The hedgehog-related gene qua-1 is required for molting in Caenorhabditis elegans

Activated hedgehog and insulin ligands by decreased transcriptional factor DAF-16 mediate transgenerational nanoplastic toxicity in Caenorhabditis elegans

In Caenorhabditis elegans, transcriptional factor DAF-16 in insulin signaling pathway played important role in regulating transgenerational nanoplastic toxicity. Activation of insulin signals mediated transgenerational toxicity of polystyrene nanoparticle (PS-NP) by inhibiting DAF-16. Among identified germline ligands, expression of wrt-3 encoding hedgehog ligand was increased by RNAi of daf-16 in PS-NP exposed C. elegans. In PS-NP exposed C. elegans, expressions of 4 other germline hedgehog ligand genes and 10 hedgehog receptor genes were increased by daf-16 RNAi. Among these candidate genes, expressions of hedgehog ligand genes (grl-15, grl-16, qua-1, and wrt-1) and hedgehog receptor genes (ptr-23, scp-1, ptd-2, and ncr-1) could be increased by PS-NP (1-100 μg/L), and their transgenerational expressions were observed after PS-NP exposure. RNAi of grl-15, grl-16, qua-1, wrt-1, ptr-23, scp-1, ptd-2, and ncr-1 caused resistance to transgenerational PS-NP toxicity. In nematodes exposed to PS-NPs, RNAi of wrt-3, grl-15, grl-16, qua-1, and wrt-1 at parental generation (P0-G) inhibited expressions of ptr-23, scp-1, ptd-2, and ncr-1 in their offspring. Moreover, we observed increased expressions of insulin peptides genes (ins-3, ins-39, and daf-28) in PS-NP exposed daf-16(RNAi) nematodes, suggesting formation of feedback loop. We raise the molecular basis for formation of toxicity on multiple generations after nanoplastic exposure at P0-G.

Caenorhabditis elegans Hedgehog-related proteins are tissue- and substructure-specific components of the cuticle and precuticle

In Caenorhabditis elegans, expanded families of divergent Hedgehog-related and patched-related proteins promote numerous processes ranging from epithelial and sense organ development to pathogen responses to cuticle shedding during the molt cycle. The molecular functions of these proteins have been mysterious since nematodes lack a canonical Hedgehog signaling pathway. Here we show that Hedgehog-related proteins are components of the cuticle and precuticle apical extracellular matrices that coat, shape, and protect external epithelia. Of four Hedgehog-related proteins imaged, two (GRL-2 and GRL-18) stably associated with the cuticles of specific tubes and two (GRL-7 and WRT-10) labeled precuticle substructures such as furrows or alae. We found that wrt-10 mutations disrupt cuticle alae ridges, consistent with a structural role in matrix organization. We hypothesize that most nematode Hedgehog-related proteins are apical extracellular matrix components, a model that could explain many of the reported functions for this family. These results highlight ancient connections between Hedgehog proteins and the extracellular matrix and suggest that any signaling roles of C. elegans Hedgehog-related proteins will be intimately related to their matrix association.

The Caenorhabditis elegans cuticle and precuticle: a model for studying dynamic apical extracellular matrices in vivo

Apical extracellular matrices (aECMs) coat the exposed surfaces of animal bodies to shape tissues, influence social interactions, and protect against pathogens and other environmental challenges. In the nematode Caenorhabditis elegans, collagenous cuticle and zona pellucida protein-rich precuticle aECMs alternately coat external epithelia across the molt cycle and play many important roles in the worm's development, behavior, and physiology. Both these types of aECMs contain many matrix proteins related to those in vertebrates, as well as some that are nematode-specific. Extensive differences observed among tissues and life stages demonstrate that aECMs are a major feature of epithelial cell identity. In addition to forming discrete layers, some cuticle components assemble into complex substructures such as ridges, furrows, and nanoscale pillars. The epidermis and cuticle are mechanically linked, allowing the epidermis to sense cuticle damage and induce protective innate immune and stress responses. The C. elegans model, with its optical transparency, facilitates the study of aECM cell biology and structure/function relationships and all the myriad ways by which aECM can influence an organism.

C. elegans Hedgehog-related proteins are tissue- and substructure-specific components of the cuticle and pre-cuticle

In C. elegans, divergent Hedgehog-related (Hh-r) and Patched-related (PTR) proteins promote numerous processes ranging from epithelial and sense organ development to pathogen responses to cuticle shedding during the molt cycle. Here we show that Hh-r proteins are actual components of the cuticle and pre-cuticle apical extracellular matrices (aECMs) that coat, shape, and protect external epithelia. Different Hh-r proteins stably associate with the aECMs of specific tissues and with specific substructures such as furrows and alae. Hh-r mutations can disrupt matrix structure. These results provide a unifying model for the function of nematode Hh-r proteins and highlight ancient connections between Hh proteins and the extracellular matrix.

GRD-1/PTR-11, the C. elegans hedgehog/patched-like morphogen-receptor pair, modulates developmental rate

Both hedgehog (Hh) and target of rapamycin complex 2 (TORC2) are central, evolutionarily conserved signaling pathways that regulate development and metabolism. In C. elegans, loss of the essential TORC2 component RICTOR (rict-1) causes delayed development, shortened lifespan, reduced brood, small size and increased fat. Here, we report that knockdown of both the hedgehog-related morphogen grd-1 and its patched-related receptor ptr-11 rescues delayed development in TORC2 loss-of-function mutants, and grd-1 and ptr-11 overexpression delays wild-type development to a similar level to that in TORC2 loss-of-function animals. These findings potentially indicate an unexpected role for grd-1 and ptr-11 in slowing developmental rate downstream of a nutrient-sensing pathway. Furthermore, we implicate the chronic stress transcription factor pqm-1 as a key transcriptional effector in this slowing of whole-organism growth by grd-1 and ptr-11. We propose that TORC2, grd-1 and ptr-11 may act linearly or converge on pqm-1 to delay organismal development.

