Regulation of cerebral cortex development by Rho GTPases: insights from in vivo studies

F-BAR proteins CIP4 and FBP17 function in cortical neuron radial migration and process outgrowth

Neurite initiation from newly born neurons is a critical step in neuronal differentiation and migration. Neuronal migration in the developing cortex is accompanied by dynamic extension and retraction of neurites as neurons progress through bipolar and multipolar states. However, there is a relative lack of understanding regarding how the dynamic extension and retraction of neurites is regulated during neuronal migration. In recent work we have shown that CIP4, a member of the F-BAR family of membrane bending proteins, inhibits cortical neurite formation in culture, while family member FBP17 induces premature neurite outgrowth. These results beg the question of how CIP4 and FBP17 function in radial neuron migration and differentiation in vivo, including the timing and manner of neurite extension and retraction. Indeed, the regulation of neurite outgrowth is essential for the transitions between bipolar and multipolar states during radial migration. To examine the effects of modulating expression of CIP4 and FBP17 in vivo, we used in utero electroporation, in combination with our published Double UP technique, to compare knockdown or overexpression cells with control cells within the same mouse tissue of either sex. We show that either knockdown or overexpression of CIP4 and FBP17 results in the marked disruption of radial neuron migration by modulating neuronal morphology and neurite outgrowth, consistent with our findings in culture. Our results demonstrate that the F-BAR proteins CIP4 and FBP17 are essential for proper radial migration in the developing cortex and thus play a key role in cortical development.

Gnao1 is a molecular switch that regulates the Rho signaling pathway in differentiating neurons

GNAO1 encodes G protein subunit alpha O1 (Gαo). Pathogenic variations in GNAO1 cause developmental delay, intractable seizures, and progressive involuntary movements from early infancy. Because the functional role of GNAO1 in the developing brain remains unclear, therapeutic strategies are still unestablished for patients presenting with GNAO1-associated encephalopathy. We herein report that siRNA-mediated depletion of Gnao1 perturbs the expression of transcripts associated with Rho GTPase signaling in Neuro2a cells. Consistently, siRNA treatment hampered neurite outgrowth and extension. Growth cone formation was markedly disrupted in monolayer neurons differentiated from iPSCs from a patient with a pathogenic variant of Gαo (p.G203R). This variant disabled neuro-spherical assembly, acquisition of the organized structure, and polarized signals of phospho-MLC2 in cortical organoids from the patient's iPSCs. We confirmed that the Rho kinase inhibitor Y27632 restored these morphological phenotypes. Thus, Gαo determines the self-organizing process of the developing brain by regulating the Rho-associated pathway. These data suggest that Rho GTPase pathway might be an alternative target of therapy for patients with GNAO1-associated encephalopathy.

Interneuron odyssey: molecular mechanisms of tangential migration

Cortical GABAergic interneurons are critical components of neural networks. They provide local and long-range inhibition and help coordinate network activities involved in various brain functions, including signal processing, learning, memory and adaptative responses. Disruption of cortical GABAergic interneuron migration thus induces profound deficits in neural network organization and function, and results in a variety of neurodevelopmental and neuropsychiatric disorders including epilepsy, intellectual disability, autism spectrum disorders and schizophrenia. It is thus of paramount importance to elucidate the specific mechanisms that govern the migration of interneurons to clarify some of the underlying disease mechanisms. GABAergic interneurons destined to populate the cortex arise from multipotent ventral progenitor cells located in the ganglionic eminences and pre-optic area. Post-mitotic interneurons exit their place of origin in the ventral forebrain and migrate dorsally using defined migratory streams to reach the cortical plate, which they enter through radial migration before dispersing to settle in their final laminar allocation. While migrating, cortical interneurons constantly change their morphology through the dynamic remodeling of actomyosin and microtubule cytoskeleton as they detect and integrate extracellular guidance cues generated by neuronal and non-neuronal sources distributed along their migratory routes. These processes ensure proper distribution of GABAergic interneurons across cortical areas and lamina, supporting the development of adequate network connectivity and brain function. This short review summarizes current knowledge on the cellular and molecular mechanisms controlling cortical GABAergic interneuron migration, with a focus on tangential migration, and addresses potential avenues for cell-based interneuron progenitor transplants in the treatment of neurodevelopmental disorders and epilepsy.

