|Ph.D Student||Shklyar Boris|
|Subject||Molecular Mechanisms of Apoptotic Cell Clearance during|
|Department||Department of Medicine||Supervisor||Dr. Estee Kurant|
|Full Thesis text|
Apoptosis, the most eminent type of programmed cell death (PCD), occurs in a variety of tissues as part of normal embryonic development and tissue homeostasis in the adult. The final step of apoptosis in vivo is clearance of apoptotic cells through phagocytosis, a highly evolutionarily conserved process. During Drosophila development, most of the cell death occurs in the nervous system, where about third the neurons are eliminated by apoptosis and subsequent phagocytosis by glial cells. Proper removal of apoptotic cells is crucial for the establishment of an intact and functioning central nervous system (CNS), therefore, it should be accurately regulated. In this work, we address two main questions: how phagocytes discriminate between living and dying cells and how phagocytic proficiency of glial cells is established.
To answer these questions we focus on the mode of action of the transmembrane phagocytic receptor Six Microns Under (SIMU), which is specifically expressed in phagocytic glia during Drosophila embryogenesis and required for recognition and engulfment of apoptotic neurons. Structure-function analysis of SIMU demonstrates that SIMU binds an ‘eat-me’ signal phosphatidylserine (PS) on apoptotic cell via its N-terminal EMI (EMILIN) domain and Nimrod repeats (NIM1 and NIM2), a type of epidermal growth factor (EGF)-like repeats. Interestingly, the NIM3 and NIM4 domains are not required for SIMU binding but control its affinity to PS. Based on these data we propose a novel mechanism that prevents phagocytosis of living cells transiently externalizing PS on their membranes and we show that in addition to PS exposure, caspase activity is required for clearance of apoptotic cells by phagocytes. We also demonstrate that the phagocytic ability of glial cells depends on the expression of SIMU and an additional phagocytic receptor Draper (Drpr) that is required for degradation of apoptotic cells inside phagocytes. This specific expression of SIMU and Drpr is not affected by apoptosis per se but it is part of the developmental program responsible for glial cell fate determination and consequently for the phagocytic ability of the embryonic glia.
In conclusion, our work shades an additional light on the molecular and cellular mechanisms underlying the process of apoptotic cell clearance during Drosophila CNS development and adds to our understanding of glial phagocytic function in evolutionary advanced organisms.