A Circular RNA (circRNA) ciRS‐7 in Alzheimer's disease (AD) targets miRNA‐7 trafficking and promotes deficits in the expression of the ubiquitin conjugase (UBE2A) and the epidermal growth factor receptor (EGFR)

Abstract

Circular RNAs (circRNAs) represent a recently described category of novel, single stranded ribonucleic acid (ssRNA) abundantly represented in the eukaryotic transcriptome. Non‐polyadenylated and intrinsically resistant to exonucleolytic attack, circRNAs are a highly stable ssRNA species significantly enriched in human brain and retinal tissues. The biological evolution and function of most central nervous system (CNS) circRNAs, however, remains poorly understood. Established methods of ssRNA detection and quantitation that require a free 3′ or 5′ ribonucleotide end may have significantly underestimated circRNA abundance and importance in CNS function in both health and disease. One highly represented circRNA in the human brain and retina is a ~1400 nucleotide (nt) ciRS‐7 that contains about ~70 tandem anti‐miRNA‐7 sequences; ciRS‐7 thereby acts as a kind of endogenous, competing, anti‐complementary miRNA inhibitor or “sponge” to adsorb, and hence quench, the normal function(s) of miRNA‐7 (hsa‐miRNA‐7‐1; 23 nt; chr 9q21.32). Using DNA and miRNA arrays, RNA‐sequencing, enhanced LED‐Northern, ELISA and Western blot hybridization, the circularity‐sensitive probe RNaseR, energy of association (EA) indexing and miRNA‐mRNA bioinformatics algorithms, we here provide initial evidence of a mis‐regulated miRNA‐7‐anti‐miRNA‐7‐circRNA system in sporadic Alzheimer's disease (AD) brain. Deficits in ciRS‐7, and ciRS‐7 “sponging activities” might be expected to increase ambient miRNA‐7 levels in AD‐affected tissues, as is observed, and these ultimately contribute to the down‐regulation of selective miRNA‐7‐sensitive messenger RNA (mRNA) targets. We report here that the up‐regulation of ambient miRNA‐7 in AD brain, due to a deficiency in ciRS‐7 and ciRS‐7‐mediated “sponging” effects, in turn down‐regulates several AD‐relevant mRNA targets and their expression, including the ubiquitin conjugase protein (UBE2A; miRNA‐7‐UBE2A mRNA 3′UTR EA = −22.9 kcal/mol) and the epidermal growth factor receptor (EGFR; miRNA‐7‐EGFR mRNA 3′UTR EA = −23.2 kcal/mol). UBE2A (chr Xq24; ~17kDa), a central effector in the ubiquitination cycle that helps to orchestrate the clearance of amyloid peptides via phagocytosis, is depleted in sporadic AD brain and retina and contributes to amyloidogenesis. Similarly, decreased levels of the transmembrane glycoprotein EGFR (chr 7p11.1; ~134kDa) found in AD appears to be linked to deficits in brain cell proliferation, cytoskeletal rearrangement and angiogenesis. Our conceptual understanding of the highly specialized functions for non‐coding ssRNA such as circRNAs in the human CNS continues to evolve and their presence and activities underscore the increasing complexity of miRNA‐mRNA‐mediated genetic regulatory networks.Support or Funding InformationResearch on miRNA signaling in the Lukiw laboratory involving the innate‐immune response in AD, AMD and in other forms of neurological or retinal disease, amyloidogenesis, ubiquitination, synaptogenesis and brain inflammation was supported through an unrestricted grant to the LSU Eye Center from Research to Prevent Blindness (RPB); the Louisiana Biotechnology Research Network (LBRN) and NIH grants NEI EY006311 and NIA AG038834 (WJL).