Localization of plectin and other related proteins along the sarcolemma in smooth muscle cells of rat colon

Dystrophin in the Neonatal and Adult Rat Intestine

Background: Gastrointestinal (GI) complaints are frequently noted in aging dystrophinopathy patients, yet their underlying molecular mechanisms are largely unknown. As dystrophin protein isoform 71 (Dp71) is particularly implicated in the development of smooth muscle cells, we evaluated its distribution in the neonatal and adult rat intestine in this study. Methods: Dp71 expression levels were assessed in the proximal (duodenum, jejunum and ileum) and distal (caecum, colon and rectum) intestine by Western blotting and qPCR. In addition, the cellular distribution of total Dp was evaluated in the duodenum and colon by immunohistochemical colocalization studies with alpha-smooth muscle actin (aSMA), Hu RNA binding proteins C and D (HuC/HuD) for neurons and vimentin (VIM) for interstitial cells. Results: In neonatal and adult rats, the distal intestine expressed 2.5 times more Dp71 protein than the proximal part (p < 0.01). This regional difference was not observed in Dp71 mRNA. During both stages, Dp-immunoreactivity was predominant in the muscularis propria, where it co-localized with aSMA and HuC/HuD. Conclusions: In neonatal and adult rats, Dp71 was expressed highest in the distal intestine. Together with the observation that Dp may be expressed by myenteric neurons, this warrants a paradigm shift in the treatment of GI comorbidities.

Expression of smooth muscle-like effectors and core cardiomyocyte regulators in the contractile papillae of Ciona

BACKGROUND

The evolution of vertebrate smooth muscles is obscured by lack of identifiable smooth muscle-like cells in tunicates, the invertebrates most closely related to vertebrates. A recent evolutionary model was proposed in which smooth muscles arose before the last bilaterian common ancestor, and were later diversified, secondarily lost or modified in the branches leading to extant animal taxa. However, there is currently no data from tunicates to support this scenario.

METHODS AND RESULTS

Here, we show that the axial columnar cells, a unique cell type in the adhesive larval papillae of the tunicate Ciona, are enriched for orthologs of vertebrate smooth/non-muscle-specific effectors of contractility, in addition to developing from progenitors that express conserved cardiomyocyte regulatory factors. We show that these cells contract during the retraction of the Ciona papillae during larval settlement and metamorphosis.

CONCLUSIONS

We propose that the axial columnar cells of Ciona are a myoepithelial cell type required for transducing external stimuli into mechanical forces that aid in the attachment of the motile larva to its final substrate. Furthermore, they share developmental and functional features with vertebrate myoepithelial cells, vascular smooth muscle cells, and cardiomyocytes. We discuss these findings in the context of the proposed models of vertebrate smooth muscle and cardiomyocyte evolution.

β4 integrin is not essential for localization of hemidesmosome proteins plectin and CD151 in cerebral vessels

Objective:

In the central nervous system (CNS), β4 integrin is predominantly expressed by endothelial cells lining arterioles. As β4 integrin plays an essential role in epithelial tissues, organizing structural proteins into specialized adhesive structures called hemidesmosomes (HD), the aim of this study was to determine whether it plays a similar role in CNS endothelium.

Methods:

Dual-immunofluorescence was used to examine the relationship between β4 integrin expression and co-expression of the HD proteins plectin and CD151 in frozen sections of mouse brain, both under normoxic (control) conditions and following chronic mild hypoxia. The requirement of β4 integrin for the localization of HD proteins was examined in transgenic mice lacking β4 integrin expression specifically in endothelial cells (β4-EC-KO mice).

Results:

Immunofluorescence revealed that in the normal adult CNS, plectin and CD151 strongly co-localized with β4 integrin in arterioles. However, in the chronic mild hypoxia model, in which extensive cerebrovascular remodeling is observed, plectin and CD151 were strongly upregulated on all cerebral vessels, but surprisingly, in capillaries, this occurred in a β4 integrin-independent manner. Unexpectedly, absence of endothelial β4 integrin (in β4-EC-KO mice) had no impact on the expression level or distribution pattern of plectin and CD151 within stable or remodeling cerebral vessels.

