DSulfatase-1 fine-tunes Hedgehog patterning activity through a novel regulatory feedback loop

Sulfs are secreted sulfatases that catalyse removal of sulfate from Heparan Sulfate Proteoglycans (HSPGs) in the extracellular space. These enzymes are well known to regulate a number of crucial signalling pathways during development. In this study, we report that DSulfatase-1 (DSulf1), the unique Drosophila Sulf protein, is a regulator of Hedgehog (Hh) signalling during wing development. DSulf1 activity is required in both Hh source and Hh receiving cells for proper positioning of Hh target gene expression boundaries. As assessed by loss- and gain-of-function experiments in specific compartments, DSulf1 displays dual functions with respect to Hh signalling, acting as a positive regulator in Hh producing cells and a negative regulator in Hh receiving cells. In either domain, DSulf1 modulates Hh distribution by locally lowering the concentration of the morphogen at the apical pole of wing disc cells. Thus, we propose that DSulf1, by its desulfation catalytic activity, lowers Hh/HSPG interaction in both Hh source and target fields, thereby enhancing Hh release from its source of production and reducing Hh signalling activity in responding cells. Finally, we show that Dsulf1 pattern of expression is temporally regulated and depends on EGFR signalling, a Hh-dependent secondary signal in this tissue. Our data reveal a novel Hh regulatory feedback loop, involving DSulf1, which contributes to maintain and stabilise expression domains of Hh target genes during wing disc development.     

Expression of zebrafish hip: Response to Hedgehog signalling,comparison with ptc1 expression,and possible role in otic patterning

In zebrafish, Hedgehog (Hh) signalling is required to specify posterior otic identity. This presents a conundrum, as the nearest source of Hh to the developing inner ear is the ventral midline, in the notochord and floorplate. How can a source of Hh that is ostensibly constant with respect to the anteroposterior axis of the otic vesicle specify posterior otic identity? One possibility is that localised inhibition of Hh signalling is involved. Here we show that genes coding for three inhibitors of Hh signalling, su(fu), dzip1 and hip, are expressed in and around the developing otic vesicle. su(fu) and dzip1 are ubiquitously expressed and unaffected by Hh levels. The expression of hip, however, is positively regulated by Hh signalling and has a complex, dynamic pattern. It is detectable in the neural tube, otic vesicle, statoacoustic ganglion, brain, fin buds, mouth, somites, pronephros and branchial arches. These expression domains bear some similarity, but are not identical, to those of ptc1, a Hh receptor gene that is also positively regulated by Hh signalling. In the neural tube, for instance, hip is expressed in a subset of the ptc1 expression domain, while in other regions, including the otic vesicle, hip and ptc1 expression domains differ. Significantly, we find that initial expression of hip is higher in and adjacent to anterior otic regions, while ptc1 expression becomes progressively restricted to the posterior of the ear. Hip-mediated inhibition of Hh signalling may therefore be important in restricting the effects of Hh to posterior regions of the developing inner ear.     

The Suppressor of fused gene, involved in Hedgehog signal transduction in Drosophila, is conserved in mammals

 The Suppressor of fused [Su(fu)] gene of Drosophila melanogaster encodes a protein containing a PEST sequence [sequence enriched in proline (P), glutamic acid (E), serine (S) and threonine (T)] which acts as an antagonist to the serine-threonine kinase Fused in Hedgehog (Hh) signal transduction during embryogenesis. The Su(fu) gene isolated from a distantly related Drosophila species, D. virilis, shows significantly high homology throughout its protein sequence with its D. melanogaster counterpart. We show that these two Drosophila homologs of Su(fu) are functionally interchangeable in enhancing the fused phenotype. We have also isolated mammalian homologs of Su(fu). The absence of the PEST sequence in the mammalian Su(fu) protein suggests a different regulation for this product between fly and vertebrates. Using the yeast two-hybrid method, we show that the murine Su(fu) protein can interact directly with the Fused and Cubitus interruptus proteins, known partners of Su(fu) in Drosophila. These data are discussed in the light of their evolutionary relationships. Received: 11 September 1998 / Accepted: 9 December 1998     

