Control of actin reorganization by Slingshot, a family of phosphatases that dephosphorylate ADF/cofilin

Planar cell polarity (PCP) proteins and spermatogenesis

In adult mammalian testes, spermatogenesis is comprised of several discrete cellular events that work in tandem to support the transformation and differentiation of diploid spermatogonia to haploid spermatids in the seminiferous epithelium during the seminiferous epithelial cycle. These include: self-renewal of spermatogonial stem cells via mitosis and their transformation into differentiated spermatogonia, meiosis I/II, spermiogenesis and the release of sperms at spermiation. Studies have shown that these cellular events are under precise and coordinated controls of multiple proteins and signaling pathways. These events are also regulated by polarity proteins that are known to confer classical apico-basal (A/B) polarity in other epithelia. Furthermore, spermatid development is likely supported by planar cell polarity (PCP) proteins since polarized spermatids are aligned across the plane of seminiferous epithelium in an orderly fashion, analogous to hair cells in the cochlea of the inner ear. Thus, the maximal number of spermatids can be packed and supported by a fixed population of differentiated Sertoli cells in the limited space of the seminiferous epithelium in adult testes. In this review, we briefly summarize recent findings regarding the role of PCP proteins in the testis. This information should be helpful in future studies to better understand the role of PCP proteins in spermatogenesis.

Caveolin-1-mediated Japanese encephalitis virus entry requires a two-step regulation of actin reorganization

Aim: To investigate the detailed mechanism of Japanese encephalitis virus (JEV) cell entry. Materials & methods: Utilize a siRNA library targeting cellular membrane trafficking genes to identify key molecules that mediate JEV entry into human neuronal cells. Results: JEV enters human neuronal cells by caveolin-1-mediated endocytosis, which depends on a two-step regulation of actin cytoskeleton remodeling triggered by RhoA and Rac1: RhoA activation promoted the phosphorylation of caveolin-1, and then Rac1 activation facilitated caveolin-associated viral internalization. Specifically, virus attachment activates the EGFR–PI3K signaling pathway, thereby leading to RhoA activation. Conclusion: This work provides a detailed picture of the entry route and intricate cellular events following the entry of JEV into human neuronal cells, and promotes a better understanding of JEV entry.

Regulation of pulmonary endothelial barrier function by kinases

The pulmonary endothelium is the target of continuous physiological and pathological stimuli that affect its crucial barrier function. The regulation, defense, and repair of endothelial barrier function require complex biochemical processes. This review examines the role of endothelial phosphorylating enzymes, kinases, a class with profound, interdigitating influences on endothelial permeability and lung function.

Cellular functions of the ADF/cofilin family at a glance

The actin depolymerizing factor (ADF)/cofilin family comprises small actin-binding proteins with crucial roles in development, tissue homeostasis and disease. They are best known for their roles in regulating actin dynamics by promoting actin treadmilling and thereby driving membrane protrusion and cell motility. However, recent discoveries have increased our understanding of the functions of these proteins beyond their well-characterized roles. This Cell Science at a Glance article and the accompanying poster serve as an introduction to the diverse roles of the ADF/cofilin family in cells. The first part of the article summarizes their actions in actin treadmilling and the main mechanisms for their intracellular regulation; the second part aims to provide an outline of the emerging cellular roles attributed to the ADF/cofilin family, besides their actions in actin turnover. The latter part discusses an array of diverse processes, which include regulation of intracellular contractility, maintenance of nuclear integrity, transcriptional regulation, nuclear actin monomer transfer, apoptosis and lipid metabolism. Some of these could, of course, be indirect consequences of actin treadmilling functions, and this is discussed.

The M. tuberculosis HAD phosphatase (Rv3042c) interacts with host proteins and is inhibited by Clofazimine

Mycobacterium tuberculosis codes for a HAD-phosphatase, Rv3042c (MtSerB2), that has earlier been characterized as a metabolic enzyme. Here we demonstrate that MtSerB2 is secreted into the cytosol of infected macrophages and is found in bronchoalveolar lavage samples of tuberculosis patients. MtSerB2 induces significant cytoskeleton rearrangements through cofilin activation and affects the expression of genes that regulate actin dynamics. It specifically interacts with HSP90, HSP70 and HSP27 that block apoptotic pathways and not with other HSPs. It actively dephosphorylates MAPK-p38 and NF-kappa B p65 that play crucial roles in inflammatory and immune responses. This in turn leads to down-regulation of Interleukin 8, a chemotactic and inflammatory cytokine. Finally, during evaluation of inhibitors against MtSerB2 we found that Clofazimine, a drug being evaluated for XDR and MDR tuberculosis, inhibits MtSerB2 phosphatase activity and reverses the above effects and interactions with host proteins. Overall, the study identifies that MtSerB2 has new functions that might help the pathogen to evade the host's immune response.

Organization and dynamics of the actin cytoskeleton during dendritic spine morphological remodeling

In the central nervous system, most excitatory post-synapses are small subcellular structures called dendritic spines. Their structure and morphological remodeling are tightly coupled to changes in synaptic transmission. The F-actin cytoskeleton is the main driving force of dendritic spine remodeling and sustains synaptic plasticity. It is therefore essential to understand how changes in synaptic transmission can regulate the organization and dynamics of actin binding proteins (ABPs). In this review, we will provide a detailed description of the organization and dynamics of F-actin and ABPs in dendritic spines and will discuss the current models explaining how the actin cytoskeleton sustains both structural and functional synaptic plasticity.

Modifications to the composition of the hyphal outer layer of Aspergillus fumigatus modulates HUVEC proteins related to inflammatory and stress responses

Aspergillus fumigatus, the main etiologic agent causing invasive aspergillosis, can induce an inflammatory response and a prothrombotic phenotype upon contact with human umbilical vein endothelial cells (HUVECs). However, the fungal molecules involved in this endothelial response remain unknown. A. fumigatus hyphae produce an extracellular matrix composed of galactomannan, galactosaminogalactan and α-(1,3)-glucan. In this study, we investigated the consequences of UGM1 gene deletion in A. fumigatus, which produces a mutant with increased galactosaminogalactan production. The ∆ugm1 mutant exhibited an HUVEC-hyperadhesive phenotype and induced increased endothelial TNF-α secretion and tissue factor mRNA overexpression in this "semi-professional" immune host cell. Using a shotgun proteomics approach, we show that the A. fumigatus ∆ugm1 strain can modulate the levels of proteins in important endothelial pathways related to the inflammatory response mediated by TNF-α and to stress response pathways. Furthermore, a purified galactosaminogalactan fraction was also able to induce TNF-α secretion and the coincident HUVEC pathways regulated by the ∆ugm1 mutant, which overexpresses this component, as demonstrated by fluorescence microscopy. This work contributes new data regarding endothelial mechanisms in response to A. fumigatus infection.

