Michael P. Coleman

Axon degeneration mechanisms: commonality amid diversity

A wide range of insults can trigger axon degeneration, and axons respond with diverse morphology, topology and speed. However, recent genetic, immunochemical, morphological and pharmacological investigations point to convergent degeneration mechanisms. The principal convergence points — poor axonal transport, mitochondrial dysfunction and an increase in intra-axonal calcium — have been identified by rescuing axons with the slow Wallerian degeneration gene (WldS) and studies with blockers of sodium or calcium influx. By understanding how the pathways fit together, we can combine our knowledge of mechanisms, and potentially also treatment strategies, from different axonal disorders.

Wallerian degeneration of injured axons and synapses is delayed by a Ube4b/Nmnat chimeric gene

Axons and their synapses distal to an injury undergo rapid Wallerian degeneration, but axons in the C57BL/WldS mouse are protected. The degenerative and protective mechanisms are unknown. We identified the protective gene, which encodes an N-terminal fragment of ubiquitination factor E4B (Ube4b) fused to nicotinamide mononucleotide adenylyltransferase (Nmnat), and showed that it confers a dose-dependent block of Wallerian degeneration. Transected distal axons survived for two weeks, and neuromuscular junctions were also protected. Surprisingly, the Wld protein was located predominantly in the nucleus, indicating an indirect protective mechanism. Nmnat enzyme activity, but not NAD+ content, was increased fourfold in WldS tissues. Thus, axon protection is likely to be mediated by altered ubiquitination or pyridine nucleotide metabolism.

Axon pathology in neurological disease: a neglected therapeutic target

In the C57BL/Wld(S) mouse, a dominant mutation dramatically delays Wallerian degeneration in injury and disease, possibly by influencing multi-ubiquitination. Studies on this mouse show that axons and synapses degenerate by active and regulated mechanisms that are akin to apoptosis. Axon loss contributes to neurological symptoms in disorders as diverse as multiple sclerosis, stroke, traumatic brain and spinal cord injury, peripheral neuropathies and chronic neurodegenerative diseases, but it has been largely neglected in neuroprotective strategies. Defects in axonal transport, myelination or oxygenation could trigger such mechanisms of active axon degeneration. Understanding how these diverse insults might initiate an axon-degeneration process could lead to new therapeutic interventions.

Wallerian Degeneration, WldS, and Nmnat

Traditionally, researchers have believed that axons are highly dependent on their cell bodies for long-term survival. However, recent studies point to the existence of axon-autonomous mechanism(s) that regulate rapid axon degeneration after axotomy. Here, we review the cellular and molecular events that underlie this process, termed Wallerian degeneration. We describe the biphasic nature of axon degeneration after axotomy and our current understanding of how Wld(S)--an extraordinary protein formed by fusing a Ube4b sequence to Nmnat1--acts to protect severed axons. Interestingly, the neuroprotective effects of Wld(S) span all species tested, which suggests that there is an ancient, Wld(S)-sensitive axon destruction program. Recent studies with Wld(S) also reveal that Wallerian degeneration is genetically related to several dying back axonopathies, thus arguing that Wallerian degeneration can serve as a useful model to understand, and potentially treat, axon degeneration in diverse traumatic or disease contexts.

dSarm/Sarm1 Is Required for Activation of an Injury-Induced Axon Death Pathway

Sarm-Assisted Suicide Neurodegenerative disease or nerve lesions cause axons and synapses to disintegrate through a process known as Wallerian degeneration, which may involve an active “axon death program.” Osterloh et al. (p. 481, published online 7 June; see the Perspective by Yu and Luo) identify loss-of-function mutations in Drosophila dSarm that are capable of blocking the degeneration of severed axons for the fly life span. Deletion of mouse Sarm1 provides similar protection to severed axons for weeks after injury, which suggests that Sarm is part of an ancient axonal death signaling cascade. Mutations in a scaffold protein block the Wallerian degeneration of axons in flies and mice. Axonal and synaptic degeneration is a hallmark of peripheral neuropathy, brain injury, and neurodegenerative disease. Axonal degeneration has been proposed to be mediated by an active autodestruction program, akin to apoptotic cell death; however, loss-of-function mutations capable of potently blocking axon self-destruction have not been described. Here, we show that loss of the Drosophila Toll receptor adaptor dSarm (sterile α/Armadillo/Toll-Interleukin receptor homology domain protein) cell-autonomously suppresses Wallerian degeneration for weeks after axotomy. Severed mouse Sarm1 null axons exhibit remarkable long-term survival both in vivo and in vitro, indicating that Sarm1 prodegenerative signaling is conserved in mammals. Our results provide direct evidence that axons actively promote their own destruction after injury and identify dSarm/Sarm1 as a member of an ancient axon death signaling pathway.

Endogenous Nmnat2 is an essential survival factor for maintenance of healthy axons.

We conclude that endogenous Nmnat2 prevents spontaneous degeneration of healthy axons and propose that, when present, the more long-lived, functionally related WldS protein substitutes for Nmnat2 loss after axon injury. Endogenous Nmnat2 represents an exciting new therapeutic target for axonal disorders.

