Caldesmon inhibits active crossbridges in unstimulated vascular smooth muscle: an antisense oligodeoxynucleotide approach

[Caldesmon and tumor growth: prospects for optimizing diagnosis and targeted therapy]

Tumor invasion plays a key role in the progression of tumors. This process is regulated by the interactions of cells and tissues, in which physical, cellular and molecular determinants undergo changes throughout the entire period of progression of tumor growth. Tumor invasion is triggered and maintained by specialized signal cascades that control the dynamic state of the cytoskeleton in tumor cells, the processes of rearrangement of cell-matrix and intercellular connections, followed by cell migration to neighboring tissues. Studying the mechanisms of regulation of cell motor activity and determining its main regulators is an important task for understanding the pathophysiology of tumor growth. Caldesmon is an actin, myosin and calmodulin binding protein. It is involved in the regulation of smooth muscle contraction by inhibiting actin and myosin binding, in the formation of actin stress fibers, and in the transport of intracellular granules. Currently, caldesmon is considered as a potential biomarker of tumor cell invasion, migration, and metastasis. The study of signaling molecules involved in tumor progression, such as caldesmon, is necessary to predict response to chemotherapy and radiotherapy. This review highlights the main functions of caldesmon and analyzes its role in oncological pathology.

Vascular aging, the vascular cytoskeleton and aortic stiffness

Vascular aging, aortic stiffness and hypertension are mechanistically interrelated. The perspective presented here will focus mainly on the molecular mechanisms of age-associated increases in the stiffness of the vascular smooth muscle cell (VSMC). This review will highlight the mechanisms by which the VSMC contributes to disorders of vascular aging. Distinct functional sub-components of the vascular cell and the molecular mechanisms of the protein-protein interactions, signaling mechanisms and intracellular trafficking processes in the setting of the aging aorta will be detailed.

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.

Calmodulin-binding proteins: A journey of 40 years

The proteins which bind to calmodulin in a Ca²⁺-dependent and reversible manner are known as calmodulin-binding proteins. These proteins are involved in a multitude of processes in which Ca²⁺ and calmodulin play crucial roles. Our group elucidated the mechanism and importance of these proteins in normal and diseased conditions. Various calmodulin-binding proteins were discovered and purified from bovine tissue including a heat stable calmodulin-binding protein 70, calmodulin-dependent protein kinase VI and a high molecular weight calmodulin-binding protein (HMWCaMBP). We observed a complex interplay occurs between these and other Ca²⁺ and calmodulin-binding proteins during cardiac ischemia and reperfusion. Purified cardiac HMWCaMBP is a homolog form of calpastatin and an inhibitor of the Ca²⁺-activated cysteine proteases, calpains and therefore can have cardioprotective role in ischemic conditions. Calcineurin is a Ca²⁺ and calmodulin-dependent serine/threonine protein phosphatase showed increased phosphatase activity in ischemic heart through its direct interaction with Hsp70 and expression of calcineurin following ischemia suggests self-repair and favorable survival outcomes. Calcineurin was also found to be present in other tissues including the eye; where its expression and calcineurin phosphatase activity varied. In neurons, calcineurin may play a key role in initiating apoptosis-related pathways especially in epilepsy. In colorectal cancer we demonstrated high calcineurin phosphatase activity and simultaneous overexpression of calcineurin. The impact of calcineurin signaling on neuronal apoptosis in epilepsy and its use as a diagnostic marker for colorectal cancer requires in-depth study.

Regulation of mitogen-activated protein kinase by protein kinase C and mitogen-activated protein kinase phosphatase-1 in vascular smooth muscle

Vascular smooth muscle contraction is primarily regulated by phosphorylation of myosin light chain. There are also modulatory pathways that control the final level of force development. We tested the hypothesis that protein kinase C (PKC) and mitogen-activated protein (MAP) kinase modulate vascular smooth muscle activity via effects on MAP kinase phosphatase-1 (MKP-1). Swine carotid arteries were mounted for isometric force recording and subjected to histamine stimulation in the presence and absence of inhibitors of PKC [bisindolylmaleimide-1 (Bis)], MAP kinase kinase (MEK) (U0126), and MKP-1 (sanguinarine) and flash frozen for measurement of MAP kinase, PKC-potentiated myosin phosphatase inhibitor 17 (CPI-17), and caldesmon phosphorylation levels. CPI-17 was phosphorylated in response to histamine and was inhibited in the presence of Bis. Caldesmon phosphorylation levels increased in response to histamine stimulation and were decreased in response to MEK inhibition but were not affected by the addition of Bis. Inhibition of PKC significantly increased p42 MAP kinase, but not p44 MAP kinase. Inhibition of MEK with U0126 inhibited both p42 and p44 MAP kinase activity. Inhibition of MKP-1 with sanguinarine blocked the Bis-dependent increase of MAP kinase activity. Sanguinarine alone increased MAP kinase activity due to its effects on MKP-1. Sanguinarine increased MKP-1 phosphorylation, which was inhibited by inhibition of MAP kinase. This suggests that MAP kinase has a negative feedback role in inhibiting MKP-1 activity. Therefore, PKC catalyzes MKP-1 phosphorylation, which is reversed by MAP kinase. Thus the fine tuning of vascular contraction is due to the concerted effort of PKC, MAP kinase, and MKP-1.

Cortical actin regulation modulates vascular contractility and compliance in veins

Most cardiovascular research focuses on arterial mechanisms of disease, largely ignoring venous mechanisms. Here we examine ex vivo venous stiffness, spanning tissue to molecular levels, using biomechanics and magnetic microneedle technology, and show for the first time that venous stiffness is regulated by a molecular actin switch within the vascular smooth muscle cell in the wall of the vein. This switch connects the contractile apparatus within the cell to adhesion structures and facilitates stiffening of the vessel wall, regulating blood flow return to the heart. These studies also demonstrate that passive stiffness, the component of total stiffness not attributable to vascular smooth muscle activation, is severalfold lower in venous tissue than in arterial tissue. We show here that the activity of the smooth muscle cells plays a dominant role in determining total venous stiffness and regulating venous return. The literature on arterial mechanics is extensive, but far less is known about mechanisms controlling mechanical properties of veins. We use here a multi-scale approach to identify subcellular sources of venous stiffness. Portal vein tissue displays a severalfold decrease in passive stiffness compared to aortic tissues. The α-adrenergic agonist phenylephrine (PE) increased tissue stress and stiffness, both attenuated by cytochalasin D (CytoD) and PP2, inhibitors of actin polymerization and Src activity, respectively. We quantify, for the first time, cortical cellular stiffness in freshly isolated contractile vascular smooth muscle cells using magnetic microneedle technology. Cortical stiffness is significantly increased by PE and CytoD inhibits this increase but, surprisingly, PP2 does not. No detectable change in focal adhesion size, measured by immunofluorescence of FAK and zyxin, accompanies the PE-induced changes in cortical stiffness. Probing with phospho-specific antibodies confirmed activation of FAK/Src and ERK pathways and caldesmon phosphorylation. Thus, venous tissue stiffness is regulated both at the level of the smooth muscle cell cortex, via cortical actin polymerization, and by downstream smooth muscle effectors of Src/ERK signalling pathways. These findings identify novel potential molecular targets for the modulation of venous capacitance and venous return in health and disease.

