Functional and structural diversity of the human Dickkopf gene family

Wnt Signaling: A Key Regulator of Bone Mass

The identification of a link between bone mass in humans and gain- [high bone mass (HBM) trait] or loss-of-function [osteoporosis pseudoglioma (OPPG) syndrome] mutations in the Wnt coreceptor lipoprotein receptor-related protein (LRP)5 or in the Wnt antagonist sclerostin (sclerosteosis, Van Buchem syndrome) has called the attention of academic and industry scientists and clinicians to the importance of this signaling pathway in skeletal biology and disease. Multiple genetic and pharmacological manipulations of Wnt signaling in mice have since then confirmed the central role of this pathway in both the establishment of peak bone mass and its maintenance throughout life. Wnt signaling appears to be located downstream of bone morphogenetic proteins (BMPs), itself induced by Hedgehog (Hh) signaling, suggesting that it is the successive recruitment of these three intracellular signaling cascades that allow the full expression of the genetic patterns that characterize the osteoblast, the cell responsible for the formation of bone.

Effects of parathyroid hormone on Wnt signaling pathway in bone

The Wnt signaling pathway has recently been demonstrated to play an important role in bone cell function. In previous studies using DNA microarray analyses, we observed a change in some of the molecular components of the canonical Wnt pathway namely, frizzled-1 (FZD-1) and axil, in response to continuous parathyroid hormone (PTH) treatment in rats. In the present study, we further explored other components of the Wnt signaling pathway in rat distal metaphyseal bone in vivo, and rat osteoblastic osteosarcoma cells (UMR 106) in culture. Several Wnt pathway components, including low-density lipoprotein-receptor-related protein 5 (LRP5), LRP6, FZD-1, Dickkopf-1 (Dkk-1), and Kremen-1 (KRM-1), were expressed in bone in vivo and in osteoblasts in vitro. Continuous exposure to PTH (1-38) both in vivo and in vitro upregulated the mRNA expression of LRP6 and FZD-1 and decreased LRP5 and Dkk-1. These effects in UMR 106 cells were associated with an increase in beta-catenin as measured by Western blots and resulted in functional activation (three to six-fold) of a downstream Wnt responsive TBE6-luciferase (TCF/LEF-binding element) reporter gene. Activation of the TBE6-luciferase reporter gene by PTH (1-38) in UMR 106 cells was inhibited by the protein kinase A (PKA) inhibitor, H89. Activation was mimicked by PTH (1-31), PTH-related protein (1-34), and forskolin, but both PTH (3-34) and (7-34) had no effect. These findings suggest that the effect of PTH on the canonical Wnt signaling pathway occurs at least in part via the cAMP-PKA pathway through the differential regulation of the receptor complex proteins (FZD-1/LRP5 or LRP6) and the antagonist (Dkk-1). Taken together, these results reveal a possible role for the Wnt signaling pathway in PTH actions in bone.

Systematic search for gastric cancer-specific genes based on SAGE data: melanoma inhibitory activity and matrix metalloproteinase-10 are novel prognostic factors in patients with gastric cancer

Gastric cancer (GC) is one of the most common malignancies worldwide. Genes expressed only in cancer tissue will be useful molecular markers for diagnosis and may also be good therapeutic targets. However, little is known about cancer-specific genes, at least in GC. In this study, we searched for GC-specific genes by serial analysis of gene expression (SAGE) data analysis and quantitative reverse transcription (RT)-PCR. Comparing GC SAGE libraries with those of various normal tissues in the SAGEmap database, we identified 54 candidate GC-specific genes. Quantitative RT-PCR analysis of these candidates revealed that APin protein (APIN), taxol resistance-associated gene 3 (TRAG3), cytochrome P450, family 2, subfamily W, polypeptide 1 (CYP2W1), melanoma inhibitory activity (MIA), matrix metalloproteinase-10 (MMP-10), dickkopf homolog 4 (DKK4), GW112, regenerating islet-derived family, member 4 (REGIV), and HORMA domain-containing 1 (HORMAD1) were expressed much more highly in GC than in 14 kinds of normal tissues. Immunohistochemical staining for MIA, MMP-10, and DKK4 was found in 47 (31.1%), 68 (45.0%), and two (1.3%) of 151 GCs, respectively, and staining for both MIA and MMP-10 was correlated with poor prognosis in advanced GC (P=0.0001 and 0.0141, respectively). Moreover, enzyme-linked immunosorbent assay showed high levels of MMP-10 (65/69, 94.2%) in serum samples from patients with GC. Levels of MIA were raised in a small proportion of serum samples from patients with GC (4/69, 5.8%). In Boyden chamber invasion assays, MIA-transfected GC cells were up to three times more invasive than cells transfected with empty vector. Taken together, these results suggest that MMP-10 is a good marker for the detection of GC and that MIA and MMP-10 are prognostic factors for GC. As expression of MIA and MMP-10 is narrowly restricted in cancer, these two molecules may be good therapeutic targets for GC.

Array-based comparative genomic hybridization identifies localized DNA amplifications and homozygous deletions in pancreatic cancer

Pancreatic cancer, the fourth leading cause of cancer death in the United States, is frequently associated with the amplification and deletion of specific oncogenes and tumor-suppressor genes (TSGs), respectively. To identify such novel alterations and to discover the underlying genes, we performed comparative genomic hybridization on a set of 22 human pancreatic cancer cell lines, using cDNA microarrays measuring approximately 26,000 human genes (thereby providing an average mapping resolution of <60 kb). To define the subset of amplified and deleted genes with correspondingly altered expression, we also profiled mRNA levels in parallel using the same cDNA microarray platform. In total, we identified 14 high-level amplifications (38-4934 kb in size) and 15 homozygous deletions (46-725 kb). We discovered novel localized amplicons, suggesting previously unrecognized candidate oncogenes at 6p21, 7q21 (SMURF1, TRRAP), 11q22 (BIRC2, BIRC3), 12p12, 14q24 (TGFB3), 17q12, and 19q13. Likewise, we identified novel polymerase chain reaction-validated homozygous deletions indicating new candidate TSGs at 6q25, 8p23, 8p22 (TUSC3), 9q33 (TNC, TNFSF15), 10q22, 10q24 (CHUK), 11p15 (DKK3), 16q23, 18q23, 21q22 (PRDM15, ANKRD3), and Xp11. Our findings suggest candidate genes and pathways, which may contribute to the development or progression of pancreatic cancer.

Mesenchymal-epithelial interactions in the skin: aiming for site-specific tissue regeneration

Since trunk skin (or non-palmoplantar skin) is less durable under mechanical stress than sole skin (palm, plantar or palmoplantar skin), conventional trunk-derived skin grafts (including the trunk dermis) commonly result in erosion and ulceration when transplanted on to plantar wounds caused by various injuries including, diabetes mellitus or collagen diseases (including systemic sclerosis, polyarthritis nodosa and rheumatoid arthritis). However, trunk-derived epidermis can adopt a plantar phenotype, characterized by keratin 9 expression, hypopigmentation and thick suprabasal layers, through factors derived from plantar dermal fibroblasts in the wounds. Thus, intractable plantar wounds with exposed bones can be treated with the combination of bone marrow exposure, occlusive dressing and epidermal grafting. The higher expression of dickkopf 1 (DKK1), an inhibitor of canonical Wnt/beta-catenin signals, in the plantar dermis partly explains these phenomena. Thus, mesenchymal-epithelial interactions play important roles not only in embryogenesis (the embryonic development) but also in maintaining the homeostasis of adult tissue. The topographical (site-specific) interactions of growth factors and substances, including DKKs, fibroblast growth factors (FGFs) and transforming growth factor-beta (TGF-beta) family proteins including bone morphogenetic proteins (BMPs), may explain the site-specific differences in the skin in addition to the expression patterns of HOX genes and sonic hedgehogs (Shhs). We review the importance of dermal-epidermal interactions in tissue homeostasis and regeneration, especially in palms and soles.

