Phosphorylation of cofilin by LIM-kinase is necessary for semaphorin 3A-induced growth cone collapse

Mitosis-specific activation of LIM motif-containing protein kinase and roles of cofilin phosphorylation and dephosphorylation in mitosis

Actin filament dynamics play a critical role in mitosis and cytokinesis. LIM motif-containing protein kinase 1 (LIMK1) regulates actin reorganization by phosphorylating and inactivating cofilin, an actin-depolymerizing and -severing protein. To examine the role of LIMK1 and cofilin during the cell cycle, we measured cell cycle-associated changes in the kinase activity of LIMK1 and in the level of cofilin phosphorylation. Using synchronized HeLa cells, we found that LIMK1 became hyperphosphorylated and activated in prometaphase and metaphase, then gradually returned to the basal level as cells entered into telophase and cytokinesis. Although Rho-associated kinase and p21-activated protein kinase phosphorylate and activate LIMK1, they are not likely to be involved in mitosis-specific activation and phosphorylation of LIMK1. Immunoblot and immunofluorescence analyses using an anti-phosphocofilin-specific antibody revealed that the level of cofilin phosphorylation, similar to levels of LIMK1 activity, increased during prometaphase and metaphase then gradually declined in telophase and cytokinesis. Ectopic expression of LIMK1 increased the level of cofilin phosphorylation throughout the cell cycle and induced the formation of multinucleate cells. These results suggest that LIMK1 is involved principally in control of mitosis-specific cofilin phosphorylation and that dephosphorylation and reactivation of cofilin at later stages of mitosis play a critical role in cytokinesis of mammalian cells.

Phosphatidylinositol 4-phosphate 5-kinase is essential for ROCK-mediated neurite remodeling

Phosphatidylinositol 4-phosphate 5-kinase (PIP-5kin) regulates actin cytoskeletal reorganization through its product phosphatidylinositol 4,5-bisphosphate. In the present study we demonstrate that PIP-5kin is essential for neurite remodeling, which is regulated by actin cytoskeletal reorganization in neuroblastoma N1E-115 cells. Overexpression of wild-type mouse PIP-5kin-alpha inhibits the neurite formation that is normally stimulated by serum depletion, whereas a lipid kinase-defective mutant of PIP-5kin-alpha, D266A, triggers neurite extension even in the presence of serum and blocks lysophosphatidic acid-induced neurite retraction. These results phenocopy those previously reported for the small GTPase RhoA and its effector p160 Rho-associated coiled coil-forming protein kinase (ROCK). However, the ROCK-specific inhibitor Y-27632 failed to block the inhibition by PIP-5kin-alpha of neurite extension, whereas D266A did block the neurite retraction induced by overexpression of ROCK. These results, taken together, suggest that PIP-5kin-alpha functions as a downstream effector for RhoA/ROCK to couple lysophosphatidic acid signaling to neurite retraction presumably through its product phosphatidylinositol 4,5-bisphosphate.

An inactive pool of GSK-3 at the leading edge of growth cones is implicated in Semaphorin 3A signaling

Glycogen synthase kinase (GSK)-3 is a serine/threonine kinase that has been implicated in several aspects in embryonic development and several growth factor signaling cascades. We now report that an inactive phosphorylated pool of the enzyme colocalizes with F-actin in both neuronal and nonneuronal cells. Semaphorin 3A (Sema 3A), a molecule that inhibits axonal growth, activates GSK-3 at the leading edge of neuronal growth cones and in Sema 3A-responsive human breast cancer cells, suggesting that GSK-3 activity might play a role in coupling Sema 3A signaling to changes in cell motility. We show that three different GSK-3 antagonists (LiCl, SB-216763, and SB-415286) can inhibit the growth cone collapse response induced by Sema 3A. These studies reveal a novel compartmentalization of inactive GSK-3 in cells and demonstrate for the first time a requirement for GSK-3 activity in the Sema 3A signal transduction pathway.

