Reactivation of phosphorylated actin depolymerizing factor and identification of the regulatory site

Ca(2+) influx through P2X receptors induces actin cytoskeleton reorganization by the formation of cofilin rods in neurites

In physiological and pathological events, extracellular ATP plays an important role by controlling several types of purinergic receptors and changing cytoskeleton dynamics. To know the process of ATP-dependent cytoskeleton remodeling, we focused on cofilin, a key regulator of actin cytoskeleton, and investigated the dynamics of cofilin in PC12 cells through fluorescent protein-labeled cofilin and actin, Ca(2+) imaging, and fluorescence resonance energy transfer (FRET) techniques. As a result, ATP induced intracellular Ca(2+) increase, following cofilin rods' formation. ATP-induced cofilin rods' formation was not observed in cells expressing unphosphorylatable variant of cofilin. A P2X receptor agonist, but not P2Y, induced the formation of cofilin rods, whereas calmodulin and calcineurin inhibitors suppressed it. These results indicate that Ca(2+) influx through P2X receptors induces the formation of cofilin rods via calcineurin-dependent dephosphorylation of cofilin. This pathway might be one candidate to explain the effects of ATP on neuronal development and injury.

Activity of cofilin can be regulated by a mechanism other than phosphorylation/dephosphorylation in muscle cells in culture

Cofilin plays a critical role in actin filament dynamics in a variety of eukaryotic cells. Its activity is regulated by phosphorylation/dephosphorylation of a Ser3 residue on the N-terminal side and/or its binding to a phosphoinositide, PIP(2). To clarify how cofilin activity is regulated in muscle cells, we generated analogues of the unphosphorylated form (A3-cofilin) and phosphorylated form (D3-cofilin) by converting the phosphorylation site (Ser3) of cofilin to Ala and Asp, respectively. These mutated proteins, as well as the cofilin having Ser3 residue (S3-cofilin), were produced in an E. coli expression system and conjugated with fluorescent dyes. In an in vitro functional assay, A3-cofilin retained the ability to bind to F-actin. Upon injection into cultured muscle cells, A3-cofilin and S3-cofilin promptly disrupted actin filaments in the cytoplasm, and many cytoplasmic rods containing both the exogenous cofilin and actin were generated, while D3-cofilin was simply diffused in the cytoplasm without affecting actin filaments. Several hours after the injection, however, the activity of A3-cofilin and S3-cofilin was suppressed: the actin-A3-cofilin (or S3-cofilin) rods disappeared, the cofilin diffused in the cytoplasm like D3-cofilin, and actin filaments reformed. Both GFP-fused A3-cofilin and S3-cofilin that were produced by cDNA transfection were also suppressed in the cytoplasm of muscle cells in culture. Thus, some mechanism(s) other than phosphorylation can suppress A3-cofilin activity. We observed that PIP(2) can bind to A3-cofilin just as to S3-cofilin and inhibits the interaction of A3-cofilin with actin. Our results suggest that the activity of A3-cofilin and also S3-cofilin can be regulated by PIP(2) in the cytoplasm of muscle cells.

Agrin induced morphological and structural changes in growth cones of cultured hippocampal neurons

The role of agrin in synaptogenesis has been extensively studied. On the other hand, little is known about the function of this extracellular matrix protein during developmental processes that precede the formation of synapses. Recently, agrin was shown to regulate the rate of neurite elongation and the behavior of growth cones in hippocampal and spinal neurons, respectively. However, the molecular mechanisms underlying these effects have not been completely elucidated. In the present study, we analyzed the morphological and molecular changes induced by agrin in growth cones of hippocampal neurons that developed in culture. Morphometric analysis showed a significant enlargement of growth cones of hippocampal neurons cultured in the presence of agrin. These agrin-induced growth cone changes were accompanied by the formation of loops of microtubules highly enriched in acetylated tubulin and an increase in the content of the microtubule-associated protein (MAP)1B. Together, these data provide further insights into the potential molecular mechanisms underlying the effects of agrin on neurite outgrowth in rat central neurons.

Barley MLO modulates actin-dependent and actin-independent antifungal defense pathways at the cell periphery

Cell polarization is a crucial process during plant development, as well as in plant-microbe interactions, and is frequently associated with extensive cytoskeletal rearrangements. In interactions of plants with inappropriate fungal pathogens (so-called non-host interactions), the actin cytoskeleton is thought to contribute to the establishment of effective barriers at the cell periphery against fungal ingress. Here, we impeded actin cytoskeleton function in various types of disease resistance using pharmacological inhibitors and genetic interference via ectopic expression of an actin-depolymerizing factor-encoding gene, ADF. We demonstrate that barley (Hordeum vulgare) epidermal cells require actin cytoskeleton function for basal defense to the appropriate powdery mildew pathogen Blumeria graminis f. sp. hordei and for mlo-mediated resistance at the cell wall, but not for several tested race-specific immune responses. Analysis of non-host resistance to two tested inappropriate powdery mildews, Erysiphe pisi and B. graminis f. sp. tritici, revealed the existence of actin-dependent and actin-independent resistance pathways acting at the cell periphery. These pathways act synergistically and appear to be under negative control by the plasma membrane-resident MLO protein.

Mutations in the cofilin partner Aip1/Wdr1 cause autoinflammatory disease and macrothrombocytopenia

A pivotal mediator of actin dynamics is the protein cofilin, which promotes filament severing and depolymerization, facilitating the breakdown of existing filaments, and the enhancement of filament growth from newly created barbed ends. It does so in concert with actin interacting protein 1 (Aip1), which serves to accelerate cofilin's activity. While progress has been made in understanding its biochemical functions, the physiologic processes the cofilin/Aip1 complex regulates, particularly in higher organisms, are yet to be determined. We have generated an allelic series for WD40 repeat protein 1 (Wdr1), the mammalian homolog of Aip1, and report that reductions in Wdr1 function produce a dramatic phenotype gradient. While severe loss of function at the Wdr1 locus causes embryonic lethality, macrothrombocytopenia and autoinflammatory disease develop in mice carrying hypomorphic alleles. Macrothrombocytopenia is the result of megakaryocyte maturation defects, which lead to a failure of normal platelet shedding. Autoinflammatory disease, which is bone marrow-derived yet nonlymphoid in origin, is characterized by a massive infiltration of neutrophils into inflammatory lesions. Cytoskeletal responses are impaired in Wdr1 mutant neutrophils. These studies establish an essential requirement for Wdr1 in megakaryocytes and neutrophils, indicating that cofilin-mediated actin dynamics are critically important to the development and function of both cell types.

Coronin 1B coordinates Arp2/3 complex and cofilin activities at the leading edge

Actin filament formation and turnover within the treadmilling actin filament array at the leading edge of migrating cells are interdependent and coupled, but the mechanisms coordinating these two activities are not understood. We report that Coronin 1B interacts simultaneously with Arp2/3 complex and Slingshot (SSH1L) phosphatase, two regulators of actin filament formation and turnover, respectively. Coronin 1B inhibits filament nucleation by Arp2/3 complex and this inhibition is attenuated by phosphorylation of Coronin 1B at Serine 2, a site targeted by SSH1L. Coronin 1B also directs SSH1L to lamellipodia where SSH1L likely regulates Cofilin activity via dephosphorylation. Accordingly, depleting Coronin 1B increases phospho-Cofilin levels, and alters lamellipodial dynamics and actin filament architecture at the leading edge. We conclude that Coronin 1B's coordination of filament formation by Arp2/3 complex and filament turnover by Cofilin is required for effective lamellipodial protrusion and cell migration.

