These mice accumulate myoblasts that are arrested in their terminal differentiation program

These mice accumulate myoblasts that are arrested in their terminal differentiation program. from wild type myocytes (p<0.005) 40 h after differentiation induction. In regenerating muscle, the expression of myogenin in the -syntrophin null muscle was reduced to approximately 25% that of wild type muscle (p<0.005). Conversely, myogenin expression is enhanced in primary cultures of myoblasts isolated from a transgenic mouse over-expressing -syntrophin and in Sol8 cells transfected with a vector to over-express -syntrophin. Moreover, we find that myogenin mRNA is reduced in the absence of -syntrophin and increased by -syntrophin over-expression. Immunofluorescence microscopy shows that -syntrophin is localized to the nuclei of differentiating myoblasts. Finally, immunoprecipitation experiments demonstrate that -syntrophin associates with Mixed-Lineage Leukemia 5, a regulator of myogenin expression. == Conclusions == We conclude that -syntrophin plays an important role in regulating myogenesis by modulating myogenin expression. == Introduction == The basic helix-loop-helix family of transcription regulatory factors is critical for proper development of skeletal muscle. MyoD and Myf5 are important for myogenic determination while myogenin and Mrf4 promote terminal differentiation of muscle precursor cells to mature skeletal muscle. Myogenin regulates the expression of a myriad of muscle specific genes and is necessary for normal muscle development[1]. Myogenin knockout mice have a severe muscle deformity at birth[2][3]. These mice accumulate myoblasts that are arrested in their terminal differentiation program. During normal muscle development, myogenin is not expressed RIEG in undifferentiating myoblasts but accumulates at the onset of differentiation[4]. Expression of myogenin is induced by factors that stimulate the myogenic program, such as the decrease in growth factors or presence of insulin-like growth factor-1 (IGF-1), a potent activator of differentiation[5][8]. -Syntrophin has been widely studied for its role as a molecular adapter protein that recruits signaling proteins to the sarcolemmal dystrophin scaffold in mature muscle. -Syntrophin binds ion channels[9][11], G-protein coupled receptors[12], water channels[13], kinases and associated proteins[14][17], and neuronal nitric oxide synthase[18]. Nearly all studies of skeletal muscle -syntrophin have been performed using mature muscle. Very little is known about the role of -syntrophin during early muscle development, even though myoblasts express -syntrophin before terminal differentiation[19]. One study has shown that -syntrophin localizes diacylglycerol kinase- (DGK during several stages of myoblast fusion[19]. The most compelling evidence for a role of -syntrophin in muscle development comes from investigation of muscle regeneration in a knockout mouse lacking of the -syntrophin[20]. Regenerating skeletal Deramciclane muscle in this mouse showed several defects including Deramciclane fiber splitting, fiber type shifts, increased IGF-1 mRNA, and decreased muscle strength 7 days after regeneration. The regeneration defects in the absence of -syntrophin and the observation that -syntrophin is present in undifferentiated myoblasts prompted us to investigate the role of -syntrophin in muscle development. In this study we focus on the relationship between -syntrophin and myogenin expression. We explore myogenin expression usingin vivoandin vitrosystems that vary in their level of -syntrophin expression. Furthermore we investigate the temporal expression and spatial localization of -syntrophin during myoblast differentiation to facilitate the understanding of -syntrophin’s role in muscle development. == Materials and Methods == == Deramciclane Materials == Isoform-specific antibodies to syntrophins were previously prepared[21]. All other antibodies were purchased as follows; anti-myogenin from BD Bioscience (San Jose, CA) or Santa Cruz Deramciclane Biotechnology (Santa Cruz, CA), antibodies to dystrophin and -actin from Sigma (St. Louis, MO), anti-MLL-5 antibody from Orbigen (San Diego, CA), antibodies to GAPDH and actin from Cell Signaling (Danvers, PA). -Syntrophin-specific siRNA and control siRNA were from Santa Cruz Biotechnology (Santa Cruz, CA). Cardiotoxin (Naja nigricollis) was obtained from Calbiochem (La Jolla, CA). Rabbit TrueBlot beads and antibody were obtained from eBioscience (San Diego, CA). All the cell culture reagents, 0.25% Trypsin-EDTA, Lipofectamine 2000 and pcDNA 3.1 were from Invitrogen (Carlsbad, CA). RNA-SPIN total RNA extraction kit, Maxim RT premix kit, and other.