The Tim gene family in efferocytosis
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Genes & Genomics https://doi.org/10.1007/s13258-020-00969-x
MINIREVIEW
The Tim gene family in efferocytosis Deokhwan Kim1,2 · Sang‑Ah Lee1,2 · Hyunji Moon1,2 · Kwanhyeong Kim1 · Daeho Park1,2 Received: 22 June 2020 / Accepted: 5 July 2020 © The Genetics Society of Korea 2020
Abstract One of the key features of the plasma membrane is the asymmetrical distribution of phospholipids across it. Especially, phosphatidylserine (PS) exclusively locates on its inner leaflet. Thus, the exposure of PS on the surface of cells could function as a signal initiating various cellular processes such as phagocytosis of apoptotic cells called efferocytosis, blood clotting, muscle formation, and viral entry. Indeed, PS on apoptotic cells stimulates phagocytes to engulf them and functions as an essential ligand for efferocytosis. Due to the importance of PS in efferocytosis, the existence of the PS receptor had been conceived. However, the PS receptor had not been revealed for a long time. Thus, the first identification of the PS receptor was significant excitement. Tim-4, a member of the T cell immunoglobulin and mucin domain containing family of genes, was one of PS receptors which first identified and received the greatest attention due to its expression in macrophages and relevance to autoimmune and allergic diseases. This review will serve to provide a comprehensive overview of Tim proteins as PS receptors. Keywords Phosphatidylserine · Phosphatidylserine receptor · Efferocytosis · Timd · Tim-4
Introduction Phagocytosis of apoptotic cells, called efferocytosis, is an essential cellular process that removes apoptotic cells generated during development and in tissue homeostasis in multicellular organisms (Doran et al. 2020; Morioka et al. 2019). Apoptotic cells are specifically and efficiently removed by phagocytes, which is achieved by well-orchestrated engulfment machinery (Savill and Fadok 2000). Phosphatidylserine (PS), which locates in the inner leaflet of the plasma membrane in live cells, is exposed on apoptotic cells and a best-characterized ligand on apoptotic cells for efferocytosis (Segawa and Nagata 2015). During efferocytosis, PS on apoptotic cells is directly or indirectly recognized by phagocytes through interaction between PS and PS binding proteins including PS receptors on phagocytes (Ravichandran Deokhwan Kim, Sang-Ah Lee and Hyunji Moon equally contributed to this work. * Daeho Park [email protected] 1
School of Life Sciences, Gwangju Institute of Science and Technology, Gwangju 61005, Korea
Center for Cell Mechanobiology, Gwangju Institute of Science and Technology, Gwangju 61005, Korea
2
and Lorenz 2007). It is considered that PS on apoptotic cells is essential but not sufficient for efferocytosis because blocking PS on apoptotic cells using Annexin V or Mfge8 abrogates efferocytosis but exposure of PS on live cells fails to promote engulfment of the live cells (Hanayama et al. 2002; Park et al. 2007; Segawa et al. 2011; Tsai and Discher 2008). Studies conducted over the past several decades
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