p57(KIP2) regulates radial glia and intermediate precursor cell cycle dynamics and lower layer neurogenesis in developing cerebral cortex

Protein restriction during pregnancy alters Cdkn1c silencing, dopamine circuitry and offspring behaviour without changing expression of key neuronal marker genes

We tracked the consequences of in utero protein restriction in mice throughout their development and life course using a luciferase-based allelic reporter of imprinted Cdkn1c. Exposure to gestational low-protein diet (LPD) results in the inappropriate expression of paternally inherited Cdkn1c in the brains of embryonic and juvenile mice. These animals were characterised by a developmental delay in motor skills, and by behavioural alterations indicative of reduced anxiety. Exposure to LPD in utero resulted in significantly more tyrosine hydroxylase positive (dopaminergic) neurons in the midbrain of adult offspring as compared to age-matched, control-diet equivalents. Positron emission tomography (PET) imaging revealed an increase in striatal dopamine synthesis capacity in LPD-exposed offspring, where elevated levels of dopamine correlated with an enhanced sensitivity to cocaine. These data highlight a profound sensitivity of the developing epigenome to gestational protein restriction. Our data also suggest that loss of Cdkn1c imprinting and p57KIP2 upregulation alters the cellular composition of the developing midbrain, compromises dopamine circuitry, and thereby provokes behavioural abnormalities in early postnatal life. Molecular analyses showed that despite this phenotype, exposure to LPD solely during pregnancy did not significantly change the expression of key neuronal- or dopamine-associated marker genes in adult offspring.

Dysregulation of mTOR signaling mediates common neurite and migration defects in both idiopathic and 16p11.2 deletion autism neural precursor cells

Autism spectrum disorder (ASD) is defined by common behavioral characteristics, raising the possibility of shared pathogenic mechanisms. Yet, vast clinical and etiological heterogeneity suggests personalized phenotypes. Surprisingly, our iPSC studies find that six individuals from two distinct ASD subtypes, idiopathic and 16p11.2 deletion, have common reductions in neural precursor cell (NPC) neurite outgrowth and migration even though whole genome sequencing demonstrates no genetic overlap between the datasets. To identify signaling differences that may contribute to these developmental defects, an unbiased phospho-(p)-proteome screen was performed. Surprisingly despite the genetic heterogeneity, hundreds of shared p-peptides were identified between autism subtypes including the mTOR pathway. mTOR signaling alterations were confirmed in all NPCs across both ASD subtypes, and mTOR modulation rescued ASD phenotypes and reproduced autism NPC-associated phenotypes in control NPCs. Thus, our studies demonstrate that genetically distinct ASD subtypes have common defects in neurite outgrowth and migration which are driven by the shared pathogenic mechanism of mTOR signaling dysregulation.

Transcription factors in microcephaly

Higher cognition in humans, compared to other primates, is often attributed to an increased brain size, especially forebrain cortical surface area. Brain size is determined through highly orchestrated developmental processes, including neural stem cell proliferation, differentiation, migration, lamination, arborization, and apoptosis. Disruption in these processes often results in either a small (microcephaly) or large (megalencephaly) brain. One of the key mechanisms controlling these developmental processes is the spatial and temporal transcriptional regulation of critical genes. In humans, microcephaly is defined as a condition with a significantly smaller head circumference compared to the average head size of a given age and sex group. A growing number of genes are identified as associated with microcephaly, and among them are those involved in transcriptional regulation. In this review, a subset of genes encoding transcription factors (e.g., homeobox-, basic helix-loop-helix-, forkhead box-, high mobility group box-, and zinc finger domain-containing transcription factors), whose functions are important for cortical development and implicated in microcephaly, are discussed.

Impaired neurogenesis and neural progenitor fate choice in a human stem cell model of SETBP1 disorder

BACKGROUND

Disruptions of SETBP1 (SET binding protein 1) on 18q12.3 by heterozygous gene deletion or loss-of-function variants cause SETBP1 disorder. Clinical features are frequently associated with moderate to severe intellectual disability, autistic traits and speech and motor delays. Despite the association of SETBP1 with neurodevelopmental disorders, little is known about its role in brain development.

METHODS

Using CRISPR/Cas9 genome editing technology, we generated a SETBP1 deletion model in human embryonic stem cells (hESCs) and examined the effects of SETBP1-deficiency in neural progenitors (NPCs) and neurons derived from these stem cells using a battery of cellular assays, genome-wide transcriptomic profiling and drug-based phenotypic rescue.

RESULTS

Neural induction occurred efficiently in all SETBP1 deletion models as indicated by uniform transition into neural rosettes. However, SETBP1-deficient NPCs exhibited an extended proliferative window and a decrease in neurogenesis coupled with a deficiency in their ability to acquire ventral forebrain fate. Genome-wide transcriptome profiling and protein biochemical analysis revealed enhanced activation of Wnt/β-catenin signaling in SETBP1 deleted cells. Crucially, treatment of the SETBP1-deficient NPCs with a small molecule Wnt inhibitor XAV939 restored hyper canonical β-catenin activity and restored both cortical and MGE neuronal differentiation.

LIMITATIONS

The current study is based on analysis of isogenic hESC lines with genome-edited SETBP1 deletion and further studies would benefit from the use of patient-derived iPSC lines that may harbor additional genetic risk that aggravate brain pathology of SETBP1 disorder.

CONCLUSIONS

We identified an important role for SETBP1 in controlling forebrain progenitor expansion and neurogenic differentiation. Our study establishes a novel regulatory link between SETBP1 and Wnt/β-catenin signaling during human cortical neurogenesis and provides mechanistic insights into structural abnormalities and potential therapeutic avenues for SETBP1 disorder.

IGF2 interacts with the imprinted gene Cdkn1c to promote terminal differentiation of neural stem cells

Adult neurogenesis is supported by multipotent neural stem cells (NSCs) with unique properties and growth requirements. Adult NSCs constitute a reversibly quiescent cell population that can be activated by extracellular signals from the microenvironment in which they reside in vivo. Although genomic imprinting plays a role in adult neurogenesis through dose regulation of some relevant signals, the roles of many imprinted genes in the process remain elusive. Insulin-like growth factor 2 (IGF2) is encoded by an imprinted gene that contributes to NSC maintenance in the adult subventricular zone through a biallelic expression in only the vascular compartment. We show here that IGF2 additionally promotes terminal differentiation of NSCs into astrocytes, neurons and oligodendrocytes by inducing the expression of the maternally expressed gene cyclin-dependent kinase inhibitor 1c (Cdkn1c), encoding the cell cycle inhibitor p57. Using intraventricular infusion of recombinant IGF2 in a conditional mutant strain with Cdkn1c-deficient NSCs, we confirm that p57 partially mediates the differentiation effects of IGF2 in NSCs and that this occurs independently of its role in cell-cycle progression, balancing the relationship between astrogliogenesis, neurogenesis and oligodendrogenesis.

