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Evolutionary Expansion of Human Cerebellar Germinal Zones
TINS 1568 No. of Pages 3
Trends in Neurosciences
Spotlight
Evolutionary Expansion of Human Cerebellar Germinal Zones
the cellular processes that give rise to the cerebellum have been understudied and human cerebellar development remains poorly understood despite a long history of studies on model Matthew G. Keefe1,2 and vertebrates. Haldipur et al. have revealed distinct features of human cereTomasz J. Nowakowski1,2,3,* bellar development that may have Haldipur et al. explored the devel- significant implications for future underopmental origins of the human cer- standing of cerebellar organization and ebellum, which has gained growing diseases [5].
appreciation for its involvement in human cognition. The authors discovered human-unique expansion and maintenance of cerebellar germinal zones, reminiscent of processes in the developing human cerebral cortex necessary for generating expanded neuronal populations. The human brain contains an astonishing diversity of cell types distributed across hundreds of anatomically and functionally defined structures. Comparative studies of brain evolution have largely focused on anatomically unique structures, such as the disproportionately expanded cerebral cortex which represents a major adaptation that enabled increased cognitive capacity in hominins. Other structures that are more anatomically conserved across mammals have been assumed to be functionally conserved, and therefore have received less attention. One such structure, the cerebellum, has recently been reexamined with functional brain imaging studies, identifying a wide range of association functions linked to intellect and social cognition [1,2] in addition to its roles in motor circuits. New studies of the cerebellum, fueled by interest in its cognitive functions, have shown that cerebellar association zones may be disproportionately expanded in humans [2,3]. Defects in cerebellar development are thought to underlie a range of disorders ranging from ataxia to autism spectrum disorders [4], but
The developmental processes underlying the emergence of forebrain structures, in particular the cerebral cortex, are well understood and provide useful context for the authors’ findings. In the developing cerebral cortex, neurogenesis begins with progenitor cells known as ventricular radial glia, which have cell bodies located near the ventricular surface and long processes that physically span the ventricular to pial limits of the tissue. In both humans and mice, ventricular radial glia give rise to a secondary proliferative population that resides in the subventricular zone (SVZ); an anatomically distinct germinal zone that is expanded in human and primate development. The human SVZ contains outer radial glia that possess long fibers extending to the pial surface, but no ventricular contacts, and has been hypothesized to underlie the expansion of neuronal populations in the developing human cerebral cortex [6–8]. This emergence of an anatomically distinct subventricular zone and the unique ecosystems of secondary progenitors have been largely unexplored outside of the forebrain. In mice and humans, the cerebellar anlage develops in the most anterior segment of the hindbrain (Figure 1A) and gives rise to an astonishing number of cerebellar neurons – as many as 80% of human neurons reside in the cerebellum [9]. Cerebellar neurons emerge from two major germinal zones: the cerebellar ventricular zone (VZ), where GABAergic neurons are born, and the rhombic lip, which gives rise to
glutamatergic neuron progenitors that populate the future granule layers and the cerebellar nuclei (Figure 1B,I). Haldipur et al. sought to compare these two germinal zones between mouse and human brains, starting with a broad characterization of the morphology and organization at different stages of development. In striking contrast to the simple organization of mouse cerebellar ventricular and rhombic lip radial glia into single proliferative layers along the ventricular surface, the germinal zones of the human cerebellum and rhombic lip expand beyond a simple VZ to form defined SVZs (Figure 1G,H). Gene expression analysis of the human rhombic lip has identified persistent expression of genes related to organ size control, such as the HIPPO and WNT signaling pathways, and genes involved in the maturation and expansion of cortical radial glia and neural progenitors, including key outer radial glia genes. Cells of the human rhombic lip SVZ are morphologically similar to cerebral cortical outer radial glia, with unipolar processes that extend away from the ventricle and no apparent ventricle-contacting fiber (Figure 1H). Together, the shared gene expression profiles and morphological characteristics between human rhombic lip SVZ progenitors and cortical outer radial glia suggest that the same molecular pathways that support the developmental expansion of cortical progenitors may have also been co-opted during evolution to support the expansion of the human cerebellar ventricular and rhombic lip germinal zones. While these findings identify novel similarities between cerebral and cerebellar cortex progenitors, the cerebellum remains unique in terms of developmental timing. The cerebellum develops far earlier than the cerebral cortex, with Purkinje cells generated from the cerebellar VZ arising before 8 weeks post conception (pcw), placing them among the earliestborn neurons of the CNS; by contrast, peak neurogenesis in the cerebral Trends in Neurosciences, Month 2019, Vol. xx, No. xx
