news and views
Exome sequencing uncovers hidden pathways in familial and sporadic ALS Conceição Bettencourt & Henry Houlden
The most significant advances in our understanding of the etiology of neurodegenerative disorders, and perhaps all neurological disorders, have come from the identification of disease-causing genes. Most Mendelian genes have now been identified, forming a framework of disease pathways. Yet there are major challenges ahead in the investigation of genetically diverse, more common disorders such as ALS that are likely to be caused by an intricate combination of rare and common genetic variation, together with environmental and stochastic factors. ALS is a rapidly progressive and fatal degenerative disorder of the motor system, with a median survival time of just 3–5 years1. Our knowledge of the genetic factors behind ALS has improved substantially over the last few years (reviewed in refs. 2,3), and these discoveries have been made possible by large international collaborations and the parallel advances in genomic technologies (Fig. 1). These two factors have been effectively applied in two recent independent studies, by Cirulli et al.4 in Science and Freischmidt et al.5 in this issue of Nature Neuroscience. They add to the mix a new Mendelian risk gene, TBK1. Notably, the protein encoded by this gene connects to two essential cellular pathways of emerging interest in ALS research, namely autophagy (degradation of damaged cellular components) and inflammation (biological response to harmful stimuli)4.
Analysis strategies
Linkage analysis, candidate gene analysis
Exome sequencing
GWAS
Genome sequencing
TBK1 60 MATR3 CHCHD10 PFN1 SIGMAR1
40 C9orf72
SQSTM1
UBQLN2 VCP
20
SPG11
OPTN FUS TARDBP SOD1
Conceição Bettencourt and Henry Houlden are in the Department of Molecular Neuroscience and the Medical Research Council Centre for Neuromuscular Diseases, The National Hospital for Neurology and Neurosurgery and University College London Institute of Neurology, London, UK. e-mail: [email protected]
OPTN. Mutations in this gene were initially identified in a few ALS-bearing families6, but OPTN variants also appear to contribute to sporadic disease4. The genetic search for disease variants in ALS has relied on the development of new sequencing technologies, and this has dictated the recent boom in discoveries (Fig. 1). These technologies are providing unprecedented throughput, scalability and speed. Wholeexome sequencing (WES), which obtains single base-pair resolution of variation in all of the protein-coding regions, or exons, of all
As in other genetic neurodegenerative disorders, there are familial forms of ALS, representing ~10% of cases, with several possible means of transmission from one generation to the next (such as dominant, recessive or X-linked), as well as sporadic forms, with no history of the disease in close relatives. The effect of genetic factors is expected to be very strong in the former and likely milder in the latter, but the same genes can be involved in both. The well-known C9orf72 repeat expansion, for example, is present in both familial ALS (38%) and sporadic ALS (6%)3. Another example is
Genes associated with ALS
npg
© 2015 Nature America, Inc. All rights reserved.
Amyotrophic lateral sclerosis (ALS) is a complex and as yet untreatable neurodegenerative disorder. We discuss two examples of exome sequencing in large international collections of familial and sporadic ALS cases that are revealing new and potentially treatable pathways, such as those involving autophagy and neuroinflammation.
1990
1995
ALS2
2000
SETX
VAPB
DCTN1 2005 Year
2010
2015
Figure 1 Progress of genetic findings related to ALS etiology and pathogenesis. Mendelian ALS genes are shown, with the sizes of the circles indicating the relative contribution of each gene toward the explanation of ALS cases. The effect of TBK1 is likely to expand, as indicated by the dashed circle outline, in the near future as additional studies emerge.
nature neuroscience volume 18 | number 5 | May 2015
611
npg
© 2015 Nature America, Inc. All rights reserved.
