Differential expression of ventricular-like myosin heavy chain mRNA in developing and regenerating avian skeletal muscles.

Based on previous immunological data, cross-reactivity of myosin heavy chain (MHC) with the ventricular (V) isoform was observed in primordia of avian skeletal muscles and in regenerating adult anterior latissimus dorsi (ALD) muscle. To determine whether this primordial (P) MHC is identical to adult V-MHC gene product, we have cloned and characterized the 3' portion of MHC cDNA that is expressed in ALD muscle at 3 d of regeneration. Comparison of nucleotide sequences between adult V-MHC and P-MHC cDNAs revealed more than 98% homology in the 3'-untranslated (UT) portions of these genes. The expression pattern of P-MHC was analyzed in adult regenerating muscles using total RNA from two fast muscles, posterior latissimus dorsi (PLD) and pectoralis major (PM), as well as from slow ALD and mixed fast/slow gastrocnemius muscles at 0, 1, 3, 4, 6, 9, and 14 d after cold injury. Identical results were obtained by RNase protection assays using either a probe specifying the coding region of adult V-MHC or a P-MHC probe encoding the carboxy end plus the 3'-UT region. The expected protected fragments were detected early from day 2 up to day 6 in ALD muscle. Similar rate of appearance, reaching the highest level at day 3, was observed in PLD, PM, and gastrocnemius muscles. However, the amount and the kinetics of disappearance differed among the various muscles analyzed. In contrast, during development, steady-state levels and kinetics of V-MHC mRNA expression were found to be alike in axial and appendicular muscles. These data strongly suggest the identity of P-MHC as the ventricular isoform and support the concept that expression of P-MHC mRNA is a common feature of developing as well as of all regenerating adult skeletal muscles. Interestingly, no expression of cardiac specific myosin light chain (MLC) 2A was observed after cold injury, suggesting independent regulatory pathways for the two kinds of myosin subunits.

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