Filamin A regulates neuronal migration through brefeldin A-inhibited guanine exchange factor 2-dependent Arf1 activation

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.

Unraveling the pathogenic mechanism of a novel filamin a frameshift variant in periventricular nodular heterotopia

Background

Periventricular nodular heterotopia (PVNH) is a neuronal migration disorder caused by the inability of neurons to move to the cortex. Patients with PVNH often experience epilepsy due to ectopic neuronal discharges. Most cases of PVNH are associated with variations in filamin A (FLNA), which encodes an actin-binding protein. However, the underlying pathological mechanisms remain unclear.

Methods

Next-generation sequencing was performed to detect variants in the patient with PVNH, and the findings were confirmed using Sanger sequencing. Iterative threading assembly refinement was used to predict the structures of the variant proteins, and the search tool for the retrieval of interacting genes/proteins database was used to determine the interactions between FLNA and motility-related proteins. An induced pluripotent stem cell (iPSC) line was generated as a disease model by reprogramming human peripheral blood mononuclear cells. The FLNA expression in iPSCs was assessed using western blot and quantitative real-time polymerase chain reaction (qRT-PCR). Immunofluorescence analysis was performed to determine the arrangement of F-actin.

Results

A novel FLNA frameshift variant (NM_001456.3: c.1466delG, p. G489Afs*9) was identified in a patient with PVNH and epilepsy. Bioinformatic analysis indicated that this variation was likely to impair FLNA function. Western blot and qRT-PCR analysis of iPSCs derived from the patient's peripheral blood mononuclear cells revealed the absence of FLNA protein and mRNA. Immunofluorescence analysis suggested an irregular arrangement and disorganization of F-actin compared to that observed in healthy donors.

Conclusion

Our findings indicate that the frameshift variant of FLNA (NM_001456.3: c.1466delG, p. G489Afs*9) impairs the arrangement and organization of F-actin, potentially influencing cell migration and causing PVNH.

Novel insight into the role of A-kinase anchoring proteins (AKAPs) in ischemic stroke and therapeutic potentials

Ischemic stroke, a devastating disease associated with high mortality and disability worldwide, has emerged as an urgent public health issue. A-kinase anchoring proteins (AKAPs) are a group of signal-organizing molecules that compartmentalize and anchor a wide range of receptors and effector proteins and have a major role in stabilizing mitochondrial function and promoting neurodevelopmental development in the central nervous system (CNS). Growing evidence suggests that dysregulation of AKAPs expression and activity is closely associated with oxidative stress, ion disorder, mitochondrial dysfunction, and blood-brain barrier (BBB) impairment in ischemic stroke. However, the underlying mechanisms remain inadequately understood. This review provides a comprehensive overview of the composition and structure of A-kinase anchoring protein (AKAP) family members, emphasizing their physiological functions in the CNS. We explored in depth the molecular and cellular mechanisms of AKAP complexes in the pathological progression and risk factors of ischemic stroke, including hypertension, hyperglycemia, lipid metabolism disorders, and atrial fibrillation. Herein, we highlight the potential of AKAP complexes as a pharmacological target against ischemic stroke in the hope of inspiring translational research and innovative clinical approaches.

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.

A neurodevelopmental disorder associated with an activating de novo missense variant in ARF1

ADP-ribosylation factor 1 (ARF1) is a small GTPase that regulates membrane traffic at the Golgi apparatus and endosomes through recruitment of several coat proteins and lipid-modifying enzymes. Here, we report a pediatric patient with an ARF1-related disorder because of a monoallelic de novo missense variant (c.296 G > A; p.R99H) in the ARF1 gene, associated with developmental delay, hypotonia, intellectual disability and motor stereotypies. Neuroimaging revealed a hypoplastic corpus callosum and subcortical white matter abnormalities. Notably, this patient did not exhibit periventricular heterotopias previously observed in other patients with ARF1 variants (including p.R99H). Functional analysis of the R99H-ARF1 variant protein revealed that it was expressed at normal levels and properly localized to the Golgi apparatus; however, the expression of this variant caused swelling of the Golgi apparatus, increased the recruitment of coat proteins such as coat protein complex I, adaptor protein complex 1 and GGA3 and altered the morphology of recycling endosomes. In addition, we observed that the expression of R99H-ARF1 prevented dispersal of the Golgi apparatus by the ARF1-inhibitor brefeldin A. Finally, protein interaction analyses showed that R99H-ARF1 bound more tightly to the ARF1-effector GGA3 relative to wild-type ARF1. These properties were similar to those of the well-characterized constitutively active Q71L-ARF1 mutant, indicating that the pathogenetic mechanism of the R99H-ARF1 variant involves constitutive activation with resultant Golgi and endosomal alterations. The absence of periventricular nodular heterotopias in this R99H-ARF1 subject also indicates that this finding may not be a consistent phenotypic expression of all ARF1-related disorders.

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.

LUZP1: A new player in the actin-microtubule cross-talk

LUZP1 (leucine zipper protein 1) was first described as being important for embryonic development. Luzp1 null mice present defective neural tube closure and cardiovascular problems, which cause perinatal death. Since then, LUZP1 has also been implicated in the etiology of diseases like the 1p36 and the Townes-Brocks syndromes, and the molecular mechanisms involving this protein started being uncovered. Proteomics studies placed LUZP1 in the interactomes of the centrosome-cilium interface, centriolar satellites, and midbody. Concordantly, LUZP1 is an actin and microtubule-associated protein, which localizes to the centrosome, the basal body of primary cilia, the midbody, actin filaments and cellular junctions. LUZP1, like its interactor EPLIN, is an actin-stabilizing protein and a negative regulator of primary cilia formation. Moreover, through the regulation of actin, LUZP1 has been implicated in the regulation of cell cycle progression, cell migration and epithelial cell apical constriction. This review discusses the latest findings concerning LUZP1 molecular functions and implications in disease development.

