Tbx5 and Tbx4 genes determine the wing/leg identity of limb buds

Specification and determination of limb identity: evidence for inhibitory regulation of Tbx gene expression

Limb-type-specific expression of Tbx5/Tbx4 plays a key role in drawing distinction between a forelimb and a hindlimb. Here, we show insights into specification and determination during commitment of limb-type identity, in particular that median tissues regulate Tbx expressions. By using the RT-PCR technique on chick embryos, the onset of specific Tbx5/Tbx4 expression in the wing/leg region was estimated to be stage 13. Specification of the limb-type identity is thought to occur before stage 9, since all explants from stage 9 through 14 expressed the intrinsic Tbx gene autonomously in a simple culture medium. The results of transplantation experiments revealed that axial structures medial to the lateral plate mesoderm at the level of the wing region are capable of transforming leg identity to wing identity, suggesting that a factor(s) from the median tissues is involved in the limb-type determination. Nevertheless, the transplanted wing region was not converted to leg identity. The results of the transplantation experiments also suggested that wing-type identity is determined much earlier than is leg-type identity. Finally, we also found that inhibitory effects of median tissues mediate the specific expression of Tbx5/Tbx4 in the presumptive wing/leg region. We propose a model for limb-type identification in which inhibitory regulation is involved in restricting one Tbx gene expression by masking the other Tbx expression there.

Noggin and retinoic acid transform the identity of avian facial prominences

The signals that determine body part identity in vertebrate embryos are largely unknown, with some exceptions such as those for teeth and digits. The vertebrate face is derived from small buds of tissue, facial prominences, that surround the embryonic oral cavity. In chicken embryos, the skeleton of the upper beak is derived from the frontonasal mass and maxillary prominences. Here we show that bone morphogenetic proteins (Bmps) and the vitamin A derivative, retinoic acid (RA), are used to specify the identity of the frontonasal mass and maxillary prominences. Implanting two beads adjacent to the stage-15 presumptive maxillary field, one soaked in the Bmp antagonist Noggin and one soaked in RA, induces a duplicate set of frontonasal mass skeletal elements in place of maxillary bones. We also show that the duplicated beak is due to transformation of the maxillary prominence into a second frontonasal mass and not due to ectopic migration of cells or splitting of the normal frontonasal mass. Thus the levels of Bmp and RA determine whether specific regions of the face form maxillary or frontonasal mass derivatives.

Leaping lopsided: a review of the current hypotheses regarding etiologies of limb malformations in frogs

Recent progress in the investigation of limb malformations in free-living frogs has underlined the wide range in the types of limb malformations and the apparent spatiotemporal clustering of their occurrence. Here, we review the current understanding of normal and abnormal vertebrate limb development and regeneration and discuss some of the molecular events that may bring about limb malformation. Consideration of the differences between limb development and regeneration in amphibians has led us to the hypothesis that some of the observed limb malformations come about through misdirected regeneration. We report the results of a pilot study that supports this hypothesis. In this study, the distal aspect of the right hindlimb buds of X. laevis tadpoles was amputated at the pre-foot paddle stage. The tadpoles were raised in water from a pond in Minnesota at which 7% of surveyed newly metamorphosed feral frogs had malformations. Six percent (6 of 100) of the right limbs of the tadpoles raised in pond water developed abnormally. One truncated right limb was the only malformation in the control group, which was raised in dechlorinated municipal water. All unamputated limbs developed normally in both groups. Three major factors under consideration for effecting the limb malformations are discussed. These factors include environmental chemicals (primarily agrichemicals), encysted larvae (metacercariae) of trematode parasites, and increased levels of ultraviolet light. Emphasis is placed on the necessary intersection of environmental stressors and developmental events to bring about the specific malformations that are observed in free-living frog populations.

