Smooth Muscle Myosin Light Chain Research Articles

Network pharmacology combined with an animal model to reveal the material basis and mechanism of Amomum villosum in alleviating constipation in mice

Constipation is a prevalent gastrointestinal disorder, with its prevalence showing an annual upward trend. There are many factors involved in the occurrence of constipation, such as abnormal smooth muscle contraction and disorders of gastrointestinal hormone secretion. Amomum villosum (A. villosum) has been proven to be effective in improving digestive system diseases, but there is no report on improving constipation. Therefore, we used network pharmacology prediction combined with animal experiments to explore the key active components of A. villosum and their pharmacological mechanisms. The results of network pharmacological prediction showed that β-sitosterol was the key laxative compound of A. villosum, which may play a laxative role by activating the adrenoceptor alpha 1 A-myosin light chain (ADRA1A-MLC) pathway. Further animal experiments showed that β-sitosterol could significantly shorten the time to first black stool; increase faecal weight, faecal number, and faecal water content; and promote gastrointestinal motility. β-sitosterol may promote intestinal motility by upregulating the expression of ADRA1A and myosin light chain 9 (Myl9) mRNA and protein in the colon, thereby activating the ADRA1A-MLC signalling pathway. In addition, it is possible to improve constipation symptoms by regulating serum neurotransmitters and gastrointestinal motility-related factors, such as the serum content of 5-hydroxytryptamine (5-HT) and acetylcholinesterase (AchE) and the mRNA expression of 5-hydroxytryptamine receptor 4 (5-HT4), stem cell factor (SCF), stem cell factor receptor (c-Kit) and smooth muscle myosin light chain kinase (smMLCK) in the colon. These results lay a foundation for the application of A. villosum and β-sitosterol in constipation.

Cymbopogon citratus (DC.) Stapf aqueous extract ameliorates loperamide-induced constipation in mice by promoting gastrointestinal motility and regulating the gut microbiota.

Slow transit constipation (STC) is the most common type of functional constipation. Drugs with good effects and few side effects are urgently needed form the treatment of STC. Cymbopogon citratus (DC.) Stapf (CC) is an important medicinal and edible spice plant. The wide range of biological activities suggested that CC may have laxative effects, but thus far, it has not been reported. In this study, the loperamide-induced STC mouse model was used to evaluate the laxative effect of the aqueous extract of CC (CCAE), and the laxative mechanism was systematically explored from the perspectives of the enteric nervous system (ENS), neurotransmitter secretion, gastrointestinal motility factors, intestinal inflammation, gut barrier and gut microbiota. The results showed that CCAE not only decreased the serum vasoactive intestinal polypeptide (VIP), induced nitric oxide synthases (iNOS), and acetylcholinesterase (AchE) in STC mice but also increased the expression of gastrointestinal motility factors in colonic interstitial cells of Cajal (ICCs) and smooth muscle cells (SMCs), thereby significantly shortening the defecation time and improving the gastrointestinal transit rate. The significantly affected gastrointestinal motility factors included stem cell factor receptor (c-Kit), stem cell factor (SCF), anoctamin 1 (Ano1), ryanodine receptor 3 (RyR3), smooth muscle myosin light chain kinase (smMLCK) and Connexin 43 (Cx43). Meanwhile, CCAE could repair loperamide-induced intestinal inflammation and intestinal barrier damage by reducing the expression of the pro-inflammatory factor IL-1β and increasing the expression of the anti-inflammatory factor IL-10, chemical barrier (Muc-2) and mechanical barrier (Cldn4, Cldn12, Occludin, ZO-1, and ZO-2). Interestingly, CCAE could also partially restore loperamide-induced gut microbial dysbiosis in various aspects, such as microbial diversity, community structure and species composition. Importantly, we established a complex but clear network between gut microbiota and host parameters. Muribaculaceae, Lachnospiraceae and UCG-010 showed the most interesting associations with the laxative phenotypes; several other specific taxa showed significant associations with serum neurotransmitters, gastrointestinal motility factors, intestinal inflammation, and the gut barrier. These findings suggested that CCAE might promote intestinal motility by modulating the ENS-ICCs-SMCs network, intestinal inflammation, intestinal barrier and gut microbiota. CC may be an effective and safe therapeutic choice for STC.