Functional conservation of the human <Emphasis Type="Italic">EXT1</Emphasis> tumor suppressor gene and its <Emphasis Type="Italic">Drosophila</Emphasis> homolog <Emphasis Type="Italic">tout</Emphasis><Emphasis Type="Italic">velu</Emphasis>

Heparan sulfate proteoglycans play a vital role in signaling of various growth factors in both Drosophila and vertebrates. In Drosophila, mutations in the tout velu (ttv) gene, a homolog of the mammalian EXT1 tumor suppressor gene, leads to abrogation of glycosaminoglycan (GAG) biosynthesis. This impairs distribution and signaling activities of various morphogens such as Hedgehog (Hh), Wingless (Wg), and Decapentaplegic (Dpp). Mutations in members of the exostosin (EXT) gene family lead to hereditary multiple exostosis in humans leading to bone outgrowths and tumors. In this study, we provide genetic and biochemical evidence that the human EXT1 (hEXT1) gene is conserved through species and can functionally complement the ttv mutation in Drosophila. The hEXT1 gene was able to rescue a ttv null mutant to adulthood and restore GAG biosynthesis.     

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.     

Overgrowth of a mouse model of Simpson– Golabi–Behmel syndrome is partly mediated by Indian Hedgehog

Loss‐of‐function mutations of Glypican 3 (Gpc3) cause the Simpson–Golabi–Behmel overgrowth syndrome (SGBS), and developmental overgrowth is observed in Gpc3‐null mice, a mouse model for SGBS. We recently reported that GPC3 inhibits Hedgehog (Hh) signalling by inducing its endocytosis and degradation. Here, we show that the developmental overgrowth observed in Gpc3‐null mice is, at least in part, a consequence of the hyperactivation of the Hh pathway. We bred Gpc3‐null mice with mice that are Hh signalling‐deficient owing to the lack of Indian Hh (Ihh), one of the three mammalian Hhs. We found that the Gpc3‐null mice showed a 29.9% overgrowth in an Ihh wild‐type background, whereas an Ihh‐null background partly rescues the overgrowth caused by the lack of Gpc3 as the double mutants were 19.8% bigger than the Ihh‐null mice. Consistent with the role of GPC3 in Hh endocytosis and degradation, the Gpc3‐null mice show increased levels of Ihh protein and signalling, but similar levels of Ihh messenger RNA.     

Cdkn1c drives muscle differentiation through a positive feedback loop with Myod

Differentiation often requires conversion of analogue signals to a stable binary output through positive feedback. Hedgehog (Hh) signalling promotes myogenesis in the vertebrate somite, in part by raising the activity of muscle regulatory factors (MRFs) of the Myod family above a threshold. Hh is known to enhance MRF expression. Here we show that Hh is also essential at a second step that increases Myod protein activity, permitting it to promote Myogenin expression. Hh acts by inducing expression of cdkn1c (p57^Kip2) in slow muscle precursor cells, but neither Hh nor Cdkn1c is required for their cell cycle exit. Cdkn1c co-operates with Myod to drive differentiation of several early zebrafish muscle fibre types. Myod in turn up-regulates cdkn1c, thereby providing a positive feedback loop that switches myogenic cells to terminal differentiation.     

Developmental roles of heparan sulfate proteoglycans in Drosophila

The formation of complex patterns in multi-cellular organisms is regulated by a number of signaling pathways. In particular, the Wnt and Hedgehog (Hh) pathways have been identified as critical organizers of pattern in many tissues. Although extensive biochemical and genetic studies have elucidated the central components of the signal transduction pathways regulated by these secreted molecules, we still do not understand fully how they organize gradients of gene activities through field of cells. Studies in Drosophila have implicated a role for heparan sulfate proteoglycans (HSPGs) in regulating the signaling activities and distribution of both Wnt and Hh. Here we review these findings and discuss various models by which HSPGs regulate the distributions of Wnt and Hh morphogens. Published in 2003.     

