The PAR-6 polarity protein regulates dendritic spine morphogenesis through p190 RhoGAP and the Rho GTPase

Loss of the polarity protein Par3 promotes dendritic spine neoteny and enhances learning and memory

The Par3 polarity protein is critical for subcellular compartmentalization in different developmental processes. Variants of PARD3, encoding PAR3, are associated with intelligence and neurodevelopmental disorders. However, the role of Par3 in glutamatergic synapse formation and cognitive functions in vivo remains unknown. Here, we show that forebrain-specific Par3 conditional knockout leads to increased long, thin dendritic spines in vivo. In addition, we observed a decrease in the amplitude of miniature excitatory postsynaptic currents. Surprisingly, loss of Par3 enhances hippocampal-dependent spatial learning and memory and repetitive behavior. Phosphoproteomic analysis revealed proteins regulating cytoskeletal dynamics are significantly dysregulated downstream of Par3. Mechanistically, we found Par3 deletion causes increased Rac1 activation and dysregulated microtubule dynamics through CAMSAP2. Together, our data reveal an unexpected role for Par3 as a molecular gatekeeper in regulating the pool of immature dendritic spines, a rate-limiting step of learning and memory, through modulating Rac1 activation and microtubule dynamics in vivo.

Loss of the polarity protein Par3 promotes dendritic spine neoteny and enhances learning and memory

The Par3 polarity protein is critical for subcellular compartmentalization in different developmental processes. Variants of PARD3 , which encodes PAR3, are associated with intelligence and neurodevelopmental disorders. However, the role of Par3 in glutamatergic synapse formation and cognitive functions in vivo remains unknown. Here, we show that forebrain conditional knockout of Par3 leads to an increase in long, thin dendritic spines without significantly impacting mushroom spines in vivo . In addition, we observed a decrease in the amplitude of miniature excitatory postsynaptic currents. Surprisingly, loss of Par3 in vivo enhances hippocampal- dependent spatial learning. Phosphoproteomic analysis revealed proteins regulating cytoskeletal dynamics are significantly dysregulated downstream of Par3. Mechanistically, we found Par3 deletion causes increased activation of the Rac1 pathway. Together, our data reveal an unexpected role for Par3 as a molecular gatekeeper in regulating the pool of immature dendritic spines, a rate-limiting step of learning and memory, through modulating Rac1 activation in vivo .

Polarity proteins: Shaping dendritic spines and memory

The morphogenesis and plasticity of dendritic spines are associated with synaptic strength, learning, and memory. Dendritic spines are highly compartmentalized structures, which makes proteins involved in cellular polarization and membrane compartmentalization likely candidates regulating their formation and maintenance. Indeed, recent studies suggest polarity proteins help form and maintain dendritic spines by compartmentalizing the spine neck and head. Here, we review emerging evidence that polarity proteins regulate dendritic spine plasticity and stability through the cytoskeleton, scaffolding molecules, and signaling molecules. We specifically analyze various polarity complexes known to contribute to different forms of cell polarization processes and examine the essential conceptual context linking these groups of polarity proteins to dendritic spine morphogenesis, plasticity, and cognitive functions.

Genome-wide translation control analysis of developing human neurons

During neuronal differentiation, neuroprogenitor cells become polarized, change shape, extend axons, and form complex dendritic trees. While growing, axons are guided by molecular cues to their final destination, where they establish synaptic connections with other neuronal cells. Several layers of regulation are integrated to control neuronal development properly. Although control of mRNA translation plays an essential role in mammalian gene expression, how it contributes temporarily to the modulation of later stages of neuronal differentiation remains poorly understood. Here, we investigated how translation control affects pathways and processes essential for neuronal maturation, using H9-derived human neuro progenitor cells differentiated into neurons as a model. Through Ribosome Profiling (Riboseq) combined with RNA sequencing (RNAseq) analysis, we found that translation control regulates the expression of critical hub genes. Fundamental synaptic vesicle secretion genes belonging to SNARE complex, Rab family members, and vesicle acidification ATPases are strongly translationally regulated in developing neurons. Translational control also participates in neuronal metabolism modulation, particularly affecting genes involved in the TCA cycle and glutamate synthesis/catabolism. Importantly, we found translation regulation of several critical genes with fundamental roles regulating actin and microtubule cytoskeleton pathways, critical to neurite generation, spine formation, axon guidance, and circuit formation. Our results show that translational control dynamically integrates important signals in neurons, regulating several aspects of its development and biology.

The Roles of Par3, Par6, and aPKC Polarity Proteins in Normal Neurodevelopment and in Neurodegenerative and Neuropsychiatric Disorders

Normal neural circuits and functions depend on proper neuronal differentiation, migration, synaptic plasticity, and maintenance. Abnormalities in these processes underlie various neurodevelopmental, neuropsychiatric, and neurodegenerative disorders. Neural development and maintenance are regulated by many proteins. Among them are Par3, Par6 (partitioning defective 3 and 6), and aPKC (atypical protein kinase C) families of evolutionarily conserved polarity proteins. These proteins perform versatile functions by forming tripartite or other combinations of protein complexes, which hereafter are collectively referred to as "Par complexes." In this review, we summarize the major findings on their biophysical and biochemical properties in cell polarization and signaling pathways. We next summarize their expression and localization in the nervous system as well as their versatile functions in various aspects of neurodevelopment, including neuroepithelial polarity, neurogenesis, neuronal migration, neurite differentiation, synaptic plasticity, and memory. These versatile functions rely on the fundamental roles of Par complexes in cell polarity in distinct cellular contexts. We also discuss how cell polarization may correlate with subcellular polarization in neurons. Finally, we review the involvement of Par complexes in neuropsychiatric and neurodegenerative disorders, such as schizophrenia and Alzheimer's disease. While emerging evidence indicates that Par complexes are essential for proper neural development and maintenance, many questions on their in vivo functions have yet to be answered. Thus, Par3, Par6, and aPKC continue to be important research topics to advance neuroscience.

Calcitonin gene-related peptide receptor antagonist BIBN4096BS regulates synaptic transmission in the vestibular nucleus and improves vestibular function via PKC/ERK/CREB pathway in an experimental chronic migraine rat model

Background

Vestibular symptoms are frequently reported in patients with chronic migraine (CM). However, whether vestibular symptoms arise through overlapping neurobiology of migraine remains to be elucidated. The neuropeptide calcitonin gene-related peptide (CGRP) and CGRP1 receptor play important pathological roles in facilitating central sensitization in CM. Therefore, we aimed to investigate whether CGRP1 receptor contributes to vestibular dysfunction after CM by improving synaptic transmission in the vestibular nucleus (VN).

Methods

A CM rat model was established by recurrent intermittent administration of nitroglycerin (NTG). Migraine- and vestibular-related behaviors were assessed. CGRP1 receptor specific antagonist, BIBN4096BS, and protein kinase C (PKC) inhibitor chelerythrine chloride (CHE) were administered intracerebroventricularly. The expressions of CGRP and CGRP1 receptor components, calcitonin receptor-like receptor (CLR) and receptor activity modifying protein 1 (RAMP1) were evaluated by western blot, immunofluorescent staining and quantitative real-time polymerase chain reaction in the vestibular nucleus (VN). Synaptic associated proteins and synaptic morphological characteristics were explored by western blot, transmission electron microscope, and Golgi-cox staining. The expressions of PKC, phosphorylated extracellular signal regulated kinase (p-ERK), phosphorylated cAMP response element-binding protein at serine 133 site (p-CREB-S133) and c-Fos were detected using western blot or immunofluorescent staining.

