Targeted expression of the dominant-negative FGFR4a in the eye using Xrx1A regulatory sequences interferes with normal retinal development

Injury-induced MAPK activation triggers body axis formation in Hydra by default Wnt signaling

Hydra's almost unlimited regenerative potential is based on Wnt signaling, but so far it is unknown how the injury stimulus is transmitted to discrete patterning fates in head and foot regenerates. We previously identified mitogen-activated protein kinases (MAPKs) among the earliest injury response molecules in Hydra head regeneration. Here, we show that three MAPKs-p38, c-Jun N-terminal kinases (JNKs), and extracellular signal-regulated kinases (ERKs)-are essential to initiate regeneration in Hydra, independent of the wound position. Their activation occurs in response to any injury and requires calcium and reactive oxygen species (ROS) signaling. Phosphorylated MAPKs hereby exhibit cross talk with mutual antagonism between the ERK pathway and stress-induced MAPKs, orchestrating a balance between cell survival and apoptosis. Importantly, Wnt3 and Wnt9/10c, which are induced by MAPK signaling, can partially rescue regeneration in tissues treated with MAPK inhibitors. Also, foot regenerates can be reverted to form head tissue by a pharmacological increase of β-catenin signaling or the application of recombinant Wnts. We propose a model in which a β-catenin-based stable gradient of head-forming capacity along the primary body axis, by differentially integrating an indiscriminate injury response, determines the fate of the regenerating tissue. Hereby, Wnt signaling acquires sustained activation in the head regenerate, while it is transient in the presumptive foot tissue. Given the high level of evolutionary conservation of MAPKs and Wnts, we assume that this mechanism is deeply embedded in our genome.

miR-199 plays both positive and negative regulatory roles in Xenopus eye development

To investigate microRNA (miR) functions in early eye development, we asked whether eye field transcription factors (EFTFs) are targets of miR-dependent regulation in Xenopus embryos. Argonaute (AGO) ribonucleoprotein complexes, including miRs and targeted mRNAs, were coimmunoprecipitated from transgenic embryos expressing myc-tagged AGO under the control of the rax1 promoter; mRNAs for all EFTFs coimmunoprecipitated with Ago in late neurulae. Computational predictions of miR binding sites within EFTF 3'UTRs identified miR-199a-3p ("miR-199") as a candidate regulator of EFTFs, and miR-199 was shown to regulate rax1 in vivo. Targeted overexpression of miR-199 led to small eyes, a reduction in EFTF expression, and reduced cell proliferation. Inhibition of interactions between mir-199 and the rax1 3'UTR reversed the small eye phenotype. Although targeted knockdown of miR-199 left the eye field intact, it reduced optic cup outgrowth and disrupted eye formation. Computational identification of candidate miR-199 targets within the Xenopus transcriptome led to the identification of ptk7 as a candidate regulator. Targeted overexpression of ptk7 resulted in abnormal optic cup formation and a reduction or loss of eye development, recapitulating the range of eye phenotypes seen following miR-199 knockdown. Our results indicate that miR-199 plays both positive and negative regulatory roles in eye development.

Autoregulation of retinal homeobox (rax) gene promoter activity through a highly conserved genomic element

The Retinal homeobox (rax) gene is expressed in vertebrate retinal progenitor and stem cells and is essential for retinal development. In frogs, rax is expressed in the ciliary marginal zone (CMZ), a region containing retinal progenitor and stem cells at the anterior of the eye. Little is known regarding regulation of rax transcription and regulation of transcription of rax targets. We found that three ultra-conserved genomic elements (UCEs) flanking the rax coding region regulate expression of a rax promoter-GFP transgene in Xenopus tadpoles. One of these elements, UCE1, regulates expression of the transgene in the dorsal CMZ. UCE1 contains a Rax binding site, PCE-1. We demonstrate that rax regulates expression of the transgene through the PCE-1 site found in UCE1. Therefore, rax transcription in the CMZ is controlled, in part, by autoregulatory mechanisms.

