Signalling by hedgehog family proteins in Drosophila and vertebrate development

The morphogen Hedgehog is essential for proper adult morphogenesis in Bombyx mori

The well-known morphogen Hedgehog (Hh) is indispensable for embryo patterning and organ development from invertebrates to vertebrates. The role of Hh signaling pathway has been extensively investigated in the model organism Drosophila melanogaster, whereas its biological functions are still poorly understood in non-drosophilid insects. In the current study, we describe comprehensive investigation of Hh biological roles in the model lepidopteran insect Bombyx mori by using both CRISPR/Cas9-mediated gene ablation and Gal4/UAS-mediated ectopic expression. Direct injection of Cas9 protein and Hh-specific sgRNAs into preblastoderm embryos induced complete lethality. In contrast, Hh mutants obtained by the binary transgenic CRISPR/Cas9 system showed no deleterious phenotypes during embryonic and larval stages. However, mutants showed abnormalities from the pupal stage and most of adult body appendages exhibited severe developmental defects. Molecular analysis focused on wing development reveal that Hh signaling, Imd signaling and Wnt signaling pathways were distorted in Hh mutant wings. Ectopic expression by using the binary Gal4/UAS system induce early larval lethality. On contrary, moderate overexpression of Hh by using a unitary transgenic system resulted in severe defects in adult leg and antenna development. Our data directly provide genetic evidence that Hh plays vital roles in imaginal discs development and proper adult morphogenesis in B. mori.

Cholesterol and Hedgehog Signaling: Mutual Regulation and Beyond

The Hedgehog (HH) signaling is one of the key agents that govern the precisely regulated developmental processes of multicellular organisms in vertebrates and invertebrates. The HH pathway in the receiving cell includes Patched1, a twelve-pass transmembrane receptor, and Smoothened, a seven-transmembrane G-protein coupled receptor (GPCR), and the downstream GLI family of three transcriptional factors (GLI1-GLI3). Mutations of HH gene and the main components in HH signaling are also associated with numerous types of diseases. Before secretion, the HH protein undergoes post-translational cholesterol modification to gain full activity, and cholesterol is believed to be essential for proper HH signaling transduction. In addition, results from recent studies show the reciprocal effect that HH signaling functions in cholesterol metabolism as well as in cholesterol homeostasis, which provides feedback to HH pathway. Here, we hope to provide new insights into HH signaling function by discussing the role of cholesterol in HH protein maturation, secretion and HH signaling transduction, and the potential role of HH in regulation of cholesterol as well.

A Phase I Study of FOLFIRINOX Plus IPI-926, a Hedgehog Pathway Inhibitor, for Advanced Pancreatic Adenocarcinoma

OBJECTIVES

In mouse models of pancreatic cancer, IPI-926, an oral Hedgehog inhibitor, increases chemotherapy delivery by depleting tumor-associated stroma. This multicenter phase Ib study evaluated IPI-926 in combination with FOLFIRINOX (5-fluorouracil, leucovorin, irinotecan, oxaliplatin) in patients with advanced pancreatic cancer.

METHODS

Patients were treated with once-daily IPI-926 plus FOLFIRINOX. A 3 + 3 dose escalation design was used, with cohort expansion at the maximum tolerated dose. A subset of patients underwent perfusion computed tomography to assess changes in tumor perfusion.

RESULTS

The maximum tolerated dose was identified 1 dose level below standard FOLFIRINOX. Common treatment-related adverse events included liver function test abnormalities, neuropathy, nausea/vomiting, and diarrhea. Objective response rate was high (67%), and patients receiving IPI-926 maintenance showed further declines in CA19-9 levels even after FOLFIRINOX discontinuation. Treatment did not result in consistent increases in tumor perfusion. The study closed early when a separate phase II trial of IPI-926 plus gemcitabine indicated detrimental effects of this combination.

CONCLUSIONS

This is the first study to demonstrate the feasibility of using FOLFIRINOX as the chemotherapeutic backbone in a clinical trial design. Although robust antitumor activity and acceptable safety were observed with the addition of IPI-926 to this regimen, future development of Hedgehog inhibitors in pancreatic cancer seems unlikely.

Glioma-associated Oncogene 2 Is Essential for Trophoblastic Fusion by Forming a Transcriptional Complex with Glial Cell Missing-a

Cell-cell fusion of human villous trophoblasts, referred to as a process of syncytialization, acts as a prerequisite for the proper development and functional maintenance of the human placenta. Given the fact that the main components of the Hedgehog signaling pathway are expressed predominantly in the syncytial layer of human placental villi, in this study, we investigated the potential roles and underlying mechanisms of Hedgehog signaling in trophoblastic fusion. Activation of Hedgehog signaling by a variety of approaches robustly induced cell fusion and the expression of syncytial markers, whereas suppression of Hedgehog signaling significantly attenuated cell fusion and the expression of syncytial markers in both human primary cytotrophoblasts and trophoblast-like BeWo cells. Moreover, among glioma-associated oncogene (GLI) family transcriptional factors in Hedgehog signaling, knockdown of GLI2 but not GLI1 and GLI3 significantly attenuated Hedgehog-induced cell fusion, whereas overexpression of the GLI2 activator alone was sufficient to induce cell fusion. Finally, GLI2 not only stabilized glial cell missing-a, a pivotal transcriptional factor for trophoblastic syncytialization, but also formed a transcriptional heterodimer with glial cell missing-a to transactivate syncytin-1, a trophoblastic fusogen, and promote trophoblastic syncytialization. Taken together, this study uncovered a so far uncharacterized role of Hedgehog/GLI2 signaling in trophoblastic fusion, implicating that Hedgehog signaling, through GLI2, could be required for human placental development and pregnancy maintenance.