Yeokwisan, a Standardized Herbal Formula, Enhances Gastric Emptying via Modulation of the Ghrelin Pathway in a Loperamide-induced Functional Dyspepsia Mouse Model.

Background: Yeokwisan, a standardized herbal formula, has exhibited clinical benefit for patients suffering from refractory functional dyspepsia (FD) in Korea since 2016. However, data about the mechanism of action of this formula are yet not available.Aim of the study: To evaluate and explore the effects of Yeokwisan on gastric emptying, a major symptom of functional dyspepsia, and its underlying mechanisms of action using a mouse model.Materials and methods: BALB/C mice were pretreated with Yeokwisan (100, 200, and 400 mg/kg, po) or mosapride (3 mg/kg, po) for 5 days and then treated with loperamide (10 mg/kg, ip) after 20 h of fasting. A solution of 0.05% phenol red (500 μL) or diet of 5% charcoal (200 μL) was orally administered, followed by assessment of gastric emptying or intestinal transit. Plasma acyl-ghrelin (ELISA), C-kit (immunofluorescence and western blotting), nNOS (western blotting) and gastric contraction- and ghrelin-related gene/protein expression levels were examined in stomach and small intestine tissues.Results: Loperamide injection substantially delayed gastric emptying, while Yeokwisan pretreatment (especially 200 and 400 mg/kg Yeokwisan) significantly attenuated this peristaltic dysfunction, as evidenced by the quantity of phenol red retained in the stomach (p < 0.05 or 0.01) and stomach weight (p < 0.05 or 0.01). The levels of plasma acyl-ghrelin and expression of gastric ghrelin-related genes, such as growth hormone secretagogue receptor (GHSR), ghrelin-O-acyltransferase (GOAT), adrenergic receptor β1 (ADRB1) and somatostatin receptor (SSTR), were significantly normalized (p < 0.05 or 0.01) by Yeokwisan (400 mg/kg). Yeokwisan (400 mg/kg) significantly tempered the loperamide-induced alterations in the c-kit and nNOS levels (p < 0.01) as well as the expression of contraction- and ghrelin-related genes, such as 5-HT4 receptor (5-HT4R), anoctamin-1 (ANO1), ryanodine receptor 3 (RYR3) and smooth muscle myosin light chain kinase (smMLCK), in the stomach, but not in the small intestine.Conclusion: The present results showed the clinical relevance of Yeokwisan, in treating FD, especially in promoting gastric emptying but not small intestinal transit. The main mechanisms corresponding to these effects may involve the modulation of the ghrelin pathway and activation of interstitial cells of Cajal in stomach tissue.

An Integrative Approach to Determine 3D Protein Structures Using Sparse Paramagnetic NMR Data and Physical Modeling.

Paramagnetic nuclear magnetic resonance (NMR) methods have emerged as powerful tools for structure determination of large, sparsely protonated proteins. However traditional applications face several challenges, including a need for large datasets to offset the sparsity of restraints, the difficulty in accounting for the conformational heterogeneity of the spin-label, and noisy experimental data. Here we propose an integrative approach to structure determination combining sparse paramagnetic NMR with physical modelling to infer approximate protein structural ensembles. We use calmodulin in complex with the smooth muscle myosin light chain kinase peptide as a model system. Despite acquiring data from samples labeled only at the backbone amide positions, we are able to produce an ensemble with an average RMSD of ∼2.8 Å from a reference X-ray crystal structure. Our approach requires only backbone chemical shifts and measurements of the paramagnetic relaxation enhancement and residual dipolar couplings that can be obtained from sparsely labeled samples.

Quantitative in situ proximity ligation assays examining protein interactions and phosphorylation during smooth muscle contractions

Antibody-based in situ proximity ligation assays (isPLA) have the potential to study protein phosphorylation and protein interactions with spatial resolution in intact tissues. However, the application of isPLA at the tissue level is limited by a lack of appropriate positive and negative controls and the difficulty in accounting for changes in tissue shape. Here we demonstrate a set of experimental and computational approaches using gastric fundus smooth muscles to improve the validity of quantitative isPLA. Appropriate positive and negative biological controls and PLA technical controls were selected to ensure experimental rigor. To account for changes in morphology between relaxed and contracted smooth muscles, target PLA spots were normalized to smooth muscle myosin light chain 20 PLA spots or the cellular cross-sectional areas. We describe the computational steps necessary to filter out false-positive improperly sized spots and set the thresholds for counting true positive PLA spots to quantify the PLA signals. We tested our approach by examining protein phosphorylation and protein interactions in smooth muscle myofilament Ca2+ sensitization pathways from resting and contracted gastric fundus smooth muscles. In conclusion, our tissue-level isPLA method enables unbiased quantitation of protein phosphorylation and protein-protein interactions in intact smooth muscle tissues, suggesting the potential for quantitative isPLA applications in other types of intact tissues.

