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• W2013249463 abstract "The Rho family of small GTPases has been shown to be involved in the regulation of neuronal morphology, and Rac and Rho exert antagonistic actions in neurite formation. In this study, we have examined the cross-talk between Rac and Rho in relation to the nerve growth factor (NGF)-induced neurite outgrowth in PC12 cells. NGF induced a rapid activation of Rac1 and suppression of RhoA activity. Constitutively active RhoA, RhoAV14, or constitutively active Gα12-induced endogenous RhoA activation inhibited the NGF-induced Rac1 activation without any effect on the NGF-induced extracellular signal-regulated kinase activation. Moreover, Y-27632, an inhibitor of Rho-associated kinase, completely abolished the RhoA-induced down-regulation of the NGF-induced Rac1 activation. We also revealed that NGF induced a rapid recruitment of Rac1 to the cell surface protrusion sites and formed filamentous actin-rich protrusions. Activation of RhoA and Rho-associated kinase formed a thick ringlike structure of cortical actin filaments at the cell periphery and then inhibited the NGF-induced recruitment of Rac1 to protrusions. These results indicate that RhoA down-regulates the NGF- induced Rac1 activation through Rho-associated kinase, inhibiting the neurite formation. The Rho family of small GTPases has been shown to be involved in the regulation of neuronal morphology, and Rac and Rho exert antagonistic actions in neurite formation. In this study, we have examined the cross-talk between Rac and Rho in relation to the nerve growth factor (NGF)-induced neurite outgrowth in PC12 cells. NGF induced a rapid activation of Rac1 and suppression of RhoA activity. Constitutively active RhoA, RhoAV14, or constitutively active Gα12-induced endogenous RhoA activation inhibited the NGF-induced Rac1 activation without any effect on the NGF-induced extracellular signal-regulated kinase activation. Moreover, Y-27632, an inhibitor of Rho-associated kinase, completely abolished the RhoA-induced down-regulation of the NGF-induced Rac1 activation. We also revealed that NGF induced a rapid recruitment of Rac1 to the cell surface protrusion sites and formed filamentous actin-rich protrusions. Activation of RhoA and Rho-associated kinase formed a thick ringlike structure of cortical actin filaments at the cell periphery and then inhibited the NGF-induced recruitment of Rac1 to protrusions. These results indicate that RhoA down-regulates the NGF- induced Rac1 activation through Rho-associated kinase, inhibiting the neurite formation. nerve growth factor hemagglutinin extracellular signal-regulated kinase green fluorescent protein isopropyl-μ-d-thiogalactoside glutathioneS-transferase Cdc42/Rac interactive binding Rho-binding domain phosphate-buffered saline catalytic domain of Rho-associated kinase In the developing nervous system, formation of appropriate connections between neurons is an essential process for the establishment and maturation of neural circuits. Neurite extension and retraction are very important processes in the formation of neuronal networks, and these dynamic morphological changes of neuronal cells are largely decided by the actin cytoskeletal organization (1Tanaka E. Sabry J. Cell. 1995; 83: 171-176Abstract Full Text PDF PubMed Scopus (297) Google Scholar). The Rho family of small GTPases, consisting of Cdc42, Rac, and Rho, has been implicated in the reorganization of the actin cytoskeleton and subsequent morphological changes in various cellular functions (2Hall A. Science. 1998; 279: 509-514Crossref PubMed Scopus (5168) Google Scholar). Among them, Rac is involved in membrane ruffling and formation of lamellipodia, whereas Rho is responsible for regulating the assembly of focal adhesion and stress fiber formation in fibroblasts (3Ridley A.J. Paterson H.F. Johnston C.L. Diekmann D. Hall A. Cell. 1992; 70: 401-410Abstract Full Text PDF PubMed Scopus (3041) Google Scholar, 4Ridley A.J. Hall A. Cell. 1992; 70: 389-399Abstract Full Text PDF PubMed Scopus (3788) Google Scholar). Similar to other GTPases of the Ras superfamily, they serve as a molecular switch by cycling between an inactive GDP-bound state and an active GTP-bound state. Activation of the Rho family proteins requires GDP-GTP exchange catalyzed by various guanine nucleotide exchange factors, and their activation is regulated by GTPase-activating proteins, which stimulate the intrinsic GTPase activities of the G proteins, leading to the cessation of their actions. In addition, guanine nucleotide dissociation inhibitors inhibit the exchange of GDP for GTP and might also serve to regulate their association with membranes (5Sasaki T. Takai Y. Biochem. Biophys. Res. Commun. 1998; 245: 641-645Crossref PubMed Scopus (166) Google Scholar). In neuronal cells, Rho family proteins are involved in axon and dendrite formation in various types of neurons (6Threadgill R. Bobb K. Ghosh A. Neuron. 