Alterations in the sensing and transport of phosphate and calcium by differentiating chondrocytes

During endochondral bone formation and fracture healing, cells committed to chondrogenesis undergo a temporally restricted program of differentiation that is characterized by sequential changes in their phenotype and gene exp ression. This results in the manufacture, remodeling, and mineralization of a cartilage template on which bone is laid down. Articular chondrocytes un dergo a similar but restricted differentiation program that does not procee d to mineralization, except in pathologic conditions such as osteoarthritis . The pathogenesis of disorders of cartilage development and metabolism, in cluding osteochondrodysplasia, fracture non-union, and osteoarthritis remai n poorly defined. We used the CFK2 model to examine the potential roles of phosphate and calcium ions in the regulatory pathways that mediate chondrog enesis and cartilage maturation. Differentiation was monitored over a 4-wee k period using a combination of morphological, biochemical, and molecular m arkers that have been characterized in vivo and in vitro. CFK2 cells expres sed the type III sodium-dependent phosphate transporters Glvr-1 and Ram-1, as well as a calcium-sensing mechanism. Regulated expression and activity o f Glvr-1 by extracellular phosphate and parathyroid hormone-related protein was restricted to an early stage of CFK2 differentiation, as evidenced by expression of type II collagen, proteoglycan, and Ihh. On the other hand, r egulated expression and activity of a calcium-sensing receptor by extracell ular calcium was most evident after 2 weeks of differentiation, concomitant with an increase in type X collagen expression, alkaline phosphatase activ ity and parathyroid hormone/parathyroid hormone-related protein receptor ex pression. On the basis of these temporally restricted changes in the sensin g and transport of phosphate and calcium, we predict that extracellular pho sphate plays a role in the commitment of chondrogenic cells to differentiat ion, whereas extracellular calcium plays a role at a later stage in their d ifferentiation program.