Asymmetric production of surface-dividing and non-surface-dividing cortical progenitor cells

Pre-existing DNA methylation signatures in the prefrontal cortex of alcohol-naïve nonhuman primates define neural vulnerability for future risky ethanol consumption

Alcohol use disorder (AUD) is a highly prevalent, complex, multifactorial and heterogeneous disorder, with 11 % and 30 % of adults meeting criteria for past-year and lifetime AUD, respectively. Identification of the molecular mechanisms underlying risk for AUD would facilitate effective deployment of personalized interventions. Studies using rhesus monkeys and rats, have demonstrated that individuals with low cognitive flexibility and a predisposition towards habitual behaviors show an increased risk for future heavy drinking. Further, low cognitive flexibility is associated with reduced dorsolateral prefrontal cortex (dlPFC) function in rhesus monkeys. To explore the underlying unique molecular signatures that increase risk for chronic heavy drinking, a genome-wide DNA methylation (DNAm) analysis of the alcohol-naïve dlPFC-A46 biopsy prior to chronic alcohol self-administration was conducted. The DNAm profile provides a molecular snapshot of the alcohol-naïve dlPFC, with mapped genes and associated signaling pathways that vary across individuals. The analysis identified 1,463 differentially methylated regions (DMRs) related to unique genes that were strongly associated with average ethanol intake consumed over 6 months of voluntary self-administration. These findings translate behavioral phenotypes into neural markers of risk for AUD, and hold promise for parallel discoveries in risk for other disorders involving impaired cognitive flexibility. SIGNIFICANCE: Alcohol use disorder (AUD) is a highly prevalent and heterogeneous disorder. Prevention strategies to accurately identify individuals with a high risk for AUD, would help reduce the prevalence, and severity of AUD. Our novel epigenomic analysis of the alcohol-naïve nonhuman primate cortex provides a molecular snapshot of the vulnerable brain, pointing to circuitry and molecular mechanisms associated with cortical development, synaptic functions, glutamatergic signaling and coordinated signaling pathways. With a complex disorder like AUD, having the ability to identify the molecular mechanisms underlying AUD risk is critical for better development of personalized effective treatments.

Asxl1 loss in mice leads to microcephaly by regulating neural stem cell survival

Additional sex comb-like 1 (ASXL1) is a chromatin-associated factor essential for transcriptional regulation. De novo truncating mutations in the ASXL1 gene are linked to Bohring-Opitz syndrome, a developmental disorder characterized by microcephaly; however, the role of Asxl1 in brain development remains unclear. In this study, we demonstrate that Asxl1 deletion in mice induces microcephaly, primarily caused by a reduction in the size and number of cortical neurons. Asxl1 ablation disrupts neural stem cell (NSC) maintenance, as evidenced by decreased proliferation and increased apoptosis. Transcriptomic analysis of Asxl1-deficient NSCs revealed 4,635 differentially expressed genes, including 2,262 upregulated and 2,373 downregulated genes. Gene ontology analysis indicated that Asxl1 regulates NSC survival through the histone methyltransferase Ezh2, a core component of the Polycomb Repressive Complex 2 (PRC2). Inhibition of H3K27me3 using GSK343 significantly reduced the viability of wild-type NSCs, but had a markedly diminished effect on Asxl1-deficient NSCs. Furthermore, Ezh2 target genes associated with apoptosis, such as Epha7 and Osr1, were upregulated in wild-type NSCs following GSK343 treatment but not significantly affected in Asxl1-deficient NSCs. These findings establish Asxl1 as a critical regulator of NSC survival and neurogenesis via Ezh2-mediated chromatin modification and provide insights into the mechanisms underlying microcephaly in developmental disorders.

A dyad of human-specific NBPF14 and NOTCH2NLB orchestrates cortical progenitor abundance crucial for human neocortex expansion

We determined the roles of two coevolved and coexpressed human-specific genes, NBPF14 and NOTCH2NLB, on the abundance of the cortical progenitors that underlie the evolutionary expansion of the neocortex, the seat of higher cognitive abilities in humans. Using automated microinjection into apical progenitors (APs) of embryonic mouse neocortex and electroporation of APs in chimpanzee cerebral organoids, we show that NBPF14 promotes the delamination of AP progeny, by promoting oblique cleavage plane orientation during AP division, leading to increased abundance of the key basal progenitor type, basal radial glia. In contrast, NOTCH2NLB promotes AP proliferation, leading to expansion of the AP pool. When expressed together, NBPF14 and NOTCH2NLB exert coordinated effects, resulting in expansion of basal progenitors while maintaining self-renewal of APs. Hence, these two human-specific genes orchestrate the behavior of APs, and the lineages of their progeny, in a manner essential for the evolutionary expansion of the human neocortex.

Orderly specification and precise laminar deployment of mouse cortical projection neuron types through intermediate progenitors

The cerebral cortex comprises diverse types of glutamatergic projection neurons (PNs) generated from radial glial progenitors (RGs) through either direct neurogenesis (dNG) or indirect neurogenesis (iNG) via intermediate progenitors (IPs). A foundational concept in corticogenesis is the "inside-out" model, whereby successive generations of PNs sequentially migrate first to deep and then progressively to more superficial layers. However, its biological significance remains unclear, and the role of iNG in this process is unknown. Using genetic strategies linking PN birth dating to projection mapping in mice, we found that the laminar deployment of IP-derived PNs substantially deviates from a stringent inside-out rule: PNs destined to non-consecutive layers are generated at the same time, and different PN types of the same layer are generated at non-contiguous times. The overarching scheme of iNG is the sequential specification and precise laminar deployment of projection-defined PN types, which may contribute to the orderly assembly of cortical output channels and processing streams.

The microcephaly-associated transcriptional regulator AUTS2 cooperates with Polycomb complex PRC2 to produce upper-layer neurons in mice

AUTS2 syndrome is characterized by intellectual disability and microcephaly, and is often associated with autism spectrum disorder, but the underlying mechanisms, particularly concerning microcephaly, remain incompletely understood. Here, we analyze mice mutated for the transcriptional regulator AUTS2, which recapitulate microcephaly. Their brains exhibit reduced division of intermediate progenitor cells (IPCs), leading to fewer neurons and decreased thickness in the upper-layer cortex. Increased expression of the AUTS2 transcriptional target Robo1 in the mutant animals suppresses IPC division, and transcriptomic and chromatin profiling shows that AUTS2 primarily represses transcription of genes like Robo1 in IPCs. Regions around the transcriptional start sites of AUTS2 target genes are enriched for the repressive histone modification H3K27me3, which is reduced in Auts2 mutants. Furthermore, we find that AUTS2 interacts with Polycomb complex PRC2, with which it cooperates to promote IPC division. These findings shed light on the microcephaly phenotype observed in the AUTS2 syndrome.

Pax6 regulates neuronal migration and cell proliferation via interacting with Wnt3a during cortical development

The paired box 6 (Pax6) gene encodes a highly conserved transcription factor, involved in the development of eyes, brain, and endocrine glands. Homozygous loss of Pax6 resulted in neonatal death in mice, plus loss of eyes and malformation of cerebral cortex. In patients with heterozygous Pax6 mutations, a reduction in thickness of the frontoparietal cortex was detected, which was also observed in small eye mice. In this study, we found that Pax6 overexpression increased the cortical thickness, especially in the intermediate zone of the cortex, which conflicts with the report of Manuel et al. Pax6 overexpression appears to detain neurons in the intermediate zone while promoting cell proliferation. It is worth noting that the impact of Pax6 overexpression on cortical thickness and neuronal migration was temporal, explaining the differences with other reports. We postulated that the alteration of Pax6 isoform ratio by autoregulation might be responsible for this. JASPAR analysis together with the results of qPCR, Western blot, CUT&Tag, and rescue experiments revealed that Pax6 regulates neuronal migration and cell proliferation by indirectly mediating Wnt3a expression. Therefore, we propose that Pax6 participates in corticogenesis via interaction with Wnt3a in regulating neuronal migration and cell proliferation.

