Six3-mediated auto repression and eye development requires its interaction with members of the Groucho-related family of co-repressors

Glutamate regulates retinal progenitors cells proliferation during development

The precise coordination of cell cycle exit and cell fate specification is essential for generating the correct proportion of retinal cell types during development. The decision to exit the cell cycle is regulated by intrinsic and extrinsic cues. There is growing evidence that neurotransmitters can regulate cell proliferation and cell fate specification during the early stages of CNS development prior to the formation of synaptic connections. We found that the excitatory neurotransmitter glutamate regulates retinal progenitor cell proliferation during embryonic development of the mouse. AMPA/kainate and N-methyl-d-aspartate receptors are expressed in embryonic retinal progenitor cells. Addition of exogenous glutamate leads to a dose-dependent decrease in cell proliferation without inducing cell death or activating the p53 pathway. Activation of AMPA/kainate receptors induced retinal progenitor cells to prematurely exit the cell cycle. Using a replication-incompetent retrovirus to follow the clonal expansion of individual retinal progenitor cells, it was observed that blockade of AMPA/kainate receptors increased the proportion of large clones, showing that modulation of endogenous glutamatergic activity can have long-term consequences on retinal cell proliferation. Real time reverse transcriptase-polymerase chain reaction and immunoblot analyses demonstrated that glutamate does not alter the levels of the mRNA and proteins that regulate the G1/S-phase transition. Instead, the activity of the Cdk2 kinase is reduced in the presence of glutamate. These data indicate that glutamate regulates retinal progenitor cell proliferation by post-translational modulation of cyclin/Cdk2 kinase activity.

Induction and specification of cranial placodes

Cranial placodes are specialized regions of the ectoderm, which give rise to various sensory ganglia and contribute to the pituitary gland and sensory organs of the vertebrate head. They include the adenohypophyseal, olfactory, lens, trigeminal, and profundal placodes, a series of epibranchial placodes, an otic placode, and a series of lateral line placodes. After a long period of neglect, recent years have seen a resurgence of interest in placode induction and specification. There is increasing evidence that all placodes despite their different developmental fates originate from a common panplacodal primordium around the neural plate. This common primordium is defined by the expression of transcription factors of the Six1/2, Six4/5, and Eya families, which later continue to be expressed in all placodes and appear to promote generic placodal properties such as proliferation, the capacity for morphogenetic movements, and neuronal differentiation. A large number of other transcription factors are expressed in subdomains of the panplacodal primordium and appear to contribute to the specification of particular subsets of placodes. This review first provides a brief overview of different cranial placodes and then synthesizes evidence for the common origin of all placodes from a panplacodal primordium. The role of various transcription factors for the development of the different placodes is addressed next, and it is discussed how individual placodes may be specified and compartmentalized within the panplacodal primordium. Finally, tissues and signals involved in placode induction are summarized with a special focus on induction of the panplacodal primordium itself (generic placode induction) and its relation to neural induction and neural crest induction. Integrating current data, new models of generic placode induction and of combinatorial placode specification are presented.

Eya1 is required for lineage-specific differentiation, but not for cell survival in the zebrafish adenohypophysis

The homeodomain transcription factor Six1 and its modulator, the protein phosphatase Eya1, cooperate to promote cell differentiation and survival during mouse organ development. Here, we studied the effects caused by loss of eya1 and six1 function on pituitary development in zebrafish. eya1 and six1 are co-expressed in all adenohypophyseal cells. Nevertheless, eya1 (aal, dog) mutants show lineage-specific defects, defining corticotropes, melanotropes, and gonadotropes as an Eya1-dependent lineage, which is complementary to the Pit1 lineage. Furthermore, eya1 is required for maintenance of pit1 expression, leading to subsequent loss of cognate hormone gene expression in thyrotropes and somatotropes of mutant embryos, whereas prolactin expression in lactotropes persists. In contrast to other organs, adenohypophyseal cells of eya1 mutants do not become apoptotic, and the adenohypophysis remains at rather normal size. Also, cells do not trans-differentiate, as in the case of pit1 mutants, but display morphological features characteristic for nonsecretory cells. Some of the adenohypophyseal defects of eya1 mutants are moderately enhanced in combination with antisense-mediated loss of Six1 function, which per se does not affect pituitary cell differentiation. In conclusion, this is the first report of an essential role of Eya1 during pituitary development in vertebrates. Eya1 is required for lineage-specific differentiation of adenohypophyseal cells, but not for their survival, thereby uncoupling the differentiation-promoting and anti-apoptotic effects of Eya proteins seen in other tissues.

