Slow cycling of unphosphorylated myosin is inhibited by calponin, thus keeping smooth muscle relaxed

Mechanics of Vascular Smooth Muscle

Vascular smooth muscle (VSM; see Table 1 for a list of abbreviations) is a heterogeneous biomaterial comprised of cells and extracellular matrix. By surrounding tubes of endothelial cells, VSM forms a regulated network, the vasculature, through which oxygenated blood supplies specialized organs, permitting the development of large multicellular organisms. VSM cells, the engine of the vasculature, house a set of regulated nanomotors that permit rapid stress-development, sustained stress-maintenance and vessel constriction. Viscoelastic materials within, surrounding and attached to VSM cells, comprised largely of polymeric proteins with complex mechanical characteristics, assist the engine with countering loads imposed by the heart pump, and with control of relengthening after constriction. The complexity of this smart material can be reduced by classical mechanical studies combined with circuit modeling using spring and dashpot elements. Evaluation of the mechanical characteristics of VSM requires a more complete understanding of the mechanics and regulation of its biochemical parts, and ultimately, an understanding of how these parts work together to form the machinery of the vascular tree. Current molecular studies provide detailed mechanical data about single polymeric molecules, revealing viscoelasticity and plasticity at the protein domain level, the unique biological slip-catch bond, and a regulated two-step actomyosin power stroke. At the tissue level, new insight into acutely dynamic stress-strain behavior reveals smooth muscle to exhibit adaptive plasticity. At its core, physiology aims to describe the complex interactions of molecular systems, clarifying structure-function relationships and regulation of biological machines. The intent of this review is to provide a comprehensive presentation of one biomachine, VSM.

UNC-87 isoforms, Caenorhabditis elegans calponin-related proteins, interact with both actin and myosin and regulate actomyosin contractility

Two UNC-87 isoforms with seven calponin-like repeats are expressed widely in muscle and nonmuscle cells in Caenorhabditis elegans. They bind to actin and myosin and inhibit actomyosin motility in vitro. unc-87 mutation enhances contraction of nonstriated muscle in vivo, suggesting that UNC-87 isoforms are negative regulators of actomyosin contractility.

Calponin-related proteins are widely distributed among eukaryotes and involved in signaling and cytoskeletal regulation. Calponin-like (CLIK) repeat is an actin-binding motif found in the C-termini of vertebrate calponins. Although CLIK repeats stabilize actin filaments, other functions of these actin-binding motifs are unknown. The Caenorhabditis elegans unc-87 gene encodes actin-binding proteins with seven CLIK repeats. UNC-87 stabilizes actin filaments and is essential for maintenance of sarcomeric actin filaments in striated muscle. Here we show that two UNC-87 isoforms, UNC-87A and UNC-87B, are expressed in muscle and nonmuscle cells in a tissue-specific manner by two independent promoters and exhibit quantitatively different effects on both actin and myosin. Both UNC-87A and UNC-87B have seven CLIK repeats, but UNC-87A has an extra N-terminal extension of ∼190 amino acids. Both UNC-87 isoforms bind to actin filaments and myosin to induce ATP-resistant actomyosin bundles and inhibit actomyosin motility. UNC-87A with an N-terminal extension binds to actin and myosin more strongly than UNC-87B. UNC-87B is associated with actin filaments in nonstriated muscle in the somatic gonad, and an unc-87 mutation causes its excessive contraction, which is dependent on myosin. These results strongly suggest that proteins with CLIK repeats function as a negative regulator of actomyosin contractility.

Cytoskeletal reorganization evoked by Rho-associated kinase- and protein kinase C-catalyzed phosphorylation of cofilin and heat shock protein 27, respectively, contributes to myogenic constriction of rat cerebral arteries

Our understanding of the molecular events contributing to myogenic control of diameter in cerebral resistance arteries in response to changes in intravascular pressure, a fundamental mechanism regulating blood flow to the brain, is incomplete. Myosin light chain kinase and phosphatase activities are known to be increased and decreased, respectively, to augment phosphorylation of the 20-kDa regulatory light chain subunits (LC20) of myosin II, which permits cross-bridge cycling and force development. Here, we assessed the contribution of dynamic reorganization of the actin cytoskeleton and thin filament regulation to the myogenic response and serotonin-evoked constriction of pressurized rat middle cerebral arteries. Arterial diameter and the levels of phosphorylated LC(20), calponin, caldesmon, cofilin, and HSP27, as well as G-actin content, were determined. A decline in G-actin content was observed following pressurization from 10 mm Hg to between 40 and 120 mm Hg and in three conditions in which myogenic or agonist-evoked constriction occurred in the absence of a detectable change in LC20 phosphorylation. No changes in thin filament protein phosphorylation were evident. Pressurization reduced G-actin content and elevated the levels of cofilin and HSP27 phosphorylation. Inhibitors of Rho-associated kinase and PKC prevented the decline in G-actin; reduced cofilin and HSP27 phosphoprotein content, respectively; and blocked the myogenic response. Furthermore, phosphorylation modulators of HSP27 and cofilin induced significant changes in arterial diameter and G-actin content of myogenically active arteries. Taken together, our findings suggest that dynamic reorganization of the cytoskeleton involving increased actin polymerization in response to Rho-associated kinase and PKC signaling contributes significantly to force generation in myogenic constriction of cerebral resistance arteries.

Altered vascular smooth muscle cell differentiation in the endometrial vasculature in menorrhagia

STUDY QUESTION

How does the smooth muscle content and differentiation stage of vascular smooth muscle cells (VSMCs) in endometrial blood vessels change according to the different phases of the menstrual cycle and is this altered in women with menorrhagia?

SUMMARY ANSWER

The smooth muscle content (as a proportion of the vascular cross-sectional area) of endometrial blood vessels remained unchanged during the normal menstrual cycle and in menorrhagia; however, expression of the VSMC differentiation markers, smoothelin and calponin, was dysregulated in endometrial blood vessels in samples from women with menorrhagia compared with controls.

WHAT IS KNOWN ALREADY

Menorrhagia affects 30% of women of reproductive age and is the leading indication for hysterectomy. Previous studies have suggested important structural and functional roles for endometrial blood vessels, including impaired vascular contractility. Differentiation of VSMC from a synthetic to contractile state is associated with altered cellular phenotype that contributes to normal blood flow and pressure. This vascular maturation process has been little studied in endometrium both across the normal menstrual cycle and in menorrhagia.

