Calponin phosphorylation does not accompany contraction of various smooth muscles

Calponin isoforms CNN1, CNN2 and CNN3: Regulators for actin cytoskeleton functions in smooth muscle and non-muscle cells

Calponin is an actin filament-associated regulatory protein expressed in smooth muscle and many types of non-muscle cells. Three homologous genes, CNN1, CNN2 and CNN3, encoding calponin isoforms 1, 2, and 3, respectively, are present in vertebrate species. All three calponin isoforms are actin-binding proteins with functions in inhibiting actin-activated myosin ATPase and stabilizing the actin cytoskeleton, while each isoform executes different physiological roles based on their cell type-specific expressions. Calponin 1 is specifically expressed in smooth muscle cells and plays a role in fine-tuning smooth muscle contractility. Calponin 2 is expressed in both smooth muscle and non-muscle cells and regulates multiple actin cytoskeleton-based functions. Calponin 3 participates in actin cytoskeleton-based activities in embryonic development and myogenesis. Phosphorylation has been extensively studied for the regulation of calponin functions. Cytoskeleton tension regulates the transcription of CNN2 gene and the degradation of calponin 2 protein. This review summarizes our knowledge learned from studies over the past three decades, focusing on the evolutionary lineage of calponin isoform genes, their tissue- and cell type-specific expressions, structure-function relationships, and mechanoregulation.

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).

Altered expression of thin filament-associated proteins in hypertrophied urinary bladder smooth muscle

AIMS

Obstruction of the urinary bladder outlet induces detrusor smooth muscle (DSM) hypertrophy. The goal of this study was to determine whether the composition of thin filament-associated proteins, known to play important roles in cytoskeletal structure and/or the regulation of contraction, is altered in DSM during hypertrophy.

METHODS

DSM hypertrophy was induced in male rabbits by partial ligation of the urethra. Sham-operated rabbits served as a control. Reverse transcriptase-polymerase chain reaction (RT-PCR) and real-time PCR revealed a significant increase in the expression of mRNAs for basic (h1) calponin (CaP), and alpha-isoform of tropomyosin (Tm) in hypertrophied DSM compared to controls. Western blotting and two-dimensional (2-D) gel electrophoresis showed enhanced expression of these proteins and also a significant increase in the expression of beta-non muscle and gamma-smooth muscle actin in the DSM from obstructed bladders, while alpha-actin remained constant.

RESULTS

Enhanced expression of these proteins in the DSM from obstructed bladders was confirmed by immunofluorescence microscopy. Double immunostaining with Cap/Tm and alpha/beta-actin-specific antibodies showed co-localization of these proteins in myocytes. Colocalization of smooth muscle specific myosin and CaP to cytoplasmic filaments in cells dissociated from the hypertrophied DSM indicated that these cells are differentiated smooth muscle cells.

CONCLUSIONS

The change in the isoforms of actin, Cap, and Tm may be part of the molecular mechanism for bladder compensation in increased urethral resistance.

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.

Urinary bladder contraction and relaxation: physiology and pathophysiology

The detrusor smooth muscle is the main muscle component of the urinary bladder wall. Its ability to contract over a large length interval and to relax determines the bladder function during filling and micturition. These processes are regulated by several external nervous and hormonal control systems, and the detrusor contains multiple receptors and signaling pathways. Functional changes of the detrusor can be found in several clinically important conditions, e.g., lower urinary tract symptoms (LUTS) and bladder outlet obstruction. The aim of this review is to summarize and synthesize basic information and recent advances in the understanding of the properties of the detrusor smooth muscle, its contractile system, cellular signaling, membrane properties, and cellular receptors. Alterations in these systems in pathological conditions of the bladder wall are described, and some areas for future research are suggested.

Exchange of the actin-bound nucleotide in intact arterial smooth muscle

The actin-bound ADP was separated from cytoplasmic nucleotides by treatment of intact arterial smooth muscle with 50% ethanol. In (32)P-labeled smooth muscle the actin-bound ADP and phosphate readily exchanged with the cytoplasmic [gamma,beta-(32)P]ATP; the specific radioactivity of actin-bound ADP was equal to that of the beta-phosphate of cytoplasmic ATP and the specific radioactivity of actin-bound phosphate was equal to that of the gamma-phosphate of cytoplasmic ATP. In contrast, the exchange of the actin-bound ADP in skeletal muscle was very slow. The presence of cytoplasmic ATP was required for the exchange of the actin-bound ADP and phosphate; if ATP synthesis was inhibited the exchange was also inhibited. The extent of exchange was reduced in muscles contracted by histamine or K(+), as compared with resting muscles. The exchange was also shown in other mammalian smooth muscles, uterus, urinary bladder, and stomach. The data indicate a dynamic state of actin in smooth muscle. The data also suggest that polymerization-depolymerization of actin is part of the contraction-relaxation cycle 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.

