β-Catenin activation regulates tissue growth non-cell autonomously in the hair stem cell niche

Wnt/β-catenin signaling is critical for tissue regeneration. However, it is unclear how β-catenin controls stem cell behaviors to coordinate organized growth. Using live imaging, we show that activation of β-catenin specifically within mouse hair follicle stem cells generates new hair growth through oriented cell divisions and cellular displacement. β-Catenin activation is sufficient to induce hair growth independently of mesenchymal dermal papilla niche signals normally required for hair regeneration. Wild-type cells are co-opted into new hair growths by β-catenin mutant cells, which non-cell autonomously activate Wnt signaling within the neighboring wild-type cells via Wnt ligands. This study demonstrates a mechanism by which Wnt/β-catenin signaling controls stem cell-dependent tissue growth non-cell autonomously and advances our understanding of the mechanisms that drive coordinated regeneration.

KLF5 regulates the integrity and oncogenicity of intestinal stem cells

The intestinal epithelium maintains homeostasis by a self-renewal process involving resident stem cells, including Lgr5(+) crypt-base columnar cells, but core mechanisms and their contributions to intestinal cancer are not fully defined. In this study, we examined a hypothesized role for KLF5, a zinc-finger transcription factor that is critical to maintain the integrity of embryonic and induced pluripotent stem cells, in intestinal stem-cell integrity and cancer in the mouse. Klf5 was indispensable for the integrity and oncogenic transformation of intestinal stem cells. In mice, inducible deletion of Klf5 in Lgr5(+) stem cells suppressed their proliferation and survival in a manner associated with nuclear localization of β-catenin (Catnb), generating abnormal apoptotic cells in intestinal crypts. Moreover, production of lethal adenomas and carcinomas by specific expression of an oncogenic mutant of β-catenin in Lgr5(+) stem cells was suppressed completely by Klf5 deletion in the same cells. Given that activation of the Wnt/β-catenin pathway is the most frequently altered pathway in human colorectal cancer, our results argue that KLF5 acts as a fundamental core regulator of intestinal oncogenesis at the stem-cell level, and they suggest KLF5 targeting as a rational strategy to eradicate stem-like cells in colorectal cancer.

β-Catenin activation contributes to the pathogenesis of adenomyosis through epithelial-mesenchymal transition

Adenomyosis is defined by the presence of endometrial glands and stroma within the myometrium. Despite its frequent occurrence, the precise aetiology and physiopathology of adenomyosis is still unknown. WNT/β-catenin signalling molecules are important and should be tightly regulated for uterine function. To investigate the role of β-catenin signalling in adenomyosis, the expression of β-catenin was examined. Nuclear and cytoplasmic β-catenin expression was significantly higher in epithelial cells of human adenomyosis compared to control endometrium. To determine whether constitutive activation of β-catenin in the murine uterus leads to development of adenomyosis, mice that expressed a dominant stabilized β-catenin in the uterus were used by crossing PR-Cre mice with Ctnnb1(f(ex3)/+) mice. Uteri of PR(cre) (/+) Ctnnb1(f(ex3)/+) mice displayed an abnormal irregular structure and highly active proliferation in the myometrium, and subsequently developed adenomyosis. Interestingly, the expression of E-cadherin was repressed in epithelial cells of PR(cre) (/+) Ctnnb1(f(ex3)/+) mice compared to control mice. Repression of E-cadherin is one of the hallmarks of epithelial-mesenchymal transition (EMT). The expression of SNAIL and ZEB1 was observed in some epithelial cells of the uterus in PR(cre) (/+) Ctnnb1(f(ex3)/+) mice but not in control mice. Vimentin and COUP-TFII, mesenchymal cell markers, were expressed in some epithelial cells of PR(cre) (/+) Ctnnb1(f(ex3)/+) mice. In human adenomyosis, the expression of E-cadherin was decreased in epithelial cells compared to control endometrium, while CD10, an endometrial stromal marker, was expressed in some epithelial cells of human adenomyosis. These results suggest that abnormal activation of β-catenin contributes to adenomyosis development through the induction of EMT.

