Suppression of epithelial folding at actomyosin-enriched compartment boundaries downstream of Wingless signalling in Drosophila

Epithelial folding shapes embryos and tissues during development. Here, we investigate the coupling between epithelial folding and actomyosin-enriched compartmental boundaries. The mechanistic relationship between the two is unclear, because actomyosin-enriched boundaries are not necessarily associated with folds. Also, some cases of epithelial folding occur independently of actomyosin contractility. We investigated the shallow folds called parasegment grooves that form at boundaries between anterior and posterior compartments in the early Drosophila embryo. We demonstrate that formation of these folds requires the presence of an actomyosin enrichment along the boundary cell-cell contacts. These enrichments, which require Wingless signalling, increase interfacial tension not only at the level of the adherens junctions but also along the lateral surfaces. We find that epithelial folding is normally under inhibitory control because different genetic manipulations, including depletion of the Myosin II phosphatase Flapwing, increase the depth of folds at boundaries. Fold depth correlates with the levels of Bazooka (Baz), the Par-3 homologue, along the boundary cell-cell contacts. Moreover, Wingless and Hedgehog signalling have opposite effects on fold depth at the boundary that correlate with changes in Baz planar polarity.

The role of the calponin homology domain of smoothelin-like 1 (SMTNL1) in myosin phosphatase inhibition and smooth muscle contraction

In this study, we provide further insight into the contribution of the smoothelin-like 1 (SMTNL1) calponin homology (CH)-domain on myosin light chain phosphatase (SMPP-1M) activity and smooth muscle contraction. SMTNL1 protein was shown to have inhibitory effects on SMPP-1M activity but not on myosin light chain kinase (MLCK) activity. Treatment of beta-escin permeabilized rabbit, ileal smooth muscle with SMTNL1 had no effect on the time required to reach half-maximal force (t(1/2)) during stimulation with pCa6.3 solution. The addition of recombinant SMTNL1 protein to permeabilized, smooth muscle strips caused a significant decrease in contractile force. While the calponin homology (CH)-domain was essential for maximal SMTNL1-associated relaxation, it alone did not cause significant changes in force. SMTNL1 was poorly dephosphorylated by PP-1C in the presence of the myosin targeting subunit (MYPT1), suggesting that phosphorylated SMTNL1 does not possess "substrate trapping" properties. Moreover, while full-length SMTNL1 could suppress SMPP-1M activity toward LC(20) in vitro, truncated SMTNL1 lacking the CH-domain was ineffective. In summary, our findings suggest an important role for the CH-domain in mediating the effects of SMTNL1 on smooth muscle contraction.

The mechanism of vasorelaxation induced by Schisandra chinensis extract in rat thoracic aorta

AIM OF THE STUDY

Schisandra chinensis (SC) is a known medical herb for the treatment of cardiovascular symptoms associated with menopausal symptoms in Korea. However, the pharmacological action mechanisms involved have not been well studied. This study was aimed to investigate the vascular effects of SC in rat thoracic aorta.

MATERIALS AND METHODS

We isolated the hexane, chloroform, and methanol extracts from SC and evaluated their vasodilatory effects in the rat thoracic aorta.

RESULTS

Hexane extracts of SC (SCHE, 5 x 10(-5) to 10(-3) g/L) caused a concentration-dependent relaxation in both endothelium-intact and -denuded aortas. The relaxant effect of SCHE on the endothelium-intact aorta was more prominent than on the endothelium-denuded aorta. The former was significantly attenuated by L-NAME, a nitric oxide synthase inhibitor, and ODQ, a soluble guanyl cyclase inhibitor, but not by tetraethylammonium, a nonselective blocker of K(+) channels, and indomethacin, a cyclooxygenase inhibitor. Furthermore, SCHE caused nitrite production as well as eNOS activation in aortic segments, suggesting implication of NO signal pathway in SCHE-induced relaxation. In endothelium-denuded aorta, SCHE-induced vasorelaxation was also attenuated by calyculin A, an inhibitor of myosin light chain (MLC) phosphatase, but not by ML-9, a MLC kinase inhibitor, suggestive of implication of MLC phosphatase activation. Phenylephrine-enhanced MLC phosphorylation ratio was significantly attenuated by SCHE, which was recovered to the control level by pretreatment with calyculin A.

CONCLUSIONS

Taken collectively, these findings suggest that the vascular relaxation evoked by SCHE was mediated by not only endothelium dependent NO pathway but also direct effect on vascular smooth muscle cell via dephosphorylation of MLC.

Slow motility in hair cells of the frog amphibian papilla: myosin light chain-mediated shape change

Using video, fluorescence and confocal microscopy, quantitative analysis and modeling, we investigated intracellular processes mediating the calcium/calmodulin (Ca(2+)/CaM)-dependent slow motility in hair cells dissociated from the rostral region of amphibian papilla, one of the two auditory organs in frogs. The time course of shape changes in these hair cells during the period of pretreatment with several specific inhibitors, as well as their response to the calcium ionophore, ionomycin, were recorded and compared. These cells respond to ionomycin with a tri-phasic shape change: an initial phase of iso-volumetric length decrease; a period of concurrent shortening and swelling; and the final phase of increase in both length and volume. We found that both the myosin light chain kinase inhibitor, ML-7, and antagonists of the multifunctional Ca(2+)/CaM-dependent kinases, KN-62 and KN-93, inhibit the iso-volumetric shortening phase of the response to ionomycin. The type 1 protein phosphatase inhibitors, calyculin A and okadaic acid induce minor shortening on their own, but do not significantly alter phase 1 response. However, they appear to counter effects of the inhibitors of Ca(2+)/CaM-dependent kinases. We hypothesize that an active actomyosin-based process mediates the iso-volumetric shortening in the frog rostral amphibian papillar hair cells.