Glial Hedgehog signalling and lipid metabolism regulate neural stem cell proliferation in Drosophila

The final size and function of the adult central nervous system (CNS) are determined by neuronal lineages generated by neural stem cells (NSCs) in the developing brain. In Drosophila, NSCs called neuroblasts (NBs) reside within a specialised microenvironment called the glial niche. Here, we explore non‐autonomous glial regulation of NB proliferation. We show that lipid droplets (LDs) which reside within the glial niche are closely associated with the signalling molecule Hedgehog (Hh). Under physiological conditions, cortex glial Hh is autonomously required to sustain niche chamber formation. Upon FGF‐mediated cortex glial overgrowth, glial Hh non‐autonomously activates Hh signalling in the NBs, which in turn disrupts NB cell cycle progression and its ability to produce neurons. Glial Hh’s ability to signal to NB is further modulated by lipid storage regulator lipid storage droplet‐2 (Lsd‐2) and de novo lipogenesis gene fatty acid synthase 1 (Fasn1). Together, our data suggest that glial‐derived Hh modified by lipid metabolism mechanisms can affect the neighbouring NB’s ability to proliferate and produce neurons.     

Tow (Target of Wingless), a novel repressor of the Hedgehog pathway in Drosophila

Hedgehog (Hh) signalling plays a crucial role in the development and patterning of many tissues in both vertebrates and invertebrates. Aberrations in this pathway lead to severe developmental defects and cancer. Hh signal transduction in receiving cells is a well studied phenomenon; however questions still remain concerning the mechanism of repression of the pathway activator Smoothened (Smo) in the absence of Hh. Here we describe a novel repressor of the Hh pathway, Target of Wingless (Tow). Tow represents the Drosophila homolog of a conserved uncharacterised protein family. We show that Tow acts in Hh receiving cells, where its overexpression represses all levels of Hh signalling, and that this repression occurs upstream or at the level of Smo and downstream of the Hh receptor Patched (Ptc). In addition, we find that like Ptc, overexpression of Tow causes an accumulation of lipophorin in the wing disc. We demonstrate that loss of tow enhances different ptc alleles in a similar manner to another pathway repressor, Suppressor of Fused (SuFu), possibly through mediating Ptc dependant lipophorin internalisation. Combined, these results demonstrate that Tow is an important novel regulator of the Hh pathway in the wing imaginal disc, and may shed light on the mechanism of Ptc repression of Smo.     

Non-cell autonomous control of apoptosis by ligand-independent Hedgehog signaling in Drosophila

Hedgehog (Hh) signaling is important for development and homeostasis in vertebrates and invertebrates. Ligand-independent, deregulated Hh signaling caused by loss of negative regulators such as Patched causes excessive cell proliferation, leading to overgrowth in Drosophila and tumors in humans, including basal-cell carcinoma and medulloblastoma. We show that in Drosophila deregulated Hh signaling also promotes cell survival by increasing the resistance to apoptosis. Surprisingly, cells with deregulated Hh activity do not protect themselves from apoptosis; instead, they promote cell survival of neighboring wild-type cells. This non-cell autonomous effect is mediated by Hh-induced Notch signaling, which elevates the protein levels of Drosophila inhibitor of apoptosis protein-1 (Diap-1), conferring resistance to apoptosis. In summary, we demonstrate that deregulated Hh signaling not only promotes proliferation but also cell survival of neighboring cells. This non-cell autonomous control of apoptosis highlights an underappreciated function of deregulated Hh signaling, which may help to generate a supportive micro-environment for tumor development.     

