Although many proteins -- both damaged and normal -- have a tendency to aggregate, only some are capable of dividing and propagating. What does it take to turn a protein aggregate into an infectious prion?

Many damaged and mutant polypeptides, as well as some normal proteins, have a tendency to aggregate in cells. Some protein aggregates are capable of “dividing” and propagating in cells, leading to formation of similar aggregates in daughter cells or even in neighboring cells due to “infection.” These self-propagating protein aggregates are called prions and constitute the basis of prion diseases. The infectious agent in these diseases is an abnormal conformation of the PrP protein (PrP Sc ), which makes it protease-resistant and initiates its aggregation ( Prusiner 1998 ). The abnormal aggregated species can recruit normal soluble PrP molecules into aggregates, thus inactivating them. The aggregates of PrP Sc  can proliferate within cells and be transmitted to other cells and tissues, leading to the spread of neurotoxicity.Prion Domains While so far only one prion protein is known in mammals, several prion-like proteins capable of forming self-propagating aggregates have been found in various yeast species. The common structural feature of yeast prion proteins is the so-called prion domain, characterized by the high content of glutamines (Q) and asparagines (N) ( DePace et al. 1998 ;  Michelitsch and Weissman 2000 ), also known as the Q/N-rich domain. The prion domains are the major structural determinants that are solely responsible for the polypeptide aggregation and propagation of the aggregates. Interestingly, the mammalian PrP Sc  is fundamentally different from yeast prions, since it lacks a Q/N-rich domain, indicating that distinct structural features are responsible for its ability to form self-propagating aggregates. The Q/N-rich domains in yeast prions are transferable in that, when fused to a heterologous polypeptide, they confer prion properties to this polypeptide. With a low probability, soluble proteins with prion domains can change conformation to form self-propagating aggregates, which can be transmitted to daughter cells ( Lindquist 1997 ) ( Figure 1 ). As with PrP Sc , yeast prions efficiently recruit soluble molecules of the same species, thus inactivating them ( Lindquist 1997 ;  Chernoff 2001 ;  Wickner et al. 2001 ). Also with low probability, the aggregation-prone conformation of yeast prion proteins can reverse to a soluble functional conformation. Certain yeast prion proteins, when in soluble conformation, function in important pathways; e.g., Sup35 (forming [PSI + ] prion) controls termination of translation, and Ure2 (forming [URE3 + ] prion) controls some membrane transporter systems. Aggregation of these proteins leads to phenotypes (e.g., suppression of nonsense mutations or transport defects) inherited in a non-Mendelian fashion owing to the nonchromosomal basis of the inheritance. Figure 1 Aggregation, Division, and Transfer of Prions in Yeast
Prion Domains
While so far only one prion protein is known in mammals, several prion-like proteins capable of forming self-propagating aggregates have been found in various yeast species. The common structural feature of yeast prion proteins is the so-called prion domain, characterized by the high content of glutamines (Q) and asparagines (N) ( DePace et al. 1998 ;  Michelitsch and Weissman 2000 ), also known as the Q/N-rich domain. The prion domains are the major structural determinants that are solely responsible for the polypeptide aggregation and propagation of the aggregates. Interestingly, the mammalian PrP Sc  is fundamentally different from yeast prions, since it lacks a Q/N-rich domain, indicating that distinct structural features are responsible for its ability to form self-propagating aggregates. The Q/N-rich domains in yeast prions are transferable in that, when fused to a heterologous polypeptide, they confer prion properties to this polypeptide. With a low probability, soluble proteins with prion domains can change conformation to form self-propagating aggregates, which can be transmitted to daughter cells ( Lindquist 1997 ) ( Figure 1 ). As with PrP Sc , yeast prions efficiently recruit soluble molecules of the same species, thus inactivating them ( Lindquist 1997 ;  Chernoff 2001 ;  Wickner et al. 2001 ). Also with low probability, the aggregation-prone conformation of yeast prion proteins can reverse to a soluble functional conformation. Certain yeast prion proteins, when in soluble conformation, function in important pathways; e.g., Sup35 (forming [PSI + ] prion) controls termination of translation, and Ure2 (forming [URE3 + ] prion) controls some membrane transporter systems. Aggregation of these proteins leads to phenotypes (e.g., suppression of nonsense mutations or transport defects) inherited in a non-Mendelian fashion owing to the nonchromosomal basis of the inheritance.Inheriting Variations A remarkable feature of yeast prion proteins is their ability to produce distinct inherited “variants” of the prion. For example, [PSI + ] prion could exist in several distinct forms that suppress termination of translation to different degrees. These “variants” of yeast prions are analogous to different prion “strains” of PrP Sc , which cause versions of the disease with different incubation periods and different patterns of brain pathology. The molecular nature of distinct PrP Sc  strains is determined by specific stable conformations of PrP. Similarly, “variants” of yeast prions are explained by different stable conformation states of the corresponding prion proteins ( Chien et al. 2003 ). Strict conformation requirements for aggregate formation can also explain interspecies transmission barriers, where prion domains of Sup35 derived from other yeast species cannot cause formation of [PSI + ] prion in Saccharomyces cerevisiae, in spite of a high degree of homology. This observation is very intriguing, especially in light of a recent finding that prion conformation of some proteins is required for formation of prions by the other proteins. For example, for de novo formation of [PSI + ] prion, a distinct prion [RNQ + ] should be present in a cell ( Derkatch et al. 2001 ;  Osherovich and Weissman 2001 ), probably in order to cross-seed Sup35 aggregates. This is in spite of relatively limited homology between the prion domains of these proteins. The apparent contradiction between the interspecies transmission barriers of very homologous prion proteins and possible cross-seeding of aggregates by prion proteins with more limited homology represents an interesting biological problem. On the other hand, this apparent contradiction may indicate that prion formation is a more complicated process than we currently think and that it may involve many cellular factors.
Inheriting Variations
A remarkable feature of yeast prion proteins is their ability to produce distinct inherited “variants” of the prion. For example, [PSI + ] prion could exist in several distinct forms that suppress termination of translation to different degrees. These “variants” of yeast prions are analogous to different prion “strains” of PrP Sc , which cause versions of the disease with different incubation periods and different patterns of brain pathology. The molecular nature of distinct PrP Sc  strains is determined by specific stable conformations of PrP. Similarly, “variants” of yeast prions are explained by different stable conformation states of the corresponding prion proteins ( Chien et al. 2003 ). Strict conformation requirements for aggregate formation can also explain interspecies transmission barriers, where prion domains of Sup35 derived from other yeast species cannot cause formation of [PSI + ] prion in Saccharomyces cerevisiae, in spite of a high degree of homology. This observation is very intriguing, especially in light of a recent finding that prion conformation of some proteins is required for formation of prions by the other proteins. For example, for de novo formation of [PSI + ] prion, a distinct prion [RNQ + ] should be present in a cell ( Derkatch et al. 2001 ;  Osherovich and Weissman 2001 ), probably in order to cross-seed Sup35 aggregates. This is in spite of relatively limited homology between the prion domains of these proteins. The apparent contradiction between the interspecies transmission barriers of very homologous prion proteins and possible cross-seeding of aggregates by prion proteins with more limited homology represents an interesting biological problem. On the other hand, this apparent contradiction may indicate that prion formation is a more complicated process than we currently think and that it may involve many cellular factors.What Do Prions Do? Although yeast prions have been studied for almost ten years, very little is known about their biological significance. We do not know the functions of the majority of proteins that can exist as prions. Even if a function of prion proteins, such as with Sup35 or Ure2, is known, we do not understand the biological significance of their “prionization,” i.e., that they aggregate and propagate in the aggregated form. A very intriguing and unexpected finding was that formation of [PSI + ] prion causes a wide variety of phenotypic alterations, which depend on the strain background ( True and Lindquist 2000 ). In fact, comparison of yeast strains of different origin, each with and without [PSI + ] prion, showed that certain strains with [PSI + ] prion have different sensitivity to stresses and antibiotics than their non-prion derivatives, despite their genetic identity. In some strains, cells with [PSI + ] prion demonstrated better survival than their non-prion counterparts in the presence of inhibitors of translation or microtubules, heavy metals, low pH, and other deleterious conditions, which of course gives a strong advantage to the [PSI + ] cells. It is likely that some genomic mutations could be suppressed and therefore become silent when termination of translation by Sup35 is partially inactivated in [PSI + ] prion cells ( Lindquist 2000 ;  True and Lindquist 2000 ). [PSI + ] could also reveal previously silent mutations or their combinations. It was hypothesized that switches between prion and non-prion forms of Sup35 enhance survival in fluctuating environments and provide a novel instrument for evolution of new traits.
What Do Prions Do?