Differential neurogenic patterns underlie the formation of primary and secondary areas in the developing somatosensory cortex

The cerebral cortex consists of hierarchically organized areas interconnected by reciprocal axonal projections. However, the coordination of neurogenesis to optimize neuronal production and wiring between distinct cortical areas remains largely unexplored. The somatosensory cortex plays a crucial role in processing tactile information, with inputs from peripheral sensory receptors relayed through the thalamus to the primary and secondary somatosensory areas. To investigate the dynamics of neurogenesis in cortical circuit formation, we employed temporal genetic fate mapping of glutamatergic neuron cohorts across the somatosensory cortices. Our analysis revealed that neuronal production in the secondary somatosensory cortex (S2) precedes that of the primary somatosensory cortex (S1) from the deep-layer neuron production period and terminates earlier. We further revealed a progressive decline in upper-layer neuron output in S2, attributed to the attenuation of the apical ventricular surface, resulting in a reduced number of upper-layer neurons within S2. These findings support the existence of a protomap mechanism governing the area-specific assembly of primary and secondary areas in the developing neocortex.

Examining the NEUROG2 lineage and associated gene expression in human cortical organoids

Proneural genes are conserved drivers of neurogenesis across the animal kingdom. How their functions have adapted to guide human-specific neurodevelopmental features is poorly understood. Here, we mined transcriptomic data from human fetal cortices and generated from human embryonic stem cell-derived cortical organoids (COs) to show that NEUROG1 and NEUROG2 are most highly expressed in basal neural progenitor cells, with pseudotime trajectory analyses indicating that NEUROG1-derived lineages predominate early and NEUROG2 lineages later. Using ChIP-qPCR, gene silencing and overexpression studies in COs, we show that NEUROG2 is necessary and sufficient to directly transactivate known target genes (NEUROD1, EOMES, RND2). To identify new targets, we engineered NEUROG2-mCherry knock-in human embryonic stem cells for CO generation. The mCherry-high CO cell transcriptome is enriched in extracellular matrix-associated genes, and two genes associated with human-accelerated regions: PPP1R17 and FZD8. We show that NEUROG2 binds COL1A1, COL3A1 and PPP1R17 regulatory elements, and induces their ectopic expression in COs, although NEUROG2 is not required for this expression. Neurog2 similarly induces Col3a1 and Ppp1r17 in murine P19 cells. These data are consistent with a conservation of NEUROG2 function across mammalian species.

Syngap1 and the development of murine neocortical progenitor cells

SYNGAP1 is a major regulator of synaptic plasticity through its interaction with synaptic scaffold proteins and modulation of Ras and Rap GTPase signaling pathways. SYNGAP1 mutations in humans are often associated with intellectual disability, epilepsy, and autism spectrum disorder. Syngap1 heterozygous loss-of-function results in impaired LTP, premature maturation of dendritic spines, learning disabilities and seizures in mice. More recently, SYNGAP1 was shown to influence cortical neurogenesis and the proliferation of progenitors in human organoids. Here, we show that the absence or haploinsufficiency of Syngap1 does not influence the properties of neocortical progenitors and their cellular output in mice. This discrepancy highlights potential species-specific or methodological differences and raises important questions about the broader applicability of SYNGAP1's role in neurogenesis.

The Control of Cortical Folding: Multiple Mechanisms, Multiple Models

The cerebral cortex develops through a carefully conscripted series of cellular and molecular events that culminate in the production of highly specialized neuronal and glial cells. During development, cortical neurons and glia acquire a precise cellular arrangement and architecture to support higher-order cognitive functioning. Decades of study using rodent models, naturally gyrencephalic animal models, human pathology specimens, and, recently, human cerebral organoids, reveal that rodents recapitulate some but not all the cellular and molecular features of human cortices. Whereas rodent cortices are smooth-surfaced or lissencephalic, larger mammals, including humans and nonhuman primates, have highly folded/gyrencephalic cortices that accommodate an expansion in neuronal mass and increase in surface area. Several genes have evolved to drive cortical gyrification, arising from gene duplications or de novo origins, or by alterations to the structure/function of ancestral genes or their gene regulatory regions. Primary cortical folds arise in stereotypical locations, prefigured by a molecular "blueprint" that is set up by several signaling pathways (e.g., Notch, Fgf, Wnt, PI3K, Shh) and influenced by the extracellular matrix. Mutations that affect neural progenitor cell proliferation and/or neurogenesis, predominantly of upper-layer neurons, perturb cortical gyrification. Below we review the molecular drivers of cortical folding and their roles in disease.

Neuronal fate resulting from indirect neurogenesis in the mouse neocortex

Excitatory cortical neurons originate from cortical radial glial cells (RGCs). Initially, these neurons were thought to derive directly from RGCs (direct neurogenesis) and be distributed in an inside-out fashion. However, the discovery of indirect neurogenesis, whereby intermediate neuronal progenitors (INPs) generate neurons, challenged this view. To investigate the integration of neurons via these two modes, we developed a method to identify INP progeny and analyze their fate using transgenic mice expressing tamoxifen-inducible Cre recombinase under the neurogenin-2 promoter, alongside thymidine analog incorporation. Their fate was further analyzed using mosaic analysis with double markers in mice. Indirect neurogenesis was prominent during early neurogenesis, generating neuron types that would emerge slightly later than those produced via direct neurogenesis. Despite the timing difference, both neurogenic modes produced fundamentally similar neuron types, as evidenced by marker expression and cortical-depth location. Furthermore, INPs generated pairs of similar phenotype neurons. These findings suggest that indirect neurogenesis, like direct neurogenesis, generates neuron types in a temporally ordered sequence and increases the number of similar neuron types, particularly in deep layers. Thus, both neurogenic modes cooperatively generate a diverse array of neuron types in a similar order, and their progeny populate together to form a coherent cortical structure.

Neuronal lineage tracing from progenitors in human cortical organoids reveals mechanisms of neuronal production, diversity, and disease

The contribution of progenitor subtypes to generating the billions of neurons produced during human cortical neurogenesis is not well understood. We developed the cortical organoid lineage-tracing (COR-LT) system for human cortical organoids. Differential fluorescent reporter activation in distinct progenitor cells leads to permanent reporter expression, enabling the progenitor cell lineage of neurons to be determined. Surprisingly, nearly all excitatory neurons produced in cortical organoids were generated indirectly from intermediate progenitor cells. Additionally, neurons of different progenitor lineages were transcriptionally distinct. Isogenic lines made from an autistic individual with and without a likely pathogenic CTNNB1 variant demonstrated that the variant substantially altered the proportion of neurons derived from specific progenitor cell lineages, as well as the lineage-specific transcriptional profiles of these neurons, suggesting a pathogenic mechanism for this mutation. These results suggest individual progenitor subtypes play roles in generating the diverse neurons of the human cerebral cortex.

Mouse cortical organoids reveal key functions of p73 isoforms: TAp73 governs the establishment of the archetypical ventricular-like zones while DNp73 is central in the regulation of neural cell fate

Introduction

Neurogenesis is tightly regulated in space and time, ensuring the correct development and organization of the central nervous system. Critical regulators of brain development and morphogenesis in mice include two members of the p53 family: p53 and p73. However, dissecting the in vivo functions of these factors and their various isoforms in brain development is challenging due to their pleiotropic effects. Understanding their role, particularly in neurogenesis and brain morphogenesis, requires innovative experimental approaches.

