Molecular Pathways Underlying Projection Neuron Production and Migration during Cerebral Cortical Development

ADAMTS2 promotes radial migration by activating TGF-β signaling in the developing neocortex

The mammalian neocortex is formed by sequential radial migration of newborn excitatory neurons. Migrating neurons undergo a multipolar-to-bipolar transition at the subplate (SP) layer, where extracellular matrix (ECM) components are abundantly expressed. Here, we investigate the role of the ECM at the SP layer. We show that TGF-β signaling-related ECM proteins, and their downstream effector, p-smad2/3, are selectively expressed in the SP layer. We also find that migrating neurons express a disintegrin and metalloproteinase with thrombospondin motif 2 (ADAMTS2), an ECM metalloproteinase, just below the SP layer. Knockdown and knockout of Adamts2 suppresses the multipolar-to-bipolar transition of migrating neurons and disturbs radial migration. Time-lapse luminescence imaging of TGF-β signaling indicates that ADAMTS2 activates this signaling pathway in migrating neurons during the multipolar-to-bipolar transition at the SP layer. Overexpression of TGF-β2 in migrating neurons partially rescues migration defects in ADAMTS2 knockout mice. Our data suggest that ADAMTS2 secreted by the migrating multipolar neurons activates TGF-β signaling by ECM remodeling of the SP layer, which might drive the multipolar to bipolar transition.

Expression of Dystrophin Dp71 Splice Variants Is Temporally Regulated During Rodent Brain Development

Dystrophin Dp71 is the major product of the Duchenne muscular dystrophy (DMD) gene in the brain, and its loss in DMD patients and mouse models leads to cognitive impairments. Dp71 is expressed as a range of proteins generated by alternative splicing of exons 71 to 74 and 78, classified in the main Dp71d and Dp71f groups that contain specific C-terminal ends. However, it is unknown whether each isoform has a specific role in distinct cell types, brain regions, and/or stages of brain development. In the present study, we characterized the expression of Dp71 isoforms during fetal (E10.5, E15.5) and postnatal (P1, P7, P14, P21 and P60) mouse and rat brain development. We finely quantified the expression of several Dp71 transcripts by RT-PCR and cloning assays in samples from whole-brain and distinct brain structures. The following Dp71 transcripts were detected: Dp71d, Dp71d∆71, Dp71d∆74, Dp71d∆71,74, Dp71d∆71-74, Dp71f, Dp71f∆71, Dp71f∆74, Dp71f∆71,74, and Dp71fΔ71-74. We found that the Dp71f isoform is the main transcript expressed at E10.5 (> 80%), while its expression is then progressively reduced and replaced by the expression of isoforms of the Dp71d group from E15.5 to postnatal and adult ages. This major finding was confirmed by third-generation nanopore sequencing. In addition, we found that the level of expression of specific Dp71 isoforms varies as a function of postnatal stages and brain structure. Our results suggest that Dp71 isoforms have different and complementary roles during embryonic and postnatal brain development, likely taking part in a variety of maturation processes in distinct cell types.

Timing dependent neuronal migration is regulated by Cdk5-mediated phosphorylation of JIP1

The mammalian brain, especially the cerebral cortex, has evolved to increase in size and complexity. The proper development of the cerebral cortex requires the coordination of several events, such as differentiation and migration, that are essential for forming a precise six-layered structure. We have previously reported that Cdk5-mediated phosphorylation of JIP1 at T205 modulates axonal out-growth. However, the spatiotemporal expression patterns and functions of these three genes (Cdk5, Cdk5r1 or p35, and Mapk8ip1 or JIP1) in distinct cell types during cortical development remain unclear. In this study, we analyzed single-cell RNA-sequencing data of mouse embryonic cortex and discovered that Cdk5, p35, and JIP1 are dynamically expressed in intermediate progenitors (IPs). Pseudotime analysis revealed that the expression of these three genes was concomitantly upregulated in IPs during neuronal migration and differentiation. By manipulating the expression of JIP1 and phospho-mimetic JIP1 using in utero electroporation, we showed that phosphorylated JIP1 at T205 affected the temporal migration of neurons.

Connecting neurodevelopment to neurodegeneration: a spotlight on the role of kinesin superfamily protein 2A (KIF2A)

Microtubules play a central role in cytoskeletal changes during neuronal development and maintenance. Microtubule dynamics is essential to polarity and shape transitions underlying neural cell division, differentiation, motility, and maturation. Kinesin superfamily protein 2A is a member of human kinesin 13 gene family of proteins that depolymerize and destabilize microtubules. In dividing cells, kinesin superfamily protein 2A is involved in mitotic progression, spindle assembly, and chromosome segregation. In postmitotic neurons, it is required for axon/dendrite specification and extension, neuronal migration, connectivity, and survival. Humans with kinesin superfamily protein 2A mutations suffer from a variety of malformations of cortical development, epilepsy, autism spectrum disorder, and neurodegeneration. In this review, we discuss how kinesin superfamily protein 2A regulates neuronal development and function, and how its deregulation causes neurodevelopmental and neurological disorders.

Caudally pronounced deficiencies in preplate splitting and migration underly a rostro-caudal progression of cortical lamination defects in the reeler brain

In mammalian neocortex development, every cohort of newborn neurons is guided toward the marginal zone, leading to an "inside-out" organization of the 6 neocortical layers. This migratory pattern is regulated by the extracellular glycoprotein Reelin. The reeler mouse shows a homozygous mutation of the reelin gene. Using RNA in situ hybridization we could demonstrate that the Reelin-deficient mouse cortex (male and female) displays an increasing lamination defect along the rostro-caudal axis that is characterized by strong cellular intermingling, but roughly reproduces the "inside-out" pattern in rostral cortex, while caudal cortex shows a relative inversion of neuronal positioning ("outside-in"). We found that in development of the reeler cortex, preplate-splitting is also defective with an increasing severity along the rostro-caudal axis. This leads to a misplacement of subplate neurons that are crucial for a switch in migration mode within the cortical plate. Using Flash Tag labeling and nucleoside analog pulse-chasing, we found an according migration defect within the cortical plate, again with a progressive severity along the rostro-caudal axis. Thus, loss of one key player in neocortical development leads to highly area-specific (caudally pronounced) developmental deficiencies that result in multiple roughly opposite rostral versus caudal adult neocortical phenotypes.

Molecular mechanisms of corpus callosum development: a four-step journey

The Corpus Callosum (CC) is a bundle of axons connecting the cerebral hemispheres. It is the most recent structure to have appeared during evolution of placental mammals. Its development is controlled by a very complex interplay of many molecules. In humans it contains almost 80% of all commissural axons in the brain. The formation of the CC can be divided into four main stages, each controlled by numerous intracellular and extracellular molecular factors. First, a newborn neuron has to specify an axon, leave proliferative compartments, the Ventricular Zone (VZ) and Subventricular Zone (SVZ), migrate through the Intermediate Zone (IZ), and then settle at the Cortical Plate (CP). During the second stage, callosal axons navigate toward the midline within a compact bundle. Next stage is the midline crossing into contralateral hemisphere. The last step is targeting a defined area and synapse formation. This review provides an insight into these four phases of callosal axons development, as well as a description of the main molecular players involved.