Membrane potential regulates Hedgehog signalling in the Drosophila wing imaginal disc

While the membrane potential of cells has been shown to be patterned in some tissues, specific roles for membrane potential in regulating signalling pathways that function during development are still being established. In the Drosophila wing imaginal disc, Hedgehog (Hh) from posterior cells activates a signalling pathway in anterior cells near the boundary which is necessary for boundary maintenance. Here, we show that membrane potential is patterned in the wing disc. Anterior cells near the boundary, where Hh signalling is most active, are more depolarized than posterior cells across the boundary. Elevated expression of the ENaC channel Ripped Pocket (Rpk), observed in these anterior cells, requires Hh. Antagonizing Rpk reduces depolarization and Hh signal transduction. Using genetic and optogenetic manipulations, in both the wing disc and the salivary gland, we show that membrane depolarization promotes membrane localization of Smoothened and augments Hh signalling, independently of Patched. Thus, membrane depolarization and Hh‐dependent signalling mutually reinforce each other in cells immediately anterior to the compartment boundary.     

JAK-STAT signalling is required throughout telotrophic oogenesis and short-germ embryogenesis of the beetle Tribolium

In Drosophila, the JAK-STAT signalling pathway regulates a broad array of developmental functions including segmentation and oogenesis. Here we analysed the functions of Tribolium JAK-STAT signalling factors and of Suppressor Of Cytokine Signalling (SOCS) orthologues, which are known to function as negative regulators of JAK-STAT signalling, during telotrophic oogenesis and short-germ embryogenesis. The beetle Tribolium features telotrophic ovaries, which differ fundamentally from the polytrophic ovary of Drosophila. While we found the requirement for JAK-STAT signalling in specifying the interfollicular stalk to be principally conserved, we demonstrate that these genes also have early and presumably telotrophic specific functions. Moreover, we show that the SOCS genes crucially contribute to telotrophic Tribolium oogenesis, as their inactivation by RNAi results in compound follicles. During short-germ embryogenesis, JAK-STAT signalling is required in the maintenance of segment primordia, indicating that this signalling cascade acts in the framework of the segment-polarity network. In addition, we demonstrate that JAK-STAT signalling crucially contributes to early anterior patterning. We posit that this signalling cascade is involved in achieving accurate levels of expression of individual pair-rule and gap gene domains in early embryonic patterning.     

Evolution of the<Emphasis Type="Italic"> Drosophila broad</Emphasis> locus: the<Emphasis Type="Italic"> Manduca sexta broad</Emphasis> Z4 isoform has biological activity in<Emphasis Type="Italic"> Drosophila</Emphasis>

The Drosophila melanogaster broad locus is essential for normal metamorphic development. Broad encodes three genetically distinct functions (rbp, br, and 2Bc) and a family of four zinc-finger DNA-binding proteins (Z1-Z4). The Z1, Z2, and Z3 protein isoforms are primarily associated with the rbp, br, and 2Bc genetic functions respectively. The Z4 protein isoform also provides some rbp genetic function, however an essential function for the Z4 isoform in metamorphosis has not been identified. To determine the degree of conservation of Z4 function between the tobacco hornworm Manduca sexta and Drosophila we generated transgenic Drosophila expressing the Manduca broad Z4 isoform and used this transgene to rescue rbp mutant lethality during Drosophila metamorphosis. We find that the Manduca Z4 protein has significant biological activity in Drosophila with respect to rescue of rbp-associated lethality. There was also some overlap in effects on cuticle gene expression between the Manduca Z4 and Drosophila Z1 isoforms that was not shared with the Drosophila Z4 isoform. Our findings show that Z4 function has been conserved over the 260-million-year period since the divergence of Diptera and Lepidoptera, and are consistent with the hypothesis that the Drosophila Z4 and Manduca Z4 isoforms have essential roles in metamorphosis.Edited by M. Akam     

Rab11 regulates JNK and Raf/MAPK‐ERK signalling pathways during Drosophila wing development