Conclusions:

These results demonstrate that the HD proteins plectin and CD151 are closely associated with β4 integrin on arterioles in normal brain, and are strongly upregulated on remodeling blood vessels. However, unlike its described role in the epidermis, β4 integrin is not essential for localization or regulation of expression of plectin and CD151 in cerebral vessels.

Smooth muscle-protein translocation and tissue function

Smooth muscle (SM) tissue is a complex organization of multiple cell types and is regulated by numerous signaling molecules (neurotransmitters, hormones, cytokines, etc.). SM contractile function can be regulated via expression and distribution of the contractile and cytoskeletal proteins, and activation of any of the second messenger pathways that regulate them. Spatial-temporal changes in the contractile, cytoskeletal or regulatory components of SM cells (SMCs) have been proposed to alter SM contractile activity. Ca(2+) sensitization/desensitization can occur as a result of changes at any of these levels, and specific pathways have been identified at all of these levels. Understanding when and how proteins can translocate within the cytoplasm, or to-and-from the plasmalemma and the cytoplasm to alter contractile activity is critical. Numerous studies have reported translocation of proteins associated with the adherens junction and G protein-coupled receptor activation pathways in isolated SMC systems. Specific examples of translocation of vinculin to and from the adherens junction and protein kinase C (PKC) and 17 kDa PKC-potentiated inhibitor of myosin light chain phosphatase (CPI-17) to and from the plasmalemma in isolated SMC systems but not in intact SM tissues are discussed. Using both isolated SMC systems and SM tissues in parallel to pursue these studies will advance our understanding of both the role and mechanism of these pathways as well as their possible significance for Ca(2+) sensitization in intact SM tissues and organ systems.

Electron microscopic examination of podosomes induced by phorbol 12, 13 dibutyrate on the surface of A7r5 cells

The role of myosin light chain kinase (MLCK) in inducing podosomes was examined by confocal and electron microscopy. Removal of myosin from the actin core of podosomes using blebbistatin, a myosin inhibitor, resulted in the formation of smaller podosomes. Downregulation of MLCK by the transfection of MLCK small interfering RNA (siRNA) led to the failure of podosome formation. However, ML-7, an inhibitor of the kinase activity of MLCK, failed to inhibit podosome formation. Based on our previous report (Thatcher et al. J.Pharm.Sci. 116 116-127, 2011), we outlined the important role of the actin-binding activity of MLCK in producing smaller podosomes.

Blebbistatin, a myosin II inhibitor, suppresses contraction and disrupts contractile filaments organization of skinned taenia cecum from guinea pig

To explore the precise mechanisms of the inhibitory effects of blebbistatin, a potent inhibitor of myosin II, on smooth muscle contraction, we studied the blebbistatin effects on the mechanical properties and the structure of contractile filaments of skinned (cell membrane permeabilized) preparations from guinea pig taenia cecum. Blebbistatin at 10 microM or higher suppressed Ca(2+)-induced tension development at any given Ca(2+) concentration but had little effects on the Ca(2+)-induced myosin light chain phosphorylation. Blebbistatin also suppressed the 10 and 2.75 mM Mg(2+)-induced, "myosin light chain phosphorylation-independent" tension development at more than 10 microM. Furthermore, blebbistatin induced conformational change of smooth muscle myosin (SMM) and disrupted arrangement of SMM and thin filaments, resulting in inhibition of actin-SMM interaction irrespective of activation with Ca(2+). In addition, blebbistatin partially inhibited Mg(2+)-ATPase activity of native actomyosin from guinea pig taenia cecum at around 10 microM. These results suggested that blebbistatin suppressed skinned smooth muscle contraction through disruption of structure of SMM by the agent.

