Cyclic nucleotides serve as second messengers throughout kingdoms of life and regulate various pathways. Here, we review recent milestones in cyclic nucleotide biology, focusing on different enzyme folds that synthesize these signals, their regulatory mechanisms, and pleiotropic downstream signaling events. Our particular focus is on enzyme-centered approaches specifically targeting nucleotidyltransferases, which have enabled the discovery of novel cyclic nucleotides.

Cyclic nucleotides are the most diversified category of second messengers and are found in all organisms modulating diverse pathways. While cAMP and cGMP have been studied over 50 years, cyclic di-nucleotide signaling in eukaryotes emerged only recently with the anti-viral molecule 2´3´cGAMP. Recent breakthrough discoveries have revealed not only the astonishing chemical diversity of cyclic nucleotides but also surprisingly deep-rooted evolutionary origins of cyclic oligo-nucleotide signaling pathways and structural conservation of the proteins involved in their synthesis and signaling. Here we discuss how enzyme-centered approaches have paved the way for the identification of several cyclic nucleotide signals, focusing on the advantages and challenges associated with deciphering the activation mechanisms of such enzymes.