Phosphorylation of the regulatory subunit of smooth muscle protein phosphatase 1M at Thr850 induces its dissociation from myosin

Molecular mechanism of bitter taste receptor agonist-mediated relaxation of airway smooth muscle

G-protein-coupled receptors (GPCRs) belonging to the type 2 taste receptors (TAS2Rs) family are predominantly present in taste cells to allow the perception of bitter-tasting compounds. TAS2Rs have also been shown to be expressed in human airway smooth muscle (ASM), and TAS2R agonists relax ASM cells and bronchodilate airways despite elevating intracellular calcium. This calcium "paradox" (calcium mediates contraction by pro-contractile Gq-coupled GPCRs) and the mechanisms by which TAS2R agonists relax ASM remain poorly understood. To gain insight into pro-relaxant mechanisms effected by TAS2Rs, we employed an unbiased phosphoproteomic approach involving dual-mass spectrometry to determine differences in the phosphorylation of contractile-related proteins in ASM following the stimulation of cells with TAS2R agonists, histamine (an agonist of the Gq-coupled H1 histamine receptor) or isoproterenol (an agonist of the Gs-coupled β2-adrenoceptor) alone or in combination. Our study identified differential phosphorylation of proteins regulating contraction, including A-kinase anchoring protein (AKAP)2, AKAP12, and RhoA guanine nucleotide exchange factor (ARHGEF)12. Subsequent signaling analyses revealed RhoA and the T853 residue on myosin light chain phosphatase (MYPT)1 as points of mechanistic divergence between TAS2R and Gs-coupled GPCR pathways. Unlike Gs-coupled receptor signaling, which inhibits histamine-induced myosin light chain (MLC)20 phosphorylation via protein kinase A (PKA)-dependent inhibition of intracellular calcium mobilization, HSP20 and ERK1/2 activity, TAS2Rs are shown to inhibit histamine-induced pMLC20 via inhibition of RhoA activity and MYPT1 phosphorylation at the T853 residue. These findings provide insight into the TAS2R signaling in ASM by defining a distinct signaling mechanism modulating inhibition of pMLC20 to relax contracted ASM.

Myosin light chain phosphatase is a downstream target of Rho-kinase in endothelin-1-induced transactivation of the TGF-β receptor

BACKGROUND

Rho-kinase (ROCK) regulates actomyosin contraction, coronary vasospasm, and cytoskeleton dynamics. ROCK and of NADPH oxidase (NOX) play an essential role in cardiovascular disease and proteoglycan synthesis, which promotes atherosclerosis by trapping low density lipoprotein. ROCK is activated by endothelin-1 (ET1) and transactivates the transforming growth factor beta receptor (TGFβR1), intensifying Smad signaling and proteoglycan production. This study aimed to identify the role of myosin light chain phosphatase (MLCP) as a downstream target of ROCK in TβR1 transactivation.

METHODS

Vascular smooth muscle cells were treated with ET1 and inhibitors of ROCK and MLCP were added. The phosphorylation levels of Smad2C, myosin light chain (MLC), and MLCP were monitored by western blot, and the mRNA expression of chondroitin 4-O-sulfotransferase 1 (C4ST1) was assessed by quantitative real-time PCR.

RESULTS

We examined ROCK's role in ET1-induced TGFβR1 activation. ROCK phosphorylated MLCP at the MYPT1 T853 residue, blocked by the ROCK inhibitor Y27632. ROCK also increased MLC phosphorylation and actomyosin contraction in response to ET1, enhanced by the phosphatase inhibitor Calyculin A. Calyculin A also increased C4ST1 expression, GAG-chain synthesizing enzymes.

CONCLUSIONS

This work suggests that ROCK is involved in ET1-mediated TβR1 activation through increased MLCP phosphorylation, which leads to Smad2C phosphorylation and stimulates C4ST1 expression.

The Helicobacter pylori CagA oncoprotein disrupts Wnt/PCP signaling and promotes hyperproliferation of pyloric gland base cells

Helicobacter pylori strains that deliver the oncoprotein CagA into gastric epithelial cells are the major etiologic agents of upper gastric diseases including gastric cancer. CagA promotes gastric carcinogenesis through interactions with multiple host proteins. Here, we show that CagA also disrupts Wnt-dependent planar cell polarity (Wnt/PCP), which orients cells within the plane of an epithelium and coordinates collective cell behaviors such as convergent extension to enable epithelial elongation during development. Ectopic expression of CagA in Xenopus laevis embryos impaired gastrulation, neural tube formation, and axis elongation, processes driven by convergent extension movements that depend on the Wnt/PCP pathway. Mice specifically expressing CagA in the stomach epithelium had longer pyloric glands and mislocalization of the tetraspanin proteins VANGL1 and VANGL2 (VANGL1/2), which are critical components of Wnt/PCP signaling. The increased pyloric gland length was due to hyperproliferation of cells at the gland base, where Lgr5⁺ stem and progenitor cells reside, and was associated with fewer differentiated enteroendocrine cells. In cultured human gastric epithelial cells, the N terminus of CagA interacted with the C-terminal cytoplasmic tails of VANGL1/2, which impaired Wnt/PCP signaling by inducing the mislocalization of VANGL1/2 from the plasma membrane to the cytoplasm. Thus, CagA may contribute to the development of gastric cancer by subverting a Wnt/PCP-dependent mechanism that restrains pyloric gland stem cell proliferation and promotes enteroendocrine differentiation.

Phosphorylated Peptide Derived from the Myosin Phosphatase Target Subunit Is a Novel Inhibitor of Protein Phosphatase-1

Identification of specific protein phosphatase-1 (PP1) inhibitors is of special importance regarding the study of its cellular functions and may have therapeutic values in diseases coupled to signaling processes. In this study, we prove that a phosphorylated peptide of the inhibitory region of myosin phosphatase (MP) target subunit (MYPT1), R⁶⁹⁰QSRRS(pT696)QGVTL⁷⁰¹ (P-Thr696-MYPT1^690-701), interacts with and inhibits the PP1 catalytic subunit (PP1c, IC₅₀ = 3.84 µM) and the MP holoenzyme (Flag-MYPT1-PP1c, IC₅₀ = 3.84 µM). Saturation transfer difference NMR measurements established binding of hydrophobic and basic regions of P-Thr696-MYPT1^690-701 to PP1c, suggesting interactions with the hydrophobic and acidic substrate binding grooves. P-Thr696-MYPT1^690-701 was dephosphorylated by PP1c slowly (t_1/2 = 81.6-87.9 min), which was further impeded (t_1/2 = 103 min) in the presence of the phosphorylated 20 kDa myosin light chain (P-MLC20). In contrast, P-Thr696-MYPT1^690-701 (10-500 µM) slowed down the dephosphorylation of P-MLC20 (t_1/2 = 1.69 min) significantly (t_1/2 = 2.49-10.06 min). These data are compatible with an unfair competition mechanism between the inhibitory phosphopeptide and the phosphosubstrate. Docking simulations of the PP1c-P-MYPT1^690-701 complexes with phosphothreonine (PP1c-P-Thr696-MYPT1^690-701) or phosphoserine (PP1c-P-Ser696-MYPT1^690-701) suggested their distinct poses on the surface of PP1c. In addition, the arrangements and distances of the surrounding coordinating residues of PP1c around the phosphothreonine or phosphoserine at the active site were distinct, which may account for their different hydrolysis rate. It is presumed that P-Thr696-MYPT1^690-701 binds tightly at the active center but the phosphoester hydrolysis is less preferable compared to P-Ser696-MYPT1^690-701 or phosphoserine substrates. Moreover, the inhibitory phosphopeptide may serve as a template to synthesize cell permeable PP1-specific peptide inhibitors.

Regulation of myosin light-chain phosphorylation and its roles in cardiovascular physiology and pathophysiology

The regulation of muscle contraction is a critical function in the cardiovascular system, and abnormalities may be life-threatening or cause illness. The common basic mechanism in muscle contraction is the interaction between the protein filaments myosin and actin. Although this interaction is primarily regulated by intracellular Ca²⁺, the primary targets and intracellular signaling pathways differ in vascular smooth muscle and cardiac muscle. Phosphorylation of the myosin regulatory light chain (RLC) is a primary molecular switch for smooth muscle contraction. The equilibrium between phosphorylated and unphosphorylated RLC is dynamically achieved through two enzymes, myosin light chain kinase, a Ca²⁺-dependent enzyme, and myosin phosphatase, which modifies the Ca²⁺ sensitivity of contractions. In cardiac muscle, the primary target protein for Ca²⁺ is troponin C on thin filaments; however, RLC phosphorylation also plays a modulatory role in contraction. This review summarizes recent advances in our understanding of the regulation, physiological function, and pathophysiological involvement of RLC phosphorylation in smooth and cardiac muscles.

