Osmolarity modulates the de-differentiation of horse articular chondrocytes during cell expansion in vitro: implications for tissue engineering in cartilage repair

Due to the importance of joint disease and ostearthritis (OA) in equine athletes, new regenerative treatments to improve articular cartilage repair after damage are gaining relevance. Chondrocyte de-differentiation, an important pathogenetic mechanism in OA, is a limiting factor when differentiated articular chondrocytes are used for cell-based therapies. Current research focuses on the prevention of this de-differentiation and/or on the re-differentiation of chondrocytes by employing different strategies in vitro and in vivo. Articular chondrocytes normally live in a condition of higher osmolarity (350-450 mOsm/L) compared to normal physiological fluids (~ 300 mOsm/L) and some studies have demonstrated that osmolarity has a chondroprotective effect in vitro and in vivo. Therefore, the response of horse articular chondrocytes to osmolarity changes (280, 380, and 480 mOsm/L) was studied both in proliferating, de-differentiated chondrocytes grown in adhesion, and in differentiated chondrocytes grown in a 3D culture system. To this aim, cell proliferation (cell counting), morphology (optical microscopy), and differentiation (gene expression of specific markers) were monitored along with the expression of osmolyte transporters involved in volume regulation [betaine-GABA transporter (BGT-1), taurine transporter (SLC6A6), and neutral amino acid transporter (SNAT)] real-time qPCR. Proliferating chondrocytes cultured under hyperosmolar conditions showed low proliferation, spheroidal morphology, a significant reduction of de-differentiation markers [collagen type I (Col1) and RUNX2] and an increase of differentiation markers [collagen type II (Col2) and aggrecan]. Notably, a persistently high level of BGT-1 gene expression was maintained in chondrocyte cultures at 380 mOsm/L, and particularly at 480 mOsm/L both in proliferating and differentiated chondrocytes. These preliminary data encourage the study of osmolarity as a microenvironmental co-factor to promote/maintain chondrocyte differentiation in both 2D and 3D in vitro culture systems.