STUDY DESIGN, SIZE, DURATION

Endometrial biopsies were taken from hysterectomy specimens or by pipelle biopsy prior to hysterectomy in controls without endometrial pathology and in women with menorrhagia (n = 7 for each of proliferative, early-secretory, mid-secretory and late-secretory phases for both groups). Biopsies were formalin fixed and embedded in paraffin wax.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Paraffin-embedded sections were immunostained for α smooth muscle actin (αSMA), myosin heavy chain (MyHC), H-caldesmon, desmin, smoothelin and calponin (h1 or basic). VSMC content was measured in 25 αSMA(+) vascular cross sections per sample and expressed as a ratio of the muscular area:gross vascular cross-sectional area. VSMC differentiation was analysed by the presence/absence of differentiation markers compared with αSMA expression. Smoothelin and calponin expression was also analysed in relation to total number of blood vessels by double immunostaining for endothelial cell markers.

MAIN RESULTS AND THE ROLE OF CHANCE

Study of VSMC differentiation markers revealed decreased expression of calponin both in αSMA(+) vessels (P = 0.008) and in relation to total number of vessels (P = 0.001) in late secretory phase endometrium in menorrhagia compared with controls. Smoothelin expression in αSMA(+) vessels was increased (P = 0.03) in menorrhagia, although this was not significant in relation to the total number of vessels. In normal endometrium, the proportion of blood vessels expressing αSMA increased from 63% in proliferative endometrium to 81% in the late secretory phase (P = 0.002). The overall arterial muscle content did not differ between control and menorrhagia at any phase of the menstrual cycle, occupying 78-81% of gross vascular cross-sectional area during the different menstrual cycle phases.

LIMITATIONS, REASONS FOR CAUTION

This study included both straight and spiral arterioles and analysed only stratum functionalis. The VSMC differentiation with respect to αSMA expression is an observational study and the data are presented as presence or absence of the differentiation markers in each field of view, corresponding with the vascular cross sections included in the study of vascular muscle content.

WIDER IMPLICATIONS OF THE FINDINGS

Smoothelin and calponin have been widely implicated as important regulators of vascular tone, vascular contractility and rate of blood flow. Our results have uncovered a disparate pattern of calponin expression, potentially indicating a dysfunctional contraction mechanism in the endometrial blood vessels in menorrhagia, thus implicating calponin as a potential therapeutic target.

STUDY FUNDING/COMPETING INTERESTS

This study was funded by Wellbeing of Women (RG1342) and Newcastle University. There are no competing interests to declare.

TRIAL REGISTRATION NUMBER

Not applicable.

Unphosphorylated calponin enhances the binding force of unphosphorylated myosin to actin

BACKGROUND

Smooth muscle has the distinctive ability to maintain force for long periods of time and at low energy costs. While it is generally agreed that this property, called the latch-state, is due to the dephosphorylation of myosin while attached to actin, dephosphorylated-detached myosin can also attach to actin and may contribute to force maintenance. Thus, we investigated the role of calponin in regulating and enhancing the binding force of unphosphorylated tonic muscle myosin to actin.

METHODS

To measure the effect of calponin on the binding of unphosphorylated myosin to actin, we used the laser trap assay to quantify the average force of unbinding (Funb) in the absence and presence of calponin or phosphorylated calponin.

RESULTS

Funb from F-actin alone (0.12±0.01pN; mean±SE) was significantly increased in the presence of calponin (0.20±0.02pN). This enhancement was lost when calponin was phosphorylated (0.12±0.01pN). To further verify that this enhancement of Funb was due to the cross-linking of actin to myosin by calponin, we repeated the measurements at high ionic strength. Indeed, the Funb obtained at a [KCl] of 25mM (0.21±0.02pN; mean±SE) was significantly decreased at a [KCl] of 150mM, (0.13±0.01pN).

CONCLUSIONS

This study provides direct molecular level-evidence that calponin enhances the binding force of unphosphorylated myosin to actin by cross-linking them and that this is reversed upon calponin phosphorylation. Thus, calponin might play an important role in the latch-state.

GENERAL SIGNIFICANCE

This study suggests a new mechanism that likely contributes to the latch-state, a fundamental and important property of smooth muscle that remains unresolved.

Molluscan smooth catch muscle contains calponin but not caldesmon

We isolated Ca(2+)-regulated thin filaments from the smooth muscle of the mussel Crenomytilus grayanus and studied the protein composition of different preparations from this muscle: whole muscle, heat-stable extract, fractions from heat-stable extract, thin filaments and intermediate stages of thin filaments purification. Among the protein components of the above-listed preparations, we did not find caldesmon (CaD), although two isoforms of a calponin-like (CaP-like) protein, which along with CaD is characteristic of vertebrate smooth muscle, were present in thin filaments. Thus, CaD is not Ca(2+)-regulator of thin filaments of this muscle. On the other hand, the mussel CaP-like protein is also not such Ca(2+)-regulator since we have shown that this protein can be selectively removed from isolated mussel thin filaments without loss of their Ca(2+)-sensitivity. We suggest that thin filaments in the smooth catch muscle possess other type of Ca(2+)-regulation, different from that in vertebrate smooth muscles.

New insights into myosin phosphorylation during cyclic nucleotide-mediated smooth muscle relaxation

Nitrovasodilators and agonists, via an increase in intracellular cyclic nucleotide levels, can induce smooth muscle relaxation without a concomitant decrease in phosphorylation of the regulatory light chains (RLC) of myosin. However, since cyclic nucleotide-induced relaxation is associated with a decrease in intracellular [Ca²⁺], and hence, a decreased activity of MLCK, we tested the hypothesis that the site responsible for the elevated RLC phosphorylation is not Ser19. Smooth muscle strips from gastric fundus were isometrically contracted with ET-1 which induced an increase in monophosphorylation from 9 ± 1 % under resting conditions (PSS) to 36 ± 1 % determined with 2D-PAGE. Electric field stimulation induced a rapid, largely NO-mediated relaxation with a half time of 8 s, which was associated with an initial decline in RLC phosphorylation to 18 % within 2 s and a rebound to 34 % after 30 s whereas relaxation was sustained. In contrast, phosphorylation of RLC at Ser19 probed with phosphospecific antibodies declined in parallel with force. LC/MS and western blot analysis with phosphospecific antibodies against monophosphorylated Thr18 indicate that Thr18 is significantly monophosphorylated during sustained relaxation. We therefore suggest that (i) monophosphorylation of Thr18 rather than Ser19 is responsible for the phosphorylation rebound during sustained EFS-induced relaxation of mouse gastric fundus, and (ii) that relaxation can be ascribed to dephosphorylation of Ser19, the site considered to be responsible for regulation of smooth muscle tone.