Smoothelin-Like Protein 1 Regulates the Thyroid Hormone-Induced Homeostasis and Remodeling of C2C12 Cells via the Modulation of Myosin Phosphatase

The pathological elevation of the active thyroid hormone (T3) level results in the manifestation of hyperthyroidism, which is associated with alterations in the differentiation and contractile function of skeletal muscle (SKM). Myosin phosphatase (MP) is a major cellular regulator that hydrolyzes the phosphoserine of phosphorylated myosin II light chain. MP consists of an MYPT1/2 regulatory and a protein phosphatase 1 catalytic subunit. Smoothelin-like protein 1 (SMTNL1) is known to inhibit MP by directly binding to MP as well as by suppressing the expression of MYPT1 at the transcriptional level. Supraphysiological vs. physiological concentration of T3 were applied on C2C12 myoblasts and differentiated myotubes in combination with the overexpression of SMTNL1 to assess the role and regulation of MP under these conditions. In non-differentiated myoblasts, MP included MYPT1 in the holoenzyme complex and its expression and activity was regulated by SMTNL1, affecting the phosphorylation level of MLC20 assessed using semi-quantitative Western blot analysis. SMTNL1 negatively influenced the migration and cytoskeletal remodeling of myoblasts measured by high content screening. In contrast, in myotubes, the expression of MYPT2 but not MYPT1 increased in a T3-dependent and SMTNL1-independent manner. T3 treatment combined with SMTNL1 overexpression impeded the activity of MP. In addition, MP interacted with Na+/K+-ATPase and dephosphorylated its inhibitory phosphorylation sites, identifying this protein as a novel MP substrate. These findings may help us gain a better understanding of myopathy, muscle weakness and the disorder of muscle regeneration in hyperthyroid patients.

Thomas Willis Lecture: Targeting Brain Arterioles for Acute Stroke Treatment

[Figure: see text].

Mfge8 attenuates human gastric antrum smooth muscle contractions

Coordinated gastric smooth muscle contraction is critical for proper digestion and is adversely affected by a number of gastric motility disorders. In this study we report that the secreted protein Mfge8 (milk fat globule-EGF factor 8) inhibits the contractile responses of human gastric antrum muscles to cholinergic stimuli by reducing the inhibitory phosphorylation of the MYPT1 (myosin phosphatase-targeting subunit (1) subunit of MLCP (myosin light chain phosphatase), resulting in reduced LC20 (smooth muscle myosin regulatory light chain (2) phosphorylation. Mfge8 reduced the agonist-induced increase in the F-actin/G-actin ratios of β-actin and γ-actin1. We show that endogenous Mfge8 is bound to its receptor, α8β1 integrin, in human gastric antrum muscles, suggesting that human gastric antrum muscle mechanical responses are regulated by Mfge8. The regulation of gastric antrum smooth muscles by Mfge8 and α8 integrin functions as a brake on gastric antrum mechanical activities. Further studies of the role of Mfge8 and α8 integrin in regulating gastric antrum function will likely reveal additional novel aspects of gastric smooth muscle motility mechanisms.

Effector-mediated ERM activation locally inhibits RhoA activity to shape the apical cell domain

Activated ezrin-radixin-moesin (ERM) proteins link the plasma membrane to the actin cytoskeleton to generate apical structures, including microvilli. Among many kinases implicated in ERM activation are the homologues LOK and SLK. CRISPR/Cas9 was used to knock out all ERM proteins or LOK/SLK in human cells. LOK/SLK knockout eliminates all ERM-activating phosphorylation. The apical domains of cells lacking LOK/SLK or ERMs are strikingly similar and selectively altered, with loss of microvilli and with junctional actin replaced by ectopic myosin-II–containing apical contractile structures. Constitutively active ezrin can reverse the phenotypes of either ERM or LOK/SLK knockouts, indicating that a central function of LOK/SLK is to activate ERMs. Both knockout lines have elevated active RhoA with concomitant enhanced myosin light chain phosphorylation, revealing that active ERMs are negative regulators of RhoA. As RhoA-GTP activates LOK/SLK to activate ERM proteins, the ability of active ERMs to negatively regulate RhoA-GTP represents a novel local feedback loop necessary for the proper apical morphology of epithelial cells.

Regulation of Myosin Light-Chain Phosphatase Activity to Generate Airway Smooth Muscle Hypercontractility

Smooth muscle is a central structure involved in the regulation of airway tone. In addition, it plays an important role in the development of some pathologies generated by alterations in contraction, such as hypercontractility and the airway hyperresponsiveness observed in asthma. The molecular processes associated with smooth muscle contraction are centered around myosin light chain (MLC) phosphorylation, which is controlled by a balance in the activity of myosin light-chain kinase (MLCK) and myosin light-chain phosphatase (MLCP). MLCK activation depends on increasing concentrations of intracellular Ca²⁺, while MLCP activation is independent of Ca²⁺. MLCP contains a phosphatase subunit (PP1c) that is regulated through myosin phosphatase target subunit 1 (MYPT1) and other subunits, such as glycogen-associated regulatory subunit and myosin-binding subunit 85 kDa. Interestingly, MLCP inhibition may contribute to exacerbation of smooth muscle contraction by increasing MLC phosphorylation to induce hypercontractility. Many pathways inhibiting MLCP activity in airway smooth muscle have been proposed and are focused on inhibition of PP1c, inhibitory phosphorylation of MYPT1 and dissociation of the PP1c-MYPT1 complex.

Epstein-Barr Virus miRNA BART2-5p Promotes Metastasis of Nasopharyngeal Carcinoma by Suppressing RND3

Nasopharyngeal carcinoma is an Epstein-Barr virus (EBV)-related malignancy. Recently, we found that the EBV-encoded miRNA BART2-5p was increased in the serum of patients with preclinical nasopharyngeal carcinoma and that the copy number positively correlated with disease progression. In this study, we established its role in nasopharyngeal carcinoma progression and explored underlying mechanisms and clinical significance. BART2-5p was an independent unfavorable prognostic factor for progression-free survival and its circulating abundance positively associated with distant metastasis. Ectopic expression of BART2-5p promoted migration and invasion of EBV-negative nasopharyngeal carcinoma cells, whereas genetic downregulation of BART2-5p in EBV-positive nasopharyngeal carcinoma cells decreased aggressiveness. Mechanistically, BART2-5p targeted RND3, a negative regulator of Rho signaling. Downregulation of RND3 phenocopied the effect of BART2-5p and reconstitution of RND3 rescued the phenotype. By suppressing RND3, BART2-5p activated Rho signaling to enhance cell motility. These findings suggest a novel role for EBV miRNA BART2-5p in promoting nasopharyngeal carcinoma metastasis and its potential value as a prognostic indicator or therapeutic target. SIGNIFICANCE: This study shows that EBV-encoded BART2-5p miRNA suppresses expression of the RND3 Rho family GTPase, consequently promoting ROCK signaling, cell motility, and metastatic behavior of NPC cells.

Rho-associated kinase and zipper-interacting protein kinase, but not myosin light chain kinase, are involved in the regulation of myosin phosphorylation in serum-stimulated human arterial smooth muscle cells

