Calponin 1 inhibits agonist-induced ERK activation and decreases calcium sensitization in vascular smooth muscle

Smooth muscle cell (SMC) contraction and vascular tone are modulated by phosphorylation and multiple modifications of the thick filament, and thin filament regulation of SMC contraction has been reported to involve extracellular regulated kinase (ERK). Previous studies in ferrets suggest that the actin-binding protein, calponin 1 (CNN1), acts as a scaffold linking protein kinase C (PKC), Raf, MEK and ERK, promoting PKC-dependent ERK activation. To gain further insight into this function of CNN1 in ERK activation and the regulation of SMC contractility in mice, we generated a novel Calponin 1 knockout mouse (Cnn1 KO) by a single base substitution in an intronic CArG box that preferentially abolishes expression of CNN1 in vascular SMCs. Using this new Cnn1 KO mouse, we show that ablation of CNN1 has two effects, depending on the cytosolic free calcium level: (1) in the presence of elevated intracellular calcium caused by agonist stimulation, Cnn1 KO mice display a reduced amplitude of stress and stiffness but an increase in agonist-induced ERK activation; and (2) during intracellular calcium depletion, in the presence of an agonist, Cnn1 KO mice exhibit increased duration of SM tone maintenance. Together, these results suggest that CNN1 plays an important and complex modulatory role in SMC contractile tone amplitude and maintenance.

The importance of dystrophin and the dystrophin associated proteins in vascular smooth muscle

This review details the role of dystrophin and the dystrophin associated proteins (DAPs) in the vascular smooth muscle. Dystrophin is most comprehensively studied in the skeletal muscle due to serious symptoms found related to the skeletal muscle of patients with muscular dystrophy. Mutations in the dystrophin gene, or DAPs genes, result in a wide range of muscular dystrophies. In skeletal muscle, dystrophin is known to act to as a cytoskeletal stabilization protein and protects cells against contraction-induced damage. In skeletal muscle, dystrophin stabilizes the plasma membrane by transmitting forces generated by sarcomeric contraction to the extracellular matrix (ECM). Dystrophin is a scaffold that binds the dystroglycan complex (DGC) and has many associated proteins (DAPs). These DAPs include sarcoglycans, syntrophins, dystroglycans, dystrobrevin, neuronal nitric oxide synthase, and caveolins. The DAPs provide biomechanical support to the skeletal or cardiac plasma membrane during contraction, and loss of one or several of these DAPs leads to plasma membrane fragility. Dystrophin is expressed near the plasma membrane of all muscles, including cardiac and vascular smooth muscle, and some neurons. Dystrophic mice have noted biomechanical irregularities in the carotid arteries and spontaneous motor activity in portal vein altered when compared to wild type mice. Additionally, some studies suggest the vasculature of patients and animal models with muscular dystrophy is abnormal. Although the function of dystrophin and the DAPs in vascular smooth muscle is not thoroughly established in the field, this review makes the point that these proteins are expressed, and important and further study is warranted.

Ageing causes an aortic contractile dysfunction phenotype by targeting the expression of members of the extracellular signal-regulated kinase pathway

The extracellular signal-regulated kinase (ERK) pathway is a well-known regulator of vascular smooth muscle cell proliferation, but it also serves as a regulator of caldesmon, which negatively regulates vascular contractility. This study examined whether aortic contractile function requires ERK activation and if this activation is regulated by ageing. Biomechanical experiments revealed that contractile responses to the alpha1-adrenergic agonist phenylephrine are attenuated specifically in aged mice, which is associated with downregulation of ERK phosphorylation. ERK inhibition attenuates phenylephrine-induced contractility, indicating that the contractile tone is at least partially ERK-dependent. To explore the mechanisms of this age-related downregulation of ERK phosphorylation, we transfected microRNAs, miR-34a and miR-137 we have previously shown to increase with ageing and demonstrated that in A7r5 cells, both miRs downregulate the expression of Src and paxillin, known regulators of ERK signalling, as well as ERK phosphorylation. Further studies in aortic tissues transfected with miRs show that miR-34a but not miR-137 has a negative effect on mRNA levels of Src and paxillin. Furthermore, ERK phosphorylation is decreased in aortic tissue treated with the Src inhibitor PP2. Increases in miR-34a and miR-137 with ageing downregulate the expression of Src and paxillin, leading to impaired ERK signalling and aortic contractile dysfunction.

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.

Aging-induced microbleeds of the mouse thalamus compared to sensorimotor and memory defects

The aim of this study is to investigate the relationship between aging and brain vasculature health. Three groups of mice, 3, 17-18, and 24 months, comparable to young adult, middle age, and old human were studied. Prussian blue histology and fast imaging with steady precession T2∗-weighted magnetic resonance imaging were used to quantify structural changes in the brain across age groups. The novel object recognition test was used to assess behavioral changes associated with anatomical changes. This study is the first to show that the thalamus is the most vulnerable brain region in the mouse model for aging-induced vascular damage. Magnetic resonance imaging data document the timeline of accumulation of thalamic damage. Histological data reveal that the majority of vascular damage accumulates in the ventroposterior nucleus and mediodorsal thalamic nucleus. Functional studies indicate that aging-induced vascular damage in the thalamus is associated with memory and sensorimotor deficits. This study points to the possibility that aging-associated vascular disease is a factor in irreversible brain damage as early as middle age.

Non-muscle myosin II regulates aortic stiffness through effects on specific focal adhesion proteins and the non-muscle cortical cytoskeleton

Non‐muscle myosin II (NMII) plays a role in many fundamental cellular processes including cell adhesion, migration, and cytokinesis. However, its role in mammalian vascular function is not well understood. Here, we investigated the function of NMII in the biomechanical and signalling properties of mouse aorta. We found that blebbistatin, an inhibitor of NMII, decreases agonist‐induced aortic stress and stiffness in a dose‐dependent manner. We also specifically demonstrate that in freshly isolated, contractile, aortic smooth muscle cells, the non‐muscle myosin IIA (NMIIA) isoform is associated with contractile filaments in the core of the cell as well as those in the non‐muscle cell cortex. However, the non‐muscle myosin IIB (NMIIB) isoform is excluded from the cell cortex and colocalizes only with contractile filaments. Furthermore, both siRNA knockdown of NMIIA and NMIIB isoforms in the differentiated A7r5 smooth muscle cell line and blebbistatin‐mediated inhibition of NM myosin II suppress agonist‐activated increases in phosphorylation of the focal adhesion proteins FAK Y925 and paxillin Y118. Thus, we show in the present study, for the first time that NMII regulates aortic stiffness and stress and that this regulation is mediated through the tension‐dependent phosphorylation of the focal adhesion proteins FAK and paxillin.

BCL11B Regulates Arterial Stiffness and Related Target Organ Damage

Supplemental Digital Content is available in the text.

BCL11B (B-cell leukemia 11b) is a transcription factor known as an essential regulator of T lymphocytes and neuronal development during embryogenesis. A genome-wide association study showed that a gene desert region downstream of BCL11B, known to function as a BCL11B enhancer, harbors single nucleotide polymorphisms associated with increased arterial stiffness. However, a role for BCL11B in the adult cardiovascular system is unknown.

