We turnover billions of cells in the body each day as part of normal homeostasis, and many of these cells die by the process of apoptosis. The apoptotic cells are quickly cleared by phagocytes in vivo, in an 'immunologically silent' fashion. Defects in prompt removal leads to secondary necrosis, with the release of intracellular contents from the dying cells promoting chronic tissue inflammation, which has been linked to autoimmune disorders such as SLE, airway inflammation, and atherosclerosis. Thus, a better knowledge of the mechanisms of apoptotic cell recognition and clearance becomes necessary for countering these disorders. This current competitive renewal application is based on the progress made during the two previous funding periods of 4 years each. We first identified a novel cytoplasmic engulfment adapter protein ELMO1, and defined the ELMO/Dock180/Rac signaling pathway as an evolutionarily conserved module for promoting apoptotic cell engulfment. We then identified the membrane protein BAI1 as the engulfment receptor upstream of ELMO1; independently, we generated knockout mice for Elmo1 to identify its requirement in apoptotic germ cell clearance in the testes, and during neurogenesis in the brain. These works have also raised a number of exciting next set of questions on signaling via the BAI1/ELMO1 module in vivo, and how this pathway may dampen inflammation in tissues.
In Aim1 of this proposal, using mice recently generated in our laboratory where the Bai1 locus has been targeted for deletion (floxed as well as straight knockout), we will test the hypothesis that BAI1 plays a role in apoptotic cell clearance in vivo in two different tissues, the thymus and the testes. Moreover, using inducible transgenic mice that overexpress wild type or a mutant form of BAI1, we will test whether BAI1 provides unique versus redundant signals in promoting engulfment in vivo.
Aim 2 focuses on how apoptotic cell recognition translates to anti-inflammatory cytokine production, key feature of apoptotic cell clearance that is not fully understood. We will test the hypothesis that the BAI1-ELMO1-mediated signaling contributes to anti-inflammatory responses of phagocytes. Specifically, we will test how an unexpected interaction between ELMO1 and components of the transcriptional machinery (which we have discovered in our preliminary studies), contribute to the anti-inflammatory gene transcription during engulfment. We will extend these in vivo using mice deficient in Elmo2 (that we have recently generated) in a mouse model of tissue inflammation. Collectively, we expect these studies to provide exciting new information on signaling via the BAI1/Elmo1 signaling pathway in cell clearance in vivo. Since altered expression of ELMO1 and BAI1 are genetically linked to inflammatory disorders in humans, the results from the proposed studies could be relevant for therapeutic intervention in inflammatory disorders.
As part of normal homeostasis, we turnover billions of cells in the body each day. Failure to efficiently clear the dying cells has been linked autoimmunity, atherosclerosis, and defects in development, with relevance also to chemotherapies that induce cell death of tumors. This proposal aims to define the mechanistic aspects and in vivo relevance of a signaling pathway via BAI1 and ELMO1 proteins, both of which have been linked to human diseases through GWAS studies. The insights from the proposed studies are expected to lead to a better understanding of cell clearance in physiology, and how the process might be altered in certain inflammatory disease states.
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