Melatonin restores impaired contractility in aged guinea pig urinary bladder

Urinary bladder disturbances are frequent in the elderly population but the responsible mechanisms are poorly understood. This study evaluates the effects of aging on detrusor myogenic contractile responses and the impact of melatonin treatment. The contractility of bladder strips from adult, aged and melatonin-treated guinea pigs was evaluated by isometric tension recordings. Cytoplasmatic calcium concentration ([Ca(2+)](i)) was estimated by epifluorescence microscopy of fura-2-loaded isolated detrusor smooth muscle cells, and the levels of protein expression and phosphorylation were quantitated by Western blotting. Aging impairs the contractile response of detrusor strips to cholinergic and purinergic agonists and to membrane depolarization. The impaired contractility correlates with increased [Ca(2+)](i) in response to the stimuli, suggesting a reduced Ca(2+)sensitivity. Indeed, the agonist-induced contractions in adult strips were sensitive to blockade with Y27362, an inhibitor of Rho kinase (ROCK) and GF109203X, an inhibitor of protein kinase C (PKC), but these inhibitors had negligible effects in aged strips. The reduced Ca(2+) sensitivity in aged tissues correlated with lower levels of RhoA, ROCK, PKC and the two effectors CPI-17 and MYPT1, and with the absence of CPI-17 and MYPT1 phosphorylation in response to agonists. Interestingly, melatonin treatment restored impaired contractility via normalization of Ca(2+) handling and Ca(2+) sensitizations pathways. Moreover, the indoleamine restored age-induced changes in oxidative stress and mitochondrial polarity. These results suggest that melatonin might be a novel therapeutic tool to palliate aging-related urinary bladder contractile impairment.

Mechanisms of Rho kinase regulation of vascular reactivity following hemorrhagic shock in rats

Our previous research showed that Rho kinase took part in the regulation of vascular hyporeactivity after shock. The objective of the present study was to investigate its mechanism. With isolated superior mesenteric artery (SMA) from hemorrhagic shock rats, we studied the relationship of Rho kinase regulating vascular reactivity to calcium sensitivity and myosin light chain phosphatase (MLCP) and myosin light chain kinase (MLCK). The vascular reactivity and calcium sensitivity of SMA were observed by measuring the contraction initiated by accumulative norepinephrine (NE) and calcium under depolarizing condition (120 mM K(+)) with an isolated organ perfusion system. Hypoxia-treated vascular smooth muscle cells (VSMCs) were used to study the effects of Rho kinase on the activity of MLCP and MLCK and the phosphorylation of 20-kDa myosin light chain (MLC(20)). Myosin light chain (20 kDa) phosphorylation of VSMC in mesenteric artery was detected by immunoprecipitation and Western blotting. The activity of MLCP and MLCK was assayed by enzymatic catalysis. The contractile response of VSMC was measured by the ratio of accumulative infiltration of fluorescent isothiocyanate-conjugated bovine serum albumin through transwell. The results indicated that the vascular reactivity and calcium sensitivity of SMA to NE and calcium following hemorrhagic shock and the contractile response of VSMC to NE following hypoxia were significantly decreased. Angiotensin II (Ang-II), the Rho kinase stimulator, significantly improved hypoxia or hemorrhagic shock-induced decrease of vascular reactivity and calcium sensitivity. These effects of Ang-II on vascular reactivity were abolished by Y-27632, the specific Rho kinase inhibitor. Calyculin A, the MLCP inhibitor, further enhanced Ang-II-induced increase of calcium sensitivity, but ML-9, the MLCK inhibitor, had no effect. Further studies showed Ang-II reversed the hypoxia-induced increase of MLCP activity and increased the hypoxia-induced decrease of MLC(20) phosphorylation in VSMC. It was suggested that Rho kinase played an important role in the regulation of vascular reactivity after hemorrhagic shock. The mechanisms may be related to its calcium sensitivity regulation. Rho kinase up-regulates calcium sensitivity of VSMC possibly through inhibiting the activity of MLCP and increasing the phosphorylation of MLC(20).

Phosphorylation-induced conformational switching of CPI-17 produces a potent myosin phosphatase inhibitor

Phosphorylation of endogenous inhibitor proteins for type-1 Ser/Thr phosphatase (PP1) provides a mechanism for reciprocal coordination of kinase and phosphatase activities. A myosin phosphatase inhibitor protein CPI-17 is phosphorylated at Thr38 through G-protein-mediated signals, resulting in a >1000-fold increase in inhibitory potency. We show here the solution NMR structure of phospho-T38-CPI-17 with rmsd of 0.36 +/- 0.06 A for the backbone secondary structure, which reveals how phosphorylation triggers a conformational change and exposes an inhibitory surface. This active conformation is stabilized by the formation of a hydrophobic core of intercalated side chains, which is not formed in a phospho-mimetic D38 form of CPI-17. Thus, the profound increase in potency of CPI-17 arises from phosphorylation, conformational change, and hydrophobic stabilization of a rigid structure that poses the phosphorylated residue on the protein surface and restricts its hydrolysis by myosin phosphatase. Our results provide structural insights into transduction of kinase signals by PP1 inhibitor proteins.