Actin deficiency induces cofilin phosphorylation: proteome analysis of HeLa cells after beta-actin gene silencing

Actin-binding proteins regulate the dynamic structure and function of actin filaments in the cell. Much is known about how manipulation of the actin-binding proteins affects the structure and function of actin filaments; however, little is known about how manipulation of actin in the cell affects actin-binding proteins. We addressed this question by utilizing two technologies: RNA interference and 2-dimensional gel electrophoresis. We knocked down beta-actin expression in HeLa cells using short interfering RNA and applied 2-DGE to examine alterations in the HeLa cell proteome. We revealed a 2-5 fold increases of four protein spots on 2-D gels and identified these proteins by mass spectrometry. Three of the four proteins were actin-binding proteins, including cofilin, which promotes both disassembly and assembly of actin filaments but becomes inactivated when phosphorylated. Further examination revealed that the cofilin total protein level barely increased, but the phosphorylated cofilin level increased dramatically in HeLa cells after beta-actin siRNA treatment. These results suggest that in response to siRNA-induced beta-actin deficiency HeLa cells inactivate cofilin by phosphorylation rather than down-regulate its protein expression level. This study also demonstrates that the combination of RNA interference and 2-dimensional gel electrophoresis technologies provides a valuable method to study protein interactions in a specific cellular pathway.

Mapping the interaction of cofilin with subdomain 2 on actin

Cofilin, a member of the actin-depolymerizing factor (ADF)/cofilin family of proteins, is a key regulator of actin dynamics. Cofilin binds to monomer (G-) and filamentous (F-) actin, severs the filaments, and increases their turnover rate. Electron microscopy studies suggested cofilin interactions with subdomains 2 and 1/3 on adjacent actin protomers in F-actin. To probe for the presence of a cryptic cofilin binding site in subdomain 2 in G-actin, we used transglutaminase-mediated cross-linking, which targets Gln41 in subdomain 2. The cross-linking proceeded with up to 85% efficiency with skeletal alpha-actin and WT yeast actin, yielding a single product corresponding to a 1:1 actin-cofilin complex but was strongly inhibited in Q41C yeast actin (in which Q41 was substituted with cysteine). LC-MS/MS analysis of the proteolytic fragments of this complex mapped the cross-linking to Gln41 on actin and Gly1 on recombinant yeast cofilin. The actin-cofilin (AC) heterodimer was purified on FPLC for analytical ultracentrifugation and electron microscopy analysis. Sedimentation equilibrium and velocity runs revealed oligomers of AC in G-actin buffer. In the presence of excess cofilin, the covalent AC heterodimer bound a second cofilin, forming a 2:1 cofilin/actin complex, as revealed by sedimentation results. Under polymerizing conditions the cross-linked AC formed mostly short filaments, which according to image reconstruction were similar to uncross-linked actin-cofilin filaments. Although a majority of the cross-linking occurs at Gln41, a small fraction of the AC cross-linked complex forms in the Q41C yeast actin mutant. This secondary cross-linking site was sequenced by MALDI-MS/MS as linking Gln360 in actin to Lys98 on cofilin. Overall, these results demonstrate that the region around Gln41 (subdomain 2) is involved in a weak binding of cofilin to G-actin.

Mechanism of depolymerization and severing of actin filaments and its significance in cytoskeletal dynamics

The actin cytoskeleton is one of the major structural components of the cell. It often undergoes rapid reorganization and plays crucial roles in a number of dynamic cellular processes, including cell migration, cytokinesis, membrane trafficking, and morphogenesis. Actin monomers are polymerized into filaments under physiological conditions, but spontaneous depolymerization is too slow to maintain the fast actin filament dynamics observed in vivo. Gelsolin, actin-depolymerizing factor (ADF)/cofilin, and several other actin-severing/depolymerizing proteins can enhance disassembly of actin filaments and promote reorganization of the actin cytoskeleton. This review presents advances as well as a historical overview of studies on the biochemical activities and cellular functions of actin-severing/depolymerizing proteins.

A role of LIM kinase 1/cofilin pathway in regulating endocytic trafficking of EGF receptor in human breast cancer cells

We have previously shown that overexpression of LIM kinase1 (LIMK1) resulted in a marked retardation of the internalization of the receptor-mediated endocytic tracer, Texas red-labeled epidermal growth factor (EGF) in low-invasive human breast cancer cell MCF-7. We thereby postulate that LIMK1 signaling plays an important role in the regulation of ligand-induced endocytosis of EGF receptor (EGFR) in tumor cells by reorganizing and influencing actin-filament dynamics. In the present study, we further assessed the effect of wild-type LIMK1, a kinase-deficient dominant negative mutant of LIMK1 (DN-LIMK1) and an active, unphosphorylatable cofilin mutant (S3A cofilin) on internalization of EGF-EGFR in MDA-MB-231, a highly invasive human breast cancer cell line. We demonstrate here that a marked delay in the receptor-mediated internalization of Texas red-labeled EGF was observed in the wild-type LIMK1 transfectants, and that most of the internalized EGF staining were accumulated within transferrin receptor-positive early endosomes even after 30 min internalization. In contrast, the expression of dominant-negative LIMK1 mutant rescued the efficient endocytosis of Texas red-EGF, and large amounts of Texas red-EGF staining already reached LIMPII-positive late endosomes/lysosomal vacuoles after 15 min internalization. We further analyzed the effect of S3A cofilin mutant on EGFR trafficking, and found an efficient delivery of Texas red-EGF into late endosomes/lysosomes at 15-30 min after internalization. Taken together, our novel findings presented in this paper implicate that LIMK1 signaling indeed plays a pivotal role in the regulation of EGFR trafficking through the endocytic pathway in invasive tumor cells.

Integrin beta4 regulates migratory behavior of keratinocytes by determining laminin-332 organization

Whether alpha6beta4 integrin regulates migration remains controversial. beta4 integrin-deficient (JEB) keratinocytes display aberrant migration in that they move in circles, a behavior that mirrors the circular arrays of laminin (LM)-332 in their matrix. In contrast, wild-type keratinocytes and JEB keratinocytes, induced to express beta4 integrin, assemble laminin-332 in linear tracks over which they migrate. Moreover, laminin-332-dependent migration of JEB keratinocytes along linear tracks is restored when cells are plated on wild-type keratinocyte matrix, whereas wild-type keratinocytes show rotation over circular arrays of laminn-332 in JEB keratinocyte matrix. The activities of Rac1 and the actin cytoskeleton-severing protein cofilin are low in JEB keratinocytes compared with wild-type cells but are rescued following expression of wild-type beta4 integrin in JEB cells. Additionally, in wild-type keratinocytes Rac1 is complexed with alpha6beta4 integrin. Moreover, Rac1 or cofilin inactivation induces wild-type keratinocytes to move in circles over rings of laminin-332 in their matrix. Together these data indicate that laminin-332 matrix organization is determined by the alpha6beta4 integrin/actin cytoskeleton via Rac1/cofilin signaling. Furthermore, our results imply that the organizational state of laminin-332 is a key determinant of the motility behavior of keratinocytes, an essential element of skin wound healing and the successful invasion of epidermal-derived tumor cells.

