Xrx1 controls proliferation and neurogenesis in Xenopus anterior neural plate

Comparing the effects of three neonicotinoids on embryogenesis of the South African clawed frog Xenopus laevis

Neonicotinoids (NEOs) are widely used insecticides that are ubiquitous in agricultural use. Since NEOs are found in natural waters as well as in tap water and human urine in regions where NEOs are widely used, NEOs pose a potential hazard to non-target organisms such as animals and humans. Some of the commonly detected NEOs are imidacloprid (IMD), thiamethoxam (TMX), and its metabolite clothianidin (CLO). Although previously published scientific information, including an assessment of the environmental risks, particularly for bees, had resulted in a ban on the outdoor use of these three NEOs in the EU - their use is now only permitted in closed greenhouses - these NEOs continue to be used in agriculture in many other parts of the world. Therefore, a detailed study and comparison of the effects of NEOs on the embryonic development of non-target organisms is needed to further define the risk profiles. Embryos of the South African clawed frog Xenopus laevis, a well-established aquatic model, were exposed to different concentrations of IMD, TMX, or CLO (0.1-100 mg/L) to study and compare the possible effects of a single contaminant in natural water bodies on early embryogenesis. The results included a reduced body length, a smaller orbital space, impaired cranial cartilage and nerves, and an altered heart structure and function. At the molecular level, NEO exposure partially resulted in an altered expression of tissue-specific factors, which are involved in eye, cranial placode, and heart development. Our results suggest that the NEOs studied negatively affect the embryonic development of the non-target organism X. laevis. Since pesticides, especially NEOs, pollute the environment worldwide, it is suggested that they are strictly controlled and monitored in the areas where they are used. In addition, the question arises as to whether pesticide metabolites also pose a risk to the environment and need to be investigated further so that they can be taken into account when registering ingredients.

Regulation of gene expression downstream of a novel Fgf/Erk pathway during Xenopus development

Activation of Map kinase/Erk signalling downstream of fibroblast growth factor (Fgf) tyrosine kinase receptors regulates gene expression required for mesoderm induction and patterning of the anteroposterior axis during Xenopus development. We have proposed that a subset of Fgf target genes are activated in the embyo in response to inhibition of a transcriptional repressor. Here we investigate the hypothesis that Cic (Capicua), which was originally identified as a transcriptional repressor negatively regulated by receptor tyrosine kinase/Erk signalling in Drosophila, is involved in regulating Fgf target gene expression in Xenopus. We characterise Xenopus Cic and show that it is widely expressed in the embryo. Fgf overexpression or ectodermal wounding, both of which potently activate Erk, reduce Cic protein levels in embryonic cells. In keeping with our hypothesis, we show that Cic knockdown and Fgf overexpression have overlapping effects on embryo development and gene expression. Transcriptomic analysis identifies a cohort of genes that are up-regulated by Fgf overexpression and Cic knockdown. We investigate two of these genes as putative targets of the proposed Fgf/Erk/Cic axis: fos and rasl11b, which encode a leucine zipper transcription factor and a ras family GTPase, respectively. We identify Cic consensus binding sites in a highly conserved region of intron 1 in the fos gene and Cic sites in the upstream regions of several other Fgf/Cic co-regulated genes, including rasl11b. We show that expression of fos and rasl11b is blocked in the early mesoderm when Fgf and Erk signalling is inhibited. In addition, we show that fos and rasl11b expression is associated with the Fgf independent activation of Erk at the site of ectodermal wounding. Our data support a role for a Fgf/Erk/Cic axis in regulating a subset of Fgf target genes during gastrulation and is suggestive that Erk signalling is involved in regulating Cic target genes at the site of ectodermal wounding.

Foxd1-dependent induction of a temporal retinal character is required for visual function

Appropriate patterning of the retina during embryonic development is assumed to underlie the establishment of spatially localised specialisations that mediate the perception of specific visual features. For example, in zebrafish, an area involved in high acuity vision (HAA) is thought to be present in the ventro-temporal retina. Here, we show that the interplay of the transcription factor Rx3 with Fibroblast Growth Factor and Hedgehog signals initiates and restricts foxd1 expression to the prospective temporal retina, initiating naso-temporal regionalisation of the retina. Abrogation of Foxd1 results in the loss of temporal and expansion of nasal retinal character, and consequent absence of the HAA. These structural defects correlate with severe visual defects, as assessed in optokinetic and optomotor response assays. In contrast, optokinetic responses are unaffected in the opposite condition, in which nasal retinal character is lost at the expense of expanded temporal character. Our study indicates that the establishment of temporal retinal character during early retinal development is required for the specification of the HAA, and suggests a prominent role of the temporal retina in controlling specific visual functions.

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.

Biallelic sequence and structural variants in RAX2 are a novel cause for autosomal recessive inherited retinal disease

Purpose

RAX2 encodes a homeobox-containing transcription factor, in which four monoallelic pathogenic variants have been described in autosomal dominant cone-dominated retinal disease.

Methods

Exome sequencing in a European cohort with inherited retinal disease (IRD) (n = 2086) was combined with protein structure modeling of RAX2 missense variants, bioinformatics analysis of deletion breakpoints, haplotyping of RAX2 variant c.335dup, and clinical assessment of biallelic RAX2-positive cases and carrier family members.

Results

Biallelic RAX2 sequence and structural variants were found in five unrelated European index cases, displaying nonsyndromic autosomal recessive retinitis pigmentosa (ARRP) with an age of onset ranging from childhood to the mid-40s (average mid-30s). Protein structure modeling points to loss of function of the novel recessive missense variants and to a dominant-negative effect of the reported dominant RAX2 alleles. Structural variants were fine-mapped to disentangle their underlying mechanisms. Haplotyping of c.335dup in two cases suggests a common ancestry.

Conclusion

This study supports a role for RAX2 as a novel disease gene for recessive IRD, broadening the mutation spectrum from sequence to structural variants and revealing a founder effect. The identification of biallelic RAX2 pathogenic variants in five unrelated families shows that RAX2 loss of function may be a nonnegligible cause of IRD in unsolved ARRP cases.

