Multiplicity of the interactions of Wnt proteins and their receptors

Laminin gamma2 mediates Wnt5a-induced invasion of gastric cancer cells

BACKGROUND & AIMS

Wnt5a expression stimulates in vitro migration and invasion of cultured gastric cancer cells by an unknown mechanism and is also correlated with aggressiveness of gastric tumors. The aim of this study was to show that Wnt5a is involved in metastasis of gastric cancer cells in vivo and to explore the molecular mechanism by which Wnt5a regulates migration and invasion.

METHODS

In an experimental liver metastasis assay, Wnt5a-knockdown gastric cancer cells were injected into the spleens of nude mice. Microarray analyses were used to compare expression patterns between mouse fibroblast L cells that stably express wild-type and a mutant form of Wnt5a to investigate Wnt5a-dependent gene expression. The expression of genes found to be regulated by Wnt5a was investigated in cultured gastric cancer cells. Immunohistochemical analyses were performed to measure levels of Wnt-regulated gene products in 153 gastric cancer samples.

RESULTS

Knockdown of Wnt5a in gastric cancer cells reduced the number of liver metastases that formed in nude mice. Microarray analyses indicated that Wnt5a activity induced expression of the gene encoding laminin gamma2, a subunit of the epithelial basement membrane protein laminin-5. Wnt5a induced the expression of laminin gamma2 through the activation of protein kinase C and c-Jun-N-terminal kinase. The invasive activity of gastric cancer cells depended on laminin gamma2; Wnt5a expression levels correlated with those of laminin gamma2 in diffuse-scattered type gastric tumor samples from patients.

CONCLUSIONS

Wnt5a contributes to gastric cancer progression by increasing metastatic potential. Wnt5a up-regulates laminin gamma2 to mediate gastric cancer cell aggressiveness.

Endocytosis and spatial restriction of cell signaling

Endocytosis and recycling are essential components of the wiring enabling cells to perceive extracellular signals and transduce them in a temporally and spatially controlled fashion, directly influencing not only the duration and intensity of the signaling output, but also their correct location. Here, we will discuss key experimental evidence that support how different internalization routes, the generation of diverse endomembrane platforms, and cycles of internalization and recycling ensure polarized compartmentalization of signals, regulating a number of physiological and pathologically-relevant processes in which the resolution of spatial information is vital for their execution.

Human CD133+ progenitor cells promote the healing of diabetic ischemic ulcers by paracrine stimulation of angiogenesis and activation of Wnt signaling

We evaluated the healing potential of human fetal aorta-derived CD133(+) progenitor cells and their conditioned medium (CD133(+) CCM) in a new model of ischemic diabetic ulcer. Streptozotocin-induced diabetic mice underwent bilateral limb ischemia and wounding. One wound was covered with collagen containing 2x10(4) CD133(+) or CD133(-) cells or vehicle. The contralateral wound, covered with only collagen, served as control. Fetal CD133(+) cells expressed high levels of wingless (Wnt) genes, which were downregulated following differentiation into CD133(-) cells along with upregulation of Wnt antagonists secreted frizzled-related protein (sFRP)-1, -3, and -4. CD133(+) cells accelerated wound closure as compared with CD133(-) or vehicle and promoted angiogenesis through stimulation of endothelial cell proliferation, migration, and survival by paracrine effects. CD133(+) cells secreted high levels of vascular endothelial growth factor (VEGF)-A and interleukin (IL)-8. Consistently, CD133(+) CCM accelerated wound closure and reparative angiogenesis, with this action abrogated by co-administering the Wnt antagonist sFRP-1 or neutralizing antibodies against VEGF-A or IL-8. In vitro, these effects were recapitulated following exposure of high-glucose-primed human umbilical vein endothelial cells to CD133(+) CCM, resulting in stimulation of migration, angiogenesis-like network formation and induction of Wnt expression. The promigratory and proangiogenic effect of CD133(+) CCM was blunted by sFRP-1, as well as antibodies against VEGF-A or IL-8. CD133(+) cells stimulate wound healing by paracrine mechanisms that activate Wnt signaling pathway in recipients. These preclinical findings open new perspectives for the cure of diabetic ulcers.

WNT signaling in lung disease: a failure or a regeneration signal?

The WNT family of signaling proteins is essential to organ development in general and lung morphogenesis in particular. Originally identified as a developmentally active signaling pathway, the WNT pathway has recently been linked to the pathogenesis of important lung diseases, in particular lung cancer and pulmonary fibrosis. This review summarizes our current understanding about WNT signaling in lung development and disease, and is structured into three chapters. The first chapter presents an introduction to WNT signaling, outlining WNT proteins, their receptors and signaling intermediates, as well as the regulation of this complex pathway. The second chapter focuses on the role of WNT signaling in the normal embryonic and adult lung, and highlights recent findings of altered WNT signaling in lung diseases, such as lung cancer, pulmonary fibrosis, or pulmonary arterial hypertension. In the last chapter, we will discuss novel data and ideas about the biological effects of WNT signaling on the cellular level, highlighting pleiotropic effects induced by WNT ligands on distinct cell types, and how these cellular effects may be relevant to the pathogenesis of the aforementioned diseases.

Selective activation mechanisms of Wnt signaling pathways

Wnts comprise a large family of secreted, hydrophobic glycoproteins that control a variety of developmental and adult processes in all metazoan organisms, including cellular proliferation, differentiation, migration and polarity. Wnts have many receptors that are present on a variety of cell types, partly specifying which Wnt pathways are activated. Recently, evidence has been accumulating that specificity of activation downstream of Wnt is also regulated by receptor-mediated endocytosis and the presence of cofactors such as heparan sulfate proteoglycans, in addition to the formation of specific ligand-receptor pairs. Here, we describe how the different endocytic routes of Wnt receptors through caveolin and clathrin determine specificity of Wnt signaling in vertebrates.

