Beyond Wnt inhibition: new functions of secreted Frizzled-related proteins in development and disease

Role of fibroblast growth factor 2 and Wnt signaling in anabolic effects of parathyroid hormone on bone formation

Osteoporosis poses enormous health and economic burden worldwide. One of the very few anabolic agents for osteoporosis is parathyroid hormone (PTH). Although great progress has been made since the FDA approved PTH in 2002, the detailed mechanisms of the bone anabolic effects of intermittent PTH treatment is still not well understood. PTH bone anabolic effect is regulated by extracellular factors. Maximal bone anabolic effect of PTH requires fibroblast growth factor 2 (FGF2) signaling, which might be mediated by transcription factor activating transcription factor 4 (ATF4). Maximal bone anabolic effect of PTH also requires Wnt signaling. Particularly, Wnt antagonists such as sclerostin, dickkopf 1 (DKK1) and secreted frizzled related protein 1 (sFRP1) are promising targets to increase bone formation. Interestingly, FGF2 signaling modulates Wnt/β-Catenin signaling pathway in bone. Therefore, multiple signaling pathways utilized by PTH are cross talking and working together to promote bone formation. Extensive studies on the mechanisms of action of PTH will help to identify new pathways that regulate bone formation, to improve available agents to stimulate bone formation, and to identify potential new anabolic agents for osteoporosis.

Gene expression and epigenetic discovery screen reveal methylation of SFRP2 in prostate cancer

Aberrant activation of Wnts is common in human cancers, including prostate. Hypermethylation associated transcriptional silencing of Wnt antagonist genes SFRPs (Secreted Frizzled-Related Proteins) is a frequent oncogenic event. The significance of this is not known in prostate cancer. The objectives of our study were to (i) profile Wnt signaling related gene expression and (ii) investigate methylation of Wnt antagonist genes in prostate cancer. Using TaqMan Low Density Arrays, we identified 15 Wnt signaling related genes with significantly altered expression in prostate cancer; the majority of which were upregulated in tumors. Notably, histologically benign tissue from men with prostate cancer appeared more similar to tumor (r = 0.76) than to benign prostatic hyperplasia (BPH; r = 0.57, p < 0.001). Overall, the expression profile was highly similar between tumors of high (≥ 7) and low (≤ 6) Gleason scores. Pharmacological demethylation of PC-3 cells with 5-Aza-CdR reactivated 39 genes (≥ 2-fold); 40% of which inhibit Wnt signaling. Methylation frequencies in prostate cancer were 10% (2/20) (SFRP1), 64.86% (48/74) (SFRP2), 0% (0/20) (SFRP4) and 60% (12/20) (SFRP5). SFRP2 methylation was detected at significantly lower frequencies in high-grade prostatic intraepithelial neoplasia (HGPIN; 30%, (6/20), p = 0.0096), tumor adjacent benign areas (8.82%, (7/69), p < 0.0001) and BPH (11.43% (4/35), p < 0.0001). The quantitative level of SFRP2 methylation (normalized index of methylation) was also significantly higher in tumors (116) than in the other samples (HGPIN = 7.45, HB = 0.47, and BPH = 0.12). We show that SFRP2 hypermethylation is a common event in prostate cancer. SFRP2 methylation in combination with other epigenetic markers may be a useful biomarker of prostate cancer.

VEGF-directed blood vessel patterning: from cells to organism

VEGF-A signaling is required for almost every aspect of vascular development, and it is a major regulator of vessel morphogenesis and patterning. VEGF-A perturbations are associated with severe vascular defects and lethality, and the pathway is coopted in pathological scenarios, including tumor angiogenesis. This review focuses on the roles of VEGF-A signaling during vessel development and patterning. I review the impact of VEGF-A signaling on endothelial cells in developing vessels, with emphasis on the importance of spatial regulation of several pathway components. I also discuss VEGF-A signaling patterns at the level of the vessel, with a focus on how polarity is set up and maintained in several vessel axes. The role of VEGF-A in patterning vessels relative to tissues and organs is also reviewed, with emphasis on neurovascular patterning and patterning at the embryonic midline.

The association between polymorphisms in Wnt antagonist genes and bone response to hormone therapy in postmenopausal Korean women

OBJECTIVE

The aim of this study was to explore the association between polymorphisms in Wnt antagonist genes and bone response to hormone therapy (HT) in postmenopausal Korean women.

METHODS

A prospective study was conducted with 303 postmenopausal women receiving sequential estrogen plus progestogen therapy in a university hospital. The dickkopf (Dkk) 1 c.318A>G, Dkk2 c.437G>A, Dkk3 c.1003A>G, secreted frizzled-related protein (sFRP) 1 rs3242C>T, rs16890444C>T, sFRP3 c.970C>G, sFRP4 c.958C>A, c.1019G>A, and sFRP5 c.20G>C polymorphisms were analyzed, and bone mineral density (BMD) at the lumbar spine and femoral neck (FN) was measured before and after 1 year of sequential estrogen plus progestogen therapy.

RESULTS

The percentage changes in BMD of the FN after 1 year of HT were found to be significantly (P < 0.05) different according to the haplotype genotype composed of the sFRP4 c.958C>A and c.1019G>A polymorphisms after adjustment for baseline BMD. The percentage change in BMD at the FN after 1 year of HT was significantly higher in the AA/AG haplotype genotype than in the AG/CG (P < 0.01) or CG/CG (P < 0.05) haplotype genotype. However, any single and combined polymorphisms measured were not related with nonresponsiveness to HT when a nonresponder was defined as a woman who had lost more than 3% of BMD per year after HT.

CONCLUSIONS

The haplotype genotypes of sFRP4 c.958C>A and c.1019G>A polymorphisms are genetic factors that affect changes in BMD of the FN after HT in postmenopausal Korean women.

Control of osteogenesis by the canonical Wnt and BMP pathways in vivo: cooperation and antagonism between the canonical Wnt and BMP pathways as cells differentiate from osteochondroprogenitors to osteoblasts and osteocytes

Although many regulators of skeletogenesis have been functionally characterized, one current challenge is to integrate this information into regulatory networks. Here, we discuss how the canonical Wnt and Smad-dependent BMP pathways interact together and play antagonistic or cooperative roles at different steps of osteogenesis, in the context of the developing vertebrate embryo. Early on, BMP signaling specifies multipotent mesenchymal cells into osteochondroprogenitors. In turn, the function of Wnt signaling is to drive these osteochondroprogenitors towards an osteoblastic fate. Subsequently, both pathways promote osteoblast differentiation, albeit with notable mechanistic differences. In osteocytes, the ultimate stage of osteogenic differentiation, the Wnt and BMP pathways exert opposite effects on the control of bone resorption by osteoclasts. We describe how the dynamic molecular wiring of the canonical Wnt and Smad-dependent BMP signaling into the skeletal cell genetic programme is critical for the generation of bone-specific cell types during development.

Wnt antagonist SFRP1 functions as a secreted mediator of senescence

Cellular senescence has emerged as a critical tumor suppressive mechanism in recent years, but relatively little is known about how senescence occurs. Here, we report that secreted Frizzled-related protein 1 (SFRP1), a secreted antagonist of Wnt signaling, is oversecreted upon cellular senescence caused by DNA damage or oxidative stress. SFRP1 is necessary for stress-induced senescence caused by these factors and is sufficient for the induction of senescence phenotypes. We present evidence suggesting that SFRP1 functions as a secreted mediator of senescence through inhibition of Wnt signaling and activation of the retinoblastoma (Rb) pathway and that cancer-associated SFRP1 mutants are defective for senescence induction.

