Hedgehog-stimulated phosphorylation of the kinesin-related protein Costal2 is mediated by the serine/threonine kinase fused

Global and local functions of the Fused kinase ortholog CdaH in intracellular patterning in Tetrahymena

Ciliates assemble numerous microtubular structures into complex cortical patterns. During ciliate division, the pattern is duplicated by intracellular segmentation that produces a tandem of daughter cells. In Tetrahymena thermophila, the induction and positioning of the division boundary involves two mutually antagonistic factors: posterior CdaA (cyclin E) and anterior CdaI (Hippo kinase). Here, we characterized the related cdaH-1 allele, which confers a pleiotropic patterning phenotype including an absence of the division boundary and an anterior-posterior mispositioning of the new oral apparatus. CdaH is a Fused or Stk36 kinase ortholog that localizes to multiple sites that correlate with the effects of its loss, including the division boundary and the new oral apparatus. CdaH acts downstream of CdaA to induce the division boundary and drives asymmetric cytokinesis at the tip of the posterior daughter. CdaH both maintains the anterior-posterior position of the new oral apparatus and interacts with CdaI to pattern ciliary rows within the oral apparatus. Thus, CdaH acts at multiple scales, from induction and positioning of structures on the cell-wide polarity axis to local organelle-level patterning.

Protein modifications in Hedgehog signaling: Cross talk and feedback regulation confer divergent Hedgehog signaling activity

The complexity of the Hedgehog (Hh) signaling cascade has increased over the course of evolution; however, it does not suffice to accommodate the dynamic yet robust requirements of differential Hh signaling activity needed for embryonic development and adult homeostatic maintenance. One solution to solve this dilemma is to apply multiple forms of post-translational modifications (PTMs) to the core Hh signaling components, modulating their abundance, localization, and signaling activity. This review summarizes various forms of protein modifications utilized to regulate Hh signaling, with a special emphasis on crosstalk between different forms of PTMs and their feedback regulation by Hh signaling.

Phosphorylation of Ci/Gli by Fused Family Kinases Promotes Hedgehog Signaling

Hedgehog (Hh) signaling culminates in the conversion of the latent transcription factor Cubitus interruptus (Ci)/Gli into its activator form (Ci^A/Gli^A), but the underlying mechanism remains poorly understood. Here, we demonstrate that Hh stimulates the phosphorylation of Ci by the Ser/Thr kinase Fused (Fu) and that Fu-mediated phosphorylation of Ci promotes its activation. We find that Fu directly phosphorylates Ci on Ser218 and Ser1230, which primes its further phosphorylation by CK1 on adjacent sties. These phosphorylation events alter Ci binding to the pathway inhibitor Suppressor of fused (Sufu) and facilitate the recruitment of Transportion and the transcriptional coactivator CBP. Furthermore, we provide evidence that Sonic hedgehog (Shh) activates Gli2 by stimulating its phosphorylation on conserved sites through the Fu-family kinases ULK3 and mFu/STK36 in a manner depending on Gli2 ciliary localization. Hence, Fu-family kinase-mediated phosphorylation of Ci/Gli serves as a conserved mechanism that activates the Hh pathway transcription factor.

Protein association changes in the Hedgehog signaling complex mediate differential signaling strength

Hedgehog (Hh) is a conserved morphogen that controls cell differentiation and tissue patterning in metazoans. In Drosophila, the Hh signal is transduced from the G protein-coupled receptor Smoothened (Smo) to the cytoplasmic Hh signaling complex (HSC). How activated Smo is translated into a graded activation of the downstream pathway is still not well understood. In this study, we show that the last amino acids of the cytoplasmic tail of Smo, in combination with G protein-coupled receptor kinase 2 (Gprk2), bind to the regulatory domain of Fused (Fu) and highly activate its kinase activity. We further show that this binding induces changes in the association of Fu protein with the HSC and increases the proximity of the Fu catalytic domain to its substrate, the Costal2 kinesin. We propose a new model in which, depending on the magnitude of Hh signaling, Smo and Gprk2 modulate protein association and conformational changes in the HSC, which are responsible for the differential activation of the pathway.

Ubr3, a Novel Modulator of Hh Signaling Affects the Degradation of Costal-2 and Kif7 through Poly-ubiquitination

Hedgehog (Hh) signaling regulates multiple aspects of metazoan development and tissue homeostasis, and is constitutively active in numerous cancers. We identified Ubr3, an E3 ubiquitin ligase, as a novel, positive regulator of Hh signaling in Drosophila and vertebrates. Hh signaling regulates the Ubr3-mediated poly-ubiquitination and degradation of Cos2, a central component of Hh signaling. In developing Drosophila eye discs, loss of ubr3 leads to a delayed differentiation of photoreceptors and a reduction in Hh signaling. In zebrafish, loss of Ubr3 causes a decrease in Shh signaling in the developing eyes, somites, and sensory neurons. However, not all tissues that require Hh signaling are affected in zebrafish. Mouse UBR3 poly-ubiquitinates Kif7, the mammalian homologue of Cos2. Finally, loss of UBR3 up-regulates Kif7 protein levels and decreases Hh signaling in cultured cells. In summary, our work identifies Ubr3 as a novel, evolutionarily conserved modulator of Hh signaling that boosts Hh in some tissues.

Contributions of Costal 2-Fused interactions to Hedgehog signaling in Drosophila

The Drosophila kinesin-family protein Costal 2 (Cos2) and its mammalian ortholog Kif7 play dual roles in Hedgehog (Hh) signaling. In the absence of Hh, Cos2 and Kif7 contribute to proteolytic processing and silencing of the Hh-regulated transcription factors, Drosophila Cubitus interruptus (Ci) and mammalian Gli proteins. Cos2 and Kif7 are also necessary for full activation of full-length Ci-155 and Gli transcription factors in response to Hh proteins. Here, we use classical fused alleles and transgenic Cos2 products deficient for Fused (Fu) association to show that Cos2 must bind to Fu to support efficient Ci-155 processing. Residual Ci-155 processing in the absence of Cos2-Fu interaction did not require Suppressor of Fused, which has been implicated in processing mammalian Gli proteins. We also provide evidence that Cos2 binding to the CORD domain of Ci-155 contributes to both Ci-155 processing and Ci-155 silencing in the absence of Hh. In the presence of Hh, Ci-155 processing is blocked and Cos2 now promotes activation of Ci-155, which requires Fu kinase activity. Here, we show that normal Ci-155 activation by Hh requires Cos2 binding to Fu, supporting the hypothesis that Cos2 mediates the apposition of Fu molecules suitable for cross-phosphorylation and consequent full activation of Fu kinase. We also find that phosphorylation of Cos2 by Fu at two previously mapped sites, S572 and S931, which is thought to mediate Ci-155 activation, is not required for normal activation of Ci-155 by Hh or by activated Fu.