CTNNB1 in mesenchyme regulates epithelial cell differentiation during Müllerian duct and postnatal uterine development

Müllerian duct differentiation and development into the female reproductive tract is essential for fertility, but mechanisms regulating these processes are poorly understood. WNT signaling is critical for proper development of the female reproductive tract as evident by the phenotypes of Wnt4, Wnt5a, Wnt7a, and β-catenin (Ctnnb1) mutant mice. Here we extend these findings by determining the effects of constitutive CTNNB1 activation within the mesenchyme of the developing Müllerian duct and its differentiated derivatives. This was accomplished by crossing Amhr2-Cre knock-in mice with Ctnnb1 exon (ex) 3(f/f) mice. Amhr2-Cre(Δ/+); Ctnnb1 ex3(f/+) females did not form an oviduct, had smaller uteri, endometrial gland defects, and were infertile. At the cellular level, stabilization of CTNNB1 in the mesenchyme caused alterations within the epithelium, including less proliferation, delayed uterine gland formation, and induction of an epithelial-mesenchymal transition (EMT) event. This EMT event is observed before birth and is complete within 5 days after birth. Misexpression of estrogen receptor α in the epithelia correlated with the EMT before birth, but not after. These studies indicate that regulated CTNNB1 in mesenchyme is important for epithelial cell differentiation during female reproductive tract development.

Wnt4/β-catenin signaling in medullary kidney myofibroblasts

Injury to the adult kidney induces a number of developmental genes thought to regulate repair, including Wnt4. During kidney development, early nephron precursors and medullary stroma both express Wnt4, where it regulates epithelialization and controls smooth muscle fate, respectively. Expression patterns and roles for Wnt4 in the adult kidney, however, remain unclear. In this study, we used reporters, lineage analysis, and conditional knockout or activation of the Wnt/β-catenin pathway to investigate Wnt4 in the adult kidney. Proliferating, medullary, interstitial myofibroblasts strongly expressed Wnt4 during renal fibrosis, whereas tubule epithelia, except for the collecting duct, did not. Exogenous Wnt4 drove myofibroblast differentiation of a pericyte-like cell line, suggesting that Wnt4 might regulate pericyte-to-myofibroblast transition through autocrine signaling. However, conditional deletion of Wnt4 in interstitial cells did not reduce myofibroblast proliferation, cell number, or myofibroblast gene expression during fibrosis. Because the injured kidney expresses multiple Wnt ligands that might compensate for the absence of Wnt4, we generated a mouse model with constitutive activation of canonical Wnt/β-catenin signaling in interstitial pericytes and fibroblasts. Kidneys from these mice exhibited spontaneous myofibroblast differentiation in the absence of injury. Taken together, Wnt4 expression in renal fibrosis defines a population of proliferating medullary myofibroblasts. Although Wnt4 may be dispensable for myofibroblast transformation, canonical Wnt signaling through β-catenin stabilization is sufficient to drive spontaneous myofibroblast differentiation in interstitial pericytes and fibroblasts, emphasizing the importance of this pathway in renal fibrosis.

Canonical Wnt signaling induces BMP-4 to specify slow myofibrogenesis of fetal myoblasts

Background

The Wnts are secreted proteins that play important roles in skeletal myogenesis, muscle fiber type diversification, neuromuscular junction formation and muscle stem cell function. How Wnt proteins orchestrate such diverse activities remains poorly understood. Canonical Wnt signaling stabilizes β-catenin, which subsequently translocate to the nucleus to activate the transcription of TCF/LEF family genes.

Methods

We employed TCF-reporter mice and performed analysis of embryos and of muscle groups. We further isolated fetal myoblasts and performed cell and molecular analyses.