Partner specificity is essential for proper function of the SIX-type homeodomain proteins Sine oculis and Optix during fly eye development

The development of the Drosophila visual system utilizes two members of the highly conserved Six-Homeobox family of transcription factor, Sine oculis and Optix. Although in vitro studies have detected differences in DNA-binding and interactions with some co-factors, questions remain as to what extent the activity for these two transcriptional regulators is redundant or specific in vivo. In this work, we show that the SoD mutation within the Six domain does not abolish DNA-protein interactions, but alters co-factor binding specificity to resemble that of Optix. A mutation in the same region of Optix alters its activity in vivo. We propose that the dominant mutant phenotype is primarily due to an alteration in binding properties of the Sine oculis protein and that distinct partner interactions is one important mechanism in determining significant functional differences between these highly conserved factors during eye development.

Lhx2 mediates the activity of Six3 in zebrafish forebrain growth

The telencephalon shows the greatest degree of size variation in the vertebrate brain. Understanding the genetic cascade that regulates telencephalon growth is crucial to our understanding of how evolution of the normal human brain has supported such a variation in size. Here, we present a simple and quick approach to analyze this cascade that combines caged-mRNA technology and the use of antisense morpholino oligonucleotides in zebrafish embryos. Lhx2, a LIM-homeodomain protein, and Six3s (Six3b and Six3a), another homeodomain proteins, show very similar expression patterns early in forebrain development, and these are known to be involved in the growth of this part of the brain. The telencephalon of six3b and six3a double morphant (six3 morphant) embryos is markedly reduced in size due to impaired cellular proliferation. Head-specific overexpression of Lhx2 by photoactivation of a caged-lhx2 mRNA completely rescued this size reduction, whereas similar head-specific activation of Six3b could not rescue the knockdown effect of lhx2. In the forebrain of medaka embryos, Six3 facilitates cellular proliferation by sequestration of Geminin from Cdt1, a key component in the assembly of the prereplication complex. Our results suggest that Lhx2 may mediate an alternative or parallel pathway for control of cellular proliferation in the developing forebrain via Six3.

Comparative analysis of Six 3 and Six 6 distribution in the developing and adult mouse brain

Six 3 and Six 6 genes are two closely related members of the Six/sine oculis family of homeobox containing transcription factors. Their expression and function at early stages of embryonic development has been widely addressed in a variety of species. However, their mRNA distribution during late embryonic, postnatal, and adult brain barely has been analyzed. Here, we show that despite their initial overlap in the anterior neural plate, the expression of Six 3 and Six 6 progressively segregates to different regions during mammalian brain development, maintaining only few areas of partial overlap in the thalamic and hypothalamic regions. Six 3, but not Six 6, is additionally expressed in the olfactory bulb, cerebral cortex, hippocampus, midbrain, and cerebellum. These distinct patterns support the idea that Six 3 and Six 6 are differentially required during forebrain development.

Groucho corepressor proteins regulate otic vesicle outgrowth

The Groucho/Tle family of corepressor proteins is known to regulate multiple developmental pathways. Applying the dominant-negative effect of the short member Aes, we demonstrate here a critical role of this gene family also for ear development. Misexpression of Aes in medaka embryos resulted in reduced size or loss of otic vesicles, whereas overexpression of the full-length Groucho protein Tle4 gave the opposite phenotype. These results are in close agreement with phenotypes observed for eye formation, suggesting a similar role for Groucho/Tle proteins in the developmental pathways of both sensory organs. Furthermore, by using the heat-inducible HSE promoter, we observed reversible branching of the embryonic axis upon Aes misexpression, indicating a transient duplication of the organizer. Groucho proteins, therefore, are critical for organizer maintenance.

Topotecan Combination Chemotherapy in Two New Rodent Models of Retinoblastoma

Chemotherapy combined with laser therapy and cryotherapy has improved the ocular salvage rate for children with bilateral retinoblastoma. However, children with late-stage disease often experience recurrence shortly after treatment. To improve the vision salvage rate in advanced bilateral retinoblastoma, we have developed and characterized two new rodent models of retinoblastoma for screening chemotherapeutic drug combinations. The first model is an orthotopic xenograft model in which green fluorescent protein- or luciferase-labeled human retinoblastoma cells are injected into the eyes of newborn rats. The second model uses a replication-incompetent retrovirus (LIA-E(E1A)) encoding the E1A oncogene. Clonal, focal tumors arise from mouse retinal progenitor cells when LIA-E(E1A) is injected into the eyes of newborn p53-/- mice. Using these two models combined with pharmacokinetic studies and cell culture experiments, we have tested the efficacy of topotecan combined with carboplatin and of topotecan combined with vincristine for the treatment of retinoblastoma. The combination of topotecan and carboplatin most effectively halted retinoblastoma progression in our rodent models and was superior to the current triple drug therapy using vincristine, carboplatin, and etoposide. Vincristine had the lowest LC50 in culture but did not reduce tumor growth in our preclinical retinoblastoma models. Taken together, these data suggest that topotecan may be a suitable replacement for etoposide in combination chemotherapy for the treatment of retinoblastoma.

