An in vivo comparative study of sonic, desert and Indian hedgehog reveals that hedgehog pathway activity regulates epidermal stem cell homeostasis

PTCH1 +/- dermal fibroblasts isolated from healthy skin of Gorlin syndrome patients exhibit features of carcinoma associated fibroblasts

Gorlin's or nevoid basal cell carcinoma syndrome (NBCCS) causes predisposition to basal cell carcinoma (BCC), the commonest cancer in adult human. Mutations in the tumor suppressor gene PTCH1 are responsible for this autosomal dominant syndrome. In NBCCS patients, as in the general population, ultraviolet exposure is a major risk factor for BCC development. However these patients also develop BCCs in sun-protected areas of the skin, suggesting the existence of other mechanisms for BCC predisposition in NBCCS patients. As increasing evidence supports the idea that the stroma influences carcinoma development, we hypothesized that NBCCS fibroblasts could facilitate BCC occurence of the patients. WT (n = 3) and NBCCS fibroblasts bearing either nonsense (n = 3) or missense (n = 3) PTCH1 mutations were cultured in dermal equivalents made of a collagen matrix and their transcriptomes were compared by whole genome microarray analyses. Strikingly, NBCCS fibroblasts over-expressed mRNAs encoding pro-tumoral factors such as Matrix Metalloproteinases 1 and 3 and tenascin C. They also over-expressed mRNA of pro-proliferative diffusible factors such as fibroblast growth factor 7 and the stromal cell-derived factor 1 alpha, known for its expression in carcinoma associated fibroblasts. These data indicate that the PTCH1^+/− genotype of healthy NBCCS fibroblasts results in phenotypic traits highly reminiscent of those of BCC associated fibroblasts, a clue to the yet mysterious proneness to non photo-exposed BCCs in NBCCS patients.

Trafficking, development and hedgehog

Embryogenesis is mediated by a relatively small number of developmental signaling pathways, and the morphogens, receptors and transcription factors integral to these cascades are considered the master regulators of development. However, superimposed on this is an additional layer of control by complex intracellular trafficking networks. The importance of trafficking in controlling the processes of morphogenesis and development is highlighted by recent data regarding the transport and localisation of the morphogen sonic hedgehog (Shh) and the machinery that leads to its secretion, modification, cellular internalisation and signal transduction. Here we review the regulation of hedgehog signaling by intracellular trafficking, including the role of the primary cilium and lipids in mediating pathway activity.

Hedgehog signaling regulates epithelial-mesenchymal transition during biliary fibrosis in rodents and humans

Epithelial-mesenchymal transitions (EMTs) play an important role in tissue construction during embryogenesis, and evidence suggests that this process may also help to remodel some adult tissues after injury. Activation of the hedgehog (Hh) signaling pathway regulates EMT during development. This pathway is also induced by chronic biliary injury, a condition in which EMT has been suggested to have a role. We evaluated the hypothesis that Hh signaling promotes EMT in adult bile ductular cells (cholangiocytes). In liver sections from patients with chronic biliary injury and in primary cholangiocytes isolated from rats that had undergone bile duct ligation (BDL), an experimental model of biliary fibrosis, EMT was localized to cholangiocytes with Hh pathway activity. Relief of ductal obstruction in BDL rats reduced Hh pathway activity, EMT, and biliary fibrosis. In mouse cholangiocytes, coculture with myofibroblastic hepatic stellate cells, a source of soluble Hh ligands, promoted EMT and cell migration. Addition of Hh-neutralizing antibodies to cocultures blocked these effects. Finally, we found that EMT responses to BDL were enhanced in patched-deficient mice, which display excessive activation of the Hh pathway. Together, these data suggest that activation of Hh signaling promotes EMT and contributes to the evolution of biliary fibrosis during chronic cholestasis.