1997; 19: 625-634Abstract Full Text Full Text PDF PubMed Scopus (423) Google Scholar, 7Albertinazzi C. Gilardelli D. Paris S. Longhi R. de Curtis I. J. Cell Biol. 1998; 142: 815-825Crossref PubMed Scopus (83) Google Scholar, 8Lehmann M. Fournier A. Selles-Navarro I. Dergham P. Sebok A. Leclerc N. Tigyi G. McKerracher L. J. Neurosci. 1999; 19: 7537-7547Crossref PubMed Google Scholar, 9Ruchhoeft M.L. Ohnuma S. McNeill L. Holt C.E. Harris W.A. J. Neurosci. 1999; 19: 8454-8463Crossref PubMed Google Scholar), and defects in the regulation of these GTPase activities have been reported to affect the development of the nervous system (10Luo L. Liao Y.J. Jan L.Y. Jan Y.N. Genes Dev. 1994; 8: 1787-1802Crossref PubMed Scopus (799) Google Scholar, 11Luo L. Hensch T.K. Ackerman L. Barbel S. Jan L.Y. Jan Y.N. Nature. 1996; 379: 837-840Crossref PubMed Scopus (390) Google Scholar, 12Zipkin I.D. Kindt R.M. Kenyon C.J. Cell. 1997; 90: 883-894Abstract Full Text Full Text PDF PubMed Scopus (211) Google Scholar, 13Kaufmann N. Wills Z.P. Van Vactor D. Development. 1998; 125: 453-461PubMed Google Scholar). Rac has been shown to be involved in the formation of lamellipodia of a growth cone and to be required for the outgrowth of neurites (14Kozma R. Sarner S. Ahmed S. Lim L. Mol. Cell. Biol. 1997; 17: 1201-1211Crossref PubMed Scopus (534) Google Scholar). On the other hand, activation of Rho was reported to induce the collapse of the growth cone and the retraction of neurites and to inhibit the neurite outgrowth (14Kozma R. Sarner S. Ahmed S. Lim L. Mol. Cell. Biol. 1997; 17: 1201-1211Crossref PubMed Scopus (534) Google Scholar, 15Katoh H. Aoki J. Ichikawa A. Negishi M. J. Biol. Chem. 1998; 273: 2489-2492Abstract Full Text Full Text PDF PubMed Scopus (158) Google Scholar, 16Sebok A. Nusser N. Debreceni B. Guo Z. Santos M.F. Szeberenyi J. Tigyi G. J. Neurochem. 1999; 73: 949-960Crossref PubMed Scopus (108) Google Scholar). As mentioned above, Rac and Rho are thought to counteract each other's activity in neuronal cells (14Kozma R. Sarner S. Ahmed S. Lim L. Mol. Cell. Biol. 1997; 17: 1201-1211Crossref PubMed Scopus (534) Google Scholar, 17Chen X.Q. Tan I. Leung T. Lim L. J. Biol. Chem. 1999; 274: 19901-19905Abstract Full Text Full Text PDF PubMed Scopus (73) Google Scholar). Therefore, the balance between Rac and Rho activities is likely to be a crucial point for neuronal morphology. The cross-talk between Rac and Rho has been studied in a variety of cell lines, and the physiological significance of the cross-talk has been established in various cellular functions (18Burridge K. Science. 1999; 283: 2028-2029Crossref PubMed Scopus (52) Google Scholar, 19Rottner K. Hall A. Small J.V. Curr. Biol. 1999; 9: 640-648Abstract Full Text Full Text PDF PubMed Scopus (502) Google Scholar). However, little is yet known about the regulatory mechanism of the cross-talk between Rac and Rho. Rat pheochromocytoma PC12 cells have been used as a model system for neuronal differentiation and neurite outgrowth. After stimulation with nerve growth factor (NGF),1the cells stop growing and begin to extend neurites. Rac plays an important role in the regulation of the cytoskeletal changes required for neurite outgrowth in response to NGF, whereas Rho has been shown to inhibit neurite outgrowth by NGF (16Sebok A. Nusser N. Debreceni B. Guo Z. Santos M.F. Szeberenyi J. Tigyi G. J. Neurochem. 1999; 73: 949-960Crossref PubMed Scopus (108) Google Scholar). In this study, we have examined the cross-talk between Rac and Rho in PC12 cells, and we showed that NGF induced the Rac1 activation but also the inhibition of RhoA activity and that the activation of RhoA down-regulated the NGF-induced Rac1 activation via Rho-associated kinase. Y-27632 was a generous gift from Yoshitomi Pharmaceutical Industries (Saitama, Japan). Agents obtained and commercial sources were as follows: NGF, Promega; mouse monoclonal anti-Rac1 antibody, Transduction Laboratories; mouse monoclonal anti-RhoA antibody, Santa Cruz Biotechnology, Inc. (Santa Cruz, CA); mouse monoclonal anti-hemagglutinin (HA) antibody (clone 12CA5), Roche Molecular Biochemicals; rabbit antiphosphospecific extracellular signal-regulated kinase (ERK) antibody, New England Biolabs, Inc.; rabbit anti-ERK1 and mouse anti-ERK2 antibodies, Upstate Biotechnology, Inc. (Lake Placid, NY); Alexa 488-conjugated phalloidin and rhodamine-conjugated phalloidin, Molecular Probes, Inc. (Eugene, OR); rhodamine-conjugated donkey anti-mouse immunoglobulin G (IgG) antibody, Chemicon International Inc.; horseradish peroxidase-conjugated goat anti-mouse IgG antibody and horseradish peroxidase-conjugated swine anti-rabbit IgG antibody, DAKO; and chemiluminescence ECL Western blotting system, Amersham Pharmacia Biotech. Mammalian