Expression pattern of two Frizzled-related genes, Frzb-1 and Sfrp-1, during mouse embryogenesis suggests a role for modulating action of Wnt family members

Mannose controls mesoderm specification and symmetry breaking in mouse gastruloids

Patterning and growth are fundamental features of embryonic development that must be tightly coordinated. To understand how metabolism impacts early mesoderm development, we used mouse embryonic stem-cell-derived gastruloids, that co-expressed glucose transporters with the mesodermal marker T/Bra. We found that the glucose mimic, 2-deoxy-D-glucose (2-DG), blocked T/Bra expression and abolished axial elongation in gastruloids. However, glucose removal did not phenocopy 2-DG treatment despite a decline in glycolytic intermediates. As 2-DG can also act as a competitive inhibitor of mannose in protein glycosylation, we added mannose together with 2-DG and found that it could rescue the mesoderm specification both in vivo and in vitro. We further showed that blocking production and intracellular recycling of mannose abrogated mesoderm specification. Proteomics analysis demonstrated that mannose reversed glycosylation of the Wnt pathway regulator, secreted frizzled receptor Frzb. Our study showed how mannose controls mesoderm specification in mouse gastruloids.

Frizzled7 Activates β-Catenin-Dependent and β-Catenin-Independent Wnt Signalling Pathways During Developmental Morphogenesis: Implications for Therapeutic Targeting in Colorectal Cancer

Frizzled₇ activates β-catenin-dependent and β-catenin-independent Wnt signalling pathways, is highly conserved through evolution from the ancient phylum hydra to man, plays essential roles in stem cells, tissue homeostasis and regeneration in the adult, and is upregulated in diverse cancers. Much of what is known about the core components of the Wnt signalling pathways was derived from studying the function of Frizzled₇ orthologues in the development of lower organism. As we interrogate Frizzled₇ signalling and function for therapeutic targeting in cancer, it is timely to revisit lower organisms to gain insight into the context dependent and dynamic nature of Wnt signalling for effective drug design.

Ventricular, atrial, and outflow tract heart progenitors arise from spatially and molecularly distinct regions of the primitive streak

The heart develops from 2 sources of mesoderm progenitors, the first and second heart field (FHF and SHF). Using a single-cell transcriptomic assay combined with genetic lineage tracing and live imaging, we find the FHF and SHF are subdivided into distinct pools of progenitors in gastrulating mouse embryos at earlier stages than previously thought. Each subpopulation has a distinct origin in the primitive streak. The first progenitors to leave the primitive streak contribute to the left ventricle, shortly after right ventricle progenitor emigrate, followed by the outflow tract and atrial progenitors. Moreover, a subset of atrial progenitors are gradually incorporated in posterior locations of the FHF. Although cells allocated to the outflow tract and atrium leave the primitive streak at a similar stage, they arise from different regions. Outflow tract cells originate from distal locations in the primitive streak while atrial progenitors are positioned more proximally. Moreover, single-cell RNA sequencing demonstrates that the primitive streak cells contributing to the ventricles have a distinct molecular signature from those forming the outflow tract and atrium. We conclude that cardiac progenitors are prepatterned within the primitive streak and this prefigures their allocation to distinct anatomical structures of the heart. Together, our data provide a new molecular and spatial map of mammalian cardiac progenitors that will support future studies of heart development, function, and disease.

Using organoids to study human brain development and evolution

Recent advances in methods for making cerebral organoids have opened a window of opportunity to directly study human brain development and disease, countering limitations inherent in non-human-based approaches. Whether freely patterned, guided into a region-specific fate or fused into assembloids, organoids have successfully recapitulated key features of in vivo neurodevelopment, allowing its examination from early to late stages. Although organoids have enormous potential, their effective use relies on understanding the extent of their limitations in accurately reproducing specific processes and components in the developing human brain. Here we review the potential of cerebral organoids to model and study human brain development and evolution and discuss the progress and current challenges in their use for reproducing specific human neurodevelopmental processes.

Role of Sfrps in cardiovascular disease

Secreted frizzled-related proteins (Sfrps) are a family of secreted proteins that bind extracellularly to Wnt ligands and frizzled receptors. This binding modulates the Wnt signaling cascade, and Sfrps interact with their corresponding receptors. Sfrps are thought to play an important role in the pathological mechanism of cardiac disease such as myocardial infarction, cardiac remodeling, and heart failure. However, the overall role of Sfrps in cardiac disease is unknown. Some members of the Sfrps family modulate cellular apoptosis, angiogenesis, differentiation, the inflammatory process, and cardiac remodeling. In this review, we summarize the evidence of Sfrps association with cardiac disease. We also discuss how multiple mechanisms may underlie Sfrps being involved in such diverse pathologies.

Anteroposterior axis patterning by early canonical Wnt signaling during hemichordate development

The Wnt family of secreted proteins has been proposed to play a conserved role in early specification of the bilaterian anteroposterior (A/P) axis. This hypothesis is based predominantly on data from vertebrate embryogenesis as well as planarian regeneration and homeostasis, indicating that canonical Wnt (cWnt) signaling endows cells with positional information along the A/P axis. Outside of these phyla, there is strong support for a conserved role of cWnt signaling in the repression of anterior fates, but little comparative support for a conserved role in promotion of posterior fates. We further test the hypothesis by investigating the role of cWnt signaling during early patterning along the A/P axis of the hemichordate Saccoglossus kowalevskii. We have cloned and investigated the expression of the complete Wnt ligand and Frizzled receptor complement of S. kowalevskii during early development along with many secreted Wnt modifiers. Eleven of the 13 Wnt ligands are ectodermally expressed in overlapping domains, predominantly in the posterior, and Wnt antagonists are localized predominantly to the anterior ectoderm in a pattern reminiscent of their distribution in vertebrate embryos. Overexpression and knockdown experiments, in combination with embryological manipulations, establish the importance of cWnt signaling for repression of anterior fates and activation of mid-axial ectodermal fates during the early development of S. kowalevskii. However, surprisingly, terminal posterior fates, defined by posterior Hox genes, are unresponsive to manipulation of cWnt levels during the early establishment of the A/P axis at late blastula and early gastrula. We establish experimental support for a conserved role of Wnt signaling in the early specification of the A/P axis during deuterostome body plan diversification, and further build support for an ancestral role of this pathway in early evolution of the bilaterian A/P axis. We find strong support for a role of cWnt in suppression of anterior fates and promotion of mid-axial fates, but we find no evidence that cWnt signaling plays a role in the early specification of the most posterior axial fates in S. kowalevskii. This posterior autonomy may be a conserved feature of early deuterostome axis specification.

SFRP1 is a possible candidate for epigenetic therapy in non-small cell lung cancer

Background

Non-small cell lung cancer (NSCLC) remains a lethal disease despite many proposed treatments. Recent studies have indicated that epigenetic therapy, which targets epigenetic effects, might be a new therapeutic methodology for NSCLC. However, it is not clear which objects (e.g., genes) this treatment specifically targets. Secreted frizzled-related proteins (SFRPs) are promising candidates for epigenetic therapy in many cancers, but there have been no reports of SFRPs targeted by epigenetic therapy for NSCLC.

Methods

This study performed a meta-analysis of reprogrammed NSCLC cell lines instead of the direct examination of epigenetic therapy treatment to identify epigenetic therapy targets. In addition, mRNA expression/promoter methylation profiles were processed by recently proposed principal component analysis based unsupervised feature extraction and categorical regression analysis based feature extraction.

