A novel mechanism for coupling cellular intermediary metabolism to cytosolic Ca 2+ signaling via CD38/ADP‐ribosyl cyclase, a putative intracellular NAD + sensor

CD38 is an ectocyclase that converts NAD + to the Ca 2+ ‐releasing second messenger cyclic ADP‐ribose (cADPr). Here we report that in addition to CD38 ecto‐catalysis, intracellularly expressed CD38 may catalyze NAD + →cADPr conversion to cause cytosolic Ca 2+ release. High levels of CD38 were found in the plasma membranes, endoplasmic reticulum, and nuclear membranes of osteoblastic MC3T3‐E1 cells. More important, intracellular CD38 was colocalized with target ryanodine receptors. The cyclase also converted a NAD + surrogate, NGD + , to its fluorescent product, cGDPr (K m ~5.13 μM). NAD + also triggered a cytosolic Ca 2+ signal. Similar results were obtained with NIH3T3 cells, which overexpressed a CD38‐EGFP fusion protein. The Δ ‐49 ‐CD38‐EGFP mutant with a deleted amino‐terminal tail and transmembrane domain appeared mainly in the mitochondria with an expected loss of its membrane localization, but the NAD + ‐induced cytosolic Ca 2+ signal was preserved. Likewise, Ca 2+ release persisted in cells transfected with the Myr‐Δ ‐49 ‐CD38‐EGFP or Δ ‐49 ‐CD38‐EGFP‐Fan mutants, both directed to the plasma membrane but in an opposite topology to the full‐length CD38‐EGFP. Finally, ryanodine inhibited Ca 2+ signaling, indicating the downstream activation of ryanodine receptors by cADPr. We conclude that intracellularly expressed CD38 might link cellular NAD + production to cytosolic Ca 2+ signaling.—Sun, L., Adebanjo, O. A., Koval, A., Anandatheerthavarada, H. K., Iqbal, J., Wu, X. Y., Moonga, B. S., Wu, X. B., Biswas, G., Bevis, P. J. R., Kumegawa, M., Epstein, S., Huang, C. L.‐H., Avadhani, N. G., Abe, E., Zaidi, M. A novel mechanism for coupling cellular intermediary metabolism to cytosolic Ca 2+ signaling via CD38/ADP‐ribosyl cyclase, a putative intracellular NAD + sensor. FASEB J. 16, 302–314 (2002)