1
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Trends in Neurosciences
Figure 1. Overview of Human Cerebellar Development across Space and Time. (A) Schematic of a developing human embryo CNS colored by major divisions. The cerebellum arises from the most anterior region of the rhombencephalon, rhombomere I (pink). (B) Developmental trajectory showing the three major neurogenic regions of the cerebellum and their cellular output (green: GABAergic, orange: glutamatergic). (C–E) Highlight of the predominant neurogenic zone at different timepoints and the migration of their progeny. (C) Between 4 and 8 weeks post-conception the early cerebellar anlage displays a broad ventricular zone (VZ) with an elaborated subventricular zone (SVZ) and generates GABAergic neuron precursors. (D) By the second trimester, the cerebellar ventricular zone has receded to a single layer, while the rhombic lip expands and becomes internalized in the developing cerebellum. The rhombic lip generates glutamatergic neurons and granule cell progenitors, which migrate tangentially to form the external granule layer. (E) In the third trimester, the rhombic lip persists in the developing cerebellum. Neuronal progenitors derived from the rhombic lip that form the external germinal layer proliferate to generate granule cells. (F) The mature cerebellum undergoes further foliation and is comprised of distinct layers of GABAergic neurons (green) in the Purkinje cell layer and glutamatergic neurons (orange) in the internal granule layer. (G) Expansion of the cerebellar VZ to form a distinct subventricular zone. Exact progenitor composition and neuronal lineage is unknown (indicated by dotted lines and question mark). (H) Similar to G, expansion of the rhombic lip to include a proliferative subventricular zone. (I) Connectivity diagram showing organization of cells during development and in the mature cerebellum. In development, external granule cell progenitors give rise to granule cells that migrate inwards and comprise the internal granule layer of the mature cerebellar cortex. Abbreviations: BP, basal progenitor; RG, radial glia.
cortex occurs around 16 pcw. Within the human cerebellum, Haldipur et al. discovered that distinct neurogenic zones follow individual timelines (Figure 1C–E) with significant temporal expansion 2
Trends in Neurosciences, Month 2019, Vol. xx, No. xx
compared to mice [10]. The human cerebellar VZ undergoes massive expansion between 4 and 8 pcw before extinguishing its proliferative potential and receding to a single cell layer. The rhombic
lip germinal zone remains small during this period, but undergoes a significant expansion beginning around 11 pcw and persists through all developmental stages analyzed.
Trends in Neurosciences
Despite stark differences between their neurogenic timing, the developmental trajectories of human cerebellar VZ and rhombic lip germinal zones are remarkably similar in that they both involve the generation of SVZ regions and outer radial glialike cells. Haldipur et al. found that this similarity may be limited to humans. By investigating developing non-human primate tissue, the authors revealed that the macaque rhombic lip does not expand spatially nor does it become internalized into the cerebellum. Future studies will be needed to elucidate the evolutionary history and molecular pathways underlying rhombic lip expansion. Although its origins are unclear, the human-specific, long-term maintenance of the rhombic lip seems to be critical for normal development of the human cerebellum. In fact, Haldipur et al. found that in 80% of cases of Dandy– Walker malformation, which is characterized by hypoplasia of the cerebellar vermis (the region of the cerebellum linked to cognition), the rhombic lip was absent or not internalized.
including the expanded and elaborated germinal zones in the cerebellum and the rhombic lip, and the presence of outer radial glia-like cells during peak periods of neurogenesis. The similarities between cerebellar and cortical germinal zones raise exciting questions about the molecular and cellular substrates underlying the evolutionary divergence of developmental trajectories between humans and other species. More broadly, Haldipur et al. underscore the value of comparative neurodevelopmental studies in identifying divergent developmental processes that may address the role of cerebellar circuits in cognitive functions and fundamental questions about brain evolution. This characterization of human cerebellar development will inform studies seeking to develop experimentally tractable models of the developing human cerebellum, including pluripotent stem cell-derived organoids that could support disease modeling and drug discovery. Acknowledgments This work was supported by a generous gift from the
Overall, the study by Haldipur et al. identified several previously unappreciated features of human cerebellum development,
Bowes Foundation and Simons Foundation grant (SFARI 491371 to T.J.N.). We thank Kathleen Millen
1
Department of Anatomy, University of California, San Francisco, CA, USA Department of Psychiatry, University of California, San Francisco, CA, USA 3 Chan Zuckerberg Biohub, San Francisco, CA, USA 2
*Correspondence: [email protected] (T.J. Nowakowski). https://doi.org/10.1016/j.tins.2019.12.005 © 2019 Elsevier Ltd. All rights reserved.
