The variable coupling between force and myosin light chain phosphorylation in Triton-skinned chicken gizzard fibre bundles: role of myosin light chain phosphatase

Vascular oxygen sensing: detection of novel candidates by proteomics and organ culture

We have shown that the specific inhibition of hypoxia-induced relaxation by organ culture in porcine coronary arteries can be mimicked by treatment of control vessels with the protein synthesis inhibitor, cycloheximide. We hypothesize that organ culture of vascular smooth muscle results in the decreased expression of proteins that are critical for vascular oxygen sensing. Using two-dimensional gel electrophoresis and mass spectroscopy, we identified such candidate proteins. The expressions of the smooth muscle-specific protein, SM22, and tropomyosin are decreased after 24 h in organ culture. These results were confirmed by Western blot analysis. Other smooth muscle proteins (actin and calponin) exhibited little change. We also demonstrate a 50% downregulation in the small G protein, Rho, a potent modulator of Ca2+-independent force. These results indicate that organ culture preferentially inhibits the expression of certain smooth muscle proteins. This change in protein expression after organ culture correlates with the specific inhibition of hypoxic vasorelaxation. These results provide novel target pathways for investigation that are potentially important for vascular oxygen sensing.

Invited review: regulation of myosin phosphorylation in smooth muscle

Phosphorylation of the regulatory light chains of myosin II (rMLC) by the Ca(2+)/calmodulin-dependent myosin light-chain kinase (MLCK) and dephosphorylation by a type 1 phosphatase (MLCP), which is targeted to myosin by a regulatory subunit (MYPT1), are the predominant mechanisms of regulation of smooth muscle tone. The activities of both enzymes are modulated by several protein kinases. MLCK is inhibited by the Ca(2+)/calmodulin-dependent protein kinase II, whereas the activity of MLCP is increased by cGMP and perhaps also cAMP-dependent protein kinases. In either case, this results in a decrease in the Ca(2+) sensitivity of rMLC phosphorylation and force production. The activity of MLCP is inhibited by Rho-associated kinase, one of the effectors of the monomeric GTPase Rho, and protein kinase C, leading to an increase in Ca(2+) sensitivity. Hence, smooth muscle tone appears to be regulated by a network of activating and inactivating intracellular signaling cascades.

The involvement of protein kinase C in myosin phosphorylation and force development in rat tail arterial smooth muscle

Myosin light-chain phosphorylation is the primary mechanism for activating smooth-muscle contraction and occurs principally at Ser-19 of the 20 kDa light chains of myosin (LC(20)). In some circumstances, Thr-18 phosphorylation may also occur. Protein kinase C (PKC) can regulate LC(20) phosphorylation indirectly via signalling pathways leading to inhibition of myosin light-chain phosphatase. The goal of this study was to determine the relative importance of myosin light-chain kinase (MLCK) and PKC in basal and stimulated LC(20) phosphorylation in rat tail arterial smooth-muscle strips (RTA). Two MLCK inhibitors (ML-9 and wortmannin) and two PKC inhibitors (chelerythrine and calphostin C) that have different mechanisms of action were used. Results showed the following: (i) basal LC(20) phosphorylation in intact RTA is mediated by MLCK; (ii) alpha(1)-adrenoceptor stimulation increases LC(20) phosphorylation via MLCK and PKC; (iii) Ca(2+)-induced LC(20) phosphorylation in Triton X-100-demembranated RTA is catalysed exclusively by MLCK, consistent with the quantitative loss of PKCs alpha and beta following detergent treatment; (iv) very little LC(20) diphosphorylation (i.e. Thr-18 phosphorylation) occurs in intact or demembranated RTA at rest or in response to contractile stimuli; and (v) the level of LC(20) phosphorylation correlates with contraction in intact and demembranated RTA, although the steady-state tension-LC(20) phosphorylation relationship is markedly different between the two preparations such that the basal level of LC(20) phosphorylation in intact muscles is sufficient to generate maximal force in demembranated preparations. This may be due, in part, to differences in the phosphatase/kinase activity ratio, resulting from disruption of a signalling pathway leading to myosin light-chain phosphatase inhibition following detergent treatment.