news and views known human genes, has been successfully used for gene discovery in family studies (for example, see refs. 7,8) and is beginning to be used in large-scale sequencing studies of both familial and sporadic disease (for example, see refs. 4,5). WES analysis generates over 20,000 genetic variants per individual, most of which will be benign. Analysis of potentially harmful variants shared by patients in a family or casecontrol association studies then follows to discover disease-associated genes and variants. A large collaborative effort by Cirulli et al.4, involving researchers from more than two dozen laboratories, recently exome-sequenced samples from 2,874 ALS patients, mostly with sporadic ALS, and 6,405 controls. This is the largest number of ALS patients ever sequenced in a single study, and it led to the identification of a genome-wide association between rare, non-benign variants in TBK1 and ALS. This gene contributed to ALS in about 1% of the cases in this study. The TBK1 finding was confirmed in another collaborative study, by Freischmidt et al.5, in which WES was performed in 252 ALS patients, all with familial ALS, and 827 controls. Linkage analysis in the four largest families, which assesses how likely it is that the disease and the genetic marker are co-segregating in the pedigree, resulted in an aggregate LOD score of 4.6 (where >3 is considered to be significant). The finding of the same gene in two different studies with different strategies strengthens the evidence for a role of TBK1 in both familial and sporadic ALS. Both studies pinpoint a dominant effect of rare TBK1 variants in ALS; that is, only one copy of the genetic defect is needed to affect disease predisposition. This is consistent with what we know for other ALS genes. The genetic defects found can either cause a single change in a specific position of the protein (missense variants) or cause the reduction of the amount of functional protein (loss-of-function variants). The study by Freischmidt et al.5 particularly emphasizes the damaging effect of loss-offunction variants. The effect of missense variants may depend on their location in the gene. Unsurprisingly, Cirulli et al.4 also found a genome-wide significant association between SOD1 variants and ALS. Identified in 1993 as the first ALS-associated gene, SOD1 is one of the major genetic contributors to ALS, ranking second after C9orf72 (Fig. 1). Patients also showed an excess of rare variants when compared with controls in other known ALS genes (TARDBP, OPTN, VCP and SPG11). For some of these genes (for example, TARDBP and VCP), rare variants in ALS patients preferentially clustered in certain regions of the gene, again suggesting that location matters and that those regions are more vulnerable to damaging 612
effects. For instance, genetic variants in important domains of a protein may induce conformational changes that will disrupt its normal function. Larger studies are likely to be needed to detect other genes with smaller effects on ALS predisposition. Although large genome-wide association studies (GWAS) have been conducted in ALS, including one by Fogh et al.9 with 6,100 cases and 7,125 controls, they all failed to detect an association with TBK1. A caveat of those studies is that they were based on genotyping platforms of common genetic variation (singlenucleotide polymorphisms (SNPs) present in more than 1% of the population). Although those common SNPs can likely tag causal variants, they are usually benign themselves. By using WES, we are now capable of capturing rare genetic variants that are expected to have damaging effects, such as the TBK1 variants found in the recent studies4,5. Oligogenic inheritance—that is, the contribution of two or more genetic variants with additive or synergistic deleterious effects— has been reported as being relevant to ALS pathogenesis. It can result in increased risk of developing the disease and/or in an earlier age at first symptoms in people carrying variants in multiple genes10. Freischmidt et al.5 report a family with mutations in two ALS genes (TBK1 and FUS) as tending to have an earlier age at onset. The oligogenic hypothesis was not confirmed in the study by Cirulli et al.4, but it would be worth exploring in large studies. Apart from influencing disease risk, genetic