Gli2 functions in FGF signaling during antero-posterior patterning

Casein Kinase 1D Encodes a Novel Drug Target in Hedgehog-GLI-Driven Cancers and Tumor-Initiating Cells Resistant to SMO Inhibition

Simple Summary

Uncontrolled activation of hedgehog (HH)—GLI signaling contributes to the development of several human malignancies. Targeted inhibition of the HH—GLI signaling cascade with small-molecule inhibitors can reduce cancer growth, but patient relapse is very common due to the development of drug resistance. Therefore, a high unmet medical need exists for new drug targets and inhibitors to achieve efficient and durable responses. In the current study, we identified CSNK1D as a novel drug target in the HH—GLI signaling pathway. Genetic and pharmacological inhibition of CSNK1D activity leads to suppression of oncogenic HH—GLI signaling, even in cancer cells in which already approved HH inhibitors are no longer effective due to resistance mechanisms. Inhibition of CSNK1D function reduces the malignant properties of so-called tumor-initiating cells, thereby limiting cancer growth and presumably metastasis. The results of this study form the basis for the development of efficient CSNK1D inhibitors for the therapy of HH—GLI-associated cancers.

Abstract

(1) Background: Aberrant activation of the hedgehog (HH)—GLI pathway in stem-like tumor-initiating cells (TIC) is a frequent oncogenic driver signal in various human malignancies. Remarkable efficacy of anti-HH therapeutics led to the approval of HH inhibitors targeting the key pathway effector smoothened (SMO) in basal cell carcinoma and acute myeloid leukemia. However, frequent development of drug resistance and severe adverse effects of SMO inhibitors pose major challenges that require alternative treatment strategies targeting HH—GLI in TIC downstream of SMO. We therefore investigated members of the casein kinase 1 (CSNK1) family as novel drug targets in HH—GLI-driven malignancies. (2) Methods: We genetically and pharmacologically inhibited CSNK1D in HH-dependent cancer cells displaying either sensitivity or resistance to SMO inhibitors. To address the role of CSNK1D in oncogenic HH signaling and tumor growth and initiation, we quantitatively analyzed HH target gene expression, performed genetic and chemical perturbations of CSNK1D activity, and monitored the oncogenic transformation of TIC in vitro and in vivo using 3D clonogenic tumor spheroid assays and xenograft models. (3) Results: We show that CSNK1D plays a critical role in controlling oncogenic GLI activity downstream of SMO. We provide evidence that inhibition of CSNK1D interferes with oncogenic HH signaling in both SMO inhibitor-sensitive and -resistant tumor settings. Furthermore, genetic and pharmacologic perturbation of CSNK1D decreases the clonogenic growth of GLI-dependent TIC in vitro and in vivo. (4) Conclusions: Pharmacologic targeting of CSNK1D represents a novel therapeutic approach for the treatment of both SMO inhibitor-sensitive and -resistant tumors.

Functional Roles of FGF Signaling in Early Development of Vertebrate Embryos

Fibroblast growth factors (FGFs) comprise a large family of growth factors, regulating diverse biological processes including cell proliferation, migration, and differentiation. Each FGF binds to a set of FGF receptors to initiate certain intracellular signaling molecules. Accumulated evidence suggests that in early development and adult state of vertebrates, FGFs also play exclusive and context dependent roles. Although FGFs have been the focus of research for therapeutic approaches in cancer, cardiovascular disease, and metabolic syndrome, in this review, we mainly focused on their role in germ layer specification and axis patterning during early vertebrate embryogenesis. We discussed the functional roles of FGFs and their interacting partners as part of the gene regulatory network for germ layer specification, dorsal-ventral (DV), and anterior-posterior (AP) patterning. Finally, we briefly reviewed the regulatory molecules and pharmacological agents discovered that may allow modulation of FGF signaling in research.

Pigment epithelium-derived factor peptide promotes limbal stem cell proliferation through hedgehog pathway

Expansion of limbal epithelial stem cells (LSCs) is crucial for the success of limbal transplantation. Previous studies showed that pigment epithelium‐derived peptide (PEDF) short peptide 44‐mer could effectively expand LSCs and maintain them in a stem‐cell state, but the mechanism remained unclear. In the current study, we found that pharmacological inhibition of Sonic Hedgehog (SHh) activity reduced the LSC holoclone number and suppressed LSC proliferation in response to 44‐mer. In mice subjected to focal limbal injury, 44‐mer facilitated the restoration of the LSC population in damaged limbus, and such effect was impeded by the SHh or ATGL (a PEDF receptor) inhibitor. Furthermore, we showed that 44‐mer increased nuclear translocation of Gli1 and Gli3 in LSCs. Knockdown of Gli1 or Gli3 suppressed the ability of 44‐mer to induce cyclin D1 expression and LSC proliferation. In addition, ATGL inhibitor suppressed the 44‐mer‐induced phosphorylation of STAT3 at Tyr705 in LSC. Both inhibitors for ATGL and STAT3 attenuated 44‐mer‐induced SHh activation and LSC proliferation. In conclusion, our data demonstrate that SHh‐Gli pathway driven by ATGL/STAT3 signalling accounts for the 44‐mer‐mediated LSC proliferation.

Gli Proteins: Regulation in Development and Cancer

Gli proteins are transcriptional effectors of the Hedgehog signaling pathway. They play key roles in the development of many organs and tissues, and are deregulated in birth defects and cancer. We review the molecular mechanisms of Gli protein regulation in mammals, with special emphasis on posttranslational modifications and intracellular transport. We also discuss how Gli proteins interact with co-activators and co-repressors to fine-tune the expression of Hedgehog target genes. Finally, we provide an overview of the regulation of developmental processes and tissue regeneration by Gli proteins and discuss how these proteins are involved in cancer progression, both through canonical regulation via the Hedgehog pathway and through cross-talk with other signaling pathways.

A review of genetic factors contributing to the etiopathogenesis of anorectal malformations

BACKGROUND

Anorectal malformation (ARM) is a common congenital anomaly with a wide clinical spectrum. Recently, many genetic and molecular studies have been conducted worldwide highlighting the contribution of genetic factors in its etiology. We summarize the current literature on such genetic factors.

MATERIALS AND METHODS

Literature search was done using different combinations of terms related to genetics in anorectal malformations. From 2012 to June 2017, articles published in the English literature and studies conducted on human population were included.

OBSERVATIONS AND RESULTS

A paradigm shift was observed from the earlier studies concentrating on genetic aberrations in specific pathways to genome wide arrays exploring single nucleotide polymorphisms (SNPs) and copy number variations (CNVs) in ARM patients. Rare CNVs (including 79 genes) and SNPs have been found to genetically contribute to ARM. Out of disrupted 79 genes one such putative gene is DKK4. Down regulation of CDX-1 gene has also been implicated in isolated ARM patients. In syndromic ARM de novo microdeletion at 17q12 and a few others have been identified.

CONCLUSION

Major genetic aberrations proposed in the pathogenesis of ARM affect members of the Wnt, Hox (homebox) genes, Sonic hedgehog (Shh) and Gli2, Bmp4, Fgf and CDX1 signalling pathways; probable targets of future molecular gene therapy.

