FGF4, a direct target of LEF1 and Wnt signaling, can rescue the arrest of tooth organogenesis in Lef1(-/-) mice

Genetic basis of skin appendage development

Morphogenesis of hair follicles, teeth, and mammary glands depends on inductive epithelial-mesenchymal interactions mediated by a conserved set of signalling molecules. The early development of different skin appendages is remarkably similar. Initiation of organogenesis is marked by the appearance of a local epithelial thickening, a placode, which subsequently invaginates to produce a bud. These early developmental stages require many of the same genes and signalling circuits and consequently alterations in them often cause similar phenotypes in several skin appendages. After the bud stage, these organs adopt diverse patterns of epithelial growth, reflected in the usage of more divergent genes in each.

Conservation and divergence of Bmp2a, Bmp2b, and Bmp4 expression patterns within and between dentitions of teleost fishes

The diversity of tooth location in teleost fishes provides an excellent system for comparing genetic divergence between teeth in different species (phylogenetic homologs) with divergence between teeth within one species (iterative homologs). We have chosen to examine the expression of three members of the bone morphogenetic protein (Bmp) family because they are known to play multiple roles in tooth development and evolution in tetrapod vertebrates. We characterized expression of Bmp2a, Bmp2b, and Bmp4 during the development of oral and pharyngeal dentitions in three species of teleost fishes, the zebrafish (Danio rerio), Mexican tetra (Astyanax mexicanus), and Japanese medaka (Oryzias latipes). We found that expression in teleosts is generally highly conserved, with minor differences found among both iteratively homologous and phylogenetically homologous teeth. Expression of orthologous genes differs in several ways between the teeth of teleost fishes and those of the mouse, but between these vertebrate groups the summed expression pattern of Bmp genes is highly conserved. Significantly, the toothless oral region of the zebrafish lacks Bmp expression domains found in teleosts with oral teeth, implicating these genes in evolutionary tooth loss. We conclude that Bmp expression has been largely conserved in vertebrate tooth development over evolutionary time, and that loss of Bmp expression is correlated with region-specific loss of the dentition in a major group of fishes.

Continuous tooth generation in mouse is induced by activated epithelial Wnt/beta-catenin signaling

The single replacement from milk teeth to permanent teeth makes mammalian teeth different from teeth of most nonmammalian vertebrates and other epithelial organs such as hair and feathers, whose continuous replacement has been linked to Wnt signaling. Here we show that mouse tooth buds expressing stabilized beta-catenin in epithelium give rise to dozens of teeth. The molar crowns, however, are typically simplified unicusped cones. We demonstrate that the supernumerary teeth develop by a renewal process where new signaling centers, the enamel knots, bud off from the existing dental epithelium. The basic aspects of the unlocked tooth renewal can be reproduced with a computer model on tooth development by increasing the intrinsic level of activator production, supporting the role of beta-catenin pathway as an upstream activator of enamel knot formation. These results may implicate Wnt signaling in tooth renewal, a capacity that was all but lost when mammals evolved progressively more complicated tooth shapes.

Canonical Wnt signaling transiently stimulates proliferation and enhances neurogenesis in neonatal neural progenitor cultures

Canonical Wnt signaling triggers the formation of heterodimeric transcription factor complexes consisting of beta-catenin and T cell factors, and thereby controls the execution of specific genetic programs. During the expansion and neurogenic phases of embryonic neural development canonical Wnt signaling initially controls proliferation of neural progenitor cells, and later neuronal differentiation. Whether Wnt growth factors affect neural progenitor cells postnatally is not known. Therefore, we have analyzed the impact of Wnt signaling on neural progenitors isolated from cerebral cortices of newborn mice. Expression profiling of pathway components revealed that these cells are fully equipped to respond to Wnt signals. However, Wnt pathway activation affected only a subset of neonatal progenitors and elicited a limited increase in proliferation and neuronal differentiation in distinct subsets of cells. Moreover, Wnt pathway activation only transiently stimulated S-phase entry but did not support long-term proliferation of progenitor cultures. The dampened nature of the Wnt response correlates with the predominant expression of inhibitory pathway components and the rapid actuation of negative feedback mechanisms. Interestingly, in differentiating cell cultures activation of canonical Wnt signaling reduced Hes1 and Hes5 expression suggesting that during postnatal neural development, Wnt/beta-catenin signaling enhances neurogenesis from progenitor cells by interfering with Notch pathway activity.

WNT and DKK determine hair follicle spacing through a reaction-diffusion mechanism

Mathematical reaction-diffusion models have been suggested to describe formation of animal pigmentation patterns and distribution of epidermal appendages. However, the crucial signals and in vivo mechanisms are still elusive. Here we identify WNT and its inhibitor DKK as primary determinants of murine hair follicle spacing, using a combined experimental and computational modeling approach. Transgenic DKK overexpression reduces overall appendage density. Moderate suppression of endogenous WNT signaling forces follicles to form clusters during an otherwise normal morphogenetic program. These results confirm predictions of a WNT/DKK-specific mathematical model and provide in vivo corroboration of the reaction-diffusion mechanism for epidermal appendage formation.

Sprouty genes control diastema tooth development via bidirectional antagonism of epithelial-mesenchymal FGF signaling

Unlike humans, who have a continuous row of teeth, mice have only molars and incisors separated by a toothless region called a diastema. Although tooth buds form in the embryonic diastema, they regress and do not develop into teeth. Here, we identify members of the Sprouty (Spry) family, which encode negative feedback regulators of fibroblast growth factor (FGF) and other receptor tyrosine kinase signaling, as genes that repress diastema tooth development. We show that different Sprouty genes are deployed in different tissue compartments--Spry2 in epithelium and Spry4 in mesenchyme--to prevent diastema tooth formation. We provide genetic evidence that they function to ensure that diastema tooth buds are refractory to signaling via FGF ligands that are present in the region and thus prevent these buds from engaging in the FGF-mediated bidirectional signaling between epithelium and mesenchyme that normally sustains tooth development.

The mammary bud as a skin appendage: unique and shared aspects of development

Like other skin appendages, the embryonic mammary gland develops via extensive epithelial-mesenchymal interactions. Early stages in embryonic mammary development strikingly resemble analogous steps in the development of hair follicles and teeth. In each case the first morphological sign of development is a localized thickening in the surface epithelium that subsequently invaginates to form a mammary, hair follicle or tooth bud. Similar sets of intersecting signaling pathways are involved in patterning the mammary, hair follicle and dental epithelium, directing placode formation, and controlling bud invagination. Despite these similarities, subsequent events in the formation of these appendages are diverse. The mammary bud extends to form a sprout that begins to branch upon contact with the mammary fat pad. Hair follicles also extend into the underlying mesenchyme, but instead of branching, hair follicle epithelium folds around a condensation of dermal cells. In contrast, teeth undergo a more complex folding morphogenesis. Here, we review what is known of the molecular and cellular mechanisms controlling early steps in the development of these organs, attempt to unravel both common themes and unique aspects that can begin to explain the diversity of appendage formation, and discuss human genetic diseases that affect appendage morphogenesis.

