220- and 130-kDa MLCKs have distinct tissue distributions and intracellular localization patterns

Molecular mechanism of MLCK1 inducing 5-Fu resistance in colorectal cancer cells through activation of TNFR2/NF-κB pathway

BACKGROUND AND AIMS

Chemotherapy resistance in colorectal cancer have been faced with significant challenges in recent years. Particular interest is directed to tumor microenvironment function. Recent work has, identified a small molecule named Divertin that prevents myosin light chain kinase 1(MLCK1) recruitment to the perijunctional actomyosin ring(PAMR), restores barrier function after tumor necrosis factor(TNF)-induced barrier loss and prevents disease progression in experimental inflammatory bowel disease. Studies have shown that MLCK is a potential target for affecting intestinal barrier function, as well as for tumor therapy. However, the relative contributions of MLCK expression and chemotherapy resistance in colorectal cancers have not been defined.

METHODS

Statistical analysis of MYLK gene expression differences in colorectal cancer patients and normal population and prognosis results from The Cancer Genome Atlas(TCGA) data. Cell activity was detected by Cell counting Kit-8. Cell proliferation was detected by monoclonal plate. The apoptosis was detected by flow cytometry and western blot. Determine the role of MLCK1 in inducing 5-Fluorouracil(5-Fu) resistance in colorectal cancer cells was detected by overexpression of MLCK1 and knock-down expression of MLCK1.

RESULTS

MLCK1 is expressed at different levels in different colorectal cancer cells, high MLCK1 expressing cell lines are less sensitive to 5-Fu, and low MLCK1 expressing cell lines are more sensitive to 5-Fu. MLCK1 high expression enhances resistance to 5-Fu in colorectal cancer cells and the sensitivity to 5-Fu was increased after knocking down the expression of MLCK1, that might be closely correlated to TNFR2/NF-κB pathway.

CONCLUSIONS

MLCK1 high expression can enhance resistance to 5-Fu in colorectal cancer cells and the sensitivity to 5-Fu was increased after knocking down the expression of MLCK1, that might be closely correlated to TNFR2/NF-κB pathway, which will provide a new method for the treatment of colorectal cancer patients who are resistant to 5-Fu chemotherapy.

Myosin Light-Chain Kinase Inhibition Potentiates the Antitumor Effects of Avapritinib in PDGFRA D842V-Mutant Gastrointestinal Stromal Tumor

PURPOSE

To create an in vivo model of PDGFRA D842V-mutant gastrointestinal stromal tumor (GIST) and identify the mechanism of tumor persistence following avapritinib therapy.

EXPERIMENTAL DESIGN

We created a patient-derived xenograft (PDX) of PDGFRA D842V-mutant GIST and tested the effects of imatinib, avapritinib, and ML-7, an inhibitor of myosin light-chain kinase (MYLK). Bulk tumor RNA sequencing and oncogenic signaling were evaluated. Apoptosis, survival, and actin cytoskeleton were evaluated in GIST T1 cells and isolated PDX cells in vitro. Human GIST specimens were analyzed for MYLK expression.

RESULTS

The PDX was minimally responsive to imatinib but sensitive to avapritinib. Avapritinib therapy increased tumor expression of genes related to the actin cytoskeleton, including MYLK. ML-7 induced apoptosis and disrupted actin filaments in short-term cultures of PDX cells and decreased survival in GIST T1 cells in combination with imatinib or avapritinib. Combined therapy with ML-7 improved the antitumor effects of low-dose avapritinib in vivo. Furthermore, MYLK was expressed in human GIST specimens.

CONCLUSIONS

MYLK upregulation is a novel mechanism of tumor persistence after tyrosine kinase inhibition. Concomitant MYLK inhibition may enable the use of a lower dose of avapritinib, which is associated with dose-dependent cognitive side effects.

Tight junctions: from molecules to gastrointestinal diseases

Intestinal epithelium functions as a tissue barrier to prevent interaction between the internal compartment and the external milieu. Intestinal barrier function also determines epithelial polarity for the absorption of nutrients and the secretion of waste products. These vital functions require strong integrity of tight junction proteins. In fact, intestinal tight junctions that seal the paracellular space can restrict mucosal-to-serosal transport of hostile luminal contents. Tight junctions can form both an absolute barrier and a paracellular ion channel. Although defective tight junctions potentially lead to compromised intestinal barrier and the development and progression of gastrointestinal (GI) diseases, no FDA-approved therapies that recover the epithelial tight junction barrier are currently available in clinical practice. Here, we discuss the impacts and regulatory mechanisms of tight junction disruption in the gut and related diseases. We also provide an overview of potential therapeutic targets to restore the epithelial tight junction barrier in the GI tract.

CARMN Is an Evolutionarily Conserved Smooth Muscle Cell-Specific LncRNA That Maintains Contractile Phenotype by Binding Myocardin

BACKGROUND

Vascular homeostasis is maintained by the differentiated phenotype of vascular smooth muscle cells (VSMCs). The landscape of protein coding genes comprising the transcriptome of differentiated VSMCs has been intensively investigated but many gaps remain including the emerging roles of noncoding genes.

METHODS

We reanalyzed large-scale, publicly available bulk and single-cell RNA sequencing datasets from multiple tissues and cell types to identify VSMC-enriched long noncoding RNAs. The in vivo expression pattern of a novel smooth muscle cell (SMC)-expressed long noncoding RNA, Carmn (cardiac mesoderm enhancer-associated noncoding RNA), was investigated using a novel Carmn green fluorescent protein knock-in reporter mouse model. Bioinformatics and quantitative real-time polymerase chain reaction analysis were used to assess CARMN expression changes during VSMC phenotypic modulation in human and murine vascular disease models. In vitro, functional assays were performed by knocking down CARMN with antisense oligonucleotides and overexpressing Carmn by adenovirus in human coronary artery SMCs. Carotid artery injury was performed in SMC-specific Carmn knockout mice to assess neointima formation and the therapeutic potential of reversing CARMN loss was tested in a rat carotid artery balloon injury model. The molecular mechanisms underlying CARMN function were investigated using RNA pull-down, RNA immunoprecipitation, and luciferase reporter assays.

RESULTS

We identified CARMN, which was initially annotated as the host gene of the MIR143/145 cluster and recently reported to play a role in cardiac differentiation, as a highly abundant and conserved, SMC-specific long noncoding RNA. Analysis of the Carmn GFP knock-in mouse model confirmed that Carmn is transiently expressed in embryonic cardiomyocytes and thereafter becomes restricted to SMCs. We also found that Carmn is transcribed independently of Mir143/145. CARMN expression is dramatically decreased by vascular disease in humans and murine models and regulates the contractile phenotype of VSMCs in vitro. In vivo, SMC-specific deletion of Carmn significantly exacerbated, whereas overexpression of Carmn markedly attenuated, injury-induced neointima formation in mouse and rat, respectively. Mechanistically, we found that Carmn physically binds to the key transcriptional cofactor myocardin, facilitating its activity and thereby maintaining the contractile phenotype of VSMCs.

CONCLUSIONS

CARMN is an evolutionarily conserved SMC-specific long noncoding RNA with a previously unappreciated role in maintaining the contractile phenotype of VSMCs and is the first noncoding RNA discovered to interact with myocardin.

Postnatal development alters functional compartmentalization of myosin light chain kinase in ovine carotid arteries

The rate-limiting enzyme for vascular contraction, myosin light chain kinase (MLCK), phosphorylates regulatory myosin light chain (MLC₂₀) at rates that appear faster despite lower MLCK abundance in fetal compared with adult arteries. This study explores the hypothesis that greater apparent tissue activity of MLCK in fetal arteries is due to age-dependent differences in intracellular distribution of MLCK in relation to MLC₂₀. Under optimal conditions, common carotid artery homogenates from nonpregnant adult female sheep and near-term fetuses exhibited similar values of V_max and K_m for MLCK. A custom-designed, computer-controlled apparatus enabled electrical stimulation and high-speed freezing of arterial segments at exactly 0, 1, 2, and 3 s, calculation of in situ rates of MLC₂₀ phosphorylation, and measurement of time-dependent colocalization between MLCK and MLC₂₀. The in situ rate of MLC₂₀ phosphorylation divided by total MLCK abundance averaged to values 147% greater in fetal (1.06 ± 0.28) than adult (0.43 ± 0.08) arteries, which corresponded, respectively, to 43 ± 10% and 31 ± 3% of the V_max values measured in homogenates. Confocal colocalization analysis revealed in fetal and adult arteries that 33 ± 6% and 20 ± 5% of total MLCK colocalized with pMLC₂₀, and that MLCK activation was greater in periluminal than periadventitial regions over the time course of electrical stimulation in both age groups. Together, these results demonstrate that the catalytic activity of MLCK is similar in fetal and adult arteries, but that the fraction of total MLCK in the functional compartment involved in contraction is significantly greater in fetal than adult arteries.

