Expression and Distribution of β, β-Carotene Oxygenase 2 (BCO2) in the Human Retina

Abstract

Purpose : Recent single-cell RNA sequence data showed that the β, β-Carotene Oxygenase 2 (BCO2) gene, a known carotenoid cleavage enzyme, expresses much stronger in the human macula than in the peripheral retina¹. This is different from our previous finding that the BCO2 enzyme is evenly distributed in the monkey retinas by IHC². Here, we investigated the expression and distribution of BCO2 in the human retina.

Methods : The protein expressions of BCO2 in the human macula, peripheral retina, sub-macular RPE, and peripheral RPE were detected by western blots. The spatial distribution of BCO2 was examined with confocal fluorescence microscopy. The co-localization of BCO2 and carotenoids was assessed with confocal resonance Raman microscopy. An EGFP-tagged BCO2 protein was expressed in human induced pluripotent stem cell-derived retinal pigment epithelium (iPSC-RPE) cells to further investigate BCO2’s subcellular distribution by ICC.

Results : No difference was detected between BCO2 protein levels and distribution in the human macula and the peripheral retina, or between sub-macular RPE and peripheral RPE. IHC shows that BCO2 is evenly distributed in the entire human retina. Raman Microscopy demonstrated that carotenoids are localized mainly in the OPL and slightly co-localized with BCO2. The recombinant human retinal BCO2 is expressed in the human IPS-CRPE cells with no subcellular specificity.

Conclusions : Our results demonstrated that, consistent with the monkey retina, there is no difference between the expression of BCO2 in the human macula and the peripheral retina. The retinal spatial distribution of this carotenoid cleavage enzyme BCO2 is unlikely able to explain the specific tissue distribution of carotenoids in the human macula.

References
1. Voigt, A.P., et al. “Molecular Characterization of Foveal versus Peripheral Human Retina by Single-Cell RNA Sequencing.” Experimental Eye Research, vol. 184, 2019, pp. 234–242., https://doi.org/10.1016/j.exer.2019.05.001.
2. Li, Binxing, et al. “Inactivity of Human β,β-Carotene-9′,10′-Dioxygenase (BCO2) Underlies Retinal Accumulation of the Human Macular Carotenoid Pigment.” Proceedings of the National Academy of Sciences, vol. 111, no. 28, 2014, pp. 10173–10178., https://doi.org/10.1073/pnas.1402526111.

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