Multisite protein kinase A and glycogen synthase kinase 3beta phosphorylation leads to Gli3 ubiquitination by SCFbetaTrCP

Hedgehog signaling update

In vertebrate hedgehog signaling, hedgehog ligands are processed to become bilipidated and then multimerize, which allows them to leave the signaling cell via Dispatched 1 and become transported via glypicans and megalin to the responding cells. Hedgehog then interacts with a complex of Patched 1 and Cdo/Boc, which activates endocytic Smoothened to the cilium. Patched 1 regulates the activity of Smoothened (1) via Vitamin D3, which inhibits Smoothened in the absence of hedgehog ligand or (2) via oxysterols, which activate Smoothened in the presence of hedgehog ligand. Hedgehog ligands also interact with Hip1, Patched 2, and Gas1, which regulate the range as well as the level of hedgehog signaling. In vertebrates, Smoothened is shortened at its C-terminal end and lacks most of the phosphorylation sites of importance in Drosophila. Cos2, also of importance in Drosophila, plays no role in mammalian transduction, nor do its homologs Kif7 and Kif27. The cilium may provide a function analogous to that of Cos2 by linking Smoothened to the modulation of Gli transcription factors. Disorders associated with the hedgehog signaling network follow, including nevoid basal cell carcinoma syndrome, holoprosencephaly, Smith-Lemli-Opitz syndrome, Greig cephalopolysyndactyly syndrome, Pallister-Hall syndrome, Carpenter syndrome, and Rubinstein-Taybi syndrome.

A novel domain mediates insulin-induced proteasomal degradation of insulin receptor substrate 1 (IRS-1)

Insulin receptor substrate-1 (IRS-1) plays a pivotal role in insulin signaling, therefore its degradation is exquisitely regulated. Here, we show that insulin-stimulated degradation of IRS-1 requires the presence of a highly conserved Ser/Thr-rich domain that we named domain involved in degradation of IRS-1 (DIDI). DIDI (amino acids 386-430 of IRS-1) was identified by comparing the intracellular degradation rate of several truncated forms of IRS-1 transfected into CHO cells. The isolated DIDI domain underwent insulin-stimulated Ser/Thr phosphorylation, suggesting that it serves as a target for IRS-1 kinases. The effects of deletion of DIDI were studied in Fao rat hepatoma and in CHO cells expressing Myc-IRS-1(WT) or Myc-IRS-1(Δ386-430). Deletion of DIDI maintained the ability of IRS-1(Δ386-434) to undergo ubiquitination while rendering it insensitive to insulin-induced proteasomal degradation, which affected IRS-1(WT) (80% at 8 h). Consequently, IRS-1(Δ386-434) mediated insulin signaling (activation of Akt and glycogen synthesis) better than IRS-1(WT). IRS-1(Δ386-434) exhibited a significant greater preference for nuclear localization, compared with IRS-1(WT). Higher nuclear localization was also observed when cells expressing IRS-1(WT) were incubated with the proteasome inhibitor MG-132. The sequence of DIDI is conserved more than 93% across species, from fish to mammals, as opposed to approximately 40% homology of the entire IRS-1. These findings implicate DIDI as a novel, highly conserved domain of IRS-1, which mediates its cellular localization, rate of degradation, and biological activity, with a direct impact on insulin signal transduction.

Novel insights into the molecular mechanisms governing Mdm2 ubiquitination and destruction

The Mdm2/p53 pathway is compromised in more than 50% of all human cancers, therefore it is an intensive area of research to understand the upstream regulatory pathways governing Mdm2/p53 activity. Mdm2 is frequently overexpressed in human cancers while the molecular mechanisms underlying the timely destruction of Mdm2 remain unclear. We recently reported that Casein Kinase I phosphorylates Mdm2 at multiple sites to trigger Mdm2 interaction with, and subsequent ubiquitination and destruction by the SCF(β-TRCP) E3 ubiquitin ligase. We also demonstrated that the E3 ligase activity-deficient Mdm2 was still unstable in the G1 phase and could be efficiently degraded by SCF(β-TRCP). Thus our finding expands the current knowledge on how Mdm2 is tightly regulated by both self- and SCF(β-TRCP)-dependent ubiquitination to control p53 activity in response to stress. It further indicates that loss of β-TRCP or Casein Kinase I function contributes to elevated Mdm2 expression that is frequently found in various types of tumors.

SUMOylation by Pias1 regulates the activity of the Hedgehog dependent Gli transcription factors

BACKGROUND

Hedgehog (Hh) signaling, a vital signaling pathway for the development and homeostasis of vertebrate tissues, is mediated by members of the Gli family of zinc finger transcription factors. Hh signaling increases the transcriptional activity of Gli proteins, at least in part, by inhibiting their proteolytic processing. Conversely, phosphorylation by cAMP-dependent protein kinase (PKA) inhibits Gli transcriptional activity by promoting their ubiquitination and proteolysis. Whether other post-translational modifications contribute to the regulation of Gli protein activity has been unclear.

METHODOLOGY/PRINCIPAL FINDINGS

Here we provide evidence that all three Gli proteins are targets of small ubiquitin-related modifier (SUMO)-1 conjugation. Expression of SUMO-1 or the SUMO E3 ligase, Pias1, increased Gli transcriptional activity in cultured cells. Moreover, PKA activity reduced Gli protein SUMOylation. Strikingly, in the embryonic neural tube, the forced expression of Pias1 increased Gli activity and induced the ectopic expression of the Gli dependent gene Nkx2.2. Conversely, a point mutant of Pias1, that lacks ligase activity, blocked the endogenous expression of Nkx2.2.

CONCLUSIONS/SIGNIFICANCE

Together, these findings provide evidence that Pias1-dependent SUMOylation influences Gli protein activity and thereby identifies SUMOylation as a post-translational mechanism that regulates the hedgehog signaling pathway.

Phosphorylation by casein kinase I promotes the turnover of the Mdm2 oncoprotein via the SCF(beta-TRCP) ubiquitin ligase

Mdm2 is the major negative regulator of the p53 pathway. Here, we report that Mdm2 is rapidly degraded after DNA damage and that phosphorylation of Mdm2 by casein kinase I (CKI) at multiple sites triggers its interaction with, and subsequent ubiquitination and destruction, by SCF(beta-TRCP). Inactivation of either beta-TRCP or CKI results in accumulation of Mdm2 and decreased p53 activity, and resistance to apoptosis induced by DNA damaging agents. Moreover, SCF(beta-TRCP)-dependent Mdm2 turnover also contributes to the control of repeated p53 pulses in response to persistent DNA damage. Our results provide insight into the signaling pathways controlling Mdm2 destruction and further suggest that compromised regulation of Mdm2 results in attenuated p53 activity, thereby facilitating tumor progression.

