109 Oral - Neurodevelopment II/Neurobehavior II
Friday April 08, 5:45 PM - 6:00 PM

Authors:
Hui Ji ¹; Bei Wang ¹; David Labib ^1,3; Joyce Lei ^1,4; Xinchen Chen ¹; Maria Sapar ^1,3; Ana Boulanger ²; Jean-Maurice Dura ²; Chun Han ¹

Affiliations:
1) Weill Institute for Cell and Molecular Biology and Department of Molecular Biology and Genetics, Cornell University, Ithaca, NY ; 2) IGH, Centre National de la Recherche Scientifique, Université de Montpellier, Montpellier, France; 3) New York Stem Cell Foundation, New York, NY ; 4) Tisch MS Research Center of New York, New York, NY

Keywords:
b. dendrites; n. injury

Phagocytic clearance of degenerating neurites is critical for the prevention of neuroinflammation and neuronal regeneration. Phosphatidylserine (PS), an ‘eat-me’ signal exposed on degenerating neurites, and Draper (Drpr), an engulfment receptor expressed by phagocytes, are known to mediate the phagocytosis of degenerating neurites in Drosophila. However, how PS is sensed by Drpr-expressing phagocytes remains poorly understood. A novel secreted protein, Orion, was recently found to be required for axon pruning of mushroom body neurons during metamorphosis. In this study, we investigated whether Orion functions as a bridging molecule between PS and Drpr using larval dendritic arborization (da) neurons and phagocytic epidermal cells as a model of neuronal phagocytosis. First, using multiple models of dendrite degeneration, we demonstrate that Orion is required for Drpr-mediated phagocytosis of dendrite debris. orion loss-of-function (LOF) did not interfere with PS exposure on dendrites but abolished recruitment of epidermal Drpr to degenerating dendrites, suggesting that Orion acts downstream of neuronal PS exposure and upstream of Drpr. Second, we found that Orion is secreted by many non-neural tissues and shows PS-dependent binding to degenerating dendrites. Orion outcompetes Annexin V in binding to injured dendrites, suggesting a direct interaction with PS. Third, we found that Orion functionally interacts with Drpr: Orion accumulates on epidermal cells that overexpressed Drpr. Importantly, a membrane-tethered Orion induced PS-independent and Drpr-dependent dendrite degeneration when expressed in neurons but blocked engulfment of dendrites when expressed in epidermal cells. Lastly, we found that the Orion dosage modulates the sensitivity of the epidermal cell to PS: While orion heterozygosity reduced epidermal engulfment of PS-exposing dendrites, Orion overexpression caused engulfment-dependent, ectopic degeneration of wildtype dendrites.
Together, these findings argue that Orion functions as a bridging molecule between PS and Drpr and fine-tunes the competence of phagocytes in neuronal engulfment. The shared features between Orion and human chemokines that were recently demonstrated to function as “find-me” signals by binding to PS-exposing vesicles suggest conserved mechanisms between insects and humans in phagocytosis and homeostatic regulations of diverse body systems.