Review article: cyclic AMP sensors in living cells: what signals can they actually measure?

Cyclic AMP is a ubiquitous intracellular second messenger that transmits information to several proteins including cyclic nucleotide-gated ion channels and protein kinase A (PKA). In turn, these effectors regulate such diverse cellular functions as Ca2+ influx, excitability, and gene expression, as well as cell-specific processes such as glycogenolysis and lipolysis. The enzymes known to regulate cAMP levels, adenylyl cyclase and phosphodiesterase, have been studied in detail. Unfortunately, an understanding of how information is encoded within cAMP signals has been elusive, because, until recently, methods for measuring cAMP lacked both spatial and temporal resolution. In this paper, we describe two recently developed methods for detecting cAMP levels in living cells. The first method measures fluorescence energy transfer between labeled subunits of PKA. This method is particularly useful for monitoring cellular localization of PKA activity following increases in cAMP levels. However, the slow activation and deactivation rates, the necessarily high concentrations of labeled subunits, and the redistribution of labeled subunits throughout the cell, all intrinsic to this method, limit its utility as a cAMP sensor. The second method uses genetically modified cyclic nucleotidegated channels to measure plasma membrane-localized cAMP levels in either cell populations or single cells. The rapid gating kinetics of these channels allow real-time measurement of cAMP concentrations. These methods have given us the first glimpses of cAMP signals within living cells.