Regulation of Wnt gene expression

Members of the Wnt gene family play important roles in the regulation of a number of basic developmental processes. Because Wnt is such a potent morphogen, its expression must be controlled tightly and precisely. While many review papers focused on Wnt signaling downstream of the receptor, this review addresses regulations of Wnt itself on several levels, including the transcriptional level, RNA splicing, the post-transcriptional level, the translational level, and the post-translational level. It is these multiple, precise and tight regulations that guarantee that Wnts function correctly both temporally and spatially.

Mammalian Groucho homologs: redundancy or specificity?

The proteins termed TLE in humans, Grg in mice and Groucho in Drosophila constitute a family of transcriptional corepressors. In mammalians there are five different genes encoding an even larger number of proteins. Interactions between these TLE/Grg proteins and an array of transcription factors has been described. But is there any specificity? This review tries to make a case for a non-redundant function of individual TLE/Grg proteins. The specificity may be brought about by a tightly controlled temporo-spatial expression pattern, post-translational modifications, and subtle structural differences leading to distinct preferences for interacting transcription factors. A confirmation of this concept will ultimately need to come from genetic experiments.

Induction and specification of the vertebrate ectodermal placodes: precursors of the cranial sensory organs

The sensory organs of the vertebrate head derive from two embryological structures, the neural crest and the ectodermal placodes. Although quite a lot is known about the secreted and transcription factors that regulate neural crest development, until recently little was known about the molecular pathways that regulate placode development. Herein we review recent findings on the induction and specification of the pre-placodal ectoderm, and the transcription factors that are involved in regulating placode fate and initial differentiation.

The Groucho-related gene family regulates the gonadotropin-releasing hormone gene through interaction with the homeodomain proteins MSX1 and OCT1

Gonadotropin-releasing hormone (GnRH) is exclusively expressed in a unique population of hypothalamic neurons that controls reproductive function. GnRH gene expression is highly dynamic. Its transcriptional activity is regulated in a complex spatiotemporal manner during embryonic development and postnatal life. Although a variety of transcription factors have been identified as regulators of GnRH transcription, most are promiscuous in their DNA-binding requirements, and none are solely expressed in GnRH neurons. Their specific activity is probably determined by interactions with distinct cofactors. Here we find that the Groucho-related gene (GRG) family of co-repressors is expressed in a model cell line for the GnRH neuron and co-expresses with GnRH during prenatal development. GRG proteins associate in vivo with the GnRH promoter. Furthermore, GRG proteins interact with two regulators of GnRH transcription, the homeodomain proteins MSX1 and OCT1. Co-transfection experiments indicate that GRG proteins regulate GnRH promoter activity. The long GRG forms enhance MSX1 repression and counteract OCT1 activation of the GnRH gene. In contrast, the short form, GRG5, has a dominant-negative effect on MSX1-dependent repression. Taken together, these data suggest that the dynamic switch between activation and repression of GnRH transcription is mediated by recruitment of the GRG co-regulators.

Expression analysis of SIX3 and SIX6 in human tissues reveals differences in expression and a novel correlation between the expression of SIX3 and the genes encoding isocitrate dehyhrogenase and cadherin 18

SIX3 and SIX6 are transcription factors expressed during early stages of eye development. Limited expression data for SIX3 and SIX6 are available in the literature but, to date, there are no reports of the relative levels of expression of these genes throughout the human body and in adult tissues in particular. In this paper, we report extensive real-time quantitative PCR analyses of SIX3 and SIX6 expression in many different tissues of the adult human body, including ocular tissues, and a comparison of expression data with that of many other genes to identify similarity in expression. Using this powerful technique, we have detected a novel statistical correlation between the spatial distribution and the quantitative expression of SIX3 and 5 other transcripts including IDH1, the gene encoding the NADP(+)-dependent enzyme isocitrate dehydrogenase, and cadherin 18, type 2 (CDH14). Our data demonstrate that this novel technique can be used to generate hypotheses by comparison of gene expression profiles to identify possible interactions between genes or gene products.

Slc12a2 is a direct target of two closely related homeobox proteins, Six1 and Six4

Six genes are homologs of Drosophila sine oculis and encode transcription factors that are characterized by a conserved Six domain and homeodomain. Of the six family members (Six1-Six6) in mice, Six1 and Six4 show similar expression patterns during embryogenesis. Six1-/- mice show defective formation of various organs such as inner ear, nose, skeletal muscle, kidney and thymus, whereas Six4-/- mice show little anomaly in organogenesis. To understand the molecular basis for the differential function of Six1 and Six4 in vivo, we screened target genes of Six1 and Six4 and found that Six1 and Six4 differentially regulated a set of target genes. Gel-retardation assays indicated that the promoter region of one of the targets, sodium-potassium-chloride cotransporter 1 (Slc12a2), contains multiple Six1-binding sites and one common binding site of Six1 and Six4, suggesting that the DNA-binding specificity of Six1 is distinct from that of Six4. This underlies the differential regulation of common target genes by Six1 and Six4. Furthermore, in situ hybridization demonstrated that the expression of Slc12a2 was reduced in the developing dorsal root ganglia of Six1-/-/Six4-/- mice, suggesting that Six1 and Six4 regulate Slc12a2 in vivo.