Results

The Wnt/β-catenin signalling pathway was extensively enriched among 32 genes identified by feature extraction. Among the genes identified, SFRP1 was specifically indicated to target β-catenin, and thus might be targeted by epigenetic therapy in NSCLC cell lines. A histone deacetylase inhibitor might reactivate SFRP1 based upon the re-analysis of a public domain data set. Numerical computation validated the binding of SFRP1 to WNT1 to suppress Wnt signalling pathway activation in NSCLC.

Conclusions

The meta-analysis of reprogrammed NSCLC cell lines identified SFRP1 as a promising target of epigenetic therapy for NSCLC.

Electronic supplementary material

The online version of this article (doi:10.1186/s12920-016-0196-3) contains supplementary material, which is available to authorized users.

Systematic analysis of copy number variants of a large cohort of orofacial cleft patients identifies candidate genes for orofacial clefts

Orofacial clefts (OFCs) represent a large fraction of human birth defects and are one of the most common phenotypes affected by large copy number variants (CNVs). Due to the limited number of CNV patients in individual centers, CNV analyses of a large number of OFC patients are challenging. The present study analyzed 249 genomic deletions and 226 duplications from a cohort of 312 OFC patients reported in two publicly accessible databases of chromosome imbalance and phenotype in humans, DECIPHER and ECARUCA. Genomic regions deleted or duplicated in multiple patients were identified, and genes in these overlapping CNVs were prioritized based on the number of genes encompassed by the region and gene expression in embryonic mouse palate. Our analyses of these overlapping CNVs identified two genes known to be causative for human OFCs, SATB2 and MEIS2, and 12 genes (DGCR6, FGF2, FRZB, LETM1, MAPK3, SPRY1, THBS1, TSHZ1, TTC28, TULP4, WHSC1, WHSC2) that are associated with OFC or orofacial development. Additionally, we report 34 deleted and 24 duplicated genes that have not previously been associated with OFCs but are associated with the BMP, MAPK and RAC1 pathways. Statistical analyses show that the high number of overlapping CNVs is not due to random occurrence. The identified genes are not located in highly variable genomic regions in healthy populations and are significantly enriched for genes that are involved in orofacial development. In summary, we report a CNV analysis pipeline of a large cohort of OFC patients and identify novel candidate OFC genes.

Secreted Frizzled-related protein 3 (sFRP3)-mediated suppression of interleukin-6 receptor release by A disintegrin and metalloprotease 17 (ADAM17) is abrogated in the osteoarthritis-associated rare double variant of sFRP3

A disintegrin and metalloprotease 17 (ADAM17) activity and secreted Frizzled-related protein 3 (sFRP3) down-regulation or expression of its rare double variant is associated with arthritis. sFRP3 interacts with interleukin-6 receptor (IL-6R) and ADAM17 and suppresses ADAM17 activity, whereas the rare variant does not; these findings provide explanation for their opposing pathogenic associations.

Epigenetic alterations in metastatic cutaneous carcinoma

BACKGROUND

Squamous cell carcinoma (SCC) and basal cell carcinoma (BCC) are the 2 most common cutaneous carcinomas. Molecular profiles predicting metastasis of these cancers have not been identified.

METHODS

Epigenetic profiles of 37 primary cases of cutaneous SCC and BCC were quantified via the Illumina Goldengate Cancer Panel. Differential protein expression by metastatic potential was analyzed in 110 total cases by immunohistochemical (IHC) staining.

RESULTS

Unsupervised hierarchical clustering analysis revealed that metastatic BCCs had a methylation profile resembling cutaneous SCCs. Metastatic cutaneous SCCs were found to be hypermethylated at FRZB (median methylation: 46.7% vs 4.7%; p = 4 × 10(-5) ). Metastatic BCCs were found to be hypomethylated at MYCL2 (median methylation: 3.8% vs 83.4%; p = 1.9 × 10(-6) ). Immunohistochemical staining revealed few differences between metastatic and nonmetastatic cancers.

CONCLUSION

Metastatic primary BCCs and cutaneous SCCs had distinct epigenetic profiles when compared to their nonmetastatic counterparts. Epigenetic profiling may prove useful in future diagnosis and prevention of advanced nonmelanoma skin cancers.

Dynamic expression of secreted Frizzled-related protein 3 (sFRP3) in the developing mouse spinal cord and dorsal root ganglia

Wnt proteins have been implicated in regulating a variety of developmental processes in the CNS. Secreted Frizzled-related protein 3 (sFRP3) is a member of the sFRP family that can inhibit the Wnt signaling by binding directly to Wnts via their regions of homology to the Wnt-binding domain of Frizzleds. Recent studies suggested that sFRP3 plays an important role in cell proliferation and differentiation in various tissues. To understand the role of sFRP3 in neural development, we carried out detailed studies on the expression of sFRP3 in the developing nervous system. Our results revealed that sFRP3 is initially expressed in the ventricular zone of the spinal cord and dorsal root ganglia (DRG), and later in the dorsal horn of spinal cord and subpopulation of DRG neurons. The spatiotemporally dynamic expression ofsFRP3 strongly suggests that sFRP3 has potential functions in the sensory neuron genesis and sensory circuitry formation.

The WNT signaling pathway from ligand secretion to gene transcription: molecular mechanisms and pharmacological targets

Wingless/integrase-1 (WNT) signaling is a key pathway regulating various aspects of embryonic development; however it also underlies several pathological conditions in man, including various cancers and fibroproliferative diseases in several organs. Investigating the molecular processes involved in (canonical) WNT signaling will open new avenues for generating new therapeutics to specifically target diseases in which WNT signaling is aberrantly regulated. Here we describe the complexity of WNT signal transduction starting from the processes involved in WNT ligand biogenesis and secretion by WNT producing cells followed by a comprehensive overview of the molecular signaling events ultimately resulting in enhanced transcription of specific genes in WNT receiving cells. Finally, the possible targets for therapeutic intervention and the available pharmacological inhibitors for this complex signaling pathway are discussed.

Wnt signal transduction pathways

The Wnt signaling pathway is an ancient and evolutionarily conserved pathway that regulates crucial aspects of cell fate determination, cell migration, cell polarity, neural patterning and organogenesis during embryonic development. The Wnts are secreted glycoproteins and comprise a large family of nineteen proteins in humans hinting to a daunting complexity of signaling regulation, function and biological output. To date major signaling branches downstream of the Fz receptor have been identified including a canonical or Wnt/beta-catenin dependent pathway and the non-canonical or beta-catenin-independent pathway which can be further divided into the Planar Cell Polarity and the Wnt/Ca(2+) pathways, and these branches are being actively dissected at the molecular and biochemical levels. In this review, we will summarize the most recent advances in our understanding of these Wnt signaling pathways and the role of these pathways in regulating key events during embryonic patterning and morphogenesis.

Variant alleles of the Wnt antagonist FRZB are determinants of hip shape and modify the relationship between hip shape and osteoarthritis

OBJECTIVE

To test whether single-nucleotide polymorphisms (SNPs) of the FRZB gene are associated with hip shape, and to determine whether FRZB variant alleles affect the relationship between hip shape and radiographic osteoarthritis (OA) of the hip.

