Cyclic ADP-ribose signaling in sea urchin gametes: Metabolism in spermatozoa

The molecular mechanism that initiates Ca²⁺ signaling in sea urchin egg fertilization has not yet been clarified. To determine whether sea urchin sperm may generate and possibly supply cyclic ADP-ribose (cADPR) as a Ca²⁺- releasing factor in the course of sea urchin egg fertilization, we determined cADPR content and the capacity for cADPR synthesis in sea urchin sperm. cADPR content was determined using the sea urchin egg homogenate Ca²⁺-release bioassay combined with high-performance liquid chromatography (HPLC). We found that sperm homogenates synthesized cADPR from β-NAD but did not synthesize cADPR when α-NAD was the substrate. The identity of cADPR generated by sperm homogenates was verified by HPLC analysis, use of specific Ca²⁺-release antagonists, and homologous desensitization of the sea urchin egg homogenate Ca²⁺-release bioassay. The ambient content of cADPR was ~0.3 nmol cADPR/g wet wt sea urchin sperm. Our results show that sperm can synthesize cADPR and that they contain cADPR levels comparable to other tissues.