Synthesis of the Ca2+-mobilizing messengers, NAADP and cADPR, by intracellular CD38 enzyme in mouse heart: role in {beta}-adrenoceptor signaling [Signal Transduction]

Nicotinic acid adenine dinucleotide phosphate (NAADP) and cyclic ADP-ribose (cADPR) are Ca2+-mobilizing messengers important for modulating cardiac excitation-contraction coupling and pathophysiology. CD38, which belongs to the ADP-ribosyl cyclase (ARC) family, catalyzes synthesis of both NAADP and cADPR in vitro. However, it remains unclear whether this is the main enzyme for their production under physiological conditions. Here, we show that membrane fractions from WT but not CD38-/- mouse hearts supported NAADP and cADPR synthesis. Membrane permeabilization of cardiac myocytes with saponin and/or Triton X-100 increased NAADP synthesis, indicating that intracellular CD38 contributes to NAADP production. The permeabilization also permitted immunostaining of CD38, with a striated pattern in WT myocytes, while CD38-/- myocytes and non-permeabilized WT myocytes showed little or no staining, without striation. A component of $\beta$-adrenoceptor signaling in the heart involves NAADP and lysosomes. Accordingly, in the presence of isoproterenol, Ca2+ transients and contraction amplitudes were smaller in CD38-/- myocytes than WT. In addition, suppressing lysosomal function with bafilomycin A1 reduced the isoproterenol-induced increase in Ca2+ transients in cardiac myocytes from WT but not CD38-/- mice. Whole hearts isolated fromCD38-/- mice and exposed to isoproterenol showed reduced arrhythmias. SAN4825, an ARC inhibitor, that reduced cADPR and NAADP synthesis in mouse membrane fractions, was shown to bind to CD38 in docking simulations, and reduced the isoproterenol-induced arrhythmias in WT hearts. These observations support generation of NAADP and cADPR by intracellular CD38, which contributes to effects of β-adrenoceptor stimulation (to increase both Ca2+ transients and the tendency to disturb heart rhythm).

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