A lateral protrusion latticework connects neuroepithelial cells and is regulated during neurogenesis

Dynamic contacts between cells within the developing neuroepithelium are poorly understood but play important roles in cell and tissue morphology and cell signalling. Here, using live-cell imaging and electron microscopy we reveal multiple protrusive structures in neuroepithelial apical endfeet of the chick embryonic spinal cord, including sub-apical protrusions that extend laterally within the tissue, and observe similar structures in human neuroepithelium. We characterise the dynamics, shape and cytoskeleton of these lateral protrusions and distinguish them from cytonemes, filopodia and tunnelling nanotubes. We demonstrate that lateral protrusions form a latticework of membrane contacts between non-adjacent cells, depend on actin but not microtubule dynamics, and provide a lamellipodial-like platform for further extending fine actin-dependent filipodia. We find that lateral protrusions depend on the actin-binding protein WAVE1 (also known as WASF1): misexpression of mutant WAVE1 attenuated protrusion and generated a round-ended apical endfoot morphology. However, this did not alter apico-basal cell polarity or tissue integrity. During normal neuronal delamination, lateral protrusions were withdrawn, but precocious protrusion loss induced by mutant WAVE1 was insufficient to trigger neurogenesis. This study uncovers a new form of cell-cell contact within the developing neuroepithelium, regulation of which prefigures neuronal delamination. This article has an associated First Person interview with the first author of the paper.

Roots of the Malformations of Cortical Development in the Cell Biology of Neural Progenitor Cells

The cerebral cortex is a structure that underlies various brain functions, including cognition and language. Mammalian cerebral cortex starts developing during the embryonic period with the neural progenitor cells generating neurons. Newborn neurons migrate along progenitors’ radial processes from the site of their origin in the germinal zones to the cortical plate, where they mature and integrate in the forming circuitry. Cell biological features of neural progenitors, such as the location and timing of their mitoses, together with their characteristic morphologies, can directly or indirectly regulate the abundance and the identity of their neuronal progeny. Alterations in the complex and delicate process of cerebral cortex development can lead to malformations of cortical development (MCDs). They include various structural abnormalities that affect the size, thickness and/or folding pattern of the developing cortex. Their clinical manifestations can entail a neurodevelopmental disorder, such as epilepsy, developmental delay, intellectual disability, or autism spectrum disorder. The recent advancements of molecular and neuroimaging techniques, along with the development of appropriate in vitro and in vivo model systems, have enabled the assessment of the genetic and environmental causes of MCDs. Here we broadly review the cell biological characteristics of neural progenitor cells and focus on those features whose perturbations have been linked to MCDs.

The Fine Art of Writing a Message: RNA Metabolism in the Shaping and Remodeling of the Nervous System

Neuronal morphogenesis, integration into circuits, and remodeling of synaptic connections occur in temporally and spatially defined steps. Accordingly, the expression of proteins and specific protein isoforms that contribute to these processes must be controlled quantitatively in time and space. A wide variety of post-transcriptional regulatory mechanisms, which act on pre-mRNA and mRNA molecules contribute to this control. They are thereby critically involved in physiological and pathophysiological nervous system development, function, and maintenance. Here, we review recent findings on how mRNA metabolism contributes to neuronal development, from neural stem cell maintenance to synapse specification, with a particular focus on axon growth, guidance, branching, and synapse formation. We emphasize the role of RNA-binding proteins, and highlight their emerging roles in the poorly understood molecular processes of RNA editing, alternative polyadenylation, and temporal control of splicing, while also discussing alternative splicing, RNA localization, and local translation. We illustrate with the example of the evolutionary conserved Musashi protein family how individual RNA-binding proteins are, on the one hand, acting in different processes of RNA metabolism, and, on the other hand, impacting multiple steps in neuronal development and circuit formation. Finally, we provide links to diseases that have been associated with the malfunction of RNA-binding proteins and disrupted post-transcriptional regulation.

ARF1 haploinsufficiency causes periventricular nodular heterotopia with variable clinical expressivity

The primary anatomical defect leading to periventricular nodular heterotopia occurs within the neural progenitors along the neuroepithelial lining of the lateral ventricles and results from a defect in the initiation of neuronal migration, following disruption of the neuroependyma and impaired neuronal motility. Growing evidence indicates that the FLNA-dependent actin dynamics and regulation of vesicle formation and trafficking by activation of ADP-ribosylation factors (ARFs) can play an important role in this cortical malformation. We report the first inherited variant of ARF1 in a girl with intellectual disability and periventricular nodular heterotopia who inherited the variant from the father with previously undiagnosed single nodular heterotopia and mild clinical expression. Additionally, both patients presented some features suggestive of hypohidrotic ectodermal dysplasia. These clinical features showed similarities to those of three previously reported cases with ARF1 missense variants, confirming that haploinsufficiency of this gene causes a recognisable neurological disorder with abnormal neuronal migration and variable clinical expressivity.

Adenosine 5'-Monophosphate Protects from Hypoxia by Lowering Mitochondrial Metabolism and Oxygen Demand

Ischemia and reperfusion injury following severe trauma or cardiac arrest are major causes of organ damage in intensive care patients. The brain is particularly vulnerable because hypoxia rapidly damages neurons due to their heavy reliance on oxidative phosphorylation. Therapeutic hypothermia can reduce ischemia-induced brain damage, but cooling procedures are slow and technically difficult to perform in critical care settings. It has been previously reported that injection of naturally occurring adenosine 5'-monophosphate (AMP) can rapidly induce hypothermia in mice. We studied the underlying mechanisms and found that AMP transiently reduces the heart rate, respiratory rate, body temperature, and the consciousness of adult male and female C57BL/6J mice. Adding AMP to mouse or human neuronal cell cultures dose-dependently reduced the membrane potential (ΔΨm) and Ca signaling of mitochondria in these cells. AMP treatment increased intracellular AMP levels and activated AMP-activated protein kinase, which resulted in the inhibition of mammalian target of rapamycin complex 1 (mTORC1) and of mitochondrial and cytosolic Ca signaling in resting and stimulated neurons. Pretreatment with an intraperitoneal injection of AMP almost doubled the survival time of mice under hypoxic (6% O2) or anoxic (<1% O2) conditions when compared to untreated mice. These findings suggest that AMP induces a hypometabolic state that slows mitochondrial respiration, reduces oxygen demand, and delays the processes that damage mitochondria in the brain and other organs following hypoxia and reperfusion. Further examination of these mechanisms may lead to new treatments that preserve organ function in critical care patients.