Gli3-mediated somitic Fgf10 expression gradients are required for the induction and patterning of mammary epithelium along the embryonic axes

Little is known about the regulation of cell fate decisions that lead to the formation of five pairs of mammary placodes in the surface ectoderm of the mouse embryo. We have previously shown that fibroblast growth factor 10 (FGF10) is required for the formation of mammary placodes 1, 2, 3 and 5. Here, we have found that Fgf10 is expressed only in the somites underlying placodes 2 and 3, in gradients across and within these somites. To test whether somitic FGF10 is required for the formation of these two placodes, we analyzed a number of mutants with different perturbations of somitic Fgf10 gradients for the presence of WNT signals and ectodermal multilayering, markers for mammary line and placode formation. The mammary line is displaced dorsally, and formation of placode 3 is impaired in Pax3ILZ/ILZ mutants, which do not form ventral somitic buds. Mammary line formation is impaired and placode 3 is absent in Gli3Xt-J/Xt-J and hypomorphic Fgf10 mutants, in which the somitic Fgf10 gradient is shortened dorsally and less overall Fgf10 is expressed, respectively. Recombinant FGF10 rescued mammogenesis in Fgf10(-/-) and Gli3Xt-J/Xt-J flanks. We correlate increasing levels of somitic FGF10 with progressive maturation of the surface ectoderm, and show that full expression of somitic Fgf10, co-regulated by GLI3, is required for the anteroposterior pattern in which the flank ectoderm acquires a mammary epithelial identity. We propose that the intra-somitic Fgf10 gradient, together with ventral elongation of the somites, determines the correct dorsoventral position of mammary epithelium along the flank.

Microarray analysis during adipogenesis identifies new genes altered by antiretroviral drugs

OBJECTIVE

To elucidate the pathogenesis of HAART-associated lipodystrophy, by investigating the effects of antiretroviral drugs on adipocyte differentiation and gene expression profile.

DESIGN AND METHODS

Analysis of gene expression profile by DNA microarrays and quantitative RT-PCR of 3T3-L1 preadipocytes treated with the nucleoside reverse transcriptase inhibitors (NRTI) lamivudine, zidovudine, stavudine, and zalcitabine, and with the protease inhibitors (PI) indinavir, saquinavir, and lopinavir during maturation into adipocytes.

RESULTS

Under standard adipogenic differentiation protocols, PI significantly inhibited adipocyte differentiation, as demonstrated by cell viability assay and Oil Red O staining and quantification, whereas NRTI had mild effects on adipogenesis. Gene expression profile analysis showed that treatment with NRTI modulated the expression of transcription factors, such as Aebp1, Pou5f1 and Phf6, which could play a key role in the determination of the adipocyte phenotype. PI also modulated gene expression toward inhibition of adipocyte differentiation, with up-regulation of the Wnt signaling gene Wnt10a and down-regulation of the expression of genes encoding master adipogenic transcription factors (e.g., C/EBPalpha and PPARgamma), oestrogen receptor beta, and adipocyte-specific markers (e.g., Adiponectin, Leptin, Mrap, Cd36, S100A8).

CONCLUSIONS

This study identifies new genes modulated by PI and NRTI in differentiating adipocytes. Abnormal expression of these genes, which include master adipogenic transcription factors and genes involved in lipid metabolism and cell cycle control, could contribute to the understanding of the pathogenesis of HAART-associated lipodystrophy.

FGF-4 regulates neural progenitor cell proliferation and neuronal differentiation

The FGF-4 (fibroblast growth factor 4, known as HST-1) protein is an important mitogen for a variety of cell types. However, only limited information is available concerning tissue distribution and the biological role of FGF-4 in the brain. In situ hybridization analysis revealed localization of mouse Fgf-4 mRNA in the normal postnatal mouse hippocampus, subventricular zone (SVZ), and the rostral migratory stream where new neurons generate, migrate, and become incorporated into the functional circuitry of the brain. We also investigated whether FGF-4 could promote both proliferation and differentiation of the neural progenitor cells by using an in vitro neurosphere assay. The addition of recombinant FGF-4 generated large proliferative spheres that have a multipotent differentiation ability. Furthermore, recombinant FGF-4 significantly promotes neuronal differentiation in attached clonal neurosphere culture. These findings suggest that FGF-4 has an ability to promote neural stem cell proliferation and neuronal differentiation in the postnatal brain.

Lef1 is required for the transition of Wnt signaling from mesenchymal to epithelial cells in the mouse embryonic mammary gland

Inductive reciprocal signaling between mesenchymal and adjacent epithelia gives rise to skin appendages such as hair follicles and mammary glands. Lef1-mediated canonical Wnt signaling is required for morphogenesis of these skin appendages during embryogenesis. In order to define the role of canonical Wnt signaling during early embryonic mammary gland development, we determined the temporal and spatial changes in Wnt signaling during embryogenesis in wild-type and Lef1-deficient embryos harboring a Tcf/Lef1-betagal reporter (TOPGAL) transgene. In contrast to previous studies using TOPGAL mice from a distinct founder, we observe that Wnt signaling acts initially on mesenchymal cells associated with the sequential appearance of mammary placodes. As placode development progresses between 12.5 and 15.5 dpc, Wnt signaling progressively accumulates in the mammary epithelial compartment. By 18.5 dpc, betagal activity is confined to mesenchymal and epithelial cells near the nipple region. In Lef1-deficient embryos, the transition of Wnt signaling from mesenchyme to the mammary epithelia is blocked for placodes #1, 4 and 5 despite the expression of Tcf1 in epithelial cells. These placodes ultimately disappear by 15.5 dpc, while placodes 2 and 3 typically did not form in the absence of Lef1. Progressive loss of placodes 1, 4, and 5 is accompanied by increased apoptosis in mesenchymal cells adjacent to the mammary epithelial placodes. While factors important for embryonic mammary gland development, such as FGF7, are expressed normally in Lef1-deficient animals, one mediator of the Hedgehog (Hh)-signaling pathway is aberrantly expressed. Specifically, Shh, Ihh, and Gli2 are expressed in mammary epithelial cells at levels in Lef1-deficient animals similar to wild-type littermates. However, the signal for Ptc-1 is strongly reduced in mesenchymal cells surrounding the mammary placode in Lef1 mutants relative to wild-type embryos. The loss of Ptc-1, both a receptor for and transcriptional target of Hh signaling, suggests that Hh signaling is blocked in Lef1-deficient embryos. Thus, these data reveal distinct requirements of different mammary placodes for Lef1-dependent Wnt signaling. They further define dynamic changes in which cells integrate Lef1-dependent Wnt signaling during progression of embryonic mammary gland development.