Hedgehog receptors exert immune-surveillance roles in the epidermis across species

Hedgehog signaling plays pivotal roles in the development and homeostasis of epithelial barrier tissues. However, whether and how Hedgehog signaling directly regulates innate immunity in epithelial cells remains unknown. By utilizing C. elegans epidermis as the model, we found that several Hedgehog receptors are involved in cell-autonomous regulation of the innate immune response in the epidermis. Particularly, loss of the Patched family receptor induces aberrant up-regulation of epidermal antimicrobial peptides in a STAT-dependent manner. External or internal insult to the epidermis triggers rapid rearrangement of Patched distribution along the plasma membrane, indicating that the Hedgehog (Hh) receptor is likely involved in recognition and defense against epidermal damage. Loss of PTCH1 function in primary human keratinocytes and intact mouse skin also results in STAT-dependent immune activation. These findings reveal an evolutionally conserved immune-surveillance function of Hedgehog receptors and an insult-sensing and response strategy of epithelial tissues.

Condensed tannins act as anthelmintics by increasing the rigidity of the nematode cuticle

Tannins and tanniferous plant extracts have been discussed as sustainable means for helminth control in the past two decades in response to a dramatic increase of resistances towards standard anthelmintics. While their bioactivities have been broadly investigated in vitro and in vivo, less is known about their mode of action in nematodes, apart from their protein binding properties. In the current study we therefore investigated the impact of a phytochemically well characterized plant extract from Combretum mucronatum, known to contain procyanidins as the active compounds, on the model organism Caenorhabditis elegans. By different microscopic techniques, the cuticle was identified as the main binding site for tannins, whereas underlying tissues did not seem to be affected. In addition to disruptions of the cuticle structure, molting defects occurred at all larval stages. Finally, an increased rigidity of the nematodes' cuticle due to binding of tannins was confirmed by force spectroscopic measurements. This could be a key finding to explain several anthelmintic activities reported for tannins, especially impairment of molting or exsheathment as well as locomotion.

A life cycle alteration can correct molting defects in Caenorhabditis elegans

Molting is a widespread feature in the development of many invertebrates, including nematodes and arthropods. In Caenorhabditis elegans, the highly conserved protein kinases NEKL-2/NEK8/9 and NEKL-3/NEK6/7 (NEKLs) promote molting through their involvement in the uptake and intracellular trafficking of epidermal cargos. We found that the relative requirements for NEKL-2 and NEKL-3 differed at different life-cycle stages and under different environmental conditions. Most notably, the transition from the second to the third larval stage (L2→L3 molt) required a higher level of NEKL function than during several other life stages or when animals had experienced starvation at the L1 stage. Specifically, larvae that entered the pre-dauer L2d stage could escape molting defects when transiting to the (non-dauer) L3 stage. Consistent with this, mutations that promote entry into L2d suppressed nekl-associated molting defects, whereas mutations that inhibit L2d entry reduced starvation-mediated suppression. We further showed that loss or reduction of NEKL functions led to defects in the transcription of cyclically expressed molting genes, many of which are under the control of systemic steroid hormone regulation. Moreover, the timing and severity of these transcriptional defects correlated closely with the strength of nekl alleles and with their stage of arrest. Interestingly, transit through L2d rescued nekl-associated expression defects in suppressed worms, providing an example of how life-cycle decisions can impact subsequent developmental events. Given that NEKLs are implicated in the uptake of sterols by the epidermis, we propose that loss of NEKLs leads to a physiological reduction in steroid-hormone signaling and consequent defects in the transcription of genes required for molting.

Caenorhabditis elegans PTR/PTCHD PTR-18 promotes the clearance of extracellular hedgehog-related protein via endocytosis

Spatiotemporal restriction of signaling plays a critical role in animal development and tissue homeostasis. All stem and progenitor cells in newly hatched C. elegans larvae are quiescent and capable of suspending their development until sufficient food is supplied. Here, we show that ptr-18, which encodes the evolutionarily conserved patched-related (PTR)/patched domain-containing (PTCHD) protein, temporally restricts the availability of extracellular hedgehog-related protein to establish the capacity of progenitor cells to maintain quiescence. We found that neural progenitor cells exit from quiescence in ptr-18 mutant larvae even when hatched under starved conditions. This unwanted reactivation depended on the activity of a specific set of hedgehog-related grl genes including grl-7. Unexpectedly, neither PTR-18 nor GRL-7 were expressed in newly hatched wild-type larvae. Instead, at the late embryonic stage, both PTR-18 and GRL-7 proteins were first localized around the apical membrane of hypodermal and neural progenitor cells and subsequently targeted for lysosomal degradation before hatching. Loss of ptr-18 caused a significant delay in GRL-7 clearance, causing this protein to be retained in the extracellular space in newly hatched ptr-18 mutant larvae. Furthermore, the putative transporter activity of PTR-18 was shown to be required for the appropriate function of the protein. These findings not only uncover a previously undescribed role of PTR/PTCHD in the clearance of extracellular hedgehog-related proteins via endocytosis-mediated degradation but also illustrate that failure to temporally restrict intercellular signaling during embryogenesis can subsequently compromise post-embryonic progenitor cell function.

Extensive non-redundancy in a recently duplicated developmental gene family

BACKGROUND

It has been proposed that recently duplicated genes are more likely to be redundant with one another compared to ancient paralogues. The evolutionary logic underpinning this idea is simple, as the assumption is that recently derived paralogous genes are more similar in sequence compared to members of ancient gene families. We set out to test this idea by using molecular phylogenetics and exploiting the genetic tractability of the model nematode, Caenorhabditis elegans, in studying the nematode-specific family of Hedgehog-related genes, the Warthogs. Hedgehog is one of a handful of signal transduction pathways that underpins the development of bilaterian animals. While having lost a bona fide Hedgehog gene, most nematodes have evolved an expanded repertoire of Hedgehog-related genes, ten of which reside within the Warthog family.

RESULTS

We have characterised their evolutionary origin and their roles in C. elegans and found that these genes have adopted new functions in aspects of post-embryonic development, including left-right asymmetry and cell fate determination, akin to the functions of their vertebrate counterparts. Analysis of various double and triple mutants of the Warthog family reveals that more recently derived paralogues are not redundant with one another, while a pair of divergent Warthogs do display redundancy with respect to their function in cuticle biosynthesis.

CONCLUSIONS

We have shown that newer members of taxon-restricted gene families are not always functionally redundant despite their recent inception, whereas much older paralogues can be, which is considered paradoxical according to the current framework in gene evolution.

