The role of mesenchymal stem cells and osteocytes in subchondral bone changes in hip osteoarthritis

Osteoarthritis (OA) is a disease of the whole joint, and subchondral bone changes are a universal OA feature. The cellular mechanisms of subchondral bone sclerosis in late OA joints implicate altered osteoblast and osteoclast activities, however the roles of the most abundant bone cells, the osteocytes, and their immature progenitors, the mesenchymal stem cells (MSCs) are not understood. The cell fraction characterised by the CD45-CD271+ phenotype represents a reservoir of native bone-resident MSCs in humans, however it consists of topographically different subsets, whose relative contributions to bone formation in OA remain unclear. The aim of this study was to investigate gene expression profiles and topographical features of osteocytes, MSCs and their subsets in femoral head OA bone. Compared to healthy cancellous bone, CD45-CD271+ MSCs resident in OA bone were more abundant (3.4-fold, p < 0.05) and had significantly (p < 0.05) higher expression of osteogenic genes, such as runt related transcription factor 2 (RUNX2, 7-fold) and osteonectin (SPARC, 5-fold) indicating their osteogenic commitment. Furthermore, chondrogenic genes were significantly (p < 0.05) decreased (SOX9, 5-fold and UDP-glucose 6-dehydrogenase, UGDH, 14-fold). The native OA osteocytes transcriptional profile was consistent with their early, embedding stage of development, evidenced by significantly (p < 0.01) higher levels of podoplanin (E11, 9-fold) and matrix metalloproteinase-14 (MMP14, 4-fold) compared to healthy bone osteocytes. Active bone formation in OA was evident as an accumulation and co-localisation of CD271+ MSCs and immature E11+ osteocytes in areas of bone sclerosis. Additionally, transcriptional and immunohistochemical analysis demonstrated that both MSCs and osteocytes in OA-affected bone favoured bone formation by elevated expression of the osteoclastogenesis inhibitor osteoprotegerin. To define osteogenically-committed MSC subset, CD56 was chosen as a candidate molecule. Dual immunofluorescence confirmed co-localisation of CD271 and CD56 at the bone surface. CD271+CD56+ cells were significantly more abundant in OA bone compared to healthy bone (8-fold, p < 0.0001) and also accumulated in OA sclerotic areas, having the unique topography in proximity to active bone formation sites. The CD271+CD56+ MSC subset displayed significantly (p < 0.01) over 100-fold higher expression of osteogenesis- and chondrogenesis-related genes, including osteopontin (SPP1), osteocalcin (BGLAP) and SOX9, compared to a donor-matched CD271+CD146+ subset, whilst the predominant CD271+CD146-CD56- subset represented a transcriptionally 'intermediate' MSC population with the highest levels of stromal-derived factor 1 (CXCL12) transcript. While all three subsets were multipotential, their native gene expression traits were lost after in vitro culture. This study implicates native bone-resident MSCs in subchondral bone sclerosis in late-stage hip OA. It defines the CD271+CD56+ osteogenically-committed MSC subset that has bone-lining topography and therefore the closest proximity to bone formation sites. Novel MSC subsets may represent future therapeutic targets for OA and other bone associated pathologies.