Mutation analysis of the non-muscle myosin light chain kinase (MLCK) deletion constructs on CV1 fibroblast contractile activity and proliferation

The myosin light-chain kinase MLCK-1 relocalizes during Caenorhabditis elegans ovulation to promote actomyosin bundle assembly and drive contraction

We identify the Caenorhabditis elegans myosin light-chain kinase, MLCK-1, required for contraction of spermathecae. During contraction, MLCK-1 moves from the apical cell boundaries to the basal actomyosin bundles, where it stabilizes myosin downstream of calcium signaling. MLCK and ROCK act in distinct subsets of cells to coordinate the timing of contraction.

Biological roles of glial fibrillary acidic protein as a biomarker in cartilage regenerative medicine

Glial fibrillary acidic protein (GFAP) is an intermediate filament that is expressed in specifically expressed auricular chondrocytes, which are good cell sources of cartilage regenerative medicine. Although our group uses GFAP as a biomarker of matrix production in the cultured auricular chondrocytes, the biological roles of GFAP in auricular chondrocytes has remained unknown. In this study, we demonstrated the biological functions of GFAP in the human and mouse derived auricles to clarify the significance and role with the chondrocytes of GFAP in order to provide useful information for reliable and safe regenerative medicine. We examined the cell responses to stretch stress for these chondrocytes and completed a nuclear morphological analysis. Based on these results, GFAP seems to support the resistance to severe mechanical stress in the tissue which physiologically suffers from a stretch overload, and plays pivotal roles in the conservation of cell structures and functions through the maintenance of nuclear morphology.

Loss of MLCK leads to disruption of cell-cell adhesion and invasive behavior of breast epithelial cells via increased expression of EGFR and ERK/JNK signaling

Myosin light chain kinase (MLCK) expression is downregulated in breast cancer, including invasive ductal carcinoma compared with ductal breast carcinoma in situ and metastatic breast tumors. However, little is known about how loss of MLCK expression contributes to tumor progression. MLCK is a component of the actin cytoskeleton and its known role is the phosphorylation of the regulatory light chain of myosin II. To gain insights into the role of MLCK in breast cancer, we perturbed its function using small interfering RNA (siRNA) or pharmacological inhibition in untransformed breast epithelial cells (MCF10A). Loss of MLCK by siRNAs led to increased cell migration and invasion, disruption of cell-cell adhesions and enhanced formation of focal adhesions at the leading edge of migratory cells. In addition, downregulation of MLCK cooperated with HER2 in MCF10A cells to promote cell migration and invasion and low levels of MLCK is associated with a poor prognosis in HER2-positive breast cancer patients. Associated with these altered migratory behaviors were increased expression of epidermal growth factor receptor and activation of extracellular signal-regulated kinase and c-Jun N-terminal kinase signaling pathways in MLCK downregulated MCF10A cells. By contrast, inhibition of the kinase function of MLCK using pharmacological agents inhibited cell migration and invasion, and did not affect cellular adhesions. Our results show that loss of MLCK contributes to the migratory properties of epithelial cells resulting from changes in cell-cell and cell-matrix adhesions, and increased epidermal growth factor receptor signaling. These findings suggest that decreased expression of MLCK may have a critical role during tumor progression by facilitating the metastatic potential of tumor cells.

New insights into 4-amino-2-tri-fluoromethyl-phenyl ester inhibition of cell growth and migration in the A549 lung adenocarcinoma cell line

OBJECTIVE

The present study was designed to investigate the probable mechanisms of synthetic retinoid 4-amino-2-tri-fluoromethyl-phenyl ester (ATPR) inhibition of the proliferation and migration of A549 human lung carcinoma cells.

MATERIALS AND METHODS

After the A549 cells were treated with different concentrations of ATPR or all-trans retinoic acid (ATRA) for 72 h, scratch-wound assays were performed to assess migration. Immunofluorescence was used to determine the distribution of CAV1 and RXRα, while expression of CAV1, MLCK, MLC, P38, and phosphorylation of MLC and P38 were detected by Western blotting.

