A model for de novo pigmentation of amelanotic retinal pigment epithelial cells

Abstract

Purpose: Retinal Pigment Epithelial (RPE) cells perform critical functions in the visual cycle. Their melanin pigmentation, which is organized into specialized compartments - melanosomes, is highly critical for proper vision. A chemical method to induce pigmentation in a non-pigmented model of ARPE-19 cells was applied using L-DOPA as a repurposed drug from the current treatment of Parkinson's disease.

Methods: L-DOPA was optimized for its toxic effect on ARPE-19 cells along with pigmentation development. Gene expression and immunocytochemistry confirmed upregulation of melanogenesis-related genes and proteins. Melanosomes were characterized by TEM.

Results: We found 1000 μM L-DOPA to induce pigmentation of ARPE-19 cells by Day 3, and achieve full pigmentation by Day 5. By Day 5, L-DOPA at 1000 μM induced mitochondrial and nuclear DNA damage. However, the gene expression of RPE-specific markers (tyrosinase, TYRP1, CRALBP, PEDF) was significantly different in L-DOPA-treated ARPE-19 cells compared to non-treated ones. Positive expression for Tyrosinase enzyme was confirmed by ICC on both Day 3 and Day 5 of L-DOPA treatment. Transmission electron microscopy showed the de novo melanosome formation with ultrastructural features of various stages of maturity (Stage I to IV), apical-basal polarity and melanosome localization on the apical side of the L-DOPA-treated ARPE-19 cells.

Conclusion: Our study showed that L-DOPA treatment could induce de novo melanosome formation in amelanotic RPEs. We propose a newer approach of developing an ex vivo model for de novo pigmentation of RPE cells with cell-specific modification and culture condition optimization.

Keywords: L‐DOPA; RPE; in vitro model; melanosomes; pigmentation.