Single-cell analysis reveals specific neuronal transition during mouse corticogenesis

Background: Currently, the mechanism(s) underlying corticogenesis is still under characterization. Methods: We curated the most comprehensive single-cell RNA-seq (scRNA-seq) datasets from mouse and human fetal cortexes for data analysis and confirmed the findings with co-immunostaining experiments. Results: By analyzing the developmental trajectories with scRNA-seq datasets in mice, we identified a specific developmental sub-path contributed by a cell-population expressing both deep- and upper-layer neurons (DLNs and ULNs) specific markers, which occurred on E13.5 but was absent in adults. In this cell-population, the percentages of cells expressing DLN and ULN markers decreased and increased, respectively, during the development suggesting direct neuronal transition (namely D-T-U). Whilst genes significantly highly/uniquely expressed in D-T-U cell population were significantly enriched in PTN/MDK signaling pathways related to cell migration. Both findings were further confirmed by co-immunostaining with DLNs, ULNs and D-T-U specific markers across different timepoints. Furthermore, six genes (co-expressed with D-T-U specific markers in mice) showing a potential opposite temporal expression between human and mouse during fetal cortical development were associated with neuronal migration and cognitive functions. In adult prefrontal cortexes (PFC), D-T-U specific genes were expressed in neurons from different layers between humans and mice. Conclusion: Our study characterizes a specific cell population D-T-U showing direct DLNs to ULNs neuronal transition and migration during fetal cortical development in mice. It is potentially associated with the difference of cortical development in humans and mice.

Advances in Understanding the Molecular Mechanisms of Neuronal Polarity

The establishment and maintenance of neuronal polarity are important for neural development and function. Abnormal neuronal polarity establishment commonly leads to a variety of neurodevelopmental disorders. Over the past three decades, with the continuous development and improvement of biological research methods and techniques, we have made tremendous progress in the understanding of the molecular mechanisms of neuronal polarity establishment. The activity of positive and negative feedback signals and actin waves are both essential in this process. They drive the directional transport and aggregation of key molecules of neuronal polarity, promote the spatiotemporal regulation of ordered and coordinated interactions of actin filaments and microtubules, stimulate the specialization and growth of axons, and inhibit the formation of multiple axons. In this review, we focus on recent advances in these areas, in particular the important findings about neuronal polarity in two classical models, in vitro primary hippocampal/cortical neurons and in vivo cortical pyramidal neurons, and discuss our current understanding of neuronal polarity.​.

Glial functions in the blood-brain communication at the circumventricular organs

The circumventricular organs (CVOs) are located around the brain ventricles, lack a blood-brain barrier (BBB) and sense blood-derived molecules. This review discusses recent advances in the importance of CVO functions, especially glial cells transferring periphery inflammation signals to the brain. The CVOs show size-limited vascular permeability, allowing the passage of molecules with molecular weight <10,000. This indicates that the lack of an endothelial cell barrier does not mean the free movement of blood-derived molecules into the CVO parenchyma. Astrocytes and tanycytes constitute a dense barrier at the distal CVO subdivision, preventing the free diffusion of blood-derived molecules into neighboring brain regions. Tanycytes in the CVOs mediate communication between cerebrospinal fluid and brain parenchyma via transcytosis. Microglia and macrophages of the CVOs are essential for transmitting peripheral information to other brain regions via toll-like receptor 2 (TLR2). Inhibition of TLR2 signaling or depletion of microglia and macrophages in the brain eliminates TLR2-dependent inflammatory responses. In contrast to TLR2, astrocytes and tanycytes in the CVOs of the brain are crucial for initiating lipopolysaccharide (LPS)-induced inflammatory responses via TLR4. Depletion of microglia and macrophages augments LPS-induced fever and chronic sickness responses. Microglia and macrophages in the CVOs are continuously activated, even under normal physiological conditions, as they exhibit activated morphology and express the M1/M2 marker proteins. Moreover, the microglial proliferation occurs in various regions, such as the hypothalamus, medulla oblongata, and telencephalon, with a marked increase in the CVOs, due to low-dose LPS administration, and after high-dose LPS administration, proliferation is seen in most brain regions, except for the cerebral cortex and hippocampus. A transient increase in the microglial population is beneficial during LPS-induced inflammation for attenuating sickness response. Transient receptor potential receptor vanilloid 1 expressed in astrocytes and tanycytes of the CVOs is responsible for thermoregulation upon exposure to a warm environment less than 37°C. Alternatively, Nax expressed in astrocytes and tanycytes of the CVOs is crucial for maintaining body fluid homeostasis. Thus, recent findings indicate that glial cells in the brain CVOs are essential for initiating neuroinflammatory responses and maintaining body fluid and thermal homeostasis.

Migrating Pyramidal Neurons Require DSCAM to Bypass the Border of the Developing Cortical Plate

During mammalian neocortex development, nascent pyramidal neurons migrate along radial glial cells and overtake earlier-born neurons to terminate at the front of the developing cortical plate (CP), leading to the outward expansion of the CP border. While much has been learned about the cellular and molecular mechanisms that underlie the migration of pyramidal neurons, how migrating neurons bypass the preceding neurons at the end of migration to reach their final positions remains poorly understood. Here, we report that Down syndrome cell adhesion molecule (DSCAM) is required for migrating neurons to bypass their postmigratory predecessors during the expansion of the upper cortical layers. DSCAM is a type I transmembrane cell adhesion molecule. It has been linked to Down syndrome through its location on Chromosome 21 trisomy and to autism spectrum disorders through loss-of-function mutations. Ex vivo time-lapse imaging demonstrates that DSCAM is required for migrating neurons to bypass their postmigratory predecessors, crossing the CP border to expand the upper cortical layers. In DSCAM-deficient cortices, migrating neurons stop prematurely under the CP border, leading to thinner upper cortical layers with higher neuronal density. We further show that DSCAM weakens cell adhesion mediated by N-cadherin in the upper cortical plate, allowing migrating neurons to traverse the CP border and expand the CP. These findings suggest that DSCAM is required for proper migratory termination and final positioning of nascent pyramidal neurons, which may provide insight into brain disorders that exhibit thinner upper layers of the cerebral cortex without neuronal loss.SIGNIFICANCE STATEMENT Newly born neurons in the developing mammalian neocortex migrate outward toward the cortical surface, bypassing earlier born neurons to expand the developing cortex. How migrating neurons bypass the preceding neurons and terminate at the front of the expanding cortex remains poorly understood. We demonstrate that Down syndrome cell adhesion molecule (DSCAM), linked to Down syndrome and autism spectrum disorder, is required by migrating neurons to bypass their postmigratory predecessors and terminate migration in the outwardly expanding cortical layer. Migrating neurons deficient in DSCAM stop prematurely, failing to expand the cortex. We further show that DSCAM likely mediates migratory termination by weakening cell adhesion mediated by N-cadherin.

