Regulation of N-WASP and the Arp2/3 complex by Abp1 controls neuronal morphology

Ankrd26 is a retinoic acid-responsive plasma membrane-binding and -shaping protein critical for proper cell differentiation

Morphogens are important triggers for differentiation processes. Yet, downstream effectors that organize cell shape changes in response to morphogenic cues, such as retinoic acid, largely remain elusive. Additionally, derailed plasma membrane-derived signaling often is associated with cancer. We identify Ankrd26 as a critical player in cellular differentiation and as plasma membrane-localized protein able to self-associate and form clusters at the plasma membrane in response to retinoic acid. We show that Ankrd26 uses an N-terminal amphipathic structure for membrane binding and bending. Importantly, in an acute myeloid leukemia-associated Ankrd26 mutant, this critical structure was absent, and Ankrd26's membrane association and shaping abilities were impaired. In line with this, the mutation rendered Ankrd26 inactive in both gain-of-function and loss-of-function/rescue studies addressing retinoic acid/brain-derived neurotrophic factor (BDNF)-induced neuroblastoma differentiation. Our results highlight the importance and molecular details of Ankrd26-mediated organizational platforms for cellular differentiation at the plasma membrane and how impairment of these platforms leads to cancer-associated pathomechanisms involving these Ankrd26 properties.

Direct observation of cortactin protecting Arp2/3-actin filament branch junctions from GMF-mediated destabilization

How cells tightly control the formation and turnover of branched actin filament arrays to drive cell motility, endocytosis, and other cellular processes is still not well understood. Here, we investigated the mechanistic relationship between two binding partners of the Arp2/3 complex, glia maturation factor (GMF) and cortactin. Individually, GMF and cortactin have opposite effects on the stability of actin filament branches, but it is unknown how they work in concert with each other to govern branch turnover. Using TIRF microscopy, we observe that GMF's branch destabilizing activities are potently blocked by cortactin (IC50 = 1.3 nM) and that this inhibition requires direct interactions of cortactin with Arp2/3 complex. The simplest model that would explain these results is competition for binding Arp2/3 complex. However, we find that cortactin and GMF do not compete for free Arp2/3 complex in solution. Further, we use single molecule analysis to show that cortactin's on-rate (3 ×107 s-1 M-1) and off-rate (0.03 s-1) at branch junctions are minimally affected by excess GMF. Together, these results show that cortactin binds with high affinity to branch junctions, where it blocks the destabilizing effects of GMF, possibly by a mechanism that is allosteric in nature. In addition, the affinities we measure for cortactin at actin filament branch junctions (Kd = 0.9 nM) and filament sides (Kd = 206 nM) are approximately 20-fold stronger than previously reported. These observations contribute to an emerging view of molecular complexity in how Arp2/3 complex is regulated through the integration of multiple inputs.

EHBP1 Is Critically Involved in the Dendritic Arbor Formation and Is Coupled to Factors Promoting Actin Filament Formation

The coordinated action of a plethora of factors is required for the organization and dynamics of membranous structures critically underlying the development and function of cells, organs, and organisms. The evolutionary acquisition of additional amino acid motifs allows for expansion and/or specification of protein functions. We identify a thus far unrecognized motif specific for chordata EHBP1 proteins and demonstrate that this motif is critically required for interaction with syndapin I, an F-BAR domain-containing, membrane-shaping protein predominantly expressed in neurons. Gain-of-function and loss-of-function studies in rat primary hippocampal neurons (of mixed sexes) unraveled that EHBP1 has an important role in neuromorphogenesis. Surprisingly, our analyses uncovered that this newly identified function of EHBP1 did not require the domain responsible for Rab GTPase binding but was strictly dependent on EHBP1's syndapin I binding interface and on the presence of syndapin I in the developing neurons. These findings were underscored by temporally and spatially remarkable overlapping dynamics of EHBP1 and syndapin I at nascent dendritic branch sites. In addition, rescue experiments demonstrated the necessity of two additional EHBP1 domains for dendritic arborization, the C2 and CH domains. Importantly, the additionally uncovered critical involvement of the actin nucleator Cobl in EHBP1 functions suggested that not only static association with F-actin via EHBP1's CH domain is important for dendritic arbor formation but also actin nucleation. Syndapin interactions organize ternary protein complexes composed of EHBP1, syndapin I, and Cobl, and our functional data show that only together these factors give rise to proper cell shape during neuronal development.

Actin polymerization and depolymerization in developing vertebrates

Development is a complex process that occurs throughout the life cycle. F-actin, a major component of the cytoskeleton, is essential for the morphogenesis of tissues and organs during development. F-actin is formed by the polymerization of G-actin, and the dynamic balance of polymerization and depolymerization ensures proper cellular function. Disruption of this balance results in various abnormalities and defects or even embryonic lethality. Here, we reviewed recent findings on the structure of G-actin and F-actin and the polymerization of G-actin to F-actin. We also focused on the functions of actin isoforms and the underlying mechanisms of actin polymerization/depolymerization in cellular and organic morphogenesis during development. This information will extend our understanding of the role of actin polymerization in the physiologic or pathologic processes during development and may open new avenues for developing therapeutics for embryonic developmental abnormalities or tissue regeneration.

Membrane shapers from two distinct superfamilies cooperate in the development of neuronal morphology

Membrane-shaping proteins are driving forces behind establishment of proper cell morphology and function. Yet, their reported structural and in vitro properties are noticeably inconsistent with many physiological membrane topology requirements. We demonstrate that dendritic arborization of neurons is powered by physically coordinated shaping mechanisms elicited by members of two distinct classes of membrane shapers: the F-BAR protein syndapin I and the N-Ank superfamily protein ankycorbin. Strikingly, membrane-tubulating activities by syndapin I, which would be detrimental during dendritic branching, were suppressed by ankycorbin. Ankycorbin's integration into syndapin I-decorated membrane surfaces instead promoted curvatures and topologies reflecting those observed physiologically. In line with the functional importance of this mechanism, ankycorbin- and syndapin I-mediated functions in dendritic arborization mutually depend on each other and on a surprisingly specific interface mediating complex formation of the two membrane shapers. These striking results uncovered cooperative and interdependent functions of members of two fundamentally different membrane shaper superfamilies as a previously unknown, pivotal principle in neuronal shape development.

Effect of the Rho-Kinase/ROCK Signaling Pathway on Cytoskeleton Components

The mechanical properties of cells are important in tissue homeostasis and enable cell growth, division, migration and the epithelial-mesenchymal transition. Mechanical properties are determined to a large extent by the cytoskeleton. The cytoskeleton is a complex and dynamic network composed of microfilaments, intermediate filaments and microtubules. These cellular structures confer both cell shape and mechanical properties. The architecture of the networks formed by the cytoskeleton is regulated by several pathways, a key one being the Rho-kinase/ROCK signaling pathway. This review describes the role of ROCK (Rho-associated coiled-coil forming kinase) and how it mediates effects on the key components of the cytoskeleton that are critical for cell behaviour.

Regulation of actin filament assembly and disassembly in growth cone motility and axon guidance

Directed outgrowth of axons is fundamental for the establishment of neuronal networks. Axon outgrowth is guided by growth cones, highly motile structures enriched in filamentous actin (F-actin) located at the axons' distal tips. Growth cones exploit F-actin-based protrusions to scan the environment for guidance cues, and they contain the sensory apparatus to translate guidance cue information into intracellular signaling cascades. These cascades act upstream of actin-binding proteins (ABP) and thereby control assembly and disassembly of F-actin. Spatiotemporally controlled F-actin dis-/assembly in growth cones steers the axon towards attractants and away from repellents, and it thereby navigates the axon through the developing nervous system. Hence, ABP that control F-actin dynamics emerged as critical regulators of neuronal network formation. In the present review article, we will summarize and discuss current knowledge of the mechanisms that control remodeling of the actin cytoskeleton in growth cones, focusing on recent progress in the field. Further, we will introduce tools and techniques that allow to study actin regulatory mechanism in growth cones.