Fibroblast growth factor 9 has oncogenic activity and is a downstream target of Wnt signaling in ovarian endometrioid adenocarcinomas

Wnt signaling plays a key role in development and adult tissues via effects on cell proliferation, motility, and differentiation. The cellular response to Wnt ligands largely depends on their ability to stabilize beta-catenin and the ability of beta-catenin to bind and activate T-cell factor (TCF) transcription factors. Roughly 40% of ovarian endometrioid adenocarcinomas (OEA) have constitutive activation of Wnt signaling as a result of oncogenic mutations in the beta-catenin protein or inactivating mutations in key negative regulators of beta-catenin, such as the adenomatous polyposis coli and Axin tumor suppressor proteins. We used oligonucleotide microarrays to identify genes of which expression was activated in OEAs with beta-catenin dysregulation compared with OEAs lacking Wnt/beta-catenin pathway defects. Using microarray and quantitative PCR-based approaches, we found that fibroblast growth factor (FGF9) expression was increased >6-fold in primary OEAs with Wnt/beta-catenin pathway defects compared with OEAs lacking such defects. Evidence that beta-catenin and TCFs regulate FGF9 expression in several epithelial cell lines was obtained. We found FGF9 was mitogenic for epithelial cells and fibroblasts and FGF9 could stimulate invasion of epithelial and endothelial cells through Matrigel in transwell assays. Furthermore, FGF9 could promote neoplastic transformation of the E1A-immortalized RK3E epithelial cell line, and short hairpin RNA-mediated inhibition of endogenous FGF9 expression in the OEA cell line TOV112D, which carries a beta-catenin mutation, inhibited neoplastic growth properties of the cells. Our findings support the notion that FGF9 is a key factor contributing to the cancer phenotype of OEAs carrying Wnt/beta-catenin pathway defects.

Down-regulation of stem cell genes, including those in a 200-kb gene cluster at 12p13.31, is associated with in vivo differentiation of human male germ cell tumors

Adult male germ cell tumors (GCTs) comprise distinct groups: seminomas and nonseminomas, which include pluripotent embryonal carcinomas as well as other histologic subtypes exhibiting various stages of differentiation. Almost all GCTs show 12p gain, but the target genes have not been clearly defined. To identify 12p target genes, we examined Affymetrix (Santa Clara, CA) U133A+B microarray ( approximately 83% coverage of 12p genes) expression profiles of 17 seminomas, 84 nonseminoma GCTs, and 5 normal testis samples. Seventy-three genes on 12p were significantly overexpressed, including GLUT3 and REA (overexpressed in all GCTs) and CCND2 and FLJ22028 (overexpressed in all GCTs, except choriocarcinomas). We characterized a 200-kb gene cluster at 12p13.31 that exhibited coordinated overexpression in embryonal carcinomas and seminomas, which included the known stem cell genes NANOG, STELLA, and GDF3 and two previously uncharacterized genes. A search for other coordinately regulated genomic clusters of stem cell genes did not reveal any genomic regions similar to that at 12p13.31. Comparison of embryonal carcinoma with seminomas revealed relative overexpression of several stem cell-associated genes in embryonal carcinoma, including several core "stemness" genes (EBAF, TDGF1, and SOX2) and several downstream targets of WNT, NODAL, and FGF signaling (FGF4, NODAL, and ZFP42). Our results indicate that 12p gain is a functionally relevant change leading to activation of proliferation and reestablishment/maintenance of stem cell function through activation of key stem cell genes. Furthermore, the differential expression of core stem cell genes may explain the differences in pluripotency between embryonal carcinomas and seminomas.

Tissue interactions in the regulation of axon pathfinding during tooth morphogenesis

Like many other organs, the tooth develops as a result of the epithelial-mesenchymal interactions. In addition, the tooth is a well-defined peripheral target organ for sensory trigeminal nerves, which are required for the function and protection of the teeth. Dental trigeminal axon growth and patterning are tightly linked with advancing tooth morphogenesis and cell differentiation. This review summarizes recent findings on the regulation of dental axon pathfinding, which have provided evidence that the development of tooth trigeminal innervation is controlled by epithelial-mesenchymal interactions. The early dental epithelium possesses the information to instruct tooth nerve supply, and signals mediating these interactions are part of the signaling networks regulating tooth morphogenesis. Tissue interactions, thus, appear to provide a central mechanism of spatiotemporally orchestrating tooth formation and dental axon navigation and patterning.

The genetic basis of tooth development and dental defects

More than 300 genes have so far been associated with tooth development, mainly in mouse embryos. The majority of them are associated with conserved signaling pathways mediating cellular communication, in particular between epithelial and mesenchymal tissues. Necessary functions of many signals, receptors and transcription factors have been demonstrated in mice, and mutations causing dental defects in humans have been identified in several genes.

Gene trap screening as an effective approach for identification of Wnt-responsive genes in the mouse embryo

In this study, we examined whether gene trap methodology, which would be available for systematic identification and functional analysis of genes, is effective for screening of Wnt-responsive genes during mouse development. We screened out two individual clones among 794 gene-trapped embryonic stem cell lines by their in vitro response to WNT-3A proteins. One gene was mainly expressed in the ductal epithelium of several developing organs, including the kidney and the salivary glands, and the other gene was expressed in neural crest cells and the telencephalic flexure. The spatial and temporal expression of these two genes coincided well with that of several Wnt genes. Furthermore, the expression of these two genes was significantly decreased in embryos deficient for Wnts or in cultures of embryonic tissues treated with a Wnt signal inhibitor. These results indicate that the gene trap is an effective method for systematic identification of Wnt-responsive genes during embryogenesis.

Wnt signaling in lymphopoiesis

Wnt signaling elicits changes in gene expression and cell physiology through beta-catenin and LEF1/TCF proteins. The signal transduction pathway regulates many cellular and developmental processes, including cell proliferation, cell fate decisions and differentiation. In cells that have been stimulated by a Wnt protein, cytoplasmic beta-catenin is stabilized and transferred to the nucleus, where it interacts with the nuclear mediators of Wnt signaling, the LEF1/TCF proteins, to elicit a transcriptional response. Loss-of-function and gain-of-function experiments in the mouse have provided insight into the role of this signaling pathway in lymphopoiesis. The self-renewal and maintenance of hematopoietic stem cells is regulated by Wnt signals. Differentiation of T cells and natural killer cells is blocked in the absence of LEF1/TCF proteins, and pro-B cell proliferation is regulated by Wnt signaling.

Epidermal growth factor receptor: a novel target of the Wnt/beta-catenin pathway in liver

BACKGROUND & AIMS

Wnt/beta-catenin activation is observed in normal liver development, regeneration, and liver cancer. Our aim was to elucidate the regulation and mechanism of this pathway in liver.

METHODS

We report the generation and characterization of liver-specific nonmutated beta-catenin-overexpressing transgenic mice. Transgenic livers were examined for their morphology and phenotype by histology, proliferation, apoptosis, and microarray analysis.