SIGNIFICANCE

Invasive aspergillosis is the main opportunistic fungal infection described in neutropenic hematologic patients. One important clinical aspect of this invasive fungal infection is vascular thrombosis, which could be related, at least in part, to the activation of endothelial cells, as shown in previous reports from our group. It is known that direct contact between the A. fumigatus hyphal cell wall and the HUVEC cell surface is necessary to induce an endothelial prothrombotic phenotype and secretion of pro-inflammatory cytokines, though the cell surface components of this angioinvasive fungus that trigger this endothelial response are unknown. The present work employs a discovery-driven proteomics approach to reveal the role of one important cell wall polysaccharide of A. fumigatus, galactosaminogalactan, in the HUVEC interaction and the consequent mechanisms of endothelial activation. This is the first report of the overall panel of proteins related to the HUVEC response to a specific and purified cell wall component of the angioinvasive fungus A. fumigatus.

Roles of actin binding proteins in mammalian oocyte maturation and beyond

Actin nucleation factors, which promote the formation of new actin filaments, have emerged in the last decade as key regulatory factors controlling asymmetric division in mammalian oocytes. Actin nucleators such as formin-2, spire, and the ARP2/3 complex have been found to be important regulators of actin remodeling during oocyte maturation. Another class of actin-binding proteins including cofilin, tropomyosin, myosin motors, capping proteins, tropomodulin, and Ezrin-Radixin-Moesin proteins are thought to control actin cytoskeleton dynamics at various steps of oocyte maturation. In addition, actin dynamics controlling asymmetric-symmetric transitions after fertilization is a new area of investigation. Taken together, defining the mechanisms by which actin-binding proteins regulate actin cytoskeletons is crucial for understanding the basic biology of mammalian gamete formation and pre-implantation development.

Structural Basis for Noncanonical Substrate Recognition of Cofilin/ADF Proteins by LIM Kinases

Cofilin/actin-depolymerizing factor (ADF) proteins are critical nodes that relay signals from protein kinase cascades to the actin cytoskeleton, in particular through site-specific phosphorylation at residue Ser3. This is important for regulation of the roles of cofilin in severing and stabilizing actin filaments. Consequently, cofilin/ADF Ser3 phosphorylation is tightly controlled as an almost exclusive substrate for LIM kinases. Here we determine the LIMK1:cofilin-1 co-crystal structure. We find an interface that is distinct from canonical kinase-substrate interactions. We validate this previously unobserved mechanism for high-fidelity kinase-substrate recognition by in vitro kinase assays, examination of cofilin phosphorylation in mammalian cells, and functional analysis in S. cerevisiae. The interface is conserved across all LIM kinases. Remarkably, we also observe both pre- and postphosphotransfer states in the same crystal lattice. This study therefore provides a molecular understanding of how kinase-substrate recognition acts as a gatekeeper to regulate actin cytoskeletal dynamics.

Aurora kinase A induces papillary thyroid cancer lymph node metastasis by promoting cofilin-1 activity

Aurora-A (Aur-A), a member of the serine/threonine Aurora kinase family, plays an important role in ensuring genetic stability during cell division. Previous studies indicated that Aur-A possesses oncogenic activity and may be a valuable therapeutic target in cancer therapy. However, the role of Aur-A in the most common thyroid cancer, papillary thyroid cancer (PTC), remains largely unknown. In patients with PTC, cancer cell migration and invasion account for most of the metastasis, recurrence, and cancer-related deaths. Cofilin-1 (CFL-1) is the most important effector of actin polymerization and depolymerization, determining the direction of cell migration. Here, we assessed the correlation between Aur-A and CFL-1 in PTC with lymph node metastasis. Tissue microarray data showed that simultaneous overexpression of Aur-A and CFL-1 correlated with lymph node metastasis in thyroid cancer tissue. Inhibition of Aur-A suppressed thyroid cancer cell migration in vitro and decreased lymph node metastasis in nude mice. Importantly, Aur-A increased the non-phosphorylated, active form of CFL-1 in TPC-1 cells, thus promoting cancer cell migration and thyroid cancer lymph node metastasis. Our findings indicate that the combination of Aur-A and CFL-1 may be useful as a molecular prediction model for lymph node metastasis in thyroid cancer and raise the possibility of targeting Aur-A and CFL-1 for more effective treatment of thyroid cancer.

Actin dynamics and cofilin-actin rods in alzheimer disease

Cytoskeletal abnormalities and synaptic loss, typical of both familial and sporadic Alzheimer disease (AD), are induced by diverse stresses such as neuroinflammation, oxidative stress, and energetic stress, each of which may be initiated or enhanced by proinflammatory cytokines or amyloid-β (Aβ) peptides. Extracellular Aβ-containing plaques and intracellular phospho-tau-containing neurofibrillary tangles are postmortem pathologies required to confirm AD and have been the focus of most studies. However, AD brain, but not normal brain, also have increased levels of cytoplasmic rod-shaped bundles of filaments composed of ADF/cofilin-actin in a 1:1 complex (rods). Cofilin, the major ADF/cofilin isoform in mammalian neurons, severs actin filaments at low cofilin/actin ratios and stabilizes filaments at high cofilin/actin ratios. It binds cooperatively to ADP-actin subunits in F-actin. Cofilin is activated by dephosphorylation and may be oxidized in stressed neurons to form disulfide-linked dimers, required for bundling cofilin-actin filaments into stable rods. Rods form within neurites causing synaptic dysfunction by sequestering cofilin, disrupting normal actin dynamics, blocking transport, and exacerbating mitochondrial membrane potential loss. Aβ and proinflammatory cytokines induce rods through a cellular prion protein-dependent activation of NADPH oxidase and production of reactive oxygen species. Here we review recent advances in our understanding of cofilin biochemistry, rod formation, and the development of cognitive deficits. We will then discuss rod formation as a molecular pathway for synapse loss that may be common between all three prominent current AD hypotheses, thus making rods an attractive therapeutic target. © 2016 Wiley Periodicals, Inc.

