As humans and animals age, growth slows and eventually stops. Organs grow until they reach mature size; however, little is known about how growth cessation is regulated and coordinated in the body. A set of growth regulating genes, which include Ezh2, Gpc3, Mdk, Mest, Mycn, Peg3, and Plagl1, has been identified that is dependent upon growth and whose expression declines with age in organs[1, 2]. In general, when expression of these genes decreases, proliferation decreases leading to a reduction in growth. Mutations of these genes also result in reduced viability, growth abnormalities, and diseases such as rhabdomyosarcoma, a skeletal muscle cancer, and Simpson-Golabi-Behmel syndrome[3–18].
Though well characterized in organs, these genes have not been well examined in growing muscle. It is expected that expression of these genes would differ between muscle, heart, and liver because of the differences in how these tissues grow. Postnatally, muscle grows by mainly hypertrophy, an increase in cell size, and not hyperplasia, an increase in cell number. In mice, muscle fiber number becomes fixed at approximately d 7 postnatal. Hypertrophy of muscle fibers is accompanied by satellite cell activation. These cells fuse with existing muscle fibers to support muscle growth[21, 22]. Without the activation of satellite cells, postnatal muscle growth and regeneration is severely inhibited. In muscle, it is known that the expression of Ezh2, Mdk, Mest, Peg3, and Plagl1 is increased during regeneration[23–26]. In contrast, Mycn and Gpc3 have not been characterized in muscle. Mycn, however, increases proliferation of neural progenitor cells while Gpc3 is upregulated in the early and middle stages of liver regeneration[27, 28]. Because 5 of 7 of these genes are known to be upregulated with muscle regeneration, it is possible that they are involved in muscle growth by increasing activation, proliferation, or differentiation of satellite cells. With age, the expression of all of these genes is expected to decline in both organs and muscle because the growth regulating functions of these genes would be less necessary once growth has ceased.
The objectives of this study were to 1) determine if these 7 genes were downregulated during postnatal growth of skeletal muscle, 2) determine if expression differed between male and female mice during this time period and 3) determine if expression was altered by the absence of myostatin. Myostatin null mice experience increased muscle growth due to both increased hyperplasia and hypertrophy and exhibit increased satellite cell activity. Furthermore, myostatin expression is decreased with increasing age in rats, similar to the expression patterns of genes in this study. Therefore, use of myostatin null mice as a model for altered growth to determine the influence of these genes is warranted. In general, we confirm that this set of genes is downregulated with age in skeletal muscle of mice with minimal differences between males and females. Expression of Ezh2 was increased at d 70 and expression of Mdk was increased at d 21 in myostatin null mice compared with wild type, but no other genotype effects were present. These results suggest that this set of genes may globally regulate the cessation of growth in skeletal muscles as well as organs, but likely do not contribute to differences in muscularity of myostatin null mice.