Control of basal autophagy rate by <i>vacuolar peduncle</i>

Basal autophagy is as a compressive catabolic mechanism engaged in the breakdown of damaged macromolecules and organelles leading to the recycling of elementary nutrients. Thought essential to cellular refreshing, little is known about the origin of a constitutional rate of basal autophagy. Here, we found that loss of Drosophila vacuolar peduncle (vap), a presumed GAP enzyme, is associated with enhanced basal autophagy rate and physiological alterations resulting in a wasteful cell energy balance, a hallmark of overactive autophagy. By contrast, starvation-induced autophagy was disrupted in vap mutant conditions, leading to a block of maturation into autolysosomes. This phenotype stem for exacerbated biogenesis of PI(3)P-dependent endomembranes, including autophagosome membranes and ectopic fusions of vesicles. These findings shed new light on the neurodegenerative phenotype found associated to mutant vap adult brains in a former study. A partner of Vap, Sprint (Spri), acting as an endocytic GEF for Rab5, had the converse effect of leading to a reduction in PI(3)P-dependent endomembrane formation in mutants. Spri was conditional to normal basal autophagy and instrumental to the starvation-sensitivity phenotype specific of vap. Rab5 activity itself was essential for PI(3)P and for pre-autophagosome structures formation. We propose that Vap/Spri complexes promote a cell surface-derived flow of endocytic Rab5-containing vesicles, the traffic of which is crucial for the implementation of a basal autophagy rate.基础自噬（basal autophagy）是一类分解代谢机制，负责降解受损的大分子与细胞器，实现基本营养物质的循环利用。尽管基础自噬被认为对细胞稳态维持不可或缺，但目前人们对其组成型速率的起源仍知之甚少。本研究发现，果蝇（Drosophila）中假定的GAP酶（GAP enzyme）液泡柄蛋白（vacuolar peduncle，简称vap）的缺失，会导致基础自噬速率升高，并引发生理功能异常，造成细胞能量失衡——这正是自噬过度激活的典型特征。与之相反，在vap突变体条件下，饥饿诱导的自噬过程受到阻滞，无法正常成熟为自噬溶酶体（autolysosomes）。该表型源于依赖PI(3)P的内膜系统（PI(3)P-dependent endomembranes）的过度生成，包括自噬体膜的异常形成以及囊泡的异位融合。本研究结果为此前一项研究中发现的vap突变体成年大脑所伴发的神经退行性表型提供了新的阐释。Vap的结合蛋白Sprint（简称Spri）是靶向Rab5的内吞型鸟苷酸交换因子（GEF），在突变体中则发挥相反效应：抑制依赖PI(3)P的内膜系统生成。Spri对于维持正常的基础自噬至关重要，同时也是vap突变体特有的饥饿敏感性表型的关键调控因子。Rab5活性本身对于PI(3)P的生成以及自噬前体结构（pre-autophagosome structures）的形成不可或缺。我们提出，Vap/Spri复合物可促进源自细胞表面的含内吞Rab5囊泡的转运流，而该转运过程对于实现基础自噬速率具有关键作用。