Abstract 480: Non-muscle Myosin II Regulates Aortic Stiffness via Tension-dependent Phosphorylation of Specific Focal Adhesion Proteins

Non-muscle myosin II plays a role in many fundamental cellular processes including cell adhesion, migration, and cytokinesis. However, its role in vascular function is not well understood. Here, we investigated the function of non-muscle myosin II in the biomechanical properties of mouse proximal aorta. We found that blebbistatin, a specific inhibitor of non-muscle myosin II decreases agonist-induced aortic stress and stiffness in a dose-dependent manner. We also specifically demonstrate, in freshly isolated contractile aortic smooth muscle cells, using deconvolution microscopy that the NM myosin IIA isoform co-localizes with contractile filaments in the core of the cell as well as in the non-muscle cell cortex. However, the NM myosin IIB isoform is only colocalized with contractile filaments, and is excluded from the cell cortex. Furthermore, both the siRNA knockdown of NMIIA and NMIIB isoforms in a differentiated smooth muscle cell line A7r5 and blebbistatin-mediated inhibition of NM myosin II suppresses agonist-activated increases in phosphorylation of FAK Y925 and paxillin Y118. Thus, in the present study, we show, for the first time, that NM myosin II regulates aortic stiffness and that this regulation is mediated at least in part through the tension-dependent phosphorylation of focal adhesion proteins FAK and paxillin.

The brains of aged mice are characterized by altered tissue diffusion properties and cerebral microbleeds

Background

Brain aging is a major risk factor in the progression of cognitive diseases including Alzheimer’s disease (AD) and vascular dementia. We investigated a mouse model of brain aging up to 24 months old (mo).

Methods

A high field (11.7T) MRI protocol was developed to characterize specific features of brain aging including the presence of cerebral microbleeds (CMBs), morphology of grey and white matter, and tissue diffusion properties. Mice were selected from age categories of either young (3 mo), middle-aged (18 mo), or old (24 mo) and fed normal chow over the duration of the study. Mice were imaged in vivo with multimodal MRI, including conventional T2-weighted (T2W) and T2*-weighted (T2*W) imaging, followed by ex vivo diffusion-weighted imaging (DWI) and T2*W MR-microscopy to enhance the detection of microstructural features.

Results

Structural changes observed in the mouse brain with aging included reduced cortical grey matter volume and enlargement of the brain ventricles. A remarkable age-related change in the brains was the development of CMBs found starting at 18 mo and increasing in total volume at 24 mo, primarily in the thalamus. CMBs presence was confirmed with high resolution ex vivo MRI and histology. DWI detected further brain tissue changes in the aged mice including reduced fractional anisotropy, increased radial diffusion, increased mean diffusion, and changes in the white matter fibers visualized by color-coded tractography, including around a large cortical CMB.

Conclusions

The mouse is a valuable model of age-related vascular contributions to cognitive impairment and dementia (VCID). In composite, these methods and results reveal brain aging in older mice as a multifactorial process including CMBs and tissue diffusion alterations that can be well characterized by high field MRI.

Reversal of Aging-Induced Increases in Aortic Stiffness by Targeting Cytoskeletal Protein-Protein Interfaces

Background

The proximal aorta normally functions as a critical shock absorber that protects small downstream vessels from damage by pressure and flow pulsatility generated by the heart during systole. This shock absorber function is impaired with age because of aortic stiffening.

Methods and Results

We examined the contribution of common genetic variation to aortic stiffness in humans by interrogating results from the AortaGen Consortium genome‐wide association study of carotid‐femoral pulse wave velocity. Common genetic variation in the N‐WASP (WASL) locus is associated with carotid‐femoral pulse wave velocity (rs600420, P=0.0051). Thus, we tested the hypothesis that decoy proteins designed to disrupt the interaction of cytoskeletal proteins such as N‐WASP with its binding partners in the vascular smooth muscle cytoskeleton could decrease ex vivo stiffness of aortas from a mouse model of aging. A synthetic decoy peptide construct of N‐WASP significantly reduced activated stiffness in ex vivo aortas of aged mice. Two other cytoskeletal constructs targeted to VASP and talin‐vinculin interfaces similarly decreased aging‐induced ex vivo active stiffness by on‐target specific actions. Furthermore, packaging these decoy peptides into microbubbles enables the peptides to be ultrasound‐targeted to the wall of the proximal aorta to attenuate ex vivo active stiffness.

Conclusions

We conclude that decoy peptides targeted to vascular smooth muscle cytoskeletal protein‐protein interfaces and microbubble packaged can decrease aortic stiffness ex vivo. Our results provide proof of concept at the ex vivo level that decoy peptides targeted to cytoskeletal protein‐protein interfaces may lead to substantive dynamic modulation of aortic stiffness.

Abstract 512: Bcl11b is a Newly Identified Regulator of Vascular Smooth Muscle Function and Stiffness

Background: B-cell leukemia 11b (Bcl11b) is a zinc-finger transcription factor known as master regulator of T lymphocytes and neuronal development during embryogenesis. However, a role for Bcl11b in the cardiovascular system has never been described. Based on human findings from a genome-wide association study (GWAS) that a gene desert region downstream of BCL11B , known to function as BCL11B enhancer, harbors single nucleotide polymorphisms (SNPs) associated with increased arterial stiffness, we sought to examine relations between Bcl11b and arterial function.

Methods and Results: We found for the first time that Bcl11b is expressed in the vascular smooth muscle (VSM) of human and murine vasculature and transcriptionally regulates the expression of VSM contractile proteins smooth muscle myosin and smooth muscle α-actin. Lack of Bcl11b in VSM-specific Bcl11b null mice (BSMKO) resulted in an increased expression of Ca ++ -calmodulin-dependent serine/threonine phosphatase calcineurin in BSMKO VSM cells and aortas, which were inversely correlated with levels of phosphorylated VASP S239 , a calcineurin de-phosphorylation target. Decreased pVASP S239 in BSMKO aortas was associated with increased actin polymerization (F/G actin ratio), consistent with pVASP S239 ’s function as regulator of cytoskeletal actin rearrangements, and was normalized by treatment with calcineurin inhibitor cyclosporine A. Functionally, Bcl11b deletion in VSM cells translated in increased aortic force, stress and wall tension, measured ex vivo in BSMKO aortas in organ baths, and increased pulse wave velocity, the in vivo index of arterial stiffness, in BSMKO mice compared to WT littermates. Moreover, Bcl11b and pVASP S239 expression were decreased in aortas of obese and aged mice, two models of arterial stiffness. Bcl11b deletion in VSM had no effect on baseline blood pressure or angiotensin II-induced hypertension, measured in conscious WT and BSMKO mice by radiotelemetry, but dramatically increased the incidence of angII-induced aortic aneurysms in BSMKO mice.

