Interaction of protein kinase C isozymes with Rho GTPases

Evidence is provided for direct protein-protein interactions between protein kinase C (PKC) alpha, betaI, betaII, gamma, delta, epsilon, and zeta and members of the Rho family of small GTPases. Previous investigations, based on the immunoprecipitation approach, have provided evidence consistent with a direct interaction, but this remained to be proven. In the study presented here, an in vitro assay, consisting only of purified proteins and the requisite PKC activators and cofactors, was used to determine the effects of Rho GTPases on the activities of the different PKC isoforms. It was found that the activity of PKCalpha was potently enhanced by RhoA and Cdc42 and to a lesser extent by Rac1, whereas the effects on the activities of PKCbetaI, -betaII, -gamma, -delta, -epsilon, and -zeta were much reduced. These results indicate a direct interaction between PKCalpha and each of the Rho GTPases. However, the Rho GTPase concentration dependencies for the potentiating effects on PKCalpha activity differed for each Rho GTPase and were in the following order: RhoA > Cdc42 > Rac1. PKCalpha was activated in a phorbol ester- and Ca(2+)-dependent manner. This was reflected by a substantial decrease in the phorbol ester concentration requirements for activity in the presence of Ca(2+), which for each Rho GTPase was induced within a low nanomolar phorbol ester concentration range. The activity of PKCalpha also was found to be dependent on the nature of the GTP- or GDP-bound state of the Rho GTPases, suggesting that the interaction may be regulated by conformational changes in both PKCalpha and Rho GTPases. Such an interaction could result in significant cross-talk between the distinct pathways regulated by these two signaling elements.     

Competitive binding of protein kinase Calpha to membranes and Rho GTPases

Previously, we have shown that protein kinase C (PKC) forms a direct high-affinity, isozyme-specific and membrane lipid-independent interaction with Rho GTPases [Slater, S. J., Seiz, J. L., Stagliano, B. A., and Stubbs, C. D. (2001) Biochemistry 40, 4437-4445]. Since the cellular activation of PKCalpha involves an initial translocation from cytosolic to membrane compartments, the present study investigates the interdependence between the direct protein-protein interaction of PKCalpha with the Rho GTPase, Cdc42, and the protein-lipid interactions of PKCalpha with membranes. It was hypothesized that the interaction of PKCalpha with membrane-bound Cdc42 would contribute to the overall membrane-binding affinity of the kinase by providing an additional anchor. However, it was found that the incorporation of isoprenylated Cdc42 into membranes resulted in an apparent decrease in the membrane-binding affinity of PKCalpha, whereas the association of PKCbetaI, PKCdelta, PKCepsilon, and PKCzeta was each unaffected. The presence of membrane-bound Cdc42 resulted in a rightward shift in both the PS- and Ca2+-concentration response curves for PKCalpha membrane association and for the ensuing activation, whereas the maximal levels of binding and activation attained at saturating PS and Ca2+ concentrations were in each case unaffected. Overall, these findings suggest that PKCalpha undergoes a isozyme-specific interaction with membrane-bound Cdc42 to form a PKCalpha-Cdc42 complex, which possesses a membrane-binding affinity that is reduced relative to that of the individual components due to competition between Cdc42 and PS/Ca2+ for binding to PKCalpha. Consistent with this, it was found that the interaction of PKCalpha with membrane-bound Cdc42 was accompanied by the physical dissociation of the PKCalpha-Cdc42 complex from membranes. Thus, the study provides a novel mechanism by which the membrane association and activation of PKCalpha and Cdc42 may be regulated by competing protein-protein and protein-lipid interactions.     

PKC mediates cyclic stretch-induced cardiac hypertrophy through Rho family GTPases and mitogen-activated protein kinases in cardiomyocytes

