The effects of smooth muscle caldesmon on actin filament motility

Functional and Structural Properties of Cytoplasmic Tropomyosin Isoforms Tpm1.8 and Tpm1.9

The actin cytoskeleton is one of the most important players in cell motility, adhesion, division, and functioning. The regulation of specific microfilament formation largely determines cellular functions. The main actin-binding protein in animal cells is tropomyosin (Tpm). The unique structural and functional diversity of microfilaments is achieved through the diversity of Tpm isoforms. In our work, we studied the properties of the cytoplasmic isoforms Tpm1.8 and Tpm1.9. The results showed that these isoforms are highly thermostable and differ in the stability of their central and C-terminal fragments. The properties of these isoforms were largely determined by the 6th exons. Thus, the strength of the end-to-end interactions, as well as the affinity of the Tpm molecule for F-actin, differed between the Tpm1.8 and Tpm1.9 isoforms. They were determined by whether an alternative internal exon, 6a or 6b, was included in the Tpm isoform structure. The strong interactions of the Tpm1.8 and Tpm1.9 isoforms with F-actin led to the formation of rigid actin filaments, the stiffness of which was measured using an optical trap. It is quite possible that the structural and functional features of the Tpm isoforms largely determine the appearance of these isoforms in the rigid actin structures of the cell cortex.

Acidosis modifies effects of phosphorylated tropomyosin on the actin-myosin interaction in the myocardium

Phosphorylation of α-tropomyosin (Tpm1.1), a predominant Tpm isoform in the myocardium, is one of the regulatory mechanisms of the heart contractility. The Tpm 1.1 molecule has one site of phosphorylation, Ser283. The degree of the Tpm phosphorylation decreases with age and also changes in heart pathologies. Myocardial pathologies, in particular ischemia, are usually accompanied by pH lowering in the cardiomyocyte cytosol. We studied the effects of acidosis on the structural and functional properties of the pseudo-phosphorylated form of Tpm1.1 with the S283D substitution. We found that in acidosis, the interaction of the N- and C-ends of the S283D Tpm molecules decreases, whereas that of WT Tpm does not change. The pH lowering increased thermostability of the complex of F-actin with S283D Tpm to a greater extent than with WT Tpm. Using an in vitro motility assay with NEM- modified myosin as a load, we assessed the effect of the Tpm pseudo-phosphorylation on the force of the actin-myosin interaction. In acidosis, the force generated by myosin in the interaction with thin filaments containing S283D Tpm was higher than with those containing WT Tpm. Also, the pseudo-phosphorylation increased the myosin ability to resist a load. We conclude that ischemia changes the effect of the phosphorylated Tpm on the contractile function of the myocardium.

The properties of the actin-myosin interaction in the heart muscle depend on the isoforms of myosin but not of α-actin

In myocardium of mammals there are two isoforms of myosin heavy chains, α and β. In ventricle, together with ventricular isoforms of light chains they form two isomyosins: V1 and V3, homodimers of α- and β-heavy chains, respectively. In atria, α- and β-heavy chains together with atrial light chains form A1 (αα) and A2 (ββ) isomyosins. Besides in myocardium two isoforms of α-actin, skeletal and cardiac, are expressed. We assume that the differences in the amino acid sequence of cardiac and skeletal actin may affect its interaction with myosin. To test this hypothesis, we investigated characteristics of actin-myosin interactions of cardiac and skeletal isoforms of α-actin with the isoforms of cardiac myosin using an optical trap technique and an in vitro motility assay. It was found that the mechanical and kinetic characteristics of the interactions of the isoforms of cardiac myosin with actin depend on the isoforms of myosin not α-actin.

Amino acid mutations in the caldesmon COOH-terminal functional domain increase force generation in bladder smooth muscle

Caldesmon (CaD), a component of smooth muscle thin filaments, binds actin, tropomyosin, calmodulin, and myosin and inhibits actin-activated ATP hydrolysis by smooth muscle myosin. Internal deletions of the chicken CaD functional domain that spans from amino acids (aa) 718 to 731, which corresponds to aa 512-530 including the adjacent aa sequence in mouse CaD, lead to diminished CaD-induced inhibition of actin-activated ATP hydrolysis by myosin. Transgenic mice with mutations of five aa residues (Lys(523) to Gln, Val(524) to Leu, Ser(526) to Thr, Pro(527) to Cys, and Lys(529) to Ser), which encompass the ATPase inhibitory determinants located in exon 12, were generated by homologous recombination. Homozygous (-/-) animals did not develop, but heterozygous (+/-) mice carrying the expected mutations in the CaD ATPase inhibitory domain (CaD mutant) matured and reproduced normally. The peak force produced in response to KCl and electrical field stimulation by the detrusor smooth muscle from the CaD mutant was high compared with that of the wild type. CaD mutant mice revealed nonvoiding contractions during bladder filling on awake cystometry, suggesting that the CaD ATPase inhibitory domain suppresses force generation during the filling phase and this suppression is partially released by mutations in 50% of CaD in heterozygous. Our data show for the first time a functional phenotype, at the intact smooth muscle tissue and in vivo organ levels, following mutation of a functional domain at the COOH-terminal region of CaD.

The molecular basis of frictional loads in the in vitro motility assay with applications to the study of the loaded mechanochemistry of molecular motors

Molecular motors convert chemical energy into mechanical movement, generating forces necessary to accomplish an array of cellular functions. Since molecular motors generate force, they typically work under loaded conditions where the motor mechanochemistry is altered by the presence of a load. Several biophysical techniques have been developed to study the loaded behavior and force generating capabilities of molecular motors yet most of these techniques require specialized equipment. The frictional loading assay is a modification to the in vitro motility assay that can be performed on a standard epifluorescence microscope, permitting the high-throughput measurement of the loaded mechanochemistry of molecular motors. Here, we describe a model for the molecular basis of the frictional loading assay by modeling the load as a series of either elastic or viscoelastic elements. The model, which calculates the frictional loads imposed by different binding proteins, permits the measurement of isotonic kinetics, force-velocity relationships, and power curves in the motility assay. We show computationally and experimentally that the frictional load imposed by alpha-actinin, the most widely employed actin binding protein in frictional loading experiments, behaves as a viscoelastic rather than purely elastic load. As a test of the model, we examined the frictional loading behavior of rabbit skeletal muscle myosin under normal and fatigue-like conditions using alpha-actinin as a load. We found that, consistent with fiber studies, fatigue-like conditions cause reductions in myosin isometric force, unloaded sliding velocity, maximal power output, and shift the load at which peak power output occurs.

