Ectopic Sox3 activity elicits sensory placode formation

The function of tcf3 in medaka embryos: efficient knockdown with pePNAs

Background

The application of antisense molecules, such as morpholino oligonucleotides, is an efficient method of gene inactivation in vivo. We recently introduced phosphonic ester modified peptide nucleic acids (PNA) for in vivo loss-of-function experiments in medaka embryos. Here we tested novel modifications of the PNA backbone to knockdown the medaka tcf3 gene.

Results

A single tcf3 gene exists in the medaka genome and its inactivation strongly affected eye development of the embryos, leading to size reduction and anophthalmia in severe cases. The function of Tcf3 strongly depends on co-repressor interactions. We found interactions with Groucho/Tle proteins to be most important for eye development. Using a dominant negative approach for combined inactivation of all groucho/tle genes also resulted in eye phenotypes, as did interference with three individual tle genes.

Conclusions

Our results show that side chain modified PNAs come close to the knockdown efficiency of morpholino oligonucleotides in vivo. A single medaka tcf3 gene combines the function of the two zebrafish paralogs hdl and tcf3b. In combination with Groucho/Tle corepressor proteins Tcf3 acts in anterior development and is critical for eye formation.

Electronic supplementary material

The online version of this article (10.1186/s12896-017-0411-0) contains supplementary material, which is available to authorized users.

Expression analysis of sox3 during testicular development, recrudescence, and after hCG induction in catfish, Clarias batrachus

In teleosts, the expression of steroidogenic enzymes and related transcription factor genes occurs in a stage- and tissue-specific manner causing sexual development. The role of sox3, an evolutionary ancestor of SRY, has not been studied in detail. Therefore, the full-length cDNA of sox3 (1,197 kb) was cloned from catfish testis, and mRNA expression was analyzed during gonadal development, during the seasonal reproductive cycle, and after human chorionic gonadotropin (hCG) induction. Tissue distribution analysis showed that sox3 expression was higher in testis, ovary, and brain compared to other tissues analyzed. Developing and mature testis showed higher sox3 expression than ovary of corresponding stages, and more sox3 transcripts were found during the spawning phase of the seasonal reproductive cycle. Expression of sox3 was upregulated by hCG after in vivo and in vitro induction, suggesting that gonadotropins might stimulate it. In situ hybridization and immunohistochemistry showed the presence of sox3 mRNA and protein in somatic and interstitial cell layers of the testis. Sox3 could also be found in the zona radiata of developing and mature oocytes. Exposure of methyltestosterone (1 µg/l) and ethinylestradiol (1 µg/l) for 21 days during testicular development showed lower sox3 expression levels in the testis and brain, indicating a certain feedback intervention. These results suggest a possible role for Sox3 in the regulation of testicular development and function.

Co-option of Sox3 as the male-determining factor on the Y chromosome in the fish Oryzias dancena

Sex chromosomes harbour a primary sex-determining signal that triggers sexual development of the organism. However, diverse sex chromosome systems have been evolved in vertebrates. Here we use positional cloning to identify the sex-determining locus of a medaka-related fish, Oryzias dancena, and find that the locus on the Y chromosome contains a cis-regulatory element that upregulates neighbouring Sox3 expression in developing gonad. Sex-reversed phenotypes in Sox3(Y) transgenic fish, and Sox3(Y) loss-of-function mutants all point to its critical role in sex determination. Furthermore, we demonstrate that Sox3 initiates testicular differentiation by upregulating expression of downstream Gsdf, which is highly conserved in fish sex differentiation pathways. Our results not only provide strong evidence for the independent recruitment of Sox3 to male determination in distantly related vertebrates, but also provide direct evidence that a novel sex determination pathway has evolved through co-option of a transcriptional regulator potentially interacted with a conserved downstream component.

Setting appropriate boundaries: fate, patterning and competence at the neural plate border

The neural crest and craniofacial placodes are two distinct progenitor populations that arise at the border of the vertebrate neural plate. This border region develops through a series of inductive interactions that begins before gastrulation and progressively divide embryonic ectoderm into neural and non-neural regions, followed by the emergence of neural crest and placodal progenitors. In this review, we describe how a limited repertoire of inductive signals-principally FGFs, Wnts and BMPs-set up domains of transcription factors in the border region which establish these progenitor territories by both cross-inhibitory and cross-autoregulatory interactions. The gradual assembly of different cohorts of transcription factors that results from these interactions is one mechanism to provide the competence to respond to inductive signals in different ways, ultimately generating the neural crest and cranial placodes.

New insights into the mechanism of lens development using zebra fish

On the basis of recent advances in molecular biology, genetics, and live-embryo imaging, direct comparisons between zebra fish and human lens development are being made. The zebra fish has numerous experimental advantages for investigation of fundamental biomedical problems that are often best studied in the lens. The physical characteristics of visible light can account for the highly coordinated cell differentiation during formation of a beautifully transparent, refractile, symmetric optical element, the biological lens. The accessibility of the zebra fish lens for direct investigation during rapid development will result in new knowledge about basic functional mechanisms of epithelia-mesenchymal transitions, cell fate, cell-matrix interactions, cytoskeletal interactions, cytoplasmic crowding, membrane transport, cell adhesion, cell signaling, and metabolic specialization. The lens is well known as a model for characterization of cell and molecular aging. We review the recent advances in understanding vertebrate lens development conducted with zebra fish.