Involvement of JunB Proto-Oncogene in Tail Formation During Early Xenopus Embryogenesis

Integration of signaling pathways is important for the establishment of the body plan during embryogenesis. However, little is known about how the multiple signals interact to regulate morphogenesis. Here, we show that junb is expressed in the posterior neural plate and the caudal fin during Xenopus embryogenesis and that overexpression of wild-type JunB induces small head phenotypes and ectopic tail-like structures. A mutant form of JunB that lacked GSK3 and MAPK phosphorylation sites showed stronger tail-like structure-inducing activity than wild-type JunB. Moreover, the mutant JunB induced expression of tailbud and neural marker genes, but not somite and chordoneural hinge (CNH) marker genes in ectopic tail-like structures. In ectodermal explants of Xenopus embryos, overexpression of JunB increased the expression of tailbud and posterior marker genes including fgf3, xbra (t) and wnt8. These results indicate that JunB is capable of inducing the ectopic formation of tissues similar to the tailbud, and that the tailbud-inducing activity of JunB is likely to be regulated by FGF and Wnt pathways. Overall, our results suggest that JunB is a regulator of tail organization possibly through integration of several morphogen signaling pathways.

FGF signaling enhances a sonic hedgehog negative feedback loop at the initiation of spinal cord ventral patterning

A prevalent developmental mechanism for the assignment of cell identities is the production of spatiotemporal concentration gradients of extracellular signaling molecules that are interpreted by the responding cells. One of such signaling systems is the Shh gradient that controls neuronal subtype identity in the ventral spinal cord. Using loss and gain of function approaches in chick and mouse embryos, we show here that the fibroblast growth factor (FGF) signaling pathway is required to restrict the domains of ventral gene expression as neuroepithelial cells become exposed to Shh during caudal extension of the embryo. FGF signaling activates the expression of the Shh receptor and negative pathway regulator Patched 2 (Ptch2) and therefore can enhance a negative feedback loop that restrains the activity of the pathway. Thus, we identify one of the mechanisms by which FGF signaling acts as a modulator of the onset of Shh signaling activity in the context of coordination of ventral patterning and caudal axis extension. © 2015 Wiley Periodicals, Inc. Develop Neurobiol 76: 956-971, 2016.

Transcription Factors in Craniofacial Development: From Receptor Signaling to Transcriptional and Epigenetic Regulation

Craniofacial morphogenesis is driven by spatial-temporal terrains of gene expression, which give rise to stereotypical pattern formation. Transcription factors are key cellular components that control these gene expressions. They are information hubs that integrate inputs from extracellular factors and environmental cues, direct epigenetic modifications, and define transcriptional status. These activities allow transcription factors to confer specificity and potency to transcription regulation during development.

Anorectal malformation: the etiological factors

Anorectal malformation (ARM) is a congenital anomaly commonly encountered in pediatric surgery practice. Although surgical procedures correct the anatomical anomalies, the post-operative bowel function is not universally satisfactory. The etiology of ARM remains unclear. In this review, we summarize the current understanding of the genetic and epigenetic factors contributing to the pathogenesis of ARM, based on published animal models, human genetics and epidemiological researches. Appreciation of these factors may be helpful in the management of ARM in the future.

Context-dependent signal integration by the GLI code: the oncogenic load, pathways, modifiers and implications for cancer therapy

Canonical Hedgehog (HH) signaling leads to the regulation of the GLI code: the sum of all positive and negative functions of all GLI proteins. In humans, the three GLI factors encode context-dependent activities with GLI1 being mostly an activator and GLI3 often a repressor. Modulation of GLI activity occurs at multiple levels, including by co-factors and by direct modification of GLI structure. Surprisingly, the GLI proteins, and thus the GLI code, is also regulated by multiple inputs beyond HH signaling. In normal development and homeostasis these include a multitude of signaling pathways that regulate proto-oncogenes, which boost positive GLI function, as well as tumor suppressors, which restrict positive GLI activity. In cancer, the acquisition of oncogenic mutations and the loss of tumor suppressors - the oncogenic load - regulates the GLI code toward progressively more activating states. The fine and reversible balance of GLI activating GLI(A) and GLI repressing GLI(R) states is lost in cancer. Here, the acquisition of GLI(A) levels above a given threshold is predicted to lead to advanced malignant stages. In this review we highlight the concepts of the GLI code, the oncogenic load, the context-dependency of GLI action, and different modes of signaling integration such as that of HH and EGF. Targeting the GLI code directly or indirectly promises therapeutic benefits beyond the direct blockade of individual pathways.

Cis-regulatory underpinnings of human GLI3 expression in embryonic craniofacial structures and internal organs

The zinc finger transcription factor Gli3 is an important mediator of Sonic hedgehog (Shh) signaling. During early embryonic development Gli3 participates in patterning and growth of the central nervous system, face, skeleton, limb, tooth and gut. Precise regulation of the temporal and spatial expression of Gli3 is crucial for the proper specification of these structures in mammals and other vertebrates. Previously we reported a set of human intronic cis-regulators controlling almost the entire known repertoire of endogenous Gli3 expression in mouse neural tube and limbs. However, the genetic underpinning of GLI3 expression in other embryonic domains such as craniofacial structures and internal organs remain elusive. Here we demonstrate in a transgenic mice assay the potential of a subset of human/fish conserved non-coding sequences (CNEs) residing within GLI3 intronic intervals to induce reporter gene expression at known regions of endogenous Gli3 transcription in embryonic domains other than central nervous system (CNS) and limbs. Highly specific reporter expression was observed in craniofacial structures, eye, gut, and genitourinary system. Moreover, the comparison of expression patterns directed by these intronic cis-acting regulatory elements in mouse and zebrafish embryos suggests that in accordance with sequence conservation, the target site specificity of a subset of these elements remains preserved among these two lineages. Taken together with our recent investigations, it is proposed here that during vertebrate evolution the Gli3 expression control acquired multiple, independently acting, intronic enhancers for spatiotemporal patterning of CNS, limbs, craniofacial structures and internal organs.

Fgf22 regulated by Fgf3/Fgf8 signaling is required for zebrafish midbrain development

Fibroblast growth factor (Fgf) signaling plays important roles in various developmental processes including brain development. Here, we identified zebrafish fgf22 predominantly expressed in the posterior midbrain and anterior midbrain-hindbrain boundary (MHB) primordia during early embryonic brain development. To examine roles of Fgf22 in midbrain development, we analyzed fgf22 knockdown embryos. The fgf22 morphants were defective in proper formation of the MHB constriction and the midbrain. The knockdown of fgf22 caused decreased cell proliferation in the midbrain, expanded expression of roof plate and tegmental marker genes, and decreased expression of tectal marker genes, indicating that Fgf22 is required for cell proliferation, roof plate formation, and tectum specification in the midbrain. Fgf receptor 2b (Fgfr2b), a potential receptor for Fgf22, was also required, indicating that Fgf22 signaling is mediated through Fgfr2b. The floor plate and the MHB are crucial for the dorsoventral patterning of the midbrain through Hedgehog (Hh) and Fgf signaling, respectively. The fgf3/fgf8 double morphant phenotype was essentially similar to that of fgf22 morphants, whereas the phenotype caused by inhibition of Hh signaling was not. fgf3 and fgf8 were expressed earlier than fgf22 in the MHB primordium and Fgf3/Fgf8 signaling was required for fgf22 expression in the posterior midbrain. Furthermore, fgf22 partially rescued the fgf3/fgf8 double morphant phenotype. The present results indicate Fgf22 to be involved in midbrain development downstream of Fgf3 and Fgf8 in the MHB but not of Hh in the floor plate.