Fly Six-type homeodomain proteins Sine oculis and Optix partner with different cofactors during eye development

Two members from the Six class of homeobox transcription factors, Sine oculis (SO) and Optix, function during development of the fly visual system. Differences in gain-of-function phenotypes and gene expression suggest that these related factors play distinct roles in the formation of the fly eye. However, the molecular nature of their functional differences remains unclear. In this study, we report the identification of two novel factors that participate in specific partnerships with Sine oculis and Optix during photoreceptor neurons formation and in eye progenitor cells. This work shows that different cofactors likely mediate unique functions of Sine oculis and Optix during the development of the fly eye and that the repeated requirement for SO function at multiple stages of eye development reflects the activity of different SO-cofactor complexes.

Choice of either beta-catenin or Groucho/TLE as a co-factor for Xtcf-3 determines dorsal-ventral cell fate of diencephalon during Xenopus development

Co-repressor Groucho/Transducin-Like Enhancer of split (TLE) interacts with transcription factors that are expressed in the central nervous system (CNS), and regulates transcriptional activities. In this study, we examined the contribution of Groucho/TLE to CNS development in Xenopus. The functional inhibition of Groucho/TLE using the WRPW motif as a competitor resulted in the conversion of the ventral cell into the dorsal fate in the prospective diencephalon. We also found that the neural plate was expanded laterally without inhibiting neural crest development. In tailbud, the disturbance of trigeminal ganglion development was observed. These observations allow us to conclude that Groucho/TLE plays important roles in the induction and patterning of distinct CNS territories. We found that Xtcf-3 is involved in some of the patterning in these territories. We generated the variant of Xtcf-3, Xtcf-3BDN-, which is suspected to interfere with the interaction between endogenous Groucho/TLE and Xtcf-3. The transcriptional activation of the Xtcf-3-target genes in response to endogenous Wnt/beta-catenin signaling by the overexpression of Xtcf-3BDN- led to a reduction of the ventral diencephalon. This result indicates that transcriptional repression by the Groucho/TLE-Xtcf-3 complex is important for ventral diencephalon patterning. This idea is supported by the finding that the overexpression of the dominant-negative form of Xtcf-3 or axil causes the expansion of the ventral diencephalon. Based on these data, we propose that the localized activation of Wnt/beta-catenin signaling, which converts Tcf from a repressor to an activator, is required for the establishment of dorsal-ventral patterning in the prospective diencephalon.

Molecular evidence from Ciona intestinalis for the evolutionary origin of vertebrate sensory placodes

Cranial sensory placodes are focused areas of the head ectoderm of vertebrates that contribute to the development of the cranial sense organs and their associated ganglia. Placodes have long been considered a key character of vertebrates, and their evolution is proposed to have been essential for the evolution of an active predatory lifestyle by early vertebrates. Despite their importance for understanding vertebrate origins, the evolutionary origin of placodes has remained obscure. Here, we use a panel of molecular markers from the Six, Eya, Pax, Dach, FoxI, COE and POUIV gene families to examine the tunicate Ciona intestinalis for evidence of structures homologous to vertebrate placodes. Our results identify two domains of Ciona ectoderm that are marked by the genetic cascade that regulates vertebrate placode formation. The first is just anterior to the brain, and we suggest this territory is equivalent to the olfactory/adenohypophyseal placodes of vertebrates. The second is a bilateral domain adjacent to the posterior brain and includes cells fated to form the atrium and atrial siphon of adult Ciona. We show this bares most similarity to placodes fated to form the vertebrate acoustico-lateralis system. We interpret these data as support for the hypothesis that sensory placodes did not arise de novo in vertebrates, but evolved from pre-existing specialised areas of ectoderm that contributed to sensory organs in the common ancestor of vertebrates and tunicates.

LvGroucho and nuclear beta-catenin functionally compete for Tcf binding to influence activation of the endomesoderm gene regulatory network in the sea urchin embryo