The function and pathogenic mechanism of filamin A

Filamin A(FLNa) is an actin-binding protein, which participates in the formation of the cytoskeleton, anchors a variety of proteins in the cytoskeleton and regulates cell adhesion and migration. It is involved in signal transduction, cell proliferation and differentiation, pseudopodia formation, vesicle transport, tumor resistance and genetic diseases by binding with interacting proteins. In order to fully elucidate the structure, function and pathogenesis of FLNa, we summarized all substances which directly or indirectly act on FLNa so far, upstream and downstream targets which having effect on it, signaling pathways and their functions. It also recorded the expression and effect of FLNa in different diseases, including hereditary disease and tumors.

The Multifaceted Roles of Primary Cilia in the Development of the Cerebral Cortex

The primary cilium, a microtubule based organelle protruding from the cell surface and acting as an antenna in multiple signaling pathways, takes center stage in the formation of the cerebral cortex, the part of the brain that performs highly complex neural tasks and confers humans with their unique cognitive capabilities. These activities require dozens of different types of neurons that are interconnected in complex ways. Due to this complexity, corticogenesis has been regarded as one of the most complex developmental processes and cortical malformations underlie a number of neurodevelopmental disorders such as intellectual disability, autism spectrum disorders, and epilepsy. Cortical development involves several steps controlled by cell–cell signaling. In fact, recent findings have implicated cilia in diverse processes such as neurogenesis, neuronal migration, axon pathfinding, and circuit formation in the developing cortex. Here, we will review recent advances on the multiple roles of cilia during cortex formation and will discuss the implications for a better understanding of the disease mechanisms underlying neurodevelopmental disorders.

Formin 2 Regulates Lysosomal Degradation of Wnt-Associated β-Catenin in Neural Progenitors

Although neural progenitor proliferation along the ventricular zone is regulated by β-catenin through Wnt signaling, the cytoskeletal mechanisms that regulate expression and localization of these proteins are not well understood. Our prior studies have shown that loss of the actin-binding Filamin A (FlnA) and actin-nucleating protein Formin 2 (Fmn2) impairs endocytosis of low-density-lipoprotein-receptor-related protein 6 (Lrp6), thereby disrupting β-catenin activation, resulting in decreased brain size. Here, we report that activated RhoA-GTPase disengages Fmn2 N- to C-terminal binding to promote Fmn2 activation and redistribution into lysosomal vesicles. Fmn2 colocalizes with β-catenin in lysosomes and promotes its degradation. Further, Fmn2 binds the E3 ligase Smurf2, enhances Smurf2-dependent ubiquitination, and degradation of Dishevelled-2 (Dvl2), thereby initiates β-catenin degradation. Finally, Fmn2 overexpression disrupts neuroepithelial integrity, neuronal migration, and proliferation-phenotypes in E13 mouse embryos, as seen with loss of Fmn2+FlnA function. Conversely, co-expression of Dvl2 with Fmn2 rescues the proliferation defect due to Fmn2 overexpression in mouse embryos. These findings suggest that there is a homeostatic feedback mechanism in the cytoskeletal-dependent regulation of neural proliferation within the cerebral cortex. Upstream, Fmn2 promotes proliferation by stabilizing the Lrp6 receptor, leading to β-catenin activation. Downstream, RhoA-activated Fmn2 promotes lysosomal degradation of Dvl2, leading to β-catenin degradation.

Cytoskeletal Associated Filamin A and RhoA Affect Neural Progenitor Specification During Mitosis

Neural progenitor proliferation and cell fate decision from self-renewal to differentiation are crucial factors in determining brain size and morphology. The cytoskeletal dependent regulation of these processes is not entirely known. The actin-binding filamin A (FlnA) was shown to regulate proliferation of progenitors by directing changes in cell cycles proteins such as Cdk1 during G2/M phase. Here we report that functional loss of FlnA not only affects the rate of proliferation by altering cell cycle length but also causes a defect in early differentiation through changes in cell fate specification. FlnA interacts with Rho GTPase RhoA, and FlnA loss impairs RhoA activation. Disruption of either of these cytoskeletal associated proteins delays neurogenesis and promotes neural progenitors to remain in proliferative states. Aurora kinase B (Aurkb) has been implicated in cytokinesis, and peaks in expression during the G2/M phase. Inhibition of FlnA or RhoA impairs Aurkb degradation and alters its localization during mitosis. Overexpression of Aurkb replicates the same delay in neurogenesis seen with loss of FlnA or RhoA. Our findings suggest that shared cytoskeletal processes can direct neural progenitor proliferation by regulating the expression and localization of proteins that are implicated in the cell cycle progression and cell fate specification.

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.

Golgipathies in Neurodevelopment: A New View of Old Defects

The Golgi apparatus (GA) is involved in a whole spectrum of activities, from lipid biosynthesis and membrane secretion to the posttranslational processing and trafficking of most proteins, the control of mitosis, cell polarity, migration and morphogenesis, and diverse processes such as apoptosis, autophagy, and the stress response. In keeping with its versatility, mutations in GA proteins lead to a number of different disorders, including syndromes with multisystem involvement. Intriguingly, however, > 40% of the GA-related genes known to be associated with disease affect the central or peripheral nervous system, highlighting the critical importance of the GA for neural function. We have previously proposed the term "Golgipathies" in relation to a group of disorders in which mutations in GA proteins or their molecular partners lead to consequences for brain development, in particular postnatal-onset microcephaly (POM), white-matter defects, and intellectual disability (ID). Here, taking into account the broader role of the GA in the nervous system, we refine and enlarge this emerging concept to include other disorders whose symptoms may be indicative of altered neurodevelopmental processes, from neurogenesis to neuronal migration and the secretory function critical for the maturation of postmitotic neurons and myelination.

GABA type a receptor trafficking and the architecture of synaptic inhibition

Ubiquitous expression of GABA type A receptors (GABAA R) in the central nervous system establishes their central role in coordinating most aspects of neural function and development. Dysregulation of GABAergic neurotransmission manifests in a number of human health disorders and conditions that in certain cases can be alleviated by drugs targeting these receptors. Precise changes in the quantity or activity of GABAA Rs localized at the cell surface and at GABAergic postsynaptic sites directly impact the strength of inhibition. The molecular mechanisms constituting receptor trafficking to and from these compartments therefore dictate the efficacy of GABAA R function. Here we review the current understanding of how GABAA Rs traffic through biogenesis, plasma membrane transport, and degradation. Emphasis is placed on discussing novel GABAergic synaptic proteins, receptor and scaffolding post-translational modifications, activity-dependent changes in GABAA R confinement, and neuropeptide and neurosteroid mediated changes. We further highlight modern techniques currently advancing the knowledge of GABAA R trafficking and clinically relevant neurodevelopmental diseases connected to GABAergic dysfunction. © 2017 Wiley Periodicals, Inc. Develop Neurobiol 78: 238-270, 2018.

