Transitory expression of alpha cardiac myosin heavy chain in a subpopulation of secondary generation muscle fibers in the pig

Unlike the random distribution of fiber types seen in skeletal muscles of most mammals, pig muscle exhibits a rosette pattern consisting of islets of slow fibers surrounded by concentric circles of type IIA and IIB fibers. Within each islet of slow fibers, one of the central fibers is a primary myofiber, whereas all others are secondary fibers. The present study demonstrates that a subpopulation of the slow secondary fibers transiently expresses α‐myosin heavy chain (MHC). Two cDNA libraries were made from longissimus dorsi skeletal muscle of 14‐day‐old piglet and adult pig atrium; the latter muscle is mainly composed of α‐MHC. Screening of the libraries with a human anti‐α‐MHC mAb (F8812F8) demonstrated the presence of positive MHC clones in both libraries; the nucleotide sequence of the 3′‐untranslated region (3′‐UTR) was identical in both libraries. As this MHC 3′‐UTR had 75% homology with the human α‐MHC, it was identified as pig α‐MHC. Using specific cRNA probes and mAbs against pig α‐cardiac and β/slow/type I MHC, we studied the expression of these MHCs in developing pig semitendinosus muscle by combining in situ hybridization and immunocytochemistry on serial sections at 90 days of gestation, and at 1, 6, 35 days and 6 months of age. The results showed that a subpopulation of secondary fibers that directly abut primary fibers, transiently produced α‐MHC, both at the levels of the protein and its transcript. Subsequently, these fibres expressed β‐MHC. At 1 day, immunocytochemistry showed that 16% of the secondary fibers expressed α‐MHC, among which 20% did not yet express β‐MHC. At 6 days, α‐ and β‐MHCs were mostly present in the same fibers, i.e., 23% of the secondary fibers. Thereafter, the proportion of secondary fibers reacting with α‐MHC mAb decreased to 10% at 5 weeks and 0% at 6 months, whereas β‐MHC was still accumulating in about 38% of the secondary fibers. During the period studied, the distribution of α‐ and β‐MHC transcripts closely matched that of the corresponding proteins. Expression of α‐MHC was not detected in primary type I muscle fibers and slow type I secondary fibers at the periphery of the rosettes of slow fibers. This study is the first unequivocal demonstration of a transitory expression of α‐MHC in a subpopulation of secondary fibers in a limb skeletal muscle during mammalian development. Dev. Dyn. 1997;210:106–116. © 1997 Wiley‐Liss, Inc.