Cytoskeletal proteins in cortical development and disease: actin associated proteins in periventricular heterotopia

Variations associated with neurodevelopmental disorders affect ARF1 function and cortical development

ADP-ribosylation factors (ARFs) are a family of small GTPases that regulate vesicle trafficking and actin dynamics in cells. Recent genetic analyses have revealed associations between variations in ARF genes and neurodevelopmental disorders, although their pathophysiological significance remains unclear. In this study, we conducted biochemical, cell biological and in vivo analyses of ARF1 variants linked to neurodevelopmental disorders. The mant-GDP dissociation assay revealed that ARF1-p.R19C, -p.F51L, -p.R99C and -p.R99H exhibit higher GDP/GTP exchange activity compared to ARF1 wild type (WT). The GTPase-activating protein (GAP) increased the GTPase activity of WT, p.R19C, p.Y35H, p.F51L, p.P131L and p.P131R, but not of p.Y35D, p.T48I, p.R99C and p.R99H. The transient expression of p.R99C, p.R99H and p.K127E in mammalian cells resulted in the disruption of the Golgi apparatus. In utero electroporation-mediated gene transfer into the cortical neurons of embryonic mice demonstrated that p.R99C, p.R99H and p.K127E cause a migration defect. Expression of these variants resulted in the expansion of the Golgi apparatus in migrating cortical neurons. These findings suggest that the ARF1 variants linked to neurodevelopmental disorders, specifically p.R99C, p.R99H and p.K127E, disrupt the structure of the Golgi apparatus, thereby leading to a developmental defect of cortical 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.

ARF1-related disorder: phenotypic and molecular spectrum

PURPOSE

ARF1 was previously implicated in periventricular nodular heterotopia (PVNH) in only five individuals and systematic clinical characterisation was not available. The aim of this study is to provide a comprehensive description of the phenotypic and genotypic spectrum of ARF1-related neurodevelopmental disorder.

METHODS

We collected detailed phenotypes of an international cohort of individuals (n=17) with ARF1 variants assembled through the GeneMatcher platform. Missense variants were structurally modelled, and the impact of several were functionally validated.

RESULTS

De novo variants (10 missense, 1 frameshift, 1 splice altering resulting in 9 residues insertion) in ARF1 were identified among 17 unrelated individuals. Detailed phenotypes included intellectual disability (ID), microcephaly, seizures and PVNH. No specific facial characteristics were consistent across all cases, however microretrognathia was common. Various hearing and visual defects were recurrent, and interestingly, some inflammatory features were reported. MRI of the brain frequently showed abnormalities consistent with a neuronal migration disorder.

CONCLUSION

We confirm the role of ARF1 in an autosomal dominant syndrome with a phenotypic spectrum including severe ID, microcephaly, seizures and PVNH due to impaired neuronal migration.

The specific DNA methylation landscape in focal cortical dysplasia ILAE type 3D

Focal Cortical Dysplasia (FCD) is a frequent cause of drug-resistant focal epilepsy in children and young adults. The international FCD classifications of 2011 and 2022 have identified several clinico-pathological subtypes, either occurring isolated, i.e., FCD ILAE Type 1 or 2, or in association with a principal cortical lesion, i.e., FCD Type 3. Here, we addressed the DNA methylation signature of a previously described new subtype of FCD 3D occurring in the occipital lobe of very young children and microscopically defined by neuronal cell loss in cortical layer 4. We studied the DNA methylation profile using 850 K BeadChip arrays in a retrospective cohort of 104 patients with FCD 1 A, 2 A, 2B, 3D, TLE without FCD, and 16 postmortem specimens without neurological disorders as controls, operated in China or Germany. DNA was extracted from formalin-fixed paraffin-embedded tissue blocks with microscopically confirmed lesions, and DNA methylation profiles were bioinformatically analyzed with a recently developed deep learning algorithm. Our results revealed a distinct position of FCD 3D in the DNA methylation map of common FCD subtypes, also different from non-FCD epilepsy surgery controls or non-epileptic postmortem controls. Within the FCD 3D cohort, the DNA methylation signature separated three histopathology subtypes, i.e., glial scarring around porencephalic cysts, loss of layer 4, and Rasmussen encephalitis. Differential methylation in FCD 3D with loss of layer 4 mapped explicitly to biological pathways related to neurodegeneration, biogenesis of the extracellular matrix (ECM) components, axon guidance, and regulation of the actin cytoskeleton. Our data suggest that DNA methylation signatures in cortical malformations are not only of diagnostic value but also phenotypically relevant, providing the molecular underpinnings of structural and histopathological features associated with epilepsy. Further studies will be necessary to confirm these results and clarify their functional relevance and epileptogenic potential in these difficult-to-treat children.

The kinesin Kif21b regulates radial migration of cortical projection neurons through a non-canonical function on actin cytoskeleton

Completion of neuronal migration is critical for brain development. Kif21b is a plus-end-directed kinesin motor protein that promotes intracellular transport and controls microtubule dynamics in neurons. Here we report a physiological function of Kif21b during radial migration of projection neurons in the mouse developing cortex. In vivo analysis in mouse and live imaging on cultured slices demonstrate that Kif21b regulates the radial glia-guided locomotion of newborn neurons independently of its motility on microtubules. We show that Kif21b directly binds and regulates the actin cytoskeleton both in vitro and in vivo in migratory neurons. We establish that Kif21b-mediated regulation of actin cytoskeleton dynamics influences branching and nucleokinesis during neuronal locomotion. Altogether, our results reveal atypical roles of Kif21b on the actin cytoskeleton during migration of cortical projection neurons.

The apoptotic effects of soybean agglutinin were induced through three different signal pathways by down-regulating cytoskeleton proteins in IPEC-J2 cells

Soybean agglutinin (SBA) is a main anti-nutritional factor in soybean. SBA exhibits its anti-nutritional functions by binding to intestinal epithelial cells. Keratin8 (KRT8), Keratin18 (KRT18) and Actin (ACTA) are the representative SBA-specific binding proteins. Such cytoskeletal proteins act a crucial role in different cell activities. However, limited reports reveal what the signal transduction pathway of apoptosis caused by SBA when binding to KRT8, KRT18 and ACTA. We aimed to evaluate the effects of SBA on cell apoptosis and the expression of the cytoskeletal protein (KRT8, KRT18 and ACTA), reveal the roles of these cytoskeletal proteins or their combinations on SBA-induced cell apoptosis in IPEC-J2 cell line, evaluate the influences of SBA on the mitochondria, endoplasmic reticulum stress and death receptor-mediated apoptosis signal pathway and to show the roles of KRT8, KRT18 and ACTA in different apoptosis signal pathways induced by SBA. The results showed that SBA induced the IPEC-J2 cell apoptosis and decreased the mRNA expression of KRT8, KRT18 and ACTA (p < 0.05). The degree of effect of three cytoskeleton proteins on cell apoptosis was ACTA > KRT8 > KRT18. The roles of these three cytoskeletal proteins on IPEC-J2 apoptotic rates had a certain accumulation effect. SBA up-regulated mitochondrial fission variant protein (FIS1) and fusion protein (Mfn2) promoted CytC and AIF in mitochondria to enter the cytoplasm, activated caspase-9 and caspase-3, damaged or declined mitochondrial function and reduced ATP synthesis (p < 0.05). Also, SBA up-regulated the expression of GRP78, XBP-1, eIF2α, p-eIF2α and CHOP (p < 0.05), down-regulated the expression level of ASK1 protein (p < 0.05). SBA led to the recruitment of FADD to the cytoplasmic membrane and increased the expression of FasL, resulting in caspase-8 processing. SBA up-regulated the expression level of Bax protein and decreased cytosolic Bcl-2 and Bid (p < 0.05). In addition, there was a significant negative correlation between the gene expression of cytoskeleton proteins and apoptosis, as well as the expression of key proteins of apoptosis-related signal transduction pathways. In conclusion, SBA induced the activation of the mitochondria, endoplasmic reticulum stress and the death receptor-mediated apoptosis signal pathway and the crosstalk between them. The effect of SBA on these three pathways was mainly exhibited via down-regulation of the mRNA expression of the three cytoskeletal expressions. This study elucidates the molecular mechanism and signaling pathway of SBA that lead to apoptosis from the perspective of cell biology and molecular biology and provides a new perspective on the toxicity mechanism of other food-derived anti-nutrients, medical gastrointestinal health and related cancer treatment.