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.

Ablation of smooth muscle caldesmon affects the relaxation kinetics of arterial muscle

Smooth muscle caldesmon (h-CaD) is an actin- and myosin-binding protein that reversibly inhibits the actomyosin ATPase activity in vitro. To test the function of h-CaD in vivo, we eliminated its expression in mice. The h-CaD-null animals appeared normal and fertile, although the litter size was smaller. Tissues from the homozygotes lacked h-CaD and exhibited upregulation of the non-muscle isoform, l-CaD, in visceral, but not vascular tonic smooth muscles. While the Ca(2+) sensitivity of force generation of h-CaD-deficient smooth muscle remained largely unchanged, the kinetic behavior during relaxation in arteries was different. Both intact and permeabilized arterial smooth muscle tissues from the knockout animals relaxed more slowly than those of the wild type. Since this difference occurred after myosin dephosphorylation was complete, the kinetic effect most likely resulted from slower detachment of unphosphorylated crossbridges. Detailed analyses revealed that the apparently slower relaxation of h-CaD-null smooth muscle was due to an increase in the amplitude of a slower component of the biphasic tension decay. While the identity of this slower process has not been unequivocally determined, we propose it reflects a thin filament state that elicits fewer re-attached crossbridges. Our finding that h-CaD modulates the rate of smooth muscle relaxation clearly supports a role in the control of vascular tone.

Bladder smooth muscle organ culture preparation maintains the contractile phenotype

Smooth muscle cells, when subjected to culture, modulate from a contractile to a secretory phenotype. This has hampered the use of cell culture for molecular techniques to study the regulation of smooth muscle biology. The goal of this study was to develop a new organ culture model of bladder smooth muscle (BSM) that would maintain the contractile phenotype and aid in the study of BSM biology. Our results showed that strips of BSM subjected to up to 9 days of organ culture maintained their contractile phenotype, including the ability to achieve near-control levels of force with a temporal profile similar to that of noncultured tissues. The technical aspects of our organ culture preparation that were responsible, in part, for the maintenance of the contractile phenotype were a slight longitudinal stretch during culture and subjection of the strips to daily contraction-relaxation. The tissues contained viable cells throughout the cross section of the strips. There was an increase in extracellular collagenous matrix, resulting in a leftward shift in the passive length-tension relationship. There were no significant changes in the content of smooth muscle-specific α-actin, calponin, h-caldesmon, total myosin heavy chain, protein kinase G, Rho kinase-I, or the ratio of SM1 to SM2 myosin isoforms. Moreover the organ cultured tissues maintained functional voltage-gated calcium channels and large-conductance calcium-activated potassium channels. Therefore, we propose that this novel BSM organ culture model maintains the contractile phenotype and will be a valuable tool for the use in cellular/molecular biology studies of bladder myocytes.

Smooth muscle tension induces invasive remodeling of the zebrafish intestine

The signals that initiate cell invasion are not well understood, but there is increasing evidence that extracellular physical signals play an important role. Here we show that epithelial cell invasion in the intestine of zebrafish meltdown (mlt) mutants arises in response to unregulated contractile tone in the surrounding smooth muscle cell layer. Physical signaling in mlt drives formation of membrane protrusions within the epithelium that resemble invadopodia, matrix-degrading protrusions present in invasive cancer cells. Knockdown of Tks5, a Src substrate that is required for invadopodia formation in mammalian cells blocked formation of the protrusions and rescued invasion in mlt. Activation of Src-signaling induced invadopodia-like protrusions in wild type epithelial cells, however the cells did not migrate into the tissue stroma, thus indicating that the protrusions were required but not sufficient for invasion in this in vivo model. Transcriptional profiling experiments showed that genes responsive to reactive oxygen species (ROS) were upregulated in mlt larvae. ROS generators induced invadopodia-like protrusions and invasion in heterozygous mlt larvae but had no effect in wild type larvae. Co-activation of oncogenic Ras and Wnt signaling enhanced the responsiveness of mlt heterozygotes to the ROS generators. These findings present the first direct evidence that invadopodia play a role in tissue cell invasion in vivo. In addition, they identify an inducible physical signaling pathway sensitive to redox and oncogenic signaling that can drive this process.

Smooth muscle caldesmon modulates peristalsis in the wild type and non-innervated zebrafish intestine

BACKGROUND

The high molecular weight isoform of the actin-binding protein Caldesmon (h-CaD) regulates smooth muscle contractile function by modulating cross-bridge cycling of myosin heads. The normal inhibitory activity of h-CaD is regulated by the enteric nervous system; however, the role of h-CaD during intestinal peristalsis has never been studied.

METHODS

We identified a zebrafish paralog of the human CALD1 gene that encodes an h-CaD isoform expressed in intestinal smooth muscle. We examined the role of h-CaD during intestinal peristalsis in zebrafish larvae by knocking down the h-CaD protein using an antisense morpholino oligonucleotide. We also developed transgenic zebrafish that express inhibitory peptides derived from the h-CaD myosin and actin-binding domains, and examined their effect on peristalsis in wild-type zebrafish larvae and sox10 (colourless) mutant larvae that lack enteric nerves.

KEY RESULTS

Genomic analyses identified two zebrafish Caldesmon paralogs. The cald1a ortholog encoded a high molecular weight isoform generated by alternative splicing whose intestinal expression was restricted to smooth muscle. Propulsive intestinal peristalsis was increased in wild-type zebrafish larvae by h-CaD knockdown and by expression of transgenes encoding inhibitory myosin and actin-binding domain peptides. Peristalsis in the non-innervated intestine of sox10 (colourless) larvae was partially restored by h-CaD knockdown and expression of the myosin-binding peptide.

CONCLUSIONS & INFERENCES

Disruption of the normal inhibitory function of h-CaD enhances intestinal peristalsis in both wild-type zebrafish larvae and mutant larvae that lack enteric nerves, thus confirming a physiologic role for regulation of smooth muscle contraction at the actin filament.

