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

Caldesmon ablation in mice causes umbilical herniation and alters contractility of fetal urinary bladder smooth muscle

The actin-, myosin-, and calmodulin-binding protein caldesmon (CaD) is expressed in two splice isoforms: h-CaD, which is an integral part of the actomyosin domain of smooth muscle cells, and l-CaD, which is widely expressed and is involved in many cellular functions. Despite extensive research for many years, CaD's in vivo function has remained elusive. To explore the role of CaD in smooth muscle contraction in vivo, we generated a mutant allele that ablates both isoforms. Heterozygous animals were viable and had a normal life span, but homozygous mutants died perinatally, likely because of a persistent umbilical hernia. The herniation was associated with hypoplastic and dysmorphic abdominal wall muscles. We assessed mechanical parameters in isometrically mounted longitudinal strips of E18.5 urinary bladders and in ring preparations from abdominal aorta using wire myography. Ca²⁺ sensitivity was higher and relaxation rate was slower in Cald1^−/− compared with Cald1^+/+ skinned bladder strips. However, we observed no change in the content and phosphorylation of regulatory proteins of the contractile apparatus and myosin isoforms known to affect these contractile parameters. Intact fibers showed no difference in actin and myosin content, regardless of genotype, although KCl-induced force tended to be lower in homozygous and higher in heterozygous mutants than in WTs. Conversely, in skinned fibers, myosin content and maximal force were significantly lower in Cald1^−/− than in WTs. In KO abdominal aortas, resting and U46619 elicited force were lower than in WTs. Our results are consistent with the notion that CaD impacts smooth muscle function dually by (1) acting as a molecular brake on contraction and (2) maintaining the structural integrity of the contractile machinery. Most importantly, CaD is essential for resolution of the physiological umbilical hernia and ventral body wall closure.

The pivotal role of airway smooth muscle in asthma pathophysiology

Asthma is characterized by the association of airway hyperresponsiveness (AHR), inflammation, and remodelling. The aim of the present article is to review the pivotal role of airway smooth muscle (ASM) in the pathophysiology of asthma. ASM is the main effector of AHR. The mechanisms of AHR in asthma may involve a larger release of contractile mediators and/or a lower release of relaxant mediators, an improved ASM cell excitation/contraction coupling, and/or an alteration in the contraction/load coupling. Beyond its contractile function, ASM is also involved in bronchial inflammation and remodelling. Whereas ASM is a target of the inflammatory process, it can also display proinflammatory and immunomodulatory functions, through its synthetic properties and the expression of a wide range of cell surface molecules. ASM remodelling represents a key feature of asthmatic bronchial remodelling. ASM also plays a role in promoting complementary airway structural alterations, in particular by its synthetic function.

Acrylodan-labeled smooth muscle tropomyosin reports differences in the effects of troponin and caldesmon in the transition from the active state to the inactive state

Changes in the orientation of tropomyosin on actin are important for the regulation of striated muscle contraction and could also be important for smooth muscle regulation. We showed earlier that acrylodan-labeled skeletal muscle tropomyosin reports the kinetics of the reversible transitions among the active, intermediate, and inactive states when S1 is rapidly detached from actin-tropomyosin. We now show that acrylodan-labeled smooth muscle tropomyosin reports similar transitions among states of actin-tropomyosin. When S1 was rapidly detached from actin-smooth muscle tropomyosin, there was a rapid decrease in acrylodan-tropomyosin fluorescence as the intermediate state became populated. The rate constant for this process was >600 s(-1) at temperatures near 5 °C. In the presence of skeletal troponin and EGTA, the decrease in fluorescence was followed by the redevelopment of fluorescence as the inactive state became populated. The apparent rate constant for the fluorescence increase was 14 s(-1) at 5 °C. Substituting smooth muscle caldesmon for skeletal muscle troponin produced a similar decrease and re-increase in fluorescence, but the apparent rate constant for the increase was >10 times that observed with troponin. Furthermore, the fluorescence increase was correlated with an increase in the extent of caldesmon attachment as S1-ATP dissociated. Although the measured rate constant appeared to reflect the rate-limiting transition for inactivation, it is unclear if the fluorescence change resulted from caldesmon binding, the movement of tropomyosin over actin, or both.

Methods for studying mechanical control of angiogenesis by the cytoskeleton and extracellular matrix

Mechanical forces that capillary endothelial cells generate in their cytoskeleton and exert on their extracellular matrix adhesions feed back to modulate cell sensitivity to soluble angiogenic factors, and thereby control vascular development. Here we describe various genetic, biochemical, and engineering methods that can be used to study, manipulate, and probe this physical mechanism of developmental control. These techniques are useful as in vitro angiogenesis models and for analyzing the molecular and biophysical basis of vascular control.

Role of tropomyosin in the regulation of contraction in smooth muscle

Smooth muscle contraction is due to the interaction ofmyosin filaments with thin filaments. Thin filaments are composed of actin, tropomyosin, caldesmon and calmodulin in ratios 14:2:1:1. Tissue specific isoforms of act and beta tropomyosin are expressed in smooth muscle. Compared with skeletal muscle tropomyosin, the cooperative activation of actomyosin is enhanced by smooth muscle tropomyosin: cooperative unit size is 10 and the equilibrium between on and off states is shifted towards the on state. The smooth muscle-specific actin-bindingprotein caldesmon, together with calmodulin regulates the activity of the thin filament in response to Ca2+. Caldesmon and calmodulin control the tropomyosin-mediated transition between on and offactivity states.

