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

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.

An antisense oligonucleotide efficiently suppresses splicing of an alternative exon in vascular smooth muscle in vivo

Targeting alternative exons for therapeutic gain has been achieved in a few instances and potentially could be applied more broadly. The myosin phosphatase (MP) enzyme is a critical hub upon which signals converge to regulate vessel tone. Alternative exon 24 of myosin phosphatase regulatory subunit (Mypt1 E24) is an ideal target as toggling between the two isoforms sets smooth muscle sensitivity to vasodilators such as nitric oxide (NO). This study aimed to develop a gene-based therapy to suppress splicing of Mypt1 E24 thereby switching MP enzyme to the NO-responsive isoform. CRISPR/Cas9 constructs were effective at editing of Mypt1 E24 in vitro; however, targeting of vascular smooth muscle in vivo with AAV9 was inefficient. In contrast, an octo-guanidine conjugated antisense oligonucleotide targeting the 5' splice site of Mypt1 E24 was highly efficient in vivo. It reduced the percent splicing inclusion of Mypt1 E24 from 80% to 10% in mesenteric arteries. The maximal and half-maximal effects occurred at 12.5 and 6.25 mg/kg, respectively. The effect persisted for at least 1 mo without toxicity. This highly effective splice-blocking antisense oligonucleotide could be developed as a novel therapy to reverse vascular dysfunction common to diseases such as hypertension and heart failure.NEW & NOTEWORTHY Alternative exon usage is a major driver of phenotypic diversity in all cell types including smooth muscle. However, the functional significance of most of the hundreds of thousands of alternative exons has not been defined, nor in most cases even tested. If their importance to vascular function were known these alternative exons could represent novel therapeutic targets. Here, we present injection of Vivo-morpholino splice-blocking antisense oligonucleotides as a simple, efficient, and cost-effective method for suppression of alternative exon usage in vascular smooth muscle in vivo.

Myocardin regulates exon usage in smooth muscle cells through induction of splicing regulatory factors

Differentiation of smooth muscle cells (SMCs) depends on serum response factor (SRF) and its co-activator myocardin (MYOCD). The role of MYOCD for the SMC program of gene transcription is well established. In contrast, the role of MYOCD in control of SMC-specific alternative exon usage, including exon splicing, has not been explored. In the current work we identified four splicing factors (MBNL1, RBPMS, RBPMS2, and RBFOX2) that correlate with MYOCD across human SMC tissues. Forced expression of MYOCD family members in human coronary artery SMCs in vitro upregulated expression of these splicing factors. For global profiling of transcript diversity, we performed RNA-sequencing after MYOCD transduction. We analyzed alternative transcripts with three different methods. Exon-based analysis identified 1637 features with differential exon usage. For example, usage of 3´ exons in MYLK that encode telokin increased relative to 5´ exons, as did the 17 kDa telokin to 130 kDa MYLK protein ratio. Dedicated event-based analysis identified 239 MYOCD-driven splicing events. Events involving MBNL1, MCAM, and ACTN1 were among the most prominent, and this was confirmed using variant-specific PCR analyses. In support of a role for RBPMS and RBFOX2 in MYOCD-driven splicing we found enrichment of their binding motifs around differentially spliced exons. Moreover, knockdown of either RBPMS or RBFOX2 antagonized splicing events stimulated by MYOCD, including those involving ACTN1, VCL, and MBNL1. Supporting an in vivo role of MYOCD-SRF-driven splicing, we demonstrate altered Rbpms expression and splicing in inducible and SMC-specific Srf knockout mice. We conclude that MYOCD-SRF, in part via RBPMS and RBFOX2, induce a program of differential exon usage and alternative splicing as part of the broader program of SMC differentiation.

