Membrane dynamics simulation and virtual screening reveals potential dual natural inhibitors of endothelin receptors for targeting glaucomatous condition.

Glaucoma is the second leading cause of blindness in the world and is characterized by the loss of retinal ganglion cells (RGC) over a period of time, leading to complete blindness. Recently, endothelin has been identified as an important factor that influences intraocular pressure IOP, OBF, and direct RGC damage. Targeting the endothelin receptor signaling pathway in glaucoma is considered to be highly beneficial, as it can effectively modulate IOP, OBF, and RGC damage, the key factors which are essential to modulate the disease progression holistically. Currently, synthetic drugs like Bosentan, BQ-123, and prostaglandin analogues are available as endothelin receptor antagonists, which are extensively used in the treatment of cardiovascular and other conditions like systemic hypertension. However, the usage of these drugs in glaucoma is limited due to toxicity and poor bioavailability in the ocular milieu. Thus, there is a need for potential natural compounds as endothelin receptor antagonists that acts as dual inhibitors by targeting both ETA and ETB and are highly efficient with the least toxicity. Hence, this study is intended to prioritize endothelin receptor antagonists by structural bioinformatics approaches involving molecular modeling, molecular dynamics, and molecular docking studies. Subsequently, High throughput virtual screening (HTVS) vs. Natural compound databases targeting the optimal binding sites of both ETA and ETB. Following this, the common hits were subjected to binding free energy calculations (MMGBSA) and ADMETox analysis. Finally, the most potential hits were analyzed for MD based binding stability analysis and binding free energy. Similarly, the known synthetic inhibitors were also docked to the receptors and the results were analyzed. From this study, it was inferred that among the natural compounds dataset (8929 compounds), only 4 common compounds were identified as hits. Among these, only one compound ST075640 surpassed all the prioritization criteria including MMGBSA, ADMETox prediction, dual inhibitory potential (ETA & ETB), and also in structural comparative analysis with bosentan it showed similar efficiency. Thus, the validated hit shall prove to be effective in modulating endothelin mediated IOP, OBF, and RGC damage in glaucomatous condition.

Live cell PNA labelling enables erasable fluorescence imaging of membrane proteins.

DNA nanotechnology is an emerging field that promises fascinating opportunities for the manipulation and imaging of proteins on a cell surface. The key to progress is the ability to create a nucleic acid-protein junction in the context of living cells. Here we report a covalent labelling reaction that installs a biostable peptide nucleic acid (PNA) tag. The reaction proceeds within minutes and is specific for proteins carrying a 2 kDa coiled-coil peptide tag. Once installed, the PNA label serves as a generic landing platform that enables the recruitment of fluorescent dyes via nucleic acid hybridization. We demonstrate the versatility of this approach by recruiting different fluorophores, assembling multiple fluorophores for increased brightness and achieving reversible labelling by way of toehold-mediated strand displacement. Additionally, we show that labelling can be carried out using two different coiled-coil systems, with epidermal growth factor receptor and endothelin receptor type B, on both HEK293 and CHO cells. Finally, we apply the method to monitor internalization of epidermal growth factor receptor on CHO cells.

Endothelin receptor heteromerization inhibits ß-arrestin function in HEK293 cells.

The endothelin receptor A (ETA) and endothelin receptor B (ETB) are G protein-coupled receptors that are co-expressed in vascular smooth muscle cells. Endothelin-1 (ET-1) activates endothelin receptors to cause microvascular vasoconstriction. Previous studies have shown that heteromerization between ETA and ETB prolongs Ca2+ transients, leading to prolongation of Gαq-dependent signaling and sustained vasoconstriction. We hypothesized that these effects are in part mediated by the resistance of ETA/ETB heteromers to ß-arrestin recruitment and subsequent desensitization. Using bioluminescence resonance energy transfer 2 (BRET2), we found that ETB has a relatively equal affinity to form either homomers or heteromers with ETA when co-expressed in the human embryonic kidney 293 (HEK293) cells. When co-expressed, activation of ETA and ETB by ET-1 caused a heteromer-specific reduction and delay in ß-arrestin-2 recruitment with a corresponding reduction and delay in ET-1-induced ETA/ETB co-internalization. Furthermore, the co-expression of ETA and ETB inhibited ET-1-induced ß-arrestin-1-dependent extracellular signal-regulated kinase (ERK) phosphorylation while prolonging ET-1-induced Gαq-dependent ERK phosphorylation. ETA/ETB heteromerization mediates the long-lasting vasoconstrictor response to ET-1 by the prolongation of Gαq-dependent signaling and inhibition of ß-arrestin function.

