Stress fiber organization regulated by MLCK and Rho-kinase in cultured human fibroblasts

Pulmonary Endothelial Cell Barrier Enhancement by Sphingosine 1-Phosphate

We recently reported the critical importance of Rac GTPase-dependent cortical actin rearrangement in the augmentation of pulmonary endothelial cell (EC) barrier function by sphingosine 1-phosphate (S1P). We now describe functional roles for the actin-binding proteins cortactin and EC myosin light chain kinase (MLCK) in mediating this response. Antisense down-regulation of cortactin protein expression significantly inhibits S1P-induced barrier enhancement in cultured human pulmonary artery EC as measured by transendothelial electrical resistance (TER). Immunofluorescence studies reveal rapid, Rac-dependent translocation of cortactin to the expanded cortical actin band following S1P challenge, where colocalization with EC MLCK occurs within 5 min. Adenoviral overexpression of a Rac dominant negative mutant attenuates TER elevation by S1P. S1P also induces a rapid increase in cortactin tyrosine phosphorylation (within 30 s) critical to subsequent barrier enhancement, since EC transfected with a tyrosine-deficient mutant cortactin exhibit a blunted TER response. Direct binding of EC MLCK to the cortactin Src homology 3 domain appears essential to S1P barrier regulation, since cortactin blocking peptide inhibits both S1P-induced MLC phosphorylation and peak S1P-induced TER values. These data support novel roles for the cytoskeletal proteins cortactin and EC MLCK in mediating lung vascular barrier augmentation evoked by S1P.

Pitavastatin, a 3-Hydroxy-3-methylglutaryl-coenzyme A Reductase Inhibitor, Blocks Vascular Smooth Muscle Cell Populated-Collagen Lattice Contraction

Constrictive arterial remodeling plays a major role in lumen narrowing following angioplasty. We investigated the effect of pitavastatin, a 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase inhibitor, on vascular smooth muscle cell (SMC)-populated collagen lattice contraction, an in vitro model of vascular contraction. Type I collagen gel contraction by SMCs, which are cultured in collagen gel, was used as a model of vascular remodeling. Pitavastatin pretreatment inhibited 10% serum- or platelet-derived growth factor-BB (PDGF)-induced SMC-mediated collagen lattice contraction in a concentration-dependent manner. The effect of pitavastatin was prevented by mevalonate or geranylgeranyl pyrophosphate, but not by squalene, a precursor of cholesterol, or farnesyl pyrophosphate. The serum- or PDGF-induced SMC-mediated collagen gel contraction was inhibited by GGTI-298, a geranylgeranyltransferase inhibitor, C3 exoenzyme, an inhibitor of Rho, or Y27634, a Rho kinase inhibitor, but not by FTI-277, a farnesyltransferase inhibitor. Serum or PDGF treatment increased the stress fiber organization in SMCs, which was blocked by the pitavastatin pretreatment. Pitavastatin had no effect on the serum- and PDGF-induced lamelliopodia extension of SMC. These results may suggest that pitavastatin attenuates SMC-mediated collagen gel contraction probably via an inhibition of geranylgeranylated Rho protein and a disruption of actin cytoskeletal reorganization.

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.

Actin cytoskeleton remodelling via local inhibition of contractility at discrete microdomains

Activation of conventional protein kinase C by phorbol ester triggers the Src-dependent remodelling of the actin cytoskeleton and the formation of podosomes in vascular smooth muscle cells. Rearrangement of actin cytoskeleton in response to phorbol-12,13-dibutyrate is characterised by the simultaneous disassembly of peripheral actin stress fibres and focal adhesions, focal de novo actin polymerisation and actomyosin contraction in the cell center, indicating a spatially and temporally segregated, differential modulation of actin-cytoskeleton stability and turnover. Taking advantage of the prominent actin cytoskeleton in A7r5 cells we show here, that the molecular basis for the local inhibition of contractility is the specific recruitment of p190RhoGAP to specialised microdomains at the focal adhesion/stress fibre interface, which are constitutively enriched in cortactin. The microdomains contain structurally altered actin filaments inaccessible to phalloidin. However, the filaments remain decorated with high molecular weight tropomyosins. Clustering of cortactin during podosome formation causes the rapid, local dispersion of myosin and tropomyosin, and interferes with the F-actin binding of h1calponin, consistent with a RhoGAP-mediated reduction of contractility. Phorbol ester-induced podosome formation is efficiently blocked by expression of constitutively active Dia1, which leads to the dispersion of cortactin. The results provide direct evidence for the spatially restricted inhibition of contractility via the recruitment and accumulation of cortactin and p190RhoGAP.

Rac-induced increase of phosphorylation of myosin regulatory light chain in HeLa cells

The pathways by which activation of the small GTP-binding protein Rac causes cytoskeletal changes are not fully understood but are likely to involve both assembly of new actin filaments and reorganization of actin filaments driven by the actin-dependent ATPase activity of myosin II. Here we show that expression of active RacQ61 in growing HeLa cells, in addition to inducing ruffling, substantially enhances the level of phosphorylation of serine-19 of the myosin II regulatory light chain (MLC), which would increase actomyosin II ATPase and motor activities. Phosphorylated myosin was localized to RacQ61-induced ruffles and stress fibers. RacQ61-induced phosphorylation of MLC was reduced by a maximum of about 38% by an inhibitor (Tat-PAK) of p21-activated kinase (PAK), about 35% by an inhibitor (Y-27632) of Rho kinase, 51% by Tat-PAK plus Y-27632, and 10% by an inhibitor (ML7) of myosin light chain kinase. Staurosporine, a non-specific inhibitor of serine/threonine kinases, reduced RacQ61-induced phosphorylation of MLC by about 58%, at the maximum concentration that did not kill cells. Since Rac activates PAK and PAK can phosphorylate MLC, these data strongly suggest that PAK is responsible for a significant fraction of RacQ61-induced MLC phosphorylation. To our knowledge, this is the first evidence that active Rac causes phosphorylation of MLC in cells, thus implicating activation of the ATPase activity of actomyosin II as one of the ways by which Rac may induce cytoskeletal changes.

