Regulation of the association of adducin with actin filaments by Rho-associated kinase (Rho-kinase) and myosin phosphatase

Erythrocyte adducin: a structural regulator of the red blood cell membrane

Adducin is an alpha, beta heterotetramer that performs multiple important functions in the human erythrocyte membrane. First, adducin forms a bridge that connects the spectrin-actin junctional complex to band 3, the major membrane-spanning protein in the bilayer. Rupture of this bridge leads to membrane instability and spontaneous fragmentation. Second, adducin caps the fast growing (barbed) end of actin filaments, preventing the tetradecameric protofilaments from elongating into macroscopic F-actin microfilaments. Third, adducin stabilizes the association between actin and spectrin, assuring that the junctional complex remains intact during the mechanical distortions experienced by the circulating cell. And finally, adducin responds to stimuli that may be important in regulating the global properties of the cell, possibly including cation transport, cell morphology and membrane deformability. The text below summarizes the structural properties of adducin, its multiple functions in erythrocytes, and the consequences of engineered deletions of each of adducin subunits in transgenic mice.

Involvement of reactive oxygen species in thrombin-induced pulmonary vasoconstriction

RATIONALE

Pulmonary vascular thrombosis and thrombotic arteriopathy are common pathological findings in pulmonary arterial hypertension. Thrombin may thus play an important role in the pathogenesis and pathophysiology of pulmonary arterial hypertension.

OBJECTIVES

The present study aimed to elucidate the contractile effect of thrombin in the pulmonary artery and clarify its underlying mechanisms.

METHODS

The changes in cytosolic Ca²(+) concentrations ([Ca²(+)](i)), 20-kD myosin light chain (MLC20) phosphorylation, and contraction were monitored in the isolated porcine pulmonary artery. The production of reactive oxygen species (ROS) was evaluated by fluorescence imaging.

MEASUREMENTS AND MAIN RESULTS

In the presence of extracellular Ca²(+), thrombin induced a sustained contraction accompanied by an increase in [Ca²(+)](i) and the phosphorylation of MLC20. In the absence of extracellular Ca²(+), thrombin induced a contraction without either [Ca²(+)](i) elevation or MLC20 phosphorylation. This Ca²(+)- and MLC20 phosphorylation-independent contraction was mimicked by hydrogen peroxide and inhibited by N-acetyl cysteine. Fluorescence imaging revealed thrombin to induce the production of ROS. A Rho-kinase inhibitor, Y27632, inhibited not only the thrombin-induced Ca²(+)- and MLC20 phosphorylation-dependent contraction, but also the Ca²(+)- and MLC20 phosphorylation-independent contraction and the ROS production. These effects of thrombin were mimicked by a proteinase-activated receptor 1 (PAR₁)-activating peptide.

CONCLUSIONS

This study elucidated the Ca²(+)- and MLC20 phosphorylation-independent ROS-mediated noncanonical mechanism as well as Ca²(+)- and MLC20 phosphorylation-dependent canonical mechanism that are involved in the thrombin-induced PAR₁-mediated pulmonary vasoconstriction. Rho-kinase was suggested to play multiple roles in the development of thrombin-induced pulmonary vasoconstriction.

Reduction of hyaluronan-CD44-mediated growth, migration, and cisplatin resistance in head and neck cancer due to inhibition of Rho kinase and PI-3 kinase signaling

OBJECTIVES

To investigate whether hyaluronan (HA), a ligand for the transmembrane receptor CD44, and CD44, which acts through multiple signaling pathways to influence cellular behavior, promote Rho kinase- and phosphatidylinositol 3 (PI-3) kinase-mediated oncogenic signaling to alter cisplatin sensitivity and stimulate tumor cell proliferation, migration, and matrix metalloproteinase secretion in head and neck squamous cell carcinoma (HNSCC).

DESIGN

Laboratory investigation using the HNSCC cell line HSC-3.

SETTING

University laboratory.

MAIN OUTCOME MEASURES

Rho kinase and PI-3 kinase activity, myosin phosphatase and AKT phosphorylation, tumor cell growth, migration, and matrix metalloproteinase secretion were measured in the presence or absence of HA, cisplatin, and inhibitors of Rho kinase and PI-3 kinase.

RESULTS

The addition of HA, but not HA plus anti-CD44 antibody, resulted in increased Rho kinase and PI-3 kinase activity. Results of immunoblotting studies demonstrated that HA promotes Rho kinase-mediated myosin phosphatase phosphorylation and PI-3 kinase-mediated AKT phosphorylation. Hyaluronan was shown to promote migration and increased matrix metalloproteinase secretion through Rho kinase-mediated signaling. Hyaluronan treatment promoted increased tumor proliferation and resulted in a 12-fold reduced ability of cisplatin to cause HNSCC cell death. On the other hand, the presence of Y-27632, a Rho kinase inhibitor, and LY-294002, a PI-3 kinase inhibitor, blocked HA-mediated cisplatin resistance by HNSCC.

CONCLUSIONS

Our results suggest that HA and CD44 promote Rho kinase- and PI-3 kinase-mediated oncogenic signaling and cisplatin resistance. Perturbation of HA-CD44-mediated Rho kinase and PI-3 kinase signaling pathways may be a novel strategy to treat HNSCC.

Increased leukocyte rho kinase (ROCK) activity and endothelial dysfunction in cigarette smokers

Rho-associated kinases (ROCKs) have been shown to be involved in the pathogenesis of atherosclerosis. Although smoking is associated with endothelial dysfunction and ROCK inhibitors improve endothelial function in smokers, it is not known whether ROCK activity is increased in smokers and whether this correlates with endothelial dysfunction. The purpose of this study was to evaluate the relationship between ROCK activity and endothelial function in smokers. We evaluated flow-mediated vasodilatation (FMD) using ultrasonography and ROCK activity in peripheral leukocytes using western blot analysis in 14 male smokers (28.1+/-3.9 years) and 15 healthy male non-smokers (28.3+/-3.6 years). ROCK activity was defined as the ratio of phospho-myosin-binding subunit (MBS) on myosin light-chain phosphatase to total MBS. FMD was significantly less in smokers than in non-smokers (4.7+/-3.1 vs. 9.0+/-3.8%, P=0.005). Nitroglycerine-induced vasodilation was similar in the two groups. ROCK activity was greater in smokers than in non-smokers (0.78+/-0.27 vs. 0.54+/-018, P=0.012). The expression of total MBS, ROCK1 and ROCK2 were similar in the two groups. ROCK activity correlated with systolic blood pressure (r=0.42, P=0.039). Multiple regression analysis revealed that smoking is an independent predictor of ROCK activity. There was a significant correlation between FMD and ROCK activity (r=-0.42, P=0.035). No other variable was correlated with FMD. These findings suggest that ROCK activity is enhanced by smoking and is a predictor of endothelial function.

Adducin forms a bridge between the erythrocyte membrane and its cytoskeleton and regulates membrane cohesion

The erythrocyte membrane skeleton is the best understood cytoskeleton. Because its protein components have homologs in virtually all other cells, the membrane serves as a fundamental model of biologic membranes. Modern textbooks portray the membrane as a 2-dimensional spectrin-based membrane skeleton attached to a lipid bilayer through 2 linkages: band 3-ankyrin-beta-spectrin and glycophorin C-protein 4.1-beta-spectrin.(1-7) Although evidence supports an essential role for the first bridge in regulating membrane cohesion, rupture of the glycophorin C-protein 4.1 interaction has little effect on membrane stability.(8) We demonstrate the existence of a novel band 3-adducin-spectrin bridge that connects the spectrin/actin/protein 4.1 junctional complex to the bilayer. As rupture of this bridge leads to spontaneous membrane fragmentation, we conclude that the band 3-adducin-spectrin bridge is important to membrane stability. The required relocation of part of the band 3 population to the spectrin/actin junctional complex and its formation of a new bridge with adducin necessitates a significant revision of accepted models of the erythrocyte membrane.