Results

The expressions of CGRP, CLR and RAMP1 were significantly upregulated in CM rats. CLR and RAMP1 were expressed mainly in neurons. BIBN4096BS treatment and PKC inhibition alleviated mechanical allodynia, thermal hyperalgesia and vestibular dysfunction in CM rats. Additionally, BIBN4096BS treatment and PKC inhibition markedly inhibited the overexpression of synaptic associated proteins and restored the abnormal synaptic structure in VN after CM. Furthermore, BIBN4096BS treatment dysregulated the expression levels of PKC, p-ERK and p-CREB-S133, and attenuated neuronal activation in VN after CM.

Conclusions

The present study demonstrated that CGRP1 receptor inhibition improved vestibular function after CM by reversing the aberrant synaptic transmission via downregulating PKC/ERK/CREB signaling pathway. Therapeutic interventions by inhibiting CGRP/CGRP1 signaling may be a new target for the treatment of vestibular symptoms in CM.

Rac-maninoff and Rho-vel: The symphony of Rho-GTPase signaling at excitatory synapses

Synaptic connections between neurons are essential for every facet of human cognition and are thus regulated with extreme precision. Rho-family GTPases, molecular switches that cycle between an active GTP-bound state and an inactive GDP-bound state, comprise a critical feature of synaptic regulation. Rho-GTPases are exquisitely controlled by an extensive suite of activators (GEFs) and inhibitors (GAPs and GDIs) and interact with many different signalling pathways to fulfill their roles in orchestrating the development, maintenance, and plasticity of excitatory synapses of the central nervous system. Among the mechanisms that control Rho-GTPase activity and signalling are cell surface receptors, GEF/GAP complexes that tightly regulate single Rho-GTPase dynamics, GEF/GAP and GEF/GEF functional complexes that coordinate multiple Rho-family GTPase activities, effector positive feedback loops, and mutual antagonism of opposing Rho-GTPase pathways. These complex regulatory mechanisms are employed by the cells of the nervous system in almost every step of development, and prominently figure into the processes of synaptic plasticity that underlie learning and memory. Finally, misregulation of Rho-GTPases plays critical roles in responses to neuronal injury, such as traumatic brain injury and neuropathic pain, and in neurodevelopmental and neurodegenerative disorders, including intellectual disability, autism spectrum disorder, schizophrenia, and Alzheimer's Disease. Thus, decoding the mechanisms of Rho-GTPase regulation and function at excitatory synapses has great potential for combatting many of the biggest current challenges in mental health.

Manipulating oligodendrocyte intrinsic regeneration mechanism to promote remyelination

In demyelinated lesions, astrocytes, activated microglia and infiltrating macrophages secrete several factors regulating oligodendrocyte precursor cells' behaviour. What appears to be the initiation of an intrinsic mechanism of myelin repair is only leading to partial recovery and inefficient remyelination, a process worsening over the course of the disease. This failure is largely due to the concomitant accumulation of inhibitory cues in and around the lesion sites opposing to growth promoting factors. Here starts a complex game of interactions between the signalling pathways controlling oligodendrocytes migration or differentiation. Receptors of positive or negative cues are modulating Ras, PI3K or RhoGTPases pathways acting on oligodendrocyte cytoskeleton remodelling. From the description of this intricate signalling network, this review addresses the extent to which the modulation of the global response to inhibitory cues may pave the route towards novel therapeutic approaches for myelin repair.

Behavioral, circuitry, and molecular aberrations by region-specific deficiency of the high-risk autism gene Cul3

Cullin 3 (Cul3) gene, which encodes a core component of the E3 ubiquitin ligase complex that mediates proteasomal degradation, has been identified as a true high-risk factor for autism. Here, by combining behavioral, electrophysiological, and proteomic approaches, we have examined how Cul3 deficiency contributes to the etiology of different aspects of autism. Heterozygous mice with forebrain Cul3 deletion displayed autism-like social interaction impairment and sensory-gating deficiency. Region-specific deletion of Cul3 leads to distinct phenotypes, with social deficits linked to the loss of Cul3 in prefrontal cortex (PFC), and stereotypic behaviors linked to the loss of Cul3 in striatum. Correlated with these behavioral alterations, Cul3 deficiency in forebrain or PFC induces NMDA receptor hypofunction, while Cul3 loss in striatum causes a cell type-specific alteration of neuronal excitability in striatal circuits. Large-scale profiling has identified sets of misregulated proteins resulting from Cul3 deficiency in different regions, including Smyd3, a histone methyltransferase involved in gene transcription. Inhibition or knockdown of Smyd3 in forebrain Cul3-deficient mice ameliorates social deficits and restores NMDAR function in PFC. These results have revealed for the first time a potential molecular mechanism underlying the manifestation of different autism-like behavioral deficits by Cul3 deletion in cortico-striatal circuits.

Polarity signaling ensures epidermal homeostasis by coupling cellular mechanics and genomic integrity

Epithelial homeostasis requires balanced progenitor cell proliferation and differentiation, whereas disrupting this equilibrium fosters degeneration or cancer. Here we studied how cell polarity signaling orchestrates epidermal self-renewal and differentiation. Using genetic ablation, quantitative imaging, mechanochemical reconstitution and atomic force microscopy, we find that mammalian Par3 couples genome integrity and epidermal fate through shaping keratinocyte mechanics, rather than mitotic spindle orientation. Par3 inactivation impairs RhoA activity, actomyosin contractility and viscoelasticity, eliciting mitotic failures that trigger aneuploidy, mitosis-dependent DNA damage responses, p53 stabilization and premature differentiation. Importantly, reconstituting myosin activity is sufficient to restore mitotic fidelity, genome integrity, and balanced differentiation and stratification. Collectively, this study deciphers a mechanical signaling network in which Par3 acts upstream of Rho/actomyosin contractility to promote intrinsic force generation, thereby maintaining mitotic accuracy and cellular fitness at the genomic level. Disturbing this network may compromise not only epidermal homeostasis but potentially also that of other self-renewing epithelia.

Many developing tissues require Par-driven polarization, but its role in mammalian tissue maintenance is unclear. Here, the authors show that in mouse epidermis, Par3 governs tissue homeostasis not via orientation of cell division but by coupling cell mechanics with mitotic accuracy and genome integrity.

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.

p190RhoGAPs, the ARHGAP35- and ARHGAP5-Encoded Proteins, in Health and Disease

Small guanosine triphosphatases (GTPases) gathered in the Rat sarcoma (Ras) superfamily represent a large family of proteins involved in several key cellular mechanisms. Within the Ras superfamily, the Ras homolog (Rho) family is specialized in the regulation of actin cytoskeleton-based mechanisms. These proteins switch between an active and an inactive state, resulting in subsequent inhibiting or activating downstream signals, leading finally to regulation of actin-based processes. The On/Off status of Rho GTPases implicates two subsets of regulators: GEFs (guanine nucleotide exchange factors), which favor the active GTP (guanosine triphosphate) status of the GTPase and GAPs (GTPase activating proteins), which inhibit the GTPase by enhancing the GTP hydrolysis. In humans, the 20 identified Rho GTPases are regulated by over 70 GAP proteins suggesting a complex, but well-defined, spatio-temporal implication of these GAPs. Among the quite large number of RhoGAPs, we focus on p190RhoGAP, which is known as the main negative regulator of RhoA, but not exclusively. Two isoforms, p190A and p190B, are encoded by ARHGAP35 and ARHGAP5 genes, respectively. We describe here the function of each of these isoforms in physiological processes and sum up findings on their role in pathological conditions such as neurological disorders and cancers.