Generation of FLAG-tagged Arx knock-in mouse model

The Aristaless-related homeobox (ARX) is a paired-like homeodomain transcription factor playing important roles in brain development. Patients with mutations in ARX have a spectrum of neurodevelopmental disorders such as epilepsy, intellectual disability, and autism spectrum disorder, with or without structural abnormalities of the brain such as lissencephaly (smooth brain), microcephaly (small brain), and/or agenesis of the corpus callosum. Mouse models have provided important clues on the pathophysiologic roles of ARX in these disorders. However, successfully isolating specific in vivo complexes of ARX, with DNA and proteins, has remained as a challenge. To facilitate in vivo detection of ARX complexes, we generated a mouse line containing one epitope of FLAG-tag (1 × FLAG) targeted at the translational start site of the endogenous Arx gene using CRSPR/Cas9 strategy. Homozygous Flag-Arx mice are viable and fertile without gross abnormality, suggesting that the FLAG-tag does not perturb the normal function of ARX. Using a FLAG antibody, we successfully detected ARX with immunofluorescent staining and pulled down ARX in embryonic brain tissues. This Flag-Arx mouse line will be a useful tool to isolate ARX complexes from mouse tissues for many applications.

Brain Organization and Human Diseases

The cortex is a highly organized structure that develops from the caudal regions of the segmented neural tube. Its spatial organization sets the stage for future functional arealization. Here, we suggest using a developmental perspective to describe and understand the etiology of common cortical malformations and their manifestation in the human brain.

Phosphorylation of Focal Adhesion Kinase at Y925: Role in Glia-Dependent and Independent Migration through Regulating Cofilin and N-Cadherin

The adult neocortex is a six-layered structure, consisting of nearly continuous layers of neurons that are generated in a temporally strictly coordinated order. During development, cortical neurons originating from the ventricular zone migrate toward the Reelin-containing marginal zone in an inside-out arrangement. Focal adhesion kinase (FAK), one tyrosine kinase localizing to focal adhesions, has been shown to be phosphorylated at tyrosine 925 (Y925) by Src, an important downstream molecule of Reelin signaling. Up to date, the precise molecular mechanisms of FAK and its phosphorylation at Y925 during neuronal migration are still unclear. Combining in utero electroporation with immunohistochemistry and live imaging, we examined the function of FAK in regulating neuronal migration. We show that phosphorylated FAK is colocalized with Reelin positive Cajal-Retzius cells in the developing neocortex and hippocampus. Phosphorylation of FAK at Y925 is significantly reduced in reeler mice. Overexpression and dephosphorylation of FAK impair locomotion and translocation, resulting in migration inhibition and dislocation of both late-born and early-born neurons. These migration defects are highly correlated to the function of FAK in regulating cofilin phosphorylation and N-Cadherin expression, both are involved in Reelin signaling pathway. Thus, fine-tuned phosphorylation of focal adhesion kinase at Y925 is crucial for both glia-dependent and independent neuronal migration.

YAP maintains the production of intermediate progenitor cells and upper-layer projection neurons in the mouse cerebral cortex

BACKGROUND

The Hippo pathway is conserved through evolution and plays critical roles in development, tissue homeostasis and tumorigenesis. Yes-associated protein (YAP) is a transcriptional coactivator downstream of the Hippo pathway. Previous studies have demonstrated that activation of YAP promotes proliferation in the developing brain. Whether YAP is required for the production of neural progenitor cells or neurons in vivo remains unclear.

RESULTS

We demonstrated that SATB homeobox 2 (SATB2)-positive projection neurons (PNs) in upper layers, but not T-box brain transcription factor 1-positive and Coup-TF interacting protein 2-positive PNs in deep layers, were decreased in the neonatal cerebral cortex of Yap conditional knockout (cKO) mice driven by Nestin-Cre. Cell proliferation was reduced in the developing cerebral cortex of Yap-cKO. SATB2-positive PNs are largely generated from intermediate progenitor cells (IPCs), which are derived from radial glial cells (RGCs) during cortical development. Among these progenitor cells, IPCs but not RGCs were decreased in Yap-cKO. We further demonstrated that cell cycle re-entry was reduced in progenitor cells of Yap-cKO, suggesting that fewer IPCs were generated in Yap-cKO.

CONCLUSION

YAP is required for the production of IPCs and upper-layer SATB2-positive PNs during development of the cerebral cortex in mice.

Structural alignment guides oriented migration and differentiation of endogenous neural stem cells for neurogenesis in brain injury treatment

Radial glia (RG) cells that align in parallel in the embryonic brain are found to be able to guide the directed migration of neurons in response to brain injury. Therefore, biomaterials with aligned architectures are supposed to have positive effects on neural migration and neurogenic differentiation for brain injury repair that are rarely addressed, although they have been widely demonstrated in spinal cord and peripheral nerve system. Here, we present a highly biomimetic scaffold of aligned fibrin hydrogel (AFG) that mimics the oriented structure of RG fibers. Through a combination of histological, behavioral, imaging, and transcriptomic analyses, we demonstrated that transplanting the AFG scaffold into injured cortical brains promotes effective migration, differentiation, and maturation of endogenous neural stem cells, resulting in neurological functional recovery. Therefore, this study will light up a new perspective on applying an aligned scaffold to promote cortical regeneration after injury by inducing endogenous neurogenesis.

Pathophysiological Mechanisms in Neurodevelopmental Disorders Caused by Rac GTPases Dysregulation: What's behind Neuro-RACopathies

Rho family guanosine triphosphatases (GTPases) regulate cellular signaling and cytoskeletal dynamics, playing a pivotal role in cell adhesion, migration, and cell cycle progression. The Rac subfamily of Rho GTPases consists of three highly homologous proteins, Rac 1-3. The proper function of Rac1 and Rac3, and their correct interaction with guanine nucleotide-exchange factors (GEFs) and GTPase-activating proteins (GAPs) are crucial for neural development. Pathogenic variants affecting these delicate biological processes are implicated in different medical conditions in humans, primarily neurodevelopmental disorders (NDDs). In addition to a direct deleterious effect produced by genetic variants in the RAC genes, a dysregulated GTPase activity resulting from an abnormal function of GEFs and GAPs has been involved in the pathogenesis of distinctive emerging conditions. In this study, we reviewed the current pertinent literature on Rac-related disorders with a primary neurological involvement, providing an overview of the current knowledge on the pathophysiological mechanisms involved in the neuro-RACopathies.

