Myosin light chain phosphatase and kinase abnormalities in fetal sheep pulmonary hypertension

T18/S19 diphosphorylation of myosin regulatory light chain impairs pulmonary artery relaxation in monocrotaline-induced pulmonary hypertensive rats

Phosphorylation of Ser19 (S19-p) on the myosin regulatory light chain (MLC2) is critical for arterial contraction. It has been shown that elevated RhoA-dependent kinase (ROCK) activity or decreased MLC phosphatase (MLCP) activity leads to further phosphorylation of Thr18 (T18/S19-pp), which has been linked to vasospastic diseases. However, this phenomenon has not yet been studied in the context of pulmonary arterial hypertension (PAH). In the monocrotaline-induced PAH (PAH-MCT) rat model, we observed a significant delay in pulmonary artery (PA) relaxation following high potassium-induced contraction, which persisted even with the use of an L-type calcium channel blocker or in a calcium-free solution. Immunoblot analysis showed increased levels of both S19-p and T18/S19-pp in unstimulated PAs from PAH-MCT rats. Proteomics analysis revealed a reduction in soluble guanylate cyclase (sGC) and protein kinase G (PKG) levels, and immunoblotting confirmed decreased levels of MYPT1 (a component of MLCP) and increased ROCK in PAH-MCT. In the control PAs, the pharmacological inhibition of sGC with ODQ resulted in a prominent delay of relaxation and increased T18/S19-pp as in PAH-MCT. The delayed relaxation and the T18/S19-pp in PAH-MCT were reversed by ROCK inhibitor, Y27632, while not by membrane permeable 8-Br-cGMP. The delayed relaxation and T18/S19-diP in the ODQ-treated control PA were also reversed by Y27632. Taken together, the decreased sGC and MLCP, and increased ROCK increased T18/S19-pp, which leads to the decreased ability of PA to relax in PAH-MCT rats. PA specific inhibition of ROCK or activation of MLCP are expected to serve as potential drugs in the treatment of PAH.

Characterization of Contractile Machinery of Vascular Smooth Muscles in Hypertension

Hypertension is a key risk factor for cardiovascular disease and it is a growing public health problem worldwide. The pathophysiological mechanisms of vascular smooth muscle (VSM) contraction contribute to the development of hypertension. Calcium (Ca²⁺)-dependent and -independent signaling mechanisms regulate the balance of the myosin light chain kinase and myosin light chain phosphatase to induce myosin phosphorylation, which activates VSM contraction to control blood pressure (BP). Here, we discuss the mechanism of the contractile machinery in VSM, especially RhoA/Rho kinase and PKC/CPI-17 of Ca²⁺ sensitization pathway in hypertension. The two signaling pathways affect BP in physiological and pathophysiological conditions and are highlighted in pulmonary, pregnancy, and salt-sensitive hypertension.

Risk factors for persistent pulmonary hypertension of the newborn

In utero, pulmonary blood flow is closely circumscribed and oxygenation and ventilation occur via the placental circulation. Within the first few breaths of air-breathing life, the perinatal pulmonary circulation undergoes a dramatic transition as pulmonary blood flow increases 10-fold and the pulmonary arterial blood pressure decreases by 50% within 24 hours of birth. With the loss of the placental circulation, the increase in pulmonary flow enables oxygen to enter the bloodstream. The physiologic mechanisms that account for the remarkable transition of the pulmonary circulation include establishment of an air-liquid interface, rhythmic distention of the lung, an increase in shear stress and elaboration of nitric oxide from the pulmonary endothelium. If the perinatal pulmonary circulation does not dilate, blood is shunted away from the lungs at the level of the patent foramen ovale and the ductus arteriosus leading to the profound and unremitting hypoxemia that characterizes persistent pulmonary hypertension of the newborn (PPHN), a syndrome without either optimally effective preventative or treatment strategies. Despite significant advances in treatment, PPHN remains a major cause of morbidity and mortality in neonatal centers across the globe. While there is information surrounding factors that might increase the risk of PPHN, knowledge remains incomplete. Cesarean section delivery, high maternal body mass index, maternal use of aspirin, nonsteroidal anti-inflammatory agents and maternal diabetes mellitus are among the factors associated with an increased risk for PPHN. Recent data suggest that maternal use of serotonin reuptake inhibitors might represent another important risk factor for PPHN.

