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CA2+ REGULATES THE CONTENT OF TYPE II COLLAGEN AND PROTEOGLYCAN IN INTERVERTEBRAL DISCS BY ACTIVATING THE EXTRACELLULAR CALCIUM-SENSING RECEPTOR

Calcification of the intervertebral disc (IVD) has been correlated with degenerative disc disease (DDD), a common cause of low back pain. The appearance of calcium deposits has been shown to increase with age, and its occurrence has been associated with several other disorders such as hyperparathyroidism, chondrocalcinosis, and arthritis. Trauma, vertebral fusion and infection have also been shown to increase the incidence of IVD calcification. The role of IVD calcification in the development DDD is unknown. Our preliminary data suggest that ionic calcium content and expression of the extracellular calcium-sensing receptor (CaSR), a G protein-coupled receptor (GPCR) and regulator of calcium homeostasis, are increased in the degenerated discs. However, its role in DDD remains unclear. IVD Cells: Bovine and normal human IVD cells were incubated in PrimeGrowth culture medium (Wisent Bioproducts, Canada; Cat# 319–510-CL, −S1, and S2) and supplemented with various concentrations of calcium (1.0, 1.5, 2.5, 5.0 mM), a CaSR agonist [5 µM], or IL-1β [10 ng/ml] for 7 days. Accumulated matrix protein was quantitated for aggrecan and type II collagen (Col II) by Western blotting. Conditioned medium was also collected from cells treated for 24h and measured for the synthesis and release of total proteoglycan using the DMMB assay and Western blotting for Col II content. IVD Cultures: Caudal IVDs from tails of 20–24 month old steers were isolated with the PrimeGrowth Isolation kit (Wisent Bioproducts, Canada). IVDs were cultured for 4 weeks in PrimeGrowth culture medium supplemented with calcium (1.0, 2.5, or 5.0 mM), or a CaSR agonist [5 µM]. Cell viability was measured in NP and AF tissue using Live/Dead Imaging kit (ThermoFisher, Waltham, MA), to determine if Ca2+ effects cell viability end the expression of aggrecan and Col II was evaluated in the IVD tissue by Western blotting. Histological sections were prepared to determine total proteoglycan content, alkaline phosphatase expression and degree of mineralisation by von Kossa staining. The accumulation of aggrecan and Col II decreased dose-dependently in IVD cells following supplementation with calcium or the CaSR agonist. Conditioned medium also demonstrated decreases in the synthesis and release of proteoglycan and collagen with increasing Ca2+ dose or direct activation of the CaSR with agonist. A similar phenomenon was observed for total proteoglycan and aggrecan and Col II in IVDs following calcium supplementation or the CaSR agonist. In addition to decreases in Col II and aggrecan, increases in alkaline phosphatase expression and mineralisation was observed in IVDs cultured in elevated Ca2+ concentrations without affecting cell viability. Our results suggest that changes in the local concentrations of calcium are not benign, and that activation of the CaSR may be a contributing factor in IVD degeneration. Determining ways to minimise Ca2+ infiltration into the disc may mitigate disc degeneration.