Comparative expression of the extracellular calcium sensing receptor in rat, mouse and human kidney

The calcium sensing receptor (CaSR) was cloned over 20 years ago and functionally demonstrated to regulate circulating levels of parathyroid hormone by maintaining physiological serum ionized calcium (Ca2+) concentration. The receptor is highly expressed in the kidney; however, intra-renal and intra-species distribution remains controversial. Recently, additional functions of the CaSR receptor in the kidney have emerged, including parathyroid hormone independent effects. It is therefore critical to establish unequivocally the localization of the CaSR in the kidney in order to relate this to its proposed physiological roles. In this study we determined CaSR expression in mouse, rat and human kidney using in situ hybridisation, immunohistochemistry (using eight different commercially available and custom-made antibodies) and proximity ligation assays. Both in situ hybridisation and immunohistochemistry showed CaSR expression in the thick ascending limb, distal tubule and collecting duct of all species, with the thick ascending limb showing the highest levels. Within the collecting ducts there was significant heterogeneity of expression between cell types. In the proximal tubule, lower levels of immunoreactivity were detected by immunohistochemistry and proximity ligation assays. Proximity ligation assays were the only technique to demonstrate expression within glomeruli. This study demonstrated CaSR expression throughout the kidney with minimal discrepancy between species but with significant variation in the levels of expression between cell and tubule types. These findings clarify the intra-renal distribution of the CaSR and enable elucidation of the full physiological roles of the receptor within this organ