Patterns of gene expression in microarrays and expressed sequence tags from normal and cataractous lenses

In this contribution, we have examined the patterns of gene expression in normal and cataractous lenses as presented in five different papers using microarrays and expressed sequence tags. The purpose was to evaluate unique and common patterns of gene expression during development, aging and cataracts.

The peripheral sensory nervous system in the vertebrate head: a gene regulatory perspective

In the vertebrate head, crucial parts of the sense organs and sensory ganglia develop from special regions, the cranial placodes. Despite their cellular and functional diversity, they arise from a common field of multipotent progenitors and acquire distinct identity later under the influence of local signalling. Here we present the gene regulatory network that summarises our current understanding of how sensory cells are specified, how they become different from other ectodermal derivatives and how they begin to diversify to generate placodes with different identities. This analysis reveals how sequential activation of sets of transcription factors subdivides the ectoderm over time into smaller domains of progenitors for the central nervous system, neural crest, epidermis and sensory placodes. Within this hierarchy the timing of signalling and developmental history of each cell population is of critical importance to determine the ultimate outcome. A reoccurring theme is that local signals set up broad gene expression domains, which are further refined by mutual repression between different transcription factors. The Six and Eya network lies at the heart of sensory progenitor specification. In a positive feedback loop these factors perpetuate their own expression thus stabilising pre-placodal fate, while simultaneously repressing neural and neural crest specific factors. Downstream of the Six and Eya cassette, Pax genes in combination with other factors begin to impart regional identity to placode progenitors. While our review highlights the wealth of information available, it also points to the lack information on the cis-regulatory mechanisms that control placode specification and of how the repeated use of signalling input is integrated.

Molecular analysis of neurogenic placode development in a basal ray-finned fish

Neurogenic placodes are transient, thickened patches of embryonic vertebrate head ectoderm that give rise to the paired peripheral sense organs and most neurons in cranial sensory ganglia. We present the first analysis of gene expression during neurogenic placode development in a basal actinopterygian (ray-finned fish), the North American paddlefish (Polyodon spathula). Pax3 expression in the profundal placode confirms its homology with the ophthalmic trigeminal placode of amniotes. We report the conservation of expression of Pax2 and Pax8 in the otic and/or epibranchial placodes, Phox2b in epibranchial placode-derived neurons, Sox3 during epibranchial and lateral line placode development, and NeuroD in developing cranial sensory ganglia. We identify Sox3 as a novel marker for developing fields of electrosensory ampullary organs and for ampullary organs themselves. Sox3 is also the first molecular marker for actinopterygian ampullary organs. This is consistent with, though does not prove, a lateral line placode origin for actinopterygian ampullary organs.

Molecular and cellular aspects of amphibian lens regeneration

Lens regeneration among vertebrates is basically restricted to some amphibians. The most notable cases are the ones that occur in premetamorphic frogs and in adult newts. Frogs and newts regenerate their lens in very different ways. In frogs the lens is regenerated by transdifferentiation of the cornea and is limited only to a time before metamorphosis. On the other hand, regeneration in newts is mediated by transdifferentiation of the pigment epithelial cells of the dorsal iris and is possible in adult animals as well. Thus, the study of both systems could provide important information about the process. Molecular tools have been developed in frogs and recently also in newts. Thus, the process has been studied at the molecular and cellular levels. A synthesis describing both systems was long due. In this review we describe the process in both Xenopus and the newt. The known molecular mechanisms are described and compared.

Dermal morphogenesis controls lateral line patterning during postembryonic development of teleost fish

The lateral line system displays highly divergent patterns in adult teleost fish. The mechanisms underlying this variability are poorly understood. Here, we demonstrate that the lateral line mechanoreceptor, the neuromast, gives rise to a series of accessory neuromasts by a serial budding process during postembryonic development in zebrafish. We also show that accessory neuromast formation is highly correlated to the development of underlying dermal structures such as bones and scales. Abnormalities in opercular bone morphogenesis, in endothelin 1-knockdown embryos, are accompanied by stereotypic errors in neuromast budding and positioning, further demonstrating the tight correlation between the patterning of neuromasts and of the underlying dermal bones. In medaka, where scales form between peridermis and opercular bones, the lateral line displays a scale-specific pattern which is never observed in zebrafish. These results strongly suggest a control of postembryonic neuromast patterns by underlying dermal structures. This dermal control may explain some aspects of the evolution of lateral line patterns.

Making senses development of vertebrate cranial placodes

Cranial placodes (which include the adenohypophyseal, olfactory, lens, otic, lateral line, profundal/trigeminal, and epibranchial placodes) give rise to many sense organs and ganglia of the vertebrate head. Recent evidence suggests that all cranial placodes may be developmentally related structures, which originate from a common panplacodal primordium at neural plate stages and use similar regulatory mechanisms to control developmental processes shared between different placodes such as neurogenesis and morphogenetic movements. After providing a brief overview of placodal diversity, the present review summarizes current evidence for the existence of a panplacodal primordium and discusses the central role of transcription factors Six1 and Eya1 in the regulation of processes shared between different placodes. Upstream signaling events and transcription factors involved in early embryonic induction and specification of the panplacodal primordium are discussed next. I then review how individual placodes arise from the panplacodal primordium and present a model of multistep placode induction. Finally, I briefly summarize recent advances concerning how placodal neurons and sensory cells are specified, and how morphogenesis of placodes (including delamination and migration of placode-derived cells and invagination) is controlled.

