Muscle Myosin Light Chain Phosphorylation Research Articles

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.

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.

Decreased myosin phosphatase target subunit 1(MYPT1) phosphorylation via attenuated rho kinase and zipper‐interacting kinase activities in edematous intestinal smooth muscle

Intestinal edema development after trauma resuscitation inhibits intestinal motility which results in ileus, preventing enteral feeding and compromising patient outcome. We have shown previously that decreased intestinal motility is associated with decreased smooth muscle myosin light chain (MLC) phosphorylation. The purpose of the present study was to investigate the mechanism of edema-induced decreases in MLC in a rodent model of intestinal edema. Intestinal edema was induced by a combination of resuscitation fluid administration and mesenteric venous hypertension. Sham operated animals served as controls. Contractile activity and alterations in the regulation of MLC including the regulation of MLC kinase (MLCK) and MLC phosphatase (MLCP) were measured. Contraction amplitude and basal tone were significantly decreased in edematous intestinal smooth muscle compared with non-edematous tissue. Calcium sensitivity was also decreased in edematous tissue compared with non-edematous intestinal smooth muscle. Although inhibition of MLCK decreased contractile activity significantly less in edematous tissue compared with non-edematous tissue, MLCK activity in tissue lysates was not significantly different. Phosphorylation of MYPT was significantly lower in edematous tissue compared with non-edematous tissue. In addition, activities of both rho kinase and zipper-interacting kinase were significantly lower in edematous tissue. We conclude from these data that interstitial intestinal edema inhibits MLC phosphorylation predominantly by decreasing inhibitory phosphorylation of the MLC targeting subunit (MYPT1) of MLC phosphatase via decreased ROCK and ZIPK activities, resulting in more MLC phosphatase activity.

Edema‐induced Intestinal Contractile Dysfunction Is Mediated by Pathologically Increased PAK Activity

Intestinal edema and subsequent intestinal contractile dysfunction develop under various pathologic circumstances including trauma/resuscitation, surgery and heart failure. Previously, we have shown that inhibitory phosphorylation of the myosin binding subunit of myosin light chain phosphatase (MYPT1) is significantly decreased in edematous intestinal smooth muscle tissue due to increased p21-activated kinase (PAK) activity. To investigate the role of PAK in edema-induced intestinal contractile dysfunction, a human primary intestinal smooth muscle cell (hISMC) model was developed in vitro. hISMCs were subjected to either basal physiological cyclical stretch or an increasing cyclical stretch, mimicking pathologic changes in edematous tissues. Increased cyclical stretch induced significant decreases in phosphorylation of both: MYPT1 and smooth muscle myosin light chain (MLC). In addition, there was a further decrease of MYPT1 and MLC phosphorylation when cells were transfected with constitutively active PAK. Dominant negative PAK produced opposite effects. Moreover, intestinal tissue contractility was rescued by inhibition of PAK activity in vivo. Our data indicate that activation of PAK after edema development inhibits MLC phosphorylation, which may be due to increased MYPT1 activity, eventually leading to intestinal contractile dysfunction.

Sodium hydrogen exchanger as a mediator of hydrostatic edema-induced intestinal contractile dysfunction

Resuscitation-induced intestinal edema is associated with early and profound mechanical changes in intestinal tissue. We hypothesize that the sodium hydrogen exchanger (NHE), a mechanoresponsive ion channel, is a mediator of edema-induced intestinal contractile dysfunction. An animal model of hydrostatic intestinal edema was used for all experiments. NHE isoforms 1-3 mRNA and protein were evaluated. Subsequently, the effects of NHE inhibition (with 5-(N-ethyl-N-isopropyl) amiloride [EIPA]) on wet-to-dry ratios, signal transduction and activator of transcription (STAT)-3, intestinal smooth muscle myosin light chain (MLC) phosphorylation, intestinal contractile activity, and intestinal transit were measured. NHE1-3 mRNA and protein levels were increased significantly in the small intestinal mucosa with the induction of intestinal edema. The administration of EIPA, an NHE inhibitor, attenuated validated markers of intestinal contractile dysfunction induced by edema as measured by decreased STAT-3 activation, increased MLC phosphorylation, improved intestinal contractile activity, and enhanced intestinal transit. The mechanoresponsive ion channel NHE may mediate edema-induced intestinal contractile dysfunction, possibly via a STAT-3 related mechanism.

