The new paradigm: retinal pigment epithelium cells generated from embryonic or induced pluripotent stem cells

Abstract

Compared with neural crest-derived melanocytes, retinal pigment epithelium (RPE) cells in the back of the eye are pigment cells of a different kind. They are a part of the brain, form an epithelial monolayer, respond to distinct extracellular signals, and provide functions that far exceed those of a light-absorbing screen. For instance, they control nutrient and metabolite flow to and from the retina, replenish 11-cis-retinal by re-isomerizing all-trans-retinal generated during photoconversion, phagocytose daily a portion of the photoreceptors' outer segments, and secrete cytokines that locally control the innate and adaptive immune systems. Not surprisingly, RPE cell damage is a major cause of human blindness worldwide, with age-related macular degeneration a prevalent example. RPE replacement therapies using RPE cells generated from embryonic or induced pluripotent stem cells provide a novel approach to a rational treatment of such forms of blindness. In fact, RPE-like cells can be obtained relatively easily when stem cells are subjected to a two-step induction protocol, a first step that leads to a neuroectodermal fate and a second to RPE differentiation. Here, we discuss the characteristics of such cells, propose criteria they should fulfill in order to be considered authentic RPE cells, and point out the challenges one faces when using such cells in attempts to restore vision.

Figure 1

Anatomy of the camera-style mammalian eye and photoreceptor/retinal pigment epithelium (RPE) complex. Left: schematic cross-section through the human eye showing its main structural components that allow focusing of the light onto the photoreceptor layer of the retina. Top: Histologic section of an adult mouse eye showing photoreceptor nuclei in the outer nuclear layer (ONL), rod outer segments (ROS), RPE, Bruch’s membrane, and the choroid with its neural crest-derived melanocytes. Bottom: Diagram showing two RPE cells with their microvilli contacting the rod outer segments. The intimate anatomic relationship of RPE cells with photoreceptors and Bruch’s membrane allow the cells to have multiple roles as support cells for the retina (see text). The cells also carry a dense array of apical melanosomes that serve as a light screen in the back of the eye.

Figure 2

Schematic diagrams of the differentiation protocols used to obtain retinal pigment epithelium (RPE) cells from ES or induced pluripotent stem cells. While the time periods required to obtain human ES cells and iPS cells differ by several weeks, the time to obtain RPE cells from them is similar. In a first step, neuroectodermal progenitors are generated from ES/iPS cells by exposing the cells to inhibitors of the WNT signaling pathway such as Dickkopf-1 (DKK1) or Nodal, and Lefty-A. In a second step, RPE cells are generated from neuroectodermal cells upon plating onto a laminin coated dish and culturing in a medium lacking FGF but containing activin A. Note that both neuroectodermal and RPE cells can be obtained from ES/iPS cells without the addition of Dkk1/Nodal or Activin A, but the efficiency of obtaining RPE cells under such conditions is much lower compared with conditions containing the respective factors.