Cofilin contributes to phagocytosis of IgG-opsonized particles but not non-opsonized particles in RAW264 macrophages

Cofilin is an actin-binding protein that severs actin filaments. It plays a key role in regulating actin cytoskeletal remodeling, thereby contributing to diverse cellular functions. However, the involvement of cofilin in phagocytosis remains to be elucidated. We examined the spatiotemporal localization of cofilin during phagocytosis of IgG-opsonized erythrocytes, IgG-opsonized latex beads and non-opsonized latex beads. Live-cell imaging showed that GFP-cofilin accumulates in the sites of IgG-opsonized particle binding and in phagocytic cups. Moreover, immunofluorescence microscopy revealed that endogenous cofilin localizes to phagocytic cups engulfing IgG-opsonized particles, but not non-opsonized latex beads. Scanning electron microscopy demonstrated a notable difference in morphology between phagocytic structures in IgG-dependent and IgG-independent phagocytosis. In phagocytosis of IgG-opsonized particles, sheet-like pseudopodia extended along the surface of IgG-opsonized particles to form phagocytic cups. In contrast, in opsonin-independent phagocytosis, long finger-like filopodia captured non-opsonized latex beads. Importantly, non-opsonized beads sank into the cells without extending phagocytic cups. Our analysis of cofilin mutant expression demonstrates that phagocytosis of IgG-opsonized particles is enhanced in cells expressing wild-type cofilin or active mutant cofilin-S3A, whereas the uptake of non-opsonized latex beads is not. These data suggest that cofilin promotes actin cytoskeletal remodeling to form phagocytic cups by accelerating actin turnover and thereby facilitating phagosome formation. In contrast, cofilin is not involved in opsonin-independent phagocytosis of latex beads.

Role of Actin Cytoskeleton During Mammalian Sperm Acrosomal Exocytosis

Mammalian sperm require to undergo an exocytotic process called acrosomal exocytosis in order to be able to fuse with the oocyte. This ability is acquired during the course of sperm capacitation. This review is focused on one aspect related to this acquisition: the role of the actin cytoskeleton. Evidence from different laboratories indicates that actin polymerization occurs during capacitation, and the detection of several actin-related proteins suggests that the cytoskeleton is involved in important sperm functions. In other mammalian cells, the cortical actin network acts as a dominant negative clamp which blocks constitutive exocytosis but, at the same time, is necessary to prepare the cell to undergo regulated exocytosis. Thus, F-actin stabilizes structures generated by exocytosis and supports the physiological progression of this process. Is this also the case in mammalian sperm? This review summarizes what is currently known about actin and its related proteins in the male gamete, with particular emphasis on their role in acrosomal exocytosis.

Caspase-11 and caspase-1 differentially modulate actin polymerization via RhoA and Slingshot proteins to promote bacterial clearance

Inflammasomes are multiprotein complexes that include members of the NOD-like receptor family and caspase-1. Caspase-1 is required for the fusion of the Legionella vacuole with lysosomes. Caspase-11, independently of the inflammasome, also promotes phagolysosomal fusion. However, it is unclear how these proteases alter intracellular trafficking. Here, we show that caspase-11 and caspase-1 function in opposing manners to phosphorylate and dephosphorylate cofilin, respectively upon infection with Legionella. Caspase-11 targets cofilin via the RhoA GTPase, whereas caspase-1 engages the Slingshot phosphatase. The absence of either caspase-11 or caspase-1 maintains actin in the polymerized or depolymerized form, respectively and averts the fusion of pathogen-containing vacuoles with lysosomes. Therefore, caspase-11 and caspase-1 converge on the actin machinery with opposing effects to promote vesicular trafficking.

Cofilin as a Promising Therapeutic Target for Ischemic and Hemorrhagic Stroke

Neurovascular unit (NVU) is considered as a conceptual framework for investigating the mechanisms as well as developing therapeutic targets for ischemic and hemorrhagic stroke. From a molecular perspective, oxidative stress, excitotoxicity, inflammation, and disruption of the blood brain barrier are broad pathophysiological frameworks on the basis on which potential therapeutic candidates for ischemic and hemorrhagic stroke could be discussed. Cofilin is a potent actin-binding protein that severs and depolymerizes actin filaments in order to generate the dynamics of the actin cytoskeleton. Although studies of the molecular mechanisms of cofilin-induced reorganization of the actin cytoskeleton have been ongoing for decades, the multicellular functions of cofilin and its regulation in different molecular pathways are expanding beyond its primary role in actin cytoskeleton. This review focuses on the role of cofilin in oxidative stress, excitotoxicity, inflammation, and disruption of the blood brain barrier in the context of NVU as well as how and why cofilin could be studied further as a potential target for ischemic and hemorrhagic stroke.

Stable Force Balance between Epithelial Cells Arises from F-Actin Turnover

The propagation of force in epithelial tissues requires that the contractile cytoskeletal machinery be stably connected between cells through E-cadherin-containing adherens junctions. In many epithelial tissues, the cells' contractile network is positioned at a distance from the junction. However, the mechanism or mechanisms that connect the contractile networks to the adherens junctions, and thus mechanically connect neighboring cells, are poorly understood. Here, we identified the role for F-actin turnover in regulating the contractile cytoskeletal network's attachment to adherens junctions. Perturbing F-actin turnover via gene depletion or acute drug treatments that slow F-actin turnover destabilized the attachment between the contractile actomyosin network and adherens junctions. Our work identifies a critical role for F-actin turnover in connecting actomyosin to intercellular junctions, defining a dynamic process required for the stability of force balance across intercellular contacts in tissues.

Identification of para-Substituted Benzoic Acid Derivatives as Potent Inhibitors of the Protein Phosphatase Slingshot

Slingshot proteins form a small group of dual-specific phosphatases that modulate cytoskeleton dynamics through dephosphorylation of cofilin and Lim kinases (LIMK). Small chemical compounds with Slingshot-inhibiting activities have therapeutic potential against cancers or infectious diseases. However, only a few Slingshot inhibitors have been investigated and reported, and their cellular activities have not been examined. In this study, we identified two rhodanine-scaffold-based para-substituted benzoic acid derivatives as competitive Slingshot inhibitors. The top compound, (Z)-4-((4-((4-oxo-2-thioxo-3-(o-tolyl)thiazolidin-5-ylidene)methyl)phenoxy)methyl)benzoic acid (D3) had an inhibition constant (Ki) of around 4 μm and displayed selectivity over a panel of other phosphatases. Moreover, compound D3 inhibited cell migration and cofilin dephosphorylation after nerve growth factor (NGF) or angiotensin II stimulation. Therefore, our newly identified Slingshot inhibitors provide a starting point for developing Slingshot-targeted therapies.

Roles of LIM kinases in central nervous system function and dysfunction

LIM kinase 1 (LIMK1) and LIM kinase 2 (LIMK2) regulate actin dynamics by phosphorylating cofilin. In this review, we outline studies that have shown an involvement of LIMKs in neuronal function and we detail some of the pathways and molecular mechanisms involving LIMKs in neurodevelopment and synaptic plasticity. We also review the involvement of LIMKs in neuronal diseases and emphasize the differences in the regulation of LIMKs expression and mode of action. We finally present the existence of a cofilin-independent pathway also involved in neuronal function. A better understanding of the differences between both LIMKs and of the precise molecular mechanisms involved in their mode of action and regulation is now required to improve our understanding of the physiopathology of the neuronal diseases associated with LIMKs.

