The Emerging Roles of Non-Coding RNAs in Cataract

Non-coding RNAs (ncRNAs) are a large class of transcripts lacking evident protein coding potential, and play versatile roles in a diverse range of physiological and pathological processes. Mounting evidences have indicated that ncRNAs are aberrantly expressed in a wealth of diseases such as cataract. Cataract is a cloudy lens caused by radiation, age, drugs and other factors. NcRNAs, including microRNAs, long non-coding RNAs, circular RNAs, have been identified to regulate the occurrence and development of cataract. Current studies indicate that ncRNAs exert the multifaceted functions in the lens of cataract patients and have been proved as potential diagnostic biomarkers or therapeutic targets for cataracts. This review summarizes the study of rela-tionship between the lens and ncRNAs, which can provide a novel insight into the pathogenesis of cataract.


Introduction
In the coming decades, cataract and cataract blindness will continue being a leading public health issue in China due to the aging population [1]. Cataract is one of the main causes of blindness around the world. Many factors can increase the incidence of cataracts, such as inheritance, aging, radiation, injury, diabetes mellitus and medication [2] [3] [4]. Lens Opacification Classification System III (LOCS) is the most commonly used for evaluating. Cataract is divided into cortical cataract, nuclear cataract and posterior sub-capsular cataract. Phacoemulsification is a satisfactory method for the treatment of cataract, but there are also many postoperative complications [5] [6] [7]. Posterior capsular opacification (PCO) is the most common complication after a few years of surgery [8]. Therefore, the prevention and treatment of cataract has become particularly important. It is necessary to study the mechanism of age-related cataract (ARC), diabetic cataract (DC) and PCO objective to explore a new therapeutic target for cataract.
In the entire human genome, about 70 percent of the DNA are transcribed into RNA, but only 2 percent can be used to encode proteins. Although non-coding RNA does not encode a protein, such RNAs do contain information and function. These non-coding RNAs mainly include microRNAs, tsRNA (tRNA-derived small RNA), circular RNAs, long coding RNAs and pseudogene [9]. The main function of non-coding RNA include: to participate in the stability of mRNA and the regulation of translation level; to participate in the transport of protein; to participate in the procession and modification of RNA and to affect the structure of chromosomes [10]. Most of those definite functions are still unknown in cataract, so non-coding RNAs have been the focus of researchers.
Researches have shown that the abnormal expression of non-coding RNAs (miRNA, lncRNA, circRNA) can lead to the disorder of normal lens development, apoptosis of lens epithelial cells, the disorder of fiber cells and the transparency of the lens decreases, which leads to the mechanism of cataract [11] [12] [13].
In this review, we summarize the deregulations and discuss the pathological implications of miRNAs, lncRNAs and circRNAs in ophthalmology. We highlight the biological functions of non-coding RNAs and their targets, pathways in ARC, DC or PCO. Finally, we discuss the potential roles of ncRNAs as diagnostic tools and therapeutic targets in cataract.

The Function of microRNAs
MicroRNAs (miRNAs) are type of highly conserved and small, single-stranded non-coding RNAs about 20 -22 nucleotides, that regulate the expression of target genes at post-transcriptional level. Primary miRNAs (pri-miRNAs) processed sequentially via two ribonucleases Drosha and Dicer, which belongs the RNase III family, and transcribed from miRNA genes. Mature miRNA is packed into the RNA induced silencing complex (RISC) which can directs the complex to target mRNAs, giving rise to translational repression and target mRNA degradation, by binding to the 3'untranslated region (3'UTR) of mRNA in complementary sequences [14]. MiRNAs play an important regulatory role in physiological and pathological processes, which is widely found in animals, plants and viruses. Mounting studies have shown that miRNA dysregulation plays key roles in a plethora of diseases, including cataract. Certain well-known miRNAs (such as miR-31, miR-21, and miR-224-5p) have identified as master regulators of cell proliferation, differentiation, apoptosis, cell growth, stress response and autophagy [15] [16] [17]. Ample evidence has indicated that miRNA dysregulation impact the procession of cataract. In particular, miRNAs are involved in the regulation of function of lens epithelial cell [18].

The Expression and Function of miRNA in the Cataract
Current studies indicated miRNAs have identified as a group of regulators affect the cataract formation. The dysregulation of miRNAs has discovered in a diverse range of the lens, retina and other ocular tissues, and has related with poor prognosis in a wealth of clinical cases. Most studies compared differential miR-NA expression in cataract human lenses between cataract patients and normal subjects [11] [19] [20] [21] ( Table 1).
Taken together, miRNAs could repression of those targeted protein via combine the 3'untranslated region (3'UTR) of mRNA and regulation of the different function of lens epithelium cells, and then participate in the occurrence and development of cataract ( Figure 1).

The Structure and Function of lncRNAs
Long non-coding RNAs (LncRNAs) are bigger than 200 nucleotide-long RNA molecules, which lack or have limited protein-coding potential [26]. Most of lncRNAs are RNA polymerase II, whose transcription has polyA tail and 5'cap, mainly concentrated in the nucleus, which exhibit the level of evolutionary protection or expression is lower than that of mRNAs. LncRNAs regulate various aspects of gene expression via interacting with DNA, RNA or protein [27], and involved in the transcription, post-transcription of gene expression and epigenetic regulation [28]. LncRNAs play a vital role in cell differentiation regulation and many other life activities, as well as many diseases. LncRNAs are involved in regulating the ageing process, a large number of lncRNA have found to be involved in cell cycle, such as senescence proliferation and differentiation, as well as the regulation of important senescence related signaling pathways [29] [30], thereby contributing to the development of age-related diseases such as cataract.

