The Cytoplasmic MicroRNA‐Production Complex Regulates MicroRNA Sorting to Extracellular Vesicles During Synaptogenesis

ABSTRACT Extracellular vesicles (EVs) have emerged as novel players of cell communication, in part via the transfer of microRNA (miRNA) cargo to recipient cells. Although several RNA‐binding proteins (RBPs) were shown to shuttle miRNAs to EVs, how they are loaded with miRNA is unknown. This study demonstrates that the core accessory protein of the cytoplasmic miRNA production machinery TRBP interacts with several EV‐RBPs and differentially regulates miRNA secretion in small EVs. TRBP‐regulated EV‐miRNAs were highly enriched in a Syncrip‐recognition sequence motif. Moreover, we show that Syncrip associates with TRBP via its C‐terminal domain at the surface of the endoplasmic reticulum (ER), a nucleation site of the miRNA production machinery. The recruitment of Syncrip to ER‐associated TRBP was regulated by ER‐endosome membrane contact sites (MCS), whereby increased MCS formation promotes Syncrip loading to endosomes as well as Argonaute 2‐positive RNA granules. Functionally, this pathway is important for the regulation of synapse formation in developing neurons, as expression of miRNA‐ or TRBP‐binding deficient mutants of Syncrip differentially regulated synapse formation in EV‐donor and recipient neurons. Overall, this work highlights an important role of TRBP as a gatekeeper for microRNA sorting, which could be crucial for the fine‐tuning of neuronal circuits during postnatal development.