A Xenopus distal-less gene in transgenic mice: conserved regulation in distal limb epidermis and other sites of epithelial-mesenchymal interaction

Salt Dependence of DNA Binding Activity of Human Transcription Factor Dlx3

Distal-less 3 (Dlx3) is a homeobox-containing transcription factor and plays a crucial role in the development and differentiation process. Human Dlx3 consists of two transactivation domains and a homeobox domain (HD) that selectively binds to the consensus site (5'-TAATT-3') of the DNA duplex. Here, we performed chemical shift perturbation experiments on Dlx3-HD in a complex with a 10-base-paired (10-bp) DNA duplex under various salt conditions. We also acquired the imino proton spectra of the 10-bp DNA to monitor the changes in base-pair stabilities during titration with Dlx3-HD. Our study demonstrates that Dlx3-HD selectively recognizes its consensus DNA sequences through the α3 helix and L1 loop regions with a unique dynamic feature. The dynamic properties of the binding of Dlx3-HD to its consensus DNA sequence can be modulated by varying the salt concentrations. Our study suggested that this unique structural and dynamic feature of Dlx3-HD plays an important role in target DNA recognition, which might be associated with tricho-dento-osseous syndrome.

DLX3 Inhibits the Proliferation of Human Dental Pulp Cells Through Inactivation of Canonical Wnt/β-Catenin Signaling Pathway

Homeodomain gene Distal-less-3 (Dlx3) plays an important role during tooth development. Our previous studies indicate that DLX3 inhibits proliferation of human dental pulp cells (hDPCs). However, the mechanism of DLX3 regulating proliferation of hDPCs and maintaining the quiescence of the cells remain unknown. Given the importance of canonical Wnt signaling in the proliferation of dental pulp cell and tooth development, we hypothesized that DLX3 inhibited proliferation of hDPCs through inactivation of canonical Wnt signaling. With overexpression or knock-down of DLX3 in primary hDPCs, we found DLX3 down regulated canonical Wnt signaling and its downstream target genes. And when the DLX3 overexpressed-cells were treated with lithium chloride, the proliferation inhibition by DLX3 was reversed. We also found that DLX3 enhanced the expression of DKK1 and the reduced proliferation of hDPCs by DLX3 was reversed with knock-down of DKK1. Furthermore, luciferase reporter assay and chromatin immunoprecipitation assay showed DLX3 was able to bind to Dkk1 promoter region from nucleotides (nt) -1656 to -1245, and stimulated Dkk1 promoter activity. Mutagenesis studies further revealed two DLX3 responsive elements in Dkk1 promoter. Taken together, our data indicate that DLX3 inhibits proliferation of hDPCs via inactivation of Wnt/β-catenin signaling pathway by directly binding to Dkk1 promoter and increasing its expression.

Heterogeneous conservation of Dlx paralog co-expression in jawed vertebrates

Background

The Dlx gene family encodes transcription factors involved in the development of a wide variety of morphological innovations that first evolved at the origins of vertebrates or of the jawed vertebrates. This gene family expanded with the two rounds of genome duplications that occurred before jawed vertebrates diversified. It includes at least three bigene pairs sharing conserved regulatory sequences in tetrapods and teleost fish, but has been only partially characterized in chondrichthyans, the third major group of jawed vertebrates. Here we take advantage of developmental and molecular tools applied to the shark Scyliorhinus canicula to fill in the gap and provide an overview of the evolution of the Dlx family in the jawed vertebrates. These results are analyzed in the theoretical framework of the DDC (Duplication-Degeneration-Complementation) model.

Results

The genomic organisation of the catshark Dlx genes is similar to that previously described for tetrapods. Conserved non-coding elements identified in bony fish were also identified in catshark Dlx clusters and showed regulatory activity in transgenic zebrafish. Gene expression patterns in the catshark showed that there are some expression sites with high conservation of the expressed paralog(s) and other expression sites with events of paralog sub-functionalization during jawed vertebrate diversification, resulting in a wide variety of evolutionary scenarios within this gene family.

Conclusion

Dlx gene expression patterns in the catshark show that there has been little neo-functionalization in Dlx genes over gnathostome evolution. In most cases, one tandem duplication and two rounds of vertebrate genome duplication have led to at least six Dlx coding sequences with redundant expression patterns followed by some instances of paralog sub-functionalization. Regulatory constraints such as shared enhancers, and functional constraints including gene pleiotropy, may have contributed to the evolutionary inertia leading to high redundancy between gene expression patterns.

The transcription factor DLX3 regulates the osteogenic differentiation of human dental follicle precursor cells

The transcription factor DLX3 plays a decisive role in bone development of vertebrates. In neural-crest derived stem cells from the dental follicle (DFCs), DLX3 is differentially expressed during osteogenic differentiation, while other osteogenic transcription factors such as DLX5 or RUNX2 are not highly induced. DLX3 has therefore a decisive role in the differentiation of DFCs, but its actual biological effects and regulation are unknown. This study investigated the DLX3-regulated processes in DFCs. After DLX3 overexpression, DFCs acquired a spindle-like cell shape with reorganized actin filaments. Here, marker genes for cell morphology, proliferation, apoptosis, and osteogenic differentiation were significantly regulated as shown in a microarray analysis. Further experiments showed that DFCs viability is directly influenced by the expression of DLX3, for example, the amount of apoptotic cells was increased after DLX3 silencing. This transcription factor stimulates the osteogenic differentiation of DFCs and regulates the BMP/SMAD1-pathway. Interestingly, BMP2 did highly induce DLX3 and reverse the inhibitory effect of DLX3 silencing in osteogenic differentiation. However, after DLX3 overexpression in DFCs, a BMP2 supplementation did not improve the expression of DLX3 and the osteogenic differentiation. In conclusion, DLX3 influences cell viability and regulates osteogenic differentiation of DFCs via a BMP2-dependent pathway and a feedback control.

Msx1 and Dlx5 function synergistically to regulate frontal bone development

The Msx and Dlx families of homeobox proteins are important regulators for embryogenesis. Loss of Msx1 in mice results in multiple developmental defects including craniofacial malformations. Although Dlx5 is widely expressed during embryonic development, targeted null mutation of Dlx5 mainly affects the development of craniofacial bones. Msx1 and Dlx5 show overlapping expression patterns during frontal bone development. To investigate the functional significance of Msx1/Dlx5 interaction in regulating frontal bone development, we generated Msx1 and Dlx5 double null mutant mice. In Msx1(-/-) ;Dlx5(-/-) mice, the frontal bones defect was more severe than that of either Msx1(-/-) or Dlx5(-/-) mice. This aggravated frontal bone defect suggests that Msx1 and Dlx5 function synergistically to regulate osteogenesis. This synergistic effect of Msx1 and Dlx5 on the frontal bone represents a tissue specific mode of interaction of the Msx and Dlx genes. Furthermore, Dlx5 requires Msx1 for its expression in the context of frontal bone development. Our study shows that Msx1/Dlx5 interaction is crucial for osteogenic induction during frontal bone development.

Role of homeobox genes in the patterning, specification, and differentiation of ectodermal appendages in mammals

Homeobox genes are an evolutionarily conserved class of transcription factors that are key regulators during developmental processes such as regional specification, patterning, and differentiation. In this review, we summarize the expression pattern, loss- and/or gain-of-function mouse models, and naturally occurring mouse and human mutations of known homeobox genes required for the development of ectodermal appendages.