Formation of actin-ADF/cofilin rods transiently retards decline of mitochondrial potential and ATP in stressed neurons

When neurons in culture are transiently stressed by inhibition of ATP synthesis, they rapidly form within their neurites rodlike actin inclusions that disappear when the insult is removed. Oxidative stress, excitotoxic insults, and amyloid beta-peptide oligomers also induce rods. Immunostaining of neurites indicates that these rods also contain the majority of the actin filament dynamizing proteins, actin-depolymerizing factor (ADF) and cofilin (AC). If the rods reappear within 24 h after the stress is removed, the neurite degenerates distal to the rod but with no increase in neuronal death. Here, rods were generated in cultured rat E18 hippocampal cells by overexpression of a green fluorescent protein chimera of AC. Surprisingly, we have found that, for a short period (approximately 60 min) immediately after initial rod formation, the loss of mitochondrial membrane potential (Delta Psi(m)) and ATP in neurites with rods is slower than in neurites without them. The Delta Psi(m) was monitored with the fluorescent dye tetramethylrhodamine methyl ester, and ATP was monitored with the fluorescent ion indicator mag-fura 2. Actin in rods is less dynamic than is filamentous actin in other cytoskeletal structures. Because Delta Psi(m) depends on cellular ATP and because ATP hydrolysis associated with actin filament turnover is responsible for a large fraction of neuronal energy consumption (approximately 50%), the formation of rods transiently protects neurites by slowing filament turnover and its associated ATP hydrolysis.

Involvement of slingshot in the Rho-mediated dephosphorylation of ADF/cofilin during Xenopus cleavage

ADF/cofilin is a key regulator for actin dynamics during cytokinesis. Its activity is suppressed by phosphorylation and reactivated by dephosphorylation. Little is known, however, about regulatory mechanisms of ADF/cofilin function during formation of contractile ring actin filaments. Using Xenopus cycling extracts, we found that ADF/cofilin was dephosphorylated at prophase and telophase. In addition, constitutively active Rho GTPase induced dephosphorylation of ADF/cofilin in the egg extracts. This dephosphorylation was inhibited by Na(3)VO (4) but not by other conventional phosphatase-inhibitors. We cloned a Xenopus homologue of Slingshot phosphatase (XSSH), originally identified in Drosophila and human as an ADF/cofilin phosphatase, and raised antibody specific for the catalytic domain of XSSH. This inhibitory antibody significantly suppressed the Rho-induced dephosphorylation of ADF/cofilin in extracts, suggesting that the dephosphorylation at telophase is dependent on XSSH. XSSH bound to actin filaments with a dissociation constant of 0.4 microM, and the ADF/cofilin phosphatase activity was increased in the presence of F-actin. When latrunculin A, a G-actin-sequestering drug, was added to extracts, both Rho-induced actin polymerization and dephosphorylation of ADF/cofilin were markedly inhibited. Jasplakinolide, an actin-stabilizing drug, alone induced actin polymerization in the extracts and lead to dephosphorylation of ADF/cofilin. These results suggest that Rho-induced dephosphorylation of ADF/cofilin is dependent on the XSSH activation that is caused by increase in the amount of F-actin induced by Rho signaling. XSSH colocalized with both actin filaments and ADF/cofilin in the actin patches formed on the surface of the early cleavage furrow. Injection of inhibitory antibody blocked cleavage of blastomeres. Thus, XSSH may reorganize actin filaments through dephosphorylation and reactivation of ADF/cofilin at early stage of contractile ring formation.

Functional involvement of Xenopus homologue of ADF/cofilin phosphatase, slingshot (XSSH), in the gastrulation movement

ADF/cofilin is a phosphorylation-regulated protein essential for actin filament dynamics in cells. Here, we cloned two cDNAs encoding Xenopus ADF/cofilin (XAC)-specific phosphatase, slingshot (XSSH), one of which contains an extra 15 nucleotides in a coding sequence of the other, possibly generated by alternative splicing. Whole mount in situ hybridization showed XSSH transcripts in the blastopore lip and sensorial ectoderm at stage 11, and subsequently localized to developing brain, branchial arches, developing retina, otic vesicle, cement gland, and spinal chord in neurula to tailbud embryos. Immunostaining of animal-vegetal sections of gastrula embryos demonstrated that both XAC and XSSH proteins are predominant in ectodermal and involuting mesodermal cells. Microinjection of either a wild type (thus induces overexpression) or a phosphatase-defective mutant (functions as dominantly negative form) resulted in defects in gastrulation, and often generated the spina bifida phenotype with reduced head structures. Interestingly, the ratio of phosphorylated XAC to dephosphorylated XAC markedly increased from the early gastrula stage (stage 10.5), although the amount of XSSH protein markedly increased from this stage. These results suggest that gastrulation movement requires ADF/cofilin activity through dynamic regulation of its phosphorylation state.

Cdc42 participates in the regulation of ADF/cofilin and retinal growth cone filopodia by brain derived neurotrophic factor

Rho family GTPases have important roles in mediating the effects of guidance cues and growth factors on the motility of neuronal growth cones. We previously showed that the neurotrophin BDNF regulates filopodial dynamics on growth cones of retinal ganglion cell axons through activation of the actin regulatory proteins ADF and cofilin by inhibiting a RhoA-dependent pathway that phosphorylates (inactivates) ADF/cofilin. The GTPase Cdc42 has also been implicated in mediating the effects of positive guidance cues. In this article we investigated whether Cdc42 is involved in the effects of BDNF on filopodial dynamics. BDNF treatment increases Cdc42 activity in retinal neurons, and neuronal incorporation of constitutively active Cdc42 mimics the increases in filopodial number and length. Furthermore, constitutively active and dominant negative Cdc42 decreased and increased, respectively, the activity of RhoA in retinal growth cones, indicating crosstalk between these GTPases in retinal growth cones. Constitutively active Cdc42 mimicked the activation of ADF/cofilin that resulted from BDNF treatment, while dominant negative Cdc42 blocked the effects of BDNF on filopodia and ADF/cofilin. The inability of dominant negative Cdc42 to block ADF/cofilin activation and stimulation of filopodial dynamics by the ROCK inhibitor Y-27632 indicate interaction between Cdc42 and RhoA occurs upstream of ROCK. Our results demonstrate crosstalk occurs between GTPases in mediating the effects of BDNF on growth cone motility, and Cdc42 activity can promote actin dynamics via activation of ADF/cofilin.

Lim kinase regulates the development of olfactory and neuromuscular synapses

Lim Kinase (Limk) belongs to a phylogenetically conserved family of serine/threonine kinases, which have been shown to be potent regulators of the actin cytoskeleton. Despite accumulating evidence of its biochemical actions, its in vivo function has remained poorly understood. The association of the Limk1 gene with Williams Syndrome indicates that proteins of this family play a role in the nervous system. To unravel the cellular and molecular functions of Limk, we have either knocked out or activated the Limk gene in Drosophila. At the neuromuscular junction, loss of Limk leads to enlarged terminals, while increasing the activity of Limk leads to stunted terminals with fewer synaptic boutons. In the antennal lobe, loss of Limk abolishes the ability of p21-activated kinase (Pak) to alter glomerular development. In contrast, increase in Limk function leads to ectopic glomeruli, a phenotype suppressible by the coexpression of a hyperactive Cofilin gene. These results establish Limk as a critical regulator of Cofilin function and synapse development, and a downstream effector of Pak in vivo.

Transient decrease in F-actin may be necessary for translocation of proteins into dendritic spines

It remains poorly understood as to how newly synthesized proteins that are required to act at specific synapses are translocated into only selected subsets of potentiated dendritic spines. Here, we report that F-actin, a major component of the skeletal structure of dendritic spines, may contribute to the regulation of synaptic specificity of protein translocation. We found that the stabilization of F-actin blocked the translocation of GFP-CaMKII and inhibited the diffusion of 3-kDa dextran into spines (in 2-3 weeks cultures). Neuronal activation in hippocampal slices and cultured neurons led to an increase in the activation (decrease in the phosphorylation) of the actin depolymerization factor, cofilin, and a decrease in F-actin. Furthermore, the induction of long-term potentiation by tetanic stimulation induced local transient depolymerization of F-actin both in vivo and in hippocampal slices (8-10 weeks), and this local F-actin depolymerization was blocked by APV, a N-methyl-D-aspartate (NMDA) receptor antagonist. These results suggest that F-actin may play a role in synaptic specificity by allowing protein translocation into only potentiated spines, gated through its depolymerization, which is probably triggered by the activation of NMDA receptors.