BIG2-ARF1-RhoA-mDia1 Signaling Regulates Dendritic Golgi Polarization in Hippocampal Neurons

Proper dendrite development is essential for establishing neural circuitry, and Rho GTPases play key regulatory roles in this process. From mouse brain lysates, we identified Brefeldin A-inhibited guanine exchange factor 2 (BIG2) as a novel Rho GTPase regulatory protein involved in dendrite growth and maintenance. BIG2 was highly expressed during early development, and knockdown of the ARFGEF2 gene encoding BIG2 significantly reduced total dendrite length and the number of branches. Expression of the constitutively active ADP-ribosylation factor 1 ARF1 Q71L rescued the defective dendrite morphogenesis of ARFGEF2-null neurons, indicating that BIG2 controls dendrite growth and maintenance by activating ARF1. Moreover, BIG2 co-localizes with the Golgi apparatus and is required for Golgi deployment into major dendrites in cultured hippocampal neurons. Simultaneous overexpression of BIG2 and ARF1 activated RhoA, and treatment with the RhoA activator lysophosphatidic acid in neurons lacking BIG2 or ARF1 increased the number of cells with dendritic Golgi, suggesting that BIG2 and ARF1 activate RhoA to promote dendritic Golgi polarization. mDia1 was identified as a downstream effector of BIG2-ARF1-RhoA axis, mediating Golgi polarization and dendritic morphogenesis. Furthermore, in utero electroporation of ARFGEF2 shRNA into the embryonic mouse brain confirmed an in vivo role of BIG2 for Golgi deployment into the apical dendrite. Taken together, our results suggest that BIG2-ARF1-RhoA-mDia1 signaling regulates dendritic Golgi polarization and dendrite growth and maintenance in hippocampal neurons.

FilaminA and Formin2 regulate skeletal, muscular, and intestinal formation through mesenchymal progenitor proliferation

The effects of actin dependent molecular mechanisms in coordinating cellular proliferation, migration and differentiation during embryogenesis are not well-understood. We have previously shown that actin-binding Filamin A (FlnA) and actin-nucleating Formin 2 (Fmn2) influence the development of the brain causing microcephaly in mice. In this study, we broaden this phenotype to explore the effects of these two proteins in the development of extra-CNS organ systems, including the gut, muscle, and skeleton. We observed defects in rib and sternum midline closure leading to thoracoabdominal schisis in FlnA+Fmn2 knockout mice, reminiscent of the pentalogy of Cantrell syndrome. These mice exhibit shortened guts, as well as thinned thoracic muscle mass. Immunostaining showed these changes are partially caused by a decrease in the number of presumptive mesenchymal proliferating cells with loss of either FlnA or FlnA+Fmn2. This proliferation defect appears to be in part due to delayed differentiation in these regions. While both FlnA and FlnA+Fmn2 mice show reduced cell death relative to WT control, increased caspase staining was seen in the double null relative to FlnA null suggesting that this could also contribute to the FlnA+Fmn2 phenotype. Therefore FlnA and Fmn2 are likely essential to cell proliferation, differentiation and cell death in a variety of tissues and organs, further reiterating the importance of vesicle trafficking in regulation of development.

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.

Opposing FlnA and FlnB interactions regulate RhoA activation in guiding dynamic actin stress fiber formation and cell spreading

Filamins are a family of actin-binding proteins responsible for diverse biological functions in the context of regulating actin dynamics and vesicle trafficking. Disruption of these proteins has been implicated in multiple human developmental disorders. To investigate the roles of different filamin isoforms, we focused on FlnA and FlnB interactions in the cartilage growth plate, since mutations in both molecules cause chondrodysplasias. Current studies show that FlnA and FlnB share a common function in stabilizing the actin cytoskeleton, they physically interact in the cytoplasm of chondrocytes, and loss of FlnA enhances FlnB expression of chondrocytes in the growth plate (and vice versa), suggesting compensation. Prolonged FlnB loss, however, promotes actin-stress fiber formation following plating onto an integrin activating substrate whereas FlnA inhibition leads to decreased actin formation. FlnA more strongly binds RhoA, although both filamins overlap with RhoA expression in the cell cytoplasm. FlnA promotes RhoA activation whereas FlnB indirectly inhibits this pathway. Moreover, FlnA loss leads to diminished expression of β1-integrin, whereas FlnB loss promotes integrin expression. Finally, fibronectin mediated integrin activation has been shown to activate RhoA and activated RhoA leads to stress fiber formation and cell spreading. Fibronectin stimulation in null FlnA cells impairs enhanced spreading whereas FlnB inhibited cells show enhanced spreading. While filamins serve a primary static function in stabilization of the actin cytoskeleton, these studies are the first to demonstrate a dynamic and antagonistic relationship between different filamin isoforms in the dynamic regulation of integrin expression, RhoGTPase activity and actin stress fiber remodeling.

