Discovery of a small-molecule inhibitor for kidney ADP-ribosyl cyclase: Implication for intracellular calcium signal mediated by cyclic ADP-ribose

ADP-ribosyl cyclase (ADPR-cyclase) produces a Ca²⁺-mobilizing second messenger, cyclic ADP-ribose (cADPR), from β-NAD⁺. A prototype of mammalian ADPR-cyclases is a lymphocyte antigen CD38. Accumulating evidence indicates that ADPR-cyclases other than CD38 are expressed in various cells and organs. In this study, we discovered a small molecule inhibitor of kidney ADPR-cyclase. This compound inhibited kidney ADPR-cyclase activity but not CD38, spleen, heart or brain ADPR-cyclase activity in vitro. Characterization of the compound in a cell-based system revealed that an extracellular calcium-sensing receptor (CaSR)-mediated cADPR production and a later long-lasting increase in intracellular Ca²⁺ concentration ([Ca²⁺]_i) in mouse mesangial cells were inhibited by the pre-treatment with this compound. In contrast, the compound did not block CD3/TCR-induced cADPR production and the increase of [Ca²⁺] _i in Jurkat T cells, which express CD38 exclusively. The long-lasting Ca²⁺ signal generated by both receptors was inhibited by pre-treatment with an antagonistic cADPR derivative, 8-Br-cADPR, indicating that the Ca²⁺ signal is mediated by the ADPR-cyclse metabolite, cADPR. Moreover, among structurally similar compounds tested, the compound inhibited most potently the cADPR production and Ca²⁺ signal induced by CaSR. These findings provide evidence for existence of a distinct ADPR-cyclase in the kidney and basis for the development of tissue specific inhibitors.