Developmental signalling pathways are regulated by intracellular vesicle trafficking in multicellular organisms. In our earlier communication, we have shown that mutation in Rab11 (a subfamily of the Ypt/Rab gene family) results in the activation of JNK signalling pathways in Drosophila eye. Here, we report that Rab11 regulates JNK and Raf/MAPK‐ERK signalling pathways during Drosophila wing development. Using immunofluorescence and immunohistochemical analyses, we show that overexpression of Rab11 in mutant wing imaginal disc cells triggers the induction of apoptosis and activation of JNK and ERK. Further, using a genetic approach it has been shown that Rab11 interacts with the components of these pathways during Drosophila wing development. In addition to this, in Rab11 mutant wing imaginal discs JNK activity was monitored using puc^E69, a P‐lacZ enhancer‐trap line inserted in puckered (puc). A strong induction of puc in Rab11 mutant wing imaginal disc cells provided a strong support that Rab11 regulates the JNK signalling pathway during Drosophila wing development.     

研究报告: 沉默信息调节因子2对SCA3/MJD转基因果蝇的神经保护作用及其与自噬的相关性研究

为探讨沉默信息调节因子2(Sir2)在SCA3/MJD发病机制中的作用．选用GMR-GAL4 和Nrv2-GAL4驱动子,利用经典的GAL4-UAS系统,将含有78 个CAG 重复扩增的ataxin-3 蛋白片段(MJDtr-Q78)分别在果蝇眼睛和运动神经元内选择性表达,构建GMR-GAL4/UAS 和Nrv2-GAL4/UAS 系统SCA3/MJD 转基因果蝇模型,然后分别在抑制和不抑制自噬的情况下,使Sir2在SCA3/MJD 转基因果蝇眼睛和运动神经元内过表达．结果发现,Sir2过表达明显抑制了SCA3/MJD 转基因果蝇眼睛视网膜光感受神经元变性,显著改善了果蝇运动能力,而在自噬被抑制后,Sir2的作用效果明显减弱,表明Sir2对SCA3/MJD 转基因果蝇具有神经保护作用,而这种神经保护作用需要依赖自噬的功能．     

Marked enantioselective protein binding in humans of ketorolac in vitro: Elucidation of enantiomer unbound fractions following facile synthesis and direct chiral hplc resolution of tritium-labelled ketorolac

The protein binding of the enantiomers of the nonopiate analgesic, ketorolac, was investigated in vitro using human plasma and solutions of human serum albumin (HSA) at physiological pH and temperature. In order to detect the very low levels of unbound enantiomers in protein solutions, tritium-labelled rac-ketorolac was synthesised by regiospecific isotopic exchange of the parent drug with tritiated water as the isotope donor. Radio-chemical purification of this compound by reversed-phase HPLC followed by direct resolution using a chiral α₁-acid glycoprotein (Chiral-AGP) HPLC column afforded labelled enantiomers of high specific activity. The in vitro use of (R)- and (S)-[³H₄]ketorolac enabled reproducible radiometric detection of enantiomers in protein solution ultrafiltrate. The unbound fractions of (R)- and (S)-ketorolac [fu(R) and fu(S), respectively] were determined when drug was added to various plasma or albumin solutions as either the separate enantiomers or as the racemate. Over an enantiomeric plasma concentration range of 2.0—15.0 μg/ml, fu(S) (mean range: 1.572—1.795%) was more than 2-fold greater (P < 0.001) than fu(R) (mean range: 0.565—0.674%). Both fu(R) and fu(S) were constant over this concentration range, and each was unaffected by the presence of the corresponding antipode (P > 0.05). At a concentration of 2.0 μg/ml in 40.0 g/liter fatty acid-free HSA, fu(R) and fu(S) were approximately 0.5 and 1.1%, respectively, and both values declined with increasing concentrations of the long chain fatty acid, oleic acid. We have previously shown that the pharmacokinetics of ketorolac in humans are markedly enantioselective and suggest in this report that these differences are largely the result of substantial differences in the protein binding of ketorolac enantiomers. These findings stress the importance of monitoring the unbound concentrations of the enantiomers of chiral drugs if correct interpretations are to be made of enantioselective pharmacokinetic data. © 1994 Wiley-Liss, Inc.