A comparative molecular analysis of developing mouse forelimbs and hindlimbs using serial analysis of gene expression (SAGE)

The analysis of differentially expressed genes is a powerful approach to elucidate the genetic mechanisms underlying the morphological and evolutionary diversity among serially homologous structures, both within the same organism (e.g., hand vs. foot) and between different species (e.g., hand vs. wing). In the developing embryo, limb-specific expression of Pitx1, Tbx4, and Tbx5 regulates the determination of limb identity. However, numerous lines of evidence, including the fact that these three genes encode transcription factors, indicate that additional genes are involved in the Pitx1-Tbx hierarchy. To examine the molecular distinctions coded for by these factors, and to identify novel genes involved in the determination of limb identity, we have used Serial Analysis of Gene Expression (SAGE) to generate comprehensive gene expression profiles from intact, developing mouse forelimbs and hindlimbs. To minimize the extraction of erroneous SAGE tags from low-quality sequence data, we used a new algorithm to extract tags from -analyzed sequence data and obtained 68,406 and 68,450 SAGE tags from forelimb and hindlimb SAGE libraries, respectively. We also developed an improved method for determining the identity of SAGE tags that increases the specificity of and provides additional information about the confidence of the tag-UniGene cluster match. The most differentially expressed gene between our SAGE libraries was Pitx1. The differential expression of Tbx4, Tbx5, and several limb-specific Hox genes was also detected; however, their abundances in the SAGE libraries were low. Because numerous other tags were differentially expressed at this low level, we performed a 'virtual' subtraction with 362,344 tags from six additional nonlimb SAGE libraries to further refine this set of candidate genes. This subtraction reduced the number of candidate genes by 74%, yet preserved the previously identified regulators of limb identity. This study presents the gene expression complexity of the developing limb and identifies candidate genes involved in the regulation of limb identity. We propose that our computational tools and the overall strategy used here are broadly applicable to other SAGE-based studies in a variety of organisms. [SAGE data are all available at GEO (http://www.ncbi.nlm.nih.gov/geo/) under accession nos. GSM55 and GSM56, which correspond to the forelimb and hindlimb raw SAGE data.]

Determinants of T box protein specificity

Members of the T box family of transcription factors play important roles in early development. Different members of the family exert different effects and here we show that much of the specificity of the Xenopus T box proteins Xbra, VegT and Eomesodermin resides in the DNA-binding domain, or T box. Binding site selection experiments show that the three proteins bind the same core sequence, but they select paired sites that differ in their orientation and spacing. Lysine 149 of Xbra is conserved in all Brachyury homologues, while the corresponding amino acid in VegT and Eomesodermin is asparagine. Mutation of this amino acid to lysine changes the inductive abilities of VegT and Eomesodermin to resemble that of Xbra.

Vertebrate limb development--the early stages in chick and mouse

More news this year about FGFs and their roles in vertebrate limb initiation; Wnt signalling is shown for the first time to be another component of the signalling cascade involved in early limb formation. Ectodermal compartments that control apical ridge formation were previously described in chick embryos and are now shown to exist in mouse embryos; Engrailed1 is expressed in the ventral ectodermal compartment but experiments in both chick and mouse show that it is not responsible for compartment specification.

Tbx19, a tissue-selective regulator of POMC gene expression

Pituitary cell types arise in a temporally and spatially specific fashion, in response to combinatorial actions of transcription factors induced by transient signaling gradients. The critical transcriptional determinants of the two pituitary cell types that express the pro-opiomelanocortin (POMC) gene, the anterior lobe corticotropes, producing adrenocorticotropin, and the intermediate lobe melanotropes, producing melanocyte-stimulating hormone (MSH alpha), have remained unknown. Here, we report that a member of the T-box gene family, Tbx19, which is expressed only in the rostral ventral diencephalon and pituitary gland, commencing on e11.5, marks pituitary cells that will subsequently express the POMC gene and is capable of altering progression of ventral cell types and inducing adrenocorticotropin in rostral tip cells. It is suggested that Tbx19, depending on the presence of synergizing transcription factors, can activate POMC gene expression and repress the alpha glycoprotein subunit and thyroid-stimulating hormone beta promoters.

