A novel mechanism for coupling cellular intermediary metabolism to cytosolic Ca²⁺ signaling via CD38/ADP-ribosyl cyclase, a putative intracellular NAD⁺ sensor

CD38 is an ectocyclase that converts NAD⁺ to the Ca²⁺-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 Ca²⁺ 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 Ca²⁺ 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 Ca²⁺ signal was preserved. Likewise, Ca²⁺ 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 Ca²⁺ signaling, indicating the downstream activation of ryanodine receptors by cADPr. We conclude that intracellularly expressed CD38 might link cellular NAD⁺ production to cytosolic Ca²⁺ signaling.