Although yeast prions have been studied for almost ten years, very little is known about their biological significance. We do not know the functions of the majority of proteins that can exist as prions. Even if a function of prion proteins, such as with Sup35 or Ure2, is known, we do not understand the biological significance of their “prionization,” i.e., that they aggregate and propagate in the aggregated form. A very intriguing and unexpected finding was that formation of [PSI + ] prion causes a wide variety of phenotypic alterations, which depend on the strain background ( True and Lindquist 2000 ). In fact, comparison of yeast strains of different origin, each with and without [PSI + ] prion, showed that certain strains with [PSI + ] prion have different sensitivity to stresses and antibiotics than their non-prion derivatives, despite their genetic identity. In some strains, cells with [PSI + ] prion demonstrated better survival than their non-prion counterparts in the presence of inhibitors of translation or microtubules, heavy metals, low pH, and other deleterious conditions, which of course gives a strong advantage to the [PSI + ] cells. It is likely that some genomic mutations could be suppressed and therefore become silent when termination of translation by Sup35 is partially inactivated in [PSI + ] prion cells ( Lindquist 2000 ;  True and Lindquist 2000 ). [PSI + ] could also reveal previously silent mutations or their combinations. It was hypothesized that switches between prion and non-prion forms of Sup35 enhance survival in fluctuating environments and provide a novel instrument for evolution of new traits.Q/N Does Not Necessarily a Prion Make Searching genomes of various species demonstrated that a relatively large fraction of proteins (between 0.1% and 2%) contain Q/N-rich domains ( Michelitsch and Weissman 2000 ) or polyQ or polyN sequences. These domains are often found in transcription factors, protein kinases, and components of vesicular transport. The Q/N-rich domains usually are not evolutionary conserved and their functional role is largely unknown. Some of the Q/N-rich or polyQ domains facilitate aggregation of polypeptides, especially if expanded owing to mutations. Such expansion of the polyQ domains in certain neuronal proteins could cause neurodegenerative disorders, e.g., Huntington's disease or several forms of ataxia. Importantly, aggregates formed by polypeptides with the Q/N-rich or polyQ domains are not necessarily self-propagating aggregates, i.e., prions. In fact, there are additional structural properties of the polypeptides that provide the self-propagation (see below). Even if a protein with a polyQ domain does not form a prion, its aggregation may depend on certain prions. For example, recent experiments demonstrated that [RNQ + ] prion dramatically stimulated aggregation of fragments of recombinant human huntingtin or ataxin-3 with an expanded polyQ domain cloned in yeast ( Osherovich and Weissman 2001 ;  Meriin et al. 2002 ). [RNQ + ] facilitated the nucleation phase of the huntingtin fragment aggregation, suggesting that this prion can be directly involved in seeding of the aggregates. The major question now is whether there are analogous prion-like proteins in mammalian cells that are involved in aggregation of huntingtin or ataxin-3 and subsequent neurodegenerative disease. The first indication that mammalian proteins with Q/N-rich domains can form self-propagating prions came from recent work with a regulator of translation cytoplasmic polyadenylation element-binding protein (CPEB) from  Aplysia  neurons ( Si et al. 2003 ). The neuronal form of this protein has a Q/N-rich domain similar to the prion domains of yeast prions. The Q/N-rich domain from CPEB (CPEBQ), when fused to green fluorescent protein (GFP), conferred upon it prion-like properties. The CPEBQ–GFP fusion polypeptide existed in yeast cells in one of the three distinct states, i.e., soluble, many small aggregates, or few large aggregates. Mother cells almost always gave rise to daughter cells in which the CPEBQ–GFP polypeptide was in the same state, indicating the ability of these aggregates to be inherited, i.e., to self-propagate. Furthermore, full-length  Aplysia  CPEB protein, when cloned in yeast, can also exist in two distinct states, soluble and aggregated, which is an inherited feature. Very unexpectedly, unlike other prions, the aggregated state of CPEB was more functionally active than the soluble form ( Si et al. 2003 ). These data strongly suggest that metazoan proteins with Q/N-rich domains are potentially capable of forming prions. The challenge now will be to establish whether CPEB can exist as a self-propagating aggregate in  Aplysia  or mammalian neurons.
Q/N Does Not Necessarily a Prion Make
Searching genomes of various species demonstrated that a relatively large fraction of proteins (between 0.1% and 2%) contain Q/N-rich domains ( Michelitsch and Weissman 2000 ) or polyQ or polyN sequences. These domains are often found in transcription factors, protein kinases, and components of vesicular transport. The Q/N-rich domains usually are not evolutionary conserved and their functional role is largely unknown. Some of the Q/N-rich or polyQ domains facilitate aggregation of polypeptides, especially if expanded owing to mutations. Such expansion of the polyQ domains in certain neuronal proteins could cause neurodegenerative disorders, e.g., Huntington's disease or several forms of ataxia. Importantly, aggregates formed by polypeptides with the Q/N-rich or polyQ domains are not necessarily self-propagating aggregates, i.e., prions. In fact, there are additional structural properties of the polypeptides that provide the self-propagation (see below). Even if a protein with a polyQ domain does not form a prion, its aggregation may depend on certain prions. For example, recent experiments demonstrated that [RNQ + ] prion dramatically stimulated aggregation of fragments of recombinant human huntingtin or ataxin-3 with an expanded polyQ domain cloned in yeast ( Osherovich and Weissman 2001 ;  Meriin et al. 2002 ). [RNQ + ] facilitated the nucleation phase of the huntingtin fragment aggregation, suggesting that this prion can be directly involved in seeding of the aggregates. The major question now is whether there are analogous prion-like proteins in mammalian cells that are involved in aggregation of huntingtin or ataxin-3 and subsequent neurodegenerative disease.The first indication that mammalian proteins with Q/N-rich domains can form self-propagating prions came from recent work with a regulator of translation cytoplasmic polyadenylation element-binding protein (CPEB) from  Aplysia  neurons ( Si et al. 2003 ). The neuronal form of this protein has a Q/N-rich domain similar to the prion domains of yeast prions. The Q/N-rich domain from CPEB (CPEBQ), when fused to green fluorescent protein (GFP), conferred upon it prion-like properties. The CPEBQ–GFP fusion polypeptide existed in yeast cells in one of the three distinct states, i.e., soluble, many small aggregates, or few large aggregates. Mother cells almost always gave rise to daughter cells in which the CPEBQ–GFP polypeptide was in the same state, indicating the ability of these aggregates to be inherited, i.e., to self-propagate. Furthermore, full-length  Aplysia  CPEB protein, when cloned in yeast, can also exist in two distinct states, soluble and aggregated, which is an inherited feature. Very unexpectedly, unlike other prions, the aggregated state of CPEB was more functionally active than the soluble form ( Si et al. 2003 ). These data strongly suggest that metazoan proteins with Q/N-rich domains are potentially capable of forming prions. The challenge now will be to establish whether CPEB can exist as a self-propagating aggregate in  Aplysia  or mammalian neurons.AplysiaAplysiaAplysiaMystery of Propagation What makes protein aggregates in yeast propagate? The key cellular element that is critical for this process is molecular chaperone Hsp104 ( Chernoff et al. 1995 ). This factor is specifically required for maintenance of all known prions within generations and probably is not involved in prion formation (i.e., initial protein aggregation). [PSI + ] yeast cells have about 60 seeds of this prion (although this number differed in different [PSI + ] isolates), and maintenance of about this number of seeds after cell divisions requires functional Hsp104 ( Eaglestone et al. 2000 ). In fact, in the absence of Hsp104, prion aggregates continue to grow without increase in number and are rapidly lost in generations ( Wegrzyn et al. 2001 ). Since this chaperone can directly bind to protein aggregates and promote there disassembly ( Glover and Lindquist 1998 ), it was suggested that the main function of Hsp104 in prion inheritance is to disaggregate large prion aggregates to smaller elements, thus leading to formation of new seeds ( Kushnirov and Ter-Avanesyan 1998 ). Interestingly, although Hsp104 is conserved among bacteria, fungi, and plants, animal cells do not have this chaperone or its close homologs. Therefore, if yeast-type prions with Q/N-rich domains exist in animal cells, there should be alternative factors that disaggregate large prion aggregates into smaller species in order to keep the number of seeds relatively constant and thus maintain the prions. The fact that some proteins with Q/N-rich domains form self-propagating aggregates, while others can aggregate but cannot form prions, suggests that there should be some structural elements either within the Q/N-rich sequence or close to it that confer the ability to propagate. In an article in this issue of  PLoS Biology  by  Osherovich et al. (2004) , the authors examined sequence requirements for prion formation and maintenance of two prion proteins, Sup35 and New1. They noted that both prion proteins contain an oligopeptide repeat QGGYQ in close proximity to Q/N-rich sequences and examined the functional significance of the repeats for aggregation and maintenance of the prions. In New1, in contrast to a deletion of the N-rich domain, deletion of the repeat did not affect aggregation of the protein or formation of the prion, but abrogated inheritance of the prion. With Sup35, the situation was somewhat more complicated, since repeats adjacent to Q/N-rich domain affected both protein aggregation and prion maintenance while more distant repeats affected only the prion inheritance. The authors suggested that the oligopeptide repeats facilitate the division of aggregates, either by serving as binding sites for Hsp104 or by altering the conformation of the polypeptides in aggregates to promote access for Hsp104 ( Figure 2 ). Figure 2 Distinct Domains of Sup35 Are Responsible for Aggregation and Division of Aggregates The likely possibility was that the oligopeptide repeats could be interchangeable between different prions, leading to creation of novel chimeric prions. In fact, the authors constructed an F chimera, a fusion protein having the N-rich domain of New1 and the oligopeptide repeat of Sup35. This fusion polypeptide efficiently formed prion [F + ]. Furthermore, when the oligopeptide repeat sequence was added to a polyQ sequence, this fusion polypeptide also acquired the ability to form self-propagating aggregates. This work, therefore, clarifies the architecture of prions by defining two structural motifs in prion proteins that have distinct functions in aggregation and propagation. Interestingly, not all yeast prions have similar oligopeptide repeat motifs, indicating that distinct structures could confer prion properties to polypeptides that can aggregate. It would be important to identify these structures in order to understand the mechanisms of aggregate propagation. The work of  Osherovich et al. (2004)  may help to identify proteins from mammalian cells, plants, and bacteria that can potentially form prions. Finding these novel prions could be of very high significance since they may provide insight into a wide range of currently unexplained epigenetic phenomena.
