Multiple roles of mesenchymal beta-catenin during murine limb patterning

Wnt/β-catenin signaling activates bone morphogenetic protein 2 expression in osteoblasts

The BMP and Wnt/β-catenin signaling pathways cooperatively regulate osteoblast differentiation and bone formation. Although BMP signaling regulates gene expression of the Wnt pathway, much less is known about whether Wnt signaling modulates BMP expression in osteoblasts. Given the presence of putative Tcf/Lef response elements that bind β-catenin/TCF transcription complex in the BMP2 promoter, we hypothesized that the Wnt/β-catenin pathway stimulates BMP2 expression in osteogenic cells. In this study, we showed that Wnt/β-catenin signaling is active in various osteoblast or osteoblast precursor cell lines, including MC3T3-E1, 2T3, C2C12, and C3H10T1/2 cells. Furthermore, crosstalk between the BMP and Wnt pathways affected BMP signaling activity, osteoblast differentiation, and bone formation, suggesting Wnt signaling is an upstream regulator of BMP signaling. Activation of Wnt signaling by Wnt3a or overexpression of β-catenin/TCF4 both stimulated BMP2 transcription at promoter and mRNA levels. In contrast, transcription of BMP2 in osteogenic cells was decreased by either blocking the Wnt pathway with DKK1 and sFRP4, or inhibiting β-catenin/TCF4 activity with FWD1/β-TrCP, ICAT, or ΔTCF4. Using a site-directed mutagenesis approach, we confirmed that Wnt/β-catenin transactivation of BMP2 transcription is directly mediated through the Tcf/Lef response elements in the BMP2 promoter. These results, which demonstrate that the Wnt/β-catenin signaling pathway is an upstream activator of BMP2 expression in osteoblasts, provide novel insights into the nature of functional cross talk integrating the BMP and Wnt/β-catenin pathways in osteoblastic differentiation and maintenance of skeletal homeostasis.

Wnt signaling in limb organogenesis

Secreted signaling molecules of the Wnt family have been found to play a central role in controlling embryonic development of a wide range of taxa from Hydra to humans. The most extensively studied Wnt signaling pathway is the canonical Wnt pathway, which controls gene expression by stabilizing beta-catenin, and regulates a multitude of developmental processes. More recently, noncanonical Wnt pathways, which are beta-catenin-independent, have been found to be important developmental regulators. Understanding the mechanisms of Wnt signaling is essential for the development of novel preventive and therapeutic approaches of human diseases. Limb development is a paradigm to study the principles of Wnt signaling in various developmental contexts. In the developing vertebrate limb, Wnt signaling has been shown to have important functions during limb bud initiation, limb outgrowth, early limb patterning, and later limb morphogenesis events. This review provides a brief overview on the diversity of Wnt-dependent signaling events during embryonic development of the vertebrate limb.

The Polycomb group protein Ring1b is essential for pectoral fin development

Polycomb group (PcG) proteins are transcriptional repressors that mediate epigenetic gene silencing by chromatin modification. PcG-mediated gene repression is implicated in development, cell differentiation, stem-cell fate maintenance and cancer. However, analysis of the roles of PcG proteins in orchestrating vertebrate developmental programs in vivo has been hampered by the early embryonic lethality of several PcG gene knockouts in mice. Here, we demonstrate that zebrafish Ring1b, the E3 ligase in Polycomb Repressive Complex 1 (PRC1), is essential for pectoral fin development. We show that differentiation of lateral plate mesoderm (LPM) cells into presumptive pectoral fin precursors is initiated normally in ring1b mutants, but fin bud outgrowth is impaired. Fgf signaling, which is essential for migration, proliferation and cell-fate maintenance during fin development, is not sufficiently activated in ring1b mutants. Exogenous application of FGF4, as well as enhanced stimulation of Fgf signaling by overactivated Wnt signaling in apc mutants, partially restores the fin developmental program. These results reveal that, in the absence of functional Ring1b, fin bud cells fail to execute the pectoral fin developmental program. Together, our results demonstrate that PcG-mediated gene regulation is essential for sustained Fgf signaling in vertebrate limb development.

Epiprofin/Sp6 regulates Wnt-BMP signaling and the establishment of cellular junctions during the bell stage of tooth development

Epiprofin/Specificity Protein 6 (Epfn) is a Krüppel-like family (KLF) transcription factor that is critically involved in tooth morphogenesis and dental cell differentiation. However, its mechanism of action is still not fully understood. We have employed both loss-of-function and gain-of-function approaches to address the role of Epfn in the formation of cell junctions in dental cells and in the regulation of junction-associated signal transduction pathways. We have evaluated the expression of junction proteins in bell-stage incisor and molar tooth sections from Epfn(-/-) mice and in dental pulp MDPC-23 cells overexpressing Epfn. In Epfn(-/-) mice, a dramatic reduction occurs in the expression of tight junction and adherens junction proteins and of the adherens-junction-associated β-catenin protein, a major effector of canonical Wnt signaling. Loss of cell junctions and β-catenin in Epfn(-/-) mice is correlated with a clear decrease in bone morphogenetic protein 4 (BMP-4) expression, a decrease in nestin in the tooth mesenchyme, altered cell proliferation, and failure of ameloblast cell differentiation. Overexpression of Epfn in MDPC-23 cells results in an increased cellular accumulation of β-catenin protein, indicative of upregulation of canonical Wnt signaling. Together, these results suggest that Epfn enhances canonical Wnt/β-catenin signaling in the developing dental pulp mesenchyme, a condition that promotes the activity of other downstream signaling pathways, such as BMP, which are fundamental for cellular induction and ameloblast differentiation. These altered signaling events might underlie some of the most prominent dental defects observed in Epfn(-/-) mice, such as the absence of ameloblasts and enamel, and might throw light on developmental malformations of the tooth, including hyperdontia.

