Rac regulates phosphorylation of the myosin-II heavy chain, actinomyosin disassembly and cell spreading

Crosstalk between small GTPases and polarity proteins in cell polarization

Cell polarization is crucial for the development of multicellular organisms, and aberrant cell polarization contributes to various diseases, including cancer. How cell polarity is established and how it is maintained remain fascinating questions. Conserved proteins of the partitioning defective (PAR), Scribble and Crumbs complexes guide the establishment of cell polarity in various organisms. Moreover, GTPases that regulate actin cytoskeletal dynamics have been implicated in cell polarization. Recent findings provide insights into polarization mechanisms and show intriguing crosstalk between small GTPases and members of polarity complexes in regulating cell polarization in different cellular contexts and cell types.

Antagonistic regulation of neurite morphology through Gq/G11 and G12/G13

The induction of neurite retraction and growth cone collapse via G-protein-coupled receptors is involved in developmental as well as regenerative processes. The role of individual G-protein-mediated signaling processes in the regulation of neurite morphology is still incompletely understood. Using primary neurons from brains lacking Galpha(q)/Galpha(11) or Galpha(12)/Galpha(13), we show here that G(12)/G(13)-mediated signaling is absolutely required for neurite retraction and growth cone collapse induced by the blood-borne factors lysophosphatidic acid and thrombin. Interestingly, the effects of lysophosphatidic acid were mediated mainly by G(13), whereas thrombin effects required G(12). Surprisingly, lack of Galpha(q)/Galpha(11) resulted in overshooting responses to both stimuli, indicating that G(q)/G(11)-mediated signaling most likely via activation of Rac antagonizes the effects of G(12)/G(13).

Myosin IIC: a third molecular motor driving neuronal dynamics

Neuronal dynamics result from the integration of forces developed by molecular motors, especially conventional myosins. Myosin IIC is a recently discovered nonsarcomeric conventional myosin motor, the function of which is poorly understood, particularly in relation to the separate but coupled activities of its close homologues, myosins IIA and IIB, which participate in neuronal adhesion, outgrowth and retraction. To determine myosin IIC function, we have applied a comparative functional knockdown approach by using isoform-specific antisense oligodeoxyribonucleotides to deplete expression within neuronally derived cells. Myosin IIC was found to be critical for driving neuronal process outgrowth, a function that it shares with myosin IIB. Additionally, myosin IIC modulates neuronal cell adhesion, a function that it shares with myosin IIA but not myosin IIB. Consistent with this role, myosin IIC knockdown caused a concomitant decrease in paxillin-phospho-Tyr118 immunofluorescence, similar to knockdown of myosin IIA but not myosin IIB. Myosin IIC depletion also created a distinctive phenotype with increased cell body diameter, increased vacuolization, and impaired responsiveness to triggered neurite collapse by lysophosphatidic acid. This novel combination of properties suggests that myosin IIC must participate in distinctive cellular roles and reinforces our view that closely related motor isoforms drive diverse functions within neuronal cells.

Prohibitin-1 maintains the angiogenic capacity of endothelial cells by regulating mitochondrial function and senescence

Prohibitin 1 (PHB1) is a highly conserved protein that is mainly localized to the inner mitochondrial membrane and has been implicated in regulating mitochondrial function in yeast. Because mitochondria are emerging as an important regulator of vascular homeostasis, we examined PHB1 function in endothelial cells. PHB1 is highly expressed in the vascular system and knockdown of PHB1 in endothelial cells increases mitochondrial production of reactive oxygen species via inhibition of complex I, which results in cellular senescence. As a direct consequence, both Akt and Rac1 are hyperactivated, leading to cytoskeletal rearrangements and decreased endothelial cell motility, e.g., migration and tube formation. This is also reflected in an in vivo angiogenesis assay, where silencing of PHB1 blocks the formation of functional blood vessels. Collectively, our results provide evidence that PHB1 is important for mitochondrial function and prevents reactive oxygen species-induced senescence and thereby maintains the angiogenic capacity of endothelial cells.

Roles of P21-activated kinases and associated proteins in epithelial wound healing

The primary function of epithelia is to provide a barrier between the extracellular environment and the interior of the body. Efficient epithelial repair mechanisms are therefore crucial for homeostasis. The epithelial wound-healing process involves highly regulated morphogenetic changes of epithelial cells that are driven by dynamic changes of the cytoskeleton. P21-activated kinases are serine/threonine kinases that have emerged as important regulators of the cytoskeleton. These kinases, which are activated downsteam of the Rho GTPases Rac and cd42, were initially mostly implicated in the regulation of cell migration. More recently, however, these kinases were shown to have many additional functions that are relevant to the regulation of epithelial wound healing. Here, we provide an overview of the morphogenetic changes of epithelial cells during wound healing and the many functions of p21-activated kinases in these processes.

The phosphorylation of myosin II at the Ser1 and Ser2 is critical for normal platelet-derived growth factor induced reorganization of myosin filaments

Phosphorylation of the regulatory light chain of myosin II (MLC(20)) at the activation sites promotes both the motor activity and the filament formation of myosin II, thus playing an important role in various cell motile processes. In contrast, the physiological function of phosphorylation of MLC(20) at the inhibitory sites is unknown. Here we report for the first time the function of the inhibitory site phosphorylation in the cells. We successfully produced the antibodies specifically recognizing the phosphorylation sites of MLC(20) at Ser1, and the platelet-derived growth factor (PDGF)-induced change in the phosphorylation at the Ser1 was monitored. The phosphorylation of MLC(20) at the Ser1 significantly increased during the PDGF-induced actin cytoskeletal reorganization. PDGF disassembled the stress fibers, and this was attenuated with the expression of unphosphorylatable MLC(20) at the Ser1/Ser2 phosphorylation sites. The present results suggest that the down-regulation of myosin II activity achieved by the phosphorylation at the Ser1/Ser2 sites plays an important role in the normal reorganization of actomyosin filaments triggered by PDGF receptor stimulation.

Nectin and nectin-like molecules: biology and pathology

Nectins and nectin-like molecules (Necls) are structurally related transmembrane proteins primarily involved in cell adhesion. Nectins and afadin, the adaptor or anchoring protein, stabilize the epithelium and endothelium and establish apical-basal polarity of epithelial cells, independently or in cooperation with other cell adhesion molecules. Necls facilitate cell-cell communication implicated in cell movement and proliferation, immune responses, and cancer cell phenotypes. Necls interact with nectins and specific ligands at cell-cell contacts, whereas Necls associate with integrin alpha v beta 3 and growth factor receptors on the same cell surface. Besides their roles in cell adhesion, nectins regulate the activities of Rho family small G proteins which play critical roles in maintaining the apical junctions of epithelial cells through reorganization of the actin cytoskeleton. Since mice lacking the Rho GDP-dissociation inhibitor (GDI)alpha show massive proteinuria and degeneration of renal epithelial cells, nectins and other cell adhesion molecules may play roles in the structural and functional aspects of renal diseases. Here we summarize our knowledge of nectins and Necls and discuss cell adhesion biology in the kidney.