Results

We found that canonical Wnt signaling is strongly activated during fetal myogenesis and weakly activated in adult muscles limited to the slow myofibers. Muscle-specific transgenic expression of a stabilized β-catenin protein led to increased oxidative myofibers and reduced muscle mass, suggesting that canonical Wnt signaling promotes slow fiber types and inhibits myogenesis. By TCF-luciferase reporter assay, we identified Wnt-1 and Wnt-3a as potent activators of canonical Wnt signaling in myogenic progenitors. Consistent with in vivo data, constitutive overexpression of Wnt-1 or Wnt-3a inhibited the proliferation of both C2C12 and primary myoblasts. Surprisingly, Wnt-1 and Wnt-3a overexpression up-regulated BMP-4, and inhibition of BMP-4 by shRNA or recombinant Noggin protein rescued the myogenic inhibitory effect of Wnt-1 and Wnt-3a. Importantly, Wnt-3a or BMP-4 recombinant proteins promoted slow myosin heavy chain expression during myogenic differentiation of fetal myoblasts.

Conclusions

These results demonstrate a novel interaction between canonical Wnt and BMP signaling that induces myogenic differentiation towards slow muscle phenotype.

Beta-catenin (CTNNB1) induces Bmp expression in urogenital sinus epithelium and participates in prostatic bud initiation and patterning

Fetal prostate development is initiated by androgens and patterned by androgen dependent and independent signals. How these signals integrate to control epithelial cell differentiation and prostatic bud patterning is not fully understood. To test the role of beta-catenin (Ctnnb1) in this process, we used a genetic approach to conditionally delete or stabilize Ctnnb1 in urogenital sinus (UGS) epithelium from which the prostate derives. Two opposing mechanisms of action were revealed. By deleting Ctnnb1, we found it is required for separation of UGS from cloaca, emergence or maintenance of differentiated UGS basal epithelium and formation of prostatic buds. By genetically inducing a patchy subset of UGS epithelial cells to express excess CTNNB1, we found its excess abundance increases Bmp expression and leads to a global impairment of prostatic bud formation. Addition of NOGGIN partially restores prostatic budding in UGS explants with excess Ctnnb1. These results indicate a requirement for Ctnnb1 in UGS basal epithelial cell differentiation, prostatic bud initiation and bud spacing and suggest some of these actions are mediated in part through activation of BMP signaling.

Intestinal tumorigenesis initiated by dedifferentiation and acquisition of stem-cell-like properties

Cell-type plasticity within a tumor has recently been suggested to cause a bidirectional conversion between tumor-initiating stem cells and nonstem cells triggered by an inflammatory stroma. NF-κB represents a key transcription factor within the inflammatory tumor microenvironment. However, NF-κB's function in tumor-initiating cells has not been examined yet. Using a genetic model of intestinal epithelial cell (IEC)-restricted constitutive Wnt-activation, which comprises the most common event in the initiation of colon cancer, we demonstrate that NF-κB modulates Wnt signaling and show that IEC-specific ablation of RelA/p65 retards crypt stem cell expansion. In contrast, elevated NF-κB signaling enhances Wnt activation and induces dedifferentiation of nonstem cells that acquire tumor-initiating capacity. Thus, our data support the concept of bidirectional conversion and highlight the importance of inflammatory signaling for dedifferentiation and generation of tumor-initiating cells in vivo.

Dclk1 distinguishes between tumor and normal stem cells in the intestine

There is great interest in tumor stem cells (TSCs) as potential therapeutic targets; however, cancer therapies targeting TSCs are limited. A drawback is that TSC markers are often shared by normal stem cells (NSCs); thus, therapies that target these markers may cause severe injury to normal tissues. To identify a potential TSC-specific marker, we focused on doublecortin-like kinase 1 (Dclk1). Dclk1 was reported as a candidate NSC marker in the gut, but recent reports have implicated it as a marker of differentiated cells (for example, Tuft cells). Using lineage-tracing experiments, we show here that Dclk1 does not mark NSCs in the intestine but instead marks TSCs that continuously produce tumor progeny in the polyps of Apc(Min/+) mice. Specific ablation of Dclk1-positive TSCs resulted in a marked regression of polyps without apparent damage to the normal intestine. Our data suggest the potential for developing a therapy for colorectal cancer based on targeting Dclk1-positive TSCs.