Role of caldesmon in the Ca2+ regulation of smooth muscle thin filaments: evidence for a cooperative switching mechanism

Smooth muscle thin filaments are made up of actin, tropomyosin, caldesmon, and a Ca(2+)-binding protein and their interaction with myosin is Ca(2+)-regulated. We suggested that Ca(2+) regulation by caldesmon and Ca(2+)-calmodulin is achieved by controlling the state of thin filament through a cooperative-allosteric mechanism homologous to troponin-tropomyosin in striated muscles. In the present work, we have tested this hypothesis. We monitored directly the thin filament transition between the ON and OFF state using the excimer fluorescence of pyrene iodoacetamide (PIA)-labeled smooth muscle alphaalpha-tropomyosin homodimers. In steady state fluorescence measurements, myosin subfragment 1 (S1) cooperatively switches the thin filaments to the ON state, and this is exhibited as an increase in the excimer fluorescence. In contrast, caldesmon decreases the excimer fluorescence, indicating a switch of the thin filament to the OFF state. Addition of Ca(2+)-calmodulin increases the excimer fluorescence, indicating a switch of the thin filament to the ON state. The excimer fluorescence was also used to monitor the kinetics of the ON-OFF transition in a stopped-flow apparatus. When ATP induces S1 dissociation from actin-PIA-tropomyosin, the transition to the OFF state is delayed until all S1 molecules are dissociated actin. In contrast, caldesmon switches the thin filament to the OFF state in a cooperative way, and no lag is displayed in the time course of the caldesmon-induced fluorescence decrease. We have also studied caldesmon and Ca(2+)-calmodulin-caldesmon binding to actin-tropomyosin in the ON and OFF states. The results are used to discuss both caldesmon inhibition and Ca(2+)-calmodulin-caldesmon activation of actin-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.

l-Caldesmon Regulates Proliferation and Migration of Vascular Smooth Muscle Cells and Inhibits Neointimal Formation After Angioplasty

OBJECTIVE

Light-type caldesmon (l-CaD) is a potent cytostatic and antiangiogenic protein that regulates cell growth and survival via modulation of the cell shape and cytoskeleton. The aim of this study is to explore the potential value of l-CaD for use as a cytostatic agent to inhibit neointimal formation after angioplasty by suppressing vascular smooth muscle cell (VSMC) growth and migration.

METHODS AND RESULTS

We tested the cytostatic function of l-CaD in cultured VSMCs using assays for apoptosis, cell proliferation, and migration, and evaluated the expression pattern of relevant signaling proteins (focal adhesion kinase [FAK] and mitogen-activated protein kinases) in VSMCs. Transfection of adenoviral vector encoding l-CaD (Ad-l-CaD) resulted in progressive loss of actin stress fibers and cell retraction. Enzyme-linked immunosorbent assay demonstrated that Ad-l-CaD transfection increased the apoptosis rate by 75% and reduced BrdU uptake by 49%. Furthermore, transfection of Ad-l-CaD inhibited migration of VSMCs induced by platelet-derived growth factor-BB (PDGF) by 36% (P<0.05). Immunoblotting analysis revealed that l-CaD overexpression reduced PDGF-induced phosphorylation of both FAK and extracellular signal regulated-kinase (ERK). In balloon-injured rat carotid arteries, Ad-l-CaD transfection inhibited neointimal formation by 37% (P<0.05) without delaying re-endothelialization at 14 days.

CONCLUSIONS

Overexpression of l-CaD suppressed cell growth and survival in VSMCs and inhibited neointimal formation after experimental angioplasty, partly by regulating the cytoskeletal tension-FAK-ERK axis.

Mutational and genotype-phenotype correlation analyses in 28 Polish patients with Cornelia de Lange syndrome

Cornelia de Lange syndrome (CdLS) is a multisystem congenital anomaly disorder characterized by prenatal and postnatal growth retardation, developmental delay, distinctive facial dysmorphism, limb malformations, and multiple organ defects. Mutations in the NIPBL gene have been discovered recently as a major etiology for this syndrome, and were detected in 27-56% of patients. Two groups have found significant differences in the severity or penetrance of some phenotypes between mutation positive and mutation negative patients. Different clinical features have also been described among patients with missense versus truncating mutations. In this study, we identified 13 NIPBL mutations in 28 unrelated Polish CdLS patients (46.4%), 11 were novel. Mutation positive patients were more severely affected in comparison to mutation negative individuals with respect to weight, height, and mean head circumference at birth, facial dysmorphism and speech impairment. Analyses of combined data from this and the two previous studies revealed that the degree of growth, developmental delay and limb defects showed significant differences between patients with and without mutations and between patients with missense and truncating mutations, whereas only a portion of these features differed significantly in any individual study. Furthermore, bioinformatic analyses of the NIPBL protein revealed several novel domains, which may give further clues about potential functions of this protein.

