Evolutionary expansion of the human neocortex reflects increased amplification of basal progenito... more Evolutionary expansion of the human neocortex reflects increased amplification of basal progenitors in the subventricular zone, producing more neurons during fetal corticogenesis. In this work, we analyze the transcriptomes of distinct progenitor subpopulations isolated by a cell polarity-based approach from developing mouse and human neocortex. We identify 56 genes preferentially expressed in human apical and basal radial glia that lack mouse orthologs. Among these, ARHGAP11B has the highest degree of radial glia-specific expression. ARHGAP11B arose from partial duplication of ARHGAP11A (which encodes a Rho guanosine triphosphatase-activating protein) on the human lineage after separation from the chimpanzee lineage. Expression of ARHGAP11B in embryonic mouse neocortex promotes basal progenitor generation and self-renewal and can increase cortical plate area and induce gyrification. Hence, ARHGAP11B may have contributed to evolutionary expansion of human neocortex.
Graphical Abstract Highlights d Basal progenitors (BPs) in the human neocortex have six distinct ... more Graphical Abstract Highlights d Basal progenitors (BPs) in the human neocortex have six distinct morphotypes d Increasing BP proliferative capacity is correlated with increasing BP process numbers d Membrane-bound PALMD is required for and sufficient to increase BP processes d Induction of BP processes by PALMD promotes BP proliferation via integrin signaling Correspondence [email protected] In Brief Huttner and colleagues show that an increase in the number of neural progenitor cell processes underlies the proliferative capacity of this population. Expression of human membrane-bound PALMDELPHIN increases the number of progenitor processes and activates integrin signaling, leading to enhanced progenitor proliferation and an increase in upper-layer neurons.
ECM components HAPLN1, lumican and collagen I (HLC) induce human neocortex folding d HLC-induced ... more ECM components HAPLN1, lumican and collagen I (HLC) induce human neocortex folding d HLC-induced folding involves increases in ECM stiffness and requires HA/CD168/ERK d Degradation of HA reduces physiological folds in 22 GW human neocortex d HLC-induced cortical folding is impaired in certain neurodevelopmental disorders.
Understanding the molecular basis that underlies the expansion of the neocortex during primate, a... more Understanding the molecular basis that underlies the expansion of the neocortex during primate, and notably human, evolution requires the identification of genes that are particularly active in the neural stem and progenitor cells of the developing neocortex. Here, we have used existing transcriptome datasets to carry out a comprehensive screen for protein-coding genes preferentially expressed in progenitors of fetal human neocortex. We show that 15 human-specific genes exhibit such expression, and many of them evolved distinct neural progenitor cell-type expression profiles and levels compared to their ancestral paralogs. Functional studies on one such gene, NOTCH2NL, demonstrate its ability to promote basal progenitor proliferation in mice. An additional 35 human genes with progenitor-enriched expression are shown to have orthologs only in primates. Our study provides a resource of genes that are promising candidates to exert specific, and novel, roles in neocortical development during primate, and notably human, evolution.
The generation of neocortical neurons from neural progenitor cells (NPCs) is primarily controlled... more The generation of neocortical neurons from neural progenitor cells (NPCs) is primarily controlled by transcription factors binding to DNA in the context of chromatin. To understand the complex layer of regulation that orchestrates different NPC types from the same DNA sequence, epigenome maps with cell type resolution are required. Here, we present genomewide histone methylation maps for distinct neural cell populations in the developing mouse neocortex. Using different chromatin features, we identify potential novel regulators of cortical NPCs. Moreover, we identify extensive H3K27me3 changes between NPC subtypes coinciding with major developmental and cell biological transitions. Interestingly, we detect dynamic H3K27me3 changes on promoters of several crucial transcription factors, including the basal progenitor regulator Eomes. We use catalytically inactive Cas9 fused with the histone methyltransferase Ezh2 to edit H3K27me3 at the Eomes locus in vivo, which results in reduced Tbr2 expression and lower basal progenitor abundance, underscoring the relevance of dynamic H3K27me3 changes during neocortex development. Taken together, we provide a rich resource of neocortical histone methy-lation data and outline an approach to investigate its contribution to the regulation of selected genes during neocortical development .