Tbx5 associates with Nkx2-5 and synergistically promotes cardiomyocyte differentiation

The cardiac homeobox protein Nkx2-5 is essential in cardiac development, and mutations in Csx (which encodes Nkx2-5) cause various congenital heart diseases. Using the yeast two-hybrid system with Nkx2-5 as the 'bait', we isolated the T-box-containing transcription factor Tbx5; mutations in TBX5 cause heart and limb malformations in Holt-Oram syndrome (HOS). Co-transfection of Nkx2-5 and Tbx5 into COS-7 cells showed that they also associate with each other in mammalian cells. Glutathione S-transferase (GST) 'pull-down' assays indicated that the N-terminal domain and N-terminal part of the T-box of Tbx5 and the homeodomain of Nkx2-5 were necessary for their interaction. Tbx5 and Nkx2-5 directly bound to the promoter of the gene for cardiac-specific natriuretic peptide precursor type A (Nppa) in tandem, and both transcription factors showed synergistic activation. Deletion analysis showed that both the N-terminal domain and T-box of Tbx5 were important for this transactivation. A G80R mutation of Tbx5, which causes substantial cardiac defects with minor skeletal abnormalities in HOS, did not activate Nppa or show synergistic activation, whereas R237Q, which causes upper-limb malformations without cardiac abnormalities, activated the Nppa promoter to a similar extent to that of wildtype Tbx5. P19CL6 cell lines overexpressing wildtype Tbx5 started to beat earlier and expressed cardiac-specific genes more abundantly than did parental P19CL6 cells, whereas cell lines expressing the G80R mutant did not differentiate into beating cardiomyocytes. These results indicate that two different types of cardiac transcription factors synergistically induce cardiac development.

Identification and characterization of Lbh, a novel conserved nuclear protein expressed during early limb and heart development

We report the cloning, protein characterization, and expression of a novel vertebrate gene, termed Lbh (Limb-bud-and-heart), with a spatiotemporal expression pattern that marks embryologically significant domains in the developing limbs and heart. Lbh encodes a highly conserved nuclear protein, which in tissue culture cells possesses a transcriptional activator function. During limb development, expression of Lbh initiates in the ectoderm of the presumptive limb territory in the lateral body wall. As the limb buds appear, Lbh expression is restricted primarily to the distal ventral limb ectoderm and the apical ectodermal ridge, and overlaps in these ectodermal compartments with En1 and Fgf8 expression. During heart formation, Lbh is expressed as early as Nkx2.5 and dHand in the bilateral heart primordia, with the highest levels in the anterior promyocardium. After heart tube fusion and looping, Lbh expression is confined to the ventricular myocardium, with the highest intensity in the right ventricle and atrioventricular canal, as well as in the sinus venosus. Based on the molecular characteristics and the domain-specific expression pattern, it is possible that Lbh functions in synergy with other genes known to be required for heart and limb development.

Sparking new frontiers: using in vivo electroporation for genetic manipulations

In vivo electroporation is a fascinating new approach by which gene expression, regulation, and function can be studied in developmental systems. This technique offers new opportunities for manipulations in animal models that lack genetic approaches, including avians. Furthermore, this approach is applicable to other embryo populations including mice, ascidians, zebrafish, Xenopus, and Drosophila. In this review, we discuss technical aspects of in vivo electroporation, review recent studies where this approach has been utilized successfully, and identify future directions.

Nadine Dobrovolskaïa-Zavadskaïa and the dawn of developmental genetics

In one of the first genetic screens aimed at identifying induced developmental mutants, Nadine Dobrovolskaïa-Zavadskaïa, working at the Pasteur Laboratory in the 1920s, isolated and characterized a mutation affecting Brachyury, a gene that regulates tail and axial development in the mouse. Dobrovolskaïa-Zavadskaïa's analysis of Brachyury and other mutations affecting tail development were among the earliest attempts to link gene action with a tissue-specific developmental process in a vertebrate. Her analyses of genes that interacted with Brachyury led to the discovery of the t-haplotype chromosome of mouse. After 70 years, Brachyury and the multiple genes with which it interacts continue to occupy a prominent focus in developmental biology research. A goal of this review is to identify the contributions that Dobrovolskaïa-Zavadskaïa made to our current thinking about Brachyury and how she helped to shape the dawn of the field of developmental genetics. BioEssays 23:365-371, 2001.

