Axon Death Pathways Converge on Axundead to Promote Functional and Structural Axon Disassembly

Abstract

Axon degeneration is a hallmark of neurodegenerative disease and neural injury. Axotomy activates an intrinsic pro-degenerative axon death signaling cascade involving loss of the NAD biosynthetic enzyme Nmnat/Nmnat2 in axons, activation of dSarm/Sarm1, and subsequent Sarm-dependent depletion of NAD. Here we identify Axundead (Axed) as a mediator of axon death. axed mutants suppress axon death in several types of axons for the lifespan of the fly and block the pro-degenerative effects of activated dSarm in vivo. Neurodegeneration induced by loss of the sole fly Nmnat ortholog is also fully blocked by axed, but not dsarm, mutants. Thus, pro-degenerative pathways activated by dSarm signaling or Nmnat elimination ultimately converge on Axed. Remarkably, severed axons morphologically preserved by axon death pathway mutations remain integrated in circuits and able to elicit complex behaviors after stimulation, indicating that blockade of axon death signaling results in long-term functional preservation of axons.

Axonal Degeneration Is Mediated by Necroptosis Activation

“…The expression of the Wld s protein is the most studied model of delayed axonal degeneration (Mack et al, 2001;Adalbert et al, 2005;Hoopfer et al, 2006). Loss-of-function studies have also identified mutants that exhibit Wld s -like protection of severed axons, such as dSarm/ Sarm1 (Osterloh et al, 2012;Essuman et al, 2017) and Axundead (Neukomm et al, 2017), supporting the idea that Wallerian degeneration is indeed an active and regulated process.…”

PAM forms an atypical SCF ubiquitin ligase complex that ubiquitinates and degrades NMNAT2

“…NAD is an essential co-enzyme for several biological process, such as cell death, energy metabolism and calcium mobilization. NMNAT2 is highly expressed in the brain, and along with axon protection following injury (44)(45)(46)(47)(48)(49) is implicated in neurodegenerative disease (50)(51)(52)(53). While functional genetics indicate PHR proteins can inhibit NMNAT2, the biochemical mechanism by which this occurs remains untested.…”

TIR-1/SARM1 Inhibits Axon Regeneration

“…We next asked whether TIR-1 might direct multiple responses to injury by activating different downstream signaling events in each axon fragment. TIR-1 and SARM1/dSarm execute several cellular functions, including cleavage of NAD + , activation of MAPK signaling and the Axundead protein, as well as regulation of calcium signaling, gene transcription and autophagy (14,15,24,25,28). This informed a candidate approach to dissect the genetic pathways in which TIR-1 regulates both axon regeneration and degeneration.…”

“…On the distal side of the injury, the most notable endogenous regulator of axon degeneration is Sterile Alpha and Toll/interleukin-1 resistance (TIR) Motif Containing 1 (SARM1/dSarm), which is essential for injury-induced axon degeneration of Drosophila and mouse axons (12,13). SARM1/dSarm promotes axon degeneration cell-autonomously, by depleting NAD + and ATP, interacting with the BTB/BACK domain protein Axundead, and by interacting with mitogenactivated protein kinase (MAPK) signaling cascades, which in turn regulate calpain activation, SCG10/Stathmin 2 proteolysis, mitochondrial dysfunction, cytoskeletal disruption, and axon fragmentation (14)(15)(16)(17)(18)(19)(20)(21). Whether the C. elegans SARM1/dSarm homolog, TIR-1, also regulates injury-induced axon degeneration is not known; however, in the absence of injury, SARM1, dSarm, and TIR-1 promote disease-and age-associated neurodegeneration (22)(23)(24)(25).…”