Dclk1 distinguishes between tumor and normal stem cells in the intestine

There is great interest in tumor stem cells (TSCs) as potential therapeutic targets; however, cancer therapies targeting TSCs are limited. A drawback is that TSC markers are often shared by normal stem cells (NSCs); thus, therapies that target these markers may cause severe injury to normal tissues. To identify a potential TSC-specific marker, we focused on doublecortin-like kinase 1 (Dclk1). Dclk1 was reported as a candidate NSC marker in the gut, but recent reports have implicated it as a marker of differentiated cells (for example, Tuft cells). Using lineage-tracing experiments, we show here that Dclk1 does not mark NSCs in the intestine but instead marks TSCs that continuously produce tumor progeny in the polyps of Apc(Min/+) mice. Specific ablation of Dclk1-positive TSCs resulted in a marked regression of polyps without apparent damage to the normal intestine. Our data suggest the potential for developing a therapy for colorectal cancer based on targeting Dclk1-positive TSCs.

Excessive Wnt/β-catenin signaling disturbs tooth-root formation

BACKGROUND AND OBJECTIVE

Wingless-type MMTV integration site family (Wnt)/β-catenin signaling plays an essential role in cellular differentiation and matrix formation during skeletal development. However, little is known about its role in tooth-root formation. In a previous study, we found excessive formation of dentin and cementum in mice with constitutive β-catenin stabilization in the dental mesenchyme. In the present study we analyzed the molar roots of these mice to investigate the role of Wnt/β-catenin signaling in root formation in more detail.

MATERIAL AND METHODS

We generated OC-Cre:Catnb(+/lox(ex3)) mice by intercrossing Catnb(+/lox(ex3)) and OC-Cre mice, and we analyzed their mandibular molars using radiography, histomorphometry and immunohistochemistry.

RESULTS

OC-Cre:Catnb(+/lox(ex3)) mice showed impaired root formation. At the beginning of root formation in mutant molars, dental papilla cells did not show normal differentiation into odontoblasts; rather, they were prematurely differentiated and had a disorganized arrangement. Interestingly, SMAD family member 4 was upregulated in premature odontoblasts. In 4-wk-old mutant mice, molar roots were about half the length of those in their wild-type littermates. In contrast to excessively formed dentin in crown, root dentin was thin and hypomineralized in mutant mice. Biglycan and dentin sialophosphoprotein were downregulated in root dentin of mutant mice, whereas dentin matrix protein 1 and Dickkopf-related protein 1 were upregulated. Additionally, ectonucleotide pyrophosphatase/phosphodiesterase 1 was significantly downregulated in the cementoblasts of mutant molars. Finally, in the cementum of mutant mice, bone sialoprotein was downregulated but Dickkopf-related protein 2 was upregulated.

CONCLUSION

These results suggest that temporospatial regulation of Wnt/β-catenin signaling plays an important role in cell differentiation and matrix formation during root and cementum formation.

Col1a1-cre mediated activation of β-catenin leads to aberrant dento-alveolar complex formation

Wnt/β-catenin signaling plays a critical role in bone formation and regeneration. Dentin and cementum share many similarities with bone in their biochemical compositions and biomechanical properties. Whether Wnt/β-catenin signaling is involved in the dento-alveolar complex formation is unknown. To understand the roles of Wnt/β-catenin signaling in the dento-alveolar complex formation, we generated conditional β-catenin activation mice through intercross of Catnb^+/lox(ex3) mice with Col1a1-cre mice. In mutant mice, tooth formation and eruption was disturbed. Lower incisors and molars did not erupt. Bone formation was increased in the mandible but tooth formation was severely disturbed. Hypomineralized dentin was deposited in the crown but roots of molars were extremely short and distorted. In the odontoblasts of mutant molars, expression of dentin matrix proteins was obviously downregulated following the activation of β-catenin whereas that of mineralization inhibitor was increased. Cementum and periodontal ligament were hypoplastic but periodontal space was narrow due to increased alveolar bone formation. While cementum matrix proteins were decreased, bone matrix proteins were increased in the cementum and alveolar bone of mutant mice. These results indicate that local activation of β-catenin in the osteoblasts and odontoblasts leads to aberrant dento-alveolar complex formation. Therefore, appropriate inhibition of Wnt/β-catenin signaling is important for the dento-alveolar complex formation.