A pituitary cell-restricted T box factor, Tpit, activates POMC transcription in cooperation with Pitx homeoproteins

The pituitary gland has provided unique insight into molecular mechanisms and regulatory factors controlling both differentiation and gene transcription. We identified Tpit, a novel T box factor only present in the two pituitary POMC-expressing lineages, the corticotrophs and melanotrophs, and apparently in no other tissue, including hypothalamic POMC neurons. In pituitary cells, Tpit activation of POMC gene transcription requires cooperation with Pitx1, the two factors binding to contiguous sites within the same regulatory element. In gain-of-function experiments, Tpit induces POMC expression in undifferentiated pituitary cells, indicating that it can initiate differentiation into POMC-expressing lineages. TPIT gene mutations were found in patients with isolated deficiency of pituitary POMC-derived ACTH, in support of an essential role of Tpit for differentiation of the pituitary POMC lineage.

Bone development

Early development of the vertebrate skeleton depends on genes that pattern the distribution and proliferation of cells from cranial neural crest, sclerotomes, and lateral plate mesoderm into mesenchymal condensations at sites of future skeletal elements. Within these condensations, cells differentiate to chondrocytes or osteoblasts and form cartilages and bones under the control of various transcription factors. In most of the skeleton, organogenesis results in cartilage models of future bones; in these models cartilage is replaced by bone by the process of endochondral ossification. Lastly, through a controlled process of bone growth and remodeling the final skeleton is shaped and molded. Significant and exciting insights into all aspects of vertebrate skeletal development have been obtained through molecular and genetic studies of animal models and humans with inherited disorders of skeletal morphogenesis, organogenesis, and growth.

Cell biology of limb patterning

Of vertebrate organ systems, the developing limb has been especially well characterized. Morphological studies have yielded a wealth of information describing limb outgrowth and have allowed for the identification of a multitude of important factors. In terms of the latter, key signaling pathways are known to control numerous aspects of limb development, including establishment of the early limb field, determination of limb identity, elongation of the limb bud, specification of digit pattern, and sculpting of the digits. Modification of underlying signaling pathways can thus result in dramatic alterations of the limb phenotype, accounting for many of the diverse limb patterns observed in nature. Given this, it is clear that signaling pathways regulate the highly orchestrated and tightly controlled sequence of cellular events necessary for limb outgrowth; however, exactly how molecular signals interface with the cell biology of limb development remains largely a mystery. In this review we first provide an overview of a number of the morphogenetic signaling pathways that have been identified in the developing limb and then review how a subset of these signals are known to modify cell behaviors important for limb outgrowth.

TBX5 transcription factor regulates cell proliferation during cardiogenesis

Mutations in human TBX5, a member of the T-box transcription factor gene family, cause congenital cardiac septation defects and isomerism in autosomal dominant Holt-Oram syndrome. To determine the cellular function of TBX5 in cardiogenesis, we overexpressed wild-type and mutant human TBX5 isoforms in vitro and in vivo. TBX5 inhibited cell proliferation of D17 canine osteosarcoma cells and MEQC quail cardiomyocyte-like cells in vitro. Mutagenesis of the 5' end of the T-box but not the 3' end of the T-box abolished this effect. Overexpression of TBX5 in embryonic chick hearts showed that TBX5 inhibits myocardial growth and trabeculation. TBX5 effects in vivo were abolished by Gly80Arg missense mutation of the 5' end of the T-box. PCNA analysis in transgenic chick hearts revealed that TBX5 overexpression does suppress embryonic cardiomyocyte proliferation in vivo. Inhibitory effects of TBX5 on cardiomyocyte proliferation include a noncell autonomous process in vitro and in vivo. TBX5 inhibited proliferation of both nontransgenic cells cocultured with transgenic cells in vitro and nontransgenic cardiomyocytes in transgenic chick hearts with mosaic expression of TBX5 in vivo. Immunohistochemical studies of human embryonic tissues, including hearts, also demonstrated that TBX5 expression is inversely related to cellular proliferation. We propose that TBX5 can act as a cellular arrest signal during vertebrate cardiogenesis and thereby participate in modulation of cardiac growth and development.

T-targets: clues to understanding the functions of T-box proteins

Members of the T-box gene family have been identified in both vertebrates and invertebrates, where they play key roles in the regulation of embryonic development, and particularly in morphogenesis and the assignment of cell fate. T-box proteins act as transcription factors which regulate the expression of downstream effector genes. This review focuses on the identification of T-box target genes and the basis of T-box functional specificity.