The PP2A-associated protein alpha4 plays a critical role in the regulation of cell spreading and migration

Compared with kinases, the role of protein phosphatases in regulating biological functions is less well understood. Here we show that alpha4, a non-catalytic subunit of the protein phosphatase 2A, plays a major role in the control of cell spreading, migration, and cytoskeletal architecture. Fibroblasts lacking alpha4 were impaired in their ability to spread and migrate compared with wild-type cells, whereas enforced expression of alpha4 promoted cell spreading and migration. These effects were not restricted to fibroblasts. Using a T cell-specific alpha4 transgenic mouse model, increased alpha4 expression was found to increase lymphocyte motility and chemotaxis. Elevated alpha4 expression results in an increase in the GTP-bound state of Rac1, and GTP-bound Rac1 was dramatically reduced in alpha4-deficient cells. A constitutively active mutant of Rac1 rescued the defects of cell spreading and migration caused by alpha4 deletion, while inhibition of Rac1 blocked the ability of alpha4 to promote cell migration. Together, these data define a novel role for the protein phosphatase 2A regulatory subunit alpha4 in the regulation of cell spreading and migration.

RhoE participates in the stimulation of the inflammatory response induced by ethanol in astrocytes

Astroglial cells are involved in the neuropathogenesis of several inflammatory diseases of the brain, where the activation of inflammatory mediators and cytokines plays an important role. We have previously demonstrated that ethanol up-regulates inflammatory mediators in both brain and astroglial cells. Since Rho GTPases are involved in inflammatory responses of astrocytes where loss of stress fibers takes place and RhoE/Rnd3 disorganizes the actin cytoskeleton, the aim of the present study was to investigate the implication of this protein in the stimulation of inflammatory signaling induced by ethanol. Our findings show that RhoE expression induces a decrease in both RhoA and Rac. In addition, RhoE not only induces actin cytoskeleton disorganization but it also stimulates both the IRAK/ERK/NF-kappaB pathway and the COX-2 expression associated with the inflammatory response in these cells. Our results also show that ethanol exposure induces RhoE signaling in astrocytes. Preincubation of astrocytes with GF109203X, an inhibitor of PKCs, reduces the RhoE levels and abolishes the ethanol-induced activation of IRAK, NF-kappaB and the COX-2 expression. Furthermore, RhoE overexpression restores ethanol responses in astrocytes treated with the PKCs inhibitor. Altogether, our findings suggest that this small GTPase is involved in the stimulation of the inflammatory signaling induced by ethanol in astrocytes. These findings provide new insights into the molecular mechanism involved in the inflammatory responses in astrocytes.

RhoGDI-1 Modulation of the Activity of Monomeric RhoGTPase RhoA Regulates Endothelial Barrier Function in Mouse Lungs

Rho family GTPases have been implicated in the regulation of endothelial permeability via their actions on actin cytoskeletal organization and integrity of interendothelial junctions. In cell culture studies, activation of RhoA disrupts interendothelial junctions and increases endothelial permeability, whereas activation of Rac1 and Cdc42 enhances endothelial barrier function by promoting the formation of restrictive junctions. The primary regulators of Rho proteins, guanine nucleotide dissociation inhibitors (GDIs), form a complex with the GDP-bound form of the Rho family of monomeric G proteins, and thus may serve as a nodal point regulating the activation state of RhoGTPases. In the present study, we addressed the in vivo role of RhoGDI-1 in regulating pulmonary microvascular permeability using RhoGDI-1 −/− mice. We observed that basal endothelial permeability in lungs of RhoGDI-1 −/− mice was 2-fold greater than wild-type mice. This was the result of opening of interendothelial junctions in lung microvessels which are normally sealed. The activity of RhoA (but not of Rac1 or Cdc42) was significantly increased in RhoGDI-1 −/− lungs as well as in cultured endothelial cells on downregulation of RhoGDI-1 with siRNA, consistent with RhoGDI-1–mediated modulation RhoA activity. Thus, RhoGDI-1 by repressing RhoA activity regulates lung microvessel endothelial barrier function in vivo. In this regard, therapies augmenting endothelial RhoGDI-1 function may be beneficial in reestablishing the endothelial barrier and lung fluid balance in lung inflammatory diseases such as acute respiratory distress syndrome.

Redox signalling in anchorage-dependent cell growth

Current data have provided new perspectives concerning the regulation of non-transformed cell proliferation in response to both soluble growth factors and to adhesive cues. Non-transformed cells are anchorage dependent for the execution of the mitotic program and cannot avoid the concomitant signals starting from mitogenic molecules, as growth factors, and adhesive agents belonging to extracellular matrix. Reactive oxygen species play a key role during both growth factor and integrin receptor signalling and these second messengers are recognised to have a synergistic function for anchorage-dependent growth signalling. Redox regulated proteins include protein tyrosine phosphatases and protein tyrosine kinases, although with opposite regulation of their enzymatic activity, and cytoskeletal proteins as beta-actin. In this review we support a role of ROS as key second messengers granting a proper executed mitosis for anchorage-dependent cells, through redox regulation of several downstream targets. Deregulation of these redox pathways may help to guide transformed cells to elude the native apoptotic response to abolishment of signals started by cell/ECM contact, sustaining ectopic anchorage-independent cancer cell growth.

The May-Hegglin anomaly gene MYH9 is a negative regulator of platelet biogenesis modulated by the Rho-ROCK pathway

The gene implicated in the May-Hegglin anomaly and related macrothrombocytopenias, MYH9, encodes myosin-IIA, a protein that enables morphogenesis in diverse cell types. Defective myosin-IIA complexes are presumed to perturb megakaryocyte (MK) differentiation or generation of proplatelets. We observed that Myh9(-/-) mouse embryonic stem (ES) cells differentiate into MKs that are fully capable of proplatelet formation (PPF). In contrast, elevation of myosin-IIA activity, by exogenous expression or by mimicking constitutive phosphorylation of its regulatory myosin light chain (MLC), significantly attenuates PPF. This effect occurs only in the presence of myosin-IIA and implies that myosin-IIA influences thrombopoiesis negatively. MLC phosphorylation in MKs is regulated by Rho-associated kinase (ROCK), and consistent with our model, ROCK inhibition enhances PPF. Conversely, expression of AV14, a constitutive form of the ROCK activator Rho, blocks PPF, and this effect is rescued by simultaneous expression of a dominant inhibitory MLC form. Hematopoietic transplantation studies in mice confirm that interference with the putative Rho-ROCK-myosin-IIA pathway selectively decreases the number of circulating platelets. Our studies unveil a key regulatory pathway for platelet biogenesis and hint at Sdf-1/CXCL12 as one possible extracellular mediator. The unexpected mechanism for Myh9-associated thrombocytopenia may lead to new molecular approaches to manipulate thrombopoiesis.

