PPADS is a reversible competitive antagonist of the NAADP receptor

NAADP has been shown to act as a second messenger in a wide range of systems from plants to mammalian cells. Although it had always been considered as a canonical second messenger, recent work has shown that it is also active when applied extracellularly. It has also been suggested that NAADP might have a direct action on P2 receptors, based on the action of a pharmacological agent, PPADS, on Ca²⁺ signals in response to extracellular NAADP. We have therefore investigated whether PPADS can act directly on the intracellular NAADP-induced Ca²⁺-release system in the well characterised sea urchin egg homogenate system. Indeed, PPADS, and its structural analogue PPNDS were able to compete with [³²P]NAADP for the binding site and binding curves revealed that both compounds display affinities in the low micromolar range. The binding of PPADS was reversible in contrast to that of NAADP. In fluorimetric Ca²⁺-release experiments, PPADS was able to competitively antagonise NAADP-induced Ca²⁺-release with an IC₅₀ of 20 μM, while it did not affect the other Ca²⁺-release channels. This is the first report of a reversible, competitive antagonist of the sea urchin NAADP receptor. Furthermore, PPADS might reveal itself as an invaluable tool to investigate NAADP signalling and is a lead compound for the synthesis of potent and specific antagonists.