The effect of L-arginine on bladder dysfunction following ovariectomy in a rabbit model

INTRODUCTION AND HYPOTHESIS

The present study was designed to investigate the effect of nitric oxide precursor, L: -arginine, on bladder function following ovariectomy.

METHODS

Twenty-eight New Zealand white female rabbits were separated into seven groups. Groups 1 to 6 underwent ovariectomy surgery. Among them, groups 1 and 2 received ovariectomy without treating with L-arginine. Groups 3, 4, 5, and 6 were given high L-arginine diet and were sacrificed 1, 3, 7, and 14 days after ovariectomy, respectively. Group 7 served as the control group. The effects of L: -arginine on the contractility of bladder tissues were determined in response to various stimulations. In addition, L-arginine effects on the expression of Rho kinase (ROK), protein kinase C potentiated inhibitor (CPI-17), caldesmon (CaD), and calponin (CaP) were studied by immunoblotting.

RESULTS

Ovariectomy significantly decreases contractile response to all forms of stimulation. Feeding rabbits L: -arginine significantly increases contractile response at 1 day following ovariectomy, but the response decreases to the control level by 14 days. Ovariectomy increases the expressions of both isoforms of CaD, CaP, and CPI-17; L-arginine treatment induces ROK underexpression, while CaP is overexpressed in the early few days of ovariectomy but returns to the control level at 2 weeks after ovariectomy.

CONCLUSIONS

Ovariectomy appreciably reduced bladder contractility. Treatment with L-arginine reversed the ovariectomy-induced bladder dysfunction. Decreased bladder contractile response was observed in the early days following ovariectomy.

Differential proteomic responses in hepatopancreas and adductor muscles of the green-lipped mussel Perna viridis to stresses induced by cadmium and hydrogen peroxide

This study aimed to reveal the proteomic responses in the hepatopancreas and adductor muscle of a common biomonitor, Perna viridis after 14-day exposure to two model chemicals, cadmium (Cd; a toxic metal) and hydrogen peroxide (H(2)O(2); a pro-oxidant), using two-dimensional gel electrophoresis coupled with multivariate statistical analyses. Unique sets of tissue-specific protein expression signatures were revealed corresponding to the two treatment groups. In the hepatopancreas, 15 and 2 spots responded to Cd and H(2)O(2) treatments respectively. 6 and 7 spots were differentially expressed in the adductor muscle for Cd and H(2)O(2) treatments, respectively. 15 differentially expressed spots were successfully identified by MALDI-TOF/TOF MS analysis. These proteins are involved in glycolysis, amino acid metabolism, energy homeostasis, oxidative stress response, redox homeostasis and protein folding, heat-shock response, and muscle contraction modulation. This is the first time, to have demonstrated that Cd exposure not only leads to substantial oxidative stress but also results in endoplasmic reticulum stress in hepatopancreas of the mussel. Such notable stress responses may be attributable to high Cd accumulation in this tissue. Our results suggested that investigations on these stress-associated protein changes could be used as a new and complementary approach in pollution monitoring by this popular biomonitor species.

Inducible knockout mutagenesis reveals compensatory mechanisms elicited by constitutive BK channel deficiency in overactive murine bladder

The large-conductance, voltage-dependent and Ca(2+)-dependent K(+) (BK) channel links membrane depolarization and local increases in cytosolic free Ca(2+) to hyperpolarizing K(+) outward currents, thereby controlling smooth muscle contractility. Constitutive deletion of the BK channel in mice (BK(-/-)) leads to an overactive bladder associated with increased intravesical pressure and frequent micturition, which has been revealed to be a result of detrusor muscle hyperexcitability. Interestingly, time-dependent and smooth muscle-specific deletion of the BK channel (SM-BK(-/-)) caused a more severe phenotype than displayed by constitutive BK(-/-) mice, suggesting that compensatory pathways are active in the latter. In detrusor muscle of BK(-/-) but not SM-BK(-/-) mice, we found reduced L-type Ca(2+) current density and increased expression of cAMP kinase (protein kinase A; PKA), as compared with control mice. Increased expression of PKA in BK(-/-) mice was accompanied by enhanced beta-adrenoceptor/cAMP-mediated suppression of contractions by isoproterenol. This effect was attenuated by about 60-70% in SM-BK(-/-) mice. However, the Rp isomer of adenosine-3',5'-cyclic monophosphorothioate, a blocker of PKA, only partially inhibited enhanced cAMP signaling in BK(-/-) detrusor muscle, suggesting the existence of additional compensatory pathways. To this end, proteome analysis of BK(-/-) urinary bladder tissue was performed, and revealed additional compensatory regulated proteins. Thus, constitutive and inducible deletion of BK channel activity unmasks compensatory mechanisms that are relevant for urinary bladder relaxation.

Potent inhibition of arterial smooth muscle tonic contractions by the selective myosin II inhibitor, blebbistatin

Blebbistatin is reported to be a selective and specific small molecule inhibitor of the myosin II isoforms expressed by striated muscles and nonmuscle (IC(50) = 0.5-5 microM) but is a poor inhibitor of purified turkey smooth muscle myosin II (IC(50) approximately 80 microM). We found that blebbistatin potently (IC(50) approximately 3 microM) inhibited the actomyosin ATPase activities of expressed "slow" [smooth muscle myosin IIA (SMA)] and "fast" [smooth muscle myosin IIB (SMB)] smooth muscle myosin II heavy-chain isoforms. Blebbistatin also inhibited the KCl-induced tonic contractions produced by rabbit femoral and renal arteries that express primarily SMA and the weaker tonic contraction produced by the saphenous artery that expresses primarily SMB, with an equivalent potency comparable with that identified for nonmuscle myosin IIA (IC(50) approximately 5 microM). In femoral and saphenous arteries, blebbistatin had no effect on unloaded shortening velocity or the tonic increase in myosin light-chain phosphorylation produced by KCl but potently inhibited beta-escin permeabilized artery contracted with calcium at pCa 5, suggesting that cell signaling events upstream from KCl-induced activation of cross-bridges were unaffected by blebbistatin. It is noteworthy that KCl-induced contractions of chicken gizzard were less potently inhibited (IC(50) approximately 20 microM). Adult femoral, renal, and saphenous arteries did not express significant levels of nonmuscle myosin. These data together indicate that blebbistatin is a potent inhibitor of smooth muscle myosin II, supporting the hypothesis that the force-bearing structure responsible for tonic force maintenance in adult mammalian vascular smooth muscle is the cross-bridge formed from the blebbistatin-dependent interaction between actin and smooth muscle myosin II.