Diverse functions of Hedgehog signaling in formation and physiology of steroidogenic organs

Adrenal, testis, and ovary are steroidogenic organs derived from a common primordium that consists of steroidogenic factor 1 (SF1)-positive precursor cells. SF1 not only defines the steroidogenic lineages in these organs but also controls their differentiation. Recent evidence implicates the Hedgehog (Hh) signaling pathway as a downstream regulator of SF1 in the appearance of steroidogenic cells in these organs. The Hh signaling pathway serves as a common crosstalk component, yet has evolved diverse functions in the expansion and differentiation of the steroidogenic cells in a tissue-specific manner. The purpose of this review is to compare and contrast the different roles of Hh signaling in these three organs during development.

Sonic-hedgehog-mediated proliferation requires the localization of PKA to the cilium base

Cerebellar granular neuronal precursors (CGNPs) proliferate in response to the mitogenic activity of Sonic hedgehog (Shh), and this proliferation is negatively regulated by activation of cAMP-dependent protein kinase (PKA). In the basal state, the PKA catalytic subunits (C-PKA) are inactive because of their association with the regulatory subunits (R-PKA). As the level of cAMP increases, it binds to R-PKA, displacing and thereby activating the C-PKA. Here we report that, in the presence of Shh, inactive C-PKA accumulates at the cilium base of proliferative CGNPs whereas removal of Shh triggers the activation of PKA at this particular location. Furthermore, we demonstrate that the anchoring of the PKA holoenzyme to the cilium base is mediated by the specific binding of the type II PKA regulatory subunit (RII-PKA) to the A-kinase anchoring proteins (AKAPs). Disruption of the interaction between RII-PKA and AKAPs inhibits Shh activity and, therefore, blocks proliferation of CGNP cultures. Collectively, these results demonstrate that the pool of PKA localized to the cilium base of CGNP plays an essential role in the integration of Shh signal transduction.

Suppressor of fused and Spop regulate the stability, processing and function of Gli2 and Gli3 full-length activators but not their repressors

Gli2 and Gli3 are primary transcriptional regulators that mediate hedgehog (Hh) signaling. Mechanisms that stabilize and destabilize Gli2 and Gli3 are essential for the proteins to promptly respond to Hh signaling or to be inactivated following the activation. In this study, we show that loss of suppressor of fused (Sufu; an inhibitory effector for Gli proteins) results in destabilization of Gli2 and Gli3 full-length activators but not of their C-terminally processed repressors, whereas overexpression of Sufu stabilizes them. By contrast, RNAi knockdown of Spop (a substrate-binding adaptor for the cullin3-based ubiquitin E3 ligase) in Sufu mutant mouse embryonic fibroblasts (MEFs) can restore the levels of Gli2 and Gli3 full-length proteins, but not those of their repressors, whereas introducing Sufu into the MEFs stabilizes Gli2 and Gli3 full-length proteins and rescues Gli3 processing. Consistent with these findings, forced Spop expression promotes Gli2 and Gli3 degradation and Gli3 processing. The functions of Sufu and Spop oppose each other through their competitive binding to the N- and C-terminal regions of Gli3 or the C-terminal region of Gli2. More importantly, the Gli3 repressor expressed by a Gli3 mutant allele (Gli3(Delta699)) can mostly rescue the ventralized neural tube phenotypes of Sufu mutant embryos, indicating that the Gli3 repressor can function independently of Sufu. Our study provides a new insight into the regulation of Gli2 and Gli3 stability and processing by Sufu and Spop, and reveals the unexpected Sufu-independent Gli3 repressor function.

Induced, selective proteolysis of MLK3 negatively regulates MLK3/JNK signalling

MLK3 (mixed lineage kinase 3) is a MAP3K [MAPK (mitogen-activated protein kinase) kinase kinase] that activates multiple MAPK pathways, including the JNK (c-Jun N-terminal kinase) pathway. Immunoblotting of lysates from cells ectopically expressing active MLK3 revealed an additional immunoreactive band corresponding to a CTF (C-terminal fragment) of MLK3. In the present paper we provide evidence that MLK3 undergoes proteolysis to generate a stable CTF in response to different stimuli, including PMA and TNFalpha (tumour necrosis factor alpha). The cleavage site was deduced by Edman sequencing as between Gln251 and Pro252, which is within the kinase domain of MLK3. Based on our homology model of the kinase domain of MLK3, the region containing the cleavage site is predicted to reside on a flexible solvent-accessible loop. Site-directed mutagenesis studies revealed that Leu250 and Gln251 are required for recognition by the 'MLK3 protease', reminiscent of the substrate specificity of the coronavirus 3C and 3CL proteases. Whereas numerous mammalian protease inhibitors have no effect on MLK3 proteolysis, blockade of the proteasome through epoxomicin or MG132 abolishes PMA-induced production of the CTF of MLK3. This CTF is able to heterodimerize with full-length MLK3, and interact with the active form of the small GTPase Cdc42, resulting in diminished activation loop phosphorylation of MLK3 and reduced signalling to JNK. Thus this novel proteolytic processing of MLK3 may negatively control MLK3 signalling to JNK.

The output of Hedgehog signaling is controlled by the dynamic association between Suppressor of Fused and the Gli proteins

The transcriptional program orchestrated by Hedgehog signaling depends on the Gli family of transcription factors. Gli proteins can be converted to either transcriptional activators or truncated transcriptional repressors. We show that the interaction between Gli3 and Suppressor of Fused (Sufu) regulates the formation of either repressor or activator forms of Gli3. In the absence of signaling, Sufu restrains Gli3 in the cytoplasm, promoting its processing into a repressor. Initiation of signaling triggers the dissociation of Sufu from Gli3. This event prevents formation of the repressor and instead allows Gli3 to enter the nucleus, where it is converted into a labile, differentially phosphorylated transcriptional activator. This key dissociation event depends on Kif3a, a kinesin motor required for the function of primary cilia. We propose that the Sufu-Gli3 interaction is a major control point in the Hedgehog pathway, a pathway that plays important roles in both development and cancer.

Kinetics of hedgehog-dependent full-length Gli3 accumulation in primary cilia and subsequent degradation

Hedgehog (Hh) signaling in vertebrates depends on intraflagellar transport (IFT) within primary cilia. The Hh receptor Patched is found in cilia in the absence of Hh and is replaced by the signal transducer Smoothened within an hour of Hh stimulation. By generating antibodies capable of detecting endogenous pathway transcription factors Gli2 and Gli3, we monitored their kinetics of accumulation in cilia upon Hh stimulation. Localization occurs within minutes of Hh addition, making it the fastest reported readout of pathway activity, which permits more precise temporal and spatial localization of Hh signaling events. We show that the species of Gli3 that accumulates at cilium tips is full-length and likely not protein kinase A phosphorylated. We also confirmed that phosphorylation and betaTrCP/Cul1 are required for endogenous Gli3 processing and that this is inhibited by Hh. Surprisingly, however, Hh-dependent inhibition of processing does not lead to accumulation of full-length Gli3, but instead renders it labile, leading to its proteasomal degradation via the SPOP/Cul3 complex. In fact, full-length Gli3 disappears with faster kinetics than the Gli3 repressor, the latter not requiring SPOP/Cul3 or betaTrCP/Cul1. This may contribute to the increased Gli3 activator/repressor ratios found in IFT mutants.

