Mouse suppressor of fused is a negative regulator of sonic hedgehog signaling and alters the subcellular distribution of Gli1

Pax6 regulates specification of ventral neurone subtypes in the hindbrain by establishing progenitor domains

Recent studies have shown that generation of different kinds of neurones is controlled by combinatorial actions of homeodomain (HD) proteins expressed in the neuronal progenitors. Pax6 is a HD protein that has previously been shown to be involved in the differentiation of the hindbrain somatic (SM) motoneurones and V1 interneurones in the hindbrain and/or spinal cord. To investigate in greater depth the role of Pax6 in generation of the ventral neurones, we first examined the expression patterns of HD protein genes and subtype-specific neuronal markers in the hindbrain of the Pax6 homozygous mutant rat. We found that Islet2 (SM neurone marker) and En1 (V1 interneurone marker) were transiently expressed in a small number of cells, indicating that Pax6 is not directly required for specification of these neurones. We also observed that domains of all other HD protein genes (Nkx2.2, Nkx6.1, Irx3, Dbx2 and Dbx1) were shifted and their boundaries became blurred. Thus, Pax6 is required for establishment of the progenitor domains of the ventral neurones. Next, we performed Pax6 overexpression experiments by electroporating rat embryos in whole embryo culture. Pax6 overexpression in the wild type decreased expression of Nkx2.2, but ectopically increased expression of Irx3, Dbx1 and Dbx2. Moreover, electroporation of Pax6 into the Pax6 mutant hindbrain rescued the development of Islet2-positive and En1-positive neurones. To know reasons for perturbed progenitor domain formation in Pax6 mutant, we examined expression patterns of Shh signalling molecules and states of cell death and cell proliferation. Shh was similarly expressed in the floor plate of the mutant hindbrain, while the expressions of Ptc1, Gli1 and Gli2 were altered only in the progenitor domains for the motoneurones. The position and number of TUNEL-positive cells were unchanged in the Pax6 mutant. Although the proportion of cells that were BrdU-positive slightly increased in the mutant, there was no relationship with specific progenitor domains. Taken together, we conclude that Pax6 regulates specification of the ventral neurone subtypes by establishing the correct progenitor domains.

Identification of sonic hedgehog-responsive genes using cDNA microarray

Sonic hedgehog (Shh) is a secreted signaling protein that plays important roles in a variety of developmental processes and also in pathogenesis of some human cancers and congenital diseases. Molecules that function downstream of Shh, however, still remain elusive. Here we searched for Shh-responsive genes by using an in-house cDNA microarray. Two genes were newly identified to be Shh responsive in neuroepithelial cell line MNS-70: the metal-binding protein Ceruloplasmin (Cp) and the serine protease inhibitor inter-alpha-trypsine inhibitor heavy chain H3 (ITIH3). In MNS-70 cells, expression of ITIH3 was regulated by Gli zinc-finger transcription factors downstream of Shh, whereas Cp appeared to be regulated by Gli-independent pathways. Cp mRNA was detected in the developing mouse brain, where its expression domain was closely adjacent to that of Shh. These results demonstrate that microarray technology provides a useful tool for studying expression of developmentally regulated genes.

Suppressor of fused negatively regulates beta-catenin signaling

Suppressor of fused (Su(fu)) is a negative regulator of the Hedgehog signaling pathway that controls the nuclear-cytoplasmic distribution of Gli/Ci transcription factors through direct protein-protein interactions. We show here that Su(fu) is present in a complex with the oncogenic transcriptional activator beta-catenin and functions as a negative regulator of T-cell factor (Tcf)-dependent transcription. Overexpression of Su(fu) in SW480 (APC(mut)) colon cancer cells in which beta-catenin protein is stabilized leads to a reduction in nuclear beta-catenin levels and in Tcf-dependent transcription. This effect of Su(fu) overexpression can be blocked by treatment of these cells with leptomycin B, a specific inhibitor of CRM1-mediated nuclear export. Overexpression of Su(fu) suppresses growth of SW480 (APC(mut)) tumor cells in nude mice. These observations indicate that Su(fu) negatively regulates beta-catenin signaling and that CRM-1-mediated nuclear export plays a role in this regulation. Our results also suggest that Su(fu) acts as a tumor suppressor.

The fused protein kinase regulates Hedgehog-stimulated transcriptional activation in Drosophila Schneider 2 cells

The Drosophila segment polarity gene fused encodes a putative protein-serine/threonine kinase, and plays a critical role in the signal transduction for Hedgehog (Hh)-dependent gene expression. We show that the Drosophila Schneider 2 (S2) cell line has the potential to transduce the Hh-triggered intracellular signals, leading to the activation of target gene expression, when a transcription factor, Cubitus interruptus (Ci), is provided exogenously. Using S2 cells transfected with the Ci-expressing plasmid and a patched promoter reporter construct, we demonstrate that the forced expression of Fused (Fu) stimulates Hh-triggered and Ci-dependent transcriptional activation. The N-terminal kinase domain of Fu is required for this activity, but the C-terminal domain is not. Two kinase-inactive Fu mutants fail to enhance the reporter activation, indicating that the kinase catalytic activity is essential for this function. Negative components of the Hh-signaling pathway, Costal-2 and Suppressor of Fused, strongly antagonize the Fu activity, irrespective of the presence or absence of the Fu C-terminal domain, suggesting an indirect mechanism for the inhibition of Fu by these proteins. Furthermore, mutational analyses of threonine 158 and serine 159, in the activation segment of the Fu protein kinase, indicate that threonine 158 is essential for Fu activity and that phosphorylation of this threonine residue may be involved in the activation of the kinase catalytic activity upon Hh stimulation.

