ATP-driven and AMPK-independent autophagy in an early branching eukaryotic parasite

Autophagy is a catabolic cellular process required to maintain protein synthesis, energy production and other essential activities in starved cells. While the exact nutrient sensor(s) is yet to be identified, deprivation of amino acids, glucose, growth factor and other nutrients can serve as metabolic stimuli to initiate autophagy in higher eukaryotes. In the early-branching unicellular parasite Trypanosoma brucei, which can proliferate as procyclic form (PCF) in the tsetse fly or as bloodstream form (BSF) in animal hosts, autophagy is robustly triggered by amino acid deficiency but not by glucose depletion. Taking advantage of the clearly defined adenosine triphosphate (ATP) production pathways in T. brucei, we have shown that autophagic activity depends on the levels of cellular ATP production, using either glucose or proline as a carbon source. While autophagosome formation positively correlates with cellular ATP levels; perturbation of ATP production by removing carbon sources or genetic silencing of enzymes involved in ATP generation pathways, also inhibited autophagy. This obligate energy dependence and the lack of glucose starvation-induced autophagy in T. brucei may reflect an adaptation to its specialized, parasitic life style.细胞自噬（Autophagy）是维持饥饿状态下细胞蛋白质合成、能量产生及其他核心生命活动所必需的分解代谢细胞过程。尽管目前尚未明确具体的营养感受器，氨基酸、葡萄糖、生长因子及其他营养物质的匮乏可作为代谢刺激信号，启动高等真核生物的细胞自噬过程。在早期分支的单细胞寄生虫布氏锥虫（Trypanosoma brucei）中，该虫可在采采蝇体内以循环前型（procyclic form, PCF）增殖，或在动物宿主体内以血流型（bloodstream form, BSF）增殖；其细胞自噬可被氨基酸匮乏强烈诱导，但不会因葡萄糖耗尽而启动。借助布氏锥虫中已被清晰阐明的三磷酸腺苷（adenosine triphosphate, ATP）合成通路，我们的研究证实，细胞自噬活性与细胞ATP合成水平直接相关，此时细胞可利用葡萄糖或脯氨酸作为碳源。尽管自噬体（autophagosome）的形成与细胞ATP水平呈正相关，但通过移除碳源或对ATP合成通路相关酶进行基因沉默来干扰ATP产生，同样会抑制细胞自噬。布氏锥虫这种对能量的绝对依赖性，以及其不会因葡萄糖匮乏诱导细胞自噬的特性，可能反映了其对专性寄生生活方式的适应性演化。