Stem cells in the skin: waste not, Wnt not

Differential expression of a BMP4 reporter allele in anterior fungiform versus posterior circumvallate taste buds of mice

BACKGROUND

Bone Morphogenetic Protein 4 (BMP4) is a diffusible factor which regulates embryonic taste organ development. However, the role of BMP4 in taste buds of adult mice is unknown. We utilized transgenic mice with LacZ under the control of the BMP4 promoter to reveal the expression of BMP4 in the tongues of adult mice. Further we evaluate the pattern of BMP4 expression with that of markers of specific taste bud cell types and cell proliferation to define and compare the cell populations expressing BMP4 in anterior (fungiform papillae) and posterior (circumvallate papilla) tongue.

RESULTS

BMP4 is expressed in adult fungiform and circumvallate papillae, i.e., lingual structures composed of non-taste epithelium and taste buds. Unexpectedly, we find both differences and similarities with respect to expression of BMP4-driven ß-galactosidase. In circumvallate papillae, many fusiform cells within taste buds are BMP4-ß-gal positive. Further, a low percentage of BMP4-expressing cells within circumvallate taste buds is immunopositive for markers of each of the three differentiated taste cell types (I, II and III). BMP4-positive intragemmal cells also expressed a putative marker of immature taste cells, Sox2, and consistent with this finding, intragemmal cells expressed BMP4-ß-gal within 24 hours after their final mitosis, as determined by BrdU birthdating. By contrast, in fungiform papillae, BMP4-ß-gal positive cells are never encountered within taste buds. However, in both circumvallate and fungiform papillae, BMP4-ß-gal expressing cells are located in the perigemmal region, comprising basal and edge epithelial cells adjacent to taste buds proper. This region houses the proliferative cell population that gives rise to adult taste cells. However, perigemmal BMP4-ß-gal cells appear mitotically silent in both fungiform and circumvallate taste papillae, as we do not find evidence of their active proliferation using cell cycle immunomarkers and BrdU birthdating.

CONCLUSION

Our data suggest that intragemmal BMP4-ß-gal cells in circumvallate papillae are immature taste cells which eventually differentiate into each of the 3 taste cell types, whereas perigemmal BMP4-ß-gal cells in both circumvallate and fungiform papillae may be slow cycling stem cells, or belong to the stem cell niche to regulate taste cell renewal from the proliferative cell population.

Expression of Gpr177, a Wnt trafficking regulator, in mouse embryogenesis

Wls/Evi/Srt encoding a multipass transmembrane protein has been identified as a regulator for proper sorting and secretion of Wnt in flies. We have previously demonstrated that Gpr177 is the mouse ortholog required for axis determination. Gpr177 is a transcriptional target of Wnt that is activated to assist its subcellular distribution in a feedback regulatory loop. We, therefore, proposed that reciprocal regulation of Wnt and Gpr177 is essential for the Wnt-dependent developmental and pathogenic processes. Here, we examine the expression pattern of Gpr177 in mouse development. Gpr177 is expressed in a variety of tissues and cell types during organogenesis. Furthermore, Gpr177 is a glycoprotein primarily accumulating in the Golgi apparatus in signal-producing cells. The glycosylation of Gpr177 is necessary for proper transportation in the secretory pathway. Our findings suggest that the Gpr177-mediated regulation of Wnt is crucial for organogenesis in health and disease.

Expression of Notch pathway proteins correlates with albuminuria, glomerulosclerosis, and renal function

Recent studies indicate that the Notch signaling pathway plays an important role in the development of diabetic kidney disease and focal segmental glomerulosclerosis (FSGS). Here we analyzed the degree of expression and localization of Notch ligands (Jagged1 and Delta1) and activated (cleaved) receptors (Notch1 and Notch2) in healthy human kidneys and in renal biopsies from a wide variety of kidney diseases. These included patients with minimal change disease, membranous nephropathy, lupus nephritis ISN/RPS classes III/IV/V, hypertensive nephrosclerosis, crescentic glomerulonephritis, tubulointerstitial fibrosis, IgA nephropathy, diabetic kidney disease, and FSGS. We found that cleaved Notch1, Notch2, and Jagged1 are expressed on podocytes in proteinuric nephropathies and their level of expression correlated with the amount of proteinuria across all disease groups. The degree of glomerulosclerosis correlated with podocyte expression of cleaved Notch1, while the severity of tubulointerstitial fibrosis and the estimated glomerular filtration rate correlated with expression of cleaved Notch1 in the tubulointerstitium. Hence, our results raise the possibility that Notch pathway activation is a common mechanism in the pathophysiology of a wide range of acquired renal diseases.

Cyclic expression of lhx2 regulates hair formation

Hair is important for thermoregulation, physical protection, sensory activity, seasonal camouflage, and social interactions. Hair is generated in hair follicles (HFs) and, following morphogenesis, HFs undergo cyclic phases of active growth (anagen), regression (catagen), and inactivity (telogen) throughout life. The transcriptional regulation of this process is not well understood. We show that the transcription factor Lhx2 is expressed in cells of the outer root sheath and a subpopulation of matrix cells during both morphogenesis and anagen. As the HFs enter telogen, expression becomes undetectable and reappears prior to initiation of anagen in the secondary hair germ. In contrast to previously published results, we find that Lhx2 is primarily expressed by precursor cells outside of the bulge region where the HF stem cells are located. This developmental, stage- and cell-specific expression suggests that Lhx2 regulates the generation and regeneration of hair. In support of this hypothesis, we show that Lhx2 is required for anagen progression and HF morphogenesis. Moreover, transgenic expression of Lhx2 in postnatal HFs is sufficient to induce anagen. Thus, our results reveal an alternative interpretation of Lhx2 function in HFs compared to previously published results, since Lhx2 is periodically expressed, primarily in precursor cells distinct from those in the bulge region, and is an essential positive regulator of hair formation.

Hair is generated in hair follicles, complex mini-organs in the skin that are devoted to this task. All hair follicles are generated during embryonic development. The hair follicles generate a new hair shaft by cycling through stages of regression, rest, and growth continuously throughout life. The length of the growth phase determines the length of the hair. The reason(s) for this complicated regulation of hair growth is not clear, but it has been suggested that it may accommodate seasonal variations in hair growth. In this study we have identified the transcription factor Lhx2 as an important regulator of hair formation. The Lhx2 gene is active during the growth phase of the hair follicle and is turned off during the resting phase. We confirm that Lhx2 is functionally involved in hair formation, since hair follicles where Lhx2 has been inactivated are unable to make hair. Moreover, activation of the Lhx2 gene in hair follicles induced the growth phase and hence hair formation. Thus, Lhx2 is an important regulator of hair growth.

Neural Wiskott-Aldrich syndrome protein modulates Wnt signaling and is required for hair follicle cycling in mice

The Rho family GTPases Cdc42 and Rac1 are critical regulators of the actin cytoskeleton and are essential for skin and hair function. Wiskott-Aldrich syndrome family proteins act downstream of these GTPases, controlling actin assembly and cytoskeletal reorganization, but their role in epithelial cells has not been characterized in vivo. Here, we used a conditional knockout approach to assess the role of neural Wiskott-Aldrich syndrome protein (N-WASP), the ubiquitously expressed Wiskott-Aldrich syndrome-like (WASL) protein, in mouse skin. We found that N-WASP deficiency in mouse skin led to severe alopecia, epidermal hyperproliferation, and ulceration, without obvious effects on epidermal differentiation and wound healing. Further analysis revealed that the observed alopecia was likely the result of a progressive and ultimately nearly complete block in hair follicle (HF) cycling by 5 months of age. N-WASP deficiency also led to abnormal proliferation of skin progenitor cells, resulting in their depletion over time. Furthermore, N-WASP deficiency in vitro and in vivo correlated with decreased GSK-3beta phosphorylation, decreased nuclear localization of beta-catenin in follicular keratinocytes, and decreased Wnt-dependent transcription. Our results indicate a critical role for N-WASP in skin function and HF cycling and identify a link between N-WASP and Wnt signaling. We therefore propose that N-WASP acts as a positive regulator of beta-catenin-dependent transcription, modulating differentiation of HF progenitor cells.

