The Wnt/beta-catenin pathway interacts differentially with PTHrP signaling to control chondrocyte hypertrophy and final maturation

Sequential proliferation, hypertrophy and maturation of chondrocytes are required for proper endochondral bone development and tightly regulated by cell signaling. The canonical Wnt signaling pathway acts through β-catenin to promote chondrocyte hypertrophy whereas PTHrP signaling inhibits it by holding chondrocytes in proliferating states. Here we show by genetic approaches that chondrocyte hypertrophy and final maturation are two distinct developmental processes that are differentially regulated by Wnt/β-catenin and PTHrP signaling. Wnt/β-catenin signaling regulates initiation of chondrocyte hypertrophy by inhibiting PTHrP signaling activity, but it does not regulate PTHrP expression. In addition, Wnt/β-catenin signaling regulates chondrocyte hypertrophy in a non-cell autonomous manner and Gdf5/Bmp signaling may be one of the downstream pathways. Furthermore, Wnt/β-catenin signaling also controls final maturation of hypertrophic chondrocytes, but such regulation is PTHrP signaling-independent.

Overactive beta-catenin signaling causes testicular sertoli cell tumor development in the mouse

Overactive WNT/beta-catenin signaling has been found in many forms of cancer in human patients. Mouse models with mutations in different components of the WNT/beta-catenin signaling pathway have been generated to mimic tumorigenesis in humans. Mice with mutations that result in overactive WNT/beta-catenin signaling developed tumors in some tissues, such as digestive tract, skin, and ovary, but they failed to develop tumors in other tissues, such as mammary gland, liver, kidney, and primordial germ cells. To investigate whether overactive beta-catenin signaling is capable of inducing Sertoli cell tumorigenesis in testes, we generated Ctnnb1(tm1Mmt/+);Tg(AMH-cre)1Flor male mice that express a constitutively active form of beta-catenin specifically in Sertoli cells. No tumors were observed at 4 mo of age, but 70% of the mutant males developed Sertoli cell tumors at 8 mo of age. At 1 yr of age, more than 90% of the mutant males developed tumors. No instances of extratesticular spread of the tumors were found in the mutant mice. These studies show a causal link between overactive WNT/beta-catenin signaling and Sertoli cell tumor development and provide a novel mouse model for the study of Sertoli cell tumor biology.

Conditional mutations of beta-catenin and APC reveal roles for canonical Wnt signaling in lens differentiation

PURPOSE

Previous studies indicate that the Wnt/beta-catenin-signaling pathway is active and functional during murine lens development. In this study, the consequences of constitutively activating the pathway in lens during development were investigated.

METHODS

To activate Wnt/beta-catenin signaling, beta-catenin (Catnb) and adenomatous polyposis coli (Apc) genes were conditionally mutated in two Cre lines that are active in whole lens (MLR10) or only in differentiated fibers (MLR39), from E13.5. Lens phenotype in mutant lenses was investigated by histology, immunohistochemistry, BrdU labeling, quantitative RT-PCR arrays, and TUNEL.

RESULTS

Only intercrosses with MLR10 resulted in ocular phenotypes, indicating Wnt/beta-catenin signaling functions in lens epithelium and during early fiber differentiation. Mutant lenses were characterized by increased progression of epithelial cells through the cell cycle, as shown by BrdU labeling, and phosphohistone 3 and cyclin D1 labeling, and maintenance of epithelial phenotype (E-cadherin and Pax6 expression) in the fiber compartment. Fiber cell differentiation was delayed as shown by reduced expression of c-maf and beta-crystallin and delay in expression of the CDKI, p57(kip2). From E13.5, there were numerous cells undergoing apoptosis, and by E15.5, there was evidence of epithelial-mesenchymal transition with numerous cells expressing alpha-smooth muscle actin. Quantitative PCR analyses revealed large changes in expression of Wnt target genes (Lef1, Tcf7, T (Brachyury), and Ccnd1), Wnt inhibitors (Wif1, Dkk1, Nkd1, and Frzb) and also several Wnts (Wnt6, Wnt10a, Wnt8b, and Wnt11).

