Experimental regeneration in canine muscular dystrophy--2. Expression of myosin heavy chain isoforms

The sequential expression of neonatal, fast and slow myosin heavy chain isoforms was examined during the regeneration of normal and dystrophic dog muscle from 1 to 56 days, following necrosis induced by the venom of Notechis scutatis, to assess the regenerative potential of dystrophic muscle. Regeneration was equally rapid in normal and dystrophic dogs but morphological and immunocytochemical abnormalities were more apparent in the dystrophic fibres. New myotubes were formed by 3 days, and by 4 days all myosin isoforms were expressed. Neonatal myosin persisted in normal dogs after morphological restoration of the muscle and after dystrophin and beta-spectrin expression had returned to normal. Neonatal myosin was considerably reduced by 21 days in normal dogs, but persisted beyond 28 days in dystrophic dogs, suggesting a delay in maturation. At 42 and 56 days, dystrophic dogs showed a population of small fibres expressing neonatal myosin, which may represent a second cycle of degeneration and regeneration. The reciprocal pattern of fast and slow myosin was not fully restored in normal or dystrophic regenerating muscle, and co-expression persisted. Thus, dystrophic muscle retains its potential to regenerate, but maturation is slower than normal. The persistent co-expression of isoforms has implications for the long-term function of fibres formed after myoblast therapy, but the results imply that a stable state can be achieved if dystrophin expression is restored by gene or myoblast transfer therapy.