Molecular consequences of a frameshifted DLX3 mutant leading to Tricho-Dento-Osseous syndrome

The homeodomain protein Distal-less-3 (Dlx3) plays a crucial role during embryonic development. This transcription factor is known to be essential for placental formation and to be involved in skin and skeletal organogenesis. In humans, a frameshift mutation in the coding sequence of the DLX3 gene results in an ectodermal dysplasia called Tricho-Dento-Osseous syndrome (TDO). The main features of this autosomal dominant disorder are defects in hair, teeth, and bone. To investigate the functional alterations caused by the mutated DLX3(TDO) isoform ex vivo, we used tetracycline-inducible osteoblastic and keratinocyte cell lines and calvarial derived osteoblasts in which the expression of DLX3(WT) and/or DLX3(TDO) could be regulated and monitored. Immunocytochemical analysis revealed that both DLX3(WT) and DLX3(TDO) recombinant proteins are targeted to the nucleus. However, as demonstrated by electrophoresis mobility shift assay, DLX3(TDO) is not able to bind to the canonical Dlx3 binding site. Furthermore, we demonstrate that the frameshifted C-terminal domain in DLX3(TDO) is accountable for the loss of DNA binding activity because the C-terminal domain in DLX3(WT) is not required for DNA binding activity. Although DLX3(TDO) alone cannot bind to a Dlx3 responsive element, when DLX3(WT) and DLX3(TDO) are co-expressed they form a complex that can bind DNA. Concomitant with the inability to bind DNA, DLX3(TDO) has a defective transcriptional activity. Moreover, the transcriptional activity of DLX3(WT) is significantly reduced in the presence of the mutated isoform, indicating that DLX3(TDO) has a dominant negative effect on DLX3(WT) transcriptional activity.

Activation of beta-catenin signaling programs embryonic epidermis to hair follicle fate

beta-Catenin signaling is required for hair follicle development, but it is unknown whether its activation is sufficient to globally program embryonic epidermis to hair follicle fate. To address this, we mutated endogenous epithelial beta-catenin to a dominant-active form in vivo. Hair follicle placodes were expanded and induced prematurely in activated beta-catenin mutant embryos, but failed to invaginate or form multilayered structures. Eventually, the entire epidermis adopted hair follicle fate, broadly expressing hair shaft keratins in place of epidermal stratification proteins. Mutant embryonic skin was precociously innervated, and displayed prenatal pigmentation, a phenomenon never observed in wild-type controls. Thus, beta-catenin signaling programs the epidermis towards placode and hair shaft fate at the expense of epidermal differentiation, and activates signals directing pigmentation and innervation. In transcript profiling experiments, we identified elevated expression of Sp5, a direct beta-catenin target and transcriptional repressor. We show that Sp5 normally localizes to hair follicle placodes and can suppress epidermal differentiation gene expression. We identified the pigmentation regulators Foxn1, Adamts20 and Kitl, and the neural guidance genes Sema4c, Sema3c, Unc5b and Unc5c, as potential mediators of the effects of beta-catenin signaling on pigmentation and innervation. Our data provide evidence for a new paradigm in which, in addition to promoting hair follicle placode and hair shaft fate, beta-catenin signaling actively suppresses epidermal differentiation and directs pigmentation and nerve fiber growth. Controlled downregulation of beta-catenin signaling is required for normal placode patterning within embryonic ectoderm, hair follicle downgrowth, and adoption of the full range of follicular fates.

Homeobox gene Dlx3 is regulated by p63 during ectoderm development: relevance in the pathogenesis of ectodermal dysplasias

Ectodermal dysplasias (EDs) are a group of human pathological conditions characterized by anomalies in organs derived from epithelial-mesenchymal interactions during development. Dlx3 and p63 act as part of the transcriptional regulatory pathways relevant in ectoderm derivatives, and autosomal mutations in either of these genes are associated with human EDs. However, the functional relationship between both proteins is unknown. Here, we demonstrate that Dlx3 is a downstream target of p63. Moreover, we show that transcription of Dlx3 is abrogated by mutations in the sterile alpha-motif (SAM) domain of p63 that are associated with ankyloblepharon-ectodermal dysplasia-clefting (AEC) dysplasias, but not by mutations found in ectrodactylyectodermal dysplasia-cleft lip/palate (EEC), Limb-mammary syndrome (LMS) and split hand-foot malformation (SHFM) dysplasias. Our results unravel aspects of the transcriptional cascade of events that contribute to ectoderm development and pathogenesis associated with p63 mutations.

Development of fungiform papillae: patterned lingual gustatory organs

The fungiform papilla is a gustatory organ that provides a specific tissue residence for taste buds on the anterior tongue. Thus, during development there must be a progressive differentiation to acquire papilla epithelium, then taste cell progenitor epithelium, and finally taste cells within the papilla apex. Arranged in rows, the patterned distribution of fungiform papillae requires molecular regulation not only to induce papillae, but also to suppress papilla formation in the between-papilla tissue. Intact sensory innervation is not required to initiate papilla development or pattern. However, members of several molecular families have now been identified with specific localization in developing papillae. These may participate in papilla development and pattern formation, and subsequently in taste progenitor and taste cell differentiation. This review focuses on development of fungiform papillae in embryonic rat and mouse. Basic morphology, cell biology and molecular phenotypes of developing papillae are reviewed. Regulatory roles for molecules in several families are presented, and a broad schema is proposed for progressive epithelial differentiation to form taste cell progenitors in parallel with the temporal course, and participation of lingual sensory innervation.

Smad6 represses Dlx3 transcriptional activity through inhibition of DNA binding

Dlx3 (Distal-less 3) is a homeobox-containing transcription factor required for normal placental development in mice. Here we demonstrate that Dlx3 interacts with Smad6, a member of a larger family of transcriptional regulators generally thought to regulate transforming growth factor beta/bone morphogenetic protein signaling. Immunocytochemical and immunoprecipitation studies demonstrate overlapping nuclear localization and physical interaction between Dlx3 and Smad6 in human choriocarcinoma cells and in differentiated trophoblasts from human placenta. In vitro protein interaction studies mapped the Smad6 interaction domain within Dlx3 to residues 80-163, a region of Dlx3 that includes a portion of the homeodomain. Dlx3 and Dlx4 share homology within this region, and Dlx4 was also found to bind Smad6. Using the Esx1 gene promoter as a model for a Dlx3-responsive gene, studies demonstrate two near consensus Dlx3 binding sites within the proximal 2.3 kb of the transcription start site. Interestingly, binding of Dlx3 to one of these two sites was inhibited by interaction with Smad6. Consistent with this result, expression of an Esx1 promoter luciferase reporter was increased by overexpression of Dlx3; this effect was reversed with co-expression of Smad6. Further, small interference RNA-mediated knockdown of endogenous Smad6 increased Dlx3-dependent expression of the Esx1 gene promoter. Thus, Smad6 appears to functionally interact with Dlx3, altering the ability of Dlx3 to bind target gene promoters. Smad6 appears to play a modulatory role in the regulation of Dlx3-dependent gene transcription within placental trophoblasts.