Myosin regulatory light chain (LC₂₀) phosphorylation plays an important role in vascular smooth muscle contraction and cell migration. Ca²⁺/calmodulin-dependent myosin light chain kinase (MLCK) phosphorylates LC₂₀ (its only known substrate) exclusively at S19. Rho-associated kinase (ROCK) and zipper-interacting protein kinase (ZIPK) have been implicated in the regulation of LC₂₀ phosphorylation via direct phosphorylation of LC₂₀ at T18 and S19 and indirectly via phosphorylation of MYPT1 (the myosin targeting subunit of myosin light chain phosphatase, MLCP) and Par-4 (prostate-apoptosis response-4). Phosphorylation of MYPT1 at T696 and T853 inhibits MLCP activity whereas phosphorylation of Par-4 at T163 disrupts its interaction with MYPT1, exposing the sites of phosphorylation in MYPT1 and leading to MLCP inhibition. To evaluate the roles of MLCK, ROCK and ZIPK in these phosphorylation events, we investigated the time courses of phosphorylation of LC₂₀, MYPT1 and Par-4 in serum-stimulated human vascular smooth muscle cells (from coronary and umbilical arteries), and examined the effects of siRNA-mediated MLCK, ROCK and ZIPK knockdown and pharmacological inhibition on these phosphorylation events. Serum stimulation induced rapid phosphorylation of LC₂₀ at T18 and S19, MYPT1 at T696 and T853, and Par-4 at T163, peaking within 30–120 s. MLCK knockdown or inhibition, or Ca²⁺ chelation with EGTA, had no effect on serum-induced LC₂₀ phosphorylation. ROCK knockdown decreased the levels of phosphorylation of LC₂₀ at T18 and S19, of MYPT1 at T696 and T853, and of Par-4 at T163, whereas ZIPK knockdown decreased LC₂₀ diphosphorylation, but increased phosphorylation of MYPT1 at T696 and T853 and of Par-4 at T163. ROCK inhibition with GSK429286A reduced serum-induced phosphorylation of LC₂₀ at T18 and S19, MYPT1 at T853 and Par-4 at T163, while ZIPK inhibition by HS38 reduced only LC₂₀ diphosphorylation. We also demonstrated that serum stimulation induced phosphorylation (activation) of ZIPK, which was inhibited by ROCK and ZIPK down-regulation and inhibition. Finally, basal phosphorylation of LC₂₀ in the absence of serum stimulation was unaffected by MLCK, ROCK or ZIPK knockdown or inhibition. We conclude that: (i) serum stimulation of cultured human arterial smooth muscle cells results in rapid phosphorylation of LC₂₀, MYPT1, Par-4 and ZIPK, in contrast to the slower phosphorylation of kinases and other proteins involved in other signaling pathways (Akt, ERK1/2, p38 MAPK and HSP27), (ii) ROCK and ZIPK, but not MLCK, are involved in serum-induced phosphorylation of LC₂₀, (iii) ROCK, but not ZIPK, directly phosphorylates MYPT1 at T853 and Par-4 at T163 in response to serum stimulation, (iv) ZIPK phosphorylation is enhanced by serum stimulation and involves phosphorylation by ROCK and autophosphorylation, and (v) basal phosphorylation of LC₂₀ under serum-free conditions is not attributable to MLCK, ROCK or ZIPK.

Amelioration of perivascular adipose inflammation reverses vascular dysfunction in a model of nonobese prediabetic metabolic challenge: potential role of antidiabetic drugs

The onset of vascular impairment precedes that of diagnostic hyperglycemia in diabetic patients suggesting a vascular insult early in the course of metabolic dysfunction without a well-defined mechanism. Mounting evidence implicates adipose inflammation in the pathogenesis of insulin resistance and diabetes. It is not certain whether amelioration of adipose inflammation is sufficient to preclude vascular dysfunction in early stages of metabolic disease. Recent findings suggest that antidiabetic drugs, metformin, and pioglitazone, improve vascular function in prediabetic patients, without an indication if this protective effect is mediated by reduction of adipose inflammation. Here, we used a prediabetic rat model with delayed development of hyperglycemia to study the effect of metformin or pioglitazone on adipose inflammation and vascular function. At the end of the metabolic challenge, these rats were neither obese, hypertensive, nor hyperglycemic. However, they showed increased pressor responses to phenylephrine and augmented aortic and mesenteric contraction. Vascular tissues from prediabetic rats showed increased Rho-associated kinase activity causing enhanced calcium sensitization. An elevated level of reactive oxygen species was seen in aortic tissues together with increased Transforming growth factor β1 and Interleukin-1β expression. Although, no signs of systemic inflammation were detected, perivascular adipose inflammation was observed. Adipocyte hypertrophy, increased macrophage infiltration, and elevated Transforming growth factor β1 and Interleukin-1β mRNA levels were seen. Two-week treatment with metformin or pioglitazone or switching to normal chow ameliorated adipose inflammation and vascular dysfunction. Localized perivascular adipose inflammation is sufficient to trigger vascular dysfunction early in the course of diabetes. Interfering with this inflammatory process reverses this early abnormality.

p116Rip promotes myosin phosphatase activity in airway smooth muscle cells

Myosin phosphatase-Rho interacting protein (p116^Rip ) was originally found as a RhoA-binding protein. Subsequent studies by us and others revealed that p116^Rip facilitates myosin light chain phosphatase (MLCP) activity through direct and indirect manners. However, it is unclear how p116^Rip regulates myosin phosphatase activity in cells. To elucidate the role of p116^Rip in cellular contractile processes, we suppressed the expression of p116^Rip by RNA interference in human airway smooth muscle cells (HASMCs). We found that knockdown of p116^Rip in HASMCs led to increased di-phosphorylated MLC (pMLC), that is phosphorylation at both Ser19 and Thr18. This was because of a change in the interaction between MLCP and myosin, but not an alteration of RhoA/ROCK signaling. Attenuation of Zipper-interacting protein kinase (ZIPK) abolished the increase in di-pMLC, suggesting that ZIPK is involved in this process. Moreover, suppression of p116^Rip expression in HASMCs substantially increased the histamine-induced collagen gel contraction. We also found that expression of the p116^Rip was decreased in the airway smooth muscle tissue from asthmatic patients compared with that from non-asthmatic patients, suggesting a potential role of p116^Rip expression in asthma pathogenesis.

Generation of Spontaneous Tone by Gastrointestinal Sphincters

An important feature of the gastrointestinal (GI) muscularis externa is its ability to generate phasic contractile activity. However, in some GI regions, a more sustained contraction, referred to as "tone," also occurs. Sphincters are muscles oriented in an annular manner that raise intraluminal pressure, thereby reducing or blocking the movement of luminal contents from one compartment to another. Spontaneous tone generation is often a feature of these muscles. Four distinct smooth muscle sphincters are present in the GI tract: the lower esophageal sphincter (LES), the pyloric sphincter (PS), the ileocecal sphincter (ICS), and the internal anal sphincter (IAS). This chapter examines how tone generation contributes to the functional behavior of these sphincters. Historically, tone was attributed to contractile activity arising directly from the properties of the smooth muscle cells. However, there is increasing evidence that interstitial cells of Cajal (ICC) play a significant role in tone generation in GI muscles. Indeed, ICC are present in each of the sphincters listed above. In this chapter, we explore various mechanisms that may contribute to tone generation in sphincters including: (1) summation of asynchronous phasic activity, (2) partial tetanus, (3) window current, and (4) myofilament sensitization. Importantly, the first two mechanisms involve tone generation through summation of phasic events. Thus, the historical distinction between "phasic" versus "tonic" smooth muscles in the GI tract requires revision. As described in this chapter, it is clear that the unique functional role of each sphincter in the GI tract is accompanied by a unique combination of contractile mechanisms.

p90 ribosomal S6 kinase (RSK) phosphorylates myosin phosphatase and thereby controls edge dynamics during cell migration

Cell migration is essential to embryonic development, wound healing, and cancer cell dissemination. Cells move via leading-edge protrusion, substrate adhesion, and retraction of the cell's rear. The molecular mechanisms by which extracellular cues signal to the actomyosin cytoskeleton to control these motility mechanics are poorly understood. The growth factor-responsive and oncogenically activated protein extracellular signal-regulated kinase (ERK) promotes motility by signaling in actin polymerization-mediated edge protrusion. Using a combination of immunoblotting, co-immunoprecipitation, and myosin-binding experiments and cell migration assays, we show here that ERK also signals to the contractile machinery through its substrate, p90 ribosomal S6 kinase (RSK). We probed the signaling and migration dynamics of multiple mammalian cell lines and found that RSK phosphorylates myosin phosphatase–targeting subunit 1 (MYPT1) at Ser-507, which promotes an interaction of Rho kinase (ROCK) with MYPT1 and inhibits myosin targeting. We find that by inhibiting the myosin phosphatase, ERK and RSK promote myosin II–mediated tension for lamella expansion and optimal edge dynamics for cell migration. These findings suggest that ERK activity can coordinately amplify both protrusive and contractile forces for optimal cell motility.

Control of Motility in the Internal Anal Sphincter

The internal anal sphincter (IAS) plays an important role in the maintenance of fecal continence since it generates tone and is responsible for > 70% of resting anal pressure. During normal defecation the IAS relaxes. Historically, tone generation in gastrointestinal muscles was attributed to mechanisms arising directly from smooth muscle cells, ie, myogenic activity. However, slow waves are now known to play a fundamental role in regulating gastrointestinal motility and these electrical events are generated by the interstitial cells of Cajal. Recently, interstitial cells of Cajal, as well as slow waves, have also been identified in the IAS making them viable candidates for tone generation. In this review we discuss four different mechanisms that likely contribute to tone generation in the IAS. Three of these involve membrane potential, L-type Ca²⁺ channels and electromechanical coupling (ie, summation of asynchronous phasic activity, partial tetanus, and window current), whereas the fourth involves the regulation of myofilament Ca²⁺ sensitivity. Contractile activity in the IAS is also modulated by sympathetic motor neurons that significantly increase tone and anal pressure, as well as inhibitory motor neurons (particularly nitrergic and vasoactive intestinal peptidergic) that abolish contraction and assist with normal defecation. Alterations in IAS motility are associated with disorders such as fecal incontinence and anal fissures that significantly decrease the quality of life. Understanding in greater detail how tone is regulated in the IAS is important for developing more effective treatment strategies for these debilitating defecation disorders.