Postischemic administration of angiotensin II type 1 receptor blocker reduces cerebral infarction size in hypertensive rats

Lowering the blood pressure (BP) during the acute period following ischemic stroke is still a controversial treatment. In this study, we investigated the effect of postischemic treatment using the angiotensin II type 1 receptor blocker, candesartan, on brain damage in focal cerebral ischemia. Spontaneously hypertensive rats underwent transient occlusion of the middle cerebral artery for 1 h. Candesartan (0.1, 1 and 10 mg kg(-1)) or vehicle was administered orally 3 and 24 h after ischemia. Blood pressure and neurological function were monitored, and infarct volume was evaluated 48 h after occlusion. Cerebral blood flow was measured using laser Doppler flowmetry before and after treatment with candesartan. Activation of Rho-kinase in cerebral microvessels was evaluated by immunohistochemistry. Systolic blood pressure was markedly lowered with both moderate and high doses, but it did not fall with a low dose of candesartan. The infarct volume was reduced in rats treated with the low dose of candesartan but not in those treated with the moderate or high doses. Cerebral blood flow decreased in parallel with the reduction in BP 3 h after treatment using the moderate dose, but it did not change after treatment with the low dose of candesartan, compared with vehicle. Rho-kinase was activated in the brain vessels of the ischemic cortex, but treatment with candesartan suppressed it. Our results show that oral administration of candesartan after transient focal ischemia reduced infarct volume at doses that showed little effect on BP. The neurovascular protective effects of candesartan may be caused by the inhibition of Rho-kinase in brain microvessels.

Signaling to the apical membrane and to the paracellular pathway: changes in the cytosolic proteome of Aedes Malpighian tubules

Using a proteomics approach, we examined the post-translational changes in cytosolic proteins when isolated Malpighian tubules of Aedes aegypti were stimulated for 1 min with the diuretic peptide aedeskinin-III (AK-III, 10(-7) mol l(-1)). The cytosols of control (C) and aedeskinin-treated (T) tubules were extracted from several thousand Malpighian tubules, subjected to 2-D electrophoresis and stained for total proteins and phosphoproteins. The comparison of C and T gels was performed by gel image analysis for the change of normalized spot volumes. Spots with volumes equal to or exceeding C/T ratios of +/-1.5 were robotically picked for in-gel digestion with trypsin and submitted for protein identification by nanoLC/MS/MS analysis. Identified proteins covered a wide range of biological activity. As kinin peptides are known to rapidly stimulate transepithelial secretion of electrolytes and water by Malpighian tubules, we focused on those proteins that might mediate the increase in transepithelial secretion. We found that AK-III reduces the cytosolic presence of subunits A and B of the V-type H(+) ATPase, endoplasmin, calreticulin, annexin, type II regulatory subunit of protein kinase A (PKA) and rab GDP dissociation inhibitor and increases the cytosolic presence of adducin, actin, Ca(2+)-binding protein regucalcin/SMP30 and actin-depolymerizing factor. Supporting the putative role of PKA in the AK-III-induced activation of the V-type H(+) ATPase is the effect of H89, an inhibitor of PKA, on fluid secretion. H89 reverses the stimulatory effect of AK-III on transepithelial fluid secretion in isolated Malpighian tubules. However, AK-III does not raise intracellular levels of cAMP, the usual activator of PKA, suggesting a cAMP-independent activation of PKA that removes subunits A and B from the cytoplasm in the assembly and activation of the V-type H(+) ATPase. Alternatively, protein kinase C could also mediate the activation of the proton pump. Ca(2+) remains the primary intracellular messenger of the aedeskinins that signals the remodeling of the paracellular complex apparently through protein kinase C, thereby increasing transepithelial anion secretion. The effects of AK-III on active transcellular and passive paracellular transport are additive, if not synergistic, to bring about the rapid diuresis.

Endothelial function and oxidative stress in cardiovascular diseases

The vascular endothelium is involved in the release of various vasodilators, including nitric oxide (NO), prostacyclin and endothelium-derived hyperpolarizing factor, as well as vasoconstrictors. NO plays an important role in the regulation of vascular tone, inhibition of platelet aggregation, and suppression of smooth muscle cell proliferation. Endothelial dysfunction is the initial step in the pathogenesis of atherosclerosis. Cardiovascular diseases are associated with endothelial dysfunction. It is well known that the grade of endothelial function is a predictor of cardiovascular outcomes. Oxidative stress plays an important role in the pathogenesis and development of cardiovascular diseases. Several mechanisms contribute to impairment of endothelial function. An imbalance of reduced production of NO or increased production of reactive oxygen species, mainly superoxide, may promote endothelial dysfunction. One mechanism by which endothelium-dependent vasodilation is impaired is an increase in oxidative stress that inactivates NO. This review focuses on recent findings and interaction between endothelial function and oxidative stress in cardiovascular diseases.

Rho-kinase contributes to sustained RhoA activation through phosphorylation of p190A RhoGAP

RhoA is transiently activated by specific extracellular signals such as endothelin-1 (ET-1) in vascular smooth muscle cells. RhoGAP negatively regulates RhoA activity: thus, RhoA becomes the GDP-bound inactive form afterward. Sustained activation of RhoA is induced with high doses of the extracellular signals and is implicated in certain diseases such as vasospasms. However, it remains largely unknown how prolonged activation of RhoA is induced. Here we show that Rho-kinase, an effector of RhoA, phosphorylated p190A RhoGAP at Ser(1150) and attenuated p190A RhoGAP activity in COS7 cells. Binding of Rnd to p190A RhoGAP is thought to enhance its activation. Phosphorylation of p190A RhoGAP by Rho-kinase impaired Rnd binding. Stimulation of vascular smooth muscle cells with a high dose of ET-1 provoked sustained RhoA activation and p190A RhoGAP phosphorylation, both of which were prohibited by a Rho-kinase inhibitor. The phosphomimic mutation of p190A RhoGAP weakened Rnd binding and RhoGAP activities. Taken together, these results suggest that ET-1 induces Rho-kinase activation and subsequent phosphorylation of p190A RhoGAP, leading to prolonged RhoA activation.

Induction of cell retraction by the combined actions of Abl-CrkII and Rho-ROCK1 signaling

Dynamic modulation of cell adhesion is integral to a wide range of biological processes. The small guanosine triphosphatase (GTPase) Rap1 is an important regulator of cell–cell and cell–matrix adhesions. We show here that induced expression of activated Abl tyrosine kinase reduces Rap1-GTP levels through phosphorylation of Tyr221 of CrkII, which disrupts interaction of CrkII with C3G, a guanine nucleotide exchange factor for Rap1. Abl-dependent down-regulation of Rap1-GTP causes cell rounding and detachment only when the Rho–ROCK1 pathway is also activated, for example, by lysophosphatidic acid (LPA). During ephrin-A1–induced retraction of PC3 prostate cancer cells, we show that endogenous Abl is activated and disrupts the CrkII–C3G complex to reduce Rap1-GTP. Interestingly, ephrin-A1–induced PC3 cell retraction also requires LPA, which stimulates Rho to a much higher level than that is activated by ephrin-A1. Our results establish Rap1 as another downstream target of the Abl–CrkII signaling module and show that Abl–CrkII collaborates with Rho–ROCK1 to stimulate cell retraction.