Mystery of Propagation
What makes protein aggregates in yeast propagate? The key cellular element that is critical for this process is molecular chaperone Hsp104 ( Chernoff et al. 1995 ). This factor is specifically required for maintenance of all known prions within generations and probably is not involved in prion formation (i.e., initial protein aggregation). [PSI + ] yeast cells have about 60 seeds of this prion (although this number differed in different [PSI + ] isolates), and maintenance of about this number of seeds after cell divisions requires functional Hsp104 ( Eaglestone et al. 2000 ). In fact, in the absence of Hsp104, prion aggregates continue to grow without increase in number and are rapidly lost in generations ( Wegrzyn et al. 2001 ). Since this chaperone can directly bind to protein aggregates and promote there disassembly ( Glover and Lindquist 1998 ), it was suggested that the main function of Hsp104 in prion inheritance is to disaggregate large prion aggregates to smaller elements, thus leading to formation of new seeds ( Kushnirov and Ter-Avanesyan 1998 ). Interestingly, although Hsp104 is conserved among bacteria, fungi, and plants, animal cells do not have this chaperone or its close homologs. Therefore, if yeast-type prions with Q/N-rich domains exist in animal cells, there should be alternative factors that disaggregate large prion aggregates into smaller species in order to keep the number of seeds relatively constant and thus maintain the prions.The fact that some proteins with Q/N-rich domains form self-propagating aggregates, while others can aggregate but cannot form prions, suggests that there should be some structural elements either within the Q/N-rich sequence or close to it that confer the ability to propagate. In an article in this issue of  PLoS Biology  by  Osherovich et al. (2004) , the authors examined sequence requirements for prion formation and maintenance of two prion proteins, Sup35 and New1. They noted that both prion proteins contain an oligopeptide repeat QGGYQ in close proximity to Q/N-rich sequences and examined the functional significance of the repeats for aggregation and maintenance of the prions. In New1, in contrast to a deletion of the N-rich domain, deletion of the repeat did not affect aggregation of the protein or formation of the prion, but abrogated inheritance of the prion. With Sup35, the situation was somewhat more complicated, since repeats adjacent to Q/N-rich domain affected both protein aggregation and prion maintenance while more distant repeats affected only the prion inheritance. The authors suggested that the oligopeptide repeats facilitate the division of aggregates, either by serving as binding sites for Hsp104 or by altering the conformation of the polypeptides in aggregates to promote access for Hsp104 ( Figure 2 ).PLoS BiologyThe likely possibility was that the oligopeptide repeats could be interchangeable between different prions, leading to creation of novel chimeric prions. In fact, the authors constructed an F chimera, a fusion protein having the N-rich domain of New1 and the oligopeptide repeat of Sup35. This fusion polypeptide efficiently formed prion [F + ]. Furthermore, when the oligopeptide repeat sequence was added to a polyQ sequence, this fusion polypeptide also acquired the ability to form self-propagating aggregates. This work, therefore, clarifies the architecture of prions by defining two structural motifs in prion proteins that have distinct functions in aggregation and propagation. Interestingly, not all yeast prions have similar oligopeptide repeat motifs, indicating that distinct structures could confer prion properties to polypeptides that can aggregate. It would be important to identify these structures in order to understand the mechanisms of aggregate propagation. The work of  Osherovich et al. (2004)  may help to identify proteins from mammalian cells, plants, and bacteria that can potentially form prions. Finding these novel prions could be of very high significance since they may provide insight into a wide range of currently unexplained epigenetic phenomena.


Background Methodological research to support searching for those doing systematic reviews of epidemiological studies is a relatively neglected area. Our aim was to determine how many databases it is necessary to search to ensure a comprehensive coverage of the literature in diabetes epidemiology, with the aim of examining the efficiency of searching in support of systematic reviews of the epidemiology of diabetes
Background
Methodological research to support searching for those doing systematic reviews of epidemiological studies is a relatively neglected area. Our aim was to determine how many databases it is necessary to search to ensure a comprehensive coverage of the literature in diabetes epidemiology, with the aim of examining the efficiency of searching in support of systematic reviews of the epidemiology of diabetesMethods Three approaches were used. First, we defined a set of English language diabetes journals and examined their coverage in bibliographic databases. Second, we searched extensively for diabetes epidemiology articles (in all languages) to determine which are the most useful databases; and third, we analysed the scattering of these articles to determine the core journals in the area.
Methods
Three approaches were used. First, we defined a set of English language diabetes journals and examined their coverage in bibliographic databases. Second, we searched extensively for diabetes epidemiology articles (in all languages) to determine which are the most useful databases; and third, we analysed the scattering of these articles to determine the core journals in the area.Results The overlap between MEDLINE and Embase for diabetes journals was 59%. A search for diabetes epidemiology articles across both MEDLINE and Embase, showed that MEDLINE alone retrieved about 94% of the total articles. Searching for diabetes epidemiology studies beyond MEDLINE and Embase retrieved no additional English language journal articles. The only diabetes epidemiology studies found by searching beyond MEDLINE and Embase were found in LILACS, and were Spanish or Portuguese language studies from Latin America; no additional English language studies were found. Only 30% of the meeting abstracts were converted to full publication after three years. One third of journal articles were published in just six journals, with  Diabetes Care  contributing 14.3% of the articles, followed by  Diabetic Medicine  (5.0%);  Diabetes Research & Clinical Practice  (4.1%);  Diabetologia  (4.0%);  Diabetes & Metabolism  (2.4%) and  Diabetes  (2.0%).
Results
The overlap between MEDLINE and Embase for diabetes journals was 59%. A search for diabetes epidemiology articles across both MEDLINE and Embase, showed that MEDLINE alone retrieved about 94% of the total articles. Searching for diabetes epidemiology studies beyond MEDLINE and Embase retrieved no additional English language journal articles. The only diabetes epidemiology studies found by searching beyond MEDLINE and Embase were found in LILACS, and were Spanish or Portuguese language studies from Latin America; no additional English language studies were found. Only 30% of the meeting abstracts were converted to full publication after three years. One third of journal articles were published in just six journals, with  Diabetes Care  contributing 14.3% of the articles, followed by  Diabetic Medicine  (5.0%);  Diabetes Research & Clinical Practice  (4.1%);  Diabetologia  (4.0%);  Diabetes & Metabolism  (2.4%) and  Diabetes  (2.0%).Diabetes CareDiabetic MedicineDiabetes Research & Clinical PracticeDiabetologiaDiabetes & MetabolismDiabetesConclusions Our results show that when searching for articles on diabetes epidemiology, MEDLINE and Embase would suffice for English language papers, with LILACS giving some additional non-English articles from Latin America. Although a MEDLINE-only search will retrieve the vast majority of the relevant literature, Embase and LILACs should also be searched to ensure the search is comprehensive. Searching for meeting abstracts is recommended to alert reviewers to unpublished work. The low rate of full publication of meeting abstracts has the danger of producing bias in reviews. Our findings on scattering show that the core literature in this field is concentrated in just six journals.
Conclusions
Our results show that when searching for articles on diabetes epidemiology, MEDLINE and Embase would suffice for English language papers, with LILACS giving some additional non-English articles from Latin America. Although a MEDLINE-only search will retrieve the vast majority of the relevant literature, Embase and LILACs should also be searched to ensure the search is comprehensive. Searching for meeting abstracts is recommended to alert reviewers to unpublished work. The low rate of full publication of meeting abstracts has the danger of producing bias in reviews. Our findings on scattering show that the core literature in this field is concentrated in just six journals.

Background Review articles have a valuable role in the medical literature, because the volume of journals and articles is such that keeping up to date is very difficult. Reviews are much more valuable if they are systematic reviews done to internationally agreed standards, as non-systematic reviews are known to be subject to bias [ 1 , 2 ]. Dickersin noted that there was a shortage of systematic reviews in epidemiology, and called for more reviews, and more research into the methodology relating to reviews in epidemiology [ 3 ]. A study by Breslow found that more than 60% of epidemiology reviews were not methodologically systematic [ 4 ], and there has been little methodological research relating to their performance. Also, the methods have not been standardised, and the literature searching has not been supported, as done by the Cochrane Collaboration to support systematic reviews of clinical trials. One of the key quality criteria for systematic reviews is the comprehensiveness of the searching, as failure to identify all relevant reports can result in selection bias[ 5 ]. The usual first source for identifying studies for reviews is MEDLINE, which currently indexes 4780 titles [ 6 ] of the estimated 14 000 biomedical titles currently published throughout the world [ 7 ]. In addition to searching MEDLINE, it is recommended that an extensive range of additional sources are searched [ 1 ]. A study, using  Ulrich's International Periodicals Directory  as a gold standard, found that a MEDLINE search in psychiatry would retrieve only about half the relevant journals [ 8 ]. Similarly, we were interested to investigate the coverage of diabetes journals and diabetes epidemiology articles in medical databases, in order to determine how many databases it is necessary to search to ensure a comprehensive coverage of the literature in diabetes epidemiology. The scattering of the journal literature in a subject area can provide a useful insight into the number and range of journals needed to capture the key literature in a field. Bradford's Law of Scattering states that on any one subject, a small group of 'core' journals (Zone 1) will provide about one third of the articles on that subject, a medium number of less-core journals (Zone 2) will provide another third, and a large number of peripheral journals (Zone 3) will provide the final third of the articles [ 9 , 10 ]. The aim of this study was to examine the efficiency of searching in support of systematic reviews of the incidence and prevalence of diabetes by providing empirical data to answer the following questions: 1. Which databases index diabetes journals (restricted to English language)? 2. Which databases outside MEDLINE and Embase index diabetes epidemiology journal articles and grey literature? 3. How are diabetes epidemiology articles scattered across the journals, and what are the core journals in this area? Accordingly, this study was divided into three parts. (Note: This study is concerned only with searching for the epidemiology of diabetes itself, not with its complications).