Quantitative in situ proximity ligation assays examining protein interactions and phosphorylation during smooth muscle contractions

Antibody-based in situ proximity ligation assays (isPLA) have the potential to study protein phosphorylation and protein interactions with spatial resolution in intact tissues. However, the application of isPLA at the tissue level is limited by a lack of appropriate positive and negative controls and the difficulty in accounting for changes in tissue shape. Here we demonstrate a set of experimental and computational approaches using gastric fundus smooth muscles to improve the validity of quantitative isPLA. Appropriate positive and negative biological controls and PLA technical controls were selected to ensure experimental rigor. To account for changes in morphology between relaxed and contracted smooth muscles, target PLA spots were normalized to smooth muscle myosin light chain 20 PLA spots or the cellular cross-sectional areas. We describe the computational steps necessary to filter out false-positive improperly sized spots and set the thresholds for counting true positive PLA spots to quantify the PLA signals. We tested our approach by examining protein phosphorylation and protein interactions in smooth muscle myofilament Ca²⁺ sensitization pathways from resting and contracted gastric fundus smooth muscles. In conclusion, our tissue-level isPLA method enables unbiased quantitation of protein phosphorylation and protein-protein interactions in intact smooth muscle tissues, suggesting the potential for quantitative isPLA applications in other types of intact tissues.

A comparison of adult rhabdomyosarcoma and high-grade neuroendocrine carcinoma of the urinary bladder reveals novel PPP1R12A fusions in rhabdomyosarcoma

Some rhabdomyosarcomas and sarcomatoid carcinomas with heterologous rhabdomyosarcomatous elements resemble high-grade neuroendocrine carcinoma, creating a diagnostic difficulty. The purpose of this study was to characterize the overlap of adult genitourinary rhabdomyosarcomas, excluding those occurring at paratesticular sites, with high-grade neuroendocrine carcinoma and identify features helpful in their separation. Seventeen cases of rhabdomyosarcoma (11 from the urinary bladder and 3 each from kidney and prostate) were compared to 10 cases of high-grade neuroendocrine carcinoma from the urinary bladder. These tumors were analyzed by immunohistochemistry for desmin, MyoD1, myogenin, chromogranin, synaptophysin, CD56, TTF1, and ASCL1, and RNA sequencing was performed on 4 cases of bladder rhabdomyosarcoma (2 rhabdomyosarcomas and 2 sarcomatoid-rhabdomyosarcoma) and 10 cases of bladder high-grade neuroendocrine carcinoma. This was compared to public data from 414 typical urothelial carcinomas from The Cancer Genome Atlas dataset. Morphologic and immunophenotypic overlap with high-grade neuroendocrine carcinoma was seen in half of the bladder tumors, which included 4 rhabdomyosarcomas and 2 sarcomatoid rhabdomyosarcomas. RNA sequencing confirmed expression of neuroendocrine markers in these cases (2 rhabdomyosarcomas and 2 sarcomatoid rhabdomyosarcomas). Differential neuroendocrine differentiation was highlighted by ASCL1 protein expression only in high-grade neuroendocrine carcinoma. Moreover, both a pure alveolar rhabdomyosarcoma and sarcomatoid rhabdomyosarcoma of the urinary bladder demonstrated a fusion involving PPP1R12A. In summary, adult rhabdomyosarcomas of the urinary bladder are molecularly distinct from high-grade neuroendocrine carcinomas based on specific patterns of expression of myogenic and epithelial to mesenchymal transition-related transcription factors as well as the presence of a novel PPP1R12A fusion which is seen in a subset of cases.

HFpEF, a Disease of the Vasculature: A Closer Look at the Other Half

Patients with heart failure are commonly divided into those with reduced ejection fraction (EF<40%) and those with preserved ejection fraction (HFpEF; EF>50%). For heart failure with reduced EF, a number of therapies have been found to improve patient morbidity and mortality, and treatment is guideline based. However for patients with HFpEF, no treatment has been found to have clinical benefit. To objectively assess treatments for HFpEF, a comprehensive PubMed literature search was performed using the terms HFpEF, heart failure, smooth muscle, myosin, myosin phosphatase, and PKG (up to December 31, 2017), with an unbiased focus on pathophysiology, cell signaling, and therapy. This review provides evidence that could explain the lack of clinical benefit in treating patients with HFpEF with sildenafil and long-acting nitrates. Furthermore, the review highlights the vascular abnormalities present in patients with HFpEF, and these abnormalities of the vasculature could potentially contribute to the pathophysiology of HFpEF. Thus, focusing on HFpEF as a vascular disease could result in the development of novel and effective treatment paradigms.

Diversity and plasticity in signaling pathways that regulate smooth muscle responsiveness: Paradigms and paradoxes for the myosin phosphatase, the master regulator of smooth muscle contraction

A hallmark of smooth muscle cells is their ability to adapt their functions to meet temporal and chronic fluctuations in their demands. These functions include force development and growth. Understanding the mechanisms underlying the functional plasticity of smooth muscles, the major constituent of organ walls, is fundamental to elucidating pathophysiological rationales of failures of organ functions. Also, the knowledge is expected to facilitate devising innovative strategies that more precisely monitor and normalize organ functions by targeting individual smooth muscles. Evidence has established a current paradigm that the myosin light chain phosphatase (MLCP) is a master regulator of smooth muscle responsiveness to stimuli. Cellular MLCP activity is negatively and positively regulated in response to G-protein activation and cAMP/cGMP production, respectively, through the MYPT1 regulatory subunit and an endogenous inhibitor protein named CPI-17. In this article we review the outcomes from two decade of research on the CPI-17 signaling and discuss emerging paradoxes in the view of signaling pathways regulating smooth muscle functions through MLCP.

ROS-dependent activation of RhoA/Rho-kinase in pulmonary artery: Role of Src-family kinases and ARHGEF1

The role of reactive oxygen species (ROS) in smooth muscle contraction is poorly understood. We hypothesised that G-protein coupled receptor (GPCR) activation and hypoxia induce Rho-kinase activity and contraction in rat intra-pulmonary artery (IPA) via stimulation of ROS production and subsequent Src-family kinase (SrcFK) activation. The T-type prostanoid receptor agonist U46619 induced ROS production in pulmonary artery smooth muscle cells (PASMC). U46619 also induced c-Src cysteine oxidation, SrcFK auto-phosphorylation, MYPT-1 and MLC₂₀ phosphorylation and contraction in IPA, and all these responses were inhibited by antioxidants (ebselen, Tempol). Contraction and SrcFK/MYPT-1/MLC₂₀ phosphorylations were also inhibited by combined superoxide dismutase and catalase, or by the SrcFK antagonist PP2, while contraction and MYPT-1/MLC₂₀ phosphorylations were inhibited by the Rho guanine nucleotide exchange factor (RhoGEF) inhibitor Y16. H₂O₂ and the superoxide-generating quinoledione LY83583 both induced c-Src oxidation, SrcFK auto-phosphorylation and contraction in IPA. LY83583 and H₂O₂-induced contractions were inhibited by PP2, while LY83583-induced contraction was also inhibited by antioxidants and Y16. SrcFK auto-phosphorylation and MYPT-1/MLC₂₀ phosphorylation was also induced by hypoxia in IPA and this was blocked by mitochondrial inhibitors rotenone and myxothiazol. In live PASMC, sub-cellular translocation of RhoA and the RhoGEF ARHGEF1 was triggered by both U46619 and LY83583 and this translocation was blocked by antioxidants and PP2. RhoA translocation was also inhibited by an ARHGEF1 siRNA. U46619 enhanced ROS-dependent co-immunoprecipitation of ARHGEF1 with c-Src. Our results demonstrate a link between GPCR-induced cytosolic ROS or hypoxia-induced mitochondrial ROS and SrcFK activity, Rho-kinase activity and contraction. ROS and SrcFK activate RhoA via ARHGEF1.