Caldesmon effects on the actin cytoskeleton and cell adhesion in cultured HTM cells

Caldesmon is a multifunctional ubiquitous regulator of the actin cytoskeleton, which can affect both actomyosin contractility and actin polymerization. Previous studies showed that caldesmon over-expression in cultured fibroblasts produces effects that resemble those of chemical inhibitors of cellular contractility. Since these inhibitors (H-7, Y-27632, etc.) have been shown to lower intraocular pressure and increase outflow facility from the anterior chamber of the eye, we proposed that caldesmon might be used for gene therapy of glaucoma. In the present study we examined the effects of expression of adenovirus-delivered rat non-muscle caldesmon fused with green fluorescent protein (AdCaldGFP) on the actin cytoskeleton and matrix adhesions in cultured human trabecular meshwork (HTM) cells. In addition, we assessed the effect of caldesmon on the stability of cell-cell junctions in kidney epithelial MDCK cells. Cultured HTM cells demonstrate a well-developed actin cytoskeleton, comprising mainly arrays of parallel actomyosin bundles (stress fibers). Lamellipodial protrusions containing dense actin networks are also observed. Cell-matrix adhesions are dominated by focal adhesions (FAs) associated with the ends of the stress fibers, focal complexes in lamellipodia, and fibrillar adhesions in the central part of the spread cells. Treatment of HTM cells with AdCaldGFP resulted in dose-dependent morphological changes within 24-48 hr post-infection. Cells expressing moderate levels of caldesmon exhibited straight bundles containing actin and myosin II, which were considerably shorter than those in control cells. Short filament bundles in caldesmon over-expressing cells formed arrays consisting of triangular actin structures with small vinculin-positive FAs at their vertices. In addition, the fraction of cells displaying large lamellipodia increased. About 40-50% of the population of caldesmon-expressing cells demonstrated high levels of GFP-caldesmon expression and severe changes in the actin cytoskeleton, manifested by the disappearance of stress fibers and the formation of curved actin- and myosin-containing bundles. These bundles formed together a dynamic network consisting of pulsating loops filling the entire cytoplasm. Addition of thapsigargin, which increases intracellular Ca++ concentration, resulted in a straightening of the curved bundles. Another type of novel actin structures induced by caldesmon over-expression were highly dynamic circular waves that propagated over the affected cells with a velocity about 10 microm min. In cells with disrupted stress fibers, vinculin-containing FAs and tensin-rich fibrillar adhesions had also essentially vanished. However, phosphotyrosine-positive focal complexes were still prominent throughout the lamellipodia of these cells. Over-expression of caldesmon in MDCK cells reduced, in a dose dependent manner, the beta-catenin content at cell-cell adherens junctions and in some cases led to physical disruption of adherens junctions. Thus, caldesmon over-expression induces unique reorganization of the actin cytoskeleton in affected cells, accompanied by disruption of focal and fibrillar cell-matrix adhesions, and destabilization of cell-cell adherens junctions. Inducing such changes in the contractility and actin cytoskeleton of HTM cells in glaucomatous eyes in vivo could produce a therapeutically useful increase in outflow facility.

The mechanism of smooth muscle caldesmon-tropomyosin inhibition of the elementary steps of the actomyosin ATPase

Caldesmon is a component of smooth muscle thin filaments that inhibits the actomyosin ATPase via its interaction with actin-tropomyosin. We have performed a comprehensive transient kinetic characterization of the actomyosin ATPase in the presence of smooth muscle caldesmon and tropomyosin. At physiological ratios of caldesmon to actin (1 caldesmon/7 actin monomers) actomyosin ATPase is inhibited by about 75%. Inhibitory caldesmon concentrations had little effect upon the rate of S1 binding to actin, actin-S1 dissociation by ATP, and dissociation of ADP from actin-S1 x ADP; however the rate of phosphate release from the actin-S1 x ADP x P(i) complex was decreased by more than 80%. In addition the transient of phosphate release displayed a lag of up to 200 ms. The presence of a lag phase indicates that a step on the pathway prior to phosphate release has become rate-limiting. Premixing the actin-tropomyosin filaments with myosin heads resulted in the disappearance of the lag phase. We conclude that caldesmon inhibition of the rate of phosphate release is caused by the thin filament being switched by caldesmon to an inactive state. The active and inactive states correspond to the open and closed states observed in skeletal muscle thin filaments with no evidence for the existence of a third, blocked state. Taken together these data suggest that at physiological concentrations, caldesmon controls the isomerization of the weak binding complex to the strong binding complex, and this causes the inhibition of the rate of phosphate release. This inhibition is sufficient to account for the inhibition of the steady state actomyosin ATPase by caldesmon and tropomyosin.

Caldesmon restricts the movement of both C- and N-termini of tropomyosin on F-actin in ghost fibers during the actomyosin ATPase cycle

New data on the movements of tropomyosin singly labeled at alpha- or beta-chain during the ATP hydrolysis cycle in reconstituted ghost fibers have been obtained by using the polarized fluorescence technique which allowed us following the azimuthal movements of tropomyosin on actin filaments. Pronounced structural changes in tropomyosin evoked by myosin heads suggested the "rolling" of the tropomyosin molecule on F-actin surface during the ATP hydrolysis cycle. The movements of actin-bound tropomyosin correlated to the strength of S1 to actin binding. Weak binding of myosin to actin led to an increase in the affinity of the tropomyosin N-terminus to actin with simultaneous decrease in the affinity of the C-terminus. On the contrary, strong binding of myosin to actin resulted in the opposite changes of the affinity to actin of both ends of the tropomyosin molecule. Caldesmon inhibited the "rolling" of tropomyosin on the surface of the thin filament during the ATP hydrolysis cycle, drastically decreased the affinity of the whole tropomyosin molecule to actin, and "freezed" tropomyosin in the position characteristic of the weak binding of myosin to actin.