Rho-kinase-mediated Ca2+-independent contraction in rat embryo fibroblasts

Thus far, determining the relative contribution of Ca2+/calmodulin-dependent myosin light chain kinase (MLCK) and Ca2+-independent Rho-kinase pathways to myosin II activation and contraction has been difficult. In this study, we characterize the role of Rho-kinase in a rat embryo fibroblast cell line (REF-52), which contains no detectable MLCK. No endogenous MLCK could be detected in REF-52 cells by either Western or Northern blot analysis. In the presence or absence of Ca2+, thrombin or lysophosphatidic acid (LPA) increased RhoA activity and Rhokinase activity, correlating with isometric tension development and myosin II regulatory light chain (RLC) phosphorylation. Resting tension is associated with a basal phosphorylation of 0.31 +/- 0.02 mol PO4/mol RLC, whereas upon LPA or thrombin treatment myosin II RLC phosphorylation increases to 1.08 +/- 0.05 and 0.82 +/- 0.05 mol PO4/mol RLC, respectively, within 2.5 min. Ca2+ chelation has minimal effect on the kinetics and magnitude of isometric tension development and RLC phosphorylation. Treatment of REF-52 cells with the Rho-kinase-specific inhibitor Y-27632 abolished thrombin- and LPA-stimulated contraction and RLC phosphorylation. These results suggest that Rho-kinase is sufficient to activate myosin II motor activity and contraction in REF-52 cells.

Nonmuscle myosin IIb is involved in the guidance of fibroblast migration

Although myosin II is known to play an important role in cell migration, little is known about its specific functions. We have addressed the function of one of the isoforms of myosin II, myosin IIB, by analyzing the movement and mechanical characteristics of fibroblasts where this protein has been ablated by gene disruption. Myosin IIB null cells displayed multiple unstable and disorganized protrusions, although they were still able to generate a large fraction of traction forces when cultured on flexible polyacrylamide substrates. However, the traction forces were highly disorganized relative to the direction of cell migration. Analysis of cell migration patterns indicated an increase in speed and decrease in persistence, which were likely responsible for the defects in directional movements as demonstrated with Boyden chambers. In addition, unlike control cells, mutant cells failed to respond to mechanical signals such as compressing forces and changes in substrate rigidity. Immunofluorescence staining indicated that myosin IIB was localized preferentially along stress fibers in the interior region of the cell. Our results suggest that myosin IIB is involved not in propelling but in directing the cell movement, by coordinating protrusive activities and stabilizing the cell polarity.

Distribution and expression of non-muscle myosin light chain kinase in rabbit livers

AIM: To study the distribution and expression of non-muscle myosin light chain kinase (nmMLCK) in rabbit livers.

METHODS: Human nmMLCK N-terminal cDNA was amplified by polymerase chain reaction (PCR) and was inserted into pBKcmv to construct expression vectors. The recombinant plasmid was transformed into XL1-blue. Expression protein was induced by IPTG and then purified by SDS-PAGE and electroelution, which was used to prepare the polycolonal antibody to detect the distribution and expression of nmMLCK in rabbit livers with immunofluorescene techniques.

RESULTS: The polyclonal antibody was prepared, by which nmMLCK expression was detected and distributed mainly in peripheral hepatocytes.

CONCLUSION: nmMLCK can express in hepatocytes peripherally, and may play certain roles in the regulation of hepatic functions.

Uncoordinated regulation of stress fibers and focal adhesions by DAP kinase

Death-associated protein kinase (DAP kinase) is a proapoptotic, calcium/calmodulin-dependent serine/threonine kinase. Here, we report that DAP kinase phosphorylates the regulatory light chain of myosin II (MLC) both in vitro and in vivo, and that this phosphorylation occurs preferentially at residue Ser19. In quiescent fibroblasts, DAP kinase stabilizes stress fibers through phosphorylation of MLC, but it is dispensable for the formation of peripheral microfilament bundles. This cytoskeletal effect of DAP kinase occurs before the onset of apoptosis and does not require an intact death domain. In addition, DAP kinase is required for serum-induced stress-fiber formation, which is associated with the upregulation of its catalytic activity. Despite being both sufficient and necessary for the assembly or maintenance of stress fibers, DAP kinase is incapable of stimulating the formation of focal adhesions in quiescent cells. Moreover, it promotes the disassembly of focal adhesions but not stress fibers in cells receiving serum factors. Together, our results identify a novel and unique function of DAP kinase in the uncoupling of stress fibers and focal adhesions. Such uncoupling would lead to a perturbation of the balance between contractile and adhesion forces and subsequent cell detachment, which might contribute to its pro-apoptotic activity.