Signaling events, including Rho GTPases and protein kinase C (PKC), are involved in cardiac hypertrophy. However, the mechanisms by which these pathways cooperate during the hypertrophic process remain unclear. Using an in vitro cyclic stretch model with neonatal rat cardiomyocytes, we demonstrated that stretch-induced activation of RhoA, Rac1/Cdc42, and phosphorylation of Rho-guanine nucleotide dissociation inhibitor (GDI) were prevented by inhibition or depletion of PKC, using chelerythrine and phorbol 12-myristate 13-acetate, indicating that phorbol ester-sensitive PKC isozymes may be upstream regulators of Rho GTPases. Using adenoviral-mediated gene transfer of wild-type (WT) and dominant-negative (DN) mutants of PKCalpha and delta, we found that stretch-induced activation of Rho GTPases and phosphorylation of Rho-GDI were mainly regulated by PKCalpha. PKCdelta was involved in regulation of the activation of Rac1. Stretch-induced increases in [(3)H]-leucine incorporation, myofibrillar reorganization and cell size, were blocked by inhibition of Rho GTPases, or overexpression of DN PKCalpha and delta, suggesting that PKCalpha and delta are both required in stretch-induced hypertrophy, through Rho GTPases-mediated signaling pathways. The mechanism, whereby PKC and Rho GTPases regulate hypertrophy, was associated with mitogen-activated protein (MAP) kinases. Stretch-stimulated phosphorylation of MEK1/ERK1/2 and MKK4/JNK was inhibited by overexpression of DN PKCalpha and delta, and that of MKK3/p38 inhibited by DN PKCdelta. The phosphorylation of ERK and JNK induced by overexpression of WT PKCalpha, and the phosphorylation of p38 induced by WT PKCdelta, were regulated by Rho GTPases. This study represents the first evidence that PKCalpha and delta are important regulators in mediating activation of Rho GTPases and MAP kinases, in the cyclic stretch-induced hypertrophic process.     

The nature of the hydrophobic n-alkanol binding site within the C1 domains of protein kinase Calpha

The activator-binding sites within the C1 domains of protein kinase C (PKC) are also able to bind alcohols and anesthetics. In this study, the nature of the interaction of these agents with the hydrophobic region within the C1 domains was investigated and a structure-activity relationship for the alcohol effects was obtained. The effects of a series of n-alkanols on PKCalpha activity, determined using an in vitro assay system that lacked lipids, were found to be a nonlinear function of the chain length. In the absence of phorbol ester or diacylglycerol, 1-octanol potently activated PKCalpha in a concentration-dependent manner, while 1-heptanol was completely without effect, despite differing by one methylene unit. The minimal structural requirement for the activating effect corresponded to R-CH(OH)-(CH(2))(n)-CH(3), where R = H or an alkyl group and n >or= 6. Consistent with this, 2-octanol, for which n = 5, was without effect on the activity, even though this alcohol is only marginally less hydrophobic than 1-octanol, whereas 2-nonanol, for which n = 6, was able to produce activity. Importantly, it was found that PKCalpha was activated to a greater extent by R-2-nonanol than by the S enantiomer. The potentiation of phorbol ester-induced, membrane-associated PKCalpha activity by long-chain n-alkanols reported previously (Slater, S. J., Kelly, M. B., Larkin, J. D., Ho, C, Mazurek, A, Taddeo, F. J., Yeager, M. D., Stubbs, C. D. (1997) J. Biol. Chem. 272, 6167-6173), was also found here for nonmembrane associated PKC, indicating that this effect is an intrinsic property of the enzyme rather than a result of membrane perturbation. Overall, the results suggest that the alcohol-binding sites within the C1 domains of PKCalpha contain spatially distinct hydrophilic and hydrophobic regions that impose a high degree of structural specificity on the interactions of alcohols and other anesthetic compounds, as well as diacylglycerols and phorbol esters.     

Effects of ethanol on protein kinase C activity induced by filamentous actin

Protein kinase C (PKC) can be activated by interaction with filamentous actin (F-actin) in the absence of membrane lipids (S.J. Slater, S.K. Milano, B.A. Stagliano, K.J. Gergich, J.P. Curry, F.J. Taddeo and C.D. Stubbs, Biochemistry 39 (2000) 271-280). Here, the effects of ethanol on the F-actin-induced activities of a panel of PKC isoforms consisting of 'conventional' (cPKC) alpha, betaI, gamma, 'novel' (nPKC) delta, epsilon and 'atypical' (aPKC) zeta were investigated using purified PKC and F-actin. Ethanol was found to inhibit the Ca2+- and phorbol ester-dependent activities of cPKCalpha and betaI, and the Ca2+- and phorbol ester-independent activity of cPKCgamma, whereas the activities of nPKCdelta, epsilon and aPKCzeta were unaffected. Although the activities of cPKCalpha and betaI induced by saturating levels of phorbol ester were inhibited by ethanol, the binding of these isozymes to F-actin was unaffected within the same phorbol ester concentration range. Conversely, within submaximal levels of phorbol ester, cPKCalpha and betaI activities were unaffected by ethanol whereas binding to F-actin was inhibited. The potency of the inhibition of F-actin-induced cPKCbetaI activity increased with n-alkanol chain length up to n-hexanol, after which it declined. The results indicate that PKC activities associated with F-actin, and therefore cellular processes involving the actin cytoskeleton, are potential targets for ethanol action. The effects of ethanol on these processes may differ according to the particular regulating PKC isoform, its intracellular localization and the presence of activators and cofactors.     

