Mutations in the genes necessary for the structure and function of vertebrate photoreceptor cells are associated with multiple forms of inherited retinal degeneration. Mutations in the gene encoding RHO (rhodopsin) are a common cause of autosomal dominant retinitis pigmentosa (adRP), with the Pro23His variant of RHO resulting in a misfolded protein that activates endoplasmic reticulum stress and the unfolded protein response. Stimulating macroautophagy/autophagy has been proposed as a strategy for clearing misfolded RHO and reducing photoreceptor death. We found that retinas from mice heterozygous for the gene encoding the RHO variant (hereafter called P23H) exhibited elevated levels of autophagy flux, and that pharmacological stimulation of autophagy accelerated retinal degeneration. In contrast, reducing autophagy flux pharmacologically or by rod-specific deletion of the autophagy-activating gene Atg5, improved photoreceptor structure and function. Furthermore, proteasome levels and activity were reduced in the P23H retina, and increased when Atg5 was deleted. Our findings suggest that autophagy contributes to photoreceptor cell death in P23H mice, and that decreasing autophagy shifts the degradation of misfolded RHO protein to the proteasome and is protective. These observations suggest that modulating the flux of misfolded proteins from autophagy to the proteasome may represent an important therapeutic strategy for reducing proteotoxicity in adRP and other diseases caused by protein folding defects.

The P23H variant of rhodopsin results in misfolding of the protein, and is a common cause of the blinding disease autosomal dominant retinitis pigmentosa (adRP). We have recently demonstrated that degeneration of photoreceptor cells in retinas of P23H mice is due to the endoplasmic reticulum stress (ERS)-induced activation of autophagy that leads to a secondary proteasome insufficiency and activation of cell death pathways. We propose that this increased level of autophagy flux relative to proteasome activity, which we term the A:P ratio, represents a marker of altered photoreceptor cell homeostasis, and that therapies aimed at normalizing this ratio will result in increased photoreceptor cell survival. To test this postulate, we treated P23H mice with a chemical chaperone (4-phenylbutyric acid) to improve rhodopsin folding, or with a selective phosphodiesterase-4 inhibitor (rolipram) to increase proteasome activity. P23H mice treated with either of these agents exhibited reduced ERS, decreased autophagy flux, increased proteasome activity, and decreased activation of cell death pathways. In addition, rates of retinal degeneration were decreased, and photoreceptor morphology and visual function were preserved. These findings support the conclusion that normalizing the A:P ratio, either by reducing the ERS-induced activation of autophagy, or by increasing proteasome activity, improves photoreceptor survival, and suggest a potential new therapeutic strategy for the treatment of adRP caused by protein folding defects.