Smoothened signaling in vertebrates is facilitated by a G protein-coupled receptor kinase

Smoothened, a heptahelical membrane protein, functions as the transducer of Hedgehog signaling. The kinases that modulate Smoothened have been thoroughly analyzed in flies. However, little is known about how phosphorylation affects Smoothened in vertebrates, mainly, because the residues, where Smoothened is phosphorylated are not conserved from Drosophila to vertebrates. Given its molecular architecture, Smoothened signaling is likely to be regulated in a manner analogous to G protein-coupled receptors (GPCRs). Previously, it has been shown, that arrestins and GPCR kinases, (GRKs) not only desensitize G protein-dependent receptor signaling but also function as triggers for GPCR trafficking and formation of signaling complexes. Here we describe that a GRK contributes to Smoothened-mediated signaling in vertebrates. Knockdown of the zebrafish homolog of mammalian GRK2/3 results in lowered Hedgehog transcriptional responses, impaired muscle development, and neural patterning. Results obtained in zebrafish are corroborated both in cell culture, where zGRK2/3 phosphorylates Smoothened and promotes Smoothened signal transduction and in mice where deletion of GRK2 interferes with neural tube patterning. Together, these data suggest that a GRK functions as a vertebrate kinase for Smoothened, promoting Hedgehog signal transduction during early development.

A sonic hedgehog signaling domain in the arterial adventitia supports resident Sca1+ smooth muscle progenitor cells

We characterize a sonic hedgehog (Shh) signaling domain restricted to the adventitial layer of artery wall that supports resident Sca1-positive vascular progenitor cells (AdvSca1). Using patched-1 (Ptc1(lacZ)) and patched-2 (Ptc2(lacZ)) reporter mice, adventitial Shh signaling activity was first detected at embryonic day (E) 15.5, reached the highest levels between postnatal day 1 (P1) and P10, was diminished in adult vessels, and colocalized with a circumferential ring of Shh protein deposited between the media and adventitia. In Shh(-/-) mice, AdvSca1 cells normally found at the aortic root were either absent or greatly diminished in number. Using a Wnt1-cre lineage marker that identifies cells of neural crest origin, we found that neither the adventitia nor AdvSca1 cells were labeled in arteries composed of neural crest-derived smooth muscle cells (SMCs). Although AdvSca1 cells do not express SMC marker proteins in vivo, they do express transcription factors thought to be required for SMC differentiation, including serum response factor (SRF) and myocardin family members, and readily differentiate to SMC-like cells in vitro. However, AdvSca1 cells also express potent repressors of SRF-dependent transcription, including Klf4, Msx1, and FoxO4, which may be critical for maintenance of the SMC progenitor phenotype of AdvSca1 cells in vivo. We conclude that a restricted domain of Shh signaling is localized to the arterial adventitia and may play important roles in maintenance of resident vascular SMC progenitor cells in the artery wall.

Sonic hedgehog signalling in T-cell development and activation

The production of mature functional T cells in the thymus requires signals from the thymic epithelium. Here, we review recent experiments showing that one way in which the epithelium controls the production of mature T cells is by the secretion of sonic hedgehog (SHH). We consider the increasing evidence that SHH-induced signalling is not only important for the differentiation and proliferation of early thymocyte progenitors, but also for modulating T-cell receptor signalling during repertoire selection, with implications for positive selection, CD4 versus CD8 lineage commitment, and clonal deletion of autoreactive cells. We also review the influence of hedgehog signalling in peripheral T-cell activation.

The role of vascular stem cells in atherogenesis and post-angioplasty restenosis

It is well known that atherosclerosis prevails in elderly populations as ageing acts as a recognized risk factor for this disease. Although the pathogenic factors leading to atherosclerosis are highly heterogeneous, traditionally speaking, the causative risk factors include hyperlipidemia, hypertension, diabetes mellitus and smoking, which can damage to endothelial function, and subsequently promote lipid penetration and inflammatory cell infiltration. Damaged endothelial cells (ECs) may be replaced by neighboring cell division, while damaged smooth muscle cells (SMCs) may be replaced by medial SMCs emigrating into the intima during atherogenesis. However, this standpoint is challenged by recent findings that vascular progenitor/stem cells (VPCs) may contribute to atherogenesis and post-angioplasty restenosis. VPCs are a group of primitive cells that have the potential to produce mature, functional cells in the vascular wall. VPCs residing in bone marrow, vascular wall or circulating in the peripheral blood may be stimulated by a variety of pathogenic factors. These stem cells then participate in regeneration, repair and remodeling of the injured arterial wall. This new concept may bring about a great breakthrough in understanding the pathogenesis of atherosclerosis and develop novel therapeutic strategies for coronary heart disease. This article will mainly review the role of VPCs in atherogenesis, thus providing a novel understanding about the pathophysiology of atherosclerosis.

