Membrane localization of LGG-1/GABARAP is dispensable for autophagy in C. elegans.

Most of the functions of LC3/GABARAP in macroautophagy/autophagy are considered to depend on their association with the phagophore membrane through a conjugation to a lipid. Using site-directed mutagenesis, we inhibited the conjugation of LGG-1, the single homolog of GABARAP in C. elegans. Mutants that express only cytosolic forms revealed an essential role for the cleaved form of LGG-1 in autophagy but also in an autophagy-independent embryonic function.

LGG-1/GABARAP lipidation is not required for autophagy and development in Caenorhabditis elegans.

The ubiquitin-like proteins Atg8/LC3/GABARAP are required for multiple steps of autophagy, such as initiation, cargo recognition and engulfment, vesicle closure and degradation. Most of LC3/GABARAP functions are considered dependent on their post-translational modifications and their association with the autophagosome membrane through a conjugation to a lipid, the phosphatidyl-ethanolamine. Contrarily to mammals, C. elegans possesses single homologs of LC3 and GABARAP families, named LGG-2 and LGG-1. Using site-directed mutagenesis, we inhibited the conjugation of LGG-1 to the autophagosome membrane and generated mutants that express only cytosolic forms, either the precursor or the cleaved protein. LGG-1 is an essential gene for autophagy and development in C. elegans, but we discovered that its functions could be fully achieved independently of its localization to the membrane. This study reveals an essential role for the cleaved form of LGG-1 in autophagy but also in an autophagy-independent embryonic function. Our data question the use of lipidated GABARAP/LC3 as the main marker of autophagic flux and highlight the high plasticity of autophagy.

Autophagy facilitates mitochondrial rebuilding after acute heat stress via a DRP-1-dependent process.

Acute heat stress (aHS) can induce strong developmental defects in Caenorhabditis elegans larva but not lethality or sterility. This stress results in transitory fragmentation of mitochondria, formation of aggregates in the matrix, and decrease of mitochondrial respiration. Moreover, active autophagic flux associated with mitophagy events enables the rebuilding of the mitochondrial network and developmental recovery, showing that the autophagic response is protective. This adaptation to aHS does not require Pink1/Parkin or the mitophagy receptors DCT-1/NIX and FUNDC1. We also find that mitochondria are a major site for autophagosome biogenesis in the epidermis in both standard and heat stress conditions. In addition, we report that the depletion of the dynamin-related protein 1 (DRP-1) affects autophagic processes and the adaptation to aHS. In drp-1 animals, the abnormal mitochondria tend to modify their shape upon aHS but are unable to achieve fragmentation. Autophagy is induced, but autophagosomes are abnormally elongated and clustered on mitochondria. Our data support a role for DRP-1 in coordinating mitochondrial fission and autophagosome biogenesis in stress conditions.

Mitophagy during development and stress in C. elegans.

Mitochondria is a key cellular organelle, which is tightly supervised by multiple oversight cellular mechanisms regulating mitochondrial biogenesis and mitochondria maintenance and/or elimination. Selective autophagy of mitochondria, id est mitophagy, is one of the cellular mechanisms controlling mitochondria homeostasis. The nematode Caenorhabditis elegans has recently emerged as a powerful model organism to study the roles and functions of mitophagy. We present here the current knowledge on cellular and molecular mechanisms underlying the selective elimination of mitochondria by autophagy in C. elegans in the context of developmental processes, aging and adaptive responses to various stresses.