DIFFERENTIAL EXPRESSION OF TROPONIN-C GENES DURING TONGUE MYOGENESIS

Determination of muscle fiber type is related to the developmental sta ge of the tissue. Ordinarily the final distribution of fast and slow f ibers in a muscle is determined postnatally, Tongue muscle, however, i s composed solely of fast-twitch fibers that express only troponin C f ast mRNA and fast (type II) myosin heavy chain (MHC) proteins in both the adult and the one-day-old mouse. The fiber-type determination of t his muscle was examined during fetal development, Both troponin C fast and slow mRNAs were expressed at initial stages of tongue development at embryonic day 13. However, by embryonic day 16 the troponin C fast transcripts predominated. At 17 days of embryonic development, TnC fa st mRNA was 10 times more abundant than TnC slow, and at 18 days of de velopment the TnC slow mRNA was barely detectable. The tongue muscle m yotubes expressed fast, slow, and embryonic MHC isoforms during early embryonic development. At 18 days of gestation, the MHC isoform expres sed by the majority of the myotubes was the fast isoform, whereas the slow isoform was present in very few fibers, RT-PCR analysis of the MH C transcripts present throughout tongue development demonstrated expre ssion of the mdms or type IIx MHC in both late fetal and postnatal sta ges of development, In contrast, the type yp slow MHC mRNA was undetec table in the postnatal and adult tongue. The absence of TnC and MHC sl ow-isoform mRNAs in the newborn mouse tongue suggests that slow isofor m genes become dominantly repressed with the TnC-F and MHC type IIx ge nes remaining transcriptionally active, giving rise to an unusually ho mogeneous fast-twitch phenotype, The tongue muscle fibers acquire thei r specific adult-type fiber characteristics during fetal development r ather than postnatally. (C) 1997 Wiley-Liss, Inc.