T-box genes in development: from hydra to humans

The T-box gene family was uncovered less than a decade ago but has been recognized as important in controlling many and varied aspects of development in metazoans from hydra to humans. Extensive screening and database searching has revealed several subfamilies of genes with orthologs in species as diverse as Caenorhabditis elegans and humans. The defining feature of the family is a conserved sequence coding for a DNA-binding motif known as the T-box, named after the first-discovered T-box gene, T or Brachyury. Although several T-box proteins have been shown to function as transcriptional regulators, to date only a handful of downstream target genes have been discovered. Similarly, little is known about regulation of the T-box genes themselves. Although not limited to the embryo, expression of T-box genes is characteristically seen in dynamic and highly specific patterns in many tissues and organs during embryogenesis and organogenesis. The essential role of several T-box genes has been demonstrated by the developmental phenotypes of mutant animals.

The molecular control of upper extremity development: implications for congenital hand anomalies

As the molecular aspects of limb development are being unraveled, more of the congenital anomalies seen by hand surgeons in the clinical setting will have an identifiable molecular basis. The majority of the data available regarding the molecular development of the upper extremity have come from experimental animal studies, specifically the mouse and chicken. These findings are being discovered by either direct surgical and molecular manipulation of the developing limb or by production of mice deficient in specific genes. Relatively few specific human mutations that cause limb abnormalities have been identified. Hand surgeons should be aware of the basic molecular pathways controlling limb development because they are in a unique position to be able to identify patients with such deformities. In turn, detailed clinical descriptions of congenital anomalies affecting the upper extremity will advance the understanding of the cellular events controlled by the molecular pathways of limb development. This review describes the general molecular basis of limb development and correlates it with disease processes affecting the upper extremity.

Evolutionary aspects of positioning and identification of vertebrate limbs

Emerging developmental studies contribute to our understanding of vertebrate evolution because changes in the developmental process and the genes responsible for such changes provide a unique way for evaluating the evolution of morphology. Endoskeletal limbs, the locomotor organs that are unique to vertebrates, are a popular model system in the fields of palaeontology and phylogeny because their structure is highly visible and their bony pattern is easily preserved in the fossil records. Similarly, limb development has long served as an excellent model system for studying vertebrate pattern formation. In this review, the evolution of vertebrate limb development is examined in the light of the latest knowledge, viewpoints and hypotheses.

Perspectives on the evolutionary origin of tetrapod limbs

The study of the origin and evolution of the tetrapod limb has benefited enormously from the confluence of molecular and paleontological data. In the last two decades, our knowledge of the basic molecular mechanisms that control limb development has grown exponentially, and developmental biologists now have the possibility of combining molecular data with many available descriptions of the fossil record of vertebrate fins and limbs. This synthesis of developmental and evolutionary biology has the potential to unveil the sequence of molecular changes that culminated in the adoption of the basic tetrapod limb plan.

Genetic and developmental bases of serial homology in vertebrate limb evolution

Two sets of paired appendages are a characteristic feature of the body plan of jawed vertebrates. While the fossil record provides a good morphological description of limb evolution, the molecular mechanisms involved in this process are only now beginning to be understood. It is likely that the genes essential for limb development in modern vertebrates were also important players during limb evolution. In recent years, genes from a number of gene families have been described that play important roles both in limb induction and in later patterning processes. These advances facilitate inquiries into several important aspects of limb evolution such as their origin, position along the body axis, number and identity. Integrating paleontological, developmental and genetic data, we propose models to explain the evolution of paired appendages in vertebrates. Whereas previous syntheses have tended to focus on the roles of genes from a single gene family, most notably Hox genes, we emphasize the importance of considering the interactions among multiple genes from different gene families for understanding the evolution of complex developmental systems. Our models, which underscore the roles of gene duplication and regulatory ‘tinkering’, provide a conceptual framework for elucidating the evolution of serially homologous structures in general, and thus contribute to the burgeoning field seeking to uncover the genetic and developmental bases of evolution.

Vertebrate mesendoderm induction and patterning

Many of the key molecular events underlying the induction and patterning of the vertebrate mesoderm and endoderm have recently been elucidated. T-box transcription factors and TGF-beta and Wnt signaling pathways play crucial roles in the initial induction of the mesendoderm and the subdivision of the posterior mesoderm into rostral and caudal domains.