S-nitrosoglutathione-induced decrease in calcium sensitivity of airway smooth muscle

The purpose of this study was to examine whether the nitric oxide donor S-nitrosoglutathione (GSNO) relaxes canine tracheal smooth muscle (CTSM) strips by decreasing Ca(2+) sensitivity [i.e., the amount of force for a given intracellular Ca(2+) concentration ([Ca(2+)](i))]. We further investigated whether GSNO decreases Ca(2+) sensitivity by altering the relationship between regulatory myosin light chain (rMLC) phosphorylation and [Ca(2+)](i) and the relationship between force and rMLC phosphorylation. GSNO (100 microM) relaxed intact CTSM strips contracted with 45 mM KCl by decreasing Ca(2+) sensitivity in comparison to control strips without significantly decreasing [Ca(2+)](i). GSNO reduced the amount of rMLC phosphorylation for a given [Ca(2+)](i) but did not affect the relationship between isometric force and rMLC phosphorylation. These results show that in CTSM strips contracted with KCl, GSNO decreases Ca(2+) sensitivity by affecting the level of rMLC phosphorylation for a given [Ca(2+)](i), suggesting that myosin light chain kinase is inhibited or that smooth muscle protein phosphatases are activated by GSNO.

ATP hydrolysis during contraction of permeabilized airway smooth muscle

This study determined whether the time-dependent decline in the rate of ATP hydrolysis by actomyosin ATPase during sustained isometric force can occur in the absence of a time-dependent decline in regulatory myosin light chain (rMLC) phosphorylation in Triton X-100-permeabilized canine tracheal smooth muscle. Maximal activation with 10 microM Ca(2+) induced sustained increases in isometric force, stiffness, and rMLC phosphorylation; however, the increase in the ATP hydrolysis rate was initially high but then declined to a steady-state level above that of the unstimulated muscle (basal 31.8 +/- 5.8 nmol. cm(-3). s(-1); peak 81.4 +/- 11.3 nmol. cm(-3). s(-1); steady-state 62.2 +/- 9.1 nmol. cm(-3). s(-1)). Activation of strips in which the rMLC was irreversibly and maximally thiophosphorylated with adenosine 5'-O-(3-thiotriphosphate) also induced sustained increases in isometric force and stiffness but a nonsustained increase in ATP hydrolysis rate. There was no significant difference in the peak or steady-state isometric force, stiffness, or ATP hydrolysis rate or in the steady-state maximum unloaded shortening velocity between strips activated by 10 microM Ca(2+) or rMLC thiophosphorylation (0.058 +/- 0.016 and 0.047 +/- 0.011 muscle lengths/s, respectively). Mechanisms other than changes in rMLC phosphorylation contribute to the time-dependent decline in actomyosin ATPase activity during sustained activation of canine tracheal smooth muscle.

Ca2+-independent phosphorylation of myosin in rat caudal artery and chicken gizzard myofilaments