Periodic activation of Wnt/beta-catenin signaling enhances somatic cell reprogramming mediated by cell fusion

Reprogramming of nuclei allows the dedifferentiation of differentiated cells. Somatic cells can undergo epigenetic modifications and reprogramming through their fusion with embryonic stem cells (ESCs) or after overexpression of a specific blend of ESC transcription factor-encoding genes. We show here that cyclic activation of Wnt/beta-catenin signaling in ESCs with Wnt3a or the glycogen synthase kinase-3 (GSK-3) inhibitor 6-bromoindirubin-3'-oxime (BIO) strikingly enhances the ability of ESCs to reprogram somatic cells after fusion. In addition, we show that reprogramming is triggered by a dose-dependent accumulation of active beta-catenin. Reprogrammed clones express ESC-specific genes, lose somatic differentiation markers, become demethylated on Oct4 and Nanog CpG islands, and can differentiate into cardiomyocytes in vitro and generate teratomas in vivo. Our data thus demonstrate that in ESCs, periodic beta-catenin accumulation via the Wnt/beta-catenin pathway provides a specific threshold that leads to the reprogramming of somatic cells after fusion.

Pituitary gland and beta-catenin signaling: from ontogeny to oncogenesis

Although pituitary tumors are mostly benign, they share certain molecular events with more malignant neoplasia, although their precise pathogenesis is far from established. The acquisition of new functional characteristics during their evolution suggests a multistep process that leads to tumor transformation. Mutations in classical tumor suppressor genes or oncogenes are infrequently associated with pituitary tumorigenesis. However, alterations in different signaling pathways, especially those involved in pituitary gland development, have emerged as significant features in pituitary adenomas. In particular, changes in inhibitory components of the beta-catenin pathway and its relationship to the cadherin family of peptides may well play an important role in tumorigenesis. We review and assess the role of the beta-catenin signaling pathway in the pathogenesis of pituitary adenomas.

Direct inhibition of GSK3beta by the phosphorylated cytoplasmic domain of LRP6 in Wnt/beta-catenin signaling

Wnt/β-catenin signaling plays a central role in development and is also involved in a diverse array of diseases. Binding of Wnts to the coreceptors Frizzled and LRP6/5 leads to phosphorylation of PPPSPxS motifs in the LRP6/5 intracellular region and the inhibition of GSK3β bound to the scaffold protein Axin. However, it remains unknown how GSK3β is specifically inhibited upon Wnt stimulation. Here, we show that overexpression of the intracellular region of LRP6 containing a Ser/Thr rich cluster and a PPPSPxS motif impairs the activity of GSK3β in cells. Synthetic peptides containing the PPPSPxS motif strongly inhibit GSK3β in vitro only when they are phosphorylated. Microinjection of these peptides into Xenopus embryos confirms that the phosphorylated PPPSPxS motif potentiates Wnt-induced second body axis formation. In addition, we show that the Ser/Thr rich cluster of LRP6 plays an important role in LRP6 binding to GSK3β. These observations demonstrate that phosphorylated LRP6/5 both recruits and directly inhibits GSK3β using two distinct portions of its cytoplasmic sequence, and suggest a novel mechanism of activation in this signaling pathway.

To beta or not to beta: canonical beta-catenin signaling pathway and ovarian development

The mammalian embryonic gonad is a unique organ primordium in that it can adopt two different developmental fates-namely, differentiate as either a testis or an ovary-with dramatic consequences for an individual. While a molecular cascade culminating in testis development is well characterized, the ovarian pathways still remain enigmatic. The canonical Wnt/beta-catenin signaling implements a conserved mechanism of regulating gene expression that is integral to development of all metazoans. In this review, we summarize the recent evidence that suggests a central role for this signaling pathway in the development of the mammalian female.

Activation of canonical Wnt signaling pathway promotes proliferation and self-renewal of rat hepatic oval cell line WB-F344 in vitro

AIM: To investigate the effect of activation of canonical Wnt signaling pathway on the proliferation and differentiation of hepatic oval cells in vitro.

METHODS: WB-F344 cells were treated with recombinant Wnt3a (20, 40, 80, 160, 200 ng/mL) in serum-free medium for 24 h. Cell proliferation was measured by Brdu incorporation analysis; untreated WB-F344 cells were taken as controls. After treatment with Wnt3a (160 ng/mL) for 24 h, subcellular localization and protein expression of β-catenin in WB-F344 cells treated and untreated with Wnt3a were examined by immunofluorescence staining and Western blot analysis. CyclinD1 mRNA expression was determined by semi-quantitative reverse-transcript polymerase chain reaction (RT-PCR). The mRNA levels of some phenotypic markers (AFP, CK-19, ALB) and two hepatic nuclear factors (HNF-4, HNF-6) were measured by RT-PCR. Expressions of CK-19 and AFP protein were detected by Western blot analysis.

RESULTS: Wnt3a promoted proliferation of WB-F344 cells. Stimulation of WB-F344 cells with recombinant Wnt3a resulted in accumulation of the transcriptional activator β-catenin, together with its translocation into the nuclei, and up-regulated typical Wnt target gene CyclinD1. After 3 d of Wnt3a treatment in the absence of serum, WB-F344 cells retained their bipotential to express several specific phenotypic markers of hepatocytes and cholangiocytes, such as AFP and CK-19, following activation of the canonical Wnt signaling pathway.

CONCLUSION: The canonical Wnt signaling pathway promotes proliferation and self-renewal of rat hepatic oval cells.

Expression of Frizzled10 in mouse central nervous system

Frizzled transmembrane proteins (Fzd) are receptors of Wnts, and they play key roles during central nervous system (CNS) development in vertebrates. Here we report the expression pattern of Frizzled10 in mouse CNS from embryonic stages to adulthood. Frizzled10 is expressed strongly at embryonic days E8.5 and E9.5 in the neural tube and tail bud. At E10.5, Frizzled10 is expressed in the forebrain vesicle, the fourth ventricle and the dorsal spinal cord. From E12.5 to E16.5, Frizzled10 expression is mainly observed in the cortical hem/fimbria, the neuroepithelium of the third ventricular zone, midbrain, developing cerebellum, and dorsal spinal cord. At P0, with the exception of expression in the fimbria, Frizzled10 mRNA expression is limited to specific nuclei including the ventral posterior thalamic nucleus (VP) and the dorsal lateral geniculate nucleus (DLG) in the developing thalamus as well as in the proliferative ventricular zone of the developing cerebellum. From P20 to adult, Frizzled10 mRNA is detected only in the internal capsule (ic). Our data show that expression of Frizzled10 is very strong during embryonic development of the CNS and suggest that Frizzled10 may play an essential role in spatial and temporal regulation during neural development.