Altered TAOK2 activity causes autism-related neurodevelopmental and cognitive abnormalities through RhoA signaling

Atypical brain connectivity is a major contributor to the pathophysiology of neurodevelopmental disorders (NDDs) including autism spectrum disorders (ASDs). TAOK2 is one of several genes in the 16p11.2 microdeletion region, but whether it contributes to NDDs is unknown. We performed behavioral analysis on Taok2 heterozygous (Het) and knockout (KO) mice and found gene dosage-dependent impairments in cognition, anxiety, and social interaction. Taok2 Het and KO mice also have dosage-dependent abnormalities in brain size and neural connectivity in multiple regions, deficits in cortical layering, dendrite and synapse formation, and reduced excitatory neurotransmission. Whole-genome and -exome sequencing of ASD families identified three de novo mutations in TAOK2 and functional analysis in mice and human cells revealed that all the mutations impair protein stability, but they differentially impact kinase activity, dendrite growth, and spine/synapse development. Mechanistically, loss of Taok2 activity causes a reduction in RhoA activation, and pharmacological enhancement of RhoA activity rescues synaptic phenotypes. Together, these data provide evidence that TAOK2 is a neurodevelopmental disorder risk gene and identify RhoA signaling as a mediator of TAOK2-dependent synaptic development.

Dynamics of PAR Proteins Explain the Oscillation and Ratcheting Mechanisms in Dorsal Closure

We present a vertex-based model for Drosophila dorsal closure that predicts the mechanics of cell oscillation and contraction from the dynamics of the PAR proteins. Based on experimental observations of how aPKC, Par-6, and Bazooka translocate from the circumference of the apical surface to the medial domain, and how they interact with each other and ultimately regulate the apicomedial actomyosin, we formulate a system of differential equations that captures the key features of dorsal closure, including distinctive behaviors in its early, slow, and fast phases. The oscillation in cell area in the early phase of dorsal closure results from an intracellular negative feedback loop that involves myosin, an actomyosin regulator, aPKC, and Bazooka. In the slow phase, gradual sequestration of apicomedial aPKC by Bazooka clusters causes incomplete disassembly of the actomyosin network over each cycle of oscillation, thus producing a so-called ratchet. The fast phase of rapid cell and tissue contraction arises when medial myosin, no longer antagonized by aPKC, builds up in time and produces sustained contraction. Thus, a minimal set of rules governing the dynamics of the PAR proteins, extracted from experimental observations, can account for all major mechanical outcomes of dorsal closure, including the transitions between its three distinct phases.

Primary Cilia Reconsidered in the Context of Ciliopathies: Extraciliary and Ciliary Functions of Cilia Proteins Converge on a Polarity theme?

Once dismissed as vestigial organelles, primary cilia have garnered the interest of scientists, given their importance in development/signaling, and for their implication in a new disease category known as ciliopathies. However, many, if not all, "cilia" proteins also have locations/functions outside of the primary cilium. These extraciliary functions can complicate the interpretation of a particular ciliopathy phenotype: it may be a result of defects at the cilium and/or at extraciliary locations, and it could be broadly related to a unifying cellular process for these proteins, such as polarity. Assembly of a cilium has many similarities to the development of other polarized structures. This evolutionarily preserved process for the assembly of polarized cell structures offers a perspective on how the cilium may have evolved. We hypothesize that cilia proteins are critical for cell polarity, and that core polarity proteins may have been specialized to form various cellular protrusions, including primary cilia.

PAR3-PAR6-atypical PKC polarity complex proteins in neuronal polarization

Polarity is a fundamental feature of cells. Protein complexes, including the PAR3-PAR6-aPKC complex, have conserved roles in establishing polarity across a number of eukaryotic cell types. In neurons, polarity is evident as distinct axonal versus dendritic domains. The PAR3, PAR6, and aPKC proteins also play important roles in neuronal polarization. During this process, either aPKC kinase activity, the assembly of the PAR3-PAR6-aPKC complex or the localization of these proteins is regulated downstream of a number of signaling pathways. In turn, the PAR3, PAR6, and aPKC proteins control various effector molecules to establish neuronal polarity. Herein, we discuss the many signaling mechanisms and effector functions that have been linked to PAR3, PAR6, and aPKC during the establishment of neuronal polarity.

Par3 and aPKC regulate BACE1 endosome-to-TGN trafficking through PACS1

The cleavage of amyloid precursor protein (APP) by β-site APP cleaving enzyme 1 (BACE1) is the rate-limiting step in beta amyloid generation during Alzheimer's disease (AD) pathogenesis. In AD brains, BACE1 is abnormally accumulated in endocytic compartments, where the acidic pH is optimal for its activity. However, mechanisms regulating the endosome-to-trans-Golgi network (TGN) retrieval of BACE1 remain unclear. Here, we show that partitioning defective 3 (Par3) facilitates BACE1 retrograde trafficking from endosomes to the TGN. Par3 functions through aPKC-mediated phosphorylation of BACE1 on Ser498, which in turn promotes the interaction between BACE1 and phosphofurin acidic cluster sorting protein 1 and facilitates the retrograde trafficking of BACE1 to the TGN. In human AD brains, there is a significant decrease in Ser498 phosphorylation of BACE1 suggesting that defective phosphorylation-dependent retrograde transport of BACE1 is important in AD pathogenesis. Together, our studies provide mechanistic insight into a novel role for Par3 and aPKC in regulating the retrograde endosome-to-TGN trafficking of BACE1 and shed light on the mechanisms of AD pathogenesis.

The equilibrium between antagonistic signaling pathways determines the number of synapses in Drosophila

The number of synapses is a major determinant of behavior and many neural diseases exhibit deviations in that number. However, how signaling pathways control this number is still poorly understood. Using the Drosophila larval neuromuscular junction, we show here a PI3K-dependent pathway for synaptogenesis which is functionally connected with other previously known elements including the Wit receptor, its ligand Gbb, and the MAPkinases cascade. Based on epistasis assays, we determined the functional hierarchy within the pathway. Wit seems to trigger signaling through PI3K, and Ras85D also contributes to the initiation of synaptogenesis. However, contrary to other signaling pathways, PI3K does not require Ras85D binding in the context of synaptogenesis. In addition to the MAPK cascade, Bsk/JNK undergoes regulation by Puc and Ras85D which results in a narrow range of activity of this kinase to determine normalcy of synapse number. The transcriptional readout of the synaptogenesis pathway involves the Fos/Jun complex and the repressor Cic. In addition, we identified an antagonistic pathway that uses the transcription factors Mad and Medea and the microRNA bantam to down-regulate key elements of the pro-synaptogenesis pathway. Like its counterpart, the anti-synaptogenesis signaling uses small GTPases and MAPKs including Ras64B, Ras-like-a, p38a and Licorne. Bantam downregulates the pro-synaptogenesis factors PI3K, Hiw, Ras85D and Bsk, but not AKT. AKT, however, can suppress Mad which, in conjunction with the reported suppression of Mad by Hiw, closes the mutual regulation between both pathways. Thus, the number of synapses seems to result from the balanced output from these two pathways.

