Functional impact of galectins and aspects of N-glycosylation on epithelial-to-mesenchymal transition of retinal pigment epithelial cells and proliferative vitreoretinopathy formation

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Epithelial-to-mesenchymal transition of retinal pigment epithelial cells (RPE) is a key cellular event in the onset of proliferative vitreoretinopathy (PVR), but also in various other ocular pathologies. In early PVR RPE cells transdifferentiate from a highly differentiated to a myofibroblastic phenotype, which is actively dividing and migratory and then gives rise to the formation of the tractive, fibrocellular membranes found in PVR.Several proteins especially those located at the plasma membrane, such as integrins or growth factor receptors, are glycosylated. These glycans are in turn ligands for carbohydrate-binding proteins such as galectins. Upon EMT we found profound changes in the glycan expression profile of RPE cells towards an enhanced expression of high affinity glycoligands for galectins. Furthermore, we showed that galectins can modify RPE cell adhesion and migration, which was dependent on RPE-glycosylation, galectin expression levels and the presence of recombinant galectins. Thus, via binding to glycoforms upregulated upon EMT, recombinant Galectin-1 (rGal-1) and rGal-3 may allow for targeting myofibroblastic but not healthy RPE with high selectivity.In previous investigations we identified RPE-specific glycoprotein ligands for rGal-1 and rGal-3, including PDGFRB, CD44 and integrins, all of which are attributed to play a role in EMT and PVR. Interaction of these glycoproteins with recombinant galectins induces their clustering at the cell membrane, which requires β1,6-N-glycosylation. Preliminary data indicate that rGal-1 and rGal-3 also modify endocytosis of these receptors as well as their intracellular distribution and trafficking. Considering that endocytosis is the consecutive step after receptor binding and that endocytosis can lead to receptor inactivation but also activation, we assume that depending on the glycosylation of the glycoprotein receptor galectins may influence endocytosis and thereby modify the activity of signaling pathways and hence cellular function. By using cultured human RPE cells as a model for EMT in vitro the current proposal addresses the potential functional implications of PDGFRB and integrin-ß1 endocytosis in the RPE with respect to differential glycosylation and endogenous galectin expression levels as they occur in EMT. The ultimate aim of this study is to single out the functional impact of differential protein glycosylation and galectin binding on membrane persistence as well as endocytosis of the glycoprotein ligands and subsequent activity of specific signaling pathways. The role of the galectin glycoprotein interaction in endocytosis is of major interest for a further understanding of EMT as well as PVR formation, which is driven by transdifferentiated RPE cells. Furthermore, a better understanding at this pathway could contribute to the development of novel therapeutic approaches exploiting differential glycosylation of individual glycoproteins as a therapeutic target.