Decoding Ci: from partial degradation to inhibition

Hedgehog is a morphogen, which is widely involved in the regulation of cell proliferation, differentiation and tissue patterning during development in both vertebrate and invertebrate, such as in coordination of eye, brain, gonad, gut and tracheal development. In invertebrate, Cubitus interruptus (Ci) modification process is the last identified step before transcriptional activation in the Hh signaling pathway. Ci can form a truncated repressor (Ci(R) /Ci75) or act as an activator (Ci(A) /Ci155) based on Hh gradient to regulate the expressions of target genes. The activity of Ci is mediated by different mechanisms, including processing, trafficking and degradation. While in vertebrate, Glioblastomas (Glis), homologs of Ci, play similar but more complex roles in the regulation of mammals Hh pathway. Hh signaling is responsible for a wide variety of processes during embryonic development and adult tissue homeostasis. Malfunction of Hh signaling could cause various diseases including birth defects and cancers. Enormous efforts were made in the past decades to explore the Hh pathway regulation and the results have provided us important insights into disease diagnosis and therapeutic treatment. In this review, we focus on a small branch of Hh pathway regulation based on studies in the Drosophila system, mainly about Ci degradation, aiming to explain how Ci is modified by different ubiquitin ligases due to the strong or moderate Hh signals and then been subjected to complete or partial degradation by proteasomes. Overall, we intend to offer an overview on how Ci responds to and relays Hh signals in a precise manner to control target genes expressions and ensures proper Hh signal transduction.

The E3 ubiquitin ligase MID1 catalyzes ubiquitination and cleavage of Fu

SHH (Sonic Hedgehog)-GLI signaling plays an important role during embryogenesis and in tumorigenesis. The survival and growth of several types of cancer depend on autonomously activated SHH-GLI signaling. A protein complex containing the ubiquitin ligase MID1 and protein phosphatase 2A regulates the nuclear localization and transcriptional activity of GLI3, a transcriptional effector molecule of SHH, in cancer cell lines with autonomously activated SHH signaling. However, the exact molecular mechanisms that mediate the interaction between MID1 and GLI3 remained unknown. Here, we show that MID1 catalyzes the ubiquitination and proteasomal cleavage of the GLI3 regulator Fu. Our data suggest that Fu ubiquitination and cleavage is one of the key elements connecting the MID1-PP2A protein complex with GLI3 activity control.

Arabidopsis Fused kinase TWO-IN-ONE dominantly inhibits male meiotic cytokinesis

Arabidopsis Fused kinase TWO-IN-ONE (TIO) controls phragmoplast expansion through its interaction with the Kinesin-12 subfamily proteins that anchor the plus ends of interdigitating microtubules in the phragmoplast midzone. Previous analyses of loss-of-function mutants and RNA interference lines revealed that TIO positively controls both somatic and gametophytic cell cytokinesis; however, knowledge of the full spectrum of TIO functions during plant development remains incomplete. To characterize TIO functions further, we expressed TIO and a range of TIO variants under control of the TIO promoter in wild-type Arabidopsis plants. We discovered that TIO-overexpressing transgenic lines produce enlarged pollen grains, arising from incomplete cytokinesis during male meiosis, and show sporophytic abnormalities indicative of polyploidy. These phenotypes arose independently in TIO variants in which either gametophytic function or the ability of TIO to interact with Kinesin-12 subfamily proteins was abolished. Interaction assays in yeast showed TIO to bind to the AtNACK2/TETRASPORE, and plants doubly homozygous for kinesin-12a and kinesin-12b knockout mutations to produce enlarged pollen grains. Our results show TIO to dominantly inhibit male meiotic cytokinesis in a dosage-dependent manner that may involve direct binding to a component of the canonical NACK-PQR cytokinesis signaling pathway.

Hedgehog in the Drosophila testis niche: what does it do there?

Stem cell niche is a specialized microenvironment crucial to self-renewal. The testis in Drosophila contains two different types of stem cells, the germline stem cells and the somatic cyst stem cells that are sustained by their respective niche signals, thus is a good system for studying the interaction between the stem cells and their hosting niche. The JAK-STAT and BMP pathways are known to play critical roles in the self-renewal of different kinds of stem cells, but the roles of several other pathways have emerged recently in a complex signaling network in the testis niche. Reports of independent observations from three research groups have uncovered an important role of Hedgehog (Hh) in the Drosophila testis niche. In this review, we summarize these recent findings and discuss the interplay between the Hh signaling mechanisms and those of the JAK-STAT and BMP pathways. We also discuss directions for further investigation.

Decoding the phosphorylation code in Hedgehog signal transduction

Hedgehog (Hh) signaling plays pivotal roles in embryonic development and adult tissue homeostasis, and its deregulation leads to numerous human disorders including cancer. Binding of Hh to Patched (Ptc), a twelve-transmembrane protein, alleviates its inhibition of Smoothened (Smo), a seven-transmembrane protein related to G-protein-coupled receptors (GPCRs), leading to Smo phosphorylation and activation. Smo acts through intracellular signaling complexes to convert the latent transcription factor Cubitus interruptus (Ci)/Gli from a truncated repressor to a full-length activator, leading to derepression/activation of Hh target genes. Increasing evidence suggests that phosphorylation participates in almost every step in the signal relay from Smo to Ci/Gli, and that differential phosphorylation of several key pathway components may be crucial for translating the Hh morphogen gradient into graded pathway activities. In this review, we focus on the multifaceted roles that phosphorylation plays in Hh signal transduction, and discuss the conservation and difference between Drosophila and mammalian Hh signaling mechanisms.

The Hedgehog signal transduction network

Hedgehog (Hh) proteins regulate the development of a wide range of metazoan embryonic and adult structures, and disruption of Hh signaling pathways results in various human diseases. Here, we provide a comprehensive review of the signaling pathways regulated by Hh, consolidating data from a diverse array of organisms in a variety of scientific disciplines. Similar to the elucidation of many other signaling pathways, our knowledge of Hh signaling developed in a sequential manner centered on its earliest discoveries. Thus, our knowledge of Hh signaling has for the most part focused on elucidating the mechanism by which Hh regulates the Gli family of transcription factors, the so-called "canonical" Hh signaling pathway. However, in the past few years, numerous studies have shown that Hh proteins can also signal through Gli-independent mechanisms collectively referred to as "noncanonical" signaling pathways. Noncanonical Hh signaling is itself subdivided into two distinct signaling modules: (i) those not requiring Smoothened (Smo) and (ii) those downstream of Smo that do not require Gli transcription factors. Thus, Hh signaling is now proposed to occur through a variety of distinct context-dependent signaling modules that have the ability to crosstalk with one another to form an interacting, dynamic Hh signaling network.