Conclusions: Taken together, our results identify VSM Bcl11b as a novel and crucial regulator of VSM cell phenotype and vascular structural and functional integrity

A novel mechanism of ERK1/2 regulation in smooth muscle involving acetylation of the ERK1/2 scaffold IQGAP1

Ceramide, a bioactive lipid and signaling molecule associated with cardiovascular disease, is known to activate extracellular signal regulated kinases 1 and 2 (ERK1/2). Here, we determined that the effect of ceramide on ERK1/2 is mediated by ceramide signaling on an ERK scaffold protein, IQ motif containing GTPase activating protein 1 (IQGAP1). Experiments were performed with aortic smooth muscle cells using inhibitor screening, small interfering RNA (siRNA), immunoprecipitation (IP), immunoblots and bioinformatics. We report here that C6 ceramide increases serum-stimulated ERK1/2 activation in a manner dependent on the ERK1/2 scaffold IQGAP1. C6 ceramide increases IQGAP1 protein levels by preventing its cleavage. Bioinformatic analysis of the IQGAP1 amino acid sequence revealed potential cleavage sites for proteases of the proprotein convertase family that match the cleavage products. These potential cleavage sites overlap with known motifs for lysine acetylation. Deacetylase inhibitor treatment increased IQGAP1 acetylation and reduced IQGAP1 cleavage. These data are consistent with a model in which IQGAP1 cleavage is regulated by acetylation of the cleavage sites. Activation of ERK1/2 by ceramide, known to increase lysine acetylation, appears to be mediated by acetylation-dependent stabilization of IQGAP1. This novel mechanism could open new possibilities for therapeutic intervention in cardiovascular diseases.

MicroRNA-203 mimics age-related aortic smooth muscle dysfunction of cytoskeletal pathways

Increased aortic stiffness is a biomarker for subsequent adverse cardiovascular events. We have previously reported that vascular smooth muscle Src-dependent cytoskeletal remodelling, which contributes to aortic plasticity, is impaired with ageing. Here, we use a multi-scale approach to determine the molecular mechanisms behind defective Src-dependent signalling in an aged C57BL/6 male mouse model. Increased aortic stiffness, as measured in vivo by pulse wave velocity, was found to have a comparable time course to that in humans. Bioinformatic analyses predicted several miRs to regulate Src-dependent cytoskeletal remodelling. qRT-PCR was used to determine the relative levels of predicted miRs in aortas and, notably, the expression of miR-203 increased almost twofold in aged aorta. Increased miR-203 expression was associated with a decrease in both mRNA and protein expression of Src, caveolin-1 and paxillin in aged aorta. Probing with phospho-specific antibodies confirmed that overexpression of miR-203 significantly attenuated Src and extracellular signal regulated kinase (ERK) signalling, which we have previously found to regulate vascular smooth muscle stiffness. In addition, transfection of miR-203 into aortic tissue from young mice increased phenylephrine-induced aortic stiffness ex vivo, mimicking the aged phenotype. Upstream of miR-203, we found that DNA methyltransferases (DNMT) 1, 3a, and 3b are also significantly decreased in the aged mouse aorta and that DNMT inhibition significantly increases miR-203 expression. Thus, the age-induced increase in miR-203 may be caused by epigenetic promoter hypomethylation in the aorta. These findings indicate that miR-203 promotes a re-programming of Src/ERK signalling pathways in vascular smooth muscle, impairing the regulation of stiffness in aged aorta.

Mechanisms of Vascular Smooth Muscle Contraction and the Basis for Pharmacologic Treatment of Smooth Muscle Disorders

The smooth muscle cell directly drives the contraction of the vascular wall and hence regulates the size of the blood vessel lumen. We review here the current understanding of the molecular mechanisms by which agonists, therapeutics, and diseases regulate contractility of the vascular smooth muscle cell and we place this within the context of whole body function. We also discuss the implications for personalized medicine and highlight specific potential target molecules that may provide opportunities for the future development of new therapeutics to regulate vascular function.

Vascular Smooth Muscle Sirtuin-1 Protects Against Aortic Dissection During Angiotensin II-Induced Hypertension

Background

Sirtuin-1 (SirT1), a nicotinamide adenine dinucleotide⁺–dependent deacetylase, is a key enzyme in the cellular response to metabolic, inflammatory, and oxidative stresses; however, the role of endogenous SirT1 in the vasculature has not been fully elucidated. Our goal was to evaluate the role of vascular smooth muscle SirT1 in the physiological response of the aortic wall to angiotensin II, a potent hypertrophic, oxidant, and inflammatory stimulus.

Methods and Results

Mice lacking SirT1 in vascular smooth muscle (ie, smooth muscle SirT1 knockout) had drastically high mortality (70%) caused by aortic dissection after angiotensin II infusion (1 mg/kg per day) but not after an equipotent dose of norepinephrine, despite comparable blood pressure increases. Smooth muscle SirT1 knockout mice did not show any abnormal aortic morphology or blood pressure compared with wild-type littermates. Nonetheless, in response to angiotensin II, aortas from smooth muscle SirT1 knockout mice had severely disorganized elastic lamellae with frequent elastin breaks, increased oxidant production, and aortic stiffness compared with angiotensin II–treated wild-type mice. Matrix metalloproteinase expression and activity were increased in the aortas of angiotensin II–treated smooth muscle SirT1 knockout mice and were prevented in mice overexpressing SirT1 in vascular smooth muscle or with use of the oxidant scavenger tempol.

Conclusions

Endogenous SirT1 in aortic smooth muscle is required to maintain the structural integrity of the aortic wall in response to oxidant and inflammatory stimuli, at least in part, by suppressing oxidant-induced matrix metalloproteinase activity. SirT1 activators could potentially be a novel therapeutic approach to prevent aortic dissection and rupture in patients at risk, such as those with hypertension or genetic disorders, such as Marfan’s syndrome.

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.

The contribution of vascular smooth muscle to aortic stiffness across length scales

The operation of the cardiovascular system in health and disease is inherently mechanical. Clinically, aortic stiffness has proven to be of critical importance as an early biomarker for subsequent cardiovascular disease; however, the mechanisms involved in aortic stiffening are still unclear. The etiology of aortic stiffening with age has been thought to primarily involve changes in extracellular matrix protein composition and quantity, but recent studies suggest a significant involvement of the differentiated contractile vascular smooth muscle cells in the vessel wall. Here, we provide an overview of vascular physiology and biomechanics at different spatial scales. The processes involved in aortic stiffening are examined with particular attention given to recent discoveries regarding the role of vascular smooth muscle.

Aging impairs smooth muscle-mediated regulation of aortic stiffness: a defect in shock absorption function?

Increased aortic stiffness is an early and independent biomarker of cardiovascular disease. Here we tested the hypothesis that vascular smooth muscle cells (VSMCs) contribute significantly to aortic stiffness and investigated the mechanisms involved. The relative contributions of VSMCs, focal adhesions (FAs), and matrix to stiffness in mouse aorta preparations at optimal length and with confirmed VSMC viability were separated by the use of small-molecule inhibitors and activators. Using biomechanical methods designed for minimal perturbation of cellular function, we directly quantified changes with aging in aortic material stiffness. An alpha adrenoceptor agonist, in the presence of N(G)-nitro-l-arginine methyl ester (l-NAME) to remove interference of endothelial nitric oxide, increases stiffness by 90-200% from baseline in both young and old mice. Interestingly, increases are robustly suppressed by the Src kinase inhibitor PP2 in young but not old mice. Phosphotyrosine screening revealed, with aging, a biochemical signature of markedly impaired agonist-induced FA remodeling previously associated with Src signaling. Protein expression measurement confirmed a decrease in Src expression with aging. Thus we report here an additive model for the in vitro biomechanical components of the mouse aortic wall in which 1) VSMCs are a surprisingly large component of aortic stiffness at physiological lengths and 2) regulation of the VSMC component through FA signaling and hence plasticity is impaired with aging, diminishing the aorta's normal shock absorption function in response to stressors.

