Suppression of retinal degeneration in Drosophila by stimulation of ER-associated degradation

Mutations in the rhodopsin gene that disrupt the encoded protein's folding properties are a major cause of autosomal dominant retinitis pigmentosa (ADRP). This disease is faithfully modeled in Drosophila where similar mutations in the ninaE gene, encoding rhodopsin-1 (Rh-1), cause ER stress and dominantly trigger age-related retinal degeneration. In addition, mutant flies bearing certain ninaE alleles have dramatically reduced Rh-1 protein levels, but the underlying mechanism for this reduction and significance of its contribution to the ADRP phenotype remains unclear. To address this question, we specifically analyzed the role of Drosophila genes homologous to the known yeast and animal regulators of the ER-associated degradation (ERAD) pathway, a process that reduces levels of misfolded proteins in the ER through proteasomal degradation. We found that loss-of-function of these putative ERAD factors resulted in increased levels of Rh-1 in ninaE mutant flies. Conversely, in an ER stress assay where mutant or wild-type Rh-1 were overexpressed in developing imaginal discs beyond the ER protein folding capacity of those cells, co-expression of certain ERAD factors was sufficient to reduce Rh-1 protein levels and to completely suppress ER stress reporter activation. Significantly, those ERAD factors that specifically reduced misfolded Rh-1 in the imaginal disc assay also delayed age-related retinal degeneration caused by an endogenous ninaE allele, indicating that ERAD acts as a protective mechanism against retinal degeneration in the Drosophila model for ADRP. These results suggest that manipulation of ERAD may serve as a powerful therapeutic strategy against a number of diseases associated with ER stress.