Altered reactivity of tertiary mesenteric arteries following acute myocardial ischemia

Aging related decreases in NM myosin expression and contractility in a resistance vessel

Introduction: Vasodilatation in response to NO is a fundamental response of the vasculature, and during aging, the vasculature is characterized by an increase in stiffness and decrease in sensitivity to NO mediated vasodilatation. Vascular tone is regulated by the activation of smooth muscle and nonmuscle (NM) myosin, which are regulated by the activities of myosin light chain kinase (MLCK) and MLC phosphatase. MLC phosphatase is a trimeric enzyme with a catalytic subunit, myosin targeting subunit (MYPT1) and 20 kDa subunit of unknown function. Alternative mRNA splicing produces LZ+/LZ- MYPT1 isoforms and the relative expression of LZ+/LZ- MYPT1 determines the sensitivity to NO mediated vasodilatation. This study tested the hypothesis that aging is associated with changes in LZ+ MYPT1 and NM myosin expression, which alter vascular reactivity. Methods: We determined MYPT1 and NM myosin expression, force and the sensitivity of both endothelial dependent and endothelial independent relaxation in tertiary mesenteric arteries of young (6mo) and elderly (24mo) Fischer344 rats. Results: The data demonstrate that aging is associated with a decrease in both the expression of NM myosin and force, but LZ+ MYPT expression and the sensitivity to both endothelial dependent and independent vasodilatation did not change. Further, smooth muscle cell hypertrophy increases the thickness of the medial layer of smooth muscle with aging. Discussion: The reduction of NM myosin may represent an aging associated compensatory mechanism to normalize the stiffness of resistance vessels in response to the increase in media thickness observed during aging.

The vasculature in HFpEF vs HFrEF: differences in contractile protein expression produce distinct phenotypes

Both heart failure with reduced (HFrEF) and preserved (HFpEF) ejection fraction are associated with abnormalities of the vasculature, including a resting vasoconstriction and a decrease in sensitivity to nitric oxide (NO) mediated vasodilation. Vascular tone is controlled by the expression and activation of both smooth muscle (SM) and nonmuscle (NM) myosin, and NO mediated vasodilation is regulated by the expression of the leucine zipper positive (LZ+) isoform of the myosin targeting subunit (MYPT1) of myosin light chain phosphatase (MLCP). This study was designed to determine the expression of these contractile proteins in humans with HFrEF and HFpEF vs normal controls. We isolated tertiary mesenteric vessels from remnant biospecimens of patients undergoing partial or total colectomy at Mayo Clinic Rochester from August 2017 to December 2018, and examined the expression of MYPT1 and the LZ + MYPT1 isoform with immunoblots, while 2D SDS-PAGE was used to resolve the phosphorylated and nonphosphorylated regulatory light chains of NM and SM myosin. Our data show that NM myosin expression, as a percentage of total myosin, was 12 ± 3% (controls, n = 6), 7 ± 5% (HFpEF, n = 4) and 37 ± 18% (HFrEF, n = 5, p < 0.05). Total MYPT1 expression was significantly reduced (p < 0.05) in both HFpEF (70 ± 11%) and HFrEF (48 ± 6%); and in HFrEF, LZ + MYPT1 was also depressed (62 ± 19%, <0.05). These results demonstrate that HFrEF and HFpEF are distinct vascular entities, and the changes in protein expression contribute to the vascular abnormalities associated with these diseases. Further in HFpEF, the decrease in MYPT1 would explain why pharmacologic therapies that are designed to activate the NO/cGMP/PKG signaling pathway do not produce a clinical benefit.

Mechanisms underlying TNFα-induced enhancement of force generation in airway smooth muscle