Different activity regulation and subcellular localization of LIMK1 and LIMK2 during cell cycle transition

LIM kinases (LIMK1 and LIMK2) regulate actin cytoskeletal reorganization through phosphorylating and inactivating cofilin, an actin-depolymerizing factor of actin filaments. Here, we describe a detailed analysis of the cell-cycle-dependent activity of LIMK2, and a subcellular localization of LIMK1 and LIMK2. The activity of LIMK2, distinct from LIMK1, toward cofilin phosphorylation did not change in the normal cell division cycle. In contrast, LIMK2 was hyperphosphorylated and its activity was markedly increased when HeLa cells were synchronized at mitosis with nocodazole treatment. Immunofluorescence analysis showed that LIMK1 was localized at cell-cell adhesion sites in interphase and prophase, redistributed to the spindle poles during prometaphase to anaphase, and accumulated at the cleavage furrow in telophase. In contrast, LIMK2 was diffusely localized in the cytoplasm during interphase, redistributed to the mitotic spindle, and finally to the spindle midzone during anaphase to telophase. These findings suggest that LIMK2 is activated in response to microtubule disruption, and that LIMK1 and LIMK2 may play different roles in regulating for the mitotic spindle organization, chromosome segregation, and cytokinesis during the cell division cycle.

LIM kinase 2 is widely expressed in all tissues

The LIM kinase family includes two proteins: LIMK1 and LIMK2. These proteins have identical genomic structure and overall amino acid identity of 50%. Both proteins regulate actin polymerization via phosphorylation and inactivation of the actin depolymerizing factors ADF/cofilin. Although the function of endogenous LIMK1 is well established, little is known about the function of the endogenous LIMK2 protein. To understand the specific role of endogenous LIMK2 protein, we examined its expression in embryonic and adult mice using a rat monoclonal antibody, which recognizes specifically the PDZ domain of LIMK2 but not that of LIMK1. Immunoblotting and immunoprecipitation analyses of mouse tissues and human and mouse cell lines revealed widespread expression of the 75-kDa LIMK2 protein. Immunofluorescence analysis demonstrated that the cellular localization of LIMK2 is different from that of LIMK1. LIMK2 protein is found in the cytoplasm localized to punctae and is not enriched within focal adhesions like LIMK1. Immunohistochemical studies revealed that LIMK2 is widely expressed in embryonic and adult mouse tissues and that its expression pattern is similar to that of LIMK1 except in the testes. We have also demonstrated that endogenous LIMK1 and LIMK2 form heterodimers, and that LIMK2 does not always interact with the same proteins as LIMK1.

Cofilin phosphatases and regulation of actin dynamics

Cofilin is a ubiquitous actin-binding factor required for the reorganization of actin filaments in eukaryotes. The dephosphorylation of cofilin enables its actin severing and depolymerizing activity and drives directional cell motility, thus providing a simple phosphoregulatory mechanism for actin reorganization. To date, two cofilin-specific phosphatases have been identified: Slingshot and Chronophin. These cofilin phosphatases are unrelated in sequence and regulatory properties, each potentially providing a unique mechanism for cofilin activation under varying biological circumstances.

Biochemistry and biomechanics of cell motility

Cell motility is an essential cellular process for a variety of biological events. The process of cell migration requires the integration and coordination of complex biochemical and biomechanical signals. The protrusion force at the leading edge of a cell is generated by the cytoskeleton, and this force generation is controlled by multiple signaling cascades. The formation of new adhesions at the front and the release of adhesions at the rear involve the outside-in and inside-out signaling mediated by integrins and other adhesion receptors. The traction force generated by the cell on the extracellular matrix (ECM) regulates cell-ECM adhesions, and the counter force exerted by ECM on the cell drives the migration. The polarity of cell migration can be amplified and maintained by the feedback loop between the cytoskeleton and cell-ECM adhesions. Cell migration in three-dimensional ECM has characteristics distinct from that on two-dimensional ECM. The migration of cells is initiated and modulated by external chemical and mechanical factors, such as chemoattractants and the mechanical forces acting on the cells and ECM, as well as the surface density, distribution, topography, and rigidity of the ECM.

Cofilin, actin and their complex observed in vivo using fluorescence resonance energy transfer

Actin is the principal component of microfilaments. Its assembly/disassembly is essential for cell motility, cytokinesis, and a range of other functions. Recent evidence suggests that actin is present in the nucleus where it may be involved in the regulation of gene expression and that cofilin binds actin and can translocate into the nucleus during times of stress. In this report, we combine fluorescence resonance energy transfer and confocal microscopy to analyze the interactions of cofilin and G-actin within the nucleus and cytoplasm. By measuring the rate of photobleaching of fluorescein-labeled actin in the presence and absence of Cy5-labeled cofilin, we determined that almost all G-actin in the nucleus is bound to cofilin, whereas approximately (1/2) is bound in the cytoplasm. Using fluorescence resonance energy transfer imaging techniques we observed that a significant proportion of fluorescein-labeled cofilin in both the nucleus and cytoplasm binds added tetramethylrhodamine-labeled G-actin. Our data suggest there is significantly more cofilin-G-actin complex and less free cofilin in the nucleus than in the cytoplasm.

Brain-derived neurotrophic factor regulation of retinal growth cone filopodial dynamics is mediated through actin depolymerizing factor/cofilin

The molecular mechanisms by which neurotrophins regulate growth cone motility are not well understood. This study investigated the signaling involved in transducing BDNF-induced increases of filopodial dynamics. Our results indicate that BDNF regulates filopodial length and number through a Rho kinase-dependent mechanism. Additionally, actin depolymerizing factor (ADF)/cofilin activity is necessary and sufficient to transduce the effects of BDNF. Our data indicate that activation of ADF/cofilin mimics the effects of BDNF on filopodial dynamics, whereas ADF/cofilin inactivity blocks the effects of BDNF. Furthermore, BDNF promotes the activation of ADF/cofilin by reducing the phosphorylation of ADF/cofilin. Although inhibition of myosin II also enhances filopodial length, our results indicate that BDNF signaling is independent of myosin II activity and that the two pathways result in additive effects on filopodial length. Thus, filopodial extension is regulated by at least two independent mechanisms. The BDNF-dependent pathway works via regulation of ADF/cofilin, independently of myosin II activity.

Cofilin takes the lead

Cofilin has emerged as a key regulator of actin dynamics at the leading edge of motile cells. Through its actin-severing activity, it creates new actin barbed ends for polymerization and also depolymerizes old actin filaments. Its function is tightly regulated in the cell. Spatially, its activity is restricted by other actin-binding proteins, such as tropomyosin, which compete for accessibility of actin filament populations in different regions of the cell. At the molecular level, it is regulated by phosphorylation, pH and phosphatidylinositol (4,5)-bisphosphate binding downstream of signaling cascades. In addition, it also appears to be regulated by interactions with 14-3-3zeta and cyclase-associated protein. In vivo, cofilin acts synergistically with the Arp2/3 complex to amplify local actin polymerization responses upon cell stimulation, which gives it a central role in setting the direction of motility in crawling cells.