Bilirubin-Induced Transcriptomic Imprinting in Neonatal Hyperbilirubinemia

Recent findings indicated aberrant epigenetic control of the central nervous system (CNS) development in hyperbilirubinemic Gunn rats as an additional cause of cerebellar hypoplasia, the landmark of bilirubin neurotoxicity in rodents. Because the symptoms in severely hyperbilirubinemic human neonates suggest other regions as privileged targets of bilirubin neurotoxicity, we expanded the study of the potential impact of bilirubin on the control of postnatal brain development to regions correlating with human symptoms. Histology, transcriptomic, gene correlation, and behavioral studies were performed. The histology revealed widespread perturbation 9 days after birth, restoring in adulthood. At the genetic level, regional differences were noticed. Bilirubin affected synaptogenesis, repair, differentiation, energy, extracellular matrix development, etc., with transient alterations in the hippocampus (memory, learning, and cognition) and inferior colliculi (auditory functions) but permanent changes in the parietal cortex. Behavioral tests confirmed the presence of a permanent motor disability. The data correlate well both with the clinic description of neonatal bilirubin-induced neurotoxicity, as well as with the neurologic syndromes reported in adults that suffered neonatal hyperbilirubinemia. The results pave the way for better deciphering the neurotoxic features of bilirubin and evaluating deeply the efficacy of new therapeutic approaches against the acute and long-lasting sequels of bilirubin neurotoxicity.

Therapeutic strategies for autism: targeting three levels of the central dogma of molecular biology

The past decade has yielded much success in the identification of risk genes for Autism Spectrum Disorder (ASD), with many studies implicating loss-of-function (LoF) mutations within these genes. Despite this, no significant clinical advances have been made so far in the development of therapeutics for ASD. Given the role of LoF mutations in ASD etiology, many of the therapeutics in development are designed to rescue the haploinsufficient effect of genes at the transcriptional, translational, and protein levels. This review will discuss the various therapeutic techniques being developed from each level of the central dogma with examples including: CRISPR activation (CRISPRa) and gene replacement at the DNA level, antisense oligonucleotides (ASOs) at the mRNA level, and small-molecule drugs at the protein level, followed by a review of current delivery methods for these therapeutics. Since central nervous system (CNS) penetrance is of utmost importance for ASD therapeutics, it is especially necessary to evaluate delivery methods that have higher efficiency in crossing the blood-brain barrier (BBB).

Rac-deficient cerebellar granule neurons die before they migrate to the internal granule layer

Granule neurons are the most common cell type in the cerebellum. They are generated in the external granule layer and migrate inwardly, forming the internal granule layer. Small Rho GTPases play various roles during development of the nervous system and may be involved in generation, differentiation and migration of granule neurons. We deleted Rac1, a member of small Rho GTPases, by GFAP-Cre driver in cerebellar granule neurons and Bergmann glial cells. Rac1^flox/flox; Cre mice showed impaired migration and slight reduction in the number of granule neurons in the internal granule layer. Deletion of both Rac1 and Rac3 resulted in almost complete absence of granule neurons. Rac-deficient granule neurons differentiated into p27 and NeuN-expressing post mitotic neurons, but died before migration to the internal granule layer. Loss of Rac3 has little effect on granule neuron development. Rac1^flox/flox; Rac3^+/−; Cre mice showed intermediate phenotype between Rac1^flox/flox; Cre and Rac1^flox/flox; Rac3^−/−; Cre mice in both survival and migration of granule neurons. Rac3 itself seems to be unimportant in the development of the cerebellum, but has some roles in Rac1-deleted granule neurons. Conversely, overall morphology of Rac1^+/flox; Rac3^−/−; Cre cerebella was normal. One allele of Rac1 is therefore thought to be sufficient to promote development of cerebellar granule neurons.

Variant-specific changes in RAC3 function disrupt corticogenesis in neurodevelopmental phenotypes

Variants in RAC3, encoding a small GTPase RAC3 which is critical for the regulation of actin cytoskeleton and intracellular signal transduction, are associated with a rare neurodevelopmental disorder with structural brain anomalies and facial dysmorphism. We investigated a cohort of 10 unrelated participants presenting with global psychomotor delay, hypotonia, behavioural disturbances, stereotyped movements, dysmorphic features, seizures and musculoskeletal abnormalities. MRI of brain revealed a complex pattern of variable brain malformations, including callosal abnormalities, white matter thinning, grey matter heterotopia, polymicrogyria/dysgyria, brainstem anomalies and cerebellar dysplasia. These patients harboured eight distinct de novo RAC3 variants, including six novel variants (NM_005052.3): c.34G > C p.G12R, c.179G > A p.G60D, c.186_188delGGA p.E62del, c.187G > A p.D63N, c.191A > G p.Y64C and c.348G > C p.K116N. We then examined the pathophysiological significance of these novel and previously reported pathogenic variants p.P29L, p.P34R, p.A59G, p.Q61L and p.E62K. In vitro analyses revealed that all tested RAC3 variants were biochemically and biologically active to variable extent, and exhibited a spectrum of different affinities to downstream effectors including p21-activated kinase 1. We then focused on the four variants p.Q61L, p.E62del, p.D63N and p.Y64C in the Switch II region, which is essential for the biochemical activity of small GTPases and also a variation hot spot common to other Rho family genes, RAC1 and CDC42. Acute expression of the four variants in embryonic mouse brain using in utero electroporation caused defects in cortical neuron morphology and migration ending up with cluster formation during corticogenesis. Notably, defective migration by p.E62del, p.D63N and p.Y64C were rescued by a dominant negative version of p21-activated kinase 1. Our results indicate that RAC3 variants result in morphological and functional defects in cortical neurons during brain development through variant-specific mechanisms, eventually leading to heterogeneous neurodevelopmental phenotypes.

