Effects of exogenously applied calponin on Ca(2+)-regulated force in skinned smooth muscle of the rabbit mesenteric artery

A rapid degradation of calponin 2 is required for cytokinesis

Calponin 2 is an actin cytoskeleton-associated protein and plays a role in regulating cell motility-related functions such as phagocytosis, migration, and division. We previously reported that overexpression of calponin 2 inhibits the rate of cell proliferation. To investigate the underlying mechanism, our present study found that the levels of endogenous calponin 2 in NIH3T3 and HEK293 cells rapidly decreased before cell division characterized by an absence at the actin contractile ring. In cells lacking endogenous calponin 2, transfective expression of GFP-fusion calponin 2 inhibited cell proliferation similar to that of nonfusion calponin 2. Fluorescent imaging studies of mitotic cells indicated that a proper level of calponin 2 expression and effective degradation during cytokinesis are necessary for normal cell division. Computer-assisted dynamic image analysis of dividing cells revealed that overexpression of calponin 2 significantly affects motility and shape behaviors of cells only on the interval from the start of anaphase to the start of cytokinesis, i.e., the pre-cytokinesis phase, but not on the interval from the start of cytokinesis to 50% completion of cytokinesis. The pre-cytokinesis degradation of calponin 2 was attenuated by MG132 inhibition of the ubiquitin proteasome and inhibitor of protein kinase C (PKC), suggesting that PKC phosphorylation-triggered degradation of calponin 2 could determine the rate of cytokinesis. The novel role of calponin 2 in regulating the rate of cytokinesis may be targeted for therapeutic applications such as in an inhibition of malignant tumor growth.

MEKK1-dependent phosphorylation of calponin-3 tunes cell contractility

MEKK1 (also known as MAP3K1), which plays a major role in MAPK signaling, has been implicated in mechanical processes in cells, such as migration. Here, we identify the actin-binding protein calponin-3 as a new MEKK1 substrate in the signaling that regulates actomyosin-based cellular contractility. MEKK1 colocalizes with calponin-3 at the actin cytoskeleton and phosphorylates it, leading to an increase in the cell-generated traction stress. MEKK1-mediated calponin-3 phosphorylation is attenuated by the inhibition of myosin II activity, the disruption of actin cytoskeletal integrity and adhesion to soft extracellular substrates, whereas it is enhanced upon cell stretching. Our results reveal the importance of the MEKK1-calponin-3 signaling pathway to cell contractility.

Calponin control of cerebrovascular reactivity: therapeutic implications in brain trauma

Calponin (Cp) is an actin-binding protein first characterized in chicken gizzard smooth muscle (SM). This review discusses the role of Cp in mediating SM contraction, the biochemical process by which Cp facilitates SM contraction and the function of Cp in the brain. Recent work on the role of Cp in pathological states with emphasis on traumatic brain injury is also discussed. Based on past and present data, the case is presented for targeting Cp for novel genetic and pharmacological therapies aimed at improving outcome following traumatic brain injury (TBI).

Calponin in non-muscle cells

Calponin is an actin filament-associated regulatory protein expressed in smooth muscle and non-muscle cells. Calponin is an inhibitor of the actin-activated myosin ATPase. Three isoforms of calponin have been found in the vertebrates. Whereas the role of calponin in regulating smooth muscle contractility has been extensively investigated, the function and regulation of calponin in non-muscle cells is much less understood. Based on recent progresses in the field, this review focuses on the studies of calponin in non-muscle cells, especially its regulation by cytoskeleton tension and function in cell motility. The ongoing research has demonstrated that calponin plays a regulatory role in non-muscle cell motility. Therefore, non-muscle calponin is an attractive target for the control of cell proliferation, migration and phagocytosis, and the treatment of cancer metastasis.

Proteome alteration induced by hTERT transfection of human fibroblast cells

Background

Telomerase confers cellular immortality by elongating telomeres, thereby circumventing the Hayflick limit. Extended-life-span cells have been generated by transfection with the human telomerase reverse transcriptase (hTERT) gene. hTERT transfected cell lines may be of outstanding interest to monitor the effect of drugs targeting the telomerase activity. The incidence of hTERT gene transfection at the proteome level is a prerequisite to that purpose. The effect of the transfection has been studied on the proteome of human fibroblast (WI38). Cytosolic and nuclear fractions of WI38 cells, empty vector transfected WI38 (WI38-HPV) and hTERT WI38 cells were submitted to a 2D-DIGE (Two-Dimensional Differential In-Gel Electrophoresis) analysis. Only spots that had a similar abundance in WI38 and WI38-HPV, but were differentially expressed in WI38 hTERT were selected for MS identification. This method directly points to the proteins linked with the hTERT expression. Number of false positive differentially expressed proteins has been excluded by using control WI38-HPV cells. The proteome alteration induced by hTERT WI38 transfection should be taken into account in subsequent use of the cell line for anti-telomerase drugs evaluation.

