BAI1 is an engulfment receptor for apoptotic cells upstream of the ELMO/Dock180/Rac module

Abstract

Engulfment and subsequent degradation of apoptotic cells is an essential step that occurs throughout life in all multicellular organisms [1][2][3] . ELMO/Dock180/Rac proteins are a conserved signalling module for promoting the internalization of apoptotic cell corpses 4,5 ; ELMO and Dock180 function together as a guanine nucleotide exchange factor (GEF) for the small GTPase Rac, and thereby regulate the phagocyte actin cytoskeleton during engulfment [4][5][6] . However, the receptor(s) upstream of the ELMO/ Dock180/Rac module are still unknown. Here we identify brainspecific angiogenesis inhibitor 1 (BAI1) as a receptor upstream of ELMO and as a receptor that can bind phosphatidylserine on apoptotic cells. BAI1 is a seven-transmembrane protein belonging to the adhesion-type G-protein-coupled receptor family, with an extended extracellular region 7-9 and no known ligands. We show that BAI1 functions as an engulfment receptor in both the recognition and subsequent internalization of apoptotic cells. Through multiple lines of investigation, we identify phosphatidylserine, a key 'eat-me' signal exposed on apoptotic cells 10-13 , as a ligand for BAI1. The thrombospondin type 1 repeats within the extracellular region of BAI1 mediate direct binding to phosphatidylserine. As with intracellular signalling, BAI1 forms a trimeric complex with ELMO and Dock180, and functional studies suggest that BAI1 cooperates with ELMO/Dock180/Rac to promote maximal engulfment of apoptotic cells. Last, decreased BAI1 expression or interference with BAI1 function inhibits the engulfment of apoptotic targets ex vivo and in vivo. Thus, BAI1 is a phosphatidylserine recognition receptor that can directly recruit a Rac-GEF complex to mediate the uptake of apoptotic cells.Previous studies revealed two 'functional' regions within ELMO1 and its Caenorhabditis elegans homologue CED-12 during phagocytosis 5,14-17 . The amino-terminal 558 amino-acid residues (N-term) were necessary for targeting of the ELMO-Dock180 complex to the membrane 14,17 , whereas the carboxy-terminal 196 residues (C-term) were necessary for binding Dock180 and for optimal Rac activation 15,16 . Because the receptor(s) upstream of ELMO1 during engulfment were not known, we performed a yeast two-hybrid screen, with N-term as bait. After screening more than 1.1 3 10 7 colonies from a mouse embryo library, followed by several subscreens for specificity, we identified a single membrane protein, BAI1.BAI1 belongs to subgroup VII of the adhesion-type G-proteincoupled receptor (GPCR) family 7-9 , with extended extracellular termini containing multiple domains and motifs that are thought to function in cell-cell or cell-matrix interactions 9 . BAI1 (1,584 residues) has an 943-residue extracellular region, a seven-transmembrane

Immunobiology of the TAM receptors

“…Given the dependence on phosphatidylserine of TAM receptor action during phagocytosis, it will be interesting to examine potential physical interaction between TAM receptors and the recently described direct phosphatidylserine receptors TIM4 (T-cell immunoglobulin domain and mucin domain protein 4) 59 and BAI1 (brain-specific angiogenesis factor 1) 60 , particularly as a clear physiological interaction between MER and α v β 5 -integrin, which is also required for the phagocytosis of both apoptotic cells 58 and photoreceptor outer segment membranes 61 , has been demonstrated. Similarly, it will be important to assess downstream TAM signalling pathways that mediate phagocytosis 62 versus those that are required for immunosuppression 25,27,56 .…”

Do not let death do us part: ‘find-me’ signals in communication between dying cells and the phagocytes

“…Direct recognition can occur via brainspecific angiogenesis inhibitor 1 (BAI1), 12 Stabilin-2, 13 and members of the T-cell immunoglobulin mucin domain; [14][15][16] alternatively, bridging molecules such as GAS6/Protein S and milk-fat globule EGF factor 8 can bind to the PtdSer exposed on the apoptotic cells, and in turn be recognized by members of the Tyro-Axl-Mer family and α v β 3 integrin, respectively, on the phagocyte. 17,18 Although some of these receptors only serve to tether the apoptotic cells, others can initiate intracellular signaling to induce phagocytosis.…”

The phospholipid code: a key component of dying cell recognition, tumor progression and host–microbe interactions

“…For example, PS can be recognized by cell surface receptors including TIM-4 (Kobayashi et al 13 and Miyanishi et al 14 ) and BAI-1. 15 PS can also be recognized by soluble opsonins such as MFG-E8 and Gas6, that are in turn detected by cell surface molecules α V β 3 and TAM receptors, respectively [16][17][18] (Figure 2a). The recognition of PS by phagocytes through various phospholipid detectors is an important molecular step to trigger the engulfment of apoptotic cells, 3 and exemplifies how modification of the phospholipid code on the surface of apoptotic cells can promote their clearance.…”