Regulation of MDCK cell-substratum adhesion by RhoA and myosin light chain kinase after ATP depletion

The attachment of epithelial cells to the extracellular matrix substratum is essential for their differentiation and polarization. Despite this, the precise adhesion mechanism and its regulation are poorly understood. In the kidney, an ischemic insult causes renal tubular epithelial cells to detach from the basement membrane, even though they remain viable. To understand this phenomenon, and to probe the regulation of epithelial cell attachment, we used a model system consisting of newly adherent Madin-Darby canine kidney (MDCK) cells subjected to ATP depletion to mimic ischemic injury. We found that MDCK cells detach from collagen I after 60 min of ATP depletion but reattach when resupplied with glucose. Detachment is not caused by degradation or endocytosis of beta(1)-integrins, which mediate attachment to collagen I. Basal actin filaments and paxillin-containing adhesion complexes are disrupted by ATP depletion and quickly reform on glucose repletion. However, partial preservation of basal actin by overexpression of constitutively active RhoA does not significantly affect cell detachment. Furthermore, Y-27632, an inhibitor of the RhoA effector Rho-kinase, does not prevent reattachment of cells on glucose addition, even though reformation of central stress fibers and large adhesion complexes is blocked. In contrast, reattachment of ATP-depleted cells and detachment of cells not previously subjected to ATP depletion are prevented by ML-7, an inhibitor of myosin light chain kinase (MLCK). We conclude that initial adherence of MDCK cells to a collagen I substratum is mediated by peripheral actin filaments and adhesion complexes regulated by MLCK but not by stress fibers and adhesion complexes controlled by RhoA.

Rho inhibition decreases TNF-induced endothelial MAPK activation and monolayer permeability

Our laboratory previously demonstrated that MAPK activation is an important signal during cytokine-induced endothelial permeability (Nwariaku FE, Liu Z, Terada L, Duffy S, Sarosi G, and Turnage R. Shock 18: 82-85, 2002). Because GTP-binding proteins have been implicated in MAPK activation, we now hypothesize that the GTP-binding protein Rho is a mediator of TNF-induced MAPK activation and increased endothelial permeability. Transmonolayer permeability was assessed in human lung microvascular cells by measuring transmonolayer electrical resistance. MAPK activity was assessed by using a phospho-specific immunoprecipitation kinase assay and by comparing Western blots for phospho-MAPK with total MAPK. MAPK inhibitors used were SB-202190 and PD-098059, whereas Clostridium botulinum C3 transferase was used as a Rho inactivator. Rho-associated coiled-coil kinase was inhibited with Y-27632. TNF increased pulmonary endothelial permeability in vitro and caused a rapid, sustained increase in endothelial p38 and extracellular signal-regulated kinase MAPK activity. Inhibition of p38 and extracellular signal-regulated kinase MAPK with SB-202190 and PD-098059, respectively, decreased TNF-induced endothelial permeability. C3 transferase attenuated TNF-induced MAPK activation and blocked TNF-induced endothelial permeability. Finally, inhibition of Rho-associated coiled-coil kinase with Y-27632 prevented both MAPK activation and TNF-induced decreases in transmonolayer resistance. Rho acts upstream of mitogen-activated protein kinases in mediating TNF-induced pulmonary endothelial leak.

Engulfment of apoptotic cells is negatively regulated by Rho-mediated signaling

The rapid and efficient phagocytosis of apoptotic cells plays a critical role in preventing secondary necrosis, inflammation as well as in tissue remodeling and regulating immune responses. However, the molecular details of engulfment are just beginning to be elucidated. Among the Rho family GTPases, previous studies have implicated a role for Rac and Cdc42 in the uptake of apoptotic cells by phagocytes, yet the role of Rho has remained unclear. Here, we present evidence that Rho-GTP levels decrease during engulfment. RhoA seems to negatively affect basal engulfment, such that inhibition of Rho-mediated signaling in phagocytes enhanced the uptake of apoptotic targets. Activation of endogenous Rho or overexpression of constitutively active forms of Rho also inhibited engulfment. By testing mutants of RhoA that selectively activate downstream effectors, the Rho-kinase seemed to be primarily responsible for this inhibitory effect. Taken together, these data suggest that inhibition of Rho- and Rho-kinase-mediated signaling might be important during engulfment, which could have important implications for several clinical trials involving inhibition of the Rho kinase.

Different molecular motors mediate platelet-derived growth factor and lysophosphatidic acid-stimulated floating collagen matrix contraction

Fibroblast-collagen matrix contraction has been used as a model system to study how cells organize connective tissue. Previous work showed that lysophosphatidic acid (LPA)-stimulated floating collagen matrix contraction is independent of Rho kinase, whereas platelet-derived growth factor (PDGF)-stimulated contraction is Rho kinase-dependent. The current studies were carried out to learn more about the molecular motors responsible for LPA- and PDGF-stimulated contraction. We found that neither PDGF nor LPA-dependent contractile mechanisms require myosin II regulatory light chain kinase or increased phosphorylation of myosin II regulatory light chain (measured as diphosphorylation). Low concentrations of the specific myosin II inhibitor blebbistatin blocked PDGF-stimulated matrix contraction and LPA-stimulated retraction of fibroblast dendritic extensions but not LPA-stimulated matrix contraction. These data suggest that PDGF- and LPA-stimulated floating matrix contraction utilize myosin II-dependent and -independent mechanisms, respectively. LPA-dependent, Rho kinase-independent force generation also was detected during fibroblast spreading on collagen-coated coverslips.