Wnt/β-Catenin Signaling: a Promising New Target for Fibrosis Diseases

Wnt/β-catenin signaling is involved in virtually every aspect of embryonic development and also controls homeostatic self-renewal in a number of adult tissues. Recently, emerging evidence from researches of organ fibrosis suggest that sustained Wnt/β-catenin pathway reactivation is linked to the pathogenesis of fibrotic disorders. Here we focus on Wnt/β-catenin-related pathogenic effects in different organs, such as lung fibrosis, liver fibrosis, skin fibrosis and renal fibrosis. Additionally, Wnt/β-catenin signaling works in a combinatorial manner with TGF-β signaling in the process of fibrosis, and TGF-β signaling can induce expression of Wnt/β-catenin superfamily members and vice versa. Moreover, network analysis, based on pathway databases, revealed that key factors in the Wnt pathway were targeted by some differentially expressed microRNAs detected in fibrosis diseases. These findings demonstrated the crosstalks between Wnt/β-catenin pathway and TGF-β signalings, and microRNAs, highlighting the role of Wnts in organ fibrogenesis. Most importantly, nowadays there is a variety of Wnt pathway inhibitors which give us the potential therapeutic feasibility, modulation of the Wnt pathway may, therefore, present as a suitable and promising therapeutic strategy in the future.

Antagonistic cross-regulation between Wnt and Hedgehog signalling pathways controls post-embryonic retinal proliferation

Continuous neurogenesis in the adult nervous system requires a delicate balance between proliferation and differentiation. Although Wnt/β-catenin and Hedgehog signalling pathways are thought to share a mitogenic function in adult neural stem/progenitor cells, it remains unclear how they interact in this process. Adult amphibians produce retinal neurons from a pool of neural stem cells localised in the ciliary marginal zone (CMZ). Surprisingly, we found that perturbations of the Wnt and Hedgehog pathways result in opposite proliferative outcomes of neural stem/progenitor cells in the CMZ. Additionally, our study revealed that Wnt and Hedgehog morphogens are produced in mutually exclusive territories of the post-embryonic retina. Using genetic and pharmacological tools, we found that the Wnt and Hedgehog pathways exhibit reciprocal inhibition. Our data suggest that Sfrp-1 and Gli3 contribute to this negative cross-regulation. Altogether, our results reveal an unexpected antagonistic interplay of Wnt and Hedgehog signals that may tightly regulate the extent of neural stem/progenitor cell proliferation in the Xenopus retina.

Wnt/β-catenin signaling and disease

The WNT signal transduction cascade controls myriad biological phenomena throughout development and adult life of all animals. In parallel, aberrant Wnt signaling underlies a wide range of pathologies in humans. In this Review, we provide an update of the core Wnt/β-catenin signaling pathway, discuss how its various components contribute to disease, and pose outstanding questions to be addressed in the future.

Thyroid hormone's action on progenitor/stem cell biology: new challenge for a classic hormone?

BACKGROUND

Thyroid hormones are involved in developmental and homeostatic processes in several tissues. Their action results in different outcomes depending on the developmental stage, tissue and/or cellular context. Interestingly, their pleiotropic roles are conserved across vertebrates. It is largely documented that thyroid hormones act via nuclear receptors, the TRs, which are transcription factors and whose activity can be modulated by the local availability of the hormone T3. In the "classical view", the T3-induced physiological response depends on the expression of specific TR isoforms and the iodothyronine deiodinase selenoenzymes that control the local level of T3, thus TR activity.

SCOPE OF THE REVIEW

Recent data have clearly established that the functionality of TRs is coordinated and integrated with other signaling pathways, specifically at the level of stem/progenitor cell populations. Here, we summarize these data and propose a new and intriguing role for thyroid hormones in two selected examples.

MAJOR CONCLUSIONS

In the intestinal epithelium and the retina, TRα1 and TRβ2 are expressed at the level of the precursors where they induce cell proliferation and differentiation, respectively. Moreover, these different functions result from the integration of the hormone signal with other intrinsic pathways, which play a fundamental role in progenitor/stem cell physiology.

GENERAL SIGNIFICANCE

Taken together, the interaction of TRs with other signaling pathways, specifically in stem/progenitor cells, is a new concept that may have biological relevance in therapeutic approaches aimed to target stem cells such as tissue engineering and cancer. This article is part of a Special Issue entitled Thyroid hormone signalling.

Sizzled-tolloid interactions maintain foregut progenitors by regulating fibronectin-dependent BMP signaling

The liver, pancreas, and lungs are induced from endoderm progenitors by a series of dynamic growth factor signals from the mesoderm, but how the temporal-spatial activity of these signals is controlled is poorly understood. We have identified an extracellular regulatory loop required for robust bone morphogenetic protein (BMP) signaling in the Xenopus foregut. We show that BMP signaling is required to maintain foregut progenitors and induce expression of the secreted frizzled related protein Sizzled (Szl) and the extracellular metalloprotease Tolloid-like 1 (Tll1). Szl negatively regulates Tll activity to control deposition of a fibronectin (FN) matrix between the mesoderm and endoderm, which is required to maintain BMP signaling. Foregut-specific Szl depletion results in a loss of the FN matrix and failure to maintain robust pSmad1 levels, causing a loss of foregut gene expression and organ agenesis. These results have implications for BMP signaling in diverse contexts and the differentiation of foregut tissue from stem cells.

Osteoarthritis, a disease bridging development and regeneration

The osteoarthritic diseases are common disorders characterized by progressive destruction of the articular cartilage in the joints, and associated with remodeling of the subchondral bone, synovitis and the formation of bone outgrowths at the joint margins, osteophytes. From the clinical perspective, osteoarthritis leads to joint pain and loss of function. Osteoarthritis is the leading cause of progressive disability. New data from genetic, translational and basic research have demonstrated that pathways with essential roles in joint and bone development also contribute to the postnatal homeostasis of the articular cartilage and are involved in osteoarthritis, making these potential therapeutic targets. Other systems of interest are the tissue-destructive enzymes that break down the extracellular matrix of the cartilage as well as mediators of inflammation that contribute to synovitis. However, the perspective of a durable treatment over years to decades highlights the need for a personalized medicine approach encompassing a global view on the disease and its management, thereby including nonpharmaceutical approaches such as physiotherapy and advanced surgical methods. Integration of novel strategies based on their efficacy and safety with the identification of individuals at risk and optimal individual rehabilitation management remains a major challenge for the medical community in particular, as the incidence of osteoarthritis is likely to further increase with the overall aging of the population.

The Wnt inhibitor secreted Frizzled-Related Protein 1 (sFRP1) promotes human Th17 differentiation

Wnt/β-catenin signaling plays a crucial role during embryogenesis and tumorigenesis, and in T cells, promotes the differentiation of Th2 cells. However, the role of Wnt signals in the differentiation and maintenance of human Th17 cells remains poorly understood. We found that the higher levels of IL-17 in the synovial fluid of rheumatoid arthritis (RA) patients compared with that of osteoarthritis (OA) patients were associated with a higher concentration of sFRP1 (secreted Frizzled-Related Protein 1), an inhibitor of the Wnt/β-catenin pathway. The addition of sFRP1 during TCR-mediated stimulation induced a significant increase in IL-17 production by both naïve and memory CD4(+) T cells. Moreover, under Th17-differentiation conditions, the addition of sFRP1 significantly reduced the requirement for TGF-β. Mechanistically, we observed that sFRP1 significantly enhanced the phosphorylation of Smad2/3 in CD4(+) T cells upon TGF-β stimulation and that blocking TGF-β signaling abolished the Th17-promoting activity of sFRP1. Our findings reveal a novel function for sFRP1 as a potent inducer of human Th17-cell differentiation. Consequently, sFRP1 may represent a promising target for the treatment of Th17-mediated disease in humans.

SFRP1 and SFRP2 dose-dependently regulate midbrain dopamine neuron development in vivo and in embryonic stem cells

Secreted Frizzled related proteins (sFRPs) are a family of proteins that modulate Wnt signaling, which in turn regulates multiple aspects of ventral midbrain (VM) and dopamine (DA) neuron development. However, it is not known which Wnt signaling branch and what aspects of midbrain DA neuron development are regulated by sFRPs. Here, we show that sFRP1 and sFRP2 activate the Wnt/planar-cell-polarity/Rac1 pathway in DA cells. In the developing VM, sFRP1 and sFRP2 are expressed at low levels, and sFRP1-/- or sFRP2-/- mice had no detectable phenotype. However, compound sFRP1-/-;sFRP2-/- mutants revealed a Wnt/PCP phenotype similar to that previously described for Wnt5a-/- mice. This included an anteroposterior shortening of the VM, a lateral expansion of the Shh domain and DA lineage markers (Lmx1a and Th), as well as an accumulation of Nurr1+ precursors in the VM. In vitro experiments showed that, while very high concentrations of SFRP1 had a negative effect on cell survival, low/medium concentrations of sFRP1 or sFRP2 promoted the DA differentiation of progenitors derived from primary VM cultures or mouse embryonic stem cells (ESCs), mimicking the effects of Wnt5a. We thus conclude that the main function of sFRP1 and sFRP2 is to enhance Wnt/PCP signaling in DA cells and to regulate Wnt/PCP-dependent functions in midbrain development. Moreover, we suggest that low-medium concentrations of sFRPs may be used to enhance the DA differentiation of ESCs and improve their therapeutic application.

