Acetylcholine-Induced M/KCNQ Current Inhibition is regulated by CB38 and A-Kinase Anchoring Protein in Neuroblastoma Cells

The CD38 membrane protein exhibits multifunctional activities: ADP-ribosyl cyclase, cyclic ADP-ribose hydrolase, NAD+ glycohydrolase, formation of nicotinic acid adenine dinucleotide phosphate (NAADP+) by base exchange and degradation of NAADP+ to 2′-phospho-ADP-ribose. The intermediate product of ADP-ribosyl cyclase of CD38 is cyclic ADP-ribose (cADPR). The role of cADPR as a second messenger in neurons is not completely understood. The present study was performed to determine whether cADPR plays a role as a second messenger as a downstream component of muscarinic acetylcholine receptor (mAChR) signaling in neuronal cells. We performed electrophysiology experiments on non-inactivating voltage-dependent potassium M-like currents, which are currently defined as currents due to KCNQ 2/3 or Kv 7.2/7.3 potassium channels. We first examined NG108-15 neuroblastoma × glioma hybrid cells expressing both m1 mAChRs and human CD38. ACh-evoked inhibition of the M-like current was significantly enhanced in CD38-transformed cells compared to control cells. ACh-induced inhibition was greater in the (KCNQ) components with faster kinetics in M-like currents than another (HERG) component with a slow decay time constant. The CD38-enhanced M-like current inhibition was decreased in the presence of 8-bromo-cADPR. Although overexpression of A-kinase anchoring protein (AKAP) did not interfere with ACh-induced inhibition, alternative expression of mutant AKAP lacking the protein kinase C binding domain reduced ACh-induced M-like current inhibition in transformed NG108-15 cells with CD38 and mAChRs. These results indicate that overexpression and deletion of CD38 modulate M-current inhibition by ACh, and suggest that CD38 and cADPR are located downstream of mAChRs and upstream of AKAP-involved protein kinase C-induced inhibition of Kv 7.2 channels in the mAChR signaling cascade.