SUMO and ubiquitin modifications during steroid hormone synthesis and function

Steroid hormones control many aspects of animal physiology and behaviour. They are highly regulated, among other mechanisms, by post-translational modifications of the transcription factors involved in their synthesis and response. In the present review, we will focus on the influence of SUMO (small ubiquitin-related modifier) and ubiquitin modifications on the function of transcription factors involved in adrenal cortex formation, steroidogenesis and the hormonal response.

Gli2 trafficking links Hedgehog-dependent activation of Smoothened in the primary cilium to transcriptional activation in the nucleus

Stimulation by the extracellular Hedgehog (Hh) protein signal has been shown to alter ciliary localization of the mammalian Hh receptor components Smoothened (Smo) and Patched (Ptc), and mutations that disrupt the structure and function of the cilium also disrupt Hh-induced changes in gene expression. But how ciliary events affect gene expression in the nucleus is not known, and to address this question we have characterized the cellular trafficking of Gli2, the principal mediator of Hh-dependent transcriptional activation. From a combination of pharmacological and genetic manipulations we find in resting cells that both Gli2 and Smo appear to shuttle in and out of the cilium, with Gli2 but not Smo requiring intact cytoplasmic microtubules for ciliary entry and both requiring the ciliary retrograde motor, cytoplasmic dynein 2, for ciliary exit. We also find that changes in ciliary and nuclear trafficking of Gli2 are triggered by the Hh-dependent accumulation of activated Smo in the cilium, resulting in a shift from primarily cytoplasmic localization to accumulation at the distal tip of the cilium and within the nucleus. Gli2 thus functions as a dynamic monitor of Smo activity in the cilium and thereby links Hh pathway activation in the cilium to transcriptional activation in the nucleus.

Point mutations in GLI3 lead to misregulation of its subcellular localization

Background

Mutations in the transcription factor GLI3, a downstream target of Sonic Hedgehog (SHH) signaling, are responsible for the development of malformation syndromes such as Greig-cephalopolysyndactyly-syndrome (GCPS), or Pallister-Hall-syndrome (PHS). Mutations that lead to loss of function of the protein and to haploinsufficiency cause GCPS, while truncating mutations that result in constitutive repressor function of GLI3 lead to PHS. As an exception, some point mutations in the C-terminal part of GLI3 observed in GCPS patients have so far not been linked to loss of function. We have shown recently that protein phosphatase 2A (PP2A) regulates the nuclear localization and transcriptional activity a of GLI3 function.

Principal Findings

We have shown recently that protein phosphatase 2A (PP2A) and the ubiquitin ligase MID1 regulate the nuclear localization and transcriptional activity of GLI3. Here we show mapping of the functional interaction between the MID1-α4-PP2A complex and GLI3 to a region between amino acid 568-1100 of GLI3. Furthermore we demonstrate that GCPS-associated point mutations, that are located in that region, lead to misregulation of the nuclear GLI3-localization and transcriptional activity. GLI3 phosphorylation itself however appears independent of its localization and remains untouched by either of the point mutations and by PP2A-activity, which suggests involvement of an as yet unknown GLI3 interaction partner, the phosphorylation status of which is regulated by PP2A activity, in the control of GLI3 subcellular localization and activity.

Conclusions

The present findings provide an explanation for the pathogenesis of GCPS in patients carrying C-terminal point mutations, and close the gap in our understanding of how GLI3-genotypes give rise to particular phenotypes. Furthermore, they provide a molecular explanation for the phenotypic overlap between Opitz syndrome patients with dysregulated PP2A-activity and syndromes caused by GLI3-mutations.

GSK-3 phosphorylates delta-catenin and negatively regulates its stability via ubiquitination/proteosome-mediated proteolysis

Delta-catenin was first identified because of its interaction with presenilin-1, and its aberrant expression has been reported in various human tumors and in patients with Cri-du-Chat syndrome, a form of mental retardation. However, the mechanism whereby delta-catenin is regulated in cells has not been fully elucidated. We investigated the possibility that glycogen-synthase kinase-3 (GSK-3) phosphorylates delta-catenin and thus affects its stability. Initially, we found that the level of delta-catenin was greater and the half-life of delta-catenin was longer in GSK-3beta(-/-) fibroblasts than those in GSK-3beta(+/+) fibroblasts. Furthermore, four different approaches designed to specifically inhibit GSK-3 activity, i.e. GSK-3-specific chemical inhibitors, Wnt-3a conditioned media, small interfering RNAs, and GSK-3alpha and -3beta kinase dead constructs, consistently showed that the levels of endogenous delta-catenin in CWR22Rv-1 prostate carcinoma cells and primary cortical neurons were increased by inhibiting GSK-3 activity. In addition, it was found that both GSK-3alpha and -3beta interact with and phosphorylate delta-catenin. The phosphorylation of DeltaC207-delta-catenin (lacking 207 C-terminal residues) and T1078A delta-catenin by GSK-3 was noticeably reduced compared with that of wild type delta-catenin, and the data from liquid chromatography-tandem mass spectrometry analyses suggest that the Thr(1078) residue of delta-catenin is one of the GSK-3 phosphorylation sites. Treatment with MG132 or ALLN, specific inhibitors of proteosome-dependent proteolysis, increased delta-catenin levels and caused an accumulation of ubiquitinated delta-catenin. It was also found that GSK-3 triggers the ubiquitination of delta-catenin. These results suggest that GSK-3 interacts with and phosphorylates delta-catenin and thereby negatively affects its stability by enabling its ubiquitination/proteosome-mediated proteolysis.

Establishing and interpreting graded Sonic Hedgehog signaling during vertebrate neural tube patterning: the role of negative feedback

The secreted protein Sonic Hedgehog (SHH) acts in graded fashion to pattern the dorsal-ventral axis of the vertebrate neural tube. This is a dynamic process in which increasing concentrations and durations of exposure to SHH generate neurons with successively more ventral identities. Interactions between the receiving cells and the graded signal underpin the mechanism of SHH action. In particular, negative feedback, involving proteins transcriptionally induced or repressed by SHH signaling, plays an essential role in shaping the graded readout. On one hand, negative feedback controls, in a noncell-autonomous manner, the distribution of SHH across the field of receiving cells. On the other, it acts cell-autonomously to convert different concentrations of SHH into distinct durations of intracellular signal transduction. Together, these mechanisms exemplify a strategy for morphogen interpretation, which we have termed temporal adaptation that relies on the continuous processing and refinement of the cellular response to the graded signal.