ASCL1 phosphorylation and ID2 upregulation are roadblocks to glioblastoma stem cell differentiation

The growth of glioblastoma (GBM), one of the deadliest adult cancers, is fuelled by a subpopulation of stem/progenitor cells, which are thought to be the source of resistance and relapse after treatment. Re-engagement of a latent capacity of these cells to re-enter a trajectory resulting in cell differentiation is a potential new therapeutic approach for this devastating disease. ASCL1, a proneural transcription factor, plays a key role in normal brain development and is also expressed in a subset of GBM cells, but fails to engage a full differentiation programme in this context. Here, we investigated the barriers to ASCL1-driven differentiation in GBM stem cells. We see that ASCL1 is highly phosphorylated in GBM stem cells where its expression is compatible with cell proliferation. However, overexpression of a form of ASCL1 that cannot be phosphorylated on Serine–Proline sites drives GBM cells down a neuronal lineage and out of cell cycle more efficiently than its wild-type counterpart, an effect further enhanced by deletion of the inhibitor of differentiation ID2, indicating mechanisms to reverse the block to GBM cell differentiation.

Zfp57 inactivation illustrates the role of ICR methylation in imprinted gene expression during neural differentiation of mouse ESCs

ZFP57 is required to maintain the germline-marked differential methylation at imprinting control regions (ICRs) in mouse embryonic stem cells (ESCs). Although DNA methylation has a key role in genomic imprinting, several imprinted genes are controlled by different mechanisms, and a comprehensive study of the relationship between DMR methylation and imprinted gene expression is lacking. To address the latter issue, we differentiated wild-type and Zfp57^-/- hybrid mouse ESCs into neural precursor cells (NPCs) and evaluated allelic expression of imprinted genes. In mutant NPCs, we observed a reduction of allelic bias of all the 32 genes that were imprinted in wild-type cells, demonstrating that ZFP57-dependent methylation is required for maintaining or acquiring imprinted gene expression during differentiation. Analysis of expression levels showed that imprinted genes expressed from the non-methylated chromosome were generally up-regulated, and those expressed from the methylated chromosome were down-regulated in mutant cells. However, expression levels of several imprinted genes acquiring biallelic expression were not affected, suggesting the existence of compensatory mechanisms that control their RNA level. Since neural differentiation was partially impaired in Zfp57-mutant cells, this study also indicates that imprinted genes and/or non-imprinted ZFP57-target genes are required for proper neurogenesis in cultured ESCs.

Forebrain-specific deficiency of the GTPase CRAG/Centaurin-γ3 leads to immature dentate gyri and hyperactivity in mice

Mouse models of various neuropsychiatric disorders, such as schizophrenia, often display an immature dentate gyrus, characterized by increased numbers of immature neurons and neuronal progenitors and a dearth of mature neurons. We previously demonstrated that the CRMP5-associated GTPase (CRAG), a short splice variant of Centaurin-γ3/AGAP3, is highly expressed in the dentate gyrus. CRAG promotes cell survival and antioxidant defense by inducing the activation of serum response factors at promyelocytic leukemia protein bodies, which are nuclear stress-responsive domains, during neuronal development. However, the physiological role of CRAG in neuronal development remains unknown. Here, we analyzed the role of CRAG using dorsal forebrain-specific CRAG/Centaurin-γ3 knockout mice. The mice revealed maturational abnormality of the hippocampal granule cells, including increased doublecortin-positive immature neurons and decreased calbindin-positive mature neurons, a typical phenotype of immature dentate gyri. Furthermore, the mice displayed hyperactivity in the open-field test, a common measure of exploratory behavior, suggesting that these mice may serve as a novel model for neuropsychiatric disorder associated with hyperactivity. Thus, we conclude that CRAG is required for the maturation of neurons in the dentate gyrus, raising the possibility that its deficiency might promote the development of psychiatric disorders in humans.

PRDM16 regulates a temporal transcriptional program to promote progression of cortical neural progenitors

Radial glia (RG) in the neocortex sequentially generate distinct subtypes of projection neurons, accounting for the diversity and complex assembly of cortical neural circuits. Mechanisms that drive the rapid and precise temporal progression of RG are beginning to be elucidated. Here, we reveal that the RG-specific transcriptional regulator PRDM16 promotes the transition of early to late phase of neurogenesis in the mouse neocortex. Loss of Prdm16 delays the timely progression of RG, leading to defective cortical laminar organization. Our genomic analyses demonstrate that PRDM16 regulates a subset of genes that are dynamically expressed between early and late neurogenesis. We show that PRDM16 suppresses target gene expression through limiting chromatin accessibility of permissive enhancers. We further confirm that crucial target genes regulated by PRDM16 are neuronal specification genes, cell cycle regulators and molecules required for neuronal migration. These findings provide evidence to support the finding that neural progenitors temporally shift the gene expression program to achieve neural cell diversity.

Phenotypic Characterization and Brain Structure Analysis of Calcium Channel Subunit α2δ-2 Mutant (Ducky) and α2δ Double Knockout Mice

Auxiliary α2δ subunits of voltage-gated calcium channels modulate channel trafficking, current properties, and synapse formation. Three of the four isoforms (α2δ-1, α2δ-2, and α2δ-3) are abundantly expressed in the brain; however, of the available knockout models, only α2δ-2 knockout or mutant mice display an obvious abnormal neurological phenotype. Thus, we hypothesize that the neuronal α2δ isoforms may have partially specific as well as redundant functions. To address this, we generated three distinct α2δ double knockout mouse models by crossbreeding single knockout (α2δ-1 and -3) or mutant (α2δ-2/ducky) mice. Here, we provide a first phenotypic description and brain structure analysis. We found that genotypic distribution of neonatal litters in distinct α2δ-1/-2, α2δ-1/-3, and α2δ-2/-3 breeding combinations did not conform to Mendel's law, suggesting premature lethality of single and double knockout mice. Notably, high occurrences of infant mortality correlated with the absence of specific α2δ isoforms (α2Δ-2 > α2δ-1 > α2δ-3), and was particularly observed in cages with behaviorally abnormal parenting animals of α2δ-2/-3 cross-breedings. Juvenile α2δ-1/-2 and α2δ-2/-3 double knockout mice displayed a waddling gate similar to ducky mice. However, in contrast to ducky and α2δ-1/-3 double knockout animals, α2δ-1/-2 and α2δ-2/-3 double knockout mice showed a more severe disease progression and highly impaired development. The observed phenotypes within the individual mouse lines may be linked to differences in the volume of specific brain regions. Reduced cortical volume in ducky mice, for example, was associated with a progressively decreased space between neurons, suggesting a reduction of total synaptic connections. Taken together, our findings show that α2δ subunits differentially regulate premature survival, postnatal growth, brain development, and behavior, suggesting specific neuronal functions in health and disease.

