SARM1 is an enzyme with a unique, prodegenerative role in axon death 1. In specific circumstances removing SARM1 permanently rescues axons from degeneration 2. Not surprisingly, it has become a widely-studied drug target for axonopathies 3. In the last four years, there has been remarkable progress in understanding SARM1 mechanism of action and regulation. It was found to have NAD-degrading and -cyclisation activities 4, to be an octamer where the regulatory ARM domain of one subunit limits substrate access to the catalytic domain of another 5–7, and to be allosterically activated by the prodegenerative NAD precursor NMN 8–10, which binds its ARM domain 11,12. A second allosteric regulator is NAD, which competitively binds the same site, partially blocking activation. Thus, SARM1 is a metabolic sensor of the NMN:NAD ratio 12, which rises in compromised axons to trigger axon death 8,9.
In contrast, vacor is an old rodenticide whose uses appeared to be over. It was banned over 40 years ago because it is toxic to humans. Individuals who survived vacor ingestion typically developed rapid-onset, wide-ranging neuropathy 13. Most also developed diabetes, but as neuropathy began within hours, this could not have been secondary to diabetes. Other than amelioration of toxicity by its analogue, nicotinamide, and recent data suggesting inhibition of NAD metabolism 14, nothing was known about how vacor kills neurons and their axons, until now.
We show that vacor metabolite VMN is the third known allosteric regulator of SARM1. VMN is the most potent SARM1 activator, binding the same ARM domain site over twice as strongly as NMN. With the structures of all three complexes now available, this is a strong basis for rational drug design to block SARM1 activation. Neurons lacking SARM1 are completely resistant to vacor, not only failing to die after repeated dosing, but even continuing to grow. Thus, the toxic mechanism that remained unknown for so long has been revealed as highly-specific SARM1 activation.
Drug discovery is also facilitated by vacor’s membrane permeability. Although not the first membrane-permeable compound to activate SARM1 11, it is the easiest to synthesise and the only one that causes rapid SARM1-dependent axon and neuron death. This makes vacor unique for specific SARM1 activation in living cells, a crucial requirement for high-throughput drug screening.
The complete rescue of vacor-treated neurons shows that full rescue by SARM1 deletion extends beyond specific mouse gene mutation 2 to a lethal human neurotoxin. Alleviation by SARM1 deletion of axon injury models, or non-specific, chronic transport blockade, is only temporary, but our findings here, and in lifelong rescue of Nmnat2 null mice 2, demonstrate its full protective potential. It seems that activating SARM1 very specifically unmasks a capacity for full protection that other degenerative mechanisms override when the cause is less specific. It follows that drugs blocking SARM1 could be highly effective in any human diseases, or specific patients, where it plays a major role, highlighting the importance of identifying such cases.
The specificity of vacor toxicity for SARM1 raises important questions about other pyridines or related chemicals that are still widely used, for example in pesticide synthesis, solvents and, controversially, as food additives. Could any of them have similar effects? This is particularly pertinent for molecules already associated with neurodegeneration, such as paraquat and MPP+.
Finally, what do we learn about SARM1 activation? The higher potency could reflect stronger binding than NMN and/or differences in the structural response of the ARM domain. Stronger binding has been confirmed, but this does not exclude the second possibility, which is important to study further. And exactly how does a change in the ARM domain uncover the active site of the neighbouring TIR domain? Despite the rapid progress so far, it is clear that SARM1 is a complex enzyme with many activities; we have only scratched the surface of how this intriguing enzyme operates. Vacor and VMN are vital new reagents for those next steps.
References
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