p53 regulates the effects of DAPT on Rac1 activation and migration of non-small-cell lung cancer cells

The use of γ-secretase inhibitors to inhibit the activation of Notch receptors can effectively inhibit the malignant process of tumors. Here, we demonstrate that p53 can modulate the effect of DAPT (a γ-secretase inhibitor) on the activation of small GTPase Rac1, thereby affecting cell migration of non-small-cell lung cancer H1299 and A549 cells. After treatment with 20 μM DAPT, activation of Rac1 was increased in H1299 cells but not in A549 cells. We further found that the migration ability of H1299 cells was increased, whereas that of A549 cells was reduced. The effect of DAPT on H1299 migration was repressed by Rac1-T17N, a dominant inactivated mutant of Rac1. H1299 is a p53-deficient cell line. When p53 protein was overexpressed in H1299 cells with a pEGFP-p53 plasmid, DAPT treatment no longer activated Rac1 and increased migration ability. Moreover, DAPT promoted the migration of H1299 cells by increasing the activity of Rac1 through the non-canonical Notch pathway. Taken together, these results indicate that the expression of p53 protein in lung cancer cells regulates the effect of DAPT on cell migration by modulating the activation of Rac1, suggesting that p53 may affect the therapeutic effects of Notch inhibitors in lung cancer patients.

Coronin 2B Regulates Neuronal Migration via Rac1-Dependent Multipolar-Bipolar Transition

In the developing cortex, excitatory neurons migrate along the radial fibers to their final destinations and build up synaptic connection with each other to form functional circuitry. The shaping of neuronal morphologies by actin cytoskeleton dynamics is crucial for neuronal migration. However, it is largely unknown how the distribution and assembly of the F-actin cytoskeleton are coordinated. In the present study, we found that an actin regulatory protein, coronin 2B, is indispensable for the transition from a multipolar to bipolar morphology during neuronal migration in ICR mice of either sex. Loss of coronin 2B led to heterotopic accumulation of migrating neurons in the intermediate zone along with reduced dendritic complexity and aberrant neuronal activity in the cortical plate. This was accompanied by increased seizure susceptibility, suggesting the malfunction of cortical development in coronin 2B-deficient brains. Coronin 2B knockdown disrupted the distribution of the F-actin cytoskeleton at the leading processes, while the migration defect in coronin 2B-deficient neurons was partially rescued by overexpression of Rac1 and its downstream actin-severing protein, cofilin. Our results collectively reveal the physiological function of coronin 2B during neuronal migration whereby it maintains the proper distribution of activated Rac1 and the F-actin cytoskeleton.SIGNIFICANCE STATEMENT Deficits in neuronal migration during cortical development result in various neurodevelopmental disorders (e.g., focal cortical dysplasia, periventricular heterotopia, epilepsy, etc.). Most signaling pathways that control neuronal migration process converge to regulate actin cytoskeleton dynamics. Therefore, it is important to understand how actin dynamics is coordinated in the critical processes of neuronal migration. Herein, we report that coronin 2B is a key protein that regulates neuronal migration through its ability to control the distribution of the actin cytoskeleton and its regulatory signaling protein Rac1 during the multipolar-bipolar transition in the intermediate zone, providing insights into the molecular machinery that drives the migration process of newborn neurons.

Breasi-CRISPR: an efficient genome-editing method to interrogate protein localization and protein-protein interactions in the embryonic mouse cortex

In developing tissues, knowing the localization and interactors of proteins of interest is key to understanding their function. Here, we describe the Breasi-CRISPR approach (Brain Easi-CRISPR), combining Easi-CRISPR with in utero electroporation to tag endogenous proteins within embryonic mouse brains. Breasi-CRISPR enables knock-in of both short and long epitope tag sequences with high efficiency. We visualized epitope-tagged proteins with varied expression levels, such as ACTB, LMNB1, EMD, FMRP, NOTCH1 and RPL22. Detection was possible by immunohistochemistry as soon as 1 day after electroporation and we observed efficient gene editing in up to 50% of electroporated cells. Moreover, tagged proteins could be detected by immunoblotting in lysates from individual cortices. Next, we demonstrated that Breasi-CRISPR enables the tagging of proteins with fluorophores, allowing visualization of endogenous proteins by live imaging in organotypic brain slices. Finally, we used Breasi-CRISPR to perform co-immunoprecipitation mass-spectrometry analyses of the autism-related protein FMRP to discover its interactome in the embryonic cortex. Together, these data demonstrate that Breasi-CRISPR is a powerful tool with diverse applications that will propel the understanding of protein function in neurodevelopment.

Molecular mechanisms of synaptogenesis

Synapses are the basic units for information processing and storage in the nervous system. It is only when the synaptic connection is established, that it becomes meaningful to discuss the structure and function of a circuit. In humans, our unparalleled cognitive abilities are correlated with an increase in the number of synapses. Additionally, genes involved in synaptogenesis are also frequently associated with neurological or psychiatric disorders, suggesting a relationship between synaptogenesis and brain physiology and pathology. Thus, understanding the molecular mechanisms of synaptogenesis is the key to the mystery of circuit assembly and neural computation. Furthermore, it would provide therapeutic insights for the treatment of neurological and psychiatric disorders. Multiple molecular events must be precisely coordinated to generate a synapse. To understand the molecular mechanisms underlying synaptogenesis, we need to know the molecular components of synapses, how these molecular components are held together, and how the molecular networks are refined in response to neural activity to generate new synapses. Thanks to the intensive investigations in this field, our understanding of the process of synaptogenesis has progressed significantly. Here, we will review the molecular mechanisms of synaptogenesis by going over the studies on the identification of molecular components in synapses and their functions in synaptogenesis, how cell adhesion molecules connect these synaptic molecules together, and how neural activity mobilizes these molecules to generate new synapses. Finally, we will summarize the human-specific regulatory mechanisms in synaptogenesis and results from human genetics studies on synaptogenesis and brain disorders.