Synergistic activation of protein kinase Calpha, -betaI, and -gamma isoforms induced by diacylglycerol and phorbol ester: roles of membrane association and activating conformational changes

Protein kinase Calpha (PKCalpha) has been shown to contain two discrete activator sites with differing binding affinities for phorbol esters and diacylglycerols. The interaction of diacylglycerol with a low-affinity phorbol ester binding site leads to enhanced high-affinity phorbol ester binding and to a potentiated level of activity [Slater, S. J., Ho, C., Kelly, M. B., Larkin, J. D. , Taddeo, F. J., Yeager, M. D., and Stubbs, C. D. (1996) J. Biol. Chem. 271, 4627-4631]. In this study, the mechanism of this enhancement of activity was examined with respect to the Ca2+ dependences of membrane association and accompanying conformational changes that lead to activation. The association of PKCalpha with membranes containing 12-O-tetradecanoylphorbol 13-acetate (TPA) or 1, 2-dioleoylglycerol (DAG), determined from tryptophan to dansyl-PE resonance energy transfer (RET) measurements, was found to occur at relatively low Ca2+ levels (相似文献   

Stimulation of M3 muscarinic receptors induces phosphorylation of the Cdc42 effector activated Cdc42Hs-associated kinase-1 via a Fyn tyrosine kinase signaling pathway

The tyrosine kinase, activated Cdc42Hs-associated kinase-1 (ACK-1), is a specific effector of the Rho family GTPase Cdc42. GTP-bound Cdc42 has been shown to facilitate neurite outgrowth elicited by activation of muscarinic cholinergic receptors (mAChRs). Because tyrosine kinase activity is a requirement for neuritogenesis in several cell systems, we investigated whether endogenous mAChRs (principally of the M3 subtype) expressed in human SH-SY5Y neuroblastoma cells would signal to ACK-1. Incubation of cells with the cholinergic agonist oxotremorine-M (Oxo-M) induced an approximately 6-fold increase in the tyrosine phosphorylation of ACK-1 which was inhibited by atropine. ACK-1 phosphorylation was blocked by Clostridium difficile toxin B, an inhibitor of Rho family GTPases. In contrast, disruption of the actin cytoskeleton with cytochalasin D stimulated ACK-1 phosphorylation, and moreover, addition of Oxo-M to cells preincubated with this agent elicited a further increase in phosphorylation, indicating that an intact cytoskeleton is not required for mAChR signaling to ACK-1. Although stimulation of M3 mAChRs induces both an increase in intracellular Ca2+ and activation of protein kinase C (PKC), neither of these second messenger pathways was required for receptor-stimulated ACK-1 phosphorylation. Instead, inhibition of PKC resulted in a 2-fold increase in Oxo-M-stimulated ACK-1 phosphorylation, whereas acute activation of PKC with phorbol ester decreased ACK-1 phosphorylation. The agonist-induced tyrosine phosphorylation of ACK-1 was blocked by inhibitors of Src family kinases, and ACK-1 was coprecipitated with Fyn (but not Src) in an agonist-dependent manner. Finally, scrape loading cells with glutathione S-transferase fusion proteins of either the Fyn-SH2 or Fyn-SH3 domain significantly attenuated mAChR-stimulated ACK-1 tyrosine phosphorylation. The data are the first to show phosphorylation of ACK-1 after stimulation of a receptor coupled to neurite outgrowth and indicate that a Rho family GTPase (i.e. Cdc42) and Fyn are essential upstream elements of this signaling pathway.     

