The endoplasmic reticulum (ER) is the cell organelle where secreted and membrane proteins are synthesized and folded. This process requires the recruitment of ribosomes, translocation of the nascent peptides into the lumen of the ER, and a variety of post-translational modifications and folding events. When the folding capacity of the ER is impaired, the presence of misfolded proteins in the ER causes stress to the cell (“ER stress”) and activates a cellular response, the Unfolded Protein Response (UPR), to restore homeostasis in the ER. The UPR is mediated by several signaling pathways, which sense stress in the ER and activate a variety of cellular responses, such as, translational attenuation, to reduce protein synthesis and prevent further accumulation of unfolded proteins and the transcriptional upregulation of genes encoding ER chaperones and enzymes, to increase the folding capacity of the ER. However, in situations where ER stress is severe or prolonged, or when the cellular responses induced by UPR are not sufficient to overcome the origin of ER stress, cells can undergo programmed cell death (Apoptosis).
Retinitis pigmentosa (RP) is a major cause of human blindness. In this disease, the photoreceptor cells in the eye progressively degenerate over time. About 30% of autosomal dominant RP cases are caused by mutations in Rhodopsin, the light sensitive protein of photoreceptors. In Drosophila, equivalent mutations in ninaE (the gene encoding Rhodopsin 1), also cause dominant degeneration of the retina and most of these mutations produce misfolded forms of Rhodopsin 1, which are not properly processed and accumulate in the ER.
We have shown that one of the branches of the UPR, the IRE1/Xbp1 signaling pathway has a protective role against ninaE induced photoreceptor degeneration. We are currently using the tools of modern genetics, cell biology and imaging to investigate the genes downstream of the IRE1/Xbp1 signaling pathway that regulate photoreceptor degeneration.
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