Methods

To address these challenges, we developed an efficient and highly reproducible protocol to generate mouse brain organoids from pluripotent stem cells. These organoids contain neural progenitors and neurons that self-organize into rosette-like structures resembling the ventricular zone of the embryonic forebrain. Using this model, we generated organoids from p73-deficient mouse cells to investigate the roles of p73 and its isoforms (TA and DNp73) during brain development.

Results and Discussion

Organoids derived from p73-deficient cells exhibited increased neuronal apoptosis and reduced neural progenitor proliferation, linked to compensatory activation of p53. This closely mirrors previous in vivo observations, confirming that p73 plays a pivotal role in brain development. Further dissection of p73 isoforms function revealed a dual role of p73 in regulating brain morphogenesis, whereby TAp73 controls transcriptional programs essential for the establishment of the neurogenic niche structure, while DNp73 is responsible for the precise and timely regulation of neural cell fate. These findings highlight the distinct roles of p73 isoforms in maintaining the balance of neural progenitor cell biology, providing a new understanding of how p73 regulates brain morphogenesis.

Conserved transcriptional regulation by BRN1 and BRN2 in neocortical progenitors drives mammalian neural specification and neocortical expansion

The neocortex varies in size and complexity among mammals due to the tremendous variability in the number and diversity of neuronal subtypes across species. The increased cellular diversity is paralleled by the expansion of the pool of neocortical progenitors and the emergence of indirect neurogenesis during brain evolution. The molecular pathways that control these biological processes and are disrupted in neurological disorders remain largely unknown. Here we show that the transcription factors BRN1 and BRN2 have an evolutionary conserved function in neocortical progenitors to control their proliferative capacity and the switch from direct to indirect neurogenesis. Functional studies in mice and ferrets show that BRN1/2 act in concert with NOTCH and primary microcephaly genes to regulate progenitor behavior. Analysis of transcriptomics data from genetically modified macaques provides evidence that these molecular pathways are conserved in non-human primates. Our findings thus demonstrate that BRN1/2 are central regulators of gene expression programs in neocortical progenitors critical to determine brain size during evolution.

Role of cell metabolism in the pathophysiology of brain size-associated neurodevelopmental disorders

Cell metabolism is a key regulator of human neocortex development and evolution. Several lines of evidence indicate that alterations in neural stem/progenitor cell (NPC) metabolism lead to abnormal brain development, particularly brain size-associated neurodevelopmental disorders, such as microcephaly. Abnormal NPC metabolism causes impaired cell proliferation and thus insufficient expansion of NPCs for neurogenesis. Therefore, the production of neurons, which is a major determinant of brain size, is decreased and the size of the brain, especially the size of the neocortex, is significantly reduced. This review discusses recent progress understanding NPC metabolism, focusing in particular on glucose metabolism, fatty acid metabolism and amino acid metabolism (e.g., glutaminolysis and serine metabolism). We provide an overview of the contributions of these metabolic pathways to brain development and evolution, as well as to the etiology of neurodevelopmental disorders. Furthermore, we discuss the advantages and disadvantages of various experimental models to study cell metabolism in the developing brain.

The Neurog2-Tbr2 axis forms a continuous transition to the neurogenic gene expression state in neural stem cells

Neural stem cells (NSCs) differentiate into neuron-fated intermediate progenitor cells (IPCs) via cell division. Although differentiation from NSCs to IPCs is a discrete process, recent transcriptome analyses identified a continuous transcriptional trajectory during this process, raising the question of how to reconcile these contradictory observations. In mouse NSCs, Hes1 expression oscillates, regulating the oscillatory expression of the proneural gene Neurog2, while Hes1 expression disappears in IPCs. Thus, the transition from Hes1 oscillation to suppression is involved in the differentiation of NSCs to IPCs. Here, we found that Neurog2 oscillations induce the accumulation of Tbr2, which suppresses Hes1 expression, generating an IPC-like gene expression state in NSCs. In the absence of Tbr2, Hes1 expression is up-regulated, decreasing the formation of IPCs. These results indicate that the Neurog2-Tbr2 axis forms a continuous transcriptional trajectory to an IPC-like neurogenic state in NSCs, which then differentiate into IPCs via cell division.

Corticogenesis and folding process of the neopallium in the South American plains vizcacha, Lagostomus maximus

The plains vizcacha, Lagostomus maximus, is a precocial hystricomorph rodent with a gyrencephalic brain. This work aimed to perform a time-lapse analysis of the embryonic brain cortical development in the plains vizcacha to establish a species-specific temporal window for corticogenesis and the gyrencephaly onset. Additionally, a comparative examination with evolutionarily related rodents was conducted. Embryos from 40 embryonic days (ED) until the end of pregnancy ( ∼ $\sim $ 154 ED) were evaluated. The neuroanatomical examination determined transverse sulci at 80 ED and rostral lateral and caudal intraparietal sulci around 95 ED. Histological examination of corticogenesis showed emergence of the subplate at 43 ED and expansion of the subventricular zone (SVZ) and its division into inner and outer SVZs around 54 ED. The neocortical layers formation followed an inside-to-outside spatiotemporal gradient beginning with the emergence of layers VI and V at 68 ED and establishing the final six neocortical layers around 100 ED. A progressive increment of gyrencephalization index (GI) from 1.005 ± 0.003 around 70 ED, which reflects a smooth cortex, up to 1.07 ± 0.009 at the end of gestation, reflecting a gyrencephalic neuroanatomy, was determined. Contrarily, the minimum cortical thickness (MCT) progressively decreased from 61 ED up to the end of gestation. These results show that the decrease in the cortical thickness, which enables the onset of neocortical invaginations, occurs together with the expansion and subdivision of the SVZ. The temporal comparison of corticogenesis in plains vizcacha with that in relative species reflects a prenatal long process compared with other rodents that may give an evolutionary advantage to L. maximus as a precocial species.

Higher-order thalamocortical circuits are specified by embryonic cortical progenitor types in the mouse brain

The sensory cortex receives synaptic inputs from both first-order and higher-order thalamic nuclei. First-order inputs relay simple stimulus properties from the periphery, whereas higher-order inputs relay more complex response properties, provide contextual feedback, and modulate plasticity. Here, we reveal that a cortical neuron's higher-order input is determined by the type of progenitor from which it is derived during embryonic development. Within layer 4 (L4) of the mouse primary somatosensory cortex, neurons derived from intermediate progenitors receive stronger higher-order thalamic input and exhibit greater higher-order sensory responses. These effects result from differences in dendritic morphology and levels of the transcription factor Lhx2, which are specified by the L4 neuron's progenitor type. When this mechanism is disrupted, cortical circuits exhibit altered higher-order responses and sensory-evoked plasticity. Therefore, by following distinct trajectories, progenitor types generate diversity in thalamocortical circuitry and may provide a general mechanism for differentially routing information through the cortex.

hPSC-Derived Astrocytes at the Forefront of Translational Applications in Neurological Disorders

Astrocytes, the most abundant glial cell type in the brain, play crucial roles in maintaining homeostasis within the central nervous system (CNS). Impairment or abnormalities of typical astrocyte functions in the CNS serve as a causative or contributing factor in numerous neurodevelopmental, neurodegenerative, and neuropsychiatric disorders. Currently, disease-modeling and drug-screening approaches, primarily focused on human astrocytes, rely on human pluripotent stem cell (hPSC)-derived astrocytes. However, it is important to acknowledge that these hPSC-derived astrocytes exhibit notable differences across studies and when compared to their in vivo counterparts. These differences may potentially compromise translational outcomes if not carefully accounted for. This review aims to explore state-of-the-art in vitro models of human astrocyte development, focusing on the developmental processes, functional maturity, and technical aspects of various hPSC-derived astrocyte differentiation protocols. Additionally, it summarizes their successful application in modeling neurological disorders. The discussion extends to recent advancements in the large-scale production of human astrocytes and their application in developing high-throughput assays conducive to therapeutic drug discovery.

