Tim-4 functions as a scavenger receptor for phagocytosis of exogenous particles

Chanhyuk Min,Hyeokjin Cho,Jeongjun Park,Hyunji Moon,Daeho Park,Gwangrog Lee,Jihwan Park,Deokhwan Kim,Susumin Yang,Dae-Hee Lee,Byeongjin Moon,Kwanhyeong Kim,Sang-Ah Lee,Juyeon Lee,Gayoung Kim

doi:10.1038/s41419-020-02773-7

Abstract

The phosphatidylserine (PS) receptor Tim-4 mediates phagocytosis of apoptotic cells by binding to PS exposed on the surface of these cells, and thus functions as a PS receptor for apoptotic cells. Some of PS receptors are capable of recognizing other molecules, such as LPS on bacteria, besides PS on apoptotic cells. However, it is unclear whether Tim-4 perceives other molecules like the PS receptors. Here, we report that Tim-4 facilitates the phagocytosis of exogenous particles as well as apoptotic cells. Similar to the process that occurs during Tim-4-mediated efferocytosis, the uptake of exogenous E. coli and S. aureus bioparticles was promoted by overexpression of Tim-4 on phagocytes, whereas phagocytosis of the bioparticles was reduced in Tim-4-deficient cells. A truncation mutant of Tim-4 lacking the cytoplasmic tail promoted phagocytosis of the particles, but a mutant lacking the IgV or the mucin domain failed to enhance phagocytosis. However, expression of Tim-4AAA (a mutant form of Tim-4 that does not bind phosphatidylserine and does not promote efferocytosis) still promoted phagocytosis. Tim-4-mediated phagocytosis was not blocked by expression of the phosphatidylserine-binding protein Anxa5. Furthermore, binding of lipopolysaccharide (LPS), which is found in the outer membrane of Gram-negative bacteria, was higher in Tim-4-overexpressing cells than in Tim-4-deficient cells. In summary, our study suggests that Tim-4 acts as a scavenger receptor and mediates phagocytosis of exogenous particles in a phosphatidylserine-independent manner.

Highlights

Scavenger receptors are a large family of cell-surface receptors that participate in a broad range of biological functions
Tim-4 enhances phagocytosis of exogenous particles as well as apoptotic cells A number of PS receptors perceive PS on apoptotic cells and other molecules on foreign substances to phagocytose them[18]. It is not known whether Tim-4 can recognize bioparticles other than apoptotic cells, or whether phagocytosis of other recognized particles is dependent upon PS on the surface of these molecules
LR73 cells transiently overexpressing Tim-4 were incubated with fluorescently labeled E. coli and S. aureus bioparticles or apoptotic cells, and phagocytosis of the bioparticles or apoptotic cells by LR73 cells was evaluated by confocal microscopy

Summary

Introduction

Scavenger receptors are a large family of cell-surface receptors that participate in a broad range of biological functions They were categorized on the basis of their ability to bind to modified low-density lipoproteins (LDL). They recognize and remove a diverse variety of ligands, including endogenous and modified host-derived molecules (DAMPs, damage-associated molecular patterns) and microbial pathogens (PAMPs, pathogenassociated molecular patterns)[1,2]. Lipids across the plasma membrane are asymmetrically distributed. PS is exclusively located on the inner leaflet of the plasma membrane, and this asymmetrical distribution of PS across the membrane is maintained by the action of scramblases and flippases[4,5]

Methods

Results

Conclusion