Genomic organization and embryonic expression of Suppressor of Fused, a candidate gene for the split-hand/split-foot malformation type 3

The genes for human and mouse Suppressor of Fused (SU(FU)/Su(Fu)) in the Hedgehog signaling pathway were characterized and found to contain 12 exons. Human SU(FU) localized on chromosome 10q24-25 between the markers D10S192 and AFM183XB12. We detected three additional SU(FU) isoforms, two of which have lost their ability to interact with the transcription factor GLI1. Expression analysis using whole mount in situ hybridization revealed strong expression of Su(Fu) in various mouse embryonic tissues. SU(FU) was considered a candidate gene for the split-hand/split-foot malformation type 3 (SHFM3). However, no alterations in the SU(FU) gene were found in SHFM3 patients.

Genetic dissection of the Drosophila Cubitus interruptus signaling complex

Much of our understanding of the Hedgehog (Hh) signaling pathway comes from Drosophila, where a gradient of Hh signaling regulates the function of the transcription factor Cubitus interruptus (Ci) at three levels: protein stabilization, nuclear import, and activation. Regulation of Ci occurs in a cytoplasmic complex containing Ci, the kinesin-like protein Costal-2 (Cos2), the serine-threonine kinase Fused (Fu), and the Suppressor of Fused [Su(fu)] protein. The mechanisms by which this complex responds to different levels of Hh signaling and establishes distinct domains of gene expression are not fully understood. By sequentially mutating components from the Ci signaling complex, their roles in each aspect of Ci regulation can be analyzed. The Cos2-Ci core complex is able to mediate Hh-regulated activation of Ci but is insufficient to regulate nuclear import and cleavage. Addition of Su(fu) to the core complex blocks nuclear import while the addition of Fu restores Hh regulation of Ci nuclear import and proteolytic cleavage. Fu participates in two partially redundant pathways to regulate Ci nuclear import; the kinase function plays a positive role by inhibiting Su(fu), and the regulatory domain plays a negative role in conjunction with Cos2.

Genetics of morphogen gradients

The organization of cells and tissues is controlled by the action of 'form-giving' signalling molecules, or morphogens, which pattern a developmental field in a concentration-dependent manner. As the fate of each cell in the field depends on the level of the morphogen signal, the concentration gradient of the morphogen prefigures the pattern of development. In recent years, molecular genetic studies in Drosophila melanogaster have allowed tremendous progress in understanding how morphogen gradients are formed and maintained, and the mechanism by which receiving cells respond to the gradient.

A role of PDGFRalpha in basal cell carcinoma proliferation

Activation of the hedgehog pathway, through the loss of patched (PTC) or the activation of smoothened (SMO), occurs frequently in basal cell carcinoma (BCC), the most common human cancer. However, the molecular basis of this neoplastic effect is not understood. The downstream molecule Gli1 is known to mediate the biological effect of the pathway and is itself up-regulated in all BCCs. Gli1 can drive the production of BCCs in the mouse when overexpressed in the epidermis. Here we show that Gli1 can activate platelet-derived growth factor receptor alpha (PDGFRalpha) in C3H10T(1/2) cells. Functional up-regulation of PDGFRalpha by Gli1 is accompanied by activation of the ras-ERK pathway, a pathway associated with cell proliferation. The relevance of this mechanism in vivo is supported by a high level expression of PDGFRalpha in BCCs of mice and humans. In the murine BCC cell line ASZ001, in which both copies of the PTC gene are inactivated, DNA synthesis and cell proliferation can be slowed by re-expression of PTC, which down-regulates PDGFRalpha expression, or by downstream inhibition of PDGFRalpha with neutralizing antibodies. Therefore, we conclude that increased expression of PDGFRalpha may be an important mechanism by which mutations in the hedgehog pathway cause BCCs.