Xwnt11 and the regulation of gastrulation in Xenopus

The molecular basis of gastrulation is poorly understood. In this paper we address this problem by taking advantage of the observation that the transcription activator Brachyury is essential for gastrulation movements in Xenopus and mouse embryos. We infer from this observation that amongst the target genes of Brachyury are some that are involved in the regulation of gastrulation. In the course of a screen for Brachyury targets we identified Xwnt11. Use of a dominant-negative Xwntll construct confirms that signalling by this class of Wnts is essential for normal gastrulation movements, and further investigation suggests that Xwntll signals not through the canonical Wnt signalling pathway involving GSK-3 and beta-catenin but through another route, which may require small GTPases such as Rho and Rac. Future work will concentrate on elucidating the Xwnt11 signal transduction pathway and on investigating its influence on cell shape and polarity during Xenopus gastrulation.

The gene encoding the T-box factor Tbx2 is a target for the microphthalmia-associated transcription factor in melanocytes

Commitment to the melanocyte lineage is characterized by the onset of microphthalmia-associated transcription factor (Mitf) expression. Mitf plays a fundamental role in melanocyte development, with mice lacking Mitf being entirely devoid of pigment cells. In the absence of functional Mitf protein, melanoblasts expressing Mitf mRNA disappear around 2 days after their first appearance either by apoptosis or by losing their identity and adopting an alternative cell fate. The role of Mitf must therefore be to regulate genes required for melanoblast survival, proliferation, or the maintenance of melanoblast identity. Yet to date, Mitf has been shown to regulate genes such as Tyrosinase, Tyrp-1, and Dct, which are required for pigmentation, a differentiation-specific process. Because expression of these genes cannot account for the complete absence of pigment cells in Mitf-negative mice, Mitf must regulate the expression of other as yet uncharacterized genes. Here we provide several lines of evidence to suggest that Mitf may regulate the expression of the Tbx2 transcription factor, a member of the T-box family of proteins implicated in the maintenance of cell identity. First, isolation and sequencing of the entire murine Tbx2 gene revealed that the Tbx2 promoter contains a full consensus Mitf recognition element; second, Mitf could bind the promoter in vitro and activate Tbx2 expression in vivo in an E box-dependent fashion; and third, Tbx2 is expressed in melanoma cell lines expressing Mitf, but not in a line in which Mitf expression was not detectable. Taken together, with the fact that Tbx2 is expressed in Mitf-positive melanoblasts and melanocytes, but not in Mitf-negative melanoblast precursor cells, the evidence suggests that the Tbx2 gene may represent one of the first known targets for Mitf that is not a gene involved directly in the manufacture of pigment.

Ventricular expression of tbx5 inhibits normal heart chamber development

The T-box gene tbx5 is expressed in the developing heart, forelimb, eye, and liver in vertebrate embryos during critical stages of morphogenesis and patterning. In humans, mutations in the TBX5 gene have been associated with Holt-Oram syndrome, which is characterized by developmental anomalies in the heart and forelimbs. In chicken and mouse embryos, tbx5 expression is initiated at the earliest stages of heart formation throughout the heart primordia and is colocalized with other cardiac transcription factors such as nkx-2.5 and GATA4. As the heart differentiates, tbx5 expression is restricted to the posterior sinoatrial segments of the heart, consistent with the timing of atrial chamber determination. The correlation between tbx5 expression and atrial lineage determination was examined in retinoic acid (RA)-treated chicken embryos. tbx5 expression is maintained throughout the hearts of RA-treated embryos under conditions that also expand atrial-specific gene expression. The downstream effects of persistent tbx5 expression in the ventricles were examined directly in transgenic mice. Embryos that express tbx5 driven by a beta-myosin heavy chain promoter throughout the primitive heart tube were generated. Loss of ventricular-specific gene expression and retardation of ventricular chamber morphogenesis were observed in these embryos. These studies provide direct evidence for an essential role for tbx5 in early heart morphogenesis and chamber-specific gene expression.