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.

Autism-linked Cullin3 germline haploinsufficiency impacts cytoskeletal dynamics and cortical neurogenesis through RhoA signaling

E3-ubiquitin ligase Cullin3 (Cul3) is a high confidence risk gene for autism spectrum disorder (ASD) and developmental delay (DD). To investigate how Cul3 mutations impact brain development, we generated a haploinsufficient Cul3 mouse model using CRISPR/Cas9 genome engineering. Cul3 mutant mice exhibited social and cognitive deficits and hyperactive behavior. Brain MRI found decreased volume of cortical regions and changes in many other brain regions of Cul3 mutant mice starting from early postnatal development. Spatiotemporal transcriptomic and proteomic profiling of embryonic, early postnatal and adult brain implicated neurogenesis and cytoskeletal defects as key drivers of Cul3 functional impact. Specifically, dendritic growth, filamentous actin puncta, and spontaneous network activity were reduced in Cul3 mutant mice. Inhibition of small GTPase RhoA, a molecular substrate of Cul3 ligase, rescued dendrite length and network activity phenotypes. Our study identified defects in neuronal cytoskeleton and Rho signaling as the primary targets of Cul3 mutation during brain development.

In vivo Functional Genomics for Undiagnosed Patients: The Impact of Small GTPases Signaling Dysregulation at Pan-Embryo Developmental Scale

While individually rare, disorders affecting development collectively represent a substantial clinical, psychological, and socioeconomic burden to patients, families, and society. Insights into the molecular mechanisms underlying these disorders are required to speed up diagnosis, improve counseling, and optimize management toward targeted therapies. Genome sequencing is now unveiling previously unexplored genetic variations in undiagnosed patients, which require functional validation and mechanistic understanding, particularly when dealing with novel nosologic entities. Functional perturbations of key regulators acting on signals' intersections of evolutionarily conserved pathways in these pathological conditions hinder the fine balance between various developmental inputs governing morphogenesis and homeostasis. However, the distinct mechanisms by which these hubs orchestrate pathways to ensure the developmental coordinates are poorly understood. Integrative functional genomics implementing quantitative in vivo models of embryogenesis with subcellular precision in whole organisms contribute to answering these questions. Here, we review the current knowledge on genes and mechanisms critically involved in developmental syndromes and pediatric cancers, revealed by genomic sequencing and in vivo models such as insects, worms and fish. We focus on the monomeric GTPases of the RAS superfamily and their influence on crucial developmental signals and processes. We next discuss the effectiveness of exponentially growing functional assays employing tractable models to identify regulatory crossroads. Unprecedented sophistications are now possible in zebrafish, i.e., genome editing with single-nucleotide precision, nanoimaging, highly resolved recording of multiple small molecules activity, and simultaneous monitoring of brain circuits and complex behavioral response. These assets permit accurate real-time reporting of dynamic small GTPases-controlled processes in entire organisms, owning the potential to tackle rare disease mechanisms.

DNA Methylation of Fluoxetine Response in Child and Adolescence: Preliminary Results

Purpose

The search for predictors of antidepressant response is gaining increasing attention, with epigenetic markers attracting a great deal of interest. We performed a genome-wide study assessing baseline differences in DNA methylation between Responders and Non-Responders.

Patients and Methods

Twenty-two children and adolescents, receiving fluoxetine treatment for the first time, were classified as Responders or Non-Responders according to CGI-I score after 8 weeks of fluoxetine treatment. Genome-wide DNA methylation was profiled using the Illumina Infinium MethylationEPIC BeadChip Kit and analyzed using the Chip Analysis Methylation Pipeline (ChAMP).