The progressive nature of Wallerian degeneration in wild-type and slow Wallerian degeneration (WldS) nerves

BackgroundThe progressive nature of Wallerian degeneration has long been controversial. Conflicting reports that distal stumps of injured axons degenerate anterogradely, retrogradely, or simultaneously are based on statistical observations at discontinuous locations within the nerve, without observing any single axon at two distant points. As axon degeneration is asynchronous, there are clear advantages to longitudinal studies of individual degenerating axons. We recently validated the study of Wallerian degeneration using yellow fluorescent protein (YFP) in a small, representative population of axons, which greatly improves longitudinal imaging. Here, we apply this method to study the progressive nature of Wallerian degeneration in both wild-type and slow Wallerian degeneration (WldS) mutant mice.ResultsIn wild-type nerves, we directly observed partially fragmented axons (average 5.3%) among a majority of fully intact or degenerated axons 37–42 h after transection and 40–44 h after crush injury. Axons exist in this state only transiently, probably for less than one hour. Surprisingly, axons degenerated anterogradely after transection but retrogradely after a crush, but in both cases a sharp boundary separated intact and fragmented regions of individual axons, indicating that Wallerian degeneration progresses as a wave sequentially affecting adjacent regions of the axon. In contrast, most or all WldS axons were partially fragmented 15–25 days after nerve lesion, WldS axons degenerated anterogradely independent of lesion type, and signs of degeneration increased gradually along the nerve instead of abruptly. Furthermore, the first signs of degeneration were short constrictions, not complete breaks.ConclusionsWe conclude that Wallerian degeneration progresses rapidly along individual wild-type axons after a heterogeneous latent phase. The speed of progression and its ability to travel in either direction challenges earlier models in which clearance of trophic or regulatory factors by axonal transport triggers degeneration. WldS axons, once they finally degenerate, do so by a fundamentally different mechanism, indicated by differences in the rate, direction and abruptness of progression, and by different early morphological signs of degeneration. These observations suggest that WldS axons undergo a slow anterograde decay as axonal components are gradually depleted, and do not simply follow the degeneration pathway of wild-type axons at a slower rate.

Wallerian degeneration: an emerging axon death pathway linking injury and disease

Axon degeneration is a prominent early feature of most neurodegenerative disorders and can also be induced directly by nerve injury in a process known as Wallerian degeneration. The discovery of genetic mutations that delay Wallerian degeneration has provided insight into mechanisms underlying axon degeneration in disease. Rapid Wallerian degeneration requires the pro-degenerative molecules SARM1 and PHR1. Nicotinamide mononucleotide adenylyltransferase 2 (NMNAT2) is essential for axon growth and survival. Its loss from injured axons may activate Wallerian degeneration, whereas NMNAT overexpression rescues axons from degeneration. Here, we discuss the roles of these and other proposed regulators of Wallerian degeneration, new opportunities for understanding disease mechanisms and intriguing links between Wallerian degeneration, innate immunity, synaptic growth and cell death.

Neuronal death: where does the end begin?

Neurodegenerative disorders involve death of cell bodies, axons, dendrites and synapses, but it is surprisingly difficult to determine the spatiotemporal sequence of events and the causal relationships among these events. Neuronal compartments often crucially depend upon one another for survival, and molecular defects in one compartment can trigger cellular degeneration in distant parts of the neuron. Here, we consider the novel approaches used to understand these biologically complex and technically challenging questions in amyotrophic lateral sclerosis, spinal muscular atrophy, glaucoma, Alzheimers disease, Parkinsons disease and polyglutamine disorders. We conclude that there is partial understanding of what degenerates first and why, but that controversy remains the rule not the exception. Finally, we highlight strategies for resolving these fundamental issues.

NAD + and axon degeneration revisited: Nmnat1 cannot substitute for Wld S to delay Wallerian degeneration

The slow Wallerian degeneration protein (WldS), a fusion protein incorporating full-length nicotinamide mononucleotide adenylyltransferase 1 (Nmnat1), delays axon degeneration caused by injury, toxins and genetic mutation. Nmnat1 overexpression is reported to protect axons in vitro, but its effect in vivo and its potency remain unclear. We generated Nmnat1-overexpressing transgenic mice whose Nmnat activities closely match that of WldS mice. Nmnat1 overexpression in five lines of transgenic mice failed to delay Wallerian degeneration in transected sciatic nerves in contrast to WldS mice where nearly all axons were protected. Transected neurites in Nmnat1 transgenic dorsal root ganglion explant cultures also degenerated rapidly. The delay in vincristine-induced neurite degeneration following lentiviral overexpression of Nmnat1 was significantly less potent than for WldS, and lentiviral overexpressed enzyme-dead WldS still displayed residual neurite protection. Thus, Nmnat1 is significantly weaker than WldS at protecting axons against traumatic or toxic injury in vitro, and has no detectable effect in vivo. The full protective effect of WldS requires more N-terminal sequences of the protein.