N-WASP-Arp2/3 signaling controls multiple steps of dendrite maturation in Purkinje cells in vivo

During neural development, the actin filament network must be precisely regulated to form elaborate neurite structures. N-WASP tightly controls actin polymerization dynamics by activating an actin nucleator Arp2/3. However, the importance of N-WASP-Arp2/3 signaling in the assembly of neurite architecture in vivo has not been clarified. Here, we demonstrate that N-WASP-Arp2/3 signaling plays a crucial role in the maturation of cerebellar Purkinje cell (PC) dendrites in vivo in mice. N-WASP was expressed and activated in developing PCs. Inhibition of Arp2/3 and N-WASP from the beginning of dendrite formation severely disrupted the establishment of a single stem dendrite, which is a characteristic basic structure of PC dendrites. Inhibition of Arp2/3 after stem dendrite formation resulted in hypoplasia of the PC dendritic tree. Cdc42, an upstream activator of N-WASP, is required for N-WASP-Arp2/3 signaling-mediated PC dendrite maturation. In addition, overactivation of N-WASP is also detrimental to dendrite formation in PCs. These findings reveal that proper activation of N-WASP-Arp2/3 signaling is crucial for multiple steps of PC dendrite maturation in vivo.

Load adaptation by endocytic actin networks

Clathrin-mediated endocytosis (CME) robustness under elevated membrane tension is maintained by actin assembly-mediated force generation. However, whether more actin assembles at endocytic sites in response to increased load has not previously been investigated. Here actin network ultrastructure at CME sites was examined under low and high membrane tension. Actin and N-WASP spatial organization indicate that actin polymerization initiates at the base of clathrin-coated pits and that the network then grows away from the plasma membrane. Actin network height at individual CME sites was not coupled to coat shape, raising the possibility that local differences in mechanical load feed back on assembly. By manipulating membrane tension and Arp2/3 complex activity we tested the hypothesis that actin assembly at CME sites increases in response to elevated load. Indeed, in response to elevated membrane tension, actin grew higher, resulting in greater coverage of the clathrin coat, and CME slowed. When membrane tension was elevated and the Arp2/3 complex was inhibited, shallow clathrin-coated pits accumulated, indicating that this adaptive mechanism is especially crucial for coat curvature generation. We propose that actin assembly increases in response to increased load to ensure CME robustness over a range of plasma membrane tensions. [Media: see text] [Media: see text] [Media: see text] [Media: see text] [Media: see text].

Inheritance and flexibility of cell polarity: a clue for understanding human brain development and evolution

Cell polarity is fundamentally important for understanding brain development. Here, we hypothesize that the inheritance and flexibility of cell polarity during neocortex development could be implicated in neocortical evolutionary expansion. Molecular and morphological features of cell polarity may be inherited from one type of progenitor cell to the other and finally transmitted to neurons. Furthermore, key cell types, such as basal progenitors and neurons, exhibit a highly flexible polarity. We suggest that both inheritance and flexibility of cell polarity are implicated in the amplification of basal progenitors and tangential dispersion of neurons, which are key features of the evolutionary expansion of the neocortex.

Summary: We suggest that the inheritance and flexibility of cell polarity are implicated in the evolutionary expansion of the developing neocortex by promoting the amplification of neural progenitors and tangential migration of neurons.

Functional interdependence of the actin nucleator Cobl and Cobl-like in dendritic arbor development

Local actin filament formation is indispensable for development of the dendritic arbor of neurons. We show that, surprisingly, the action of single actin filament-promoting factors was insufficient for powering dendritogenesis. Instead, this required the actin nucleator Cobl and its only evolutionary distant ancestor Cobl-like acting interdependently. This coordination between Cobl-like and Cobl was achieved by physical linkage by syndapins. Syndapin I formed nanodomains at convex plasma membrane areas at the base of protrusive structures and interacted with three motifs in Cobl-like, one of which was Ca²⁺/calmodulin-regulated. Consistently, syndapin I, Cobl-like’s newly identified N terminal calmodulin-binding site and the single Ca²⁺/calmodulin-responsive syndapin-binding motif all were critical for Cobl-like’s functions. In dendritic arbor development, local Ca²⁺/CaM-controlled actin dynamics thus relies on regulated and physically coordinated interactions of different F-actin formation-promoting factors and only together they have the power to bring about the sophisticated neuronal morphologies required for neuronal network formation in mammals.

The Role of Protein Arginine Methylation as Post-Translational Modification on Actin Cytoskeletal Components in Neuronal Structure and Function

The brain encompasses a complex network of neurons with exceptionally elaborated morphologies of their axonal (signal-sending) and dendritic (signal-receiving) parts. De novo actin filament formation is one of the major driving and steering forces for the development and plasticity of the neuronal arbor. Actin filament assembly and dynamics thus require tight temporal and spatial control. Such control is particularly effective at the level of regulating actin nucleation-promoting factors, as these are key components for filament formation. Arginine methylation represents an important post-translational regulatory mechanism that had previously been mainly associated with controlling nuclear processes. We will review and discuss emerging evidence from inhibitor studies and loss-of-function models for protein arginine methyltransferases (PRMTs), both in cells and whole organisms, that unveil that protein arginine methylation mediated by PRMTs represents an important regulatory mechanism in neuritic arbor formation, as well as in dendritic spine induction, maturation and plasticity. Recent results furthermore demonstrated that arginine methylation regulates actin cytosolic cytoskeletal components not only as indirect targets through additional signaling cascades, but can also directly control an actin nucleation-promoting factor shaping neuronal cells—a key process for the formation of neuronal networks in vertebrate brains.

High-throughput kinase inhibitor screening reveals roles for Aurora and Nuak kinases in neurite initiation and dendritic branching

Kinases are essential regulators of a variety of cellular signaling processes, including neurite formation—a foundational step in neurodevelopment. Aberrant axonal sprouting and failed regeneration of injured axons are associated with conditions like traumatic injury, neurodegenerative disease, and seizures. Investigating the mechanisms underlying neurite formation will allow for identification of potential therapeutics. We used a kinase inhibitor library to screen 493 kinase inhibitors and observed that 45% impacted neuritogenesis in Neuro2a (N-2a) cells. Based on the screening, we further investigated the roles of Aurora kinases A, B, and C and Nuak kinases 1 and 2. The roles of Aurora and Nuak kinases have not been thoroughly studied in the nervous system. Inhibition or overexpression of Aurora and Nuak kinases in primary cortical neurons resulted in various neuromorphological defects, with Aurora A regulating neurite initiation, Aurora B and C regulating neurite initiation and elongation, all Aurora kinases regulating arborization, and all Nuak kinases regulating neurite initiation and elongation and arborization. Our high-throughput screening and analysis of Aurora and Nuak kinases revealed their functions and may contribute to the identification of therapeutics.

Compound heterozygous variants in Wiskott-Aldrich syndrome like (WASL) gene segregating in a family with early onset Parkinson's disease

BACKGROUND

Knowledge of genetic determinants in Parkinson's disease is still limited. Familial forms of the disease continue to provide a rich resource to capture the genetic spectrum in disease pathogenesis, and this approach is exploited in this study.

METHODS

Informative members from a three-generation family of Indian ethnicity manifesting a likely autosomal recessive mode of inheritance of Parkinson's disease were used for whole exome sequencing. Variant data analysis and in vitro functional characterisation of variant(s) segregating with the phenotype were carried out in HEK-293 and SH-SY5Y cells using gene constructs of interest.