Dlx genes, p63, and ectodermal dysplasias

Many events in vertebrate morphogenesis and organogenesis develop from epithelial/mesenchymal interactions. These processes involve a series of sequential and reciprocal interactions between the thickened epithelial sheets and underlying mesenchymal cells. Much has been learned from in vitro assays and knockout experiments in mice on the early signaling molecules that regulate the initial stages of the epithelial/mesenchymal interactions. In this review, we discuss effectors of these initial signals, specifically the p63 and Dlx families of transcription factors, that play central roles in embryonic patterning and regulation of different developmental processes, and provide a review of some of the mutations in these genes that have been associated with ectodermal dysplasias (EDs).

Building sensory receptors on the tongue

Neurotrophins, neurotrophin receptors and sensory neurons are required for the development of lingual sense organs. For example, neurotrophin 3 sustains lingual somatosensory neurons. In the traditional view, sensory axons will terminate where neurotrophin expression is most pronounced. Yet, lingual somatosensory axons characteristically terminate in each filiform papilla and in each somatosensory prominence within a cluster of cells expressing the p75 neurotrophin receptor (p75NTR), rather than terminating among the adjacent cells that secrete neurotrophin 3. The p75NTR on special specialized clusters of epithelial cells may promote axonal arborization in vivo since its over-expression by fibroblasts enhances neurite outgrowth from overlying somatosensory neurons in vitro. Two classical observations have implicated gustatory neurons in the development and maintenance of mammalian taste buds--the early arrival times of embryonic innervation and the loss of taste buds after their denervation in adults. In the modern era more than a dozen experimental studies have used early denervation or neurotrophin gene mutations to evaluate mammalian gustatory organ development. Necessary for taste organ development, brain-derived neurotrophic factor sustains developing gustatory neurons. The cardinal conclusion is readily summarized: taste buds in the palate and tongue are induced by innervation. Taste buds are unstable: the death and birth of taste receptor cells relentlessly remodels synaptic connections. As receptor cells turn over, the sensory code for taste quality is probably stabilized by selective synapse formation between each type of gustatory axon and its matching taste receptor cell. We anticipate important new discoveries of molecular interactions among the epithelium, the underlying mesenchyme and gustatory innervation that build the gustatory papillae, their specialized epithelial cells, and the resulting taste buds.

Positive and negative regulatory elements in the late lactation protein-A gene promoter from the tammar wallaby (Macropus eugenii)

Little is known about the regulation of the marsupial-specific late lactation protein-A (LLP-A) gene, first expressed at mid-lactation in the mammary gland of the tammar wallaby. A genomic clone of LLP-A was sequenced and shown to include seven exons. The LLP-A promoter region of 1969 bp ligated to a secreted alkaline phosphatase (SEAP) gene reporter was co-transfected into CHO-K1 cells with prolactin (PRL) receptor cDNA. Transfected cells cultured with insulin, cortisol and PRL did not secrete SEAP into media. Similarly, this construct was not expressed in the mammary gland of eight lines of transgenic mice. In contrast, when the LLP-A promoter region was reduced to 850 bp, the expression of the SEAP reporter in CHO-K1 cells was constitutive and PRL-independent, despite the presence of two low affinity Stat5 binding sites. The 1969 bp promoter was analyzed using nine serial deletions ligated to the SEAP gene. The expression of these constructs was PRL-independent. Five putative inhibitory elements were identified between -1969 and -1796, -1404 and -1184, -1184 and -992, -992 and -757, and -591 and -425, and a putative enhancer or core transcription element between -425 and-239. These studies indicate that the complex temporal regulation of the LLP-A gene involves elements in its 5'-regulatory region.

Transcriptional regulation of Msx2 in the AERs of developing limbs is dependent on multiple closely spaced regulatory elements

In developing limb buds, Msx2 transcripts are expressed in the apical ectodermal ridge (AER) and in various regions of the limb mesenchyme. To identify DNA sequences responsible for Msx2 expression in the AER, we characterized the expression of LacZ reporter constructs driven by chicken Msx2 regulatory sequences in transgenic mice. We have identified a 55-bp enhancer that can direct AER-specific reporter gene expression. This 55-bp enhancer contains three elements that are evolutionary conserved among five vertebrate Msx2 genomic sequences. AER expression of reporter constructs in transgenic mice is lost or reduced when mutations are introduced into each of these three regions. Moreover, changing the relative orientation by reverse complementing one of the three elements also results in loss of expression, suggesting that the relative orientations of transcription factor binding is important. To identify the transcription factor(s) binding to these elements, we conducted one-hybrid screening and identified Dlx5 and Sox11. Both Dlx5 and Sox11 are expressed in the AER, and the proteins encoded by these genes bind to separate conserved elements, supporting their possible roles in regulating Msx2 expression.

Overexpression of the calcium sensing receptor accelerates epidermal differentiation and permeability barrier formation in vivo

The calcium sensing receptor (CaSR) has emerged as an important mediator of a wide range of Ca(2+)-dependent physiological responses (Ca(2+) signaling) in various tissues. To explore the role of CaSR in the epidermis, we utilised the keratin 14 promoter to express CaSR cDNA constitutively in the basal cells of the stratified squamous epithelium of transgenic mice. Analysis of the transgenic mice revealed that a sensitized response to CaSR signaling accelerates the epidermal differentiation program with the precocious formation of the epidermal permeability barrier (EPB) during development and an accelerated hair growth at birth. Our observations indicate that overexpression of CaSR in the undifferentiated basal cells leads to changes in the differentiation program of the transgenic epidermis, including the stimulation of keratins 1 and 6 as well as the overexpression of several markers of terminal differentiation such as filaggrin, loricrin and involucrin. Our data suggest that the observed modifications in the differentiation pathway are a consequence of a CaSR-induced enhancement of Ca(2+) signaling involving cross-talk with other signaling pathways (e.g. EGF and Wnt/Ca(2+)). These studies provide new insights into the role of CaSR in epidermal differentiation including EPB development and hair follicle morphogenesis.

Expression of foreign genes in lamprey embryos: an approach to study evolutionary changes in gene regulation

Evolution in development can be viewed as a sequence of changes in gene regulation. To investigate the cross-species compatibility of 5' upstream regulatory regions, we introduced exogenous gene constructs derived from a gnathostome genome into fertilized eggs of the Japanese lamprey, Lampetra japonica, a sister group of the gnathostomes. Eggs were injected with gene constructs in which a sequence encoding the green fluorescent protein (GFP) had been located downstream of either a virus promoter or 5' regulatory regions of medaka actin genes. Reporter gene expression was recorded for more than a month starting two days after injection. Although the expression patterns were highly mosaic and differed among individuals, GFP was expressed predominantly in the striated muscles of lamprey embryos when driven by the 5' upstream regions of the medaka muscle actin genes. This implies that a pan-vertebrate muscle-specific gene regulatory mechanism may have evolved before the agnathan/gnathostome divergence. This gene-transfer technique potentially facilitates the visualization of cells in various differentiating tissues throughout development. The introduction of developmental genes of the lamprey or other animals into lamprey embryos is another potentially important application, one that could provide us with information on the evolutionary changes in functions of genes or gene cascades.