The Müllerian inhibiting substance type 2 receptor suppresses tumorigenesis in testes with sustained β-catenin signaling

Dysregulated WNT/β-catenin signaling in murine testes results in a phenotype with complete germ cell loss that resembles human Sertoli cell-only syndrome. In other systems, including the ovary, dysregulated WNT/β-catenin induces tumorigenesis but no tumors are observed in the mutant testes without deletion of a tumor suppressor, such as phosphatase and tensin homolog (PTEN). Müllerian inhibiting substance (MIS, also known as AMH), a member of the transforming growth factor-β family of growth factors responsible for Müllerian duct regression in fetal males, has been shown to inhibit tumor growth in vitro and in vivo but its role as an endogenous tumor suppressor has never been reported. We have deleted the MIS type 2 receptor (MISR2), and thus MIS signaling, in mice with dysregulated WNT/β-catenin and show that these mice develop testicular stromal tumors with 100% penetrance within a few months postnatal. The tumors are highly proliferative and have characteristics of either Sertoli cell tumors or progenitor Leydig cell tumors based on their marker profiles and histology. Phosphorylated Sma and mothers against decapentaplegic-related homolog 1/5/8 is absent in the tumors and β-catenin target genes are induced. The tumor suppressor TP53 is also highly expressed in the tumors, as is phosphorylated γH2AX, which is indicative of DNA damage. The phenotype of these tumors closely resembles those observed when PTEN is also deleted in mice with dysregulated WNT/β-catenin. Tumorigenesis in these mice provides conclusive evidence that physiological MIS signaling is a tumor suppressor mechanism and suggests that targeted treatment of MISR2-expressing cancers with therapeutic MIS should have a beneficial effect on tumor progression.

Endothelial Wnt/β-catenin signaling inhibits glioma angiogenesis and normalizes tumor blood vessels by inducing PDGF-B expression

Wnt modulates glioma vascularization by regulating PDGF-B expression.

Endothelial Wnt/β-catenin signaling is necessary for angiogenesis of the central nervous system and blood–brain barrier (BBB) differentiation, but its relevance for glioma vascularization is unknown. In this study, we show that doxycycline-dependent Wnt1 expression in subcutaneous and intracranial mouse glioma models induced endothelial Wnt/β-catenin signaling and led to diminished tumor growth, reduced vascular density, and normalized vessels with increased mural cell attachment. These findings were corroborated in GL261 glioma cells intracranially transplanted in mice expressing dominant-active β-catenin specifically in the endothelium. Enforced endothelial β-catenin signaling restored BBB characteristics, whereas inhibition by Dkk1 (Dickkopf-1) had opposing effects. By overactivating the Wnt pathway, we induced the Wnt/β-catenin–Dll4/Notch signaling cascade in tumor endothelia, blocking an angiogenic and favoring a quiescent vascular phenotype, indicated by induction of stalk cell genes. We show that β-catenin transcriptional activity directly regulated endothelial expression of platelet-derived growth factor B (PDGF-B), leading to mural cell recruitment thereby contributing to vascular quiescence and barrier function. We propose that reinforced Wnt/β-catenin signaling leads to inhibition of angiogenesis with normalized and less permeable vessels, which might prove to be a valuable therapeutic target for antiangiogenic and edema glioma therapy.

β-Catenin signaling specifies progenitor cell identity in parallel with Shh signaling in the developing mammalian thalamus

Neural progenitor cells within the developing thalamus are spatially organized into distinct populations. Their correct specification is critical for generating appropriate neuronal subtypes in specific locations during development. Secreted signaling molecules, such as sonic hedgehog (Shh) and Wnts, are required for the initial formation of the thalamic primordium. Once thalamic identity is established and neurogenesis is initiated, Shh regulates the positional identity of thalamic progenitor cells. Although Wnt/β-catenin signaling also has differential activity within the thalamus during this stage of development, its significance has not been directly addressed. In this study, we used conditional gene manipulations in mice and explored the roles of β-catenin signaling in the regional identity of thalamic progenitor cells. We found β-catenin is required during thalamic neurogenesis to maintain thalamic fate while suppressing prethalamic fate, demonstrating that regulation of regional fate continues to require extrinsic signals. These roles of β-catenin appeared to be mediated at least partly by regulating two basic helix-loop-helix (bHLH) transcription factors, Neurog1 and Neurog2. β-Catenin and Shh signaling function in parallel to specify two progenitor domains within the thalamus, where individual transcription factors expressed in each progenitor domain were regulated differently by the two signaling pathways. We conclude that β-catenin has multiple functions during thalamic neurogenesis and that both Shh and β-catenin pathways are important for specifying distinct types of thalamic progenitor cells, ensuring that the appropriate neuronal subtypes are generated in the correct locations.