Characterization of Critical Residues in the Extracellular and Transmembrane Domains of the Endothelin Type B Receptor for Propagation of the Endothelin-1 Signal.

We have previously reported the crystal structures of endothelin-1 (ET-1)-bound, ligand-free, and antagonist bosentan-bound forms of the thermostabilized ET type B receptor (ETB). Although other agonist-bound structures of ETB have been determined, the interactions for high-affinity binding and ETB receptor activation, as well as the roles of rearrangement of the hydrogen-bond network surrounding the ligand in G protein activation, remain elusive. ET-1, a 21-amino acid residue peptide, plays fundamental roles in basal vascular tone, sodium balance, cell proliferation, and stress-responsive regulation. We studied the interactions between the ET-1(8-21) peptide and ETB in the ligand binding and activation of ETB using a series of Ala-substituted ET-1(8-21) analogues and the mutated ETB. We found that while D8, L17, D18, I20, and W21 were responsible for high-affinity binding and potent G protein activation, Y13 and F14 in the helical region of ET-1 are prerequisites for the full activation of ETB via interactions near the extracellular side. Furthermore, we introduced the mutation into the residues around the ET-1 binding pocket of ETB. The results showed that while S1843.35, W3366.48, N3787.45, and S3797.46 in a conserved polar network are required for full activation, N1191.50, D1472.50, and N3827.49 are essential for G protein activation via direct interactions after rearrangement upon ET-1 binding. These results demonstrate that both interactions near the extracellular side and within the transmembrane helices with ET-1 play crucial roles in the full activation of the ETB receptor.

Crystal structure of human endothelin ETB receptor in complex with sarafotoxin S6b.

Sarafotoxins (SRTXs) are endothelin-like peptides extracted from snake venom. SRTXs stimulate the endothelin ETA and ETB receptors and enhance vasoconstriction, followed by left ventricular dysfunction and bronchoconstriction. SRTXs include four major isopeptides, S6a-d, with different subtype selectivities. Here, we report the crystal structure of the human ETB receptor in complex with the non-selective sarafotoxin S6b at 3.0 Å resolution. This structure reveals the similarities and differences between the binding modes of the endothelins and S6b. Moreover, molecular dynamics simulations based on the S6b-bound receptor provides structural insight into the subtype selectivity of the sarafotoxins. Our study clarifies the recognition mechanism of the endothelin-like peptide families.

Free-energy landscape of molecular interactions between endothelin 1 and human endothelin type B receptor: fly-casting mechanism.

The free-energy landscape of interaction between a medium-sized peptide, endothelin 1 (ET1), and its receptor, human endothelin type B receptor (hETB), was computed using multidimensional virtual-system coupled molecular dynamics, which controls the system's motions by introducing multiple reaction coordinates. The hETB embedded in lipid bilayer was immersed in explicit solvent. All molecules were expressed as all-atom models. The resultant free-energy landscape had five ranges with decreasing ET1-hETB distance: completely dissociative, outside-gate, gate, binding pocket, and genuine-bound ranges. In the completely dissociative range, no ET1-hETB interaction appeared. In the outside-gate range, an ET1-hETB attractive interaction was the fly-casting mechanism. In the gate range, the ET1 orientational variety decreased rapidly. In the binding pocket range, ET1 was in a narrow pathway with a steep free-energy slope. In the genuine-bound range, ET1 was in a stable free-energy basin. A G-protein-coupled receptor (GPCR) might capture its ligand from a distant place.

Crystal structure of human endothelin ETB receptor in complex with peptide inverse agonist IRL2500.

Endothelin receptors (ETA and ETB) are G-protein-coupled receptors activated by endothelin-1 and are involved in blood pressure regulation. IRL2500 is a peptide-mimetic of the C-terminal tripeptide of endothelin-1, and has been characterized as a potent ETB-selective antagonist, which has preventive effects against brain edema. Here, we report the crystal structure of the human ETB receptor in complex with IRL2500 at 2.7 Å-resolution. The structure revealed the different binding modes between IRL2500 and endothelin-1, and provides structural insights into its ETB-selectivity. Notably, the biphenyl group of IRL2500 penetrates into the transmembrane core proximal to D2.50, thus stabilizing the inactive conformation. Using the newly-established constitutively active mutant, we clearly demonstrate that IRL2500 functions as an inverse agonist for the ETB receptor. The current findings will expand the chemical space of ETR antagonists and facilitate the design of inverse agonists for other class A GPCRs.