RESULTS

Two compound heterozygous variants, a rare missense (c.1139C > T:p.P380L) and a novel splice variant (c.1456 + 2 delTAGA, intron10) in Wiskott-Aldrich syndrome like gene (WASL, 7q31), both predicted to be deleterious were shared among the proband and two affected siblings. WASL, a gene not previously linked to a human Mendelian disorder is known to regulate actin polymerisation via Arp2/3 complex. Based on exon trapping assay using pSPL3 vector in HEK-293 cells, the splice variant showed skipping of exon10. Characterisation of the missense variant in SH-SY5Y cells demonstrated: i) significant alterations in neurite length and number; ii) decreased reactive oxygen species tolerance in mutation carrying cells on Tetrabutylphosphonium hydroxide induction and iii) increase in alpha-synuclein protein. Screening for WASL variants in two independent PD cohorts identified four individuals with heterozygous but none with biallelic variants.

CONCLUSION

WASL, with demonstrated functional relevance in neurons may be yet another strong candidate gene for autosomal recessive PD encouraging assessment of its contribution across populations.

Direct Visualization of Actin Filaments and Actin-Binding Proteins in Neuronal Cells

Actin networks and actin-binding proteins (ABPs) are most abundant in the cytoskeleton of neurons. The function of ABPs in neurons is nucleation of actin polymerization, polymerization or depolymerization regulation, bundling of actin through crosslinking or stabilization, cargo movement along actin filaments, and anchoring of actin to other cellular components. In axons, ABP–actin interaction forms a dynamic, deep actin network, which regulates axon extension, guidance, axon branches, and synaptic structures. In dendrites, actin and ABPs are related to filopodia attenuation, spine formation, and synapse plasticity. ABP phosphorylation or mutation changes ABP–actin binding, which regulates axon or dendritic plasticity. In addition, hyperactive ABPs might also be expressed as aggregates of abnormal proteins in neurodegeneration. Those changes cause many neurological disorders. Here, we will review direct visualization of ABP and actin using various electron microscopy (EM) techniques, super resolution microscopy (SRM), and correlative light and electron microscopy (CLEM) with discussion of important ABPs in neuron.

Comparison of Multiscale Imaging Methods for Brain Research

A major challenge in neuroscience is how to study structural alterations in the brain. Even small changes in synaptic composition could have severe outcomes for body functions. Many neuropathological diseases are attributable to disorganization of particular synaptic proteins. Yet, to detect and comprehensively describe and evaluate such often rather subtle deviations from the normal physiological status in a detailed and quantitative manner is very challenging. Here, we have compared side-by-side several commercially available light microscopes for their suitability in visualizing synaptic components in larger parts of the brain at low resolution, at extended resolution as well as at super-resolution. Microscopic technologies included stereo, widefield, deconvolution, confocal, and super-resolution set-ups. We also analyzed the impact of adaptive optics, a motorized objective correction collar and CUDA graphics card technology on imaging quality and acquisition speed. Our observations evaluate a basic set of techniques, which allow for multi-color brain imaging from centimeter to nanometer scales. The comparative multi-modal strategy we established can be used as a guide for researchers to select the most appropriate light microscopy method in addressing specific questions in brain research, and we also give insights into recent developments such as optical aberration corrections.

Ankyrin repeat-containing N-Ank proteins shape cellular membranes

Cells of multicellular organisms need to adopt specific morphologies. However, the molecular mechanisms bringing about membrane topology changes are far from understood-mainly because knowledge of membrane-shaping proteins that can promote local membrane curvatures is still limited. Our analyses unveiled that several members of a large, previously unrecognised protein family, which we termed N-Ank proteins, use a combination of their ankyrin repeat array and an amino (N)-terminal amphipathic helix to bind and shape membranes. Consistently, functional analyses revealed that the N-Ank protein ankycorbin (NORPEG/RAI14), which was exemplarily characterised further, plays an important, ankyrin repeat-based and N-terminal amphipathic helix-dependent role in early morphogenesis of neurons. This function furthermore required coiled coil-mediated self-assembly and manifested as ankycorbin nanodomains marked by protrusive membrane topologies. In summary, here, we unveil a class of powerful membrane shapers and thereby assign mechanistic and cell biological functions to the N-Ank protein superfamily.

A Maternal Transcription Factor, Junction Mediating and Regulatory Protein is Required for Preimplantation Development in the Mouse

Junction-mediating and regulatory protein (JMY) is a regulator of both transcription and actin filament assembly. The actin-regulatory activity of JMY is based on a cluster of three actin-binding Wiskott-Aldrich syndrome protein homology 2 (WH2) domains that nucleate actin filaments directly and promote nucleation of the Arp2/3 complex. In addition to these activities, we examined the activity of JMY generation in early embryo of mice carrying mutations in the JMY gene by CRISPR/Cas9 mediated genome engineering. We demonstrated that JMY protein shuttled expression between the cytoplasm and the nucleus. Knockout of exon 2, CA (central domain and Arp2/3-binding acidic domain) and NLS-2 (nuclear localization signal domain) on the JMY gene by CRISPR/Cas9 system was effective and markedly impeded embryonicdevelopment. Additionally, it impaired transcription and zygotic genome activation (ZGA)-related genes. These results suggest that JMY acts as a transcription factor, which is essential for the early embryonic development in mice.

Identification of putative effectors of the Type IV secretion system from the Wolbachia endosymbiont of Brugia malayi

Wolbachia is an unculturable, intracellular bacterium that persists within an extremely broad range of arthropod and parasitic nematode hosts, where it is transmitted maternally to offspring via vertical transmission. In the filarial nematode Brugia malayi, a causative agent of human lymphatic filariasis, Wolbachia is an endosymbiont, and its presence is essential for proper nematode development, survival, and pathogenesis. While the elucidation of Wolbachia:nematode interactions that promote the bacterium’s intracellular persistence is of great importance, research has been hampered due to the fact that Wolbachia cannot be cultured in the absence of host cells. The Wolbachia endosymbiont of B. malayi (wBm) has an active Type IV secretion system (T4SS). Here, we have screened 47 putative T4SS effector proteins of wBm for their ability to modulate growth or the cell biology of a typical eukaryotic cell, Saccharomyces cerevisiae. Five candidates strongly inhibited yeast growth upon expression, and 6 additional proteins showed toxicity in the presence of zinc and caffeine. Studies on the uptake of an endocytic vacuole-specific fluorescent marker, FM4-64, identified 4 proteins (wBm0076 wBm00114, wBm0447 and wBm0152) involved in vacuole membrane dynamics. The WAS(p)-family protein, wBm0076, was found to colocalize with yeast cortical actin patches and disrupted actin cytoskeleton dynamics upon expression. Deletion of the Arp2/3-activating protein, Abp1p, provided resistance to wBm0076 expression, suggesting a role for wBm0076 in regulating eukaryotic actin dynamics and cortical actin patch formation. Furthermore, wBm0152 was found to strongly disrupt endosome:vacuole cargo trafficking in yeast. This study provides molecular insight into the potential role of the T4SS in the Wolbachia endosymbiont:nematode relationship.

Abp1 promotes Arp2/3 complex-dependent actin nucleation and stabilizes branch junctions by antagonizing GMF

Formation and turnover of branched actin networks underlies cell migration and other essential force-driven processes. Type I nucleation-promoting factors (NPFs) such as WASP recruit actin monomers to Arp2/3 complex to stimulate nucleation. In contrast, mechanisms of type II NPFs such as Abp1 (also known as HIP55 and Drebrin-like protein) are less well understood. Here, we use single-molecule analysis to investigate yeast Abp1 effects on Arp2/3 complex, and find that Abp1 strongly enhances Arp2/3-dependent branch nucleation by stabilizing Arp2/3 on sides of mother filaments. Abp1 binds dynamically to filament sides, with sub-second lifetimes, yet associates stably with branch junctions. Further, we uncover a role for Abp1 in protecting filament junctions from GMF-induced debranching by competing with GMF for Arp2/3 binding. These data, combined with EM structures of Abp1 dimers bound to Arp2/3 complex in two different conformations, expand our mechanistic understanding of type II NPFs.

