Unlocking SARM1: A Promising Drug Target for Neurodegenerative Diseases

Background

Neurodegenerative diseases are a class of disorders where neuronal structures or functions gradually deteriorate, leading to functional impairments and even cell death. These conditions pose a serious threat to human health and daily life. SARM1 (Sterile Alpha and Toll/Interleukin-1 Receptor Motif-Containing Protein 1) plays a crucial role in mediating neuronal death and morphological changes. It is associated with the occurrence of various neurodegenerative diseases and plays an important role in neuroinflammation and nervous system development. Axonal degeneration is a key feature of neurodegeneration, and SARM1 serves as a major executor of pathological axonal degeneration. It regulates nerve fiber degeneration through negative regulation of the TRIF-mediated TLR pathway. Currently, SARM1 has become an important target for drug development in the field of neurodegenerative diseases.

The Structure and Function of SARM1

The SARM1 protein consists of 724 amino acids and is primarily composed of three structural domains: an N-terminal mitochondrial localization sequence and autoinhibitory ARM domain, two central tandem sterile alpha motif (SAM) domains that mediate oligomerization, and a C-terminal Toll/interleukin-1 receptor (TIR) domain. The ARM domain can bind to nicotinamide adenine dinucleotide (NAD) or its precursor, nicotinamide mononucleotide (NMN), and inhibit the activity of the TIR domain, thereby maintaining SARM1 in a self-inhibited state and suppressing its pro-degenerative ability. SAM can mediate the formation of SARM1 dimers or oligomers. The C-terminal TIR domain is the effector domain of SARM1, and TIR must interact with SAM to exert its pro-degenerative function. While TIR domains are typically associated with Toll-like receptor signaling, the TIR domain of SARM1 is actually an NADase, capable of cleaving NAD to produce nicotinamide (Nam), adenosine diphosphate ribose (ADPR), and cyclic adenosine diphosphate ribose (cADPR). Research has shown that the glutamate residue at position 642 in human and mouse SARM1 is essential for its catalytic activity, and an alanine mutation (E682A) is sufficient to abolish its NADase activity.

The Link Between SARM1 and Diseases

SARM1 is in a self-inhibitory state in healthy neurons, regulated by the NMNAT2 gene. NMNAT2 (nicotinamide nucleotide adenylyltransferase 2) is an enzyme that converts nicotinamide mononucleotide (NMN) into nicotinamide adenine dinucleotide (NAD+). In healthy axons, SARM1 activity is restricted by continuous NMNAT2 supply. However, when axons are severed, NMNAT2 becomes depleted, leading to accumulation of NMN and activation of SARM1. This activation contributes to axonal degeneration in conditions such as amyotrophic lateral sclerosis (ALS), Alzheimer’s disease, peripheral neuropathies, and other neurodegenerative diseases. Additionally, SARM1 plays a crucial role in the mitogen-activated protein kinase (MAPK) pathway. SARM1 interacts with MAPKKK family members (DLK, MEKK4, and MLK2) and downstream MAPKKs (MKK4 and MKK7), ultimately promoting axonal degeneration.

The TIR domain of SARM1 itself possesses intrinsic NAD+ enzymatic activity, cleaving NAD+ into ADPR and cADPR. Both of these molecules enhance the activity of ryanodine receptors on calcium stores, leading to calcium release and subsequent axonal degeneration. Research also suggests that the balance between SARM1 and NMNAT2 in terms of NAD+/NMN levels can modulate axonal degeneration. Direct competition between NAD+ and NMN regulates SARM1’s enzymatic activity. Altering the NMN/NAD+ ratio by increasing NMN levels or decreasing NAD+ can trigger SARM1’s NADase activity. Specifically, a higher NMN/NAD+ ratio promotes NMN binding to a conformational site, altering the inhibitory ARM domain structure and facilitating TIR-TIR interactions, favoring SARM1 activation.