Epidermal hyperplasia and expansion of the interfollicular stem cell compartment in mutant mice with a C-terminal truncation of Patched1

Hedgehog (Hh) signaling is conserved from flies to humans and is indispensable in embryogenesis and adulthood. Patched (Ptc) encodes a receptor for Hh ligands and functions as a tumor suppressor. PTCH1 mutations in humans are found in basal cell carcinoma (BCC) and irradiated Ptc1(+/-) mice recapitulate this phenotype. However, due to embryonic lethality associated with the Ptc1 null mutation, its normal function in embryonic and adult skin remains unknown. Here we describe the epidermal phenotypes of a spontaneous and viable allele of Ptc1, Ptc1(mes), in which the C-terminal domain (CTD) is truncated. Ptc1(mes/mes) embryos display normal epidermal and hair follicle development. Postnatal Ptc1(mes/mes) skin displays severe basal cell layer hyperplasia and increased proliferation, while stratification of the suprabasal layers is mostly normal. Interestingly, truncation of the Ptc1 CTD did not result in skin tumors. However, long term labeling studies revealed a greater than three-fold increase in label-retaining cells in the interfollicular epidermis of Ptc1(mes/mes) adults, indicating possible expansion of the epidermal stem cell compartment. Increased expression of regulators of epidermal homeostasis, c-Myc and p63, was also observed in Ptc1(mes/mes) adult skin. These results suggest that the CTD of Ptc1 is involved in regulating epidermal homeostasis in mature skin.

Investigation of downstream target genes of PAX3c, PAX3e and PAX3g isoforms in melanocytes by microarray analysis

PAX3 encodes a transcription factor, which with Zic1 is necessary for induction of the neural crest during early embryonic development. There are 7 human PAX3 isoforms (a-h). PAX3e is the full length isoform comprising 10 exons. PAX3c comprises 8 exons plus 5 codons of intron 8, while PAX3g has a truncated transactivation domain. Previous studies by us indicated that these isoforms have different activities in melanocytes in vitro. In this study, a mouse gene oligo array ( approximately 7.5 k oligos), from the Human Genome Mapping Project (HGMP) Resource Centre, was used to screen for alterations in downstream gene expression in PAX3c, PAX3e and PAX3g melanocyte transfectants, compared with empty vector controls. The data analyses identified 109 genes up or downregulated, at least 2-fold, and involved in cell differentiation, proliferation, migration, adhesion, apoptosis and angiogenesis. Semi-quantitative RT-PCR and Western blotting confirmed the changes identified by microarrays for several putative targets of PAX3, including Met, MyoD and Muc18, and previously undescribed targets, including Dhh, Fgf17, Kitl and Rac1. Thus, our data reveal that PAX3 isoforms regulate distinct but overlapping sets of genes in melanocytes in vitro.

Hair follicle stem cells

The increasing use of the hair follicle as a stem cell paradigm is due in part to the complex interplay between epithelial, dermal and other cell types, each with interesting differentiation potential and prospective therapeutic applications. This review focuses on research into the environmental niche, gene expression profiles and plasticity of hair follicle stem cell populations, where many recent advances have come about through novel technological and experimental approaches. We discuss major developmental pathways involved in the establishment and control of the epithelial stem cell niche, and evidence of plasticity between stem and transit amplifying cell populations.