Low- and high-affinity phorbol ester and diglyceride interactions with protein kinase C: 1-O-alkyl-2-acyl-sn-glycerol enhances phorbol ester- and diacylglycerol-induced activity but alone does not induce activity

Phorbol ester-induced conventional protein kinase C (PKCalpha, -betaIota/IotaIota, and -gamma) isozyme activities are potentiated by 1,2-diacyl-sn-glycerol. This has been attributed to a "cooperative" interaction of the two activators with two discrete sites termed the low- and high-affinity phorbol ester binding sites, respectively [Slater, S. J., Milano, S. K., Stagliano, B. A., Gergich, K. J., Ho, C., Mazurek, A., Taddeo, F. J., Kelly, M. B., Yeager, M. D., and Stubbs, C. D. (1999) Biochemistry 38, 3804-3815]. Here, we report that the 1-O-alkyl ether diglyceride, 1-O-hexadecyl-2-acetyl-sn-glycerol (HAG), like its 1,2-diacyl counterpart, 1-oleoyl-2-acetyl-sn-glycerol (OAG), also potentiated PKCalpha, -betaI/II, and -gamma activities induced by the phorbol ester 4beta-12-O-tetradecanoylphorbol-13-acetate (TPA). Similar to OAG, HAG was found to bind to the low-affinity phorbol ester binding site and to enhance high-affinity phorbol ester binding, and to decrease the level of Ca(2+) required for phorbol ester-induced activity, while being without effect on the Ca(2+) dependence of membrane association. Thus, similar to OAG, HAG may also potentiate phorbol ester-induced activity by interacting with the low-affinity phorbol ester binding site, leading to a reduced level of Ca(2+) required for the activating conformational change. However, HAG was found not to behave like a 1,2-diacyl-sn-glycerol in that alone it did not induce PKC activity, and also in that it enhanced OAG-induced activity. The results reveal HAG to be a member of a new class of "nonactivating" compounds that modulate PKC activity by interacting with the low-affinity phorbol ester binding site.     

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.     

The C1 domain of protein kinase C as a lipid bilayer surface sensing module

The activity of membrane-associated protein kinase C (PKC) is tightly controlled by the physical properties of the membrane lipid bilayer, in particular, curvature stress, which is induced by bilayer-destabilizing lipid components. An important example of this is the weakened lipid headgroup interactions induced by phosphatidylethanolamine (PE) and cholesterol. In this work our previous observation with a mixed isoform PKC showing a biphasic dependence of activity as a function of membrane curvature stress [Slater et al. (1994) J. Biol. Chem. 269, 4866-4871] was here extended to individual isoforms. The Ca(2+)-dependent PKCalpha, PKCbeta, and PKCgamma, along with Ca(2+)-independent PKCdelta, but not PKCepsilon or PKCzeta, displayed a biphasic activity as a function of membrane PE content. The fluorescence anisotropy of N-(5-dimethylaminonaphthalene-1-sulfonyl)dioleoylphosphatidylserine (dansyl-PS), which probes the lipid environment of PKC, also followed a biphasic profile as a function of PE content for full-length PKCalpha, PKCbetaIotaIota, and PKCgamma as did the isolated C1 domain of PKCalpha. In addition, the rotational correlation time of both PKCalpha and PKCdelta C1-domain-associated sapintoxin D, a fluorescent phorbol ester, was also a biphasic function of membrane lipid PE content. These results indicate that the C1 domain acts as a sensor of the bilayer surface properties and that its conformational response to these effects may directly underlie the resultant effects on enzyme activity.     

Oncogenic Dbl, Cdc42, and p21-activated kinase form a ternary signaling intermediate through the minimum interactive domains

Activation of many Rho family GTPase pathways involves the signaling module consisting of the Dbl-like guanine nucleotide exchange factors (GEFs), the Rho GTPases, and the Rho GTPase specific effectors. The current biochemical model postulates that the GEF-stimulated GDP/GTP exchange of Rho GTPases leads to the active Rho-GTP species, and subsequently the active Rho GTPases interact with and activate the effectors. Here we report an unexpected finding that the Dbl oncoprotein, Cdc42 GTPase, and PAK1 can form a complex through their minimum functional motifs, i.e., the Dbl-homolgy (DH) and Pleckstrin-homology domains of Dbl, Cdc42, and the PBD domain of PAK1. The Dbl-Cdc42-PAK1 complex is sensitive to the nucleotide-binding state of Cdc42 since either dominant negative or constitutively active Cdc42 readily disrupts the ternary binding interaction. The complex formation depends on the interactions between the DH domain of Dbl and Cdc42 and between Cdc42 and the PBD domain of PAK1 and can be reconstituted in vitro by using the purified components. Furthermore, the Dbl-Cdc42-PAK1 ternary complex is active in generating signaling output through the activated PAK1 kinase in the complex. The GEF-Rho-effector ternary intermediate is also found in other Dbl-like GEF, Rho GTPase, and effector interactions. Finally, PAK1, through the PDB domain, is able to accelerate the GEF-induced GTP loading onto Cdc42. These results suggest that signal transduction through Cdc42 and possibly other Rho family GTPases could involve tightly coupled guanine nucleotide exchange and effector activation mechanisms and that Rho GTPase effector may have a feedback regulatory role in the Rho GTPase activation.     