BRN1/2 Function in Neocortical Size Determination and Microcephaly

The mammalian neocortex differs vastly in size and complexity between mammalian species, yet the mechanisms that lead to an increase in brain size during evolution are not known. We show here that two transcription factors coordinate gene expression programs in progenitor cells of the neocortex to regulate their proliferative capacity and neuronal output in order to determine brain size. Comparative studies in mice, ferrets and macaques demonstrate an evolutionary conserved function for these transcription factors to regulate progenitor behaviors across the mammalian clade. Strikingly, the two transcriptional regulators control the expression of large numbers of genes linked to microcephaly suggesting that transcriptional deregulation as an important determinant of the molecular pathogenesis of microcephaly, which is consistent with the finding that genetic manipulation of the two transcription factors leads to severe microcephaly.

The characteristics of excitatory lineage differentiation and the developmental conservation in Reeler neocortex

The majority of neocortical projection neurons are generated indirectly from radial glial cells (RGCs) mediated by intermediate progenitor cells (IPCs) in mice. IPCs are thought to be a great breakthrough in the evolutionary expansion of the mammalian neocortex. However, the precise ratio of neuron production from IPCs and characteristics of RGC differentiation process are still unclear. Our study revealed that direct neurogenesis was seldom observed and increased slightly at late embryonic stage. Besides, we conducted retrovirus sparse labelling combined carboxyfluorescein diacetate succinimide ester (CFSE) and Tbr2-CreER strain to reconstruct individual lineage tree in situ. The lineage trees simulated the output of RGCs at per round of division in sequence with high temporal, spatial and cellular resolution at P7. We then demonstrated that only 1.90% of neurons emanated from RGCs directly in mouse cerebral neocortex and 79.33% of RGCs contributed to the whole clones through IPCs. The contribution of indirect neurogenesis was underestimated previously because approximately a quarter of IPC-derived neurons underwent apoptosis. Here, we also showed that abundant IPCs from first-generation underwent self-renewing division and generated four neurons ultimately. We confirmed that the intermediate proliferative progenitors expressed higher Cux2 characteristically at early embryonic stage. Finally, we validated that the characteristics of neurogenetic process in lineages and developmental fate of neurons were conserved in Reeler mice. This study contributes to further understanding of neurogenesis in neocortical development.

A cell fate decision map reveals abundant direct neurogenesis bypassing intermediate progenitors in the human developing neocortex

The human neocortex has undergone strong evolutionary expansion, largely due to an increased progenitor population, the basal radial glial cells. These cells are responsible for the production of a diversity of cell types, but the successive cell fate decisions taken by individual progenitors remain unknown. Here we developed a semi-automated live/fixed correlative imaging method to map basal radial glial cell division modes in early fetal tissue and cerebral organoids. Through the live analysis of hundreds of dividing progenitors, we show that basal radial glial cells undergo abundant symmetric amplifying divisions, and frequent self-consuming direct neurogenic divisions, bypassing intermediate progenitors. These direct neurogenic divisions are more abundant in the upper part of the subventricular zone. We furthermore demonstrate asymmetric Notch activation in the self-renewing daughter cells, independently of basal fibre inheritance. Our results reveal a remarkable conservation of fate decisions in cerebral organoids, supporting their value as models of early human neurogenesis.

From Vessels to Neurons-The Role of Hypoxia Pathway Proteins in Embryonic Neurogenesis

Embryonic neurogenesis can be defined as a period of prenatal development during which divisions of neural stem and progenitor cells give rise to neurons. In the central nervous system of most mammals, including humans, the majority of neocortical neurogenesis occurs before birth. It is a highly spatiotemporally organized process whose perturbations lead to cortical malformations and dysfunctions underlying neurological and psychiatric pathologies, and in which oxygen availability plays a critical role. In case of deprived oxygen conditions, known as hypoxia, the hypoxia-inducible factor (HIF) signaling pathway is activated, resulting in the selective expression of a group of genes that regulate homeostatic adaptations, including cell differentiation and survival, metabolism and angiogenesis. While a physiological degree of hypoxia is essential for proper brain development, imbalanced oxygen levels can adversely affect this process, as observed in common obstetrical pathologies such as prematurity. This review comprehensively explores and discusses the current body of knowledge regarding the role of hypoxia and the HIF pathway in embryonic neurogenesis of the mammalian cortex. Additionally, it highlights existing gaps in our understanding, presents unanswered questions, and provides avenues for future research.

The growth factor EPIREGULIN promotes basal progenitor cell proliferation in the developing neocortex

Neocortex expansion during evolution is linked to higher numbers of neurons, which are thought to result from increased proliferative capacity and neurogenic potential of basal progenitor cells during development. Here, we show that EREG, encoding the growth factor EPIREGULIN, is expressed in the human developing neocortex and in gorilla cerebral organoids, but not in the mouse neocortex. Addition of EPIREGULIN to the mouse neocortex increases proliferation of basal progenitor cells, whereas EREG ablation in human cortical organoids reduces proliferation in the subventricular zone. Treatment of cortical organoids with EPIREGULIN promotes a further increase in proliferation of gorilla but not of human basal progenitor cells. EPIREGULIN competes with the epidermal growth factor (EGF) to promote proliferation, and inhibition of the EGF receptor abrogates the EPIREGULIN-mediated increase in basal progenitor cells. Finally, we identify putative cis-regulatory elements that may contribute to the observed inter-species differences in EREG expression. Our findings suggest that species-specific regulation of EPIREGULIN expression may contribute to the increased neocortex size of primates by providing a tunable pro-proliferative signal to basal progenitor cells in the subventricular zone.

Orderly specification and precise laminar deployment of cortical glutamatergic projection neuron types through intermediate progenitors

The cerebral cortex comprises diverse types of glutamatergic projection neurons (PNs) generated from radial glial progenitors (RGs) through either direct neurogenesis or indirect neurogenesis (iNG) via intermediate progenitors (IPs). A foundational concept in corticogenesis is the "inside-out" model whereby successive generations of PNs sequentially migrate to deep then progressively more superficial layers, but its biological significance remains unclear; and the role of iNG in this process is unknown. Using genetic strategies linking PN birth-dating to projection mapping in mice, we found that the laminar deployment of IP-derived PNs substantially deviate from an inside-out rule: PNs destined to non-consecutive layers are generated at the same time, and different PN types of the same layer are generated at non-contiguous times. The overarching scheme of iNG is the sequential specification and precise laminar deployment of projection-defined PN types, which may contribute to the orderly assembly of cortical output channels and processing streams.

HIGHLIGHTS

- Each IP is fate-restricted to generate a pair of near-identical PNs - Corticogenesis involves the orderly generation of fate-restricted IP temporal cohorts - IP temporal cohorts sequentially as well as concurrently specify multiple PN types - The deployment of PN types to specific layers does not follow an inside-out order.

