The roles of CD38 and NAADP in cardiac tissue

Nicotinic acid adenine dinucleotide phosphate (NAADP) is an endogenous molecule that releases calcium ions (Ca2+) from acidic endolysosomal stores. This Ca2+-mobilising molecule is known to play a role in modulating cardiac excitation-contraction coupling and pathophysiology. The first part of the thesis focuses on understanding the mechanistic details of NAADP actions, particularly during the flight and fight response of the heart. Paced ventricular myocytes, which had genetically- (Tpcn2-/-) or pharmacologically-impaired (bafilomycin-A1 treatment) NAADP signalling pathways, had blunted responses to isoprenaline. Further investigations revealed that NAADP actions are dependent of functional CaMKII proteins but independent of L-type Ca2+ channels. The main part of the thesis focuses on understanding the enzymatic production of NAADP in the heart. Although CD38, which belongs to the ADP-ribosyl cyclase (ARC) family, has been shown to catalyse the synthesis of NAADP in other mammalian tissue, whether CD38 is responsible for the production of NAADP in the heart remains debatable. Membrane fractions from CD38-/- mouse hearts were not able to generate NAADP through catalysing the base-exchange reaction. Experiments involving the permeabilisation of cardiac myocytes also revealed the intracellular expression of CD38 on the sarcoplasmic reticulum (SR). The SR-enriched membrane preparation (SRMP) from sheep heart is also capable of catalysing NAADP production and showed properties associated with those of CD38. Upon isoprenaline addition, paced CD38-/- myocytes had smaller Ca2+ transients and contraction amplitudes, compared to those of wild type animals. In addition, impairing NAADP signalling using bafilomycin-A1 did not affect the effect of isoprenaline on CD38-/- myocytes. highlighting the role of CD38 in coupling NAADP action to beta-adrenoceptor signalling. Lastly, a novel ARC inhibitor, SAN4825, was also shown to inhibit NAADP synthesis in both mouse and sheep cardiac preparations, through targeting CD38. CD38-/- hearts or SAN 4825-treated hearts were resistant to isoprenaline-induced arrhythmias. These observations support the hypothesis that NAADP is generated by intracellular CD38 in the heart, which contributes to the positive inotropic and pro-arrhythmic effects of the beta-adrenoceptor activation.