ECTOCELLULAR IN-VITRO AND IN-VIVO METABOLISM OF CADP-RIBOSE IN CEREBELLUM

CD38, a type II transmembrane glycoprotein predominantly expressed in blood cells, is a bifunctional ectoenzyme directly involved in the met abolism of cADP-ribose (cADPR). This is a potent Ca2+ mobilizer in sev eral types of cells. The relationship between the ectocellular site of cADPR production and its intracellular calcium-related functions is p oorly understood. Cultured rat cerebellar granule cells showed both en zymic activities of CD38, ADP-ribosyl cyclase and cADPR hydrolase, at a ratio of 16 to 1 respectively, and were immunostained by the anti (h uman CD38) monoclonal antibody IB4. In these cells externally added cA DPR and beta-NAD(+) (the precursor of cADPR), but not alpha-NAD(+) or ADP-ribose, enhanced the peak of the depolarization-induced rise in in tracellular Ca2+ concentration. This effect was inhibited by 1 mu M ry anodine, suggesting a potentiation of calcium-induced calcium release by cADPR. CD38 ectoenzyme activities, ADP-ribosyl cyclase and cADPR hy drolase, were also demonstrated in viva by microdialysis of adult rat cerebellum, where IB4 bound to granule neurons selectively. Trace amou nts (11.5+/-3.8 nM) of NAD(+) were detected by microdialysis sampling and sensitive assays in the basal interstitial fluid of the cerebellum . These results provide a link between ectocellular cADPR turnover and intracellular calcium mobilization in cerebellum.