Circular RNA Encoded Amyloid Beta peptides-A Novel Putative Player in Alzheimer's Disease.

Alzheimer's disease (AD) is an age-related detrimental dementia. Amyloid beta peptides (Aß) play a crucial role in the pathology of AD. In familial AD, Aß are generated from the full-length amyloid beta precursor protein (APP) via dysregulated proteolytic processing; however, in the case of sporadic AD, the mechanism of Aß biogenesis remains elusive. circRNAs are a class of transcripts preferentially expressed in brain. We identified a circRNA harboring the Aß-coding region of the APP gene termed circAß-a. This circular RNA was detected in the brains of AD patients and non-dementia controls. With the aid of our recently established approach for analysis of circRNA functions, we demonstrated that circAß-a is efficiently translated into a novel Aß-containing Aß175 polypeptide (19.2 KDa) in both cultured cells and human brain. Furthermore, Aß175 was shown to be processed into Aß peptides-a hallmark of AD. In summary, our analysis revealed an alternative pathway of Aß biogenesis. Consequently, circAß-a and its corresponding translation product could potentially represent novel therapeutic targets for AD treatment. Importantly, our data point to yet another evolutionary route for potentially increasing proteome complexity by generating additional polypeptide variants using back-splicing of primary transcripts that yield circular RNA templates.

A universal approach to investigate circRNA protein coding function.

Circular RNAs (circRNAs) are an emerging class of RNA molecules that have been linked to human diseases and important regulatory pathways. Their functional roles are still under investigation, often hampered by inefficient circRNA formation in and ex vivo. We generated an intron-mediated enhancement (IME) system that-in comparison to previously published methods-increases circRNA formation up to 5-fold. This strategy also revealed previously undetected translation of circRNA, e.g., circRtn4. Substantiated by Western blots and mass spectrometry we showed that in mammalian cells, translation of circRtn4 containing a potential "infinite" circular reading frame resulted in "monomers" and extended proteins, presumably "multimer" tandem repeats. In order to achieve high levels of circRNA formation and translation of other natural or recombinant circRNAs, we constructed a versatile circRNA expression vector-pCircRNA-DMo. We demonstrated the general applicability of this method by efficiently generating two additional circRNAs exhibiting high expression levels. The circRNA expression vector will be an important tool to investigate different aspects of circRNA biogenesis and to gain insights into mechanisms of circular RNA translation.