Neocortex evolutionary expansion is primarily due to increased
proliferative capacity of neural p... more Neocortex evolutionary expansion is primarily due to increased proliferative capacity of neural progenitor cells during cortical development. Exploiting insights into the cell biology of cortical progenitors gained during the past two decades, recent studies uncovered a variety of gene expression differences that underlie differential cortical progenitor behavior. These comprise both, differences between cortical areas that likely provide a molecular basis for cortical folding, and differences across species thought to be responsible for increases in neocortex size. Human-specific signatures have been identified for gene regulatory elements, non-coding gene products, and protein- encoding genes, and have been functionally examined in in vivo as well as novel in vitro model systems.
The gene ARHGAP11B promotes basal progenitor amplification and is implicated in neocortex expansi... more The gene ARHGAP11B promotes basal progenitor amplification and is implicated in neocortex expansion. It arose on the human evolutionary lineage by partial duplication of ARHGAP11A, which encodes a Rho guanosine triphosphatase– activating protein (RhoGAP). However, a lack of 55 nucleotides in ARHGAP11B mRNA leads to loss of RhoGAP activity by GAP domain truncation and addition of a human-specific carboxy-terminal amino acid sequence. We show that these 55 nucleotides are deleted by mRNA splicing due to a single C→G substitution that creates a novel splice donor site. We reconstructed an ancestral ARHGAP11B complementary DNA without this substitution. Ancestral ARHGAP11B exhibits RhoGAP activity but has no ability to increase basal progenitors during neocortex development. Hence, a single nucleotide substitution underlies the specific properties of ARHGAP11B that likely contributed to the evolutionary expansion of the human neocortex.
Only about five percent of our genome contains human-specific sequences. These sequences arose mo... more Only about five percent of our genome contains human-specific sequences. These sequences arose mostly from segmental duplications during human evolution and include genes that are unique to the human lineage. Although these genes are currently not well characterised, they may be potential key players in the development of human-specific traits, like an expanded neocortex. Here, we present an overview of the role of human-specific genes during development and evolution of the embryo - nic brain, with a focus on one of these genes, ARHGAP11B (Rho GTPase Activating Protein 11B).
Neocortex expansion in development and evolution reflects an increased and prolonged activity of ... more Neocortex expansion in development and evolution reflects an increased and prolonged activity of neural progenitor cells. Insight into key aspects of the underlying cell biology has recently been obtained. First, the restriction of apical progenitors to undergo mitosis at the ventricular surface is overcome by generation of basal progenitors, which are free to undergo mitosis at abventricular location, typically the subventricular zone. This process involves basolateral ciliogenesis, delamination from the apical adherens junction belt, and loss of apical cell polarity. Second, proliferative capacity of basal progenitors is supported by self-produced extracellular matrix constituents, which in turn promote growth factor signalling. Humans amplify these processes by characteristic alterations in expression of key regulatory genes (PAX6), and via human-specific genes (ARHGAP11B).
Apical radial glia (aRG), the stem cells in developing neocortex, are unique bipolar epithelial c... more Apical radial glia (aRG), the stem cells in developing neocortex, are unique bipolar epithelial cells, extending an apical process to the ventricle and a basal process to the basal lamina. Here, we report novel features of the Golgi apparatus, a central organelle for cell polarity, in mouse aRGs. The Golgi was confined to the apical process but not associated with apical centrosome(s). In contrast, in aRG-derived, delaminating basal progenitors that lose apical polarity, the Golgi became pericentrosomal. The aRG Golgi underwent evolutionarily conserved, accordion-like compression and extension concomitant with cell cycle-dependent nuclear migration. Importantly, in line with endoplasmic reticulum but not Golgi being present in the aRG basal process, its plasma membrane contained glycans lacking Golgi processing, consistent with direct ER-to-cell surface membrane traffic. Our study reveals hitherto unknown complexity of neural stem cell polarity, differential Golgi contribution to their specific architecture, and fundamental Golgi reorganization upon cell fate change. The primary neural stem cells from which all other cells of the mammalian central nervous system (CNS) are derived, are canonical epithelial cells 1. These cells, called neuroepithelial cells, exhibit apical-basal