Background
Review articles have a valuable role in the medical literature, because the volume of journals and articles is such that keeping up to date is very difficult. Reviews are much more valuable if they are systematic reviews done to internationally agreed standards, as non-systematic reviews are known to be subject to bias [ 1 , 2 ].Dickersin noted that there was a shortage of systematic reviews in epidemiology, and called for more reviews, and more research into the methodology relating to reviews in epidemiology [ 3 ]. A study by Breslow found that more than 60% of epidemiology reviews were not methodologically systematic [ 4 ], and there has been little methodological research relating to their performance. Also, the methods have not been standardised, and the literature searching has not been supported, as done by the Cochrane Collaboration to support systematic reviews of clinical trials.One of the key quality criteria for systematic reviews is the comprehensiveness of the searching, as failure to identify all relevant reports can result in selection bias[ 5 ]. The usual first source for identifying studies for reviews is MEDLINE, which currently indexes 4780 titles [ 6 ] of the estimated 14 000 biomedical titles currently published throughout the world [ 7 ]. In addition to searching MEDLINE, it is recommended that an extensive range of additional sources are searched [ 1 ].A study, using  Ulrich's International Periodicals Directory  as a gold standard, found that a MEDLINE search in psychiatry would retrieve only about half the relevant journals [ 8 ]. Similarly, we were interested to investigate the coverage of diabetes journals and diabetes epidemiology articles in medical databases, in order to determine how many databases it is necessary to search to ensure a comprehensive coverage of the literature in diabetes epidemiology.Ulrich's International Periodicals DirectoryThe scattering of the journal literature in a subject area can provide a useful insight into the number and range of journals needed to capture the key literature in a field. Bradford's Law of Scattering states that on any one subject, a small group of 'core' journals (Zone 1) will provide about one third of the articles on that subject, a medium number of less-core journals (Zone 2) will provide another third, and a large number of peripheral journals (Zone 3) will provide the final third of the articles [ 9 , 10 ].The aim of this study was to examine the efficiency of searching in support of systematic reviews of the incidence and prevalence of diabetes by providing empirical data to answer the following questions:1. Which databases index diabetes journals (restricted to English language)?2. Which databases outside MEDLINE and Embase index diabetes epidemiology journal articles and grey literature?3. How are diabetes epidemiology articles scattered across the journals, and what are the core journals in this area?Accordingly, this study was divided into three parts.(Note: This study is concerned only with searching for the epidemiology of diabetes itself, not with its complications).Methods For the purposes of this study, epidemiology articles were defined as studies of incidence or prevalence of diabetes, or of factors affecting those (thereby excluding studies looking at other epidemiological aspects such as mortality). Basic science studies, e.g. biological mechanisms of disease, were not included. We started from a position that both MEDLINE and Embase should always both be searched, since the overlap between these databases has been estimated to range from 10% to 87% [ 5 , 11 - 17 ]. Also, it is recognised that many relevant studies will appear in non-diabetes journals, and in sources not indexed in MEDLINE or Embase. Hence, a three-part approach was used to investigate literature searching to support systematic reviews of diabetes epidemiology, and address each of our aims in turn. Part one: defining a set of 'diabetes' journals, and determining the databases in which theses journal were indexed Diabetes journals were identified from the 'Medical Sciences – Endocrinology' section of  Ulrich's Periodical Directory  2003 (a comprehensive bibliographic database providing detailed information on periodicals published throughout the world) [ 18 ]. This was supplemented with a search of PubList.com [ 19 ] for journals with 'diabet' in title. The inclusion criteria for the journals were: i) the word stem 'diabet' in the title, ii) contains original scientific studies of an academic or scholarly nature, iii) currently in print, and iv) published in English. If inclusion could not be decided on the basis of the information provided in  Ulrich's , the contents pages for the past five years, and where necessary, abstracts or the full journal articles, were examined by a diabetes epidemiologist. Journals which fulfilled all the above criteria were then checked against the  List of Serials Indexed for Online Users  [ 6 ] to see if they were indexed in MEDLINE. To determine if a journal was indexed by Embase, a search was done of 'diabet$' in the Journal Word (JW) field using the Embase OVID search interface, and limiting to publication year 2003. If any journals were not indexed in either MEDLINE or Embase, searches of BIOSIS, BNI, CINAHL, SCI were done to determine if the journals were indexed in any of these databases. Part two: searching databases other than MEDLINE and Embase for journal articles and grey literature on diabetes epidemiology Databases searched were: AMED, ASSIA, BIOSIS (abstracts only), BNI, CINAHL, Conference Papers Index, Dissertation Abstracts US, Health Management Information Consortium (HMIC), Index to Theses UK, ISI Proceedings, PsycINFO, NLM Gateway, LILACS (Latin American and Caribbean Health Sciences), National Research Register (NRR), SIGLE, SCI (abstracts only), SSCI, and Zetoc. The search strategy used was 'diabetes and (epidemiology or incidence or prevalence)' in the Title (TI) field and restricted to publication years 1998 to 2003. The titles (and abstracts when available) of all records were checked by an expert in diabetes epidemiology, in order to determine their potential usefulness for those doing systematic reviews. Part three: investigating the scatter of diabetes epidemiology journal articles found in a search of MEDLINE and Embase, and determining the core journals in this area MEDLINE and Embase were searched using the search strategy: 'Diabetes Mellitus as the major subject heading, with the sub-heading 'Epidemiology' assigned (which includes incidence and prevalence), and restricted to publication years 1998 to 2003'. All languages were included. Duplicates found in both MEDLINE and Embase were removed. The journal titles in which the articles were found were ranked according to the number of articles contributed by each journal. The cumulated numbers of articles and journals were calculated and plotted. This was used to identify Bradford zones; that is, the number of journals needed to cover about one third, two thirds or all the relevant articles in the field.
Methods
For the purposes of this study, epidemiology articles were defined as studies of incidence or prevalence of diabetes, or of factors affecting those (thereby excluding studies looking at other epidemiological aspects such as mortality). Basic science studies, e.g. biological mechanisms of disease, were not included.We started from a position that both MEDLINE and Embase should always both be searched, since the overlap between these databases has been estimated to range from 10% to 87% [ 5 , 11 - 17 ]. Also, it is recognised that many relevant studies will appear in non-diabetes journals, and in sources not indexed in MEDLINE or Embase.Hence, a three-part approach was used to investigate literature searching to support systematic reviews of diabetes epidemiology, and address each of our aims in turn.Part one: defining a set of 'diabetes' journals, and determining the databases in which theses journal were indexed Diabetes journals were identified from the 'Medical Sciences – Endocrinology' section of  Ulrich's Periodical Directory  2003 (a comprehensive bibliographic database providing detailed information on periodicals published throughout the world) [ 18 ]. This was supplemented with a search of PubList.com [ 19 ] for journals with 'diabet' in title. The inclusion criteria for the journals were: i) the word stem 'diabet' in the title, ii) contains original scientific studies of an academic or scholarly nature, iii) currently in print, and iv) published in English. If inclusion could not be decided on the basis of the information provided in  Ulrich's , the contents pages for the past five years, and where necessary, abstracts or the full journal articles, were examined by a diabetes epidemiologist. Journals which fulfilled all the above criteria were then checked against the  List of Serials Indexed for Online Users  [ 6 ] to see if they were indexed in MEDLINE. To determine if a journal was indexed by Embase, a search was done of 'diabet$' in the Journal Word (JW) field using the Embase OVID search interface, and limiting to publication year 2003. If any journals were not indexed in either MEDLINE or Embase, searches of BIOSIS, BNI, CINAHL, SCI were done to determine if the journals were indexed in any of these databases.