Normal development of the tomato clownfish Amphiprion frenatus: live imaging and in situ hybridization analyses of mesodermal and neurectodermal development

The normal embryonic development of the tomato clownfish Amphiprion frenatus was analysed using live imaging and by in situ hybridization for detection of mesodermal and neurectodermal development. Both morphology of live embryos and tissue-specific staining revealed significant differences in the gross developmental programme of A. frenatus compared with better-known teleost fish models, in particular, initiation of somitogenesis before complete epiboly, initiation of narrowing of the neurectoderm (neurulation) before somitogenesis, relatively early pigmentation of melanophores at the 10-15 somite stage and a distinctive pattern of melanophore distribution. These results suggest evolutionary adaptability of the teleost developmental programme. The ease of obtaining eggs, in vitro culture of the embryo, in situ staining analyses and these reported characteristics make A. frenatus a potentially important model marine fish species for studying embryonic development, physiology, ecology and evolution.

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

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

Competence, specification and commitment to an olfactory placode fate

The nasal placode shares a common origin with other sensory placodes within a pre-placodal domain at the cranial neural plate border. However, little is known about early events in nasal placode development as it segregates from prospective lens, neural tube and epidermis. Here, Dlx3, Dlx5, Pax6 and the pan-neuronal marker Hu serve as molecular labels to follow the maturation of olfactory precursors over time. When competence to form olfactory placode was tested by grafting ectoderm from different axial levels to the anterior neural fold, we found that competence is initially broad for head, but not trunk, ectoderm and declines rapidly with time. Isolated olfactory precursors are specified by HH10, concomitant with their complete segregation from other placodal, epidermal and neural progenitors. Heterotopic transplantation of olfactory progenitors reveals they are capable of autonomous differentiation only 12 hours later, shortly before overt placode invagination at HH14. Taken together, these results show that olfactory placode development is a step-wise process whereby signals from adjacent tissues specify competent ectoderm at or before HH10, followed by gradual commitment just prior to morphological differentiation.

Evolution of non-coding regulatory sequences involved in the developmental process: reflection of differential employment of paralogous genes as highlighted by Sox2 and group B1 Sox genes

In higher vertebrates, the expression of Sox2, a group B1 Sox gene, is the hallmark of neural primordial cell state during the developmental processes from embryo to adult. Sox2 is regulated by the combined action of many enhancers with distinct spatio-temporal specificities. DNA sequences for these enhancers are conserved in a wide range of vertebrate species, corresponding to a majority of highly conserved non-coding sequences surrounding the Sox2 gene, corroborating the notion that the conservation of non-coding sequences mirrors their functional importance. Among the Sox2 enhancers, N-1 and N-2 are activated the earliest in embryogenesis and regulate Sox2 in posterior and anterior neural plates, respectively. These enhancers differ in their evolutionary history: the sequence and activity of enhancer N-2 is conserved in all vertebrate species, while enhancer N-1 is fully conserved only in amniotes. In teleost embryos, Sox19a/b play the major pan-neural role among the group B1 Sox paralogues, while strong Sox2 expression is limited to the anterior neural plate, reflecting the absence of posterior CNS-dedicated enhancers, including N-1. In Xenopus, neurally expressed SoxD is the orthologue of Sox19, but Sox3 appears to dominate other B1 paralogues. In amniotes, however, Sox19 has lost its group B1 Sox function and transforms into group G Sox15 (neofunctionalization), and Sox2 assumes the dominant position by gaining enhancer N-1 and other enhancers for posterior CNS. Thus, the gain and loss of specific enhancer elements during the evolutionary process reflects the change in functional assignment of particular paralogous genes, while overall regulatory functions attributed to the gene family are maintained.

Development of diverse lateral line patterns on the teleost caudal fin

The lateral line is composed of mechanoreceptors, the neuromasts, which are distributed over the body surfaces of fish. We examine the development of neuromast patterns on the caudal fins of medaka and zebrafish. In medaka, the terminal neuromast is established just prior to the caudal fin formation. The terminal neuromast subsequently gives rise to a cluster of accessory neuromasts. In zebrafish, the terminal neuromasts vary in terms of both number and position, and they achieve their final positions relative to the caudal fin structures through migration. Subsequently, they give rise to four lines of accessory neuromasts that extend along the caudal fin. We show that developmental processes similar to those observed in medaka and zebrafish may account for a large variety of patterns in other teleost species. These results establish terminal neuromast patterning as a new model for the study of the developmental mechanisms underlying diverse lateral line patterns.

Do vertebrate neural crest and cranial placodes have a common evolutionary origin?

Two embryonic tissues-the neural crest and the cranial placodes-give rise to most evolutionary novelties of the vertebrate head. These two tissues develop similarly in several respects: they originate from ectoderm at the neural plate border, give rise to migratory cells and develop into multiple cell fates including sensory neurons. These similarities, and the joint appearance of both tissues in the vertebrate lineage, may point to a common evolutionary origin of neural crest and placodes from a specialized population of neural plate border cells. However, a review of the developmental mechanisms underlying the induction, specification, migration and cytodifferentiation of neural crest and placodes reveals fundamental differences between the tissues. Taken together with insights from recent studies in tunicates and amphioxus, this suggests that neural crest and placodes have an independent evolutionary origin and that they evolved from the neural and non-neural side of the neural plate border, respectively.