Cartilage-specific β-catenin signaling regulates chondrocyte maturation, generation of ossification centers, and perichondrial bone formation during skeletal development

The WNT/β-catenin signaling pathway is a critical regulator of chondrocyte and osteoblast differentiation during multiple phases of cartilage and bone development. Although the importance of β-catenin signaling during the process of endochondral bone development has been previously appreciated using a variety of genetic models that manipulate β-catenin in skeletal progenitors and osteoblasts, genetic evidence demonstrating a specific role for β-catenin in committed growth-plate chondrocytes has been less robust. To identify the specific role of cartilage-derived β-catenin in regulating cartilage and bone development, we studied chondrocyte-specific gain- and loss-of-function genetic mouse models using the tamoxifen-inducible Col2Cre(ERT2) transgene in combination with β-catenin(fx(exon3)/wt) or β-catenin(fx/fx) floxed alleles, respectively. From these genetic models and biochemical data, three significant and novel findings were uncovered. First, cartilage-specific β-catenin signaling promotes chondrocyte maturation, possibly involving a bone morphogenic protein 2 (BMP2)-mediated mechanism. Second, cartilage-specific β-catenin facilitates primary and secondary ossification center formation via the induction of chondrocyte hypertrophy, possibly through enhanced matrix metalloproteinase (MMP) expression at sites of cartilage degradation, and potentially by enhancing Indian hedgehog (IHH) signaling activity to recruit vascular tissues. Finally, cartilage-specific β-catenin signaling promotes perichondrial bone formation possibly via a mechanism in which BMP2 and IHH paracrine signals synergize to accelerate perichondrial osteoblastic differentiation. The work presented here supports the concept that the cartilage-derived β-catenin signal is a central mediator for major events during endochondral bone formation, including chondrocyte maturation, primary and secondary ossification center development, vascularization, and perichondrial bone formation.

Disruption of a Sox9-β-catenin circuit by mutant Fgfr3 in thanatophoric dysplasia type II

Mutations in fibroblast growth factor (FGF) receptors are responsible for a variety of skeletal birth defects, but the underlying mechanisms responsible remain unclear. Using a mouse model of thanatophoric dysplasia type II in which FGFR3(K650E) expression was directed to the appendicular skeleton, we show that the mutant receptor caused a block in chondrocyte differentiation specifically at the prehypertrophic stage. The differentiation block led to a severe reduction in hypertrophic chondrocytes that normally produce vascular endothelial growth factor, which in turn was associated with poor vascularization of primary ossification centers and disrupted endochondral ossification. We show that the differentiation block and defects in joint formation are associated with persistent expression of the chondrogenic factor Sox9 and down-regulation of β-catenin levels and activity in growth plate chondrocytes. Consistent with these in vivo results, FGFR3(K650E) expression was found to increase Sox9 and decrease β-catenin levels and transcriptional activity in cultured mesenchymal cells. Coexpression of Fgfr3(K650E) and Sox9 in cells resulted in very high levels of Sox9 and cooperative suppression of β-catenin-dependent transcription. Fgfr3(K650E) had opposing effects on Sox9 and β-catenin protein stability with it promoting Sox9 stabilization and β-catenin degradation. Since both Sox9 overexpression and β-catenin deletion independently blocks hypertrophic differentiation of chondrocytes and cause chondrodysplasias similar to those caused by mutations in FGFR3, our results suggest that dysregulation of Sox9 and β-catenin levels and activity in growth plate chondrocytes is an important underlying mechanism in skeletal diseases caused by mutations in FGFR3.

Integration of the transcriptional networks regulating limb morphogenesis

The developing limb is one of the best described vertebrate systems for understanding how coordinated gene expression during embryogenesis leads to the structures present in the mature organism. This knowledge, derived from decades of research, is largely based upon gain- and loss-of-function experiments. These studies have provided limited information about how the key signaling pathways interact with each other and the downstream effectors of these pathways. We summarize our current understanding of known genetic interactions in the context of three temporally defined gene regulatory networks. These networks crystallize our current knowledge, depicting a dynamic process involving multiple feedback loops between the ectoderm and mesoderm. At the same time, they highlight the fact that many essential processes are still largely undescribed. Much of the dynamic transcriptional activity occurring during development is regulated by distal cis-regulatory elements. Modern genomic tools have provided new approaches for studying the function of cis-regulatory elements and we discuss the results of these studies in regard to understanding limb development. Ultimately, these genomic techniques will allow scientists to understand how multiple signaling pathways are integrated in space and time to drive gene expression and regulate the formation of the limb.

Sox9 directs hypertrophic maturation and blocks osteoblast differentiation of growth plate chondrocytes

The transcription factor Sox9 is necessary for early chondrogenesis, but its subsequent roles in the cartilage growth plate, a highly specialized structure that drives skeletal growth and endochondral ossification, remain unclear. Using a doxycycline-inducible Cre transgene and Sox9 conditional null alleles in the mouse, we show that Sox9 is required to maintain chondrocyte columnar proliferation and generate cell hypertrophy, two key features of functional growth plates. Sox9 keeps Runx2 expression and β-catenin signaling in check and thereby inhibits not only progression from proliferation to prehypertrophy, but also subsequent acquisition of an osteoblastic phenotype. Sox9 protein outlives Sox9 RNA in upper hypertrophic chondrocytes, where it contributes with Mef2c to directly activate the major marker of these cells, Col10a1. These findings thus reveal that Sox9 remains a central determinant of the lineage fate and multistep differentiation program of growth plate chondrocytes and thereby illuminate our understanding of key molecular mechanisms underlying skeletogenesis.

Detection of genes regulated by Lmx1b during limb dorsalization

Lmx1b is a homeodomain transcription factor that regulates dorsal identity during limb development. Lmx1b knockout (KO) mice develop distal ventral-ventral limbs. Although induction of Lmx1b is linked to Wnt7a expression in the dorsal limb ectoderm, the downstream targets of Lmx1b that accomplish limb dorsalization are unknown. To identify genes targeted by Lmx1b, we compared gene arrays from Lmx1b KO and wild type mouse limbs during limb dorsalization, i.e., 11.5, 12.5, and 13.5 days post coitum. We identified 54 target genes that were differentially expressed in all three stages. Several skeletal targets, including Emx2, Matrilin1 and Matrilin4, demonstrated a loss of scapular expression in the Lmx1b KO mice, supporting a role for Lmx1b in scapula development. Furthermore, the relative abundance of extracellular matrix-related soft tissue targets regulated by Lmx1b, such as collagens and proteoglycans, suggests a mechanism that includes changes in the extracellular matrix composition to accomplish limb dorsalization. Our study provides the most comprehensive characterization of genes regulated by Lmx1b during limb development to-date and provides targets for further investigation.