Loss of BAF (mSWI/SNF) chromatin-remodeling ATPase Brg1 causes multiple malformations of cortical development in mice

Mutations in genes encoding subunits of the BAF (BRG1/BRM-associated factor) complex cause various neurodevelopmental diseases. However, the underlying pathophysiology remains largely unknown. Here, we analyzed the function of Brg1, a core ATPase of BAF complexes, in the developing cerebral cortex. Loss of Brg1 causes several morphological defects resembling human malformations of cortical development (MCDs), including microcephaly, cortical dysplasia, cobblestone lissencephaly, and periventricular heterotopia. We demonstrated that neural progenitor cell (NPC) renewal, neuronal differentiation, neuronal migration, apoptotic cell death, pial basement membrane, and apical junctional complexes, which are associated with MCD formation, were impaired after Brg1 deletion. Furthermore, transcriptome profiling indicated that a large number of genes were deregulated. The deregulated genes were closely related to MCD formation, and most of these genes were bound by Brg1. Cumulatively, our study indicates an essential role of Brg1 in cortical development and provides a new possible pathogenesis underlying Brg1-based BAF complex-related neurodevelopmental disorders.

The Role of Hippo-YAP/TAZ Signaling in Brain Development

In order for our complex nervous system to develop normally, both precise spatial and temporal regulation of a number of different signaling pathways is critical. During both early embryogenesis and in organ development, one pathway that has been repeatedly implicated is the Hippo-YAP/TAZ signaling pathway. The paralogs YAP and TAZ are transcriptional co-activators that play an important role in cell proliferation, cell differentiation, and organ growth. Regulation of these proteins by the Hippo kinase cascade is therefore important for normal development. In this article, we review the growing field of research surrounding the role of Hippo-YAP/TAZ signaling in normal and atypical brain development. Starting from the development of the neural tube to the development and refinement of the cerebral cortex, cerebellum, and ventricular system, we address the typical role of these transcriptional co-activators, the functional consequences that manipulation of YAP/TAZ and their upstream regulators have on brain development, and where further research may be of benefit. This article is protected by copyright. All rights reserved.

Novel role of the synaptic scaffold protein Dlgap4 in ventricular surface integrity and neuronal migration during cortical development

Subcortical heterotopias are malformations associated with epilepsy and intellectual disability, characterized by the presence of ectopic neurons in the white matter. Mouse and human heterotopia mutations were identified in the microtubule-binding protein Echinoderm microtubule-associated protein-like 1, EML1. Further exploring pathological mechanisms, we identified a patient with an EML1-like phenotype and a novel genetic variation in DLGAP4. The protein belongs to a membrane-associated guanylate kinase family known to function in glutamate synapses. We showed that DLGAP4 is strongly expressed in the mouse ventricular zone (VZ) from early corticogenesis, and interacts with key VZ proteins including EML1. In utero electroporation of Dlgap4 knockdown (KD) and overexpression constructs revealed a ventricular surface phenotype including changes in progenitor cell dynamics, morphology, proliferation and neuronal migration defects. The Dlgap4 KD phenotype was rescued by wild-type but not mutant DLGAP4. Dlgap4 is required for the organization of radial glial cell adherens junction components and actin cytoskeleton dynamics at the apical domain, as well as during neuronal migration. Finally, Dlgap4 heterozygous knockout (KO) mice also show developmental defects in the dorsal telencephalon. We hence identify a synapse-related scaffold protein with pleiotropic functions, influencing the integrity of the developing cerebral cortex.

A recurrent, de novo pathogenic variant in ARPC4 disrupts actin filament formation and causes microcephaly and speech delay

We report seven affected individuals from six families with a recurrent, de novo variant in the ARPC4 gene (c.472C>T [p.Arg158Cys (GenBank: NM_005718.4)]). Core features in affected individuals include microcephaly, mild motor delays, and significant speech impairment. ARPC4 is a core subunit of the actin-related protein (ARP2/3) complex, which catalyzes the formation of F-actin networks. We show that the recurrent ARPC4 missense change is associated with a decreased amount of F-actin in cells from two affected individuals. Taken together, our results implicate heterozygous ARPC4 missense variants as a cause of neurodevelopmental disorders and microcephaly.

Association Between Genetic Risks for Obesity and Working Memory in Children

Introduction: Obesity is highly heritable, and recent evidence demonstrates that obesity is associated with cognitive deficits, specifically working memory. However, the relationship between genetic risks for obesity and working memory is not clear. In addition, whether the effect of these genetic risks on working memory in children is mediated by increased body mass index (BMI) has not been elucidated. Methods: In order to test whether the polygenic risk score (PRS) for obesity in adulthood (adulthood-BMI-PRS) is associated with working memory at 8 years of age, and whether the effect is mediated by childhood BMI, in children from the general population, participants in the Hamamatsu Birth Cohort for Mothers and Children (HBC) study in Hamamatsu, Japan, underwent testing for association of adulthood-BMI-PRS with working memory. HBC data collection began in December 2007 and is ongoing. Adulthood-BMI-PRS values were generated using summary data from the recent genome-wide association study (GWAS) undertaken in Japan, and the significance of thresholds was calculated for each outcome. Outcomes measured included the working memory index (WMI) of Weschler Intelligence Scale-4 (WISC-IV) scores and the BMI at 8 years of age. Gene-set enrichment analysis was conducted to clarify the molecular basis common to adulthood-BMI and childhood-WMI. Mediation analysis was performed to assess whether childhood-BMI of children mediated the association between adulthood-BMI-PRS and working memory. Results: A total of 734 participants (377 males, 357 females) were analyzed. Adulthood-BMI-PRS was associated with lower childhood-WMI (β[SE], -1.807 [0.668]; p = 0.010, corrected) of WISC-IV. Gene-set enrichment analyses found that regulation of neurotrophin Trk receptor signaling (β[SE], -2.020 [6.39]; p = 0.002, corrected), negative regulation of GTPase activity (β[SE], 2.001 [0.630]; p = 0.002, corrected), and regulation of gene expression epigenetic (β[SE], -2.119 [0.664]; p = 0.002, corrected) were enriched in BMI in adulthood and WMI in childhood. Mediation analysis showed that there is no mediation effect of childhood-BMI between the adulthood-BMI-PRS and working memory deficits in children. Conclusion: Adulthood-BMI-PRS was associated with working memory among children in the general population. These genetic risks were not mediated by the childhood-BMI itself and were directly associated with working memory deficits.

Local Translation Across Neural Development: A Focus on Radial Glial Cells, Axons, and Synaptogenesis

In the past two decades, significant progress has been made in our understanding of mRNA localization and translation at distal sites in axons and dendrites. The existing literature shows that local translation is regulated in a temporally and spatially restricted manner and is critical throughout embryonic and post-embryonic life. Here, recent key findings about mRNA localization and local translation across the various stages of neural development, including neurogenesis, axon development, and synaptogenesis, are reviewed. In the early stages of development, mRNAs are localized and locally translated in the endfeet of radial glial cells, but much is still unexplored about their functional significance. Recent in vitro and in vivo studies have provided new information about the specific mechanisms regulating local translation during axon development, including growth cone guidance and axon branching. Later in development, localization and translation of mRNAs help mediate the major structural and functional changes that occur in the axon during synaptogenesis. Clinically, changes in local translation across all stages of neural development have important implications for understanding the etiology of several neurological disorders. Herein, local translation and mechanisms regulating this process across developmental stages are compared and discussed in the context of function and dysfunction.

