Investigation of proteins that interact with NAADP-Gated two-pore channels.

Abstract

All living organisms respond to environmental stimuli by eliciting a sequence of signaling cascades, many of which converge in regulating [Ca2+]cyt via intracellular Ca2+ stores. Three agonist-mediated second messengers have been identified, including inositol 1,4,5-trisphosphate (IP3), cyclic ADP ribose (cADPR), and nicotinic acid adenine dinucleotide phosphate (NAADP). NAADP is the most potent calcium mobilizer identified to date, and unlike IP3 and cADPR that target ER Ca2+ stores, NAADP-mediated Ca2+ response is restricted to acidic Ca2+ stores. Several candidate Ca2+ channels expressed in the endolysosomal system have been proposed to be gated by NAADP, with recently two-pore channels (TPCs) emerging as NAADP targets. My research project utilized a radioactive photoactivable NAADP analogue, 32P-5 azido-NAADP (32P-5N3-NAADP), to perform an unbiased assay in a variety of model systems, attempting to verify NAADP targets via a direct crosslinking approach. My results revealed that 5N3-NAADP labeled protein candidate(s) were significantly smaller than predicted sizes of TPC proteins (85-100kDa) in all systems examined (22/23kDa doublet in mammals, 41kDa in sea urchin). Further, the labeling pattern and intensity of the NAADP-targeted doublet remained unchanged in TPC-overexpressing cells. Surprisingly, photoaffinity labeling (PAL) of the high affinity NAADP targets was preserved in TPC-knockout pancreatic samples, further suggesting that NAADP binding and Ca2+ release are mediated by distinct protein identities. My data contradicts current models that TPCs are direct NAADP receptors, implying an alternative cellular binding partner for NAADP on endolysosomes that then serves to indirectly regulate TPC activity. This is an important revision of current dogma, and crucial for rational design of drugs that may modulate NAADP activity. Such therapeutics may be important in disorders (diabetes, lysosomal storage disorders, and neuronal excitotoxicity) where NAADP signaling is pathologically perturbed.