The SVZ stem cell niche-components, functions, and in vitro modelling

Neocortical development depends on the intrinsic ability of neural stem and progenitor cells to proliferate and differentiate to generate the different kinds of neurons in the adult brain. These progenitor cells can be distinguished into apical progenitors, which occupy a stem cell niche in the ventricular zone and basal progenitors, which occupy a stem cell niche in the subventricular zone (SVZ). During development, the stem cell niche provided in the subventricular zone enables the increased proliferation and self-renewal of basal progenitors, which likely underlie the expansion of the human neocortex. However, the components forming the SVZ stem cell niche in the developing neocortex have not yet been fully understood. In this review, we will discuss potential components of the SVZ stem cell niche, i.e., extracellular matrix composition and brain vasculature, and their possible key role in establishing and maintaining this niche during fetal neocortical development. We will also emphasize the potential role of basal progenitor morphology in maintaining their proliferative capacity within the stem cell niche of the SVZ. Finally, we will focus on the use of brain organoids to i) understand the unique features of basal progenitors, notably basal radial glia; ii) study components of the SVZ stem cell niche; and iii) provide future directions on how to improve brain organoids, notably the organoid SVZ, and make them more reliable models of human neocortical development and evolution studies.

Shaping the brain: The emergence of cortical structure and folding

The cerebral cortex-the brain's covering and largest region-has increased in size and complexity in humans and supports higher cognitive functions such as language and abstract thinking. There is a growing understanding of the human cerebral cortex, including the diversity and number of cell types that it contains, as well as of the developmental mechanisms that shape cortical structure and organization. In this review, we discuss recent progress in our understanding of molecular and cellular processes, as well as mechanical forces, that regulate the folding of the cerebral cortex. Advances in human genetics, coupled with experimental modeling in gyrencephalic species, have provided insights into the central role of cortical progenitors in the gyrification and evolutionary expansion of the cerebral cortex. These studies are essential for understanding the emergence of structural and functional organization during cortical development and the pathogenesis of neurodevelopmental disorders associated with cortical malformations.

Reduced neural progenitor cell count and cortical neurogenesis in guinea pigs congenitally infected with Toxoplasma gondii

Toxoplasma (T.) gondii is an obligate intracellular parasite with a worldwide distribution. Congenital infection can lead to severe pathological alterations in the brain. To examine the effects of toxoplasmosis in the fetal brain, pregnant guinea pigs are infected with T. gondii oocysts on gestation day 23 and dissected 10, 17 and 25 days afterwards. We show the neocortex to represent a target region of T. gondii and the parasite to infect neural progenitor cells (NPCs), neurons and astrocytes in the fetal brain. Importantly, we observe a significant reduction in neuron number at end-neurogenesis and find a marked reduction in NPC count, indicating that impaired neurogenesis underlies the neuronal decrease in infected fetuses. Moreover, we observe focal microglioses to be associated with T. gondii in the fetal brain. Our findings expand the understanding of the pathophysiology of congenital toxoplasmosis, especially contributing to the development of cortical malformations.

MKL/SRF and Bcl6 mutual transcriptional repression safeguards the fate and positioning of neocortical progenitor cells mediated by RhoA

During embryogenesis, multiple intricate and intertwined cellular signaling pathways coordinate cell behavior. Their slightest alterations can have dramatic consequences for the cells and the organs they form. The transcriptional repressor Bcl6 was recently found as important for brain development. However, its regulation and integration with other signals is unknown. Using in vivo functional approaches combined with molecular mechanistic analysis, we identified a reciprocal regulatory loop between B cell lymphoma 6 (Bcl6) and the RhoA-regulated transcriptional complex megakaryoblastic leukemia/serum response factor (MKL/SRF). We show that Bcl6 physically interacts with MKL/SRF, resulting in a down-regulation of the transcriptional activity of both Bcl6 and MKL/SRF. This molecular cross-talk is essential for the control of proliferation, neurogenesis, and spatial positioning of neural progenitors. Overall, our data highlight a regulatory mechanism that controls neuronal production and neocortical development and reveal an MKL/SRF and Bcl6 interaction that may have broader implications in other physiological functions and in diseases.

Patient Brain Organoids Identify a Link between the 16p11.2 Copy Number Variant and the RBFOX1 Gene

Copy number variants (CNVs) that delete or duplicate 30 genes within the 16p11.2 genomic region give rise to a range of neurodevelopmental phenotypes with high penetrance in humans. Despite the identification of this small region, the mechanisms by which 16p11.2 CNVs lead to disease are unclear. Relevant models, such as human cortical organoids (hCOs), are needed to understand the human-specific mechanisms of neurodevelopmental disease. We generated hCOs from 17 patients and controls, profiling 167,958 cells with single-cell RNA-sequencing analysis, which revealed neuronal-specific differential expression of genes outside the 16p11.2 region that are related to cell-cell adhesion, neuronal projection growth, and neurodevelopmental disorders. Furthermore, 16p11.2 deletion syndrome organoids exhibited reduced mRNA and protein levels of RBFOX1, a gene that can also harbor CNVs linked to neurodevelopmental phenotypes. We found that the genes previously shown to be regulated by RBFOX1 are also perturbed in organoids from patients with the 16p11.2 deletion syndrome and thus identified a novel link between independent CNVs associated with neuronal development and autism. Overall, this work suggests convergent signaling, which indicates the possibility of a common therapeutic mechanism across multiple rare neuronal diseases.

Human-biased TMEM25 expression promotes expansion of neural progenitor cells to alter cortical structure in the developing brain

Cortical expansion has occurred during human brain evolution. By comparing human and mouse RNA-seq datasets, we found that transmembrane protein 25 (TMEM25) was much more highly expressed in human neural progenitors (NPCs). Overexpression of either human TMEM25 or mouse Tmem25 similarly promoted mouse NPC proliferation in vitro. Mimicking human-type expression of TMEM25 in mouse ventricular cortical progenitors accelerated proliferation of basal radial glia (bRG) and increased the number of upper-layer neurons in vivo. By contrast, RNA-seq analysis, and pharmacological assays showed that knockdown of TMEM25 in cultured human NPCs compromised the effects of extracellular signals, leading to cell cycle inhibition via Akt repression. Thus, TMEM25 can receive extracellular signals to expand bRG in human cortical development.

Dihydrofolate reductase activity controls neurogenic transitions in the developing neocortex

One-carbon/folate (1C) metabolism supplies methyl groups required for DNA and histone methylation, and is involved in the maintenance of self-renewal in stem cells. Dihydrofolate reductase (DHFR), a key enzyme in 1C metabolism, is highly expressed in human and mouse neural progenitors at the early stages of neocortical development. Here, we have investigated the role of DHFR in the developing neocortex and report that reducing its activity in human neural organoids and mouse embryonic neocortex accelerates indirect neurogenesis, thereby affecting neuronal composition of the neocortex. Furthermore, we show that decreasing DHFR activity in neural progenitors leads to a reduction in one-carbon/folate metabolites and correlates with modifications of H3K4me3 levels. Our findings reveal an unanticipated role for DHFR in controlling specific steps of neocortex development and indicate that variations in 1C metabolic cues impact cell fate transitions.

Neocortex neurogenesis and maturation in the African greater cane rat

BACKGROUND

Neocortex development has been extensively studied in altricial rodents such as mouse and rat. Identification of alternative animal models along the "altricial-precocial" spectrum in order to better model and understand neocortex development is warranted. The Greater cane rat (GCR, Thyronomys swinderianus) is an indigenous precocial African rodent. Although basic aspects of brain development in the GCR have been documented, detailed information on neocortex development including the occurrence and abundance of the distinct types of neural progenitor cells (NPCs) in the GCR are lacking.

METHODS

GCR embryos and fetuses were obtained from timed pregnant dams between gestation days 50-140 and their neocortex was analyzed by immunofluorescence staining using characteristic marker proteins for NPCs, neurons and glia cells. Data were compared with existing data on closely related precocial and altricial species, i.e. guinea pig and dwarf rabbit.