Postsynaptic density 95 (PSD-95) serine 561 phosphorylation regulates a conformational switch and bidirectional dendritic spine structural plasticity

Postsynaptic density 95 (PSD-95) is a major synaptic scaffolding protein that plays a key role in bidirectional synaptic plasticity, which is a process important for learning and memory. It is known that PSD-95 shows increased dynamics upon induction of plasticity. However, the underlying structural and functional changes in PSD-95 that mediate its role in plasticity remain unclear. Here we show that phosphorylation of PSD-95 at Ser-561 in its guanylate kinase (GK) domain, which is mediated by the partitioning-defective 1 (Par1) kinases, regulates a conformational switch and is important for bidirectional plasticity. Using a fluorescence resonance energy transfer (FRET) biosensor, we show that a phosphomimetic mutation of Ser-561 promotes an intramolecular interaction between GK and the nearby Src homology 3 (SH3) domain, leading to a closed conformation, whereas a non-phosphorylatable S561A mutation or inhibition of Par1 kinase activity decreases SH3-GK interaction, causing PSD-95 to adopt an open conformation. In addition, S561A mutation facilitates the interaction between PSD-95 and its binding partners. Fluorescence recovery after photobleaching imaging reveals that the S561A mutant shows increased stability, whereas the phosphomimetic S561D mutation increases PSD-95 dynamics at the synapse. Moreover, molecular replacement of endogenous PSD-95 with the S561A mutant blocks dendritic spine structural plasticity during chemical long-term potentiation and long-term depression. Endogenous Ser-561 phosphorylation is induced by synaptic NMDA receptor activation, and the SH3-GK domains exhibit a Ser-561 phosphorylation-dependent switch to a closed conformation during synaptic plasticity. Our results provide novel mechanistic insight into the regulation of PSD-95 in dendritic spine structural plasticity through phosphorylation-mediated regulation of protein dynamics and conformation.

Scaffolding proteins in the development and maintenance of the epidermal permeability barrier

The skin of mammals and other terrestrial vertebrates protects the organism against the external environment, preventing heat, water and electrolyte loss, as well as entry of chemicals and pathogens. Impairments in the epidermal permeability barrier function are associated with the genesis and/or progression of a variety of pathological conditions, including genetic inflammatory diseases, microbial and viral infections, and photodamage induced by UV radiation. In mammals, the outside-in epidermal permeability barrier is provided by the joint action of the outermost cornified layer, together with assembled tight junctions in granular keratinocytes found in the layers underneath. Tight junctions serve as both outside-in and inside-out barriers, and impede paracellular movements of ions, water, macromolecules and microorganisms. At the molecular level, tight junctions consist of integral membrane proteins that form an extracellular seal between adjacent cells, and associate with cytoplasmic scaffold proteins that serve as links with the actin cytoskeleton. In this review, we address the roles that scaffold proteins play specifically in the establishment and maintenance of the epidermal permeability barrier, and how various pathologies alter or impair their functions.

Neuralized regulates Crumbs endocytosis and epithelium morphogenesis via specific Stardust isoforms

The E3 ubiquitin ligase Neuralized is shown to interact with a subset of the Stardust isoforms to regulate the endocytosis of the apical protein Crumbs and thereby promote epithelial remodeling during Drosophila development.

Crumbs (Crb) is a conserved determinant of apical membrane identity that regulates epithelial morphogenesis in many developmental contexts. In this study, we identify the Crb complex protein Stardust (Sdt) as a target of the E3 ubiquitin ligase Neuralized (Neur) in Drosophila melanogaster. Neur interacts with and down-regulates specific Sdt isoforms containing a Neur binding motif (NBM). Using a CRISPR (clustered regularly interspaced short palindromic repeats)-induced deletion of the NBM-encoding exon, we found that Sdt is a key Neur target and that Neur acts via Sdt to down-regulate Crb. We further show that Neur promotes the endocytosis of Crb via the NBM-containing isoforms of Sdt. Although the regulation of Crb by Neur is not strictly essential, it contributes to epithelium remodeling in the posterior midgut and thereby facilitates the trans-epithelial migration of the primordial germ cells in early embryos. Thus, our study uncovers a novel regulatory mechanism for the developmental control of Crb-mediated morphogenesis.

Association of ARHGAP18 polymorphisms with schizophrenia in the Chinese-Han population

Numerous developmental genes have been linked to schizophrenia (SZ) by case-control and genome-wide association studies, suggesting that neurodevelopmental disturbances are major pathogenic mechanisms. However, no neurodevelopmental deficit has been definitively linked to SZ occurrence, likely due to disease heterogeneity and the differential effects of various gene variants across ethnicities. Hence, it is critical to examine linkages in specific ethnic populations, such as Han Chinese. The newly identified RhoGAP ARHGAP18 is likely involved in neurodevelopment through regulation of RhoA/C. Here we describe four single nucleotide polymorphisms (SNPs) in ARHGAP18 associated with SZ across a cohort of >2000 cases and controls from the Han population. Two SNPs, rs7758025 and rs9483050, displayed significant differences between case and control groups both in genotype (P = 0.0002 and P = 7.54×10⁻⁶) and allelic frequencies (P = 4.36×10⁻⁵ and P = 5.98×10⁻⁷), respectively. The AG haplotype in rs7758025−rs9385502 was strongly associated with the occurrence of SZ (P = 0.0012, OR = 0.67, 95% CI = 0.48–0.93), an association that still held following a 1000-times random permutation test (P = 0.022). In an independently collected validation cohort, rs9483050 was the SNP most strongly associated with SZ. In addition, the allelic frequencies of rs12197901 remained associated with SZ in the combined cohort (P = 0.021), although not in the validation cohort alone (P = 0.251). Collectively, our data suggest the ARHGAP18 may confer vulnerability to SZ in the Chinese Han population, providing additional evidence for the involvement of neurodevelopmental dysfunction in the pathogenesis of schizophrenia.

Rho GTPase-activating proteins: Regulators of Rho GTPase activity in neuronal development and CNS diseases

The Rho family of small GTPases was considered as molecular switches in regulating multiple cellular events, including cytoskeleton reorganization. The Rho GTPase-activating proteins (RhoGAPs) are one of the major families of Rho GTPase regulators. RhoGAPs were initially considered negative mediators of Rho signaling pathways via their GAP domain. Recent studies have demonstrated that RhoGAPs also regulate numerous aspects of neuronal development and are related to various neurodegenerative diseases in GAP-dependent and GAP-independent manners. Moreover, RhoGAPs are regulated through various mechanisms, such as phosphorylation. To date, approximately 70 RhoGAPs have been identified; however, only a small portion has been thoroughly investigated. Thus, the characterization of important RhoGAPs in the central nervous system is crucial to understand their spatiotemporal role during different stages of neuronal development. In this review, we summarize the current knowledge of RhoGAPs in the brain with an emphasis on their molecular function, regulation mechanism and disease implications in the central nervous system.

