Washington University School of Medicine in Saint Louis, Department of Genetics, St. Louis, MO, USA; Needleman Center for Neurometabolism and Axonal Therapeutics, USA.
Washington University School of Medicine in Saint Louis, Department of Genetics, St. Louis, MO, USA; Needleman Center for Neurometabolism and Axonal Therapeutics, USA.
Exp Neurol. 2021 Nov;345:113842. doi: 10.1016/j.expneurol.2021.113842. Epub 2021 Aug 14.
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，是病理性轴突丧失的核心执行者。最近，我们阐明了 SARM1 激活的分子机制，表明 SARM1 是一种代谢传感器，通过其与 SARM1 N 端 ARM 结构域内的变构位点的竞争结合，受 NAD 及其前体烟酰胺单核苷酸 (NMN) 的水平调节。在 NAD 丰富的健康神经元中，NAD 的结合阻止了 NMN 进入该变构位点。然而，在损伤或疾病时，NAD 生物合成酶 NMNAT2 的水平下降，增加了 NMN/NAD 比值，从而促进了 NMN 与 SARM1 变构位点的结合，进而诱导 SARM1 NAD 水解酶的构象变化而激活。因此，NAD 代谢物不仅调节 SARM1 的激活，而且反过来又受到 SARM1 NAD 水解酶的调节。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 代谢组的轴突保护机制的机制理解。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断轴突退化的方法，1）联合使用 NMN 生物合成抑制剂 FK866 和 NAD 前体烟酰胺核苷（NaR），2）神经元表达细菌酶 NMN 脱酰胺酶。我们发现，这些方法不仅导致 SARM1 激活剂 NMN 的水平降低，而且还导致 NAD 前体烟酰胺单核苷酸（NaMN）的水平增加。我们表明 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很重要。NaMN-ARM 结构域共晶结构的分析表明，NaMN 与 NMN 竞争结合 SARM1 变构位点，并促进 SARM1 ARM 结构域的开放、自动抑制构象。总之，这些结果表明，SARM1 变构口袋可以结合一系列不同的代谢物，包括 NMN、NAD 和 NaMN，以监测细胞 NAD 动态平衡并调节 SARM1 NAD 水解酶活性。变构位点的相对混杂性可能使开发有效的 SARM1 激活的药理学抑制剂成为治疗神经退行性疾病的可能。
