Abstract
Articular cartilage is a relatively hypoxic tissue with a unique extracellular matrix that is enriched with cations, resulting in an elevated interstitial fluid osmolarity. Several biomechanical and physicochemical stimuli are reported to influence chondrocyte metabolism. For regenerative in vitro applications, increasing the extracellular osmolarity above plasma level to more physiological valuesinduces chondrogenic marker expression and the differentiation of chondroprogenitor cells. Calcineurin inhibitor FK506 modulates the differentiation of primary chondrocytes under such conditions and its effect on cell proliferation, extracellular matrix quality, and BMP- and TGF-β signaling will be described. Supraphysiological osmolarity compromises chondrocyte proliferation, while physosmolarity or FK506 did not. Rather, the combination of the latter increased proteoglycan and collagen expression in chondrocytesin vitro and in situ, affecting expression of TGF-β-inducible protein TGFBI and chondrogenic (SOX9, Col2) as well as terminal differentiation markers (e.g., Col10). Surprisingly, expression of particularly minor collagens (e.g., Col9, Col11) was improved. Physiological osmolarity seems to promote terminal chondrogenic differentiation of progenitor cells through sensitization of TGF-β superfamily signaling at the type I receptor. While hyperosmolarity alone facilitates TGF-β superfamily signaling, FK506 seems to enhance signaling by releasing the FKBP12 break from the type I receptor to improve collagenous marker expression. Our data help explaining seemingly contradictory earlier findings and potentially benefit future cell-based cartilage repair strategies.
