Figure 6. Proposed model for norrin’s role on RGC-5 cells. Our working hypothesis is that under normal conditions, phosphorylated LRP-1 receptor prevents uPA-mediated cell death. SS, by inhibiting PKC, downregulates LRP-1 phosphorylation, and in the absence of LRP-1 phosphorylation, elevated levels of tPA and uPA cause death of RGC-5 cells. In contrast, norrin prevents tPA and uPA-mediated cell death, in part, by restoring phosphorylation of LRP-1 and, in part, by activating Wnt pathway.