Myosin, transgelin, and myosin light chain kinase: expression and function in asthma

RATIONALE

Airway smooth muscle (SM) of patients with asthma exhibits a greater velocity of shortening (Vmax) than that of normal subjects, and this is thought to contribute to airway hyperresponsiveness. A greater Vmax can result from increased myosin activation. This has been reported in sensitized human airway SM and in models of asthma. A faster Vmax can also result from the expression of specific contractile proteins that promote faster cross-bridge cycling. This possibility has never been addressed in asthma.

OBJECTIVES

We tested the hypothesis that the expression of genes coding for SM contractile proteins is altered in asthmatic airways and contributes to their increased Vmax.

METHODS

We quantified the expression of several genes that code for SM contractile proteins in mild allergic asthmatic and control human airway endobronchial biopsies. The function of these contractile proteins was tested using the in vitro motility assay.

MEASUREMENTS AND MAIN RESULTS

We observed an increased expression of the fast myosin heavy chain isoform, transgelin, and myosin light chain kinase in patients with asthma. Immunohistochemistry demonstrated the expression of these genes at the protein level. To address the functional significance of this overexpression, we purified tracheal myosin from the hyperresponsive Fisher rats, which also overexpress the fast myosin heavy chain isoform as compared with the normoresponsive Lewis rats, and found a faster rate of actin filament propulsion. Conversely, transgelin did not alter the rate of actin filament propulsion.

CONCLUSIONS

Selective overexpression of airway smooth muscle genes in asthmatic airways leads to increased Vmax, thus contributing to the airway hyperresponsiveness observed in asthma.

Modulation of actin mechanics by caldesmon and tropomyosin

The ability of cells to sense and respond to physiological forces relies on the actin cytoskeleton, a dynamic structure that can directly convert forces into biochemical signals. Because of the association of muscle actin-binding proteins (ABPs) may affect F-actin and hence cytoskeleton mechanics, we investigated the effects of several ABPs on the mechanical properties of the actin filaments. The structural interactions between ABPs and helical actin filaments can vary between interstrand interactions that bridge azimuthally adjacent actin monomers between filament strands (i.e. by molecular stapling as proposed for caldesmon) or, intrastrand interactions that reinforce axially adjacent actin monomers along strands (i.e. as in the interaction of tropomyosin with actin). Here, we analyzed thermally driven fluctuations in actin's shape to measure the flexural rigidity of actin filaments with different ABPs bound. We show that the binding of phalloidin increases the persistence length of actin by 1.9-fold. Similarly, the intrastrand reinforcement by smooth and skeletal muscle tropomyosins increases the persistence length 1.5- and 2- fold respectively. We also show that the interstrand crosslinking by the C-terminal actin-binding fragment of caldesmon, H32K, increases persistence length by 1.6-fold. While still remaining bound to actin, phosphorylation of H32K by ERK abolishes the molecular staple (Foster et al. 2004. J Biol Chem 279;53387-53394) and reduces filament rigidity to that of actin with no ABPs bound. Lastly, we show that the effect of binding both smooth muscle tropomyosin and H32K is not additive. The combination of structural and mechanical studies on ABP-actin interactions will help provide information about the biophysical mechanism of force transduction in cells.

Caldesmon is necessary for maintaining the actin and intermediate filaments in cultured bladder smooth muscle cells

Caldesmon (CaD), a component of microfilaments in all cells and thin filaments in smooth muscle cells, is known to bind to actin, tropomyosin, calmodulin, and myosin and to inhibit actin-activated ATP hydrolysis by smooth muscle myosin. Thus, it is believed to regulate smooth muscle contraction, cell motility and the cytoskeletal structure. Using bladder smooth muscle cell cultures and RNA interference (RNAi) technique, we show that the organization of actin into microfilaments in the cytoskeleton is diminished by siRNA-mediated CaD silencing. CaD silencing significantly decreased the amount of polymerized actin (F-actin), but the expression of actin was not altered. Additionally, we find that CaD is associated with 10 nm intermediate-sized filaments (IF) and in vitro binding assay reveals that it binds to vimentin and desmin proteins. Assembly of vimentin and desmin into IF is also affected by CaD silencing, although their expression is not significantly altered when CaD is silenced. Electronmicroscopic analyses of the siRNA-treated cells showed the presence of myosin filaments and a few surrounding actin filaments, but the distribution of microfilament bundles was sparse. Interestingly, the decrease in CaD expression had no effect on tubulin expression and distribution of microtubules in these cells. These results demonstrate that CaD is necessary for the maintenance of actin microfilaments and intermediate-sized filaments in the cytoskeletal structure. This finding raises the possibility that the cytoskeletal structure in smooth muscle is affected when CaD expression is altered, as in smooth muscle de-differentiation and hypertrophy seen in certain pathological conditions.

Caldesmon and the regulation of cytoskeletal functions

Caldesmon (CaD) is an extraordinary actin-binding protein, because in addition to actin, it also bindsmyosin, calmodulin and tropomyosin. As a component of the smoothmuscle and nonmuscle contractile apparatus CaD inhibits the actomyosin ATPase activity and its inhibitory action is modulated by both Ca2+ and phosphorylation. The multiplicity of binding partners and diverse biochemical properties suggest CaD is a potent and versatile regulatory protein both in contractility and cell motility. However, after decades ofinvestigation in numerous laboratories, hard evidence is still lacking to unequivocally identify its in vivo functions, although indirect evidence is mounting to support an important role in connection with the actin cytoskeleton. This chapter reviews the highlights of the past findings and summarizes the current views on this protein, with emphasis of its interaction with tropomyosin.