Mechanosensitive Ion Channels in the Development of Ileus

IntroductionApproximately 30% of trauma patients develop ileus, a decrease or cessation of intestinal motility resulting in pathological distension of the intestinal wall. Mechanosensitive ion channels play a role in both the physiological and pathological response to mechanical stimuli.ObjectiveThe objective of this study was to determine if mechanosensitive ion channels play a role in the development of ileus and the response of the intestine to pathological stretch.MethodsIn a rodent model, we induced ileus by a combination of hemodilution and mesenteric venous hypertension. We measured intestinal function using an ex vivo organ bath system. We also measured smooth muscle myosin light chain phosphorylation and intestinal wall fluid content. We collected tissue for microarray analysis.ResultsIntestinal wall fluid content increased significantly, indicating the development of intestinal edema. Edema development resulted in distension of the intestinal wall, decreased intestinal smooth muscle contractile activity, and decreased smooth muscle myosin light chain phosphorylation. We used microarray analysis to compare normal (unstretched) intestinal smooth muscle to edematous (stretched) intestinal smooth muscle to characterize changes in ion channel expression. We identified hyperpolarization‐activated cyclic nucleotide‐gated 2 (HCN2), a cyclic nucleotide gated ion channel, which is also stretch‐activated. HCN2 gene expression decreased in edematous intestinal tissue. In the ex vivo organ bath system, we tested the effects of HCN2 inhibitors on intestinal function. In preliminary experiments, HCN2 inhibition with ZD7288 resulted in decreased response to carbachol, with little change in spontaneous contractile activity or frequency.ConclusionsOur data suggest that HCN2 downregulation/inhibition can suppress intestinal contractile activity. Thus, HCN2 is a potential therapeutic target for improving intestinal motility. In future experiments, we will determine the role of HCN2 in the intestinal response to stretch.Support or Funding InformationHungarian National Research Development and Innovation Office (K120669, EFOP‐3.6.2‐16‐2017‐00006)This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Banha-sasim-tang improves gastrointestinal function in loperamide-induced functional dyspepsia mouse model

Ethnopharmacological relevanceBanha-sasim-tang (BST; Hange-shashin-to in Kampo medicine; Banxia xiexin tang in traditional Chinese medicine) is a traditional Chinese harbal medicine that has been commonly used for gastrointestinal disorders. Aim of the studyTo investigate the pharmacological effects of BST, a standardized herbal drug, on main symptoms of functional dyspepsia including delayed gastric emptying, and underlying mechanisms of action in mouse model. Methods and materialsBalb/C mice were pretreated with BST (25, 50, 100 mg/kg, po) or mosapride (3 mg/kg, po) for 3 days, and then treated with loperamide (10 mg/kg, ip) after 19 h fasting. A solution of 0.05% phenol red (500 μL) or 5% charcoal diet (200 μL) was orally administered, followed by scarifying and assessment of gastric emptying or gastro-intestinal motility. C-kit (immunofluorescence), nNOS (western blot) and gastric contraction-related gene expression were examined in stomach tissue. ResultsThe loperamide injection substantially delayed gastric emptying, while the BST pretreatment significantly attenuated this peristaltic dysfunction, as evidenced by the quantity of stomach-retained phenol red (p < 0.05 or 0.01) and stomach weight (p < 0.05 or 0.01). The BST pretreatment significantly tempered the loperamide-induced inactivation of c-kit and nNOS (p < 0.05 or 0.01) as well as the contraction-related gene expression, such as the 5HT4 receptor (5HT4R), anoctamin-1 (ANO1), ryanodine receptor 3 (RYR3) and smooth muscle myosin light chain kinase (smMLCK). The BST pretreatment also significantly attenuated the alterations in gastro-intestinal motility (p < 0.01). ConclusionOur results are the first evidence of the prokinetic agent effects of Banha-sasim-tang in a loperamide-induced FD animal model. The underlying mechanisms of action may involve the modulation of peristalsis via activation of the interstitial cells of Cajal and the smooth muscle cells in the stomach.