METHODS

A nested case-control study of Caucasian women, age ≥65 years, from the Study of Osteoporotic Fractures cohort was performed. Cases (n = 451) were defined as subjects with radiographic evidence of incident hip OA during followup, while controls (n = 601) were subjects in whom no radiographic hip OA was identified at baseline or followup. Statistical shape modeling (SSM) of the digitized hip radiographs was performed to assess the shape of the proximal femur, using 10 independent modes of shape variation generated by principal components analysis. In addition, center-edge angle and acetabular depth were assessed as geometric measurements of acetabular shape. The association of the rs288326 and rs7775 FRZB variant alleles with hip shape was analyzed using linear regression. The effect of these alleles on the relationship between hip shape and radiographic hip OA was analyzed using a logistic regression model with or without inclusion of interaction terms.

RESULTS

The rs288326 and rs7775 alleles were associated with the shape of the proximal femur (SSM mode 2). There was a significant interaction between the rs288326 SNP and proximal femur shape (SSM mode 2) in predicting radiographic hip OA (P for interaction = 0.022). Among subjects with the rs288326 variant allele, there was an increased likelihood of radiographic hip OA in association with increasing quartiles of proximal femur shape mode 2 (for the fourth quartile of mode 2, odds ratio 2.5, 95% confidence interval 1.15, 5.25; P for linear trend = 0.02).

CONCLUSION

The rs288326 and rs7775 FRZB SNPs are associated with the shape of the proximal femur. The presence of the rs288326 SNP alters the relationship between proximal femur shape and incident radiographic hip OA. These findings suggest that FRZB may serve an important role in determining hip shape and may modify the relationship between hip shape and OA.

Wnt signalling in smooth muscle cells and its role in cardiovascular disorders

Vascular smooth muscle cells (SMCs) are the major cell type within blood vessels. SMCs exhibit low rates of proliferation, migration, and apoptosis in normal blood vessels. However, increased SMC proliferation, migration, and apoptosis rates radically alter the composition and structure of the blood vessel wall and contribute to cardiovascular diseases, such as atherosclerosis, and restenosis that occur after coronary artery vein grafting and stent implantation. Consequently, therapies that modulate SMC proliferation, migration, and apoptosis may be useful for treating cardiovascular diseases. The family of Wnt proteins, which were first identified in the wingless drosophila, has a well-established role in embryogenesis and development. It is now emerging that Wnt proteins also regulate SMC proliferation, migration, and survival. In this review article, we discuss recently emerging research that has revealed that Wnt proteins are important regulators of SMC behaviour via activation of β-catenin-dependent and β-catenin-independent Wnt signalling pathways.

Regulation of SFRP-1 expression in the rat dental follicle

Tooth eruption requires osteoclastogenesis and subsequent bone resorption. Secreted frizzled-related protein-1 (SFRP-1) negatively regulates osteoclastogenesis. Our previous studies indicated that SFRP-1 is expressed in the rat dental follicle (DF), with reduced expression at days 3 and 9 close to the times for the major and minor bursts of osteoclastogenesis, respectively; but it remains unclear as to what molecules contribute to its reduced expression at these critical times. Thus, it was the aim of this study to determine which molecules regulate the expression of SFRP-1 in the DF. To that end, the DF cells were treated with cytokines that are maximally expressed at days 3 or 9, and SFRP-1 expression was determined. Our study indicated that colony-stimulating factor-1 (CSF-1), a molecule maximally expressed in the DF at day 3, down-regulated SFRP-1 expression. As to endothelial monocyte-activating polypeptide II (EMAP-II), a highly expressed molecule in the DF at day 3, it had no effect on the expression of SFRP-1. However, when EMAP-II was knocked down by siRNA, the expression of SFRP-1 was elevated, and this elevated SFRP-1 expression could be reduced by adding recombinant EMAP-II protein. This suggests that EMAP-II maintained a lower level of SFRP-1 in the DF. TNF-α is a molecule maximally expressed at day 9, and this study indicated that it also down-regulated the expression of SFRP-1 in the DF cells. In conclusion, CSF-1 and EMAP-II may contribute to the reduced SFRP-1 expression seen on day 3, while TNF-α may contribute to the reduced SFRP-1 expression at day 9.

Secreted frizzled related protein 4 reduces fibrosis scar size and ameliorates cardiac function after ischemic injury

Expression of the Wnt modulator secreted frizzled related protein 4 (Sfrp4) is upregulated after heart ischemic injury. We show that intramuscular administration of recombinant Sfrp4 to rat heart ischemic injury and recanalization models prevents further deterioration of cardiac function after the ischemic injury. The effect of Sfrp4 persisted for at least 20 weeks when Sfrp4 was administered in a slow release system (Sfrp4-polyhedra) to both acute and subacute ischemic models. The histology of the dissected heart showed that the cardiac wall was thicker and the area of acellular scarring was smaller in Sfrp4-treated hearts than in controls. Increased amounts of both the inactive serine 9-phosphorylated form of glycogen synthase kinase (GSK)-3β and the active form of β-catenin were observed by immunohistology 3 days after lateral anterior descendant ligation in control, but not in Sfrp4-treated hearts. All together, we show that administration of Sfrp4 interferes with canonical Wnt signaling that could mediate the formation of acellular scar and consequently contributes to the prevention of aggravation of cardiac function.

Osteocytes and WNT: the Mechanical Control of Bone Formation

Mechanical loading is of pivotal importance in the maintenance of skeletal homeostasis, but the players involved in the transduction of mechanical stimuli to promote bone maintenance have long remained elusive. Osteocytes, the most abundant cells in bone, possess mechanosensing appendices stretching through a system of bone canaliculi. Mechanical stimulation plays an important role in osteocyte survival and hence in the preservation of bone mechanical properties, through the maintenance of bone hydratation. Osteocytes can also control the osteoblastic differentiation of mesenchymal precursors in response to mechanical loading by modulating WNT signaling pathways, essential regulators of cell fate and commitment, through the protein sclerostin. Mutations of Sost, the sclerostin-encoding gene, have dramatic effects on the skeleton, indicating that osteocytes may act as master regulators of bone formation and localized bone remodeling. Moreover, the development of sclerostin inhibitors is opening new possibilities for bone regeneration in orthopedics and the dental field.

Genetic dissection of differential signaling threshold requirements for the Wnt/beta-catenin pathway in vivo

Contributions of null and hypomorphic alleles of Apc in mice produce both developmental and pathophysiological phenotypes. To ascribe the resulting genotype-to-phenotype relationship unambiguously to the Wnt/β-catenin pathway, we challenged the allele combinations by genetically restricting intracellular β-catenin expression in the corresponding compound mutant mice. Subsequent evaluation of the extent of resulting Tcf4-reporter activity in mouse embryo fibroblasts enabled genetic measurement of Wnt/β-catenin signaling in the form of an allelic series of mouse mutants. Different permissive Wnt signaling thresholds appear to be required for the embryonic development of head structures, adult intestinal polyposis, hepatocellular carcinomas, liver zonation, and the development of natural killer cells. Furthermore, we identify a homozygous Apc allele combination with Wnt/β-catenin signaling capacity similar to that in the germline of the Apc^min mice, where somatic Apc loss-of-heterozygosity triggers intestinal polyposis, to distinguish whether co-morbidities in Apc^min mice arise independently of intestinal tumorigenesis. Together, the present genotype–phenotype analysis suggests tissue-specific response levels for the Wnt/β-catenin pathway that regulate both physiological and pathophysiological conditions.