Cyclic AMP-independent CGRP8-37-sensitive receptors mediate adrenomedullin-induced decrease of CaCl2-contraction in pregnant rat mesenteric artery

OBJECTIVES

We tested the hypothesis that adrenomedullin reduces calcium influx independent of potassium channels in depolarized endothelium-denuded mesenteric artery from pregnant rats.

RESULTS

Adrenomedullin reduced the CaCl(2)-induced contraction, while the receptor antagonist calcitonin gene-related peptide (CGRP)(8-37), but not adrenomedullin(22-52), reversed these effects. Adenylate cyclase inhibition by SQ22536 did not prevent adrenomedullin effects on CaCl(2)-induced contraction. Adrenomedullin did not inhibit depolarization-induced calcium entry to isolated vascular smooth muscle. Inhibition of myosin light-chain (MLC) phosphatase by calyculin A reversed the effects of adrenomedullin on contraction caused by submillimolar concentrations of CaCl(2), while adrenomedullin still inhibited contraction caused by higher concentrations of CaCl(2). However, the ratio of phosphorylated to total myosin phosphatase target 1, the regulatory subunit of MLC phosphatase, did not change with adrenomedullin, indicating a lack of MLC phosphatase activation. Interestingly, sodium fluoride, a nonspecific protein phosphatase inhibitor, completely blocked the effect of adrenomedullin on CaCl(2)-induced contraction. Adrenomedullin inhibited calcium mobilization from intracellular stores induced by thapsigargin.

CONCLUSION

Adrenomedullin inhibits CaCl(2)-induced contraction, without affecting calcium influx, through a CGRP(8-37)-sensitive receptor, but not using the cyclic adenosine monophosphate pathway, probably through activation of protein phosphatases. Inhibition of intracellular calcium release is an additional role played by adrenomedullin in calcium homeostasis in vascular smooth muscle.

Postjunctional synergism of norepinephrine with ATP and diadenosine tetraphosphate in Guinea pig vas deferens. Role of protein kinase C and Myosin light chain phosphatase

In isolated guinea pig vas deferens, prior addition of norepinephrine (NE) significantly potentiated the contractile responses to adenosine-5'-triphosphate (ATP) and diadenosine tetraphosphate (AP4A) in a dose-dependent manner up to 240% of the control purine dose. The myosin light chain phosphatase (MLCP) inhibitor cantharidin at a dose of 10 micromol/l caused significant enhancement of ATP at concentrations of 1 and 3 mmol/l by 91 and 95% respectively. Similarly, cantharidin enhanced the contraction to AP4A, 30 and 100 micromol/l by 92 and 100% respectively. Inhibition of protein kinase C (PKC) by the use of chelerythrine (10 micromol/l), incubated at the vas deferens for 60 min, inhibited the NE-induced enhancement of purine-induced contraction. Chelerythrine reversed the NE-ATP and NE-AP4A synergism back close to control ATP and AP4A contraction values respectively. It can be concluded that postjunctional synergism becomes evident not only for adenine mononucleotides and NE but also for diadenosine polyphosphates presented here by AP4A in the guinea pig vas deferens. This synergism involves receptor-mediated activation of PKC and possibly PKC-induced inhibition of MLCP.

Myosin phosphatase dephosphorylates HDAC7, controls its nucleocytoplasmic shuttling, and inhibits apoptosis in thymocytes

The repressive activity of histone deacetylase 7 (HDAC7), a class IIa HDAC expressed in CD4+CD8+ double-positive thymocytes, is regulated by its nucleocytoplasmic shuttling. In resting thymocytes, HDAC7 is nuclear and functions as a transcriptional repressor. After T-cell receptor (TCR) activation, the serine/threonine kinase PKD1 phosphorylates HDAC7, resulting in its nuclear export and the derepression of its target genes. Here, we identify protein phosphatase 1beta (PP1beta) and myosin phosphatase targeting subunit 1 (MYPT1), two components of the myosin phosphatase complex, as HDAC7-associated proteins in thymocytes. Myosin phosphatase dephosphorylates HDAC7 and promotes its nuclear localization, leading to the repression of the HDAC7 target, Nur77, and the inhibition of apoptosis in CD4+CD8+ thymocytes.

Extracellular ATP induces assembly and activation of the myosin light chain phosphatase complex in endothelial cells

OBJECTIVES

Extracellular ATP stabilizes the endothelial barrier and inactivates the contractile machinery of endothelial cells. This inactivation relies on dephosphorylation of the regulatory myosin light chain (MLC) due to an activation of the MLC phosphatase (MLCP). To date, activation and function of MLCP in endothelial cells are only partially understood.

METHODS

Here, the mechanism of extracellular ATP-mediated activation of MLCP was analyzed in human endothelial cells from umbilical veins. Cells were transfected with the endogenous protein phosphatase 1 (PP1)-specific inhibitor-2 (I-2).

RESULTS

Overexpression of I-2 led to inhibition of PP1 activity and abrogation of the ATP-induced dephosphorylation of MLC. This indicates that the PP1 catalytic subunit is the principal phosphatase catalyzing the MLC dephosphorylation induced by extracellular ATP. As demonstrated by immunoprecipitation analysis, extracellular ATP recruits the PP1delta catalytic subunit and the myosin phosphatase targeting subunit (MYPT1) to form a complex. ATP stimulated dephosphorylation of MYPT1 at the inhibitory phosphorylation sites threonine 850 and 696. However, extracellular ATP failed to stimulate MYPT1 dephosphorylation in I-2-overexpressing cells.