The Calcium Signaling Mechanisms in Arterial Smooth Muscle and Endothelial Cells

The contractile state of resistance arteries and arterioles is a crucial determinant of blood pressure and blood flow. Physiological regulation of arterial contractility requires constant communication between endothelial and smooth muscle cells. Various Ca²⁺ signals and Ca²⁺ -sensitive targets ensure dynamic control of intercellular communications in the vascular wall. The functional effect of a Ca²⁺ signal on arterial contractility depends on the type of Ca²⁺ -sensitive target engaged by that signal. Recent studies using advanced imaging methods have identified the spatiotemporal signatures of individual Ca²⁺ signals that control arterial and arteriolar contractility. Broadly speaking, intracellular Ca²⁺ is increased by ion channels and transporters on the plasma membrane and endoplasmic reticular membrane. Physiological roles for many vascular Ca²⁺ signals have already been confirmed, while further investigation is needed for other Ca²⁺ signals. This article focuses on endothelial and smooth muscle Ca²⁺ signaling mechanisms in resistance arteries and arterioles. We discuss the Ca²⁺ entry pathways at the plasma membrane, Ca²⁺ release signals from the intracellular stores, the functional and physiological relevance of Ca²⁺ signals, and their regulatory mechanisms. Finally, we describe the contribution of abnormal endothelial and smooth muscle Ca²⁺ signals to the pathogenesis of vascular disorders. © 2021 American Physiological Society. Compr Physiol 11:1831-1869, 2021.

Corticosteroid enhances epithelial barrier function in intestinal organoids derived from patients with Crohn's disease

Abstract

Corticosteroids (CS), first-line therapeutics for Crohn’s disease (CD) with moderate or severe disease activity, were found to restore intestinal permeability in CD patients, whereas the underlying molecular events are still largely unknown. This study aimed to investigate the effect and mechanisms of CS prednisolone on epithelial barrier using CD patient-derived intestinal organoids. 3D intestinal organoids were generated from colon biopsies of inactive CD patients. To mimic the inflammatory microenvironment, a mixture of cytokines containing TNF-α, IFN-γ, and IL-1β were added to the organoid culture with or without pre-incubation of prednisolone or mifepristone. Epithelial permeability of the organoids was assessed by FITC-D4 flux from the basal to luminal compartment using confocal microscopy. Expression of junctional components were analyzed by qRT-PCR, immunofluorescence staining, and western blot. Activity of signaling pathways were analyzed using western blot. Exposure of the cytokines significantly disrupted epithelial barrier of the intestinal organoids, which was partially restored by prednisolone. On the molecular level, the cytokine mixture resulted in a significant reduction in E-cadherin and ILDR-1, an increase in CLDN-2, MLCK, and STAT1 phosphorylation, whereas prednisolone ameliorated the abovementioned effects induced by the cytokine mixture. This study demonstrates that prednisolone confers a direct effect in tightening the epithelial barrier, identifies novel junctional targets regulated by prednisolone, and underscores intestinal barrier restoration as a potential mechanism that contributes to the clinical efficacy of prednisolone in CD patients.

Key messages

Prednisolone confers a direct preventive effect against cytokine-induced barrier dysfunction.

Prednisolone regulates the expression of CLDN-2, E-cadherin, and ILDR-1.

The effect of prednisolone is GR-, MLCK-, and STAT1-dependent.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00109-021-02045-7.

Noncanonical function of long myosin light chain kinase in increasing ER-PM junctions and augmentation of SOCE

Increased endothelial permeability leads to excessive exudation of plasma proteins and leukocytes in the interstitium, which characterizes several vascular diseases including acute lung injury. The myosin light chain kinase long (MYLK-L) isoform is canonically known to regulate the endothelial permeability by phosphorylating myosin light chain (MLC-P). Compared to the short MYLK isoform, MYLK-L contains an additional stretch of ~919 amino acid at the N-terminus of unknown function. We show that thapsigargin and thrombin-induced SOCE was markedly reduced in Mylk-L-/- endothelial cells (EC) or MYLK-L-depleted human EC. These agonists also failed to increase endothelial permeability in MYLK-L-depleted EC and Mylk-L-/- lungs, thus demonstrating the novel role of MYLK-L-induced SOCE in increasing vascular permeability. MYLK-L augmented SOCE by increasing endoplasmic reticulum (ER)-plasma membrane (PM) junctions and STIM1 translocation to these junctions. Transduction of N-MYLK domain (amino acids 1-919 devoid of catalytic activity) into Mylk-L-/- EC rescued SOCE to the level seen in control EC in a STIM1-dependent manner. N-MYLK-induced SOCE augmented endothelial permeability without MLC-P via an actin-binding motif, DVRGLL. Liposomal-mediated delivery of N-MYLK mutant but not ∆DVRGLL-N-MYLK mutant in Mylk-L-/- mice rescued vascular permeability increase in response to endotoxin, indicating that targeting of DVRGLL motif within MYLK-L may limit SOCE-induced vascular hyperpermeability.

The Regulation of Intestinal Mucosal Barrier by Myosin Light Chain Kinase/Rho Kinases

The intestinal epithelial apical junctional complex, which includes tight and adherens junctions, contributes to the intestinal barrier function via their role in regulating paracellular permeability. Myosin light chain II (MLC-2), has been shown to be a critical regulatory protein in altering paracellular permeability during gastrointestinal disorders. Previous studies have demonstrated that phosphorylation of MLC-2 is a biochemical marker for perijunctional actomyosin ring contraction, which increases paracellular permeability by regulating the apical junctional complex. The phosphorylation of MLC-2 is dominantly regulated by myosin light chain kinase- (MLCK-) and Rho-associated coiled-coil containing protein kinase- (ROCK-) mediated pathways. In this review, we aim to summarize the current state of knowledge regarding the role of MLCK- and ROCK-mediated pathways in the regulation of the intestinal barrier during normal homeostasis and digestive diseases. Additionally, we will also suggest potential therapeutic targeting of MLCK- and ROCK-associated pathways in gastrointestinal disorders that compromise the intestinal barrier.

Absolute expressions of hypoxia-inducible factor-1 alpha (HIF1A) transcript and the associated genes in chicken skeletal muscle with white striping and wooden breast myopathies

Development of white striping (WS) and wooden breast (WB) in broiler breast meat have been linked to hypoxia, but their etiologies are not fully understood. This study aimed at investigating absolute expression of hypoxia-inducible factor-1 alpha subunit (HIF1A) and genes involved in stress responses and muscle repair using a droplet digital polymerase chain reaction. Total RNA was isolated from pectoralis major collected from male 6-week-old medium (carcass weight ≤ 2.5 kg) and heavy (carcass weight > 2.5 kg) broilers. Samples were classified as “non-defective” (n = 4), “medium-WS” (n = 6), “heavy-WS” (n = 7) and “heavy-WS+WB” (n = 3) based on abnormality scores. The HIF1A transcript was up-regulated in all of the abnormal groups. Transcript abundances of genes encoding 6-phosphofructo-2-kinase/fructose-2,6-biphosphatase 4 (PFKFB4), lactate dehydrogenase-A (LDHA), and phosphorylase kinase beta subunit (PHKB) were increased in heavy-WS but decreased in heavy-WS+WB. Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) was up-regulated in non-defective samples. The muscle-specific mu-2 isoform of glutathione S-transferases (GSTM2) was up-regulated in the abnormal samples, particularly in the heavy groups. The genes encoding myogenic differentiation (MYOD1) and myosin light chain kinase (MYLK) exhibited similar expression pattern, of which medium-WS and heavy-WS significantly increased compared to non-defective whereas expression in heavy-WS+WB was not different from either non-defective or WS-affected group. The greatest and the lowest levels of calpain-3 (CAPN3) and delta-sarcoglycan (SCGD) were observed in heavy-WS and heavy-WS+WB, respectively. Based on micrographs, the abnormal muscles primarily comprised fibers with cross-sectional areas ranging from 2,000 to 3,000 μm². Despite induced glycolysis at the transcriptional level, lower stored glycogen in the abnormal muscles corresponded with the reduced lactate and higher pH within their meats. The findings support hypoxia within the abnormal breasts, potentially associated with oversized muscle fibers. Between WS and WB, divergent glucose metabolism, cellular detoxification and myoregeneration at the transcriptional level could be anticipated.