expression vector pcDNA3 carrying cDNA for a variant of the Aequorea victoria green fluorescent protein (GFP) was obtained as described previously (20Katoh H. Yasui H. Yamaguchi Y. Aoki J. Fujita H. Mori K. Negishi M. Mol. Cell. Biol. 2000; 20: 7378-7387Crossref PubMed Scopus (123) Google Scholar). cDNA for the catalytic domain of Rho-associated kinase, ROKα (CD-ROKα; Ref. 15Katoh H. Aoki J. Ichikawa A. Negishi M. J. Biol. Chem. 1998; 273: 2489-2492Abstract Full Text Full Text PDF PubMed Scopus (158) Google Scholar), was inserted into mammalian expression vector pEF-BOS. The sources of other materials are as indicated. PC12 cells were cultured in Dulbecco's modified Eagle's medium containing 10% horse serum, 5% fetal bovine serum, 4 mm glutamine, 100 units/ml penicillin, and 0.2 mg/ml streptomycin under humidified conditions in 95% air and 5% CO2 at 37 °C. PC12 cell line expressing RhoAV14 or Gα12QL under the control of isopropyl-μ-d-thiogalactoside (IPTG) was obtained as described previously (21Yamaguchi Y. Katoh H. Yasui H. Aoki J. Nakamura K. Negishi M. J. Neurochem. 2000; 75: 708-717Crossref PubMed Scopus (24) Google Scholar). For transfection, cells were seeded onto poly-d-lysine (Sigma)-coated glass coverslips (circular, 13 mm) in 24-well plates at a density of 1 × 104cells/well and cultured for 15–18 h. Then cells were transfected with 0.8 μg of total DNA using LipofectAMINE 2000 (Life Technologies, Inc.) according to the manufacturer's instructions. Cells were fixed 24 h after transfection. After PC12 cells were rinsed briefly with PBS, they were lysed for 5 min with the respective ice-cold lysis buffers (Rac1 activity assay, 50 mm Tris-HCl, pH 7.4, 100 mm NaCl, 2 mm MgCl2, 1% Nonidet P-40, 10% glycerol, 1 mm dithiothreitol, 1 mm phenylmethylsulfonyl fluoride, 1 μg/ml aprotinin, and 1 μg/ml leupeptin; RhoA activity assay, 50 mm Tris-HCl, pH 7.4, 150 mm NaCl, 30 mm MgCl2, 0.1% Triton X-100, 10% glycerol, 1 mm dithiothreitol, 1 mm phenylmethylsulfonyl fluoride, 1 μg/ml aprotinin, and 1 μg/ml leupeptin; ERK activity assay, 50 mm Tris-HCl, pH 7.4, 200 mm NaCl, 2.5 mm MgCl2, 10 mm NaF, 1 mm Na3VO4, 10% glycerol, 1% Nonidet P-40, 250 μm phenylmethylsulfonyl fluoride, 1 μg/ml aprotinin, and 1 μg/ml leupeptin). The cell lysates were immediately centrifuged for 5 min at 10,000 × g at 4 °C, and the supernatants were separated by SDS-polyacrylamide gel electrophoresis. The proteins were transferred onto a polyvinylidene difluoride membrane (Millipore Corp.). The membrane was blocked with 3% low fat milk in Tris-buffered saline and then incubated at 4 °C with primary antibodies. The immunoblots were detected using a chemiluminescence ECL Western blotting system with horseradish peroxidase-conjugated secondary antibodies. Measurement of Rac1 and RhoA activities was performed as described previously (20Katoh H. Yasui H. Yamaguchi Y. Aoki J. Fujita H. Mori K. Negishi M. Mol. Cell. Biol. 2000; 20: 7378-7387Crossref PubMed Scopus (123) Google Scholar, 21Yamaguchi Y. Katoh H. Yasui H. Aoki J. Nakamura K. Negishi M. J. Neurochem. 2000; 75: 708-717Crossref PubMed Scopus (24) Google Scholar). Eighteen-hour serum-starved PC12 cells (Rac1, 3 × 106cells; RhoA, 2 × 107 cells) were lysed for 5 min with the respective ice-cold cell lysis buffer containing 6 μg of the glutathione S-transferase-fused Cdc42/Rac-interactive binding domain of rat αPAK (GST-CRIB) for GTP-bound Rac1 or 16 μg of GST-Rho-binding domain of mouse rhotekin (GST-RBD) for GTP-bound RhoA. For preparing suspended cells, PC12 cells were detached and kept in suspension in serum-free Dulbecco's modified Eagle's medium for 10 min before lysis. Cell lysates were then centrifuged for 5 min at 10,000 × g at 4 °C, and the supernatants were incubated with glutathione-Sepharose beads for 30 min (Rac1) or 60 min (RhoA) at 4 °C. After the beads had been washed with the cell lysis buffer, the bound proteins were resolved on 12.5% SDS-polyacrylamide gel electrophoresis and immunoblotted using a mouse monoclonal anti-Rac1 antibody (1:1000) or a mouse monoclonal anti-RhoA antibody (1:100). Densitometric analyses were performed using NIH Image software, and the amounts of GTP-bound Rac1 and RhoA were normalized to the total amounts of Rac1 and RhoA in cell lysates, respectively. PC12 cells were seeded onto poly-d-lysine-coated glass coverslips in 24-well plates at a density of 1 × 104 cells/well. All steps were carried out at room temperature, and cells were rinsed with phosphate-buffered saline (PBS) between each step. Cells on coverslips were fixed with 3.7% formaldehyde/PBS for 20 min. After residual formaldehyde had been quenched with 50 mmNH4Cl/PBS for 10 min, cells were permeabilized in 0.2% Triton X-100 for 10 min and incubated with 10% fetal bovine serum in PBS for 30 min to block nonspecific antibody binding. Endogenous Rac1 was stained with an anti-Rac1 