Glycosaminoglycan presentation-dependent regulation of glial responses by Orion during Drosophila neuronal pruning

Neuronal pruning refines neural circuits by eliminating unnecessary connections to ensure proper nervous system function. In Drosophila mushroom body gamma-neuron remodeling, the chemokine-like protein Orion is required for astrocyte infiltration and phagocytosis. This study investigated whether glial responses depend on how Orion is presented. We found that Orion interacts with glycosaminoglycans (GAG) associated to proteoglycans, which are essential for proper axon pruning. Tethering Orion to the gamma-neuron plasma membrane, mimicking glycosaminoglycan binding, was sufficient to induce astrocyte infiltration into axon bundles. However, membrane-bound Orion alone could not efficiently support phagocytosis, which additionally requires a soluble and mobile Orion pool. We also identified exosomes carrying GAG-bound Orion as potential mediators of neuronal remodeling. These findings provide the first evidence that glial infiltration can be uncoupled from phagocytosis, demonstrating that the mode of Orion presentation regulates distinct glial mechanisms during neuronal remodeling.