Spalt4 mediates invagination and otic placode gene expression in cranial ectoderm

Vertebrate placodes are regions of thickened head ectoderm that contribute to paired sensory organs and cranial ganglia. We demonstrate that the transcription factor Spalt4 (also known as Sall4) is broadly expressed in chick preplacodal epiblast and later resolves to otic, lens and olfactory placodes. Ectopic expression of Spalt4 by electroporation is sufficient to induce invagination of non-placodal head ectoderm and prevent neurogenic placodes from contributing to cranial ganglia. Conversely, loss of Spalt4 function in the otic placode results in abnormal otic vesicle development. Intriguingly, Spalt4 appears to initiate a placode program appropriate for the axial level but is not involved in later development of specific placode fates. Fgfs can regulate Spalt4, since implantation of Fgf2 beads into the area opaca induces its expression. The results suggest that Spalt4 is involved in early stages of placode development, initiating cranial ectodermal invagination and region-specific gene regulatory networks.

Teleost lens development and degeneration

The transparent properties of the lens and its ability to focus light onto the retina are critical for normal vision. Optical clarity of the lens is achieved and maintained by a unique, highly regulated integration of lens cell proliferation and differentiation that persists throughout life. Zebrafish is a powerful genetic model for studying vertebrate lens differentiation and growth because the structural organization of the lens and gene functions are largely conserved with mammals, including humans. However, some features of zebrafish lens developmental morphology and gene expression are different from those of mammals and other terrestrial vertebrates. For example, the presumptive zebrafish lens delaminates from the surface ectoderm to form a solid mass of cells, in which the primary fibers differentiate by elongating in circular fashion. Both mutational and candidate gene analyses have identified and characterized developmental gene functions of the lens in zebrafish. This chapter presents the recent morphological analysis of zebrafish lens formation. In addition, the roles of Pitx3, Foxe3, and the lens-specific protein Lengsin (LENS Glutamine SYNthetase-like) in lens development are analyzed. Selected zebrafish lens mutants defective in early developmental processes and the maintenance of lens transparency are also discussed.

The amphioxus SoxB family: implications for the evolution of vertebrate placodes

Cranial placodes are regions of thickened ectoderm that give rise to sense organs and ganglia in the vertebrate head. Homologous structures are proposed to exist in urochordates, but have not been found in cephalochordates, suggesting the first chordates lacked placodes. SoxB genes are expressed in discrete subsets of vertebrate placodes. To investigate how placodes arose and diversified in the vertebrate lineage we isolated the complete set of SoxB genes from amphioxus and analyzed their expression in embryos and larvae. We find that while amphioxus possesses a single SoxB2 gene, it has three SoxB1 paralogs. Like vertebrate SoxB1 genes, one of these paralogs is expressed in non-neural ectoderm destined to give rise to sensory cells. When considered in the context of other amphioxus placode marker orthologs, amphioxus SoxB1 expression suggests a diversity of sensory cell types utilizing distinct placode-type gene programs was present in the first chordates. Our data supports a model for placode evolution and diversification whereby the full complement of vertebrate placodes evolved by serial recruitment of distinct sensory cell specification programs to anterior pre-placodal ectoderm.

Differential expression and dynamic changes of SOX3 during gametogenesis and sex reversal in protogynous hermaphroditic fish

SOX3 has been suggested to play significant roles in gametogenesis and gonad differentiation of vertebrates, but the exact cellular localization evidence is insufficient and controversial. In this study, a protogynous hermaphrodite fish Epinephelus coioides is selected to analyze EcSox3 differential expression and the expression pattern in both processes of oogenesis and spermatogenesis by utilizing the advantages that gonad development undergoes transition from ovary to intersexual gonad and then to testis, and primordial germ cells and different stage cells during oogenesis and spermatogenesis are synchronously observed in the transitional gonads. The detailed and clear immunofluoresence localization indicates that significantly differential expression and dynamic changes of Sox3 occur in the progresses of gametogenesis and sex reversal, and EcSOX3 protein exists in the differentiating primordial germ cells, oogonia, and different stage oocytes of ovaries, and also in the differentiating primordial germ cells and the Sertoli cells of testis. One important finding is that the EcSox3 expression is a significant time point for enterable gametogenesis of primordial germ cells because EcSOX3 is obviously expressed and localized in primordial germ cells. As EcSox3 continues to express, the EcSOX3-positive primordial germ cells develop toward oogonia and then oocytes, whereas when EcSox3 expression is ceased, the EcSOX3-positive primordial germ cells develop toward spermatogonia. Therefore, the current finding of EcSOX3 in the differentiating primordial germ cells again confirms the potential regulatory role in oogenesis and germ cell differentiation. The data further suggest that SOX3, as a transcription factor, might have more important roles in oogenesis than in spermatogenesis.