Intracellular MLCK1 diversion reverses barrier loss to restore mucosal homeostasis

Epithelial barrier loss is a driver of intestinal and systemic diseases. Myosin light chain kinase (MLCK) is a key effector of barrier dysfunction and a potential therapeutic target, but enzymatic inhibition has unacceptable toxicities. Here, we show that a unique domain within the MLCK splice-variant MLCK1 directs perijunctional actomyosin ring (PAMR) recruitment. Using the domain structure and multiple screens, we identified a domain-binding small molecule (Divertin) that blocks MLCK1 recruitment without inhibiting enzymatic function. Divertin blocks acute, TNF-induced MLCK1 recruitment as well as downstream MLC phosphorylation, barrier loss, and diarrhea in vitro and in vivo. Divertin corrects barrier dysfunction and prevents disease development and progression in experimental inflammatory bowel disease. Beyond applications of Divertin in gastrointestinal disease, this general approach to enzymatic inhibition by preventing access to specific subcellular sites provides a new paradigm for safely and precisely targeting individual properties of enzymes with multiple functions.

MYLK promotes hepatocellular carcinoma progression through regulating cytoskeleton to enhance epithelial-mesenchymal transition

Myosin light chain kinase (MYLK) is found to catalyze the phosphorylation of myosin light chains (MLC) and regulate invasion and metastasis in some malignancies. However, there is little knowledge on the role of MYLK in hepatocellular carcinoma (HCC), and no studies have been conducted to investigate the mechanisms underlying MYLK-mediated promotion of HCC invasion and metastasis until now. In this study, we investigated the expression of MYLK in 50 pairs of human HCC and adjacent liver specimens. High MYLK expression was significantly correlated with aggressive clinicopathological features including tumor encapsulation, microvascular invasion and metastasis. In vitro assays showed that shRNA-induced MYLK knockdown significantly inhibited the wound-healing ability of HCC cells and the ability to migrate and invade through Matrigel. We next uncovered that MYLK knockdown resulted in a reduction in the number of F-actin stress fibers, disorganization of F-actin architectures and morphological alterations of HCC cells. Phosphorylated MLC, rather than total MLC, was found to be markedly reduced in response to downregulation of MYLK expression, and MYLK-regulated actin cytoskeleton through phosphorylating MLC in HCC cells. In addition, Western blotting assay revealed downregulation of the epithelial marker E-cadherin and upregulation of mesenchymal markers Vimentin, N-cadherin and Snail. Taken together, our findings indicate that MYLK promotes HCC progression by altering cytoskeleton to enhance epithelial-mesenchymal transition (EMT).

Activation of ROCK and MLCK tunes regional stress fiber formation and mechanics via preferential myosin light chain phosphorylation

Graded induction of regulatory light chain (RLC) activators MLCK and ROCK were used to explore the relationship between RLC phosphorylation and actin-myosin stress fiber viscoelasticity. MLCK controls peripheral stress fiber mechanics by monophosphorylation of RLC, whereas ROCK acts on central stress fibers via diphosphorylation.

The assembly and mechanics of actomyosin stress fibers (SFs) depend on myosin regulatory light chain (RLC) phosphorylation, which is driven by myosin light chain kinase (MLCK) and Rho-associated kinase (ROCK). Although previous work suggests that MLCK and ROCK control distinct pools of cellular SFs, it remains unclear how these kinases differ in their regulation of RLC phosphorylation or how phosphorylation influences individual SF mechanics. Here, we combine genetic approaches with biophysical tools to explore relationships between kinase activity, RLC phosphorylation, SF localization, and SF mechanics. We show that graded MLCK overexpression increases RLC monophosphorylation (p-RLC) in a graded manner and that this p-RLC localizes to peripheral SFs. Conversely, graded ROCK overexpression preferentially increases RLC diphosphorylation (pp-RLC), with pp-RLC localizing to central SFs. Interrogation of single SFs with subcellular laser ablation reveals that MLCK and ROCK quantitatively regulate the viscoelastic properties of peripheral and central SFs, respectively. The effects of MLCK and ROCK on single-SF mechanics may be correspondingly phenocopied by overexpression of mono- and diphosphomimetic RLC mutants. Our results point to a model in which MLCK and ROCK regulate peripheral and central SF viscoelastic properties through mono- and diphosphorylation of RLC, offering new quantitative connections between kinase activity, RLC phosphorylation, and SF viscoelasticity.

Chronic hypoxia alters fetal cerebrovascular responses to endothelin-1

In utero hypoxia influences the structure and function of most fetal arteries, including those of the developing cerebral circulation. Whereas the signals that initiate this hypoxic remodeling remain uncertain, these appear to be distinct from the mechanisms that maintain the remodeled vascular state. The present study explores the hypothesis that chronic hypoxia elicits sustained changes in fetal cerebrovascular reactivity to endothelin-1 (ET-1), a potent vascular contractant and mitogen. In fetal lambs, chronic hypoxia (3,820-m altitude for the last 110 days of gestation) had no significant effect on plasma ET-1 levels or ETA receptor density in cerebral arteries but enhanced contractile responses to ET-1 in an ETA-dependent manner. In organ culture (24 h), 10 nM ET-1 increased medial thicknesses less in hypoxic than in normoxic arteries, and these increases were ablated by inhibition of PKC (chelerythrine) in both normoxic and hypoxic arteries but were attenuated by inhibition of CaMKII (KN93) and p38 (SB203580) in normoxic but not hypoxic arteries. As indicated by Ki-67 immunostaining, ET-1 increased medial thicknesses via hypertrophy. Measurements of colocalization between MLCK and SMαA revealed that organ culture with ET-1 also promoted contractile dedifferentiation in normoxic, but not hypoxic, arteries through mechanisms attenuated by inhibitors of PKC, CaMKII, and p38. These results support the hypothesis that chronic hypoxia elicits sustained changes in fetal cerebrovascular reactivity to ET-1 through pathways dependent upon PKC, CaMKII, and p38 that cause increased ET-1-mediated contractility, decreased ET-1-mediated smooth muscle hypertrophy, and a depressed ability of ET-1 to promote contractile dedifferentiation.

Myosin Light Chain Kinase: A Potential Target for Treatment of Inflammatory Diseases

Myosin light chain kinase (MLCK) induces contraction of the perijunctional apical actomyosin ring in response to phosphorylation of the myosin light chain. Abnormal expression of MLCK has been observed in respiratory diseases, pancreatitis, cardiovascular diseases, cancer, and inflammatory bowel disease. The signaling pathways involved in MLCK activation and triggering of endothelial barrier dysfunction are discussed in this review. The pharmacological effects of regulating MLCK expression by inhibitors such as ML-9, ML-7, microbial products, naturally occurring products, and microRNAs are also discussed. The influence of MLCK in inflammatory diseases starts with endothelial barrier dysfunction. The effectiveness of anti-MLCK treatment may depend on alleviation of that primary pathological mechanism. This review summarizes evidence for the potential benefits of anti-MLCK agents in the treatment of inflammatory disease and the importance of avoiding treatment-related side effects, as MLCK is widely expressed in many different tissues.

Myosin Light Chain Kinase MYLK1: Anatomy, Interactions, Functions, and Regulation

This review discusses and summarizes the results of molecular and cellular investigations of myosin light chain kinase (MLCK, MYLK1), the key regulator of cell motility. The structure and regulation of a complex mylk1 gene and the domain organization of its products is presented. The interactions of the mylk1 gene protein products with other proteins and posttranslational modifications of the mylk1 gene protein products are reviewed, which altogether might determine the role and place of MLCK in physiological and pathological reactions of cells and entire organisms. Translational potential of MLCK as a drug target is evaluated.