Edema‐induced alterations in the regulation of myosin light chain phosphorylation in intestinal smooth muscle.

We have shown that edema decreases intestinal contractile activity via decreased intestinal smooth muscle myosin light chain (MLC) phosphorylation. The objective of this study was to determine the mechanism by which edema decreases MLC phosphorylation. Thus, regulation of MLC phosphorylation was studied in a rodent model of intestinal edema. Edema was induced by a combination of resuscitative fluid administration and induction of mesenteric venous hypertension. Edema‐induced changes in MLC kinase (MLCK) activity and phosphorylation of MYPT1 subunit of MLC phosphatase (MLCP) were measured in intestinal smooth muscle. Phosphorylation of the MYPT1 subunit in the presence of rho kinase (ROK), zipper interacting protein kinase (ZIPK), and p21 activated kinase (PAK) inhibitors, Y‐27632, SM‐1 peptide, and IPA‐3 respectively, was also measured. No significant changes in MLCK activity were observed in the edematous intestinal smooth muscle. In contrast, phosphorylation of the MYPT1 subunit of MLCP was significantly decreased in edematous intestinal smooth muscle compared to controls. Data suggest that both ROK and ZIPK were involved in edema‐induced decreases in MYPT1 phosphorylation. We conclude from these data that edema‐induced decreases in the inhibitory phosphorylation of MYPT1 subunit of MLCP via regulation of ROK and ZIPK result in decreased MLC phosphorylation in edematous intestinal smooth muscle.

The presence of an extractable substance in the CSF of humans with cerebral vasospasm after subarachnoid haemorrhage that correlates with phosphatase inhibition

The cellular events leading to cerebral vasospasm after subarachnoid haemorrhage are poorly understood, although an increase in smooth muscle myosin light chain phosphorylation has been observed. This study set out to determine if phosphatase inhibition may be involved in the pathological maintenance of tension observed during vasospasm. We found that 1 nM okadaic acid, a type 2A protein phosphatase inhibitor, elicited an increase in rate of O 2 consumption in the porcine carotid artery similar to that by cerebrospinal fluid (CSF) from vasospastic patients (CSF V, n=5) (control 0.23±0.03, CSF V 0.84±0.16 and okadaic acid 0.85±0.02 μmol min −1 g dwt −1). It was also observed that phosphatase inhibition with 1 nM okadaic acid significantly slowed relaxation after a stretch in a similar fashion to CSF V haemorrhage. CSF from vasospastic subarachnoid haemorrhage patients, but not from those without vasospasm, contains an extractable substance which modulates myosin light chain phosphorylation in vitro. A phosphatase preparation obtained from the porcine carotid artery dephosphorylated 63±2% of the phosphorylated (MLC 20) substrate in vitro, and non-vasospastic CSF treated enzyme dephosphorylated 60±2.6%. Okadaic acid inhibited phosphatase dephosphorylated only 7.5±1% of the substrate where CSF V treated enzyme dephosphorylated 22±2.8% of the substrate. We conclude that inhibition of smooth muscle phosphatase may be involved in the mechanisms associated with cerebral vasospasm after subarachnoid haemorrhage.

Myosin light chain kinase phosphorylation in swine carotid artery contraction and relaxation

We investigated the role of myosin light chain kinase (MLCK) phosphorylation in regulating the sensitivity of vascular smooth muscle myosin light chain (MLC) phosphorylation to intracellular Ca2+ concentration ([Ca2+]i). 32PO4-loaded swine carotid arteries were stimulated with histamine or high K+, MLCK was isolated, and the relative phosphorylation of tryptic peptides was measured. In nonlabeled tissues, we measured [Ca2+]i with aequorin, MLCK activity ratio, MLC phosphorylation, and force. A comparison of MLCK phosphorylation on peptide A (mol P in site A/mol MLCK) and MLCK activity ratio showed an inverse relation, suggesting that MLCK site A phosphorylation can regulate the Ca2+ sensitivity of MLCK. MLCK site A phosphorylation and MLCK activity ratio depended on [Ca2+]i. Histamine stimulation yielded greater MLC phosphorylation than high K+ stimulation over a range of [Ca2+]i; however, there were no apparent stimulus-dependent differences in MLCK phosphorylation, suggesting that stimulus-dependent differences in the Ca2+ sensitivity of MLC phosphorylation are not based on differences in MLCK phosphorylation. We also determined whether MLCK phosphorylation was involved in adenosine 3',5'-cyclic monophosphate-mediated relaxation. In histamine-contracted tissues, forskolin decreased [Ca2+]i, MLC phosphorylation, and force. MLCK phosphorylation decreased to an extent consistent with the decrease in [Ca2+]i. In KCl-stimulated tissues, forskolin did not alter [Ca2+]i or increase MLCK phosphorylation but forskolin did decrease MLC phosphorylation. Thus, in swine carotid artery, MLCK phosphorylation appears to be regulated exclusively by Ca2+ and plays little role in stimulus-dependent differences in Ca2+ sensitivity of MLC phosphorylation or in mediating forskolin-induced relaxation.