Hairless is a nuclear receptor corepressor essential for skin function

The activity of nuclear receptors is modulated by numerous coregulatory factors. Corepressors can either mediate the ability of nuclear receptors to repress transcription, or can inhibit transactivation by nuclear receptors. As we learn more about the mechanisms of transcriptional repression, the importance of repression by nuclear receptors in development and disease has become clear. The protein encoded by the mammalian Hairless (Hr) gene was shown to be a corepressor by virtue of its functional similarity to the well-established corepressors N-CoR and SMRT. Mutation of the Hr gene results in congenital hair loss in both mice and men. Investigation of Hairless function both in vitro and in mouse models in vivo has revealed a critical role in maintaining skin and hair by regulating the differentiation of epithelial stem cells, as well as a putative role in regulating gene expression via chromatin remodeling.

Wnt signalling and cancer stem cells

Intracellular signalling mediated by secreted Wnt proteins is essential for the establishment of cell fates and proper tissue patterning during embryo development and for the regulation of tissue homeostasis and stem cell function in adult tissues. Aberrant activation of Wnt signalling pathways has been directly linked to the genesis of different tumours. Here, the components and molecular mechanisms implicated in the transduction of Wnt signal, along with important results supporting a central role for this signalling pathway in stem cell function regulation and carcinogenesis will be briefl y reviewed.

Overexpression of Hr links excessive induction of Wnt signaling to Marie Unna hereditary hypotrichosis

Marie Unna hereditary hypotrichosis (MUHH) is a rare autosomal dominant hair disorder. Through the study of a mouse model, we identified a mutation in the 5'-untranslated region of the hairless (HR) gene in patients with MUHH in a Caucasian family. The corresponding mutation, named 'hairpoor', was found in mutant mice that were generated through N-ethyl-N-nitrosourea mutagenesis. Hairpoor mouse mutants display partial hair loss at an early age and progress to near alopecia, which resembles the MUHH phenotype. This mutation conferred overexpression of HR through translational derepression and, in turn, decreased the expression of Sfrp2, an inhibitor of the Wnt signaling pathway. This study indicates that the gain in function of HR also results in alopecia, as seen with the loss of function of HR, via abnormal upregulation of the Wnt signaling pathway.

mTOR mediates Wnt-induced epidermal stem cell exhaustion and aging

Epidermal integrity is a complex process established during embryogenesis and maintained throughout the organism lifespan by epithelial stem cells. Although Wnt regulates normal epithelial stem cell renewal, aberrant Wnt signaling can contribute to cancerous growth. Here, we explored the consequences of persistent expressing Wnt1 in an epidermal compartment that includes the epithelial stem cells. Surprisingly, Wnt caused the rapid growth of the hair follicles, but this was followed by epithelial cell senescence, disappearance of the epidermal stem cell compartment, and progressive hair loss. Although Wnt1 induced the activation of beta-catenin and the mTOR pathway, both hair follicle hyperproliferation and stem cell exhaustion were strictly dependent on mTOR function. These findings suggest that whereas activation of beta-catenin contributes to tumor growth, epithelial stem cells may be endowed with a protective mechanism that results in cell senescence upon the persistent stimulation of proliferative pathways that activate mTOR, ultimately suppressing tumor formation.

Aberrant activation of hedgehog signaling pathway contributes to endometrial carcinogenesis through beta-catenin

The hedgehog and Wnt signaling pathways play important roles in human cancers with possible interaction. This study aimed at analysis and correlation of the expression of Gli1, a transcriptional factor and target gene of hedgehog signaling pathway, with clinicopathological parameters and expression of beta-catenin, an important member of the Wnt pathway, in normal, hyperplastic and malignant endometrium. Immunohistochemical study on 15 normal endometrium, 14 simple and complex hyperplasia without atypia, 37 atypical complex hyperplasia and 80 endometrial cancers showed significant Gli1 overexpression and beta-catenin nuclear immunoreactivity in endometrial cancers and atypical endometrial hyperplasia when compared with normal endometrium (P<0.05). Overexpression of Gli1 in endometrial cancers correlated with well-differentiated histological grade (P<0.001), non-myometrial invasion (P=0.004) and superficial myometrial invasion (P=0.041). beta-Catenin nuclear immunoreactivity was also associated with well-differentiated histology (P=0.013). Gli1 overexpression positively correlated with beta-catenin nuclear immunoreactivity in atypical complex hyperplasia (P=0.013) and endometrial carcinoma (P=0.017). Similar Gli1 and beta-catenin protein expression pattern was observed in normal and endometrial cancer cell lines by western blotting. We further showed a complex formation between Gli1 and beta-catenin protein in endometrial cancer cell lines in an immunoprecipitation study. Ectopic overexpression of Gli1 into endometrial cancer cells led to reduced expression of beta-catenin in cell cytoplasm and increased expression of beta-catenin in the nuclei. In summary, overexpression of Gli1 was an early event in endometrial carcinogenesis. Aberrant activation of hedgehog pathway may play important roles in endometrial cancer through beta-catenin nuclear accumulation.

Wnt signaling promotes proliferation and stemness regulation of spermatogonial stem/progenitor cells

Spermatogonial stem cells (SSCs) self-renew throughout life to produce progenitor cells that are able to differentiate into spermatozoa. However, the mechanisms underlying the cell fate determination between self-renewal and differentiation have not yet been delineated. Culture conditions and growth factors essential for self-renewal and proliferation of mouse SSCs have been investigated, but no information is available related to growth factors that affect fate determination of human spermatogonia. Wnts form a large family of secreted glycoproteins, the members of which are involved in cell proliferation, differentiation, organogenesis, and cell migration. Here, we show that Wnts and their receptors Fzs are expressed in mouse spermatogonia and in the C18-4 SSC line. We demonstrate that WNT3A induces cell proliferation, morphological changes, and cell migration in C18-4 cells. Furthermore, we show that beta-catenin is activated during testis development in 21-day-old mice. In addition, our study demonstrates that WNT3A sustained adult human embryonic stem (ES)-like cells derived from human germ cells in an undifferentiated stage, expressing essential human ES cell transcription factors. These results demonstrate for the first time that Wnt/beta-catenin pathways, especially WNT3A, may play an important role in the regulation of mouse and human spermatogonia.

Loss of TGF-beta or Wnt5a results in an increase in Wnt/beta-catenin activity and redirects mammary tumour phenotype

Introduction

The tumour-suppressive effects of transforming growth factor-beta (TGF-β) are well documented; however, the mechanistic basis of these effects is not fully understood. Previously, we showed that a non-canonical member of the Wingless-related protein family, Wnt5a, is required for TGF-β-mediated effects on mammary development. Several lines of evidence support the hypothesis that Wnt5a acts as a tumour suppressor. In addition, it has been shown that Wnt5a can antagonise canonical Wnt/β-catenin signalling in various cell types. Here we test the hypothesis that TGF-β and Wnt5a can antagonise Wnt/β-catenin signalling and redirect mammary tumour phenotype. The results provide a new mechanism for the tumour-suppressive effects of TGF-β.

