In:
Proceedings of the National Academy of Sciences, Proceedings of the National Academy of Sciences, Vol. 110, No. 6 ( 2013-02-05)
Abstract:
In addition to retinal defects, miR-183/96/182 –mutant mice exhibited a circling behavior typical of defects in vestibular organs, suggesting the presence of syndromic sensory defects and that the requirement for the miR-183/96/182 gene for normal functions of sensory organs other than the retina. Studies on genetic variations around miR-183/96/182 gene are warranted in patients with progressive, sensory syndromic retinal dysfunction and other neurological conditions. In mouse, both rod and cone photoreceptor differentiation and synaptogenesis occur postnatally starting from postnatal day 4–5 ( 4 ), coincident with developmental up-regulation of the miR-183/96/182 gene ( 1 ). Our data suggest that miR-183/96/182 is required for postnatal differentiation of photoreceptors and establishment of fully functional synaptic connections with their downstream neurons by directly and indirectly regulating genes important for synaptic functions and the phototransduction pathway. Inactivation of the miR-183/96/182 gene disrupts the homeostasis and functions of photoreceptors, leading to progressive retinal dysfunction and degeneration ( Fig. P1 E ). Many genes known to be involved in phototransduction, a process by which light signals are transformed into neuroelectrical impulses in photoreceptors, also were dysregulated. We confirmed that the expression of red/green opsin was decreased significantly. Quantification of red/green opsin-positive and total cones in the whole-mount retina showed that both the number and the percentage of red/green opsin-positive cones were decreased significantly in the superior retina, but in the inferior retina the total number of cones and the number of red/green opsin-positive cones did not change significantly, suggesting that red/green cones were the most affected photoreceptors and that the greater vulnerability of red/green cones contributed to more severe defects in photopic ERGs and more degeneration in the superior retina in miR-183 / 96/182 –mutant mice. Gene-expression profiling revealed global gene-expression changes in retinas of 5-wk-old miR-183/96/182 –mutant mice. Genes important for synaptogenesis, nerve-impulse transmission, and synaptic remodeling were enriched significantly in the dysregulated genes, consistent with ERG data indicating abnormal synaptic transmission of photoreceptors. Electron microscopy and immunostaining with synaptic markers confirmed that the sizes of synaptic ribbons were decreased significantly in the mutant mice as early as age 5 wk. A light-damage experiment showed significantly increased degeneration in the retinas of the mutant mice as compared with wild-type littermates after a 2-h exposure to 10,000-lx cool, white fluorescent light. In this experiment, as with aging, the superior retina was more damaged than the inferior retina, consistent with a recent report in a miR-183/96/182 knockdown mouse model ( 2 ). Histologically, despite significant ERG abnormalities, retinas of 5-wk-old mutant mice were indistinguishable from littermate controls. By 6 mo, however, the mutant mice showed significant retinal degeneration, which progressed at 1 y of age ( Fig. P1 D ) . Intriguingly, the superior retina, which is enriched with red/green cones, was more affected than the inferior retina, which is enriched with blue cones. To study the functions of miR-183/96/182 , we generated a mouse model in which the miR-183/96/182 gene is inactivated. Functionally, inactivation of the miR-183/96/182 gene resulted in early-onset, progressive electroretinogram (ERG) abnormalities. The most prominent defect, decreased b -wave amplitude, was common to both dark-adapted and light-adapted ERGs in the mutant mice ( Fig. P1 B and C ) and was detectable as early as 5 wk of age. The ERG b wave is derived mostly from activities of secondary neuron-bipolar cells; defects in bipolar cells and photoreceptor synaptic transmission can produce defective b waves ( 3 ). Because miR-183/96/182 is expressed predominantly in photoreceptors, reduction in b -wave amplitude likely reflects synaptic transmission defects in photoreceptors of miR-183/96/182 –mutant mice. The amplitude, latency, and implicit time of the a wave, which reflects the activities of photoreceptors, appeared normal in 5-wk-old mutant mice but became defective with age. In all parameters, light-adapted ERG was more affected than dark-adapted ERG, suggesting more severe defects in the cone pathways. We first dissected the genomic structure of the gene encoding miR-183/96/182 and showed that the miR-183/96/182 gene produces at least two primary RNA molecules, the transcripts ( Fig. P1 A ). The structure of the second transcript clearly positions miR-183/96/182 in the intron of the gene. The second transcript has four predicted protein-coding regions, suggesting that the miR-183/96/182 gene may encode one or more peptides. Previously, we identified a cluster of miRNAs—small, noncoding, regulatory RNAs—that appears to be evolutionarily conserved and specifically expressed in sensory organs ( 1 ). Expression of this miR-183/96/182 cluster (also referred to as “ miR-183/96/182 ”) is minimal in embryonic retina but increases dramatically after birth and peaks in adult retina with specific expression in photoreceptors ( 1 ). Reducing the expression of miR-183/96/182 in rod photoreceptors increased light damage to the mouse retina ( 2 ), but no histological or functional defects of the retina were observed under normal lighting conditions ( 2 ). Thus, in vivo functions of miR-183/96/182 in the retina remain uncertain.
Type of Medium:
Online Resource
ISSN:
0027-8424
,
1091-6490
DOI:
10.1073/pnas.1212655110
Language:
English
Publisher:
Proceedings of the National Academy of Sciences
Publication Date:
2013
detail.hit.zdb_id:
209104-5
detail.hit.zdb_id:
1461794-8
SSG:
11
SSG:
12
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