Hypoxic Pulmonary Vasoconstriction

It has been known for more than 60 years, and suspected for over 100, that alveolar hypoxia causes pulmonary vasoconstriction by means of mechanisms local to the lung. For the last 20 years, it has been clear that the essential sensor, transduction, and effector mechanisms responsible for hypoxic pulmonary vasoconstriction (HPV) reside in the pulmonary arterial smooth muscle cell. The main focus of this review is the cellular and molecular work performed to clarify these intrinsic mechanisms and to determine how they are facilitated and inhibited by the extrinsic influences of other cells. Because the interaction of intrinsic and extrinsic mechanisms is likely to shape expression of HPV in vivo, we relate results obtained in cells to HPV in more intact preparations, such as intact and isolated lungs and isolated pulmonary vessels. Finally, we evaluate evidence regarding the contribution of HPV to the physiological and pathophysiological processes involved in the transition from fetal to neonatal life, pulmonary gas exchange, high-altitude pulmonary edema, and pulmonary hypertension. Although understanding of HPV has advanced significantly, major areas of ignorance and uncertainty await resolution.

Role of rho kinase in the functional and dysfunctional tonic smooth muscles

Tonic smooth muscles play pivotal roles in the pathophysiology of debilitating diseases of the gastrointestinal and cardiovascular systems. Tonic smooth muscles differ from phasic smooth muscles in the ability to spontaneously develop myogenic tone. This ability has been primarily attributed to the local production of specific neurohumoral substances that can work in conjunction with calcium sensitization via signal transduction events associated with the Ras homolog gene family, member A (RhoA)/Rho-associated, coiled-coil containing protein kinase 2 (ROCK II) pathways. In this article, we discuss the molecular pathways involved in the myogenic properties of tonic smooth muscles, particularly the contribution of protein kinase C vs the RhoA/ROCK II pathway in the genesis of basal tone, pathophysiology and novel therapeutic approaches for certain gastrointestinal and cardiovascular diseases. Emerging evidence suggests that manipulation of RhoA/ROCK II activity through inhibitors or silencing of RNA interface techniques could represent a new therapeutic approach for various gastrointestinal and cardiovascular diseases.

The association of caveolae, actin, and the dystrophin-glycoprotein complex: a role in smooth muscle phenotype and function?

Smooth muscle cells exhibit phenotypic and mechanical plasticity. During maturation, signalling pathways controlling actin dynamics modulate contractile apparatus-associated gene transcription and contractile apparatus remodelling resulting from length change. Differentiated myocytes accumulate abundant caveolae that evolve from the structural association of lipid rafts with caveolin-1, a protein with domains that confer unique functional properties. Caveolae and caveolin-1 modulate and participate in receptor-mediated signalling, and thus contribute to functional diversity of phenotypically similar myocytes. In mature smooth muscle, caveolae are partitioned into discrete linear domains aligned with structural proteins that tether actin to the extracellular matrix. Caveolin-1 binds with beta-dystroglycan, a subunit of the dystrophin glycoprotein complex (DGC), and with filamin, an actin binding protein that organizes cortical actin, to which integrins and focal adhesion complexes are anchored. The DGC is linked to the actin cytoskeleton by a dystrophin subunit and is a receptor for extracellular laminin. Thus, caveolae and caveolin-associated signalling proteins and receptors are linked via structural proteins to a dynamic filamentous actin network. Despite development of transgenic models to investigate caveolins and membrane-associated actin-linking proteins in skeletal and cardiac muscle function, only superficial understanding of this association in smooth muscle phenotype and function has emerged.

Rat pulmonary arterial smooth muscle myosin light chain kinase and phosphatase activities decrease with age

We and others have shown that the fetal pulmonary arterial smooth muscle potential for contraction and relaxation is significantly reduced compared with the adult. Whether these developmental changes relate to age differences in the expression and/or activity of key enzymes regulating the smooth muscle mechanical properties has not been previously evaluated. Therefore, we studied the catalytic activities and expression of myosin light chain kinase (MLCK) and myosin light chain phosphatase (MLCP) catalytic (PP1cδ) and regulatory (MYPT) subunits in late fetal, early newborn, and adult rat intrapulmonary arterial tissues. In keeping with the greater force development and relaxation of adult pulmonary artery, Western blot analysis showed that the MLCK, MYPT, and PP1cδ contents increased significantly with age and were highest in the adult rat. In contrast, their specific activities (activity/enzyme content) were significantly higher in the fetal compared with the adult tissue. The fetal and newborn pulmonary arterial muscle relaxant response to the Rho-kinase inhibitor Y-27632 was greater than the adult tissue. In addition to the 130-kDa isoform of MLCK, we documented the presence of minor higher-molecular-weight embryonic isoforms in the fetus and newborn. During fetal life, the lung pulmonary arterial MLCK- and MLCP-specific activities are highest and appear to be related to Rho-kinase activation during lung morphogenesis.