Ontogenesis of myosin light chain phosphorylation in guinea pig tracheal smooth muscle

Increased airway responsiveness occurs in normal young individuals compared to adults. A maturation of airway smooth muscle (ASM) contractility is likely a mechanism of this juvenile airway hyperresponsiveness. Indeed, we showed in guinea pig tracheal smooth muscle (TSM) that maximum shortening velocity decreases dramatically after the first 3 weeks of life. Because the phosphorylation of the 20-kDa myosin light chain (MLC(20)) was shown to be a key event in ASM contractility, in the present work we sought to investigate it during ontogenesis. In three age groups (1-week-old, 3-week-old, and adult guinea pigs), we assessed the amount of MLC(20) phosphorylation achieved either in TSM crude protein homogenates exposed to Mg(2+) . ATP . CaCl(2) or in tracheal strips during electrical field stimulation (EFS). Phosphorylated and unphosphorylated MLC(20) were separated on nondenaturing 10% polyacrylamide gels, and the ratio of phosphorylation was obtained by densitometric analysis of chemiluminescent Western immunoblots. Maximum MLC(20) phosphorylation (% of total MLC(20)) in TSM tissue homogenate was, respectively, 32.6 +/- 5.7, 32.2 +/- 5.7, and 46.8 +/- 5.8 in 1-week, 3-week, and adult guinea pigs. Interestingly, in nonstimulated intact tracheal strips, we found a substantial degree of MLC(20) phosphorylation: respectively, 42.2 +/- 5.8, 36.5 +/- 7.8, and 46.4 +/- 4.7 in 1-week, 3-week, and adult guinea pigs. Maximal EFS-induced MLC(20) phosphorylation (% increase over baseline) in the 3-week age group was attained after 3 sec of EFS, and was 161.2 +/- 17.6, while in 1-week and adult guinea pigs, it was attained at 1.5 sec of EFS and was, respectively, 133.3 +/- 9.3 and 110.2 +/- 3.9 (P < 0.05). We conclude that MLC(20) phosphorylation in guinea pig intact tracheal strips correlates with ontogenetic changes in shortening velocity and changes in myosin light chain kinase content. These results further suggest that the maturation of ASM contractile properties plays a role in the greater airway responsiveness reported in children and young animals.

Ontogenesis of myosin light chain kinase mRNA and protein content in guinea pig tracheal smooth muscle

We previously reported in guinea pig tracheal smooth muscle that maximal shortening velocity decreases from 3 weeks of age to adulthood. It is not known whether myosin light chain kinase (MLCK), a key enzyme determining the velocity of smooth muscle contraction, undergoes maturational changes. In the present work, we investigated MLCK protein content and mRNA expression in 1-week-old, 3-week-old, and adult guinea pigs. We extracted either proteins or RNA from isolated tracheal smooth muscle. The content of MLCK was assessed by Western immunoblots. MLCK mRNA was evaluated by Northern analysis and by quantitative real time reverse transcriptase-polymerase chain reaction (RT-PCR). The content of MLCK increased 3-fold at 3 weeks of age and then decreased in adults, being 0.116 +/- 0.042, 0.330 +/- 0.125 (P < 0.05), and 0.153 +/- 0.054 microg/mg of total protein, respectively, in 1-week, 3-week, and adult animals. Quantitative RT-PCR revealed that MLCK mRNA increased with age to 135 +/- 35% and 177 +/- 23% (P < 0.01) in 3-week and adult animals, respectively, compared to 1-week animals. The transient increase of MLCK content in juvenile guinea pig tracheal smooth muscle may contribute to the increased shortening velocity at this age. We suggest that this increased content of MLCK is one of the mechanisms leading to maturation of airway smooth muscle contractility, which in turn contributes to the airway hyperresponsiveness reported in children and young animals.

Essential role of rho kinase in the Ca2+ sensitization of prostaglandin F(2alpha)-induced contraction of rabbit aortae

Inhibition of dephosphorylation of the 20 kDa myosin light chain (MLC(20)) is an important mechanism for the Ca(2+)-induced sensitization of vascular smooth muscle contraction. We investigated whether this mechanism operates in prostaglandin F(2alpha) (PGF(2alpha))-induced contraction of rabbit aortic smooth muscle and, if so, whether protein kinase C (PKC) or rho-associated kinase (rho kinase) contribute to the inhibition of dephosphorylation. In normal medium, PGF(2alpha) (10 microM) increased the phosphorylation of MLC(20) and developed tension. The rho-kinase inhibitors fasudil and hydroxyfasudil inhibited these changes, despite having no effect on a phorbol-ester-induced MLC(20) phosphorylation. After treatment with verapamil or chelation of external Ca(2+) with EGTA, PGF(2alpha) increased the MLC(20) phosphorylation and tension without an increase in [Ca(2+)](i), all of which were sensitive to fasudil and hydroxyfasudil. ML-9, a MLC kinase inhibitor, quickly reversed the KCl-induced MLC(20) phosphorylation and contraction to the resting level. However, fractions of PGF(2alpha)-induced contraction and MLC(20) phosphorylation were resistant to ML-9 but were sensitive to fasudil. Ro31-8220 (10 microM), a PKC inhibitor, did not affect the phosphorylation of MLC(20) and the tension caused by PGF(2alpha), thus excluding the possibility of the involvement of PKC in the PGF(2alpha)-induced MLC(20) phosphorylation. PGF(2alpha) increased phosphorylation at Thr654 of the myosin binding subunit (MBS) of myosin phosphatase, which is a target of rho kinase, and fasudil decreased the phosphorylation. These data suggest that the PGF(2alpha)-induced contraction is accompanied by the inhibition of MLC(20) dephosphorylation through rho kinase-induced MBS phosphorylation, leading to Ca(2+) sensitization of contraction. An actin-associated mechanism may also be involved in the PGF(2alpha)-induced sensitization.