Gene expression oscillations in C. elegans underlie a new developmental clock

During C. elegans larval development, thousands of genes, accounting for >20% of the transcriptome, exhibit oscillatory expression with large amplitudes. The time of peaking varies for different genes, but expression generally peaks once per larval stage, with both the oscillation period and larval stage duration varying in concert with temperature. This and other evidence support the existence of a gene expression oscillator that functions as a developmental clock. In this article, we review what is known about the biology, architecture and possible mechanisms of this clock. We compare it to other oscillators, and highlight tools and approaches suited to its study. Finally, we point out implications of these wide-spread and dynamic changes of gene expression on any type of gene expression profiling experiment in C. elegans larvae and how such experiments need to be controlled.

Periodic subcellular structures undergo long-range synchronized reorganization during C. elegans epidermal development

Periodic pattern formation on the cellular and tissue scale is an important process and has been extensively studied. However, periodic pattern formation at the subcellular level still remains poorly understood. The C. elegans epidermis displays a highly ordered parallel stripe pattern as part of its subcellular structure, making it an ideal model to study the formation and reorganization of periodic patterns within cells. Here, we show that the initial formation of periodic striped patterns in the C. elegans epidermis is dependent on actin and spectrin, and requires the apical membrane attachment structures for maintenance. The periodic subcellular structures do not accommodate cell growth by continuously making new stripes. Instead, they increase the number of stripes by going through one round of uniform duplication, which is independent of the increasing epidermal length or the developmental cycles. This long-range synchronized reorganization of subcellular structures is achieved by physical links established by extracellular collagens together with extension forces generated from epidermal cell growth. Our studies uncover a novel strategy employed by evenly spaced and interlinked subcellular structures to maintain their integrity and equidistribution during cell growth and tissue development.

SLC17A6/7/8 Vesicular Glutamate Transporter Homologs in Nematodes

Members of the superfamily of solute carrier (SLC) transmembrane proteins transport diverse substrates across distinct cellular membranes. Three SLC protein families transport distinct neurotransmitters into synaptic vesicles to enable synaptic transmission in the nervous system. Among them is the SLC17A6/7/8 family of vesicular glutamate transporters, which endows specific neuronal cell types with the ability to use glutamate as a neurotransmitter. The genome of the nematode Caenorhabditis elegans encodes three SLC17A6/7/8 family members, one of which, eat-4/VGLUT, has been shown to be involved in glutamatergic neurotransmission. Here, we describe our analysis of the two remaining, previously uncharacterized SLC17A6/7/8 family members, vglu-2 and vglu-3 These two genes directly neighbor one another and are the result of a recent gene duplication event in C. elegans, but not in other Caenorhabditis species. Compared to EAT-4, the VGLU-2 and VGLU-3 protein sequences display a more distant similarity to canonical, vertebrate VGLUT proteins. We tagged both genomic loci with gfp and detected no expression of vglu-3 at any stage of development in any cell type of both C. elegans sexes. In contrast, vglu-2::gfp is dynamically expressed in a restricted set of distinct cell types. Within the nervous system, vglu-2::gfp is exclusively expressed in a single interneuron class, AIA, where it localizes to vesicular structures in the soma, but not along the axon, suggesting that VGLU-2 may not be involved in synaptic transport of glutamate. Nevertheless, vglu-2 mutants are partly defective in the function of the AIA neuron in olfactory behavior. Outside the nervous system, VGLU-2 is expressed in collagen secreting skin cells where VGLU-2 most prominently localizes to early endosomes, and to a lesser degree to apical clathrin-coated pits, the trans-Golgi network, and late endosomes. On early endosomes, VGLU-2 colocalizes most strongly with the recycling promoting factor SNX-1, a retromer component. Loss of vglu-2 affects the permeability of the collagen-containing cuticle of the worm, and based on the function of a vertebrate VGLUT1 protein in osteoclasts, we speculate that vglu-2 may have a role in collagen trafficking in the skin. We conclude that C. elegans SLC17A6/7/8 family members have diverse functions within and outside the nervous system.

Hedgehog-related genes regulate reactivation of quiescent neural progenitors in Caenorhabditis elegans

The animal body contains various types of stem and progenitor cells. These undifferentiated cells coordinate the balance between quiescence and proliferation with dynamics of various physiological conditions such as the developmental stage, food availability, and injury. Although regulation of such coordination plays a critical role in maintaining tissue homeostasis, controlling the growth rate and regeneration, much of its mechanism remains elusive. Newly hatched Caenorhabditis elegans larvae possess quiescent stem and progenitor cells in several tissues, and these cells are reactivated by the insulin/insulin-like growth factor (IGF) signaling (IIS) pathway only when sufficient food is supplied. Maintenance of the quiescence of neuronal and mesodermal progenitor cells requires microRNA (miRNA), miR-235, which is upregulated under the starvation. On the other hand, feeding ample food downregulates the miRNA via the activity of the IIS pathway. As miR-235 in the hypodermis can non-autonomously regulate quiescence of neuronal and mesodermal progenitor cells, a cell-cell signaling pathway has been hypothesized to act downstream of the miRNA. Here, we provide evidence that two hedgehog-related (hh-r) genes, grl-5 and grl-7, are targets of miR-235 that promote reactivation of quiescent neuroblasts. These grl genes possess an miR-235 binding site on 3'UTRs of their transcripts, and are upregulated in starved mir-235 mutant larvae. grl-5 and grl-7 promoters can continuously drive the expression of GFP-pest reporter protein in the hypodermis under the fed condition. However, expression of these reporters is strikingly downregulated under the starvation condition after hatching. We found that miR-235 can repress expression of reporter genes via the predicted miR-235 binding sites on the grl-5 and grl-7 3'UTRs. Furthermore, activity of grl-5 and grl-7 genes are required for reactivation of neural progenitor cells in starved mir-235 mutant larvae. These findings suggest that the IIS pathway-miR-235 signaling in the hypodermis non-autonomously regulates quiescence of neural progenitor cells, partly via grl-5 and grl-7.