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.

Reconstitution of protein kinase C-induced contractile Ca2+ sensitization in triton X-100-demembranated rabbit arterial smooth muscle

1. Triton X-100-demembranated smooth muscle loses Ca2+-sensitizing responsiveness to protein kinase C (PKC) activators while intact and alpha-toxin-permeabilized smooth muscles remain responsive. We attempted to reconstitute the contractile Ca2+ sensitization by PKC in the demembranated preparations. 2. Western blot analyses showed that the content of the PKC alpha-isoform (PKCalpha) was markedly reduced and that the smooth muscle-specific protein phosphatase-1 inhibitor protein CPI-17 was not detectable, while the amount of calponin and actin still remained similar to those of intact strips. 3. Unphosphorylated recombinant CPI-17 alone induced a small but significant contraction at constant Ca2+. Isoform-selective PKC inhibitors inhibited unphosphorylated but not pre-thiophosphorylated CPI-17-induced contraction, suggesting that in situ conventional PKC isoform(s) can phosphorylate CPI-17. 4. Exogenously replenishing PKCalpha alone did not induce potentiation of contraction and only slowly increased myosin light chain (MLC) phosphorylation at submaximal Ca2+. 5. PKC in the presence of CPI-17, but not the [T38A]-CPI mutant, markedly induced potentiation of both contraction and MLC phosphorylation. CPI-17 itself was phosphorylated. 6. In in vitro experiments, CPI-17 was a much better substrate for PKCalpha than calponin, caldesmon, MLC and myosin. 7. Our results indicate that PKC requires CPI-17 phosphorylation at Thr-38 but not calponin for reconstitution of the contractile Ca2+ sensitization in the demembranated arterial smooth muscle.

Activation of protein kinases in canine basilar artery in vasospasm

Subarachnoid hemorrhage (SAH) often leads to a long-term narrowing of cerebra! artery called vasospasm. To understand the molecular mechanisms in vasospasm, signal transduction of tyrosine kinase pathway and phosphorylation of myosin light chain (MLC) and calponin (CaP) in the basilar artery were studied. Vasospasm was produced in the canine basilar artery by a two-hemorrhage method, and vasocontraction was induced by a local application of KCI or serotonin to the basilar artery after a transclival exposure. Intracellular substrates of tyrosine kinase pathway, including Shc, Rafl, and extracellular-regulated kinases in the basilar artery, were activated after SAH, and the activation of Shc suggests stimulation of signal transductions from tyrosine kinase receptors, G-coupled receptors, or both. The activation of tyrosine kinase pathway in vasospasm also was supported by dose-dependent dilation of the spastic basilar artery on days 0 and 7 by topical application of genistein, a tyrosine kinase inhibitor, and associated marked inhibition of tyrosine phosphorylation of intracellular substrates, including Shc. In addition, the generation of protein kinase M, catalytic fragment of protein kinase C(alpha) (PKC alpha), in vasospasm on days 0 and 7 was inhibited in response to genistein, indicating an inactivation of mu-calpain. It is suggested, therefore, that the reversal of vasospasm by genistein is closely associated with the restoration of intracellular Ca2+ levels. However, the increased activities of Raf1 and extracellular-regulated kinases in vasospasm were declined on day 7 compared with those on day 0 or 2, suggesting that the activation of tyrosine kinase pathway is more closely associated with the early stage of vasospasm than with the late stage of vasospasm. The analysis by pyrophosphate polyacrylamide gel electrophoresis (PPi-PAGE) demonstrated three MLC bands in vasospasm on days 2 and 7, as well as in KCI- and serotonin-induced vasocontraction. Since PPi-PAGE resolves smooth muscle MLC into three bands in the MLC kinase (MLCK)-mediated phosphorylation and into a single band in the PKC-mediated phosphorylation based on the phosphorylation state, the current results suggest that MLC in vasospasm is phosphorylated by MLCK but not by PKC. In basilar artery, CaP was significantly down-regulated, and in addition, significantly phosphorylated on serine and threonine residues only in vasospasm on days 2 and 7. Although the significance of CaP phosphorylations in vivo still is controversial, CaP down-regulation and phosphorylation may attenuate the inhibition of Mg(2+)-ATPase activity by CaP and induce a potential enhancement of smooth muscle contractility in delayed vasospasm. Since CaP is phosphorylated in vivo by PKC, activated PKC in vasospasm may phosphorylate CaP. Thus, SAH stimulates tyrosine kinase pathway to increase intracellular Ca2+ and activate PKC, and the former activates MLCK to phosphorylate MLC, whereas the latter phosphorylates CaP but not MLC.

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.