Abp1, a type II actin nucleation promoting factor, is a known component of branched actin networks but its mechanism remains poorly understood. Here, the authors find that Abp1 enhances Arp2/3-mediated actin branch formation, and blocks ‘debranching’ by GMF, making it a pro-branching factor.

Network Analysis of UBE3A/E6AP-Associated Proteins Provides Connections to Several Distinct Cellular Processes

Perturbations in activity and dosage of the UBE3A ubiquitin-ligase have been linked to Angelman syndrome and autism spectrum disorders. UBE3A was initially identified as the cellular protein hijacked by the human papillomavirus E6 protein to mediate the ubiquitylation of p53, a function critical to the oncogenic potential of these viruses. Although a number of substrates have been identified, the normal cellular functions and pathways affected by UBE3A are largely unknown. Previously, we showed that UBE3A associates with HERC2, NEURL4, and MAPK6/ERK3 in a high-molecular-weight complex of unknown function that we refer to as the HUN complex (HERC2, UBE3A, and NEURL4). In this study, the combination of two complementary proteomic approaches with a rigorous network analysis revealed cellular functions and pathways in which UBE3A and the HUN complex are involved. In addition to finding new UBE3A-associated proteins, such as MCM6, SUGT1, EIF3C, and ASPP2, network analysis revealed that UBE3A-associated proteins are connected to several fundamental cellular processes including translation, DNA replication, intracellular trafficking, and centrosome regulation. Our analysis suggests that UBE3A could be involved in the control and/or integration of these cellular processes, in some cases as a component of the HUN complex, and also provides evidence for crosstalk between the HUN complex and CAMKII interaction networks. This study contributes to a deeper understanding of the cellular functions of UBE3A and its potential role in pathways that may be affected in Angelman syndrome, UBE3A-associated autism spectrum disorders, and human papillomavirus-associated cancers.

Cobl-like promotes actin filament formation and dendritic branching using only a single WH2 domain

Local actin filament formation powers the development of the signal-receiving arbor of neurons. In this study, Izadi et al. demonstrate that Cobl-like, which bears only a single WH2 domain, mediates dendritic branching by coordinating with the F-actin–binding protein Abp1 in a Ca²⁺/CaM-controlled manner to control actin dynamics.

Local actin filament formation powers the development of the signal-receiving arbor of neurons that underlies neuronal network formation. Yet, little is known about the molecules that drive these processes and may functionally connect them to the transient calcium pulses observed in restricted areas in the forming dendritic arbor. Here we demonstrate that Cordon-Bleu (Cobl)–like, an uncharacterized protein suggested to represent a very distantly related, evolutionary ancestor of the actin nucleator Cobl, despite having only a single G-actin–binding Wiskott–Aldrich syndrome protein Homology 2 (WH2) domain, massively promoted the formation of F-actin–rich membrane ruffles of COS-7 cells and of dendritic branches of neurons. Cobl-like hereby integrates WH2 domain functions with those of the F-actin–binding protein Abp1. Cobl-like–mediated dendritic branching is dependent on Abp1 as well as on Ca²⁺/calmodulin (CaM) signaling and CaM association. Calcium signaling leads to a promotion of complex formation with Cobl-like’s cofactor Abp1. Thus, Ca²⁺/CaM control of actin dynamics seems to be a much more broadly used principle in cell biology than previously thought.

Dendritic Actin Cytoskeleton: Structure, Functions, and Regulations

Actin is a versatile and ubiquitous cytoskeletal protein that plays a major role in both the establishment and the maintenance of neuronal polarity. For a long time, the most prominent roles that were attributed to actin in neurons were the movement of growth cones, polarized cargo sorting at the axon initial segment, and the dynamic plasticity of dendritic spines, since those compartments contain large accumulations of actin filaments (F-actin) that can be readily visualized using electron- and fluorescence microscopy. With the development of super-resolution microscopy in the past few years, previously unknown structures of the actin cytoskeleton have been uncovered: a periodic lattice consisting of actin and spectrin seems to pervade not only the whole axon, but also dendrites and even the necks of dendritic spines. Apart from that striking feature, patches of F-actin and deep actin filament bundles have been described along the lengths of neurites. So far, research has been focused on the specific roles of actin in the axon, while it is becoming more and more apparent that in the dendrite, actin is not only confined to dendritic spines, but serves many additional and important functions. In this review, we focus on recent developments regarding the role of actin in dendrite morphology, the regulation of actin dynamics by internal and external factors, and the role of F-actin in dendritic protein trafficking.

Crucial roles of the Arp2/3 complex during mammalian corticogenesis

The polarity and organization of radial glial cells (RGCs), which serve as both stem cells and scaffolds for neuronal migration, are crucial for cortical development. However, the cytoskeletal mechanisms that drive radial glial outgrowth and maintain RGC polarity remain poorly understood. Here, we show that the Arp2/3 complex – the unique actin nucleator that produces branched actin networks – plays essential roles in RGC polarity and morphogenesis. Disruption of the Arp2/3 complex in murine RGCs retards process outgrowth toward the basal surface and impairs apical polarity and adherens junctions. Whereas the former is correlated with an abnormal actin-based leading edge, the latter is consistent with blockage in membrane trafficking. These defects result in altered cell fate, disrupted cortical lamination and abnormal angiogenesis. In addition, we present evidence that the Arp2/3 complex is a cell-autonomous regulator of neuronal migration. Our data suggest that Arp2/3-mediated actin assembly might be particularly important for neuronal cell motility in a soft or poorly adhesive matrix environment.

Summary: During mouse cortical development, the Arp2/3 actin branching complex regulates process formation and the maintenance of radial glial cell polarity, as well as affecting neuronal migration.

Shank-cortactin interactions control actin dynamics to maintain flexibility of neuronal spines and synapses

The family of Shank scaffolding molecules (comprising Shank1, 2 and 3) are core components of the postsynaptic density (PSD) in neuronal synapses. Shanks link surface receptors to other scaffolding molecules within the PSD, as well as to the actin cytoskeleton. However, determining the function of Shank proteins in neurons has been complicated because the different Shank isoforms share a very high degree of sequence and domain homology. Therefore, to control Shank content while minimizing potential compensatory effects, a miRNA-based knockdown strategy was developed to reduce the expression of all synaptically targeted Shank isoforms simultaneously in rat hippocampal neurons. Using this approach, a strong (>75%) reduction in total Shank protein levels was achieved at individual dendritic spines, prompting an approximately 40% decrease in mushroom spine density. Furthermore, Shank knockdown reduced spine actin levels and increased sensitivity to the actin depolymerizing agent Latrunculin A. A SHANK2 mutant lacking the proline-rich cortactin-binding motif (SHANK2-ΔPRO) was unable to rescue these defects. Furthermore, Shank knockdown reduced cortactin levels in spines and increased the mobility of spine cortactin as measured by single-molecule tracking photoactivated localization microscopy, suggesting that Shank proteins recruit and stabilize cortactin at the synapse. Furthermore, it was found that Shank knockdown significantly reduced spontaneous remodelling of synapse morphology that could not be rescued by the SHANK2-ΔPRO mutant. It was concluded that Shank proteins are key intermediates between the synapse and the spine interior that, via cortactin, permit the actin cytoskeleton to dynamically regulate synapse morphology and function.