Secretome of the carcinogenic helminth Spirocerca lupi reveals specific parasite proteins associated with its different life stages

Spirocerca lupi is a parasitic and carcinogenic nematode of canids distributed in tropical and subtropical regions around the world. The excretion-secretion proteins (PES) of S. lupi have been suggested to play a role in the pathogenesis of its infection. We aimed to identify the PES of different stages of S. lupi and search for proteins that would be useful for diagnostic, therapeutic and vaccination purposes as well as understand their functions. A nano-UPLC mass spectrometry de novo analysis was performed on proteins collected from cultures of S. lupi L3 larvae, L4 females, adult females and adult males from naturally infected hosts. A total of 211 proteins were identified in all cultures. Accordingly, 117, 130, 99 and 116 proteins were detected in L3 larva, L4 females, adult females and adult males, respectively, with a strong correlation in the biological replicates (Pearson coefficients > 0.73). Fourty-four proteins were detected in all developmental stages, 64 were stage-specific and 49 were exclusively identified in L4 females. Cell compartment enrichment analysis revealed that proteins common to all stages were cytoplasmatic (p < 9.x10^-6), whereas L4 unique proteins were in collagen trimers, and macromolecular complexes (p < 0.00001). Functional enrichment analysis of proteins showed significant enrichment in lipid metabolism in L3-unique proteins (p<0.00005), in mannose metabolism and protein de-glycosylation for L4-unique proteins (p < 0.00004), and in phosphorus metabolism in proteins shared by all stages (p <  2.1 x10^-9). Interestingly, annexin 6, associated with cancer in humans, was detected in all life stages, but in a larger abundance in L4 females and adults. These findings indicate that S. lupi establishes complex interactions with its hosts by an arsenal of proteins expressed in different patterns in each life stage which influence the pathogenesis and oncogenesis of S. lupi and may be used as potential targets for diagnostic assays, drug targets or vaccine candidates.

How Weird is The Worm? Evolution of the Developmental Gene Toolkit in Caenorhabditis elegans

Comparative developmental biology and comparative genomics are the cornerstones of evolutionary developmental biology. Decades of fruitful research using nematodes have produced detailed accounts of the developmental and genomic variation in the nematode phylum. Evolutionary developmental biologists are now utilising these data as a tool with which to interrogate the evolutionary basis for the similarities and differences observed in Nematoda. Nematodes have often seemed atypical compared to the rest of the animal kingdom-from their totally lineage-dependent mode of embryogenesis to their abandonment of key toolkit genes usually deployed by bilaterians for proper development-worms are notorious rule breakers of the bilaterian handbook. However, exploring the nature of these deviations is providing answers to some of the biggest questions about the evolution of animal development. For example, why is the evolvability of each embryonic stage not the same? Why can evolution sometimes tolerate the loss of genes involved in key developmental events? Lastly, why does natural selection act to radically diverge toolkit genes in number and sequence in certain taxa? In answering these questions, insight is not only being provided about the evolution of nematodes, but of all metazoans.

Molting in C. elegans

Molting is an essential developmental process for the majority of animal species on Earth. During the molting process, which is a specialized form of extracellular matrix (ECM) remodeling, the old apical ECM, or cuticle, is replaced with a new one. Many of the genes and pathways identified as important for molting in nematodes are highly conserved in vertebrates and include regulators and components of vesicular trafficking, steroid-hormone signaling, developmental timers, and hedgehog-like signaling. In this review, we discuss what is known about molting, with a focus on studies in Caenorhabditis elegans. We also describe the key structural elements of the cuticle that must be released, newly synthesized, or remodeled for proper molting to occur.

Conserved Ankyrin Repeat Proteins and Their NIMA Kinase Partners Regulate Extracellular Matrix Remodeling and Intracellular Trafficking in Caenorhabditis elegans

Molting is an essential developmental process in nematodes during which the epidermal apical extracellular matrix, the cuticle, is remodeled to accommodate further growth. Using genetic approaches, we identified a requirement for three conserved ankyrin repeat-rich proteins, MLT-2/ANKS6, MLT-3/ANKS3, and MLT-4/INVS, in Caenorhabditis elegans molting. Loss of mlt function resulted in severe defects in the ability of larvae to shed old cuticle and led to developmental arrest. Genetic analyses demonstrated that MLT proteins functionally cooperate with the conserved NIMA kinase family members NEKL-2/NEK8 and NEKL-3/NEK6/NEK7 to promote cuticle shedding. MLT and NEKL proteins were specifically required within the hyp7 epidermal syncytium, and fluorescently tagged mlt and nekl alleles were expressed in puncta within this tissue. Expression studies further showed that NEKL-2-MLT-2-MLT-4 and NEKL-3-MLT-3 colocalize within largely distinct assemblies of apical foci. MLT-2 and MLT-4 were required for the normal accumulation of NEKL-2 at the hyp7-seam cell boundary, and loss of mlt-2 caused abnormal nuclear accumulation of NEKL-2 Correspondingly, MLT-3, which bound directly to NEKL-3, prevented NEKL-3 nuclear localization, supporting the model that MLT proteins may serve as molecular scaffolds for NEKL kinases. Our studies additionally showed that the NEKL-MLT network regulates early steps in clathrin-mediated endocytosis at the apical surface of hyp7, which may in part account for molting defects observed in nekl and mlt mutants. This study has thus identified a conserved NEKL-MLT protein network that regulates remodeling of the apical extracellular matrix and intracellular trafficking, functions that may be conserved across species.

LRP2, an auxiliary receptor that controls sonic hedgehog signaling in development and disease

To fulfill their multiple roles in organ development and adult tissue homeostasis, hedgehog (HH) morphogens act through their receptor Patched (PTCH) on target cells. However, HH actions also require HH binding proteins, auxiliary cell surface receptors that agonize or antagonize morphogen signaling in a context-dependent manner. Here, we discuss recent findings on the LDL receptor-related protein 2 (LRP2), an exemplary HH binding protein that modulates sonic hedgehog activities in stem and progenitor cell niches in embryonic and adult tissues. LRP2 functions are crucial for developmental processes in a number of tissues, including the brain, the eye, and the heart, and defects in this receptor pathway are the cause of devastating congenital diseases in humans. Developmental Dynamics 245:569-579, 2016. © 2016 Wiley Periodicals, Inc.