1. Smooth muscle contraction is activated primarily by the Ca2+-calmodulin (CaM)-dependent phosphorylation of the 20 kDa light chains (LC20) of myosin. Activation can also occur in some instances without a change in intracellular free [Ca2+] or indeed in a Ca2+-independent manner. These signalling pathways often involve inhibition of myosin light chain phosphatase and unmasking of basal kinase activity leading to LC20 phosphorylation and contraction. 2. We have used demembranated rat caudal arterial smooth muscle strips and isolated chicken gizzard myofilaments in conjunction with the phosphatase inhibitor microcystin-LR to investigate the mechanism of Ca2+-independent phosphorylation of LC20 and contraction. 3. Treatment of Triton X-100-demembranated rat caudal arterial smooth muscle strips with microcystin at pCa 9 triggered a concentration-dependent contraction that was slower than that induced by pCa 4.5 or 6 but reached comparable steady-state levels of tension. 4. This Ca2+-independent, microcystin-induced contraction correlated with phosphorylation of LC20 at serine-19 and threonine-18. 5. Whereas Ca2+-dependent LC20 phosphorylation and contraction were inhibited by a synthetic peptide (AV25) based on the autoinhibitory domain of myosin light chain kinase (MLCK), Ca2+-independent, microcystin-induced LC20 phosphorylation and contraction were resistant to AV25. 6. Ca2+-independent LC20 kinase activity was also detected in chicken gizzard smooth muscle myofilaments and catalysed phosphorylation of endogenous myosin LC20 at serine-19 and/or threonine-18. This is in contrast to MLCK which phosphorylates threonine-18 only after prior phosphorylation of serine-19. 7. Gizzard Ca2+-independent LC20 kinase could be separated from MLCK by differential extraction from myofilaments and by CaM affinity chromatography. Its activity was resistant to AV25. 8. We conclude that inhibition of smooth muscle myosin light chain phosphatase (MLCP) unmasks the activity of a Ca2+-independent LC20 kinase associated with the myofilaments and distinct from MLCK. This kinase, therefore, probably plays a role in Ca2+ sensitization and Ca2+-independent contraction of smooth muscle in response to stimuli that act via Ca2+-independent pathways, leading to inhibition of MLCP.

Cross-bridge cycling in smooth muscle: a short review

This review is focused on the cross-bridge interaction of the organized contractile system of smooth muscle fibres. By using chemically skinned preparations the different enzymatic reactions of actin-myosin interaction have been associated with mechanical events. A rigor state has been identified in smooth muscle and the binding of ATP causes dissociation of rigor cross-bridges at rates slightly slower than those in skeletal muscle, but fast enough not to be rate-limiting for cross-bridge turn over in the muscle fibre. The release of inorganic phosphate (Pi) is associated with force generation, and this process is not rate-limiting for maximal shortening velocity (Vmax) in the fully activated muscle. The binding of ADP to myosin is strong in the smooth muscle contractile system, a property that might be associated with the generally slow cross-bridge turn over. Both force and Vmax are modulated by the extent of myosin light chain phosphorylation. Low levels of activation are considered to be associated with the recruitment of slowly cycling dephosphorylated cross-bridges which reduces shortening velocity. The attachment of these cross-bridge states in skinned smooth muscles can be regulated by cooperative mechanisms and thin filament associated systems. Smooth muscles exhibit a large diversity in their Vmax and the individual smooth muscle tissue can alter its Vmax under physiological conditions. The diversity and the long-term modulation of phenotype are associated with changes in myosin heavy and light chain isoform expression.

Control of cross-bridge cycling by myosin light chain phosphorylation in mammalian smooth muscle

This review focuses on experiments in which the single turnover of myosin-bound ADP is used to characterize the regulation of the cross-bridge cycle by myosin light chain phosphorylation in mammalian smooth muscle. Under isometric conditions, at rest, when the myosin light chain is not phosphorylated, myosin cycles very slowly (about 0.004 s-1), while phosphorylation of the light chain results in a 50-fold increase in cycling rate of 0.2 s-1. Experiments consistently show that some myosin does not increase its cycling rate although its light chain is phosphorylated. Studies at low levels of myosin light chain phosphorylation show that phosphorylation also induces an increase in the cycling rate of unphosphorylated myosin. The fast cycling phosphorylated myosin is the main determinant of suprabasal myosin ATPase activity, while the cycling rate of cooperatively activated unphosphorylated myosin is slow and appears to depend on the extent of phosphorylation of the entire thick filament. Single turnover experiments measuring the rate of phosphorylation and dephosphorylation of myosin light chain show that the turnover of light chain phosphate can be very rapid (0.3-0.4 s-1) at suprabasal calcium concentrations. The expected effect of such a rapid turnover of light chain phosphorylation on the turnover of myosin-bound ADP is not observed. The effects of low levels of myosin light chain phosphorylation on the single turnover of myosin suggest that the same small pool of myosin remains phosphorylated for relatively long periods of time rather than the entire pool of myosin spending a small fraction of its cycle time in the phosphorylated state.

