Mertk Interacts with Tim-4 to Enhance Tim-4-Mediated Efferocytosis.

Byeongjin Moon,Daeho Park,Hyunji Moon,Chanhyuk Min,Susumin Yang,Young-Eun Joo,Deokhwan Kim,Sang-Ah Lee,Juyeon Lee,Jaeseon Jeon,Heera Moon

doi:10.3390/cells9071625

Byeongjin Moon, Daeho Park + Show 9 more

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https://doi.org/10.3390/cells9071625

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Abstract

Apoptotic cells expressing phosphatidylserine (PS) on their cell surface are directly or indirectly recognized by phagocytes through PS-binding proteins. The PS-binding protein Tim-4 secures apoptotic cells to phagocytes to facilitate the engulfment of apoptotic cells. However, the molecular mechanism by which Tim-4 transduces signals to phagocytes during Tim-4-mediated efferocytosis is incompletely understood. Here, we report that Tim-4 collaborates with Mertk during efferocytosis through a biochemical interaction with Mertk. Proximal localization between the two proteins in phagocytes was observed by immunofluorescence and proximal ligation assays. Physical association between Tim-4 and Mertk, which was mediated by an interaction between the IgV domain of Tim-4 and the fibronectin type-III domain of Mertk, was also detected with immunoprecipitation. Furthermore, the effect of Mertk on Tim-4-mediated efferocytosis was abolished by GST-MertkFnIII, a soluble form of the fibronectin type-III domain of Mertk that disrupts the interaction between Tim-4 and Mertk. Taken together, the results from our study suggest that a physical interaction between Tim-4 and Mertk is necessary for Mertk to enhance efferocytosis mediated by Tim-4.

Highlights

Apoptotic cells generated during development and in homeostasis are efficiently removed by a process called efferocytosis [1]
Mertk possesses two fibronectin type-III (FnIII) domains in its extracellular region (Figure 1A), and it is reported that Mertk functionally collaborates with Tim-4 to facilitate engulfment of apoptotic cells [21,30]
We confirmed the effect of Mertk on Tim-4-mediated efferocytosis under serum-free conditions, which did not contain bridging molecules such as Gas6 required for Mertk-mediated efferocytosis and excluded the effect of Mertk itself on efferocytosis: efferocytosis mediated by cells overexpressing both Tim-4 and Mertk was superior to that by cells overexpressing Tim-4 alone (Figure 1B), and this superior efferocytosis was not due to an increase in the number of apoptotic cells binding to phagocytes (Figure 1C)

Summary

Introduction

Apoptotic cells generated during development and in homeostasis are efficiently removed by a process called efferocytosis [1]. To facilitate this process, phagocytes have developed a unique apparatus to distinguish apoptotic cells from live cells and to phagocytose apoptotic cells [2,3]. PS exposed on apoptotic cells is recognized by phagocytes through. PS-binding proteins can be cytoplasmic proteins, transmembrane proteins, or secreted proteins. Transmembrane and secreted PS-binding proteins are the types predominately involved in recognition of apoptotic cells by phagocytes due to their subcellular locations and ability to bind to PS on apoptotic cells [7]

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