Phosphorylation-dependent autoinhibition of myosin light chain phosphatase accounts for Ca2+ sensitization force of smooth muscle contraction

LUZP1 regulates the maturation of contractile actomyosin bundles

Contractile actomyosin bundles play crucial roles in various physiological processes, including cell migration, morphogenesis, and muscle contraction. The intricate assembly of actomyosin bundles involves the precise alignment and fusion of myosin II filaments, yet the underlying mechanisms and factors involved in these processes remain elusive. Our study reveals that LUZP1 plays a central role in orchestrating the maturation of thick actomyosin bundles. Loss of LUZP1 caused abnormal cell morphogenesis, migration, and the ability to exert forces on the environment. Importantly, knockout of LUZP1 results in significant defects in the concatenation and persistent association of myosin II filaments, severely impairing the assembly of myosin II stacks. The disruption of these processes in LUZP1 knockout cells provides mechanistic insights into the defective assembly of thick ventral stress fibers and the associated cellular contractility abnormalities. Overall, these results significantly contribute to our understanding of the molecular mechanism involved in actomyosin bundle formation and highlight the essential role of LUZP1 in this process.

Pulses of RhoA signaling stimulate actin polymerization and flow in protrusions to drive collective cell migration

In animals, cells often move as collectives to shape organs, close wounds, or-in the case of disease-metastasize. To accomplish this, cells need to generate force to propel themselves forward. The motility of singly migrating cells is driven largely by an interplay between Rho GTPase signaling and the actin network. Whether cells migrating as collectives use the same machinery for motility is unclear. Using the zebrafish posterior lateral line primordium as a model for collective cell migration, we find that active RhoA and myosin II cluster on the basal sides of the primordium cells and are required for primordium motility. Positive and negative feedbacks cause RhoA and myosin II activities to pulse. These pulses of RhoA signaling stimulate actin polymerization at the tip of the protrusions and myosin-II-dependent actin flow and protrusion retraction at the base of the protrusions and deform the basement membrane underneath the migrating primordium. This suggests that RhoA-induced actin flow on the basal sides of the cells constitutes the motor that pulls the primordium forward, a scenario that likely underlies collective migration in other contexts.

Pulses of RhoA Signaling Stimulate Actin Polymerization and Flow in Protrusions to Drive Collective Cell Migration

In animals, cells often move as collectives to shape organs, close wounds, or-in the case of disease-metastasize. To accomplish this, cells need to generate force to propel themselves forward. The motility of singly migrating cells is driven largely by an interplay between Rho GTPase signaling and the actin network (Yamada and Sixt, 2019). Whether cells migrating as collectives use the same machinery for motility is unclear. Using the zebrafish posterior lateral line primordium as a model for collective cell migration, we find that active RhoA and myosin II cluster on the basal sides of the primordium cells and are required for primordium motility. Positive and negative feedbacks cause RhoA and myosin II activities to pulse. These pulses of RhoA signaling stimulate actin polymerization at the tip of the protrusions and myosin II-dependent actin flow and protrusion retraction at the base of the protrusions, and deform the basement membrane underneath the migrating primordium. This suggests that RhoA-induced actin flow on the basal sides of the cells constitutes the motor that pulls the primordium forward, a scenario that likely underlies collective migration in other-but not all (Bastock and Strutt, 2007; Lebreton and Casanova, 2013; Matthews et al., 2008)-contexts.

Serine/Threonine Protein Phosphatases 1 and 2A in Lung Endothelial Barrier Regulation

Vascular barrier dysfunction is characterized by increased permeability and inflammation of endothelial cells (ECs), which are prominent features of acute lung injury (ALI), acute respiratory distress syndrome (ARDS), and sepsis, and a major complication of the SARS-CoV-2 infection and COVID-19. Functional impairment of the EC barrier and accompanying inflammation arises due to microbial toxins and from white blood cells of the lung as part of a defensive action against pathogens, ischemia-reperfusion or blood product transfusions, and aspiration syndromes-based injury. A loss of barrier function results in the excessive movement of fluid and macromolecules from the vasculature into the interstitium and alveolae resulting in pulmonary edema and collapse of the architecture and function of the lungs, and eventually culminates in respiratory failure. Therefore, EC barrier integrity, which is heavily dependent on cytoskeletal elements (mainly actin filaments, microtubules (MTs), cell-matrix focal adhesions, and intercellular junctions) to maintain cellular contacts, is a critical requirement for the preservation of lung function. EC cytoskeletal remodeling is regulated, at least in part, by Ser/Thr phosphorylation/dephosphorylation of key cytoskeletal proteins. While a large body of literature describes the role of phosphorylation of cytoskeletal proteins on Ser/Thr residues in the context of EC barrier regulation, the role of Ser/Thr dephosphorylation catalyzed by Ser/Thr protein phosphatases (PPases) in EC barrier regulation is less documented. Ser/Thr PPases have been proposed to act as a counter-regulatory mechanism that preserves the EC barrier and opposes EC contraction. Despite the importance of PPases, our knowledge of the catalytic and regulatory subunits involved, as well as their cellular targets, is limited and under-appreciated. Therefore, the goal of this review is to discuss the role of Ser/Thr PPases in the regulation of lung EC cytoskeleton and permeability with special emphasis on the role of protein phosphatase 1 (PP1) and protein phosphatase 2A (PP2A) as major mammalian Ser/Thr PPases. Importantly, we integrate the role of PPases with the structural dynamics of the cytoskeleton and signaling cascades that regulate endothelial cell permeability and inflammation.

Dual thick and thin filament linked regulation of stretch- and L-NAME-induced tone in young and senescent murine basilar artery

Stretch-induced vascular tone is an important element of autoregulatory adaptation of cerebral vasculature to maintain cerebral flow constant despite changes in perfusion pressure. Little is known as to the regulation of tone in senescent basilar arteries. We tested the hypothesis, that thin filament mechanisms in addition to smooth muscle myosin-II regulatory-light-chain-(MLC20)-phosphorylation and non-muscle-myosin-II, contribute to regulation of stretch-induced tone. In young BAs (y-BAs) mechanical stretch does not lead to spontaneous tone generation. Stretch-induced tone in y-BAs appeared only after inhibition of NO-release by L-NAME and was fully prevented by treatment with 3 μmol/L RhoA-kinase (ROK) inhibitor Y27632. L-NAME-induced tone was reduced in y-BAs from heterozygous mice carrying a point mutation of the targeting-subunit of the myosin phosphatase, MYPT1 at threonine696 (MYPT1-T696A/+). In y-BAs, MYPT1-T696A-mutation also blunted the ability of L-NAME to increase MLC20-phosphorylation. In contrast, senescent BAs (s-BAs; >24 months) developed stable spontaneous stretch-induced tone and pharmacological inhibition of NO-release by L-NAME led to an additive effect. In s-BAs the MYPT1-T696A mutation also blunted MLC20-phosphorylation, but did not prevent development of stretch-induced tone. In s-BAs from both lines, Y27632 completely abolished stretch- and L-NAME-induced tone. In s-BAs phosphorylation of non-muscle-myosin-S1943 and PAK1-T423, shown to be down-stream effectors of ROK was also reduced by Y27632 treatment. Stretch- and L-NAME tone were inhibited by inhibition of non-muscle myosin (NM-myosin) by blebbistatin. We also tested whether the substrate of PAK1 the thin-filament associated protein, caldesmon is involved in the regulation of stretch-induced tone in advanced age. BAs obtained from heterozygotes Cald1+/− mice generated stretch-induced tone already at an age of 20–21 months old BAs (o-BA). The magnitude of stretch-induced tone in Cald1+/− o-BAs was similar to that in s-BA. In addition, truncation of caldesmon myosin binding Exon2 (CaD-▵Ex2−/−) did not accelerate stretch-induced tone. Our study indicates that in senescent cerebral vessels, mechanisms distinct from MLC20 phosphorylation contribute to regulation of tone in the absence of a contractile agonist. While in y-and o-BA the canonical pathways, i.e., inhibition of MLCP by ROK and increase in pMLC20, predominate, tone regulation in senescence involves ROK regulated mechanisms, involving non-muscle-myosin and thin filament linked mechanisms involving caldesmon.