cell polarity and contact a basal lamina with their basal plasma membrane and the lumen of the brain ventricles and spinal cord central canal with their apical plasma membrane. Apical and basolateral plasma membrane domains are separated from each other by a belt of cell junctions at the apical-most end of the basolateral membrane that are crucial for maintaining the cells integrated into the neuroepithelium 2. During interphase, a primary cilium protrudes from the apical plasma membrane of neuroepithelial cells into the lumen. The membrane association of the ciliary basal body, that is, the mother centriole, is responsible for the interphase centrosome(s) being tethered at the apical plasma membrane 3. Early in CNS development, the neuroepithelium consists of a single layer of neuroepithelial cells that exhibits pseudostratification because the nuclei occupy various positions along the apical-basal axis. This reflects a process called interkinetic nuclear migration (INM) 4,5. Following mitosis just beneath the apical plasma membrane, nuclei migrate basally during the G1 phase of the cell cycle such that S-phase takes place near the basal lamina. During G2, nuclei migrate in the opposite direction towards the apically tethered centrosomes and then undergo again apical mitosis. At the early developmental stage, all divisions of neuroepithelial cells are symmetric prolif-erative, that is, both daughters are neuroepithelial cells. With the onset of neurogenesis, neuroepithelial cells transform into a highly related, but nonetheless distinct, cell type called apical radial glia (aRG) 6. As not only neuroepithelial cells, but also aRGs undergo apical mitosis, they are collectively referred to as apical progenitors (APs). The transformation from neuroepithelial cells to aRGs is accompanied by several substantial changes that are most pronounced in the developing neocortex and pertain to the mode of cell division and daughter cell fate, and consequently to the cell biology, INM, and tissue architecture. Specifically, neuroepithelial cells and subsequently aRGs switch to asymmetric self-renewing division, which generates an aRG daughter and a daughter cell with a different fate that delaminates from the apical surface and junctional belt, loses apical cell polarity features, and migrates basally to generate additional cell layers. In the developing neocortex, this basal daughter cell can be a neuron, but in most cases is a secondary type of stem or progenitor cell, collectively referred to as basal progenitors (BPs) 7–9 , which ultimately generate most cortical neurons 10 .
Cerebral organoids—3D cultures of human cerebral tissue derived from pluripotent stem cells—have ... more Cerebral organoids—3D cultures of human cerebral tissue derived from pluripotent stem cells—have emerged as models of human cortical development. However, the extent to which in vitro organoid systems recapitulate neural progenitor cell proliferation and neuronal differentiation programs observed in vivo remains unclear. Here we use single-cell RNA sequencing (scRNA-seq) to dissect and compare cell composition and progenitor-to-neuron lineage relationships in human cerebral organoids and fetal neocortex. Covariation network analysis using the fetal neocortex data reveals known and previously unidentified interactions among genes central to neural progenitor proliferation and neuronal differentiation. In the organoid, we detect diverse progenitors and differentiated cell types of neuronal and mesenchymal lineages and identify cells that derived from regions resembling the fetal neocortex. We find that these organoid cortical cells use gene expression programs remarkably similar to those of the fetal tissue to organize into cerebral cortex-like regions. Our comparison of in vivo and in vitro cortical single-cell transcriptomes illuminates the genetic features underlying human cortical development that can be studied in organoid cultures.
Evolutionary expansion of the human neocortex reflects increased amplification of basal progenito... more Evolutionary expansion of the human neocortex reflects increased amplification of basal progenitors in the subventricular zone, producing more neurons during fetal corticogenesis. Here, we analyze the transcriptomes of distinct progenitor subpopulations isolated by a cell polarity-based approach from developing mouse and human neocortex. We identify 56 genes preferentially expressed in human apical and basal radial glia that lack mouse orthologs. Among these, ARHGAP11B has the highest degree of radial glia-specific expression. ARHGAP11B arose from partial duplication of the Rho GTPase-activating-protein–encoding ARHGAP11A on the human lineage after separation from the chimpanzee lineage. Expression of ARHGAP11B in embryonic mouse neocortex promotes basal progenitor generation and self-renewal, and can increase cortical plate area and induce gyrification. Hence, ARHGAP11B may have contributed to evolutionary expansion of human neocortex.
