Voltage-gated sodium channels in retinal pigment epithelium – a real deal or an artifact?

Abstract

Purpose : Cells of retinal pigment epithelium (RPE) have been thought to undergo epithelial to neuronal transition by expressing voltage-gated sodium channels (Na_v) in culture. Because Na_v currents have not been recorded ex vivo, their presence in vitro is not considered to reflect the physiology of the native RPE. We revisited this question by investigating the presence of the Na_v channels in mouse and human embryonic stem cell (hESC) derived RPE. In this study, the plausible roles of Na_v channels in RPE were studied during development and in phagocytosis of the photoreceptor outer segments (POS).

Methods : Patch clamp recordings were conducted from intact monolayers of hESC-RPE and freshly dissociated cells at various stages of RPE development with and without the presence of Na_v channel blockers. Immunolabelling from both hESC-derived and mouse RPE was used to investigate the channel subtype distribution and localization as well as their changes during development. The phagocytosis was assayed in vitro by incubating the purified porcine outer segments on hESC-RPE and ex vivo by preparing the mouse eyes with the retina intact at various timepoints of the circadian cycle.

Results : Recordings from the monolayer showed the presence of Na_V current in hESC-RPE that was inhibited by TTX and QX-314. The current was mainly carried by subtypes Na_v1.4-Na_v1.6 and Na_v1.8, and these channels were observed throughout the development. The set of channel subtypes we identified in the RPE were not characteristic of the neural retina and, notably, the main neuronal subtypes Na_v1.1 and Na_v1.2 were not observed in mature RPE. Interestingly, the currents identified in isolated RPE cells were infrequent and had reduced amplitudes. The immunolabeling indicated that dissociation causes the channels to concentrate to cellular infoldings where they might not be accessible to pass ionic currents. Finally, the phagocytosis assays revealed a possible role for Na_v1.4 and Na_v1.8 in POS uptake and indicated the accumulation of these channels to the phagosomes.

Conclusions : Our data strongly indicates that Na_V channels are present in RPE throughout development and that they are relevant for its physiology. The findings from mouse RPE challenge the notion that these channels appear because of the epithelial to neuronal transition in culture.

This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.