Patterns of tropomyosin and troponin-T isoform expression in jaw-closing muscles of mammals and reptiles that express masticatory myosin

Abstract

SUMMARYWe recently reported that masticatory ('superfast') myosin is expressed in jaw-closing muscles of some rodent species. Most mammalian limb muscle fibers express tropomyosin- (Tm-), along with fast-type or slow-type tropomyosin-a (Tm-a), but jawclosing muscle fibers in members of Carnivora express a unique isoform of Tm [Tm-masticatory (Tm-M)] and little or no Tm-. The goal of this study was to determine patterns of Tm and troponin-T (TnT) isoform expression in the jaw-closing muscles of rodents and other vertebrate species that express masticatory myosin, and compare the results to those from members of Carnivora. Comparisons of electrophoretic mobility, immunoblotting and mass spectrometry were used to probe the Tm and fast-type TnT isoform composition of jaw-closing and limb muscles of six species of Carnivora, eight species of Rodentia, five species of Marsupialia, big brown bat, long-tailed macaque and six species of Reptilia. Extensive heterogeneity exists in Tm and TnT isoform expression in jaw-closing muscles between phylogenetic groups, but there are fairly consistent patterns within each group. We propose that the differences in Tm and TnT isoform expression patterns between phylogenetic groups, which share the expression of masticatory myosin, may impart fundamental differences in thin-filament-mediated muscle activation to accommodate markedly different feeding styles that may require high force generation in some species (e.g. many members of Carnivora) and high speed in others (e.g. Rodentia).

Nucleotide and protein sequences for dog masticatory tropomyosin identify a novel Tpm4 gene product

“…However, the complement of TnT isoforms in jaw-closing muscles that express masticatory myosin is very different from that in limb muscles in some species (Bicer et al 2011) and in one species, long-tailed macaque, one relatively abundant fast-type TnT isoform in the temporalis muscle was not detected in limb fast muscle. There do not appear to have been any reports describing troponin-I (TnI) isoforms in muscles that express masticatory myosin.…”

“…It was initially observed that the Tpm expressed in dog masseter co-migrates with Tpm1.1 on gels without urea (Bicer et al 2011). We, therefore, sought a gel format that yielded clear separation of the masseter Tpm from the sarcomeric Tpm isoforms known to be expressed in skeletal muscle—Tpm1.1, Tpm2.2 and Tpm3.12.…”

“…1 of the present study, which may be non-specific background stain, a band corresponding to Tpm4.3 was never observed in limb muscles on gels or immunoblots in which all Tpm isoforms were separated (e.g., Figs. 2, 4). …”

“…For most species that have been studied, it appears that isoforms of troponin-C (TnC) and troponin-T (TnT) that are expressed in muscles that express masticatory myosin are the same as those in limb fast muscle fibers (Rowlerson et al 1983a; Bicer et al 2011). However, the complement of TnT isoforms in jaw-closing muscles that express masticatory myosin is very different from that in limb muscles in some species (Bicer et al 2011) and in one species, long-tailed macaque, one relatively abundant fast-type TnT isoform in the temporalis muscle was not detected in limb fast muscle.…”

Polymorphism in tropomyosin structure and function

“…The skββ-Tm can be assembled in vitro but has low thermal stability (Lehrer and Joseph 1987) and is not found in most muscle tissues. An excess of β-Tm over α-Tm is reported in some specific muscle tissue which requires the formation of ββ-Tm dimers, notably the rabbit tongue (Bronson and Schachat 1982), mouse tibialis anterior (Corbett et al 2005) and the temporalis of some squirrels, chipmunks and turtles (Bicer et al 2011). Minor Tm components are also present in cardiac sarcomeres, κ-Tm (skα1-1 Tm from gene TPM1)(Rajan et al 2010).…”