Neuroligin-2 is ubiquitinated by Nedd4l to control developmental astrocyte morphogenesis

Central nervous system astrocytes have an intricate, highly branched morphology. Proper development of perisynaptic astrocyte processes is necessary for tripartite synapse formation and function. However, cellular pathways orchestrating this development are largely unknown. Neuroligins (NLs) 1-3 regulate astrocyte morphogenesis via transcellular adhesions with neuronal neurexins. Here, we found an astrocytic NL2-based mechanism governing morphogenesis. Through structure and function studies, we identified a WW-binding motif within the NL2 intracellular domain required for astrocyte morphogenesis. Using cell-specific in vivo proximity labeling (iBioID), we found that each NL displays distinct protein-protein interactions within astrocytes, distinct from the neuronal NL2-binding partners. From these data, we identified a role for WW domain-containing E3 ubiquitin ligase Nedd4l in astrocyte morphogenesis. Biochemical assays revealed Nedd4l ubiquitinates and stabilizes NL2, and this ubiquitination is required for astrocyte morphogenesis. This study shows that NLs have nonoverlapping roles in controlling astrocyte growth and uncovers a molecular mechanism of how NL2 mediates astrocyte morphogenesis.