Clathrin and AP2 Are Required for Phagocytic Receptor-Mediated Apoptotic Cell Clearance in Caenorhabditis elegans

Didi Chen,Chonglin Yang,Lianwan Chen,Hongwei Du,Xuezhao Liu,Wei Zou,Xiaying Qi,Guangshuo Ou,Yongping Chai,Yingchun Wang,Jingjing Liang,Yuanya Zhang,Youli Jian,Long Miao

doi:10.1371/journal.pgen.1003517

Abstract

Clathrin and the multi-subunit adaptor protein complex AP2 are central players in clathrin-mediated endocytosis by which the cell selectively internalizes surface materials. Here, we report the essential role of clathrin and AP2 in phagocytosis of apoptotic cells. In Caenorhabditis elegans, depletion of the clathrin heavy chain CHC-1 and individual components of AP2 led to a significant accumulation of germ cell corpses, which resulted from defects in both cell corpse engulfment and phagosome maturation required for corpse removal. CHC-1 and AP2 components associate with phagosomes in an inter-dependent manner. Importantly, we found that the phagocytic receptor CED-1 interacts with the α subunit of AP2, while the CED-6/Gulp adaptor forms a complex with both CHC-1 and the AP2 complex, which likely mediates the rearrangement of the actin cytoskeleton required for cell corpse engulfment triggered by the CED-1 signaling pathway. In addition, CHC-1 and AP2 promote the phagosomal association of LST-4/Snx9/18/33 and DYN-1/dynamin by forming a complex with them, thereby facilitating the maturation of phagosomes necessary for corpse degradation. These findings reveal a non-classical role of clathrin and AP2 and establish them as indispensable regulators in phagocytic receptor-mediated apoptotic cell clearance.

Highlights

Phagocytosis of apoptotic cells is critical to tissue remodeling, suppression of inflammation and control of immune responses [1,2]
In clathrin-mediated endocytosis (CME), the recognition of plasma membrane receptors by adaptor proteins such as the adaptor protein 2 (AP2) complex triggers the formation of clathrin-coated vesicles (CCVs) with diameters ranging from 10–200 nm
It is not known whether clathrin and AP2 play a role in phagocytosis of apoptotic cells that are much larger than CCVs

Summary

Introduction

Phagocytosis of apoptotic cells is critical to tissue remodeling, suppression of inflammation and control of immune responses [1,2]. A non-canonical Wnt pathway consisting of the MOM-5 receptor, GSK-3 kinase and APC/APR-1 may act through CED-2 to regulate CED-10 activity for cell corpse engulfment during early embryo development [10]. The phagocytic receptor CED-1, which shares homology with the human scavenger receptor SREC, LRP/CD91 and MEGF10, and Drosophila Draper and Six-microns-under (SIMU) [11,12,13,14,15], recognizes apoptotic cells by interacting with TTR-52, a PtdSer-binding protein secreted from engulfing cells [16]. The adaptor protein CED-6/Gulp likely acts downstream of CED-1 to transduce engulfing signals to other effectors including the large GTPase DYN-1/dynamin, resulting in cell corpse engulfment and formation of phagosomes [14,17,18]. Recent studies suggest that CED-7 acts with TTR-52 and NRF-5, another secreted PtdSer-binding

Methods

Results

Discussion

Conclusion