Distinct roles of Fgf8, Foxi1, Dlx3b and Pax8/2 during otic vesicle induction and maintenance in medaka

The development of the vertebrate inner ear is a complex process that has been investigated in several model organisms. In this work, we examined genetic interactions regulating early development of otic structures in medaka. We demonstrate that misexpression of Fgf8, Dlx3b and Foxi1 during early gastrulation is sufficient to produce ectopic otic vesicles. Combined misexpression strongly increases the appearance of this phenotype. By using a heat-inducible promoter we were furthermore able to separate the regulatory interactions among Fgf8, Foxi1, Dlx3b, Pax8 and Pax2 genes, which are active during different stages of early otic development. In the preplacodal stage we suggest a central position of Foxi1 within a regulatory network of early patterning genes including Dlx3b and Pax8. Different pathways are active after the placodal stage with Dlx3b playing a central role. There Dlx3b regulates members of the Pax-Six-Eya-Dach network and also strongly affects the early dorsoventral marker genes Otx1 and Gbx2.

Initial specification of the epibranchial placode in zebrafish embryos depends on the fibroblast growth factor signal

In vertebrates, cranial sensory ganglia are mainly derived from ectodermal placodes, which are focal thickenings at characteristic positions in the embryonic head. Here, we provide the first description of the early development of the epibranchial placode in zebrafish embryos using sox3 as a molecular marker. By the one-somite stage, we saw a pair of single sox3-expressing domains appear lateral to the future hindbrain. The sox3 domain, which is referred to here as the early lateral placode, is segregated during the early phase of segmentation to form a pax2a-positive medial area and a pax2a-negative lateral area. The medial area subsequently developed to form the otic placode, while the lateral area was further segregated along the anteroposterior axis, giving rise to four sox3-positive subdomains by 26 hr postfertilization. Given their spatial relationship with the expression of the markers for the epibranchial ganglion, as well as their positions and temporal changes, we propose that these four domains correspond to the facial, glossopharyngeal, vagal, and posterior lateral line placodes in an anterior-to-posterior order. The expression of sox3 in the early lateral placode was absent in mutants lacking functional fgf8, while implantation of fibroblast growth factor (FGF) beads restored the sox3 expression. Using SU5402, which inhibits the FGF signal, we were able to demonstrate that formation of both the early lateral domains and later epibranchial placodes depends on the FGF signal operating at the beginning of somitogenesis. Together, these data provide evidence for the essential role of FGF signals in the development of the epibranchial placodes.

Foxe view of lens development and disease

The recent identification of a mutation in Foxe3 that causes congenital primary aphakia in humans marks an important milestone. Congenital primary aphakia is a rare developmental disease in which the lens does not form. Previously, Foxe3 had been shown to play a crucial role in vertebrate lens formation and this gene is one of the earliest integrators of several signaling pathways that cooperate to form a lens. In this review, we highlight recent advances that have led to a better understanding of the developmental processes and gene regulatory networks involved in lens development and disease.

Lens induction in vertebrates: Variations on a conserved theme of signaling events

This review provides an overview of our current understanding of signaling mechanisms involved in lens induction, which are presented in context of the major stages of lens induction (competence, bias, inhibition and specification). Although the process of lens induction is generally well conserved, we highlight aspects of induction that vary among species. Finally, this review identifies future challenges in forming an integrated network of signaling pathways involved in lens induction.

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.

Comparative genomic and expression analysis of group B1soxgenes in zebrafish indicates their diversification during vertebrate evolution

Group B1 Sox genes encode HMG domain transcription factors that play major roles in neural development. We have identified six zebrafish B1 sox genes, which include pan-vertebrate sox1a/b, sox2, and sox3, and also fish-specific sox19a/b. SOX19A/B proteins show a transcriptional activation potential that is similar to other B1 SOX proteins. The expression of sox19a and sox3 begins at approximately the 1,000-cell stage during embryogenesis and becomes confined to the future ectoderm by the shield stage. This is reminiscent of the epiblastic expression of Sox2 and/or Sox3 in amniotes. As development progresses, these six B1 sox genes display unique expression patterns that overlap distinctly from one region to another. sox19a expression is widespread in the early neuroectoderm, resembling pan-neural Sox2 expression in amniotes, whereas zebrafish sox2 shows anterior-restricted expression. Comparative genomics suggests that sox19a/b and mammalian Sox15 (group G) have an orthologous relationship and that the B1/G Sox genes arose from a common ancestral gene through two rounds of genome duplication. It seems likely, therefore, that each B1/G Sox gene has gained a distinct expression profile and function during vertebrate evolution.

Development and evolution of lateral line placodes in amphibians I. Development

Lateral line placodes are specialized regions of the ectoderm that give rise to the receptor organs of the lateral line system as well as to the sensory neurons innervating them. The development of lateral line placodes has been studied in amphibians since the early 1900s. This paper reviews these older studies and tries to integrate them with more recent findings. Lateral line placodes are probably induced in a multistep process from a panplacodal area surrounding the neural plate. The time schedule of these inductive processes has begun to be unravelled, but little is known yet about their molecular basis. Subsequent pattern formation, morphogenesis and differentiation of lateral line placodes proceeds in most respects relatively autonomously: Onset and polarity of migration of lateral line primordia, the type, spacing, size and number of receptor organs formed, as well as the patterned differentiation of different cell types occur normally even in ectopic locations. Only the pathways for migration of lateral line primordia depend on external cues. Thus, lateral line placodes act as integrated and relatively context-insensitive developmental modules.

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.