Caldesmon inhibits the actin–myosin interaction by changing its spatial orientation and mobility during the ATPase activity cycle

Orientation and mobility of acrylodan fluorescent probe specifically bound to caldesmon Cys580 incorporated into muscle ghost fibers decorated with myosin S1 and containing tropomyosin was studied in the presence or absence of MgADP, MgAMP-PNP, MgATPgammaS or MgATP. Modeling of various intermediate states of actomyosin has shown discrete changes in orientation and mobility of the dye dipoles which is the evidence for multistep changes in the structural changes of caldesmon during the ATPase hydrolysis cycle. It is suggested that S1 interaction with actin results in nucleotide-dependent displacement of the C-terminal part of caldesmon molecule and changes in its mobility. Thus inhibition of the actomyosin ATPase activity may be due to changes in caldesmon position on the thin filament and its interaction with actin. Our new findings described in the present paper as well as those published recently elsewhere might conciliate the two existing models of molecular mechanism of inhibition of the actomyosin ATPase by caldesmon.

Bronchospasm and its biophysical basis in airway smooth muscle

Airways hyperresponsiveness is a cardinal feature of asthma but remains unexplained. In asthma, the airway smooth muscle cell is the key end-effector of bronchospasm and acute airway narrowing, but in just the past five years our understanding of the relationship of responsiveness to muscle biophysics has dramatically changed. It has become well established, for example, that muscle length is equilibrated dynamically rather than statically, and that non-classical features of muscle biophysics come to the forefront, including unanticipated interactions between the muscle and its time-varying load, as well as the ability of the muscle cell to adapt rapidly to changes in its dynamic microenvironment. These newly discovered phenomena have been described empirically, but a mechanistic basis to explain them is only beginning to emerge.

Macrophage caldesmon is an actin bundling protein

A rapid purification procedure was developed for the isolation of caldesmon (CaD) from rabbit alveolar macrophage. The purified protein migrated with an apparent M(r) of 74,000 +/- 4000 on SDS-PAGE and cross-reacted with anti-gizzard CaD antibodies. A higher M(r) isoform was isolated from chicken gizzard. Their actin-binding parameters and effects on actomyosin-ATPase activity were investigated under identical experimental conditions. Electron microscope studies revealed that macrophage CaD was able to cross-link actin filaments into both networks and bundles. Compact F-actin bundles were predominantly or exclusively seen at cross-linker to actin molar ratios in the 1:20 to 1:10 range. Apparent K(a) at extrapolated saturation of the CaD-binding sites on F-actin was 1.2 x 10(6) M(-1) for macrophage CaD and 1.6 x 10(6) M(-1) for chicken gizzard CaD. CaD from either source was able to stimulate the actin-activated ATPase activity of macrophage myosin. Unexpectedly, chicken gizzard CaD also increased the ATPase activity of gizzard myosin. The degree of stimulation was approximately doubled in the presence of a large excess of Ca(2+)-calmodulin but was unaffected by the presence of macrophage tropomyosin. However, macrophage CaD did not behave as a Ca(2+)- and calmodulin-regulated actin-binding protein. These results, together with published data on other well-characterized actin bundling proteins, suggest that nonmuscle CaD could be essentially involved in the formation and organization of actin bundles at adhesion sites and cell surface projections. However, they afforded no evidence that the macrophage isoform might play a specific role in the Ca(2+)-dependent regulation of actin and myosin II interactions.

Casein kinase II phosphorylation of caldesmon downregulates myosin-caldesmon interactions

It is well-known that caldesmon (CaD) is a substrate for casein kinase II (CKII), and the phosphorylation of CaD by CKII regulates the interaction of CaD with myosin. However, the functionally relevant CKII phosphorylation site(s) on CaD and the precise role of CaD phosphorylation by CKII in mediating CaD's function have remained elusive. In this study, we demonstrate that Ser-26 is the major CKII phosphorylation site on CaD, while Ser-73 is of relatively minor importance. Moreover, the phosphorylation of Ser-26 and Ser-73 reduced CaD's ability to bind myosin by 45% and 27%, respectively, suggesting that the interaction of CaD with myosin is downregulated, at least in part, by the phosphorylation of these serine residues by CKII. Our results also demonstrate that there are at least four myosin-binding motifs within the amino-terminal region of CaD, located between residues 1-23, 34-43, 44-53, and 86-115, respectively. The myosin-binding motif between residues 44-53 contributes to strong myosin binding, while the three other myosin-binding motifs are responsible for weak myosin binding. The sequences between residues 24-33 and 54-85 on CaD are not required for the binding of CaD to myosin; thus, both Ser-26 and Ser-73 are located outside of the myosin-binding motifs. It is therefore likely that the downregulation of myosin-CaD interactions by CKII phosphorylation is due to phosphorylation-induced conformational changes in the adjacent myosin-binding motifs on CaD, rather than by the direct modification of these myosin-binding motifs by CKII.

The major myosin-binding site of caldesmon resides near its N-terminal extreme

The primary myosin-binding site of caldesmon was thought to be in the N-terminal region of the molecule, but the exact nature of the caldesmon-myosin interaction has not been well characterized. A caldesmon fragment that encompasses residues 1-240 (N240) was found to bind full-length smooth muscle myosin on the basis of co-sedimentation experiments. The interaction between myosin and N240 was not affected by phosphorylation of myosin, but it was weakened by the presence of Ca(2+)/calmodulin. To locate the myosin-binding site, we have designed several synthetic peptides based on the N-terminal caldesmon sequence. We found that a peptide stretch corresponding to the first 27 residues (Met-1 to Tyr-27), but not that of the first 22 residues (Met-1 to Ala-22), exhibited a moderate affinity toward myosin. We also found that a peptide containing the segment from Ile/Leu-25 to Lys-53 bound both myosin and heavy meromyosin more strongly and was capable of displacing caldesmon from myosin. Our results demonstrate that the sequence near the N-terminal extreme of caldesmon harbors a major myosin-binding site of caldesmon, in which both the nonpolar residues and clusters of positively and negatively charged residues confer the specificity and affinity of the caldesmon-myosin interaction.