CONCLUSIONS

These data indicate that the Wnt/beta-catenin pathway plays key roles in regulating proliferation of lens stem/progenitor cells during early stages of fiber cell differentiation.

Imaging cyclooxygenase-2 (Cox-2) gene expression in living animals with a luciferase knock-in reporter gene

The cyclooxygenase-2 (Cox-2) gene plays a role in a variety of normal and pathophysiological conditions. Expression of the Cox-2 gene is induced in a broad range of cells, in response to many distinct stimuli. The ability to monitor and quantify Cox-2 expression noninvasively in vivo may facilitate a better understanding of the role of Cox-2, both in normal physiology and in different diseases. We generated a "knock-in" mouse in which the firefly luciferase reporter enzyme is expressed at the start site of translation of the endogenous Cox-2 gene. Correlation of luciferase and Cox-2 expression was confirmed in heterozygous Cox-2luc/+ mouse embryonic fibroblasts isolated from the knock-in mouse. In an acute sepsis model, following injection of interferon gamma and endotoxin, ex vivo imaging and Western blotting demonstrated coordinate Cox-2 and luciferase induction in multiple organs. Using both paw and air pouch inflammation models, we can monitor repeatedly localized luciferase expression in the same living mouse. Cox-2luc/+ knock-in mice should provide a valuable tool to analyze Cox-2 expression in many disease models.

Genetic interactions of the androgen and Wnt/beta-catenin pathways for the masculinization of external genitalia

In most mammals, the sexually dimorphic development of embryos is typically achieved by the differentiation of the external genitalia. Hence, the sexual distinction of mammalian newborns is based on the external genital structure. Although it was shown in the 1940s and 1950s that androgen from the testes establishes the male sexual characteristics, the involvement of nongonadal and locally produced masculine effectors remains totally unknown. It is noteworthy that the disorders of fetal masculinization, including hypospadias, one of the most frequent birth defects, occur at a high frequency. Furthermore, their causative factors remain unclear. In this study, the involvement of the coordinated actions of androgen and the growth factor systems was genetically analyzed for the first time on mammalian reproductive organ formation. The results demonstrated that the Wnt/beta-catenin pathway is indispensable masculine factor for the external genital development. The bilateral mesenchymal region adjacent to the urethral plate epithelium displayed a sexually dimorphic activity of Wnt/beta-catenin signaling. Loss- and gain-of-function beta-catenin mutants displayed altered sexual development of the external genitalia. These results indicate the novel functions of the Wnt/beta-catenin pathway as a locally expressed masculine effector. This could be the first genetic study analyzing the roles of the genetic interactions between androgen and locally expressed growth factor signaling during the development of reproductive organs. These results also shed new insight on the reproductive genetics and the causative factors of genital disorders.

Embryonic hair follicle fate change by augmented beta-catenin through Shh and Bmp signaling

beta-catenin signaling is one of the key factors regulating the fate of hair follicles (HFs). To elucidate the regulatory mechanism of embryonic HF fate determination during epidermal development/differentiation, we analyzed conditional mutant mice with keratinocytes expressing constitutively active beta-catenin (K5-Cre Catnb(ex3)fl/+). The mutant mice developed scaly skin with a thickened epidermis and showed impaired epidermal stratification. The hair shaft keratins were broadly expressed in the epidermis but there was no expression of the terminal differentiation markers K1 and loricrin. Hair placode markers (Bmp2 and Shh) and follicular dermal condensate markers (noggin, patched 1 and Pdgfra) were expressed throughout the epidermis and the upper dermis, respectively. These results indicate that the embryonic epidermal keratinocytes have switched extensively to the HF fate. A series of genetic studies demonstrated that the epidermal switching to HF fate was suppressed by introducing the conditional mutation K5-Cre Catnb(ex3)fl/+Shhfl/- (with additional mutation of Shh signaling) or K5-Cre Catnb(ex3)fl/+BmprIAfl/fl (with additional mutation of Bmp signaling). These results demonstrate that Wnt/beta-catenin signaling relayed through Shh and Bmp signals is the principal regulatory mechanism underlying the HF cell fate change. Assessment of Bmp2 promoter activities suggested a putative regulation by beta-catenin signaling relayed by Shh signaling towards Bmp2. We also found that Shh protein expression was increased and expanded in the epidermis of K5-Cre Catnb(ex3)fl/+BmprIAfl/fl mice. These results indicate the presence of growth factor signal cross-talk involving beta-catenin signaling, which regulates the HF fate.