Germline or somatic mutations in genes are the underlying cause of many human diseases, most notably cancer. Interestingly though, even in situations where every cell of every tissue of an organism carries the same mutation (as is the case for germline mutations), some tissues are more susceptible to the development of disease over time than others. For example, in familial adenomatous polyposis (FAP), affected persons carry different germline mutations in the APC gene and are prone to developing cancers of the colon and the rectum—and, less frequently, cancers in other tissues such as stomach, liver, and bones. Here we utilize a panel of mutant mice with truncating or hypomorphic mutations in the Apc gene, resulting in different levels of activation of the Wnt/β-catenin pathway. Our results reveal that different pathophysiological outcomes depend on different permissive signaling thresholds in embryonic, intestinal, and liver tissues. Importantly, we demonstrate that reducing Wnt pathway activation by 50% is enough to prevent the manifestation of embryonic abnormalities and disease in the adult mouse. This raises the possibility of developing therapeutic strategies that modulate the activation levels of this pathway rather than trying to “repair” the mutation in the gene itself.

Is Wnt signalling the final common pathway leading to bone formation?

Since the discovery of the link between mutations in the LRP5 gene and human bone mass, considerable progress has been made in our understanding of Wnt signalling and bone formation. The connection between canonical Wnt signalling and bone formation is convincing, and there is evidence of interaction between the Wnt signalling pathway and key growth factors, transcriptional factors and systemic hormones. More recently, the role of the non-canonical pathway in bone metabolism has also started to be explored as well as potential bone-gut interactions. This review focuses on the role of the Wnt pathway in osteoblast differentiation as well as the interplay between Wnt signalling and other pathways involved in bone formation.

Neural differentiation of embryonic stem cells in vitro: a road map to neurogenesis in the embryo

Background

The in vitro generation of neurons from embryonic stem (ES) cells is a promising approach to produce cells suitable for neural tissue repair and cell-based replacement therapies of the nervous system. Available methods to promote ES cell differentiation towards neural lineages attempt to replicate, in different ways, the multistep process of embryonic neural development. However, to achieve this aim in an efficient and reproducible way, a better knowledge of the cellular and molecular events that are involved in the process, from the initial specification of neuroepithelial progenitors to their terminal differentiation into neurons and glial cells, is required.

Methodology/Principal Findings

In this work, we characterize the main stages and transitions that occur when ES cells are driven into a neural fate, using an adherent monolayer culture system. We established improved conditions to routinely produce highly homogeneous cultures of neuroepithelial progenitors, which organize into neural tube-like rosettes when they acquire competence for neuronal production. Within rosettes, neuroepithelial progenitors display morphological and functional characteristics of their embryonic counterparts, namely, apico-basal polarity, active Notch signalling, and proper timing of production of neurons and glia. In order to characterize the global gene activity correlated with each particular stage of neural development, the full transcriptome of different cell populations that arise during the in vitro differentiation protocol was determined by microarray analysis. By using embryo-oriented criteria to cluster the differentially expressed genes, we define five gene expression signatures that correlate with successive stages in the path from ES cells to neurons. These include a gene signature for a primitive ectoderm-like stage that appears after ES cells enter differentiation, and three gene signatures for subsequent stages of neural progenitor development, from an early stage that follows neural induction to a final stage preceding terminal differentiation.

Conclusions/Significance

Overall, our work confirms and extends the cellular and molecular parallels between monolayer ES cell neural differentiation and embryonic neural development, revealing in addition novel aspects of the genetic network underlying the multistep process that leads from uncommitted cells to differentiated neurons.

The Wnt antagonists Frzb-1 and Crescent locally regulate basement membrane dissolution in the developing primary mouth

The primary mouth forms from ectoderm and endoderm at the extreme anterior of the embryo, a conserved mesoderm-free region. In Xenopus, a very early step in primary mouth formation is loss of the basement membrane between the ectoderm and endoderm. In an unbiased microarray screen, we defined genes encoding the sFRPs Frzb-1 and Crescent as transiently and locally expressed in the primary mouth anlage. Using antisense oligonucleotides and ;face transplants', we show that frzb-1 and crescent expression is specifically required in the primary mouth region at the time this organ begins to form. Several assays indicate that Frzb-1 and Crescent modulate primary mouth formation by suppressing Wnt signaling, which is likely to be mediated by beta-catenin. First, a similar phenotype (no primary mouth) is seen after loss of Frzb-1/Crescent function to that seen after temporally and spatially restricted overexpression of Wnt-8. Second, overexpression of either Frzb-1 or Dkk-1 results in an enlarged primary mouth anlage. Third, overexpression of Dkk-1 can restore a primary mouth to embryos in which Frzb-1/Crescent expression has been inhibited. We show that Frzb-1/Crescent function locally promotes basement membrane dissolution in the primary mouth primordium. Consistently, Frzb-1 overexpression decreases RNA levels of the essential basement membrane genes fibronectin and laminin, whereas Wnt-8 overexpression increases the levels of these RNAs. These data are the first to connect Wnt signaling and basement membrane integrity during primary mouth development, and suggest a general paradigm for the regulation of basement membrane remodeling.

A Wnt survival guide: from flies to human disease

It has been two decades since investigators discovered the link between the Drosophila wingless (Wg) gene and the vertebrate oncogene int-1, thus establishing the family of signaling proteins known as Wnts. Since the inception of the Wnt signaling field, there have been 19 Wnt isoforms identified in humans. These secreted glycoproteins can activate at least two distinct signaling pathways in vertebrate cells, leading to cellular changes that regulate a vast array of biological processes, including embryonic development, cell fate, cell proliferation, cell migration, stem cell maintenance, tumor suppression, and oncogenesis. In certain contexts, one subset of Wnt isoforms activates the canonical Wnt/beta-catenin pathway that is characterized by the activation of certain beta-catenin-responsive target genes in response to the binding of Wnt ligand to its cognate receptors. Similarly, a second subset of Wnt isoforms activates beta-catenin-independent pathways, including the Wnt/calcium (Wnt/Ca) pathway and the Wnt/planar cell polarity (Wnt/PCP) pathway, in certain cellular contexts. In addition, research has identified several secreted proteins known to regulate Wnt signaling, including the Dickkopf (DKK) family, secreted Frizzled-related proteins (sFRPs), and Wnt inhibitory factor-1 (WIF-1). The advent of technologies that can provide genome-wide expression data continues to implicate Wnts and proteins that regulate Wnt signaling pathways in a growing number of disease processes. The aim of this review is to provide a context on the Wnt field that will facilitate the interpretation and study of Wnt signaling in the context of human disease.

Microarray analysis of Foxa2 mutant mouse embryos reveals novel gene expression and inductive roles for the gastrula organizer and its derivatives

Background

The Spemann/Mangold organizer is a transient tissue critical for patterning the gastrula stage vertebrate embryo and formation of the three germ layers. Despite its important role during development, there are still relatively few genes with specific expression in the organizer and its derivatives. Foxa2 is a forkhead transcription factor that is absolutely required for formation of the mammalian equivalent of the organizer, the node, the axial mesoderm and the definitive endoderm (DE). However, the targets of Foxa2 during embryogenesis, and the molecular impact of organizer loss on the gastrula embryo, have not been well defined.