Zizimin1, a novel Cdc42 activator,reveals a new GEF domain for Rho proteins

The Rho family GTPases Rac, Rho and Cdc42 are critical in regulating the actin-based cytoskeleton, cell migration, growth, survival and gene expression. These GTPases are activated by guanine nucleotide-exchange factors (GEFs). A biochemical search for Cdc42 activators led to the cloning of zizimin1, a new protein whose overexpression induces Cdc42 activation. Sequence comparison combined with mutational analysis identified a new domain, which we named CZH2, that mediates direct interaction with Cdc42. CZH2-containing proteins constitute a new superfamily that includes the so-called 'CDM' proteins that bind to and activate Rac. Together, the results suggest that CZH2 is a new GEF domain for the Rho family of proteins.     

RhoGAP18B Isoforms Act on Distinct Rho-Family GTPases and Regulate Behavioral Responses to Alcohol via Cofilin

Responses to the effects of ethanol are highly conserved across organisms, with reduced responses to the sedating effects of ethanol being predictive of increased risk for human alcohol dependence. Previously, we described that regulators of actin dynamics, such as the Rho-family GTPases Rac1, Rho1, and Cdc42, alter Drosophila’s sensitivity to ethanol-induced sedation. The GTPase activating protein RhoGAP18B also affects sensitivity to ethanol. To better understand how different RhoGAP18B isoforms affect ethanol sedation, we examined them for their effects on cell shape, GTP-loading of Rho-family GTPase, activation of the actin-severing cofilin, and actin filamentation. Our results suggest that the RhoGAP18B-PA isoform acts on Cdc42, while PC and PD act via Rac1 and Rho1 to activate cofilin. In vivo, a loss-of-function mutation in the cofilin-encoding gene twinstar leads to reduced ethanol-sensitivity and acts in concert with RhoGAP18B. Different RhoGAP18B isoforms, therefore, act on distinct subsets of Rho-family GTPases to modulate cofilin activity, actin dynamics, and ethanol-induced behaviors.     

Transforming growth factor beta activates Rac1 and Cdc42Hs GTPases and the JNK pathway in skeletal muscle cells

The transforming growth factor beta (TGFbeta) plays an important role in cell growth and differentiation. However, the intracellular signaling pathways through which TGFbeta inhibits skeletal myogenesis remain largely undefined. By measuring GTP-loading of Rho GTPases and the organization of the F-actin cytoskeleton and the plasma membrane, we analyzed the effect of TGFbeta addition on the activity of three GTPases, Rac1, Cdc42Hs and RhoA. We report that TGFbeta activates Rac1 and Cdc42Hs in skeletal muscle cells, two GTPases previously described to inhibit skeletal muscle cell differentiation whereas it inactivates RhoA, a positive regulator of myogenesis. We further show that TGFbeta activates the C-jun N-terminal kinases (JNK) pathway in myoblastic cells through Rac1 and Cdc42Hs GTPases. We propose that the activation of Rho family proteins Rac1 and Cdc42Hs which subsequently regulate JNK activity participates in the inhibition of myogenesis by TGFbeta.     

Cdc42p and Rho1p are sequentially activated and mechanistically linked to vacuole membrane fusion

Small monomeric GTPases act as molecular switches, regulating many biological functions via activation of membrane localized signaling cascades. Activation of their switch function is controlled by GTP binding and hydrolysis. Two Rho GTPases, Cdc42p and Rho1p, are localized to the yeast vacuole where they regulate membrane fusion. Here, we define a method to directly examine vacuole membrane Cdc42p and Rho1p activation based on their affinity to probes derived from effectors. Cdc42p and Rho1p showed unique temporal activation which aligned with distinct subreactions of in vitro vacuole fusion. Cdc42p was rapidly activated in an ATP-independent manner while Rho1p activation was kinetically slower and required ATP. Inhibitors that are known to block vacuole membrane fusion were examined for their effect on Cdc42p and Rho1p activation. Rdi1p, which inhibits the dissociation of GDP from Rho proteins, blocked both Cdc42p and Rho1p activation. Ligands of PI(4,5)P₂ specifically inhibited Rho1p activation while pre-incubation with U73122, which targets Plc1p function, increased Rho1p activation. These results define unique activation mechanisms for Cdc42p and Rho1p, which may be linked to the vacuole membrane fusion mechanism.     