Wnt signalling pathway: a new target for the treatment of osteoporosis

The prevention and treatment of osteoporosis traditionally involves the use of antiresorptive agents that target osteoclast function. Antiresorptive therapy is not associated with a significant increase in bone mass and, thus, only partially reduces the risk of fractures. For that reason, the search for anabolic agents, which target osteoblast function, represents an urgent medical need. The first approved bone anabolic drug for the treatment of osteoporosis was teriparatide (human parathyroid hormone 1-34). Recently, both human genetics and animal studies have pointed out the role of the Wnt/LRP5 pathway as a major regulator of bone mass accrual. Wnts are secreted glycoproteins that bind to receptor complexes including low-density lipoprotein receptor-related protein (LRP)-5/6 and Frizzled proteins. A subsequent intracellular cascade of events stabilises beta-catenin, leading to its translocation into the nucleus where, associated with Tcf/Lef transcription factors, it triggers gene expression. The existence of many potential pharmacological targets in this pathway makes it attractive for bone anabolic drug discovery.

Cross-talk between Wnt and bone morphogenetic protein 2 (BMP-2) signaling in differentiation pathway of C2C12 myoblasts

Loss of function of the Wnt co-receptor, lipoprotein receptor-related protein 5, decreases bone formation, and a point mutation in this gene results in high bone mass, indicating the importance of this signaling pathway in bone formation. However, the exact mechanism is currently unknown. We examined a potential role for Wnt signaling and functional cross-talk of bone morphogenetic protein 2 (BMP-2) in osteoblast differentiation. To assess the contribution of Wnt, we generated C2C12 cells over-expressing Wnt3a or Wnt5a and treated these with BMP-2. We showed that expression of matrix extracellular phosphoglycoprotein was induced by BMP-2 in Wnt3a over-expressing C2C12 cells but not in Wnt5a over-expressing C2C12 cells. Over-expression of Wnt3a blocked BMP-2-induced inhibition of myotube formation in C2C12 cells when switched to low mitogen medium. In these cultures, expression of inhibitor of DNA binding/differentiation (Id) 1, a helix-loop-helix protein induced by BMP-2, decreased in stable Wnt3a- but not in Wnt5a-expressing cells. This suppression is mediated by a GC-rich region of the BMP-2-responsive element of the Id1 gene promoter, and interaction between Smad1/4 and beta-catenin is crucial for Wnt-mediated suppression of the BMP-2 response in C2C12 cells. Over-expression of the inhibitor of canonical Wnt signaling, Dickkopf, inhibits this suppression. In contrast, BMP-2 or Smad1/4 up-regulated Wnt3a or activated beta-catenin-induced lymphoid-enhancing factor 1/T cell factor-dependent transcriptional activity. These findings identify functional cross-talk of Id1 expression between Wnt and BMP signaling and demonstrate a novel mechanism for Wnt regulation of the BMP-2 response, linking Id1 expression to Wnt/beta-catenin signaling.

Large-scale reprogramming of cranial neural crest gene expression by retinoic acid exposure

Although retinoic acid (RA), the active form of vitamin A, is required for normal embryonic growth and development, it is also a powerful teratogen. Infants born to mothers exposed to retinoids during pregnancy have a 25-fold increased risk for malformations, nearly exclusively of cranial neural crest-derived tissues. To characterize neural crest cell responses to RA, we exposed murine crest cultures to teratogenic levels of RA and subjected their RNA to microarray-based gene expression profile analysis using Affymetrix MG-U74Av2 GeneChips. RNAs were isolated from independent cultures treated with 10(-6) M RA for 6, 12, 24, or 48 h. Statistical analyses of gene expression profile data facilitated identification of the 205 top-ranked differentially regulated genes whose expression was reproducibly changed by RA over time. Cluster analyses of these genes across the independently treated sample series revealed distinctive kinetic patterns of altered gene expression. The largest group was transiently affected within the first 6 h of exposure, representing early responding genes. Group 2 showed sustained induction by RA over all times, whereas group 3 was characterized by the suppression of a time-dependent expression increase normally seen in untreated cells. Additional patterns demonstrated time-dependent increased or decreased expression among genes not normally regulated to a significant extent. Gene function analysis revealed that more than one-third of all RA-regulated genes were associated with developmental regulation, including both canonical and noncanonical Wnt signaling pathways. Multiple genes associated with cell adhesion and cell cycle regulation, recognized targets for the biological effects of RA, were also affected. Taken together, these results support the hypothesis that the teratogenic effects of RA derive from reprogramming gene expression of a host of genes, which play critical roles during embryonic development regulating pathways that determine subsequent differentiation of cranial neural crest cells.

Cancer/testis antigens, gametogenesis and cancer

Cancer/testis (CT) antigens, of which more than 40 have now been identified, are encoded by genes that are normally expressed only in the human germ line, but are also expressed in various tumour types, including melanoma, and carcinomas of the bladder, lung and liver. These immunogenic proteins are being vigorously pursued as targets for therapeutic cancer vaccines. CT antigens are also being evaluated for their role in oncogenesis--recapitulation of portions of the germline gene-expression programme might contribute characteristic features to the neoplastic phenotype, including immortality, invasiveness, immune evasion, hypomethylation and metastatic capacity.

Expression pattern of Wnt signaling components in the adult intestine

BACKGROUND & AIMS

In the intestine, the canonical Wnt signaling cascade plays a crucial role in driving the proliferation of epithelial cells. Furthermore, aberrant activation of Wnt signaling is strongly associated with the development of colorectal cancer. Despite this evidence, little is known about the precise identity and localization of Wnts and their downstream effectors in the adult intestine. To address this issue, we examined the expression pattern of all Wnts, Frizzleds (Fzs), low-density lipoprotein receptor-related proteins, Wnt antagonists, and T-cell factors in the murine small intestine and colon and adenomas.

METHODS

Embryonic, postnatal, and adult intestinal samples were subjected to in situ hybridization by using specific RNA probes for the various genes tested.

RESULTS

Our analysis showed high expression of several signaling components (including Wnt-3, Wnt-6, Wnt-9b, Frizzled 4, Frizzled 6, Frizzled 7, low-density lipoprotein receptor-related protein 5, and secreted Frizzled-related protein 5) in crypt epithelial cells. We also detected Wnt-2b, Wnt-4, Wnt-5a, Wnt-5b, Frizzled 4, and Frizzled 6 in differentiated epithelial and mesenchymal cells of the small intestine and colon. Finally, several factors (Frizzled 4, T-cell factor 1, lymphoid enhancer factor, Dickkopf 2, Dickkopf 3, and Wnt-interacting factor) displayed differential expression in normal vs neoplastic tissue.

CONCLUSIONS

Our study predicts a much broader role for Wnt signaling in gut development and homeostasis than was previously anticipated from available genetic studies and identifies novel factors likely involved in promoting canonical and noncanonical Wnt signals in the intestine.

Dkk2 has a role in terminal osteoblast differentiation and mineralized matrix formation

Human and mouse genetic and in vitro evidence has shown that canonical Wnt signaling promotes bone formation, but we found that mice lacking the canonical Wnt antagonist Dickkopf2 (Dkk2) were osteopenic. We reaffirmed the finding that canonical Wnt signaling stimulates osteogenesis, including the differentiation from preosteoblasts to osteoblasts, in cultured osteoblast differentiation models, but we also found that canonical Wnts upregulated the expression of Dkk2 in osteoblasts. Although exogenous overexpression of Dkk before the expression of endogenous canonical Wnt (Wnt7b) suppressed osteogenesis in cultures, its expression after peak Wnt7b expression induced a phenotype resembling terminal osteoblast differentiation leading to mineralization. In addition, osteoblasts from Dkk2-null mice were poorly mineralized upon osteogenic induction in cultures, and Dkk2 deficiency led to attenuation of the expression of osteogenic markers, which could be partially reversed by exogenous expression of Dkk2. Taken together with the finding that Dkk2-null mice have increased numbers of osteoids, these data indicate that Dkk2 has a role in late stages of osteoblast differentiation into mineralized matrices. Because expression of another Wnt antagonist, FRP3, differs from Dkk2 expression in rescuing Dkk2 deficiency and regulating osteoblast differentiation, the effects of Dkk2 on terminal osteoblast differentiation may not be entirely mediated by its Wnt signaling antagonistic activity.

