Age is the most important risk factor for primary osteoarthritis (OA). Members of the TGF-β superfamily play a crucial role in chondrocyte differentiation and maintenance of healthy articular cartilage.
We have investigated whether age-related changes in TGF-β superfamily signaling components play a role in the relationship between OA-related cartilage degradation and aging.
The relationship between age, OA and TGF-β superfamily signaling was studied using murine experimental OA models, aging mice, bovine articular cartilage and human OA cartilage. The effects of TGF-β on cartilage homeostasis was studied with immunohistochemistry, Q-RT-PCR and signaling pathway analysis with Western blotting and the application of specific TGF-β inhibitors.
We have found that TGF-β loses its protective effects in old cartilage. Moreover, we found that on chondrocytes, TGF-β not only signals via the canonical type I receptor ALK5 (TGFBR1) but also via the ALK1 (ACVRL1) receptor. Remarkably, signaling via ALK5 (Smad2/3 route) results in protective while ALK1 signaling (Smad1/5/8 route) results in deleterious responses in articular chondrocytes. In cartilage of aging mice it was detected that the ALK1/ALK5 ratio is significantly increased, favoring TGF-β signaling via the Smad1/5/8 route, inducing changes in chondrocyte differentiation and matrix metalloproteinase-13 (MMP-13) expression. Moreover, human OA cartilage showed a significant correlation between ALK1 and MMP-13 expression. Since in mice aging and OA in often goes hand in hand, we also analyzed age-related expression of TGF-β superfamily related signaling molecules in healthy bovine cartilage in an age range from 6 months to 14 years. In this cohort of aging cartilage, we found that mainly signaling receptors determining the Smad2/3 pathway were decreased with age while Smad1/5/8-related signaling molecules did not alter, confirming our findings in aging mice.
Old cartilage appears to be less protected by TGF-β and shows significant alterations in TGF-β signaling pathways. Loss of the protective Smad2/3 pathway during aging can provide an explanation for the relationship between OA and aging.