Ovarian development in mice requires the GATA4-FOG2 transcription complex

We have demonstrated previously that mammalian sexual differentiation requires both the GATA4 and FOG2 transcriptional regulators to assemble the functioning testis. Here we have determined that the sexual development of female mice is profoundly affected by the loss of GATA4-FOG2 interaction. We have also identified the Dkk1 gene, which encodes a secreted inhibitor of canonical β-catenin signaling, as a target of GATA4-FOG2 repression in the developing ovary. The tissue-specific ablation of theβ-catenin gene in the gonads disrupts female development. In Gata4ki/ki; Dkk1-/- or Fog2-/-;Dkk1-/- embryos, the normal ovarian gene expression pattern is partially restored. Control of ovarian development by the GATA4-FOG2 complex presents a novel insight into the cross-talk between transcriptional regulation and extracellular signaling that occurs in ovarian development.

Wnt5a and Wnt11 interact in a maternal Dkk1-regulated fashion to activate both canonical and non-canonical signaling in Xenopus axis formation

Wnt signaling in development and adult tissue homeostasis requires tight regulation to prevent patterning abnormalities and tumor formation. Here, we show that the maternal Wnt antagonist Dkk1 downregulates both the canonical and non-canonical signaling that are required for the correct establishment of the axes of the Xenopus embryo. We find that the target Wnts of Dkk activity are maternal Wnt5a and Wnt11, and that both Wnts are essential for canonical and non-canonical signaling. We determine that Wnt5a and Wnt11 form a previously unrecognized complex. This work suggests a new aspect of Wnt signaling: two Wnts acting in a complex together to regulate embryonic patterning.

Wnts acting through canonical and noncanonical signaling pathways exert opposite effects on hippocampal synapse formation

Background

Wnt proteins comprise a large class of signaling molecules that regulate a variety of developmental processes, including synapse formation. Previous studies have shown Wnts to be involved in both the induction and prevention of synapses in a number of different organisms. However, it is not clear whether the influence of Wnts on synapses is a result of Wnts' behavior in different organisms or differences in the activity of different Wnt ligands.

Results

We used in situ hybridization to show that several Wnt ligands (Wnt3, Wnt5a, Wnt7a, and Wnt7b) and their receptors, Frizzled, are expressed in the developing hippocampus during the period of synapse formation in rodents. We used recombinant Wnt protein or Wnt conditioned media to explore the effects of Wnts on synapses in hippocampal cultures. We found that Wnt7a and Wnt7b activate canonical signaling, whereas Wnt5a activates a noncanonical pathway. The activation of the canonical pathway, either through pathway manipulations or through Wnt stimulation, increases presynaptic inputs. In contrast, exposure to Wnt5a, which activates a noncanonical signaling pathway, decreases the number of presynaptic terminals.

Conclusion

Our observations suggest that the pro- and antisynaptogenic effects of Wnt proteins are associated with the activation of the canonical and noncanonical Wnt signaling pathways.

Molecular specification and patterning of progenitor cells in the lateral and medial ganglionic eminences

We characterized intrinsic and extrinsic specification of progenitors in the lateral and medial ganglionic eminences (LGE and MGE). We identified seven genes whose expression is enriched or restricted in either the LGE [biregional cell adhesion molecule-related/downregulated by oncogenes binding protein (Boc), Frizzled homolog 8 (Fzd8), Ankrd43 (ankyrin repeat domain-containing protein 43), and Ikzf1 (Ikaros family zinc finger 1)] or MGE [Map3k12 binding inhibitory protein 1 (Mbip); zinc-finger, SWIM domain containing 5 (Zswim5); and Adamts5 [a disintegrin-like and metallopeptidase (reprolysin type) with thrombospondin type 1 motif, 5]]. Boc, Fzd8, Mbip, and Zswim5 are apparently expressed in LGE or MGE progenitors, whereas the remaining three are seen in the postmitotic mantle zone. Relative expression levels are altered and regional distinctions are lost for each gene in LGE or MGE cells propagated as neurospheres, indicating that these newly identified molecular characteristics of LGE or MGE progenitors depend on forebrain signals not available in the neurosphere assay. Analyses of Pax6(Sey/Sey), Shh(-/-), and Gli3(XtJ/XtJ) mutants suggests that LGE and MGE progenitor identity does not rely exclusively on previously established forebrain-intrinsic patterning mechanisms. Among a limited number of additional potential patterning mechanisms, we found that extrinsic signals from the frontonasal mesenchyme are essential for Shh- and Fgf8-dependent regulation of LGE and MGE genes. Thus, extrinsic and intrinsic forebrain patterning mechanisms cooperate to establish LGE and MGE progenitor identity, and presumably their capacities to generate distinct classes of neuronal progeny.

