Early loss of endogenous NAD+ following rotenone treatment leads to mitochondrial dysfunction and Sarm1 induction 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 nicotinamide adenine dinucleotide(+) (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 PARP inhibitor, PJ34 restored mitochondrial complex I activity and also 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 uncover 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.

Automated summary

This study reveals the regulation of Sarm1 activation by endogenous NAD(+) levels, showing that the early loss of endogenous NAD(+) levels acts as a "biological trigger" for Sarm1 activation following rotenone treatment. Moreover, replenishing NAD(+) through PARP inhibition restores mitochondrial complex I activity and prevents Sarm1 activation in rotenone-treated cells.