Phosphorylation of caldesmon by ERK MAP kinases in smooth muscle

Phosphorylation of h-caldesmon has been proposed to regulate airway smooth muscle contraction. Both extracellular signal-regulated kinase (ERK) and p38 mitogen-activated protein (MAP) kinases phosphorylate h-caldesmon in vitro. To determine whether both enzymes phosphorylate caldesmon in vivo, phosphorylation-site-selective antibodies were used to assay phosphorylation of MAP kinase consensus sites. Stimulation of cultured tracheal smooth muscle cells with ACh or platelet-derived growth factor increased caldesmon phosphorylation at Ser789 by about twofold. Inhibiting ERK MAP kinase activation with 50 microM PD-98059 blocked agonist-induced caldesmon phosphorylation completely. Inhibiting p38 MAP kinases with 25 microM SB-203580 had no effect on ACh-induced caldesmon phosphorylation. Carbachol stimulation increased caldesmon phosphorylation at Ser789 in intact tracheal smooth muscle, which was blocked by the M(2) antagonist AF-DX 116 (1 microM). AF-DX 116 inhibited carbachol-induced isometric contraction by 15 +/- 1.4%, thus dissociating caldesmon phosphorylation from contraction. Activation of M(2) receptors leads to activation of ERK MAP kinases and phosphorylation of caldesmon with little or no functional effect on isometric force. P38 MAP kinases are also activated by muscarinic agonists, but they do not phosphorylate caldesmon in vivo.

Regulation of contraction in striated muscle

Ca(2+) regulation of contraction in vertebrate striated muscle is exerted primarily through effects on the thin filament, which regulate strong cross-bridge binding to actin. Structural and biochemical studies suggest that the position of tropomyosin (Tm) and troponin (Tn) on the thin filament determines the interaction of myosin with the binding sites on actin. These binding sites can be characterized as blocked (unable to bind to cross bridges), closed (able to weakly bind cross bridges), or open (able to bind cross bridges so that they subsequently isomerize to become strongly bound and release ATP hydrolysis products). Flexibility of the Tm may allow variability in actin (A) affinity for myosin along the thin filament other than through a single 7 actin:1 tropomyosin:1 troponin (A(7)TmTn) regulatory unit. Tm position on the actin filament is regulated by the occupancy of NH-terminal Ca(2+) binding sites on TnC, conformational changes resulting from Ca(2+) binding, and changes in the interactions among Tn, Tm, and actin and as well as by strong S1 binding to actin. Ca(2+) binding to TnC enhances TnC-TnI interaction, weakens TnI attachment to its binding sites on 1-2 actins of the regulatory unit, increases Tm movement over the actin surface, and exposes myosin-binding sites on actin previously blocked by Tm. Adjacent Tm are coupled in their overlap regions where Tm movement is also controlled by interactions with TnT. TnT also interacts with TnC-TnI in a Ca(2+)-dependent manner. All these interactions may vary with the different protein isoforms. The movement of Tm over the actin surface increases the "open" probability of myosin binding sites on actins so that some are in the open configuration available for myosin binding and cross-bridge isomerization to strong binding, force-producing states. In skeletal muscle, strong binding of cycling cross bridges promotes additional Tm movement. This movement effectively stabilizes Tm in the open position and allows cooperative activation of additional actins in that and possibly neighboring A(7)TmTn regulatory units. The structural and biochemical findings support the physiological observations of steady-state and transient mechanical behavior. Physiological studies suggest the following. 1) Ca(2+) binding to Tn/Tm exposes sites on actin to which myosin can bind. 2) Ca(2+) regulates the strong binding of M.ADP.P(i) to actin, which precedes the production of force (and/or shortening) and release of hydrolysis products. 3) The initial rate of force development depends mostly on the extent of Ca(2+) activation of the thin filament and myosin kinetic properties but depends little on the initial force level. 4) A small number of strongly attached cross bridges within an A(7)TmTn regulatory unit can activate the actins in one unit and perhaps those in neighboring units. This results in additional myosin binding and isomerization to strongly bound states and force production. 5) The rates of the product release steps per se (as indicated by the unloaded shortening velocity) early in shortening are largely independent of the extent of thin filament activation ([Ca(2+)]) beyond a given baseline level. However, with a greater extent of shortening, the rates depend on the activation level. 6) The cooperativity between neighboring regulatory units contributes to the activation by strong cross bridges of steady-state force but does not affect the rate of force development. 7) Strongly attached, cycling cross bridges can delay relaxation in skeletal muscle in a cooperative manner. 8) Strongly attached and cycling cross bridges can enhance Ca(2+) binding to cardiac TnC, but influence skeletal TnC to a lesser extent. 9) Different Tn subunit isoforms can modulate the cross-bridge detachment rate as shown by studies with mutant regulatory proteins in myotubes and in in vitro motility assays. (ABSTRACT TRUNCATED)

Caldesmon inhibits nonmuscle cell contractility and interferes with the formation of focal adhesions