Regulatory and essential light chains of myosin rotate equally during contraction of skeletal muscle

Myosin head consists of a globular catalytic domain and a long alpha-helical regulatory domain. The catalytic domain is responsible for binding to actin and for setting the stage for the main force-generating event, which is a "swing" of the regulatory domain. The proximal end of the regulatory domain contains the essential light chain 1 (LC1). This light chain can interact through the N and C termini with actin and myosin heavy chain. The interactions may inhibit the motion of the proximal end. In consequence the motion of the distal end (containing regulatory light chain, RLC) may be different from the motion of the proximal end. To test this possibility, the angular motion of LC1 and RLC was measured simultaneously during muscle contraction. Engineered LC1 and RLC were labeled with red and green fluorescent probes, respectively, and exchanged with native light chains of striated muscle. The confocal microscope was modified to measure the anisotropy from 0.3 microm(3) volume containing approximately 600 fluorescent cross-bridges. Static measurements revealed that the magnitude of the angular change associated with transition from rigor to relaxation was less than 5 degrees for both light chains. Cross-bridges were activated by a precise delivery of ATP from a caged precursor. The time course of the angular change consisted of a fast phase followed by a slow phase and was the same for both light chains. These results suggest that the interactions of LC1 do not inhibit the angular motion of the proximal end of the regulatory domain and that the whole domain rotates as a rigid body.

Altered mechanical properties in smooth muscle of mice with a mutated calponin locus

The mechanical properties of smooth muscles in aorta and vas deferens were studied in mice with a mutated basic calponin locus to learn the physiological function of calponin. The intact smooth muscles were stimulated with high KCl and the force development was compared between calponin deficient (knockout, KO) mice and wild type (WT) ones. The isometric force induced by various concentrations of high KCl was lower in KO than in WT both in aorta and in vas deferens. The length-force relations were compared between KO and WT. The active isometric force in KO was significantly lower at most muscle lengths examined than in WT without the change in resting force both in aorta and in vas deferens. In vas deferens, the rate of force development after quick release in length at the peak force was significantly faster in KO than in WT. The above results show that the force development is lower and the rate of cross-bridge cycle is faster in KO mice than in WT ones, suggesting that calponin plays basic roles in the control of the contraction of smooth muscle.

Calponin is required for agonist-induced signal transduction--evidence from an antisense approach in ferret smooth muscle

1. The present study was undertaken to determine whether calponin (CaP) participates in the regulation of vascular smooth muscle contraction and, if so, to investigate the mechanism. 2. By PCR homology cloning, the cDNA sequence of ferret basic (h1) CaP was determined and phosphorothioate antisense and random oligonucleotides were synthesized and introduced into strips of ferret aorta by a chemical loading procedure. 3. Treatment of ferret aorta with CaP antisense oligonucleotides resulted in a decrease in protein levels of CaP to 54% of that in random sequence-loaded muscles, but no change in the protein levels of caldesmon (CaD), actin, desmin or extracellular regulated protein kinase (ERK). 4. Contraction in response to phenylephrine or a phorbol ester was significantly decreased in antisense-treated muscles compared to random sequence-loaded controls. Neither basal intrinsic tone nor the contraction in response to 51 mM KCl was significantly affected by antisense treatment. 5. During phenylephrine contractions, phospho-ERK levels increased, as did myosin light chain (LC20) phosphorylation. Phenylephrine-induced ERK phosphorylation and CaD phosphorylation at an ERK site were significantly decreased by CaP antisense. Increases in myosin light chain phosphorylation were unaffected. 6. The data indicate that CaP plays a significant role in the regulation of contraction and suggest that in a tonically active smooth muscle CaP may function as a signalling protein to facilitate ERK-dependent signalling, but not as a direct regulator of actomyosin interactions at the myofilament level.

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 myosin cross-bridge cycle and its control by twitchin phosphorylation in catch muscle

The anterior byssus retractor muscle of Mytilus edulis was used to characterize the myosin cross-bridge during catch, a state of tonic force maintenance with a very low rate of energy utilization. Addition of MgATP to permeabilized muscles in high force rigor at pCa > 8 results in a rapid loss of some force followed by a very slow rate of relaxation that is characteristic of catch. The fast component is slowed 3-4-fold in the presence of 1 mM MgADP, but the distribution between the fast and slow (catch) components is not dependent on [MgADP]. Phosphorylation of twitchin results in loss of the catch component. Fewer than 4% of the myosin heads have ADP bound in rigor, and the time course (0.2-10 s) of ADP formation following release of ATP from caged ATP is similar whether or not twitchin is phosphorylated. This suggests that MgATP binding to the cross-bridge and subsequent splitting are independent of twitchin phosphorylation, but detachment occurs only if twitchin is phosphorylated. A similar dependence of detachment on twitchin phosphorylation is seen with AMP-PNP and ATPgammaS. Single turnover experiments on bound ADP suggest an increase in the rate of release of ADP from the cross-bridge when catch is released by phosphorylation of twitchin. Low [Ca(2+)] and unphosphorylated twitchin appear to cause catch by 1) markedly slowing ADP release from attached cross-bridges and 2) preventing detachment following ATP binding to the rigor cross-bridge.

Regulation of asymmetric smooth muscle myosin II molecules

The emerging view of smooth/nonmuscle myosin regulation suggests that the attainment of the completely inhibited state requires numerous weak interactions between components of the two heads and the myosin rod. To further examine the nature of the structural requirements for regulation, we engineered smooth muscle heavy meromyosin molecules that contained one complete head and truncations of the second head. These truncations eliminated the motor domain but retained two, one, or no light chains. All constructs contained 37 heptads of rod sequence. None of the truncated constructs displayed complete regulation of both ATPase and motility, reinforcing the idea that interactions between motor domains are necessary for complete regulation. Surprisingly, the rate of ADP release was slowed by regulatory light chain dephosphorylation of the truncated construct that contained all four light chains and one motor domain. These data suggest that there is a second step (ADP release) in the smooth muscle myosin-actin-activated ATPase cycle that is modulated by regulatory light chain phosphorylation. This may be part of the mechanism underlying "latch" in smooth muscle.

Contractile properties and proteins of smooth muscles of a calponin knockout mouse

The role of h1-calponin in regulating the contractile properties of smooth muscle was investigated in bladder and vas deferens of mice carrying a targeted mutation in both alleles designed to inactivate the basic calponin gene. These calponin knockout (KO) mice displayed no detectable h1-calponin in their smooth muscles. The amplitudes of Ca2+ sensitization, force and Ca2+ sensitivity were not significantly different in permeabilized smooth muscle of KO compared with wild-type (WT) mice, nor were the delays in onset and half-times of Ca2+ sensitization, initiated by flash photolysis of caged GTPgammaS, different. The unloaded shortening velocity (Vus) of thiophosphorylated fibres was significantly (P<0.05) faster in the smooth muscle of KO than WT animals, but could be slowed by exogenous calponin to approximate WT levels; the concentration dependence of exogenous calponin slowing of Vus was proportional to its actomyosin binding in situ. Actin expression was reduced by 25-50%, relative to that of myosin heavy chain, in smooth muscle of KO mice, without any change in the relative distribution of the actin isoforms. We conclude that the faster Vus of smooth muscle of the KO mouse is consistent with, but does not prove without further study, physiological regulation of the crossbridge cycle by calponin. Our results show no detectable role of calponin in the signal transduction of the Ca2+-sensitization pathways in smooth muscle.