The Actin Nucleator Cobl Is Controlled by Calcium and Calmodulin

Actin nucleation triggers the formation of new actin filaments and has the power to shape cells but requires tight control in order to bring about proper morphologies. The regulation of the members of the novel class of WASP Homology 2 (WH2) domain-based actin nucleators, however, thus far has largely remained elusive. Our study reveals signal cascades and mechanisms regulating Cordon-Bleu (Cobl). Cobl plays some, albeit not fully understood, role in early arborization of neurons and nucleates actin by a mechanism that requires a combination of all three of its actin monomer–binding WH2 domains. Our experiments reveal that Cobl is regulated by Ca²⁺ and multiple, direct associations of the Ca²⁺ sensor Calmodulin (CaM). Overexpression analyses and rescue experiments of Cobl loss-of-function phenotypes with Cobl mutants in primary neurons and in tissue slices demonstrated the importance of CaM binding for Cobl’s functions. Cobl-induced dendritic branch initiation was preceded by Ca²⁺ signals and coincided with local F-actin and CaM accumulations. CaM inhibitor studies showed that Cobl-mediated branching is strictly dependent on CaM activity. Mechanistic studies revealed that Ca²⁺/CaM modulates Cobl’s actin binding properties and furthermore promotes Cobl’s previously identified interactions with the membrane-shaping F-BAR protein syndapin I, which accumulated with Cobl at nascent dendritic protrusion sites. The findings of our study demonstrate a direct regulation of an actin nucleator by Ca²⁺/CaM and reveal that the Ca²⁺/CaM-controlled molecular mechanisms we discovered are crucial for Cobl’s cellular functions. By unveiling the means of Cobl regulation and the mechanisms, by which Ca²⁺/CaM signals directly converge on a cellular effector promoting actin filament formation, our work furthermore sheds light on how local Ca²⁺ signals steer and power branch initiation during early arborization of nerve cells—a key process in neuronal network formation.

The calcium sensor calmodulin directly regulates the actin filament-promoting factor Cobl to help shape the complex architecture of neurons underlying neuronal network formation.

The organization and the formation of new actin filaments by polymerization of actin monomers has the power to shape cells. The rate-limiting step in actin polymerization is “nucleation”—a process during which the first actin monomers are assembled with the help of actin nucleators. This nucleation step requires tight temporal and spatial control in order to achieve proper cell morphologies. Here, we analyse signaling cascades and mechanisms regulating the actin nucleator Cobl, which is crucial for the formation of dendritic arbors of nerve cells—a key process in neuronal network formation. We show that the calcium (Ca²⁺)-binding signaling component calmodulin (CaM) binds to Cobl and regulates its functions. Using 3-D time-lapse analyses of developing neurons, we visualized how Cobl works. We observed local accumulation of CaM, Cobl, actin, and syndapin I—a membrane-shaping protein—at dendritic branch initiation sites. We find that Ca²⁺/CaM modulates Cobl’s actin-binding properties and promotes its interactions with syndapin I, which then serves as a membrane anchor for Cobl. In summary, we i) show a direct regulation of the actin nucleator Cobl by Ca²⁺/CaM, ii) demonstrate that the molecular mechanisms we discovered are crucial for shaping nerve cells, and iii) underscore how local Ca²⁺ signals steer and power branch initiation during early arborization of neurons.

Extracellular and Intracellular Signaling for Neuronal Polarity

Neurons are one of the highly polarized cells in the body. One of the fundamental issues in neuroscience is how neurons establish their polarity; therefore, this issue fascinates many scientists. Cultured neurons are useful tools for analyzing the mechanisms of neuronal polarization, and indeed, most of the molecules important in their polarization were identified using culture systems. However, we now know that the process of neuronal polarization in vivo differs in some respects from that in cultured neurons. One of the major differences is their surrounding microenvironment; neurons in vivo can be influenced by extrinsic factors from the microenvironment. Therefore, a major question remains: How are neurons polarized in vivo? Here, we begin by reviewing the process of neuronal polarization in culture conditions and in vivo. We also survey the molecular mechanisms underlying neuronal polarization. Finally, we introduce the theoretical basis of neuronal polarization and the possible involvement of neuronal polarity in disease and traumatic brain injury.

Cytoskeletal and signaling mechanisms of neurite formation

The formation of a neurite, the basis for axons and dendrites, begins with the concerted accumulation and organization of actin and microtubules. Whereas much is known about the proteins that play a role in these processes, because they perform similar functions in axon branching and filopodia formation, much remains to be discovered concerning the interaction of these individual cytoskeletal regulators during neurite formation. Here, we review the literature regarding various models of filopodial formation and the way in which proteins that control actin organization and polymerization induce neurite formation. Although several different regulators of actin polymerization are involved in neurite initiation, redundancy occurs between these regulators, as the effects of the loss of a single regulator can be mitigated by the addition of neurite-promoting substrates and proteins. Similar to actin dynamics, both microtubule stabilizing and destabilizing proteins play a role in neurite initiation. Furthermore, interactions between the actin and microtubule cytoskeleton are required for neurite formation. Several lines of evidence indicate that the interactions between these two components of the cytoskeleton are needed for force generation and for the localization of microtubules at sites of nascent neurites. The general theme that emerges is the existence of several central regulatory pathways on which extracellular cues converge to control and organize both actin and microtubules to induce the formation of neurites.

Actin-binding protein 1 links B-cell antigen receptors to negative signaling pathways

Prolonged or uncontrolled B-cell receptor (BCR) signaling is associated with autoimmunity. We previously demonstrated a role for actin in BCR signal attenuation. This study reveals that actin-binding protein 1 (Abp1/HIP-55/SH3P7) is a negative regulator of BCR signaling and links actin to negative regulatory pathways of the BCR. In both Abp1(-/-) and bone marrow chimeric mice, in which only B cells lack Abp1 expression, the number of spontaneous germinal center and marginal zone B cells and the level of autoantibody are significantly increased. Serum levels of T-independent antibody responses and total antibody are elevated, whereas T-dependent antibody responses are markedly reduced and fail to undergo affinity maturation. Upon activation, surface BCR clustering is enhanced and B-cell contraction delayed in Abp1(-/-) B cells, concurrent with slow but persistent increases in F-actin at BCR signalosomes. Furthermore, BCR signaling is enhanced in Abp1(-/-) B cells compared with wild-type B cells, including Ca(2+) flux and phosphorylation of B-cell linker protein, the mitogen-activated protein kinase kinase MEK1/2, and ERK, coinciding with reductions in recruitment of the inhibitory signaling molecules hematopoietic progenitor kinase 1 and SH2-containing inositol 5-phosphatase to BCR signalosomes. Our results indicate that Abp1 negatively regulates BCR signaling by coupling actin remodeling to B-cell contraction and activation of inhibitory signaling molecules, which contributes to the regulation of peripheral B-cell development and antibody responses.

Terminal axonal arborization and synaptic bouton formation critically rely on abp1 and the arp2/3 complex

Neuronal network formation depends on properly timed and localized generation of presynaptic as well as postsynaptic structures. Although of utmost importance for understanding development and plasticity of the nervous system and neurodegenerative diseases, the molecular mechanisms that ensure the fine-control needed for coordinated establishment of pre- and postsynapses are still largely unknown. We show that the F-actin-binding protein Abp1 is prominently expressed in the Drosophila nervous system and reveal that Abp1 is an important regulator in shaping glutamatergic neuromuscular junctions (NMJs) of flies. STED microscopy shows that Abp1 accumulations can be found in close proximity of synaptic vesicles and at the cell cortex in nerve terminals. Abp1 knock-out larvae have locomotion defects and underdeveloped NMJs that are characterized by a reduced number of both type Ib synaptic boutons and branches of motornerve terminals. Abp1 is able to indirectly trigger Arp2/3 complex-mediated actin nucleation and interacts with both WASP and Scar. Consistently, Arp2 and Arp3 loss-of-function also resulted in impairments of bouton formation and arborization at NMJs, i.e. fully phenocopied abp1 knock-out. Interestingly, neuron- and muscle-specific rescue experiments revealed that synaptic bouton formation critically depends on presynaptic Abp1, whereas the NMJ branching defects can be compensated for by restoring Abp1 functions at either side. In line with this presynaptic importance of Abp1, also presynaptic Arp2 and Arp3 are crucial for the formation of type Ib synaptic boutons. Interestingly, presynaptic Abp1 functions in NMJ formation were fully dependent on the Arp2/3 complex, as revealed by suppression of Abp1-induced synaptic bouton formation and branching of axon terminals upon presynaptic Arp2 RNAi. These data reveal that Abp1 and Arp2/3 complex-mediated actin cytoskeletal dynamics drive both synaptic bouton formation and NMJ branching. Our data furthermore shed light on an intense bidirectional functional crosstalk between pre- and postsynapses during the development of synaptic contacts.