Results

2D-DIGE experiment shows that 57 spots out of 2246 are significantly differentially expressed in the cytosolic fraction due to hTERT transfection, and 38 were confidently identified. In the nuclear fraction, 44 spots out of 2172 were selected in the differential proteome analysis, and 14 were identified. The results show that, in addition to elongating telomeres, hTERT gene transfection has other physiological roles, among which an enhanced ER capacity and a potent cell protection against apoptosis.

Conclusion

We show that the methodology reduces the complexity of the proteome analysis and highlights proteins implicated in other processes than telomere elongation. hTERT induced proteome changes suggest that telomerase expression enhances natural cell repair mechanisms and stress resistance probably required for long term resistance of immortalized cells. Thus, hTERT transfected cells can not be only consider as an immortal equivalent to parental cells but also as cells which are over-resistant to stresses. These findings are the prerequisite for any larger proteomics aiming to evaluate anti-telomerase drugs proteome alteration and thus therapeutics induced cell reactions.

Effects of h1-calponin ablation on the contractile properties of bladder versus vascular smooth muscle in mice lacking SM-B myosin

The functional significance of smooth muscle-specific h1-calponin up-regulation in the smooth muscle contractility of SM-B null mice was studied by generating double knockout mice lacking both h1-calponin and SM-B myosin. The double knockout mice appear healthy, reproduce well and do not show any smooth muscle pathology. Loss of h1-calponin in the SM-B null mice bladder resulted in increased maximal shortening velocity (V(max)) and steady-state force generation. The force dilatation pressure, which was decreased in the SM-B null mesenteric vessels, was restored to wild-type levels in the double knockout vessels. In contrast, the half-time to maximal constriction was significantly increased in the double knockout vessels similar to that of SM-B null mice and indicating decreased shortening velocity in the double knockout vessels. Biochemical analyses showed that there is a significant reduction in smooth muscle alpha-actin levels, whereas h-caldesmon levels are increased in the double knockout bladder and mesenteric vessels, suggesting that these changes may also partly contribute to the altered contractile function. Taken together, our studies suggest that up-regulation of h1-calponin in the SM-B null mice may be necessary to maintain a reduced level of cross-bridge cycling over time in the absence of SM-B myosin and play an important role in regulating the smooth muscle contraction.

Role of the basic C-terminal half of caldesmon in its regulation of F-actin: comparison between caldesmon and calponin

We previously reported that caldesmon (CaD), together with tropomyosin (TM), effectively protects actin filaments from gelsolin, an actin-severing protein. To elucidate the structure/function relationship of CaD, we dissected the functional domain of CaD required for the protection. The basic C-terminal half of rat nonmuscle CaD (D3) inhibits gelsolin activity to the same degree as intact CaD, although a smaller C-terminal region of D3 does not. This smaller C-terminal region contains the minimum regulatory domain responsible for the inhibition of actomyosin ATPase, and for the binding to actin, calmodulin and TM. These results suggest that the domain responsible for the inhibition of gelsolin activity lies outside the minimum regulatory domain, and that the positive charge possessed by the C-terminal half of CaD is important for its interaction with actin. Moreover, while the D3 fragment promotes the aggregation of F-actin into bundles as reported previously, this bundle formation is inhibited by the acidic N-terminal half of CaD, as well as by poly-l-glutamate. It seems likely that the acidic N-terminal half of CaD neutralizes the superfluous basic feature of the C-terminal half. A comparison between D3 and calponin, another actin-binding protein that is also basic and has similar actin-regulatory activities, is also discussed.

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.

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.

Ca2+-Sensitization of contraction in the h1 calponin-deficient smooth muscle

Role of h1 calponin on Ca2+-sensitivity of smooth muscle contraction was investigated using h1 calponin gene-deficient mice (CP -/-) and wild type mice (CP +/+). PGF2. induced a comparable force in intact aorta of CP +/+ and CP -/-. DPB showed similar effects to PGF2alpha. In membrane-permeabilized ileal smooth muscle, PDBu enhanced Ca2+-sensitivity of contraction comparably in CP +/+ and CP -/-. GTPgamma-S showed similar effects. Our results suggest that h1 calponin does not regulate Ca2+-sensitivity in the contractile mechanism of smooth muscle.

Identification of calponin as a novel substrate of Rho-kinase

Calponin, an F-actin-associated protein implicated in the regulation of smooth muscle contraction, is known to be phosphorylated in vitro by protein kinase C (PKC) and Ca(2+)/calmodulin dependent protein kinase II (CaM kinase II). Unphosphorylated calponin binds to F-actin and inhibits the actin-activated myosin ATPase activity; these properties are lost on phosphorylation. In the present study, we found that Rho-kinase phosphorylated basic calponin stoichiometrically in vitro. We identified the sites of phosphorylation of calponin by Rho-kinase as Thr-170, Ser-175, Thr-180, Thr-184, and Thr-259, and prepared antibodies that specifically recognized calponin phosphorylated at Thr-170 and Thr-184. We showed that the phosphorylation of calponin by Rho-kinase inhibited the binding of calponin to F-actin. Taken together, these results suggest that calponin is a substrate of Rho-kinase and that Rho-kinase regulates the interaction of calponin with F-actin.