Arabidopsis Fused kinase and the Kinesin-12 subfamily constitute a signalling module required for phragmoplast expansion

The conserved Fused kinase plays vital but divergent roles in many organisms from Hedgehog signalling in Drosophila to polarization and chemotaxis in Dictyostelium. Previously we have shown that Arabidopsis Fused kinase termed TWO-IN-ONE (TIO) is essential for cytokinesis in both sporophytic and gametophytic cell types. Here using in vivo imaging of GFP-tagged microtubules in dividing microspores we show that TIO is required for expansion of the phragmoplast. We identify the phragmoplast-associated kinesins, PAKRP1/Kinesin-12A and PAKRP1L/Kinesin-12B, as TIO-interacting proteins and determine TIO-Kinesin-12 interaction domains and their requirement in male gametophytic cytokinesis. Our results support the role of TIO as a functional protein kinase that interacts with Kinesin-12 subfamily members mainly through the C-terminal ARM repeat domain, but with a contribution from the N-terminal kinase domain. The interaction of TIO with Kinesin proteins and the functional requirement of their interaction domains support the operation of a Fused kinase signalling module in phragmoplast expansion that depends upon conserved structural features in diverse Fused kinases.

Ligand-independent activation of the Hedgehog pathway displays non-cell autonomous proliferation during eye development in Drosophila

Deregulation of the Hedgehog (Hh) signaling pathway is associated with the development of human cancer including medullobastoma and basal cell carcinoma. Loss of Patched or activation of Smoothened in mouse models increases the occurrence of tumors. Likewise, in a Drosophila eye model, deregulated Hedgehog signaling causes overgrowth of eye and head tissues. Surprisingly, we show that cells with deregulated Hh signaling do not or only little contribute to the tissue overgrowth. Instead, they become more sensitive to apoptosis and may eventually be eliminated. Nevertheless, these mutant cells increase proliferation in the adjacent wild-type tissue, i.e., in a non-cell autonomous manner. This non-cell autonomous effect is position-dependent and restricted to mutant cells in the anterior portion of the eye. We also observe precocious non-cell autonomous differentiation in genetic mosaics with deregulated Hh signaling. Together, these non-cell autonomous growth and differentiation phenotypes in the Drosophila eye model reveal another strategy by which oncogenes may generate a supportive micro-environment for tumor growth.

The complexities of G-protein-coupled receptor kinase function in Hedgehog signaling

Hedgehog (Hh) signaling is essential for proper tissue patterning and maintenance and has a substantial impact on human disease. While many of the main components and mechanisms involved in transduction of the Hh signal have been identified, the details of how the pathway functions are continually being refined. One aspect that has attracted much attention recently is the involvement of G-protein-coupled receptor kinases (GRKs) in the pathway. These regulators of G-protein-coupled receptor (GPCR) signaling have an evolutionarily-conserved function in promoting high-threshold Hh target gene expression through regulation of Smoothened (Smo), a GPCR family member that activates intracellular Hh signaling. Several models of how GRKs impact on Smo to increase downstream signaling have been proposed. Recently, we demonstrated that these kinases have surprisingly complex and conflicting roles, acting to limit signaling through the pathway while also promoting Smo activity. In addition to the previously described direct effects of Gprk2 on Smo activation, Gprk2 also indirectly affects Hh signaling by controlling production of the second messenger cyclic AMP to influence Protein kinase A activity.

Unconventional functions of microtubule motors

With the functional characterization of proteins advancing at fast pace, the notion that one protein performs different functions - often with no relation to each other - emerges as a novel principle of how cells work. Molecular motors are no exception to this new development. Here, we provide an account on recent findings revealing that microtubule motors are multifunctional proteins that regulate many cellular processes, in addition to their main function in transport. Some of these functions rely on their motor activity, but others are independent of it. Of the first category, we focus on the role of microtubule motors in organelle biogenesis, and in the remodeling of the cytoskeleton, especially through the regulation of microtubule dynamics. Of the second category, we discuss the function of microtubule motors as static anchors of the cargo at the destination, and their participation in regulating signaling cascades by modulating interactions between signaling proteins, including transcription factors. We also review atypical forms of transport, such as the cytoplasmic streaming in the oocyte, and the movement of cargo by microtubule fluctuations. Our goal is to provide an overview of these unexpected functions of microtubule motors, and to incite future research in this expanding field.

Distinct phosphorylations on kinesin costal-2 mediate differential hedgehog signaling strength

The graded Hedgehog (Hh) signal is transduced by the transmembrane Smoothened (Smo) proteins in both vertebrates and invertebrates. In Drosophila, associations between Smo and the Fused (Fu)/Costal-2 (Cos2)/Cubitus Interruptus (Ci) cytoplasmic complex lead to pathway activation, but it remains unclear how the cytoplasmic complex responds to and transduces different levels of Hh signaling. We show here that, within the Hh gradient field, low- and high-magnitude Smo activations control differentially the phosphorylation of Cos2 on two distinct serines. We also provide evidence that these phosphorylations depend on the Fu kinase activity and lead to a shift of Cos2 distribution from the cytoplasm to the plasma membrane. Moreover, the distinct Cos2 phosphorylation states mediate differential Hh signaling magnitude, suggesting that phosphorylation and relocation of Cos2 to the plasma membrane facilitate high-level Hh signaling through the control of Ci nuclear translocation and transcriptional activity.

Phosphorylation regulation of Hedgehog signaling

The study of posttranslational regulation of proteins has occupied biochemists for well over a half century. Understanding balanced phosphorylation and dephosphorylation of the proteins may be the key to meeting some of the most pressing scientific challenges. A detailed examination of the phosphorylation of many components in the Hedgehog (Hh) pathway leads to a better understanding of the Hh signaling mechanisms. This chapter describes the precise phosphorylation that evolves during the phosphorylation/dephosphorylation of players in the Hh signaling cascade, including the signal transducer Smoothened and the transcription factor Ci/Gli.

Mammalian homologues of Drosophila fused kinase

Sonic Hedgehog (Shh) signaling pathway is implicated in various developmental and postnatal processes. Much of the current knowledge about the mechanisms of Shh signal transduction in vertebrates comes from the investigations of the respective pathway in fruit fly Drosophila melanogaster. In Drosophila, serine/threonine kinase fused is involved in all aspects of regulation of the Hh-dependent transcription factor cubitus interruptus possessing both catalytic and regulatory functions. Two proteins, Stk36 and Ulk3, share similarity with fu and have been suggested as mammalian fu homologues. However, in vivo data clarify that Stk36 is not required for embryonic development in mice and participates in Shh-independent genesis of motile cilia. Even if Stk36 is associated with any pathological or physiological aspect of postnatal Shh signaling in mammals, it has perhaps only regulatory functions since its catalytic activity seems to be lost during evolution. In contrast to Stk36, Ulk3 is an active kinase. In non-stimulated cells, Ulk3 catalytic activity is blocked, and it is involved in negative control of Gli proteins, mediators of Shh signaling. In response to Shh, Ulk3 positively regulates Gli proteins by directly phosphorylating them. Thus, Ulk3 is able to recapitulate both positive and negative roles of fu in vitro. However, Ulk3 functioning in vivo remains to be investigated.