variants may have other effects on clinical features, such as the age at disease onset or the survival time after the appearance of the disease. Cirulli et al.4 suggest that patients with variants in the d-amino acid oxidase gene (DAO) may have shorter survival times. The product of this gene is required for the clearance of an important signaling molecule in the brain called d-serine, which seems to accumulate in ALS and may lead to neurotoxicity11. d-amino acid oxidase might therefore be a potential therapeutic target in ALS. WES is offering rapid progress toward a complete genetic landscape of ALS (for example, refs. 4,5) and other heterogeneous neurological diseases (for example, ref. 12). However, not all types of genetic variation can be captured by WES. For instance, genetic variants outside of protein coding regions, large expansions of repeat motifs or large genomic rearrangements would be missed by WES. Evolving sequencing methods will enhance our ability to further hunt disease-associated genetic variants. Massive sequencing of gene panels (for example, ref. 13), with the possibility of adding noncoding regions, or even
sequencing the whole genome can overcome some of those limitations. Work by our group14 and by others15 supports the idea that, in genetically heterogeneous neurodegenerative diseases, genes associated with similar clinical phenotypes tend to converge on a few key molecular pathways. Notably, the new ALS gene (TBK1) converges, together with previously known ALS genes (such as OPTN and SQSTM1) on autophagy and neuroinflammation pathways4. We believe these pathways provide reservoirs of additional genes to explore for association with the disease, but the relevance of these findings goes well beyond knowing more about disease biology. They provide possible points for therapeutic intervention and should become the focus of targeted drug-development efforts. This new ALS-associated gene is particularly promising, as compounds that affect TBK1 signaling have already been developed for use in cancer given its involvement in tumor cell survival. Many ALS-associated genes are associated with a wide spectrum of diseases. This is the case for the well-known genetic overlap between ALS and frontotemporal dementia. Genetic variants in C9orf72, TARDBP (also known as TDP-43), FUS, VCP and now TBK1 can be involved in both diseases. Within some families, the same genetic defect can manifest as ALS in one individual, as frontotemporal dementia in another, or even as both in others. In some cases, the overlap goes beyond the brain and crosses to the other side of neuromuscular junction (for example, VCP mutations are also seen in muscle diseases)3. The more we study these genes, the wider the disease spectrum is expected to be, and the more we hope can be amenable to therapeutic intervention. Collaborative and multidisciplinary efforts forming open-access data ‘silos’ are certainly the way forward in our understanding of heterogeneous diseases. With the formation of initiatives such as the Genomics England 100,000 Genomes Project and the Clinical Interpretation Partnerships (GeCIPs) to concentrate and focus expertise, there will undoubtedly be a further raft of disease-associated genes delivered by genome sequencing. COMPETING FINANCIAL INTERESTS The authors declare no competing financial interests. 1. Vucic, S., Rothstein, J.D. & Kiernan, M.C. Trends Neurosci. 37, 433–442 (2014). 2. Marangi, G. & Traynor, B.J. Brain Res. published online, doi:10.1016/j.brainres.2014.10.009 (12 October 2014). 3. Leblond, C.S., Kaneb, H.M., Dion, P.A. & Rouleau, G.A. Exp. Neurol. 262, 91–101 (2014). 4. Cirulli, E.T. et al. Science 347, 1436–1441 (2015). 5. Freischmidt, A. et al. Nat. Neurosci. 18, 631–636 (2015). 6. Maruyama, H. et al. Nature 465, 223–226 (2010). 7. Johnson, J.O. et al. Neuron 68, 857–864 (2010). 8. Bannwarth, S., et al. Brain 137, 2329–2345 (2014).
volume 18 | number 5 | May 2015 nature neuroscience
news and views 9. Fogh, I. et al. Hum. Mol. Genet. 23, 2220–2231 (2014). 10. Cady, J. et al. Ann. Neurol. 77, 100–113 (2015). 11. Iguchi, Y., Katsuno, M., Ikenaka, K., Ishigaki, S. & Sobue, G. J. Neurol. 260, 2917–2927 (2013).
12. Cottenie, E. et al. Am. J. Hum. Genet. 95, 590–601 (2014). 13. Morgan, S. et al. Neurobiol. Aging 36, 1600.e5–1600.e8 (2015).