CONCLUSIONS

The present study shows for the first time that, in endothelial cells, extracellular ATP causes activation of MLCP through recruitment of PP1delta and MYPT1 into a MLCP holoenzyme complex and PP1-mediated reduction of the inhibitory phosphorylation of MYPT1.

Hyperoxic conditions inhibit airway smooth muscle myosin phosphatase in rat pups

Exposure of rat pups to 100% oxygen is a model for studying neonatal lung injury. Airway reactivity is increased in this model, in part due to impaired airway smooth muscle (ASM) relaxation. We compared biochemical determinants of ASM contractility in rat pups exposed to 100% oxygen for 7 days vs. littermates raised in room air. The baseline quantities of ASM contractile proteins, extent of phosphorylation of the 20-kDa myosin regulatory light chain (LC20), and amount of the myosin-binding subunit of smooth muscle myosin phosphatase (MYPT) were all comparable between the two groups. Bethanechol-induced contraction increased the extent of phosphorylation of both LC20 and MYPT in the hyperoxic group (45% and 70% over control, respectively). Relaxation after electrical field stimulation demonstrated greater phosphorylation of both LC20 and MYPT in the hyperoxic group compared with controls (67% and 84%, respectively). To determine if hyperoxia induced changes in the isoforms of MYPT, isoform expression was also compared but differences were not found. To determine potential mechanisms whereby MYPT phosphorylation was increased by hyperoxia, separate tracheas were treated with the Rho kinase inhibitor Y-27632. This treatment completely eliminated differences in MYPT phosphorylation between the groups. Because phosphorylation of MYPT impairs the phosphatase activity of myosin phosphatase, these data suggest that hyperoxic conditioning during early postnatal life impairs relaxation through prolonging LC20 phosphorylation. This mechanism might contribute to increased ASM reactivity seen in bronchopulmonary dysplasia.

Contraction of myofibroblasts in granulation tissue is dependent on Rho/Rho kinase/myosin light chain phosphatase activity

During wound healing and fibrocontractive diseases fibroblasts acquire a smooth muscle cell-like phenotype by differentiating into contractile force generating myofibroblasts. We examined whether regulation of myofibroblast contraction in granulation tissue is dominated by Ca2+-induced phosphorylation of myosin light chain kinase or by Rho/Rho kinase (ROCK)-mediated inhibition of myosin light chain phosphatase, similar to that of cultured myofibroblasts. Strips of granulation tissue obtained from rat granuloma pouches were stimulated with endothelin-1 (ET-1), serotonin, and angiotensin-II and isometric force generation was measured. We here investigated ET-1 in depth, because it was the only agonist that produced a long-lasting and strong response. The ROCK inhibitor Y27632 completely inhibited ET-1-promoted contraction and the phosphatase inhibitor calyculin elicited contraction in the absence of any other agonists, suggesting that activation of the Rho/ROCK/myosn light chain phosphatase pathway is critical in regulating in vivo myofibroblast contraction. Membrane depolarization with K+ also stimulated a long-lasting contraction of granulation tissue; however, the amount of force generated was significantly less compared to ET-1. Moreover, K+-induced contraction was inhibited by Y27632. These results are consistent with inhibition of myosin light chain phosphatase by the Rho/ROCK signaling pathway, which would account for the long-duration contraction of myofibroblasts necessary for wound closure.

Histamine-induced phosphorylation of the regulatory light chain of myosin II disrupts the barrier integrity of corneal endothelial cells

PURPOSE

To investigate histamine-induced changes in the phosphorylation of myosin light chain (MLC) and its influence on the barrier integrity of corneal endothelial cells through altered contractility of the actin cytoskeleton.

METHODS

Experiments were performed in cultured bovine corneal endothelial cells (BCECs). Phosphorylation of MLC, which increases contractility of the actin cytoskeleton through actomyosin interaction, was assessed by urea-glycerol gel electrophoresis and Western blot analysis. Immunocytochemistry was used to locate phosphorylated MLC in relation to tight junctions. Phosphorylation of the 17-kDa PKC-potentiated inhibitory protein of type 1 protein phosphatase (CPI-17), which inhibits MLC phosphatase, was studied using Western blot analysis. The cortical actin cytoskeleton was visualized by staining with Texas-red phalloidin. Barrier integrity was determined by quantifying horseradish peroxidase (HRP; 44 kDa) flux across cells grown on porous filters.

RESULTS

RT-PCR and Western blot analysis confirmed the expression of Galphaq/11-coupled H1 receptors in BCECs. Exposure to histamine (100 microM; 10 minutes) led to phosphorylation of MLC (134% relative to untreated cells) and of CPI-17. Histamine also increased the flux of HRP by sevenfold and disrupted the assembly of the dense cortical actin found in resting cells. PKC activation by phorbol 12-myristate 13-acetate (PMA; 100 nM; 30 minutes) caused phosphorylation of both MLC and CPI-17. The histamine-induced MLC phosphorylation was reduced by pre-exposure to either ML-7 (50 microM), an MLCK (MLC kinase) inhibitor, or chelerythrine (10 microM), an inhibitor of PKC. Cotreatment with agents that elevate cAMP in BCECs prevented the histamine-induced MLC phosphorylation and the disruption of the actin cytoskeleton, and increased HRP flux. Phosphorylated MLC in response to histamine or PMA was found in a punctate form in close proximity to ZO-1, a marker of the tight junctional complex.