Regulation of Cell Polarity by PAR-1/MARK Kinase

PAR-1/MARK kinases are conserved serine/threonine kinases that are essential regulators of cell polarity. PAR-1/MARK kinases localize and function in opposition to the anterior PAR proteins to control the asymmetric distribution of factors in a wide variety polarized cells. In this review, we discuss the mechanisms that control the localization and activity of PAR-1/MARK kinases, including their antagonistic interactions with the anterior PAR proteins. We focus on the role PAR-1 plays in the asymmetric division of the Caenorhabditis elegans zygote, in the establishment of the anterior/posterior axis in the Drosophila oocyte and in the control of microtubule dynamics in mammalian neurons. In addition to conserved aspects of PAR-1 biology, we highlight the unique ways in which PAR-1 acts in these distinct cell types to orchestrate their polarization. Finally, we review the connections between disruptions in PAR-1/MARK function and Alzheimer's disease and cancer.

The polarity protein Par3 regulates APP trafficking and processing through the endocytic adaptor protein Numb

The processing of amyloid precursor protein (APP) into β-amyloid peptide (Aβ) is a key step in the pathogenesis of Alzheimer's disease (AD), and trafficking dysregulations of APP and its secretases contribute significantly to altered APP processing. Here we show that the cell polarity protein Par3 plays an important role in APP processing and trafficking. We found that the expression of full length Par3 is significantly decreased in AD patients. Overexpression of Par3 promotes non-amyloidogenic APP processing, while depletion of Par3 induces intracellular accumulation of Aβ. We further show that Par3 functions by regulating APP trafficking. Loss of Par3 decreases surface expression of APP by targeting APP to the late endosome/lysosome pathway. Finally, we show that the effects of Par3 are mediated through the endocytic adaptor protein Numb, and Par3 functions by interfering with the interaction between Numb and APP. Together, our studies show a novel role for Par3 in regulating APP processing and trafficking.

Developmental RacGAP α2-Chimaerin Signaling Is a Determinant of the Morphological Features of Dendritic Spines in Adulthood

Morphological characteristics of dendritic spines form the basis of cognitive ability. However, molecular mechanisms involved in fine-tuning of spine morphology during development are not fully understood. Moreover, it is unclear whether, and to what extent, these developmental mechanisms determine the normal adult spine morphological features. Here, we provide evidence that α2-isoform of Rac-specific GTPase-activating protein α-chimaerin (α2-chimaerin) is involved in spine morphological refinement during late postnatal period, and furthermore show that this developmental α2-chimaerin function affects adult spine morphologies. We used a series of mice with global and conditional knock-out of α-chimaerin isoforms (α1-chimaerin and α2-chimaerin). α2-Chimaerin disruption, but not α1-chimaerin disruption, in the mouse results in an increased size (and density) of spines in the hippocampus. In contrast, overexpression of α2-chimaerin in developing hippocampal neurons induces a decrease of spine size. Disruption of α2-chimaerin suppressed EphA-mediated spine morphogenesis in cultured developing hippocampal neurons. α2-Chimaerin disruption that begins during the juvenile stage results in an increased size of spines in the hippocampus. Meanwhile, spine morphologies are unaltered when α2-chimaerin is deleted only in adulthood. Consistent with these spine morphological results, disruption of α2-chimaerin beginning in the juvenile stage led to an increase in contextual fear learning in adulthood; whereas contextual learning was recently shown to be unaffected when α2-chimaerin was deleted only in adulthood. Together, these results suggest that α2-chimaerin signaling in developmental stages contributes to determination of the morphological features of adult spines and establishment of normal cognitive ability.

SIGNIFICANCE STATEMENT

Recent studies of neurodevelopmental disorders in humans and their animal models have led to an attractive hypothesis that spine morphogenesis during development forms the basis of adult cognition. In particular, the roles of Rac and its regulators, such as Rac-specific GTPase-activating proteins (RacGAPs) and Rac guanine nucleotide exchange factors, are a topic of focus in spine morphogenesis and cognitive ability. Using a series of mice with global and conditional knock-out (KO) of RacGAP α-chimaerin isoforms (α1-chimaerin and α2-chimaerin), we provide compelling evidence demonstrating that α2-chimaerin is involved in spine morphological refinement during late postnatal development and that this developmental α2-chimaerin function affects adult spine morphologies. Furthermore, our results clearly showed that α2-chimaerin signaling during late postnatal development contributes to normal cognitive ability in adult mice.

Polarity Determinants in Dendritic Spine Development and Plasticity

The asymmetric distribution of various proteins and RNAs is essential for all stages of animal development, and establishment and maintenance of this cellular polarity are regulated by a group of conserved polarity determinants. Studies over the last 10 years highlight important functions for polarity proteins, including apical-basal polarity and planar cell polarity regulators, in dendritic spine development and plasticity. Remarkably, many of the conserved polarity machineries function in similar manners in the context of spine development as they do in epithelial morphogenesis. Interestingly, some polarity proteins also utilize neuronal-specific mechanisms. Although many questions remain unanswered in our understanding of how polarity proteins regulate spine development and plasticity, current and future research will undoubtedly shed more light on how this conserved group of proteins orchestrates different pathways to shape the neuronal circuitry.

The Rho family GEF Asef2 regulates cell migration in three dimensional (3D) collagen matrices through myosin II

Cell migration is fundamental to a variety of physiological processes, including tissue development, homeostasis, and regeneration. Migration has been extensively studied with cells on 2-dimensional (2D) substrates, but much less is known about cell migration in 3D environments. Tissues and organs are 3D, which is the native environment of cells in vivo, pointing to a need to understand migration and the mechanisms that regulate it in 3D environments. To investigate cell migration in 3D environments, we developed microfluidic devices that afford a controlled, reproducible platform for generating 3D matrices. Using these devices, we show that the Rho family guanine nucleotide exchange factor (GEF) Asef2 inhibits cell migration in 3D type I collagen (collagen I) matrices. Treatment of cells with the myosin II (MyoII) inhibitor blebbistatin abolished the decrease in migration by Asef2. Moreover, Asef2 enhanced MyoII activity as shown by increased phosphorylation of serine 19 (S19). Furthermore, Asef2 increased activation of Rac, which is a Rho family small GTPase, in 3D collagen I matrices. Inhibition of Rac activity by treatment with the Rac-specific inhibitor NSC23766 abrogated the Asef2-promoted increase in S19 MyoII phosphorylation. Thus, our results indicate that Asef2 regulates cell migration in 3D collagen I matrices through a Rac-MyoII-dependent mechanism.

NGL-2 Is a New Partner of PAR Complex in Axon Differentiation

Neuronal polarization is pivotal for neural network formation during brain development. Axon differentiation is a hallmark of initial neuronal polarization. Here, we report that the leucine-rich repeat-containing protein netrin-G ligand-2 (NGL-2) as a polarity regulator that localizes asymmetrically in rat hippocampal neurons and is required for differentiation of the future axon. NGL-2 was associated with PAR complex, and this interaction resulted in local stabilization of axonal microtubules. Further study showed that the C terminal of NGL-2 binds to the PDZ domain of PAR6, and NGL-2 interacts with PAR3 and atypical PKCζ (aPKCζ), with PAR6 acting as a bridge or modifier. Then, NGL-2 regulates the local stabilization of microtubules and promotes axon differentiation by the aPKCζ/microtubule affinity-regulating kinase 2 pathway. These findings reveal the critical role of NGL-2 in regulating axon differentiation in rat hippocampal neurons and reveal a novel partner of the PAR complex.

Rho1- and Pkc1-dependent phosphorylation of the F-BAR protein Syp1 contributes to septin ring assembly

Septins often form filaments and rings at the neck of cellular appendages. Assembly of these structures must be coordinated with membrane remodeling. In budding yeast, the Rho1 GTPase and its effector, Pkc1, play a role in septin ring stabilization during budding at least partly through phosphorylation of the bud neck–associated F-BAR protein Syp1.