Methods

Wnt/β-catenin signalling was measured in tumours with altered TGF-β (dominant-negative TGF-β type II receptor, DNIIR) or Wnt5a (Wnt5a^-/-) signalling as the accumulation of nuclear β-catenin using both confocal microscopy and cell fractionation. RT-PCR was used to measure the expression of Wnt/β-catenin target genes. Sca1 expression was determined by western blot and keratin (K) 6- and K14-positive populations were determined by immunohistochemistry.

Results

Loss of TGF-β or Wnt5a signalling resulted in stabilisation of nuclear β-catenin and expression of Wnt/β-catenin target genes suggesting that TGF-β and Wnt5a act to inhibit Wnt/β-catenin signalling in mammary epithelium. Increased expression of Sca-1 was observed in developing DNIIR and Wnt5a-/- mammary glands. DNIIR and Wnt5a-/- tumours demonstrated an expanded population of K6- and K14-expressing cells typically seen in Wnt/β-catenin-induced tumours.

Conclusions

The key findings here are that: TGF-β and Wnt5a regulate Wnt/β-catenin activity; and loss of TGF-β and Wnt5a redirect the phenotype of tumours so that they resemble tumours induced by activation of Wnt/β-catenin. The findings suggest a new mechanism for the tumour-suppressive effects of TGF-β.

Scaffold Attachment Factor B1 (SAFB1) heterozygosity does not influence Wnt-1 or DMBA-induced tumorigenesis

Background

Scaffold Attachment Factor B1 (SAFB1) is a multifunctional protein which has been implicated in breast cancer previously. We recently generated SAFB1 knockout mice (SAFB1^-/-), but pleiotropic phenotypes including high lethality, dwarfism associated with low IGF-I levels, and infertility and subfertility in male and female mice, respectively, do not allow for straightforward tumorigenesis studies in these mice. Therefore, we asked whether SAFB1 heterozygosity would influence tumor development and progression in MMTV-Wnt-1 oncomice or DMBA induced tumorigenicity, in a manner consistent with haploinsufficiency of the remaining allele.

Methods

We crossed female SAFB1^+/- (C57B6/129) mice with male MMTV-Wnt-1 (C57B6/SJL) mice to obtain SAFB1^+/+/Wnt-1, SAFB1^+/-/Wnt-1, and SAFB1^+/- mice. For the chemical induced tumorigenesis study we treated 8 weeks old SAFB1^+/- and SAFB^+/+ BALB/c mice with 1 mg DMBA once per week for 6 weeks. Animals were monitored for tumor incidence and tumor growth. Tumors were characterized by performing H&E, and by staining for markers of proliferation and apoptosis.

Results

We did not detect significant differences in tumor incidence and growth between SAFB1^+/+/Wnt-1 and SAFB1^+/-/Wnt-1 mice, and between DMBA-treated SAFB1^+/+ and SAFB1^+/-mice. Histological evaluation of tumors showed that SAFB1 heterozygosity did not lead to changes in proliferation or apoptosis. There were, however, significant differences in the distribution of tumor histologies with an increase in papillary and cribriform tumors, and a decrease in squamous tumors in the SAFB1^+/-/Wnt-1 compared to the SAFB1^+/+/Wnt-1 tumors. Of note, DMBA treatment resulted in shortened survival of SAFB1^+/- mice compared to their wildtype littermates, however this trend did not reach statistical significance.

Conclusion

Our data show that SAFB1 heterozygosity does not influence Wnt-1 or DMBA-induced mammary tumorigenesis.

The stem cell niche

Virtually every tissue of the adult organism maintains a population of putatively slowly-cycling stem cells that maintain homeostasis of the tissue and respond to injury when challenged. These cells are regulated and supported by the surrounding microenvironment, referred to as the stem cell 'niche'. The niche includes all cellular and non-cellular components that interact in order to control the adult stem cell, and these interactions can often be broken down into one of two major mechanistic categories--physical contact and diffusible factors. The niche has been studied directly and indirectly in a number of adult stem cell systems. Herein, we will first focus on the most well-understood niches supporting the germline stem cells in the lower organisms Caenorhabditis elegans and Drosophila melanogaster before concentrating on the more complex, less well-understood mammalian niches supporting the neural, epidermal, haematopoietic and intestinal stem cells.

Dynamic complexes of A-type lamins and emerin influence adipogenic capacity of the cell via nucleocytoplasmic distribution of beta-catenin

It is well documented that adipogenic differentiation of the cell is associated with downregulation of Wnt/beta-catenin signalling. Using preadipocytes and dermal fibroblasts, we have found that activation of the adipogenic program was associated with marked changes in the expression of nuclear beta-catenin-interacting partners, emerin and lamins A/C, to influence expression and activation of peroxisome proliferators-activated receptors gamma (PPARgamma). In addition, silencing of protein expression with siRNA revealed that beta-catenin and emerin influenced each other's levels of expression and the onset of adipogenesis, suggesting that changes in the expression of nuclear lamina proteins were intimately linked to the stability of beta-catenin. By contrast, dermal fibroblasts, which are emerin null, demonstrated increased nuclear accumulation of stable beta-catenin and constant lamin expression. This was also associated with an unusual adipogenic capacity of the cells, with adipogenesis occurring in the presence of activated beta-catenin but declining upon silencing of the protein expression with siRNA. We propose that the process of adipogenesis is affected by a dynamic link between complexes of emerin and lamins A/C at the nuclear envelope and nucleocytoplasmic distribution of beta-catenin, to influence cellular plasticity and differentiation.

Loss-of-function mutations of an inhibitory upstream ORF in the human hairless transcript cause Marie Unna hereditary hypotrichosis

Marie Unna hereditary hypotrichosis (MUHH) is an autosomal dominant form of genetic hair loss. In a large Chinese family carrying MUHH, we identified a pathogenic initiation codon mutation in U2HR, an inhibitory upstream ORF in the 5' UTR of the gene encoding the human hairless homolog (HR). U2HR is predicted to encode a 34-amino acid peptide that is highly conserved among mammals. In 18 more families from different ancestral groups, we identified a range of defects in U2HR, including loss of initiation, delayed termination codon and nonsense and missense mutations. Functional analysis showed that these classes of mutations all resulted in increased translation of the main HR physiological ORF. Our results establish the link between MUHH and U2HR, show that fine-tuning of HR protein levels is important in control of hair growth, and identify a potential mechanism for preventing hair loss or promoting hair removal.

Mesodermal fate decisions of a stem cell: the Wnt switch

Stem cells are a powerful resource for cell-based transplantation therapies in osteodegenerative disorders, but before some kinds of stem cells can be applied clinically, several aspects of their expansion and differentiation need to be better controlled. Wnt molecules and members of the Wnt signaling cascade have been ascribed a role in both these processes in vitro as well as normal development in vivo. However some results are controversial. In this review we will present the hypothesis that both canonical and non-canonical signaling are involved in mesenchymal cell fate regulation, such as adipogenesis, chondrogenesis and osteogenesis, and that in vitro it is a timely switch between the two that specifies the identity of the differentiating cell. We will specifically focus on the in vitro differentiation of adipocytes, chondrocytes and osteoblasts contrasting embryonic and mesenchymal stem cells as well as the role of Wnts in mesenchymal fate specification during embryogenesis.

