In animals, cells undergoing apoptosis and necrosis are rapidly internalized by other cells via phagocytosis (engulfment) and degraded inside phagocytes. The removal of these dying cells provides a safe means for eliminating unwanted and dangerous cells from the body and actively modulates immune responses. The study of apoptotic cell removal will shed light on inflammatory and auto-immune disorders, many of which are associated with an inefficient clearance of apoptotic cells from the human body. This study is also closely related to cancer research and treatment. My long-term objective is to understand the molecular mechanism that controls the recognition, engulfment, and degradation of apoptotic and necrotic cells. I use the nematode Caenorhabditis elegans, a small round worm as a model organism to identify genes and delineate pathways that control these evolutionarily conserved events and will apply the knowledge to understand similar processes in mammals, including humans. We have identified novel signaling events both upstream and downstream of C. elegans phagocytic receptor CED-1, which have provided conceptual advances in our understanding of how apoptotic cells are removed by engulfing cells. In the next project period we propose to broaden our research scope by investigating the mechanisms behind these events. We have identified phosphatidylserine (PS) as one of the """"""""eat me"""""""" signals exposed on the surface of apoptotic cells and activate CED-1. We will further study the molecular mechanisms for the exposure and recognition of PS and other potential """"""""eat me"""""""" signals in both apoptotic and engulfing cells (Aim 1). We will expand our study to the recognition and phagocytosis of necrotic touch neurons, which die in a caspase-independent manner and may employ unique mechanisms to attract engulfing cells (Aim 1). We have discovered that phagocytic receptor CED-1 not only initiates the engulfment, but also promotes the degradation of apoptotic cells through the recruitment and activation of downstream mediator DYN-1, the C. elegans ortholog of mammalian large GTPase dynamin. We will study the mechanism leading to the transient enrichment of DYN-1 to the surface of extending pseudopodia and phagosomes, which is important for its functions in both engulfment and phagosome maturation. One particular hypothesis to test is that the CED-1 pathway recruits DYN-1 via the collaborative efforts of CED-6 and phosphatidylinositol 4,5- phosphate (PI(4,5)P2) (Aim 2). We have established C. elegans as a unique model system for studying the degradation of apoptotic cells inside phagosomes, a process not well studied, and have identified the specific functions of two Rab GTPases, a PI3 kinase and its product phosphatidylinositol 3-phosphate (PI(3)P) in phagosome maturation. To reveal the mechanism of PI(3)P function and the relationship between PI(3)P and Rab GTPases, we will identify downstream effectors of PI(3)P and the events that they regulate (Aim 3).

Public Health Relevance

This project studies how unwanted cells (cells that commit suicide or die due to injuries) are recognized, internalized, and digested by their engulfing cells, a process that protects human bodies from their harmful effects. Understanding the mechanisms controlling this process will have important therapeutic implications, since many inflammatory and auto-immune diseases are closely related to defects in removing dying cells from human bodies, and will further help develop new strategies to specifically eliminate tumor cells. This project will be conducted in a small round worm the nematode C. elegans, which uses evolutionarily conserved mechanisms to control cell death, and provides a powerful means to reveal the principle for biological actions in a relatively simple system.

National Institute of Health (NIH)
National Institute of General Medical Sciences (NIGMS)
Research Project (R01)
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Study Section
Cellular Signaling and Regulatory Systems Study Section (CSRS)
Program Officer
Hagan, Ann A
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Baylor College of Medicine
Schools of Medicine
United States
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Li, Zao; Venegas, Victor; Nagaoka, Yuji et al. (2015) Necrotic Cells Actively Attract Phagocytes through the Collaborative Action of Two Distinct PS-Exposure Mechanisms. PLoS Genet 11:e1005285
Huang, Shuyi; Jia, Kailiang; Wang, Ying et al. (2013) Autophagy genes function in apoptotic cell corpse clearance during C. elegans embryonic development. Autophagy 9:138-49
Li, Zao; Lu, Nan; He, Xiangwei et al. (2013) Monitoring the clearance of apoptotic and necrotic cells in the nematode Caenorhabditis elegans. Methods Mol Biol 1004:183-202
Shen, Qian; He, Bin; Lu, Nan et al. (2013) Phagocytic receptor signaling regulates clathrin and epsin-mediated cytoskeletal remodeling during apoptotic cell engulfment in C. elegans. Development 140:3230-43
Sun, Lin; Liu, Ou; Desai, Jigar et al. (2012) CED-10/Rac1 regulates endocytic recycling through the RAB-5 GAP TBC-2. PLoS Genet 8:e1002785
Lu, Nan; Shen, Qian; Mahoney, Timothy R et al. (2012) Two PI 3-kinases and one PI 3-phosphatase together establish the cyclic waves of phagosomal PtdIns(3)P critical for the degradation of apoptotic cells. PLoS Biol 10:e1001245
Lu, Nan; Zhou, Zheng (2012) Membrane trafficking and phagosome maturation during the clearance of apoptotic cells. Int Rev Cell Mol Biol 293:269-309
Lu, Nan; Shen, Qian; Mahoney, Timothy R et al. (2011) Three sorting nexins drive the degradation of apoptotic cells in response to PtdIns(3)P signaling. Mol Biol Cell 22:354-74
He, Bin; Yu, Xiaomeng; Margolis, Moran et al. (2010) Live-cell imaging in Caenorhabditis elegans reveals the distinct roles of dynamin self-assembly and guanosine triphosphate hydrolysis in the removal of apoptotic cells. Mol Biol Cell 21:610-29
He, Bin; Lu, Nan; Zhou, Zheng (2009) Cellular and nuclear degradation during apoptosis. Curr Opin Cell Biol 21:900-12

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