Hes1 overexpression leads to expansion of embryonic neural stem cell pool and stem cell reservoir in the postnatal brain

Neural stem cells (NSCs) gradually alter their characteristics during mammalian neocortical development, resulting in the production of various neurons and glial cells, and remain in the postnatal brain as a source of adult neurogenesis. Notch-Hes signaling is a key regulator of stem cell properties in the developing and postnatal brain, and Hes1 is a major effector that strongly inhibits neuronal differentiation and maintains NSCs. To manipulate Hes1 expression levels in NSCs, we generated transgenic (Tg) mice using the Tet-On system. In Hes1-overexpressing Tg mice, NSCs were maintained in both embryonic and postnatal brains, and generation of later-born neurons was prolonged until later stages in the Tg neocortex. Hes1 overexpression inhibited the production of Tbr2⁺ intermediate progenitor cells but instead promoted the generation of basal radial glia-like cells in the subventricular zone (SVZ) at late embryonic stages. Furthermore, Hes1-overexpressing Tg mice exhibited the expansion of NSCs and enhanced neurogenesis in the SVZ of adult brain. These results indicate that Hes1 overexpression expanded the embryonic NSC pool and led to the expansion of the NSC reservoir in the postnatal and adult brain.

Functional Versatility of the CDK Inhibitor p57Kip2

The cyclin/CDK inhibitor p57^Kip2 belongs to the Cip/Kip family, with p21^Cip1 and p27^Kip1, and is the least studied member of the family. Unlike the other family members, p57^Kip2 has a unique role during embryogenesis and is the only CDK inhibitor required for embryonic development. p57^Kip2 is encoded by the imprinted gene CDKN1C, which is the gene most frequently silenced or mutated in the genetic disorder Beckwith-Wiedemann syndrome (BWS), characterized by multiple developmental anomalies. Although initially identified as a cell cycle inhibitor based on its homology to other Cip/Kip family proteins, multiple novel functions have been ascribed to p57^Kip2 in recent years that participate in the control of various cellular processes, including apoptosis, migration and transcription. Here, we will review our current knowledge on p57^Kip2 structure, regulation, and its diverse functions during development and homeostasis, as well as its potential implication in the development of various pathologies, including cancer.

Visualizing Cell Cycle Phase Organization and Control During Neural Lineage Elaboration

In neural precursors, cell cycle regulators simultaneously control both progression through the cell cycle and the probability of a cell fate switch. Precursors act in lineages, where they transition through a series of cell types, each of which has a unique molecular identity and cellular behavior. Thus, investigating links between cell cycle and cell fate control requires simultaneous identification of precursor type and cell cycle phase, as well as an ability to read out additional regulatory factor expression or activity. We use a combined FUCCI-EdU labelling protocol to do this, and then applied it to the embryonic olfactory neural lineage, in which the spatial position of a cell correlates with its precursor identity. Using this integrated model, we find the CDKi p27KIP1 has different regulation relative to cell cycle phase in neural stem cells versus intermediate precursors. In addition, Hes1, which is the principle transcriptional driver of neural stem cell self-renewal, surprisingly does not regulate p27KIP1 in this cell type. Rather, Hes1 indirectly represses p27KIP1 levels in the intermediate precursor cells downstream in the lineage. Overall, the experimental model described here enables investigation of cell cycle and cell fate control linkage from a single precursor through to a lineage systems level.

The Role of CDKs and CDKIs in Murine Development

Cyclin-dependent kinases (CDKs) and their inhibitors (CDKIs) play pivotal roles in the regulation of the cell cycle. As a result of these functions, it may be extrapolated that they are essential for appropriate embryonic development. The twenty known mouse CDKs and eight CDKIs have been studied to varying degrees in the developing mouse, but only a handful of CDKs and a single CDKI have been shown to be absolutely required for murine embryonic development. What has become apparent, as more studies have shone light on these family members, is that in addition to their primary functional role in regulating the cell cycle, many of these genes are also controlling specific cell fates by directing differentiation in various tissues. Here we review the extensive mouse models that have been generated to study the functions of CDKs and CDKIs, and discuss their varying roles in murine embryonic development, with a particular focus on the brain, pancreas and fertility.

FGF2 modulates simultaneously the mode, the rate of division and the growth fraction in cultures of radial glia

Radial glial progenitors in the mammalian developing neocortex have been shown to follow a deterministic differentiation program restricted to an asymmetric-only mode of division. This feature seems incompatible with their well-known ability to increase in number when cultured in vitro, driven by fibroblast growth factor 2 and other mitogenic signals. The changes in their differentiation dynamics that allow this transition from in vivo asymmetric-only division mode to an in vitro self-renewing culture have not been fully characterized. Here, we combine experiments of radial glia cultures with numerical models and a branching process theoretical formalism to show that fibroblast growth factor 2 has a triple effect by simultaneously increasing the growth fraction, promoting symmetric divisions and shortening the length of the cell cycle. These combined effects partner to establish and sustain a pool of rapidly proliferating radial glial progenitors in vitro. We also show that, in conditions of variable proliferation dynamics, the branching process tool outperforms other commonly used methods based on thymidine analogs, such as BrdU and EdU, in terms of accuracy and reliability.

Highlighted Article: When mode and/or rate of division are changing, a branching process, rather than a thymidine analog method, provides temporal resolution, it is more robust and does not interfere with cell homeostasis.

Heterogeneous nuclear ribonucleoprotein A3 controls mitotic progression of neural progenitors via interaction with cohesin

Cortex development is controlled by temporal patterning of neural progenitor (NP) competence with sequential generation of deep and superficial layer neurons, but underlying mechanisms remain elusive. Here, we report a role for heterogeneous nuclear ribonucleoprotein A3 (HNRNPA3) in regulating the division of early cortical NPs that mainly give rise to deep-layer neurons via direct neurogenesis. HNRNPA3 is expressed at high levels in NPs of mouse and human cortex at early stages, with a unique peri-chromosome pattern. Intriguingly, downregulation of HNRNPA3 caused chromosome disarrangement, which hindered normal separation of chromosomes during NP division, leading to mitotic delay. Furthermore, HNRNPA3 is associated with the cohesin-core subunit SMC1A and controls its association with chromosomes, implicating a mechanism for the role of HNRNPA3 in regulating chromosome segregation in dividing NPs. Hnrnpa3-deficient mice exhibited reduced cortical thickness, especially of deep layers. Moreover, downregulation of HNRNPA3 in cultured human cerebral organoids led to marked reduction in NPs and deep-layer neurons. Thus, this study has identified a crucial role for HNRNPA3 in NP division and highlighted the relationship between mitosis progression and early neurogenesis.

p57Kip2 is a master regulator of human adipose derived stem cell quiescence and senescence

Although human adipose derived stem cells (hADSCs) hold great promises for regenerative medicine, their key biological properties remain poorly understood. In particular, proliferation defects resulted from deep quiescence (dormancy) and senescence represent a major hurdle in hADSC production and clinical application. We have developed a model system for mechanistic dissection of hADSC quiescence and senescence. p57^Kip2, a major CDK inhibitor, was highly expressed in quiescent and senescent hADSCs but its level quickly declined upon stem cell activation. p57^Kip2 overexpression induced quiescence in spite of proliferative signals and its knockdown promoted cell cycle reentry even with induction of quiescence presumably through modulating the CDK2-CyclinE1 complex. Given its key role in quiescence and senescence, p57^Kip2 may be exploited for innovative strategies to amplify hADSCs of high quality for clinics.

