Heterogeneous activation of a slow myosin gene in proliferating myoblasts and differentiated single myofibers.

Each skeletal muscle contains a fixed ratio of fast and slow myofibers that are distributed in a stereotyped pattern to achieve a specific motor function. How myofibers are specified during development and regeneration is poorly understood. Here we address this question using transgenic reporter mice that indelibly mark the myofiber lineages based on activation of fast or slow myosin. Lineage tracing indicates that during development all muscles have activated the fast myosin gene Myl1, but not the slow myosin gene Myh7, which is activated in all slow but a subset of fast myofibers. Similarly, most nascent myofibers do not activate Myh7 during fast muscle regeneration, but the ratio and pattern of fast and slow myofibers are restored at the completion of regeneration. At the single myofiber level, most mature fast myofibers are heterogeneous in nuclear composition, manifested by mosaic activation of Myh7. Strikingly, Myh7 is activated in a subpopulation of proliferating myoblasts that co-express the myogenic progenitor marker Pax7. When induced to differentiate, the Myh7-activated myoblasts differentiate more readily than the non-activated myoblasts, and have a higher tendency, but not restricted, to become slow myotubes. Together, our data reveal significant nuclear heterogeneity within a single myofiber, and challenge the conventional view that myosin genes are only expressed after myogenic differentiation. These results provide novel insights into the regulation of muscle fiber type specification.

The role of integrin-ß/FAK in cyclic mechanical stimulation in MG-63 cells.

OBJECTIVE: This study aims to explore the function of Integrin-ß/FAK in the mechanical signal transduction and the connection with downstream ERK signal pathways. METHODS: Human osteosarcoma MG63 cell lines were used in this study. The effects of mechanical strain on the Integrin-ß1 expression, FAK and ERK signal pathway in Human osteosarcoma MG63 cells were detected using RT-PCR and Western-blotting methods. The localization of FAK in Human osteosarcoma MG63 cells were determined using immunofluorescent method. The interaction between Integrin-ß1 and FAK were detected by using co-immunoprecipitation method. RESULTS: The expression of Integrin-ß1 shows a notable bimodel distribution, mechanical strain stimulation can promote Integrin-ß1 expression and the phosphorylation of FAK and ERK, mechanical strain activated FAK and ERK mediated by Integrin-ß1. CONCLUSION: Integrin-ß1 may play an important role in osteoblast proliferation differentiation process, it might feel external strain stimulation through ECM composition and makes FAK phosphated through the interaction with FAK, thus causing a series of activation of signal molecules. Finally it reduces MAPK (ERK) activation and cellular responses to finish mechanical signal transduction.