In the sea urchin embryo, specification of the endomesoderm is accomplished by the activity of a network of regulatory genes in the vegetal hemisphere, called the endomesoderm gene regulatory network (GRN). The activation of this network is mediated primarily through the activity of the Wnt pathway, though details of pathway activation remain unclear. To gain further insight into control of endomesoderm GRN activation, we have identified a sea urchin homologue of the co-repressor Groucho (LvGroucho) that has been shown to antagonize beta-catenin/Tcf activation complexes during Wnt signaling in other systems. Groucho functions by recruiting the histone deacetylase Rpd3 to the DNA template via interaction with site-specific transcription factors, resulting in localized chromatin condensation and transcriptional silencing. Our results show that the LvGroucho protein localizes to all nuclei throughout embryonic development. Interaction assays demonstrate that LvGroucho interacts with Tcf via both the Q and the WD domains of the protein. LvGroucho interacts with Tcf to antagonize the expression of key endomesoderm regulatory genes. Assays demonstrate that LvGroucho and n beta-catenin functionally compete for binding to Tcf as a major mechanism by which the Tcf-control switch is regulated. Functional analysis of the N-terminal AES197 domain of LvGroucho shows that it is sufficient to recapitulate the function of full-length LvGroucho. This finding strongly supports the conclusion that the effects of LvGro overexpression are due primarily to its interactions with Tcf and not other Groucho interacting partners, since Tcf is the only protein present in the sea urchin known to interact with AES197. Because the Q domain is unable to bind Rpd3, it was expected to behave as a dominant negative LvGroucho. Unexpectedly, overexpression of the Q domain gave functional results similar to LvGroucho and the AES197 domain. This is the first evidence for an inherent repressive function for the Q domain alone. Together, our results indicate that LvGroucho functionally competes with beta-catenin for Tcf binding, and this competitive mechanism regulates one of the earliest steps in the initiation of the sea urchin endomesoderm GRN.

Novel dominant-negative mutation within the six domain of the conserved eye specification gene sine oculis inhibits eye development in Drosophila

The development of the compound eye of Drosophila is controlled, in part, by the concerted actions of several nuclear proteins that form an intricate regulatory system. One member of this network is sine oculis (so), the founding member of the Six gene family. Mutations within so affect the entire visual system, including the compound eye. The vertebrate homologs Six3 and Six6 also appear to play crucial roles in retinal formation. Mutations in Six3 inhibit retinal formation in chickens and fish, whereas those in Six6 are the underlying cause of bilateral anophthalmia in humans. Together, these phenotypes suggest a conserved role for the Six genes in eye development. In this report, we describe the effects of a dominant-negative mutation of sine oculis on the development of the compound eye of Drosophila. The mutation resides within the Six domain and may have implications for eye development and disease.

Transcriptional control of precursor proliferation in the early phases of pituitary development

The anterior pituitary is derived from Rathke's pouch arising from the oral ectoderm. The initial apparently uniform precursor cells proliferate and differentiate into six different cell types that are present in mature gland by integrative interactions between different signaling molecules and transcription factors. This system provides an opportunity to understand gene regulation in the cellular processes of precursor cell proliferation, determination, and differentiation events during organogenesis. Recent studies have made significant advances in our appreciation of the molecular mechanisms by which transcription factors regulate these cellular processes.

Hierarchy of regulatory events in sensory placode development

Cranial placodes are a uniquely vertebrate characteristic; they form the paired sense organs of the eyes, ears and nose, in addition to the distal parts of some of the cranial sensory ganglia. These focal ectodermal thickenings have been studied from an embryological perspective in a diversity of organisms, revealing tissue interactions that are crucial for the morphological formation of the different placodes. In recent times, there has been a renewed interest in understanding the induction and differentiation of these deceptively simple ectodermal regions. This has led to a wealth of information on the molecular cues governing these processes. In particular, the integration of signals at the level of 'placode-specific' enhancers is beginning to provide a glimpse into the complexity of genetic networks that function within this embryonic cell population to generate key components of the peripheral nervous system.

Molecular evidence from ascidians for the evolutionary origin of vertebrate cranial sensory placodes

Cranial sensory placodes are specialised areas of the head ectoderm of vertebrate embryos that contribute to the formation of the cranial sense organs and associated ganglia. Placodes are often considered a vertebrate innovation, and their evolution has been hypothesised as one key adaptation underlying the evolution of active predation by primitive vertebrates. Here, we review recent molecular evidence pertinent to understanding the evolutionary origin of placodes. The development of vertebrate placodes is regulated by numerous genes, including members of the Pax, Six, Eya, Fox, Phox, Neurogenin and Pou gene families. In the sea squirt Ciona intestinalis (a basal chordate and close relative of the vertebrates), orthologues of these genes are deployed in the development of the oral and atrial siphons, structures used for filter feeding by the sessile adult. Our interpretation of these findings is that vertebrate placodes and sea squirt siphon primordia have evolved from the same patches of specialised ectoderm present in the common ancestor of the chordates.

Retinal degeneration in Aipl1-deficient mice: a new genetic model of Leber congenital amaurosis