The neocortex is the seat of higher cognitive functions and, in evolutionary terms, is the younge... more The neocortex is the seat of higher cognitive functions and, in evolutionary terms, is the youngest part of the mammalian brain. Since its origin, the neocortex has expanded in several mammalian lineages, and this is particularly notable in humans. This expansion reflects an increase in the number of neocortical neurons, which is determined during development and primarily reflects the number of neurogenic divisions of distinct classes of neural progenitor cells. Consequently, the evolutionary expansion of the neocortex and the concomitant increase in the numbers of neurons produced during development entail interspecies differences in neural progenitor biology. Here, we review the diversity of neocortical neural progenitors, their interspecies variations and their roles in determining the evolutionary increase in neuron numbers and neocortex size.
The cerebellar primordium arises between embryonic days 8.5 and 9.5 from dorsal rhombomere 1, adj... more The cerebellar primordium arises between embryonic days 8.5 and 9.5 from dorsal rhombomere 1, adjacent to the fourth ventricle. Cerebellar patterning requires the concerted action of morphogens secreted by the rhombic lip and roof plate, and leads to the formation of two main neurogenetic centers, the upper rhombic lip and ventricular zone, from which glutamatergic and GABAergic neurons arise, respectively. These territories contain gene expression microdomains that are partially overlapping and, among others, express proneural genes. This gene family is tightly conserved in evolution and encodes basic helix-loop-helix transcription factors implicated in many neurogenetic events, ranging from cell-fate specification to terminal differentiation of a variety of neuronal types across the embryonic nervous system. The present paper deals with the established or suggested roles of proneural genes in cerebellar neurogenesis. Of the proneural genes examined in this chapter, Atoh1 plays a quintessential role in the specification and development of granule cells and other cerebellar glutamatergic neurons. Besides playing key roles at early stages in these early developmental events, Atoh1 is a key player in the clonal expansion of GC progenitors of the external granule layer. NeuroD, formerly regarded as a proneural gene, acts as a master gene in granule cell differentiation, survival, and dendrite formation. Ascl1 participates in GABA interneuron and cerebellar nuclei neurons generation, and suppresses astrogliogenesis. Conversely, little is known to date about the role(s) of Neurog1 and Neurog2 in cerebellar neurogenesis, and a combination of loss- and gain-of-function studies is required to elucidate their role, if any, in cerebellar neurogenesis.
By serving as the sole output of the cerebellar cortex, integrating a myriad of afferent stimuli,... more By serving as the sole output of the cerebellar cortex, integrating a myriad of afferent stimuli, Purkinje cells (PCs) constitute the principal neuron in cerebellar circuits. Several neurodegenerative cerebellar ataxias feature a selective cell-autonomous loss of PCs, warranting the development of regenerative strategies. To date, very little is known as to the regulatory cascades controlling PC development. During central nervous system development, the proneural gene neurogenin 2 (Neurog2) contributes to many distinct neuronal types by specifying their fate and/or dictating development of their morphological features. By analyzing a mouse knock-in line expressing Cre recombinase under the control of Neurog2 cis-acting sequences we show that, in the cerebellar primordium, Neurog2 is expressed by cycling progenitors cell-autonomously fated to become PCs, even when transplanted heterochronically. During cerebellar development, Neurog2 is expressed in G1 phase by progenitors poised to exit the cell cycle. We demonstrate that, in the absence of Neurog2, both cell-cycle progression and neuronal output are significantly affected, leading to an overall reduction of the mature cerebellar volume. Although PC fate identity is correctly specified, the maturation of their dendritic arbor is severely affected in the absence of Neurog2, as null PCs develop stunted and poorly branched dendrites, a defect evident from the early stages of dendritogenesis. Thus, Neurog2 represents a key regulator of PC development and maturation.
Evolutionary expansion of the human neocortex reflects increased amplification of basal progenito... more Evolutionary expansion of the human neocortex reflects increased amplification of basal progenitors in the subventricular zone, producing more neurons during fetal corticogenesis. In this work, we analyze the transcriptomes of distinct progenitor subpopulations isolated by a cell polarity-based approach from developing mouse and human neocortex. We identify 56 genes preferentially expressed in human apical and basal radial glia that lack mouse orthologs. Among these, ARHGAP11B has the highest degree of radial glia-specific expression. ARHGAP11B arose from partial duplication of ARHGAP11A (which encodes a Rho guanosine triphosphatase-activating protein) on the human lineage after separation from the chimpanzee lineage. Expression of ARHGAP11B in embryonic mouse neocortex promotes basal progenitor generation and self-renewal and can increase cortical plate area and induce gyrification. Hence, ARHGAP11B may have contributed to evolutionary expansion of human neocortex.