RESULTS

The primary sequence of neuro- and gliogenesis, and neuronal maturation is preserved in the prenatal GCR neocortex. We show that the GCR exhibits a relatively long period of cortical neurogenesis of 70 days. The subventricular zone becomes the major NPC pool during mid-end stages of neurogenesis with Pax6 + NPCs constituting the major basal progenitor subtype in the GCR neocortex. Whereas dendrite formation in the GCR cortical plate appears to initiate immediately after the onset of neurogenesis, major aspects of axon formation and maturation, and astrogenesis do not begin until mid-neurogenesis. Similar to the guinea pig, the GCR neocortex exhibits a high maturation status, containing neurons with well-developed dendrites and myelinated axons and astrocytes at birth, thus providing further evidence for the notion that a great proportion of neocortex growth and maturation in precocial mammals occurs before birth.

CONCLUSIONS

Together, this work has deepened our understanding of neocortex development of the GCR, of the timing and the cellular differences that regulate brain growth and development within the altricial-precocial spectrum and its suitability as a research model for neurodevelopmental studies. The timelines of brain development provided by this study may serve as empirical reference data and foundation in future studies in order to model and better understand neurodevelopment and associated alterations.

PCDH12 loss results in premature neuronal differentiation and impeded migration in a cortical organoid model

Protocadherins (PCDHs) are cell adhesion molecules that regulate many essential neurodevelopmental processes related to neuronal maturation, dendritic arbor formation, axon pathfinding, and synaptic plasticity. Biallelic loss-of-function variants in PCDH12 are associated with several neurodevelopmental disorders (NDDs). Despite the highly deleterious outcome resulting from loss of PCDH12, little is known about its role during brain development and disease. Here, we show that PCDH12 loss severely impairs cerebral organoid development, with reduced proliferative areas and disrupted laminar organization. 2D models further show that neural progenitor cells lacking PCDH12 prematurely exit the cell cycle and differentiate earlier when compared with wild type. Furthermore, we show that PCDH12 regulates neuronal migration and suggest that this could be through a mechanism requiring ADAM10-mediated ectodomain shedding and/or membrane recruitment of cytoskeleton regulators. Our results demonstrate a critical involvement of PCDH12 in cortical organoid development, suggesting a potential cause for the pathogenic mechanisms underlying PCDH12-related NDDs.

Deletion of ARGLU1 causes global defects in alternative splicing in vivo and mouse cortical malformations primarily via apoptosis

Haploinsufficient mutation in arginine and glutamine-rich protein 1 (Arglu1), a newly identified pre-mRNA splicing regulator, may be linked to neural developmental disorders associated with mental retardation and epilepsy in human patients, but the underlying causes remain elusive. Here we show that ablation of Arglu1 promotes radial glial cell (RG) detachment from the ventricular zone (VZ), leading to ectopic localized RGs in the mouse embryonic cortex. Although they remain proliferative, ectopic progenitors, as well as progenitors in the VZ, exhibit prolonged mitosis, p53 upregulation and cell apoptosis, leading to reduced neuron production, neuronal loss and microcephaly. RNA seq analysis reveals widespread changes in alternative splicing in the mutant mouse embryonic cortex, preferentially affecting genes involved in neuronal functions. Mdm2 and Mdm4 are found to be alternatively spliced at the exon 3 and exon 5 respectively, leading to absence of the p53-binding domain and nonsense-mediated mRNA decay (NMD) and thus relieve inhibition of p53. Removal of p53 largely rescues the microcephaly caused by deletion of Arglu1. Our findings provide mechanistic insights into cortical malformations of human patients with Arglu1 haploinsufficient mutation.

Direct and indirect neurogenesis generate a mosaic of distinct glutamatergic projection neuron types in cerebral cortex

Variations in size and complexity of the cerebral cortex result from differences in neuron number and composition, rooted in evolutionary changes in direct and indirect neurogenesis (dNG and iNG) that are mediated by radial glia and intermediate progenitors (IPs), respectively. How dNG and iNG differentially contribute to neuronal number, diversity, and connectivity are unknown. Establishing a genetic fate-mapping method to differentially visualize dNG and iNG in mice, we found that while both dNG and iNG contribute to all cortical structures, iNG contributes the largest relative proportions to the hippocampus and neocortex. Within the neocortex, whereas dNG generates all major glutamatergic projection neuron (PN) classes, iNG differentially amplifies and diversifies PNs within each class; the two pathways generate distinct PN types and assemble fine mosaics of lineage-based cortical subnetworks. Our results establish a ground-level lineage framework for understanding cortical development and evolution by linking foundational progenitor types and neurogenic pathways to PN types.

An adhesion signaling axis involving Dystroglycan, β1-Integrin, and Cas adaptor proteins regulates the establishment of the cortical glial scaffold

The mature mammalian cortex is composed of 6 architecturally and functionally distinct layers. Two key steps in the assembly of this layered structure are the initial establishment of the glial scaffold and the subsequent migration of postmitotic neurons to their final position. These processes involve the precise and timely regulation of adhesion and detachment of neural cells from their substrates. Although much is known about the roles of adhesive substrates during neuronal migration and the formation of the glial scaffold, less is understood about how these signals are interpreted and integrated within these neural cells. Here, we provide in vivo evidence that Cas proteins, a family of cytoplasmic adaptors, serve a functional and redundant role during cortical lamination. Cas triple conditional knock-out (Cas TcKO) mice display severe cortical phenotypes that feature cobblestone malformations. Molecular epistasis and genetic experiments suggest that Cas proteins act downstream of transmembrane Dystroglycan and β1-Integrin in a radial glial cell-autonomous manner. Overall, these data establish a new and essential role for Cas adaptor proteins during the formation of cortical circuits and reveal a signaling axis controlling cortical scaffold formation.

Bacterial TLR2/6 Ligands Block Ciliogenesis, Derepress Hedgehog Signaling, and Expand the Neocortex

Microbial components have a range of direct effects on the fetal brain. However, little is known about the cellular targets and molecular mechanisms that mediate these effects. Neural progenitor cells (NPCs) control the size and architecture of the brain and understanding the mechanisms regulating NPCs is crucial to understanding brain developmental disorders. We identify ventricular radial glia (vRG), the primary NPC, as the target of bacterial cell wall (BCW) generated during the antibiotic treatment of maternal pneumonia. BCW enhanced proliferative potential of vRGs by shortening the cell cycle and increasing self-renewal. Expanded vRGs propagated to increase neuronal output in all cortical layers. Remarkably, Toll-like receptor 2 (TLR2), which recognizes BCW, localized at the base of primary cilia in vRGs and the BCW-TLR2 interaction suppressed ciliogenesis leading to derepression of Hedgehog (HH) signaling and expansion of vRGs. We also show that TLR6 is an essential partner of TLR2 in this process. Surprisingly, TLR6 alone was required to set the number of cortical neurons under healthy conditions. These findings suggest that an endogenous signal from TLRs suppresses cortical expansion during normal development of the neocortex and that BCW antagonizes that signal through the TLR2/cilia/HH signaling axis changing brain structure and function. IMPORTANCE Fetal brain development in early gestation can be impacted by transplacental infection, altered metabolites from the maternal microbiome, or maternal immune activation. It is less well understood how maternal microbial subcomponents that cross the placenta, such as bacterial cell wall (BCW), directly interact with fetal neural progenitors and neurons and affect development. This scenario plays out in the clinic when BCW debris released during antibiotic therapy of maternal infection traffics to the fetal brain. This study identifies the direct interaction of BCW with TLR2/6 present on the primary cilium, the signaling hub on fetal neural progenitor cells (NPCs). NPCs control the size and architecture of the brain and understanding the mechanisms regulating NPCs is crucial to understanding brain developmental disorders. Within a window of vulnerability before the appearance of fetal immune cells, the BCW-TLR2/6 interaction results in the inhibition of ciliogenesis, derepression of Sonic Hedgehog signaling, excess proliferation of neural progenitors, and abnormal cortical architecture. In the first example of TLR signaling linked to Sonic Hedgehog, BCW/TLR2/6 appears to act during fetal brain morphogenesis to play a role in setting the total cell number in the neocortex.