Sufu recruits GSK3beta for efficient processing of Gli3

Hedgehog (Hh) signaling activates the transcription factor Gli by suppressing the function of the suppressor of fused (Sufu) protein in mammals. Here, a novel role of mammalian Sufu is identified where it mediates the phosphorylation of Gli3 by GSK3beta, essential for Gli3 processing to generate a transcriptional repressor for Hh-target genes. Studies using Sufu(-/-) mouse embryonic fibroblasts and siRNA targeting Sufu demonstrate the requirement of Sufu for Gli3 processing. In addition, Sufu can bind to GSK3beta as well as Gli3, and mediates formation of the trimolecular complex Gli3/Sufu/GSK3beta. Thus, Sufu stimulates Gli3 phosphorylation by GSK3beta and Gli3 processing. Furthermore, Sonic Hh stimulation dissociates the Sufu/GSK3beta complex from Gli3, resulting in the blockade of Gli3 processing. Collectively, Sufu presumably functions as a GSK3beta recruiter for Hh-dependent regulation of Gli3 processing. Such a function is very similar to that of Costal2 in Drosophila, suggesting a functional complementation through evolution.

Sonic hedgehog signalling inhibits palatogenesis and arrests tooth development in a mouse model of the nevoid basal cell carcinoma syndrome

Nevoid basal cell carcinoma syndrome (NBCCS) is an autosomal dominant or spontaneous disorder characterized by multiple cutaneous basal cell carcinomas, odontogenic keratocysts, skeletal anomalies and facial dysmorphology, including cleft lip and palate. Causative mutations for NBCCS occur in the PTCH1 gene on chromosome 9q22.3-q31, which encodes the principle receptor for the Hedgehog signalling pathway. We have investigated the molecular basis of craniofacial defects seen in NBCCS using a transgenic mouse model expressing Shh in basal epithelium under a Keratin-14 promoter. These mice have an absence of flat bones within the skull vault, hypertelorism, open-bite malocclusion, cleft palate and arrested tooth development. Significantly, increased Hedgehog signal transduction in these mice can influence cell fate within the craniofacial region. In medial edge epithelium of the palate, Shh activity prevents apoptosis and subsequent palatal shelf fusion. In contrast, high levels of Shh in odontogenic epithelium arrests tooth development at the bud stage, secondary to a lack of cell proliferation in this region. These findings illustrate the importance of appropriately regulated Hedgehog signalling during early craniofacial development and demonstrate that oro-facial clefting and hypodontia seen in NBCCS can occur as a direct consequence of increased Shh signal activity within embryonic epithelial tissues.

Thiazolidinediones mimic glucose starvation in facilitating Sp1 degradation through the up-regulation of beta-transducin repeat-containing protein

This study investigated the mechanism by which the transcription factor Sp1 is degraded in prostate cancer cells. We recently developed a thiazolidinedione derivative, (Z)-5-(4-hydroxy-3-trifluoromethylbenzylidene)-3-(1-methylcyclohexyl)-thiazolidine-2,4-dione (OSU-CG12), that induces Sp1 degradation in a manner paralleling that of glucose starvation. Based on our finding that thiazolidinediones suppress beta-catenin and cyclin D1 by up-regulating the E3 ligase SCF(beta-TrCP), we hypothesized that beta-transducin repeat-containing protein (beta-TrCP) targets Sp1 for proteasomal degradation in response to glucose starvation or OSU-CG12. Here we show that either treatment of LNCaP cells increased specific binding of Sp1 with beta-TrCP. This direct binding was confirmed by in vitro pull-down analysis with bacterially expressed beta-TrCP. Although ectopic expression of beta-TrCP enhanced the ability of OSU-CG12 to facilitate Sp1 degradation, suppression of endogenous beta-TrCP function by a dominant-negative mutant or small interfering RNA-mediated knockdown blocked OSU-CG12-facilitated Sp1 ubiquitination and/or degradation. Sp1 contains a C-terminal conventional DSG destruction box ((727)DSGAGS(732)) that mediates beta-TrCP recognition and encompasses a glycogen synthase kinase 3beta (GSK3beta) phosphorylation motif (SXXXS). Pharmacological and molecular genetic approaches and mutational analyses indicate that extracellular signal-regulated kinase-mediated phosphorylation of Thr739 and GSK3beta-mediated phosphorylation of Ser728 and Ser732 were critical for Sp1 degradation. The ability of OSU-CG12 to mimic glucose starvation to activate beta-TrCP-mediated Sp1 degradation has translational potential to foster novel strategies for cancer therapy.

Modeling the spatio-temporal network that drives patterning in the vertebrate central nervous system

In this review, we discuss the gene regulatory network underlying the patterning of the ventral neural tube during vertebrate embryogenesis. The neural tube is partitioned into domains of distinct cell fates by inductive signals along both anterior-posterior and dorsal-ventral axes. A defining feature of the dorsal-ventral patterning is the graded distribution of Sonic hedgehog (Shh), which acts as a morphogen to specify several classes of ventral neurons in a concentration-dependent fashion. These inductive signals translate into patterned expressions of transcription factors that define different neural progenitor subtypes. Progenitor boundaries are sharpened by repressive interactions between these transcription factors. The progenitor-expressed transcription factors induce another set of transcription factors that are thought to contribute to neural identities in post-mitotic neural precursors. Thus, the gene regulatory network of the ventral neural tube patterning is characterized by hierarchical expression [inductive signal-->progenitor specifying factors (mitotic)--> precursor specifying factors (post mitotic)--> differentiated neural markers] and cross-repression between progenitor-expressed regulatory factors. Although a number of transcriptional regulators have been identified at each hierarchical level, their precise regulatory relationships are not clear. Here we discuss approaches aimed at clarifying and extending our understanding of the formation and propagation of this network.

Mouse hitchhiker mutants have spina bifida, dorso-ventral patterning defects and polydactyly: identification of Tulp3 as a novel negative regulator of the Sonic hedgehog pathway

The mammalian Sonic hedgehog (Shh) signalling pathway is essential for embryonic development and the patterning of multiple organs. Disruption or activation of Shh signalling leads to multiple birth defects, including holoprosencephaly, neural tube defects and polydactyly, and in adults results in tumours of the skin or central nervous system. Genetic approaches with model organisms continue to identify novel components of the pathway, including key molecules that function as positive or negative regulators of Shh signalling. Data presented here define Tulp3 as a novel negative regulator of the Shh pathway. We have identified a new mouse mutant that is a strongly hypomorphic allele of Tulp3 and which exhibits expansion of ventral markers in the caudal spinal cord, as well as neural tube defects and preaxial polydactyly, consistent with increased Shh signalling. We demonstrate that Tulp3 acts genetically downstream of Shh and Smoothened (Smo) in neural tube patterning and exhibits a genetic interaction with Gli3 in limb development. We show that Tulp3 does not appear to alter expression or processing of Gli3, and we demonstrate that transcriptional regulation of other negative regulators (Rab23, Fkbp8, Thm1, Sufu and PKA) is not affected. We discuss the possible mechanism of action of Tulp3 in Shh-mediated signalling in light of these new data.