Abnormal mechanosensing and cofilin activation promote the progression of ascending aortic aneurysms in mice

Smooth muscle cells (SMCs) and the extracellular matrix (ECM) are intimately associated in the aortic wall. Fbln4(SMKO) mice with an SMC-specific deletion of the Fbln4 gene, which encodes the vascular ECM component fibulin-4, develop ascending aortic aneurysms that have increased abundance of angiotensin-converting enzyme (ACE); inhibiting angiotensin II signaling within the first month of life prevents aneurysm development. We used comparative proteomics analysis of Fbln4(SMKO) aortas from postnatal day (P) 1 to P30 mice to identify key molecules involved in aneurysm initiation and expansion. At P14, the actin depolymerizing factor cofilin was dephosphorylated and thus activated, and at P7, the abundance of slingshot-1 (SSH1) phosphatase, an activator of cofilin, was increased, leading to actin cytoskeletal remodeling. Also, by P7, biomechanical changes and underdeveloped elastic lamina-SMC connections were evident, and the abundance of early growth response 1 (Egr1), a mechanosensitive transcription factor that stimulates ACE expression, was increased, which was before the increases in ACE abundance and cofilin activation. Postnatal deletion of Fbln4 in SMCs at P7 prevented cofilin activation and aneurysm formation, suggesting that these processes required disruption of elastic lamina-SMC connections. Phosphoinositide 3-kinase (PI3K) is involved in the angiotensin II-mediated activation of SSH1, and administration of PI3K inhibitors from P7 to P30 decreased SSH1 abundance and prevented aneurysms. These results suggest that aneurysm formation arises from abnormal mechanosensing of SMCs resulting from the loss of elastic lamina-SMC connections and from increased SSH1 and cofilin activity, which may be potential therapeutic targets for treating ascending aortic aneurysms.

Competition and collaboration between different actin assembly pathways allows for homeostatic control of the actin cytoskeleton

Tremendous insight into actin-associated proteins has come from careful biochemical and cell biological characterization of their activities and regulation. However, many studies of their cellular behavior have only considered each in isolation. Recent efforts reveal that assembly factors compete for polymerization-competent actin monomers, suggesting that actin is homeostatically regulated. It seems that a major regulatory component is competition between Arp2/3-activating nucleation promoting factors and profilin for actin monomers. The result is differential delivery of actin to different pathways, allowing for simultaneous assembly of competing F-actin structures and collaborative building of higher order cellular structures. Although there are likely to be additional factors that regulate actin homeostasis, especially in a cell type-dependent fashion, we advance the notion that competition between actin assembly factors results in a tunable system that can be adjusted according to extracellular and intracellular cues.

Reorganization of actin filaments by ADF/cofilin is involved in formation of microtubule structures during Xenopus oocyte maturation

We examined the reorganization of actin filaments and microtubules during Xenopus oocyte maturation. Surrounding the germinal vesicle (GV) in immature oocytes, the cytoplasmic actin filaments reorganized to accumulate beneath the vegetal side of the GV, where the microtubule-organizing center and transient microtubule array (MTOC-TMA) assembled, just before GV breakdown (GVBD). Immediately after GVBD, both Xenopus ADF/cofilin (XAC) and its phosphatase Slingshot (XSSH) accumulated into the nuclei and intranuclear actin filaments disassembled from the vegetal side with the shrinkage of the GV. As the MTOC-TMA developed well, cytoplasmic actin filaments were retained at the MTOC-TMA base region. Suppression of XAC dephosphorylation by anti-XSSH antibody injection inhibited both actin filament reorganization and proper formation and localization of both the MTOC-TMA and meiotic spindles. Stabilization of actin filaments by phalloidin also inhibited formation of the MTOC-TMA and disassembly of intranuclear actin filaments without affecting nuclear shrinkage. Nocodazole also caused the MTOC-TMA and the cytoplasmic actin filaments at its base region to disappear, which further impeded disassembly of intranuclear actin filaments from the vegetal side. XAC appears to reorganize cytoplasmic actin filaments required for precise assembly of the MTOC and, together with the MTOC-TMA, regulate the intranuclear actin filament disassembly essential for meiotic spindle formation.

Cofilin phosphorylation is elevated after F-actin disassembly induced by Rac1 depletion

Cytoskeletal reorganization is essential to keratinocyte function. Rac1 regulates cytoskeletal reorganization through signaling pathways such as the cofilin cascade. Cofilin severs actin filaments after activation by dephosphorylation. Rac1 was knocked out in mouse keratinocytes and it was found that actin filaments disassembled. In the epidermis of mice in which Rac1 was knocked out only in keratinocytes, cofilin phosphorylation was aberrantly elevated, corresponding to repression of the phosphatase slingshot1 (SSH1). These effects were independent of the signaling pathways for p21-activated kinase/LIM kinase (Pak/LIMK), protein kinase C, or protein kinase D or generation of reactive oxygen species. Similarly, when actin polymerization was specifically inhibited or Rac1 was knocked down, cofilin phosphorylation was enhanced and SSH1 was repressed. Repression of SSH1 partially blocked actin depolymerization induced by Rac1 depletion. Therefore, aberrant cofilin phosphorylation that induces actin polymerization might be a consequence of actin disassembly induced by the absence of Rac1.

ADF/cofilin: a crucial regulator of synapse physiology and behavior

Actin filaments (F-actin) are the major structural component of excitatory synapses, being present in presynaptic terminals and in postsynaptic dendritic spines. In the last decade, it has been appreciated that actin dynamics, the assembly and disassembly of F-actin, is crucial not only for the structure of excitatory synapses, but also for pre- and postsynaptic physiology. Hence, regulators of actin dynamics take a central role in mediating neurotransmitter release, synaptic plasticity, and ultimately behavior. Actin depolymerizing proteins of the ADF/cofilin family are essential regulators of actin dynamics, and a number of recent studies highlighted their crucial functions in excitatory synapses. In dendritic spines, ADF/cofilin activity is required for spine enlargement during initial long-term potentiation (LTP), but needs to be switched off during spine stabilization and LTP consolidation. Conversely, active ADF/cofilin is needed for spine pruning during long-term depression (LTD). Moreover, ADF/cofilin controls activity-induced synaptic availability of glutamate receptors, and exocytosis of synaptic vesicles. These data show that the activity of ADF/cofilin in synapses needs to be spatially and temporally tightly controlled through several upstream regulatory pathways, which have been identified recently. Hence, ADF/cofilin-controlled actin dynamics emerged as a critical and central regulator of synapse physiology. In this review, I will summarize and discuss our current knowledge on the roles of ADF/cofilin in synapse physiology and behavior, by focusing on excitatory synapses of the mammalian central nervous system.

