Fragile X syndrome (FXS) is the most frequent monogenic cause of autism spectrum disorder (ASD). It is characterized by a variety of symptoms, including epileptic seizures, hyperactivity and autistic traits. FXS patients lack fragile X messenger ribonucleoprotein (FMRP) due to transcriptional silencing of the fragile X messenger ribonucleoprotein 1 (FMR1) gene. In FXS, loss of FMRP alters mRNA translation and axonal transport and synapse structure and function, thus affecting neuronal circuit formation, cognition and behavior. The Appelbaum lab showed increased axonal branching and synaptic density as well as hyper-locomotor activity in a zebrafish model of FXS (fmr1-/-). Intriguingly, RNA-sequencing of the fmr1-/- synaptosome revealed differential expression of circular RNAs (circRNAs) in fmr1-/- brains. circRNA is a conserved class of non-coding RNA that is generated by the back-splicing of exons and introns. circRNAs are enriched in synapses, are highly stable, and accumulate with age, but their role in regulating neurogenesis, synaptic activity and behavior in both health and psychiatric disorders is unclear. Gad Vatine, Lior Appelbaum and colleague Shahar Alon postulate that loss of FMRP alters circRNA expression and trafficking, particularly in synapses,  to affect the function of neuronal circuit and ultimately behavior in FXS. Supporting this hypothesis, the expression of circmGluR5b (named CIRCGRM5 in humans), which is derived from the metabotropic glutamate receptor mGluR5b (named GRM5 in humans), is markedly increased in fmr1-/- zebrafish brains. The team will use fmr1-/- zebrafish and FXS human induced pluripotent stem cells (iPSCs) differentiated into neurons (iNeurons) in Brain-on-Chip (BoC) assays, and will combine super-resolution spatially-resolved expansion sequencing (ExSeq), genetic manipulations of circRNAs, two-photon live imaging, and video tracking of behavior to profile the subcellular localization of circRNAs and to understand their role in FXS. The functional characterization of synaptic circRNAs could point to epigenetic mechanisms and new therapeutic targets for the treatment of FXS and ASDs more generally.