Inhibition of plasminogen activators attenuates the death of differentiated retinal ganglion cells and stabilizes their neurite network in vitro.

PURPOSE: Although previous studies have indicated that elevated levels of the tissue plasminogen activator (tPA) and the urokinase plasminogen activator (uPA) associate with the death of retinal ganglion cells (RGCs), it was unclear whether these proteases directly cause cell death. With the use of a transformed and undifferentiated retinal ganglion cell line, RGC-5, which does not express tPA, and by treating this cell line with staurosporine, which induces not only the differentiation of RGC-5 cells but also the expression of uPA and tPA in other neuronal cells, the authors sought to determine whether these proteases regulate the differentiation of RGC-5 cells and whether elevated levels of these proteases directly cause the death of RGC-5 cells.
METHODS: Transformed RGC-5 cells were cultured in serum-free medium and were treated with 0.5 muM to 2.0 muM staurosporine to induce their differentiation. Neurite outgrowth was assessed by phase-contrast microscopy and calcein AM staining and quantified with imaging software. Proteolytic activities of tPA and uPA were determined by zymography assays. Cell viability was determined by LIVE/DEAD viability assay kit.
RESULTS: Compared with untreated RGC-5 cells, cells treated with staurosporine differentiated as early as 1 to 6 hours. However, proteolytic activities of neither tPA nor uPA were observed within this time frame. Differentiated RGC-5 cells expressed detectable levels of uPA proteolytic activity starting at 24 hours and tPA proteolytic activity only at 48 hours. RGC-5 cells synthesized and secreted uPA and tPA into the conditioned medium, depending on staurosporine concentration and treatment time. At lower concentrations of staurosporine, differentiated RGC-5 cells had longer neurites and expressed lower levels of tPA and uPA. At higher concentrations of staurosporine, differentiated RGC-5 cells expressed higher levels of tPA and uPA, had smaller neurites, and most of them died. In contrast, when RGC-5 cells were treated with staurosporine along with inhibitors specific to tPA and uPA, proteolytic activities of both PAs were significantly reduced. Under these conditions, a significant number of RGC-5 cells survived, showed increased neurite outgrowth, and established their neurite network in vitro.
CONCLUSIONS: Results presented in this study indicate that RGC-5 cells do not require tPA and tPA for their differentiation. In fact, differentiated RGC-5 cells synthesize elevated levels of tPA and uPA, and elevated levels of these proteases acting in an autocrine-fashion in turn lead to the death of RGC-5 cells.