Intermediate Progenitor Cohorts Differentially Generate Cortical Layers and Require Tbr2 for Timely Acquisition of Neuronal Subtype Identity

Regional differences in progenitor metabolism shape brain growth during development

Mammals have particularly large forebrains compared with other brain parts, yet the developmental mechanisms underlying this regional expansion remain poorly understood. Here, we provide a single-cell-resolution birthdate atlas of the mouse brain (www.neurobirth.org), which reveals that while hindbrain neurogenesis is transient and restricted to early development, forebrain neurogenesis is temporally sustained through reduced consumptive divisions of ventricular zone progenitors. This atlas additionally reveals region-specific patterns of direct and indirect neurogenesis. Using single-cell RNA sequencing, we identify evolutionarily conserved cell-cycle programs and metabolism-related molecular pathways that control regional temporal windows of proliferation. We identify the late neocortex-enriched mitochondrial protein FAM210B as a key regulator using in vivo gain- and loss-of-function experiments. FAM210B elongates mitochondria and increases lactate production, which promotes progenitor self-replicative divisions and, ultimately, the larger clonal size of their progeny. Together, these findings indicate that spatiotemporal heterogeneity in mitochondrial function regulates regional progenitor cycling behavior and associated clonal neuronal production during brain development.

Cell-type-specific roles of FOXP1 in the excitatory neuronal lineage during early neocortical murine development

Forkhead box protein P1 (FOXP1), a transcription factor enriched in the neocortex, is associated with autism spectrum disorders (ASDs) and FOXP1 syndrome. Emx1Cre/+;Foxp1fl/fl conditional deletion (Foxp1 conditional knockout [cKO]) in the mouse cortex leads to overall reduced cortex thickness, alterations in cortical lamination, and changes in the relative thickness of cortical layers. However, the developmental and cell-type-specific mechanisms underlying these changes remained unclear. We find that Foxp1 deletion results in accelerated pseudo-age during early neurogenesis, increased cell cycle exit during late neurogenesis, altered gene expression and chromatin accessibility, and selective migration deficits in a subset of upper-layer neurons. These data explain the postnatal differences observed in cortical layers and relative cortical thickness. We also highlight genes regulated by FOXP1 and their enrichment with high-confidence ASD or synaptic genes. Together, these results underscore a network of neurodevelopmental-disorder-related genes that may serve as potential modulatory targets for postnatal modification relevant to ASDs and FOXP1 syndrome.

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.

Cell type-specific roles of FOXP1 in the excitatory neuronal lineage during early neocortical murine development

FOXP1, a transcription factor enriched in the neocortex, is associated with autism spectrum disorders (ASD) and FOXP1 syndrome. Emx1 Cre/+ ;Foxp1 fl/fl conditional deletion (Foxp1 cKO) in the mouse cortex leads to overall reduced cortex thickness, alterations in cortical lamination, and changes in the relative thickness of cortical layers. However, the developmental and cell type-specific mechanisms underlying these changes remained unclear. We find that Foxp1 deletion results in accelerated pseudo-age during early neurogenesis, increased cell cycle exit during late neurogenesis, altered gene expression and chromatin accessibility, and selective migration deficits in a subset of upper-layer neurons. These data explain the postnatal differences observed in cortical layers and relative cortical thickness. We also highlight genes regulated by FOXP1 and their enrichment with high-confidence ASD or synaptic genes. Together, these results underscore a network of neurodevelopmental disorder-related genes that may serve as potential modulatory targets for postnatal modification relevant to ASD and FOXP1 syndrome.

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.

Dynamic culture of cerebral organoids using a pillar/perfusion plate for the assessment of developmental neurotoxicity

Despite the potential toxicity of commercial chemicals to the development of the nervous system (known as developmental neurotoxicity or DNT), conventionalin vitrocell models have primarily been employed for the assessment of acute neuronal toxicity. On the other hand, animal models used for the assessment of DNT are not physiologically relevant due to the heterogenic difference between humans and animals. In addition, animal models are low-throughput, time-consuming, expensive, and ethically questionable. Recently, human brain organoids have emerged as a promising alternative to assess the detrimental effects of chemicals on the developing brain. However, conventional organoid culture systems have several technical limitations including low throughput, lack of reproducibility, insufficient maturity of organoids, and the formation of the necrotic core due to limited diffusion of nutrients and oxygen. To address these issues and establish predictive DNT models, cerebral organoids were differentiated in a dynamic condition in a unique pillar/perfusion plate, which were exposed to test compounds to evaluate DNT potential. The pillar/perfusion plate facilitated uniform, dynamic culture of cerebral organoids with improved proliferation and maturity by rapid, bidirectional flow generated on a digital rocker. Day 9 cerebral organoids in the pillar/perfusion plate were exposed to ascorbic acid (DNT negative) and methylmercury (DNT positive) in a dynamic condition for 1 and 3 weeks, and changes in organoid morphology and neural gene expression were measured to determine DNT potential. As expected, ascorbic acid did not induce any changes in organoid morphology and neural gene expression. However, exposure of day 9 cerebral organoids to methylmercury resulted in significant changes in organoid morphology and neural gene expression. Interestingly, methylmercury did not induce adverse changes in cerebral organoids in a static condition, thus highlighting the importance of dynamic organoid culture in DNT assessment.

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.

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.

Indirect neurogenesis in space and time

During central nervous system (CNS) development, neural progenitor cells (NPCs) generate neurons and glia in two different ways. In direct neurogenesis, daughter cells differentiate directly into neurons or glia, whereas in indirect neurogenesis, neurons or glia are generated after one or more daughter cell divisions. Intriguingly, indirect neurogenesis is not stochastically deployed and plays instructive roles during CNS development: increased generation of cells from specific lineages; increased generation of early or late-born cell types within a lineage; and increased cell diversification. Increased indirect neurogenesis might contribute to the anterior CNS expansion evident throughout the Bilateria and help to modify brain-region size without requiring increased NPC numbers or extended neurogenesis. Increased indirect neurogenesis could be an evolutionary driver of the gyrencephalic (that is, folded) cortex that emerged during mammalian evolution and might even have increased during hominid evolution. Thus, selection of indirect versus direct neurogenesis provides a powerful developmental and evolutionary instrument that drives not only the evolution of CNS complexity but also brain expansion and modulation of brain-region size, and thereby the evolution of increasingly advanced cognitive abilities. This Review describes indirect neurogenesis in several model species and humans, and highlights some of the molecular genetic mechanisms that control this important process.