Down-regulation of Dickkopf 3, a regulator of the Wnt signalling pathway, in elderly schizophrenic subjects

The aetiology of schizophrenia is complex and the pathological mechanisms involved are still not fully understood. The aim of this project was to gain insight into the underlying molecular changes occurring in schizophrenia through the analysis of gene expression. Using suppression subtractive hybridization to isolate differentially expressed genes in superior temporal cortex (BA22), we detected one prominent sequence with reduced expression in schizophrenia and represented in at least nine clones. This was then selected for further validation. This 190-bp partial transcript showed identity to part of the Dickkopf-3 (Dkk3) gene sequence. Differential expression was initially confirmed in BA22 by slot blot hybridization where expression was decreased by 35% (p < 0.026). These results were further authenticated in a larger panel (12 control and 11 schizophrenia cases) using SYBR Green I real-time quantitative RT-PCR, in which a 41% decrease in expression of Dkk3 mRNA in schizophrenia was obtained (p < 0.012). Furthermore, using in situ hybridization, Dkk3 mRNA was shown to be abundantly expressed in cortical neurones, with prominent expression in layers II/III and V/VI of BA22. Dkk3 belongs to a novel family of Dkk proteins, which have been shown to be potent inhibitors of the neurodevelopmental wingless (Wnt) signalling pathway, and is therefore a putative candidate for further investigation into the aetiology of schizophrenia.

Expression of secreted Wnt antagonists in gastrointestinal tissues: potential role in stem cell homeostasis

BACKGROUND

Wnt signalling dysregulation has been implicated in cancer, including colon and gastric cancer. Initiation of Wnt signalling is modulated by soluble Wnt antagonists (sWAs), including soluble frizzled related proteins, dickkopf (Dkk) proteins, and Wnt inhibitory factor-1 (Wif1).

AIMS

To evaluate the role of sWAs in upper (gastric) and lower (colon) gastrointestinal tract tumorigenesis.

METHODS

Dkk1-3, Wif1, and FrzB expression was evaluated by in situ RNA hybridisation on normal and malignant human gastric and colon tissues. Expression was graded semiquantitatively.

RESULTS

Wif1, Dkk1, and Dkk2 were not expressed in normal gastric tissue. Dkk3 was expressed in some samples, with stronger expression in deep gastric glands. FrzB was expressed in several normal gastric samples, but not in matched tumour specimens. In contrast, Dkk1 and FrzB were not expressed in normal colon. Wif1 was expressed in most colon samples, with stronger expression at crypt bases. Dkk3 and Dkk2 expression was also concentrated at crypt bases. There were no differences between sWA expression in malignant colon and matched normal tissue.

CONCLUSIONS

sWA expression differed between upper and lower gastrointestinal tract. The loss of FrzB in gastric cancer suggests that it acts as a tumour suppressor. The graded expression of Dkk3 in gastric tissue, and Dkk2, Dkk3, and Wif1 in colon tissue, with increased expression in the deep gastric glands/colonic crypt bases, where gastrointestinal stem cells reside, suggests that sWAs may be crucial Wnt signalling regulators in these tissues, and may contribute to stem cell pool maintenance. sWAs are important components of the gastrointestinal proliferative regulatory network.

SOST is a ligand for LRP5/LRP6 and a Wnt signaling inhibitor

Sclerosteosis is an autosomal recessive disease that is characterized by overgrowth of bone tissue and is linked to mutations in the gene encoding the secreted protein SOST. Sclerosteosis shares remarkable similarities with "high bone mass" diseases caused by "gain-of-function" mutations in the LRP5 gene, which encodes a coreceptor for Wnt signaling proteins. We show here that SOST antagonizes Wnt signaling in Xenopus embryos and mammalian cells by binding to the extracellular domain of the Wnt coreceptors LRP5 and LRP6 and disrupting Wnt-induced Frizzled-LRP complex formation. Our findings suggest that SOST is an antagonist for Wnt signaling and that the loss of SOST function likely leads to the hyperactivation of Wnt signaling that underlies bone overgrowth seen in sclerosteosis patients.

Sclerostin binds to LRP5/6 and antagonizes canonical Wnt signaling

The loss of the SOST gene product sclerostin leads to sclerosteosis characterized by high bone mass. In this report, we found that sclerostin could antagonize canonical Wnt signaling in human embryonic kidney A293T cells and mouse osteoblastic MC3T3 cells. This sclerostin-mediated antagonism could be reversed by overexpression of Wnt co-receptor low density lipoprotein receptor-related protein (LRP) 5. In addition, we found that sclerostin bound to LRP5 as well as LRP6 and identified the first two YWTD-EGF repeat domains of LRP5 as being responsible for the binding. Although these two repeat domains are required for transduction of canonical Wnt signals, canonical Wnt did not appear to compete with sclerostin for binding to LRP5. Examination of the expression of sclerostin and Wnt7b, an autocrine canonical Wnt, during primary calvarial osteoblast differentiation revealed that sclerostin is expressed at late stages of osteoblast differentiation coinciding with the expression of osteogenic marker osteocalcin and trailing after the expression of Wnt7b. Given the plethora of evidence indicating that canonical Wnt signaling stimulates osteogenesis, we believe that the high bone mass phenotype associated with the loss of sclerostin may be attributed, at least in part, to an increase in canonical Wnt signaling resulting from the reduction in sclerostin-mediated Wnt antagonism.

Dickkopf-3/REIC functions as a suppressor gene of tumor growth

To identify putative tumor suppressor genes in hepatocarcinogenesis, we combined the representational difference analysis and reverse northern blot identifying downregulated genes in human hepatoma tissues. One of them was Dkk-3/REIC. Dkk-3/REIC was downregulated in 11 out of the 20 human hepatoma tissues as compared to their counterparts of noncancerous liver tissues by northern blot analysis. It was also downregulated in 29 out of 48 human cancer samples including the kidney, urinary bladder, prostate, pancreas and lung cancers. Its gene product, Dkk-3/REIC, was found to be N-glycosylated and have two isoforms, the 55 kDa in the cytosol and 50 kDa secreted in the medium. Ectopic expression of Dkk-3/REIC in HeLa, Hep3B and Huh 7 cells led to suppression of cell growth, which was primarily attributable to induction of cell apoptosis. The suppression phenomenon was found to be cell-type related (most prominent in HeLa and least in Hep3B cells) and cell-density dependent (attenuated as the cell density increased). Transduction of Dkk-3/REIC into HeLa and Hep3B cells caused suppression on colony formation in vitro and reduced tumor growth rate in inoculated athymic nude mice. In conclusion, these data indicate that Dkk-3/REIC functions as a suppressor for human tumor growth.

Wnt-independent activation of β-catenin mediated by a Dkk1-Fz5 fusion protein

An XWnt8-Fz5 fusion protein synergizes with LRP6 to potently activate beta-catenin-dependent signaling. Here, we generated a fusion in which XWnt8 was fused to the N-terminus of LRP6 and show it synergizes with both Fz4 and Fz5 to potently transactivate beta-catenin-dependent Wnt signaling. Based on this, we hypothesized that the main function of Wnt is to nucleate the formation of a physical complex between LRP6 and a Frizzled. Dkk1, but not the related Dkk3, binds LRP6 and inhibits canonical Wnt signaling by blocking the interaction of Wnt and LRP6. Therefore, we reasoned that a covalent fusion of Dkk1 to Fz5 (Dkk1-Fz5) would mimic Wnt ligand by nucleating the formation of a complex containing Fz5 and LRP6, while Dkk3 (Dkk3-Fz5) would not. We found that Dkk1-Fz5, but not Dkk3-Fz5, potently synergized with LRP6 to activate signaling in a dishevelled-dependent manner.

Cancer/testis antigens and gametogenesis: a review and "brain-storming" session

Genes expressed both in normal testis and in malignancies (Cancer/ Testis associated genes - CTA) have become the most extensively studied antigen group in the field of tumour immunology. Despite this, many fundamentally important questions remain unanswered: what is the connection between germ-cell specific genes and tumours? Is the expression of these genes yet another proof for the importance of genome destabilisation in the process of tumorigenesis?, or maybe activation of these genes is not quite random but instead related to some programme giving tumours a survival advantage?This review collates most of the recent information available about CTAs expression, function, and regulation. The data suggests a programme related to ontogenesis, mostly to gametogenesis. In the "brain-storming" part, facts in conflict with the hypothesis of random CTA gene activation are discussed. We propose a programme borrowed from organisms phylogenetically much older than humans, which existed before the differentiation of sexes. It is a programme that has served as a life cycle with prominent ploidy changes, and from which, as we know, the germ-cell ploidy cycle - meiosis - has evolved. Further work may show whether this hypothesis can lead to a novel anti-tumour strategy.