Increased phosphorylation of myosin light chain associated with slow-to-fast transition in rat soleus

In striated muscles myosin light chain (MLC)2 phosphorylation regulates calcium sensitivity and mediates sarcomere organization. Little is known about the changes in MLC2 phosphorylation in relation to skeletal muscle plasticity. We studied changes in MLC2 phosphorylation in rats receiving three treatment conditions causing slow-to-fast transitions: 1) atrophy induced by 14 days of hindlimb suspension (HS), 2) hypertrophy induced by 14 days of clenbuterol administration (CB), and 3) 14 days of combined treatment (CB-HS). Three variants of the slow (MLC2s) and two variants of the fast MLC2 (MLC2f) isoform were separated with two-dimensional electrophoresis and identified with monoclonal and polyclonal antibodies specific for MLC2; their relative proportions were densitometrically quantified. In control soleus muscle MLC2s predominated over MLC2f (91.4 +/- 3.9% vs. 8.5 +/- 3.9%) and was separated into two spots, the less acidic spot being 73.5 +/- 4.3% of the total. All treatments caused a decrease of the less acidic unphosphorylated spot of MLC2s (CB: 64.1 +/- 5.6%, HS: 62.4 +/- 6.8%, CB-HS: 56.4 +/- 4.4%), the appearance of a third more acidic variant of MLC2s (representing 3.9-5.9% of total MLC2s), an increase of MLC2f (CB: 30.9 +/- 3.1%, HS: 23.9 +/- 3.3%, CB-HS: 25.3 +/- 3.9%), and the phosphorylation of a large fraction of MLC2f (CB: 30.4 +/- 6.7%, HS: 28.7 +/- 6.5%, CB-HS: 21.8 +/- 2.1%). Treatment with alkaline phosphatase or with protein phosphatase 1 (PP1) removed the most acidic spots of both MLC2f and MLC2s. We conclude that in rat skeletal muscles an increase of MLC2 phosphorylation is associated with the slow-to-fast transition regardless of whether hypertrophy or atrophy develops.

LFA-1-induced T cell migration on ICAM-1 involves regulation of MLCK-mediated attachment and ROCK-dependent detachment

This study analyzes signaling events initiated through binding of the leukocyte integrin LFA-1 to ICAM-1, which leads to T cell attachment, polarization and random migration. These events are critically dependent on dynamic changes in the acto-myosin cytoskeleton under the regulation of myosin light chain kinase and ROCK (Rho kinase). A key finding is that the activity of these two kinases is spatially segregated. Myosin light chain kinase (MLCK) must operate at the leading edge of the T cell because blocking its activity causes the polarized T cell to retract from the front of the cell. These activities are mirrored by inhibiting calmodulin, the activator of MLCK. In contrast inhibition of ROCK (and RhoA) has the effect of preventing detachment of the T cell trailing edge, showing that this kinase operates at the rear of the cell. This compartmentalized activity of the two kinases is reflected in their localization within the T cell. Myosin light chain kinase is concentrated at the leading edge, overlapping F-actin, whereas ROCK is more widely distributed in the trailing edge of the T cell. Thus these two kinases perform two different functions in the migrating T cell, with myosin light chain kinase activity important for attachment and movement at the leading edge and ROCK activity required for the detachment of the trailing edge. These two actomyosin-dependent processes operate coordinately to cause forward migration of a T cell.

Astrocyte stellation induced by Rho kinase inhibitors in culture

To understand the role of Rho kinases in regulation of astrocyte morphology, we investigated the effects of Rho kinase inhibitors on the morphology of cultured rat cortical astrocytes. Cultured astrocytes exhibited flattened, polygonal morphology in the absence of stimulation, but changed into process-bearing stellate cells following treatment with the selective Rho kinase inhibitor Y-27632 (1-10 microM). The Y-27632-induced astrocyte stellation was abolished by treatment with colchicine, indicating that the response requires reorganization of cytoskeletal elements. The effect of Y-27632 was mimicked by another Rho kinase inhibitor HA1077, but not by the protein kinase C inhibitor GF-109203X or the protein kinase A inhibitor KT5720. These results suggest that Rho kinases are in an activated state in the absence of stimuli and contribute to the maintenance of polygonal morphology of cultured astrocytes.

Apoptotic and necrotic blebs in epithelial cells display similar neck diameters but different kinase dependency

Apoptotic and necrotic blebs elicited by H(2)O(2) were compared in terms of dynamics, structure and underlying biochemistry in HeLa cells and Clone 9 cells. Apoptotic blebs appeared in a few minutes and required micromolar peroxide concentrations. Necrotic blebs appeared much later, prior to cell permeabilization, and required millimolar peroxide concentrations. Strikingly, necrotic blebs grew at a constant rate, which was unaffected throughout successive cycles of budding and detachment. At 1 microm diameter, the necks of necrotic and apoptotic blebs were almost identical. ATP depletion was discarded as a major factor for both types of bleb. Inhibition of ROCK-I, MLCK and p38MAPK strongly decreased apoptotic blebbing but had no effect on necrotic blebbing. Taken together, these data suggest the existence of a novel structure of fixed dimensions at the neck of both types of plasma membrane blebs in epithelial cells. However, necrotic blebs can be distinguished from apoptotic blebs in their susceptibility to actomyosin kinase inhibition.

