Coordinating cerebral cortical construction and connectivity: Unifying influence of radial progenitors

New aspects of a small GTPase RAB35 in brain development and function

In eukaryotic cells, organelles in the secretory, lysosomal, and endocytic pathways actively exchange biological materials with each other through intracellular membrane trafficking, which is the process of transporting the cargo of proteins, lipids, and other molecules to appropriate compartments via transport vesicles or intermediates. These processes are strictly regulated by various small GTPases such as the RAS-like in rat brain (RAB) protein family, which is the largest subfamily of the RAS superfamily. Dysfunction of membrane trafficking affects tissue homeostasis and leads to a wide range of diseases, including neurological disorders and neurodegenerative diseases. Therefore, it is important to understand the physiological and pathological roles of RAB proteins in brain function. RAB35, a member of the RAB family, is an evolutionarily conserved protein in metazoans. A wide range of studies using cultured mammalian cells and model organisms have revealed that RAB35 mediates various processes such as cytokinesis, endocytic recycling, actin bundling, and cell migration. RAB35 is also involved in neurite outgrowth and turnover of synaptic vesicles. We generated brain-specific Rab35 knockout mice to study the physiological roles of RAB35 in brain development and function. These mice exhibited defects in anxiety-related behaviors and spatial memory. Strikingly, RAB35 is required for the precise positioning of pyramidal neurons during hippocampal development, and thereby for normal hippocampal lamination. In contrast, layer formation in the cerebral cortex occurred superficially, even in the absence of RAB35, suggesting a predominant role for RAB35 in hippocampal development rather than in cerebral cortex development. Recent studies have suggested an association between RAB35 and neurodegenerative diseases, including Parkinson's disease and Alzheimer's disease. In this review, we provide an overview of the current understanding of subcellular functions of RAB35. We also provide insights into the physiological role of RAB35 in mammalian brain development and function, and discuss the involvement of RAB35 dysfunction in neurodegenerative diseases.

β-PIX-d, a Member of the ARHGEF7 Guanine Nucleotide Exchange Factor Family, Activates Rac1 and Induces Neuritogenesis in Primary Cortical Neurons

β-PIX, a Rac1/Cdc42-specific guanine nucleotide exchange factor, is known to regulate actin cytoskeleton remodeling during cell migration. In this study, we investigated the effects of β-PIX-d, an isoform of β-PIX, on neocortical development and neuritogenesis. Overexpression of β-PIX-d in the embryonic neocortex induced increased cell clusters and enhanced neurite outgrowth in cortical neurons. Following in utero electroporation of β-PIX-d expression vectors into neuronal progenitor cells at embryonic day 13.5 (E13.5), histological analysis at postnatal day 0 (P0) revealed the presence of clustered neurons and neurites outside of the marginal zone (MZ). Immunofluorescence staining with the neuronal marker TuJ1 confirmed that the clustered structures were predominantly composed of neurons. Layer-specific marker analysis further demonstrated the misplacement of layer V-VI neurons into layer I and the subarachnoid space. In primary neocortical cultures, β-PIX-d overexpression promoted neuritogenesis and increased Rac1 activity, as detected by pull-down assays. These findings suggest that β-PIX-d and Rac1 interactions play a critical role in the formation of neocortical clustering and the regulation of neuritogenesis.

Brain development and bioenergetic changes

Bioenergetics describe the biochemical processes responsible for energy supply in organisms. When these changes become dysregulated in brain development, multiple neurodevelopmental diseases can occur, implicating bioenergetics as key regulators of neural development. Historically, the discovery of disease processes affecting individual stages of brain development has revealed critical roles that bioenergetics play in generating the nervous system. Bioenergetic-dependent neurodevelopmental disorders include neural tube closure defects, microcephaly, intellectual disability, autism spectrum disorders, epilepsy, mTORopathies, and oncogenic processes. Developmental timing and cell-type specificity of these changes determine the long-term effects of bioenergetic disease mechanisms on brain form and function. Here, we discuss key metabolic regulators of neural progenitor specification, neuronal differentiation (neurogenesis), and gliogenesis. In general, transitions between glycolysis and oxidative phosphorylation are regulated in early brain development and in oncogenesis, and reactive oxygen species (ROS) and mitochondrial maturity play key roles later in differentiation. We also discuss how bioenergetics interface with the developmental regulation of other key neural elements, including the cerebrospinal fluid brain environment. While questions remain about the interplay between bioenergetics and brain development, this review integrates the current state of known key intersections between these processes in health and disease.

