SMB myosin heavy chain knockout enhances tonic contraction and reduces the rate of force generation in ileum and stomach antrum

Visceral myopathy: clinical syndromes, genetics, pathophysiology, and fall of the cytoskeleton

Visceral smooth muscle is a crucial component of the walls of hollow organs like the gut, bladder, and uterus. This specialized smooth muscle has unique properties that distinguish it from other muscle types and facilitate robust dilation and contraction. Visceral myopathies are diseases where severe visceral smooth muscle dysfunction prevents efficient movement of air and nutrients through the bowel, impairs bladder emptying, and affects normal uterine contraction and relaxation, particularly during pregnancy. Disease severity exists along a spectrum. The most debilitating defects cause highly dysfunctional bowel, reduced intrauterine colon growth (microcolon), and bladder-emptying defects requiring catheterization, a condition called megacystis-microcolon-intestinal hypoperistalsis syndrome (MMIHS). People with MMIHS often die early in childhood. When the bowel is the main organ affected and microcolon is absent, the condition is known as myopathic chronic intestinal pseudo-obstruction (CIPO). Visceral myopathies like MMIHS and myopathic CIPO are most commonly caused by mutations in contractile apparatus cytoskeletal proteins. Here, we review visceral myopathy-causing mutations and normal functions of these disease-associated proteins. We propose molecular, cellular, and tissue-level models that may explain clinical and histopathological features of visceral myopathy and hope these observations prompt new mechanistic studies.

Gastric smooth muscle cells manifest an abnormal phenotype in Parkinson's disease rats with gastric dysmotility

Gastroparesis is a common symptom in Parkinson's disease (PD) and whether any change occurs in gastric smooth muscle cells (SMCs) of PD patients is unclear. We previously reported that rats with bilateral substantia nigra lesions induced by 6-hydroxydopamine (6-OHDA), referred to as 6-OHDA rats, manifest typical gastroparesis. In the present study, we further investigate the underlying mechanism. By means of an organ bath system and an implantable radiotelemetry system, both a weakened contractile force of gastric circular smooth muscle and gastric myoelectric activity were detected in the 6-OHDA rats and phasic and tonic contractions elicited by carbachol or high concentration of potassium were significantly reduced in gastric circular muscle strips. A thickened smooth muscle layer was observed under a light microscope and an ultrastructure of hypertrophic SMCs, with increased caveolae and decreased dense bodies, was observed under transmission electron microscope. Furthermore, the mRNA and protein expression levels of contractile markers (myosin light chain 20, myosin heavy chain 11 and α-smooth muscle actin) and the transcription factor serum response factor (SRF) were significantly decreased, while the TNFα and IL-1β content was increased in the 6-OHDA rats. These results suggest that the decreased contractile force in 6-OHDA rats may be associated with the phenotypic abnormality observed in SMCs, which is due to downregulated contractile proteins induced by decreased SRF expression in the inflammatory muscular microenvironment.

Rac1 regulates myosin II phosphorylation through regulation of myosin light chain phosphatase

Phosphorylation of regulatory light chain (MLC) activates myosin II, which enables it to promote contractile and motile activities of cells. We report here a novel signaling mechanism that activates MLC phosphorylation and smooth muscle contraction. Contractile agonists activated Rac1, and Rac1 inhibition diminished agonist-induced MLC phosphorylation, thus inhibiting smooth muscle contraction. Rac1 inhibits the activity of MLC phosphatase (MLCP) but not that of MLC kinase, through a phosphatase that targets MYPT1 (a regulatory subunit of MLCP) and CPI-17 (a MLCP specific inhibitor) rather than through the RhoA-Rho dependent kinase (ROCK) pathway. Rac1 inhibition decreased the activity of protein kinase C (PKC), which also contributes to the change in CPI-17 phosphorylation. We propose that activation of Rac1 increases the activity of PKC, which increases the phosphorylation of CPI-17 and MYPT1 by inhibiting the phosphatase that targets these proteins, thereby decreasing the activity of MLCP and increasing phosphorylation of MLC. Our results suggest that Rac1 coordinates with RhoA to increase MLC phosphorylation by inactivation of CPI-17/MYPT1 phosphatase, which decreases MLCP activity thus promoting MLC phosphorylation and cell contraction.

Inhaled corticosteroids modulate the (+)insert smooth muscle myosin heavy chain in the equine asthmatic airways

RATIONALE

Overexpression of the (+)insert smooth muscle myosin heavy chain (SMMHC) isoform could contribute to airway bronchospasm by increasing the velocity of contraction. Whether the (+)insert isoform is present in the small airways and its expression is reversible in asthma are unknown.

OBJECTIVES

To determine the anatomical location and the expression kinetics of the (+)insert SMMHC isoform in airways of horses with heaves and to evaluate its modulation in response to disease status.

METHODS

We evaluated the (+)insert SMMHC isoform in the airways of horses with heaves during disease exacerbation and remission, and in controls. The expression kinetics of the SMMHC (+)insert was then assessed at multiple time points in two studies: first, in horses with heaves treated for a 1-year period with antigen avoidance alone, inhaled corticosteroids alone or both; second, in horses with heaves before and after a 30-day natural antigen exposure. Gene expression analysis was assessed by quantitative PCR and protein expression was confirmed by targeted mass spectrometry.

MEASUREMENTS AND MAIN RESULTS

The (+)insert SMMHC isoform was significantly increased in central and peripheral airways, but not in the trachea of heaves-affected horses in clinical exacerbation when compared horses with heaves in remission and controls. Both corticosteroid administration and antigen avoidance led to a significant reduction of the (+)insert expression in the airways. The (+)insert SMMHC isoform was not significantly increased in airways after 1 month of antigenic re-exposure.