monoclonal antibody in PBS at a 1:1000 dilution for 1 h followed by incubation with a rhodamine-conjugated donkey anti-mouse IgG in PBS at a 1:500 dilution for 1 h. Filamentous actin (F-actin) was stained with Alexa 488-conjugated phalloidin in PBS (0.5 units/ml) or rhodamine-conjugated phalloidin in PBS (1 unit/ml) for 1 h. Cells were mounted in 90% glycerol containing 0.1% p-phenylenediamine dihydrochloride in PBS. Confocal microscopy was performed using an MRC-1024 laser-scanning confocal imaging system (Bio-Rad) equipped with a Nicon Eclipse E800 microscope and a Nicon Plan Apo 60 × 1.4 oil immersion objective. Rac has been shown to be involved in the NGF-induced neurite outgrowth (17Chen X.Q. Tan I. Leung T. Lim L. J. Biol. Chem. 1999; 274: 19901-19905Abstract Full Text Full Text PDF PubMed Scopus (73) Google Scholar, 22Lamoureux P. Altun-Gultekin Z.F. Lin C. Wagner J.A. Heidemann S.R. J. Cell Sci. 1997; 110: 635-641Crossref PubMed Google Scholar), while active Rho has been reported to prevent the NGF-induced neuritogenesis (16Sebok A. Nusser N. Debreceni B. Guo Z. Santos M.F. Szeberenyi J. Tigyi G. J. Neurochem. 1999; 73: 949-960Crossref PubMed Scopus (108) Google Scholar). To assess more directly modulation of Rac1 and RhoA activities by NGF, we measured the amounts of cellular GTP-bound Rac1 and RhoA using the GST-fused CRIB domain of αPAK (GST-CRIB) and GST-fused Rho-binding domain of rhotekin (GST-RBD), respectively. As shown in Fig. 1, NGF induced a rapid increase in the amount of cellular GTP-bound Rac1, the elevation reaching a maximum at 3 min. The level decreased gradually but remained above the basal level for over 60 min after the stimulation. On the other hand, NGF induced a rapid decrease in the amount of cellular GTP-bound RhoA, the level reaching a minimum at 3–10 min, and it took at least 30 min to regain the basal level. Therefore, NGF oppositely regulated the activities of Rac1 and RhoA, activation of Rac1 and suppression of RhoA. Active Rho has been known to prevent NGF-induced neuritogenesis (16Sebok A. Nusser N. Debreceni B. Guo Z. Santos M.F. Szeberenyi J. Tigyi G. J. Neurochem. 1999; 73: 949-960Crossref PubMed Scopus (108) Google Scholar). We next examined the effect of RhoA activation on NGF-induced Rac1 activation, using the PC12 cell line expressing constitutively active HA-tagged RhoA, HA-RhoAV14, under the control of IPTG, which we had previously established (referred to as “RhoAV14-inducible PC12 cells”; Ref. 21Yamaguchi Y. Katoh H. Yasui H. Aoki J. Nakamura K. Negishi M. J. Neurochem. 2000; 75: 708-717Crossref PubMed Scopus (24) Google Scholar). The expression of RhoAV14 by IPTG completely inhibited the NGF-induced Rac1 activation without any effect on the basal Rac1 activity in RhoAV14-inducible PC12 cells (Fig. 2). We recently demonstrated that the G12 family of heterotrimeric G proteins induced activation of endogenous RhoA, causing neurite retraction in PC12 cells (21Yamaguchi Y. Katoh H. Yasui H. Aoki J. Nakamura K. Negishi M. J. Neurochem. 2000; 75: 708-717Crossref PubMed Scopus (24) Google Scholar). We then examined the effect of endogenous RhoA activation on the NGF-induced Rac1 activation using the PC12 cell line expressing constitutively active Gα12, Gα12QL, under the control of IPTG (21Yamaguchi Y. Katoh H. Yasui H. Aoki J. Nakamura K. Negishi M. J. Neurochem. 2000; 75: 708-717Crossref PubMed Scopus (24) Google Scholar). The expression of Gα12QL by IPTG completely inhibited the NGF-induced Rac1 activation (Fig. 3), indicating that the RhoA activation pathway is a negative regulator of the NGF-induced Rac1 activation.Figure 3Inhibition of the NGF-induced Rac1 activation by Gα12QL. A, serum-starved Gα12QL-inducible PC12 cells were treated with or without 5 mm IPTG for 12 h, and then they were stimulated with or without 50 ng/ml NGF for 3 min. The cell lysates were incubated with GST-CRIB, and the amounts of GTP-bound Rac1 were determined by immunoblotting using a monoclonal antibody against Rac1 (upper panel). Total amounts of Rac1 in cell lysates (middle panel) and expression of HA-tagged Gα12QL (bottom panel,arrow) are also shown. The results shown are representative of three independent experiments that yielded similar results.B, quantification of the effect of Gα12QL on Rac1 activity. The Rac1 activity is indicated by the amount of GST-CRIB-bound Rac1 normalized to the amount of Rac1 in whole cell lysates, and values of Rac1 activity are expressed as -fold increase over the value of cells that were not treated with either IPTG or NGF. Data are the means ± S.E. of triplicate experiments.View Large Image Figure ViewerDownload Hi-res image Download (PPT) Recent studies have shown that cell adhesion to the extracellular matrix induces activation of Rac (23Clark E.A. King W.G. Brugge J.S. Symons M. Hynes R.O. J. Cell Biol. 1998; 142: 573-586Crossref PubMed Scopus (526) Google Scholar, 24Price L.S. Leng J. Schwartz M.A. Bokoch G.M. Mol. Biol. Cell. 1998; 9: 1863-1871Crossref PubMed Scopus (523) Google Scholar). On the other hand, activation of Rho has been shown to induce cell rounding in PC12 cells, rendering the cells less adherent (21Yamaguchi Y. Katoh H. Yasui H. Aoki J. Nakamura K. Negishi M. J. Neurochem. 