C. elegans NIMA-related kinases NEKL-2 and NEKL-3 are required for the completion of molting

Caenorhabditis elegans molting is a process during which the apical extracellular matrix of the epidermis, the cuticle, is remodeled through a process of degradation and re-synthesis. Using a genetic approach, we identified nekl-3 as essential for the completion of molting. NEKL-3 is highly similar to the mammalian NEK kinase family members NEK6 and NEK7. Animals homozygous for a hypomorphic mutation in nekl-3, sv3, had a novel molting defect in which the central body region, but not the head or tail, was unable to shed the old cuticle. In contrast, a null mutation in nekl-3, gk506, led to complete enclosure within the old cuticle. nekl-2, which is most similar to mammalian NEK8, was also essential for molting. Mosaic analyses demonstrated that NEKL-2 and NEKL-3 were specifically required within the large epidermal syncytium, hyp7, to facilitate molting. Consistent with this, NEKL-2 and NEKL-3 were expressed at the apical surface of hyp7 where they localized to small spheres or tubular structures. Inhibition of nekl-2, but not nekl-3, led to the mislocalization of LRP-1/megalin, a cell surface receptor for low-density lipoprotein (LDL)-binding proteins. In addition, nekl-2 inhibition led to the mislocalization of several other endosome-associated proteins. Notably, LRP-1 acts within hyp7 to facilitate completion of molting, suggesting at least one mechanism by which NEKL-2 may influence molting. Notably, our studies failed to reveal a requirement for NEKL-2 or NEKL-3 in cell division, a function reported for several mammalian NEKs including NEK6 and NEK7. Our findings provide the first genetic and in vivo evidence for a role of NEK family members in endocytosis, which may be evolutionarily conserved.

Dauer-independent insulin/IGF-1-signalling implicates collagen remodelling in longevity

Interventions that delay ageing mobilize mechanisms that protect and repair cellular components, but it is unknown how these interventions might slow the functional decline of extracellular matrices, which are also damaged during ageing. Reduced insulin/IGF-1 signalling (rIIS) extends lifespan across the evolutionary spectrum, and in juvenile Caenorhabditis elegans also allows the transcription factor DAF-16/FOXO to induce development into dauer, a diapause that withstands harsh conditions. It has been suggested that rIIS delays C. elegans ageing through activation of dauer-related processes during adulthood, but some rIIS conditions confer robust lifespan extension unaccompanied by any dauer-like traits. Here we show that rIIS can promote C. elegans longevity through a program that is genetically distinct from the dauer pathway, and requires the Nrf (NF-E2-related factor) orthologue SKN-1 acting in parallel to DAF-16. SKN-1 is inhibited by IIS and has been broadly implicated in longevity, but is rendered dispensable for rIIS lifespan extension by even mild activity of dauer-related processes. When IIS is decreased under conditions that do not induce dauer traits, SKN-1 most prominently increases expression of collagens and other extracellular matrix genes. Diverse genetic, nutritional, and pharmacological pro-longevity interventions delay an age-related decline in collagen expression. These collagens mediate adulthood extracellular matrix remodelling, and are needed for ageing to be delayed by interventions that do not involve dauer traits. By genetically delineating a dauer-independent rIIS ageing pathway, our results show that IIS controls a broad set of protective mechanisms during C. elegans adulthood, and may facilitate elucidation of processes of general importance for longevity. The importance of collagen production in diverse anti-ageing interventions implies that extracellular matrix remodelling is a generally essential signature of longevity assurance, and that agents promoting extracellular matrix youthfulness may have systemic benefit.

Nuclear receptors in nematode development: Natural experiments made by a phylum

The development of complex multicellular organisms is dependent on regulatory decisions that are necessary for the establishment of specific differentiation and metabolic cellular states. Nuclear receptors (NRs) form a large family of transcription factors that play critical roles in the regulation of development and metabolism of Metazoa. Based on their DNA binding and ligand binding domains, NRs are divided into eight NR subfamilies from which representatives of six subfamilies are present in both deuterostomes and protostomes indicating their early evolutionary origin. In some nematode species, especially in Caenorhabditis, the family of NRs expanded to a large number of genes strikingly exceeding the number of NR genes in vertebrates or insects. Nematode NRs, including the multiplied Caenorhabditis genes, show clear relation to vertebrate and insect homologues belonging to six of the eight main NR subfamilies. This review summarizes advances in research of nematode NRs and their developmental functions. Nematode NRs can reveal evolutionarily conserved mechanisms that regulate specific developmental and metabolic processes as well as new regulatory adaptations. They represent the results of a large number of natural experiments with structural and functional potential of NRs for the evolution of the phylum. The conserved and divergent character of nematode NRs adds a new dimension to our understanding of the general biology of regulation by NRs. This article is part of a Special Issue entitled: Nuclear receptors in animal development.

The Caenorhabditis elegans epidermis as a model skin. II: differentiation and physiological roles

The Caenorhabditis elegans epidermis forms one of the principal barrier epithelia of the animal. Differentiation of the epidermis begins in mid embryogenesis and involves apical-basal polarization of the cytoskeletal and secretory systems as well as cellular junction formation. Secretion of the external cuticle layers is one of the major developmental and physiological specializations of the epidermal epithelium. The four post-embryonic larval stages are separated by periodic moults, in which the epidermis generates a new cuticle with stage-specific characteristics. The differentiated epidermis also plays key roles in endocrine signaling, fat storage, and ionic homeostasis. The epidermis is intimately associated with the development and function of the nervous system, and may have glial-like roles in modulating neuronal function. The epidermis provides passive and active defenses against skin-penetrating pathogens and can repair small wounds. Finally, age-dependent deterioration of the epidermis is a prominent feature of aging and may affect organismal aging and lifespan.

Inactivation of conserved C. elegans genes engages pathogen- and xenobiotic-associated defenses

The nematode C. elegans is attracted to nutritious bacteria and is repelled by pathogens and toxins. Here we show that RNAi and toxin-mediated disruption of core cellular activities, including translation, respiration, and protein turnover, stimulate behavioral avoidance of normally attractive bacteria. RNAi of these and other essential processes induces expression of detoxification and innate immune effectors, even in the absence of toxins or pathogens. Disruption of core processes in non-neuronal tissues was sufficient to stimulate aversion behavior, revealing a neuroendocrine axis of control that additionally required serotonergic and Jnk kinase signaling pathways. We propose that surveillance pathways overseeing core cellular activities allow animals to detect invading pathogens that deploy toxins and virulence factors to undermine vital host functions. Variation in cellular surveillance and endocrine pathways controlling behavior, detoxification, and immunity selected by past toxin or microbial interactions could underlie aberrant responses to foods, medicines, and microbes.

NHR-23 dependent collagen and hedgehog-related genes required for molting

NHR-23, a conserved member of the nuclear receptor family of transcription factors, is required for normal development in Caenorhabditis elegans where it plays a critical role in growth and molting. In a search for NHR-23 dependent genes, we performed whole genome comparative expression microarrays on both control and nhr-23 inhibited synchronized larvae. Genes that decreased in response to nhr-23 RNAi included several collagen genes. Unexpectedly, several hedgehog-related genes were also down-regulated after nhr-23 RNAi. A homozygous nhr-23 deletion allele was used to confirm the RNAi knockdown phenotypes and the changes in gene expression. Our results indicate that NHR-23 is a critical co-regulator of functionally linked genes involved in growth and molting and reveal evolutionary parallels among the ecdysozoa.

