MYOSIN HEAVY-CHAIN MESSENGER-RNA AND PROTEIN EXPRESSION IN SINGLE FIBERS OF THE RAT SOLEUS FOLLOWING REINNERVATION

Myosin heavy chain (MHC) expression is regulated by many factors inclu ding neural input. To gain a better understanding of myosin transforma tion following reinnervation we examined both MHC protein and mRNA in single fibers of the soleus. A midthigh sciatic nerve lesion resulted in reinnervation of the soleus by motoneurons from both the original a nd foreign motor pools. MHC expression was examined in individual fibe rs 8 and 16 weeks post injury using in situ histochemistry and immunoh istochemistry. Following a sciatic nerve lesion, the reinnervated sole us underwent a transformation from slow toward fast based on physiolog ic and biochemical measurements, At 8 weeks, fast MHC mRNA isoforms (I Ia and IIx) were upregulated and slow mRNA was downregulated, however, the predominant protein isoform was MHC I. At both 8 and 16 weeks, ma ny fibers expressed multiple mRNA isoforms. At 16 weeks there was limi ted co-expression of slow and fast MHC mRNAs, but continued co-express ion of fast MHC mRNAs. Sixteen weeks following reinnervation the predo minant fast mRNA and protein in the soleus was IIx MHC.