Functional diversity between Rho-kinase- and MLCK-mediated cytoskeletal actions in a myofibroblast-like hepatic stellate cell line

Using a rat myofibroblast-like hepatic stellate cell line, we studied the actomyosin-based cytoskeletal actions mediated by Rho-kinase and/or myosin light chain kinase (MLCK). Calmodulin/MLCK inhibitors W-7 and ML-7 attenuated cell migration dose-relatedly at concentrations from 10(-6) to 10(-4)M and collagen gel-contraction by the cells at 10(-4)M, respectively. Rho-kinase inhibitors Y-27632 and HA1077 attenuated the gel-contraction at concentrations from 10(-6) to 10(-4) M, respectively. These Rho-kinase inhibitors attenuated cell migration at 10(-7)M but enhanced the migration at 10(-4)M, respectively. They altered cell morphology showing prominent peripheral actin bundles and sparse central stress fibers, in comparison with the calmodulin/MLCK inhibitors. Both ML-7 and Y-27632 attenuated phosphorylation of myosin regulatory light chain and cell attachment to extracellular substrate. ML-7 attenuated the activation of GTP-binding protein Rac, while Y-27632 did not. These findings suggest that the actomyosin-based cytoskeletal actions can be functionally diverse depending on the Rho-kinase-mediated pathway and the MLCK-mediated pathway.

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.

Nucleus alignment and cell signaling in fibroblasts: response to a micro-grooved topography

Cellular response to scaffold materials is of great importance in cellular and tissue engineering, and it is perhaps the initial cell contact with the scaffold that determines development of new tissue. Material surface morphology has strong effects on cell cytoskeleton and morphology, and it is thought that cells may react to the topography of collagen and surrounding cells during tissue embryology. A poorly understood area is, however, gene-level responses to topography. Thus, this paper used microarray to probe for consistent gene changes in response to lithographically produced topography (12.5 x 2-microm grooves) with time. The results showed many initial gene changes and also down-regulation of gene response with time. Cell and nucleus morphology were also considered, with nuclear deformation linked to cell signaling.

Spike Formation by Fibroblasts Adhering to Fibrillar Collagen I Gel

The fibrillar collagen I gel induced the formation of numerous dendritic cell-like protrusions (cell spikes) from the cell body, whereas monomeric collagen I induced typical cell spreading with filopodia and lamellipodia in skin fibroblasts. Peripheral, not central stress fibers appeared upon adhesion to fibrillar collagen gel, whereas both types of fibers were evident upon adhesion to monomeric collagen. Microtubules and vimentin filaments were elongated inside stress fibers along the terminal tip of cell spikes. Spike formation was totally inhibited by nocodazole and severely delayed by cytochalasin D. This suggests that cell spike formation is dependent on microtubules rather than on F-actin. We then investigated the intracellular signaling responsible for cytoskeleton organization to identify the key factor that induces cell spike morphology. During cell spike formation, FAK and CAS were activated. More CAS was activated in cells on fibrillar collagen gel than on the monomeric form, whereas FAK was activated to the same level on either. At 90 min of culture, Rac1 was activated in cells on monomeric collagen I, whereas Cdc42, Rac1 and RhoA were activated in cells on fibrillar collagen gel. These results suggest that microtubule organization via CAS and small GTPases is important for the cell spike formation that is involved in collagen gel contraction and in wound retraction in skin.

Lysophosphatidic acid induces focal adhesion assembly through Rho/Rho-associated kinase pathway in human ovarian cancer cells

OBJECTIVE

The level of lysophosphatidic acid (LPA) is elevated in patients with ovarian cancer, and LPA has been reported to have a pivotal role in cancer dissemination. In the current study, the effect of LPA on the motility of ovarian cancer cells was investigated.

METHODS

We analyzed the effects of LPA on the migration activity, the focal adhesion formation, and the tyrosine phosphorylation of focal adhesion proteins in human ovarian cancer cell lines Caov-3 and OVCAR-3. Inhibitors of the small GTPase Rho, one of its downstream effectors (Rho-associated kinase (ROCK)), myosin light chain kinase (MLCK), and myosin light chain (MLC) phosphatase were used to examine the mechanism of LPA-induced cellular effects.

RESULTS

LPA enhanced the migration of ovarian cancer cells and facilitated their invasion. Rho and ROCK played essential roles in the migratory process, as evidenced by the inhibition of migration and focal adhesion formation of cancer cells by Clostridium botulinum C3 exoenzyme (C3), an inhibitor of Rho, or Y-27632, an inhibitor of ROCK. LPA also evoked the formation of focal adhesions and tyrosine phosphorylation of focal adhesion kinase and paxillin, all of which were inhibited by C3 or Y-27632.

CONCLUSION

These results suggest that LPA induced the migration of ovarian cancer cells, at least in part, through accelerated formation of focal adhesions mediated by Rho/ROCK-induced actomyosin contractility. This study may provide the basis for new therapies to control the metastasis of ovarian cancer.

