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      Rho-ROCK Signal Pathway Regulates Microtubule-Based Process Formation of Cultured Podocytes – Inhibition of ROCK Promoted Process Elongation

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          Background: Podocytes, renal glomerular visceral epithelial cells, have two kinds of processes, namely major processes containing microtubules (MTs) and foot processes with actin filaments (AFs). The present study investigated how MTs are organized by the Rho-ROCK signal transduction pathway during process formation of podocytes. Method: After induction of differentiation, podocytes of the conditionally immortalized mouse cell line were treated with Y-27632, a specific inhibitor of ROCK, and exoenzyme C3, an inhibitor of RhoA, as well as with forskolin whose effects include inhibition of RhoA, in order to inhibit the Rho-ROCK pathway. Results: Inhibition of ROCK significantly enhanced the formation of thick processes containing MT bundles. Y-27632 promoted process formation even in the presence of latrunculin A which disrupts AFs, strongly suggesting that ROCK directly regulates MT assembly. Treatment with Y-27632 increased MT stability, and stabilized MTs preferentially localized in podocyte processes. Moreover, when treated with a combination of Y-27632 and forskolin, and with Y-27632 and C3 as well, podocytes developed not only MT-based thick processes but also AF-based thin projections. Conclusions: These data indicate a contribution of ROCK in MT organization to promote podocyte process formation, although it was originally thought to regulate AF assembly. AF-based thin projections seem to be induced mainly by inhibition of RhoA and ROCK. The present study reveals a significant role of the Rho-ROCK signal pathway in the reorganization of both MTs and AFs during process formation of podocytes.

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          Most cited references 11

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          Molecular Dissection of the Rho-associated Protein Kinase (p160ROCK)-regulated Neurite Remodeling in Neuroblastoma N1E-115 Cells

          A critical role for the small GTPase Rho and one of its targets, p160ROCK (a Rho-associated coiled coil-forming protein kinase), in neurite remodeling was examined in neuroblastoma N1E-115 cells. Using wild-type and a dominant-negative form of p160ROCK and a p160ROCK-specific inhibitor, Y-27632, we show here that p160ROCK activation is necessary and sufficient for the agonist-induced neurite retraction and cell rounding. The neurite retraction was accompanied by elevated phosphorylation of myosin light chain and the disassembly of the intermediate filaments and microtubules. Y-27632 blocked both neurite retraction and the elevation of myosin light chain phosphorylation in a similar concentration-dependent manner. On the other hand, suppression of p160ROCK activity by expression of a dominant-negative form of p160ROCK induced neurites in the presence of serum by inducing the reassembly of the intermediate filaments and microtubules. The neurite outgrowth by the p160ROCK inhibition was blocked by coexpression of dominant-negative forms of Cdc42 and Rac, indicating that p160ROCK constitutively and negatively regulates neurite formation at least in part by inhibiting activation of Cdc42 and Rac. The assembly of microtubules and intermediate filaments to form extended processes by inhibitors of the Rho–ROCK pathway was also observed in Swiss 3T3 cells. These results indicate that Rho/ROCK-dependent tonic inhibition of cell process extension is exerted via activation of the actomysin-based contractility, in conjunction with a suppression of assembly of intermediate filaments and microtubules in many cell types including, but not exclusive to, neuronal cells.
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            Serine phosphorylation negatively regulates RhoA in vivo.

            Previous work indicates that RhoA phosphorylation on Ser188 by cAMP or cGMP-dependent kinases inhibits its activity. However, these studies lacked the possibility to directly study phosphorylated RhoA activity in vivo. Therefore, we created RhoA proteins containing phosphomimetic residues in place of the cAMP/cGMP-dependent kinase phosphorylation site. RhoA phosphorylation or phosphomimetic substitution did not affect Rho guanine nucleotide exchange factor, GTPase activating protein, or geranylgeranyl transferase activity in vitro but promoted binding to the Rho guanine-dissociation inhibitor as measured by exchange factor competition assays. The in vitro similarities between RhoA phosphomimetic proteins and phosphorylated RhoA allowed us to study function of phosphorylated RhoA in vivo. RhoA phosphomimetic proteins display depressed GTP loading when transiently expressed in NIH 3T3 cells. Stable-expressing RhoA and RhoA(S188A) clones spread significantly slower than mock-transfected or RhoA(S188E) clones. RhoA(S188A) clones were protected from the morphological effects of a cAMP agonist, whereas phosphomimetic clones exhibit stress fiber disassembly similar to control cells. Together, these data provide in vivo evidence that addition of a charged group to Ser188 upon phosphorylation negatively regulates RhoA activity and indicates that this occurs through enhanced Rho guanine-dissociation inhibitor interaction rather than direct perturbation of guanine nucleotide exchange factor, GTPase activating protein, or geranylgeranyl transferase activity.
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              Progressive impairment of kidneys and reproductive organs in mice lacking Rho GDIalpha.

              The Rho small G protein family members regulate various actin cytoskeleton-dependent cell functions. The Rho GDI (GDP dissociation inhibitor) family, consisting of Rho GDIalpha, -beta, and -gamma, is a regulator that keeps the Rho family members in the cytosol as the GDP-bound inactive form and translocates the GDP-bound form from the membranes to the cytosol after the GTP-bound form accomplishes their functions. Rho GDIalpha is ubiquitously expressed in mouse tissues and shows GDI activity on all the Rho family members in vitro. We have generated mice lacking Rho GDIalpha by homologous recombination to clarify its in vivo function. Rho GDIalpha -/- mice showed several abnormal phenotypes. Firstly, Rho GDIalpha -/- mice were initially viable but developed massive proteinuria mimicking nephrotic syndrome, leading to death due to renal failure within a year. Histologically, degeneration of tubular epithelial cells and dilatation of distal and collecting tubules were readily detected in the kidneys. Secondly, Rho GDIalpha -/- male mice were infertile and showed impaired spermatogenesis with vacuolar degeneration of seminiferous tubules in their testes. Thirdly, Rho GDIalpha -/- embryos derived from Rho GDIalpha -/- female mice were defective in the postimplantation development. In addition, these morphological and functional abnormalities showed age-dependent progression. These results suggest that the signaling pathways of the Rho family members regulated by Rho GDIalpha play important roles in maintaining the structure and physiological function of at least kidneys and reproductive systems in adult mice.

                Author and article information

                Nephron Exp Nephrol
                Cardiorenal Medicine
                S. Karger AG
                June 2004
                17 November 2004
                : 97
                : 2
                : e49-e61
                Department of aAnatomy and Embryology, bDepartment of Hygiene, School of Medicine, and cDepartment of Biological Resources, Integrated Center for Science, Ehime University, Ehime, Japan
                78406 Nephron Exp Nephrol 2004;97:e49–e61
                © 2004 S. Karger AG, Basel

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                Figures: 7, References: 38, Pages: 1
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