Emerging concepts in oligodendrocyte and myelin formation, inputs from the zebrafish model

Oligodendrocytes (OLs) are the myelinating cells of the central nervous system (CNS), which are derived from OL precursor cells. Myelin insulates axons allowing the saltatory conduction of action potentials and also provides trophic and metabolic supports to axons. Interestingly, oligodendroglial cells have the capacity to sense neuronal activity, which regulates myelin sheath formation via the vesicular release of neurotransmitters. Neuronal activity-dependent regulation of myelination is mediated by specialized interaction between axons and oligodendroglia, involving both synaptic and extra-synaptic modes of communications. The zebrafish has provided key advantages for the study of the myelination process in the CNS. External development and transparent larval stages of this vertebrate specie combined with the existence of several transgenic reporter lines provided key advances in oligodendroglial cell biology, axo-glial interactions and CNS myelination. In this publication, we reviewed and discussed the most recent knowledge on OL development and myelin formation, with a focus on mechanisms regulating these fundamental biological processes in the zebrafish. Especially, we highlighted the critical function of axons and oligodendroglia modes of communications and calcium signaling in myelin sheath formation and growth. Finally, we reviewed the relevance of these knowledge's in demyelinating diseases and drug discovery of pharmacological compounds favoring myelin regeneration.

Ethanol exposure disrupted the formation of radial glial processes and impaired the generation and migration of outer radial glial cells in forebrain organoids derived from human embryonic stem cells

Radial glial cells (RGCs) play a pivotal role in cerebral cortical development by functioning as a source of new neurons and by supporting the migration of newborn neurons. These functions are primarily dependent on the apical-basolateral structures of radial glial processes. This study aims to investigate the effects of ethanol exposure on the development of radial glial processes and the generation, migration, and transformation of outer radial glial cells (oRGCs). For this purpose, forebrain organoids were developed from human embryonic stem cells. These forebrain organoids contain abundant neural progenitor cells (SOX2+), express high levels of neural epithelial markers β-catenin and PKCλ, and dorsal forebrain marker PAX6, and display well-organized cortical architectures containing abundant apical and basal RGCs, intermediate progenitors (IPCs), and neurons. Exposure of forebrain organoids to ethanol resulted in a significant increase in apoptosis in Nestin-positive radial glial cells. Ethanol exposure also remarkably decreased the levels of radial glial process-associated proteins, including Nestin, GFAP, and Vimentin, in radial glial cells and distinctly impaired the integrity and morphologies of radial glial processes. In addition, the ethanol-induced impairment of the radial glial processes is associated with decreased migration and proliferation of radial glial cells, reduction in the generation of HOPX+ oRGCs, and the accelerated transformation of oRGCs into astrocytes. These results demonstrate that ethanol exposure can disrupt cerebral cortex development by impairing the formation of radial glial processes and the generation, migration, and transformation of oRGCs.

Heterogeneity in the size of the apical surface of cortical progenitors

BACKGROUND

The apical surface (AS) of epithelial cells is highly specialized; it is important for morphogenetic processes that are essential to shape organs and tissues and it plays a role in morphogen and growth factor signaling. Apical progenitors in the mammalian neocortex are pseudoepithelial cells whose apical surface lines the ventricle. Whether changes in their apical surface sizes are important for cortical morphogenesis and/or other aspects of neocortex development has not been thoroughly addressed.

RESULTS

Here we show that apical progenitors are heterogeneous with respect to their apical surface area. In Efnb1 mutants, the size of the apical surface is modified and this correlates with discrete alterations of tissue organization without impacting apical progenitors proliferation.

CONCLUSIONS

Altogether, our data reveal heterogeneity in apical progenitors AS area in the developing neocortex and shows a role for Ephrin B1 in controlling AS size. Our study also indicates that changes in AS size do not have strong repercussion on apical progenitor behavior.

Increased Nuclear FOXP2 Is Related to Reduced Neural Stem Cell Number and Increased Neurogenesis in the Dorsal Telencephalon of Embryos of Diabetic Rats through Histamine H1 Receptors

Diabetic rat embryos have increased cortical neurogenesis and neuron maturation, and their offspring presented altered neuron polarity, lamination, and diminished neuron excitability. The FOXP2 overexpression results in higher cortical neurogenesis by increasing the transition of radial glia to the intermediate progenitor. Similarly, histamine through H₁-receptor activation increases cortical neuron differentiation. Indeed, blocking the H₁-receptor by the systemic administration of chlorpheniramine to diabetic pregnant rats prevents increased neurogenesis. Here, we explore the relationship between the H₁-receptor and FOXP2 on embryo neurogenesis from diabetic dams. Through qRT-PCR, Western blot, immunohistofluorescence, and flow cytometry, we showed an increased FOXP2 expression and nuclear localization, a reduced Nestin expression and -positive cells number, and a higher PKCα expression in the cortical neuroepithelium of fourteen-day-old embryos from diabetic rats. Interestingly, this scenario was prevented by the chlorpheniramine systemic administration to diabetic pregnant rats at embryo day twelve. These data, together with the bioinformatic analysis, suggest that higher H₁-receptor activity in embryos under high glucose increases FOXP2 nuclear translocation, presumably through PKCα phosphorylation, impairing the transition of radial glia to intermediate progenitor and increasing neuron differentiation in embryos of diabetic rats.

Impaired migration and premature differentiation underlie the neurological phenotype associated with PCDH12 loss of function

Protocadherins (PCDHs) are cell adhesion molecules that regulate many essential neurodevelopmental processes related to neuronal maturation, dendritic arbor formation, axon pathfinding, and synaptic plasticity. Bi-allelic loss-of-function variants in PCDH12 are associated with several neurodevelopmental disorders (NDDs) such as diencephalic-mesencephalic dysplasia syndrome, cerebral palsy, cerebellar ataxia, and microcephaly. Despite the highly deleterious outcome resulting from loss of PCDH12, little is known about its role during brain development and disease. Here, we show that PCDH12 loss severely impairs cerebral organoid development with reduced proliferative areas and disrupted laminar organization. 2D models further show that neural progenitor cells lacking PCDH12 prematurely exit cell cycle and differentiate earlier when compared to wildtype. Furthermore, we show that PCDH12 regulates neuronal migration through a mechanism requiring ADAM10-mediated ectodomain shedding and membrane recruitment of cytoskeleton regulators. Our data demonstrate a critical and broad involvement of PCDH12 in cortical development, revealing the pathogenic mechanisms underlying PCDH12-related NDDs.

How mechanisms of stem cell polarity shape the human cerebral cortex

Apical-basal progenitor cell polarity establishes key features of the radial and laminar architecture of the developing human cortex. The unique diversity of cortical stem cell populations and an expansion of progenitor population size in the human cortex have been mirrored by an increase in the complexity of cellular processes that regulate stem cell morphology and behaviour, including their polarity. The study of human cells in primary tissue samples and human stem cell-derived model systems (such as cortical organoids) has provided insight into these processes, revealing that protein complexes regulate progenitor polarity by controlling cell membrane adherence within appropriate cortical niches and are themselves regulated by cytoskeletal proteins, signalling molecules and receptors, and cellular organelles. Studies exploring how cortical stem cell polarity is established and maintained are key for understanding the features of human brain development and have implications for neurological dysfunction.