Graphical Abstract Highlights d Basal progenitors (BPs) in the human neocortex have six distinct ... more Graphical Abstract Highlights d Basal progenitors (BPs) in the human neocortex have six distinct morphotypes d Increasing BP proliferative capacity is correlated with increasing BP process numbers d Membrane-bound PALMD is required for and sufficient to increase BP processes d Induction of BP processes by PALMD promotes BP proliferation via integrin signaling Correspondence [email protected] In Brief Huttner and colleagues show that an increase in the number of neural progenitor cell processes underlies the proliferative capacity of this population. Expression of human membrane-bound PALMDELPHIN increases the number of progenitor processes and activates integrin signaling, leading to enhanced progenitor proliferation and an increase in upper-layer neurons.
ECM components HAPLN1, lumican and collagen I (HLC) induce human neocortex folding d HLC-induced ... more ECM components HAPLN1, lumican and collagen I (HLC) induce human neocortex folding d HLC-induced folding involves increases in ECM stiffness and requires HA/CD168/ERK d Degradation of HA reduces physiological folds in 22 GW human neocortex d HLC-induced cortical folding is impaired in certain neurodevelopmental disorders.
Understanding the molecular basis that underlies the expansion of the neocortex during primate, a... more Understanding the molecular basis that underlies the expansion of the neocortex during primate, and notably human, evolution requires the identification of genes that are particularly active in the neural stem and progenitor cells of the developing neocortex. Here, we have used existing transcriptome datasets to carry out a comprehensive screen for protein-coding genes preferentially expressed in progenitors of fetal human neocortex. We show that 15 human-specific genes exhibit such expression, and many of them evolved distinct neural progenitor cell-type expression profiles and levels compared to their ancestral paralogs. Functional studies on one such gene, NOTCH2NL, demonstrate its ability to promote basal progenitor proliferation in mice. An additional 35 human genes with progenitor-enriched expression are shown to have orthologs only in primates. Our study provides a resource of genes that are promising candidates to exert specific, and novel, roles in neocortical development during primate, and notably human, evolution.
The generation of neocortical neurons from neural progenitor cells (NPCs) is primarily controlled... more The generation of neocortical neurons from neural progenitor cells (NPCs) is primarily controlled by transcription factors binding to DNA in the context of chromatin. To understand the complex layer of regulation that orchestrates different NPC types from the same DNA sequence, epigenome maps with cell type resolution are required. Here, we present genomewide histone methylation maps for distinct neural cell populations in the developing mouse neocortex. Using different chromatin features, we identify potential novel regulators of cortical NPCs. Moreover, we identify extensive H3K27me3 changes between NPC subtypes coinciding with major developmental and cell biological transitions. Interestingly, we detect dynamic H3K27me3 changes on promoters of several crucial transcription factors, including the basal progenitor regulator Eomes. We use catalytically inactive Cas9 fused with the histone methyltransferase Ezh2 to edit H3K27me3 at the Eomes locus in vivo, which results in reduced Tbr2 expression and lower basal progenitor abundance, underscoring the relevance of dynamic H3K27me3 changes during neocortex development. Taken together, we provide a rich resource of neocortical histone methy-lation data and outline an approach to investigate its contribution to the regulation of selected genes during neocortical development .
Neocortex evolutionary expansion is primarily due to increased
proliferative capacity of neural p... more Neocortex evolutionary expansion is primarily due to increased proliferative capacity of neural progenitor cells during cortical development. Exploiting insights into the cell biology of cortical progenitors gained during the past two decades, recent studies uncovered a variety of gene expression differences that underlie differential cortical progenitor behavior. These comprise both, differences between cortical areas that likely provide a molecular basis for cortical folding, and differences across species thought to be responsible for increases in neocortex size. Human-specific signatures have been identified for gene regulatory elements, non-coding gene products, and protein- encoding genes, and have been functionally examined in in vivo as well as novel in vitro model systems.