Mechanism of calponin stabilization of cross-linked actin networks

The actin-binding protein calponin has been previously implicated in actin cytoskeletal regulation and is thought to act as an actin stabilizer, but the mechanism of its function is poorly understood. To investigate this underlying physical mechanism, we studied an in vitro model system of cross-linked actin using bulk rheology. Networks with basic calponin exhibited a delayed onset of strain stiffening (10.0% without calponin, 14.9% with calponin) and were able to withstand a higher maximal strain before failing (35% without calponin, 56% with calponin). Using fluorescence microscopy to study the mechanics of single actin filaments, we found that calponin increased the flexibility of actin filaments, evident as a decrease in persistence length from 17.6 μm without to 7.7 μm with calponin. Our data are consistent with current models of affine strain behavior in semiflexible polymer networks, and suggest that calponin stabilization of actin networks can be explained purely by changes in single-filament mechanics. We propose a model in which calponin stabilizes actin networks against shear through a reduction of persistence length of individual filaments.

Regulation of PKC autophosphorylation by calponin in contractile vascular smooth muscle tissue

Protein kinase C (PKC) is a key enzyme involved in agonist-induced smooth muscle contraction. In some cases, regulatory phosphorylation of PKC is required for full activation of the enzyme. However, this issue has largely been ignored with respect to PKC-dependent regulation of contractile vascular smooth muscle (VSM) contractility. The first event in PKC regulation is a transphosphorylation by PDK at a conserved threonine in the activation loop of PKC, followed by the subsequent autophosphorylation at the turn motif and hydrophobic motif sites. In the present study, we determined whether phosphorylation of PKC is a regulated process in VSM and also investigated a potential role of calponin in the regulation of PKC. We found that calponin increases the level of in vitro PKCα phosphorylation at the PDK and hydrophobic sites, but not the turn motif site. In vascular tissues, phosphorylation of the PKC hydrophobic site, but not turn motif site, as well as phosphorylation of PDK at S241 increased in response to phenylephrine. Calponin knockdown inhibits autophosphorylation of cellular PKC in response to phenylephrine, confirming results with recombinant PKC. Thus these results show that autophosphorylation of PKC is regulated in dVSM and calponin is necessary for autophosphorylation of PKC in VSM.

Hierarchical scaffolding of an ERK1/2 activation pathway

Background

Scaffold proteins modulate cellular signaling by facilitating assembly of specific signaling pathways. However, there is at present little information if and how scaffold proteins functionally interact with each other.

Results

Here, we show that two scaffold proteins, caveolin-1 and IQGAP1, are required for phosphorylation of the actin associated pool of extracellular signal regulated kinase 1 and 2 (ERK1/2) in response to protein kinase C activation. We show by immunofluorescence and proximity ligation assays, that IQGAP1 tethers ERK1/2 to actin filaments. Moreover, siRNA experiments demonstrate that IQGAP1 is required for activation of actin-bound ERK1/2. Caveolin-1 is also necessary for phosphorylation of actin-bound ERK1/2 in response to protein kinase C, but is dispensible for ERK1/2 association with actin. Simultaneous knock down of caveolin-1 and IQGAP1 decreases total phorbol ester-induced ERK1/2 phosphorylation to the same degree as single knock down of either caveolin-1 or IQGAP1, indicating that caveolin-1 and IQGAP1 operate in the same ERK activation pathway. We further show that caveolin-1 knock down, but not IQGAP1 knock down, reduces C-Raf phosphorylation in response to phorbol ester stimulation.

Conclusions

Based on our data, we suggest that caveolin-1 and IQGAP1 assemble distinct signaling modules, which are then linked in a hierarchical arrangement to generate a functional ERK1/2 activation pathway.

Vasoconstrictor-induced endocytic recycling regulates focal adhesion protein localization and function in vascular smooth muscle

Turnover of focal adhesions (FAs) is known to be critical for cell migration and adhesion of proliferative vascular smooth muscle (VSM) cells. However, it is often assumed that FAs in nonmigratory, differentiated VSM (dVSM) cells embedded in the wall of healthy blood vessels are stable structures. Recent work has demonstrated agonist-induced actin polymerization and Src-dependent FA phosphorylation in dVSM cells, suggesting that agonist-induced FA remodeling occurs. However, the mechanisms and extent of FA remodeling are largely unknown in dVSM. Here we show, for the first time, that a distinct subpopulation of dVSM FA proteins, but not the entire FA, remodels in response to the α-agonist phenylephrine. Vasodilator-stimulated phosphoprotein and zyxin displayed the largest redistributions, while β-integrin and FA kinase showed undetectable redistribution. Vinculin, metavinculin, Src, Crk-associated substrate, and paxillin displayed intermediate degrees of redistribution. Redistributions into membrane fractions were especially prominent, suggesting endosomal mechanisms. Deconvolution microscopy, quantitative colocalization analysis, and Duolink proximity ligation assays revealed that phenylephrine increases the association of FA proteins with early endosomal markers Rab5 and early endosomal antigen 1. Endosomal disruption with the small-molecule inhibitor primaquine inhibits agonist-induced redistribution of FA proteins, confirming endosomal recycling. FA recycling was also inhibited by cytochalasin D, latrunculin B, and colchicine, indicating that the redistribution is actin- and microtubule-dependent. Furthermore, inhibition of endosomal recycling causes a significant inhibition of the rate of development of agonist-induced dVSM contractions. Thus these studies are consistent with the concept that FAs in dVSM cells, embedded in the wall of the aorta, remodel during the action of a vasoconstrictor.

Cinnamyl alcohol attenuates vasoconstriction by activation of K⁺ channels via NO-cGMP-protein kinase G pathway and inhibition of Rho-kinase

Cinnamyl alcohol (CAL) is known as an antipyretic, and a recent study showed its vasodilatory activity without explaining the mechanism. Here we demonstrate the vasodilatory effect and the mechanism of action of CAL in rat thoracic aorta. The change of tension in aortic strips treated with CAL was measured in an organ bath system. In addition, vascular strips or human umbilical vein endothelial cells (HUVECs) were used for biochemical experiments such as Western blot and nitrite and cyclic guanosine monophosphate (cGMP) measurements. CAL attenuated the vasoconstriction of phenylephrine (PE, 1 µM)-precontracted aortic strips in an endothelium-dependent manner. CAL-induced vasorelaxation was inhibited by pretreatment with N^G-nitro-L-arginine methyl ester (L-NAME; 10^-4 M), methylene blue (MB; 10^-5 M) and 1 H-[1,2,4]-oxadiazolole-[4,3-a] quinoxalin-10one, (ODQ; 10^-6 or 10^-7 M) in the endothelium-intact aortic strips. Atrial natriuretic peptide (ANP; 10^-8 or 10^-9 M) did not affect the vasodilatory effect of CAL. The phosphorylation of endothelial nitric oxide synthase (eNOS) and generation of nitric oxide (NO) were stimulated by CAL treatment in HUVECs and inhibited by treatment with L-NAME. In addition, cGMP and PKG1 activation in aortic strips treated with CAL were also significantly inhibited by L-NAME. Furthermore, CAL relaxed Rho-kinase activator calpeptin-precontracted aortic strips, and the vasodilatory effect of CAL was inhibited by the ATP-sensitive K⁺ channel inhibitor glibenclamide (Gli; 10^-5 M) and the voltage-dependent K⁺ channel inhibitor 4-aminopyridine (4-AP; 2 × 10^-4 M). These results suggest that CAL induces vasorelaxation by activating K⁺ channels via the NO-cGMP-PKG pathway and the inhibition of Rho-kinase.