Part one: defining a set of 'diabetes' journals, and determining the databases in which theses journal were indexed
Diabetes journals were identified from the 'Medical Sciences – Endocrinology' section of  Ulrich's Periodical Directory  2003 (a comprehensive bibliographic database providing detailed information on periodicals published throughout the world) [ 18 ]. This was supplemented with a search of PubList.com [ 19 ] for journals with 'diabet' in title.Ulrich's Periodical DirectoryThe inclusion criteria for the journals were: i) the word stem 'diabet' in the title, ii) contains original scientific studies of an academic or scholarly nature, iii) currently in print, and iv) published in English. If inclusion could not be decided on the basis of the information provided in  Ulrich's , the contents pages for the past five years, and where necessary, abstracts or the full journal articles, were examined by a diabetes epidemiologist.Ulrich'sJournals which fulfilled all the above criteria were then checked against the  List of Serials Indexed for Online Users  [ 6 ] to see if they were indexed in MEDLINE. To determine if a journal was indexed by Embase, a search was done of 'diabet$' in the Journal Word (JW) field using the Embase OVID search interface, and limiting to publication year 2003.List of Serials Indexed for Online UsersIf any journals were not indexed in either MEDLINE or Embase, searches of BIOSIS, BNI, CINAHL, SCI were done to determine if the journals were indexed in any of these databases.Part two: searching databases other than MEDLINE and Embase for journal articles and grey literature on diabetes epidemiology Databases searched were: AMED, ASSIA, BIOSIS (abstracts only), BNI, CINAHL, Conference Papers Index, Dissertation Abstracts US, Health Management Information Consortium (HMIC), Index to Theses UK, ISI Proceedings, PsycINFO, NLM Gateway, LILACS (Latin American and Caribbean Health Sciences), National Research Register (NRR), SIGLE, SCI (abstracts only), SSCI, and Zetoc. The search strategy used was 'diabetes and (epidemiology or incidence or prevalence)' in the Title (TI) field and restricted to publication years 1998 to 2003. The titles (and abstracts when available) of all records were checked by an expert in diabetes epidemiology, in order to determine their potential usefulness for those doing systematic reviews.
Part two: searching databases other than MEDLINE and Embase for journal articles and grey literature on diabetes epidemiology
Databases searched were: AMED, ASSIA, BIOSIS (abstracts only), BNI, CINAHL, Conference Papers Index, Dissertation Abstracts US, Health Management Information Consortium (HMIC), Index to Theses UK, ISI Proceedings, PsycINFO, NLM Gateway, LILACS (Latin American and Caribbean Health Sciences), National Research Register (NRR), SIGLE, SCI (abstracts only), SSCI, and Zetoc.The search strategy used was 'diabetes and (epidemiology or incidence or prevalence)' in the Title (TI) field and restricted to publication years 1998 to 2003.The titles (and abstracts when available) of all records were checked by an expert in diabetes epidemiology, in order to determine their potential usefulness for those doing systematic reviews.Part three: investigating the scatter of diabetes epidemiology journal articles found in a search of MEDLINE and Embase, and determining the core journals in this area MEDLINE and Embase were searched using the search strategy: 'Diabetes Mellitus as the major subject heading, with the sub-heading 'Epidemiology' assigned (which includes incidence and prevalence), and restricted to publication years 1998 to 2003'. All languages were included. Duplicates found in both MEDLINE and Embase were removed. The journal titles in which the articles were found were ranked according to the number of articles contributed by each journal. The cumulated numbers of articles and journals were calculated and plotted. This was used to identify Bradford zones; that is, the number of journals needed to cover about one third, two thirds or all the relevant articles in the field.
Part three: investigating the scatter of diabetes epidemiology journal articles found in a search of MEDLINE and Embase, and determining the core journals in this area
MEDLINE and Embase were searched using the search strategy: 'Diabetes Mellitus as the major subject heading, with the sub-heading 'Epidemiology' assigned (which includes incidence and prevalence), and restricted to publication years 1998 to 2003'. All languages were included. Duplicates found in both MEDLINE and Embase were removed. The journal titles in which the articles were found were ranked according to the number of articles contributed by each journal. The cumulated numbers of articles and journals were calculated and plotted. This was used to identify Bradford zones; that is, the number of journals needed to cover about one third, two thirds or all the relevant articles in the field.Results Part one: defining a set of 'diabetes' journals, and determining the databases in which theses journal were indexed Searches of  Ulrich's Periodicals Directory  and  PubList.com  initially identified four English language journals that were of potential interest but not indexed by MEDLINE or Embase. On closer inspection, three of these,  Clinical Diabetes, Journal of Diabetes Nursing , and  Diabetes and Primary Care  were excluded as they did not appear to contain any primary research; the articles were mainly educational, professional news and views, opinions, or narrative reviews. The fourth journal,  The Diabetic Foot , contained primary research, but did not appear to contain studies useful for epidemiological reviews of diabetes itself (as opposed to complications). It is indexed by CINAHL only. As shown in Table  1 , 27 English language diabetes were covered collectively by MEDLINE and Embase in 2003. Seventy-four percent (20) were indexed in MEDLINE, 85% (23) in Embase, and 59% (16) were in both MEDLINE and Embase. Table 1 English language diabetes journals indexed in either MEDLINE or Embase in 2003 Indexed in Acta Diabetologica MEDLINE & Embase Diabetes MEDLINE & Embase Diabetes & Metabolism MEDLINE & Embase Diabetes Care MEDLINE & Embase Diabetes Research MEDLINE & Embase Diabetes Research & linical Practice MEDLINE & Embase Diabetes Technology & Therapeutics MEDLINE & Embase Diabetes, Nutrition & Metabolism – Clinical & Experimental MEDLINE & Embase Diabetes, Obesity & Metabolism MEDLINE & Embase Diabetes/Metabolism Research Reviews. MEDLINE & Embase Diabetic Medicine MEDLINE & Embase Diabetologia MEDLINE & Embase Experimental & Clinical Endocrinology & Diabetes MEDLINE & Embase Experimental Diabesity Research MEDLINE & Embase Journal of Diabetes & its Complications MEDLINE & Embase Pediatric Diabetes MEDLINE & Embase Current Diabetes Reports MEDLINE Diabetes Educator MEDLINE Diabetes Forecast MEDLINE Diabetes Self-Management MEDLINE British Journal of Diabetes & Vascular Disease Embase Canadian Journal of Diabetes Embase Cme Bulletin Endocrinology & Diabetes. Embase Current Opinion in Endocrinology & Diabetes Embase Diabetologia Croatica Embase Journal of Endocrinology, Metabolism & Diabetes of South Africa Embase Practical Diabetes International. Embase The four diabetes journals unique to MEDLINE were all published in the USA. By contrast, only one of the seven journals unique to Embase was published in the USA. Part two results: searching databases other than MEDLINE and Embase for journal articles and grey literature on diabetes epidemiology The results are summarised below: Journal articles (English language) No English language journals articles that were not also indexed in MEDLINE or Embase were identified. Journal articles (non-English language) There were 23 Spanish and Portuguese language articles identified in LILACs. On the basis of the English translation of the titles, they all reported studies done in Latin America. Grey literature We defined grey literature as any literature not published in a peer reviewed journal. After removing duplicates, there were 51 dissertations identified from searches of Dissertations Abstracts US, Index to Theses UK, and SIGLE. The research presented in the vast majority (92%) of the dissertations appeared to have been written up as articles in journals indexed in MEDLINE or Embase. No grey literature studies of any format other than dissertations were retrieved from SIGLE, so there was very little additional information gained by these searches. Research in progress The National Research Register (a database of ongoing and recently completed research projects funded by, or of interest to, the United Kingdom's National Health Service) gave brief details of 18 projects in progress that had not been otherwise identified. Searching the NRR might be useful if unpublished results could be included in the review, but its main value would be to indicate when the review was likely to need updating. Meeting abstracts and conference proceedings The search of the Conference Proceedings Index retrieved 25 articles, none of which appeared to have been published as journal articles after five years. The Zetoc Conference Search found eight articles, of which 50% had been published as full journal articles in MEDLINE or Embase. The search of Science Citation Index (SCI), restricted to meeting abstracts only, found 171 relevant studies. The time to publication of the SCI abstracts was examined by checking how many had subsequently been published as journal articles indexed in MEDLINE or Embase. It was found that 30% had reached full publication after three years. A search of BIOSIS, restricted to meeting abstracts only, retrieved 71 additional relevant abstracts that were not in SCI. Most (65%) of these 71 abstracts came from the supplements of  Diabetes and Metabolism  and  Diabetes Research and Clinical Practice . Of these, 11 (12%) had been published in journals. The average time delay from the date of publication of the abstract to full publication was 1.4 years. Databases searched where no articles not in MEDLINE or Embase were found These included AMED, BNI, HMIC, NLM Gateway meeting abstracts, PsychINFO, and SSCI, In summary, the data indicate that when searching for English language journal articles on diabetes epidemiology, searches of MEDLINE and Embase would suffice. The exception would be for studies from Latin America, where LILACS should also be searched. Searching for meeting abstracts may alert reviewers to forthcoming or unpublished work. Part three: investigation of the scatter of diabetes epidemiology journal articles found in a search of MEDLINE and Embase, and determination of the core journals in this area The searches for diabetes epidemiology articles in MEDLINE and Embase resulted in 2923 articles being found in 696 different journal titles; 39% were found to be in 'diabetes journals' and 14% were non-English language. Figure  1  shows the distribution of all 696 articles retrieved across the journals. Figure 1 Distribution of diabetes epidemiology articles across journals Applying Bradford's Law of Scattering gives three zones, each providing one-third of the articles. Zone 1 The first one-third of articles were from six journals. They are in rank order:  Diabetes Care  (contained 14.3% of the articles);  Diabetic Medicine  (5.0%);  Diabetes Research & Clinical Practice  (4.1%);  Diabetologia  (4.0%);  Diabetes & Metabolism  (2.4%),  Diabetes  (2.0%) These six journals represent 0.9% (of the 696) total journals, and all are indexed in MEDLINE. Zone 2 The second one-third of articles were from 62 journals, representing 9.1% of the total journals. The four journals in Embase only were:  Practical Diabetes International ;  Diabetologia Polska ;  Diabetes und Stoffwechsel  and  Journal of the Japan Diabetes Society . Hence, 94% of Zone 2 journals are covered by MEDLINE. Zone 3 The final one-third of articles were in 628 journals, representing 90.2% of the total journals. MEDLINE indexed 88% of these journals. Overall, for the three zones, the search of MEDLINE and Embase for diabetes epidemiology articles revealed that MEDLINE indexed 89% of the total journals, and these contained 94% of the articles.