Expression of genes encoding smooth muscle contractile proteins in vaginal tissue of women with and without pelvic organ prolapse

AIMS

We hypothesize that the expression of genes encoding vaginal smooth muscle (SM) contractile proteins is altered in patients with pelvic organ prolapse (POP) and is influenced by age and menopausal status. We aim to analyze the expression of SM-myosin heavy chain (MHY11), caldesmon (CALD1), SM gamma-actin (ACTG2), and tropomyosin (TPM1), in premenopausal and postmenopausal women with advanced POP and asymptomatic controls.

METHODS

During total hysterectomy we collected anterior vaginal wall biopsy samples from 55 women, 37 premenopausal (23 patients and 14 controls), and 18 postmenopausal women (13 patients and 5 controls). Total mRNA from the tissues was quantified by real-time RT-PCR.

RESULTS

MHY11 gene expression was down-regulated in premenopausal POP patients compared to premenopausal controls (fivefold, P = 0.002). In the postmenopausal groups, we observed a sixfold increase in the CALD1 gene expression in POP patients compared to asymptomatic controls (P = 0.03). The gene expression of CALD1, ACTG2, and TPM1 was significantly down-regulated in vaginal tissue of healthy women after menopause (P < 0.05).

CONCLUSION

Dysregulation of the vaginal SM content in POP patients involves alteration of different cellular pathways according to age and menopausal status.

From molecule to man: integrating molecular biology with whole organ physiology in studying respiratory disease

Chronic lung diseases such as asthma, chronic obstructive pulmonary disease (COPD), and idiopathic pulmonary fibrosis (IPF) are all characterized by structural changes of the airways and/or lungs that limit airflow and/or gas exchange. Currently, there is no therapy available that adequately targets the structural remodeling of the airways and lungs in these diseases. This underscores the great need for insight into the mechanisms that underpin the development of airway remodeling, fibrosis and emphysema in these diseases, in order to identify suitable drug targets. It is increasingly evident that structural cell-cell communication within the lung is central to the development of remodeling, indicating that a more integrative approach should be considered when studying molecular and cellular mechanisms of remodeling. Therefore, there is a great need to study molecular and cellular physiological and pathophysiological mechanisms in as much detail as possible, but with as little as possible loss of the physiological context. Here, we will review the use of models such as cellular co-culture, tissue culture, and lung slice culture, in which cell-cell communication and tissue architecture are better preserved or mimicked than in cell culture, and zoom in on the usefulness of molecular and cellular biological tools in these complex model systems to read out or control signaling and gene/protein regulation.

Caldesmon regulates the motility of vascular smooth muscle cells by modulating the actin cytoskeleton stability

Background

Migration of vascular smooth muscle cells (SMCs) from the media to intima constitutes a critical step in the development of proliferative vascular diseases. To elucidate the regulatory mechanism of vacular SMC motility, the roles of caldesmon (CaD) and its phosphorylation were investigated.

Methods

We have performed Transwell migration assays, immunofluorescence microscopy, traction microscopy and cell rounding assays using A7r5 cells transfected with EGFP (control), EGFP-wtCaD or phosphomimetic CaD mutants, including EGFP-A1A2 (the two PAK sites Ser452 and Ser482 converted to Ala), EGFP-A3A4 (the two Erk sites Ser497 and Ser527 converted to Ala), EGFP-A1234 (both PAK- and Erk-sites converted to Ala) and EGFP-D1234 (both PAK- and Erk-sites converted to Asp).

Results

We found that cells transfected with wtCaD, A1A2 or A3A4 mutants of CaD migrated at a rate approximately 50% more slowly than those EGFP-transfected cells. The migration activity for A1234 cells was only about 13% of control cells. Thus it seems both MAPK and PAK contribute to the motility of A7r5 cells and the effects are comparable and additive. The A1234 mutant also gave rise to highest strain energy and lowest rate of cell rounding. The migratory and contractile properties of these cells are consistent with stabilized actin cytoskeletal structures. Indeed, the A1234 mutant cells exhibited most robust stress fibers, whereas cells transfected with wtCaD or A3A4 (and A1A2) had moderately reinforced actin cytoskeleton. The control cells (transfected with EGFP alone) exhibited actin cytoskeleton that was similar to that in untransfected cells, and also migrated at about the same speed as the untransfected cells.

Conclusions

These results suggest that both the expression level and the level of MAPK- and/or PAK-mediated phosphorylation of CaD play key roles in regulating the cell motility by modulating the actin cytoskeleton stability in dedifferentiated vascular SMCs such as A7r5.

MEK modulates force-fluctuation-induced relengthening of canine tracheal smooth muscle

Tidal breathing, and especially deep breathing, is known to antagonise bronchoconstriction caused by airway smooth muscle (ASM) contraction; however, this bronchoprotective effect of breathing is impaired in asthma. Force fluctuations applied to contracted ASM in vitro cause it to relengthen, force-fluctuation-induced relengthening (FFIR). Given that breathing generates similar force fluctuations in ASM, FFIR represents a likely mechanism by which breathing antagonises bronchoconstriction. Thus it is of considerable interest to understand what modulates FFIR, and how ASM might be manipulated to exploit this phenomenon. It was demonstrated previously that p38 mitogen-activated protein kinase (MAPK) signalling regulates FFIR in ASM strips. Here, it was hypothesised that the MAPK kinase (MEK) signalling pathway also modulates FFIR. In order to test this hypothesis, changes in FFIR were measured in ASM treated with the MEK inhibitor, U0126 (1,4-diamino-2,3-dicyano-1,4-bis[2-aminophenylthio]butadiene). Increasing concentrations of U0126 caused greater FFIR. U0126 reduced extracellular signal-regulated kinase 1/2 phosphorylation without affecting isotonic shortening or 20-kDa myosin light chain and p38 MAPK phosphorylation. However, increasing concentrations of U0126 progressively blunted phosphorylation of high-molecular-weight caldesmon (h-caldesmon), a downstream target of MEK. Thus changes in FFIR exhibited significant negative correlation with h-caldesmon phosphorylation. The present data demonstrate that FFIR is regulated through MEK signalling, and suggest that the role of MEK is mediated, in part, through caldesmon.