Caldesmon transgene expression disrupts focal adhesions in HTM cells and increases outflow facility in organ-cultured human and monkey anterior segments

Cytoskeleton modulating compounds have been shown to lower intraocular pressure (IOP) and increase outflow facility. Caldesmon is one protein that is involved in the regulation of actin stress fiber formation. The effects of rat non-muscle caldesmon (Cald) gene over-expression on focal adhesions in human trabecular meshwork (HTM) cells and on outflow facility in organ-cultured human and monkey anterior segments were determined. Treatment of HTM cells with adenovirus-delivered caldesmon (AdCaldGFP) resulted in characteristic changes in the actin cytoskeleton and matrix adhesions within 24-48 hr post-transduction. Stress fibers gradually disappeared and novel actin structures were formed (see manuscript by Grosheva et al., this issue). In cells with disrupted stress fibers, vinculin-containing focal adhesions were also disrupted. In organ-cultured anterior segments, baseline outflow facility (microl min-1 mmHg-1) for all anterior segments averaged (mean+/-sem): human, 0.19+/-0.03 (n=12); monkey, 0.36+/-0.02 (n=19). In human anterior segments, transduction with 10(7) plaque forming units of AdGFPCald increased outflow facility by 43+/-21% (p<or=0.11, n=6) at 66 hr compared to baseline and corrected for the changes in outflow facility of the contralateral vehicle treated segment. Using the same time point, i.e. 2-3 days after injection, outflow facility in monkey anterior segments, transduced with 1.5x10(7) plaque forming units of AdGFPCald was increased by 35+/-18%, p<0.2, n=10 compared to baseline and corrected for the change in outflow facility in the contralateral AdGFP treated segment. Combining human (66 hr) and monkey (2-3 days) data, outflow facility was increased by 38+/-13%, p<0.02, n=16. Additional analysis of maximum responses in monkey anterior segments from 1 to 6 days after transduction showed outflow facility was increased by 66+/-18%, p<0.01, n=10. Caldesmon over-expression, which relaxes cultured HTM cells and disrupts their actin cytoskeleton and cell-matrix adhesions, also appears to increase outflow facility in organ-cultured human and monkey anterior segments. This suggests that over-expression of the caldesmon gene in the TM may be an effective approach for the gene therapy of glaucoma.

Caldesmon is an integral component of podosomes in smooth muscle cells

Podosomes are highly dynamic actin-based structures commonly found in motile and invasive cells such as macrophages, osteoclasts and vascular smooth muscle cells. Here, we have investigated the role of caldesmon, an actin-binding protein, in the formation of podosomes in aortic smooth muscle A7r5 cells induced by the phorbol ester PDBu. We found that endogenous low molecular weight caldesmon (l-caldesmon), which was normally localised to actin-stress fibres and membrane ruffles, was recruited to the actin cores of PDBu-induced podosomes. Overexpression of l-caldesmon in A7r5 cells caused dissociation of actin-stress fibres and disruption of focal adhesion complexes, and significantly reduced the ability of PDBu to induce podosome formation. By contrast, siRNA interference of caldesmon expression enhanced PDBu-induced formation of podosomes. The N-terminal fragment of l-caldesmon, CaD40, which contains the myosin-binding site, did not label stress fibres and was not translocated to PDBu-induced podosomes. Cad39, the C-terminal fragment housing the binding sites for actin, tropomyosin and calmodulin, was localised to stress fibres and was translocated to podosomes induced by PDBu. The caldesmon mutant, CadCamAB, which does not interact with Ca2+/calmodulin, was not recruited to PDBu-induced podosomes. These results show that (1) l-caldesmon is an integral part of the actin-rich core of the podosome; (2) overexpression of l-caldesmon suppresses podosome formation, whereas siRNA knock-down of l-caldesmon facilitates its formation; and (3) the actin-binding and calmodulin-binding sites on l-caldesmon are essential for the translocation of l-caldesmon to the podosomes. In summary, this data suggests that caldesmon may play a role in the regulation of the dynamics of podosome assembly and that Ca2+/calmodulin may be part of a regulatory mechanism in podosome formation.

The latch-bridge hypothesis of smooth muscle contraction

In contrast to striated muscle, both normalized force and shortening velocities are regulated functions of cross-bridge phosphorylation in smooth muscle. Physiologically this is manifested as relatively fast rates of contraction associated with transiently high levels of cross-bridge phosphorylation. In sustained contractions, Ca2+, cross-bridge phosphorylation, and ATP consumption rates fall, a phenomenon termed "latch". This review focuses on the Hai and Murphy (1988a) model that predicted the highly non-linear dependence of force on phosphorylation and a directly proportional dependence of shortening velocity on phosphorylation. This model hypothesized that (i) cross-bridge phosphorylation was obligatory for cross-bridge attachment, but also that (ii) dephosphorylation of an attached cross-bridge reduced its detachment rate. The resulting variety of cross-bridge cycles as predicted by the model could explain the observed dependencies of force and velocity on cross-bridge phosphorylation. New evidence supports modifications for more general applicability. First, myosin light chain phosphatase activity is regulated. Activation of myosin phosphatase is best demonstrated with inhibitory regulatory mechanisms acting via nitric oxide. The second modification of the model incorporates cooperativity in cross-bridge attachment to predict improved data on the dependence of force on phosphorylation. The molecular basis for cooperativity is unknown, but may involve thin filament proteins absent in striated muscle.