β-Catenin stabilization in skeletal muscles, but not in motor neurons, leads to aberrant motor innervation of the muscle during neuromuscular development in mice

β-Catenin, a key component of the Wnt signaling pathway, has been implicated in the development of the neuromuscular junction (NMJ) in mice, but its precise role in this process remains unclear. Here we use a β-catenin gain-of-function mouse model to stabilize β-catenin selectively in either skeletal muscles or motor neurons. We found that β-catenin stabilization in skeletal muscles resulted in increased motor axon number and excessive intramuscular nerve defasciculation and branching. In contrast, β-catenin stabilization in motor neurons had no adverse effect on motor innervation pattern. Furthermore, stabilization of β-catenin, either in skeletal muscles or in motor neurons, had no adverse effect on the formation and function of the NMJ. Our findings demonstrate that β-catenin levels in developing muscles in mice are crucial for proper muscle innervation, rather than specifically affecting synapse formation at the NMJ, and that the regulation of muscle innervation by β-catenin is mediated by a non-cell autonomous mechanism.

Wnt9b-dependent FGF signaling is crucial for outgrowth of the nasal and maxillary processes during upper jaw and lip development

Outgrowth and fusion of the lateral and medial nasal processes and of the maxillary process of the first branchial arch are integral to lip and primary palate development. Wnt9b mutations are associated with cleft lip and cleft palate in mice; however, the cause of these defects remains unknown. Here, we report that Wnt9b(-/-) mice show significantly retarded outgrowth of the nasal and maxillary processes due to reduced proliferation of mesenchymal cells, which subsequently results in a failure of physical contact between the facial processes that leads to cleft lip and cleft palate. These cellular defects in Wnt9b(-/-) mice are mainly caused by reduced FGF family gene expression and FGF signaling activity resulting from compromised canonical WNT/β-catenin signaling. Our study has identified a previously unknown regulatory link between WNT9B and FGF signaling during lip and upper jaw development.

WNT signaling increases proliferation and impairs differentiation of stem cells in the developing cerebellum

The WNT pathway plays multiple roles in neural development and is crucial for establishment of the embryonic cerebellum. In addition, WNT pathway mutations are associated with medulloblastoma, the most common malignant brain tumor in children. However, the cell types within the cerebellum that are responsive to WNT signaling remain unknown. Here we investigate the effects of canonical WNT signaling on two important classes of progenitors in the developing cerebellum: multipotent neural stem cells (NSCs) and granule neuron precursors (GNPs). We show that WNT pathway activation in vitro promotes proliferation of NSCs but not GNPs. Moreover, mice that express activated β-catenin in the cerebellar ventricular zone exhibit increased proliferation of NSCs in that region, whereas expression of the same protein in GNPs impairs proliferation. Although β-catenin-expressing NSCs proliferate they do not undergo prolonged expansion or neoplastic growth; rather, WNT signaling markedly interferes with their capacity for self-renewal and differentiation. At a molecular level, mutant NSCs exhibit increased expression of c-Myc, which might account for their transient proliferation, but also express high levels of bone morphogenetic proteins and the cyclin-dependent kinase inhibitor p21, which might contribute to their altered self-renewal and differentiation. These studies suggest that the WNT pathway is a potent regulator of cerebellar stem cell growth and differentiation.