In many cell types, septins assemble into filaments and rings at the neck of cellular appendages and/or at the cleavage furrow to help compartmentalize the plasma membrane and support cytokinesis. How septin ring assembly is coordinated with membrane remodeling and controlled by mechanical stress at these sites is unclear. Through a genetic screen, we uncovered an unanticipated link between the conserved Rho1 GTPase and its effector protein kinase C (Pkc1) with septin ring stability in yeast. Both Rho1 and Pkc1 stabilize the septin ring, at least partly through phosphorylation of the membrane-associated F-BAR protein Syp1, which colocalizes asymmetrically with the septin ring at the bud neck. Syp1 is displaced from the bud neck upon Pkc1-dependent phosphorylation at two serines, thereby affecting the rigidity of the new-forming septin ring. We propose that Rho1 and Pkc1 coordinate septin ring assembly with membrane and cell wall remodeling partly by controlling Syp1 residence at the bud neck.

Mechanisms for spatiotemporal regulation of Rho-GTPase signaling at synapses

Synapses mediate information flow between neurons and undergo plastic changes in response to experience, which is critical for learning and memory. Conversely, synaptic defects impair information processing and underlie many brain pathologies. Rho-family GTPases control synaptogenesis by transducing signals from extracellular stimuli to the cytoskeleton and nucleus. The Rho-GTPases Rac1 and Cdc42 promote synapse development and the growth of axons and dendrites, while RhoA antagonizes these processes. Despite its importance, many aspects of Rho-GTPase signaling remain relatively unknown. Rho-GTPases are activated by guanine nucleotide exchange factors (GEFs) and inhibited by GTPase-activating proteins (GAPs). Though the number of both GEFs and GAPs greatly exceeds that of Rho-GTPases, loss of even a single GEF or GAP often has profound effects on cognition and behavior. Here, we explore how the actions of specific GEFs and GAPs give rise to the precise spatiotemporal activation patterns of Rho-GTPases in neurons. We consider the effects of coupling GEFs and GAPs targeting the same Rho-GTPase and the modular pathways that connect specific cellular stimuli with a given Rho-GTPase via different GEFs. We discuss how the creation of sharp borders between Rho-GTPase activation zones is achieved by pairing a GEF for one Rho-GTPase with a GAP for another and the extensive crosstalk between different Rho-GTPases. Given the importance of synapses for cognition and the fundamental roles that Rho-GTPases play in regulating them, a detailed understanding of Rho-GTPase signaling is essential to the progress of neuroscience.

Spatiotemporal 16p11.2 protein network implicates cortical late mid-fetal brain development and KCTD13-Cul3-RhoA pathway in psychiatric diseases

The psychiatric disorders autism and schizophrenia have a strong genetic component, and copy number variants (CNVs) are firmly implicated. Recurrent deletions and duplications of chromosome 16p11.2 confer a high risk for both diseases, but the pathways disrupted by this CNV are poorly defined. Here we investigate the dynamics of the 16p11.2 network by integrating physical interactions of 16p11.2 proteins with spatiotemporal gene expression from the developing human brain. We observe profound changes in protein interaction networks throughout different stages of brain development and/or in different brain regions. We identify the late mid-fetal period of cortical development as most critical for establishing the connectivity of 16p11.2 proteins with their co-expressed partners. Furthermore, our results suggest that the regulation of the KCTD13-Cul3-RhoA pathway in layer 4 of the inner cortical plate is crucial for controlling brain size and connectivity and that its dysregulation by de novo mutations may be a potential determinant of 16p11.2 CNV deletion and duplication phenotypes.

The guanine nucleotide exchange factor (GEF) Asef2 promotes dendritic spine formation via Rac activation and spinophilin-dependent targeting

Dendritic spines are actin-rich protrusions that establish excitatory synaptic contacts with surrounding neurons. Reorganization of the actin cytoskeleton is critical for the development and plasticity of dendritic spines, which is the basis for learning and memory. Rho family GTPases are emerging as important modulators of spines and synapses, predominantly through their ability to regulate actin dynamics. Much less is known, however, about the function of guanine nucleotide exchange factors (GEFs), which activate these GTPases, in spine and synapse development. In this study we show that the Rho family GEF Asef2 is found at synaptic sites, where it promotes dendritic spine and synapse formation. Knockdown of endogenous Asef2 with shRNAs impairs spine and synapse formation, whereas exogenous expression of Asef2 causes an increase in spine and synapse density. This effect of Asef2 on spines and synapses is abrogated by expression of GEF activity-deficient Asef2 mutants or by knockdown of Rac, suggesting that Asef2-Rac signaling mediates spine development. Because Asef2 interacts with the F-actin-binding protein spinophilin, which localizes to spines, we investigated the role of spinophilin in Asef2-promoted spine formation. Spinophilin recruits Asef2 to spines, and knockdown of spinophilin hinders spine and synapse formation in Asef2-expressing neurons. Furthermore, inhibition of N-methyl-d-aspartate receptor (NMDA) activity blocks spinophilin-mediated localization of Asef2 to spines. These results collectively point to spinophilin-Asef2-Rac signaling as a novel mechanism for the development of dendritic spines and synapses.

A transcriptional cross-talk between RhoA and c-Myc inhibits the RhoA/Rock-dependent cytoskeleton

The GTPase RhoA and the transcriptional factor c-Myc are closely intertwined in cancer cells. Although this cross-talk results in potent synergistic effects that favor the transformed phenotype of cancer cells, recent results from our laboratory indicate that c-Myc also participates in a negative feed-back loop that blocks specific RhoA signaling branches connected to the induction of stress fibers, focal adhesions and actomyosin contractility. Using microarray analysis, we have unveiled a RhoA/c-Myc-dependent gene signature in charge of this negative cross-talk. This signature is composed of upregulated and repressed transcripts encoding cytoskeletal modulators located downstream of both RhoA and Rock. Our results also indicate that this negative feed-back loop modifies the invasion and adhesion properties of RhoA-transformed cells, suggesting that it may be important to ensure fluid cytoskeletal dynamics of cancer cells. Preliminary data indicate that c-Myc may also use a different transcriptional program to interfere with the RhoA/Rock-dependent cytoskeletal branch in non-transformed cells.

A novel function of the cell polarity-regulating kinase PAR-1/MARK in dendritic spines

Dendritic spines are postsynaptic structures that receive excitatory synaptic signals from presynaptic terminals in neurons. Because the morphology of spines has been considered to be a crucial factor for the efficiency of synaptic transmission, understanding the mechanisms regulating their morphology is important for neuroscience. Actin filaments and their regulatory proteins are known to actively maintain spine morphology; recent studies have also shown an essential role of microtubules (MTs). Live imaging of the plus-ends of MTs in mature neurons revealed that MTs stochastically enter spines and mediate accumulation of p140Cap, which regulates reorganization of actin filaments. However, the molecular mechanism by which MT dynamics is controlled has remained largely unknown. A cell polarity-regulating serine/threonine kinase, partitioning-defective 1 (PAR-1), phosphorylates classical MAPs and inhibits their binding to MTs. Because the interaction of MAPs with MTs can decrease MT dynamic instability, PAR-1 is supposed to activate MT dynamics through its MAP/MT affinity-regulating kinase (MARK) activity, although there is not yet any direct evidence for this. Here, we review recent findings on the localization of PAR-1b in the dendrites of mouse hippocampal neurons, and its novel function in the maintenance of mature spine morphology by regulating MT dynamics.