Chronophin coordinates cell leading edge dynamics by controlling active cofilin levels

Cofilin, a critical player of actin dynamics, is spatially and temporally regulated to control the direction and force of membrane extension required for cell locomotion. In carcinoma cells, although the signaling pathways regulating cofilin activity to control cell direction have been established, the molecular machinery required to generate the force of the protrusion remains unclear. We show that the cofilin phosphatase chronophin (CIN) spatiotemporally regulates cofilin activity at the cell edge to generate persistent membrane extension. We show that CIN translocates to the leading edge in a PI3-kinase-, Rac1-, and cofilin-dependent manner after EGF stimulation to activate cofilin, promotes actin free barbed end formation, accelerates actin turnover, and enhances membrane protrusion. In addition, we establish that CIN is crucial for the balance of protrusion/retraction events during cell migration. Thus, CIN coordinates the leading edge dynamics by controlling active cofilin levels to promote MTLn3 cell protrusion.

Non-muscle tropomyosin (Tpm3) is crucial for asymmetric cell division and maintenance of cortical integrity in mouse oocytes

Tropomyosins are actin-binding cytoskeletal proteins that play a pivotal role in regulating the function of actin filaments in muscle and non-muscle cells; however, the roles of non-muscle tropomyosins in mouse oocytes are unknown. This study investigated the expression and functions of non-muscle tropomyosin (Tpm3) during meiotic maturation of mouse oocytes. Tpm3 mRNA was detected at all developmental stages in mouse oocytes. Tpm3 protein was localized at the cortex during the germinal vesicle and germinal vesicle breakdown stages. However, the overall fluorescence intensity of Tpm3 immunostaining was markedly decreased in metaphase II oocytes. Knockdown of Tpm3 impaired asymmetric division of oocytes and spindle migration, considerably reduced the amount of cortical actin, and caused membrane blebbing during cytokinesis. Expression of a constitutively active cofilin mutant and Tpm3 overexpression confirmed that Tpm3 protects cortical actin from depolymerization by cofilin. The data indicate that Tpm3 plays crucial roles in maintaining cortical actin integrity and asymmetric cell division during oocyte maturation, and that dynamic regulation of cortical actin by Tpm3 is critical to ensure proper polar body protrusion.

Actin-Modulating Protein Cofilin Is Involved in the Formation of Measles Virus Ribonucleoprotein Complex at the Perinuclear Region

In measles virus (MV)-infected cells, the ribonucleoprotein (RNP) complex, comprised of the viral genome and the nucleocapsid (N) protein, phosphoprotein (P protein), and large protein, assembles at the perinuclear region and synthesizes viral RNAs. The cellular proteins involved in the formation of the RNP complex are largely unknown. In this report, we show that cofilin, an actin-modulating host protein, interacts with the MV N protein and aids in the formation of the RNP complex. Knockdown of cofilin using the short hairpin RNA reduces the formation of the RNP complex after MV infection and that of the RNP complex-like structure after plasmid-mediated expression of MV N and P proteins. A lower level of formation of the RNP complex results in the reduction of viral RNA synthesis. Cofilin phosphorylation on the serine residue at position 3, an enzymatically inactive form, is increased after MV infection and the phosphorylated form of cofilin is preferentially included in the complex. These results indicate that cofilin plays an important role in MV replication by increasing formation of the RNP complex and viral RNA synthesis.

IMPORTANCE

Many RNA viruses induce within infected cells the structure called the ribonucleoprotein (RNP) complex in which viral RNA synthesis occurs. It is comprised of the viral genome and proteins that include the viral RNA polymerase. The cellular proteins involved in the formation of the RNP complex are largely unknown. In this report, we show that cofilin, an actin-modulating host protein, binds to the measles virus (MV) nucleocapsid protein and plays an important role in the formation of the MV RNP complex and MV RNA synthesis. The level of the phosphorylated form of cofilin, enzymatically inactive, is increased after MV infection, and the phosphorylated form is preferentially associated with the RNP complex. Our findings determined with cofilin will help us better understand the mechanism by which the RNP complex is formed in virus-infected cells and develop new antiviral drugs targeting the RNP complex.

Transcriptional profiling of GBM invasion genes identifies effective inhibitors of the LIM kinase-Cofilin pathway

Malignant gliomas are highly proliferative and invasive neoplasms where total surgical resection is often impossible and effective local radiation therapy difficult. Consequently, there is a need to develop a greater understanding of the molecular events driving invasion and to identify novel treatment targets. Using microarray analysis comparing normal brain samples and mesenchymal glioblastoma multiforme (GBM), we identified over 140 significant genes involved in cell migration and invasion. The cofilin (CFL) pathway, which disassembles actin filaments, was highly up-regulated compared to normal brain. Up-regulation of LIM domain kinase 1 and 2 (LIMK1/2), that phosphorylates and inactivates cofilin, was confirmed in an additional independent data set comparing normal brain to GBM. We identified and utilized two small molecule inhibitors BMS-5 and Cucurbitacin I directed against the cofilin regulating kinases, LIMK1 and LIMK2, to target this pathway. Significant decreases in cell viability were observed in glioma cells treated with BMS-5 and Cucurbitacin I, while no cytotoxic effects were seen in normal astrocytes that lack LIMK. BMS-5 and Cucurbitacin I promoted increased adhesion in GBM cells, and decreased migration and invasion. Collectively, these data suggest that use of LIMK inhibitors may provide a novel way to target the invasive machinery in GBM.

PKA-dependent phosphorylation of LIMK1 and Cofilin is essential for mouse sperm acrosomal exocytosis

Mammalian sperm must acquire their fertilizing ability after a series of biochemical modifications in the female reproductive tract collectively called capacitation to undergo acrosomal exocytosis, a process that is essential for fertilization. Actin dynamics play a central role in controlling the process of exocytosis in somatic cells as well as in sperm from several mammalian species. In somatic cells, small GTPases of the Rho family are widely known as master regulators of actin dynamics. However, the role of these proteins in sperm has not been studied in detail. In the present work we characterized the participation of small GTPases of the Rho family in the signaling pathway that leads to actin polymerization during mouse sperm capacitation. We observed that most of the proteins of this signaling cascade and their effector proteins are expressed in mouse sperm. The activation of the signaling pathways of cAMP/PKA, RhoA/C and Rac1 is essential for LIMK1 activation by phosphorylation on Threonine 508. Serine 3 of Cofilin is phosphorylated by LIMK1 during capacitation in a transiently manner. Inhibition of LIMK1 by specific inhibitors (BMS-3) resulted in lower levels of actin polymerization during capacitation and a dramatic decrease in the percentage of sperm that undergo acrosomal exocytosis. Thus, we demonstrated for the first time that the master regulators of actin dynamics in somatic cells are present and active in mouse sperm. Combining the results of our present study with other results from the literature, we have proposed a working model regarding how LIMK1 and Cofilin control acrosomal exocytosis in mouse sperm.