CaSR stimulates secretion of Wnt5a from colonic myofibroblasts to stimulate CDX2 and sucrase-isomaltase using Ror2 on intestinal epithelia

To understand whether extracellular calcium-sensing receptor (CaSR) expression on colonic myofibroblast cells (18Co) contributed to epithelial homeostasis, we activated the CaSR with 5 mM Ca(2+), screened by RT-PCR Wnt family members, and measured their secretion. Transcripts for Wnt 1, 2, 2b, 3a, 4, and 7a were either absent or unchanged whereas Wnt3 decreased and Wnt5a increased. We assessed Wnt5a secretion by Western blot. High Ca(2+) (5 mM) substantially increased Wnt5a secretion; small interfering RNA (siRNA) against the CaSR reduced this to constitutive amounts. Expression of Wnt5a plasmid but not Wnt1 or Wnt3a increased caudal homeodomain factor CDX2 transcripts and protein in HT-29 adenocarcinoma cells. Wnt5a increased activity of a sucrase-isomaltase (SI) promoter in Caco-2BBE cells. Wnt5a protein stimulation of CDX2 transcripts and protein and SI reporter were increased by overexpression of wild-type Ror2, a Wnt5a receptor, and reduced with siRNA against Ror2. CaSR activation of HT-29 cells increased Ror2 protein expression. Ror2 protein was expressed in mouse jejunum from crypt base to villus tip and in the colon on surface epithelia. Our results show that activation of a G protein-coupled receptor, the CaSR, stimulates secretion of Wnt5a from myofibroblasts. Stimulation of epithelia by the CaSR increased the expression of a receptor for Wnt5a, the tyrosine kinase Ror2, suggesting existence of a unique paracrine relationship for CDX2 homoeostasis in the intestine and revealing new contributions of CaSR-activated myofibroblasts to intestinal stem cell niche microenvironments.

Wnt and Frizzled RNA expression in human mesenchymal and embryonic (H7) stem cells

Background

Wnt signals are important for embryonic stem cells renewal, growth and differentiation. Although 19 Wnt, 10 Frizzled genes have been identified in mammals, their expression patterns in stem cells were largely unknown.

Results

We conducted RNA expression profiling for the Wnt ligands, their cellular receptors "Frizzleds" and co-receptors LRP5/6 in human embryonic stem cells (H7), human bone marrow mesenchymal cells, as well as mouse totipotent F9 teratocarcinoma embryonal cells. Except failing to express Wnt2 gene, totipotent F9 cells expressed RNA for all other 18 Wnt genes as well as all 10 members of Frizzled gene family. H7 cells expressed RNA for each of the 19 Wnt genes. In contrast, human mesenchymal cells did not display detectable RNA expression of Wnt1, Wnt8a, Wnt8b, Wnt9b, Wnt10a, and Wnt11. Analysis of Frizzled RNAs in H7 and human mesechymal cells revealed expression of 9 members of the receptor gene family, except Frizzled8. Expression of the Frizzled co-receptor LRP5 and LRP6 genes were detected in all three cell lines. Human H7 and mouse F9 cells express nearly a full complement of both Wnts and Frizzleds genes. The human mesenchymal cells, in contrast, have lost the expression of six Wnt ligands, i.e. Wnt1, 8a, 8b, 9b, 10a and 11.

Conclusion

Puripotent human H7 and mouse F9 embryonal cells express the genes for most of the Wnts and Frizzleds. In contrast, multipotent human mesenchymal cells are deficient in expression of Frizzled-8 and of 6 Wnt genes.

Non-canonical Wnt signaling through Wnt5a/b and a novel Wnt11 gene, Wnt11b, regulates cell migration during avian gastrulation

Knowledge of the molecular mechanisms regulating cell ingression, epithelial-mesenchymal transition and migration movements during amniote gastrulation is steadily improving. In the frog and fish embryo, Wnt5 and Wnt11 ligands are expressed around the blastopore and play an important role in regulating cell movements associated with gastrulation. In the chicken embryo, although Wnt5a and Wnt5b are expressed in the primitive streak, the known Wnt11 gene is expressed in paraxial and intermediate mesoderm, and in differentiated myocardial cells, but not in the streak. Here, we identify a previously uncharacterized chicken Wnt11 gene, Wnt11b, that is orthologous to the frog Wnt11 and zebrafish Wnt11 (silberblick) genes. Chicken Wnt11b is expressed in the primitive streak in a pattern similar to chicken Wnt5a and Wnt5b. When non-canonical Wnt signaling is blocked using a Dishevelled dominant-negative protein, gastrulation movements are inhibited and cells accumulate in the primitive streak. Furthermore, disruption of non-canonical Wnt signaling by overexpression of full-length or dominant-negative Wnt11b or Wnt5a constructions abrogates normal cell migration through the primitive streak. We conclude that non-canonical Wnt signaling, mediated in part by Wnt11b, is important for regulation of gastrulation cell movements in the avian embryo.

Functional Wnt signaling is increased in idiopathic pulmonary fibrosis

Background

Idiopathic pulmonary fibrosis (IPF) is a fatal lung disease, characterized by distorted lung architecture and loss of respiratory function. Alveolar epithelial cell injury and hyperplasia, enhanced extracellular matrix deposition, and (myo)fibroblast activation are features of IPF. Wnt/β-catenin signaling has been shown to determine epithelial cell fate during development. As aberrant reactivation of developmental signaling pathways has been suggested to contribute to IPF pathogenesis, we hypothesized that Wnt/β-catenin signaling is activated in epithelial cells in IPF. Thus, we quantified and localized the expression and activity of the Wnt/β-catenin pathway in IPF.

Methodology/Principal Findings

The expression of Wnt1, 3a, 7b, and 10b, the Wnt receptors Fzd1-4, Lrp5-6, as well as the intracellular signal transducers Gsk-3β, β-catenin, Tcf1, 3, 4, and Lef1 was analyzed in IPF and transplant donor lungs by quantitative real-time (q)RT-PCR. Wnt1, 7b and 10b, Fzd2 and 3, β-catenin, and Lef1 expression was significantly increased in IPF. Immunohistochemical analysis localized Wnt1, Wnt3a, β-catenin, and Gsk-3β expression largely to alveolar and bronchial epithelium. This was confirmed by qRT-PCR of primary alveolar epithelial type II (ATII) cells, demonstrating a significant increase of Wnt signaling in ATII cells derived from IPF patients. In addition, Western blot analysis of phospho-Gsk-3β, phospho-Lrp6, and β-catenin, and qRT-PCR of the Wnt target genes cyclin D1, Mmp 7, or Fibronectin 1 demonstrated increased functional Wnt/β-catenin signaling in IPF compared with controls. Functional in vitro studies further revealed that Wnt ligands induced lung epithelial cell proliferation and (myo)fibroblast activation and collagen synthesis.

