Biomechanical Characterization of Retinal Pigment Epitheliums Derived from hPSCs Using Atomic Force Microscopy

The retinal pigment epithelium (RPE), a multifunctional cell monolayer located at the back of the eye, plays a crucial role in the survival and homeostasis of photoreceptors. Dysfunction or death of RPE cells leads to retinal degeneration and subsequent vision loss, such as in Age-related macular degeneration and some forms of Retinitis Pigmentosa. Therefore, regenerative medicine that aims to replace RPE cells by new cells obtained from the differentiation of human pluripotent stem cells, is the focus of intensive research. However, despite their critical interest in therapy, there is a lack of biomechanical RPE surface description. Such biomechanical properties are tightly related to their functions. Herein, we used atomic force microscopy (AFM) to analyze both the structural and mechanical properties of RPEs obtained from four cell lines and at different stages of epithelial formation. To characterize epitheliums, we used apical markers in immunofluorescence and showed the increase of transepithelial resistance, as well as the ability to secrete cytokines with an apico-basal polarity. Then, we used AFM to scan the apical surface of living or fixed RPE cells. We show that RPE monolayers underwent softening of apical cell center as well as stiffening of cell borders over epithelial formation. We also observed apical protrusions that depend on actin network, suggesting the formation of microvilli at the surface of RPE epitheliums. These RPE cell characteristics are essential for their functions into the retina and AFM studies may improve the characterization of the RPE epithelium suitable for cell therapy.