Opposing roles of p190RhoGAP and Ect2 RhoGEF in regulating cytokinesis

Evidence suggests that p190RhoGAP (p190), a GTPase activating protein (GAP) specific for Rho, plays a role in cytokinesis. First, ectopic expression of p190 induces a multinucleated cellular phenotype. Second, endogenous p190 localizes to the cleavage furrow of dividing cells. Lastly, its levels are reduced in late mitosis by ubiquitin-mediated proteasomal degradation, consistent with the idea that low levels of p190 and high levels of active Rho are required for completion of cytokinesis. As with p190, RhoA and the RhoGEF, ECT2, have been localized to the cleavage furrow. These findings raise the question of whether p190 and ECT2 cooperate antagonistically to regulate the activity of Rho and contraction of the actomyosin ring during cytokinesis. Here we demonstrate ECT2 can, in a dose-dependent manner, reduce multinucleation induced by p190. Furthermore, endogenous p190 and ECT2 colocalize at the cleavage furrow of dividing cells and stably associate with one another in co-immunoprecipitation assays. Functional and physical interactions between p190 and ECT2 are reflected in the levels of Rho activity, as assessed by Rho pull-down assays. Together, these results suggest that co-regulation of Rho activity by p190RhoGAP and ECT2 in the cleavage furrow determines whether cells properly complete cytokinesis.

Targeted deletion of alpha-adducin results in absent beta- and gamma-adducin, compensated hemolytic anemia, and lethal hydrocephalus in mice

In the red blood cell (RBC), adducin is present primarily as tetramers of alpha- and beta-subunits at spectrin-actin junctions, or junctional complexes. Mouse RBCs also contain small amounts of gamma-adducin. Platelets contain alpha- and gamma-adducin only. Adducin functions as a barbed-end actin capping protein to regulate actin filament length and recruits spectrin to the ends of actin filaments. To further define adducin's role in vivo, we generated alpha-adducin knockout mice. alpha-Adducin is absent in all tissues examined in homozygous null mice. In RBCs, beta- and gamma-adducin are also absent, indicating that alpha-adducin is the limiting subunit in tetramer formation at the spectrin-actin junction. Similarly, gamma-adducin is absent in alpha-null platelets. alpha-Adducin-null mice display compensated hemolytic anemia with features characteristic of RBCs in hereditary spherocytosis (HS), including spherocytes with significant loss of surface area, decreased mean corpuscular volume (MCV), cell dehydration, and increased osmotic fragility. Platelets maintain their normal discoid shape, and bleeding times are normal. alpha-Adducin-null mice show growth retardation at birth and throughout adulthood. Approximately 50% develop lethal communicating hydrocephalus with striking dilation of the lateral, third, and fourth ventricles. These data indicate that adducin plays a role in RBC membrane stability and in cerebrospinal fluid homeostasis.

alpha-Adducin mutations increase Na/K pump activity in renal cells by affecting constitutive endocytosis: implications for tubular Na reabsorption

Genetic variation in alpha-adducin cytoskeletal protein is implicated in the polymerization and bundling of actin and alteration of the Na/K pump, resulting in abnormal renal sodium transport and hypertension in Milan hypertensive rats and humans. To investigate the molecular involvement of alpha-adducin in controlling Na/K pump activity, wild-type or mutated rat and human alpha-adducin forms were, respectively, transfected into several renal cell lines. Through multiple experimental approaches (microscopy, enzymatic assays, coimmunoprecipitation), we showed that rat and human mutated forms increased Na/K pump activity and the number of pump units; moreover, both variants coimmunoprecipitate with Na/K pump. The increased Na/K pump activity was not due to changes in its basolateral localization, but to an alteration of Na/K pump residential time on the plasma membrane. Indeed, both rat and human mutated variants reduced constitutive Na/K pump endocytosis and similarly affected transferrin receptor trafficking and fluid-phase endocytosis. In fact, alpha-adducin was detected in clathrin-coated vesicles and coimmunoprecipitated with clathrin. These results indicate that adducin, besides its modulatory effects on actin cytoskeleton dynamics, might play a direct role in clathrin-dependent endocytosis. The constitutive reduction of the Na/K pump endocytic rate induced by mutated adducin variants may be relevant in Na-dependent hypertension.

A method for measuring Rho kinase activity in tissues and cells

The Rho-associated kinases (ROCKs) can regulate cell shape and function by modulating the actin cytoskeleton. ROCKs are serine-threonine protein kinases that can phosphorylate adducin, ezrin-radixin-moesin proteins, LIM kinase, and myosin light chain phosphatase. In the cardiovascular system, the RhoA/ROCK pathway has been implicated in angiogenesis, atherosclerosis, cerebral and coronary vasospasm, cerebral ischemia, hypertension, myocardial hypertrophy, and neointima formation after vascular injury. ROCKs consist of two isoforms: ROCK1 and ROCK2. They share overall 65% homology in their amino acid sequence and 92% homology in their amino kinase domains. However, these two isoforms have different subcellular localizations and exert biologically different functions. In particular, ROCK1 appears to be more important for immunological functions, whereas ROCK2 is more important for endothelial and vascular smooth muscle function. Thus, the ability to measure ROCK activity in tissues and cells would be important for understanding mechanisms underlying cardiovascular disease. This chapter describes a method for measuring ROCK activity in peripheral blood, tissues, and cells.

Green tea polyphenol epigallocatechin-3-gallate signaling pathway through 67-kDa laminin receptor

(-)-Epigallocatechin-3-gallate (EGCG), the principal polyphenol in green tea, has been shown to be a potent chemopreventive agent. Recently, 67-kDa laminin receptor (67LR) has been identified as a cell surface receptor for EGCG that mediates the anticancer activity of EGCG. Indeed, expression of 67LR confers EGCG responsiveness to tumor cells; however, the molecular basis for the anticancer activity of EGCG in vivo is not entirely understood. Here we show that (i) using a direct genetic screen, eukaryotic translation elongation factor 1A (eEF1A) is identified as a component responsible for the anticancer activity of EGCG; (ii) through both eEF1A and 67LR, EGCG induces the dephosphorylation of myosin phosphatase targeting subunit 1 (MYPT1) at Thr-696 and activates myosin phosphatase; and (iii) silencing of 67LR, eEF1A, or MYPT1 in tumor cells results in abrogation of EGCG-induced tumor growth inhibition in vivo. Additionally, we found that eEF1A is up-regulated by EGCG through 67LR. Overall, these findings implicate both eEF1A and MYPT1 in EGCG signaling for cancer prevention through 67LR.

ROCK and Rho: biochemistry and neuronal functions of Rho-associated protein kinases

Rho-associated protein kinases (ROCKs) play key roles in mediating the control of the actin cytoskeleton by Rho family GTPases in response to extracellular signals. Such signaling pathways contribute to diverse neuronal functions from cell migration to axonal guidance to dendritic spine morphology to axonal regeneration to cell survival. In this review, the authors summarize biochemical knowledge of ROCK function and categorize neuronal ROCK-dependent signaling pathways. Further study of ROCK signal transduction mechanisms and specificities will enhance our understanding of brain development, plasticity, and repair. The ROCK pathway also provides a potential site for therapeutic intervention to promote neuronal regeneration and to limit degeneration.

Regulation of vascular endothelial cell barrier function and cytoskeleton structure by protein phosphatases of the PPP family

Reversible phosphorylation of cytoskeletal and cytoskeleton-associated proteins is a significant element of endothelial barrier function regulation. Therefore, understanding the mechanisms of phosphorylation/dephosphorylation of endothelial cell cytoskeletal proteins is vital to the treatment of severe lung disorders such as high permeability pulmonary edema. In vivo, there is a controlled balance between the activities of protein kinases and phosphatases. Due to various external or internal signals, this balance may be shifted. The actual balances at a given time alter the phosphorylation level of certain proteins with appropriate physiological consequences. The latest information about the structure and regulation of different types of Ser/Thr protein phosphatases participating in the regulation of endothelial cytoskeletal organization and barrier function will be reviewed here.