Spine architecture and synaptic plasticity

Many forms of mental retardation and cognitive disability are associated with abnormalities in dendritic spine morphology. Visualization of spines using live-imaging techniques provides convincing evidence that spine morphology is altered in response to certain forms of LTP-inducing stimulation. Thus, information storage at the cellular level appears to involve changes in spine morphology that support changes in synaptic strength produced by certain patterns of synaptic activity. Because the structure of a spine is determined by its underlying actin cytoskeleton, there has been much effort to identify signaling pathways linking synaptic activity to control of actin polymerization. This review, part of the TINS Synaptic Connectivity series, discusses recent studies that implicate EphB and NMDA receptors in the regulation of actin-binding proteins through modulation of Rho family small GTPases.

LIM kinase 1 coordinates microtubule stability and actin polymerization in human endothelial cells

Microtubule (MT) destabilization promotes the formation of actin stress fibers and enhances the contractility of cells; however, the mechanism involved in the coordinated regulation of MTs and the actin cytoskeleton is poorly understood. LIM kinase 1 (LIMK1) regulates actin polymerization by phosphorylating the actin depolymerization factor, cofilin. Here we report that LIMK1 is also involved in the MT destabilization. In endothelial cells endogenous LIMK1 co-localizes with MTs and forms a complex with tubulin via the PDZ domain. MT destabilization induced by thrombin or nocodazole resulted in a decrease of LIMK1 colocalization with MTs. Overexpression of wild type LIMK1 resulted in MT destabilization, whereas the kinase-dead mutant of LIMK1 (KD) did not affect MT stability. Importantly, down-regulation of endogenous LIMK1 by small interference RNA resulted in abrogation of the thrombin-induced MTs destabilization and the inhibition of thrombin-induced actin polymerization. Expression of Rho kinase 2, which phosphorylates and activates LIMK1, dramatically decreases the interaction of LIMK1 with tubulin but increases its interaction with actin. Interestingly, expression of KD-LIMK1 or small interference RNA-LIMK1 prevents thrombin-induced microtubule destabilization and F-actin formation, suggesting that LIMK1 activity is required for thrombin-induced modulation of microtubule destabilization and actin polymerization. Our findings indicate that LIMK1 may coordinate microtubules and actin cytoskeleton.

Integrins control motile strategy through a Rho-cofilin pathway

During wound healing, angiogenesis, and tumor invasion, cells often change their expression profiles of fibronectin-binding integrins. Here, we show that beta1 integrins promote random migration, whereas beta3 integrins promote persistent migration in the same epithelial cell background. Adhesion to fibronectin by alpha(v)beta3 supports extensive actin cytoskeletal reorganization through the actin-severing protein cofilin, resulting in a single broad lamellipod with static cell-matrix adhesions at the leading edge. Adhesion by alpha5beta1 instead leads to the phosphorylation/inactivation of cofilin, and these cells fail to polarize their cytoskeleton but extend thin protrusions containing highly dynamic cell-matrix adhesions in multiple directions. The activity of the small GTPase RhoA is particularly high in cells adhering by alpha5beta1, and inhibition of Rho signaling causes a switch from a beta1- to a beta3-associated mode of migration, whereas increased Rho activity has the opposite effect. Thus, alterations in integrin expression profiles allow cells to modulate several critical aspects of the motile machinery through Rho GTPases.

Cellular localization of activated N-WASP using a conformation-sensitive antibody

The main regulators of Arp2/3 activity appear to be N-WASP and the other members of the Scar/WAVE family of proteins. We show here that after EGF stimulation, N-WASP is recruited to the nucleation zone of the dynamic leading edge compartment of carcinoma cells, with maximal recruitment of N-WASP within 1 min after EGF stimulation. The timing of N-WASP recruitment mirrors the timing of barbed-end formation at the leading edge. To determine the cellular activation of N-WASP after EGF stimulation, we made a conformation-sensitive antibody (CSA) against the CRIB domain of N-WASP that is predicted to recognize N-WASP in its open, active conformation, but not in its closed, inactive conformation. The ability of CSA to detect only active N-WASP was demonstrated by in vitro experiments using immunoprecipitation of active N-WASP from EGF-stimulated cells and Cdc42 activation of N-WASP activity. In cell staining experiments, N-WASP is maximally accessible to CSA 40 sec after EGF stimulation and this activated N-WASP is in the nucleation zone. These results indicate that active N-WASP is present at the leading edge of lamellipods, an unexpected finding given its reported involvement in filopod formation. This work establishes the feasibility of using antibodies directed against specific conformations or epitopes with changing accessibilities as a window on the status and localization of activity.

AIP1/WDR1 supports mitotic cell rounding

The actin cytoskeleton plays a fundamental role in configuring cell shapes and movements. Actin interacting protein 1 (AIP1)/tryptophan-aspartate-repeat protein 1 (WDR1) induces actin severing and disassembly cooperating with ADF/cofilin. We found that mitotic cell flattening but not rounding was manifested by suppression of AIP1/WDR1 in cells. This mitotic cell flattening was not due to any changes in phosphorylation and distribution of cofilin in cells. We carried out a direct observation of actin filament severing/disassembly assay and found that phosphorylated cofilin still somewhat severs/disassembles actin filaments and that AIP1/WDR1 effaces this in vitro. We suggest that the phosphorylation of ADF/cofilin will be insufficient to completely inhibit actin turnover during mitosis, and that AIP1/WDR1 could abort the severing/disassembly activity somewhat still carried out due to phosphorylated ADF/cofilin. This mechanism could be required to induce cell morphologic changes, especially mitotic cell rounding.

Calcium signal-induced cofilin dephosphorylation is mediated by Slingshot via calcineurin

Cofilin, an essential regulator of actin filament dynamics, is inactivated by phosphorylation at Ser-3 and reactivated by dephosphorylation. Although cofilin undergoes dephosphorylation in response to extracellular stimuli that elevate intracellular Ca2+ concentrations, signaling mechanisms mediating Ca2+-induced cofilin dephosphorylation have remained unknown. We investigated the role of Slingshot (SSH) 1L, a member of a SSH family of protein phosphatases, in mediating Ca2+-induced cofilin dephosphorylation. The Ca2+ ionophore A23187 and Ca2+-mobilizing agonists, ATP and histamine, induced SSH1L activation and cofilin dephosphorylation in cultured cells. A23187- or histamine-induced SSH1L activation and cofilin dephosphorylation were blocked by calcineurin inhibitors or a dominant-negative form of calcineurin, indicating that calcineurin mediates Ca2+-induced SSH1L activation and cofilin dephosphorylation. Importantly, knockdown of SSH1L expression by RNA interference abolished A23187- or calcineurin-induced cofilin dephosphorylation. Furthermore, calcineurin dephosphorylated SSH1L and increased the cofilin-phosphatase activity of SSH1L in cell-free assays. Based on these findings, we suggest that Ca2+-induced cofilin dephosphorylation is mediated by calcineurin-dependent activation of SSH1L.

Rho GTPases regulate axon growth through convergent and divergent signaling pathways

Rho GTPases are essential regulators of cytoskeletal reorganization, but how they do so during neuronal morphogenesis in vivo is poorly understood. Here we show that the actin depolymerization factor cofilin is essential for axon growth in Drosophila neurons. Cofilin function in axon growth is inhibited by LIM kinase and activated by Slingshot phosphatase. Dephosphorylating cofilin appears to be the major function of Slingshot in regulating axon growth in vivo. Genetic data provide evidence that Rho or Rac/Cdc42, via effector kinases Rok or Pak, respectively, activate LIM kinase to inhibit axon growth. Importantly, Rac also activates a Pak-independent pathway that promotes axon growth, and different RacGEFs regulate these distinct pathways. These genetic analyses reveal convergent and divergent pathways from Rho GTPases to the cytoskeleton during axon growth in vivo and suggest that different developmental outcomes could be achieved by biases in pathway selection.