Regulation of Pulmonary Vascular Smooth Muscle Contractility in Pulmonary Arterial Hypertension: Implications for Therapy

There are two primary components that produce pulmonary arterial hypertension (PAH); aberrant structural changes (smooth muscle cell proliferation, smooth muscle cell hypertrophy, and the deposition of matrix proteins within the media of pulmonary arterial vessels), and excess vasoconstriction. However, in PAH, the target and aim of all current therapeutic agents is to reduce the contractility of the pulmonary vasculature; prostaglandins, phosphodiesterase inhibitors, guanylate cyclase stimulators, endothelin antagonists, NO inhalation and Rho kinase inhibitors all influence signaling pathways in the pulmonary vascular smooth muscle to decrease vasoconstriction, and hence, pulmonary vascular resistance (PVR). This review will therefore primarily focus on discussing the signaling pathways regulating contractility in pulmonary vascular smooth muscle, the mechanism for current treatments, as well as highlighting potential targets for the development of novel therapies.

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.

Effects of eupatilin on the contractility of corpus cavernosal smooth muscle through nitric oxide-independent pathways

Eupatilin (5,7-dihydroxy-3,4,6-trimethoxyflavone) is one of the main compounds present in Artemisia species. Eupatilin has both antioxidative and anti-inflammatory properties and a relaxation effect on vascular contraction regardless of endothelial function. We evaluated the relaxant effects of eupatilin on the corpus cavernosum (CC) of rabbits and the underlying mechanisms of its activity in human corpus cavernosum smooth muscle (CCSM) cells. Isolated rabbit CC strips were mounted in an organ bath system. A conventional whole-cell patch clamp technique was used to measure activation of calcium-sensitive K⁺ -channel currents in human CCSM cells. The relaxation effect of eupatilin was evaluated by cumulative addition (10^-5  m ~ 3 × 10^-4  m) to CC strips precontracted with 10^-5  m phenylephrine. Western blotting analysis was performed to measure myosin phosphatase targeting subunit 1 (MYPT1) and protein kinase C-potentiated inhibitory protein for heterotrimeric myosin light chain phosphatase of 17-kDa (CPI-17) expression and to evaluate the effect of eupatilin on the RhoA/Rho-kinase pathway. Eupatilin effectively relaxed the phenylephrine-induced tone in the rabbit CC strips in a concentration-dependent manner with an estimated EC₅₀ value of 1.2 ± 1.6 × 10^-4  m (n = 8, p < 0.05). Iberiotoxin and tetraethylammonium significantly reduced the relaxation effect (n = 8, p < 0.001 and p = 0.003, respectively). Removal of the endothelium or the presence of L-NAME or indomethacin did not affect the relaxation effect of eupatilin. In CCSM cells, the extracellular application of eupatilin 10^-4  m significantly increased the outward currents, and the eupatilin-stimulated currents were significantly attenuated by treatment with 10^-7  m iberiotoxin (n = 13, p < 0.05). Eupatilin reduced the phosphorylation level of MYPT1 at Thr853 of MLCP and CPI-17 at Thr38. Eupatilin-induced relaxation of the CCSM cells via NO-independent pathways. The relaxation effects of eupatilin on CCSM cells were partially due to activation of BK_Ca channels and inhibition of RhoA/Rho-kinase.

Protein phosphatases 1 and 2A and their naturally occurring inhibitors: current topics in smooth muscle physiology and chemical biology

Protein phosphatases 1 and 2A (PP1 and PP2A) are the most ubiquitous and abundant serine/threonine phosphatases in eukaryotic cells. They play fundamental roles in the regulation of various cellular functions. This review focuses on recent advances in the functional studies of these enzymes in the field of smooth muscle physiology. Many naturally occurring protein phosphatase inhibitors with different relative PP1/PP2A affinities have been discovered and are widely used as powerful research tools. Current topics in the chemical biology of PP1/PP2A inhibitors are introduced and discussed, highlighting the identification of the gene cluster responsible for the biosynthesis of calyculin A in a symbiont microorganism of a marine sponge.

Analysis of microtubule growth dynamics arising from altered actin network structure and contractility in breast tumor cells

The periphery of epithelial cells is shaped by opposing cytoskeletal physical forces generated predominately by two dynamic force generating systems-growing microtubule ends push against the boundary from the cell center, and the actin cortex contracts the attached plasma membrane. Here we investigate how changes to the structure and dynamics of the actin cortex alter the dynamics of microtubules. Current drugs target actin polymerization and contraction to reduce cell division and invasiveness; however, the impacts on microtubule dynamics remain incompletely understood. Using human MCF-7 breast tumor cells expressing GFP-tagged microtubule end-binding-protein-1 (EB1) and coexpression of cytoplasmic fluorescent protein mCherry, we map the trajectories of growing microtubule ends and cytoplasmic boundary respectively. Based on EB1 tracks and cytoplasmic boundary outlines, we calculate the speed, distance from cytoplasmic boundary, and straightness of microtubule growth. Actin depolymerization with Latrunculin-A reduces EB1 growth speed as well as allows the trajectories to extend beyond the cytoplasmic boundary. Blebbistatin, a direct myosin-II inhibitor, reduced EB1 speed and yielded less straight EB1 trajectories. Inhibiting signaling upstream of myosin-II contractility via the Rho-kinase inhibitor, Y-27632, altered EB1 dynamics differently from Blebbistatin. These results indicate that reduced actin cortex integrity can induce distinct alterations in microtubule dynamics. Given recent findings that tumor stem cell characteristics are increased by drugs which reduce actin contractility or stabilize microtubules, it remains important to clearly define how cytoskeletal drugs alter the interactions between these two filament systems in tumor cells.

Unfair competition governs the interaction of pCPI-17 with myosin phosphatase (PP1-MYPT1)

The small phosphoprotein pCPI-17 inhibits myosin light-chain phosphatase (MLCP). Current models postulate that during muscle relaxation, phosphatases other than MLCP dephosphorylate and inactivate pCPI-17 to restore MLCP activity. We show here that such hypotheses are insufficient to account for the observed rapidity of pCPI-17 inactivation in mammalian smooth muscles. Instead, MLCP itself is the critical enzyme for pCPI-17 dephosphorylation. We call the mutual sequestration mechanism through which pCPI-17 and MLCP interact inhibition by unfair competition: MLCP protects pCPI-17 from other phosphatases, while pCPI-17 blocks other substrates from MLCP’s active site. MLCP dephosphorylates pCPI-17 at a slow rate that is, nonetheless, both sufficient and necessary to explain the speed of pCPI-17 dephosphorylation and the consequent MLCP activation during muscle relaxation.

DOI:http://dx.doi.org/10.7554/eLife.24665.001

Rho Kinases and Cardiac Remodeling

Hypertensive cardiac remodeling is characterized by left ventricular hypertrophy and interstitial fibrosis, which can lead to heart failure with preserved ejection fraction. The Rho-associated coiled-coil containing kinases (ROCKs) are members of the serine/threonine protein kinase family, which mediates the downstream effects of the small GTP-binding protein RhoA. There are 2 isoforms: ROCK1 and ROCK2. They have different functions in different types of cells and tissues. There is growing evidence that ROCKs contribute to the development of cardiovascular diseases, including cardiac fibrosis, hypertrophy, and subsequent heart failure. Recent experimental studies using ROCK inhibitors, such as fasudil, have shown the benefits of ROCK inhibition in cardiac remodeling. Mice lacking each ROCK isoform also exhibit reduced myocardial fibrosis in a variety of pathological models of cardiac remodeling. Indeed, clinical studies with fasudil have suggested that ROCKs could be potential novel therapeutic targets for cardiovascular diseases. In this review, we summarize the current understanding of the roles of ROCKs in the development of cardiac fibrosis and hypertrophy and discuss their therapeutic potential for deleterious cardiac remodeling. (Circ J 2016; 80: 1491-1498).

Specialized Functional Diversity and Interactions of the Na,K-ATPase

Na,K-ATPase is a protein ubiquitously expressed in the plasma membrane of all animal cells and vitally essential for their functions. A specialized functional diversity of the Na,K-ATPase isozymes is provided by molecular heterogeneity, distinct subcellular localizations, and functional interactions with molecular environment. Studies over the last decades clearly demonstrated complex and isoform-specific reciprocal functional interactions between the Na,K-ATPase and neighboring proteins and lipids. These interactions are enabled by a spatially restricted ion homeostasis, direct protein-protein/lipid interactions, and protein kinase signaling pathways. In addition to its "classical" function in ion translocation, the Na,K-ATPase is now considered as one of the most important signaling molecules in neuronal, epithelial, skeletal, cardiac and vascular tissues. Accordingly, the Na,K-ATPase forms specialized sub-cellular multimolecular microdomains which act as receptors to circulating endogenous cardiotonic steroids (CTS) triggering a number of signaling pathways. Changes in these endogenous cardiotonic steroid levels and initiated signaling responses have significant adaptive values for tissues and whole organisms under numerous physiological and pathophysiological conditions. This review discusses recent progress in the studies of functional interactions between the Na,K-ATPase and molecular microenvironment, the Na,K-ATPase-dependent signaling pathways and their significance for diversity of cell function.