The PAR complex controls the spatiotemporal dynamics of F-actin and the MTOC in directionally migrating leukocytes

Inflammatory cells acquire a polarized phenotype to migrate towards sites of infection or injury. A conserved polarity complex comprising PAR-3, PAR-6 and atypical protein kinase C (aPKC) relays extracellular polarizing cues to control cytoskeletal and signaling networks affecting morphological and functional polarization. However, there is no evidence that myeloid cells use PAR signaling to migrate vectorially in three-dimensional (3D) environments in vivo. Using genetically encoded bioprobes and high-resolution live imaging, we reveal the existence of F-actin oscillations in the trailing edge and constant repositioning of the microtubule organizing center (MTOC) to direct leukocyte migration in wounded medaka fish larvae (Oryzias latipes). Genetic manipulation in live myeloid cells demonstrates that the catalytic activity of aPKC and the regulated interaction with PAR-3 and PAR-6 are required for consistent F-actin oscillations, MTOC perinuclear mobility, aPKC repositioning and wound-directed migration upstream of Rho kinase (also known as ROCK or ROK) activation. We propose that the PAR complex coordinately controls cytoskeletal changes affecting both the generation of traction force and the directionality of leukocyte migration to sites of injury.

Establishment of epithelial polarity--GEF who's minding the GAP?

Cell polarization is a fundamental process that underlies epithelial morphogenesis, cell motility, cell division and organogenesis. Loss of polarity predisposes tissues to developmental disorders and contributes to cancer progression. The formation and establishment of epithelial cell polarity is mediated by the cooperation of polarity protein complexes, namely the Crumbs, partitioning defective (Par) and Scribble complexes, with Rho family GTPases, including RhoA, Rac1 and Cdc42. The activation of different GTPases triggers distinct downstream signaling pathways to modulate protein-protein interactions and cytoskeletal remodeling. The spatio-temporal activation and inactivation of these small GTPases is tightly controlled by a complex interconnected network of different regulatory proteins, including guanine-nucleotide-exchange factors (GEFs), GTPase-activating proteins (GAPs), and guanine-nucleotide-dissociation inhibitors (GDIs). In this Commentary, we focus on current understanding on how polarity complexes interact with GEFs and GAPs to control the precise location and activation of Rho GTPases (Crumbs for RhoA, Par for Rac1, and Scribble for Cdc42) to promote apical-basal polarization in mammalian epithelial cells. The mutual exclusion of GTPase activities, especially that of RhoA and Rac1, which is well established, provides a mechanism through which polarity complexes that act through distinct Rho GTPases function as cellular rheostats to fine-tune specific downstream pathways to differentiate and preserve the apical and basolateral domains. This article is part of a Minifocus on Establishing polarity.

Overexpression of atypical protein kinase C in HeLa cells facilitates macropinocytosis via Src activation

Atypical protein kinase C (aPKC) is the first recognized kinase oncogene. However, the specific contribution of aPKC to cancer progression is unclear. The pseudosubstrate domain of aPKC is different from the other PKC family members, and therefore a synthetic peptide corresponding to the aPKC pseudosubstrate (aPKC-PS) sequence, which specifically blocks aPKC kinase activity, is a valuable tool to assess the role of aPKC in various cellular processes. Here, we learned that HeLa cells incubated with membrane permeable aPKC-PS peptide displayed dilated heterogeneous vesicles labeled with peptide that were subsequently identified as macropinosomes. A quantitative membrane binding assay revealed that aPKC-PS peptide stimulated aPKC recruitment to membranes and activated Src. Similarly, aPKC overexpression in transfected HeLa cells activated Src and induced macropinosome formation. Src-aPKC interaction was essential; substitution of the proline residues in aPKC that associate with the Src-SH3 binding domain rendered the mutant kinase unable to induce macropinocytosis in transfected cells. We propose that aPKC overexpression is a contributing factor to cell transformation by interacting with and consequently promoting Src activation and constitutive macropinocytosis, which increases uptake of extracellular factors, required for altered cell growth and accelerated cell migration.

Rho GTPases in collective cell migration

The family of Rho GTPases are intracellular signal transducers that link cell surface signals to multiple intracellular responses. They are best known for their role in regulating actin dynamics required for cell migration, but in addition control cell-cell adhesion, polarization, vesicle trafficking, and the cell cycle. The roles of Rho GTPases in single mesenchymal cell migration are well established and rely on Cdc42- and Rac-dependent cell protrusion of a leading edge, coupled to Rho-dependent contractility required to move the cell body forward. In cells migrating collectively, cell-cell junctions are maintained, and migrating leader cells are mechanically coupled to, and coordinate, migration with follower cells. Recent evidence suggests that Rho GTPases provide multifunctional input to collective cell polarization, cell-cell interaction, and migration. Here, we discuss the role of Rho GTPases in initiating and maintaining front-rear, apical-basal cell polarization, mechanotransduction, and cell-cell junction stability between leader and follower cells, and how these roles are integrated in collective migration. Thereby, spatiotemporal fine-tuning of Rho GTPases within the same cell and among cells in the cell group are crucial in controlling potentially conflicting, divergent cell adhesion and cytoskeletal functions to achieve supracellular coordination and mechanocoupling.

Dendritic spines: the locus of structural and functional plasticity

The introduction of high-resolution time lapse imaging and molecular biological tools has changed dramatically the rate of progress towards the understanding of the complex structure-function relations in synapses of central spiny neurons. Standing issues, including the sequence of molecular and structural processes leading to formation, morphological change, and longevity of dendritic spines, as well as the functions of dendritic spines in neurological/psychiatric diseases are being addressed in a growing number of recent studies. There are still unsettled issues with respect to spine formation and plasticity: Are spines formed first, followed by synapse formation, or are synapses formed first, followed by emergence of a spine? What are the immediate and long-lasting changes in spine properties following exposure to plasticity-producing stimulation? Is spine volume/shape indicative of its function? These and other issues are addressed in this review, which highlights the complexity of molecular pathways involved in regulation of spine structure and function, and which contributes to the understanding of central synaptic interactions in health and disease.

RalA promotes a direct exocyst-Par6 interaction to regulate polarity in neuronal development

Cell polarization is essential for neuronal development in both the embryonic and postnatal brain. Here, using primary cultures, in vivo postnatal electroporation and conditional genetic ablation, we show that the Ras-like small GTPase RalA and its effector, the exocyst, regulate the morphology and polarized migration of neural progenitors derived from the subventricular zone, a major neurogenic niche in the postnatal brain. Active RalA promotes the direct binding between the exocyst subunit Exo84 and the PDZ domain of Par6 through a non-canonical PDZ-binding motif. Blocking the Exo84-Par6 interaction impairs polarization in postnatal neural progenitors and cultured embryonic neurons. Our results provide the first in vivo characterization of RalA function in the mammalian brain and highlight a novel molecular mechanism for cell polarization. Given that the exocyst and the Par complex are conserved in many tissues, the functional significance of their interaction and its regulation by RalA are likely to be important in a wide range of polarization events.

