For RNA expression analyses, joint articular cartilage total RNA from the four flight and ground control samples were isolated. Since the function of microRNAs (miRNAs) in cartilage has received substantial research attention and several appear essential to normal development and homeostasis each of the eight samples were subjected to miRNA microarray analysis using the Affymetrix Genechip miRNA array. Samples were also hybridized to the Affymetrix Murine Gene 1.0 ST GeneChip microarray for mRNA expression.

RESULTS:
miRNA data: Bioinformatic analysis of the miRNA data demonstrated that 10 miRNA were upregluated (p>0.001) and 26 were downregulated in flight compared to control mice. Interestingly, miR-35, which was downregulated 1.4-fold, is a known mechanoresponsive microRNA in chondrocytes. It has been reported that miR-35 was upregluated following cyclical loading of chick sternal chondrocytes and that it plays a role in chondrocytee proliferation and differentiation. Furthermore, miR365 binds to and inhibits HDAC4, a negative regulator of chondrocyte hypertrophy. Our finding that miR-365 is downregulated in the reduced biomechanical unloading of spaceflight is consistent with a role in mechano-sensing. Reduced levels of miR-365 would be expected to reduce chondrocyte proliferation thereby contributing to an OA phenotype.

Thirteen mRNA were upregulated>1.5-fold in flight compared to the ground control animals and one downgraded>1.5 fold. Of the 13 upregulated genes, 7 have been implicated in either osteoarthritis, rheumatoid arthritis or both. There are Vdr, Nfkbia, Cited2, Traf6, Mtl, Txnip, and Cdknla. The highest upregulated gene was Vdr which encodes the vitamin D receptor. Vdr is a key genetic and biochemical marker of osteoarthritis (OA). Several genetic association studies on large populations of OA patients have linked the Vdr gene locus with knee OA. The link between Vdr and OA is further highlighted by the finding that Vdr mRNA ius upregulated in severe osteoarthritis. Consistent with this is the finding that, vitamin D, the Vdr ligand, upregulates matrix metalloprteinase (MMP) production. MMPs are the major class of matrix-degrading enzymes in cartilage.