Constitutive activation of Beta-catenin in uterine stroma and smooth muscle leads to the development of mesenchymal tumors in mice

Leiomyomas and other mesenchymally derived tumors are the most common neoplasms of the female reproductive tract. Presently, very little is known about the etiology and progression of these tumors, which are the primary indication for hysterectomies. Dysregulated WNT signaling through beta-catenin is a well-established mechanism for tumorigenesis. We have developed a mouse model that expresses constitutively activated beta-catenin in uterine mesenchyme driven by the expression of Cre recombinase knocked into the Müllerian-inhibiting substance type II receptor promoter locus to investigate its effects on uterine endometrial stroma and myometrium. These mice show myometrial hyperplasia and develop mesenchymal tumors with 100% penetrance that exhibit histological and molecular characteristics of human leiomyomas and endometrial stromal sarcomas. By immunohistochemistry, we also show that both transforming growth factor beta and the mammalian target of rapamycin are induced by constitutive activation of beta-catenin. The prevalence of the tumors was greater in multiparous mice, suggesting that their development may be a hormonally driven process or that changes in uterine morphology during pregnancy and after parturition induce injury and repair mechanisms that stimulate tumorigenesis from stem/progenitor cells, which normally do not express constitutively activated beta-catenin. Additionally, adenomyosis and endometrial gland hyperplasia were occasionally observed in some mice. These results show evidence suggesting that dysregulated, stromal, and myometrial WNT/beta-catenin signaling has pleiotropic effects on uterine function and tumorigenesis.

Stabilized beta-catenin in thymic epithelial cells blocks thymus development and function

Thymic T cell development is dependent on a specialized epithelial microenvironment mainly composed of cortical and medullary thymic epithelial cells (TECs). The molecular programs governing the differentiation and maintenance of TECs remain largely unknown. Wnt signaling is central to the development and maintenance of several organ systems but a specific role of this pathway for thymus organogenesis has not yet been ascertained. In this report, we demonstrate that activation of the canonical Wnt signaling pathway by a stabilizing mutation of beta-catenin targeted exclusively to TECs changes the initial commitment of endodermal epithelia to a thymic cell fate. Consequently, the formation of a correctly composed and organized thymic microenvironment is prevented, thymic immigration of hematopoietic precursors is restricted, and intrathymic T cell differentiation is arrested at a very early developmental stage causing severe immunodeficiency. These results suggest that a precise regulation of canonical Wnt signaling in thymic epithelia is essential for normal thymus development and function.

Wnt/beta-catenin signaling controls development of the blood-brain barrier

The blood–brain barrier (BBB) is confined to the endothelium of brain capillaries and is indispensable for fluid homeostasis and neuronal function. In this study, we show that endothelial Wnt/β-catenin (β-cat) signaling regulates induction and maintenance of BBB characteristics during embryonic and postnatal development. Endothelial specific stabilization of β-cat in vivo enhances barrier maturation, whereas inactivation of β-cat causes significant down-regulation of claudin3 (Cldn3), up-regulation of plamalemma vesicle-associated protein, and BBB breakdown. Stabilization of β-cat in primary brain endothelial cells (ECs) in vitro by N-terminal truncation or Wnt3a treatment increases Cldn3 expression, BBB-type tight junction formation, and a BBB characteristic gene signature. Loss of β-cat or inhibition of its signaling abrogates this effect. Furthermore, stabilization of β-cat also increased Cldn3 and barrier properties in nonbrain-derived ECs. These findings may open new therapeutic avenues to modulate endothelial barrier function and to limit the devastating effects of BBB breakdown.