Caldesmon is known to inhibit the ATPase activity of actomyosin in a Ca(2+)-calmodulin-regulated manner. Although a nonmuscle isoform of caldesmon is widely expressed, its functional role has not yet been elucidated. We studied the effects of nonmuscle caldesmon on cellular contractility, actin cytoskeletal organization, and the formation of focal adhesions in fibroblasts. Transient transfection of nonmuscle caldesmon prevents myosin II-dependent cell contractility and induces a decrease in the number and size of tyrosine-phosphorylated focal adhesions. Expression of caldesmon interferes with Rho A-V14-mediated formation of focal adhesions and stress fibers as well as with formation of focal adhesions induced by microtubule disruption. This inhibitory effect depends on the actin- and myosin-binding regions of caldesmon, because a truncated variant lacking both of these regions is inactive. The effects of caldesmon are blocked by the ionophore A23187, thapsigargin, and membrane depolarization, presumably because of the ability of Ca(2+)-calmodulin or Ca(2+)-S100 proteins to antagonize the inhibitory function of caldesmon on actomyosin contraction. These results indicate a role for nonmuscle caldesmon in the physiological regulation of actomyosin contractility and adhesion-dependent signaling and further demonstrate the involvement of contractility in focal adhesion formation.

cGMP modulation of Ca2+ sensitivity in airway smooth muscle

A beta-escin-permeabilized canine tracheal smooth muscle preparation was used to test the hypothesis that cGMP decreases Ca2+ sensitivity in airway smooth muscle primarily by inhibiting the membrane receptor-coupled mechanisms that regulate Ca2+ sensitivity and not by inhibiting Ca2+/calmodulin activation of the contractile proteins. 8-Bromo-cGMP (100 microM) had no effect on the free Ca2+ concentration-response curves generated in the absence of muscarinic receptor stimulation. In the presence of 100 microM ACh plus 10 microM GTP, 8-bromo-cGMP (100 microM) caused a rightward shift of the free Ca2+ concentration-response curve, significantly increasing the EC50 for free Ca2+ from 0.35 +/- 0.03 to 0.75 +/- 0.06 microM; this effect of 8-bromo-cGMP was concentration dependent from 1 to 100 microM. 8-Bromo-cGMP (100 microM) decreased the level of regulatory myosin light chain (rMLC) phosphorylation for a given cytosolic Ca2+ concentration but had no effect on the amount of isometric force produced for a given level of rMLC phosphorylation. These findings suggest that cGMP decreases Ca2+ sensitivity in canine tracheal smooth muscle primarily by inhibiting the membrane receptor-coupled mechanisms that modulate the relationship between cytosolic Ca2+ concentration and rMLC phosphorylation.

Caldesmon: binding to actin and myosin and effects on elementary steps in the ATPase cycle

The actin binding protein caldesmon inhibits the actin-activation of myosin ATPase activity. The steps in the cycle of ATP hydrolysis that caldesmon could inhibit include: (1) the binding of myosin to actin, (2) the transition between any two actin-myosin states and (3) the distribution between inactive and active states of actin. The analysis of these possibilities is complicated because caldesmon binds to both myosin and actin and because each caldesmon molecule binds to several actin monomers. This paper reviews procedures for analysing these interactions and summarizes current information on the stability and dynamics of the interaction of caldesmon with actin and myosin. Possible effects of caldesmon on transitions within the ATPase cycle of actomyosin are also discussed.

Caldesmon-actin-tropomyosin contains two types of binding sites for myosin S1

Caldesmon inhibits the activation of myosin ATPase activity by actin-tropomyosin. Caldesmon also inhibits the binding of myosin to actin. There is disagreement as to the degree to which competitive displacement of myosin subfragment binding to actin is responsible for the inhibition of ATPase activity. We have examined the possibility that one or more molecules of S1 may bind to actin-tropomyosin-caldesmon without having the normal actin activation of ATPase activity. The effect of caldesmon on the binding and ATPase activity of S1 was measured at several initial levels of saturation of S1 to determine if a fraction of the bound S1 was resistant to displacement by caldesmon. In the case of both unmodified S1 and rhoPDM-modified S1, most, but not all, of the S1 was displaced by caldesmon. The results are consistent with a single molecule of S1 binding with low affinity for each seven actin monomers that are fully saturated with caldesmon and tropomyosin. This single S1 is not necessarily bound directly to actin but may be attached to the NH2-terminal region of caldesmon.

Augmentation of aortic ring contractions by angiotensin II antisense peptide

Previous biochemical experiments have revealed two antisense peptide antagonists to human angiotensin II (Ang II), one encoded in the cDNA in the antiparallel reading, the other in the parallel reading. Neither peptide's ability to produce physiological antagonism has been demonstrated previously. Both peptides were tested for their ability to antagonize Ang II-induced contractions on rabbit aorta smooth muscle. Neither peptide had any direct contractile activity. The antiparallel Ang II peptide had physiological antagonism to Ang II contractions at a lower sensitivity than reported in biochemical studies, and its antagonist activity was partially blocked by Ang II antiserum, suggesting that it is not an antipeptide but an Ang II homologue. The parallel Ang II antipeptide also required high concentrations for physiological inhibition. Its contractile inhibition was not affected by Ang II antiserum and diminished the Ang II contraction at high micromolar concentrations, findings consistent with physicochemical data showing that it is an Ang II complement. The concentration of either peptide required to produce an antagonistic physiological effect was too high to predict any pharmacological usefulness. The parallel antipeptide, however, significantly increased the force of muscle contractions at high nanomolar concentrations, thus displaying a unique dual augmentation/antagonist activity. This antipeptide seems to have highly sequence-specific activity because other similar parallel antipeptides had no activity. The parallel antipeptide augmentation mimics the shift in the Ang II dose-response curve produced in hypertension studies of the slow pressor effect of Ang II and may be useful in deducing the currently unknown cause of the slow pressor effect. It may also have some uses in migraine studies.