Regulation of shortening velocity by calponin in intact contracting smooth muscles

To elucidate the function of calponin in intact contracting smooth muscle cells in vivo, we generated mice with a mutated basic calponin (h1) locus (Yoshikawa et al., Genes Cells 3, 685-695, 1998). Crossbridge cycling rates were estimated in aortic smooth muscle by the force redevelopment following an isometric step shortening as a function of time after K(+) depolarization. Evidence is presented that calponin is involved in the inhibition of shortening velocity in the tonic phase of contraction. The phosphorylation levels of myosin regulatory light chain and cytosolic calcium concentrations were not significantly different in paired comparisons between calponin-deficient (-/-) and wild-type (+/+) muscles at any time point after stimulation. The force-velocity relationships in vas deferens smooth muscle showed that the maximum shortening velocity of -/- muscle was significantly faster than that of +/+ muscle. There was no change in the length-force relationships in both -/- and +/+ muscles of aorta and vas deferens. The results suggest that calponin plays a role in regulation of the crossbridge cycling and that it may be responsible for reduced shortening velocity during a maintained contraction of mammalian smooth muscle.

The maximal velocity of vascular smooth muscle shortening is independent of the expression of calponin

In smooth muscle, the phosphorylation/dephosphorylation of the 20-kDa regulatory light chain of myosin (MLC20) is known to regulate actomyosin interaction and force. However, a thin filament based regulatory system for actomyosin interaction has been suggested to exist in parallel to MLC20 phosphorylation. Calponin is a thin filament associated protein that in vitro inhibits actomyosin interaction, and has been suggested to reduce maximal shortening velocity (vmax). Using antibodies to h1- and h2-calponin, we demonstrated that calponin was present in smooth muscle from Sprague Dawley (SD) rats, while calponin was not detectable in the smooth muscle from Wistar Kyoto (WKY) rats. vmax determined from the force vs. velocity relationship at maximal Ca2+ activation was not different for either the aorta or the portal vein of SD vs. WKY rats. These results suggest that physiological levels of calponin do not contribute to a thin filament-based secondary regulation to inhibit smooth muscle contraction.

Actin and the smooth muscle regulatory proteins: a structural perspective

The structural details of the smooth muscle acto-myosin interaction and its functional implications have been much discussed in recent years, however other, smooth muscle specific, actin-binding proteins have received much less attention. With increasing technical advances in structural biology a great deal of structural information is now coming to light, information that can provide useful insight into the mechanism of action for many important nonmotor actin-binding proteins. The purpose of the review is to instill the current knowledge on the structure, and interaction sites on F-actin, of the major, non-motor actin-binding proteins from smooth muscle, proposed to have a role in regulation. In the light of the recent structural studies the probable roles of the various actin-binding proteins will be discussed with particular reference to structure function relationships.

Thin-filament linked regulation of smooth muscle myosin

Phosphorylation of the regulatory light chain subunit of smooth muscle myosin is sufficient, but not necessary for muscle contraction. It has been suggested that thin-filament regulation may also contribute to the regulation of contraction. A hallmark feature of regulated thin filaments, previously described for vertebrate skeletal muscle, is the capacity of strong-binding or rigor-like cross bridges to "turn-on" the actin filament. Turned-on thin filaments stimulate cross-bridge attachment even in the absence of calcium. The present study utilized an in vitro sliding-filament motility assay to test for thin-filament regulation of both unphosphorylated and phosphorylated smooth muscle myosins. Regulated thin-filaments were reconstituted from skeletal muscle actin and chicken gizzard smooth muscle tropomyosin (TmCG), and then turned-on either (1) by rigor cross bridges at low concentrations of MgATP, or (2) by adding N-ethyl-maleimide-modified skeletal subfragment S1(NEM-S1), which forms rigor-like bonds in the presence of MgATP. For control actin.TmCG filaments, force production by unphosphorylated myosin was 0.5% of that produced by thiophosphorylated myosin. The force exerted on actin.Tm filaments by both unphosphorylated and phosphorylated myosins was increased by reducing the [MgATP] to 10-100 microM MgATP (rigor-dependent activation). Force was also increased by actin.TmCG filaments that had been turned-on by NEM-S1 binding, with force production by unphosphorylated myosin increased 80-fold vs. 2.3-fold for thiophosphorylated myosin. TmCG was required for increased force production with both low MgATP and NEM-S1. Unloaded filament velocity for NEM-S1-activated thin filaments was 0.72 micron/sec with unphosphorylated myosin compared to 1.24 microns/sec with thiophosphorylated myosin. Taken together, these results suggest that thin-filament regulation may play a role in the activation of both unphosphorylated and phosphorylated smooth muscle myosins and suggest a possible mechanism for activation of slowly cycling unphosphorylated cross bridges (i.e. latch-state) during tonic contractions of smooth muscle.

Pharmacomechanical coupling: the role of calcium, G-proteins, kinases and phosphatases

The concept of pharmacomechanical coupling, introduced 30 years ago to account for physiological mechanisms that can regulate contraction of smooth muscle independently of the membrane potential, has since been transformed from a definition into what we now recognize as a complex of well-defined, molecular mechanisms. The release of Ca2+ from the SR by a chemical messenger, InsP3, is well known to be initiated not by depolarization, but by agonist-receptor interaction. Furthermore, this G-protein-coupled phosphatidylinositol cascade, one of many processes covered by the umbrella of pharmacomechanical coupling, is part of complex and general signal transduction mechanisms also operating in many non-muscle cells of diverse organisms. It is also clear that, although the major contractile regulatory mechanism of smooth muscle, phosphorylation/dephosphorylation of MLC20, is [Ca2+]-dependent, the activity of both the kinase and the phosphatase can also be modulated independently of [Ca2+]i. Sensitization to Ca2+ is attributed to inhibition of SMPP-1M, a process most likely dominated by activation of the monomeric GTP-binding protein RhoA that, in turn, activates Rho-kinase that phosphorylates the regulatory subunit of SMPP-1M and inhibits its myosin phosphatase activity. It is likely that the tonic phase of contraction activated by a variety of excitatory agonists is, at least in part, mediated by this Ca(2+)-sensitizing mechanism. Desensitization to Ca2+ can occur either through inhibitory phosphorylation of MLCK by other kinases or autophosphorylation and by activation of SMPP-1M by cyclic nucleotide-activated kinases, probably involving phosphorylation of a phosphatase activator. Based on our current understanding of the complexity of the many cross-talking signal transduction mechanisms that operate in cells, it is likely that, in the future, our current concepts will be refined, additional mechanisms of pharmacomechanical coupling will be recognized, and those contributing to the pathologenesis diseases, such as hypertension and asthma, will be identified.