14. Bettencourt, C. et al. JAMA Neurol. 71, 831–839 (2014). 15. Novarino, G. et al. Science 343, 506–511 (2014).
Stem cells: slow and steady wins the race Wieland B Huttner
There are at least two neurogenic niches in the adult mammalian brain: the subependymal zone (SEZ) of the lateral ventricle and the subgranular zone (SGZ) of the hippocampal dentate gyrus. These generate neurons that contribute to various cognitive functions and innate behaviors1. It has remained unclear, however, how continuous neurogenesis can be maintained throughout life in the adult brain. In the SGZ, some adult NSCs divide over a long period of time, whereas others divide a limited number of times over a short period and then undergo terminal differentiation2,3. Quiescence has been proposed to be important for the long-term maintenance of adult NSCs, as these cells undergo premature exhaustion when cyclin-dependent kinase (CDK) inhibitors are deleted4,5. A recent study based on clonal analysis in the SEZ revealed that actively dividing adult NSCs are short-lived and rapidly generate multiple waves of expanding progeny before becoming exhausted6. An NSC division may therefore result in the loss of the NSC, with cell division likely being traded off against loss of NSC self-renewal. An important question then arises: how are quiescent adult NSCs generated and maintained during development without exhaustion, given that embryonic neural progenitor cells (NPCs), which exhibit some stem cell–like features, need to actively divide to build the brain? In this issue of Nature Neuroscience, Furutachi et al.7 provide an answer by showing that the embryonic origin of adult NSCs in the mouse SEZ is a distinct, slowly dividing subpopulation of embryonic NPCs (Fig. 1). The researchers took advantage of GFPtagged histone 2B (H2B) dilution analysis to label slowly dividing cells, an approach that was originally used in other stem cell systems8. H2B-GFP was expressed transiently in mouse embryos using a tetracycline-inducible Wieland B. Huttner is at the Max Planck Institute of Molecular Cell Biology and Genetics, Dresden, Germany. e-mail: [email protected]
promoter system at embryonic day (E) 9.5, and the presence of H2B-GFP label–retaining cells (LRCs) was examined later during development. Strikingly, LRCs were detected in the SEZ even at postnatal day 28, with this LRC population accounting for more than 70% of NSCs (as defined by cell morphology and marker expression) in the lateral wall of the SEZ. Moreover, most NSCs activated by injury (treatment with cytosine arabinoside) were also found to be LRCs derived from the slowly dividing embryonic NPCs. These observations demonstrate that the cells from which adult NSCs originate become quiescent between E13 and E15, much earlier than previously assumed, and are set aside from other, rapidly cycling NPCs during development7. Embryo
Embryonic NPCs sequentially produce neurons and glial cells during brain development1. Given that adult NSCs show some features typical of glial cells, such as expression of the astrocyte markers GFAP and GLAST, these NSCs have been thought to belong to the same lineage as late-stage radial glial cells, which mostly produce glial cell types1. Indeed, a population-level study revealed that postnatal radial glial cells (labeled with a thymidine analog at birth), which produce parenchymal astrocytes, oligodendrocytes and ependymal cells after having earlier generated olfactory interneurons, subsequently give rise to NSCs in the adult SEZ1,9. However, it has been unclear whether these postnatal radial glial cells (that is, NPCs) in the SEZ are homogeneous or heterogeneous. Furutachi Adult
Embryonic NPCs: slowly dividing
Adult NSCs: slowly dividing
p57
Embryonic NPCs: rapidly dividing
Neuron
Marina Corral Spence/Nature Publishing Group
npg
© 2015 Nature America, Inc. All rights reserved.
How are quiescent adult neural stem cells (NSCs) generated during development? A study now identifies a reserve population of p57-expressing, slowly dividing embryonic neural progenitors that later give rise to adult NSCs.