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

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

Stabilization of beta-catenin in XY gonads causes male-to-female sex-reversal

During mammalian sex determination, expression of the Y-linked gene Sry shifts the bipotential gonad toward a testicular fate by upregulating a feed-forward loop between FGF9 and SOX9 to establish SOX9 expression in somatic cells. We previously proposed that these signals are mutually antagonistic with counteracting signals in XX gonads and that a shift in the balance of these factors leads to either male or female development. Evidence in mice and humans suggests that the male pathway is opposed by the expression of two signals, WNT4 and R-SPONDIN-1 (RSPO1), that promote the ovarian fate and block testis development. Both of these ligands can activate the canonical Wnt signaling pathway. Duplication of the distal portion of chromosome 1p, which includes both WNT4 and RSPO1, overrides the male program and causes male-to-female sex reversal in XY patients. To determine whether activation of beta-catenin is sufficient to block the testis pathway, we have ectopically expressed a stabilized form of beta-catenin in the somatic cells of XY gonads. Our results show that activation of beta-catenin in otherwise normal XY mice effectively disrupts the male program and results in male-to-female sex-reversal. The identification of beta-catenin as a key pro-ovarian and anti-testis signaling molecule will further our understanding of the mechanisms controlling sex determination and the molecular mechanisms that lead to sex-reversal.

Stromal fibroblasts activated by tumor cells promote angiogenesis in mouse gastric cancer

Myofibroblasts, also known as activated fibroblasts, constitute an important niche for tumor development through the promotion of angiogenesis. However, the mechanism of stromal fibroblast activation in tumor tissues has not been fully understood. A gastric cancer mouse model (Gan mice) was recently constructed by simultaneous activation of prostaglandin (PG) E2 and Wnt signaling in the gastric mucosa. Because both the PGE2 and Wnt pathways play a role in human gastric tumorigenesis, the Gan mouse model therefore recapitulates the molecular etiology of human gastric cancer. Microvessel density increased significantly in Gan mouse tumors. Moreover, the expression of vascular endothelial growth factor A (VEGFA) was predominantly induced in the stromal cells of gastric tumors. Immunohistochemistry suggested that VEGFA-expressing cells in the stroma were alpha-smooth muscle actin-positive myofibroblasts. Bone marrow transplantation experiments indicated that a subset of gastric myofibroblasts is derived from bone marrow. Importantly, the alpha-smooth muscle actin index in cultured fibroblasts increased significantly when stimulated with the conditioned medium of Gan mouse tumor cells, indicating that gastric tumor cells activate stromal fibroblasts. Furthermore, conditioned medium of Gan mouse tumor cells induced VEGFA expression both in embryonic and gastric fibroblasts, which further accelerated the tube formation of human umbilical vein endothelial cells in vitro. Notably, stimulation of fibroblasts with PGE2 and/or Wnt1 did not induce VEGFA expression, thus suggesting that factors secondarily induced by PGE2 and Wnt signaling in the tumor cells are responsible for activation of stromal fibroblasts. Such tumor cell-derived factors may therefore be an effective target for chemoprevention against gastric cancer.