Vinculin-mediated axon growth requires interaction with actin but not talin in mouse neocortical neurons

The actin-binding protein vinculin is a major constituent of focal adhesion, but its role in neuronal development is poorly understood. We found that vinculin deletion in mouse neocortical neurons attenuated axon growth both in vitro and in vivo. Using functional mutants, we found that expression of a constitutively active vinculin significantly enhanced axon growth while the head-neck domain had an inhibitory effect. Interestingly, we found that vinculin-talin interaction was dispensable for axon growth and neuronal migration. Strikingly, expression of the tail domain delayed migration, increased branching, and stunted axon. Inhibition of the Arp2/3 complex or abolishing the tail domain interaction with actin completely reversed the branching phenotype caused by tail domain expression without affecting axon length. Super-resolution microscopy showed increased mobility of actin in tail domain expressing neurons. Our results provide novel insights into the role of vinculin and its functional domains in regulating neuronal migration and axon growth.

Oscillation of Cdc20-APC/C-mediated CAMDI stability is critical for cortical neuron migration

Radial migration during cortical development is required for formation of the six-layered structure of the mammalian cortex. Defective migration of neurons is linked to several developmental disorders such as autism and schizophrenia. A unique swollen structure called the dilation is formed in migrating neurons and is required for movement of the centrosome and nucleus. However, the detailed molecular mechanism by which this dilation forms is unclear. We report that CAMDI, a gene whose deletion is associated with psychiatric behavior, is degraded by cell division cycle protein 20 (Cdc20)–anaphase-promoting complex/cyclosome (APC/C) cell-cycle machinery after centrosome migration into the dilation in mouse brain development. We also show that CAMDI is restabilized in the dilation until the centrosome enters the dilation, at which point it is once again immediately destabilized. CAMDI degradation is carried out by binding to Cdc20–APC/C via the destruction box degron of CAMDI. CAMDI destruction box mutant overexpression inhibits dilation formation and neuronal cell migration via maintaining the stabilized state of CAMDI. These results indicate that CAMDI is a substrate of the Cdc20–APC/C system and that the oscillatory regulation of CAMDI protein correlates with dilation formation for proper cortical migration.

The Formin Fmn2b Is Required for the Development of an Excitatory Interneuron Module in the Zebrafish Acoustic Startle Circuit

The formin family member Fmn2 is a neuronally enriched cytoskeletal remodeling protein conserved across vertebrates. Recent studies have implicated Fmn2 in neurodevelopmental disorders, including sensory processing dysfunction and intellectual disability in humans. Cellular characterization of Fmn2 in primary neuronal cultures has identified its function in the regulation of cell-substrate adhesion and consequently growth cone translocation. However, the role of Fmn2 in the development of neural circuits in vivo, and its impact on associated behaviors have not been tested. Using automated analysis of behavior and systematic investigation of the associated circuitry, we uncover the role of Fmn2b in zebrafish neural circuit development. As reported in other vertebrates, the zebrafish ortholog of Fmn2 is also enriched in the developing zebrafish nervous system. We find that Fmn2b is required for the development of an excitatory interneuron pathway, the spiral fiber neuron, which is an essential circuit component in the regulation of the Mauthner cell (M-cell)-mediated acoustic startle response. Consistent with the loss of the spiral fiber neurons tracts, high-speed video recording revealed a reduction in the short latency escape events while responsiveness to the stimuli was unaffected. Taken together, this study provides evidence for a circuit-specific requirement of Fmn2b in eliciting an essential behavior in zebrafish. Our findings underscore the importance of Fmn2 in neural development across vertebrate lineages and highlight zebrafish models in understanding neurodevelopmental disorders.

Conservation of neural progenitor identity and the emergence of neocortical neuronal diversity

One paramount challenge for neuroscientists over the past century has been to identify the embryonic origins of the enormous diversity of cortical neurons found in the adult human neocortex and to unravel the developmental processes governing their emergence. In all mammals, including humans, the radial glia lining the ventricles of the embryonic telencephalon, more recently reclassified as apical radial glia (aRGs), have been identified as the neural progenitors giving rise to all excitatory neurons and inhibitory interneurons of the six-layered cortex. In this review, we explore the fundamental molecular and cellular mechanisms that regulate aRG function and the generation of neuronal diversity in the dorsal telencephalon. We survey the key structural features essential for the retention of the highly polarized aRG morphology and therefore impose aRG identity after cytokinesis. We discuss how these structures and associated molecular signaling complexes influence aRG proliferative capacity and the decision to undergo proliferative self-renewing symmetric or neurogenic asymmetric divisions. We also explore the intriguing and complex question of how the extensive neuronal diversity within the adult neocortex arises from the small aRG population located within the cortical proliferative zone. We further highlight the recent clonal lineage tracing and single-cell transcriptomic profiling studies providing compelling evidence that individual neuronal identity emerges as a consequence of exposure to temporally regulated extrinsic cues which coordinate waves of transcriptional activity that evolve over time to drive neuronal commitment and maturation.

Developmental malformations in Huntington disease: neuropathologic evidence of focal neuronal migration defects in a subset of adult brains

Neuropathologic hallmarks of Huntington Disease (HD) include the progressive neurodegeneration of the striatum and the presence of Huntingtin (HTT) aggregates that result from abnormal polyQ expansion of the HTT gene. Whether the pathogenic trinucleotide repeat expansion of the HTT gene causes neurodevelopmental abnormalities has garnered attention in both murine and human studies; however, documentation of discrete malformations in autopsy brains of HD individuals has yet to be described. We retrospectively searched the New York Brain Bank (discovery cohort) and an independent cohort (validation cohort) to determine whether developmental malformations are more frequently detected in HD versus non-HD brains and to document their neuropathologic features. One-hundred and thirty HD and 1600 non-HD whole brains were included in the discovery cohort and 720 HD and 1989 non-HD half brains were assessed in the validation cohort. Cases with developmental malformations were found at 6.4–8.2 times greater frequency in HD than in non-HD brains (discovery cohort: OR 8.68, 95% CI 3.48–21.63, P=4.8 × 10^-5; validation cohort: OR 6.50, 95% CI 1.83–23.17, P=0.0050). Periventricular nodular heterotopias (PNH) were the most frequent malformations and contained HTT and p62 aggregates analogous to the cortex, whereas cortical malformations with immature neuronal populations did not harbor such inclusions. HD individuals with malformations had heterozygous HTT CAG expansions between 40 and 52 repeats, were more frequently women, and all were asymmetric and focal, aside from one midline hypothalamic hamartoma. Using two independent brain bank cohorts, this large neuropathologic series demonstrates an increased occurrence of developmental malformations in HD brains. Since pathogenic HTT gene expansion is associated with genomic instability, one possible explanation is that neuronal precursors are more susceptible to somatic mutation of genes involved in cortical migration. Our findings further support emerging evidence that pathogenic trinucleotide repeat expansions of the HTT gene may impact neurodevelopment.

