Mask mitigates MAPT- and FUS-induced degeneration by enhancing autophagy through lysosomal acidification

Accumulation of intracellular misfolded or damaged proteins is associated with both normal aging and late-onset degenerative diseases. Two cellular clearance mechanisms, the ubiquitin-proteasome system (UPS) and the macroautophagy/autophagy-lysosomal pathway, work in concert to degrade harmful protein aggregates and maintain protein homeostasis. Here we show that Mask, an Ankyrin-repeat and KH-domain containing protein, plays a key role in promoting autophagy flux and mitigating degeneration caused by protein aggregation or impaired UPS function. In Drosophila eye models of human tauopathy or amyotrophic lateral sclerosis diseases, loss of Mask function enhanced, while gain of Mask function mitigated, eye degenerations induced by eye-specific expression of human pathogenic MAPT/TAU or FUS proteins. The fly larval muscle, a more accessible tissue, was then used to study the underlying molecular mechanisms in vivo. We found that Mask modulates the global abundance of K48- and K63-ubiquitinated proteins by regulating autophagy-lysosome-mediated degradation, but not UPS function. Indeed, upregulation of Mask compensated the partial loss of UPS function. We further demonstrate that Mask promotes autophagic flux by enhancing lysosomal function, and that Mask is necessary and sufficient for promoting the expression levels of the proton-pumping vacuolar (V)-type ATPases in a TFEB-independent manner. Moreover, the beneficial effects conferred by Mask expression on the UPS dysfunction and neurodegenerative models depend on intact autophagy-lysosomal pathway. Our findings highlight the importance of lysosome acidification in cellular surveillance mechanisms and establish a model for exploring strategies to mitigate neurodegeneration by boosting lysosomal function.

细胞内错误折叠或受损蛋白质的积累与正常衰老以及迟发性退行性疾病密切相关。两种细胞清除机制——泛素-蛋白酶体系统（ubiquitin-proteasome system, UPS）和巨自噬/自噬-溶酶体途径（macroautophagy/autophagy-lysosomal pathway）——协同作用以降解有害蛋白质聚集体，维持蛋白质稳态。本研究证实，含有锚蛋白重复序列和KH结构域的蛋白质Mask在促进自噬流、缓解由蛋白质聚集或泛素-蛋白酶体系统功能受损所引发的退行性病变中发挥关键作用。在人类tau蛋白病或肌萎缩侧索硬化症的果蝇眼部模型中，Mask功能缺失会加剧由眼部特异性表达人类致病性MAPT/TAU或FUS蛋白所诱导的眼部退行性病变，而Mask功能过表达则可缓解该病变。研究进一步采用更易操作的果蝇幼虫肌肉组织，在活体水平探究其潜在分子机制。我们发现，Mask通过调控自噬-溶酶体介导的降解途径（而非泛素-蛋白酶体系统），调节K48泛素化和K63泛素化蛋白质的整体丰度。确实，Mask的过表达可补偿泛素-蛋白酶体系统的部分功能缺失。我们进一步证实，Mask通过增强溶酶体功能促进自噬流，且Mask以不依赖转录因子EB（TFEB）的方式，对质子泵型液泡V-ATP酶（proton-pumping vacuolar (V)-type ATPases）的表达水平具有必需且充分的调控作用。此外，Mask表达对泛素-蛋白酶体系统功能障碍及神经退行性疾病模型所产生的有益效应，依赖于完整的自噬-溶酶体途径。本研究结果凸显了溶酶体酸化在细胞监视机制中的重要性，并为通过增强溶酶体功能以缓解神经退行性病变的策略探索建立了研究模型。