The age-regulated zinc finger factor ZNF367 is a new modulator of neuroblast proliferation during embryonic neurogenesis

Global population aging is one of the major social and economic challenges of contemporary society. During aging the progressive decline in physiological functions has serious consequences for all organs including brain. The age-related incidence of neurodegenerative diseases coincides with the sharp decline of the amount and functionality of adult neural stem cells. Recently, we identified a short list of brain age-regulated genes by means of next-generation sequencing. Among them znf367 codes for a transcription factor that represents a central node in gene co-regulation networks during aging, but whose function in the central nervous system (CNS), is completely unknown. As proof of concept, we analysed the role of znf367 during Xenopus laevis neurogenesis. By means of a gene loss of function approach limited to the CNS, we suggested that znf367 might act as a key controller of the neuroblast cell cycle, particularly in the progression of mitosis and spindle checkpoint. A candidate gene approach based on a weighted-gene co-expression network analysis, revealed fancd2 and ska3 as possible targets of znf367. The age-related decline of znf367 correlated well with its role during embryonic neurogenesis, opening new lines of investigation also in adult neurogenesis to improved maintenance and even repair of neuronal function.

The role of the Rx homeobox gene in retinal progenitor proliferation and cell fate specification

The Retinal homeobox gene (Rx; also Rax) plays a crucial role in the early development of the vertebrate eye. Germline deletion of Rx in mice results in the failure of optic vesicle formation, leading to anophthalmia. Recent research using conditional mouse knockout models provides some clues to the role of Rx in eye development following optic vesicle formation. However, the functions of Rx in embryonic retinogenesis are still not fully understood. We investigated the function of Rx in the mouse neural retina using a conditional knockout where the Pax6α-Cre driver deletes Rx activity in early retinal progenitors. The deletion of Rx activity causes a loss of retinal lamination, a depletion of retinal progenitors, and a change in retinal cell fate in our conditional knockout model. The deletion of Rx leads to an absence of late-born retinal neurons (rods and bipolar cells) and Müller glia at postnatal ages, as well as a loss of the early-born cone photoreceptors. Decreased BrdU labeling in the Rx-deleted portion of the retina suggests a loss of retinal progenitors via early cell cycle exit, which likely prevents the formation of late-born cells. As early-born cells, cone photoreceptors should not be as affected by early cell cycle exit of retinal progenitors. However, embryonic cone photoreceptor labeling is also markedly reduced in Rx-deleted retinas. Together these data demonstrate the importance of Rx for retinal progenitor proliferation and a specific requirement of Rx for cone formation in mice.

Evolution of the Rax family of developmental transcription factors in vertebrates

Rax proteins comprise a small family of paired-type, homeodomain-containing transcription factors with essential functions in eye and forebrain development. While invertebrates possess only one Rax gene, vertebrates can have several Rax paralogue genes, but the evolutionary history of the members of the family has not been studied in detail. Here, we present a thorough analysis of the evolutionary relationships between vertebrate Rax genes and proteins available in diverse genomic databases. Phylogenetic and synteny analyses indicate that Rax genes went through a duplication in an ancestor of all jawed vertebrates (Gnathostomata), giving rise to the ancestral vertebrate Rax1 and Rax2 genes. This duplication event is likely related to the proposed polyploidisations that occurred during early vertebrate evolution. Subsequent genome-wide duplications in the lineage of ray-finned fish (Actinopterygii) originated new Rax2 paralogues in the genomes of teleosts. In the lobe-finned fish lineage (Sarcopterygii), the N-terminal octapeptide domain of Rax2 was lost in a common ancestor of tetrapods, giving rise to a shorter version of Rax2 in this lineage. Within placental mammals, the Rax2 gene was lost altogether in an ancestor of rodents and lagomorphs (Glires). Finally, we discuss the scientific literature in the light of Rax gene evolution and propose new avenues of research on the function of this important family of transcriptional regulators.

Activation of Wnt/ß-catenin signaling in ESC promotes rostral forebrain differentiation in vitro

Wnt/ß-catenin signaling is crucial for maintenance of pluripotent state of embryonic stem cell (ESC). However, it is unclear how Wnt/ß-catenin signaling affects the differentiation ability of ESC, especially with regard to rostral forebrain cells. Here, using Rax, rostral forebrain marker, and Wnt/ß-catenin reporter lines, we report ratio of Rax⁺ and Wnt responding tissue (Wnt⁺) patterns, which were affected by seeding number of ESC in three-dimensional culture system. Surprisingly, we found ß-catenin level and localization are heterogeneous in ESC colony by immunostaining and time-laps imaging of ß-catenin-mEGFP signals. Moreover, activation of Wnt signaling in ESC promoted expression level and nuclear localization of ß-catenin, and mRNA levels of Wnt antagonists, axin2 and dkk1, leading to upregulating Wnt/ß-catenin reporter in ESC state and Rax expression at differentiation culture day 7. Together, our results suggest that activation of Wnt signaling in ESC promotes the differentiation efficacy of rostral forebrain cells. Wnt-priming culture method may provide a useful tool for applications in the areas of basic science and molecular therapeutics for regenerative medicine.

Noggin-Mediated Retinal Induction Reveals a Novel Interplay Between Bone Morphogenetic Protein Inhibition, Transforming Growth Factor β, and Sonic Hedgehog Signaling

It has long been known that the depletion of bone morphogenetic protein (BMP) is one of the key factors necessary for the development of anterior neuroectodermal structures. However, the precise molecular mechanisms that underlie forebrain regionalization are still not completely understood. Here, we show that Noggin1 is involved in the regionalization of anterior neural structures in a dose-dependent manner. Low doses of Noggin1 expand prosencephalic territories, while higher doses specify diencephalic and retinal regions at the expense of telencephalic areas. A similar dose-dependent mechanism determines the ability of Noggin1 to convert pluripotent cells in prosencephalic or diencephalic/retinal precursors, as shown by transplant experiments and molecular analyses. At a molecular level, the strong inhibition of BMP signaling exerted by high doses of Noggin1 reinforces the Nodal/transforming growth factor (TGF)β signaling pathway, leading to activation of Gli1 and Gli2 and subsequent activation of Sonic Hedgehog (SHH) signaling. We propose a new role for Noggin1 in determining specific anterior neural structures by the modulation of TGFβ and SHH signaling.