Fgfr signaling is required as the early eye field forms to promote later patterning and morphogenesis of the eye

BACKGROUND

A major step in eye morphogenesis is the transition from optic vesicle to optic cup, which occurs as a ventral groove forms along the base of the optic vesicle. A ventral gap in the eye, or coloboma, results when this groove fails to close. Extrinsic signals, such as fibroblast growth factors (Fgfs), play a critical role in the development and morphogenesis of the vertebrate eye. Whether these extrinsic signals are required throughout eye development, or within a defined critical period remains an unanswered question.

RESULTS

Here we show that an early Fgf signal, required as the eye field is first emerging, drives eye morphogenesis. In addition to triggering coloboma, inhibition of this early Fgf signal results in defects in dorsal-ventral patterning of the neural retina, particularly in the nasal retina, and development of the periocular mesenchyme (POM). These processes are unaffected by inhibition of Fgfr signaling at later time points.

CONCLUSIONS

We propose that Fgfs act within an early critical period as the eye field forms to promote development of the neural retina and POM, which subsequently drive eye morphogenesis.

Loss of cell-extracellular matrix interaction triggers retinal regeneration accompanied by Rax and Pax6 activation

The whole retina regenerates from retinal pigmented epithelial (RPE) cells by transdifferentiation in the adult newt and Xenopus laevis when it is surgically removed. We produced a transgenic animal line, in which EGFP expression is under the control of Rax pomotor. Using F1 and F2 generations, we analyzed Rax-EGFP expression during retinal regeneration in a tissue culture model. In the culture, 4 zones were distinguished as RPE cells migrating outwards from the periphery of the explant: the explant zone, epithelial zone, transition zone and differentiation zone. Expression of transcription factors such as Pax6 and Rax-EGFP was observed in different zones. Rax-EGFP expression preceded Pax6 expression, and the expression of both genes occurred in RPE cells that had lost contact with the basement membrane facing the choroid. We have developed a new culture method in which RPE tissues are embedded in Matrigel. This method has many advantages over the previous gel-overlay method to reproduce construction of 3D-retinal structures and clearly showed that RPE cells need to be detached from the choroid before entering the regeneration pathway. The present results indicate that the temporal changes in cell-cell and cell-extracellular matrix interactions regulate transdifferentiation.

Social Network: JAZ Protein Interactions Expand Our Knowledge of Jasmonate Signaling

Members of the family of JASMONATE ZIM-DOMAIN (JAZ) proteins are key regulators of the jasmonate (JA) hormonal response. The 12-member family is characterized by three conserved domains, an N-terminal domain, a TIFY-containing ZINC-FINGER EXPRESSED IN INFLORESCENCE MERISTEM domain, and a C-terminal Jas domain. JAZ proteins regulate JA-responsive gene transcription by inhibiting DNA-binding transcription factors in the absence of JA. JAZ proteins interact in a hormone-dependent manner with CORONATINE INSENSITIVE 1 (COI1), the recognition component of the E3 ubiquitin ligase, SCF(COI1), resulting in the ubiquitination and subsequent degradation of JAZs via the 26S proteasome pathway. Since their discovery in 2007, JAZ proteins have been implicated in protein-protein interactions with multiple transcription factors. These studies have shed light on the mechanism by which JAZs repress transcription, are targeted for degradation, modulate the JA signaling response, and participate in crosstalk with other hormone signaling pathways. In this review, we will take a close look at the recent discoveries made possible by the characterization JAZ protein-protein interactions.

FGF signaling regulates rod photoreceptor cell maintenance and regeneration in zebrafish

Fgf signaling is required for many biological processes involving the regulation of cell proliferation and maintenance, including embryonic patterning, tissue homeostasis, wound healing, and cancer progression. Although the function of Fgf signaling is suggested in several different regeneration models, including appendage regeneration in amphibians and fin and heart regeneration in zebrafish, it has not yet been studied during zebrafish photoreceptor cell regeneration. Here we demonstrate that intravitreal injections of FGF-2 induced rod precursor cell proliferation and photoreceptor cell neuroprotection during intense light damage. Using the dominant-negative Tg(hsp70:dn-fgfr1) transgenic line, we found that Fgf signaling was required for homeostasis of rod, but not cone, photoreceptors. Even though fgfr1 is expressed in both rod and cone photoreceptors, we found that Fgf signaling differentially affected the regeneration of cone and rod photoreceptors in the light-damaged retina, with the dominant-negative hsp70:dn-fgfr1 transgene significantly repressing rod photoreceptor regeneration without affecting cone photoreceptors. These data suggest that rod photoreceptor homeostasis and regeneration is Fgf-dependent and that rod and cone photoreceptors in adult zebrafish are regulated by different signaling pathways.