Fesselin binds to actin and myosin and inhibits actin-activated ATPase activity

Fesselin is an actin binding protein that bundles actin filaments and accelerates nucleation of actin polymerization. The effect of fesselin on actin polymerization is regulated by Ca(++)-calmodulin. Because actin filaments serve both structural and contractile functions we also examined the effect of fesselin on activation of myosin S1 ATPase activity. Fesselin inhibited the activation of S1-catalyzed ATP hydrolysis in a similar manner in both the presence and absence of tropomyosin. This inhibition was unaffected by Ca(++)-calmodulin. Fesselin inhibited the binding of myosin-S1 to actin during steady-state ATP hydrolysis. Fesselin also displaced caldesmon from actin. S1 displaced fesselin from actin in the absence of nucleotide when the affinity of S1 for actin was much greater than the affinity of fesselin for actin. It is likely that fesselin and S1 share common binding sites on F-actin. We also observed that fesselin could bind to smooth muscle myosin with muM affinity. Fesselin shares some similarities to caldesmon in binding to several other proteins and having multiple potential functions.

Over expression of smooth muscle thin filament associated proteins in the bladder wall of diabetics

PURPOSE

The thin filament associated proteins caldesmon, tropomyosin and calponin have been shown to modulate actin-myosin interaction, actomyosin adenosine triphosphatase and contraction in smooth muscle. This study was performed to determine whether the expression of these proteins is altered in diabetes induced decrease in the contractility of bladder wall smooth muscle.

MATERIALS AND METHODS

Detrusor samples were obtained from New Zealand White male rabbits with alloxan induced diabetes, and from age and sex matched control rabbits. In addition, a bladder myocyte cell line, which continues to express smooth muscle phenotype, was exposed to either normal (5 mM) or high (50 mM) concentrations of glucose. The levels of expression of the thin filament associated proteins were determined at the mRNA and protein levels by reverse transcriptase-polymerase chain reaction and Western blotting, respectively.

RESULTS

Detrusor smooth muscle tissue from rabbits with alloxan induced diabetes showed over expression of thin filament associated proteins, calponin, tropomyosin and caldesmon when compared with that of the control. Similar up-regulation was seen also in bladder myocytes in cultures treated with 50 mM glucose, indicating that the high glucose induced the changes.

CONCLUSIONS

Our results suggest that the increased expression of thin filament proteins, calponin, tropomyosin and caldesmon in diabetic rabbits might alter the contractile and cytoskeletal structure in bladder myocytes. The over expression of these thin filament associated proteins, which suppresses actin-myosin interaction and actomyosin adenosine triphosphatase, and the enhancement of this suppression by tropomyosin are likely to have an effect on the relationship between force and myosin light chain phosphorylation, requiring higher levels of phosphorylation in diabetic detrusor compared with that of control. The downstream effects of high glucose (eg oxidative stress) appear to modulate the transcriptional regulation of thin filament mediated regulatory proteins in bladder smooth muscle.

Effect of Caldesmon on the Position and Myosin-induced Movement of Smooth Muscle Tropomyosin Bound to Actin

It is known that the actin-binding protein caldesmon inhibits actomyosin ATPase activity and might in this way take part in the thin filament regulation of smooth muscle contraction. Although the molecular mechanism of this inhibition is unknown, it is clear that the presence of actin-bound tropomyosin is necessary for full inhibition. Recent evidence also suggests that the myosin-induced movement of tropomyosin plays a key role in regulation. In this work, fluorescence studies provide evidence to show that caldesmon interacts with and alters the position of tropomyosin in a reconstituted actin thin filament and thereby limits the ability of myosin heads to move tropomyosin. Caldesmon interacts with the Cys-190 region in the COOH-terminal half of tropomyosin, resulting in the movement of this part of tropomyosin to a new position on actin. Additionally, this constrains the myosin-induced movement of this region of tropomyosin. On the other hand, caldesmon does not appear to interact with the Cys-36 region in the NH2-terminal half of tropomyosin and neither alters the position of nor significantly constrains the myosin-induced movement of this part of tropomyosin. The ability of caldesmon to limit the myosin-induced movement of tropomyosin provides a possible molecular basis for the inhibitory function of caldesmon. The different movements of the two halves of tropomyosin indicate that actin-bound tropomyosin moves as a flexible molecule and not as a rigid rod. Interestingly, caldesmon, which inhibits tropomyosin's potentiation of actomyosin ATPase activity, moves tropomyosin in one direction, whereas myosin heads, which enhance potentiation, move tropomyosin in the opposite direction.