RESULTS

Transgenic livers displayed a significant increase in cytoplasmic, membranous, and nuclear beta-catenin in hepatocytes as compared with their wild-type littermates, which display a predominant membranous localization only. A 15%-20% increase in the liver weight-body weight ratio was evident in transgenic mice secondary to increased hepatocyte proliferation. Microarray analysis showed differential expression of approximately 400 genes in the transgenic livers. Epidermal growth factor receptor RNA and protein and increased levels of activated epidermal growth factor receptor and Stat3 were observed in the transgenic livers. Epidermal growth factor receptor promoter analysis showed a T-cell factor-binding site, and subsequent reporter assay confirmed epidermal growth factor receptor activation in response to Wnt-3A treatment that was abrogated by frizzled related protein 1, a known Wnt antagonist. Epidermal growth factor receptor inhibition successfully decreased liver size in transgenic mice. Next, 7 of 10 hepatoblastomas displayed simultaneous beta-catenin and epidermal growth factor receptor up-regulation, thus suggesting a strong relationship between these 2 proteins in tumors.

CONCLUSIONS

beta-Catenin transgenic mice show an in vivo hepatotrophic effect secondary to increased basal hepatocyte proliferation. Epidermal growth factor receptor seems to be a direct target of the pathway, and epidermal growth factor receptor activation might contribute toward some mitogenic effects of increased beta-catenin in liver: epidermal growth factor receptor inhibition might be useful in such states.

The role of Axin2 in calvarial morphogenesis and craniosynostosis

Axin1 and its homolog Axin2/conductin/Axil are negative regulators of the canonical Wnt pathway that suppress signal transduction by promoting degradation of beta-catenin. Mice with deletion of Axin1 exhibit defects in axis determination and brain patterning during early embryonic development. We show that Axin2 is expressed in the osteogenic fronts and periosteum of developing sutures during skull morphogenesis. Targeted disruption of Axin2 in mice induces malformations of skull structures, a phenotype resembling craniosynostosis in humans. In the mutants, premature fusion of cranial sutures occurs at early postnatal stages. To elucidate the mechanism of craniosynostosis, we studied intramembranous ossification in Axin2-null mice. The calvarial osteoblast development is significantly affected by the Axin2 mutation. The Axin2 mutant displays enhanced expansion of osteoprogenitors, accelerated ossification, stimulated expression of osteogenic markers and increases in mineralization. Inactivation of Axin2 promotes osteoblast proliferation and differentiation in vivo and in vitro. Furthermore, as the mammalian skull is formed from cranial skeletogenic mesenchyme, which is derived from mesoderm and neural crest, our data argue for a region-specific effect of Axin2 on neural crest dependent skeletogenesis. The craniofacial anomalies caused by the Axin2 mutation are mediated through activation of beta-catenin signaling, suggesting a novel role for the Wnt pathway in skull morphogenesis.

Defining the impact of beta-catenin/Tcf transactivation on epithelial stem cells

Wnt signaling has been implicated in stem cell (SC) biology, but little is known about how stabilized beta-catenin functions within native SC niches. We address this by defining the impact of beta-catenin stabilization on maintenance, proliferation, and lineage commitment of multipotent follicle SCs when in their native niche and in culture. We employ gain of function mutations and inducible loss of function mutations to demonstrate that beta-catenin stabilization is essential for promoting the transition between SC quiescence and conversion to proliferating transit amplifying (TA) progeny. We transcriptionally profile purified SCs isolated directly from wild-type and elevated beta-catenin follicles in both resting and activated states to uncover the discrete set of genes whose expression in native SCs is dependent upon beta-catenin stabilization. Finally, we address the underlying mechanism and show that in the SC niche, Wnt signaling and beta-catenin stabilization transiently activate Lef1/Tcf complexes and promote their binding to target genes that promote TA cell conversion and proliferation to form the activated cells of the newly developing hair follicle. We also show that these changes precede subsequent Wnt signals that impact on the TA progeny to specify the differentiation lineages of the follicle.

Common mutations of beta-catenin in adamantinomatous craniopharyngiomas but not in other tumours originating from the sellar region

Dysregulation of the Wnt signalling pathway contributes to developmental abnormalities and carcinogenesis of solid tumours. Here, we examined beta-catenin and adenomatous polyposis coli (APC) by mutational analysis in pituitary adenomas (n=60) and a large series of craniopharyngiomas (n=41). Furthermore, the expression pattern of beta-catenin was immunohistochemically analysed in a cohort of tumours and cysts of the sellar region including pituitary adenomas (n=58), craniopharyngiomas (n=57), arachnoidal cysts (n=8), Rathke's cleft cysts (n=10) and xanthogranulomas (n=6). Whereas APC mutations were not detectable in any tumour entity, beta-catenin mutations were present in 77% of craniopharyngiomas, exclusively of the adamantinomatous subtype. All mutations affected exon 3, which encodes the degradation targeting box of beta-catenin compatible with an accumulation of nuclear beta-catenin protein. In addition, a novel 81-bp deletion of this exonic region was detected in one case. Immunohistochemical analysis confirmed a shift from membrane-bound to nuclear accumulation of beta-catenin in 94% of the adamantinomatous tumours. Aberrant distribution patterns of beta-catenin were never observed in the other tumour entities under study. We conclude that beta-catenin mutations and/or nuclear accumulation serve as diagnostic hallmarks of the adamantinomatous variant, setting it apart from the papillary variant of craniopharyngioma.

Expression of enamel proteins and LEF1 in adamantinomatous craniopharyngioma: evidence for its odontogenic epithelial differentiation

AIMS

Adamantinomatous craniopharyngioma (ACP) resembles histologically some odontogenic tumours, such as ameloblastoma and calcifying odontogenic cyst. However, there has been no evidence that ACP differentiates also functionally as odontogenic epithelium. The aim of this study was to gain evidence of odontogenic epithelial differentiation in ACP by means of immunohistochemistry. Among normal human tissues, enamel proteins are expressed exclusively in teeth, and lymphoid enhancer factor 1 (LEF1), in co-operation with beta-catenin, play an important role in tooth development. The expression of these proteins is therefore indicative of odontogenic epithelial differentiation.

METHODS AND RESULTS

The expression of enamel proteins and LEF1 was examined in 10 adamantinomatous and six papillary craniopharyngiomas. All the ACPs showed a variable degree of enamel protein expression, including amelogenin, enamelin and enamelysin, mainly in ghost cells. LEF1 was also heterogeneously expressed in ACPs; remarkably, its expression pattern was identical to that of nuclear beta-catenin accumulation. In contrast, none of the papillary craniopharyngiomas expressed enamel proteins or LEF1.

CONCLUSIONS

These results suggest that ACP consistently shows odontogenic epithelial differentiation. Since ACPs harbour beta-catenin mutation, the inappropriate activation of beta-catenin/LEF1 complex-dependent transcription may play a critical role in ACP tumorigenesis.

Sox2 induction by FGF and FGFR2 activating mutations inhibits Wnt signaling and osteoblast differentiation

Activating mutations in fibroblast growth factor receptor 2 (FGFR2) cause several craniosynostosis syndromes by affecting the proliferation and differentiation of osteoblasts, which form the calvarial bones. Osteoblasts respond to FGF with increased proliferation and inhibition of differentiation. We analyzed the gene expression profiles of osteoblasts expressing FGFR2 activating mutations (C342Y or S252W) and found a striking down-regulation of the expression of many Wnt target genes and a concomitant induction of the transcription factor Sox2. Most of these changes could be reproduced by treatment of osteoblasts with exogenous FGF. Wnt signals promote osteoblast function and regulate bone mass. Sox2 is expressed in calvarial osteoblasts in vivo and we show that constitutive expression of Sox2 inhibits osteoblast differentiation and causes down-regulation of the expression of numerous Wnt target genes. Sox2 associates with β-catenin in osteoblasts and can inhibit the activity of a Wnt responsive reporter plasmid through its COOH-terminal domain. Our results indicate that FGF signaling could control many aspects of osteoblast differentiation through induction of Sox2 and regulation of the Wnt–β-catenin pathway.