The Wnt signaling pathway in development and disease

Tight control of cell-cell communication is essential for the generation of a normally patterned embryo. A critical mediator of key cell-cell signaling events during embryogenesis is the highly conserved Wnt family of secreted proteins. Recent biochemical and genetic analyses have greatly enriched our understanding of how Wnts signal, and the list of canonical Wnt signaling components has exploded. The data reveal that multiple extracellular, cytoplasmic, and nuclear regulators intricately modulate Wnt signaling levels. In addition, receptor-ligand specificity and feedback loops help to determine Wnt signaling outputs. Wnts are required for adult tissue maintenance, and perturbations in Wnt signaling promote both human degenerative diseases and cancer. The next few years are likely to see novel therapeutic reagents aimed at controlling Wnt signaling in order to alleviate these conditions.

Wnt signaling in osteoblasts and bone diseases

Recent revelations that the canonical Wnt signaling pathway promotes postnatal bone accrual are major advances in our understanding of skeletal biology and bring tremendous promise for new therapeutic treatments for osteoporosis and other diseases of altered bone mass. Wnts are soluble glycoproteins that engage receptor complexes composed of Lrp5/6 and Frizzled proteins. A subgroup of Wnts induces a cascade of intracellular events that stabilize beta-catenin, facilitating its transport to nuclei where it binds Lef1/Tcf transcription factors and alters gene expression to promote osteoblast expansion and function. Natural extracellular Wnt antagonists, Dickkopfs and secreted frizzled-related proteins, impair osteoblast function and block bone formation. In several genetic disorders of altered skeletal mass, mutations in LRP5 create gain-of-function or loss-of-function receptors that are resistant to normal regulatory mechanisms and cause higher or lower bone density, respectively. In this review, we summarize the available molecular, cellular, and genetic data that demonstrate how Lrp5 and other components of the Wnt signaling pathway influence osteoblast proliferation, function, and survival. We also discuss regulatory mechanisms discovered in developmental and tumor models that may provide insights into novel therapies for bone diseases.

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

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

An In Vitro Analysis of Myocardial Potential Indicates That Phenotypic Plasticity Is an Innate Property of Early Embryonic Tissue

Explants from gastrula-stage avian embryos have provided an important culture model for examining the formation of the vertebrate heart. Explants harvested from anterior regions containing the precardiac mesoderm faithfully recapitulate cardiogenesis and generate contractile tissue in culture. Posterior regions of the early embryo do not supply cellular material to the developing heart in situ, and thus have been commonly employed as negative control tissues for studying cardiogenic induction. To begin to understand the cellular mechanisms that account for the distinct cell fates of precardiac and posterior tissue within the embryo, we undertook a comprehensive investigation on the myocardial potential of presumptive noncardiac tissue. Myocardial differentiation was assayed by expression of the myocardium-associated transcription factor gene Nkx2.5 and positive immunostaining for sarcomeric myosin, muscle alpha-actinin, and smooth muscle alpha-actin. Our results demonstrate that regions of the early embryo that do not provide a cellular contribution to the myocardium in situ are capable of generating myocardial tissue when removed from their normal embryonic environment and placed in culture under nontreated conditions. Although treatment with the presumptive cardiac inducer Dickkopf-1 increased the frequency that cardiac tissue appeared within cultures of posterior tissue, no difference was observed in either the size or morphology of the myocardium-positive areas among treated and nontreated explants. These findings suggest that progenitor cells within the early embryo possess an innate phenotypic plasticity and that presumptive cardiac inducing signals do not induce cardiac differentiation but instead augment a pre-existing cardiac potential of embryonic tissue.

Molecular definition of an in vitro niche for dendritic cell development

OBJECTIVE

Although dendritic cell (DC) precursors have been isolated from many lymphoid sites, the regulation and location of early DC development is still poorly understood. Here we describe a splenic microenvironment that supports DC hematopoiesis in vitro and identify gene expression specific for that niche.

METHODS

The DC supportive function of the STX3 splenic stroma and the lymph node-derived 2RL22 stroma for overlaid bone marrow cells was assessed by coculture over 2 weeks. The DC supportive function of SXT3 was identified in terms of specific gene expression in STX3 and not 2RL22 using Affymetrix microchips.

RESULTS

STX3 supports DC differentiation from overlaid bone marrow precursors while 2RL22 does not. A dataset of 154 genes specifically expressed in STX3 and not 2RL22 was retrieved from Affymetrix results. Functional annotation has led to selection of 26 genes as candidate regulators of the microenvironment supporting DC hematopoiesis. Specific expression of 14 of these genes in STX3 and not 2RL22 was confirmed by reverse transcription-polymerase chain reaction.

CONCLUSION

Some genes specifically expressed in STX3 have been previously associated with hematopoietic stem cell niches. A high proportion of genes encode growth factors distinct from those commonly used for in vitro development of DC from precursors. Potential regulators of a DC microenvironment include genes involved in angiogenesis, hematopoiesis, and development, not previously linked to DC hematopoiesis.

Determinants of Endometrial Receptivity

Understanding the molecular changes that occur during the window of implantation is fundamental to our knowledge of human reproduction. Lately, the development of microarray technology has allowed this process to be studied from a global molecular perspective. In the last 2 years, researchers have focused their efforts on throwing light on the gene expression profile of the receptive endometrium. The genes hold the key to the development of the endometrium at any stage, and we have focused our work on the window of implantation. The four most recently published works in this field have revealed a long list of genes that are up- or downregulated at the time of implantation. Although these studies have been conducted using varying approaches, collectively these studies identify new candidate markers that can be used to accurately diagnose the receptive state of the endometrium. The next step is to perform functional analysis for confirming the importance of these genes. In this article, we gather together these recent findings to provide an overview of the current knowledge regarding the genetic functioning of human endometrial receptivity and related processes.

Transcription and translation of dickkopf-1 in endometrium of pregnant mice during the peri-implantation period

To study the expression of Dickkopf-1 (DKK-1) in endometrium of pregnant mice during the peri-implantation period and the role of DKK-1 during the embryo implantation in mice. Immunohistochemical technique was employed to determine the location of DKK-1 protein in endometrium, and semiquantitative reverse transcriptase polymerase chain reaction (RT-PCR) was utilized to determine the levels of DKK-1 mRNA. Our results showed that the expressions of DKK1 mRNA and protein were higher in experimental groups than in control group (P<0.01) and it increased significantly on day 3 and reached its peak on day 4, and then decreased gradually on day 5-7. The levels of DKK-1 mRNA and protein on day 4 was significantly higher than those of other groups (P<0.01). It is concluded that DKK-1 probably plays an important role in signal transudation of embryo implantation and its high expression indicates the opening of implantation window.

Characterisation of the Wnt antagonists and their response to conditionally activated Wnt signalling in the developing mouse forebrain

In the present work, the expression patterns of the Wnt antagonists of the Dickkopf (Dkk) family were characterized in the developing mouse forebrain. In situ hybridisation on sections from E12 embryos showed an expression of dkk2 in the thalamus and dkk3 in the cortical hem and thalamus. At later developmental stages (E15.5, E17.5, and P0), little or no expression of dkk1, dkk2, and dkk4 was found in the forebrain, while dkk3 expression was detected in the ventricular zone (VZ) of the lateral and III ventricles, cortical neurons, migrating cells of the primary and secondary dentate migration, and the neuroblastic layer of the eye. In the adult forebrain, dkk3 expression was detected in the lateral VZ, pyramidal neurons of the hippocampus, and cortical neurons. We also provide evidence indicating that only dkk1 and dkk4, along with two other Wnt antagonists axin2 and wif1, but not dkk2 and dkk3, are involved in a feedback mechanism to restrain Wnt signalling in transgenic mice carrying a conditional augmentation of beta-catenin in the forebrain.