Results

We identified 21 CpG sites significantly (FDR<0.05) associated with fluoxetine response that showed meaningful differences (Δβ> ±0.2) in methylation level between Responders and Non-Responders. Two genes, RHOJ (Ras Homolog Family Member J) and OR2L13 (Olfactory Receptor family 2 subfamily L member 13), presented more than one significant CpG sites.

Conclusion

Our findings provide new insights into the molecular mechanisms underlying the complex phenotype of antidepressant response, indicating that methylation at specific genes could be a promising biomarker that needs further replication in large cohorts.

B3GNT3 acts as a carcinogenic factor in endometrial cancer via facilitating cell growth, invasion and migration through regulating RhoA/RAC1 pathway-associated markers

BACKGROUND

Aberrant expression of beta-1,3-N-acetylglucosaminyltransferase-3 (B3GNT3) has been frequently clarified in various cancers, however, its role in endometrial cancer (EC) has not been assessed in detail.

PURPOSE

This study aimed to investigate the biological role of B3GNT3 in EC and simply explored the detailed mechanism.

METHODS

The EC RNA-Seq dataset from TCGA database was applied to evaluate the expression of B3GNT3 and assess its role on prognostic value. HEC-1-A and KLE cell lines of EC were used to perform loss- and gain-of-function B3GNT3 assays respectively. Quantitative real-time PCR (qRT-PCR) and western blot were used to measure the mRNA and protein levels of indicated molecules respectively. Cell counting kit-8, clone formation tests, and Transwell assay served to determine the changes of proliferative, invasive and migratory abilities of EC cells after altering the expression of B3GNT3.

RESULTS

B3GNT3 was found to be highly expressed in EC tissues compared to normal tissues according to the online public databases, which confirmed by the following qRT-PCR in 3 EC cell lines. Besides, high B3GNT3 expression presented a worse overall survival in EC patients as compared with low B3GNT3 expression group. Furthermore, functional experiments in vitro indicated that B3GNT3 could facilitate the cell growth, invasion and migration. Moreover, we found that downregulation of B3GNT3 significantly reduced the expression level of GTP-RhoA and GTP-RAC1, whereas upregulation of B3GNT3 presented the opposite results.

CONCLUSION

The results of current study demonstrate that B3GNT3 acts as an oncogene that promotes EC cells growth, invasion and migration possibly through regulating the RhoA/RAC1 signaling pathway-related markers, suggesting that B3GNT3 may be a candidate biomarker for EC therapeutic intervention.

Mutated RAP1GDS1 causes a new syndrome of dysmorphic feature, intellectual disability & speech delay

Background

RAP1GDS1 (RAP1, GTP‐GDP dissociation stimulator 1), also known as SmgGDS, is a guanine nucleotide exchange factor (GEF) that regulates small GTPases, including, RHOA, RAC1, and KRAS. RAP1GDS1 was shown to be highly expressed in different tissue types including the brain. However, mutations in the RAP1GDS1 gene associated with human diseases have not previously been reported.

Methods

We report on four affected individuals, presenting intellectual disability, global developmental delay (GDD), and hypotonia. The probands’ DNA was subjected to whole‐genome sequencing, revealing a homozygous splice acceptor site mutation in the RAP1GDS1 gene (1444‐1G > A). Sanger sequencing was performed to confirm the segregation of the variant in two Saudi families. The possible aberrant splicing in the patients’ RNA was investigated using RT‐PCR and changes in mRNA expression of the patients were confirmed using qRT‐PCR.

Results

The identified splice variant was found to segregate within the two families. RT‐PCR showed that the mutation affected RAP1GDS1 gene splicing, resulting in the production of aberrant transcripts in the affected individuals. Quantitative gene expression analysis demonstrated that the RAP1GDS1 mRNA expression in all the probands was significantly decreased compared to that of the control, and Sanger sequencing of the probands’ cDNA revealed skipping of exon 13, further strengthening the pathogenicity of this variant.

Conclusion

We are the first to report the mutation of the RAP1GDS1 gene as a potential cause of GDD and hypotonia. However, further investigations into the molecular mechanisms involved are required to confirm the role of RAP1GDS1 gene in causing GDD and hypotonia.