Cytoskeletal signaling in TGFβ-induced epithelial-mesenchymal transition

Epithelial-mesenchymal transition (EMT) is a physiological process that plays an important role in embryonic development and wound healing and is appropriated during pathological conditions including fibrosis and cancer metastasis. EMT can be initiated by a variety of factors, including transforming growth factor (TGF)-β, and is characterized by loss of epithelial features including cell-cell contacts and apicobasal polarity and acquisition of a motile, mesenchymal phenotype. A key feature of EMT is reorganization of the cytoskeleton and recent studies have elucidated regulation mechanisms governing this process. This review describes changes in gene expression patterns of cytoskeletal associated proteins during TGFβ-induced EMT. It further reports TGFβ-induced intracellular signaling cascades that regulate cytoskeletal reorganization during EMT. Finally, it highlights how changes in cytoskeletal architecture during EMT can regulate gene expression, thus further promoting EMT progression.

Capsaicin alleviates abnormal intestinal motility through regulation of enteric motor neurons and MLCK activity: Relevance to intestinal motility disorders

SCOPE

Capsaicin is an active component of chili peppers, having diverse effects. However, the effects of capsaicin on intestinal motility are still controversial. The present study aimed to investigate the effects of capsaicin on intestinal motility disorder and uncover related mechanisms.

MATERIALS AND RESULTS

A rat model with intestinal motility disorder was established in vitro through adding different stimuli into tissue bath; in vivo using constipation and diarrhea model, respectively. Capsaicin exerted dual effects on intestinal motility, i.e. the relaxation and contraction of jejunum induced by corresponding stimulus were, respectively, regulated to be normal contraction by capsaicin. The mechanisms underlined capsaicin-induced dual effects were investigated using Western blotting, qRT-PCR, and whole-cell patch clamp, respectively. Results showed that cholinergic excitatory nerves, adrenergic nerves, and neurons containing nitric oxide synthase, which are the main muscle motor neurons in enteric nervous system (ENS), are involved in capsaicin-induced dual effects. The competition for regulation of Ca(2+) influx by capsaicin induced the interaction with components of the ENS. Capsaicin significantly increased myosin light chain kinase (MLCK) expression and myosin phosphorylation extent in jejunal segments of constipation-prominent rats and significantly decreased MLCK expression and myosin phosphorylation extent in jejunal segments of diarrhea-prominent rats.

CONCLUSION

In summary, capsaicin alleviates abnormal intestinal motility through regulating enteric motor neurons and MLCK activity, which is beneficial for the treatment of gastrointestinal motility disorders.

Myosin light chain kinase (MLCK) regulates cell migration in a myosin regulatory light chain phosphorylation-independent mechanism

Myosin light chain kinase (MLCK) has long been implicated in the myosin phosphorylation and force generation required for cell migration. Here, we surprisingly found that the deletion of MLCK resulted in fast cell migration, enhanced protrusion formation, and no alteration of myosin light chain phosphorylation. The mutant cells showed reduced membrane tether force and fewer membrane F-actin filaments. This phenotype was rescued by either kinase-dead MLCK or five-DFRXXL motif, a MLCK fragment with potent F-actin-binding activity. Pull-down and co-immunoprecipitation assays showed that the absence of MLCK led to attenuated formation of transmembrane complexes, including myosin II, integrins and fibronectin. We suggest that MLCK is not required for myosin phosphorylation in a migrating cell. A critical role of MLCK in cell migration involves regulating the cell membrane tension and protrusion necessary for migration, thereby stabilizing the membrane skeleton through F-actin-binding activity. This finding sheds light on a novel regulatory mechanism of protrusion during cell migration.

Myosins 1 and 6, myosin light chain kinase, actin and microtubules cooperate during antibody-mediated internalisation and trafficking of membrane-expressed viral antigens in feline infectious peritonitis virus infected monocytes

Monocytes infected with feline infectious peritonitis virus, a coronavirus, express viral proteins in their plasma membranes. Upon binding of antibodies, these proteins are quickly internalised through a new clathrin- and caveolae-independent internalisation pathway. By doing so, the infected monocytes can escape antibody-dependent cell lysis. In the present study, we investigated which kinases and cytoskeletal proteins are of importance during internalisation and subsequent intracellular transport. The experiments showed that myosin light chain kinase (MLCK) and myosin 1 are crucial for the initiation of the internalisation. With co-localisation stainings, it was found that MLCK and myosin 1 co-localise with antigens even before internalisation started. Myosin 6 co-localised with the internalising complexes during passage through the cortical actin, were it might play a role in moving or disintegrating actin filaments, to overcome the actin barrier. One minute after internalisation started, vesicles had passed the cortical actin, co-localised with microtubules and association with myosin 6 was lost. The vesicles were further transported over the microtubules and accumulated at the microtubule organising centre after 10 to 30 min. Intracellular trafficking over microtubules was mediated by MLCK, myosin 1 and a small actin tail. Since inhibiting MLCK with ML-7 was so efficient in blocking the internalisation pathway, this target can be used for the development of a new treatment for FIPV.

Regulation of 130-kDa smooth muscle myosin light chain kinase expression by an intronic CArG element

The mylk1 gene encodes a 220-kDa nonmuscle myosin light chain kinase (MLCK), a 130-kDa smooth muscle MLCK (smMLCK), as well as the non-catalytic product telokin. Together, these proteins play critical roles in regulating smooth muscle contractility. Changes in their expression are associated with many pathological conditions; thus, it is important to understand the mechanisms regulating expression of mylk1 gene transcripts. Previously, we reported a highly conserved CArG box, which binds serum response factor, in intron 15 of mylk1. Because this CArG element is near the promoter that drives transcription of the 130-kDa smMLCK, we examined its role in regulating expression of this transcript. Results show that deletion of the intronic CArG region from a β-galactosidase reporter gene abolished transgene expression in mice in vivo. Deletion of the CArG region from the endogenous mylk1 gene, specifically in smooth muscle cells, decreased expression of the 130-kDa smMLCK by 40% without affecting expression of the 220-kDa MLCK or telokin. This reduction in 130-kDa smMLCK expression resulted in decreased phosphorylation of myosin light chains, attenuated smooth muscle contractility, and a 24% decrease in small intestine length that was associated with a significant reduction of Ki67-positive smooth muscle cells. Overall, these data show that the CArG element in intron 15 of the mylk1 gene is necessary for maximal expression of the 130-kDa smMLCK and that the 130-kDa smMLCK isoform is specifically required to regulate smooth muscle contractility and small intestine smooth muscle cell proliferation.

Contribution of increased VEGF receptors to hypoxic changes in fetal ovine carotid artery contractile proteins

Recent studies suggest that vascular endothelial growth factor (VEGF) can modulate smooth muscle phenotype and, consequently, the composition and function of arteries upstream from the microcirculation, where angiogenesis occurs. Given that hypoxia potently induces VEGF, the present study explores the hypothesis that, in fetal arteries, VEGF contributes to hypoxic vascular remodeling through changes in abundance, organization, and function of contractile proteins. Pregnant ewes were acclimatized at sea level or at altitude (3,820 m) for the final 110 days of gestation. Endothelium-denuded carotid arteries from full-term fetuses were used fresh or after 24 h of organ culture in a physiological concentration (3 ng/ml) of VEGF. After 110 days, hypoxia had no effect on VEGF abundance but markedly increased abundance of the Flk-1 (171%) and Flt-1 (786%) VEGF receptors. Hypoxia had no effect on smooth muscle α-actin (SMαA), decreased myosin light chain (MLC) kinase (MLCK), and increased 20-kDa regulatory MLC (MLC(20)) abundances. Hypoxia also increased MLCK-SMαA, MLC(20)-SMαA, and MLCK-MLC(20) colocalization. Compared with hypoxia, organ culture with VEGF produced the same pattern of changes in contractile protein abundance and colocalization. Effects of VEGF on colocalization were blocked by the VEGF receptor antagonists vatalanib (240 nM) and dasatinib (6.3 nM). Thus, through increases in VEGF receptor density, hypoxia can recruit VEGF to help mediate remodeling of fetal arteries upstream from the microcirculation. The results support the hypothesis that VEGF contributes to hypoxic vascular remodeling through changes in abundance, organization, and function of contractile proteins.

Cell population-specific expression analysis of human cerebellum

Background

Interpreting gene expression profiles obtained from heterogeneous samples can be difficult because bulk gene expression measures are not resolved to individual cell populations. We have recently devised Population-Specific Expression Analysis (PSEA), a statistical method that identifies individual cell types expressing genes of interest and achieves quantitative estimates of cell type-specific expression levels. This procedure makes use of marker gene expression and circumvents the need for additional experimental information like tissue composition.

