Co-localization and polarized distribution of Na,K-ATPase α3 and β2 subunits in photoreceptor cells

Na,K-ATPase plays a central role in the visual sensitivity of photoreceptors by driving the dark current of vision. The α3 and β2 isoforms of Na,K-ATPase were previously shown to be the major α and β subunit mRNAs expressed in photoreceptors. Here we compared the distribution of β-subunits of the enzyme in the retina and kidney, using electron microscopic immunocytochemistry with specific antibodies against α3, β1, and β2 isoforms as well as with an antibody (Ax2) that binds to α2 and/or α3 isoforms. Both the α3 and β2 isoforms were localized to photoreceptor inner segments at highest labeling density between the base of the connecting cilium and the outer limiting membrane (OLM). Quantitative analysis of Ax2 antibody binding to α3 revealed a significant decrease in labeling density below the OLM and above the base of the connecting cilium. Although the β2-subunit has been reported to have adhesive functions in glial cells in cerebellum, we detected β2 in the photoreceptor, a cell of neural origin, but not in the Mueller cell, the glial cell of the retina. Moreover, anti-β2 antibodies bound maximally to portions of photoreceptor cells not involved in cell-cell contact.