RhoA Controls Axon Extension Independent of Specification in the Developing Brain

The specification of an axon and its subsequent outgrowth are key steps during neuronal polarization, a prerequisite to wire the brain. The Rho-guanosine triphosphatase (GTPase) RhoA is believed to be a central player in these processes. However, its physiological role has remained undefined. Here, genetic loss- and gain-of-function experiments combined with time-lapse microscopy, cell culture, and in vivo analysis show that RhoA is not involved in axon specification but confines the initiation of neuronal polarization and axon outgrowth during development. Biochemical analysis and super-resolution microscopy together with molecular and pharmacological manipulations reveal that RhoA restrains axon growth by activating myosin-II-mediated actin arc formation in the growth cone to prevent microtubules from protruding toward the leading edge. Through this mechanism, RhoA regulates the duration of axon growth and pause phases, thus controlling the tightly timed extension of developing axons. Thereby, this work unravels physiologically relevant players coordinating actin-microtubule interactions during axon growth.

SVCT2 Promotes Neural Stem/Progenitor Cells Migration Through Activating CDC42 After Ischemic Stroke

Ischemic stroke is one of the most leading diseases causing death/long-term disability worldwide. Activating endogenous neural stem/progenitors cells (NSPCs), lining in the subventricular zone (SVZ) and dentate gyrus, facilitates injured brain tissue recovery in both short and long-term experimental settings. While, only a few proliferated NSPCs migrate toward the lesions to enhance endogenous repair after ischemia. Here, the results indicated that the functional recovery was evidently improved and the infarct volume was significantly reduced with ascorbic acid (AA) treatment in a dose-dependent manner from 125 to 500 mg/Kg, and the suitable therapeutic concentration was 250 mg/Kg. The possible mechanism might be due to activating sodium-vitamin C cotransporter 2 (SVCT2), which was down-regulated in SVZ after ischemia. Furthermore, immunostaining images depicted the number of migrated NSPCs from SVZ were significantly increased with 250 mg/Kg AA treatment or SVCT2 overexpression under the physiological and pathological condition in vivo. Besides, the data also represented that 250 mg/Kg AA or SVCT2 overexpression facilitated NSPCs migration via promoting F-actin assembling in the manner of up-regulating CDC42 expression using oxygen-glucose deprivation in vitro. Collectively, the present study indicates that SVCT2 promotes NSPCs migration through CDC42 activation to facilitate F-actin assembling, which enlarges the therapeutic scope of AA and the role of SVCT2 in NSPCs migration after brain injury.

Rab23 Regulates Radial Migration of Projection Neurons via N-cadherin

Radial migration of cortical projection neurons is a prerequisite for shaping a distinct multilayered cerebral cortex during mammalian corticogenesis. Members of Rab GTPases family were reported to regulate radial migration. Here, in vivo conditional knockout or in utero knockdown (KD) of Rab23 in mice neocortex causes aberrant polarity and halted migration of cortical projection neurons. Further investigation of the underlying mechanism reveals down-regulation of N-cadherin in the Rab23-deficient neurons, which is a cell adhesion protein previously known to modulate radial migration. (Shikanai M, Nakajima K, Kawauchi T. 2011. N-cadherin regulates radial glial fiber-dependent migration of cortical locomoting neurons. Commun Integr Biol. 4:326–330.) Interestingly, pharmacological inhibition of extracellular signal-regulated kinases (ERK1/2) also decreases the expression of N-cadherin, implicating an upstream effect of ERK1/2 on N-cadherin and also suggesting a link between Rab23 and ERK1/2. Further biochemical studies show that silencing of Rab23 impedes activation of ERK1/2 via perturbed platelet-derived growth factor-alpha (PDGFRα) signaling. Restoration of the expression of Rab23 or N-cadherin in Rab23-KD neurons could reverse neuron migration defects, indicating that Rab23 modulates migration through N-cadherin. These studies suggest that cortical neuron migration is mediated by a molecular hierarchy downstream of Rab23 via N-cadherin.

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.

The Ras Superfamily of Small GTPases in Non-neoplastic Cerebral Diseases

The small GTPases from the Ras superfamily play crucial roles in basic cellular processes during practically the entire process of neurodevelopment, including neurogenesis, differentiation, gene expression, membrane and protein traffic, vesicular trafficking, and synaptic plasticity. Small GTPases are key signal transducing enzymes that link extracellular cues to the neuronal responses required for the construction of neuronal networks, as well as for synaptic function and plasticity. Different subfamilies of small GTPases have been linked to a number of non-neoplastic cerebral diseases such as Alzheimer’s disease (AD), Parkinson’s disease (PD), intellectual disability, epilepsy, drug addiction, Huntington’s disease (HD), amyotrophic lateral sclerosis (ALS) and a large number of idiopathic cerebral diseases. Here, we attempted to make a clearer illustration of the relationship between Ras superfamily GTPases and non-neoplastic cerebral diseases, as well as their roles in the neural system. In future studies, potential treatments for non-neoplastic cerebral diseases which are based on small GTPase related signaling pathways should be explored further. In this paper, we review all the available literature in support of this possibility.