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.

Contraction kinetics of isolated human myometrium during menstrual cycle and pregnancy

OBJECTIVE

To investigate the interaction between actin and myosin in the myometrium by studying the contraction kinetics of isolated samples of human myometrium.

DESIGN

Experimental and observational cross-sectional study.

SETTING

Eppendorf University Hospital, Hamburg.

SAMPLES

Myometrium samples were taken from women in the follicular phase (n = 6) or luteal phase (n = 6) of the menstrual cycle and during pregnancy at term (n = 25).

METHODS

The frequency, extent and rate of force development were determined in spontaneously active myometrial preparations. From a resting force of 2 mN, sustained tonic contractions were induced by K+-depolarisation (124 mM), or by protein kinase C activation (19 x 9 microM indolactam). The steady force was reversibly interrupted by rapid length changes (100 Hz sinus vibrations lasting 1 s, 5% of muscle length). Extent (steady plateau), as well as rate of force increase after cessation of vibrations, were derived from bi-exponential functions fitted to the time course of force recovery.

RESULTS

Frequency of spontaneous contractions was higher in the follicular phase [mean (SD) 18 x 3 contractions/hour (1 x 0)] than in the luteal phase [13 x 4 contractions/hour (8 x 1)] or in pregnancy at term [8 x 8 contractions/hour (7 x 6)]. During indolactam treatment, steady force in pregnancy at term was significantly increased [8 x 8 mN (4 x 0)], compared with the follicular phase [3 x 7 mN (0 x 9)]. Force recovery was distinctly slower in pregnancy at term during indolactam treatment [time constant 99 x 2 s (57 x 9); P < 0 x 005] than during K+-depolarisation [time constant 29 x 1 s (5 x 9)], whereas in the follicular phase the rate of force recovery was faster with indolactam [16 x 8 s (7 x 1)] than with K+ depolarisation [24 x 4 s (5 x 9); P < 0 x 005].

CONCLUSIONS

The responses of human myometrium to contraction stimuli differ according to the reproductive state. Membrane depolarisation causes similar responses in all myometrial strips. In contrast, near term stimulation of protein kinase C generates a large tonic force and slow contraction kinetics, whereas early in the menstrual cycle contraction kinetics are fast.

Calponin interaction with alpha-actinin-actin: evidence for a structural role for calponin

The purpose of this study was to address the paradox of calponin localization with alpha-actinin and filamin, two proteins with tandem calponin homology (CH) domains, by determining the effect of these proteins on the binding of calponin to actin. The results show that actin can accommodate near-saturating concentrations of either calponin and alpha-actinin or calponin and filamin with little change or no change in ligand affinity. Little direct interaction occurred between alpha-actinin and calponin in the absence of actin, so this effect is not likely to explain the co-distribution of these proteins. Calponin, like alpha-actinin, induced elastic gel formation when added to actin. When alpha-actinin was added to newly formed calponin/actin gels, no change was seen in the mechanical properties of the gel compared to calponin and actin alone. However, when calponin was added to newly formed alpha-actinin/actin gels, the resulting gel was much stronger than the gels formed by either ligand alone. Furthermore, gels formed by the addition of calponin to alpha-actinin/actin exhibited a phenomenon known as strain hardening, a characteristic of mechanically resilient gels. These results add weight to the concept that one of the functions of calponin is to stabilize the actin cytoskeleton.

Protein kinase C promotes spontaneous relaxation of streptolysin-O-permeabilized smooth muscle cells from the guinea-pig stomach

Isolated single smooth muscle cells from the fundus of a guinea-pig stomach were permeabilized by use of streptolysin-O (0.5 U/ml). Most of the permeabilized cells responded to 0.6 microM Ca2+, but not to 0.2 microM Ca2+, with a resulting maximal cell shortening to approximately 71% of the resting cell length. These cells were relaxed again by washing with the Ca2+-free solution (2.5 nM free Ca2+) for 3-5 min. Addition of 10 microM acetylcholine (ACh) resulted in both a marked decrease in the concentration of Ca2+ required to trigger a threshold response and an increase in the maximal cell shortening, indicating that the cells retained the muscarinic receptor function. When the cell treated with a protein kinase C (PKC) inhibitor, K-252b (1 microM), for 3 min was exposed to 10 microM ACh in the presence of K-252b, the cell shortened within 2 min with a maximal cell shortening. When the cell shortening was induced by 10 microM ACh plus 1 microM Ca2+ in the presence of K-252b (1 microM) or more selective PKC inhibitors, such as calphostin C (1 microM) or PKC pseudosubstrate peptide (100 microM), the extension of the shortened cells, by washing with the Ca2+-free solution, was significantly inhibited. In contrast, K-252b (1 microM) did not inhibit the relaxation of Ca2+-induced shortened cells. These results suggest that the receptor-mediated activation of PKC in the process of ACh-induced cell shortening plays a role in the subsequent relaxation of the shortened cells.