Visualization of caldesmon on smooth muscle thin filaments

Caldesmon, a narrow, elongated actin-binding protein, is found in both nonmuscle and smooth muscle cells. It inhibits actomyosin ATPase and filament severing in vitro, and is thus a putative regulatory protein. To elucidate its function, we have used electron microscopy and three-dimensional image reconstruction to reveal the location of caldesmon on isolated smooth muscle thin filaments. Caldesmon density was clearly delineated in reconstructions and found to occur peripherally, on the extreme outer edge of actin subdomains-1 and 2, without making obvious contacts with tropomyosin strands on the inner domains of actin. When the reconstructions were fitted to the atomic model of F-actin, caldesmon appeared to cover potentially weak sites of myosin interaction with actin, while, together with tropomyosin, it flanked strong sites of myosin interaction, without covering them. These interactions are unlike those of troponin-tropomyosin and therefore inhibition of actomyosin ATPase by caldesmon-tropomyosin and by troponin-tropomyosin cannot occur in the same way. The location of caldesmon would allow it to compete with a number of cellular actin-binding proteins, including those known to sever or sequester actin.

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.

Arrangement of the COOH-terminal and NH2-terminal domains of caldesmon bound to actin

Smooth muscle caldesmon is a single polypeptide chain with its NH2- and COOH-terminal domains separated by a long alpha-helix. Caldesmon was labeled at either Cys-153 in the NH2 domain or Cys-580 in the COOH domain with a variety of fluorescence probes. Fluorescence intensity, peak position, and polarization of probes on Cys-580 were very sensitive to the binding to actin (with or without tropomyosin), whereas for probes on Cys-153, there was a lack of response, in reconstituted or native actin thin filaments. From fluorescence resonance energy transfer from donor labels on either caldesmon cysteine to acceptor labels on Cys-374 of actin, the distance between the donor and acceptor was estimated to be 27 A for the donor at Cys-580 and 65-80 A for the donor at Cys-153. These findings were the same for caldesmon prepared with or without heat treatment and with striated or smooth muscle actin. These results, together with previous knowledge that COOH-terminal fragments of caldesmon bind to actin whereas NH2-terminal fragments do not, indicate that, while the COOH domain of caldesmon is bound to actin, the NH2 domain is largely dissociated. Fluorescence quenching studies showed that actin binding to caldesmon greatly decreased the accessibility of probes at caldesmon Cys-580 to the quencher, whereas for probes at Cys-153, actin afforded much less, but significant, protection from quenching. Consequently, it appears that, although the NH2 domain is mostly dissociated, it spends some time in the vicinity of actin, through either a weak interaction with actin or collisions with actin and/or because of restricted flexibility which constrains the NH2 domain to be close to the actin filament. Since the NH2 domain of caldesmon binds to the neck region of myosin, a dissociated NH2 domain may account for caldesmon's ability to link myosin and actin filaments.

Myosin light chain kinase: an actin-binding protein that regulates an ATP-dependent interaction with myosin

Myosin light chain kinase (MLCK) is a key regulator of smooth muscle contraction. The most conspicuous form of regulation is achieved by phosphorylation of the myosin light chain, allowing myosin to interact with actin. This interaction is regulated by actin-binding proteins that modulate actin filaments. In this review Kazuhiro Kohama and colleagues consider MLCK as an actin-binding protein and attempt to shed light on the cross-talk between the different kinds of regulation of the actin-myosin interaction in smooth muscle. An understanding of these mechanisms will assist the development of compounds with therapeutic importance in muscular disorders.

A simple method for automatic tracking of actin filaments in the motility assay

A great deal of quantitative information about the actomyosin interaction can be obtained from the basic Kron and Spudich in vitro motility assay provided that care is taken to obtain consistency between experiments and that the data is examined comprehensively and not selectively. From observations of filament movement under a wide variety of conditions we have formulated the hypothesis that a large number of filaments moving over a short time period is indistinguishable from fewer filaments moving over a longer sequence of frames. This has been used to devise a simple automation of filament detection procedures. A sequence of images is digitized through a frame-grabber. If successive pairs of frames are compared the program will search for and detect the new position of every filament and show its vector on screen. Velocity is calculated and shown as a frequency histogram. The program regularly detects over 100 filaments moving in each pair of frames; usually a sequence of up to 15 pairs of frames are studied yielding 500-1000 vectors in total. The algorithm cannot deal with filaments that meet, cross or divide, however, when filaments are moving less than 2 microns between frames this is only a small proportion of the whole. The program outputs fraction of filaments motile, mean velocity with standard deviation and density of filaments (filaments microns-2). A cumulative frequency histogram gives an immediate visual indication of the performance of the population of filaments. Direct comparisons show that the data produced by automatic tracking is indistinguishable from manual tracking apart from the small apparent velocity of non-mobile filaments. The detection process takes about 5 min and requires little skill or judgement. This can lead to great increases in the rate of data analysis in motility work.

Structure-function relationships in smooth muscle: the missing links

Smooth muscle cells have developed a contractile machinery that allows them to exert tension on the surrounding extracellular matrix over their entire length. This has been achieved by coupling obliquely organized contractile filaments to a more-or-less longitudinal framework of cytoskeletal elements. Earlier structural data suggested that the cytoskeleton was composed primarily of intermediate filaments and played only a passive role. More recent findings highlight the segregation of actin isotypes and of actin-associated proteins between the contractile and cytoskeletal domains and raise the possibility that the cytoskeleton performs a more active function. Current efforts focus on defining the relative contributions of myosin cross-bridge cycling and actin-associated protein interactions to the maintenance of tension in smooth muscle tissue.