Drosophila G-protein-coupled receptor kinase 2 regulates cAMP-dependent Hedgehog signaling

G-protein-coupled receptor kinases (GRKs) play a conserved role in Hedgehog (Hh) signaling. In several systems, GRKs are required for efficient Hh target gene expression. Their principal target appears to be Smoothened (Smo), the intracellular signal-generating component of the pathway and a member of the G-protein-coupled receptor (GPCR) protein family. In Drosophila, a GRK called Gprk2 is needed for internalization and downregulation of activated Smo, consistent with the typical role of these kinases in negatively regulating GPCRs. However, Hh target gene activation is strongly impaired in gprk2 mutant flies, indicating that Gprk2 must also positively regulate Hh signaling at some level. To investigate its function in signaling, we analyzed several different readouts of Hh pathway activity in animals or cells lacking Gprk2. Surprisingly, although target gene expression was impaired, Smo-dependent activation of downstream components of the signaling pathway was increased in the absence of Gprk2. This suggests that Gprk2 does indeed play a role in terminating Smo signaling. However, loss of Gprk2 resulted in a decrease in cellular cAMP concentrations to a level that was limiting for Hh target gene activation. Normal expression of target genes was restored in gprk2 mutants by stimulating cAMP production or activating the cAMP-dependent Protein kinase A (Pka). Our results suggest that direct regulation of Smo by Gprk2 is not absolutely required for Hh target gene expression. Gprk2 is important for normal cAMP regulation, and thus has an indirect effect on the activity of Pka-regulated components of the Hh pathway, including Smo itself.

The Hedgehog-induced Smoothened conformational switch assembles a signaling complex that activates Fused by promoting its dimerization and phosphorylation

Hedgehog (Hh) transduces signal by regulating the subcellular localization and conformational state of the GPCR-like protein Smoothened (Smo) but how Smo relays the signal to cytoplasmic signaling components remains poorly understood. Here, we show that Hh-induced Smo conformational change recruits Costal2 (Cos2)/Fused (Fu) and promotes Fu kinase domain dimerization. We find that induced dimerization through the Fu kinase domain activates Fu by inducing multi-site phosphorylation of its activation loop (AL) and phospho-mimetic mutations of AL activate the Hh pathway. Interestingly, we observe that graded Hh signals progressively increase Fu kinase domain dimerization and AL phosphorylation, suggesting that Hh activates Fu in a dose-dependent manner. Moreover, we find that activated Fu regulates Cubitus interruptus (Ci) by both promoting its transcriptional activator activity and inhibiting its proteolysis into a repressor form. We provide evidence that activated Fu exerts these regulations by interfering with the formation of Ci-Sufu and Ci-Cos2-kinase complexes that normally inhibit Ci activity and promote its processing. Taken together, our results suggest that Hh-induced Smo conformational change facilitates the assembly of active Smo-Cos2-Fu signaling complexes that promote Fu kinase domain dimerization, phosphorylation and activation, and that Fu regulates both the activator and repressor forms of Ci.

Transduction of the Hedgehog signal through the dimerization of Fused and the nuclear translocation of Cubitus interruptus

The Hedgehog (Hh) family of secreted proteins is essential for development in both vertebrates and invertebrates. As one of main morphogens during metazoan development, the graded Hh signal is transduced across the plasma membrane by Smoothened (Smo) through the differential phosphorylation of its cytoplasmic tail, leading to pathway activation and the differential expression of target genes. However, how Smo transduces the graded Hh signal via the Costal2 (Cos2)/Fused (Fu) complex remains poorly understood. Here we present a model of the cell response to a Hh gradient by translating Smo phosphorylation information to Fu dimerization and Cubitus interruptus (Ci) nuclear localization information. Our findings suggest that the phosphorylated C-terminus of Smo recruits the Cos2/Fu complex to the membrane through the interaction between Smo and Cos2, which further induces Fu dimerization. Dimerized Fu is phosphorylated and transduces the Hh signal by phosphorylating Cos2 and Suppressor of Fu (Su(fu)). We further show that this process promotes the dissociation of the full-length Ci (Ci155) and Cos2 or Su(fu), and results in the translocation of Ci155 into the nucleus, activating the expression of target genes.

The Fused/Smurf complex controls the fate of Drosophila germline stem cells by generating a gradient BMP response

In the Drosophila ovary, germline stem cells (GSCs) are maintained primarily by bone morphogenetic protein (BMP) ligands produced by the stromal cells of the niche. This signaling represses GSC differentiation by blocking the transcription of the differentiation factor Bam. Remarkably, bam transcription begins only one cell diameter away from the GSC in the daughter cystoblasts (CBs). How this steep gradient of response to BMP signaling is formed has been unclear. Here, we show that Fused (Fu), a serine/threonine kinase that regulates Hedgehog, functions in concert with the E3 ligase Smurf to regulate ubiquitination and proteolysis of the BMP receptor Thickveins in CBs. This regulation generates a steep gradient of BMP activity between GSCs and CBs, allowing for bam expression on CBs and concomitant differentiation. We observed similar roles for Fu during embryonic development in zebrafish and in human cell culture, implying broad conservation of this mechanism.

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.

Non-cell-autonomous signaling by Shh in tumors: challenges and opportunities for therapeutic targets

IMPORTANCE OF THE FIELD

The Hedgehog (Hh) pathway is required during many developmental events; in adults the Hedgehog pathway is involved in the maintenance of several stem cell niches. It is therefore not surprising that aberrantly regulated Hh pathway activity can cause birth defects in the developing organism, as well as neoplastic disease later in life.

AREAS COVERED IN THIS REVIEW

As a consequence of the involvement in pathogenesis, the Hh pathway components are subject to an intense scrutiny as potential targets for therapeutic agents. We aim to provide an overview of the biology of the Hh proteins and the cellular response, in conjunction with potential therapeutic interventions.

WHAT THE READER WILL GAIN

Specifically, we focus on the recently discovered non-cell-autonomous Shh signaling used by tumors and the implications of this for the design of treatment strategies. This should provide the reader with up-to-date knowledge on the role of the Hh pathway in tumor progression and the options to treat these malignancies.

TAKE HOME MESSAGE

An important concept that we advocate in this review is the need to recognize the need to target both the stromal and the tumor compartment in malignancies that rely on paracrine Shh signaling.

