microRNA expression in the eyes and their significance in relation to functions

microRNAs (miRNAs) are endogenous, small, non-coding, regulatory RNAs, approximately 22 nucleotides (nts) in size. Since the first discovery of miRNAs in 1993 in Caenorhabditis elegans, miRNAs have been shown to be widely expressed in metazoans and plants in tissue-specific and developmental stage-specific manners. miRNAs target their downstream messenger RNAs (mRNAs) by base pairing to their target sites with sequence complementarity, mainly in the 3' untranslated region (UTR), and induce the breakdown of the targeted mRNAs and/or inhibition of translation from the mRNAs. Approximately 30% of the protein-coding genes are estimated to be regulated by miRNAs. One miRNA can target hundreds of downstream target mRNAs, while one mRNA can be targeted by multiple miRNAs. miRNAs have been recognized as a major level of post-transcriptional regulation of the fine-tuning of gene expression, playing important roles in cellular proliferation, differentiation, and cell death and are involved in all aspects of the biological processes investigated thus far. Mutations in miRNAs and/or the target sites in the transcripts of their downstream target genes and dysregulation of miRNA biogenesis can result in a wide variety of diseases, including cancers. In the past few years, we and other groups have investigated miRNA expression in the retina and other ocular tissues. The miRNA transcriptomes of the retina, lens, and cornea have been established. Many miRNAs showed unique tissue-specific and developmental stage-specific expression patterns, suggesting potential unique functions in the retina and other ocular tissues. In spite of this progress, the roles of retinal and other ocular miRNAs in the development, normal functions, and diseases of the retina and other ocular tissues are still largely unknown. This review aims to provide an extensive summary of the current status and recent progress of our understanding of the miRNAs in retinal and other ocular tissues and speculate on future directions to define the roles of miRNAs in the retina and other ocular tissues and diseases.