Transcriptional profiling of putative human epithelial stem cells

Background

Human interfollicular epidermis is sustained by the proliferation of stem cells and their progeny, transient amplifying cells. Molecular characterization of these two cell populations is essential for better understanding of self renewal, differentiation and mechanisms of skin pathogenesis. The purpose of this study was to obtain gene expression profiles of alpha 6⁺/MHCI⁺, transient amplifying cells and alpha 6⁺/MHCI^-, putative stem cells, and to compare them with existing data bases of gene expression profiles of hair follicle stem cells. The expression of Major Histocompatibility Complex (MHC) class I, previously shown to be absent in stem cells in several tissues, and alpha 6 integrin were used to isolate MHCI positive basal cells, and MHCI low/negative basal cells.

Results

Transcriptional profiles of the two cell populations were determined and comparisons made with published data for hair follicle stem cell gene expression profiles. We demonstrate that presumptive interfollicular stem cells, alpha 6⁺/MHCI^- cells, are enriched in messenger RNAs encoding surface receptors, cell adhesion molecules, extracellular matrix proteins, transcripts encoding members of IFN-alpha family proteins and components of IFN signaling, but contain lower levels of transcripts encoding proteins which take part in energy metabolism, cell cycle, ribosome biosynthesis, splicing, protein translation, degradation, DNA replication, repair, and chromosome remodeling. Furthermore, our data indicate that the cell signaling pathways Notch1 and NF-κB are downregulated/inhibited in MHC negative basal cells.

Conclusion

This study demonstrates that alpha 6⁺/MHCI^- cells have additional characteristics attributed to stem cells. Moreover, the transcription profile of alpha 6⁺/MHCI^- cells shows similarities to transcription profiles of mouse hair follicle bulge cells known to be enriched for stem cells. Collectively, our data suggests that alpha 6⁺/MHCI^- cells may be enriched for stem cells. This study is the first comprehensive gene expression profile of putative human epithelial stem cells and their progeny that were isolated directly from neonatal foreskin tissue. Our study is important for understanding self renewal and differentiation of epidermal stem cells, and for elucidating signaling pathways involved in those processes. The generated data base may serve those working with other human epithelial tissue progenitors.

Expression profiles of Wnt genes during neural differentiation of mouse embryonic stem cells

The Wnt family of secreted signaling proteins regulates many aspects of animal development and the behavior of several types of stem cells, including embryonic stem (ES) cells. Activation of canonical Wnt signaling has been shown to either inhibit or promote the differentiation of ES cells into neurons, depending on the stage of differentiation. Here, we describe the expression of all 19 mouse Wnt genes during this process. Using the well-established retinoic acid induction protocol we found that all Wnt genes except Wnt8b are expressed as ES cells differentiate into neurons, many of them in dynamic patterns. The expression pattern of 12 Wnt genes was analyzed quantitatively at 2-day intervals throughout neural differentiation, showing that multiple Wnt genes are expressed at each stage. A large proportion of these, including both canonical and noncanonical Wnts, are expressed at highest levels during later stages of differentiation. The complexity of the patterns observed indicates that disentangling specific roles for individual Wnt genes in the differentiation process will be a significant challenge.

Pygopus and the Wnt signaling pathway: a diverse set of connections

Identification of Pygopus in Drosophila as a dedicated component of the Wg (fly homolog of mammalian Wnt) signaling cascade initiated many inquiries into the mechanism of its function. Surprisingly, the nearly exclusive role for Pygopus in Wg signal transduction in flies is not seen in mice, where Pygopus appears to have both Wnt-related and Wnt-independent functions. This review addresses the initial findings of Pygopus as a Wg/Wnt co-activator in light of recent data from both fly and mammalian studies. We compare and contrast the developmental phenotypes of pygopus mutants to those characterized for known Wg/Wnt transducers and explore the data regarding a role for mammalian Pygopus 2 in tumorigenesis. We further analyze the roles of the two conserved domains of Pygopus proteins in transcription, and propose a model for the molecular mechanism of Pygopus function in both Wg/Wnt signaling and Wnt-independent transcriptional regulation.

APC mutant zebrafish uncover a changing temporal requirement for wnt signaling in liver development

Developmental signaling pathways hold the keys to unlocking the promise of adult tissue regeneration, and to inhibiting carcinogenesis. Patients with mutations in the Adenomatous Polyposis Coli (APC) gene are at increased risk of developing hepatoblastoma, an embryonal form of liver cancer, suggesting that Wnt affects hepatic progenitor cells. To elucidate the role of APC loss and enhanced Wnt activity in liver development, we examined APC mutant and wnt inducible transgenic zebrafish. APC(+/-) embryos developed enlarged livers through biased induction of hepatic gene programs and increased proliferation. Conversely, APC(-/-) embryos formed no livers. Blastula transplantations determined that the effects of APC loss were cell autonomous. Induction of wnt modulators confirmed biphasic consequences of wnt activation: endodermal pattern formation and gene expression required suppression of wnt signaling in early somitogenesis; later, increased wnt activity altered endodermal fate by enhancing liver growth at the expense of pancreas formation; these effects persisted into the larval stage. In adult APC(+/-) zebrafish, increased wnt activity significantly accelerated liver regeneration after partial hepatectomy. Similarly, liver regeneration was significantly enhanced in APC(Min/+) mice, indicating the conserved effect of Wnt pathway activation in liver regeneration across vertebrate species. These studies reveal an important and time-dependent role for wnt signaling during liver development and regeneration.

Down-regulation of Wnt signaling could promote bone marrow-derived mesenchymal stem cells to differentiate into hepatocytes

Bone marrow-derived mesenchymal stem cells (BMSCs) have been demonstrated to be able to differentiate into hepatocytes, but the precise mechanisms controlling this process are unclear. Our aim is try to explore the role of Wnt signaling on the differentiation of BMSCs into hepatocytes. Our study demonstrated that BMSCs could successfully differentiate into hepatocytes under in vitro induction of the tissue extract of damaged liver. The mRNA level of Wnt-1, Wnt-5a, Frizzled1, DSH (disheveled), GSK-3beta (glycogen synthase kinase 3 beta) and beta-catenin on day 21 when the differentiation direction was determined, was lower than that on days 0, 7, and 11. Furthermore, blocking Wnt-1 signaling by treating BMSCs with Dkk1 could induce BMSCs to express albumin earlier and up-regulation of Wnt signaling by treating BMSCs with Wnt-1 could inhibit BMSCs to differentiate into hepatocytes. Above results indicated that inhibition on Wnt signaling can promote BMSCs to differentiate into hepatocytes.

Prospective isolation and functional analysis of stem and differentiated cells from the mouse mammary gland

Prospective isolation and in vitro and in vivo analysis of primary mouse mammary epithelial cells has been used to separate cell subpopulations and identify stem, progenitor and differentiated cell compartments. Progress has been made from cell separation strategies based on a single marker of the luminal epithelial or myoepithelial compartments to use of markers that allow simultaneous isolation of non-epithelial, basal/myoepithelial and luminal epithelial cells. Transplant analysis has shown that mammary stem cells are found in the basal/myoepithelial compartment, whereas in vitro colony progenitors are found in the luminal compartment. A basal population enriched for stem cell activity can be purified from the myoepithelial cells and the most recent data shows that the luminal population can now be prospectively split into estrogen receptor positive and estrogen receptor negative cells. Future work aims to molecularly characterise these populations to identify new drug targets, which can be used to specifically kill breast cancer stem cells.