Morphological classification of radial glia-like cells in the postnatal mouse subventricular zone

The subventricular zone (SVZ) is one of the neurogenic regions of the adult mammalian brain. Neural stem cells (NSCs) in the SVZ have certain key features: they express glial fibrillary acidic protein (GFAP), proliferate slowly, have a radial glia-like (RG-L) morphology, and are in contact with the cerebrospinal fluid (CSF). NSCs have been isolated by FACS to analyse them, but their morphology has not been systematically examined. To address this knowledge gap, we sparsely labelled RG-L cells in the SVZ of neonatal mice by introducing via electroporation a plasmid expressing fluorescent protein under the control of the GFAP promoter. We then classified RG-L cells into three types (RG-L1, 2, and 3) based on their morphologies. RG-L1 cells had a basal process with some branches and numerous fine processes. RG-L2 cells had a basal process, but fewer branches and fine processes than RG-L1 cells. RG-L3 cells had one basal process that was almost free of branches and fine processes. Importantly, regardless of the cell type, about half of their somata resided on the basal side of the SVZ. Based on changes in their proportions during postnatal development and their expression of GFAP and cell proliferation markers at the adult stage, we speculated that NSCs change their morphologies during development/maturation and not all NSCs must always be in the apical SVZ or in contact with the CSF. Our results indicate that in addition to expression of markers for NSCs, the morphology is a critical feature to identify NSCs.

Dynamic changes in mitochondrial localization in human neocortical basal radial glial cells during cell cycle

Mitochondria play critical roles in neural stem/progenitor cell proliferation and fate decisions. The subcellular localization of mitochondria in neural stem/progenitor cells during mitosis potentially influences the distribution of mitochondria to the daughter cells and thus their fates. Therefore, understanding the spatial dynamics of mitochondria provides important knowledge about brain development. In this study, we analyzed the subcellular localization of mitochondria in the fetal human neocortex with a particular focus on the basal radial glial cells (bRGCs), a neural stem/progenitor cell subtype attributed to the evolutionary expansion of the human neocortex. During interphase, bRGCs exhibit a polarized localization of mitochondria that is localized at the base of the process or the proximal part of the process. Thereafter, mitochondria in bRGCs at metaphase show unpolarized distribution in which the mitochondria are randomly localized in the cytoplasm. During anaphase and telophase, mitochondria are still localized evenly, but mainly in the periphery of the cytoplasm. Mitochondria start to accumulate at the cleavage furrow during cytokinesis. These results suggest that the mitochondrial localization in bRGCs is tightly regulated during the cell cycle, which may ensure the proper distribution of mitochondria to the daughter cells and, thus in turn, influence their fates.

ESCRT-I protein UBAP1 controls ventricular expansion and cortical neurogenesis via modulating adherens junctions of radial glial cells

Intricate cerebral cortex formation is orchestrated by the precise behavior and division dynamics of radial glial cells (RGCs). Endocytosis functions in the recycling and remodeling of adherens junctions (AJs) in response to changes in RGC activity and function. Here, we show that conditional disruption of ubiquitin-associated protein 1 (UBAP1), a component of endosomal sorting complex required for transport (ESCRT), causes severe brain dysplasia and prenatal ventriculomegaly. UBAP1 depletion disrupts the AJs and polarity of RGCs, leading to failure of apically directed interkinetic nuclear migration. Accordingly, UBAP1 knockout or knockdown results in reduced proliferation and precocious differentiation of neural progenitor cells. Mechanistically, UBAP1 regulates the expression and surface localization of cell adhesion molecules, and β-catenin over-expression significantly rescues the phenotypes of Ubap1 knockdown in vivo. Our study reveals a critical physiological role of the ESCRT machinery in cortical neurogenesis by regulating AJs of RGCs.