References 1. Schmahmann, J.D. (2019) The cerebellum and cognition. Neurosci. Lett. 688, 62–75 2. Buckner, R.L. et al. (2011) The organization of the human cerebellum estimated by intrinsic functional connectivity. J. Neurophysiol. 106, 2322–2345 3. Buckner, R.L. and Krienen, F.M. (2013) The evolution of distributed association networks in the human brain. Trends Cogn. Sci. 17, 648–665 4. Wang, S.S.-H. et al. (2014) The cerebellum, sensitive periods, and autism. Neuron 83, 518–532 5. Haldipur, P. et al. (2019) Spatiotemporal expansion of primary progenitor zones in the developing human cerebellum. Science 366, 454–460 6. Cadwell, C.R. et al. (2019) Development and arealization of the cerebral cortex. Neuron 103, 980–1004 7. Nowakowski, T.J. et al. (2016) Transformation of the radial glia scaffold demarcates two stages of human cerebral cortex development. Neuron 91, 1219–1227 8. Smart, I.H.M. et al. (2002) Unique morphological features of the proliferative zones and postmitotic compartments of the neural epithelium giving rise to striate and extrastriate cortex in the monkey. Cereb. Cortex 12, 37–53 9. Herculano-Houzel, S. (2010) Coordinated scaling of cortical and cerebellar numbers of neurons. Front. Neuroanat. 4, 12 10. Leto, K. et al. (2016) Consensus paper: cerebellar development. Cerebellum 15, 789–828
and Parthiv Haldipur for their insight and feedback in preparing the manuscript.
Trends in Neurosciences, Month 2019, Vol. xx, No. xx
3
Trends in Neurosciences
Spotlight
Evolutionary Expansion of Human Cerebellar Germinal Zones
the cellular processes that give rise to the cerebellum have been understudied and human cerebellar development remains poorly understood despite a long history of studies on model Matthew G. Keefe1,2 and vertebrates. Haldipur et al. have revealed distinct features of human cereTomasz J. Nowakowski1,2,3,* bellar development that may have Haldipur et al. explored the devel- significant implications for future underopmental origins of the human cer- standing of cerebellar organization and ebellum, which has gained growing diseases [5].
appreciation for its involvement in human cognition. The authors discovered human-unique expansion and maintenance of cerebellar germinal zones, reminiscent of processes in the developing human cerebral cortex necessary for generating expanded neuronal populations. The human brain contains an astonishing diversity of cell types distributed across hundreds of anatomically and functionally defined structures. Comparative studies of brain evolution have largely focused on anatomically unique structures, such as the disproportionately expanded cerebral cortex which represents a major adaptation that enabled increased cognitive capacity in hominins. Other structures that are more anatomically conserved across mammals have been assumed to be functionally conserved, and therefore have received less attention. One such structure, the cerebellum, has recently been reexamined with functional brain imaging studies, identifying a wide range of association functions linked to intellect and social cognition [1,2] in addition to its roles in motor circuits. New studies of the cerebellum, fueled by interest in its cognitive functions, have shown that cerebellar association zones may be disproportionately expanded in humans [2,3]. Defects in cerebellar development are thought to underlie a range of disorders ranging from ataxia to autism spectrum disorders [4], but
The developmental processes underlying the emergence of forebrain structures, in particular the cerebral cortex, are well understood and provide useful context for the authors’ findings. In the developing cerebral cortex, neurogenesis begins with progenitor cells known as ventricular radial glia, which have cell bodies located near the ventricular surface and long processes that physically span the ventricular to pial limits of the tissue. In both humans and mice, ventricular radial glia give rise to a secondary proliferative population that resides in the subventricular zone (SVZ); an anatomically distinct germinal zone that is expanded in human and primate development. The human SVZ contains outer radial glia that possess long fibers extending to the pial surface, but no ventricular contacts, and has been hypothesized to underlie the expansion of neuronal populations in the developing human cerebral cortex [6–8]. This emergence of an anatomically distinct subventricular zone and the unique ecosystems of secondary progenitors have been largely unexplored outside of the forebrain. In mice and humans, the cerebellar anlage develops in the most anterior segment of the hindbrain (Figure 1A) and gives rise to an astonishing number of cerebellar neurons – as many as 80% of human neurons reside in the cerebellum [9]. Cerebellar neurons emerge from two major germinal zones: the cerebellar ventricular zone (VZ), where GABAergic neurons are born, and the rhombic lip, which gives rise to
