σ-1 receptors protect RGC-5 cells from apoptosis by regulating intracellular calcium, bax levels, and caspase-3 activation

PURPOSE. σ-1 Receptor ligands prevent neuronal death associated with glutamate excitotoxicity both in vitro and in vivo. However, the molecular mechanism of the neuroprotective effect remains to be elucidated. The present study was undertaken to determine whether σ-1 receptor agonists provide neuroprotection by decreasing glutamate-induced calcium mobilization and preventing apoptotic gene expression. METHODS. Cell death was measured by using a calcein-AM/propidium iodide cell-survival assay. Western blot analysis determined the expression levels of Bax in normal RGC-5 cells. Caspase-3 activation after glutamate treatment was determined with a carboxyfluorescein caspase-3 detection kit. Glutamate-induced intracellular calcium mobilization was measured by using ratiometric calcium imaging. RESULTS. σ-1 Receptor-overexpressing RGC-5 (RGC-5-S1R) cells had lower glutamate-induced intracellular calcium mobilization than did normal RGC-5 cells, and the σ-1 receptor ligand (+)-SKF10047 reduced the glutamate calcium response in normal and RGC-5-S1R cells. (+)-SKF10047 protected RGC-5 cells from glutamate-induced cell death, and the RGC-5-S1R cells showed a significant resistance to glutamate-induced apoptosis compared with the control RGC-5 cells. BD1047, a σ-1 receptor antagonist, blocked the protective effect of (+)-SKF10047. Western blot analysis showed that (+)-SKF10047 inhibited the increase in Bax after glutamate treatments. Glutamate-mediated cell death involved activation of caspase-3, and σ-1 receptor activation prevented an increase in caspase-3 expression. CONCLUSIONS. The results suggest that σ-1 receptors regulate intracellular calcium levels and prevent activation of proapoptotic genes, thus promoting retinal ganglion cell survival. The σ-1 ligands appear to be neuroprotective and are a potential target for neuroprotective therapeutics.