Gene transfer into chicken embryos as an effective system of analysis in developmental biology

Chicken embryos have been used as a model animal in developmental biology since the time of comparative and experimental embryology. Recent application of gene transfer techniques to the chicken embryo increases their value as an experimental animal. Today, gene transfer into chicken cells is performed by three major systems, lipofection, electroporation and the virus-mediated method. Each system has its own features and applicability. In this overview and the associated four minireviews, the methods and application of each system will be presented.

Applications of microelectroporation for studies of chick embryogenesis

A technique by which genes can be introduced into the cells and tissues of developing embryos has great potential for studying the roles of genes during vertebrate embryogenesis. The 'microelectroporation' technique, in which DNA is introduced into cells within a restricted area of developing chick embryos with high reproducibility, was developed by the authors. In this review, the advantages and applications of this microelectroporation technique for developmental studies and functional analysis of genes in chick embryos is discussed.

Misexpression of genes in brain vesicles by in ovo electroporation

Transfection to living chick embryos in ovo by electroporation has been recently developed. In this mini-review, misexpression in brain vesicles is introduced. To transfect, expression plasmid is inserted in the brain vesicle, and the square pulse of 25 V, 50 ms was charged five times. The translation product of the transfected gene is detected 2 h after electroporation, and reaches the peak at 24 h after electroporation. Transfection is so effective that this method is contributing greatly to the study of the molecular mechanisms of morphogenesis.

FGF and genes encoding transcription factors in early limb specification

SnR, twist and Fgf10 are expressed in presumptive limb territories of early chick embryos. When FGF-2/FGF-8 beads are implanted in chick flank, an ectopic limb develops and SnR is irreversibly activated as early as 1 h. Ectopic Fgf10 and twist expression are activated much later at 17 and 20 h, respectively. FGF-10 can also induce SnR, but much later, and in this case activation occurs simultaneously with that of twist and Fgf10 via the Fgf8- expressing ridge. Tbx-4 and Tbx-5 are expressed in leg and wing forming regions, respectively, in a similar pattern to SnR and twist. FGF-2 leads to ectopic expression of Tbx-4 and Tbx-5 as rapidly as ectopic expression of SnR, but the patterns of ectopic transcripts suggest that induction of SnR and Tbx gene expression occur via different pathways.

A novel transcription factor, T-bet, directs Th1 lineage commitment

Naive T helper cells differentiate into two subsets, Th1 and Th2, each with distinct functions and cytokine profiles. Here, we report the isolation of T-bet, a Th1-specific T box transcription factor that controls the expression of the hallmark Th1 cytokine, IFNgamma. T-bet expression correlates with IFNgamma expression in Th1 and NK cells. Ectopic expression of T-bet both transactivates the IFNgamma gene and induces endogenous IFNgamma production. Remarkably, retroviral gene transduction of T-bet into polarized Th2 and Tc2 primary T cells redirects them into Th1 and Tc1 cells, respectively, as evidenced by the simultaneous induction of IFNgamma and repression of IL-4 and IL-5. Thus, T-bet initiates Th1 lineage development from naive Thp cells both by activating Th1 genetic programs and by repressing the opposing Th2 programs.

A conserved role for H15-related T-box transcription factors in zebrafish and Drosophila heart formation

T-box transcription factors are critical regulators of early embryonic development. We have characterized a novel zebrafish T-box transcription factor, hrT (H15-related T box) that is a close relative of Drosophila H15 and a recently identified human gene. We show that Drosophila H15 and zebrafish hrT are both expressed early during heart formation, in strong support of previous work postulating that vertebrate and arthropod hearts are homologous structures with conserved regulatory mechanisms. The timing and regulation of zebrafish hrT expression in anterior lateral plate mesoderm suggest a very early role for hrT in the differentiation of the cardiac precursors. hrT is coexpressed with gata4 and nkx2.5 not only in anterior lateral plate mesoderm but also in noncardiac mesoderm adjacent to the tail bud, suggesting that a conserved regulatory pathway links expression of these three genes in cardiac and noncardiac tissues. Finally, we analyzed hrT expression in pandora mutant embryos, since these have defects in many of the tissues that express hrT, including the heart. hrT expression is much reduced in the early heart fields of pandora mutants, whereas it is ectopically expressed subsequently. Using hrT expression as a marker, we describe a midline patterning defect in pandora affecting the anterior hindbrain and associated midline mesendodermal derivatives. We discuss the possibility that the cardiac ventricular defect previously described in pandora and the midline defects described here are related.