A transgenic, mesodermal specific, Dkk1 mouse model recapitulates a spectrum of human congenital limb reduction defects

Congenital limb reduction defects occurring in isolation of other developmental abnormalities continue to be an important medical problem in which little progress has been made. Herein we generated transgenic mice expressing Dkk1 in an appendicular mesodermal pattern. Prx1-Dkk1 mice recapitulate a full spectrum of human congenital limb reduction defects, without other developmental issues, and have normal life-spans. Importantly, a close examination of the inheritance pattern suggests that there is a significant degree of incomplete penetrance as progeny of phenotypically positive or phenotypically negative, but genotypically positive Prx1-Dkk1 mice, consistently give rise to both phenotypically positive mice and phenotypically normal-appearing mice. Thus, this heterogeneous phenotype is reproducible with each generation regardless of the phenotype of the parents. We further go on to identify that mesenchymal stem cells from Prx1-Dkk1 mice have limited proliferative ability, but normal differentiation potential, which may explain the mechanism for the limb reduction defects observed. We believe Prx1-Dkk1 mice may prove useful in the future to study the mechanisms underlying the development of congenital limb reduction defects.

Wls-mediated Wnts differentially regulate distal limb patterning and tissue morphogenesis

Wnt proteins are diffusible morphogens that play multiple roles during vertebrate limb development. However, the complexity of Wnt signaling cascades and their overlapping expression prevent us from dissecting their function in limb patterning and tissue morphogenesis. Depletion of the Wntless (Wls) gene, which is required for the secretion of various Wnts, makes it possible to genetically dissect the overall effect of Wnts in limb development. In this study, the Wls gene was conditionally depleted in limb mesenchyme and ectoderm. The loss of mesenchymal Wls prevented the differentiation of distal mesenchyme and arrested limb outgrowth, most likely by affecting Wnt5a function. Meanwhile, the deletion of ectodermal Wls resulted in agenesis of distal limb tissue and premature regression of the distal mesenchyme. These observations suggested that Wnts from the two germ layers differentially regulate the pool of undifferentiated distal limb mesenchyme cells. Cellular behavior analysis revealed that ectodermal Wnts sustain mesenchymal cell proliferation and survival in a manner distinct from Fgf. Ectodermal Wnts were also shown for the first time to be essential for distal tendon/ligament induction, myoblast migration and dermis formation in the limb. These findings provide a comprehensive view of the role of Wnts in limb patterning and tissue morphogenesis.

Nuclear translocation of β-catenin synchronized with loss of E-cadherin in oral epithelial dysplasia with a characteristic two-phase appearance

AIMS

One of the important histopathological characteristics of oral epithelial dysplasia is a two-phase appearance of rete processes, comprising an upper layer of keratinized cells and a lower layer of basaloid cells, and thereby creating a sharp contrast between these two separate cell populations. The aim of this study was to determine the cellular adhesion status of the basaloid cells.

METHODS AND RESULTS

Immunohistochemistry for β-catenin, E-cadherin and their related molecules was carried out in surgical specimens of two-phase epithelial dysplasia of the oral mucosa. The lower-half basaloid cells and the upper keratinized cells were microdissected separately, and extracted DNA samples were subjected to methylation-specific polymerase chain reaction amplification for E-cadherin. β-Catenin was immunolocalized within the nuclei and cytoplasm of Ki67-positive lower-half basaloid cells, as well as on the cell membrane of upper parakeratotic cells. The basaloid cells of the lower-half were also positive for matrix metalloproteinase-7 and cyclin D1, β-catenin target gene products, α-dystroglycan, tenascin-C, and perlecan, but not for E-cadherin. The promoter region of the E-cadherin gene was hypermethylated.

CONCLUSIONS

The solid proliferation of lower-half E-cadherin-free basaloid cells is enhanced by Wnt signalling cascades, as well as by the intraepithelial extracellular matrix or its bound growth factors.

Adherens junctions in mammalian development, homeostasis and disease: lessons from mice

Mice have proven to be a particularly powerful model to study molecular mechanisms of development and disease. The reason for this is the close evolutionary relationship between rodents and humans, similarities in physiological mechanisms in mice and human, and the large number of techniques available to study gene functions in mice. A large number of mice mutations, either germ line, conditional or inducible, have been generated in the past years for adherens junctions components, and the number is still increasing. In this review we will discuss mice models that have contributed to understanding the developmental and physiological role of adherens junctions and their components in mammals and have revealed novel mechanistic aspects of how adherens junctions regulate morphogenesis and tissue homeostasis.

Cartilage biology in osteoarthritis--lessons from developmental biology

The cellular and molecular mechanisms responsible for the initiation and progression of osteoarthritis (OA), and in particular cartilage degeneration in OA, are not completely understood. Increasing evidence implicates developmental processes in OA etiology and pathogenesis. Herein, we review this evidence. We first examine subtle changes in cartilage development and the specification and formation of joints, which predispose to OA development, and second, we review the switch from an articular to a hypertrophic chondrocyte phenotype that is thought to be part of the OA pathological process ultimately resulting in cartilage degeneration. The latest studies are summarized and we discuss the concepts emerging from these findings in cartilage biology, in the light of our understanding of the developmental processes involved.

Canonical Wnt signalling requires the BMP pathway to inhibit oligodendrocyte maturation

OLs (oligodendrocytes) are the myelinating cells of the CNS (central nervous system), wrapping axons in conductive sheathes to ensure effective transmission of neural signals. The regulation of OL development, from precursor to mature myelinating cell, is controlled by a variety of inhibitory and inductive signalling factors. The dorsal spinal cord contains signals that inhibit OL development, possibly to prevent premature and ectopic precursor differentiation. The Wnt and BMP (bone morphogenic protein) signalling pathways have been identified as dorsal spinal cord signals with overlapping temporal activity, and both have similar inhibitory effects on OL differentiation. Both these pathways feature prominently in many developmental processes and demyelinating events after injury, and they are known to interact in complex inductive, inhibitive and synergistic manners in many developing systems. The interaction between BMP and Wnt signalling in OL development, however, has not been extensively explored. In the present study, we examine the relationship between the canonical Wnt and BMP pathways. We use pharmacological and genetic paradigms to show that both Wnt3a and BMP4 will inhibit OL differentiation in vitro. We also show that when the canonical BMP signalling pathway is blocked, neither Wnt3a nor BMP4 have inhibitory effects on OL differentiation. In contrast, abrogating the Wnt signalling pathway does not alter the actions of BMP4 treatment. Our results indicate that the BMP signalling pathway is necessary for the canonical Wnt signalling pathway to exert its effects on OL development, but not vice versa, suggesting that Wnt signals upstream of BMP.