Leber congenital amaurosis (LCA) is the most severe inherited retinopathy, with the earliest age of onset. Because this currently incurable disease is present from birth and is a relatively rare disorder, the development of animal models that closely resemble the phenotype in patients is especially important. Our previous genetic analyses of LCA patients identified mutations in the aryl-hydrocarbon interacting protein-like 1 (AIPL1) gene. Here we present development of an animal model of AIPL1-associated LCA, the Aipl1-deficient mouse. Aipl1 is expressed at low levels throughout human and mouse retinal development and is rapidly upregulated as photoreceptors differentiate. The mouse displays rapid retinal degeneration and massive Müler cell gliosis, resembling the phenotype of the rd mouse, which is caused by a mutation in the gene for the beta-subunit of the rod-specific phosphodiesterase. We confirm that this phenotype is consistent with the human disease using electroretinograms, and document the disease pathogenesis by analyzing the development of all retinal cell types and synaptogenesis during retinal histogenesis. Ectopic expression of AIPL1 led to deregulated retinal progenitor cell proliferation and alterations in cell fate specification; however, no gross abnormalities of proliferation during retinal development were detected. Data from analysis of proliferation and cell fate specification during retinal development of Aipl1-deficient mice suggests that there may be redundancy or compensation for Aipl1 loss by other related proteins. Because this mouse model closely mimics the human retinopathy caused by homozygous mutations in this gene, it provides a preclinical model for testing therapies to rescue the vision of children whose blindness is caused by AIPL1 mutations.

Transcriptional regulation of mouse alphaB- and gammaF-crystallin genes in lens: opposite promoter-specific interactions between Pax6 and large Maf transcription factors

Mammalian alphaB-crystallin is highly expressed both in lens epithelium and lens fibers. In contrast, gammaF-crystallin is highly expressed in the lens fiber cells. Crystallin gene expression in lens is regulated at the level of transcription by a sparse number of specific DNA-binding transcription factors. Here, we report studies on transcriptional regulation of mouse alphaB- and gammaF-crystallin promoters by specific combinations of Pax6/Pax6(5a), large Mafs (MafA, MafB, c-Maf, and NRL), Sox1, Sox2, Six3, and RARbeta/RXRbeta. Two sets of these factors, co-expressed both in lens epithelium and in lens fibers, were tested in co-transfection assays using cultured lens and non-lens cells. Regulation of alphaB-crystallin was studied in the presence of lens epithelial-factors Pax6, MafB, and RARbeta/RXRbeta, and lens fiber-factors Pax6, MafA, c-Maf, and NRL. Pax6 proteins activated the alphaB-crystallin promoter (-162 to +45) with any combination of Mafs. Addition of RARbeta/RXRbeta further increased its promoter activity. Gel shift assays using lens nuclear extracts demonstrated interactions of Pax6, Maf, and retinoic acid nuclear receptor proteins with two lens-specific regions, the distal LSR1 (-147/-118) and proximal LSR2 (-78/-40), of the alphaB-crystallin promoter. In contrast, Pax6 proteins acted as repressors of gammaF-crystallin promoter activity elicited by a combination of large Mafs, Sox, and RARbeta/RXRbeta proteins in transiently transfected lens and non-lens cells. The results show that Pax6 conversely regulates these two lens crystallin promoters. We propose that the opposite roles of Pax6 in crystallin gene regulation are results of different promoter architectures of the alphaB- and gammaF-crystallin genes, developmentally regulated association of transcription factors with the corresponding cis-regulatory sites, and specific recruitment of transcriptional co-activators and co-repressors by Pax6.

Functional characterization of SIX3 homeodomain mutations in holoprosencephaly: interaction with the nuclear receptor NR4A3/NOR1

Holoprosencephaly (HPE) is a relatively common brain malformation resulting in an incomplete separation of the two cerebral hemispheres. A number of mutations in different genes have been linked to this malformation, including three missense mutations in the homeodomain of the transcription factor SIX3. In this study, we investigated the functional consequences of these SIX3 mutations with respect to the ability of the protein to interact with and stimulate the transcriptional activity of the nuclear receptor NOR1 (NR4A3). Using glutathione S-transferase fusion protein pull-down assays and transient cotransfections of Neuro-2a cells with expression and reporter vectors, we found that one mutation, c.676C>G (p.L226V), does not alter the properties of SIX3 toward NOR1. Another mutation, c.749T>C (p.V250A), results in the production of a highly unstable protein in Neuro-2a cells. The third mutation, c.770G>C (p.R257P), results in a mutant SIX3 protein that no longer interacts with NOR1 in vivo. These observations suggest that different SIX3 mutations in HPE2 may affect different signaling pathways, and that one of these pathways may involve the nuclear receptor NOR1.

Cell-intrinsic regulators of proliferation in vertebrate retinal progenitors

The proliferative expansion of retinal progenitor cells (RPCs) is a fundamental mechanism of growth during vertebrate retinal development. Over the past couple of years, significant progress has been made in identifying genes expressed in RPCs that are essential for their proliferation, and the molecular mechanisms are beginning to be resolved. In this review, we highlight recent studies that have identified regulatory components of the RPC cell cycle machinery and implicate a set of homeobox genes as key regulators of proliferative expansion in the retina.