siRNA-mediated knockdown of h-caldesmon in vascular smooth muscle

Smooth muscle contraction involves phosphorylation of the regulatory myosin light chain. However, this thick-filament system of regulation cannot account for all aspects of a smooth muscle contraction. An alternate site of contractile regulation may be in the thin-filament-associated proteins, in particular caldesmon. Caldesmon has been proposed to be an inhibitory protein that acts either as a brake to stop any increase in resting or basal tone, or as a modulatory protein during contraction. The goal of this study was to use short interfering RNA technology to decrease the levels of the smooth muscle-specific isoform of caldesmon in intact vascular smooth muscle tissue to determine more carefully what role(s) caldesmon has in smooth muscle regulation. Intact strips of vascular tissue depleted of caldesmon produced significant levels of shortening velocity, indicative of cross-bridge cycling, in the unstimulated tissue and exhibited lower levels of contractile force to histamine. Our results also suggest that caldesmon does not play a role in the cooperative activation of unphosphorylated cross bridges by phosphorylated cross bridges. The velocity of shortening of the constitutively active tissue and the high basal values of myosin light chain phosphorylation suggest that h-caldesmon in vivo acts as a brake against contractions due to basally phosphorylated myosin. It is also possible that phosphorylation of h-caldesmon alone in the resting state may be a mechanism to produce increases in force without stimulation and increases in calcium. Disinhibition of h-caldesmon by phosphorylation would then allow force to be developed by activated myosin in the resting state.

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.

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.

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.

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.

Regulation of alpha1-adrenoceptor-mediated contractions of the uterine artery by protein kinase C: role of the thick- and thin-filament regulatory pathways

Previously we demonstrated that activation of protein kinase C (PKC) enhanced alpha(1)-adrenoceptor-induced contractions in nonpregnant uterine arteries (NPUA) by increasing the Ca(2+) sensitivity but that it inhibited the contractions in pregnant uterine arteries (PUA) by decreasing intracellular Ca(2+) mobilization. The present study tested the hypothesis that PKC activation differentially regulated the thick- and thin-filament regulatory pathways in alpha(1)-adrenoceptor-induced contractions of NPUA and PUA in sheep. Simultaneous measurements of contractions and phosphorylation levels of 20-kDa regulatory myosin light chain (LC(20)) in the same tissue revealed that the PKC activator phorbol-12,13-dibutyrate (PDBu) inhibited phenylephrine-induced phosphorylation of LC(20) and contractions in PUA. In NPUA, PDBu significantly potentiated phenylephrine-induced contractions without significantly changing phosphorylation levels of LC(20). Further studies in NPUA demonstrated that PDBu-mediated potentiation of phenylephrine-induced contractions was associated with a significant increase in phosphorylation levels of extracellular signal-regulated kinase (ERK(42/44)) and caldesmon-Ser(789), measured simultaneously with the tension in the same tissue. In addition, the ERK(42/44) inhibitor PD98059 [2-(2-amino-3-methoxyphenyl)-4H-1-benzopyran-4-one] and the actin polymerization inhibitor cytochalasin B produced a concentration-dependent inhibition of PDBu-mediated potentiation of phenylephrine-induced contractions in NPUA. The results suggest that activation of PKC inhibits alpha(1)-adrenoceptor-mediated contractions in PUA through down-regulation of the thick-filament pathway and decreased myosin light chain phosphorylation, but that it enhances the contractions in NPUA through its effect on the thin-filament regulatory pathway and activation of ERK/caldesmon and actin polymerization.

siRNA knock down of casein kinase 2 increases force and cross-bridge cycling rates in vascular smooth muscle

Contraction of smooth muscle involves myosin light chain (MLC) kinase catalyzed phosphorylation of the regulatory MLC, activation of myosin, and the development of force. However, this cannot account for all aspects of a smooth muscle contraction, suggesting that other regulatory mechanisms exist. One potentially important technique to study alternative sites of contractile regulation is the use of small interfering RNA (siRNA). The goal of this study was to determine whether siRNA technology can decrease the levels of a specific protein and allow for the determination of how that protein affects contractile regulation. To achieve this goal, we tested the hypothesis that casein kinase 2 (CK2) is part of the complex regulatory scheme present in vascular smooth muscle. Using intact strips of swine carotid artery, we determined that siRNA against CK2 produced a tissue that resulted in a ∼60% knockdown after 4 days in organ culture. Intact strips of vascular tissue depleted of CK2 produced greater levels of force and exhibited an increased sensitivity to all stimuli tested. This was accompanied by an increase in cross-bridge cycling rates but not by a change in MLC phosphorylation levels. α-Toxin-permeabilized vascular tissue depleted of CK2 also showed an increased sensitivity to calcium compared with control tissues. Our results demonstrate that siRNA is a viable technique with which to study regulatory pathways in intact smooth muscle tissue. Our results also demonstrate that CK2 plays an important role in the mechanism(s) responsible for the development of force and cross-bridge cycling by a MLC phosphorylation-independent pathway.

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.

A mechanical model for adjustable passive stiffness in rabbit detrusor

Strips of rabbit detrusor smooth muscle (DSM) exhibit adjustable passive stiffness characterized by strain softening: a loss of stiffness on stretch to a new length distinct from viscoelastic behavior. At the molecular level, strain softening appears to be caused by cross-link breakage and is essentially irreversible when DSM is maintained under passive conditions (i.e., when cross bridges are not cycling to produce active force). However, on DSM activation, strain softening is reversible and likely due to cross-link reformation. Thus DSM displays adjustable passive stiffness that is dependent on the history of both muscle strain and activation. The present study provides empirical data showing that, in DSM, 1) passive isometric force relaxation includes a very slow component requiring hours to approach steady state, 2) the level of passive force maintained at steady state is less if the tissue has previously been strain softened, and 3) tissues subjected to a quick-release protocol exhibit a biphasic response consisting of passive force redevelopment followed by force relaxation. To explain these and previously identified characteristics, a mechanical model for adjustable passive stiffness is proposed based on the addition of a novel cross-linking element to a hybrid Kelvin/Voigt viscoelastic model.

Proteomic approach to identify changes in protein expression modified by 17β-oestradiol in bovine vascular smooth muscle cells

The aim of the present study was to use proteomics to analyse modifications in the level of expression of different proteins in BVSMCs (bovine vascular smooth muscle cells) incubated in the absence and presence of 17β-oestradiol. By using two-dimensional electrophoresis with a pH range of 4–7, we identified several areas on the gels in which the level of expression of proteins were different between control BVSMCs and cells incubated for 24 h with 17β-oestradiol. Changes in several isoforms of α-enolase, HSP60 (heat-shock protein 60), vimentin and PDI (protein disulphide-isomerase) were observed in BVSMCs. The expression of α-enolase isoform 1 was enhanced after 17β-oestradiol treatment. The expression of HSP60 isoform 3, vimentin isoforms 2 and 3 and caldesmon was reduced by 17β-oestradiol. Finally, the expression of PDI isoforms was reduced by 17β-oestradiol. In summary, 17β-oestradiol modified the expression of isoforms of proteins associated with smooth muscle cell proliferation (α-enolase, vimentin and HSP-60), cell contraction (vimentin and caldesmon) and cell redox modulation (PDI). These findings confirm that 17β-oestradiol may modulate a wide range of signalling pathways in vascular smooth muscle cells.

