Differential modulation of α, β and γ enolase isoforms in regenerating mouse skeletal muscle

Nothing is known about the expression of the glycolytic enzyme enolase in skeletal 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, whereas a transition towards specific isoforms occurs during ontogenesis in two cell types with high energy requirements: alpha gamma and gamma gamma in neurons, alpha beta and beta beta in striated muscle cells. During murine myogenesis, beta enolase transcripts are detected early in the forming muscles, and the beta gene is further upregulated at specific stages of muscle development. The alpha and beta subunits exhibit characteristic developmental microheterogeneity patterns. High levels of beta enolase subunits characterize the glycolytic fast-twitch fibers of adult muscles. We have investigated the expression of enolase subunits in a mouse experimental model of muscle 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 activity that correlated with the degeneration of myofibers. Concomitant with the regeneration of new fibers, beta subunit levels began to increase, reaching normal values by 30 days after injury. Changes in the embryonic and ubiquitous subunit, alpha, mimicked those occurring during development by two aspects: modifications in electrophoretic variants and redistribution between soluble and insoluble compartments of muscle extracts. Imunocytochemical analyses of alpha and beta enolase subunits first revealed a homogeneous labeling within myofibers. Striations characteristic of normal adult muscle tissue were visible again by day 14 after injury. A perinuclear alpha and beta immunoreactivity was often observed in regenerating myofibers but its functional significance remains to be elucidated. Double labeling experiments with anti-gamma enolase and FITC-alpha bungarotoxin allowed us to follow the neuromuscular junction remodeling that occurs during muscle regeneration despite the absence of nerve injury.