Inhibiting actin depolymerization enhances osteoblast differentiation and bone formation in human stromal stem cells

Remodeling of the actin cytoskeleton through actin dynamics is involved in a number of biological processes, but its role in human stromal (skeletal) stem cells (hMSCs) differentiation is poorly understood. In the present study, we demonstrated that stabilizing actin filaments by inhibiting gene expression of the two main actin depolymerizing factors (ADFs): Cofilin 1 (CFL1) and Destrin (DSTN) in hMSCs, enhanced cell viability and differentiation into osteoblastic cells (OB) in vitro, as well as heterotopic bone formation in vivo. Similarly, treating hMSC with Phalloidin, which is known to stabilize polymerized actin filaments, increased hMSCs viability and OB differentiation. Conversely, Cytocholasin D, an inhibitor of actin polymerization, reduced cell viability and inhibited OB differentiation of hMSC. At a molecular level, preventing Cofilin phosphorylation through inhibition of LIM domain kinase 1 (LIMK1) decreased cell viability and impaired OB differentiation of hMSCs. Moreover, depolymerizing actin reduced FAK, p38 and JNK activation during OB differentiation of hMSCs, while polymerizing actin enhanced these signaling pathways. Our results demonstrate that the actin dynamic reassembly and Cofilin phosphorylation loop is involved in the control of hMSC proliferation and osteoblasts differentiation.

Drosophila Imp iCLIP identifies an RNA assemblage coordinating F-actin formation

Background

Post-transcriptional RNA regulons ensure coordinated expression of monocistronic mRNAs encoding functionally related proteins. In this study, we employ a combination of RIP-seq and short- and long-wave individual-nucleotide resolution crosslinking and immunoprecipitation (iCLIP) technologies in Drosophila cells to identify transcripts associated with cytoplasmic ribonucleoproteins (RNPs) containing the RNA-binding protein Imp.

Results

We find extensive binding of Imp to 3′ UTRs of transcripts that are involved in F-actin formation. A common denominator of the RNA–protein interface is the presence of multiple motifs with a central UA-rich element flanked by CA-rich elements. Experiments in single cells and intact flies reveal compromised actin cytoskeletal dynamics associated with low Imp levels. The former shows reduced F-actin formation and the latter exhibits abnormal neuronal patterning. This demonstrates a physiological significance of the defined RNA regulon.

Conclusions

Our data imply that Drosophila Imp RNPs may function as cytoplasmic mRNA assemblages that encode proteins which participate in actin cytoskeletal remodeling. Thus, they may facilitate coordinated protein expression in sub-cytoplasmic locations such as growth cones.

Electronic supplementary material

The online version of this article (doi:10.1186/s13059-015-0687-0) contains supplementary material, which is available to authorized users.

Genome-wide RNAi screen for nuclear actin reveals a network of cofilin regulators

Nuclear actin plays an important role in many processes that regulate gene expression. Cytoplasmic actin dynamics are tightly controlled by numerous actin-binding proteins, but regulation of nuclear actin has remained unclear. Here, we performed a genome-wide RNA interference (RNAi) screen in Drosophila cells to identify proteins that influence either nuclear polymerization or import of actin. We validate 19 factors as specific hits, and show that Chinmo (known as Bach2 in mammals), SNF4Aγ (Prkag1 in mammals) and Rab18 play a role in nuclear localization of actin in both fly and mammalian cells. We identify several new regulators of cofilin activity, and characterize modulators of both cofilin kinases and phosphatase. For example, Chinmo/Bach2, which regulates nuclear actin levels also in vivo, maintains active cofilin by repressing the expression of the kinase Cdi (Tesk in mammals). Finally, we show that Nup98 and lamin are candidates for regulating nuclear actin polymerization. Our screen therefore reveals new aspects of actin regulation and links nuclear actin to many cellular processes.

Skeletal muscle microRNA and messenger RNA profiling in cofilin-2 deficient mice reveals cell cycle dysregulation hindering muscle regeneration

Congenital myopathies are rare skeletal muscle diseases presenting in early age with hypotonia and weakness often linked to a genetic defect. Mutations in the gene for cofilin-2 (CFL2) have been identified in several families as a cause of congenital myopathy with nemaline bodies and cores. Here we explore the global messenger and microRNA expression patterns in quadriceps muscle samples from cofillin-2-null mice and compare them with sibling-matched wild-type mice to determine the molecular pathways and mechanisms involved. Cell cycle processes are markedly dysregulated, with altered expression of genes involved in mitotic spindle formation, and evidence of loss of cell cycle checkpoint regulation. Importantly, alterations in cell cycle, apoptosis and proliferation pathways are present in both mRNA and miRNA expression patterns. Specifically, p21 transcript levels were increased, and the expression of p21 targets, such as cyclin D and cyclin E, was decreased. We therefore hypothesize that deficiency of cofilin-2 is associated with interruption of the cell cycle at several checkpoints, hindering muscle regeneration. Identification of these pathways is an important step towards developing appropriate therapies against various congenital myopathies.

Cofilin-2 phosphorylation and sequestration in myocardial aggregates: novel pathogenetic mechanisms for idiopathic dilated cardiomyopathy

BACKGROUND

Recently, tangles and plaque-like aggregates have been identified in certain cases of dilated cardiomyopathy (DCM), traditionally labeled idiopathic (iDCM), where there is no specific diagnostic test or targeted therapy. This suggests a potential underlying cause for some of the iDCM cases. [Corrected]

OBJECTIVES

This study sought to identify the make-up of myocardial aggregates to understand the molecular mechanisms of these cases of DCM; this strategy has been central to understanding Alzheimer's disease.

METHODS

Aggregates were extracted from human iDCM samples with high congophilic reactivity (an indication of plaque presence), and the findings were validated in a larger cohort of samples. We tested the expression, distribution, and activity of cofilin in human tissue and generated a cardiac-specific knockout mouse model to investigate the functional impact of the human findings. We also modeled cofilin inactivity in vitro by using pharmacological and genetic gain- and loss-of-function approaches.

RESULTS

Aggregates in human myocardium were enriched for cofilin-2, an actin-depolymerizing protein known to participate in neurodegenerative diseases and nemaline myopathy. Cofilin-2 was predominantly phosphorylated, rendering it inactive. Cardiac-specific haploinsufficiency of cofilin-2 in mice recapitulated the human disease's morphological, functional, and structural phenotype. Pharmacological stimulation of cofilin-2 phosphorylation and genetic overexpression of the phosphomimetic protein promoted the accumulation of "stress-like" fibers and severely impaired cardiomyocyte contractility.

CONCLUSIONS

Our study provides the first biochemical characterization of prefibrillar myocardial aggregates in humans and the first report to link cofilin-2 to cardiomyopathy. The findings suggest a common pathogenetic mechanism connecting certain iDCMs and other chronic degenerative diseases, laying the groundwork for new therapeutic strategies.

