Programmed axon degeneration: from mouse to mechanism to medicine

Abstract

Wallerian degeneration is a widespread mechanism of programmed axon degeneration. In the three decades since the discovery of the Wallerian degeneration slow (Wld^S) mouse, research has generated extensive knowledge of the molecular mechanisms underlying Wallerian degeneration, demonstrated its involvement in non-injury disorders and found multiple ways to block it. Recent developments have included: the detection of NMNAT2 mutations that implicate Wallerian degeneration in rare human diseases; the capacity for lifelong rescue of a lethal condition related to Wallerian degeneration in mice; the discovery of 'druggable' enzymes, including SARM1 and MYCBP2 (also known as PHR1), in Wallerian pathways; and the elucidation of protein structures to drive further understanding of the underlying mechanisms and drug development. Additionally, new data have indicated the potential of these advances to alleviate a number of common disorders, including chemotherapy-induced and diabetic peripheral neuropathies, traumatic brain injury, and amyotrophic lateral sclerosis.

Fig. 1 |. Activation of Wallerian degeneration in injury and disease.

The figure shows three ways in which the Wallerian degeneration mechanism can be activated. a | In classical Wallerian degeneration paradigms, axon injury prevents the delivery of nicotinamide mononucleotide adenylyltransferase 2 (NMNAT2) from the soma along with all other cargoes. Because of its short half-life, NMNAT2 is quickly lost in the distal stump, which triggers sterile-α and Toll/interleukin 1 receptor (TIR) motif containing protein 1 (SARM1-dependent degeneration. b | Axonal transport impairment (for example, due to a toxin, a protein defect or ageing) limits the supply of all cargoes to distal axons. Delivery of NMNAT2 to distal ends of the axon is particularly affected because of its short half-life. When NMNAT2 falls below the threshold for survival, the distal terminal dies back. c | Mutation of the Nmnat2 gene leads to loss or dysfunction of the protein^,,. Without it, the axon cannot grow, causing a developmental disorder (unless rescued through a Sarm1 deletion, for example). If the mutation causes only a partial loss of function, the axon grows but is more susceptible to degeneration.

Fig. 2 |. Wallerian degeneration timeline.

Prior to discovery of the Wallerian degeneration slow (Wld^S) mouse, there was wide acceptance that axonal transport was required for axon survival and that axon injury could serve as a model for disease, albeit with no molecular understanding of the process. The identification of the axon-protective gene encoded by Wld^S made it possible to address this question and led to the identification of nicotinamide mononucleotide adenylyltransferase 2 (NMNAT2) as the best match for the previously proposed ‘Wallerian degeneration inhibitor’. Conservation of the Wallerian mechanism in Drosophila melanogaster has enabled the discovery of other major players in the pathway, including sterile-α and TIR motif containing protein 1 (SARM1). More recently, human genetic studies have started to identify the clinical disorders involving this preventable degeneration pathway, with promising leads in the quest to develop drugs to block it^,. ALS, amyotrophic lateral sclerosis; CIPN, chemotherapy-induced peripheral neuropathy; DLK, dual leucine zipper kinase; GWAS, genome-wide association studies; HFD, high-fat diet; NMN, nicotinamide mononucleotide; SOD1, superoxide dismutase 1; TIR, Toll/interleukin 1 receptor.

Fig. 3 |. A working model of the Wallerian degeneration pathway.

The figure shows current knowledge of the factors that determine nicotinamide mononucleotide adenylyltransferase 2 (NMNAT2) activity in axons, how NMNAT2 loss activates sterile-α and Toll/interleukin 1 receptor (TIR) motif containing protein 1 (SARM1) and events downstream of SARM1 leading to axon degeneration. Various factors disrupt the delivery of functional NMNAT2 to axons. NMNAT2 turnover is mediated by the mitogen-activated protein kinases (MAPK) dual leucine zipper kinase (DLK) and leucine zipper-bearing kinase (LZK), as well as a ubiquitin ligase complex consisting of MYC-binding protein 2 (MYCBP2), s-phase kinase-associated protein 1A (SKP1A) and F-box protein 45 (FBXO45). In combination, this reduces NMNAT2 levels and causes downstream activation of SARM1, probably as a result of increased levels of nicotinamide mononucleotide (NMN) but potentially also in other ways. The combined effect of decreased NMNAT2 and increased activation of SARM1 leads to a great decrease in axonal nicotinamide adenine dinucleotide (NAD), which may itself cause axon degeneration through ATP synthesis failure. Alternatively, other SARM1 substrates or its calcium mobilizing products could be important for the later stages of Wallerian degeneration. In Drosophila melanogaster, there is also an as-yet undefined role for Axundead (Axed) in Wallerian axon degeneration. ADPR, ADP-ribose; cADPR, cyclic ADP-ribose.

Fig. 4 |. Activation of SARM1 by NMN.

The figure summarizes evidence supporting activation of sterile-α and Toll/interleukin 1 receptor (TIR) motif containing protein 1 (SARM1) by nicotinamide mononucleotide (NMN). NMN accumulates after axon injury because the enzyme that normally removes it, nicotinamide mononucleotide adenylyltransferase 2 (NMNAT2), is quickly degraded. If left unchecked, this activates SARM1, leading to axon degeneration. However, NMN accumulation can be prevented in three ways (dark blue): through inhibition of its synthetic enzyme, nicotinamide phosphoribosyl transferase (NAMPT)^,–; by overexpression, or increased axonal targeting, of any isoform of NMNAT^,, its normal processing enzyme in mammals; or by ectopic expression of bacterial NMN deamidase^,. Conversely, its membrane-permeable analogue CZ-48 activates SARM1 directly. Thus, NMN, or a close analogue, appears to be necessary and sufficient for SARM1 activation. ADPR, ADP-ribose; cADPR, cyclic ADP-ribose; NAD, nicotinamide adenine dinucleotide; NAMN, nicotinic acid mononucleotide; Nm, nicotinamide; WLD^S, Wallerian degeneration slow.

Fig. 5 |. An axon vulnerability spectrum in humans and mice.

The figure shows the range of known axon survival phenotypes in mice and proposed analogous phenotypes in the human population for which evidence is growing. The line graph depicts the likely prevalence in the human population and the gradients underneath depict protein and enzyme activity levels of nicotinamide mononucleotide adenylyltransferase 2 (NMNAT2) and sterile-α and Toll/interleukin 1 receptor (TIR) motif containing protein 1 (SARM1) in axons. The pink and blue boxes show five distinct phenotypes in humans and mice, respectively, all of which have been extensively characterized in mice^,,. In humans, the first two phenotypes (non-viable and pathogenic) have been linked to NMNAT2 mutation^,. Causes of the proposed disease-modifying effects indicated towards the right side, corresponding to lower axon vulnerability, may include (1) SARM1 splicing alleles that are present in the Genome Aggregation Database (gnomAD), which are likely to disrupt the function of this pro-degeneration protein and (2) higher than average expression levels of NMNAT2, which are likely to result in gain of function of this pro-survival protein.