Regulation of pulmonary arterial myosin phosphatase activity in neonatal circulatory transition and in hypoxic pulmonary hypertension: a role for CPI-17

Neonatal circulatory transition is dependent upon tightly regulated pulmonary circuit relaxation. Persistent pulmonary hypertension of the newborn (PPHN) is characterized by pulmonary arterial myocyte relaxation failure. We examined the effect of short course (72 hour) in vivo normobaric hypoxia in newborn swine on smooth muscle contractile enzyme activity and regulatory phosphoprotein abundance, in tissue homogenates of 2nd to 4th generation pulmonary arteries. Myosin light chain kinase (MLCK) and phosphatase (MLCP) protein contents were unchanged in hypoxic pulmonary arteries compared to controls. MLCP activity increased in normoxic animals from birth to day 3. This was ablated by hypoxia; phosphatase activity, measured as in vitro myosin light chain dephosphorylation, was decreased significantly (P < 0.005) in the hypoxic group. Inhibitory site phosphorylations of MLCP myosin binding subunit at threonines 696 and 850 were similar in both hypoxic and normoxic subjects, suggesting that downregulation of MLCP in hypoxia does not involve this pathway. However, content of regulatory protein CPI-17 (protein kinase C-related phosphatase inhibitor) increased from birth in hypoxic subjects (P < 0.05); active (phosphorylated) CPI-17 protein abundance declined after birth in normals, but increased in hypoxic arteries (P < 0.05). This corresponded with the decrease in phosphatase activity. We speculate that CPI-17 may play a role in myosin phosphatase upregulation during neonatal circulatory transition, and in hypoxic inhibition of pulmonary phosphatase activity in PPHN.

Regulation of myosin light chain phosphatase and pulmonary arterial relaxation

Neonatal circulatory transition is dependent upon tightly regulated pulmonary circuit relaxation. Persistent pulmonary hypertension (PPHN), a rapidly progressive disease of pulmonary arterial vasospasm and remodelling, may be characterized by pulmonary arterial myocyte relaxation failure. A key regulator of vascular tone is myocyte calcium sensitivity, determined by the relative stoichiometry of myosin light chain phosphorylation and dephosphorylation. We have recently reported downregulation of myosin light chain phosphatase activity in a hypoxic model of neonatal pulmonary hypertension. This review examines the recognized pathways of regulation governing myosin light chain phosphatase activity, including targeting subunit isoform switching, targeting unit phosphorylation and catalytic site inhibition. In light of the reviewed literature, further speculation is proposed on the potential contributions of these mechanisms to the pathophysiology of the perinatal pulmonary arterial relaxation defect in PPHN.Key words: smooth muscle, pulmonary hypertension, myosin light chain phosphatase, CPI-17, MYPT, review.

Pathophysiologic mechanisms of persistent pulmonary hypertension of the newborn

Persistent pulmonary hypertension of the newborn (PPHN), among the most rapidly progressive and potentially fatal of vasculopathies, is a disorder of vascular transition from fetal to neonatal circulation, manifesting as hypoxemic respiratory failure. PPHN represents a common pathway of vascular injury activated by numerous perinatal stresses: hypoxia, hypoglycemia, cold stress, sepsis, and direct lung injury. As with other multifactorial diseases, a single inciting event may be augmented by multiple concurrent/subsequent phenomena that result in differing courses of disease progression. I review the various mechanisms of vascular injury involved in neonatal pulmonary hypertension: endothelial dysfunction, inflammation, hypoxia, and mechanical strain, in the context of downstream effects on pulmonary vascular endothelial-myocyte interactions and myocyte phenotypic plasticity.