PAR1 specifies ciliated cells in vertebrate ectoderm downstream of aPKC

Partitioning-defective 1 (PAR1) and atypical protein kinase C (aPKC) are conserved serine/threonine protein kinases implicated in the establishment of cell polarity in many species from yeast to humans. Here we investigate the roles of these protein kinases in cell fate determination in Xenopus epidermis. Early asymmetric cell divisions at blastula and gastrula stages give rise to the superficial (apical) and the deep (basal) cell layers of epidermal ectoderm. These two layers consist of cells with different intrinsic developmental potential, including superficial epidermal cells and deep ciliated cells. Our gain- and loss-of-function studies demonstrate that aPKC inhibits ciliated cell differentiation in Xenopus ectoderm and promotes superficial cell fates. We find that the crucial molecular substrate for aPKC is PAR1, which is localized in a complementary domain in superficial ectoderm cells. We show that PAR1 acts downstream of aPKC and is sufficient to stimulate ciliated cell differentiation and inhibit superficial epidermal cell fates. Our results suggest that aPKC and PAR1 function sequentially in a conserved molecular pathway that links apical-basal cell polarity to Notch signaling and cell fate determination. The observed patterning mechanism may operate in a wide range of epithelial tissues in many species.

Wnt/beta-catenin signaling in murine hepatic transit amplifying progenitor cells

BACKGROUND & AIMS

Oval cells are postnatal hepatic progenitors with high proliferative potential and bipotent differentiation ability to become hepatocytes and cholangiocytes. Because Wnt/beta-catenin signaling is a known regulatory pathway for liver development and regeneration, we studied the role of Wnt signaling in oval cells using a mouse model of chronic liver injury.

METHODS

A 3,5-diethoxycarbonyl-1,4-dihydrocollidine (DDC)-enriched diet was used to stimulate oval cell proliferation. Livers were harvested for histologic analysis and determination of Wnt family gene expression by quantitative reverse transcription-polymerase chain reaction and in situ hybridization. The transgenic beta-catenin reporter mouse (TOPGAL) was use to confirm canonical Wnt/beta-catenin signal transduction in proliferating oval cells within atypical ductal proliferations (ADPs). Confocal fluorescence microscopy and immunohistochemistry was used to confirm colocalization of beta-catenin with the oval cell antigen A-6.

RESULTS

Several Wnt ligands were significantly induced in the liver of DDC-fed mice and localized to proliferating cells in and adjacent to the ADPs. Oval cells isolated from DDC-fed mouse livers showed the presence of active beta-catenin in the nucleus along with cell-cycle entry in response to purified Wnt3a in vitro. Moreover, Wnt3a-induced beta-catenin/T-cell factor/lymphoid enhancer factor (TCF/LEF) transcriptional activation was quantified by TCF/LEF luciferase reporter assays.

CONCLUSIONS

From these data, we conclude that oval cells respond to Wnt ligands (Wnt3a) in vitro with an increase in amino-terminus dephosphorylated beta-catenin and cell-cycle entry and that canonical Wnt/beta-catenin/TCF signaling is active in proliferating facultative hepatic progenitor cells in vivo. These findings may lend insight to the consequences of increased canonical Wnt signaling during periods of chronic liver injury.

Increased nuclear beta-catenin in suprabasal involved psoriatic epidermis

BACKGROUND

Psoriasis is a common inflammatory skin disease characterized by abnormal keratinocyte proliferation and differentiation, increased angiogenesis and inflammation. There is evidence that some keratinocyte differentiation events are controlled by changes in cell-cell adhesion. beta-catenin is a 94-kDa protein which has a dual function as a component of intercellular adherens junctions and also as a transcription factor as part of the Wnt signalling pathway. beta-catenin is not required for keratinocyte proliferation but has been shown to regulate keratinocyte stem cells and hair follicle morphogenesis.

OBJECTIVES

To investigate the distribution and function of beta-catenin in involved psoriatic epidermis and in epidermal keratinocytes.

METHODS

Biopsies were obtained from patients with psoriasis and from normal controls. The distribution of beta-catenin was investigated using antibodies to both total and unphosphorylated active beta-catenin. Luciferase assays were used to measure transcriptional activation of transglutaminase 1 (TGase 1) and involucrin and to investigate the functional role of beta-catenin in interfollicular keratinocytes.

RESULTS

Increased nuclear beta-catenin was seen in lesional suprabasal psoriatic epidermis compared with uninvolved or normal skin. Increased active unphosphorylated beta-catenin was also detected within the differentiating compartment of involved psoriatic epidermis. Expression of TGase 1 overlapped with beta-catenin in suprabasal lesional psoriasis. The TGase 1 promoter was positively regulated by activated beta-catenin and by the glycogen synthase kinase binding protein, suggesting that beta-catenin and glycogen synthase kinase 3beta may regulate TGase 1 expression.

CONCLUSIONS

This is the first report to convincingly demonstrate increased beta-catenin in involved psoriasis and to implicate beta-catenin in the regulation of TGase 1. This evidence suggests a role for beta-catenin signalling in regulating keratinocyte differentiation in interfollicular skin in addition to previously reported functions in stem cell fate determination, hair follicle regulation and skin tumorigenesis.

Bioengineering skin using mechanisms of regeneration and repair

The development and use of artificial skin in treating acute and chronic wounds has, over the last 30 years, advanced from a scientific concept to a series of commercially viable products. Many important clinical milestones have been reached and the number of artificial skin substitutes licensed for clinical use is growing, but they have yet to replace the current "gold standard" of an autologous skin graft. Currently available skin substitutes often suffer from a range of problems that include poor integration (which in many cases is a direct result of inadequate vascularisation), scarring at the graft margins and a complete lack of differentiated structures. The ultimate goal for skin tissue engineers is to regenerate skin such that the complete structural and functional properties of the wounded area are restored to the levels before injury. New synthetic biomaterials are constantly being developed that may enable control over wound repair and regeneration mechanisms by manipulating cell adhesion, growth and differentiation and biomechanics for optimal tissue development. In this review, the clinical developments in skin bioengineering are discussed, from conception through to the development of clinically viable products. Central to the discussion is the development of the next generation of skin replacement therapy, the success of which is likely to be underpinned with our knowledge of wound repair and regeneration.

Stem cell niches in mammals

Stem cells safeguard tissue homeostasis and guarantee tissue repair throughout life. The decision between self-renewal and differentiation is influenced by a specialized microenvironment called stem cell niche. Physical and molecular interactions with niche cells and orientation of the cleavage plane during stem cell mitosis control the balance between symmetric and asymmetric division of stem cells. Here we highlight recent progress made on the anatomical and molecular characterization of mammalian stem cell niches, focusing particularly on bone marrow, tooth and hair follicle. The knowledge of the regulation of stem cells within their niches in health and disease will be instrumental to develop novel therapies that target stem cell niches to achieve tissue repair and re-establish tissue homeostasis.

Loss of alpha6 integrins in keratinocytes leads to an increase in TGFbeta and AP1 signaling and in expression of differentiation genes

Mice lacking the alpha6 integrin chain die at birth with severe skin blistering. To further study the function of alpha6 integrin in skin, we generated conditionally immortalized cell lines from the epidermis of wild-type and alpha6 deficient mouse embryos. Mutant cells presented a decreased adhesion on laminin 5, the major component of the basement membrane in the skin, and on laminins 10/11 and 2. A DNA array analysis revealed alterations in the expression of extracellular matrix (ECM) components including laminin 5, cytoskeletal elements, but also membrane receptors like the hemidesmosomal components integrin beta4 and collagen XVII, or growth factors and signaling molecules of the TGFbeta, EGF, and Wnt pathways. Finally, an increase of several epidermal differentiation markers was observed in cells and tissue at the protein level. Further examination of the mutant tissue revealed alterations in the filaggrin signal. These differences may be linked to an upregulation of the TGFbeta and the Jun/Fos pathways in mutant keratinocytes. These results are in favor of a role for integrin alpha6beta4 in the maintenance of basal keratinocyte properties and epidermal homeostasis.