Conclusions/Significance

Our study demonstrates that the Wnt/β-catenin pathway is expressed and operative in adult lung epithelium. Increased Wnt/β-catenin signaling may be involved in epithelial cell injury and hyperplasia, as well as impaired epithelial-mesenchymal cross-talk in IPF. Thus, modification of Wnt signaling may represent a therapeutic option in IPF.

Intracellular trafficking of LRP9 is dependent on two acidic cluster/dileucine motifs

LDL receptor-related protein 9 (LRP9) is a distant member of the low-density lipoprotein receptor (LDLR) superfamily. To date, there are no reports on the cellular distribution of LRP9 or the signals responsible for its localization. Here, we investigated the intracellular localization and trafficking of LRP9. Using confocal microscopy, we demonstrated that LRP9 was not present at the plasma membrane but co-localized with various markers of the trans-Golgi network (TGN) and endosomes. This co-localization was dependent on the presence of two acidic cluster/dileucine (DXXLL) motifs in the cytoplasmic tail of LRP9, which interact with GGA proteins, clathrin adaptors involved in transport between the TGN and endosomes. LRP9 is the first example of a transmembrane protein with an internal GGA-binding sequence in addition to the usual C-terminal motif. An inactivating mutation (LL --> AA) in both DXXLL motifs, which completely inhibited the interaction of LRP9 with GGA proteins, led to an intracellular redistribution of LRP9 from the TGN to early endosomes and the cell surface, indicating that the two DXXLL motifs are essential sorting determinants of LRP9. In conclusion, our results suggest that LRP9 cycles between the TGN, endosomes and the plasma membrane through a GGA dependent-trafficking mechanism.

Apical accumulation of Rho in the neural plate is important for neural plate cell shape change and neural tube formation

Although Rho-GTPases are well-known regulators of cytoskeletal reorganization, their in vivo distribution and physiological functions have remained elusive. In this study, we found marked apical accumulation of Rho in developing chick embryos undergoing folding of the neural plate during neural tube formation, with similar accumulation of activated myosin II. The timing of accumulation and biochemical activation of both Rho and myosin II was coincident with the dynamics of neural tube formation. Inhibition of Rho disrupted its apical accumulation and led to defects in neural tube formation, with abnormal morphology of the neural plate. Continuous activation of Rho also altered neural tube formation. These results indicate that correct spatiotemporal regulation of Rho is essential for neural tube morphogenesis. Furthermore, we found that a key morphogenetic signaling pathway, the Wnt/PCP pathway, was implicated in the apical accumulation of Rho and regulation of cell shape in the neural plate, suggesting that this signal may be the spatiotemporal regulator of Rho in neural tube formation.

Roles of Wnt signalling in bone growth, remodelling, skeletal disorders and fracture repair

Wnt signalling has an essential role in regulating bone formation and remodelling during embryonic development and throughout postnatal and adult life. Specifically, Wnt signalling regulates bone formation by controlling embryonic cartilage development and postnatal chondrogenesis, osteoblastogenesis, osteoclastogenesis, endochondral bone formation, and bone remodelling. Abnormalities in the function of Wnt genes give rise to or contribute to the development of several pathological bone conditions, including abnormal bone mass, osteosarcomas and bone loss in multiple myeloma. Furthermore, Wnt signalling is activated during bone fracture repair and plays a crucial role in regulating bone regeneration.

Regulation of glycogen synthase kinase 3beta functions by modification of the small ubiquitin-like modifier

Modification of the Small Ubiquitin-like Modifier (SUMO) (SUMOylation) appears to regulate diverse cellular processes, including nuclear transport, signal transduction, apoptosis, autophagy, cell cycle control, ubiquitin-dependent degradation and gene transcription. Glycogen synthase kinase 3beta (GSK 3beta) is a serine/threonine kinase that is thought to contribute to a variety of biological events, including embryonic development, metabolism, tumorigenesis, and cell death. GSK 3beta is a constitutively active kinase that regulates many intracellular signaling pathways by phosphorylating substrates such as beta-catenin. We noticed that the putative SUMOylation sites are localized on K(292 )residueof (291)FKFPQ(295) in GSK 3beta based on analysis of the SUMOylation consensus sequence. In this report, we showed that the SUMOylation of GSK 3beta occurs on its K(292) residue, and this modification promotes its nuclear localization in COS-1. Additionally, our data showed that the GSK 3beta SUMO mutant (K292R) decreased its kinase activity and protein stability, affecting cell death. Therefore, our observations at first time suggested that SUMOylation on the K(292) residue of GSK 3beta might be a GSK 3beta regulation mechanism for its kinase activation, subcellular localization, protein stability, and cell apoptosis.

Non-conventional Frizzled ligands and Wnt receptors

The Wnt family of secreted signaling factors plays numerous roles in embryonic development and in stem cell biology. In the adult, Wnt signaling is involved in tissue homeostasis and mutations that lead to the overexpression of Wnt can be linked to cancer. Wnt signaling is transduced intracellularly by the Frizzled (Fzd) family of receptors. In the canonical pathway, accumulation of beta-catenin and the subsequent formation of a complex with T cell factors (TCF) or lymphoid enhancing factors (Lef) lead to target gene activation. The identification of Ryk as an alternative Wnt receptor and the discovery of the novel Fzd ligands Norrie disease protein (NDP) and R-Spondin, changed the traditional view of Wnts binding to Fzd receptors. Mouse R-Spondin cooperates with Wnt signaling and Low density lipoprotein (LDL) receptor related protein (LRP) to activate beta-catenin dependent gene expression and is involved in processes such as limb and placental development in the mouse. NDP is the product of the Norrie disease gene and controls vascular development in the retina, inner ear and in the female reproductive system during pregnancy. In this review a functional overview of the interactions of the different Wnt and non-Wnt ligands with the Fzd receptors is given as well as a survey of Wnts binding to Ryk and we discuss the biological significance of these interactions.