THE EFFECTS OF 5-AZA-2'-DEOXYCYTIDINE (D-AZA) ON SONIC HEDGEHOG EXPRESSION IN MOUSE EMBRYONIC LIMB BUDS

5-Aza-2'-deoxycytidine (d-AZA) causes temporally-related defects in the mouse. At 1.0 mg/kg on gestational day (GD) 10, d-AZA causes hindlimb phocomelia. Sonic hedgehog (Shh) plays a significant role in the normal development of limbs in rodent species. Sonic hedgehog peptides, found in the posterior mesenchyme of limb buds, are involved in patterning functions and in the regulation of both anterior-posterior polarity and proximal-distal outgrowth of the limb. The objective of the present study was to analyze alterations in Shh expression subsequent to d-AZA exposure. Pregnant mice were treated with d-AZA via intraperitonlal injection on GD 10. Controls were untreated. The reverse transcription-polymerase chain reaction (RT-PCR), whole mount in situ hybridization (ISH), and whole mount immunohistochemistry (WMI) were used to analyze expression patterns of Shh . For RT-PCR, embryonic hindlimb buds (buds) were taken 0, 4, 8, 12, or 24 hr after exposure. Cyclophilin was used as the baseline monitor. RNA was transcribed to cDNA and used as template with Shh specific primers for amplification. Whole embryos were collected 12 and 24 hr posttreatment for ISH. An antisense primer specific for Shh was used in an oligo-based ISH protocol. Whole embryos were collected 36 and 48 hr posttreatment for WMI. The antibody corresponding to the amino terminal subunit of the Shh peptide was used. There was a treatment related up-regulation of Shh transcripts by 12 and 24 hr posttreatment. The protein response of up-regulation was detectable by 36 and 48 hr posttreatment. Our data suggest that 5-aza-2'-deoxycytidine-induced hindlimb defects may be associated with alterations in the level of Shh expression. This may be part of a cascade of signaling events involved in d-AZA-induced hindlimb defects. Work is ongoing to determine the relationship of other gene species that may cooperate with Shh in the induction of the hindlimb defects.

Desert hedgehog promotes ischemia-induced angiogenesis by ensuring peripheral nerve survival

RATIONALE

Blood vessel growth and patterning have been shown to be regulated by nerve-derived signals. Desert hedgehog (Dhh), one of the Hedgehog family members, is expressed by Schwann cells of peripheral nerves.

OBJECTIVE

The purpose of this study was to investigate the contribution of Dhh to angiogenesis in the setting of ischemia.

METHODS AND RESULTS

We induced hindlimb ischemia in wild-type and Dhh(-/-) mice. First, we found that limb perfusion is significantly impaired in the absence of Dhh. This effect is associated with a significant decrease in capillary and artery density in Dhh(-/-). By using mice in which the Hedgehog signaling pathway effector Smoothened was specifically invalidated in endothelial cells, we demonstrated that Dhh does not promote angiogenesis by a direct activation of endothelial cells. On the contrary, we found that Dhh promotes peripheral nerve survival in the ischemic muscle and, by doing so, maintains the pool of nerve-derived proangiogenic factors. Consistently, we found that denervation of the leg, immediately after the onset of ischemia, severely impairs ischemia-induced angiogenesis and decreases expression of vascular endothelial growth factor A, angiopoietin 1, and neurotrophin 3 in the ischemic muscle.

CONCLUSIONS

This study demonstrates the crucial roles of nerves and factors regulating nerve physiology in the setting of ischemia-induced angiogenesis.

Targeting metastatic upper gastrointestinal adenocarcinomas

Upper gastrointestinal (GI) tumors, including adenocarcinoma of the esophagus, stomach, pancreas, and biliary tree, have traditionally been difficult to treat with cytotoxic chemotherapeutic agents. There has been little drug development success in treating these cancers over the last 20 years, perhaps a reflection of a combination of the aggressive biology of these tumors, the void in effective and specific drug development for these varied tumors, and the lack of properly designed, biologically-based clinical trials. Recently, so called "targeted agents" have risen to the forefront in the care of cancer patients and have made strong impacts in many areas of oncology, particularly gastrointestinal stromal tumors (GIST), colon, breast, and lung cancers. Unfortunately, slow progress has been made using such agents in upper GI tumors. However, more recently, trials in some tumor types have demonstrated gains in progression free survival and overall survival. In this review, we discuss the drugs and pathways that have been most successful in the treatment of upper GI tumors and present the relevant data supporting their use for each tumor site. Additionally, we will explore a few novel pathways that may prove effective in the treatment of upper GI malignancies in the near future.

Mesodermal deletion of transforming growth factor-beta receptor II disrupts lung epithelial morphogenesis: cross-talk between TGF-beta and Sonic hedgehog pathways

In vertebrates, Sonic hedgehog (Shh) and transforming growth factor-beta (TGF-beta) signaling pathways occur in an overlapping manner in many morphogenetic processes. In vitro data indicate that the two pathways may interact. Whether such interactions occur during embryonic development remains unknown. Using embryonic lung morphogenesis as a model, we generated transgenic mice in which exon 2 of the TbetaRII gene, which encodes the type II TGF-beta receptor, was deleted via a mesodermal-specific Cre. Mesodermal-specific deletion of TbetaRII (TbetaRII(Delta/Delta)) resulted in embryonic lethality. The lungs showed abnormalities in both number and shape of cartilage in trachea and bronchi. In the lung parenchyma, where epithelial-mesenchymal interactions are critical for normal development, deletion of mesenchymal TbetaRII caused abnormalities in epithelial morphogenesis. Failure in normal epithelial branching morphogenesis in the TbetaRII(Delta/Delta) lungs caused cystic airway malformations. Interruption of the TbetaRII locus in the lung mesenchyme increased mRNA for Patched and Gli-1, two downstream targets of Shh signaling, without alterations in Shh ligand levels produced in the epithelium. Therefore, we conclude that TbetaRII-mediated signaling in the lung mesenchyme modulates transduction of Shh signaling that originates from the epithelium. To our knowledge, this is the first in vivo evidence for a reciprocal and novel mode of cross-communication between Shh and TGF-beta pathways during embryonic development.

Hedgehog signaling plays a cell-autonomous role in maximizing cardiac developmental potential

Elucidation of the complete roster of signals required for myocardial specification is crucial to the future of cardiac regenerative medicine. Prior studies have implicated the Hedgehog (Hh) signaling pathway in the regulation of multiple aspects of heart development. However, our understanding of the contribution of Hh signaling to the initial specification of myocardial progenitor cells remains incomplete. Here, we show that Hh signaling promotes cardiomyocyte formation in zebrafish. Reduced Hh signaling creates a cardiomyocyte deficit, and increased Hh signaling creates a surplus. Through fate-mapping, we find that Hh signaling is required at early stages to ensure specification of the proper number of myocardial progenitors. Genetic inducible fate mapping in mouse indicates that myocardial progenitors respond directly to Hh signals, and transplantation experiments in zebrafish demonstrate that Hh signaling acts cell autonomously to promote the contribution of cells to the myocardium. Thus, Hh signaling plays an essential early role in defining the optimal number of cardiomyocytes, making it an attractive target for manipulation of multipotent progenitor cells.