Mutations in Frizzled 6 cause isolated autosomal-recessive nail dysplasia

Inherited and isolated nail malformations are rare and heterogeneous conditions. We identified two consanguineous pedigrees in which some family members were affected by isolated nail dysplasia that suggested an autosomal-recessive inheritance pattern and was characterized by claw-shaped nails, onychauxis, and onycholysis. Genome-wide SNP array analysis of affected individuals from both families showed an overlapping and homozygous region of 800 kb on the long arm of chromosome 8. The candidate region spans eight genes, and DNA sequence analysis revealed homozygous nonsense and missense mutations in FZD(6), the gene encoding Frizzled 6. FZD(6) belongs to a family of highly conserved membrane-bound WNT receptors involved in developmental processes and differentiation through several signaling pathways. We expressed the FZD(6) missense mutation and observed a quantitative shift in subcellular distribution from the plasma membrane to the lysosomes, where the receptor is inaccessible for signaling and presumably degraded. Analysis of human fibroblasts homozygous for the nonsense mutation showed an aberrant response to both WNT-3A and WNT-5A stimulation; this response was consistent with an effect on both canonical and noncanonical WNT-FZD signaling. A detailed analysis of the Fzd(6)(-/-) mice, previously shown to have an altered hair pattern, showed malformed claws predominantly of the hind limbs. Furthermore, a transient Fdz6 mRNA expression was observed in the epidermis of the digital tips at embryonic day 16.5 during early claw morphogenesis. Thus, our combined results show that FZD6 mutations can result in severe defects in nail and claw formation through reduced or abolished membranous FZD(6) levels and several nonfunctional WNT-FZD pathways.

HMGB factors are required for posterior digit development through integrating signaling pathway activities

The chromatin factors Hmgb1 and Hmgb2 have critical roles in cellular processes, including transcription and DNA modification. To identify the function of Hmgb genes in embryonic development, we generated double mutants of Hmgb1;Hmgb2 in mice. While double null embryos arrest at E9.5, Hmgb1(-/-) ; Hmgb2(+/-) embryos exhibit a loss of digit5, the most posterior digit, in the forelimb. We show that Hmgb1(-/-) ; Hmgb2(+/-) forelimbs have a reduced level of Shh signaling, as well as a clear downregulation of Wnt and BMP target genes in the posterior region. Moreover, we demonstrate that hmgb1 and hmgb2 in zebrafish embryos enhance Wnt signaling in a variety of tissues, and that double knockdown embryos have reduced Wnt signaling and shh expression in pectoral fin buds. Our data show that Hmgb1 and Hmgb2 function redundantly to enhance Wnt signaling in embryos, and further suggest that integrating Wnt, Shh, and BMP signaling regulates the development of digit5 in forelimbs.

Mechanisms of digit formation: Human malformation syndromes tell the story

Identifying the genetic basis of human limb malformation disorders has been instrumental in improving our understanding of limb development. Abnormalities of the hands and/or feet include defects affecting patterning, establishment, elongation, and segmentation of cartilaginous condensations, as well as growth of the individual skeletal elements. While the phenotype of such malformations is highly diverse, the mutations identified to date cluster in genes implicated in a limited number of molecular pathways, namely hedgehog, Wnt, and bone morphogenetic protein. The latter pathway appears to function as a key molecular network regulating different phases of digit and joint development. Studies in animal models not only extended our insight into the pathogenesis of these conditions, but have also contributed to our understanding of the in vivo functions and interactions of these key players. This review is aimed at integrating the current understanding of human digit malformations into the increasing knowledge of the molecular mechanisms of digit development. Developmental Dynamics 240:990-1004, 2011. © 2011 Wiley-Liss, Inc.

Disruption of PCP signaling causes limb morphogenesis and skeletal defects and may underlie Robinow syndrome and brachydactyly type B

Brachydactyly type B (BDB1) and Robinow syndrome (RRS) are two skeletal disorders caused by mutations in ROR2, a co-receptor of Wnt5a. Wnt5a/Ror2 can activate multiple branches of non-canonical Wnt signaling, but it is unclear which branch(es) mediates Wnt5a/Ror2 function in limb skeletal development. Here, we provide evidence implicating the planar cell polarity (PCP) pathway as the downstream component of Wnt5a in the limb. We show that a mutation in the mouse PCP gene Vangl2 causes digit defects resembling the clinical phenotypes in BDB1, including loss of phalanges. Halving the dosage of Wnt5a in Vangl2 mutants enhances the severity and penetrance of the digit defects and causes long bone defects reminiscent of RRS, suggesting that Wnt5a and Vangl2 function in the same pathway and disruption of PCP signaling may underlie both BDB1 and RRS. Consistent with a role for PCP signaling in tissue morphogenesis, mutation of Vangl2 alters the shape and dimensions of early limb buds: the width and thickness are increased, whereas the length is decreased. The digit pre-chondrogenic condensates also become wider, thicker and shorter. Interestingly, altered limb bud dimensions in Vangl2 mutants also affect limb growth by perturbing the signaling network that regulates the balance between Fgf and Bmp signaling. Halving the dosage of Bmp4 partially suppresses the loss of phalanges in Vangl2 mutants, supporting the hypothesis that an aberrant increase in Bmp signaling is the cause of the brachydactyly defect. These findings provide novel insight into the signaling mechanisms of Wnt5a/Ror2 and the pathogenesis in BDB1 and RRS.

Wnt/β-catenin signaling in the dental mesenchyme regulates incisor development by regulating Bmp4

Loss- and gain-of function approaches modulating canonical Wnt/β-catenin activity have established a role for the Wnt/β-catenin pathway during tooth development. Here we show that Wnt/β-catenin signaling is required in the dental mesenchyme for normal incisor development, as locally restricted genetic inactivation of β-catenin results in a splitting of the incisor placode, giving rise to two incisors. Molecularly this is first associated with down-regulation of Bmp4 and subsequent splitting of the Shh domain at a subsequent stage. The latter phenotype can be mimicked by ectopic application of the BMP antagonist Noggin. Conditional genetic inactivation of Bmp4 in the mesenchyme reveals that mesenchymal BMP4 activity is required for maintenance of Shh expression in the dental ectoderm. Taken together our results indicate that β-catenin together with Lef1 and Tcf1 are required to activate Bmp4 expression in order to maintain Shh expression in the dental ectoderm. This provides a mechanism whereby the number of incisors arising from one placode can be varied through local alterations of a mesenchymal signaling circuit involving β-catenin, Lef1, Tcf1 and Bmp4.

