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

Cell death & disease 2020 Jul 23

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The phosphatidylserine (PS) receptor Tim-4 mediates phag ocytosis 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.

Citation

Chanhyuk Min, Jeongjun Park, Gayoung Kim, Hyunji Moon, Sang-Ah Lee, Deokhwan Kim, Byeongjin Moon, Susumin Yang, Juyeon Lee, Kwanhyeong Kim, Hyeokjin Cho, Jihwan Park, Dae-Hee Lee, Gwangrog Lee, Daeho Park. Tim-4 functions as a scavenger receptor for phagocytosis of exogenous particles. Cell death & disease. 2020 Jul 23;11(7):561