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.

Starving a Cell Promotes Airway Smooth Muscle Relaxation: Inhibition of Glycolysis Attenuates Excitation-Contraction Coupling

Bronchomotor tone modulated by airway smooth muscle shortening represents a key mechanism that increases airway resistance in asthma. Altered glucose metabolism in inflammatory and airway structural cells is associated with asthma. Although these observations suggest a causal link between glucose metabolism and airway hyperresponsiveness, the mechanisms are unclear. We hypothesized that glycolysis modulates excitation-contraction coupling in human airway smooth muscle (HASM) cells. Cultured HASM cells from human lung donors were subject to metabolic screenings using Seahorse XF cell assay. HASM cell monolayers were treated with vehicle or PFK15 (1-(Pyridin-4-yl)-3-(quinolin-2-yl)prop-2-en-1-one), an inhibitor of PFKFB3 (PFK-1,6-phosphofructo-2-kinase/fructose-2,6-biphosphatase 3) that generates an allosteric activator for glycolysis rate-limiting enzyme PFK1 (phosphofructokinase 1), for 5-240 minutes, and baseline and agonist-induced phosphorylation of MLC (myosin light chain), MYPT1 (myosin phosphatase regulatory subunit 1), Akt, RhoA, and cytosolic Ca2+ were determined. PFK15 effects on metabolic activity and contractile agonist-induced bronchoconstriction were determined in human precision-cut lung slices. Inhibition of glycolysis attenuated carbachol-induced excitation-contraction coupling in HASM cells. ATP production and bronchodilator-induced cAMP concentrations were also attenuated by glycolysis inhibition in HASM cells. In human small airways, glycolysis inhibition decreased mitochondrial respiration and ATP production and attenuated carbachol-induced bronchoconstriction. The findings suggest that energy depletion resulting from glycolysis inhibition is a novel strategy for ameliorating HASM cell shortening and bronchoprotection of human small airways.

Strategy for sodium-salt substitution: On the relationship between hypertension and dietary intake of cations

Chronic diseases, especially cardiovascular diseases (CVD), have become one of the main causes affecting human health. Hypertension is a prominent representative of CVD. The formation and development of hypertension is closely related to people's daily diet. A large number of studies have shown that excessive intake of salt (NaCl) could increase the risk of hypertension. In recent years, more and more investigations have focused on other cations that may be contained in edible salt, exploring whether they have an effect on hypertension and the underlying mechanism. This article focuses on the relationship between four metal elements (potassium, calcium, magnesium, and zinc) and hypertension, by discussing the main metabolic pathway, the impact of diet intake on blood pressure, and especially the regulation mechanisms on blood pressure in detail. At the same time, some opinions and suggestions are put forward, combined with the current hot topics "salt reduction" and "salt substitution".

MARK2 regulates directed cell migration through modulation of myosin II contractility and focal adhesion organization

Cancer cell migration during metastasis is mediated by a highly polarized cytoskeleton. MARK2 and its invertebrate homolog Par1B are kinases that regulate the microtubule cytoskeleton to mediate polarization of neurons in mammals and embryos in invertebrates. However, the role of MARK2 in cancer cell migration is unclear. Using osteosarcoma cells, we found that in addition to its known localizations on microtubules and the plasma membrane, MARK2 also associates with the actomyosin cytoskeleton and focal adhesions. Cells depleted of MARK proteins demonstrated that MARK2 promotes phosphorylation of both myosin II and the myosin phosphatase targeting subunit MYPT1 to synergistically drive myosin II contractility and stress fiber formation in cells. Studies with isolated proteins showed that MARK2 directly phosphorylates myosin II regulatory light chain, while its effects on MYPT1 phosphorylation are indirect. Using a mutant lacking the membrane-binding domain, we found that membrane association is required for focal adhesion targeting of MARK2, where it specifically enhances cell protrusion by promoting FAK phosphorylation and formation of focal adhesions oriented in the direction of migration to mediate directionally persistent cell motility. Together, our results define MARK2 as a master regulator of the actomyosin and microtubule cytoskeletal systems and focal adhesions to mediate directional cancer cell migration.

Identification of the Kinase-Substrate Recognition Interface between MYPT1 and Rho-Kinase

Protein kinases exert physiological functions through phosphorylating their specific substrates; however, the mode of kinase–substrate recognition is not fully understood. Rho-kinase is a Ser/Thr protein kinase that regulates cytoskeletal reorganization through phosphorylating myosin light chain (MLC) and the myosin phosphatase targeting subunit 1 (MYPT1) of MLC phosphatase (MLCP) and is involved in various diseases, due to its aberrant cellular contraction, morphology, and movement. Despite the importance of the prediction and identification of substrates and phosphorylation sites, understanding of the precise regularity in phosphorylation preference of Rho-kinase remains far from satisfactory. Here we analyzed the Rho-kinase–MYPT1 interaction, to understand the mode of Rho-kinase substrate recognition and found that the three short regions of MYPT1 close to phosphorylation sites (referred to as docking motifs (DMs); DM1 (DLQEAEKTIGRS), DM2 (KSQPKSIRERRRPR), and DM3 (RKARSRQAR)) are important for interactions with Rho-kinase. The phosphorylation levels of MYPT1 without DMs were reduced, and the effects were limited to the neighboring phosphorylation sites. We further demonstrated that the combination of pseudosubstrate (PS) and DM of MYPT1 (PS1 + DM3 and PS2 + DM2) serves as a potent inhibitor of Rho-kinase. The present information will be useful in identifying new substrates and developing selective Rho-kinase 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.

Functional and Molecular Characterisation of Heart Failure Progression in Mice and the Role of Myosin Regulatory Light Chains in the Recovery of Cardiac Muscle Function

Heart failure (HF) as a result of myocardial infarction (MI) is a major cause of fatality worldwide. However, the cause of cardiac dysfunction succeeding MI has not been elucidated at a sarcomeric level. Thus, studying the alterations within the sarcomere is necessary to gain insights on the fundamental mechansims leading to HF and potentially uncover appropriate therapeutic targets. Since existing research portrays regulatory light chains (RLC) to be mediators of cardiac muscle contraction in both human and animal models, its role was further explored In this study, a detailed characterisation of the physiological changes (i.e., isometric force, calcium sensitivity and sarcomeric protein phosphorylation) was assessed in an MI mouse model, between 2D (2 days) and 28D post-MI, and the changes were related to the phosphorylation status of RLCs. MI mouse models were created via complete ligation of left anterior descending (LAD) coronary artery. Left ventricular (LV) papillary muscles were isolated and permeabilised for isometric force and Ca²⁺ sensitivity measurement, while the LV myocardium was used to assay sarcomeric proteins’ (RLC, troponin I (TnI) and myosin binding protein-C (MyBP-C)) phosphorylation levels and enzyme (myosin light chain kinase (MLCK), zipper interacting protein kinase (ZIPK) and myosin phosphatase target subunit 2 (MYPT2)) expression levels. Finally, the potential for improving the contractility of diseased cardiac papillary fibres via the enhancement of RLC phosphorylation levels was investigated by employing RLC exchange methods, in vitro. RLC phosphorylation and isometric force potentiation were enhanced in the compensatory phase and decreased in the decompensatory phase of HF failure progression, respectively. There was no significant time-lag between the changes in RLC phosphorylation and isometric force during HF progression, suggesting that changes in RLC phosphorylation immediately affect force generation. Additionally, the in vitro increase in RLC phosphorylation levels in 14D post-MI muscle segments (decompensatory stage) enhanced its force of isometric contraction, substantiating its potential in HF treatment. Longitudinal observation unveils potential mechanisms involving MyBP-C and key enzymes regulating RLC phosphorylation, such as MLCK and MYPT2 (subunit of MLCP), during HF progression. This study primarily demonstrates that RLC phosphorylation is a key sarcomeric protein modification modulating cardiac function. This substantiates the possibility of using RLCs and their associated enzymes to treat HF.

