The human retina, as a part of the central nervous system, is distinctively designated to the initiation of the visual processing. In accordance with its complex anatomy and physiology, a broad number of coding and noncoding regions of the genome have been implicated in the heterogeneous pathogenesis of several retinal degenerative diseases, i.e. monogenic disorders, such as the various Mendelian-inherited forms of retinitis pigmentosa (RP), and multifactorial polygenic/environmental disorders, such as age-related macular degeneration (AMD) and pathologic myopia (PM). Both occurrence and progression of these chronic vision-threatening diseases can be influenced by genetic and epigenetic factors, affecting different retinal and sub-retinal structures: the neural retina (NR), composed by the inner neurosensory layer with its vascular capillary nets and the outer photoreceptor cells (PRCs) layer with the underlying retinal pigment epithelium (RPE), which is separated from choriocapillaris by a modified basement stratum named Bruch’s membrane (BM), while the supporting glial Müller cells span the NR from external to inner limiting membranes sealing the axonal fibers of the retinal ganglion cells from the vitreous. The diverse retinal degenerations primarily involve a specific target-structure: (i) RP phenotypes are primary caused by damages of PRCs; (ii) AMD lesions depend on degenerative RPE modifications; (iii) PM-related scleral expansion irreversibly modifies the RPE-BM complex. Both health and function of the human retina rely on a collaborative interaction among diverse classes of molecular regulators. Starting from 1993, small non-coding endogenous RNAs of approximately 21-25 nucleotides in length, named microRNAs (miRNAs), have been recognized as key factors of posttranscriptional gene regulation in mammalian genomes. At present, our knowledge about either origin or functions of circulating exosomal miRNAs is going to be implemented, and there are several investigative methods that allow the identification of miRNAs contribution to heterogeneous disorders. The perceived opportunity appears especially large in neoplastic diseases, even if the evaluation of diagnostic specificity and reproducibility of miRNAs assessment remains a work in progress. In the last years, transcriptome analyses have documented the presence of several miRNAs expressed in the human adult retina, indicating the pivotal role of miRNAs as regulators for homeostasis, function and survival of the differentiated cell types at the level of mature retinal and sub-retinal tissues. The demanding physiologic tasks of the retina postulate the action of a continuous gene regulation to render its perennial post-mitotic cells less vulnerable to premature damage or death secondary to the most frequent causes of irreversible low-vision and blindness, i.e. AMD, PM, and RP. Moreover, because miRNAs are involved in transferring inflammatory signals among cells, they may have a crucial prognostic value in the above-mentioned diseases, whose modalities of onset and/or progression are invariably related to complex para-inflammatory processes. Particularly, in Alzheimer’s disease and AMD, a group of five miRNAs (miRNA-9, -34a, -125b, -146a and -155) has been found up-regulated during several independent experimental tests, indicating the presence of progressive inflammatory damages in both pathologic patterns of these age-related neurodegenerative disorders. In an all-embracing view, this scenario highlights the potential relevance of miRNAs to advance the clinical management of the most severe retinal diseases. Notionally, the pathogenic mechanisms of each different retinal disorder express specific miRNA-genes inside pathologic cells. Part of these expressed miRNAs are then secreted by exosomes and become circulating miRNAs, which can be detected in patients’ serum samples as biomarkers of vision-threatening diseases. It has been shown that altered miRNA expression patterns in the human fluids might be the result of various eye disorders. Recent studies are beginning to document the possible role of circulating miRNAs as biomarkers for both pathologic conditions and severity of disease progression. It is very challenging to approach the question whether each cell type of the retinal complex has its own miRNA phenotype, since one miRNA can target different gene products that might be expressed in different microstructures. Although some patterns of miRNAs expression have been tentatively labeled as distinctive signatures of specific retinal cell types, no unequivocal data have been hitherto obtained in patients on the correlations between dysregulated miRNA profiles and pathologic retinal phenotypes, i.e. AMD, PM, and RP. The miRNAs identified in human serum and plasma are known to be relatively stable, as they have been found to be resistant to RNAase degradation, even in stored samples. This remarkable extracellular stability has made miRNAs desirable candidates for epidemiological and clinical studies, indicating their potential role as non- or minimal-invasive biomarkers particularly because small serum or plasma samples are needed for miRNAs profiling. However, despite a multitude of investigations of basic research, there are only few independent validation studies on blood-born miRNAs as disease biomarkers. Toward clinical applications in ophthalmology numerous obstacles still need to be overcome, starting from the lack of data resulting from trials conducted on homogeneous study groups. Because no study has been hitherto conducted in patients with retinal degenerative disorders to assess the degree of intra-individual consistency in correlating the clinical changes in diseases’ expressivity with putative modifications of miRNAs profile, this comparative approach of investigation should be recommended to advance our levels of knowledge and to provide concrete answers about the translational potentialities of miRNAs, as recently advocated by Andreas Keller and Eckart Meese emphasizing a very challenging question: "Can circulating miRNAs live up to the promise of being minimal invasive biomarkers in clinical settings?". Circulating miRNAs vary in their expression and concentration in serum samples, and these variations indicate the potential role of miRNAs as biomarkers of pathologic status vs. health condition. Indeed, several investigations have been recently conducted on patients suffering from cancers, infections, cardiovascular, metabolic, neurodegenerative or genetic diseases, in which miRNAs were found variably dysregulated compared to those detected in healthy controls. Expression of circulating miRNAs may change in the course of the pathologies exemplified above and/or their complications. In this context, it is important to note that, during the period of illness, circulating miRNAs can be modified either at the level of their panel expression or at the single-miRNA concentration. The detection of dysregulated circulating miRNAs could have application in early diagnosis and progression of retinal degenerations, as already shown for other degenerative disorders of the central nervous system. Just for example, starting from the available knowledge about a multifactorial degenerative disease characterized by both environmental and gene-related pathogenetic mechanisms such as AMD, the future steps for translational miRNAs application might be focused on the ability of correlating differences in miRNAs profiles to: (i) status of AMD patient versus matched healthy control; (ii) morpho-functional deteriorations of RPE cells and/or PRCs layers characterizing the dry AMD forms; (iii) occurrence of choroidal neovascularization complicating the wet AMD forms. Considering both technology and cost of the analyses, the miRNAs detection in serum samples of patients with disorders of neural ocular structures has the potential to become an affordable approach. It may be especially helpful when the eye disease is asymptomatic or paucisymptomatic and/or when it is characterized by critical patterns of heterogeneity. In these cases, the detection of one or more dysregulated circulating miRNAs could be used as a specific parameter to facilitate the phenotypic and/or genotypic interpretation of the disease, and the ophthalmologists may benefit of these new diagnostic parameters/tools. At the same time, miRNAs are now investigated as potential target for innovative therapies. One of the most promising experimental strategies foresees the use of liposome-like structures loaded with anti-sense miRNAs when a specific miRNA is over-expressed. On the other hand, the same strategy with liposomes loaded with miRNAs can be used when a specific miRNA is down-expressed, with the aim to re-establish the normal level of miRNAs. It is noteworthy to recall that miRNAs play an important role during the expression process of different genes. It has been shown that more than 2,000 genes encode for miRNAs which, in turn, recognize distinct and multiple mRNAs as targets. This biological process is crucial for the cell because the protein production is affected by the amount of mRNA, which is translated at the ribosomes level. In this context, the protein concentration and bioavailability into the cell is determined by the accessible amount of the specific mRNA. More recently, it has been shown that miRNAs can be "secreted" by cells with exosomes, which are liposome-like particles filled of specific miRNAs and circulating into the blood stream. This loop resembles the so-called autocrine and paracrine loops, employed by the cells for their cell-to-cell talks with growth/differentiation factors. It represents one the main mechanism used by a living cell to control its development and functions, as well as to maintain its homeostasis. Similarly, miRNAs are released by cells with exosomes to reach new cells, in distant or near tissues, using the blood/serum/plasma as a vehicle. This mechanism ensures a sort of equilibrium mediated, at distance, by miRNAs. In case of altered equilibrium, a specific pathology may occur. This alteration can be used to recognize a serum miRNA as parameter/biomarker for diagnostic and/or prognostic purposes. Indeed, by improving our understanding about the pathogenesis of AMD, PM and RP, together with the clinical laboratory-based screening and the forthcoming development of new therapeutic strategies, it will be possible to ameliorate the clinical management of numerous patients at high risk of low-vision and blindness. In the near future, new miRNA-based assays, which are highly sensitive and specific, might be able to attain a "customized" classification of the different eye diseases. The study of miRNA biomarkers implicating in pathogenesis, diagnosis and prognosis of retinal degenerations represents a translational topic of increased research interest, especially because miRNAs themselves might be considered as therapeutic targets or even therapeutic agents as anti-miRNAs. In view of these elements of miRNA-related exploratory context, additional studies will help us in assessing miRNAs potential as upstream regulators and/or downstream targets of the common vision-disabling diseases.
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|Titolo:||MicroRNAs as potential biomarkers and innovative therapeutic targets in retinal degenerations|
|Data di pubblicazione:||2016|
|Appare nelle tipologie:||03.1 Articolo su rivista|