Airway diseases such as asthma are triggered by inflammation and mediated by proinflammatory cytokines such as tumor necrosis factor alpha (TNFα). Our goal was to systematically examine the potential mechanisms underlying the effect of TNFα on airway smooth muscle (ASM) contractility. Porcine ASM strips were incubated for 24 h with and without TNFα. Exposure to TNFα increased maximum ASM force in response to acetylcholine (Ach), with an increase in ACh sensitivity (hyperreactivity), as reflected by a leftward shift in the dose-response curve (EC50 ). At the EC50 , the [Ca2+ ]cyt response to ACh was similar between TNFα and control ASM, while force increased; thus, Ca2+ sensitivity appeared to increase. Exposure to TNFα increased the basal level of regulatory myosin light chain (rMLC) phosphorylation in ASM; however, the ACh-dependent increase in rMLC phosphorylation was blunted by TNFα with no difference in the extent of rMLC phosphorylation at the EC50 ACh concentration. In TNFα-treated ASM, total actin and myosin heavy chain concentrations increased. TNFα exposure also enhanced the ACh-dependent polymerization of G- to F-actin. The results of this study confirm TNFα-induced hyperreactivity to ACh in porcine ASM. We conclude that the TNFα-induced increase in ASM force, cannot be attributed to an enhanced [Ca2+ ]cyt response or to an increase in rMLC phosphorylation. Instead, TNFα increases Ca2+ sensitivity of ASM force generation due to increased contractile protein content (greater number of contractile units) and enhanced cytoskeletal remodeling (actin polymerization) resulting in increased tethering of contractile elements to the cortical cytoskeleton and force translation to the extracellular matrix.

Smoothelin-like 1 deletion enhances myogenic reactivity of mesenteric arteries with alterations in PKC and myosin phosphatase signaling

The role of the smoothelin-like 1 (SMTNL1) protein in mediating vascular smooth muscle contractile responses to intraluminal pressure was examined in resistance vessels. Mesenteric arterioles from wild type (WT) and SMTNL1 global knock-out (KO) mice were examined with pressure myography. SMTNL1 deletion was associated with enhanced myogenic tone in vessels isolated from male, but not female, mice. Intraluminal pressures greater than 40 mmHg generated statistically significant differences in myogenic reactivity between WT and KO vessels. No overt morphological differences were recorded for vessels dissected from KO animals, but SMTNL1 deletion was associated with loss of myosin phosphatase-targeting protein MYPT1 and increase in the myosin phosphatase inhibitor protein CPI-17. Additionally, we observed altered contractile responses of isolated arteries from SMTNL1 KO mice to phenylephrine, KCl-dependent membrane depolarization and phorbol 12,13-dibutyrate (PDBu). Using pharmacological approaches, myogenic responses of both WT and KO vessels were equally affected by Rho-associated kinase (ROCK) inhibition; however, augmented protein kinase C (PKC) signaling was found to contribute to the increased myogenic reactivity of SMTNL1 KO vessels across the 60–120 mmHg pressure range. Based on these findings, we conclude that deletion of SMTNL1 contributes to enhancement of pressure-induced contractility of mesenteric resistance vessels by influencing the activity of myosin phosphatase.

MYPT1 isoforms expressed in HEK293T cells are differentially phosphorylated after GTPγS treatment

Agonist stimulation of smooth muscle is known to activate RhoA/Rho kinase signaling, and Rho kinase phosphorylates the myosin targeting subunit (MYPT1) of myosin light chain (MLC) phosphatase at Thr696 and Thr853, which inhibits the activity of MLC phosphatase to produce a Ca²⁺ independent increase in MLC phosphorylation and force (Ca²⁺ sensitization). Alternative mRNA splicing produces four MYPT1 isoforms, which differ by the presence or absence of a central insert (CI) and leucine zipper (LZ). This study was designed to determine if Rho kinase differentially phosphorylates MYPT1 isoforms. In HEK293T cells expressing each of the four MYPT1 isoforms, we could not detect a change in Thr853 MYPT1 phosphorylation following GTPγS treatment. However, there is differential phosphorylation of MYPT1 isoforms at Thr696; GTPγS treatment increases MYPT1 phosphorylation for the CI+LZ- and CI-LZ- MYPT1 isoforms, but not the CI+LZ+ or CI-LZ+ MYPT1 isoforms. These data could suggest that in smooth muscle Rho kinase differentially phosphorylates MYPT1 isoforms.