Abnormal hair development and apparent follicular transformation to mammary gland in the absence of hedgehog signaling

We show that removing the Shh signal tranducer Smoothened from skin epithelium secondarily results in excess Shh levels in the mesenchyme. Moreover, the phenotypes we observe reflect decreased epithelial Shh signaling, yet increased mesenchymal Shh signaling. For example, the latter contributes to exuberant hair follicle (HF) induction, while the former depletes the resulting follicular stem cell niches. This disruption of the niche apparently also allows the remaining stem cells to initiate hair formation at inappropriate times. Thus, the temporal structure of the hair cycle may depend on the physical structure of the niche. Finally, we find that the ablation of epithelial Shh signaling results in unexpected transformations: the follicular outer root sheath takes on an epidermal character, and certain HFs disappear altogether, having adopted a strikingly mammary gland-like fate. Overall, our study uncovers a multifaceted function for Shh in sculpting and maintaining the integrity and identity of the developing HF.

The contribution of epidermal stem cells to skin cancer

Tumors arising from the skin are of multiple phenotypes, with differing degrees of malignant potential. In mouse models of skin carcinogenesis, tumors of squamous phenotype are the most common; however, human disease indicates that multiple phenotypes may arise from a common pool of stem cells that are then influenced by epigenetic factors. The use of transgenic and knockout gene technologies with mice is unraveling some of the specific genes regulating fate determination in stem cells other than squamous lineage, including basal cell carcinoma and sebaceous adenomas. The following review examines the evidence for the stem cell origin of epidermal tumors and the contribution of some specific gene families toward stem cell fate decisions during epidermal tumor progression.

Hedgehog: an attribute to tumor regrowth after chemoradiotherapy and a target to improve radiation response

PURPOSE

Despite aggressive chemotherapy, radiotherapy, surgery, or combination approaches, the survival rate of patients with esophageal cancer remains poor. Recent studies have suggested that constitutive activation of the Hedgehog (Hh) pathway in cancers of the digestive tract may contribute to the growth and maintenance of cancer. However, the relationship between Hh signaling and therapeutic response is unknown.

EXPERIMENTAL DESIGN

The expression and temporal kinetics of Hh signaling and proliferation biomarkers after chemoradiotherapy were examined in esophageal tumor xenografts. Additionally, immunohistochemical analysis of Sonic Hh (Shh) and Gli-1 expression were done on residual tumors from patients who received neoadjuvant chemoradiotherapy followed by surgery. The ability of Shh signaling to induce proliferation in esophageal cell lines was determined. Expression of cell cycle checkpoint proteins was analyzed in cells in which Hh signaling was activated or inhibited. We further determined the effect of inhibiting Hh signaling in sensitizing esophageal tumors to radiation.

RESULTS

We showed that the Shh signaling pathway was extensively activated in esophageal cancer xenografts and residual tumors after chemoradiotherapy and the temporal kinetics of Hh signaling preceded increases in proliferation biomarker expression and tumor size during tumor regrowth. We further showed that Hh pathway activity influences proliferation rates of esophageal cancer cell lines through up-regulation of the G1-cyclin-Rb axis. Additionally, we found that blocking Hh signaling enhanced radiation cytotoxicity of esophageal cancer cells.

CONCLUSIONS

These results suggest that activation of the Hh pathway may promote tumor repopulation after chemoradiotherapy and contribute to chemoradiation resistance in esophageal cancers.

Hedgehog signal pathway is activated in ovarian carcinomas, correlating with cell proliferation: it's inhibition leads to growth suppression and apoptosis

