Abstract
Properties of muscle fibers, i.e., their type, number and size, are important determinants of functional characteristics of skeletal muscle, and of the quality of meat in livestock. Genetic factors play an important role in determining variation in fiber properties, however, specific genes remain largely elusive. We examined histological properties of soleus muscle fibers in two strains of mice exhibiting a twofold difference in muscle mass, LG/J and SM/J, and their F2 intercross. The total number of muscle fibers (555 ± 106; mean ± SD) did not differ between the strains or between males and females. A higher percentage of type I fibers was observed in the LG/J compared to the SM/J strain (P < 0.001) in both males (45 ± 3 vs. 37 ± 4%) and females (58 ± 4 vs. 41 ± 3%). Across strains, females had a higher percentage of type I fibers than males (P < 0.001), and the sex effect was greater in the LG/J strain (strain-by-sex interaction, P < 0.001). The cross-sectional area (CSA) did not differ between type I and type IIA fibers, but was greater in the LG/J than the SM/J strain (1365 ± 268 vs. 825 ± 229 μm2, P < 0.001). Three significant quantitative trait locus (QTL) affecting CSA for type I and type IIA fibers mapped to chromosomes (Chr) 1, 6, and 11 and three suggestive QTL for percentage of type I fibers mapped to Chr 2, 3, and 4. Within each significant QTL, regions of conserved synteny were also implicated in variation of similar traits in an analogous study in pigs. Our results provide the evidence that the intercross between the SM/J and LG/J strains is a promising model to search for genes affecting muscle fiber properties.
Highlights
Skeletal muscle has numerous biological functions including locomotion, thermoregulation, respiration, postural support, protection of bones and viscera, and serves as a repository of amino acids in times of starvation or disease
The aim of this study was to initiate the search for genes affecting variation in the number of fibers, their size, and proportion of type I fibers in soleus muscle between the LG/J and SM/J strains of mice
cross-sectional area (CSA) of muscle fibers The CSA was similar between type I and type IIA fibers (P = 0.516), and larger in the LG/J compared to the SM/J strain (P < 0.0001)
Summary
Skeletal muscle has numerous biological functions including locomotion, thermoregulation, respiration, postural support, protection of bones and viscera, and serves as a repository of amino acids in times of starvation or disease. Properties of skeletal muscle are determined mainly by the number, type, and size of muscle fibers. Muscle fibers are heterogeneous and can vary in size and functional properties, such as their ability to develop contractile power and resistance to fatigue (Bottinelli and Reggiani, 2000). Human skeletal muscles are mainly comprised of a mixture of type I, IIA, and IIX muscle fibers (Schiaffino, 2010). Varying proportions of those fiber types affect functional properties of the muscle. Postural muscles, such as the soleus, predominantly consist of oxidative, fatigue resistant, type I fibers, whereas the proportion of glycolytic, fast twitch, type II fibers is higher in the phasic muscles (Johnson et al, 1973)
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