The focal adhesion: a regulated component of aortic stiffness

Increased aortic stiffness is an acknowledged predictor and cause of cardiovascular disease. The sources and mechanisms of vascular stiffness are not well understood, although the extracellular matrix (ECM) has been assumed to be a major component. We tested here the hypothesis that the focal adhesions (FAs) connecting the cortical cytoskeleton of vascular smooth muscle cells (VSMCs) to the matrix in the aortic wall are a component of aortic stiffness and that this component is dynamically regulated. First, we examined a model system in which magnetic tweezers could be used to monitor cellular cortical stiffness, serum-starved A7r5 aortic smooth muscle cells. Lysophosphatidic acid (LPA), an activator of myosin that increases cell contractility, increased cortical stiffness. A small molecule inhibitor of Src-dependent FA recycling, PP2, was found to significantly inhibit LPA-induced increases in cortical stiffness, as well as tension-induced increases in FA size. To directly test the applicability of these results to force and stiffness development at the level of vascular tissue, we monitored mouse aorta ring stiffness with small sinusoidal length oscillations during agonist-induced contraction. The alpha-agonist phenylephrine, which also increases myosin activation and contractility, increased tissue stress and stiffness in a PP2- and FAK inhibitor 14-attenuated manner. Subsequent phosphotyrosine screening and follow-up with phosphosite-specific antibodies confirmed that the effects of PP2 and FAK inhibitor 14 in vascular tissue involve FA proteins, including FAK, CAS, and paxillin. Thus, in the present study we identify, for the first time, the FA of the VSMC, in particular the FAK-Src signaling complex, as a significant subcellular regulator of aortic stiffness and stress.

Src modulates contractile vascular smooth muscle function via regulation of focal adhesions

Src is a known regulator of focal adhesion turnover in migrating cells; but, in contrast, Src is generally assumed to play little role in differentiated, contractile vascular smooth muscle (dVSM). The goal of the present study was to determine if Src-family kinases regulate focal adhesion proteins and how this might affect contractility of non-proliferative vascular smooth muscle. We demonstrate here, through the use of phosphotyrosine screening, deconvolution microscopy imaging, and differential centrifugation, that the activity of Src family kinases in aorta is regulated by the alpha agonist and vasoconstrictor phenylephrine, and leads to focal adhesion protein phosphorylation and remodeling in dVSM. Furthermore, Src inhibition via morpholino knockdown of Src or by the small molecule inhibitor PP2 prevents phenylephrine-induced adhesion protein phosphorylation, markedly slows the tissue's ability to contract, and decreases steady state contractile force amplitude. Significant vasoconstrictor-induced and Src-dependent phosphorylation of Cas pY-165, FAK pY-925, paxillin pY-118, and Erk1/2 were observed. However, increases in FAK 397 phosphorylation were not seen, demonstrating differences between cells in tissue versus migrating, proliferating cells. We show here that Src, in a cause and effect manner, regulates focal adhesion protein function and, consequently, modulates contractility during the action of a vasoconstrictor. These data point to the possibility that vascular focal adhesion proteins may be useful drug discovery targets for novel therapeutic approaches to cardiovascular disease.

Nonredundant roles of cytoplasmic β- and γ-actin isoforms in regulation of epithelial apical junctions

The functional effects of cytoplasmic actins on epithelial junctions are examined by using isoform-specific siRNAs and cell-permeable inhibitory peptides. Unique roles of cytoplasmic actin isoforms in regulating structure and remodeling of adherens and tight junctions are revealed.

Association with the actin cytoskeleton is critical for normal architecture and dynamics of epithelial tight junctions (TJs) and adherens junctions (AJs). Epithelial cells express β-cytoplasmic (β-CYA) and γ-cytoplasmic (γ-CYA) actins, which have different cellular localization and functions. This study elucidates the roles of cytoplasmic actins in regulating structure and remodeling of AJs and TJs in model intestinal epithelia. Immunofluorescence labeling and latrunculin B treatment reveal affiliation of dynamic β-CYA filaments with newly assembled and mature AJs, whereas an apical γ-CYA pool is composed of stable perijunctional bundles and rapidly turning-over nonjunctional filaments. The functional effects of cytoplasmic actins on epithelial junctions are examined by using isoform-specific small interfering RNAs and cell-permeable inhibitory peptides. These experiments demonstrate unique roles of β-CYA and γ-CYA in regulating the steady-state integrity of AJs and TJs, respectively. Furthermore, β-CYA is selectively involved in establishment of apicobasal cell polarity. Both actin isoforms are essential for normal barrier function of epithelial monolayers, rapid AJ/TJ reassembly, and formation of three-dimensional cysts. Cytoplasmic actin isoforms play unique roles in regulating structure and permeability of epithelial junctions.

Deciphering actin cytoskeletal function in the contractile vascular smooth muscle cell

This review focuses on the vascular smooth muscle cells present in the medial layer of the blood vessels wall in the fully differentiated state (dVSMCs). The dVSMC contractile phenotype enables these cells to respond in a highly regulated manner to changes in extracellular stimuli. Through modulation of vascular contractile force and vascular compliance dVSMCs regulate blood pressure and blood flow. The cellular and molecular mechanisms by which vascular smooth muscle contractile functions are regulated are not completely elucidated. Recent studies have documented a critical role for actin polymerization and cytoskeletal dynamics in the regulation of contractile function. Here we will review the current understanding of actin cytoskeletal dynamics and focal adhesion function in dVSMCs in order to better understand actin cytoskeleton connections to the extracellular matrix and the effects of cytoskeletal remodelling on vascular contractility and vascular stiffness in health and disease.

Stimulus-specific activation and actin dependency of distinct, spatially separated ERK1/2 fractions in A7r5 smooth muscle cells

A proliferative response of smooth muscle cells to activation of extracellular signal regulated kinases 1 and 2 (ERK1/2) has been linked to cardiovascular disease. In fully differentiated smooth muscle, however, ERK1/2 activation can also regulate contraction. Here, we use A7r5 smooth muscle cells, stimulated with 12-deoxyphorbol 13-isobutylate 20-acetate (DPBA) to induce cytoskeletal remodeling or fetal calf serum (FCS) to induce proliferation, to identify factors that determine the outcomes of ERK1/2 activation in smooth muscle. Knock down experiments, immunoprecipitation and proximity ligation assays show that the ERK1/2 scaffold caveolin-1 mediates ERK1/2 activation in response to DPBA, but not FCS, and that ERK1/2 is released from caveolin-1 upon DPBA, but not FCS, stimulation. Conversely, ERK1/2 associated with the actin cytoskeleton is significantly reduced after FCS, but not DPBA stimulation, as determined by Triton X fractionation. Furthermore, cytochalasin treatment inhibits DPBA, but not FCS-induced ERK1/2 phosphorylation, indicating that the actin cytoskeleton is not only a target but also is required for ERK1/2 activation. Our results show that (1) at least two ERK1/2 fractions are regulated separately by specific stimuli, and that (2) the association of ERK1/2 with the actin cytoskeleton regulates the outcome of ERK1/2 signaling.