Reexpression of tumor suppressor, sFRP1, leads to antitumor synergy of combined HDAC and methyltransferase inhibitors in chemoresistant cancers

Metastatic solid tumors are aggressive and mostly drug resistant, leading to few treatment options and poor prognosis as seen with clear cell renal cell carcinoma (ccRCC) and triple-negative breast cancer (TNBC). Therefore, the identification of new therapeutic regimes for the treatment of metastatic disease is desirable. ccRCC and TNBC cell lines were treated with the HDAC inhibitor romidepsin and the methyltransferase inhibitor decitabine, two epigenetic modifying drugs approved by the U.S. Food and Drug Administration for the treatment of various hematologic malignancies. Cell proliferation analysis, flow cytometry, quantitative PCR, and immunoblotting techniques were used to evaluate the antitumor synergy of this drug combination and identify the reexpression of epigenetically silenced tumor suppressor genes. Combinatorial treatment of metastatic TNBC and stage IV ccRCC cell lines with romidepsin/decitabine leads to synergistic inhibition of cell growth and induction of apoptosis above levels of individual drug treatments alone. Synergistic reexpression of the tumor suppressor gene secreted frizzled-related protein one (sFRP1) was observed in combinatorial drug-treated groups. Silencing sFRP1 (short hairpin RNA) before combinatorial drug treatment showed that sFRP1 mediates the growth inhibitory and apoptotic activity of combined romidepsin/decitabine. Furthermore, addition of recombinant sFRP1 to ccRCC or TNBC cells inhibits cell growth in a dose-dependent manner through the induction of apoptosis, identifying that epigenetic silencing of sFRP1 contributes to renal and breast cancer cell survival. Combinatorial treatment with romidepsin and decitabine in drug resistant tumors is a promising treatment strategy. Moreover, recombinant sFRP1 may be a novel therapeutic strategy for cancers with suppressed sFRP1 expression.

Sizzled is unique among secreted frizzled-related proteins for its ability to specifically inhibit bone morphogenetic protein-1 (BMP-1)/tolloid-like proteinases

BMP-1/tolloid-like proteinases (BTPs) are major enzymes involved in extracellular matrix assembly and activation of bioactive molecules, both growth factors and anti-angiogenic molecules. Although the control of BTP activity by several enhancing molecules is well established, the possibility that regulation also occurs through endogenous inhibitors is still debated. Secreted frizzled-related proteins (sFRPs) have been studied as possible candidates, with highly contradictory results, after the demonstration that sizzled, a sFRP found in Xenopus and zebrafish, was a potent inhibitor of Xenopus and zebrafish tolloid-like proteases. In this study, we demonstrate that mammalian sFRP-1, -2, and -4 do not modify human BMP-1 activity on several of its known substrates including procollagen I, procollagen III, pN-collagen V, and prolysyl oxidase. In contrast, Xenopus sizzled appears as a tight binding inhibitor of human BMP-1, with a K(i) of 1.5 ± 0.5 nM, and is shown to strongly inhibit other human tolloid isoforms mTLD and mTLL-1. Because sizzled is the most potent inhibitor of human tolloid-like proteinases known to date, we have studied its mechanism of action in detail and shown that the frizzled domain of sizzled is both necessary and sufficient for inhibitory activity and that it acts directly on the catalytic domain of BMP-1. Residues in sizzled required for inhibition include Asp-92, which is shared by sFRP-1 and -2, and also Phe-94, Ser-43, and Glu-44, which are specific to sizzled, thereby providing a rational basis for the absence of inhibitory activity of human sFRPs.

Repeated evolution of identical domain architecture in metazoan netrin domain-containing proteins

The majority of proteins in eukaryotes are composed of multiple domains, and the number and order of these domains is an important determinant of protein function. Although multidomain proteins with a particular domain architecture were initially considered to have a common evolutionary origin, recent comparative studies of protein families or whole genomes have reported that a minority of multidomain proteins could have appeared multiple times independently. Here, we test this scenario in detail for the signaling molecules netrin and secreted frizzled-related proteins (sFRPs), two groups of netrin domain-containing proteins with essential roles in animal development. Our primary phylogenetic analyses suggest that the particular domain architectures of each of these proteins were present in the eumetazoan ancestor and evolved a second time independently within the metazoan lineage from laminin and frizzled proteins, respectively. Using an array of phylogenetic methods, statistical tests, and character sorting analyses, we show that the polyphyly of netrin and sFRP is well supported and cannot be explained by classical phylogenetic reconstruction artifacts. Despite their independent origins, the two groups of netrins and of sFRPs have the same protein interaction partners (Deleted in Colorectal Cancer/neogenin and Unc5 for netrins and Wnts for sFRPs) and similar developmental functions. Thus, these cases of convergent evolution emphasize the importance of domain architecture for protein function by uncoupling shared domain architecture from shared evolutionary history. Therefore, we propose the terms merology to describe the repeated evolution of proteins with similar domain architecture and discuss the potential of merologous proteins to help understanding protein evolution.

WNT secretion and signalling in human disease

Wnt signalling, a key pathway involved in various aspects of embryonic development, also underlies many human diseases, in particular, cancer. Research focused on signal transduction within signal-receiving cells led to the discovery of many Wnt pathway components, but study of the secretion of Wnt ligands themselves was neglected until recently. Attention was drawn to this highly regulated process by the association of aberrant Wnt levels with an increasing number of diseases. Studying the biogenesis and processing of active Wnt ligands will open new avenues for generating therapeutics to specifically target aberrant Wnt signalling. Here we review the proteins required for Wnt secretion and signalling at the plasma membrane, ending with a discussion on potential therapeutic approaches to treat Wnt-induced diseases.

Multispecies model of cell lineages and feedback control in solid tumors

We develop a multispecies continuum model to simulate the spatiotemporal dynamics of cell lineages in solid tumors. The model accounts for protein signaling factors produced by cells in lineages, and nutrients supplied by the microenvironment. Together, these regulate the rates of proliferation, self-renewal and differentiation of cells within the lineages, and control cell population sizes and distributions. Terminally differentiated cells release proteins (e.g., from the TGFβ superfamily) that feedback upon less differentiated cells in the lineage both to promote differentiation and decrease rates of proliferation (and self-renewal). Stem cells release a short-range factor that promotes self-renewal (e.g., representative of Wnt signaling factors), as well as a long-range inhibitor of this factor (e.g., representative of Wnt inhibitors such as Dkk and SFRPs). We find that the progression of the tumors and their response to treatment is controlled by the spatiotemporal dynamics of the signaling processes. The model predicts the development of spatiotemporal heterogeneous distributions of the feedback factors (Wnt, Dkk and TGFβ) and tumor cell populations with clusters of stem cells appearing at the tumor boundary, consistent with recent experiments. The nonlinear coupling between the heterogeneous expressions of growth factors and the heterogeneous distributions of cell populations at different lineage stages tends to create asymmetry in tumor shape that may sufficiently alter otherwise homeostatic feedback so as to favor escape from growth control. This occurs in a setting of invasive fingering, and enhanced aggressiveness after standard therapeutic interventions. We find, however, that combination therapy involving differentiation promoters and radiotherapy is very effective in eradicating such a tumor.

