A circRNA-derived IRES-like element enables programmable translational activation of the endogenous chaperone HSP90

Circular RNAs (circRNAs) are covalently closed transcripts that lack a 5′ cap, consequently their translation depends on cap-independent initiation, in most cases through Internal Ribosome Entry Sites (IRESs). Whereas IRES-mediated translation has been studied almost exclusively in cis , the use of IRES-containing circRNAs to activate translation of an independent mRNA in trans has only recently begun to be explored. Here, we characterize SynFarsa , a synthetic circRNA-derived IRES-like element that promotes cap-independent translation in mouse and human cells. We engineered SynFarsa into a trans -acting regulator by fusing it to a short antisense sequence complementary to a selected region of a target mRNA. This reproduces the architecture of a circular SINEUP, in which IRES activity promotes translation in trans of a transcript recognized through antisense complementarity. Directing this construct to the hsp90 3′UTR increased HSP90 protein levels approximately 1.6-fold without altering hsp90 mRNA abundance, whereas the hsp70 3′UTR and the 5′UTRs of both transcripts were not permissive, indicating that activity depends strongly on target site selection. In a FUS ^P525L neuroblastoma model of Amyotrophic Lateral Sclerosis, this increase in HSP90 was accompanied by a reduction in the number and size of cytoplasmic FUS aggregates. SynFarsa thus provides a circRNA-derived effector domain for targeted translational upregulation of an endogenous protein.