Choriocapillaris Opsin 3 is a master switch for oxygen demand-supply balance in the outer retina.

Abstract

Purpose : The choroid contributes approximately 50% of the O₂ consumed by the whole retina in the dark, while light reduces its O₂ consumption. However, lack of regulation of the choroid by O₂ may be detrimental to the photoreceptors under conditions when choroidal PO₂ decreases. Tissue O₂ consumption is known to be modulated by Nitric Oxide (NO) produced by the constitutive NO synthase. So, how does choroid maintain O₂ levels in the outer retina under dark-light conditions? We hypothesize that choriocapillaris fine-tunes O₂ consumption of the outer retina via an intrinsic light sensitive Opsin 3 (OPN3)-Nitric Oxide signaling.

Methods : The choroid OPN3 expression, Ca²⁺ imaging and NO release were studied in a flat-mount choroid preparation using OPN3-eGFP reporter mouse, cdh5-Gcamp6f mice and DAF2-diacetate, respectively. We also studied choroid hemodynamics in our ex-vivo arterially perfused mouse eye model using confocal microscopy.

Results : We also discovered an expression of OPN3 in choriocapillaris endothelium. We also found that activation of endothelial OPN3 via blue light (470 nm) initiates Ca²⁺ signaling that is sensitive to PTX (G_i inhibitor), G_q inhibitor -YM254890-, PLC inhibitor -U73122-, Suramin -P2Y blocker-, Apyrase -ATP hydrolysis- and TRPC4/5 channel blocker -ML204-. We hypothesized that light co-activates both the G_i coupled (OPN3) and the G_q coupled P2Y receptor signaling, which together potentiate endothelial cell Ca²⁺ influx via TRPC4/5 activation. We also found a bimodal action of OPN3 stimulation. Under low basal endothelial Ca²⁺ condition (dark adaptation), OPN3 stimulation (using cis-retinal) abrogates the light induced Ca²⁺ response while under high basal Ca²⁺ conditions (during light adaptation) OPN3 stimulation potentiates Ca²⁺ response via TRPC4/5 activation. Using DAF2-diacetate we observed a Ca²⁺ dependent and light induced release of NO from choriocapillaris endothelium. Finally, we discovered that the extremely high flow in the choroid enhances its NO bioavailability.

Conclusions : Choriocapillaris endothelium is intrinsically sensitive to light via OPN3. Light stimulation of endothelial OPN3 has a bimodal action leading to either termination or potentiation of the light induced endothelial Ca²⁺ response. The OPN3 bimodal action is dependent on endothelial basal Ca²⁺ levels. Finally, light-induced choroid NO release might regulate O₂ distribution in the outer retina.

This abstract was presented at the 2023 ARVO Annual Meeting, held in New Orleans, LA, April 23-27, 2023.