Functional characterization of a heterozygous GLI2 missense mutation in patients with multiple pituitary hormone deficiency

CONTEXT

The GLI2 transcription factor is a major effector protein of the sonic hedgehog pathway and suggested to play a key role in pituitary development. Genomic GLI2 aberrations that mainly result in truncated proteins have been reported to cause holoprosencephaly or holoprosencephaly-like features, sometimes associated with hypopituitarism.

OBJECTIVE

Our objective was to determine the frequency of GLI2 mutations in patients with multiple pituitary hormone deficiency (MPHD).

DESIGN

Patients were selected from participants in the Genetics and Neuroendocrinology of Short Stature International Study (GeNeSIS) program. Patients with mutations within established candidate genes were excluded.

PATIENTS

A total of 165 patients with MPHD defined as GH deficiency and at least 1 additional pituitary hormone deficiency were studied regardless of the presence of extrapituitary clinical manifestations.

MAIN OUTCOME MEASURES

Prevalence of GLI2 variations in MPHD patients was assessed and detailed phenotypic characterization is given. Transcriptional activity of identified GLI2 variants was evaluated by functional reporter assays.

RESULTS

In 5 subjects, 4 heterozygous missense variants were identified, of which 2 are unpublished so far. One variant, p.R516P, results in vitro in a complete loss of protein function. In addition to GH deficiency, the carrier of the mutation demonstrates deficiency of thyrotrope and gonadotrope function, a maldescended posterior pituitary lobe, and polydactyly, but no midline defects.

CONCLUSIONS

For the first time, we show that heterozygous amino acid substitutions within GLI2 may lead to MPHD with mild extrapituitary findings. The phenotype of GLI2 mutations is variable, and penetrance is incomplete. GLI2 mutations are associated with anterior pituitary hypoplasia, and frequently, ectopy of the posterior lobe occurs.

The abluminal endothelial membrane in neurovascular remodeling in health and disease

After brain injury, blood-brain barrier (BBB) integrity can be compromised as a consequence of the breakdown of cell-cell interactions in the neurovascular unit, resulting in the loss of the characteristic endothelial luminal-to-abluminal structure. During the process of restoration of the BBB and vascularization, the endothelial cells are continuously reshaped, with both the luminal and abluminal membranes serving as sites of signaling. Here, we focus on the bidirectional signaling processes that are rapidly initiated between endothelial and perivascular cells and occur in certain brain diseases or in response to injury. The goal of these processes is (i) the reemergence of endothelial cell polarity, (ii) the remodeling of extracellular matrix interactions, (iii) the realignment of pericytes and astrocytic endfeet with endothelial cells, and (iv) the restitution of a well-organized and stable BBB. This abluminal membrane exemplifies how the brain vasculature responds to stressors and may represent promising targets for therapeutic interventions of brain diseases.

Crosstalk between TGF-β and hedgehog signaling in cancer

Hedgehog (HH) and TGF-β signals control various aspects of embryonic development and cancer progression. While their canonical signal transduction cascades have been well characterized, there is increasing evidence that these pathways are able to exert overlapping activities that challenge efficient therapeutic targeting. We herein review the current knowledge on HH signaling and summarize the recent findings on the crosstalks between the HH and TGF-β pathways in cancer.

Gli2 and MEF2C activate each other's expression and function synergistically during cardiomyogenesis in vitro

The transcription factors Gli2 (glioma-associated factor 2), which is a transactivator of Sonic Hedgehog (Shh) signalling, and myocyte enhancer factor 2C (MEF2C) play important roles in the development of embryonic heart muscle and enhance cardiomyogenesis in stem cells. Although the physiological importance of Shh signalling and MEF2 factors in heart development is well known, the mechanistic understanding of their roles is unclear. Here, we demonstrate that Gli2 and MEF2C activated each other's expression while enhancing cardiomyogenesis in differentiating P19 EC cells. Furthermore, dominant-negative mutant proteins of either Gli2 or MEF2C repressed each other's expression, while impairing cardiomyogenesis in P19 EC cells. In addition, chromatin immunoprecipitation (ChIP) revealed association of Gli2 to the Mef2c gene, and of MEF2C to the Gli2 gene in differentiating P19 cells. Finally, co-immunoprecipitation studies showed that Gli2 and MEF2C proteins formed a complex, capable of synergizing on cardiomyogenesis-related promoters containing both Gli- and MEF2-binding elements. We propose a model whereby Gli2 and MEF2C bind each other's regulatory elements, activate each other's expression and form a protein complex that synergistically activates transcription, enhancing cardiac muscle development. This model links Shh signalling to MEF2C function during cardiomyogenesis and offers mechanistic insight into their in vivo functions.

Hedgehog-EGFR cooperation response genes determine the oncogenic phenotype of basal cell carcinoma and tumour-initiating pancreatic cancer cells

Inhibition of Hedgehog (HH)/GLI signalling in cancer is a promising therapeutic approach. Interactions between HH/GLI and other oncogenic pathways affect the strength and tumourigenicity of HH/GLI. Cooperation of HH/GLI with epidermal growth factor receptor (EGFR) signalling promotes transformation and cancer cell proliferation in vitro. However, the in vivo relevance of HH-EGFR signal integration and the critical downstream mediators are largely undefined. In this report we show that genetic and pharmacologic inhibition of EGFR signalling reduces tumour growth in mouse models of HH/GLI driven basal cell carcinoma (BCC). We describe HH-EGFR cooperation response genes including SOX2, SOX9, JUN, CXCR4 and FGF19 that are synergistically activated by HH-EGFR signal integration and required for in vivo growth of BCC cells and tumour-initiating pancreatic cancer cells. The data validate EGFR signalling as drug target in HH/GLI driven cancers and shed light on the molecular processes controlled by HH-EGFR signal cooperation, providing new therapeutic strategies based on combined targeting of HH-EGFR signalling and selected downstream target genes.