Brief report: Rx1 defines retinal precursor identity by repressing alternative fates through the activation of TLE2 and Hes4

The molecular mechanisms underlying the acquisition of retinal precursor identity are scarcely defined. Although the homeobox gene Rx1 (also known as Rax) plays a major role in specifying retinal precursors and maintaining their multipotent state, the involved mechanisms remain to be largely deciphered. Here, following a highthroughput screen for genes regulated by Rx1, we found that this transcription factor specifies the fate of retinal progenitors by repressing genes normally activated in adjacent ectodermal territories. Unexpectedly, we also observed that Rx1, mainly through the activation of the transcriptional repressors TLE2 and Hes4, is necessary and sufficient to inhibit endomesodermal gene expression in retinal precursors of the eye field. In particular, Rx1 knockdown leads retinogenic blastomeres to adopt an endomesodermal fate, indicating a previously undescribed function for Rx1 in preventing the expression of endomesoderm determinants known to inhibit retinal fate. Altogether these data suggest that an essential requirement to establish a retinal precursor identity is the active inhibition of pathways leading to alternative fates.

Xenopus mutant reveals necessity of rax for specifying the eye field which otherwise forms tissue with telencephalic and diencephalic character

The retinal anterior homeobox (rax) gene encodes a transcription factor necessary for vertebrate eye development. rax transcription is initiated at the end of gastrulation in Xenopus, and is a key part of the regulatory network specifying anterior neural plate and retina. We describe here a Xenopus tropicalis rax mutant, the first mutant analyzed in detail from a reverse genetic screen. As in other vertebrates, this nonsense mutation results in eyeless animals, and is lethal peri-metamorphosis. Tissue normally fated to form retina in these mutants instead forms tissue with characteristics of diencephalon and telencephalon. This implies that a key role of rax, in addition to defining the eye field, is in preventing alternative forebrain identities. Our data highlight that brain and retina regions are not determined by the mid-gastrula stage but are by the neural plate stage. An RNA-Seq analysis and in situ hybridization assays for early gene expression in the mutant revealed that several key eye field transcription factors (e.g. pax6, lhx2 and six6) are not dependent on rax activity through neurulation. However, these analyses identified other genes either up- or down-regulated in mutant presumptive retinal tissue. Two neural patterning genes of particular interest that appear up-regulated in the rax mutant RNA-seq analysis are hesx1 and fezf2. These genes were not previously known to be regulated by rax. The normal function of rax is to partially repress their expression by an indirect mechanism in the presumptive retina region in wildtype embryos, thus accounting for the apparent up-regulation in the rax mutant. Knock-down experiments using antisense morpholino oligonucleotides directed against hesx1 and fezf2 show that failure to repress these two genes contributes to transformation of presumptive retinal tissue into non-retinal forebrain identities in the rax mutant.

Rax regulates hypothalamic tanycyte differentiation and barrier function in mice

The wall of the ventral third ventricle is composed of two distinct cell populations: tanycytes and ependymal cells. Tanycytes regulate many aspects of hypothalamic physiology, but little is known about the transcriptional network that regulates their development and function. We observed that the retina and anterior neural fold homeobox transcription factor (Rax) is selectively expressed in hypothalamic tanycytes, and showed a complementary pattern of expression to markers of hypothalamic ependymal cells, such as Rarres2 (retinoic acid receptor responder [tazarotene induced] 2). To determine whether Rax controls tanycyte differentiation and function, we generated Rax haploinsufficient mice and examined their cellular and molecular phenotype in adulthood. These mice appeared grossly normal, but careful examination revealed a thinning of the third ventricular wall and reduction of both tanycyte and ependymal markers. These experiments show that Rax is required for hypothalamic tanycyte and ependymal cell differentiation. Rax haploinsufficiency also resulted in the ectopic presence of ependymal cells in the α2 tanycytic zone, where few ependymal cells are normally found, suggesting that Rax is selectively required for α2 tanycyte differentiation. These changes in the ventricular wall were associated with reduced diffusion of Evans Blue tracer from the ventricle to the hypothalamic parenchyma, with no apparent repercussion on the gross anatomical or behavioral phenotype of these mice. In conclusion, we have provided evidence that Rax is required for the normal differentiation and patterning of hypothalamic tanycytes and ependymal cells, as well as for maintenance of the cerebrospinal fluid-hypothalamus barrier.

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.

Hypothalamic subependymal niche: a novel site of the adult neurogenesis

The discovery of undifferentiated, actively proliferating neural stem cells (NSCs) in the mature brain opened a brand new chapter in the contemporary neuroscience. Adult neurogenesis appears to occur in specific brain regions (including hypothalamus) throughout vertebrates’ life, being considered an important player in the processes of memory, learning, and neural plasticity. In the adult mammalian brain, NSCs are located mainly in the subgranular zone (SGZ) of the hippocampal dentate gyrus and in the subventricular zone (SVZ) of the lateral ventricle ependymal wall. Besides these classical regions, hypothalamic neurogenesis occurring mainly along and beneath the third ventricle wall seems to be especially well documented. Neurogenic zones in SGZ, SVZ, and in the hypothalamus share some particular common features like similar cellular cytoarchitecture, vascularization pattern, and extracellular matrix properties. Hypothalamic neurogenic niche is formed mainly by four special types of radial glia-like tanycytes. They are characterized by distinct expression of some neural progenitor and stem cell markers. Moreover, there are numerous suggestions that newborn hypothalamic neurons have a significant ability to integrate into the local neural pathways and to play important physiological roles, especially in the energy balance regulation. Newly formed neurons in the hypothalamus can synthesize and release food intake regulating neuropeptides and they are sensitive to the leptin. On the other hand, high-fat diet positively influences hypothalamic neurogenesis in rodents. The nature of this intriguing new site of adult neurogenesis is still so far poorly studied and requires further investigations.