CONCLUSIONS

Histamine induces MLC phosphorylation by activating MLCK and partly inhibiting MLC phosphatase. The latter is facilitated by the phosphorylation of CPI-17. Localization of phosphorylated MLC in proximity to ZO-1 suggests increased contractility of the cortical actin at the tight junctional complex. This contractility oppose the tethering forces and lead to a breakdown of the barrier integrity. Last, elevated cAMP prevents histamine-induced loss of the barrier integrity, not only by blocking inactivation of MLC phosphatase but also by inactivating MLCK.

Linalyl acetate as a major ingredient of lavender essential oil relaxes the rabbit vascular smooth muscle through dephosphorylation of myosin light chain

In a preliminary experiment, we found that lavender essential oil relaxes vascular smooth muscle. Thus, the present experiments were designed to investigate the relaxation mechanism of linalyl acetate as the major ingredient of lavender essential oil in rabbit carotid artery specimens. Linalyl acetate produced sustained and progressive relaxation during the contraction caused by phenylephrine. The relaxation effect of linalyl acetate at a concentration near the EC50 was partially but significantly attenuated by nitroarginine as an inhibitor of nitric oxide synthase, 1H-[1,2,4]oxadiazolo[4,3-a]quinoxaline-1-one as an inhibitor of guanylyl cyclase, or by the denudation of endothelial cells. In specimens without endothelium, the phenylephrine-induced contraction and phosphorylation of myosin light chain (MLC) were significantly attenuated after the pretreatment with linalyl acetate. The relaxation caused by linalyl acetate in the endothelium-denuded specimens was clearly inhibited by calyculin A as an inhibitor of MLC phosphatase, although not by ML-9 as an inhibitor of MLC kinase. Furthermore, suppression of the phenylephrine-induced contraction and MLC phosphorylation with linalyl acetate was canceled by the pretreatment with calyculin A. These results suggest that linalyl acetate relaxes the vascular smooth muscle through partially activation of nitric oxide/cyclic guanosine monophosphate pathway, and partially MLC dephosphorylation via activating MLC phosphatase.

Involvement of reduced sensitivity to Ca in beta-adrenergic action on airway smooth muscle

BACKGROUND

It is well known that beta-adrenoceptor agonists (beta-agonists) cause relaxation in airway smooth muscle mediated by a reduction in the concentration of intracellular Ca2+ ([Ca2+](i)). However, little is currently known regarding whether reduced sensitization to Ca2+ is involved in the beta-adrenergic relaxation.

OBJECTIVE

This study was designed to determine the intracellular mechanisms underlying suppression of Ca2+ sensitization in beta-adrenergic relaxation (Ca(2+)-independent relaxation by beta-agonists). Methods Isometric tension and [Ca2+](i) were simultaneously measured in fura-2-loaded strips isolated from guinea-pig tracheal smooth muscles. The relationships between tension and [Ca2+](i) were examined in the inhibitory action of isoprenaline (ISO) and other cAMP-related agents against methacholine-induced contraction.

RESULTS

The concentration-inhibition curve for ISO against methacholine in tension was significantly dissociated from the curve for ISO in [Ca2+](i). In ISO-induced relaxation, a reduction in tension was significantly greater than that in [Ca2+](i.) This phenomenon was mimicked by other cAMP-related agents: forskolin and dibutyryl-cAMP. In contrast, the inhibitory action of SKF-96365, a non-selective inhibitor of Ca(2+) channels, was associated with that in [Ca2+](i). In the presence of Rp-cAMPS, an inhibitor of protein kinase A (PKA), ISO caused an equivalent relaxation with less reduction in [Ca2+](i). The effects of ISO were not affected by Y-27632, an inhibitor of Rho-kinase, or by bisindolylmaleimide, an inhibitor of protein kinase C. ISO failed to inhibit contraction elicited by calyculin A, an inhibitor of myosin phosphatase. Conclusion beta-Adrenergic action antagonizes not only Ca2+ mobilization but also Ca2+ sensitization in methacholine-induced contraction. The cAMP/PKA-independent, G(s)-direct action is more potent in Ca(2+)-independent relaxation by beta-agonists than the cAMP/PKA-dependent pathway. Moreover, myosin phosphatase is a fundamentally affected protein in the reduced response to Ca2+ mediated by beta-agonist. Our results may provide evidence that this Ca2+ desensitization is a novel target for a reliever medication using rapid-acting beta-agonists in acute asthma management.

Agonist- and depolarization-induced signals for myosin light chain phosphorylation and force generation of cultured vascular smooth muscle cells

Phosphorylation of myosin light chain (MLC) and contraction of differentiated smooth muscle cells in vascular walls are regulated by Ca2+ -dependent activation of MLC kinase, and by Rho-kinase- or protein-kinases-C-dependent inhibition of MLC phosphatase (MLCP). We examined regulatory pathways for MLC kinase and MLCP in cultured vascular smooth muscle cells (VSMCs), and for isometric force generation of VSMCs reconstituted in collagen fibers. Protein levels of RhoA, Rho-kinase and MYPT1 (a regulatory subunit of MLCP) were upregulated in cultured VSMCs, whereas a MLCP inhibitor protein, CPI-17, was downregulated. Endothelin-1 evoked a steady rise in levels of Ca2+, MLC phosphorylation and the contractile force of VSMCs, whereas angiotensin-II induced transient signals. Also, Thr853 phosphorylation of MYPT1 occurred in response to stimuli, but neither agonist induced phosphorylation of MYPT1 at Thr696. Unlike fresh aortic tissues, removal of Ca2+ or addition of voltage-dependent Ca2+ -channel blocker did not inhibit contractions of reconstituted VSMC fibers induced by agonists or even high concentrations of extracellular K+ ions. Inhibitors of Ins(1,4,5)P3-receptor and Rho-kinase antagonized agonist-induced or high-K+ -induced contraction in both reconstituted fibers and fresh tissues. These results indicate that both Ins(1,4,5)P3-induced Ca2+ release and Rho-kinase-induced MYPT1 phosphorylation at Thr853 play pivotal roles in MLC phosphorylation of cultured VSMCs where either Ca2+ -influx or CPI-17-MLCP signaling is downregulated.

