Emergence of SARM1 as a Potential Therapeutic Target for Wallerian-type Diseases

Abstract

Wallerian degeneration is a neuronal death pathway that is triggered in response to injury or disease. Death was thought to occur passively until the discovery of a mouse strain, i.e., Wallerian degeneration slow (WLD^S), which was resistant to degeneration. Given that the WLD^S mouse encodes a gain-of-function fusion protein, its relevance to human disease was limited. The later discovery that SARM1 (sterile alpha and toll/interleukin receptor [TIR] motif-containing protein 1) promotes Wallerian degeneration suggested the existence of a pathway that might be targeted therapeutically. More recently, SARM1 was found to execute degeneration by hydrolyzing NAD⁺. Notably, SARM1 knockdown or knockout prevents neuron degeneration in response to a range of insults that lead to peripheral neuropathy, traumatic brain injury, and neurodegenerative disease. Here, we discuss the role of SARM1 in Wallerian degeneration and the opportunities to target this enzyme therapeutically.

Keywords: NAD(+); SARM1; neurodegeneration; therapeutics.

Figure 1.. Phases of Wallerian degeneration, WLD^S and NMNAT isozymes, and the NNMNAT-dependent enzymatic reaction.

A. Wallerian degeneration is characterized by 3 phases: 1) the initiation phase where injury occurs; 2) a latent phase during which the axon remains intact and the expression of cytokines, chemokines, and growth factors increases; and 3) an execution phase characterized by abrupt fragmentation and granular disintegration of the axon portion distal to the site of lesion. B. The WLD^S chimera is composed of the N-terminal 70 amino acids of UBE4B, 18 amino acids from the untranslated region of NMNAT1 and the full length NMNAT1 gene. The three isozymes are also shown with their subcellular localizations highlighted. C. Reaction catalyzed by NMNATs.

Figure 2.. The domain architecture of the five TIR domain-containing adaptor proteins in humans, NAD⁺ hydrolase mechanism catalyzed by SARM1, and Proposed Activation Mechanism.

A. Schematic depicting the domain architecture of MyD88, MAL, TRIF, TRAM and SARM1. MyD88 is comprised of a death domain (DD), an intermediary domain (ID), and a C-terminal TIR domain. MAL consists of a phosphatidylinositol-4,5-bisphosphate (PtdIns(4,5)P2)-binding motif (PIP2) and a C-terminal TIR domain. TRIF has a tumor-necrosis-factor-receptor-associated factor (TRAF6)-binding motif (T6BM), a TIR domain and a receptor-interacting protein (RIP) homotypic interaction motif (RHIM). TRAM is solely composed of a TIR domain. SARM1 consists of an N-terminal HEAT/Armadillo (ARM) domain, two SAM domains and a C-terminal TIR domain. B. SARM1 cleaves NAD⁺ to form nicotinamide and a mixture of ADPR and cADPR. C. A stimulus relieves inhibition by the ARM domain allowing for dimerization and activation of NAD⁺ hydrolase activity.

Figure 3.. NAD⁺ biosynthetic pathways and model for SARM1-mediated degeneration.

A. The metabolic pathways that contribute to the synthesis and degradation of NAD⁺, highlight the roles of NMNAT and SARM1, in axonal protection and destruction, respectively. (NA = nicotinic acid; NR = nicotinamide riboside; NAM = nicotinamide; NNMT = nicotinamide N-methyl transferase; MNA = 1-methyl nicotinamide; NAPRT = nicotinate phosphoribosyltransferase; NRK = nicotinamide riboside kinase; NAMPT = nicotinamide phosphoribosyltransferase; NaMN = nicotinic acid mononucleotide; NMN = nicotinamide mononucleotide; NMNAT = nicotinamide mononucleotide adenyltransferase isoforms 1, 2, and 3; NaAD = nicotinic acid adenine dinucleotide; NADS = NAD synthase) B. SARM1-mediated cleavage of NAD⁺ is proposed to hold a pivotal role in the execution of degeneration. Under normal conditions, NMNAT2 is constantly transported by anterograde transport. Injury or disease disrupts the microtubule assembly, transport ceases and the levels of the labile isoform fall, activating SARM1. SARM1 activation leads to cleavage of NAD⁺ and accumulation of ADPR and cADPR, which can activate Ca²⁺ channels. Increasing calcium levels activates calcium-activated cysteine proteases, calpains, which can cleave microtubules and neurofilaments leading to degeneration. C. Pathway depicting upstream triggers that lead to SARM1 activation and Wallerian degeneration.

Figure 4.. Insights into SARM1 Structure.

A. SARM1 TIR domain with the BB loop (blue), DD loop (orange) and SS loop (magenta) regions highlighted (PDBID: 6O0Q). B. Amino acid sequence alignments of enzymatically active and inactive TIR domains highlighting the presence of the SS loop (identical residues are highlighted in blue, similar residues in orange and different in green). TIR domains lacking the SS loop do not possess enzymatic activity C. Overlay of SARM1 (Orange), CMP hydrolase (green), and CD38 (blue) highlighting their catalytic residues (PDBID: 6O0Q, 2I65 and 4JEM). D. The SAM domains of SARM1 adopt an octameric structure according to the crystal structure (PDBID: 6QWV). E. Proposed catalytic mechanism. E642 has been proposed to be a nucleophile that attacks the anomeric carbon leading to release of nicotinamide and the formation of a covalent intermediate. Nucleophilic attack by water or N1 of the adenosine moiety regenerates the enzyme. Alternatively, E642 could stabilize the formation of an electrophilic oxocarbenium ion.