The gene ARHGAP11B promotes basal progenitor amplification and is implicated in neocortex expansi... more The gene ARHGAP11B promotes basal progenitor amplification and is implicated in neocortex expansion. It arose on the human evolutionary lineage by partial duplication of ARHGAP11A, which encodes a Rho guanosine triphosphatase– activating protein (RhoGAP). However, a lack of 55 nucleotides in ARHGAP11B mRNA leads to loss of RhoGAP activity by GAP domain truncation and addition of a human-specific carboxy-terminal amino acid sequence. We show that these 55 nucleotides are deleted by mRNA splicing due to a single C→G substitution that creates a novel splice donor site. We reconstructed an ancestral ARHGAP11B complementary DNA without this substitution. Ancestral ARHGAP11B exhibits RhoGAP activity but has no ability to increase basal progenitors during neocortex development. Hence, a single nucleotide substitution underlies the specific properties of ARHGAP11B that likely contributed to the evolutionary expansion of the human neocortex.
Only about five percent of our genome contains human-specific sequences. These sequences arose mo... more Only about five percent of our genome contains human-specific sequences. These sequences arose mostly from segmental duplications during human evolution and include genes that are unique to the human lineage. Although these genes are currently not well characterised, they may be potential key players in the development of human-specific traits, like an expanded neocortex. Here, we present an overview of the role of human-specific genes during development and evolution of the embryo - nic brain, with a focus on one of these genes, ARHGAP11B (Rho GTPase Activating Protein 11B).
Neocortex expansion in development and evolution reflects an increased and prolonged activity of ... more Neocortex expansion in development and evolution reflects an increased and prolonged activity of neural progenitor cells. Insight into key aspects of the underlying cell biology has recently been obtained. First, the restriction of apical progenitors to undergo mitosis at the ventricular surface is overcome by generation of basal progenitors, which are free to undergo mitosis at abventricular location, typically the subventricular zone. This process involves basolateral ciliogenesis, delamination from the apical adherens junction belt, and loss of apical cell polarity. Second, proliferative capacity of basal progenitors is supported by self-produced extracellular matrix constituents, which in turn promote growth factor signalling. Humans amplify these processes by characteristic alterations in expression of key regulatory genes (PAX6), and via human-specific genes (ARHGAP11B).
Apical radial glia (aRG), the stem cells in developing neocortex, are unique bipolar epithelial c... more Apical radial glia (aRG), the stem cells in developing neocortex, are unique bipolar epithelial cells, extending an apical process to the ventricle and a basal process to the basal lamina. Here, we report novel features of the Golgi apparatus, a central organelle for cell polarity, in mouse aRGs. The Golgi was confined to the apical process but not associated with apical centrosome(s). In contrast, in aRG-derived, delaminating basal progenitors that lose apical polarity, the Golgi became pericentrosomal. The aRG Golgi underwent evolutionarily conserved, accordion-like compression and extension concomitant with cell cycle-dependent nuclear migration. Importantly, in line with endoplasmic reticulum but not Golgi being present in the aRG basal process, its plasma membrane contained glycans lacking Golgi processing, consistent with direct ER-to-cell surface membrane traffic. Our study reveals hitherto unknown complexity of neural stem cell polarity, differential Golgi contribution to their specific architecture, and fundamental Golgi reorganization upon cell fate change. The primary neural stem cells from which all other cells of the mammalian central nervous system (CNS) are derived, are canonical epithelial cells 1. These cells, called neuroepithelial cells, exhibit apical-basal cell polarity and contact a basal lamina with their basal plasma membrane and the lumen of the brain ventricles and spinal cord central canal with their apical plasma membrane. Apical and basolateral plasma membrane domains are separated from each other by a belt of cell junctions at the apical-most end of the basolateral membrane that are crucial for maintaining the cells integrated into the neuroepithelium 2. During interphase, a primary cilium protrudes from the apical plasma membrane of neuroepithelial cells into the lumen. The membrane association of the ciliary basal body, that is, the mother centriole, is responsible for the interphase centrosome(s) being tethered at the apical plasma membrane 3. Early in CNS development, the neuroepithelium consists of a single layer of neuroepithelial cells that exhibits pseudostratification because the nuclei occupy various positions along the apical-basal axis. This