Engrailed transcription factors direct excitatory cerebellar neuron diversity and survival

The neurons of the three cerebellar nuclei (CN) are the primary output neurons of the cerebellum. The excitatory neurons (e) of the medial (m) CN (eCNm) were recently divided into molecularly defined subdomains in the adult; however, how they are established during development is not known. We define molecular subdomains of the mouse embryonic eCNm using single-cell RNA-sequencing and spatial expression analysis, showing that they evolve during embryogenesis to prefigure the adult. Furthermore, eCNm are transcriptionally divergent from cells in the other nuclei by embryonic day 14.5. We previously showed that loss of the homeobox genes En1 and En2 leads to loss of approximately half of the embryonic eCNm. We demonstrate that mutation of En1/2 in the embryonic eCNm results in death of specific posterior eCNm molecular subdomains and downregulation of TBR2 (EOMES) in an anterior embryonic subdomain, as well as reduced synaptic gene expression. We further reveal a similar function for EN1/2 in mediating TBR2 expression, neuron differentiation and survival in the other excitatory neurons (granule and unipolar brush cells). Thus, our work defines embryonic eCNm molecular diversity and reveals conserved roles for EN1/2 in the cerebellar excitatory neuron lineage.

Making Ramón y Cajal proud: Development of cell identity and diversity in the cerebral cortex

Since the beautiful images of Santiago Ramón y Cajal provided a first glimpse into the immense diversity and complexity of cell types found in the cerebral cortex, neuroscience has been challenged and inspired to understand how these diverse cells are generated and how they interact with each other to orchestrate the development of this remarkable tissue. Some fundamental questions drive the field's quest to understand cortical development: what are the mechanistic principles that govern the emergence of neuronal diversity? How do extrinsic and intrinsic signals integrate with physical forces and activity to shape cell identity? How do the diverse populations of neurons and glia influence each other during development to guarantee proper integration and function? The advent of powerful new technologies to profile and perturb cortical development at unprecedented resolution and across a variety of modalities has offered a new opportunity to integrate past knowledge with brand new data. Here, we review some of this progress using cortical excitatory projection neurons as a system to draw out general principles of cell diversification and the role of cell-cell interactions during cortical development.

Tcf4 dysfunction alters dorsal and ventral cortical neurogenesis in Pitt-Hopkins syndrome mouse model showing sexual dimorphism

Pitt-Hopkins syndrome (PTHS) is a neurodevelopmental disorder caused by haploinsufficiency of transcription factor 4 (TCF4). In this work, we focused on the cerebral cortex and investigated in detail the progenitor cell dynamics and the outcome of neurogenesis in a PTHS mouse model. Labeling and quantification of progenitors and newly generated neurons at various time points during embryonic development revealed alterations affecting the dynamic of cortical progenitors since the earliest stages of cortex formation in PTHS mice. Consequently, establishment of neuronal populations and layering of the cortex were found to be altered in heterozygotes subjects at birth. Interestingly, defective layering process of pyramidal neurons was partially rescued by reintroducing TCF4 expression using focal in utero electroporation in the cerebral cortex. Coincidentally with a defective dorsal neurogenesis, we found that ventral generation of interneurons was also defective in this model, which may lead to an excitation/inhibition imbalance in PTHS. Overall, sex-dependent differences were detected with more marked effects evidenced in males compared with females. All of this contributes to expand our understanding of PTHS, paralleling the advances of research in autism spectrum disorder and further validating the PTHS mouse model as an important tool to advance preclinical studies.

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.

KCTD10 regulates brain development by destabilizing brain disorder-associated protein KCTD13

KCTD10 belongs to the KCTD (potassiumchannel tetramerization domain) family, many members of which are associated with neuropsychiatric disorders. However, the biological function underlying the association with brain disorders remains to be explored. Here, we reveal that Kctd10 is highly expressed in neuronal progenitors and layer V neurons throughout brain development. Kctd10 deficiency triggers abnormal proliferation and differentiation of neuronal progenitors, reduced deep-layer (especially layer V) neurons, increased upper-layer neurons, and lowered brain size. Mechanistically, we screened and identified a unique KCTD10-interacting protein, KCTD13, associated with neurodevelopmental disorders. KCTD10 mediated the ubiquitination-dependent degradation of KCTD13 and KCTD10 ablation resulted in a considerable increase of KCTD13 expression in the developing cortex. KCTD13 overexpression in neuronal progenitors led to reduced proliferation and abnormal cell distribution, mirroring KCTD10 deficiency. Notably, mice with brain-specific Kctd10 knockout exhibited obvious motor deficits. This study uncovers the physiological function of KCTD10 and provides unique insights into the pathogenesis of neurodevelopmental disorders.

Dynamic culture of cerebral organoids using a pillar/perfusion plate for the assessment of developmental neurotoxicity

Despite the potential toxicity of commercial chemicals to the development of the nervous system (known as developmental neurotoxicity or DNT), conventional in vitro cell models have primarily been employed for the assessment of acute neuronal toxicity. On the other hand, animal models used for the assessment of DNT are not physiologically relevant due to the heterogenic difference between humans and animals. In addition, animal models are low-throughput, time-consuming, expensive, and ethically questionable. Recently, human brain organoids have emerged as a promising alternative to assess the detrimental effects of chemicals on the developing brain. However, conventional organoid culture systems have several technical limitations including low throughput, lack of reproducibility, insufficient maturity of organoids, and the formation of the necrotic core due to limited diffusion of nutrients and oxygen. To address these issues and establish predictive DNT models, cerebral organoids were differentiated in a dynamic condition in a unique pillar/perfusion plate, which were exposed to test compounds to evaluate DNT potential. The pillar/perfusion plate facilitated uniform, dynamic culture of cerebral organoids with improved proliferation and maturity by rapid, bidirectional flow generated on a digital rocker. Day 9 cerebral organoids in the pillar/perfusion plate were exposed to ascorbic acid (DNT negative) and methylmercury (DNT positive) in a dynamic condition for 1 and 3 weeks, and changes in organoid morphology and neural gene expression were measured to determine DNT potential. As expected, ascorbic acid didn't induce any changes in organoid morphology and neural gene expression. However, exposure of day 9 cerebral organoids to methylmercury resulted in significant changes in organoid morphology and neural gene expression. Interestingly, methylmercury did not induce adverse changes in cerebral organoids in a static condition, thus highlighting the importance of dynamic organoid culture in DNT assessment.

