Altered nucleocytoplasmic export of adenosine (A)-rich circRNAs by PABPC1 contributes to neuronal function

Circular RNAs (circRNAs) are upregulated during neurogenesis. Where and how circRNAs are localized, and what roles they play during this process have remained elusive. By comparing the nuclear and cytoplasmic circRNAs between H9 cells and H9-derived forebrain neurons (FB), we found a subset of adenosine (A)-rich circRNAs are restricted in H9 nuclei but exported to cytosols upon forebrain neuron differentiation. This differentiation-coupled circRNA subcellular relocation is modulated by the poly(A)-binding protein PABPC1. In the nucleus, newly produced (A)-rich circRNAs are bound by PABPC1 and the nuclear basket protein TPR to prevent their nucleocytoplasmic export. Modulation of (A)-rich motifs in circRNAs remarkably alters their subcellular localization. Enforced (A)-rich circRNAs in cytosols result in mRNA translation suppression. Furthermore, decreased nuclear PABPC1 upon neuronal differentiation enables the export of (A)-rich circRNAs including circRTN4(2,3), which is required for neurite outgrowth. These findings uncover subcellular localization features of circRNAs, linking their processing and function during neurogenesis.