β-catenin/Wnt signaling controls progenitor fate in the developing and regenerating zebrafish retina

Abstract

BackgroundThe zebrafish retina maintains two populations of stem cells: first, the germinal zone or ciliary marginal zone (CMZ) contains multipotent retinal progenitors that add cells to the retinal periphery as the fish continue to grow; second, radial glia (Müller cells) occasionally divide asymmetrically to generate committed progenitors that differentiate into rod photoreceptors, which are added interstitially throughout the retina with growth. Retinal injury stimulates Müller glia to dedifferentiate, re-enter the cell cycle, and generate multipotent retinal progenitors similar to those in the CMZ to replace missing neurons. The specific signals that maintain these two distinct populations of endogenous retinal stem cells are not understood.ResultsWe used genetic and pharmacological manipulation of the β-catenin/Wnt signaling pathway to show that it is required to maintain proliferation in the CMZ and that hyperstimulation of β-catenin/Wnt signaling inhibits normal retinal differentiation and expands the population of proliferative retinal progenitors. To test whether similar effects occur during regeneration, we developed a method for making rapid, selective photoreceptor ablations in larval zebrafish with intense light. We found that dephosphorylated β-catenin accumulates in Müller glia as they re-enter the cell cycle following injury, but not in Müller glia that remain quiescent. Activation of Wnt signaling is required for regenerative proliferation, and hyperstimulation results in loss of Müller glia from the INL as all proliferative cells move into the ONL.Conclusionsβ-catenin/Wnt signaling is thus required for the maintenance of retinal progenitors during both initial development and lesion-induced regeneration, and is sufficient to prevent differentiation of those progenitors and maintain them in a proliferative state. This suggests that the β-catenin/Wnt cascade is part of the shared molecular circuitry that maintains retinal stem cells for both homeostatic growth and epimorphic regeneration.

Analysis of DNA methylation reveals a partial reprogramming of the Müller glia genome during retina regeneration

“…S1A). MGPC formation is accompanied by the increased expression of pluripotency factors (14) and other key signaling molecules (15)(16)(17)(18)(19)(20)(21). The induction of pluripotency genes, along with the finding that the putative cytidine deaminases Apobec2a and Apobec2b (Apobec2a,2b) are necessary for MGPC formation (22), is consistent with the idea that active modification of the DNA methylation landscape may underlie the reprogramming of MG to MGPCs.…”

N-cadherin-based adherens junction regulates the maintenance, proliferation, and differentiation of neural progenitor cells during development

“…68,84,[120][121][122] Co-localization of b-catenin with N-cadherin is observed in the apical domain of NPCs in the cerebral cortex and midbrain (Fig. 2).…”

“…34,84 It has been observed that hyperstimulation of b-catenin/Wnt signaling inhibits normal retinal differentiation and expands the population of proliferative retinal progenitors in zebra fish. 120 Downregulation of b-catenin signaling by loss of function of N-cadherin by the shRNA is associated with losing the identity of NPCs and premature differentiation in the developing mouse cerebral cortex. 84 Furthermore, loss of b-catenin in NPCs of mouse ventral midbrain during development perturbs the integrity of niche formation and leads to a severe reduction of dopamine neurogenesis.…”

A self-renewing division of zebrafish Müller glial cells generates neuronal progenitors that require N-cadherin to regenerate retinal neurons

“…Damage to photoreceptors disrupts the adherens junctions, which are important regulators of differentiation and proliferation in radial glia (Götz and Huttner, 2005). In particular, β-catenin links the intracellular domain of cadherins at the adherens junctions, and separately functions as the intracellular mediator of canonical Wnt signaling; β-cateninmediated signal transduction regulates radial glia proliferation in the developing mouse cortex (Johansson et al, 2010) as well as progenitors in the regenerating retina (Wan et al, 2012;Meyers et al, 2012).…”