Mechanisms of cyclic dinucleotide signaling: from prokaryotes to humans

Cyclic dinucleotides (cdNs) are novel second messengers that are synthesized directly from two “molecular units of energy” in the cell, namely ATP and GTP. In prokaryotes, all three combinations of cdNs have been discovered: cyclic di-AMP, cyclic AMP-GMP, and cyclic di-GMP. Their effects on bacterial host physiology have been shown to be important and diverse, impacting processes ranging from biofilm formation to motility to sporulation. Many of their prokaryotic effectors are protein-based; however, cyclic dinucleotides have also been shown to exert genetic expression control at the RNA level by binding to mRNA regulatory elements called riboswitches. While cyclic dinucleotide signaling in prokaryotes has been appreciated for over 25 years, it was not until the last few years that cyclic dinucleotide signaling has been shown to exist and have a profound impact in mammals. cGAMP (a non-canonically linked cyclic AMP-GMP) has been discovered endogenously within humans and found to be a critical second messenger for signaling pathogen invasion to the host. cGAS, a cytosolic mammalian cyclic dinucleotide synthase, produces cGAMP upon binding pathogen DNA. Subsequently, cGAMP binds to the ER-resident transmembrane protein STING, which triggers a downstream cascade of interferon, cytokine, and chemokine production. Therefore from prokaryotes to humans, cyclic dinucleotide signaling spans billions of years in evolution, and today has led to great interest in its therapeutic targeting.This work details the discovery of cyclic AMP-GMP and its cognate riboswitch within the bacteria Geobacter sulfurreducens. Interestingly, cyclic AMP-GMP signaling through its riboswitch plays a previously unappreciated role in exoelectrogenesis within Geobacter. The structural elucidation and signaling role of cyclic AMP-GMP (cGAMP) in humans is subsequently described, showing that humans utilize a novel regioisomer of cyclic AMP-GMP containing mixed 2′-5′ and 3′-5′ phosphodiester bonds. The ancient origin of human cGAMP signaling is illuminated by the discovery of a functional cGAS-cdN-STING pathway in Nematostella vectensis, an anemone species >500 million years diverged from humans. The structure of anemone STING is highly conserved compared to human STING, yet anemone cGAS appears to produce only a 3′-5′ linked cdN. Finally, the signaling space of the cGAS-cdN-STING pathway is examined with a focus on its potential functions in lower organisms. Taken holistically, this work details the signaling mechanisms of cyclic dinucleotides in multiple organisms, while profiling the chemical evolution of a unique second messenger