Expression of MHC-2B in skeletal muscles of large mammals (pig and cow)

Myosin heavy chain 2B is the most abundant MHC isoform in skeletal muscles of mice and rats and is also present in rabbit muscles. In human muscles there is no evidence of expression of MHC-2B protein, although the gene has been identified and the corresponding mRNA dectected in some specialized muscles (Horton et al., 2001, Arch Oral Biol, 46: 1039). In the pig, MHC-2B is expressed in skeletal muscles (Chang et al., 1997, DNA and Cell Biol 16: 1429). In this study, we confirmed the expression of MHC-2B in limb, trunk and extraocular muscles of the pig with RT-PCR and, using a novel protocol for MHC isoform electrophoretic separation, we found MHC-2B, always coexpressed with MHC-2X, in hybrid fibres of longissimus dorsi and semitendinosus muscles and, expressed alone, in fibres of retractor bulbi. This identification allowed the determination of the mechanical properties of porcine pure 2B fibres. To extend the analysis to bovine muscles, MHC-2B gene was identified and partially sequenced and specific primers were designed for RT-PCR: MHC-2B was not found expressed in limb, trunk or extraocular muscles, but was detected in substantial amount in retractor bulbi. A specific electrophoretic protocol was developed for bovine MHC isoform separation and confirmed the results obtained at mRNA level. The results obtained suggest that the pig is the only large mammal where MHC-2B is abundantly expressed in some skeletal muscles.