**1. Introduction**

Non-coding RNAs (ncRNAs) are a broad spectrum of functional RNA molecules that are transcribed from DNA but not translated into proteins [1]. The discovery of microRNAs (miRNAs) in 1993 followed by developments and discoveries in small RNA biology have hinted the importance of RNA in the post-transcriptional regulation of genes, especially in eukaryotes [2, 3]. With the development of highthroughput RNA sequencing technologies and bioinformatics methods, thousands of new ncRNAs have been discovered. Circular RNAs (circular RNAs, circRNAs) are a novel class of highly conservative endogenous ncRNA generated by premRNA back splicing, which is characterized by a covalently closed-loop structure (**Figure 1**) [4]. It was originally discovered that circRNAs are universally present in human and mouse, but then they were found to be common across essentially all eukaryotes [5]. Although most circRNAs are generally expressed at low levels, some of them are more abundant than their linear counterparts and often exhibit

**Figure 1.** *Back-splicing and canonical splicing of a single pre-mRNA.*

cell-type-specific and tissue-specific patterns [6, 7]. CircRNA levels are highly enriched in the brain, both embryonic and adult stage, and many of them play important roles in synaptic plasticity and neuronal function [8, 9]. Accumulating data suggest key roles for circRNAs in Alzheimer's disease (AD) by affecting mechanisms such as neuronal plasticity, autophagy, apoptosis, and inflammation [10]. This review covers the expression and function of circRNAs in the brain, as well as their role involvement in initiation and progression of AD.