Retinal stem cells and regeneration of vision system

The vertebrate retina is a well-characterized model for studying neurogenesis. Retinal neurons and glia are generated in a conserved order from a pool of mutlipotent progenitor cells. During retinal development, retinal stem/progenitor cells (RPC) change their competency over time under the influence of intrinsic (such as transcriptional factors) and extrinsic factors (such as growth factors). In this review, we summarize the roles of these factors, together with the understanding of the signaling pathways that regulate eye development. The information about the interactions between intrinsic and extrinsic factors for retinal cell fate specification is useful to regenerate specific retinal neurons from RPCs. Recent studies have identified RPCs in the retina, which may have important implications in health and disease. Despite the recent advances in stem cell biology, our understanding of many aspects of RPCs in the eye remains limited. PRCs are present in the developing eye of all vertebrates and remain active in lower vertebrates throughout life. In mammals, however, PRCs are quiescent and exhibit very little activity and thus have low capacity for retinal regeneration. A number of different cellular sources of RPCs have been identified in the vertebrate retina. These include PRCs at the retinal margin, pigmented cells in the ciliary body, iris, and retinal pigment epithelium, and Müller cells within the retina. Because PRCs can be isolated and expanded from immature and mature eyes, it is possible now to study these cells in culture and after transplantation in the degenerated retinal tissue. We also examine current knowledge of intrinsic RPCs, and human embryonic stems and induced pluripotent stem cells as potential sources for cell transplant therapy to regenerate the diseased retina.

Maturin is a novel protein required for differentiation during primary neurogenesis

Proliferation and differentiation are tightly controlled during neural development. In the embryonic neural plate, primary neurogenesis is driven by the proneural pathway. Here we report the characterization of Maturin, a novel, evolutionarily conserved protein that is required for normal primary neurogenesis. Maturin is detected throughout the early nervous system, yet it is most strongly expressed in differentiating neurons of the embryonic fish, frog and mouse nervous systems. Maturin expression can be induced by the proneural transcription factors Neurog2, Neurod1, and Ebf3. Maturin overexpression promotes neurogenesis, while loss-of-function inhibits the differentiation of neuronal progenitors, resulting in neural plate expansion. Maturin knockdown blocks the ability of Neurog2, Neurod1, and Ebf3 to drive ectopic neurogenesis. Maturin and Pak3, are both required for, and can synergize to promote differentiation of the primary neurons in vivo. Together, our results suggest that Maturin functions during primary neurogenesis and is required for the proneural pathway to regulate neural differentiation.

Rax is a selector gene for mediobasal hypothalamic cell types

The brain plays a central role in controlling energy, glucose, and lipid homeostasis, with specialized neurons within nuclei of the mediobasal hypothalamus, namely the arcuate (ARC) and ventromedial (VMH), tasked with proper signal integration. Exactly how the exquisite cytoarchitecture and underlying circuitry becomes established within these nuclei remains largely unknown, in part because hypothalamic developmental programs are just beginning to be elucidated. Here, we demonstrate that the Retina and anterior neural fold homeobox (Rax) gene plays a key role in establishing ARC and VMH nuclei in mice. First, we show that Rax is expressed in ARC and VMH progenitors throughout development, consistent with genetic fate mapping studies demonstrating that Rax+ lineages give rise to VMH neurons. Second, the conditional ablation of Rax in a subset of VMH progenitors using a Shh::Cre driver leads to a fate switch from a VMH neuronal phenotype to a hypothalamic but non-VMH identity, suggesting that Rax is a selector gene for VMH cellular fates. Finally, the broader elimination of Rax throughout ARC/VMH progenitors using Six3::Cre leads to a severe loss of both VMH and ARC cellular phenotypes, demonstrating a role for Rax in both VMH and ARC fate specification. Combined, our study illustrates that Rax is required in ARC/VMH progenitors to specify neuronal phenotypes within this hypothalamic brain region. Rax thus provides a molecular entry point for further study of the ontology and establishment of hypothalamic feeding circuits.

An essential role for Rax in retina and neuroendocrine system development

In vertebrates, the central nervous system (CNS) develops as a highly hierarchical, patterned organ with a vast diversity of neuronal and glial cell types. The vertebrate retina is developmentally a part of the CNS. Establishment of the vertebrate retina requires a series of developmental steps including specification of the anterior neural plate, evagination of the optic vesicles from the ventral forebrain, and differentiation of cells. The transcription factor RAX is a paired-type homeoprotein that plays a critical role in the eye and forebrain development of vertebrate species. Rax is initially expressed in the anterior neural region of developing mouse embryos, and later in the retina, pituitary gland, hypothalamus, and pineal gland. The targeted deletion of Rax in the mouse results in no eye formation and abnormal forebrain formation. In humans, mutations in the RAX gene lead to anophthalmia and microphthalmia. These observations indicate that RAX plays a pivotal role in the establishment of the retina. In addition, recent studies have reported that retina and pituitary gland tissues can be induced in a culture system from embryonic stem cells, using RAX expression as an indicator of neuronal progenitor cells in the induced tissue, and suggesting that the Rax gene is a key factor in neuronal regeneration. This review highlights the biological functions and molecular mechanisms of RAX in retina, pituitary, hypothalamus, and pineal gland development.

Transcription factors involved in lens development from the preplacodal ectoderm

Lens development is a stepwise process accompanied by the sequential activation of transcription factors. Transcription factor genes can be classified into three groups according to their functions: the first group comprises preplacodal genes, which are implicated in the formation of the preplacodal ectoderm that serves as a common primordium for cranial sensory tissues, including the lens. The second group comprises lens-specification genes, which establish the lens-field within the preplacodal ectoderm. The third group comprises lens-differentiation genes, which promote lens morphogenesis after the optic vesicle makes contact with the presumptive lens ectoderm. Analyses of the regulatory interactions between these genes have provided an overview of lens development, highlighting crucial roles for positive cross-regulation in fate specification and for feed-forward regulation in the execution of terminal differentiation. This overview also sheds light upon the mechanisms of how preplacodal gene activities lead to the activation of genes involved in lens-specification.

Simple, fast, tissue-specific bacterial artificial chromosome transgenesis in Xenopus

We have developed a method of injecting bacterial artificial chromosome (BAC) DNA into Xenopus embryos that is simple and efficient, and results in consistent and tissue-specific expression of transgenes cloned into BAC vectors. Working with large pieces of DNA, as can be accommodated by BACs, is necessary when studying large or complex genes and conducive to studying the function of long-range regulatory elements that act to control developmentally restricted gene expression. We recombineered fluorescent reporters into three Xenopus tropicalis BAC clones targeting three different genes and report that up to 60% of injected embryos express the reporter in a manner consistent with endogenous expression. The behavior of these BACs, which are replicated after injection, contrasts with that of smaller plasmids, which degrade relatively quickly when injected as circular molecules and generally fail to recapitulate endogenous expression when not integrated into the Xenopus genome.

