Numerous rhodopsin mutations have been implicated in night blindness and retinal degeneration, often with unclear etiology. D190N-rhodopsin (D190N-Rho) is a well-known inherited human mutation causing retinitis pigmentosa. Both higher-than-normal spontaneous-isomerization activity and misfolding/mistargeting of the mutant protein have been proposed as causes of the disease, but neither explanation has been thoroughly examined. We replaced wild-type rhodopsin (WT-Rho) in Rho^D190N/WT mouse rods with a largely “functionally silenced” rhodopsin mutant to isolate electrical responses triggered by D190N-Rho activity, and found that D190N-Rho at the single-molecule level indeed isomerizes more frequently than WT-Rho by over an order of magnitude. Importantly, however, this higher molecular dark activity does not translate into an overall higher cellular dark noise, owing to diminished D190N-Rho content in the rod outer segment. Separately, we found that much of the degeneration and shortened outer-segment length of Rho^D190N/WT mouse rods was not averted by ablating rod transducin in phototransduction—also consistent with D190N-Rho’s higher isomerization activity not being the primary cause of disease. Instead, the low pigment content, shortened outer-segment length, and a moderate unfolded protein response implicate protein misfolding as the major pathogenic problem. Finally, D190N-Rho also provided some insight into the mechanism of spontaneous pigment excitation.

许多视紫红质突变与夜盲症和视网膜变性有关，通常病因不明。D190N-视紫红质 (D190N-Rho) 是一种众所周知的遗传性人类突变，可引起色素性视网膜炎。高于正常水平的自发异构化活性和突变蛋白的错误折叠/错误定位都被认为是该疾病的原因，但两种解释都没有得到彻底检查。我们在Rho ^D190N/WT中替换了野生型视紫红质 (WT-Rho)具有很大程度上“功能沉默”的视紫红质突变体的小鼠杆，以隔离由 D190N-Rho 活动触发的电响应，并发现单分子水平的 D190N-Rho 确实比 WT-Rho 异构化更频繁超过一个数量级。然而，重要的是，由于杆外段中 D190N-Rho 含量减少，这种较高的分子暗活性不会转化为整体较高的细胞暗噪声。另外，我们发现Rho ^{D190N/WT的大部分退化和缩短的外段长度}在光转导中消融杆状转导蛋白并不能避免小鼠杆状细胞——这也与 D190N-Rho 较高的异构化活性不是疾病的主要原因一致。相反，低色素含量、缩短的外段长度和适度的未折叠蛋白质反应表明蛋白质错误折叠是主要的致病问题。最后，D190N-Rho 还提供了一些关于自发色素激发机制的见解。