Dual function of UNC-51-like kinase 3 (Ulk3) in the Sonic hedgehog signaling pathway

The Sonic hedgehog (Shh) signaling pathway controls a variety of developmental processes and is implicated in tissue homeostasis maintenance and neurogenesis in adults. Recently, we identified Ulk3 as an active kinase able to positively regulate Gli proteins, mediators of the Shh signaling in mammals. Here, we provide several lines of evidence that Ulk3 participates in the transduction of the Shh signal also independently of its kinase activity. We demonstrate that Ulk3 through its kinase domain interacts with Suppressor of Fused (Sufu), a protein required for negative regulation of Gli proteins. Sufu blocks Ulk3 autophosphorylation and abolishes its ability to phosphorylate and positively regulate Gli proteins. We show that Shh signaling destabilizes the Sufu-Ulk3 complex and induces the release of Ulk3. We demonstrate that the Sufu-Ulk3 complex, when co-expressed with Gli2, promotes generation of the Gli2 repressor form, and that reduction of the Ulk3 mRNA level in Shh-responsive cells results in higher potency of the cells to transmit the Shh signal. Our data suggests a dual function of Ulk3 in the Shh signal transduction pathway and propose an additional way of regulating Gli proteins by Sufu, through binding to and suppression of Ulk3.

Dynamic phosphorylation of the kinesin Costal-2 in vivo reveals requirement of fused kinase activity for all levels of hedgehog signalling

Hedgehog (Hh) proteins are secreted molecules that play an essential role in development and tumorigenesis. In Drosophila cultured cells, phosphorylation of the kinesin-like Costal2 (Cos2) protein at Ser572 is triggered by the kinase fused (Fu) upon Hh pathway activation. Here, we validate the first phospho-antibody for one of the Hh pathway components, Cos2, as a universal in situ readout of Hh signal transduction. For the first time, this tool allows the visualisation of a gradient of signalling activity and therefore the range of the activating Hh ligand in different tissues. We also show that, in vivo, Fu kinase is activated by and necessary to transduce all levels of intracellular Hh signalling. Our study fills a gap in the understanding of the Hh pathway by showing that the molecular cascade leading to Cos2 phosphorylation is conserved in all cells activated by Hh. Therefore, we propose that the extracellular Hh information is conveyed to an intracellular signal through graded Fu kinase activity.

The role of kinases in the Hedgehog signalling pathway

The Hedgehog (Hh) signalling pathway has a crucial role in several developmental processes and is aberrantly activated in a variety of cancers. In Drosophila, many of the canonical Hh pathway components are phosphorylated, yet the precise role of these phosphorylation events in the regulation of Hh signal transduction is unclear. Furthermore, the Hh pathway receives input from several kinases that have well-described roles in other cellular functions, some of which have both positive and negative effects on Hh signalling. Several recent studies have characterized the role of specific phosphorylation events in the Hh pathway, and have begun to shed light on how phosphorylation of Hh signalling components affects their subcellular location, stability and activity to mediate the transcriptional response to the Hh gradient.

Costal2 functions as a kinesin-like protein in the hedgehog signal transduction pathway

The Hedgehog (Hh) signaling pathway initiates an evolutionarily conserved developmental program required for the proper patterning of many tissues [1]. Although Costal2 (Cos2) is a requisite component of the Hh pathway, its mechanistic role is not well understood. Because of its primary sequence, Cos2 was initially predicted to function as a kinesin-like protein [2]. However, evidence showing that Cos2 function might require kinesin-like properties has been lacking [2-6]. Thus, the prevailing dogma in the field is that Cos2 functions solely as a scaffolding protein [7, 8]. Here, we show that Cos2 motility is required for its biological function and that this motility may be Hh regulated. We show that Cos2 motility requires an active motor domain, ATP, and microtubules. Additionally, Cos2 recruits and transports other components of the Hh signaling pathway, including the transcription factor Cubitus interruptus (Ci). Drosophila expressing cos2 mutations that encode proteins that lack motility are attenuated in their ability to regulate Ci activity and exhibit phenotypes consistent with attenuated Cos2 function [9]. Combined, these results demonstrate that Cos2 motility plays an important role in its function, regulating the amounts and activity of Ci that ultimately interpret the level of Hh to which cells are exposed.

A screen for modifiers of hedgehog signaling in Drosophila melanogaster identifies swm and mts

Signaling by Hedgehog (Hh) proteins shapes most tissues and organs in both vertebrates and invertebrates, and its misregulation has been implicated in many human diseases. Although components of the signaling pathway have been identified, key aspects of the signaling mechanism and downstream targets remain to be elucidated. We performed an enhancer/suppressor screen in Drosophila to identify novel components of the pathway and identified 26 autosomal regions that modify a phenotypic readout of Hh signaling. Three of the regions include genes that contribute constituents to the pathway-patched, engrailed, and hh. One of the other regions includes the gene microtubule star (mts) that encodes a subunit of protein phosphatase 2A. We show that mts is necessary for full activation of Hh signaling. A second region includes the gene second mitotic wave missing (swm). swm is recessive lethal and is predicted to encode an evolutionarily conserved protein with RNA binding and Zn(+) finger domains. Characterization of newly isolated alleles indicates that swm is a negative regulator of Hh signaling and is essential for cell polarity.

Fused-Costal2 protein complex regulates Hedgehog-induced Smo phosphorylation and cell-surface accumulation

The seven-transmembrane protein Smoothened (Smo) acts as a signal transducer in the Hedgehog (Hh) pathway that mediates many key developmental processes. In Drosophila, Hh-induced phosphorylation promotes Smo cell-surface accumulation and signaling activity; however, the mechanisms controlling Smo phosphorylation and cell-surface accumulation are still unknown. The intracellular signaling complex containing Fused (Fu) and Costal2 (Cos2) is thought to transduce the Hh signal downstream from Smo. Here, we identify a novel feedback mechanism that regulates Smo through the Fu-Cos2 complex. We found that Hh-induced Smo accumulation is inhibited in fu mutant clones or by expressing a dominant-negative form of Fu, and such inhibition is alleviated by removal of Cos2. Conversely, overexpressing Cos2 blocks Smo accumulation, which is reversed by coexpressing Fu. Cos2 blocks Smo accumulation through its C-terminal Smo-interacting domain, and Fu antagonizes Cos2 by phosphorylating Cos2 at Ser572. Furthermore, we found that Ser572 phosphorylation attenuates the Cos2-Smo interaction and promotes Cos2 instability. Finally, we provided evidence that Fu and Cos2 control Smo cell-surface accumulation by regulating Smo phosphorylation. Our data suggest that Cos2-Smo interaction blocks Hh-induced Smo phosphorylation, and that Fu promotes Smo phosphorylation by antagonizing Cos2.