Modes of Caldesmon Binding to Actin

Smooth muscle caldesmon binds actin and inhibits actomyosin ATPase activity. Phosphorylation of caldesmon by extracellular signal-regulated kinase (ERK) reverses this inhibitory effect and weakens actin binding. To better understand this function, we have examined the phosphorylation-dependent contact sites of caldesmon on actin by low dose electron microscopy and three-dimensional reconstruction of actin filaments decorated with a C-terminal fragment, hH32K, of human caldesmon containing the principal actin-binding domains. Helical reconstruction of negatively stained filaments demonstrated that hH32K is located on the inner portion of actin subdomain 1, traversing its upper surface toward the C-terminal segment of actin, and forms a bridge to the neighboring actin monomer of the adjacent long pitch helical strand by connecting to its subdomain 3. Such lateral binding was supported by cross-linking experiments using a mutant isoform, which was capable of cross-linking actin subunits. Upon ERK phosphorylation, however, the mutant no longer cross-linked actin to polymers. Three-dimensional reconstruction of ERK-phosphorylated hH32K indeed indicated loss of the interstrand connectivity. These results, together with fluorescence quenching data, are consistent with a phosphorylation-dependent conformational change that moves the C-terminal end segment of caldesmon near the phosphorylation site but not the upstream region around Cys(595), away from F-actin, thus neutralizing its inhibitory effect on actomyosin interactions. The binding pattern of hH32K suggests a mechanism by which unphosphorylated, but not ERK-phosphorylated, caldesmon could stabilize actin filaments and resist F-actin severing or depolymerization in both smooth muscle and nonmuscle cells.

Cooperative attachment of cross bridges predicts regulation of smooth muscle force by myosin phosphorylation

Serine 19 phosphorylation of the myosin regulatory light chain (MRLC) appears to be the primary determinant of smooth muscle force development. The relationship between MRLC phosphorylation and force is nonlinear, showing that phosphorylation is not a simple switch regulating the number of cycling cross bridges. We reexamined the MRLC phosphorylation-force relationship in slow, tonic swine carotid media; fast, phasic rabbit urinary bladder detrusor; and very fast, tonic rat anococcygeus. We found a sigmoidal dependence of force on MRLC phosphorylation in all three tissues with a threshold for force development of approximately 0.15 mol P(i)/mol MRLC. This behavior suggests that force is regulated in a highly cooperative manner. We then determined whether a model that employs both the latch-bridge hypothesis and cooperative activation could reproduce the relationship between Ser(19)-MRLC phosphorylation and force without the need for a second regulatory system. We based this model on skeletal muscle in which attached cross bridges cooperatively activate thin filaments to facilitate cross-bridge attachment. We found that such a model describes both the steady-state and time-course relationship between Ser(19)-MRLC phosphorylation and force. The model required both cooperative activation and latch-bridge formation to predict force. The best fit of the model occurred when binding of a cross bridge cooperatively activated seven myosin binding sites on the thin filament. This result suggests cooperative mechanisms analogous to skeletal muscle that will require testing.

Smooth muscle hypertrophy following partial bladder outlet obstruction is associated with overexpression of non-muscle caldesmon

Partial bladder outlet obstruction (PBOO) induces remodeling of urinary bladder smooth muscle (detrusor). We demonstrate an increase in bladder wall mass, muscle bundle size, and a threefold increase in the cross-sectional area of detrusor myocytes following PBOO in male New Zealand White rabbits compared to that of controls. Some bladders with detrusor hypertrophy function close to normal (compensated), whereas others were dysfunctional (decompensated), showing high intravesical pressure, large residual urine volume, and voiding difficulty. We analyzed the expression of smooth muscle-specific caldesmon (h-CaD) and non-muscle (l-CaD) by Western blotting, RT-PCR, and real-time PCR. The expression of l-CaD is increased significantly at the mRNA and protein levels in the decompensated bladders compared to that of normal and compensated bladders. The CaD was also co-localized with myosin containing cytoplasmic fibrils in cells dissociated from obstructed bladders and cultured overnight. Our data show that the inability of decompensated bladders to empty, despite detrusor hypertrophy, is associated with an overexpression of l-CaD. The level of l-CaD overexpression might be a useful marker to estimate the degree of detrusor remodeling and contractile dysfunction in PBOO.

Troponin I inhibitory peptide suppresses the force generation in smooth muscle by directly interfering with cross-bridge formation

To explore possible mechanisms involving the thin filament-linked regulation of contraction in living smooth muscles, we studied the effects of a synthetic peptide of rabbit cardiac troponin I [residues 136-147] (TnIp), which is a minimal sequence required to inhibit striated muscle acto-tropomyosin-myosin ATPase activity, on the mechanical properties of beta-escin skinned preparations of taenia caeci from guinea pig. TnIp reversibly suppressed the Ca(2+)-activated force without significant effects on the Ca(2+) sensitivity and on the phosphorylation level of myosin regulatory light chain (MLC(20)). TnIp also reversibly suppressed the Ca(2+)/calmodulin-independent contraction induced by 30mM Mg(2+). An analogue of TnIp, which lost inhibiting action on acto-tropomyosin-myosin ATPase activity, affected neither Ca(2+)-activated nor 30mM Mg(2+)-induced contraction. These results indicate that TnIp suppresses the force generation in smooth muscle by directly interfering with cross-bridge formation rather than inhibiting the Ca(2+)/calmodulin-dependent thick and thin filament activating processes.

Differential gene expression in pubococcygeus muscle from patients with pelvic organ prolapse

OBJECTIVE

This study was undertaken to compare differential gene expression in the pubococcygeus muscle in patients with pelvic organ prolapse and controls.

