Defining the functional role of miR-210 in the visual system of Drosophila melanogaster and mammals

miR-210 is one of the most evolutionarily conserved microRNAs (miRNAs) and is involved in several physiological and pathological processes. Recently, new roles of this microRNA are emerging in eye and visual system homeostasis. Specifically, in Drosophila melanogaster, the loss of miR-210 has been shown to result in a gradual retinal degeneration, which was associated with lipid droplets accumulation and alterations in lipid metabolism. However, the possible conservation of miR-210 role in preserving mammalian retina homeostasis remained to be investigated. In this work, we further characterized the miR-210 KO fly model, and, at the same time, we generated and characterized a fly model in which miR-210 was overexpressed specifically at the level of the eye. We further investigated the reported alterations in lipid metabolism, dissecting both lipid anabolism and catabolism and identifying significant alterations in the expression levels of multiple genes within these pathways. We also looked for possible miR-210 target genes that could be responsible for these alterations, and we were able to identify a direct target of the microRNA, Acbp1. However, considering the substantial changes in various lipid metabolism pathways, which are unlikely solely caused by the effects of a single miRNA target gene, we speculated that multiple other target genes of miR-210, not solely associated with lipid metabolism, might be implicated. Additionally, it appeared that lipid droplet accumulation could be more of a secondary phenotype rather than the primary cause of the observed retinal degeneration. In accordance with this, we demonstrated that, when subjected to starvation, the triacylglycerols consumption in the heads of miR-210 KO flies was similar to that in wild type flies. This observation suggests the restoration of normal lipid metabolism in miR-210 KO flies under stressful conditions, indicating that lipid droplet accumulation does not overwhelm the organism's metabolic demands. In addition, to investigate the possible conservation of miR-210 role(s) in the mammalian retina, in this work we provide the first morphological characterization of the retinas of miR-210 KO and OE mice. Notably, in miR-210 KO mice we reported an ongoing photoreceptor degeneration, with an extensive disorganization in the ultrastructure of rod outer segments. To explore other potential parallels between miR-210 KO fly and mouse models, we examined their lipid metabolism and circadian behaviour, but no similarities were found. We therefore conducted an RNA-seq experiment on the retinas of miR-210 KO mice, which resulted in a small amount of DEGs, most of which linked to chloride channels activity and extracellular matrix structural constituents, that are known to play crucial roles in retinal physiology. Consistent with this, the direct or indirect regulation of miR-210 on these pathways may represent the cause for the retinal effects of miR-210 depletion. Conversely, another plausible scenario is that the retinal phenotype may result from an upstream disruption in central nervous system homeostasis. With this perspective in mind, we conducted an RNA-seq analysis on the brains of miR-210 KO and WT flies. We identified a substantial number of differentially expressed genes, with the downregulated genes predominantly enriched for the detection and transduction of light stimuli. This outcome suggests that the alterations observed in miR-210 KO flies extend beyond the eye and may be linked to neuronal deficiencies in signal detection and transduction. Additionally, among the upregulated genes, the few enriched pathways pertained to collagen, thereby reinforcing the possibility of a shared upstream mechanism that underlies the retinal degeneration induced by miR-210 KO in both fruit flies and mammals. These results encourage us to further investigate towards a complete understanding of the functional role of miR-210 in the mammalian visual system.