C. elegans patched-3 is an essential gene implicated in osmoregulation and requiring an intact permease transporter domain

The nematode Caenorhabditis elegans has retained a rudimentary Hedgehog (Hh) signalling pathway; Hh and Smoothened (Smo) homologs are absent, but two highly related Patched gene homologs, ptc-1 and ptc-3, and 24 ptc-related (ptr) genes are present. We previously showed that ptc-1 is essential for germ line cytokinesis. Here, we report that ptc-3 is also an essential gene; the absence of ptc-3 results in a late embryonic lethality due to an apparent defect in osmoregulation. Rescue of a ptc-3 mutant with a ptc-3::gfp translational reporter reveals that ptc-3 is dynamically expressed in multiple tissues across development. Consistent with this pattern of expression, ptc-3(RNAi) reveals an additional postembryonic requirement for ptc-3 activity. Tissue-specific promoter studies indicate that hypodermal expression of ptc-3 is required for normal development. Missense changes in key residues of the sterol sensing domain (SSD) and the permease transporter domain GxxxD/E motif reveal that the transporter domain is essential for PTC-3 activity, whereas an intact SSD is dispensable. Taken together, our studies indicate that PTC proteins have retained essential roles in C. elegans that are independent of Smoothened (Smo). These observations reveal novel, and perhaps ancestral, roles for PTC proteins.

Selenoprotein TRXR-1 and GSR-1 are essential for removal of old cuticle during molting in Caenorhabditis elegans

Selenoproteins, in particular thioredoxin reductase, have been implicated in countering oxidative damage occurring during aging but the molecular functions of these proteins have not been extensively investigated in different animal models. Here we demonstrate that TRXR-1 thioredoxin reductase, the sole selenoprotein in Caenorhabditis elegans, does not protect against acute oxidative stress but functions instead together with GSR-1 glutathione reductase to promote the removal of old cuticle during molting. We show that the oxidation state of disulfide groups in the cuticle is tightly regulated during the molting cycle, and that when trxr-1 and gsr-1 function is reduced, disulfide groups in the cuticle remain oxidized. A selenocysteine-to-cysteine TRXR-1 mutant fails to rescue molting defects. Furthermore, worms lacking SELB-1, the C. elegans homolog of Escherichia coli SelB or mammalian EFsec, a translation elongation factor known to be specific for selenocysteine in E. coli, fail to incorporate selenocysteine, and display the same phenotype as those lacking trxr-1. Thus, TRXR-1 function in the reduction of old cuticle is strictly selenocysteine dependent in the nematode. Exogenously supplied reduced glutathione reduces disulfide groups in the cuticle and induces apolysis, the separation of old and new cuticle, strongly suggesting that molting involves the regulated reduction of cuticle components driven by TRXR-1 and GSR-1. Using dauer larvae, we demonstrate that aged worms have a decreased capacity to molt, and decreased expression of GSR-1. Together, our results establish a function for the selenoprotein TRXR-1 and GSR-1 in the removal of old cuticle from the surface of epidermal cells.

MLT-10 defines a family of DUF644 and proline-rich repeat proteins involved in the molting cycle of Caenorhabditis elegans

Molting of nematodes involves the synthesis and removal of a collagen-rich exoskeleton. We describe Caenorhabditis elegans MLT-10, which defines a large family of DUF644 and proline-rich repeat proteins. We show that MLT-10 is released from the epidermis during molting and that MLT-10 is involved in renewal of the exoskeleton and development of the epidermis.

The molting cycle of nematodes involves the periodic synthesis and removal of a collagen-rich exoskeleton, but the underlying molecular mechanisms are not well understood. Here, we describe the mlt-10 gene of Caenorhabditis elegans, which emerged from a genetic screen for molting-defective mutants sensitized by low cholesterol. MLT-10 defines a large family of nematode-specific proteins comprised of DUF644 and tandem P-X₂-L-(S/T)-P repeats. Conserved nuclear hormone receptors promote expression of the mlt-10 gene in the hypodermis whenever the exoskeleton is remade. Further, a MLT-10::mCherry fusion protein is released from the hypodermis to the surrounding matrices and fluids during molting. The fusion protein is also detected in strands near the surface of animals. Both loss-of-function and gain-of-function mutations of mlt-10 impede the removal of old cuticles. However, the substitution mutation mlt-10(mg364), which disrupts the proline-rich repeats, causes the most severe phenotype. Mutations of mlt-10 are also associated with abnormalities in the exoskeleton and improper development of the epidermis. Thus, mlt-10 encodes a secreted protein involved in three distinct but interconnected aspects of the molting cycle. We propose that the molting cycle of C. elegans involves the dynamic assembly and disassembly of MLT-10 and possibly the paralogs of MLT-10.

Secretion of Hedgehog-related peptides and WNT during Caenorhabditis elegans development

There is growing awareness that endocytic trafficking plays a critical role in cell-cell communication during animal development. We are beginning to understand how endocytosis can initiate, modulate or terminate signaling. In contrast, our knowledge of the mechanisms involved in secreting signaling peptides remains more limited, particularly when it comes to secretion at the apical surface in epithelial cells. In this study, we review the mechanisms that control secretion in Caenorhabditis elegans, focusing on the role of Patched family members and the V0 complex of the vacuolar-adenosine triphosphatase (V-ATPase) in secreting Hedgehog-related peptides and of MIG-14/Wls and the retromer complex in secreting EGL-20/WNT.