LEF1 is a critical epithelial survival factor during tooth morphogenesis

LEF1 is a cell-type-specific transcription factor and mediates Wnt signaling pathway by association with its co-activator beta-catenin. Wnt signaling is known to be critical for the specification of cranial neural crest (CNC) cells and may regulate the fate diversity of the CNC during craniofacial morphogenesis. Loss of Lef1 results in arrested tooth development at the late bud stage and LEF1 is required for a relay of a Wnt signaling to a cascade of FGF signaling activities to mediate the epithelial-mesenchymal interaction during tooth morphogenesis. It remains unclear, however, what is the cellular mechanism of LEF1 signaling in regulating tooth morphogenesis. To test the hypothesis that LEF1 signaling regulates the fate of the dental epithelial and the CNC-derived mesenchymal cells during tooth morphogenesis, we investigated and compared the cellular migration, proliferation, and apoptotic activity within the tooth germ between the wild-type and Lef1 null mutant mice. Using the Wnt1-Cre/R26R transgenic system for indelibly marking the progenies of CNC cells, we show that there is no CNC migration defect in the Lef1 null mutant mice, indicating that the arrest in tooth development is not the result of shortage of the CNC contribution into the first branchial arch in the Lef1 mutant. Furthermore, there is no alteration in cell proliferation or condensation of the CNC-derived dental mesenchyme in the Lef1 null mutant, suggesting that LEF1 may not affect the cell cycle progression of the multipotential CNC cells during tooth morphogenesis. Importantly, apoptotic activity is significantly increased within the dental epithelium in the Lef1 null mutant mice. As the result of this increased cell death, the bud stage tooth germ fails to advance to the cap stage in the absence of Lef1. Inhibition of apoptotic activity by FGF4 rescues the tooth development in the Lef1 null mutant. Our studies suggest that LEF1 is a critical survival factor for the dental epithelial cells during tooth morphogenesis.

Mechanisms underlying differential responses to FGF signaling

Fibroblast growth factors (FGFs) are key regulators of several developmental processes in which cell fate and differentiation to various tissue lineages are determined. The importance of the proper spatial and temporal regulation of FGF signals is evident from human and mouse genetic studies which show that mutations leading to the dysregulation of FGF signals cause a variety of developmental disorders including dominant skeletal diseases and cancer. The FGF ligands signal via a family of receptor tyrosine kinases and, depending on the cell type or stage of maturation, produce diverse biological responses that include proliferation, growth arrest, differentiation or apoptosis. A central issue in FGF biology is to understand how these diverse cellular responses are determined and how similar signaling inputs can generate distinct patterns of gene expression that govern the specificity of the cellular response. In this review we draw upon studies from the past fifteen years and attempt to construct a molecular picture of the different levels of regulation by which such specific cellular responses could be achieved by FGF signals. We discuss whether specificity could lie in the nature of the ligand, the particular receptor, the signal transduction pathways utilized, or the transcriptional regulation of specific genes. Finally, we also discuss how the interplay of FGF signals with other signaling systems could contribute to the cellular response. In particular we focus on the interaction with the Wnt pathway since FGF/Wnt cross-talk is emerging as an important nexus in regulating a variety of biological processes.

Dynamic expression of Wnt signaling-related Dickkopf1, -2, and -3 mRNAs in the developing mouse tooth

Wnt signaling is essential for tooth formation. Members of the Dickkopf (Dkk) family modulate the Wnt signaling pathway by binding to the Wnt receptor complex. Comparison of Dkk1, -2, and -3 mRNA expression during mouse tooth formation revealed that all three genes showed distinct spatiotemporally regulated expression patterns. Dkk1 was prominently expressed in the distal, incisor-bearing mesenchyme area of the mandibular process during the initial stages of tooth formation. During molar morphogenesis Dkk1 was detected in the dental mesenchyme, including the preodontoblasts. Dkk2 was seen in the dental papilla, whereas Dkk3 was specifically expressed in the putative epithelial signaling centers, the primary and secondary enamel knots. Postnatally, Dkk1 was prominently expressed in the preodonto- and odontoblasts, while Dkk3 mRNAs were transiently seen in the preameloblasts before the onset of enamel matrix secretion. These results suggest that modulation of Wnt-signaling by Dkks may serve important functions in patterning of dentition as well as in crown morphogenesis and dental hard-tissue formation.

Coordination of trigeminal axon navigation and patterning with tooth organ formation: epithelial-mesenchymal interactions, and epithelial Wnt4 and Tgfbeta1 regulate semaphorin 3a expression in the dental mesenchyme

During development, trigeminal nerve fibers navigate and establish their axonal projections to the developing tooth in a highly spatiotemporally controlled manner. By analyzing Sema3a and its receptor Npn1 knockout mouse embryos, we found that Sema3a regulates dental trigeminal axon navigation and patterning, as well as the timing of the first mandibular molar innervation, and that the effects of Sema3a appear to be mediated by Npn1 present in the axons. By performing tissue recombinant experiments and analyzing the effects of signaling molecules, we found that early oral and dental epithelia, which instruct tooth formation, and epithelial Wnt4 induce Sema3a expression in the presumptive dental mesenchyme before the arrival of the first dental nerve fibers. Later, at the bud stage, epithelial Wnt4 and Tgfbeta1 regulate Sema3a expression in the dental mesenchyme. In addition, Wnt4 stimulates mesenchymal expression of Msx1 transcription factor, which is essential for tooth formation, and Tgfbeta1 proliferation of the dental mesenchymal cells. Thus, epithelial-mesenchymal interactions control Sema3a expression and may coordinate axon navigation and patterning with tooth formation. Moreover, our results suggest that the odontogenic epithelium possesses the instructive information to control the formation of tooth nerve supply.

LEF1-mediated regulation of Delta-like1 links Wnt and Notch signaling in somitogenesis

Wnt signaling, which is mediated by LEF1/TCF transcription factors, has been placed upstream of the Notch pathway in vertebrate somitogenesis. Here, we examine the molecular basis for this presumed hierarchy and show that a targeted mutation of Lef1, which abrogates LEF1 function and impairs the activity of coexpressed TCF factors, affects the patterning of somites and the expression of components of the Notch pathway. LEF1 was found to bind multiple sites in the Dll1 promoter in vitro and in vivo. Moreover, mutations of LEF1-binding sites in the Dll1 promoter impair expression of a Dll1-LacZ transgene in the presomitic mesoderm. Finally, the induced expression of LEF1-beta-catenin activates the expression of endogenous Dll1 in fibroblastic cells. Thus, Wnt signaling can affect the Notch pathway by a LEF1-mediated regulation of Dll1.