Rac activity is polarized and regulates meiotic spindle stability and anchoring in mammalian oocytes

Mammalian meiotic divisions are asymmetrical and generate a large oocyte and two small polar bodies. This asymmetry results from the anchoring of the meiotic spindle to the oocyte cortex and subsequent cortical reorganization, but the mechanisms involved are poorly understood. We investigated the role of Rac in oocyte meiosis by using a fluorescent reporter for Rac-GTP. We find that Rac-GTP is polarized in the cortex overlying the meiotic spindle. Polarization of Rac activation occurs during spindle migration and is promoted by the proximity of chromatin to the cortex. Inhibition of Rac during oocyte maturation caused a permanent block at prometaphase I and spindle elongation. In metaphase II-arrested oocytes, Rac inhibition caused the spindle to detach from the cortex and prevented polar body emission after activation. These results demonstrate that Rac-GTP plays a major role in oocyte meiosis, via the regulation of spindle stability and anchoring to the cortex.

Myosin II and mechanotransduction: a balancing act

Adherent cells respond to mechanical properties of the surrounding extracellular matrix. Mechanical forces, sensed at specialized cell-matrix adhesion sites, promote actomyosin-based contraction within the cell. By manipulating matrix rigidity and adhesion strength, new roles for actomyosin contractility in the regulation of basic cellular functions, including cell proliferation, migration and stem cell differentiation, have recently been discovered. These investigations demonstrate that a balance of forces between cell adhesion on the outside and myosin II-based contractility on the inside of the cell controls many aspects of cell behavior. Disturbing this balance contributes to the pathogenesis of various human diseases. Therefore, elaborate signaling networks have evolved that modulate myosin II activity to maintain tensional homeostasis. These include signaling pathways that regulate myosin light chain phosphorylation as well as myosin II heavy chain interactions.

Rac1 and Rac3 have opposing functions in cell adhesion and differentiation of neuronal cells

Rac1 and Rac3 are highly homologous members of the Rho small GTPase family. Rac1 is ubiquitously expressed and regulates cell adhesion, migration and differentiation in various cell types. Rac3 is primarily expressed in brain and may therefore have a specific function in neuronal cells. We found that depletion of Rac1 by short interference RNA leads to decreased cell-matrix adhesions and cell rounding in neuronal N1E-115 cells. By contrast, depletion of Rac3 induces stronger cell adhesions and dramatically increases the outgrowth of neurite-like protrusions, suggesting opposite functions for Rac1 and Rac3 in neuronal cells. Consistent with this, overexpression of Rac1 induces cell spreading, whereas overexpression of Rac3 results in a contractile round morphology. Rac1 is mainly found at the plasma membrane, whereas Rac3 is predominantly localized in the perinuclear region. Residues 185-187, present in the variable polybasic rich region at the carboxyl terminus are responsible for the difference in phenotype induced by Rac1 and Rac3 as well as for their different intracellular localization. The Rac1-opposing function of Rac3 is not mediated by or dependent on components of the RhoA signaling pathway. It rather seems that Rac3 exerts its function through negatively affecting integrin-mediated cell-matrix adhesions. Together, our data reveal that Rac3 opposes Rac1 in the regulation of cell adhesion and differentiation of neuronal cells.

Big roles for small GTPases in the control of directed cell movement

Small GTPases are involved in the control of diverse cellular behaviours, including cellular growth, differentiation and motility. In addition, recent studies have revealed new roles for small GTPases in the regulation of eukaryotic chemotaxis. Efficient chemotaxis results from co-ordinated chemoattractant gradient sensing, cell polarization and cellular motility, and accumulating data suggest that small GTPase signalling plays a central role in each of these processes as well as in signal relay. The present review summarizes these recent findings, which shed light on the molecular mechanisms by which small GTPases control directed cell migration.

Cells move when ions and water flow

Cell migration is a process that plays an important role throughout the entire life span. It starts early on during embryogenesis and contributes to shaping our body. Migrating cells are involved in maintaining the integrity of our body, for instance, by defending it against invading pathogens. On the other side, migration of tumor cells may have lethal consequences when tumors spread metastatically. Thus, there is a strong interest in unraveling the cellular mechanisms underlying cell migration. The purpose of this review is to illustrate the functional importance of ion and water channels as part of the cellular migration machinery. Ion and water flow is required for optimal migration, and the inhibition or genetic ablation of channels leads to a marked impairment of migration. We briefly touch cytoskeletal mechanisms of migration as well as cell-matrix interactions. We then present some general principles by which channels can affect cell migration before we discuss each channel group separately.

Phosphorylation of nonmuscle myosin heavy chain IIA on Ser1917 is mediated by protein kinase C beta II and coincides with the onset of stimulated degranulation of RBL-2H3 mast cells

Dynamic remodeling of the actinomyosin cytoskeleton is integral to many biological processes. It is regulated, in part, by myosin phosphorylation. Nonmuscle myosin H chain IIA is phosphorylated by protein kinase C (PKC) on Ser(1917). Our aim was to determine the PKC isoform specificity of this phosphorylation event and to evaluate its potential role in regulated secretion. Using an Ab against the phosphorylated form of Ser(1917), we show that this site is not phosphorylated in unstimulated RBL-2H3 mast cells. The physiological stimulus, Ag, or the pharmacological activators, PMA plus A23187, induced Ser(1917) phosphorylation with a time course coincident with the onset of granule mediator secretion. Dephosphorylation at this site occurred as Ag-stimulated secretion declined from its peak, but dephosphorylation was delayed in cells activated with PMA plus A23187. Phosphate incorporation was also enhanced by PMA alone and by inhibition of protein phosphatase 2A. Gö6976, an inhibitor of conventional PKC isoforms, abolished secretion and Ser(1917) phosphorylation with similar dose dependencies consistent with involvement of either PKCalpha or PKCbeta. Phorbol ester-stimulated Ser(1917) phosphorylation was reconstituted in HEK-293 cells (which lack endogenous PKCbeta) by overexpression of both wild-type and constitutively active PKCbetaII but not the corresponding PKCbetaI or PKCalpha constructs. A similar selectivity for PKCbetaII overexpression was also observed in MIN6 insulinoma cells infected with recombinant PKC wild-type adenoviruses. Our results implicate PKC-dependent phosphorylation of myosin H chain IIA in the regulation of secretion in mast cells and suggest that Ser(1917) phosphorylation might be a marker of PKCbetaII activation in diverse cell types.

Nonmuscle myosin IIA-dependent force inhibits cell spreading and drives F-actin flow

Nonmuscle myosin IIA (NMM-IIA) is involved in the formation of focal adhesions and neurite retraction. However, the role of NMM-IIA in these functions remains largely unknown. Using RNA interference as a tool to decrease NMM-IIA expression, we have found that NMM-IIA is the major myosin involved in traction force generation and retrograde F-actin flow in mouse embryonic fibroblast cells. Quantitative analyses revealed that approximately 60% of traction force on fibronectin-coated surfaces is contributed by NMM-IIA and approximately 30% by NMM-IIB. The retrograde F-actin flow decreased dramatically in NMM-IIA-depleted cells, but seemed unaffected by NMM-IIB deletion. In addition, we found that depletion of NMM-IIA caused cells to spread at a higher rate and to a greater area on fibronectin substrates during the early spreading period, whereas deletion of NMM-IIB appeared to have no effect on spreading. The distribution of NMM-IIA was concentrated on the dorsal surface and approached the ventral surface in the periphery, whereas NMM-IIB was primarily concentrated around the nucleus and to a lesser extent at the ventral surface in cell periphery. Our results suggest that NMM-IIA is involved in generating a coherent cytoplasmic contractile force from one side of the cell to the other through the cross-linking and the contraction of dorsal actin filaments.

