**2. Characteristics of circRNAs**

## **2.1 Abundance of circRNAs**

In the 1970s, circRNAs were first discovered in eukaryotic cytoplasm, but due to their low abundance and circular isoforms, these RNAs were perceived as mis-splicing occurrences [11]. With the advancement of high-throughput sequencing technology, circRNAs have been found to be abundant and widespread not only in metazoans including mice, Drosophila, and zebrafish but also in protists, fungi, and plants [7]. Although most of these covalently linked transcripts generally are expressed at low levels, in some cases, their abundance can exceed that of related linear mRNAs due to higher expression accumulation [12, 13]. For example, the expression of CDR1as within the brain is highly and independent of the expression of its linear isoform [14]. You et al. [15] discovered that the expression of some circRNAs exhibited strong upregulation in brain during development, and their expression independent of their host linear transcripts. Jeck et al. [16] found that the abundance of some circRNAs exceeded associated linear mRNA by >10-fold in human fibroblasts.

### **2.2 Stability of circRNAs**

CircRNAs are found mostly in the cytoplasm, and most undesired splicing products accumulate at the transcription site [17]. Because they do not have 5′-3′ polarities and polyadenylated tails, which make them much more stable than

*The circRNA and Role in Alzheimer's Disease: From Regulation to Therapeutic and Diagnostic… DOI: http://dx.doi.org/10.5772/intechopen.99893*

linear RNA and resistant to RNase R, an exonuclease that efficiently degrades linear RNAs [13]. The average half-life of circRNAs in cells exceeds 48 hours, while mRNAs only maintaine for the average of 10 hours [14]. Besides, circRNAs may also be sensitive to many other RNases, such as RNase A, RNase T1, and RNase T2 [18], which suggests that circRNA may serve as an ideal biomarker for a variety of disease. Recent studies have shown that circRNAs are enriched and stable in exosomes, which can be transported to distant tissues and organs via exosomes [19]. Moreover, circRNAs can be detected in blood cells like red blood cells, white blood cells, and platelets [20]. Besides blood, circRNAs can also be detected in other bodily fluids, such as saliva and seminal plasma [14, 21].

## **2.3 Profile and localization of circRNA**

The expression of circRNA has tissue specificity, subcellular location specificity and developmental stage specificity. It is reported that circRNAs in *Drosophila*, mice, and humans are highly enriched in the nervous system [22]. RNA sequencing of human adult and fetal tissues showed that up to 50% of circRNAs were tissue-specific and development-specific fashion, and the number and expression levels of circRNAs were higher in fetal tissue than adult tissue [23]. Notably, the expression of circRNAs within the brain is highly specific and increases during neuronal differentiation and development, which may be involved in brain diseases [24]. CircRNAs are mostly found in the cytoplasm, and many of them in neurons localized to axons, dendrites, and synaptosomes, which is modulated by neuronal activity [15]. Reports show that compared with total brain RNA, circRNAs in murine synaptoneurosome fractions and micro-dissected neuropil from hippocampal slices were more enriched in cytoplasmic RNA [25]. This is supported by the detection of circRNAs in cultured hippocampal neurons and hippocampal slices [15]. Moreover, some circRNAs show a regulated switch in their nuclear and cytoplasmic positioning during development [24].

### **2.4 Classification of circRNAs**

According to different combinations of sequences and domains, circRNAs can be divided into three categories: exonic circRNAs (EciRNAs), intronic circRNAs (CiRNAs) and exon-intron circRNAs (EIciRNA) (**Figure 2**) [26]. There are three hypothetical models explaining the formation of exonic circRNAs. Most circRNAs are formed by exon skipping during pre-mRNA transcription to produce specific regions, called lariat structures. Lariat structures contain exons, in which the intron sequence is then removed by splicing [27]. Other than exon skipping, due to the presence of reverse complement sequences in introns of pre-mRNA, circular structures can be formed by base-pairing between two introns, and some introns are then removed [28].

During the biogenesis of circRNAs, some RNA binding proteins (RBPs) are considered to participate in the circularization of circRNA, such as Quaking, Muscleblind and Fused-in sarcoma [29]. In some cases, during the formation of EciRNAs, introns that surround the exons are not removed, and EIciRNAs are generated [30]. The formation of CiRNAs depends on a consensus motif containing a conserved 7-nucleotide GU-rich motif at the 5′ splicing site and the 11-nucleotide C-rich motif at the 3′-branch site. it bypasses the action of the debranching enzyme, then generates a linear intron and form a circularized RNA lariat, leading to the production of ciRNA [9, 31]. EciRNAs mainly locate in the cytoplasm, which is the focus of current research, accounting for almost 80% of the total circRNAs [6]. CiRNAs and EIciRNAs locate in the nucleus, and regulate the expression of their parental genes [32].