Results
Part one: defining a set of 'diabetes' journals, and determining the databases in which theses journal were indexed Searches of  Ulrich's Periodicals Directory  and  PubList.com  initially identified four English language journals that were of potential interest but not indexed by MEDLINE or Embase. On closer inspection, three of these,  Clinical Diabetes, Journal of Diabetes Nursing , and  Diabetes and Primary Care  were excluded as they did not appear to contain any primary research; the articles were mainly educational, professional news and views, opinions, or narrative reviews. The fourth journal,  The Diabetic Foot , contained primary research, but did not appear to contain studies useful for epidemiological reviews of diabetes itself (as opposed to complications). It is indexed by CINAHL only. As shown in Table  1 , 27 English language diabetes were covered collectively by MEDLINE and Embase in 2003. Seventy-four percent (20) were indexed in MEDLINE, 85% (23) in Embase, and 59% (16) were in both MEDLINE and Embase. Table 1 English language diabetes journals indexed in either MEDLINE or Embase in 2003 Indexed in Acta Diabetologica MEDLINE & Embase Diabetes MEDLINE & Embase Diabetes & Metabolism MEDLINE & Embase Diabetes Care MEDLINE & Embase Diabetes Research MEDLINE & Embase Diabetes Research & linical Practice MEDLINE & Embase Diabetes Technology & Therapeutics MEDLINE & Embase Diabetes, Nutrition & Metabolism – Clinical & Experimental MEDLINE & Embase Diabetes, Obesity & Metabolism MEDLINE & Embase Diabetes/Metabolism Research Reviews. MEDLINE & Embase Diabetic Medicine MEDLINE & Embase Diabetologia MEDLINE & Embase Experimental & Clinical Endocrinology & Diabetes MEDLINE & Embase Experimental Diabesity Research MEDLINE & Embase Journal of Diabetes & its Complications MEDLINE & Embase Pediatric Diabetes MEDLINE & Embase Current Diabetes Reports MEDLINE Diabetes Educator MEDLINE Diabetes Forecast MEDLINE Diabetes Self-Management MEDLINE British Journal of Diabetes & Vascular Disease Embase Canadian Journal of Diabetes Embase Cme Bulletin Endocrinology & Diabetes. Embase Current Opinion in Endocrinology & Diabetes Embase Diabetologia Croatica Embase Journal of Endocrinology, Metabolism & Diabetes of South Africa Embase Practical Diabetes International. Embase The four diabetes journals unique to MEDLINE were all published in the USA. By contrast, only one of the seven journals unique to Embase was published in the USA.
Part one: defining a set of 'diabetes' journals, and determining the databases in which theses journal were indexed
Searches of  Ulrich's Periodicals Directory  and  PubList.com  initially identified four English language journals that were of potential interest but not indexed by MEDLINE or Embase. On closer inspection, three of these,  Clinical Diabetes, Journal of Diabetes Nursing , and  Diabetes and Primary Care  were excluded as they did not appear to contain any primary research; the articles were mainly educational, professional news and views, opinions, or narrative reviews. The fourth journal,  The Diabetic Foot , contained primary research, but did not appear to contain studies useful for epidemiological reviews of diabetes itself (as opposed to complications). It is indexed by CINAHL only.Ulrich's Periodicals DirectoryPubList.comClinical Diabetes, Journal of Diabetes NursingDiabetes and Primary CareThe Diabetic FootAs shown in Table  1 , 27 English language diabetes were covered collectively by MEDLINE and Embase in 2003. Seventy-four percent (20) were indexed in MEDLINE, 85% (23) in Embase, and 59% (16) were in both MEDLINE and Embase.English language diabetes journals indexed in either MEDLINE or Embase in 2003Indexed inThe four diabetes journals unique to MEDLINE were all published in the USA. By contrast, only one of the seven journals unique to Embase was published in the USA.Part two results: searching databases other than MEDLINE and Embase for journal articles and grey literature on diabetes epidemiology The results are summarised below: Journal articles (English language) No English language journals articles that were not also indexed in MEDLINE or Embase were identified. Journal articles (non-English language) There were 23 Spanish and Portuguese language articles identified in LILACs. On the basis of the English translation of the titles, they all reported studies done in Latin America. Grey literature We defined grey literature as any literature not published in a peer reviewed journal. After removing duplicates, there were 51 dissertations identified from searches of Dissertations Abstracts US, Index to Theses UK, and SIGLE. The research presented in the vast majority (92%) of the dissertations appeared to have been written up as articles in journals indexed in MEDLINE or Embase. No grey literature studies of any format other than dissertations were retrieved from SIGLE, so there was very little additional information gained by these searches. Research in progress The National Research Register (a database of ongoing and recently completed research projects funded by, or of interest to, the United Kingdom's National Health Service) gave brief details of 18 projects in progress that had not been otherwise identified. Searching the NRR might be useful if unpublished results could be included in the review, but its main value would be to indicate when the review was likely to need updating. Meeting abstracts and conference proceedings The search of the Conference Proceedings Index retrieved 25 articles, none of which appeared to have been published as journal articles after five years. The Zetoc Conference Search found eight articles, of which 50% had been published as full journal articles in MEDLINE or Embase. The search of Science Citation Index (SCI), restricted to meeting abstracts only, found 171 relevant studies. The time to publication of the SCI abstracts was examined by checking how many had subsequently been published as journal articles indexed in MEDLINE or Embase. It was found that 30% had reached full publication after three years. A search of BIOSIS, restricted to meeting abstracts only, retrieved 71 additional relevant abstracts that were not in SCI. Most (65%) of these 71 abstracts came from the supplements of  Diabetes and Metabolism  and  Diabetes Research and Clinical Practice . Of these, 11 (12%) had been published in journals. The average time delay from the date of publication of the abstract to full publication was 1.4 years. Databases searched where no articles not in MEDLINE or Embase were found These included AMED, BNI, HMIC, NLM Gateway meeting abstracts, PsychINFO, and SSCI, In summary, the data indicate that when searching for English language journal articles on diabetes epidemiology, searches of MEDLINE and Embase would suffice. The exception would be for studies from Latin America, where LILACS should also be searched. Searching for meeting abstracts may alert reviewers to forthcoming or unpublished work.
Part two results: searching databases other than MEDLINE and Embase for journal articles and grey literature on diabetes epidemiology
The results are summarised below:Journal articles (English language) No English language journals articles that were not also indexed in MEDLINE or Embase were identified.
Journal articles (English language)
No English language journals articles that were not also indexed in MEDLINE or Embase were identified.Journal articles (non-English language) There were 23 Spanish and Portuguese language articles identified in LILACs. On the basis of the English translation of the titles, they all reported studies done in Latin America.
Journal articles (non-English language)
There were 23 Spanish and Portuguese language articles identified in LILACs. On the basis of the English translation of the titles, they all reported studies done in Latin America.Grey literature We defined grey literature as any literature not published in a peer reviewed journal. After removing duplicates, there were 51 dissertations identified from searches of Dissertations Abstracts US, Index to Theses UK, and SIGLE. The research presented in the vast majority (92%) of the dissertations appeared to have been written up as articles in journals indexed in MEDLINE or Embase. No grey literature studies of any format other than dissertations were retrieved from SIGLE, so there was very little additional information gained by these searches.
Grey literature
We defined grey literature as any literature not published in a peer reviewed journal. After removing duplicates, there were 51 dissertations identified from searches of Dissertations Abstracts US, Index to Theses UK, and SIGLE. The research presented in the vast majority (92%) of the dissertations appeared to have been written up as articles in journals indexed in MEDLINE or Embase. No grey literature studies of any format other than dissertations were retrieved from SIGLE, so there was very little additional information gained by these searches.Research in progress The National Research Register (a database of ongoing and recently completed research projects funded by, or of interest to, the United Kingdom's National Health Service) gave brief details of 18 projects in progress that had not been otherwise identified. Searching the NRR might be useful if unpublished results could be included in the review, but its main value would be to indicate when the review was likely to need updating.
Research in progress
The National Research Register (a database of ongoing and recently completed research projects funded by, or of interest to, the United Kingdom's National Health Service) gave brief details of 18 projects in progress that had not been otherwise identified. Searching the NRR might be useful if unpublished results could be included in the review, but its main value would be to indicate when the review was likely to need updating.Meeting abstracts and conference proceedings The search of the Conference Proceedings Index retrieved 25 articles, none of which appeared to have been published as journal articles after five years. The Zetoc Conference Search found eight articles, of which 50% had been published as full journal articles in MEDLINE or Embase. The search of Science Citation Index (SCI), restricted to meeting abstracts only, found 171 relevant studies. The time to publication of the SCI abstracts was examined by checking how many had subsequently been published as journal articles indexed in MEDLINE or Embase. It was found that 30% had reached full publication after three years. A search of BIOSIS, restricted to meeting abstracts only, retrieved 71 additional relevant abstracts that were not in SCI. Most (65%) of these 71 abstracts came from the supplements of  Diabetes and Metabolism  and  Diabetes Research and Clinical Practice . Of these, 11 (12%) had been published in journals. The average time delay from the date of publication of the abstract to full publication was 1.4 years.