Mesenchymal-epithelial interactions in the skin: increased expression of dickkopf1 by palmoplantar fibroblasts inhibits melanocyte growth and differentiation

We investigated whether or not the topographic regulation of melanocyte differentiation is determined by mesenchymal–epithelial interactions via fibroblast-derived factors. The melanocyte density in palmoplantar human skin (i.e., skin on the palms and the soles) is five times lower than that found in nonpalmoplantar sites. Palmoplantar fibroblasts significantly suppressed the growth and pigmentation of melanocytes compared with nonpalmoplantar fibroblasts. Using cDNA microarray analysis, fibroblasts derived from palmoplantar skin expressed high levels of dickkopf 1 (DKK1; an inhibitor of the canonical Wnt signaling pathway), whereas nonpalmoplantar fibroblasts expressed higher levels of DKK3. Transfection studies revealed that DKK1 decreased melanocyte function, probably through β-catenin–mediated regulation of microphthalmia-associated transcription factor activity, which in turn modulates the growth and differentiation of melanocytes. Thus, our results provide a basis to explain why skin on the palms and the soles is generally hypopigmented compared with other areas of the body, and might explain why melanocytes stop migrating in the palmoplantar area during human embryogenesis.

Neural system-enriched gene expression: relationship to biological pathways and neurological diseases

To understand the commitment of the genome to nervous system differentiation and function, we sought to compare nervous system gene expression to that of a wide variety of other tissues by gene expression database construction and mining. Gene expression profiles of 10 different adult nervous tissues were compared with that of 72 other tissues. Using ANOVA, we identified 1,361 genes whose expression was higher in the nervous system than other organs and, separately, 600 genes whose expression was at least threefold higher in one or more regions of the nervous system compared with their median expression across all organs. Of the 600 genes, 381 overlapped with the 1,361-gene list. Limited in situ gene expression analysis confirmed that identified genes did represent nervous system-enriched gene expression, and we therefore sought to evaluate the validity and significance of these top-ranked nervous system genes using known gene literature and gene ontology categorization criteria. Diverse functional categories were present in the 381 genes, including genes involved in intracellular signaling, cytoskeleton structure and function, enzymes, RNA metabolism and transcription, membrane proteins, as well as cell differentiation, death, proliferation, and division. We searched existing public sites and identified 110 known genes related to mental retardation, neurological disease, and neurodegeneration. Twenty-one of the 381 genes were within the 110-gene list, compared with a random expectation of 5. This suggests that the 381 genes provide a candidate set for further analyses in neurological and psychiatric disease studies and that as a field, we are as yet, far from a large-scale understanding of the genes that are critical for nervous system structure and function. Together, our data indicate the power of profiling an individual biologic system in a multisystem context to gain insight into the genomic basis of its structure and function.

RFX2 is a potential transcriptional regulatory factor for histone H1t and other genes expressed during the meiotic phase of spermatogenesis

H1t is a novel linker histone variant synthesized in mid- to late pachytene spermatocytes. Its regulatory region is of interest because developmentally specific expression has been impressed on an otherwise ubiquitously expressed promoter. Using competitive band-shift assays and specific antisera, we have now shown that the H1t-60 CCTAGG palindrome motif region binds members of the RFX family of transcriptional regulators. The testis-specific binding complex contains RFX2, probably as a homodimer. Other DNA-protein complexes obtained from testis as well as somatic organs contain RFX1, primarily as a heterodimer. Western blots confirmed that RFX2 expression is greatly enhanced in adult testis and that RFX2 is equally prominent in highly enriched populations of late pachytene spermatocytes and round spermatids. Immunohistochemistry carried out on mouse testis showed that RFX2 is strongly expressed in pachytene spermatocytes, remains high in early round spermatids, and declines only in advance of nuclear condensation. Maximum expression correlates well with the appearance of H1t. In contrast, RFX1 immunoreactivity in germ cells was only detected in late round spermatids. RFX-specific band complexes were also identified for both the mouse lamin C2 and Sgy promoters, using either testis nuclear extracts or in vitro-synthesized RFX2. These results call attention to RFX2 as a transcription factor with obvious potential for the regulation of gene expression during meiosis and the early development of spermatids.

Dickkopf 3 inhibits invasion and motility of Saos-2 osteosarcoma cells by modulating the Wnt-beta-catenin pathway

Osteosarcoma (OS) is a primary malignancy of bone with a tendency to metastasize early. Despite intensive chemotherapy and surgical resection, approximately 30% of patients still develop distant metastasis. Our previous work using clinical OS samples suggested that expression of the Wnt receptor LRP5 might be associated with tumor metastasis. In the present study, we used a Dickkopf (Dkk) family member and a dominant-negative LRP5 receptor construct to modulate Wnt signaling in OS cells. Saos-2 cells, which ectopically express Dkk-3, do not undergo apoptosis and exhibit enhanced resistance to serum starvation and chemotherapy-induced cytotoxicity. Transfection of Dkk-3 and dominant-negative LRP5 into Saos-2 cells significantly reduces invasion capacity and cell motility. This blockade is associated with changes in cell morphology consistent with a less invasive phenotype. In addition, Dkk-3 and dominant-negative LRP5 also induce changes in beta-catenin localization consistent with an increase in cell-cell adhesion. Taken together, these results support a possible role for Wnt signaling in the pathobiology and progression of human OS.

Glucocorticoid enhances the expression of dickkopf-1 in human osteoblasts: novel mechanism of glucocorticoid-induced osteoporosis

To clarify the underlying mechanism of glucocorticoid-induced osteoporosis, we investigated the effect of glucocorticoid on the expression of dickkopf-1 (Dkk-1), an antagonist of Wnt signaling, in primary cultured human osteoblasts. Dexamethasone markedly induced the expression of mRNA for Dkk-1 in a dose- and time-dependent manner. The expression of Kremen1, a receptor for Dkk, did not change by the treatment with dexamethasone, while that of low-density lipoprotein receptor-related protein 5 (LRP5), a Wnt coreceptor, slightly decreased by the treatment with dexamethasone. Dexamethasone increased the transcriptional activity of the Dkk-1 gene promoter in human osteoblasts. Serial deletion and mutation analyses of the Dkk-1 promoter showed that one putative glucocorticoid responsive element-like sequence located from -788 to -774bp is essential for the enhancement of the Dkk-1 promoter activity by dexamethasone in human osteoblasts. Since the Wnt signal is now recognized as a crucial regulator for bone formation, the Dkk-1 enhanced by glucocorticoid may inhibit the Wnt signal in osteoblasts, which may be involved in the pathogenesis of glucocorticoid-induced osteoporosis.

LDL receptor-related proteins 5 and 6 in Wnt/beta-catenin signaling: arrows point the way

Wnt signaling through the canonical beta-catenin pathway plays essential roles in development and disease. Low-density-lipoprotein receptor-related proteins 5 and 6 (Lrp5 and Lrp6) in vertebrates, and their Drosophila ortholog Arrow, are single-span transmembrane proteins that are indispensable for Wnt/beta-catenin signaling, and are likely to act as Wnt co-receptors. This review highlights recent progress and unresolved issues in understanding the function and regulation of Arrow/Lrp5/Lrp6 in Wnt signaling. We discuss Arrow/Lrp5/Lrp6 interactions with Wnt and the Frizzled family of Wnt receptors, and with the intracellular beta-catenin degradation apparatus. We also discuss the regulation of Lrp5/Lrp6 by other extracellular ligands, and LRP5 mutations associated with familial osteoporosis and other disorders.

The LRP5 high-bone-mass G171V mutation disrupts LRP5 interaction with Mesd

The mechanism by which the high-bone-mass (HBM) mutation (G171V) of the Wnt coreceptor LRP5 regulates canonical Wnt signaling was investigated. The mutation was previously shown to reduce DKK1-mediated antagonism, suggesting that the first YWTD repeat domain where G171 is located may be responsible for DKK-mediated antagonism. However, we found that the third YWTD repeat, but not the first repeat domain, is required for DKK1-mediated antagonism. Instead, we found that the G171V mutation disrupted the interaction of LRP5 with Mesd, a chaperone protein for LRP5/6 that is required for transport of the coreceptors to cell surfaces, resulting in fewer LRP5 molecules on the cell surface. Although the reduction in the number of cell surface LRP5 molecules led to a reduction in Wnt signaling in a paracrine paradigm, the mutation did not appear to affect the activity of coexpressed Wnt in an autocrine paradigm. Together with the observation that osteoblast cells produce autocrine canonical Wnt, Wnt7b, and that osteocytes produce paracrine DKK1, we think that the G171V mutation may cause an increase in Wnt activity in osteoblasts by reducing the number of targets for paracrine DKK1 to antagonize without affecting the activity of autocrine Wnt.