Structure and dynamics of the actin-based smooth muscle contractile and cytoskeletal apparatus

The thin filaments of differentiated smooth muscle cells are composed of actin and tropomyosin isoforms and numerous ancillary actin-binding proteins that assemble together into distinct thin filament classes. These different filament classes are segregated in smooth muscle cells into structurally and functionally separated contractile and cytoskeletal cellular domains. Typically, thin filaments in smooth muscle cells have been considered to be relatively stable structures like those in striated cells. However, recent efforts have shown that smooth muscle thin filaments indeed are dynamic and that remodeling of the actin cytoskeleton, in particular, regulates smooth muscle function. Thus, the cytoskeleton of differentiated smooth muscle cells appears to function midway between that of less dynamic striated muscle cells and that of very plastic proliferative cells such as fibroblasts. Michael and Kate Bárány keenly followed and participated in some of these studies, consistent with their broad interest in actin function and smooth muscle mechanisms. As a way of honoring the memory of these two pioneer members of the muscle research community, we review data on distribution and remodeling of thin filaments in smooth muscle cells, one of the many research topics that intrigued them.

Effects of basic calponin on the flexural mechanics and stability of F-actin

The cellular actin cytoskeleton plays a central role in the ability of cells to properly sense, propagate, and respond to external stresses and other mechanical stimuli. Calponin, an actin-binding protein found both in muscle and non-muscle cells, has been implicated in actin cytoskeletal organization and regulation. In this work, we studied the mechanical and structural interaction of actin with basic calponin, a differentiation marker in smooth muscle cells, on a single filament level. We imaged fluorescently labeled thermally fluctuating actin filaments and found that at moderate calponin binding densities, actin filaments were more flexible, evident as a reduction in persistence length from 8.0 to 5.8 μm. When calponin-decorated actin filaments were subjected to shear, we observed a marked reduction of filament lengths after decoration with calponin, which we argue was due to shear-induced filament rupture rather than depolymerization. This increased shear susceptibility was exacerbated with calponin concentration. Cryo-electron microscopy results confirmed previously published negative stain electron microscopy results and suggested alterations in actin involving actin subdomain 2. A weakening of F-actin intermolecular association is discussed as the underlying cause of the observed mechanical perturbations.

Scaffolding proteins and non-proliferative functions of ERK1/2

Studies of ERK1/2 generally focus on the regulation of nuclear ERK1/2 function mainly related to proliferation, whereas less attention has been drawn to the role ERK1/2 plays in the cytosol. Scaffolding proteins for ERK1/2 have been shown to control the time point and also the intracellular location of ERK1/2 activation. Hence, by concentrating ERK1/2 within subcellular compartments, scaffolding proteins restrict the substrate specificity of ERK1/2 and thus optimize the cell response for specific signal transduction programs in order to manipulate specific cellular functions. We have presented evidence that the F-actin binding protein calponin represents a new type of ERK1/2 scaffold, controlling the activation of a subfraction of ERK1/2 which is connected solely to contractile and/or migratory events in a cell.

Enhanced contractility and myosin phosphorylation induced by Ca(2+)-independent MLCK activity in hypertensive rats

AIMS

The role of Ca(2+) sensitization induced by a Ca(2+)-independent myosin light chain kinase (MLCK) in hypertension has not been determined. The aim of this study was to clarify the role of possible Ca(2+)-independent MLCK activity in hypertension.

METHODS AND RESULTS

We compared increases in contractile force and phosphorylation of myosin light chain (MLC) evoked by calyculin A, a phosphatase inhibitor, in β-escin-permeabilized mesenteric arteries at pCa 9.0 between spontaneously hypertensive rat (SHR) and Wistar Kyoto rat (WKY). We found that there was no detectable phosphorylation of MLC at pCa 9.0, but that the administration of 1 μM calyculin A gradually increased force and mono- and di-phosphorylation of MLC. This contraction was inhibited by staurosporine but not by wortmannin, Y-27632, or calphostin-C. The calyculin A-induced contraction was significantly greater in the SHR than in the WKY and was associated with an increase in mono- and di-phosphorylation of MLC. SM-1, a zipper-interacting protein kinase (ZIPK)-inhibiting peptide, significantly inhibited the amplitude of the calyculin A-induced contraction and di-phosphorylation. Total ZIPK expression (54 + 32 kDa) was greater in the SHR than in the WKY. Phosphorylation of myosin phosphatase target subunit at Thr(697), but not at Thr(855), was consistently stronger in the SHR than in the WKY in calyculin A-treated tissues at pCa 9.0.

CONCLUSIONS

Our results suggest that Ca(2+)-independent MLCK activity is enhanced in the SHR, and that ZIPK plays, at least in part, an important role as a candidate for this kinase in rat mesenteric arteries.

Tropomyosin variants describe distinct functional subcellular domains in differentiated vascular smooth muscle cells

Tropomyosin (Tm) is known to be an important gatekeeper of actin function. Tm isoforms are encoded by four genes, and each gene produces several variants by alternative splicing, which have been proposed to play roles in motility, proliferation, and apoptosis. Smooth muscle studies have focused on gizzard smooth muscle, where a heterodimer of Tm from the α-gene (Tmsm-α) and from the β-gene (Tmsm-β) is associated with contractile filaments. In this study we examined Tm in differentiated mammalian vascular smooth muscle (dVSM). Liquid chromatography-tandem mass spectrometry (LC MS/MS) analysis and Western blot screening with variant-specific antibodies revealed that at least five different Tm proteins are expressed in this tissue: Tm6 (Tmsm-α) and Tm2 from the α-gene, Tm1 (Tmsm-β) from the β-gene, Tm5NM1 from the γ-gene, and Tm4 from the δ-gene. Tm6 is by far most abundant in dVSM followed by Tm1, Tm2, Tm5NM1, and Tm4. Coimmunoprecipitation and coimmunofluorescence studies demonstrate that Tm1 and Tm6 coassociate with different actin isoforms and display different intracellular localizations. Using an antibody specific for cytoplasmic γ-actin, we report here the presence of a γ-actin cortical cytoskeleton in dVSM cells. Tm1 colocalizes with cortical cytoplasmic γ-actin and coprecipitates with γ-actin. Tm6, on the other hand, is located on contractile bundles. These data indicate that Tm1 and Tm6 do not form a classical heterodimer in dVSM but rather describe different functional cellular compartments.