Glia
Figure 1 The embryonic origin of adult NSCs in the SEZ. Expression of p57 converts rapidly dividing NPCs to slowly dividing NPCs that then turn into slowly dividing adult NSCs.
nature neuroscience volume 18 | number 5 | May 2015
613
Exome sequencing uncovers hidden pathways in familial and sporadic ALS Conceição Bettencourt & Henry Houlden
The most significant advances in our understanding of the etiology of neurodegenerative disorders, and perhaps all neurological disorders, have come from the identification of disease-causing genes. Most Mendelian genes have now been identified, forming a framework of disease pathways. Yet there are major challenges ahead in the investigation of genetically diverse, more common disorders such as ALS that are likely to be caused by an intricate combination of rare and common genetic variation, together with environmental and stochastic factors. ALS is a rapidly progressive and fatal degenerative disorder of the motor system, with a median survival time of just 3–5 years1. Our knowledge of the genetic factors behind ALS has improved substantially over the last few years (reviewed in refs. 2,3), and these discoveries have been made possible by large international collaborations and the parallel advances in genomic technologies (Fig. 1). These two factors have been effectively applied in two recent independent studies, by Cirulli et al.4 in Science and Freischmidt et al.5 in this issue of Nature Neuroscience. They add to the mix a new Mendelian risk gene, TBK1. Notably, the protein encoded by this gene connects to two essential cellular pathways of emerging interest in ALS research, namely autophagy (degradation of damaged cellular components) and inflammation (biological response to harmful stimuli)4.
Analysis strategies
Linkage analysis, candidate gene analysis
Exome sequencing
GWAS
Genome sequencing
TBK1 60 MATR3 CHCHD10 PFN1 SIGMAR1
40 C9orf72
SQSTM1
UBQLN2 VCP
20
SPG11
OPTN FUS TARDBP SOD1
Conceição Bettencourt and Henry Houlden are in the Department of Molecular Neuroscience and the Medical Research Council Centre for Neuromuscular Diseases, The National Hospital for Neurology and Neurosurgery and University College London Institute of Neurology, London, UK. e-mail: [email protected]
OPTN. Mutations in this gene were initially identified in a few ALS-bearing families6, but OPTN variants also appear to contribute to sporadic disease4. The genetic search for disease variants in ALS has relied on the development of new sequencing technologies, and this has dictated the recent boom in discoveries (Fig. 1). These technologies are providing unprecedented throughput, scalability and speed. Wholeexome sequencing (WES), which obtains single base-pair resolution of variation in all of the protein-coding regions, or exons, of all
As in other genetic neurodegenerative disorders, there are familial forms of ALS, representing ~10% of cases, with several possible means of transmission from one generation to the next (such as dominant, recessive or X-linked), as well as sporadic forms, with no history of the disease in close relatives. The effect of genetic factors is expected to be very strong in the former and likely milder in the latter, but the same genes can be involved in both. The well-known C9orf72 repeat expansion, for example, is present in both familial ALS (38%) and sporadic ALS (6%)3. Another example is
Genes associated with ALS
npg
© 2015 Nature America, Inc. All rights reserved.
Amyotrophic lateral sclerosis (ALS) is a complex and as yet untreatable neurodegenerative disorder. We discuss two examples of exome sequencing in large international collections of familial and sporadic ALS cases that are revealing new and potentially treatable pathways, such as those involving autophagy and neuroinflammation.
1990
1995
ALS2
2000
SETX
VAPB
DCTN1 2005 Year
2010
2015
Figure 1 Progress of genetic findings related to ALS etiology and pathogenesis. Mendelian ALS genes are shown, with the sizes of the circles indicating the relative contribution of each gene toward the explanation of ALS cases. The effect of TBK1 is likely to expand, as indicated by the dashed circle outline, in the near future as additional studies emerge.
nature neuroscience volume 18 | number 5 | May 2015
611
npg
© 2015 Nature America, Inc. All rights reserved.