Activated Galphaq family members induce Rho GTPase activation and Rho-dependent actin filament assembly

Rho GTPase is required for actin filament assembly and serum response element (SRE)-dependent gene transcription. Certain G protein-coupled receptors (GPCRs) induce Rho-dependent responses, but the intermediary signaling steps are poorly understood. The heterotrimeric Galpha12 family can induce Rho-dependent responses. In contrast, there are conflicting reports on the role of the Galphaq family in Rho signaling. We report that expression of activated Galphaq members, or activation of endogenous Galphaq via GPCR stimulation, induces SRE reporter activation via Rho, and increased GTP-Rho levels. Moreover, microinjection of activated Galphaq in fibroblasts induces actin stress fiber formation via Rho. Galphaq functionally cooperates with Lbc Rho guanine nucleotide exchange factor. Overall, these findings indicate that Galphaq family signals are sufficient to induce Rho-dependent cellular responses.

Stress fibres-a Ca2+ -independent store for annexins?

Annexins belong to a family of lipid-binding proteins that are implicated in membrane organization. Several members are capable of binding to actin and, in smooth muscle cells, annexin 6 is known to form a Ca(2+)-dependent, plasmalemmal complex with actin filaments. Annexins can also associate with F-actin containing stress fibres within cultured smooth muscle cells or fibroblasts in a Ca(2+)-independent manner. Depolymerization of stress-fibre systems with cytochalasin D leads to the translocation of actin-bound annexin 2 from the cytoplasm to the plasma membrane at high intracellular levels of Ca(2+). This type of Ca(2+)-dependent annexin mobility is observed only in cells of mesenchymal phenotype, which have a well-developed stress-fibre system; not in epithelial cells.

Myosin light chain kinase inhibitors induce retraction of mature oligodendrocyte processes

Mature oligodendrocytes emit numerous myelinating processes. Force generating molecules are required for process outgrowth and spreading. We have analyzed the effect of the myosin II light chain kinase inhibitors ML-7 and ML-9 in cultured oligodendrocytes. Both drugs affect oligodendrocyte cell shape, provoking a retraction of high order processes. Our results suggest that the adhesion of the myelinating processes to the substrate depends on MLC phosphorylation, thus likely implicating myosin IIA.

Modulation of fibroblast morphology and adhesion during collagen matrix remodeling

When fibroblasts are placed within a three-dimensional collagen matrix, cell locomotion results in translocation of the flexible collagen fibrils of the matrix, a remodeling process that has been implicated in matrix morphogenesis during development and wound repair. In the current experiments, we studied formation and maturation of cell-matrix interactions under conditions in which we could distinguish local from global matrix remodeling. Local remodeling was measured by the movement of collagen-embedded beads towards the cells. Global remodeling was measured by matrix contraction. Our observations show that no direct relationship occurs between protrusion and retraction of cell extensions and collagen matrix remodeling. As fibroblasts globally remodel the collagen matrix, however, their overall morphology changes from dendritic to stellate/bipolar, and cell-matrix interactions mature from punctate to focal adhesion organization. The less well organized sites of cell-matrix interaction are sufficient for translocating collagen fibrils, and focal adhesions only form after a high degree of global remodeling occurs in the presence of growth factors. Rho kinase activity is required for maturation of fibroblast morphology and formation of focal adhesions but not for translocation of collagen fibrils.

Characterization of Ca2+/calmodulin-dependent protein kinase I as a myosin II regulatory light chain kinase in vitro and in vivo

Ca(2+)/calmodulin (CaM)-dependent protein kinase I (CaM-KI), which is a member of the multifunctional CaM-K family, is thought to be involved in various Ca(2+)-signalling pathways. In this report, we demonstrate that CaM-KI activated by an upstream kinase (CaM-K kinase), but not unactivated CaM-KI, phosphorylates myosin II regulatory light chain (MRLC) efficiently ( K (cat), 1.7 s(-1)) and stoichiometrically (approximately 0.8 mol of phosphate/mol) in a Ca(2+)/CaM-dependent manner in vitro. One-dimensional phosphopeptide mapping and mutational analysis of MRLC revealed that the activated CaM-KI monophosphorylates only Ser-19 in MRLC. Transient expression of the Ca(2+)/CaM-independent form of CaM-KI (CaM-KI(1-293)) in HeLa cells induced Ser-19 phosphorylation of myosin, II accompanied by reorganization of actin filaments in the peripheral region of the cells. CaM-KI-induced reorganization of actin filaments was suppressed by co-expression of non-phosphorylatable MRLC mutants (S19A and T18AS19A). Furthermore, a kinase-negative form of CaM-KI (CaM-KI(1-293,K49E)) significantly reduced reorganization of actin filaments, indicating a dominant negative effect. This is the first demonstration that the activation of the CaM-KI cascade induces myosin II phosphorylation, resulting in regulation of actin filament organization in mammalian cells.

Stiffness changes in cultured airway smooth muscle cells

Airway smooth muscle (ASM) cells in culture stiffen when exposed to contractile agonists. Such cell stiffening may reflect activation of the contractile apparatus as well as polymerization of cytoskeletal biopolymers. Here we have assessed the relative contribution of these mechanisms in cultured ASM cells stimulated with serotonin (5-hydroxytryptamine; 5-HT) in the presence or absence of drugs that inhibit either myosin-based contraction or polymerization of filamentous (F) actin. Magnetic twisting cytometry was used to measure cell stiffness, and associated changes in structural organization of actin cytoskeleton were evaluated by confocal microscopy. We found that 5-HT increased cell stiffness in a dose-dependent fashion and also elicited rapid formation of F-actin as marked by increased intensity of FITC-phalloidin staining in these cells. A calmodulin antagonist (W-7), a myosin light chain kinase inhibitor (ML-7) and a myosin ATPase inhibitor (BDM) each ablated the stiffening response but not the F-actin polymerization induced by 5-HT. Agents that inhibited the formation of F-actin (cytochalasin D, latrunculin A, C3 exoenzyme, and Y-27632) attenuated both baseline stiffness and the extent of cell stiffening in response to 5-HT. Together, these data suggest that agonist-evoked stiffening of cultured ASM cells requires actin polymerization as well as myosin activation and that neither actin polymerization nor myosin activation by itself is sufficient to account for the cell stiffening response.