G protein-independent cell-based assays for drug discovery on seven-transmembrane receptors

Conventional cell-based assays for seven-transmembrane receptors, also known as G protein-coupled receptors, rely on the coupling of the ligand-bound receptor to heterotrimeric G proteins. New assay methods have become available that are not based on G protein activation, but that apply the molecular mechanism underlying the attenuation of G protein signaling mediated by beta-arrestin. beta-arrestin is a cytoplasmic protein that targets receptors to clathrin-coated endocytotic vesicles for degradation or recycling. This process has been visualized and quantified in high-content imaging assays using receptor- or beta-arrestin-chimeras with green fluorescent protein. Other assay methods use bioluminescence resonance energy transfer, enzyme fragment complementation, or a protease-activated transcriptional reporter gene, to measure receptor-beta-arrestin proximity. beta-arrestin recruitment assays have been applied successfully for receptors coupling to Galpha(q), Galpha(s) and Galpha(i) proteins, thus providing a generic assay platform for drug discovery on G protein-coupled receptors. The best understood signal transduction pathway elicited by the seven-transmembrane Frizzled receptors does not involve G proteins. The activation of Frizzleds by their cognate ligands of the Wnt family recruits the phosphoprotein dishevelled. Dishevelled regulates a protein complex involved in the destruction of beta-catenin. Activation of Frizzled blocks degradation of beta-catenin, which translocates to the nucleus to activate transcription of Wnt-responsive genes. The cytoplasm-to-nuclear translocation of beta-catenin forms the basis of several high-content assays to measure Wnt/Frizzled signal transduction. Interestingly, Frizzled receptors have recently been shown to internalize and to recruit beta-arrestin. This suggests that beta-arrestin recruitment assays may be applied for drug discovery on seven-transmembrane receptors beyond G protein-coupled receptors.

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).

Expression of the human FUSED protein in Drosophila

The Drosophila segment polarity gene fused, which encodes a serine threonine kinase, is required to transmit the Hedgehog (Hh) signal in imaginal discs. To explore the functional homology between the human protein FUSED (hFU) and the Drosophila protein fused (dFu), we have subjected hFU to a precise and well-defined Hh signalling assay of Drosophila wing development. In the wildtype, hFU affects the expression of Hh target genes leading thus to defects in adult wings. In fu mutants, overexpression of hFU cannot rescue the fu phenotype. These results suggest that hFU in Drosophila interferes with endogenous Hh signalling probably by competing with endogenous dFu when binding its partners but cannot perform the normal Fu function.

Role ofSonic hedgehog in patterning of tracheal-bronchial cartilage and the peripheral lung

Sonic hedgehog (Shh) was conditionally deleted in respiratory epithelial cells of the embryonic lung in vivo. Deletion of Shh before embryonic day (E) 13.5 resulted in respiratory failure at birth. While lobulation was not perturbed, the lungs were hypoplastic, with reduced branching of peripheral lung tubules, evident from E13.5. Smooth muscle and endothelial cells were absent or reduced, the latter in relationship to the loss of peripheral lung parenchyma. Tracheal-bronchial ring abnormalities occurred when Shh was deleted between E8.5 and E12.5. Deletion of Shh later in gestation (after E13.5) caused mild abrogation of peripheral branching morphogenesis but did not disrupt tracheal-bronchial development. Defects in branching morphogenesis and vascularization seen in Shh null mutant (Shh(-/-)) mice were substantially corrected when SHH was ectopically expressed in the respiratory epithelium; however, peripheral expression of SHH failed to correct cartilage abnormalities in the trachea and bronchi, indicating a spatial requirement for SHH expression near sites of cartilage formation. Expression of SHH by the respiratory epithelium plays an important role in the patterning of tracheal-bronchial mesenchyme required for formation of cartilage rings in conducting airways. SHH regulates branching morphogenesis and influences differentiation of the peripheral lung mesenchyme required for formation of bronchial and vascular smooth muscle.

Characterization of the physical interaction of Gli proteins with SUFU proteins

The Hedgehog signaling pathway is involved in both development and cancer induction in a wide range of organisms. The end point of the Hedgehog signal-transduction cascade is the Gli/Ci, zinc-finger transcription factors. Proteins such as Fused, Suppressor of fused (SUFU), Costal-2, and protein kinase A are essential for regulation of Gli/Ci processing, activity, and localization. Coimmunoprecipitation and Far Western assays, coupled with truncation analysis and mutagenesis have been used to define the region of interaction between Gli proteins and SUFU. We identify a novel motif SYGH in Gli/Ci family proteins, which is required for the interaction with SUFU. Mutational studies revealed that Gly(122) and His(123) are crucial for binding to SUFU, suggesting the importance of hydrophobicity for the correct binding conformation. Functional analysis revealed that the activity of GLI transcription factors with mutations in this motif is no longer suppressed by co-expression of SUFU. Moreover, we have found that a C-terminal 19-amino acid deletion in SUFU (delta465) is sufficient to abrogate interaction with GLI1. Interestingly, this SUFU mutant localizes in the nucleus, most probably because it is not efficiently sequestered in the cytoplasm. Taken together, we identified a novel motif in the Gli/Ci family of proteins that is essential both for protein-protein interaction with SUFU and for functional repression of GLI1 by SUFU.

Shaggy/GSK3 antagonizes Hedgehog signalling by regulating Cubitus interruptus

The Drosophila protein Shaggy (Sgg, also known as Zeste-white3, Zw3) and its vertebrate orthologue glycogen synthase kinase 3 (GSK3) are inhibitory components of the Wingless (Wg) and Wnt pathways. Here we show that Sgg is also a negative regulator in the Hedgehog (Hh) pathway. In Drosophila, Hh acts both by blocking the proteolytic processing of full-length Cubitus interruptus, Ci (Ci155), to generate a truncated repressor form (Ci75), and by stimulating the activity of accumulated Ci155 (refs 2-6). Loss of sgg gene function results in a cell-autonomous accumulation of high levels of Ci155 and the ectopic expression of Hh-responsive genes including decapentaplegic (dpp) and wg. Simultaneous removal of sgg and Suppressor of fused, Su(fu), results in wing duplications similar to those caused by ectopic Hh signalling. Ci is phosphorylated by GSK3 after a primed phosphorylation by protein kinase A (PKA), and mutating GSK3-phosphorylation sites in Ci blocks its processing and prevents the production of the repressor form. We propose that Sgg/GSK3 acts in conjunction with PKA to cause hyperphosphorylation of Ci, which targets it for proteolytic processing, and that Hh opposes Ci proteolysis by promoting its dephosphorylation.