Senescent murine femoral arteries undergo vascular remodelling associated with accelerated stress-induced contractility and reactivity to nitric oxide

This work explored the mechanism of augmented stress-induced vascular reactivity of senescent murine femoral arteries (FAs). Mechanical and pharmacological reactivity of young (12-25 weeks, y-FA) and senescent (>104 weeks, s-FAs) femoral arteries was measured by wire myography. Expression and protein phosphorylation of selected regulatory proteins were studied by western blotting. Expression ratio of the Exon24 in/out splice isoforms of the regulatory subunit of myosin phosphatase, MYPT1 (MYPT1-Exon24 in/out), was determined by polymerase chain reaction (PCR). While the resting length-tension relationship showed no alteration, the stretch-induced-tone increased to 8.3 ± 0.9 mN in s-FA versus only 4.6 ± 0.3 mN in y-FAs. Under basal conditions, phosphorylation of the regulatory light chain of myosin at S19 was 19.2 ± 5.8% in y-FA versus 49.2 ± 12.6% in s-FA. Inhibition of endogenous NO release raised tone additionally to 10.4 ± 1.2 mN in s-FA, whereas this treatment had a negligible effect in y-FAs (4.8 ± 0.3 mN). In s-FAs, reactivity to NO donor was augmented (pD₂  = -4.5 ± 0.3 in y-FA vs. -5.2 ± 0.1 in senescent). Accordingly, in s-FAs, MYPT1-Exon24-out-mRNA, which is responsible for expression of the more sensitive to protein-kinase G, leucine-zipper-positive MYPT1 isoform, was increased. The present work provides evidence that senescent murine s-FA undergoes vascular remodelling associated with increases in stretch-activated contractility and sensitivity to NO/cGMP/PKG system.

The mechanism of electroacupuncture for treating spinal cord injury rats by mediating Rho/Rho-associated kinase signaling pathway

Objective: To determine the changes of gene and protein expression through Rho/ROCK signaling pathway in EA treated spinal cord injury (SCI) rats and to unveil the possible underlying mechanism.Design: Animal study.Setting: Affiliated Hospital of Jiangxi University of Traditional Chinese Medicine.Participants: Eighty Male Sprague Dawley rats.Interventions: Electroacupuncture at Yaoyangguan (GV3), Dazhui (GV14), Zusanli (ST36) and Ciliao (BL32) and/or blocking agent Y27632 treatment.Outcome Measures: Protein expression was detected by ELISA and Western blotting, mRNA expression was detected by quantitative PCR and in situ hybridization. Morphological changes in spinal cord were evaluated by HE-staining and Nissl staining. Hindlimb motor function in the rats was evaluated by Basso-Beattie-Bresnahan (BBB) assessment methods.Results: Compared with injured rats in SCI group, EA, blocking agent Y27632 and EA + blocking agent Y27632 treatment had significantly reduced mRNA and protein expression levels of RhoA and ROCKII, decreased p-MLC protein expression and p-MLC/MLC ratio, suppressed cPLA2 activity and PGE₂ level, improved spinal cord tissue morphology and BBB score of lower limb movement function at 7 days and at 14 days (P < 0.01 or <0.05).Conclusion: Similar to the blocking agent Y27632, EA may have a notable inhibitory effect on the Rho/ROCK signaling pathway after SCI, therefore reducing the inhibition of axonal growth and inflammatory reaction may be a key mechanism of EA treatment for SCI.

The lipid phosphatase-like protein PLPPR1 associates with RhoGDI1 to modulate RhoA activation in response to axon growth inhibitory molecules

Phospholipid Phosphatase-Related Protein Type 1 (PLPPR1) is a member of a family of lipid phosphatase related proteins, integral membrane proteins characterized by six transmembrane domains. This family of proteins is enriched in the brain and recent data indicate potential pleiotropic functions in several different contexts. An inherent ability of this family of proteins is to induce morphological changes, and we have previously reported that members of this family interact with each other and may function co-operatively. However, the function of PLPPR1 is not yet understood. Here we show that the expression of PLPPR1 reduces the inhibition of neurite outgrowth of cultured mouse hippocampal neurons by chondroitin sulfate proteoglycans and the retraction of neurites of Neuro-2a cells by lysophosphatidic acid (LPA). Further, we show that PLPPR1 reduces the activation of Ras homolog family member A (RhoA) by LPA in Neuro-2a cells, and that this is because of an association of PLPPR1with the Rho-specific guanine nucleotide dissociation inhibitor (RhoGDI1). These results establish a novel signaling pathway for the PLPPR1 protein.

EPAC in Vascular Smooth Muscle Cells

Vascular smooth muscle cells (VSMCs) are major components of blood vessels. They regulate physiological functions, such as vascular tone and blood flow. Under pathological conditions, VSMCs undergo a remodeling process known as phenotypic switching. During this process, VSMCs lose their contractility and acquire a synthetic phenotype, where they over-proliferate and migrate from the tunica media to the tunica interna, contributing to the occlusion of blood vessels. Since their discovery as effector proteins of cyclic adenosine 3′,5′-monophosphate (cAMP), exchange proteins activated by cAMP (EPACs) have been shown to play vital roles in a plethora of pathways in different cell systems. While extensive research to identify the role of EPAC in the vasculature has been conducted, much remains to be explored to resolve the reported discordance in EPAC’s effects. In this paper, we review the role of EPAC in VSMCs, namely its regulation of the vascular tone and phenotypic switching, with the likely involvement of reactive oxygen species (ROS) in the interplay between EPAC and its targets/effectors.

Sarcoplasmic reticulum and calcium signaling in muscle cells: Homeostasis and disease

The sarco/endoplasmic reticulum is an extensive, dynamic and heterogeneous membranous network that fulfills multiple homeostatic functions. Among them, it compartmentalizes, stores and releases calcium within the intracellular space. In the case of muscle cells, calcium released from the sarco/endoplasmic reticulum in the vicinity of the contractile machinery induces cell contraction. Furthermore, sarco/endoplasmic reticulum-derived calcium also regulates gene transcription in the nucleus, energy metabolism in mitochondria and cytosolic signaling pathways. These diverse and overlapping processes require a highly complex fine-tuning that the sarco/endoplasmic reticulum provides by means of its numerous tubules and cisternae, specialized domains and contacts with other organelles. The sarco/endoplasmic reticulum also possesses a rich calcium-handling machinery, functionally coupled to both contraction-inducing stimuli and the contractile apparatus. Such is the importance of the sarco/endoplasmic reticulum for muscle cell physiology, that alterations in its structure, function or its calcium-handling machinery are intimately associated with the development of cardiometabolic diseases. Cardiac hypertrophy, insulin resistance and arterial hypertension are age-related pathologies with a common mechanism at the muscle cell level: the accumulation of damaged proteins at the sarco/endoplasmic reticulum induces a stress response condition termed endoplasmic reticulum stress, which impairs proper organelle function, ultimately leading to pathogenesis.