PKC-dependent signalling mechanisms in differentiated smooth muscle

Protein kinase C (PKC) is now known to play an important physiological role in essentially all cell types. This review will focus on what is known about the kinase in contractile differentiated smooth muscle. Current knowledge on the molecular structure of PKC isoforms will be discussed as they relate to mechanisms of translocation and targeting of the kinase within smooth muscle cells. Studies performed on PKC-dependent signalling pathways in differentiated smooth muscle cells will be discussed with emphasis on studies form our laboratory, especially discussing thin filament linked pathways. Thick filament linked PKC-dependent pathways will be described in more detail elsewhere in this monograph.

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.

Regulatory mechanisms of calcium sensitization of contractile elements in smooth muscle

It is evident that smooth muscle contraction is regulated not only by the Ca2+/calmodulin/myosin light chain kinase system but also by modulation of Ca2+ sensitivity. Changes in free calmodulin concentrations, myosin light chain phosphorylation elicited by rho/rho-kinase, regulation of myosin phosphatase activity and thin filament-linked mechanisms are the possible mechanisms for regulation of Ca2+ sensitivity.

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.

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.

Effects of calponin on isometric force and shortening velocity in permeabilized taenia coli smooth muscle

Calponin, a thin filament-associated protein, inhibits actomyosin adenosinetriphosphatase in solution and has been suggested to modulate smooth muscle contractility. We used permeabilized guinea pig taenia coli smooth muscle to investigate whether calponin can modulate actin-myosin interaction in a more organized contractile system. Fibers were permeabilized with Triton X-100 and glycerol, which permit access of large macromolecules to the contractile apparatus. For contractures elicited by Ca2+ (6.6 microM + 0.1 microM calmodulin), the recombinant alpha-isoform of chicken gizzard calponin (CaP) decreased isometric force (Fo) and unloaded shortening velocity (Vus) in a dose-dependent manner; 1 microM CaP had minimal effects on force (< 10%) but reduced Vus by approximately 50% and 10 microM CaP reduced Fo to 27% of control and Vus to near zero levels. To eliminate any effects of the binding of calmodulin by CaP and consequent inhibition of myosin light chain kinase activity, we also studied fibers activated by thiophosphorylation of the myosin regulatory light chain. Fo was only moderately inhibited, remaining at approximately 75% of control in the presence of CaP (10 microM), whereas Vus was reduced to 32% of control. A similar inhibition was obtained with a mutant (CaPcys175) that retains the ability to bind to actin. CaP phosphorylated by protein kinase C and CaPcys175 mutant labeled with 1,5-IAEDANS, which bind actin poorly, were not effective inhibitors. Our results indicate that 1) CaP more strongly inhibits Vus (approximately cross-bridge cycle rate) than Fo (approximately number of activated cross bridges) and 2) the effects of CaP are related to its binding to actin. Thus the function of CaP in regulation of smooth muscle contractility may be more strongly related to its function as a modulator of velocity, as related to the "latch state," than as an "on-off" switch.

Expression and epitopic conservation of calponin in different smooth muscles and during development

Calponin is a thin filament associated protein found in smooth muscle as a potential modulator of contraction. Five mouse monoclonal antibodies (mAbs CP1, CP3, CP4, CP7, and CP8) were prepared against chicken gizzard alpha-calponin. The CP1 epitopic structure is conserved in smooth muscles across vertebrate phyla and is highly sensitive to CNBr cleavage in contrast with the chicken-specific CP4 and the avian-mammalian-specific CP8 epitopes that are resistant to CNBr fragmentation. Using this panel of mAbs against multiple epitopes, only alpha-calponin was detected in adult chicken smooth muscles and throughout development of the gizzard. Western blotting showed that the calponin content varied among different smooth muscle tissues and correlated with that of h-caldesmon. In contrast with the constitutive expression of calponin in phasic smooth muscle of the digestive tract, very low levels of calponin were detected in adult avian tracheas and no calponin expression was detected in embryonic and young chick tracheas. These results provide information on the structural conservation of calponins and suggest a relationship between calponin expression and smooth muscle functional states.

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+.

Physical characterization of calponin. A circular dichroism, analytical ultracentrifuge, and electron microscopy study

Calponin is a thin filament-associated smooth muscle protein that has been suggested to play a role in the regulation of smooth muscle contraction. We have used circular dichroism spectroscopy, electron microscopy, and analytical ultracentrifugation to study the physical properties of recombinant chicken gizzard alpha-calponin. The alpha-helix content of alpha-calponin was estimated from its circular dichroism spectrum to be approximately 13%, alpha-Calponin melts with a single sharp transition at approximately 57 degrees C. Rotary shadowing electron micrographs of alpha-calponin reveal diverse shapes ranging from elongated rods to collapsed coils. The lengths of the rod-shaped structures are approximately 18 nm. Analytical ultracentrifugation studies found alpha-calponin to be homogeneous with a monomer molecular mass of 31.4 kDa, and a s20,w value of 2.34 S. These data could be used to model alpha-calponin as a prolate ellipsoid of revolution with an axial ratio of 6.16, a length of 16.2 nm, and a diameter of 2.6 nm. Taken together, our results indicate that calponin is a flexible, elongated molecule whose contour length is sufficient to span three actin subunits along the long pitch helix of an F-actin filament.

