Programmed cell death occurs in many tissues of our bodies throughout life. The quick and efficient removal of dying cells is a critical step that protects neighboring cells, and is also an important part of tissue remodeling and wound healing. The failure to remove apoptotic bodies has been implicated as a cause for certain types of chronic inflammation and autoimmune disorders. In mammals, macrophages and dendritic cells, as well as many other cells types can carry out the engulfment of apoptotic corpses. The cloning of the engulfment genes in the model organism C. elegans, and the identification of their homologues in mammals, has provided an exciting opportunity to delineate this process in better detail. Our work during the previous funding period defined a role for a novel engulfment protein ELMO in regulating the cytoskeletal reorganization to mediate engulfment. Specifically, ELMO associates with another protein Dock180 and together they function as a novel type of Rac-GEF to activate Rac, and thereby modify the cytoskeleton. Using a combination of biochemical analyses and phagocytosis assays in mammalian cells, we have shown that the ELMO/Dock180/Rac pathway represents an evolutionary conserved mode of signaling that promotes engulfment. We also identified two new players (RhoG and TRIO) that function together with ELMO during engulfment. However, a number of key questions remain. The upstream receptor(s) on phagocytes that links to the ELMO/Dock180/Rac module during engulfment is not known.
In Aim 1 we will characterize a novel type of receptor that we have cloned through a yeast two hybrid screen with ELMO that promotes phagocytosis of apoptotic targets.
Aim2 will address how phagocytes are attracted to apoptotic cells in vitro and in vivo, and the role of phagocyte proteins in this process, including ELMO and Dock180. Thus far, the relative importance of the ELMO/Dock180/Rac signaling module in a mammalian model system, such as in mouse models, have not been addressed.
Aim3 will address the importance of ELMO in engulfment in vivo, using conditional knockout of ELMO1 and conditional dominant negative ELMO1 transgenic mice we have generated. Taken together, through these studies, we expect to gain a better molecular understanding of the fundamentally important process of clearing apoptotic cells and define the role of specific players during engulfment.
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