Lighting the fat furnace without SFRP5

WNT signaling plays a central role in the regulation of cellular growth and differentiation. In this issue of the JCI, Mori et al. link WNT signaling to the oxidative capacity of adipocytes during obesity. They show that secreted frizzled-related protein 5 is an extracellular matrix-residing protein that is highly induced during obesity and inhibits oxidative phosphorylation in a tissue-autonomous manner, possibly by sequestering WNT3a. These results implicate local WNT signaling as an attractive target for combating obesity.

Secreted frizzled-related protein 5 suppresses adipocyte mitochondrial metabolism through WNT inhibition

Preadipocytes secrete several WNT family proteins that act through autocrine/paracrine mechanisms to inhibit adipogenesis. The activity of WNT ligands is often decreased by secreted frizzled-related proteins (SFRPs). Sfrp5 is strongly induced during adipocyte differentiation and increases in adipocytes during obesity, presumably to counteract WNT signaling. We tested the hypothesis that obesity-induced Sfrp5 expression promotes the development of new adipocytes by inhibiting endogenous suppressors of adipogenesis. As predicted, mice that lack functional SFRP5 were resistant to diet-induced obesity. However, counter to our hypothesis, we found that adipose tissue of SFRP5-deficient mice had similar numbers of adipocytes, but a reduction in large adipocytes. Transplantation of adipose tissue from SFRP5-deficient mice into leptin receptor-deficient mice indicated that the effects of SFRP5 deficiency are tissue-autonomous. Mitochondrial gene expression was increased in adipose tissue and cultured adipocytes from SFRP5-deficient mice. In adipocytes, lack of SFRP5 stimulated oxidative capacity through increased mitochondrial activity, which was mediated in part by PGC1α and mitochondrial transcription factor A. WNT3a also increased oxygen consumption and the expression of mitochondrial genes. Thus, our findings support a model of adipogenesis in which SFRP5 inhibits WNT signaling to suppress oxidative metabolism and stimulate adipocyte growth during obesity.

Wnt/β-catenin signalling in prostate cancer

The Wnts are secreted cysteine-rich glycoproteins that have important roles in the developing embryo as well as in tissue homeostasis in adults. Dysregulation of Wnt signalling can lead to several types of cancer, including prostate cancer. A hallmark of the signalling pathway is the stabilization of the transcriptional co-activator β-catenin, which not only regulates expression of many genes implicated in cancer but is also an essential component of cadherin cell adhesion complexes. β-catenin regulates gene expression by binding members of the T-cell-specific transcription factor/lymphoid enhancer-binding factor 1 (TCF/LEF-1) family of transcription factors. In addition, β-catenin associates with the androgen receptor, a key regulator of prostate growth that drives prostate cancer progression. Wnt/β-catenin signalling can be controlled by secreted Wnt antagonists, many of which are downregulated in cancer. Activation of the Wnt/β-catenin pathway has effects on prostate cell proliferation, differentiation and the epithelial-mesenchymal transition, which is thought to regulate the invasive behaviour of tumour cells. However, whether targeting Wnt/β-catenin signalling is a good therapeutic option for prostate cancer remains unclear.

Dvl2-dependent activation of Daam1 and RhoA regulates Wnt5a-induced breast cancer cell migration

BACKGROUND

The Dishevelled (Dvl) and Dishevelled-associated activator of morphogenesis 1 (Daam1) pathway triggered by Wnt5a regulates cellular polarity during development and tissue homoeostasis. However, Wnt5a signaling in breast cancer progression remains poorly defined.

METHODOLOGY/PRINCIPAL FINDINGS

We showed here that Wnt5a activated Dvl2, Daam1 and RhoA, and promoted migration of breast cancer cells, which was, however, abolished by Secreted Frizzled-related protein 2 (sFRP2) pretreatment. Dominant negative Dvl2 mutants or Dvl2 siRNA significantly decreased Wnt5a-induced Daam1/RhoA activation and cell migration. Ectopic expression of N-Daam1, a dominant negative mutant, or Daam1 siRNA remarkably inhibited Wnt5a-induced RhoA activation, stress fiber formation and cell migration. Ectopic expression of dominant negative RhoA (N19) or C3 exoenzyme transferase, a Rho inhibitor, decreased Wnt5a-induced stress fiber formation and cell migration.

CONCLUSIONS/SIGNIFICANCE

Taken together, we demonstrated for the first time that Wnt5a promotes breast cancer cell migration via Dvl2/Daam1/RhoA.

WNTs in synapse formation and neuronal circuitry

Wnt proteins play important roles in wiring neural circuits. Wnts regulate many aspects of neural circuit generation through their receptors and distinct signalling pathways. In this review, we discuss recent findings on the functions of Wnts in various aspects of neural circuit formation, including neuronal polarity, axon guidance, synapse formation, and synaptic plasticity in vertebrate and invertebrate nervous systems.

The WNT inhibitor Dickkopf 1 and bone morphogenetic protein 4 rescue adipogenesis in hypertrophic obesity in humans

Overweight characterized by inappropriate expansion of adipose cells (hypertrophic obesity) is associated with the metabolic syndrome and is caused by an inability to recruit and differentiate new precursor cells. We examined the role of bone morphogenetic protein 4 (BMP4) and WNT activation in the regulation of human adipose cell differentiation. Cluster of differentiation (CD)14(+)/45(+) and CD31(+) cells were first removed before the remaining stromal vascular cells of human subcutaneous biopsy specimens were differentiated with/without different WNT inhibitors and/or BMP4. Inhibition of WNT and induction of Dickkopf 1 (DKK1) were markers of precursor cells undergoing excellent differentiation. The addition of DKK1 inhibited WNT activation and promoted adipogenesis in cells with a low degree of differentiation. The positive effect of DKK1, inhibiting cellular WNT activation by binding to the Kremen/LDL receptor-related protein receptors, was not seen with inhibitors of secreted WNT ligands. BMP4 increased differentiation, and BMP4 in the presence of DKK1 produced an additive effect. There was an apparent cross-talk between differentiation and commitment because BMP4 expression increased in differentiating adipocytes, and the addition of the BMP4 inhibitor, Noggin, reduced precursor cell differentiation. Thus, differentiated human adipose cells can promote adipogenesis via endogenous BMP4 activation, and the impaired adipogenesis in hypertrophic obesity is mainly due to an inability to suppress canonical WNT and to induce DKK1.

A new validated mathematical model of the Wnt signalling pathway predicts effective combinational therapy by sFRP and Dkk

The Wnt signalling pathway controls cell proliferation and differentiation, and its deregulation is implicated in different diseases including cancer. Learning how to manipulate this pathway could substantially contribute to the development of therapies. We developed a mathematical model describing the initial sequence of events in the Wnt pathway, from ligand binding to β-catenin accumulation, and the effects of inhibitors, such as sFRPs (secreted Frizzled-related proteins) and Dkk (Dickkopf). Model parameters were retrieved from experimental data reported previously. The model was retrospectively validated by accurately predicting the effects of Wnt3a and sFRP1 on β-catenin levels in two independent published experiments (R2 between 0.63 and 0.91). Prospective validation was obtained by testing the model's accuracy in predicting the effect of Dkk1 on Wnt-induced β-catenin accumulation (R2≈0.94). Model simulations under different combinations of sFRP1 and Dkk1 predicted a clear synergistic effect of these two inhibitors on β-catenin accumulation, which may point towards a new treatment avenue. Our model allows precise calculation of the effect of inhibitors applied alone or in combination, and provides a flexible framework for identifying potential targets for intervention in the Wnt signalling pathway.

Spatial and temporal localization of WNT signaling proteins in a mouse model of distraction osteogenesis

While the surgical procedure of distraction osteogenesis (DO) is very successful in the treatment of orthopedic conditions, its major limitation of slow bone formation in the distracted gap has prompted numerous attempts to understand and accelerate this slow bone formation. Interestingly, WNT/FZD signaling has been identified as a critical pathway in mediating bone formation and regeneration but has not yet been studied in the context of DO. The objective of this study was to determine the spatial and temporal localization of endogenous WNT signaling proteins at various times of bone formation in a wild-type mouse model of DO. In this study, the DO protocol performed on mice consisted of three phases: latency (5 days), distraction (12 days), and consolidation (34 days). Our immunohistochemical findings of distracted bone specimens show an increased expression of WNT ligands (WNT4 and WNT10A), receptors (FZD1 and 2, LRP5 and 6), β-catenin, and pathway antagonizers (DKK1; CTBP1 and 2; sFRP1, 2, and 4) during the distraction phase, which were then down-regulated during consolidation. This is the first published report to show an activation of the WNT pathway in DO and could help identify WNT as a potential therapeutic target in accelerating bone regeneration during DO.