Phosphorylation of the atypical kinesin Costal2 by the kinase Fused induces the partial disassembly of the Smoothened-Fused-Costal2-Cubitus interruptus complex in Hedgehog signalling

The Hedgehog (Hh) family of secreted proteins is involved both in developmental and tumorigenic processes. Although many members of this important pathway are known, the mechanism of Hh signal transduction is still poorly understood. In this study, we analyse the regulation of the kinesin-like protein Costal2 (Cos2) by Hh. We show that a residue on Cos2, serine 572 (Ser572), is necessary for normal transduction of the Hh signal from the transmembrane protein Smoothened (Smo) to the transcriptional mediator Cubitus interruptus (Ci). This residue is located in the serine/threonine kinase Fused (Fu)-binding domain and is phosphorylated as a consequence of Fu activation. Although Ser572 does not overlap with known Smo- or Ci-binding domains, the expression of a Cos2 variant mimicking constitutive phosphorylation and the use of a specific antibody to phosphorylated Ser572 showed a reduction in the association of phosphorylated Cos2 with Smo and Ci, both in vitro and in vivo. Moreover, Cos2 proteins with an Ala or Asp substitution of Ser572 were impaired in their regulation of Ci activity. We propose that, after activation of Smo, the Fu kinase induces a conformational change in Cos2 that allows the disassembly of the Smo-Fu-Cos2-Ci complex and consequent activation of Hh target genes. This study provides new insight into the mechanistic regulation of the protein complex that mediates Hh signalling and a unique antibody tool for directly monitoring Hh receptor activity in all activated cells.

The cell biology of Smo signalling and its relationships with GPCRs

The Smoothened (Smo) signalling pathway participates in many developmental processes, contributing to the regulation of gene expression by controlling the activity of transcription factors belonging to the Gli family. The key elements of the pathway were identified by means of genetic screens carried out in Drosophila, and subsequent analysis in other model organisms revealed a high degree of conservation in both the proteins involved and in their molecular interactions. Recent analysis of the pathway, using a combination of biochemical and cell biological approaches, is uncovering the intricacies of Smo signalling, placing its elements in particular cellular compartments and qualifying the molecular processes involved. These include the synthesis, secretion and diffusion of the ligand, the activation of the receptor and the modifications in the activity of nuclear effectors. In this review we discuss recent advances in understanding biochemical and cellular aspects of Smo signalling, with particular focus in the similarities in the mechanism of signal transduction between Smo and other transmembrane proteins belonging to the G-Protein coupled receptors superfamily (GPCR).

A large complex containing Patched and Smoothened initiates Hedgehog signaling in Drosophila

Hedgehog acts as an organizer during development. Its signaling involves the receptor Patched, signal transducer Smoothened and a cytoplasmic complex containing the transcription factor Cubitus interruptus tethered to the Smoothened carboxyl tail. Without Hedgehog, Patched represses Smoothened resulting in proteolysis of Cubitus interruptus to its repressor form. With Hedgehog, Patched repression of Smoothened is relieved and Cubitus interruptus is activated. Sex-lethal, the master switch for sex determination in Drosophila, has been shown to associate with Cubitus interruptus and the cytoplasmic components of the Hedgehog signaling pathway. Additionally, Sex-lethal responds to the presence of Hedgehog in a Patched-dependent manner. The latter prompted us to examine the role of Patched in signaling. We find that Cubitus interruptus, Sex-lethal, Patched and Smoothened co-immunoprecipitate and co-fractionate, suggesting a large complex of both membrane and cytoplasmic components of the Hedgehog pathway. The entire complex is present at the plasma membrane and the association of Patched changes depending on the activation state of the pathway; it also is not female specific. Colocalization analyses suggest that Sex-lethal alters the endocytic cycling of the Hedgehog components and may augment the Hedgehog signal in females by decreasing the proteolytic cleavage of Cubitus interruptus, availing more of it for activation.

The last 59 amino acids of Smoothened cytoplasmic tail directly bind the protein kinase Fused and negatively regulate the Hedgehog pathway

The Hedgehog (HH) signaling pathway is crucial for the development of many organisms and its inappropriate activation is involved in numerous cancers. HH signal controls the traffic and activity of the seven-pass transmembrane protein Smoothened (SMO), leading to the transcriptional regulation of HH-responsive genes. In Drosophila, the intracellular transduction events following SMO activation depend on cytoplasmic multimeric complexes that include the Fused (FU) protein kinase. Here we show that the regulatory domain of FU physically interacts with the last 52 amino acids of SMO and that the two proteins colocalize in vivo to vesicles. The deletion of this region of SMO leads to a constitutive activation of SMO, promoting the ectopic transcription of HH target genes. This activation is partially dependent of FU activity. Thus, we identify a novel link between SMO and the cytoplasmic complex(es) and reveal a negative role of the SMO C-terminal region that interacts with FU. We propose that FU could act as a switch, activator in presence of HH signal or inhibitor in absence of HH.

Modulation of the Suppressor of fused protein regulates the Hedgehog signaling pathway in Drosophila embryo and imaginal discs

The Suppressor of fused (Su(fu)) protein is known to be a negative regulator of Hedgehog (Hh) signal transduction in Drosophila imaginal discs and embryonic development. It is antagonized by the kinase Fused (Fu) since Su(fu) null mutations fully suppress the lack of Fu kinase activity. In this study, we overexpressed the Su(fu) gene in imaginal discs and observed opposing effects depending on the position of the cells, namely a repression of Hh target genes in cells receiving Hh and their ectopic expression in cells not receiving Hh. These effects were all enhanced in a fu mutant context and were suppressed by cubitus interruptus (Ci) overexpression. We also show that the Su(fu) protein is poly-phosphorylated during embryonic development and these phosphorylation events are altered in fu mutants. This study thus reveals an unexpected role for Su(fu) as an activator of Hh target gene expression in absence of Hh signal. Both negative and positive roles of Su(fu) are antagonized by Fused. Based on these results, we propose a model in which Su(fu) protein levels and isoforms are crucial for the modulation of the different Ci states that control Hh target gene expression.

A genome-wide RNA interference screen in Drosophila melanogaster cells for new components of the Hh signaling pathway

Members of the Hedgehog (Hh) family of signaling proteins are powerful regulators of developmental processes in many organisms and have been implicated in many human disease states. Here we report the results of a genome-wide RNA interference screen in Drosophila melanogaster cells for new components of the Hh signaling pathway. The screen identified hundreds of potential new regulators of Hh signaling, including many large protein complexes with pleiotropic effects, such as the coat protein complex I (COPI) complex, the ribosome and the proteasome. We identified the multimeric protein phosphatase 2A (PP2A) and two new kinases, the D. melanogaster orthologs of the vertebrate PITSLRE and cyclin-dependent kinase-9 (CDK9) kinases, as Hh regulators. We also identified a large group of constitutive and alternative splicing factors, two nucleoporins involved in mRNA export and several RNA-regulatory proteins as potent regulators of Hh signal transduction, indicating that splicing regulation and mRNA transport have a previously unrecognized role in Hh signaling. Finally, we showed that several of these genes have conserved roles in mammalian Hh signaling.

