The Vici Syndrome Protein EPG5 Is a Rab7 Effector that Determines the Fusion Specificity of Autophagosomes with Late Endosomes/Lysosomes

Abstract

Mutations in the human autophagy gene EPG5 cause the multisystem disorder Vici syndrome. Here we demonstrated that EPG5 is a Rab7 effector that determines the fusion specificity of autophagosomes with late endosomes/lysosomes. EPG5 is recruited to late endosomes/lysosomes by direct interaction with Rab7 and the late endosomal/lysosomal R-SNARE VAMP7/8. EPG5 also binds to LC3/LGG-1 (mammalian and C. elegans Atg8 homolog, respectively) and to assembled STX17-SNAP29 Qabc SNARE complexes on autophagosomes. EPG5 stabilizes and facilitates the assembly of STX17-SNAP29-VAMP7/8 trans-SNARE complexes, and promotes STX17-SNAP29-VAMP7-mediated fusion of reconstituted proteoliposomes. Loss of EPG5 activity causes abnormal fusion of autophagosomes with various endocytic vesicles, in part due to elevated assembly of STX17-SNAP25-VAMP8 complexes. SNAP25 knockdown partially suppresses the autophagy defect caused by EPG5 depletion. Our study reveals that EPG5 is a Rab7 effector involved in autophagosome maturation, providing insight into the molecular mechanism underlying Vici syndrome.

New insights into autophagosome–lysosome fusion

“…Loss of EPG5 causes abnormal fusion of autophagosomes with various endocytic vesicles, partly due to increased assembly of the STX17-SNAP25-VAMP8 complex. Consistent with this, SNAP25 knockdown partially suppresses the effect on autophagy because of EPG5 depletion (Wang et al, 2016). These findings have, thus, begun to answer the question as to why autophagosomes specifically fuse with late endosomes and lysosomes.…”

“…Ectopic P granules protein 5 (EPG5) was originally identified in a C. elegans genetic screen and is another Rab7 effector and tethering factor that determines the fusion specificity of autophagosomes with endosomes/lysosomes (Tian et al, 2010;Wang et al, 2016) (Fig. 3).…”

Impaired retrograde transport of axonal autophagosomes contributes to autophagic stress in Alzheimer’s disease neurons

“…The significantly reduced but not fully abolished DIC recruitment in the Snapin cKO mouse brains may suggest (1) a compensatory role of other dynein adaptors in LE-dynein coupling, or (2) the remaining Snapin expressed in other types of cells in mouse brains. Interestingly, from the purified LEs in Snapin cKO mouse brains, we also detected increased LC3-II, and syntaxin 17 (Stx17) (LC3-II: p=0.0014707; Stx17: p=0.013641) (Figure 6E,F), an autophagosome-targeted protein mediating the fusion with late endosomes/lysosomes by forming the SNARE fusion complex with SNAP29 and VAMP8 (Cheng et al, 2015a; Guo et al, 2014; Itakura et al, 2012; Wang et al, 2016). This study further confirms that Snapin is required for dynein motor recruitment to amphisomes, and the subsequent removal of AVs from distal axons and synapses.…”

DNA Damage Response and Autophagy: A Meaningful Partnership

“…Indeed, there is now significant evidence to suggest that autophagy is required for the function of ‘error-proof’ HR and NER (Liu et al, 2015; Park et al, 2015; Hewitt et al, 2016; Qiang et al, 2016; Wang et al, 2016). Conversely, autophagy-deficient cells rely mostly on the error-prone NHEJ repair process, which may explain the genomic instability observed in experimental mouse models with defective autophagy (Karantza-Wadsworth et al, 2007; Mathew et al, 2007b) and the observation that in human breast, ovarian and prostate cancers, beclin-1 is monoallelically deleted (Aita et al, 1999).…”

“…This role has been attributed to a nuclear pool of p62 that accumulates upon autophagy blockade and binds RNF168, inhibiting its E3 ubiquitin ligase activity toward histone H2A. The ensued reduction in chromatin ubiquitination hinders the recruitment of DNA repair proteins such as BRCA1, RAD51, and RAP80 to sites of DSBs and impacts on their ability to repair radiation-induced DNA damage (Wang et al, 2016). RAD51 is also regulated by SQSTM1/p62 through filamin A which physiologically responds to DNA damage by recruiting RAD51 to DSBs.…”