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.

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.

HA1077, a protein kinase inhibitor, inhibits calponin phosphorylation on Ser175 in porcine coronary artery

Calponin is a thin filament-associated protein which has been implicated in the modulation of the contractile state of smooth muscle via its interaction with actin and inhibition of the actin-activated myosin Mg-ATPase. This inhibitory effect is alleviated by phosphorylation of calponin at Ser175 in vitro by protein kinase C. The issue of calponin phosphorylation in intact smooth muscle in response to agonists that activate protein kinase C is controversial. We have produced a monoclonal antibody that specifically recognizes calponin phosphorylated at Ser175 and used it to analyze calponin phosphorylation in porcine coronary arterial smooth muscle stimulated with prostaglandin F2alpha or phorbol 12,13-dibutylate (PDB). Calponin phosphorylation increased rapidly in response to prostaglandin F2alpha concomitant with the increase in tension. Calponin was then dephosphorylated while force was maintained. Tension development in response to PDB was significantly slower, but again calponin phosphorylation paralleled force development. In this case, calponin dephosphorylation was very slow, consistent with prolonged activation of protein kinase C. The protein kinase inhibitors, HA1077 (1-5-(isoquinoline sulfonyl)-homopiperazine HCl) and HA1100 (1-hydroxy HA1077; 1-(hydroxy-5-isoquinoline sulfonyl-homopiperazine), inhibited tension development and calponin phosphorylation in a concentration-dependent manner with similar ED50 values in response to prostaglandin F2alpha and PDB. These results support physiological roles for calponin in force development in smooth muscle in response to agonists which trigger protein kinase C activation and in the latch state, i.e., force maintenance at low energy cost. Furthermore, the vasodilator effect of HA1077 and HA1100 is more likely due to inhibition of protein kinase C than of myosin light chain kinase.

Protein kinase C--catalyzed calponin phosphorylation in swine carotid arterial homogenate

Calponin, a thin filament-associated protein, inhibits actin-activated myosin ATPase activity, and this inhibition is reversed by phosphorylation. Calponin phosphorylation by protein kinase C and Ca2+/calmodulin-dependent protein kinase II has been shown in purified protein systems but has been difficult to demonstrate in more physiological preparations. We have previously shown that calponin is phosphorylated in a cell-free homogenate of swine carotid artery. The goal of this study was to determine whether protein kinase C and/or Ca2+/calmodulin-dependent protein kinase II catalyzes calponin phosphorylation. Ca2+-dependent calponin phosphorylation was not inhibited by calmodulin antagonists. In contrast, both Ca2+- and phorbol dibutyrate/1-oleoyl-2-acetyl-sn-glycerol dependent calponin phosphorylation were inhibited by the pseudosubstrate inhibitor of protein kinase C and staurosporine. Our results also demonstrate that stimulation with either Ca2+, phorbol dibutyrate, or 1-oleoyl-2-acetyl-sn-glycerol activates endogenous protein kinase C. We interpret our results as clearly demonstrating that the physiological kinase for calponin phosphorylation is protein kinase C and not Ca2+/calmodulin-dependent protein kinase II. We also present data showing that the direct measurement of 32P incorporation into calponin and the indirect measurement of calponin phosphorylation using nonequilibrium pH gradient gel electrophoresis provide similar quantitative values of calponin phosphorylation.

h1- and h2-calponins are not essential for norepinephrine- or sodium fluoride-induced contraction of rat aortic smooth muscle

To investigate the controversial issue concerning the role of calponin in smooth muscle contraction, this study examined the relationship between smooth muscle calponin and the contraction of aortic rings from different strains of rats: Sprague-Dawley (SD), Wistar, and Wistar Kyoto (WKY). Western blot analysis demonstrated that h1- and h2-calponins are present in aortic smooth muscle from adult SD rats but not Wistar or WKY rats. Nevertheless, h1-calponin is detectable in stomach from Wistar rats, although at a much lower level compared with that in the SD rat stomach. This suggests that a repressed expression of the gene, instead of a simple null mutation, may have caused its absence from the aortic smooth muscle. Despite the presence or absence of calponin, the aortic smooth muscles from the different strains of rats all develop contractions in response to the physiological agonist norepinephrine (NE) and following activation with the plasma membrane receptor-independent NaF induction. The data indicate that h1- and h2-calponins are not essential for NE- and NaF-induced contractions in aortic smooth muscle. The calponin-positive adult SD rat aorta was found to be more sensitive in contractile response to NE and NaF inductions compared with the calponin-negative rat aortae. This may imply a potential modulator function of calponin in the contraction of smooth muscle, whereas other contractile protein isoform differences between these rat strains may also play a role.