Characterization of magnesium-induced contractions in detergent-skinned swine carotid media

Ca(2+)-calmodulin-dependent phosphorylation of the 20-kDa smooth muscle myosin light chain (MLC) results in high shortening velocities and rapid stress development. The stress maintained after a reduction in Ca2+ is associated with a decrease in MLC phosphorylation and velocity of shortening. This Ca(2+)-dependent stress without proportional MLC phosphorylation has been termed "latch" and has been postulated to reflect a population of dephosphorylated noncycling cross bridges or "latch bridges." Mg2+ is necessary for contraction of smooth muscle, and in high concentrations, Mg2+ elicits contractions that are MLC phosphorylation independent. The purpose of this study was to test the hypothesis that high concentrations of Mg2+ directly induce latch-bridge formation. This was accomplished by comparing the characteristics of Mg(2+)-induced contractions of Triton X-100-skinned swine carotid media with the known characteristics of the Ca(2+)-dependent latch state. In the absence of Ca2+, free Mg2+ (3-20 mM) caused an increase in the velocity of shortening and a concentration-dependent increase in stress, with no detectable increase in MLC phosphorylation. Mg(2+)-induced contractions could be supported by CTP, which is a substrate for the actin-activated myosin adenosinetriphosphatase but not the MLC kinase. Stress development in response to Mg2+ was abolished at long tissue lengths, which also inhibit the expression of latch bridges. The calmodulin antagonist, trifluoperazine (TFP), inhibited the MLC phosphorylation-independent contractions elicited by Mg2+. TFP also inhibited the latch state. The results of this study support the existence of a regulatory system in vascular smooth muscle that is independent of the MLC phosphorylation system and can be directly activated by pharmacological levels of Mg2+.

N-(6-phenylhexyl)-5-chloro-1-naphthalenesulfonamide, a novel activator of protein kinase C

Naphthalenesulfonamide derivatives were used to study the mechanism of regulation of Ca2+-dependent smooth muscle myosin light chain phosphorylation catalyzed by Ca2+-activated, phospholipid-dependent protein kinase (protein kinase C) and myosin light chain kinase. Derivatives such as N-(6-phenylhexyl)-5-chloro-1-naphthalenesulfonamide (SC-9), with a hydrophobic residue at the end of a hydrocarbon chain, stimulated Ca2+-activated, phospholipid-dependent myosin light chain phosphorylation in a Ca2+-dependent fashion. There was no significant effect of these compounds on Ca2+-calmodulin (CaM) dependent myosin light chain phosphorylation. On the other hand, derivatives with the guanidino or amino residue at the same position had an inhibitory effect on both Ca2+-phospholipid- and Ca2+-CaM-dependent myosin light chain phosphorylation. These observations suggest that activation of Ca2+-activated, phospholipid-dependent myosin light chain phosphorylation by naphthalenesulfonamide derivatives depends on the chemical structure at the end of hydrocarbon chain of each compound. SC-9 was similar to phosphatidylserine with regard to activation, and the apparent Km values for Ca2+ of the enzyme with this compound and phosphatidylserine were 40 microM and 80 microM, respectively. Kinetic analysis indicated that 12-O-tetradecanoylphorbol 13-acetate increased the affinity of the enzyme with SC-9 for calcium ion. However, kinetic constants revealed that the Km value of protein kinase C activated by SC-9 for substrate myosin light chain was 5.8 microM, that is, about 10 times lower than that of the enzyme with phosphatidylserine, and that the Vmax value with SC-9 was 0.13 nmol X min-1, that is, 3-fold smaller than that seen with phosphatidylserine.(ABSTRACT TRUNCATED AT 250 WORDS)