SARM1 is an inducible NAD⁺ hydrolase that is the central executioner of pathological axon loss. Recently, we elucidated the molecular mechanism of SARM1 activation, demonstrating that SARM1 is a metabolic sensor regulated by the levels of NAD⁺ and its precursor, nicotinamide mononucleotide (NMN), via their competitive binding to an allosteric site within the SARM1 N-terminal ARM domain. In healthy neurons with abundant NAD⁺, binding of NAD⁺ blocks access of NMN to this allosteric site. However, with injury or disease the levels of the NAD⁺ biosynthetic enzyme NMNAT2 drop, increasing the NMN/ NAD⁺ ratio and thereby promoting NMN binding to the SARM1 allosteric site, which in turn induces a conformational change activating the SARM1 NAD⁺ hydrolase. Hence, NAD⁺ metabolites both regulate the activation of SARM1 and, in turn, are regulated by the SARM1 NAD⁺ hydrolase. This dual upstream and downstream role for NAD⁺ metabolites in SARM1 function has hindered mechanistic understanding of axoprotective mechanisms that manipulate the NAD⁺ metabolome. Here we reevaluate two methods that potently block axon degeneration via modulation of NAD⁺ related metabolites, 1) the administration of the NMN biosynthesis inhibitor FK866 in conjunction with the NAD⁺ precursor nicotinic acid riboside (NaR) and 2) the neuronal expression of the bacterial enzyme NMN deamidase. We find that these approaches not only lead to a decrease in the levels of the SARM1 activator NMN, but also an increase in the levels of the NAD⁺ precursor nicotinic acid mononucleotide (NaMN). We show that NaMN inhibits SARM1 activation, and demonstrate that this NaMN-mediated inhibition is important for the long-term axon protection induced by these treatments. Analysis of the NaMN-ARM domain co-crystal structure shows that NaMN competes with NMN for binding to the SARM1 allosteric site and promotes the open, autoinhibited configuration of SARM1 ARM domain. Together, these results demonstrate that the SARM1 allosteric pocket can bind a diverse set of metabolites including NMN, NAD⁺, and NaMN to monitor cellular NAD⁺ homeostasis and regulate SARM1 NAD⁺ hydrolase activity. The relative promiscuity of the allosteric site may enable the development of potent pharmacological inhibitors of SARM1 activation for the treatment of neurodegenerative disorders.

SARM1 是一种诱导型 NAD ⁺水解酶，是病理性轴突损失的中心执行者。最近，我们阐明了 SARM1 激活的分子机制，证明 SARM1 是一种代谢传感器，通过 NAD ⁺及其前体烟酰胺单核苷酸 (NMN) 的水平与 SARM1 N 端 ARM 内的变构位点竞争性结合来调节。领域。在具有丰富 NAD ^{+ 的}健康神经元中，NAD ⁺的结合会阻止 NMN 进入该变构位点。然而，随着损伤或疾病，NAD ⁺生物合成酶 NMNAT2 的水平下降，增加 NMN/NAD ⁺比率，从而促进 NMN 与 SARM1 变构位点结合，进而诱导激活 SARM1 NAD ⁺水解酶的构象变化。因此，NAD ⁺代谢物既调节 SARM1 的激活，又受 SARM1 NAD ⁺水解酶调节。 NAD ⁺代谢物在 SARM1 功能中的这种双重上游和下游作用阻碍了对操纵 NAD ⁺代谢组的轴保护机制的机械理解。在这里，我们重新评估了两种通过调节 NAD ⁺相关代谢物来有效阻止轴突变性的方法，1）联合使用 NMN 生物合成抑制剂 FK866 和 NAD ⁺前体烟酸核苷（NaR），2）细菌的神经元表达NMN 脱酰胺酶。 我们发现这些方法不仅导致 SARM1 激活剂 NMN 水平降低，而且 NAD ⁺前体烟酸单核苷酸 (NaMN) 水平增加。我们证明 NaMN 抑制 SARM1 激活，并证明这种 NaMN 介导的抑制对于这些治疗诱导的长期轴突保护非常重要。对NaMN-ARM结构域共晶结构的分析表明，NaMN与NMN竞争与SARM1变构位点的结合，并促进SARM1 ARM结构域的开放、自抑制构型。总之，这些结果表明，SARM1 变构口袋可以结合多种代谢物，包括 NMN、NAD ⁺和 NaMN，以监测细胞 NAD ⁺稳态并调节 SARM1 NAD ⁺水解酶活性。变构位点的相对混杂性可能有助于开发 SARM1 激活的有效药理学抑制剂，用于治疗神经退行性疾病。