Dominant activation of the hedgehog signaling pathway in the ovary alters theca development and prevents ovulation

The role of the hedgehog (HH) signaling pathway in ovarian function was examined in transgenic mice in which expression of a dominant active allele of the signal transducer smoothened (SmoM2) was directed to the ovary and Müllerian duct by cre-mediated recombination (Amhr2(cre/+)SmoM2). Mutant mice were infertile and had ovarian and reproductive tract defects. Ovaries contained follicles of all sizes and corpora lutea (CL), but oocytes were rarely recovered from the oviducts of superovulated mice and remained trapped in preovulatory follicles. Measures of luteinization did not differ. Cumulus expansion appeared disorganized, and in vitro analyses confirmed a reduced expansion index. Microarray analysis indicated that expression levels of genes typical of smooth muscle were reduced in mutant mice, and RT-PCR showed that levels of expression of muscle genes were reduced in the nongranulosa, theca-interstitial cell-enriched fraction. Whereas a layer of cells in the outer theca was positively stained for smooth muscle actin in control ovaries, this staining was reduced or absent in mutant ovaries. Expression of a number of genes in granulosa cells that are known to be important for ovulation did not differ in mutants and controls. Expression of components of the HH pathway was observed in both granulosa cells and in the nongranulosa, residual ovarian tissue and changed in response to treatment with equine chorionic gonadotropin/human gonadotropin. The results show that appropriate signaling through the HH pathway is required for development of muscle cells within the theca and that impaired muscle development is associated with failure to release the oocyte at ovulation.

Phosphorylation of Gli2 by protein kinase A is required for Gli2 processing and degradation and the Sonic Hedgehog-regulated mouse development

In mice, Gli2 and Gli3 are the transcription factors that mediate the initial Hedgehog (Hh) signaling. In the absence of Hh signaling, the majority of the full-length Gli3 protein undergoes proteolytic processing into a repressor, while only a small fraction of the full-length Gli2 protein is processed. Gli3 processing is dependent on phosphorylation of the first four of the six protein kinase A (PKA) sites at its C-terminus. However, whether the same phosphorylation of Gli2 by PKA is required for Gli2 processing and, if so, whether such phosphorylation regulates additional Gli2 function are unknown. To address these questions, we mutated these PKA sites in the mouse Gli2 locus to create the Gli2(P1-4) allele. Gli2(P1-4) homozygous embryos die in utero and exhibit exencephaly, defects in neural tube closure, enlarged craniofacial structures, and an extra anterior digit. Analysis of spinal cord patterning shows that domains of motoneurons and V2, V1, and V0 interneurons are expanded to different degrees in both Gli2(P1-4) single and Gli2(P1-4);Shh double mutants. Furthermore, Gli2(P1-4) expression prevents massive cell death and promotes cell proliferation in Shh mutant. Analysis of Gli2(P1-4) protein in vivo reveals that the mutant protein is not processed and is twice as stable as wild type Gli2 protein. We also show that the Gli2 repressor can effectively antagonize Gli2P1-4 activity. Together, these results indicate that phosphorylation of Gli2 by PKA induces Gli2 processing and destabilization in vivo and plays an important role in the Hh-regulated mouse embryonic patterning.

Protein kinase Cdelta negatively regulates hedgehog signaling by inhibition of Gli1 activity

Constitutive activation of the hedgehog pathway is implicated in the development of many human malignancies; hedgehog targets, PTCH1 and Gli1, are markers of hedgehog signaling activation and are expressed in most hedgehog-associated tumors. Protein kinase Cdelta (PKCdelta) generally slows proliferation and induces cell cycle arrest of various cell lines. In this study, we show that activated PKCdelta (wild-type PKCdelta stimulated by phorbol 12-myristate 13-acetate or constitutively active PKCdelta) decreased Gli-luciferase reporter activity in NIH/3T3 cells, as well as the endogenous hedgehog-responsive gene PTCH1. In human hepatoma (i.e. Hep3B) cells, wild-type PKCdelta and constitutively active PKCdelta decreased the expression levels of endogenous Gli1 and PTCH1. In contrast, PKCdelta siRNA increased the expression levels of these target genes. Silencing of PKCdelta by siRNA rescued the inhibition of cell growth by KAAD-cyclopamine, an antagonist of hedgehog signaling element Smoothened, suggesting that PKCdelta acts downstream of Smoothened. The biological relevance of our study is shown in hepatocellular carcinoma where we found that hepatocellular carcinoma with detectable hedgehog signaling had weak or no detectable expression of PKCdelta, whereas PKCdelta highly expressing tumors had no detectable hedgehog signaling. Our results demonstrate that PKCdelta alters hedgehog signaling by inhibition of Gli protein transcriptional activity. Furthermore, our findings suggest that, in certain cancers, PKCdelta plays a role as a negative regulator of tumorigenesis by regulating hedgehog signaling.

A unique protection signal in Cubitus interruptus prevents its complete proteasomal degradation

The limited proteolysis of Cubitus interruptus (Ci), the transcription factor for the developmentally and medically important Hedgehog (Hh) signaling pathway, triggers a critical switch between transcriptional repressor and activator forms. Ci repressor is formed when the C terminus of full-length Ci is degraded by the ubiquitin-proteasome pathway, an unusual reaction since the proteasome typically completely degrades its substrates. We show that several regions of Ci are required for generation of the repressor form: the zinc finger DNA binding domain, a single lysine residue (K750) near the degradation end point, and a 163-amino-acid region at the C terminus. Unlike other proteins that are partially degraded by the proteasome, dimerization is not a key feature of Ci processing. Using a pulse-chase assay in cultured Drosophila cells, we distinguish between regions required for initiation of degradation and those required for the protection of the Ci N terminus from degradation. We present a model whereby the zinc finger region and K750 together form a unique protection signal that prevents the complete degradation of Ci by the proteasome.

