Neural stem cell properties of Müller glia in the mammalian retina: Regulation by Notch and Wnt signaling

Abstract

The retina in adult mammals, unlike those in lower vertebrates such as fish and amphibians, is not known to support neurogenesis. However, when injured, the adult mammalian retina displays neurogenic changes, raising the possibility that neurogenic potential may be evolutionarily conserved and could be exploited for regenerative therapy. Here, we show that Müller cells, when retrospectively enriched from the normal retina, like their radial glial counterparts in the central nervous system (CNS), display cardinal features of neural stem cells (NSCs), i.e., they self-renew and generate all three basic cell types of the CNS. In addition, they possess the potential to generate retinal neurons, both in vitro and in vivo. We also provide direct evidence, by transplanting prospectively enriched injury-activated Müller cells into normal eye, that Müller cells have neurogenic potential and can generate retinal neurons, confirming a hypothesis, first proposed in lower vertebrates. This potential is likely due to the NSC nature of Müller cells that remains dormant under the constraint of non-neurogenic environment of the adult normal retina. Additionally, we demonstrate that the mechanism of activating the dormant stem cell properties in Müller cells involves Wnt and Notch pathways. Together, these results identify Müller cells as latent NSCs in the mammalian retina and hence, may serve as a potential target for cellular manipulation for treating retinal degeneration.

Notch activity permits retinal cells to progress through multiple progenitor states and acquire a stem cell property

“…When examined at later stages of development, however, the cells exhibited expression of an inappropriate mixture of late progenitor and glial genes. When such cells were taken from the adult retina and cultured in EGF/FGF2, neurospheres were formed, as is observed with normal retinal stem cells taken from the retinal periphery (25), or Müller glia cultured in certain conditions (31). In contrast, activation of the Notch pathway in newly postmitotic cells led to a subset of cells with glial gene expression and proper glial morphology, in the absence of progenitor or stem cell characteristics, suggesting that proper glial differentiation requires a release from Notch activation.…”

“…Large, retrovirally marked clones in the murine or chick retina contain cell types that are born exclusively early and exclusively late in development, e.g., ganglion and Müller glia cells (31,32). Such clones can arise only if an initially infected progenitor cell makes a postmitotic daughter(s) early and a progenitor daughter(s) that progresses to a late state of competence.…”

“…Some of these cells may become spatially restricted to the peripheral retina from which they can be cultured as neurospheres (17). In addition, Müller glia in the avian and mouse retina can proliferate and retain multipotency in situ in response to injury (41,42), and a small fraction of murine cells can exhibit such properties after a culture period (31).…”

Stem Cells for Retinal Replacement Therapy

“…Mueller glial cells actively regenerate damaged retina in lower animals and can be similarly activated in mice by application of growth factors [20]. Although the Mueller cells reside in the retina and have properties of RSCs, they also produce all major neural lineages, in which there are multipotent neural stem cells [21].…”

“…Neural stem cells are found in many regions of the embryonic nervous system, including the retina [21], with the richest source for transplantation being forebrain-derived NSCs (for more detail see Temple [39]). Green fluorescent protein-expressing forebrain NSCs survive and display limited morphologic characteristic of retinal neurons with limited integration after transplantation into the mouse retina.…”