Calbindin-D28K dynamically controls TRPV5-mediated Ca2+ transport

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Abstract
In Ca2+‐transporting epithelia, calbindin‐D28K (CaBP28K) facilitates Ca2+ diffusion from the luminal Ca2+ entry side of the cell to the basolateral side, where Ca2+ is extruded into the extracellular compartment. Simultaneously, CaBP28K provides protection against toxic high Ca2+ levels by buffering the cytosolic Ca2+ concentration ([Ca2+]i) during high Ca2+ influx. CaBP28K consistently colocalizes with the epithelial Ca2+ channel TRPV5, which constitutes the apical entry step in renal Ca2+‐transporting epithelial cells. Here, we demonstrate using protein‐binding analysis, subcellular fractionation and evanescent‐field microscopy that CaBP28K translocates towards the plasma membrane and directly associates with TRPV5 at a low [Ca2+]i. 45Ca2+ uptake measurements, electrophysiological recordings and transcellular Ca2+ transport assays of lentivirus‐infected primary rabbit connecting tubule/distal convolute tubule cells revealed that associated CaBP28K tightly buffers the flux of Ca2+ entering the cell via TRPV5, facilitating high Ca2+ transport rates by preventing channel inactivation. In summary, CaBP28K acts in Ca2+‐transporting epithelia as a dynamic Ca2+ buffer, regulating [Ca2+] in close vicinity to the TRPV5 pore by direct association with the channel.