Paired-type homeodomain transcription factors are imported into the nucleus by karyopherin 13

We report that the paired homeodomain transcription factor Pax6 is imported into the nucleus by the Karyopherin beta family member Karyopherin 13 (Kap13). Pax6 was identified as a potential cargo for Kap13 by a yeast two-hybrid screen. Direct binding of Pax6 to Kap13 was subsequently confirmed by in vitro assays with recombinant proteins, and binding in vivo was shown by coimmunoprecipitation. Ran-dependent import of Pax6 by Kap13 was shown to occur by using a digitonin-permeabilized cells assay. Kap13 binds to Pax6 via a nuclear localization sequence (NLS), which is located within a segment of 80 amino acid residues that includes the homeodomain. Kap13 showed reduced binding to Pax6 when either region located at each end of the homeodomain (208 to 214 and 261 to 267) was deleted. The paired-type homeodomain transcription factor family includes more than 20 members. All members contain a region similar to the NLS found in Pax6 and are therefore likely to be imported by Kap13. We confirmed this hypothesis for Pax3 and Crx, which bind to and are imported by Kap13.

Direct interaction of geminin and Six3 in eye development

Organogenesis in vertebrates requires the tight control of cell proliferation and differentiation. The homeobox-containing transcription factor Six3 plays a pivotal role in the proliferation of retinal precursor cells. In a yeast two-hybrid screen, we identified the DNA replication-inhibitor geminin as a partner of Six3. Geminin inhibits cell-cycle progression by sequestering Cdt1 (refs 4, 5), the key component for the assembly of the pre-replication complex. Here, we show that Six3 efficiently competes with Cdt1 directly to bind to geminin, which reveals how Six3 can promote cell proliferation without transcription. In common with Six3 inactivation, overexpression of the geminin gene (Gem; also known as Gmn) in medaka (Oryzias latipes) induces specific forebrain and eye defects that are rescued by Six3. Conversely, loss of Gem (in common with gain of Six3 (ref. 1)) promotes retinal precursor-cell proliferation and results in expanded optic vesicles, markedly potentiating Six3 gain-of-function phenotypes. Our data indicate that the transcription factor Six3 and the replication-initiation inhibitor geminin act antagonistically to control the balance between proliferation and differentiation during early vertebrate eye development.

Eya protein phosphatase activity regulates Six1-Dach-Eya transcriptional effects in mammalian organogenesis

The precise mechanistic relationship between gene activation and repression events is a central question in mammalian organogenesis, as exemplified by the evolutionarily conserved sine oculis (Six), eyes absent (Eya) and dachshund (Dach) network of genetically interacting proteins. Here, we report that Six1 is required for the development of murine kidney, muscle and inner ear, and that it exhibits synergistic genetic interactions with Eya factors. We demonstrate that the Eya family has a protein phosphatase function, and that its enzymatic activity is required for regulating genes encoding growth control and signalling molecules, modulating precursor cell proliferation. The phosphatase function of Eya switches the function of Six1-Dach from repression to activation, causing transcriptional activation through recruitment of co-activators. The gene-specific recruitment of a co-activator with intrinsic phosphatase activity provides a molecular mechanism for activation of specific gene targets, including those regulating precursor cell proliferation and survival in mammalian organogenesis.

Neurogenesis and the cell cycle

For a long time, it has been understood that neurogenesis is linked to proliferation and thus to the cell cycle. Recently, the gears that mediate this linkage have become accessible to molecular investigation. This review describes some of the progress that has been made in understanding how the molecular machinery of the cell cycle is used in the processes of size regulation in the brain, histogenesis, neuronal differentiation, and the maintenance of stem cells.

The genetic and molecular basis of congenital eye defects

The mature eye is a complex organ that develops through a highly organized process during embryogenesis. Alterations in its genetic programming can lead to severe disorders that become apparent at birth or shortly afterwards; for example, one-half of the cases of blindness in children have a genetic cause. This review outlines the genetic basis of eye development, as determined by mutation analysis in patients and in model organisms. A better understanding of how this intricate organ develops at the genetic and cellular level is central to our understanding of the pathologies that afflict it.

Functional dissection of eyes absent reveals new modes of regulation within the retinal determination gene network

The retinal determination (RD) gene network encodes a group of transcription factors and cofactors necessary for eye development. Transcriptional and posttranslational regulation of RD family members is achieved through interactions within the network and with extracellular signaling pathways, including epidermal growth factor receptor/RAS/mitogen-activated protein kinase (MAPK), transforming growth factor beta/DPP, Wingless, Hedgehog, and Notch. Here we present the results of structure-function analyses that reveal novel aspects of Eyes absent (EYA) function and regulation. We find that the conserved C-terminal EYA domain negatively regulates EYA transactivation potential, and that GROUCHO-SINE OCULIS (SO) interactions provide another mechanism for negative regulation of EYA-SO target genes. We have mapped the transactivation potential of EYA to an internal proline-, serine-, and threonine-rich region that includes the EYA domain 2 (ED2) and two MAPK phosphorylation consensus sites and demonstrate that activation of the RAS/MAPK pathway potentiates transcriptional output of EYA and the EYA-SO complex in certain contexts. Drosophila S2 cell two-hybrid assays were used to describe a novel homotypic interaction that is mediated by EYA's N terminus. Our data suggest that EYA requires homo- and heterotypic interactions and RAS/MAPK signaling responsiveness to ensure context-appropriate RD gene network activity.