A new method for direct detection of the sites of actin polymerization in intact cells and its application to differentiated vascular smooth muscle

Here we report and validate a new method, suitable broadly, for use in differentiated cells and tissues, for the direct visualization of actin polymerization under physiological conditions. We have designed and tested different versions of fluorescently labeled actin, reversibly attached to the protein transduction tag TAT, and have introduced this novel reagent into intact differentiated vascular smooth muscle cells (dVSMCs). A thiol-reactive version of the TAT peptide was synthesized by adding the amino acids glycine and cysteine to its NH(2)-terminus and forming a thionitrobenzoate adduct: viz. TAT-Cys-S-STNB. This peptide reacts readily with G-actin, and the complex is rapidly taken up by freshly enzymatically isolated dVSMC, as indicated by the fluorescence of a FITC tag on the TAT peptide. By comparing different versions of the construct, we determined that the optimal construct for biological applications is a nonfluorescently labeled TAT peptide conjugated to rhodamine-labeled actin. When TAT-Cys-S-STNB-tagged rhodamine actin (TSSAR) was added to live, freshly enzymatically isolated cells, we observed punctae of incorporated actin at the cortex of the cell. The punctae are indistinguishable from those we have previously reported to occur in the same cell type when rhodamine G-actin is added to permeabilized cells. Thus this new method allows the delivery of labeled G-actin into intact cells without disrupting the native state and will allow its further use to study the effect of physiological intracellular Ca(2+) concentration transients and signal transduction on actin dynamics in intact cells.

h3/Acidic calponin: an actin-binding protein that controls extracellular signal-regulated kinase 1/2 activity in nonmuscle cells

Migration of fibroblasts is important in wound healing. Here, we demonstrate a role and a mechanism for h3/acidic calponin (aCaP, CNN3) in REF52.2 cell motility, a fibroblast line rich in actin filaments. We show that the actin-binding protein h3/acidic calponin associates with stress fibers in the absence of stimulation but is targeted to the cell cortex and podosome-like structures after stimulation with a phorbol ester, phorbol-12,13-dibutyrate (PDBu). By coimmunoprecipitation and colocalization, we show that extracellular signal-regulated kinase (ERK)1/2 and protein kinase C (PKC)alpha constitutively associate with h3/acidic calponin and are cotargeted with h3/acidic calponin in the presence of PDBu. This targeting can be blocked by a PKC inhibitor but does not require phosphorylation of h3/acidic calponin at the PKC sites S175 or T184. Knockdown of h3/acidic calponin results in a loss of PDBu-mediated ERK1/2 targeting, whereas PKCalpha targeting is unaffected. Caldesmon is an actin-binding protein that regulates actomyosin interactions and is a known substrate of ERK1/2. Both ERK1/2 activity and nonmuscle l-caldesmon phosphorylation are blocked by h3/acidic calponin knockdown. Furthermore, h3/acidic calponin knockdown inhibits REF52.2 migration in an in vitro wound healing assay. Our findings are consistent with a model whereby h3/acidic calponin controls fibroblast migration by regulation of ERK1/2-mediated l-caldesmon phosphorylation.

Par-4: a new activator of myosin phosphatase

We show here for the first time that the pro-apoptotic protein Par-4 binds to and activates myosin phosphatase (MP). During agonist stimulation, Par-4 facilitates ZIPK targeting and inhibitory phosphorylation of MP, however, phosphorylation of Par-4 is required for MP inhibition. Our model presents Par-4 as an amplifier of the MP activity range.

Myosin phosphatase (MP) is a key regulator of myosin light chain (LC20) phosphorylation, a process essential for motility, apoptosis, and smooth muscle contractility. Although MP inhibition is well studied, little is known about MP activation. We have recently demonstrated that prostate apoptosis response (Par)-4 modulates vascular smooth muscle contractility. Here, we test the hypothesis that Par-4 regulates MP activity directly. We show, by proximity ligation assays, surface plasmon resonance and coimmunoprecipitation, that Par-4 interacts with the targeting subunit of MP, MYPT1. Binding is mediated by the leucine zippers of MYPT1 and Par-4 and reduced by Par-4 phosphorylation. Overexpression of Par-4 leads to increased phosphatase activity of immunoprecipitated MP, whereas small interfering RNA knockdown of endogenous Par-4 significantly decreases MP activity and increases MYPT1 phosphorylation. LC20 phosphorylation assays demonstrate that overexpression of Par-4 reduces LC20 phosphorylation. In contrast, a phosphorylation site mutant, but not wild-type Par-4, interferes with zipper-interacting protein kinase (ZIPK)-mediated MP inhibition. We conclude from our results Par-4 operates through a “padlock” model in which binding of Par-4 to MYPT1 activates MP by blocking access to the inhibitory phosphorylation sites, and inhibitory phosphorylation of MYPT1 by ZIPK requires “unlocking” of Par-4 by phosphorylation and displacement of Par-4 from the MP complex.

Actin polymerization in differentiated vascular smooth muscle cells requires vasodilator-stimulated phosphoprotein

Our group has previously shown that vasoconstrictors increase net actin polymerization in differentiated vascular smooth muscle cells (dVSMC) and that increased actin polymerization is linked to contractility of vascular tissue (Kim et al., Am J Physiol Cell Physiol 295: C768-778, 2008). However, the underlying mechanisms are largely unknown. Here, we evaluated the possible functions of the Ena/vasodilator-stimulated phosphoprotein (VASP) family of actin filament elongation factors in dVSMC. Inhibition of actin filament elongation by cytochalasin D decreases contractility without changing myosin light-chain phosphorylation levels, suggesting that actin filament elongation is necessary for dVSM contraction. VASP is the only Ena/VASP protein highly expressed in aorta tissues, and VASP knockdown decreased smooth muscle contractility. VASP partially colocalizes with alpha-actinin and vinculin in dVSMC. Profilin, known to associate with G actin and VASP, also colocalizes with alpha-actinin and vinculin, potentially identifying the dense bodies and the adhesion plaques as hot spots of actin polymerization. The EVH1 domain of Ena/VASP is known to target these proteins to their sites of action. Introduction of an expressed EVH1 domain as a dominant negative inhibits stimulus-induced increases in actin polymerization. VASP phosphorylation, known to inhibit actin polymerization, is decreased during phenylephrine stimulation in dVSMC. We also directly visualized, for the first time, rhodamine-labeled actin incorporation in dVSMC and identified hot spots of actin polymerization in the cell cortex that colocalize with VASP. These results indicate a role for VASP in actin filament assembly, specifically at the cell cortex, that modulates contractility in dVSMC.

Smooth muscle archvillin is an ERK scaffolding protein

ERK influences a number of pathways in all cells, but how ERK activities are segregated between different pathways has not been entirely clear. Using immunoprecipitation and pulldown experiments with domain-specific recombinant fragments, we show that smooth muscle archvillin (SmAV) binds ERK and members of the ERK signaling cascade in a domain-specific, stimulus-dependent, and pathway-specific manner. MEK binds specifically to the first 445 residues of SmAV. B-Raf, an upstream regulator of MEK, constitutively interacts with residues 1-445 and 446-1250. Both ERK and 14-3-3 bind to both fragments, but in a stimulus-specific manner. Phosphorylated ERK is associated only with residues 1-445. An ERK phosphorylation site was determined by mass spectrometry to reside at Ser132. A phospho-antibody raised to this site shows that the site is phosphorylated during alpha-agonist-mediated ERK activation in smooth muscle tissue. Phosphorylation of SmAV by ERK decreases the association of phospho-ERK with SmAV. These results, combined with previous observations, indicate that SmAV serves as a new ERK scaffolding protein and provide a mechanism for regulation of ERK binding, activation, and release from the signaling complex.