RESULTS

ATPR could block the migration of A549 cells. The relative migration rate of ML-7 group had significantly decreased compared with control group. In addition, ATPR decreased the expression of a migration related proteins, MLCK, and phosphorylation of MLC and P38. ATPR could also influence the expression of RARs or RXRs. At the same time, CAV1 accumulated at cell membranes, and RXRα relocated to the nucleus after ATPR treatment.

CONCLUSIONS

Caveolae may be implicate in the transport of ATPR to the nucleus. Change in the expression and distribution of RXRα may be implicated in ATPR inhibition of A549 cell proliferation. The mechanisms of ATPR reduction in A549 cell migration may be associated with expression of MLCK and phosphorylation of MLC and P38.

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.

α(1) adrenergic receptor agonist, phenylephrine, actively contracts early rat rib fracture callus ex vivo

Early, soft fracture callus that links fracture ends together is smooth muscle-like in nature. We aimed to determine if early fracture callus could be induced to contract and relax ex vivo by similar pathways to smooth muscle, that is, contraction via α(1) adrenergic receptor (α(1) AR) activation with phenylephrine (PE) and relaxation via β(2) adrenergic receptor (β(2) AR) stimulation with terbutaline. A sensitive force transducer quantified 7 day rat rib fracture callus responses in modified Krebs-Henseliet (KH) solutions. Unfractured ribs along with 7, 14, and 21 day fracture calluses were analyzed for both α(1) AR and β(2) AR gene expression using qPCR, whilst 7 day fracture callus was examined via immunohistochemistry for both α(1) AR and β(2) AR- immunoreactivity. In 7 day callus, PE (10(-6)  M) significantly induced an increase in force that was greater than passive force generated in calcium-free KH (n = 8, mean 51% increase, 95% CI: 26-76%). PE-induced contractions in calluses were attenuated by the α(1) AR antagonist, prazosin (10(-6)  M; n = 7, mean 5% increase, 95% CI: 2-11%). Terbutaline did not relax callus. Gene expression of α(1) ARs was constant throughout fracture healing; however, β(2) AR expression was down-regulated at 7 days compared to unfractured rib (p < 0.01). Furthermore, osteoprogenitor cells of early fibrous callus displayed considerable α(1) AR-like immunoreactivity but not β(2) AR-like immunoreactivity. Here, we demonstrate for the first time that early fracture callus can be pharmacologically induced to contract. We propose that increased concentrations of α(1) AR agonists such as noradrenaline may tonically contract callus in vivo to promote osteogenesis.

Quantitative distribution and colocalization of non-muscle myosin light chain kinase isoforms and cortactin in human lung endothelium

Vascular barrier regulation is intimately linked to alterations in the distribution and configuration of the endothelial cell (EC) cytoskeleton in response to angiogenic and edemagenic agonists. Critical actin cytoskeletal rearrangement includes spatially directed increases in myosin light chain (MLC) phosphorylation, catalyzed by Ca(2+)/calmodulin-dependent non-muscle myosin light chain kinase variants (nmMLCK1- and -2), as well as association of nmMLCK with the actin-binding protein, cortactin. As these associations have proven difficult to quantify in a spatially specific manner, we now describe the utility of intensity correlation image analysis and the intensity correlation quotient (ICQ) to quantify colocalization in fixed and live cell imaging assays in human pulmonary artery EC. From baseline ICQ values averaging 0.216 reflecting colocalization of cortactin-DsRed with EGFP-nmMLCK fusion proteins in resting EC, thrombin-induced EC contraction significantly reduced cortactin-DsRed-EGFP-nmMLCK colocalization (nmMLCK1: ICQ=0.118; nmMLCK2: ICQ=0.091) whereas the potent EC barrier-protective agonist, sphingosine 1-phosphate (S1P), significantly increased nmMLCK-cortactin colocalization within lamellipodia (nmMLCK1: ICQ=0.275; nmMLCK2: ICQ=0.334). Over-expression of a cortactin-DsRed mutant fusion protein lacking the SH3 domain, known to be essential for cortactin-nmMLCK association, reduced baseline and S1P-mediated live cell colocalization with each nmMLCK variant (nmMLCK1: ICQ=0.160; nmMLCK2: ICQ=0.157). Similarly, expression of a truncated EGFP-nmMLCK2 mutant lacking cortactin- and actin-binding domains, markedly reduced basal localization in lamellipodia and abolished colocalization with cortactin-DsRed in lamellipodia after S1P (ICQ=-0.148). These data provide insights into the molecular basis for vascular barrier-regulatory cytoskeletal responses and support the utility of sophisticated imaging analyses and methodological assessment to quantify the critical nmMLCK and cortactin interaction during vascular barrier regulation.