Rho-Kinase and RGS-Containing RhoGEFs as Molecular Targets for the Treatment of Erectile Dysfunction

Erectile dysfunction (ED) is a highly prevalent and often under-treated condition. Erection is basically a spinal reflex that can be initiated by recruitment of penile afferents but also by visual, olfactory and imaginary stimuli. The generated nervous signals will influence the balance between contractile and relaxant factors, which control the degree of contraction of penile corporal cavernosal smooth muscles and, thus, determine the erectile state of the penis. The different steps involved in neurotransmission, impulse propagation and intracellular transduction of neural signals may be changed in different types of ED. Recent studies have revealed important roles for the small GTPase RhoA and its effector, Rho-kinase in regulating cavernosal smooth muscle tone. The RhoA/Rho-kinase pathway modulates the level of phosphorylation of the myosin light chain, mainly through inhibition of myosin phosphatase, and contributes to agonist-induced Ca(2+)-sensitization in smooth muscle contraction. Changes in this pathway may contribute to ED in various patient subgroups (e.g. hypertension, diabetes, hypogonadism). This review summarizes the importance of Rho-kinase signaling in the erectile response and introduces the evidence pointing to RGS-containing Rho-guanine nucleotide exchange factors (GEFs) as critical mediators of RhoA-GTPase activation in cavernosal smooth muscle and its possible compartmentalization in the caveolae. In addition, we suggest that the design of selective inhibitors of these GEFs might represent a novel class of pharmacological agents to treat ED.

Phosphorylation of Thr695 and Thr850 on the myosin phosphatase target subunit: inhibitory effects and occurrence in A7r5 cells

Major sites for Rho-kinase on the myosin phosphatase target subunit (MYPT1) are Thr695 and Thr850. Phosphorylation of Thr695 inhibits phosphatase activity but the role of phosphorylation at Thr850 is not clear and is evaluated here. Phosphorylation of both Thr695 and Thr850 by Rho-kinase inhibited activity of the type 1 phosphatase catalytic subunit. Rates of phosphorylation of the two sites were similar and efficacy of inhibition following phosphorylation was equivalent for each site. Phosphorylation of each site on MYPT1 was detected in A7r5 cells, but Thr850 was preferred by Rho-kinase and Thr695 was phosphorylated by an unidentified kinase(s).

PHI-1 induced enhancement of myosin phosphorylation in chicken smooth muscle

Herein, we provide evidence that in chicken smooth muscle, G-protein stimulation by a Rho-kinase pathway leads to an increase in myosin light chain phosphorylation. Additionally, G-protein stimulation did not increase MYPT1 phosphorylation at Thr695 or Thr850, and CPI-17, was not expressed in chicken smooth muscle. However, PHI-1 was present in chicken smooth muscle tissues. Both agonist and GTP(gamma)S stimulation result in an increase in PHI-1 phosphorylation, which is inhibited by inhibitors to both Rho-kinase (Y-27632) and (PKC) GF109203x. These data suggest that PHI-1 may act as a CPI-17 analog in chicken smooth muscle and inhibit myosin phosphatase activity during G-protein stimulation to produce Ca2+ sensitization.

The role of calcium desensitization in vascular hyporeactivity and its regulation after hemorrhagic shock in the rat

The objectives of the present study were to investigate the role of calcium desensitization in vascular hyporeactivity, and the regulatory effects of Rho-kinase, protein kinase C (PKC), and protein kinase G (PKG) on calcium sensitivity. The vascular reactivity and calcium sensitivity with superior mesenteric artery (SMA) from hemorrhagic shock rat were observed by measuring the contraction initiated by norepinephrine (NE) and Ca2+ under depolarizing conditions (120 mmol/L K) in an isolated organ perfusion system. Angiotensin II (Ang-II) and Fasudil, the Rho-kinase agonist and inhibitor, phorbol 12-myristate 13-acetate (PMA) and staurosporine, the PKC agonist and inhibitor, 8Br-cGMP and KT-5823, the PKG agonist and inhibitor, and Calyculin A, myosin light chain phosphatase (MLCP) inhibitor were used as tool agents. The results indicated that vascular reactivity and calcium sensitivity were decreased after hemorrhagic shock. The cumulative dose-response curve of SMA to NE and Ca2+ after shock was shifted to the right. Ang-II (10 mol/L) could improve the decreased vascular reactivity by increasing the calcium sensitivity of SMA, and insulin (100 nmol/L) could further decrease the vascular reactivity by decreasing the calcium sensitivity of SMA. These results suggested that the vasculature after shock was desensitized to calcium, which played an important role in the onset of vascular hyporeactivity after shock. PMA and KT-5823 could increase the sensitivity of SMA to Ca2+ and made the cumulative dose-response curve shift to the left. In contrast, Fasudil, staurosporine, and 8Br-cGMP decreased the sensitivity of SMA to Ca2+ and made the cumulative dose-response curve of Ca2+ shift to the right. Calyculin A (10 mol/L) pretreatment further enhanced Ang-II, and PMA induced increase of calcium sensitivity, yet weakened the 8Br-cGMP-induced decrease of calcium sensitivity. Taken together, the data suggest that Rho-kinase, PKC, and PKG are involved in the regulation of calcium sensitivity of vascular smooth muscle after hemorrhagic shock, and their regulatory effects on calcium sensitivity of vasculature are possibly related to MLCP.