Regulation of Dlx3 gene expression in visceral arches by evolutionarily conserved enhancer elements

The mammalian Distal-less (Dlx) clusters (Dlx1-2, Dlx5-6, and Dlx3-7) have a nested expression pattern in developing visceral (branchial) arches. Genetic regulatory mechanisms controlling Dlx spatial expression within the visceral arches have not yet been defined. Here we show that an enhancer in the Dlx3-7 cluster can regulate the visceral arch specific expression pattern of the Dlx3 gene. We have used a 79-kb transgene construct containing the entire Dlx3-7 bigene cluster with a LacZ reporter inserted in frame in the first exon of the Dlx3 gene. Visceral arch expression is absent when a 4-kb element located within the Dlx3-7 intergenic region is deleted. A 245-bp element (I37-2) whose DNA sequence is highly conserved between human and mouse located within the 4kb-deleted region can drive visceral arch expression when fused to a hsp68-lacZ reporter transgene construct. Reporter expression is detected in 9.5 and 10.5 days postcoitum transgenic embryos in a manner consistent with the endogenous Dlx3 expression pattern in the mesenchyme of the first and second visceral arches. Thus the I37-2 element is both necessary and sufficient for Dlx3 expression. The I37-2 element contains several putative binding sites for several transcription factors including Dlx and other homeodomain proteins within the evolutionarily conserved region. Significantly, the I37-2 element shows a sequence-match including a Dlx binding site to a cis-element in the Dlx5-6 intermediate region designated mI56i [Zerucha, T., Stuhmer, T., Hatch, G., Park, B. K., Long, Q., Yu, G., Gambarotta, A., Schultz, J. R., Rubenstein, J. L. & Ekker, M. (2000) J. Neurosci. 20, 709-721], despite distant phylogenetic relationship between these clusters. Our results provide evidence for a concerted role for DLX auto- and cross-regulation in the establishment of a nested expression pattern for Dlx3-7 and Dlx5-6 clusters within the visceral arches.

Role of homeobox genes in normal mammary gland development and breast tumorigenesis

The role of homeobox-containing genes in embryogenesis and organogenesis is well documented. Also, a sizeable body of evidence has accumulated and supports the fact that homeobox genes, when dysregulated, are involved in tumorigenesis. However, the precise mechanisms of homeobox gene functions are largely unknown. The mammary gland, in which most maturation occurs postnatally, provides an ideal model for studying the functions of homeobox genes in both development and tumorigenesis. The expression of many homeobox genes has been detected in both normal mammary gland and neoplastic breast tissues. In the normal mammary gland, the expression of homeobox genes is coordinately regulated by hormone and extracellular matrix (ECM) and other unknown factors in a spatial and temporal manner in both stromal and epithelial cells. Animals with misexpressed homeobox genes displayed different extents of defects in ductal proliferation, side branching, and alveoli formation, implying that homeobox genes are important for normal mammary gland development. Recent studies of homeobox genes in breast cancer cells and primary tumors indicate that they may also play a contributory or causal role in tumorigenesis by regulating the cell cycle, apoptosis, angiogenesis, and/or metastasis.

Patterns of gene expression: homology or homocracy?

Numerous papers over the years have stated that the original meaning of the term homology is historical and morphological and denotes organs/structures in two or more species derived from the same structure in their latest common ancestor. However, several more recent papers have extended the use of the term to cover organs/structures which are organised through the expression of homologous genes. This usage has created an ambiguity about the meaning of the term, and we propose to remove this by proposing a new term, homocracy, for organs/structures which are organised through the expression of identical patterning genes. We want to emphasise that the terms homologous and homocratic are not mutually exclusive. Many homologous structures are in all probability homocratic, whereas only a small number of homocratic structures are homologous.

Cyclopamine and jervine in embryonic rat tongue cultures demonstrate a role for Shh signaling in taste papilla development and patterning: fungiform papillae double in number and form in novel locations in dorsal lingual epithelium

From time of embryonic emergence, the gustatory papilla types on the mammalian tongue have stereotypic anterior and posterior tongue locations. Furthermore, on anterior tongue, the fungiform papillae are patterned in rows. Among the many molecules that have potential roles in regulating papilla location and pattern, Sonic hedgehog (Shh) has been localized within early tongue and developing papillae. We used an embryonic, tongue organ culture system that retains temporal, spatial, and molecular characteristics of in vivo taste papilla morphogenesis and patterning to study the role of Shh in taste papilla development. Tongues from gestational day 14 rat embryos, when papillae are just beginning to emerge on dorsal tongue, were maintained in organ culture for 2 days. The steroidal alkaloids, cyclopamine and jervine, that specifically disrupt the Shh signaling pathway, or a Shh-blocking antibody were added to the standard culture medium. Controls included tongues cultured in the standard medium alone, and with addition of solanidine, an alkaloid that resembles cyclopamine structurally but that does not disrupt Shh signaling. In cultures with cyclopamine, jervine, or blocking antibody, fungiform papilla numbers doubled on the dorsal tongue with a distribution that essentially eliminated inter-papilla regions, compared with tongues in standard medium or solanidine. In addition, fungiform papillae developed on posterior oral tongue, just in front of and beside the single circumvallate papilla, regions where fungiform papillae do not typically develop. The Shh protein was in all fungiform papillae in embryonic tongues, and tongue cultures with standard medium or cyclopamine, and was conspicuously localized in the basement membrane region of the papillae. Ptc protein had a similar distribution to Shh, although the immunoproduct was more diffuse. Fungiform papillae did not develop on pharyngeal or ventral tongue in cyclopamine and jervine cultures, or in the tongue midline furrow, nor was development of the single circumvallate papilla altered. The results demonstrate a prominent role for Shh in fungiform papilla induction and patterning and indicate differences in morphogenetic control of fungiform and circumvallate papilla development and numbers. Furthermore, a previously unknown, broad competence of dorsal lingual epithelium to form fungiform papillae on both anterior and posterior oral tongue is revealed.

Mouse embryonic mammogenesis as a model for the molecular regulation of pattern formation

In this review we describe how mouse embryonic mammogenesis depends on a continuous communication between the epithelial and mesenchymal compartment of the mammary rudiment. Although the functions of only a few genes in the regulation of these epithelio-mesenchymal interactions during mouse mammary development are known so far, key roles are suggested for WNT, FGF and PTHrP signaling. However, the exact mechanism of action of these signaling pathways and their possible cross-talk in the induction of mammary development are not clear, nor does our current knowledge suffice to explain how the number and positions of the mammary rudiments are so well defined. Nonetheless, by the description of aberrant induction and/or maintenance of the mammary rudiments in a variety of inbred mouse strains and mutants, we have accumulated data demonstrating that the mammary rudiments develop independently of each other at these positions. In addition, each rudiment pair responds differently to altered levels of gene expression. This not only clarifies the unique identity of each placode, but the different molecular requirement of each placode also suggests that different molecular mechanisms may underlie the formation of such identical structures. For future investigations in the field, such a unique molecular identity of each mammary rudiment should be of critical concern.

Developmental functions of theDistal-less/Dlx homeobox genes

Distal-less is the earliest known gene specifically expressed in developing insect limbs; its expression is maintained throughout limb development. The homeodomain transcription factor encoded by Distal-less is required for the elaboration of proximodistal pattern elements in Drosophila limbs and can initiate proximodistal axis formation when expressed ectopically. Distal-less homologs, the Dlx genes, are expressed in developing appendages in at least six phyla, including chordates, consistent with requirements for Dlx function in normal appendage development across the animal kingdom. Recent work implicates the Dlx genes of vertebrates in a variety of other developmental processes ranging from neurogenesis to hematopoiesis. We review what is known about the invertebrate and vertebrate Dll/Dlx genes and their varied roles during development. We propose revising the vertebrate nomenclature to reflect phylogenetic relationships among the Dlx genes.