The acceleration of implant osseointegration by liposomal Wnt3a

The strength of a Wnt-based strategy for tissue regeneration lies in the central role that Wnts play in healing. Tissue injury triggers local Wnt activation at the site of damage, and this Wnt signal is required for the repair and/or regeneration of almost all tissues including bone, neural tissues, myocardium, and epidermis. We developed a biologically based approach to create a transient elevation in Wnt signaling in peri-implant tissues, and in doing so, accelerated bone formation around the implant. Our subsequent molecular and cellular analyses provide mechanistic insights into the basis for this pro-osteogenic effect. Given the essential role of Wnt signaling in bone formation, this protein-based approach may have widespread application in implant osseointegration.

Dynamic expression of a LEF-EGFP Wnt reporter in mouse development and cancer

We have characterized a transgenic mouse line in which enhanced green fluorescent protein (EGFP) is expressed under the control of multimerized LEF-1 responsive elements. In embryos, EGFP was detected in known sites of Wnt activation, including the primitive streak, mesoderm, neural tube, somites, heart, limb buds, mammary placodes, and whisker follicles. In vitro cultured transgenic embryonic fibroblasts upregulated EGFP expression in response to activation of Wnt signaling by GSK3beta inhibition. Mammary tumor cell lines derived from female LEF-EGFP transgenic mice treated with the carcinogen 7, 12-dimethylbenz[a]anthracene (DMBA) also express EGFP. Thus, this transgenic line is useful for ex vivo and in vitro studies of Wnt signaling in development and cancer.

Genetic evidence that SOST inhibits WNT signaling in the limb

SOST is a negative regulator of bone formation, and mutations in human SOST are responsible for sclerosteosis. In addition to high bone mass, sclerosteosis patients occasionally display hand defects, suggesting that SOST may function embryonically. Here we report that overexpression of SOST leads to loss of posterior structures of the zeugopod and autopod by perturbing anterior-posterior and proximal-distal signaling centers in the developing limb. Mutant mice that overexpress SOST in combination with Grem1 and Lrp6 mutations display more severe limb defects than single mutants alone, while Sost(-/-) significantly rescues the Lrp6(-/-) skeletal phenotype, signifying that SOST gain-of-function impairs limb patterning by inhibiting the WNT signaling through LRP5/6.

Comparative gene expression analysis of genital tubercle development reveals a putative appendicular Wnt7 network for the epidermal differentiation

Here we describe the first detailed catalog of gene expression in the developing lower urinary tract (LUT), including epithelial and mesenchymal portions of the developing bladder, urogenital sinus, urethra, and genital tubercle (GT) at E13 and E14. Top compartment-specific genes implicated by the microarray data were validated using whole-mount in situ hybridization (ISH) over the entire LUT. To demonstrate the potential of this resource to implicate developmentally critical features, we focused on gene expression patterns and pathways in the sexually indeterminate, androgen-independent GT. GT expression patterns reinforced the proposed similarities between development of GT, limb, and craniofacial prominences. Comparison of spatial expression patterns predicted a network of Wnt7a-associated GT-enriched epithelial genes, including Gjb2, Dsc3, Krt5, and Sostdc1. Known from other contexts, these genes are associated with normal epidermal differentiation, with disruptions in Dsc3 and Gjb2 showing palmo-plantar keratoderma in the limb. We propose that this gene network contributes to normal foreskin, scrotum, and labial development. As several of these genes are known to be regulated by, or contain cis elements responsive to retinoic acid, estrogen, or androgen, this implicates this pathway in the later androgen-dependent development of the GT.

Crosstalk between Wnt and bone morphogenic protein signaling: a turbulent relationship

The Wnt and the bone morphogenic protein (BMP) pathways are evolutionarily conserved and essentially independent signaling mechanisms, which, however, often regulate similar biological processes. Wnt and BMP signaling are functionally integrated in many biological processes, such as embryonic patterning in Drosophila and vertebrates, formation of kidney, limb, teeth and bones, maintenance of stem cells, and cancer progression. Detailed inspection of regulation in these and other tissues reveals that Wnt and BMP signaling are functionally integrated in four fundamentally different ways. The molecular mechanism evolved to mediate this integration can also be summarized in four different ways. However, a fundamental aspect of functional and mechanistic interaction between these pathways relies on tissue-specific mechanisms, which are often not conserved and cannot be extrapolated to other tissues. Integration of the two pathways contributes toward the sophisticated means necessary for creating the complexity of our bodies and the reliable and healthy function of its tissues and organs.

Immunohistochemical Analysis of Osteoblasts in Zygapophyseal Joints of Patients with Ankylosing Spondylitis Reveal Repair Mechanisms Similar to Osteoarthritis

Objective.

New bone formation of the spine is a typical feature of ankylosing spondylitis (AS). It is unknown whether new bone formation is part of a physiological repair process or a unique pathological entity of the disease.

Methods.

We analyzed zygapophyseal joints from patients with AS and osteoarthritis (OA) undergoing spinal surgery for rigid hyperkyphosis (AS) or radiculopathy caused by severe OA. In 17 patients with AS, 11 with OA, and 5 controls we performed immunohistochemical analysis of osteoprotegerin (OPG), nuclear factor-κB ligand (RANKL), and osteocalcin (OC) expression in osteoblasts and determined the trabecular thickness in AS and OA patients and controls. Osteoclasts were detected by tartrate-resistant alkaline phosphatase (TRAP) staining.

Results.

Trabecular thickness was significantly lower in patients with AS compared to OA (p = 0.01). The absolute number of CD56+ osteoblasts (p < 0.001) and OC+ (p = 0.002), OPG+ (p = 0.003), and RANKL+ osteoblasts (p = 0.03) in AS patients was also significantly lower than in OA patients. The percentages of OC+, OPG+, and RANKL+ osteoblasts did not differ between AS and OA (p > 0.05 in all cases). In controls, the percentages of OPG+ (p = 0.013) and OC+ (p = 0.034) but not RANKL+ (p > 0.05) osteoblasts were significantly lower compared to AS patients. The frequency of TRAP+ osteoclasts in AS patients was significantly lower compared to OA (p < 0.001), but higher compared to controls.