Bi-compartmental communication contributes to the opposite proliferative behavior of Notch1-deficient hair follicle and epidermal keratinocytes

Notch1-deficient epidermal keratinocytes become progressively hyperplastic and eventually produce tumors. By contrast, Notch1-deficient hair matrix keratinocytes have lower mitotic rates, resulting in smaller follicles with fewer cells. In addition, the ratio of melanocytes to keratinocytes is greatly reduced in hair follicles. Investigation into the underlying mechanism for these phenotypes revealed significant changes in the Kit, Tgfbeta and insulin-like growth factor (IGF) signaling pathways, which have not been previously shown to be downstream of Notch signaling. The level of Kitl (Scf) mRNA produced by Notch1-deficient follicular keratinocytes was reduced when compared with wild type, resulting in a decline in melanocyte population. Tgfbeta ligands were elevated in Notch1-deficient keratinocytes, which correlated with elevated expression of several targets, including the diffusible IGF antagonist Igfbp3 in the dermal papilla. Diffusible stromal targets remained elevated in the absence of epithelial Tgfbeta receptors, consistent with paracrine Tgfbeta signaling. Overexpression of Igf1 in the keratinocyte reversed the phenotype, as expected if Notch1 loss altered the IGF/insulin-like growth factor binding protein (IGFBP) balance. Conversely, epidermal keratinocytes contained less stromal Igfbp4 and might thus be primed to experience an increase in IGF signaling as animals age. These results suggest that Notch1 participates in a bi-compartmental signaling network that controls homeostasis, follicular proliferation rates and melanocyte population within the skin.

Immunolocalization of enzymes, binding proteins, and receptors sufficient for retinoic acid synthesis and signaling during the hair cycle

Retinoic acid (RA) is essential for maintenance of most epithelial tissues. One RA biosynthesis pathway consists of cellular retinol-binding protein (Crbp), retinol dehydrogenase (Dhrs9/eRoldh), retinal dehydrogenase 1-3 (Aldh1a1-3), and cellular RA-binding protein 2 (Crabp2). Previously, we localized Aldh1a2 and Aldh1a3 to both epithelial and mesenchymal cells within the hair follicle throughout the hair cycle. This study expands that observation by examining the complete pathway of RA biosynthesis and signaling via RA receptors alpha, beta, and gamma by immunohistochemistry in C57BL/6J mice wax-stripped to initiate and synchronize the cycle. This pathway of RA biosynthesis and signaling localized to the majority of layers of the hair follicle, sebaceous gland, and interfollicular epidermis in a hair cycle-dependent manner, suggesting that RA biosynthesis within the hair follicle is regulated in both a spatial and temporal manner. This localization pattern also revealed insights into epithelial-mesenchymal interactions and differentiation state differences within the RA biosynthesis and signaling pathway, as well as novel observations on nuclear versus cytoplasmic localization of Crabp2 and RA receptors. This complex pattern of RA biosynthesis and signaling identified by immunolocalization suggests that endogenous RA regulates specific aspects of hair follicle growth, differentiation, and cycling.

Tissue engineering of replacement skin: the crossroads of biomaterials, wound healing, embryonic development, stem cells and regeneration

Advanced therapies combating acute and chronic skin wounds are likely to be brought about using our knowledge of regenerative medicine coupled with appropriately tissue-engineered skin substitutes. At the present time, there are no models of an artificial skin that completely replicate normal uninjured skin. Natural biopolymers such as collagen and fibronectin have been investigated as potential sources of biomaterial to which cells can attach. The first generation of degradable polymers used in tissue engineering were adapted from other surgical uses and have drawbacks in terms of mechanical and degradation properties. This has led to the development of synthetic degradable gels primarily as a way to deliver cells and/or molecules in situ, the so-called smart matrix technology. Tissue or organ repair is usually accompanied by fibrotic reactions that result in the production of a scar. Certain mammalian tissues, however, have a capacity for complete regeneration without scarring; good examples include embryonic or foetal skin and the ear of the MRL/MpJ mouse. Investigations of these model systems reveal that in order to achieve such complete regeneration, the inflammatory response is altered such that the extent of fibrosis and scarring is diminished. From studies on the limited examples of mammalian regeneration, it may also be possible to exploit such models to further clarify the regenerative process. The challenge is to identify the factors and cytokines expressed during regeneration and incorporate them to create a smart matrix for use in a skin equivalent. Recent advances in the use of DNA microarray and proteomic technology are likely to aid the identification of such molecules. This, coupled with recent advances in non-viral gene delivery and stem cell technologies, may also contribute to novel approaches that would generate a skin replacement whose materials technology was based not only upon intelligent design, but also upon the molecules involved in the process of regeneration.

Hair follicle stem cells: walking the maze

The discovery of epithelial stem cells (eSCs) in the bulge region of the outer root sheath of hair follicles in mice and man has encouraged research into utilizing the hair follicle as a therapeutic source of stem cells (SCs) for regenerative medicine, and has called attention to the hair follicle as a highly instructive model system for SC biology. Under physiological circumstances, bulge eSCs serve as cell pool for the cyclic regeneration of the anagen hair bulb, while they can also regenerate the sebaceous gland and the epidermis after injury. More recently, melanocyte SCs, nestin+, mesenchymal and additional, as yet ill-defined "stem cell" populations, have also been identified in or immediately adjacent to the hair follicle epithelium, including in the specialized hair follicle mesenchyme (connective tissue sheath), which is crucial to wound healing. Thus the hair follicle and its adjacent tissue environment contain unipotent, multipotent, and possibly even pluripotent SC populations of different developmental origin. It provides an ideal model system for the study of central issues in SC biology such as plasticity and SC niches, and for the identification of reliable, specific SC markers, which distinguish them from their immediate progeny (e.g. transient amplifying cells). The current review attempts to provide some guidance in this growing maze of hair follicle-associated SCs and their progeny, critically reviews potential or claimed hair follicle SC markers, highlights related differences between murine and human hair follicles, and defines major unanswered questions in this rapidly advancing field.

Regeneration of squamous epithelia from stem cells of cultured grafts

The only cultured cell types extensively used for tissue regeneration are the keratinocyte and the chondrocyte. Cultured autologous keratinocytes derived from the epidermis have been used for many years to produce grafts that regenerate an epidermis over a full-thickness wound, such as a third-degree burn. But there have been many failures of engraftment, and in the absence of criteria for the quality of the cultures, the causes of failure cannot be analyzed. It has become clear that the essential feature of the graft is the presence of an adequate number of stem cells. This article describes the criteria for estimating that number. Advances in graft preparation, combining better preservation of stem cells with ease of application of the graft, are also described. These improvements have been applied to cultures of ocular limbal cells, which contain the keratinocyte stem cells of the corneal epithelium. Cultures meeting the criteria of stem cell number have been grafted to 116 patients suffering from chemical destruction of the limbus. The procedure has been highly successful in the alleviation of suffering and the restoration of vision.

Vitamin D receptor is essential for normal keratinocyte stem cell function

The major physiological role of the vitamin D receptor (VDR) is the maintenance of mineral ion homeostasis. Mutation of the VDR, in humans and mice, results in alopecia. Unlike the effects of the VDR on mineral ion homeostasis, the actions of the VDR that prevent alopecia are ligand-independent. Although absence of the VDR does not prevent the development of a keratinocyte stem cell niche in the bulge region of the hair follicle, it results in an inability of these stem cells to regenerate the lower portion of the hair follicle in vivo and impairs keratinocyte stem cell colony formation in vitro. VDR ablation is associated with a gradual decrease in keratinocyte stem cells, accompanied by an increase in sebaceous activity, a phenotype analogous to that seen with impaired canonical Wnt signaling. Transient gene expression assays demonstrate that the cooperative transcriptional effects of beta-catenin and Lef1 are abolished in keratinocytes isolated from VDR-null mice, revealing a role for the unliganded VDR in canonical Wnt signaling. Thus, absence of the VDR impairs canonical Wnt signaling in keratinocytes and leads to the development of alopecia due to a defect in keratinocyte stem cells.