The hedgehog (Hh) signal pathway has recently been shown to be activated in human malignancies. However, little is known about its role in the development or patient prognosis of epithelial ovarian carcinoma. In the present study, we examined in vivo and in vitro the expression and functional role of Hh signal molecules in epithelial ovarian tumors and normal ovarian surface epithelial (OSE) cells. The expression of Shh, Dhh, Ptch, Smo and Gli1 proteins was not observed in normal OSE, but was increased stepwise in benign, borderline and malignant neoplasms. In addition, immunoreactivity for Shh, Dhh, Ptch, Smo and Gli1 was highly correlated with cell proliferation assessed by Ki-67. Blocking the Hh signal using either the Hh pathway inhibitor cyclopamine or Gli1 siRNA led to remarkably decreased cell proliferation in ovarian carcinoma cells. Treatment with cyclopamine induced not only G, arrest but also apoptosis along with the downregulation of cyclin A and cyclin D1, and the upregulation of p21 and p27. Among the Hh signal molecules, Dhh expression was correlated with poor prognosis of ovarian carcinoma patients. These findings suggest that the Hh signal pathway plays an important role in ovarian tumorigenesis as well as in the activation of cell proliferation in ovarian carcinomas. Thus, the Hh signal pathway is a possible molecular target of new treatment strategies for ovarian carcinoma.

Epidermal stem cells of the skin

The skin constantly renews itself throughout adult life, and the hair follicle undergoes a perpetual cycle of growth and degeneration. Stem cells (SCs) residing in the epidermis and hair follicle ensure the maintenance of adult skin homeostasis and hair regeneration, but they also participate in the repair of the epidermis after injuries. We summarize here the current knowledge of epidermal SCs of the adult skin. We discuss their fundamental characteristics, the methods recently designed to isolate these cells, the genes preferentially expressed in the multipotent SC niche, and the signaling pathways involved in SC niche formation, SC maintenance, and activation. Finally, we speculate on how the deregulation of these pathways may lead to cancer formation.

Mice with a targeted mutation of patched2 are viable but develop alopecia and epidermal hyperplasia

Hedgehog (Hh) signaling plays pivotal roles in tissue patterning and development in Drosophila melanogaster and vertebrates. The Patched1 (Ptc1) gene, encoding the Hh receptor, is mutated in nevoid basal cell carcinoma syndrome, a human genetic disorder associated with developmental abnormalities and increased incidences of basal cell carcinoma (BCC) and medulloblastoma (MB). Ptc1 mutations also occur in sporadic forms of BCC and MB. Mutational studies with mice have verified that Ptc1 is a tumor suppressor. We previously identified a second mammalian Patched gene, Ptc2, and demonstrated its distinct expression pattern during embryogenesis, suggesting a unique role in development. Most notably, Ptc2 is expressed in an overlapping pattern with Shh in the epidermal compartment of developing hair follicles and is highly expressed in the developing limb bud, cerebellum, and testis. Here, we describe the generation and phenotypic analysis of Ptc2(tm1/tm1) mice. Our molecular analysis suggests that Ptc2(tm1) likely represents a hypomorphic allele. Despite the dynamic expression of Ptc2 during embryogenesis, Ptc2(tm1/tm1) mice are viable, fertile, and apparently normal. Interestingly, adult Ptc2(tm1/tm1) male animals develop skin lesions consisting of alopecia, ulceration, and epidermal hyperplasia. While functional compensation by Ptc1 might account for the lack of a strong mutant phenotype in Ptc2-deficient mice, our results suggest that normal Ptc2 function is required for adult skin homeostasis.

SP analysis may be used to identify cancer stem cell populations

Side populations (SP), as defined by Hoechst exclusion in flow cytometry, have been described a few years ago. While they represent only a small fraction of the whole cell population, their properties confer an important place in several investigations. SP cells express high levels of various members of ABC transporters family, such as MDR1 and BCRP, which are responsible for drug resistance. Targeting SP could improve cancer therapy by blocking these transporters. In addition, SP appear to be enriched in stem cells, cells that play a pivotal role in normal development and cancer biology. Thus, they could provide a useful tool and a readily accessible source for stem cell studies in both the normal and cancerous settings. However, these cells are poorly defined and pose challenges in their identification and isolation, particularly since they are few in number. Thus, better characterization of SP will advance our understanding of stem cells and will provide us an accessible target for drug resistance in cancer therapy.