Conclusion.

Immunohistochemical analysis of zygapophyseal joints suggested that osteoblast activity is similar in AS and OA, indicating that new bone formation is possibly a physiological function of repair in both diseases.

The regulation of Dkk1 expression during embryonic development

During embryogenesis, the Dkk1 mediated Wnt inhibition controls the spatiotemporal dynamics of cell fate determination, cell differentiation and cell death. Furthermore, the Dkk1 dose is critical for the normal Wnt homeostasis, as alteration of the Dkk1 activity is associated with various diseases. We investigated the regulation of Dkk1 expression during embryonic development. We identified nine conserved non-coding elements (CNEs), located 3' to the Dkk1 locus. Analyses of the regulatory potential revealed that four of these CNEs in combination drive reporter expression very similar to Dkk1 expression in several organs of transgenic embryos. We extended the knowledge of Dkk1 expression during hypophysis, external genitalia and kidney development, suggesting so far to unexplored functions of Dkk1 during the development of these organs. Characterization of the regulatory potential of four individual CNEs revealed that each of these promotes Dkk1 expression in brain and kidney. In combination, two enhancers are responsible for expression in the pituitary and the genital tubercle. Furthermore, individual CNEs mediates craniofacial, optic cup and limb specific Dkk1 regulation. Our study substantially improves the knowledge of Dkk1 regulation during embryonic development and thus might be of high relevance for therapeutic approaches.

Role of Epiprofin, a zinc-finger transcription factor, in limb development

The formation and maintenance of the apical ectodermal ridge (AER) is critical for the outgrowth and patterning of the vertebrate limb. In the present work, we have investigated the role of Epiprofin (Epfn/Sp6), a member of the SP/KLF transcription factor family that is expressed in the limb ectoderm and the AER, during limb development. Epfn mutant mice have a defective autopod that shows mesoaxial syndactyly in the forelimb and synostosis (bony fusion) in the hindlimb and partial bidorsal digital tips. Epfn mutants also show a defect in the maturation of the AER that appears flat and broad, with a double ridge phenotype. By genetic analysis, we also show that Epfn is controlled by WNT/b-CATENIN signaling in the limb ectoderm. Since the less severe phenotypes of the conditional removal of b-catenin in the limb ectoderm strongly resemble the limb phenotype of Epfn mutants, we propose that EPFN very likely functions as a modulator of WNT signaling in the limb ectoderm.

Genetic manipulations reveal dynamic cell and gene functions: Cre-ating a new view of myogenesis

Development of multicellular organisms is temporally and spatially complex. The Cre/loxP and Flp/FRT systems for genetic manipulation in mammals now enable researchers to explicitly examine in vivo the temporal and spatial role of cells and genes during development via cell lineage and ablation studies and conditional gene inactivation and activation. Recently we have used these methods to genetically dissect the role of Pax3(+) and Pax7(+) progenitor populations and the function of beta-catenin, an important regulator of myogenesis, in vertebrate limb myogenesis. Our lineage and ablation studies of Pax3(+) and Pax7(+) progenitors revealed surprising insights into myogenesis not apparent from Pax3 and Pax7 expression and functional studies. In addition, conditional inactivation and activation of beta-catenin in different progenitor populations and their progeny demonstrated that beta-catenin plays several cell-autonomous roles in myogenesis. Our studies highlight the hierarchical (i.e., genes versus cells), temporal and spatial complexity of development and demonstrate that manipulations of both cells and genes will be required to obtain a full understanding of the development of multicellular organisms.

Adhesive and signaling functions of cadherins and catenins in vertebrate development

Properly regulated intercellular adhesion is critical for normal development of all metazoan organisms. Adherens junctions play an especially prominent role in development because they link the adhesive function of cadherin-catenin protein complexes to the dynamic forces of the actin cytoskeleton, which helps to orchestrate a spatially confined and very dynamic assembly of intercellular connections. Intriguingly, in addition to maintaining intercellular adhesion, cadherin-catenin proteins are linked to several major developmental signaling pathways crucial for normal morphogenesis. In this article we will highlight the key genetic studies that uncovered the role of cadherin-catenin proteins in vertebrate development and discuss the potential role of these proteins as molecular biosensors of external cellular microenvironment that may spatially confine signaling molecules and polarity cues to orchestrate cellular behavior throughout the complex process of normal morphogenesis.

Essential mesenchymal role of small GTPase Rac1 in interdigital programmed cell death during limb development

Developing vertebrate limbs are often utilized as a model for studying pattern formation and morphogenetic cell death. Herein, we report that conditional deletion of Rac1, a member of the Rho family of proteins, in mouse limb bud mesenchyme led to skeletal deformities in the autopod and soft tissue syndactyly, with the latter caused by a complete absence of interdigital programmed cell death. Furthermore, the lack of interdigital programmed cell death and associated syndactyly was related to down-regulated gene expression of Bmp2, Bmp7, Msx1, and Msx2, which are known to promote apoptosis in the interdigital mesenchyme. Our findings from Rac1 conditional mutants indicate crucial roles for Rac1 in limb bud morphogenesis, especially interdigital programmed cell death.

Building limb morphology through integration of signalling modules

Growth and patterning of the vertebrate limb relies on signals produced by three discrete signalling centres: the Apical Ectodermal Ridge (AER), the Zone of Polarising Activity (ZPA) and the dorsal ectoderm. The molecular identities of these signals and their associated downstream pathways have begun to be uncovered. In this review, we focus on recent work that has highlighted the importance of cross-talk between these signalling centres and how mesenchymal progenitors integrate multiple signalling inputs. We also discuss recent evidence suggesting how modulations of key signalling pathways have been used to generate the morphological diversity seen between different vertebrate limb appendages.

Growth and patterning in the limb: signaling gradients make the decision

The vertebrate limb bud provides a unique system to investigate the coordinated regulation of growth and patterning, two key processes that govern the formation of a complex multicellular organism from a fertilized egg. Two studies have advanced our understanding of limb development by elucidating that signaling gradients from the limb ectoderm, including the apical ectoderm ridge (AER), act in concert to establish a basic pattern of tissue layers by coordinating cell proliferation and cell fate determination. These studies reveal that cell proliferation and fate determination in development can be two faces of the same coin in that they are regulated by the same signaling pathways. Alterations in the duration and range of the signaling gradients may underlie many of the morphological differences in the evolution of vertebrate limbs.

