Effect of calcitriol and calcium on basal ganglia calcification in hypoparathyroidism: experimental models

Abstract

Basal ganglia calcification (BGC) is a common complication in hypoparathyroid patients, linked to hyperphosphatemia and altered vitamin-D and calcium homeostasis following conventional therapy. The pathogenesis of BGC in hypoparathyroidism is not clear. Recently, we developed an ex vivo model of BGC using rat-striatal cell culture in 10.0 mmol/L of β-glycerophosphate (31.8 mg/dL phosphate). However, the effect of 1,25(OH)2 D, calcium, and milder phosphate excess on BGC in hypoparathyroidism is not known. This study describes two modified ex vivo models investigating pathogenesis of BGC in ‘drug-naïve’ and ‘conventionally treated’ hypoparathyroid state. The first modification involved striatal cells cultured in low concentration 1,25(OH)2D (16.0 pg/mL), ionized calcium(0.99 mmol/L), hPTH(1-34) (6.0 pg/mL), and 2.68 mmol/L (8.3 mg/dL) of phosphate akin to ‘drug-naïve’ state for 24 days. In second modification, striatal cells were exposed to 46.0 pg/mL of 1,25(OH)2D, normal ionized calcium of 1.17 mmol/L, and 2.20 mmol/L (6.8 mg/dL) of phosphate akin to ‘conventionally treated’ state. Striatal cell culture under ‘drug-naïve’ state showed that even 16.0 pg/mL of 1,25(OH)2D enhanced the calcification. In ‘conventionally treated’ model, striatal cell calcification was enhanced in 54% cases over ‘drug-naïve’ state. Calcification in ‘conventionally treated’ state further increased on increasing phosphate to 8.3 mg/dL, suggesting importance of phosphatemic control in hypoparathyroid patients. Striatal cells in ‘drug-naïve’ state showed increased mRNA expression of pro-osteogenic Wnt3a, Cd133,Vglut-1-neuronal phosphate-transporters, calcium-ion channel-Trvp2,Alp, and Collagen-1α and decreased expression of Ca-II. These models suggest that in ‘drug-naïve’ state, 1,25(OH)2D along with moderately elevated phosphate increases the expression of pro-osteogenic molecules to induce BGC. Although normalization of calcium in ‘conventionally treated’ state increased the expression of Opg, Osterix, Alp, and Cav2, calcification increased only in a subset, akin to variation in progression of BGC in hypoparathyroid patients on conventional therapy.