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.

Regulation of smooth muscle actin-myosin interaction and force by calponin

Smooth muscle contraction is regulated primarily by the reversible phosphorylation of myosin triggered by an increase in sarcoplasmic free Ca2+ concentration ([Ca2+]i). Contraction can, however, be modulated by other signal transduction pathways, one of which involves the thin filament-associated protein calponin. The h1 (basic) isoform of calponin binds to actin with high affinity and is expressed specifically in smooth muscle at a molar ratio to actin of 1:7. Calponin inhibits (i) the actin-activated MgATPase activity of smooth muscle myosin (the cross-bridge cycling rate) via its interaction with actin, (ii) the movement of actin filaments over immobilized myosin in the in vitro motility assay, and (iii) force development or shortening velocity in permeabilized smooth muscle strips and single cells. These inhibitory effects of calponin can be alleviated by protein kinase C (PKC)-catalysed phosphorylation and restored following dephosphorylation by a type 2A phosphatase. Three physiological roles of calponin can be considered based on its in vitro functional properties: (i) maintenance of relaxation at resting [Ca2+]i, (ii) energy conservation during prolonged contractions, and (iii) Ca(2+)-independent contraction mediated by phosphorylation of calponin by PKC epsilon, a Ca(2+)-independent isoenzyme of PKC.

The cytoskeleton of the vertebrate smooth muscle cell

Smooth muscle cells possess a structural lattice composed of two primary parts: the 'cytoskeleton' that pervades the cytoplasm and the 'membrane skeleton' that provides anchorage for the cytoskeleton and contractile apparatus at the cell surface. The cytoskeleton contains two major components: first, a complement of actin filaments that links the cytoplasmic dense bodies at equispaced intervals in longitudinal fibrils; and second, a network of desmin intermediate filaments that co-distributes with the cytoskeletal actin. The actin filaments of the contractile apparatus are presumed to interface with the cytoskeleton at the cytoplasmic dense bodies and with the longitudinal rib-like arrays of dense plaques of the membrane skeleton that couple to the extracellular matrix. The present report focuses attention on the functional role of intermediate filaments and on the molecular domain structure of the protein calponin, which is found both in the cytoskeleton and the contractile apparatus. New information about the role of intermediate filaments in smooth muscle has come from studies of transgenic mice in which desmin expression has been ablated. These have shown that while desmin is dispensable for normal development and viability its absence has significant consequences for the mechanical properties of muscle tissue. Thus, the visceral smooth muscles develop only 40% of the normal contractile force and the maximal shortening velocity is reduced by 25-40%. Intermediate filaments therefore play an active role in force transmission and do not contribute solely to cell shape maintenance, as has hitherto been presumed. Recent studies on calponin have revealed a second actin binding domain at the C-terminus of the molecule and have also pinpointed an N-terminal domain that shares homology with a growing family of actin binding and signalling molecules. How these newly identified features of calponin relate to its function in vivo remains to be established.

Inhibitory mechanisms for cross-bridge cycling: the nitric oxide-cGMP signal transduction pathway in smooth muscle relaxation

Relaxation follows sequestration of Ca2+ mobilized by an excitatory stimulus in striated muscle. Removal of excitatory stimuli also relaxes smooth muscle in vitro after reductions in the myoplasmic [Ca2+] and dephosphorylation of the myosin regulatory light chains. However, there are several experimental procedures that produce relaxation in the presence of excitatory stimuli and elevated Ca(2+)-dependent cross-bridge phosphorylation. Of potential widespread physiological importance are treatments that increase myoplasmic [cGMP] owing to the ubiquity of nitric oxide (NO) as a signalling molecule for endothelial-mediated vasodilation and inhibitory nerves in most types of smooth muscle. Several mechanisms are implicated in the NO-cGMP mediated relaxation. Most studies support reductions in myoplasmic Ca2+. However, there is evidence that increases in cGMP also lower the Ca(2+)-sensitivity of cross-bridge phosphorylation. This would contribute to a decline in force through actions on the myosin light chain kinase/phosphatase system. In addition, changes in the dependence of force on phosphorylation are observed in tissues partially relaxed by treatments that elevate cGMP. This demonstrates that either the attachment and cycling of phosphorylated cross-bridges is impaired or blocked, or that the formation of dephosphorylated, force-generating cross-bridges ('latch-bridges') is reduced. Protein kinase G-catalysed phosphorylation of either a thin filament protein that blocks attachment of cross-bridges or a protein that inhibits myosin light chain phosphatase may explain the NO-induced relaxation with elevated cross-bridge phosphorylation.

Regulation of the cross-bridge cycle: the effects of MgADP, LC17 isoforms and telokin

This review summarizes the role of MgADP in force maintenance by dephosphorylated cross-bridges in smooth muscle and a potential physiological role for telokin. In tonic, compared with phasic, smooth muscles the affinity of cross-bridges in approximately 5 times higher for MgADP and the apparent second-order rate constant for MgATP is approximately 3 times lower. This gives rise to a large population of dephosphorylated cross-bridges in tonic smooth muscle. Such cross-bridges are thought to be major determinants of the different relaxation kinetics of the two types of smooth muscle and contribute to force maintenance at low levels of MLC20 phosphorylation, termed 'catch-like state' (Somlyo & Somlyo 1967) or 'latch' (Dillon et al. 1981). The molecular basis of the different affinities for MgADP and MgATP between tonic and phasic smooth muscle myosin was explored by exchange of essential myosin light chain (LC17) isoforms. In phasic bladder smooth muscle the exchange of LC17b for LC17a caused a significant decrease in the unloaded shortening velocity of non-phosphorylated, slowly cycling cross-bridges, suggesting that the LC17 isoforms contribute to the nucleotide affinity of latch bridges. The role of telokin in Ca(2+)-desensitization in phasic smooth muscle is reviewed. Telokin, the independently expressed C-terminus of myosin light chain kinase, is extensively phosphorylated during forskolin- and 8-br-cGMP-induced relaxation in situ. Telokin accelerated dephosphorylation of the regulatory myosin light chain and relaxed rabbit ileum smooth muscle. The results suggest that telokin contributes to cAMP and/or cGMP kinase-mediated Ca(2+)-desensitization of phasic smooth muscles.