In vitro motility analysis of smooth muscle caldesmon control of actin-tropomyosin filament movement

We have used the in vitro motility assay to investigate the effect of caldesmon on the movement of actin-tropomyosin filaments over thiophosphorylated smooth muscle myosin and skeletal muscle heavy meromyosin. Using either motor, incorporation of up to 8 nM caldesmon inhibited filament movement by decreasing the proportion of filaments motile from > 85% to < 30%. There was a minimal effect on filament attachment and a modest decrease in motile filament velocity in this concentration range. The reduction in the proportion of filaments motile could be completely reversed by incorporation of an excess of calmodulin at pCa 4.5. The expressed C-terminal fragment, 606C, which retains caldesmon's inhibitory capacity but does not bind to myosin, decreased the proportion of filaments motile but had no effect on velocity. We conclude that the velocity reduction by whole caldesmon is due to actin-myosin cross-linking. A significant decrease in filament attachment was observed when caldesmon was added to an excess over actin (> 10 nM). In the absence of tropomyosin, addition of an excess of caldesmon caused a similar decrease in the filament density, but there was no effect on the proportion of filaments that were motile. Our results demonstrate that caldesmon can switch actin-tropomyosin from motile to non-motile states without controlling velocity of movement or weak binding affinity and show the inhibitory action of caldesmon in the motility assay to be functionally indistinguishable from that reported for troponin.

Activation of mitogen-activated protein kinase in porcine carotid arteries

The thin-filament protein h-caldesmon (the high molecular weight isoform of caldesmon) is phosphorylated in resting and contracted porcine carotid arteries. Phosphorylation of h-caldesmon in intact tissue occurs at sites that are covalently modified by mitogen-activated protein kinase (MAPK) in vitro. In this study, we have evaluated MAPK activation in arteries in response to mechanical load and pharmacological stimulation. MAPK was extracted from resting and stimulated porcine carotid arteries and then partially purified by anion-exchange fast-performance liquid chromatography. MAPK activity was separated into two peaks corresponding to the tyrosine-phosphorylated 42- and 44-kD isoforms of MAPK (p42MAPK and p44MAPK, respectively). Of the total MAPK activity, 42% was associated with p42MAPK, and 58% was associated with p44MAPK, this percentage was not altered by stimulation of the muscles with either KCl (110 mmol/L) or phorbol 12,13-dibutyrate (PDBu, 1 mumol/L). Both p42MAPK and p44MAPK, purified from porcine carotid arteries, phosphorylated h-caldesmon at the same sites and to levels approaching or > 1 mol phosphate per mole protein. In unloaded muscle strips, MAPK activity was 39 pmol.min-1.mg protein-1 when assayed with the peptide substrate APRTPG-GRR. MAPK activity increased in response to incremental mechanical loading to a maximum of 99 pmol.min-1.mg protein-1 at 16 x 10(3) N/m2. MAPK activity could be further increased in loaded muscles by pharmacological stimulation. With KCl stimulation, MAPK activities rose to a peak of 205 pmol.min-1.mg protein-1 at 10 minutes and then declined to basal values at 30 and 60 minutes.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of unphosphorylated smooth muscle myosin on caldesmon-mediated regulation of actin filament velocity

The effect of smooth muscle myosin at different levels of light chain phosphorylation on caldesmon-mediated movement of actin filaments was investigated using an in vitro motility assay. Myosin at different levels of phosphorylation was obtained by mixing different proportions of fully phosphorylated and unphosphorylated myosin in monomeric form, while keeping the total myosin concentration constant. The average velocity of actin filaments containing tropomyosin was 1.20 +/- 0.046 microns s-1 at 30 degrees C with fully phosphorylated myosin. This velocity was not altered when the percentage of unphosphorylated myosin coated on the nitrocellulose surface was increased to 80%; further increases lowered the velocity. When the actin filaments with caldesmon bound at stoichiometric levels were used, filament velocity was unaffected until 50% of the myosin was unphosphorylated, but further increases in the percentage of unphosphorylated myosin induced a decrease in the velocity, and at 95% unphosphorylated myosin, filament movement had ceased. The decreased filament velocity in the presence of caldesmon was also observed when phosphorylated myosin was mixed with myosin rod instead of unphosphorylated myosin, but was not observed when the 38 kDa caldesmon C-terminal fragment, which lacks the myosin-binding domain, was used instead of intact caldesmon. These data indicate that the decreased filament velocity in the presence of caldesmon reflects the mechanical load produced by the tethering of actin to myosin through the interaction of the caldesmon N-terminal domain and the myosin S-2 region. The tethering effect mediated by caldesmon may play a role in smooth muscle contraction when a large number of myosin heads are dephosphorylated, as in force maintenance.

Over-expression of smooth muscle caldesmon in mouse fibroblasts

Caldesmon is an actin, calmodulin, tropomyosin, and myosin binding protein implicated in the regulation of actomyosin interactions. We have investigated the effect of overexpression of the higher molecular weight smooth muscle isoform of caldesmon on mouse L cell physiology. Mouse L(TK-) cells were transfected stably with plasmids carrying the TK+ gene and a full length human smooth muscle caldesmon cDNA under control of the adenovirus major late promoter. Two clones displaying four and eight times the level of the endogenous mouse high molecular weight caldesmon were isolated. These cells acquire a distinct phenotype characterized by an altered morphology, including an increased number of processes and larger area due to enhanced cell spreading, and a significantly slower growth rate than that of untransfected control cells, or cells transfected with the TK+ gene alone. The majority of the overexpressed caldesmon appears to be active and localized on cytoskeleton structures as determined by detergent lysis. Immunofluorescence analysis of the clones revealed that the caldesmon is localized as punctate staining on stress-fibers and in membrane ruffles. The immunofluorescence images suggest that caldesmon overexpressing cells have more total filaments than control cells. The effects of excess caldesmon on cell mobility are ambiguous: one clone displayed increased motility compared to the control, while the motility of the second clone was decreased relative to the control.

Molecular aspects of bladder outlet obstruction

In an animal model of obstruction, increasing load induces significant smooth muscle hypertrophy which is associated with a down-regulation of myosin heavy chain expression. This undoubtedly contributes to the decreased smooth muscle contractility seen in this model. Moreover, obstruction-induced hypertrophy leads to the development of a dedifferentiated smooth muscle phenotype, as evidenced by a revision of the cell to fetal (of non-muscle) gene expression patterns. Similar alterations are seen in atherosclerotic vessels and other pathologic smooth muscle systems. In these systems, dedifferentiation is also associated with significant alterations in extracellular matrix expression. It seems likely that obstruction in the bladder induces dedifferentiation of the smooth muscle cell which alters contractility as well as extracellular matrix expression, leading to altered bladder performance and decreased compliance.