Early development of the cortical layers in the human brain

The cortical plate (CP) first appears at seven postconceptional weeks (pcw), when it splits the preexisting preplate into two layers, the marginal zone and the presubplate (pSP). Although three-dimensional (3D) analysis using fetal magnetic resonance imaging and two-dimensional tissue observations have been reported, there have been no studies analyzing the early development of the layer structure corresponding to the pSP stage in 3D. Here, we reconstructed 3-D models of the brain with a focus on the cortical layers in pSP stage. To achieve this, we digitized serial tissue sections of embryos between CS20 and CS23 from the Kyoto Collection (n = 7, approximately 7-8.5 pcw), and specimens at early fetal phase from the Blechschmidt Collection (n = 2, approximately 9.5-12 pcw, crown rump length [CRL] 39 and 64 mm). We observed tissue sections and 3D images and performed quantitative analysis of the thickness, surface area, and volume. Because the boundary between pSP and the intermediate zone (IZ) could not be distinguished in hematoxylin and eosin-stained sections, the two layers were analyzed together as a single layer in this study. The histology of the layers was observed from CS21 and became distinct at CS22. Subsequently, we observed the 3-D models; pSP-IZ was present in a midlateral region of the cerebral wall at CS21, and an expansion centered around this region was observed after CS22. We observed it over the entire cerebral hemisphere at early fetal phase (CRL 39 mm). The thickness of pSP-IZ was visible in 3D and was greater in the midlateral region. At the end of the pSP stage (CRL 64 mm), the thick region expanded to lateral, superior, and posterior regions around the primordium of the insula. While, the region near the basal ganglia was not included in the thickest 10% of the pSP-IZ area. Middle cerebral artery was found in the midlateral region of the cerebral wall, near the area where pSP-IZ was observed. Feature of layer structure growth was revealed by quantitative assessment as thickness, surface area, and volume. The maximum thickness value of pSP-IZ and CP increased significantly according to CRL, whereas the median value increased slightly. The layer structure appeared to grow and spread thin, rather than thickening during early development, which is characteristic during pSP stages. The surface area of the cerebral total tissue, CP, and pSP-IZ increased in proportion to the square of CRL. The surface area of CP and pSP-IZ approached that of the total tissue at the end of the pSP stage. Volume of each layer increased in proportion to the cube of CRL. pSP-IZ and CP constituted over 50% of the total tissue in volume at the end of the pSP stages. We could visualize the growth of pSP-IZ in 3D and quantify it during pSP stage. Our approach allowed us to observe the process of rapid expansion of pSP-IZ from the midlateral regions of the cerebral wall, which subsequently becomes the insula.

Induced pluripotent stem cells for 2D and 3D modelling the biological basis of schizophrenia and screening possible therapeutics

Induced pluripotent stem cells (iPSCs) are providing unprecedented insight into complex neuropsychiatric disorders such as schizophrenia (SZ). Here we review the use of iPSCs for investigating the etiopathology and treatment of SZ, beginning with conventional in vitro two-dimensional (2D; monolayer) cell modelling, through to more advanced 3D tissue studies. With the advent of 3D modelling, utilising advanced differentiation paradigms and additive manufacturing technologies, inclusive of patient-specific cerebral/neural organoids and bioprinted neural tissues, such live disease-relevant tissue systems better recapitulate "within-body" tissue function and pathobiology. We posit that by enabling better understanding of biological causality, these evolving strategies will yield novel therapeutic targets and accordingly, drug candidates.

Changes in Wnt-Dependent Neuronal Morphology Underlie the Anatomical Diversification of Neocortical Homologs in Amniotes

The six-layered neocortex is a shared characteristic of all mammals, but not of non-mammalian species, and its formation requires an inside-out pattern of neuronal migration. The extant reptilian dorsal cortex is thought to represent an ancestral form of the neocortex, although how the reptilian three-layered cortex is formed is poorly understood. Here, we show unique patterns of lamination and neuronal migration in the developing reptilian cortex. While the multipolar-to-bipolar transition of migrating neurons is essential for mammalian cortical development, the reptilian cortex lacks bipolar-shaped migrating neurons, resulting in an outside-in pattern of cortical development. Furthermore, dynamic regulation of Wnt signal strengths contributes to neuronal morphological changes, which is conserved across species. Our data preclude the idea that the six-layered mammalian neocortex emerged by simple addition to the reptilian dorsal cortex but suggest that the acquisition of a novel neuronal morphology based on conserved developmental programs contributed to neocortical evolution.

Integrative analysis of somatic mutations and differential expression profiles in glioblastoma based on aging acceleration

BACKGROUND

Glioblastoma (GBM) is an aggressive brain tumor and the mechanisms of progression are very complex. Accelerated aging is a driving factor of GBM. However, there has not been thorough research about the mechanisms of GBM progression based on aging acceleration.

METHODS

The aging predictor was modeled based on normal brain samples. Then an aging acceleration background network was constructed to explore GBM mechanisms.

RESULTS

The accelerated aging-related mechanisms provided an innovative way to study GBM, wherein integrative analysis of somatic mutations and differential expression revealed key pathologic characteristics. Moreover, the influence of the immune system, the nervous system and other critical factors on GBM were identified. The survival analysis also disclosed crucial GBM markers.

CONCLUSION

An integrative analysis of multi-omics data based on aging acceleration identified new driving factors for GBM.

Critical aspects of neurodevelopment

Organisms have the unique ability to adapt to their environment by making use of external inputs. In the process, the brain is shaped by experiences that go hand-in-hand with optimisation of neural circuits. As such, there exists a time window for the development of different brain regions, each unique for a particular sensory modality, wherein the propensity of forming strong, irreversible connections are high, referred to as a critical period of development. Over the years, this domain of neurodevelopmental research has garnered considerable attention from many scientists, primarily because of the intensive activity-dependent nature of development. This review discusses the cellular, molecular, and neurophysiological bases of critical periods of different sensory modalities, and the disorders associated in cases the regulators of development are dysfunctional. Eventually, the neurobiological bases of the behavioural abnormalities related to developmental pathologies are discussed. A more in-depth insight into the development of the brain during the critical period of plasticity will eventually aid in developing potential therapeutics for several neurodevelopmental disorders that are categorised under critical period disorders.

Neuroserpin Is Strongly Expressed in the Developing and Adult Mouse Neocortex but Its Absence Does Not Perturb Cortical Lamination and Synaptic Proteome

Neuroserpin is a serine protease inhibitor that regulates the activity of tissue-type plasminogen activator (tPA) in the nervous system. Neuroserpin is strongly expressed during nervous system development as well as during adulthood, when it is predominantly found in regions eliciting synaptic plasticity. In the hippocampus, neuroserpin regulates developmental neurogenesis, synaptic maturation and in adult mice it modulates synaptic plasticity and controls cognitive and social behavior. High expression levels of neuroserpin in the neocortex starting from prenatal stage and persisting during adulthood suggest an important role for the serpin in the formation of this brain region and in the maintenance of cortical functions. In order to uncover neuroserpin function in the murine neocortex, in this work we performed a comprehensive investigation of its expression pattern during development and in the adulthood. Moreover, we assessed the role of neuroserpin in cortex formation by comparing cortical lamination and neuronal maturation between neuroserpin-deficient and control mice. Finally, we evaluated a possible regulatory role of neuroserpin at cortical synapses in neuroserpin-deficient mice. We observed that neuroserpin is expressed starting from the beginning of corticogenesis until adulthood throughout the neocortex in several classes of glutamatergic projection neurons and GABA-ergic interneurons. However, in the absence of neuroserpin we did not detect any alteration either in cortical layer formation, or in neuronal soma size and dendritic length. Furthermore, no significant quantitative changes were observed in the proteome of cortical synapses upon neuroserpin deficiency. We conclude that, although strongly expressed in the neocortex, absence of neuroserpin does not lead to gross developmental abnormalities, and does not perturb the composition of the cortical synaptic proteome.