Epithelial stem cells: a folliculocentric view

Putative epithelial stem cells were identified in the hair follicle bulge as quiescent "label retaining cells". The study of these cells was hindered until the identification of bulge cell molecular markers, such as CD34 expression and K15 promoter activity. This allowed for the isolation and characterization of bulge cells from mouse follicles. Bulge cells possess stem cell characteristics, including multipotency, high proliferative potential, and their cardinal feature of quiescence. Lineage analysis demonstrated that all epithelial layers within the adult follicle and hair originated from bulge cells. Bulge cells only contribute to the epidermis during wound healing, but after isolation, when combined with neonatal dermal cells, they regenerate new hair follicles, epidermis, and sebaceous glands. Bulge cells maintain their stem cell characteristics after propagation in vitro, thus ultimately they may be useful for tissue engineering applications. Understanding the signals important for directing movement and differentiation of bulge cells into different lineages will be important for developing treatments based on stem cells as well as clarifying their role in skin disease.

Interfollicular Epidermal Stem Cells: Identification, Challenges, Potential

Homeostatic epidermal tissue renewal is the result of the combined activity of rare but potent stem cells, and a large pool of short-lived progenitor cells termed transit amplifying cells. Although the existence of epidermal stem cells has been known for some decades, their specific role in the processes of wound repair, skin diseases, and carcinogenesis remains unelucidated. Nevertheless, significant advances have been made in the identification and functional characterization of both murine and human epidermal stem cells, which place investigators in an exciting position to gain further insights into the fundamental processes of tissue renewal and repair in the epidermis.

Hedgehog signaling and response to cyclopamine differ in epithelial and stromal cells in benign breast and breast cancer

The hedgehog pathway regulates epithelial-mesenchymal interactions, differentiation, proliferation and survival during development. Stimulation of hedgehog signaling induces carcinogenesis or promotes cell survival in cancers of multiple organs. Using real-time, quantitative PCR, laser capture microdissection, and immunohistochemistry, distinctive patterns of expression of the hedgehog pathway members patched 1 (PTCH1), smoothened, GLI1, GLI2 and the 3 hedgehog ligands were identified for epithelial cells and stromal fibroblasts in benign breast and breast cancer. Hedgehog ligands were expressed at higher levels in some cancer epithelial cell lines compared to noncancerous epithelial cells. Correspondingly, expression of GLI1, a transcription factor and transcriptional product of hedgehog signaling, was increased 8-fold in cancer epithelial cell lines; however, PTCH1, also a transcriptional target of hedgehog signaling in many cell types, was not increased. GLI1 protein and mRNA, and PTCH1 and sonic hedgehog (SHH) proteins were elevated in 3 of 10 breast cancers; however, PTCH1 transcripts were not consistently increased. Hedgehog-mediated transcription, as indicated by a reporter of GLI-dependent promoter activity and by expression of GLI1 transcripts, was reduced by the hedgehog pathway inhibitor cyclopamine in both MDA-MB-435 cancer epithelial cells and MCF10AT epithelial cells, a cell line derived from benign breast. However, cyclopamine reduced viability of cancer epithelial cell lines, including MDA-MB-435, but did not specifically affect fibroblasts or epithelial cells from benign breast, including MCF10AT. Treatment with sonic hedgehog ligand diminished the cyclopamine-induced reduction in GLI-dependent promoter activity in MCF10AT and MDA-MB-435 and viability of MDA-MB-435. These results demonstrate modulation of GLI-mediated transcription in both cancer and benign-derived epithelial cells by cyclopamine and sonic hedgehog, and further suggest that hedgehog signaling contributes to the survival of only the cancer epithelial cells. Determination as to whether the increase in GLI1 and SHH expression in breast cancer indicates a significant increase in hedgehog signaling will require further evaluation.

Role of sonic hedgehog in maintaining a pool of proliferating stem cells in the human fetal epidermis

BACKGROUND

The mammalian epidermis is maintained by the ongoing proliferation of a subpopulation of keratinocytes known as epidermal stem cells. Sonic hedgehog (Shh) can regulate morphogenesis of hair follicles and several types of skin cancer, but the effect of Shh on proliferation of human putative epidermal stem cells (HPESCs) is poorly understood.