Redox regulation of beta-actin during integrin-mediated cell adhesion

Redox sensitivity of actin toward an exogenous oxidative stress has recently been reported. We report here the first evidence of in vivo actin redox regulation by a physiological source of reactive oxygen species, specifically those species generated by integrin receptors during cell adhesion. Actin oxidation takes place via the formation of a mixed disulfide between cysteine 374 and glutathione; this modification is essential for spreading and for cytoskeleton organization. Impairment of actin glutathionylation, either through GSH depletion or expression of the C374A redox-insensitive mutant, greatly affects cell spreading and the formation of stress fibers, leading to inhibition of the disassembly of the actinomyosin complex. These data suggest that actin glutathionylation is essential for cell spreading and cytoskeleton organization and that it plays a key role in disassembly of actinomyosin complex during cell adhesion.

Beta8 integrin binds Rho GDP dissociation inhibitor-1 and activates Rac1 to inhibit mesangial cell myofibroblast differentiation

Alpha(v)beta8 integrin expression is restricted primarily to kidney, brain, and placenta. Targeted alpha(v) or beta8 deletion is embryonic lethal due to defective placenta and brain angiogenesis, precluding investigation of kidney alpha(v)beta8 function. We find that kidney beta8 is localized to glomerular mesangial cells, and expression is decreased in mouse models of glomerulosclerosis, suggesting that beta8 regulates normal mesangial cell differentiation. To interrogate beta8 signaling pathways, yeast two-hybrid and co-precipitation studies demonstrated beta8 interaction with Rho guanine nucleotide dissociation inhibitor-1 (GDI). Selective beta8 stimulation enhanced beta8-GDI interaction as well as Rac1 (but not RhoA) activation and lamellipodia formation. Mesangial cells from itgb8-/- mice backcrossed to a genetic background that permitted survival, or gdi-/- mice, which develop glomerulosclerosis, demonstrated RhoA (but not Rac1) activity and alpha-smooth muscle actin assembly, which characterizes mesangial cell myofibroblast transformation in renal disease. To determine whether Rac1 directly modulates RhoA-associated myofibroblast differentiation, mesangial cells were transduced with inhibitory Rac peptide fused to human immunodeficiency virus-Tat, resulting in enhanced alpha-smooth muscle actin organization. We conclude that the beta8 cytosolic tail in mesangial cells organizes a signaling complex that culminates in Rac1 activation to mediate wild-type differentiation, whereas decreased beta8 activation shifts mesangial cells toward a RhoA-dependent myofibroblast phenotype.

Rac1 and RhoA promote neurite outgrowth through formation and stabilization of growth cone point contacts

Growth cone advance depends on coordinated membrane protrusion and adhesion to the extracellular matrix. Although many studies have addressed the mechanisms responsible for membrane protrusion, the assembly of integrin-dependent adhesion sites known as point contacts remains poorly understood in growth cones. We show balanced Rac1 activity controls both leading edge protrusion and point contact dynamics during neurite outgrowth. Immunocytochemistry and live imaging of paxillin-green fluorescent protein (GFP) showed that inhibiting Rac1 blocked point contact formation, whereas Rac1 overactivation produced small, unstable point contacts. Both inhibition and overactivation of Rac1 reduced the persistence of lamellar protrusions and neurite outgrowth. Inhibition of ROCK (Rho kinase), a RhoA effector, perturbed protrusion and point contact dynamics similar to Rac1 overactivation. Moreover, the repulsive guidance cue Semaphorin 3A, which signals through Rac1, destabilizes point contacts. Together, our data suggest that coordinated Rho GTPase activities regulate neurite outgrowth through point contact formation and stabilization of membrane protrusion.

PAK1 and aPKCzeta regulate myosin II-B phosphorylation: a novel signaling pathway regulating filament assembly

Many signaling pathways regulate the function of the cellular cytoskeleton. Yet we know very little about the proteins involved in the cross-talk between the signaling and the cytoskeletal systems. Here we show that myosin II-B, an important cytoskeletal protein, resides in a complex with p21-activated kinase 1 (PAK1) and atypical protein kinase C (PKC) zeta (aPKCzeta) and that the interaction between these proteins is EGF-dependent. We further show that PAK1 is involved in aPKCzeta phosphorylation and that aPKCzeta phosphorylates myosin II-B directly on a specific serine residue in an EGF-dependent manner. This latter phosphorylation is specific to isoform B of myosin II, and it leads to slower filament assembly of myosin II-B. Furthermore, a decrease in aPKCzeta expression in the cells alters myosin II-B cellular organization. Our finding of a new signaling pathway involving PAK1, aPKCzeta, and myosin II-B, which is implicated in myosin II-B filament assembly and cellular organization, provides an important link between the signaling system and cytoskeletal dynamics.

p190-RhoGAP as an integral component of the Tiam1/Rac1-induced downregulation of Rho

The Rho family of small GTPases plays a central role in intracellular signal transduction, particularly in reorganization of the actin cytoskeleton. Rho activity induces cell contractility, whereas Rac promotes cellular protrusion, which counteracts Rho signaling. In this regard, the reciprocal balance between these GTPases determines cell morphology and migratory behavior. Here we demonstrate that Tiam1/Rac1 signaling is able to antagonize Rho activity directly at the GTPase level in COS-7 cells. p190-RhoGAP plays a central regulatory role in this signaling pathway. Interfering with its activation by Src-kinase-dependent tyrosine phosphorylation or its recruitment to the membrane through interaction with the SH2 domains of p120-RasGAP blocks the Tiam1-mediated rapid downregulation of Rho. This process is mediated by Rac1, but not by Rac2 or Rac3 isoforms. Our data provide evidence for a biochemical pathway of the reciprocal regulation of two related small GTPases, which are key elements in cell migration.

FRET imaging in nerve growth cones reveals a high level of RhoA activity within the peripheral domain

Rho-family GTPases play a central role in the regulation of neuronal morphogenesis. In growth cones, for example, Rho GTPases transduce extracellular stimuli into structural changes such as filopodia and lamellipodia. Although it is generally accepted that Rac1/Cdc42 and RhoA are positive and negative regulators of neurite outgrowth, respectively, the role of each Rho-family member in neuronal morphogenesis may change according to the cell context. At present, the mechanism underlying this complexity is largely unknown. In growth cones, this is partly due to a lack of information on the distribution of active Rho GTPases. Here, we visualized RhoA/Rac1/Cdc42 activities during laminin-induced growth cone advance of DRG neurons and N1E-115 neuroblastoma cells using probes based on fluorescence/Förster resonance energy transfer. The Rac1 and Cdc42 activities were high in the peripheral domain (P-domain) of growth cones. Active Rac1 was uniformly detected throughout the P-domain, whereas Cdc42 activity increased gradually toward the growth cone edge. Against a model involving RhoA down-regulation at the periphery of protruding growth cones, we found that the RhoA activity was higher in the P-domain than in the central domain and axon shaft, and that a high level of RhoA activity was maintained in the extending part of growth cones. In lysophosphatidic acid-treated N1E-115 cells, well-developed neurites with growth cones showed RhoA activation, but sustained their extended morphology until they were drawn toward the contracting somata. On the other hand, suppression of RhoA activity by C3 exoenzyme led to loss or deformation of actin bundles in the growth cones. Thus, RhoA activation in the shaft results in neurite retraction, whereas high RhoA activity in the P-domain is necessary to retain the spread morphology of nerve growth cone.