STUDY DESIGN

We performed microarray analysis on individual pubococcygeus muscle biopsy specimens from five patients with stage III or IV pelvic organ prolapse and five control subjects without prolapse. This study received full Institutional Review Board approval. Total RNA was extracted, purified, and probed on the Human Genome U95A Array for each individual sample. RNA from patients and controls was not pooled. For microarray analysis, 7 microg of total RNA was used to synthesize complementary DNA that was then biotinylated. Arrays were hybridized for 16 hours in the GeneChip Fluidics Station 400 and were washed and scanned with the Hewlett-Packard GeneArray Scanner. Affymetrix GeneChip 5.0 software was used for scanning and data analysis.

RESULTS

Of the 12626 total genes compared, 257 genes were more than 2-fold underexpressed, 20 genes were more than 5-fold underexpressed, and 3 genes were more than 10-fold underexpressed in patients with pelvic organ prolapse compared with control subjects. Myosin-binding protein H was 24.7 times underexpressed in patients with prolapse (normalized signal intensity [NSI]: 0.46 [0.2-0.6]) compared with controls (NSI: 11.4 [0.2-31.3]). Skeletal muscle myosin heavy polypeptide 3 was 17.4 times underexpressed in patients with prolapse (NSI: 0.85 [0.7-0.9]) compared with controls (NSI: 14.8 [1.5-38.3]). Of the 12,626 genes compared, 479 genes were more than 2-fold overexpressed, 18 genes were more than 5-fold overexpressed, and 2 genes were more than 10-fold overexpressed in patients with pelvic organ prolapse compared with controls. Many of these overexpressed genes were related to actin and myosin proteins. Smooth muscle myosin heavy chain was 11.8 times overexpressed in patients (NSI: 5.21 [0.25-22.71]) compared with controls (NSI 0.44 [0.11-0.71]). Myosin light-chain kinase was 5.8 times overexpressed in patients (NSI: 7.9 [0.5-36.1]) compared with controls (NSI: 1.37 [0.38-1.8]). Extracellular matrix proteins were also differentially regulated. Cartilage oligomeric matrix protein precursor was found to be 6.0 times underexpressed, whereas tenascin-C (hexabrachion) was 5.1 times overexpressed in prolapse patients.

CONCLUSION

These data suggest that the differences between patients with advanced pelvic organ prolapse and controls may be related to differential gene expression of structural proteins that are related to actin and myosin as well as extracellular matrix proteins in the pubococcygeus muscle. Studies are ongoing to confirm these findings and to further characterize the role of these genes in prolapse.

Influence of ionic strength, actin state, and caldesmon construct size on the number of actin monomers in a caldesmon binding site

There is no consensus on the mechanism of inhibition of actin-myosin ATPase activity by caldesmon. Various models are based on different assumptions for the number of actin monomers that constitute a caldesmon binding site. Differences in binding behavior may be due to variations in the assay, the range of caldesmon concentrations, the type of caldesmon, and the method of data analysis used. We have evaluated these factors by measuring binding in the presence and absence of tropomyosin with both intact caldesmon and a recombinant 35 kDa actin binding fragment and with actin initially in the polymerized state or monomeric state. In all cases caldesmon binding could be simulated with a model having one class of binding sites. However, the number of actin monomers constituting a site was variable. Binding to F-actin at 165 mM ionic strength was best described with 7 actin monomers per site. When caldesmon bound to actin during the polymerization of G-actin, the size of the binding site was 3. Binding of the expressed truncated fragment, Cad35, could be described with 3 monomers per site. A simple interpretation of the data is that caldesmon binds tightly to 2-3 actin monomers. Additional parts of caldesmon bind less tightly to actin, causing caldesmon to cover approximately 7 actin monomers. The appendix contains an analysis of several binding curves with multiple binding site models. There is no compelling evidence for two classes of binding sites.

Theoretical studies on competitive binding of caldesmon and myosin S1 to actin: prediction of apparent cooperativity in equilibrium and slow-down in kinetics of S1 binding by caldesmon

Several laboratories have reported cooperative binding of S1 to actin in the presence of caldesmon. This cooperative binding has been interpreted with a model similar to that proposed for the binding of S1 to regulated actins in which the binding affinity of S1 is controlled by the position of the tropomyosin filaments. In a recent paper [Sen, A., Chen, Y., Yan, B., and Chalovich, J. M. (2001) Biochemistry 40, 5757-64], we showed qualitatively that S1 binding resulted in rapid dissociation of caldesmon from actin or actin-tropomyosin. This suggests that the cooperativity observed in the case of caldesmon is not due to a conformational change in actin-caldesmon but to the displacement of caldesmon. We show in this paper that the pure competitive binding model, in which both S1 and caldesmon are competing for the same binding sites on actin, can simulate quantitatively the effect of caldesmon on both the equilibrium and the kinetics of S1 binding to actin. This model successfully predicts an apparent cooperativity for the binding of S1 to actin-caldesmon without the need to assume multiple actin-caldesmon structures and produces a decreased rate of S1 binding to actin in the presence of caldesmon. This suggests that the inhibitory action of caldesmon on the actin-activated ATPase activity of myosin in solution and on the generation of active force in a contracting muscle may be simply due to the blocking of myosin binding sites on actin by caldesmon.

Canine mesenteric artery and vein convey no difference in the content of major contractile proteins

BACKGROUND

Mesenteric arteries and veins are composed of tonic smooth muscles and serve distinct functions in the peripheral circulation. However, the basis for the functional disparity of the resistive and capacitative parts of the mesenteric circulation is poorly understood. We studied potential differences in the expression levels of six contractile proteins in secondary and tertiary branches of the inferior mesenteric artery and vein along with differences in the vessel wall morphology.