Androgens down-regulate myosin light chain kinase in human prostate cancer cells

Androgens play a major role in the growth and survival of primary prostate tumors. The molecular mechanisms involved in prostate cancer progression are not fully understood but genes that are regulated by androgens clearly influence this process. We searched for new androgen-regulated genes using the Affymetrix GeneChip Human Genome U95 Set in the androgen-sensitive LNCaP prostate cancer cell line. Analysis of gene expression profiles revealed that myosin light chain kinase (MLCK) mRNA levels were markedly down-regulated by the synthetic androgen R1881. The microarray data were confirmed by ribonuclease protection assays. RNA and protein analyses revealed that LNCaP cells express both long (non-muscle) and short (smooth muscle) isoforms, and that both isoforms are down-regulated by androgens. Taken together, these data identify MLCK as a novel downstream target of the androgen signalling pathway in prostate cells.

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.

Migration of Zebrafish Primordial Germ Cells: A Role for Myosin Contraction and Cytoplasmic Flow

The molecular and cellular mechanisms governing cell motility and directed migration in response to the chemokine SDF-1 are largely unknown. Here, we demonstrate that zebrafish primordial germ cells whose migration is guided by SDF-1 generate bleb-like protrusions that are powered by cytoplasmic flow. Protrusions are formed at sites of higher levels of free calcium where activation of myosin contraction occurs. Separation of the acto-myosin cortex from the plasma membrane at these sites is followed by a flow of cytoplasm into the forming bleb. We propose that polarized activation of the receptor CXCR4 leads to a rise in free calcium that in turn activates myosin contraction in the part of the cell responding to higher levels of the ligand SDF-1. The biased formation of new protrusions in a particular region of the cell in response to SDF-1 defines the leading edge and the direction of cell migration.

Focal adhesions: paradigm for a signaling nexus

The vascular wall contains intimal endothelium and medial smooth muscle that act as contiguous tissues with tight spatial and functional coordination in response to tonic and episodic input from the bloodstream and the surrounding parenchyma. Focal adhesions are molecular bridges between the intracellular and extracellular spaces that integrate a variety of environmental stimuli and mediate 2-way crosstalk between the extracellular matrix and the cytoskeleton. Focal adhesion components are targets for biochemical and mechanical stimuli that evoke crucial developmental and injury response mechanisms including cell growth, movement, and differentiation, and tailoring of the extracellular microenvironment. Focal adhesions provide the vascular wall constituents with flexible and specific tools for exchanging cues in a complex system. The molecular mechanisms that underlie these vital communications are detailed in this review with the goal of defining future targets for vascular tissue engineering and for the therapeutic modulation of disordered vascular growth, inflammation, thrombosis, and angiogenesis.

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.