TRPM7, a novel regulator of actomyosin contractility and cell adhesion

Actomyosin contractility regulates various cell biological processes including cytokinesis, adhesion and migration. While in lower eukaryotes, alpha-kinases control actomyosin relaxation, a similar role for mammalian alpha-kinases has yet to be established. Here, we examined whether TRPM7, a cation channel fused to an alpha-kinase, can affect actomyosin function. We demonstrate that activation of TRPM7 by bradykinin leads to a Ca(2+)- and kinase-dependent interaction with the actomyosin cytoskeleton. Moreover, TRPM7 phosphorylates the myosin IIA heavy chain. Accordingly, low overexpression of TRPM7 increases intracellular Ca2+ levels accompanied by cell spreading, adhesion and the formation of focal adhesions. Activation of TRPM7 induces the transformation of these focal adhesions into podosomes by a kinase-dependent mechanism, an effect that can be mimicked by pharmacological inhibition of myosin II. Collectively, our results demonstrate that regulation of cell adhesion by TRPM7 is the combined effect of kinase-dependent and -independent pathways on actomyosin contractility.

Effects of constitutively active GTPases on fibroblast behavior

The GTP-binding proteins RhoA, Cdc42 and Rac1 regulate the organization and turnover of the cytoskeleton and cell-matrix adhesions, structures bridging cells to their support, and translating forces, external or generated within the cell. To investigate the specific requirements of Rho GTPases for biomechanical activities of clonal cell populations, we compared side-by-side stable lines of human fibroblasts expressing constitutively active (CA) RhoA, Cdc42 or Rac1. There was no marked effect of any CA GTPase on cell adhesion to different extracellular matrix proteins. Cell spreading was CA Rho GTPase specific and independent of the extracellular matrix proteins allowing adhesion. Mechanical properties were dramatically restricted by CA RhoA on bi- and in tri-dimensional surroundings, were boosted by CA Rac1 on bi-dimensional surroundings only, and were not or marginally affected by CA Cdc42. In conclusion, the action of Rho GTPases appears to depend on the task cells are performing.

Laminin alpha5 is required for dental epithelium growth and polarity and the development of tooth bud and shape

In tooth development, the oral ectoderm and mesenchyme coordinately and reciprocally interact through the basement membrane for their growth and differentiation to form the proper shape and size of the tooth. Laminin alpha5 subunit-containing laminin-10/11 (LM-511/521) is the major laminin in the tooth germ basement membrane. Here, we have examined the role of laminin alpha5 (Lama5) in tooth development using laminin alpha5-null mouse primary dental epithelium and tooth germ organ cultures. Lama5-null mice develop a small tooth germ with defective cusp formation and have reduced proliferation of dental epithelium. Also, cell polarity and formation of the monolayer of the inner dental epithelium are disturbed. The enamel knot, a signaling center for tooth germ development, is defective, and there is a significant reduction of Shh and Fgf4 expression in the dental epithelium. In the absence of laminin alpha5, the basement membrane in the inner dental epithelium becomes discontinuous. In normal mice, integrin alpha6beta4, a receptor for laminin alpha5, is strongly localized at the basal layer of the epithelium, whereas in mutant mice, integrin alpha6beta4 is expressed around the cell surface. In primary dental epithelium culture, laminin-10/11 promotes cell growth, spreading, and filopodia-like microspike formation. This promotion is inhibited by anti-integrin alpha6 and beta4 antibodies and by phosphatidylinositol 3-kinase inhibitors and dominant negative Rho-GTPase family proteins Cdc42 and Rac. In organ culture, anti-integrin alpha6 antibody and wortmannin reduce tooth germ size and shape. Our studies demonstrate that laminin alpha5 is required for the proliferation and polarity of basal epithelial cells and suggest that the interaction between laminin-10/11-integrin alpha6beta4 and the phosphatidylinositol 3-kinase-Cdc42/Rac pathways play an important role in determining the size and shape of tooth germ.

Evidences that β1 integrin and Rac1 are involved in the overriding effect of laminin on myelin-associated glycoprotein inhibitory activity on neuronal cells

During neurite elongation, migrating growth cones encounter both permissive and inhibitory substrates, such as laminin and MAG (myelin-associated glycoprotein), respectively. Here, we demonstrated on two neuronal cell lines (PC12 and N1E-115), that laminin and collagen hampered, in a dose-dependent manner, MAG inhibitory activity on several integrin functions, i.e., neurite growth, cell adhesion and cell spreading. Using a function blocking antibody, in PC12 cells, we showed that alpha1beta1 integrin is required in these phenomena. In parallel, we observed that MAG perturbs actin dynamics and lamellipodia formation during early steps of cell spreading. This seemed to be independent of RhoA activation, but dependent of Rac-1 inhibition by MAG. Laminin overrode MAG activity on actin and prevented MAG inhibition NGF-induced Rac1 activation. In conclusion, we evidenced antagonistic signaling between MAG receptors and beta1 integrins, in which Rac-1 may have a central function.

Regulation of phagocytosis by Rho GTPases

Phagocytosis is the mechanism of internalization used by specialized cells such as macrophages, dendritic cells, and neutrophils to internalize, degrade, and eventually present peptides derived from particulate antigens. The phagocytic process comprises several sequential and complex events initiated by the recognition ofligands on the surface of the particles by specific receptors on the surface of the phagocytic cells. Receptor clustering at the attachment site generates a phagocytic signal that in turn leads to local polymerization of actin filaments and to particle internalization. Depending on the particles and receptors involved, it appears that the structures and mechanisms associated with particle ingestion are diverse. However, work during the past few years has highlighted the importance of small GTP-binding proteins of the Rho family in various types of phagocytosis. As reviewed here, Rho family GTPases, their activators, and their downstream effectors control the local reorganization of the actin cytoskeleton beneath bound particles.

Mechanisms of T cell motility and arrest: deciphering the relationship between intra- and extracellular determinants

T lymphocytes are capable of rapid motility in vitro and in vivo. Upon antigen recognition, they may stop crawling and form a stable cell-cell contact called the 'immunological synapse' (IS). However, it is becoming clear that this outcome may not occur with the reliability that was once presumed. T cells, particularly naïve cells, are apparently triggered partly 'on the fly' during short contacts with peptide-MHC (pMHC) bearing antigen-presenting cells (APCs) and are also influenced in both activity and synapse duration by a multitude of external cues. Underlying the emerging issues is a paucity of data concerning the cell biology of T lymphocytes. Here, we review the molecular mechanisms of crawling and adhesion versus the various potential modes of 'stopping' in T lymphocytes. Both motility and arrest involve similar processes: adhesion, actin elongation and internal tension control, but with different coordination. We will attempt to integrate this with the known and potential external cues that signal for T cell motility versus stopping to form a synapse in vivo. Finally, we discuss how this interplay may give rise to unexpectedly complex motile and morphological behavior.