Time-resolved phosphoproteomic analysis elucidates hepatic 11,12-Epoxyeicosatrienoic acid signaling pathways

Epoxyeicosatrienoic acids (EETs) are potent lipid mediators with well-established effects in vascular tissues. Recent studies indicated an emerging role of these eicosanoids in metabolic diseases and the EET signaling pathway was shown to be involved in hepatic insulin sensitivity. However, compared to vascular tissues, there is only limited knowledge about the underlying signaling pathways in the liver. Therefore, we employed an LC-MS/MS-based time-resolved phosphoproteomics approach to characterize 11,12-EET-mediated signaling events in the liver cell line Hepa 1-6. 11,12-EET treatment resulted in the time-dependent regulation of phosphopeptides involved in processes as yet unknown to be affected by EETs, including RNA processing, splicing and translation regulation. Pathway analysis combined with western blot-based validation revealed enhanced AKT/mTOR/p70S6K signaling as demonstrated by increased acute phosphorylation of AKT (Ser473) and p70S6K (Thr389). In addition, 11,12-EET treatment led to differential regulation of phosphopeptides including important mediators of the DNA damage response and we observed a prolonged induction of the etoposide-induced DNA damage marker γH2AX in response to 11,12-EET. In summary, our findings extend current knowledge of 11,12-EET signaling events and emphasize the importance of the AKT/mTOR/p70S6K pathway in hepatic 11,12-EET signaling. Based on the results presented in this study, we furthermore propose a novel role of EET signaling in the regulation of the DNA damage response.

Rebalancing of actomyosin contractility enables mammary tumor formation upon loss of E-cadherin

E-cadherin (CDH1) is a master regulator of epithelial cell adherence junctions and a well-established tumor suppressor in Invasive Lobular Carcinoma (ILC). Intriguingly, somatic inactivation of E-cadherin alone in mouse mammary epithelial cells (MMECs) is insufficient to induce tumor formation. Here we show that E-cadherin loss induces extrusion of luminal MMECs to the basal lamina. Remarkably, E-cadherin-deficient MMECs can breach the basal lamina but do not disseminate into the surrounding fat pad. Basal lamina components laminin and collagen IV supported adhesion and survival of E-cadherin-deficient MMECs while collagen I, the principle component of the mammary stromal micro-environment did not. We uncovered that relaxation of actomyosin contractility mediates adhesion and survival of E-cadherin-deficient MMECs on collagen I, thereby allowing ILC development. Together, these findings unmask the direct consequences of E-cadherin inactivation in the mammary gland and identify aberrant actomyosin contractility as a critical barrier to ILC formation.

Vascular smooth muscle contraction in hypertension

Hypertension is a major risk factor for many common chronic diseases, such as heart failure, myocardial infarction, stroke, vascular dementia, and chronic kidney disease. Pathophysiological mechanisms contributing to the development of hypertension include increased vascular resistance, determined in large part by reduced vascular diameter due to increased vascular contraction and arterial remodelling. These processes are regulated by complex-interacting systems such as the renin-angiotensin-aldosterone system, sympathetic nervous system, immune activation, and oxidative stress, which influence vascular smooth muscle function. Vascular smooth muscle cells are highly plastic and in pathological conditions undergo phenotypic changes from a contractile to a proliferative state. Vascular smooth muscle contraction is triggered by an increase in intracellular free calcium concentration ([Ca²⁺]_i), promoting actin–myosin cross-bridge formation. Growing evidence indicates that contraction is also regulated by calcium-independent mechanisms involving RhoA-Rho kinase, protein Kinase C and mitogen-activated protein kinase signalling, reactive oxygen species, and reorganization of the actin cytoskeleton. Activation of immune/inflammatory pathways and non-coding RNAs are also emerging as important regulators of vascular function. Vascular smooth muscle cell [Ca²⁺]_i not only determines the contractile state but also influences activity of many calcium-dependent transcription factors and proteins thereby impacting the cellular phenotype and function. Perturbations in vascular smooth muscle cell signalling and altered function influence vascular reactivity and tone, important determinants of vascular resistance and blood pressure. Here, we discuss mechanisms regulating vascular reactivity and contraction in physiological and pathophysiological conditions and highlight some new advances in the field, focusing specifically on hypertension.

TIMAP inhibits endothelial myosin light chain phosphatase by competing with MYPT1 for the catalytic protein phosphatase 1 subunit PP1cβ

Transforming growth factor-β membrane associated protein (TIMAP) is an endothelial cell (EC)-predominant PP1 regulatory subunit and a member of the myosin phosphatase target (MYPT) protein family. The MYPTs preferentially bind the catalytic protein phosphatase 1 subunit PP1cβ, forming myosin phosphatase holoenzymes. We investigated whether TIMAP/PP1cβ could also function as a myosin phosphatase. Endogenous PP1cβ, myosin light chain 2 (MLC2), and myosin IIA heavy chain coimmunoprecipitated from EC lysates with endogenous TIMAP, and endogenous MLC2 colocalized with TIMAP in EC projections. Purified recombinant GST-TIMAP interacted directly with purified recombinant His-MLC2. However, TIMAP overexpression in EC enhanced MLC2 phosphorylation, an effect not observed with a TIMAP mutant that does not bind PP1cβ. Conversely, MLC2 phosphorylation was reduced in lung lysates from TIMAP-deficient mice and upon silencing of endogenous TIMAP expression in ECs. Ectopically expressed TIMAP slowed the rate of MLC2 dephosphorylation, an effect requiring TIMAP-PP1cβ interaction. The association of MYPT1 with PP1cβ was profoundly reduced in the presence of excess TIMAP, leading to proteasomal MYPT1 degradation. In the absence of TIMAP, MYPT1-associated PP1cβ readily bound immobilized microcystin-LR, an active-site inhibitor of PP1c. By contrast, TIMAP-associated PP1cβ did not interact with microcystin-LR, indicating that the active site of PP1cβ is blocked when it is bound to TIMAP. Thus, TIMAP inhibits myosin phosphatase activity in ECs by competing with MYPT1 for PP1cβ and blocking the PP1cβ active site.

Protein Serine/Threonine Phosphatases: Keys to Unlocking Regulators and Substrates

Protein serine/threonine phosphatases (PPPs) are ancient enzymes, with distinct types conserved across eukaryotic evolution. PPPs are segregated into types primarily on the basis of the unique interactions of PPP catalytic subunits with regulatory proteins. The resulting holoenzymes dock substrates distal to the active site to enhance specificity. This review focuses on the subunit and substrate interactions for PPP that depend on short linear motifs. Insights about these motifs from structures of holoenzymes open new opportunities for computational biology approaches to elucidate PPP networks. There is an expanding knowledge base of posttranslational modifications of PPP catalytic and regulatory subunits, as well as of their substrates, including phosphorylation, acetylation, and ubiquitination. Cross talk between these posttranslational modifications creates PPP-based signaling. Knowledge of PPP complexes, signaling clusters, as well as how PPPs communicate with each other in response to cellular signals should unlock the doors to PPP networks and signaling "clouds" that orchestrate and coordinate different aspects of cell physiology.

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.

