Link to bioRxiv paper: http://biorxiv.org/cgi/content/short/2020.06.07.087221v1?rss=1 Authors: Lim, T. K. Y., Ruthazer, E. S. Abstract: Partial phagocytosis - called trogocytosis - of axons by microglia has been documented in ex vivo preparations but has yet to be observed in vivo. Fundamental questions regarding the mechanisms that modulate axon trogocytosis as well as its function in neural circuit development remain unanswered. Here we used 2-photon live imaging of the developing Xenopus laevis retinotectal circuit to observe axon trogocytosis by microglia in vivo. Amphibian regulator of complement activation 3 (aRCA3) was identified as a neuronally expressed, synapse-associated complement inhibitory molecule. Overexpression of aRCA3 enhanced axonal arborization and inhibited trogocytosis, while expression of VAMP2-C3, a complement-enhancing fusion protein tethered to the axon surface, reduced axonal arborization. Depletion of microglia also enhanced axonal arborization and reversed the stereotypical escape behaviors to dark and bright looming stimuli. These findings demonstrate that microglia remodel axon morphology through the complement system and that neurons may control this process through expression of complement inhibitory proteins. Copy rights belong to original authors. Visit the link for more info