Intracellular interaction of myosin light chain kinase with macrophage migration inhibition factor (MIF) in endothelium

The endothelial cell Ca2+/calmodulin (CaM)-dependent myosin light chain kinase isoform (EC MLCK) is a multifunctional contractile effector involved in vascular barrier regulation, leukocyte diapedesis, apoptosis, and angiogenesis. The EC MLCK isoform and its splice variants contain a unique N-terminal sequence not present in the smooth muscle MLCK isoform (SM MLCK), which allows novel upregulation of MLCK activation by signaling cascades including p60src. The yeast two-hybrid assay system using the entire EC MLCK1 N-terminus (922 aa) as bait, identified additional stable MLCK binding partners including the 12 KDa macrophage migration inhibitory factor (MIF). This finding was confirmed by cross immunoprecipitation assays under non-denaturing conditions and by GST pull down experiments using GST-N-terminal MLCK (#1-923) and MLCK N-terminal deletion mutants in TNFalpha- and thrombin-stimulated endothelium. This EC MLCK-MIF interaction was shown biochemically and by immunofluorescent microscopy to be enhanced in TNFalpha- and thrombin-stimulated endothelium, both of which induce increased MLCK activity. Thrombin induced the colocalization of an epitope-tagged, full-length MIF fusion protein with phosphorylated MLC along peripheral actin stress fibers. Together these studies suggest that the novel interaction between MIF and MLCK may have important implications for the regulation of both non-muscle cytoskeletal dynamics as well as pathobiologic vascular events that involve MLCK.

Ethanol-Induced Activation of Myosin Light Chain Kinase Leads to Dysfunction of Tight Junctions and Blood-Brain Barrier Compromise

BACKGROUND

Brain endothelial cells form the blood-brain barrier (BBB) that regulates solute and macromolecule flux in and out of the brain, leukocyte migration, and maintains the homeostasis of the central nervous system. BBB dysfunction is associated with disruption of tight junctions (TJ) in the brain endothelium. We propose that alcohol abuse may impair BBB permeability through TJ modification.

METHODS

Primary cultured bovine brain microvascular endothelial cells (BBMEC) were treated with 50 mM ethanol (EtOH), and monolayer tightness was assessed by measurement of transendothelial electrical resistance (TEER). Changes in TEER were correlated with alterations in TJ protein distribution [occludin, zonula occludens-1 (ZO-1), claudin-5] using immunofluorescence (IF). Expression of myosin light chain (MLC) kinase (MLCK), ZO-1, claudin-5, and phosphorylated MLC, occludin and claudin-5 were determined by immunoprecipitation and Western blot. EtOH-induced changes in monocyte migration across in vitro BBB constructs were also examined.

RESULTS

EtOH induced a decrease in TEER of BBMEC monolayers that was reversed by EtOH withdrawal. Treatment of BBMEC with EtOH or its metabolite, acetaldehyde, prior to monocyte application resulted in a 2-fold increase in monocyte migration across the BBB. IF demonstrated decrease in claudin-5 staining, occludin translocation from cell borders to cytoplasm and gap formation in EtOH-treated BBMEC monolayer. These changes paralleled significant increase in phosphorylation of MLC, occludin and claudin-5. EtOH-treated BBMEC showed reduction of total occludin and claudin-5 without changes in ZO-1 or MLC. TEER decrease, changes in occludin/claudin staining, increase in MLC, occludin and claudin-5 phosphorylation and enhanced monocyte migration across the BBB were all reversed by inhibition of MLCK. Inhibition of EtOH metabolism in BBMEC also reversed these events.

CONCLUSION

These results suggest that EtOH activates MLCK leading to phosphorylation of MLC, occludin and claudin-5. Cytoskeletal alterations (MLC) and TJ changes (occludin and claudin-5 phosphorylation) result in BBB impairment (decrease in TEER). TJ compromise is associated with increased monocyte migration across the BBB.