Angiotensin II-Regulated Autophagy Is Required for Vascular Smooth Muscle Cell Hypertrophy

Hypertension is a disease associated to increased plasma levels of angiotensin II (Ang II). Ang II can regulate proliferation, migration, ROS production and hypertrophy of vascular smooth muscle cells (VSMCs). However, the mechanisms by which Ang II can affect VSMCs remain to be fully elucidated. In this context, autophagy, a process involved in self-digestion of proteins and organelles, has been described to regulate vascular remodeling. Therefore, we sought to investigate if Ang II regulates VSMC hypertrophy through an autophagy-dependent mechanism. To test this, we stimulated A7r5 cell line and primary rat aortic smooth muscle cells with Ang II 100 nM and measured autophagic markers at 24 h by Western blot. Autophagosomes were quantified by visualizing fluorescently labeled LC3 using confocal microscopy. The results showed that treatment with Ang II increases Beclin-1, Vps34, Atg-12-Atg5, Atg4 and Atg7 protein levels, Beclin-1 phosphorylation, as well as the number of autophagic vesicles, suggesting that this peptide induces autophagy by activating phagophore initiation and elongation. These findings were confirmed by the assessment of autophagic flux by co-administering Ang II together with chloroquine (30 μM). Pharmacological antagonism of the angiotensin type 1 receptor (AT1R) with losartan and RhoA/Rho Kinase inhibition prevented Ang II-induced autophagy. Moreover, Ang II-induced A7r5 hypertrophy, evaluated by α-SMA expression and cell size, was prevented upon autophagy inhibition. Taking together, our results suggest that the induction of autophagy by an AT1R/RhoA/Rho Kinase-dependent mechanism contributes to Ang II-induced hypertrophy in VSMC.

Augmented contractility of murine femoral arteries in a streptozotocin diabetes model is related to increased phosphorylation of MYPT1

Diabetes mellitus (DM) is a metabolic disorder with high prevalence, and a major risk factor for macro- and microvascular abnormalities. This study was undertaken to explore the mechanisms of hypercontractility of murine femoral arteries (FA) obtained from mice with streptozotocin (STZ)-induced diabetes and its relation to the phosphorylation profile of the myosin phosphatase target subunit 1, MYPT1. The immunoreactivity of MYPT1 toward phospho-MYPT1-T696, MYPT1-T853, or MYPT1-S695, used as a read out for MYPT1 phosphorylation, has been studied by Western Blotting. Contractile activity of FA from control and STZ mice has been studied by wire myography. At basal conditions (no treatment), the immunoreactivity of MYPT1-T696/T853 was ~2-fold higher in the STZ arteries compared with controls. No changes in MYPT1-T696/853 phosphorylation were observed after stimulation with the Thromboxan-A2 analog, U46619. Neither basal nor U46619-stimulated phosphorylation of MYPT1 at S695 was affected by STZ treatment. Mechanical distensibility and basal tone of FA obtained from STZ animals were similar to controls. Maximal force after treatment of FA with the contractile agonists phenylephrine (10 μmol/L) or U46619 (1 μmol/L) was augmented in the arteries of STZ mice by ~2- and ~1.5-fold, respectively. In summary, our study suggests that development of a hypercontractile phenotype in murine FA in STZ diabetes is at least partially related to an increase in phosphorylation of MLCP at MYPT1-T696/853. Interestingly, the phosphorylation at S695 site was not altered in STZ-induced diabetes, supporting the view that S695 may serve as a sensor for mechanical activity which is not directly involved in tone regulation.

Electroacupuncture Promoting Axonal Regeneration in Spinal Cord Injury Rats via Suppression of Nogo/NgR and Rho/ROCK Signaling Pathway

PURPOSE

To observe the changes of Nogo/NgR and Rho/ROCK signaling pathway-related gene and protein expression in rats with spinal cord injury (SCI) treated with electroacupuncture (EA) and to further investigate the possible mechanism of EA for treating SCI.

METHODS

Allen's method was used to create the SCI rat model. Sixty-four model rats were further subdivided into four subgroups, namely, the SCI model group (SCI), EA treatment group (EA), blocking agent Y27632 treatment group (Y27632) and EA+blocking agent Y27632 treatment group (EA+Y), according to the treatment received. The rats were subjected to EA and/or blocking agent Y27632 treatment. After 14 days, injured spinal cord tissue was extracted for analysis. The mRNA and protein expression levels were determined by real-time fluorescence quantitative PCR and Western blotting, respectively. Cell apoptosis changes in the spinal cord were evaluated by in situ hybridization. Hindlimb motor function in the rats was evaluated by Basso-Beattie-Bresnahan assessment methods.

RESULTS

Except for RhoA protein expression, compared with the SCI model group, EA, blocking agent Y27632 and EA+blocking agent Y27632 treatment groups had significantly reduced mRNA and protein expression of Nogo-A, NgR, LINGO-1, RhoA and ROCK II in spinal cord tissues, increased mRNA and protein expression of MLCP, decreased p-MYPT1 protein expression and p-MYPT1/MYPT1 ratio, and caspase3 expression, and improved lower limb movement function after treatment for 14 days (P<0.01 or <0.05). The combination of EA and the blocking agent Y27632 was superior to EA or blocking agent Y27632 treatment alone (P < 0.01 or <0.05).

CONCLUSION

EA may have an obvious inhibitory effect on the Nogo/NgR and Rho/ROCK signaling pathway after SCI, thereby reducing the inhibition of axonal growth, which may be a key mechanism of EA treatment for SCI.

Differential mechanisms of adenosine- and ATPγS-induced microvascular endothelial barrier strengthening

Maintenance of the endothelial cell (EC) barrier is critical to vascular homeostasis and a loss of barrier integrity results in increased vascular permeability. While the mechanisms that govern increased EC permeability have been under intense investigation over the past several decades, the processes regulating the preservation/restoration of the EC barrier remain poorly understood. Herein we show that the extracellular purines, adenosine (Ado) and adenosine 5'-[γ-thio]-triphosphate (ATPγS) can strengthen the barrier function of human lung microvascular EC (HLMVEC). This ability involves protein kinase A (PKA) activation and decreases in myosin light chain 20 (MLC20) phosphorylation secondary to the involvement of MLC phosphatase (MLCP). In contrast to Ado, ATPγS-induced PKA activation is accompanied by a modest, but significant decrease in cyclic adenosine monophosphate (cAMP) levels supporting the existence of an unconventional cAMP-independent pathway of PKA activation. Furthermore, ATPγS-induced EC barrier strengthening does not involve the Rap guanine nucleotide exchange factor 3 (EPAC1) which is directly activated by cAMP but is instead dependent upon PKA-anchor protein 2 (AKAP2) expression. We also found that AKAP2 can directly interact with the myosin phosphatase-targeting protein MYPT1 and that depletion of AKAP2 abolished ATPγS-induced increases in transendothelial electrical resistance. Ado-induced strengthening of the HLMVEC barrier required the coordinated activation of PKA and EPAC1 in a cAMP-dependent manner. In summary, ATPγS-induced enhancement of the EC barrier is EPAC1-independent and is instead mediated by activation of PKA which is then guided by AKAP2, in a cAMP-independent mechanism, to activate MLCP which dephosphorylates MLC20 resulting in reduced EC contraction and preservation.

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

AIM

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

METHODS

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

RESULTS

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

CONCLUSION

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

Rho kinase collaborates with p21-activated kinase to regulate actin polymerization and contraction in airway smooth muscle

KEY POINTS

The mechanisms by which Rho kinase (ROCK) regulates airway smooth muscle contraction were determined in tracheal smooth muscle tissues. ROCK may mediate smooth muscle contraction by inhibiting myosin regulatory light chain (RLC) phosphatase. ROCK can also regulate F-actin dynamics during cell migration, and actin polymerization is critical for airway smooth muscle contraction. Our results show that ROCK does not regulate airway smooth muscle contraction by inhibiting myosin RLC phosphatase or by stimulating myosin RLC phosphorylation. We find that ROCK regulates airway smooth muscle contraction by activating the serine-threonine kinase Pak, which mediates the activation of Cdc42 and neuronal Wiskott-Aldrich syndrome protein (N-WASp). N-WASP transmits signals from Cdc42 to the Arp2/3 complex for the nucleation of actin filaments. These results demonstrate a novel molecular function for ROCK in the regulation of Pak and Cdc42 activation that is critical for the processes of actin polymerization and contractility in airway smooth muscle.

