Alterations in the Sensing and Transport of Phosphate and Calcium by Differentiating Chondrocytes

Abstract

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 expression. This results in the manufacture, remodeling, and mineralization of a cartilage template on which bone is laid down. Articular chondrocytes undergo a similar but restricted differentiation program that does not proceed to mineralization, except in pathologic conditions such as osteoarthritis. The pathogenesis of disorders of cartilage development and metabolism, including osteochondrodysplasia, fracture non-union, and osteoarthritis remain poorly defined. We used the CFK2 model to examine the potential roles of phosphate and calcium ions in the regulatory pathways that mediate chondrogenesis and cartilage maturation. Differentiation was monitored over a 4-week period using a combination of morphological, biochemical, and molecular markers that have been characterized in vivo and in vitro. CFK2 cells expressed the type III sodiumdependent phosphate transporters Glvr-1 and Ram-1, as well as a calcium-sensing mechanism. Regulated expression and activity of 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, regulated expression and activity of a calcium-sensing receptor by extracellular calcium was most evident after 2 weeks of differentiation, concomitant with an increase in type X collagen expression, alkaline phosphatase activity and parathyroid hormone/parathyroid hormone-related protein receptor expression. On the basis of these temporally restricted changes in the sensing and transport of phosphate and calcium, we predict that extracellular phosphate plays a role in the commitment of chondrogenic cells to differentiation, whereas extracellular calcium plays a role at a later stage in their differentiation program.Bones of the mammalian skeleton develop by two distinct mechanisms. Intramembranous bone, exemplified by the frontal and parietal bones of the cranial vault, is formed when mesenchymal stem cells differentiate directly into bone-forming osteoblasts. The bones at the base of the skull, including parts of the temporal and occipital bones, as well as the vertebral column and the appendicular skeleton, are formed by endochondral ossification. These "cartilage bones" are formed when mesenchymal precursors condense and differentiate into chondrogenic cells that deposit and mineralize a cartilage matrix (1), which is remodeled by catabolic cells (chondroclasts) brought in by the vasculature (2). The remodeled cartilage then forms the template on which bone is deposited by osteoblastic cells. The same process of endochondral ossification occurs in the adult during fracture healing. Although chondrocytes involved in endochondral ossification and those present in articular cartilage arise from the same lineage and ...

Hypophosphatemia leads to rickets by impairing caspase-mediated apoptosis of hypertrophic chondrocytes

“…These observations implicate a threshold in the circulating phosphate levels, below which impairment of hypertrophic chondrocyte apoptosis is observed. Although calcium has been shown to play a role in chondrocyte differentiation (27,28), our investigations demonstrate that wild-type mice rendered hypercalcemic and hypophosphatemic by dietary means develop rickets, demonstrating that hypercalcemia cannot maintain normal growth-plate maturation in the presence of a 50% reduction in circulating phosphate levels. Therefore, our studies eliminate calcium and PTH as modulators of apoptosis.…”

Distinct Transglutaminase 2-independent and Transglutaminase 2-dependent Pathways Mediate Articular Chondrocyte Hypertrophy

“…Alternatively, the capacity of TG2 to regulate Phospholipase C d1 signaling (22), and signal transduction and P i generation through GTPase and ATPase activities of TG2 (22), could modulate TG2 involvement in retinoid signaling, hypertrophy, and calcification in chondrocytes. In this regard, it is noteworthy that the accumulation of intracellular P i at critical times during chondrocyte differentiation induces mineralization and increases expression of Cbfa1 and type X collagen (54).…”

Biophysical aspects of biomineralization

“…PiT-1 plays important roles in the regulation of P i homeostasis in bone and cartilage in vitro (Cecil et al 2005;Solomon et al 2007;Sugita et al 2011;Wang et al 2005), and extracellular P i , epinephrine, insulin-like growth factor 1 and bone morphogenic protein 2 are known to regulate PiT-1 levels in osteoblast-like cells (Suzuki et al 2006;VillaBellosta et al 2009b). Moreover, it has been demonstrated that P i modulates chondrocyte differentiation (Cecil et al 2005;Fujita et al 2001;Guicheux et al 2000;Montessuit et al 1991;Wang et al 2001;Wu et al 2002) and apoptosis (Magne et al 2003;Mansfield et al 2001). Sugita et al (2011) suggested that ATP synthesis mediated by P i influx via PiT-1 is critical for the regulation of late chondrogenesis, including apoptosis and mineralization, as the differentiation of cartilage is an ATP-dependent event.…”