RhoA-Rho kinase pathway mediates thrombin- and U-46619-induced phosphorylation of a myosin phosphatase inhibitor, CPI-17, in vascular smooth muscle cells

Protein kinase C-potentiated phosphatase inhibitor of 17 kDa (CPI-17) mediates some agonist-induced smooth muscle contraction by suppressing the myosin phosphatase in a phosphorylation-dependent manner. The physiologically relevant kinases that phosphorylate CPI-17 remain to be identified. Several previous studies have shown that some agonist-induced CPI-17 phosphorylation in smooth muscle tissues was attenuated by the Rho kinase (ROCK) inhibitor Y-27632, suggesting that ROCK is involved in agonist-induced CPI-17 phosphorylation. However, Y-27632 has recently been found to inhibit protein kinase C (PKC)-delta, a well-recognized CPI-17 kinase. Thus the role of ROCK in agonist-induced CPI-17 phosphorylation remains uncertain. The present study was designed to address this important issue. We selectively activated the RhoA pathway using inducible adenovirus-mediated expression of a constitutively active mutant RhoA (V14RhoA) in primary cultured rabbit aortic vascular smooth muscle cells (VSMCs). V14RhoA caused expression level-dependent CPI-17 phosphorylation at Thr38 as well as myosin phosphatase phosphorylation at Thr853. Importantly, we have shown that V14RhoA-induced CPI-17 phosphorylation was not affected by the PKC inhibitor GF109203X but was abolished by Y-27632, suggesting that ROCK but not PKC was involved. Furthermore, we have shown that the contractile agonists thrombin and U-46619 induced CPI-17 phosphorylation in VSMCs. Similarly to V14RhoA-induced CPI-17 phosphorylation, thrombin-induced CPI-17 phosphorylation was not affected by inhibition of PKC with GF109203X, but it was blocked by inhibition of RhoA with adenovirus-mediated expression of exoenzyme C3 as well as by Y-27632. Taken together, our present data provide the first clear evidence indicating that ROCK is responsible for thrombin- and U-46619-induced CPI-17 phosphorylation in primary cultured VSMCs.

Inhibition by Rho-kinase and protein kinase C of myosin phosphatase is involved in thrombin-induced shape change of megakaryocytic leukemia cell line UT-7/TPO

Thrombin induced a shape change of UT-7/TPO, a thrombopoietin-dependent human megakaryocytic cell line. Expression of myosin light chain (MLC) kinase was negligible in UT-7/TPO cells, while Rho-kinase and protein kinase C (PKC) were detected. Thrombin stimulated both monophosphorylation at Ser19 and diphosphorylation at Thr18 and Ser19 of 20 kDa MLC, as well as phosphorylation of myosin-binding subunit (MBS) and PKC-potentiated inhibitory phosphoprotein of myosin phosphatase (CPI). The Rho-kinase inhibitor Y-27632 [(+)-(R)-trans-(1-aminoethyl)-N-(4-phynidyl) cyclohexane-carboxamide dihydrochloride, monohydrade] strongly inhibited thrombin-induced shape change, MBS phosphorylation, and mono- and diphosphorylation of MLC. The PKC inhibitor GF109203X (2-[1-(3-dimethylaminopropyl)-1H-indol-3-yl]-3-(1H-indol-3-yl)-maleimide) partially inhibited thrombin-induced shape change and MLC diphosphorylation even at the concentration that completely inhibited thrombin-induced CPI phosphorylation. In shape-changed UT-7/TPO cells induced by thrombin, phosphorylated MBS and CPI were colocalized with diphosphorylated MLC at pseudopods, whereas monophosphorylated MLC was mainly located in the cortical region. The accumulation of diphosphorylated MLC was blocked by preincubation with either Y-27632 or GF109203X. These results suggest that Rho-kinase is responsible for the induction of MLC phosphorylation in thrombin-induced shape change of UT-7/TPO cells and that myosin phosphatase inactivation through Rho-kinase-MBS and PKC-CPI pathways could be necessary for enhancement of MLC diphosphorylation which promote the pseudopod formation.

Distinct roles of MLCK and ROCK in the regulation of membrane protrusions and focal adhesion dynamics during cell migration of fibroblasts

We examined the role of regulatory myosin light chain (MLC) phosphorylation of myosin II in cell migration of fibroblasts. Myosin light chain kinase (MLCK) inhibition blocked MLC phosphorylation at the cell periphery, but not in the center. MLCK-inhibited cells did not assemble zyxin-containing adhesions at the periphery, but maintained focal adhesions in the center. They generated membrane protrusions all around the cell, turned more frequently, and migrated less effectively. In contrast, Rho-associated kinase (ROCK) inhibition blocked MLC phosphorylation in the center, but not at the periphery. ROCK-inhibited cells assembled zyxin-containing adhesions at the periphery, but not focal adhesions in the center. They moved faster and more straight. On the other hand, inhibition of myosin phosphatase increased MLC phosphorylation and blocked peripheral membrane ruffling, as well as turnover of focal adhesions and cell migration. Our results suggest that myosin II activated by MLCK at the cell periphery controls membrane ruffling, and that the spatial regulation of MLC phosphorylation plays critical roles in controlling cell migration of fibroblasts.