A splice variant of the myosin phosphatase regulatory subunit tunes arterial reactivity and suppresses response to salt loading

The cGMP activated kinase cGK1α is targeted to its substrates via leucine zipper (LZ)-mediated heterodimerization and thereby mediates vascular smooth muscle (VSM) relaxation. One target is myosin phosphatase (MP), which when activated by cGK1α results in VSM relaxation even in the presence of activating calcium. Variants of MP regulatory subunit Mypt1 are generated by alternative splicing of the 31 nt exon 24 (E24), which, by changing the reading frame, codes for isoforms that contain or lack the COOH-terminal LZ motif (E24+/LZ-; E24-/LZ+). Expression of these isoforms is vessel specific and developmentally regulated, modulates in disease, and is proposed to confer sensitivity to nitric oxide (NO)/cGMP-mediated vasorelaxation. To test this, mice underwent Tamoxifen-inducible and smooth muscle-specific knockout of E24 (E24 cKO) after weaning. Deletion of a single allele of E24 (shift to Mypt1 LZ+) enhanced vasorelaxation of first-order mesenteric arteries (MA1) to diethylamine-NONOate (DEA/NO) and to cGMP in permeabilized and calcium-clamped arteries and lowered blood pressure. There was no further effect of deletion of both E24 alleles, indicating high sensitivity to shift of Mypt1 isoforms. However, a unique property of MA1s from homozygous E24 cKOs was significantly reduced force generation to α-adrenergic activation. Furthermore 2 wk of high-salt (4% NaCl) diet increased MA1 force generation to phenylephrine in control mice, a response that was markedly suppressed in the E24 cKO homozygotes. Thus Mypt1 E24 splice variants tune arterial reactivity and could be worthy targets for lowering vascular resistance in disease states.

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.

Myosin phosphatase isoforms as determinants of smooth muscle contractile function and calcium sensitivity of force production

The dephosphorylation of myosin by the MP causes smooth muscle relaxation. MP is also a key target of signals that regulate vascular tone and thus blood flow and pressure. Here, we review studies from the past two decades that support the hypothesis that the regulated expression of MP subunits is a critical determinant of smooth muscle responses to constrictor and dilator signals. In particular, the highly regulated splicing of the regulatory subunit Mypt1 Exon 24 is proposed to tune sensitivity to NO/cGMP-mediated relaxation. The regulated transcription of the MP inhibitory subunit CPI-17 is proposed to determine sensitivity to agonist-mediated constriction. The expression of these subunits is specific in the microcirculation and varies in developmental and disease contexts. To date, the relationship between MP subunit expression and vascular function in these different contexts is correlative; confirmation of the hypothesis will require the generation of genetically engineered mice to test the role of MP subunits and their isoforms in the specificity of vascular smooth muscle responses to constrictor and dilator signals.

Redox signaling and splicing dependent change in myosin phosphatase underlie early versus late changes in NO vasodilator reserve in a mouse LPS model of sepsis

Microcirculatory dysfunction may cause tissue malperfusion and progression to organ failure in the later stages of sepsis, but the role of smooth muscle contractile dysfunction is uncertain. Mice were given intraperitoneal LPS, and mesenteric arteries were harvested at 6-h intervals for analyses of gene expression and contractile function by wire myography. Contractile (myosin and actin) and regulatory [myosin light chain kinase and phosphatase subunits (Mypt1, CPI-17)] mRNAs and proteins were decreased in mesenteric arteries at 24 h concordant with reduced force generation to depolarization, Ca(2+), and phenylephrine. Vasodilator sensitivity to DEA/nitric oxide (NO) and cGMP under Ca(2+) clamp were increased at 24 h after LPS concordant with a switch to Mypt1 exon 24- splice variant coding for a leucine zipper (LZ) motif required for PKG-1α activation of myosin phosphatase. This was reproduced by smooth muscle-specific deletion of Mypt1 exon 24, causing a shift to the Mypt1 LZ+ isoform. These mice had significantly lower resting blood pressure than control mice but similar hypotensive responses to LPS. The vasodilator sensitivity of wild-type mice to DEA/NO, but not cGMP, was increased at 6 h after LPS. This was abrogated in mice with a redox dead version of PKG-1α (Cys42Ser). Enhanced vasorelaxation in early endotoxemia is mediated by redox signaling through PKG-1α but in later endotoxemia by myosin phosphatase isoform shifts enhancing sensitivity to NO/cGMP as well as smooth muscle atrophy. Muscle atrophy and modulation may be a novel target to suppress microcirculatory dysfunction; however, inactivation of inducible NO synthase, treatment with the IL-1 antagonist IL-1ra, or early activation of α-adrenergic signaling did not suppressed this response.

