Impaired Mineral Ion Metabolism in a Mouse Model of Targeted Calcium-Sensing Receptor (CaSR) Deletion from Vascular Smooth Muscle Cells.

BACKGROUND: Impaired mineral ion metabolism is a hallmark of CKD-metabolic bone disorder. It can lead to pathologic vascular calcification and is associated with an increased risk of cardiovascular mortality. Loss of calcium-sensing receptor (CaSR) expression in vascular smooth muscle cells exacerbates vascular calcification in vitro. Conversely, vascular calcification can be reduced by calcimimetics, which function as allosteric activators of CaSR. METHODS: To determine the role of the CaSR in vascular calcification, we characterized mice with targeted Casr gene knockout in vascular smooth muscle cells ( SM22α CaSR Δflox/Δflox ). RESULTS: Vascular smooth muscle cells cultured from the knockout (KO) mice calcified more readily than those from control (wild-type) mice in vitro. However, mice did not show ectopic calcifications in vivo but they did display a profound mineral ion imbalance. Specifically, KO mice exhibited hypercalcemia, hypercalciuria, hyperphosphaturia, and osteopenia, with elevated circulating fibroblast growth factor 23 (FGF23), calcitriol (1,25-D3), and parathyroid hormone levels. Renal tubular α-Klotho protein expression was increased in KO mice but vascular α-Klotho protein expression was not. Altered CaSR expression in the kidney or the parathyroid glands could not account for the observed phenotype of the KO mice. CONCLUSIONS: These results suggest that, in addition to CaSR's established role in the parathyroid-kidney-bone axis, expression of CaSR in vascular smooth muscle cells directly contributes to total body mineral ion homeostasis.

Calcimimetics increase CaSR expression and reduce mineralization in vascular smooth muscle cells: mechanisms of action.

AIMS: Vascular calcification (VC) contributes to morbidity and mortality in patients with chronic kidney disease (CKD). Allosteric modulators of the calcium (Ca)-sensing receptor (CaSR) may slow the progression of VC in CKD patients either by reducing serum parathyroid hormone (PTH), Ca, and phosphate levels or by a direct effect on the vessel wall. The aim of this study was to examine the effects of calcimimetics on CaSR expression, cell phenotype, and mineral deposition in human vascular smooth muscle cells (h-VSMCs). METHODS AND RESULTS: Primary h-VSMCs were exposed for 14 days to increasing concentrations of Ca(2+) (from 1.8 to 5 mmol/L) in the presence or absence of calcimimetics R-568 or AMG 641 (0.1 µmol/L). Mineralization was detected by Alizarin red staining, and the cell phenotype was assessed using immunocytochemistry and qRT-PCR. CaSR expression was evaluated using flow cytometry. Short- and long-term exposure (1 day to 14 days) of h-VSMCs to calcimimetics promoted CaSR protein transport from the endoplasmic reticulum to the plasma membrane with enhanced CaSR expression on the cell surface, together with an increase in total cell CaSR expression due to enhanced biosynthesis. In pro-mineralizing conditions, exposure to calcimimetics counteracted the Ca(2+)-dependent reduction of CaSR expression, decreased matrix collagen secretion, and mineral deposition by ~90%. These effects involved CaSR activation since it could be inhibited by CaSR siRNA, but not scrambled siRNA. CONCLUSIONS: The calcimimetic-dependent increase in biosynthesis and activation of the CaSR in h-VSMCs probably play a key role in the protection against calcium-induced VC.