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SARM1 and NAD+: the enzyme behind axon self-destruction, explained
Prepared by: Research Desk
Prepared by: Research Desk. Sources are linked for verification.
Most writing about NAD+ focuses on how cells make it and how levels change with age. SARM1 is a different part of the story. It is an enzyme that, once switched on, destroys NAD+ inside nerve fibres, and that loss of NAD+ is part of how a damaged axon takes itself apart. This explainer covers the biochemistry only: what the enzyme is, how it is activated, and what laboratory research on blocking it has reported. Nothing here concerns supplements or human use.
Wallerian degeneration and the WldS clue
When an axon is cut, the segment separated from the cell body does not simply wither. After a delay it breaks down in an organised, active way. This is called Wallerian degeneration, and reviews of the field describe it as a regulated program rather than passive decay [1].
The first strong clue came from a mouse strain called WldS (Wallerian degeneration slow). In these animals, cut axons stay intact far longer than normal. The protective WldS protein is a fusion that contains the enzyme domain of NMNAT1, an enzyme that makes NAD+ from NMN [1][2]. That finding tied axon survival to NAD+ metabolism years before SARM1's chemistry was understood.
NMNAT2: the short-lived survival factor
Healthy axons depend on a related enzyme, NMNAT2. It performs the final step of the salvage route, converting NMN into NAD+ (the earlier step, making NMN, is handled by NAMPT; see NAMPT explained and the NAD+ salvage pathway).
The key property of NMNAT2 is that it is labile. It is made in the cell body and shipped down the axon, and it turns over quickly. A 2010 study in cultured neurons and mice identified endogenous Nmnat2 as an essential survival factor for axons: when its supply from the cell body is cut off, it disappears from the axon and degeneration follows [2]. A 2013 study reported that where Nmnat2 sits inside the cell affects both its stability and how well it protects axons [3]. Mouse work in 2019 found that reduced NMNAT2 levels compromise both axon development and survival [4]. A 2026 study reported that MEK-ERK signalling regulates Nmnat2 transcription in a neuron-subtype-specific way, suggesting that how much NMNAT2 a neuron produces is itself controlled [5].
SARM1 is an NAD+-cleaving enzyme
SARM1 (sterile alpha and TIR motif-containing 1) was known to be required for injury-induced axon degeneration before anyone knew what it did chemically [1]. The answer arrived in 2017, when a Neuron paper showed that the TIR domain of SARM1 has intrinsic NADase activity. An accompanying commentary called this an "unpredicted NADase" controlling axonal degeneration [6].
Activated SARM1 cleaves NAD+ into nicotinamide plus ADP-ribose (ADPR) and cyclic ADP-ribose (cADPR) [7]. Because SARM1 consumes NAD+ rather than recycling it, it belongs alongside other NAD+-using enzymes such as sirtuins (see NAD+ and sirtuins), though its role is destructive. Reviewers have noted that the SARM1 TIR NADase resembles TIR domains used in bacterial defence against phages and in toxin-antitoxin systems, a sign that NAD+ depletion is an ancient way for cells to shut themselves down [7].
Structural work published in 2022 described how SARM1 recognises its substrate and how small molecules can occupy its active sites [8].
How SARM1 is switched on: the NMN/NAD+ ratio
This is where NMNAT2 and SARM1 meet. A 2021 Neuron study reported that SARM1 acts as a metabolic sensor activated by an increased NMN/NAD+ ratio, not by either molecule alone [9]. The sequence in an injured axon runs roughly like this:
Injury halts delivery of NMNAT2 from the cell body.
Existing NMNAT2 in the axon is degraded.
NMN is no longer converted to NAD+, so NMN rises while NAD+ falls.
The higher NMN/NAD+ ratio activates SARM1 [9].
SARM1 cleaves NAD+, deepening the NAD+ loss, and the axon degenerates [1].
Structurally, SARM1's regulatory region binds NAD+ in its resting state, and NMN binding at that site favours activation [8]. A 2026 study in Nature Chemical Biology proposed that activation also involves a two-step phase transition of the protein [10]. Not every proposed trigger has held up: a 2026 PNAS paper reported that biochemical and cellular evidence does not support direct activation of SARM1 by double-stranded DNA [11].
For background on NMN as a molecule in its own right, see NMN vs NAD+.
Beyond cut axons
Research has extended SARM1's role past simple injury. A 2026 Neuron paper reported that SARM1 executes neuronal parthanatos and contributes to excitotoxic cell death in experimental models [12]. A 2025 study examined SARM1 in models of neurotropic viral infection [13]. These are laboratory findings in cells and animals; their relevance to people has not been established in the sources cited here.
Inhibitor research: where it stands
Because SARM1 sits at a single chokepoint, it has drawn drug-discovery interest. All of the work in the sources below is preclinical:
Adduct-forming inhibitors (2022). A Neuron study described uncompetitive SARM1 inhibitors that form an adduct with NAD+-derived chemistry and reported neuroprotective effects in preclinical models of nerve injury and disease [14].
Paclitaxel model (2021). A Brain study reported that pharmacological SARM1 inhibition protected axon structure and function in a rodent model of paclitaxel-induced peripheral neuropathy [15].
Structural basis (2022). Crystal and cryo-EM structures mapped how small molecules bind SARM1 [8].
None of the cited sources reports an approved SARM1 inhibitor. The accurate description is research-stage. For how U.S. rules separate approved drugs from other NAD+-related products, see our legal-status overview; for the wider metabolism, see the NAD+ guide.
Frequently asked questions
What is SARM1?
SARM1 is a protein in neurons whose TIR domain cleaves NAD+ [6]. When activated after axon damage, it depletes NAD+ and is required for the axon to degenerate [1].
Why does the NMN/NAD+ ratio matter?
A 2021 study reported that SARM1 senses the ratio of NMN to NAD+ rather than either level alone [9]. Losing NMNAT2 pushes that ratio upward, which links NAD+ salvage directly to the decision to dismantle an axon [2][9].
Are there approved SARM1 inhibitors?
Not in the sources cited here. The published inhibitor studies used cell and animal models [14][15].
Is SARM1 related to bacterial proteins?
Its TIR NADase domain shows mechanistic similarities to bacterial phage-defence and toxin-antitoxin systems, according to a 2021 review [7].
Further reading
SARM1's NADase activity was only described in 2017, so any textbook printed earlier will cover Wallerian degeneration without the enzyme now central to it. When choosing a book, check the edition date first.
Useful criteria for comparing titles: whether the enzymology chapter covers NAD+-consuming enzymes and not just redox roles; whether chapters carry primary-literature references to check; whether a recent edition is available at a transparent price; and whether the format (print, e-book, institutional access) suits close reading of pathway diagrams.
A neuroscience text that explains axonal transport well also makes the NMNAT2 story much easier to follow, since the whole mechanism depends on delivery from the cell body.
Related on this site
NAD+ explained: biochemistry, precursors, testing and human evidence
NAD+ and sirtuins: how the enzymes use NAD+ and what research examined
NAD+ books: an evidence-graded reading list on sirtuins and aging
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Last updated: 26 October 2026.
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