Tre1/s1pr1 is an evolutionarily conserved regulator of astrocyte development

Astrocytes are specialized glial cells that possess an intricate morphology with highly ramified cellular processes interacting closely with individual synapses and other brain cells. Astrocytes perform a range of essential developmental and homeostatic functions in neural circuits, and it is thought that these functions depend on their remarkable morphological complexity and close association with neural elements. However, we know surprisingly little about the molecular and genetic mechanisms that underlie astrocyte morphogenesis or how astrocytes modulate brain circuits in vivo. By leveraging a combination of molecular-genetic tools and live-imaging techniques in Drosophila and zebrafish, we sought to identify evolutionarily conserved signaling pathways that regulate astrocyte development and function in vivo. In Drosophila, we found that RNAi-mediated depletion of Trapped in endoderm 1 (Tre1) GPCR in astrocytes resulted in severely reduced astrocyte morphological complexity. We showed that endogenous Tre1 is highly enriched in astrocytes and validated the astrocyte morphology phenotypes in Tre1 genetic mutants. We demonstrated that Tre1 acts cell autonomously to direct the elaboration of fine astrocyte processes into the synaptic neuropil, by genetically interacting with small GTPase Rac1 to regulate the actin cytoskeleton. Furthermore, we found this is a conserved signaling modality across species, as loss of the functional vertebrate GPCR analog sphingosine-1-phosphate receptor 1 (s1pr1) in zebrafish led to disrupted astrocyte morphology similar to that observed in Drosophila mutants. Using a combination of live-imaging and pharmacology approaches in zebrafish, we found that S1pr1 is required throughout astrocyte development to control astrocyte process outgrowth and dynamics. Together, our data reveal a crucial and conserved Tre1/S1pr1-signaling in governing astrocyte morphogenesis in vivo. None.