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.

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.

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

A key unanswered question in smooth muscle biology is whether phosphorylation of the myosin regulatory light chain (RLC) is sufficient for regulation of contraction, or if thin-filament-based regulatory systems also contribute to this process. To address this issue, the endogenous RLC was extracted from single smooth muscle cells and replaced with either a thiophosphorylated RLC or a mutant RLC (T18A/S19A) that cannot be phosphorylated by myosin light chain kinase. The actin-binding protein calponin was also extracted. Following photolysis of caged ATP, cells without calponin that contained a nonphosphorylatable RLC shortened at 30% of the velocity and produced 65% of the isometric force of cells reconstituted with the thiophosphorylated RLC. The contraction of cells reconstituted with nonphosphorylatable RLC was, however, specifically suppressed in cells that contained calponin. These results indicate that calponin is required to maintain cells in a relaxed state, and that in the absence of this inhibition, dephosphorylated cross-bridges can slowly cycle and generate force. These findings thus provide a possible framework for understanding the development of latch contraction, a widely studied but poorly understood feature of smooth muscle.

Receptor-dependent G protein-mediated Ca2+ sensitization in canine airway smooth muscle

To determine the mechanisms of receptor-dependent Ca2+ sensitization in airway smooth muscle, canine tracheal smooth muscle (CTSM) was permeabilized with alpha-toxin or beta-escin. Although the effects of 5-hydroxytryptamine (100 microM), histamine (100 microM), and the thromboxane A2 analogue U-46619 (100 microM) were negligible, carbachol (100 microM) and endothelin-1 (ET-1, 1 microM) evoked additional contractions of 47.0 +/- 5.90% and 25.0 +/- 5.37% (n = 6) at pCa 6.7 with GTP (3 microM) (normalized to the maximum contraction at pCa 4.5) in alpha-toxin-permeabilized CTSM. GDP-beta-S (1 mM) reversed the carbachol and ET-1 responses completely. GTP-gamma-S (30 microM) and 4 beta-phorbol 12,13-dibutyrate (PDBu, 3 microM) increased the Ca2+ sensitivity (median effective pCa) of contraction by 1.8- and 4.4-fold, respectively (n = 4-11, P < 0.05). The effects of saturating concentrations of GTP-gamma-S and PDBu were additive. A synthetic peptide (T2) corresponding to the actin-binding site of calponin caused a dose-dependent contraction of beta-escin permeabilized CTSM, with the peak effect (25 +/- 4%, n = 4) at 1200 microM, PDBu (3 microM) caused contraction of the T2 peptide-treated CTSM. In conclusion, Ca2+ sensitization of CTSM depends on receptor type and is mediated by G proteins and protein kinase C whose effects are additive, with a partial contribution by calponin.

Electrostatic effects of smooth muscle calponin on actin assembly

The contribution of electrostatic interactions to the effects of chicken gizzard calponin on the kinetics of actin polymerization and the bundling of F-actin were characterized by a combination of fluorescence, light-scattering, co-sedimentation, and electron-microscopic methods. Stoichiometric amounts of calponin accelerate actin polymerization in low-ionic-strength solutions, but this effect is diminished at [KCI] = 150 mM. At low ionic strengths, micromolar concentrations of calponin induce the formation of large bundles of actin filaments, and lower concentrations of calponin quench the fluorescence of pyrene-labeled F-actin. The latter effect is related to binding of calponin to F-actin rather than to bundling of the filaments. The concentration of calponin required to bundle a fixed concentration of actin filaments increases with increasing ionic strength, as the average diameter of the bundles decreases. Millimolar concentrations of ATP, GTP or ITP are equally efficient at dispersing actin bundles to single filaments or smaller aggregates, even though a significant fraction of calponin remains bound to F-actin. Our findings show that the binding of calponin to actin is determined at least in part by electrostatic interactions, and that the polycationic nature of calponin is primarily responsible for the formation of F-actin bundles via its ability to reduce the electrostatic repulsion between the negatively charged actin filaments.

Calponin inhibits actin-activated MgATPase of myosin subfragment 1 (S1) without displacing S1 from its binding site on actin

Calponin is a smooth-muscle thin-filament protein implicated in the regulation of contraction. Its binding to actin is a prerequisite for inhibition of actin-activated myosin MgATPase. Investigating the molecular mechanism of this inhibition, it was found that titration of acto-myosin subfragment 1 with calponin in the presence of either ADP or ATP does not displace weakly or strongly bound myosin subfragment 1 (S1) from actin. S1.ADP, however, is able to release about two-thirds of the calponin from saturated (equimolar) complexes of actin-calponin. The remaining calponin is sufficient for almost full inhibition of acto-S1 MgATPase activity. Bunding of actin filaments by calponin takes place at a higher ratio calponin/actin (above 1:3) and, therefore, is not responsible for inhibition of the ATPase. Bundle formation is inhibited by S1.ADP. These results suggest the existence of two calponin-binding sites on actin; one, that is insensitive to S1, which is responsible for inhibition of the ATPase, the other, from which calponin is readily displaced by S1.