Actin-mediated feedback loops in B-cell receptor signaling

Upon recognizing cognate antigen, B cells mobilize multiple cellular apparatuses to propagate an optimal response. Antigen binding is transduced into cytoplasmic signaling events through B-cell antigen receptor (BCR)-based signalosomes at the B-cell surface. BCR signalosomes are dynamic and transient and are subsequently endocytosed for antigen processing. The function of BCR signalosomes is one of the determining factors for the fate of B cells: clonal expansion, anergy, or apoptosis. Accumulating evidence underscores the importance of the actin cytoskeleton in B-cell activation. We have begun to appreciate the role of actin dynamics in regulating BCR-mediated tonic signaling and the formation of BCR signalosomes. Our recent studies reveal an additional function of the actin cytoskeleton in the downregulation of BCR signaling, consequently contributing to the generation and maintenance of B-cell self-tolerance. In this review, we discuss how actin remodels its organization and dynamics in close coordination with BCR signaling and how actin remodeling in turn amplifies the activation and subsequent downregulation process of BCR signaling, providing vital feedback for optimal BCR activation.

The differential hippocampal phosphoproteome of Apodemus sylvaticus paralleling spatial memory retrieval in the Barnes maze

Protein phosphorylation is a well-known and well-documented mechanism in memory processes. Although a large series of protein kinases involved in memory processes have been reported, information on phosphoproteins is limited. It was therefore the aim of the study to determine a partial and differential phosphoproteome along with the corresponding network in hippocampus of a wild caught mouse strain with excellent performance in several paradigms of spatial memory. Apodemus sylvaticus mice were trained in the Barnes maze, a non-invasive test system for spatial memory and untrained mice served as controls. Animals were sacrificed 6h following memory retrieval, hippocampi were taken, proteins extracted and in-solution digestion was carried out with subsequent iTRAQ double labelling. Phosphopeptides were enriched by a TiO2-based method and semi-quantified using two fragmentation principles on the LTQ-orbitrap Velos. In hippocampi of trained animals phosphopeptide levels representing signalling, neuronal, synaptosomal, cytoskeletal and metabolism proteins were at least twofold reduced or increased. Furthermore, a network revealing a link to pathways of ubiquitination, the androgen receptor, small GTPase Rab5 and MAPK signaling as well as synucleins was constructed. This work is relevant for interpretation of previous work and the design of future studies on protein phosphorylation in spatial memory.

Mutations in KPTN cause macrocephaly, neurodevelopmental delay, and seizures

The proper development of neuronal circuits during neuromorphogenesis and neuronal-network formation is critically dependent on a coordinated and intricate series of molecular and cellular cues and responses. Although the cortical actin cytoskeleton is known to play a key role in neuromorphogenesis, relatively little is known about the specific molecules important for this process. Using linkage analysis and whole-exome sequencing on samples from families from the Amish community of Ohio, we have demonstrated that mutations in KPTN, encoding kaptin, cause a syndrome typified by macrocephaly, neurodevelopmental delay, and seizures. Our immunofluorescence analyses in primary neuronal cell cultures showed that endogenous and GFP-tagged kaptin associates with dynamic actin cytoskeletal structures and that this association is lost upon introduction of the identified mutations. Taken together, our studies have identified kaptin alterations responsible for macrocephaly and neurodevelopmental delay and define kaptin as a molecule crucial for normal human neuromorphogenesis.

Syndapin--a membrane remodelling and endocytic F-BAR protein

Syndapin [also called PACSIN (protein kinase C and casein kinase II interacting protein)] is an Fes-CIP4 homology Bin-amphiphysin-Rvs161/167 (F-BAR) and Src-homology 3 domain-containing protein. Three genes give rise to three main isoforms in mammalian cells. They each function in different endocytic and vesicle trafficking pathways and provide critical links between the cytoskeletal network in different cellular processes, such as neuronal morphogenesis and cell migration. The membrane remodelling activity of syndapin via its F-BAR domain and its interaction partners, such as dynamin and neural Wiskott-Aldrich syndrome protein binding to its Src-homology 3 domain, are important with respect to its function. Its various partner proteins provide insights into its mechanism of action, as well as its differential roles in these cellular processes. Signalling pathways leading to the regulation of syndapin function by phosphorylation are now contributing to our understanding of the broader functions of this family of proteins.

The actin nucleator Cobl is crucial for Purkinje cell development and works in close conjunction with the F-actin binding protein Abp1

Cortical actin dynamics shapes cells. To generate actin filaments, cells rely on actin nucleators. Cobl is a novel, brain-enriched, WH2 domain-based actin nucleator, yet, its functions remained largely elusive. Here, we reveal that Cobl plays a crucial role in Purkinje cell development using gene gun transfections within intact murine cerebellar contexts. Cobl deficiency impaired proper dendritic arborization of Purkinje cells and led to low-complexity arbors. Branch point numbers and density and especially higher order branching were strongly affected. Our efforts to reveal how Cobl is physically and functionally integrated into the cortical actin cytoskeleton showed that all Cobl loss-of-function phenotypes were exactly mirrored by knockdown of the F-actin-binding protein Abp1. By subcellular fractionations, protein interaction analyses, subcellular reconstitutions of protein complexes, colocalization studies in cells and tissues, and by functional analyses in neuronal morphogenesis we demonstrate that both proteins associate and work with each other closely. Cobl-mediated dendritic branch induction in hippocampal neurons critically relied on Abp1. Our study highlights that the functions of Abp1 are distinct from those of the Cobl-binding protein syndapin I. The importance of Cobl/Abp1 complex formation and of Abp1-mediated F-actin association was highlighted by functional rescue experiments demonstrating that a Cobl mutant deficient for Abp1 binding and an Abp1 mutant supporting Cobl association but lacking the F-actin binding ability failed to rescue the respective loss-of-function phenotypes. Thus, F-actin-anchored Cobl/Abp1 complexes seem crucial for neuromorphogenesis processes, particularly for the postnatal arborization of Purkinje cells representing the source for all motor coordination in the cerebellar cortex.

Transcriptional insights on the regenerative mechanics of axotomized neurons in vitro

Axotomized neurons have the innate ability to undergo regenerative sprouting but this is often impeded by the inhibitory central nervous system environment. To gain mechanistic insights into the key molecular determinates that specifically underlie neuronal regeneration at a transcriptomic level, we have undertaken a DNA microarray study on mature cortical neuronal clusters maintained in vitro at 8, 15, 24 and 48 hrs following complete axonal severance. A total of 305 genes, each with a minimum fold change of ±1.5 for at least one out of the four time points and which achieved statistical significance (one-way ANOVA, P < 0.05), were identified by DAVID and classified into 14 different functional clusters according to Gene Ontology. From our data, we conclude that post-injury regenerative sprouting is an intricate process that requires two distinct pathways. Firstly, it involves restructuring of the neurite cytoskeleton, determined by compound actin and microtubule dynamics, protein trafficking and concomitant modulation of both guidance cues and neurotrophic factors. Secondly, it elicits a cell survival response whereby genes are regulated to protect against oxidative stress, inflammation and cellular ion imbalance. Our data reveal that neurons have the capability to fight insults by elevating biological antioxidants, regulating secondary messengers, suppressing apoptotic genes, controlling ion-associated processes and by expressing cell cycle proteins that, in the context of neuronal injury, could potentially have functions outside their normal role in cell division. Overall, vigilant control of cell survival responses against pernicious secondary processes is vital to avoid cell death and ensure successful neurite regeneration.