The cell\u2013cell junctions of mammalian testes: II. The lamellar smooth muscle monolayer cells of the peritubular wall are laterally connected by vertical adherens junctions\u2014a novel architectonic cell\u2013cell junction system

The testes of sexually mature males of six mammalian species (men, bulls, boars, rats, mice, guinea pigs) have been studied using biochemical as well as light and electron microscopical techniques, in particular immunolocalizations. In these tissues, the peritubular walls represent lamellar encasement structures wrapped around the seminiferous tubules as a bandage system of extracellular matrix layers, alternating with monolayers of very flat polyhedral “lamellar smooth muscle cells” (LSMCs), the number of which varies in different species from 1 to 5 or 6. These LSMCs are complete SMCs containing smooth muscle α-actin (SMA), myosin light and heavy chains, α-actinin, tropomyosin, smoothelin, intermediate-sized filament proteins desmin and/or vimentin, filamin, talin, dystrophin, caldesmon, calponin, and protein SM22α, often also cytokeratins 8 and 18. In the monolayers, the LSMCs are connected by adherens junctions (AJs) based on cadherin-11, in some species also with P-cadherin and/or E-cadherin, which are anchored in cytoplasmic plaques containing β-catenin and other armadillo proteins, in some species also striatin family proteins, protein myozap and/or LUMA. The LSMC cytoplasm is rich in myofilament bundles, which in many regions are packed in paracrystalline arrays, as well as in “dense bodies,” “focal adhesions,” and caveolae. In addition to some AJ-like end-on-end contacts, the LSMCs are laterally connected by numerous vertical AJ-like junctions located in variously sized and variously shaped, overlapping (alter super alterum) lamelliform cell protrusions. Consequently, the LSMCs of the peritubular wall monolayers are SMCs sensu stricto which are laterally connected by a novel architectonic system of arrays of vertical AJs located in overlapping cell protrusions.

Regulation of Smooth Muscle Myosin Light Chain Phosphatase by Multisite Phosphorylation of the Myosin Targeting Subunit, MYPT1.

Smooth muscle contraction is triggered primarily by activation of Ca2+/calmodulin-dependent myosin light chain kinase leading to phosphorylation of the regulatory light chains of myosin II. Numerous contractile stimuli also induce inhibition of myosin light chain phosphatase thereby prolonging the contractile response. The phosphatase is a trimeric enzyme containing a catalytic subunit, a regulatory, myosin-binding subunit (MYPT1) and a third subunit of uncertain function. MYPT1 is phosphorylated at multiple sites by several kinases, which regulate phosphatase activity, protein-protein interactions and subcellular localization. The best-characterized phosphorylation events involve phosphorylation by Rho-associated coiled-coil kinase (ROCK) at T697 and T855, which inhibits phosphatase activity, and phosphorylation by cAMP- or cGMPdependent protein kinases (PKA and PKG, respectively) at S696, T697, S854 and T855, which has no effect on phosphatase activity. Furthermore, phosphorylation by ROCK at T697 and T855 prevents phosphorylation by PKA or PKG at the neighboring serine residues. Some 30 phosphorylation sites have been identified in MYPT1 with many more suggested by large-scale phosphoproteomic studies. It is important to gain as complete understanding as possible of the complex phosphorylation-mediated mechanisms of regulation of MYPT1 functions in part because of their involvement in pathological processes. For example, dysfunctional MYPT1 phosphorylation has been implicated in the pathogenesis of several vascular disorders, including type 2 diabetes. Much effort is now being devoted to the development of novel therapeutics targeting MYPT1 and specific kinases involved in the phosphorylation of MYPT1.

