Multiple cellular processes regulate expression of slow myosin heavy chain isoforms during avian myogenesis in vitro.

We analyzed the expression of two slow myosin heavy chain isoforms (slow MHC1 and slow MHC2) in myotubes formed from embryonic and fetal chicken myoblasts in vitro and found that the myotubes formed from myoblasts of these two developmental periods had different patterns of expression of slow MHCs. In cultures of myogenic cells from the Embryonic Day 5 (E5) hindlimb, immunoblot analysis showed that two slow MHCs with the immunological and electrophoretic properties like those of slow MHC1 and slow MHC2 were expressed on Day 3 of culture and that both slow MHCs continued to be expressed through 10 days of culture. In cultures of myogenic cells from the fetal (E12) thigh, in contrast, slow MHC1 was not expressed on Day 3 of culture, but was expressed after Day 7; slow MHC2 was never expressed by myotubes formed from fetal myoblasts. Immunocytochemistry was used to further analyze slow MHC1 and slow MHC2 expression in individual myotubes formed from embryonic and fetal myoblasts in both clonal cultures and high density, cytosine arabinoside-treated cultures. These analyses showed that (i) E5 embryonic myoblasts were of two principal types, those that formed myotubes that expressed isoforms like slow MHC1 and MHC2 throughout the life of the myotube, and those that formed myotubes that did not express slow MHCs at any time; and (ii) E12 fetal myoblasts formed myotubes that at first expressed only fast MHC but expressed both fast MHC and slow MHC1--but not slow MHC2--as culture duration was lengthened. Thus, the expression patterns of slow MHC1 and slow MHC2-like isoforms appeared to be regulated by different cellular processes in myotubes formed from embryonic and fetal myoblasts.