Rho-kinase phosphorylates eNOS at threonine 495 in endothelial cells

Endothelial nitric oxide synthase (eNOS) produces nitric oxide (NO), which is involved in various physiological functions of the cardiovascular system. eNOS is activated by dephosphorylation at Thr495 and phosphorylation at Ser1177. Inhibition of Rho-kinase, an effector of the small GTPase RhoA, leads to activation of Akt/PKB, which phosphorylates eNOS at Ser1177 and thereby promotes NO production. However, little is known about the effects of Rho-kinase on phosphorylation of Thr495. We here found that the constitutively active form of Rho-kinase phosphorylated eNOS at Thr495 in vitro. Expression of the constitutively active form of RhoA or Rho-kinase increased this phosphorylation in COS-7 cells. Addition of thrombin to cultured human umbilical vein endothelial cells induced phosphorylation of eNOS at Thr495. Treatment with Y27632, a Rho-kinase inhibitor, suppressed thrombin-induced phosphorylation at Thr495. These results indicate that Rho-kinase can directly phosphorylate eNOS at Thr495 to suppress NO production in endothelium.

Rho-kinase dependent organization of stress fibers and focal adhesions in cultured fibroblasts

The activation of Rho-kinase is known to modulate the organization of the actin-based cytoskeletal systems, including the formation of stress fibers and focal adhesions. Rho-kinase likely plays a more crucial and complex role in the organization of actin-based cytoskeletal systems than in that of myosin light chain kinase (MLCK). In order to understand the role of Rho-kinase in the organization of stress fibers and focal adhesions, we treated cultured fibroblasts with a Rho-kinase inhibitor and analyzed the stress fiber and focal adhesion organization under conventional fluorescence microscopy and replica electron microscopy. Some of the cells were transfected with GFP-labeled paxillin, actin or alpha-actinin, and the effects of the inhibitor were monitored in the living cells. The Rho-kinase inhibitor caused disassembly of the stress fibers and focal adhesions in the central portion of the cell within 1 h. However, the stress fibers and focal adhesions located in the cell periphery were not as severely affected by the Rho-kinase inhibitor. The time-lapse video recording revealed that when these cells were washed with a fresh medium in order to remove the Rho-kinase inhibitor, the stress fibers and focal adhesions located in the center of the cell gradually reorganized and, within 1.5-2 h, the cells completely recovered. This observation strongly suggests that the activation of Rho-kinase plays an important role in the organization of the central stress fibers and focal adhesions.

Rho-kinase activation in endothelial cells contributes to expansion of infarction after focal cerebral ischemia

Microcirculatory disturbances contribute to the expansion of infarct lesions after focal cerebral ischemia. Recently, it was shown that Rho-kinase involves in endothelial dysfunction via down-regulation of endothelial nitric oxide synthase function in a rodent stroke model. However, it is not clear whether endothelial Rho-kinase is activated in vivo or Rho-kinase activation contributes to microcirculatory disturbances after cerebral ischemia. In this study, we assessed the temporal and spatial profiles of Rho-kianse activity and the effect of the Rho-kinase inhibitor fasudil on microcirculatory disturbances in the focal brain ischemia. Rho-kinase activation was evaluated by analyzing the phosphorylation of adducin, a substrate of Rho-kinase, by immunohistochemistry. Staining for p-adducin was found in endothelia in the ischemic area 6 hr after induction of ischemia. Microcirculatory disturbances and increased endothelial cell staining for von Willebrand factor (vWF) were observed in the same area. Postischemic treatment with fasudil suppressed endothelial Rho-kinase activation, preserved microcirculation, and inhibited endothelial cell vWF staining. These effects resulted in inhibition of infarct expansion and improvement of neurologic deficits. These findings indicate that Rho-kinase is activated in the endothelial cells and contributes to microcirculatory disturbances in cerebral ischemia. The vascular protective effect of Rho-kinase inhibitors may be useful in the treatment of the acute phase of ischemic stroke.

Rac GTPases regulate the morphology and deformability of the erythrocyte cytoskeleton

Actin oligomers are a significant structural component of the erythrocyte cytoskeleton. Rac1 and Rac2 GTPases regulate actin structures and have multiple overlapping as well as distinct roles in hematopoietic cells; therefore, we studied their role in red blood cells (RBCs). Conditional gene targeting with a loxP-flanked Rac1 gene allowed Crerecombinase-induced deletion of Rac1 on a Rac2 null genetic background. The Rac1(-/-);Rac2(-/-) mice developed microcytic anemia with a hemoglobin drop of about 20% and significant anisocytosis and poikilocytosis. Reticulocytes increased more than 2-fold. Rac1(-/-);Rac2(-/-) RBCs stained with rhodamine-phalloidin demonstrated F-actin meshwork gaps and aggregates under confocal microscopy. Transmission electron microscopy of the cytoskeleton demonstrated junctional aggregates and pronounced irregularity of the hexagonal spectrin scaffold. Ektacytometry confirmed that these cytoskeletal changes in Rac1(-/-);Rac2(-/-) erythrocytes were associated with significantly decreased cellular deformability. The composition of the cytoskeletal proteins was altered with an increased actin-to-spectrin ratio and increased phosphorylation (Ser724) of adducin, an F-actin capping protein. Actin and phosphorylated adducin of Rac1(-/-);Rac2(-/-) erythrocytes were more easily extractable by Triton X-100, indicating weaker association to the cytoskeleton. Thus, deficiency of Rac1 and Rac2 GTPases in mice alters actin assembly in RBCs and causes microcytic anemia with reticulocytosis, implicating Rac GTPases as dynamic regulators of the erythrocyte cytoskeleton organization.

Physiological role of ROCKs in the cardiovascular system

Rho-associated kinases (ROCKs), the immediate downstream targets of RhoA, are ubiquitously expressed serine-threonine protein kinases that are involved in diverse cellular functions, including smooth muscle contraction, actin cytoskeleton organization, cell adhesion and motility, and gene expression. Recent studies have shown that ROCKs may play a pivotal role in cardiovascular diseases such as vasospastic angina, ischemic stroke, and heart failure. Indeed, inhibition of ROCKs by statins or other selective inhibitors leads to the upregulation and activation of endothelial nitric oxide synthase (eNOS) and reduction of vascular inflammation and atherosclerosis. Thus inhibition of ROCKs may contribute to some of the cholesterol-independent beneficial effects of statin therapy. Currently, two ROCK isoforms have been identified, ROCK1 and ROCK2. Because ROCK inhibitors are nonselective with respect to ROCK1 and ROCK2 and also, in some cases, may be nonspecific with respect to other ROCK-related kinases such as myristolated alanine-rich C kinase substrate (MARCKS), protein kinase A, and protein kinase C, the precise role of ROCKs in cardiovascular disease remains unknown. However, with the recent development of ROCK1- and ROCK2-knockout mice, further dissection of ROCK signaling pathways is now possible. Herein we review what is known about the physiological role of ROCKs in the cardiovascular system and speculate about how inhibition of ROCKs could provide cardiovascular benefits.

Slow motility, electromotility and lateral wall stiffness in the isolated outer hair cells

Slow motile length changes of isolated, apical turn outer hair cells (OHCs) (n=36) were induced by perfusion of saline (flow rate: 0.6 microl/min) as a mechanical challenge or by perfusion of 12.5 mM KCl solution for 90 s as a chemical and mechanical challenge with and without ocadaic acid (OA), a serine/threonine protein phosphatase inhibitor. Electromotility was evoked by square pulses from +/-35 mV to +/-240 mV during the slow shortening and recovery period (n=36). Stiffness of the lateral wall was measured by the micropipette aspiration technique (n=20). Saline perfusion caused a reversible shortening of 774+/-87 nm (n=9) as well as K+ of 1465+/-159 nm (n=9). Slow shortening increased lateral wall stiffness (1.25+/-0.02 to 1.52+/-0.03 nN/microm) (n=5-5). Simultaneously, electromotility magnitude decreased (n=9). Ocadaic acid blocked slow shortening, increased lateral wall stiffness, and decreased the magnitude of electromotility. Mechanical or mechanical+chemical stimulation of ocadaic acid treated OHCs do not further change stiffness or electromotility. Isolated OHCs respond with slow shortening and consutive cell stiffness increase to mechanical insult. This phenomenon seems operating with calcium-, and phosphorylation-dependent modifications of the cytoskeletal proteins. The subsequent electromotility gain decrease suggests a slow OHC shortening driven regulation of the cochlear amplifier with simultaneous safety control of the auditory periphery against overstimulation.