Abstract 621: Vascular Smooth Muscle EphA2 Signaling Regulates Fibrillar Adhesion Formation to Drive Fibronectin Deposition

During atherosclerosis, synthetic vascular smooth muscle cells drive development of the fibrous cap through extracellular matrix deposition. Deposition of the provisional matrix protein fibronectin appears to critically regulate fibrous cap development by promoting smooth muscle recruitment and by providing a critical scaffold for assembly of fibrillar collagens. We previously showed that deletion of the receptor tyrosine kinase EphA2 in ApoE knockout mice attenuated plaque size and progression, characterized by a reduction of both smooth muscle content and fibrosis, and we further demonstrated that EphA2 depletion attenuates smooth muscle fibronectin deposition in vitro. Despite this, the mechanisms by which EphA2 regulates smooth muscle matrix deposition remains unknown. Fibronectin fibrillogenesis involves both the formation of α5β1 integrin-rich fibrillar adhesions and contractility-dependent fibronectin unfolding. We now show EphA2 localizes to integrin adhesions and appears to regulate matrix deposition through multiple mechanisms. Knockdown of EphA2 reduces the formation of fibrillar adhesions characterized by a loss of α5β1 integrins and tensin, whereas EphA2 overexpression is sufficient to induce fibrillar adhesion formation in contractile smooth muscle cells. In addition, vascular smooth muscle cells show diminished collagen gel contraction following EphA2 knockdown, and EphA2 depletion significantly reduces smooth muscle myosin light chain phosphorylation, suggesting that EphA2 expression is required to generate the contractive forces required for matrix remodeling in synthetic smooth muscle cells. Together these data identify a novel role for EphA2 in the regulation of smooth muscle matrix deposition through effects on both integrin adhesion structure and cytoskeletal contractility.

ML-7 attenuates airway inflammation and remodeling via inhibiting the secretion of Th2 cytokines in mice model of asthma.

Previous studies have indicated that smooth muscle myosin light chain kinase (MLCK) has a prominent role in the regulation of smooth muscle contraction, which tends to be upregulated in asthma. In recent years, numerous studies have reported that MLCK is intimately connected with the immunoregulatory mechanism of T cells. The imbalance of T helper type 1 cells (Th1)/Th2 constitutes the immune-associated pathological basis of chronic asthma. Th2-associated cytokines, including interleukin-4, −5, −13, −25 and −33, are involved in airway inflammation, hyperresponsiveness and remodeling, which leads to a progressive decline in lung function. The purpose of the present study was to verify whether inhibition of bronchial MLCK attenuated the expression Th2-associated cytokines in asthmatic mice, including the above-mentioned ones. Female BALB/c mice were used to establish an ovalbumin (OVA)-induced model of asthma, of which one group was treated with the MLCK inhibitor (5-iodonaphthalene-1-sulfonyl) homopiperazine (ML-7). The inhibitor of MLCK, ML-7 attenuated airway inflammation and remodeling by reducing inflammatory cell infiltration and the secretion of Th2 cytokines in mice model of asthma, which may represent a promising therapeutic strategy for asthma.

Detailed Evidence for an Unparalleled Interaction Mode between Calmodulin and Orai Proteins.

Calmodulin (CaM) binds most of its targets by wrapping around an amphipathic α-helix. The N-terminus of Orai proteins contains a conserved CaM-binding segment but the binding mechanism has been only partially characterized. Here, microscale thermophoresis (MST), surface plasmon resonance (SPR), and atomic force microscopy (AFM) were employed to study the binding equilibria, the kinetics, and the single-molecule interaction forces involved in the binding of CaM to the conserved helical segments of Orai1 and Orai3. The results consistently indicated stepwise binding of two separate target peptides to the two lobes of CaM. An unparalleled high affinity was found when two Orai peptides were dimerized or immobilized at high lateral density, thereby mimicking the close proximity of the N-termini in native Orai oligomers. The analogous experiments with smooth muscle myosin light chain kinase (smMLCK) showed only the expected 1:1 binding, confirming the validity of our methods.

Increasing evidence of mechanical force as a functional regulator in smooth muscle myosin light chain kinase.

Mechanosensitive proteins are key players in cytoskeletal remodeling, muscle contraction, cell migration and differentiation processes. Smooth muscle myosin light chain kinase (smMLCK) is a member of a diverse group of serine/threonine kinases that feature cytoskeletal association. Its catalytic activity is triggered by a conformational change upon Ca2+/calmodulin (Ca2+/CaM) binding. Due to its significant homology with the force-activated titin kinase, smMLCK is suspected to be also regulatable by mechanical stress. In this study, a CaM-independent activation mechanism for smMLCK by mechanical release of the inhibitory elements is investigated via high throughput AFM single-molecule force spectroscopy. The characteristic pattern of transitions between different smMLCK states and their variations in the presence of different substrates and ligands are presented. Interaction between kinase domain and regulatory light chain (RLC) substrate is identified in the absence of CaM, indicating restored substrate-binding capability due to mechanically induced removal of the auto-inhibitory regulatory region.

