Transcriptional regulation of the myosin heavy chain IIb gene in inactive rat soleus.

The myosin heavy chain (MHC) isoform composition of skeletal muscle is dependent, in part, on the functional demands of the muscle. The rat soleus muscle primarily expresses the slow-contracting type I MHC; however, chronic inactivity increases expression of the faster-contracting type II MHC isoforms. The purpose of this study was to identify the type IIb MHC promoter region(s) that regulate de novo transcription during chronic inactivity of the soleus induced by spinal cord isolation (SI; complete mid-thoracic and high sacral spinal cord transections plus deafferentation). Seven days after SI, transcription of IIb MHC was evidenced by increases in IIb pre-mRNA and mRNA. The activity of an approximately 2.2-kb IIb promoter-firefly luciferase reporter plasmid increased in SI soleus over control as compared to that of a promoterless plasmid. Deletion analyses indicated that the regions encompassing -2237 to -1431, -1048 to -461, and -192 to -161 basepairs (bp) each contributed to the increase in transcriptional activity. Moreover, deletions or mutations of AT-rich regions in the proximal -192 bp region abolished the increased promoter activity. These results provide important insights related to how proximal IIb MHC promoter elements regulate the increased expression of the IIb MHC gene in response to inactivity of a predominantly slow postural muscle as it undergoes a remodeling of its phenotype and functional characteristics.