Results

To systematically assess the performance of statistical deconvolution, we applied PSEA to gene expression profiles from cerebellum tissue samples and compared with parallel, experimental separation methods. Owing to the particular histological organization of the cerebellum, we could obtain cellular expression data from in situ hybridization and laser-capture microdissection experiments and successfully validated computational predictions made with PSEA. Upon statistical deconvolution of whole tissue samples, we identified a set of transcripts showing age-related expression changes in the astrocyte population.

Conclusions

PSEA can predict cell-type specific expression levels from tissues homogenates on a genome-wide scale. It thus represents a computational alternative to experimental separation methods and allowed us to identify age-related expression changes in the astrocytes of the cerebellum. These molecular changes might underlie important physiological modifications previously observed in the aging brain.

TLR4 activation of TRPC6-dependent calcium signaling mediates endotoxin-induced lung vascular permeability and inflammation

Lung vascular endothelial barrier disruption and the accompanying inflammation are primary pathogenic features of acute lung injury (ALI); however, the basis for the development of both remains unclear. Studies have shown that activation of transient receptor potential canonical (TRPC) channels induces Ca(2+) entry, which is essential for increased endothelial permeability. Here, we addressed the role of Toll-like receptor 4 (TLR4) intersection with TRPC6-dependent Ca(2+) signaling in endothelial cells (ECs) in mediating lung vascular leakage and inflammation. We find that the endotoxin (lipopolysaccharide; LPS) induces Ca(2+) entry in ECs in a TLR4-dependent manner. Moreover, deletion of TRPC6 renders mice resistant to endotoxin-induced barrier dysfunction and inflammation, and protects against sepsis-induced lethality. TRPC6 induces Ca(2+) entry in ECs, which is secondary to the generation of diacylglycerol (DAG) induced by LPS. Ca(2+) entry mediated by TRPC6, in turn, activates the nonmuscle myosin light chain kinase (MYLK), which not only increases lung vascular permeability but also serves as a scaffold to promote the interaction of myeloid differentiation factor 88 and IL-1R-associated kinase 4, which are required for NF-κB activation and lung inflammation. Our findings suggest that TRPC6-dependent Ca(2+) entry into ECs, secondary to TLR4-induced DAG generation, participates in mediating both lung vascular barrier disruption and inflammation induced by endotoxin.

Myosin light chain kinase signaling in endothelial barrier dysfunction

Microvascular barrier dysfunction is a serious problem that occurs in many inflammatory conditions, including sepsis, trauma, ischemia-reperfusion injury, cardiovascular disease, and diabetes. Barrier dysfunction permits extravasation of serum components into the surrounding tissue, leading to edema formation and organ failure. The basis for microvascular barrier dysfunction is hyperpermeability at endothelial cell-cell junctions. Endothelial hyperpermeability is increased by actomyosin contractile activity in response to phosphorylation of myosin light chain by myosin light chain kinase (MLCK). MLCK-dependent endothelial hyperpermeability occurs in response to inflammatory mediators (e.g., activated neutrophils, thrombin, histamine, tumor necrosis factor alpha, etc.), through multiple cell signaling pathways and signaling molecules (e.g., Ca(++) , protein kinase C, Src kinase, nitric oxide synthase, etc.). Other signaling molecules protect against MLCK-dependent hyperpermeability (e.g., sphingosine-1-phosphate or cAMP). In addition, individual MLCK isoforms play specific roles in endothelial barrier dysfunction, suggesting that isoform-specific inhibitors could be useful for treating inflammatory disorders and preventing multiple organ failure. Because endothelial barrier dysfunction depends upon signaling through MLCK in many instances, MLCK-dependent signaling comprises multiple potential therapeutic targets for preventing edema formation and multiple organ failure. The following review is a discussion of MLCK-dependent mechanisms and cell signaling events that mediate endothelial hyperpermeability.

The role of molecular regulation and targeting in regulating calcium/calmodulin stimulated protein kinases

Calcium/calmodulin-stimulated protein kinases can be classified as one of two types - restricted or multifunctional. This family of kinases contains several structural similarities: all possess a calmodulin binding motif and an autoinhibitory region. In addition, all of the calcium/calmodulin-stimulated protein kinases examined in this chapter are regulated by phosphorylation, which either activates or inhibits their kinase activity. However, as the multifunctional calcium/calmodulin-stimulated protein kinases are ubiquitously expressed, yet regulate a broad range of cellular functions, additional levels of regulation that control these cell-specific functions must exist. These additional layers of control include gene expression, signaling pathways, and expression of binding proteins and molecular targeting. All of the multifunctional calcium/calmodulin-stimulated protein kinases examined in this chapter appear to be regulated by these additional layers of control, however, this does not appear to be the case for the restricted kinases.

Smooth muscle myosin light chain kinase efficiently phosphorylates serine 15 of cardiac myosin regulatory light chain

Specific phosphorylation of the human ventricular cardiac myosin regulatory light chain (MYL2) modifies the protein at S15. This modification affects MYL2 secondary structure and modulates the Ca(2+) sensitivity of contraction in cardiac tissue. Smooth muscle myosin light chain kinase (smMLCK) is a ubiquitous kinase prevalent in uterus and present in other contracting tissues including cardiac muscle. The recombinant 130 kDa (short) smMLCK phosphorylated S15 in MYL2 in vitro. Specific modification of S15 was verified using the direct detection of the phospho group on S15 with mass spectrometry. SmMLCK also specifically phosphorylated myosin regulatory light chain S15 in porcine ventricular myosin and chicken gizzard smooth muscle myosin (S20 in smooth muscle) but failed to phosphorylate the myosin regulatory light chain in rabbit skeletal myosin. Phosphorylation kinetics, measured using a novel fluorescence method eliminating the use of radioactive isotopes, indicates similar Michaelis-Menten V(max) and K(M) for regulatory light chain S15 phosphorylation rates in MYL2, porcine ventricular myosin, and chicken gizzard myosin. These data demonstrate that smMLCK is a specific and efficient kinase for the in vitro phosphorylation of MYL2, cardiac, and smooth muscle myosin. Whether smMLCK plays a role in cardiac muscle regulation or response to a disease causing stimulus is unclear but it should be considered a potentially significant kinase in cardiac tissue on the basis of its specificity, kinetics, and tissue expression.

MLCK regulates Schwann cell cytoskeletal organization, differentiation and myelination

Signaling through cyclic AMP (cAMP) has been implicated in the regulation of Schwann cell (SC) proliferation and differentiation. In quiescent SCs, elevation of cAMP promotes the expression of proteins associated with myelination such as Krox-20 and P0, and downregulation of markers associated with the non-myelinating SC phenotype. We have previously shown that the motor protein myosin II is required for the establishment of normal SC-axon interactions, differentiation and myelination, however, the mechanisms behind these effects are unknown. Here we report that the levels and activity of myosin light chain kinase (MLCK), an enzyme that regulates MLC phosphorylation in non-muscle cells, are dramatically downregulated in SCs after cAMP treatment, in a similar pattern to that of c-Jun, a known inhibitor of myelination. Knockdown of MLCK in SCs mimics the effect of cAMP elevation, inducing plasma membrane expansion and expression of Krox-20 and myelin proteins. Despite activation of myelin gene transcription these cells fail to make compact myelin when placed in contact with axons. Our data indicate that myosin II activity is differentially regulated at various stages during myelination and that in the absence of MLCK the processes of SC differentiation and compact myelin assembly are uncoupled.

Biochemistry of smooth muscle myosin light chain kinase

The smooth muscle isoform of myosin light chain kinase (MLCK) is a Ca(2+)-calmodulin-activated kinase that is found in many tissues. It is particularly important for regulating smooth muscle contraction by phosphorylation of myosin. This review summarizes selected aspects of recent biochemical work on MLCK that pertains to its function in smooth muscle. In general, the focus of the review is on new findings, unresolved issues, and areas with the potential for high physiological significance that need further study. The review includes a concise summary of the structure, substrates, and enzyme activity, followed by a discussion of the factors that may limit the effective activity of MLCK in the muscle. The interactions of each of the many domains of MLCK with the proteins of the contractile apparatus, and the multi-domain interactions of MLCK that may control its behaviors in the cell are summarized. Finally, new in vitro approaches to studying the mechanism of phosphorylation of myosin are introduced.