Hypomorphic expression of Dkk1 in the doubleridge mouse: dose dependence and compensatory interactions with Lrp6

doubleridge is a transgene-induced mouse mutation displaying forelimb postaxial polysyndactyly. We have cloned the doubleridge transgene insertion site and demonstrate that doubleridge acts in cis from a distance of 150 kb to reduce the expression of dickkopf 1 (Dkk1), the secreted Wnt antagonist. Expression of Dkk1 from the doubleridge allele ranges from 35% of wild-type level in E7.0 head to <1% of wild type in E13.5 tail. doubleridge homozygotes and doubleridge/null compound heterozygotes are viable. An allelic series combining the wild-type, doubleridge and null alleles of Dkk1 demonstrates the effect of varying Dkk1 concentration on development of limb, head and vertebrae. Decreasing expression of Dkk1 results in hemivertebral fusions in progressively more anterior positions, with severity increasing from tail kinks to spinal curvature. We demonstrated interaction between Dkk1 and the Wnt coreceptors Lrp5 and Lrp6 by analysis of several types of double mutants. The polydactyly of Dkk1(d/d) mice was corrected by reduced expression of Lrp5 or Lrp6. The posterior digit loss and axial truncation characteristic of Lrp6 null mice was partially corrected by reduction of Dkk1. Similarly, the anterior head truncation characteristic of Dkk1 null mice was rescued by reduction of Lrp6. These compensatory interactions between Dkk1 and Lrp6 demonstrate the importance of correctly balancing positive and negative regulation of Wnt signaling during mammalian development.

The Wnt antagonist sFRP1 in colorectal tumorigenesis

Regions of the short arm of chromosome 8 are deleted frequently in a range of solid tumors, indicating that tumor suppressor genes reside at these loci. In this study, we have examined the properties of the Wnt signaling antagonist secreted frizzled-related protein (sFRP) 1 as a candidate for this role at c8p11.2. An initial survey of 10 colorectal tumors, selected by the presence of isolated short deletions of the 8p11.2 region, identified three chain-terminating mutations, all within the first exon, which encodes the cysteine-rich domain. None of these tumors exhibited microsatellite instability, indicating intact mismatch repair gene function. The preserved sFRP1 alleles in the remaining seven tumors each contained a polymorphic three-base insertion in the signal sequence, but in a broader study, no association was found between this and the development of colorectal cancer. Epigenetic inhibition of sFRP1 transcription was investigated, and increased methylation of the promotor region was demonstrated in an additional cohort of 51 locally advanced colorectal cancers. Hypermethylation was identified in 40 of 49 (82%) cancers and in only 11 of 36 (30%) matched normal mucosal samples (P < 0.001). Semiquantitative analysis, by real-time PCR, of mRNA expression in 37 of the same cohort of 51 cancers revealed that sFRP1 mRNA expression was down-regulated in 28 (76%) cases compared with matched normal large bowel mucosa. The 3' end of the sFRP1 mRNA also was found to be alternatively spliced, compared with the prototype liver and lung forms, in the colon and a number of other tissues, yielding an extended COOH terminus, which may influence its activity in a tissue-specific manner. The inactivation and down-regulation of sFRP1 observed are consistent with it acting as a tumor suppressor gene in colorectal carcinogenesis. Because beta-catenin is constitutively active in the majority of colorectal tumors, it is unlikely that sFRP1 can act in the canonical Wnt response pathway. Therefore, we propose that the reduced activity or absence of sFRP1 allows the transduction of noncanonical Wnt signals, which contribute to tumor progression.

Decreased expression of REIC/Dkk-3 in human renal clear cell carcinoma

PURPOSE

We examined the expression of REIC/Dkk-3, a possible candidate for a tumor suppressor gene, in human renal clear cell carcinoma (RCCC) cell lines and sporadic RCCC surgical specimens.

MATERIALS AND METHODS

Human RCCC cell lines (Caki-1, Caki-2, ACHN and KPK-1) and several control cell lines were used to examine the expression of REIC/Dkk-3 mRNA and characterize a newly raised antibody specific for REIC/Dkk-3 protein. Pairs of cancerous and adjacent noncancerous tissues were obtained from 20 patients with RCCC. Of them 17 and 7 cases were analyzed by real-time quantitative reverse transcriptase-polymerase chain reaction, and by Western blot analysis and/or immunohistochemical analysis, respectively.

RESULTS

The decreased expression of REIC/Dkk-3 mRNA and protein in human RCCC cell lines, and the specificity of the new antibody were confirmed. In a real-time quantitative reverse transcriptase-polymerase chain reaction study using 17 pairs of RCCC and adjacent normal tissues REIC/Dkk-3 mRNA levels were significantly decreased in carcinoma tissues (by 25% to approximately 95% in 15 pairs). Western blot analysis and immunohistochemistry revealed a significant decrease in REIC/Dkk-3 protein levels in 6 of the 7 and 13 of the 14 RCCC cases analyzed, respectively.

CONCLUSIONS

The decrease in REIC/Dkk-3 mRNA and protein levels was observed irrespective of tumor grade and stage, indicating the involvement of REIC/Dkk-3 in an initial step of malignant conversion. Consequently REIC/Dkk-3 could be a new molecular target for therapeutic measures against RCCC.

Structural determinants required for the bioactivities of prokineticins and identification of prokineticin receptor antagonists

Prokineticins are cysteine-rich secreted proteins that regulate diverse biological processes, including gastrointestinal motility, angiogenesis, and circadian rhythms. Two closely related G protein-coupled receptors that mediate signal transduction of prokineticins have recently been cloned. The structural elements required for prokineticins' bioactivities are still unknown. We show here that both the N-terminal hexapeptide (AVITGA) and C-terminal cysteine-rich domains are critical for the bioactivities of prokineticins. Substitutions, deletions, and insertions to the conserved N-terminal hexapeptides result in the loss of agonist activity. Mutant prokineticins with the substitution of the first N-terminal alanine with methionine or the addition of a methionine to the N terminus inhibit the activation of prokineticin receptors and thus are considered as antagonists of prokineticin receptors. We have further shown that mutations in selected cysteine residues in the C-terminal domain result in prokineticins without biological activity. The essential role of C-terminal domain is reinforced by two observations: that peptides without the carboxyl domain and proteins with the N-terminal hexapeptide fused to the carboxyl domains of colipase or dickkopf are devoid of biological activity. We demonstrate that limited structural changes of C-terminal cysteine-rich regions of prokineticins are tolerable because chimeric prokineticins with swapped cysteine-rich domains between prokineticin 1 and prokineticin 2, as well as a splice variant of prokineticin 2 that contains extra 21 residue insertion in its C-terminal domain, are biologically active.

A Dickkopf- 3-related gene is expressed in differentiating nematocytes in the basal metazoan Hydra

In vertebrate development the Dickkopf protein family carries out multiple functions and is represented by at least four different genes with distinct biological activities. In invertebrates such as Drosophila and Caenorhabditis, Dickkopf genes have so far not been identified. Here we describe the identification and characterization of a Dickkopf gene with a deduced amino acid sequence closely related to that of chicken Dkk-3 in the basal metazoan Hydra. HyDkk-3 appears to be the only Dickkopf gene in Hydra. The gene is expressed in the gastric region in nematocytes at a late differentiation stage. In silico searches of EST and genome databases indicated the absence of Dkk genes from the protostomes Drosophila and Caenorhabditis, whereas within the deuterostomes, a Dkk-3 gene could be identified in the genome of the urochordate Ciona intestinalis. The results indicate that at an early stage of evolution of multicellularity Dickkopf proteins have already played important roles as developmental signals. They also suggest that vertebrate Dkk-1, 2 and 4 may have originated from a common ancestor gene of Dkk-3.