Multiple N-cadherin enhancers identified by systematic functional screening indicate its Group B1 SOX-dependent regulation in neural and placodal development

Neural plate and sensory placodes share the expression of N-cadherin and Group B1 Sox genes, represented by Sox2. A 219-kb region of the chicken genome centered by the N-cadherin gene was scanned for neural and placodal enhancers. Random subfragments of 4.5 kb average length were prepared and inserted into tkEGFP reporter vector to construct a library with threefold coverage of the region. Each clone was then transfected into N-cadherin-positive (lens, retina and forebrain) or -negative embryonic cells, or electroporated into early chicken embryos to examine enhancer activity. Enhancers 1-4 active in the CNS/placode derivatives and non-specific Enhancer 5 were identified by transfection, while electroporation of early embryos confirmed enhancers 2-4 as having activity in the early CNS and/or sensory placodes but with unique spatiotemporal specificities. Enhancers 2-4 are dependent on SOX-binding sites, and misexpression of Group B1 Sox genes in the head ectoderm caused ectopic development of placodes expressing N-cadherin, indicating the involvement of Group B1 Sox functions in N-cadherin regulation. Enhancers 1, 2 and 4 correspond to sequence blocks conserved between the chicken and mammalian genomes, but enhancers 3 and 5 are unique to the chicken.

Evolutionary origins of vertebrate placodes: insights from developmental studies and from comparisons with other deuterostomes

Ectodermal placodes comprise the adenohypophyseal, olfactory, lens, profundal, trigeminal, otic, lateral line, and epibranchial placodes. The first part of this review presents a brief overview of placode development. Placodes give rise to a variety of cell types and contribute to many sensory organs and ganglia of the vertebrate head. While different placodes differ with respect to location and derivative cell types, all appear to originate from a common panplacodal primordium, induced at the anterior neural plate border by a combination of mesodermal and neural signals and defined by the expression of Six1, Six4, and Eya genes. Evidence from mouse and zebrafish mutants suggests that these genes promote generic placodal properties such as cell proliferation, cell shape changes, and specification of neurons. The common developmental origin of placodes suggests that all placodes may have evolved in several steps from a common precursor. The second part of this review summarizes our current knowledge of placode evolution. Although placodes (like neural crest cells) have been proposed to be evolutionary novelties of vertebrates, recent studies in ascidians and amphioxus have proposed that some placodes originated earlier in the chordate lineage. However, while the origin of several cellular and molecular components of placodes (e.g., regionalized expression domains of transcription factors and some neuronal or neurosecretory cell types) clearly predates the origin of vertebrates, there is presently little evidence that these components are integrated into placodes in protochordates. A scenario is presented according to which all placodes evolved from an adenohypophyseal-olfactory protoplacode, which may have originated in the vertebrate ancestor from the anlage of a rostral neurosecretory organ (surviving as Hatschek's pit in present-day amphioxus).

An interstitial deletion-insertion involving chromosomes 2p25.3 and Xq27.1, near SOX3, causes X-linked recessive hypoparathyroidism

X-linked recessive hypoparathyroidism, due to parathyroid agenesis, has been mapped to a 906-kb region on Xq27 that contains 3 genes (ATP11C, U7snRNA, and SOX3), and analyses have not revealed mutations. We therefore characterized this region by combined analysis of single nucleotide polymorphisms and sequence-tagged sites. This identified a 23- to 25-kb deletion, which did not contain genes. However, DNA fiber-FISH and pulsed-field gel electrophoresis revealed an approximately 340-kb insertion that replaced the deleted fragment. Use of flow-sorted X chromosome-specific libraries and DNA sequence analyses revealed that the telomeric and centromeric breakpoints on X were, respectively, approximately 67 kb downstream of SOX3 and within a repetitive sequence. Use of a monochromosomal somatic cell hybrid panel and metaphase-FISH mapping demonstrated that the insertion originated from 2p25 and contained a segment of the SNTG2 gene that lacked an open reading frame. However, the deletion-insertion [del(X)(q27.1) inv ins (X;2)(q27.1;p25.3)], which represents a novel abnormality causing hypoparathyroidism, could result in a position effect on SOX3 expression. Indeed, SOX3 expression was demonstrated, by in situ hybridization, in the developing parathyroid tissue of mouse embryos between 10.5 and 15.5 days post coitum. Thus, our results indicate a likely new role for SOX3 in the embryonic development of the parathyroid glands.

A balance of FGF, BMP and WNT signalling positions the future placode territory in the head

The sensory nervous system in the vertebrate head arises from two different cell populations: neural crest and placodal cells. By contrast, in the trunk it originates from neural crest only. How do placode precursors become restricted exclusively to the head and how do multipotent ectodermal cells make the decision to become placodes or neural crest? At neural plate stages,future placode cells are confined to a narrow band in the head ectoderm, the pre-placodal region (PPR). Here, we identify the head mesoderm as the source of PPR inducing signals, reinforced by factors from the neural plate. We show that several independent signals are needed: attenuation of BMP and WNT is required for PPR formation. Together with activation of the FGF pathway, BMP and WNT antagonists can induce the PPR in naïve ectoderm. We also show that WNT signalling plays a crucial role in restricting placode formation to the head. Finally, we demonstrate that the decision of multipotent cells to become placode or neural crest precursors is mediated by WNT proteins:activation of the WNT pathway promotes the generation of neural crest at the expense of placodes. This mechanism explains how the placode territory becomes confined to the head, and how neural crest and placode fates diversify.

