Genetic analysis of metamorphic and premetamorphic Xenopus ciliary marginal zone

dnmt1 function is required to maintain retinal stem cells within the ciliary marginal zone of the zebrafish eye

The ciliary marginal zone (CMZ) of the zebrafish retina contains a population of actively proliferating resident stem cells, which generate retinal neurons throughout life. The maintenance methyltransferase, dnmt1, is expressed within the CMZ. Loss of dnmt1 function results in gene misregulation and cell death in a variety of developmental contexts, however, its role in retinal stem cell (RSC) maintenance is currently unknown. Here, we demonstrate that zebrafish dnmt1^s872 mutants possess severe defects in RSC maintenance within the CMZ. Using a combination of immunohistochemistry, in situ hybridization, and a transgenic reporter assay, our results demonstrate a requirement for dnmt1 activity in the regulation of RSC proliferation, gene expression and in the repression of endogenous retroelements (REs). Ultimately, cell death is elevated in the dnmt1^−/− CMZ, but in a p53-independent manner. Using a transgenic reporter for RE transposition activity, we demonstrate increased transposition in the dnmt1^−/− CMZ. Taken together our data identify a critical role for dnmt1 function in RSC maintenance in the vertebrate eye.

Retinal Degeneration and Regeneration-Lessons From Fishes and Amphibians

PURPOSE OF REVIEW

Retinal degenerative diseases have immense socio-economic impact. Studying animal models that recapitulate human eye pathologies aids in understanding the pathogenesis of diseases and allows for the discovery of novel therapeutic strategies. Some non-mammalian species are known to have remarkable regenerative abilities and may provide the basis to develop strategies to stimulate self-repair in patients suffering from these retinal diseases.

RECENT FINDINGS

Non-mammalian organisms, such as zebrafish and Xenopus, have become attractive model systems to study retinal diseases. Additionally, many fish and amphibian models of retinal cell ablation and cell lineage analysis have been developed to study regeneration. These investigations highlighted several cellular sources for retinal repair in different fish and amphibian species. Moreover, major differences in repair mechanisms have been reported in these animal models.

SUMMARY

This review aims to emphasize first on the importance of zebrafish and Xenopus models in studying the pathogenesis of retinal diseases and, second, on the different modes of regeneration processes in these model organisms.

Characterization of the Rx1-dependent transcriptome during early retinal development

BACKGROUND

The transcription factor Rx1, also known as Rax, controls key properties of retinal precursors including migration behavior, proliferation, and maintenance of multipotency. However, Rx1 effector genes are largely unknown.

RESULTS

To identify genes controlled by Rx1 in early retinal precursors, we compared the transcriptome of Xenopus embryos overexpressing Rx1 to that of embryos in which Rx1 was knocked-down. In particular, we selected 52 genes coherently regulated, i.e., actived in Rx1 gain of function and repressed in Rx1 loss of function experiments, or vice versa. RT-qPCR and in situ hybridization confirmed the trend of regulation predicted by microarray data for the selected genes. Most of the genes upregulated by Rx1 are coexpressed with this transcription factor, while downregulated genes are either not expressed or expressed at very low levels in the early developing retina. Putative direct Rx1 target genes, activated by GR-Rx1 in the absence of protein synthesis, include Ephrin B1 and Sh2d3c, an interactor of ephrinB1 receptor, which represent candidate novel effectors for the migration promoting activity of Rx1.

CONCLUSIONS

This study identifies previously undescribed Rx1 regulated genes mainly involved in transcription regulation, cell migration/adhesion, and cell proliferation that contribute to delineate the molecular mechanisms underlying Rx1 activities.

Gene expression is dynamically regulated in retinal progenitor cells prior to and during overt cellular differentiation

The retina is comprised of one glial and six neuronal populations that are generated from a multipotent pool of retinal progenitor cells (RPCs) during development. To give rise to these different cell types, RPCs undergo temporal identity transitions, displaying distinct gene expression profiles at different stages of differentiation. Little, however, is known about temporal differences in RPC identities prior to the onset of overt cellular differentiation, during the period when a retinal identity is gradually acquired. Here we examined the sequential onset of expression of regional markers (i.e., homeodomain transcription factors) and cell fate determinants (i.e., basic-helix-loop-helix transcription factors and neurogenic genes) in RPCs from the earliest appearance of a morphologically-distinct retina. By performing a comparative analysis of the expression of a panel of 27 homeodomain, basic-helix-loop-helix and Notch pathway genes between embryonic day (E) 8.75 and postnatal day (P) 9, we identified six distinct RPC molecular profiles. At E8.75, the earliest stage assayed, murine RPCs expressed five homeodomain genes and a single neurogenic gene (Pax6, Six3, Six6, Rx, Otx2, Hes1). This early gene expression profile was remarkably similar to that of 'early' RPCs in the amphibian ciliary marginal zone (CMZ), where RPCs are compartmentalised according to developmental stage, and homologs of Pax6, Six3 and Rx are expressed in the 'early' stem cell zone. As development proceeds, expression of additional homeodomain, bHLH and neurogenic genes was gradually initiated in murine RPCs, allowing distinct genetic profiles to also be defined at E9.5, E10.5, E12.5, E15.5 and P0. In addition, RPCs in the postnatal ciliary margin, where retinal stem cells are retained throughout life, displayed a unique molecular signature, expressing all of the early-onset genes as well as several late-onset markers, indicative of a 'mixed' temporal identity. Taken together, the identification of temporal differences in gene expression in mammalian RPCs during pre-neurogenic developmental stages leads to new insights into how regional identities are progressively acquired during development, while comparisons at later stages highlight the dynamic nature of gene expression in temporally distinct RPC pools.