The human Frizzled 6 (HFz6) acts as a negative regulator of the canonical Wnt. beta-catenin signaling cascade

Previously we have cloned the human Frizzled 1 (HFz1) and shown that it transmits the Wnt-3a-induced canonical pathway. We also cloned the human Frizzled 6 (HFz6) and show in the present study that, as opposed to HFz1, HFz6 did not activate the canonical Wnt pathway following exposure to various Wnts, whether belonging to the Wnt-1 or to the Wnt-5a group. Moreover we show that HFz6 repressed Wnt-3a-induced canonical signaling when co-expressed with HFz1. HFz6 repressed the canonical Wnt cascade activated also by various Wnt signaling intracellular mediators such as Dishevelled-1, a stabilized beta-catenin(S33Y) mutant, and LiCl-mediated repression of glycogen synthase kinase-3beta activity. Removal of HFz6 N'- or C'-terminal sequences abolished HFz6 repressive activity. As the HFz6 repressive effect was not associated with a decrease in the level of beta-catenin, it is suggested that HFz6 does not affect beta-catenin stabilization, implying that HFz6 transmits a repressive signaling that cross-talks with and inhibits the canonical Wnt pathway downstream of beta-catenin destruction complex. HFz6 did not affect the level of nuclear T-cell factor 4 (TCF4) nor did it affect beta-catenin.TCF4 complex formation. However, electrophoretic mobility shift assays indicated that HFz6 repressed the binding of TCF/lymphoid enhancer factor transcription factors to target DNA. Moreover we present data suggesting that HFz6 activates the transforming growth factor-beta-activated kinase-NEMO-like kinase pathway that blocks TCF/lymphoid enhancer factor binding to target promoters, thereby inhibiting the ability of beta-catenin to activate transcription of Wnt target genes.

The low density lipoprotein receptor-1, LRP1, interacts with the human frizzled-1 (HFz1) and down-regulates the canonical Wnt signaling pathway

Members of the low density lipoprotein receptor family (LDLR), LRP5/6, were shown to interact with the Frizzled (Fz) receptors and to function as Wnt coreceptors. Here we show that mLRP4T100, a minireceptor of LRP1, another member of the LDLR family, interacts with the human Fz-1 (HFz1), previously shown to serve as a receptor transmitting the canonical Wnt-3a-induced signaling cascade. However, in contrast to LRP5/6, mLRP4T100, as well as the full-length LRP1, did not cooperate with HFz1 in transmitting the Wnt-3a signaling but rather repressed it. mLRP4T100 inhibitory effect was displayed also by endocytosis-defective mLRP4T100 mutants, suggesting that LRP1 repressive effect is not attributable to LRP1-mediated enhanced HFz1 internalization and subsequent degradation. Enforced expression of mLRP4T100 decreased the capacity of HFz1 cysteine-rich domain (CRD) to interact with LRP6, in contrast to HFz1-CRD/Wnt-3a interaction that was not disrupted by overexpressing mLRP4T100. These data suggest that LRP1, by sequestering HFz1, disrupts the receptor/coreceptor complex formation, leading to the repression of the canonical Wnt signaling. Thus, this study implies that the ability to interact with Fz receptors is shared by several members of the LDLR family. However, whereas some members of the LDLR family, such as LRP5/6, interact with Fz and serve as Wnt coreceptors, others negatively regulate Wnt signaling, presumably by sequestering Fz.

Expression profiling in tuberous sclerosis complex (TSC) knockout mouse astrocytes to characterize human TSC brain pathology

Individuals with tuberous sclerosis complex (TSC) exhibit a variety of neurologic abnormalities, including mental retardation, epilepsy, and autism. Examination of human TSC brains demonstrate dysplastic astrocytes and neurons, areas of abnormal neuronal migration (tubers), and hamartomatous growths, termed subependymal nodules, which can progress to subependymal giant cell astrocytomas (SEGA). Previous studies have suggested that these neuropathologic features may result from abnormal neuroglial cell differentiation. In an effort to provide support for this hypothesis and to identify specific markers of aberrant neuroglial cell differentiation in TSC, we employed gene expression profiling on Tsc1 conditional knockout (Tsc1(GFAP)CKO) mouse astrocytes. We identified several transcripts implicated in central nervous system development that are differentially expressed in Tsc1(-/-) astrocytes compared to wild-type astrocytes. We validated the differential expression of select transcripts on the protein level both in primary cultures of Tsc1(-/-) astrocytes and in Tsc1(GFAP)CKO mouse brains. Moreover, we show that these markers are also differentially expressed within cortical tubers, but not in adjacent normal tissue from TSC patient brains. This study provides supportive evidence for a developmental defect in neuroglial cell differentiation relevant to the genesis of TSC nervous system pathology and underscores the utility of mouse modeling for understanding the molecular pathogenesis of human disease.

A functional genomics approach for the identification of putative tumor suppressor genes: Dickkopf-1 as suppressor of HeLa cell transformation

We described previously the isolation and characterization of two non-tumorigenic revertants from the HeLa cervical carcinoma cell line, and demonstrated that loss of the transformed phenotype in these cells was the result of dominant somatic mutations. The goal of the present study was to use cDNA microarrays to identify candidate tumor suppressors among the set of genes whose increased expression correlated with loss of tumorigenicity in both revertants. Among the genes with significantly increased expression levels in both HA and HF revertants we identified Insulin Growth Factor Binding Protein-3 (IGFBP-3) and the Dickkopf-1 (DKK-1) genes. Both of these genes encode secreted proteins implicated in the modulation cell growth and differentiation, and IGFBP-3 was shown previously to have tumor suppressing activity. To test the hypothesis that increased expression of IGFBP-3 or the DKK-1 genes could have contributed to the suppression of tumorigenicity in the revertants, we expressed IGFBP-3 or DKK-1 in HeLa cells, and assessed their effects on anchorage dependent and independent growth, and tumor formation in athymic nude mice. Ectopic expression of IGFBP-3 or DKK-1 resulted in significantly decreased growth in soft agar. HeLa cells expressing ectopic IGFBP-3 or DKK-1 showed statistically significant differences in the kinetics of tumor formation. In any tumors that arose in animals injected with the IGFBP-3 expressing cells, there was a complete loss of IGFBP-3 activity, as measured by binding to IGF-1 and IGF-2 proteins. All tumors that arose after injection of cells expressing DKK-1, invariably showed almost a complete loss of ectopic DKK-1 expression. The observations that loss of DKK-1 expression or IGFBP-3 activity was required for tumorigenicity suggested that both proteins encode putative tumor suppressor genes. We also show that while DKK-1 expression does not affect cell growth in vitro, the protein does sensitize cells to apoptosis. We also demonstrated that effect of DKK-1 was not due to inhibition of beta-catenin/TCF4-regulated transcription. Taken together, our results indicate that somatic cell genetics combining with gene expression profiling may be a useful approach for the identification of functional suppressors of malignant cell growth.

Functional characterization of WNT7A signaling in PC12 cells: interaction with A FZD5 x LRP6 receptor complex and modulation by Dickkopf proteins

WNT factors represent key mediators of many processes in animal development and homeostasis and act through a receptor complex comprised of members of the Frizzled and low density lipoprotein-related receptors (LRP). In mammals, 19 genes encoding Wingless and Int-related factor (WNTs), 10 encoding Frizzled, and 2 encoding LRP proteins have been identified, but little is known of the identities of individual Frizzled-LRP combinations mediating the effects of specific WNT factors. Additionally, several secreted modulators of WNT signaling have been identified, including at least three members of the Dickkopf family. WNT7A is a WNT family member expressed in the vertebrate central nervous system capable of modulating aspects of neuronal plasticity. Gene knock-out models in the mouse have revealed that WNT7A plays a role in cerebellar maturation, although its function in the development of distal limb structures and of the reproductive tract have been more intensely studied. To identify a receptor complex for this WNT family member, we have analyzed the response of the rat pheochromocytoma cell line PC12 to WNT7A. We find that PC12 cells are capable of responding to WNT7A as measured by increased beta-catenin stability and activation of a T-cell factor-based luciferase reporter construct and that these cells express three members of the Frizzled family (Frizzled-2, -5, and -7) and LRP6. Our functional analysis indicates that WNT7A can specifically act via a Frizzled-5.LRP6 receptor complex in PC12 cells and that this activity can be antagonized by Dickkopf-1 and Dickkopf-3.

Secreted antagonists of the Wnt signalling pathway

The extracellular antagonists of the Wnt signalling pathway can be divided into two broad classes. Both classes of molecule prevent ligand-receptor interactions, but by different mechanisms: members of the first class, which include the sFRP (secreted Frizzled-related protein) family, WIF (Wnt inhibitory factor)-1 and Cerberus, primarily bind to Wnt proteins; the second class comprises certain members of the Dickkopf (Dkk) family, which bind to one subunit of the Wnt receptor complex. In addition, there are other protein interactions that contribute to Wnt antagonist function. Moreover, certain sFRPs and Dkks do not antagonise Wnt function, which suggests that these families have as-yet-undiscovered functions.