Neuronal Delamination and Outer Radial Glia Generation in Neocortical Development

During neocortical development, many neuronally differentiating cells (neurons and intermediate progenitor cells) are generated at the apical/ventricular surface by the division of neural progenitor cells (apical radial glial cells, aRGs). Neurogenic cell delamination, in which these neuronally differentiating cells retract their apical processes and depart from the apical surface, is the first step of their migration. Since the microenvironment established by the apical endfeet is crucial for maintaining neuroepithelial (NE)/aRGs, proper timing of the detachment of the apical endfeet is critical for the quantitative control of neurogenesis in cerebral development. During delamination, the microtubule-actin-AJ (adherens junction) configuration at the apical endfeet shows dynamic changes, concurrent with the constriction of the AJ ring at the apical endfeet and downregulation of cadherin expression. This process is mediated by transcriptional suppression of AJ-related molecules and multiple cascades to regulate cell adhesion and cytoskeletal architecture in a posttranscriptional manner. Recent advances have added molecules to the latter category: the interphase centrosome protein AKNA affects microtubule dynamics to destabilize the microtubule-actin-AJ complex, and the microtubule-associated protein Lzts1 inhibits microtubule assembly and activates actomyosin systems at the apical endfeet of differentiating cells. Moreover, Lzts1 induces the oblique division of aRGs, and loss of Lzts1 reduces the generation of outer radial glia (oRGs, also called basal radial glia, bRGs), another type of neural progenitor cell in the subventricular zone. These findings suggest that neurogenic cell delamination, and in some cases oRG generation, could be caused by a spectrum of interlinked mechanisms.

C11orf95-RELA reprograms 3D epigenome in supratentorial ependymoma

Supratentorial ependymoma (ST-EPN) is a type of malignant brain tumor mainly seen in children. Since 2014, it has been known that an intrachromosomal fusion C11orf95-RELA is an oncogenic driver in ST-EPN [Parker et al. Nature 506:451–455 (2014); Pietsch et al. Acta Neuropathol 127:609–611 (2014)] but the molecular mechanisms of oncogenesis are unclear. Here we show that the C11orf95 component of the fusion protein dictates DNA binding activity while the RELA component is required for driving the expression of ependymoma-associated genes. Epigenomic characterizations using ChIP-seq and HiChIP approaches reveal that C11orf95-RELA modulates chromatin states and mediates chromatin interactions, leading to transcriptional reprogramming in ependymoma cells. Our findings provide important characterization of the molecular underpinning of C11orf95-RELA fusion and shed light on potential therapeutic targets for C11orf95-RELA subtype ependymoma.

The Role of ADF/Cofilin in Synaptic Physiology and Alzheimer's Disease

Actin-depolymerization factor (ADF)/cofilin, a family of actin-binding proteins, are critical for the regulation of actin reorganization in response to various signals. Accumulating evidence indicates that ADF/cofilin also play important roles in neuronal structure and function, including long-term potentiation and depression. These are the most extensively studied forms of long-lasting synaptic plasticity and are widely regarded as cellular mechanisms underlying learning and memory. ADF/cofilin regulate synaptic function through their effects on dendritic spines and the trafficking of glutamate receptors, the principal mediator of excitatory synaptic transmission in vertebrates. Regulation of ADF/cofilin involves various signaling pathways converging on LIM domain kinases and slingshot phosphatases, which phosphorylate/inactivate and dephosphorylate/activate ADF/cofilin, respectively. Actin-depolymerization factor/cofilin activity is also regulated by other actin-binding proteins, activity-dependent subcellular distribution and protein translation. Abnormalities in ADF/cofilin have been associated with several neurodegenerative disorders such as Alzheimer’s disease. Therefore, investigating the roles of ADF/cofilin in the brain is not only important for understanding the fundamental processes governing neuronal structure and function, but also may provide potential therapeutic strategies to treat brain disorders.

The actin binding protein α-actinin-2 expression is associated with dendritic spine plasticity and migrating granule cells in the rat dentate gyrus following pilocarpine-induced seizures

α-actinin-2 (α-actn-2) is an F-actin-crosslinking protein, localized in dendritic spines. In vitro studies suggested that it is involved in spinogenesis, morphogenesis, actin organization, cell migration and anchoring of the NR1 subunit of the N-methyl-D-aspartate (NMDA) receptors in dendritic spines. However, little is known regarding its function in vivo. We examined the levels of α-actn-2 expression within the dentate gyrus (DG) during the development of chronic limbic seizures (epileptogenesis) induced by pilocarpine in rats. In this model, plasticity of the DG glutamatergic granule cells including spine loss, spinogenesis, morphogenesis, neo-synaptogenesis, aberrant migration, and alterations of NMDA receptors have been well characterized. We showed that α-actn-2 immunolabeling was reduced in the inner molecular layer at 1-2 weeks post-status epilepticus (SE), when granule cell spinogenesis and morphogenesis occur. This low level persisted at the chronic stage when new functional synapses are established. This decreased of α-actn-2 protein is concomitant with the recovery of drebrin A (DA), another actin-binding protein, at the chronic stage. Indeed, we demonstrated in cultured cells that in contrast to DA, α-actn-2 did not protect F-actin destabilization and DA inhibited α-actn-2 binding to F-actin. Such alteration could affect the anchoring of NR1 in dendritic spines. Furthermore, we showed that the expression of α-actn-2 and NR1 are co-down-regulated in membrane fractions of pilocarpine animals at chronic stage. Last, we showed that α-actn-2 is expressed in migrating newly born granule cells observed within the hilus of pilocarpine-treated rats. Altogether, our results suggest that α-actn-2 is not critical for the structural integrity and stabilization of granule cell dendritic spines. Instead, its expression is regulated when spinogenesis and morphogenesis occur and within migrating granule cells. Our data also suggest that the balance between α-actn-2 and DA expression levels may modulate NR1 anchoring within dendritic spines.

Neuronal migration and disorders - an update

This review highlights genes, proteins and subcellular mechanisms, recently shown to influence cortical neuronal migration. A current view on mechanisms which become disrupted in a diverse array of migration disorders is presented. The microtubule (MT) cytoskeleton is a major player in migrating neurons. Recently, variable impacts on MTs have been revealed in different cell compartments. Thus there are a multiplicity of effects involving centrosomal, microtubule-associated, as well as motor proteins. However, other causative factors also emerge, illuminating cortical neuronal migration research. These include disruptions of the actin cytoskeleton, the extracellular matrix, different adhesion molecules and signaling pathways, especially revealed in disorders such as periventricular heterotopia. These recent advances often involve the use of human in vitro models as well as model organisms. Focusing on cell-type specific knockouts and knockins, as well as generating omics and functional data, all seem critical for an integrated view on neuronal migration dysfunction.

Extracellular Control of Radial Glia Proliferation and Scaffolding During Cortical Development and Pathology

During the development of the cortex, newly generated neurons migrate long-distances in the expanding tissue to reach their final positions. Pyramidal neurons are produced from dorsal progenitors, e.g., radial glia (RGs) in the ventricular zone, and then migrate along RG processes basally toward the cortex. These neurons are hence dependent upon RG extensions to support their migration from apical to basal regions. Several studies have investigated how intracellular determinants are required for RG polarity and subsequent formation and maintenance of their processes. Fewer studies have identified the influence of the extracellular environment on this architecture. This review will focus on extracellular factors which influence RG morphology and pyramidal neuronal migration during normal development and their perturbations in pathology. During cortical development, RGs are present in different strategic positions: apical RGs (aRGs) have their cell bodies located in the ventricular zone with an apical process contacting the ventricle, while they also have a basal process extending radially to reach the pial surface of the cortex. This particular conformation allows aRGs to be exposed to long range and short range signaling cues, whereas basal RGs (bRGs, also known as outer RGs, oRGs) have their cell bodies located throughout the cortical wall, limiting their access to ventricular factors. Long range signals impacting aRGs include secreted molecules present in the embryonic cerebrospinal fluid (e.g., Neuregulin, EGF, FGF, Wnt, BMP). Secreted molecules also contribute to the extracellular matrix (fibronectin, laminin, reelin). Classical short range factors include cell to cell signaling, adhesion molecules and mechano-transduction mechanisms (e.g., TAG1, Notch, cadherins, mechanical tension). Changes in one or several of these components influencing the RG extracellular environment can disrupt the development or maintenance of RG architecture on which neuronal migration relies, leading to a range of cortical malformations. First, we will detail the known long range signaling cues impacting RG. Then, we will review how short range cell contacts are also important to instruct the RG framework. Understanding how RG processes are structured by their environment to maintain and support radial migration is a critical part of the investigation of neurodevelopmental disorders.