Onecut is a direct neural-specific transcriptional activator of Rx in Ciona intestinalis

Retinal homeobox (Rx) genes play a crucial and conserved role in the development of the anterior neural plate of metazoans. During chordate evolution, they have also acquired a novel function in the control of eye formation and neurogenesis. To characterize the Rx genetic cascade and shed light on the mechanisms that led to the acquisition of this new role in eye development, we studied Rx transcriptional regulation using the ascidian, Ciona intestinalis. Through deletion analysis of the Ci-Rx promoter, we have identified two distinct enhancer elements able to induce Ci-Rx specific expression in the anterior part of the CNS and in the photosensory organ at tailbud and larva stages. Bioinformatic analysis highlighted the presence of two Onecut binding sites contained in these enhancers, so we explored the role of this transcription factor in the regulation of Ci-Rx. By in situ hybridization, we first confirmed that these genes are co-expressed in the same cells. Through a series of in vivo and in vitro experiments, we then demonstrated that the two Onecut sites are responsible for enhancer activation in Ci-Rx endogenous territories. We also demonstrated in vivo that Onecut misexpression is able to induce ectopic activation of the Rx promoter. Finally, we demonstrated that Ci-Onecut is able to promote Ci-Rx expression in the sensory vesicle. Together, these results support the conclusion that in Ciona embryogenesis, Ci-Rx expression is under the control of the Onecut transcription factor and that this factor is necessary and sufficient to specifically activate Ci-Rx through two enhancer elements.

The Retinal Homeobox (Rx) gene is necessary for retinal regeneration

The Retinal Homeobox (Rx) gene is essential for vertebrate eye development. Rx function is required for the specification and maintenance of retinal progenitor cells (RPCs). Loss of Rx function leads to a lack of eye development in a variety of species. Here we show that Rx function is also necessary during retinal regeneration. We performed a thorough characterization of retinal regeneration after partial retinal resection in pre-metamorphic Xenopus laevis. We show that after injury the wound is repopulated with retinal progenitor cells (RPCs) that express Rx and other RPC marker genes. We used an shRNA-based approach to specifically silence Rx expression in vivo in tadpoles. We found that loss of Rx function results in impaired retinal regeneration, including defects in the cells that repopulate the wound and the RPE at the wound site. We show that the regeneration defects can be rescued by provision of exogenous Rx. These results demonstrate for the first time that Rx, in addition to being essential during retinal development, also functions during retinal regeneration.

Eye field specification in Xenopus laevis

Vertebrate eyes begin as a small patch of cells at the most anterior end of the early brain called the eye field. If these cells are removed from an amphibian embryo, the eyes do not form. If the eye field is transplanted to another location on the embryo or cultured in a dish, it forms eyes. These simple cut and paste experiments were performed at the beginning of the last century and helped to define the embryonic origin of the vertebrate eye. The genes necessary for eye field specification and eventual eye formation, by contrast, have only recently been identified. These genes and the molecular mechanisms regulating the initial formation of the Xenopus laevis eye field are the subjects of this review.

Nlcam modulates midline convergence during anterior neural plate morphogenesis

During development, different cell types must undergo distinct morphogenetic programs so that tissues develop the right dimensions in the appropriate place. In early eye morphogenesis, retinal progenitor cells (RPCs) move first towards the midline, before turning around to migrate out into the evaginating optic vesicles. Neighbouring forebrain cells, however, converge rapidly and then remain at the midline. These differential behaviours are regulated by the transcription factor Rx3. Here, we identify a downstream target of Rx3, the Ig-domain protein Nlcam, and characterise its role in regulating cell migration during the initial phase of optic vesicle morphogenesis. Through sophisticated live imaging and comprehensive cell tracking experiments in zebrafish, we show that ectopic expression of Nlcam in RPCs, as is observed in Rx3 mutants, causes enhanced convergence of these cells. Expression levels of Nlcam therefore regulate the migratory properties of RPCs. Our results provide evidence that the two phases of optic vesicle morphogenesis: slowed convergence and outward-directed migration, are under different genetic control. We propose that Nlcam forms part of the guidance machinery directing rapid midline migration of forebrain precursors, where it is normally expressed, and that its ectopic expression upon loss of Rx3 imparts these migratory characteristics upon RPCs.

Shaping the vertebrate eye

For over a century, the vertebrate eye has served as a paradigm for organogenesis. It forms through a complex sequence of morphogenetic events, involving the lateral evagination of the optic vesicles and their subsequent folding into the optic cups. Through intensive studies by experimental embryologists, anatomical descriptions of the process were available since many decades. Recent genetic and molecular work has illuminated essential features of the stereotyped cellular behaviour driving eye morphogenesis. The first pieces of the molecular machinery operating in each individual progenitor cell have been identified. These studies now set the groundwork for a system-wide approach towards understanding the cellular and molecular mechanisms involved in shaping the vertebrate eye.

Frizzled-10 promotes sensory neuron development in Xenopus embryos

Formation of the vertebrate nervous system requires coordinated cell-cell interactions, intracellular signalling events, gene transcription, and morphogenetic cell movements. Wnt signalling has been involved in regulating a wide variety of biological processes such as embryonic patterning, cell proliferation, cell polarity, motility, and the specification of cell fate. Wnt ligands associate with their receptors, members of the frizzled family (Fz). In Xenopus, five members of the frizzled family are expressed in the early nervous system. We have investigated the role of Xenopus frizzled-10 (Fz10) in neural development. We show that Fz10 is expressed in the dorsal neural ectoderm and neural folds in the region where primary sensory neurons develop. Fz10 mediates canonical Wnt signalling and interacts with Wnt1 and Wnt8 but not Wnt3a as shown in synergy assays. We find that Fz10 is required for the late stages of sensory neuron differentiation. Overexpression of Fz10 in Xenopus leads to an increase in the number of sensory neurons. Loss of Fz10 function using morpholinos inhibits the development of sensory neurons in Xenopus at later stages of neurogenesis and this can be rescued by co-injection of modified Fz10B and beta-catenin. In mouse P19 cells induced by retinoic acid to undergo neural differentiation, overexpression of Xenopus Fz10 leads to an increase in the number of neurons generated while siRNA knockdown of endogenous mouse Fz10 inhibits neurogenesis. Thus we propose Fz10 mediates Wnt1 signalling to determine sensory neural differentiation in Xenopus in vivo and in mouse cell culture.