Calmodulin and the regulation of smooth muscle contraction

Calmodulin, the ubiquitous and multifunctional Ca(2+)-binding protein, mediates many of the regulatory effects of Ca2+, including the contractile state of smooth muscle. The principal function of calmodulin in smooth muscle is to activate crossbridge cycling and the development of force in response to a [Ca2+]i transient via the activation of myosin light-chain kinase and phosphorylation of myosin. A distinct calmodulin-dependent kinase, Ca2+/calmodulin-dependent protein kinase II, has been implicated in modulation of smooth-muscle contraction. This kinase phosphorylates myosin light-chain kinase, resulting in an increase in the calmodulin concentration required for half-maximal activation of myosin light-chain kinase, and may account for desensitization of the contractile response to Ca2+. In addition, the thin filament-associated proteins, caldesmon and calponin, which inhibit the actin-activated MgATPase activity of smooth-muscle myosin (the cross-bridge cycling rate), appear to be regulated by calmodulin, either by the direct binding of Ca2+/calmodulin or indirectly by phosphorylation catalysed by Ca2+/calmodulin-dependent protein kinase II. Another level at which calmodulin can regulate smooth-muscle contraction involves proteins which control the movement of Ca2+ across the sarcolemmal and sarcoplasmic reticulum membranes and which are regulated by Ca2+/calmodulin, e.g. the sarcolemmal Ca2+ pump and the ryanodine receptor/Ca2+ release channel, and other proteins which indirectly regulate [Ca2+]i via cyclic nucleotide synthesis and breakdown, e.g. NO synthase and cyclic nucleotide phosphodiesterase. The interplay of such regulatory mechanisms provides the flexibility and adaptability required for the normal functioning of smooth-muscle tissues.

Calponin is localised in both the contractile apparatus and the cytoskeleton of smooth muscle cells

Calponin and caldesmon are two thin filament-binding proteins found in smooth muscle that have both been attributed a role in modulating the interaction of actin and myosin. Using high-resolution dual-label immunocytochemistry we have determined the distribution of calponin relative to the contractile and cytoskeletal compartments of the smooth muscle cell. We show, using chicken gizzard smooth muscle, that calponin occurs in the cytoskeleton, with beta-cytoplasmic actin, filamin and desmin, as well as in the contractile apparatus, with myosin and caldesmon. According to the observed labelling intensities, calponin was more concentrated in the cytoskeleton and it was additionally localised in the cytoplasmic dense bodies as well as in the adhesion plaques at the cell surface, which both harbour the beta-cytoplasmic isoform of actin. It is probable that these results explain earlier conflicting reports on the composition of smooth muscle thin filaments and suggest that calponin, together with a Ca(2+)-receptor protein, could just as likely serve a role in the cytoskeleton of smooth muscle as in the contractile apparatus.

Stimulation of the ATP-dependent interaction between actin and myosin by a myosin-binding fragment of smooth muscle caldesmon

We reported previously that smooth muscle caldesmon stimulates the ATP-dependent interaction between actin and phosphorylated smooth muscle myosin, as monitored by ATPase measurement and in vitro motility assay. Furthermore, this effect changes from stimulatory to inhibitory with increasing concentrations of caldesmon [Ishikawa et al., 1991: J. Biol. Chem. 266:21784-21790]. The N-terminal (myosin-binding) fragment and the C-terminal (actin-binding) fragment were purified from digests of caldesmon. The effects of the myosin-binding fragment and the actin-binding fragment on the interaction were stimulatory and inhibitory, respectively, indicating that stimulatory and inhibitory domains are localized in the myosin-binding domain and actin-binding domain of caldesmon, respectively. The effect of the myosin-binding fragment on the interaction was exclusively stimulatory when the interaction was challenged by caldesmon, both at lower and higher concentrations. However, the actin-binding fragment had no effect on the interaction at lower concentrations and inhibited the interaction at higher concentrations. Thus, the stimulatory effect of caldesmon that is observed at lower concentrations can be explained by the hypothesis that the stimulatory effect of the myosin-binding domain predominates over the inhibitory effect of the actin-binding domain when the concentration of caldesmon is low. With uncleaved caldesmon, we also emphasized the role of the myosin-binding domain in the stimulation as follows; the stimulatory effect of caldesmon became obscured when binding of caldesmon to myosin was competed by the exogenous caldesmon-binding fragment of myosin.

The caldesmon content of vertebrate smooth muscle

Caldesmon and tropomyosin can be selectively and quantitatively extracted from vascular and visceral smooth muscle following heat treatment; all other smooth muscle proteins are precipitated by this procedure. Estimates of the caldesmon/tropomyosin molar ratio in heat-extracts determined by SDS-PAGE densitometry are 1 caldesmon:5.1-5.3 tropomyosin for rabbit and sheep aorta, and 1 caldesmon:5.9 tropomyosin for rabbit stomach and chicken gizzard. If the assumption is made that tropomyosin serves as a true reference of thin-filament content in intact muscle, it follows that the relative caldesmon contents in the above tissues are similar to each other. Caldesmon in heat extracts was identified by Western blotting, by its anomalous migration on several different SDS-PAGE systems and by its position on two-dimensional PAGE. Values of caldesmon contents in unfractionated total tissue homogenates were found to be similar to those cited above. Smooth muscles contain different thin-filament classes and only one type appears to possess caldesmon. By comparing values for the molar composition of caldesmon-specific filaments (1 caldesmon:2 tropomyosin:14 actin) with the values above determined for intact tissue, we conclude that the caldesmon filaments account for approx. 35-45% of the total thin-filament pool in arterial smooth muscle and slightly less in visceral muscles.