Role of the imprinted allele of the Cdkn1c gene in mouse neocortical development

Imprinted genes are expressed from only one allele in a parent of origin-specific manner. The cyclin-dependent kinase inhibitor p57kip2 is encoded by an imprinted gene Cdkn1c, with the paternal allele being silenced. The possible expression and function of the paternal allele of Cdkn1c have remained little studied, however. We now show that the paternal allele of the Cdkn1c gene is expressed at a low level in the developing mouse neocortex. Surprisingly, the central nervous system-specific conditional deletion of the paternal allele (pat cKO) at the Cdkn1c locus resulted in a marked reduction in brain size. Furthermore, pat cKO gradually reduced the number of neural stem-progenitor cells (NPCs) during neocortical development, and thus reduced the number of upper-layer neurons, which were derived from late-stage NPCs. Our results thus show that the paternal allele of the Cdkn1c locus plays a key role in maintenance of NPCs during neocortical development.

Imprinted Cdkn1c genomic locus cell-autonomously promotes cell survival in cerebral cortex development

The cyclin-dependent kinase inhibitor p57^KIP2 is encoded by the imprinted Cdkn1c locus, exhibits maternal expression, and is essential for cerebral cortex development. How Cdkn1c regulates corticogenesis is however not clear. To this end we employ Mosaic Analysis with Double Markers (MADM) technology to genetically dissect Cdkn1c gene function in corticogenesis at single cell resolution. We find that the previously described growth-inhibitory Cdkn1c function is a non-cell-autonomous one, acting on the whole organism. In contrast we reveal a growth-promoting cell-autonomous Cdkn1c function which at the mechanistic level mediates radial glial progenitor cell and nascent projection neuron survival. Strikingly, the growth-promoting function of Cdkn1c is highly dosage sensitive but not subject to genomic imprinting. Collectively, our results suggest that the Cdkn1c locus regulates cortical development through distinct cell-autonomous and non-cell-autonomous mechanisms. More generally, our study highlights the importance to probe the relative contributions of cell intrinsic gene function and tissue-wide mechanisms to the overall phenotype.

How the imprinted Cdkn1c locus regulates corticogenesis is unclear. Here, the authors dissect the level of cell-autonomy of imprinted Cdkn1c gene function in mouse corticogenesis and identify this as regulating radial glial progenitor cell and projection neuron survival.

Chromatin Remodeling Complex NuRD in Neurodevelopment and Neurodevelopmental Disorders

The nucleosome remodeling and deacetylase (NuRD) complex presents one of the major chromatin remodeling complexes in mammalian cells. Here, we discuss current evidence for NuRD's role as an important epigenetic regulator of gene expression in neural stem cell (NSC) and neural progenitor cell (NPC) fate decisions in brain development. With the formation of the cerebellar and cerebral cortex, NuRD facilitates experience-dependent cerebellar plasticity and regulates additionally cerebral subtype specification and connectivity in postmitotic neurons. Consistent with these properties, genetic variation in NuRD's subunits emerges as important risk factor in common polygenic forms of neurodevelopmental disorders (NDDs) and neurodevelopment-related psychiatric disorders such as schizophrenia (SCZ) and bipolar disorder (BD). Overall, these findings highlight the critical role of NuRD in chromatin regulation in brain development and in mental health and disease.

Divide or Commit - Revisiting the Role of Cell Cycle Regulators in Adult Hippocampal Neurogenesis

The adult dentate gyrus continuously generates new neurons that endow the brain with increased plasticity, helping to cope with changing environmental and cognitive demands. The process leading to the birth of new neurons spans several precursor stages and is the result of a coordinated series of fate decisions, which are tightly controlled by extrinsic signals. Many of these signals act through modulation of cell cycle (CC) components, not only to drive proliferation, but also for linage commitment and differentiation. In this review, we provide a comprehensive overview on key CC components and regulators, with emphasis on G₁ phase, and analyze their specific functions in precursor cells of the adult hippocampus. We explore their role for balancing quiescence versus self-renewal, which is essential to maintain a lifelong pool of neural stem cells while producing new neurons “on demand.” Finally, we discuss available evidence and controversies on the impact of CC/G₁ length on proliferation versus differentiation decisions.

Epigenetic cues modulating the generation of cell-type diversity in the cerebral cortex

The cerebral cortex is composed of a large variety of distinct cell-types including projection neurons, interneurons, and glial cells which emerge from distinct neural stem cell lineages. The vast majority of cortical projection neurons and certain classes of glial cells are generated by radial glial progenitor cells in a highly orchestrated manner. Recent studies employing single cell analysis and clonal lineage tracing suggest that neural stem cell and radial glial progenitor lineage progression are regulated in a profound deterministic manner. In this review we focus on recent advances based mainly on correlative phenotypic data emerging from functional genetic studies in mice. We establish hypotheses to test in future research and outline a conceptual framework how epigenetic cues modulate the generation of cell-type diversity during cortical development.

Histone demethylase KDM2B promotes triple negative breast cancer proliferation by suppressing p15INK4B, p16INK4A, and p57KIP2 transcription

H3K4me3 and H3K36me2 histone demethylase KDM2B is an epigenetic regulatory factor involved in cell proliferation in numerous cells including breast cancer cells, however, the regulatory mechanism of KDM2B in cell proliferation of breast cancer cells, specifically in triple negative breast cancer (TNBC), remains largely unknown. In this study, we showed that higher expression level of KDM2B was associated with poor prognosis in TNBC. Using cell proliferation assay, we found that KDM2B promoted TNBC cell proliferation by suppressing the transcription of the cell cycle inhibitors p15INK4B, p16INK4A, and p57KIP2. Chromatin immunoprecipitation assay results showed that KDM2B bound to the promoters of these genes and thereby reduced the H3K4me3 and H3K36me2 levels, leading to the suppression of gene transcription in a histone demethylation activity-dependent manner. Silencing of p15INK4B, p16INK4A, and p57KIP2 in TNBC cells was shown to restore the promoting effect of KDM2B on TNBC cell proliferation. The present study reveals a novel cell regulatory mechanism through which KDM2B promotes TNBC cell proliferation by binding to the promoters of p15INK4B, p16INK4A, and p57KIP2, which reduces H3K4me3 and H3K36me2 levels to suppress gene transcription.