Nonmyelinating Schwann cells maintain hematopoietic stem cell hibernation in the bone marrow niche

Hematopoietic stem cells (HSCs) reside and self-renew in the bone marrow (BM) niche. Overall, the signaling that regulates stem cell dormancy in the HSC niche remains controversial. Here, we demonstrate that TGF-β type II receptor-deficient HSCs show low-level Smad activation and impaired long-term repopulating activity, underlining the critical role of TGF-β/Smad signaling in HSC maintenance. TGF-β is produced as a latent form by a variety of cells, so we searched for those that express activator molecules for latent TGF-β. Nonmyelinating Schwann cells in BM proved responsible for activation. These glial cells ensheathed autonomic nerves, expressed HSC niche factor genes, and were in contact with a substantial proportion of HSCs. Autonomic nerve denervation reduced the number of these active TGF-β-producing cells and led to rapid loss of HSCs from BM. We propose that glial cells are components of a BM niche and maintain HSC hibernation by regulating activation of latent TGF-β.

β-catenin signaling controls metastasis in Braf-activated Pten-deficient melanomas

Malignant melanoma is characterized by frequent metastasis, however, specific changes that regulate this process have not been clearly delineated. Although it is well known that Wnt signaling is frequently dysregulated in melanoma, the functional implications of this observation are unclear. By modulating β-catenin levels in a mouse model of melanoma that is based on melanocyte-specific Pten loss and Braf(V600E) mutation, we demonstrate that β-catenin is a central mediator of melanoma metastasis to the lymph nodes and lungs. In addition to altering metastasis, β-catenin levels control tumor differentiation and regulate both MAPK/Erk and PI3K/Akt signaling. Highly metastatic tumors with β-catenin stabilization are very similar to a subset of human melanomas. Together these findings establish Wnt signaling as a metastasis regulator in melanoma.

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.

RSPO1/β-catenin signaling pathway regulates oogonia differentiation and entry into meiosis in the mouse fetal ovary

Differentiation of germ cells into male gonocytes or female oocytes is a central event in sexual reproduction. Proliferation and differentiation of fetal germ cells depend on the sex of the embryo. In male mouse embryos, germ cell proliferation is regulated by the RNA helicase Mouse Vasa homolog gene and factors synthesized by the somatic Sertoli cells promote gonocyte differentiation. In the female, ovarian differentiation requires activation of the WNT/β-catenin signaling pathway in the somatic cells by the secreted protein RSPO1. Using mouse models, we now show that Rspo1 also activates the WNT/β-catenin signaling pathway in germ cells. In XX Rspo1^−/− gonads, germ cell proliferation, expression of the early meiotic marker Stra8, and entry into meiosis are all impaired. In these gonads, impaired entry into meiosis and germ cell sex reversal occur prior to detectable Sertoli cell differentiation, suggesting that β-catenin signaling acts within the germ cells to promote oogonial differentiation and entry into meiosis. Our results demonstrate that RSPO1/β-catenin signaling is involved in meiosis in fetal germ cells and contributes to the cellular decision of germ cells to differentiate into oocyte or sperm.

Wnt/β-catenin pathway in podocytes integrates cell adhesion, differentiation, and survival

Diabetic kidney disease (DKD) is the single most common cause of albuminuria and end-stage kidney disease in the United States. We found increased expression of Wnt/β-catenin (Ctnnb1) pathway transcripts and proteins in glomeruli and podocytes of patients and mouse models of DKD. Mice with podocyte-specific expression of stabilized Ctnnb1 exhibited basement membrane abnormalities, albuminuria, and increased susceptibility to glomerular injury. Mice with podocyte-specific deletion of Ctnnb1 or podocyte-specific expression of the canonical Wnt inhibitor Dickkopf-related protein 1 (Dkk1) also showed increased susceptibility to DKD. Podocytes with stabilized Ctnnb1 were less motile and less adhesive to different matrices. Deletion of Ctnnb1 in cultured podocytes increased the expression of podocyte differentiation markers and enhanced cell motility; however, these cells were more susceptible to apoptosis. These results indicate that Wnt/Ctnnb1 signaling in podocytes plays a critical role in integrating cell adhesion, motility, cell death, and differentiation. Balanced Ctnnb1 expression is critical for glomerular filtration barrier maintenance.