Physiological Functions of Cyclic ADP-Ribose and NAADP as Calcium Messengers

Cyclic ADP-ribose (cADPR) and nicotinic acid adenine dinucleotide phosphate (NAADP) are two Ca²⁺ messengers derived from NAD and NADP, respectively. Although NAADP is a linear molecule, structurally distinct from the cyclic cADPR, it is synthesized by similar enzymes, ADP-ribosyl cyclase and its homolog, CD38. The crystal structure of the cyclase has been solved and its active site identified. These two novel nucleotides have now been shown to be involved in a wide range of cellular functions including: cell cycle regulation in Euglena, a protist; gene expression in plants; and in animal systems, from fertilization to neurotransmitter release and long-term depression in brain. A battery of pharmacological reagents have been developed, providing valuable tools for elucidating the physiological functions of these two novel Ca²⁺ messengers. This article reviews these recent results and explores the implications of the existence of multiple Ca²⁺ messengers and Ca²⁺ stores in cells.