Mice deficient in the fused homolog do not exhibit phenotypes indicative of perturbed hedgehog signaling during embryonic development

Hedgehog (Hh) signaling plays a major role in multiple aspects of embryonic development. To understand how a single Hh signal is capable of generating distinct readouts in Hh-responsive cells requires elucidation of the signal transduction cascade at the molecular level. Key components that mediate Hh signal transduction downstream of the receptor include Fused (Fu), Suppressor of fused (Sufu), and Costal-2 (Cos2) or the vertebrate homologs Kif27/Kif7. Studies with both invertebrates and vertebrates have led to a model in which a protein complex composed of Fu, Sufu, and Cos2 controls the processing, activity, and subcellular distribution of the Ci/Gli transcription factors responsible for Hh target gene activation. These converging results obtained with different species reaffirm the prevailing view of pathway conservation during evolution. Genetic studies of Fu, Sufu, and Kif27/Kif7 in mice are required to provide further verification of Hh pathway conservation. To this end, we generated a gene-targeted allele of Fu in mice. Surprisingly, our analysis indicates that Fu-deficient mice do not exhibit any embryonic phenotypes indicative of perturbed Hh signaling. This could be due to either functional redundancy or Hh pathway divergence and clearly indicates greater complexity of Hh signaling in vertebrates.

Differential regulation of Hedgehog target gene transcription by Costal2 and Suppressor of Fused

The mechanism by which the secreted signaling molecule Hedgehog (Hh) elicits concentration-dependent transcriptional responses from cells is not well understood. In the Drosophila wing imaginal disc, Hh signaling differentially regulates the transcription of target genes decapentaplegic (dpp), patched (ptc) and engrailed (en) in a dose-responsive manner. Two key components of the Hh signal transduction machinery are the kinesin-related protein Costal2 (Cos2) and the nuclear protein trafficking regulator Suppressor of Fused [Su(fu)]. Both proteins regulate the activity of the transcription factor Cubitus interruptus (Ci) in response to the Hh signal. We have analyzed the activities of mutant forms of Cos2 in vivo and found effects on differential target gene transcription. A point mutation in the motor domain of Cos2 results in a dominant-negative form of the protein that derepresses dpp but not ptc. Repression of ptc in the presence of the dominant-negative form of Cos2 requires Su(fu), which is phosphorylated in response to Hh in vivo. Overexpression of wild-type or dominant-negative cos2 represses en. Our results indicate that differential Hh target gene regulation can be accomplished by differential sensitivity of Cos2 and Su(Fu) to Hh.

An intramolecular association between two domains of the protein kinase Fused is necessary for Hedgehog signaling

The protein kinase Fused (Fu) is an integral member of the Hedgehog (Hh) signaling pathway. Although genetic studies demonstrate that Fu is required for the regulation of the Hh pathway, the mechanistic role that it plays remains largely unknown. Given our difficulty in developing an in vitro kinase assay for Fu, we reasoned that the catalytic activity of Fu might be highly regulated. Several mechanisms are known to regulate protein kinases, including self-association in either an intra- or an intermolecular fashion. Here, we provide evidence that Hh regulates Fu through intramolecular association between its kinase domain (DeltaFu) and its carboxyl-terminal domain (Fu-tail). We show that DeltaFu and Fu-tail can interact in trans, with or without the kinesin-related protein Costal 2 (Cos2). However, since the majority of Fu is found associated with Cos2 in vivo, we hypothesized that Fu-tail, which binds Cos2 directly, would be able to tether DeltaFu to Cos2. We demonstrate that DeltaFu colocalizes with Cos2 in the presence of Fu-tail and that this colocalization occurs on a subset of membrane vesicles previously characterized to be important for Hh signal transduction. Additionally, expression of Fu-tail in fu mutant flies that normally express only the kinase domain rescues the fu wing phenotype. Therefore, reestablishing the association between these two domains of Fu in trans is sufficient to restore Hh signal transduction in vivo. In such a manner we validate our hypothesis, demonstrating that Fu self-associates and is functional in an Hh-dependent manner. Our results here enhance our understanding of one of the least characterized, yet critical, components of Hh signal transduction.

The G12 family of heterotrimeric G proteins and Rho GTPase mediate Sonic hedgehog signalling

Sonic hedgehog (Shh) is a secreted morphogen crucial for cell fate decision, cellular proliferation, and patterning during vertebrate development. The intracellular Shh signalling is transduced by Smoothened (Smo), a seven-transmembrane spanning protein that belongs to the G-protein coupled receptor family. Among four families of Galpha subunits, Galphai has been thought to be responsible for transducing Shh signalling, while several lines of evidence indicated that other signalling pathways may be involved. We found that the G12 family of heterotrimeric G proteins and the small GTPase RhoA are involved in Shh/Smo-mediated cellular responses, including stimulation of target gene promoter and inhibition of neurite outgrowth of neuroblastoma cells. We also found that the G12/RhoA pathway is responsible for Smo-induced nuclear import of GLI3 which is thought to transduce Shh signals to nucleus. Furthermore, misexpression of a G12-specific GTPase-activating protein in rat neural tubes leads to pertubation of motor neurone and interneurone development, mimicking the effects of decreased Shh signalling. These results show that Shh signalling is mediated in part by activating G12 family coupled signalling pathways. The participation of RhoA, a pivotal molecular switch in many signal transduction pathways, may help explain how Shh can trigger a variety of cellular responses.

Hedgehog: an unusual signal transducer

Hedgehog proteins are of pivotal importance for development and maintenance of tissue patterns in adult organisms. Despite the role of Hedgehogs in differentiation and tumorigenesis, signal transduction of Hedgehog remains a relatively uncharted area of signalling biochemistry. For proper Hedgehog distribution into tissues, two highly unusual covalent modifications are necessary, palmitoylation of a secreted protein and the attachment of a cholesterol group, making Hedgehog the only established sterolated protein in nature. Hedgehog exerts its function via two membrane-bound receptors, Patched and Smoothened; presumably, Patched transports a cholesterol derivate out of cells which inhibits Smoothened. Binding of Hedgehog to Patched impedes this proposed pump function and thus Inhibition of Smoothened, which leads to expression of genetic Hedgehog targets via relief of transcriptional repression. These atypical features make Hedgehog physiology unique in biology and may explain why this field has attracted such significant attention.