Ghrelin Promotes Cortical Neurites Growth in Late Stage After Oxygen-Glucose Deprivation/Reperfusion Injury

Acyl ghrelin, a novel brain-gut peptide, is an endogenous ligand for the growth hormone secretagogue receptor. Accumulated research data have shown that acyl ghrelin exercises a significant neuroprotective effect against cerebral ischemia/reperfusion (I/R) injury in animal models and in cultured neurons during the acute phase, usually, 1 day after cerebral ischemia. The chronic effects of acyl ghrelin 1 week after brain ischemia remain largely unknown. In this study, we explored the effects of acyl ghrelin on cultured organotypic brain slices and cortical neurons which were injured by oxygen-glucose deprivation/reperfusion(OGD/R) for 7 days. The underlying molecular mechanisms were deciphered based on label-free proteomic analysis. Acyl ghrelin treatment promoted neurite (axons and dendrites) growth and alleviated the neuronal damage in both cultured brain slices and neurons. Proteomic analysis showed that cell division control protein 42 (Cdc42) mediates the effects of acyl ghrelin on neurite growth. Acyl ghrelin stimulated the expression of Cdc42 and neurite growth in cultured neurons comparing with OGD/R group. Inhibition of Cdc42 attenuated the effects of acyl ghrelin. These results suggest that acyl ghrelin promotes neurite growth during the later stage after OGD/R injury via Cdc42. Our study suggests that acyl ghrelin may be promising to restore the neuronal structure in the late phase after stroke.

Contactin-1/F3 Regulates Neuronal Migration and Morphogenesis Through Modulating RhoA Activity

During neocortical development, newborn neurons migrate along radial fibers from the germinal ventricular zone (VZ) toward the cortical plate (CP) to populate the cerebral cortex. This radial migration requires adhesion activities between neurons and radial fibers; however, past research has identified only a limited number of adhesion molecules involved in this process. Contactin-1/F3 (Cntn1), a cell adhesion molecule expressed in the developing nervous system is essential for many key developmental events including neural cell adhesion, neurite outgrowth, axon guidance and myelination. However, the potential role of Cntn1 in neuronal migration during cortical development has not been investigated. Here we used in utero electroporation to introduce short hairpin RNA (shRNA) to knock down (KD) Cntn1 in neural stem cells in vivo. We found that Cntn1 KD led to a delay in neuronal migration. The arrested cells presented abnormal morphology in their leading process and more multipolar cells were observed in the deep layers of the brain, suggestive of dysregulation in process formation. Intriguingly, Cntn1 KD also resulted in upregulation of RhoA, a negative regulator for neuronal migration. Interference of RhoA by expression of the dominant-negative RhoA^N19 partially rescued the neuronal migration defects caused by Cntn1 KD. Our results showed that Cntn1 is a novel adhesion protein that is essential for neuronal migration and regulates process formation of newborn cortical neurons through modulating RhoA signaling pathway.

Rho GTPases in Intellectual Disability: From Genetics to Therapeutic Opportunities

Rho-class small GTPases are implicated in basic cellular processes at nearly all brain developmental steps, from neurogenesis and migration to axon guidance and synaptic plasticity. GTPases are key signal transducing enzymes that link extracellular cues to the neuronal responses required for the construction of neuronal networks, as well as for synaptic function and plasticity. Rho GTPases are highly regulated by a complex set of activating (GEFs) and inactivating (GAPs) partners, via protein:protein interactions (PPI). Misregulated RhoA, Rac1/Rac3 and cdc42 activity has been linked with intellectual disability (ID) and other neurodevelopmental conditions that comprise ID. All genetic evidences indicate that in these disorders the RhoA pathway is hyperactive while the Rac1 and cdc42 pathways are consistently hypoactive. Adopting cultured neurons for in vitro testing and specific animal models of ID for in vivo examination, the endophenotypes associated with these conditions are emerging and include altered neuronal networking, unbalanced excitation/inhibition and altered synaptic activity and plasticity. As we approach a clearer definition of these phenotype(s) and the role of hyper- and hypo-active GTPases in the construction of neuronal networks, there is an increasing possibility that selective inhibitors and activators might be designed via PPI, or identified by screening, that counteract the misregulation of small GTPases and result in alleviation of the cognitive condition. Here we review all knowledge in support of this possibility.