Hes4 controls proliferative properties of neural stem cells during retinal ontogenesis

The retina of fish and amphibian contains genuine neural stem cells located at the most peripheral edge of the ciliary marginal zone (CMZ). However, their cell-of-origin as well as the mechanisms that sustain their maintenance during development are presently unknown. We identified Hes4 (previously named XHairy2), a gene encoding a bHLH-O transcriptional repressor, as a stem cell-specific marker of the Xenopus CMZ that is positively regulated by the canonical Wnt pathway and negatively by Hedgehog signaling. We found that during retinogenesis, Hes4 labels a small territory, located first at the pigmented epithelium (RPE)/neural retina (NR) border and later in the retinal margin, that likely gives rise to adult retinal stem cells. We next addressed whether Hes4 might impart this cell subpopulation with retinal stem cell features: inhibited RPE or NR differentiation programs, continuous proliferation, and slow cell cycle speed. We could indeed show that Hes4 overexpression cell autonomously prevents retinal precursor cells from commitment toward retinal fates and maintains them in a proliferative state. Besides, our data highlight for the first time that Hes4 may also constitute a crucial regulator of cell cycle kinetics. Hes4 gain of function indeed significantly slows down cell division, mainly through the lengthening of G1 phase. As a whole, we propose that Hes4 maintains particular stemness features in a cellular cohort dedicated to constitute the adult retinal stem cell pool, by keeping it in an undifferentiated and slowly proliferative state along embryonic retinogenesis. Stem Cells 2012;30:2784–2795

A large scale screen for neural stem cell markers in Xenopus retina

Neural stem cell research suffers from a lack of molecular markers to specifically assess stem or progenitor cell properties. The organization of the Xenopus ciliary marginal zone (CMZ) in the retina allows the spatial distinction of these two cell types: stem cells are confined to the most peripheral region, while progenitors are more central. Despite this clear advantage, very few genes specifically expressed in retinal stem cells have been discovered so far in this model. To gain insight into the molecular signature of these cells, we performed a large-scale expression screen in the Xenopus CMZ, establishing it as a model system for stem cell gene profiling. Eighteen genes expressed specifically in the CMZ stem cell compartment were retrieved and are discussed here. These encode various types of proteins, including factors associated with proliferation, mitotic spindle organization, DNA/RNA processing, and cell adhesion. In addition, the publication of this work in a special issue on Xenopus prompted us to give a more general illustration of the value of large-scale screens in this model species. Thus, beyond neural stem cell specific genes, we give a broader highlight of our screen outcome, describing in particular other retinal cell markers that we found. Finally, we present how these can all be easily retrieved through a novel module we developed in the web-based annotation tool XenMARK, and illustrate the potential of this powerful searchable database in the context of the retina.

A role for Xvax2 in controlling proliferation of Xenopus ventral eye and brain progenitors

The Vax2 homeobox gene plays a crucial role in early dorsoventral patterning of the eye. However, although Vax2 transcripts have been detected in later differentiating eye and brain regions, its possible roles at these stages are still unclear. By immunohistochemistry and in situ hybridization, we extensively compared the expression patterns of Xenopus Vax2 (Xvax2) mRNA and protein. Expression of Xvax2 protein was found to be largely overlapping but more restricted than that of mRNA, suggesting that Xvax2 expression may be also regulated at posttranscriptional levels. During eye and brain neurogenesis, Xvax2 protein was detected in proliferating neural progenitors and postmitotic differentiating cells in ventral regions of both structures. Overexpression of Xvax2 in Xenopus embryos by mRNA microinjection and DNA lipofection appeared to inhibit proliferation in both eye and brain cells, thus pointing to a new potential role for Vax2 in controlling the proliferative properties of ventral eye and brain progenitors.

Cloning and developmental expression of the Xenopus homeobox gene Xvsx1

In contrast to the high degree of evolutionary conservation of the Vsx2/Chx10 gene family, vertebrate orthologues of Vsx1 display more divergent sequences and spatio-temporal expression patterns. Here, we report the cloning and expression pattern of Xenopus laevis Vsx1. Differently from the mouse and zebrafish orthologues, Xvsx1 transcription is activated at early neurula both in the evaginating eye vesicles and in the presumptive spinal cord. Compared to other retinal homeobox genes, such as Xrx1, Xsix3 and Xpax6, Xvsx1 is activated at a later stage; in addition, its anterior expression appears to be more specifically restricted to the retina. At tail bud stage, Xvsx1 expression in retinal progenitors persists, and its neural tube expression, which in the spinal cord corresponds to interneurons, progressively expands anteriorly reaching the midbrain-hindbrain boundary. During retinal neurogenesis, Xvsx1 expression is maintained in retinal progenitors and in a peripheral region of the ciliary marginal zone, while in the central retina, it becomes restricted to differentiated bipolar cells.