title: The phosphatidylserine receptor TIM4 utilizes integrins as coreceptors to effect phagocytosis.
abstract: 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: 1) we compared bone marrow-derived macrophages from wild-type and TIM4(-/-) mice, and 2) we 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 by others, 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. Of interest, TIM4-driven phagocytosis depends on the activation of integrins and involves stimulation of Src-family kinases and focal adhesion kinase, as well as the localized accumulation of phosphatidylinositol 3,4,5-trisphosphate. These mediators promote 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 in which TIM4 engages integrins as coreceptors to evoke the signal transduction needed to internalize PtdSer-bearing targets such as apoptotic cells.
authors.affiliation: Program in Cell Biology, Hospital for Sick Children, Toronto, ON M5G 1X8, Canada Faculty of Dentistry, University of Toronto, Toronto, ON M5G 1G6, Canada Keenan Research Centre of the Li Ka Shing Knowledge Institute, St. Michael's Hospital, Toronto, ON M5C 1N8, Canada.
authors.forename: Ronald S
authors.initials: RS
authors.lastname: Flannagan
authors.forename: Johnathan
authors.initials: J
authors.lastname: Canton
authors.forename: Wendy
authors.initials: W
authors.lastname: Furuya
authors.forename: Michael
authors.initials: M
authors.lastname: Glogauer
authors.forename: Sergio
authors.initials: S
authors.lastname: Grinstein
mesh_terms: D000818:Animals; D002460:Cell Line; D051417:Focal Adhesion Kinase 1 / Q000378:metabolism; D006801:Humans; D016023:Integrins / Q000502:physiology*; D008264:Macrophages / Q000502:physiology; D008565:Membrane Proteins / Q000502:physiology*; D018345:Mice, Knockout; D010587:Phagocytosis; D019869:Phosphatidylinositol 3-Kinases / Q000378:metabolism; D010718:Phosphatidylserines / Q000378:metabolism; D011485:Protein Binding; D051567:Proto-Oncogene Proteins c-vav / Q000378:metabolism; D020741:rho GTP-Binding Proteins / Q000378:metabolism; D019061:src-Family Kinases / Q000378:metabolism
chemical_list: D016023:Integrins; D008565:Membrane Proteins; D010718:Phosphatidylserines; D051567:Proto-Oncogene Proteins c-vav; C500399:TIM-4 protein, mouse; C496622:Vav3 protein, mouse; D019869:Phosphatidylinositol 3-Kinases; D051417:Focal Adhesion Kinase 1; C496051:Ptk2 protein, mouse; D019061:src-Family Kinases; D020741:rho GTP-Binding Proteins