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.

Transcriptional regulation in the development and dysfunction of neocortical projection neurons

Glutamatergic projection neurons generate sophisticated excitatory circuits to integrate and transmit information among different cortical areas, and between the neocortex and other regions of the brain and spinal cord. Appropriate development of cortical projection neurons is regulated by certain essential events such as neural fate determination, proliferation, specification, differentiation, migration, survival, axonogenesis, and synaptogenesis. These processes are precisely regulated in a tempo-spatial manner by intrinsic factors, extrinsic signals, and neural activities. The generation of correct subtypes and precise connections of projection neurons is imperative not only to support the basic cortical functions (such as sensory information integration, motor coordination, and cognition) but also to prevent the onset and progression of neurodevelopmental disorders (such as intellectual disability, autism spectrum disorders, anxiety, and depression). This review mainly focuses on the recent progress of transcriptional regulations on the development and diversity of neocortical projection neurons and the clinical relevance of the failure of transcriptional modulations.

FEZ1 participates in human embryonic brain development by modulating neuronal progenitor subpopulation specification and migrations

Mutations in the human fasciculation and elongation protein zeta 1 (FEZ1) gene are found in schizophrenia and Jacobsen syndrome patients. Here, using human cerebral organoids (hCOs), we show that FEZ1 expression is turned on early during brain development and is detectable in both neuroprogenitor subtypes and immature neurons. FEZ1 deletion disrupts expression of neuronal and synaptic development genes. Using single-cell RNA sequencing, we detected abnormal expansion of homeodomain-only protein homeobox (HOPX)- outer radial glia (oRG), concurrent with a reduction of HOPX+ oRG, in FEZ1-null hCOs. HOPX- oRGs show higher cell mobility as compared to HOPX+ oRGs. Ectopic localization of neuroprogenitors to the outer layer is seen in FEZ1-null hCOs. Anomalous encroachment of TBR2+ intermediate progenitors into CTIP2+ deep layer neurons further indicated abnormalities in cortical layer formation these hCOs. Collectively, our findings highlight the involvement of FEZ1 in early cortical brain development and how it contributes to neurodevelopmental disorders.

Engrailed transcription factors direct excitatory cerebellar neuron diversity and survival

The excitatory neurons of the three cerebellar nuclei (eCN) form the primary output for the cerebellar circuit. The medial eCN (eCNm) were recently divided into molecularly defined subdomains in the adult, however how they are established during development is not known. We define molecular subdomains of the eCNm using scRNA-seq and spatial expression analysis and show they evolve during embryogenesis to resemble the adult. Furthermore, the eCNm is transcriptionally divergent from the rest of the eCN by E14.5. We previously showed that loss of the homeobox genes En1 and En2 leads to death of a subset of embryonic eCNm. We demonstrate that mutation of En1/2 in embryonic eCNm results in cell death of specific posterior eCNm molecular subdomains and loss of TBR2 (EOMES) expression in an anterior subdomain, as well as reduced synaptic gene expression. We further reveal a similar function for EN1/2 in mediating TBR2 expression, neuron differentiation and survival in the two other cerebellar excitatory neuron types. Thus, our work defines embryonic eCNm molecular diversity and reveals conserved roles for EN1/2 in the cerebellar excitatory neuron lineage.

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.

The progenitor basis of cortical projection neuron diversity

Diverse glutamatergic projection neurons (PNs) mediate myriad processing streams and output channels of the cerebral cortex. Yet, how different types of neural progenitors, such as radial glia (RGs) and intermediate progenitors (IPs), produce PN diversity, and hierarchical organization remains unclear. A fundamental issue is whether RGs constitute a homogeneous, multipotent lineage capable of generating all major PN types through a temporally regulated developmental program, or whether RGs comprise multiple transcriptionally heterogenous pools, each fated to generate a subset of PNs. Beyond RGs, the role of IPs in PN diversification remains underexplored. Addressing these questions requires tracking PN developmental trajectories with cell-type resolution - from transcription factor-defined RGs and IPs to their PN progeny, which are defined not only by laminar location but also by projection patterns and gene expression. Advances in cell-type resolution genetic fate mapping, axon tracing, and spatial transcriptomics may provide the technical capability for answering these fundamental questions.

Cortical inhibitory but not excitatory synaptic transmission and circuit refinement are altered after the deletion of NMDA receptors during early development

Neurons in the cerebral cortex form excitatory and inhibitory circuits with specific laminar locations. The mechanisms underlying the development of these spatially specific circuits is not fully understood. To test if postsynaptic N-methyl-D-aspartate (NMDA) receptors on excitatory neurons are required for the development of specific circuits to these neurons, we genetically ablated NMDA receptors from a subset of excitatory neurons in the temporal association cortex (TeA) through in utero electroporation and assessed the intracortical circuits connecting to L5 neurons through in vitro whole-cell patch clamp recordings coupled with laser-scanning photostimulation (LSPS). In NMDAR knockout neurons, α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor-mediated connections were largely intact. In contrast both LSPS and mini-IPSC recordings revealed that γ-aminobutyric acid type A (GABAA) receptor-mediated connections were impaired in NMDAR knockout neurons. These results suggest that postsynaptic NMDA receptors are important for the development of GABAergic circuits.

