Balance between S-nitrosylation and denitrosylation modulates myoblast proliferation independently of soluble guanylyl cyclase activation.

Nitric oxide (NO) contributes to myogenesis by regulating the transition between myoblast proliferation and fusion through cGMP signaling. NO can form S-nitrosothiols (RSNO), which control signaling pathways in many different cell types. However, neither the role of RSNO content nor its regulation by the denitrosylase activity of S-nitrosoglutathione reductase (GSNOR) during myogenesis is understood. Here, we used primary cultures of chick embryonic skeletal muscle cells to investigate whether changes in intracellular RSNO alter proliferation and fusion of myoblasts in the presence and absence of cGMP. Cultures were grown to fuse most of the myoblasts into myotubes, with and without S-nitrosocysteine (CysNO), 8-Br-cGMP, DETA-NO, or inhibitors for NO synthase (NOS), GSNOR, soluble guanylyl cyclase (sGC), or a combination of these, followed by analysis of GSNOR activity, protein expression, RSNO, cGMP, and cell morphology. Although the activity of GSNOR increased progressively over 72 h, inhibiting GSNOR (by GSNOR inhibitor - GSNORi - or by knocking down GSNOR with siRNA) produced an increase in RSNO and in the number of myoblasts and fibroblasts, accompanied by a decrease in myoblast fusion index. This was also detected with CysNO supplementation. Enhanced myoblast number was proportional to GSNOR inhibition. Effects of the GSNORi and GSNOR knockdown were blunted by NOS inhibition, suggesting their dependence on NO synthesis. Interestingly, GSNORi and GSNOR knockdown reversed the attenuated proliferation obtained with sGC inhibition in myoblasts, but not in fibroblasts. Hence myoblast proliferation is enhanced by increasing RSNO, and regulated by GSNOR activity, independently of cGMP production and signaling.

Cholesterol depletion induces transcriptional changes during skeletal muscle differentiation.

BACKGROUND: Myoblasts undergo major changes in their plasma membrane during the initial steps of skeletal muscle differentiation, including major alterations in the distribution of cholesterol. Cholesterol is involved in crucial membrane functions, such as fluidity, and permeability, and in the organization of specialized membrane microdomains (or lipid rafts). We have previously shown that alterations in cholesterol levels in myoblasts induce changes in proliferation and differentiation, which involves activation of Wnt/beta-catenin signaling pathway. In this study we used methyl-ß-cyclodextrin (MbCD) to extract cholesterol from the membrane of chick skeletal muscle cells grown in culture. Using Ion Torrent-based sequencing, we compared the transcriptome of untreated and MbCD treated cells. Our aim was to define the genes that are expressed in these two conditions and relate their expression to cellular functions. RESULTS: Over 5.7 million sequences were obtained, representing 671.38 Mb of information. mRNA transcriptome profiling of myogenic cells after cholesterol depletion revealed alterations in transcripts involved in the regulation of apoptosis, focal adhesion, phagosome, tight junction, cell cycle, lysosome, adherens junctions, gap junctions, p53 signaling pathway, endocytosis, autophagy and actin cytoskeleton. Lim domain only protein 7 mRNA was found to be the highest up-regulated feature after cholesterol depletion. CONCLUSIONS: This is the first study on the effects of membrane cholesterol depletion in mRNA expression in myogenic cells. Our data shows that alterations in the availability of plasma membrane cholesterol lead to transcriptional changes in myogenic cells. The knowledge of the genes involved in the cellular response to cholesterol depletion could contribute to our understanding of skeletal muscle differentiation.

Knockdown of Lmo7 inhibits chick myogenesis.

The multifunctional protein Lmo7 has been implicated in some aspects of myogenesis in mammals. Here we studied the distribution and expression of Lmo7 and the effects of Lmo7 knockdown in primary cultures of chick skeletal muscle cells. Lmo7 was localized within the nuclei of myoblasts and at the perinuclear region of myotubes. Knockdown of Lmo7 using siRNA specific to chick reduces the number and width of myotubes and the number of MyoD positive-myoblasts. Both Wnt3a enriched medium and Bio, activators of the Wnt/beta-catenin pathway, could rescue the effects of the Lmo7 knockdown suggesting a crosstalk between the Wnt/beta-catenin and Lmo7-mediated signaling pathways. Our data shows a role of Lmo7 during the initial events of chick skeletal myogenesis, particularly in myoblast survival.