Wnt signaling pathway and lung disease

The Wnt pathway plays an important role in development and in regulating adult stem cell systems. A variety of cellular processes is mediated by Wnt signaling, which includes cellular proliferation, differentiation, survival, apoptosis, and cell motility. Loss of regulation of these pathways can lead to tumorigenesis, and the Wnt pathway has been implicated in the development of several types of cancers, including colon, lung, leukemia, breast, thyroid, and prostate. The Wnt pathway has also been associated with other lung diseases such as interstitial lung disease (ILD) and asthma. Our increasing understanding of the Wnt pathway offers great hope that new molecular-based screening tests and pharmaceutical agents that selectively target this pathway will be developed to diagnose and treat these diseases in the future.

Palmitoylation and ubiquitination regulate exit of the Wnt signaling protein LRP6 from the endoplasmic reticulum

Canonical Wnt signaling is initiated by binding of Wnt proteins to members of the Frizzled family and subsequent complex formation with lipoprotein receptor-related proteins 5/6 (LRP5/6). Here, we show that LRP6 is palmitoylated on a juxtamembranous cysteine and that palmitoylation is required for exit from the endoplasmic reticulum (ER). We propose that palmitoylation serves to tilt the long, 23-residue transmembrane domain of LRP6 with respect to the plane of membrane to prevent a hydrophobic mismatch and subsequent recognition by the ER quality control. In support of this model, a palmitoylation-deficient LRP6 mutant could be rescued from ER retention by deletion of two to four residues in the transmembrane domain. Importantly, we found that palmitoylation-deficient LRP6 was retained in the ER by a completely novel monoubiquitination-dependent ER retention mechanism. Mutation of a specific lysine indeed abolished ubiquitination of palmitoylation-deficient LRP6 and led to a rescue from ER retention. Finally, at the cell surface, we found that interplay between palmitoylation and ubiquitination was necessary for efficient Wnt signaling.

Tumor formation due to abnormalities in the beta-catenin-independent pathway of Wnt signaling

Wnt signaling is a complex pathway in which beta-catenin is typically viewed as a central mediator in regulating cell proliferation and differentiation. The significance of Wnt signaling in human cancer has been elucidated by the identification of mutations in genes coding for the beta-catenin-dependent pathway components, adenomatous polyposis coli gene product, beta-catenin, and Axin. Within the past 15 years, evidence has been growing of a beta-catenin-independent pathway in Wnt signaling. It is likely that this pathway activates several intracellular signaling systems to regulate cell migration, adhesion, and polarity. The beta-catenin-independent pathway has also been shown to play an important role in tumor biology. In contrast to the beta-catenin-dependent pathway, which is upregulated in many cancers and serves as a tumor promoter, the role of the beta-catenin-independent pathway is still controversial. Here we review recent developments in both the functions and mechanisms of the beta-catenin-independent pathway, with an emphasis on its functional contribution to human tumor progression.

Ror2 modulates the canonical Wnt signaling in lung epithelial cells through cooperation with Fzd2

Background

Wnt signaling is mediated through 1) the beta-catenin dependent canonical pathway and, 2) the beta-catenin independent pathways. Multiple receptors, including Fzds, Lrps, Ror2 and Ryk, are involved in Wnt signaling. Ror2 is a single-span transmembrane receptor-tyrosine kinase (RTK). The functions of Ror2 in mediating the non-canonical Wnt signaling have been well established. The role of Ror2 in canonical Wnt signaling is not fully understood.

Results

Here we report that Ror2 also positively modulates Wnt3a-activated canonical signaling in a lung carcinoma, H441 cell line. This activity of Ror2 is dependent on cooperative interactions with Fzd2 but not Fzd7. In addition, Ror2-mediated enhancement of canonical signaling requires the extracellular CRD, but not the intracellular PRD domain of Ror2. We further provide evidence that the positive effect of Ror2 on canonical Wnt signaling is inhibited by Dkk1 and Krm1 suggesting that Ror2 enhances an Lrp-dependent STF response.

Conclusion

The current study demonstrates the function of Ror2 in modulating canonical Wnt signaling. These findings support a functional scheme whereby regulation of Wnt signaling is achieved by cooperative functions of multiple mediators.

Wnt-frizzled interactions in Xenopus

The Wnt signaling cascades are regulatory modules which are involved in embryonic patterning, cell differentiation, morphogenesis, and diseases. The Wnt pathways are activated when secreted Wnt ligands interact with 7-trans-membrane receptors of the Frizzled (Fz) family. Specific readouts are determined by the ligand/receptor combinations and the cellular context. Here we describe two methods for the analysis of Wnt/Frizzled interactions in Xenopus embryos. Physical interaction of ligand and receptor are demonstrated by co-immunoprecipitation assays. The activation of Wnt targets in Xenopus animal cap tissue provides a versatile test system for activating and inhibitory components of the Wnt/beta -catenin pathway.

Expression of Wnt4 in human pituitary adenomas regulates activation of the beta-catenin-independent pathway

The Wnt signaling pathway has been implicated in the genesis of numerous human cancers. A member of the Wnt family of genes, Wnt4, has been known to regulate proliferation of anterior pituitary cell types in the mouse during embryonic development. In order to elucidate the roles of Wnt signaling in human pituitary adenomas, we examined the expression of Wnt4 and its putative receptor Frizzled6 (Fzd6) by immunohistochemistry in pituitary adenomas and normal pituitaries. Expression of Wnt4 was higher in growth hormone-producing adenomas (GHomas), prolactin-producing adenomas (PRLomas), and thyroid-stimulating hormone-producing adenomas (TSHomas) than in the normal pituitary. Fzd6 was widely expressed in GHomas, PRLomas, TSHomas, and gonadotropin subunit (GnSU)-positive adenomas. In normal pituitary glands, Wnt4 and Fzd6 were colocalized predominantly in follicle-stimulating hormone-, luteinizing hormone-, and alpha-subunits of glycoprotein hormone-positive cells. The canonical Wnt/beta-catenin signaling pathway was analyzed by beta-catenin immunohistochemistry. beta-Catenin was localized at the cell membrane in all pituitary adenomas, but not in the nuclei. On the other hand, Erk1/2 was highly activated in GHomas and TSHomas. These results suggested that activation of Wnt4/Fzd6 signaling through a "beta-catenin-independent" pathway played a role in proliferation and survival of the pituitary adenoma cells. Detailed involvement of transcription factors including Pit-1 remains to be further investigated.