Plectin defects in epidermolysis bullosa simplex with muscular dystrophy

Epidermolysis bullosa simplex with muscular dystrophy (EBS-MD, MIM 226670) is caused by plectin defects. We performed mutational analysis and immunohistochemistry using EBS-MD (n = 3 cases) and control skeletal muscle to determine pathogenesis. Mutational analysis revealed a novel homozygous plectin-exon32 rod domain mutation (R2465X). All plectin/HD1-121 antibodies stained the control skeletal muscle membrane. However, plectin antibodies stained the cytoplasm of type II control muscle fibers (as confirmed by ATPase staining), whereas HD1-121 stained the cytoplasm of type I fibers. EBS-MD samples lacked membrane (n = 3) but retained cytoplasmic HD1-121 (n = 1) and plectin staining in type II fibers (n = 3). Ultrastructurally, EBS-MD demonstrated widening and vacuolization adjacent to the membrane and disorganization of Z-lines (n = 2 of 3) compared to controls (n = 5). Control muscle immunogold labeling colocalized plectin and desmin to filamentous bridges between Z-lines and the membrane that were disrupted in EBS-MD muscle. We conclude that fiber-specific plectin expression is associated with the desmin-cytoskeleton, Z-lines, and crucially myocyte membrane linkage, analogous to hemidesmosomes in skin.

Adherens junction-associated protein distribution differs in smooth muscle tissue and acutely isolated cells

This study was designed to examine how smooth muscle (SM) cell (SMC) isolation affects the distribution of some adherens junction (AJ) complex-associated proteins. Immunofluorescence procedures for identifying protein distribution were used on gastrointestinal and tracheal SM tissues and freshly isolated SMCs from dogs and rabbits. As confirmed by force measurements, relaxation, Ca(2+) depletion, and cholinergic activation of SM tissues do not cause significant redistribution of the AJ-associated proteins vinculin, talin, or fibronectin away from the plasma membrane. Unlike SMCs in tissue, freshly isolated SMCs show a variable peripheral/cytoplasmic vinculin and talin distribution that is not altered by activation. Enzymatic treatment of SM tissues (as done for the first step of SMC isolation) results in loss of fibronectin immunoreactivity in SMCs still in the tissue but fails to cause redistribution of vinculin, talin, or caveolin away from the periphery. The loss of fibronectin immunofluorescence with enzymatic digestion correlates significantly with loss of tissue force production. These results confirm that the AJ-associated proteins vinculin and talin do not redistribute throughout SMCs in tissues when relaxed, when generating force, or after enzymatic digestion. In addition, in freshly isolated SMCs, the distribution of these proteins is significantly altered in approximately 50% of the SMCs. The cause of this redistribution is currently unknown, as is the impact on intracellular signaling and mechanics of these cells. Use of these two systems (SMCs in tissues vs. freshly isolated SMCs) provides an ideal situation for studying the role of the AJ in SMC signaling and mechanics.

Blebbistatin inhibits sphingosylphosphorylcholine-induced contraction of collagen-gel fiber populated by vascular smooth-muscle cells

We prepared a cell-populated collagen-gel fiber including GbaSM-4 cells derived from the basilar artery of guinea pigs. This fiber tended to be a differentiated contractile phenotype in electron-microscope observations. Sphingosylphosphorylcholine (SPC) can induce contraction of the fiber (EC50 = 0.70 +/- 0.05 microM), and blebbistatin can inhibit the SPC-induced contraction (IC50 = 22.8 +/- 1.26 microM). Phosphorylation of the 20 kD myosin light chain (MLC20) significantly increased in GbaSM-4 cells provided with 1 microM SPC (P<0.05), which was maintained in the presence of 1 to 100 microM blebbistatin. These results suggest that vascular smooth muscle can relax even if MLC20 is phosphorylated.