Beyond Axon Guidance: Roles of Slit-Robo Signaling in Neocortical Formation

The formation of the neocortex relies on intracellular and extracellular signaling molecules that are involved in the sequential steps of corticogenesis, ranging from the proliferation and differentiation of neural progenitor cells to the migration and dendrite formation of neocortical neurons. Abnormalities in these steps lead to disruption of the cortical structure and circuit, and underly various neurodevelopmental diseases, including dyslexia and autism spectrum disorder (ASD). In this review, we focus on the axon guidance signaling Slit-Robo, and address the multifaceted roles of Slit-Robo signaling in neocortical development. Recent studies have clarified the roles of Slit-Robo signaling not only in axon guidance but also in progenitor cell proliferation and migration, and the maturation of neocortical neurons. We further discuss the etiology of neurodevelopmental diseases, which are caused by defects in Slit-Robo signaling during neocortical formation.

Modeling human neuronal migration deficits in 3D

During the past few decades, we have witnessed an impressive gain in the knowledge regarding the basic mechanisms underlying human neuronal migration disorders by the usage of mouse models. Nevertheless, despite the remarkable conservation both in the genetic encoded information and the developmental processes, there are still numerous important differences between human and mouse. This may explain the vast excitement following the realization that technological breakthroughs enabled generating tissue-like human-based organoids for modeling human neuronal migration diseases. This review will provide a short introduction on human and mouse neuronal migration processes, and highlight human brain organoid models of neuronal migration diseases.

How Do Electric Fields Coordinate Neuronal Migration and Maturation in the Developing Cortex?

During development the vast majority of cells that will later compose the mature cerebral cortex undergo extensive migration to reach their final position. In addition to intrinsically distinct migratory behaviors, cells encounter and respond to vastly different microenvironments. These range from axonal tracts to cell-dense matrices, electrically active regions and extracellular matrix components, which may all change overtime. Furthermore, migrating neurons themselves not only adapt to their microenvironment but also modify the local niche through cell-cell contacts, secreted factors and ions. In the radial dimension, the developing cortex is roughly divided into dense progenitor and cortical plate territories, and a less crowded intermediate zone. The cortical plate is bordered by the subplate and the marginal zone, which are populated by neurons with high electrical activity and characterized by sophisticated neuritic ramifications. Neuronal migration is influenced by these boundaries resulting in dramatic changes in migratory behaviors as well as morphology and electrical activity. Modifications in the levels of any of these parameters can lead to alterations and even arrest of migration. Recent work indicates that morphology and electrical activity of migrating neuron are interconnected and the aim of this review is to explore the extent of this connection. We will discuss on one hand how the response of migrating neurons is altered upon modification of their intrinsic electrical properties and whether, on the other hand, the electrical properties of the cellular environment can modify the morphology and electrical activity of migrating cortical neurons.

FLRTing Neurons in Cortical Migration During Cerebral Cortex Development

During development, two coordinated events shape the morphology of the mammalian cerebral cortex, leading to the cortex's columnar and layered structure: the proliferation of neuronal progenitors and cortical migration. Pyramidal neurons originating from germinal zones migrate along radial glial fibers to their final position in the cortical plate by both radial migration and tangential dispersion. These processes rely on the delicate balance of intercellular adhesive and repulsive signaling that takes place between neurons interacting with different substrates and guidance cues. Here, we focus on the function of the cell adhesion molecules fibronectin leucine-rich repeat transmembrane proteins (FLRTs) in regulating both the radial migration of neurons, as well as their tangential spread, and the impact these processes have on cortex morphogenesis. In combining structural and functional analysis, recent studies have begun to reveal how FLRT-mediated responses are precisely tuned - from forming different protein complexes to modulate either cell adhesion or repulsion in neurons. These approaches provide a deeper understanding of the context-dependent interactions of FLRTs with multiple receptors involved in axon guidance and synapse formation that contribute to finely regulated neuronal migration.

Involvement of JNK1 in Neuronal Polarization During Brain Development

The c-Jun N-terminal Kinases (JNKs) are a group of regulatory elements responsible for the control of a wide array of functions within the cell. In the central nervous system (CNS), JNKs are involved in neuronal polarization, starting from the cell division of neural stem cells and ending with their final positioning when migrating and maturing. This review will focus mostly on isoform JNK1, the foremost contributor of total JNK activity in the CNS. Throughout the text, research from multiple groups will be summarized and discussed in order to describe the involvement of the JNKs in the different steps of neuronal polarization. The data presented support the idea that isoform JNK1 is highly relevant to the regulation of many of the processes that occur in neuronal development in the CNS.

Experience-Dependent Development of Dendritic Arbors in Mouse Visual Cortex

The dendritic arbor of neurons constrains the pool of available synaptic partners and influences the electrical integration of synaptic currents. Despite these critical functions, our knowledge of the dendritic structure of cortical neurons during early postnatal development and how these dendritic structures are modified by visual experience is incomplete. Here, we present a large-scale dataset of 849 3D reconstructions of the basal arbor of pyramidal neurons collected across early postnatal development in visual cortex of mice of either sex. We found that the basal arbor grew substantially between postnatal day 7 (P7) and P30, undergoing a 45% increase in total length. However, the gross number of primary neurites and dendritic segments was largely determined by P7. Growth from P7 to P30 occurred primarily through extension of dendritic segments. Surprisingly, comparisons of dark-reared and typically reared mice revealed that a net gain of only 15% arbor length could be attributed to visual experience; most growth was independent of experience. To examine molecular contributions, we characterized the role of the activity-regulated small GTPase Rem2 in both arbor development and the maintenance of established basal arbors. We showed that Rem2 is an experience-dependent negative regulator of dendritic segment number during the visual critical period. Acute deletion of Rem2 reduced directionality of dendritic arbors. The data presented here establish a highly detailed, quantitative analysis of basal arbor development that we believe has high utility both in understanding circuit development as well as providing a framework for computationalists wishing to generate anatomically accurate neuronal models.SIGNIFICANCE STATEMENT Dendrites are the sites of the synaptic connections among neurons. Despite their importance for neural circuit function, only a little is known about the postnatal development of dendritic arbors of cortical pyramidal neurons and the influence of experience. Here we show that the number of primary basal dendritic arbors is already established before eye opening, and that these arbors primarily grow through lengthening of dendritic segments and not through addition of dendritic segments. Surprisingly, visual experience has a modest net impact on overall arbor length (15%). Experiments in KO animals revealed that the gene Rem2 is positive regulator of dendritic length and a negative regulator of dendritic segments.

Mechanisms of axon polarization in pyramidal neurons

Neurons are highly polarized cells that have specialized regions for synaptic input, the dendrites, and synaptic output, the axons. This polarity is critical for appropriate neural circuit formation and function. One of the central gaps in our knowledge is understanding how developing neurons initiate axon polarity. Given the critical nature of this polarity on neural circuit formation and function, neurons have evolved multiple mechanisms comprised of extracellular and intracellular cues that allow them to initiate and form axons. These mechanisms engage a variety of signaling cascades that provide positive and negative cues to ensure axon polarization. This review highlights our current knowledge of the molecular underpinnings of axon polarization in pyramidal neurons and their relevance to the development of the brain.