WISp39 binds phosphorylated Coronin 1B to regulate Arp2/3 localization and Cofilin-dependent motility

We previously identified Waf1 Cip1 stabilizing protein 39 (WISp39) as a binding partner for heat shock protein 90 (Hsp90). We now report that WISp39 has an essential function in the control of directed cell migration, which requires WISp39 interaction with Hsp90. WISp39 knockdown (KD) resulted in the loss of directional motility of mammalian cells and profound changes in cell morphology, including the loss of a single leading edge. WISp39 binds Coronin 1B, known to regulate the Arp2/3 complex and Cofilin at the leading edge. WISp39 preferentially interacts with phosphorylated Coronin 1B, allowing it to complex with Slingshot phosphatase (SSH) to dephosphorylate and activate Cofilin. WISp39 also regulates Arp2/3 complex localization at the leading edge. WISp39 KD-induced morphological changes could be rescued by overexpression of Coronin 1B together with a constitutively active Cofilin mutant. We conclude that WISp39 associates with Hsp90, Coronin 1B, and SSH to regulate Cofilin activation and Arp2/3 complex localization at the leading edge.

A RhoA Signaling Pathway Regulates Dendritic Golgi Outpost Formation

The neuronal Golgi apparatus (GA) localizes to the perinuclear region and dendrites as tubulo-vesicular structures designated Golgi outposts (GOPs). Current evidence suggests that GOPs shape dendrite morphology and serve as platforms for the local delivery of synaptic receptors. However, the mechanisms underlying GOP formation remain a mystery. Using live-cell imaging and confocal microscopy in cultured hippocampal neurons, we now show that GOPs destined to major "apical" dendrites are generated from the somatic GA by a sequence of events involving: (1) generation of a GA-derived tubule; (2) tubule elongation and deployment into the dendrite; (3) tubule fission; and (4) transport and condensation of the fissioned tubule. A RhoA-Rock signaling pathway involving LIMK1, PKD1, slingshot, cofilin, and dynamin regulates polarized GOP formation by controlling the tubule fission. Our observations identify a mechanism underlying polarized GOP biogenesis and provide new insights regarding involvement of RhoA in dendritic development and polarization.

The diaphanous gene of Drosophila interacts antagonistically with multiple wing hairs and plays a key role in wing hair morphogenesis

The Drosophila wing is covered by an array of distally pointing hairs that has served as a key model system for studying planar cell polarity (PCP). The adult cuticular hairs are formed in the pupae from cell extensions that contain extensive actin filaments and microtubules. The importance of the actin cytoskeleton for hair growth and morphogenesis is clear from the wide range of phenotypes seen in mutations in well-known actin regulators. Formin proteins promote the formation of long actin filaments of the sort thought to be important for hair growth. We report here that the formin encoding diaphanous (dia) gene plays a key role in hair morphogenesis. Both loss of function mutations and the expression of a constitutively active Dia led to cells forming both morphologically abnormal hairs and multiple hairs. The conserved frizzled (fz)/starry night (stan) PCP pathway functions to restrict hair initiation and activation of the cytoskeleton to the distal most part of wing cells. It also ensures the formation of a single hair per cell. Our data suggest that the localized inhibition of Dia activity may be part of this mechanism. We found the expression of constitutively active Dia greatly expands the region for activation of the cytoskeleton and that dia functions antagonistically with multiple wing hairs (mwh), the most downstream member of the fz/stan pathway. Further we established that purified fragments of Dia and Mwh could be co-immunoprecipitated suggesting the genetic interaction could reflect a direct physical interaction.

Rho-kinase regulates human platelet activation induced by thromboxane A2 independently of p38 MAP kinase

We have previously demonstrated that ristocetin, an activator of GPIb/IX/V, induces the release of soluble CD40 ligand (sCD40L) via thromboxane A2 production in human platelets. It has been shown that thromboxane A2 induces the activation of Rho-kinase, a downstream effector of Rho, in human platelets. In the present study, we investigated the exact roles of Rho-kinase in thromboxane A2-induced platelet activation. We found that U46619, a thromboxane receptor (TP) agonist, induced the phosphorylation of cofilin, a target of Rho-kinase signaling, and that the cofilin phosphorylation by U46619 was suppressed by Y27632 or fasudil, specific inhibitors of Rho-kinase. Y27632 and fasudil markedly decreased large platelet aggregate formation by U46619. The release of sCD40L and secretion of platelet-derived growth factor (PDGF)-AB stimulated by U46619 were inhibited by Y27632 and fasudil. SB203580, a specific inhibitor of p38 mitogen-activated protein (MAP) kinase, reduced the sCD40L release and PDGF-AB secretion. Y27632 and fasudil failed to affect the phosphorylation of p38 MAP kinase whereas SB203580 had little effect on the phosphorylation of cofilin induced by U46619. In conclusion, our results strongly suggest that Rho-kinase regulates thromboxane A2-induced human platelet activation independently of p38 MAP kinase.

Slingshot-Cofilin activation mediates mitochondrial and synaptic dysfunction via Aβ ligation to β1-integrin conformers

The accumulation of amyloid-β protein (Aβ) is an early event associated with synaptic and mitochondrial damage in Alzheimer's disease (AD). Recent studies have implicated the filamentous actin (F-actin) severing protein, Cofilin, in synaptic remodeling, mitochondrial dysfunction, and AD pathogenesis. However, whether Cofilin is an essential component of the AD pathogenic process and how Aβ impinges its signals to Cofilin from the neuronal surface are unknown. In this study, we found that Aβ42 oligomers (Aβ42_O, amyloid-β protein 1–42 oligomers) bind with high affinity to low or intermediate activation conformers of β1-integrin, resulting in the loss of surface β1-integrin and activation of Cofilin via Slingshot homology-1 (SSH1) activation. Specifically, conditional loss of β1-integrin prevented Aβ42_O-induced Cofilin activation, and allosteric modulation or activation of β1-integrin significantly reduced Aβ42_O binding to neurons while blocking Aβ42_O-induced reactive oxygen species (ROS) production, mitochondrial dysfunction, depletion of F-actin/focal Vinculin, and apoptosis. Cofilin, in turn, was required for Aβ42_O-induced loss of cell surface β1-integrin, disruption of F-actin/focal Talin–Vinculin, and depletion of F-actin-associated postsynaptic proteins. SSH1 reduction, which mitigated Cofilin activation, prevented Aβ42_O-induced mitochondrial Cofilin translocation and apoptosis, while AD brain mitochondria contained significantly increased activated/oxidized Cofilin. In mechanistic support in vivo, AD mouse model (APP (amyloid precursor protein)/PS1) brains contained increased SSH1/Cofilin and decreased SSH1/14-3-3 complexes, indicative of SSH1–Cofilin activation via release of SSH1 from 14-3-3. Finally, genetic reduction in Cofilin rescued APP/Aβ-induced synaptic protein loss and gliosis in vivo as well as deficits in long-term potentiation (LTP) and contextual memory in APP/PS1 mice. These novel findings therefore implicate the essential involvement of the β1-integrin–SSH1–Cofilin pathway in mitochondrial and synaptic dysfunction in AD.