Regulation of retinal homeobox gene transcription by cooperative activity among cis-elements

The retinal homeobox (Rx/rax) gene is essential for the development of the eye. Rax is among the earliest genes expressed during eye development, beginning in the prospective eye fields in the anterior neural plate. Additionally Rax expression persists in retinal progenitor cells and in differentiated photoreceptors. We have isolated and characterized a 2.8 kb genomic DNA fragment that regulates expression of Rax in the developing and maturing retina. We have discovered and characterized cis-acting elements that function to specifically control spatial and temporal Rax expression during retinal development. We have found that the regulation of Rax2A promoter activity requires cooperative interactions between positive and negative regulatory elements. Further, a highly conserved genomic element containing SOX, OTX, and POU transcription factor binding sites is necessary but not sufficient for promoter activity in retinal progenitor or stem cells. Finally, a putative binding element for forkhead transcription factors is necessary for promoter activity and can cooperate with other cis-acting elements to drive Rax2A promoter activity.

Temporal requirement of the protein tyrosine phosphatase Shp2 in establishing the neuronal fate in early retinal development

FGF signaling is critical in the development of the vertebrate retina, which differentiates in a wave-like pattern similar to that found in the Drosophila eye. In this study, we investigated the mechanism of FGF signaling in vertebrate eye development by identifying Shp2, a protein tyrosine phosphatase, as a novel factor in orchestrating retinal morphogenesis. Using a series of Shp2 conditional mutants, we have shown that Shp2 is specifically required for the initiation of retinal neurogenesis but not for the maintenance of the retinal differentiation program. By mosaic deletion of Shp2, we further demonstrated that Shp2 ablation did not prevent the spreading of the retinal differentiation wave. Shp2 instead controls the patterning of the optic vesicle by regulating the retinal progenitor factors and cell proliferation. In ex vivo culture models and genetic rescue experiments, we showed that Shp2 acts downstream to FGF signaling in retinal development and that it can be functionally substituted by activated Ras signaling. Together, these results demonstrate that Shp2 mediates FGF-Ras signaling to control retinal progenitor cell fate.

Regulation of photoreceptor gene expression by the retinal homeobox (Rx) gene product

The retinal homeobox (Rx) gene product is essential for eye development. However little is known about its molecular function. It has been demonstrated that Rx binds to photoreceptor conserved element (PCE-1), a highly conserved element found in the promoter region of photoreceptor-specific genes such as rhodopsin and red cone opsin. We verify that Rx is co-expressed with rhodopsin and red cone opsin in maturing photoreceptors and demonstrate that Rx binds to the rhodopsin and red cone opsin promoters in vivo. We also find that Rx can cooperate with the Xenopus analogs of Crx and Nrl, otx5b and XLMaf (respectively), to activate a Xenopus opsin promoter-dependent reporter. Finally, we demonstrate that reduction of Rx expression in tadpoles results in decreases in expression of several PCE-1 containing photoreceptor genes, abnormal photoreceptor morphology, and impaired vision. Our data suggests that Rx, in combination with other transcription factors, is necessary for normal photoreceptor gene expression, maintenance, and function. This establishes a direct role for Rx in regulation of genes expressed in a differentiated cell type.

A directional Wnt/beta-catenin-Sox2-proneural pathway regulates the transition from proliferation to differentiation in the Xenopus retina

Progenitor cells in the central nervous system must leave the cell cycle to become neurons and glia, but the signals that coordinate this transition remain largely unknown. We previously found that Wnt signaling, acting through Sox2, promotes neural competence in the Xenopus retina by activating proneural gene expression. We now report that Wnt and Sox2 inhibit neural differentiation through Notch activation. Independently of Sox2, Wnt stimulates retinal progenitor proliferation and this, when combined with the block on differentiation, maintains retinal progenitor fates. Feedback inhibition by Sox2 on Wnt signaling and by the proneural transcription factors on Sox2 mean that each element of the core pathway activates the next element and inhibits the previous one, providing a directional network that ensures retinal cells make the transition from progenitors to neurons and glia.