The co-workers of actin filaments: from cell structures to signals

Cells have various surface architectures, which allow them to carry out different specialized functions. Actin microfilaments that are associated with the plasma membrane are important for generating these cell-surface specializations, and also provide the driving force for remodelling cell morphology and triggering new cell behaviour when the environment is modified. This phenomenon is achieved through a tight coupling between cell structure and signal transduction, a process that is modulated by the regulation of actin-binding proteins.

Efficient Salmonella entry requires activity cycles of host ADF and cofilin

Entry of Salmonella into mammalian cells is strictly dependent on the reorganization of actin cytoskeleton induced by a panel of Salmonella type III secreted proteins. Although several factors have been identified to be responsible for inducing the actin polymerization and stability, little is known about how the actin depolymerization contributes to Salmonella-induced actin rearrangements. We report here that activity cycles of host actin depolymerizing factor (ADF and cofilin) are modulated by Salmonella during bacterial entry. Efficient Salmonella internalization involves an initial dephosphorylation of ADF and cofilin followed by phosphorylation, suggesting that ADF and cofilin activities are increased briefly. Expression of a kinase dead form of an ADF/cofilin kinase (LIM kinase 1) or a catalytically inactive ADF/cofilin phosphatase (Slingshot), but not constitutively active LIM kinase 1 or wild-type Slingshot, resulted in decreased invasion. These data suggest that ADF/cofilin activities play a key role in the actin polymerization/depolymerization process induced by Salmonella. The activation of ADF/cofilin is brief and has to be reversed to facilitate efficient bacterial entry. Surprisingly, co-expression of constitutive active ADF and cofilin prevented efficient Salmonella entry, whereas expression of either one alone had no effect. We propose that ADF and cofilin actin-dynamizing activities and their activity cycling via phosphorylation are required for efficient Salmonella internalization.

In Vitro Activity Differences between Proteins of the ADF/Cofilin Family Define Two Distinct Subgroups†

The actin depolymerizing factor (ADF)/cofilins are an essential group of proteins that are important regulators of actin filament turnover in vivo. Although protists and yeasts express only a single member of this family, metazoans express two or more members in many cell types. In cells expressing both ADF and cofilin, differences have been reported in the regulation of their expression, their pH sensitivity, and their intracellular distribution. Each member has qualitatively similar interactions with actin, but quantitative differences have been noted. Here we compared quantitative differences between chick ADF and chick cofilin using several assays that measure G-actin binding, actin filament length distribution, and assembly/disassembly dynamics. Quantitative differences were measured in the critical concentrations of the complexes required for assembly, in the effects of nucleotide and divalent metal on actin monomer binding, in pH-dependent severing, in enhancement of filament minus end off-rates, and in steady-state filament length distributions generated in similar mixtures. Some of these assays were used to compare the activities of several ADF/cofilins from across phylogeny, most of which fall into one of two groups based upon their behavior. The ADF-like group has higher affinities for Mg(2+)-ATP-G-actin than the cofilin-like group and a greater pH-dependent depolymerizing activity.

LIM kinase 1, a key regulator of actin dynamics, is widely expressed in embryonic and adult tissues

The expression of endogenous LIM kinase 1 (LIMK1) protein was investigated in embryonic and adult mice using a rat monoclonal antibody (mAb), which recognizes specifically the PDZ domain of LIMK1 and not LIMK2. Immunoblotting analysis revealed widespread expression of LIMK1 existing as a 70-kDa protein in tissues and in cell lines, with a higher mass form (approximately 75 kDa) present in some tissues and cell lines. Smaller isoforms of approximately 50 kDa were also occasionally evident. Immunofluorescence analysis demonstrated LIMK1 subcellular localization at focal adhesions in fibroblasts as revealed by co-staining with actin, paxillin and vinculin in addition to perinuclear (Golgi) and occasional nuclear localization. Furthermore, an association between LIMK1 and paxillin but not vinculin was identified by co-immunoprecipitation analysis. LIMK1 is enriched in both axonal and dendritic growth cones of E18 rat hippocampal pyramidal neurons where it is found in punctae that extend far out into filopodia, as well as in a perinuclear region identified as Golgi. In situ, we identify LIMK1 protein expression in all embryonic and adult tissues examined, albeit at different levels and in different cell populations. The rat monoclonal LIMK1 antibody recognizes proteins of similar size in cell and tissue extracts from numerous species. Thus, LIMK1 is a widely expressed protein that exists as several isoforms.

CD29 integrin- and LIMK1/cofilin-mediated actin reorganization regulates the migration of haematopoietic progenitor cells underneath bone marrow stromal cells

Migration and successive homing of haematopoietic stem/progenitor cells (HS/PCs) into haematopoietic microenvironments are critical to their proliferation and differentiation. To investigate molecular mechanisms underlying HS/PC migration, we used a human erythroleukaemia (HEL) cell line which has been characterized as a haematopoietic progenitor cell line and displays high migratory properties underneath the haematopoietic-supportive stromal cell line, HESS-M28. HEL cell migration is mediated by the adhesion of the CD29 integrin on HEL cells to HESS-28 cells which leads to the localization of filamentous actin and formation of cell polarity at membrane protrusions via actin cytoskeleton reorganization. HEL cell migration is inhibited by both dominant negative forms of the Rho-GTPase family members and a cell permeable inhibitor of LIMK1, S3 peptide. Expression of constitutively active- or inactive-forms of cofilin also inhibits HEL cell migration and phosphorylated cofilin is localized to the front protrusions of HEL cells. These results suggest that cytoskeleton reorganization mediated by a Rho-GTPase/LIMK1/cofilin pathway plays a critical role in the migration of HEL cells underneath HESS-M28 cells.

Regulation of the neuronal actin cytoskeleton by ADF/cofilin

Actin and microtubules are major cytoskeletal elements of most cells including neurons. In order for a cell to move and change shape, its cytoskeleton must undergo rearrangements that involve breaking down and reforming filaments. Many recent reviews have focused on the signaling pathways emanating from receptors that ultimately affect axon growth and growth cone steering. This particular review will address changes in the actin cytoskeleton modulated by the family of actin dynamizing proteins known as actin depolymerizing factor (ADF)/cofilin or AC proteins. Though much is known about inactivation of AC proteins through phosphorylation at ser3 by LIM or TES kinases, new mechanisms of regulation of AC have recently emerged. A novel phosphatase, slingshot (SSH), and the 14-3-3 family of regulatory proteins have also been found to affect AC activity. The potential role of AC proteins in modulating the actin organizational changes that accompany neurite initiation, axonogenesis, growth cone guidance, and dendritic spine formation will be discussed.

Signal transduction cascades underlying de novo protein synthesis required for neuronal morphogenesis in differentiating neurons

Differentiating neurons must acquire many unique morphological and functional characteristics in creating the precise neural circuits of the mature nervous system. The phenomenon of 'neuronal differentiation' includes a special set of simple, separate processes, that is, neuritogenesis, neurite outgrowth, pathfinding, targeting and synaptogenesis. All of these processes are critically dependent on the reorganization of actin cytoskeleton by many actin-binding proteins that function downstream of Rho-family GTPases. Furthermore, de novo synthesis of key proteins are critically involved in the reorganization of actin cytoskeleton during neuronal differentiation. In this article, we review recent progresses in the general mechanisms that control actin dynamics by various actin-binding proteins in differentiating neurons, including a series of recent studies from our laboratory on de novo synthesis of several key proteins that are essential for actin reorganization induced by second messengers. We demonstrated that dual regulation of cyclic AMP and Ca2+ determines cofilin (an actin-binding protein) phosphorylation states and LIM kinase 1 (a cofilin kinase) expression level during neuritogenesis.

