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Saumil Sethna, Patrick A Scott, Arnaud Giese, Mary A Johnson, Saima Riazuddin, Steven L Bernstein, Maureen A McCall, Zubair Ahmed; Usher Syndrome protein, CIB2, is essential for murine retinal sensory cells maintenance and function. Invest. Ophthalmol. Vis. Sci. 2017;58(8):270.
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© ARVO (1962-2015); The Authors (2016-present)
Previously, we identified missense allele of CIB2, encoding the calcium and integrin binding protein 2, responsible for Usher syndrome type I (autosomal recessive deaf-blindness) in a large human family. In the mouse retina, CIB2 expression was found within the retinal pigment epithelium (RPE) amongst other retinal cell types. Here, we developed and characterized a mouse model to understand the function and subcellular localization of the CIB2 in the eye.
Subcellular localization of CIB2 in the retina was determined by immunogold electron microscopy. Visual deficits were determined by temporal electroretinogram (ERG) at 1, 3, 6, and 9 mth in Cib2ko/ko and control mice. Light microscopy and transmission electron microscopy (TEM) on 8 mth old mice ascertained retinal pathology. We quantified the circadian, LC3-associated phagocytosis (LAP) of photoreceptor (PR) outer segments (OS) in the RPE, in situ, using localization of opsin-phagosomes in whole mount RPE. Statistics: One-way ANOVA or student’s t-test.
CIB2 antibody-coated gold particles were concentrated within RPE subcellular structures called retinosomes and within interphotoreceptor matrix (IPM). We found age-dependent decline in the ERG a- and b-waves amplitude between Cib2ko/+ and Cib2ko/ko mice as compared to WT. Light microscopy reveled no gross damage and thickness of OS and PR nuclear layers (p<0.05) were comparable in WT, Cib2ko/+, and Cib2ko/ko mice at 8 mths. However, at 8 mths, TEM micrographs showed accumulation of large vacuoles in RPE of Cib2ko/+ and Cib2ko/ko mutant mice but not in WT mice. In 3 mth old mice, at 8 and 12 hrs after light onset we found significantly more opsin-phagosomes in Cib2ko/+ and Cib2ko/ko mutant RPE cells as compare to WT, but similar number of phagosomes at 1 hr after light onset.
Loss of CIB2 leads to progressive blindness in mice without degeneration of PR due to dysregulation of LAP leading to aberrant visual cycle (VS). Expanded retinosomes, likely due to accumulation of VS byproducts, appear as vacuoles in TEM images. This is the first mouse model which replicates human USH retinal phenotype. Currently, we are investigating Ca2+ signaling pathways dysregulation in Cib2 mutant mice.
This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.
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