Endothelial cell myosin light chain kinase (MLCK) regulates TNF?-induced NF?B activity

Tumor necrosis factor (TNFalpha-) generates both apoptotic and survival signals with endothelial cell (EC) survival dependent on nuclear factor kappa-B (NFkappaB) activation, a regulator of anti-apoptotic genes. We previously demonstrated that increased EC contractility, rearrangement of the actin cytoskeleton, and increased myosin light chain (MLC) phosphorylation occurs as a consequence of TNFalpha-induced activation of EC MLC kinase (EC MLCK) and is required for bovine lung EC apoptosis. As the association between MLCK and pro-survival signals such as NFkappaB activation is unknown, we studied the role of MLCK in the regulation of NFkappaB-dependent transactivation in bovine pulmonary artery EC. Both TNFalpha-induced increase in NFkappaB dependent transactivation measured by NFkappaB luciferase reporter assay (approximately fivefold) and nuclear translocation of NFkappaB were significantly inhibited by MLCK-selective inhibitors, KT5926 (60% inhibition of luciferase activity) and ML7 (50% decrease). Furthermore, our data revealed that inhibition of MLCK attenuated the TNFalpha-induced IkappaB phosphorylation, translocation of p65, NFkappaB-DNA binding, and NFkappaB transcriptional activity. Molecular approaches to either reduce EC MLCK expression (AdV EC MLCK antisense construct) or to reduce kinase activity (kinase-dead EC MLCK ATPdel mutant) produced similar attenuation of the TNFalpha-induced NFkappaB response. In contrast, a constitutively active MLCK mutant (EC MLCK1745) enhanced TNFalpha-induced luciferase activity. Together, these novel observations indicate that TNFalpha-induced cytoskeletal rearrangement driven by MLCK activity is necessary for TNFalpha-dependent NFkappaB activation and amplification of pro-survival signals.

Myosin light chain kinase (210 kDa) is a potential cytoskeleton integrator through its unique N-terminal domain

Recently discovered 210-kDa myosin light chain kinase (MLCK-210) is identical to 108-130 kDa MLCK, the principal regulator of the myosin II molecular motor, except for the presence of a unique amino terminal extension. Our in vitro experiments and transfected cell studies demonstrate that the N-terminal half of MLCK-210 unique tail domain has novel microfilament and microtubule binding activity. Consistent with this activity, the MLCK-210 domain codistributes with microfilaments and microtubules in cultured cells and with soluble tubulin in nocodazole-treated cells. This domain is capable of aggregating tubulin dimers in vitro, causing bundling and branching of microtubules induced by taxol. The N-terminal actin-binding region of MLCK-210 has lower affinity to actin (K(d) = 7.4 microM) than its central D(F/V)RXXL repeat-based actin-binding site and does not protect stress fibers from disassembly triggered by MLCK inhibition in transfected cells. Obtained results suggest that while being resident on microfilaments, MLCK-210 may interact with other cytoskeletal components through its N-terminal domain. Based on available evidence, we propose a model in which MLCK-210 could organize cell motility by simultaneous control of cytoskeleton architecture and actomyosin activation through the novel protein scaffold function of the unique tail domain and the classical MLCK catalytic function of the kinase domain.

Distribution and expression of non-muscle myosin light chain kinase in rabbit livers

AIM: To study the distribution and expression of non-muscle myosin light chain kinase (nmMLCK) in rabbit livers.

METHODS: Human nmMLCK N-terminal cDNA was amplified by polymerase chain reaction (PCR) and was inserted into pBKcmv to construct expression vectors. The recombinant plasmid was transformed into XL1-blue. Expression protein was induced by IPTG and then purified by SDS-PAGE and electroelution, which was used to prepare the polycolonal antibody to detect the distribution and expression of nmMLCK in rabbit livers with immunofluorescene techniques.

RESULTS: The polyclonal antibody was prepared, by which nmMLCK expression was detected and distributed mainly in peripheral hepatocytes.

CONCLUSION: nmMLCK can express in hepatocytes peripherally, and may play certain roles in the regulation of hepatic functions.