Carbachol-induced signaling through Thr696-phosphorylation of myosin phosphatase-targeting subunit 1 (MYPT1) in rat bladder smooth muscle cells

PURPOSE

Lines of evidence suggest that Rho-associated protein kinase (ROCK)-mediated myosin phosphatase-targeting subunit 1 (MYPT1) phosphorylation plays a central role in smooth muscle contraction. However, the physiological significance of MYPT1 phosphorylation at Thr696 catalyzed by ROCK in bladder smooth muscle remains controversial. We attempt to directly observe the quantitative protein expression of Rho A/ROCK and phosphorylation of MYPT1 at Thr696 after carbachol administration in rat bladder smooth muscle cells (RBMSCs).

MATERIALS AND METHODS

Primary cultured smooth muscle cells were obtained from rat bladders. The effects of both concentration and time-course induced by the muscarinic agonist carbachol were investigated by assessing the expression of Rho A/ROCK and MYPT1 phosphorylation at Thr696 using Western blot.

RESULTS

In the dose-course studies, carbachol showed significant increase in phosphorylation of MYPT1 at Thr696 (p-MYPT1) from concentrations of 15-100 μM based on Western blot results (p < 0.05, ANOVA test). In the time-course studies, treatment of cells with 15 μM of carbachol significantly enhanced the expression of p-MYPT1 from 3 to 15 h (p < 0.05, ANOVA test) and induced the expression of Rho A from 10 to 120 min (p < 0.05, ANOVA test).

CONCLUSIONS

Carbachol can induce the expression of ROCK pathway, leading to MYPT1 phosphorylation at Thr696 and thereby sustained RBSMCs contraction.

Airway Hyperresponsiveness in Asthma Model Occurs Independently of Secretion of β1 Integrins in Airway Wall and Focal Adhesions Proteins Down Regulation

The extracellular domains of some membrane proteins can be shed from the cell. A similar phenomenon occurs with β1 integrins (α1β1 and α2β1) in guinea pig. The putative role of β1 integrin subunit alterations due to shedding in airway smooth muscle (ASM) in an allergic asthma model was evaluated. Guinea pigs were sensitized and challenged with antigen. Antigenic challenges induced bronchoobstruction and hyperresponsiveness at the third antigenic challenge. Immunohistochemistry and immunoelectronmicroscopy studies showed that the cytosolic and extracellular domains of the β1 integrin subunit shared the same distribution in airway structures in both groups. Various polypeptides with similar molecular weights were detected with both the cytosolic and extracellular β1 integrin subunit antibodies in isolated airway myocytes and the connective tissue that surrounds the ASM bundle. Flow cytometry and Western blot studies showed that the expression of cytosolic and extracellular β1 integrin subunit domains in ASM was similar between groups. An increment of ITGB1 mRNA in ASM was observed in the asthma model group. RACE-PCR of ITGB1 in ASM did not show splicing variants. The expression levels of integrin-linked kinase (ILK) and paxillin diminished in the asthma model, but not talin. The levels of phosphorylation of myosin phosphatase target subunit 1 (MYPT1) at Thr(696) increased in asthma model. Our work suggests that β1 integrin is secreted in guinea pig airway wall. This secretion is not altered in asthma model; nevertheless, β1 integrin cytodomain assembly proteins in focal cell adhesions in which ILK and paxillin are involved are altered in asthma model. J. Cell. Biochem. 117: 2385-2396, 2016. © 2016 Wiley Periodicals, Inc.

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.

Physiological signalling to myosin phosphatase targeting subunit-1 phosphorylation in ileal smooth muscle

KEY POINTS

The extent of myosin regulatory light chain phosphorylation (RLC) necessary for smooth muscle contraction depends on the respective activities of Ca(2+) /calmodulin-dependent myosin light chain kinase and myosin light chain phosphatase (MLCP), which contains a regulatory subunit MYPT1 bound to the phosphatase catalytic subunit and myosin. MYPT1 showed significant constitutive T696 and T853 phosphorylation, which is predicted to inhibit MLCP activity in isolated ileal smooth muscle tissues, with additional phosphorylation upon pharmacological treatment with the muscarinic agonist carbachol. Electrical field stimulation (EFS), which releases ACh from nerves, increased force and RLC phosphorylation but not MYPT1 T696 or T853 phosphorylation. The conditional knockout of MYPT1 or the knockin mutation T853A in mice had no effect on the frequency-maximal force responses to EFS in isolated ileal tissues. Physiological RLC phosphorylation and force development in ileal smooth muscle depend on myosin light chain kinase and MLCP activities without changes in constitutive MYPT1 phosphorylation.

ABSTRACT

Smooth muscle contraction initiated by myosin regulatory light chain (RLC) phosphorylation is dependent on the relative activities of Ca(2+) /calmodulin-dependent myosin light chain kinase (MLCK) and myosin light chain phosphatase (MLCP). We have investigated the physiological role of the MLCP regulatory subunit MYPT1 in ileal smooth muscle in adult mice with (1) smooth muscle-specific deletion of MYPT1; (2) non-phosphorylatable MYPT1 containing a T853A knockin mutation; and (3) measurements of force and protein phosphorylation responses to cholinergic neurostimulation initiated by electric field stimulation. Isolated MYPT1-deficient tissues from MYPT1(SM-/-) mice contracted and relaxed rapidly with moderate differences in sustained responses to KCl and carbachol treatments and washouts, respectively. Similarly, measurements of regulatory proteins responsible for RLC phosphorylation during contractions also revealed moderate changes. There were no differences in contractile or RLC phosphorylation responses to carbachol between tissues from normal mice vs. MYPT1 T853A knockin mice. Quantitatively, there was substantial MYPT1 T696 and T853 phosphorylation in wild-type tissues under resting conditions, predicting a high extent of MLCP phosphatase inhibition. Reduced PP1cδ activity in MYPT1-deficient tissues may be similar to attenuated MLCP activity in wild-type tissues resulting from constitutively phosphorylated MYPT1. Electric field stimulation increased RLC phosphorylation and force development in tissues from wild-type mice without an increase in MYPT1 phosphorylation. Thus, physiological RLC phosphorylation and force development in ileal smooth muscle appear to be dependent on MLCK and MLCP activities without changes in constitutive MYPT1 phosphorylation.

Analysis of phosphorylation of the myosin-targeting subunit of myosin light chain phosphatase by Phos-tag SDS-PAGE

Phosphorylation of the myosin-targeting subunit 1 of myosin light chain phosphatase (MYPT1) plays an important role in the regulation of smooth muscle contraction, and several sites of phosphorylation by different protein Ser/Thr kinases have been identified. Furthermore, in some instances, phosphorylation at specific sites affects phosphorylation at neighboring sites, with functional consequences. Characterization of the complex phosphorylation of MYPT1 in tissue samples at rest and in response to contractile and relaxant stimuli is, therefore, challenging. We have exploited Phos-tag SDS-PAGE in combination with Western blotting using antibodies to MYPT1, including phosphospecific antibodies, to separate multiple phosphorylated MYPT1 species and quantify MYPT1 phosphorylation stoichiometry using purified, full-length recombinant MYPT1 phosphorylated by Rho-associated coiled-coil kinase (ROCK) and cAMP-dependent protein kinase (PKA). This approach confirmed that phosphorylation of MYPT1 by ROCK occurs at Thr(697)and Thr(855), PKA phosphorylates these two sites and the neighboring Ser(696)and Ser(854), and prior phosphorylation at Thr(697)and Thr(855)by ROCK precludes phosphorylation at Ser(696)and Ser(854)by PKA. Furthermore, phosphorylation at Thr(697)and Thr(855)by ROCK exposes two other sites of phosphorylation by PKA. Treatment of Triton-skinned rat caudal arterial smooth muscle strips with the membrane-impermeant phosphatase inhibitor microcystin or treatment of intact tissue with the membrane-permeant phosphatase inhibitor calyculin A induced slow, sustained contractions that correlated with phosphorylation of MYPT1 at 7 to ≥10 sites. Phos-tag SDS-PAGE thus provides a suitable and convenient method for analysis of the complex, multisite MYPT1 phosphorylation events involved in the regulation of myosin light chain phosphatase activity and smooth muscle contraction.