Canonical and noncanonical Hedgehog/GLI signaling in hematological malignancies

The highly conserved Hedgehog/GLI signaling pathway regulates multiple aspects of embryonic development and plays a decisive role in tissue homeostasis and the hematopoietic system by controlling cell fate decisions, stem cell self-renewal, and activation. Loss of negative control of Hedgehog signaling contributes to tumor pathogenesis and progression. In the classical view of canonical Hedgehog signaling, Hedgehog ligand binding to its receptor Patched culminates in the activation of the key pathway activator Smoothened, followed by activation of the GLI transcription factors. Its essential function and druggability render Smoothened well suited to therapeutic intervention. However, recent evidence suggests a critical role of Smoothened-independent regulation of GLI activity by several other signaling pathways including the PI3K/AKT and RAS/RAF/MEK/ERK axes. In addition, the contribution of canonical Hedgehog signaling via Patched and Smoothened to normal and malignant hematopoiesis has been the subject of recent controversies. In this review, we discuss the current understanding and controversial findings of canonical and noncanonical GLI activation in hematological malignancies in light of the current therapeutic strategies targeting the Hedgehog pathway.

Hedgehog signaling and the Gli code in stem cells, cancer, and metastases

The Hedgehog (Hh)-Gli signaling pathway is an essential pathway involved in development and cancer. It controls the Gli code-the sum of all activator and repressor functions of the Gli transcription factors. Through the Gli code, and Gli1 in particular, it modulates the fate and behavior of stem and cancer stem cells, as well as tumor growth and survival in many human cancer types. It also affects recurrence and metastasis and is enhanced in advanced tumors, where it promotes an embryonic stem (ES) cell-like gene expression signature. A central component of this signature, Nanog, is critical for glioblastoma and cancer stem cell survival and expansion. Gli1 activity is also enhanced by several oncogenic proteins, including Ras, Myc, and Akt, and by loss of tumor suppressors, such as p53 and PTEN. The oncogenic load boosts Gli1 levels, which supports tumor progression, and promotes a critical threshold of Gli1 activity that allows cells to enter the metastatic transition. In colon cancers, this transition is defined by enhanced Hh-Gli and, surprisingly, by repressed Wnt-Tcf signaling. Together our data support a model in which the Gli code, and Gli1 in particular, acts as a key sensor that responds to both Hh signals and the oncogenic load. We hypothesize that, in turn, the Gli-regulated ES-like factors induce a reprogramming event in cancer stem cells that promotes high invasion, growth and/or metastasis. Targeting the Gli code, the autoregulatory Gli1-Nanog module and interacting partners and pathways thus offers new therapeutic possibilities.

Akt1 interacts with epidermal growth factor receptors and hedgehog signaling to increase stem/transit amplifying cells in the embryonic mouse cortex

A subset of precursors in the embryonic mouse cortex and in neurospheres expresses a higher level of the serine/threonine kinase Akt1 than neighboring precursors. We reported previously that the functional significance of high Akt1 expression was enhanced Akt1 activity, resulting in an increase in survival, proliferation, and self-renewal of multipotent stem/transit amplifying cells. Akt1 can interact with a number of signaling pathways, but the extrinsic factors that are required for specific effects of elevated Akt1 expression have not been identified. In this study we addressed the contributions of signaling via epidermal growth factor (EGF) and hedgehog (Hh) receptors. In EGF receptor-null precursors or following transient inhibition of EGF receptor tyrosine kinase activity, elevating Akt1 by retroviral transduction could still increase survival and proliferation but could not increase self-renewal. We also found that elevated Akt1 expression induced the expression of EGF receptors (EGFRs) in wild-type precursors. Several extrinsic factors, including Shh, can induce EGFR expression by cortical precursors, and we found that elevating Akt1 allowed them to respond to a subthreshold concentration of Shh to induce EGFRs. In precursors that lack the Hh receptor smoothened, however, elevating Akt1 did not increase EGFR expression or self-renewal, though it could still stimulate proliferation. These findings suggest that a subset of precursors in the embryonic cortex that express an elevated level of Akt1 can respond to lower concentrations of Shh than neighboring precursors, resulting in an increase in their expression of EGFRs. Signaling via EGFRs is required for their self-renewal.

TGF-β/SMAD/GLI2 signaling axis in cancer progression and metastasis

The Hedgehog (HH) and TGF-β signaling pathways represent essential regulators of cell proliferation and differentiation during embryogenesis. Pathway deregulation is a characteristic of various cancers. Recently, evidence for a convergence of these pathways at the level of the GLI2 transcription factor in the context of tumor initiation and progression to metastasis has emerged. This short review summarizes recent knowledge about GLI2 function and mechanisms of action downstream of TGF-β in cancer.

Conditional ablation of Pten in osteoprogenitors stimulates FGF signaling

Phosphatase and tensin homolog deleted on chromosome ten (PTEN) is a direct antagonist of phosphatidylinositol 3 kinase. Pten is a well recognized tumor suppressor and is one of the most commonly mutated genes in human malignancies. More recent studies of development and stem cell behavior have shown that PTEN regulates the growth and differentiation of progenitor cells. Significantly, PTEN is found in osteoprogenitor cells that give rise to bone-forming osteoblasts; however, the role of PTEN in bone development is incompletely understood. To define how PTEN functions in osteoprogenitors during bone development, we conditionally deleted Pten in mice using the cre-deleter strain Dermo1cre, which targets undifferentiated mesenchyme destined to form bone. Deletion of Pten in osteoprogenitor cells led to increased numbers of osteoblasts and expanded bone matrix. Significantly, osteoblast development and synthesis of osteoid in the nascent bone collar was uncoupled from the usual tight linkage to chondrocyte differentiation in the epiphyseal growth plate. The expansion of osteoblasts and osteoprogenitors was found to be due to augmented FGF signaling as evidenced by (1) increased expression of FGF18, a potent osteoblast mitogen, and (2) decreased expression of SPRY2, a repressor of FGF signaling. The differentiation of osteoblasts was autonomous from the growth plate chondrocytes and was correlated with an increase in the protein levels of GLI2, a transcription factor that is a major mediator of hedgehog signaling. We provide evidence that increased GLI2 activity is also a consequence of increased FGF signaling through downstream events requiring mitogen-activated protein kinases. To test whether FGF signaling is required for the effects of Pten deletion, we deleted one allele of fibroblast growth factor receptor 2 (FGFR2). Significantly, deletion of FGFR2 caused a partial rescue of the Pten-null phenotype. This study identifies activated FGF signaling as the major mediator of Pten deletion in osteoprogenitors.

The transcriptional repressor REST/NRSF modulates hedgehog signaling

The spatial and temporal control of gene expression is key to generation of specific cellular fates during development. Studies of the transcriptional repressor REST/NRSF (RE1 Silencing Transcription Factor or Neural Restrictive Silencing Factor) have provided important insight into the role that epigenetic modifications play in differential gene expression. However, the precise function of REST during embryonic development is not well understood. We have discovered a novel interaction between zebrafish Rest and the Hedgehog (Hh) signaling pathway. We observed that Rest knockdown enhances or represses Hh signaling in a context-dependant manner. In wild-type embryos and embryos with elevated Hh signaling, Rest knockdown augments transcription of Hh target genes. Conversely, in contexts where Hh signaling is diminished, Rest knockdown has the opposite effect and Hh target gene expression is further attenuated. Epistatic analysis revealed that Rest interacts with the Hh pathway at a step downstream of Smo. Furthermore, we present evidence implicating the bifunctional, Hh signaling component Gli2a as key to the Rest modulation of the Hh response. The role of Rest as a regulator of Hh signaling has broad implications for many developmental contexts where REST and Hh signaling act.