Nogos and the Nogo-66 receptor: factors inhibiting CNS neuron regeneration

The recently cloned gene Nogo, whose alternative splice products correspond to the antigenic target of the central nervous system (CNS) regeneration enhancing monoclonal antibody IN-1, codes for membrane proteins enriched in brain, particularly in oligodendrocytes. The 66-amino acid extracellular domain of Nogo (Nogo-66) interacts with a high-affinity receptor (NgR), a glycosylphosphatidylinositol (GPI)-linked protein with multiple leucine-rich repeats. The amino terminal cytoplasmic domain of Nogo appears to have a general cellular growth inhibitory effect. Nogo-66, on the other hand, specifically retards neurite outgrowth and induces growth cone collapse, possibly through its interaction with NgR and as yet unidentified transmembrane coreceptors. Recent results also suggest that Nogo expression may induce apoptosis in tumor cells. Together, these proteins provide new molecular handles for the design of therapeutic interventions for CNS injuries and neurodegenerative diseases, as well as possible leads to anticancer strategies.

Collapsin response mediator protein-2 inhibits neuronal phospholipase D(2) activity by direct interaction

Although the functional significance of neuronal phospholipase D (PLD) is being recognized, little is known about its regulatory role in neuronal cells. To elucidate the regulatory mechanism of neuronal PLD, we investigated PLD(2)-binding neuronal protein from rat brain cytosol. During the fractionation of rat brain cytosol by four-column chromatography, a 62-kDa PLD(2)-interacting protein was detected by PLD(2) overlay assay and identified as collapsin response mediator protein-2 (CRMP-2), which controls neuronal axon guidance and outgrowth. Using bacterially expressed glutathione S-transferase fusion proteins, we found that two regions (amino acids 65-192 (the phagocytic oxidase domain) and 724-825) of PLD(2) and a single region (amino acids 243-300) of CRMP-2 are required for the direct binding of both proteins. A co-immunoprecipitation study in COS-7 cells also showed an in vivo interaction between CRMP-2 and PLD(2). Interestingly, CRMP-2 was found to potently inhibit PLD(2) activity in a concentration-dependent manner (IC(50) = 30 nm). Overexpression studies also showed that CRMP-2 is an in vivo inhibitor of PLD(2) in PC12 cells. Moreover, increasing the concentration of semaphorin 3A, one of the repulsive axon guidance cues, showed that PLD(2) activity can be inhibited in PC12 cells. Immunocytochemistry further revealed that PLD(2) is co-localized with CRMP-2 in the distal tips of neurites, its possible action site, in differentiated PC12 cells. Taken together, our results indicate that CRMP-2 may interact directly with and inhibit neuronal PLD(2), suggesting that this inhibitory mode of regulation may play a role in neuronal pathfinding during the developmental stage.

Knowing how to navigate: mechanisms of semaphorin signaling in the nervous system

Neuronal connections are made during embryonic development with astonishing precision to ultimately form the physical basis for the central nervous system's main capacity: information processing. Over the past few decades, much has been learned about the general principles of axon guidance. A key finding to emerge is that extracellular cues play decisive roles in establishing the connections. One family of such cues, the semaphorin proteins, was first identified as repellents for navigating axons during brain wiring. Recent studies have implicated these molecules in many other processes of neuronal development, including axonal fasciculation, target selection, neuronal migration, and dendritic guidance, as well as in the remodeling and repair of the adult nervous system. It appears that responding neuronal processes sense these semaphorin signals by a family of transmembrane molecules, namely the plexins, even though neuropilins were also found to be required for mediating the interaction between plexins and class 3 semaphorins. Our understanding of the intracellular signaling machinery linking the receptors to the cytoskeleton machinery is still incomplete, but several molecules have been implicated in mediating or modulating semaphorin-induced responses. Adding to the complexity of semaphorin biology, new findings implicate semaphorins in functioning not only as signaling ligands, but also as signal-transducing receptors. Thus, semaphorins may serve as important probes for exploring the mechanisms of intercellular communication during the development and function of the nervous system.

Winging it--actin on the fly

How actin dynamics might be regulated to generate specific cellular structures during development remains something of a mystery. New insights may be gained from the recent identification of a conserved cofilin phosphatase, Slingshot, which modulates actin dynamics to help control Drosophila wing hair morphogenesis.

