Cyclic ADP-ribose Enhances Coupling between Voltage-gated Ca²⁺ Entry and Intracellular Ca²⁺ Release*

Abstract

Ca²⁺ release from intracellular stores can be activated in neurons by influx of Ca²⁺through voltage-gated Ca²⁺ channels. This process, called Ca²⁺-induced Ca²⁺ release, relies on the properties of the ryanodine receptor and represents a mechanism by which Ca²⁺ influx during neuronal activity can be amplified into large intracellular Ca²⁺ signals. In a differentiated neuroblastoma cell line, we show that caffeine, a pharmacological activator of the ryanodine receptor, released Ca²⁺ from intracellular stores in a Ca²⁺-dependent and ryanodine-sensitive manner. The pyridine nucleotide, cyclic ADP-ribose, thought to be an endogenous modulator of ryanodine receptors also amplified Ca²⁺-induced Ca²⁺ release in these neurons. Cyclic ADP-ribose enhanced the total cytoplasmic Ca²⁺ levels during controlled Ca²⁺ influx through voltage gated channels, in a concentration-dependent and ryanodine-sensitive manner and also increased the sensitivity with which a small amount of Ca²⁺ influx could trigger additional release from the ryanodine-sensitive intracellular Ca²⁺ stores. Single cell imaging showed that following the Ca²⁺ influx, cyclic ADP-ribose enhanced the spatial spread of the Ca²⁺ signal from the edge of the cell into its center. These powerful actions suggest a role for cyclic ADP-ribose in the functional coupling of neuronal depolarization, Ca²⁺entry, and global intracellular Ca²⁺ signaling.