Lactose supports Muller cell protein expression patterns in the absence ofthe retinal pigment epithelium

PURPOSE: We have previously shown that lactose, but not mannose, promotes t he assembly of nascent photoreceptor outer segments in the absence of the r etinal pigment epithelium (RPE). The purpose of the present study was to de termine if, in addition to the improved outer segment assembly observed in the presence of lactose, biosynthetic changes in Muller cells could also be detected. METHODS: The RPE was removed from intact isolated Xenopus embryonic eyes th at were allowed to complete differentiation in Niu-Twitty medium, Niu-Twitt y with mannose, or Niu-Twitty with lactose. Control retinas matured in vitr o with an adherent RPE. Retinal morphology was evaluated for organized fold ing of outer segment membranes and cell loss. In addition, the expression o f three Muller cell proteins, glial fibrillary acidic protein (GFAP), cellu lar retinaldehyde-binding protein (CRALBP), and glutamine synthetase, was e xamined. RESULTS: In control retinas, GFAP is undetectable, CRALBP heavily immunolab els Muller cells, and radial patterns of glutamine synthetase immunoreactiv ity are present. In the absence of the RPE, Muller cells upregulate GFAP ex pression, CRALBP labeling is present at a slightly reduced level, and gluta mine synthetase immunolabeling is negligible. Neither mannose nor lactose m odify significantly the expression of CRALBP. Similarly, both compounds com pletely prevent the upregulation of GFAP. However, normal glutamine synthet ase expression was observed only in the presence of lactose, but not in the presence of mannose. Statistical analyses of slot blot-based protein quant ification confirmed our immunochemical results. CONCLUSIONS: In RPE-deprived retinas supplemented with lactose, Muller cell s were morphologically normal. The proper photoreceptor outer segment morph ogenesis observed under these conditions was uniquely associated with norma l levels of glutamine synthetase expression. The exact significance of this finding with respect to photoreceptor outer segment morphogenesis is unkno wn. We suggest that glutamine synthetase may be a marker of Muller cell met abolic or structural integrity that may reflect the enhanced ability of the se cells, in the presence of lactose, to support photoreceptor outer segmen t morphogenesis.