The decoupling of Smoothened from Galphai proteins has little effect on Gli3 protein processing and Hedgehog-regulated chick neural tube patterning

The Hedgehog (Hh) signal is transmitted by two receptor molecules, Patched (Ptc) and Smoothened (Smo). Ptc suppresses Smo activity, while Hh binds Ptc and alleviates the suppression, which results in activation of Hh targets. Smo is a seven-transmembrane protein with a long carboxyl terminal tail. Vertebrate Smo has been previously shown to be coupled to Galpha(i) proteins, but the biological significance of the coupling in Hh signal transduction is not clear. Here we show that although inhibition of Galpha(i) protein activity appears to significantly reduce Hh pathway activity in Ptc(-/-) mouse embryonic fibroblasts and the NIH3T3-based Shh-light cells, it fails to derepress Shh- or a Smo-agonist-induced inhibition of Gli3 protein processing, a known in vivo indicator of Hh signaling activity. The inhibition of Galpha(i) protein activity also cannot block the Sonic Hedgehog (Shh)-dependent specification of neural progenitor cells in the neural tube. Consistent with these results, overexpression of a constitutively active Galpha(i) protein, Galpha(i2)QL, cannot ectopically specify the neural cell types in the spinal cord, whereas an active Smo, SmoM2, can. Thus, our results indicate that the Smo-induced Galpha(i) activity plays an insignificant role in the regulation of Gli3 processing and Shh-regulated neural tube patterning.

UV-induced degradation of securin is mediated by SKP1-CUL1-beta TrCP E3 ubiquitin ligase

Securin is a chaperone protein with bifunctional properties. It binds to separase to inhibit premature sister chromatid separation until the onset of anaphase, and it also takes part in cell-cycle arrest after UV irradiation. At metaphase-to-anaphase transition, securin is targeted for proteasomal destruction by the anaphase-promoting complex or cyclosome (APC/C), allowing activation of separase. However, although securin is reported to undergo proteasome-dependent degradation after UV irradiation, the ubiquitin ligase responsible for securin ubiquitylation has not been well characterized. In this study, we show that UV radiation induced a marked reduction of securin in both the nucleus and cytoplasm. Moreover, we show that GSK-3beta inhibitors prevent securin degradation, and that CUL1 and betaTrCP are involved in this depletion. We also confirmed that SKP1-CUL1-betaTrCP (SCF(betaTrCP)) ubiquitylates securin in vivo, and identified a conserved and unconventional betaTrCP recognition motif (DDAYPE) in the securin primary amino acid sequence of humans, nonhuman primates and rodents. Furthermore, downregulation of betaTrCP caused an accumulation of securin in non-irradiated cells. We conclude that SCF(betaTrCP) is the E3 ubiquitin ligase responsible for securin degradation after UV irradiation, and that it is involved in securin turnover in nonstressed cells.

Controlling hair follicle signaling pathways through polyubiquitination

Hair follicle development and maintenance require precise reciprocal signaling interactions between the epithelium and underlying dermis. Three major developmental signaling pathways, Wnt, Sonic hedgehog, and NF-kappaB/Edar, are indispensable for this process and, when aberrantly activated, can lead to skin and appendage neoplasms. Recent data point to protein polyubiquitination as playing a central role in regulating the timing, duration, and location of signaling. Here we review how polyubiquitination regulates the stability and interaction of key signaling components that control hair follicle development and regeneration.

The primary cilium at the crossroads of mammalian hedgehog signaling

Cilia function as critical sensors of extracellular information, and ciliary dysfunction underlies diverse human disorders including situs inversus, polycystic kidney disease, retinal degeneration, and Bardet-Biedl syndrome. Importantly, mammalian primary cilia have recently been shown to mediate transduction of Hedgehog (Hh) signals, which are involved in a variety of developmental processes. Mutations in several ciliary components disrupt the patterning of the neural tube and limb bud, tissues that rely on precisely coordinated gradients of Hh signal transduction. Numerous components of the Hh pathway, including Patched, Smoothened, and the Gli transcription factors, are present within primary cilia, indicating that key steps of Hh signaling may occur within the cilium. Because dysregulated Hh signaling promotes the development of a variety of human tumors, cilia may also have roles in cancer. Together, these findings have shed light on one mechanism by which primary cilia transduce signals critical for both development and disease.

Hedgehog signaling in development and homeostasis of the gastrointestinal tract

The Hedgehog family of secreted morphogenetic proteins acts through a complex evolutionary conserved signaling pathway to regulate patterning events during development and in the adult organism. In this review I discuss the role of Hedgehog signaling in the development, postnatal maintenance, and carcinogenesis of the gastrointestinal tract. Three mammalian hedgehog genes, sonic hedgehog (Shh), indian hedgehog (Ihh), and desert hedgehog (Dhh) have been identified. Shh and Ihh are important endodermal signals in the endodermal-mesodermal cross-talk that patterns the developing gut tube along different axes. Mutations in Shh, Ihh, and downstream signaling molecules lead to a variety of gross malformations of the murine gastrointestinal tract including esophageal atresia, tracheoesophageal fistula, annular pancreas, midgut malrotation, and duodenal and anal atresia. These congenital malformations are also found in varying constellations in humans, suggesting a possible role for defective Hedgehog signaling in these patients. In the adult, Hedgehog signaling regulates homeostasis in several endoderm-derived epithelia, for example, the stomach, intestine, and pancreas. Finally, growth of carcinomas of the proximal gastrointestinal tract such as esophageal, gastric, biliary duct, and pancreatic cancers may depend on Hedgehog signaling offering a potential avenue for novel therapy for these aggressive cancers.

Evidence for the Direct Involvement of the Proteasome in the Proteolytic Processing of the Aspergillus nidulans Zinc Finger Transcription Factor PacC

The 72-kDa zinc finger transcription factor PacC, distantly related to Ci/Gli developmental regulators, undergoes two-step proteolytic processing in response to alkaline ambient pH. "Signaling protease" cleavage of PacC(72) removes a processing-inhibitory C-terminal domain, making its truncated PacC(53) product accessible to a second "processing" protease, yielding PacC(27). Features of the processing proteolysis suggested the proteasome as a candidate protease. We constructed, using gene replacements, two missense active site mutations in preB, the Aspergillus nidulans orthologue of Saccharomyces cerevisiae PRE2 encoding the proteasome beta5 subunit. preB1(K101A) is lethal. Viable preB2(K101R) impairs growth and, like its equivalent pre2(K108R) in yeast, impairs chymotryptic activity. pre2(K108R) and preB2(K101R) active site mutations consistently shift position of the scissile bonds when PacC is processed in S. cerevisiae and A. nidulans, respectively, indicating that PacC must be a direct substrate of the proteasome. preB2(K101R) leads to a 2-3-fold elevation in NimE mitotic cyclin levels but appears to result in PacC instability, suggesting an altered balance between processing and degradation. preB2(K101R) compensates the marked impairment in PacC(27) formation resulting from deletion of the processing efficiency determinant in PacC, further indicating direct proteasomal involvement in the formation of PacC(27). Deletion of a Gly-Pro-Ala-rich region within this processing efficiency determinant markedly destabilizes PacC. Arg substitutions of Lys residues within this efficiency determinant and nearby show that they cooperate to promote PacC processing. A quadruple Lys-to-Arg substitution (4K-->R) impairs formation of PacC(27) and leads to persistence of PacC(53). Wild-type PacC(53) becomes multiply phosphorylated upon alkaline pH exposure. Processing-impaired 4K-->R PacC(53) becomes excessively phosphorylated.

