Nicotinic acid adenine dinucleotide phosphate (NAADP) is the most potent Ca 2؉-mobilizing intracellular messenger and is linked to a variety of stimuli and cell surface receptors. However, the enzyme responsible for endogenous NAADP synthesis in vivo is unknown, and it has been proposed that another enzyme differing from ADP-ribosyl cyclase family members may exist. The ectoenzyme CD38, involved in many functions as diverse as cell proliferation and social behavior, represents an important alternative. In pancreatic acinar cells, the hormone cholecystokinin (CCK) stimulates NAADP production evoking Ca 2؉ signals by discharging acidic Ca 2؉ stores and leading to digestive enzyme secretion. From cells derived from CD38؊/؊ mice, we provide the first physiological evidence that CD38 is required for endogenous NAADP generation in response to CCK stimulation. Furthermore, CD38 expression in CD38-deficient pancreatic AR42J cells remodels Ca 2؉-signaling pathways in these cells by restoring Ca 2؉ mobilization from lysosomes during CCK-induced Ca 2؉ signaling. In agreement with an intracellular site for messenger synthesis, we found that CD38 is expressed in endosomes. These CD38-containing vesicles, likely of endosomal origin, appear to be proximal to lysosomes but not colocalized with them. We propose that CD38 is an NAADP synthase required for coupling receptor activation to NAADP-mediated Ca 2؉ release from lysosomal stores in pancreatic acinar cells.The Ca 2ϩ -mobilizing messenger nicotinic acid adenine dinucleotide phosphate (NAADP) 5 is the most potent of the three established Ca 2ϩ -mobilizing messengers. NAADP was first shown to evoke Ca 2ϩ release from intracellular stores in sea urchin eggs (1). It has been proposed that NAADP initially releases Ca 2ϩ from lysosomal related organelles (2-6), which may then be amplified by Ca 2ϩ -induced Ca 2ϩ release mechanisms in the endoplasmic reticulum (ER) (7). The recent demonstration that NAADP mobilizes Ca 2ϩ from endolysosomal stores by targeting two-pore channels (1, 8, 9) has intensified interest in this messenger and its signaling pathways (10, 11). Two-pore channel-mediated Ca 2ϩ release may be coupled to further Ca 2ϩ release by inositol trisphosphate (IP 3 ) receptors or ryanodine receptors (1,8). To assign NAADP as a second messenger, it was important to show that endogenous levels of NAADP could be controlled by external stimuli, and now NAADP has been shown to be linked to various stimuli and cell surface receptors (12, 13). Dramatic increases in NAADP occur during sea urchin egg fertilization due to production in sperm upon contacting egg jelly (14). Increases in NAADP levels have been also reported in pancreatic ␤-cells after glucose (15) and glucagon-like peptide-1 stimulation (17), in pancreatic acinar cells after CCK stimulation (16), in pulmonary smooth muscle following exposure to endothelin-1 (3), in response to histamine in human myometrial cells (18), and glutamate in neurones (19). However, the enzymes responsible for endogenous NAADP synthesis i...

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