Regulation of mandibular growth and morphogenesis

The development of the vertebrate face is a dynamic process that starts with the formation of facial processes/prominences. Facial processes are small buds made up of mesenchymal masses enclosed by an epithelial layer that surround the primitive mouth. The 2 maxillary processes, the 2 lateral nasal processes, and the frontonasal processes form the upper jaw. The lower jaw is formed by the 2 mandibular processes. Although the question of the embryonic origin of facial structures has received considerable attention, the mechanisms that control differential growth of the facial processes and patterning of skeletal tissues within these structures have been difficult to study and still are not well-understood. This has been partially due to the lack of readily identifiable morphologically discrete regions in the developing face that regulate patterning of the face. Nonetheless, in recent years there has been significant progress in the understanding of the signaling network controlling the patterning and development of the face (for review, see Richman et al., 1991; Francis-West et al., 1998). This review focuses on current understanding of the processes and signaling molecules that are involved in the formation of the mandibular arch.

Cloning and characterization of two promoters for the human HSAL2 gene and their transcriptional repression by the Wilms tumor suppressor gene product

HSAL2 is a member of a gene family that encodes a group of putative developmental transcription factors. The HSAL gene complex was originally identified on the basis of DNA sequence homology to a region-specific homeotic gene (SAL) in Drosophila. This study reveals a novel, functional 5' exon for HSAL2 and demonstrates that two distinct HSAL2 gene transcripts arise from two overlapping transcription units, resulting in proteins that differ by 25 amino acids. By utilizing functional luciferase reporter assays, two distinct promoters for HSAL2, P1 for the proximal promoter (upstream of exon 1) and P2 for the distal promoter (upstream of exon 1A), were identified. Evaluation of mRNA prevalence and tissue specificity, with particular focus on adult tissues, revealed that production of mRNA from P1 was selective and relatively rare. Production of mRNA from P2 was demonstrably higher and was expressed by a greater number of tissues. In contradistinction, HSAL2 expression directed by P2 was undetectable in some malignant populations as opposed to their normal human counterparts, suggesting a potential role as a tumor suppressor gene. Consensus-binding sites were identified for several transcriptional factors, with multiple sites for WT-1, and Hox-1.3 present within both the P1 and P2 regions. In transient transfection assays, transcription from both HSAL2 P1 and P2 was strikingly repressed by the WT-1 tumor suppressor protein. These findings suggest that an intracellular WT-1/HSAL2 pathway may play a role in development and hematopoiesis.

Immunolocalization of sonic hedgehog (Shh) in developing mouse lung

Expression of sonic hedgehog (Shh) is required for normal development of the lung during embryogenesis. Loss of Shh expression in mice results in tracheoesophageal fistula, lung hypoplasia, and abnormal lung lobulation. To determine whether Shh may play a role later in lung morphogenesis, immunostaining for Shh was performed in mouse lung from embryonic day (E) 10.5 to postnatal day (PD) 24. Shh was detected in the distal epithelium of the developing mouse lung from E10.5 to E16.5. From E16.5 until PD15, Shh was present in epithelial cells in both the peripheral and conducting airways. Although all cells of the developing epithelium uniformly expressed Shh at E10.5, Shh expression was restricted to subsets of epithelial cells by E16.5. Between E16.5 and PD15, non-uniform Shh staining of epithelial cells was observed in the conducting airways in a pattern consistent with the distribution of non-ciliated bronchiolar cells (i.e., Clara cells) and the Clara cell marker CCSP. Shh did not co-localize with hepatocyte nuclear factor/forkhead homologue-4 (HFH-4), beta-tubulin, or with the presence of cilia. These results support the concept that Shh plays a distinct regulatory role in the lung later in morphogenesis, when it may influence formation or cytodifferentiation of the conducting airways.

Sightless has homology to transmembrane acyltransferases and is required to generate active Hedgehog protein

Proteins of the Hedgehog (Hh) family act as important developmental signals in a variety of species [1]. Hh proteins are synthesized as full-length precursors that are autocatalytically cleaved by their C-terminal domains to release the signaling N-terminal domains [2]. The addition of a cholesterol molecule to the C terminus of the signaling domain is concomitant with cleavage [3]. Vertebrate Sonic hedgehog (Shh) proteins have also been shown to acquire a fatty acid chain on the N-terminal cysteine of this domain [4], which is required for a subset of their in vivo functions [5, 6]. A mutation of the corresponding cysteine in Drosophila Hh transforms it into a dominant-negative protein [6]. We have identified a novel gene, sightless (sit), which is required for the activity of Drosophila Hh in the eye and wing imaginal discs and in embryonic segmentation. sit acts in the cells that produce Hh, but does not affect hh transcription, Hh cleavage, or the accumulation of Hh protein. sit encodes a conserved transmembrane protein with homology to a family of membrane-bound acyltransferases. The Sit protein could act by acylating Hh or by promoting other modifications or trafficking events necessary for its function.

Hedgehog induces opposite changes in turnover and subcellular localization of patched and smoothened

Secreted signaling proteins of the Hedgehog family organize spatial pattern during animal development. Two integral membrane proteins have been identified with distinct roles in Hedgehog signaling. Patched functions in Hedgehog binding, and Smoothened functions in transducing the signal. Current models view Patched and Smoothened as a preformed receptor complex that is activated by Hedgehog binding. Here we present evidence that Patched destabilizes Smoothened in the absence of Hedgehog. Hedgehog binding causes removal of Patched from the cell surface. In contrast, Hedgehog causes phosphorylation, stabilization, and accumulation of Smoothened at the cell surface. Comparable effects can be produced by removing Patched from cells by RNA-mediated interference. These findings raise the possibility that Patched acts indirectly to regulate Smoothened activity.

