Bioinformatics and Experimental Identification of circ_0001535 Associated with Diagnosis and Development of Alzheimer's Disease

Alzheimer's disease (AD) is a type of disease frequently occurs in the elderly population. Diagnosis and treatment methods for this disease are still lacking, and more research is required. In addition, little is known about the function of the circular RNAs (circRNAs) in AD.In this research, RNA expression data of AD from the Gene Expression Omnibus (GEO) database were downloaded. The expression levels of circRNAs in cerebrospinal fluid samples of healthy participants and AD patients were measured by reverse transcription‑quantitative PCR (RT-qPCR). The diagnosed value of differential expressed circRNAs was analyzed with the Receiver operating characteristic curve (ROC). Pathways related to circ_0001535 were found using gene set enrichment analysis (GSEA) and Metascape. The direct interactions between circ_0001535 and E2F transcription factor 1 (E2F1) or E2F1 and dihydrofolate reductase (DHFR) were verified using Chromatin immunoprecipitation (ChIP) and RNA Binding Protein Immunoprecipitation (RIP) assays. Cell Counting Kit-8 (CCK8) and flow cytometry were used to identify the function of circ_0001535/E2F1/DHFR axis on the proliferation and apoptosis of AD cells.In total, 12 circRNAs have been linked to AD diagnosis. The expression levels of 7 circRNAs differed between AD patients and control groups. Circ_0001535 had the most diagnose value among these circRNAs. Hence, circ_0001535 was regarded as a key circRNA in the present study. E2F1/DHFR axis was predicted to be regulated by circ_0001535. In addition, IP assays experiment results showed that E2F1 could bind to the promoter region of DHFR and be regulated by circ_0001535. In vitro results showed that circ_0001535 overexpression could promote DHFR expression, while E2F1 knock down could inhibit DHFR expression in SH-SY5Y cells. Finally, rescue experiments suggested that circ_0001535 could reduce Aβ25-35-induced SH-SY5Y cell proliferation and facilitate apoptosis through E2F1/DHFR axis.Our research in AD circRNA can offer important information regarding the role of specific circRNAs in the AD environment and point to specific future areas of therapeutic intervention in AD.