Morphogenesis and cytodifferentiation of the avian retinal pigmented epithelium require downregulation of Group B1 Sox genes

The optic vesicle is a multipotential primordium of the retina, which becomes subdivided into the neural retina and retinal pigmented epithelium domains. Although the roles of several paracrine factors in patterning the optic vesicle have been studied extensively, little is known about cell-autonomous mechanisms that regulate coordinated cell morphogenesis and cytodifferentiation of the retinal pigmented epithelium. Here we demonstrate that members of the SoxB1 gene family, Sox1, Sox2 and Sox3, are all downregulated in the presumptive retinal pigmented epithelium. Constitutive maintenance of SoxB1 expression in the presumptive retinal pigmented epithelium both in vivo and in vitro resulted in the absence of cuboidal morphology and pigmentation, and in concomitant induction of neural differentiation markers. We also demonstrate that exogenous Fgf4 inhibits downregulation all SoxB1 family members in the presumptive retinal pigment epithelium. These results suggest that retinal pigment epithelium morphogenesis and cytodifferentiation requires SoxB1 downregulation, which depends on the absence of exposure to an FGF-like signal.

Extracellular regulation of developmental cell signaling by XtSulf1

Heparan sulfate proteoglycans (HSPGs) are synthesised and modified in the Golgi before they are presented at the cell surface. Modifications include the addition of sulfate groups at specific positions on sugar residues along the heparan sulfate (HS) chain which results in a structural heterogeneity that underpins the ability of HSPGs to bind with high affinity to many different proteins, including growth factors and their receptors. Sulf1 codes for a 6-0-endosulfatase that is present and active extracellularly, providing a further mechanism to generate structural diversity through the post-synthetic remodelling of HS. Here we use Xenopus embryos to demonstrate in vivo that Xtsulf1 plays an important role in modulating cell signaling during development. We show that while XtSulf1 can enhance the axis-inducing activity of Wnt11, XtSulf1 acts during embryogenesis to restrict BMP and FGF signaling.

Molecular links among the causative genes for ocular malformation: Otx2 and Sox2 coregulate Rax expression

The neural-related genes Sox2, Pax6, Otx2, and Rax have been associated with severe ocular malformations such as anophthalmia and microphthalmia, but it remains unclear as to how these genes are linked functionally. We analyzed the upstream signaling of Xenopus Rax (also known as Rx1) and identified the Otx2 and Sox2 proteins as direct upstream regulators of Rax. We revealed that endogenous Otx2 and Sox2 proteins bound to the conserved noncoding sequence (CNS1) located approximately 2 kb upstream of the Rax promoter. This sequence is conserved among vertebrates and is required for potent transcriptional activity. Reporter assays showed that Otx2 and Sox2 synergistically activated transcription via CNS1. Furthermore, the Otx2 and Sox2 proteins physically interacted with each other, and this interaction was affected by the Sox2-missense mutations identified in these ocular disorders. These results demonstrate that the direct interaction and interdependence between the Otx2 and Sox2 proteins coordinate Rax expression in eye development, providing molecular linkages among the genes responsible for ocular malformation.

FGF-mediated induction of ciliary body tissue in the chick eye

Upon morphogenesis, the simple neuroepithelium of the optic vesicle gives rise to four basic tissues in the vertebrate optic cup: pigmented epithelium, sensory neural retina, secretory ciliary body and muscular iris. Pigmented epithelium and neural retina are established through interactions with specific environments and signals: periocular mesenchyme/BMP specifies pigmented epithelium and surface ectoderm/FGF specifies neural retina. The anterior portions (iris and ciliary body) are specified through interactions with lens although the molecular mechanisms of induction have not been deciphered. As lens is a source of FGF, we examined whether this factor was involved in inducing ciliary body. We forced the pigmented epithelium of the embryonic chick eye to express FGF4. Infected cells and their immediate neighbors were transformed into neural retina. At a distance from the FGF signal, the tissue transitioned back into pigmented epithelium. Ciliary body tissue was found in the transitioning zone. The ectopic ciliary body was never in contact with the lens tissue. In order to assess the contribution of the lens on the specification of normal ciliary body, we created optic cups in which the lens had been removed while still pre-lens ectoderm. Ciliary body tissue was identified in the anterior portion of lens-less optic cups. We propose that the ciliary body may be specified at optic vesicle stages, at the same developmental stage when the neural retina and pigmented epithelium are specified and we present a model as to how this could be accomplished through overlapping BMP and FGF signals.

