Modification of the dystrophic phenotype after transient neonatal denervation: role of MHC isoforms

Activity-dependent induction of slow myosin gene expression in isolated fast-twitch mouse muscle

We demonstrate that direct electrical stimulation of isolated fast-twitch muscle in an organ culture system can induce expression of the slow myosin heavy chain (beta-MHC) gene, indicative of a phenotype transformation. Pairs of extensor digitorum longus (EDL) muscles were isolated from adult mice, incubated at resting length in separate chambers, and superfused with the same recirculated media One muscle was subjected to twitch stimulation (5-s trains of 5-Hz pulses at supramaximal voltage every minute), and force was recorded to assess function. The contralateral muscle was incubated without stimulation, to control for effects of the experimental preparation. Both muscle were rapidly frozen for RNA purification and oligo(dT)-primed reverse transcription; serial studies were carried out to 36 h. Polymerase chain reaction was performed utilizing primers specific for cytoplasmic beta-actin (beta-actin), a constitutive marker, and beta-MHC, a gene that is either inactive or expressed at very low levels in control EDL. After 30 h of stimulation, beta-MHC was consistently detected at a level severalfold higher in stimulated EDL than in incubated control EDL when band intensities were normalized to those of beta-actin. These results show that signals or fiber-specific transformations reside within the muscle and that this shift begins rapidly after induction of continuous stimulation.

Inherited muscular disorder in mutant Japanese quail (Coturnix coturnix japonica): an immunohistochemical study

Cryostat sections of myofibres from the Musculus pectoralis thoracicus of a newly established mutant strain (LWC) of Japanese quail with a myotonic dystrophy-like myopathy were labelled with antibody against myosin heavy chain (MHC) isoforms and neural cell adhesion molecule (N-CAM). The characteristic lesions found in sections of muscle of LWC quail stained with haematoxylin and eosin were type 2B fibre atrophy, sarcoplasmic masses, and ring fibres. Immunohistochemical examination failed to distinguish type 2A and 2B fibres in the LWC quail. Antibody to adult fast MHC, which reacted only with type 2A fibres in normal quail, reacted in LWC quail with type 2B fibres, and to a limited degree with type 2A fibres. Sarcoplasmic masses reacted with both fast and slow MHC antibodies. Some masses also reacted with NCAM antibody, but apparently independently of similar reactions in fibres. These findings suggest that the changes observed in the myofibres of the LWC quail were not neurogenic but represented defects in both the plasma membrane and intermediate filaments.