Collaboration between Phosphatidylserine receptors

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Abstract Apoptosis, or programed cell death, is a common occurrence during development, pathogenesis and tissue homeostasis. The efficient removal of apoptotic cell corpses is crucial for preventing inflammation and a potentially damaging immune response. For this reason, enhancing apoptotic corpse clearance may be an effective strategy to resolve damaging inflammation following infection or aggressive cancer therapies that overwhelm the endogenous system for removing apoptotic cells. Developing these therapies requires a deeper understanding for how apoptotic corpse clearance pathways are activated. Dying cells recruit phagocytes through `eat me' signals, the most widespread of which is phosphatidylserine (PS). Several PS-recognizing receptors have been implicated in apoptotic corpse clearance but many aspects of their function are still unclear: does each receptor regulate distinct aspects of the corpse clearance pathway or do they additively converge on the same downstream targets? How are the PS receptors activated and how is this activation translated into F-actin reorganization and engulfment? In particular, the phosphatidylserine receptor (PSR) plays a highly conserved role in promoting clearance of apoptotic corpses, but the mechanism by which PSR promotes corpse clearance is not known. I will examine the collaboration between two conserved PS- recognizing receptors, the PS receptor (PSR) and CED-1/Draper, using a combination of in vivo imaging in C. elegans and reconstituting engulfment pathways in Drosophila S2 cells. In the C. elegans germline, I will use a dual inverted selective plane microscope (diSPIM) to examine phagocytic cup formation, F-actin dynamics and efficiency of corpse clearance. This will determine which steps in the apoptotic corpse clearance program are regulated by PSR-1, CED-1 or both receptors. To rapidly dissect the mechanism for PSR function, I will reconstitute the apoptotic corpse clearance pathway in Drosophila S2 cells. My preliminary data shows that ectopically expressing dPSR transforms S2 cells from poor phagocytes into efficient engulfers of PS-coated glass microspheres. I will determine if PSR partitions with Draper or F-actin into microdomains of active signaling in the plasma membrane, whether nuclear or only membrane-bound PSR signaling is required to promote engulfment, and if PSR is activated by oligomerization. Together, these studies will give insight into the function of PSR, a highly conserved yet poorly understood PS receptor, and elucidate how multiple PS receptors collaborate to promote clearance of apoptotic cell corpses.