The role of secreted frizzled-related protein 2 expression in prostate cancer

AIMS

Improved prostate cancer (PCa)-specific biomarkers are urgently required to distinguish between indolent and aggressive disease, in order to avoid overtreatment. In this study, we investigated the prostatic tissue expression of secreted frizzled-related protein (SFRP)-2.

METHODS AND RESULTS

Following immunohistochemical analysis on PCa tissue microarrays with samples from 216 patients, strong/moderate SFRP-2 expression was observed in epithelial cells of benign prostatic hyperplasia, and negative/weak SFRP-2 expression was observed in the majority of tumour epithelia. However, among Gleason grade 5 carcinomas, 40% showed strong/moderate SFRP-2 expression and 60% showed negative SFRP-2 expression in epithelial cells. Further microscopic evaluation of Gleason grade 5 tumours revealed different morphological patterns, corresponding with differential SFRP-2 expression. The first subgroup (referred to as Type A) appeared to have a morphologically solid growth pattern, whereas the second subgroup (referred to as Type B) appeared to have a more diffuse pattern. Furthermore, 100% (4/4) of Type A patients experienced biochemical recurrence, as compared with 0% (0/6) of Type B patients.

CONCLUSIONS

These results imply: (i) that there is a loss of SFRP-2 expression from benign to malignant prostate glands; and (ii) differential SFRP-2 expression among two possible subgroups of Gleason grade 5 tumours.

Genistein, a soya isoflavone, prevents azoxymethane-induced up-regulation of WNT/β-catenin signalling and reduces colon pre-neoplasia in rats

The present study aimed to explore the role(s) of the soya isoflavone genistein (GEN) in preventing the development of colon pre-neoplasia, using Wingless/int (WNT)/β-catenin as a molecular marker of colon abnormality. Specifically, the effects on the WNT/β-catenin signalling pathway from GEN were examined by using an azoxymethane (AOM)-induced rat colon cancer model. Male Sprague-Dawley rats were fed a control (CTL), a soya protein isolate (SPI) or a GEN diet from gestation to 13 weeks of age. The first sampling was conducted at 7 weeks of age for pre-AOM analysis. The remaining rats were injected with AOM at 7 weeks of age. The descending colon was collected 6 weeks later for the evaluation of aberrant crypt foci (ACF), gene expression and nuclear protein accumulation. AOM injection induced aberrant nuclear accumulation of β-catenin in the CTL group but not in the SPI or GEN group. Moreover, the WNT target genes Cyclin D1 and c-Myc were repressed by SPI and GEN. Meanwhile, SPI and GEN suppressed the expression of WNT signalling genes including Wnt5a, Sfrp1, Sfrp2 and Sfrp5 to the similar level to that of the pre-AOM period. Rats fed SPI and GEN had a decreased number of total aberrant crypts. GEN feeding also resulted in a reduced number of ACF with N = 3 per foci. The reduction of WNT/β-catenin signalling was correlated with the decrease in total aberrant crypts. By testing WNT/β-catenin signalling as a biomarker of colon carcinogenic potential, we showed the novel role of GEN as a suppressor of carcinogen-induced WNT/β-catenin signalling in preventing the development of early colon neoplasia.

Wnt/β-catenin signalling in adrenal physiology and tumour development

Wnt/β-catenin signalling plays essential roles during embryonic development and in adult tissue homeostasis. Canonical signalling through Wnt secreted ligands relies on the control of β-catenin cytoplasmic accumulation and translocation to the nucleus. In this compartment, β-catenin serves as a transcription coactivator for transcription factors such as Lef/Tcf or some nuclear receptors. Constitutive Wnt signalling resulting from inactivation of inhibitors of the pathway or from activating mutations in β-catenin, triggers tumour development in a number of tissues. Analysis of patients' samples and genetically engineered mouse models has shown that Wnt signalling was involved in adrenal development and tumourigenesis. This review will summarise all these recent findings and will focus on some of the mechanisms that may lead to aberrant accumulation of β-catenin in adrenocortical tumours.

sizzled function and secreted factor network dynamics

Studies on the role of the E-box binding transcription factor Snail2 (Slug) in the induction of neural crest by mesoderm (Shi et al., 2011) revealed an unexpected increase in the level of sizzled RNA in the dorsolateral mesodermal zone (DMLZ) of morphant Xenopus embryos. sizzled encodes a secreted protein with both Wnt and BMP inhibitor activities. Morpholino-mediated down-regulation of sizzled expression in one cell of two cell embryos or the C2/C3 blastomeres of 32-cell embryos, which give rise to the DLMZ, revealed decreased expression of the mesodermal marker brachyury and subsequent defects in neural crest induction, pronephros formation, and muscle patterning. Loss of sizzled expression led to decreases in RNAs encoding the secreted Wnt inhibitor SFRP2 and the secreted BMP inhibitor Noggin; the sizzled morphant phenotype could be rescued by co-injection of RNAs encoding Noggin and either SFRP2 or Dickkopf (a mechanistically distinct Wnt inhibitor). Together, these observations reveal that sizzled, in addition to its established role in dorsal-ventral patterning, is also part of a dynamic BMP and Wnt signaling network involved in both mesodermal patterning and neural crest induction.

Adipocyte-secreted factors increase osteoblast proliferation and the OPG/RANKL ratio to influence osteoclast formation

Several studies have reported a positive relationship of the body fat mass and bone density. However, it is not clear whether adipocyte-derived signaling molecules directly act on osteoblasts or osteoclasts. Therefore, we investigated the effect of fat cell-secreted factors on the proliferation and differentiation of preosteoblasts and the molecular mechanisms involved. This stimulation led to an increased proliferation of MC3T3-E1 and primary preosteoblastic cells (2.8-fold and 1.5-fold, respectively; p<0.0001), which could be reduced with inhibitors of protein tyrosine kinases, FGFR1 and PI3K. Concordantly, we found human adipocytes to secrete bFGF and bFGF to mimic the effect of adipocyte-secreted factors. The ratio of OPG/RANKL secretion in primary human preosteoblasts increased 9-fold (mRNA and protein) when stimulated with adipocyte-secreted factors. Moreover, osteoblasts which were prestimulated with adipocyte-secreted factors inhibited the formation of osteoclasts. In conclusion, human adipocytes secrete factors that directly act on preosteoblasts and alter their crosstalk with osteoclasts. These in vitro findings reflect the higher bone mass in obese people and attribute it to effects of adipocyte-secreted factors on bone formation.

Wnt5a is strongly expressed at the leading edge in non-melanoma skin cancer, forming active gradients, while canonical Wnt signalling is repressed

Wnt5a is one of the so-called non-canonical Wnt ligands which do not act through β-catenin. In normal development, Wnt5a is secreted and directs the migration of target cells along concentration gradients. The effect of Wnt5a on target cells is regulated by many factors, including the expression level of inhibitors and receptors. Dysregulated Wnt5a signalling facilitates invasion of multiple tumor types into adjacent tissue. However, the expression and distribution of Wnt5a in cutaneous squamous cell carcinoma (SCC) and basal cell carcinoma (BCC), as well as the effect of Wnt5a on keratinocyte migration has not been studied in detail to date. We here report that Wnt5a is upregulated in SCC and BCC and localised to the leading edge of tumors, as well as tumor-associated fibroblasts. The Wnt5a-triggered bundling of its receptor Fzd3 provides evidence of Wnt5a concentration gradients projecting into the tumor. In vitro migration assays show that Wnt5a concentration gradients determine its effect on keratinoctye migration: While chemotactic migration is inhibited by Wnt5a present in homogenous concentrations, it is enhanced in the presence of a Wnt5a gradient. Expression profiling of the Wnt pathway shows that the upregulation of Wnt5a in SCC is coupled to repression of canonical Wnt signalling. This is confirmed by immunohistochemistry showing lack of nuclear β-catenin, as well as absent accumulation of Axin2. Since both types of Wnt signalling act mutually antogonistically at multiple levels, the concurrent repression of canonical Wnt signalling suggests hyper-active Wnt5a signal transduction. Significantly, this combination of gene dysregulation is not observed in the benign hyperproliferative inflammatory skin disease psoriasis. Collectively, our data strongly suggest that Wnt5a signalling contributes to tissue invasion by non-melanoma skin cancer.