Definitions and classification of malformations of cortical development: practical guidelines

Malformations of cortical development are a group of rare disorders commonly manifesting with developmental delay, cerebral palsy or seizures. The neurological outcome is extremely variable depending on the type, extent and severity of the malformation and the involved genetic pathways of brain development. Neuroimaging plays an essential role in the diagnosis of these malformations, but several issues regarding malformations of cortical development definitions and classification remain unclear. The purpose of this consensus statement is to provide standardized malformations of cortical development terminology and classification for neuroradiological pattern interpretation. A committee of international experts in paediatric neuroradiology prepared systematic literature reviews and formulated neuroimaging recommendations in collaboration with geneticists, paediatric neurologists and pathologists during consensus meetings in the context of the European Network Neuro-MIG initiative on Brain Malformations (https://www.neuro-mig.org/). Malformations of cortical development neuroimaging features and practical recommendations are provided to aid both expert and non-expert radiologists and neurologists who may encounter patients with malformations of cortical development in their practice, with the aim of improving malformations of cortical development diagnosis and imaging interpretation worldwide.

Periventricular nodular heterotopia in a Chihuahua

Periventricular nodular heterotopia is a common neuronal malformation in humans, often leading to epilepsy and other neurologic diseases. A 2‐month‐old female Chihuahua weighing 750 g was examined because of a history of epileptic seizures and abnormalities in gait and behavior. Results of the clinical examination were consistent with a multifocal neurologic disease with localization in the forebrain and spinovestibular system. The magnetic resonance imaging showed multiple bilateral periventricular nodules isointense to gray matter and ventriculomegaly. Histopathological and immunohistological examination of the brain revealed that periventricular nodules consisted of neurons, fewer astrocytes, and some oligodendroglia consistent with periventricular nodular heterotopias.

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.

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STAND protein Nwd1 interacts with Paics to regulate the purinosome formation

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Dysregulated expression of Nwd1 induced the premature differentiation of NSPCs

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Nwd1 KD repressed the neuronal migration, causing the periventricular heterotopia

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Tightly regulated assembly of purinosome components is crucial for corticogenesis

Adherens Junctions: Guardians of Cortical Development

Apical radial glia comprise the pseudostratified neuroepithelium lining the embryonic lateral ventricles and give rise to the extensive repertoire of pyramidal neuronal subtypes of the neocortex. The establishment of a highly apicobasally polarized radial glial morphology is a mandatory prerequisite for cortical development as it governs neurogenesis, neural migration and the integrity of the ventricular wall. As in all epithelia, cadherin-based adherens junctions (AJs) play an obligate role in the maintenance of radial glial apicobasal polarity and neuroepithelial cohesion. In addition, the assembly of resilient AJs is critical to the integrity of the neuroepithelium which must resist the tensile forces arising from increasing CSF volume and other mechanical stresses associated with the expansion of the ventricles in the embryo and neonate. Junctional instability leads to the collapse of radial glial morphology, disruption of the ventricular surface and cortical lamination defects due to failed neuronal migration. The fidelity of cortical development is therefore dependent on AJ assembly and stability. Mutations in genes known to control radial glial junction formation are causative for a subset of inherited cortical malformations (neuronal heterotopias) as well as perinatal hydrocephalus, reinforcing the concept that radial glial junctions are pivotal determinants of successful corticogenesis. In this review we explore the key animal studies that have revealed important insights into the role of AJs in maintaining apical radial glial morphology and function, and as such, have provided a deeper understanding of the aberrant molecular and cellular processes contributing to debilitating cortical malformations. We highlight the reciprocal interactions between AJs and the epithelial polarity complexes that impose radial glial apicobasal polarity. We also discuss the critical molecular networks promoting AJ assembly in apical radial glia and emphasize the role of the actin cytoskeleton in the stabilization of cadherin adhesion – a crucial factor in buffering the mechanical forces exerted as a consequence of cortical expansion.

Persistent Cyfip1 Expression Is Required to Maintain the Adult Subventricular Zone Neurogenic Niche

Neural stem cells (NSCs) persist throughout life in the subventricular zone (SVZ) neurogenic niche of the lateral ventricles as Type B1 cells in adult mice. Maintaining this population of NSCs depends on the balance between quiescence and self-renewing or self-depleting cell divisions. Interactions between B1 cells and the surrounding niche are important in regulating this balance, but the mechanisms governing these processes have not been fully elucidated. The cytoplasmic FMRP-interacting protein (Cyfip1) regulates apical-basal polarity in the embryonic brain. Loss of Cyfip1 during embryonic development in mice disrupts the embryonic niche and affects cortical neurogenesis. However, a direct role for Cyfip1 in the regulation of adult NSCs has not been established. Here, we demonstrate that Cyfip1 expression is preferentially localized to B1 cells in the adult mouse SVZ. Loss of Cyfip1 in the embryonic mouse brain results in altered adult SVZ architecture and expansion of the adult B1 cell population at the ventricular surface. Furthermore, acute deletion of Cyfip1 in adult NSCs results in a rapid change in adherens junction proteins as well as increased proliferation and number of B1 cells at the ventricular surface. Together, these data indicate that Cyfip1 plays a critical role in the formation and maintenance of the adult SVZ niche; furthermore, deletion of Cyfip1 unleashes the capacity of adult B1 cells for symmetric renewal to increase the adult NSC pool.SIGNIFICANCE STATEMENT Neural stem cells (NSCs) persist in the subventricular zone of the lateral ventricles in adult mammals, and the size of this population is determined by the balance between quiescence and self-depleting or renewing cell division. The mechanisms regulating these processes are not fully understood. This study establishes that the cytoplasmic FMRP interacting protein 1 (Cyfip1) regulates NSC fate decisions in the adult subventricular zone and adult NSCs that are quiescent or typically undergo self-depleting divisions retain the ability to self-renew. These results contribute to our understanding of how adult NSCs are regulated throughout life and has potential implications for human brain disorders.

Intraventricular Ectopic Cerebellum

BACKGROUND

Cerebellar ectopy is a rare finding, with few cases previously reported. Intraventricular localized cerebellar ectopy was described in only 1 case within the fourth ventricle.

CASE DESCRIPTION

A 9-year-old girl suffered for 2 years from bilateral frontoparietal headaches, sometimes accompanied by vomiting and photophobia. Magnetic resonance imaging demonstrated an oval-shaped lesion within the left lateral ventricle, characterized by well-defined margins without a clear cleavage plane from the adjacent choroid plexus. The mass presented an intermediate signal on T1- and T2-weighted sequences, similar to gray matter, and reduced ADC values on ADC maps compared with white matter, with no enhancement after gadolinium-based contrast injection. After resection, macroscopic examination revealed an organoid structure with leptomeningeal lining and a clear-cut cortex and white matter components. Histology demonstrated normal cerebellum with a double-layered cortex and normal underlying white matter. The cerebellar ectopy was focally covered by bundles of capillary vascular structures covered by a monostratified ependymal cell lining, consistent with choroid plexus.