Multifaceted role of Rho proteins in angiogenesis

The Rho family of GTPases is part of the Ras superfamily. The Rho, Rac, and Cdc42 members of the family are present in mammalian cells and have been the subject of attention of researchers due to their vast spectrum of functions. Rac 1, Cdc42, and RhoA are well-known for their role in the regulation of the actin cytoskeleton in promoting the formation of lamellipodia, filopodia, and stress fibers, respectively. The Rho proteins also participate in the control of cell growth, motility, cell-cell adhesions, morphogenesis, cytoskeletal dynamics, and cellular trafficking. The mechanisms for eliciting these functions have become clearer during the last decade. Concordant with their roles in multiple processes of cellular control, the Rho proteins have been shown to be involved in tumor growth, progression, metastasis, and now angiogenesis.

Signaling via the angiotensin-converting enzyme results in the phosphorylation of the nonmuscle myosin heavy chain IIA

The phosphorylation of the short C-terminal cytoplasmic domain of the somatic angiotensin-converting enzyme (ACE) is involved in the regulation of enzyme shedding. We determined whether the phosphorylation of the cytoplasmic domain of ACE (ACEct) on Ser1270 regulates the cleavage/secretion of the enzyme by affecting its association with other proteins. ACE was associated with beta-actin and the nonmuscle myosin heavy chain IIA (MYH9) in endothelial cells, as determined by coimmunoprecipitation experiments as well as an ACEct affinity column. The ACE-associated MYH9 immunoprecipitated from (32)P-labeled endothelial cells was basally phosphorylated and cell stimulation with ACE inhibitors, or with bradykinin, increased the phosphorylation of MYH9. Casein kinase 2 (CK2) but not protein kinase C phosphorylated MYH9 in vitro, CK2 coprecipitated with MYH9 from endothelial cells and the phosphorylation of MYH9 in intact cells paralleled the phosphorylation of ACE on Ser1270 by CK2. The CK2 inhibitor 5,6-dichloro-1-beta-d-ribofuranosylbenzimidazole attenuated the phosphorylation of ACE and MYH9, disrupted their association, and enhanced the cleavage/secretion of ACE from the plasma membrane. Cytochalasin D decreased the interaction between ACE and MYH9 and stimulated ACE shedding. Although MYH9 was still able to associate with residual amounts of a nonphosphorylatable S1270A ACE mutant, no ACE inhibitor-induced increase in MYH9 phosphorylation could be detected in S1270A-expressing cells. These data indicate that the interaction of ACE with MYH9 determines ACE shedding and is modulated by phosphorylation processes. Furthermore, because ACE inhibitors affect the phosphorylation of MYH9, the phosphorylation of this class II myosin might contribute to the phenomenon of ACE signaling in endothelial cells.

Cytotoxic necrotizing factor 1 hinders skeletal muscle differentiation in vitro by perturbing the activation/deactivation balance of Rho GTPases

The current knowledge assigns a crucial role to the Rho GTPases family (Rho, Rac, Cdc42) in the complex transductive pathway leading to skeletal muscle cell differentiation. Their exact function in myogenesis, however, remains largely undefined. The protein toxin CNF1 was herein employed as a tool to activate Rho, Rac and Cdc42 in the myogenic cell line C2C12. We demonstrated that CNF1 impaired myogenesis by affecting the muscle regulatory factors MyoD and myogenin and the structural protein MHC expressions. This was principally driven by Rac/Cdc42 activation whereas Rho apparently controlled only the fusion process. More importantly, we proved that a controlled balance between Rho and Rac/Cdc42 activation/deactivation state was crucial for the correct execution of the differentiation program, thus providing a novel view for the role of Rho GTPases in muscle cell differentiation. Also, the use of Rho hijacking toxins can represent a new strategy to pharmacologically influence the differentiative process.

RAC1 inhibition targets amyloid precursor protein processing by gamma-secretase and decreases Abeta production in vitro and in vivo

beta-Amyloid peptides (Abeta) that form the senile plaques of Alzheimer disease consist mainly of 40- and 42-amino acid (Abeta 40 and Abeta 42) peptides generated from the cleavage of the amyloid precursor protein (APP). Generation of Abeta involves beta-secretase and gamma-secretase activities and is regulated by membrane trafficking of the proteins involved in Abeta production. Here we describe a new small molecule, EHT 1864, which blocks the Rac1 signaling pathways. In vitro, EHT 1864 blocks Abeta 40 and Abeta 42 production but does not impact sAPPalpha levels and does not inhibit beta-secretase. Rather, EHT 1864 modulates APP processing at the level of gamma-secretase to prevent Abeta 40 and Abeta 42 generation. This effect does not result from a direct inhibition of the gamma-secretase activity and is specific for APP cleavage, since EHT 1864 does not affect Notch cleavage. In vivo, EHT 1864 significantly reduces Abeta 40 and Abeta 42 levels in guinea pig brains at a threshold that is compatible with delaying plaque accumulation and/or clearing the existing plaque in brain. EHT 1864 is the first derivative of a new chemical series that consists of candidates for inhibiting Abeta formation in the brain of AD patients. Our findings represent the first pharmacological validation of Rac1 signaling as a target for developing novel therapies for Alzheimer disease.

Cell adhesion status-dependent histone acetylation is regulated through intracellular contractility-related signaling activities

Although histone acetylation is important for epigenetic gene transcription, histone acetylation regulation by extracellular cues has rarely been evidenced. Here, we examined whether and how histone acetylation is regulated by cell adhesion-mediated signaling. Gastric carcinoma cells in suspension showed a higher histone acetylation, compared with fibronectin-adherent cells. This difference was supported by a decreased histone deacetylases activity. Furthermore, trichostatin A (TSA)-mediated histone acetylation was significantly increased only in suspended, but not in fibronectin-adherent, cells. Pharmacological inhibition of intracellular contractility-related myosin light chain kinase or RhoA-kinase (ROCK) or expression of ROCK1 small interfering RNA, dominant negative RhoA, or active Rac1 decreased basal and TSA-mediated histone H3 acetylations in suspended cells,whereas inhibition of calmodulin-dependent protein kinase II or transient overexpression of wild type myosin light chain kinase enhanced the acetylations. Meanwhile, chromatin immunoprecipitation showed higher basal and TSA-enhanced associations of ROCK1 promoter regions with Lys(9)-acetylated histone 3 in suspended cells than in fibronectin-adherent cells and expression of ROCK1 was higher and further increased by TSA treatment in suspension. In addition, phosphorylation of myosin light chain was further increased by TSA in suspension and higher in anchorage-independent cells over adherently growing cells, indicating an inverse relationship between ROCK1 expression-mediated contractility and cell adhesion abilities. Cell adhesion analysis showed that pharmacological activation of intracellular contractility-related signaling activities decreased cell adhesion abilities, whereas inhibition of them increased the adhesion. Taken together, these observations suggest that cell adhesion-related signal transduction regulates histone acetylation, presumably through a close functional linkage between intracellular contractility and histone deacetylases activity/histone acetylation.