Crystal structures of human ETB receptor provide mechanistic insight into receptor activation and partial activation.

Endothelin receptors (ETA and ETB) are class A GPCRs activated by vasoactive peptide endothelins, and are involved in blood pressure regulation. ETB-selective signalling induces vasorelaxation, and thus selective ETB agonists are expected to be utilized for improved anti-tumour drug delivery and neuroprotection. Here, we report the crystal structures of human ETB receptor in complex with ETB-selective agonist, endothelin-3 and an ETB-selective endothelin analogue IRL1620. The structure of the endothelin-3-bound receptor reveals that the disruption of water-mediated interactions between W6.48 and D2.50 is critical for receptor activation, while these hydrogen-bonding interactions are partially preserved in the IRL1620-bound structure. Consistently, functional analysis reveals the partial agonistic effect of IRL1620. The current findings clarify the detailed molecular mechanism for the coupling between the orthosteric pocket and the G-protein binding, and the partial agonistic effect of IRL1620, thus paving the way for the design of improved agonistic drugs targeting ETB.

Molecular Determinants for Ligand Selectivity of the Cell-Free Synthesized Human Endothelin B Receptor.

Extracellular domains of G-protein-coupled receptors act as initial molecular selectivity filters for subtype specific ligands and drugs. Chimeras of the human endothelin-B receptor containing structural units from the extracellular domains of the endothelin-A receptor were analyzed after their co-translational insertion into preformed nanodiscs. A short ß-strand and a linker region in the second extracellular loop as well as parts of the extracellular N-terminal domain were identified as molecular discrimination sites for the endothelin-B receptor-selective agonists IRL1620, sarafotoxin 6c, 4Ala-ET-1 and ET-3, but not for the non-selective agonist ET-1 recognized by both endothelin receptors. A proposed second disulfide bridge in the endothelin-B receptor tethering the N-terminal domain with the third extracellular loop was not essential for ET-1 recognition and binding, but increased the receptor thermostability. We further demonstrate an experimental approach with cell-free synthesized engineered agonists to analyze the differential discrimination of peptide ligand topologies by the two endothelin receptors. The study is based on the engineering and cell-free insertion of G-protein-coupled receptors into defined membranes and may become interesting also for other targets as an alternative platform to reveal molecular details of ligand selectivity and ligand binding mechanisms.

Lipopolysaccharides (LPS) Induced Angiogenesis During Chicken Embryogenesis is Abolished by Combined ETA/ETB Receptor Blockade.

BACKGROUND/AIMS: Angiogenesis plays a key role during embryonic development. The vascular endothelin (ET) system is involved in the regulation of angiogenesis. Lipopolysaccharides (LPS) could induce angiogenesis. The effects of ET blockers on baseline and LPS-stimulated angiogenesis during embryonic development remain unknown so far. METHODS: The blood vessel density (BVD) of chorioallantoic membranes (CAMs), which were treated with saline (control), LPS, and/or BQ123 and the ETB blocker BQ788, were quantified and analyzed using an IPP 6.0 image analysis program. Moreover, the expressions of ET-1, ET-2, ET3, ET receptor A (ETRA), ET receptor B (ETRB) and VEGFR2 mRNA during embryogenesis were analyzed by semi-quantitative RT-PCR. RESULTS: All components of the ET system are detectable during chicken embryogenesis. LPS increased angiogenesis substantially. This process was completely blocked by the treatment of a combination of the ETA receptor blockers-BQ123 and the ETB receptor blocker BQ788. This effect was accompanied by a decrease in ETRA, ETRB, and VEGFR2 gene expression. However, the baseline angiogenesis was not affected by combined ETA/ETB receptor blockade. CONCLUSION: During chicken embryogenesis, the LPS-stimulated angiogenesis, but not baseline angiogenesis, is sensitive to combined ETA/ETB receptor blockade.

Up-regulation of contractile endothelin receptors by airborne fine particulate matter in rat mesenteric arteries via activation of MAPK pathway.