Context-dependent regulation of the GLI code in cancer by HEDGEHOG and non-HEDGEHOG signals

A surprisingly large and unrelated number of human tumors depend on sustained HEDGEHOG-GLI (HH-GLI) signaling for growth. This includes cancers of the skin, brain, colon, lungs, prostate, blood and pancreas among others. The basis of such commonality is not obvious. HH-GLI signaling has also been shown to be active in and required for cancer stem cell survival and expansion in different cancer types, and its activity is essential not only for tumor growth but also for recurrence and metastatic growth, two key medical problems. Here we review recent data on the role of HH-GLI signaling in cancer focusing on the role of the GLI code, the regulated combinatorial and cooperative function of repressive and activating forms of all Gli transcription factors, as a signaling nexus that integrates not only HH signals but also those of multiple tumor suppressors and oncogenes. Recent data support the view that the context-dependent regulation of the GLI code by oncogenes and tumor suppressors constitutes a basis for the widespread involvement of GLI1 in human cancers, representing a perversion of its normal role in the control of stem cell lineages during normal development and homeostasis.

A GLI1-p53 inhibitory loop controls neural stem cell and tumour cell numbers

How cell numbers are determined is not understood. Hedgehog-Gli activity is involved in precursor cell proliferation and stem cell self-renewal, and its deregulation sustains the growth of many human tumours. However, it is not known whether GLI1, the final mediator of Hh signals, controls stem cell numbers, and how its activity is restricted to curtail tumourigenesis. Here we have altered the levels of GLI1 and p53, the major tumour suppressor, in multiple systems. We show that GLI1 expression in Nestin+ neural progenitors increases precursor and clonogenic stem cell numbers in vivo and in vitro. In contrast, p53 inhibits GLI1-driven neural stem cell self-renewal, tumour growth and proliferation. Mechanistically, p53 inhibits the activity, nuclear localisation and levels of GLI1 and in turn, GLI1 represses p53, establishing an inhibitory loop. We also find that p53 regulates the phosphorylation of a novel N' truncated putative activator isoform of GLI1 in human cells. The balance of GLI1 and p53 functions, thus, determines cell numbers, and prevalence of p53 restricts GLI1-driven stem cell expansion and tumourigenesis.

K, S. D. Branchless and Hedgehog operate in a positive feedback loop to regulate the initiation of neuroblast division in the Drosophila larval brain

The Drosophila central nervous system is produced by two rounds of neurogenesis: one during embryogenesis to form the larval brain and one during larval stages to form the adult central nervous system. Neurogenesis caused by the activation of neural stem division in the larval brain is essential for the proper patterning and functionality of the adult central nervous system. Initiation of neuroblast proliferation requires signaling by the Fibroblast Growth Factor homolog Branchless and by the Hedgehog growth factor. We show here that the Branchless and Hedgehog pathways form a positive feedback loop to regulate the onset of neuroblast division. This feedback loop is initiated during embryogenesis. Our genetic and molecular studies demonstrate that the absolute level of Branchless and Hedgehog signaling is critical to fully activate stem cell division. Furthermore, over-expression and mutant studies establish that signaling by Branchless is the crucial output of the feedback loop that stimulates neuroblast division and that Branchless signaling is necessary for initiating the division of all mitotically regulated neuroblasts in the brain lobes. These studies establish the molecular mechanism through which Branchless and Hedgehog signaling interface to regulate the activation of neural stem cell division.

The Gli code: an information nexus regulating cell fate, stemness and cancer

The Gli code hypothesis postulates that the three vertebrate Gli transcription factors act together in responding cells to integrate intercellular Hedgehog (Hh) and other signaling inputs, resulting in the regulation of tissue pattern, size and shape. Hh and other inputs are then just ways to modify the Gli code. Recent data confirm this idea and suggest that the Gli code regulates stemness and also tumor progression and metastatic growth, opening exciting possibilities for both regenerative medicine and novel anticancer therapies.

Induction of sonic hedgehog mediators by transforming growth factor-beta: Smad3-dependent activation of Gli2 and Gli1 expression in vitro and in vivo

Hedgehog (Hh) and transforming growth factor-beta (TGF-beta) family members are involved in numerous overlapping processes during embryonic development, hair cycle, and cancer. Herein, we show that TGF-beta induces the expression of the Hh signaling molecules Gli1 and Gli2 in various human cell types, including normal fibroblasts and keratinocytes, as well as various cancer cell lines. Gli2 induction by TGF-beta is rapid, independent from Hh receptor signaling, and requires a functional Smad pathway. Gli1 expression is subsequently activated in a Gli2-dependent manner. In transgenic mice overexpressing TGF-beta1 in the skin, Gli1 and Gli2 expression is also elevated and depends on Smad3. In pancreatic adenocarcinoma cell lines resistant to Hh inhibition, pharmacologic blockade of TGF-beta signaling leads to repression of cell proliferation accompanied with a reduction in Gli2 expression. We thus identify TGF-beta as a potent transcriptional inducer of Gli transcription factors. Targeting the cooperation of Hh and TGF-beta signaling may provide new therapeutic opportunities for cancer treatment.

Ultraconserved non-coding sequence element controls a subset of spatiotemporal GLI3 expression

The zinc-finger transcription factor GLI3 acts during vertebrate development in a combinatorial, context-dependent fashion as a primary transducer of sonic hedgehog (SHH) signaling. In humans, mutations affecting this key regulator of development are associated with GLI3-morphopathies, a group of congenital malformations in which forebrain and limb development are preferentially affected. We show that a non-coding element from intron two of GLI3, ultraconserved in mammals and highly conserved in the pufferfish Fugu, is a transcriptional enhancer. In transient transfection assays, it activates reporter gene transcription in human cell cultures expressing endogenous GLI3 but not in GLI3 negative cells. The identified enhancer element is predicted to contain conserved binding sites for transcription factors crucial for developmental steps in which GLI3 is involved. The regulatory potential of this element is conserved and was used to direct tissue-specific expression of a green fluorescent protein reporter gene in zebrafish embryos and of a beta-galactosidase reporter in transgenic mouse embryos. Time, location, and quantity of reporter gene expression are congruent with part of the pattern previously reported for endogenous GLI3 transcription.

Human GLI3 intragenic conserved non-coding sequences are tissue-specific enhancers

The zinc-finger transcription factor GLI3 is a key regulator of development, acting as a primary transducer of Sonic hedgehog (SHH) signaling in a combinatorial context dependent fashion controlling multiple patterning steps in different tissues/organs. A tight temporal and spatial control of gene expression is indispensable, however, cis-acting sequence elements regulating GLI3 expression have not yet been reported. We show that 11 ancient genomic DNA signatures, conserved from the pufferfish Takifugu (Fugu) rubripes to man, are distributed throughout the introns of human GLI3. They map within larger conserved non-coding elements (CNEs) that are found in the tetrapod lineage. Full length CNEs transiently transfected into human cell cultures acted as cell type specific enhancers of gene transcription. The regulatory potential of these elements is conserved and was exploited to direct tissue specific expression of a reporter gene in zebrafish embryos. Assays of deletion constructs revealed that the human-Fugu conserved sequences within the GLI3 intronic CNEs were essential but not sufficient for full-scale transcriptional activation. The enhancer activity of the CNEs is determined by a combinatorial effect of a core sequence conserved between human and teleosts (Fugu) and flanking tetrapod-specific sequences, suggesting that successive clustering of sequences with regulatory potential around an ancient, highly conserved nucleus might be a possible mechanism for the evolution of cis-acting regulatory elements.

