Reprogramming Müller Glia to Regenerate Retinal Neurons

Abstract

In humans, various genetic defects or age-related diseases, such as diabetic retinopathies, glaucoma, and macular degeneration, cause the death of retinal neurons and profound vision loss. One approach to treating these diseases is to utilize stem and progenitor cells to replace neurons in situ, with the expectation that new neurons will create new synaptic circuits or integrate into existing ones. Reprogramming non-neuronal cells in vivo into stem or progenitor cells is one strategy for replacing lost neurons. Zebrafish have become a valuable model for investigating cellular reprogramming and retinal regeneration. This review summarizes our current knowledge regarding spontaneous reprogramming of Müller glia in zebrafish and compares this knowledge to research efforts directed toward reprogramming Müller glia in mammals. Intensive research using these animal models has revealed shared molecular mechanisms that make Müller glia attractive targets for cellular reprogramming and highlighted the potential for curing degenerative retinal diseases from intrinsic cellular sources.

Keywords: Notch; cytokines; epigenetics; growth factors; retina; zebrafish.

Figure 1

Time course of retinal regeneration in zebrafish. (a) Schematic of an uninjured retina. In the injured retina, (b) apoptotic cells are cleared by microglia and MG, and a subset of MG dedifferentiate or reprogram (light orange), (c) resulting in reentry into the cell cycle, indicated by the upregulation of PCNA. (d) MG undergo interkinetic nuclear migration to the ONL, (e) where they divide into MG-derived progenitors and postmitotic MG that return to the INL. (f) MG-derived progenitors continue to proliferate, generating clusters of progenitor cells that (g) ultimately differentiate into neurons that replace those that were ablated. Abbreviations: AC, amacrine cell; BC, bipolar cell; GC, ganglion cell; GCL, ganglion cell layer; HC, horizontal cell; INL, inner nuclear layer; MG, Müller glia; ONL, outer nuclear layer; PCNA, proliferating cell nuclear antigen. Figure adapted with permission from Lahne & Hyde (2017).

Figure 2

Intercellular and transcriptional regulation of Müller glia reprogramming. (a) In the uninjured retina, neurotransmitter- and cell–cell-contact-mediated signaling through Notch receptors and their ligands maintain Müller glia in a quiescent state (left panel). The loss of neurotransmitter- and cell-contact-mediated signaling, in combination with increased paracrine signaling, initiates the reprogramming of Müller glia (right panel). Tumor necrosis factor alpha (TNFα) and nucleotides released from dying neurons directly act on Müller glia and/or stimulate microglia to release cytokines that induce reprogramming (right panel). (b) Müller glia also release growth factors and cytokines that act in an autocrine fashion. (c) Cytokines including TNFα stimulate Jak-mediated phosphorylation of Stat3, which leads to its nuclear translocation and transcription of its target gene, ascl1a (left). Ascl1a then initiates the expression of insm1a, lin28, and wnt4a, and downstream, Insm1a represses the expression of the Wnt signaling inhibitor, dkkb (right).

Figure 3

Comparison of the injury response in zebrafish and mice: schematic of a quiescent Müller glia that becomes activated or gliotic following retinal injury, displaying a hypertrophied cell body and processes in both mice and zebrafish. In mice, the activated or gliotic Müller glia returns toward quiescence based on gene expression and morphology, whereas in zebrafish, the activated or gliotic Müller glia reprograms and proliferates.