RESULTS

Bright field and electron microscopy showed that both vessel walls had the same major structural elements. The arterial walls, however, had greater number, and more tightly assembled, smooth muscle cell layers compared to vein walls. The content of actin, myosin heavy chain, myosin light chain, and calponin was similar in the two blood vessels. The artery expressed higher amount of the actin-binding protein caldesmon than the vein (41.86 +/- 2.33 and 30.13 +/- 3.37 microg/mg respectively, n = 12). Although the total tropomyosin content was almost identical in both blood vessels, the alpha isoform dominated in the artery, while the beta isoform prevailed in the vein.

CONCLUSIONS

Canine mesenteric artery and vein differ in vessel wall morphology but do not convey differences in the expression levels of actin, myosin light chain, myosin heavy chain and calponin. The two vascular networks express distinct amounts of caldesmon and tropomyosin, which might contribute to the fine tuning of the contractile machinery in a manner consistent with the physiological functions of the two vascular networks.

Myosin light chain kinase binding to a unique site on F-actin revealed by three-dimensional image reconstruction

Ca2+-calmodulin-dependent phosphorylation of myosin regulatory light chains by the catalytic COOH-terminal half of myosin light chain kinase (MLCK) activates myosin II in smooth and nonmuscle cells. In addition, MLCK binds to thin filaments in situ and F-actin in vitro via a specific repeat motif in its NH2 terminus at a stoichiometry of one MLCK per three actin monomers. We have investigated the structural basis of MLCK-actin interactions by negative staining and helical reconstruction. F-actin was decorated with a peptide containing the NH2-terminal 147 residues of MLCK (MLCK-147) that binds to F-actin with high affinity. MLCK-147 caused formation of F-actin rafts, and single filaments within rafts were used for structural analysis. Three-dimensional reconstructions showed MLCK density on the extreme periphery of subdomain-1 of each actin monomer forming a bridge to the periphery of subdomain-4 of the azimuthally adjacent actin. Fitting the reconstruction to the atomic model of F-actin revealed interaction of MLCK-147 close to the COOH terminus of the first actin and near residues 228-232 of the second. This unique location enables MLCK to bind to actin without interfering with the binding of any other key actin-binding proteins, including myosin, tropomyosin, caldesmon, and calponin.

Caldesmon reduces the apparent rate of binding of myosin S1 to actin-tropomyosin

Equilibrium measurements of the rate of binding of caldesmon and myosin S1 to actin-tropomyosin from different laboratories have yielded different results and have led to different models of caldesmon function. An alternate approach to answering these questions is to study the kinetics of binding of both caldesmon and S1 to actin. We observed that caldesmon decreased the rate of binding of S1 to actin in a concentration-dependent manner. The inhibition of the rate of S1 binding was enhanced by tropomyosin, but the effect of tropomyosin on the binding was small. Premixing actin with S1 reduced the amplitude (extent) of caldesmon binding in proportion to the fraction of actin that contained bound S1, but the rate of binding of caldesmon to free sites was not greatly altered. No evidence for a stable caldesmon-actin-tropomyosin-S1 complex was observed, although S1 did apparently bind to gaps between caldesmon molecules. These results indicate that experiments involving caldesmon, actin, tropomyosin, and myosin are inherently complex. When the concentration of either S1 or caldesmon is varied, the amount of the other component bound to actin-tropomyosin cannot be assumed to remain fixed. The results are not readily explained by a mechanism in which caldesmon acts only by stabilizing an inactive state of actin-tropomyosin. The results support regulatory mechanisms that involve changes in the actin-S1 interaction.

Tropomyosin and actin isoforms modulate the localization of tropomyosin strands on actin filaments

Tropomyosin is present in virtually all eucaryotic cells, where it functions to modulate actin-myosin interaction and to stabilize actin filament structure. In striated muscle, tropomyosin regulates contractility by sterically blocking myosin-binding sites on actin in the relaxed state. On activation, tropomyosin moves away from these sites in two steps, one induced by Ca(2+) binding to troponin and a second by the binding of myosin to actin. In smooth muscle and non-muscle cells, where troponin is absent, the precise role and structural dynamics of tropomyosin on actin are poorly understood. Here, the location of tropomyosin on F-actin filaments free of troponin and other actin-binding proteins was determined to better understand the structural basis of its functioning in muscle and non-muscle cells. Using electron microscopy and three-dimensional image reconstruction, the association of a diverse set of wild-type and mutant actin and tropomyosin isoforms, from both muscle and non-muscle sources, was investigated. Tropomyosin position on actin appeared to be defined by two sets of binding interactions and tropomyosin localized on either the inner or the outer domain of actin, depending on the specific actin or tropomyosin isoform examined. Since these equilibrium positions depended on minor amino acid sequence differences among isoforms, we conclude that the energy barrier between thin filament states is small. Our results imply that, in striated muscles, troponin and myosin serve to stabilize tropomyosin in inhibitory and activating states, respectively. In addition, they are consistent with tropomyosin-dependent cooperative switching on and off of actomyosin-based motility. Finally, the locations of tropomyosin that we have determined suggest the possibility of significant competition between tropomyosin and other cellular actin-binding proteins. Based on these results, we present a general framework for tropomyosin modulation of motility and cytoskeletal modelling.

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.