CONCLUSIONS

We describe, for the first time to our knowledge, the case of a child with ectopic cerebellar tissue harboring the supratentorial ventricular system. Plausible etiologic mechanism consists in the herniation of the cerebellar germinal tissue into the ventricular system through the ependyma, allowing cell migration to the supratentorial compartment, followed by maturation into the normal cerebellum.

Cortical Malformations: Lessons in Human Brain Development

Creating a functional cerebral cortex requires a series of complex and well-coordinated developmental steps. These steps have evolved across species with the emergence of cortical gyrification and coincided with more complex behaviors. The presence of diverse progenitor cells, a protracted timeline for neuronal migration and maturation, and diverse neuronal types are developmental features that have emerged in the gyrated cortex. These factors could explain how the human brain has expanded in size and complexity. However, their complex nature also renders new avenues of vulnerability by providing additional cell types that could contribute to disease and longer time windows that could impact the composition and organization of the cortical circuit. We aim to discuss the unique developmental steps observed in human corticogenesis and propose how disruption of these species-unique processes could lead to malformations of cortical development.

Loss of Tiparp Results in Aberrant Layering of the Cerebral Cortex

2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-inducible poly-ADP-ribose polymerase (TIPARP) is an enzyme that adds a single ADP-ribose moiety to itself or other proteins. Tiparp is highly expressed in the brain; however, its function in this organ is unknown. Here, we used Tiparp^–/– mice to determine Tiparp’s role in the development of the prefrontal cortex.

2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-inducible poly-ADP-ribose polymerase (TIPARP) is an enzyme that adds a single ADP-ribose moiety to itself or other proteins. Tiparp is highly expressed in the brain; however, its function in this organ is unknown. Here, we used Tiparp^–/– mice to determine Tiparp’s role in the development of the prefrontal cortex. Loss of Tiparp resulted in an aberrant organization of the mouse cortex, where the upper layers presented increased cell density in the knock-out mice compared with wild type. Tiparp loss predominantly affected the correct distribution and number of GABAergic neurons. Furthermore, neural progenitor cell proliferation was significantly reduced. Neural stem cells (NSCs) derived from Tiparp^–/– mice showed a slower rate of migration. Cytoskeletal components, such as α-tubulin are key regulators of neuronal differentiation and cortical development. α-tubulin mono-ADP ribosylation (MAR) levels were reduced in Tiparp^–/– cells, suggesting that Tiparp plays a role in the MAR of α-tubulin. Despite the mild phenotype presented by Tiparp^–/– mice, our findings reveal an important function for Tiparp and MAR in the correct development of the cortex. Unravelling Tiparp’s role in the cortex, could pave the way to a better understanding of a wide spectrum of neurological diseases which are known to have increased expression of TIPARP.

Role of adherens junctions and apical-basal polarity of neural stem/progenitor cells in the pathogenesis of neurodevelopmental disorders: a novel perspective on congenital Zika syndrome

Radial glial cells (RGCs) are the neural stem/progenitor cells (NSPCs) that give rise to most of neurons and glial cells that constitute the adult central nervous system. A hallmark of RGCs is their polarization along the apical-basal axis. They extend a long basal process that contacts the pial surface and a short apical process to the ventricular surface. Adherens junctions (AJs) are organized as belt-like structures at the most-apical lateral plasma membrane of the apical processes. These junctional complexes anchor RGCs to each other and allow the recruitment of cytoplasmic proteins that act as apical-basal determinants. It has been proposed that disruption of AJs underlies the onset of different neurodevelopmental disorders. In fact, studies performed in different animal models indicate that loss of function of AJs-related proteins in NSPCs can disrupt cell polarity, imbalance proliferation and/or differentiation rates and increase cell death, which, in turn, lead to disruption of the cytoarchitecture of the ventricular zone, protrusion of non-polarized cells into the ventricles, cortical thinning, and ventriculomegaly/hydrocephalus, among other neuropathological findings. Recent Zika virus (ZIKV) outbreaks and the high comorbidity of ZIKV infection with congenital neurodevelopmental defects have led to the World Health Organization to declare a public emergency of international concern. Thus, noteworthy advances have been made in clinical and experimental ZIKV research. This review summarizes the current knowledge regarding the function of AJs in normal and pathological corticogenesis and focuses on the neuropathological and cellular mechanisms involved in congenital ZIKV syndrome, highlighting the potential role of cell-to-cell junctions between NSPCs in the etiopathogenesis of such syndrome.

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.

A novel role for CAMKIIβ in the regulation of cortical neuron migration: implications for neurodevelopmental disorders

Perturbation of CaMKIIβ expression has been associated with multiple neuropsychiatric diseases, highlighting CaMKIIβ as a gene of interest. Yet, in contrast to CaMKIIα, the specific functions of CaMKIIβ in the brain remain poorly explored. Here, we reveal a novel function for this CaMKII isoform in vivo during neuronal development. By using in utero electroporation, we show that CaMKIIβ is an important regulator of radial migration of projection neurons during cerebral cortex development. Knockdown of CaMKIIβ causes accelerated migration of nascent pyramidal neurons, whereas overexpression of CaMKIIβ inhibits migration, demonstrating that precise regulation of CaMKIIβ expression is required for correct neuronal migration. More precisely, CaMKIIβ controls the multipolar-bipolar transition in the intermediate zone and locomotion in the cortical plate through its actin-binding and -bundling activities. In addition, our data indicate that a fine-tuned balance between CaMKIIβ and cofilin activities is necessary to ensure proper migration of cortical neurons. Thus, our findings define a novel isoform-specific function for CaMKIIβ, demonstrating that CaMKIIβ has a major biological function in the developing brain.

Shaping neurodevelopment: distinct contributions of cytoskeletal proteins

Development of a neuron critically depends on the organization of its cytoskeleton. Cytoskeletal components, such as tubulins and actins, have the remarkable ability to organize themselves into filaments and networks to support specialized and compartmentalized functions. Alterations in cytoskeletal proteins have long been associated with a variety of neurodevelopmental disorders. This review focuses on recent findings, primarily from forward genetic screens in Caenorhabditis elegans that illustrate how different tubulin protein isotypes can play distinct roles in neuronal development and function. Additionally, we discuss studies revealing new regulators of the actin cytoskeleton, and highlight recent technological advances in in vivo imaging and functional dissection of the neuronal cytoskeleton.

De novo and inherited private variants in MAP1B in periventricular nodular heterotopia

Periventricular nodular heterotopia (PVNH) is a malformation of cortical development commonly associated with epilepsy. We exome sequenced 202 individuals with sporadic PVNH to identify novel genetic risk loci. We first performed a trio-based analysis and identified 219 de novo variants. Although no novel genes were implicated in this initial analysis, PVNH cases were found overall to have a significant excess of nonsynonymous de novo variants in intolerant genes (p = 3.27x10-7), suggesting a role for rare new alleles in genes yet to be associated with the condition. Using a gene-level collapsing analysis comparing cases and controls, we identified a genome-wide significant signal driven by four ultra-rare loss-of-function heterozygous variants in MAP1B, including one de novo variant. In at least one instance, the MAP1B variant was inherited from a parent with previously undiagnosed PVNH. The PVNH was frontally predominant and associated with perisylvian polymicrogyria. These results implicate MAP1B in PVNH. More broadly, our findings suggest that detrimental mutations likely arising in immediately preceding generations with incomplete penetrance may also be responsible for some apparently sporadic diseases.