Control of basement membrane remodeling and epithelial branching morphogenesis in embryonic lung by Rho and cytoskeletal tension

Local alterations in the mechanical compliance of the basement membrane that alter the level of isometric tension in the cell have been postulated to influence tissue morphogenesis. To explore whether cell tension contributes to tissue pattern formation in vivo, we modulated cytoskeletal force generation in embryonic mouse lung (embryonic days 12-14) rudiments using inhibitors of Rho-associated kinase (ROCK), myosin light chain kinase, myosin ATPase, and microfilament integrity, or a Rho stimulator (cytotoxic necrotizing factor-1). Tension inhibition resulted in loss of normal differentials in basement membrane thickness, inhibition of new terminal bud formation, and disorganization of epithelial growth patterns as well as disruption of capillary blood vessels. In contrast, increasing cell tension through Rho activation, as confirmed by quantitation of myosin light chain phosphorylation and immunohistocytochemical analysis of actin organization, accelerated lung branching and increase capillary elongation. These data suggest that changes in cytoskeletal tension mediated by Rho signaling through ROCK may play an important role in the establishment of the spatial differentials in cell growth and extracellular matrix remodeling that drive embryonic lung development.

Rho-kinase induces association of adducin with the cytoskeleton in platelet activation

We examined whether adducin function is regulated through Rho-kinase after agonist stimulation in platelets. A variety of stimuli such as thrombin, STA(2) (a stable analog of TXA(2)), Ca(2+) ionophore, phorbol diester, and shear stress induced phosphorylation of alpha-adducin at Thr445. Preincubation with the Rho-kinase inhibitor Y-27632 in platelets inhibited agonist-induced phosphorylation of alpha-adducin. STA(2) stimulation led to a redistribution of adducin from Triton-insoluble (high speed) fraction (membrane skeleton) to Triton-insoluble (low speed) fraction (cytoskeleton) and detergent-soluble fraction. Phosphoadducin at Thr445 was selectively isolated in the cytoskeletal fraction, whereas phosphoadducin at Ser726 was mainly present in the Triton-soluble fraction. Y-27632 inhibition of STA(2)-induced alpha-adducin phosphorylation at Thr445 inhibited incorporation of alpha-adducin and spectrin into the platelet cytoskeleton, although Y-27632 did not affect phosphorylation of alpha-adducin at Ser726. These results suggest that Rho-kinase regulates the association of alpha-adducin and spectrin with the actin cytoskeleton in platelet activation.

GSK3 and PKB/Akt are associated with integrin-mediated regulation of PTHrP, IL-6 and IL-8 expression in FG pancreatic cancer cells

We have demonstrated recently that PTHrP is upregulated in pancreatic adenocarcinoma and that the ECM exerts regulatory control, at least in part, over PTHrP expression. In our present study, we examined the potential signaling interactions between these 2 pathways. Our results demonstrate that, under serum-free conditions, adhesion of FG pancreatic adenocarcinoma cells on Fn is mediated by the alpha5beta1 integrin, whereas adhesion to Type I collagen is mediated by the alpha2beta1 integrin. alpha5beta1 integrin-mediated adhesion to Fn results in a phenotype that includes a reduction in cell proliferation, increased E-cadherin localization in cell-cell contacts, increased beta-catenin localization throughout the cell, inhibition of haptokinetic cell migration, and increased expression of PTHrP, IL-6 and IL-8 relative to alpha2beta1 integrin-mediated adhesion on Type I collagen. A phosphoprotein immunoblotting screen of FG pancreatic cancer cells grown on either Fn or Type I collagen indicates that GSK3 and PKB/Akt are differentially phosphorylated on these 2 substrates. These results implicate GSK3 and PKB/Akt in the integrin-mediated regulation of PTHrP, IL-6 and IL-8 in pancreatic cancer.

Interaction of Rho-kinase with myosin II at stress fibres

Rho-kinase and myosin phosphatase cooperatively regulate the phosphorylation level of myosin light chain and are involved in the formation of stress fibres and smooth muscle contraction. Rho-kinase has been known to be localized at stress fibres, but little is known about the mechanism of its localization. Here we identified non-muscle myosin heavy chain IIA and IIB as the pleckstrin homology domain-interacting molecules by affinity column chromatography. The pleckstrin homology domain of Rho-kinase binds to myosin II directly in in vitro cosedimentation assay. The C-terminal region of the pleckstrin homology domain was important for this interaction, and the point mutations in the pleckstrin homology domain mutant (W1170A, W1340L) resulted in a decrease in the binding. We also found that the pleckstrin homology domain, but not the pleckstrin homology domain mutant (W1170A, W1340L), was localized at stress fibres in fibroblasts. These results indicate that Rho-kinase is localized at stress fibres through binding of the pleckstrin homology domain to myosin II.

Myosin phosphatase: structure, regulation and function

Phosphorylation of myosin II plays an important role in many cell functions, including smooth muscle contraction. The level of myosin II phosphorylation is determined by activities of myosin light chain kinase and myosin phosphatase (MP). MP is composed of 3 subunits: a catalytic subunit of type 1 phosphatase, PPlc; a targeting subunit, termed myosin phosphatase target subunit, MYPT; and a smaller subunit, M20, of unknown function. Most of the properties of MP are due to MYPT and include binding of PP1c and substrate. Other interactions are discussed. A recent discovery is the existence of an MYPT family and members include, MYPT1, MYPT2, MBS85, MYPT3 and TIMAP. Characteristics of each are outlined. An important discovery was that the activity of MP could be regulated and both activation and inhibition were reported. Activation occurs in response to elevated cyclic nucleotide levels and various mechanisms are presented. Inhibition of MP is a major component of Ca2+-sensitization in smooth muscle and various molecular mechanisms are discussed. Two mechanisms are cited frequently: (1) Phosphorylation of an inhibitory site on MYPT1, Thr696 (human isoform) and resulting inhibition of PP1c activity. Several kinases can phosphorylate Thr696, including Rho-kinase that serves an important role in smooth muscle function; and (2) Inhibition of MP by the protein kinase C-potentiated inhibitor protein of 17 kDa (CPI-17). Examples where these mechanisms are implicated in smooth muscle function are presented. The critical role of RhoA/Rho-kinase signaling in various systems is discussed, in particular those vascular smooth muscle disorders involving hypercontractility.

The COOH-terminal End of R-Ras Alters the Motility and Morphology of Breast Epithelial Cells through Rho/Rho-Kinase

R-Ras has a high degree of sequence homology with Ras and other members of the Ras subfamily, including Rap, TC21, and M-Ras. Although R-Ras has been suggested to regulate cell adhesion, migration, and invasion, the biological mechanism has not been well assessed. In this report, we show that constitutively active R-Ras (38V) induces a more rounded cell shape and redistribution of focal adhesion, and enhances the phosphorylation of focal adhesion kinase and paxillin. Active R-Ras (38V) induces cell adhesion to type I collagen, but inhibits cell motility. In active R-Ras (38V) cells, the activity of RhoA is increased and accompanied with translocation to plasma membrane, but not that of Rac1 or Cdc42. In parallel, dominant-negative RhoA (N19RhoA) and Y27632, a specific inhibitor of Rho-associated kinase, dramatically reverse the rounded cell morphology to a spread cell shape and enhance motility. Furthermore, coincident with the formation of cortical actin filaments in active R-Ras (38V) cells, myosin light chain and Ser-19-phosphorylated myosin light chain mainly accumulate at the peripheral region, which is inhibited by the treatment of Y27632. Using H-Ras/R-Ras and R-Ras/H-Ras hybrid constructs, we show that the COOH-terminal region of R-Ras contains the specific signal for inducing changes in motility and morphology. Our results suggest that R-Ras in breast epithelial cells disrupts cell polarity and motility through the Rho/Rho–associated kinase pathway triggered by a signal from the COOH-terminal end of R-Ras.

