Early loss of endogenous NAD+ following rotenone treatment leads to Sarm1 activation that is ameliorated by PARP inhibition

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.