Nwd1 Regulates Neuronal Differentiation and Migration through Purinosome Formation in the Developing Cerebral Cortex

Engagement of neural stem/progenitor cells (NSPCs) into proper neuronal differentiation requires the spatiotemporally regulated generation of metabolites. Purines are essential building blocks for many signaling molecules. Enzymes that catalyze de novo purine synthesis are assembled as a huge multienzyme complex called “purinosome.” However, there is no evidence of the formation or physiological function of the purinosome in the brain. Here, we showed that a signal transduction ATPases with numerous domains (STAND) protein, NACHT and WD repeat domain-containing 1 (Nwd1), interacted with Paics, a purine-synthesizing enzyme, to regulate purinosome assembly in NSPCs. Altered Nwd1 expression affected purinosome formation and induced the mitotic exit and premature differentiation of NSPCs, repressing neuronal migration and periventricular heterotopia. Overexpression/knockdown of Paics or Fgams, other purinosome enzymes, in the developing brain resulted in a phenocopy of Nwd1 defects. These findings indicate that strict regulation of purinosome assembly/disassembly is crucial for maintaining NSPCs and corticogenesis.

•

STAND protein Nwd1 interacts with Paics to regulate the purinosome formation

•

Dysregulated expression of Nwd1 induced the premature differentiation of NSPCs

•

Nwd1 KD repressed the neuronal migration, causing the periventricular heterotopia

•

Tightly regulated assembly of purinosome components is crucial for corticogenesis

Subplate Neurons as an Organizer of Mammalian Neocortical Development

Subplate neurons (SpNs) are one of the earliest born and matured neurons in the developing cerebral cortex and play an important role in the early development of the neocortex. It has been known that SpNs have an essential role in thalamocortical axon (TCA) pathfinding and the establishment of the first neural circuit from the thalamus towards cortical layer IV. In addition to this function, it has recently been revealed in mouse corticogenesis that SpNs play an important role in the regulation of radial neuronal migration during the mid-embryonic stage. Moreover, accumulating studies throw light on the possible roles of SpNs in adult brain functions and also their involvement in psychiatric or other neurological disorders. As SpNs are unique to mammals, they may have contributed to the evolution of the mammalian neocortex by efficiently organizing cortical formation during the limited embryonic period of corticogenesis. By increasing our knowledge of the functions of SpNs, we will clarify how SpNs act as an organizer of mammalian neocortical formation.

Human antigen R-regulated mRNA metabolism promotes the cell motility of migrating mouse neurons

Neocortex development during embryonic stages requires the precise control of mRNA metabolism. Human antigen R (HuR) is a well-studied mRNA-binding protein that regulates mRNA metabolism, and it is highly expressed in the neocortex during developmental stages. Deletion of HuR does not impair neural progenitor cell proliferation or differentiation, but it disturbs the laminar structure of the neocortex. We report that HuR is expressed in postmitotic projection neurons during mouse brain development. Specifically, depletion of HuR in these neurons led to a mislocalization of CDP⁺ neurons in deeper layers of the cortex. Time-lapse microscopy showed that HuR was required for the promotion of cell motility in migrating neurons. PCR array identified profilin 1 (Pfn1) mRNA as a major binding partner of HuR in neurons. HuR positively mediated the stability of Pfn1 mRNA and influenced actin polymerization. Overexpression of Pfn1 successfully rescued the migration defects of HuR-deleted neurons. Our data reveal a post-transcriptional mechanism that maintains actin dynamics during neuronal migration.

Summary: Maintaining actin dynamics is crucial for cell motility. Post-transcriptional regulation plays a pivotal role in supporting actin dynamics during neuronal migration.

Resolving the Synaptic versus Developmental Dichotomy of Autism Risk Genes

Genes that are mutated in Autism Spectrum Disorders (ASD) can be classified broadly as either synaptic or developmental. But what if this is a false distinction? A recent spate of publications has provided evidence for developmental mechanisms that rely on neural activity for proper cortical development. Conversely, a growing body of evidence indicates a role for developmental mechanisms, particularly chromatin remodeling, during learning or in response to neural activity. Here, we review these recent publications and propose a model in which genes that confer ASD risk operate in signal transduction networks critical for both cortical development and synaptic homeostasis.

Neural stem cell phenotype of tanycyte-like ependymal cells in the circumventricular organs and central canal of adult mouse brain

Tanycyte is a subtype of ependymal cells which extend long radial processes to brain parenchyma. The present study showed that tanycyte-like ependymal cells in the organum vasculosum of the lamina terminalis, subfornical organ and central canal (CC) expressed neural stem cell (NSC) marker nestin, glial fibrillar acidic protein and sex determining region Y. Proliferation of these tanycyte-like ependymal cells was promoted by continuous intracerebroventricular infusion of fibroblast growth factor-2 and epidermal growth factor. Tanycytes-like ependymal cells in the CC are able to form self-renewing neurospheres and give rise mostly to new astrocytes and oligodendrocytes. Collagenase-induced small medullary hemorrhage increased proliferation of tanycyte-like ependymal cells in the CC. These results demonstrate that these tanycyte-like ependymal cells of the adult mouse brain are NSCs and suggest that they serve as a source for providing new neuronal lineage cells upon brain damage in the medulla oblongata.

A Mouse Model System to Study Peroxisomal Roles in Neurodegeneration of Peroxisome Biogenesis Disorders

Fourteen PEX genes are currently identified as genes responsible for peroxisome biogenesis disorders (PBDs). Patients with PBDs manifest as neurodegenerative symptoms such as neuronal migration defect and malformation of the cerebellum. To address molecular mechanisms underlying the pathogenesis of PBDs, mouse models for the PBDs have been generated by targeted disruption of Pex genes. Pathological phenotypes and metabolic abnormalities in Pex-knockout mice well resemble those of the patients with PBDs. The mice with tissue- or cell type-specific inactivation of Pex genes have also been established by using a Cre-loxP system. The genetically modified mice reveal that pathological phenotypes of PBDs are mediated by interorgan and intercellular communications. Despite the illustrations of detailed pathological phenotypes in the mutant mice, mechanistic insights into pathogenesis of PBDs are still underway. In this chapter, we overview the phenotypes of Pex-inactivated mice and the current understanding of the pathogenesis underlying PBDs.