Increased phospho-adducin immunoreactivity in a murine model of amyotrophic lateral sclerosis

Adducins alpha, beta and gamma are proteins that link spectrin and actin in the regulation of cytoskeletal architecture and are substrates for protein kinase C and other signaling molecules. Previous studies have shown that expressions of phosphorylated adducin (phospho-adducin) and protein kinase C are increased in spinal cord tissue from patients who died with amyotrophic lateral sclerosis, a neurodegenerative disorder of motoneurons and other cells. However, the distribution of phospho-adducin immunoreactivity has not been described in the mammalian spinal cord. We have evaluated the distribution of immunoreactivity to serine/threonine-dependent phospho-adducin at a region corresponding to the myristoylated alanine-rich C kinase substrate-related domain of adducin in spinal cords of mice over-expressing mutant human superoxide dismutase, an animal model of amyotrophic lateral sclerosis, and in control littermates. We find phospho-adducin immunoreactivity in control spinal cord in ependymal cells surrounding the central canal, neurons and astrocytes. Phospho-adducin immunoreactivity is localized to the cell bodies, dendrites and axons of some motoneurons, as well as to astrocytes in the gray and white matter. Spinal cords of mutant human superoxide dismutase mice having motoneuron loss exhibit significantly increased phospho-adducin immunoreactivity in ventral and dorsal horn spinal cord regions, but not in ependyma surrounding the central canal, compared with control animals. Increased phospho-adducin immunoreactivity localizes predominantly to astrocytes and likely increases as a consequence of the astrogliosis that occurs in the mutant human superoxide dismutase mouse with disease progression. These findings demonstrate increased immunoreactivity against phosphorylated adducin at the myristoylated alanine-rich C kinase substrate domain in a murine model of amyotrophic lateral sclerosis. As adducin is a substrate for protein kinase C at the myristoylated alanine-rich C kinase substrate domain, the increased phospho-adducin immunoreactivity is likely a consequence of protein kinase C activation in neurons and astrocytes of the spinal cord and evidence for aberrant phosphorylation events in mutant human superoxide dismutase mice that may affect neuron survival.

Localization of myosin phosphatase target subunit 1 in rat brain and in primary cultures of neuronal cells

Myosin phosphatase (PP1M) is composed of the delta isoform of the PP1 catalytic subunit (PP1cdelta), the myosin phosphatase target subunit (MYPT), and a 20 kDa subunit. Western blots detected higher amounts of the MYPT1 isoform compared to MYPT2 in whole brain extracts. The localization of MYPT1 was studied in rat brain and in primary cell cultures of neurons using specific antibodies. Analysis of lysates of brain regions for MYPT1 and PP1M by Western blots using anti-MYPT1 antibodies and by phosphatase assays with myosin as substrate suggested a ubiquitous distribution. Immunohistochemistry of tissue sections revealed that MYPT1 was distributed in all areas of the brain, with staining observed in many different cell types. Depending on the method used for fixation, the MYPT1 appeared with varying intensity in nuclei, in nucleoli, and in the cytoplasm. In primary hippocampal cultures, MYPT1 was identified by confocal microscopy in the cytoplasm and in the nucleus, whereas a predominantly cytoplasmic localization was found in cochlear nucleus cells. In cultured cells, MYPT1 and PP1cdelta colocalized with synaptophysin. PP1M activity was high in synaptosomes isolated from the cerebral cortex, but was relatively low in the postsynaptic densities. The interaction of MYPT1 with synaptophysin and with known partners (Rho-kinase, PP1cdelta) in brain extracts was shown by immunoprecipitation with anti-MYPT1. Pull-down assays from synaptosomes, using GST-MYPT1, also confirmed these interactions. In conclusion, the widespread cellular and subcellular localization of MYPT1 implies that PP1M may play an important role in the dephosphorylation of key regulatory proteins in neuronal cells.

Functional diversity of protein phosphatase-1, a cellular economizer and reset button

The protein serine/threonine phosphatase protein phosphatase-1 (PP1) is a ubiquitous eukaryotic enzyme that regulates a variety of cellular processes through the dephosphorylation of dozens of substrates. This multifunctionality of PP1 relies on its association with a host of function-specific targetting and substrate-specifying proteins. In this review we discuss how PP1 affects the biochemistry and physiology of eukaryotic cells. The picture of PP1 that emerges from this analysis is that of a "green" enzyme that promotes the rational use of energy, the recycling of protein factors, and a reversal of the cell to a basal and/or energy-conserving state. Thus PP1 promotes a shift to the more energy-efficient fuels when nutrients are abundant and stimulates the storage of energy in the form of glycogen. PP1 also enables the relaxation of actomyosin fibers, the return to basal patterns of protein synthesis, and the recycling of transcription and splicing factors. In addition, PP1 plays a key role in the recovery from stress but promotes apoptosis when cells are damaged beyond repair. Furthermore, PP1 downregulates ion pumps and transporters in various tissues and ion channels that are involved in the excitation of neurons. Finally, PP1 promotes the exit from mitosis and maintains cells in the G1 or G2 phases of the cell cycle.

Rho and Rac take center stage

Many features of cell behavior are regulated by Rho family GTPases, but the most profound effects of these proteins are on the actin cytoskeleton and it was these that first drew attention to this family of signaling proteins. Focusing on Rho and Rac, we will discuss how their effectors regulate the actin cytoskeleton. We will describe how the activity of Rho proteins is regulated downstream from growth factor receptors and cell adhesion molecules by guanine nucleotide exchange factors and GTPase activating proteins. Additionally, we will discuss how there is signaling crosstalk between family members and how various bacterial pathogens have developed strategies to manipulate Rho protein activity so as to enhance their own survival.

Caveolin-1 regulates contractility in differentiated vascular smooth muscle

Caveolin is a principal component of caveolar membranes. In the present study, we utilized a decoy peptide approach to define the degree of involvement of caveolin in PKC-dependent regulation of contractility of differentiated vascular smooth muscle. The primary isoform of caveolin in ferret aorta vascular smooth muscle is caveolin-1. Chemical loading of contractile vascular smooth muscle tissue with a synthetic caveolin-1 scaffolding domain peptide inhibited PKC-dependent increases in contractility induced by a phorbol ester or an alpha agonist. Peptide loading also resulted in a significant inhibition of phorbol ester-induced adducin Ser662 phosphorylation, an intracellular monitor of PKC kinase activity, ERK1/2 activation, and Ser789 phosphorylation of the actin binding protein caldesmon. alpha-Agonist-induced ERK1-1/2 activation was also inhibited by the caveolin-1 peptide. Scrambled peptide-loaded tissues or sham-loaded tissues were unaffected with respect to both contractility and signaling. Depolarization-induced activation of contraction was not affected by caveolin peptide loading. Similar results with respect to contractility and ERK1/2 activation during exposure to the phorbol ester or the alpha-agonist were obtained with the cholesterol-depleting agent methyl-beta-cyclodextrin. These results are consistent with a role for caveolin-1 in the coordination of signaling leading to the regulation of contractility of smooth muscle.