Activation of beta-catenin signaling programs embryonic epidermis to hair follicle fate

beta-Catenin signaling is required for hair follicle development, but it is unknown whether its activation is sufficient to globally program embryonic epidermis to hair follicle fate. To address this, we mutated endogenous epithelial beta-catenin to a dominant-active form in vivo. Hair follicle placodes were expanded and induced prematurely in activated beta-catenin mutant embryos, but failed to invaginate or form multilayered structures. Eventually, the entire epidermis adopted hair follicle fate, broadly expressing hair shaft keratins in place of epidermal stratification proteins. Mutant embryonic skin was precociously innervated, and displayed prenatal pigmentation, a phenomenon never observed in wild-type controls. Thus, beta-catenin signaling programs the epidermis towards placode and hair shaft fate at the expense of epidermal differentiation, and activates signals directing pigmentation and innervation. In transcript profiling experiments, we identified elevated expression of Sp5, a direct beta-catenin target and transcriptional repressor. We show that Sp5 normally localizes to hair follicle placodes and can suppress epidermal differentiation gene expression. We identified the pigmentation regulators Foxn1, Adamts20 and Kitl, and the neural guidance genes Sema4c, Sema3c, Unc5b and Unc5c, as potential mediators of the effects of beta-catenin signaling on pigmentation and innervation. Our data provide evidence for a new paradigm in which, in addition to promoting hair follicle placode and hair shaft fate, beta-catenin signaling actively suppresses epidermal differentiation and directs pigmentation and nerve fiber growth. Controlled downregulation of beta-catenin signaling is required for normal placode patterning within embryonic ectoderm, hair follicle downgrowth, and adoption of the full range of follicular fates.

Berberine and its more biologically available derivative, dihydroberberine, inhibit mitochondrial respiratory complex I: a mechanism for the action of berberine to activate AMP-activated protein kinase and improve insulin action

OBJECTIVE

Berberine (BBR) activates AMP-activated protein kinase (AMPK) and improves insulin sensitivity in rodent models of insulin resistance. We investigated the mechanism of activation of AMPK by BBR and explored whether derivatization of BBR could improve its in vivo efficacy.

RESEARCH DESIGN AND METHODS

AMPK phosphorylation was examined in L6 myotubes and LKB1(-/-) cells, with or without the Ca(2+)/calmodulin-dependent protein kinase kinase (CAMKK) inhibitor STO-609. Oxygen consumption was measured in L6 myotubes and isolated muscle mitochondria. The effect of a BBR derivative, dihydroberberine (dhBBR), on adiposity and glucose metabolism was examined in rodents fed a high-fat diet. RESULTS; We have made the following novel observations: 1) BBR dose-dependently inhibited respiration in L6 myotubes and muscle mitochondria, through a specific effect on respiratory complex I, similar to that observed with metformin and rosiglitazone; 2) activation of AMPK by BBR did not rely on the activity of either LKB1 or CAMKKbeta, consistent with major regulation at the level of the AMPK phosphatase; and 3) a novel BBR derivative, dhBBR, was identified that displayed improved in vivo efficacy in terms of counteracting increased adiposity, tissue triglyceride accumulation, and insulin resistance in high-fat-fed rodents. This effect is likely due to enhanced oral bioavailability.

CONCLUSIONS

Complex I of the respiratory chain represents a major target for compounds that improve whole-body insulin sensitivity through increased AMPK activity. The identification of a novel derivative of BBR with improved in vivo efficacy highlights the potential importance of BBR as a novel therapy for the treatment of type 2 diabetes.

Activation of beta-catenin signaling by Rspo1 controls differentiation of the mammalian ovary

The sex of an individual is determined by the fate of the gonad. While the expression of Sry and Sox9 is sufficient to induce male development, we here show that female differentiation requires activation of the canonical beta-catenin signaling pathway. beta-catenin activation is controlled by Rspo1 in XX gonads and Rspo1 knockout mice show masculinized gonads. Molecular analyses demonstrate an absence of female-specific activation of Wnt4 and as a consequence XY-like vascularization and steroidogenesis. Moreover, germ cells of XX knockout embryos show changes in cellular adhesions and a failure to enter XX specific meiosis. Sex cords develop around birth, when Sox9 becomes strongly activated. Thus, a balance between Sox9 and beta-catenin activation determines the fate of the gonad, with Rspo1 acting as a crucial regulator of canonical beta-catenin signaling required for female development.

