Mitochondrial impairment activates the Wallerian pathway through depletion of NMNAT2 leading to SARM1-dependent axon degeneration

Andrea Loreto,Ciaran S Hill,Victoria L Hewitt,Giuseppe Orsomando,Carlo Angeletti,Jonathan Gilley,Cristiano Lucci,Alvaro Sanchez-Martinez,Alexander J Whitworth,Laura Conforti,Federico Dajas-Bailador,Michael P Coleman

doi:10.1016/j.nbd.2019.104678

Abstract

Wallerian degeneration of physically injured axons involves a well-defined molecular pathway linking loss of axonal survival factor NMNAT2 to activation of pro-degenerative protein SARM1. Manipulating the pathway through these proteins led to the identification of non-axotomy insults causing axon degeneration by a Wallerian-like mechanism, including several involving mitochondrial impairment. Mitochondrial dysfunction is heavily implicated in Parkinson's disease, Charcot-Marie-Tooth disease, hereditary spastic paraplegia and other axonal disorders. However, whether and how mitochondrial impairment activates Wallerian degeneration has remained unclear. Here, we show that disruption of mitochondrial membrane potential leads to axonal NMNAT2 depletion in mouse sympathetic neurons, increasing the substrate-to-product ratio (NMN/NAD) of this NAD-synthesising enzyme, a metabolic fingerprint of Wallerian degeneration. The mechanism appears to involve both impaired NMNAT2 synthesis and reduced axonal transport. Expression of WLDS and Sarm1 deletion both protect axons after mitochondrial uncoupling. Blocking the pathway also confers neuroprotection and increases the lifespan of flies with Pink1 loss-of-function mutation, which causes severe mitochondrial defects. These data indicate that mitochondrial impairment replicates all the major steps of Wallerian degeneration, placing it upstream of NMNAT2 loss, with the potential to contribute to axon pathology in mitochondrial disorders.

Highlights

Studies of axon degeneration following axotomy (Wallerian degeneration) and of the axon-protective protein WLDS have led to the discovery of critical endogenous regulators of the mechanisms resulting in axon degeneration (Conforti et al, 2014; Gerdts et al, 2016; Llobet Rosell and Neukomm, 2019)
The current model predicts that the pathway regulating Wallerian degeneration (Wallerian pathway) is activated by the loss in the axon of the labile nicotinamide mononucleotide adenylyl-transferase 2 (NMNAT2), a nicotinamide adenine dinucleotide (NAD)-synthesising enzyme
Previous work by us and others demonstrated that sympathetic and sensory primary neurons exposed to Carbonyl cyanide mchlorophenyl hydrazone (CCCP) undergo disruption of mitochondrial membrane potential and axon degeneration (Summers et al, 2014; Loreto et al, 2015), providing a good experimental model to study mitochondrial dysfunction leading to axon degeneration

Summary

Introduction

Studies of axon degeneration following axotomy (Wallerian degeneration) and of the axon-protective protein WLDS have led to the discovery of critical endogenous regulators of the mechanisms resulting in axon degeneration (Conforti et al, 2014; Gerdts et al, 2016; Llobet Rosell and Neukomm, 2019). The current model predicts that the pathway regulating Wallerian degeneration (Wallerian pathway) is activated by the loss in the axon of the labile nicotinamide mononucleotide adenylyl-transferase 2 (NMNAT2), a nicotinamide adenine dinucleotide (NAD)-synthesising enzyme. Downstream of NMNAT2 depletion, the pro-degenerative protein sterile alpha and TIR motif-containing protein 1 (SARM1) executes the degeneration program (Osterloh et al, 2012; Gilley et al, 2015; Gerdts et al, 2015; Loreto et al, 2015). There is still debate about how NMNAT2 loss leads to SARM1 activation but the rise in its substrate, NMN, appears to be important (Di Stefano et al, 2015, 2017; Loreto et al, 2015; Cohen, 2017; Zhao et al, 2019) as well as the fall in its product, NAD (Gerdts et al, 2015; Sasaki et al, 2016; Essuman et al, 2017)

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