Global Metabolomic Profiles and Viability of Encapsulated Chondrocytes Exposed to Short-Term Simulated Microgravity
Shifts from physiological loading conditions, such as overloading or reduced loading, can lead to an imbalance that increases catabolic pathways resulting in cartilage degeneration consistent with that of osteoarthritis (OA). The risk of developing OA and the mechanism by which chondrocytes respond to reduced mechanical loading remains unclear. This is of particular concern in space, where reduced mechanical forces during prolonged microgravity (10-6 g) exposure could lead to OA, compromising flight crew mobility and leading to reduced quality of life post-spaceflight. We encapsulated human chondrocytes in an agarose gel of similar stiffness to the pericellular matrix to mimic the cartilage microenvironment and exposed encapsulated chondrocyte constructs to SM using a rotating wall vessel (RWV) bioreactor to better assess the cartilage health risks associated with spaceflight. Global metabolomic profiling detected a total of 1205 metabolite features across all samples, with 497 significant metabolite features by ANOVA (FDR-corrected p-value < 0.05). Unsupervised statistical analyses did not show clear separation between SM and control cohorts, suggesting that short-term (< 4 days) exposure to microgravity does not induce large-scale shifts in chondrocyte metabolism. However, we detected specific metabolic shifts in response to simulated microgravity (SM) exposure by identifying clusters of co-regulated metabolites in a HCA clustergram and by ranking metabolites by OPLS-DA VIP scores. Microgravity-induced metabolic shifts mapped to histidine, amino sugars, fatty acid, butanoate, phosphatidyl phosphate, methionine, and cysteine metabolism, valine, leucine, and isoleucine degradation; glycine, serine, threonine, methionine, cysteine, glutathione, arginine, proline, and glutamate metabolism. The specific metabolic shifts that occurred in response to microgravity exposure were consistent with early osteoarthritic metabolomic profiles in human synovial fluid, which suggests that even short-term exposure to microgravity (or other reduced mechanical loading environments) may lead to the development of OA.