Tissue-specific expression of myosin phosphatase subunits and isoforms in smooth muscle of mice and humans

Alternative splicing of exon24 (E24) of myosin phosphatase targeting subunit 1 (Mypt1) by setting sensitivity to nitric oxide (NO)/cGMP-mediated relaxation is a key determinant of smooth muscle function. Here we defined expression of myosin phosphatase (MP) subunits and isoforms by creation of new genetic mouse models, assay of human and mouse tissues, and query of public databases. A Mypt1-LacZ reporter mouse revealed that Mypt1 transcription is turned on early in development during smooth muscle differentiation. Mypt1 is not as tightly restricted in its expression as smooth muscle myosin heavy chain (Myh11) and its E6 splice variant. Mypt1 is enriched in mature smooth versus nonmuscle cells. The E24 splice variant and leucine zipper minus protein isoform that it encodes is enriched in phasic versus tonic smooth muscle. In the vascular system, E24 splicing increases as vessel size decreases. In the gastrointestinal system, E24 splicing is most predominant in smooth muscle of the small intestine. Tissue-specific expression of MP subunits and Mypt1 E24 splicing is conserved in humans, whereas a splice variant of the inhibitory subunit (CPI-17) is unique to humans. A Mypt1 E24 mini-gene splicing reporter mouse generated to define patterns of E24 splicing in smooth muscle cells (SMCs) dispersed throughout the organ systems was unsuccessful. In summary, expression of Mypt1 and splicing of E24 is part of the program of smooth muscle differentiation, is further enhanced in phasic smooth muscle, and is conserved in humans. Its low-level expression in nonmuscle cells may confound its measurement in tissue samples.

Editing of the myosin phosphatase regulatory subunit suppresses angiotensin II induced hypertension via sensitization to nitric oxide mediated vasodilation

Alternative splicing of exon 24 (E24) of the myosin phosphatase regulatory subunit (Mypt1) tunes smooth muscle sensitivity to NO/cGMP-mediated vasorelaxation and thereby controls blood pressure (BP) in otherwise normal mice. This occurs via the toggling in or out of a C-terminal leucine zipper (LZ) motif required for hetero-dimerization with and activation by cGMP-dependent protein kinase cGK1α. Here we tested the hypothesis that editing (deletion) of E24, by shifting to the LZ positive isoform of Mypt1, would suppress the hypertensive response to angiotensin II (AngII). To test this, mice underwent tamoxifen-inducible and smooth muscle-specific deletion of E24 (E24 cKO) at age 6 weeks followed by a chronic slow-pressor dose of AngII (400 ng/kg/min) plus additional stressors. E24 cKO suppressed the hypertensive response to AngII alone or with the addition of a high salt diet. This effect was not a function of altered salt balance as there were no differences in intake or renal excretion of sodium. This effect was NO dependent as L-NAME in the drinking water caused an exaggerated hypertensive response in the E24cKO mice. E24cKO mouse mesenteric arteries were more sensitive to DEA/NO-induced vasorelaxation and less responsive to AngII- and α-adrenergic-induced vasoconstriction at baseline. Only the latter two effects were still present after 2 weeks of chronic AngII treatment. We conclude that editing of Mypt1 E24, by shifting the expression of naturally occurring isoforms and sensitizing to NO-mediated vasodilation, could be a novel approach to the treatment of human hypertension.

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.

The involvement of phosphorylation of myosin phosphatase targeting subunit 1 (MYPT1) and MYPT1 isoform expression in NO/cGMP mediated differential vasoregulation of cerebral arteries compared to systemic arteries

AIM

Constitutive release of NO blunts intrinsic and stimulated contractile activity in cerebral arteries (CA). Here, we explored whether phosphorylation and expression levels of the PKG-sensitive, leucine zipper positive (LZ⁺ ) splice variants of the regulatory subunit of myosin phosphatase (MYPT1) are involved and whether its expression is associated with higher cGMP sensitivity.

METHODS

Vascular contractility was investigated by wire myography. Phosphorylation of MYPT1 was determined by Western blotting.

