Tropomyosin and Troponin-T Isoform Differences in Jaw-Closing Muscles of Rodentia and Carnivora that Express Masticatory Myosin

We recently reported that, contrary to prevailing dogma, masticatory (“superfast”) myosin is expressed in jaw-closing muscles of some members of Rodentia (Reiser et al., J. Exp. Biol. 212:2511-2519, 2009). Whereas virtually all mammalian limb muscle fibers express tropomyosin-β (Tm-β), along with fast-type or slow-type Tm-α, jaw-closing muscle fibers that express masticatory myosin in members of Carnivora express a unique isoform of Tm-α and do not express Tm-β (Rowlerson et al., Biochem. Biophys. Res. Commun. 113:519-525, 1983; Hoh et al., Proc. Aust. Physiol. Pharmacol. Soc. 20:192P, 1989). The goal of this study was to examine thin filament protein isoform composition in jaw-closing muscles of rodents that express masticatory myosin and compare the results to those from members of Carnivora. Tm or troponin-T (TnT) specific antibodies and immunoblotting were used to probe homogenates of limb and jaw-closing muscles of six species of Rodentia and three species of Carnivora. The results verify the almost exclusive expression of Tm-α, presumably the unique isoform reported earlier, in the jaw-closing muscles of Carnivora and reveal that members of Rodentia express both Tm-α and Tm-β in jaw-closing muscles, similar to limb muscles in the same species. The results also indicate that the same complement of TnT isoforms are expressed in jaw-closing and limb muscles of Carnivora, but differ markedly between the two muscle groups in Rodentia. Fast-type TnC and TnI appear to be expressed in jaw-closing muscles of both orders. It is postulated that the differences in Tm and TnT isoform expression patterns between Carnivora and Rodentia may impart fundamental differences in calcium-sensitivity of force generation to accommodate markedly different feeding styles, with shared expression of masticatory myosin, between these two animal orders. Supported by the National Science Foundation.