Regulation of Hedgehog signaling: a complex story

The Hedgehog (Hh) signal transduction pathway plays critical instructional roles during development. Activating mutations in human Hh signaling components predispose to a variety of tumor types, and have been observed in sporadic tumors occurring in a wide range of organs. Multiple insights into the regulation of Hh signaling have been achieved through studies using Drosophila melanogaster as a model organism. In Drosophila, regulation of the transcription factor Cubitus interruptus (Ci) is the ultimate target of the Hh pathway. Ci is regulated through communication of the membrane proteins Patched (Ptc) and Smoothened (Smo) to the intracellular Hedgehog Signaling Complex (HSC) in response to a graded concentration of Hh ligand. The HSC consists of the Kinesin Related Protein, Costal2 (Cos2), the serine-threonine protein kinase. Fused (Fu) and Ci. In the absence of Hh stimulation, the HSC is involved in processing of Ci to a truncated repressor protein. In response to Hh binding to Ptc, processing of Ci is blocked to allow for accumulation of full-length Ci activator protein(s). Differential concentrations of Hh ligand stimulate production of Ci transcriptional activators of varying strength, which facilitate activation of distinct subsets of target genes. The mechanism(s) by which Ptc and Smo communicate with the HSC in response to differential ligand concentrations to regulate Ci function are not yet fully elucidated. Here, we review what is known about regulation of individual Hh signaling components, concentrating on the mechanisms by which the Hh signal is propagated through Smo to the HSC.

A positive role for Patched in Hedgehog signaling revealed by the intracellular trafficking of Sex-lethal, the Drosophila sex determination master switch

The sex determination master switch, Sex-lethal(Sxl), controls sexual development as a splicing and translational regulator. Hedgehog (Hh) is a secreted protein that specifies cell fate during development. We show that Sxl is in a complex that contains all of the known Hh cytoplasmic components, including Cubitus interruptus (Ci)the only known target of Hh signaling. Hh promotes the entry of Sxl into the nucleus in the wing disc. In the anterior compartment, the Hh receptor Patched(Ptc) is required for this effect, revealing Ptc as a positive effector of Hh. Some of the downstream components of the Hh signaling pathway also alter the rate of Sxl nuclear entry. Mutations in Suppressor of Fused or Fused with altered ability to anchor Ci are also impaired in anchoring Sxl in the cytoplasm. The levels, and consequently, the ability of Sxl to translationally repress downstream targets in the sex determination pathway, can also be adversely affected by mutations in Hh signaling genes. Conversely,overexpression of Sxl in the domain that Hh patterns negatively affects wing patterning. These data suggest that the Hh pathway impacts on the sex determination process and vice versa and that the pathway may serve more functions than the regulation of Ci.

The Kinesin-related protein Costal2 associates with membranes in a Hedgehog-sensitive, Smoothened-independent manner

In Drosophila, Hedgehog (Hh) signal transduction has been shown to require a multiprotein complex (Hedgehog signaling complex (HSC)), which includes the Kinesin-related protein Costal2 (Cos2), the serine/threonine protein kinase Fused (Fu), and the transcription factor Cubitus interruptus (Ci). We present evidence that a biologically relevant fraction of the HSC is found in association with cellular membranes. We demonstrate that Cos2 is capable of tethering an exogenous protein to vesicular membranes and that Cos2 association with membranes is Hh-sensitive. In addition, we demonstrate that Cos2 associates with membranes in cells that lack the transmembrane protein Smoothened (Smo) through a domain of Cos2 distinct from its recently characterized Smo binding domain. We suggest that an Hh-regulated membrane binding activity of Cos2 is part of the mechanism by which Cos2 contributes to Hh signaling. We propose a model in which there are two distinct HSCs with discrete subcellular localizations and activities: one is endosome-associated and facilitates production of a repressor form of Ci (HSC-R), and one is Smo-associated and promotes Ci activation (HSC-A). In response to Hh and through interaction with Cos2, Smo mediates both inhibition of the endosome-associated HSC-R and activation of HSC-A at the plasma membrane.

Smoothened translates Hedgehog levels into distinct responses

In the Drosophila wing, Hedgehog is made by cells of the posterior compartment and acts as a morphogen to pattern cells of the anterior compartment. High Hedgehog levels instruct L3/4 intervein fate, whereas lower levels instruct L3 vein fate. Transcriptional responses to Hedgehog are mediated by the balance between repressor and activator forms of Cubitus interruptus, CiR and CiA. Hedgehog regulates this balance through its receptor, Patched, which acts through Smoothened and thence a regulatory complex that includes Fused, Costal, Suppressor of Fused and Cubitus interruptus. It is not known how the Hedgehog signal is relayed from Smoothened to the regulatory complex nor how responses to different levels of Hedgehog are implemented. We have used chimeric and deleted forms of Smoothened to explore the signaling functions of Smoothened. A Frizzled/Smoothened chimera containing the Smo cytoplasmic tail (FFS) can induce the full spectrum of Hedgehog responses but is regulated by Wingless rather than Hedgehog. Smoothened whose cytoplasmic tail is replaced with that of Frizzled (SSF) mimics fused mutants, interfering with high Hedgehog responses but with no effect on low Hedgehog responses. The cytoplasmic tail of Smoothened with no transmembrane or extracellular domains (SmoC) interferes with high Hedgehog responses and allows endogenous Smoothened to constitutively initiate low responses. SmoC mimics costal mutants. Genetic interactions suggest that SSF interferes with high signaling by titrating out Smoothened, whereas SmoC drives constitutive low signaling by titrating out Costal. These data suggest that low and high signaling (1) are qualitatively different, (2) are mediated by distinct configurations of the regulatory complex and (3) are initiated by distinct activities of Smoothened. We present a model where low signaling is initiated when a Costal inhibitory site on the Smoothened cytoplasmic tail shifts the regulatory complex to its low state. High signaling is initiated when cooperating Smoothened cytoplasmic tails activate Costal and Fused, driving the regulatory complex to its high state. Thus, two activities of Smoothened translate different levels of Hedgehog into distinct intracellular responses.

The Hedgehog signaling pathway--implications for drug targets in cancer and neurodegenerative disorders

The Hedgehog (Hh) pathway is a highly conserved signaling cascade involved in many developmental processes. Among others, these include patterning of the ventral neural tube and establishment of left-right asymmetry of the embryo. Additionally, the pathway regulates the development of numerous tissues and cell types. Mutations in elements of the pathway are associated with congenital diseases and defects, and ectopic Hh signaling activity is implicated in the development of a number of neoplasms. While little is known of Hh signaling function in the adult organism, a role of the pathway in maintenance of adult organs and cell types, including several neuronal subtypes in the central nervous system, is beginning to emerge. Elements of the Hh pathway are therefore potential drug targets for the treatment of cancers and degenerative diseases like Parkinson's disease, and the recent isolation of synthetic molecules capable of modulating the activity of the Hh cascade through a direct interaction with elements of the pathway is promising.