Intraflagellar transport/Hedgehog-related signaling components couple sensory cilium morphology and serotonin biosynthesis in Caenorhabditis elegans

Intraflagellar transport in cilia has been proposed as a crucial mediator of Hedgehog signal transduction during embryonic pattern formation in both vertebrates and invertebrates. Here, we show that the Hh receptor Patched-related factor DAF-6 and intraflagellar transport modulate serotonin production in Caenorhabditis elegans animals, by remodeling the architecture of dendritic cilia of a pair of ADF serotonergic chemosensory neurons. Wild-type animals under aversive environment drastically reduce DAF-6 expression in glia-like cells surrounding the cilia of chemosensory neurons, resulting in cilium structural remodeling and upregulation of the serotonin-biosynthesis enzyme tryptophan hydroxylase tph-1 in the ADF neurons. These cellular and molecular modifications are reversed when the environment improves. Mutants of daf-6 or intraflagellar transport constitutively upregulate tph-1 expression. Epistasis analyses indicate that DAF-6/intraflagellar transport and the OCR-2/OSM-9 TRPV channel act in concert, regulating two layers of activation of tph-1 in the ADF neurons. The TRPV signaling turns on tph-1 expression under favorable and aversive conditions, whereas inactivation of DAF-6 by stress results in further upregulation of tph-1 independently of OCR-2/OSM-9 activity. Behavioral analyses suggest that serotonin facilitates larval animals resuming development when the environment improves. Our study revealed the cilium structure of serotonergic neurons as a trigger of regulated serotonin production, and demonstrated that a Hedgehog-related signaling component is dynamically regulated by environment and underscores neuroplasticity of serotonergic neurons in C. elegans under stress and stress recovery.

Evolution of hedgehog and hedgehog-related genes, their origin from Hog proteins in ancestral eukaryotes and discovery of a novel Hint motif

Background

The Hedgehog (Hh) signaling pathway plays important roles in human and animal development as well as in carcinogenesis. Hh molecules have been found in both protostomes and deuterostomes, but curiously the nematode Caenorhabditis elegans lacks a bona-fide Hh. Instead a series of Hh-related proteins are found, which share the Hint/Hog domain with Hh, but have distinct N-termini.

Results

We performed extensive genome searches such as the cnidarian Nematostella vectensis and several nematodes to gain further insights into Hh evolution. We found six genes in N. vectensis with a relationship to Hh: two Hh genes, one gene with a Hh N-terminal domain fused to a Willebrand factor type A domain (VWA), and three genes containing Hint/Hog domains with distinct novel N-termini. In the nematode Brugia malayi we find the same types of hh-related genes as in C. elegans. In the more distantly related Enoplea nematodes Xiphinema and Trichinella spiralis we find a bona-fide Hh. In addition, T. spiralis also has a quahog gene like C. elegans, and there are several additional hh-related genes, some of which have secreted N-terminal domains of only 15 to 25 residues. Examination of other Hh pathway components revealed that T. spiralis - like C. elegans - lacks some of these components. Extending our search to all eukaryotes, we recovered genes containing a Hog domain similar to Hh from many different groups of protists. In addition, we identified a novel Hint gene family present in many eukaryote groups that encodes a VWA domain fused to a distinct Hint domain we call Vint. Further members of a poorly characterized Hint family were also retrieved from bacteria.

Conclusion

In Cnidaria and nematodes the evolution of hh genes occurred in parallel to the evolution of other genes that contain a Hog domain but have different N-termini. The fact that Hog genes comprising a secreted N-terminus and a Hog domain are found in many protists indicates that this gene family must have arisen in very early eukaryotic evolution, and gave rise eventually to hh and hh-related genes in animals. The results indicate a hitherto unsuspected ability of Hog domain encoding genes to evolve new N-termini. In one instance in Cnidaria, the Hh N-terminal signaling domain is associated with a VWA domain and lacks a Hog domain, suggesting a modular mode of evolution also for the N-terminal domain. The Hog domain proteins, the inteins and VWA-Vint proteins are three families of Hint domain proteins that evolved in parallel in eukaryotes.

Muscle-epidermis interactions affect exoskeleton patterning in Caenorhabditis elegans

The C. elegans hypodermis is a single epithelial cell layer separated from the musculature by a thin basement membrane on its basal surface. The hypodermis secretes the extracellular material of the cuticle from its apical surface. The regulation of cuticle synthesis and apical secretion is not well understood. UNC-95 is a component of the muscle dense bodies and M-lines, which are integrin-based adhesion complexes required for force transduction to the cuticle. Using gene expression profiling and in vivo assays, we show that, in unc-95 mutant worms, there is an increase in expression levels of a group of hypodermal and pharyngeal genes related to cuticle structure and molting. Moreover, the cuticle structure of unc-95 mutant adult is impaired. Our findings suggest that aberrant force transduction from the structurally impaired muscle attachments across the basement membrane to the underlying hypodermis elicits intercellular signaling that plays a role in regulating cuticle synthesis and patterning.

The Hedgehog gene family of the cnidarian, Nematostella vectensis, and implications for understanding metazoan Hedgehog pathway evolution

Hedgehog signaling is an important component of cell-cell communication during bilaterian development, and abnormal Hedgehog signaling contributes to disease and birth defects. Hedgehog genes are composed of a ligand ("hedge") domain and an autocatalytic intein ("hog") domain. Hedgehog (hh) ligands bind to a conserved set of receptors and activate downstream signal transduction pathways terminating with Gli/Ci transcription factors. We have identified five intein-containing genes in the anthozoan cnidarian Nematostella vectensis, two of which (NvHh1 and NvHh2) contain definitive hedgehog ligand domains, suggesting that to date, cnidarians are the earliest branching metazoan phylum to possess definitive Hh orthologs. Expression analysis of NvHh1 and NvHh2, the receptor NvPatched, and a downstream transcription factor NvGli (a Gli3/Ci ortholog) indicate that these genes may have conserved roles in planar and trans-epithelial signaling during gut and germline development, while the three remaining intein-containing genes (NvHint1,2,3) are expressed in a cell-type-specific manner in putative neural precursors. Metazoan intein-containing genes that lack a hh ligand domain have previously only been identified within nematodes. However, we have identified intein-containing genes from both Nematostella and in two newly annotated lophotrochozoan genomes. Phylogenetic analyses suggest that while nematode inteins may be derived from an ancestral true hedgehog gene, the newly identified cnidarian and lophotrochozoan inteins may be orthologous, suggesting that both true hedgehog and hint genes may have been present in the cnidarian-bilaterian ancestor. Genomic surveys of N. vectensis suggest that most of the components of both protostome and deuterostome Hh signaling pathways are present in anthozoans and that some appear to have been lost in ecdysozoan lineages. Cnidarians possess many bilaterian cell-cell signaling pathways (Wnt, TGFbeta, FGF, and Hh) that appear to act in concert to pattern tissues along the oral-aboral axis of the polyp. Cnidarians represent a diverse group of animals with a predominantly epithelial body plan, and perhaps selective pressures to pattern epithelia resulted in the ontogeny of the hedgehog pathway in the common ancestor of the Cnidaria and Bilateria.