Suppression of beta-catenin signaling by liver X receptor ligands

The nuclear receptors liver X receptor (LXR) alpha and LXRbeta serve as oxysterol receptors and play an important role in the regulation of lipid metabolism. We investigated the potential effects of LXRs on pathways of colon carcinogenesis and found that LXR activation suppresses the transactivation activity of beta-catenin, a key molecule in Wnt signaling. LXRalpha and LXRbeta inhibited beta-catenin transactivation of T cell factor-mediated transcription in a ligand-dependent manner. LXR activation suppressed an oncogenic beta-catenin, which has phosphorylation site mutations, and did not change beta-catenin protein expression in cells. In contrast, beta-catenin enhanced LXR transactivation activity. Nuclear LXRs and beta-catenin were coimmunoprecipitated in colon cancer HCT116 cells, and in vitro experiments showed that LXRs bind directly to the Armadillo repeat region of beta-catenin in a ligand-independent manner. LXR ligand decreased mRNA expression of beta-catenin targets, MYC, MMP7 and BMP4, and recruited LXRs to MYC and MMP7 promoters. Transfection of a dominant negative LXR to HCT116 cells and experiments using LXR-null cells showed the involvement of cellular LXRs in beta-catenin suppression and proliferation inhibition. The results show lipid-sensing receptor LXRs regulate the beta-catenin activity and cellular proliferation.

Deciphering the function of canonical Wnt signals in development and disease: conditional loss- and gain-of-function mutations of beta-catenin in mice

Wnt signaling is one of a handful of powerful signaling pathways that play crucial roles in the animal life by controlling the genetic programs of embryonic development and adult homeostasis. When disrupted, these signaling pathways cause developmental defects, or diseases, among them cancer. The gateway of the canonical Wnt pathway, which contains >100 genes, is an essential molecule called beta-catenin (Armadillo in Drosophila). Conditional loss- and gain-of-function mutations of beta-catenin in mice provided powerful tools for the functional analysis of canonical Wnt signaling in many tissues and organs. Such studies revealed roles of Wnt signaling that were previously not accessible to genetic analysis due to the early embryonic lethality of conventional beta-catenin knockout mice, as well as the redundancy of Wnt ligands, receptors, and transcription factors. Analysis of conditional beta-catenin loss- and gain-of-function mutant mice demonstrated that canonical Wnt signals control progenitor cell expansion and lineage decisions both in the early embryo and in many organs. Canonical Wnt signaling also plays important roles in the maintenance of various embryonic or adult stem cells, and as recent findings demonstrated, in cancer stem cell types. This has opened new opportunities to model numerous human diseases, which have been associated with deregulated Wnt signaling. Our review summarizes what has been learned from genetic studies of the Wnt pathway by the analysis of conditional beta-catenin loss- and gain-of-function mice.

Wnt and hedgehog signaling pathways in bone development

Cell-cell signaling is a major strategy that vertebrate embryos employ to coordinately control cell proliferation, differentiation, and survival in many developmental processes. Similar cell signaling pathways also control adult tissue regeneration and repair. We demonstrated in the developing skeletal system that the Wnt/beta-catenin signaling controls the differentiation of progenitor cells into either osteoblasts or chondrocytes. Genetic ablation of beta-catenin in the developing mouse embryo resulted in ectopic formation of chondrocytes at the expense of osteoblast differentiation during both intramembranous and endochondral ossification. Conversely, ectopic upregulation of the canonical Wnt signaling led to suppression of chondrocyte formation and enhanced ossification. As other signaling pathways also play critical roles in controlling skeletal development, to gain a full picture of the molecular regulatory network of skeletal development, we investigated how the Wnt/beta-catenin signaling is integrated with Indian hedgehog (Ihh) signaling in controlling various aspects of skeletal development. We found that Wnt signaling acts downstream of Ihh signaling and is required in osteoblasts after Osterix expression to promote osteoblast maturation during endochondral bone formation. Since similar controlling mechanisms of osteoblast proliferation and differentiation may be employed by adult mesenchymal progenitor cells during fracture repair, these studies suggest that, to enhance fracture repair or bone formation, Ihh signaling needs to be enhanced at early stages, whereas Wnt signaling should be upregulated slightly later in differentiated osteoblasts.

Tissue-specific transgenic, conditional knockout and knock-in mice of genes in the canonical Wnt signaling pathway

The Wnt signaling pathway plays key roles in the development and homeostasis of a number of organs such as the brain, lung, liver, heart, gastrointestinal tract, mammary gland, skin, and bone, as well as of the immune system. Studies on conventional knockout mice of the genes in the Wnt signaling pathway have revealed its essential roles in these tissues; however, most of these knockout mice die during embryogenesis or soon after birth. Through more advanced techniques such as Cre/loxP and tetracycline-inducible systems, a gene of interest can be expressed or inactivated in a tissue-specific and time-controlled manner. Here we review recent papers on the tissue-specific transgenic, conditional knockout and knock-in mice of the genes in the Wnt signaling pathway In addition to such engineered mice, we also list reporter mice that have been generated to determine the activity of the canonical Wnt signaling pathway in mouse tissues.

Glycogen synthase kinase 3 alpha and 3 beta have distinct functions during cardiogenesis of zebrafish embryo

Background

Glycogen synthase kinase 3 (GSK3) encodes a serine/threonine protein kinase, is known to play roles in many biological processes. Two closely related GSK3 isoforms encoded by distinct genes: GSK3α (51 kDa) and GSK3β (47 kDa). In previously studies, most GSK3 inhibitors are not only inhibiting GSK3, but are also affecting many other kinases. In addition, because of highly similarity in amino acid sequence between GSK3α and GSK3β, making it difficult to identify an inhibitor that can be selective against GSK3α or GSK3β. Thus, it is relatively difficult to address the functions of GSK3 isoforms during embryogenesis. At this study, we attempt to specifically inhibit either GSK3α or GSK3β and uncover the isoform-specific roles that GSK3 plays during cardiogenesis.