news and views known human genes, has been successfully used for gene discovery in family studies (for example, see refs. 7,8) and is beginning to be used in large-scale sequencing studies of both familial and sporadic disease (for example, see refs. 4,5). WES analysis generates over 20,000 genetic variants per individual, most of which will be benign. Analysis of potentially harmful variants shared by patients in a family or casecontrol association studies then follows to discover disease-associated genes and variants. A large collaborative effort by Cirulli et al.4, involving researchers from more than two dozen laboratories, recently exome-sequenced samples from 2,874 ALS patients, mostly with sporadic ALS, and 6,405 controls. This is the largest number of ALS patients ever sequenced in a single study, and it led to the identification of a genome-wide association between rare, non-benign variants in TBK1 and ALS. This gene contributed to ALS in about 1% of the cases in this study. The TBK1 finding was confirmed in another collaborative study, by Freischmidt et al.5, in which WES was performed in 252 ALS patients, all with familial ALS, and 827 controls. Linkage analysis in the four largest families, which assesses how likely it is that the disease and the genetic marker are co-segregating in the pedigree, resulted in an aggregate LOD score of 4.6 (where >3 is considered to be significant). The finding of the same gene in two different studies with different strategies strengthens the evidence for a role of TBK1 in both familial and sporadic ALS. Both studies pinpoint a dominant effect of rare TBK1 variants in ALS; that is, only one copy of the genetic defect is needed to affect disease predisposition. This is consistent with what we know for other ALS genes. The genetic defects found can either cause a single change in a specific position of the protein (missense variants) or cause the reduction of the amount of functional protein (loss-of-function variants). The study by Freischmidt et al.5 particularly emphasizes the damaging effect of loss-offunction variants. The effect of missense variants may depend on their location in the gene. Unsurprisingly, Cirulli et al.4 also found a genome-wide significant association between SOD1 variants and ALS. Identified in 1993 as the first ALS-associated gene, SOD1 is one of the major genetic contributors to ALS, ranking second after C9orf72 (Fig. 1). Patients also showed an excess of rare variants when compared with controls in other known ALS genes (TARDBP, OPTN, VCP and SPG11). For some of these genes (for example, TARDBP and VCP), rare variants in ALS patients preferentially clustered in certain regions of the gene, again suggesting that location matters and that those regions are more vulnerable to damaging 612
effects. For instance, genetic variants in important domains of a protein may induce conformational changes that will disrupt its normal function. Larger studies are likely to be needed to detect other genes with smaller effects on ALS predisposition. Although large genome-wide association studies (GWAS) have been conducted in ALS, including one by Fogh et al.9 with 6,100 cases and 7,125 controls, they all failed to detect an association with TBK1. A caveat of those studies is that they were based on genotyping platforms of common genetic variation (singlenucleotide polymorphisms (SNPs) present in more than 1% of the population). Although those common SNPs can likely tag causal variants, they are usually benign themselves. By using WES, we are now capable of capturing rare genetic variants that are expected to have damaging effects, such as the TBK1 variants found in the recent studies4,5. Oligogenic inheritance—that is, the contribution of two or more genetic variants with additive or synergistic deleterious effects— has been reported as being relevant to ALS pathogenesis. It can result in increased risk of developing the disease and/or in an earlier age at first symptoms in people carrying variants in multiple genes10. Freischmidt et al.5 report a family with mutations in two ALS genes (TBK1 and FUS) as tending to have an earlier age at onset. The oligogenic hypothesis was not confirmed in the study by Cirulli et al.4, but it would be worth exploring in large studies. Apart from influencing disease risk, genetic