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, αα, remains expressed in most adult tissues, whereas a transition towards specific isoforms occurs during ontogenesis in two cell types with high energy requirements: αγ and γγ in neurons, αβ and ββ in striated muscle cells. During murine myogenesis, β enolase transcripts are detected early in the forming muscles, and the β gene is further upregulated at specific stages of muscle development. The α and β subunits exhibit characteristic developmental microheterogeneity patterns. High levels of β 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 β, accounting for the drop in total enolase activity that correlated with the degeneration of myofibers. Concomitant with the regeneration of new fibers, β subunit levels began to increase, reaching normal values by 30 days after injury. Changes in the embryonic and ubiquitous subunit, α, 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 α and β 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 α and β immunoreactivity was often observed in regenerating myofibers but its functional significance remains to be elucidated. Double labeling experiments with anti‐γ enolase and FITC‐α bungarotoxin allowed us to follow the neuromuscular junction remodeling that occurs during muscle regeneration despite the absence of nerve injury.