FGF-20 and DKK1 are transcriptional targets of beta-catenin and FGF-20 is implicated in cancer and development

beta-catenin is the major effector of the canonical Wnt signaling pathway. Mutations in components of the pathway that stabilize beta-catenin result in augmented gene transcription and play a major role in many human cancers. We employed microarrays to identify transcriptional targets of deregulated beta-catenin in a human epithelial cell line (293) engineered to produce mutant beta-catenin and in ovarian endometrioid adenocarcinomas characterized with respect to mutations affecting the Wnt/beta-catenin pathway. Two genes strongly induced in both systems-FGF20 and DKK1-were studied in detail. Elevated levels of FGF20 RNA were also observed in adenomas from mice carrying the Apc(Min)allele. Both XFGF20 and Xdkk-1 are expressed early in Xenopus embryogenesis under the control of the Wnt signaling pathway. Furthermore, FGF20 and DKK1 appear to be direct targets for beta-catenin/TCF transcriptional regulation via LEF/TCF-binding sites. Finally, by using small inhibitory RNAs specific for FGF20, we show that continued expression of FGF20 is necessary for maintenance of the anchorage-independent growth state in RK3E cells transformed by beta-catenin, implying that FGF-20 may be a critical element in oncogenesis induced by the Wnt signaling pathway.

Fgf signaling is required for zebrafish tooth development

We have investigated fibroblast growth factor (FGF) signaling during the development of the zebrafish pharyngeal dentition with the goal of uncovering novel roles for FGFs in tooth development as well as phylogenetic and topographic diversity in the tooth developmental pathway. We found that the tooth-related expression of several zebrafish genes is similar to that of their mouse orthologs, including both epithelial and mesenchymal markers. Additionally, significant differences in gene expression between zebrafish and mouse teeth are indicated by the apparent lack of fgf8 and pax9 expression in zebrafish tooth germs. FGF receptor inhibition with SU5402 at 32 h blocked dental epithelial morphogenesis and tooth mineralization. While the pharyngeal epithelium remained intact as judged by normal pitx2 expression, not only was the mesenchymal expression of lhx6 and lhx7 eliminated as expected from mouse studies, but the epithelial expression of dlx2a, dlx2b, fgf3, and fgf4 was as well. This latter result provides novel evidence that the dental epithelium is a target of FGF signaling. However, the failure of SU5402 to block localized expression of pitx2 suggests that the earliest steps of tooth initiation are FGF-independent. Investigations of specific FGF ligands with morpholino antisense oligonucleotides revealed only a mild tooth shape phenotype following fgf4 knockdown, while fgf8 inhibition revealed only a subtle down-regulation of dental dlx2b expression with no apparent effect on tooth morphology. Our results suggest redundant FGF signals target the dental epithelium and together are required for dental morphogenesis. Further work will be required to elucidate the nature of these signals, particularly with respect to their origins and whether they act through the mesenchyme.

WNT and beta-catenin signalling: diseases and therapies

WNT signalling has been studied primarily in developing embryos, in which cells respond to WNTs in a context-dependent manner through changes in survival and proliferation, cell fate and movement. But WNTs also have important functions in adults, and aberrant signalling by WNT pathways is linked to a range of diseases, most notably cancer. What is the full range of diseases that involve WNT pathways? Can inhibition of WNT signalling form the basis of an effective therapy for some cancers? Could activation of WNT signalling provide new therapies for other clinical conditions? Finally, on the basis of recent experiments, might WNTs normally participate in self-renewal, proliferation or differentiation of stem cells? If so, altering WNT signalling might be beneficial to the use of stem cells for therapeutic means.

Interactions between FGF and Wnt signals and Tbx3 gene expression in mammary gland initiation in mouse embryos

Interactions between Wnts, Fgfs and Tbx genes are involved in limb initiation and the same gene families have been implicated in mammary gland development. Here we explore how these genes act together in mammary gland initiation. We compared expression of Tbx3, the gene associated with the human condition ulnar-mammary syndrome, expression of the gene encoding the dual-specificity MAPK phosphatase Pyst1/MKP3, which is an early response to FGFR1 signalling (as judged by sensitivity to the SU5402 inhibitor), and expression of Lef1, encoding a transcription factor mediating Wnt signalling and the earliest gene so far known to be expressed in mammary gland development. We found that Tbx3 is expressed earlier than Lef1 and that Pyst1 is also expressed early but only transiently. Patterns of expression of Tbx3, Pyst1 and Lef1 in different glands suggest that the order of mammary gland initiation is 3, 4, 1, 2 and 5. Consistent with expression of Pyst1 in the mammary gland, we detected expression of Fgfr1b, Fgf8 and Fgf9 in both surface ectoderm and mammary bud epithelium, and Fgf4 and Fgf17 in mammary bud epithelium. Beads soaked in FGF-8 applied to the flank of mouse embryos, at a stage just prior to mammary bud initiation, induce expression of Pyst1 and Lef1 and maintain Tbx3 expression in flank tissue surrounding the bead. Grafting beads soaked in the FGFR1 inhibitor, SU5402, abolishes Tbx3, Pyst1 and Lef1 expression, supporting the idea that FGFR1 signalling is required for early mammary gland initiation. We also showed that blocking Wnt signalling abolishes Tbx3 expression but not Pyst1 expression. These data, taken together with previous findings, suggest a model in which Tbx3 expression is induced and maintained in early gland initiation by both Wnt and Fgf signalling through FGFR1.

Fibroblast growth factor enriches the embryonic liver cultures for hepatic progenitors

Fibroblast growth factors (FGFs) play an important role in hepatic induction during development. The aim of our study was to investigate the effect of exogenous FGFs on ex vivo liver development. We begin our analysis by examining FGF signaling during early mouse liver development. Phospho-FGF receptor (Tyr653/654) was detected in embryonic day 10 (E10) to E12 livers only. Next, E10 livers were cultured in the presence of FGF1, FGF4, or FGF8 for 72 hours and examined for histology, proliferation, apoptosis, and differentiation. FGFs especially FGF8 promoted sheet-like architecture, cell proliferation, and survival as compared to the control. All FGFs induced a striking increase in the number of c-kit and alpha-fetoprotein-positive progenitors, without altering albumin staining. However these progenitors were CK-19-positive (biliary and bipotential progenitor marker) only in the presence of FGF1 or FGF4 and not FGF8. FGFs also induced beta-catenin, a stem cell renewal factor in these cultures. In conclusion, the presence of activated FGFR indicates a physiological role of FGF during early liver development. FGF1 and FGF4 enrich the embryonic liver cultures for bipotential hepatic progenitors. FGF8 promotes such enrichment and induces a one-step differentiation toward a unipotential hepatocyte progenitor. Thus, FGFs might be useful for enrichment and propagation of developmental hepatic progenitors.