Rapid Detection of Neurodevelopmental Phenotypes in Human Neural Precursor Cells (NPCs)

Human brain development proceeds through a series of precisely orchestrated processes, with earlier stages distinguished by proliferation, migration, and neurite outgrowth; and later stages characterized by axon/dendrite outgrowth and synapse formation. In neurodevelopmental disorders, often one or more of these processes are disrupted, leading to abnormalities in brain formation and function. With the advent of human induced pluripotent stem cell (hiPSC) technology, researchers now have an abundant supply of human cells that can be differentiated into virtually any cell type, including neurons. These cells can be used to study both normal brain development and disease pathogenesis. A number of protocols using hiPSCs to model neuropsychiatric disease use terminally differentiated neurons or use 3D culture systems termed organoids. While these methods have proven invaluable in studying human disease pathogenesis, there are some drawbacks. Differentiation of hiPSCs into neurons and generation of organoids are lengthy and costly processes that can impact the number of experiments and variables that can be assessed. In addition, while post-mitotic neurons and organoids allow the study of disease-related processes, including dendrite outgrowth and synaptogenesis, they preclude the study of earlier processes like proliferation and migration. In neurodevelopmental disorders, such as autism, abundant genetic and post-mortem evidence indicates defects in early developmental processes. Neural precursor cells (NPCs), a highly proliferative cell population, may be a suitable model in which to ask questions about ontogenetic processes and disease initiation. We now extend methodologies learned from studying development in mouse and rat cortical cultures to human NPCs. The use of NPCs allows us to investigate disease-related phenotypes and define how different variables (e.g., growth factors, drugs) impact developmental processes including proliferation, migration, and differentiation in only a few days. Ultimately, this toolset can be used in a reproducible and high-throughput manner to identify disease-specific mechanisms and phenotypes in neurodevelopmental disorders.

Neuronal Migration and Lamination in the Vertebrate Retina

In the retina, like in most other brain regions, developing neurons are arranged into distinct layers giving the mature tissue its stratified appearance. This process needs to be highly controlled and orchestrated, as neuronal layering defects lead to impaired retinal function. To achieve successful neuronal layering and lamination in the retina and beyond, three main developmental steps need to be executed: First, the correct type of neuron has to be generated at a precise developmental time. Second, as most retinal neurons are born away from the position at which they later function, newborn neurons have to move to their final layer within the developing tissue, a process also termed neuronal lamination. Third, these neurons need to connect to their correct synaptic partners. Here, we discuss neuronal migration and lamination in the vertebrate retina and summarize our knowledge on these aspects of retinal development. We give an overview of how lamination emerges and discuss the different modes of neuronal translocation that occur during retinogenesis and what we know about the cell biological machineries driving them. In addition, retinal mosaics and their importance for correct retinal function are examined. We close by stating the open questions and future directions in this exciting field.

Primary Cilia in Brain Development and Diseases

The primary cilium, a sensory appendage that is present in most mammalian cells, plays critical roles in signaling pathways and cell cycle progression. Mutations that affect the structure or function of primary cilia result in ciliopathies, a group of developmental and degenerative diseases that affect almost all organs and tissues. Our understanding of the constituents, development, and function of primary cilia has advanced considerably in recent years, revealing pathogenic mechanisms that potentially underlie ciliopathies. In the brain, the primary cilia are crucial for early patterning, neurogenesis, neuronal maturation and survival, and tumorigenesis, mostly through regulating cell cycle progression, Hedgehog signaling, and WNT signaling. We review these advances in our knowledge of primary cilia, focusing on brain development, and discuss the mechanisms that may underlie brain abnormalities in ciliopathies.

Mouse Parthenogenetic Embryonic Stem Cells with Biparental-Like Expression of Imprinted Genes Generate Cortical-Like Neurons That Integrate into the Injured Adult Cerebral Cortex

One strategy for stem cell-based therapy of the cerebral cortex involves the generation and transplantation of functional, histocompatible cortical-like neurons from embryonic stem cells (ESCs). Diploid parthenogenetic Pg-ESCs have recently emerged as a promising source of histocompatible ESC derivatives for organ regeneration but their utility for cerebral cortex therapy is unknown. A major concern with Pg-ESCs is genomic imprinting. In contrast with biparental Bp-ESCs derived from fertilized oocytes, Pg-ESCs harbor two maternal genomes but no sperm-derived genome. Pg-ESCs are therefore expected to have aberrant expression levels of maternally expressed (MEGs) and paternally expressed (PEGs) imprinted genes. Given the roles of imprinted genes in brain development, tissue homeostasis and cancer, their deregulation in Pg-ESCs might be incompatible with therapy. Here, we report that, unexpectedly, only one gene out of 7 MEGs and 12 PEGs was differentially expressed between Pg-ESCs and Bp-ESCs while 13 were differentially expressed between androgenetic Ag-ESCs and Bp-ESCs, indicating that Pg-ESCs but not Ag-ESCs, have a Bp-like imprinting compatible with therapy. In vitro, Pg-ESCs generated cortical-like progenitors and electrophysiologically active glutamatergic neurons that maintained the Bp-like expression levels for most imprinted genes. In vivo, Pg-ESCs participated to the cortical lineage in fetal chimeras. Finally, transplanted Pg-ESC derivatives integrated into the injured adult cortex and sent axonal projections in the host brain. In conclusion, mouse Pg-ESCs generate functional cortical-like neurons with Bp-like imprinting and their derivatives properly integrate into both the embryonic cortex and the injured adult cortex. Collectively, our data support the utility of Pg-ESCs for cortical therapy. Stem Cells 2018;36:192-205.

The E2A splice variant E47 regulates the differentiation of projection neurons via p57(KIP2) during cortical development

During corticogenesis, distinct classes of neurons are born from progenitor cells located in the ventricular and subventricular zones, from where they migrate towards the pial surface to assemble into highly organized layer-specific circuits. However, the precise and coordinated transcriptional network activity defining neuronal identity is still not understood. Here, we show that genetic depletion of the basic helix-loop-helix (bHLH) transcription factor E2A splice variant E47 increased the number of Tbr1-positive deep layer and Satb2-positive upper layer neurons at E14.5, while depletion of the alternatively spliced E12 variant did not affect layer-specific neurogenesis. While ChIP-Seq identified a big overlap for E12- and E47-specific binding sites in embryonic NSCs, including sites at the cyclin-dependent kinase inhibitor (CDKI) Cdkn1c gene locus, RNA-Seq revealed a unique transcriptional regulation by each splice variant. E47 activated the expression of the CDKI Cdkn1c through binding to a distal enhancer. Finally, overexpression of E47 in embryonic NSCs in vitro impaired neurite outgrowth, and overexpression of E47 in vivo by in utero electroporation disturbed proper layer-specific neurogenesis and upregulated p57(KIP2) expression. Overall, this study identifies E2A target genes in embryonic NSCs and demonstrates that E47 regulates neuronal differentiation via p57(KIP2).

