MicroRNA-mediated gene silencing is a fundamental mechanism in the regulation of gene expression. It remains unclear how the efficiency of RNA silencing could be influenced by RNA-binding proteins associated with the microRNA-induced silencing complex (miRISC). Here we report that fused in sarcoma (FUS), an RNA-binding protein linked to neurodegenerative diseases including amyotrophic lateral sclerosis (ALS), interacts with the core miRISC component AGO2 and is required for optimal microRNA-mediated gene silencing. FUS promotes gene silencing by binding to microRNA and mRNA targets, as illustrated by its action on miR-200c and its target ZEB1. A truncated mutant form of FUS that leads its carriers to an aggressive form of ALS, R495X, impairs microRNA-mediated gene silencing. The C. elegans homolog fust-1 also shares a conserved role in regulating the microRNA pathway. Collectively, our results suggest a role for FUS in regulating the activity of microRNA-mediated silencing.

Long-lasting forms of synaptic plasticity and memory require de novo protein synthesis. Yet, how learning triggers this process to form memory is unclear. Translin/trax is a candidate to drive this learning-induced memory mechanism by suppressing microRNA-mediated translational silencing at activated synapses. We find that mice lacking translin/trax display defects in synaptic tagging, which requires protein synthesis at activated synapses, and long-term memory. Hippocampal samples harvested from these mice following learning show increases in several disease-related microRNAs targeting the activin A receptor type 1C (ACVR1C), a component of the transforming growth factor-b receptor superfamily. Furthermore, the absence of translin/trax abolishes synaptic upregulation of ACVR1C protein after learning. Finally, synaptic tagging and long-term memory deficits in mice lacking translin/trax are mimicked by ACVR1C inhibition. Thus, we define a new memory mechanism by which learning reverses microRNA-mediated silencing of the novel plasticity protein ACVR1C via translin/trax.