In the nervous system, the close association between neurons and glial cells suggests that gene expression in these cells is likely to be influenced by mutual interactions. We are specifically interested in understanding how neuron-glia interactions influence gene expression in retinal glial cells. Experiments recently carried out in our laboratory demonstrate that photoreceptor degeneration results in induction of the glial fibrillary acidic protein (GFAP) in Muller (glial) cells in the mouse retina, and that GFAP expression is regulated mainly at the level of transcription. The molecular mechanisms responsible for changes in transcriptional activity of the GFAP gene are not yet known. These could involve modifications such as methylation at the gene level, changes in chromatin organization, or both. We will map the DNase I hypersensitive sites that appear during GFAP gene activation. Changes in methylation pattern of the gene will also be examined in retinas with photoreceptor degeneration. DNA sequences from a GFAP genomic clone that we have isolated will be used in transfection assays with cultured Muller cells to define the cis-acting sequences needed for cell type-specific expression of GFAP in Muller cells. The developmental expression of GFAP gene will also be investigated. In addition, we will examine the role of macrophage-derived 'growth factors' in GFAP induction. These studies will be carried out by a combination of biochemical and in situ hybridization techniques. In order to facilitate the identification of Muller cells both in vivo and in vitro, we plan to obtain monoclonal antibodies that bind specifically to Muller cells. Since Muller cell dysfunction has been observed in retinal diseases such as cystoid macular edema and retinitis pigmentosa, a knowledge of the interactions between retinal neurons and Muller cells is fundamental to our understanding of the cellular basis of retinal diseases.
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