Conserved and Distinct Functions of the Autism-Related Chromatin Remodeler CHD8 in Embryonic and Adult Forebrain Neurogenesis

The chromatin remodeler CHD8 represents a high-confidence risk factor in autism, a multistage progressive neurologic disorder, however the underlying stage-specific functions remain elusive. In this study, by analyzing Chd8 conditional knock-out mice (male and female), we find that CHD8 controls cortical neural stem/progenitor cell (NSC) proliferation and survival in a stage-dependent manner. Strikingly, inducible genetic deletion reveals that CHD8 is required for the production and fitness of transit-amplifying intermediate progenitors (IPCs) essential for upper-layer neuron expansion in the embryonic cortex. p53 loss of function partially rescues apoptosis and neurogenesis defects in the Chd8-deficient brain. Further, transcriptomic and epigenomic profiling indicates that CHD8 regulates the chromatin accessibility landscape to activate neurogenesis-promoting factors including TBR2, a key regulator of IPC neurogenesis, while repressing DNA damage- and p53-induced apoptotic programs. In the adult brain, CHD8 depletion impairs forebrain neurogenesis by impeding IPC differentiation from NSCs in both subventricular and subgranular zones; however, unlike in embryos, it does not affect NSC proliferation and survival. Treatment with an antidepressant approved by the Federal Drug Administration (FDA), fluoxetine, partially restores adult hippocampal neurogenesis in Chd8-ablated mice. Together, our multistage functional studies identify temporally specific roles for CHD8 in developmental and adult neurogenesis, pointing to a potential strategy to enhance neurogenesis in the CHD8-deficient brain.SIGNIFICANCE STATEMENT The role of the high-confidence autism gene CHD8 in neurogenesis remains incompletely understood. Here, we identify a stage-specific function of CHD8 in development of NSCs in developing and adult brains by conserved, yet spatiotemporally distinct, mechanisms. In embryonic cortex, CHD8 is critical for the proliferation, survival, and differentiation of both NSC and IPCs during cortical neurogenesis. In adult brain, CHD8 is required for IPC generation but not the proliferation and survival of adult NSCs. Treatment with FDA-approved antidepressant fluoxetine partially rescues the adult neurogenesis defects in CHD8 mutants. Thus, our findings help resolve CHD8 functions throughout life during embryonic and adult neurogenesis and point to a potential avenue to promote neurogenesis in CHD8 deficiency.

Lack of placental neurosteroid alters cortical development and female somatosensory function

Placental endocrine function is essential to fetal brain development. Placental hormones include neurosteroids such as allopregnanolone (ALLO), a regulator of neurodevelopmental processes via positive allosteric modulation of the GABAA receptor (GABAA-R). Using a mouse model (plKO) in which the gene encoding the ALLO synthesis enzyme is specifically deleted in trophoblasts, we previously showed that placental ALLO insufficiency alters cerebellar white matter development and leads to male-specific autistic-like behavior. We now demonstrate that the lack of placental ALLO causes female-predominant alterations of cortical development and function. Placental ALLO insufficiency disrupts cell proliferation in the primary somatosensory cortex (S1) in a sex-linked manner. Early changes are seen in plKO embryos of both sexes, but persist primarily in female offspring after birth. Adolescent plKO females show significant reduction in pyramidal neuron density, as well as somatosensory behavioral deficits as compared with plKO males and control littermates. Assessment of layer-specific markers in human postmortem cortices suggests that preterm infants may also have female-biased abnormalities in cortical layer specification as compared with term infants. This study establishes a novel and fundamental link between placental function and sex-linked long-term neurological outcomes, emphasizing the importance of the growing field of neuroplacentology.

Young transposable elements rewired gene regulatory networks in human and chimpanzee hippocampal intermediate progenitors

The hippocampus is associated with essential brain functions, such as learning and memory. Human hippocampal volume is significantly greater than expected compared with that of non-human apes, suggesting a recent expansion. Intermediate progenitors, which are able to undergo multiple rounds of proliferative division before a final neurogenic division, may have played a role in evolutionary hippocampal expansion. To investigate the evolution of gene regulatory networks underpinning hippocampal neurogenesis in apes, we leveraged the differentiation of human and chimpanzee induced pluripotent stem cells into TBR2 (or EOMES)-positive hippocampal intermediate progenitor cells (hpIPCs). We found that the gene networks active in hpIPCs are significantly different between humans and chimpanzees, with ∼2500 genes being differentially expressed. We demonstrate that species-specific transposon-derived enhancers contribute to these transcriptomic differences. Young transposons, predominantly endogenous retroviruses and SINE-Vntr-Alus (SVAs), were co-opted as enhancers in a species-specific manner. Human-specific SVAs provided substrates for thousands of novel TBR2-binding sites, and CRISPR-mediated repression of these SVAs attenuated the expression of ∼25% of the genes that are upregulated in human intermediate progenitors relative to the same cell population in the chimpanzee.

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.

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.

Evolution of genetic mechanisms regulating cortical neurogenesis

The size of the cerebral cortex increases dramatically across amniotes, from reptiles to great apes. This is primarily due to different numbers of neurons and glial cells produced during embryonic development. The evolutionary expansion of cortical neurogenesis was linked to changes in neural stem and progenitor cells, which acquired increased capacity of self‐amplification and neuron production. Evolution works via changes in the genome, and recent studies have identified a small number of new genes that emerged in the recent human and primate lineages, promoting cortical progenitor proliferation and increased neurogenesis. However, most of the mammalian genome corresponds to noncoding DNA that contains gene‐regulatory elements, and recent evidence precisely points at changes in expression levels of conserved genes as key in the evolution of cortical neurogenesis. Here, we provide an overview of basic cellular mechanisms involved in cortical neurogenesis across amniotes, and discuss recent progress on genetic mechanisms that may have changed during evolution, including gene expression regulation, leading to the expansion of the cerebral cortex.