2000; 75: 708-717Crossref PubMed Scopus (24) Google Scholar, 25Katoh H. Aoki J. Yamaguchi Y. Kitano Y. Ichikawa A. Negishi M. J. Biol. Chem. 1998; 273: 28700-28707Abstract Full Text Full Text PDF PubMed Scopus (156) Google Scholar). Therefore, down-regulation of the NGF-induced Rac1 activation by RhoAV14 might be a secondary effect of RhoAV14-induced reduction of cell adhesion activity. To address this issue, we examined the effect of RhoAV14 on the NGF-induced Rac1 activation in suspended RhoAV14-inducible PC12 cells. NGF induced a rapid increase in the amount of GTP-Rac1 in suspended PC12 cells as well, but the level reached a maximum at 1 min (data not shown). As shown in Fig.4, the expression of RhoAV14by IPTG inhibited the NGF-induced Rac1 activation in the suspended PC12 cells. Thus, the RhoAV14-induced down-regulation of the Rac1 activation is not due to the reduction of cell adhesion activity. NGF has been known to differentiate PC12 cells through the ERK signal transduction pathway (26Boulton T.G. Nye S.H. Robbins D.J. Ip N.Y. Radziejewska E. Morgenbesser S.D. DePinho R.A. Panayotatos N. Cobb M.H. Yancopoulos G.D. Cell. 1991; 65: 663-675Abstract Full Text PDF PubMed Scopus (1473) Google Scholar, 27Traverse S. Gomez N. Paterson H. Marshall C. Cohen P. Biochem. J. 1992; 288: 351-355Crossref PubMed Scopus (802) Google Scholar). RhoAV14-induced down-regulation of the Rac1 activation might result from perturbation of the NGF-induced ERK activation. To exclude this possibility, we examined the effect of RhoAV14 on the NGF-induced ERK activation by visualizing phosphorylation of endogenous ERK with an antiphospho-ERK antibody. As shown in Fig.5, the expression of RhoAV14by IPTG did not affect the NGF-induced ERK activation in RhoAV14-inducible PC12 cells, indicating that the RhoAV14-induced down-regulation occurred irrespective of the ERK signal transduction pathway. Rho-associated kinase, one of the downstream targets of RhoA, has been shown to be involved in a variety of actions of Rho (28Kaibuchi K. Kuroda S. Amano M. Annu. Rev. Biochem. 1999; 68: 459-486Crossref PubMed Scopus (883) Google Scholar). To determine whether Rho-associated kinase was involved in the RhoAV14-induced down-regulation of the Rac1 activation, the cells were treated with a Rho-associated kinase-selective inhibitor, Y-27632 (29Uehata M. Ishizaki T. Satoh H. Ono T. Kawahara T. Morishita T. Tamakawa H. Yamagami K. Inui J. Maekawa M. Narumiya S. Nature. 1997; 389: 990-994Crossref PubMed Scopus (2507) Google Scholar). As shown in Fig.6, Y-27632 had little effect on the basal and NGF-induced Rac1 activation. However, Y-27632 completely abolished the RhoAV14-induced down-regulation of the Rac1 activation in both adherent and suspended RhoAV14-inducible PC12 cells. In addition, Y-27632 also abolished the Gα12QL-induced down-regulation of the Rac1 activation in Gα12QL-inducible PC12 cells (data not shown). On the other hand, Y-27632 did not affect the time course profile of the NGF-induced Rac1 activation (Fig. 6 E). These results indicate that the RhoAV14-induced down-regulation of the Rac1 activation is mediated by Rho-associated kinase. We examined the subcellular distribution of Rac1 and F-actin after the stimulation with NGF. Immunofluorescence staining of Rac1 using an anti-Rac1 antibody revealed that in unstimulated RhoAV14-inducible PC12 cells, Rac1 was present mainly in the cytoplasm (Fig.7). At 3 min after the addition of NGF, RhoAV14-inducible PC12 cells produced a few protrusions from the cell membrane, and Rac1 and F-actin were accumulated at these protrusions. We recently showed that the constitutively active Rac1, Rac1V12, induced membrane ruffles where Rac1V12and F-actin were accumulated, but Rac1V12 failed to induce protrusions in PC12 cells (20Katoh H. Yasui H. Yamaguchi Y. Aoki J. Fujita H. Mori K. Negishi M. Mol. Cell. Biol. 2000; 20: 7378-7387Crossref PubMed Scopus (123) Google Scholar). Furthermore, Rac1V12 also induced membrane ruffles in NGF-treated cells and abrogated the NGF-induced straightforward neurite formation (data not shown). However, the expression of dominant negative Rac1, Rac1N17, blocked the NGF-induced F-actin redistribution and the protrusion formation, indicating that Rac1 activation is required for the formation of F-actin-rich protrusion (data not shown). On the other hand, the expression of RhoAV14 gave rise to cell rounding accompanied by formation of a thick ringlike structure of cortical actin filaments at the cell