Fibre-type distribution and subcellular localisation of alpha and beta enolase in mouse striated muscle.

Enolase is a dimeric glycolytic enzyme exhibiting tissue specific isoforms. During ontogenesis, a transition occurs from the embryonic alphaalpha towards the specific alphabeta, and betabeta isoforms in striated muscle. Immunocytochemical analyses on transverse sections of adult mouse gastrocnemius muscle, allowed us to compare the expression of alpha and beta subunits to that of myosin heavy chain (MHC) isoforms. Levels of beta immunoreactivity followed the order IIB > IIX > IIA > I. This gradient parallels the ATPase activity associated to MHC isoforms, indicating that the expression of beta enolase in myofibres is finely regulated as a function of energetic requirements. By contrast, variations in alpha immunolabelling intensity appeared independent of fibre types. Longitudinal muscle sections exhibited a striated pattern of alpha immunoreactivity. Confocal microscopy analyses demonstrated that alpha was localised at the M band. Most beta immunoreactivity was diffuse all over the sarcoplasm. However, some beta immunoreactivity was striated and localized at both Z and M bands. Thus, betabeta enolase could participate to multi-enzyme complexes present at the I band, and involved with local ATP production. Our results support the notion that isozymes differ in their ability to interact with other macromolecules, thus segregating to different subcellular sites where they would respond to specific functional demands.

A binding site for nuclear receptors is required for the differential expression of the aldolase A fast-twitch muscle promoter in body and head muscles.

In hind limb muscles, the aldolase A muscle-specific promoter is specifically expressed in glycolytic fast-twitch fibers. Here, we show that in addition, it is expressed at higher levels in trunk and limb muscles than in neck and head muscles independent of their fiber-type content. We have identified by analysis of transgenic mice a DNA element that is required for this differential expression and, to a lesser extent, for fiber-type specificity. We show that members of the nuclear receptor superfamily bind this element in skeletal muscle nuclear extracts. Interestingly, in gel mobility shift assays, different complexes were formed with this sequence in tongue nuclear extracts compared with limb or trunk muscle nuclear extracts. Therefore, binding of distinct nuclear receptors to a single regulatory sequence appears to be associated with the location-dependent expression of the aldolase A muscle-specific promoter.