Thirty Years' History since the Discovery of Pax6: From Central Nervous System Development to Neurodevelopmental Disorders

Pax6 is a sequence-specific DNA binding transcription factor that positively and negatively regulates transcription and is expressed in multiple cell types in the developing and adult central nervous system (CNS). As indicated by the morphological and functional abnormalities in spontaneous Pax6 mutant rodents, Pax6 plays pivotal roles in various biological processes in the CNS. At the initial stage of CNS development, Pax6 is responsible for brain patterning along the anteroposterior and dorsoventral axes of the telencephalon. Regarding the anteroposterior axis, Pax6 is expressed inversely to Emx2 and Coup-TF1, and Pax6 mutant mice exhibit a rostral shift, resulting in an alteration of the size of certain cortical areas. Pax6 and its downstream genes play important roles in balancing the proliferation and differentiation of neural stem cells. The Pax6 gene was originally identified in mice and humans 30 years ago via genetic analyses of the eye phenotypes. The human PAX6 gene was discovered in patients who suffer from WAGR syndrome (i.e., Wilms tumor, aniridia, genital ridge defects, mental retardation). Mutations of the human PAX6 gene have also been reported to be associated with autism spectrum disorder (ASD) and intellectual disability. Rodents that lack the Pax6 gene exhibit diverse neural phenotypes, which might lead to a better understanding of human pathology and neurodevelopmental disorders. This review describes the expression and function of Pax6 during brain development, and their implications for neuropathology.

Long Non-Coding RNA Lacuna Regulates Neuronal Differentiation of Neural Stem Cells During Brain Development

Although long non-coding RNAs (lncRNAs) is one of the most abundant classes of RNAs encoded within the mammalian genome and are highly expressed in the adult brain, they remain poorly characterized and their roles in the brain development are not well understood. Here we identify the lncRNA Lacuna (also catalogued as NONMMUT071331.2 in NONCODE database) as a negative regulator of neuronal differentiation in the neural stem/progenitor cells (NSCs) during mouse brain development. In particular, we show that Lacuna is transcribed from a genomic locus near to the Tbr2/Eomes gene, a key player in the transition of intermediate progenitor cells towards the induction of neuronal differentiation. Lacuna RNA expression peaks at the developmental time window between E14.5 and E16.5, consistent with a role in neural differentiation. Overexpression experiments in ex vivo cultured NSCs from murine cortex suggest that Lacuna is sufficient to inhibit neuronal differentiation, induce the number of Nestin+ and Olig2+ cells, without affecting proliferation or apoptosis of NSCs. CRISPR/dCas9-KRAB mediated knockdown of Lacuna gene expression leads to the opposite phenotype by inducing neuronal differentiation and suppressing Nestin+ and Olig2+ cells, again without any effect on proliferation or apoptosis of NSCs. Interestingly, despite the negative action of Lacuna on neurogenesis, its knockdown inhibits Eomes transcription, implying a simultaneous, but opposite, role in facilitating the Eomes gene expression. Collectively, our observations indicate a critical function of Lacuna in the gene regulation networks that fine tune the neuronal differentiation in the mammalian NSCs.

An epigenetic circuit controls neurogenic programs during neocortex development

The production and expansion of intermediate progenitors (IPs) are essential for neocortical neurogenesis during development and over evolution. Here, we have characterized an epigenetic circuit that precisely controls neurogenic programs, particularly properties of IPs, during neocortical development. The circuit comprises a long non-coding RNA (LncBAR) and the BAF (SWI/SNF) chromatin-remodeling complex, which transcriptionally maintains the expression of Zbtb20. LncBAR knockout neocortex contains more deep-layer but fewer upper-layer projection neurons. Intriguingly, loss of LncBAR promotes IP production, but paradoxically prolongs the duration of the cell cycle of IPs during mid-later neocortical neurogenesis. Moreover, in LncBAR knockout mice, depletion of the neural progenitor pool at embryonic stage results in fewer adult neural progenitor cells in the subventricular zone of lateral ventricles, leading to a failure in adult neurogenesis to replenish the olfactory bulb. LncBAR binds to BRG1, the core enzymatic component of the BAF chromatin-remodeling complex. LncBAR depletion enhances association of BRG1 with the genomic locus of, and suppresses the expression of, Zbtb20, a transcription factor gene known to regulate both embryonic and adult neurogenesis. ZBTB20 overexpression in LncBAR-knockout neural precursors reverses compromised cell cycle progressions of IPs.

Low folate induces abnormal neuronal maturation and DNA hypomethylation of neuronal differentiation-related genes in cultured mouse neural stem and progenitor cells

Folate deficiency in a fetus is well known to cause neurodevelopment defects and development disorders. A low level of folate is also thought to be a risk for depression in adults. We have previously shown that post-weaning low folate induces neuronal immaturity in the dentate gyrus in mice, which suggests that low folate causes neuropsychological disorders via inhibition of neuronal maturation. In this study, we examined the effects of low folate on expression and epigenetic modification of genes involved in neuronal differentiation and maturation in primary mouse neural stem/progenitor cells (NSPCs) in vitro. An increase in Nestin (NSPC marker)-positive cells was observed in cells differentiated in a low folate medium for 3 days. An increase in βIII-tubulin (Tuj1: immature neuron marker)-positive cells and a decrease in microtubule-associated protein 2 (MAP2: mature neuron marker)-positive cells were observed in cells differentiated in a low folate medium for 7 days. In these cells, mRNA levels for genes involved in neuronal differentiation and maturation were altered. Hypomethylation of DNA, but not of histone proteins, was also observed at some promoters of these neuronal genes. The level of S-adenosylmethionine (SAM), a methyl donor, was decreased in these cells. The abnormalities in neural maturation and changes in gene expression in culture under low folate conditions were partially normalized by addition of SAM (5 μM). Based on these results, decreased SAM may induce DNA hypomethylation at genes involved in neuronal differentiation and maturation under low folate conditions, and this hypomethylation may be associated with low folate-induced neuronal immaturity.

