Cyclosporin-A inhibits stretch-induced changes in myosin heavy chain expression in C2C12 skeletal muscle cells

Molecular and Biomechanical Adaptations to Mechanical Stretch in Cultured Myotubes

Myotubes are mature muscle cells that form the basic structural element of skeletal muscle. When stretching skeletal muscles, myotubes are subjected to passive tension as well. This lead to alterations in myotube cytophysiology, which could be related with muscular biomechanics. During the past decades, much progresses have been made in exploring biomechanical properties of myotubes in vitro. In this review, we integrated the studies focusing on cultured myotubes being mechanically stretched, and classified these studies into several categories: amino acid and glucose uptake, protein turnover, myotube hypertrophy and atrophy, maturation, alignment, secretion of cytokines, cytoskeleton adaption, myotube damage, ion channel activation, and oxidative stress in myotubes. These biomechanical adaptions do not occur independently, but interconnect with each other as part of the systematic mechanoresponse of myotubes. The purpose of this review is to broaden our comprehensions of stretch-induced muscular alterations in cellular and molecular scales, and to point out future challenges and directions in investigating myotube biomechanical manifestations.

The effects of age upon the expression of three miRNAs in muscle stem cells isolated from two different porcine skeletal muscles

Aging is associated with a gradual loss of skeletal muscle mass and an impaired ability of this tissue to compensate for trauma. Studies in rodents and humans have also shown that resident stem cells within muscle have a reduced ability to proliferate and differentiate. In this study muscle stem cells have been isolated from two muscles, the diaphragm (DIA) and the semimembranosus (SM), from young and old pigs. The levels of three micro-RNAs (miRNAs) were measured when cells were in a proliferative phase and after 24 and 72h in differentiation medium. All three miRNAs are abundant in skeletal muscle with miR-1 and miR-206 known to regulate myogenic differentiation and miR-24 is involved in cell cycle regulation. The levels of expression of Pax7 and the myogenic regulatory factors MyoD and myogenin were also measured. There were marked differences in expression of all three miRNAs between the two age groups. Both miR-1 and miR-206 were reduced in the cells from the older animals. In contrast miR-24 expression was significantly higher in cells from older animals under differentiation conditions. There were also significant differences in the relative expression of all three miRNAs between cells from the SM and DIA in both young and old animals. The changes in miRNA expression described in this study that relate to age, may play a role in the impaired differentiation capacity of older muscle stem cells.

Calcineurin is critical for sodium-induced neointimal formation in normotensive and hypertensive rats

It is well known that excessive intake of sodium chloride (sodium) is a risk factor for cardiovascular disease because it raises blood pressure. However, sodium loading reportedly promotes cardiovascular disease independently of its effect on blood pressure. To examine the mechanisms by which sodium loading promotes vascular inflammation independently of its effect on blood pressure, we examined the role of calcineurin in sodium loading-induced vascular inflammation using a wire injury model of the rat femoral artery. Calcineurin mRNA expression in the wire-injured femoral artery was significantly higher in sodium-loaded normotensive rats, such as Wistar-Kyoto (WKY) rats, than that in control WKY rats. Neointimal formation was also significantly enhanced in sodium-loaded WKY rats compared with control WKY rats. Gene transfer of an adenovirus expressing a dominant negative mutant of calcineurin (AdCalAΔC92Q) significantly suppressed neointimal formation in sodium-loaded WKY rats to a level similar to that observed in control WKY rats. Calcineurin expression and neointimal formation were more significantly enhanced in hypertensive rats, such as spontaneously hypertensive rats (SHRs), than those in control WKY rats. AdCalAΔC92Q infection significantly suppressed neointimal formation in SHRs to a level similar to that observed in control WKY rats. These results suggest that sodium loading promotes neointimal formation, even in normotensive rats, and that hypertension further stimulates neointimal formation. These results also suggest that calcineurin plays a pivotal role in this process.

Biomechanical signals upregulate myogenic gene induction in the presence or absence of inflammation

Inflammation of the muscle invariably leads to muscle cell damage and impaired regeneration. Biomechanical signals play a vital role in the regulation of myogenesis in healthy and inflamed muscle. We hypothesized that biomechanical signals counteract the actions of proinflammatory mediators and upregulate the basic helix-loop-helix and MADS box transcription enhancer factor 2 (MEF2) families of transcription factors, leading to increased myogenesis in inflamed muscle cells. For this purpose, C2C12 cells plated on collagenized silastic membranes were subjected to equibiaxial cyclic tensile strain (CTS) in the presence or absence of TNF-alpha, and the myogenic gene induction was examined over a period of 72 h. Exposure of cells to CTS resulted in a significant upregulation of mRNA expressions and synthesis of myogenic regulatory factors, MYOD1, myogenin (MYOG), MEF2A, and cyclin-dependent kinase inhibitor 1A (CDKN1A; p21) as well as muscle structural proteins like myosin heavy chain (MYHC) isoforms (MYH1, MYH2, and MYH4) and alpha-tropomyosin (TPM1), eventually leading to an increase in myotube formation. Contrarily, TNF-alpha suppressed the expression of all of the above differentiation-inducing factors in C2C12 cells. Further results revealed that simultaneous exposure of C2C12 cells to CTS and TNF-alpha abrogated the TNF-alpha-mediated downregulation of myogenic differentiation. In fact, the mRNA expression and protein synthesis of all myogenic factors (Myod1, Myog, Mef2a, Cdkn1a, Myh1, Myh2, Myh4, and Tpm1) were increased in stretched C2C12 cells despite the sustained presence of TNF-alpha. These results demonstrate that mechanotransduction regulates multiple signaling molecules involved in C2C12 cell differentiation. On one hand, these signals are potent transducers of myotube phenotype in myoblasts; on the other, these signals counteract catabolic actions of proinflammatory cytokines like TNF-alpha and allow the expression of myogenic genes to upregulate muscle cell differentiation.

Cytoskeleton/stretch-activated ion channel interaction regulates myogenic differentiation of skeletal myoblasts

In the present study, we investigated the functional interaction between stress fibers (SFs) and stretch-activated channels (SACs) and its possible role in the regulation of myoblast differentiation induced by switch to differentiation culture in the presence or absence of sphingosine 1-phosphate. It was found that there was a clear temporal correlation between SF formation and SAC activation in differentiating C2C12 myoblasts. Inhibition of actin polymerization with the specific Rho kinase inhibitor Y-27632, significantly decreased SAC sensitivity in these cells, suggesting a role for Rho-dependent actin remodeling in the regulation of the channel opening. The alteration of cytoskeletal/SAC functional correlation had also deleterious effects on myogenic differentiation of C2C12 cells as judged by combined confocal immunofluorescence, biochemical and electrophysiological analyses. Indeed, the treatment with Y-27632 or with DHCB, an actin disrupting agent, inhibited the expression of the myogenic markers (myogenin and sarcomeric proteins) and myoblast-myotube transition. The treatment with the channel blocker, GdCl(3), also affected myogenesis in these cells. It impaired, in fact, myoblast phenotypic maturation (i.e., reduced the expression of alpha-sarcomeric actin and skeletal myosin and the activity of creatine kinase) but did not modify promoter activity and protein expression levels of myogenin. The results of this study, together with being in agreement with the general idea that cytoskeletal remodeling is essential for muscle differentiation, describe a novel pathway whereby the formation of SFs and their contraction, generate a mechanical tension to the plasma membrane, activate SACs and trigger Ca(2+)-dependent signals, thus influencing the phenotypic maturation of myoblasts.