Sigma receptor 1 modulates endoplasmic reticulum stress in retinal neurons

To investigate the mechanism of σ receptor 1 (σR1) neuroprotection in retinal neurons.

Oxidative stress, which is implicated in diabetic retinopathy, was induced in mouse primary ganglion cells (GCs) and RGC-5 cells, and the effect of the σR1 ligand (+)-pentazocine on pro- and anti-apoptotic and endoplasmic reticulum (ER) stress gene expression was examined. Binding of σR1 to BiP, an ER chaperone protein, and σR1 phosphorylation status were examined by immunoprecipitation. Retinas were harvested from Ins2Akita/+ diabetic mice treated with (+)-pentazocine, and the expression of ER stress genes and of the retinal transcriptome was evaluated.

Oxidative stress induced the death of primary GCs and RGC-5 cells. The effect was decreased by the application of (+)-pentazocine. Stress increased σR1 binding to BiP and enhanced σR1 phosphorylation in RGC-5 cells. BiP binding was prevented, and σR1 phosphorylation decreased in the presence of (+)-pentazocine. The ER stress proteins PERK, ATF4, ATF6, IRE1α, and CHOP were upregulated in RGC-5 cells during oxidative stress, but decreased in the presence of (+)-pentazocine. A similar phenomenon was observed in retinas of Ins2Akita/+ diabetic mice. Retinal transcriptome analysis of Ins2Akita/+ mice compared with wild-type revealed differential expression of the genes critically involved in oxidative stress, differentiation, and cell death. The expression profile of those genes was reversed when the Ins2Akita/+ mice were treated with (+)-pentazocine.

In retinal neurons, the molecular chaperone σR1 binds BiP under stressful conditions; (+)-pentazocine may exert its effects by dissociating σR1 from BiP. As stress in retinal cells increases, phosphorylation of σR1 is increased, which is attenuated when agonists bind to the receptor.