Mislocalization of neuronal mitochondria reveals regulation of Wallerian degeneration and NMNAT/WLDS-mediated axon protection independent of axonal mitochondria

Abstract

Axon degeneration is a common and often early feature of neurodegeneration that correlates with the clinical manifestations and progression of neurological disease. Nicotinamide mononucleotide adenylytransferase (NMNAT) is a neuroprotective factor that delays axon degeneration following injury and in models of neurodegenerative diseases suggesting a converging molecular pathway of axon self-destruction. The underlying mechanisms have been under intense investigation and recent reports suggest a central role for axonal mitochondria in both degeneration and NMNAT/WLD(S) (Wallerian degeneration slow)-mediated protection. We used dorsal root ganglia (DRG) explants and Drosophila larval motor neurons (MNs) as models to address the role of mitochondria in Wallerian degeneration (WD). We find that expression of Drosophila NMNAT delays WD in human DRG neurons demonstrating evolutionary conservation of NMNAT function. Morphological comparison of mitochondria from WLD(S)-protected axons demonstrates that mitochondria shrink post-axotomy, though analysis of complex IV activity suggests that they retain their functional capacity despite this morphological change. To determine whether mitochondria are a critical site of regulation for WD, we genetically ablated mitochondria from Drosophila MN axons via the mitochondria trafficking protein milton. Milton loss-of-function did not induce axon degeneration in Drosophila larval MNs, and when axotomized WD proceeded stereotypically in milton distal axons although with a mild, but significant delay. Remarkably, the protective effects of NMNAT/WLD(S) were also maintained in axons devoid of mitochondria. These experiments unveil an axon self-destruction cascade governing WD that is not initiated by axonal mitochondria and for the first time illuminate a mitochondria-independent mechanism(s) regulating WD and NMNAT/WLD(S)-mediated axon protection.

Axons Degenerate in the Absence of Mitochondria in C. elegans

“…Alternatively, mitochondria can protect axons from stress and insults, for example by buffering calcium [8]. Recent studies manipulating mitochondria lend support to both of these models [9–13]. Here, we identify a C. elegans mutant, ric-7 , in which mitochondria are unable to exit the neuron cell bodies, similar to the kinesin-1/ unc-116 mutant.…”

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.…”

Local axonal protection by WldS as revealed by conditional regulation of protein stability

“…Remarkably, expression of the Wallerian degeneration slow (WldS) transgene delays nerve degeneration in these events, and the protection is conserved across many species, including rats (1), Drosophila (2, 3), and even in human neurons (4). Thus, identifying the molecular components of the degeneration pathway with which WldS interferes provides a window of opportunity to understand how axons are normally lost after injury.…”