Reduction of phagosomes in the vitiligo (C57BL/6-mivit/mivit) mouse model of retinal degeneration.

PURPOSE: The vitiligo (C57BL/6-mivit/mivit) mouse has a slowly progressing retinal degeneration, in which photoreceptor cell nuclei are gradually lost and the retinal pigment epithelium (RPE) is unevenly pigmented. The purpose of the present study was to assess the phagocytic ability of the RPE in the vitiligo mouse by determining whether and when a phagocytic burst occurs in affected mice and whether the number of phagosomes varies between control and affected animals. METHODS: Eyes of control and vitiligo mice 4 to 20 weeks of age were embedded in Spurr. Thin sections were cut and examined by electron microscopy to confirm the presence of phagosomes, particularly in the affected animals. Thick (1 micron) sections were cut, and quantitative morphometry was performed at the light microscope level. The length of RPE was determined, and phagosomes were counted in RPE cytoplasmic and microvillous areas. Data were expressed as phagosomes per 1000 microns. RESULTS: The vitiligo mouse has a peak phagocytic episode approximately 2 hours after light onset. The number of phagosomes in 4-week-old affected mice was significantly less than that in controls (13 phagosomes per 1000 microns compared to 30 phagosomes per 1000 microns). By week 8, the number was reduced to approximately 5 per 1000 microns. Phagosome number was not reduced further between weeks 8 and 20 in the affected animal. Macrophage-like cells containing pigment granules and phagosomes were observed in the subretinal space in areas where the rod outer segments had been separated from the RPE. CONCLUSIONS: The vitiligo mouse RPE contains phagosomes, but there are significantly fewer than in controls. It is not known whether a defect in RPE phagocytosis is the direct cause of the retinal defect in this model.

Effects of dark-rearing on the retinal degeneration of the C57BL/6-mivit/mivit mouse.

The C57BL/6J-mivit/mivit mouse has a retinal degeneration in which photoreceptor cells are lost slowly at a rate of about one row per month beginning at 8 weeks. ROS are severely disrupted by 4 months, but inner segments maintain a normal length through 6 months. In addition to photoreceptor changes, the retinal pigment epithelium is unevenly pigmented. The present study utilized histological and biochemical techniques to assess the effects of dark-rearing on the progression of the retinal degeneration in the mivit/mivit mouse at ages 4, 6, 8, 12, 16, 20, 24, and 28 weeks. Results of systematic morphometric evaluation indicated that the rate of loss of photoreceptor cells did not differ significantly from the rate determined for mivit/mivit animals reared in a standard light cycle. Furthermore, retinal detachment from RPE, the displacement of darkly-staining cells into the subretinal space and the influx of macrophage-like cells in the area of the ROS were still present in mivit/mivit animals reared in darkness. ROS of mivit/mivit seemed to be preserved for a slightly longer period of time in the dark-rearing condition. Rhodopsin levels in 4-week dark-reared mivit/mivit mice were 0.32 +/- 0.04 nmol per retina which was comparable to mivit/mivit mice reared under standard lighting conditions. At 20 and 28 weeks, rhodopsin levels decreased in mivit/mivit retinas to a similar level regardless of their lighting history. The findings of the study suggest that light deprivation does not retard the degeneration mivit/mivit retina. Results are discussed in comparison with effects of dark-rearing on other models of retinal degeneration.

Opsin synthesis in the C57BL/6-mivit/mivit mouse model of retinal degeneration.

The capacity of photoreceptor cells to synthesize opsin was evaluated in a newly-described mouse model of retinal degeneration, the C57BL/6-mivit/mivit. The mivit/mivit mouse loses photoreceptor cells at a rate of about one row per month beginning at 8 weeks, ROS are severely disrupted at 4 months, RPE is unevenly pigmented. Retinas of affected and control mice ages 4, 6, 8, 12, 16, 20, 24, 28, 32 and 52 weeks were incubated for 2 hours in medium containing [3H] leucine. Homogenates of retina samples were subjected to SDS-PAGE using disc gels. The gels were sliced and counted by scintillation. The incorporation of [3H] leucine into opsin was compared with its incorporation into other retinal proteins. During the early time points studied, mivit/mivit retinas incorporated proportionately similar amounts of [3H] leucine into opsin versus other retinal proteins as did controls. At 12 weeks, the percentage was about 80% and it continued to decline over the succeeding weeks studied. By 1 year, the proportion of leucine incorporated into opsin versus other proteins was only about 23% the amount incorporated in controls. The results of the present study suggest that the mivit/mivit photoreceptor cells are able to synthesize opsin and the gradual decline in synthetic ability follows the gradual loss of cells and is not correlated with the disruption of ROS.