Base exchange inhibitors of SARM1 form mononucleotide adducts and activate SARM1 in vivo
Activation of the NAD hydrolase SARM1 causes neuronal pathology. NAD binding to an allosteric site maintains SARM1 autoinhibition while NMN binding enables NADase activity. We evaluated SARM1 base exchange inhibitors (BEI) that exchange with nicotinamide in the SARM1 catalytic site to form inhibitor adducts. We found that BEI paradoxically activated SARM1 in some contexts, elevating both the SARM1 product cADPR and ratio to substrate, cADPR/NAD, in uninjured sciatic nerve and in healthy animals. Catalytically inactive E642A SARM1 knock-in mice were protected from elevated cADPR and cADPR/NAD in sciatic nerve after BEI dosing. Like the SARM1 activating forms of the neurotoxins Vacor and 3-acetylpyridine, BEI formed mononucleotide (MN) adducts in vivo. BEI also formed MN adducts in human THP-1 cells, as did the pyridine-containing drugs indinavir and chloroquine. These findings underscore the promiscuity of nicotinamide-related metabolism and potential for neurotoxicity arising from SARM1s susceptibility to activation by exogenous agents.