Expression of multiple nebulin isoforms in human skeletal muscle and brain

INTRODUCTION

Nebulin is a large actin-binding protein of the skeletal muscle sarcomere. Multiple isoforms of nebulin are produced from the 183-exon-containing nebulin gene (NEB). Mutations in NEB cause nemaline myopathy, distal myopathy, and core-rod myopathy.

METHODS

Nebulin mRNA expression was assessed by microarrays and RT-PCR in 21 human leg muscle and 2 brain samples. Protein expression was assessed by immunohistochemistry in 5 regions of 1 brain sample.

RESULTS

Nebulin isoform diversity is as high in brain as in skeletal muscle. Isoforms with more than 22 super repeats seem to be more common than previously anticipated. Immunohistochemistry showed nebulin expression predominantly in the cytoplasm of pyramidal neurons but also in the cytoplasm of mainly subcortical endothelial cells.

CONCLUSIONS

Nebulin, as in skeletal muscle, may have a role as an actin filament stabilizer or length regulator in neurons of the human brain, although patients with NEB mutations usually have normal cognition.

Arp2/3 complex-dependent actin networks constrain myosin II function in driving retrograde actin flow

The Arp2/3 complex nucleates actin filaments to generate networks at the leading edge of motile cells. Nonmuscle myosin II produces contractile forces involved in driving actin network translocation. We inhibited the Arp2/3 complex and/or myosin II with small molecules to investigate their respective functions in neuronal growth cone actin dynamics. Inhibition of the Arp2/3 complex with CK666 reduced barbed end actin assembly site density at the leading edge, disrupted actin veils, and resulted in veil retraction. Strikingly, retrograde actin flow rates increased with Arp2/3 complex inhibition; however, when myosin II activity was blocked, Arp2/3 complex inhibition now resulted in slowing of retrograde actin flow and veils no longer retracted. Retrograde flow rate increases induced by Arp2/3 complex inhibition were independent of Rho kinase activity. These results provide evidence that, although the Arp2/3 complex and myosin II are spatially segregated, actin networks assembled by the Arp2/3 complex can restrict myosin II-dependent contractility with consequent effects on growth cone motility.

Myosin 1E coordinates actin assembly and cargo trafficking during clathrin-mediated endocytosis

An actin-dependent role is shown for Myo1E in the trafficking of newly internalized cargo to early endosomes during CME. The results establish for mammalian cells, similar to budding yeast, interdependence in the recruitment of type I myosins, WIP/WIRE, and N-WASP to endocytic sites to assemble F-actin as endocytic vesicles are being formed.

Myosin 1E (Myo1E) is recruited to sites of clathrin-mediated endocytosis coincident with a burst of actin assembly. The recruitment dynamics and lifetime of Myo1E are similar to those of tagged actin polymerization regulatory proteins. Like inhibition of actin assembly, depletion of Myo1E causes reduced transferrin endocytosis and a significant delay in transferrin trafficking to perinuclear compartments, demonstrating an integral role for Myo1E in these actin-mediated steps. Mistargeting of GFP-Myo1E or its src-homology 3 domain to mitochondria results in appearance of WIP, WIRE, N-WASP, and actin filaments at the mitochondria, providing evidence for Myo1E's role in actin assembly regulation. These results suggest for mammalian cells, similar to budding yeast, interdependence in the recruitment of type I myosins, WIP/WIRE, and N-WASP to endocytic sites for Arp2/3 complex activation to assemble F-actin as endocytic vesicles are being formed.

Barfly: sculpting membranes at the Drosophila neuromuscular junction

The ability of a cell to change the shape of its membranes is intrinsic to many cellular functions. Proteins that can alter or recognize curved membrane structures and those that can act to recruit other proteins which stabilize the membrane curvature are likely to be essential in cell functions. The BAR (Bin, amphiphysin, RVS167 homology) domain is a protein domain that can either induce lipidic membranes to curve or can sense curved membranes. BAR domains are found in several proteins at neuronal synapses. We will review BAR domain structure and the role that BAR domain containing proteins play in regulating the morphology and function of the Drosophila neuromuscular junction. In flies the BAR domain containing proteins, endophilin and syndapin affect synaptic vesicle endocytosis, whereas CIP4, dRich, nervous wreck and syndapin affect synaptic morphology. We will review the growing evidence implicating mutations in BAR domain containing proteins being the cause of human pathologies.

hnRNP Q regulates Cdc42-mediated neuronal morphogenesis

The RNA-binding protein hnRNP Q has been implicated in neuronal mRNA metabolism. Here, we show that knockdown of hnRNP Q increased neurite complexity in cultured rat cortical neurons and induced filopodium formation in mouse neuroblastoma cells. Reexpression of hnRNP Q1 in hnRNP Q-depleted cells abrogated the morphological changes of neurites, indicating a specific role for hnRNP Q1 in neuronal morphogenesis. A search for mRNA targets of hnRNP Q1 identified functionally coherent sets of mRNAs encoding factors involved in cellular signaling or cytoskeletal regulation and determined its preferred binding sequences. We demonstrated that hnRNP Q1 bound to a set of identified mRNAs encoding the components of the actin nucleation-promoting Cdc42/N-WASP/Arp2/3 complex and was in part colocalized with Cdc42 mRNA in granules. Using subcellular fractionation and immunofluorescence, we showed that knockdown of hnRNP Q reduced the level of some of those mRNAs in neurites and redistributed their encoded proteins from neurite tips to soma to different extents. Overexpression of dominant negative mutants of Cdc42 or N-WASP compromised hnRNP Q depletion-induced neurite complexity. Together, our results suggest that hnRNP Q1 may participate in localization of mRNAs encoding Cdc42 signaling factors in neurites, and thereby may regulate actin dynamics and control neuronal morphogenesis.

NESH regulates dendritic spine morphology and synapse formation

Background

Dendritic spines are small membranous protrusions on the neuronal dendrites that receive synaptic input from axon terminals. Despite their importance for integrating the enormous information flow in the brain, the molecular mechanisms regulating spine morphogenesis are not well understood. NESH/Abi-3 is a member of the Abl interactor (Abi) protein family, and its overexpression is known to reduce cell motility and tumor metastasis. NESH is prominently expressed in the brain, but its function there remains unknown.

Methodology/Principal Findings

NESH was strongly expressed in the hippocampus and moderately expressed in the cerebral cortex, cerebellum and striatum, where it co-localized with the postsynaptic proteins PSD95, SPIN90 and F-actin in dendritic spines. Overexpression of NESH reduced numbers of mushroom-type spines and synapse density but increased thin, filopodia-like spines and had no effect on spine density. siRNA knockdown of NESH also reduced mushroom spine numbers and inhibited synapse formation but it increased spine density. The N-terminal region of NESH co-sedimented with filamentous actin (F-actin), which is an essential component of dendritic spines, suggesting this interaction is important for the maturation of dendritic spines.

Conclusions/Significance

NESH is a novel F-actin binding protein that likely plays important roles in the regulation of dendritic spine morphogenesis and synapse formation.