reflects a process called interkinetic nuclear migration (INM) 4,5. Following mitosis just beneath the apical plasma membrane, nuclei migrate basally during the G1 phase of the cell cycle such that S-phase takes place near the basal lamina. During G2, nuclei migrate in the opposite direction towards the apically tethered centrosomes and then undergo again apical mitosis. At the early developmental stage, all divisions of neuroepithelial cells are symmetric prolif-erative, that is, both daughters are neuroepithelial cells. With the onset of neurogenesis, neuroepithelial cells transform into a highly related, but nonetheless distinct, cell type called apical radial glia (aRG) 6. As not only neuroepithelial cells, but also aRGs undergo apical mitosis, they are collectively referred to as apical progenitors (APs). The transformation from neuroepithelial cells to aRGs is accompanied by several substantial changes that are most pronounced in the developing neocortex and pertain to the mode of cell division and daughter cell fate, and consequently to the cell biology, INM, and tissue architecture. Specifically, neuroepithelial cells and subsequently aRGs switch to asymmetric self-renewing division, which generates an aRG daughter and a daughter cell with a different fate that delaminates from the apical surface and junctional belt, loses apical cell polarity features, and migrates basally to generate additional cell layers. In the developing neocortex, this basal daughter cell can be a neuron, but in most cases is a secondary type of stem or progenitor cell, collectively referred to as basal progenitors (BPs) 7–9 , which ultimately generate most cortical neurons 10 .
Cerebral organoids—3D cultures of human cerebral tissue derived from pluripotent stem cells—have ... more Cerebral organoids—3D cultures of human cerebral tissue derived from pluripotent stem cells—have emerged as models of human cortical development. However, the extent to which in vitro organoid systems recapitulate neural progenitor cell proliferation and neuronal differentiation programs observed in vivo remains unclear. Here we use single-cell RNA sequencing (scRNA-seq) to dissect and compare cell composition and progenitor-to-neuron lineage relationships in human cerebral organoids and fetal neocortex. Covariation network analysis using the fetal neocortex data reveals known and previously unidentified interactions among genes central to neural progenitor proliferation and neuronal differentiation. In the organoid, we detect diverse progenitors and differentiated cell types of neuronal and mesenchymal lineages and identify cells that derived from regions resembling the fetal neocortex. We find that these organoid cortical cells use gene expression programs remarkably similar to those of the fetal tissue to organize into cerebral cortex-like regions. Our comparison of in vivo and in vitro cortical single-cell transcriptomes illuminates the genetic features underlying human cortical development that can be studied in organoid cultures.
Evolutionary expansion of the human neocortex reflects increased amplification of basal progenito... more Evolutionary expansion of the human neocortex reflects increased amplification of basal progenitors in the subventricular zone, producing more neurons during fetal corticogenesis. Here, we analyze the transcriptomes of distinct progenitor subpopulations isolated by a cell polarity-based approach from developing mouse and human neocortex. We identify 56 genes preferentially expressed in human apical and basal radial glia that lack mouse orthologs. Among these, ARHGAP11B has the highest degree of radial glia-specific expression. ARHGAP11B arose from partial duplication of the Rho GTPase-activating-protein–encoding ARHGAP11A on the human lineage after separation from the chimpanzee lineage. Expression of ARHGAP11B in embryonic mouse neocortex promotes basal progenitor generation and self-renewal, and can increase cortical plate area and induce gyrification. Hence, ARHGAP11B may have contributed to evolutionary expansion of human neocortex.
The neocortex is the seat of higher cognitive functions and, in evolutionary terms, is the younge... more The neocortex is the seat of higher cognitive functions and, in evolutionary terms, is the youngest part of the mammalian brain. Since its origin, the neocortex has expanded in several mammalian lineages, and this is particularly notable in humans. This expansion reflects an increase in the number of neocortical neurons, which is determined during development and primarily reflects the number of neurogenic divisions of distinct classes of neural progenitor cells. Consequently, the evolutionary expansion of the neocortex and the concomitant increase in the numbers of neurons produced during development entail interspecies differences in neural progenitor biology. Here, we review the diversity of neocortical neural progenitors, their interspecies variations and their roles in determining the evolutionary increase in neuron numbers and neocortex size.
