Effects of chronic exposure to simulated microgravity on skeletal muscle cell proliferation and differentiation

Cell culture models that mimic long-term exposure to microgravity provide i mportant insights into the cellular biological adaptations of human skeleta l muscle to long-term residence in space. We developed insert scaffolding f or the NASA-designed rotating cell culture system (RCCS) in order to study the effects of time-averaged microgravity on the proliferation and differen tiation of anchorage-dependent skeletal muscle myocytes. We hypothesized th at prolonged microgravity exposure would result in the retardation of myocy te differentiation. Microgravity exposure in the RCCS resulted in increased cellular proliferation. Despite shifting to media conditions promoting cel lular differentiation, 5 d later, there was an increase in cell number of a pproximately 62%, increases in total cellular protein (52%), and cellular p roliferating cell nuclear antigen (PCNA) content (2.7 times control), and o nly a modest (insignificant) decrease (10%) in sarcomeric myosin protein ex pression. We grew cells in an inverted orientation on membrane inserts. Cha nges in cell number and PCNA content were the converse to those observed fo r cells in the RCCS. We also grew cells on inserts at unit gravity with con stant mixing. Mixing accounted for part, but not all, of the effects of mic rogravity exposure on skeletal muscle cell cultures (53% of the RCCS effect on PCNA at 4-6 d). In summary, the mechanical effects of simulated microgr avity exposure in the RCCS resulted in the maintenance of cellular prolifer ation. manifested as increases in cell number and expression of PCNA relati ve to control conditions, with only a modest reciprocal inhibition of cellu lar differentiation. Therefore, this model provides conditions wherein cell ular differentiation and proliferation appear to be uncoupled.