glutamatergic neuron progenitors that populate the future granule layers and the cerebellar nuclei (Figure 1B,I). Haldipur et al. sought to compare these two germinal zones between mouse and human brains, starting with a broad characterization of the morphology and organization at different stages of development. In striking contrast to the simple organization of mouse cerebellar ventricular and rhombic lip radial glia into single proliferative layers along the ventricular surface, the germinal zones of the human cerebellum and rhombic lip expand beyond a simple VZ to form defined SVZs (Figure 1G,H). Gene expression analysis of the human rhombic lip has identified persistent expression of genes related to organ size control, such as the HIPPO and WNT signaling pathways, and genes involved in the maturation and expansion of cortical radial glia and neural progenitors, including key outer radial glia genes. Cells of the human rhombic lip SVZ are morphologically similar to cerebral cortical outer radial glia, with unipolar processes that extend away from the ventricle and no apparent ventricle-contacting fiber (Figure 1H). Together, the shared gene expression profiles and morphological characteristics between human rhombic lip SVZ progenitors and cortical outer radial glia suggest that the same molecular pathways that support the developmental expansion of cortical progenitors may have also been co-opted during evolution to support the expansion of the human cerebellar ventricular and rhombic lip germinal zones. While these findings identify novel similarities between cerebral and cerebellar cortex progenitors, the cerebellum remains unique in terms of developmental timing. The cerebellum develops far earlier than the cerebral cortex, with Purkinje cells generated from the cerebellar VZ arising before 8 weeks post conception (pcw), placing them among the earliestborn neurons of the CNS; by contrast, peak neurogenesis in the cerebral Trends in Neurosciences, Month 2019, Vol. xx, No. xx
1
Trends in Neurosciences
Trends in Neurosciences
Figure 1. Overview of Human Cerebellar Development across Space and Time. (A) Schematic of a developing human embryo CNS colored by major divisions. The cerebellum arises from the most anterior region of the rhombencephalon, rhombomere I (pink). (B) Developmental trajectory showing the three major neurogenic regions of the cerebellum and their cellular output (green: GABAergic, orange: glutamatergic). (C–E) Highlight of the predominant neurogenic zone at different timepoints and the migration of their progeny. (C) Between 4 and 8 weeks post-conception the early cerebellar anlage displays a broad ventricular zone (VZ) with an elaborated subventricular zone (SVZ) and generates GABAergic neuron precursors. (D) By the second trimester, the cerebellar ventricular zone has receded to a single layer, while the rhombic lip expands and becomes internalized in the developing cerebellum. The rhombic lip generates glutamatergic neurons and granule cell progenitors, which migrate tangentially to form the external granule layer. (E) In the third trimester, the rhombic lip persists in the developing cerebellum. Neuronal progenitors derived from the rhombic lip that form the external germinal layer proliferate to generate granule cells. (F) The mature cerebellum undergoes further foliation and is comprised of distinct layers of GABAergic neurons (green) in the Purkinje cell layer and glutamatergic neurons (orange) in the internal granule layer. (G) Expansion of the cerebellar VZ to form a distinct subventricular zone. Exact progenitor composition and neuronal lineage is unknown (indicated by dotted lines and question mark). (H) Similar to G, expansion of the rhombic lip to include a proliferative subventricular zone. (I) Connectivity diagram showing organization of cells during development and in the mature cerebellum. In development, external granule cell progenitors give rise to granule cells that migrate inwards and comprise the internal granule layer of the mature cerebellar cortex. Abbreviations: BP, basal progenitor; RG, radial glia.
cortex occurs around 16 pcw. Within the human cerebellum, Haldipur et al. discovered that distinct neurogenic zones follow individual timelines (Figure 1C–E) with significant temporal expansion 2
Trends in Neurosciences, Month 2019, Vol. xx, No. xx
compared to mice [10]. The human cerebellar VZ undergoes massive expansion between 4 and 8 pcw before extinguishing its proliferative potential and receding to a single cell layer. The rhombic
lip germinal zone remains small during this period, but undergoes a significant expansion beginning around 11 pcw and persists through all developmental stages analyzed.