Structural and thermodynamic basis of a frontometaphyseal dysplasia mutation in filamin A

Filamin-mediated linkages between transmembrane receptors (TR) and the actin cytoskeleton are crucial for regulating many cytoskeleton-dependent cellular processes such as cell shape change and migration. A major TR binding site in the immunoglobulin repeat 21 (Ig21) of filamin is masked by the adjacent repeat Ig20, resulting in autoinhibition. The TR binding to this site triggers the relief of Ig20 and protein kinase A (PKA)-mediated phosphorylation of Ser-2152, thereby dynamically regulating the TR-actin linkages. A P2204L mutation in Ig20 reportedly cause frontometaphyseal dysplasia, a skeletal disorder with unknown pathogenesis. We show here that the P2204L mutation impairs a hydrophobic core of Ig20, generating a conformationally fluctuating molten globule-like state. Consequently, unlike in WT filamin, where PKA-mediated Ser-2152 phosphorylation is ligand-dependent, the P2204L mutant is readily accessible to PKA, promoting ligand-independent phosphorylation on Ser-2152. Strong TR peptide ligands from platelet GP1bα and G-protein-coupled receptor MAS effectively bound Ig21 by displacing Ig20 from autoinhibited WT filamin, but surprisingly, the capacity of these ligands to bind the P2204L mutant was much reduced despite the mutation-induced destabilization of the Ig20 structure that supposedly weakens the autoinhibition. Thermodynamic analysis indicated that compared with WT filamin, the conformationally fluctuating state of the Ig20 mutant makes Ig21 enthalpically favorable to bind ligand but with substantial entropic penalty, resulting in total higher free energy and reduced ligand affinity. Overall, our results reveal an unusual structural and thermodynamic basis for the P2204L-induced dysfunction of filamin and frontometaphyseal dysplasia disease.

Filamin A- and formin 2-dependent endocytosis regulates proliferation via the canonical Wnt pathway

Actin-associated proteins regulate multiple cellular processes, including proliferation and differentiation, but the molecular mechanisms underlying these processes are unclear. Here, we report that the actin-binding protein filamin A (FlnA) physically interacts with the actin-nucleating protein formin 2 (Fmn2). Loss of FlnA and Fmn2 impairs proliferation, thereby generating multiple embryonic phenotypes, including microcephaly. FlnA interacts with the Wnt co-receptor Lrp6. Loss of FlnA and Fmn2 impairs Lrp6 endocytosis, downstream Gsk3β activity, and β-catenin accumulation in the nucleus. The proliferative defect in Flna and Fmn2 null neural progenitors is rescued by inhibiting Gsk3β activity. Our findings thus reveal a novel mechanism whereby actin-associated proteins regulate proliferation by mediating the endocytosis and transportation of components in the canonical Wnt pathway. Moreover, the Fmn2-dependent signaling in this pathway parallels that seen in the non-canonical Wnt-dependent regulation of planar cell polarity through the Formin homology protein Daam. These studies provide evidence for integration of actin-associated processes in directing neuroepithelial proliferation.

Cytoskeletal Filamin A Differentially Modulates RNA Polymerase III Gene Transcription in Transformed Cell Lines

Cytoskeletal filamin A (FLNA) is an important protein involved in multiple cellular processes. Previous studies have shown that FLNA can promote or inhibit cancer growth and development; however, the mechanisms underlying these events are not fully understood. Here we show that, in both 293T and SaOS2 cells, knockdown of FLNA significantly enhanced transcription of RNA polymerase (pol) III-transcribed genes except for a subset of tRNA genes. In contrast, re-expression of FLNA in an FLNA-deficient melanoma cell line (A7) repressed transcription of all pol III-transcribed genes, suggesting that FLNA inhibits pol III transcription in a cell type-specific manner. Chromatin immunoprecipitation assays revealed that the repression of pol III gene transcription by FLNA correlates with the decreased occupancy of the RNA pol III transcription machinery at promoters. Immunofluorescence microscopy and coimmunoprecipitation assays revealed that FLNA can associate with the RNA pol III transcription machinery through its actin-binding domain within nuclei. Mechanistic analysis revealed that FLNA suppresses pol III gene transcription by confining the recruitment of the RNA pol III transcription machinery at the promoters of the genes that are sensitive to the alteration of FLNA expression. These findings not only extend the understanding of FLNA function in cells but also provide novel insights into the mechanism by which FLNA represses cell proliferation.

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.

Essential role of the nuclear isoform of RBFOX1, a candidate gene for autism spectrum disorders, in the brain development

Gene abnormalities in RBFOX1, encoding an mRNA-splicing factor, have been shown to cause autism spectrum disorder and other neurodevelopmental disorders. Since pathophysiological significance of the dominant nuclear isoform in neurons, RBFOX1-isoform1 (iso1), remains to be elucidated, we performed comprehensive analyses of Rbfox1-iso1 during mouse corticogenesis. Knockdown of Rbfox1-iso1 by in utero electroporation caused abnormal neuronal positioning during corticogenesis, which was attributed to impaired migration. The defects were found to occur during radial migration and terminal translocation, perhaps due to impaired nucleokinesis. Axon extension and dendritic arborization were also suppressed in vivo in Rbfox1-iso1-deficient cortical neurons. In addition, electrophysiology experiments revealed significant defects in the membrane and synaptic properties of the deficient neurons. Aberrant morphology was further confirmed by in vitro analyses; Rbfox1-iso1-konckdown in hippocampal neurons resulted in the reduction of primary axon length, total length of dendrites, spine density and mature spine number. Taken together, this study shows that Rbfox1-iso1 plays an important role in neuronal migration and synapse network formation during corticogenesis. Defects in these critical processes may induce structural and functional defects in cortical neurons, and consequently contribute to the pathophysiology of neurodevelopmental disorders with RBFOX1 abnormalities.

The Oxygen Sensor PHD2 Controls Dendritic Spines and Synapses via Modification of Filamin A

Neuronal function is highly sensitive to changes in oxygen levels, but how hypoxia affects dendritic spine formation and synaptogenesis is unknown. Here we report that hypoxia, chemical inhibition of the oxygen-sensing prolyl hydroxylase domain proteins (PHDs), and silencing of Phd2 induce immature filopodium-like dendritic protrusions, promote spine regression, reduce synaptic density, and decrease the frequency of spontaneous action potentials independently of HIF signaling. We identified the actin cross-linker filamin A (FLNA) as a target of PHD2 mediating these effects. In normoxia, PHD2 hydroxylates the proline residues P2309 and P2316 in FLNA, leading to von Hippel-Lindau (VHL)-mediated ubiquitination and proteasomal degradation. In hypoxia, PHD2 inactivation rapidly upregulates FLNA protein levels because of blockage of its proteasomal degradation. FLNA upregulation induces more immature spines, whereas Flna silencing rescues the immature spine phenotype induced by PHD2 inhibition.