Behavioural abnormalities in a novel mouse model for Silver Russell Syndrome

Silver Russell Syndrome (SRS) syndrome is an imprinting disorder involving low birth weight with complex genetics and diagnostics. Some rare SRS patients carry maternally inherited microduplications spanning the imprinted genes CDKN1C, PHLDA2, SLC22A18 and KCNQ1, suggesting that overexpression of one of more of these genes contributes to the SRS phenotype. While this molecular alteration is very rare, feeding difficulties are a very common feature of this condition. Given that SRS children also have very low body mass index, understanding the underpinning biology of the eating disorder is important, as well as potential co-occurring behavioural alterations. Here, we report that a mouse model of this microduplication exhibits a number of behavioural deficits. The mice had a blunted perception of the palatability of a given foodstuff. This perception may underpin the fussiness with food. We additionally report hypoactivity, unrelated to anxiety or motoric function, and a deficit in the appropriate integration of incoming sensory information. Importantly, using a second genetic model, we were able to attribute all altered behaviours to elevated expression of a single gene, Cdkn1c. This is the first report linking elevated Cdkn1c to altered behaviour in mice. Importantly, the findings from our study may have relevance for SRS and highlight a potentially underreported aspect of this disorder.

CyclinD1 and p57kip2 as biomarkers in differentiation, metastasis and prognosis of gastric cardia adenocarcinoma

Objective

This study aims to investigate the expression and significance of p57^kip2 and cyclinD1 in gastric cardia adenocarcinoma (GCA). p57^kip2 is a negative regulator in the cell cycle. On the contrary, cyclinD1 is a positive regulator of cell cycle progression.

Methods

Thirty-two cases of GCA tissues and adjacent non-cancerous tissues were collected for this study. Immunohistochemistry and fluorescence qualitative PCR was used to determine the level of p57^kip2 and cyclinD1 in GCA and its adjacent non-cancerous tissues. Furthermore, the correlation between the mRNA/protein and GCA clinical pathologic parameters were analyzed, and the relationship of p57^kip2 and cyclinD1 in GCA were also evaluated.

Results

The expression of p57^kip2 significantly lower in GCA (P = 0.036), and there was a significant correlation in the different degrees of differentiation (P < 0.05). Furthermore, median survival time was 41 months for patients with high mRNA expression of p57^kip2. This was longer compared to patients with low mRNA expression of P57^kip2 (37 months, X² = 4.788, P = 0.029).The expression of cyclinD1 was significantly higher in GCA(P = 0.002), and was significant correlated to clinical stage(P<0.05). Median survival time was 34 months in patients with high mRNA expression of cyclinD1, which was shorter than in patients with low expression of cyclinD1 mRNA (41 months, X² = 4.071, P = 0.044). The protein expression of p57^kip2 was not correlated to the protein expression of cyclinD1 (P = 0.55).

Conclusion

The expression of p57^kip2 and cyclinD1 are likely to suppress or promote the tumorigenesis and progression of GCA.

Chd8 Mutation Leads to Autistic-like Behaviors and Impaired Striatal Circuits

Autism spectrum disorder (ASD) is a heterogeneous disease, but genetically defined models can provide an entry point to studying the molecular underpinnings of this disorder. We generated germline mutant mice with loss-of-function mutations in Chd8, a de novo mutation strongly associated with ASD, and demonstrate that these mice display hallmark ASD behaviors, macrocephaly, and craniofacial abnormalities similar to patient phenotypes. Chd8^+/- mice display a broad, brain-region-specific dysregulation of major regulatory and cellular processes, most notably histone and chromatin modification, mRNA and protein processing, Wnt signaling, and cell-cycle regulation. We also find altered synaptic physiology in medium spiny neurons of the nucleus accumbens. Perturbation of Chd8 in adult mice recapitulates improved acquired motor learning behavior found in Chd8^+/- animals, suggesting a role for CHD8 in adult striatal circuits. These results support a mechanism linking chromatin modification to striatal dysfunction and the molecular pathology of ASD.

In Vitro Corticogenesis from Embryonic Stem Cells Recapitulates the In Vivo Epigenetic Control of Imprinted Gene Expression

In vitro corticogenesis from embryonic stem cells (ESCs) is an attractive model of cortical development and a promising tool for cortical therapy. It is unknown to which extent epigenetic mechanisms crucial for cortex development and function, such as parental genomic imprinting, are recapitulated by in vitro corticogenesis. Here, using genome-wide transcriptomic and methylation analyses on hybrid mouse tissues and cells, we find a high concordance of imprinting status between in vivo and ESC-derived cortices. Notably, in vitro corticogenesis strictly reproduced the in vivo parent-of-origin-dependent expression of 41 imprinted genes (IGs), including Mest and Cdkn1c known to control corticogenesis. Parent-of-origin-dependent DNA methylation was also conserved at 14 of 18 imprinted differentially methylated regions. The least concordant imprinted locus was Gpr1-Zdbf2, where the aberrant bi-allelic expression of Zdbf2 and Adam23 was concomitant with a gain of methylation on the maternal allele in vitro. Combined, our data argue for a broad conservation of the epigenetic mechanisms at imprinted loci in cortical cells derived from ESCs. We propose that in vitro corticogenesis helps to define the still poorly understood mechanisms that regulate imprinting in the brain and the roles of IGs in cortical development.

Valproic acid stimulates proliferation of glial precursors during cortical gliogenesis in developing rat

Valproic acid (VPA) is a neurotherapeutic drug prescribed for seizures, bipolar disorder, and migraine, including women of reproductive age. VPA is a well-known teratogen that produces congenital malformations in many organs including the nervous system, as well as later neurodevelopmental disorders, including mental retardation and autism. In developing brain, few studies have examined VPA effects on glial cells, particularly astrocytes. To investigate effects on primary glial precursors, we developed new cell culture and in vivo models using frontal cerebral cortex of postnatal day (P2) rat. In vitro, VPA exposure elicited dose-dependent, biphasic effects on DNA synthesis and proliferation. In vivo VPA (300 mg/kg) exposure from P2 to P4 increased both DNA synthesis and cell proliferation, affecting primarily astrocyte precursors, as >75% of mitotic cells expressed brain lipid-binding protein. Significantly, the consequence of early VPA exposure was increased astrocytes, as both S100-β+ cells and glial fibrillary acidic protein were increased in adolescent brain. Molecularly, VPA served as an HDAC inhibitor in vitro and in vivo as enhanced proliferation was accompanied by increased histone acetylation, whereas it elicited changes in culture in cell-cycle regulators, including cyclin D1 and E, and cyclin-dependent kinase (CDK) inhibitors, p21 and p27. Collectively, these data suggest clinically relevant VPA exposures stimulate glial precursor proliferation, though at higher doses can elicit inhibition through differential regulation of CDK inhibitors. Because changes in glial cell functions are proposed as mechanisms contributing to neuropsychiatric disorders, these observations suggest that VPA teratogenic actions may be mediated through changes in astrocyte generation during development. © 2015 Wiley Periodicals, Inc. Develop Neurobiol 76: 780-798, 2016.