variants may have other effects on clinical features, such as the age at disease onset or the survival time after the appearance of the disease. Cirulli et al.4 suggest that patients with variants in the d-amino acid oxidase gene (DAO) may have shorter survival times. The product of this gene is required for the clearance of an important signaling molecule in the brain called d-serine, which seems to accumulate in ALS and may lead to neurotoxicity11. d-amino acid oxidase might therefore be a potential therapeutic target in ALS. WES is offering rapid progress toward a complete genetic landscape of ALS (for example, refs. 4,5) and other heterogeneous neurological diseases (for example, ref. 12). However, not all types of genetic variation can be captured by WES. For instance, genetic variants outside of protein coding regions, large expansions of repeat motifs or large genomic rearrangements would be missed by WES. Evolving sequencing methods will enhance our ability to further hunt disease-associated genetic variants. Massive sequencing of gene panels (for example, ref. 13), with the possibility of adding noncoding regions, or even
sequencing the whole genome can overcome some of those limitations. Work by our group14 and by others15 supports the idea that, in genetically heterogeneous neurodegenerative diseases, genes associated with similar clinical phenotypes tend to converge on a few key molecular pathways. Notably, the new ALS gene (TBK1) converges, together with previously known ALS genes (such as OPTN and SQSTM1) on autophagy and neuroinflammation pathways4. We believe these pathways provide reservoirs of additional genes to explore for association with the disease, but the relevance of these findings goes well beyond knowing more about disease biology. They provide possible points for therapeutic intervention and should become the focus of targeted drug-development efforts. This new ALS-associated gene is particularly promising, as compounds that affect TBK1 signaling have already been developed for use in cancer given its involvement in tumor cell survival. Many ALS-associated genes are associated with a wide spectrum of diseases. This is the case for the well-known genetic overlap between ALS and frontotemporal dementia. Genetic variants in C9orf72, TARDBP (also known as TDP-43), FUS, VCP and now TBK1 can be involved in both diseases. Within some families, the same genetic defect can manifest as ALS in one individual, as frontotemporal dementia in another, or even as both in others. In some cases, the overlap goes beyond the brain and crosses to the other side of neuromuscular junction (for example, VCP mutations are also seen in muscle diseases)3. The more we study these genes, the wider the disease spectrum is expected to be, and the more we hope can be amenable to therapeutic intervention. Collaborative and multidisciplinary efforts forming open-access data ‘silos’ are certainly the way forward in our understanding of heterogeneous diseases. With the formation of initiatives such as the Genomics England 100,000 Genomes Project and the Clinical Interpretation Partnerships (GeCIPs) to concentrate and focus expertise, there will undoubtedly be a further raft of disease-associated genes delivered by genome sequencing. COMPETING FINANCIAL INTERESTS The authors declare no competing financial interests. 1. Vucic, S., Rothstein, J.D. & Kiernan, M.C. Trends Neurosci. 37, 433–442 (2014). 2. Marangi, G. & Traynor, B.J. Brain Res. published online, doi:10.1016/j.brainres.2014.10.009 (12 October 2014). 3. Leblond, C.S., Kaneb, H.M., Dion, P.A. & Rouleau, G.A. Exp. Neurol. 262, 91–101 (2014). 4. Cirulli, E.T. et al. Science 347, 1436–1441 (2015). 5. Freischmidt, A. et al. Nat. Neurosci. 18, 631–636 (2015). 6. Maruyama, H. et al. Nature 465, 223–226 (2010). 7. Johnson, J.O. et al. Neuron 68, 857–864 (2010). 8. Bannwarth, S., et al. Brain 137, 2329–2345 (2014).
volume 18 | number 5 | May 2015 nature neuroscience
news and views 9. Fogh, I. et al. Hum. Mol. Genet. 23, 2220–2231 (2014). 10. Cady, J. et al. Ann. Neurol. 77, 100–113 (2015). 11. Iguchi, Y., Katsuno, M., Ikenaka, K., Ishigaki, S. & Sobue, G. J. Neurol. 260, 2917–2927 (2013).
12. Cottenie, E. et al. Am. J. Hum. Genet. 95, 590–601 (2014). 13. Morgan, S. et al. Neurobiol. Aging 36, 1600.e5–1600.e8 (2015).