Pαx6 expression in postmitotic neurons mediates the growth of axons in response to SFRP1

During development, the mechanisms that specify neuronal subclasses are coupled to those that determine their axonal response to guidance cues. Pax6 is a homedomain transcription factor required for the specification of a variety of neural precursors. After cell cycle exit, Pax6 expression is often shut down in the precursor progeny and most postmitotic neurons no longer express detectable levels of the protein. There are however exceptions and high Pax6 protein levels are found, for example, in postmitotic retinal ganglion cells (RGCs), dopaminergic neurons of the olfactory bulb and the limbic system in the telencephalon. The function of Pax6 in these differentiating neurons remains mostly elusive. Here, we demonstrate that Pax6 mediates the response of growing axons to SFRP1, a secreted molecule expressed in several Pax6-positive forebrain territories. Forced expression of Pax6 in cultured postmitotic cortical neurons, which do not normally express Pax6, was sufficient to increment axonal length. Growth was blocked by the addition of anti-SFRP1 antibodies, whereas exogenously added SFRP1 increased axonal growth of Pax6-transfected neurons but not that of control or untransfected cortical neurons. In the reverse scenario, shRNA-mediated knock-down of Pax6 in mouse retinal explants specifically abolished RGCs axonal growth induced by SFRP1, but had no effect on RGCs differentiation and it did not modify the effect of Shh or Netrin on axon growth. Taken together these results demonstrate that expression of Pax6 is necessary and sufficient to render postmitotic neurons competent to respond to SFRP1. These results reveal a novel and unexpected function of Pax6 in postmitotic neurons and situate Pax6 and SFRP1 as pair regulators of axonal connectivity.

Mef2c deletion in osteocytes results in increased bone mass

Myocyte enhancer factors 2 (MEF2) are required for expression of the osteocyte bone formation inhibitor Sost in vitro, implying these transcription factors in bone biology. Here, we analyzed the in vivo function of Mef2c in osteocytes in male and female mice during skeletal growth and aging. Dmp1-Cre-induced Mef2c deficiency led to progressive decreases in Sost expression by 40% and 70% in femoral cortical bone at 3.5 months and 5 to 6 months of age. From 2 to 3 months onward, bone mass was increased in the appendicular and axial skeleton of Mef2c mutant relative to control mice. Cortical thickness and long bone and vertebral trabecular density were elevated. To assess whether the increased bone mass was related to the decreased Sost expression, we characterized 4-month-old heterozygous Sost-deficient mice. Sost heterozygotes displayed similar increases in long bone mass and density as Mef2c mutants, but the relative increases in axial skeletal parameters were mostly smaller. At the cellular level, bone formation parameters were normal in 3.5-month-old Mef2c mutant mice, whereas bone resorption parameters were significantly decreased. Correspondingly, cortical expression of the anti-osteoclastogenic factor and Wnt/β-catenin target gene osteoprotegerin (OPG) was increased by 70% in Mef2c mutant males. Furthermore, cortical expression of the Wnt signaling modulators Sfrp2 and Sfrp3 was strongly deregulated in both sexes. In contrast, heterozygous Sost deficient males displayed mildly increased osteoblastic mineral apposition rate, but osteoclast surface and cortical expression of osteoclastogenic regulators including OPG were normal and Sfrp2 and Sfrp3 were not significantly changed. Together, our data demonstrate that Mef2c regulates cortical Sfrp2 and Sfrp3 expression and is required to maintain normal Sost expression in vivo. Yet, the increased bone mass phenotype of Mef2c mutants is not directly related to the reduced Sost expression. We identified a novel function for Mef2c in control of adult bone mass by regulation of osteoclastic bone resorption.

Subtype-specific alterations of the Wnt signaling pathway in breast cancer: clinical and prognostic significance

The aim of the study is to understand the importance of the Wnt/β-catenin pathway in the development of breast cancer (BC) and its association with different clinicopathological parameters. Alterations (deletion/methylation/expression) of some Wnt/β-catenin pathway antagonists like APC, SFRP1/2, CDH1 and activator β-catenin (CTNNB1) were analyzed in primary BC in Indian patients. High frequencies (65-70%) of overall alterations (deletion/methylation) of the antagonists were seen in the BC samples. Also, 99% (156/158) of the samples showed alterations in any one of the genes, indicating the importance of this pathway in the development of this tumor. Co-alterations of these genes were observed in 30% of samples, with significantly high alterations in late-onset (37%) and estrogen receptor (ER)-/progesterone receptor (PR)- (37%) BC compared with early onset (21%) and ER/PR+ (18%) BC samples, respectively. Significantly high (P-value = 0.001-0.02) alterations of APC and CDH1 genes were seen in ER-/PR- BC compared with ER/PR+ BC. Immunohistochemical analysis showed reduced expression of the Wnt antagonists in BC concordant with their molecular alterations. Nuclear localization of β-catenin showed significant association with alterations in the antagonists and was also significantly high in the ER-/PR- BC samples. Alterations of SFRP2 coupled with a late clinical stage and low/nulliparity predicted the worst prognosis in BC patients. Therefore, the present study suggests that cumulative alterations in more than one Wnt antagonist along with increased nuclear accumulation of β-catenin play an important role in the development of BC and have significant clinical as well as prognostic importance.

Radiofrequency ablation for dysplasia in Barrett's esophagus restores β-catenin activation within esophageal progenitor cells

BACKGROUND AND AIMS

Endoscopic therapies for Barrett's esophagus (BE) associated dysplasia, particularly radiofrequency ablation (RFA), are popular alternatives to surgery. The effect of such therapies on dysplastic stem/progenitor cells (SPC) is unknown. Recent studies suggest that AKT phosphorylation of β-Catenin occurs in SPCs and may be a marker of activated SPCs. We evaluate the effect of RFA in restoring AKT-mediated β-Catenin signaling in regenerative epithelium.

METHODS

Biopsies were taken from squamous, non-dysplastic BE, dysplastic BE and esophageal adenocarcinoma (EAC). Also, post-RFA, biopsies of endoscopically normal appearing neosquamous epithelium were taken at 3, 6, and 12 months after successful RFA. Immunohistochemistry and Western blot analysis was performed for Pβ-Catenin(552) (Akt-mediated phosphorylation of β-Catenin), Ki-67 and p53.

RESULTS

There was no difference in Pβ-Catenin552 in squamous, GERD, small bowel and non-dysplastic BE. There was a fivefold increase in Pβ-Catenin(552) in dysplasia and EAC compared to non-dysplastic BE (P < 0.05). Also, there was a persistent threefold increase in Pβ-Catenin(552) in neosquamous epithelium 3 months after RFA compared to native squamous epithelium (P < 0.05) that correlated with increased Ki-67. Six months after RFA, Pβ-Catenin(552) and Ki-67 are similar to native squamous epithelium.

CONCLUSIONS

Enhanced AKT-mediated β-Catenin activation is seen in BE-associated carcinogenesis. Three months after RFA, squamous epithelial growth from SPC populations exhibited increased levels of Pβ-Catenin(552). This epithelial response becomes quiescent at 6 months after RFA. These data suggest that elevated Pβ-Catenin(552) after RFA denotes a repair response in the neosquamous epithelium 3 months post-RFA.