The sea urchin animal pole domain is a Six3-dependent neurogenic patterning center

Two major signaling centers have been shown to control patterning of sea urchin embryos. Canonical Wnt signaling in vegetal blastomeres and Nodal signaling in presumptive oral ectoderm are necessary and sufficient to initiate patterning along the primary and secondary axes, respectively. Here we define and characterize a third patterning center, the animal pole domain (APD), which contains neurogenic ectoderm, and can oppose Wnt and Nodal signaling. The regulatory influence of the APD is normally restricted to the animal pole region, but can operate in most cells of the embryo because, in the absence of Wnt and Nodal, the APD expands throughout the embryo. We have identified many constituent APD regulatory genes expressed in the early blastula and have shown that expression of most of them requires Six3 function. Furthermore, Six3 is necessary for the differentiation of diverse cell types in the APD, including the neurogenic animal plate and immediately flanking ectoderm, indicating that it functions at or near the top of several APD gene regulatory networks. Remarkably, it is also sufficient to respecify the fates of cells in the rest of the embryo, generating an embryo consisting of a greatly expanded, but correctly patterned, APD. A fraction of the large group of Six3-dependent regulatory proteins are orthologous to those expressed in the vertebrate forebrain, suggesting that they controlled formation of the early neurogenic domain in the common deuterostome ancestor of echinoderms and vertebrates.

Cell-autonomous requirement for rx function in the mammalian retina and posterior pituitary

Rx is a paired-like homeobox gene that is required for vertebrate eye formation. Mice lacking Rx function do not develop eyes or the posterior pituitary. To determine whether Rx is required cell autonomously in these tissues, we generated embryonic chimeras consisting of wild type and Rx−/− cells. We found that in the eye, Rx-deficient cells cannot participate in the formation of the neuroretina, retina pigment epithelium and the distal part of the optic stalk. In addition, in the ventral forebrain, Rx function is required cell autonomously for the formation of the posterior pituitary. Interestingly, Rx−/− and wild type cells segregate before the morphogenesis of these two tissues begins. Our observations suggest that Rx function is not only required for the morphogenesis of the retina and posterior pituitary, but also prior to morphogenesis, for the sorting out of cells to form distinct fields of retinal/pituitary cells.

Retinal homeobox 1 is required for retinal neurogenesis and photoreceptor differentiation in embryonic zebrafish

Retinal homeobox (Rx/Rax) genes are essential for the organogenesis of the vertebrate eye. These genes are dynamically expressed in a tissue-specific manner during eye development, suggesting pleiotropic roles. We use a temporally-selective gene knockdown approach to identify endogenous functions for the zebrafish rx genes, rx1 and rx2. Depletion of rx1 over the period of eye organogenesis resulted in severely reduced proliferation of retinal progenitors, the loss of expression of the transcription factor pax6, delayed retinal neurogenesis, and extensive retinal cell death. In contrast, depletion of rx2 over the same developmental time resulted in reduced expression of pax6 in the eye anlage, but only modest effects on retinal cell survival. Knockdown of rx1 specifically during photoreceptor development inhibited the expression of multiple photoreceptor-specific genes, while knockdown of rx2 over this time selectively inhibited the expression of a subset of these genes. Our findings support a function for rx2 in regulating pax6 within the optic primordia, a function for rx1 in maintaining the pluripotent, retinal progenitor cell state during retinal development, as well as selective functions for rx1 and rx2 in regulating photoreceptor differentiation.

The developmental sequence of gene expression within the rod photoreceptor lineage in embryonic zebrafish

In postembryonic zebrafish, rod photoreceptors are continuously generated from progenitors in the inner nuclear layer, which are derived from radial Müller glia that express the transcription factor pax6. We used BrdU incorporation, in combination with in situ hybridization for cell-specific transcription factors, to establish the patterns of gene expression during rod lineage maturation in the embryonic zebrafish. Downregulation of pax6 expression was accompanied by sporadic upregulation of expression of the transcription factors NeuroD/nrd, rx1, crx, and Nr2e3/pnr. As cells of the rod lineage entered the outer nuclear layer, they became homogeneous, coordinately expressing NeuroD, rx1, crx, and Nr2e3. Postmitotic, maturing rods also expressed nrl, rod opsin, and rod transducin/gnat1. The presence of rx1 within the rod lineage and in maturing rods indicates that rx1 is not cone-specific, as previously reported, and suggests a high degree of molecular similarity between rod and cone progenitor populations in the zebrafish.

Confirmation of RAX gene involvement in human anophthalmia

Microphthalmia and anophthalmia are at the severe end of the spectrum of abnormalities in ocular development. Mutations in several genes have been involved in syndromic and non-syndromic anophthalmia. Previously, RAX recessive mutations were implicated in a single patient with right anophthalmia, left microphthalmia and sclerocornea. In this study, we report the findings of novel compound heterozygous RAX mutations in a child with bilateral anophthalmia. Both mutations are located in exon 3. c.664delT is a frameshifting deletion predicted to introduce a premature stop codon (p.Ser222ArgfsX62), and c.909C>G is a nonsense mutation with similar consequences (p.Tyr303X). This is the second report of a patient with anophthalmia caused by RAX mutations. These findings confirm that RAX plays a major role in the early stages of eye development and is involved in human anophthalmia.

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.