Modulation of actomyosin contractility by myosin light chain phosphorylation/dephosphorylation through Rho GTPases signaling specifies axon formation in neurons

Actin depolymerization through Rho GTPases or exogenous mechanical tension has been suggested as a key determinant for the first step of neuronal polarization, the axonogenesis, in which one of the neurites starts to grow becoming the axon. The underlying mechanism and the relationship between two forces in the cells, however, are mostly unknown. Here, we report that the myosin-dependent contractility is a common effector between two forces and a critical determinant in axonogenesis and neuronal polarization. We have found that inhibition of myosin ATPase activity and modulation of myosin light chain phosphorylation/dephosphorylation through Rho GTPases signaling induced multiple axons. Moreover, overexpression of wild-type myosin light chain kinase dramatically increased filopodial structures and produced multi-axonal structures. Our results suggest that MLC phosphorylation/dephosphorylation through Rho GTPases signaling modulates the actomyosin contractility, and then in turn provides a physiological tension in neurons to induce axon.

Phosphoprotein inhibitor CPI-17 specificity depends on allosteric regulation of protein phosphatase-1 by regulatory subunits

Inhibition of myosin phosphatase is critical for agonist-induced contractility of vascular smooth muscle. The protein CPI-17 is a phosphorylation-dependent inhibitor of myosin phosphatase and, in response to agonists, Thr-38 is phosphorylated by protein kinase C, producing a >1,000-fold increase in inhibitory potency. Here, we addressed how CPI-17 could selectively inhibit myosin phosphatase among other protein phosphatase-1 (PP1) holoenzymes. PP1 in cell lysates was separated by sequential affinity chromatography into at least two fractions, one bound specifically to thiophospho-CPI-17, and another bound specifically to inhibitor-2. The MYPT1 regulatory subunit of myosin phosphatase was concentrated only in the fraction bound to thiophospho-CPI-17. This binding was eliminated by addition of excess microcystin-LR to the lysate, showing that binding at the active site of PP1 is required. Phospho-CPI-17 failed to inhibit glycogen-bound PP1 from skeletal muscle, composed primarily of PP1 with the striated muscle glycogen-targeting subunit (G(M)) regulatory subunit. Phospho-CPI-17 was dephosphorylated during assay of glycogen-bound PP1, not MYPT1-associated PP1, even though these two holoenzymes have the same PP1 catalytic subunit. Phosphorylation of CPI-17 in rabbit arteries was enhanced by calyculin A but not okadaic acid or fostriecin, consistent with PP1-mediated dephosphorylation. We propose that CPI-17 binds at the PP1 active site where it is dephosphorylated, but association of MYPT1 with PP1C allosterically retards this hydrolysis, resulting in formation of a complex of MYPT1.PP1C.P-CPI-17, leading to an increase in smooth muscle contraction.

CPI-17-deficient smooth muscle of chicken

Ca(2+) sensitivity of arterial contractility is governed by regulating myosin phosphatase activity in response to agonist stimuli. CPI-17, a myosin phosphatase inhibitor phosphoprotein, is phosphorylated concomitantly with agonist-induced contractile Ca(2+) sensitization in mammalian artery. CPI-17 has not been detected in chicken artery, but is readily detectable in pigeon artery. To evaluate a role of CPI-17, we compared contractility of the arteries of 'CPI-17-deficient' chicken with those of CPI-17-rich rabbit and pigeon, and studied the effect of CPI-17-reconstitution in chicken artery. Other major regulatory/contractile proteins for Ca(2+) sensitization are expressed in both chicken and rabbit arteries. Agonists, such as an alpha(1)-agonist and endothelin-1, produced significant contraction in arteries of all species under physiological Ca(2+)-containing conditions. Depletion of Ca(2+) abolished these contractions in chicken but partially inhibited them in rabbit and pigeon arteries. Unlike CPI-17-rich tissues, chicken arteries exerted little Ca(2+) sensitization in response to alpha(1)-agonist or endothelin-1. GTPgammaS produced a slight Ca(2+) sensitizing effect in chicken artery, but this was significantly smaller compared with CPI-17-rich tissues. A PKC activator (PDBu) did not generate but rather reduced a contraction in both intact and alpha-toxin-permeabilized chicken artery in contrast to a large contraction in CPI-17-rich arteries. Myosin light chain phosphorylation was reduced by PDBu in chicken but elevated in rabbit artery. Addition of recombinant CPI-17 into beta-escin-permeabilized chicken artery restored PDBu-induced and enhanced GTPgammaS-induced Ca(2+) sensitization. Thus, CPI-17 is essential for G protein/PKC-mediated Ca(2+) sensitization in smooth muscle.

Platelets play an essential role in the aetiology of cerebral vasospasm after subarachnoid haemorrhage

Platelets have long been implicated in the aetiology of cerebral vasospasm (CV) after subarachnoid haemorrhage (SAH). It was noticed that vasospastic CSF (CSF(V)) could be formed in vitro by the mixing of control blood (with platelets) and non-SAH CSF. We also propose a hypothesis for the aetiology of CV after SAH based on this and previous research. This study also aims to determine which blood fraction is responsible for the stimulation of O(2) consumption and vasospasm of blood vessels. Control blood was separated into various fractions and mixed with non-SAH CSF. The activity of the resulting mixture and the blood fraction alone were assessed. Only the fractions containing platelets mixed with CSF showed vasoactivity. These data suggest that platelets plus some component in the CSF produce vasoactive factors with actions similar to CSF(V). This study may help to elucidate the aetiology of CV after SAH.