Concerted action of twodlxparalogs in sensory placode formation

Sensory placodes are ectodermal thickenings that give rise to elements of the vertebrate cranial sensory nervous system, including the inner ear and nose. Although mutations have been described in humans, mice and zebrafish that perturb ear and nose development, no mutation is known to prevent sensory placode formation. Thus, it has been postulated that a functional redundancy exists in the genetic mechanisms that govern sensory placode development. We describe a zebrafish deletion mutation, b380, which results in a lack of both otic and olfactory placodes.

The b380 deletion removes several known genes and expressed sequence tags, including dlx3 and dlx7, two transcription factors that share a homoeobox domain similar in sequence to the Drosophila Distal-less gene. dlx3 and dlx7 are expressed in an overlapping pattern in the regions that produce the otic and olfactory placodes in zebrafish. We present evidence suggesting that it is specifically the removal of these two genes that leads to the otic and olfactory phenotype of b380 mutants. Using morpholinos, antisense oligonucleotides that effectively block translation of target genes, we find that functional reduction of both dlx genes contributes to placode loss. Expression patterns of the otic marker pax2.1, olfactory marker anxV and eya1, a marker of both placodes, in morpholino-injected embryos recapitulate the reduced expression of these genes seen in b380 mutants. We also examine expression of dlx3 and dlx7 in the morpholino-injected embryos and present evidence for existence of auto- and cross-regulatory control of expression among these genes.

We demonstrate that dlx3 is necessary and sufficient for proper otic and olfactory placode development. However, our results indicate that dlx3 and dlx7 act in concert and their importance in placode formation is only revealed by inactivating both paralogs.

Development of anterior gustatory epithelia in the palate and tongue requires epidermal growth factor receptor

We characterized the gustatory phenotypes of neonatal mice having null mutations for epidermal growth factor receptor (egfr(-/-)), brain-derived neurotrophic factor (bdnf(-/-)), or both. We counted the number and diameter of fungiform taste buds, the prevalence of poorly differentiated or missing taste cells, and the incidence of ectopic filiform-like spines, each as a function of postnatal age and anterior/posterior location. Egfr(-/-) mice and bdnf(-/-) mice had similar reductions in the total number of taste buds on the anterior portions of the tongue and palate. Nonetheless, there were significant differences in their gustatory phenotypes. EGFR deficiency selectively impaired the development of anterior gustatory epithelia in the mouth. Only bdnf(-/-) mice had numerous taste buds missing from the foliate, vallate, and posterior fungiform papillae. Only egfr(-/-) fungiform taste papillae had robust gustatory innervation, markedly reduced cytokeratin 8 expression in taste cells, and a high incidence of a filiform-like spine. Egfr/bdnf double-null mutant mice had a higher frequency of failed fungiform taste bud differentiation. In bdnf(-/-) mice taste cell development failed because of sparse gustatory innervation. In contrast, in young egfr(-/-) mice the abundance of axons innervating fungiform papillae and the normal numbers of geniculate ganglion neurons implicate gustatory epithelial defects rather than neural defects.

MSX2 expression in the apical ectoderm ridge is regulated by an MSX2 and Dlx5 binding site

The apical ectodermal ridge (AER) is a specialized ectodermal region essential for limb outgrowth. Msx2 expression patterns in limb development strongly suggest an important role for Msx2 in the AER. Our previous studies identified a 348-bp fragment of the chicken Msx2 gene with AER enhancer activity. In this study, the functions of four potential homeodomain binding TAAT sites in this enhancer were studied using transgenic mice and in vitro protein-DNA interactions. Transgenic studies indicate that the four TAAT sites are not redundant and that only the B-TAAT site is critical for AER enhancer activity. The expression patterns of Msx2 and Dlx5 genes in the AER suggest that they might be involved in the regulation of Msx2. In support of this hypothesis, we found that Msx2 and Dlx5 can bind to the B-TAAT site as well as to a fragment containing the D- and E-TAAT sites in the Msx2 AER enhancer sequences. (c)2002 Elsevier Science (USA).

Bone morphogenetic protein-2 (BMP-2) transactivates Dlx3 through Smad1 and Smad4: alternative mode for Dlx3 induction in mouse keratinocytes

Expression of the Dlx3 homeodomain gene is induced in terminally differentiated epidermal cells. Dlx3 regulates gene expression in skin and plays important roles in patterning of the embryonic ectoderm through differential sensitivity to bone morphogenetic protein (BMP) signaling. We analyzed the expression of BMP family members in murine keratinocytes; BMP-2 is expressed in proliferative basal and differentiated suprabasal keratinocytes. BMP-2 induced transcription of Dlx3 within 12 h of treatment of keratinocytes cultured in vitro. We proceeded to delineate the BMP-2-responsive region to an area between -1917 and -1747 in the Dlx3 promoter. Gel shift assays with recombinant Smad1 and Smad4 demonstrated that this DNA fragment (-1917 to -1747) was competent in the formation of protein-DNA complexes. By deletion and mutational analyses we localized a Smad1/Smad4-binding site containing a GCAT motif, which showed similarity to other TGF-beta family responsive elements. Supershift assays with keratinocyte nuclear extracts and antibodies against members of the Smad family showed that this motif was able to form a complex with Smad1. Mutation of the Smad1/Smad4-binding site inhibited transcriptional activation of the Dlx3 gene by BMP-2. In the hair follicle, where Dlx3 is expressed in the hair matrix cells, BMP-2 also activates Dlx3 transcription. These results provide a possible mechanism of action for the BMP signaling pathway on the regulation of Dlx3.

Cranial neural crest-cell migration in the direct-developing frog, Eleutherodactylus coqui: molecular heterogeneity within and among migratory streams

Direct development is a specialized reproductive mode that has evolved repeatedly in many different lineages of amphibians, especially anurans. A fully formed, albeit miniature adult hatches directly from the egg; there is no free-living larva. In many groups, the evolution of direct development has had profound consequences for cranial development and morphology, including many components that are derived from the embryonic neural crest. Yet, the developmental bases of these effects remain poorly known. In order to more fully characterize these changes, we used three molecular markers to analyze cranial neural crest-cell emergence and migration in the direct-developing frog, Eleutherodactylus coqui: HNK-1 immunoreactivity, Dlx protein expression, and cholinesterase activity. Our study validates and extends earlier results showing that the comprehensive changes in embryonic cranial patterning, differentiation, and developmental timing that are associated with direct development in Eleutherodactylus have not affected gross features of cranial neural crest biology: the relative timing of crest emergence and the number, configuration and identity of the principal migratory streams closely resemble those seen in metamorphic anurans. The three markers are variably expressed within and among neural crest-cell populations. This variation suggests that determination of cranial neural crest-cells may already have begun at or soon after the onset of migration, when the cells emerge from the neural tube. It is not known how or even if this variation correlates with differential cell lineage or fate. Finally, although HNK-1 expression is widely used to study neural crest migration in teleost fishes and amniotes, E. coqui is the only amphibian known in which it effectively labels migrating neural crest-cells. There are not enough comparative data to determine whether this feature is functionally associated with direct development or is instead unrelated to reproductive mode.