Generation of a cell line with smooth muscle phenotype from hypertrophied urinary bladder

We have established a cell line from hypertrophied rabbit urinary bladder smooth muscle (SM) that stably expresses SM myosin (SMM). These cells, termed BSM, are spindle shaped and form swirls, similar to the "hills and valleys" described for cultured aortic SM cells. Western blotting revealed that BSM expresses the amino-terminal SMM heavy chain isoform SM-B, the carboxy-terminal SM1 and SM2 isoforms, and SM alpha-actin. In addition, they express cGMP-dependent protein kinase G, made by contractile SM cells in vitro but not by noncontractile cells synthesizing extracellular matrix. Immunofluorescence studies indicate a homogeneous population of cells expressing alpha-actin and SMM, including the SM-B isoform, and karyotyping demonstrates a stable 4N chromosomal pattern. These cells also express calcium-dependent myosin light chain kinase and phosphatase activity and contract in response to the muscarinic agonist bethanechol. To our knowledge, BSM is the first visceral SM cell line that expresses the SM-B isoform and might serve as a useful model to study the transcriptional regulation of tissue-specific SMM isoforms in differentiation and pathological SM.

Role of protein kinase C and phosphatases in the pulmonary vasculature of neonatal piglets

OBJECTIVE

Persistent pulmonary hypertension of the newborn is characterized by the presence of intense vasoconstriction and vascular remodeling. Protein tyrosine phosphorylation has been recognized as a critical regulatory element in signal transduction, because it is dynamically regulated by the opposing actions of protein tyrosine kinases and protein tyrosine phosphatases. The objectives of this study were to investigate the role of protein kinase C and phosphatases in the neonatal pulmonary vasculature of normoxic and chronically hypoxic piglets.

DESIGN

Prospective, randomized, unblinded study.

SETTING

Hospital research laboratory.

SUBJECTS

Newborn Yorkshire-Landrace piglets.

INTERVENTIONS

Normoxic animals were 3-6 days old. Hypoxic animals were exposed to hypoxia (Fio2 0.10) between 1 and 15 days of age to induce pulmonary hypertension and then were studied.

MEASUREMENTS AND MAIN RESULTS

In isolated perfused lungs from normoxic piglets, we measured the perfusion pressure to assess the vasoconstrictor response to protein kinase C activation with phorbol 12,13-dibutyrate or 1-oleyl-2-acetyl-glycerol. We also assessed the effect of protein kinase C inhibition with staurosporine (2 x 10-6M) and chelerythrine (5 x 10-5M) on endothelin-1-induced pulmonary vasoconstriction. We then examined the effect of chelerythrine and phosphatase inhibition with phenylarsine oxide on the baseline perfusion pressure of normoxic and chronically hypoxic piglets. Phorbol 12,13-dibutyrate and 1-oleyl-2-acetyl-glycerol caused a sustained, dose-dependent increase in perfusion pressure, with relative potencies about 100- and 1000-fold less than endothelin-1, respectively. Protein kinase C inhibitors, chelerythrine and staurosporine, decreased the constrictor response to endothelin-1. Chelerythrine did not affect baseline perfusion pressure in the normoxic animal, whereas it lowered pulmonary vascular tone in chronically hypoxic animals. With respect to phosphatases, phenylarsine oxide significantly increased perfusion pressure in normoxia as well as in hypoxia.

CONCLUSIONS

These findings confirm that protein kinase C activation causes sustained vasoconstriction in the neonatal pulmonary vasculature and mediates the vasoconstrictor action of potent peptides, like endothelin-1. These findings also confirm that protein kinase C activation could be induced by hypoxic exposure in the neonatal piglet pulmonary vasculature. Phosphatases appear to modulate pulmonary vascular tone in the normoxic and hypoxic newborn piglet.

Molecular cloning and analysis of the 5'-flanking region of the human MYPT1 gene

We have cloned and sequenced a 5 kb genomic fragment in the 5'-flanking region of the human myosin phosphatase target subunit 1. The transcription initiation site (+1) was 268 bp upstream from the translation start site. In this promoter there are no canonical TATA or CAAT box elements but there is a high GC-rich sequence. Basal promoter activity was due to the GC-rich region that contained one Sp1 transcription factor binding site, thus demonstrating that the MYPT1 gene is a housekeeping gene. Luciferase reporter assays showed the presence of two regions for positive elements and one for a negative element.