CONCLUSIONS

The (+)insert SMMHC expression is increased throughout the bronchial tree in horses with heaves and reversible by corticosteroids administration and antigen avoidance.

Jun kinase-induced overexpression of leukemia-associated Rho GEF (LARG) mediates sustained hypercontraction of longitudinal smooth muscle in inflammation

The signaling pathways mediating sustained contraction of mouse colonic longitudinal smooth muscle and the mechanisms involved in hypercontractility of this muscle layer in response to cytokines and TNBS-induced colitis have not been fully explored. In control longitudinal smooth muscle cells, ACh acting via m3 receptors activated sequentially Gα12, RhoGEF (LARG), and the RhoA/Rho kinase pathway. There was abundant expression of MYPT1, minimal expression of CPI-17, and a notable absence of a PKC/CPI-17 pathway. LARG expression was increased in longitudinal muscle cells isolated from muscle strips cultured for 24 h with IL-1β or TNF-α or obtained from the colon of TNBS-treated mice. The increase in LARG expression was accompanied by a significant increase in ACh-stimulated Rho kinase and ZIP kinase activities, and sustained muscle contraction. The increase in LARG expression, Rho kinase and ZIP kinase activities, and sustained muscle contraction was abolished in cells pretreated with the Jun kinase inhibitor, SP600125. Expression of the MLCP activator, telokin, and MLCP activity were also decreased in longitudinal muscle cells from TNBS-treated mice or from strips treated with IL-1β or TNF-α. In contrast, previous studies had shown that sustained contraction in circular smooth muscle is mediated by sequential activation of Gα13, p115RhoGEF, and dual RhoA-dependent pathways involving phosphorylation of MYPT1 and CPI-17. In colonic circular smooth muscle cells isolated from TNBS-treated mice or from strips treated with IL-1β or TNF-α, CPI-17 expression and sustained muscle contraction were decreased. The disparate changes in the two muscle layers contribute to intestinal dysmotility during inflammation.

Tonic and phasic smooth muscle contraction is not regulated by the PKCα - CPI-17 pathway in swine stomach antrum and fundus

Regulation of myosin light chain phosphatase (MLCP) via protein kinase C (PKC) and the 17 kDa PKC-potentiated inhibitor of myosin light chain phosphatase (CPI-17) has been reported as a Ca²⁺ sensitization signaling pathway in smooth muscle (SM), and thus may be involved in tonic vs. phasic contractions. This study examined the protein expression and spatial-temporal distribution of PKCα and CPI-17 in intact SM tissues. KCl or carbachol (CCh) stimulation of tonic stomach fundus SM generates a sustained contraction while the phasic stomach antrum generates a transient contraction. In addition, the tonic fundus generates greater relative force than phasic antrum with 1 µM phorbol 12, 13-dibutyrate (PDBu) stimulation which is reported to activate the PKCα – CPI-17 pathway. Western blot analyses demonstrated that this contractile difference was not caused by a difference in the protein expression of PKCα or CPI-17 between these two tissues. Immunohistochemical results show that the distribution of PKCα in the longitudinal and circular layers of the fundus and antrum do not differ, being predominantly localized near the SM cell plasma membrane. Stimulation of either tissue with 1 µM PDBu or 1 µM CCh does not alter this peripheral PKCα distribution. There are no differences between these two tissues for the CPI-17 distribution, but unlike the PKCα distribution, CPI-17 appears to be diffusely distributed throughout the cytoplasm under relaxed tissue conditions but shifts to a primarily peripheral distribution at the plasma membrane with stimulation of the tissues with 1 µM PDBu or 1 µM CCh. Results from double labeling show that neither PKCα nor CPI-17 co-localize at the adherens junction (vinculin/talin) at the membrane but they do co-localize with each other and with caveoli (caveolin) at the membrane. This lack of difference suggests that the PKCα - CPI-17 pathway is not responsible for the tonic vs. phasic contractions observed in stomach fundus and antrum.

Differential expression of multidrug resistance protein 5 and phosphodiesterase 5 and regulation of cGMP levels in phasic and tonic smooth muscle

Previous studies have identified differences in the expression of proteins that regulate myosin light chain phosphorylation and contraction in tonic and phasic smooth muscle. cGMP plays a critical role in smooth muscle relaxation and is important for optimal function of phasic and tonic smooth muscle. The intracellular cGMP levels are regulated by its hydrolysis via phosphodiesterase 5 (PDE5) and efflux via novel multidrug resistance protein 5 (MRP5). In the present study we tested the hypothesis that the differences in the phasic and tonic behavior of smooth muscles may be related to differences in mechanisms that terminate cGMP signaling. Expression of PDE5 and MRP5 was significantly (more than 2-fold) higher in fundus compared with antrum. The NO donor S-nitrosoglutathione (GSNO) caused an increase in PDE5 activity and intra- and extracellular cGMP levels in both fundus and antrum. Stimulation of PDE5 activity and increase in extracellular cGMP were significantly higher in fundus, whereas increase in intracellular cGMP was significantly higher in antrum. GSNO-induced increase in extracellular cGMP was blocked in dispersed cells by the cyclic nucleotide export blocker probenecid and in cultured muscle cells by depletion of ATP or suppression of MRP5 by siRNA, providing evidence that cGMP efflux was mediated by ATP-dependent export via MRP5. Consistent with the higher expression and activity levels of PDE5 and MRP5, GSNO-induced PKG activity and muscle relaxation were significantly lower in muscle cells from fundus compared with antrum. Thus higher expression of PDE5 and MRP5 in muscle cells from fundus correlates with tonic phenotype of muscle.