Roles of imprinted genes in neural stem cells

Imprinted genes and neural stem cells (NSC) play an important role in the developing and mature brain. A central theme of imprinted gene function in NSCs is cell survival and G1 arrest to control cell division, cell-cycle exit, migration and differentiation. Moreover, genomic imprinting can be epigenetically switched off at some genes to ensure stem cell quiescence and differentiation. At the genome scale, imprinted genes are organized in dynamic networks formed by interchromosomal interactions and transcriptional coregulation of imprinted and nonimprinted genes. Such multilayered networks may synchronize NSC activity with the demand from the niche resembling their roles in adjusting fetal size.

The outer subventricular zone and primate-specific cortical complexification

Evolutionary expansion and complexification of the primate cerebral cortex are largely linked to the emergence of the outer subventricular zone (OSVZ), a uniquely structured germinal zone that generates the expanded primate supragranular layers. The primate OSVZ departs from rodent germinal zones in that it includes a higher diversity of precursor types, inter-related in bidirectional non-hierarchical lineages. In addition, primate-specific regulatory mechanisms are operating in primate cortical precursors via the occurrence of novel miRNAs. Here, we propose that the origin and evolutionary importance of the OSVZ is related to genetic changes in multiple regulatory loops and that cell-cycle regulation is a favored target for evolutionary adaptation of the cortex.

Defective pericyte recruitment of villous stromal vessels as the possible etiologic cause of hydropic change in complete hydatidiform mole

The pathogenetic mechanism underlying the hydropic change in complete hydatidiform moles (CHMs) is poorly understood. A growing body of data suggests that pericytes play a role in vascular maturation. Since maturation of villous stromal vessels in CHMs is markedly impaired at early stages, we postulated that a defect in pericytes around stromal vessels in chorionic villi might cause vascular immaturity and subsequent hydropic change. To investigate this, we examined several markers of pericytes, namely, α-smooth muscle actin (α-SMA), platelet-derived growth factor receptor-β (PDGFR-β), and desmin, in 61 normally developing placentas and 41 CHMs with gestational ages of 4-12 weeks. The ultrastructure of villous stromal vessels was also examined. Mature blood vessels from normal placentas show patent vascular lumens and formed hematopoietic components in the villous stroma. α-SMA and PDGFR-β expression in the villous stroma gradually increased and extended from the chorionic plate to peripheral villous branches. The labeled cells formed a reticular network in the villous stroma and, after week 7, encircled villous stromal vessels. In comparison, α-SMA and PDGFR-β expression in the villous stroma and stromal vessels of CHMs was significantly lower (p<0.05). Ultrastructurally, endothelial cells in villous stromal vessels in normal placentas were consistently attached by pericytes after week 7 when the vessels formed distinct lumen, whereas the villous stromal vessels in CHMs consisted of linear chains of endothelial cells, often disclosing primitive clefts without hematopoietic cells inside, and neither pericytes nor basal lamina surrounded the endothelial cells at any gestational age studied. This suggests that pericytes recruitment around villous stromal vessels is defective in CHMs and links to the persistent vascular immaturity of the villous stroma in CHMs, which in turns leads to hydropic villi.

Imprinted Zac1 in neural stem cells

Neural stem cells (NSCs) and imprinted genes play an important role in brain development. On historical grounds, these two determinants have been largely studied independently of each other. Recent evidence suggests, however, that NSCs can reset select genomic imprints to prevent precocious depletion of the stem cell reservoir. Moreover, imprinted genes like the transcriptional regulator Zac1 can fine tune neuronal vs astroglial differentiation of NSCs. Zac1 binds in a sequence-specific manner to pro-neuronal and imprinted genes to confer transcriptional regulation and furthermore coregulates members of the p53-family in NSCs. At the genome scale, Zac1 is a central hub of an imprinted gene network comprising genes with an important role for NSC quiescence, proliferation and differentiation. Overall, transcriptional, epigenomic, and genomic mechanisms seem to coordinate the functional relationships of NSCs and imprinted genes from development to maturation, and possibly aging.

A systems-level approach to parental genomic imprinting: the imprinted gene network includes extracellular matrix genes and regulates cell cycle exit and differentiation

Genomic imprinting is an epigenetic mechanism that restrains the expression of ∼ 100 eutherian genes in a parent-of-origin-specific manner. The reason for this selective targeting of genes with seemingly disparate molecular functions is unclear. In the present work, we show that imprinted genes are coexpressed in a network that is regulated at the transition from proliferation to quiescence and differentiation during fibroblast cell cycle withdrawal, adipogenesis in vitro, and muscle regeneration in vivo. Imprinted gene regulation is not linked to alteration of DNA methylation or to perturbation of monoallelic, parent-of-origin-dependent expression. Overexpression and knockdown of imprinted gene expression alters the sensitivity of preadipocytes to contact inhibition and adipogenic differentiation. In silico and in cellulo experiments showed that the imprinted gene network includes biallelically expressed, nonimprinted genes. These control the extracellular matrix composition, cell adhesion, cell junction, and extracellular matrix-activated and growth factor-activated signaling. These observations show that imprinted genes share a common biological process that may account for their seemingly diverse roles in embryonic development, obesity, diabetes, muscle physiology, and neoplasm.

aPKC phosphorylates p27Xic1, providing a mechanistic link between apicobasal polarity and cell-cycle control

During the development of the nervous system, apicobasally polarized stem cells are characterized by a shorter cell cycle than nonpolar progenitors, leading to a lower differentiation potential of these cells. However, how polarization might be directly linked to the kinetics of the cell cycle is not understood. Here, we report that apicobasally polarized neuroepithelial cells in Xenopus laevis have a shorter cell cycle than nonpolar progenitors, consistent with mammalian systems. We show that the apically localized serine/threonine kinase aPKC directly phosphorylates an N-terminal site of the cell-cycle inhibitor p27Xic1 and reduces its ability to inhibit the cyclin-dependent kinase 2 (Cdk2), leading to shortening of G1 and S phases. Overexpression of activated aPKC blocks the neuronal differentiation-promoting activity of p27Xic1. These findings provide a direct mechanistic link between apicobasal polarity and the cell cycle, which may explain how proliferation is favored over differentiation in polarized neural stem cells.

