Dynamic changes in expression of myosin phosphatase in a model of portal hypertension

School of Medicine, Case Western Reserve University, Cleveland, Ohio, United States
AJP Heart and Circulatory Physiology (Impact Factor: 3.84). 06/2004; 286(5):H1801-10. DOI: 10.1152/ajpheart.00696.2003
Source: PubMed


Myosin phosphatase is a target for signaling pathways that modulate calcium sensitivity of force production in smooth muscle. Myosin phosphatase targeting subunit 1 (MYPT1) isoforms are generated by cassette-type alternative splicing of exons in the central and 3' portion of the transcript. Exclusion of the 3' alternative exon, coding for the leucine zipper (LZ)-positive MYPT1 isoform, is associated with the ability to desensitize to calcium (relax) in response to NO/cGMP-dependent signaling. We examined expression of MYPT1 isoforms and smooth muscle phenotype in normal rat vessels and in a prehepatic model of portal hypertension characterized by arteriolar dilation. The large capacitance vessels, aorta, pulmonary artery, and inferior vena cava expressed predominantly the 3' exon-out/LZ-positive MYPT1 isoform. The first-order mesenteric resistance artery (MA1) and portal vein (PV) expressed severalfold higher levels of MYPT1 with predominance of the 3' exon-included/LZ-negative isoform. There was minor variation in the presence of the MYPT1 central alternative exons. Myosin heavy and light chain splice variants in part cosegregated with MYPT1 isoforms. In response to portal hypertension induced by PV ligature, abundance of MYPT1 in PV and MA1 was significantly reduced and switched to the LZ-positive isoform. These changes were evident within 1 day of PV ligature and were maintained for up to 10 days before reverting to control values at day 14. Alteration of MYPT1 expression was part of a complex change in protein expression that can be generalized as a modulation from a phasic (fast) to a tonic (slow) contractile phenotype. Implications of vascular smooth muscle phenotypic diversity and reversible phenotypic modulation in portal hypertension with regards to regulation of blood flow are discussed.

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Available from: Michael Payne
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    • "Thus, changes in LZ+MYPT1 expression contribute to the mechanism that regulates the sensitivity of the vasculature to NO-mediated relaxation [14] [15] [16]. Further, LZ+MYPT1 isoform expression decreases in heart failure, pre-eclampsia, and portal hypertension [15] [16] [17] [18], and thus, a decrease in LZ+MYPT1 expression contributes to the decrease in sensitivity to NO associated with these diseases. "
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    ABSTRACT: 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) myosin, 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 (15±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.
    Full-text · Article · Oct 2013 · Journal of Molecular and Cellular Cardiology
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    • "A number of groups have demonstrated that the sensitivity to cGMP-mediated smooth muscle cell relaxation correlates with the relative expression of LZ+/LZ− MYPT1 isoforms [6], [17], [18], [19], [20], suggesting that the relative expression of LZ+/LZ- MYPT1 isoforms could determine the sensitivity of the smooth muscle to NO mediated vasodilatation [12]. However, not only does the relative expression of LZ+/LZ− MYPT1 isoforms correlate with the sensitivity of cGMP-mediated relaxation; we have demonstrated that changes in LZ+/LZ− MYPT1 expression, in isolation, cause changes in the sensitivity to cGMP-mediated smooth muscle relaxation [21]. "
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    ABSTRACT: Heart failure is associated with impairment in nitric oxide (NO) mediated vasodilatation, which has been demonstrated to result from a reduction in the relative expression of the leucine zipper positive (LZ+) isoform of the myosin targeting subunit (MYPT1) of myosin light chain phosphatase. Further, captopril preserves normal LZ+ MYPT1 expression, the sensitivity to cGMP-mediated vasodilatation and modulates the expression of genes in the p42/44 MAPK and p38 MAPK signaling cascades. This study tests whether angiotensin receptor blockade (ARB) with losartan decreases p42/44 MAPK or p38 MAPK signaling and preserves LZ+ MYPT1 expression in a rat infarct model of heart failure. In aortic smooth muscle, p42/44 MAPK activation increases and LZ+ MYPT1 expression falls after LAD ligation. Losartan treatment decreases the activation of p42/44 MAPK to the uninfarcted control level and preserves normal LZ+ MYPT1 expression. The expression and activation of p38 MAPK, however, is low and does not change following LAD ligation or with losartan therapy. These data suggest that either reducing or blocking the effects of circulating angiotensin II, both decreases the activation of the p42/44 MAPK signaling cascade and preserves LZ+ MYPT1 expression. Thus, the ability of ACE-inhibitors and ARBs to modulate the vascular phenotype, to preserve normal flow mediated vasodilatation may explain the beneficial effects of these drugs compared to other forms of afterload reduction in the treatment of heart failure.
    Full-text · Article · Feb 2009 · PLoS ONE
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    • "Article RA muscle relaxation and MYPT1 is a key target of signals that regulate smooth muscle tone (for review, see Ito et al. 2004). The regulated expression of MP subunits has been proposed as a key determinant of the diversity in smooth muscle contractile properties and in signaling responses in development and disease (Khatri et al. 2001; Payne et al. 2004). Isoforms of MYPT1 in both birds and mammals are generated by the cassette type alternative splicing of exons in central and 3 0 portions of the transcript (Fig.1A; Shimizu et al. 1994; Johnson et al. 1997; Dirksen et al. 2000). "
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    ABSTRACT: A considerable amount of smooth muscle phenotypic diversity is generated by tissue-specific and developmentally regulated splicing of alternative exons. The control mechanisms are unknown. We are using a myosin phosphatase targeting subunit-1 (MYPT1) alternative exon as a model to investigate this question. In the present study, we show that the RNA binding proteins TIA and PTB function as antagonistic enhancers and suppressors of splicing of the alternative exon, respectively. Each functions through a single U-rich element, containing two UCUU motifs, just downstream of the alternative exon 5' splice site. Tissue-specific down-regulation of TIA protein in the perinatal period allows PTB to bind to the U-rich element and suppress splicing of the alternative exon as the visceral smooth muscle acquires the fast-phasic smooth muscle contractile phenotype. This provides a novel role for PTB in the tissue-specific regulation of splicing of alternative exons during the generation of smooth muscle phenotypic diversity.
    Full-text · Article · Dec 2005 · RNA
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