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A Reichenbach, H Wolburg, B Biedermann, I Iandiev, K Franze, O Uckermann, A Bringmann, H Schrewe, A Gehrig, BH F Weber; Müller Cells in the Retina of Mice with a Targeted Inactivation of the Murine X-linked Juvenile Retinoschisis Gene, Rs1h . Invest. Ophthalmol. Vis. Sci. 2002;43(13):3662.
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Purpose: X-linked juvenile retinoschsis (RS) is thought to involve anomalies of the Müller cells, the principal retinal glia. To prove this hypothesis, we studied the Müller cells in knock-out mice deficient in Rs1h, the murine ortholog of RS1, encoding retinoschisin. Methods: The retinae of hemizygous male mice were assessed by histology, transmission and freeze-fracture electron microscopy and immunofluorescence labeling. Results: Histological sections of Rs1h-/Y retinae revealed local splitting of the inner nuclear layer as the key feature of retinoschisis. In addition to degenerative changes in several types of retinal neurons, severe alterations of the Müller cells were observed. The most striking feature was the occurrence of elements of the 'outer limiting membrane' in the inner retinal layers and also adjacent to the intraretinal cleft. Zonulae adherentes and microvilli were scattered between the Müller cell endfeet, at some places less than one micrometer distant from the vitread basal lamina. Some of the Müller cell endfeet were atrophic. Generally the Müller cell stem processes were very thin and appeared fragile though they contained many intermediate filaments. Immunocytochemistry revelaed the expression of glial fibrillary acidic protein as marker for glial reactivity, and an increased expression of vimentin while the beta-tubulin content of the cells was apparently normal. In the Rs1h-/Y retinae the Müller cells showed an up-regulation and re-distribution of the water channel protein, aquaporin-4, and of the inward-rectifying K+ channel proteins, Kir2.1 and Kir4.1. Freeze-fracture studies revealed the presence of a high density of orthogonal particle arrays in the vitread endfoot membrane of Müller cells. Conclusions: Our findings in the knock-out mouse model suggest that retinoschisin may be required to organize the polarity of Müller cells. Furthermore, the Müller cells display severe changes in their ultrastructure, as well in the expression of functionally important membrane proteins. These changes may be involved in the development of the clinical phenotype.
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