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aPKC shortens G1 and S phases of cell cycle by phosphorylating p27Xic1

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Phosphorylated p27Xic1 exhibits weaker binding to and inhibition of Cdk2

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p27Xic1 promotes neuronal differentiation and elongates cell cycle via G1 phase

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Effects of p27Xic1 on neuronal differentiation are rescued by activated aPKC

Taspase1-dependent TFIIA cleavage coordinates head morphogenesis by limiting Cdkn2a locus transcription

Head morphogenesis requires complex signal relays to enable precisely coordinated proliferation, migration, and patterning. Here, we demonstrate that, during mouse head formation, taspase1-mediated (TASP1-mediated) cleavage of the general transcription factor TFIIA ensures proper coordination of rapid cell proliferation and morphogenesis by maintaining limited transcription of the negative cell cycle regulators p16Ink4a and p19Arf from the Cdkn2a locus. In mice, loss of TASP1 function led to catastrophic craniofacial malformations that were associated with inadequate cell proliferation. Compound deficiency of Cdkn2a, especially p16Ink4a deficiency, markedly reduced the craniofacial anomalies of TASP1-deficent mice. Furthermore, evaluation of mice expressing noncleavable TASP1 targets revealed that TFIIA is the principal TASP1 substrate that orchestrates craniofacial morphogenesis. ChIP analyses determined that noncleaved TFIIA accumulates at the p16Ink4a and p19Arf promoters to drive transcription of these negative regulators. In summary, our study elucidates a regulatory circuit comprising proteolysis, transcription, and proliferation that is pivotal for construction of the mammalian head.

Deficiency of the chromatin regulator BRPF1 causes abnormal brain development

Epigenetic mechanisms are important in different neurological disorders, and one such mechanism is histone acetylation. The multivalent chromatin regulator BRPF1 (bromodomain- and plant homeodomain-linked (PHD) zinc finger-containing protein 1) recognizes different epigenetic marks and activates three histone acetyltransferases, so it is both a reader and a co-writer of the epigenetic language. The three histone acetyltransferases are MOZ, MORF, and HBO1, which are also known as lysine acetyltransferase 6A (KAT6A), KAT6B, and KAT7, respectively. The MORF gene is mutated in four neurodevelopmental disorders sharing the characteristic of intellectual disability and frequently displaying callosal agenesis. Here, we report that forebrain-specific inactivation of the mouse Brpf1 gene caused early postnatal lethality, neocortical abnormalities, and partial callosal agenesis. With respect to the control, the mutant forebrain contained fewer Tbr2-positive intermediate neuronal progenitors and displayed aberrant neurogenesis. Molecularly, Brpf1 loss led to decreased transcription of multiple genes, such as Robo3 and Otx1, important for neocortical development. Surprisingly, elevated expression of different Hox genes and various other transcription factors, such as Lhx4, Foxa1, Tbx5, and Twist1, was also observed. These results thus identify an important role of Brpf1 in regulating forebrain development and suggest that it acts as both an activator and a silencer of gene expression in vivo.

Hbp1 regulates the timing of neuronal differentiation during cortical development by controlling cell cycle progression

In the developing mammalian brain, neural stem cells (NSCs) initially expand the progenitor pool by symmetric divisions. NSCs then shift from symmetric to asymmetric division and commence neurogenesis. Although the precise mechanisms regulating the developmental timing of this transition have not been fully elucidated, gradual elongation in the length of the cell cycle and coinciding accumulation of determinants that promote neuronal differentiation may function as a biological clock that regulates the onset of asymmetric division and neurogenesis. We conducted gene expression profiling of embryonic NSCs in the cortical regions and found that expression of high mobility group box transcription factor 1 (Hbp1) was upregulated during neurogenic stages. Induced conditional knockout mice of Hbp1 generated by crossing with Nestin-CreERT2 mice exhibited a remarkable dilatation of the telencephalic vesicles with a tangentially expanded ventricular zone and a thinner cortical plate containing reduced numbers of neurons. In these Hbp1-deficient mouse embryos, neural stem/progenitor cells continued to divide with a shorter cell cycle length. And downstream target genes of the Wnt signaling, such as cyclin D1 and c-jun, were upregulated in the germinal zone of the cortical regions. These results indicate that Hbp1 plays a critical role in regulating the timing of cortical neurogenesis by elongating the cell cycle and is essential for normal cortical development.

Cell cycle regulation of proliferation versus differentiation in the central nervous system

Formation of the central nervous system requires a period of extensive progenitor cell proliferation, accompanied or closely followed by differentiation; the balance between these two processes in various regions of the central nervous system gives rise to differential growth and cellular diversity. The correlation between cell cycle lengthening and differentiation has been reported across several types of cell lineage and from diverse model organisms, both in vivo and in vitro. Furthermore, different cell fates might be determined during different phases of the preceding cell cycle, indicating direct cell cycle influences on both early lineage commitment and terminal cell fate decisions. Significant advances have been made in the last decade and have revealed multi-directional interactions between the molecular machinery regulating the processes of cell proliferation and neuronal differentiation. Here, we first introduce the modes of proliferation in neural progenitor cells and summarise evidence linking cell cycle length and neuronal differentiation. Second, we describe the manner in which components of the cell cycle machinery can have additional and, sometimes, cell-cycle-independent roles in directly regulating neurogenesis. Finally, we discuss the way that differentiation factors, such as proneural bHLH proteins, can promote either progenitor maintenance or differentiation according to the cellular environment. These intricate connections contribute to precise coordination and the ultimate division versus differentiation decision.

Growth and folding of the mammalian cerebral cortex: from molecules to malformations

The size and extent of folding of the mammalian cerebral cortex are important factors that influence a species' cognitive abilities and sensorimotor skills. Studies in various animal models and in humans have provided insight into the mechanisms that regulate cortical growth and folding. Both protein-coding genes and microRNAs control cortical size, and recent progress in characterizing basal progenitor cells and the genes that regulate their proliferation has contributed to our understanding of cortical folding. Neurological disorders linked to disruptions in cortical growth and folding have been associated with novel neurogenetic mechanisms and aberrant signalling pathways, and these findings have changed concepts of brain evolution and may lead to new medical treatments for certain disorders.

Nervous decision-making: to divide or differentiate

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Multiple mechanisms coordinate the cell cycle and neuronal differentiation.

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Lengthening of G1 phase is functionally important for differentiation.

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Cell cycle components can directly and independently affect neurogenesis.

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Differentiation factors can directly affect the cell cycle structure and machinery.

The intricate balance between proliferation and differentiation is of fundamental importance in the development of the central nervous system (CNS). The division versus differentiation decision influences both the number and identity of daughter cells produced, thus critically shaping the overall microstructure and function of the CNS. During the past decade, significant advances have been made to characterise the changes in the cell cycle during differentiation, and to uncover the multiple bidirectional links that coordinate these two processes. Here, we explore the nature and mechanistic basis of these links in the context of the developing CNS, highlighting new insights into transcriptional, post-translational, and epigenetic levels of interaction.