Biochemistry of smooth muscle myosin light chain kinase

The smooth muscle isoform of myosin light chain kinase (MLCK) is a Ca(2+)-calmodulin-activated kinase that is found in many tissues. It is particularly important for regulating smooth muscle contraction by phosphorylation of myosin. This review summarizes selected aspects of recent biochemical work on MLCK that pertains to its function in smooth muscle. In general, the focus of the review is on new findings, unresolved issues, and areas with the potential for high physiological significance that need further study. The review includes a concise summary of the structure, substrates, and enzyme activity, followed by a discussion of the factors that may limit the effective activity of MLCK in the muscle. The interactions of each of the many domains of MLCK with the proteins of the contractile apparatus, and the multi-domain interactions of MLCK that may control its behaviors in the cell are summarized. Finally, new in vitro approaches to studying the mechanism of phosphorylation of myosin are introduced.

Vascular smooth muscle phenotypic diversity and function

The control of force production in vascular smooth muscle is critical to the normal regulation of blood flow and pressure, and altered regulation is common to diseases such as hypertension, heart failure, and ischemia. A great deal has been learned about imbalances in vasoconstrictor and vasodilator signals, e.g., angiotensin, endothelin, norepinephrine, and nitric oxide, that regulate vascular tone in normal and disease contexts. In contrast there has been limited study of how the phenotypic state of the vascular smooth muscle cell may influence the contractile response to these signaling pathways dependent upon the developmental, tissue-specific (vascular bed) or disease context. Smooth, skeletal, and cardiac muscle lineages are traditionally classified into fast or slow sublineages based on rates of contraction and relaxation, recognizing that this simple dichotomy vastly underrepresents muscle phenotypic diversity. A great deal has been learned about developmental specification of the striated muscle sublineages and their phenotypic interconversions in the mature animal under the control of mechanical load, neural input, and hormones. In contrast there has been relatively limited study of smooth muscle contractile phenotypic diversity. This is surprising given the number of diseases in which smooth muscle contractile dysfunction plays a key role. This review focuses on smooth muscle contractile phenotypic diversity in the vascular system, how it is generated, and how it may determine vascular function in developmental and disease contexts.

Myosin light chain kinase in microvascular endothelial barrier function

Microvascular barrier dysfunction is implicated in the initiation and progression of inflammation, posttraumatic complications, sepsis, ischaemia-reperfusion injury, atherosclerosis, and diabetes. Under physiological conditions, a precise equilibrium between endothelial cell-cell adhesion and actin-myosin-based centripetal tension tightly controls the semi-permeability of microvascular barriers. Myosin light chain kinase (MLCK) plays an important role in maintaining the equilibrium by phosphorylating myosin light chain (MLC), thereby inducing actomyosin contractility and weakening endothelial cell-cell adhesion. MLCK is activated by numerous physiological factors and inflammatory or angiogenic mediators, causing vascular hyperpermeability. In this review, we discuss experimental evidence supporting the crucial role of MLCK in the hyperpermeability response to key cell signalling events during inflammation. At the cellular level, in vitro studies of cultured endothelial monolayers treated with MLCK inhibitors or transfected with specific inhibiting peptides have demonstrated that induction of endothelial MLCK activity is necessary for hyperpermeability. Ex vivo studies of live microvessels, enabled by development of the isolated, perfused venule method, support the importance of MLCK in endothelial permeability regulation in an environment that more closely resembles in vivo tissues. Finally, the role of MLCK in vascular hyperpermeability has been confirmed with in vivo studies of animal disease models and the use of transgenic MLCK210 knockout mice. These approaches provide a more complete view of the role of MLCK in vascular barrier dysfunction.

Characterization of tightly associated smooth muscle myosin-myosin light-chain kinase-calmodulin complexes

A current popular model to explain phosphorylation of smooth muscle myosin (SMM) by myosin light-chain kinase (MLCK) proposes that MLCK is bound tightly to actin but weakly to SMM. We found that MLCK and calmodulin (CaM) co-purify with unphosphorylated SMM from chicken gizzard, suggesting that they are tightly bound. Although the MLCK:SMM molar ratio in SMM preparations was well below stoichiometric (1:73+/-9), the ratio was approximately 23-37% of that in gizzard tissue. Fifteen to 30% of MLCK was associated with CaM at approximately 1 nM free [Ca(2+)]. There were two MLCK pools that bound unphosphorylated SMM with K(d) approximately 10 and 0.2 microM and phosphorylated SMM with K(d) approximately 20 and 0.2 microM. Using an in vitro motility assay to measure actin sliding velocities, we showed that the co-purifying MLCK-CaM was activated by Ca(2+) and phosphorylation of SMM occurred at a pCa(50) of 6.1 and at a Hill coefficient of 0.9. Similar properties were observed from reconstituted MLCK-CaM-SMM. Using motility assays, co-sedimentation assays, and on-coverslip enzyme-linked immunosorbent assays to quantify proteins on the motility assay coverslip, we provide strong evidence that most of the MLCK is bound directly to SMM through the telokin domain and some may also be bound to both SMM and to co-purifying actin through the N-terminal actin-binding domain. These results suggest that this MLCK may play a role in the initiation of contraction.

Nonkinase activity of MLCK in elongated filopodia formation and chemotaxis of vascular smooth muscle cells toward sphingosylphosphorylcholine

The actin-myosin interaction of vascular smooth muscle cells (VSMCs) is regulated by myosin light chain kinase (MLCK), which is a fusion protein of the central catalytic domain with the N-terminal actin-binding and C-terminal myosin-binding domains. In addition to the regulatory role of kinase activity mediated by the catalytic domain, nonkinase activity that derives from both terminals is able to exert a regulatory role as reviewed by Nakamura et al. (32). We previously showed that nonkinase activity mediated the filopodia upon the stimulation by sphingosylphosphorylcholine (SPC) (25). To explore the regulatory role of nonkinase activity in chemotaxis, we constructed VSMCs where the expression of MLCK was totally abolished by using a lentivirus-mediated RNAi system. We hypothesized that the MLCK-downregulated VSMCs were unable to form filopodia and to migrate upon SPC stimulation and confirmed the hypothesis. We further constructed a kinase-inactive mutant from bovine cDNA coding wild-type (WT) MLCK by mutating the ATP-binding sites located in the catalytic domain, followed by confirming the presence (absence) of the kinase activity of WT (kinase-inactive mutant). We transfected WT and the mutant into MLCK-downregulated VSMCs. We expected that the transfected VSMCs will recover the ability to induce filopodia and chemotaxis toward SPC and found both constructs rescued the ability. Because they share the actin- and myosin-binding domains, we concluded nonkinase activity plays a major role for SPC-induced migration.

Dependence of myoblast fusion on a cortical actin wall and nonmuscle myosin IIA

Cell-cell fusion is a fundamental cellular process that is essential for development as well as fertilization. Myoblast fusion to form multinucleated skeletal muscle myotubes is a well studied, yet incompletely understood example of cell-cell fusion that is essential for formation of contractile skeletal muscle tissue. Studies in this report identify several novel cytoskeletal events essential to an early phase of myoblast fusion among cultured murine myoblasts. During myoblast pairing and alignment, cortical actin filaments organize into a dense actin wall structure that parallels and extends the length of the plasma membrane of the bipolar, aligned cells. As fusion progresses, gaps appear within the actin wall at sites of vesicle accumulation, the vesicles pair across the aligned myoblasts, cell-cell contacts and fusion pores form. Inhibition of nonmuscle myosin IIA (NM-MHC-IIA) motor activity prevents formation of this cortical actin wall, as well as the appearance of vesicles at a membrane proximal location, and myoblast fusion. These results suggest that early formation of a subplasmalemmal actin wall during myoblast alignment is a critical event for myoblast fusion that supports bipolar membrane alignment and temporally regulates trafficking of vesicles to the nascent fusion sites during skeletal muscle myoblast differentiation.

Myosin light chain kinase is central to smooth muscle contraction and required for gastrointestinal motility in mice

BACKGROUND & AIMS

Smooth muscle is essential for maintaining homeostasis for many body functions and provides adaptive responses to stresses imposed by pathologic disorders. Identified cell signaling networks have defined many potential mechanisms for initiating smooth muscle contraction with or without myosin regulatory light chain (RLC) phosphorylation by myosin light chain kinase (MLCK). We generated tamoxifen-inducible and smooth muscle-specific MLCK knockout (KO) mice and provide direct loss-of-function evidence that shows the primary importance of MLCK in phasic smooth muscle contractions.