Cdc42 Regulates Neuronal Polarity during Cerebellar Axon Formation and Glial-Guided Migration

CNS cortical histogenesis depends on polarity signaling pathways that regulate cell adhesion and motility. Here we report that conditional deletion of the Rho GTPase Cdc42 in cerebellar granule cell precursors (GCPs) results in abnormalities in cerebellar foliation revealed by iDISCO clearing methodology, a loss of columnar organization of proliferating GCPs in the external germinal layer (EGL), disordered parallel fiber organization in the molecular layer (ML), and a failure to extend a leading process and form a neuron-glial junction during migration along Bergmann glia (BG). Notably, GCPs lacking Cdc42 had a multi-polar morphology and slowed migration rate. In addition, secondary defects occurred in BG development and organization, especially in the lateral cerebellar hemispheres. By phosphoproteomic analysis, affected Cdc42 targets included regulators of the cytoskeleton, cell adhesion and polarity. Thus, Cdc42 signaling pathways are critical regulators of GCP polarity and the formation of neuron-glial junctions during cerebellar development.

•

Conditional deletion of Cdc42 in GCPs perturbs cerebellar cortical histogenesis

•

Loss of Cdc42 in GCPs disrupts GCP neuron-glial junctions

•

Cdc42 deficiency causes a loss of GCP polarity and slows their migration

•

Phosphoproteomics reveals changes in cytoskeletal, adhesion, and polarity proteins

The RacGAP β-Chimaerin is essential for cerebellar granule cell migration

During mammalian cerebellar development, postnatal granule cell progenitors proliferate in the outer part of the External Granule Layer (EGL). Postmitotic granule progenitors migrate tangentially in the inner EGL before switching to migrate radially inward, past the Purkinje cell layer, to achieve their final position in the mature Granule Cell Layer (GCL). Here, we show that the RacGAP β-chimaerin is expressed by a small population of late-born, premigratory granule cells. β-chimaerin deficiency causes a subset of granule cells to become arrested in the EGL, where they differentiate and form ectopic neuronal clusters. These clusters of granule cells are able to recruit aberrantly projecting mossy fibers. Collectively, these data suggest a role for β-chimaerin as an intracellular mediator of Cerebellar Granule Cell radial migration.

In Vitro, Ex Vivo and In Vivo Techniques to Study Neuronal Migration in the Developing Cerebral Cortex

Neuronal migration is a fundamental biological process that underlies proper brain development and neuronal circuit formation. In the developing cerebral cortex, distinct neuronal populations, producing excitatory, inhibitory and modulatory neurotransmitters, are generated in different germinative areas and migrate along various routes to reach their final positions within the cortex. Different technical approaches and experimental models have been adopted to study the mechanisms regulating neuronal migration in the cortex. In this review, we will discuss the most common in vitro, ex vivo and in vivo techniques to visualize and study cortical neuronal migration.

Meningeal retinoic acid contributes to neocortical lamination and radial migration during mouse brain development

Retinoic acid (RA) is a diffusible molecule involved in early forebrain patterning. Its later production in the meninges by the retinaldehyde dehydrogenase RALDH2 coincides with the time of cortical neuron generation. A function of RA in this process has not been adressed directly as Raldh2^−/− mouse mutants are embryonic lethal. Here, we used a conditional genetic strategy to inactivate Raldh2 just prior to onset of its expression in the developing meninges. This inactivation does not affect the formation of the cortical progenitor populations, their rate of division, or timing of differentiation. However, migration of late-born cortical neurons is delayed, with neurons stalling in the intermediate zone and exhibiting an abnormal multipolar morphology. This suggests that RA controls the multipolar-to-bipolar transition that occurs in the intermediate zone and allows neurons to start locomotion in the cortical plate. Our work also shows a role for RA in cortical lamination, as deep layers are expanded and a subset of layer IV neurons are not formed in the Raldh2-ablated mutants. These data demonstrate that meninges are a source of extrinsic signals important for cortical development.

Summary: Involvement of the signalling molecule retinoic acid in neurogenesis of the developing cerebral cortex is shown through targeted deletion of its synthesizing enzyme.