Fine particle matters (PM2.5) is a well-known risk factor for cardiovascular diseases. However, the underlying molecular mechanisms are largely unknown. Vascular hyper-reactivity plays an important roles in the pathogenesis of cardiovascular diseases. The present study was designed to investigate a hypothesis that PM2.5 up-regulated endothelin receptors in mesenteric artery and the potential underlying mechanisms. Rat mesenteric arteries were cultured with PM2.5. The artery contractile responses were recorded by a sensitive myograph. ETB and ETA receptor expressions of mRNA and protein were assessed by quantitative real-time PCR, Western blotting, and immunohistochemistry, respectively. Results showed that ETB receptor agonist, sarafotoxin 6c induced a negligible contraction in fresh artery segments, while ETA receptor agonist, ET-1 induced an obvious contraction. After organ culture, the contraction curve mediated by ETB and ETA receptors were shifted toward the left. PM2.5 1.0 µg/ml cultured for 16 h further enhanced ETB and ETA receptor-mediated contractile responses with a markedly increased maximal contraction. The organ culture enhanced ETB and ETA receptor mRNA and protein levels from fresh arteries, which were further increased by PM2.5. The U0126 (MEK/ERK1/2 inhibitor) and SB203580 (p38 inhibitor) significantly attenuated both organ cultured-induced and PM2.5-induced up-regulation of ETB receptor. U0126 also suppressed organ culture-increased and PM2.5-increased expressions of ETA receptor. SB203580 only suppressed PM2.5-induced enhanced expressions of ETA receptor In conclusion, airborne PM2.5 up-regulates ETB and ETA receptors of mesenteric artery via p38 MAPK and MEK/ERK1/2 MAPK pathways.

Systematic optimization of cell-free synthesized human endothelin B receptor folding.

Cell-free production of G-protein coupled receptors is becoming attractive for biochemical characterization, ligand screening or even structural purposes. However, despite high production levels within the range of mg/mL, the fraction of functionally folded receptor is frequently below 1%. In synthetic cell-free reactions, numerous factors that affect the efficient folding and stability of translated membrane proteins can be addressed by the appropriate design of the synthetic expression environment. We demonstrate the systematic quality optimization of the cell-free synthesized human endothelin B receptor by a combined approach of lipid screening, redox optimization, and molecular engineering. Key parameters for receptor folding are the implementation of nanodiscs, the selection of suitable lipid environments for co-translational solubilization, as well as providing an optimized redox system for essential disulfide bridge formation. In addition, enrichment with chaperones as well as receptor engineering by thermostabilization further supported the folding into ligand binding conformation. In summary, we provide evidence that the initial co-translational folding process rather than long-term stability of the receptor is limiting. The folding efficiency could be improved by more than 103-fold and under optimized conditions, up to 1.6 nmol or ∼100 µg of ligand binding competent receptor could be produced per mL of reaction mixture in a timescale of less than 24 h. The identified parameters affect rather common characteristics of G-protein receptors and are thus likely to improve the folding of similar targets as well. The optimized process provides full-length receptors embedded in defined membrane environments and in quantities and quality sufficient for throughput screening applications.

Reduction of Uterine Perfusion Pressure Induced Redistribution of Endothelin Receptor Type-B Between the Intima and Media Contributes to the Pathogenesis of Pregnancy-Induced Hypertension.

BACKGROUND/AIMS: Studies have shown that a change in endothelin receptor expression in the artery is related to pregnancy-induced hypertension (PIH). However, the mechanism underlying this change remains unclear. METHODS: To test whether the distribution of endothelin receptor type-A (ETAR) and type-B (ETBR) plays an important role in PIH, a reduction of uterine perfusion pressure (RUPP) rat model was used to mimic some of the features of PIH; the resulting variable endothelin receptor expression was investigated in the media and intima of the aorta. Single vascular smooth muscle cells (VSMCs) were isolated from RUPP and normal pregnant (NP) rats to study the effect of ETAR and ETBR in smooth muscle cells. RESULTS: Compared with NP rats, RUPP rats had a significant redistribution of ETBR expression in the intima and media, while there was no significant difference in ETAR expression between the two groups. ETBR upregulation in VSMCs enhanced cellular contraction and contributed to PIH. The TNF-α plasma levels in RUPP rats were two-fold higher than those of NP rats, which upregulated the expression of ETBR in VSMCS through the NF-κB pathways in RUPP rats. CONCLUSION: Redistribution of ETBR between the media and intima played an important role in the pathogenesis of PIH.

X-ray structures of endothelin ETB receptor bound to clinical antagonist bosentan and its analog.