Modulation of actin conformation and inhibition of actin filament velocity by calponin

Calponin, an actin/calmodulin-binding protein present in smooth muscle thin filaments, modulates the actin-myosin interaction and actomyosin ATPase activity of smooth muscle myosin II. Binding of myosin heads to actin under conditions that produce weak or strong binding induces conformational changes in actin. Polarized fluorimetric measurements of rhodamine-phalloidin complex and 1,5-IAEDANS specifically linked to actin in myosin-free muscle fibers (ghost fibers) and to Cys-707 in myosin head, respectively, revealed conformational changes, as determined from the changes in orientation and mobility of fluorescent probes, upon addition of calponin to ghost fibers. The effect of calponin on conformational changes produced upon binding of phosphorylated or dephosphorylated heavy meromyosin (HMM) was also determined. Subfragment-1 preparation modified with NEM (NEM-S1) or pPDM (pPDM-S1) were used as models of strong and weak binding, respectively. Calponin changed both the orientation of fluorophores on the actin and the flexibility of the actin filaments, as determined from the angle between an actin filament and the fiber axis. Changes in the flexibility of actin filaments and the orientation of fluorophores produced by phosphorylated smooth muscle HMM were similar to those seen with NEM-S1, which formed a strong-binding association with actin and caused the transition of actin monomers to the "on" state; calponin markedly inhibited this effect. In contrast, pPDM-S1 and dephosphorylated HMM induced weak binding and the transition of actin monomers to the "of" state, and these effects were enhanced by calponin. Furthermore, calponin decreased the velocity of actin filament movement over skeletal muscle myosin O gamma phosphorylated smooth muscle myosin heads in an in vitro motility assay. These results suggest that calponin induces modulation of smooth muscle contraction by inhibiting the force-producing (strong-binding) state of cross-bridges and involves changes in actin conformation.

Inhibition by calponin of isometric force in demembranated vascular smooth muscle strips: the critical role of serine-175

alpha-Calponin is a thin-filament-associated protein which has been implicated in the regulation of smooth muscle contraction. Quantification of the tissue content of rat tail arterial smooth muscle revealed approximately half the amount of alpha-calponin relative to actin compared with chicken gizzard and other smooth muscles, suggesting that this tissue would be particularly suitable for investigation of the effects of exogenous alpha-calponin on the contractile properties of permeabilized muscle strips. Rat tail arterial strips demembranated with Triton X-100 retained approximately 90% of their complement of alpha-calponin, and exogenous chicken gizzard alpha-calponin (which conveniently has a slightly lower molecular mass than the rat arterial protein) bound to the permeabilized muscle, presumably through its high affinity for actin. Exogenous alpha-calponin inhibited force in demembranated muscle strips in a concentration-dependent manner when added at the peak of a submaximal Ca(2+)-induced contraction, with a half-maximal effect at approximately 3 microM alpha-calponin. Pretreatment of demembranated muscle strips with alpha-calponin inhibited subsequent force development at all concentrations of Ca2+ examined over the activation range. The inhibitory effect of alpha-calponin was shown to be Ca(2+)-independent, since exogenous alpha-calponin also inhibited force in the absence of Ca2+ in demembranated muscle strips containing thiophosphorylated myosin. Phosphorylation of alpha-calponin on Ser-175 by protein kinase C has been suggested to alleviate the inhibitory effect of alpha-calponin on smooth muscle contraction. To test this hypothesis, the effects on Ca(2+)-induced and Ca(2+)-independent contractions of demembranated muscle strips of phosphorylated alpha-calponin and three site-specific mutants of alpha-calponin (in which Ser-175 was replaced by Ala, Asp or Thr) were compared with the effects of unphosphorylated tissue-purified and recombinant wild-type alpha-calponins. The recombinant wild-type protein behaved identically to the unphosphorylated tissue-purified protein, as did the S175T mutant, which is known to bind actin with high affinity and to inhibit the actin-activated myosin MgATPase in vitro. On the other hand, phosphorylated alpha-calponin and the S175A and S175D mutants, which bind weakly to actin and have little effect on the actin-activated myosin MgATPase in vitro, failed to cause significant inhibition of force induced by Ca2+ or myosin thiophosphorylation. These results support a role for alpha-calponin in the regulation of smooth muscle contraction and indicate the functional importance of Ser-175 of alpha-calponin as a regulatory site of phosphorylation.