The Caenorhabditis elegans distal-less ortholog ceh-43 is required for development of the anterior hypodermis

Homeobox genes of the Distal-less (Dll) class are expressed in developing appendages as well as in the central nervous system in invertebrates and vertebrates. Mutant analyses in mice and Drosophila have implicated these genes in outgrowth of structures, cell adhesion, cell migration, and cell fate decisions. We have investigated the expression and function of ceh-43, the Dll ortholog from the nematode Caenorhabditis elegans, by using gfp reporter constructs and double-stranded RNA-mediated interference (RNAi). Our results show that, as in the fly, the C. elegans Dll ortholog seems to play a role in cell adhesion. An antibody against the butterfly Distal-less homeodomain stains the nervous system of C. elegans embryos (Panganiban et al. [1997] Proc Natl Acad Sci USA. 94:5162-5166). GFP expression under the control of the ceh-43 promoter looks similar, although strong expression is primarily confined to the head hypodermis and to neuronal support cells. ceh-43(RNAi) results in 100% lethality at embryonic or early larval stages. At the beginning of morphogenesis, ceh-43(RNAi) embryos start to lose cells through a hole in the head hypodermis. They either rupture anteriorly as elongation proceeds, or they elongate normally to threefold egg length with the pharynx not connected to the mouth. Elongated ceh-43(RNAi) animals die before or soon after hatching with a fluid-filled pseudocoel and large vacuoles. These phenotypes suggest a role for ceh-43 in development of adhesive properties in the head hypodermis that connects the epithelia of the skin and the digestive tract. Furthermore, possible defects in the excretory system may result at least in part from a requirement for ceh-43 in the CAN neurons where ceh-43:gfp is also expressed.

Cavefish as a model system in evolutionary developmental biology

The Mexican tetra Astyanax mexicanus has many of the favorable attributes that have made the zebrafish a model system in developmental biology. The existence of eyed surface (surface fish) and blind cave (cavefish) dwelling forms in Astyanax also provides an attractive system for studying the evolution of developmental mechanisms. The polarity of evolutionary changes and the environmental conditions leading to the cavefish phenotype are known with certainty, and several different cavefish populations have evolved constructive and regressive changes independently. The constructive changes include enhancement of the feeding apparatus (jaws, taste buds, and teeth) and the mechanosensory system of cranial neuromasts. The homeobox gene Prox 1, which is expressed in the expanded taste buds and cranial neuromasts, is one of the genes involved in the constructive changes in sensory organ development. The regressive changes include loss of pigmentation and eye degeneration. Although adult cavefish lack functional eyes, small eye primordia are formed during embryogenesis, which later arrest in development, degenerate, and sink into the orbit. Apoptosis and lens signaling to other eye parts, such as the cornea, iris, and retina, result in the arrest of eye development and ultimate optic degeneration. Accordingly, an eye with restored cornea, iris, and retinal photoreceptor cells is formed when a surface fish lens is transplanted into a cavefish optic cup, indicating that cavefish optic tissues have conserved the ability to respond to lens signaling. Genetic analysis indicates that multiple genes regulate eye degeneration, and molecular studies suggest that Pax6 may be one of the genes controlling cavefish eye degeneration. Further studies of the Astyanax system will contribute to our understanding of the evolution of developmental mechanisms in vertebrates.

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.

The Dlx3 protein harbors basic residues required for nuclear localization, transcriptional activity and binding to Msx1

The murine Dlx3 protein is a putative transcriptional activator that has been implicated during development and differentiation of epithelial tissue. Dlx3 contains a homeodomain and mutational analysis has revealed two regions, one N-terminal and one C-terminal to the homeodomain, that act as transcriptional activators in a yeast one-hybrid assay. In addition to transactivation, data are presented to demonstrate specific DNA binding and an association between Dlx3 and the Msx1 protein in vitro. Immunohistochemical analysis confirmed coexpression of Dlx3 and Msx1 proteins in the differentiated layers of murine epidermal tissues. Transcription factor function requires nuclear localization. In this study, the intracellular localization of the green fluorescent protein fused to Dlx3 was examined in keratinocytes induced to differentiate by calcium and is shown to localize to the nucleus. A bipartite nuclear localization signal (NLS) was identified by mutational analysis and shown to be sufficient for nuclear localization. This was demonstrated by insertion of the Dlx3 bipartite NLS sequence into a cytoplasmic fusion protein, GFP-keratin 14, which functionally redirected GFP-keratin 14 expression to the nucleus. Further analysis of Dlx3 NLS mutants revealed that the Dlx3 NLS sequences are required for specific DNA binding, transactivation potential and interactions with the Msx1 protein.

Distal-less-related homeobox genes of vertebrates: Evolution, function, and regulation

Homeobox genes of the Distal-less family have been identified in virtually all metazoan groups where they play roles in the ontogeny of these animals. The vertebrate Distal-less related genes (Dlx genes) are thought to have arisen as a result of a tandem gene duplication event followed by a number of larger genomic scale duplications and thus represent an interesting model with which to study the evolution of clustered gene families. Dlx genes are involved in the development of the forebrain, branchial arches, sensory organs, and limbs. Here we describe the current state of knowledge of the Dlx genes in terms of their developmental expression, how this expression is regulated and how the products of these genes function, once expressed. We highlight a number of recent studies that have shed light on the transcriptional regulation of this gene family. These findings have not only contributed to our understanding of the selective pressures involved in the maintenance of familial gene clustering in genomes, but also to our understanding of how genes may diverge in function during the course of evolution as a result of divergence of regulatory mechanisms.Key words: genome, homeodomain, inner ear, olfactory placode, transcription.

Regulation and function of Dlx3 in vertebrate development

Dlx3 is a homeodomain transcription factor in vertebrates, related to Distal-less in Drosophila, that is expressed in differentiating epidermal cells, in neural crest, hair follicles, dental epithelium and mesenchyme, the otic and olfactory placodes, limb bud, placenta, and in the cement gland, which is located in the extreme anterior neural plate in Xenopus embryos. This factor behaves as a transcriptional activator, and positively regulates gene expression in the skin, and negatively regulates central nervous system markers in Xenopus epidermis and anterior neural plate. A mutation in the DLX3 gene is associated with a hereditary syndrome in humans, and loss of Dlx3 function is a developmental lethal in gene-targeted mice, where it is essential for proper modeling of the labyrinthine layer of the placenta. In this review, we discuss the evolution, expression, regulation, and function of Dlx3 in mouse, amphibians, and zebrafish. Published 2000 Wiley-Liss, Inc.

Roles for Msx and Dlx homeoproteins in vertebrate development

This review provides a comparative analysis of the expression patterns, functions, and biochemical properties of Msx and Dlx homeobox genes. These comprise multi-gene families that are closely related with respect to sequence features as well as expression patterns during vertebrate development. Thus, members of the Msx and Dlx families are expressed in overlapping, but distinct, patterns and display complementary or antagonistic functions, depending upon the context. A common theme shared among Msx and Dlx genes is that they are required during early, middle, and late phases of development where their differential expression mediates patterning, morphogenesis, and histogenesis of tissues in which they are expressed. With respect to their biochemical properties, Msx proteins function as transcriptional repressors, while Dlx proteins are transcriptional activators. Moreover, their ability to oppose each other's transcriptional actions implies a mechanism underlying their complementary or antagonistic functions during development.