Developmental regulation of Tbx5 in zebrafish embryogenesis

T-box (tbx) genes constitute a large family of transcriptional regulators involved in developmental patterning processes. In tetrapods, tbx5 has been implicated in specifying forelimb type identity. Here, we report the cloning of the zebrafish tbx5.1 gene and characterise its expression during zebrafish embryogenesis and early larval development of wild type and mutant embryos that affect pectoral fin patterning. tbx5.1 is expressed during development of the heart, the pectoral fins and the eye. Notably, its expression in the lateral plate mesoderm defines a single and continuous region of heart and pectoral fin precursor cells, and constitutes the earliest specific marker for pectoral fin development in the zebrafish.

Tbx5 and the retinotectum projection

Dorsal and ventral aspects of the eye are distinct from the early stages of development. The developing eye cup grows dorsally, and the choroidal fissure is formed on its ventral side. Retinal axons from the dorsal and ventral retina project to the ventral and dorsal tectum, respectively. Misexpression of the Tbx5 gene induced dorsalization of the ventral side of the eye and altered projections of retinal ganglion cell axons. Thus, Tbx5 is involved in eye morphogenesis and is a topographic determinant of the visual projections between retina and tectum.

Highly efficient electro-gene therapy of solid tumor by using an expression plasmid for the herpes simplex virus thymidine kinase gene

We report successful electro-gene therapy (EGT) by using plasmid DNA for tumor-bearing mice. Subcutaneously inoculated CT26 tumor was subjected to EGT, which consists of intratumoral injection of a naked plasmid encoding a marker gene or a therapeutic gene, followed by in vivo electroporation (EP). When this treatment modality is carried out with the plasmid DNA for the green fluorescent protein gene, followed by in vivo EP with the optimized pulse parameters, numerous intensely bright green fluorescent signals appeared within the tumor. EGT, by using the "A" fragment of the diphtheria toxin gene significantly inhibited the growth of tumors, by about 30%, on the flank of mice. With the herpes simplex virus thymidine kinase gene, followed by systemic injection of ganciclovir, EGT was far more effective in retarding tumor growth, varying between 50% and 90%, compared with the other controls. Based on these results, it appears that EGT can be used successfully for treating murine solid tumors.

Regulation of a muscle-specific transgene by persistent expression of Hox genes in postnatal murine limb muscle

Homeobox genes are necessary for the generation of the embryonic body plan in both invertebrate and vertebrate organisms. To investigate the potential function of homeodomain proteins in normal and regenerating skeletal muscle, we analyzed patterns of clustered homeobox gene expression in neonatal and adult muscle tissue. Transcripts encoding 5' genes in the HoxA cluster were detected in muscles from both the fore- and hindlimbs of neonatal and adult mice, whereas expression of HoxC gene transcripts was generally restricted to the muscles of the hindlimb. In contrast, transcripts encoding genes of the HoxB or HoxD clusters were not detected in muscles from either fore- or hindlimbs. Although ectopic expression of select HOX proteins in muscle cell cultures had modest effects upon the activity of a co-transfected myosin light chain (MLC) enhancer, mutation of a Hox binding site in this enhancer elicited increased linked reporter gene expression. Induction of muscle damage and regeneration was accompanied by the down-regulation of at least one Hox gene, concurrent with the activation of the regenerative program. Moreover, targeted ablation of the Hoxc-8 gene, normally expressed in mature fore- and hindlimb muscles, resulted in reduced expression of an MLC enhancer-driven transgene only in specific leg muscles. These results indicate that members of the HoxA and C clusters may, in combination, mediate various aspects of differentiation and patterning in adult musculature.

'Shocking' developments in chick embryology: electroporation and in ovo gene expression

Efficient gene transfer by electroporation of chick embryos in ovo has allowed the development of new approaches to the analysis of gene regulation, function and expression, creating an exciting opportunity to build upon the classical manipulative advantages of the chick embryonic system. This method is applicable to other vertebrate embryos and is an important tool with which to address cell and developmental biology questions. Here we describe the technical aspects of in ovo electroporation, its different applications and future perspectives.