STAU2 binds a complex RNA cargo that changes temporally with production of diverse intermediate progenitor cells during mouse corticogenesis

STAU2 is a double-stranded RNA-binding protein enriched in the nervous system. During asymmetric divisions in the developing mouse cortex, STAU2 preferentially distributes into the intermediate progenitor cell (IPC), delivering RNA molecules that can impact IPC behavior. Corticogenesis occurs on a precise time schedule, raising the hypothesis that the cargo STAU2 delivers into IPCs changes over time. To test this, we combine RNA-immunoprecipitation with sequencing (RIP-seq) over four stages of mouse cortical development, generating a comprehensive cargo profile for STAU2. A subset of the cargo was 'stable', present at all stages, and involved in chromosome organization, macromolecule localization, translation and DNA repair. Another subset was 'dynamic', changing with cortical stage, and involved in neurogenesis, cell projection organization, neurite outgrowth, and included cortical layer markers. Notably, the dynamic STAU2 cargo included determinants of IPC versus neuronal fates and genes contributing to abnormal corticogenesis. Knockdown of one STAU2 target, Taf13, previously linked to microcephaly and impaired myelination, reduced oligodendrogenesis in vitro. We conclude that STAU2 contributes to the timing of corticogenesis by binding and delivering complex and temporally regulated RNA cargo into IPCs.

Mercury Toxicity and Neurogenesis in the Mammalian Brain

The mammalian brain is formed from billions of cells that include a wide array of neuronal and glial subtypes. Neural progenitor cells give rise to the vast majority of these cells during embryonic, fetal, and early postnatal developmental periods. The process of embryonic neurogenesis includes proliferation, differentiation, migration, the programmed death of some newly formed cells, and the final integration of differentiated neurons into neural networks. Adult neurogenesis also occurs in the mammalian brain, but adult neurogenesis is beyond the scope of this review. Developing embryonic neurons are particularly susceptible to neurotoxicants and especially mercury toxicity. This review focused on observations concerning how mercury, and in particular, methylmercury, affects neurogenesis in the developing mammalian brain. We summarized information on models used to study developmental mercury toxicity, theories of pathogenesis, and treatments that could be used to reduce the toxic effects of mercury on developing neurons.

Cell-Type-Specific Gene Expression in Developing Mouse Neocortex: Intermediate Progenitors Implicated in Axon Development

Cerebral cortex projection neurons (PNs) are generated from intermediate progenitors (IPs), which are in turn derived from radial glial progenitors (RGPs). To investigate developmental processes in IPs, we profiled IP transcriptomes in embryonic mouse neocortex, using transgenic Tbr2-GFP mice, cell sorting, and microarrays. These data were used in combination with in situ hybridization to ascertain gene sets specific for IPs, RGPs, PNs, interneurons, and other neural and non-neural cell types. RGP-selective transcripts (n = 419) included molecules for Notch receptor signaling, proliferation, neural stem cell identity, apical junctions, necroptosis, hippo pathway, and NF-κB pathway. RGPs also expressed specific genes for critical interactions with meningeal and vascular cells. In contrast, IP-selective genes (n = 136) encoded molecules for activated Delta ligand presentation, epithelial-mesenchymal transition, core planar cell polarity (PCP), axon genesis, and intrinsic excitability. Interestingly, IPs expressed several “dependence receptors” (Unc5d, Dcc, Ntrk3, and Epha4) that induce apoptosis in the absence of ligand, suggesting a competitive mechanism for IPs and new PNs to detect key environmental cues or die. Overall, our results imply a novel role for IPs in the patterning of neuronal polarization, axon differentiation, and intrinsic excitability prior to mitosis. Significantly, IPs highly express Wnt-PCP, netrin, and semaphorin pathway molecules known to regulate axon polarization in other systems. In sum, IPs not only amplify neurogenesis quantitatively, but also molecularly “prime” new PNs for axogenesis, guidance, and excitability.

Human intermediate progenitor diversity during cortical development

Studies of the spatiotemporal, transcriptomic, and morphological diversity of radial glia (RG) have spurred our current models of human corticogenesis. In the developing cortex, neural intermediate progenitor cells (nIPCs) are a neuron-producing transit-amplifying cell type born in the germinal zones of the cortex from RG. The potential diversity of the nIPC population, that produces a significant portion of excitatory cortical neurons, is understudied, particularly in the developing human brain. Here we explore the spatiotemporal, transcriptomic, and morphological variation that exists within the human nIPC population and provide a resource for future studies. We observe that the spatial distribution of nIPCs in the cortex changes abruptly around gestational week (GW) 19/20, marking a distinct shift in cellular distribution and organization during late neurogenesis. We also identify five transcriptomic subtypes, one of which appears at this spatiotemporal transition. Finally, we observe a diversity of nIPC morphologies that do not correlate with specific transcriptomic subtypes. These results provide an analysis of the spatiotemporal, transcriptional, and morphological diversity of nIPCs in developing brain tissue and provide an atlas of nIPC subtypes in the developing human cortex that can benchmark in vitro models of human development such as cerebral organoids and help inform future studies of how nIPCs contribute to cortical neurogenesis.

Molecular Profiling Reveals Involvement of ESCO2 in Intermediate Progenitor Cell Maintenance in the Developing Mouse Cortex

Intermediate progenitor cells (IPCs) are neocortical neuronal precursors. Although IPCs play crucial roles in corticogenesis, their molecular features remain largely unknown. In this study, we aimed to characterize the molecular profile of IPCs. We isolated TBR2-positive (+) IPCs and TBR2-negative (-) cell populations in the developing mouse cortex. Comparative genome-wide gene expression analysis of TBR2⁺ IPCs versus TBR2^- cells revealed differences in key factors involved in chromatid segregation, cell-cycle regulation, transcriptional regulation, and cell signaling. Notably, mutation of many IPC genes in human has led to intellectual disability and caused a wide range of cortical malformations, including microcephaly and agenesis of corpus callosum. Loss-of-function experiments in cortex-specific mutants of Esco2, one of the novel IPC genes, demonstrate its critical role in IPC maintenance, and substantiate the identification of a central genetic determinant of IPC biogenesis. Our data provide novel molecular characteristics of IPCs in the developing mouse cortex.

