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.

Sarm1 induction and accompanying inflammatory response mediates age-dependent susceptibility to rotenone-induced neurotoxicity.

Aging is a complex biological process and environmental risk factors like pesticide exposure have been implicated in the increased incidence of age-related neurodegenerative diseases like Parkinson's disease (PD) but the etiology remains unknown. There is also lack of a proper animal model system to study the progressive effect of these environmental toxins on age-associated neurodegeneration. In this study, we established a drosophila model of aging to study the age-dependent vulnerability to the environmental toxin rotenone that has been implicated in sporadic cases of PD. We demonstrate that age plays a determining role in the increased susceptibility to chronic rotenone exposure that is accompanied by severe locomotor deficits, decreased lifespan and loss of dopaminergic (DA) neurons. Chronic low dose exposure to rotenone results in the rapid induction of the neurodegenerative molecule SARM1/dSarm. Further, the age-dependent dSarm induction is accompanied by a heightened inflammatory response (increased expression of Eiger and Relish) that is independent of reactive oxygen species (ROS) generation in the observed rotenone-induced neurotoxicity. dSarm induction and subsequent locomotor deficits is reversed in the presence of the anti-inflammatory molecule resveratrol. Thus, dSarm and heightened inflammatory responses may play a crucial role in age-dependent vulnerability to the pesticide rotenone thus making it an attractive target to help develop cost-effective therapeutic strategies to prevent ongoing dopaminergic neuronal loss as seen in PD.