METHODS AND RESULTS

We first found that Shh, its receptors Patched1 (Ptc1) as well as Smoothened (Smo) and its downstream transcription factor Gli-1 were expressed in the basal layer of human fetal epidermis and freshly sorted HPESCs. Next, treatment of HPESCs with media conditioned by Shh-N-expressing cells promoted cell proliferation, whereas inhibition of Shh by cyclopamine, a specific inhibitor of Shh signalling, had an opposite effect. Interestingly, the mitogenic effect of epidermal growth factor (EGF) on HPESCs was efficiently abolished by cyclopamine. Finally, bone morphogenetic protein 4 (BMP-4), a potential downstream effector of Shh signalling, increased HPESC proliferation in a concentration-dependent manner.

CONCLUSIONS

Shh is an important regulator of HPESC proliferation in the basal layer of human fetal epidermis and modulates the cell responsiveness to EGF, which will assist to unravel the mechanisms that regulate stem cell proliferation and neoplasia in the human epidermis.

Association of Shh and Ptc with keratin localization in the initiation of the formation of circumvallate papilla and von Ebner's gland

The development of gustatory papillae in mammalian embryos requires the coordination of a series of morphological events, such as proliferation, differentiation and innervation. In mice, the circumvallate papilla (CVP) is a specialized structure that develops in a characteristic spatial and temporal pattern in the posterior region of the tongue dorsal surface. The distinct expression patterns of Shh and Ptc, which play important roles in the development of other epithelial appendages, have been localized in the trench wall that gives rise to von Ebner's gland (VEG). To define the cellular mechanisms responsible for morphogenesis and differentiation during early development of CVP and VEG, the localization patterns of keratins (cytokeratins) K7, K8, K18, K19, K14 and connexin-43, which are dependent on Shh expression in other developmental systems, have been examined in detail. The distinct localization of keratins K7, K8, K18, K19, K14 and connexin-43 in the epithelium giving rise to the CVP and VEG suggests that cytodifferentiation is established prior to morphological changes. Interestingly, the localization of proliferating cell nuclear antigen, a marker for cell proliferation, is similar to that of Shh. An understanding of the regulatory roles of cell-cell interactions and signalling molecules in orchestrating a mutual network will bring us nearer to defining the molecular and cellular mechanisms underlying morphogenesis in mammalian taste bud development.

Patched1 Functions as a Gatekeeper by Promoting Cell Cycle Progression

Mutations in the Hedgehog receptor, Patched 1 (Ptch1), have been linked to both familial and sporadic forms of basal cell carcinoma (BCC), leading to the hypothesis that loss of Ptch1 function is sufficient for tumor progression. By combining conditional knockout technology with the inducible activity of the Keratin6 promoter, we provide in vivo evidence that loss of Ptch1 function from the basal cell population of mouse skin is sufficient to induce rapid skin tumor formation, reminiscent of human BCC. Elimination of Ptch1 does not promote the nuclear translocation of beta-catenin and does not induce ectopic activation or expression of Notch pathway constituents. In the absence of Ptch1, however, a large proportion of basal cells exhibit nuclear accumulation of the cell cycle regulators cyclin D1 and B1. Collectively, our data suggest that Ptch1 likely functions as a tumor suppressor by inhibiting G1-S phase and G2-M phase cell cycle progression, and the rapid onset of tumor progression clearly indicates Ptch1 functions as a "gatekeeper." In addition, we note the high frequency and rapid onset of tumors in this mouse model makes it an ideal system for testing therapeutic strategies, such as Patched pathway inhibitors.

Communicating with Hedgehogs

Signalling by secreted Hedgehog (Hh) proteins is important for the development of many tissues and organs. Damage to components of the Hh signal-transduction pathway can lead to birth defects and cancer. The Hh proteins are distributed in tissues in a gradient, and cells respond to different thresholds of Hh with distinct responses. The cellular machinery that is responsible for the unique molecular mechanisms of Hh signal transduction has been largely conserved during metazoan evolution.