Rac2 regulates neutrophil chemotaxis, superoxide production, and myeloid colony formation through multiple distinct effector pathways

Polymorphonuclear neutrophils (PMN) are an important component of the innate immune system. We have shown previously that migration and superoxide (O2*-) production, as well as some kinase signaling pathways are compromised in mice deficient in the Ras-related Rho GTPase Rac2. In this study, we demonstrate that Rac2 controls chemotaxis and superoxide production via distinct pathways and is critical for development of myeloid colonies in vitro. The Rac2 mutants V36A, F37A, and N39A all bind to both Pak1 and p67(phox), yet are unable to rescue superoxide production and chemotaxis when expressed in Rac2-/- PMN. In contrast, the N43A mutant, which binds to Por1 (Arfaptin 2), p67phox, and Pak1, is able to rescue superoxide production but not chemotaxis. The F37A mutant, demonstrated to have reduced binding to Por1, shows reduced rescue of fMLP-induced chemotaxis. Finally, the Rac2Y40C mutant that is defective in binding to all three potential downstream effectors (Pak1, p67phox, and Por1) is unable to rescue chemotaxis, motility, or superoxide production, but is able to rescue defective growth of myeloid colonies in vitro. These findings suggest that binding to any single effector is not sufficient to rescue the distinct cellular phenotypes of Rac2-/- PMN, implicating multiple, distinct, and potentially parallel effector pathways.

c-Cbl regulates migration of v-Abl-transformed NIH 3T3 fibroblasts via Rac1

Cellular events like cell adhesion and migration involve complex rearrangements of the actin cytoskeleton. We have previously shown that the multidomain adaptor protein c-Cbl facilitates actin cytoskeletal reorganizations that result in the adhesion of v-Abl-transformed NIH 3T3 fibroblasts. In this report, we demonstrate that c-Cbl also enhances migration of v-Abl-transformed NIH 3T3 fibroblasts. This effect of c-Cbl depends on its tyrosine phosphorylation, specifically on phosphorylation of its Tyr-731, which is required for binding of PI-3' kinase to c-Cbl. Furthermore, we demonstrate that the effect of c-Cbl on migration of v-Abl-transformed fibroblasts is mediated by active PI-3' kinase and the small GTPase Rac1. Our results also indicate that ubiquitin ligase activity of c-Cbl is required, while spatial localization of c-Cbl to the pseudopodia is not required for the observed effects of c-Cbl on cell migration.

Control of lymphocyte shape and the chemotactic response by the GTP exchange factor Vav

Rho GTPases control many facets of cell polarity and migration; namely, the reorganization of the cellular cytoskeleton to extracellular stimuli. Rho GTPases are activated by GTP exchange factors (GEFs), which induce guanosine diphosphate (GDP) release and the stabilization of the nucleotide-free state. Thus, the role of GEFs in the regulation of the cellular response to extracellular cues during cell migration is a critical step of this process. In this report, we have analyzed the activation and subcellular localization of the hematopoietic GEF Vav in human peripheral blood lymphocytes stimulated with the chemokine stromal cell-derived factor-1 (SDF-1alpha). We show a robust activation of Vav and its redistribution to motility-associated subcellular structures, and we provide biochemical evidence of the recruitment of Vav to the membrane of SDF-1alpha-activated human lymphocytes, where it transiently interacts with the SDF-1alpha receptor CXCR4. Overexpression of a dominant negative form of Vav abolished lymphocyte polarization, actin polymerization, and migration. SDF-1alpha-mediated cell polarization and migration also were impaired by overexpression of an active, oncogenic Vav, although the mechanism appears to be different. Together, our data postulate a pivotal role for Vav in the transmission of the migratory signal through the chemokine receptor CXCR4.

Guiding neuronal growth cones using Ca2+ signals

Pathfinding by growing axons in the developing or regenerating nervous system is guided by gradients of molecular guidance cues. The neuronal growth cone, located at the ends of axons, uses surface receptors to sense these cues and to transduce guidance information to cellular machinery that mediates growth and turning responses. Cytoplasmic Ca2+ signals have key roles in regulating this motility. Global growth cone Ca2+ signals can regulate cytoskeletal elements and membrane dynamics to control elongation, whereas Ca2+ signals localized to one side of the growth cone can cause asymmetric activation of effector enzymes to steer the growth cone. Modulating Ca2+ levels in the growth cone might overcome inhibitory signals that normally prevent regeneration in the central nervous system.

Contribution of cellular contractility to spatial and temporal variations in cellular stiffness

Scanning probe microscopy and immunofluorescence observations indicated that cellular stiffness was attributed to a contractile network structure consisting of stress fibers. We measured temporal variations in cellular stiffness when cellular contractility was regulated by dosing with lysophosphatidic acid or Y-27632. This experiment revealed a clear relation between cellular stiffness and contractility: Increases in contractility caused cells to stiffen. On the other hand, decreases in contractility reduced cellular stiffness. In both cases, not only the stiffness of the stress fibers but also that of the whole of the cell varied. Immunofluorescence observations of myosin II and vinculin indicated that the stiffness variations induced by the regulation of cellular contractility were mainly due to rearrangements of the contractile actin network on the dorsal surface. Taken together, our findings provide evidence that the actin cytoskeletal network and its contractility features provide and modulate the mechanical stability of adherent cells.

Effects of angiotensin II receptor signaling during skin wound healing

The tissue angiotensin (Ang) system, which acts independently of the circulating renin Ang system, is supposed to play an important role in tissue repair in the heart and kidney. In the skin, the role of the system for wound healing has remained to be ascertained. Our study demonstrated that oral administration of selective AngII type-1 receptor (AT(1)) blocker suppressed keratinocyte re-epithelization and angiogenesis during skin wound healing in rats. Immunoprecipitation and Western blot analysis indicated the existence of AT(1) and AngII type-2 receptor (AT(2)) in cultured keratinocytes and myofibroblasts. In a bromodeoxyuridine incorporation study, induction of AT(1) signaling enhanced the incorporation into keratinocytes and myofibroblasts. Wound healing migration assays revealed that induction of AT(1) signaling accelerated keratinocyte re-epithelization and myofibroblasts recovering. In these experiments, induction of AT(2) signaling acted vice versa. Taken together, our study suggests that skin wound healing is regulated by balance of opposing signals between AT(1) and AT(2).

Dictyostelium PAKc is required for proper chemotaxis

We have identified a new Dictyostelium p21-activated protein kinase, PAKc, that we demonstrate to be required for proper chemotaxis. PAKc contains a Rac-GTPase binding (CRIB) and autoinhibitory domain, a PAK-related kinase domain, an N-terminal phosphatidylinositol binding domain, and a C-terminal extension related to the Gbetagamma binding domain of Saccharomyces cerevisiae Ste20, the latter two domains being required for PAKc transient localization to the plasma membrane. In response to chemoattractant stimulation, PAKc kinase activity is rapidly and transiently activated, with activity levels peaking at approximately 10 s. pakc null cells exhibit a loss of polarity and produce multiple lateral pseudopodia when placed in a chemoattractant gradient. PAKc preferentially binds the Dictyostelium Rac protein RacB, and point mutations in the conserved CRIB that abrogate this binding result in misregulated kinase activation and chemotaxis defects. We also demonstrate that a null mutation lacking the PAK family member myosin I heavy chain kinase (MIHCK) shows mild chemotaxis defects, including the formation of lateral pseudopodia. A null strain lacking both PAKc and the PAK family member MIHCK exhibits severe loss of cell movement, suggesting that PAKc and MIHCK may cooperate to regulate a common chemotaxis pathway.