Results

To identify genes specific to the Spemann/Mangold organizer, we performed a microarray-based screen that compared wild-type and Foxa2 mutant embryos at late gastrulation stage (E7.5). We could detect genes that were consistently down-regulated in replicate pools of mutant embryos versus wild-type, and these included a number of known node and DE markers. We selected 314 genes without previously published data at E7.5 and screened for expression by whole mount in situ hybridization. We identified 10 novel expression patterns in the node and 5 in the definitive endoderm. We also found significant reduction of markers expressed in secondary tissues that require interaction with the organizer and its derivatives, such as cardiac mesoderm, vasculature, primitive streak, and anterior neuroectoderm.

Conclusion

The genes identified in this screen represent novel Spemann/Mangold organizer genes as well as potential Foxa2 targets. Further investigation will be needed to define these genes as novel developmental regulatory factors involved in organizer formation and function. We have placed these genes in a Foxa2-dependent genetic regulatory network and we hypothesize how Foxa2 may regulate a molecular program of Spemann/Mangold organizer development. We have also shown how early loss of the organizer and its inductive properties in an otherwise normal embryo, impacts on the molecular profile of surrounding tissues.

Genomic deletions correlate with underexpression of novel candidate genes at six loci in pediatric pilocytic astrocytoma

The molecular pathogenesis of pediatric pilocytic astrocytoma (PA) is not well defined. Previous cytogenetic and molecular studies have not identified nonrandom genetic aberrations. To correlate differential gene expression and genomic copy number aberrations (CNAs) in PA, we have used Affymetrix GeneChip HG_U133A to generate gene expression profiles of 19 pediatric patients and the SpectralChip 2600 to investigate CNAs in 11 of these tumors. Hierarchical clustering according to expression profile similarity grouped tumors and controls separately. We identified 1844 genes that showed significant differential expression between tumor and normal controls, with a large number clearly influencing phosphatidylinositol and mitogen-activated protein kinase signaling in PA. Most CNAs identified in this study were single-clone alterations. However, a small region of loss involving up to seven adjacent clones at 7q11.23 was observed in seven tumors and correlated with the underexpression of BCL7B. Loss of four individual clones was also associated with reduced gene expression including SH3GL2 at 9p21.2-p23, BCL7A (which shares 90% sequence homology with BCL7B) at 12q24.33, DRD1IP at 10q26.3, and TUBG2 and CNTNAP1 at 17q21.31. Moreover, the down-regulation of FOXG1B at 14q12 correlated with loss within the gene promoter region in most tumors. This is the first study to correlate differential gene expression with CNAs in PA.

Wnt signaling determines ventral spinal cord cell fates in a time-dependent manner

The identity of distinct cell types in the ventral neural tube is generally believed to be specified by sonic hedgehog (Shh) in a concentration-dependent manner. However, recent studies have questioned whether Shh is the sole signaling molecule determining ventral neuronal cell fates. Here we provide evidence that canonical Wnt signaling is involved in the generation of different cell types in the ventral spinal cord. We show that Wnt signaling is active in the mouse ventral spinal cord at the time when ventral cell types are specified. Furthermore, using an approach that stabilizes beta-catenin protein in small patches of ventral spinal cord cells at different stages, we show that Wnt signaling activates different subsets of target genes depending on the time when Wnt signaling is amplified. Moreover, disruption of Wnt signaling results in the expansion of ventrally located progenitors. Finally, we show genetically that Wnt signaling interacts with Hh signaling at least in part through regulating the transcription of Gli3. Our results reveal a novel mechanism by which ventral patterning is achieved through a coordination of Wnt and Shh signaling.

Brachy-syndactyly caused by loss of Sfrp2 function

Wnt signaling pathways are regulated both at the intracellular and extracellular levels. During embryogenesis, the in vivo effects of the secreted frizzled-related protein (Sfrp) family of Wnt inhibitors are poorly understood. Here, we show that inactivation of Sfrp2 results in subtle limb defects in mice with mesomelic shortening and consistent shortening of all autopodal elements that is clinically manifested as brachydactyly. In addition, there is soft-tissue syndactyly of the hindlimb. The brachydactyly is caused by decreased chondrocyte proliferation and delayed differentiation in distal limb chondrogenic elements. These data suggest that Sfrp2 can regulate both chondrogenesis and regression of interdigital mesenchyme in distal limb. Sfrp2 can also repress canonical Wnt signaling by Wnt1, Wnt9a, and Wnt4 in vitro. Sfrp2-/- and TOPGAL/Sfrp2-/- mice have a mild increase in beta-catenin and beta-galactosidase staining, respectively, in some phalangeal elements. This however does not exclude a potential concurrent effect on non-canonical Wnt signaling in the growth plate. In combination with what is known about BMP and Wnt signaling in human brachydactylies, our data establish a critical role for Sfrp2 in proper distal limb formation and suggest SFPR2 could be a novel candidate gene for human brachy-syndactyly defects.

Expression of secreted frizzled related protein 1, a Wnt antagonist, in brain, kidney, and skeleton is dispensable for normal embryonic development

Secreted frizzled related protein-1 (sFRP1), an antagonist of Wnt signaling, regulates cell proliferation, differentiation and apoptosis and negatively regulates bone formation. The spatial and temporal pattern of endogenous sFRP1 expression and loss-of-function were examined in the sFRP1-LacZ knock-in mouse (sFRP1-/-) during embryonic development and post-natal growth. beta-gal activity representing sFRP1 expression is robust in brain, skeleton, kidney, eye, spleen, abdomen, heart and somites in early embryos, but sFRP1 gene inactivation in these tissues did not compromise normal embryonic and post-natal development. Kidney histology revealed increased numbers of glomeruli in KO mice, observed after 5 years of breeding. In the skeleton, we show sFRP1 expression is found in relation to the mineralizing front of bone tissue during skeletal development from E15.5 to birth. Trabecular bone volume and bone mineral density in the sFRP1-/- mouse compared to WT was slightly increased during post-natal growth. Calvarial osteoblasts from newborn sFRP1-/- mice exhibited a 20% increase in cell proliferation and differentiation at the early stages of osteoblast maturation. sFRP1 expression was observed in osteoclasts, but this did not affect osteoclast number or activity. These findings have identified functions for sFRP1 in kidney and bone that are not redundant with other sFRPs. In summary, the absence of major organ abnormalities, the enhanced bone formation and a normal life span with no detection of spontaneous tumors suggests that targeting sFRP1 can be used as a therapeutic strategy for increasing bone mass in metabolic bone disorders or promoting fracture healing by modulating Wnt signaling.

Molecular effects of lithium exposure during mouse and chick gastrulation and subsequent valve dysmorphogenesis

BACKGROUND

Lithium (Li) has been associated with cardiac teratogenicity in the developing fetus. We took advantage of the association of therapeutic administration of Li with an increase in heart defects to gain insight into both normal and pathological heart and valve development with GSK-3 inhibition. The objective of this study was to define whether Li mimicry of canonical Wnt/beta-catenin signaling induces cardiac valve defects.

METHODS

Li was administered by a single intraperitoneal injection to the pregnant mouse on embryonic day E6.75, much earlier than heretofore analyzed. On E15.5 developing heart defects were defined by Doppler ultrasound. The embryonic hearts were analyzed for changes in patterning of active canonical Wnt expression and nuclear factor of the activated T cells-c1 (NFATc1), both key regulators of valve development. Li-exposed chick embryos were used to define the early cell populations during gastrulation that are susceptible to GSK-3 inhibition and may relate to valve formation.