Differential role of 14-3-3 family members in Xenopus development

The 14-3-3 proteins are intracellular dimeric phosphoserine/threonine binding molecules that participate in signal transduction, checkpoint control, nutrient sensing, and cell survival pathways. Previous work established that 14-3-3 proteins are required in early Xenopus laevis development by modulating fibroblast growth factor signaling. Although this general requirement for 14-3-3 proteins in Xenopus early embryogenesis is established, there is no information about the specific role of individual 14-3-3 genes. Botanical studies previously demonstrated functional specificity among 14-3-3 genes during plant development. In this study, an antisense morpholino oligo microinjection approach was used to characterize the requirement for six specific 14-3-3 family members in Xenopus embryogenesis. Microinjection experiments followed by Western blot analysis showed that morpholinos reduced specific 14-3-3 protein levels. Embryos lacking specific 14-3-3 isoforms displayed unique phenotypic defects. In particular, reduction of 14-3-3 tau (tau) protein, and to a lesser extent, 14-3-3 epsilon (epsilon), resulted in embryos with prominent gastrulation and axial patterning defects and reduced mesodermal marker gene expression. In contrast, reduction of 14-3-3 zeta (zeta) protein caused no obvious phenotypic abnormalities. Reduction of 14-3-3 gamma (gamma) protein resulted in eye defects without gastrulation abnormalities. Therefore, individual 14-3-3 genes have separable functions in vertebrate embryonic development.

Pbx1 and Meis1 regulate activity of the Xenopus laevis Zic3 promoter through a highly conserved region

Xenopus Zic3 (zinc finger in the cerebellum-3) is expressed in the dorsal neural tube of tailbud embryos and tadpoles. We have isolated a 3.1kb DNA fragment from the Xenopus laevis Zic3 locus that drives proper expression of a GFP reporter in transgenic frog tailbud embryos and tadpoles. This fragment contains regions that are highly similar among frogs, mice, and humans. One extremely conserved region contains a predicted Pbx binding site. We found that the transcription factors Pbx1b and Meis1 can bind this site and synergistically transactivate expression of a reporter containing the conserved region. Finally, we found that an intact Pbx site is essential for normal Zic3 promoter activity in transgenic frog embryos. Our data strongly suggest that a highly conserved region of the Zic3 promoter functions by direct interaction with Pbx1b and Meis1.

Recombineered Xenopus tropicalis BAC expresses a GFP reporter under the control of Arx transcriptional regulatory elements in transgenic Xenopus laevis embryos

The aristaless-related homeobox (Arx) gene is expressed in a dynamic pattern in the developing vertebrate forebrain. We identified a bacterial artificial chromosome (BAC) containing the Xenopus tropicalis Arx gene and replaced a portion of the first coding exon with a green fluorescent protein (GFP) expression cassette by homologous recombination in bacteria (recombineering). Transgenic X. laevis embryos obtained by microinjecting the modified BAC expressed GFP in the developing forebrain in a pattern identical to that of the endogenous Arx gene. Thus, this BAC contains transcriptional regulatory elements necessary for regulating proper expression in transgenic frogs. This work demonstrates that transgenesis using recombineered BACs is a viable technique in Xenopus.