periphery. When the cells expressing RhoAV14 were stimulated with NGF, the thick ring of cortical actin filaments remained unchanged, and formation of protrusions enriched with Rac1 and F-actin was not observed. We next examined the effect of Rho-associated kinase on the NGF-induced accumulation of Rac1 and F-actin by transient transfection of the constitutively active form of Rho-associated kinase, CD-ROKα. Transfected cells were identified by cotransfection of GFP. As shown in Fig. 8, CD-ROKα also induced the cell rounding with cortical actin filaments and inhibited the NGF-induced accumulation of Rac1 and F-actin at the protrusions. Moreover, Y-27632 completely abolished the RhoAV14-induced inhibition of the accumulation of Rac1 and F-actin (Fig.9). These results indicate that RhoAV14 inhibited the NGF-induced accumulation of Rac1 and F-actin at the protrusions and eventually suppressed the NGF-induced neurite outgrowth via Rho-associated kinase.Figure 8Effect of CD-ROKα on the NGF-induced subcellular distribution of endogenous Rac1 and F-actin. PC12 cells were transiently transfected with an empty vector (a--d) or an expression vector encoding CD-ROKα (e--h) along with an expression vector encoding GFP. At 24 h after transfection, cells were stimulated with 50 ng/ml NGF for 3 min and were fixed. Transfected cells were identified by the fluorescence of GFP (a, c,e, and g). Cells were stained with an anti-Rac1 monoclonal antibody (b and f) or rhodamine-conjugated phalloidin (d and h). The results shown are representative of three independent experiments.Bar, 10 μm.View Large Image Figure ViewerDownload Hi-res image Download (PPT)Figure 9Effect of a Rho-associated kinase inhibitor on the RhoAV14-induced inhibition of endogenous Rac1 and F-actin distribution. Serum-starved RhoAV14-inducible PC12 cells were treated with or without 5 mm IPTG for 12 h in the presence of 10 μm Y-27632. The cells stimulated or not stimulated with 50 ng/ml NGF for 3 min were fixed and double-stained with an anti-Rac1 monoclonal antibody (left panels) and Alexa 488-conjugated phalloidin (right panels). The results shown are representative of three independent experiments. Bar, 10 μm.View Large Image Figure ViewerDownload Hi-res image Download (PPT) The cross-talk between Rac and Rho GTPases has been extensively studied (18Burridge K. Science. 1999; 283: 2028-2029Crossref PubMed Scopus (52) Google Scholar, 30Evers E.E. Zondag G.C. Malliri A. Price L.S. ten Klooster J.P. van der Kammen R.A. Collard J.G. Eur. J. Cancer. 2000; 36: 1269-1274Abstract Full Text Full Text PDF PubMed Scopus (192) Google Scholar), and the actions of Rac and Rho have been shown to counteract each other in a variety of cell lines (14Kozma R. Sarner S. Ahmed S. Lim L. Mol. Cell. Biol. 1997; 17: 1201-1211Crossref PubMed Scopus (534) Google Scholar, 31Moorman J.P. Luu D. Wickham J. Bobak D.A. Hahn C.S. Oncogene. 1999; 18: 47-57Crossref PubMed Scopus (91) Google Scholar, 32Ory S. Munari-Silem Y. Fort P. Jurdic P. J. Cell Sci. 2000; 113: 1177-1188Crossref PubMed Google Scholar, 33van Leeuwen F.N. Kain H.E. van der Kammen R.A. Michiels F. Kranenburg O.W. Collard J.G. J. Cell Biol. 1997; 139: 797-807Crossref PubMed Scopus (317) Google Scholar). Contractility by myosin activation has been shown to be antagonistically regulated by Rac1 and RhoA through the action of PAK on myosin light chain kinase and that of Rho-associated kinase on myosin phosphatase, respectively (18Burridge K. Science. 1999; 283: 2028-2029Crossref PubMed Scopus (52) Google Scholar). Thus, the cross-talk is coordinated regulation of downstream functions of Rac1 and RhoA, usually referred to as signal convergence (34Bar-Sagi D. Hall A. Cell. 2000; 103: 227-238Abstract Full Text Full Text PDF PubMed Scopus (694) Google Scholar). Such signal-convergent regulation by Rac1 and RhoA was also reported in neuronal cells. Activation of Rho promotes contractility by activation of Rho-associated kinase, leading to neurite retraction, cell rounding, and the inhibition of neurite outgrowth, while activation of Rac enhances cell spreading and neurite extension and interferes with Rho-mediated cell rounding through phosphorylation of the myosin-II heavy chain, suggesting antagonistic roles for Rho and Rac in the control of neuronal morphology (35van Leeuwen F.N. van Delft S. Kain H.E. van der Kammen R.A. Collard J.G. Nat. Cell Biol. 1999; 1: 242-248Crossref PubMed Scopus (181) Google Scholar). In contrast to the signal-convergent system, we here demonstrated that constitutively active RhoA or Gα12QL-induced endogenous RhoA activation down-regulated the NGF-induced Rac1 activation via a Rho-associated kinase-dependent pathway. This down-regulation of Rac1 activity by RhoA can be categorized into the different regulatory mechanisms, upstream signal regulation, referred to as signal divergence (34Bar-Sagi D. Hall A. Cell. 2000; 103: 227-238Abstract Full Text Full Text PDF PubMed Scopus (694) Google Scholar). Concerning this system, down-regulation of Rho activity by Rac has been reported in NIH3T3 cells (36Sander E.E. ten Klooster J.P. van Delft S. van der Kammen R.A. Collard J.G. J. Cell Biol. 1999; 147: 1009-1021Crossref PubMed Scopus (729) Google Scholar). However, this is the first report of down-regulation of Rac1 activity by RhoA. Rho activation induces cell rounding, decreasing cell adhesion activity in PC12 cells (21Yamaguchi Y. Katoh H. Yasui H. Aoki J. Nakamura K. Negishi M. J. Neurochem. 