Anteroposterior patterning is required within segments for somite boundary formation in developing zebrafish

Somite formation involves the establishment of a segmental prepattern in the presomitic mesoderm, anteroposterior patterning of each segmental primordium and formation of boundaries between adjacent segments. How these events are co-ordinated remains uncertain. In this study, analysis of expression of zebrafish mesp-a reveals that each segment acquires anteroposterior regionalisation when located in the anterior presomitic mesoderm. Thus anteroposterior patterning is occurring after the establishment of a segmental prepattern in the paraxial mesoderm and prior to somite boundary formation. Zebrafish fss(−), bea(−), des(−) and aei(−) embryos all fail to form somites, yet we demonstrate that a segmental prepattern is established in the presomitic mesoderm of all these mutants and hox gene expression shows that overall anteroposterior patterning of the mesoderm is also normal. However, analysis of various molecular markers reveals that anteroposterior regionalisation within each segment is disturbed in the mutants. In fss(−), there is a loss of anterior segment markers, such that all segments appear posteriorized, whereas in bea(−), des(−) and aei(−), anterior and posterior markers are expressed throughout each segment. Since somite formation is disrupted in these mutants, correct anteroposterior patterning within segments may be a prerequisite for somite boundary formation. In support of this hypothesis, we show that it is possible to rescue boundary formation in fss(−) through the ectopic expression of EphA4, an anterior segment marker, in the paraxial mesoderm. These observations indicate that a key consequence of the anteroposterior regionalisation of segments may be the induction of Eph and ephrin expression at segment interfaces and that Eph/ephrin signalling subsequently contributes to the formation of somite boundaries.

Analysis of PTCH/SMO/SHH pathway genes in medulloblastoma

Inactivation of the PTCH tumor suppressor gene occurs in a subset of sporadic medulloblastomas, suggesting that alterations in the PTCH pathway may be important in the development of this tumor. In order to address the frequency of genetic alterations affecting genes in this pathway, we used a combination of loss of heterozygosity (LOH) analysis, single-stranded conformational polymorphism (SSCP) analysis, and direct sequencing of DNA samples from sporadic primitive neuroectodermal tumors (PNETs). To identify alterations in the PTCH gene, we performed LOH analysis on 37 tumor DNA samples. Of those with matched constitutional DNA samples, one demonstrated LOH. Of those without matched constitutional DNA, six were homozygous with all markers. All exons of the PTCH gene were sequenced in these seven tumors, and three mutations were found. To identify alterations in the SHH and SMO genes, we analyzed all exons of both genes in 24 tumors with SSCP and sequenced any exons that showed aberrant band patterns. No mutations were found in either SHH or SMO in any tumor. We also identified the following genes as candidate tumor suppressors based on their roles in controlling hh/ptc signaling in Drosophila: EN-1 and EN-2, deletion of which results in a lack of cerebellar development in mice; SMAD family members 1-7, and protein kinase A subunits RIalpha, RIbeta, RIIbeta, Calpha, and Cbeta. Each of these genes was investigated in a panel of 24 matched constitutional and tumor DNA samples. Our search revealed no mutations in any of these genes. Thus, PTCH is the only gene in this complex pathway that is mutated with notable frequency in PNET. Genes Chromosomes Cancer 27:44-51, 2000.

Plasticity of axial identity among somites: cranial somites can generate vertebrae without expressing Hox genes appropriate to the trunk

Classic studies have shown that the presomitic mesoderm is already committed to a specific morphological fate, for example, the ability to generate a rib. Hox gene expression in the paraxial mesoderm has also been shown to be fixed early and not susceptible to modulation by an ectopic environment. This is in contrast to the plasticity of Hox expression in neuroectodermal derivatives. We reexamine here the potential of somites for morphological plasticity by transplanting the cranial (occipital) somites 1-4, that normally produce small contributions to the skull, to the trunk of avian embryos. Surprisingly, the transposed cranial somites are able to form reasonably normal vertebral anlage. In addition, the cranial somitic mesoderm produces intervertebral disks, structures not normally found in the skull. These somites are however unable to generate some elements of the vertebrae, such as the costal process. In contrast to the morphogenetic plasticity of the occipital somites, their characteristic inability to support survival of dorsal root ganglia was not significantly modified by posterior transplantation. Dorsal root ganglia initially developed and then degenerated with the same morphological stages as normally observed. In striking contrast to the plasticity of morphology, we found that all four members of the of the fourth paralogous group of Hox genes that are expressed endogenously at the level of the graft are not upregulated in the caudad-transposed cranial mesoderm. It therefore appears that genes other than those of the Hox family normally expressed at this axial level control the position-specific morphogenesis of ectopic vertebrae formed from cranial somites. In evolutionary terms, the present results imply that occipital somites that were incorporated into the "New Head" retain the ability to develop according to their original morphogenetic fate, into vertebrae.

Characterization of the human suppressor of fused, a negative regulator of the zinc-finger transcription factor Gli

Drosophila Suppressor of fused (Su(fu)) encodes a novel 468-amino-acid cytoplasmic protein which, by genetic analysis, functions as a negative regulator of the Hedgehog segment polarity pathway. Here we describe the primary structure, tissue distribution, biochemical and functional analyses of a human Su(fu) (hSu(fu)). Two alternatively spliced isoforms of hSu(fu) were identified, predicting proteins of 433 and 484 amino acids, with a calculated molecular mass of 48 and 54 kDa, respectively. The two proteins differ only by the inclusion or exclusion of a 52-amino-acid extension at the carboxy terminus. Both isoforms were expressed in multiple embryonic and adult tissues, and exhibited a developmental profile consistent with a role in Hedgehog signaling. The hSu(fu) contains a high-scoring PEST-domain, and exhibits an overall 37% sequence identity (63% similarity) with the Drosophila protein and 97% sequence identity with the mouse Su(fu). The hSu(fu) locus mapped to chromosome 10q24-q25, a region which is deleted in glioblastomas, prostate cancer, malignant melanoma and endometrial cancer. HSu(fu) was found to repress activity of the zinc-finger transcription factor Gli, which mediates Hedgehog signaling in vertebrates, and to physically interact with Gli, Gli2 and Gli3 as well as with Slimb, an F-box containing protein which, in the fly, suppresses the Hedgehog response, in part by stimulating the degradation of the fly Gli homologue. Coexpression of Slimb with Su(fu) potentiated the Su(fu)-mediated repression of Gli. Taken together, our data provide biochemical and functional evidence for the hypothesis that Su(fu) is a key negative regulator in the vertebrate Hedgehog signaling pathway. The data further suggest that Su(fu) can act by binding to Gli and inhibiting Gli-mediated transactivation as well as by serving as an adaptor protein, which links Gli to the Slimb-dependent proteasomal degradation pathway.