Trends in Neurosciences
Despite stark differences between their neurogenic timing, the developmental trajectories of human cerebellar VZ and rhombic lip germinal zones are remarkably similar in that they both involve the generation of SVZ regions and outer radial glialike cells. Haldipur et al. found that this similarity may be limited to humans. By investigating developing non-human primate tissue, the authors revealed that the macaque rhombic lip does not expand spatially nor does it become internalized into the cerebellum. Future studies will be needed to elucidate the evolutionary history and molecular pathways underlying rhombic lip expansion. Although its origins are unclear, the human-specific, long-term maintenance of the rhombic lip seems to be critical for normal development of the human cerebellum. In fact, Haldipur et al. found that in 80% of cases of Dandy– Walker malformation, which is characterized by hypoplasia of the cerebellar vermis (the region of the cerebellum linked to cognition), the rhombic lip was absent or not internalized.
including the expanded and elaborated germinal zones in the cerebellum and the rhombic lip, and the presence of outer radial glia-like cells during peak periods of neurogenesis. The similarities between cerebellar and cortical germinal zones raise exciting questions about the molecular and cellular substrates underlying the evolutionary divergence of developmental trajectories between humans and other species. More broadly, Haldipur et al. underscore the value of comparative neurodevelopmental studies in identifying divergent developmental processes that may address the role of cerebellar circuits in cognitive functions and fundamental questions about brain evolution. This characterization of human cerebellar development will inform studies seeking to develop experimentally tractable models of the developing human cerebellum, including pluripotent stem cell-derived organoids that could support disease modeling and drug discovery. Acknowledgments This work was supported by a generous gift from the
Overall, the study by Haldipur et al. identified several previously unappreciated features of human cerebellum development,
Bowes Foundation and Simons Foundation grant (SFARI 491371 to T.J.N.). We thank Kathleen Millen
1
Department of Anatomy, University of California, San Francisco, CA, USA Department of Psychiatry, University of California, San Francisco, CA, USA 3 Chan Zuckerberg Biohub, San Francisco, CA, USA 2
*Correspondence: [email protected] (T.J. Nowakowski). https://doi.org/10.1016/j.tins.2019.12.005 © 2019 Elsevier Ltd. All rights reserved.
References 1. Schmahmann, J.D. (2019) The cerebellum and cognition. Neurosci. Lett. 688, 62–75 2. Buckner, R.L. et al. (2011) The organization of the human cerebellum estimated by intrinsic functional connectivity. J. Neurophysiol. 106, 2322–2345 3. Buckner, R.L. and Krienen, F.M. (2013) The evolution of distributed association networks in the human brain. Trends Cogn. Sci. 17, 648–665 4. Wang, S.S.-H. et al. (2014) The cerebellum, sensitive periods, and autism. Neuron 83, 518–532 5. Haldipur, P. et al. (2019) Spatiotemporal expansion of primary progenitor zones in the developing human cerebellum. Science 366, 454–460 6. Cadwell, C.R. et al. (2019) Development and arealization of the cerebral cortex. Neuron 103, 980–1004 7. Nowakowski, T.J. et al. (2016) Transformation of the radial glia scaffold demarcates two stages of human cerebral cortex development. Neuron 91, 1219–1227 8. Smart, I.H.M. et al. (2002) Unique morphological features of the proliferative zones and postmitotic compartments of the neural epithelium giving rise to striate and extrastriate cortex in the monkey. Cereb. Cortex 12, 37–53 9. Herculano-Houzel, S. (2010) Coordinated scaling of cortical and cerebellar numbers of neurons. Front. Neuroanat. 4, 12 10. Leto, K. et al. (2016) Consensus paper: cerebellar development. Cerebellum 15, 789–828
and Parthiv Haldipur for their insight and feedback in preparing the manuscript.
Trends in Neurosciences, Month 2019, Vol. xx, No. xx
3