Resolving Compartmentalized Signals with a cAMP Sensor

Abstract

Changes in the intracellular concentration of 3′,5′-adenosine monophosphate (cAMP) represent a key element in many signaling pathways. Although cAMP is a small, potentially freely diffusible molecule, the evidence is mounting that it can be spatially and temporally restricted (see the review by Xiao that discusses this issue in cardiac cells). Rich et al . used a simple fibroblast cell line (HEK293 cells) expressing a mutated cyclic nucleotide-gated channel engineered to open in response to binding cAMP and the subsequent influx of Ca 2+ ions to monitor changes in cAMP in single cells and populations. Stimulation of the transfected cells, either as a population or as single cells, with prostaglandin E 1 (PGE 1 ) (a ligand for an endogenous receptor) produced a transient activation of the channel. In contrast, the total cellular cAMP production was not transient, but reached a plateau, suggesting that the concentration of cAMP near the channels was decreasing more rapidly than the total cellular cAMP concentration. In the presence of inhibitors of phosphodiesterases (PDEs), there was increased flux through the channels, and the change in channel activity was no longer transient, but plateaued and strongly resembled the temporal profile for changes in total cellular cAMP concentration. When cAMP was elevated by activation of adenylate cyclase with forskolin, the transient nature of the Ca 2+ influx was again lost, suggesting that PGE 1 results in the activation not only of adenylate cyclase, but also of PDE. Mathematical modeling with a two-compartment model reproduces the data if the membrane-proximal compartment has an enhanced rate of cAMP degradation due to activation of PDE. Thus, even in a morphologically "simple" cell, cAMP signals can be spatially and temporally limited, allowing for multiple signals from a single second messenger. T. C. Rich, K. A. Fagan, T. E. Tse, J. Schaack, D. M. F. Cooper, J. W. Karpen, A uniform extracellular stimulus triggers distinct cAMP signals in different compartments of a simple cell. Proc. Natl. Acad. Sci. U.S.A. 98 , 13049-13054 (2001). [Abstract] [Full Text]