Stromal cell-derived factor 1alpha activates LIM kinase 1 and induces cofilin phosphorylation for T-cell chemotaxis

Stromal cell-derived factor 1 alpha (SDF-1alpha), the ligand for G-protein-coupled receptor CXCR4, is a chemotactic factor for T lymphocytes. LIM kinase 1 (LIMK1) phosphorylates cofilin, an actin-depolymerizing and -severing protein, at Ser-3 and regulates actin reorganization. We investigated the role of cofilin phosphorylation by LIMK1 in SDF-1alpha-induced chemotaxis of T lymphocytes. SDF-1alpha significantly induced the activation of LIMK1 in Jurkat human leukemic T cells and peripheral blood lymphocytes. SDF-1alpha also induced cofilin phosphorylation, actin reorganization, and activation of small GTPases, Rho, Rac, and Cdc42, in Jurkat cells. Pretreatment with pertussis toxin inhibited SDF-1alpha-induced LIMK1 activation, thus indicating that Gi protein is involved in LIMK1 activation. Expression of dominant negative Rac (DN-Rac), but not DN-Rho or DN-Cdc42, blocked SDF-1alpha-induced activation of LIMK1, which means that SDF-1alpha-induced LIMK1 activation is mediated by Rac but not by Rho or Cdc42. We used a cell-permeable peptide (S3 peptide) that contains the phosphorylation site (Ser-3) of cofilin to inhibit the cellular function of LIMK1. S3 peptide inhibited the kinase activity of LIMK1 in vitro. Treatment of Jurkat cells with S3 peptide inhibited the SDF-1alpha-induced cofilin phosphorylation, actin reorganization, and chemotactic response of Jurkat cells. These results suggest that the phosphorylation of cofilin by LIMK1 plays a critical role in the SDF-1alpha-induced chemotactic response of T lymphocytes.

Antagonistic effects of Rnd1 and RhoD GTPases regulate receptor activity in Semaphorin 3A-induced cytoskeletal collapse

The semaphorins are a large protein family that is involved in the patterning of neuronal connections in the developing nervous system of both vertebrates and invertebrates. The chemorepulsive axon guidance signal Semaphorin 3A (Sema3A) induces the depolymerization of actin filaments and the collapse of sensory growth cones by activating a receptor complex that contains a plexin as the signal-transducing subunit. Here we show that, of a large number of GTPases tested, only Rnd1 and RhoD bind the cytoplasmic domain of Plexin-A1. Recruitment of active Rnd1 is sufficient to trigger signaling by Plexin-A1, even in the absence of Sema3A, and initiates cytoskeletal collapse by activating its cytoplasmic domain. RhoD, in contrast, blocks Plexin-A1 activation by Rnd1 and repulsion of sympathetic axons by Sema3A. Thus, the antagonism of two GTPases regulates the activity of the Sema3A receptor, and activation by Rnd1 appears to be an essential step in signaling by Plexin-A1.

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

The ADF (actin-depolymerizing factor)/cofilin family is a stimulus-responsive mediator of actin dynamics. In contrast to the mechanisms of inactivation of ADF/cofilin by kinases such as LIM-kinase 1 (LIMK1), much less is known about its reactivation through dephosphorylation. Here we report Slingshot (SSH), a family of phosphatases that have the property of F actin binding. In Drosophila, loss of ssh function dramatically increased levels of both F actin and phospho-cofilin (P cofilin) and disorganized epidermal cell morphogenesis. In mammalian cells, human SSH homologs (hSSHs) suppressed LIMK1-induced actin reorganization. Furthermore, SSH and the hSSHs dephosphorylated P cofilin in cultured cells and in cell-free assays. Our results strongly suggest that the SSH family plays a pivotal role in actin dynamics by reactivating ADF/cofilin in vivo.

Collapsin response mediator protein switches RhoA and Rac1 morphology in N1E-115 neuroblastoma cells and is regulated by Rho kinase

The formation and directional guidance of neurites involves dynamic regulation of Rho family GTPases. Rac and Cdc42 promote neurite outgrowth, whereas Rho activation causes neurite retraction. Here we describe a role for collapsin response mediator protein (Crmp-2), a neuronal protein implicated in axonal outgrowth and a component of the semaphorin 3A pathway, in switching GTPase signaling when expressed in combination with either dominant active Rac or Rho. In neuroblastoma N1E-115 cells, co-expression of Crmp-2 with dominant active RhoA V14 induced Rac morphology, cell spreading and ruffling (and the formation of neurites). Conversely, co-expression of Crmp-2 with dominant active Rac1 V12 inhibited Rac morphology, and in cells already expressing Rac1 V12, Crmp-2 caused localized peripheral collapse, involving Rho (and Cdc42) activation. Rho kinase was a pivotal regulator of Crmp-2; Crmp-2 phosphorylation was required for Crmp-2/Rac1 V12 inhibition, but not Crmp-2/RhoA V14 induction, of Rac morphology. Thus Crmp-2, regulated by Rho kinase, promotes outgrowth and collapse in response to active Rho and Rac, respectively, reversing their usual morphological effects and providing a mechanism for dynamic modulation of growth cone guidance.