Development and evolution of the primate neocortex from a progenitor cell perspective

The generation of neurons in the developing neocortex is a major determinant of neocortex size. Crucially, the increase in cortical neuron numbers in the primate lineage, notably in the upper-layer neurons, contributes to increased cognitive abilities. Here, we review major evolutionary changes affecting the apical progenitors in the ventricular zone and focus on the key germinal zone constituting the foundation of neocortical neurogenesis in primates, the outer subventricular zone (OSVZ). We summarize characteristic features of the OSVZ and its key stem cell type, the basal (or outer) radial glia. Next, we concentrate on primate-specific and human-specific genes, expressed in OSVZ-progenitors, the ability of which to amplify these progenitors by targeting the regulation of the cell cycle ultimately underlies the evolutionary increase in upper-layer neurons. Finally, we address likely differences in neocortical development between present-day humans and Neanderthals that are based on human-specific amino acid substitutions in proteins operating in cortical progenitors.

Neuroestradiol and neuronal development: Not an exclusive male tale anymore

The brain synthesizes a variety of neurosteroids, including neuroestradiol. Inhibition of neuroestradiol synthesis results in alterations in basic neurodevelopmental processes, such as neurogenesis, neuroblast migration, neuritogenesis and synaptogenesis. Although the neurodevelopmental actions of neuroestradiol are exerted in both sexes, some of them are sex-specific, such as the well characterized effects of neuroestradiol derived from the metabolism of testicular testosterone during critical periods of male brain development. In addition, recent findings have shown sex-specific actions of neuroestradiol on neuroblast migration, neuritic growth and synaptogenesis in females. Among other factors, the epigenetic regulation exerted by X linked genes, such as Kdm6a/Utx, may determine sex-specific actions of neuroestradiol in the female brain. This review evidences the impact of neuroestradiol on brain formation in both sexes and highlights the interaction of neural steriodogenesis, hormones and sex chromosomes in sex-specific brain development.

Subcellular mRNA localization and local translation of Arhgap11a in radial glial progenitors regulates cortical development

mRNA localization and local translation enable exquisite spatial and temporal control of gene expression, particularly in polarized, elongated cells. These features are especially prominent in radial glial cells (RGCs), which are neural and glial precursors of the developing cerebral cortex and scaffolds for migrating neurons. Yet the mechanisms by which subcellular RGC compartments accomplish their diverse functions are poorly understood. Here, we demonstrate that mRNA localization and local translation of the RhoGAP ARHGAP11A in the basal endfeet of RGCs control their morphology and mediate neuronal positioning. Arhgap11a transcript and protein exhibit conserved localization to RGC basal structures in mice and humans, conferred by the 5' UTR. Proper RGC morphology relies upon active Arhgap11a mRNA transport and localization to the basal endfeet, where ARHGAP11A is locally synthesized. This translation is essential for positioning interneurons at the basement membrane. Thus, local translation spatially and acutely activates Rho signaling in RGCs to compartmentalize neural progenitor functions.

Embryonic mouse medial neocortex as a model system for studying the radial glial scaffold in fetal human neocortex