14. Bettencourt, C. et al. JAMA Neurol. 71, 831–839 (2014). 15. Novarino, G. et al. Science 343, 506–511 (2014).
Stem cells: slow and steady wins the race Wieland B Huttner
There are at least two neurogenic niches in the adult mammalian brain: the subependymal zone (SEZ) of the lateral ventricle and the subgranular zone (SGZ) of the hippocampal dentate gyrus. These generate neurons that contribute to various cognitive functions and innate behaviors1. It has remained unclear, however, how continuous neurogenesis can be maintained throughout life in the adult brain. In the SGZ, some adult NSCs divide over a long period of time, whereas others divide a limited number of times over a short period and then undergo terminal differentiation2,3. Quiescence has been proposed to be important for the long-term maintenance of adult NSCs, as these cells undergo premature exhaustion when cyclin-dependent kinase (CDK) inhibitors are deleted4,5. A recent study based on clonal analysis in the SEZ revealed that actively dividing adult NSCs are short-lived and rapidly generate multiple waves of expanding progeny before becoming exhausted6. An NSC division may therefore result in the loss of the NSC, with cell division likely being traded off against loss of NSC self-renewal. An important question then arises: how are quiescent adult NSCs generated and maintained during development without exhaustion, given that embryonic neural progenitor cells (NPCs), which exhibit some stem cell–like features, need to actively divide to build the brain? In this issue of Nature Neuroscience, Furutachi et al.7 provide an answer by showing that the embryonic origin of adult NSCs in the mouse SEZ is a distinct, slowly dividing subpopulation of embryonic NPCs (Fig. 1). The researchers took advantage of GFPtagged histone 2B (H2B) dilution analysis to label slowly dividing cells, an approach that was originally used in other stem cell systems8. H2B-GFP was expressed transiently in mouse embryos using a tetracycline-inducible Wieland B. Huttner is at the Max Planck Institute of Molecular Cell Biology and Genetics, Dresden, Germany. e-mail: [email protected]
promoter system at embryonic day (E) 9.5, and the presence of H2B-GFP label–retaining cells (LRCs) was examined later during development. Strikingly, LRCs were detected in the SEZ even at postnatal day 28, with this LRC population accounting for more than 70% of NSCs (as defined by cell morphology and marker expression) in the lateral wall of the SEZ. Moreover, most NSCs activated by injury (treatment with cytosine arabinoside) were also found to be LRCs derived from the slowly dividing embryonic NPCs. These observations demonstrate that the cells from which adult NSCs originate become quiescent between E13 and E15, much earlier than previously assumed, and are set aside from other, rapidly cycling NPCs during development7. Embryo
Embryonic NPCs sequentially produce neurons and glial cells during brain development1. Given that adult NSCs show some features typical of glial cells, such as expression of the astrocyte markers GFAP and GLAST, these NSCs have been thought to belong to the same lineage as late-stage radial glial cells, which mostly produce glial cell types1. Indeed, a population-level study revealed that postnatal radial glial cells (labeled with a thymidine analog at birth), which produce parenchymal astrocytes, oligodendrocytes and ependymal cells after having earlier generated olfactory interneurons, subsequently give rise to NSCs in the adult SEZ1,9. However, it has been unclear whether these postnatal radial glial cells (that is, NPCs) in the SEZ are homogeneous or heterogeneous. Furutachi Adult
Embryonic NPCs: slowly dividing
Adult NSCs: slowly dividing
p57
Embryonic NPCs: rapidly dividing
Neuron
Marina Corral Spence/Nature Publishing Group
npg
© 2015 Nature America, Inc. All rights reserved.
How are quiescent adult neural stem cells (NSCs) generated during development? A study now identifies a reserve population of p57-expressing, slowly dividing embryonic neural progenitors that later give rise to adult NSCs.
Glia
Figure 1 The embryonic origin of adult NSCs in the SEZ. Expression of p57 converts rapidly dividing NPCs to slowly dividing NPCs that then turn into slowly dividing adult NSCs.
nature neuroscience volume 18 | number 5 | May 2015
613
