Compartmentation of Cellular cGMP‐signalling: A Mechanism for Spatial and Temporal Regulation of cGMP Mediated Effects

Abstract

Cyclic GMP (cGMP) is a ubiquitous intracellular messenger that acts to integrate and translate the information encoded by extracellular messenger molecules including nitric oxide (NO) and natriuretic peptides. Recent evidence suggests that selective activation of the NO‐sensitive guanylyl cyclase results in a localized increase of cGMP which is selectively hydrolyzed by the cGMP specific phosphodiesterase (PDE5). To investigate the role of PDE5 in maintaining this pool of cGMP, we performed selective immunoprecipitations in human platelets and discovered that PDE5 exists in two functional compartments. One compartment is soluble and enzymatically active in resting platelets, while the other fraction exists in a cGMP signaling complex with Protein Kinase G (PKG) and is not active. Interestingly, while the PKG integrated fraction of PDE5 was not activated upon addition of cGMP alone, it was markedly activated upon addition of cGMP and ATP, resulting in PKG dependent phosphorylation of Serine 92. In addition, biochemical fractionation of platelets showed that this PKG complex containing PDE5 was present on the sarcoplasmic reticulum and allows local cGMP‐mediated control of IP3‐stimualted Ca2+ release. These findings may give insight into the specific effects of nitric oxide and present new targets for sildenafil, a potent PDE5 inhibitor and active ingredient in VIAGRATM.