Buckling of actin stress fibers: a new wrinkle in the cytoskeletal tapestry

Intracellular tension is considered an important determinant of cytoskeletal architecture and cell function. However, many details about cytoskeletal tension remain poorly understood because these forces cannot be directly measured in living cells. Therefore, we have developed a method to characterize the magnitude and distribution of pre-extension of actin stress fibers (SFs) due to resting tension in the cytoskeleton. Using a custom apparatus, human aortic endothelial cells (HAECs) were cultured on a pre-stretched silicone substrate coated with a fibronectin-like polymer. Release of the substrate caused SFs aligned in the shortening direction in adhered cells to buckle when compressed rapidly (5% shortening per second or greater) beyond their unloaded slack length. Subsequently, the actin cytoskeleton completely disassembled in 5 sec and reassembled within 60 sec. Quantification of buckling in digital fluorescent micrographs of cells fixed and stained with rhodamine phalloidin indicated a nonuniform distribution of 0-26% pre-extension of SFs in non-locomoting HAECs. Local variability suggests heterogeneity of cytoskeletal tension and/or stiffness within individual cells. These findings provide new information about the magnitude and distribution of cytoskeletal tension and the dynamics of actin stress fibers, and the approach offers a novel method to elucidate the role of specific cytoskeletal elements and crosslinking proteins in the force generating apparatus of non-muscle cells.

The molecular basis for the autoregulation of calponin by isoform-specific C-terminal tail sequences

The three genetic isoforms of calponin (CaP), h1, h2 and acidic, are distinguished mostly by their individual C-terminal tail sequences. Deletion of these sequences beyond the last homologous residue Cys273 increases actin filament association for all three isoforms, indicating a negative regulatory role for the unique tail regions. We have tested this hypothesis by constructing a series of deletion and substitution mutants for all three CaP isoforms. Here we demonstrate that the C-terminal sequences regulate actin association by altering the function of the second actin-binding site, ABS2, in CaP comprised of the three 29-residue calponin repeats. Removal of the inhibitory tail resulted in an increased binding and bundling activity, and caused a prominent re-localization of h2 CaP from the peripheral actin network to the central actin stress fibers in transfected A7r5 smooth muscle cells. Domain-swap experiments demonstrated that the tail sequence of h2 CaP can downregulate cytoskeletal association efficiently in all three CaP isoforms, whereas the tail of the smooth-muscle-specific h1 CaP variant had little effect. Site-directed mutagenesis further revealed that the negatively charged residues within the tail region are essential for this regulatory function. Finally we demonstrate that the tail sequences regulate the second actin-binding site (ABS2) and not the strong actin-binding ABS1 region in CaP.

Activation of K(+) channels and increased migration of differentiated intestinal epithelial cells after wounding

Early mucosal restitution occurs by epithelial cell migration to reseal superficial wounds after injury. Differentiated intestinal epithelial cells induced by forced expression of the Cdx2 gene migrate over the wounded edge much faster than undifferentiated parental cells in an in vitro model. This study determined whether these differentiated intestinal epithelial cells exhibit increased migration by altering voltage-gated K(+) (Kv) channel expression and cytosolic free Ca(2+) concentration ([Ca(2+)](cyt)). Stable Cdx2-transfected IEC-6 cells (IEC-Cdx2L1) with highly differentiated phenotype expressed higher basal levels of Kv1.1 and Kv1.5 mRNAs and proteins than parental IEC-6 cells. Neither IEC-Cdx2L1 cells nor parental IEC-6 cells expressed voltage-dependent Ca(2+) channels. The increased expression of Kv channels in differentiated IEC-Cdx2L1 cells was associated with an increase in whole cell K(+) currents, membrane hyperpolarization, and a rise in [Ca(2+)](cyt). The migration rates in differentiated IEC-Cdx2L1 cells were about four times those of parental IEC-6 cells. Inhibition of Kv channel expression by polyamine depletion decreased [Ca(2+)](cyt), reduced myosin stress fibers, and inhibited cell migration. Elevation of [Ca(2+)](cyt) by ionomycin promoted myosin II stress fiber formation and increased cell migration. These results suggest that increased migration of differentiated intestinal epithelial cells is mediated, at least partially, by increasing Kv channel activity and Ca(2+) influx during restitution.

Fibroblast signaling events in response to nanotopography: a gene array study

When considering the complicated nature of cell/tissue interactions with biomaterials, especially materials with nanometric surface features, observation of changes in one or two selected genes or proteins may not be sufficient. To get a fuller understanding of the scope of responses effected by nanotopography on cells, many genes need to be surveyed. Recent developments in molecular biology have lead to the commercial production of microarrays. Microarray presents a powerful tool by which many genes (up to many thousands) can be probed simultaneously. In this study, 1718 gene arrays have been used to measure human fibroblast response to 13-nm-high polymer demixed islands. The results have shown many changes in genes involved in signaling, cytoskeleton, extracellular matrix, gene transcription, and protein translation; these results have been used to build a more complete overview of fibroblast response to the islands. The use of microarray has expanded the range of observations possible using established microscopical and biochemical techniques.