Frizzled 5 signaling governs the neural potential of progenitors in the developing Xenopus retina

Progenitors in the developing central nervous system acquire neural potential and proliferate to expand the pool of precursors competent to undergo neuronal differentiation. The formation and maintenance of neural-competent precursors are regulated by SoxB1 transcription factors, and evidence that their expression is regionally regulated suggests that specific signals regulate neural potential in subdomains of the developing nervous system. We show that the frizzled (Fz) transmembrane receptor Xfz5 selectively governs neural potential in the developing Xenopus retina by regulating the expression of Sox2. Blocking either Xfz5 or canonical Wnt signaling within the developing retina inhibits Sox2 expression, reduces cell proliferation, inhibits the onset of proneural gene expression, and biases individual progenitors toward a nonneural fate, without altering the expression of multiple progenitor markers. Blocking Sox2 function mimics these effects. Rescue experiments indicate that Sox2 is downstream of Xfz5. Thus, Fz signaling can regulate the neural potential of progenitors in the developing nervous system.

Expression of FoxE4 and Rx Visualizes the Timing and Dynamics of Critical Processes Taking Place during Initial Stages of Vertebrate Eye Development

Several transcription factors have a critical function during initial stages of vertebrate eye formation. In this paper, we discuss the role of the Rx subfamily of homeobox-containing genes in retinal development, and the role of the Foxe3 and FoxE4 subfamily of forkhead box-containing genes in lens development. Rx genes are expressed in the initial stages of retinal development and they play a critical role in eye formation. Elimination of Rx function in mice results in lack of eye formation. Abnormal eye development observed in the mouse mutation eyeless (ey1), the medakatemperature-sensitive mutation eyeless (el), and the zebrafish mutation chokh are caused by abnormal regulation or function of Rx genes. In humans, a mutation in Rx leads to anophthalmia. In contrast, Foxe3 and FoxE4 genes are expressed in the lens and they play an essential role in its formation. Mutations in the Foxe3 gene are the cause of the mouse mutation dysgenetic lens (dyl) and in humans, mutation in FOXE3 leads to anterior segment dysgenesis and cataracts. Since Rx and FoxE4 are expressed in the earliest stages of retina and lens development, their expression visualizes the timing and dynamics of the crucial processes that comprise eye formation. In this paper we present a model of eye development based on the expression pattern of these two genes.

Function and regulation of FoxF1 during Xenopus gut development

Development of the visceral mesoderm is a critical process in the organogenesis of the gut. Elucidation of function and regulation of genes involved in the development of visceral mesoderm is therefore essential for an understanding of gut organogenesis. One of the genes specifically expressed in the lateral plate mesoderm, and later in its derivative, the visceral mesoderm, is the Fox gene FoxF1. Its function is critical for Xenopus gut development, and embryos injected with FoxF1 morpholino display abnormal gut development. In the absence of FoxF1 function, the lateral plate mesoderm, and later the visceral mesoderm, does not proliferate and differentiate properly. Region- and stage-specific markers of visceral mesoderm differentiation, such as Xbap and alpha-smooth muscle actin, are not activated. The gut does not elongate and coil. These experiments provide support for the function of FoxF1 in the development of visceral mesoderm and the organogenesis of the gut. At the molecular level, FoxF1 is a downstream target of BMP4 signaling. BMP4 can activate FoxF1 transcription in animal caps and overexpression of FoxF1 can rescue twinning phenotypes, which results from the elimination of BMP4 signaling. The cis-regulatory elements of FoxF1 are located within a 2 kb DNA fragment upstream of the coding region. These sequences can drive correct temporal-spatial expression of a GFP reporter gene in transgenic Xenopus tadpoles. These sequences represent a unique tool, which can be used to specifically alter gene expression in the lateral plate mesoderm.

Roles of cell-extrinsic growth factors in vertebrate eye pattern formation and retinogenesis

Formation of the vertebrate visual system involves complex interplays of cell-extrinsic cues and cell-intrinsic determinants. Studies in several vertebrate species demonstrate that multiple classes of signaling molecules participate in pattern formation of the eye and neurogenesis of the retina. Certain signals, such as hedgehog, BMP, and FGF molecules, are repeatedly deployed at varying concentration thresholds and in different cellular contexts. Accumulating evidence reveals a striking conservation of molecular mechanisms regulating the neurogenic process between Drosophila and vertebrate retinas. The remaining challenge is to understand how these well-characterized signaling pathways are activated and integrated to impact eye morphogenesis and retinal progenitor cell fate determination.