Intracellular CD38 Mediates Cardiac Synthesis of NAADP and CADPR

Previously, we have reported that nicotinic acid adenine dinucleotide phosphate (NAADP) and cyclic ADP-ribose (cADPR) are calcium mobilising messengers that have significant roles in excitation-contraction coupling and disease development of the heart. The enzyme responsible for producing these molecules is a potential therapeutic target, but its identity remains unclear. CD38 is an ADP-ribosyl cyclase (ARC) that catalyses synthesis of NAADP and cADPR in various tissues, with one caveat: the targets of NAADP and cADPR are intracellular while the active site of CD38 is generally thought to be located on the extracellular surface, making the physiological relevance of this ecto-enzyme questionable. Permeabilised cardiac myocytes showed greater ARC activity than intact cells, suggesting the presence of intracellular ARCs. Immunostaining experiments showed that cell permeabilisation was critical for labelling CD38 with a striated staining pattern. We further showed that sarcoplasmic reticulum enriched membrane preparations from sheep hearts exhibited ARC activity with CD38-like properties. Genetic knock-out of CD38 in mice suppressed NAADP and cADPR synthesis. Isolated ventricular myocytes from CD38-/- mice showed reduced peak calcium transients and contractions, both under basal conditions and when stimulated by the β-adrenoceptor agonist, isoprenaline. CD38-/- hearts were more resistant to arrhythmias during over-stimulation of β-adrenoceptor pathway. A novel ARC inhibitor, SAN4825, suppressed the in vitro synthesis of NAADP and cADPR, and in blind docking simulations was shown to target the active site of CD38. The observations are consistent with an important role for CD38 in producing cADPR and NAADP intracellularly under basal conditions; the production of both these substances is increased by β-adrenoceptor stimulation and the elevated levels enhance arrhythmogenicity. In conclusion, we provide evidence that CD38 generates NAADP and cADPR in situ intracellularly and this enzymatic activity plays a role in both physiology and pathophysiology of the heart.