Autophagosomes fuse to phagosomes and facilitate the degradation of apoptotic cells in Caenorhabditis elegans

Omar Peña-Ramos,Tianyou Yao,Henry He,Xiaomeng Yu,Lucia Chiao,Zheng Zhou,Xianghua Liu

doi:10.7554/elife.72466

Abstract

Autophagosomes are double-membrane intracellular vesicles that degrade protein aggregates, intracellular organelles, and other cellular components. During the development of the nematode Caenorhabditis elegans, many somatic and germ cells undergo apoptosis. These cells are engulfed and degraded by their neighboring cells. We discovered a novel role of autophagosomes in facilitating the degradation of apoptotic cells using a real-time imaging technique. Specifically, the double-membrane autophagosomes in engulfing cells are recruited to the surfaces of phagosomes containing apoptotic cells and subsequently fuse to phagosomes, allowing the inner vesicle to enter the phagosomal lumen. Mutants defective in the production of autophagosomes display significant defects in the degradation of apoptotic cells, demonstrating the importance of autophagosomes to this process. The signaling pathway led by the phagocytic receptor CED-1, the adaptor protein CED-6, and the large GTPase dynamin (DYN-1) promotes the recruitment of autophagosomes to phagosomes. Moreover, the subsequent fusion of autophagosomes with phagosomes requires the functions of the small GTPase RAB-7 and the HOPS complex components. Further observations suggest that autophagosomes provide apoptotic cell-degradation activities in addition to and in parallel of lysosomes. Our findings reveal that, unlike the single-membrane, LC3-associated phagocytosis (LAP) vesicles reported for mammalian phagocytes, the canonical double-membrane autophagosomes facilitate the clearance of C. elegans apoptotic cells. These findings add autophagosomes to the collection of intracellular organelles that contribute to phagosome maturation, identify novel crosstalk between the autophagy and phagosome maturation pathways, and discover the upstream signaling molecules that initiate this crosstalk.

Highlights

During metazoan development and adulthood, a large number of cells undergo apoptosis or cell suicide; these dying cells are engulfed by phagocytes and degraded inside phagosomes, vacuoles composed of the lipid bilayers originated from the plasma membrane [1,2]
C. elegans has two LC3/GABARAP family members, LGG-1 and LGG-2, which belong to the LC3 and GABARAP subfamilies, respectively (Fig 1A) [27]
C. elegans embryos, we constructed the GFP-tagged LGG-1 and LGG-2 reporters that were expressed under the control of the ced-1 promoter (Pced-1), a well-documented engulfing cell-specific promoter 127 [21,28]

Summary

Introduction

During metazoan development and adulthood, a large number of cells undergo apoptosis or cell suicide; these dying cells are engulfed by phagocytes and degraded inside phagosomes, vacuoles composed of the lipid bilayers originated from the plasma membrane [1,2]. Swift engulfment and degradation of apoptotic cells are critical for tissue remodeling, the resolution of the wound area, and the prevention and suppression of harmful inflammatory and autoimmune responses induced by the content of the dying cells [2]. Critical to the degradation of phagosomal contents is the fusion of intracellular organelles, including lysosomes and early endosomes, to phagosomes, which results in the delivery of the content of these organelles to the phagosomal lumen [3]. Lysosomes, which contribute many kinds of hydrolytic enzymes, including proteases, nucleases, lipases, and hydrolyzing enzymes for polysaccharides to the lumen of phagosomes, are the most pivotal organelles that support phagosomal degradation [3]. Whether other kinds of intracellular organelles fuse to phagosomes and contribute to the degradation of the apoptotic cells inside remains elusive

Methods

Results

Conclusion