Cell Junction Dynamics in the Testis: Sertoli-Germ Cell Interactions and Male Contraceptive Development

Spermatogenesis is an intriguing but complicated biological process. However, many studies since the 1960s have focused either on the hormonal events of the hypothalamus-pituitary-testicular axis or morphological events that take place in the seminiferous epithelium. Recent advances in biochemistry, cell biology, and molecular biology have shifted attention to understanding some of the key events that regulate spermatogenesis, such as germ cell apoptosis, cell cycle regulation, Sertoli-germ cell communication, and junction dynamics. In this review, we discuss the physiology and biology of junction dynamics in the testis, in particular how these events affect interactions of Sertoli and germ cells in the seminiferous epithelium behind the blood-testis barrier. We also discuss how these events regulate the opening and closing of the blood-testis barrier to permit the timely passage of preleptotene and leptotene spermatocytes across the blood-testis barrier. This is physiologically important since developing germ cells must translocate across the blood-testis barrier as well as traverse the seminiferous epithelium during their development. We also discuss several available in vitro and in vivo models that can be used to study Sertoli-germ cell anchoring junctions and Sertoli-Sertoli tight junctions. An in-depth survey in this subject has also identified several potential targets to be tackled to perturb spermatogenesis, which will likely lead to the development of novel male contraceptives.

Rho-dependent agonist-induced spatio-temporal change in myosin phosphorylation in smooth muscle cells

Agonist-induced translocation of RhoA and the spatio-temporal change in myosin regulatory light chain (MLC20) phosphorylation in smooth muscle was clarified at the single cell level. We expressed green fluorescent protein-tagged RhoA in the differentiated tracheal smooth muscle cells and visualized the translocation of RhoA in a living cell with three-dimensional digital imaging analysis. The stimulation of the cells by carbachol initiated the translocation of green fluorescent protein-tagged wild type RhoA to the plasma membrane within a minute. The change in MLC20 phosphorylation level after carbachol stimulation was monitored by using phospho-Ser-19-specific antibody recognizing the phosphorylated MLC20 in single cells. Cells expressing the dominant negative form (T19N) of RhoA significantly suppressed sustained MLC20 phosphorylation during the prolonged phase (>300 s), whereas the maximum phosphorylation level (reached at 10 s after stimulation) of these cells was not significantly different from the control cells. The kinetics of RhoA translocation was consistent with that of sustained myosin phosphorylation, suggesting the involvement of a RhoA pathway. Carbachol stimulation increased myosin phosphorylation within a minute both at the cortical and the central region. On the other hand, during prolonged phase, myosin phosphorylation was sustained at the cortical region of the cells but not at the central fibers. A myosin light chain kinase-specific inhibitor, ML-9, diminished myosin phosphorylation at the central region of the cells after the stimulation but not at the cortical area. On the other hand, Y-27632, a Rho kinase-specific inhibitor, diminished myosin phosphorylation at the cortical region but not the central region. The results clearly show that the myosin light chain kinase pathway and the Rho pathway distinctly change myosin phosphorylation in smooth muscle cells in both a temporal and spatial manner.

Preliminary research on myosin light chain kinase in rabbit liver

AIM: To study preliminarily the properties of myosin light chain kinase (MLCK) in rabbit liver.

METHODS: The expression of MLCK was detected by reverse transcription-polymerase chain reaction (RT-PCR); the MLCK was obtained from rabbit liver, and its activity was analyzed by γ-³² P incorporation technique to detect the phosphorylation of myosin light chain.

RESULTS: MLCK was expressed in rabbit liver, and the activity of the enzyme was similar to rabbit smooth muscle MLCK, and calmodulin- dependent. When the concentration was 0.65 mg •L¯¹, the activity was at the highest level.

CONCLUSION: MLCK expressed in rabbit liver may catalyze the phosphorylation of myosin light chain, which may play important roles in the regulation of hepatic cell functions.

Rho-kinase--mediated contraction of isolated stress fibers

It is widely accepted that actin filaments and the conventional double-headed myosin interact to generate force for many types of nonmuscle cell motility, and that this interaction occurs when the myosin regulatory light chain (MLC) is phosphorylated by MLC kinase (MLCK) together with calmodulin and Ca(2+). However, recent studies indicate that Rho-kinase is also involved in regulating the smooth muscle and nonmuscle cell contractility. We have recently isolated reactivatable stress fibers from cultured cells and established them as a model system for actomyosin-based contraction in nonmuscle cells. Here, using isolated stress fibers, we show that Rho-kinase mediates MLC phosphorylation and their contraction in the absence of Ca(2+). More rapid and extensive stress fiber contraction was induced by MLCK than was by Rho-kinase. When the activity of Rho-kinase but not MLCK was inhibited, cells not only lost their stress fibers and focal adhesions but also appeared to lose cytoplasmic tension. Our study suggests that actomyosin-based nonmuscle contractility is regulated by two kinase systems: the Ca(2+)-dependent MLCK and the Rho-kinase systems. We propose that Ca(2+) is used to generate rapid contraction, whereas Rho-kinase plays a major role in maintaining sustained contraction in cells.