Using the Xenopus Developmental Eye Regrowth System to Distinguish the Role of Developmental Versus Regenerative Mechanisms

Abstract

A longstanding challenge in regeneration biology is to understand the role of developmental mechanisms in restoring lost or damaged tissues and organs. As these body structures were built during embryogenesis, it is not surprising that a number of developmental mechanisms are also active during regeneration. However, it remains unclear whether developmental mechanisms act similarly or differently during regeneration as compared to development. Since regeneration is studied in the context of mature, differentiated tissues, it is difficult to evaluate comparative studies with developmental processes due to the latter’s highly proliferative environment. We have taken a more direct approach to study regeneration in a developmental context (regrowth). Xenopus laevis , the African clawed frog, is a well-established model for both embryology and regeneration studies, especially for the eye. Xenopus eye development is well-defined. Xenopus is also an established model for retinal and lens regeneration studies. Previously, we demonstrated that Xenopus tailbud embryo can successfully regrow a functional eye that is morphologically indistinguishable from an age-matched control eye. In this study, we assessed the temporal regulation of retinal differentiation and patterning restoration during eye regrowth. Our findings showed that during regrowth, cellular patterning and retinal layer formation was delayed by approximately 1 day but was restored by 3 days when compared to eye development. An assessment of the differentiation of ganglion cells, photoreceptor cells, and Müller glia indicated that the retinal birth order generated during regrowth was consistent with that observed for eye development. Thus, retina differentiation and patterning during regrowth is similar to endogenous eye development. We used this eye regrowth model to assess the role of known mechanisms in development versus regrowth. Loss-of-function studies showed that Pax6 was required for both eye development and regrowth whereas apoptosis was only required for regrowth. Together, these results revealed that the mechanisms required for both development and regrowth can be distinguished from regrowth-specific ones. Our study highlights this developmental model of eye regrowth as a robust platform to systematically and efficiently define the molecular mechanisms that are required for regeneration versus development.

MAPK/ERK Pathway as a Central Regulator in Vertebrate Organ Regeneration

“…Retinal pigmented epithelium cells (RPE) [67,69,119] in newt, Xenopus, and embryonic chicks, and Müller glia cells (MG) [62,63,120,121] in teleost fish, Xenopus, and post-hatched chicks are the major cell sources for retina regeneration through transdifferentiation and proliferation. During transdifferentiaton, quiescent RPE/MG become stimulated to dedifferentiate and proliferate into multipotent neuroepithelial cells, which continue to differentiate into all cell types required to rebuild the retina [62,122,123].…”

Zic5 stabilizes Gli3 via a non-transcriptional mechanism during retinal development

“…The expression of all three marker genes were largely not affected in the absence of Zic5 (Figures 1F and 1G), indicating that Zic5 is not involved in early eye induction and specification. We then tested whether Zic5 plays a role in eye differentiation by examining the RPE marker RPE65, the rod photoreceptor layer marker rhodopsin, and the INL marker islet1 (Kha et al, 2019). Interestingly, knockdown of Zic5 significantly reduced RPE65 and rhodopsin expression in the eye, which was restored by introducing Zic5-GR MOR along with subsequent dexamethasone treatment (Figures 2A and 2B;Videos S1,S2,and S3).…”

Xenopus leads the way: Frogs as a pioneering model to understand the human brain

“…Frogs undergo metamorphosis, a well‐characterized and major developmental event driven by circulating hormones whose production is regulated by the brain (Furlow & Neff, 2006); this provides a powerful model to understand interactions between the brain, gonads, hormones, and the rest of the body, which has already become the subject of enthusiastic investigation in frogs (Buchholz, 2015; Buchholz, 2017). Frog embryos and larvae also exhibit high regenerative capacity (Kakebeen & Wills, 2019a, 2019b; Kha, Guerin, & Tseng, 2019; Lee‐Liu, Méndez‐Olivos, Muñoz, & Larraín, 2017; Slack, Lin, & Chen, 2008; Tseng & Levin, 2008), including of neural tissues (e.g., the spinal cord and elements of the limb), and this capacity decreases after metamorphosis (Slack et al, 2008; Slack, Beck, Gargioli, & Christen, 2004). Both the ability to regenerate and the loss of this ability provide attractive opportunities to study regeneration and to contrast it with the case of humans, who exhibit little or no regeneration of most tissues across their lifetime.…”