Connexin-43 Mediates Fcγ Receptor-Induced Calcium Signaling by Acting as an NAD/Cyclic ADP-Ribose Transporter.

Cyclic ADP-ribose (cADPR) is produced from NAD by CD38, a leukocyte receptor and ectoenzyme with its catalytic domain positioned outside of cells, and mobilizes Ca²⁺ from ryanodine sensitive Ca²⁺ stores. It has long been questioned how NAD approach CD38 and extracellularly generated cADPR reaches its intracellular target Ca²⁺ stores. Here we address this question by identifying connexin 43 hemichannels (Cx43) as a transporter of cyclic ADP-ribose as well as NAD in Fcg receptor (FcgR) triggering-induced calcium signaling. FcgR clustering triggers a Ca²⁺ transient via three sequential steps, Cx43-mediated NAD export, CD38-mediated cADPR production and subsequent Cx43-mediated cADPR import in murine macrophages. FcgR clustering induces Cx43 phosphorylation, thereby opening Cx43 and transporting NAD and cADPR. After cADPR-mediated Ca²⁺ transient, Cx43 is immediately dephosphorylated and closed, blocking further cADPR transport. This finding is the first evidence showing the involvement of Cx43 in cADPR-mediated Ca²⁺ release induced by physiological ligand where the apparent topological paradox of extracellular cADPR synthesis and its intracellular action exist. Our results provide new insight into the mechanism of intracellular calcium release triggered by the extracellularly generated calcium mobilizer cADPR, which is involved in (FcgR)-mediated calcium signaling in murine macrophages.