Involvement of small GTPases in the regulation of smooth muscle contraction

Neurohumoral stimulation of smooth muscle leads to an increased responsiveness of the myofilaments to Ca2+. This review provides a summary of the data that suggest that the signalling from the membrane-bound serpentine receptors to the contractile apparatus leading to the increase in Ca(2+)-sensitivity requires the activation of the Ras-related low molecular mass GTPase Rho. In smooth muscle permeabilized with alpha-toxin or beta-escin, the increase in force elicited by different agonists at fixed [Ca2+] (Ca(2+)-sensitization) can be inhibited by bacterial toxins (EDIN, and exoenzyme C3) which ADP-ribosylate and inactivate Rho proteins. Moreover, the agonist-induced increase in Ca(2+)-sensitivity can be mimicked by constitutively active recombinant Rho proteins. The physiological relevance of this mechanism is suggested by the fact that toxins that are internalized into intact cells (toxin B from C. difficile and a chimeric toxin (DC3B) consisting of C3 and the (non-catalytic) B fragment of diphteria toxin (inhibit the tonic phase of an agonist-induced contraction. Toxin B inhibits contraction without affecting the intracellular Ca(2+)-transient determined with fura-2. However, it inhibits phosphorylation of the regulatory light chains of myosin (MLC). Rho has been suggested to activate a Rho-associated kinase which in turn phosphorylates the myosin binding subunit of the myosin light chain phosphatase. This would lead to an increase in phosphorylation of MLC and hence of force at constant Ca2+. The Ca(2+)-sensitizing effect of agonists is also inhibited by tyrosine kinase inhibitors. This suggests the possibility that in smooth muscle, like in non-muscle cells, there is a cross-talk between Rho and tyrosine kinases.

Acceleration of myosin light chain dephosphorylation and relaxation of smooth muscle by telokin. Synergism with cyclic nucleotide-activated kinase

Incorporation of 32P into telokin, a smooth muscle-specific, 17-18-kDa, acidic (pI 4.2-4.4) protein, was increased by forskolin (20 microM) in intact rabbit ileum smooth muscle (ileum) and by 8-bromo-cyclic GMP (100 microM) in alpha-toxin-permeabilized ileum. Native telokin (5-20 microM), purified from turkey gizzard, and recombinant rabbit telokin, expressed in Escherichia coli and purified to >90% purity, induced dose-dependent relaxation, associated with a significant decrease in regulatory myosin light chain phosphorylation, without affecting the rate of thiophosphorylation of regulatory myosin light chain of ileum permeabilized with 0.1% Triton X-100. Endogenous telokin was lost from ileum during prolonged permeabilization (>20 min) with 0.1% Triton X-100, and the time course of loss was correlated with the loss of 8-bromo-cyclic GMP-induced calcium desensitization. Recombinant and native gizzard telokins were phosphorylated, in vitro, by the catalytic subunit of cAMP-dependent protein kinase, cGMP-dependent protein kinase, and p42/44 mitogen-activated protein kinase; the recombinant protein was also phosphorylated by calmodulin-dependent protein kinase II. Exogenous cGMP-dependent protein kinase (0.5 microM) activated by 8-bromo-cyclic GMP (50 microM) phosphorylated recombinant telokin (10 microM) when added concurrently to ileum depleted of its endogenous telokin, and their relaxant effects were mutually potentiated. Forskolin (20 microM) also increased phosphorylation of telokin in intact ileum. We conclude that telokin induces calcium desensitization in smooth muscle by enhancing myosin light chain phosphatase activity, and cGMP- and/or cAMP-dependent phosphorylation of telokin up-regulates its relaxant effect.