Cooperative action between L-Maf and Sox2 on delta-crystallin gene expression during chick lens development

Lens development is regulated by a variety of transcription factors with distinct properties. The lens-specific transcription factor, L-Maf, is essential for lens formation and induces lens-specific markers, such as the crystallin genes. In this study, we analyzed the mechanism by which L-Maf regulates delta-crystallin expression. Misexpression of L-Maf in the head ectoderm of lens placode-forming embryos by in ovo electroporation induced delta-crystallin only in the region surrounding the lens. To define this restricted expression, we misexpressed L-Maf together with other transcription factors implicated in delta-crystallin expression. Sox2 plus L-Maf expanded the delta-crystallin-inducible domain to the entire head ectoderm and simultaneously increased the quantity of delta-crystallin mRNA expressed. In contrast, co-expression of L-Maf with other factors such as Pax6, Six3 and Prox1 had little or no effect on delta-crystallin. We also observed that L-Maf and Sox2 cooperatively enhanced the transactivation of a reporter gene bearing the delta-crystallin enhancer in ovo, implying that L-Maf and Sox2 can induce delta-crystallin through the same enhancer. In conclusion, we report here that L-Maf and Sox2 cooperatively regulate the expression of delta-crystallin during chick lens development.

Six3 repression of Wnt signaling in the anterior neuroectoderm is essential for vertebrate forebrain development

In vertebrate embryos, formation of anterior neural structures requires suppression of Wnt signals emanating from the paraxial mesoderm and midbrain territory. In Six3(-/-) mice, the prosencephalon was severely truncated, and the expression of Wnt1 was rostrally expanded, a finding that indicates that the mutant head was posteriorized. Ectopic expression of Six3 in chick and fish embryos, together with the use of in vivo and in vitro DNA-binding assays, allowed us to determine that Six3 is a direct negative regulator of Wnt1 expression. These results, together with those of phenotypic rescue of headless/tcf3 zebrafish mutants by mouse Six3, demonstrate that regionalization of the vertebrate forebrain involves repression of Wnt1 expression by Six3 within the anterior neuroectoderm. Furthermore, these results support the hypothesis that a Wnt signal gradient specifies posterior fates in the anterior neural plate.

Meis homeoproteins directly regulate Pax6 during vertebrate lens morphogenesis

Pax6 is a pivotal regulator of eye development throughout Metazoa, but the direct upstream regulators of vertebrate Pax6 expression are unknown. In vertebrates, Pax6 is required for formation of the lens placode, an ectodermal thickening that precedes lens development. Here we show that the Meis1 and Meis2 homeoproteins are direct regulators of Pax6 expression in prospective lens ectoderm. In mice, Meis1 and Meis2 are developmentally expressed in a pattern remarkably similar to Pax6 and their expression is Pax6-independent. Biochemical and transgenic experiments reveal that Meis1 and Meis2 bind a specific sequence in the Pax6 lens placode enhancer that is required for its activity. Furthermore, Pax6 and Meis2 exhibit a strong genetic interaction in lens development, and Pax6 expression is elevated in lenses of Meis2-overexpressing transgenic mice. When expressed in embryonic lens ectoderm, dominant-negative forms of Meis down-regulate endogenous Pax6. These results contrast with those in Drosophila, where the single Meis homolog, Homothorax, has been shown to negatively regulate eye formation. Therefore, despite the striking evolutionary conservation of Pax6 function, Pax6 expression in the vertebrate lens is uniquely regulated.

The homeobox gene Six3 is a potential regulator of anterior segment formation in the chick eye

The anterior segment of the vertebrate eye consists of highly organized and specialized ocular tissues critical for normal vision. The periocular mesenchyme, originating from the neural crest, contributes extensively to the anterior segment. During chick eye morphogenesis, the homeobox gene Six3 is expressed in a subset of periocular mesenchymal cells and in differentiating anterior segment tissues. Retrovirus-mediated misexpression of Six3 causes eye anterior segment malformation, including corneal protrusion and opacification, ciliary body and iris hypoplasia, and trabecular meshwork dysgenesis. Histological and molecular marker analyses demonstrate that Six3 misexpression disrupts the integrity of the corneal endothelium and the expression of extracellular matrix components critical for corneal transparency. Six3 misexpression also leads to a reduction of the periocular mesenchymal cell population expressing Lmx1b, Pitx2, and Pax6, transcription factors critical for eye anterior segment morphogenesis. Moreover, elevated levels of Six3 attenuate proliferation of periocular mesenchymal cells in vitro and differentiating anterior segment tissues in vivo. These results suggest that, in addition to its function in eye primordium determination, Six3 plays a role in regulating the development of the vertebrate eye anterior segment.