Results

We blocked gsk3α and gsk3β translations by injection of morpholino antisense oligonucleotides (MO). Both gsk3α- and gsk3β-MO-injected embryos displayed similar morphological defects, with a thin, string-like shaped heart and pericardial edema at 72 hours post-fertilization. However, when detailed analysis of the gsk3α- and gsk3β-MO-induced heart defects, we found that the reduced number of cardiomyocytes in gsk3α morphants during the heart-ring stage was due to apoptosis. On the contrary, gsk3β morphants did not exhibit significant apoptosis in the cardiomyocytes, and the heart developed normally during the heart-ring stage. Later, however, the heart positioning was severely disrupted in gsk3β morphants. bmp4 expression in gsk3β morphants was up-regulated and disrupted the asymmetry pattern in the heart. The cardiac valve defects in gsk3β morphants were similar to those observed in axin1 and apc^mcr mutants, suggesting that GSK3β might play a role in cardiac valve development through the Wnt/β-catenin pathway. Finally, the phenotypes of gsk3α mutant embryos cannot be rescued by gsk3β mRNA, and vice versa, demonstrating that GSK3α and GSK3β are not functionally redundant.

Conclusion

We conclude that (1) GSK3α, but not GSK3β, is necessary in cardiomyocyte survival; (2) the GSK3β plays important roles in modulating the left-right asymmetry and affecting heart positioning; and (3) GSK3α and GSK3β play distinct roles during zebrafish cardiogenesis.

The role of Hox genes during vertebrate limb development

The potential role of Hox genes during vertebrate limb development was brought into focus by gene expression analyses in mice (P Dolle, JC Izpisua-Belmonte, H Falkenstein, A Renucci, D Duboule, Nature 1989, 342:767-772), at a time when limb growth and patterning were thought to depend upon two distinct and rather independent systems of coordinates; one for the anterior-to-posterior axis and the other for the proximal-to-distal axis (see D Duboule, P Dolle, EMBO J 1989, 8:1497-1505). Over the past years, the function and regulation of these genes have been addressed using both gain-of-function and loss-of-function approaches in chick and mice. The use of multiple mutations either in cis-configuration in trans-configuration or in cis/trans configurations, has confirmed that Hox genes are essential for proper limb development, where they participate in both the growth and organization of the structures. Even though their molecular mechanisms of action remain somewhat elusive, the results of these extensive genetic analyses confirm that, during the development of the limbs, the various axes cannot be considered in isolation from each other and that a more holistic view of limb development should prevail over a simple cartesian, chess grid-like approach of these complex structures. With this in mind, the functional input of Hox genes during limb growth and development can now be re-assessed.

Wnt signaling and the regulation of bone mass

Human genetic studies have firmly established a link between bone mass in humans and gain-of-function or loss-of-function mutations in a Wnt coreceptor, low-density lipoprotein receptor-related protein 5 (LRP5), or in the Wnt antagonist sclerostin, and several molecular genetic studies in mice have consistently confirmed the critical importance of the Wnt signaling pathway in skeletal biology and disease. In what may be a novel paradigm, the ubiquitous nature of LRP5/6 and Wnt signaling is counterbalanced by the bone-restricted and regulated expression of Wnt antagonists such as sclerostin and Dickkopf-1 (Dkk1) in adult tissues, offering new and potentially safe therapeutic means of intervention to stimulate bone formation.

Interactions between HOXD and Gli3 genes control the limb apical ectodermal ridge via Fgf10

The development of the vertebrate limb is dependent upon two signaling centers, the apical ectodermal ridge (AER), which provides the underlying mesenchyme with essential growth factors, and the zone of polarizing activity (ZPA), the source of the Sonic hedgehog (SHH) product. Recent work involving gain and loss of function of Hox genes has emphasized their impact both on AER maintenance and Shh transcriptional activation. Here, we describe antagonistic interactions between posterior Hoxd genes and Gli3, suggesting that the latter product protects the AER from the deleterious effect of the formers, and we present evidence that Fgf10 is the mediator of HOX-dependent AER expansion. Furthermore, the striking similarity between some of the hereby observed Hox/Gli3-dependent morphogenetic defects and those displayed by fetuses with severely altered retinoic acid metabolism suggests a tight connection between these various pathways. The nature of these potential interactions is discussed in the context of proximal-distal growth and patterning.

Genetic analysis of the roles of BMP2, BMP4, and BMP7 in limb patterning and skeletogenesis

Bone morphogenetic protein (BMP) family members, including BMP2, BMP4, and BMP7, are expressed throughout limb development. BMPs have been implicated in early limb patterning as well as in the process of skeletogenesis. However, due to complications associated with early embryonic lethality, particularly for Bmp2 and Bmp4, and with functional redundancy among BMP molecules, it has been difficult to decipher the specific roles of these BMP molecules during different stages of limb development. To circumvent these issues, we have constructed a series of mouse strains lacking one or more of these BMPs, using conditional alleles in the case of Bmp2 and Bmp4 to remove them specifically from the limb bud mesenchyme. Contrary to earlier suggestions, our results indicate that BMPs neither act as secondary signals downstream of Sonic Hedghog (SHH) in patterning the anteroposterior axis nor as signals from the interdigital mesenchyme in specifying digit identity. We do find that a threshold level of BMP signaling is required for the onset of chondrogenesis, and hence some chondrogenic condensations fail to form in limbs deficient in both BMP2 and BMP4. However, in the condensations that do form, subsequent chondrogenic differentiation proceeds normally even in the absence of BMP2 and BMP7 or BMP2 and BMP4. In contrast, we find that the loss of both BMP2 and BMP4 results in a severe impairment of osteogenesis.

A group of related signaling molecules called bone morphogenetic proteins (BMPs) are known to play important roles in the formation of the structures such as the limbs. However, because different members of this group often have similar effects on target cells and are produced in overlapping regions of the embryo and hence can be redundant with one another, removal of any single member of the BMP family may not reveal the full extent of the roles they play during development. We have therefore improved on this type of analysis by removing pairs of these factors (BMP2 and BMP4 or BMP2 and BMP7) specifically from the developing limb. Although some have speculated that these signals play an early role in organizing or “patterning” the different tissues of the limb, we find no evidence for such a role. We do find, however, that a minimal amount of BMP signal is required to form cartilage, and hence some cartilaginous elements fail to form in limbs deficient in both BMP2 and BMP4. Moreover, in the absence of these two BMP family members, there is a severe impairment in the development of bone tissue, resulting in severely deformed limbs. This study gives important new insight into the roles of these BMP signals in making skeletal tissues in the embryo.