Melanomas require HEDGEHOG-GLI signaling regulated by interactions between GLI1 and the RAS-MEK/AKT pathways

Melanoma is one of the most aggressive cancers, and its incidence is increasing. These tumors derive from the melanocyte lineage and remain incurable after metastasis. Here we report that SONIC HEDGEHOG (SHH)-GLI signaling is active in the matrix of human hair follicles, and that it is required for the normal proliferation of human melanocytes in culture. SHH-GLI signaling also regulates the proliferation and survival of human melanomas: the growth, recurrence, and metastasis of melanoma xenografts in mice are prevented by local or systemic interference of HH-GLI function. Moreover, we show that oncogenic RAS-induced melanomas in transgenic mice express Gli1 and require Hh-Gli signaling in vitro and in vivo. Finally, we provide evidence that endogenous RAS-MEK and AKT signaling regulate the nuclear localization and transcriptional activity of GLI1 in melanoma and other cancer cells. Our data uncover an unsuspected role of HH-GLI signaling in melanocytes and melanomas, demonstrate a role for this pathway in RAS-induced tumors, suggest a general integration of the RAS/AKT and HH-GLI pathways, and open a therapeutic approach for human melanomas.

PTHrP regulates growth plate chondrocyte differentiation and proliferation in a Gli3 dependent manner utilizing hedgehog ligand dependent and independent mechanisms

Growth plate chondrocytes undergo a tightly regulated process of differentiation, allowing for the longitudinal growth of bones. Although it is known that parathyroid hormone related protein (PTHrP) and Indian hedgehog regulate the differentiation of growth plate chondrocytes, how these pathways interact to regulate chondrocyte development is not fully elucidated. We examined how the interaction between PTHrP and the hedgehog activated transcription factors, Gli2 and Gli3, regulates growth plate chondrocyte differentiation and proliferation. Analysis of fetal limbs showed that Gli2 is a negative regulator and Gli3 a positive regulator of type X collagen expression. Limb explant cultures showed that PTHrP treatment inhibited type X collagen expression and increased chondrocyte proliferation. This effect was substantially enhanced in Gli2-/- limbs, was blocked in Gli3-/- limbs, and was only partially inhibited by hedgehog ligand blockade. PTHrP negatively regulated Gli mediated transcription in cell cultures, and regulated the level of the repressor form of Gli3 in a PKA dependent manner. These results show that PTHrP regulates growth plate chondrocyte proliferation and differentiation in part through the activity of Gli3, suggesting a crucial role for Gli3 in growth plate chondrocyte development.

The role of Shh transcription activator Gli2 in chick cloacal development

Patterning and differentiation along the dorsal-ventral (D-V) axis lead to cloacal partitioning into ventral urinary and dorsal alimentary tracts in most mammals, but not birds and fish. We previously reported that the major activator of Sonic hedgehog (Shh) signaling transcription factor Gli2 plays an essential role in cloacal partitioning along the D-V axis in a mouse model. Here, we report that chick cloacal patterning and differentiation is along the anterior-posterior axis. During chick cloacal formation, Shh is expressed strongly in hindgut endoderm; Gli2 is very weakly detected in the surrounding hindgut mesoderm. In the mesoderm of the cloacal region, the over-expression of the constitutively active form of mouse Gli2 has been shown to: not induce cloacal partitioning along the D-V axis; induce expression of Ptch1, Gli2, bmp4, wnt5a, and hoxd-13, which have been previously shown to play a role in hindgut patterning; increase cell proliferation; and reduce apoptosis. Interestingly, p63 expression in the cloacal endoderm is also up-regulated, suggesting an interaction between the Shh and p63 pathways. In conclusion, Gli2 alone is insufficient to induce partitioning along the D-V axis in the chick embryo. However, Gli2 regulates both epithelial and mesenchymal cell proliferation and apoptosis during cloacal development.

Sonic hedgehog signaling in forebrain development and its interactions with pathways that modify its effects

During the development of the nervous system and other organs in the embryo, a limited set of master signaling pathways are used repeatedly for induction, patterning and growth. Among these, the Sonic hedgehog (Shh) pathway is crucial for the development of many structures in the brain. How the context-specific interplay between these various signaling pathways produces distinct temporal and spatial outcomes is not clear. Resolving this problem is a major goal in the study of cell and organ development. Here, we focus on signaling events during dorso-ventral patterning of the embryonic forebrain in vertebrates. In particular, we discuss the role of the Shh pathway in this process and on its interactions with the FGF, retinoic acid and Nodal pathways and other information cascades that modify its effects.

Zic3 is critical for early embryonic patterning during gastrulation

Mutations in the zinc finger transcription factor ZIC3 are associated with human left-right patterning abnormalities (X-linked heterotaxy, HTX1, MIM 306955), and mice null for Zic3 show a similar phenotype. However, the developmental function of Zic3 is largely unknown and its expression in early embryonic development suggests a role prior to organ formation. The current study of Zic3 null mice identifies a novel function for Zic3 in the gastrula-stage embryo. Analysis of Zic3 function at early embryonic stages shows that it ensures the fidelity of embryonic patterning, including patterning of the anterior visceral endoderm, the initiation of gastrulation, and positioning of the primitive streak. At later stages, deficiency of Zic3 results in abnormal mesoderm allocation. These results indicate a requirement for Zic3 during early embryogenesis prior to cardiac and visceral organ patterning.

Oncogenic Met receptor induces ectopic structures in Xenopus embryos

When aberrantly expressed or activated, the Met receptor tyrosine kinase is involved in tumor invasiveness and metastasis. In this study, we have used the Xenopus embryonic system to define the role of various Met proximal-binding partners and downstream signaling pathways in regulating an induced morphogenetic event. We show that expression of an oncogenic derivative of the Met receptor (Tpr-Met) induces ectopic morphogenetic structures during Xenopus embryogenesis. Using variant forms of Tpr-Met that are engineered to recruit a specific signaling molecule of choice, we demonstrate that the sole recruitment of either the Grb2 or the Shc adaptor protein is sufficient to induce ectopic structures and anterior reduction, while the recruitment of PI-3Kinase (PI-3K) is necessary but not sufficient for this effect. In contrast, the recruitment of PLCgamma can initiate the induction, but fails to maintain or elongate supernumerary structures. Finally, evidence indicates that the Ras/Raf/MAPK pathway is necessary, but not sufficient to induce these structures. This study also emphasizes the importance of examining signaling molecules in the regulatory context that is provided by receptor/effector interactions when assessing a role in cell growth and differentiation.