Structure-function relations of smooth muscle calponin. The critical role of serine 175

Calponin has been implicated in the regulation of smooth muscle contraction through its interaction with F-actin and inhibition of the actin-activated MgATPase activity of phosphorylated myosin. Both properties are lost following phosphorylation (primarily at serine 175) by protein kinase C or calmodulin-dependent protein kinase II. To evaluate further the functional importance of serine 175, wild-type calponin and three site-specific mutants (S175A, S175D, and S175T) were expressed in Escherichia coli and compared with calponin purified from chicken gizzard smooth muscle in terms of actin binding, actomyosin MgATPase inhibition, and phosphorylation by protein kinase C and calmodulin-dependent protein kinase II. The affinities of skeletal muscle F-actin for wild-type and S175T calponins were similar to that for the tissue-purified protein (Kd = 0.8, 1.3, and 1.0 microM, respectively), whereas the affinities for S175A and S175D calponins were much lower (Kd = 26.8 and 44.2 microM, respectively). Tissue-purified, wild-type, and S175T calponins displayed comparable inhibition of the smooth muscle actin-activated myosin MgATPase, whereas S175A and S175D calponins were much less effective. Phosphorylation confirmed serine 175 as the principal site of phosphorylation by both kinases. These results indicate that the hydroxyl side chain at position 175 of calponin plays a critical role in the binding of calponin to actin and inhibition of the cross-bridge cycling rate.

Protein kinase C mediation of Ca(2+)-independent contractions of vascular smooth muscle

Tumour-promoting phorbol esters induce slow, sustained contractions of vascular smooth muscle, suggesting that protein kinase C (PKC) may play a role in the regulation of smooth muscle contractility. In some cases, e.g., ferret aortic smooth muscle, phorbol ester induced contractions occur without a change in [Ca2+]i or myosin phosphorylation. Direct evidence for the involvement of PKC came from the use of single saponin-permeabilized ferret aortic cells. A constitutively active catalytic fragment of PKC induced a slow, sustained contraction similar to that triggered by phenylephrine. Both responses were abolished by a peptide inhibitor of PKC. Contractions of similar magnitude occurred even when the [Ca2+] was reduced to close to zero, implicating a Ca(2+)-independent isoenzyme of PKC. Of the two Ca(2+)-independent PKC isoenzymes, epsilon and zeta, identified in ferret aorta, PKC epsilon is more likely to mediate the contractile response because (i) PKC epsilon, but not PKC zeta, is responsive to phorbol esters; (ii) upon stimulation with phenylephrine, PKC epsilon translocates from the sarcoplasm to the sarcolemma, whereas PKC zeta, translocates from a perinuclear localization to the interior of the nucleus; and (iii) when added to permeabilized single cells of the ferret aorta at pCa 9, PKC epsilon, but not PKC zeta, induced a contractile response similar to that induced by phenylephrine. A possible substrate of PKC epsilon is the smooth muscle specific, thin filament associated protein, calponin. Calponin is phosphorylated in intact smooth muscle strips in response to carbachol, endothelin-1, phorbol esters, or okadaic acid. Phosphorylation of calponin in vitro by PKC (a mixture of alpha, beta, and gamma isoenzymes) dramatically reduces its affinity for F-actin and alleviates its inhibition of the cross-bridge cycling rate. Calponin is phosphorylated in vitro by PKC epsilon but is a very poor substrate of PKC zeta. A signal transduction pathway is proposed to explain Ca(2+)-independent contraction of ferret aorta whereby extracellular signals trigger diacylglycerol production without a Ca2+ transient. The consequent activation of PKC epsilon would result in calponin phosphorylation, its release from the thin filaments, and alleviation of inhibition of cross-bridge cycling. Slow, sustained contraction then results from a slow rate of cross-bridge cycling because of the basal level of myosin light chain phosphorylation (approximately 0.1 mol Pi/mol light chain). We also suggest that signal transduction through PKC epsilon is a component of contractile responses triggered by agonists that activate phosphoinositide turnover; this may explain why smooth muscles often develop more force in response, e.g., to alpha 1-adrenergic agonists than to K+.

A calponin peptide enhances Ca2+ sensitivity of smooth muscle contraction without affecting myosin light chain phosphorylation

In permeabilized smooth muscle, exogenously applied calponin binds to myofibrils and reduces Ca(2+)-activated tension (Itoh, T., Suzuki, S., Suzuki, A., Nakamura, F., Naka, M., and Tanaka, T. (1994) Pflügers Arch. Eur. J. Physiol. 427, 301-308). A calponin peptide (calponin Phe173-Arg185), which inhibits the binding of calponin to actin, blocks the action of calponin and enhances the contraction induced by submaximal Ca2+ in permeabilized vascular smooth muscle. Unlike calmodulin, this peptide enhances the Ca(2+)-induced contraction without a corresponding increase in the level of myosin light chain phosphorylation. These results suggest that calponin decreases the sensitivity of smooth muscle to Ca2+ at a given level of myosin light chain phosphorylation.