Adenomatous polyposis coli (APC): a multi-functional tumor suppressor gene

The adenomatous polyposis coli (APC) gene is a key tumor suppressor gene. Mutations in the gene have been found not only in most colon cancers but also in some other cancers, such as those of the liver. The APC gene product is a 312 kDa protein that has multiple domains, through which it binds to various proteins, including beta-catenin, axin, CtBP, Asefs, IQGAP1, EB1 and microtubules. Studies using mutant mice and cultured cells have demonstrated that APC suppresses canonical Wnt signalling, which is essential for tumorigenesis, development and homeostasis of a variety of cell types, such as epithelial and lymphoid cells. Further studies have suggested that APC plays roles in several other fundamental cellular processes. These include cell adhesion and migration, organization of the actin and microtubule networks, spindle formation and chromosome segregation. Deregulation of these processes caused by mutations in APC is implicated in the initiation and expansion of colon cancer.

Wt1 negatively regulates beta-catenin signaling during testis development

beta-Catenin, as an important effector of the canonical Wnt signaling pathway and as a regulator of cell adhesion, has been demonstrated to be involved in multiple developmental processes and tumorigenesis. beta-Catenin expression was found mainly on the Sertoli cell membrane starting from embryonic day 15.5 in the developing testes. However, its potential role in Sertoli cells during testis formation has not been examined. To determine the function of beta-catenin in Sertoli cells during testis formation, we either deleted beta-catenin or expressed a constitutively active form of beta-catenin in Sertoli cells. We found that deletion caused no detectable abnormalities. However, stabilization caused severe phenotypes, including testicular cord disruption, germ cell depletion and inhibition of Müllerian duct regression. beta-Catenin stabilization caused changes in Sertoli cell identity and misregulation of inter-Sertoli cell contacts. As Wt1 conditional knockout in Sertoli cells causes similar phenotypes to our stabilized beta-catenin mutants, we then investigated the relationship of Wt1 and beta-catenin in Sertoli cells and found Wt1 inhibits beta-catenin signaling in these cells during testis development. Wt1 deletion resulted in upregulation of beta-catenin expression in Sertoli cells both in vitro and in vivo. Our study indicates that Sertoli cell expression of beta-catenin is dispensable for testis development. However, the suppression of beta-catenin signaling in these cells is essential for proper testis formation and Wt1 is a negative regulator of beta-catenin signaling during this developmental process.

Sustained epithelial beta-catenin activity induces precocious hair development but disrupts hair follicle down-growth and hair shaft formation

During embryonic and postnatal development, Wnt/beta-catenin signaling is involved in several stages of hair morphogenesis from placode formation to hair shaft differentiation. Using a transgenic approach, we have investigated further the role of beta-catenin signaling in embryonic hair development. Forced epithelial stabilization of beta-catenin resulted in precocious and excessive induction of hair follicles even in the absence of Eda/Edar signaling, a pathway essential for primary hair placode formation. In addition, the spacing and size of the placodes was randomized. Surprisingly, the down-growth of follicles was suppressed and hair shaft production was severely impaired. Gene and reporter expression analyses revealed elevated mesenchymal Wnt activity, as well as increased BMP signaling, throughout the skin that was accompanied by upregulation of Sostdc1 (Wise, ectodin) expression. Our data suggest that BMPs are downstream of Wnt/beta-catenin and that their interplay may be a critical component in establishing correct patterning of hair follicles through the reaction-diffusion mechanism.

Tissue-specific transgenic, conditional knockout and knock-in mice of genes in the canonical Wnt signaling pathway

The Wnt signaling pathway plays key roles in the development and homeostasis of a number of organs such as the brain, lung, liver, heart, gastrointestinal tract, mammary gland, skin, and bone, as well as of the immune system. Studies on conventional knockout mice of the genes in the Wnt signaling pathway have revealed its essential roles in these tissues; however, most of these knockout mice die during embryogenesis or soon after birth. Through more advanced techniques such as Cre/loxP and tetracycline-inducible systems, a gene of interest can be expressed or inactivated in a tissue-specific and time-controlled manner. Here we review recent papers on the tissue-specific transgenic, conditional knockout and knock-in mice of the genes in the Wnt signaling pathway In addition to such engineered mice, we also list reporter mice that have been generated to determine the activity of the canonical Wnt signaling pathway in mouse tissues.

