Cyclic Nucleotide Analogs as Pharmacological Tools for Studying Signaling Pathways

Since the discovery of adenosine-3′,5′-cyclic monophosphate (cAMP)1 and guanosine3′,5′-cyclic monophosphate (cGMP)2 nearly 60 years ago, cyclic nucleotides have been envisioned as one of the most universal and versatile second messengers and have been detected from elephants down to E. coli. In the following years, the corresponding cAMP-dependent protein kinases (PKA I and II; for review, see Kim et al.3) and the cGMP-dependent protein kinases (PKG Iα, Iβ, and II; for review, see Francis et al.4) were identied and characterized.