The homeobox genes MSX2 and MOX2 are candidates for regulating epithelial-mesenchymal cell interactions in the human placenta

Homeobox genes of the Msx and Mox families are coexpressed in the vertebrate embryo in regions of epithelial-mesenchymal interactions. Here we show that a member of each family is expressed in extra-embryonic structures where epithelial and mesenchymal cell layers contact. In situ hybridization studies on first trimester human placental sections reveal that MSX2 and MOX2 are expressed predominantly in the cytotrophoblast cell layer. In term placenta, MSX2 and MOX2 are expressed in the syncytiotrophoblast. This is the first study to describe the expression of MOX2 in human tissues and to show that members of the Msx and Mox families of homeobox genes are expressed where epithelial and mesenchymal cell layers contact in the human placenta. A combinatorial code of homeobox genes that includes members of the Msx, Mox and Dlx families has been predicted to regulate epithelial-mesenchymal cell interactions in the vertebrate embryo. We have shown that MSX2, MOX2, DLX4 and the HB24 homeobox gene are expressed in the epithelial and mesenchymal cell types that form the placenta. We predict that this combination of homeobox genes is involved in regulating epithelial-mesenchymal cell interactions in extraembryonic tissues.

Expression of Distal-less in molluscan eggs, embryos, and larvae

Distal-less (Dll) is best known as a transcription factor involved with "limb patterning" in Drosophila melanogaster. Observations of both deuterostome and protostome phyla have led to the suggestion that some aspect of this gene's function in "appendage" or proximal-distal "outgrowth" development is conserved. Here we explore the possibility of other conserved roles operating earlier in development. We examine the expression of DLL protein during the early development of two molluscan classes, Polyplacophora (chiton) and Gastropoda (snail). Using an antibody approach, we find DLL expression in the oocytes of a chiton (Mopalia muscosa) and in the pregastrulae through early veliger larvae of a marine snail (Kelletia kelletii). We observe antibody localization in the oocyte, nuclear expression in all cells of the pregastrulae, and predominant expression in the ectoderm of postgastrulae and early veliger larvae. Comparison of our observations on spiralian taxa, thought to have conservative development with previous work, primarily on deuterostomes, suggests the possibility of an ancient role(s) for DLL in early development. Possible functions appear to include maternal and zygotic involvement in the establishment of embryonic polarity, involvement in the process of germ layer formation, and a role in the specification and/or differentiation of ectoderm/epithelia. We note that the exploration of conserved gene function in early development may be clarified by examining taxa whose early development has putatively not been subject to dramatic evolutionary change.

Regulation of the Dlx3 homeobox gene upon differentiation of mouse keratinocytes

The Distal-less Dlx3 homeodomain gene is expressed in terminally differentiated murine epidermal cells, and there is evidence to support an essential role as a transcriptional regulator of the terminal differentiation process in these cells. In an attempt to determine the factors that induce Dlx3 gene expression, we have cloned the 1.2-kilobase pair proximal region of murine gene and analyzed its cis-regulatory elements and potential trans-acting factors. The proximal region of the Dlx3 gene has a canonical TATA box and CCAAT box, and the transcription start site was located 205 base pairs upstream from the initiation of translation site. Serial deletion analysis showed that the region between -84 and -34 confers the maximal promoter activity both in undifferentiated and differentiated primary mouse keratinocytes. Gel retardation assays and mutational analysis demonstrated that the transcriptional regulator NF-Y (also referred to as CBF) binds to a CCAAT box motif within this region and is responsible for the majority of the Dlx3 promoter activity. In addition, an Sp1-binding site was located immediately upstream of transcription start site that acts as a positive regulatory element of the Dlx3 promoter, independent of the CCAAT box motif. Importantly, elements residing between +30 to +60 of the Dlx3 gene are responsible for the Ca(2+)-dependent induction of Dlx3 during keratinocyte differentiation.

Transcriptional activation by the homeodomain protein distal-less 3

PCR-based methods and mobility shift competition assays were used to determine the basic biochemical features of the homeodomain transcription factor Distal-less 3 (Dlx3), including an optimal DNA binding site, the binding constant and dissociation rates of this protein. Expression of Dlx3 protein in either HeLa cells or Xenopus embryos resulted in strong activation of a model target gene construct containing three tandem copies of the Dlx3 binding site upstream from the TATA element. In addition, deletion analysis revealed that transcriptional activation by Dlx3 depends on two subdomains located on either side of the homeobox: removal of either subdomain resulted in complete loss of Dlx3 function. These observations provide new insight regarding the function of Dlx3 in vertebrate development and tissue differentiation and also suggest a mechanism for the dominant inheritance pattern of a hereditary disease resulting from mutation of the DLX3 gene in human.

Taste neurons have multiple inductive roles in mammalian gustatory development

The embryonic loss of brain-derived neurotropic factor (BDNF)-dependent taste axons in bdnf null mutant mice secondary impairs the development of gustatory epithelia and taste buds. In normal mice gustatory development continues for at least two weeks postnatally as axons promote taste bud formation. We conclude that taste axons in the fungiform, foliate, vallate and nasopalate papillae: i) promote papilla development, and ii) establish competent gustatory cells and iii) mature taste buds. Hence, gustatory innervation contributes critically to at least three of the multiple inductive interactions controlling the development of mammalian gustatory structures.

The role of innervation in induction and differentiation of taste organs: introduction and background

To establish lingual receptive fields that are the basic unit of taste function, ganglion cells must extend neurites of peripheral and central targets and form connections. This symposium concerns developmental interactions between the geniculate, trigeminal and petrosal ganglia and peripheral taste organs, the gustatory papillae and resident taste buds. Investigators present data from organ and tissue culture, from mice with targeted gene deletions and from grafting experiments, in pursuit of principles that direct early innervation of the taste system. The lingual ganglia and the taste papillae initially develop independently, but then become reciprocally dependent as ganglia drive neurotrophin support from gustatory papillae and the papillae require sensory innervation for growth and morphogenesis. The issue of subsequent taste bud induction is discussed with results from amphibian and mammalian models, yielding conclusions that are not yet totally convergent. However, an essential role for sensory innervation in mammalian taste bud differentiation and acquisition of appropriate quantitative relations between ganglion cells and target organs is clearly demonstrated. A working outline is presented for periods of ganglion cell/target organ independence and interdependence during early innervation of the peripheral taste system.

Expression of the Emx-1 and Dlx-1 homeobox genes define three molecularly distinct domains in the telencephalon of mouse, chick, turtle and frog embryos: implications for the evolution of telencephalic subdivisions in amniotes

Homologies between vertebrate forebrain subdivisions are still uncertain. In particular the identification of homologs of the mammalian neocortex or the dorsal ventricular ridge (DVR) of birds and reptiles is still a matter of dispute. To get insight about the organization of the primordia of the main telencephalic subdivisions along the anteroposterior axis of the neural tube, a fate map of the dorsal prosencephalon was obtained in avian chimeras at the 8- to 9-somite stage. At this stage, the primordia of the pallium, DVR and striatum were located on the dorsal aspect of the prosencephalon and ordered caudorostrally along the longitudinal axis of the brain. Expression of homeobox-containing genes of the Emx, Dlx and Pax families were used as markers of anteroposterior developmental subdivisions of the forebrain in mouse, chick, turtle and frog. Their expression domains delineated three main telencephalic subdivisions in all species at the onset of neurogenesis: the pallial, intermediate and striatal neuroepithelial domains. The fate of the intermediate subdivisions diverged, however, between species at later stages of development. Homologies between forebrain subdivisions are proposed based on the conservation and divergence of these gene expression patterns.

