Reduced sensitivity to nitric oxide limits satellite cell activity on cultured myofibers from aged mice.

摘要

Although aging-induced deficits in satellite cell proliferation and muscle regeneration are well-characterized, our investigation provides novel evidence of an age-related deficit in satellite cell activation, independent of proliferation, that is intrinsic to the muscle fiber. Mechanical activation of muscle fibers induces enzymatic production of nitric oxide (NO) resulting in proteolytic release of hepatocyte growth factor (HGF) from its extracellular tethering, and subsequent activation of satellite cells. We hypothesized that aging would reduce in situ satellite cell activation, and that L-arginine or nitric oxide (DETA-NO) supplementation would improve satellite cell activity in old muscle. Single intact myofibers were isolated from the gastrocnemius muscles of young (2 mo), adult (10 mo), and aged (22 mo) male C57 mice. Fibers were centrifuged to stimulate satellite cell activation and incubated in serum-free media with 0.002% 5-bromodeoxyuridine (BrdU) and either 2 mM L-arginine, 10 muM DETA-NO, or control media for 48 h. The total number of satellite cells per fiber (i.e., pax7+) was unchanged with aging. Nevertheless, the number of activated satellite cells (BrdU+) following centrifugation was reduced in the aged fibers. L-arginine or DETA-NO treatment to the aged fibers restored satellite cell activation to young-control levels. However, these treatments also increased satellite cell activation in young and adult fibers, such that an age-dependent deficit in satellite cell activity persisted within treatment groups. Expression of neuronal and endothelial nitric oxide synthase (NOS) proteins in the plantaris muscles did not differ between age groups. Further, contraction-induced NO release from the extensor digitorum longus (EDL) did not differ between adult and old mice. These data suggest that impairment of NO production does not account for reduced satellite cell activity in aged muscle fibers. Also, treatment with exogenous HGF was equally effective in activating satellite cells across age groups, indicating that events downstream of HGF release are intact in the aged muscle. Therefore, these data imply a reduced sensitivity of aged muscle to NO. Insight gained from these experiments will augment therapeutic strategies to maintain skeletal muscle mass in the elderly, and to enhance muscle regeneration in this population following disuse or acute injury.