Identification of Tau and MAP2 as novel substrates of Rho-kinase and myosin phosphatase

Rho-kinase and myosin phosphatase are implicated in the phosphorylation-state of myosin light chain downstream of Rho, which is thought to induce smooth muscle contraction and stress fibre formation in non-muscle cells. Here, we found that microtubule-associated proteins, Tau and MAP2, interacted with the myosin-binding subunit (MBS) of myosin phosphatase, and were the possible substrates of both Rho-kinase and myosin phosphatase. We determined the phosphorylation sites of Tau (Thr245, Thr377, Ser409) and MAP2 (Ser1796) by Rho-kinase. We also found that Rho-kinase phosphorylated Tau at Ser262 to some extent. Phosphorylation by Rho-kinase decreased the activity of Tau to promote microtubule assembly in vitro. Substitutions of Ala for Ser/Thr at the phosphorylation sites of Tau (Tau-AAA) did not affect the activity to promote microtubule assembly, while substitutions of Asp for Ser/Thr (Tau-DDD), which are expected to mimic the phosphorylation-state of Tau, slightly reduced the activity. When Tau, or mutated forms of Tau, were expressed in PC12 cells, followed by treatment with cytochalasin D, they promoted extension of the cell process in a cytochalasin-dependent manner. However, Tau-DDD showed the weaker activity in this capacity than wild-type Tau or Tau-AAA. These results suggest that the phosphorylation-state of these residues of Tau affects its activity both in vitro and in vivo. Thus, it is likely that the Rho-kinase/MBS pathway regulates not only the actin-myosin system but also microtubule dynamics.

Parallel coiled-coil association of the RhoA-binding domain in Rho-kinase

Rho-kinase is a serine/threonine protein kinase that regulates cytoskeletal events in cells. The enzyme activity of Rho-kinase is auto-inhibited in the free state but is activated through direct binding to the small GTPase Rho in the GTP-bound form. The crystal structure of the Rho-binding domain (RhoBD) of Rho-kinase has been determined at 1.8-A resolution by the multi-wavelength anomalous dispersion technique. The structure shows that RhoBD dimerizes to form a parallel coiled-coil with long consecutive alpha-helices extended to approximately 97 A and suggests that free Rho-kinase can also form a dimer through parallel self-association. At the middle region of the coiled-coil, the polypeptide chains are flexible and display loose "knobs-into-holes" packing of the side chains from both chains. RhoBD residues that have been shown to be critical for Rho-binding are spread in the positively charged C-terminal region. The parallel coiled-coil structure of our Rho-kinase RhoBD in the free form is different from the anti-parallel coiled-coil structure of RhoBD of protein kinase N when complexed with RhoA. Implications derived from these structural studies in relation to the mechanism of Rho-kinase activation will be addressed with previously reported experimental data.

Adducin expression in cutaneous and oral lesions: alpha- and beta-adducin transcripts down-regulate with keratinocyte differentiation in stratified epithelia

Adducin is a heterodimer of alpha- with beta- or gamma-subunits that regulates the assembly of the spectrin-based membrane skeleton in erythrocytes. Although adducin has been identified in various non-erythroid cells and tissues, it has been localized at intercellular junctions only in keratinocytes and epidermis. However, no data are available yet on the regulation of individual adducin genes in differentiating versus hyperproliferating keratinocytes. Due to the unavailability of mono-specific antibodies for individual adducins, this study has used RT-PCR and in situ hybridization to investigate the expression of alpha- and beta-adducins in cultured cells and in stratified epithelia including cutaneous and oral lesions. Using RT-PCR, the alpha-transcripts were consistently expressed in all cell lines tested, as well as in normal interfollicular epidermis, whereas the beta-transcripts were more variable and were strongly expressed in K562, A431, and primary keratinocytes. However, in normal skin, oral mucosa, and attached gingivae, the levels of alpha-transcripts closely paralleled those for the beta-subunit. In most normal tissues, adducin expression was observed primarily in the proliferating compartments including the basal layer and lower suprabasal layers. Expression of both genes was also up-regulated in skin diseases characterized by increased cell proliferation and keratinocyte activation, such as basal cell carcinoma (BCC), squamous cell carcinoma (SCC), and psoriasis. It was observed that, in most cases, the expression of both alpha- and beta-adducin was accompanied by increased expression of the proliferation marker Ki-67 and keratins K6 and K16. Differentiating keratinocytes in normal epithelia as well as in tumours appear to suppress the expression of adducin transcripts. The data suggest that the expression of adducin genes may be linked to cell proliferation and starts to down-regulate at the onset of differentiation.

Do inflammatory mediators influence the contribution of airway smooth muscle contraction to airway hyperresponsiveness in asthma?

It is now accepted that a host of cytokines, chemokines, growth factors, and other inflammatory mediators contributes to the development of nonspecific airway hyperresponsiveness in asthma. Yet, relatively little is known about how inflammatory mediators might promote airway structural remodeling or about the molecular mechanisms by which they might exaggerate smooth muscle shortening as observed in asthmatic airways. Taking a deep inspiration, which provides relief of bronchodilation in normal subjects, is less effective in asthmatic subjects, and some have speculated that this deficiency stems directly from an abnormality of airway smooth muscle and results in airway hyperresponsiveness to constrictor agonists. Here, we consider some of the mechanisms by which inflammatory mediators might acutely or chronically induce changes in the contractile apparatus that in turn might contribute to hyperresponsive airways in asthma.

Sertoli-germ cell adherens junction dynamics in the testis are regulated by RhoB GTPase via the ROCK/LIMK signaling pathway

During spermatogenesis, cell-cell actin-based adherens junctions (AJs), such as ectoplasmic specializations (ESs), between Sertoli and germ cells undergo extensive restructuring in the seminiferous epithelium to facilitate germ cell movement across the epithelium. Although the mechanism(s) that regulates AJ dynamics in the testis is virtually unknown, Rho GTPases have been implicated in the regulation of these events in other epithelia. Studies have shown that the in vitro assembly of the Sertoli-germ cell AJs but not of the Sertoli cell tight junctions (TJs) is associated with a transient but significant induction of RhoB. Immunohistochemistry has shown that the localization of RhoB in the seminiferous epithelium is stage specific, being lowest in stages VII-VIII prior to spermiation, and displays cell-specific association during the epithelial cycle. Throughout the cycle, RhoB was localized near the site of basal and apical ESs but was restricted to the periphery of the nuclei in elongating (but not elongated) spermatids, spermatocytes, and Sertoli cells. However, RhoB was not detected near the site of apical ESs at stages VII-VIII. Furthermore, disruption of AJs in Sertoli-germ cell cocultures either by hypotonic treatment or by treatment with 1-(2,4-dichlorobenzyl)-indazole-3-carbohydrazide (AF-2364) also induced RhoB expression. When adult rats were treated with AF-2364 to perturb Sertoli-germ cell AJs in vivo, a approximately 4-fold induction in RhoB in the testis, but not in kidney and brain, was detected within 1 h, at least approximately 1-4 days before germ cell loss from the epithelium could be detected by histological analysis. The signaling pathway(s) by which AF-2364 perturbed the Sertoli-germ cell AJs apparently began with an initial activation of integrin, which in turn activated RhoB, ROCK1, (Rho-associated protein kinase 1, also called ROKbeta), LIMK1 (LIM kinase 1, also called lin-11 isl-1 mec3 kinase 1), and cofilin but not p140mDia and profilin via phosphorylation. Immunoprecipitation and immunoblots revealed that the induction of LIMK1 was mediated via an increase in its phospho-Ser but not phospho-Tyr content. Furthermore, Y-27632 ([(R)-(+)-trans-N-(4-pyridyl)-4-(1-aminoethyl)-cyclohexane-carboxamide, 2HCl]), a specific ROCK inhibitor, could effectively delay the AF-2364-induced germ cell loss from the seminiferous epithelium in vivo, illustrating that the integrin/RhoB/ROCK/LIMK pathway indeed plays a crucial role in the regulation of Sertoli-germ cell AJ dynamics. The fact that the RhoB pathway in the kidney and brain was not activated suggests that AF-2364 exerts its effects primarily at the testis-specific ES multiprotein complex structures between Sertoli cells and spermatids. In summary, this report illustrates that Sertoli germ cell AJ dynamics are regulated, at least in part, via the integrin/ROCK/LIMK/cofilin signaling pathway.

Rocks: multifunctional kinases in cell behaviour

ROCKs, or Rho kinases, are serine/threonine kinases that are involved in many aspects of cell motility, from smooth-muscle contraction to cell migration and neurite outgrowth. Recent experiments have defined new functions of ROCKs in cells, including centrosome positioning and cell-size regulation, which might contribute to various physiological and pathological states.