RESULTS

Li exposure during gastrulation decreased the number of prechordal plate (PP) cells that reached the anterior intestinal portal, a region associated with valve development. Li decreased expression of Hex, an endoderm cardiac inducing molecule, normally also expressed by the PP cells, and of Sox 4 at the anterior intestinal portal and NFAT, critical factors in valvulogenesis.

CONCLUSIONS

Cells existing already during gastrulation are associated with valve formation days later. The Wnt/beta-catenin signaling in PP cells is normally repressed by Wnt antagonists and Hex is up-regulated. The antagonism occurring at the receptor level is bypassed by Li exposure by its intracellular inactivation of GSK-3 directly to augment Wnt signaling.

Dkk1andWnt3interact to control head morphogenesis in the mouse

Loss of Dkk1 results in ectopic WNT/β-catenin signalling activity in the anterior germ layer tissues and impairs cell movement in the endoderm of the mouse gastrula. The juxtaposition of the expression domains of Dkk1 and Wnt3 is suggestive of an antagonist-agonist interaction. The downregulation of Dkk1 when Wnt3 activity is reduced reveals a feedback mechanism for regulating WNT signalling. Compound Dkk1;Wnt3 heterozygous mutant embryos display head truncation and trunk malformation, which are not found in either Dkk1+/- or Wnt3+/- embryos. Reducing the dose of Wnt3 gene in Dkk1-/- embryos partially rescues the truncated head phenotype. These findings highlight that head development is sensitive to the level of WNT3 signalling and that DKK1 is the key antagonist that modulates WNT3 activity during anterior morphogenesis.

Sfrp1, Sfrp2, and Sfrp5 regulate the Wnt/beta-catenin and the planar cell polarity pathways during early trunk formation in mouse

Sfrp is a secreted Wnt antagonist that directly interacts with Wnt ligand. We show here that inactivation of Sfrp1, Sfrp2, and Sfrp5 leads to fused somites formation in early-somite mouse embryos, simultaneously resulting in defective convergent extension (CE), which causes severe shortening of the anteroposterior axis. These observations indicate the redundant roles of Sfrp1, Sfrp2, and Sfrp5 in early trunk formation. The roles of the Sfrps were genetically distinguished in terms of the regulation of Wnt pathways. Genetic analysis combining Sfrps mutants and Loop-tail mice revealed the involvement of Sfrps in CE through the regulation of the planar cell polarity pathway. Furthermore, Dkk1-deficient embryos carrying Sfrp1 homozygous and Sfrp2 heterozygous mutations display irregular somites and indistinct intersomitic boundaries, which indicates that Sfrps-mediated inhibition of the Wnt/beta-catenin pathway is necessary for somitogenesis. Our results suggest that Sfrps regulation of the canonical and noncanonical pathways is essential for proper trunk formation.

Mouse homologues of Shisa antagonistic to Wnt and Fgf signalings

In an effort to identify Otx2 targets in mouse anterior neuroectoderm we identified a gene, mShisa, which is homologous to xShisa1 that we previously reported as a head inducer in Xenopus. mShisa encodes an antagonist against both Wnt and Fgf signalings; it inhibits these signalings cell-autonomously as xShisa1 does. The mShisa expression is lost or greatly reduced in Otx2 mutant visceral endoderm, anterior mesendoderm and anterior neuroectoderm. However, mShisa mutants exhibited no defects in head development. Shisa is composed of five subfamilies, but normal head development in mShisa mutants is unlikely to be explained in terms of the compensation of mShisa deficiency by its paralogues or by known Wnt antagonists in anterior visceral endoderm and/or anterior mesendoderm.

Investigation of mRNA expression for secreted frizzled-related protein 2 (sFRP2) in chick embryos

The roles of secreted frizzled-related protein 2 (sFRP2) in organ development of vertebrate animals are not well understood. We investigated expression of sFRP2 during embryogenesis of Arbor Acre broiler chicken eggs. Expression of sFRP2 was detected in the folds and lateral layer of developing brains. The sFRP2 signals in the developing eye were marked as a circle along the orbit. In younger embryos on days 3-6, the sFRP2 signals were consistent with growth of the sclerotome, suggesting that sFRP2 may be associated with somite development. Furthermore, with the exception of bones, sFRP2 mRNA was detectable in the interdigital tissue of embryos older than eight days as the limbs matured. This revealed that sFRP2 might play a role in myogenesis. In situ hybridization was also used to analyze the expression of sFRP2 in day 3-10 chick embryos. Signals were expressed in the gray matter of the developing brain coelom, including the optic lobe, metencephalon, myelencephalon, mesencephalon and diencephalon. The developing eyes contained an intercellular distribution of sFRP2 in the pigmented layer of the retina and photoreceptors. Furthermore, sFRP2 was expressed in the mantle layer of the neural tube and notochord. Based on these findings, it seems reasonable to suggest that sFRP2 may play an active role in embryogenesis, especially in development of the neural system, eyes, muscles and limbs.

Wnt signal transduction and the formation of the myocardium

Soon after fertilization, vertebrate embryos grow very rapidly. Thus, early in gestation, a sizeable yet underdeveloped organism requires circulating blood. This need dictates the early appearance of a contractile heart, which is the first functional organ in both the avian and mammalian embryo. The heart arises from paired mesodermal regions within the anterior half of the embryo. As development proceeds, these bilateral precardiac fields merge at the midline to give rise to the primary heart tube. How specific areas of nondifferentiated mesoderm organize into myocardial tissue has been a question that has long intrigued developmental biologists. In recent years, the regulation of Wnt signal transduction has been implicated as an important event that initiates cardiac development. While initial reports in Drosophila and the bird had implicated Wnt proteins as promoters of cardiac tissue formation, subsequent findings that the WNT inhibitors Dkk1 and crescent possess cardiac-inducing activities led to the contrary hypothesis that WNTs actively inhibit cardiogenesis. This seeming contradiction has been resolved, in part, by more recent information indicating that Wnts stimulate multiple signal transduction pathways. In this review, we will examine what is presently known about the importance of regulated Wnt activity for the formation of the heart and the development of the myocardium and discuss this information in context of the emerging complexity of Wnt signal transduction.

Canonical WNT signaling promotes osteogenesis by directly stimulating Runx2 gene expression

Both activating and null mutations of proteins required for canonical WNT signaling have revealed the importance of this pathway for normal skeletal development. However, tissue-specific transcriptional mechanisms through which WNT signaling promotes the differentiation of bone-forming cells have yet to be identified. Here, we address the hypothesis that canonical WNT signaling and the bone-related transcription factor RUNX2/CBFA1/AML3 are functionally linked components of a pathway required for the onset of osteoblast differentiation. Our findings show that, in bone of the SFRP1 (secreted frizzled-related protein-1)-null mouse, which exhibits activated WNT signaling and a high bone mass phenotype, there is a significant increase in expression of T-cell factor (TCF)-1, Runx2, and the RUNX2 target gene osteocalcin. We demonstrate by mutational analysis that a functional TCF regulatory element responsive to canonical WNT signaling resides in the promoter of the Runx2 gene (-97 to -93). By chromatin immunoprecipitation, recruitment of beta-catenin and TCF1 to the endogenous Runx2 gene is shown. Coexpression of TCF1 with canonical WNT proteins resulted in a 2-5-fold activation of Runx2 promoter activity and a 7-8-fold induction of endogenous mRNA in mouse pluripotent mesenchymal and osteoprogenitor cells. This enhancement was abrogated by SFRP1. Taken together, our results provide evidence for direct regulation of Runx2 by canonical WNT signaling and suggest that Runx2 is a target of beta-catenin/TCF1 for the stimulation of bone formation. We propose that WNT/TCF1 signaling, like bone morphogenetic protein/transforming growth factor-beta signaling, activates Runx2 gene expression in mesenchymal cells for the control of osteoblast differentiation and skeletal development.