Novel roles for APC family members and Wingless/Wnt signaling during Drosophila brain development

Construction of the brain is one of the most complex developmental challenges. Wnt signals shape all tissues, including the brain, and the tumor suppressor adenomatous polyposis coli (APC) is a key negative regulator of Wnt/Wingless (Wg) signaling. We carried out the first assessment of the role of APC proteins in brain development, simultaneously inactivating both APC1 and APC2 in clones of cells in the Drosophila larval optic lobe. We focused on the medulla, where epithelial neural progenitors shift from symmetric to asymmetric divisions across the lateral-medial axis. Loss of both APCs triggers dramatic defects in optic lobe development. Double mutant cells segregate from wild-type neighbors, while double mutant neurons form tangled axonal knots, suggesting changes in cell adhesion. Strikingly, phenotypes are graded along the anterior-posterior axis. Activation of Wg signaling downstream of APC mimics these phenotypes, a dominant-negative TCF blocks them, and a known Wg target, decapentaplegic, is activated in double mutant clones, strongly suggesting that the phenotypes result from activated Wg signaling. We also explored the roles of classic cadherins in differential adhesion. Finally, we propose a model suggesting that Wg signaling regulates fine scale cell fates along the anterior-posterior axis, in part by creating an adhesion gradient and consider possible alternate explanations for our observations.

Viable mice with compound mutations in the Wnt/Dvl pathway antagonists nkd1 and nkd2

Gradients of Wnt/beta-catenin signaling coordinate development and physiological homeostasis in metazoan animals. Proper embryonic development of the fruit fly Drosophila melanogaster requires the Naked cuticle (Nkd) protein to attenuate a gradient of Wnt/beta-catenin signaling across each segmental anlage. Nkd inhibits Wnt signaling by binding the intracellular protein Dishevelled (Dsh). Mice and humans have two nkd homologs, nkd1 and nkd2, whose encoded proteins can bind Dsh homologs (the Dvl proteins) and inhibit Wnt signaling. To determine whether nkd genes are necessary for murine development, we replaced nkd exons that encode Dvl-binding sequences with IRES-lacZ/neomycin cassettes. Mutants homozygous for each nkd(lacZ) allele are viable with slightly reduced mean litter sizes. Surprisingly, double-knockout mice are viable, with subtle alterations in cranial bone morphology that are reminiscent of mutation in another Wnt/beta-catenin antagonist, axin2. Our data show that nkd function in the mouse is dispensable for embryonic development.

Delayed healing of chronic leg ulcers can result from impaired trafficking of bone marrow-derived precursors of keratinocytes to the skin

In this paper, it is hypothesized that in chronic wounds the process of homing of bone marrow-derived precursors of keratinocytes is disturbed, and that the interaction between cutaneous T-cell attracting chemokine (CTACK/CCL27) and soluble P-selectin glycoprotein ligand-1 (PSGL-1) can be the cause of this impairment. Several studies have revealed that bone marrow-derived cells (BMDC) trans-differentiate into various cellular lineages, and probably they participate also in healing of wounded skin. Recent studies have demonstrated that BMDC can engraft into the epidermis, and probably they do not engraft into epidermis as keratinocyte stem cells, but rather as transient amplifying cells. So, bone marrow-derived keratinocytes build provisional epidermal layer, and later they are replaced by keratinocytes migrating from surrounding skin. Probably after injury BMDC are recruited by pro-inflammatory cytokines, like granulocyte-macrophage colony stimulating factor. Further homing to the skin is mediated by CTACK/CCL27. This chemokine is exclusively secreted by keratinocytes. In chronic wounds the recruitment of BMDC seems to be impaired. Inhibition of CTACK/CCL27 by as yet not determined factor could be the cause of delayed healing. PSGL-1 appears to be a good candidate for such inhibiting factor. PSGL-1 is expressed by several populations of leukocytes, and can be released from surface of activated neutrophils. It was demonstrated that soluble PSGL-1 binds CTACK/CCL27, and inhibits chemotaxis mediated by this chemokine. Because there are many activated neutrophils in the wound, it should be expected that wound exudate contains large amount of soluble PSGL-1. Thus, CTACK/CCL27 in the wound would be inhibited, and homing of bone marrow-derived precursors of keratinocytes would be disturbed. If this interaction were found to be the main cause of wound chronicity, above-mentioned molecules could be the potential targets for pharmaceutical agents.

Lhx2--decisive role in epithelial stem cell maintenance, or just the "tip of the iceberg"?

Stem cell self renewal, maintenance and differentiation are influenced by the convergence of intrinsic cellular signals and extrinsic microenvironmental cues from the surrounding stem cell niche. However, the specific signals involved are often still poorly understood. This is also true for skin epithelial stem cells. Recently, by transcriptionally profiling of embryonic hair progenitors in mice, Rhee et al. have managed to define how murine hair follicle epithelial stem cells are specified and maintained in an undifferentiated state. These authors have identified Lhx2 as a transcription factor functionally positioned downstream of signals necessary to specify hair follicle stem cells such as p63 or NFkappaB, but upstream of signals like Wnt/beta-catenin, Bmp or Shh that are required to drive activated stem cells via the production of transient amplifying cells into terminal differentiation.

Id2 gene-targeted crosstalk between Wnt and retinoid signaling regulates proliferation in human keratinocytes

We investigated the effect of all-trans-retinoic acid (atRA) on proliferation in several human skin cell lines and found that antiproliferative potency of atRA correlated with the endogenous activity of canonical Wnt signaling. In HaCaT keratinocytes, we found that atRA significantly suppressed the expression of Id2, a member of the inhibitor of differentiation family of transcription factors that regulate cell growth and differentiation. However, no apparent change in the expression of other Wnt targets, like c-Myc or cyclin D1, was observed. Retinoid-induced Id2 gene suppression was associated with decreased levels of histone H3 and H4 acetylation and histone H3 Lys-4 methylation, and with recruitment of the LSD1 demethylase at the Wnt-response element (WRE) (TCF/LEF-binding site), in the Id2 gene promoter. None of such changes was detected at the WRE of c-Myc and cyclin D1 gene promoters. Inhibition of Id2 by short interfering RNA (siRNA) had a similar effect on the proliferation of HaCaT cells as exposure to atRA, whereas anti-beta-catenin siRNA significantly inhibited its antiproliferative effect. These data suggest that downregulation of Id2 gene expression through transcriptional convergence between Wnt and retinoid signaling pathways underlies the antiproliferative effect of retinoids in keratinocytes, and provide evidence of gene-targeted crosstalk between signaling pathways.