The potential role of rho GTPases in Alzheimer's disease pathogenesis

Alzheimer's disease (AD) is characterized by a wide loss of synapses and dendritic spines. Despite extensive efforts, the molecular mechanisms driving this detrimental alteration have not yet been determined. Among the factors potentially mediating this loss of neuronal connectivity, the contribution of Rho GTPases is of particular interest. This family of proteins is classically considered a key regulator of actin cytoskeleton remodeling and dendritic spine maintenance, but new insights into the complex dynamics of its regulation have recently determined how its signaling cascade is still largely unknown, both in physiological and pathological conditions. Here, we review the growing evidence supporting the potential involvement of Rho GTPases in spine loss, which is a unanimously recognized hallmark of early AD pathogenesis. We also discuss some new insights into Rho GTPase signaling framework that might explain several controversial results that have been published. The study of the connection between AD and Rho GTPases represents a quite unchartered avenue that holds therapeutic potential.

Transduction of extracellular cues into cell polarity: the role of the transmembrane proteoglycan NG2

Resident progenitor cells expressing nerve/glial antigen 2 (NG2) such as oligodendrocyte precursor cells (OPC) and pericytes persist in the adult brain. The transmembrane proteoglycan NG2 regulates migration of both these cell types in response to growth factors or specific components of the extracellular matrix. This role of NG2 is linked to the control of cell polarity. The polarization of OPC toward an acute lesion in the brain is impaired in NG2-deficient mice, supporting this concept. A review of the signaling pathways impinged on by NG2 reveals key proteins of cell polarity: phosphatidylinositol 3-kinase, focal adhesion kinase, Rho GTPases, and polarity complex proteins. In the scope of cell migration, I discuss here how the interplay of NG2 with signaling transmitted by extracellular cues can control the establishment of cell polarity, and I propose a model to integrate the apparent opposite effects of NG2 on cellular dynamics.

Loss of aPKCλ in differentiated neurons disrupts the polarity complex but does not induce obvious neuronal loss or disorientation in mouse brains

Cell polarity plays a critical role in neuronal differentiation during development of the central nervous system (CNS). Recent studies have established the significance of atypical protein kinase C (aPKC) and its interacting partners, which include PAR-3, PAR-6 and Lgl, in regulating cell polarization during neuronal differentiation. However, their roles in neuronal maintenance after CNS development remain unclear. Here we performed conditional deletion of aPKCλ, a major aPKC isoform in the brain, in differentiated neurons of mice by camk2a-cre or synapsinI-cre mediated gene targeting. We found significant reduction of aPKCλ and total aPKCs in the adult mouse brains. The aPKCλ deletion also reduced PAR-6β, possibly by its destabilization, whereas expression of other related proteins such as PAR-3 and Lgl-1 was unaffected. Biochemical analyses suggested that a significant fraction of aPKCλ formed a protein complex with PAR-6β and Lgl-1 in the brain lysates, which was disrupted by the aPKCλ deletion. Notably, the aPKCλ deletion mice did not show apparent cell loss/degeneration in the brain. In addition, neuronal orientation/distribution seemed to be unaffected. Thus, despite the polarity complex disruption, neuronal deletion of aPKCλ does not induce obvious cell loss or disorientation in mouse brains after cell differentiation.

Rho-GTPase-activating protein interacting with Cdc-42-interacting protein 4 homolog 2 (Rich2): a new Ras-related C3 botulinum toxin substrate 1 (Rac1) GTPase-activating protein that controls dendritic spine morphogenesis

Development of dendritic spines is important for synaptic function, and alteration in spine morphogenesis is often associated with mental disorders. Rich2 was an uncharacterized Rho-GAP protein. Here we searched for a role of this protein in spine morphogenesis. We found that it is enriched in dendritic spines of cultured hippocampal pyramidal neurons during early stages of development. Rich2 specifically stimulated the Rac1 GTPase in these neurons. Inhibition of Rac1 by EHT 1864 increased the size and decreased the density of dendritic spines. Similarly, Rich2 overexpression increased the size and decreased the density of dendritic spines, whereas knock-down of the protein by specific si-RNA decreased both size and density of spines. The morphological changes were reflected by the increased amplitude and decreased frequency of miniature EPSCs induced by Rich2 overexpression, while si-RNA treatment decreased both amplitude and frequency of these events. Finally, treatment of neurons with EHT 1864 rescued the phenotype induced by Rich2 knock-down. These results suggested that Rich2 controls dendritic spine morphogenesis and function via inhibition of Rac1.

Bazooka inhibits aPKC to limit antagonism of actomyosin networks during amnioserosa apical constriction

Cell shape changes drive tissue morphogenesis during animal development. An important example is the apical cell constriction that initiates tissue internalisation. Apical constriction can occur through a phase of cyclic assembly and disassembly of apicomedial actomyosin networks, followed by stabilisation of these networks. Delayed negative-feedback mechanisms typically underlie cyclic behaviour, but the mechanisms regulating cyclic actomyosin networks remain obscure, as do mechanisms that transform overall network behaviour. Here, we show that a known inhibitor of apicomedial actomyosin networks in Drosophila amnioserosa cells, the Par-6-aPKC complex, is recruited to the apicomedial domain by actomyosin networks during dorsal closure of the embryo. This finding establishes an actomyosin-aPKC negative-feedback loop in the system. Additionally, we find that aPKC recruits Bazooka to the apicomedial domain, and phosphorylates Bazooka for a dynamic interaction. Remarkably, stabilising aPKC-Bazooka interactions can inhibit the antagonism of actomyosin by aPKC, suggesting that Bazooka acts as an aPKC inhibitor, and providing a possible mechanism for delaying the actomyosin-aPKC negative-feedback loop. Our data also implicate an increasing degree of Par-6-aPKC-Bazooka interactions as dorsal closure progresses, potentially explaining a developmental transition in actomyosin behaviour from cyclic to persistent networks. This later impact of aPKC inhibition is supported by mathematical modelling of the system. Overall, this work illustrates how shifting chemical signals can tune actomyosin network behaviour during development.

The Rac-GAP Bcr is a novel regulator of the Par complex that controls cell polarity

The Par complex (Par3, Par6, and PKCζ) controls cell polarity, which is essential for many biological processes. Here we identify the Rac1 GTPase-activating protein Bcr as an integral member of the Par complex that regulates polarized cell migration by locally restricting both Rac1 and PKCζ function.

Cell polarization is essential for many biological processes, including directed cell migration, and loss of polarity contributes to pathological conditions such as cancer. The Par complex (Par3, Par6, and PKCζ) controls cell polarity in part by recruiting the Rac-specific guanine nucleotide exchange factor T-lymphoma invasion and metastasis 1 (Tiam1) to specialized cellular sites, where Tiam1 promotes local Rac1 activation and cytoskeletal remodeling. However, the mechanisms that restrict Par-Tiam1 complex activity to the leading edge to maintain cell polarity during migration remain unclear. We identify the Rac-specific GTPase-activating protein (GAP) breakpoint cluster region protein (Bcr) as a novel regulator of the Par-Tiam1 complex. We show that Bcr interacts with members of the Par complex and inhibits both Rac1 and PKCζ signaling. Loss of Bcr results in faster, more random migration and striking polarity defects in astrocytes. These polarity defects are rescued by reducing PKCζ activity or by expressing full-length Bcr, but not an N-terminal deletion mutant or the homologous Rac-GAP, Abr, both of which fail to associate with the Par complex. These results demonstrate that Bcr is an integral member of the Par-Tiam1 complex that controls polarized cell migration by locally restricting both Rac1 and PKCζ function.