Neocortex is the evolutionarily newest region in the brain, and is a structure with diversified size and morphology among mammalian species. Humans have the biggest neocortex compared to the body size, and their neocortex has many foldings, that is, gyri and sulci. Despite the recent methodological advances in in vitro models such as cerebral organoids, mice have been continuously used as a model system for studying human neocortical development because of the accessibility and practicality of in vivo gene manipulation. The commonly studied neocortical region, the lateral neocortex, generally recapitulates the developmental process of the human neocortex, however, there are several important factors missing in the lateral neocortex. First, basal (outer) radial glia (bRG), which are the main cell type providing the radial scaffold to the migrating neurons in the fetal human neocortex, are very few in the mouse lateral neocortex, thus the radial glial scaffold is different from the fetal human neocortex. Second, as a consequence of the difference in the radial glial scaffold, migrating neurons might exhibit different migratory behavior and thus distribution. To overcome those problems, we propose the mouse medial neocortex, where we have earlier revealed an abundance of bRG similar to the fetal human neocortex, as an alternative model system. We found that similar to the fetal human neocortex, the radial glial scaffold, neuronal migration and neuronal distribution are tangentially scattered in the mouse medial neocortex. Taken together, the embryonic mouse medial neocortex could be a suitable and accessible in vivo model system to study human neocortical development and its pathogenesis.

Evaluation of advances in cortical development using model systems

Compared with that of even the closest primates, the human cortex displays a high degree of specialization and expansion that largely emerges developmentally. Although decades of research in the mouse and other model systems has revealed core tenets of cortical development that are well preserved across mammalian species, small deviations in transcription factor expression, novel cell types in primates and/or humans, and unique cortical architecture distinguish the human cortex. Importantly, many of the genes and signaling pathways thought to drive human-specific cortical expansion also leave the brain vulnerable to disease, as the misregulation of these factors is highly correlated with neurodevelopmental and neuropsychiatric disorders. However, creating a comprehensive understanding of human-specific cognition and disease remains challenging. Here, we review key stages of cortical development and highlight known or possible differences between model systems and the developing human brain. By identifying the developmental trajectories that may facilitate uniquely human traits, we highlight open questions in need of approaches to examine these processes in a human context and reveal translatable insights into human developmental disorders.

Pediatric Brain Maturation and Migration Disorders

Neurodevelopmental disorders, including neuronal migration disorders, are best understood in the context of altered normal development. Neurons normally migrate from their site of origin to their (usually cortical) destination using a wide range of molecular and cellular signaling as a guide. In the case of abnormal migration neurons: (1) do not migrate and remain at their site of origin; (2) incompletely migrate and remain within the white matter; (3) migrate to the cortex but fail to organize correctly; or (4) over-migrate, beyond the cortex. In this review, we discuss normal brain development, along with the malformations that result from these different migration abnormalities.

Integration of structural MRI and epigenetic analyses hint at linked cellular defects of the subventricular zone and insular cortex in autism: Findings from a case study

Introduction

Autism Spectrum Disorder (ASD) is a neurodevelopmental disorder characterized by deficits in social interaction, communication and repetitive, restrictive behaviors, features supported by cortical activity. Given the importance of the subventricular zone (SVZ) of the lateral ventrical to cortical development, we compared molecular, cellular, and structural differences in the SVZ and linked cortical regions in specimens of ASD cases and sex and age-matched unaffected brain.

Methods

We used magnetic resonance imaging (MRI) and diffusion tractography on ex vivo postmortem brain samples, which we further analyzed by Whole Genome Bisulfite Sequencing (WGBS), Flow Cytometry, and RT qPCR.

Results

Through MRI, we observed decreased tractography pathways from the dorsal SVZ, increased pathways from the posterior ventral SVZ to the insular cortex, and variable cortical thickness within the insular cortex in ASD diagnosed case relative to unaffected controls. Long-range tractography pathways from and to the insula were also reduced in the ASD case. FACS-based cell sorting revealed an increased population of proliferating cells in the SVZ of ASD case relative to the unaffected control. Targeted qPCR assays of SVZ tissue demonstrated significantly reduced expression levels of genes involved in differentiation and migration of neurons in ASD relative to the control counterpart. Finally, using genome-wide DNA methylation analyses, we identified 19 genes relevant to neurological development, function, and disease, 7 of which have not previously been described in ASD, that were significantly differentially methylated in autistic SVZ and insula specimens.

Conclusion

These findings suggest a hypothesis that epigenetic changes during neurodevelopment alter the trajectory of proliferation, migration, and differentiation in the SVZ, impacting cortical structure and function and resulting in ASD phenotypes.