Plexin B mediates axon guidance in Drosophila by simultaneously inhibiting active Rac and enhancing RhoA signaling

Plexins are neuronal receptors for the repulsive axon guidance molecule Semaphorins. Previous studies showed that Plexin B (PlexB) binds directly to the active, GTP-bound form of the Rac GTPase. Here, we define a seven amino acid sequence in PlexB required for Rac(GTP) binding. The interaction of PlexB with Rac(GTP) is necessary for Plexin-mediated axon guidance in vivo. A different region of PlexB binds to RhoA. Dosage-sensitive genetic interactions suggest that PlexB suppresses Rac activity and enhances RhoA activity. Biochemical evidence indicates that PlexB sequesters Rac(GTP) from its downstream effector PAK. These results suggest a model whereby PlexB mediates repulsion by coordinately regulating two small GTPases in opposite directions: PlexB binds to Rac(GTP) and downregulates its output by blocking its access to PAK and, at the same time, binds to and increases the output of RhoA.

Plexin signaling via off-track and rho family GTPases

Two papers in this issue of Neuron examine new aspects of Semaphorin signaling via Plexin receptors. Winberg et al. present evidence that the transmembrane protein Off-track (OTK) interacts biochemically and genetically with Plexin A and is important for Sema 1a repulsive signaling. Hu et al. examine the coupling of Plexin B to Rac and RhoA and propose that Plexin B signaling involves inhibition of Rac function by direct sequestration and simultaneous activation of RhoA.

Semaphorin-mediated axonal guidance via Rho-related G proteins

For many growing axons, interaction with an extracelluar Semaphorin signal leads to growth cone collapse and axon repulsion. Semaphorin receptors composed of Neuropilins and Plexins transduce extracellular cues into changes in the growth cone actin cytoskeleton. The data implicating Rho family G proteins in Semaphorin signaling and in other axon guidance events are considered here. Recent work makes it clear that Rac1 is required for this process. In particular, there is intriguing new evidence that the Plexin receptors communicate directly with members of the Rho family GTPases, although uncertainties remain concerning how Plexins alter Rac1 function. The CRMP (collapsin response mediator protein) family is also required for Plexin-based Semaphorin signaling, and new data demonstrate direct links to Rho and Rac1-based signaling.

Cell-type-specific expression of a TESK1 promoter-linked lacZ gene in transgenic mice

Testicular protein kinase 1 (TESK1) is a serine/threonine kinase highly expressed in testicular germ cells and has the potential to phosphorylate cofilin and induce actin cytoskeletal reorganization. We examined the expression of a lacZ reporter gene linked to a 9.0-kb 5'-flanking region of TESK1 gene in transgenic mice. A high level of lacZ expression was observed in testicular germ cells only at stages after pachytene spermatocytes, the expression patterns being similar to those of TESK1 mRNA in rat testis, determined by in situ hybridization. Expression of lacZ was also detected in renal proximal tubules, cardiac myocytes, and specific neurons in the central nervous system in adult transgenic mice. Whole-mount staining revealed the expression of lacZ in neural tissues in embryonic mice. These results suggest the cell-type- and stage-specific expression of TESK1 gene and the diverse and specific physiological functions of TESK1, including those in spermatogenesis and neural development.

Inhibition of neurite extension by overexpression of individual domains of LIM kinase 1

Lin-11, Isl-1 and Mec-3 (LIM) kinases are serine/threonine kinases that phosphorylate cofilin, an actin depolymerizing protein. LIM kinases have a highly modular structure composed of two N-terminal LIM domains (LIM 1/2), a PSD-95, Dlg and ZO-1 (PDZ) domain and a C-terminal protein kinase domain. Here, we overexpressed individual domains of mouse LIM kinase 1 (LIMK1) in PC12 cells and investigated their effects on neurite outgrowth. Although none of the LIMK1 domains had an effect on spontaneous neurite outgrowth, the N-terminal LIM 1/2 domains strongly inhibited differentiation of PC12 cells after stimulation with both nerve growth factor (NGF) and the Rho-kinase inhibitor Y-27632. In contrast, the overexpressed PDZ domain reduced neurite outgrowth only when differentiation had been induced by Y-27632, but not by NGF. Our data suggest that the different non-catalytic N-terminal domains of LIMK1 contribute to the regulation of neurite extension by using distinct signal transduction pathways.