ABSTRACT

Rho kinase (ROCK), a RhoA GTPase effector, can regulate the contraction of airway and other smooth muscle tissues. In some tissues, ROCK can inhibit myosin regulatory light chain (RLC) phosphatase, which increases the phosphorylation of myosin RLC and promotes smooth muscle contraction. ROCK can also regulate cell motility and migration by affecting F-actin dynamics. Actin polymerization is stimulated by contractile agonists in airway smooth muscle tissues and is required for contractile tension development in addition to myosin RLC phosphorylation. We investigated the mechanisms by which ROCK regulates the contractility of tracheal smooth muscle tissues by expressing a kinase-inactive mutant of ROCK, ROCK-K121G, in the tissues or by treating them with the ROCK inhibitor H-1152P. Our results show no role for ROCK in the regulation of non-muscle or smooth muscle myosin RLC phosphorylation during contractile stimulation in this tissue. We found that ROCK regulates airway smooth muscle contraction by mediating activation of p21-activated kinase (Pak), a serine-threonine kinase, to promote actin polymerization. Pak catalyses paxillin phosphorylation on Ser273 and coupling of the GIT1-βPIX-Pak signalling module to paxillin, which activates the guanine nucleotide exchange factor (GEF) activity of βPIX towards Cdc42. Cdc42 is required for the activation of neuronal Wiskott-Aldrich syndrome protein (N-WASp), which transmits signals from Cdc42 to the Arp2/3 complex for the nucleation of actin filaments. Our results demonstrate a novel molecular function for ROCK in the regulation of Pak and Cdc42 activation that is critical for the processes of actin polymerization and contractility in airway smooth muscle.

NADPH Oxidase 5 Is a Pro-Contractile Nox Isoform and a Point of Cross-Talk for Calcium and Redox Signaling-Implications in Vascular Function

BACKGROUND

NADPH Oxidase 5 (Nox5) is a calcium-sensitive superoxide-generating Nox. It is present in lower forms and higher mammals, but not in rodents. Nox5 is expressed in vascular cells, but the functional significance remains elusive. Given that contraction is controlled by calcium and reactive oxygen species, both associated with Nox5, we questioned the role of Nox5 in pro-contractile signaling and vascular function.

METHODS AND RESULTS

Transgenic mice expressing human Nox5 in a vascular smooth muscle cell-specific manner (Nox5 mice) and Rhodnius prolixus, an arthropod model that expresses Nox5 endogenoulsy, were studied. Reactive oxygen species generation was increased systemically and in the vasculature and heart in Nox5 mice. In Nox5-expressing mice, agonist-induced vasoconstriction was exaggerated and endothelium-dependent vasorelaxation was impaired. Vascular structural and mechanical properties were not influenced by Nox5. Vascular contractile responses in Nox5 mice were normalized by N-acetylcysteine and inhibitors of calcium channels, calmodulin, and endoplasmic reticulum ryanodine receptors, but not by GKT137831 (Nox1/4 inhibitor). At the cellular level, vascular changes in Nox5 mice were associated with increased vascular smooth muscle cell [Ca2+]i, increased reactive oxygen species and nitrotyrosine levels, and hyperphosphorylation of pro-contractile signaling molecules MLC20 (myosin light chain 20) and MYPT1 (myosin phosphatase target subunit 1). Blood pressure was similar in wild-type and Nox5 mice. Nox5 did not amplify angiotensin II effects. In R. prolixus, gastrointestinal smooth muscle contraction was blunted by Nox5 silencing, but not by VAS2870 (Nox1/2/4 inhibitor).

CONCLUSIONS

Nox5 is a pro-contractile Nox isoform important in redox-sensitive contraction. This involves calcium-calmodulin and endoplasmic reticulum-regulated mechanisms. Our findings define a novel function for vascular Nox5, linking calcium and reactive oxygen species to the pro-contractile molecular machinery in vascular smooth muscle cells.

Inhibition of human prostate smooth muscle contraction by the LIM kinase inhibitors, SR7826 and LIMKi3

BACKGROUND AND PURPOSE

In men with benign prostatic hyperplasia, increased smooth muscle tone in the prostate may lead to bladder outlet obstruction and subsequent lower urinary tract symptoms. Consequently, medical treatment aims to inhibit prostate smooth muscle contraction. However, the efficacy of the treatment options available is limited, and improved understanding of mechanisms of prostate smooth muscle contraction and identification of new targets for medical intervention are mandatory. Several studies suggest that LIM kinases (LIMKs) promote smooth muscle contraction; however, this has not yet been examined. Here, we studied effects of the LIMK inhibitors on prostate smooth muscle contraction.

EXPERIMENTAL APPROACH

Human prostate tissues were obtained from radical prostatectomy. Phosphorylation of cofilin, a LIMK substrate, was examined using a phospho-specific antibody. Smooth muscle contractions were studied in organ bath experiments.

KEY RESULTS

Real-time PCR, Western blot and immunofluorescence suggested LIMKs are expressed in smooth muscle cells of prostate tissues. Two different LIMK inhibitors, SR7826 (1 μM) and LIMKi3 (1 μM), inhibited contractions of prostate strips, which were induced by electrical field stimulation, α₁ -adrenoceptor agonists phenylephrine and methoxamine and the TXA₂ analogue, U46619. LIMK inhibition in prostate tissues and cultured stromal cells (WPMY-1) was confirmed by cofilin phosphorylation, which was reduced by SR7826 and LIMKi3. In WPMY-1 cells, SR7826 and LIMKi3 caused breakdown of actin filaments and reduced viability.

CONCLUSIONS AND IMPLICATIONS

Smooth muscle tone in the hyperplastic human prostate may underlie the effects of LIMKs, which promote contraction. Contraction of prostate strips can be inhibited by small molecule LIMK inhibitors.

Overexpression of Myosin Phosphatase Target Subunit 1 (MYPT1) Inhibits Tumor Progression and Metastasis of Gastric Cancer

Background

Myosin phosphatase target subunit 1 (MYPT1) serves as a subgroup of myosin phosphatases, and is frequently low-expressed in human cancers. However, little is known about the effects of MYPT1 in gastric cancer (GC).

Material/Methods

In our study, MYPT1 expression was detected by quantitative real-time reverse transcription PCR (qRT-PCR) in GC tissues, different advanced pathological stages of GC tissues, and preoperative and postoperative patients. Kaplan-Meier analysis was used to measure the overall survival of GC patients. MYPT1 expression was analyzed by qRT-PCR and Western blot assays in GES-1 cells and GC cells. Cell proliferation, cycle, and migration and invasion abilities were detected by CCK-8, flow cytometry, and Transwell assays. E-cadherin, TIMP-2, MMP-2, MMP-9 RhoA, and p-RhoA expressions were assessed by qRT-PCR and Western blot assays in treated SNU-5 cells.

Results

Our results indicated that MYPT1 was down-regulated in GC tissues and cells, and is related to clinical stages and overall survival of GC. Functional research demonstrated that overexpression of MYPT1 can inhibit cell proliferation, cell cycle progression, and migration and invasion of GC cells. Many studies on mechanisms reported that overexpression of MYPT1 dramatically improved the expression levels of cell cycle-related genes (Cyclin D1 and c-myc), significantly increased epithelial marker (E-cadherin) expression, and decreased invasion-associated genes (TIMP-2 and MMP-2) expressions in SNU-5 cells. In addition, we found that MYPT1 suppressed RhoA phosphorylation.

Conclusions

We verified that MYPT1 inhibits GC cell proliferation and metastasis by regulating RhoA phosphorylation.

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

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

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.