A conserved enhancer element that drives FGF4 gene expression in the embryonic myotomes is synergistically activated by GATA and bHLH proteins

FGF4 is the earliest member of the fibroblast growth factor (FGF) family expressed during embryogenesis where it plays essential roles in post-implantation development and limb growth and patterning. The expression of the Fgf4 gene in specific developmental stages, including the ICM of the blastocyst, the myotomes, and the limb bud AER, is regulated by distinct enhancer elements (Hom) in the 3' UTR. We previously identified the Hom3a region as the major DNA element responsible for Fgf4 expression in the myotomes and AER, and showed that a conserved E-box is a target for the myogenic bHLH transcription factors MYF5 and MYOD. To further define the cis- and trans-acting elements that determine Hom3a activity, we conducted a mutational analysis of the ability of the Hom3a region to drive lacZ expression in the myotomes of transgenic mice. We identified a minimal enhancer of 226nt that contains four elements, including the E-box, necessary to drive gene expression in the myotomes. One of these elements is a binding site for the GATA family of transcription factors, and we show here that GATA 1-4 and 6 can synergize with MYF5 or MYOD to activate transcription of a reporter plasmid driven by a portion of the Hom3a enhancer including the GATA site and the E-box. In line with this finding, we could show a direct interaction between MYF5/MYOD and GATA-3 or GATA-4, mediated by the N-terminal and bHLH domains of MYF5/MYOD and the C-terminal zing finger domain of GATA-3/4. To further study the role of the Hom3a enhancer in directing Fgf4 expression and the function of FGF4 in limb and muscle development, we generated mutant mice in which the Fgf4 Hom3a region had been deleted (Delta3a). In situ hybridization analysis of sections from Delta3a/ Delta3a embryos at E11.5 showed a drastically reduced expression of Fgf4 mRNA in the myotomes and AER. However, these mice developed normally and show no limb or muscle defects, and the same was true of heterozygous mice in which one Fgf4 allele carried the Hom3a deletion and the other was a null allele (Delta3a/Fgf4(-)). Together, these results show that Hom3a is the major DNA enhancer element directing Fgf4 expression in myotomes and limb bud AER, and that its activity in the myotomes results at least in part from the synergistic action of GATA and bHLH myogenic factors that bind to evolutionary conserved sequences in the Hom3a enhancer. However, expression of Fgf4 in the myotomes or AER of murine embryos does not appear to be essential for muscle or limb development.

Runx2 mediates FGF signaling from epithelium to mesenchyme during tooth morphogenesis

Runx2 (Cbfa1) is a runt domain transcription factor that is essential for bone development and tooth morphogenesis. Teeth form as ectodermal appendages and their development is regulated by interactions between the epithelium and mesenchyme. We have shown previously that Runx2 is expressed in the dental mesenchyme and regulated by FGF signals from the epithelium, and that tooth development arrests at late bud stage in Runx2 knockout mice [Development 126 (1999) 2911]. In the present study, we have continued to clarify the role of Runx2 in tooth development and searched for downstream targets of Runx2 by extensive in situ hybridization analysis. The expression of Fgf3 was downregulated in the mesenchyme of Runx2 mutant teeth. FGF-soaked beads failed to induce Fgf3 expression in Runx2 mutant dental mesenchyme whereas in wild-type mesenchyme they induced Fgf3 in all explants indicating a requirement of Runx2 for transduction of FGF signals. Fgf3 was absent also in cultured Runx2-/- calvarial cells and it was induced by overexpression of Runx2. Furthermore, Runx2 was downregulated in Msx1 mutant tooth germs, indicating that it functions in the dental mesenchyme between Msx1 and Fgf3. Shh expression was absent from the epithelial enamel knot in lower molars of Runx2 mutant and reduced in upper molars. However, other enamel knot marker genes were expressed normally in mutant upper molars, while reduced or missing in lower molars. These differences between mutant upper and lower molars may be explained by the substitution of Runx2 function by Runx3, another member of the runt gene family that was upregulated in upper but not lower molars of Runx2 mutants. Shh expression in mutant enamel knots was not rescued by FGFs in vitro, indicating that in addition to Fgf3, Runx2 regulates other mesenchymal genes required for early tooth morphogenesis. Also, exogenous FGF and SHH did not rescue the morphogenesis of Runx2 mutant molars. We conclude that Runx2 mediates the functions of epithelial FGF signals regulating Fgf3 expression in the dental mesenchyme and that Fgf3 may be a direct target gene of Runx2.

The presence of FGF2 signaling determines whether beta-catenin exerts effects on proliferation or neuronal differentiation of neural stem cells

Neural stem cells proliferate and maintain multipotency when cultured in the presence of FGF2, but subsequent lineage commitment by the cells is nevertheless influenced by the exposure to FGF2. Here we show that FGF2 effects on neural stem cells are mediated, in part, by beta-catenin. Conversely, the effects of beta-catenin in neural stem cells depend in part upon whether there is concurrent fibroblast growth factor (FGF) signaling. FGF2 increases beta-catenin signaling through several different mechanisms including increased expression of beta-catenin mRNA, increased nuclear translocation of beta-catenin, increased phosphorylation of GSK-3beta, and tyrosine phosphorylation of beta-catenin. Overexpression of beta-catenin in the presence of FGF2 helps to maintain neural progenitor cells in a proliferative state. However, overexpression of beta-catenin in the absence of FGF2 enhances neuronal differentiation. Further, chromatin immunoprecipitation (ChIP) assays demonstrate that both beta-catenin and Lef1 bind directly to the neurogenin promoter, and luciferase reporter assays demonstrate that beta-catenin is directly involved in the regulation of neurogenin 1 and possibly other proneural genes when neural stem cells are cultured in the presence of FGF2. We suggest that the balance between the mitogenic effects and the proneural effects of beta-catenin is determined by the presence of FGF signaling.

Immunolocalization of BMP-2/-4, FGF-4, and WNT10b in the developing mouse first lower molar

Intercellular signaling controls all steps of odontogenesis. The purpose of this work was to immunolocalize in the developing mouse molar four molecules that play major roles during odontogenesis: BMP-2, -4, FGF-4, and WNT10b. BMP-2 and BMP-4 were detected in the epithelium and mesenchyme at the bud stage. Staining for BMP-2 markedly increased at the cap stage. The relative amount of BMP-4 strongly increased from E14 to E15. At E15, BMP-4 was detected in the internal part of the enamel knot where apoptosis was intense. In contrast to TGFbeta1, BMP-2 and -4 did not show accumulation at the epithelial-mesenchymal junction where the odontoblast started differentiation. When odontoblasts became functional, BMP-2 and BMP-4 were detected at the apical and basal poles of preameloblasts. BMP-2, which induces ameloblast differentiation in vitro, may also be involved physiologically. The decrease in FGF-4 from E14 to E15 supports a possible role for the growth factor in the control of mesenchymal cell proliferation. The relative amount of FGF-4 was maximal at E17. The subsequent decrease at E19 showed correlation with the withdrawal of odontoblasts and ameloblasts from the cell cycle. WNT10b might also stimulate cell proliferation. At E14-15, WNT10b was present in the mesenchyme and epithelium except for the enamel knot, where the mitotic activity was very low. At E19 there was a decreasing gradient of staining from the cervical loop where cells divide to the tip of the cusp in the inner dental epithelium where cells become postmitotic. The target cells for FGF-4 and WNT10b appeared different.