Development of an endoplasmic reticulum calcium sensor based on fluorescence resonance energy transfer

Endoplasmic reticulum (ER) is an organelle critically involved in Ca2+ signaling by regulating the intracellular Ca2+ gradient. In the present study, we have developed an ER Ca2+ sensor based on fluorescence resonance energy transfer (FRET), capable of monitoring dynamic Ca2+ signals within the ER. This genetically-encoded ER sensor contains a mutated version of calmodulin (mCaM) and its binding peptide derived from smooth muscle myosin light chain kinase (m-smMLCK) as the Ca2+-responsive elements, thereby enabling an effective ER Ca2+ sensing without perturbation by endogenous CaM. The Ca2+ sensitivity, including detection limit, of the sensor was evaluated through in vitro characterization. In cell-based assay, our biosensor displayed high accuracy to ER Ca2+ sensing in response to pharmacological stimuli, indicating that this sensor is reliable for visualization of ER Ca2+ signals at high spatiotemporal resolutions in live cells.

Characterization of the Orai-Calmodulin Interaction as Potential Mediator of Calcium-Dependent Orai-Channel Inactivation

Calcium is an important second messenger involved in many cellular processes such as cell proliferation, T-cell activation, muscle contraction, egg fertilization, or apoptosis. Calcium entry into non-excitable cells is mainly carried by store-operated Ca2+ release-activated Ca2+ (CRAC) channels, whereby Orai was found to be the calcium channel in the plasma membrane and the stromal interaction molecule (STIM) to act as a calcium sensor in the endoplasmatic reticulum (ER), and as an activator of Orai channels. A reduction of Ca2+ in the ER causes STIM to oligomerize, enabling its accumulation at the ER-plasma membrane junctions where it binds directly to Orai to open the Ca2+-channel. Compared with activation, much less is known about the mechanisms underlying Ca2+-dependent inactivation (CDI) processes. It has been proposed that binding of calmodulin (CaM) to a highly conserved N-terminal segment of Orai1 adjacent to its first transmembrane helix is critical for CDI. Hereby it is assumed that CaM acts in concert with STIM1 and the N terminus of Orai1 to evoke rapid CRAC channel inactivation. However, the molecular basis of these interactions remains unclear. In this study, the interaction between calmodulin and the N-terminal fragment of Orai1 and its homologue Orai3 was studied by Single Molecule Recognition Force Spectroscopy, Surface Plasmon Resonance, and Microscale Thermophoresis. Both Orai fragments showed a calcium-dependent and highly specific interaction with calmodulin, with slightly longer bond lifetimes and lower dissociation rate constants for Orai3 than for Orai1. Our data support a switch between a 1:1 and a 1:2 binding stoichiometry between CaM and Orai1/3. The latter occurs at low CaM concentration, has much higher affinity, and takes much longer to disscociate after removal of Ca2+ by EGTA. By comparison, only 1:1 binding with high affinity was found between CaM and smooth muscle myosin light chain kinase. This emphasizes the importance of the unique amino acid sequence of the conserved N-terminal Orai segment for high CaM affinity and CDI. This work was supported by the state of Upper Austria (project DK NanoCell).

Cardiac myosin light chain is phosphorylated by Ca2+/calmodulin-dependent and -independent kinase activities

The well-known, muscle-specific smooth muscle myosin light chain kinase (MLCK) (smMLCK) and skeletal muscle MLCK (skMLCK) are dedicated protein kinases regulated by an autoregulatory segment C terminus of the catalytic core that blocks myosin regulatory light chain (RLC) binding and phosphorylation in the absence of Ca(2+)/calmodulin (CaM). Although it is known that a more recently discovered cardiac MLCK (cMLCK) is necessary for normal RLC phosphorylation in vivo and physiological cardiac performance, information on cMLCK biochemical properties are limited. We find that a fourth uncharacterized MLCK, MLCK4, is also expressed in cardiac muscle with high catalytic domain sequence similarity with other MLCKs but lacking an autoinhibitory segment. Its crystal structure shows the catalytic domain in its active conformation with a short C-terminal "pseudoregulatory helix" that cannot inhibit catalysis as a result of missing linker regions. MLCK4 has only Ca(2+)/CaM-independent activity with comparable Vmax and Km values for different RLCs. In contrast, the Vmax value of cMLCK is orders of magnitude lower than those of the other three MLCK family members, whereas its Km (RLC and ATP) and KCaM values are similar. In contrast to smMLCK and skMLCK, which lack activity in the absence of Ca(2+)/CaM, cMLCK has constitutive activity that is stimulated by Ca(2+)/CaM. Potential contributions of autoregulatory segment to cMLCK activity were analyzed with chimeras of skMLCK and cMLCK. The constitutive, low activity of cMLCK appears to be intrinsic to its catalytic core structure rather than an autoinhibitory segment. Thus, RLC phosphorylation in cardiac muscle may be regulated by two different protein kinases with distinct biochemical regulatory properties.

