The Wilmer Eye Institute, The Johns Hopkins University School of Medicine, 600 N. Wolfe Street, Baltimore, MD 21287, USA.
J Cell Sci. 2011 Feb 15;124(Pt 4):523-31. doi: 10.1242/jcs.078790. Epub 2011 Jan 25.
Phagocytosis of the shed outer segment discs of photoreceptors is a major function of the retinal pigmented epithelium (RPE). We demonstrate for the first time that βA3/A1-crystallin, a major structural protein of the ocular lens, is expressed in RPE cells. Further, by utilizing the Nuc1 rat, in which the βA3/A1-crystallin gene is mutated, we show that this protein is required by RPE cells for proper degradation of outer segment discs that have been internalized in phagosomes. We also demonstrate that in wild-type RPE, βA3/A1-crystallin is localized to the lysosomes. However, in the Nuc1 RPE, βA3/A1-crystallin fails to translocate to the lysosomes, perhaps because misfolding of the mutant protein masks sorting signals required for proper trafficking. The digestion of phagocytized outer segments requires a high level of lysosomal enzyme activity, and cathepsin D, the major enzyme responsible for proteolysis of the outer segments, is decreased in mutant RPE cells. Interestingly, our results also indicate a defect in the autophagy process in the Nuc1 RPE, which is probably also linked to impaired lysosomal function, because phagocytosis and autophagy might share common mechanisms in degradation of their targets. βA3/A1-crystallin is a novel lysosomal protein in RPE, essential for degradation of phagocytosed material.
光感受器的脱落外节盘的吞噬作用是视网膜色素上皮（RPE）的主要功能。我们首次证明，βA3/A1-晶体蛋白，眼部晶状体的主要结构蛋白，在 RPE 细胞中表达。此外，我们利用 Nuc1 大鼠（其βA3/A1-晶体蛋白基因发生突变）表明，该蛋白是 RPE 细胞正确降解已被内吞入吞噬体的外节盘所必需的。我们还表明，在野生型 RPE 中，βA3/A1-晶体蛋白定位于溶酶体。然而，在 Nuc1 RPE 中，βA3/A1-晶体蛋白未能转运到溶酶体，可能是因为突变蛋白的错误折叠掩盖了正确运输所需的分拣信号。吞噬的外节段的消化需要高水平的溶酶体酶活性，并且负责外节段蛋白水解的主要酶组织蛋白酶 D 在突变型 RPE 细胞中减少。有趣的是，我们的结果还表明 Nuc1 RPE 中的自噬过程存在缺陷，这可能也与溶酶体功能受损有关，因为吞噬作用和自噬可能在其靶标的降解过程中共享共同的机制。βA3/A1-晶体蛋白是 RPE 中的一种新型溶酶体蛋白，对于吞噬作用的物质的降解是必需的。