Protein kinase C-delta and mitogen-activated protein/extracellular signal-regulated kinase-1 control GLI activation in hedgehog signaling

One third of all lethal cancers are associated with excessive activation of the Hedgehog (HH) pathway by mutations of its signaling components or by increased responsiveness of cells to the HH ligand. HH signaling through the GLI transcription factors leads to increased cell proliferation by up-regulation of the extracellular regulated kinase (ERK) pathway and by expression of S phase cyclins. In this study, we have tested the hypothesis that the HH pathway can integrate ERK signaling to modulate the activity of GLI. Using NIH 3T3 cells, we show that phorbol esters, acting through protein kinase C-delta (PKCdelta) and mitogen-activated protein/extracellular signal-regulated kinase-1 (MEK-1), fully stimulate the transcriptional activity of endogenous and overexpressed GLI proteins, as assessed by GLI-luciferase reporter assays, and induce the expression of endogenous GLI1 and PTCH-1 target genes, as assessed by reverse transcription-PCR. Moreover, activation of GLI elicited by Sonic Hedgehog also requires PKCdelta and MEK-1 function. Remarkably, coexpression of activated MEK-1 and GLI1 or GLI2 induced a 10-fold synergistic increase in GLI-luciferase activity that was totally blocked by PD98059. The NH(2)-terminal region of GLI1 (amino acids 1-130) is required for sensing the ERK pathway, as deletion of this domain produces active GLI1 protein with greatly reduced response to activation by MEK-1. Basic fibroblast growth factor activation of the ERK pathway also stimulated GLI1 activity through its NH(2)-terminal domain. Our results identify PKCdelta and MEK-1 as essential, positive regulators of GLI-mediated HH signaling. Furthermore, our findings suggest that tumors with deregulated HH and ERK synergize to stimulate cell proliferation pathways.

Fgf19 regulated by Hh signaling is required for zebrafish forebrain development

Fibroblast growth factor (Fgf) signaling plays important roles in brain development. Fgf3 and Fgf8 are crucial for the formation of the forebrain and hindbrain. Fgf8 is also required for the midbrain to form. Here, we identified zebrafish Fgf19 and examined its roles in brain development by knocking down Fgf19 function. We found that Fgf19 expressed in the forebrain, midbrain and hindbrain was involved in cell proliferation and cell survival during embryonic brain development. Fgf19 was also essential for development of the ventral telencephalon and diencephalon. Regional specification is linked to cell type specification. Fgf19 was also essential for the specification of gamma-aminobutyric acid (GABA)ergic interneurons and oligodendrocytes generated in the ventral telencephalon and diencephalon. The cross talk between Fgf and Hh signaling is critical for brain development. In the forebrain, Fgf19 expression was down-regulated on inhibition of Hh but not of Fgf3/Fgf8, and overexpression of Fgf19 rescued partially the phenotype on inhibition of Hh. The present findings indicate that Fgf19 signaling is crucial for forebrain development by interacting with Hh and provide new insights into the roles of Fgf signaling in brain development.

Neptune is involved in posterior axis and tail formation inXenopus embryogenesis

In order to elucidate the molecular mechanisms underlying the posterior axis and tail formation in embryogenesis, the function of Neptune, a zinc-finger transcription factor, in Xenopus laevis embryos was investigated. Injection of neptune mRNA into the animal pole area of embryos resulted in the formation of an additional tail structure that included a neural tube and muscle tissue. This activity required FGF signaling since coinjection of a dominant-negative FGF receptor RNA (XFD) completely blocked the formation of a tail structure. A loss-of-function experiment using a fusion construct of neptune and Drosophila engrailed (en-neptune) RNA showed that endogenous Neptune is necessary for formation of the posterior trunk and tail. Furthermore, activity of Neptune was necessary for the endogenous expression of brachyury and fgf-8 at the late gastrula stage. These findings demonstrate a novel function of Neptune in the process of anterior-posterior axis formation through the FGF and brachyury signaling cascades. An experiment using a combination explant with ventral and dorsal marginal tissues showed that cooperation of these two distinct tissues is important for the tail formation and that expression of Neptune in prospective ventral cells may be involved in the activation of the process of tail formation.

Activation of the BCL2 promoter in response to Hedgehog/GLI signal transduction is predominantly mediated by GLI2

Aberrant activation of the Hedgehog (HH)/GLI signaling pathway has been implicated in the development of basal cell carcinoma (BCC). The zinc finger transcription factors GLI1 and GLI2 are considered mediators of the HH signal in epidermal cells, although their tumorigenic nature and their relative contribution to tumorigenesis are only poorly understood. To shed light on the respective role of these transcription factors in epidermal neoplasia, we screened for genes preferentially regulated either by GLI1 or GLI2 in human epidermal cells. We show here that expression of the key antiapoptotic factor BCL2 is predominantly activated by GLI2 compared with GLI1. Detailed promoter analysis and gel shift assays identified three GLI binding sites in the human BCL2 cis-regulatory region. We found that one of these binding sites is critical for conferring GLI2-specific activation of the human BCL2 promoter and that the selective induction of BCL2 expression depends on the zinc finger DNA binding domain of GLI2. In vivo, GLI2 and BCL2 were coexpressed in the outer root sheath of hair follicles and BCC and in plasma cells that infiltrated BCC tumor islands. On the basis of the latter observation, we analyzed plasma cell-derived tumors and found strong expression of GLI2 and BCL2 in neoplastic cells of plasmacytoma patients, implicating HH/GLI signaling in the development of plasma cell-derived malignancies. The results reveal a central role for GLI2 in activating the prosurvival factor BCL2, which may represent an important mechanism in the development or maintenance of cancers associated with inappropriate HH signaling.

GLI2 is expressed in normal human epidermis and BCC and induces GLI1 expression by binding to its promoter

Sonic hedgehog (Shh) binds to its receptor patched (PTCH), leading to the activation and repression of target genes via the GLI family of zinc-finger transcription factors. Deregulation of the Shh pathway is associated with basal cell carcinoma (BCC) due to upregulation of GLI1 and GLI2. We recently demonstrated a positive feedback loop between GLI1 and GLI2, which revealed that GLI1 may be a direct target of GLI2. Using band shift and luciferase reporter assays, we now show that GLI2 binds the GLI-binding consensus sequence in the GLI1 promoter. These data suggest that GLI2 directly activates GLI1 and that retrovirally expressed GLI2 induces expression of endogenous GLI1 in human primary keratinocytes. Finally, using in situ hybridization, we show that GLI2 is expressed in the interfollicular epidermis and the outer root sheath of hair follicles in normal skin as well as in BCC tumor islands. These results suggest an important role for GLI2 in regulating epidermal proliferation and skin tumorigenesis.