A Peptide Link Between Human Cytomegalovirus Infection, Neuronal Migration, and Psychosis

Alongside biological, psychological, and social risk factors, psychotic syndromes may be related to disturbances of neuronal migration. This highly complex process characterizes the developing brain of the fetus, the early postnatal brain, and the adult brain, as reflected by changes within the subventricular zone and the dentate gyrus of the hippocampus, where neurogenesis persists throughout life. Psychosis also appears to be linked to human cytomegalovirus (HCMV) infection. However, little is known about the connection between psychosis, HCMV infection, and disruption of neuronal migration. The present study addresses the hypothesis that HCMV infection may lead to mental disorders through mechanisms of autoimmune cross-reactivity. Searching for common peptides that underlie immune cross-reactions, the analyses focus on HCMV and human proteins involved in neuronal migration. Results demonstrate a large overlap of viral peptides with human proteins associated with neuronal migration, such as ventral anterior homeobox 1 and cell adhesion molecule 1 implicated in GABAergic and glutamatergic neurotransmission. The present findings support the possibility of immune cross-reactivity between HCMV and human proteins that-when altered, mutated, or improperly functioning-may disrupt normal neuronal migration. In addition, these findings are consistent with a molecular and mechanistic framework for pathological sequences of events, beginning with HCMV infection, followed by immune activation, cross-reactivity, and neuronal protein variations that may ultimately contribute to the emergence of mental disorders, including psychosis.

Neurogenesis, neuronal migration, and axon guidance

Development of the central nervous system (CNS) is a complex, dynamic process that involves a precisely orchestrated sequence of genetic, environmental, biochemical, and physical factors from early embryonic stages to postnatal life. Duringthe past decade, great strides have been made to unravel mechanisms underlying human CNS development through the employment of modern genetic techniques and experimental approaches. In this chapter, we review the current knowledge regarding the main developmental processes and signaling mechanisms of (i) neurogenesis, (ii) neuronal migration, and (iii) axon guidance. We discuss mechanisms related to neural stem cells proliferation, migration, terminal translocation of neuronal progenitors, and axon guidance and pathfinding. For each section, we also provide a comprehensive overview of the underlying regulatory processes, including transcriptional, posttranscriptional, and epigenetic factors, and a myriad of signaling pathways that are pivotal to determine the fate of neuronal progenitors and newly formed migrating neurons. We further highlight how impairment of this complex regulating system, such as mutations in its core components, may cause cortical malformation, epilepsy, intellectual disability, and autism in humans. A thorough understanding of normal human CNS development is thus crucial to decipher mechanisms responsible for neurodevelopmental disorders and in turn guide the development of effective and targeted therapeutic strategies.

Terminal neuron localization to the upper cortical plate is controlled by the transcription factor NEUROD2

Excitatory neurons of the mammalian cerebral cortex are organized into six functional layers characterized by unique patterns of connectivity, as well as distinctive physiological and morphological properties. Cortical layers appear after a highly regulated migration process in which cells move from the deeper, proliferative zone toward the superficial layers. Importantly, defects in this radial migration process have been implicated in neurodevelopmental and psychiatric diseases. Here we report that during the final stages of migration, transcription factor Neurogenic Differentiation 2 (Neurod2) contributes to terminal cellular localization within the cortical plate. In mice, in utero knockdown of Neurod2 resulted in reduced numbers of neurons localized to the uppermost region of the developing cortex, also termed the primitive cortical zone. Our ChIP-Seq and RNA-Seq analyses of genes regulated by NEUROD2 in the developing cortex identified a number of key target genes with known roles in Reelin signaling, a critical regulator of neuronal migration. Our focused analysis of regulation of the Reln gene, encoding the extracellular ligand REELIN, uncovered NEUROD2 binding to conserved E-box elements in multiple introns. Furthermore, we demonstrate that knockdown of NEUROD2 in primary cortical neurons resulted in a strong increase in Reln gene expression at the mRNA level, as well as a slight upregulation at the protein level. These data reveal a new role for NEUROD2 during the late stages of neuronal migration, and our analysis of its genomic targets offers new genes with potential roles in cortical lamination.

Semaphorin 6A-Plexin A2/A4 Interactions with Radial Glia Regulate Migration Termination of Superficial Layer Cortical Neurons

Precise regulation of neuronal migration termination is crucial for the establishment of brain cytoarchitectures. However, little is known about how neurons terminate migration. Here we focused on interactions between migrating cortical neurons and their substrates, radial glial (RG) cells, and analyzed the role of Plexin A2 and A4 (PlxnA2/A4) receptors and their repulsive ligand, Semaphorin 6A (Sema6A), for this process. In both PlxnA2/A4 double-knockout and Sema6A mutant mice, the outermost cortical plate neurons ectopically invade layer 1 at a stage when they should reach their destinations. PlxnA2/A4 proteins are abundantly expressed on their leading processes, whereas Sema6A mRNA is enriched in RG cell somata. Cell-targeted gene expression and conditional knockouts indicate critical roles for these molecules. We hypothesize that the timely appearance of repulsive signaling mediated by Sema6A-PlxnA2/A4 weakens migrating neuron-RG cell interactions, leading to migration termination.

Dynein activating adaptor BICD2 controls radial migration of upper-layer cortical neurons in vivo

For the proper organization of the six-layered mammalian neocortex it is required that neurons migrate radially from their place of birth towards their designated destination. The molecular machinery underlying this neuronal migration is still poorly understood. The dynein-adaptor protein BICD2 is associated with a spectrum of human neurological diseases, including malformations of cortical development. Previous studies have shown that knockdown of BICD2 interferes with interkinetic nuclear migration in radial glial progenitor cells, and that Bicd2-deficient mice display an altered laminar organization of the cerebellum and the neocortex. However, the precise in vivo role of BICD2 in neocortical development remains unclear. By comparing cell-type specific conditional Bicd2 knock-out mice, we found that radial migration in the cortex predominantly depends on BICD2 function in post-mitotic neurons. Neuron-specific Bicd2 cKO mice showed severely impaired radial migration of late-born upper-layer neurons. BICD2 depletion in cortical neurons interfered with proper Golgi organization, and neuronal maturation and survival of cortical plate neurons. Single-neuron labeling revealed a specific role of BICD2 in bipolar locomotion. Rescue experiments with wildtype and disease-related mutant BICD2 constructs revealed that a point-mutation in the RAB6/RANBP2-binding-domain, associated with cortical malformation in patients, fails to restore proper cortical neuron migration. Together, these findings demonstrate a novel, cell-intrinsic role of BICD2 in cortical neuron migration in vivo and provide new insights into BICD2-dependent dynein-mediated functions during cortical development.

Cortical Neuron Migration and Dendrite Morphology are Regulated by Carboxypeptidase E

Higher brain function relies on proper development of the cerebral cortex, including correct positioning of neurons and dendrite morphology. Disruptions in these processes may result in various neurocognitive disorders. Mutations in the CPE gene, which encodes carboxypeptidase E (CPE), have been linked to depression and intellectual disability. However, it remains unclear whether CPE is involved in early brain development and in turn contributes to the pathophysiology of neurocognitive disorders. Here, we investigate the effects of CPE knockdown on early brain development and explore the functional significance of the interaction between CPE and its binding partner p150Glued. We demonstrate that CPE is required for cortical neuron migration and dendrite arborization. Furthermore, we show that expression of CPE-C10 redistributes p150Glued from the centrosome and that disruption of CPE interaction with p150Glued leads to abnormal neuronal migration and dendrite morphology, suggesting that a complex between CPE and p150Glued is necessary for proper neurodevelopment.

