NAD+ repletion by the PARP inhibitor PJ34 prevents Sarm1 activation and rotenone-induced cell death

Sarm1 is an evolutionary conserved innate immune adaptor protein that has emerged as a primary regulator of programmed axonal degeneration over the past decade. In vitro structural insights have revealed that although Sarm1 induces energy depletion by breaking down NAD+, it is also allosterically inhibited by NAD+. However, how NAD+ levels modulate the activation of intracellular Sarm1 has not been elucidated so far. This study focuses on understanding the events leading to Sarm1 activation in both neuronal and non-neuronal cells using the mitochondrial complex I inhibitor rotenone. Here we report the regulation of rotenone-induced cell death by loss of NAD+ that may act as a "biological trigger" of Sarm1 activation. Our study revealed that early loss of endogenous NAD+ levels arising due to PARP1 hyperactivation preceded Sarm1 induction following rotenone treatment. Interestingly, replenishing NAD+ levels by the PARP1 inhibitor, PJ34 restored mitochondrial homeostasis and prevented subsequent Sarm1 activation in rotenone treated cells. These cellular data were further validated in Drosophila melanogaster where a significant reduction in rotenone mediated loss of locomotor abilities and reduced dSarm expression was observed in the flies following PARP inhibition. Taken together, these observations not only uncovers a novel regulation of Sarm1 induction by endogenous NAD+ levels but also point towards an important understanding on how PARP inhibitors could be repurposed in the treatment of mitochondrial complex I deficiency disorders mediated by Sarm1.