Plasmid map.

Sterile alpha and TIR motif-containing 1 (SARM1) is a protein involved in programmed death of injured axons. Following axon injury or a drug-induced insult, the TIR domain of SARM1 degrades the essential molecule nicotinamide adenine dinucleotide (NAD+), leading to a form of axonal death called Wallerian degeneration. Degradation of NAD+ by SARM1 is essential for the Wallerian degeneration process, but accumulating evidence suggest that other activities of SARM1, beyond the mere degradation of NAD+, may be necessary for programmed axonal death. In this study we show that the TIR domains of both human and fruit fly SARM1 produce 1′′–2′ and 1′′–3′ glycocyclic ADP-ribose (gcADPR) molecules as minor products. As previously reported, we observed that SARM1 TIR domains mostly convert NAD+ to ADPR (for human SARM1) or cADPR (in the case of SARM1 from Drosophila melanogaster). However, we now show that human and Drosophila SARM1 additionally convert ~0.1–0.5% of NAD+ into gcADPR molecules. We find that SARM1 TIR domains produce gcADPR molecules both when purified in vitro and when expressed in bacterial cells. Given that gcADPR is a second messenger involved in programmed cell death in bacteria and likely in plants, we propose that gcADPR may play a role in SARM1-induced programmed axonal death in animals.

含无菌α基序与TIR基序的蛋白1（Sterile alpha and TIR motif-containing 1, SARM1）是一类参与损伤轴突程序性死亡的蛋白。当轴突发生损伤或受到药物诱导的刺激时，SARM1的TIR结构域会降解关键分子烟酰胺腺嘌呤二核苷酸（NAD+），进而引发一种被称为瓦勒里安变性（Wallerian degeneration）的轴突死亡形式。SARM1介导的NAD+降解是瓦勒里安变性过程的核心必需环节，但越来越多的研究证据表明，SARM1除单纯降解NAD+之外的其他活性，或许也是程序性轴突死亡所不可或缺的。
本研究证实，人类与黑腹果蝇（Drosophila melanogaster）SARM1的TIR结构域均可产生1''–2'与1''–3'糖环化ADP核糖（glycocyclic ADP-ribose, gcADPR）分子作为次要产物。正如既往研究所报道的那样，我们观察到SARM1的TIR结构域主要将NAD+转化为ADP核糖（ADPR，对应人类SARM1）或环化ADP核糖（cADPR，对应黑腹果蝇SARM1）。但本研究新发现，人类与黑腹果蝇SARM1还可将约0.1%~0.5%的NAD+转化为gcADPR分子。实验证实，SARM1的TIR结构域无论是在体外纯化条件下，还是在细菌细胞中异源表达时，均可生成gcADPR分子。鉴于gcADPR是一类参与细菌且可能也参与植物程序性细胞死亡的第二信使，我们推测gcADPR可能在动物体内SARM1诱导的程序性轴突死亡过程中发挥关键作用。