Advancements in SARM1 Inhibitors Research

Currently, research on SARM1 enzyme inhibitors for neurodegenerative diseases is quite active, although progress in related inhibitor studies has been slow. Dehydronitrosonisoldipine, a derivative of Nisoldipine, is a reversible and cell-permeable SARM1 inhibitor. It primarily works by blocking SARM1 activation rather than inhibiting its enzymatic activity, making it suitable for studying neurodegenerative diseases. Benfotiamine hydrochloride and its derivatives act as TRPM8 antagonists and competitive SARM1 inhibitors. They can be used in research related to traumatic brain injury, peripheral neuropathy, and neurodegenerative diseases, although their inhibitory effects are less potent. DSRM-3716 (5-Iodoisoquinoline) has been reported as an effective and selective small-molecule inhibitor of SARM1 NADase activity. It exhibits greater selectivity for SARM1 compared to NAMPT and NMNAT, which are other NAD+ processing enzymes, receptors, and transport proteins. DSRM-3716 demonstrates robust axonal protection.

Assays at ICE Bioscience

ICE Bioscience has developed SARM1 enzymatic assays for high-throughput screening. The assays utilize the AMP-Glo approach to quantitatively assess the effects of SARM1 inhibitors. Additionally, a recent development includes an NAD/NADH-Glo assay, which evaluates SARM1 inhibitor efficacy based on substrate NAD consumption.

NAD+ is an essential coenzyme in cell redox reactions, widely involved in regulating various biological processes such as cell growth, differentiation, energy metabolism, and apoptosis. Increasing research suggests a close association between NAD+ levels and aging-related diseases. Currently, ICE Bioscience offers several NAD+ related target screening services. The summarized assays are as follows:

Summary

The discovery of NADase activity in the TIR domain of SARM1 has led to investigations into its functional mechanisms. We believe that ongoing research by scientists and the study of relevant inhibitors will yield new insights in the challenging field of neurodegenerative diseases.

References

[1] GERDTS J, BRACE E, SASAKI Y, et al. SARM1 activation triggers axon degeneration locally via NAD? destruction [J]. Sci ence, 2015, 348(6233): 453-7.

[2] DiAntonio, Aaron et al. “The SARM1 TIR NADase: Mechanistic Similarities to Bacterial Phage Defense and Toxin-Antitoxin Systems.” Frontiers in immunology vol. 12 752898. 23 Sep. 2021, doi:10.3389/fimmu.2021.752898

[3] Gerdts, Josiah et al. “Axon Self-Destruction: New Links among SARM1, MAPKs, and NAD+ Metabolism.” Neuron vol. 89,3 (2016): 449-60. doi:10.1016/j.neuron.2015.12.023.

[4] A phase transition enhances the catalytic activity of SARM1, an NAD+ glycohydrolase involved in neurodegeneration.

[5] Shanahan, Katharine A et al. “SARM1 regulates NAD+-linked metabolism and select immune genes in macrophages.” The Journal of biological chemistry vol. 300,2 (2024): 105620. doi:10.1016/j.jbc.2023.105620.

[6] Kulikova, V A, and A A Nikiforov. “Role of NUDIX Hydrolases in NAD and ADP-Ribose Metabolism in Mammals.” Biochemistry. Biokhimiia vol. 85,8 (2020): 883-894. doi:10.1134/S0006297920080040.

[7] Shi, Yun et al. “Structural basis of SARM1 activation, substrate recognition, and inhibition by small molecules.” Molecular cell vol. 82,9 (2022): 1643-1659.e10. doi:10.1016/j.molcel.2022.03.007.

[8] Figley, Matthew D et al. “SARM1 is a metabolic sensor activated by an increased NMN/NAD+ ratio to trigger axon degeneration.” Neuron vol. 109,7 (2021): 1118-1136.e11. doi:10.1016/j.neuron.2021.02.009.

[9] Loring, Heather S, and Paul R Thompson. “Emergence of SARM1 as a Potential Therapeutic Target for Wallerian-type Diseases.” Cell chemical biology vol. 27,1 (2020): 1-13. doi:10.1016/j.chembiol.2019.11.002

[10]Sambashivan, Shilpa, and Marc R Freeman. “SARM1 signaling mechanisms in the injured nervous system.” Current opinion in neurobiology vol. 69 (2021): 247-255. doi:10.1016/j.conb.2021.05.004.