Mining predicted essential genes of Brugia malayi for nematode drug targets

We report results from the first genome-wide application of a rational drug target selection methodology to a metazoan pathogen genome, the completed draft sequence of Brugia malayi, a parasitic nematode responsible for human lymphatic filariasis. More than 1.5 billion people worldwide are at risk of contracting lymphatic filariasis and onchocerciasis, a related filarial disease. Drug treatments for filariasis have not changed significantly in over 20 years, and with the risk of resistance rising, there is an urgent need for the development of new anti-filarial drug therapies. The recent publication of the draft genomic sequence for B. malayi enables a genome-wide search for new drug targets. However, there is no functional genomics data in B. malayi to guide the selection of potential drug targets. To circumvent this problem, we have utilized the free-living model nematode Caenorhabditis elegans as a surrogate for B. malayi. Sequence comparisons between the two genomes allow us to map C. elegans orthologs to B. malayi genes. Using these orthology mappings and by incorporating the extensive genomic and functional genomic data, including genome-wide RNAi screens, that already exist for C. elegans, we identify potentially essential genes in B. malayi. Further incorporation of human host genome sequence data and a custom algorithm for prioritization enables us to collect and rank nearly 600 drug target candidates. Previously identified potential drug targets cluster near the top of our prioritized list, lending credibility to our methodology. Over-represented Gene Ontology terms, predicted InterPro domains, and RNAi phenotypes of C. elegans orthologs associated with the potential target pool are identified. By virtue of the selection procedure, the potential B. malayi drug targets highlight components of key processes in nematode biology such as central metabolism, molting and regulation of gene expression.

Unravelling the moulting degradome: new opportunities for chemotherapy?

Replacement of the nematode cuticle with a newly synthesized cuticle (a process known as moulting) occurs four times during larval development. Therefore, the key components of this essential developmental process represent attractive targets for new chemotherapeutic strategies. Recent advances in understanding the molecular genetics of nematode moulting should stimulate and facilitate development of novel drugs that target the essential molecules of the moulting cycle. In particular, we argue that further understanding of the moulting degradome and its key peptidase members offers an important opportunity for the development of novel antinematode agents.

Genes required for osmoregulation and apical secretion in Caenorhabditis elegans

Few studies have investigated whether or not there is an interdependence between osmoregulation and vesicular trafficking. We previously showed that in Caenorhabditis elegans che-14 mutations affect osmoregulation, cuticle secretion, and sensory organ development. We report the identification of seven lethal mutations displaying che-14-like phenotypes, which define four new genes, rdy-1-rdy-4 (rod-like larval lethality and dye-filling defective). rdy-1, rdy-2, and rdy-4 mutations affect excretory canal function and cuticle formation. Moreover, rdy-1 and rdy-2 mutations reduce the amount of matrix material normally secreted by sheath cells in the amphid channel. In contrast, rdy-3 mutants have short cystic excretory canals, suggesting that it acts in a different process. rdy-1 encodes the vacuolar H+-ATPase a-subunit VHA-5, whereas rdy-2 encodes a new tetraspan protein. We suggest that RDY-1/VHA-5 acts upstream of RDY-2 and CHE-14 in some tissues, since it is required for their delivery to the epidermal, but not the amphid sheath, apical plasma membrane. Hence, the RDY-1/VHA-5 trafficking function appears essential in some cells and its proton pump function essential in others. Finally, we show that RDY-1/VHA-5 distribution changes prior to molting in parallel with that of actin microfilaments and propose a model for molting whereby actin provides a spatial cue for secretion.

Hedgehog Morphogen in Cardiovascular Disease

In this review, we focus on the basic biology of the important developmental Hedgehog (Hh) protein family, its general function in development, pathway mechanisms, and gene discovery and nomenclature. Hh function in cardiovascular development and recent findings concerning Hh signaling in ischemia models are discussed in more detail, and future perspectives are proposed. In light of the recent discovery of Hh transport by insect lipophorin, we also hypothesize a role for low-density lipoprotein (LDL) in mammalian Hh transport, creating a surprising role for LDL in cardiovascular disease.

Comprehensive analysis of gene expression patterns of hedgehog-related genes

BACKGROUND

The Caenorhabditis elegans genome encodes ten proteins that share sequence similarity with the Hedgehog signaling molecule through their C-terminal autoprocessing Hint/Hog domain. These proteins contain novel N-terminal domains, and C. elegans encodes dozens of additional proteins containing only these N-terminal domains. These gene families are called warthog, groundhog, ground-like and quahog, collectively called hedgehog (hh)-related genes. Previously, the expression pattern of seventeen genes was examined, which showed that they are primarily expressed in the ectoderm.

RESULTS

With the completion of the C. elegans genome sequence in November 2002, we reexamined and identified 61 hh-related ORFs. Further, we identified 49 hh-related ORFs in C. briggsae. ORF analysis revealed that 30% of the genes still had errors in their predictions and we improved these predictions here. We performed a comprehensive expression analysis using GFP fusions of the putative intergenic regulatory sequence with one or two transgenic lines for most genes. The hh-related genes are expressed in one or a few of the following tissues: hypodermis, seam cells, excretory duct and pore cells, vulval epithelial cells, rectal epithelial cells, pharyngeal muscle or marginal cells, arcade cells, support cells of sensory organs, and neuronal cells. Using time-lapse recordings, we discovered that some hh-related genes are expressed in a cyclical fashion in phase with molting during larval development. We also generated several translational GFP fusions, but they did not show any subcellular localization. In addition, we also studied the expression patterns of two genes with similarity to Drosophila frizzled, T23D8.1 and F27E11.3A, and the ortholog of the Drosophila gene dally-like, gpn-1, which is a heparan sulfate proteoglycan. The two frizzled homologs are expressed in a few neurons in the head, and gpn-1 is expressed in the pharynx. Finally, we compare the efficacy of our GFP expression effort with EST, OST and SAGE data.

CONCLUSION

No bona-fide Hh signaling pathway is present in C. elegans. Given that the hh-related gene products have a predicted signal peptide for secretion, it is possible that they constitute components of the extracellular matrix (ECM). They might be associated with the cuticle or be present in soluble form in the body cavity. They might interact with the Patched or the Patched-related proteins in a manner similar to the interaction of Hedgehog with its receptor Patched.