Role of Circular Tau RNA's in Tauopathies

Microtuble 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 that contain the alternative exon 10. CircRNAs are formed through a backsplicing mechanism when pre-mRNA forms loops, which in humans is aided by primate-specific Alu elements. 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, some of which could be translated into proteins containing multimers of microtubule binding sites. Further studies will use tissue culture for molecular mechanistic studies, correlated with assessments in human brain tissue across the disease spectrum from pathologically-confirmed normal, MCI (mild cognitive impairment), and AD cases. We will use (a) minigenes to determine cis and trans-acting factors forming circular tau RNAs, test whether they are translated into proteins after RNA methylation and test their interaction with other RNAs and proteins. To (b) test their physiological relevance, we generated zebrafish lines expressing genomic parts of MAPT that form circular RNAs in neurons, and observe abnormal neurons in FTDP-17 mutants. We will determine the expression of circRNA encoded proteins, possible sequestration of other RNAs and proteins and determine changes in neuron morphology, tau pathology and behavior after the fish have been stressed. These are the first reports and studies of human circular tau RNAs. These studies are significant, as they could provide a novel 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 Dr. Nelson, the Co-I and the zebrafish expertise of Co-I Dr. Blackburn.