Endothelin receptors (ETRs) have crucial roles in vascular control and are targets for drugs designed to treat circulatory-system diseases and cancer progression. The nonpeptide dual-ETR antagonist bosentan is the first oral drug approved to treat pulmonary arterial hypertension. Here we report crystal structures of human endothelin ETB receptor bound to bosentan and to the ETB-selective analog K-8794, at 3.6-Å and 2.2-Å resolution, respectively. The K-8794-bound structure reveals the detailed water-mediated hydrogen-bonding network at the transmembrane core, which could account for the weak negative allosteric modulation of ETB by Na+ ions. The bosentan-bound structure reveals detailed interactions with ETB, which are probably conserved in the ETA receptor. A comparison of the two structures shows unexpected similarity between antagonist and agonist binding. Despite this similarity, bosentan sterically prevents the inward movement of transmembrane helix 6 (TM6), and thus exerts its antagonistic activity. These structural insights will facilitate the rational design of new ETR-targeting drugs.

A novel antihypertension agent, sargachromenol D from marine brown algae, Sargassum siliquastrum, exerts dual action as an L-type Ca2+ channel blocker and endothelin A/B2 receptor antagonist.

We isolated the novel vasoactive marine natural products, (5E,10E)-14-hydroxy-2,6,10-trimethylpentadeca-5,10-dien-4-one (4) and sargachromenol D (5), from Sargassum siliquastrum collected from the coast of the East Sea in South Korea by using activity-guided HPLC purification. The compounds effectively dilated depolarization (50mMK+)-induced basilar artery contraction with EC50 values of 3.52±0.42 and 1.62±0.63µM, respectively, but only sargachromenol D (5) showed a vasodilatory effect on endothelin-1 (ET-1)-induced basilar artery contraction (EC50=9.8±0.6µM). These results indicated that sargachromenol D (5) could act as a dual antagonist of l-type Ca2+ channel and endothelin A/B2 receptors. Moreover, sargachromenol D (5) lowered blood pressure in spontaneous hypertensive rats (SHRs) 2h after oral treatment at a dose of 80mg/kg dose and the effect was maintained for 24h. Based on our ex vivo and in vivo experiments, we propose that sargachromenol D (5) is a strong candidate for the treatment of hypertension that is not controlled by conventional drugs, in particular, severe-, type II diabetes-, salt-sensitive, and metabolic disease-induced hypertension.

Qualifying a eukaryotic cell-free system for fluorescence based GPCR analyses.

Membrane proteins are key elements in cell-mediated processes. In particular, G protein-coupled receptors (GPCRs) have attracted increasing interest since they affect cellular signaling. Furthermore, mutations in GPCRs can cause acquired and inheritable diseases. Up to date, there still exist a number of GPCRs that has not been structurally and functionally analyzed due to difficulties in cell-based membrane protein production. A promising approach for membrane protein synthesis and analysis has emerged during the last years and is known as cell-free protein synthesis (CFPS). Here, we describe a simply portable method to synthesize GPCRs and analyze their ligand-binding properties without the requirement of additional supplements such as liposomes or nanodiscs. This method is based on eukaryotic cell lysates containing translocationally active endogenous endoplasmic reticulum-derived microsomes where the insertion of GPCRs into biologically active membranes is supported. In this study we present CFPS in combination with fast fluorescence-based screening methods to determine the localization, orientation and ligand-binding properties of the endothelin B (ET-B) receptor upon expression in an insect-based cell-free system. To determine the functionality of the cell-free synthesized ET-B receptor, we analyzed the binding of its ligand endothelin-1 (ET-1) in a qualitative fluorescence-based assay and in a quantitative radioligand binding assay.

Activation mechanism of endothelin ETB receptor by endothelin-1.

Endothelin, a 21-amino-acid peptide, participates in various physiological processes, such as regulation of vascular tone, humoral homeostasis, neural crest cell development and neurotransmission. Endothelin and its G-protein-coupled receptor are involved in the development of various diseases, such as pulmonary arterial hypertension, and thus are important therapeutic targets. Here we report crystal structures of human endothelin type B receptor in the ligand-free form and in complex with the endogenous agonist endothelin-1. The structures and mutation analysis reveal the mechanism for the isopeptide selectivity between endothelin-1 and -3. Transmembrane helices 1, 2, 6 and 7 move and envelop the entire endothelin peptide, in a virtually irreversible manner. The agonist-induced conformational changes are propagated to the receptor core and the cytoplasmic G-protein coupling interface, and probably induce conformational flexibility in TM6. A comparison with the M2 muscarinic receptor suggests a shared mechanism for signal transduction in class A G-protein-coupled receptors.