A zebrafish LMO4 ortholog limits the size of the forebrain and eyes through negative regulation of six3b and rx3

The Six3 and Rx3 homeodomain proteins are essential for the specification and proliferation of forebrain and retinal precursor cells of the vertebrate brain, and the regulatory networks that control their expression are beginning to be elucidated. We identify the zebrafish lmo4b gene as a negative regulator of forebrain growth that acts via restriction of six3 and rx3 expression during early segmentation stages. Loss of lmo4b by morpholino knockdown results in enlargement of the presumptive telencephalon and optic vesicles and an expansion of the post-gastrula expression domains of six3 and rx3. Overexpression of lmo4b by mRNA injection causes complementary phenotypes, including a reduction in the amount of anterior neural tissue, especially in the telencephalic, optic and hypothalamic primordia, and a dosage-sensitive reduction in six3 and rx3 expression. We suggest that lmo4b activity is required at the neural boundary to restrict six3b expression, and later within the neural plate to for attenuation of rx3 expression independently of its effect on six3 transcription. We propose that lmo4b has an essential role in forebrain development as a modulator of six3 and rx3 expression, and thus indirectly influences neural cell fate commitment, cell proliferation and tissue growth in the anterior CNS.

Emerging roles for zic genes in early development

Members of the Zic family of zinc finger transcription factors play critical roles in a variety of developmental processes. They are involved in development of neural tissues and the neural crest, in left-right axis patterning, in somite development, and in formation of the cerebellum. In addition to their roles in cell-fate specification, zic genes also promote cell proliferation. Further, they are expressed in postmitotic cells of the cerebellum and in retinal ganglion cells. Efforts to determine the role of individual zic genes within an array of developmental and cellular processes are complicated by overlapping patterns of zic gene expression and strong sequence conservation within this gene family. Nevertheless, substantial progress has been made. This review summarizes our knowledge of the molecular events that govern the activities of zic family members, including emerging relationships between upstream signaling pathways and zic genes. In addition, advancements in our understanding of the molecular events downstream of Zic transcription factors are reviewed. Despite significant progress, however, much remains to be learned regarding the mechanisms through which zic genes exert their function in a variety of different contexts.

Expression of Xenopus laevis Lhx2 during eye development and evidence for divergent expression among vertebrates

Members of the LIM homeodomain (LIM-HD) family of proteins are double zinc-finger containing transcription factors with important functions in pattern formation and cell lineage determination. The LIM-HD family member Lhx2 is required for normal eye, liver, and central nervous system formation. Lhx2(-/-) mice lack eyes, and experiments in Xenopus predict that Lhx2 forms a regulatory network with other eye field transcription factors to specify the eye field during eye formation. Here, we describe the structure and developmental expression pattern of the Xenopus laevis homologue, XLhx2. We show that XLhx2 shares significant amino acid sequence identity with other vertebrate Lhx2 proteins and Drosophila apterous (ap). The expression patterns of XLhx2 in the early neural plate and during eye development are consistent with a role in eye field specification and retinal differentiation. Despite highly similar expression patterns in the mouse and Xenopus central nervous system, divergent expression patterns were also observed. Phylogenetic analysis confirmed the identity of the isolated cDNA as a Xenopus ortholog of Lhx2. Therefore, in spite of structural similarities, the mouse and Xenopus Lhx2 expression patterns differ, suggesting potential functional differences in these species.

Stage- and tissue-specific patterns of cell division in embryonic and larval tissues of amphioxus during normal development

The distribution of dividing cells is described for embryos and larvae of amphioxus (Branchiostoma floridae) pulse labeled with bromodeoxyuridine. Because cell division is assessed for all of the developing tissues, this is the first comprehensive study of developmental cell proliferation for an animal lacking a stereotyped cell lineage. In amphioxus, cell divisions are virtually synchronous during cleavage, but become asynchronous at the blastula stage. Starting at the neurula stage, after the origin of the mesoderm, the proportion of dividing cells progressively declines in the somitic mesoderm and notochord. Other tissues, however, deviate from this pattern. For example, in the mid-neurula, there is a brief, intense burst of mitosis at the anterior end of the neural plate. Also, from the neurula through the early larval stage, all of the ectoderm cells cease dividing and develop cilia that propel the animal through the water; subsequently, in the epidermis of later larvae, mitosis resumes and the proportion of ciliated cells declines as muscular undulation gradually replaces ciliation for swimming. Finally, in the early larvae, there is a terminal arrest of cell division in three cell types that differentiate early to participate in feeding as soon as the mouth opens-namely the ciliated pharyngeal cells that produce the feeding current and the secretory cells of the club-shaped gland and endostyle that export food-trapping mucus into the pharynx. In sum, these stage- and tissue-specific changes in cell proliferation intensity illustrate how the requirements of embryonic and larval natural history can shape developmental programs.

An obligatory caravanserai stop on the silk road to neural induction: Inhibition of BMP/GDF signaling

Work in Xenopus laevis produced the first molecular explanation for neural specification, the default model, where inactivation of the BMP pathway in ectodermal cells changes fates from epidermal to neural. This review covers the present status of our understanding of neural specification, with emphasis on Xenopus, but including relevant facts in other model systems. While recent experiments have increased the complexity of the molecular picture, they have also provided additional support for the default model and the central position of the BMP pathway. We conclude that synergy between accumulated knowledge and technical progress will maintain Xenopus at the forefront of research in neural development.

Nucleosome regulator Xhmgb3 is required for cell proliferation of the eye and brain as a downstream target of Xenopus rax/Rx1

Rax/Rx is a paired-type homeodomain-containing transcription factor that is essential for cell proliferation in the developing eye and brain. The molecular mechanisms that regulate cell proliferation by rax, however, are largely unknown. Here, we identify the high mobility group B3 gene (hmgb3) as a downstream target of Xenopus rax (Xrax/XRx1). Overexpression of Xhmgb3 results in an increase in eye and brain sizes due to promoted cell proliferation, while morpholino-oligo-mediated knock down of Xhmgb3 reduces eye and brain sizes. In addition, ChIP assays showed that Xhmgb3 is recruited around the promoter region of c-myc to enhance c-myc transcription. We also found that XOptx2 requires rax for its initial expression. Furthermore, we show that Xhmgb3 and XOptx2 are required for retinal development mainly at different developmental stages. Our findings reveal a novel aspect of progenitor cell proliferation during embryonic central nervous system (CNS) development.

