Changes in Cellular Crosstalk between Skeletal Muscle Myoblasts and Bone Osteoblasts with Aging

Musculoskeletal function declines with aging, resulting in an increased incidence of trips and falls. Both bone and muscle experience age-related losses in tissue mass that alter their mechanical interactions in a well characterized manner, but changes in the biochemical interactions between bone and muscle with aging are not well understood. Of note, insulin-like growth factor 1 (IGF-1), a potent growth factor for bone and muscle, can be negatively altered with aging and may help explain losses in these tissues. We recently developed a co-culture system for simultaneous growth of bone mesenchymal stem cells (MSCs) and muscle satellite cells (SCs) to investigate the biochemical crosstalk between the two cell types. Here, we utilized an aging rat model to study cellular changes between young and old rat MSCs and SCs, in particular whether 1) young MSCs and SCs have increased proliferation and differentiation compared to old MSCs and SCs; 2) young cells have increased IGF-1 and collagen expression as a measure of crosstalk compared to old cells; and 3) young cells can mitigate the aging phenotype of old cells in co-culture. Rat MSCs and SCs were either mono- or co-cultured in Transwell(R) plates, grown to confluence, and allowed to differentiate for 14 days. Across the 14 days, cell proliferation was measured, with differentiation and crosstalk measurements evaluated at 14 days. The results suggest that in both young and old, proliferation is greater in mono-cultures compared to co-cultures, yet age and cell type did not have a significant effect. Differentiation did not differ between young and old cells, yet MSCs and SCs demonstrated the greatest amount of differentiation in co-culture. Finally, age, cell type, and culture type did not have a significant effect on collagen or IGF-1 expression. These results suggest co-culture may have a controlling effect, with the two cell types acting together to promote differentiation more than in mono-cultures, yet this response was not altered by age. In general, results for old cells had higher variability, suggesting a wider variety in the aging phenotypes demonstrated in these animals. This study was the first to use this rat aging model to investigate changes between bone and skeletal muscle cells, however further investigations are required to determine what signaling changes occur in response to age. Determining these signaling changes could lead to new targets for mitigating the progression of aging.