In vivo regulation of the beta-myosin heavy chain gene in soleus muscle ofsuspended and weight-bearing rats

In the weight-bearing hindlimb soleus muscle of the rat, similar to 90% of muscle fibers express the beta-myosin heavy chain (beta-MHC) isoform protei n. Hindlimb suspension (HS) causes the MHC isoform population to shift from beta toward the fast MHC isoforms. Our aim was to establish a model to tes t the hypothesis that this shift in expression is transcriptionally regulat ed through specific cis elements of the beta-MHC promoter. With the use of a direct gene transfer approach, we determined the activity of different le ngth beta-MHC promoter fragments, linked to a firefly luciferase reporter g ene, in soleus muscle of control and HS rats. In weight-bearing rats, the r elative luciferase activity of the longest beta-promoter fragment (-3500 bp ) was threefold higher than the shorter promoter constructs, which suggests that an enhancer sequence is present in the upstream promoter region. Afte r 1 wk of HS, the reporter activities of the -3500-, -914-, and -408-bp pro moter constructs were significantly reduced (similar to 40%), compared with the control muscles. However, using the -215-bp construct, no differences in promoter activity were observed between HS and control muscles, which in dicates that the response to HS in the rodent appears to be regulated withi n the -408 and -215 bp of the promoter.