Over- and underdosage of SOX3 is associated with infundibular hypoplasia and hypopituitarism

Duplications of Xq26-27 have been implicated in the etiology of X-linked hypopituitarism associated with mental retardation (MR). Additionally, an expansion of a polyalanine tract (by 11 alanines) within the transcription factor SOX3 (Xq27.1) has been reported in patients with growth hormone deficiency and variable learning difficulties. We report a submicroscopic duplication of Xq27.1, the smallest reported to date (685.6 kb), in two siblings with variable hypopituitarism, callosal abnormalities, anterior pituitary hypoplasia (APH), an ectopic posterior pituitary (EPP), and an absent infundibulum. This duplication contains SOX3 and sequences corresponding to two transcripts of unknown function; only Sox3 is expressed in the infundibulum in mice. Next, we identified a novel seven-alanine expansion within a polyalanine tract in SOX3 in a family with panhypopituitarism in three male siblings with an absent infundibulum, severe APH, and EPP. This mutation led to reduced transcriptional activity, with impaired nuclear localization of the mutant protein. We also identified a novel polymorphism (A43T) in SOX3 in another child with hypopituitarism. In contrast to findings in previous studies, there was no evidence of MR or learning difficulties in our patients. We conclude that both over- and underdosage of SOX3 are associated with similar phenotypes, consisting of infundibular hypoplasia and hypopituitarism but not necessarily MR.

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.

Early development of the cranial sensory nervous system: from a common field to individual placodes

Sensory placodes are unique columnar epithelia with neurogenic potential that develop in the vertebrate head ectoderm next to the neural tube. They contribute to the paired sensory organs and the cranial sensory ganglia generating a wide variety of cell types ranging from lens fibres to sensory receptor cells and neurons. Although progress has been made in recent years to identify the molecular players that mediate placode specification, induction and patterning, the processes that initiate placode development are not well understood. One hypothesis suggests that all placode precursors arise from a common territory, the pre-placodal region, which is then subdivided to generate placodes of specific character. This model implies that their induction begins through molecular and cellular mechanisms common to all placodes. Embryological and molecular evidence suggests that placode induction is a multi-step process and that the molecular networks establishing the pre-placodal domain as well as the acquisition of placodal identity are surprisingly similar to those used in Drosophila to specify sensory structures.

Ectopic expression and knockdown of a zebrafish sox21 reveal its role as a transcriptional repressor in early development

Sox proteins are DNA-binding proteins belonging to the HMG box superfamily and they play key roles in animal embryonic development. Zebrafish Sox21a is part of group B Sox proteins and its chicken and mouse orthologs have been described as transcriptional repressor and activator, respectively, in two different target gene contexts. Zebrafish sox21a is present as a maternal transcript in the oocyte and is mainly expressed at the developing midbrain-hindbrain boundary from the onset of neurulation. In order to understand its role in vivo, we ectopically expressed sox21a by microinjection. Ectopic expression of full length sox21a leads to dorsalization of the embryos. A subset of the dorsalized embryos shows a partial axis splitting, and hence an ectopic neural tube, as an additional phenotype. At gastrulation, injected embryos show expansion of the expression domains of organizer-specific genes, such as chordin and goosecoid. Molecular markers used in somitogenesis highlight that sox21a-injected embryos have shortened AP axis, undulating axial structures, enlarged or even radialized paraxial territory. The developmental abnormalities caused by ectopic expression of sox21a are suggestive of defects in convergence-extension morphogenetic movements. Antisense morpholino oligonucleotides, designed to functionally knockdown sox21a, cause ventralization of the embryos. Moreover, gain-of-function experiments with chimeric constructs, where Sox21a DNA-binding domain is fused to a transcriptional activator (VP16) or repressor (EnR) domain, suggests that zebrafish Sox21a acts as a repressor in dorso-ventral patterning.

Molecular anatomy of placode development in Xenopus laevis

We analyzed the spatiotemporal pattern of expression of 15 transcription factors (Six1, Six4, Eya1, Sox3, Sox2, Pax6, Pax3, Pax2, Pax8, Dlx3, Msx1, FoxI1c, Tbx2, Tbx3, Xiro1) during placode development in Xenopus laevis from neural plate to late tail bud stages. Out of all genes investigated, only the expression of Eya1, Six1, and Six4 is maintained in all types of placode (except the lens) throughout embryonic development, suggesting that they may promote generic placodal properties and that their crescent-shaped expression domain surrounding the neural plate defines a panplacodal primordium from which all types of placode originate. Double-labeling procedures were employed to reveal the precise position of this panplacodal primordium relative to neural plate, neural crest, and other placodal markers. Already at neural plate stages, the panplacodal primordium is subdivided into several subregions defined by particular combinations of transcription factors allowing us to identify the approximate regions of origin of various types of placode. Whereas some types of placode were already prefigured by molecularly distinct areas at neural plate stages, the epibranchial, otic, and lateral line placodes arise from a common posterior placodal area (characterized by Pax8 and Pax2 expression) and acquire differential molecular signatures only after neural tube closure. Our findings argue for a multistep mechanism of placode induction, support a combinatorial model of placode specification, and suggest that different placodes evolved from a common placodal primordium by successive recruitment of new inducers and target genes.