The Expression and Function of lncRNAs in the Cataract
Emerging studies reveal that the dysregulation of lncRNAs are impact the development and progression of cataract [ [38]. The significant differential expression of lncRNAs in age-related cataract human lenses indicates that lncRNAs might play a crucial role in cataract formation (Table 2). Current researches have indicated lncRNAs exert multifaceted roles in various biological processes and disorders of cataract. Cytoplasmic lncRNAs regulate expression at the post-transcriptional level by "sponging" miRNA or interacting with RNA-binding proteins. Jin et al. demonstrated that miR-214 has a potential binding site for both KCNQ1OT1 and caspase-1 3'-UTR, KCNQ1OT1 acted as endogenous miRNA "sponge" to degrade the transcription of target genes and bind miR-214 to suppressed the miR-214 expression, consequently the HLECs impact pyroptosis, which is a type of programmed cell death by caspase-1 mediated via inflammatory cytokines [12]. LncRNA MIAT acts as a competing endogenous RNA (ceRNA) to sponge miR-150-5p and accelerates the proliferation and migration of HLECs [34]. LncH19 was up-regulated in ARC and act as ceRNA via H19/miR-29a/TDG pathway, which may be a promising target for the treatment [36]. Another study discovered that H19/miR-675 axis may affected CRYAA expression involved in the pathogenesis of nuclear ARC [33]. CRYAA encodes the predominant structural protein participate in refractive properties and the maintenance of lens clarity. The imbalanced expression of alpha-crystallin could accumulate damage and cripple its protective effect to the lens. Therefore, H19 could be a useful marker in ARC. LncPLCD3-OT1 may act as a ceRNA to regulate the expression of PLCD3 by combining with mi-224-5p and also plays a protective role in ARC [38]. Highly expressed TUG1 promotes lens epithelial cell apoptosis via sponging miR-421 [37]. LncRNAs also shown to regulate gene expression by interacting with a diverse range of proteins. As an example, the elevated KCNQ1OT1 positively regulated SMAD4 expression to promote LEC proliferation and EMT in PCO [31]. LncFEZF1-AS1 promoted the proliferation and migration via upregulating FEZF1 expression in PCO [39]. Upregulated MALAT1 can activate p38, a component of mitogen-activated protein kinases (MAPKs) pathway, resulting in HLECs apoptosis in DC. Collectively, aberrant expression of lncRNAs contribute to cataract via different regulatory mechanisms and signaling pathways (Figure 2).

The Structure and Function of circRNAs
Circular RNAs (circRNAs) are a novel class of non-coding RNAs generated from back splicing and the covalently closed loop structure linking the 3' and 5' ends. CircRNAs are commonly classified into three types: exonic circRNA, exon-intronic circRNA, and circular intronic RNA [40]. The pre-mRNA was changed from a chain structure to a ring structure, and then the introns in the ring were further cut to form circRNA which has broadly studied in recent years also can serve as useful markers in many cancers [41]. Although circRNA can't achieve biological function by encoding functional protein and have a low expression level, it revealed its work by combining with miRNA as molecule sponges [42]. The miRNA sponge effect of circRNA is only the unique function of some circRNAs, rather than the general characteristics of circRNAs. Some circRNAs molecules can also bind to RNA binding protein as protein sponges [43].

CircRNAs in the Cataract
Circular RNAs stably expressed in a variety of biological cells, and have tissue specificity and other characteristics. Circular RNAs participate in a wide range of developmental and physiological processes, including cell apoptosis, proliferation and differentiation. Accumulating evidence have proved their crucial regulation in respective biological processes and ocular diseases. Recently, the studies on circRNAs have shown that many circRNAs play roles as microRNA sponges, RNA-binding protein sponges, transcription regulators [44]. These roles give circRNA a great potential in biological applications. CircKMT2E acted as a sponge molecule of miR-204-5p and involved in the pathogenesis of DC [45].
Liu et al. reported that CircHIPK3 are the potential regulators to regulate HLECs function in cataractogenesis through miR-193a-mediated CRYAA expression [13]. CircHIPK3 acted as a molecular sponge to adsorb miRNA-193a to inhibit the production of crystalline proteins-CRYAA. CRYAA is a protein that keeps the crystal clearly. CircHIPK3 could regulate HLECs function provide a novel insight into ARC (Figure 3).

Conclusions and Future Perspectives
In conclusion, this review mainly summarized and analyzed the physiological and pathological effects of these non-coding RNAs in the lens. The expression of non-coding RNAs modulated the diverse proteins, RNAs and signaling pathways to impair the proliferation, migration, apoptosis, pyroptosis, autophagy and abnormal EMT of HLECs. ARC is commonly caused by apoptosis, pyroptosis, autophagy of the lens epithelial cell or unbalanced expression of crystalline proteins. However, PCO is commonly caused by proliferation, metastasis, and EMT of the lens epithelial cell. In DC, non-coding RNAs may be the etiology and inducing factors to trigger the cataract, resulting in its earlier occurrence and faster progress. With a growing number of ncRNAs being identified and characterized in human diseases, the emerging researches in the pre-clinical setting have shown the promise of ncRNAs as diagnostic and prognostic biomarkers. However, targeting ncRNAs in ophthalmology is still in its infancy, and improved techniques and approaches are expected in this field. In future efforts, focus should be put on a comprehensive molecular mechanism of ncRNA networks to decipher the precise roles of ncRNAs in cataract.