Smooth muscle adherens junctions associated proteins are stable at the cell periphery during relaxation and activation

This study was performed to determine the stability of the adherens junction (AJ)-associated proteins at the smooth muscle cell (SMC) plasma membrane during relaxing and activating conditions. Dog stomach, ileum, colon, and trachea tissues were stored in Ca2+-free PSS or regular PSS or were activated in 10 μM carbachol in PSS before rapid freezing. The tissues were subsequently sectioned and immunoreacted using antibodies for vinculin, talin, fibronectin, and caveolin to determine their cellular distribution in these tissues under these conditions. In all four tissues and under all three conditions, the distribution of these four proteins remained localized to the periphery of the cell. In transverse tissue sections, the AJ-associated proteins formed a distinct punctate pattern around the periphery of the SMCs at the plasma membrane. These domains alternated with the caveolae (as identified by the presence of caveolin). In longitudinal tissue sections, the AJ-associated proteins formed continuous tracks or staves, while the caveolae remained punctate in this dimension as well. Caveolin is not present in the tapered ends of the SMCs, where the AJ-associated proteins appear continuous around the periphery. Densitometry of the fluorophore distribution of these proteins showed no shift in their localization from the SMC periphery when the tissues were relaxed or when they were activated before freezing. These results suggest that under physiologically relaxing and activating conditions, AJ-associated proteins remain stably localized at the plasma membrane.

Vanilloid receptor expressed in the sarcoplasmic reticulum of rat skeletal muscle

Vanilloid receptor subtype 1 (VR1) was cloned as a capsaicin receptor from neuronal cells of dorsal root ganglia. VR1 was subsequently found in a few non-neuronal tissues, including skeletal muscle [Onozawa et al., Tissue distribution of capsaicin receptor in the various organs of rats, Proc. Jpn. Acad. Ser. B 76 (2000) 68-72]. We confirmed the expression of VR1 in muscle cells using the RT-PCR method and Western blot analysis. Immunostaining studies with a confocal microscope and an electron microscope indicated that VR1 was present in the sarcoplasmic reticulum (SR), a store of Ca2+. The SR releases Ca2+ to cause a contraction when a muscle is excited. However, SR still releases a small amount of Ca2+ under relaxed conditions. We found that this leakage was enhanced by capsaicin and was antagonized by capsazepine, a capsaicin blocker, indicating that leakage of Ca2+ occurs through a channel composed of VR1.

Regulation of smooth muscle calcium sensitivity: KCl as a calcium-sensitizing stimulus

KCl has long been used as a convenient stimulus to bypass G protein-coupled receptors (GPCR) and activate smooth muscle by a highly reproducible and relatively “simple” mechanism involving activation of voltage-operated Ca2+channels that leads to increases in cytosolic free Ca2+([Ca2+]i), Ca2+-calmodulin-dependent myosin light chain (MLC) kinase activation, MLC phosphorylation and contraction. This KCl-induced stimulus-response coupling mechanism is a standard tool-set used in comparative studies to explore more complex mechanisms generated by activation of GPCRs. One area where this approach has been especially productive is in studies designed to understand Ca2+sensitization, the relationship between [Ca2+]iand force produced by GPCR agonists. Studies done in the late 1980s demonstrated that a unique relationship between stimulus-induced [Ca2+]iand force does not exist: for a given increase in [Ca2+]i, GPCR activation can produce greater force than KCl, and relaxant agents can produce the opposite effect to cause Ca2+desensitization. Such changes in Ca2+sensitivity are now known to involve multiple cell signaling strategies, including translocation of proteins from cytosol to plasma membrane, and activation of enzymes, including RhoA kinase and protein kinase C. However, recent studies show that KCl can also cause Ca2+sensitization involving translocation and activation of RhoA kinase. Rather than complicating the Ca2+sensitivity story, this surprising finding is already providing novel insights into mechanisms regulating Ca2+sensitivity of smooth muscle contraction. KCl as a “simple” stimulus promises to remain a standard tool for smooth muscle cell physiologists, whose focus is to understand mechanisms regulating Ca2+sensitivity.