Modulation of activin/bone morphogenetic protein signaling by follistatin is required for the morphogenesis of mouse molar teeth

Teeth form as ectodermal appendages, and their morphogenesis is regulated by conserved signaling pathways. The shape of the tooth crown results from growth and folding of inner dental epithelium, and the cusp patterning is regulated by transient signaling centers, the enamel knots. Several signal proteins in the transforming growth factor-beta (TGF beta) superfamily are required for tooth development. Follistatin is an extracellular inhibitor of TGF beta signaling. To investigate the roles of follistatin during tooth development, we analyzed in detail the expression patterns of follistatin, activin beta A, as well as Bmp2, Bmp4, and Bmp7 during tooth morphogenesis. We also examined the tooth phenotypes of follistatin knockout mice and of transgenic mice overexpressing follistatin in the epithelium under the keratin 14 (K14) promoter. The folding of the dental epithelium was aberrant in the molars of follistatin knockout mice, and the cusps were shallow with reduced cell proliferation and lack of anteroposterior polarization. The functions of both primary and secondary enamel knots were apparently disturbed. In K14-follistatin transgenic mice, the molar cusp pattern was also seriously affected (although different from the follistatin knockouts) and the occlusal surfaces of the molars were whorled. Their enamel was prematurely worn. In addition, all of the third molars were missing. Our results indicate that follistatin regulates morphogenesis and shaping of the tooth crown. We propose that finely tuned antagonistic effects between follistatin and TGF beta superfamily signals are critical for enamel knot formation and function, as well as for patterning of tooth cusps.

The neural crest: Basic biology and clinical relationships in the craniofacial and enteric nervous systems

The highly migratory, mesenchymal neural crest cell population was discovered over 100 years ago. Proposals of these cells' origin within the neuroepithelium, and of the tissues they gave rise to, initiated decades-long heated debates, since these proposals challenged the powerful germ-layer theory. Having survived this storm, the neural crest is now regarded as a pluripotent stem cell population that makes vital contributions to an astounding array of both neural and non-neural organ systems. The earliest model systems for studying the neural crest were amphibian, and these pioneering contributions have been ably refined and extended by studies in the chick, mouse, and more recently the fish to provide detailed understanding of the cellular and molecular mechanisms regulating and regulated by the neural crest. The key questions regarding control of craniofacial morphogenesis and innervation of the gut illustrate the wide range of developmental contexts in which the neural crest plays an important role. These questions also focus attention on common issues such as the role of growth factor signaling in neural crest cell development and highlight the central role of the neural crest in human congenital disease.

Identification of a secreted BMP antagonist, ectodin, integrating BMP, FGF, and SHH signals from the tooth enamel knot

We have identified mouse and human cDNAs encoding a novel secreted BMP inhibitor, which we have named ectodin. It is most homologous (approximately 37% amino acid identity) to sclerostin that is a secreted BMP antagonist. Recombinant ectodin protein produced in cultured cells was efficiently secreted as a antagonist. Ectodin inhibited the activity of BMP2, BMP4, BMP6, and BMP7 for mouse preosteoblastic MC3T3-E1 cells, and bound to these BMPs with high affinity. Ectodin is intensely expressed in developing ectodermal organs, including teeth, vibrissae, and hair follicles. However, it is absent from the hair placodes and from the enamel knot signaling centers in teeth. In addition, several cell layers surrounding the enamel knots were completely devoid of ectodin transcripts. We analyzed the regulation and function of ectodin in tooth germs. Recombinant ectodin protein antagonized the BMP-mediated induction of Msx2 expression in cultured tooth explants, indicating that ectodin is a secreted BMP inhibitor. BMP2 and BMP7 stimulated ectodin expression in tooth explants, showing that it is part of a feedback mechanism controlling the activity of BMPs. The stimulation of ectodin expression by BMP was prevented by SHH and FGF4 but not by Wnt6. Hence, the feedback mechanism whereby BMPs upregulate their own inhibitor is counteracted by signals coexpressed with BMPs in the enamel knot. We conclude that ectodin is a novel BMP inhibitor which integrates BMP signaling with the SHH and FGF signal pathways and contributes in defining the exact spatiotemporal domain of BMP target field around the ectodermal signaling centers.

Mechanisms of ectodermal organogenesis

All ectodermal organs, e.g. hair, teeth, and many exocrine glands, originate from two adjacent tissue layers: the epithelium and the mesenchyme. Similar sequential and reciprocal interactions between the epithelium and mesenchyme regulate the early steps of development in all ectodermal organs. Generally, the mesenchyme provides the first instructive signal, which is followed by the formation of the epithelial placode, an early signaling center. The placode buds into or out of the mesenchyme, and subsequent proliferation, cell movements, and differentiation of the epithelium and mesenchyme contribute to morphogenesis. The molecular signals regulating organogenesis, such as molecules in the FGF, TGFbeta, Wnt, and hedgehog families, regulate the development of all ectodermal appendages repeatedly during advancing morphogenesis and differentiation. In addition, signaling by ectodysplasin, a recently identified member of the TNF family, and its receptor Edar is required for ectodermal organ development across vertebrate species. Here the current knowledge on the molecular regulation of the initiation, placode formation, and morphogenesis of ectodermal organs is discussed with emphasis on feathers, hair, and teeth.

Thymic generation and regeneration

The thymus is a complex epithelial organ in which thymocyte development is dependent upon the sequential contribution of morphologically and phenotypically distinct stromal cell compartments. It is these microenvironments that provide the unique combination of cellular interactions, cytokines, and chemokines to induce thymocyte precursors to undergo a differentiation program that leads to the generation of functional T cells. Despite the indispensable role of thymic epithelium in the generation of T cells, the mediators of this process and the differentiation pathway undertaken by the primordial thymic epithelial cells are not well defined. There is a lack of lineage-specific cell-surface-associated markers, which are needed to characterize putative thymic epithelial stem cell populations. This review explores the role of thymic stromal cells in T-cell development and thymic organogenesis, as well as the molecular signals that contribute to the growth and expansion of primordial thymic epithelial cells. It highlights recent advances in these areas, which have allowed for a lineage relationship amongst thymic epithelial cell subsets to be proposed. While many fundamental questions remain to be addressed, collectively these works have broadened our understanding of how the thymic epithelium becomes specialized in the ability to support thymocyte differentiation. They should also facilitate the development of novel, rationally based therapeutic strategies for the regeneration and manipulation of thymic function in the treatment of many clinical conditions in which defective T cells have an important etiological role.

A Wnt-Wnt situation

A recent Juan March Foundation workshop on "wnt genes and Wnt signaling" brought developmental and cancer biologists together to share some of the latest advances in Wnt research. Discussion topics included molecular, genetic, and genomic dissections of wnt genes in embryogenesis and cancer, Wnt signaling components and downstream targets, interactions with other signaling pathways, cell biological aspects of Wnt signaling, and a first glimpse of a purified Wnt protein.