New Candidates for the PS Receptor

Abstract

Prompt removal of cells that die by programmed cell death, or apoptosis, is a key mechanism by which this intentional cell death response does not lead to inflammation. Phosphatidylserine (PS) is a membrane lipid that is exposed on the surface of apoptotic cells, where it is believed to be a key signal for the recognition of such dying cells by phagocytosing cells. Two groups report the identification of proteins that bind PS and appear to be important for recognition and phagocytosis of apoptotic cells. Park et al . identified brain-specific angiogenesis inhibitor 1 (BAI1)--a seven-transmembrane-domain protein with an extended extracellular region containing thrombospondin repeats (TSR) and an RGD domain--in a yeast two-hybrid screen for proteins that interacted with Rac and its guanine nucleotide exchange factor (GNEF), the EMLO and Dock180 complex, which are part of the signaling cascade that triggers the changes in the actin cytoskeleton associated with phagocytosis. Forced expression of BAI1 in nonphagocytic cells or in macrophages enhanced uptake of apoptotic thymocytes or beads mimicking apoptotic cells. (BAI1 was detected in untransfected human monocytes and macrophages, macrophage cell lines, spleen, and bone marrow.) Annexin V, which binds PS, prevented BAI1-stimulated phagocytosis of apoptotic cells, as did exposure of the cells to a BAI1 fragment containing the RGD and TSR domains, but not one lacking the TSR domain. The BAI1 TSR domain fragment also blocked phagocytosis of labeled beads when co-injected into the peritoneum of mice. Depletion of BAI1 in primary mouse astrocytes decreased phagocytosis of the beads or apoptotic thymocytes. Miyanishi et al . screened a panel of antibodies against mouse peritoneal macrophages and found that an antibody that recognized T cell immunoglobulin- and mucin-domain-containing molecule 4 (Tim4) inhibited PS-dependent engulfment of apoptotic cells. Tim4 mRNA was detected in peritoneal cells, spleen, lymph nodes, and salivary glands, and forced expression of Tim4, which has a single transmembrane domain, in nonphagocytic cells promoted the phagocytosis of apoptotic cells. Injection of the antibody that recognized Tim4 in mice followed by treatment to induce apoptosis in the thymus resulted in impaired phagocytosis of the apoptotic cells and the development of autoantibodies. A fusion protein between the Tim4 extracellular domain and the Fc domain of antibody receptors (Tim4-Fc) bound to apoptotic cells or to PS spots on nitrocellulose filters. Fusion proteins containing the Tim4 or Tim1 IgV domain bound to microtiter plates coated in PS. However, the same domains of the Tim2 and Tim3 family members showed no such binding, nor did the full-length proteins promote apoptotic cell phagocytosis when overexpressed. Exosomes, which are membrane fragments released from cells that may mediate cell communication, also have PS on the surface, and cells expressing either Tim4 or Tim1 bound exosome-like vesicles (based on gold-labeling and electron microscopy). PS appeared to promote heterophilic and homophilic interactions between Tim4 and Tim1. The authors suggest that Tim1 and Tim4 may serve as PS receptors involved in recognition of both apoptotic cells and exosomes. D. Park, A.-C. Tosello-Trampont, M. R. Elliott, M. Lu, L. B. Haney, Z. Ma, A. L. Klibanov, J. W. Mandell, K. S. Ravichandran, BAI1 is an engulfment receptor for apoptotic cells upstream of the ELMO/Dock180/Rac module. Nature 450 , 430-434 (2007). [PubMed] M. Miyanishi, K. Tada, M. Koike, Y. Uchiyama, T. Kitamura, S. Nagata, Identification of Tim4 as a phosphatidylserine receptor. Nature 450 , 435-439 (2007). [PubMed]