Deactivation of ROCK-II by Y-27632 enhances basolateral pancreatic enzyme secretion and acute pancreatitis induced by CCK analogues

In isolated rat pancreatic acini, protein expression of RhoA and Rho-associated kinase, ROCK-II, and the formation of immunocomplex of RhoA with ROCK-II were enhanced by CCK-8, carbachol, and the phorbol ester TPA. The ROCK-specific inhibitor, Y-27632, did not alter basal amylase secretion, whereas it potentiated CCK-stimulated pancreatic enzyme secretion in vitro. During caerulein-induced pancreatitis occurring in mice in vivo, Y-27632 enhanced serum amylase levels and the formation of interstitial edema and vacuolization at 12-18h after the first injection of caerulein. Y-27632 in turn inhibited the recovery of protein expression of ROCK-II at 18h after the first caerulein injection. These results suggest that RhoA and ROCK-II assemble normal CCK-stimulated pancreatic enzyme secretion and prevent caerulein-induced acute pancreatitis.

Identification and characterization of Aplysia adducin, an Aplysia cytoskeletal protein homologous to mammalian adducins: increased phosphorylation at a protein kinase C consensus site during long-term synaptic facilitation

Structural changes at synapses are associated with long-term facilitation (LTF) of synaptic transmission between sensory and motor neurons in Aplysia. We have cloned a cDNA encoding Aplysia adducin (ApADD), the Aplysia homolog of mammalian adducins that are regulatory components of the membrane cytoskeleton. ApADD is recovered in the particulate fraction of nervous system extracts and is localized predominantly in the submembraneous region of Aplysia neurons. ApADD is phosphorylated in vitro by protein kinase C (PKC) at a site homologous to the in vivo PKC phosphorylation site in mammalian adducins. Phosphorylation of ApADD at this site is also detected in vivo in the intact Aplysia nervous system and is increased 18 hr after serotonin-induced LTF. In contrast, there is no change in phosphorylation during short-term facilitation or 1 hr after initial LTF induction. Thus, ApADD is modulated specifically with later phases of LTF and provides an attractive candidate protein that contributes to structural changes accompanying long-lasting synaptic alteration.

Alpha-adducin dissociates from F-actin and spectrin during platelet activation

Aspectrin-based skeleton uniformly underlies and supports the plasma membrane of the resting platelet, but remodels and centralizes in the activated platelet. alpha-Adducin, a phosphoprotein that forms a ternary complex with F-actin and spectrin, is dephosphorylated and mostly bound to spectrin in the membrane skeleton of the resting platelet at sites where actin filaments attach to the ends of spectrin molecules. Platelets activated through protease-activated receptor 1, FcgammaRIIA, or by treatment with PMA phosphorylate adducin at Ser726. Phosphoadducin releases from the membrane skeleton concomitant with its dissociation from spectrin and actin. Inhibition of PKC blunts adducin phosphorylation and release from spectrin and actin, preventing the centralization of spectrin that normally follows cell activation. We conclude that adducin targets actin filament ends to spectrin to complete the assembly of the resting membrane skeleton. Dissociation of phosphoadducin releases spectrin from actin, facilitating centralization of spectrin, and leads to the exposure of barbed actin filament ends that may then participate in converting the resting platelet's disc shape into its active form.

Shear stress-induced endothelial cell polarization is mediated by Rho and Rac but not Cdc42 or PI 3-kinases

Shear stress induces endothelial polarization and migration in the direction of flow accompanied by extensive remodeling of the actin cytoskeleton. The GTPases RhoA, Rac1, and Cdc42 are known to regulate cell shape changes through effects on the cytoskeleton and cell adhesion. We show here that all three GTPases become rapidly activated by shear stress, and that each is important for different aspects of the endothelial response. RhoA was activated within 5 min after stimulation with shear stress and led to cell rounding via Rho-kinase. Subsequently, the cells respread and elongated within the direction of shear stress as RhoA activity returned to baseline and Rac1 and Cdc42 reached peak activation. Cell elongation required Rac1 and Cdc42 but not phosphatidylinositide 3-kinases. Cdc42 and PI3Ks were not required to establish shear stress-induced polarity although they contributed to optimal migration speed. Instead, Rho and Rac1 regulated directionality of cell movement. Inhibition of Rho or Rho-kinase did not affect the cell speed but significantly increased cell displacement. Our results show that endothelial cells reorient in response to shear stress by a two-step process involving Rho-induced depolarization, followed by Rho/Rac-mediated polarization and migration in the direction of flow.

RhoA is required for cortical retraction and rigidity during mitotic cell rounding

Mitotic cell rounding is the process of cell shape change in which a flat interphase cell becomes spherical at the onset of mitosis. Rearrangement of the actin cytoskeleton, de-adhesion, and an increase in cortical rigidity accompany mitotic cell rounding. The molecular mechanisms that contribute to this process have not been defined. We show that RhoA is required for cortical retraction but not de-adhesion during mitotic cell rounding. The mitotic increase in cortical rigidity also requires RhoA, suggesting that increases in cortical rigidity and cortical retraction are linked processes. Rho-kinase is also required for mitotic cortical retraction and rigidity, indicating that the effects of RhoA on cell rounding are mediated through this effector. Consistent with a role for RhoA during mitotic entry, RhoA activity is elevated in rounded, preanaphase mitotic cells. The activity of the RhoA inhibitor p190RhoGAP is decreased due to its serine/threonine phosphorylation at this time. Cumulatively, these results suggest that the mitotic increase in RhoA activity leads to rearrangements of the cortical actin cytoskeleton that promote cortical rigidity, resulting in mitotic cell rounding.

Sphingosine 1-phosphate induces cell contraction via calcium-independent/Rho-dependent pathways in undifferentiated skeletal muscle cells

We have previously shown that sphingosine 1-phosphate (S1P) can induce intracellular Ca(2+) mobilization and cell contraction in C2C12 myoblasts and that the two phenomena are temporally unrelated. Although Ca(2+)-independent mechanisms of cell contraction have been the focus of numerous studies on Ca(2+) sensitization of smooth muscle, comparatively less studies have focused on the role that these mechanisms play in the regulation of skeletal muscle contractility. Phosphorylation and activation of myosin by Rho-dependent kinase mediate most of Ca(2+)-independent contractile responses. In the present study, we examined the potential role of Rho/Rho-kinase cascade activation in S1P-induced C2C12 cell contraction. First, we showed that depletion of Ca(2+), by pre-treatment with BAPTA, did not affect S1P-induced myoblastic contractility, whereas it abolished S1P-induced Ca(2+) transients. These results correlated with the absence of troponin C and with the immature cytoskeletal organization of these cells. Experimental evidence demonstrating the involvement of Rho pathway in S1P-stimulated myoblast contraction included: the activation/translocation of RhoA to the membrane in response to agonist-stimulation in cells depleted of Ca(2+) and the inhibition of dynamic changes of the actin cytoskeleton in cells where Rho functions had been inhibited either by overexpression of RhoGDI, a physiological inhibitor of GDP dissociation from Rho proteins, or by pretreatment with Y-27632, a specific Rho kinase inhibitor. Contribution of protein kinase C in this cytoskeletal rearrangement was also evaluated. However, the pretreatment with Gö6976 or rottlerin, specific inhibitors of PKC alpha and PKC delta, respectively, failed to inhibit the agonist-induced myoblastic contraction. Single particle tracking of G-actin fluorescent probe was performed to statistically evaluate actin cytoskeletal dynamics in response to S1P. Stimulation with S1P was also able to increase the phosphorylation level of myosin light chain II. In conclusion, our results strongly suggest that Ca(2+)-independent/Rho-Rho kinase-dependent pathways may exert an important role in S1P-induced myoblastic cell contraction.