Alterations in the actin cytoskeleton of renal tubular epithelial cells during periods of ischemic injury and recovery have important consequences for normal cell and kidney function. Myosin II has been demonstrated to be an important effector in organizing basal actin structures in some cell types. ATP depletion in vitro has been demonstrated to recapitulate alterations of the actin cytoskeleton in renal tubular epithelial cells observed during renal ischemia in vivo. We utilized this reversible cell culture model of ischemia to examine the correlation of the activation state and cellular distribution of myosin II with disruption of actin stress fibers in Madin-Darby canine kidney (MDCK) cells during ATP depletion and recovery from ATP depletion. We found that myosin II inactivation occurs rapidly and precedes dissociation of myosin II from actin stress fibers during ATP depletion. Myosin II activation temporally correlates with colocalization of myosin II to reorganizing stress fibers during recovery from ATP depletion. Furthermore, myosin activation and actin stress fiber formation were found to be Rho-associated Ser/Thr protein kinase dependent during recovery from ATP depletion.
"This NM II-dependent regulatory mechanism may operate in neurite branching and angiogenic sprouting , where precise regulation of contractility and actin dynamics is critically important. Moreover, the depletion of cellular ATP induces actomyosin disassembly , which would result in an uncontrolled release of GEFs from NM II, accompanied by Rho GTPase activation. Hence, the NM II-dependent release of GEFs may contribute to the pathogenesis of cerebral ischemia by the Rac1-dependent generation of reactive oxygen species  and postischemic morphological changes . "
[Show abstract][Hide abstract] ABSTRACT: Non-muscle myosin II (NM II) regulates a wide range of cellular functions, including neuronal differentiation, which requires precise spatio-temporal activation of Rho GTPases. The molecular mechanism underlying the NM II-mediated activation of Rho GTPases is poorly understood. The present study explored the possibility that NM II regulates neuronal differentiation, particularly morphological changes in growth cones and the distal axon, through guanine nucleotide exchange factors (GEFs) of the Dbl family.
NM II colocalized with GEFs, such as βPIX, kalirin and intersectin, in growth cones. Inactivation of NM II by blebbistatin (BBS) led to the increased formation of short and thick filopodial actin structures at the periphery of growth cones. In line with these observations, FRET analysis revealed enhanced Cdc42 activity in BBS-treated growth cones. BBS treatment also induced aberrant targeting of various GEFs to the distal axon where GEFs were seldom observed under physiological conditions. As a result, numerous protrusions and branches were generated on the shaft of the distal axon. The disruption of the NM II-GEF interactions by overexpression of the DH domains of βPIX or Tiam1, or by βPIX depletion with specific siRNAs inhibited growth cone formation and induced slender axons concomitant with multiple branches in cultured hippocampal neurons. Finally, stimulation with nerve growth factor induced transient dissociation of the NM II-GEF complex, which was closely correlated with the kinetics of Cdc42 and Rac1 activation.
Our results suggest that NM II maintains proper morphology of neuronal growth cones and the distal axon by regulating actin dynamics through the GEF-Rho GTPase signaling pathway.
PLoS ONE 04/2014; 9(4):e95212. DOI:10.1371/journal.pone.0095212 · 3.23 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Membrane-cytoskeleton interactions have been shown to be crucial to modulate polarity, cell shape and the paracellular pathway in epithelial MDCK cell monolayers. In particular, actin organization and myosin-dependent contractility play an important role in the regulation of these functions. Participation of myosin in vectorial transport, expressed as formation of domes, was investigated in confluent monolayers of high transepithelial electrical resistance (TER) plated on non-permeable supports. Cells exposed to 2,3-butanedione monoxime, a selective inhibitor of myosin ATPase, showed a remarkable increase in the number of domes. Replacement of extracellular Na+ and Cl- and inhibition of Na+-K+-ATPase blocked the induction of domes. The monoxime also caused a reduction of the TER leading to an increase in the paracellular flux of small molecular weight dextran. However, immunofluorescence microscopy of drug-treated cells showed that the localization and staining pattern of tight junction proteins ZO-1, occludin, and claudin 1, or the actin-myosin ring at the zonula adherens, were not modified. Treatment with the drug produced striking re-arrangements of actin filaments at the microvilli and at the basal level of the cells. Our data show that disruption of actin-myosin interaction at several cellular sites contributed importantly to the increased transport activity and the formation of the domes. These results point to the relevant role or actin-myosin dynamics and actin organization in the regulation of ion and water channel activity in these cells.
Journal of Muscle Research and Cell Motility 02/2002; 23(3):223-34. DOI:10.1023/A:1020979203141 · 2.09 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Rho GTPases are critical for actin cytoskeletal regulation, and alterations in their activity may contribute to altered cytoskeletal organization that characterizes many pathological conditions, including ischemia. G protein activity is a function of the ratio of GTP-bound (active) to GDP-bound (inactive) protein, but the effect of altered energy metabolism on Rho protein activity has not been determined. We used antimycin A and substrate depletion to induce depletion of intracellular ATP and GTP in the kidney proximal tubule cell line LLC-PK10 and measured the activity of RhoA, Rac1, and Cdc42 with GTPase effector binding domains fused to glutathione S-transferase. RhoA activity decreased in parallel with the concentration of ATP and GTP during depletion, so that by 60 min there was no detectable RhoA-GTP, and recovered rapidly when cells were returned to normal culture conditions. Dissociation of the membrane-actin linker ezrin, a target of RhoA signaling, from the cytoskeletal fraction paralleled the decrease in RhoA activity and was augmented by treatment with the Rho kinase inhibitor Y27632. The activity of Cdc42 did not decrease significantly during depletion or recovery. Rac1 activity decreased moderately to a minimum at 30 min of depletion but then increased from 30 to 90 min of depletion, even as ATP and GTP levels continued to fall. Our data are consistent with a principal role for RhoA in cytoskeletal reorganization during ischemia and demonstrate that the activity of Rho GTPases can be maintained even at low GTP concentrations.
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