Smooth muscle signalling pathways in health and disease

Smooth muscle contractile activity is a major regulator of function of the vascular system, respiratory system, gastrointestinal system and the genitourinary systems. Malfunction of contractility in these systems leads to a host of clinical disorders, and yet, we still have major gaps in our understanding of the molecular mechanisms by which contractility of the differentiated smooth muscle cell is regulated. This review will summarize recent advances in the molecular understanding of the regulation of smooth muscle myosin activity via phosphorylation/dephosphorylation of myosin, the regulation of the accessibility of actin to myosin via the actin-binding proteins calponin and caldesmon, and the remodelling of the actin cytoskeleton. Understanding of the molecular 'players' should identify target molecules that could point the way to novel drug discovery programs for the treatment of smooth muscle disorders such as cardiovascular disease, asthma, functional bowel disease and pre-term labour.

Smooth muscle signalling pathways in health and disease

Smooth muscle contractile activity is a major regulator of function of the vascular system, respiratory system, gastrointestinal system and the genitourinary systems. Malfunction of contractility in these systems leads to a host of clinical disorders, and yet, we still have major gaps in our understanding of the molecular mechanisms by which contractility of the differentiated smooth muscle cell is regulated. This review will summarize recent advances in the molecular understanding of the regulation of smooth muscle myosin activity via phosphorylation/dephosphorylation of myosin, the regulation of the accessibility of actin to myosin via the actin-binding proteins calponin and caldesmon, and the remodelling of the actin cytoskeleton. Understanding of the molecular 'players' should identify target molecules that could point the way to novel drug discovery programs for the treatment of smooth muscle disorders such as cardiovascular disease, asthma, functional bowel disease and pre-term labour.

Cytoskeletal remodeling in differentiated vascular smooth muscle is actin isoform dependent and stimulus dependent

Dynamic remodeling of the actin cytoskeleton plays an essential role in the migration and proliferation of vascular smooth muscle cells. It has been suggested that actin remodeling may also play an important functional role in nonmigrating, nonproliferating differentiated vascular smooth muscle (dVSM). In the present study, we show that contractile agonists increase the net polymerization of actin in dVSM, as measured by the differential ultracentrifugation of vascular smooth muscle tissue and the costaining of single freshly dissociated cells with fluorescent probes specific for globular and filamentous actin. Furthermore, induced alterations of the actin polymerization state, as well as actin decoy peptides, inhibit contractility in a stimulus-dependent manner. Latrunculin pretreatment or actin decoy peptides significantly inhibit contractility induced by a phorbol ester or an alpha-agonist, but these procedures have no effect on contractions induced by KCl. Aorta dVSM expresses alpha-smooth muscle actin, beta-actin, nonmuscle gamma-actin, and smooth muscle gamma-actin. The incorporation of isoform-specific cell-permeant synthetic actin decoy peptides, as well as isoform-specific probing of cell fractions and two-dimensional gels, demonstrates that actin remodeling during alpha-agonist contractions involves the remodeling of primarily gamma-actin and, to a lesser extent, beta-actin. Taken together, these results show that net isoform- and agonist-dependent increases in actin polymerization regulate vascular contractility.

The pro-apoptotic protein Par-4 facilitates vascular contractility by cytoskeletal targeting of ZIPK

Par-4 (prostate apoptosis response 4) is a pro-apoptotic protein and tumour suppressor that was originally identified as a gene product up-regulated during apoptosis in prostate cancer cells. Here, we show, for the first time, that Par-4 is expressed and co-localizes with the actin filament bundles in vascular smooth muscle. Furthermore, we demonstrate that targeting of ZIPK to the actin filaments, as observed upon PGF-2α stimulation, is inhibited by the presence of a cell permeant Par-4 decoy peptide. The same decoy peptide also significantly inhibits PGF-2α induced contractions of smooth muscle tissue. Moreover, knockdown of Par-4 using antisense morpholino nucleotides results in significantly reduced contractility, and myosin light chain and myosin phosphatase target subunit phosphorylation. These results indicate that Par-4 facilitates contraction by targeting ZIPK to the vicinity of its substrates, myosin light chain and MYPT, which are located on the actin filaments. These results identify Par-4 as a novel regulator of myosin light chain phosphorylation in differentiated, contractile vascular smooth muscle.

Myometrial mechanoadaptation during pregnancy: implications for smooth muscle plasticity and remodelling

The smooth muscle of the uterus during pregnancy presents a unique circumstance of physiological mechanotransduction as the tissue remodels in response to stretches imposed by the growing foetus(es), yet the nature of the molecular and functional adaptations remain unresolved. We studied, in myometrium isolated from non-pregnant (NP) and pregnant mice, the active and passive length–tension curves by myography and the expression and activation by immunoblotting of focal adhesion-related proteins known in other systems to participate in mechanosensing and mechanotransduction. In situ uterine mass correlated with pup number and weight throughout pregnancy. In vitro myometrial active, and passive, length-tension curves shifted significantly to the right during pregnancy indicative of altered mechanosensitivity; at term, maximum active tension was generated following 3.94 ± 0.33-fold stretch beyond slack length compared to 1.91 ± 0.12-fold for NP mice. Moreover, mechanotransduction was altered during pregnancy as evidenced by the progressive increase in absolute force production at each optimal stretch. Pregnancy was concomitantly associated with an increased expression of the dense plaque-associated proteins FAK and paxillin, and elevated activation of FAK, paxillin, c-Src and extracellular signal-regulated kinase (ERK1/2) which reversed 1 day post-partum. Electron microscopy revealed close appositioning of neighbouring myometrial cells across a narrow extracellular cleft adjoining plasmalemmal dense plaques. Collectively, these results suggest a physiological basis of myometrial length adaptation, long known to be a property of many smooth muscles, whereupon plasmalemmal dense plaque proteins serve as molecular signalling and structural platforms contributing to functional (contractile) remodelling in response to chronic stretch.

CaMKIIT287 and T305 regulate history-dependent increases in alpha agonist-induced vascular tone

CaMKII is a calcium and calmodulin-activated kinase that has been shown to regulate learning and memory in the brain, and contractility in blood vessels. Following Ca activation, CaMKII autophosphorylates, gaining a calcium-independent autonomous activity that reflects a molecular memory of having previously come into contact with calcium. The present study addresses whether the molecular memory properties of CaMKII are involved in the modulation of sustained vascular tone. We demonstrate a history-dependence of α agonist-induced vascular tone and show that CaMKII activation in vascular cells is also history dependent. Autophosphorylation of Thr287, which is classically associated with autonomous activity, does not persist during tone maintenance after transient increases in intracellular calcium levels. However, we have found that another site, Thr305, known from in vitro studies to be inhibitory, is regulated by α agonists in that the inhibitory action is removed, thus leading to a delayed reactivation of CaMKII as measured by Thr287 phosphorylation. By the use of a small molecule CaMKII inhibitor (KN93) as well as a decoy peptide (autoinhibitory peptide; AIP) we show a cause and effect relationship between CaMKII reactivation and sustained vascular tone maintenance. Thus, it appears that a complex interplay between the regulation of Thr305 and Thr287 provides a novel mechanism by which a history-dependence is developed and contributes to a new facet of molecular memory for CaMKII of relevance to vascular tone maintenance.