Dystroglycan is required for proper retinal layering

Dystroglycan (DG) is a transmembrane receptor linking the extracellular matrix to the internal cytoskeleton. Its structural function has been mainly characterized in muscle fibers, but DG plays signaling and developmental roles also in different tissues and cell types. We have investigated the effects of dystroglycan depletion during eye development of Xenopus laevis. We have injected a specific morpholino (Mo) antisense oligonucleotide in the animal pole of one dorsal blastomere of embryos at four cells stage. Mo-mediated loss of DG function caused disruption of the basal lamina layers, increased apoptosis and reduction of the expression domains of specific retinal markers, at early stages. Later in development, morphants displayed unilateral ocular malformations, such as microphtalmia and retinal delayering with photoreceptors and ganglion cells scattered throughout the retina or aggregated in rosette-like structures. These results recall the phenotypes observed in specific human diseases and suggest that DG presence is crucial at early stages for the organization of retinal architecture.

Expression of p27BBP/eIF6 is highly modulated duringXenopus laevis embryogenesis

Protein p27BBP/eIF6 is necessary for ribosomal function of all cells. Previous data showed that from mammals to yeast p27BBP/eIF6 is involved in the biogenesis of ribosomal subunit 60S and its association with the 60S prevents premature 80S formation regulated by PKC signaling, indicating that phosphorylation of p27BBP/eIF6 is needed for translation to occur. While in vitro p27BBP/eIF6 is constitutively expressed, and it has a high level of expression in cycling cells, in vivo its expression varies according to tissues and appears regulated by factors up to now unknown. p27BBP/eIF6 has never been investigated in developing organisms where its upregulation can be correlated with tissue growth and differentiation. In this study we have sequenced p27BBP/eIF6 cDNA and studied its expression during development of Xenopus laevis, as the first step for studying its regulation. The amino acid sequence is highly conserved with two putative PKC phosphorylation sites in serine, one site being typical of Xenopus. At the end of gastrulation, the p27BBP/eIF6 riboprobe localizes in the neural plate and in the paraxial mesoderm. In particular, from stage 24, a clear-cut localization occurs in the perspective head. In embryos exposed to teratogens, the localization of p27BBP/eIF6 riboprobe varies according to the change of head size caused by the treatment. p27BBP/eIF6 expression is particularly evident in differentiating olfactory pits, the lens, otic vesicles, and in branchial arches. Features of particular interest are p27BBP/eIF6 high level of expression in the eye field, and in the mid-hindbrain-boundary, two regions with high proliferative activity. Altogether, data indicate that a modulated expression of p27BBP/eIF6 occurs in developing anlagens in addition to a basal level of expression, and may suggest a correlation between p27BBP/eIF6 and proliferative activity. Moreover, the X. laevis cDNA isolation and characterization offer new hints for further studies in relation to potential p27BBP/eIF6 phosphorylation.

chokh/rx3 specifies the retinal pigment epithelium fate independently of eye morphogenesis

Despite the importance of the retinal pigment epithelium (RPE) for vision, the molecular processes involved in its specification are poorly understood. We identified two new mutant alleles for the zebrafish gene chokh (chk), which display a reduction or absence of the RPE. Unexpectedly, the neural retina (NR) in chk is specified and laminated, indicating that the regulatory network leading to NR development is largely independent of the RPE. Genetic mapping and molecular characterization revealed that chk encodes Rx3. Expression analyses show that otx2 and mitfb are not expressed in the prospective RPE of chk, indicating that the retinal homeobox gene rx3 acts upstream of the molecular network controlling RPE specification. Cellular transplantations demonstrate that rx3 function is autonomously required to specify the prospective RPE. Though rx2 is also absent in chk, neither rx2 nor rx1 is required for RPE development. Thus, our data provide the first indication that, in addition to controlling optic lobe evagination and proliferation, chk/rx3 also determines cellular fate.

The role of combinational coding by homeodomain and bHLH transcription factors in retinal cell fate specification

Two major families of transcription factors (TFs), basic helix-loop-helix (bHLH) and homeodomain (HD), are known to be involved in cell fate identity. Some recent findings suggest that these TFs are used combinatorially to code for cellular determination in the retina. However, neither the extent nor the efficiency of such a combinatorial coding mechanism has been tested. To look systematically for interactions between these two TF types that would address these questions, we used a matrix analysis. We co-expressed each of six retinally expressed bHLH TFs (XNeuroD; XNgnr-1; Xath3; Xath5; Xash1; Xash3) with each of eight retinally expressed HD TFs (XRx1; XOptx2; XSix3; XPax6; XOtx2; XOtx5b; XBH; XChx10) in retinal progenitors of Xenopus laevis using targeted lipofection. The effects of each of these combinations were assayed on the six major cell types in the retina: Retinal ganglion cells (GCs), Amacrines (ACs), Bipolars (BCs), Horizontals (HCs), Photoreceptors (PRs), and Muller cells (MCs), creating 288 result categories. Multiple-way ANOVA indicated that in 14 categories, there were interactions between the two TFs that produced significantly more or less of a particular cell type than either of the components alone. However, even the most effective combinations were incapable of generating more than 65% of any particular cell type. We therefore used the same techniques to misexpress selected combinations of three TFs in retinal progenitors, but found no further enhancements of particular cell fates, indicating that other factors are probably involved in cell type specification. To test whether particular combinations were essential for horizontal fates, we made VP16 and EnR fusion constructs of some of the factors to provide dominant negative transcriptional activities. Our results confirmed that normal activities of certain combinations were sufficient, and that individually these activities were important for this fate.

Genetic analysis of metamorphic and premetamorphic Xenopus ciliary marginal zone

A major event affecting the eye during amphibian metamorphosis is an asymmetrical growth of the ventrotemporal portion of the retina compared with its dorsonasal counterpart. This event is due to an increased proliferation of the precursors of the ventral ciliary marginal zone (CMZ). Here, we analyze the expression patterns of several key homeobox genes implicated in eye development (Xrx1, Xvax2, Xsix3, Xpax6, Xchx10, Xotx2) to understand whether they are active at the time in which the metamorphic changes of the retina occur. We also analyze their expression patterns in the ventral and dorsal CMZ and compare them with bromodeoxyuridine incorporation in the CMZ. Our results suggest that the metamorphic CMZ maintains the functional subdivisions described during embryonic development. Moreover, we find that genes involved in proliferation and cell type determination of the embryonic retina are actively transcribed in the proliferating CMZ, thus indicating a potential regulatory role for these genes in the metamorphic retina.