TNFα-induced airway smooth muscle cell proliferation depends on endothelin receptor signaling, GM-CSF and IL-6.

UNLABELLED: Pathological proliferation of human airway smooth muscle cells (HASMCs) causes hyperplasia in chronic lung diseases. Signaling pathways that link airway inflammation to HASMC proliferation might provide therapeutic targets for the prevention of airway remodeling and chronic lung diseases. Endothelin-1 (ET-1) signals via endothelin-A- and B-receptors (ETAR, ETBR) to perpetuate HASMC-associated and TNFα-dependent inflammatory processes. HYPOTHESIS: endothelin receptor antagonists (ERAs) suppress HASMC proliferation induced by inflammatory cytokines. HASMCs were stimulated ex vivo with cytokines in the presence or absence of ERAs (ETAR-specific/selective: BQ123, ambrisentan; ETBR-specific: BQ788; non-selective: bosentan, macitentan, ACT-132577) or cytokine-blocking antibodies. Cell counts, DNA-synthesis (BrdU-incorporation assay), cytokine production (ELISA) and ETBR expression (whole-genome microarray data, western blot) were analyzed. ET-1-induced HASMC proliferation and DNA-synthesis were reduced by protein kinase inhibitors and ETAR-specific/selective ERAs but not by BQ788. TNFα-induced HASMC proliferation and DNA-synthesis were reduced by all ERAs. TNFα induced ET-1 and ETBR expression. TNFα- and ET-1-induced GM-CSF releases were both reduced by BQ123 and BQ788. TNFα- and ET-1-induced IL-6 releases were both reduced by BQ123 but not by BQ788. Combined but not single blockade of GM-CSF-receptor-α-chain and IL-6 reduced TNFα- and ET-1-induced HASMC proliferation and DNA-synthesis. Combined but not single treatment with GM-CSF and IL-6 induced HASMC proliferation and DNA-synthesis in the presence of ET-1. In conclusion, TNFα induces HASMC proliferation via ET-1/GM-CSF/IL-6. ETBR requires up-regulation by TNFα to mediate ET-1 effects on HASMC proliferation. This signaling cascade links airway inflammation to HASMC-associated remodeling processes and is sensitive to ERAs. Therefore, ERAs could prevent inflammation-induced airway smooth muscle hyperplasia.

Expression and purification of an engineered human endothelin receptor B in a monomeric form.

In humans, two endothelin receptors, ETa and ETb, are activated by three endogenous 21-mer cyclic peptides, ET-1, ET-2, and ET-3, which control various physiological processes, including vasoconstriction, vasodilation, and stimulation of cell proliferation. The first stage of this study it to produce a stable solubilized and purified receptor in a monodisperse state. This article is focused on the engineering, expression, purification, and characterization of the endothelin receptor B for subsequent structural and functional studies.

Hirschsprung disease is associated with an L286P mutation in the fifth transmembrane domain of the endothelin-B receptor in the N-ethyl-N-nitrosourea-induced mutant line.

Hirschsprung disease (HSCR), or colonic aganglionosis, is a congenital disorder characterized by the absence of intramural ganglia along variable lengths of the colon, resulting in intestinal obstruction. It is the most common cause of congenital intestinal obstruction, with an incidence of 1 in 5,000 live births. N-ethyl-N-nitrosourea (ENU)-induced mutagenesis is a powerful tool for the study of gene function and the generation of human disease models. In the current study, a novel mutant mouse with aganglionic megacolon and coat color spotting was generated by ENU-induced mutagenesis. Histological and acetylcholinesterase (AChE) whole-mount staining analysis showed a lack of ganglion cells in the colon in mutant mice. The mutation was mapped to chromosome 14 between markers rs30928624 and D14Mit205 (Chr 14 positions 103723921 bp and 105054651 bp). The Ednrb (Chr 14 position 103814625-103844173 bp) was identified as a potential candidate gene in this location. Mutation analysis revealed a T>C missense mutation at nucleotide 857 of the cDNA encoding endothelin receptor B (EDNRB) in which a proline was substituted for the highly conserved Lys-286 residue (L286P) in the fifth transmembrane (TM V) domain of this G protein-coupled receptor. The mutant mouse was named Ednrb(m1yzcm) (Ednrb; mutation 1, Yangzhou University Comparative Medicine Center). The results of the present study implicate the structural importance of the TM V domain in Ednrb function, and the Ednrb(m1yzcm) mouse represents a valuable model for the study of HSCR in humans.