Human-specific ARHGAP11B ensures human-like basal progenitor levels in hominid cerebral organoids

The human-specific gene ARHGAP11B has been implicated in human neocortex expansion. However, the extent of ARHGAP11B's contribution to this expansion during hominid evolution is unknown. Here we address this issue by genetic manipulation of ARHGAP11B levels and function in chimpanzee and human cerebral organoids. ARHGAP11B expression in chimpanzee cerebral organoids doubles basal progenitor levels, the class of cortical progenitors with a key role in neocortex expansion. Conversely, interference with ARHGAP11B's function in human cerebral organoids decreases basal progenitors down to the chimpanzee level. Moreover, ARHGAP11A or ARHGAP11B rescue experiments in ARHGAP11A plus ARHGAP11B double-knockout human forebrain organoids indicate that lack of ARHGAP11B, but not of ARHGAP11A, decreases the abundance of basal radial glia-the basal progenitor type thought to be of particular relevance for neocortex expansion. Taken together, our findings demonstrate that ARHGAP11B is necessary and sufficient to ensure the elevated basal progenitor levels that characterize the fetal human neocortex, suggesting that this human-specific gene was a major contributor to neocortex expansion during human evolution.

PAR3 restricts the expansion of neural precursor cells by regulating hedgehog signaling

During brain development, neural precursor cells (NPCs) expand initially, and then switch to generating stage-specific neurons while maintaining self-renewal ability. Because the NPC pool at the onset of neurogenesis crucially affects the final number of each type of neuron, tight regulation is necessary for the transitional timing from the expansion to the neurogenic phase in these cells. However, the molecular mechanisms underlying this transition are poorly understood. Here, we report that the telencephalon-specific loss of PAR3 before the start of neurogenesis leads to increased NPC proliferation at the expense of neurogenesis, resulting in disorganized tissue architecture. These NPCs demonstrate hyperactivation of hedgehog signaling in a smoothened-dependent manner, as well as defects in primary cilia. Furthermore, loss of PAR3 enhanced ligand-independent ciliary accumulation of smoothened and an inhibitor of smoothened ameliorated the hyperproliferation of NPCs in the telencephalon. Thus, these findings support the idea that PAR3 has a crucial role in the transition of NPCs from the expansion phase to the neurogenic phase by restricting hedgehog signaling through the establishment of ciliary integrity.

A kinase-independent function of cyclin-dependent kinase 6 promotes outer radial glia expansion and neocortical folding

The neocortex, the center for higher brain function, first emerged in mammals and has become massively expanded and folded in humans, constituting almost half the volume of the human brain. Primary microcephaly, a developmental disorder in which the brain is smaller than normal at birth, results mainly from there being fewer neurons in the neocortex because of defects in neural progenitor cells (NPCs). Outer radial glia (oRGs), NPCs that are abundant in gyrencephalic species but rare in lissencephalic species, are thought to play key roles in the expansion and folding of the neocortex. However, how oRGs expand, whether they are necessary for neocortical folding, and whether defects in oRGs cause microcephaly remain important questions in the study of brain development, evolution, and disease. Here, we show that oRG expansion in mice, ferrets, and human cerebral organoids requires cyclin-dependent kinase 6 (CDK6), the mutation of which causes primary microcephaly via an unknown mechanism. In a mouse model in which increased Hedgehog signaling expands oRGs and intermediate progenitor cells and induces neocortical folding, CDK6 loss selectively decreased oRGs and abolished neocortical folding. Remarkably, this function of CDK6 in oRG expansion did not require its kinase activity, was not shared by the highly similar CDK4 and CDK2, and was disrupted by the mutation causing microcephaly. Therefore, our results indicate that CDK6 is conserved to promote oRG expansion, that oRGs are necessary for neocortical folding, and that defects in oRG expansion may cause primary microcephaly.

Corticogenesis across species at single-cell resolution

The neocortex (or pallium) consists of diverse cell types that are organized in a highly species-specific manner under strict spatiotemporal control during development. Many of the cell types are present transiently throughout development but contribute to permanent species-specific cortical features that are acquired through evolution. Therefore, capturing cell type-specific biological information has always been an important quest in the field of neurodevelopment. The progress in achieving fine cellular resolution has been slow due to technical challenges. However, with recent advancements in single-cell and multi-omics technologies, many laboratories have begun to successfully interrogate cellular and molecular mechanisms driving corticogenesis at single-cell resolution. In this review, we provide summarized results from many primary publications and several in-depth review articles that utilize or address single-cell genomics techniques to understand important topics, such as cellular and molecular mechanisms governing cortical progenitor proliferation, cell lineage progression, neuronal specification, and arealization, across multiple gyrencephalic (i.e., human and non-human primates) and lissencephalic species (i.e., mouse, reptiles, and songbirds). We also examine findings from recent studies involving epigenomic and posttranscriptional regulation of corticogenesis. In the discussion section, we provide our insights on the challenges the field currently faces as well as promising future applications of single cell technologies.

Glyphosate-based herbicide induces long-lasting impairment in neuronal and glial differentiation

Glyphosate-based herbicides (GBH) are among the most sold pesticides in the world. There are several formulations based on the active ingredient glyphosate (GLY) used along with other chemicals to improve the absorption and penetration in plants. The final composition of commercial GBH may modify GLY toxicological profile, potentially enhancing its neurotoxic properties. The developing nervous system is particularly susceptible to insults occurring during the early phases of development, and exposure to chemicals in this period may lead to persistent impairments on neurogenesis and differentiation. The aim of this study was to evaluate the long-lasting effects of a sub-cytotoxic concentration, 2.5 parts per million of GBH and GLY, on the differentiation of human neuroepithelial stem cells (NES) derived from induced pluripotent stem cells (iPSC). We treated NES cells with each compound and evaluated the effects on key cellular processes, such as proliferation and differentiation in daughter cells never directly exposed to the toxicants. We found that GBH induced a more immature neuronal profile associated to increased PAX6, NESTIN and DCX expression, and a shift in the differentiation process toward glial cell fate at the expense of mature neurons, as shown by an increase in the glial markers GFAP, GLT1, GLAST and a decrease in MAP2. Such alterations were associated to dysregulation of key genes critically involved in neurogenesis, including PAX6, HES1, HES5, and DDK1. Altogether, the data indicate that subtoxic concentrations of GBH, but not of GLY, induce long-lasting impairments on the differentiation potential of NES cells.

Mid1 is associated with androgen-dependent axonal vulnerability of motor neurons in spinal and bulbar muscular atrophy

Spinal and bulbar muscular atrophy (SBMA) is an adult-onset hereditary neurodegenerative disease caused by the expansions of CAG repeats in the androgen receptor (AR) gene. Androgen-dependent nuclear accumulation of pathogenic AR protein causes degeneration of lower motor neurons, leading to progressive muscle weakness and atrophy. While the successful induction of SBMA-like pathology has been achieved in mouse models, mechanisms underlying motor neuron vulnerability remain unclear. In the present study, we performed a transcriptome-based screening for genes expressed exclusively in motor neurons and dysregulated in the spinal cord of SBMA mice. We found upregulation of Mid1 encoding a microtubule-associated RNA binding protein which facilitates the translation of CAG-expanded mRNAs. Based on the finding that lower motor neurons begin expressing Mid1 during embryonic stages, we developed an organotypic slice culture system of the spinal cord obtained from SBMA mouse fetuses to study the pathogenic role of Mid1 in SBMA motor neurons. Impairment of axonal regeneration arose in the spinal cord culture in SBMA mice in an androgen-dependent manner, but not in mice with non-CAG-expanded AR, and was either exacerbated or ameliorated by Mid1 overexpression or knockdown, respectively. Hence, an early Mid1 expression confers vulnerability to motor neurons, at least by inducing axonogenesis defects, in SBMA.