The Function of Rho-Associated Kinases ROCK1 and ROCK2 in the Pathogenesis of Cardiovascular Disease

Rho-associated kinases ROCK1 and ROCK2 are serine/threonine kinases that are downstream targets of the small GTPases RhoA, RhoB, and RhoC. ROCKs are involved in diverse cellular activities including actin cytoskeleton organization, cell adhesion and motility, proliferation and apoptosis, remodeling of the extracellular matrix and smooth muscle cell contraction. The role of ROCK1 and ROCK2 has long been considered to be similar; however, it is now clear that they do not always have the same functions. Moreover, depending on their subcellular localization, activation, and other environmental factors, ROCK signaling can have different effects on cellular function. With respect to the heart, findings in isoform-specific knockout mice argue for a role of ROCK1 and ROCK2 in the pathogenesis of cardiac fibrosis and cardiac hypertrophy, respectively. Increased ROCK activity could play a pivotal role in processes leading to cardiovascular diseases such as hypertension, pulmonary hypertension, angina pectoris, vasospastic angina, heart failure, and stroke, and thus ROCK activity is a potential new biomarker for heart disease. Pharmacological ROCK inhibition reduces the enhanced ROCK activity in patients, accompanied with a measurable improvement in medical condition. In this review, we focus on recent findings regarding ROCK signaling in the pathogenesis of cardiovascular disease, with a special focus on differences between ROCK1 and ROCK2 function.

Role of Telokin in Regulating Murine Gastric Fundus Smooth Muscle Tension

Telokin phosphorylation by cyclic GMP-dependent protein kinase facilitates smooth muscle relaxation. In this study we examined the relaxation of gastric fundus smooth muscles from basal tone, or pre-contracted with KCl or carbachol (CCh), and the phosphorylation of telokin S13, myosin light chain (MLC) S19, MYPT1 T853, T696, and CPI-17 T38 in response to 8-Bromo-cGMP, the NO donor sodium nitroprusside (SNP), or nitrergic neurotransmission. We compared MLC phosphorylation and the contraction and relaxation responses of gastric fundus smooth muscles from telokin-/- mice and their wild-type littermates to KCl or CCh, and 8-Bromo-cGMP, SNP, or nitrergic neurotransmission, respectively. We compared the relaxation responses and telokin phosphorylation of gastric fundus smooth muscles from wild-type mice and W/WV mice which lack ICC-IM, to 8-Bromo-cGMP, SNP, or nitrergic neurotransmission. We found that telokin S13 is basally phosphorylated and that 8-Bromo-cGMP and SNP increased basal telokin phosphorylation. In muscles pre-contracted with KCl or CCh, 8-Bromo-cGMP and SNP had no effect on CPI-17 or MYPT1 phosphorylation, but increased telokin phosphorylation and reduced MLC phosphorylation. In telokin-/- gastric fundus smooth muscles, basal tone and constitutive MLC S19 phosphorylation were increased. Pre-contracted telokin-/- gastric fundus smooth muscles have increased contractile responses to KCl, CCh, or cholinergic neurotransmission and reduced relaxation to 8-Bromo-cGMP, SNP, and nitrergic neurotransmission. However, basal telokin phosphorylation was not increased when muscles were stimulated with lower concentrations of SNP or when the muscles were stimulated by nitrergic neurotransmission. SNP, but not nitrergic neurotransmission, increased telokin Ser13 phosphorylation in both wild-type and W/WV gastric fundus smooth muscles. Our findings indicate that telokin may play a role in attenuating constitutive MLC phosphorylation and provide an additional mechanism to augment gastric fundus mechanical responses to inhibitory neurotransmission.

Myotonic dystrophy kinase-related Cdc42-binding kinases (MRCK), the ROCK-like effectors of Cdc42 and Rac1

Cdc42 is a member of the Rho GTPase protein family that plays key roles in local F-actin organization through a number of kinase and non-kinase effector proteins. The myotonic dystrophy kinase-related Cdc42-binding kinases (MRCKs), and the RhoA binding coiled-coil containing kinases (ROCKs) are widely expressed members of the Dystrophia myotonica protein kinase (DMPK) family. The MRCK proteins are ∼190 kDa multi-domain proteins expressed in all cells and coordinate certain acto-myosin networks. Notably MRCK is a key regulator of myosin18A and myosin IIA/B, and through phosphorylation of their common regulatory light chains (MYL9 or MLC2) to promote actin stress fiber contractility. The MRCK kinases are regulated by Cdc42, which is required for cell polarity and directional migration; MRCK links to the acto-myosin complex through interaction with a coiled-coil containing adaptor proteins LRAP35a/b. The biological activities of MRCK in model organisms such as worms and flies confirm it as a myosin II activator. In mammalian cell culture MRCK can be critical for cancer cell migration and neurite outgrowth. We review the current literatures regarding MRCK and highlight the similarities and differences between MRCK and ROCK kinases.

The Rho kinases: critical mediators of multiple profibrotic processes and rational targets for new therapies for pulmonary fibrosis

Idiopathic pulmonary fibrosis (IPF) is characterized by progressive lung scarring, short median survival, and limited therapeutic options, creating great need for new pharmacologic therapies. IPF is thought to result from repetitive environmental injury to the lung epithelium, in the context of aberrant host wound healing responses. Tissue responses to injury fundamentally involve reorganization of the actin cytoskeleton of participating cells, including epithelial cells, fibroblasts, endothelial cells, and macrophages. Actin filament assembly and actomyosin contraction are directed by the Rho-associated coiled-coil forming protein kinase (ROCK) family of serine/threonine kinases (ROCK1 and ROCK2). As would therefore be expected, lung ROCK activation has been demonstrated in humans with IPF and in animal models of this disease. ROCK inhibitors can prevent fibrosis in these models, and more importantly, induce the regression of already established fibrosis. Here we review ROCK structure and function, upstream activators and downstream targets of ROCKs in pulmonary fibrosis, contributions of ROCKs to profibrotic cellular responses to lung injury, ROCK inhibitors and their efficacy in animal models of pulmonary fibrosis, and potential toxicities of ROCK inhibitors in humans, as well as involvement of ROCKs in fibrosis in other organs. As we discuss, ROCK activation is required for multiple profibrotic responses, in the lung and multiple other organs, suggesting ROCK participation in fundamental pathways that contribute to the pathogenesis of a broad array of fibrotic diseases. Multiple lines of evidence therefore indicate that ROCK inhibition has great potential to be a powerful therapeutic tool in the treatment of fibrosis, both in the lung and beyond.

Platelet myosin light chain phosphatase: keeping it together

MLCP (myosin light chain phosphatase) regulates platelet function through its ability to control myosin IIa phosphorylation. Recent evidence suggests that MLCP is a de facto target for signalling events stimulated by cAMP. In the present mini-review, we discuss the mechanisms by which cAMP signalling maintains MLCP in an active state to control platelet contractile machinery.

Compressive stress induces dephosphorylation of the myosin regulatory light chain via RhoA phosphorylation by the adenylyl cyclase/protein kinase A signaling pathway

Mechanical stress that arises due to deformation of the extracellular matrix (ECM) either stretches or compresses cells. The cellular response to stretching has been actively studied. For example, stretching induces phosphorylation of the myosin regulatory light chain (MRLC) via the RhoA/RhoA-associated protein kinase (ROCK) pathway, resulting in increased cellular tension. In contrast, the effects of compressive stress on cellular functions are not fully resolved. The mechanisms for sensing and differentially responding to stretching and compressive stress are not known. To address these questions, we investigated whether phosphorylation levels of MRLC were affected by compressive stress. Contrary to the response in stretching cells, MRLC was dephosphorylated 5 min after cells were subjected to compressive stress. Compressive loading induced activation of myosin phosphatase mediated via the dephosphorylation of myosin phosphatase targeting subunit 1 (Thr853). Because myosin phosphatase targeting subunit 1 (Thr853) is phosphorylated only by ROCK, compressive loading may have induced inactivation of ROCK. However, GTP-bound RhoA (active form) increased in response to compressive stress. The compression-induced activation of RhoA and inactivation of its effector ROCK are contradictory. This inconsistency was due to phosphorylation of RhoA (Ser188) that reduced affinity of RhoA to ROCK. Treatment with the inhibitor of protein kinase A that phosphorylates RhoA (Ser188) induced suppression of compression-stimulated MRLC dephosphorylation. Incidentally, stretching induced phosphorylation of MRLC, but did not affect phosphorylation levels of RhoA (Ser188). Together, our results suggest that RhoA phosphorylation is an important process for MRLC dephosphorylation by compressive loading, and for distinguishing between stretching and compressing cells.

