Identification of a gene network contributing to hypertrophy in callipyge skeletal muscle

The callipyge mutation in sheep results in postnatal skeletal muscle hypertrophy in the pelvic limbs and loins with little or no effect on anterior skeletal muscles. Associated with the phenotype are changes in the expression of a number of imprinted genes flanking the site of the mutation, which lies in an intergenic region at the telomeric end of ovine chromosome 18. The manner in which these local changes in gene expression are translated into muscle hypertrophy is not known. Microarray-based transcriptional profiling was used to identify differentially expressed genes in longissimus dorsi skeletal muscle samples taken at birth and 12 wk of age from callipyge and wild-type sheep. The phenotype was only expressed at the latter developmental time and associated with decreased type 1 fibers (slow oxidative) and a shift toward type IIx and IIb fibers (fast-twitch glycolytic). We have identified 131 genes in the samples taken at 12 wk of age that were differentially expressed as a function of genotype but not due to the fiber type changes. The gene expression changes occurring as a function of genotype in the samples taken at birth indicated that the transcriptional framework underpinning the phenotype was emerging prior to expression of the phenotype. Eight genes were differentially expressed as a function of genotype at both developmental times. A model is proposed describing a core network of genes and histone epigenetic modifications that is likely to underpin the fiber type changes and muscle hypertrophy characteristic of callipyge sheep.

Growth and differentiation of cultured satellite cells from callipyge and normal lambs

We tested whether the muscle enlargement found in callipyge sheep is linked to increased proliferation, differentiation, and protein accrual of cultured satellite cells. Satellite cells were isolated from the longissimus muscle of three callipyge and three normal lambs. The satellite cells were grown using serum from horse, normal lambs, and callipyge lambs and cultured under conditions that promoted differentiation into myotubes and accumulation of myofibrillar protein. There was no difference in the population doubling times (PDT) or fusion percentages for callipyge and normal satellite cells, but PDT was longer (P < 0.05) for satellite cells grown in callipyge serum (pooled mean PDT = 22 h) than for cells grown in normal sheep serum (PDT = 20 h) or horse serum (PDT = 18 h). The protein:DNA ratios of the cultures increased during 96 h in differentiation media (P < 0.01), but there was no difference in protein:DNA ratios between cells from callipyge and normal lambs. These results suggest that muscle hypertrophy of callipyge sheep is not linked to the inherent capacity of their cultured satellite cells to proliferate, fuse or accrue protein, but hypertrophy may be linked to the influence of serum-born factors on satellite cell proliferation. Key words: Satellite cell, callipyge, sheep, cell culture