Analysis of phosphorylation of the myosin targeting subunit of smooth muscle myosin light chain phosphatase by Phos‐tag SDS‐PAGE

Phosphorylation of the myosin targeting subunit (MYPT1) of myosin light chain phosphatase plays an important role in the regulation of smooth muscle contraction, and several sites of phosphorylation by different protein Ser/Thr kinases have been identified. Furthermore, in some instances, phosphorylation at specific sites affects phosphorylation at neighboring sites, with functional consequences. Characterization of the complex phosphorylation of MYPT1 in tissue samples at rest and in response to contractile and relaxant stimuli is, therefore, challenging. We have exploited Phos‐tag SDS‐PAGE in combination with western blotting using antibodies to MYPT1 to separate multiple phosphorylated MYPT1 species and quantify MYPT1 phosphorylation stoichiometry using purified, full‐length recombinant MYPT1 phosphorylated by Rho‐associated coiled‐coil kinase (ROCK) and cyclic AMP‐dependent protein kinase (PKA). This approach confirmed that phosphorylation of MYPT1 by ROCK occurs at Thr697 and Thr855, PKA phosphorylates these two sites and the neighboring Ser696 and Ser854, and prior phosphorylation at Thr697 and Thr855 by ROCK precludes phosphorylation at Ser696 and Ser854 by PKA. Furthermore, phosphorylation at Thr697 and Thr855 by ROCK exposes two other sites of phosphorylation by PKA. Treatment of Triton‐skinned rat caudal arterial smooth muscle strips with the membrane‐impermeant phosphatase inhibitor microcystin, or of intact tissue with the membrane‐permeant calyculin A, induced slow, sustained contractions that correlated with phosphorylation of MYPT1 at 7 to ≥10 sites. Phos‐tag SDS‐PAGE thus provides a suitable and convenient method for analysis of the complex, multi‐site MYPT1 phosphorylation events involved in the regulation of myosin light chain phosphatase activity and smooth muscle contraction.Support or Funding InformationFunded by the Canadian Institutes of Health Research (MOP‐111262)

Involvement of gap junctions between smooth muscle cells in sustained hypoxic pulmonary vasoconstriction development: a potential role for 15-HETE and 20-HETE.

In response to hypoxia, the pulmonary artery normally constricts to maintain optimal ventilation-perfusion matching in the lung, but chronic hypoxia leads to the development of pulmonary hypertension. The mechanisms of sustained hypoxic pulmonary vasoconstriction (HPV) remain unclear. The aim of this study was to determine the role of gap junctions (GJs) between smooth muscle cells (SMCs) in the sustained HPV development and involvement of arachidonic acid (AA) metabolites in GJ-mediated signaling. Vascular tone was measured in bovine intrapulmonary arteries (BIPAs) using isometric force measurement technique. Expression of contractile proteins was determined by Western blot. AA metabolites in the bath fluid were analyzed by mass spectrometry. Prolonged hypoxia elicited endothelium-independent sustained HPV in BIPAs. Inhibition of GJs by 18β-glycyrrhetinic acid (18β-GA) and heptanol, nonspecific blockers, and Gap-27, a specific blocker, decreased HPV in deendothelized BIPAs. The sustained HPV was not dependent on Ca(2+) entry but decreased by removal of Ca(2+) and by Rho-kinase inhibition with Y-27632. Furthermore, inhibition of GJs decreased smooth muscle myosin heavy chain (SM-MHC) expression and myosin light chain phosphorylation in BIPAs. Interestingly, inhibition of 15- and 20-hydroxyeicosatetraenoic acid (HETE) synthesis decreased HPV in deendothelized BIPAs. 15-HETE- and 20-HETE-stimulated constriction of BIPAs was inhibited by 18β-GA and Gap-27. Application of 15-HETE and 20-HETE to BIPAs increased SM-MHC expression, which was also suppressed by 18β-GA and by inhibitors of lipoxygenase and cytochrome P450 monooxygenases. More interestingly, 15,20-dihydroxyeicosatetraenoic acid and 20-OH-prostaglandin E2, novel derivatives of 20-HETE, were detected in tissue bath fluid and synthesis of these derivatives was almost completely abolished by 18β-GA. Taken together, our novel findings show that GJs between SMCs are involved in the sustained HPV in BIPAs, and 15-HETE and 20-HETE, through GJs, appear to mediate SM-MHC expression and contribute to the sustained HPV development.

