Role of tau circular RNAs in tauopathies

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Microtubule associated protein tau (encoded by the MAPT gene) is a critical component in the etiology of Alzheimer's Disease (AD) and other tauopathies in which tau aggregates in neurofibrillary tangles. However, the basic regulation of MAPT gene expression is still imperfectly understood. It is not clear what causes the formation of neurofibrillary tangles, but tauopathic conditions show changes in tau mRNA isoforms, generated through alternative pre-mRNA processing. Furthermore, mutations changing alternative splicing of MAPT cause frontotemporal dementia, underscoring the importance of tau pre-mRNA processing. We have identified human-specific circular RNAs (circRNAs) formed by the tau-encoding mRNA from the MAPT locus generated by backsplicing of exon 10. Backsplicing forming circRNAs occurs when pre-mRNA forms loops, which is aided by primate-specific Alu elements in humans. The ground-breaking preliminary data demonstrate conclusive evidence of tau circRNAs derived from human brain samples. We hypothesize that Alu elements help forming human specific MAPT circRNAs, that likely after RNA methylation could be translated into proteins containing multimers of microtubule binding sites. We will use (a) minigenes in tissue culture for molecular mechanistic studies, to test whether MAPT circRNAs are translated into proteins after RNA methylation and to test tau circRNA's interaction with other RNAs and proteins. These data will be correlated with assessments in human brain tissue across the disease spectrum from pathologically-confirmed normal, MCI (mild cognitive impairment), and AD cases. To (b) test tau circRNA's physiological relevance, we have generated zebrafish lines expressing genomic parts of human MAPT that form circular RNAs in neurons, and observed abnormal neurons after introduction of FTDP-17 mutants in constructs that express only circRNAs. We will determine the expression of transgenic circRNAs, their encoded proteins, possible sequestration of other RNAs and proteins, changes in neuron morphology, tau pathology and correlate these data with spatial memory tests that take stress into account. These are the first reports and studies of human tau circular RNAs. These studies are significant, as they could provide a novel, human-specific molecular mechanism to alter MAPT gene expression, and possibly a new mechanism to promote the formation of tau aggregates. Our studies capitalize on the excellent brain bank at the University of Kentucky, headed by Co-I Dr. Nelson, and the zebrafish expertise of Co- I Dr. Blackburn.