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The oxygen sensor PHD2 is present in dendritic spines

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PHD2 inhibition by hypoxia reduces spine maturation, synaptic density, and activity

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Through hydroxylation, PHD2 targets filamin A for proteasomal degradation

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Filamin A stabilization promotes dendritic spine remodeling

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

Glutamatergic neurons of the mammalian cerebral cortex originate from radial glia (RG) progenitors in the ventricular zone (VZ). During corticogenesis, neuroblasts migrate toward the pial surface using two different migration modes. One is multipolar (MP) migration with random directional movement, and the other is locomotion, which is a unidirectional movement guided by the RG fiber. After reaching their final destination, the neurons finalize their migration by terminal translocation, which is followed by maturation via dendrite extension to initiate synaptogenesis and thereby complete neural circuit formation. This switching of migration modes during cortical development is unique in mammals, which suggests that the RG-guided locomotion mode may contribute to the evolution of the mammalian neocortical 6-layer structure. Many factors have been reported to be involved in the regulation of this radial neuronal migration process. In general, the radial migration can be largely divided into four steps; (1) maintenance and departure from the VZ of neural progenitor cells, (2) MP migration and transition to bipolar cells, (3) RG-guided locomotion, and (4) terminal translocation and dendrite maturation. Among these, many different gene mutations or knockdown effects have resulted in failure of the MP to bipolar transition (step 2), suggesting that it is a critical step, particularly in radial migration. Moreover, this transition occurs at the subplate layer. In this review, we summarize recent advances in our understanding of the molecular mechanisms underlying each of these steps. Finally, we discuss the evolutionary aspects of neuronal migration in corticogenesis.

Genes and brain malformations associated with abnormal neuron positioning

Neuronal positioning is a fundamental process during brain development. Abnormalities in this process cause several types of brain malformations and are linked to neurodevelopmental disorders such as autism, intellectual disability, epilepsy, and schizophrenia. Little is known about the pathogenesis of developmental brain malformations associated with abnormal neuron positioning, which has hindered research into potential treatments. However, recent advances in neurogenetics provide clues to the pathogenesis of aberrant neuronal positioning by identifying causative genes. This may help us form a foundation upon which therapeutic tools can be developed. In this review, we first provide a brief overview of neural development and migration, as they relate to defects in neuronal positioning. We then discuss recent progress in identifying genes and brain malformations associated with aberrant neuronal positioning during human brain development.

Cellullar insights into cerebral cortical development: focusing on the locomotion mode of neuronal migration

The mammalian brain consists of numerous compartments that are closely connected with each other via neural networks, comprising the basis of higher order brain functions. The highly specialized structure originates from simple pseudostratified neuroepithelium-derived neural progenitors located near the ventricle. A long journey by neurons from the ventricular side is essential for the formation of a sophisticated brain structure, including a mammalian-specific six-layered cerebral cortex. Neuronal migration consists of several contiguous steps, but the locomotion mode comprises a large part of the migration. The locomoting neurons exhibit unique features; a radial glial fiber-dependent migration requiring the endocytic recycling of N-cadherin and a neuron-specific migration mode with dilation/swelling formation that requires the actin and microtubule organization possibly regulated by cyclin-dependent kinase 5 (Cdk5), Dcx, p27(kip1), Rac1, and POSH. Here I will introduce the roles of various cellular events, such as cytoskeletal organization, cell adhesion, and membrane trafficking, in the regulation of the neuronal migration, with particular focus on the locomotion mode.

Neuronal migration disorders: Focus on the cytoskeleton and epilepsy

A wide spectrum of focal, regional, or diffuse structural brain abnormalities, collectively known as malformations of cortical development (MCDs), frequently manifest with intellectual disability (ID), epilepsy, and/or autistic spectrum disorder (ASD). As the acronym suggests, MCDs are perturbations of the normal architecture of the cerebral cortex and hippocampus. The pathogenesis of these disorders remains incompletely understood; however, one area that has provided important insights has been the study of neuronal migration. The amalgamation of human genetics and experimental studies in animal models has led to the recognition that common genetic causes of neurodevelopmental disorders, including many severe epilepsy syndromes, are due to mutations in genes regulating the migration of newly born post-mitotic neurons. Neuronal migration genes often, though not exclusively, code for proteins involved in the function of the cytoskeleton. Other cellular processes, such as cell division and axon/dendrite formation, which similarly depend on cytoskeletal functions, may also be affected. We focus here on how the susceptibility of the highly organized neocortex and hippocampus may be due to their laminar organization, which involves the tight regulation, both temporally and spatially, of gene expression, specialized progenitor cells, the migration of neurons over large distances and a birthdate-specific layering of neurons. Perturbations in neuronal migration result in abnormal lamination, neuronal differentiation defects, abnormal cellular morphology and circuit formation. Ultimately this results in disorganized excitatory and inhibitory activity leading to the symptoms observed in individuals with these disorders.

Filamin A is required in injured axons for HDAC5 activity and axon regeneration

Microtubule dynamics are important for axon growth during development as well as axon regeneration after injury. We have previously identified HDAC5 as an injury-regulated tubulin deacetylase that functions at the injury site to promote axon regeneration. However, the mechanisms involved in the spatial control of HDAC5 activity remain poorly understood. Here we reveal that HDAC5 interacts with the actin binding protein filamin A via its C-terminal domain. Filamin A plays critical roles in HDAC5-dependent tubulin deacetylation because, in cells lacking filamin A, the levels of acetylated tubulin are elevated markedly. We found that nerve injury increases filamin A axonal expression in a protein synthesis-dependent manner. Reducing filamin A levels or interfering with the interaction between HDAC5 and filamin A prevents injury-induced tubulin deacetylation as well as HDAC5 localization at the injured axon tips. In addition, neurons lacking filamin A display reduced axon regeneration. Our findings suggest a model in which filamin A local translation following axon injury controls localized HDAC5 activity to promote axon regeneration.

Significance of filamin A in mTORC2 function in glioblastoma

Background

Glioblastoma multiforme (GBM) is one of the most highly metastatic cancers. GBM has been associated with a high level of the mechanistic target of rapamycin complex 2 (mTORC2) activity. We aimed to observe roles of mTORC2 in GBM cells especially on actin cytoskeleton reorganization, cell migration and invasion, and further determine new important players involved in the regulation of these cellular processes.

