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

Park D, Tosello-Trampont AC, Elliott MR, Lu M, Haney LB, Ma Z, Klibanov AL, Mandell JW, Ravichandran KS
Source: Nature
Publication Date: (2007)
Issue: 450(7168): 430-434
Research Area:
Cancer Research/Cell Biology
Immunotherapy / Hematology
Cells used in publication:
Species: mouse
Tissue Origin: ascites
Astrocyte, mouse
Species: mouse
Tissue Origin: brain
Nucleofector® I/II/2b
J-774: stable overexpression of BAI1; stable knockdown of ELMO1 by shRNA construct, co-transfection with BAI-1-GFP construct - apoptosis assays; also transient transfection of BAI1 siRNA (Dharmacon SmartPool), knock-down 20 %. Murine Astrocytes: Transfection of BAI1 siRNA, knock-down 46 %, resulting in decreased uptake of apoptotic thymocytes.
Engulfment and subsequent degradation of apoptotic cells is an essential step that occurs throughout life in all multicellular organisms. ELMO/Dock180/Rac proteins are a conserved signalling module for promoting the internalization of apoptotic cell corpses; 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. However, the receptor(s) upstream of the ELMO/Dock180/Rac module are still unknown. Here we identify brain-specific 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 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, 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.