SORDD1介导的内质网相关蛋白降解在视网膜色素变性疾病中的机制研究

Retinitis pigmentosa (RP) characterized by abnormal function of optic rod neurons is one of the main hereditary retinal degenerative diseases that cause blindness. Although mutations in more than 50 genes have been identified in RP patients, there is effective treatment for RP. The accumulation of misfolded rhodopsin in endoplasmic reticulum (ER) and associated ER stress caused by the mutations of the rhodopsin gene are the most common cause of RP. In response, photoreceptor neurons can degrade misfolded rhodopsin accumulated on ER through Endoplasmic Reticulum Associated protein Degradation (ERAD) pathway, thereby relieving the ER stress and maintaining normal function of cells. We established a retinal degeneration model caused by Rh1G69D mutation in Drosophila, and carried out genome-wide screens for suppressors. As results, we identified a formerly uncharacterized E3 ubiquitin ligase SORDD1, which effectively alleviates ER stress and suppresses cell death caused by Rh1G69D mutation. We further demonstrated that SORDD1 mediates a new ERAD pathway independent of Hrd1 complex. Compared with Hrd1, the SORDD1 pathway is more specific and effective in degradation of the mutant rhodopsin and reduction of ER stress. Therefore, we plan to systematically study the molecular mechanisms of SORDD1-mediated ERAD by combination of genetic screening and proteomics methods, and further investigate the biological function of SORDD1 in rhodopsin metabolism and maintaining the normal physiological properties of photoreceptor cells using cell, fly and mice models. This series of studies will eventually provide a novel therapeutic strategy and therapeutic targets for treating RP.

视紫红质基因突变导致的蛋白内质网积累和内质网应激是视网膜色素变性最常见的原因。细胞通过内质网相关蛋白降解通路降解内质网上积累的视紫红质，缓解内质网应激，维持细胞的正常功能。我们在果蝇中建立了紫红质突变体Rh1G69D引发的视网膜退变模型，并进行了全基因组的遗传筛选，鉴定到一个能有效缓解Rh1G69D突变导致的内质网应激及细胞死亡的E3泛素连接酶SORDD1。进一步的研究揭示SORDD1介导了一个不依赖于经典的Hrd1复合体介导的内质网相关蛋白降解途径。相对于Hrd1，SORDD1通路更专一、有效地降解突变体紫红质并减轻内质网应激。我们计划通过遗传筛选结合蛋白组学等方法系统地研究SORDD1介导的内质网相关蛋白降解的分子机制，并利用细胞、果蝇和小鼠的模型揭示SORDD1在视紫红质的代谢以及维持视神经正常生理功能中的作用。这一系列研究成果将会对视网膜色素变性的治疗提供新的方向和药物靶点。

常染色体显性色素性视网膜炎（adRP, autosomal dominant retinitis pigmentosa）等多种神经退行性疾病是膜蛋白的错误折叠并在内质网滞留引发内质网应激(Endoplasmic Reticulum Stress)导致的。通过内质网蛋白降解（ERAD, Endoplasmic Reticulum Associated Degradation）通路对错误折叠的蛋白质降解，可以有效地减少蛋白质的积累及其引发的内质网应激，从而起到关键的保护作用。我们通过在果蝇感光神经元中表达一种错误折叠并在内质网积累的视紫红质突变体（Rh1G69D）建立了显性色素性视网膜炎的动物模型，并通过全基因组功能增益筛选这种视网膜细胞死亡的抑制因子，以此鉴定到了两种为被报道高度同源的内质网E3泛素连接酶，命名为SORDD1（Suppression Of Retinal Degeneration Disease 1 upon overexpression）和SORDD2。SORDD1/2的表达有效抑制表达Rh1G69D引发的视网膜细胞死亡，体内体外生化实验证明SORDD1/2催化泛素链转移到错误折叠的Rh1上，促进Rh1突变体蛋白的降解，从而抑制其造成的感光神经元异常。随后通过全基因组RNAi筛选鉴定了多个SORDD1介导的Rh1G69D降解的必需因子，揭示了VCP复合物和蛋白酶体在SORDD1/2下游发挥作用。此外，SORDD1/2对底物的降解，而不依赖于经典的HRD1/HRD3 E3泛素连接酶和逆向易位通道复合体。进一步的研究表明sorrd1/2和hrd1的双突变导致光感受器神经元的功能障碍和退行性死亡而单突变体表现正常，这些结果揭示SORDD1/2和HRD1独立降解错误折叠的视紫红质并共同维持感光细胞的功能和完整性。SORDD1/2的人类同源基因RNF185也可以抑制了Rh1G69D介导的视网膜病变。这些发现鉴定了一种与HRD1平行的ERAD途径，对SORDD1/2的同源基因RNF185的调节可能成为治疗色素性视网膜炎有效手段。这一系列研究工作加深了对视网膜退行性疾病的发生机理和对内质网错误折叠蛋白降解机制的理解，并且为色素性视网膜炎等视网膜退行性疾病的治疗有重要的参考价值。.期间研究团队在该基金的资助下在国际重要期刊发表有影响的SCI文章7篇，研究工作对视网膜