Hedgehog activates the EGF receptor pathway during Drosophila head development

The Hedgehog (Hh) and Epidermal growth factor receptor (EGFR) signaling pathways play critical roles in pattern formation and cell proliferation in invertebrates and vertebrates. In this study, we demonstrate a direct link between these two pathways in Drosophila melanogaster. Hh and EGFR signaling are each required for the formation of a specific region of the head of the adult fruitfly. We show that hh and vein (vn), which encodes a ligand of the Drosophila EGFR (Schnepp, B., Grumbling, G., Donaldson, T. and Simcox, A. (1996) Genes Dev. 10, 2302–13), are expressed in adjacent domains within the imaginal primordium of this region. Using loss- and gain-of-function approaches, we demonstrate that Hh activates vn expression. We also show that Hh activation of vn is mediated through the gene cubitus interruptus (ci) and that this activation requires the C-terminal region of the Ci protein. Finally, we demonstrate that wingless (wg) represses vn expression, thereby limiting the domain of EGFR signaling.

Segment polarity genes in neuroblast formation and identity specification during Drosophila neurogenesis

The relatively simple central nervous system (CNS) of the Drosophila embryo provides a useful model system for investigating the mechanisms that generate and pattern complex nervous systems. Central to the generation of different types of neurons by precursor neuroblasts is the initial specification of neuroblast identity and the Drosophila segment polarity genes, genes that specify regions within a segment or repeating unit of the Drosophila embryo, have emerged recently as significant players in this process. During neurogenesis the segment polarity genes are expressed in the neuroectodermal cells from which neuroblasts delaminate and they continue to be expressed in neuroblasts and their progeny. Loss-of-function mutations in these genes lead to a failure in the formation of neuroblasts and/or specification of neuroblast identity. Results from several recent studies suggest that regulatory interactions between segment polarity genes during neurogenesis lead to an increase in the number of neuroblasts and specification of different identities to neuroblasts within a population of cells.

SWiP-1: novel SOCS box containing WD-protein regulated by signalling centres and by Shh during development

We describe a novel chick WD-protein, cSWiP-1, expressed in somitic mesoderm and developing limb buds as well as in other embryonic structures where Hedgehog signalling has been shown to play a role. Using embryonic manipulations we show that in somites cSWiP-1 expression integrates two signals originating from structures adjacent to the segmental mesoderm: a positive signal from the notochord and a negative signal from intermediate and/or lateral mesoderm. In explant cultures of somitic mesoderm, Shh protein induces cSWiP-1, while a blocking antibody to Shh inhibits the induction of cSWiP-1 by the notochord. These results show that the positive signal from the notochord is mediated by Shh. We also show that in limb buds cSWiP-1 is upregulated by ectopic Shh. This occurs in about the same time period as upregulation of BMP2, placing cSWiP-1 among the earliest markers for the change of limb pattern caused by ectopic Shh. We also describe a human homologue of cSWiP-1 and a mouse gene, mSWiP-2, that is more distantly related to SWiP-1, suggesting that SWiP-1 belongs to a novel subfamily of WD-proteins.

Mosaic analysis in the drosophila ovary reveals a common hedgehog-inducible precursor stage for stalk and polar cells

The fates of two small subgroups of the ovarian follicle cells appear to be linked: mutations in Notch, Delta, fs(1)Yb, or hedgehog cause simultaneous defects in the specification of stalk cells and polar cells. Both of these subgroups are determined in the germarium, and both cease division early in oogenesis. To test the possibility that these subgroups are related by lineage, we generated dominantly marked mitotic clones in ovaries. Small, restricted clones in stalk cells and polar cells were found adjacent to each other at a frequency much too high to be explained by independent induction. We therefore propose a model in which stalk cells and polar cells are derived from a precursor population that is distinct from the precursors for other follicle cells. We support and extend this model by characterization of mutants that affect stalk and polar cell formation. We find that ectopic expression of Hedgehog can induce both polar and stalk cell fate, presumably by acting on the precursor stage. In contrast, we find that stall affects neither the induction of the precursors nor the decision between the stalk cell and polar cell fate but, rather, some later differentiation step of stalk cells. In addition, we show that ectopic polar and stalk cells disturb the anterior-posterior polarity of the underlying oocyte.

In vivo evidence that Patched and Smoothened constitute distinct binding and transducing components of a Hedgehog receptor complex

During Drosophila development, cells belonging to the posterior compartment of each segment organize growth and patterning by secreting Hedgehog (Hh), a protein which induces a thin strip of adjacent cells in the anterior compartment to express the morphogens Decapentaplegic (Dpp) and Wingless (Wg). Hedgehog is bound and transduced by a receptor complex that includes Smoothened (Smo), a member of the Frizzled (Fz) family of seven-pass transmembrane receptors, as well as the multiple-pass transmembrane protein Patched (Ptc). Ptc is required for the binding of Hh to the complex as well as for the Hh-dependent activation of Smo within the complex. Here, we identify a likely null allele of the smo gene and use it to determine whether Hh is bound by Ptc alone, or by Smo in concert with Ptc. We find that cells devoid of Smo can sequester Hh, but that their ability to do so depends, as in wild-type cells, on the expression of high levels of Ptc protein. These results suggest that Ptc normally binds Hh without any help from Smo and hence favor a mechanism of signal transduction in which Hh binds specifically to Ptc and induces a conformational change leading to the release of latent Smo activity.

Specification of motor neuron identity by the MNR2 homeodomain protein

Sonic hedgehog signaling controls the differentiation of motor neurons in the ventral neural tube, but the intervening steps are poorly understood. A differential screen of a cDNA library derived from a single Shh-induced motor neuron has identified a novel homeobox gene, MNR2, expressed by motor neuron progenitors and transiently by postmitotic motor neurons. The ectopic expression of MNR2 in neural cells initiates a program of somatic motor neuron differentiation characterized by the expression of homeodomain proteins, by neurotransmitter phenotype, and by axonal trajectory. Our results suggest that the Shh-mediated induction of a single transcription factor, MNR2, is sufficient to direct somatic motor neuron differentiation.

