Structural basis for SARM1 inhibition and activation under energetic stress

Abstract

SARM1, an executor of axonal degeneration, displays NADase activity that depletes the key cellular metabolite, NAD+, in response to nerve injury. The basis of SARM1 inhibition and its activation under stress conditions are still unknown. Here, we present cryo-EM maps of SARM1 at 2.9 and 2.7 Å resolutions. These indicate that SARM1 homo-octamer avoids premature activation by assuming a packed conformation, with ordered inner and peripheral rings, that prevents dimerization and activation of the catalytic domains. This inactive conformation is stabilized by binding of SARM1’s own substrate NAD+ in an allosteric location, away from the catalytic sites. This model was validated by mutagenesis of the allosteric site, which led to constitutively active SARM1. We propose that the reduction of cellular NAD+ concentration contributes to the disassembly of SARM1's peripheral ring, which allows formation of active NADase domain dimers, thereby further depleting NAD+ to cause an energetic catastrophe and cell death.

Permeant fluorescent probes visualize the activation of SARM1 and uncover an anti-neurodegenerative drug candidate

“…They are all analogs of NAD and are cell-impermeant, such as εNAD. The fact that NAD is now shown to be an inhibitor of SARM1 ( Jiang et al, 2020 ; Sporny et al, 2020 ) makes the use of these analog probes problematic, as they may affect SARM1 activity as well. PC6 has no such drawback as it is a pyridine, not an NAD analog.…”

“…It is self-inhibitory and is activated by nicotinamide mononucleotide (NMN) ( Zhao et al, 2019 ), resulting in depletion of the intracellular NAD-pool ( Essuman et al, 2017 ; Zhao et al, 2019 ). However, recent studies suggest that nicotinamide adenine dinucleotide (NAD) itself is an inhibitor of SARM1 activation and the balance between NMN and NAD controls the activation of SARM1 ( Figley et al, 2021 ; Jiang et al, 2020 ; Sporny et al, 2020 ).…”

Genetic inactivation of SARM1 axon degeneration pathway improves outcome trajectory after experimental traumatic brain injury based on pathological, radiological, and functional measures

“…Mitochondrial stress, trauma or other insults lead to loss of nicotinamide mononucleotide adenylyl-transferase 2 (NMNAT2), an NAD-synthesizing enzyme, in axons that depletes NAD+ and raises the level of its precursor nicotinamide mononucleotide (NMN) [20, 47]. Low NAD+ and high NMN levels regulate release of auto-inhibition of SARM1 NADase activity, which further depletes NAD+ and axon energy stores [11, 21, 38, 73, 76]. Axons with swollen mitochondria were significantly increased at 3 days after TBI in Sarm1 WT and Sarm1 KO mice [51].…”

“…After injury or mitochondrial dysfunction, the SARM1 (sterile alpha and Toll/interleukin-1 receptor motif-containing 1) protein executes a highly conserved molecular axon death pathway [26, 47, 67, 80]. Injury releases SARM1 from an auto-inhibited state that is found in healthy axons [21, 73, 76]. Active SARM1 is a glycohydrolase which depletes nicotinamide adenine dinucleotide (NAD+) that is critical for energy stores in axons [17, 22, 25].…”

Genetic inactivation of SARM1 axon degeneration pathway improves outcome trajectory after experimental traumatic brain injury based on pathological, radiological, and functional measures

“…After injury or mitochondrial dysfunction, the SARM1 (sterile alpha and Toll/interleukin-1 receptor motif-containing 1) protein executes a highly conserved molecular axon degeneration pathway [ 27 , 48 , 70 , 84 ]. Injury releases SARM1 from an auto-inhibited state that is found in healthy axons [ 22 , 76 , 80 ]. Active SARM1 is a glycohydrolase which depletes nicotinamide adenine dinucleotide (NAD +) that is critical for energy stores in axons [ 18 , 23 , 26 ].…”