Neuronal mTORC1 inhibition promotes longevity without suppressing anabolic growth and reproduction in C. elegans

mTORC1 (mechanistic target of rapamycin complex 1) is a metabolic sensor that promotes growth when nutrients are abundant. Ubiquitous inhibition of mTORC1 extends lifespan in multiple organisms but also disrupts several anabolic processes resulting in stunted growth, slowed development, reduced fertility, and disrupted metabolism. However, it is unclear if these pleotropic effects of mTORC1 inhibition can be uncoupled from longevity. Here, we utilize the auxin-inducible degradation (AID) system to restrict mTORC1 inhibition to C. elegans neurons. We find that neuron-specific degradation of RAGA-1, an upstream activator of mTORC1, or LET-363, the ortholog of mammalian mTOR, is sufficient to extend lifespan in C. elegans. Unlike raga-1 loss of function genetic mutations or somatic AID of RAGA-1, neuronal AID of RAGA-1 robustly extends lifespan without impairing body size, developmental rate, brood size or neuronal function. Moreover, while somatic degradation of RAGA-1 alters the expression of thousands of genes, demonstrating the widespread effects of mTORC1 inhibition, neuronal degradation of RAGA-1 only results in around 200 differentially expressed genes with a specific enrichment in metabolism and stress response. Notably, our work demonstrates that targeting mTORC1 specifically in the nervous system in C. elegans uncouples longevity from growth and reproductive impairments, and that many canonical effects of low mTORC1 activity are not required to promote healthy aging. These data challenge previously held ideas about the mechanisms of mTORC1 lifespan extension and underscore the potential of promoting longevity by neuron-specific mTORC1 modulation. To compare the effects of knocking down RAGA-1 in either the neurons or all somatic tissues, we generated C. elegans strains in which endogenous raga-1 was tagged with an auxin-inducible degron tag. We then expressed the degrading enzyme, TIR1, in either the neurons (1250) or all somatic (1438) tissues using the rab-3 and eft-3 promoters, respectively. In these two strains along with wild type worms (N2), we began auxin treatment at day 1 of adulthood to induce RAGA-1 degradation in either the neurons or in all somatic tissues and then harvested RNA from whole worm lysate 2 days later. We then performed RNA sequencing and differential gene expression analysis to compare gene expression changes between auxin-treated (A) and untreated samples (C) from each genotype. 4 replicates of all 6 samples were performed.

mTORC1（mechanistic target of rapamycin complex 1，雷帕霉素机制靶标复合物1）是一类代谢传感器，可在营养充足时促进生物体生长。全身性mTORC1抑制可延长多种生物的寿命，但同时会破坏多种合成代谢过程，引发生长迟滞、发育迟缓、生育力下降及代谢紊乱。目前尚不清楚mTORC1抑制的这类多效性效应是否可与延寿效应解耦。
本研究利用植物生长素诱导降解系统（auxin-inducible degradation, AID），将mTORC1抑制限定在秀丽隐杆线虫（C. elegans）的神经元中。研究发现，对mTORC1上游激活因子RAGA-1，或哺乳动物mTOR的同源蛋白LET-363进行神经元特异性降解，足以延长秀丽隐杆线虫的寿命。
与raga-1功能丧失性基因突变或体细胞RAGA-1的AID降解不同，神经元特异性RAGA-1的AID降解可显著延长线虫寿命，且不会对其体型、发育速率、产卵量及神经元功能造成损害。此外，尽管体细胞RAGA-1降解会改变数千个基因的表达，体现出mTORC1抑制的广泛调控效应，但神经元RAGA-1降解仅导致约200个差异表达基因，且这些基因显著富集于代谢与应激反应通路。
值得注意的是，本研究证实，在秀丽隐杆线虫的神经系统中特异性靶向mTORC1，可将延寿效应与生长及生殖损伤解耦，且低mTORC1活性的诸多经典效应并非促进健康衰老所必需。上述数据挑战了此前关于mTORC1延寿机制的主流认知，并凸显了通过神经元特异性调控mTORC1以延长寿命的潜力。
为比较在神经元或全部体细胞中敲低raga-1的效应差异，我们构建了内源性raga-1被植物生长素诱导降解标签标记的秀丽隐杆线虫品系。随后分别利用rab-3和eft-3启动子，在神经元（品系1250）或全部体细胞（品系1438）中表达降解酶TIR1。
在上述两个品系及野生型线虫（N2）中，我们从成虫第1天开始施加植物生长素处理，以诱导神经元或全部体细胞中的RAGA-1降解，并于2天后收集全虫裂解物的RNA。随后开展RNA测序及差异基因表达分析，比较各基因型中生长素处理组（A）与未处理组（C）的基因表达变化。所有6个样本均设置4次重复。