Methods

To further investigate the significance of mTORC2 in GBM, we treated GBM cells with PP242, an ATP-competitive inhibitor of mTOR, and used RICTOR siRNA to knock down mTORC2 activity. Effects on actin cytoskeleton, focal adhesion, migration, and invasion of GBM cells were examined. To gain insight into molecular basis of the mTORC2 effects on cellular cytoskeletal arrangement and motility/invasion, we affinity purified mTORC2 from GBM cells and identified proteins of interest by mass spectrometry. Characterization of the protein of interest was performed.

Results

In addition to the inhibition of mTORC2 activity, we demonstrated significant alteration of actin distribution as revealed by the use of phalloidin staining. Furthermore, vinculin staining was altered which suggests changes in focal adhesion. Inhibition of cell migration and invasion was observed with PP242. Two major proteins that are associated with this mTORC2 multiprotein complex were found. Mass spectrometry identified one of them as Filamin A (FLNA). Association of FLNA with RICTOR but not mTOR was demonstrated. Moreover, in vitro, purified mTORC2 can phosphorylate FLNA likewise its known substrate, AKT. In GBM cells, colocalization of FLNA with RICTOR was observed, and the overall amounts of FLNA protein as well as phosphorylated FLNA are high. Upon treatments of RICTOR siRNA or PP242, phosphorylated FLNA levels at the regulatory residue (Ser2152) decreased. This treatment also disrupted colocalization of Actin filaments and FLNA.

Conclusions

Our results support FLNA as a new downstream effector of mTORC2 controlling GBM cell motility. This new mTORC2-FLNA signaling pathway plays important roles in motility and invasion of glioblastoma cells.

Electronic supplementary material

The online version of this article (doi:10.1186/s12943-015-0396-z) contains supplementary material, which is available to authorized users.

Malformations of cortical development and epilepsy

Malformations of cortical development (MCDs) are an important cause of epilepsy and an extremely interesting group of disorders from the perspective of brain development and its perturbations. Many new MCDs have been described in recent years as a result of improvements in imaging, genetic testing, and understanding of the effects of mutations on the ability of their protein products to correctly function within the molecular pathways by which the brain functions. In this review, most of the major MCDs are reviewed from a clinical, embryological, and genetic perspective. The most recent literature regarding clinical diagnosis, mechanisms of development, and future paths of research are discussed.

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

The actin cytoskeleton regulates many important cellular processes in the brain, including cell division and proliferation, migration, and cytokinesis and differentiation. These developmental processes can be regulated through actin dependent vesicle and organelle movement, cell signaling, and the establishment and maintenance of cell junctions and cell shape. Many of these processes are mediated by extensive and intimate interactions of actin with cellular membranes and proteins. Disruption in the actin cytoskeleton in the brain gives rise to periventricular heterotopia (PH), a malformation of cortical development, characterized by abnormal neurons clustered deep in the brain along the lateral ventricles. This disorder can give rise to seizures, dyslexia and psychiatric disturbances. Anatomically, PH is characterized by a smaller brain (impaired proliferation), heterotopia (impaired initial migration) and disruption along the neuroependymal lining (impaired cell-cell adhesion). Genes causal for PH have also been implicated in actin-dependent processes. The current review provides mechanistic insight into actin cytoskeletal regulation of cortical development in the context of this malformation of cortical development.

Hindsight regulates photoreceptor axon targeting through transcriptional control of jitterbug/Filamin and multiple genes involved in axon guidance in Drosophila

During axon targeting, a stereotyped pattern of connectivity is achieved by the integration of intrinsic genetic programs and the response to extrinsic long and short-range directional cues. How this coordination occurs is the subject of intense study. Transcription factors play a central role due to their ability to regulate the expression of multiple genes required to sense and respond to these cues during development. Here we show that the transcription factor HNT regulates layer-specific photoreceptor axon targeting in Drosophila through transcriptional control of jbug/Filamin and multiple genes involved in axon guidance and cytoskeleton organization.Using a microarray analysis we identified 235 genes whose expression levels were changed by HNT overexpression in the eye primordia. We analyzed nine candidate genes involved in cytoskeleton regulation and axon guidance, six of which displayed significantly altered gene expression levels in hnt mutant retinas. Functional analysis confirmed the role of OTK/PTK7 in photoreceptor axon targeting and uncovered Tiggrin, an integrin ligand, and Jbug/Filamin, a conserved actin- binding protein, as new factors that participate of photoreceptor axon targeting. Moreover, we provided in silico and molecular evidence that supports jbug/Filamin as a direct transcriptional target of HNT and that HNT acts partially through Jbug/Filamin in vivo to regulate axon guidance. Our work broadens the understanding of how HNT regulates the coordinated expression of a group of genes to achieve the correct connectivity pattern in the Drosophila visual system. © 2015 Wiley Periodicals, Inc. Develop Neurobiol 75: 1018-1032, 2015.

Morphological and functional aspects of progenitors perturbed in cortical malformations

In this review, we discuss molecular and cellular mechanisms important for the function of neuronal progenitors during development, revealed by their perturbation in different cortical malformations. We focus on a class of neuronal progenitors, radial glial cells (RGCs), which are renowned for their unique morphological and behavioral characteristics, constituting a key element during the development of the mammalian cerebral cortex. We describe how the particular morphology of these cells is related to their roles in the orchestration of cortical development and their influence on other progenitor types and post-mitotic neurons. Important for disease mechanisms, we overview what is currently known about RGC cellular components, cytoskeletal mechanisms, signaling pathways and cell cycle characteristics, focusing on how defects lead to abnormal development and cortical malformation phenotypes. The multiple recent entry points from human genetics and animal models are contributing to our understanding of this important cell type. Combining data from phenotypes in the mouse reveals molecules which potentially act in common pathways. Going beyond this, we discuss future directions that may provide new data in this expanding area.

Steering cell migration: lamellipodium dynamics and the regulation of directional persistence

Membrane protrusions at the leading edge of cells, known as lamellipodia, drive cell migration in many normal and pathological situations. Lamellipodial protrusion is powered by actin polymerization, which is mediated by the actin-related protein 2/3 (ARP2/3)-induced nucleation of branched actin networks and the elongation of actin filaments. Recently, advances have been made in our understanding of positive and negative ARP2/3 regulators (such as the SCAR/WAVE (SCAR/WASP family verprolin-homologous protein) complex and Arpin, respectively) and of proteins that control actin branch stability (such as glial maturation factor (GMF)) or actin filament elongation (such as ENA/VASP proteins) in lamellipodium dynamics and cell migration. This Review highlights how the balance between actin filament branching and elongation, and between the positive and negative feedback loops that regulate these activities, determines lamellipodial persistence. Importantly, directional persistence, which results from lamellipodial persistence, emerges as a critical factor in steering cell migration.