Effect of KC399, a newly synthesized K+ channel opener, on acetylcholine-induced electrical and mechanical activities in rabbit tracheal smooth muscle

1. Effects of KC399, an opener of ATP-sensitive K+ channels were investigated on membrane potential, isometric force and intracellular Ca2+ ([Ca2+]i) mobilization induced by acetylcholine (ACh) in smooth muscle from the rabbit trachea. 2. In these smooth muscle cells, ACh (0.1 and 1 microM) depolarized the membrane in a concentration-dependent manner, KC399 (1-100 nM) hyperpolarized the membrane whether in the presence or absence of ACh. When the concentration of ACh was increased, the absolute values of the membrane potential induced by the maximum concentration of KC399 were less negative. 3. ACh (0.1 to 10 microM) concentration-dependently produced a phasic, followed by a tonic increase in both [Ca2+]i and force. KC399 (above 3 nM) lowered the resting [Ca2+]i and attenuated the ACh-induced phasic and tonic increases in [Ca2+]i and force, in a concentration-dependent manner. The magnitude of the inhibition was greater for the ACh-induced tonic responses than for the phasic ones. Nicardipine (0.3 microM), a blocker of the L-type Ca2+ channel, attenuated the ACh-induced tonic, but not phasic, increases in [Ca2+]i and force. KC399 further attenuated the ACh-induced tonic responses in the presence of nicardipine. 4. In beta-escin-skinned strips, Ca2+ (0.3-10 microM) produced a contraction in a concentration-dependent manner. KC399 (0.1 microM) had no effect on the Ca(2+)-force relationship in the presence or absence of ATP with GTP. However, at a very high concentration (1 microM), this agent slightly shifted the relationship to the right and attenuated the maximum Ca(2+)-induced contraction. 5. We conclude that, in rabbit tracheal smooth muscle, the membrane hyperpolarization induced byKC399 attenuates the ACh-induced tonic increase in [Ca2+], through an inhibition of nicardipinesensitive and -insensitive Ca2+-influxes, thus causing an inhibition of the ACh-induced tonic contraction. The ACh-induced phasic increase in [Ca2+]i and force are also inhibited, but less effectively than the tonic ones, suggesting that the action of such K+ channel openers on agonist-induced responses may be slightly different in tracheal from vascular smooth muscle.

Calponin reduces shortening velocity in skinned taenia coli smooth muscle fibres

Calponin (4.1-5.9 microM, pig stomach) inhibited maximal shortening velocity (Vmax) by 20-25% with only minor influence on force in skinned smooth muscle from guinea-pig taenia coli activated at different Ca2+ levels and with thiophosphorylation. Similar results were obtained with a fragment of the N-terminal 1-228 amino acids engineered using a mouse cDNA construct (5.4 microM). Both the native calponin and the fragment inhibited actin filament sliding in a graded manner in an in vitro motility assay. We conclude that calponin influences the kinetics of the actin-myosin interaction in the organised smooth muscle contractile system and that engineered fragments of calponin can be used to probe its action in muscle fibres. The effects can be due to an introduction of an internal load during filament sliding, possibly by decreasing the detachment rates and increasing the cross-bridge time spent in the attached state.

Characterization of the regulatory domain of gizzard calponin. Interactions of the 145-163 region with F-actin, calcium-binding proteins, and tropomyosin

Earlier, we proposed that the interaction of gizzard calponin with F-actin, promoting the inhibition of the actomyosin ATPase activity, involves the NH2-terminal portion of the calponin segment Ala145-Tyr182 (Mezgueldi, M., Fattoum, A., Derancourt, J., and Kassab, R. (1992) J. Biol. Chem. 267, 15943-15951). In this work, we have directly probed this region for actin binding sites using five peptide analogs covering different stretches of the sequence Thr133-Ile163. Co-sedimentation with F-actin, actomyosin ATPase measurements, and zero-length cross-linking reactions demonstrated that the 19-residue sequence Ala145-Ile163 is essential for actin interaction and ATPase inhibition. Furthermore, each peptide was tested for binding to the Ca(2+)-dependent proteins, caltropin and calmodulin, in both an actomyosin ATPase assay and an affinity chromatographic assay. The results revealed the 11-residue segment Gln153-Ile163, representing the COOH-terminal moiety of the F-actin binding sequence, as a crucial region for the high affinity binding of these regulatory proteins with concomitant removal of the ATPase inhibition. The 153-163 stretch contained also interactive sites for tropomyosin as assessed by affinity chromatography and spectrofluorometry. Collectively, the data support our initial results and highlight the ability of the multifunctional 145-163 region to serve as a potent regulatory domain of the smooth muscle calponin.