Integrin-mediated adhesion regulates cell polarity and membrane protrusion through the Rho family of GTPases

Integrin-mediated adhesion is a critical regulator of cell migration. Here we demonstrate that integrin-mediated adhesion to high fibronectin concentrations induces a stop signal for cell migration by inhibiting cell polarization and protrusion. On fibronectin, the stop signal is generated through alpha 5 beta 1 integrin-mediated signaling to the Rho family of GTPases. Specifically, Cdc42 and Rac1 activation exhibits a biphasic dependence on fibronectin concentration that parallels optimum cell polarization and protrusion. In contrast, RhoA activity increases with increasing substratum concentration. We find that cross talk between Cdc42 and Rac1 is required for substratum-stimulated protrusion, whereas RhoA activity is inhibitory. We also show that Cdc42 activity is inhibited by Rac1 activation, suggesting that Rac1 activity may down-regulate Cdc42 activity and promote the formation of stabilized rather than transient protrusion. Furthermore, expression of RhoA down-regulates Cdc42 and Rac1 activity, providing a mechanism whereby RhoA may inhibit cell polarization and protrusion. These findings implicate adhesion-dependent signaling as a mechanism to stop cell migration by regulating cell polarity and protrusion via the Rho family of GTPases.     

Cross-talk-dependent cortical patterning of Rho GTPases during cell repair

Rho GTPases such as Rho, Rac, and Cdc42 are important regulators of the cortical cytoskeleton in processes including cell division, locomotion, and repair. In these processes, Rho GTPases assume characteristic patterns wherein the active GTPases occupy mutually exclusive “zones” in the cell cortex. During cell wound repair, for example, a Rho zone encircles the wound edge and is in turn encircled by a Cdc42 zone. Here we evaluated the contributions of cross-talk between Rho and Cdc42 to the patterning of their respective zones in wounded Xenopus oocytes using experimental manipulations in combination with mathematical modeling. The results show that the position of the Cdc42 zone relative to the Rho zone and relative to the wound edge is controlled by the level of Rho activity. In contrast, the outer boundary of the Rho zone is limited by the level of Cdc42 activity. Models based on positive feedback within zones and negative feedback from Rho to the GEF-GAP Abr to Cdc42 capture some, but not all, of the observed behaviors. We conclude that GTPase zone positioning is controlled at the level of Rho activity and we speculate that the Cdc42 zone or something associated with it limits the spread of Rho activity.     

A role for the small molecular weight GTPases, Rho and Cdc42, in muscarinic receptor signaling to focal adhesion kinase

An enhanced tyrosine phosphorylation of focal adhesion kinase (FAK) is elicited during neuronal growth cone remodeling and requires the maintenance of agonist-sensitive pools of phosphatidylinositol 4,5-bisphosphate (PIP2). Rho family GTPases are putative regulators of both PIP2 synthesis and growth cone remodeling, including neurite outgrowth elicited by muscarinic cholinergic receptor (mAChR) stimulation. In this study, we investigated the interrelationships among Rho family GTPases, PIP2 synthesis, and mAChR signaling to FAK in SH-SY5Y neuroblastoma cells. Preincubation with Clostridium difficile toxin B (Tox B), an inhibitor of Rho, Rac, and Cdc42, attenuated mAChR-stimulated FAK and paxillin tyrosine phosphorylation and lysophosphatidic acid (LPA)-induced FAK phosphorylation to a similar extent (75% decreases at 200 pg/ml Tox B) but did not affect mitogen-activated protein kinase activation elicited by either phorbol ester or an mAChR agonist. In contrast, preincubation with selective inhibitors of either Rho (C3 exoenzyme) or Rho kinase (HA-1 077) resulted in 80-90% reductions in LPA-induced FAK phosphorylation but only 40-50% decreases in mAChR-stimulated phosphorylation. Moreover, mAChR-mediated FAK phosphorylation was significantly attenuated in cells scrape-loaded with dominant-negative N17Cdc42 but not N17Rac1. Tox B had little or no effect on agonist-sensitive pools of PIP2 but inhibited mAChR-driven actin cytoskeletal remodeling. The results suggest that the Rho family GTPases, Rho and Cdc42, link mAChR stimulation to increases in FAK phosphorylation independently of effects on PIP2 synthesis.