A role for the tyrosine kinase Pyk2 in depolarization-induced contraction of vascular smooth muscle

Depolarization of the vascular smooth muscle cell membrane evokes a rapid (phasic) contractile response followed by a sustained (tonic) contraction. We showed previously that the sustained contraction involves genistein-sensitive tyrosine phosphorylation upstream of the RhoA/Rho-associated kinase (ROK) pathway leading to phosphorylation of MYPT1 (the myosin-targeting subunit of myosin light chain phosphatase (MLCP)) and myosin regulatory light chains (LC20). In this study, we addressed the hypothesis that membrane depolarization elicits activation of the Ca(2+)-dependent tyrosine kinase Pyk2 (proline-rich tyrosine kinase 2). Pyk2 was identified as the major tyrosine-phosphorylated protein in response to membrane depolarization. The tonic phase of K(+)-induced contraction was inhibited by the Pyk2 inhibitor sodium salicylate, which abolished the sustained elevation of LC20 phosphorylation. Membrane depolarization induced autophosphorylation (activation) of Pyk2 with a time course that correlated with the sustained contractile response. The Pyk2/focal adhesion kinase (FAK) inhibitor PF-431396 inhibited both phasic and tonic components of the contractile response to K(+), Pyk2 autophosphorylation, and LC20 phosphorylation but had no effect on the calyculin A (MLCP inhibitor)-induced contraction. Ionomycin, in the presence of extracellular Ca(2+), elicited a slow, sustained contraction and Pyk2 autophosphorylation, which were blocked by pre-treatment with PF-431396. Furthermore, the Ca(2+) channel blocker nifedipine inhibited peak and sustained K(+)-induced force and Pyk2 autophosphorylation. Inhibition of Pyk2 abolished the K(+)-induced translocation of RhoA to the particulate fraction and the phosphorylation of MYPT1 at Thr-697 and Thr-855. We conclude that depolarization-induced entry of Ca(2+) activates Pyk2 upstream of the RhoA/ROK pathway, leading to MYPT1 phosphorylation and MLCP inhibition. The resulting sustained elevation of LC20 phosphorylation then accounts for the tonic contractile response to membrane depolarization.

Rho kinase as a target for cerebral vascular disorders

The development of novel pharmaceutical treatments for disorders of the cerebral vasculature is a serious unmet medical need. These vascular disorders are typified by a disruption in the delicate Rho signaling equilibrium within the blood vessel wall. In particular, Rho kinase overactivation in the smooth muscle and endothelial layers of the vessel wall results in cytoskeletal modifications that lead to reduced vascular integrity and abnormal vascular growth. Rho kinase is thus a promising target for the treatment of cerebral vascular disorders. Indeed, preclinical studies indicate that Rho kinase inhibition may reduce the formation/growth/rupture of both intracranial aneurysms and cerebral cavernous malformations.

ERK and p38MAPK pathways regulate myosin light chain phosphatase and contribute to Ca2+ sensitization of intestinal smooth muscle contraction

BACKGROUND

Mitogen-activated protein kinases (MAPKs), including extracellular signal-regulated protein kinase (ERK) and p38MAPK, are known regulators of smooth muscle contractility. The contraction of smooth muscle is mainly regulated by the phosphorylation of regulatory light chains of myosin II (LC20), which is driven by the balance between myosin light chain kinase (MLCK) and myosin light chain phosphatase (MLCP). We hypothesized that one possible mechanism for MAPK-dependent modulation of intestinal smooth muscle contractility is via the regulation of MLCP activity.

METHODS

Contractile responses to carbachol (CCh) and effects of MAPK inhibitors on CCh-induced contractions were assessed with isolated rat ileal longitudinal smooth muscle strips. Biochemical assessments of MLCP activity and myosin phosphatse targeting subunit (MYPT1) and CPI-17 phosphorylations were completed.

KEY RESULTS

Treatment of ileal smooth muscle with PD98059 (10 μM; MEK inhibitor) or SB203580 (10 μM; p38MAPK inhibitor) significantly inhibited CCh-induced contractile force. Decreased MLCP activity was observed during sustained contractions induced by CCh; the MLCP activity was recovered by treatment with PD98059 and SB203580. However, MYPT1 (Thr697 and Thr855) and CPI-17 (Thr38) phosphorylations were not affected. Application of ML-7 (MLCK inhibitor) during CCh-induced sustained contraction elicited an MLCP-dependent relaxation, the rate of which was accelerated by application of PD98059 and SB203580 with proportional changes in LC20 phosphorylation levels but not MYPT1 phosphorylation (Thr697 or Thr855).

CONCLUSIONS & INFERENCES

ERK and p38MAPK contribute to CCh-induced sustained contraction in a LC20 phosphorylation dependent manner. Moreover, both kinases inhibit MLCP activity possibly by a novel mechanism.

In vivo roles for myosin phosphatase targeting subunit-1 phosphorylation sites T694 and T852 in bladder smooth muscle contraction

KEY POINTS

Force production and maintenance in smooth muscle is largely controlled by myosin regulatory light chain (RLC) phosphorylation, which relies on a balance between Ca(2+)/calmodulin-dependent myosin light chain kinase (MLCK) and myosin light chain phosphatase (MLCP) activities. MYPT1 is the regulatory subunit of MLCP that biochemically inhibits MLCP activity via T694 or T852 phosphorylation in vitro. Here we separately investigated the contribution of these two phosphorylation sites in bladder smooth muscles by establishing two single point mutation mouse lines, T694A and T852A, and found that phosphorylation of MYPT1 T694, but not T852, mediates force maintenance via inhibition of MLCP activity and enhancement of RLC phosphorylation in vivo. Our findings reveal the role of MYPT1 T694/T852 phosphorylation in vivo in regulation of smooth muscle contraction.

ABSTRACT

Force production and maintenance in smooth muscle is largely controlled by different signalling modules that fine tune myosin regulatory light chain (RLC) phosphorylation, which relies on a balance between Ca(2+)/calmodulin-dependent myosin light chain kinase (MLCK) and myosin light chain phosphatase (MLCP) activities. To investigate the regulation of MLCP activity in vivo, we analysed the role of two phosphorylation sites on MYPT1 (regulatory subunit of MLCP) that biochemically inhibit MLCP activity in vitro. MYPT1 is constitutively phosphorylated at T694 by unidentified kinases in vivo, whereas the T852 site is phosphorylated by RhoA-associated protein kinase (ROCK). We established two mouse lines with alanine substitution of T694 or T852. Isolated bladder smooth muscle from T852A mice displayed no significant changes in RLC phosphorylation or force responses, but force was inhibited with a ROCK inhibitor. In contrast, smooth muscles containing the T694A mutation showed a significant reduction of force along with reduced RLC phosphorylation. The contractile responses of T694A mutant smooth muscle were also independent of ROCK activation. Thus, phosphorylation of MYPT1 T694, but not T852, is a primary mechanism contributing to inhibition of MLCP activity and enhancement of RLC phosphorylation in vivo. The constitutive phosphorylation of MYPT1 T694 may provide a mechanism for regulating force maintenance of smooth muscle.

Asef controls vascular endothelial permeability and barrier recovery in the lung

This is the first report of Asef involvement in the regulation of endothelial vascular permeability in vitro and in vivo. Asef activation in endothelial cells by hepatocyte growth factor suppressed the Rho-dependent pathway of agonist-induced endothelial permeability and promoted Rac1-dependent endothelial barrier recovery.

Increased levels of hepatocyte growth factor (HGF) in injured lungs may reflect a compensatory response to diminish acute lung injury (ALI). HGF-induced activation of Rac1 GTPase stimulates endothelial barrier protective mechanisms. This study tested the involvement of Rac-specific guanine nucleotide exchange factor Asef in HGF-induced endothelial cell (EC) cytoskeletal dynamics and barrier protection in vitro and in a two-hit model of ALI. HGF induced membrane translocation of Asef and stimulated Asef Rac1-specific nucleotide exchange activity. Expression of constitutively activated Asef mutant mimicked HGF-induced peripheral actin cytoskeleton enhancement. In contrast, siRNA-induced Asef knockdown or expression of dominant-negative Asef attenuated HGF-induced Rac1 activation evaluated by Rac-GTP pull down and FRET assay with Rac1 biosensor. Molecular inhibition of Asef attenuated HGF-induced peripheral accumulation of cortactin, formation of lamellipodia-like structures, and enhancement of VE-cadherin adherens junctions and compromised HGF-protective effect against thrombin-induced RhoA GTPase activation, Rho-dependent cytoskeleton remodeling, and EC permeability. Intravenous HGF injection attenuated lung inflammation and vascular leak in the two-hit model of ALI induced by excessive mechanical ventilation and thrombin signaling peptide TRAP6. This effect was lost in Asef^−/− mice. This study shows for the first time the role of Asef in HGF-mediated protection against endothelial hyperpermeability and lung injury.