2000; 75: 708-717Crossref PubMed Scopus (24) Google Scholar, 25Katoh H. Aoki J. Yamaguchi Y. Kitano Y. Ichikawa A. Negishi M. J. Biol. Chem. 1998; 273: 28700-28707Abstract Full Text Full Text PDF PubMed Scopus (156) Google Scholar). However, the RhoA-induced down-regulation of Rac1 activity is not a secondary effect of this RhoA-mediated reduction of cell adhesion activity, because the down-regulation was observed with suspended PC12 cells as well. Therefore, the site of action of RhoA is on a step(s), involved in the NGF-induced Rac1 activation signaling pathway. Ras is a critical component in the signaling pathway of the NGF-induced neurite outgrowth in PC12 cells (37Szeberenyi J. Cai H. Cooper G.M. Mol. Cell. Biol. 1990; 10: 5324-5332Crossref PubMed Scopus (276) Google Scholar, 38Thomas S.M. DeMarco M. D'Arcangelo G. Halegoua S. Brugge J.S. Cell. 1992; 68: 1031-1040Abstract Full Text PDF PubMed Scopus (501) Google Scholar, 39Wood K.W. Sarnecki C. Roberts T.M. Blenis J. Cell. 1992; 68: 1041-1050Abstract Full Text PDF PubMed Scopus (656) Google Scholar), and activation of Ras was shown to induce neurite outgrowth (20Katoh H. Yasui H. Yamaguchi Y. Aoki J. Fujita H. Mori K. Negishi M. Mol. Cell. Biol. 2000; 20: 7378-7387Crossref PubMed Scopus (123) Google Scholar, 40Noda M. Ko M. Ogura A. Liu D.G. Amano T. Takano T. Ikawa Y. Nature. 1985; 318: 73-75Crossref PubMed Scopus (348) Google Scholar, 41Bar-Sagi D. Feramisco J.R. Cell. 1985; 42: 841-848Abstract Full Text PDF PubMed Scopus (566) Google Scholar). Recently, Rac1 was revealed to act downstream of Ras in neurite outgrowth independent of Ras-mediated ERK activation in N1E-115 neuroblastoma cells (42Sarner S. Kozma R. Ahmed S. Lim L. Mol. Cell. Biol. 2000; 20: 158-172Crossref PubMed Scopus (107) Google Scholar). Furthermore, NGF was shown to activate ERK in PC12 cells in the presence of RhoAV14 expression (16Sebok A. Nusser N. Debreceni B. Guo Z. Santos M.F. Szeberenyi J. Tigyi G. J. Neurochem. 1999; 73: 949-960Crossref PubMed Scopus (108) Google Scholar). We also observed here that active RhoA did not perturb the NGF-induced ERK activation. Therefore, active RhoA may modulate the signaling pathway from Ras to Rac1 but not from Ras to ERK. The activity of Rac1 is known to be regulated by a variety of proteins, such as guanine nucleotide exchange factors, GTPase-activating proteins, and guanine nucleotide dissociation inhibitors (28Kaibuchi K. Kuroda S. Amano M. Annu. Rev. Biochem. 1999; 68: 459-486Crossref PubMed Scopus (883) Google Scholar). A Rac guanine nucleotide exchange factor, Sos, was shown to be implicated in coupling Ras to Rac (43Nimnual A.S. Yatsula B.A. Bar-Sagi D. Science. 1998; 279: 560-563Crossref PubMed Scopus (387) Google Scholar, 44Scita G. Nordstrom J. Carbone R. Tenca P. Giardina G. Gutkind S. Bjarnegard M. Betsholtz C. Di Fiore P.P. Nature. 1999; 401: 290-293Crossref PubMed Scopus (283) Google Scholar). RhoA may modulate the activities of these molecules, such as Sos. Molecular mechanisms of the RhoA-induced down-regulation of Rac1 activity are currently under investigation in our laboratory. We further investigated the effect of active RhoA on actin reorganization. We revealed here that NGF recruited Rac1 to the protrusion sites and accumulated F-actin, initiating the neurite formation. Active RhoA inhibited the NGF-induced Rac1 recruitment to the protrusion sites and resultant process formation through Rho-associated kinase. Our result showed that the cytoskeletal action of RhoA and its downstream effector, Rho-associated kinase, was the stabilization of cortical actin filaments in the cell periphery of PC12 cells. Since Rho-associated kinase is known to increase phosphorylation of myosin light chain, consequently elevating contractile activity of myosin, this myosin activation may enhance the stabilization of cortical actin filaments. In neuronal cells, disruption of the cortical actin filament network was shown to induce neurite formation, suggesting that contractile activity by cortical actin filaments inhibits the neuritogenesis and neurite extension (45Spero D.A. Roisen F.J. Brain Res. 1985; 355: 155-159Crossref PubMed Scopus (24) Google Scholar, 46Bradke F. Dotti C.G. Science. 