Angiotensin II, a unique vasoactive agent dissociates myosin light chain phosphorylation from contraction

Angiotensin II (100 nM) induced bi-phasic increases in cytosolic Ca²⁺ level ([Ca²⁺]_i) through the activation of angiotensin II type 1 receptor. Pharmacological examinations using 10 µM verapamil, 30 µM La³⁺, and 1 µM thapsigargin indicated that the first phase of the [Ca²⁺]_i-increase was mediated by Ca²⁺ release from sarcoplasmic reticulum (SR) and Ca²⁺ influx independently of voltage dependent Ca²⁺ channel (VDC). In contrast, the second phase of [Ca²⁺]_i-increase was mediated by Ca²⁺ influx through VDC. Although both [Ca²⁺]_i and myosin light chain (MLC)-phosphorylation at the first phase was apparently exceeded the threshold for contraction as estimated by high K⁺-induced responses, there was no appreciable contraction, indicating the dissociation between MLC phosphorylation and force during this phase. In contrast, the second phase of [Ca²⁺]_i was associated with the increases in both MLC phosphorylation and force. These results suggest that angiotensin II is a unique agonist which dissociates MLC-phosphorylation from muscle force during the Ca²⁺ releases from SR.

Metabolic Stress-Induced Activation of AMPK and Inhibition of Constitutive Phosphoproteins Controlling Smooth Muscle Contraction: Evidence for Smooth Muscle Fatigue?

Metabolic stress diminishes smooth muscle contractile strength by a poorly defined mechanism. To test the hypothesis that metabolic stress activates a compensatory cell signaling program to reversibly downregulate contraction, arterial rings and bladder muscle strips in vitro were deprived of O₂ and glucose for 30 and 60 min (“starvation”) to induce metabolic stress, and the phosphorylation status of proteins involved in regulation of contraction and metabolic stress were assessed in tissues under basal and stimulated conditions. A 15–30 min recovery period (O₂ and glucose repletion) tested whether changes induced by starvation were reversible. Starvation decreased basal phosphorylation of myosin regulatory light chain (MLC-pS19) and of the rho kinase (ROCK) downstream substrates cofilin (cofilin-pS3) and myosin phosphatase targeting subunit MYPT1 (MYPT1-pT696 and MYPT1-pT853), and abolished the ability of contractile stimuli to cause a strong, sustained contraction. Starvation increased basal phosphorylation of AMPK (AMPK-pT172) and 3 downstream AMPK substrates, acetyl-CoA carboxylase (ACC-pS79), rhoA (rhoA-pS188), and phospholamban (PLB-pS16). Increases in rhoA-pS188 and PLB-pS16 would be expected to inhibit contraction. Recovery restored basal AMPK-pT172 and MLC-pS19 to control levels, and restored contraction. In AMPKα₂ deficient mice (AMPKα2-/-), the basal level of AMPK-pT172 was reduced by 50%, and MLC-pS19 was elevated by 50%, but AMPKα2-/- did not prevent starvation-induced contraction inhibition nor enhance recovery from starvation. These results indicate that constitutive AMPK activity participates in constitutive regulation of contractile proteins, and suggest that AMPK activation is necessary, but may not be sufficient, to cause smooth muscle contraction inhibition during metabolic stress.

Physiological vs. pharmacological signalling to myosin phosphorylation in airway smooth muscle

KEY POINTS

Smooth muscle myosin regulatory light chain (RLC) is phosphorylated by Ca²⁺ /calmodulin-dependent myosin light chain kinase and dephosphorylated by myosin light chain phosphatase (MLCP). Tracheal smooth muscle contains significant amounts of myosin binding subunit 85 (MBS85), another myosin phosphatase targeting subunit (MYPT) family member, in addition to MLCP regulatory subunit MYPT1. Concentration/temporal responses to carbachol demonstrated similar sensitivities for bovine tracheal force development and phosphorylation of RLC, MYPT1, MBS85 and paxillin. Electrical field stimulation releases ACh from nerves to increase RLC phosphorylation but not MYPT1 or MBS85 phosphorylation. Thus, nerve-mediated muscarinic responses in signalling modules acting on RLC phosphorylation are different from pharmacological responses with bath added agonist. The conditional knockout of MYPT1 or the knock-in mutation T853A in mice had no effect on muscarinic force responses in isolated tracheal tissues. MLCP activity may arise from functionally shared roles between MYPT1 and MBS85, resulting in minimal effects of MYPT1 knockout on contraction.

ABSTRACT

Ca²⁺ /calmodulin activation of myosin light chain kinase (MLCK) initiates myosin regulatory light chain (RLC) phosphorylation for smooth muscle contraction with subsequent dephosphorylation for relaxation by myosin light chain phosphatase (MLCP) containing regulatory (MYPT1) and catalytic (PP1cδ) subunits. RLC phosphorylation-dependent force development is regulated by distinct signalling modules involving protein phosphorylations. We investigated responses to cholinergic agonist treatment vs. neurostimulation by electric field stimulation (EFS) in bovine tracheal smooth muscle. Concentration/temporal responses to carbachol demonstrated tight coupling between force development and RLC phosphorylation but sensitivity differences in MLCK, MYPT1 T853, MYPT1 T696, myosin binding subunit 85 (MBS85), paxillin and CPI-17 (PKC-potentiated protein phosphatase 1 inhibitor protein of 17 kDa) phosphorylations. EFS increased force and phosphorylation of RLC, CPI-17 and MLCK. In the presence of the cholinesterase inhibitor neostigmine, EFS led to an additional increase in phosphorylation of MYPT1 T853, MYPT1 T696, MBS85 and paxillin. Thus, there were distinct pharmacological vs. physiological responses in signalling modules acting on RLC phosphorylation and force responses, probably related to degenerate G protein signalling networks. Studies with genetically modified mice were performed. Expression of another MYPT1 family member, MBS85, was enriched in mouse, as well as bovine tracheal smooth muscle. Carbachol concentration/temporal-force responses were similar in trachea from MYPT1^SM+/+ , MYPT1^SM-/- and the knock-in mutant mice containing nonphosphorylatable MYPT1 T853A with no differences in RLC phosphorylation. Thus, MYPT1 T853 phosphorylation was not necessary for regulation of RLC phosphorylation in tonic airway smooth muscle. Furthermore, MLCP activity may arise from functionally shared roles between MYPT1 and MBS85, resulting in minimal effects of MYPT1 knockout on contraction.

cGMP signaling inhibits platelet shape change through regulation of the RhoA-Rho Kinase-MLC phosphatase signaling pathway

Essentials Platelet shape change requires cytoskeletal rearrangement via myosin-mediated actin contraction. We investigated whether nitric oxide (NO) affected thrombin-induced platelet shape change. NO inhibits shape change, RhoA/ROCK signalling and myosin light chain (MLC) phosphorylation. NO promotes MLC phosphatase activity, thus prevents MLC phosphorylation and shape change.

SUMMARY

Background Platelet shape change, spreading and thrombus stability require activation of the actin cytoskeleton contractile machinery. The mechanisms controlling actin assembly to prevent unwanted platelet activation are unclear. Objectives We examined the effects of nitric oxide on the signaling pathways regulating platelet actin-myosin activation. Results S-nitrosoglutathione (GSNO) inhibited thrombin-induced platelet shape change and myosin phosphorylation of the myosin light chain (MLC). Because thrombin stimulates phospho-MLC through the RhoA/ ROCK dependent inhibition of MLC phosphatase (MLCP) we examined the effects of NO on this pathway. Thrombin caused the GTP loading and activation of RhoA, leading to the ROCK-mediated phosphorylation of MLCP on threonine 853 (thr853 ), which is known to inhibit phosphatase activity. Treatment of platelets with GSNO blocked ROCK-mediated increases in phosphoMLCP-thr853 induced by thrombin. This effect was mimicked by the direct activator of protein kinase G, 8-pCPT-PET-cGMP, and blocked by the inhibition of guanylyl cyclase, but not inhibitors of protein kinase A. Further exploration of the mechanism demonstrated that GSNO stimulated the association of RhoA with protein kinase G (PKG) and the inhibitory phosphorylation (serine188) of RhoA in a cGMP-dependent manner. Consistent with these observations, in vitro experiments revealed that recombinant PKG caused direct phosphorylation of RhoA. The inhibition of RhoA by GSNO prevented ROCK-mediated phosphorylation and inhibition of MLCP activity. Conclusions These data suggest novel crosstalk between the NO-cGMP-PKG and RhoA/ROCK signaling pathways to control platelet actin remodeling.