Periventricular nodular heterotopia and dystonia due to an ARFGEF2 mutation

BACKGROUND

Heterotopias are a neuronal migration disorder caused by extrinsic factors or by genetic mutations. When the location is periventricular, the most frequent genetic cause is the mutation in the "filamin A2 gene", which is X-linked. New genes for periventricular nodular heterotopia with an autosomal inheritance pattern have been recently discovered.

PATIENTS

We describe two siblings. The girl, who was prenatally diagnosed ventriculomegaly, had delayed development. At 6 months, she had no head control and variable muscle tone, alternating low axial tone with jerking movements. She became microcephalic. Magnetic resonance imaging at 12 months of age revealed enlarged lateral ventricles, periventricular nodular heterotopia, thin corpus callosum, a T2-hyperintensity of the putamen and the thalamus, and a loss of volume of lenticular nucleus. At 18 months, she developed sporadic myoclonic seizures that were well controlled with valproic acid. Her younger brother also developed progressive microcephaly and psychomotor delay by 6 months. He exhibited axial hypotonia with a prominent dystonic-athetoid component. Magnetic resonance imaging at 15 months of age revealed asymmetric ventriculomegaly plus diffuse nodules lining the temporal horns, a thin corpus callosum, and hyperintensity signal in putamens. He had no seizures.

RESULTS

Because of the association of microcephaly, developmental delay with dystonic movements, the imaging results, and the probable autosomal recessive inheritance pattern, genetic analysis was requested. This detected a homozygous nonsense mutation in ARFGEF2 gene, at the DNA level c.388C>T in exon 4.

CONCLUSIONS

The presence of dyskinetic movements in individuals with acquired microcephaly could be a manifestation of periventricular nodular heterotopia due to ARFGEF2 mutation.

Filamin A mediated Big2 dependent endocytosis: From apical abscission to periventricular heterotopia

Periventricular heterotopia (PH) is one of the most common malformations of cortical development (MCD). Nodules along the lateral ventricles of the brain, disruption of the ventricular lining, and a reduced brain size are hallmarks of this disorder. PH results in a disruption of the neuroependyma, inhibition of neural proliferation and differentiation, and altered neuronal migration. Human mutations in the genes encoding the actin-binding Filamin A (FLNA) and the vesicle trafficking Brefeldin A-associated guanine exchange factor 2 (BIG2 is encoded by the ARFGEF2 gene) proteins are implicated in PH formation. Recent studies have shown that the transition from proliferating neural progenitors to post-mitotic neurons relies on apical abscission along the neuroepithelium. This mechanism involves an actin dependent contraction of the apical portion of a neural progenitor along the ventricular lining to complete abscission. Actin also maintains stability of various cell adhesion molecules along the neuroependyma. Loss of cadherin directs disassembly of the primary cilium, which transduces sonic-hedgehog (Shh) signaling. Shh signaling is required for continued proliferation. In this context, apical abscission regulates neuronal progenitor exit and migration from the ventricular zone by detachment from the neuroependyma, relies on adhesion molecules that maintain the integrity of the neuroepithelial lining, and directs neural proliferation. Each of these processes is disrupted in PH, suggesting that genes causal for this MCD, may fundamentally mediate apical abscission in cortical development. Here we discuss several recent reports that demonstrate a coordinated role for actin and vesicle trafficking in modulating neural development along the neurepithelium, and potentially the neural stem cell to neuronal transition.

Arf Proteins and Their Regulators: At the Interface Between Membrane Lipids and the Protein Trafficking Machinery

The Arf small GTP-binding (G) proteins regulate membrane traffic and organelle structure in eukaryotic cells through a regulated cycle of GTP binding and hydrolysis. The first function identified for Arf proteins was recruitment of cytosolic coat complexes to membranes to mediate vesicle formation. However, subsequent studies have uncovered additional functions, including roles in plasma membrane signalling pathways, cytoskeleton regulation, lipid droplet function, and non-vesicular lipid transport. In contrast to other families of G proteins, there are only a few Arf proteins in each organism, yet they function specifically at many different cellular locations. Part of this specificity is achieved by formation of complexes with their guanine nucleotide-exchange factors (GEFs) and GTPase activating proteins (GAPs) that catalyse GTP binding and hydrolysis, respectively. Because these regulators outnumber their Arf substrates by at least 3-to-1, an important aspect of understanding Arf function is elucidating the mechanisms by which a single Arf protein is incorporated into different GEF, GAP, and effector complexes. New insights into these mechanisms have come from recent studies showing GEF–effector interactions, Arf activation cascades, and positive feedback loops. A unifying theme in the function of Arf proteins, carried out in conjunction with their regulators and effectors, is sensing and modulating the properties of the lipids that make up cellular membranes.

Filamin A and Big2: a shared endocytic pathway

Neural proliferation, migration and differentiation require reorganization of the actin cytoskeleton and regulation of vesicle trafficking to provide stability in maintaining cell adhesions, allow for changes in cell shape, and establishing cell polarity. Human disorders involving the actin-binding Filamin A (FLNA) and vesicle trafficking Brefeldin-associated guanine exchange factor 2 (BIG2 is encoded by the ARFGEF2 gene) proteins are implicated in these various developmental processes, resulting in a malformation of cortical development called periventricular heterotopia (nodules along the ventricular lining) and microcephaly (small brain). Here we discuss several recent reports from our laboratory that demonstrate a shared role for both proteins in actin-associated vesicle trafficking, which is required to maintain the expression and stability of cell adhesion and cell cycle associated molecules during cortical development. While changes in FLNA and BIG2 have first been linked to disorders involving the central nervous system, increasing reports suggest they are associated with aberrant development of various other organ systems in the body. These studies suggest that vesicle trafficking defects in FLN-GEF dependent pathways may contribute to a much broader phenotype than previously realized.