Mutation in the βA3/A1-crystallin gene impairs phagosome degradation in the retinal pigmented epithelium of the rat

Abstract

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.

Defects in retinal pigment epithelial cell proteolysis and the pathology associated with age-related macular degeneration

“…Cytoskeletal rearrangements occur through the evolutionarily conserved ELMO1 (engulfment and cell motility protein 1)-DOCK180 (dedicator of cytokinesis)-RAC complex (Elliott and Ravichandran, 2010). We have reported that some of these proteins, such as ELMO1, DOCK 180 and RAC1 are also expressed in the RPE cells of the rat, indicating that a similar signaling pathway may be involved in phagosome maturation in RPE (Zigler et al, 2011). We have also shown that Rab5 GTPase, which in macrophages is known to be crucial in the final maturation process before the degradation by lysosomal enzymes occurs, is also important in the maturation process in RPE (Zhou and Yu, 2008; Zigler et al, 2011).…”

“…We have reported that some of these proteins, such as ELMO1, DOCK 180 and RAC1 are also expressed in the RPE cells of the rat, indicating that a similar signaling pathway may be involved in phagosome maturation in RPE (Zigler et al, 2011). We have also shown that Rab5 GTPase, which in macrophages is known to be crucial in the final maturation process before the degradation by lysosomal enzymes occurs, is also important in the maturation process in RPE (Zhou and Yu, 2008; Zigler et al, 2011). Information regarding the role of the cytoskeleton during the RPE degradation process is relatively sparse.…”

Congenital Cataract-Causing Mutation G129C in γC-Crystallin Promotes the Accumulation of Two Distinct Unfolding Intermediates That Form Highly Toxic Aggregates

“…The most widely understood mechanism is the disruption of crystallin solubility and/or stability directly by mutations/modifications or indirectly by changes in cellular homeostasis. It is worth noting that mutations in some of the crystallin genes have been associated with non-cataract abnormities such as neurodegenerative diseases, cardiomyopathy, tumorigenesis and retinal and vascular disorders [23][24][25][26]. Similar to proteins involved in neurodegenerative diseases, crystallins can also form amyloidlike fibrils under certain solution conditions [27][28][29][30][31][32][33].…”

New Biomarkers in the Retina and RPE Under Oxidative Stress

“…Recently, crystallin location and function in the retina and RPE have been described [Sakaguchi et al, 2003;Organisciak et al, 2011;Zigler et al, 2011;Sreekumar et al, 2010;Gangalum et al, 2011]. A-and B-crystallin are located in the ganglion cell layer and photoreceptor layer, whereas -crystallin is detected in all nuclear layers of the retina [Xi et al, 2003].…”