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.

Alpha1-adrenoceptor-mediated phosphorylation of MYPT-1 and CPI-17 in the uterine artery: role of ERK/PKC

We previously demonstrated that ERK/PKC signaling pathways play a key role in regulation of Ca(2+) sensitivity and contractility of the uterine artery. The present study tested the hypothesis that ERK and PKC differentially regulated myosin light chain phosphatase activity by phosphorylation of myosin phosphatase target protein-1 (MYPT-1) and CPI-17. Agonist-induced contractions and phosphorylation of MYPT-1/Thr(696), MYPT-1/Thr(850), and CPI-17/Thr(38) were measured simultaneously in the same tissues of isolated near-term pregnant ovine uterine arteries. Phenylephrine produced time-dependent concurrent increases in the phosphorylation of ERK(44/42) and MYPT-1/Thr(850) that preceded contractions. In addition, phenylephrine induced phosphorylation of CPI-17/Thr(38) that was concurrent with the contractions. In contrast, phenylephrine did not induce phosphorylation of MYPT-1/Thr(696) in the uterine artery. PD-098059 inhibited phosphorylation of ERK(44/42) and the initial peak phosphorylation of MYPT-1/Thr(850) but did not affect CPI-17/Thr(38) phosphorylation. Activation of PKC by phorbol 12,13-dibutyrate induced a time-dependent phosphorylation of CPI-17/Thr(38) that preceded contractions of the uterine artery. In addition, phorbol 12,13-dibutyrate activated PKC-alpha and induced a coimmunoprecipitation of PKC-alpha with caldesmon. The results suggest that phosphorylation of MYPT-1/Thr(850) and CPI-17/Thr(38) play important roles in regulation of agonist-mediated Ca(2+) sensitivity in the uterine artery, in part by ERK and PKC, respectively. In addition, phosphorylated CPI-17 may regulate Ca(2+) sensitivity by interacting with caldesmon and reversing its inhibitory effect on myosin ATPase.

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.

Inhibition of protein kinase C-mediated contraction by Rho kinase inhibitor fasudil in rabbit aorta

Protein kinase C (PKC) activation by a phorbol ester increases myosin light chain (MLC(20)) phosphorylation through inhibition of MLC phosphatase (MLCP) and enhances contraction of vascular smooth muscle. We investigated whether Rho kinase, which is known to inhibit MLCP, is involved in the MLC(20) phosphorylation caused by a phorbol ester, 12-deoxyphorbol 13-isobutyrate (DPB), in rabbit aortas. DPB (1 microM) increased MLC(20) phosphorylation and tension. The Rho kinase inhibitor fasudil (10 microM) inhibited the DPB-induced contraction and decreased the MLC(20) phosphorylation at Ser19, a site phosphorylated by MLC kinase, although it did not affect the phosphorylation of total MLC(20). Rinsing a 65.4 mM KCl-contracted aorta with Ca(2+)-free, EGTA solution caused rapid dephosphorylation of MLC(20) and relaxation. When DPB was present in the rinsing solution, the MLC(20) dephosphorylation and the relaxation were inhibited. In this protocol, Ro31-8220 (10 microM), a PKC inhibitor, suppressed the phosphorylation of total MLC(20) and Ser19 induced by DPB. Fasudil also inhibited the Ser19 phosphorylation to a degree similar to Ro31-8220 and accelerated relaxation, which was less than the relaxation caused by Ro31-8220. The phospholipase A(2) inhibitor ONO-RS-082 (5 microM) inhibited the DPB-induced Ser19 phosphorylation but only transiently decreased the tension, suggesting the involvement of arachidonic acid in the phosphorylation and the existence of a MLC(20) phosphorylation-independent mechanism. When fasudil was combined with ONO-RS-082, fasudil exerted additional inhibition of the tension without further inhibition of the Ser19 phosphorylation. DPB phosphorylated the 130 kDa myosin binding subunit (MBS) of MLCP and fasudil inhibited the phosphorylation. These data suggest that the inhibition by fasudil of DPB-induced contraction and phosphorylation of MLC(20) at the MLC kinase-targeted site is a result of inhibition of Rho kinase. Thus, the PKC-dependent Ca(2+)-sensitization of vascular smooth muscle involves Rho kinase. A MLC(20) phosphorylation-independent mechanism is also involved in the Ca(2+)-sensitization.

Diabetes decreases rabbit bladder smooth muscle contraction while increasing levels of myosin light chain phosphorylation

The effect of diabetes mellitus on the regulation of urinary bladder smooth muscle contraction was studied. Diabetes was induced in the rabbit by alloxan injection followed by 16 wk of housing. The bladder was harvested and strips of wall devoid of both mucosa and serosa were examined. Intact strips of bladder smooth muscle from diabetic animals produced less stress in response to membrane depolarization than muscle from control animals; sensitivity to KCl was not changed. Carbachol responses were similar in muscle strips from the two animal groups. Basal myosin light chain (MLC) phosphorylation levels were significantly elevated in response to most stimuli in muscle strips from diabetic animals, although levels of stress were either unchanged or lower. alpha-Toxin-permeabilized strips that allow for control of the intracellular environment while maintaining excitation-contraction coupling showed increased levels of MLC phosphorylation but decreased sensitivity to activator Ca2+ in smooth muscle from diabetic animals. MLC phosphatase contents were similar in smooth muscle from the two animal groups; however, MLC phosphatase activity was greater in muscle from control compared with diabetic animals. These results suggest that diabetes mellitus uncouples basal MLC phosphorylation from force in the bladder smooth muscle cell.

