Repression of the embryonic myosin heavy chain phenotype in regenerating chicken slow muscle is dependent on innervation

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Abstract
Ventricular‐like and fast myosin heavy chains (VL‐MHC and FMHC) are transiently expressed during slow skeletal muscle development. The influence of innervation on repression of these MHC isoforms is investigated over an 84‐day time course in: (1) normal anterior latissimus dorsi (N‐ALD) muscles, (2) regenerating ALD (R‐ALD) muscles, (3) denervated ALD (D‐ALD) muscles, and (4) regenerating and denervated ALD (RD‐ALD) muscles. Western blotting demonstrates that the VL‐MHC is expressed in R‐, D‐, and RD‐ALD muscles, but not in N‐ALD muscles. Expression of the VL‐MHC is transient in R‐ALD musles. In contrast, VL‐MHC expression persists in RD‐ALD muscles, and appears with time in D‐ALD muscles. FMHC was not detected in N‐ALD muscles by Western blotting. Two FMHCs are seen in R‐ALD and RD‐ALD muscles, and in 13‐day emryonic ALD muscles. The slower migrating FMHC (FMHCA) comigrates with developmentally regulated FMHCs in fast pectoralis muscle, while the faster migrating FMHC (FMHCB) comigrates with the faster migrating FMHC in embryonic ALD muscle (13 days in ovo). FMHCB decreases in amount over the time course in R‐ALD muscles, while FMHCA persists. In contrast, substantial levels of both FMHCs persist in RD‐ALD muscles, and appear with time in D‐ALD muscles. The cellular distribution of MHCs is followed by immunocytochemistry. Regenerating cells expressing VL‐MHC and FMHC are replaced by a mature population in R‐ALD muscles. Some of the mature myofibers in R‐ALD muscles express FMHC, but not VL‐MHC. In RD‐ALD and D‐ALD muscles, both regenerating and mature muscle cells are seen which express VL‐MHC and FMHC. Our results indicate that innervation is required for the repression of VL‐MHC and FMHCB during regneration of slow muscle.