Cortical Neural Stem Cell Lineage Progression Is Regulated by Extrinsic Signaling Molecule Sonic Hedgehog

Neural stem cells (NSCs) in the prenatal neocortex progressively generate different subtypes of glutamatergic projection neurons. Following that, NSCs have a major switch in their progenitor properties and produce γ-aminobutyric acid (GABAergic) interneurons for the olfactory bulb (OB), cortical oligodendrocytes, and astrocytes. Herein, we provide evidence for the molecular mechanism that underlies this switch in the state of neocortical NSCs. We show that, at around E16.5, mouse neocortical NSCs start to generate GSX2-expressing (GSX2⁺) intermediate progenitor cells (IPCs). In vivo lineage-tracing study revealed that GSX2⁺ IPC population gives rise not only to OB interneurons but also to cortical oligodendrocytes and astrocytes, suggesting that they are a tri-potential population. We demonstrated that Sonic hedgehog signaling is both necessary and sufficient for the generation of GSX2⁺ IPCs by reducing GLI3R protein levels. Using single-cell RNA sequencing, we identify the transcriptional profile of GSX2⁺ IPCs and the process of the lineage switch of cortical NSCs.

Zhang et al. reveal that cortical radial glia-derived GSX2⁺ cells at the late embryonic stage are tri-potential intermediate progenitors, which give rise to a subset of cortical oligodendrocytes, astrocytes, and olfactory bulb interneurons. SHH signaling is crucial for the generation of GSX2⁺ cells by reducing GLI3R protein level.

A Highly Conserved Circular RNA Is Required to Keep Neural Cells in a Progenitor State in the Mammalian Brain

circSLC45A4 is the main RNA splice isoform produced from its genetic locus and one of the highest expressed circRNAs in the developing human frontal cortex. Knockdown of this highly conserved circRNA in a human neuroblastoma cell line is sufficient to induce spontaneous neuronal differentiation, measurable by increased expression of neuronal marker genes. Depletion of circSlc45a4 in the developing mouse cortex causes a significant reduction of the basal progenitor pool and increases the expression of neurogenic regulators. Furthermore, knockdown of circSlc45a4a induces a significant depletion of cells in the cortical plate. In addition, deconvolution of the bulk RNA-seq data with the help of single-cell RNA-seq data validates the depletion of basal progenitors and reveals an increase in Cajal-Retzius cells. In summary, we present a detailed study of a highly conserved circular RNA that is necessary to maintain the pool of neural progenitors in vitro and in vivo.

Early Chronic Intermittent Maternal Hyperoxygenation Impairs Cortical Development by Inhibition of Pax6-Positive Apical Progenitor Cell Proliferation

Maternal hyperoxygenation is a feasible, noninvasive method to treat fetal diseases, such as heart hypoplasia, but effects of maternal hyperoxygenation on the developing brain remain poorly understood. Previous studies showed that short-term maternal hyperoxygenation during midneurogenic phase (E14-E16) but not in earlier development (E10-E12) increases oxygen tension and enhances neurogenesis in the developing mouse cortex. We investigated effects of early chronic maternal hyperoxygenation (CMH) as a potential clinical treatment. Pregnant C57BL/6J mice were housed in a chamber at 75% atmospheric oxygen and the brains of E16 fetuses were analyzed using immunohistochemistry. The mitosis marker phH3 showed a significant reduction of proliferation in the dorsolateral cortices of CMH-treated E16 fetuses. Numbers of Tbr2-positive intermediate progenitor cells were unaffected whereas numbers of Pax6-positive apical progenitor cells were significantly reduced in CMH-treated mice. This resulted in altered cortical plate development with fewer Satb2-positive upper layer neurons but more Tbr1-positive neurons corresponding to the deeper layer 6. Thus, maternal hyperoxygenation affects the developing cortex depending on timing and length of applied oxygen. Early CMH causes a severe reduction of neuroprogenitor proliferation likely affecting cortical development. Further studies are needed to investigate the mechanisms underlying these findings and to assess the clinical and neurodevelopmental outcomes of the pups.

Unipolar (Dendritic) Brush Cells Are Morphologically Complex and Require Tbr2 for Differentiation and Migration

Previous studies demonstrated specific expression of transcription factor Tbr2 in unipolar brush cells (UBCs) of the cerebellum during development and adulthood. To further study UBCs and the role of Tbr2 in their development we examined UBC morphology in transgenic mouse lines (reporter and lineage tracer) and also examined the effects of Tbr2 deficiency in Tbr2 (MGI: Eomes) conditional knock-out (cKO) mice. In Tbr2 reporter and lineage tracer cerebellum, UBCs exhibited more complex morphologies than previously reported including multiple dendrites, bifurcating dendrites, and up to four dendritic brushes. We propose that “dendritic brush cells” (DBCs) may be a more apt nomenclature. In Tbr2 cKO cerebellum, mature UBCs were completely absent. Migration of UBC precursors from rhombic lip to cerebellar cortex and other nuclei was impaired in Tbr2 cKO mice. Our results indicate that UBC migration and differentiation are sensitive to Tbr2 deficiency. To investigate whether UBCs develop similarly in humans as in rodents, we studied Tbr2 expression in mid-gestational human cerebellum. Remarkably, Tbr2⁺ UBC precursors migrate along the same pathways in humans as in rodent cerebellum and disperse to create the same “fountain-like” appearance characteristic of UBCs exiting the rhombic lip.

Developmental exposure to diesel exhaust upregulates transcription factor expression, decreases hippocampal neurogenesis, and alters cortical lamina organization: relevance to neurodevelopmental disorders

Background

Exposure to traffic-related air pollution (TRAP) during development and/or in adulthood has been associated in many human studies with both neurodevelopmental and neurodegenerative diseases, such as autism spectrum disorder (ASD) and Alzheimer’s disease (AD) or Parkinson’s disease (PD).

Methods

In the present study, C57BL/6 J mice were exposed to environmentally relevant levels (250+/−50 μg/m³) of diesel exhaust (DE) or filtered air (FA) during development (E0 to PND21). The expression of several transcription factors relevant for CNS development was assessed on PND3. To address possible mechanistic underpinnings of previously observed behavioral effects of DE exposure, adult neurogenesis in the hippocampus and laminar organization of neurons in the somatosensory cortex were analyzed on PND60. Results were analyzed separately for male and female mice.