Novel Plasma Proteins in Nepalese School-aged Children are Associated with a Small Head Size at Birth

Fetal growth restriction increases the risk of poor childhood growth and development and chronic disease in adulthood. Yet, little is known about biological pathways that mediate the long-lasting effects of suboptimal intrauterine growth. We explored the plasma proteome in a cohort of 500 Nepalese children 6–8 years of age to identify plasma proteins associated with multiple anthropometric size indicators at birth. Among 982 proteins analyzed, no proteins differed by birth weight, length, or weight-for-length indicators. However, 25 proteins were differentially abundant in children with a small vs normal head circumference at birth (<−2 vs. ≥−2 z-scores of the WHO growth standards). Angiopoietin-like 6 was 19.4% more abundant and the other 24 proteins were 7–21% less abundant in children with a small vs normal head circumference at birth, adjusted for potential confounders. The less abundant proteins included actins, actin filament organizing proteins (α-actinin, talin, filamin, cofilin, profilin, and vinculin), proteins involved in muscle contraction, and glycolytic enzymes, which were all positively correlated with each other. A novel cluster of childhood plasma proteins involved in angiogenesis and cytoskeleton dynamics was associated with a small head size at birth. The prognostic value of an altered proteomic phenotype remains to be investigated.

Inter-dependent apical microtubule and actin dynamics orchestrate centrosome retention and neuronal delamination

Detachment of newborn neurons from the neuroepithelium is required for correct neuronal architecture and functional circuitry. This process, also known as delamination, involves adherens-junction disassembly and acto-myosin-mediated abscission, during which the centrosome is retained while apical/ciliary membranes are shed. Cell-biological mechanisms mediating delamination are, however, poorly understood. Using live-tissue and super-resolution imaging, we uncover a centrosome-nucleated wheel-like microtubule configuration, aligned with the apical actin cable and adherens-junctions within chick and mouse neuroepithelial cells. These microtubules maintain adherens-junctions while actin maintains microtubules, adherens-junctions and apical end-foot dimensions. During neuronal delamination, acto-myosin constriction generates a tunnel-like actin-microtubule configuration through which the centrosome translocates. This movement requires inter-dependent actin and microtubule activity, and we identify drebrin as a potential coordinator of these cytoskeletal dynamics. Furthermore, centrosome compromise revealed that this organelle is required for delamination. These findings identify new cytoskeletal configurations and regulatory relationships that orchestrate neuronal delamination and may inform mechanisms underlying pathological epithelial cell detachment.

Rho GTPases as therapeutic targets in cancer (Review)

Rho GTPases are key molecular switches controlling the transduction of external signals to cytoplasmic and nuclear effectors. In the last few years, the development of genetic and pharmacological tools has allowed a more precise definition of the specific roles of Rho GTPases in cancer. The aim of the present review is to describe the cellular functions regulated by these proteins with focus in deregulated signals present in malignant tumors. Finally, we describe the state of the art in search of different experimental therapeutic strategies with Rho GTPases as molecular targets.

Llgl1 Connects Cell Polarity with Cell-Cell Adhesion in Embryonic Neural Stem Cells

Malformations of the cerebral cortex (MCCs) are devastating developmental disorders. We report here that mice with embryonic neural stem-cell-specific deletion of Llgl1 (Nestin-Cre/Llgl1^fl/fl), a mammalian ortholog of the Drosophila cell polarity gene lgl, exhibit MCCs resembling severe periventricular heterotopia (PH). Immunohistochemical analyses and live cortical imaging of PH formation revealed that disruption of apical junctional complexes (AJCs) was responsible for PH in Nestin-Cre/Llgl1^fl/fl brains. While it is well known that cell polarity proteins govern the formation of AJCs, the exact mechanisms remain unclear. We show that LLGL1 directly binds to and promotes internalization of N-cadherin, and N-cadherin/LLGL1 interaction is inhibited by atypical protein kinase C-mediated phosphorylation of LLGL1, restricting the accumulation of AJCs to the basolateral-apical boundary. Disruption of the N-cadherin-LLGL1 interaction during cortical development in vivo is sufficient for PH. These findings reveal a mechanism responsible for the physical and functional connection between cell polarity and cell-cell adhesion machineries in mammalian cells.

Missense-depleted regions in population exomes implicate ras superfamily nucleotide-binding protein alteration in patients with brain malformation

Genomic sequence interpretation can miss clinically relevant missense variants for several reasons. Rare missense variants are numerous in the exome and difficult to prioritise. Affected genes may also not have existing disease association. To improve variant prioritisation, we leverage population exome data to identify intragenic missense-depleted regions (MDRs) genome-wide that may be important in disease. We then use missense depletion analyses to help prioritise undiagnosed disease exome variants. We demonstrate application of this strategy to identify a novel gene association for human brain malformation. We identified de novo missense variants that affect the GDP/GTP-binding site of ARF1 in three unrelated patients. Corresponding functional analysis suggests ARF1 GDP/GTP-activation is affected by the specific missense mutations associated with heterotopia. These findings expand the genetic pathway underpinning neurologic disease that classically includes FLNA. ARF1 along with ARFGEF2 add further evidence implicating ARF/GEFs in the brain. Using functional ontology, top MDR-containing genes were highly enriched for nucleotide-binding function, suggesting these may be candidates for human disease. Routine consideration of MDR in the interpretation of exome data for rare diseases may help identify strong genetic factors for many severe conditions, infertility/reduction in reproductive capability, and embryonic conditions contributing to preterm loss.

Actin dynamics provides membrane tension to merge fusing vesicles into the plasma membrane

Vesicle fusion is executed via formation of an Ω-shaped structure (Ω-profile), followed by closure (kiss-and-run) or merging of the Ω-profile into the plasma membrane (full fusion). Although Ω-profile closure limits release but recycles vesicles economically, Ω-profile merging facilitates release but couples to classical endocytosis for recycling. Despite its crucial role in determining exocytosis/endocytosis modes, how Ω-profile merging is mediated is poorly understood in endocrine cells and neurons containing small ∼30–300 nm vesicles. Here, using confocal and super-resolution STED imaging, force measurements, pharmacology and gene knockout, we show that dynamic assembly of filamentous actin, involving ATP hydrolysis, N-WASP and formin, mediates Ω-profile merging by providing sufficient plasma membrane tension to shrink the Ω-profile in neuroendocrine chromaffin cells containing ∼300 nm vesicles. Actin-directed compounds also induce Ω-profile accumulation at lamprey synaptic active zones, suggesting that actin may mediate Ω-profile merging at synapses. These results uncover molecular and biophysical mechanisms underlying Ω-profile merging.

As vesicles fuse to the plasma membrane, they form intermediate Ω-shaped structures followed by either closure of the pore or full merging with the plasma membrane. Here Wen et al. show that dynamic actin assembly provides membrane tension to promote Ω merging in neuroendocrine cells and synapses.