Both type-I hemidesmosomes and adherens-type junctions contribute to the cell–substratum adhesion system in myoepithelial cells

Myoepithelial cells present in exocrine glands cause secretion from the glands by contraction. They have mixed characteristics with regard to cytoskeletal elements, containing both epithelial-type intermediate filaments and smooth muscle-type myofilaments. For further characterization, myoepithelial cells from bovine apocrine sweat glands and tracheal glands were here examined with special attention to the cell-substratum adhesion system. Immunofluorescence microscopy using a panel of antibodies against adherens-type junctional and hemidesmosomal proteins demonstrated two types of cell-substratum junctions in myoepithelial cells from both glands. Type-I hemidesmosomes (HDs) consisting of plectin, BP230, integrin alpha6beta4, and BP180 were thus observed as punctate arrays longitudinally arranged along myoepithelial cell surfaces, while adherens-type junctions were similarly evident as linear rib-like structures. Double-label immunofluoresence revealed the two junctions to be distributed in a mutually exclusive or independent manner. Electron microscopy further demonstrated that apocrine myoepithelial cells surround secretory epithelial cells completely, without any gaps, HDs being abundant along the basement membrane, but with no distinct structures in the inter-hemidesmosomal regions. Immunoelectron microscopy, however, revealed an interhemidesmosomal localization of vinculin, pointing to the existence of adherens-type junctions. Secretory epithelial cells in tracheal glands were found not to be completely covered with myoepithelial cells, so that more than half of them are directly attached to the basement membrane, where they form type II-HDs lacking BP230 and BP180, but no detectable adherens junctions, like epidermal basal cells and sebaceous gland cells. These observations demonstrate that, in addition to their cytoskeleton, myoepithelial cells have both epithelial- and smooth muscle-type cell-substratum adhesion structures, i.e. HDs and dense plaque-like adherens junctions.

Nicotinic Acetylcholine Receptor α7 Subunit Mediates Migration of Vascular Smooth Muscle Cells Toward Nicotine

GbaSM-4 cells, vascular smooth muscle cells (VSMCs) derived from brain basilar arteries, were shown to migrate toward d-nicotine by augmenting the actin cytoskeleton in their cell bodies and lamellipodia, and expression of nicotinic acetylcholine receptor (alpha7-nAChR) was detected in GbaSM-4 cells. Their chemotaxis was antagonized by an alpha7-nAChR antagonist of methyllycaconitine. It was also antagonized by inhibiting myosin light chain (MLC) kinase and by down-regulating MLC kinase. However, the changes in MLC phosphorylation were not associated with the nicotine treatment, suggesting the involvement of non-kinase activity of MLC kinase as reviewed by Gao et al. (IUBMB Life. 2001;51:337). This plot may work to induce arteriosclerosis during cigarette smoking.

Immunolocalization of caveolin-1 and caveolin-3 in monkey skeletal, cardiac and uterine smooth muscles

Caveolin, a 20-24 kDa integral membrane protein, is a principal component of caveolar domains. Caveolin-1 is expressed predominantly in endothelial cells, fibroblasts, and adipocytes, while the expression of caveolin-3 is confined to muscle cells. However, their localization in various muscles has not been well documented. Using double-immunofluorescence labeling and confocal laser microscopy, we examined the localization of caveolins-1 and 3 in adult monkey skeletal, cardiac and uterine smooth muscles and the co-immunolocalization of these caveolins with dystrophin, which is a product of the Duchenne muscular dystrophy gene. In the skeletal muscle tissue, caveolin-3 was localized along the sarcolemma except for the transverse tubules, and co-immunolocalized with dystrophin, whereas caveolin-1 was absent except in the blood vessels of the muscle tissue. In cardiac muscle cells, caveolins-1 and -3 and dystrophin were co-immunolocalized on the sarcolemma and transverse tubules. In uterine smooth muscle cells, caveolin-1, but not caveolin-3, was co-immunolocalized with dystrophin on the sarcolemma.