Renal ischemia induces tropomyosin dissociation-destabilizing microvilli microfilaments

Ischemic-induced cell injury results in rapid duration-dependent actin-depolymerizing factor (ADF)/cofilin-mediated disruption of the apical microvilli microfilament cores. Because intestinal microvillar microfilaments are bound and stabilized in the terminal web by the actin-binding protein tropomyosin, we questioned whether a protective effect of tropomyosin localization to the terminal web of the proximal tubule microfilament cores is disrupted during ischemic injury. With tropomyosin-specific antibodies, we examined rat cortical sections under physiological conditions and following ischemic injury by confocal microscopy. In addition, Western blot analysis of cortical extracts and urine was undertaken. Our studies demonstrated the presence of tropomyosin isoforms in the proximal tubule microvillar terminal web under physiological conditions and their dissociation in response to 25 min of ischemic injury. This correlated with the excretion of tropomyosin-containing plasma membrane vesicles in urine from ischemic rats. In addition, we noted increased tropomyosin Triton X-100 solubility following ischemia in cortical extracts. These studies suggest tropomyosin binds to and stabilizes the microvillar microfilament core in the terminal web under physiological conditions. With the onset of ischemic injury, we propose that tropomyosin dissociates from the microfilament core providing access to microfilaments in the terminal web for F-actin binding, severing and depolymerizing actions of ADF/cofilin proteins.

Solution structure of human cofilin: actin binding, pH sensitivity, and relationship to actin-depolymerizing factor

Human actin-depolymerizing factor (ADF) and cofilin are pH-sensitive, actin-depolymerizing proteins. Although 72% identical in sequence, ADF has a much higher depolymerizing activity than cofilin at pH 8. To understand this, we solved the structure of human cofilin using nuclear magnetic resonance and compared it with human ADF. Important sequence differences between vertebrate ADF/cofilins were correlated with unique structural determinants in the F-actin-binding site to account for differences in biochemical activities of the two proteins. Cofilin has a short beta-strand at the C terminus, not found in ADF, which packs against strands beta3/beta4, changing the environment around Lys96, a residue essential for F-actin binding. A salt bridge involving His133 and Asp98 (Glu98 in ADF) may explain the pH sensitivity of human cofilin and ADF; these two residues are fully conserved in vertebrate ADF/cofilins. Chemical shift perturbations identified residues that (i) differ in their chemical environments between wild type cofilin and mutants S3D, which has greatly reduced G-actin binding, and K96Q, which does not bind F-actin; (ii) are affected when G-actin binds cofilin; and (iii) are affected by pH change from 6 to 8. Many residues affected by G-actin binding also show perturbation in the mutants or in response to pH. Our evidence suggests the involvement of residues 133-138 of strand beta5 in all of the activities examined. Because residues in beta5 are perturbed by mutations that affect both G-actin and F-actin binding, this strand forms a "boundary" or "bridge" between the proposed F- and G-actin-binding sites.

Regulation of Actin Filament Dynamics by Actin Depolymerizing Factor/Cofilin and Actin-Interacting Protein 1: New Blades for Twisted Filaments

Actin depolymerizing factor (ADF)/cofilin enhances turnover of actin filaments by severing and depolymerizing filaments. A number of proteins functionally interact with ADF/cofilin to modulate the dynamics of actin filaments. Actin-interacting protein 1 (AIP1) has emerged as a conserved WD-repeat protein that specifically enhances ADF/cofilin-induced actin dynamics. Interaction of AIP1 with actin was originally characterized by a yeast two-hybrid system. However, biochemical studies revealed its unique activity on ADF/cofilin-bound actin filaments. AIP1 alone has negligible effects on actin filament dynamics, whereas in the presence of ADF/cofilin, AIP1 enhances filament fragmentation by capping ends of severed filaments. Studies in model organisms demonstrated that AIP1 genetically interacts with ADF/cofilin and participates in several actin-dependent cellular events. The crystal structure of AIP1 revealed its unique structure with two seven-bladed beta-propeller domains. Thus, AIP1 is a new class of actin regulatory proteins that selectively enhances ADF/cofilin-dependent actin filament dynamics.

Actin filament disassembling activity of Caenorhabditis elegans actin-interacting protein 1 (UNC-78) is dependent on filament binding by a specific ADF/cofilin isoform

Actin-interacting protein 1 (AIP1) is a conserved WD-repeat protein that enhances actin filament disassembly only in the presence of actin depolymerizing factor (ADF)/cofilin. In the nematode Caenorhabditis elegans, an AIP1 ortholog is encoded by the unc-78 gene that is required for organized assembly of muscle actin filaments. We produced bacterially expressed UNC-78 protein and found that it enhances actin filament disassembly preferentially in the presence of a specific ADF/cofilin isoform. Extensive and rapid filament disassembly by UNC-78 was observed in the presence of UNC-60B, a muscle-specific C. elegans ADF/cofilin isoform. UNC-78 also reduced the rate of spontaneous polymerization and enhanced subunit dissociation from filaments in the presence of UNC-60B. However, in the presence of UNC-60A, a non-muscle C. elegans ADF/cofilin isoform, UNC-78 only slightly enhanced filament disassembly. Interestingly, UNC-78 failed to enhance disassembly by mouse muscle-type cofilin. Using mutant forms of UNC-60B, we demonstrated that the F-actin-specific binding site of UNC-60B at the C terminus is required for filament disassembly by UNC-78. UNC-78 was expressed in body wall muscle and co-localized with actin where UNC-60B was also present. Surprisingly, UNC-78 was co-localized with actin in unc-60B null mutants, suggesting that the AIP1-actin interaction is not dependent on ADF/cofilin in muscle. These results suggest that UNC-78 closely collaborates with UNC-60B to regulate actin dynamics in muscle cells.

Differential activities, subcellular distribution and tissue expression patterns of three members of Slingshot family phosphatases that dephosphorylate cofilin

BACKGROUND

Cofilin, a key regulator of actin filament dynamics, is inactivated by phosphorylation at Ser-3 by LIM-kinases and is reactivated by dephosphorylation by a family of protein phosphatases, termed Slingshot (SSH).

RESULTS

We have identified two novel isoforms of SSHs, termed SSH-2L and SSH-3L and characterized them in comparison with SSH-1L that was previously reported. SSH-1L and SSH-2L, but not SSH-3L, tightly bound to and co-localized with actin filaments. When expressed in cultured cells, SSH-1L, SSH-2L and SSH-3L decreased the level of Ser-3-phosphorylated cofilin (P-cofilin) in cells and suppressed LIM-kinase-induced actin reorganization, although SSH-3L was less effective than SSH-1L and SSH-2L. In cell-free assays, SSH-1L and SSH-2L efficiently dephosphorylated P-cofilin, whereas SSH-3L did do so only weakly. Using deleted mutants of SSH-1L and SSH-2L, we found that the N-terminal and C-terminal extracatalytic regions are critical for cofilin-phosphatase and F-actin-binding activities, respectively. In situ hybridization analyses revealed characteristic patterns of expression of each of the mouse Ssh genes in both neuronal and non-neuronal tissues; in particular, expression of Ssh-3 in epithelial tissues is evident.