ERK-mediated uterine artery contraction: role of thick and thin filament regulatory pathways

We have demonstrated that extracellular signal-regulated kinase (ERK) plays an important role in the regulation of uterine artery contraction. The present study tested the hypothesis that ERK regulates thick and thin filament regulatory pathways in the uterine artery. Isometric tension, intracellular free Ca2+ concentration ([Ca2+]i), and 20-kDa myosin light chain (LC20) phosphorylation were measured simultaneously in uterine arteries isolated from near-term (140 days gestation) pregnant sheep. Phenylephrine produced time-dependent increases in [Ca2+]i and LC20 phosphorylation that preceded the contraction, which were inhibited by the MEK (ERK) inhibitor PD-098059. In addition, PD-098059 decreased the intercept of the regression line of LC20 phosphorylation vs. [Ca2+]i but increased the rate of tension development vs. LC20 phosphorylation. In contrast to phenylephrine, phorbol 12,13-bibutyrate (PDBu) produced contractions without changing [Ca2+]i or LC20 phosphorylation. PD-098059 potentiated PDBu-induced contractions without affecting [Ca2+]i and LC20 phosphorylation. PDBu produced time-dependent increases in phosphorylation of p42 and p44 ERK and ERK-dependent phosphorylation of caldesmon at Ser789 in the uterine artery. PD-098059 blocked PDBu-mediated phosphorylation of p42 and p44 ERK and caldesmon. The results indicate that ERK may regulate force by a dual regulation of thick and thin filaments in uterine artery smooth muscle. ERK potentiates the thick filament regulatory pathway by enhancing LC20 phosphorylation via increases in [Ca2+]i and Ca2+ sensitivity of LC20 phosphorylation. In contrast, ERK attenuates the thin filament regulatory pathway and suppresses contractions independent of changes in LC20 phosphorylation in the uterine artery.

Alpha1-adrenergic signaling mechanisms in contraction of resistance arteries

Our goal in this review is to provide a comprehensive, integrated view of the numerous signaling pathways that are activated by alpha(1)-adrenoceptors and control actin-myosin interactions (i.e., crossbridge cycling and force generation) in mammalian arterial smooth muscle. These signaling pathways may be categorized broadly as leading either to thick (myosin) filament regulation or to thin (actin) filament regulation. Thick filament regulation encompasses both "Ca(2+) activation" and "Ca(2+)-sensitization" as it involves both activation of myosin light chain kinase (MLCK) by Ca(2+)-calmodulin and regulation of myosin light chain phosphatase (MLCP) activity. With respect to Ca(2+) activation, adrenergically induced Ca(2+) transients in individual smooth muscle cells of intact arteries are now being shown by high resolution imaging to be sarcoplasmic reticulum-dependent asynchronous propagating Ca(2+) waves. These waves differ from the spatially uniform increases in [Ca(2+)] previously assumed. Similarly, imaging during adrenergic activation has revealed the dynamic translocation, to membranes and other subcellular sites, of protein kinases (e.g., Ca(2+)-activated protein kinases, PKCs) that are involved in regulation of MLCP and thus in "Ca(2+) sensitization" of contraction. Thin filament regulation includes the possible disinhibition of actin-myosin interactions by phosphorylation of CaD, possibly by mitogen-activated protein (MAP) kinases that are also translocated during adrenergic activation. An hypothesis for the mechanisms of adrenergic activation of small arteries is advanced. This involves asynchronous Ca(2+) waves in individual SMC, synchronous Ca(2+) oscillations (at high levels of adrenergic activation), Ca(2+) sparks, "Ca(2+)-sensitization" by PKC and Rho-associated kinase (ROK), and thin filament mechanisms.

Caldesmon phosphorylation is catalyzed by two kinases in permeabilized and intact vascular smooth muscle

Smooth muscle contraction is initiated by myosin light chain (MLC) phosphorylation catalyzed by the Ca(2+) dependent MLC kinase. However, many aspects of smooth muscle contraction cannot be accounted for by MLC phosphorylation. One hypothesis that has received experimental support involves the thin filament protein caldesmon. Caldesmon inhibits myosin ATPase activity; phosphorylation of caldesmon relieves this inhibitory effect. The primary candidates for catalysis of caldesmon phosphorylation are the p42/p44 ERK MAP kinases. However, we and others have shown that inhibition of the ERK MAP kinases has no effect on many smooth muscles. The goal of this study was to determine if evidence for a second endogenous caldesmon kinase may be obtained. We used Triton X-100 skinned and intact tissues of the swine carotid artery to address this goal. Caldesmon phosphorylation was evident in resting and Ca(2+) stimulated Triton X-100 skinned fibers. Ca(2+)-dependent caldesmon phosphorylation was partially sensitive to the ERK MAP kinase inhibitor PD98059, whereas all caldesmon phosphorylation was sensitive to the general kinase inhibitor, staurosporine. Histamine increased caldesmon phosphorylation levels in intact swine carotid artery, which was sensitive to both PD98059 and staurosporine. Histamine increased ERK MAP kinase activity, which was reversed by PD98059, staurosporine, and EGTA. Histamine-induced contractions were inhibited by staurosporine but not by PD98059. We interpret these results to suggest that although ERK MAP kinases catalyze caldesmon phosphorylation, a second staurosporine sensitive kinase is also important in caldesmon phosphorylation and it is this pathway that may be more important in contractile regulation.

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.

Effect of partial outlet obstruction on rabbit urinary bladder smooth muscle function

Bladder outlet obstruction secondary to benign prostate hyperplasia is associated with many cellular changes. This study was designed to determine whether these changes involve the contractile apparatus. Bladder smooth muscles from rabbits subjected to partial outlet obstruction for 2 wk were mounted for isometric force, isotonic shortening velocity, and myosin light chain (MLC) phosphorylation levels. Muscle strips from obstructed bladders exhibited spontaneous phasic activity; muscle strips from control bladders did not. Muscle strips from obstructed bladders exhibited increased sensitivity and higher levels of stress in response to the cumulative addition of KCl or carbachol compared with control. During noncumulative addition of KCl or carbachol, no differences in sensitivity were noted. Muscle strips from obstructed bladders had elevated basal MLC phosphorylation levels and stimulation produced small increases in MLC phosphorylation compared with control. V(max) during KCl stimulation of muscle strips from obstructed bladders was 10-fold lower than control. Our results suggest that bladder outlet obstruction produces a muscle cell that develops higher levels of force but with greatly reduced cross bridge cycling rates.

The smooth muscle myosin seven amino acid heavy chain insert's kinetic role in the crossbridge cycle for mouse bladder

The seven amino acid insert in the smooth muscle myosin heavy chain is thought to regulate the kinetics of contraction, contributing to the differences between fast and slow smooth muscle. The effects of this insert on force and stiffness were determined in bladder tissue of a transgenic mouse line expressing the insert SMB at one of three levels: an SMB wild type (+/+), an SMA homozygous type (-/-) and a heterozygous type (+/-). For skinned muscle, an increase in MgADP or inorganic phosphate (Pi) should shift the distribution of crossbridges in the actomyosin ATPase (AMATPase) to increase the relative population of the crossbridge state prior to ADP release and Pi release, respectively. Exogenous ADP increased force and stiffness in a manner consistent with increasing the Ca2+ concentration in both the +/+ and +/- mouse types. However, the -/- type showed a significantly greater increase in force than in stiffness suggesting that immediately prior to ADP release, the AMATPase either has an additional force producing isomerization state or a slower ADP dissociation rate for the -/- type compared to the +/+ or +/- types. Exogenous Pi led to a significantly greater decrease in stiffness than in force for all three mouse types suggesting that there is a force producing state prior to Pi release. In addition, the increase in Pi showed similar changes in the +/+ and -/- types whereas in the +/- type the decreases in both force and stiffness were greater than the other two mouse types indicating that the insert can affect the cooperativity between myosin heads. In conclusion, the seven amino acid insert modulates the kinetics and/or states of the AMATPase, which could lead to differences in the kinetics of contraction between fast and slow smooth muscle.