A bioinformatic and computational study of myosin phosphatase subunit diversity

Variability in myosin phosphatase (MP) subunits may provide specificity in signaling pathways that regulate muscle tone. We utilized public databases and computational algorithms to investigate the phylogenetic diversity of MP regulatory (PPP1R12A-C) and inhibitory (PPP1R14A-D) subunits. The comparison of exonic coding sequences and expression data confirmed or refuted the existence of isoforms and their tissue-specific expression in different model organisms. The comparison of intronic and exonic sequences identified potential expressional regulatory elements. As examples, smooth muscle MP regulatory subunit (PPP1R12A) is highly conserved through evolution. Its alternative exon E24 is present in fish through mammals with two invariant features: 1) a reading frame shift generating a premature termination codon and 2) a hexanucleotide sequence adjacent to the 3' splice site hypothesized to be a novel suppressor of exon splicing. A characteristic of the striated muscle MP regulatory subunit (PPP1R12B) locus is numerous and phylogenetically variable transcriptional start sites. In fish this locus only codes for the small (M21) subunit, suggesting the primordial function of this gene. Inhibitory subunits show little intragenic variability; their diversity is thought to have arisen by expansion and tissue-specific expression of different gene family members. We demonstrate differences in the regulatory landscape between smooth muscle enriched (PPP1R14A) and more ubiquitously expressed (PPP1R14B) family members and identify deeply conserved intronic sequence and predicted transcriptional cis-regulatory elements. This bioinformatic and computational study has uncovered a number of attributes of MP subunits that supports selection of ideal model organisms and testing of hypotheses regarding their physiological significance and regulated expression.

Smooth muscle contractile diversity in the control of regional circulations

Each regional circulation has unique requirements for blood flow and thus unique mechanisms by which it is regulated. In this review we consider the role of smooth muscle contractile diversity in determining the unique properties of selected regional circulations and its potential influence on drug targeting in disease. Functionally smooth muscle diversity can be dichotomized into fast versus slow contractile gene programs, giving rise to phasic versus tonic smooth muscle phenotypes, respectively. Large conduit vessel smooth muscle is of the tonic phenotype; in contrast, there is great smooth muscle contractile diversity in the other parts of the vascular system. In the renal circulation, afferent and efferent arterioles are arranged in series and determine glomerular filtration rate. The afferent arteriole has features of phasic smooth muscle, whereas the efferent arteriole has features of tonic smooth muscle. In the splanchnic circulation, the portal vein and hepatic artery are arranged in parallel and supply blood for detoxification and metabolism to the liver. Unique features of this circulation include the hepatic-arterial buffer response to regulate blood flow and the phasic contractile properties of the portal vein. Unique features of the pulmonary circulation include the low vascular resistance and hypoxic pulmonary vasoconstriction, the latter attribute inherent to the smooth muscle cells but the mechanism uncertain. We consider how these unique properties may allow for selective drug targeting of regional circulations for therapeutic benefit and point out gaps in our knowledge and areas in need of further investigation.

The role of pulmonary vascular contractile protein expression in pulmonary arterial hypertension

Pulmonary arterial hypertension (PAH) is associated with refractory vasoconstriction and impaired NO-mediated vasodilatation of the pulmonary vasculature. Vascular tone is regulated by light chain (LC) phosphorylation of both nonmuscle (NM) and smooth muscle (SM) myosins, which are determined by the activities of MLC kinase and MLC phosphatase. Further, NO mediated vasodilatation requires the expression of a leucine zipper positive (LZ+) isoform of the myosin targeting subunit (MYPT1) of MLC phosphatase. The objective of this study was to define contractile protein expression in the pulmonary arterial vasculature and vascular reactivity in PAH. In severe PAH, compared to controls, relative LZ+MYPT1 expression was decreased (100 ± 14% vs. 60 ± 6%, p<0.05, n=7-8), and NM myosin expression was increased (1 5 ± 4% vs. 53 ± 5% of total myosin, p<0.05, n=4-6). These changes in contractile protein expression should alter vascular reactivity; following activation with Ang II, force activation and relaxation were slowed, and sustained force was increased. Further, the sensitivity to ACh-mediated relaxation was reduced. These results demonstrate that changes in the pulmonary arterial SM contractile protein expression may participate in the molecular mechanism producing both the resting vasoconstriction and the decreased sensitivity to NO-mediated vasodilatation associated with PAH.