Autoregulatory Control of Smooth Muscle Myosin Light Chain Kinase Promoter by Notch Signaling

Smooth muscle myosin light chain kinase (SM-MLCK) is the key enzyme responsible for phosphorylation of regulatory myosin light chain (MLC20), resulting in actin-myosin cross-bridging and force generation in vascular smooth muscle required for physiological vasoreactivity and blood pressure control. In this study, we investigated the combinatorial role of myocardin/serum response factor (SRF) and Notch signaling in the transcriptional regulation of MLCK gene expression. Promoter reporter analyses in rat A10 smooth muscle cells revealed a bimodal pattern of MLCK promoter activity and gene expression upon stimulation with constitutively active Notch1 in presence of myocardin or by Jagged1 ligand stimulation. An initial Notch1-induced increase in MLCK transcription was followed by loss in promoter sensitivity, which could be restored with further Notch1 dose escalation. Real-time PCR analyses revealed that endogenous levels of Hairy Related Transcription (HRT) factor 2 (HRT2) peaked concurrently with inhibitory concentrations of Notch1. Forced expression of HRT2 demonstrated simultaneous repression of both myocardin- and Notch1-induced MLCK promoter activity. HRT2-mediated repression was further confirmed by HRT2 truncations and siHRT2 treatments that rescued MLCK promoter activity and gene expression. Chromatin immunoprecipitation studies revealed both Jagged1 ligand- and Notch1-enhanced myocardin/SRF complex formation at the promoter CArG element. In contrast, heightened levels of HRT2 concomitantly disrupted myocardin/SRF and Notch transcription complex formation at respective CArG and CSL binding elements. Taken together, SM-MLCK promoter activity appears highly sensitive to the relative levels of Notch1 signaling, HRT2, and myocardin. These findings identify a novel Notch-dependent HRT2 autoregulatory circuit coordinating transcriptional regulation of SM-MLCK.

Diffusion of myosin light chain kinase on actin: A mechanism to enhance myosin phosphorylation rates in smooth muscle.

Smooth muscle myosin (SMM) light chain kinase (MLCK) phosphorylates SMM, thereby activating the ATPase activity required for muscle contraction. The abundance of active MLCK, which is tightly associated with the contractile apparatus, is low relative to that of SMM. SMM phosphorylation is rapid despite the low ratio of MLCK to SMM, raising the question of how one MLCK rapidly phosphorylates many SMM molecules. We used total internal reflection fluorescence microscopy to monitor single molecules of streptavidin-coated quantum dot-labeled MLCK interacting with purified actin, actin bundles, and stress fibers of smooth muscle cells. Surprisingly, MLCK and the N-terminal 75 residues of MLCK (N75) moved on actin bundles and stress fibers of smooth muscle cell cytoskeletons by a random one-dimensional (1-D) diffusion mechanism. Although diffusion of proteins along microtubules and oligonucleotides has been observed previously, this is the first characterization to our knowledge of a protein diffusing in a sustained manner along actin. By measuring the frequency of motion, we found that MLCK motion is permitted only if acto-myosin and MLCK-myosin interactions are weak. From these data, diffusion coefficients, and other kinetic and geometric considerations relating to the contractile apparatus, we suggest that 1-D diffusion of MLCK along actin (a) ensures that diffusion is not rate limiting for phosphorylation, (b) allows MLCK to locate to areas in which myosin is not yet phosphorylated, and (c) allows MLCK to avoid getting "stuck" on myosins that have already been phosphorylated. Diffusion of MLCK along actin filaments may be an important mechanism for enhancing the rate of SMM phosphorylation in smooth muscle.