Inhibition of Wnt/β-catenin signaling by a soluble collagen-derived frizzled domain interacting with Wnt3a and the receptors frizzled 1 and 8

The Wnt/β-catenin pathway controls cell proliferation, death and differentiation. Several families of extracellular proteins can antagonize Wnt/β-catenin signaling, including the decoy receptors known as secreted frizzled related proteins (SFRPs), which have a cysteine-rich domain (CRD) structurally similar to the extracellular Wnt-binding domain of the frizzled receptors. SFRPs inhibit Wnt signaling by sequestering Wnts through the CRD or by forming inactive complexes with the frizzled receptors. Other endogenous molecules carrying frizzled CRDs inhibit Wnt signaling, such as V3Nter, which is proteolytically derived from the cell surface component collagen XVIII and contains a biologically active frizzled domain (FZC18) inhibiting in vivo cell proliferation and tumor growth in mice. We recently showed that FZC18 expressing cells deliver short-range signals to neighboring cells, decreasing their proliferation in vitro and in vivo through the Wnt/β-catenin signaling pathway. Here, using low concentrations of soluble FZC18 and Wnt3a, we show that they physically interact in a cell-free system. In addition, soluble FZC18 binds the frizzled 1 and 8 receptors' CRDs, reducing cell sensitivity to Wnt3a. Conversely, inhibition of Wnt/β-catenin signaling was partially rescued by the expression of full-length frizzled 1 and 8 receptors, but enhanced by the expression of a chimeric cell-membrane-tethered frizzled 8 CRD. Moreover, soluble, partially purified recombinant FZC18_CRD inhibited Wnt3a-induced β-catenin activation. Taken together, the data indicate that collagen XVIII-derived frizzled CRD shifts Wnt sensitivity of normal cells to a lower pitch and controls their growth.

Tight regulation of wingless-type signaling in the articular cartilage - subchondral bone biomechanical unit: transcriptomics in Frzb-knockout mice

Introduction

The aim of this research was to study molecular changes in the articular cartilage and subchondral bone of the tibial plateau from mice deficient in frizzled-related protein (Frzb) compared to wild-type mice by transcriptome analysis.

Methods

Gene-expression analysis of the articular cartilage and subchondral bone of three wild-type and three Frzb^-/- mice was performed by microarray. Data from three wild-type and two Frzb^-/- samples could be used for pathway analysis of differentially expressed genes and were explored with PANTHER, DAVID and GSEA bioinformatics tools. Activation of the wingless-type (WNT) pathway was analysed using Western blot. The effects of Frzb gain and loss of function on chondrogenesis and cell proliferation was examined using ATDC5 micro-masses and mouse ribcage chondrocytes.

Results

Extracellular matrix-associated integrin and cadherin pathways, as well as WNT pathway genes were up-regulated in Frzb^-/- samples. Several WNT receptors, target genes and other antagonists were up-regulated, but no difference in active β-catenin was found. Analysis of ATDC5 cell micro-masses overexpressing FRZB indicated an up-regulation of aggrecan and Col2a1, and down-regulation of molecules related to damage and repair in cartilage, Col3a1 and Col5a1. Silencing of Frzb resulted in down-regulation of aggrecan and Col2a1. Pathways associated with cell cycle were down-regulated in this transcriptome analysis. Ribcage chondrocytes derived from Frzb^-/- mice showed decreased proliferation compared to wild-type cells.

Conclusions

Our analysis provides evidence for tight regulation of WNT signalling, shifts in extracellular matrix components and effects on cell proliferation and differentiation in the articular cartilage - subchondral bone unit in Frzb^-/- mice. These data further support an important role for FRZB in joint homeostasis and highlight the complex biology of WNT signaling in the joint.

SFRP1 promoter methylation and expression in human trabecular meshwork cells

Glaucoma is a leading cause of blindness worldwide. In primary open angle glaucoma (POAG) patients, impaired trabecular meshwork (TM) function results in elevated intraocular pressure (IOP), which is the primary risk factor of developing optic neuropathy. Our previous studies showed that Wnt signaling pathway components are expressed in the human TM (HTM), and the Wnt inhibitor, secreted frizzled-related protein 1 (SFRP1) is elevated in the glaucomatous TM (GTM). Elevated SFRP1 increased IOP in mice eyes and in perfusion cultured anterior segments of the human eye. However, the cause of elevated SFRP1 in the GTM remains unknown. Promoter methylation plays a key role in regulating SFRP1 expression in certain cancer cells. In light of this, we studied whether promoter methylation is also involved in SFRP1 differential expression in the TM. Two normal TM (NTM) and two GTM cell strains were cultured for an additional 7 days after they were confluent. RNA and genomic DNA (gDNA) were isolated simultaneously to compare SFRP1 expression levels by quantitative PCR (qPCR) and to determine SFRP1 promoter methylation status by bisulfite conversion and methylation-sensitive high resolution melting analysis (MS-HRM). To study whether DNA methylation inhibitors affect SFRP1 expression in TM cells, the four TM cell strains were treated with or without 2 μM 5-aza-2'-deoxycytidine (AZA-dC) for 4 days. RNA was isolated to compare SFRP1 expression by qPCR. In addition, a human cancer cell line, NCI-H460, was used as a positive control. We found that the two GTM cell strains had significantly higher expression levels of SFRP1 than the two NTM cell strains. However, the SFRP1 promoter of all four TM cell strains was unmethylated. In addition, AZA-dC treatment did not affect SFRP1 expression in any of the TM cell strains (n = 3, p > 0.05). In contrast, the hypermethylated SFRP1 promoter of NCI-H460 cells was partially demethylated by the same treatment. AZA-dC treatment also elevated SFRP1 expression by approximately two fold in NCI-H460 cells (n = 3, p < 0.01). Our data suggest that the differential expression of SFRP1 in HTM cells is not due to differential promoter methylation.

The Wnt/Frizzled pathway as a therapeutic target for cardiac hypertrophy: where do we stand?

Cardiac hypertrophy is an enlargement of the heart muscle in response to wall stress. This hypertrophic response often leads to heart failure. In recent years, several studies have shown the involvement of Wnt signalling in hypertrophic growth. In this review, the role of Wnt signalling and the possibilities for therapeutic interventions are discussed. In healthy adult heart tissue, Wnt signalling is very low. However, under pathological condition such as hypertension, Wnt signalling is activated. In recent years, it has become clear that both β-catenin-dependent signalling and β-catenin-independent signalling are involved in hypertrophic growth. Several studies, both in vitro and in vivo, have shown that genetic interventions in Wnt signalling at different levels resulted in an attenuated or diminished hypertrophic response. Therefore, inhibition of Wnt signalling could provide a new therapeutic strategy for cardiac hypertrophy, but further research on the Wnts and Frizzleds involved in the different forms of cardiac hypertrophy will be needed to achieve this goal.

Dual roles for Wnt signalling during the formation of the vertebrate neuromuscular junction

Wnt proteins play prominent roles in different aspects of neuronal development culminating with the formation of complex neuronal circuits. Here, we discuss new studies addressing the function of Wnt signalling at the peripheral neuromuscular junction (NMJ). In both, invertebrate and vertebrate organisms, Wnt signalling promotes and also inhibits the assembly of the neuromuscular synapse. Here, we focus our attention on recent studies at the vertebrate NMJ that demonstrate that some Wnt proteins collaborate with the Agrin-MuSK signalling to induce post-synaptic differentiation. In contrast, Wnts that activate the Wnt/β-catenin signalling inhibit post-synaptic differentiation. The dual function of different Wnts might finely modulate the proper apposition of the pre- and post-synaptic terminals during NMJ formation and growth.

Secreted frizzled-related proteins are required for Wnt/β-catenin signalling activation in the vertebrate optic cup

Secreted frizzled-related proteins (Sfrps) are considered Wnt signalling antagonists but recent studies have shown that specific family members enhance Wnt diffusion and thus positively modulate Wnt signalling. Whether this is a general and physiological property of all Sfrps remains unexplored. It is equally unclear whether disruption of Sfrp expression interferes with developmental events mediated by Wnt signalling activation. Here, we have addressed these questions by investigating the functional consequences of Sfrp disruption in the canonical Wnt signalling-dependent specification of the mouse optic cup periphery. We show that compound genetic inactivation of Sfrp1 and Sfrp2 prevents Wnt/β-catenin signalling activation in this structure, which fails to be specified and acquires neural retina characteristics. Consistent with a positive role of Sfrps in signalling activation, Wnt spreading is impaired in the retina of Sfrp1(-/-);Sfrp2(-/-) mice. Conversely, forced expression of Sfrp1 in the wing imaginal disc of Drosophila, the only species in which the endogenous Wnt distribution can be detected, flattens the Wg gradient, suppresses the expression of high-Wg target genes but expands those typically activated by low Wg concentrations. Collectively, these data demonstrate that, in vivo, the levels of Wnt signalling activation strongly depend on the tissue distribution of Sfrps, which should be viewed as multifunctional regulators of Wnt signalling.

