Masticatory (`superfast') myosin heavy chain and embryonic/atrial myosin light chain 1 in rodent jaw-closing muscles

Abstract

SUMMARY Masticatory myosin is widely expressed among several vertebrate classes. Generally, the expression of masticatory myosin has been associated with high bite force for a carnivorous feeding style (including capturing/restraining live prey), breaking down tough plant material and defensive biting in different species. Masticatory myosin expression in the largest mammalian order, Rodentia, has not been reported. Several members of Rodentia consume large numbers of tree nuts that are encased in very hard shells, presumably requiring large forces to access the nutmeat. We, therefore, tested whether some rodent species express masticatory myosin in jaw-closing muscles. Myosin isoform expression in six Sciuridae species was examined, using protein gel electrophoresis, immunoblotting, mass spectrometry and RNA analysis. The results indicate that masticatory myosin is expressed in some Sciuridae species but not in other closely related species with similar diets but having different nut-opening strategies. We also discovered that the myosin light chain 1 isoform associated with masticatory myosin heavy chain, in the same four Sciuridae species, is the embryonic/atrial isoform. We conclude that rodent speciation did not completely eliminate masticatory myosin and that its persistent expression in some rodent species might be related to not only diet but also to feeding style.

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

“…Mice and rats express primarily myosin isoforms in jawclosing muscles that are characteristic of adult limb skeletal muscles (d 'Albis et al, 1986;Eason et al, 2000;Widmer et al, 2002). All of the other species express masticatory myosin in jaw-closing muscles, except red squirrels and southern flying squirrels (Reiser et al, 2009). Masticatory myosin expression in jaw-closing muscles of some of these species has also been reported by others (Rowlerson et al, 1981;Rowlerson et al, 1983a;Rowlerson et al, 1983b;Sciote et al, 1995;Sciote and Rowlerson, 1998;Hoh, 2002).…”

“…Consistent results were obtained when multiple individuals of a given species were studied. The predominant or exclusive myosin isoforms in jawclosing muscles of all of these species, except mouse and rat, were described in two recent reports (Reiser et al, 2009;Reiser et al, 2010). Mice and rats express primarily myosin isoforms in jawclosing muscles that are characteristic of adult limb skeletal muscles (d 'Albis et al, 1986;Eason et al, 2000;Widmer et al, 2002).…”

“…Masticatory MHC (MHC-M) and MLC2 (MLC2-M) are unique isoforms that are restricted primarily to jaw-closing muscles in those species in which they are expressed (Rowlerson et al, 1981;Rowlerson et al, 1983a;Rowlerson et al, 1983b;Qin et al, 1994;Hoh, 2002;Hoh et al, 2006;Reiser et al, 2009;Reiser et al, 2010). The MLC1 isoform of masticatory myosin is an isoform that is also widely expressed in embryonic skeletal muscle and in adult atrial tissue (embryonic/atrial MLC1) (Reiser et al, 2009;Reiser et al, 2010).…”

“…non-human primates) (reviewed in Hoh, 2002). An apparent paradox arose with the recent discovery of masticatory myosin in several rodent species (Reiser et al, 2009). Whereas rodents typically feed primarily by rapid nibbling of grasses, grains and nuts, many other species that express masticatory myosin, such as most members of Carnivora, typically strangulate prey (requiring sustained force generation), crush bones during feeding and frequently engulf large food items without extensive chewing.…”

“…Differences in Tm and TnT isoform expression could reconcile the apparent paradox that masticatory myosin is expressed in species that use different styles of chewing. Multiple species, all of which express masticatory myosin (Reiser et al, 2009;Reiser et al, 2010), from several mammalian and reptilian orders were studied to determine whether there are consistent differences in the patterns of Tm and TnT isoform expression in jaw-closing muscles between groups of animals that differ with respect to feeding styles. Our results support the view that differences in Tm and TnT isoform expression patterns may impart fundamental differences in thinfilament-mediated muscle activation to accommodate markedly different feeding styles.…”