The cerebellar primordium arises between embryonic days 8.5 and 9.5 from dorsal rhombomere 1, adj... more The cerebellar primordium arises between embryonic days 8.5 and 9.5 from dorsal rhombomere 1, adjacent to the fourth ventricle. Cerebellar patterning requires the concerted action of morphogens secreted by the rhombic lip and roof plate, and leads to the formation of two main neurogenetic centers, the upper rhombic lip and ventricular zone, from which glutamatergic and GABAergic neurons arise, respectively. These territories contain gene expression microdomains that are partially overlapping and, among others, express proneural genes. This gene family is tightly conserved in evolution and encodes basic helix-loop-helix transcription factors implicated in many neurogenetic events, ranging from cell-fate specification to terminal differentiation of a variety of neuronal types across the embryonic nervous system. The present paper deals with the established or suggested roles of proneural genes in cerebellar neurogenesis. Of the proneural genes examined in this chapter, Atoh1 plays a quintessential role in the specification and development of granule cells and other cerebellar glutamatergic neurons. Besides playing key roles at early stages in these early developmental events, Atoh1 is a key player in the clonal expansion of GC progenitors of the external granule layer. NeuroD, formerly regarded as a proneural gene, acts as a master gene in granule cell differentiation, survival, and dendrite formation. Ascl1 participates in GABA interneuron and cerebellar nuclei neurons generation, and suppresses astrogliogenesis. Conversely, little is known to date about the role(s) of Neurog1 and Neurog2 in cerebellar neurogenesis, and a combination of loss- and gain-of-function studies is required to elucidate their role, if any, in cerebellar neurogenesis.
By serving as the sole output of the cerebellar cortex, integrating a myriad of afferent stimuli,... more By serving as the sole output of the cerebellar cortex, integrating a myriad of afferent stimuli, Purkinje cells (PCs) constitute the principal neuron in cerebellar circuits. Several neurodegenerative cerebellar ataxias feature a selective cell-autonomous loss of PCs, warranting the development of regenerative strategies. To date, very little is known as to the regulatory cascades controlling PC development. During central nervous system development, the proneural gene neurogenin 2 (Neurog2) contributes to many distinct neuronal types by specifying their fate and/or dictating development of their morphological features. By analyzing a mouse knock-in line expressing Cre recombinase under the control of Neurog2 cis-acting sequences we show that, in the cerebellar primordium, Neurog2 is expressed by cycling progenitors cell-autonomously fated to become PCs, even when transplanted heterochronically. During cerebellar development, Neurog2 is expressed in G1 phase by progenitors poised to exit the cell cycle. We demonstrate that, in the absence of Neurog2, both cell-cycle progression and neuronal output are significantly affected, leading to an overall reduction of the mature cerebellar volume. Although PC fate identity is correctly specified, the maturation of their dendritic arbor is severely affected in the absence of Neurog2, as null PCs develop stunted and poorly branched dendrites, a defect evident from the early stages of dendritogenesis. Thus, Neurog2 represents a key regulator of PC development and maturation.
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Papers by Marta Florio
proliferative capacity of neural progenitor cells during cortical
development. Exploiting insights into the cell biology of cortical
progenitors gained during the past two decades, recent studies
uncovered a variety of gene expression differences that underlie
differential cortical progenitor behavior. These comprise both,
differences between cortical areas that likely provide a
molecular basis for cortical folding, and differences across
species thought to be responsible for increases in neocortex
size. Human-specific signatures have been identified for gene
regulatory elements, non-coding gene products, and protein-
encoding genes, and have been functionally examined in in vivo
as well as novel in vitro model systems.
proliferative capacity of neural progenitor cells during cortical
development. Exploiting insights into the cell biology of cortical
progenitors gained during the past two decades, recent studies
uncovered a variety of gene expression differences that underlie
differential cortical progenitor behavior. These comprise both,
differences between cortical areas that likely provide a
molecular basis for cortical folding, and differences across
species thought to be responsible for increases in neocortex
size. Human-specific signatures have been identified for gene
regulatory elements, non-coding gene products, and protein-
encoding genes, and have been functionally examined in in vivo
as well as novel in vitro model systems.