Developmental Expression and 5′ Cloning of the Porcine 2x and 2b Myosin Heavy Chain Genes

Muscle research on farm animals suffers from a lack of basic molecular probes and immunochemical reagents. To complete the cloning of a comprehensive range of isoform-specific postnatal myosin heavy chain (MyHC) probes from the pig, we report here on the 5′-end cDNA isolation, identification, and in vivo characterization of the porcine equivalent of the 2x (fast oxidative-glycolytic) and 2b (fast glycolytic) MyHC isoforms. Comparative analyses of primary nucleotide and deduced amino acid sequences of the porcine slow, 2a, 2x, and 2b cDNAs showed that the 2x and 2b isoforms were most homologous with each other and the slow isoform was the least similar. At or just after birth, at least 3 postnatal muscle fiber types (slow/β, 2a, and 2x) were present. MyHC 2x expressing fibers were arranged distinctively as large outer rings, each surrounding an inner ring of 2a fibers, which in turn circumscribed one or more central slow fibers. Subsequently, different muscles expressed at different times the MyHC 2b isoform such that by 10 weeks, 2b fibers became the most dominant fiber type, in size and number in nearly all muscles examined except for its notable absence in the soleus muscle. Co-expression of two or more MyHC isoforms within the same fiber was not a feature of porcine muscles unlike that observed in rat and human muscles. For the first time, porcine muscle fibers can now be classified into 4 adult phenotypes (slow/β, 2a, 2x, and 2b) according to the expression of specific MyHC isoforms.