Abp1 utilizes the Arp2/3 complex activator Scar/WAVE in bristle development

Many developmental processes rely on cortical actin dynamics; however, the mechanisms of its fine control at the cell cortex are still largely unknown. Our analyses demonstrate that the lipid- and F-actin-binding protein Abp1 is crucial for actin-driven bristle development in Drosophila melanogaster. Combined genetic, cell biological and biochemical analyses reveal that Abp1 triggers cortical Arp2/3-mediated actin nucleation by complex formation with Scar in bristle development. The role of the plasma-membrane-associated Abp1 subpool was highlighted by constitutively membrane-anchored Abp1. Such gain-of-function experiments led to a severe split-bristle phenotype, which was negatively correlated with bristle length. This phenotype was dependent on Scar but not on WASP and required the Scar-interacting SH3 domain of Abp1. Strikingly, knockout of abp1 led to defects in both microchaete and macrochaete bristle integrity. Importantly, Arp2- and Scar-deficient flies displayed similar bristle phenotypes. Microchaetes of flies deficient for Abp1, Arp2 and Scar functions had kinks, whereas those of wasp heterozygous flies did not. Electron microscopy analyses revealed that abp1 knockout, Arp2 RNAi and Scar RNAi all led to distorted macrochaetes with an excessive number of ridges. Interestingly, despite the physical association of Abp1 with Scar and its ability to use the Arp2/3 complex activator as an effector, abp1 knockout did not affect Scar stability. This is in contrast to classical Scar complex components, such as Kette or Sra-1. Our work reveals that Abp1 is an important, Scar-interacting factor controlling cortical Arp2/3-mediated actin nucleation and unravels a novel layer of complexity in the scrupulous control of cortical actin nucleation during sensory organ formation.

The Caenorhabditis elegans Eph receptor activates NCK and N-WASP, and inhibits Ena/VASP to regulate growth cone dynamics during axon guidance

The Eph receptor tyrosine kinases (RTKs) are regulators of cell migration and axon guidance. However, our understanding of the molecular mechanisms by which Eph RTKs regulate these processes is still incomplete. To understand how Eph receptors regulate axon guidance in Caenorhabditis elegans, we screened for suppressors of axon guidance defects caused by a hyperactive VAB-1/Eph RTK. We identified NCK-1 and WSP-1/N-WASP as downstream effectors of VAB-1. Furthermore, VAB-1, NCK-1, and WSP-1 can form a complex in vitro. We also report that NCK-1 can physically bind UNC-34/Enabled (Ena), and suggest that VAB-1 inhibits the NCK-1/UNC-34 complex and negatively regulates UNC-34. Our results provide a model of the molecular events that allow the VAB-1 RTK to regulate actin dynamics for axon guidance. We suggest that VAB-1/Eph RTK can stop axonal outgrowth by inhibiting filopodia formation at the growth cone by activating Arp2/3 through a VAB-1/NCK-1/WSP-1 complex and by inhibiting UNC-34/Ena activity.

The correct wiring of the nervous system depends on the ability of axons to properly interpret extracellular cues that guide them to their targets. The Eph receptor tyrosine kinases (RTKs) have roles in guiding axons, but their signaling pathways are not completely understood. In this study, we used the nematode Caenorhabditis elegans to study how the VAB-1 Eph RTK regulates the growth cone structure for axon guidance. Genetic and molecular data show that VAB-1 regulates the conserved molecules NCK-1, WSP-1/N-WASP, and UNC-34/Ena. Our study provides a model of how the VAB-1 Eph RTK modulates the growth cone structure to inhibit axonal outgrowth. We show that activated VAB-1 can inhibit an NCK-1/UNC-34 interaction by binding to the NCK-1 SH2 domain. We also show that NCK-1 and WSP-1 can physically interact and that VAB-1/NCK-1 and WSP-1 form a complex in vitro. We suggest that the VAB-1 Eph RTK can contribute to the termination of axon outgrowth by two methods: 1) The VAB-1/NCK-1/WSP-1 complex activates ARP-2/3 to change the actin growth cone dynamics to that of a branched structure thus reducing the number of filopodia, and 2) VAB-1 inhibits axon extension by inhibiting UNC-34/Ena's function in actin polymerization.

Src-mediated phosphorylation of mammalian Abp1 (DBNL) regulates podosome rosette formation in transformed fibroblasts

Podosomes are dynamic actin-based structures that mediate adhesion to the extracellular matrix and localize matrix degradation to facilitate cell motility and invasion. Drebrin-like protein (DBNL), which is homologous to yeast mAbp1 and is therefore known as mammalian actin-binding protein 1 (mAbp1), has been implicated in receptor-mediated endocytosis, vesicle recycling and dorsal ruffle formation. However, it is not known whether mAbp1 regulates podosome formation or cell invasion. In this study, we found that mAbp1 localizes to podosomes and is necessary for the formation of podosome rosettes in Src-transformed fibroblasts. Despite their structural similarity, mAbp1 and cortactin play distinct roles in podosome regulation. Cortactin was necessary for the formation of podosome dots, whereas mAbp1 was necessary for the formation of organized podosome rosettes in Src-transformed cells. We identified specific Src phosphorylation sites, Tyr337 and Tyr347 of mAbp1, which mediate the formation of podosome rosettes and degradation of the ECM. In contrast to dorsal ruffles, the interaction of mAbp1 with WASP-interacting protein (WIP) was not necessary for the formation of podosome rosettes. Finally, we showed that depletion of mAbp1 increased invasive cell migration, suggesting that mAbp1 differentially regulates matrix degradation and cell invasion. Collectively, our findings identify a role for mAbp1 in podosome rosette formation and cell invasion downstream of Src.

Developmental plasticity of the dendritic compartment: focus on the cytoskeleton

Plasticity, the ability to undergo lasting changes in response to a stimulus, is an important attribute of neurons. It allows proper development and underlies learning, memory, and the recovery of the nervous system after severe injuries. Often, an outcome of neuronal plasticity is a structural plasticity manifested as a change of neuronal morphology. In this chapter, we focus on the structural plasticity of dendritic arbors and spines during development. Dendrites receive and compute synaptic inputs from other neurons. The number of dendrites and their branching pattern are strictly correlated with the function of a particular neuron and the geometry of the connections it receives. The development of proper dendritic tree morphology depends on the interplay between genetic programming and extracellular signals. Spines are tiny actin-rich dendritic protrusions that harbor excitatory synapses. No consensus has been reached regarding how dendritic spines form, and several models of spine morphogenesis exist. Nevertheless, most researchers agree that spinogenesis is an important target for structural plasticity. In this chapter, we discuss examples of such plasticity and describe the principles and molecular mechanisms underlying this process, focusing mostly on the regulation of the cytoskeleton during dendrito- and spinogenesis.

SHANK3 mutations identified in autism lead to modification of dendritic spine morphology via an actin-dependent mechanism

Genetic mutations of SHANK3 have been reported in patients with intellectual disability, autism spectrum disorder (ASD) and schizophrenia. At the synapse, Shank3/ProSAP2 is a scaffolding protein that connects glutamate receptors to the actin cytoskeleton via a chain of intermediary elements. Although genetic studies have repeatedly confirmed the association of SHANK3 mutations with susceptibility to psychiatric disorders, very little is known about the neuronal consequences of these mutations. Here, we report the functional effects of two de novo mutations (STOP and Q321R) and two inherited variations (R12C and R300C) identified in patients with ASD. We show that Shank3 is located at the tip of actin filaments and enhances its polymerization. Shank3 also participates in growth cone motility in developing neurons. The truncating mutation (STOP) strongly affects the development and morphology of dendritic spines, reduces synaptic transmission in mature neurons and also inhibits the effect of Shank3 on growth cone motility. The de novo mutation in the ankyrin domain (Q321R) modifies the roles of Shank3 in spine induction and morphology, and actin accumulation in spines and affects growth cone motility. Finally, the two inherited mutations (R12C and R300C) have intermediate effects on spine density and synaptic transmission. Therefore, although inherited by healthy parents, the functional effects of these mutations strongly suggest that they could represent risk factors for ASD. Altogether, these data provide new insights into the synaptic alterations caused by SHANK3 mutations in humans and provide a robust cellular readout for the development of knowledge-based therapies.