Inhibiting autophagy reduces retinal degeneration caused by protein misfolding

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^P23H 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 <i>Atg5</i>, improved photoreceptor structure and function. Furthermore, proteasome levels and activity were reduced in the P23H retina, and increased when <i>Atg5</i> 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.

脊椎动物感光细胞结构与功能必需基因的突变，与多种遗传性视网膜变性疾病相关。编码视紫红质（rhodopsin, RHO）的基因发生突变，是常染色体显性遗传性视网膜色素变性（autosomal dominant retinitis pigmentosa, adRP）的常见病因；其中RHO的Pro23His变异体可产生错折叠蛋白质，激活内质网应激与未折叠蛋白反应。此前有研究提出，激活巨自噬（macroautophagy）/自噬（autophagy）可作为清除错折叠RHO、减少感光细胞死亡的治疗策略。本研究发现，携带RHO^P23H变异体编码基因的杂合子小鼠（以下简称P23H小鼠）的视网膜组织中，自噬流（autophagy flux）水平升高；且通过药理学手段激活自噬，会加速视网膜变性进程。与之相反，通过药理学手段抑制自噬流，或通过视杆细胞特异性敲除自噬激活基因<i>Atg5</i>，则可改善感光细胞的结构与功能。此外，P23H小鼠视网膜中的蛋白酶体（proteasome）水平与活性均有所降低，而敲除<i>Atg5</i>后则可恢复升高。本研究结果表明，在P23H小鼠中，自噬会促进感光细胞死亡；而抑制自噬可将错折叠RHO蛋白的降解途径转向蛋白酶体，从而发挥保护作用。上述研究结果提示，调控错折叠蛋白从自噬途径向蛋白酶体途径的流转，或可成为降低adRP及其他蛋白质折叠缺陷相关疾病中蛋白毒性（proteotoxicity）的重要治疗策略。