Neurotrophin Signaling and Stem Cells-Implications for Neurodegenerative Diseases and Stem Cell Therapy

Neurotrophins (NTs) are members of a neuronal growth factor protein family whose action is mediated by the tropomyosin receptor kinase (TRK) receptor family receptors and the p75 NT receptor (p75NTR), a member of the tumor necrosis factor (TNF) receptor family. Although NTs were first discovered in neurons, recent studies have suggested that NTs and their receptors are expressed in various types of stem cells mediating pivotal signaling events in stem cell biology. The concept of stem cell therapy has already attracted much attention as a potential strategy for the treatment of neurodegenerative diseases (NDs). Strikingly, NTs, proNTs, and their receptors are gaining interest as key regulators of stem cells differentiation, survival, self-renewal, plasticity, and migration. In this review, we elaborate the recent progress in understanding of NTs and their action on various stem cells. First, we provide current knowledge of NTs, proNTs, and their receptor isoforms and signaling pathways. Subsequently, we describe recent advances in the understanding of NT activities in various stem cells and their role in NDs, particularly Alzheimer's disease (AD) and Parkinson's disease (PD). Finally, we compile the implications of NTs and stem cells from a clinical perspective and discuss the challenges with regard to transplantation therapy for treatment of AD and PD.

Disruption of ArhGAP15 results in hyperactive Rac1, affects the architecture and function of hippocampal inhibitory neurons and causes cognitive deficits

During brain development, the small GTPases Rac1/Rac3 play key roles in neuronal migration, neuritogenesis, synaptic formation and plasticity, via control of actin cytoskeleton dynamic. Their activity is positively and negatively regulated by GEFs and GAPs molecules, respectively. However their in vivo roles are poorly known. The ArhGAP15 gene, coding for a Rac-specific GAP protein, is expressed in both excitatory and inhibitory neurons of the adult hippocampus, and its loss results in the hyperactivation of Rac1/Rac3. In the CA3 and dentate gyrus (DG) regions of the ArhGAP15 mutant hippocampus the CR+, PV+ and SST+ inhibitory neurons are reduced in number, due to reduced efficiency and directionality of their migration, while pyramidal neurons are unaffected. Loss of ArhGAP15 alters neuritogenesis and the balance between excitatory and inhibitory synapses, with a net functional result consisting in increased spike frequency and bursts, accompanied by poor synchronization. Thus, the loss of ArhGAP15 mainly impacts on interneuron-dependent inhibition. Adult ArhGAP15^−/− mice showed defective hippocampus-dependent functions such as working and associative memories. These findings indicate that a normal architecture and function of hippocampal inhibitory neurons is essential for higher hippocampal functions, and is exquisitely sensitive to ArhGAP15-dependent modulation of Rac1/Rac3.

Hsc70 chaperone activity underlies Trio GEF function in axon growth and guidance induced by netrin-1

During development, netrin-1 is both an attractive and repulsive axon guidance cue and mediates its attractive function through the receptor Deleted in Colorectal Cancer (DCC). The activation of Rho guanosine triphosphatases within the extending growth cone facilitates the dynamic reorganization of the cytoskeleton required to drive axon extension. The Rac1 guanine nucleotide exchange factor (GEF) Trio is essential for netrin-1-induced axon outgrowth and guidance. Here, we identify the molecular chaperone heat shock cognate protein 70 (Hsc70) as a novel Trio regulator. Hsc70 dynamically associated with the N-terminal region and Rac1 GEF domain of Trio. Whereas Hsc70 expression supported Trio-dependent Rac1 activation, adenosine triphosphatase-deficient Hsc70 (D10N) abrogated Trio Rac1 GEF activity and netrin-1-induced Rac1 activation. Hsc70 was required for netrin-1-mediated axon growth and attraction in vitro, whereas Hsc70 activity supported callosal projections and radial neuronal migration in the embryonic neocortex. These findings demonstrate that Hsc70 chaperone activity is required for Rac1 activation by Trio and this function underlies netrin-1/DCC-dependent axon outgrowth and guidance.

Regulating Rac in the nervous system: molecular function and disease implication of Rac GEFs and GAPs

Rho family GTPases, including RhoA, Rac1, and Cdc42 as the most studied members, are master regulators of actin cytoskeletal organization. Rho GTPases control various aspects of the nervous system and are associated with a number of neuropsychiatric and neurodegenerative diseases. The activity of Rho GTPases is controlled by two families of regulators, guanine nucleotide exchange factors (GEFs) as the activators and GTPase-activating proteins (GAPs) as the inhibitors. Through coordinated regulation by GEFs and GAPs, Rho GTPases act as converging signaling molecules that convey different upstream signals in the nervous system. So far, more than 70 members of either GEFs or GAPs of Rho GTPases have been identified in mammals, but only a small subset of them have well-known functions. Thus, characterization of important GEFs and GAPs in the nervous system is crucial for the understanding of spatiotemporal dynamics of Rho GTPase activity in different neuronal functions. In this review, we summarize the current understanding of GEFs and GAPs for Rac1, with emphasis on the molecular function and disease implication of these regulators in the nervous system.