Modulation of actomyosin contractility by myosin light chain phosphorylation/dephosphorylation through Rho GTPases signaling specifies axon formation in neurons

Actin depolymerization through Rho GTPases or exogenous mechanical tension has been suggested as a key determinant for the first step of neuronal polarization, the axonogenesis, in which one of the neurites starts to grow becoming the axon. The underlying mechanism and the relationship between two forces in the cells, however, are mostly unknown. Here, we report that the myosin-dependent contractility is a common effector between two forces and a critical determinant in axonogenesis and neuronal polarization. We have found that inhibition of myosin ATPase activity and modulation of myosin light chain phosphorylation/dephosphorylation through Rho GTPases signaling induced multiple axons. Moreover, overexpression of wild-type myosin light chain kinase dramatically increased filopodial structures and produced multi-axonal structures. Our results suggest that MLC phosphorylation/dephosphorylation through Rho GTPases signaling modulates the actomyosin contractility, and then in turn provides a physiological tension in neurons to induce axon.

Involvement of myosin II in dopamine-induced reorganization of the lactotroph cell's actin cytoskeleton

We have shown that dopamine (DA), an inhibitor of prolactin secretion from anterior pituitary lactotrophs, stabilizes the cortical actin cytoskeleton. DA-induced cortical actin stabilization is accompanied by cytoplasmic actin cable disassembly and cell rounding up. Our aim was to identify the mechanisms involved in DA-induced stabilization of the lactotroph's actin cytoskeleton. Here we show that DA increased the association of myosin II with the cell cortex, suggesting that DA facilitates actin-myosin interaction to stabilize cortical actin filaments. This notion was supported by the finding that inhibitors of actin-myosin interaction blocked DA-evoked morphological responses. In addition, our results showed that DA-induced myosin association with the cell periphery may be mediated by inhibition of Rac1/Cdc42-dependent pathways, whereas, DA-induced cytoplasmic actin filament disassembly may be mediated by the inhibition of MLCK- and RhoA-dependent pathways. In conclusion, the present results provide evidence that myosin II is involved in the DA-induced remodeling of actin filaments in lactotrophs, and that DA-induced cortical actin filament assembly and stabilization involve the translocation of myosin II to the cell cortex. This effect requires, among other things, inhibition of the Rac1/Cdc42-dependent signaling pathway.

Role of the cytoskeleton during leukocyte responses

The cytoskeleton is a cellular network of structural, adaptor and signalling molecules that regulates most cellular functions that are related to the immune response, including migration, extravasation, antien recognition, activation and phagocytosis by different subsets of leukocytes. Recently, a large number of regulatory elements and structural constituents of the leukocyte cytoskeleton have been identified. In this review, we discuss the composition and regulation of the different cytoskeletal elements and their role in immune responses.

Mitogen-activated protein kinase feedback phosphorylation regulates MEK1 complex formation and activation during cellular adhesion

Cell adhesion and spreading depend on activation of mitogen-activated kinase, which in turn is regulated both by growth factor and integrin signaling. Growth factors, such as epidermal growth factor, are capable of activating Ras and Raf, but integrin signaling is required to couple Raf to MEK and MEK to extracellular signal-regulated protein kinase (ERK). It was previously shown that Rac-p21-activated kinase (PAK) signaling regulated the physical association of MEK1 with ERK2 through phosphorylation sites in the proline-rich sequence (PRS) of MEK1. It was also shown that activation of MEK1 and ERK by integrins depends on PAK phosphorylation of S298 in the PRS. Here we report a novel MEK1-specific regulatory feedback mechanism that provides a means by which activated ERK can terminate continued PAK phosphorylation of MEK1. Activated ERK can phosphorylate T292 in the PRS, and this blocks the ability of PAK to phosphorylate S298 and of Rac-PAK signaling to enhance MEK1-ERK complex formation. Preventing ERK feedback phosphorylation on T292 during cellular adhesion prolonged phosphorylation of S298 by PAK and phosphorylation of S218 and S222, the MEK1 activating sites. We propose that activation of ERK during adhesion creates a feedback system in which ERK phosphorylates MEK1 on T292, and this in turn blocks additional S298 phosphorylation in response to integrin signaling.

Vascular endothelial-cadherin regulates cytoskeletal tension, cell spreading, and focal adhesions by stimulating RhoA

Changes in vascular endothelial (VE)-cadherin-mediated cell-cell adhesion and integrin-mediated cell-matrix adhesion coordinate to affect the physical and mechanical rearrangements of the endothelium, although the mechanisms for such cross talk remain undefined. Herein, we describe the regulation of focal adhesion formation and cytoskeletal tension by intercellular VE-cadherin engagement, and the molecular mechanism by which this occurs. Increasing the density of endothelial cells to increase cell-cell contact decreased focal adhesions by decreasing cell spreading. This contact inhibition of cell spreading was blocked by disrupting VE-cadherin engagement with an adenovirus encoding dominant negative VE-cadherin. When changes in cell spreading were prevented by culturing cells on a micropatterned substrate, VE-cadherin-mediated cell-cell contact paradoxically increased focal adhesion formation. We show that VE-cadherin engagement mediates each of these effects by inducing both a transient and sustained activation of RhoA. Both the increase and decrease in cell-matrix adhesion were blocked by disrupting intracellular tension and signaling through the Rho-ROCK pathway. In all, these findings demonstrate that VE-cadherin signals through RhoA and the actin cytoskeleton to cross talk with cell-matrix adhesion and thereby define a novel pathway by which cell-cell contact alters the global mechanical and functional state of cells.

Rac1-deficient macrophages exhibit defects in cell spreading and membrane ruffling but not migration

Rac GTPases are activated by extracellular stimuli and contribute to cellular responses including cytoskeletal changes and cell migration. Dominant-negative Rac1 has been used to implicate Rac GTPases in these responses, but which of the three mammalian Rac isoforms it inhibits is not known. We show that mouse bone marrow-derived macrophages express Rac1, low levels of Rac2 but not Rac3. As Rac1-null mice die early in development, we have used mice with a loxP-flanked allele of Rac1 and the type I interferon-inducible Mx1-Cre transgene to address for the first time the specific role of Rac1 in cell motility. Bone marrow-derived macrophages isolated from mice treated with polyIC to induce interferon lack detectable Rac1, and there is no compensatory increase in Rac2 or Cdc42 expression. Rac1-deficient macrophages have an altered morphology: they are significantly more elongated than control cells and have a reduced adhesive area. Re-expression of Rac1 reverts the morphology to that of control cells. Loss of Rac1 reduces but does not completely prevent membrane ruffling in response to CSF-1. However, Rac1-deficient macrophages show normal migration and chemotaxis. Thus in macrophages Rac1 is primarily responsible for regulating cell morphology, contributes to membrane ruffling, but is not required for migration.