Expression of all Wnt genes and their secreted antagonists during mouse blastocyst and postimplantation development

In this extensive study, real-time reverse transcriptase-polymerase chain reaction was used to analyze the expression levels of all 19 Wnt genes and their 11 potential antagonists in mouse blastocysts, pregastrula, gastrula, and neurula stages. By complementing these results with in situ hybridization, we revealed new expression domains for Wnt2b and Sfrp1, respectively, in the future primitive streak at the posterior side and in the anterior visceral endoderm before the initiation of gastrulation. Moreover, the anterior visceral endoderm expresses three secreted Wnt antagonists (Sfrp1, Sfrp5, and Dkk1) in partially overlapping domains. We also identified expression patterns for the Wnt1, Wnt3a, Wnt6, Wnt7b, Wnt9a, Wnt10b, and Sfrp1 genes at the blastocyst stage. In particular, the expression of Wnt1 and Sfrp1 predominantly in the inner cell mass and of Wnt9a in the mural trophoblast and inner cell mass cells surrounding the blastocoele suggests new roles for the Wnt pathway in preimplantation development. This article is the first report on the regional expression of Wnt genes in the mouse blastocyst.

Wnt/beta-catenin signaling in mesenchymal progenitors controls osteoblast and chondrocyte differentiation during vertebrate skeletogenesis

Chondrocytes and osteoblasts are two primary cell types in the skeletal system that are differentiated from common mesenchymal progenitors. It is believed that osteoblast differentiation is controlled by distinct mechanisms in intramembranous and endochondral ossification. We have found that ectopic canonical Wnt signaling leads to enhanced ossification and suppression of chondrocyte formation. Conversely, genetic inactivation of beta-catenin, an essential component transducing the canonical Wnt signaling, causes ectopic formation of chondrocytes at the expense of osteoblast differentiation during both intramembranous and endochondral ossification. Moreover, inactivation of beta-catenin in mesenchymal progenitor cells in vitro causes chondrocyte differentiation under conditions allowing only osteoblasts to form. Our results demonstrate that beta-catenin is essential in determining whether mesenchymal progenitors will become osteoblasts or chondrocytes regardless of regional locations or ossification mechanisms. Controlling Wnt/beta-catenin signaling is a common molecular mechanism underlying chondrocyte and osteoblast differentiation and specification of intramembranous and endochondral ossification.

Dissecting Wnt/beta-catenin signaling during gastrulation using RNA interference in mouse embryos

Differential gene regulation integrated in time and space drives developmental programs during embryogenesis. To understand how the program of gastrulation is regulated by Wnt/beta-catenin signaling, we have used genome-wide expression profiling of conditional beta-catenin mutant embryos. Known Wnt/beta-catenin target genes, known components of other signaling pathways, as well as a number of uncharacterized genes were downregulated in these mutants. To further narrow down the set of differentially expressed genes, we used whole-mount in situ screening to associate gene expression with putative domains of Wnt activity. Several potential novel target genes were identified by this means and two, Grsf1 and Fragilis2, were functionally analyzed by RNA interference (RNAi) in completely embryonic stem (ES) cell-derived embryos. We show that the gene encoding the RNA-binding factor Grsf1 is important for axial elongation, mid/hindbrain development and axial mesoderm specification, and that Fragilis2, encoding a transmembrane protein, regulates epithelialization of the somites and paraxial mesoderm formation. Intriguingly, the knock-down phenotypes recapitulate several aspects of Wnt pathway mutants, suggesting that these genes are components of the downstream Wnt response. This functional genomic approach allows the rapid identification of functionally important components of embryonic development from large datasets of putative targets.

Developmental regulation of Wnt/beta-catenin signals is required for growth plate assembly, cartilage integrity, and endochondral ossification

Studies have suggested that continuous Wnt/beta-catenin signaling in nascent cartilaginous skeletal elements blocks chondrocyte hypertrophy and endochondral ossification, whereas signaling starting at later stages stimulates hypertrophy and ossification, indicating that Wnt/beta-catenin roles are developmentally regulated. To test this conclusion further, we created transgenic mice expressing a fusion mutant protein of beta-catenin and LEF (CA-LEF) in nascent chondrocytes. Transgenic mice had severe skeletal defects, particularly in limbs. Growth plates were totally disorganized, lacked maturing chondrocytes expressing Indian hedgehog and collagen X, and failed to undergo endochondral ossification. Interestingly, the transgenic cartilaginous elements were ill defined, intermingled with surrounding connective and vascular tissues, and even displayed abnormal joints. However, when activated beta-catenin mutant (delta-beta-catenin) was expressed in chondrocytes already engaged in maturation such as those present in chick limbs, chondrocyte maturation and bone formation were greatly enhanced. Differential responses to Wnt/beta-catenin signaling were confirmed in cultured chondrocytes. Activation in immature cells blocked maturation and actually de-stabilized their phenotype, as revealed by reduced expression of chondrocyte markers, abnormal cytoarchitecture, and loss of proteoglycan matrix. Activation in mature cells instead stimulated hypertrophy, matrix mineralization, and expression of terminal markers such as metalloprotease (MMP)-13 and vascular endothelial growth factor. Because proteoglycans are crucial for cartilage function, we tested possible mechanisms for matrix loss. Delta-beta-catenin expression markedly increased expression of MMP-2, MMP-3, MMP-7, MMP-9, MT3-MMP, and ADAMTS5. In conclusion, Wnt/beta-catenin signaling regulates chondrocyte phenotype, maturation, and function in a developmentally regulated manner, and regulated action by this pathway is critical for growth plate organization, cartilage boundary definition, and endochondral ossification.

Lmx1b expression during joint and tendon formation: localization and evaluation of potential downstream targets

The tetrapod limb exhibits distinct dorsoventral joint, tendon, and muscle asymmetry. The LIM-homeodomain transcription factor, Lmx1b, is required to achieve the dorsal character of these structures, but the mechanism by which Lmx1b orchestrates this asymmetrical development is unknown. To identify target tissues and genes regulated by Lmx1b, we examined Lmx1b expression during joint, tendon and muscle formation (9.5-16.5 dpc) and the expression of several genes spatially restricted to developing joints and associated tissues in normal and Lmx1b knockout (KO) mice including: Gdf-5, sFrp2, sFrp3, Six1 and Six2. Lmx1b was diffusely expressed in the undifferentiated dorsal mesoderm of the emerging limb bud (E9.5-E11.5). With progressive proximal to distal differentiation, Lmx1b expression localized to dorsal joint-forming regions, to developing tendons and ligaments, but not to migrating myocytes (E13.5-15.5). By E16.5, mature tendon and ligament associations were evident and Lmx1b expression had regressed. The expression patterns of Gdf-5 and sFrp3 at E15.5 were symmetrical along the dorsoventral axis in normal and Lmx1b KO mice. sFrp2, Six1 and Six2 exhibited asymmetrical dorsoventral expression and in Lmx1b KO mice, this asymmetry is lost; however, none were solely restricted to or excluded from dorsal Lmx1b expressing tissues.