Meeting abstracts and conference proceedings
The search of the Conference Proceedings Index retrieved 25 articles, none of which appeared to have been published as journal articles after five years. The Zetoc Conference Search found eight articles, of which 50% had been published as full journal articles in MEDLINE or Embase.The search of Science Citation Index (SCI), restricted to meeting abstracts only, found 171 relevant studies. The time to publication of the SCI abstracts was examined by checking how many had subsequently been published as journal articles indexed in MEDLINE or Embase. It was found that 30% had reached full publication after three years.A search of BIOSIS, restricted to meeting abstracts only, retrieved 71 additional relevant abstracts that were not in SCI. Most (65%) of these 71 abstracts came from the supplements of  Diabetes and Metabolism  and  Diabetes Research and Clinical Practice . Of these, 11 (12%) had been published in journals. The average time delay from the date of publication of the abstract to full publication was 1.4 years.Diabetes and MetabolismDiabetes Research and Clinical PracticeDatabases searched where no articles not in MEDLINE or Embase were found These included AMED, BNI, HMIC, NLM Gateway meeting abstracts, PsychINFO, and SSCI, In summary, the data indicate that when searching for English language journal articles on diabetes epidemiology, searches of MEDLINE and Embase would suffice. The exception would be for studies from Latin America, where LILACS should also be searched. Searching for meeting abstracts may alert reviewers to forthcoming or unpublished work.
Databases searched where no articles not in MEDLINE or Embase were found
These included AMED, BNI, HMIC, NLM Gateway meeting abstracts, PsychINFO, and SSCI,In summary, the data indicate that when searching for English language journal articles on diabetes epidemiology, searches of MEDLINE and Embase would suffice. The exception would be for studies from Latin America, where LILACS should also be searched. Searching for meeting abstracts may alert reviewers to forthcoming or unpublished work.Part three: investigation of the scatter of diabetes epidemiology journal articles found in a search of MEDLINE and Embase, and determination of the core journals in this area The searches for diabetes epidemiology articles in MEDLINE and Embase resulted in 2923 articles being found in 696 different journal titles; 39% were found to be in 'diabetes journals' and 14% were non-English language. Figure  1  shows the distribution of all 696 articles retrieved across the journals. Figure 1 Distribution of diabetes epidemiology articles across journals Applying Bradford's Law of Scattering gives three zones, each providing one-third of the articles. Zone 1 The first one-third of articles were from six journals. They are in rank order:  Diabetes Care  (contained 14.3% of the articles);  Diabetic Medicine  (5.0%);  Diabetes Research & Clinical Practice  (4.1%);  Diabetologia  (4.0%);  Diabetes & Metabolism  (2.4%),  Diabetes  (2.0%) These six journals represent 0.9% (of the 696) total journals, and all are indexed in MEDLINE. Zone 2 The second one-third of articles were from 62 journals, representing 9.1% of the total journals. The four journals in Embase only were:  Practical Diabetes International ;  Diabetologia Polska ;  Diabetes und Stoffwechsel  and  Journal of the Japan Diabetes Society . Hence, 94% of Zone 2 journals are covered by MEDLINE. Zone 3 The final one-third of articles were in 628 journals, representing 90.2% of the total journals. MEDLINE indexed 88% of these journals. Overall, for the three zones, the search of MEDLINE and Embase for diabetes epidemiology articles revealed that MEDLINE indexed 89% of the total journals, and these contained 94% of the articles.
Part three: investigation of the scatter of diabetes epidemiology journal articles found in a search of MEDLINE and Embase, and determination of the core journals in this area
The searches for diabetes epidemiology articles in MEDLINE and Embase resulted in 2923 articles being found in 696 different journal titles; 39% were found to be in 'diabetes journals' and 14% were non-English language.Figure  1  shows the distribution of all 696 articles retrieved across the journals.Applying Bradford's Law of Scattering gives three zones, each providing one-third of the articles.Zone 1 The first one-third of articles were from six journals. They are in rank order:  Diabetes Care  (contained 14.3% of the articles);  Diabetic Medicine  (5.0%);  Diabetes Research & Clinical Practice  (4.1%);  Diabetologia  (4.0%);  Diabetes & Metabolism  (2.4%),  Diabetes  (2.0%) These six journals represent 0.9% (of the 696) total journals, and all are indexed in MEDLINE.
Zone 1
The first one-third of articles were from six journals. They are in rank order:  Diabetes Care  (contained 14.3% of the articles);  Diabetic Medicine  (5.0%);  Diabetes Research & Clinical Practice  (4.1%);  Diabetologia  (4.0%);  Diabetes & Metabolism  (2.4%),  Diabetes  (2.0%) These six journals represent 0.9% (of the 696) total journals, and all are indexed in MEDLINE.Diabetes CareDiabetic MedicineDiabetes Research & Clinical PracticeDiabetologiaDiabetes & MetabolismDiabetesZone 2 The second one-third of articles were from 62 journals, representing 9.1% of the total journals. The four journals in Embase only were:  Practical Diabetes International ;  Diabetologia Polska ;  Diabetes und Stoffwechsel  and  Journal of the Japan Diabetes Society . Hence, 94% of Zone 2 journals are covered by MEDLINE.
Zone 2
The second one-third of articles were from 62 journals, representing 9.1% of the total journals. The four journals in Embase only were:  Practical Diabetes International ;  Diabetologia Polska ;  Diabetes und Stoffwechsel  and  Journal of the Japan Diabetes Society . Hence, 94% of Zone 2 journals are covered by MEDLINE.Practical Diabetes InternationalDiabetologia PolskaDiabetes und StoffwechselJournal of the Japan Diabetes SocietyZone 3 The final one-third of articles were in 628 journals, representing 90.2% of the total journals. MEDLINE indexed 88% of these journals. Overall, for the three zones, the search of MEDLINE and Embase for diabetes epidemiology articles revealed that MEDLINE indexed 89% of the total journals, and these contained 94% of the articles.
Zone 3
The final one-third of articles were in 628 journals, representing 90.2% of the total journals. MEDLINE indexed 88% of these journals.Overall, for the three zones, the search of MEDLINE and Embase for diabetes epidemiology articles revealed that MEDLINE indexed 89% of the total journals, and these contained 94% of the articles.Discussion Our results showed that there was an overlap of only 59% in current English language 'diabetes journals' indexed by both MEDLINE and Embase. Also, a search for diabetes epidemiology articles across both MEDLINE and Embase showed that MEDLINE alone retrieved about 94% of the total articles; therefore, both databases should be searched. Embase appears to index more diabetes journals published outside the USA. Therefore, if searching is limited to MEDLINE only (Embase being less accessible and more expensive than MEDLINE, which is free via PubMED) this could potentially introduce a bias. Also, duplication of searching can be useful, as due to differences in indexing practices, a search of one database may retrieve something missed by the other. We also found that despite a wide range of additional databases searched after MEDLINE and Embase, no additional English language journal articles on diabetes epidemiology were identified. The LILACS database was a useful source of Spanish and Portuguese language articles on the epidemiology of diabetes in Latin American countries. Meeting abstracts appeared to be valuable sources of information on forthcoming studies, but their inclusion in systematic reviews is contentious. Some reviewers exclude abstracts on the grounds that the quality of the study cannot be judged because of the inevitably limited detail. However, others include them on the grounds that abstracts provide the most up-to-date information. Nearly all the dissertations identified had been published as journal articles. However, it was found that only 30% of the meeting abstracts were converted to full publication after three years, which is considerably lower than the figure for RCTs, which is 56% [ 20 ]. This has the danger of producing bias in systematic reviews, if failure to publish is based on the size and direction of study results. Scattering of the diabetes epidemiology articles revealed that the 'core' literature in this field is concentrated in just six journals, with  Diabetes Care  alone containing about 14% of the articles. A similar concentration effect in journals was also shown in a study of 3400 science journals in the SCI database, where just 100 journals accounted for 22% of the published articles and 100 journals also accounted for 44% of cited articles [ 21 ]. This study has a number of limitations. The search to identify diabetes journals was restricted to English language journals only, as we were unable to assess articles in other languages. We did not compare the quality of the articles identified from databases outside MEDLINE and Embase. Also, when searching for articles, we were necessarily limited to the range of databases available to us. Finally, there may be databases inaccessible or unknown to us that cover foreign language and regional journals not indexed in MEDLINE and Embase. Such journals may carry studies of incidence which may seem of primarily local interest, but which may be useful contributions to the international body of evidence because they may show large variations in incidence, or in its relationship to possible aetiological risk factors. It is often useful to study the epidemiology of a disease where it is rare, as well as where it is common. However it is likely that studies which report high incidence are more likely to be published than those which report low incidence. Similarly with risk factors; a study which finds no link between factor x and disease y may be less likely to be published than one which does show a correlation [ 22 , 23 ]. There is a need for further research to see whether our findings apply to searching for epidemiological reviews of other diseases, and on measuring the sensitivity and specificity of various search filters to retrieve epidemiological studies in MEDLINE and Embase. We endorse Dickersin's suggestion of an international collaborative effort to establish an 'epidemiological Cochrane-like database' to identify all relevant studies and to begin systematically reviewing available data for important epidemiological questions [ 3 ].