Identification of two teleost homologs of the Drosophila sex determination factor, transformer-2 in medaka (Oryzias latipes)

Transformer-2 (Tra2), an RNA-binding protein, is an important regulator in Drosophila sex determination. In vertebrates, however, the role of Tra2 homologues is not known. We identified two teleost homologues of Tra2, which we named Tra2a and Tra2b, in medaka (Oryzias latipes). Furthermore, we demonstrated that both Tra2 mRNAs were predominantly expressed in germ cells of both sexes before the onset of sex differentiation, suggesting that both Tra2 homologues might be involved in the sex differentiation in medaka.

Pax6 autoregulation mediated by direct interaction of Pax6 protein with the head surface ectoderm-specific enhancer of the mouse Pax6 gene

The Pax6 gene plays crucial roles in eye development and encodes a transcription factor containing both a paired domain and a homeodomain. During embryogenesis, Pax6 is expressed in restricted tissues under the direction of distinct cis-regulatory regions. The head surface ectoderm-specific enhancer of mouse Pax6 directs reporter expression in the derivatives of the ectoderm in the eye, such as lens and cornea, but the molecular mechanism of its control remains largely unknown. We identified a Pax6 protein-responsive element termed LE9 (52 bp in length) within the head surface ectoderm-specific enhancer. LE9, a sequence well conserved across vertebrates, acted as a highly effective enhancer in reporter analyses. Pax6 protein formed in vitro a complex with the distal half of LE9 in a manner dependent on the paired domain. The proximal half of the LE9 sequence contains three plausible sites of HMG domain recognition, and HMG domain-containing transcription factors Sox2 and Sox3 activated LE9 synergistically with Pax6. A scanning mutagenesis experiment indicated that the central site is most important among the three presumptive HMG domain recognition sites. Furthermore, Pax6 and Sox2 proteins formed a complex when they were expressed together. Based on these findings, we propose a model in which Pax6 protein directly and positively regulates its own gene expression, and Sox2 and Sox3 proteins interact with Pax6 protein, resulting in modification of the transcriptional activation by Pax6 protein.

Expression of Sox3 throughout the developing central nervous system is dependent on the combined action of discrete, evolutionarily conserved regulatory elements

SOX3 is one of the earliest neural markers in vertebrates and is thought to play a role in specifying neuronal fate. To investigate the regulation of Sox3 expression we identified cis-regulatory regions in the Sox3 promoter that direct tissue-specific heterologous marker gene expression in transgenic mice. Our results show that an 8.3 kb fragment, comprising 3 kb upstream and 3 kb downstream of the Sox3 transcriptional unit, is sufficient in a lacZ reporter construct to reproduce most aspects of Sox3 expression during CNS development from headfold to midgestation stages. The apparently uniform expression of Sox3 in the neural tube depends, however, on the combined action of distinct regulatory modules within this 8.3 kb region. Each of these gives expression in a subdomain of the complete expression pattern. These are restricted along both the rostral-caudal and dorso-ventral axes and can be quite specific, one element giving expression largely confined to V2 interneuron precursors. We also find that at least some of the regulatory sequences are able to drive expression of the transgene in the CNS Xenopus laevis embryos in a manner that reflects the endogenous Sox3 expression pattern. These results imply that the underlying mechanism regulating early CNS patterning is conserved, despite several substantial differences in neurogenesis between mammals and amphibians.

The cellular and molecular bases of vertebrate lens regeneration

Lens regeneration takes place in some vertebrates through processes of cellular dedifferentiation and transdifferentiation, processes by which certain differentiated cell types can give rise to others. This review describes the principal forms of lens regeneration that occur in vivo as well as related in vitro systems of transdifferentiation. Classic experimental studies are reviewed that define the tissue interactions that trigger these events in vivo. Recent molecular analyses have begun to identify the genes associated with these processes. These latter studies generally reveal tremendous similarities between embryonic lens development and lens regeneration. Different models are proposed to describe basic molecular pathways that define the processes of lens regeneration and transdifferentiation. Finally, studies are discussed suggesting that fibroblast growth factors play key roles in supporting the process of lens regeneration. Retinoids, such as retinoic acid, may also play important roles in this process.

Transcription factor SOX3 is involved in X-linked mental retardation with growth hormone deficiency

Physical mapping of the breakpoints of a pericentric inversion of the X chromosome (46,X,inv[X][p21q27]) in a female patient with mild mental retardation revealed localization of the Xp breakpoint in the IL1RAPL gene at Xp21.3 and the Xq breakpoint near the SOX3 gene (SRY [sex determining region Y]-box 3) (GenBank accession number NM_005634) at Xq26.3. Because carrier females with microdeletion in the IL1RAPL gene do not present any abnormal phenotype, we focused on the Xq breakpoint. However, we were unable to confirm the involvement of SOX3 in the mental retardation in this female patient. To validate SOX3 as an X-linked mental retardation (XLMR) gene, we performed mutation analyses in families with XLMR whose causative gene mapped to Xq26-q27. We show here that the SOX3 gene is involved in a large family in which affected individuals have mental retardation and growth hormone deficiency. The mutation results in an in-frame duplication of 33 bp encoding for 11 alanines in a polyalanine tract of the SOX3 gene. The expression pattern during neural and pituitary development suggests that dysfunction of the SOX3 protein caused by the polyalanine expansion might disturb transcription pathways and the regulation of genes involved in cellular processes and functions required for cognitive and pituitary development.