Limb development and evolution: a frog embryo with no apical ectodermal ridge (AER)

The treefrog Eleutherodactylus coqui is a direct developer--it has no tadpole stage. The limb buds develop earlier than in metamorphosing species (indirect developers, such as Xenopus laevis). Previous molecular studies suggest that at least some mechanisms of limb development in E. coqui are similar to those of other vertebrates and we wished to see how limb morphogenesis in this species compares with that in other vertebrates. We found that the hind limb buds are larger and more advanced than the forelimbs at all stages examined, thus differing from the typical amniote pattern. The limb buds were also small compared to those in the chick. Scanning and transmission electron microscopy showed that although the apical ectoderm is thickened, there was no apical ectodermal ridge (AER). In addition, the limb buds lacked the dorsoventral flattening seen in many amniotes. These findings could suggest a mechanical function for the AER in maintaining dorsoventral flattening, although not all data are consistent with this view. Removal of distal ectoderm from E. coqui hindlimb buds does not stop outgrowth, although it does produce anterior defects in the skeletal pattern. The defects are less severe when the excisions are performed earlier. These results contrast with the chick, in which AER excision leads to loss of distal structures. We suggest that an AER was present in the common ancestor of anurans and amniotes and has been lost in at least some direct developers including E. coqui.

Analysis of the genetic pathway leading to formation of ectopic apical ectodermal ridges in mouse Engrailed-1 mutant limbs

The apical ectodermal ridge (AER), a rim of thickened ectodermal cells at the interface between the dorsal and ventral domains of the limb bud, is required for limb outgrowth and patterning. We have previously shown that the limbs of En1 mutant mice display dorsal-ventral and proximal-distal abnormalities, the latter being reflected in the appearance of a broadened AER and formation of ectopic ventral digits. A detailed genetic analysis of wild-type, En1 and Wnt7a mutant limb buds during AER development has delineated a role for En1 in normal AER formation. Our studies support previous suggestions that AER maturation involves the compression of an early broad ventral domain of limb ectoderm into a narrow rim at the tip and further show that En1 plays a critical role in the compaction phase. Loss of En1 leads to a delay in the distal shift and stratification of cells in the ventral half of the AER. At later stages, this often leads to development of a secondary ventral AER, which can promote formation of an ectopic digit. The second AER forms at the juxtaposition of the ventral border of the broadened mutant AER and the distal border of an ectopic Lmx1b expression domain. Analysis of En1/Wnt7a double mutants demonstrates that the dorsalizing gene Wnt7a is required for the formation of the ectopic AERs in En1 mutants and for ectopic expression of Lmx1b in the ventral mesenchyme. We suggest a model whereby, in En1 mutants, ectopic ventral Wnt7a and/or Lmx1b expression leads to the transformation of ventral cells in the broadened AER to a more dorsal phenotype. This leads to induction of a second zone of compaction ventrally, which in some cases goes on to form an autonomous secondary AER.

Gene expression during amphibian limb regeneration

Limb regeneration in adult urodeles is an important phenomenon that poses fundamental questions both in biology and in medicine. In this review, we focus on recent advances in the characterization of the regeneration blastema at cellular and molecular levels and on the current understanding of the molecular basis of limb regeneration and its relationship to development. In particular, we discuss (i) the spatiotemporal distribution of genes and gene products in the mesenchyme and wound epidermis of the regenerating limb, (ii) how growth is controlled in the regeneration blastema, and (iii) molecules that are likely to be involved in patterning the regenerating limb such as homeobox genes and retinoids.

The development of the vertebrate inner ear

The inner ear is a complex sensory organ responsible for balance and sound detection in vertebrates. It originates from a transient embryonic structure, the otic vesicle, that contains all of the information to develop autonomously into the mature inner ear. We review here the development of the otic vesicle, bringing together classical embryological experiments and recent genetic and molecular data. The specification of the prospective ectoderm and its commitment to the otic fate are very early events and can be related to the expression of genes with restricted expression domains. A combinatorial gene expression model for placode specification and diversification, based on classical embryological evidence and gene expression patterns, is discussed. The formation of the otic vesicle is dependent on inducing signals from endoderm, mesoderm and neuroectoderm. Ear induction consists of a sequence of discrete instructions from those tissues that confer its final identity on the otic field, rather than a single all-or-none process. The important role of the neural tube in otic development is highlighted by the abnormalities observed in mouse mutants for the Hoxa1, kreisler and fgf3 genes and those reported in retinoic acid-deficient quails. Still, the nature of the relation between the neural tube and otic development remains unclear. Gene targeting experiments in the mouse have provided evidence for genes potentially involved in regional and cell-fate specification in the inner ear. The disruption of the mouse Brn3.1 gene identifies the first mutation affecting sensory hair-cell specification, and mutants for Pax2 and Nkx5.1 genes show their requirement for the development of specific regions of the otic vesicle. Several growth-factors contribute to the patterned cell proliferation of the otic vesicle. Among these, IGF-I and FGF-2 are expressed in the otic vesicle and may act in an autocrine manner. Finally, little is known about early mechanisms involved in guiding ear innervation. However, targeted disruption of genes coding for neurotrophins and Trk receptors have shown that once synaptic contacts are established, they depend on specific trophic interactions that involve these two gene families. The accessibility of new cellular and molecular approaches are opening new perspectives in vertebrate development and are also starting to be applied to ear development. This will allow this classical and attractive model system to see a rapid progress in the near future.

Amphibians provide new insights into taste-bud development

Until recently, the predominant model of taste-bud development was one of neural induction: ingrowing sensory fibers were thought to induce taste-bud differentiation late in embryonic development. Recent experimental studies, however, show that the development of taste buds is independent of their innervation. In amphibian embryos, the ability to generate taste buds is an intrinsic feature of the oropharyngeal epithelium long before the region becomes innervated. These studies indicate that patterning of the oropharyngeal epithelium occurs during gastrulation, and suggest that taste buds or their progenitors play the dominant role in the development of their own innervation.

Relationship between the genomic organization and the overlapping embryonic expression patterns of the zebrafish dlx genes

To understand the relationship between the expression and the genomic organization of the zebrafish dlx genes, we have determined the genomic structure of the dlx2 and dlx4 loci. This led to the identification of the zebrafish dlx1 and dlx6 genes, which are closely linked to dlx2 and dlx4, respectively. Therefore, the inverted convergent configuration of Dlx genes is conserved among vertebrates. Analysis of the expression patterns of dlx1 and dlx6 showed striking similarities to those of dlx2 and dlx4, respectively, the genes to which they are linked. Furthermore, the expression patterns of dlx3 and dlx7, which likely constitute a third pair of convergently transcribed genes, are indistinguishable. Thus, the overlapping expression patterns of linked Dlx genes during embryonic development suggest that they share cis-acting sequences that control their spatiotemporal expression. The evolutionary conservation of the genomic organization and combinatorial expression of Dlx genes in distantly related vertebrates suggest tight control mechanisms that are essential for their function during development.