CONCLUSION

SSH-1L, SSH-2L and SSH-3L have the potential to dephosphorylate P-cofilin, but subcellular distribution, F-actin-binding activity, specific phosphatase activity and expression patterns significantly differ, which suggests that they have related but distinct functions in various cellular and developmental events.

Dual regulation of LIM kinase 1 expression by cyclic AMP and calcium determines cofilin phosphorylation states during neuritogenesis in NG108-15 cells

The present study was designed to elucidate the cellular and molecular mechanisms of neuritogenesis in differentiating neurons. For this purpose, we used pharmacological and immunochemical techniques to determine the intracellular signal transduction pathways that regulate actin dynamics during neuritogenesis. We confirmed that a rise in intracellular cyclic AMP (cAMP) concentration stimulated cells to increase their neurite numbers, and that this increase of neurites was suppressed by activation of calcineurin induced by a Ca2+ influx through voltage-dependent Ca2+ channels. Expression of a specific cofilin kinase (LIM kinase 1) was increased and decreased by cAMP and Ca2+ cascades, respectively. The phosphorylation state, but not the level of expression, of a potent regulator of actin dynamics (cofilin) was strongly correlated with the expression level of LIM kinase 1. Our results suggest that polymerization and depolymerization of actin by cofilin phosphorylation is necessary for neuritogenesis in differentiating neurons.

The structure of Aip1p, a WD repeat protein that regulates Cofilin-mediated actin depolymerization

Actin-interacting protein 1 (Aip1p) is a 67-kDa WD repeat protein known to regulate the depolymerization of actin filaments by cofilin and is conserved in organisms ranging from yeast to mammals. The crystal structure of Aip1p from Saccharomyces cerevisiae was determined to a 2.3-A resolution and a final crystallographic R-factor of 0.204. The structure reveals that the overall fold is formed by two connected seven-bladed beta-propellers and has important implications for the structure of Aip1 from other organisms and WD repeat-containing proteins in general. These results were unexpected because a maximum of 10 WD repeats had been reported in the literature for this protein using sequence data. The surfaces of the beta-propellers formed by the D-A and B-C loops are positioned adjacent to one another, giving Aip1p a shape that resembles an open "clamshell." The mapping of conserved residues to the structure of Aip1p reveals dense patches of conserved residues on the surface of one beta-propeller and at the interface of the two beta-propellers. These two patches of conserved residues suggest a potential binding site for F-actin on Aip1p and that the orientation of the beta-propellers with respect to one another plays a role in binding an actin-cofilin complex. In addition, the conserved interface between the domains is mediated by a number of interactions that appear to impart rigidity between the two domains of Aip1p and may make a large substrate-induced conformational change difficult.

Detection and characterization of a rat parotid gland protein kinase that catalyzes phosphorylation of matured destrin at Ser-2

Destrin, an actin-binding protein, is partly phosphorylated at Ser-2 (numbering on the matured form) in the resting rat parotid gland, and beta-adrenergic or cholinergic stimulation of this gland induces its dephosphorylation. In this study, we searched for the protein kinase involved in phosphorylation of destrin. We developed an assay method for the kinase, using an antibody specific to destrin phosphorylated at Ser-2, and detected the kinase in the rat parotid homogenate. This enzyme was predominantly (93%) present in the soluble fraction, and the enzyme in this fraction was characterized. It had an optimum pH at 6.8 and required 3-5 mM Mg2+ for its maximum activity. Ca2+ (1 mM) had no effect whereas Mn2+ (5 mM) inhibited the enzyme activity by 75%. The apparent Km values for destrin and ATP were 92 microg/ml and 170 microM, respectively. GTP was an inefficient phosphate donor, and cAMP had no effect. Heat-denatured destrin was poorly phosphorylated. Two-dimensional PAGE analysis of destrin phosphorylated with the soluble fraction indicated that it was exclusively phosphorylated at Ser-2. None of the protein kinase inhibitors tested here was specific to this enzyme. At 1 mM, ML-7, Y-27632, KN-93, HA-1077, H-7, and H-8 inhibited the activity by 88, 61, 58, 49, 46, and 42%, respectively. Staurosporine (2 microM) and H-89 (50 microM) inhibited the activity by 48 and 33%, respectively. Heparin (30 microg/ml) had no effect. These results suggest that the rat parotid gland contains a novel, constitutively active, soluble protein kinase catalyzing specific phosphorylation of destrin at Ser-2.

Cell cycle-associated changes in Slingshot phosphatase activity and roles in cytokinesis in animal cells

During cytokinesis the actomyosin-based contractile ring is formed at the equator, constricted, and then disassembled prior to cell abscission. Cofilin stimulates actin filament disassembly and is implicated in the regulation of contractile ring dynamics. However, little is known about the mechanism controlling cofilin activity during cytokinesis. Cofilin is inactivated by phosphorylation on Ser-3 by LIM-kinase-1 (LIMK1) and is reactivated by a protein phosphatase Slingshot-1 (SSH1). Here we show that the phosphatase activity of SSH1 decreases in the early stages of mitosis and is elevated in telophase and cytokinesis in HeLa cells, a time course correlating with that of cofilin dephosphorylation. SSH1 co-localizes with F-actin and accumulates onto the cleavage furrow and the midbody. Expression of a phosphatase-inactive SSH1 induces aberrant accumulation of F-actin and phospho-cofilin near the midbody in the final stage of cytokinesis and frequently leads to the regression of the cleavage furrow and the formation of multinucleate cells. Co-expression of cofilin rescued the inhibitory effect of phosphatase-inactive SSH1 on cytokinesis. These results suggest that SSH1 plays a critical role in cytokinesis by dephosphorylating and reactivating cofilin in later stages of mitosis.

Effects of rho kinase and actin stress fibers on sustained extracellular signal-regulated kinase activity and activation of G(1) phase cyclin-dependent kinases

We recently reported that Rho kinase is required for sustained ERK signaling and the consequent mid-G(1) phase induction of cyclin D1 in fibroblasts. The results presented here indicate that these Rho kinase effects are mediated by the formation of stress fibers and the consequent clustering of alpha5beta1 integrin. Mechanistically, alpha5beta1 signaling and stress fiber formation allowed for the sustained activation of MEK, and this effect was mediated upstream of Ras-GTP loading. Interestingly, disruption of stress fibers with ML-7 led to G(1) phase arrest while comparable disruption of stress fibers with Y27632 (an inhibitor of Rho kinase) or dominant-negative Rho kinase led to a more rapid progression through G(1) phase. Inhibition of either MLCK or Rho kinase blocked sustained ERK signaling, but only Rho kinase inhibition allowed for the induction of cyclin D1 and activation of cdk4 via Rac/Cdc42. The levels of cyclin E, cdk2, and their major inhibitors, p21(cip1) and p27(kip1), were not affected by inhibition of MLCK or Rho kinase. Overall, our results indicate that Rho kinase-dependent stress fiber formation is required for sustained activation of the MEK/ERK pathway and the mid-G(1) phase induction of cyclin D1, but not for other aspects of cdk4 or cdk2 activation. They also emphasize that G(1) phase cell cycle progression in fibroblasts does not require stress fibers if Rac/Cdc42 signaling is allowed to induce cyclin D1.

Rocks: multifunctional kinases in cell behaviour

ROCKs, or Rho kinases, are serine/threonine kinases that are involved in many aspects of cell motility, from smooth-muscle contraction to cell migration and neurite outgrowth. Recent experiments have defined new functions of ROCKs in cells, including centrosome positioning and cell-size regulation, which might contribute to various physiological and pathological states.