1999; 283: 1931-1934Crossref PubMed Scopus (412) Google Scholar). Therefore, Rho-associated kinase-induced formation of the ringlike structure of cortical actin filaments in PC12 cells may interfere with the NGF-induced Rac1 recruitment and F-actin reorganization, which are initial events for the neurite formation. In this study, we showed that Y-27632, a Rho-associated kinase inhibitor, completely abolished the RhoA-induced down-regulation of the NGF-induced Rac1 activation, suggesting that the RhoA-induced down-regulation is mediated by Rho-associated kinase. The inhibition of Rac1 recruitment by Rho-associated kinase may contribute to the RhoA-induced down-regulation of the Rac1 activation. Studies using a dominant negative Rac1 show that Rac1 is involved in the NGF-induced neurite outgrowth in PC12 cells (17Chen X.Q. Tan I. Leung T. Lim L. J. Biol. Chem. 1999; 274: 19901-19905Abstract Full Text Full Text PDF PubMed Scopus (73) Google Scholar, 22Lamoureux P. Altun-Gultekin Z.F. Lin C. Wagner J.A. Heidemann S.R. J. Cell Sci. 1997; 110: 635-641Crossref PubMed Google Scholar). Furthermore, activation of Rac was shown to produce a loss of contractility associated with cell spreading and the formation of neurite-like extensions in N1E-115 cells (42Sarner S. Kozma R. Ahmed S. Lim L. Mol. Cell. Biol. 2000; 20: 158-172Crossref PubMed Scopus (107) Google Scholar). On the other hand, active Rho suppresses the NGF-induced neurite outgrowth in PC12 cells (16Sebok A. Nusser N. Debreceni B. Guo Z. Santos M.F. Szeberenyi J. Tigyi G. J. Neurochem. 1999; 73: 949-960Crossref PubMed Scopus (108) Google Scholar). Therefore, Rac and Rho exert antagonistic actions in neurite formation. We here demonstrated that NGF activated Rac1 but inversely suppressed the RhoA activity in PC12 cells. In the light of antagonistic actions of Rac1 and RhoA in neuronal morphology, the NGF-induced opposite regulation of Rac1 and RhoA activities may play an important role in the NGF-induced neurite outgrowth. The expression of the active RhoA did not affect the basal level of Rac1 activity, and Y-27632 did not increase the basal Rac1 activity in PC12 cells. Effects of RhoA and Y-27632 appear to be restricted to the NGF-induced Rac1 activation. Although the regulation of this basal Rac1 activity is not understood, the activation of Rac1 by NGF and the basal activity may be regulated by different signaling mechanisms. Suppression of RhoA activity by NGF may enhance the duration of the NGF-induced Rac1 activation. Recently, NGF was shown to abolish Rho activation by binding to the neurotrophin receptor p75 (47Yamashita T. Tucker K.L. Barde Y.A. Neuron. 1999; 24: 585-593Abstract Full Text Full Text PDF PubMed Scopus (436) Google Scholar). Considering the expression of p75 in PC12 cells (48Bilderback T.R. Gazula V.R. Lisanti M.P. Dobrowsky R.T. J. Biol. Chem. 1999; 274: 257-263Abstract Full Text Full Text PDF PubMed Scopus (145) Google Scholar), the NGF-induced down-regulation of RhoA may be mediated by p75. Furthermore, inactivation of Rho signaling pathway has been reported to promote axon regeneration in the central nervous system (8Lehmann M. Fournier A. Selles-Navarro I. Dergham P. Sebok A. Leclerc N. Tigyi G. McKerracher L. J. Neurosci. 1999; 19: 7537-7547Crossref PubMed Google Scholar), and inhibition of the Rho/Rho-associated kinase pathway was suggested to act as a gate critical for the initiation of axonal outgrowth (49Bito H. Furuyashiki T. Ishihara H. Shibasaki Y. Ohashi K. Mizuno K. Maekawa M. Ishizaki T. Narumiya S. Neuron. 2000; 26: 431-441Abstract Full Text Full Text PDF PubMed Scopus (257) Google Scholar). Thus, negative regulation of Rho activity is critical for neurite outgrowth. In sharp contrast to NGF, ephrin-A5 was shown to induce collapse of growth cone by opposite regulation of Rho and Rac, Rho activation and Rac down-regulation (50Wahl S. Barth H. Ciossek T. Aktories K. Mueller B.K. J. Cell Biol. 2000; 149: 263-270Crossref PubMed Scopus (334) Google Scholar). Neuronal morphology appears to be critically balanced between Rho and Rac activities. In conclusion, we demonstrate here that NGF oppositely regulates Rac1 and RhoA activities, activation of Rac1 and suppression of RhoA, and that activation of RhoA blocks the NGF-induced neurite formation by the inhibition of Rac1 activity via Rho-associated kinase in PC12 cells. This work takes a close-up of an important role of Rho family GTPases in neurite formation and will help to elucidate the molecular mechanism of neurite outgrowth. We thank Yoshitomi Pharmaceutical Industries and Dr. S. Nagata of Osaka University for supplying Y-27632 and pEF-BOS, respectively." @default.
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• W2013249463 title "RhoA Inhibits the Nerve Growth Factor-induced Rac1 Activation through Rho-associated Kinase-dependent Pathway" @default.
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