PGAM-M expression is regulated pretranslationally in hindlimb muscles and under altered loading conditions

Enzymatic activity from the muscle-specific isoform of phosphoglycerate mut ase (PGAM-M) is higher within glycolytic skeletal muscles than in oxidative muscles. The hypothesis that PGAM-M is regulated pretranslationally among muscles of the hindlimb was tested using enzymatic assays, Western blots, a nd Northern blots. We further investigated the regulatory level(s) at which PGAM-M gene expression is controlled during hindlimb unweighting. PGAM-M m RNA and immunoreactive protein levels were fourfold lower in the rat soleus muscle than in the tibialis anterior (TA), plantaris, and extensor digitor um longus muscles. Four weeks of unweighting induced a 2.5-fold increase in PGAM enzymatic activity within the soleus muscle, a 1.8-fold increase in P GAM-M immunoreactivity; and a 3.5-fold Increase in PGAM-M mRNA. To examine potential transcriptional regulatory mechanisms, the proximal 400 bp of the rat PGAM-M promoter were linked to a firefly luciferase and injected into normal and unweighted TA and soleus muscles. Firefly luciferase activity wa s elevated two- to threefold in the TA and the unweighted soleus over the n ormal soleus muscle. These data suggest that PGAM-M expression is pretransl ationally regulated among muscle types and within unweighted slow-twitch mu scle. Furthermore, the proximal 400 bp of the PGAM-M promoter contains cis- acting sequences to allow muscle-type-specific expression of a reporter gen e and responsiveness to soleus muscle unweighting.