Regulation of Ci-SCFSlimb binding, Ci proteolysis, and hedgehog pathway activity by Ci phosphorylation

Hedgehog (Hh) proteins signal by inhibiting the proteolytic processing of Ci/Gli family transcription factors and by increasing Ci/Gli-specific activity. When Hh is absent, phosphorylation of Ci/Gli triggers binding to SCF ubiquitin ligase complexes and consequent proteolysis. Here we show that multiple successively phosphorylated CK1 sites on Ci create an atypical extended binding site for the SCF substrate recognition component Slimb. GSK3 enhances binding primarily through a nearby region of Ci, which might contact an SCF component other than Slimb. Studies of Ci variants with altered CK1 and GSK3 sites suggest that the large number of phosphorylation sites that direct SCF(Slimb) binding confers a sensitive and graded proteolytic response to Hh, which collaborates with changes in Ci-specific activity to elicit a morphogenetic response. We also show that when Ci proteolysis is compromised, its specific activity is limited principally by Su(fu), and not by Cos2 cytoplasmic tethering or PKA phosphorylation.

Different electrostatic potentials define ETGE and DLG motifs as hinge and latch in oxidative stress response

Nrf2 is the regulator of the oxidative/electrophilic stress response. Its turnover is maintained by Keap1-mediated proteasomal degradation via a two-site substrate recognition mechanism in which two Nrf2-Keap1 binding sites form a hinge and latch. The E3 ligase adaptor Keap1 recognizes Nrf2 through its conserved ETGE and DLG motifs. In this study, we examined how the ETGE and DLG motifs bind to Keap1 in a very similar fashion but with different binding affinities by comparing the crystal complex of a Keap1-DC domain-DLG peptide with that of a Keap1-DC domain-ETGE peptide. We found that these two motifs interact with the same basic surface of either Keap1-DC domain of the Keap1 homodimer. The DLG motif works to correctly position the lysines within the Nrf2 Neh2 domain for efficient ubiquitination. Together with the results from calorimetric and functional studies, we conclude that different electrostatic potentials primarily define the ETGE and DLG motifs as a hinge and latch that senses the oxidative/electrophilic stress.

A hypermorphic mouse Gli3 allele results in a polydactylous limb phenotype

Gli3 protein processing to generate the Gli3 repressor is mediated by proteasome and inhibited by Hedgehog signaling. The Gli3 repressor concentration is graded along the anterior-posterior axis of the developing vertebrate limb due to posteriorly restricted Sonic hedgehog expression. In this study, we created a small deletion at the Gli3 locus (Gli3(Delta68)), which causes a half reduction in the Gli3 repressor levels and a slightly increased activity of full-length mutant protein in the limb. Mice homozygous for Gli3(Delta68) develop one to two extra partial digits in the anterior of the limb, while mice carrying one copy of the Gli3(Delta68) allele die soon after birth and display seven digits. These phenotypes are more severe than those found in mice lacking one wild-type Gli3 allele. The expression of dHand, Hoxd12, and Hoxd13 is anteriorly expanded in the limb, even though no up-regulation of Gli1 and Ptc RNA expression is detected. These findings suggest that a decrease in the Gli3 repressor level in combination with an increase in Gli3 full-length activity results in more severe digit patterning abnormalities than those caused by a loss of one wild-type Gli3 allele.

Role of glycogen synthase kinase-3 in cell fate and epithelial-mesenchymal transitions

Epithelial cells usually exist as sheets of immotile, tightly packed, well-coupled, polarized cells with distinct apical, basal and lateral surfaces. Remarkably, these cells can dramatically alter their morphology to become motile, fibroblast-like mesenchymal cells in a process of epithelial-mesenchymal transition (EMT). This process and the reverse, mesenchymal-epithelial transition, occur repeatedly during normal embryonic development. A phenomenon similar to physiological EMT occurs during the pathophysiological progression of some cancers. Tumours of epithelial origin, as they transform to malignancy, appear to exploit the innate plasticity of epithelial cells, with EMT conferring increased invasiveness and metastatic potential. Key to the maintenance of epithelial cell identity is the expression of E-cadherin, a protein that is required for tight intercellular adhesion along the lateral surfaces of adjacent epithelial cells. Loss of functional E-cadherin is a critical event in EMT. An important regulator of E-cadherin expression is the protein Snail, a zinc-finger transcriptional repressor. Snail contains several consensus sites for the kinase, glycogen synthase kinase-3 (GSK-3), and accumulating evidence indicates that it is a GSK-3 substrate. Phosphorylation of Snail by GSK-3 facilitates its proteasomal degradation. Conversely, inhibition of GSK-3 leads to Snail accumulation, E-cadherin downregulation, and development of EMT in cultured epithelial cells. Several signalling pathways implicated in the progression of EMT, including the Wnt and phosphoinositide 3-kinase pathways, use GSK-3 to mediate their responses. In these pathways, GSK-3's regulation of other transcriptional effectors like beta-catenin works in concert with changes in Snail to orchestrate the EMT process. This review focuses on the emerging role of GSK-3 as a modulator of cell fate and EMT in the contexts of development, in vitro cell culture and cancer.

The Shh-independent activator function of the full-length Gli3 protein and its role in vertebrate limb digit patterning

Anterior-posterior (A/P) limb patterning in vertebrates is determined by the counteraction between the Sonic Hedgehog (Shh) and the Gli3 transcription factor. Shh exerts its effect on Gli3 by regulating the full-length Gli3 protein processing to generate a Gli3 repressor gradient along the A/P axis of the limb. However, it is not clear whether the full-length Gli3 is an activator in vivo and plays any role in the limb patterning. Here we show that mouse limbs expressing only a Gli3 repressor form exhibit mild polysyndactyly and a partial loss of digit identity, while limbs expressing only a full-length Gli3 protein display severe polysyndactyly and a complete loss of digit identity. Interestingly, when the full-length Gli3 and the repressor are equally expressed in the limb, the digit patterning is overall normal except for an extra anterior digit. Furthermore, in the presence of one Gli3 wild type allele, a Gli3 mutant allele that expresses only the full-length form can rescue the Shh mutant digit phenotype to a great extent. The full-length Gli3 protein can also activate Shh target gene expression without Shh. Thus, our data indicate that the full-length Gli3 protein is an activator in vivo and that the ratio of the Gli3 activator to repressor, but neither the Gli3 repressor gradient nor the Gli3 activator/repressor ratio gradient, determines limb digit patterning.

