REGULATION OF MYOSIN HEAVY-CHAIN GENE-EXPRESSION AFTER SHORT-TERM DIAPHRAGM INACTIVATION

Although prolonged diaphragm denervation (DNV) produces myofiber atrop hy and a loss of type I myosin heavy chain (MHC) expression, short-ter m DNV leads to significant diaphragm hypertrophy. The purpose of this study was to explore the regulation of MHC isoform expression and musc le remodeling during DNV hypertrophy of the diaphragm. Both unilateral and bilateral DNV led to similar changes, with a significant increase in total RNA content and muscle mass but no change in dry-to-wet weig ht ratio. Sarcomere number was also increased in diaphragm myofibers a fter DNV (similar to 20%), suggesting an adaptive response to muscle s tretch. There was hypertrophy of type I myofibers and increased coexpr ession of type I and type II MHCs within single myofibers by immunocyt ochemistry as well as increased type I MHC (25-46%) and decreased type IIb MHC (14-39%) by SDS-PAGE. Contractility parameters were also cons istent with a type II-to-type I MHC phenotype transformation. Importan tly, DNV-induced modulation of MHC isoform mRNA transcript levels did not correspond to changes in their cognate proteins, suggesting a majo r degree of posttranscriptional control. We conclude that DNV hypertro phy of the diaphragm is associated with reciprocal changes in type I a nd type II MHC isoforms that are directly opposed to the type I-to-typ e II MHC phenotype transformation reported in the diaphragm DNV atroph y model. Furthermore, in contradistinction to most hypertrophy models, control of MHC gene expression and myofibrillar remodeling after shor t-term DNV appears to entail major involvement of posttranscriptional regulatory mechanisms.