by database. Section 3. Examples of websites to be searched for grey literature. Section 4. Details of data extraction.


Cellular plasticity is a transformation of a terminally differentiated cell into another cell type, which has been long known to occur in disease and regeneration. However, white adipocytes (fat cells) have only recently been observed to undergo different types of cellular plasticity. Adipocyte transdifferentiation into myofibroblasts and cancer-associated fibroblasts occurs in fibrosis and cancer, respectively. On the other hand, reversible adipocyte dedifferentiation into adipocyte progenitor cells (preadipocytes) has been demonstrated in mammary gland and in dermal adipose tissue. Here we discuss the research on adipocyte plasticity, including the experimental approaches that allowed to detect and study it, the current state of the knowledge, major research questions which remain to be addressed, and the advances required to stimulate adipocyte plasticity research. In the future, the knowledge of the molecular mechanisms of adipocyte plasticity can be utilized both to prevent adipocyte plasticity in disease and to stimulate it for use in regenerative medicine. In the traditional view of cell differentiation, cells follow a differentiation trajectory in discrete developmental stages, beginning with stem cells and culminating with a terminally differentiated state . The terSome types of cellular plasticity, for example, the regeneration of lens in newt , have beAdipose (fat) tissue is a major endocrine organ whose primary function is the long-term storage of energy in the form of lipids. It is distributed across multiple depots in the body and is comprised of fat cells (adipocytes), adipose progenitor cells, and other non-adipogenic cell types, including fibroblasts, endothelial cells, and neurons. There are two major types of adipose tissue present in mammals. White adipocyte tissue (WAT) contains large adipocytes filled with single large lipid droplets and its primary function is long-term energy storage and insulation. In contrast, brown adipose tissue (BAT) is comprised of adipocytes with multiple small lipid droplets capable of producing heat (thermogenesis). Adipose (fat) tissue has long been viewed as relatively dispensable, given that its primary function—long-term storage of energy—seems excessive in the age of permanent calorie excess. Even though we currently understand that fat tissue has many additional functions, for example, as a major endocrine organ , the facIn general, cellular plasticity either follows the reverse of differentiation trajectory or occurs between closely related lineages. For example, in the pancreas different types of conversions have been observed between acinar cells, α cells, β cells, and δ cells, all of which are derived from a common multipotent progenitor cell . In caseBrown adipocytes can produce heat (thermogenesis) through uncoupling protein-1 (UCP1) which dissociates electron transport from ATP production in mitochondria . ProlongThe mammary gland, which contains a high percentage of white adipocytes, undergoes dramatic remodeling firstly during lactation and later again during mammary gland involution. In lactation, fat tissue gives way to the expanding mammary epithelium, accompanied by dedifferentiation of adipocytes to preadipocytes. The dedifferentiated preadipocytes can re-enter the cell cycle and proliferate, as well as re-differentiate into adipocytes during the involution of the mammary gland . In addition to dedifferentiation into preadipocytes, previous research indicated that adipocytes in the mammary gland can transdifferentiate into mammary epithelial cells ,21,22, waP2 gene was widely used to lineage-trace adipocytes [aP2 is expressed in a fraction of undifferentiated adipogenic cells, as well as in macrophages which are present in fat tissue [Retn [Cdh5 [Adipoq:Cre mouse models) led to the increased reliability of the adipocyte lineage tracing [Adipoq:Cre lineage tracing challenged not only the possibility of the adipocyte-epithelial cell transdifferentiation in the mammary gland, but also of endothelial and myeloid sources of adipocytes [Discrepancies between the results of earlier and more recent studies on the possibility of adipocyte-epithelial cell transdifferentiation are most likely due to the differences between mouse genetic models used for the tracing of adipocyte lineage. Lineage tracing is an experimental approach in which irreversible genetic change occurs only in cells of a specific cell type (lineage), leading to the permanent expression of a marker protein in particular types of marker-expressing cells and all cells derived from them . Therefoipocytes . Howevert tissue . A similue [Retn  and Cdh5tn [Cdh5 . The dev tracing ,30. The ipocytes ,32,33. HDermal adipose tissue is located within the skin and is molecularly distinct from the adjacent subcutaneous adipose tissue . In humaFibrosis is a highly progressive disease characterized by the hardening of tissue, which is attributed to excessive deposition of extracellular matrix components, such as collagens. The progress of fibrosis is largely driven by myofibroblasts, cells which exhibit intermediate features between smooth muscle cells and fibroblasts . Under pRecently through the use of adiponectin-dependent lineage tracing it was shown that myofibroblasts can develop from