Toll-like receptor ligands sensitize B-cell receptor signalling by reducing actin-dependent spatial confinement of the receptor

Integrating signals from multiple receptors allows cells to interpret the physiological context in which a signal is received. Here we describe a mechanism for receptor crosstalk in which receptor-induced increases in actin dynamics lower the threshold for signalling by another receptor. We show that the Toll-like receptor ligands lipopolysaccharide and CpG DNA, which are conserved microbial molecules, enhance signalling by the B-cell antigen receptor (BCR) by activating the actin-severing protein cofilin. Single-particle tracking reveals that increased severing of actin filaments reduces the spatial confinement of the BCR within the plasma membrane and increases BCR mobility. This allows more frequent collisions between BCRs and greater signalling in response to low densities of membrane-bound antigen. These findings implicate actin dynamics as a means of tuning receptor signalling and as a mechanism by which B cells distinguish inert antigens from those that are accompanied by indicators of microbial infection.

Microbial pathogens can activate both innate and adaptive receptors, and integration of these signals may enhance the sensitivity of the immune response. Freeman et al. show that innate microbial cues sensitize B cells to antigen by increasing actin dynamics and reducing the actin-dependent confinement of the B-cell receptor.

GPCR-mediated PLCβγ/PKCβ/PKD signaling pathway regulates the cofilin phosphatase slingshot 2 in neutrophil chemotaxis

Chemotaxis requires precisely coordinated polymerization and depolymerization of the actin cytoskeleton at leading fronts of migrating cells. However, GPCR activation-controlled F-actin depolymerization remains largely elusive. Here, we reveal a novel signaling pathway, including Gαi, PLC, PKCβ, protein kinase D (PKD), and SSH2, in control of cofilin phosphorylation and actin cytoskeletal reorganization, which is essential for neutrophil chemotaxis. We show that PKD is essential for neutrophil chemotaxis and that GPCR-mediated PKD activation depends on PLC/PKC signaling. More importantly, we discover that GPCR activation recruits/activates PLCγ2 in a PI3K-dependent manner. We further verify that PKCβ specifically interacts with PKD1 and is required for chemotaxis. Finally, we identify slingshot 2 (SSH2), a phosphatase of cofilin (actin depolymerization factor), as a target of PKD1 that regulates cofilin phosphorylation and remodeling of the actin cytoskeleton during neutrophil chemotaxis.

EZH2 regulates cofilin activity and colon cancer cell migration by targeting ITGA2 gene

Reorganization of cytoskeleton via actin remodeling is a basic step of cell locomotion. Although cell migration of normal and cancer cells can be stimulated by a variety of intra- and extra-cellular factors, all paths ultimate on the regulation of cofilin activity. Cofilin is a small actin-binding protein able to bind both forms of actin, globular and filament, and is regulated by phosphorylation at Serine 3. Following phosphorylation at serine 3 cofilin is inactive, therefore cannot bind actin molecules and cytoskeleton remodeling is impaired. The histone methyltransferase EZH2 is frequently over expressed in many tumour types including colorectal cancer (CRC). EZH2 over activity, which results in epigenetic gene-silencing, has been associated with many tumour properties including invasion, angiogenesis and metastasis but little is known about the underneath molecular mechanisms. Herein, we report that EZH2 is able to control cofilin activity and consequently cell locomotion of CRC cell lines through a non-conventional novel axis that involves integrin signaling. Indeed, we show how genetic and pharmacological inhibition (DZNep and GSK343) of EZH2 function produces hyper phosphorylation of cofilin and reduces cell migration. We previously demonstrated by chromatin immuno-precipitation that Integrin alpha 2 (ITGα2) expression is regulated by EZH2. In the present study we provide evidence that in EZH2-silenced cells the signaling activity of the de-repressed ITGα2 is able to increase cofilin phosphorylation, which in turn reduces cell migration. This study also proposes novel mechanisms that might provide new anti-metastatic strategies for CRC treatment based on the inhibition of the epigenetic factor EZH2 and/or its target gene.

The NAV2 homolog Sickie regulates F-actin-mediated axonal growth in Drosophila mushroom body neurons via the non-canonical Rac-Cofilin pathway

The Rac-Cofilin pathway is essential for cytoskeletal remodeling to control axonal development. Rac signals through the canonical Rac-Pak-LIMK pathway to suppress Cofilin-dependent axonal growth and through a Pak-independent non-canonical pathway to promote outgrowth. Whether this non-canonical pathway converges to promote Cofilin-dependent F-actin reorganization in axonal growth remains elusive. We demonstrate that Sickie, a homolog of the human microtubule-associated protein neuron navigator 2, cell-autonomously regulates axonal growth of Drosophila mushroom body (MB) neurons via the non-canonical pathway. Sickie was prominently expressed in the newborn F-actin-rich axons of MB neurons. A sickie mutant exhibited axonal growth defects, and its phenotypes were rescued by exogenous expression of Sickie. We observed phenotypic similarities and genetic interactions among sickie and Rac-Cofilin signaling components. Using the MARCM technique, distinct F-actin and phospho-Cofilin patterns were detected in developing axons mutant for sickie and Rac-Cofilin signaling regulators. The upregulation of Cofilin function alleviated the axonal defect of the sickie mutant. Epistasis analyses revealed that Sickie suppresses the LIMK overexpression phenotype and is required for Pak-independent Rac1 and Slingshot phosphatase to counteract LIMK. We propose that Sickie regulates F-actin-mediated axonal growth via the non-canonical Rac-Cofilin pathway in a Slingshot-dependent manner.

Cofilin Phosphorylation Decreased by Serum-free Starvation with Low Glucose in the L6 Myoblasts

[Purpose] Many studies have been using cell culture models of muscle cells with exogenous cytokines or glucocorticoids to mimic atrophy in in vivo and in vitro tests. However, the changes in the phosphorylation of atrophy-related cofilin are still poorly understood in starved skeletal muscle cells. In this study, we first examined whether or not phosphorylation of cofilin is altered in L6 myoblasts after 3, 6, 12, 24, 48, and 72 hours of serum-free starvation with low glucose. [Methods] We used Western blotting to exam protein expression and phosphorylation in atrophied L6 myoblasts. [Results] L6 cell sizes and numbers were diminished as a result of serum-free starvation in a time-dependent manner. Serum-free starvation for 3, 6, 12, 24, 48, and 72 hours significantly decreased the phosphorylation of cofilin, respectively. [Conclusion] These results suggest that starvation-induced atrophy may be in part related to changes in the phosphorylation of cofilin in L6 myoblasts.