Polyacrylamide gel electrophoretic methods in the separation of structural muscle proteins

Polyacrylamide gel electrophoresis plays a major role in analyzing the function of muscle structural proteins. This review describes one- and two-dimensional gel electrophoretic methods for qualitative and quantitative investigation of the muscle proteins, with special emphasis on determination of protein phosphorylation. The electrophoretic studies established the subunit structures of the muscle proteins, characterized their multiple forms, revealed changes in subunit composition or shifts in isoform distribution of specific proteins during development, upon stimulation or denervation of the muscle. Protein phosphorylation during muscle contraction is preferentially studied by two-dimensional gel electrophoresis. The same method demonstrated protein alterations in human neuromuscular diseases.

Two domains of interaction with calcium binding proteins can be mapped using fragments of calponin

Native calponin is able to bind 2 mol of calcium binding protein (CaBP) per mole calponin. This study extends this observation to define the 2 domains of interaction, one of which is near the actin binding site, and the other in the amino-terminal region of calponin. Also, the first evidence for a differentiation in the response of calponin to interaction with caltropin versus calmodulin is demonstrated. The binding of caltropin to cleavage and recombinant fragments of calponin was determined by 3 techniques: tryptophan fluorescence of the fragments, CD measurements to determine secondary structure changes, and analytical ultracentrifugation. In order to delineate the sites of interaction, 3 fragments of calponin have been studied. From a cyanogen bromide cleavage of calponin, residues 2-51 were isolated. This fragment is shown to bind to CaBPs and the affinity for caltropin is slightly higher than that for calmodulin. A carboxyl-terminal truncated mutant of calponin comprising residues 1-228 (CP 1-228) has been produced by recombinant techniques. Analytical ultracentrifugation has shown that CP 1-228, like the parent calponin, is able to bind 2 mol of caltropin per mol of 1-228 in a Ca(2+)-dependent fashion, indicating that there is a second site of interaction between residues 52-228. Temperature denaturation of the carboxyl-terminal truncated fragment compared with whole calponin show that the carboxyl-terminal region does not change the temperature at which calponin melts; however, there is greater residual secondary structure with whole calponin versus the fragment. A second mutant produced through recombinant techniques comprises residues 45-228 and is also able to bind caltropin, thus mapping the location of the second site of interaction to near the actin binding site.

Signal transduction in vascular smooth muscle: diacylglycerol second messengers and PKC action

Agonist-stimulated phospholipid turnover can generate diacylglycerol (DAG), an intracellular second messenger that activates protein kinase C (PKC). DAG can be produced from the hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP2) by a phosphoinositide-specific phospholipase C and by the degradation of phosphatidylcholine (PC) by a phospholipase C or the concerted actions of phospholipase D and phosphatidate phosphohydrolase. In vascular smooth muscle, agonist-stimulated DAG accumulation is biphasic; PIP2 hydrolysis produces a transient increase in DAG, which is followed by a sustained phase of DAG accumulation from PC degradation. Metabolism of DAG attenuates PKC activation and thus results in signal termination. The metabolic fates for DAG include 1) ATP-dependent phosphorylation to form phosphatidic acid (DAG kinase), 2) hydrolysis to release fatty acids and glycerol (DAG and monoacylglycerol lipases), 3) synthesis of triacylglycerol (DAG acyltransferase), and 4) synthesis of PC (choline phosphotransferase). Hydrolysis through the lipase pathway is the predominant metabolic fate of DAG in vascular smooth muscle. Activation of PKC in vascular smooth muscle modulates agonist-stimulated phospholipid turnover, produces an increase in contractile force, and regulates cell growth and proliferation. Further research is required to investigate cross talk between signal transduction mechanisms involving lipid second messengers. In addition, spatial considerations such as nuclear PKC activation and the influence of diradylglycerol generation on the duration of PKC activation are important issues.

The biochemical basis of the regulation of smooth-muscle contraction

The primary signal for smooth-muscle contraction is an increase in sarcoplasmic free Ca2+ concentration ([Ca2+]i). This triggers activation of calmodulin-dependent myosin light-chain kinase, which catalyses myosin phosphorylation, thereby activating crossbridge cycling and the development of force or contraction of the muscle cell. Restoration of resting [Ca2+]i deactivates the kinase; myosin is dephosphorylated by myosin light-chain phosphatase and the muscle relaxes. Recent evidence suggests that other signal-transduction pathways can modulate the contractile state of a smooth-muscle cell by affecting specific steps in the myosin phosphorylation-dephosphorylation mechanism.