The phosphatidylserine receptor TIM4 utilizes integrins as coreceptors to effect phagocytosis

Ronald S. Flannagan, Johnathan Canton, Wendy Furuya, Michael Glogauer, and Sergio Grinstein
T-cell Immunoglobulin Mucin protein 4 (TIM4), a phosphatidylserine (PtdSer)-binding receptor, mediates the phagocytosis of apoptotic cells. How TIM4 exerts its function is unclear and conflicting data have emerged. To define the mode of action of TIM4 we used two distinct but complementary approaches: (i) we compared bone marrow-derived macrophages from wild-type and TIM4−/− mice, and (ii) heterologously expressed TIM4 in epithelioid AD293 cells, which rendered them competent for engulfment of PtdSer-bearing targets. Using these systems, we demonstrate that rather than serving merely as a tether, as proposed earlier (Toda et al. 2012), TIM4 is an active participant in the phagocytic process. Furthermore, we find that TIM4 operates independently of lactadherin, which had been proposed to act as a bridging molecule. Interestingly, TIM4-driven phagocytosis is dependent on the activation of integrins and involves stimulation of Src-family kinases and FAK, as well as the localized accumulation of phosphatidylinositol 3,4,5-Trisphosphate. These mediators promote the recruitment of the nucleotide-exchange factor Vav3, which in turn activates small Rho-family GTPases. Gene silencing or ablation experiments demonstrated that RhoA, Rac1 and Rac2 act synergistically to drive the remodeling of actin that underlies phagocytosis. Single-particle detection experiments demonstrated that TIM4 and β1 integrins associate upon receptor clustering. These findings support a model where TIM4 engages integrins as coreceptors to evoke the signal transduction needed to internalize PtdSer-bearing targets such as apoptotic cells.

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