METHODS

We used the Cre-loxP system to establish Mlck floxed mice in which exons 23, 24, and 25 were flanked by 2 loxP sites. Smooth muscle-specific MLCK KO mice were generated by crossing Mlck floxed mice with SM-CreER(T2) (ki) mice followed by tamoxifen treatment. The phenotype was assessed by histologic, biochemical, molecular, cell biological, and physiologic analyses.

RESULTS

Targeted deletion of MLCK in adult mouse smooth muscle resulted in severe gut dysmotility characterized by weak peristalsis, dilation of the digestive tract, and reduction of feces excretion and food intake. There was also abnormal urinary bladder function and lower blood pressure. Isolated muscles showed a loss of RLC phosphorylation and force development induced by K(+)-depolarization. The kinase knockout also markedly reduced RLC phosphorylation and force development with acetylcholine which activates Ca(2+)-sensitizing signaling pathways.

CONCLUSIONS

MLCK and its phosphorylation of RLC are required physiologically for smooth muscle contraction and are essential for normal gastrointestinal motility.

Identification of cardiac-specific myosin light chain kinase

Two myosin light chain (MLC) kinase (MLCK) proteins, smooth muscle (encoded by mylk1 gene) and skeletal (encoded by mylk2 gene) MLCK, have been shown to be expressed in mammals. Even though phosphorylation of its putative substrate, MLC2, is recognized as a key regulator of cardiac contraction, a MLCK that is preferentially expressed in cardiac muscle has not yet been identified. In this study, we characterized a new kinase encoded by a gene homologous to mylk1 and -2, named cardiac MLCK, which is specifically expressed in the heart in both atrium and ventricle. In fact, expression of cardiac MLCK is highly regulated by the cardiac homeobox protein Nkx2-5 in neonatal cardiomyocytes. The overall structure of cardiac MLCK protein is conserved with skeletal and smooth muscle MLCK; however, the amino terminus is quite unique, without significant homology to other known proteins, and its catalytic activity does not appear to be regulated by Ca(2+)/calmodulin in vitro. Cardiac MLCK is phosphorylated and the level of phosphorylation is increased by phenylephrine stimulation accompanied by increased level of MLC2v phosphorylation. Both overexpression and knockdown of cardiac MLCK in cultured cardiomyocytes revealed that cardiac MLCK is likely a new regulator of MLC2 phosphorylation, sarcomere organization, and cardiomyocyte contraction.

Supervillin slows cell spreading by facilitating myosin II activation at the cell periphery

During cell migration, myosin II modulates adhesion, cell protrusion and actin organization at the leading edge. We show that an F-actin- and membrane-associated scaffolding protein, called supervillin (SV, p205), binds directly to the subfragment 2 domains of nonmuscle myosin IIA and myosin IIB and to the N-terminus of the long form of myosin light chain kinase (L-MLCK). SV inhibits cell spreading via an MLCK- and myosin II-dependent mechanism. Overexpression of SV reduces the rate of cell spreading, and RNAi-mediated knockdown of endogenous SV increases it. Endogenous and EGFP-tagged SV colocalize with, and enhance the formation of, cortical bundles of F-actin and activated myosin II during early cell spreading. The effects of SV are reversed by inhibition of myosin heavy chain (MHC) ATPase (blebbistatin), MLCK (ML-7) or MEK (U0126), but not by inhibiting Rho-kinase with Y-27632. Flag-tagged L-MLCK co-localizes in cortical bundles with EGFP-SV, and kinase-dead L-MLCK disorganizes these bundles. The L-MLCK- and myosin-binding site in SV, SV1-171, rearranges and co-localizes with mono- and di-phosphorylated myosin light chain and with L-MLCK, but not with the short form of MLCK (S-MLCK) or with myosin phosphatase. Thus, the membrane protein SV apparently contributes to myosin II assembly during cell spreading by modulating myosin II regulation by L-MLCK.

Increased phosphorylation of myosin light chain prevents in vitro decidualization

Differentiation of stromal cells into decidual cells, which is critical to successful pregnancy, represents a complex transformation requiring changes in cytoskeletal architecture. We demonstrate that in vitro differentiation of human uterine fibroblasts into decidual cells includes down-regulation of alpha-smooth muscle actin and beta-tubulin, phosphorylation of focal adhesion kinase, and redistribution of vinculin. This is accompanied by varied adhesion to fibronectin and a modified ability to migrate. Cytoskeletal organization is determined primarily by actin-myosin II interactions governed by the phosphorylation of myosin light chain (MLC20). Decidualization induced by cAMP [with estradiol-17beta (E) and medroxyprogesterone acetate (P)] results in a 40% decrease in MLC20 phosphorylation and a 55% decline in the long (214 kDa) form of myosin light-chain kinase (MLCK). Destabilization of the cytoskeleton by inhibitors of MLCK (ML-7) or myosin II ATPase (blebbistatin) accelerates decidualization induced by cAMP (with E and P) but inhibits decidualization induced by IL-1beta (with E and P). Adenoviral infection of human uterine fibroblast cells with a constitutively active form of MLCK followed by decidualization stimuli leads to a 30% increase in MLC20 phosphorylation and prevents decidualization. These data provide evidence that the regulation of cytoskeletal dynamics by MLC20 phosphorylation is critical for decidualization.

Tumor Necrosis Factor-induced Long Myosin Light Chain Kinase Transcription Is Regulated by Differentiation-dependent Signaling Events

Myosin light chain kinase (MLCK) is expressed as long and short isoforms from unique transcriptional start sites within a single gene. Tumor necrosis factor (TNF) augments intestinal epithelial long MLCK expression, which is critical to cytoskeletal regulation. We found that TNF increases long MLCK mRNA transcription, both in human enterocytes in vitro and murine enterocytes in vivo.5'-RACE identified two novel exons, 1A and 1B, which encode alternative long MLCK transcriptional start sites. Chromatin immunoprecipitation (ChIP) and site-directed mutagenesis identified two essential Sp1 sites upstream of the exon 1A long MLCK transcriptional start site. Analysis of deletion and truncation mutants showed that a 102-bp region including these Sp1 sites was necessary for basal transcription. A promoter construct including 4-kb upstream of exon 1A was responsive to TNF, AP-1, or NFkappaB, but all except NFkappaB responses were absent in a shorter 2-kb construct, and all responses were absent in a 1-kb construct. Electrophoretic mobility shift assays, ChIP, and site-directed mutagenesis explained these data by identifying three functional AP-1 sites between 2- and 4-kb upstream of exon 1A and two NFkappaB sites between 1- and 2-kb upstream of exon 1A. Analysis of differentiating epithelia showed that only well differentiated enterocytes activated the 4-kb long MLCK promoter in response to TNF, and consensus promoter reporters demonstrated that TNF-induced NFkappaB activation decreased during differentiation while TNF-induced AP-1 activation increased. Thus either AP-1 or NFkappaB can up-regulate long MLCK transcription, but the mechanisms by which TNF up-regulates intestinal epithelial long MLCK transcription from exon 1A are differentiation-dependent.

Microfilament-binding properties of N-terminal extension of the isoform of smooth muscle long myosin light chain kinase

Myosin light chain kinases (MLCK) phosphorylate the regulatory light chain of myosin II in thick filaments and bind to F-actin-containing thin filaments with high affinity. The ability of short myosin light chain kinase (S-MLCK) to bind F-actin is structurally attributed to the DFRXXL regions in its N-terminus. The long myosin light chain kinase (L-MLCK) has two additional DFRXXL motifs and six Ig-like modules in its N-terminal extension. The six Ig-like modules are capable of binding to stress fibers independently. Our results from the imaging analysis demonstrated that the first two intact Ig-like modules (2Ig) in N-terminal extension of L-MLCK is the minimal binding module required for microfilament binding. Binding assay confirmed that F-actin was able to bind 2Ig. Stoichiometries of 2Ig peptide were similar for myofilament or pure F-actin. The binding affinities were slightly lower than 5DFRXXL peptide as reported previously. Similar to DFRXXL peptides, the 2Ig peptide also caused efficient F-actin bundle formation in vitro. In the living cell, over-expression of 2Ig fragment increased "spike"-like protrusion formation with over-bundled F-actin. Our results suggest that L-MLCK may act as a potent F-actin bundling protein via its DFRXXL region and the 2Ig region, implying that L-MLCK plays a role in cytoskeleton organization.