FOXE1, a new transcriptional target of GLI2 is expressed in human epidermis and basal cell carcinoma

Sonic hedgehog (Hh) signaling plays a key role in epidermal development and skin cancer. Mutational inactivation of the tumor suppressor gene patched (PTCH) leads to constitutive activation of the Hh signaling pathway, resulting in activation of target gene transcription by the zinc finger transcription factors GLI1 and GLI2. Recent experiments in mice point to GLI2 as the key mediator of Hh signaling in skin. We have concentrated on the identification of candidate mediators of GLI2 function in the human epidermis. We show here that the forkhead/winged-helix domain transcription factor FOXE1 is likely to be a direct GLI2 target gene. The kinetics of FOXE1 induction are similar to the known direct target PTCH, and a 2.5 kb upstream fragment containing five GLI-binding sites activates transcription in a reporter assay. We show by in situ hybridization that FOXE1 is expressed in the outer root sheath of the hair follicle, where murine Gli2 is also expressed. FOXE1 expression is also found in basal keratinocytes of the human epidermis and basal cell carcinoma (BCC). These data point to a putative role of FOXE1 in mediating Hh signaling in the human epidermis downstream of GLI2.

Ectopic EphA4 receptor induces posterior protrusions via FGF signaling in Xenopus embryos

The Eph family of receptor tyrosine kinases regulates numerous biological processes. To examine the biochemical and developmental contributions of specific structural motifs within Eph receptors, wild-type or mutant forms of the EphA4 receptor were ectopically expressed in developing Xenopus embryos. Wild-type EphA4 and a mutant lacking both the SAM domain and PDZ binding motif were constitutively tyrosine phosphorylated in vivo and catalytically active in vitro. EphA4 induced loss of cell adhesion, ventro-lateral protrusions, and severely expanded posterior structures in Xenopus embryos. Moreover, mutation of a conserved SAM domain tyrosine to phenylalanine (Y928F) enhanced the ability of EphA4 to induce these phenotypes, suggesting that the SAM domain may negatively regulate some aspects of EphA4 activity in Xenopus. Analysis of double mutants revealed that the Y928F EphA4 phenotypes were dependent on kinase activity; juxtamembrane sites of tyrosine phosphorylation and SH2 domain-binding were required for cell dissociation, but not for posterior protrusions. The induction of protrusions and expansion of posterior structures is similar to phenotypic effects observed in Xenopus embryos expressing activated FGFR1. Furthermore, the budding ectopic protrusions induced by EphA4 express FGF-8, FGFR1, and FGFR4a. In addition, antisense morpholino oligonucleotide-mediated loss of FGF-8 expression in vivo substantially reduced the phenotypic effects in EphA4Y928F expressing embryos, suggesting a connection between Eph and FGF signaling.

Opposing FGF and retinoid pathways: a signalling switch that controls differentiation and patterning onset in the extending vertebrate body axis

Construction of the trunk/caudal region of the vertebrate embryo involves a set of distinct molecules and processes whose relationships are just coming into focus. In addition to the subdivision of the embryo into head and trunk domains, this "caudalisation" process requires the establishment and maintenance of a stem zone. This sequentially generates caudal tissues over a long period which then undergo differentiation and patterning in the extending body axis. Here we review recent studies that show that changes in the signalling properties of the paraxial mesoderm act as a switch that controls onset of differentiation and pattern in the spinal cord. These findings identify distinct roles for different caudalising factors; in particular, Fibroblast Growth Factor (FGF) inhibits differentiation in the caudal stem zone, while Retinoic acid (RA) provided rostrally by somitic mesoderm is required for neuronal differentiation and establishment of ventral neural pattern. Furthermore, the mutual opposition of FGF and RA pathways controls not only neural differentiation but also mesoderm segmentation and might also underlie the progressive assignment of rostrocaudal identity by regulating Hox gene availability and activation.

The emergent design of the neural tube: prepattern, SHH morphogen and GLI code

The Sonic hedgehog (Shh) pathway plays an important role in the development of many tissues and organs. The secreted ligand Shh has been shown to act as a mitogen, morphogen and survival factor in different contexts whereas the three Gli transcription factors act as Shh mediators in a context-dependent combinatorial fashion. The common wisdom has been that Gli protein function is subject to Shh signaling. One can ask how Gli proteins act and what the nature of Shh signaling during CNS dorsal-ventral patterning is. Is it possible that Hedgehog signals are only one of several ways to regulate Gli activity? Moreover, in light of the partial rescue of the neural tube phenotype of Shh or Smoothened mutant embryos in Shh(-/-);Gli3(-/-), Smoothened(-/-);Gli3(-/-), and Shh(-/-);Rab23(-/-) double null embryos, one can consider the roles that the Shh-Gli pathway may have taken to orchestrate congruent prepattern and growth, and the importance of creating the correct number of precursors in patterning mechanisms.

Morphogenesis and dysmorphogenesis of the appendicular skeleton

Cartilage patterning and differentiation are prerequisites for skeletal development through endochondral ossification (EO). Multipotential mesenchymal cells undergo a complex process of cell fate determination to become chondroprogenitors and eventually differentiate into chondrocytes. These developmental processes require the orchestration of cell-cell and cell-matrix interactions. In this review, we present limb bud development as a model for cartilage patterning and differentiation. We summarize the molecular and cellular events and signaling pathways for axis patterning, cell condensation, cell fate determination, digit formation, interdigital apoptosis, EO, and joint formation. The interconnected nature of these pathways underscores the effects of genetic and teratogenic perturbations that result in skeletal birth defects. The topics reviewed also include limb dysmorphogenesis as a result of genetic disorders and environmental factors, including FGFR, GLI3, GDF5/CDMP1, Sox9, and Cbfa1 mutations, as well as thalidomide- and alcohol-induced malformations. Understanding the complex interactions involved in cartilage development and EO provides insight into mechanisms underlying the biology of normal cartilage, congenital disorders, and pathologic adult cartilage.

The roles of Hedgehogs and Fibroblast Growth Factors in eye development and retinal cell rescue

Knowledge of normal eye development is crucial for the development of retinal rescue strategies. I shall focus on two signalling pathways that affect retinal development. Fibroblast growth factors function in retinal cell proliferation, retinal ganglion cell axon guidance and target recognition, craniofacial patterning and lens induction. Hedgehog proteins are required for progression of the neurogenic wave, cell proliferation, photoreceptor differentiation, retinal ganglion cell axon growth and craniofacial patterning. These signalling pathways have pleiotropic effects, can interact and have the potential to be used therapeutically. The zebrafish model organism may be well suited to studying how signalling pathways interact.

Progression of vertebrate limb development through SHH-mediated counteraction of GLI3

Distal limb development and specification of digit identities in tetrapods are under the control of a mesenchymal organizer called the polarizing region. Sonic Hedgehog (SHH) is the morphogenetic signal produced by the polarizing region in the posterior limb bud. Ectopic anterior SHH signaling induces digit duplications and has been suspected as a major cause underlying congenital malformations that result in digit polydactyly. Here, we report that the polydactyly of Gli3-deficient mice arises independently of SHH signaling. Disruption of one or both Gli3 alleles in mouse embryos lacking Shh progressively restores limb distal development and digit formation. Our genetic analysis indicates that SHH signaling counteracts GLI3-mediated repression of key regulator genes, cell survival, and distal progression of limb bud development.