Expression, subcellular localization, and regulation of sigma receptor in retinal Müller cells

PURPOSE. Sigma receptors (σrRs) are nonopioid, nonphencyclidine binding sites with robust neuroprotective properties. Type 1 σrR1 (σTR1) is expressed in brain oligodendrocytes, but its expression and binding capacity have not been analyzed in retinal glial cells. This study examined the expression, subcellular localization, binding activity, and regulation of σR1 in retinal Müller cells. METHODS. Primary mouse Müller cells (MCs) were analyzed by RT-PCR, immunoblotting, and immunocytochemistry for the expression of σ-R1, and data were compared with those of the rat Müller cell line (rMC-1) and the rat ganglion cell line (RGC-5). Confocal microscopy was used to determine the subcellular σR1 location in primary mouse MCs. Membranes prepared from these cells were used for binding assays with [³H]-pentazocine (PTZ). The kinetics of binding, the ability of various σR1 ligands to compete with σR1 binding, and the effects of donated nitric oxide (NO) and reactive oxygen species (ROS) on binding were examined. RESULTS. σR1 is expressed in primary mouse MCs and is localized to the nuclear and endoplasmic reticulum membranes. Binding assays showed that in primary mouse MCs, rMC-1, and RGC-5, the binding of PTZ was saturable. [³H]-PTZ bound with high affinity in RGC-5 and rMC-1 cells, and the binding was similarly robust in primary mouse MCs. Competition studies showed marked inhibition of [³H]-PTZ binding in the presence of σR1-specific ligands. Incubation of cells with NO and ROS donors markedly increased σR1 binding activity. CONCLUSIONS. MCS express σR1 and demonstrate robust σR1 binding activity, which is inhibited by σR1 ligands and is stimulated during oxidative stress. The potential of Müller cells to bind σR1 ligands may prove beneficial in retinal degenerative diseases such as diabetic retinopathy.