Probing pathophysiology of extracellular cGAMP with substrate-selective ENPP1
The biology of the immune second messenger cGAMP depends on its cellular localization. cGAMP, which is synthesized in response to cytosolic double-stranded DNA, also exists in the extracellular space as a paracrine immunotransmitter that enhances the anticancer immune response. However, the role of extracellular cGAMP is unexplored outside of cancer due to a lack of tools to systemically manipulate it. The extracellular enzyme ENPP1, the only known hydrolase of cGAMP, is a promising target. However, because ENPP1 also degrades extracellular ATP, using genetic knockouts of ENPP1 to study extracellular cGAMP leads to confounding effects. Here we report the H362A point mutation in ENPP1, the dominant cGAMP hydrolase, which selectively abolishes ENPP1s ability to degrade cGAMP, while retaining activity toward other substrates. H362 is not necessary for binding cGAMP or the catalytically-essential zinc atoms but instead supports the in-line reaction geometry. H362 is evolutionarily conserved down to bacteria, suggesting an ancient origin for extracellular cGAMP biology. Enpp1H362A mice do not display the systemic calcification seen in Enpp1-/- mice, highlighting the substrate-specific phenotype of ENPP1. Remarkably, Enpp1H362A mice were resistant to HSV-1 infection, demonstrating the antiviral role of endogenous extracellular cGAMP. The ENPP1H362A mutation is the first genetic tool to enable exploration of extracellular cGAMP biology in a wide range of tissues and diseases.