EST analysis of gene expression in the tentacle of Cyanea capillata

Jellyfish, Cyanea capillata, has an important position in head patterning and ion channel evolution, in addition to containing a rich source of toxins. In the present study, 2153 expressed sequence tags (ESTs) from the tentacle cDNA library of C. capillata were analyzed. The initial ESTs consisted of 198 clusters and 818 singletons, which revealed approximately 1016 unique genes in the data set. Among these sequences, we identified several genes related to head and foot patterning, voltage-dependent anion channel gene and genes related to biological activities of venom. Five kinds of proteinase inhibitor genes were found in jellyfish for the first time, and some of them were highly expressed with unknown functions.

Divergent expression of type 2 deiodinase and the putative thyroxine-binding protein p29, in rat brain, suggests that they are functionally unrelated proteins

Deiodinases (D1, D2, and D3) are selenoproteins involved in thyroid hormone metabolism. Generation of the active hormone T(3), from T(4), is carried out by D1 and D2, whereas D3 degrades both hormones. The identity of the cloned D2 as a selenoprotein is well supported by biochemical and physiological data. However, an alternative view has proposed that type 2 deiodinase is a nonselenoprotein complex containing a putative T(4) binding subunit called p29, with an almost identity in sequence with the Dickkopf protein Dkk3. To explore a possible functional relationship between p29 and D2, we have compared their mRNA expression patterns in the rat brain. In brain, parenchyma p29 was expressed in neurons. High expression levels were found in all the regions of the blood-cerebrospinal fluid (CSF) barrier. p29 was present in different types of cells than D2, with the exception of the tanycytes. Our data do not support that p29 has a functional relationship with D2. On the other hand, expression of p29 in the blood-CSF barrier suggests that it might be involved in T(4) transport to and from the CSF, but further studies are needed to substantiate this hypothesis.

Overexpression of human Dickkopf-1, an antagonist of wingless/WNT signaling, in human hepatoblastomas and Wilms' tumors

Hepatoblastomas (HBs) represent the most frequent malignant liver tumors of childhood; yet little is known about the molecular pathogenesis and the alterations in expression patterns of these tumors. We used a suppression subtractive hybridization approach to identify new candidate genes that may play a role in HB tumorigenesis. cDNA species derived from corresponding liver and fetal liver were subtracted from HB cDNAs, and a series of interesting candidates were isolated that were differentially expressed. One of the transcripts overexpressed in HB was derived from the human Dickkopf-1 (hDkk-1) gene, which encodes a secreted protein acting as a potent inhibitor of the wingless/WNT signaling pathway. We examined the hDkk-1 expression levels in 32 HB biopsy specimens and in the corresponding liver samples, in 4 HB cell lines, and in a panel of other tumors and normal tissues using a differential PCR approach and Northern blotting. Eighty-one percent of the HBs but none of the normal pediatric or fetal liver tissues showed hDkk-1 expression. hDkk-1 transcripts were also present in 5 of 6 Wilms' tumors but only weakly detectable in 2 of 20 hepatocellular carcinoma samples and in 1 of 5 medulloblastoma cell lines; transcripts were absent in malignant gliomas and breast cancer. The central effector molecule in the WNT developmental control pathway is the beta-catenin protein. Interestingly, activating mutations of the beta-catenin gene have previously been identified in 48% of HBs, and more than 85% of HBs show accumulation of beta-catenin protein as the indicator for an activated pathway. The overexpression of the inhibitor Dkk-1 may therefore be related to uncontrolled wingless/WNT signaling and may represent a negative feedback mechanism. hDkk-1 expression represents a novel marker for HBs and Wilms' tumors.

Kremen2 modulates Dickkopf2 activity during Wnt/LRP6 signaling

Dickkopf1 (Dkk1) is a secreted antagonist of the Wnt/beta-catenin signaling pathway that acts by direct binding to and inhibiting the Wnt co-receptor LRP6. The related Dkk2, however, can function either as LRP6 agonist or antagonist, depending on the cellular context, suggesting that its activity is modulated by unknown co-factors. We have recently identified the transmembrane proteins Kremen1 and -2 as additional Dkk receptors, which bind to both Dkk1 and Dkk2 with high affinity. Here we show that Kremen2 (Krm2) regulates Dkk2 activity during Wnt signaling. In human 293 fibroblasts transfected dkk2 activates LRP6 signaling. However, co-transfection of krm2 blocks the ability of Dkk2 to activate LRP6 and enhances inhibition of Wnt/Frizzled signaling. Krm2 also co-operates with Dkk4 to inhibit Wnt signaling, but not with Dkk3, which has no effect on Wnt signaling. Likewise, in Xenopus embryos, Dkk2 and Krm2 co-operate in Wnt inhibition leading to anteriorized embryos. Finally, we show that interaction with Krm2 is mediated by the second cysteine-rich domain of Dkks. These results suggest that Krm2 can function as a switch that turns Dkk2 from an activator into an inhibitor of Wnt/lRP6 signaling.

Kremen proteins interact with Dickkopf1 to regulate anteroposterior CNS patterning

A gradient of Wnt/beta-catenin signalling formed by posteriorising Wnts and anteriorising Wnt antagonists regulates anteroposterior (AP) patterning of the central nervous system (CNS) during Xenopus gastrulation. In this process, the secreted Wnt antagonist Dkk1 functions in the Spemann organiser and its anterior derivatives by blocking Wnt receptors of the lipoprotein receptor-related protein (LRP) 5 and 6 class. In addition to LRP6, Dkk1 interacts with another recently identified receptor class, the transmembrane proteins Kremen1 (Krm1) and Kremen2 (Krm2) to synergistically inhibit LRP6. We have investigated the role of Krm1 and Krm2 during early Xenopus embryogenesis. Consistent with a role in zygotic Wnt inhibition, overexpressed Krm anteriorises embryos and rescues embryos posteriorised by Wnt8. Antisense morpholino oligonucleotide (Mo) knockdown of Krm1 and Krm2 leads to deficiency of anterior neural development. In this process, Krm proteins functionally interact with Dkk1: (1) in axis duplication assays krm2 synergises with dkk1 in inhibiting Wnt/LRP6 signalling; (2) krm2 rescues microcephalic embryos induced by injection of inhibitory anti-Dkk1 antibodies; and (3) injection of krm1/2 antisense Mo enhances microcephaly induced by inhibitory anti-Dkk1 antibodies. The results indicate that Krm proteins function in a Wnt inhibition pathway regulating early AP patterning of the CNS.

Expression patterns of Wnts, Frizzleds, sFRPs, and misexpression in transgenic mice suggesting a role for Wnts in pancreas and foregut pattern formation

It is well established that gut and pancreas development depend on epithelial-mesenchymal interactions. We show here that several Wnt, Frizzled, and secreted frizzled-related protein (sFRP) encoding mRNAs are present during mouse pancreatic morphogenesis. Wnt5a and 7b mRNA is broadly expressed in foregut mesenchyme starting around embryonic day 10 in mice. Other members expressed are Wnt2b, Wnt5b, and Wnt11. In addition, genes for the Wnt receptors, Frizzled2, 3, 4, 5, 6, 7, 8, and 9 are expressed. To understand potential Wnt functions in pancreas and foregut development in vivo, we analyzed transgenic F0 mouse fetuses expressing Wnt1 and 5a cDNAs under control of the PDX-1 gene promoter. In PDX-Wnt1 fetuses, the foregut region normally comprising the proximal duodenum instead resembles a posterior extension of the stomach, often associated with complete pancreatic and splenic agenesis. Furthermore, the boundary between expression domains of gastric and duodenal markers is shifted in a posterior direction. In PDX-Wnt5a fetuses, several structures derived from the proximal foregut are reduced in size, including the pancreas, spleen, and stomach, without any apparent shift in the stomach to duodenum transition. In these fetuses, overall pancreatic morphology is changed and the pancreatic epithelium is dense and compact, consistent with Wnt5A effects on cell movements and/or attachment. Taken together, these results suggest that Wnt genes participate in epithelial-mesenchymal signaling and may specify region identity in the anterior foregut.