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

PURPOSE. Sigma receptors (sigma Rs) are nonopioid, nonphencyclidine binding sites with robust neuroprotective properties. Type 1 sigma R1 (sigma R1) 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 sigma R1 in retinal Muller cells. METHODS. Primary mouse Muller cells (MCs) were analyzed by RT-PCR, immunoblotting, and immunocytochemistry for the expression of sigma R1, and data were compared with those of the rat Muller cell line (rMC-1) and the rat ganglion cell line (RGC-5). Confocal microscopy was used to determine the subcellular sigma R1 location in primary mouse MCs. Membranes prepared from these cells were used for binding assays with [H-3]-pentazocine (PTZ). The kinetics of binding, the ability of various sigma R1 ligands to compete with sigma R1 binding, and the effects of donated nitric oxide (NO) and reactive oxygen species (ROS) on binding were examined. RESULTS. sigma R1 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. [H-3]-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 [H-3]- PTZ binding in the presence of sigma R1-specific ligands. Incubation of cells with NO and ROS donors markedly increased sigma R1 binding activity. CONCLUSIONS. MCs express sigma R1 and demonstrate robust sigma R1 binding activity, which is inhibited by sigma R1 ligands and is stimulated during oxidative stress. The potential of Muller cells to bind sigma R1 ligands may prove beneficial in retinal degenerative diseases such as diabetic retinopathy.