Coordinate regulation of neural tube patterning and proliferation by TGFbeta and WNT activity

Pattern formation and growth must be tightly coupled during embryonic development. In vertebrates, however, little is known of the molecules that serve to link these two processes. Here we show that bone morphogenetic proteins (BMP) coordinate the acquisition of pattern information and the stimulation of proliferation in the embryonic spinal neural tube. We have blocked BMP and transforming growth factor-beta superfamily (TGFbeta) function in the chick embryo using Noggin, a BMP antagonist, and siRNA against Smad4. We show that BMPs/TGFbetas are necessary to regulate pattern formation and the specification of neural progenitor populations in the dorsal neural tube. BMPs also serve to establish discrete expression domains of Wnt ligands, receptors, and antagonists along the dorsal-ventral axis of the neural tube. Using the extracellular domain of Frizzled 8 to block Wnt signaling and Wnt3a ligand misexpression to activate WNT signaling, we demonstrate that the Wnt pathway acts mitogenically to expand the populations of neuronal progenitor cells specified by BMP. Thus, BMPs, acting through WNTs, couple patterning and growth to generate dorsal neuronal fates in the appropriate proportions within the neural tube.

Gastrula organiser and embryonic patterning in the mouse

Embryonic patterning of the mouse during gastrulation and early organogenesis engenders the specification of anterior versus posterior structures and body laterality by the interaction of signalling and modulating activities. A group of cells in the mouse gastrula, characterised by the expression of a repertoire of "organiser" genes, acts as a source and the conduit for allocation of the axial mesoderm, floor plate and definitive endoderm. The organiser and its derivatives provide the antagonistic activity that modulates WNT and TGFbeta signalling. Recent findings show that the organiser activity is augmented by morphogenetic activity of the extraembryonic and embryonic endoderm, suggesting embryonic patterning is not solely the function of the organiser.

Wnt 6 regulates the epithelialisation process of the segmental plate mesoderm leading to somite formation

In higher vertebrates, the paraxial mesoderm undergoes a mesenchymal to epithelial transformation to form segmentally organised structures called somites. Experiments have shown that signals originating from the ectoderm overlying the somites or from midline structures are required for the formation of the somites, but their identity has yet to be determined. Wnt6 is a good candidate as a somite epithelialisation factor from the ectoderm since it is expressed in this tissue. In this study, we show that injection of Wnt6-producing cells beneath the ectoderm at the level of the segmental plate or lateral to the segmental plate leads to the formation of numerous small epithelial somites. Ectopic expression of Wnt6 leads to sustained expression of markers associated with the epithelial somites and reduced or delayed expression of markers associated with mesenchymally organised somitic tissue. More importantly, we show that Wnt6-producing cells are able to rescue somite formation after ectoderm ablation. Furthermore, injection of Wnt6-producing cells following the isolation of the neural tube/notochord from the segmental plate was able to rescue somite formation at both the structural (epithelialisation) and molecular level, as determined by the expression of marker genes like Paraxis or Pax-3. We show that Wnts are indeed responsible for the epithelialisation of somites by applying Wnt antagonists, which result in the segmental plate being unable to form somites. These results show that Wnt6, the only known member of this family to be localised to the chick paraxial ectoderm, is able to regulate the development of epithelial somites and that cellular organisation is pivotal in the execution of the differentiation programmes. We propose a model in which the localisation of Wnt6 and its antagonists regulates the process of epithelialisation in the paraxial mesoderm.

Secreted frizzled-related protein 4 is a potent tumor-derived phosphaturic agent

Tumors associated with osteomalacia elaborate the novel factor(s), phosphatonin(s), which causes phosphaturia and hypophosphatemia by cAMP-independent pathways. We show that secreted frizzled-related protein-4 (sFRP-4), a protein highly expressed in such tumors, is a circulating phosphaturic factor that antagonizes renal Wnt-signaling. In cultured opossum renal epithelial cells, sFRP-4 specifically inhibited sodium-dependent phosphate transport. Infusions of sFRP-4 in normal rats over 2 hours specifically increased renal fractional excretion of inorganic phosphate (FEPi) from 14% +/- 2% to 34% +/- 5% (mean +/- SEM, P < 0.01). Urinary cAMP and calcium excretion were unchanged. In thyro-parathyroidectomized rats, sFRP-4 increased FEPi from 0.7% +/- 0.2% to 3.8% +/- 1.2% (P < 0.05), demonstrating that sFRP-4 inhibits renal inorganic phosphate reabsorption by PTH-independent mechanisms. Administration of sFRP-4 to intact rats over 8 hours increased FEPi, decreased serum phosphate (1.95 +/- 0.1 to 1.53 +/- 0.09 mmol/l, P < 0.05) but did not alter serum 1alpha, 25-dihydroxyvitamin D, renal 25-hydroxyvitamin D 1alpha-hydroxylase cytochrome P450, and sodium-phosphate cotransporter mRNA concentrations. Infusion of sFRP-4 antagonizes Wnt action as demonstrated by reduced renal beta-catenin and increased phosphorylated beta-catenin concentrations. The sFRP-4 is detectable in normal human serum and in the serum of a patient with tumor-induced osteomalacia. Thus, sFRP-4 displays phosphatonin-like properties, because it is a circulating protein that promotes phosphaturia and hypophosphatemia and blunts compensatory increases in 1alpha, 25-dihydroxyvitamin D.

Functional ablation of the mouse Ldb1 gene results in severe patterning defects during gastrulation

The LIM domain-binding protein 1 (Ldb1) is found in multi-protein complexes containing various combinations of LIM-homeodomain, LIM-only, bHLH, GATA and Otx transcription factors. These proteins exert key functions during embryogenesis. Here we show that targeted deletion of the Ldb1 gene in mice results in a pleiotropic phenotype. There is no heart anlage and head structures are truncated anterior to the hindbrain. In about 40% of the mutants, posterior axis duplication is observed. There are also severe defects in mesoderm-derived extraembryonic structures, including the allantois, blood islands of the yolk sack, primordial germ cells and the amnion. Abnormal organizer gene expression during gastrulation may account for the observed axis defects in Ldb1 mutant embryos. The expression of several Wnt inhibitors is curtailed in the mutant, suggesting that Wnt pathways may be involved in axial patterning regulated by Ldb1.

Wnt regulation of chondrocyte differentiation

The Wnt family of growth factors are important regulators of several developmental processes including skeletogenesis. To further investigate the role of Wnts we analysed their expression in the developing chick limb and performed functional analyses in vivo and in vitro. We found that Wnt5b and Wnt11 are restricted within the prehypertrophic chondrocytes of the cartilage elements, Wnt5a is found in the joints and perichondrium, while Wnt4 is expressed in the developing joints and, in some bones, a subset of the hypertrophic chondrocytes. These Wnts mediate distinct effects on the initiation of chondrogenesis and differentiation of chondrocytes in vitro and in vivo. Wnt4 blocks the initiation of chondrogenesis and accelerates terminal chondrocyte differentiation in vitro. In contrast, Wnt5a and Wnt5b promote early chondrogenesis in vitro while inhibiting terminal differentiation in vivo. As Wnt5b and Wnt11 expression overlaps with and appears after Indian hedgehog (Ihh), we also compared their effects with Ihh to see if they mediate aspects of Ihh signalling. This showed that Ihh and Wnt5b and Wnt11 control chondrogenesis in parallel pathways.