Pathways of signal transduction employed by vertebrate Hedgehogs

Signalling by Hh (Hedgehog) proteins is among the most actively studied receptor-mediated phenomena relevant to development and post-embryonic homoeostatic events. The impact of signalling by the Hh proteins is profound, and work pertaining to the presentation of these proteins and the pathways engaged by them continues to yield unique insights into basic aspects of morphogenic signalling. We review here the mechanisms of signalling relevant to the actions of Hh proteins in vertebrates. We emphasize findings within the past several years on the recognition of, in particular, Sonic hedgehog by target cells, pathways of transduction employed by the seven-pass transmembrane protein Smoothened and end points of action, as manifest in the regulation of the Gli transcription factors. Topics of extended interest are those regarding the employment of heterotrimeric G-proteins and G-protein-coupled receptor kinases by Smoothened. We also address the pathways, insofar as known, linking Smoothened to the expression and stability of Gli1, Gli2 and Gli3. The mechanisms by which Hh proteins signal have few, if any, parallels. It is becoming clear in vertebrates, however, that several facets of signalling are shared in common with other venues of signalling. The challenge in understanding both the actions of Hh proteins and the overlapping forms of regulation will be in understanding, in molecular terms, both common and divergent signalling events.

Involvement of the betaTrCP in the ubiquitination and stability of the HIV-1 Vpu protein

The human immunodeficiency virus type 1 (HIV-1) Vpu protein binds to the CD4 receptor and targets it to the proteasome for degradation. This process requires the recruitment of human betaTrCP, a component of the Skp1-Cullin-F box (SCF) ubiquitin ligase complex, that interacts with phosphorylated Vpu molecules. Vpu, unlike other ligands of betaTrCP, has never been reported to be degraded. We provide evidence that Vpu, itself, is ubiquitinated and targeted for degradation by the proteasome. We demonstrate that the mutant Vpu2.6, which cannot interact with betaTrCP, is stable and, unlike wild-type Vpu, is not polyubiquitinated. These results suggest that betaTrCP is involved in Vpu polyubiquitination.

Phorbol esters inhibit the Hedgehog signalling pathway downstream of Suppressor of Fused, but upstream of Gli

The developmentally important Hedgehog (Hh) signal transduction pathway, which has recently been implicated in several forms of cancer, is subject to regulation by several protein kinases. Here, we address the role of protein kinase Cdelta in pathway inhibition and show that cellular depletion or pharmacological inhibition of this kinase isoform results in a blockade of signalling between Suppressor of Fused and the Gli transcription factors. We further provide evidence that the observed pathway inhibition is independent of primary cilia and the mitogen-activated protein kinase kinase (Mek1) kinase. These findings allowed for the rapid dissection of downstream Hh pathway activation mechanisms in human tumour cells and demonstrate a surprising variation in how cells can activate signalling in a ligand- and receptor-independent manner.

Shifting paradigms in Hedgehog signaling

Hedgehog (Hh) signaling proteins regulate multiple developmental and adult homeostatic processes. A defining feature of Hh signaling is that relatively small changes in the concentration of Hh ligand elicit dramatically different cellular responses. As a result, the processing, release and trafficking of Hh ligands must be tightly regulated to ensure proper signaling. In addition, sensitive and specific intracellular signaling cascades are needed to interpret subtle differences in the level of Hh signal to execute an appropriate response. A detailed understanding of the mechanisms that regulate these responses is critical to shaping our view of this key regulatory system.

A novel protein-processing domain in Gli2 and Gli3 differentially blocks complete protein degradation by the proteasome

The proteasome usually completely degrades its target proteins, but it can also degrade a handful of proteins in a limited and site-specific manner. The molecular mechanism for such limited degradation is unknown. The repressor forms of Gli2 and Gli3 transcription factors are generated from their full-length proteins through limited proteasome-mediated protein degradation. In this study, we have taken advantage of the fact that Gli3 is efficiently processed, whereas Gli2 is not, and identified a region of approximately 200 residues in their C termini that determine differential processing of the two proteins. This region, named processing determinant domain, functions as a signal for protein processing in the context of not only Gli2 and Gli3 protein sequences but also a heterologous hybrid protein, which would otherwise be completely degraded by the proteasome. Thus, the processing determinant domain constitutes a novel domain that functions independently. Our findings explain, at the molecular level, why Gli2 and Gli3 are differentially processed and, more importantly, may help understand a probably general mechanism by which the proteasome degrades some of its target proteins partially rather than completely.

GSK3β positively regulates Hedgehog signaling through Sufu in mammalian cells

Hedgehog signaling plays important roles in embryonic patterning of multicellular organisms. This pathway is ultimately transmitted by the zinc-finger transcriptional factor Gli, of which activity is suppressed by Sufu, a negative regulator of this signaling. To clarify this regulation to more detail, we screened for Sufu-binding proteins. We identified GSK3beta as a specific binding partner of Sufu by mass spectrometric analysis. GSK3beta bound to Sufu both in vitro and in vivo. Down-regulation of GSK3beta expression by RNAi in Hedgehog-responsive cells attenuated Hedgehog signaling, suggesting that GSK3beta functions as a positive regulator of Hedgehog signaling. In addition, an in vitro kinase assay showed that GSK3beta phosphorylates Sufu and phosphorylation-mimicking mutant of Sufu showed significantly decreased ability to bind Gli1 and could not suppress the Gli-mediated expression of a reporter gene efficiently. These results strongly suggest that GSK3beta phosphorylates Sufu to positively regulate Hedgehog signaling in mammalian cells.

Cilia and developmental signaling

Recent studies have revealed unexpected connections between the mammalian Hedgehog (Hh) signal transduction pathway and the primary cilium, a microtubule-based organelle that protrudes from the surface of most vertebrate cells. Intraflagellar transport proteins, which are required for the construction of cilia, are essential for all responses to mammalian Hh proteins, and proteins required for Hh signal transduction are enriched in primary cilia. The phenotypes of different mouse mutants that affect ciliary proteins suggest that cilia may act as processive machines that organize sequential steps in the Hh signal transduction pathway. Cilia on vertebrate cells are likely to be important in additional developmental signaling pathways and are required for PDGF receptor alpha signaling in cultured fibroblasts. Cilia are not essential for either canonical or noncanonical Wnt signaling, although cell-type-specific modulation of cilia components may link cilia and Wnt signaling in some tissues. Because ciliogenesis in invertebrates is limited to a very small number of specialized cell types, the role of cilia in developmental signaling pathways is likely a uniquely vertebrate phenomenon.