intradermal adipocytes in a mouse model of dermal fibrosis . InteresAlthough the adipocyte-myofibroblast transition has only been observed in dermal fibrosis so far, certain cells which can give rise to myofibroblasts in other tissues share molecular characteristics with adipocytes. In a mouse model of lung fibrosis the source cell for myofibroblasts are lipogenic fibroblasts, which are interstitial fibroblasts containing lipid droplets . In expeIn addition to myofibroblasts, adipocytes can transdifferentiate into cancer-associated fibroblasts (CAFs), which, phenotypically, closely resemble myofibroblasts . A signiAltogether, the conversion of adipocytes to myofibroblasts and CAFs likely represents a transition to a cellular state which under physiological conditions is temporary. However, the altered microenvironment of the fat tissue which initially likely stimulates the transdifferentiation may also be responsible for the long-term maintenance of myofibroblasts and CAFs, leading to a permanent cell state change.Liposarcomas (LPSs) are the most common type of soft-tissue cancer. Their categorization is based on cell morphology, and distinguishes between well-differentiated liposarcoma (WDLPS), dedifferentiated liposarcoma (DDLPS), myxoid/round cell liposarcoma (MLPS), and pleomorphic liposarcoma (PLPS) . The celCEBPA, which encodes one of the key adipogenic transcription factors CCAAT/enhancer binding protein α (C/EBPα), is observed in 24% of DDLPS patients [Adipocyte dedifferentiation in the pathogenesis of human liposarcomas is supported by the fact that methylation of the gene patients . Interespatients . It is apatients ,63, whicpatients . The conpatients .Given that cellular plasticity allows for cells to transfer into ontogenically close lineages, one would expect that white adipocytes may be able to give rise to other mesenchymal cell types, such as muscle cells or osteoblasts, which would make them an important potential cellular source for regeneration medicine, especially since large numbers of adipocytes can be obtained during a biopsy . Due to their large size and low density because of their high lipid content, primary adipocytes are challenging to culture in vitro. However, adipocytes can be cultured using a so-called ceiling culture method, in which mature adipocytes adhere to the top surface of a tissue culture flask which is completely filled with culture media . After aThe transcriptional regulation of adipocyte differentiation and adipocyte function has been relatively well described compared to the mechanisms which maintain adipocyte state and prevent adipocyte plasticity under normal conditions. Adipogenesis is driven by a coordinated function of many transcription factors, including key early regulators CCAAT/enhancer binding proteins (C/EBPs) and the master regulator of adipogenesis PPARγ ,76. PPARThe signals inducing adipocyte plasticity in various physiological and disease processes are similarly not well understood. Several cytokines and secretory proteins have been implicated in driving adipocyte plasticity . PropertThe development of lineage tracing tools greatly advanced our ability to detect adipocyte plasticity in experimental mouse models. However, a certain level of heterogeneity exists between different fat depots in the body, both in terms of gene expression  and cytoOn the other hand, significant molecular differences between adipocytes within a single fat depot may also exist and affect a cell’s propensity for plasticity. The intra-depot adipocyte heterogeneity could be addressed by single-cell-omics approaches, which have been applied to uncover the heterogeneity of many other tissues and cell types ,98. Due In recent years different research groups elegantly demonstrated that adipocytes undergo plasticity. The research was greatly aided by the development of experimental tools which determine whether a particular cell in vivo originated from a mature adipocyte. However, our current knowledge of the extent of adipocyte plasticity is based on isolated findings in different disease models, rather than on a systematic identification of all possible cell types which adipocytes can transdifferentiate into. One outstanding mystery regarding adipocyte transdifferentiation is the ability of adipocytes to convert into other closely related mesodermal cell types such as osteoblasts or chondrocytes. Myofibroblasts, although usually derived from fibroblasts, are thought to represent a cell state rather than a true cell lineage in the traditional sense. On the other hand, DFAT cells, which show multipotent potential both in vitro and in vivo, have not been proven to derive from differentiated adipocytes using the golden standard of lineage tracing. Another unresolved question is the identity of plasticity-inducing molecular signals. Adipocyte transdifferentiation, which currently seems to occur uniquely in disease conditions, such as fibrosis and cancer, is mediated by the microenvironmental cues, such as cytokines . That isIn summary, recent findings indicate that adipocyte plasticity plays a role in several physiological and pathological processes. Understanding the precise mechanisms of adipocyte trans- and dedifferentiation will enable novel approaches for the treatment of diseases, such as cancer and fibrosis. It will also clarify whether adipocytes can find use in regenerative medicine.