Clostridium difficile toxin B inhibits carbachol-induced force and myosin light chain phosphorylation in guinea-pig smooth muscle: role of Rho proteins

1. Clostridium difficile toxin B glucosylates the Ras-related low molecular mass GTPases of the Rho subfamily thereby inactivating them. In the present report, toxin B was applied as a tool to test whether Rho proteins participate in the carbachol-induced increase in the Ca2+ sensitivity of force and myosin light chain (MLC) phosphorylation in intact intestinal smooth muscle. 2. Small strips of the longitudinal muscle of guinea-pig small intestine were incubated in toxin B (40 ng ml-1) overnight. Carbachol-induced force and intracellular [Ca2+], and, in a separate series, force and MLC phosphorylation, were determined. 3. Carbachol induced a biphasic contraction: an initial rapid increase in force (peak 1) followed by a partial relaxation and a second delayed increase in force (peak 2). The peak of the Ca2+ signal measured with fura-2 preceded peak 1 of force and then declined to a lower suprabasal steady-state level. Peak 2 was not associated with a significant increase in [Ca2+]. Toxin B nearly completely inhibited peak 2 while peak 1 was not significantly inhibited. Toxin B had no effect on the Ca2+ transient. 4. In control strips, MLC phosphorylation at peak 2 was 27.7% which was significantly higher than the resting value (18.6%). The inhibition of the second, delayed, rise in force induced by toxin B was associated with complete inhibition of the increase in MLC phosphorylation. The resting MLC phosphorylation was not significantly different from that of the control strips. 5. The initial increase in MLC phosphorylation determined 3 s after exposure to carbachol was 54% in the control strips. Toxin B also inhibited this initial phosphorylation peak despite the fact that the Ca2+ transient and the initial increase in force were not inhibited by toxin B. This suggests that Rho proteins play an important role in setting the balance between MLC phosphorylation and dephosphorylation reactions even at high levels of intracellular Ca2+. 6. These findings are consistent with the hypothesis that the delayed rise in force elicited by carbachol is due to an increase in the Ca2+ sensitivity of MLC phosphorylation mediated by Rho proteins.

Energetic cost of activation processes during contraction of swine arterial smooth muscle

1. The objective of this study was to partition the increase in ATP consumption during contraction of swine carotid arterial smooth muscle estimated from suprabasal oxygen consumption (suprabasal JO2) and lactate release (Jlactate) into a component associated with cross-bridge cycling (JX) and one reflecting activation (JA). 2. Two experimental approaches-varying length under constant activation, and varying activation at a long length (1.8 times the optimal length for force development (Lo)) where force generation is minimal-revealed a linear dependence of JO2 and activation energy (JA) on cross-bridge phosphorylation. Protocols inducing a large increase in myosin regulatory light chain (MRLC) phosphorylation at 1.8 Lo resulted in significant elevations of JO2 and marked reductions in the economy of force maintenance. Our evidence suggests that this is primarily due to the increased cost of cross-bridge phosphorylation. 3. The extrapolated estimate of JA during maximal K(+)-induced depolarization made by varying length was 16%, while at 1.8 Lo it was 33% of the suprabasal JO2 at Lo. Calculated activation energies ranged from 17 to 45% of the suprabasal JO2 at Lo and from 72 to 87% of the suprabasal JO2 at 1.8 Lo under stimulation conditions that varied steady-state MRLC phosphorylation from 15 to 50%. 4. The results suggest that the kinetics of cross-bridge phosphorylation-dephosphorylation can rival those of cross-bridge cycling during isometric contractions in swine arterial smooth muscle.

Myosin isoforms and functional diversity in vertebrate smooth muscle

The expression of fast and slow myosin isoforms in individual cells is associated with differences in shortening velocities and power output in fully differentiated vertebrate striated muscle. This paradigm in which shortening velocity is determined by the myosin isoform (and load) is inappropriate for smooth muscle. Smooth muscle tissues express multiple myosin heavy and light chain isoforms, and it is not currently possible to separate and identify chemically distinct native myosin hexamers (i.e., isoforms). It is not known if different isoforms are localized in subpopulations of cells or in specific cellular domains nor whether they combine preferentially to form a small number of native myosin hexamer isoforms. Potentially, thick filaments are aggregates of many different combinations of heavy and light chain isoforms that may or may not exhibit different kinetics. Shortening velocities in smooth muscle are regulated by Ca(2+)-dependent crossbridge phosphorylation of the myosin regulatory light chains. Much of the observed diversity in power output in smooth muscle may be attributed to regulatory mechanisms modulating crossbridge cycling rates rather than contractile protein isoform expression.