Distinct association between aberrant methylation of Wnt inhibitors and genetic alterations in acute myeloid leukaemia

Background:

Aberrant activation of Wnt signalling through hypermethylation of Wnt inhibitor genes is involved in several human malignancies, including acute myeloid leukaemia (AML). It remains unclear whether hypermethylation of Wnt inhibitors is associated with molecular gene mutations in the development of AML.

Methods:

We investigated the association of the promoter hypermethylation of six Wnt inhibitors (Wif-1, SFRP1, SFRR2, SFRP4, SFRP5, and DKK1) with gene aberrations in the leukaemogenesis of 269 AML patients.

Results:

In total, 166 patients (61.7%) had hypermethylation of at least one Wnt inhibitor. The majority (68.5%) of patients with Wnt inhibitor hypermethylation had concurrent Class II gene mutations that affect transcription factors or cofactors. There was a close association of Wif-1 hypermethylation with t(15;17) (P=0.0005) and CEBPA mutation (P<0.0001), DKK1 hypermethylation with t(8;21) (P<0.0001) and ASXL1 mutation (P=0.0078), SFRP-1 hypermethylation with t(8;21) (P<0.0001), SFRP-2 hypermethylation with AML1/RUNX1 mutation (P=0.0012), and SFRP-5 hypermethylation with MLL/PTD (P=0.0505). On the other side, hypermethylation of Wnt inhibitors was always negatively associated with NPM1 mutation and FLT3/ITD.

Conclusion:

There was distinct association between hypermethylation of individual Wnt inhibitors and specific gene aberrations, especially Class II mutations. The Wnt inhibitor hypermethylation might interact with genetic alterations in the leukaemogenesis.

Wnt signaling has opposing roles in the developing and the adult brain that are modulated by Hipk1

The canonical Wnt/Wingless pathway is implicated in regulating cell proliferation and cell differentiation of neural stem/progenitor cells. Depending on the context, β-Catenin, a key mediator of the Wnt signaling pathway, may regulate either cell proliferation or differentiation. Here, we show that β-Catenin signaling regulates the differentiation of neural stem/progenitor cells in the presence of the β-Catenin interactor Homeodomain interacting protein kinase-1 gene (Hipk1). On one hand, Hipk1 is expressed at low levels during the entire embryonic forebrain development, allowing β-Catenin to foster proliferation and to inhibit differentiation of neural stem/progenitor cells. On the other hand, Hipk1 expression dramatically increases in neural stem/progenitor cells, residing within the subventricular zone (SVZ), at the time when the canonical Wnt signaling induces cell differentiation. Analysis of mouse brains electroporated with Hipk1, and the active form of β-Catenin reveals that coexpression of both genes induces proliferating neural stem/progenitor cells to escape the cell cycle. Moreover, in SVZ derive neurospheres cultures, the overexpression of both genes increases the expression of the cell-cycle inhibitor P16Ink4. Therefore, our data confirm that the β-Catenin signaling plays a dual role in controlling cell proliferation/differentiation in the brain and indicate that Hipk1 is the crucial interactor able to revert the outcome of β-Catenin signaling in neural stem/progenitor cells of adult germinal niches.

Genome-wide detection of gene extinction in early mammalian evolution

Detecting gene losses is a novel aspect of evolutionary genomics that has been made feasible by whole-genome sequencing. However, research to date has concentrated on elucidating evolutionary patterns of genomic components shared between species, rather than identifying disparities between genomes. In this study, we searched for gene losses in the lineage leading to eutherian mammals. First, as a pilot analysis, we selected five gene families (Wnt, Fgf, Tbx, TGFβ, and Frizzled) for molecular phylogenetic analyses, and identified mammalian lineage-specific losses of Wnt11b, Tbx6L/VegT/tbx16, Nodal-related, ADMP1, ADMP2, Sizzled, and Crescent. Second, automated genome-wide phylogenetic screening was implemented based on this pilot analysis. As a result, we detected 147 chicken genes without eutherian orthologs, which resulted from 141 gene loss events. Our inventory contained a group of regulatory genes governing early embryonic axis formation, such as Noggins, and multiple members of the opsin and prolactin-releasing hormone receptor (“PRLHR”) gene families. Our findings highlight the potential of genome-wide gene phylogeny (“phylome”) analysis in detecting possible rearrangement of gene networks and the importance of identifying losses of ancestral genomic components in analyzing the molecular basis underlying phenotypic evolution.

Differential expression of Wnts after spinal cord contusion injury in adult rats

BACKGROUND

Spinal cord injury is a major cause of disability that has no clinically accepted treatment. Functional decline following spinal cord injury is caused by mechanical damage, secondary cell death, reactive gliosis and a poor regenerative capacity of damaged axons. Wnt proteins are a family of secreted glycoproteins that play key roles in different developmental processes although little is known of the expression patterns and functions of Wnts in the adult central nervous system in normal or diseased states.

FINDINGS

Using qRT-PCR analysis, we demonstrate that mRNA encoding most Wnt ligands and soluble inhibitors are constitutively expressed in the healthy adult spinal cord. Strikingly, contusion spinal cord injury induced a time-dependent increase in Wnt mRNA expression from 6 hours until 28 days post-injury, and a narrow peak in the expression of soluble Wnt inhibitors between 1 and 3 days post-injury. These results are consistent with the increase in the migration shift, from day 1 to 7, of the intracellular Wnt signalling component, Dishevelled-3. Moreover, after an initial decrease by 1 day, we also found an increase in phosphorylation of the Wnt co-receptor, low-density lipoprotein receptor-related protein 6, and an increase in active β-catenin protein, both of which suffer a dramatic change, from a homogeneous expression pattern in the grey matter to a disorganized injury-induced pattern.

CONCLUSIONS

Our results suggest a role for Wnts in spinal cord homeostasis and injury. We demonstrate that after injury Wnt signalling is activated via the Wnt/β-catenin and possibly other pathways. These findings provide an important foundation to further address the function of individual Wnt proteins in vivo and the pathophysiology of spinal cord injury.

Secreted-frizzled related protein 1 is a transcriptional repression target of the t(8;21) fusion protein in acute myeloid leukemia

Secreted-frizzled related proteins (SFRPs) are modulators of the Wnt signaling pathway that is closely involved in normal and malignant hematopoiesis. Epigenetic deregulation of Wnt modulators leading to aberrant signaling has been reported in adult patients with acute myeloid leukemia (AML), but its occurrence in childhood patients with AML and the role of individual modulators are unclear. In this study, we examined SFRP1, SFRP2, SFRP4, and SFRP5 promoter methylation in 83 patients with AML (59 children and 24 adults) and found preferential SFRP1 methylation and mRNA down-regulation in the prognostically favorable subgroup of AML with t(8;21) translocation. Among the 4 genes, SFRP1 methylation independently predicted prolonged event-free and relapse-free survivals in childhood patients with nonacute promyelocytic leukemia with nonadverse cytogenetics. Mechanistically, we further demonstrated that RUNX1-ETO, the t(8;21) fusion product, specifically bound the SFRP1 promoter and repressed its transcription via a consensus RUNX binding site. In t(8;21)-leukemia cells, SFRP1 selectively inhibited canonical Wnt signaling and cellular proliferation that were associated with concomitant down-regulation of Wnt/β-catenin target genes, including CCND1 and MYC. Taken together, we identified SFRP1 as a transcriptional repression target of the t(8;21) fusion protein and demonstrated a novel mechanism of Wnt activation in a specific subtype of AML.