Wnt3a/beta-catenin signaling controls posterior body development by coordinating mesoderm formation and segmentation

Somitogenesis is thought to be controlled by a segmentation clock, which consists of molecular oscillators in the Wnt3a, Fgf8 and Notch pathways. Using conditional alleles of Ctnnb1 (beta-catenin), we show that the canonical Wnt3a/beta-catenin pathway is necessary for molecular oscillations in all three signaling pathways but does not function as an integral component of the oscillator. Small, irregular somites persist in abnormally posterior locations in the absence of beta-catenin and cycling clock gene expression. Conversely, Notch pathway genes continue to oscillate in the presence of stabilized beta-catenin but boundary formation is delayed and anteriorized. Together, these results suggest that the Wnt3a/beta-catenin pathway is permissive but not instructive for oscillating clock genes and that it controls the anterior-posterior positioning of boundary formation in the presomitic mesoderm (PSM). The Wnt3a/beta-catenin pathway does so by regulating the activation of the segment boundary determination genes Mesp2 and Ripply2 in the PSM through the activation of the Notch ligand Dll1 and the mesodermal transcription factors T and Tbx6. Spatial restriction of Ripply2 to the anterior PSM is ensured by the Wnt3a/beta-catenin-mediated repression of Ripply2 in posterior PSM. Thus, Wnt3a regulates somitogenesis by activating a network of interacting target genes that promote mesodermal fates, activate the segmentation clock, and position boundary determination genes in the anterior PSM.

Keeping out the bad guys: gateway to cellular target therapy

Tumor-stromal interaction is implicated in many stages of tumor development, although it remains unclear how genetic lesions in tumor cells affect stromal cells. We have recently shown that inactivation of transforming growth factor-beta family signaling within colon cancer epithelium increases chemokine CC chemokine ligand 9 (CCL9) and promotes recruitment of the matrix metalloproteinase (MMP)-expressing stromal cells that carry CC chemokine receptor 1 (CCR1), the cognate receptor for CCL9. We have further shown that lack of CCR1 prevents the accumulation of MMP-expressing cells at the invasion front and suppresses tumor invasion. These results provide the possibility of a novel therapeutic strategy for advanced cancer--prevention of the recruitment of MMP-expressing cells by chemokine receptor antagonist.

Wnt/beta-catenin signaling promotes expansion of Isl-1-positive cardiac progenitor cells through regulation of FGF signaling

The anterior heart field (AHF), which contributes to the outflow tract and right ventricle of the heart, is defined in part by expression of the LIM homeobox transcription factor Isl-1. The importance of Isl-1-positive cells in cardiac development and homeostasis is underscored by the finding that these cells are required for cardiac development and act as cardiac stem/progenitor cells within the postnatal heart. However, the molecular pathways regulating these cells' expansion and differentiation are poorly understood. We show that Isl-1-positive AHF progenitor cells in mice were responsive to Wnt/beta-catenin signaling, and these responsive cells contributed to the outflow tract and right ventricle of the heart. Loss of Wnt/beta-catenin signaling in the AHF caused defective outflow tract and right ventricular development with a decrease in Isl-1-positive progenitors and loss of FGF signaling. Conversely, Wnt gain of function in these cells led to expansion of Isl-1-positive progenitors with a concomitant increase in FGF signaling through activation of a specific set of FGF ligands including FGF3, FGF10, FGF16, and FGF20. These data reveal what we believe to be a novel Wnt-FGF signaling axis required for expansion of Isl-1-positive AHF progenitors and suggest future therapies to increase the number and function of these cells for cardiac regeneration.