Myosin essential light chain isoforms modulate the velocity of shortening propelled by nonphosphorylated cross-bridges

The differential effects of essential light chain isoforms (LC17a and LC17b) on the mechanical properties of smooth muscle were determined by exchanging recombinant for endogenous LC17 in permeabilized smooth muscle treated with trifluoperazine (TFP). Co-precipitation with endogenous myosin heavy chain verified that 40-60% of endogenous LC17a could be exchanged for recombinant LC17a or LC17b. Upon addition of MgATP in Ca2+-free solution, recombinant LC17 exchange induced slow contractions unaccompanied by regulatory light chain (RLC) phosphorylation only in TFP-treated, but not in untreated, permeabilized smooth muscle; the shortening velocity and rate of force development were approximately 1.5 and 2 times faster, respectively, in response to LC17a than LC17b. Additional incubation with recombinant, thiophosphorylated RLC increased the shortening velocity, independent of the LC17 isoform exchanged. The LC17-induced contractions of TFP-treated muscles were abolished by prior addition of nonphosphorylated RLC. We suggest that LC17 stiffens the lever arm of myosin and, in the absence of regulation by RLC, permits cross-bridge cycling without requiring RLC phosphorylation. Our results are compatible with nonphosphorylated RLC acting as a repressor and with LC17 isoforms modulating the MgADP affinity and, consequently, rate of cooperative cycling of nonphosphorylated cross-bridges.

Regulation and tuning of smooth muscle myosin

Smooth muscle myosin is regulated by phosphorylation of one of the two myosin light chains. This phosphorylation causes an unfolding of the myosin that allows it to interact with actin to produce force. The inactive state involves trapping the myosin in a conformation wherein the myosin heads interact with a segment of the myosin rod. Phosphorylation of the regulatory light chain weakens these interactions and allows the myosin to be activated. Smooth muscle myosin has a large movement of its light chain binding domain that is coupled to ADP release. This structural change may be necessary for the generation of "latch." Smooth muscle myosin has three different regions that vary to generate different isoforms: (1) an alternative insertion within the myosin head; (2) two possible essential light chains; and (3) an alternative tail at the end of the myosin rod. There is substantial evidence that the insertion in the myosin head increases the enzymatic activity of the myosin and leads to greater shortening velocity. The function of the other two variants is as yet unclear.

Structural and kinetic studies of phosphorylation-dependent regulation in smooth muscle myosin

In this study, we have examined the mechanism of phosphorylation-dependent regulation in smooth muscle myosin through the use of structural and kinetic methodologies applied to several myosin fragments. Fluorescence anisotropy decay measurements demonstrate that regulatory light chain phosphorylation significantly reduces the rotational correlation time of regulatable myosin preparations, whereas minimally regulated ones show little effect in this assay. Sedimentation equilibrium studies show that the regulatory domain can dimerize with a dissociation constant that is unaffected by regulatory light chain phosphorylation. Finally, kinetic studies on the interactions of myosin-ADP constructs with actin are also consistent with a model in which interactions occur between the two heads, which are lost with regulatory light chain phosphorylation. We propose that in the absence of regulatory light chain phosphorylation, the two heads of myosin interact with each other, due to a weak intrinsic dimerization of the regulatory domains that is significantly stabilized by the proximal rod. Regulatory light chain phosphorylation abolishes the stabilizing effect of the proximal rod, leading to a loss of this interaction.

Cytoskeletal targeting of calponin in differentiated, contractile smooth muscle cells of the ferret

1. Biochemical and quantitative image analysis methods were used to investigate the anatomical basis for the previously described agonist-induced redistribution of calponin. 2. At 140 nm resolution, the quantitative distribution of calponin in resting cells was statistically indistinguishable from that of filament bundles containing alpha-smooth muscle actin and myosin, but was significantly different from that of filaments containing beta-non-muscle actin. Conversely, in stimulated cells, the distribution of calponin was not significantly different from that of beta-actin filaments in the subplasmalemmal cell cortex but was significantly different from the distribution of alpha-actin- and myosin-containing filamentous bundles. 3. The distribution of calponin significantly differed from that of the intermediate filament proteins vimentin and desmin as well as that of the dense body protein alpha-actinin either by ratio analysis of the subcellular distribution or by colocalization analysis. 4. The imaging results, although limited to 140 nm spatial resolution, suggested the hypothesis that the agonist-induced redistribution involves the binding of calponin to isoform-specific actin filaments. This hypothesis was tested by quantifying the relative affinity of calponin for purified alpha- and beta-actin. Light scattering measurements showed that calponin induces bundle formation with beta-actin more readily than alpha-actin, indicating that calponin may be preferentially sequestered by beta-actin under appropriate conditions. 5. These results are consistent with a model whereby agonist activation decreases calponin's binding to filaments, but the tighter binding to beta-actin filaments results in a spatial redistribution of calponin to the submembranous cortex.

3-D image reconstruction of reconstituted smooth muscle thin filaments containing calponin: visualization of interactions between F-actin and calponin

Calponin is a putative thin filament regulatory protein of smooth muscle that inhibits actomyosin ATPase in vitro. We have used electron microscopy and three-dimensional reconstruction to elucidate the structural organization of calponin on actin and actin-tropomyosin filaments. Calponin density was clearly delineated in the reconstructions and found to occur peripherally along the long-pitch actin-helix. The main calponin mass was located over sub-domain 2 of actin, and connected axially adjacent actin monomers by binding to the "upper" and "lower" edges of sub-domains 1 of each actin. When the reconstructions were fitted to the atomic model of F-actin, calponin appeared to contact actin near the N terminus and at residues 349 to 352 close to the C terminus of sub-domain 1 on one monomer. It also touched residues 92 to 95 of sub-domain 1 on the axially neighboring actin and continued up the side of this monomer as far as residues 43 to 48 of sub-domain 2. These positions are consensus binding sites for a number of actin-associated proteins and are also near to sites of weak myosin interaction. Calponin did not appear to block strong myosin binding sites on actin. In contrast to the calponin mass which appeared monomeric in reconstructions, tropomyosin formed a continuous strand of added density along F-actin. When added to tropomyosin-containing filaments, calponin caused a shift of tropomyosin away from sub-domain 1 towards sub-domain 3 of actin, exposing strong myosin-binding sites that were previously covered by tropomyosin. This structural effect is unlike that of troponin and therefore inhibition of actomyosin ATPase by calponin and troponin cannot be strictly analogous. The location of calponin would allow it to directly compete or interact with a number of actin-binding proteins.