Mechanisms of Autophagy

已知自噬是真核细胞抵御营养饥饿所必需的，它通过调节细胞成分的降解来为ATP的生成提供底物，从而帮助细胞存活，直到细胞外营养恢复。自噬还通过吞噬和降解蛋白质聚集体和受损的线粒体来调节细胞内的质量控制形式，从而减轻小鼠的神经退化。自噬底物被双层膜结构包裹，然后与溶酶体融合，以调节货物的降解。自噬小体是如何形成的，它们的底物是如何被吞噬的，目前还知之甚少。虽然在帕金森病中突变的两种基因产物PINK1和Parkin在触发哺乳动物有丝分裂方面发挥了核心作用，但自噬前隔离膜如何选择性和准确地吞噬受损的线粒体仍不清楚。RABGEF1是内体Rab GTP酶级联的上游因子，通过Parkin下游的泛素结合被招募到受损的线粒体。RABGEF1将下游的Rab蛋白RAB5和RAB7A定向到受损的线粒体，线粒体的关联进一步受到线粒体Rab-Gap的调节。TbC1D15是一种线粒体Rab GTP酶激活蛋白(Rab-GAP)，控制着自噬小体的生物发生和形态。为了将自噬小体的形态发生限制在货物的形态发生上，TBC1D15抑制Rab7的活性，并通过与LC3/GABARAP家族成员的相互作用与Fis1和隔离膜结合来与线粒体结合。TBC家族的另一成员TBC1D17也参与有丝分裂，并与TBC1D15形成同源二聚体和异源二聚体。这些结果表明，TBC1D15和TBC1D17通过调节线粒体与隔离膜交界处的Rab7活性来调节线粒体的适当自噬包裹，而受损线粒体上的内体Rab循环是线粒体有丝分裂的关键调节因子。

Autophagy is known to be required for eukaryotic cells to withstand nutrient starvation by mediating the degradation of cellular components to supply substrates for ATP generation thereby helping cells survive until extracellular nutrient availability returns. Autophagy also mediates forms of quality control within cells by engulfing and degrading protein aggregates and damaged mitochondria that mitigates neurodegeneration in mice. Autophagy substrates become encapsulated by a double membrane structure that then fuses with lysosomes to mediate degradation of the cargo. How autophagosomes form and how their substrates are recognized for engulfment remain poorly understood. Although two gene products mutated in Parkinson's disease, PINK1, and Parkin have been found to play a central role in triggering mitophagy in mammals, how the pre-autophagosomal isolation membrane selectively and accurately engulfs damaged mitochondria remains unclear. RABGEF1, an upstream factor of the endosomal Rab GTPase cascade, is recruited to damaged mitochondria via ubiquitin binding downstream of Parkin. RABGEF1 directs the downstream Rab proteins, RAB5 and RAB7A, to damaged mitochondria, whose associations are further regulated by mitochondrial Rab-GAPs. TBC1D15, a mitochondrial Rab GTPase-activating protein (Rab-GAP), governs autophagosome biogenesis and morphology. To constrain autophagosome morphogenesis to that of the cargo, TBC1D15 inhibits Rab7 activity and associates with both the mitochondria through binding Fis1 and the isolation membrane through the interactions with LC3/GABARAP family members. Another TBC family member TBC1D17, also participates in mitophagy and forms homodimers and heterodimers with TBC1D15. These results demonstrate that TBC1D15 and TBC1D17 mediate proper autophagic encapsulation of mitochondria by regulating Rab7 activity at the interface between mitochondria and isolation membranes and that endosomal Rab cycles on damaged mitochondria are crucial regulators of mitophagy.