ROCK1 is a novel Rac1 effector to regulate tubular endocytic membrane formation during clathrin-independent endocytosis

Clathrin-dependent and -independent pathways contribute for β1-integrin endocytosis. This study defines a tubular membrane clathrin-independent endocytic network, induced with the calmodulin inhibitor W13, for β1-integrin internalization. This pathway is dependent on increased phosphatidylinositol 4,5-bisphosphate (PI(4,5)P₂) levels and dynamin activity at the plasma membrane. Exogenous addition of PI(4,5)P₂ or phosphatidylinositol-4-phosphate 5-kinase (PIP5K) expression mimicked W13-generated-tubules which are inhibited by active Rac1. Therefore, the molecular mechanisms downstream of Rac1, that controls this plasma membrane tubulation, were analyzed biochemically and by the expression of different Rac1 mutants. The results indicate that phospholipase C and ROCK1 are the main Rac1 effectors that impair plasma membrane invagination and tubule formation, essentially by decreasing PI(4,5)P₂ levels and promoting cortical actomyosin assembly respectively. Interestingly, among the plethora of proteins that participate in membrane remodeling, this study revealed that ROCK1, the well-known downstream RhoA effector, has an important role in Rac1 regulation of actomyosin at the cell cortex. This study provides new insights into Rac1 functioning on plasma membrane dynamics combining phosphatidylinositides and cytoskeleton regulation.

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.

The interplay between histone deacetylases and rho kinases is important for cancer and neurodegeneration

Rho associated coiled-coil containing kinases (ROCKs) respond to defined extra- and intracellular stimuli to control cell migration, cell proliferation, and apoptosis. Histone deacetylases (HDACs) are epigenetic modifiers that regulate nuclear and cytoplasmic signaling through the deacetylation of histones and non-histone proteins. ROCK and HDAC functions are important compounds of basic and applied research interests. Recent evidence suggests a physiologically important interplay between HDACs and ROCKs in various cells and organisms. Here we summarize the crosstalk between these enzymatic families and its implications for cancer and neurodegeneration.

Activation of G protein-coupled estrogen receptor 1 induces coronary artery relaxation via Epac/Rap1-mediated inhibition of RhoA/Rho kinase pathway in parallel with PKA

Previously, we reported that cAMP/PKA signaling is involved in GPER-mediated coronary relaxation by activating MLCP via inhibition of RhoA pathway. In the current study, we tested the hypothesis that activation of GPER induces coronary artery relaxation via inhibition of RhoA/Rho kinase pathway by cAMP downstream targets, exchange proteins directly activated by cAMP (Epac) as well as PKA. Our results show that Epac inhibitors, brefeldin A (BFA, 50 μM), or ESI-09 (20 μM), or CE3F4 (100 μM), all partially inhibited porcine coronary artery relaxation response to the selective GPER agonist, G-1 (0.3–3 μM); while concurrent administration of BFA and PKI (5 μM), a PKA inhibitor, almost completely blocked the relaxation effect of G-1. The Epac specific agonist, 8-CPT-2Me-cAMP (007, 1–100 μM), induced a concentration-dependent relaxation response. Furthermore, the activity of Ras-related protein 1 (Rap1) was up regulated by G-1 (1 μM) treatment of porcine coronary artery smooth muscle cells (CASMCs). Phosphorylation of vasodilator-stimulated phosphoprotein (p-VASP) was elevated by G-1 (1 μM) treatment, but not by 007 (50 μM); and the effect of G-1 on p-VASP was blocked by PKI, but not by ESI-09, an Epac antagonist. RhoA activity was similarly down regulated by G-1 and 007, whereas ESI-09 restored most of the reduced RhoA activity by G-1 treatment. Furthermore, G-1 decreased PGF2α-induced p-MYPT1, which was partially reversed with either ESI-09 or PKI; whereas, concurrent administration of ESI-09 and PKI totally prevented the inhibitory effect of G-1. The inhibitory effects of G-1 on p- MLC levels in CASMCs were mostly restored by either ESI-09 or PKI. These results demonstrate that activation of GPER induces coronary artery relaxation via concurrent inhibition of RhoA/Rho kinase by Epac/Rap1 and PKA. GPER could be a potential drug target for preventing and treating cardiovascular diseases.

Biogenesis and activity regulation of protein phosphatase 1

Protein phosphatase 1 (PP1) is expressed in all eukaryotic cells and catalyzes a substantial fraction of phosphoserine/threonine dephosphorylation reactions. It forms stable complexes with PP1-interacting proteins (PIPs) that guide the phosphatase throughout its life cycle and control its fate and function. The diversity of PIPs is huge (≈200 in vertebrates), and most of them combine short linear motifs to form large and unique interaction interfaces with PP1. Many PIPs have separate domains for PP1 anchoring, PP1 regulation, substrate recruitment and subcellular targeting, which enable them to direct associated PP1 to a specific subset of substrates and mediate acute activity control. Hence, PP1 functions as the catalytic subunit of a large number of multimeric holoenzymes, each with its own subset of substrates and mechanism(s) of regulation.

Role of myosin light chain phosphatase in cardiac physiology and pathophysiology

Maintenance of contractile performance of the heart is achieved in part by the constitutive 40% phosphorylation of myosin regulatory light chain (RLC) in sarcomeres. The importance of this extent of RLC phosphorylation for optimal cardiac performance becomes apparent when various mouse models and resultant phenotypes are compared. The absence or attenuation of RLC phosphorylation results in poor performance leading to heart failure, whereas increased RLC phosphorylation is associated with cardiac protection from stresses. Although information is limited, RLC phosphorylation appears compromised in human heart failure which is consistent with data from mouse studies. The extent of cardiac RLC phosphorylation is determined by the balanced activities of cardiac myosin light chain kinases and phosphatases, the regulatory mechanisms of which are now emerging. This review thusly focuses on kinases that may participate in phosphorylating RLC to make the substrate for cardiac myosin light chain phosphatases, in addition to providing perspectives on the family of myosin light chain phosphatases and involved signaling mechanisms. Because biochemical and physiological information about cardiac myosin light chain phosphatase is sparse, such studies represent an emerging area of investigation in health and disease.

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.

Made to measure - keeping Rho kinase at a distance

The Rho-associated coiled-coil containing kinases (ROCK) were first identified as effectors of the small GTPase RhoA, hence their nomenclature. Since their discovery, two decades ago, scientists have sought to unravel the structure, regulation, and function of these essential kinases. During that time, a consensus model has formed, in which ROCK activity is regulated via both Rho-dependent and independent mechanisms. However, recent findings have raised significant questions regarding this model. In their recent publication in Nature Communications, Truebestein and colleagues present the structure of a full-length Rho kinase for the first time. In contrast to previous reports, the authors could find no evidence for autoinhibition, RhoA binding, or regulation of kinase activity by phosphorylation. Instead, they propose that ROCK functions as a molecular ruler, in which the central coiled-coil bridges the membrane-binding regulatory domains to the kinase domains at a fixed distance from the plasma membrane. Here, we explore the consequences of the new findings, re-examine old data in the context of this model, and emphasize outstanding questions in the field.

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.