RESULTS

Constitutive phosphorylation of MYPT1-T696 and T853 was lower and that of S695 and S668 was higher in cerebral arteries from the circulus arteriosus (CA-w) than in femoral arteries (FA), while total MYPT1 expression was not different. In CA-w but not in FA, L-NAME lowered phosphorylation of S695/S668 and increased phosphorylation of T696/T853 and of MLC₂₀ -S19, plus basal tone. The increase in basal tone was attenuated in CA-w and basilar arteries (BA) from heterozygous MYPT1-T696A/+ mice. Compared to FA, expression of the LZ⁺ -isoform was ~2-fold higher in CA-w coincident with a higher sensitivity to DEA-NONOate, cinaciguat and Y27632 in BA and 8-Br-cGMP (1 μmol/L) in pre-constricted (pCa 6.1) α-toxin permeabilized CAs. In contrast, 6-Bnz-cAMP (10 μmol/L) relaxed BA and FA similarly by ~80%.

CONCLUSION

Our results indicate that (i) regulation of the intrinsic contractile activity in CA involves phosphorylation of MYPT1 at T696 and S695/S668, (ii) the higher NO/cGMP/PKG sensitivity of CAs can be ascribed to the higher expression level of the LZ⁺ -MYPT1 isoform and (iii) relaxation by cAMP/PKA pathway is less dependent on the expression level of the LZ⁺ splice variants of MYPT1.

Overexpression of heart-specific small subunit of myosin light chain phosphatase results in heart failure and conduction disturbance

Mutations in genes encoding components of the sarcomere cause cardiomyopathy, which is often associated with abnormal Ca²⁺ sensitivity of muscle contraction. We have previously shown that a heart-specific myosin light chain phosphatase small subunit (hHS-M₂₁) increases the Ca²⁺ sensitivity of muscle contraction. The aim of the present study was to investigate the function of hHS-M₂₁ in vivo and the causative role of abnormal Ca²⁺ sensitivity in cardiomyopathy. We generated transgenic mice with cardiac-specific overexpression of hHS-M₂₁. We confirmed that hHS-M₂₁ increased the Ca²⁺ sensitivity of cardiac muscle contraction in vivo, which was not followed by an increased phosphorylation of myosin light chain 2 isoforms. hHS-M₂₁ transgenic mice developed severe systolic dysfunction with myocardial fibrosis and degeneration of cardiomyocytes in association with sinus bradycardia and atrioventricular conduction defect. The contractile dysfunction and cardiac fibrosis were improved by treatment with the Rho kinase inhibitor fasudil. Our findings suggested that the overexpression of hHS-M₂₁ results in cardiac dysfunction and conduction disturbance via non-myosin light chain 2 phosphorylation-dependent regulation. NEW & NOTEWORTHY The present study is the first to develop mice with transgenic overexpression of a heart-specific myosin light chain phosphatase small subunit (hHS-M₂₁) and to examine the effects of hHS-M₂₁ on cardiac function. Elevation of hHS-M₂₁ induced heart failure with myocardial fibrosis and degeneration of cardiomyocytes accompanied by supraventricular arrhythmias.

Smooth Muscle Phenotypic Diversity: Effect on Vascular Function and Drug Responses

At its simplest resistance to blood flow is regulated by changes in the state of contraction of the vascular smooth muscle (VSM), a function of the competing activities of the myosin kinase and phosphatase determining the phosphorylation and activity of the myosin ATPase motor protein. In contrast, the vascular system of humans and other mammals is incredibly complex and highly regulated. Much of this complexity derives from phenotypic diversity within the smooth muscle, reflected in very differing power outputs and responses to signaling pathways that regulate vessel tone, presumably having evolved over the millennia to optimize vascular function and its control. The highly regulated nature of VSM tone, described as pharmacomechanical coupling, likely underlies the many classes of drugs in clinical use to alter vascular tone through activation or inhibition of these signaling pathways. This review will first describe the phenotypic diversity within VSM, followed by presentation of specific examples of how molecular diversity in signaling, myofilament, and calcium cycling proteins impacts arterial smooth muscle function and drug responses.