Differential modulation of alpha, beta and gamma enolase isoforms in regenerating mouse skeletal muscle

Nothing is known about the expression of the glycolytic enzyme enolase in s keletal muscle alterations such as myofiber degeneration and regeneration. Enolase is a dimeric enzyme which exhibits cell type specific isoforms. The embryonic form, alpha alpha, remains expressed in most adult tissues, wher eas a transition towards specific isoforms occurs during ontogenesis in two cell types with high energy requirements: alpha gamma and gamma gamma in n eurons, alpha beta and beta beta in striated muscle cells. During murine my ogenesis, beta enolase transcripts are detected early in the forming muscle s, and the beta gene is further upregulated at specific stages of muscle de velopment. The alpha and beta subunits exhibit characteristic developmental microheterogeneity patterns. High levels of beta enolase subunits characte rize the glycolytic fast-twitch fibers of adult muscles. We have investigat ed the expression of enolase subunits in a mouse experimental model of musc le regeneration. Following a single intramuscular injection of the necrotic agent cardiotoxin, we observed a rapid decrease in the level of the major muscle enolase subunit beta, accounting for the drop in total enolase activ ity that correlated with the degeneration of myofibers. Concomitant with th e regeneration of new fibers, beta subunit levels began to increase, reachi ng normal values by 30 days after injury. Changes in the embryonic and ubiq uitous subunit, alpha, mimicked those occurring during development by two a spects: modifications in electrophoretic variants and redistribution betwee n soluble and insoluble compartments of muscle extracts. Imunocytochemical analyses of alpha and beta enolase subunits first revealed a homogeneous la beling within myofibers. Striations characteristic of normal adult muscle t issue were visible again by day 14 after injury. A perinuclear alpha and be ta immunoreactivity was often observed in regenerating myofibers but its fu nctional significance remains to be elucidated. Double labeling experiments with anti-gamma enolase and FITC-alpha bungarotoxin allowed us to follow t he neuromuscular junction remodeling that occurs during muscle regeneration despite the absence of nerve injury.