Emerin expression at the early stages of myogenic differentiation

Emerin is an ubiquitous protein localized at the nuclear membrane of most c ell types including muscle cells. The protein is absent in most patients af fected by the X-linked form of Emery-Dreifuss muscular dystrophy, a disease characterized by slowly progressive muscle wasting and weakness, early con tractures of the elbows, Achilles tendons, and post-cervical muscles, and c ardiomyopathy. Besides the nuclear localization, emerin cytoplasmic distrib ution has been suggested in several cell types. We studied the expression a nd the subcellular distribution of emerin in mouse cultured C2C12 myoblasts and in primary cultures of human myoblasts induced to differentiate or spo ntaneously differentiating in the culture medium. In differentiating myobla sts transiently transfected with a cDNA encoding the complete emerin sequen ce, the protein localized at the nuclear rim of all transfected cells and a lso in the cytoplasm of some myoblasts and myotubes. Cytoplasmic emerin was also observed in detergent-treated myotubes, as determined by electron mic roscopy observation. Both immunofluorescence and biochemical analysis showe d, that upon differentiation of C2C12 cells, emerin expression was decrease d in the resting myoblasts but the protein was highly represented in the de veloping myotubes at the early stage of cell fusion. Labeling with specific markers of myogenesis such as troponin-Tr and myogenin permitted the corre lation of increased emerin expression with the onset of muscle differentiat ion. These data suggest a role for emerin during proliferation of activated satellite cells and at the early stages of differentiation.