Discussion
Our results showed that there was an overlap of only 59% in current English language 'diabetes journals' indexed by both MEDLINE and Embase. Also, a search for diabetes epidemiology articles across both MEDLINE and Embase showed that MEDLINE alone retrieved about 94% of the total articles; therefore, both databases should be searched. Embase appears to index more diabetes journals published outside the USA. Therefore, if searching is limited to MEDLINE only (Embase being less accessible and more expensive than MEDLINE, which is free via PubMED) this could potentially introduce a bias. Also, duplication of searching can be useful, as due to differences in indexing practices, a search of one database may retrieve something missed by the other.We also found that despite a wide range of additional databases searched after MEDLINE and Embase, no additional English language journal articles on diabetes epidemiology were identified. The LILACS database was a useful source of Spanish and Portuguese language articles on the epidemiology of diabetes in Latin American countries.Meeting abstracts appeared to be valuable sources of information on forthcoming studies, but their inclusion in systematic reviews is contentious. Some reviewers exclude abstracts on the grounds that the quality of the study cannot be judged because of the inevitably limited detail. However, others include them on the grounds that abstracts provide the most up-to-date information.Nearly all the dissertations identified had been published as journal articles. However, it was found that only 30% of the meeting abstracts were converted to full publication after three years, which is considerably lower than the figure for RCTs, which is 56% [ 20 ]. This has the danger of producing bias in systematic reviews, if failure to publish is based on the size and direction of study results.Scattering of the diabetes epidemiology articles revealed that the 'core' literature in this field is concentrated in just six journals, with  Diabetes Care  alone containing about 14% of the articles. A similar concentration effect in journals was also shown in a study of 3400 science journals in the SCI database, where just 100 journals accounted for 22% of the published articles and 100 journals also accounted for 44% of cited articles [ 21 ].Diabetes CareThis study has a number of limitations. The search to identify diabetes journals was restricted to English language journals only, as we were unable to assess articles in other languages. We did not compare the quality of the articles identified from databases outside MEDLINE and Embase. Also, when searching for articles, we were necessarily limited to the range of databases available to us.Finally, there may be databases inaccessible or unknown to us that cover foreign language and regional journals not indexed in MEDLINE and Embase. Such journals may carry studies of incidence which may seem of primarily local interest, but which may be useful contributions to the international body of evidence because they may show large variations in incidence, or in its relationship to possible aetiological risk factors.It is often useful to study the epidemiology of a disease where it is rare, as well as where it is common. However it is likely that studies which report high incidence are more likely to be published than those which report low incidence. Similarly with risk factors; a study which finds no link between factor x and disease y may be less likely to be published than one which does show a correlation [ 22 , 23 ].There is a need for further research to see whether our findings apply to searching for epidemiological reviews of other diseases, and on measuring the sensitivity and specificity of various search filters to retrieve epidemiological studies in MEDLINE and Embase.We endorse Dickersin's suggestion of an international collaborative effort to establish an 'epidemiological Cochrane-like database' to identify all relevant studies and to begin systematically reviewing available data for important epidemiological questions [ 3 ].Conclusions Searching MEDLINE and Embase appears to provide comprehensive coverage of the English language journal literature in diabetes epidemiology. LILACs is a useful source of Spanish and Portuguese language articles on diabetes epidemiology done in Latin American countries and published in regional journals not indexed in MEDLINE and Embase. Searching for meeting abstracts is recommended to alert reviewers to unpublished work. The volume of literature on diabetes epidemiology makes it impossible for one person to read everything. However the provision of systematic reviews makes keeping up with research manageable, and more reviews are needed. Our findings on scattering shows that the core literature in diabetes epidemiology is concentrated in a small number of core journals, and that in the absence of reviews, one can follow the field by reading these journals. It may also be reassuring that a good MEDLINE-only search will retrieve the vast majority of the relevant literature.
Conclusions
Searching MEDLINE and Embase appears to provide comprehensive coverage of the English language journal literature in diabetes epidemiology. LILACs is a useful source of Spanish and Portuguese language articles on diabetes epidemiology done in Latin American countries and published in regional journals not indexed in MEDLINE and Embase. Searching for meeting abstracts is recommended to alert reviewers to unpublished work.The volume of literature on diabetes epidemiology makes it impossible for one person to read everything. However the provision of systematic reviews makes keeping up with research manageable, and more reviews are needed. Our findings on scattering shows that the core literature in diabetes epidemiology is concentrated in a small number of core journals, and that in the absence of reviews, one can follow the field by reading these journals. It may also be reassuring that a good MEDLINE-only search will retrieve the vast majority of the relevant literature.Competing interests The author(s) declare that they have no competing interests.
Competing interests
The author(s) declare that they have no competing interests.Authors' contributions PR and NW conceived the study and drafted the initial manuscript. PR and LB collected and analysed the data. All authors contributed to and approved the final manuscript.
Authors' contributions
PR and NW conceived the study and drafted the initial manuscript. PR and LB collected and analysed the data. All authors contributed to and approved the final manuscript.Pre-publication history The pre-publication history for this paper can be accessed here:
Pre-publication history
The pre-publication history for this paper can be accessed here:


Calcium makes up just 2% of the human body, and 99% of it is sequestered in bones and teeth. The remainder exists within and around cells, influencing a variety of cellular processes, from transcription and cell survival to nerve signaling and muscle contraction. Calcium inhabits the intra- and extracellular space as a gradient, with extracellular concentrations some 1,000 times greater than those inside the cell. These gradients are maintained by calcium pumps. Calcium signaling operates mostly through voltage- and ligand-gated calcium channels (ligands are signaling molecules), both in the plasma membrane of the cell and in the membranes enclosing intracellular organelles. Signaling is typically initiated by an influx of calcium across the plasma membrane or by the release of calcium from an organelle, such as the endoplasmic reticulum. In neuron-to-neuron signaling, calcium signaling helps convert electrical signals into chemical signals in the form of neurotransmitters. The arrival of an electrical signal at a nerve terminal opens the many calcium channels in the nerve terminal, admitting a stream of calcium ions. The increased intracellular calcium concentration in turn releases the resident neurotransmitters accumulated in the nerve terminal, converting the electrical signal into a chemical signal. As neurotransmitters bind to their receptors in the next target neuron, they change the cell's membrane potential, prompting the neuron to generate an electrical signal, thereby converting the chemical signal back into an electrical one and completing the signaling circuit. Since calcium dynamics mediates most neuronal information flow, it can be used as a general measure of neural activity.Through an elaborate network of electrical activity, the brain encodes, combines, and interprets signals to process information about the world. Simultaneous measurement of this activity in multiple brain locations has provided valuable insight into how neural networks function. But since electrical recordings can't pinpoint activity in the fine branches of individual neurons or pick up biochemical (nonelectrical) signals, researchers are increasingly turning to approaches that measure calcium concentrations as a proxy for neuron activity. Now Mazahir Hasan et al. have created mice engineered to stably express two different kinds of fluorescent calcium indicator proteins (FCIPs) in the brain (the fluorescence produced by these proteins can be seen when the brain is viewed with a two-photon microscope). Because the indicators are incorporated into the mouse genomes, this approach offers the possibility of targeting specific cells (by using promoters that specify which cells the genes should be activated in), allowing researchers to map the activity of select neuron populations.Fluorescent proteins can be incorporated into a gene of interest to help researchers track that gene's protein in living tissue. FCIPs report calcium concentrations by changing fluorescence when they bind to calcium. Introducing calcium indicators into neural tissues was largely impractical, often failing to target specific cell types, until these genetically engineered indicators were developed in the late 1990s, allowing the desired specificity. While FCIPs have been used to good effect in worms, fruitflies, and zebrafish—and just recently in mouse muscle—they had not been stably and functionally expressed in the mammalian brain until now.To deliver the FCIPs to the mouse brain, Hasan et al. used a regulatory promoter called Ptet (in the tetracycline system), which offers the possibility of targeting the expression of the FCIPs in different neural populations. To test the functionality of the proteins, they used fluorescence microscopy to analyze neurons from mouse lines and found high levels of FCIP expression. The real test, however, was whether the FCIPs could fulfill their promise as a probe for calcium activity. When the authors electrically stimulated brain slices from a mouse line expressing moderate to high levels of FCIP (electrical stimulation is known to increase intracellular calcium concentration), fluorescence increased rapidly following the stimulus. Significant changes in FCIP fluorescence were also observed when live mice responded to odor stimulation. That “fast and robust” FCIP signals were detected in live animals responding to sensory stimulation, the authors argue, proves the promise of FCIPs as a reporter on the activity of select neural populations in living systems. And since these indicator proteins retain stable functional expression over time (8- to 12-week-old mice continued to express the proteins), they could help researchers track neuronal activity over extended periods.While a variety of bugs remain to be worked out with FCIPs—it's unclear, for example, why only the Ptet promoter generates high levels of FCIP expression in the brain and why not all neurons in a given population express the proteins—Hasan et al. demonstrate that the tetracycline system supports stable expression of the calcium indicators. The FCIP approach avoids the complications of invasive techniques like surgically administering dyes and produces a more interpretable signal, since the cell populations are already known. Because FCIPS can be used in living animals, they can reveal where and when neurons are firing. And because FCIP mice can be crossed with mice containing mutations in genes important for neural function, this method could reveal how specific genes contribute to the construction of neural networks.