Spatiotemporal Regulation of Rho GTPases in Neuronal Migration

Neuronal migration is essential for the orchestration of brain development and involves several contiguous steps: interkinetic nuclear movement (INM), multipolar–bipolar transition, locomotion, and translocation. Growing evidence suggests that Rho GTPases, including RhoA, Rac, Cdc42, and the atypical Rnd members, play critical roles in neuronal migration by regulating both actin and microtubule cytoskeletal components. This review focuses on the spatiotemporal-specific regulation of Rho GTPases as well as their regulators and effectors in distinct steps during the neuronal migration process. Their roles in bridging extracellular signals and cytoskeletal dynamics to provide optimal structural support to the migrating neurons will also be discussed.

Linking Cell Polarity to Cortical Development and Malformations

Cell polarity refers to the asymmetric distribution of signaling molecules, cellular organelles, and cytoskeleton in a cell. Neural progenitors and neurons are highly polarized cells in which the cell membrane and cytoplasmic components are compartmentalized into distinct functional domains in response to internal and external cues that coordinate polarity and behavior during development and disease. In neural progenitor cells, polarity has a prominent impact on cell shape and coordinate several processes such as adhesion, division, and fate determination. Polarity also accompanies a neuron from the beginning until the end of its life. It is essential for development and later functionality of neuronal circuitries. During development, polarity governs transitions between multipolar and bipolar during migration of postmitotic neurons, and directs the specification and directional growth of axons. Once reaching final positions in cortical layers, neurons form dendrites which become compartmentalized to ensure proper establishment of neuronal connections and signaling. Changes in neuronal polarity induce signaling cascades that regulate cytoskeletal changes, as well as mRNA, protein, and vesicle trafficking, required for synapses to form and function. Hence, defects in establishing and maintaining cell polarity are associated with several neural disorders such as microcephaly, lissencephaly, schizophrenia, autism, and epilepsy. In this review we summarize the role of polarity genes in cortical development and emphasize the relationship between polarity dysfunctions and cortical malformations.

Promyelocytic leukemia zinc finger is involved in the formation of deep layer cortical neurons

BACKGROUND

Promyelocytic leukemia zinc finger (Plzf), a transcriptional regulator involved in a lot of important biological processes during development, has been implied to maintain neural stem cells and inhibit their differentiation into neurons. However, the effects of Plzf on brain structures and functions are still not clarified.

RESULTS

We showed that Plzf expression was detected as early as embryonic day (E) 9.5 in Pax6⁺ cells in the mouse brain, and was completely disappeared in telencephalon before the initiation of cortical neurogenesis. Loss of Plzf resulted in a smaller cerebral cortex with a decrease in the number of Tbr1⁺ deep layer neurons due to a decrease of mitotic cell number in the ventricular zone of forebrain at early developmental stage. Microarray, qRT-PCR, and flow cytometry analysis identified dysregulation of Mash1 proneural gene expression. We also observed an impairment of recognition memory in Plzf-deficient mice.

CONCLUSIONS

Plzf is expressed at early stages of brain development and involved in the formation of deep layer cortical neurons. Loss of Plzf results in dysregulation of Mash1, microcephaly with reduced numbers of early-born neurons, and impairment of recognition memory.

Regulation of brain development and brain function by the transcriptional repressor RP58

The mechanisms regulating the formation of the cerebral cortex have been well studied. In the developing cortex, (also known Znf238, Zfp238, and Zbtb18), which encodes a sequence-specific transcriptional repressor, is expressed in glutamatergic projection neurons and progenitor cells. Targeted deletion of Rp58 leads to dysplasia of the neocortex and hippocampus, a reduction in the number of mature cortical neurons, and defects in laminar organization due to abnormal neuronal migration within the cortical plate. During late embryogenesis, Rp58-deficient mice have larger numbers of progenitor cells due to a delay in cell cycle exit. RP58 represses all four Id genes (Id1-Id4), which regulate cell cycle exit in the developing cerebral cortex, and is essential for transcriptional repression of Ngn2 and Rnd2, which regulate the multipolar-to-bipolar transition during neuronal migration independently of its role in cell cycle exit.

Estradiol and the Development of the Cerebral Cortex: An Unexpected Role?

The cerebral cortex undergoes rapid folding in an "inside-outside" manner during embryonic development resulting in the establishment of six discrete cortical layers. This unique cytoarchitecture occurs via the coordinated processes of neurogenesis and cell migration. In addition, these processes are fine-tuned by a number of extracellular cues, which exert their effects by regulating intracellular signaling pathways. Interestingly, multiple brain regions have been shown to develop in a sexually dimorphic manner. In many cases, estrogens have been demonstrated to play an integral role in mediating these sexual dimorphisms in both males and females. Indeed, 17β-estradiol, the main biologically active estrogen, plays a critical organizational role during early brain development and has been shown to be pivotal in the sexually dimorphic development and regulation of the neural circuitry underlying sex-typical and socio-aggressive behaviors in males and females. However, whether and how estrogens, and 17β-estradiol in particular, regulate the development of the cerebral cortex is less well understood. In this review, we outline the evidence that estrogens are not only present but are engaged and regulate molecular machinery required for the fine-tuning of processes central to the cortex. We discuss how estrogens are thought to regulate the function of key molecular players and signaling pathways involved in corticogenesis, and where possible, highlight if these processes are sexually dimorphic. Collectively, we hope this review highlights the need to consider how estrogens may influence the development of brain regions directly involved in the sex-typical and socio-aggressive behaviors as well as development of sexually dimorphic regions such as the cerebral cortex.

The Candidate Schizophrenia Risk Gene DGCR2 Regulates Early Steps of Corticogenesis

BACKGROUND

Alterations in early steps of cortical circuit assembly are thought to play a critical role in vulnerability to schizophrenia (SZ), but the pathogenic impact of SZ-risk mutations on corticogenesis remains to be determined. DiGeorge syndrome critical region 2 (DGCR2) is located in the 22q11.2 locus, whose deletion is a major risk factor for SZ. Moreover, exome sequencing of individuals with idiopathic SZ identified a rare missense mutation in DGCR2, further suggesting that DGCR2 is involved in SZ.

METHODS

Here we investigated the function of Dgcr2 and the pathogenic impact of the SZ-risk DGCR2 mutation in mouse corticogenesis using in utero electroporation targeted to projection neurons.

RESULTS

Dgcr2 knockdown impaired radial locomotion and final translocation of projection neurons, leading to persistent laminar positioning alterations. The DGCR2 missense SZ-risk mutation had a pathogenic impact on projection neuron laminar allocation by reducing protein expression. Mechanistically, we identified Dgcr2 as a novel member of the Reelin complex, regulating the phosphorylation of Reelin-dependent substrates and the expression of Reelin-dependent transcriptional targets.

CONCLUSIONS

Overall, this study provides biological evidence that the SZ-risk gene DGCR2 regulates critical steps of early corticogenesis possibly through a Reelin-dependent mechanism. Additionally, we found that the SZ-risk mutation in DGCR2 has a pathogenic impact on cortical formation by reducing protein expression level, suggesting a functional role for DGCR2 haploinsufficiency in the 22q11.2 deletion syndrome.