Investigating circular RNA‐mediated neurotoxicity in tauopathies

Background Circular RNAs (circRNA) are a subclass of non‐coding RNAs with a covalently closed loop structure that are formed via non‐canonical splicing 1 . CircRNAs are specifically enriched in the brain among different species and further accumulate in the brain as a function of physiological aging 2 . CircRNA biogenesis is regulated by N6‐methyladenosine (m 6 A), a post transcriptional RNA modification 3 . Elevated levels of circRNAs and abnormal m 6 A modification significantly correlate with clinical diagnosis and development of Alzheimer’s disease, respectively 4, 5 . Alzheimer’s disease and related “tauopathies” are pathologically defined by various forms of tau aggregates in the brains of affected individuals. We have previously reported that tau‐induced dysfunction of lamin, a nuclear scaffold protein, causes nuclear polymorphisms including invaginations and blebs 6, 7 . We have also found that polyadenylated RNA accumulates within nuclear invaginations in the context of tauopathy and that genetic and pharmacologic reduction of RNA export reduce RNA accumulation within invaginations and suppresses tau neurotoxicity 8 . While dysfunction of the nuclear envelope and consequent aberrant RNA export mediate tau‐induced neurotoxicity, the identity of RNAs that accumulate within nuclear blebs, the role of m 6 A in circRNA accumulation, and their relationship with aging and tauopathy are currently unknown. Based on the association between nuclear polymorphism and RNA export, alongside the global enrichment of circRNAs and disrupted RNA methylation in Alzheimer’s disease brains, I hypothesize that m 6 A‐dependent accumulation of circRNAs in tauopathies sequester complementary RNAs and RNA binding proteins into large inclusions that trigger RNA export via nuclear blebbing. Methods RNA‐seq, digital PCR, TUNEL, electron microscopy and immunofluorescence. Results I find that circRNAs accumulate in brains of a Drosophila model of tauopathy and that RNAi‐mediated reduction of mbl, which is particularly enriched in its circular form in the brain, significantly suppresses tau‐induced neurotoxicity. RNAi‐mediated reduction of an m 6 A writer and reader significantly reduces circMbl biogenesis and neuronal death in tau transgenic Drosophila . I find that circMbl lines the nuclear blebs that contain large inclusions in brains of tau transgenic Drosophila . Conclusions Tau‐induced accumulation of circRNA is mediated by aberrant m 6 A modification. Presence of circMbl‐lined nuclear buds suggests a potential role of nuclear blebs in circRNA nuclear export.