BETA-MHC AND SMLC1 TRANSGENE INDUCTION IN OVERLOADED SKELETAL-MUSCLE OF TRANSGENIC MICE

The hypertrophic response of white fast-twitch muscle to mechanical ov erload has been investigated using transgenic mice. After 7 wk of over load, endogenous beta-myosin heavy chain (MHC) and slow myosin light c hain 1 and 2 (SMLC1, SMLC2) proteins were increased in the overloaded plantaris (OP) muscle compared with sham-operated control plantaris (C P) muscle. Concurrently, the levels of endogenous beta-MHC, SMLC1, SML C2, and cardiac/slow troponin C (cTnC) mRNA transcripts were significa ntly increased in OP muscles, whereas skeletal troponin C (sTnC) mRNA transcript levels decreased. As an initial attempt to locate DNA seque nce(s) that governs beta-MHC induction in response to mechanical overl oad, multiple independent transgenic lines harboring four different hu man beta-MHC transgenes (beta 1286, beta 988, beta 450, beta 141) were generated. Except for transgene beta 141, muscle-specific expression and induction (3- to 22-fold) in OP muscles were observed by measuring chloramphenicol acetyltransferase activity (CAT assay). Induction of a SMLC1 transgene (3920SMLC1) in OP muscles was also observed. Collect ively, these in vivo data provide evidence that 1) a mechanical overlo ad inducible element(s) is located between nucleotides -450 and +120 o f the human beta-MHC transgene, 2) 3,900 bp of 5' sequence is sufficie nt to confer mechanical overload induction to a SMLC1 transgene, and 3 ) the increased expression of slow/type I isomyosin (beta-MHC, SMLC1, SMLC2) in response to mechanical overload is regulated, in part, trans criptionally.