Understanding signaling cascades in melanoma

Understanding regulatory pathways involved in melanoma development and progression has advanced significantly in recent years. It is now appreciated that melanoma is the result of complex changes in multiple signaling pathways that affect growth control, metabolism, motility and the ability to escape cell death programs. Here we review the major signaling pathways currently known to be deregulated in melanoma with an implication to its development and progression. Among these pathways are Ras, B-Raf, MEK, PTEN, phosphatidylinositol-3 kinase (PI3Ks) and Akt which are constitutively activated in a significant number of melanoma tumors, in most cases due to genomic change. Other pathways discussed in this review include the [Janus kinase/signal transducer and activator of transcription (JAK/STAT), transforming growth factor-beta pathways which are also activated in melanoma, although the underlying mechanism is not yet clear. As a paradigm for remodeled signaling pathways, melanoma also offers a unique opportunity for targeted drug development.

The functions and possible significance of Kremen as the gatekeeper of Wnt signalling in development and pathology

Kremen (Krm) was originally discovered as a novel transmembrane protein containing the kringle domain. Both Krm1 (the first identified Krm) and its relative Krm2 were later identified to be the high-affinity receptors for Dickkopf (Dkk), the inhibitor of Wnt/β-catenin signalling. The formation of a ternary complex composed of Krm, Dkk, and Lrp5/6 (the coreceptor of Wnt) inhibits Wnt/β-catenin signalling. In Xenopus gastrula embryos, Wnt/β-catenin signalling regulates anterior-posterior patterning, with low-signalling in anterior regions. Inhibition of Krm1/2 induces embryonic head defects. Together with anterior localization of Krms and Dkks, the inhibition of Wnt signalling by Dkk-Krm action seems to allow anterior embryonic development. During mammalian development, krm1 mRNA expression is low in the early stages, but gradually and continuously increases with developmental progression and differentiation. In contrast with the wide, strong expression of krm1 mRNA in mature tissues, expression of krm1 is diminished in a variety of human tumor cells. Since stem cells and undifferentiated cells rely on Wnt/β-catenin signalling for maintenance in a low differentiation state, the physiological shutdown of Wnt/β-catenin signalling by Dkk-Krm is likely to set cells on a divergent path toward differentiation. In tumour cells, a deficit of Krm may increase the susceptibility to tumourigenic transformation. Both positive and negative regulation of Wnt/β-catenin signalling definitively contributes to diverse developmental and physiological processes, including cell-fate determination, tissue patterning and stem cell regulation. Krm is quite significant in these processes as the gatekeeper of the Wnt/β-catenin signalling pathway.

A soluble and active form of Wnt-3a protein is involved in myogenic differentiation after cholesterol depletion

Cholesterol is one of the major lipids of plasma membranes. Recently, we have shown that cholesterol depletion by methyl-beta-cyclodextrin (M beta CD) induces the activation of the Wnt/beta-catenin pathway and enhances myogenic differentiation. Here, we show that M beta CD-conditioned media accelerates myogenesis in a similar way as M beta CD does, suggesting that the effects induced by M beta CD could be caused by soluble factors present in the culture medium. Soluble Wnt-3 protein is significantly enhanced in M beta CD-conditioned medium. Wnt-3a-enriched media induces myogenesis as much as M beta CD does, whereas Wnt-5a-enriched media inhibits. We suggest that Wnt-3a is involved in the myogenic induction observed after cholesterol depletion.

Differential mediation of the Wnt canonical pathway by mammalian Dishevelleds-1, -2, and -3

In the Drosophila, a single copy of the phosphoprotein Dishevelled (Dsh) is found. In the genomes of higher organism (including mammals), three genes encoding isoforms of Dishevelled (Dvl1, Dvl2, and Dvl3) are present. In the fly, Dsh functions in the Wnt-sensitive stabilization of intracellular beta-catenin and activation of the Lef/Tcf-sensitive transcriptional response known as the Wnt "canonical" pathway. In the current work we explore the expression of Dishevelleds in mammalian cells and provide an estimate of the relative cellular abundance of each Dvl. In mouse F9 cells, all three Dvls are expressed. Dvl2 constitutes more than 95% of the total pool, the sum of Dvl1 and Dvl3 constituting the remainder. Similarly, Dvl2 constitutes more than 80% of the Dvl1-3 pool in mouse P19 and human HEK 293 cells. siRNA-induced knock-down of individual Dvls was performed using Wnt3a-sensitive canonical pathway in F9 cells as the read-out. Activation of the canonical signaling pathway by Wnt3a was dependent upon the presence of Dvl1, Dvl2, and Dvl3, but to a variable extent. Wnt3a-sensitive canonical transcription was suppressible, by knock-down of Dvl1, Dvl2, or Dvl3. Conversely, the overexpression of any one of the three Dvls individually was found to be capable of promoting Lef/Tcf-sensitive transcriptional activation, in the absence of Wnt3a, i.e., overexpression of Dvl1, Dvl2, or Dvl3 is Wnt3a-mimetic. Graded suppression of individual Dvl isoforms by siRNA was employed to test if the three Dvls could be distinguished from one another with regard to mediation of the canonical pathway. Canonical signaling was most sensitive to changes in the abundance of either Dvl3 or Dvl1. Changes in expression of Dvl2, the most abundant of the three isoforms, resulted in the least effect on canonical signaling. Dvl-based complexes were isolated by pull-downs from whole-cell extracts with isoform-specific antibodies and found to include all three Dvl isoforms. Rescue experiments were conducted in which depletion of either Dvl3 or Dvl1 suppresses Wnt3a activation of the canonical pathway and the ability of a Dvl isoform to rescue the response evaluated. Rescue of Wnt3a-stimulated transcriptional activation in these siRNA-treated cells occurred only by the expression of the very same Dvl isoform depleted by the siRNA. Thus, Dvls appear to function cooperatively as well as uniquely with respect to mediation of Wnt3a-stimulated canonical signaling. The least abundant (Dvl1, 3) plays the most obvious role, whereas the most abundant (Dvl2) plays the least obvious role, suggesting that individual Dvl isoforms in mammals may operate as a network with some features in common and others rather unique.