Signals transmitted along retinal axons in Drosophila: Hedgehog signal reception and the cell circuitry of lamina cartridge assembly

The arrival of retinal axons in the brain of Drosophila triggers the assembly of glial and neuronal precursors into a ‘neurocrystalline’ array of lamina synaptic ‘cartridges’. Hedgehog, a secreted protein, is an inductive signal delivered by retinal axons for the initial steps of lamina differentiation. In the development of many tissues, Hedgehog acts in a signal relay cascade via the induction of secondary secreted factors. Here we show that lamina neuronal precursors respond directly to Hedgehog signal reception by entering S-phase, a step that is controlled by the Hedgehog-dependent transcriptional regulator Cubitus interruptus. The terminal differentiation of neuronal precursors and the migration and differentiation of glia appear to be controlled by other retinal axon-mediated signals. Thus retinal axons impose a program of developmental events on their postsynaptic field utilizing distinct signals for different precursor populations.

Relevance of extracellular matrix, its receptors, and cell adhesion molecules in mammalian nephrogenesis

Mammalian nephrogenesis begins by the reciprocal interaction of the ureteric bud with the undifferentiated mesenchyme. The mesenchyme differentiates into an epithelial phenotype with the development of the glomerulus and proximal and distal tubules. At the same time, the mesenchyme stimulates the branching morphogenesis of the ureteric bud that differentiates into the collecting ducts. These inductive interactions and differentiation events are modulated by a number of macromolecules, including the extracellular matrix (ECM), integrin receptors, and cell adhesion molecules. Many of these macromolecules exhibit spatiotemporal developmental regulation in the metanephros. Some are expressed in the mesenchyme, whereas others appear in the ureteric bud epithelia. The molecules expressed in the mesenchyme or at the epithelial:mesenchymal interface may serve as ligands while those in the epithelia serve as the receptors. In such a scenario the ligand and the receptor would be ideally suited for epithelial:mesenchymal paracrine/juxtacrine interactions that are also influenced by RGD sequences and Ca2+ binding domains of the ECM proteins and their receptors. This review addresses the role of such interactions in metanephric development.

Underlying assumptions of developmental models

These 10 obvious propositions make a model of the specification of form, intended to expose underlying assumptions of developmental biology for examination and future experimentation. (I) The control of development is by means of local interactions, rather than global control mechanisms. (II) A macromolecule near a specific site will bind by mass action. (III) Starting with a precursor cell, all cells are assembled automatically by specifically binding macromolecules. (IV) At the surface of cells are specific adhesion sites that determine how all cells bind to each other. (V) An organism will assemble automatically from parts (macromolecules, structures, and cells) specified by nuclear control factors. (VI) The nuclear control factors in each cell are from precursor cells and factors derived by signaling from other cells. (VII) The macromolecules that determine specific binding, cell adhesion, and signaling are controlled by nuclear control factors, and in a grand feedback the cell adhesion and signaling systems determine the nuclear factor patterns. (VIII) The embryonic precursor cells for organs, termed "precursor groups," are linked by adhesion and signaling relationships. (IX) The precursor groups include precursors for regions of an organ and boundary cells between regions having few cell types, growing without additional specific cell-to-cell relationships. (X) Organs are held together by cell adhesion in functional relationships. Thus the form and function of the organism is specified entirely by local control mechanisms. Without global control systems, information for form is in the genes for structural proteins, adhesion molecules, control factors, signaling molecules, and their control regions.

Identification and analysis of a gene that is essential for morphogenesis and prespore cell differentiation in Dictyostelium

We have identified a gene (PslA) that is expressed throughout Dictyostelium development and encodes a novel protein that is required for proper aggregation and subsequent cell-type differentiation and morphogenesis. pslA null (pslA-) cells produce large aggregation streams under conditions in which wild-type cells form discrete aggregates. Tips form along the stream, elongate to produce a finger, and eventually form a terminal structure that lacks a true sorus (spore head). More than half of the cells remain as a mass at the base of the developing fingers. The primary defect in the pslA- strain is the inability to induce prespore cell differentiation. Analyses of gene expression show a complete lack of prespore-specific gene expression and no mature spores are produced. In chimeras with wild-type cells, pslA- cells form the prestalk domain and normal, properly proportioned fruiting bodies can be produced. This indicates that pslA- cells are able to interact with wild-type cells and regulate patterning, even though pslA- cells are unable to express prespore cell-type-specific genes, do not participate in prespore cell differentiation and do not produce pslA- spores in the chimeras. While pslA- cells produce mature, vacuolated stalk cells during multicellular development, pslA- cells are unable to do so in vitro in response to exogenous DIF (a morphogen required for prestalk and stalk cell differentiation). These results indicate that pslA- cells exhibit a defect in the prestalk/stalk cell pathways under these experimental conditions. Our results suggest that PslA's primary function is to regulate prespore cell determination very early in the prespore pathway via a cell-autonomous mechanism, possibly at the time of the initial prestalk/prespore cell-fate decision. Indirect immunofluorescence of myc-tagged PslA localizes the protein to the nucleus, suggesting that PslA may function to control the prespore pathway at the level of transcription.

Experimental analysis of character coupling across a complex life cycle: pigment pattern metamorphosis in the tiger salamander, Ambystoma tigrinum tigrinum

Developmental relationships among characters are expected to bias patterns of morphological variation at the population level. Studies of character development thus can provide insights into processes of adaptation and the evolutionary diversification of morphologies. Here I use experimental manipulations to test whether larval and adult pigment patterns are coupled across metamorphosis in the tiger salamander, Ambystoma tigrinum tigrinum (Ambystomatidae). Previous investigations showed that the early larval pigment pattern depends on interactions between pigment cells and the lateral line sensory system. In contrast, the results of this study demonstrate that the major features of the adult pigment pattern develop largely independently of both the early larval pattern and the lateral lines. These results suggest that ontogenetic changes that occur across metamorphosis decouple larval and adult pigment patterns and could thereby facilitate independent evolutionary modifications to the patterns during different stages of the life cycle.

