The RNA-binding protein FUS participates in several RNA biosynthetic processes and has been linked to the pathogenesis of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia. Here we report that FUS controls back-splicing reactions leading to circular RNA (circRNA) production. We identified circRNAs expressed in in vitro-derived mouse motor neurons (MNs) and determined that the production of a considerable number of these circRNAs is regulated by FUS. Using RNAi and overexpression of wild-type and ALS-associated FUS mutants, we directly correlate the modulation of circRNA biogenesis with alteration of FUS nuclear levels and with putative toxic gain of function activities. We also demonstrate that FUS regulates circRNA biogenesis by binding the introns flanking the back-splicing junctions and that this control can be reproduced with artificial constructs. Most circRNAs are conserved in humans and specific ones are deregulated in human-induced pluripotent stem cell-derived MNs carrying the FUSP525L mutation associated with ALS.

While the physiologic functions of the RNA-binding protein FUS still await thorough characterization, the pathonegetic role of FUS mutations in amyotrophic lateral sclerosis (ALS) is clearly established. Here we find that a human FUS mutation that leads to increased protein expression, and was identified in two ALS patients with severe outcome, maps to the seed sequence recognized by miR-141 and miR-200a in the 3 0 -UTR of FUS. We demonstrate that FUS and these microRNAs are linked by a feed-forward regulatory loop where FUS upregulates miR-141/200a, which in turn impact FUS protein synthesis. We also show that Zeb1, a target of miR-141/200a and transcriptional repressor of these two microRNAs, is part of the circuitry and reinforces it. Our results reveal a possible correlation between deregulation of this regulatory circuit and ALS pathogenesis, and open interesting perspectives in the treatment of these mutations through ad hoc-modified microRNAs.