The Wnt signal transduction pathway in stem cells and cancer cells: influence on cellular invasion

The regulative network conducting adult stem cells in endogenous tissue repair is of prime interest for understanding organ regeneration as well as preventing degenerative and malignant diseases. One major signal transduction pathway which is involved in the control of these (patho)physiological processes is the Wnt pathway. Recent results obtained in our laboratories showed for the first time that canonical Wnt signaling is critically involved in the control of the migration/invasion behaviour of human mesenchymal stem cells (hMSC). In the first part of this review, we describe that the regenerative state is closely linked to the activation of the Wnt pathway. Central hallmarks of activated stem cells are recapitulated in a similar way also in cancer metastasis, where the acquisition of an invasive cancer stem cell phenotype is associated with the induction of Wnt-mediated epithelial to mesenchymal transition (EMT). In the second part, the influence of proinflammatory cytokines such as transforming growth factor (TGF-)beta1, interleukin (Il-)1beta, and tumor necrosis factor (TNF-)alpha is discussed with regard to the invasive characteristics of hMSC. In this context, special attention has been paid on the role of matrix metalloproteinases (MMPs), such as MMP-2, MMP-9 and membrane type 1 (MT1)-MMP, as well as on the tissue inhibitors of metalloproteinases TIMP-1 and TIMP-2. Putative cross-talks between different signal transduction pathways that may amplify the invasive capacity of this stem cell population are also discussed. Finally, the consequences towards future drug-mediated therapeutical modifications of Wnt signaling in stem cells and tumor cells are highlighted.

Gene profiling of pooled single neuronal cell bodies from laser capture microdissected vervet monkey lateral geniculate nucleus hybridized to the Rhesus Macaque Genome Array

This report is based on an ongoing study to examine gene expression differences in monkey lateral geniculate nucleus (LGN). Here, samples from an Old World species, the vervet monkey (Cercopithecus aethiops), were cross-hybridized to the Rhesus Macaque Genome Array (Affymetrix). Microarray analysis was performed using laser capture microdissected populations of individual neuronal cell bodies isolated from the LGN compared to heterogeneous samples from whole lamina. Our results indicated that cross-species hybridization of microdissected brain tissue samples from vervet monkeys to the Rhesus array produced reliable and biologically relevant data sets. We present the first list of genes enriched in the large neuronal cell bodies of the LGN. We found that these cell bodies are concentrated with genes involved in metabolic processes and protein synthesis, whereas signaling molecules including chemokines and integrins were expressed at higher levels within heterogeneous samples. Our data set also provides support for a contribution of Wnt signaling in adult monkey LGN.

The Frizzled family of unconventional G-protein-coupled receptors

The Frizzled (FZD) family of receptors is critically involved in embryogenesis, and there is substantial evidence that members of this family also regulate tissue homeostasis in many different organs in the adult. FZD receptors have seven transmembrane-spanning domains and are activated by the WNT family of lipoglycoproteins. Many aspects of FZD signal transduction and pharmacology are still unclear. In this review, we summarize recent advances and some of the key questions about the molecular pharmacology of FZDs, FZD-associated proteins and signal transduction. We also discuss what little is known about the pharmacological binding profiles and the degree of selectivity of WNTs and other extracellular ligands for FZDs. Finally, we focus on signaling events that occur as a direct consequence of FZD activation, signaling via the central phosphoprotein Dishevelled (DVL) and FZD coupling to heterotrimeric G proteins. Here, we outline the current state of knowledge on FZDs and FZD signal transduction and pinpoint aspects of debate and future directions.

Wnt signaling and neural stem cells: caught in the Wnt web

Wnt proteins have now been identified as major physiological regulators of multiple aspects of stem cell biology, from self-renewal and pluripotency to precursor cell competence and terminal differentiation. Neural stem cells are the cellular building blocks of the developing nervous system and provide the basis for continued neurogenesis in the adult mammalian central nervous system. Here, we outline the most recent advances in the field about the critical factors and regulatory networks involved in Wnt signaling and discuss recent findings on how this increasingly intricate pathway contributes to the shaping of the developing and adult nervous system on the level of the neural stem cell.

Analysis of endogenous LRP6 function reveals a novel feedback mechanism by which Wnt negatively regulates its receptor

The canonical Wnt pathway plays a crucial role in embryonic development, and its deregulation is involved in human diseases. The LRP6 single-span transmembrane coreceptor is essential for transmission of canonical Wnt signaling. However, due to the lack of immunological reagents, our understanding of LRP6 structure and function has relied on studies involving its overexpression, and regulation of the endogenous receptor by the Wnt ligand has remained unexplored. Using a highly sensitive and specific antibody to LRP6, we demonstrate that the endogenous receptor is modified by N-glycosylation and is phosphorylated in response to Wnt stimulation in a sustained yet ligand-dependent manner. Moreover, following triggering by Wnt, endogenous LRP6 is internalized and recycled back to the cellular membrane within hours of the initial stimulus. Finally, we have identified a novel feedback mechanism by which Wnt, acting through beta-catenin, negatively regulates LRP6 at the mRNA level. Together, these findings contribute significantly to our understanding of LRP6 function and uncover a new level of regulation of Wnt signaling. In light of the direct role that the Wnt pathway plays in human bone diseases and malignancies, our findings may support the development of novel therapeutic approaches that target Wnt signaling through LRP6.