Induction of autophagy in ESCRT mutants is an adaptive response for cell survival in C. elegans

Abstract

SummaryEndosomes and autophagosomes are two vesicular compartments involved in the degradation and recycling of cellular material. They both undergo a maturation process and finally fuse with the lysosome. In mammals, the convergence between endosomes and autophagosomes is a multistep process that can generate intermediate vesicles named amphisomes. Using knockdowns and mutants of the ESCRT machinery (ESCRT-0-ESCRT-III, ATPase VPS-4) and the autophagic pathway (LGG-1, LGG-2, ATG-7, TOR), we analyzed in vivo the functional links between endosomal maturation and autophagy in Caenorhabditis elegans. We report here that, despite a strong heterogeneity of their developmental phenotypes, all ESCRT mutants present an accumulation of abnormal endosomes and autophagosomes. We show that this accumulation of autophagosomes is secondary to the formation of enlarged endosomes and is due to the induction of the autophagic flux and not a blockage of fusion with lysosomes. We demonstrate that the induction of autophagy is not responsible for the lethality of ESCRT mutants but has a protective role on cellular degradation. We also show that increasing the basal level of autophagy reduces the formation of enlarged endosomes in ESCRT mutants. Together, our data indicate that the induction of autophagy is a protective response against the formation of an abnormal vesicular compartment.

The Endosomal Protein CHARGED MULTIVESICULAR BODY PROTEIN1 Regulates the Autophagic Turnover of Plastids in Arabidopsis

“…Prior studies demonstrated that when autophagic reporters labeled with GFP are delivered to the vacuole, the GFP moiety is cleaved and accumulates in a free form due to its stability inside the vacuole (Djeddi et al, 2012;Li et al, 2014). In this way, the ratio of free GFP to GFP-tagged protein can be used as a semiquantitative measure of autophagic flux and whether this delivery changes with environmental or genetic perturbations.…”

“…These phenomena, together with the presence of arrested phagophores and the reduced autophagy-mediated degradation of the plastid marker RECA-GFP, clearly point to a direct role for CHMP1 in plastid autophagy. Like the Arabidopsis chmp1 mutant, C. elegans ESCRT mutants (Djeddi et al, 2012) show increased autophagy as measured by GFP cleavage from the autophagic reporter GFP-ATG8. This observation led to the conclusion that the ESCRT machinery is not required for autophagosome maturation and that the accumulation of autophagosomes in ESCRT mutants is due to the upregulation of the autophagy pathway (Djeddi et al, 2012).…”

“…Like the Arabidopsis chmp1 mutant, C. elegans ESCRT mutants (Djeddi et al, 2012) show increased autophagy as measured by GFP cleavage from the autophagic reporter GFP-ATG8. This observation led to the conclusion that the ESCRT machinery is not required for autophagosome maturation and that the accumulation of autophagosomes in ESCRT mutants is due to the upregulation of the autophagy pathway (Djeddi et al, 2012). However, in Arabidopsis, the autophagic degradation of plastid components is greatly reduced in the absence of CHMP1 even though more GFP-ATG8 is delivered to the vacuole.…”

“…One of these complexes, ESCRT-III, associates directly with endosomal membranes and is thought to mediate changes in the membrane architecture that ultimately lead to intraluminal vesicle scission (Schuh and Audhya, 2014). Studies with Drosophila melanogaster, Caenorhabditis elegans, plants, and mammalian cells showing that ESCRT mutants hyperaccumulate autophagosomes (Roudier et al, 2005;Filimonenko et al, 2007;Lee et al, 2007;Rusten et al, 2007;Rusten and Stenmark, 2009;Djeddi et al, 2012;Katsiarimpa et al, 2013) implied that the ESCRT machinery plays a role in autophagy. Given that the scission of nascent intraluminal vesicles at endosomes and the closure of phagophores into sealed autophagosomes are topologically similar, ESCRT proteins could participate directly in phagophore closure.…”

Dual roles of an Arabidopsis ESCRT component FREE1 in regulating vacuolar protein transport and autophagic degradation

“…New light has been thrown onto this situation through the discovery that ESCRT is also involved in autophagy. More and more studies on nematodes, flies, mammals, and even on plants provide evidence to support the conclusion that the inactivation of ESCRT machinery causes an accumulation of autophagosomes (9)(10)(11)(12)(13). Different models, such as the induction of autophagy or the disruption of autophagosome-endosome/lysosome fusion, have been proposed to explain the observation that autophagosomes accumulate in ESCRT-depleted cells (14,15).…”