ERK inhibition attenuates 5-HT-induced contractions in fetal and adult ovine carotid arteries

Growth and differentiation-related pathways are much more active in immature than in mature, fully differentiated smooth muscle. Because mitogen-activated protein kinases (MAPK) are intimately involved with growth and differentiation, and the extracellular signal-regulated kinase (ERK) subfamily of MAPKs are involved in some contractile responses, the present studies examined the hypothesis that ERKs play an important and age-dependent role in smooth muscle contraction. The MAPK inhibitors PD098059 and UO126 both inhibited serotonin (5-HT) concentration-response relations more effectively in carotid arteries from term fetal lambs, than in corresponding arteries from mature non-pregnant adult sheep. This inhibition involved significant decreases in both the pD2 (adult: 2-fold; fetus: 4- to 15-fold) and the maximum efficacy (adult: 15-19%; fetus: 34-39%) of 5-HT. Accompanying this age-dependent effect on contraction, quantitative Western blot assays revealed that ERK1 and ERK2 abundances were 39% and 164% greater, respectively, in fetal than in adult carotid arteries. The abundance of the putative ERK target, caldesmon, however, was about 7-fold greater in adult than in fetal arteries. Together, the present results support the view that ERK abundance and activity is upregulated in fetal relative to adult arteries, and that one consequence of this upregulation is that the contribution of ERKs to contraction, at least that initiated by 5-HT2a receptors, is greater in fetal than adult carotid arteries. Whereas the phosphorylation mechanisms through which ERKs augment contraction remain uncertain and controversial, the present results suggest that emphasis should be shifted away from caldesmon and toward other critical contractile proteins, and how these proteins may contribute differently to development of agonist-induced contractile force in immature and mature arteries.

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

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

Quinine inhibits vascular contraction independent of effects on calcium or myosin phosphorylation

This report contains results of studies designed to determine whether quinine has direct effects on myofilament Ca2+ sensitization in addition to effects on Ca2+. Quinine decreased the EC50 value and maximal contraction of intact arterial strips to histamine. Incubation of arterial strips with indomethacin or 1H-[1,2,4]oxadiazole[4,3-alpha]quinoxalin-1-one did not alter quinine inhibition, suggesting that the effect is not mediated via cyclooxygenase or cGMP. Pretreatment of strips with quinine had no effect on the histamine-dependent increases in myosin light chain phosphorylation levels. Quinine inhibited Ca2+-induced contraction in alpha-toxin permeabilized strips, but not the Ca2+-induced contraction in Triton X-100 permeabilized strips. Pretreatment of the alpha-toxin permeabilized strips with quinine before stimulation with guanosine-5'-O-(3-thio)triphosphate (GTPgammaS) did not have any effect on the response. In conclusion, quinine inhibited Ca2+-dependent contractions of the alpha-toxin permeabilized strips, which retain modulatory pathways both upstream and downstream from the contractile proteins but did not inhibit GTPgammaS-dependent contraction of the alpha-toxin permeabilized preparation important in upstream modulation of the contraction. Moreover, quinine did not inhibit the Ca2+-dependent contractions of the Triton X-100 permeabilized strips, which are devoid of all modulatory pathways. This suggests that quinine does not act upstream from or directly on the contractile proteins. A more likely site of action may be downstream of the contractile proteins and specifically at the coupling of the contractile proteins with the physiological endpoint of force development.

Signal transduction pathways in cerebral vasospasm

Cerebral vasospasm is a deadly complication following the rupture of intracranial aneurysms. The time course of cerebral vasospasm is unique in that it is slow developing, usually takes 4-7 days to peak, but lasts up to 2-3 weeks, and is resistant to most known vasodilators. These special features make cerebral vasospasm the most important determinant in the outcome of patients suffering subarachnoid hemorrhage. The available treatment strategies include mechanical dilation of spastic cerebral arteries (angioplasty) and non-selective vasodilatation such as by Ca(2+) channel blockers. One new development in the experimental treatment of cerebral vasospasm is the looming target of signaling pathways. Understanding vasospastic signals in cerebral arteries might offer a new avenue for selective treatment of cerebral vasospasm in the future.

ERK1/2-mediated phosphorylation of myometrial caldesmon during pregnancy and labor

We used a timed-pregnant rat model to track changes in myometrial contractility during pregnancy and labor and to correlate these changes with upstream signaling events. Myometrium was harvested from CO(2)-euthanized rats. Although contraction amplitudes increased at 16 and 20 days of pregnancy, contraction incidence and area under the force curve were inhibited, consistent with the myometrial quiescence of pregnancy. The Ca(2+) sensitivity of contraction was decreased at 20 days of pregnancy and this was partially reversed in labor. The protein content of h-caldesmon (h-CaD) was increased in pregnancy. A 40-fold increase in the signal from a phospho-CaD antibody specific for phosphorylation at an ERK1/2 site occurred during labor. ERK1/2 activation increased significantly at the onset of labor. Myosin light chain phosphorylation (LC20-P) increased significantly in labor compared with the nonpregnant state. Thus we conclude that the increase in CaD protein content during pregnancy may contribute to a suppression of the contractility of pregnant myometrium. Conversely, CaD phosphorylation, through an ERK1/2-mediated signaling pathway, as well as an increase in basal LC20-P, is suggested to contribute to the reversal of inhibition and promote contraction of the uterus during labor.

Organ culture as a useful method for studying the biology of blood vessels and other smooth muscle tissues

The benefit of organ culture is to retain the original structural relationship between various cell species and their interactions and enable us to study the long-term effects of exogenous stimuli. Organ culture methods have been used especially in the studies of the proliferative vascular diseases, such as atherosclerosis and restenosis. We describe here that organ culture is a useful in vitro method to study the biology of vascular and other smooth muscle organs.

