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J. D. Wisard, T. Waxweiler, W. Waxweiler, C. J. Johnson, C. M. Donmoyer, C. M. Craft, G. I. Liou, J. M. Nickerson; Apoptosis and Additional Phenotypes in the IRBP Knockout Mouse. Invest. Ophthalmol. Vis. Sci. 2009;50(13):3002.
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© ARVO (1962-2015); The Authors (2016-present)
Interphotoreceptor retinoid binding protein (IRBP) may protect the retina from damaging levels of retinoids/fatty acids. IRBP knockout (KO) mice experience a slow retinal degeneration that continues throughout life. To elucidate a functional role of IRBP in retina development and determine by which mechanism the slow degeneration transpires in IRBP KO mice, we examined: 1) when the degeneration begins; 2) which cells are lost; 3) where loss occurs; and 4) changes in eye shape/size/dimension.
IRBP KO and C57BL/6 (WT) mice were sacrificed from Postnatal(P) day 2 to 30 and eyes embedded in plastic or paraffin (n=3-6). Sections were cut through the center of the globe, to the optic nerve head on a superior/inferior plane. P2-P30 paraffin embedded eyes were used for TUNEL. P30 eyes were used for cone staining with LUMIj anti-cone arrestin antibody and for total globe circumference by measuring the outside edge of the cornea and continuing around the sclera. P2-P30 Plastic embedded eyes were used for counting INL nuclei and calculating the circumference of the entire retina by measuring length of the OPL. Results were analyzed with statistical t-tests.
From P2 to P12, high numbers of apoptotic cells were equally observed in retinas of WT and KO mice, consistent with known developmental remodeling. From P14 to P20, fewer apoptotic nuclei were found. At P22/23 and peaking at P25, TUNEL positive ONL cells in the IRBP KO were significantly greater than in the WT. Apoptosis sufficiently accounted for the measured 40% loss of photoreceptor cells in the KO. KO mice at P30 had ~16% fewer cone arrestin stained cells and 18% fewer INL nuclei than the WT. Beginning at P10, IRBP KO retinas were ~1000µm longer than the WT, with the KO still having a significantly longer pathlength/larger total globe circumference at P30.
Apoptosis accounts for photoreceptor cell loss in the IRBP KO mouse retina. In the KO, apoptosis of the ONL began at P22/P23, climaxed at P25, and continued to P30, in addition to a 20% loss of cone photoreceptors. Starting at P10, loss of IRBP caused an increase in the circumference of the retina even though 18% fewer cells were observed in the INL. The increased circumference suggests an increase in the radius of the globe, or an oblong/oblate/misshapen globe. Testing is in progress using photo-refraction and keratometry. Abnormal growth from P10-P30 and death of retinal cells shortly after weaning suggest a developmental role for IRBP and its potential ligands.
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