Results

Developmental DE exposure caused a male-specific upregulation of Pax6, Tbr1, Tbr2, Sp1, and Creb1 on PND3. In contrast, in both males and females, Tbr2⁺ intermediate progenitor cells in the PND60 hippocampal dentate gyrus were decreased, as an indication of reduced adult neurogenesis. In the somatosensory region of the cerebral cortex, laminar distribution of Trb1, calbindin, and parvalbumin (but not of Ctip2 or Cux1) was altered by developmental DE exposure.

Conclusions

These results provide additional evidence to previous findings indicating the ability of developmental DE exposure to cause biochemical/molecular and behavioral alterations that may be involved in neurodevelopmental disorders such as ASD.

Transcriptional regulation of MGE progenitor proliferation by PRDM16 controls cortical GABAergic interneuron production

The mammalian cortex is populated by neurons derived from neural progenitors located throughout the embryonic telencephalon. Excitatory neurons are derived from the dorsal telencephalon, whereas inhibitory interneurons are generated in its ventral portion. The transcriptional regulator PRDM16 is expressed by radial glia, neural progenitors present in both regions; however, its mechanisms of action are still not fully understood. It is unclear whether PRDM16 plays a similar role in neurogenesis in both dorsal and ventral progenitor lineages and, if so, whether it regulates common or unique networks of genes. Here, we show that Prdm16 expression in mouse medial ganglionic eminence (MGE) progenitors is required for maintaining their proliferative capacity and for the production of proper numbers of forebrain GABAergic interneurons. PRDM16 binds to cis-regulatory elements and represses the expression of region-specific neuronal differentiation genes, thereby controlling the timing of neuronal maturation. PRDM16 regulates convergent developmental gene expression programs in the cortex and MGE, which utilize both common and region-specific sets of genes to control the proliferative capacity of neural progenitors, ensuring the generation of correct numbers of cortical neurons.

Neurogenesis From Embryo to Adult - Lessons From Flies and Mice

The human brain is composed of billions of cells, including neurons and glia, with an undetermined number of subtypes. During the embryonic and early postnatal stages, the vast majority of these cells are generated from neural progenitors and stem cells located in all regions of the neural tube. A smaller number of neurons will continue to be generated throughout our lives, in localized neurogenic zones, mainly confined at least in rodents to the subependymal zone of the lateral ventricles and the subgranular zone of the hippocampal dentate gyrus. During neurogenesis, a combination of extrinsic cues interacting with temporal and regional intrinsic programs are thought to be critical for increasing neuronal diversity, but their underlying mechanisms need further elucidation. In this review, we discuss the recent findings in Drosophila and mammals on the types of cell division and cell interactions used by neural progenitors and stem cells to sustain neurogenesis, and how they are influenced by glia.

Transcriptional Regulators and Human-Specific/Primate-Specific Genes in Neocortical Neurogenesis

During development, starting from a pool of pluripotent stem cells, tissue-specific genetic programs help to shape and develop functional organs. To understand the development of an organ and its disorders, it is important to understand the spatio-temporal dynamics of the gene expression profiles that occur during its development. Modifications in existing genes, the de-novo appearance of new genes, or, occasionally, even the loss of genes, can greatly affect the gene expression profile of any given tissue and contribute to the evolution of organs or of parts of organs. The neocortex is evolutionarily the most recent part of the brain, it is unique to mammals, and is the seat of our higher cognitive abilities. Progenitors that give rise to this tissue undergo sequential waves of differentiation to produce the complete sets of neurons and glial cells that make up a functional neocortex. We will review herein our understanding of the transcriptional regulators that control the neural precursor cells (NPCs) during the generation of the most abundant class of neocortical neurons, the glutametergic neurons. In addition, we will discuss the roles of recently-identified human- and primate-specific genes in promoting neurogenesis, leading to neocortical expansion.

Coexpression enrichment analysis at the single-cell level reveals convergent defects in neural progenitor cells and their cell-type transitions in neurodevelopmental disorders

A large number of genes have been implicated in neurodevelopmental disorders (NDDs), but their contributions to NDD pathology are difficult to decipher without understanding their diverse roles in different brain cell types. Here, we integrated NDD genetics with single-cell RNA sequencing data to assess coexpression enrichment patterns of various NDD gene sets. We identified midfetal cortical neural progenitor cell development—more specifically, the ventricular radial glia-to-intermediate progenitor cell transition at gestational week 10—as a key point of convergence in autism spectrum disorder (ASD) and epilepsy. Integrated Gene Ontology–based analysis further revealed that ASD genes activate neural differentiation and inhibit cell cycle during the transition, whereas epilepsy genes function as downstream effectors in the same processes, offering one possible explanation for the high comorbidity rate of the two disorders. This approach provides a framework for investigating the cell-type-specific pathophysiology of NDDs.

Acute Lengthening of Progenitor Mitosis Influences Progeny Fate during Cortical Development in vivo

BACKGROUND/AIMS

Prenatal microcephaly is posited to arise from aberrant mitosis of neural progenitors, which disrupts both neuronal production and survival. Although microcephaly has both a genetic and environmental etiology, the mechanisms by which dysregulation of mitosis causes microcephaly are poorly understood. We previously discovered that prolonged mitosis of mouse neural progenitors, either ex vivo or in vitro, directly alters progeny cell fate, -resulting in precocious differentiation and apoptosis. This raises questions as to whether prolonged progenitor mitosis affects cell fate and neurogenesis in vivo, and what are the underlying mechanisms?

METHODS/RESULTS

Towards addressing these knowledge gaps, we developed an in vivo model of mitotic delay. This uses pharmacological inhibition to acutely and reversibly prolong mitosis during cortical development, and fluorescent dyes to label direct progeny. Using this model, we discovered that a causal relationship between mitotic delay of neural progenitors and altered progeny cell fate is evident in vivo. Using transcriptome analyses to investigate the state of delayed cells and their progeny, we uncovered potential molecular mechanisms by which prolonged mitosis induces altered cell fates, including DNA damage and p53 signaling. We then extended our studies to human neural progenitors, demonstrating that lengthened mitosis duration also directly alters neuronal cell fate.

CONCLUSIONS

This study establishes a valuable new experimental paradigm towards understanding mechanisms whereby lengthened mitosis duration may explain some cases of microcephaly.