Regulation of myosin light chain kinase and telokin expression in smooth muscle tissues

The mylk1 gene is a large gene spanning approximately 250 kb and comprising at least 31 exons. The mylk1 gene encodes at least four protein products: two isoforms of the 220-kDa myosin light chain kinase (MLCK), a 130-kDa MLCK, and telokin. Transcripts encoding these products are derived from four independent promoters within the mylk1 gene. The kinases expressed from the mylk1 gene have been extensively characterized and function to regulate the activity of nonmuscle and smooth muscle myosin II. Activation of these myosin motors by MLCK modulates a variety of contractile processes, including smooth muscle contraction, cell adhesion, migration, and proliferation. Dysregulation of these processes contributes to a number of diseases. The noncatalytic gene product telokin also has been shown to modulate contraction in smooth muscle cells through its ability to inhibit myosin light chain phosphatase. Given the crucial role of the products of the mylk1 gene in regulating numerous contractile processes, it seems intuitive that alterations in the transcriptional activity of the mylk1 gene also will have a significant impact on many physiological and pathological processes. In this review we highlight some of the recent studies that have described the transcriptional regulation of mylk1 gene products in smooth muscle tissues and discuss the implications of these findings for regulation of expression of other smooth muscle-specific genes.

130-kDa smooth muscle myosin light chain kinase is transcribed from a CArG-dependent, internal promoter within the mouse mylk gene

The 130-kDa smooth muscle myosin light chain kinase (smMLCK) is a Ca2+/CaM-regulated enzyme that plays a pivotal role in the initiation of smooth muscle contraction and regulation of cellular migration and division. Despite the critical importance of smMLCK in these processes, little is known about the mechanisms regulating its expression. In this study, we have identified the proximal promoter of smMLCK within an intron of the mouse mylk gene. The mylk gene encodes at least two isoforms of MLCK (130 and 220 kDa) and telokin. Luciferase reporter gene assays demonstrated that a 282-bp fragment (−167 to +115) of the smMLCK promoter was sufficient for maximum activity in A10 smooth muscle cells and 10T1/2 fibroblasts. Deletion of the 16 bp between −167 and −151, which included a CArG box, resulted in a nearly complete loss of promoter activity. Gel mobility shift assays and chromatin immunoprecipitation assays demonstrated that serum response factor (SRF) binds to this CArG box both in vitro and in vivo. SRF knockdown by short hairpin RNA decreased endogenous smMLCK expression in A10 cells. Although the SRF coactivator myocardin induced smMLCK expression in 10T1/2 cells, myocardin activated the promoter only two- to fourfold in reporter gene assays. Addition of either intron 1 or 6 kb of the 5′ upstream sequence did not lead to any further activation of the promoter by myocardin. The proximal smMLCK promoter also contains a consensus GATA-binding site that bound GATA-6. GATA-6 binding to this site decreased endogenous smMLCK expression, inhibited promoter activity in smooth muscle cells, and blocked the ability of myocardin to induce smMLCK expression. Altogether, these data suggest that SRF and SRF-associated factors play a key role in regulating the expression of smMLCK.

Beneficial and harmful effects of oscillatory mechanical strain on airway smooth muscle

Airway smooth muscle (ASM) cells are constantly under mechanical strain as the lung cyclically expands and deflates, and this stretch is now known to modulate the contractile function of ASM. However, depending on the experimental conditions, stretch is either beneficial or harmful limiting or enhancing contractile force generation, respectively. Stretch caused by a deep inspiration is known to be beneficial in limiting or reversing airway constriction in healthy individuals, and oscillatory stretch lowers contractile force and stiffness or lengthens muscle in excised airway tissue strips. Stretch in ASM culture has generally been reported to cause increased contractile function through increases in proliferation, contractile protein content, and organization of the cell cytoskeleton. Recent evidence indicates the type of stretch is critically important. Growing cells on flexible membranes where stretch is non-uniform and anisotropic leads to pro-contractile changes, whereas uniform biaxial stretch causes the opposite effects. Furthermore, the role of contractile tone might be important in modulating the response to mechanical stretch in cultured cells. This report will review the contrasting evidence for modulation of contractile function of ASM, both in vivo and in vitro, and summarize the recent evidence that mechanical stress applied either acutely within 2 h or chronically over 11 d is a potent stimulus for cytoskeletal remodelling and stiffening. We will also point to new data suggesting that perhaps some of the difference in response to stretch might lie with one of the fundamental differences in the ASM environment in asthma and in culture--the presence of elevated contractile tone.

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

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

Effects of ML-7 and Y-27632 on carbachol- and endothelin-1-induced contraction of bovine trabecular meshwork

The trabecular meshwork is considered a smooth muscle like tissue contributing to aqueous outflow regulation and thus to regulation of intraocular pressure. An elevation in intraocular pressure is one of the greatest risk factors for most forms of glaucoma. We assume that contraction of trabecular meshwork reduces aqueous humor outflow and thus enhances intraocular pressure, whereas relaxation exerts the opposite effect. The present paper supports the hypothesis of the trabecular meshwork being a smooth muscle-like tissue. We perform measurements of isometric force in isolated bovine trabecular meshwork strips. Contractility of this tissue is induced by carbachol or endothelin-1. The contractile force is successfully inhibited by ML-7, a highly specific inhibitor of myosin light chain kinase. The contraction is also reduced in the presence of the RhoA kinase inhibitor Y-27632. We further describe the protein expression of smooth muscle myosin and its regulatory kinase, the myosin light chain kinase, in human and bovine trabecular meshwork cells. Additionally, the serine phosphorylation of myosin light chain kinase is shown. These data indicate that the trabecular meshwork expresses major contractility regulating proteins which are involved in tissue function. Inhibition of the signaling pathways which lead to myosin phosphorylation causes inhibition of contractile force in trabecular meshwork. According to our concept of aqueous humor outflow regulation, trabecular meshwork relaxing substances appear to be ideal antiglaucomatous drugs, leading to increased outflow facility.

Epithelial myosin light chain kinase-dependent barrier dysfunction mediates T cell activation-induced diarrhea in vivo

Disruption of the intestinal epithelial barrier occurs in many intestinal diseases, but neither the mechanisms nor the contribution of barrier dysfunction to disease pathogenesis have been defined. We utilized a murine model of T cell-mediated acute diarrhea to investigate the role of the epithelial barrier in diarrheal disease. We show that epithelial barrier dysfunction is required for the development of diarrhea. This diarrhea is characterized by reversal of net water flux, from absorption to secretion; increased leak of serum protein into the intestinal lumen; and altered tight junction structure. Phosphorylation of epithelial myosin II regulatory light chain (MLC), which has been correlated with tight junction regulation in vitro, increased abruptly after T cell activation and coincided with the development of diarrhea. Genetic knockout of long myosin light chain kinase (MLCK) or treatment of wild-type mice with a highly specific peptide MLCK inhibitor prevented epithelial MLC phosphorylation, tight junction disruption, protein leak, and diarrhea following T cell activation. These data show that epithelial MLCK is essential for intestinal barrier dysfunction and that this barrier dysfunction is critical to pathogenesis of diarrheal disease. The data also indicate that inhibition of epithelial MLCK may be an effective non-immunosuppressive therapy for treatment of immune-mediated intestinal disease.

Epithelial myosin light chain kinase-dependent barrier dysfunction mediates T cell activation-induced diarrhea in vivo

Disruption of the intestinal epithelial barrier occurs in many intestinal diseases, but neither the mechanisms nor the contribution of barrier dysfunction to disease pathogenesis have been defined. We utilized a murine model of T cell-mediated acute diarrhea to investigate the role of the epithelial barrier in diarrheal disease. We show that epithelial barrier dysfunction is required for the development of diarrhea. This diarrhea is characterized by reversal of net water flux, from absorption to secretion; increased leak of serum protein into the intestinal lumen; and altered tight junction structure. Phosphorylation of epithelial myosin II regulatory light chain (MLC), which has been correlated with tight junction regulation in vitro, increased abruptly after T cell activation and coincided with the development of diarrhea. Genetic knockout of long myosin light chain kinase (MLCK) or treatment of wild-type mice with a highly specific peptide MLCK inhibitor prevented epithelial MLC phosphorylation, tight junction disruption, protein leak, and diarrhea following T cell activation. These data show that epithelial MLCK is essential for intestinal barrier dysfunction and that this barrier dysfunction is critical to pathogenesis of diarrheal disease. The data also indicate that inhibition of epithelial MLCK may be an effective non-immunosuppressive therapy for treatment of immune-mediated intestinal disease.