Suppressor of fused links fused and Cubitus interruptus on the hedgehog signalling pathway

The Hedgehog (Hh) family of signalling proteins [1] mediate inductive interactions either directly or by controlling the transcription of other secreted proteins through the action of Gli transcription factors, such as Cubitus interruptus (Ci) [2]. In Drosophila, the transcription of Hh targets requires the activation of the protein kinase Fused (Fu) and the inactivation of both Suppressor of fused (Su(fu)) and Costal-2 (Cos-2) [3]. Fu is required for Hh signalling in the embryo and in the wing imaginal disc and acts also as an antitumorigen in ovaries [4]. All fu- phenotypes are suppressed by the loss of function of Su(fu) [5]. Fu, Cos-2 and Ci are co-associated in vivo in large complexes that are bound to microtubules in a Hh-dependent manner [6,7]. Here we investigate the role of Su(fu) in the intracellular part of the Hh signalling pathway. Using the yeast two-hybrid method and an in vitro binding assay, we show that Su(fu), Ci and Fu can interact directly to form a trimolecular complex, with Su(fu) binding to both its partners simultaneously. Su(fu) and Ci also co-immunoprecipitate from embryo extracts. We propose that, in the absence of Hh signalling, Su(fu) inhibits Ci by binding to it and that, upon reception of the Hh signal, Fu is activated and counteracts Su(fu), leading to the activation of Ci.

The role of the hedgehog/patched signaling pathway in epithelial stem cell proliferation: from fly to human

The hedgehog-patched (hh-ptc) intercellular signaling pathway has recently been shown to control the proliferation of epithelial stem cells in both Drosophila and vertebrates. Mutant and ectopic expression analyses in Drosophila suggest that the HH protein diffuses from the signaling cells to promote the proliferation of nearby ovarian somatic stem cells by antagonizing the suppression of its receptor PTC towards the CI transcription factor in the stem cells. Consequently, the transcription of CI-dependent genes leads to stem cell proliferation. This regulatory pathway appears to function also in vertebrates, where defects in ptc cause basal cell carcinoma, tumors of epidermal stem cell origin. Basal cell carcinoma can also be induced by ectopic expression of Sonic hedgehog (shh) or Gli1, the vertebrate homolog of ci. These studies suggest the conservation of the hh signaling pathway in controlling epithelial stem cell divisions among different organisms.

The patched gene in development and cancer

The cloning of vertebrate homologues of the Drosophila segment polarity gene patched has led to confirmation of a role for the multipass transmembrane protein which it encodes as a receptor for secreted signalling proteins of the Hedgehog family. In addition, human patched has been identified as a tumour suppressor gene implicated in basal cell carcinomas and medullablastomas.

Regulation of the Hedgehog and Wingless signalling pathways by the F-box/WD40-repeat protein Slimb

Members of the Hedgehog (Hh) and Wnt/Wingless (Wg) families of secreted proteins control many aspects of growth and patterning during animal development. Hh signal transduction leads to increased stability of a transcription factor, Cubitus interruptus (Ci), whereas Wg signal transduction causes increased stability of Armadillo (Arm/beta-catenin), a possible co-factor for the transcriptional regulator Lef1/TCF. Here we describe a new gene, slimb (for supernumerary limbs), which negatively regulates both of these signal transduction pathways. Loss of function of slimb results in a cell-autonomous accumulation of high levels of both Ci and Arm, and the ectopic expression of both Hh- and Wg- responsive genes. The slimb gene encodes a conserved F-box/WD40-repeat protein related to Cdc4p, a protein in budding yeast that targets cell-cycle regulators for degradation by the ubiquitin/proteasome pathway. We propose that Slimb protein normally targets Ci and Arm for processing or degradation by the ubiquitin/proteasome pathway, and that Hh and Wg regulate gene expression at least in part by inducing changes in Ci and Arm, which protect them from Slimb-mediated proteolysis.

Dual pathways for induction of wingless expression by protein kinase A and Hedgehog in Drosophila embryos

The secreted Drosophila Hedgehog (Hh) protein induces transcription of specific genes by an unknown mechanism that requires the serpentine transmembrane protein Smoothened (Smo) and the transcription factor Cubitus interruptus (Ci). Protein kinase A (PKA) has been implicated in the mechanism of Hh signal transduction because it acts to repress Hh target genes in imaginal disc cells that express Ci. Changes in Ci protein levels, detected by an antibody that recognizes an epitope in the carboxy-terminal half of Ci, have been suggested to mediate the positive effects of Hh and the negative effects of PKA on Hh target gene expression in imaginal discs. Here we show that PKA inhibition, like Hh, leads to increased "carboxy-terminal" Ci staining and Hh target gene expression in embryos. In addition, we find that Hh and Smo can stimulate target gene expression at constant Ci levels and that increased PKA activity can induce ectopic Hh target gene expression in a manner that requires Smo and Ci activities but does not involve changes in Ci protein concentration. This suggests a branching pathway of Hh signal transduction downstream of Smo and that PKA exerts opposite effects on the two branches. Finally we show that Hh signaling in embryos does not depend on cAMP-dependent regulation of PKA activity.

Hedgehog elicits signal transduction by means of a large complex containing the kinesin-related protein costal2

The hedgehog gene of Drosophila melanogaster encodes a secreted protein (HH) that plays a vital role in cell fate and patterning. Here we describe a protein complex that mediates signal transduction from HH. The complex includes the products of at least three genes: fused (a protein-serine/threonine kinase), cubitus interruptus (a transcription factor), and costal2 (a kinesin-like protein). The complex binds with great affinity to microtubules in the absence of HH, but binding is reversed by HH. Mutations in the extracatalytic domain of FU abolish both the biological function of the protein and its association with COS2. We conclude that the complex may facilitate signaling from HH by governing access of the cubitus interruptus protein to the nucleus.

The Drosophila G-protein-coupled receptor kinase homologue Gprk2 is required for egg morphogenesis

G protein signaling is a widely utilized form of extracellular communication that is mediated by a family of serpentine receptors containing seven transmembrane domains. In sensory neurons, cardiac muscle and other tissues, G protein-coupled receptors are desensitized through phosphorylation by a family of kinases, the G protein-coupled receptor kinases (GRKs). Desensitization allows a cell to decrease its response to a given signal, in the continued presence of that signal. We have identified a Drosophila mutant, gprk2(6936) that disrupts expression of a putative member of the GRK family, the G protein-coupled receptor kinase 2 gene (Gprk2). This mutation affects Gprk2 gene expression in the ovaries and renders mutant females sterile. The mutant eggs contain defects in several anterior eggshell structures that are produced by specific subsets of migratory follicle cells. In addition, rare eggs that become fertilized display gross defects in embryogenesis. These observations suggest that developmental signals transduced by G protein-coupled receptors are regulated by receptor phosphorylation. Based on the known functions of G protein-coupled receptor kinases, we speculate that receptor desensitization assists cells that are migrating or undergoing shape changes to respond rapidly to changing external signals.