Wnt/beta-catenin signaling in cancer stemness and malignant behavior

Stem cells are defined by their intrinsic capacity to self-renew and differentiate. Cancer stem cells retain both these features but have lost homeostatic mechanisms which maintain normal cell numbers. The canonical Wnt/beta-catenin signaling pathway plays a central role in modulating the delicate balance between stemness and differentiation in several adult stem cell niches such as the hair follicles in the skin, the mammary gland, and the intestinal crypt. Accordingly, constitutive Wnt signaling activation, resulting from mutations in genes encoding its downstream components, underlies tumorigenesis in these tissues. In the majority of sporadic colorectal cancer cases, the rate-limiting event is either loss of APC function or oncogenic beta-catenin mutations. However, although the presence of these initiating mutations would predict nuclear beta-catenin accumulation throughout the tumor mass, heterogeneous intracellular distributions of this key Wnt signaling molecule are observed within primary tumors and their metastases. In particular, tumor cells located at the invasive front and those migrating into the adjacent stromal tissues show nuclear beta-catenin staining. Hence, different levels of Wnt signaling activity reflect tumor heterogeneity and are likely to account for distinct cellular activities such as proliferation and epithelial-mesenchymal transitions, which prompt tumor growth and malignant behavior, respectively. Several intrinsic (cell-autonomous and/or autocrine) and extrinsic (paracrine, derived from the tumor microenvironment) factors may explain this heterogeneity of Wnt/beta-catenin signaling activity within the tumor mass.

Gli1 acts through Snail and E-cadherin to promote nuclear signaling by beta-catenin

The Hedgehog pathway transcription factor Gli1 induces transformation of epithelial cells via induction of Snail, a repressor of E-cadherin (E-cad). E-cad is normally complexed with beta-catenin at the cell membrane. Loss of E-cad during developmental epithelial-mesenchymal transitions can switch beta-catenin from its role at adherens junctions to its role in nuclear transcription. During tumorigenesis it is unclear which pathways trigger this switch. In the current study, gain- and loss-of-function approaches identified E-cad as a selective inhibitor of transformation by Gli1, and Snail knockdown was rescued by downregulation of E-cad. Gli1 induced relocalization of beta-catenin from the cell membrane to the nucleus. The ability of wild-type or mutant alleles of E-cad to modulate transformation by Gli1 correlated with their ability to regulate localization of beta-catenin. Inhibition of Wnt-beta-catenin signaling by dominant negative Tcf4 selectively blocked in vitro transformation by Gli1. In Gli1-transgenic mice, infiltrating skin tumor cells expressed active, unphosphorylated beta-catenin. Our studies identify E-cad as a selective suppressor of transformation by Gli1 and point to the Sonic Hedgehog-Gli1 pathway as a key regulator of the beta-catenin switch in epithelial cells and cancers.

Transdetermination: Drosophila imaginal disc cells exhibit stem cell-like potency

Drosophila imaginal discs, the primordia of the adult fly appendages, are an excellent system for studying developmental plasticity. Cells in the imaginal discs are determined for their disc-specific fate (wingness, legness) during embryogenesis. Disc cells maintain their determination during larval development, a time of extensive growth and proliferation. Only when prompted to regenerate do disc cells exhibit lability in their determined identity. Regeneration in the disc is mediated by a localized region of cell division, known as the regeneration blastema. Most regenerating disc cells strictly adhere to their disc-specific identity; some cells however, switch fate in a phenomenon known as transdetermination. Similar regeneration and transdetermination events can be induced in situ by misexpression of the signaling molecule wingless. Recent studies indicate that the plasticity of disc cells during regeneration is associated with high morphogen activity and the reorganization of chromatin structure. Here we provide both a historical perspective of imaginal disc transdetermination, as well as discuss recent findings on how imaginal disc cells acquire developmental plasticity and multipotency. We also highlight how an understanding of imaginal disc transdetermination can enhance an understanding of developmental potency exhibited by stem cells.

The potential role of endogenous stem cells in regeneration of the inner ear

Stem cells in various mammalian organs retain the capacity to renew themselves and may be able to restore damaged tissue. Their existence has been proven by genetic tracer studies that demonstrate their differentiation into multiple tissue types and by their ability to self-renew through proliferation. Stem cells from the adult nervous system proliferate to form clonal floating colonies called spheres in vitro, and recent studies have demonstrated sphere formation by cells in the cochlea in addition to the vestibular system and the auditory ganglia, indicating that these tissues contain cells with stem cell properties. The presence of stem cells in the inner ear raises the hope of regeneration of mammalian inner ear cells but is difficult to correlate with the lack of spontaneous regeneration seen in the inner ear after tissue damage. Loss of stem cells postnatally in the cochlea may correlate with the loss of regenerative capacity and may limit our ability to stimulate regeneration. Retention of sphere forming ability in adult vestibular tissues suggests that the limited capacity for repair may be attributed to the continued presence of progenitor cells. Future strategies for regeneration must consider the distribution of endogenous stem cells in the inner ear and whether the tissue retains cells with the capacity for regeneration.

Promoting human embryonic stem cell renewal or differentiation by modulating Wnt signal and culture conditions

We previously showed that Wnt3a could stimulate human embryonic stem (hES) cell proliferation and affect cell fate determination. In the absence of feeder cell-derived factors, hES cells cultured under a feeder-free condition survived and proliferated poorly. Adding recombinant Wnt3a in the absence of feeder cell derived-factors stimulated hES cell proliferation but also differentiation. In the present study, we further extended our analysis to other Wnt ligands such as Wnt1 and Wnt5a. While Wnt1 displayed a similar effect on hES cells as Wnt3a, Wnt5a had little effect in this system. Wnt3a and Wnt1 enhanced proliferation of undifferentiated hES cells when feeder-derived self-renewal factors and bFGF are also present. To explore the possibility to promote the proliferation of undifferentiated hES cells by activating the Wnt signaling, we overexpressed Wnt3a or Wnt1 gene in immortalized human adult fibroblast (HAFi) cells that are superior in supporting long-term growth of undifferentiated hES cells than primary mouse embryonic fibroblasts. HAFi cells with or without a Wnt transgene can be propagated indefinitely. Over-expression of the Wnt3a gene significantly enhanced the ability of HAFi feeder cells to support the undifferentiated growth of 3 different hES cell lines we tested. Co-expression of three commonly-used drug selection genes in Wnt3a-overpressing HAFi cells further enabled us to select rare hES clones after stable transfection or transduction. These immortalized engineered feeder cells (W3R) that co-express growth-promoting genes such as Wnt3a and three drug selection genes should empower us to efficiently make genetic modified hES cell lines for basic and translational research.

Molecular regulation of kidney development: is the answer blowing in the Wnt?

Development of the metanephric kidney is a complicated process regulated by reciprocal signals from the ureteric bud and the metanephric mesenchyme that regulate tubule formation and epithelial branching morphogenesis. Over the past several years, several studies have suggested that Wnt signaling is involved in multiple aspects of normal kidney development as well as injury response and cancer progression. We will review these data here.

WNTS and WNT receptors as therapeutic tools and targets in human disease processes

The body of scientific literature linking Wnts and Wnt-associated proteins to human disease processes continues to grow in parallel with new discoveries from basic science laboratories that further characterize the elaborate cellular events following the binding of Wnts to their receptors. While Wnt-mediated signaling has long been known to play a major role in human carcinogenesis, accumulating evidence indicates that Wnts are also important mediators of inflammation and recovery from injury. The binding of secreted Wnt ligands to their receptors offers an attractive and accessible target for therapeutic regulation of these signaling pathways. Several promising preliminary studies have already addressed potential avenues for the manipulation of Wnt signaling in disease processes. This review will focus on disease processes involving the regulation of Wnt signaling at the level of Wnt binding to its target receptors. Wnt proteins, Wnt receptors, and secreted Wnt inhibitors are attractive as potential therapeutic agents and targets due to their extracellular location. In addition, since Wnt signaling results in a diverse array of downstream intracellular events, many of which are not fully understood, the targeting of this pathway at the most upstream site of pathway activation also provides a strategic advantage for therapy. As the list of Wnt-related diseases continues to grow, advances in our understanding of the biochemical and molecular mechanisms underlying Wnt signaling may ultimately translate into innovative ways to treat Wnt-related disease processes in patients.