Inhibition of ERK attenuates force development by lowering myosin light chain phosphorylation

Phosphorylation of the actin-associated protein caldesmon (CaD) by extracellular signal-regulated kinases (ERK1/2) is purported to participate in force maintenance by vascular smooth muscle. We examined the interrelationship among ERK1/2 activity, phosphorylation of the high molecular weight isoform of CaD (h-CaD) and the 20-kDa myosin light chain (LC(20)), and isometric force in strips of porcine carotid artery stimulated with endothelin-1 (ET-1; 50 nM). After an initial delay, ERK1/2 activity increased in parallel with ET-1-mediated force; h-CaD phosphorylation increased modestly. 2-(2'-Amino-3'-methoxyphenyl)-ox-anaphthalen-4-one (PD-098059; 50 microM), an ERK1/2 kinase inhibitor, significantly reduced basal ERK1/2 activity within 1 h, but only partially attenuated h-CaD phosphorylation at 3 h. The mechanisms underlying the temporal dissociation of ERK1/2 activity from h-CaD phosphorylation are unknown, but include the possibility that a kinase other than ERK1/2 phosphorylates h-CaD or, more likely, that phosphate turnover in h-CaD is very slow. PD-098059 partially inhibited the development of ET-1-stimulated force only in Ca(2+)-replete physiological saline solution, primarily by reducing LC(20) phosphorylation, yet had no effect on myosin light chain kinase in vitro. These inhibitory effects were most evident during the early phase of force production. The inhibitory effect of PD-098059 on force could not be correlated with a corresponding effect on ERK1/2-mediated h-CaD phosphorylation because force in arterial strips stimulated with ET-1 in the absence or presence of PD-098059 tended to approximate each other over time despite significant differences in the level of h-CaD phosphorylation. Force and LC(20) phosphorylation in response to KCl depolarization were unaffected by PD-098059. These results show that ERK1/2 may regulate force in arterial smooth muscle, but suggest that the mechanism for this effect is by inhibiting LC(20) phosphorylation.

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

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

Smooth muscle myosin heavy chain and caldesmon expression in the anterior vaginal wall of women with and without pelvic organ prolapse

OBJECTIVE

The aim of this study was to quantify the expression of smooth muscle myosin heavy chain (SM-MHC) and caldesmon in the anterior vaginal wall of women with and without pelvic organ prolapse.

STUDY DESIGN

Immunoblot analysis was conducted on protein extracts from the vaginal muscularis of 15 women with and 11 women without pelvic organ prolapse by using specific antibodies for SM-MHC, nonmuscle MHC-B, and caldesmon. The fraction of muscularis containing smooth muscle was determined by morphometric analysis of histologic cross sections. Reverse transcriptase-polymerase chain reaction was used to amplify SM-MHC isoforms produced by alternative splicing in the myosin head.

RESULTS

Whereas the expression of SM-MHC was increased modestly (2-fold), expression of smooth muscle caldesmon was increased 6- to 7-fold in vaginal muscularis from women with prolapse. The relative distribution of SM-MHC isoforms was similar in both groups.

CONCLUSIONS

Caldesmon is increased substantially in vaginal smooth muscle of women with pelvic organ prolapse. Caldesmon inhibits actin-activated myosin magnesium adenosine triphosphatase activity and inhibits the maintenance of contractile force. Thus, this disproportionate increase in caldesmon, relative to myosin, may result in inhibition of vaginal smooth muscle contractility and force maintenance.

Calcium sensitization produced by G protein activation in airway smooth muscle

We determined whether activation of G proteins can affect the force developed for a given intracellular Ca(2+) concentration ([Ca(2+)]; i.e., the Ca(2+) sensitivity) by mechanisms in addition to changes in regulatory myosin light chain (rMLC) phosphorylation. Responses in alpha-toxin-permeabilized canine tracheal smooth muscle were determined with Ca(2+) alone or in the presence of ACh, endothelin-1 (ET-1), or aluminum fluoride (AlF; acute or 1-h exposure). Acute exposure to each compound increased Ca(2+) sensitivity without changing the response to high [Ca(2+)] (maximal force). However, chronic exposure to AlF, but not to chronic ACh or ET-1, increased maximal force by increasing the force produced for a given rMLC phosphorylation. Studies employing thiophosphorylation of rMLC showed that the increase in force produced by chronic AlF exposure required Ca(2+) during activation to be manifest. Unlike the acute response to receptor agonists, which is mediated solely by increases in rMLC phosphorylation, chronic direct activation of G proteins further increases Ca(2+) sensitivity in airways by additional mechanisms that are independent of rMLC phosphorylation.

Mitogen-activated protein kinase plays an important role in hemolysate-induced contraction in rabbit basilar artery

OBJECT

Mitogen-activated protein kinase (MAPK) is an important signaling factor in the vascular proliferation and contraction, the two features of cerebral vasospasm following subarachnoid hemorrhage. We studied the possible involvement of MAPK in hemolysate-induced signal transduction and contraction in rabbit basilar artery.

METHODS

Isometric tension was used to record the contractile response of rabbit basilar artery to hemolysate. Western blots using antibodies for MAPK were conducted. 1) Hemolysate produced a concentration-dependent contraction of rabbit basilar artery. Pre-incubation of arteries with MAPK kinase inhibitor PD-98059 markedly reduced the contraction induced by hemolysate. PD-98059 also relaxed, in a concentration-dependent fashion, the sustained contraction induced by hemolysate (10%). 2) Hemolysate produced a time-dependent elevation of MAPK immunoreactivity in Western blot in rabbit basilar artery. MAPK was enhanced 3 min after hemolysate exposure and the effect reached maximum at 5 min. The immunoreactivity of MAPK decayed slowly with time, but the level of MAPK was still higher than the basal level even at two hours after exposure to hemolysate. 3) Pre-incubation of arteries with MAPK kinase inhibitor PD-98059 abolished the effect of hemolysate on MAPK immunoreactivity.

CONCLUSION

Hemolysate produced contraction of rabbit basilar artery possibly by activation of MAPK. MAPK inhibitors may be useful in the treatment of cerebral vasospasm.

Invited review: cross-bridge regulation by thin filament-associated proteins

This minireview will cover current concepts on the identity and mechanistic function of smooth muscle actin binding proteins that may regulate actin-myosin interactions. The potential roles of tropomyosin, caldesmon, calponin, and SM22 will be discussed. The review, for purposes of brevity, will be nonexhaustive but will give an overview of available information on the in vitro biochemistry and potential in vivo function of these proteins. Preterm labor is discussed as a possible example of where thin filament regulation may be relevant. Considerable controversy surrounds the putative physiological significance of these proteins, and emphasis will be placed on the need for more experimental work to determine the degree to which tissue- and species-specific effects have clouded the interpretation of functional data.