The morphogen Sonic hedgehog is an indirect angiogenic agent upregulating two families of angiogenic growth factors

Sonic hedgehog (Shh) is a prototypical morphogen known to regulate epithelial/mesenchymal interactions during embryonic development. We found that the hedgehog-signaling pathway is present in adult cardiovascular tissues and can be activated in vivo. Shh was able to induce robust angiogenesis, characterized by distinct large-diameter vessels. Shh also augmented blood-flow recovery and limb salvage following operatively induced hind-limb ischemia in aged mice. In vitro, Shh had no effect on endothelial-cell migration or proliferation; instead, it induced expression of two families of angiogenic cytokines, including all three vascular endothelial growth factor-1 isoforms and angiopoietins-1 and -2 from interstitial mesenchymal cells. These findings reveal a novel role for Shh as an indirect angiogenic factor regulating expression of multiple angiogenic cytokines and indicate that Shh might have potential therapeutic use for ischemic disorders.

Physical and functional interactions between Zic and Gli proteins

Zic and Gli family proteins are transcription factors that share similar zinc finger domains. Recent studies indicate that Zic and Gli collaborate in neural and skeletal development. We provide evidence that the Zic and Gli proteins physically and functionally interact through their zinc finger domains. Moreover, Gli proteins were translocated to cell nuclei by coexpressed Zic proteins, and both proteins regulated each other's transcriptional activity. Our result suggests that the physical interaction between Zic and Gli is the molecular basis of their antagonistic or synergistic features in developmental contexts and that Zic proteins are potential modulators of the hedgehog-mediated signaling pathway.

Sonic hedgehog promotes G(1) cyclin expression and sustained cell cycle progression in mammalian neuronal precursors

Sonic hedgehog (Shh) signal transduction via the G-protein-coupled receptor, Smoothened, is required for proliferation of cerebellar granule neuron precursors (CGNPs) during development. Activating mutations in the Hedgehog pathway are also implicated in basal cell carcinoma and medulloblastoma, a tumor of the cerebellum in humans. However, Shh signaling interactions with cell cycle regulatory components in neural precursors are poorly understood, in part because appropriate immortalized cell lines are not available. We have utilized primary cultures from neonatal mouse cerebella in order to determine (i) whether Shh initiates or maintains cell cycle progression in CGNPs, (ii) if G(1) regulation by Shh resembles that of classical mitogens, and (iii) whether individual D-type cyclins are essential components of Shh proliferative signaling in CGNPs. Our results indicate that Shh can drive continued cycling in immature, proliferating CGNPs. Shh treatment resulted in sustained activity of the G(1) cyclin-Rb axis by regulating levels of cyclinD1, cyclinD2, and cyclinE mRNA transcripts and proteins. Analysis of CGNPs from cyclinD1(-/-) or cyclinD2(-/-) mice demonstrates that the Shh proliferative pathway does not require unique functions of cyclinD1 or cyclinD2 and that D-type cyclins overlap functionally in this regard. In contrast to many known mitogenic pathways, we show that Shh proliferative signaling is mitogen-activated protein kinase independent. Furthermore, protein synthesis is required for early effects on cyclin gene expression. Together, our results suggest that Shh proliferative signaling promotes synthesis of regulatory factor intermediates that upregulate or maintain cyclin gene expression and activity of the G(1) cyclin-Rb axis in proliferating granule neuron precursors.

Cyclopamine inhibition of Sonic hedgehog signal transduction is not mediated through effects on cholesterol transport

Cyclopamine is a teratogenic steroidal alkaloid that causes cyclopia by blocking Sonic hedgehog (Shh) signal transduction. We have tested whether this activity of cyclopamine is related to disruption of cellular cholesterol transport and putative secondary effects on the Shh receptor, Patched (Ptc). First, we report that the potent antagonism of Shh signaling by cyclopamine is not a general property of steroidal alkaloids with similar structure. The structural features of steroidal alkaloids previously associated with the induction of holoprosencephaly in whole animals are also associated with inhibition of Shh signaling in vitro. Second, by comparing the effects of cyclopamine on Shh signaling with those of compounds known to block cholesterol transport, we show that the action of cyclopamine cannot be explained by inhibition of intracellular cholesterol transport. However, compounds that block cholesterol transport by affecting the vesicular trafficking of the Niemann-Pick C1 protein (NPC1), which is structurally similar to Ptc, are weak Shh antagonists. Rather than supporting a direct link between cholesterol homeostasis and Shh signaling, our findings suggest that the functions of both NPC1 and Ptc involve a common vesicular transport pathway. Consistent with this model, we find that Ptc and NPC1 colocalize extensively in a vesicular compartment in cotransfected cells.

hedgehog and wing development in Drosophila: a morphogen at work?

In Drosophila, Hedgehog (Hh) is a key regulator of limb development and activates decapentaplegic (dpp), a gene encoding a TGFbeta-related factor that controls growth and patterning of the limbs. During wing development, Hh also has morphogen-like Dpp-independent functions, controlling the morphogenesis of the central part of the wing through the activation of the evolutionarily conserved transcription factors encoded by the iroquois and collier genes. The ways in which Hh forms an activity gradient to lay the basis of patterning of the adult wing are described here. As the signal transduction pathway of Hh is strongly conserved during evolution and human Hh may be implicated in congenital diseases and cancers, these observations provide important advances which may help in understanding the function of Hh proteins in normal and pathological development and tumourigenesis in humans.