Recombinase-mediated cassette exchange reveals the selective use of Gq/G11-dependent and -independent endothelin 1/endothelin type A receptor signaling in pharyngeal arch development

The role of endothelin and its receptors in cardiomyopathy: From molecular mechanisms to therapeutic insights

Cardiomyopathy is an anatomical and pathologic condition that is related to the cardiac muscle or left ventricular failure. A diverse range of illnesses known as cardiomyopathies often result in progressive heart failure with high morbidity and death rates. Primary cardiomyopathies are hereditary, mixed, or adopted. Secondary cardiomyopathies are infiltrative, harmful, or pathogenic. The activation of many paracrine, autocrine, and neuroendocrine factors is closely linked to pathological left ventricular (LV) deformation. After the myocardial injury, in the context of higher LV wall pressure and haemodynamic disturbance, these variables are raised. New therapy techniques have been focused on these novel targets after recent studies revealed that endothelin, nitric oxide or cytokines may be implicated in the remodelling process. Vasoconstrictive peptide endothelin-1 (ET-1) is mostly generated in the endothelium and works by binding to the ETA- and ETB-endothelin receptors (ET-Rs). The expression of both ET-Rs is widespread in cardiac tissues. Heart failure, pulmonary arterial hypertension, hypertension, cardiomyopathy, and coronary artery disease are just a few of the cardiovascular disorders for which the endothelin system has been shown to play a crucial role over the years. The occurrence, pathogenesis, and natural history of endothelin antagonists in cardiomyopathies are currently not well understood, and specific aspects of their treatment responses have not received comprehensive attention. Therefore, in this study, we address the variable degrees of success that have been achieved in treating cardiomyopathy using endothelin-targeting treatments, such as endothelin receptor antagonists.

The Gq/11 family of Gα subunits is necessary and sufficient for lower jaw development

Vertebrate jaw development is coordinated by highly conserved ligand-receptor systems such as the peptide ligand Endothelin 1 (Edn1) and Endothelin receptor type A (Ednra), which are required for patterning of lower jaw structures. The Edn1/Ednra signaling pathway establishes the identity of lower jaw progenitor cells by regulating expression of numerous patterning genes, but the intracellular signaling mechanisms linking receptor activation to gene regulation remain poorly understood. As a first step towards elucidating this mechanism, we examined the function of the Gq/11 family of Gα subunits in zebrafish using pharmacological inhibition and genetic ablation of Gq/11 activity, and transgenic induction of a constitutively active Gq protein in edn1-/- embryos. Genetic loss of Gq/11 activity fully recapitulated the edn1-/- phenotype, with genes encoding G11 being most essential. Furthermore, inducing Gq activity in edn1-/- embryos not only restored Edn1/Ednra-dependent jaw structures and gene expression signatures but also caused homeosis of the upper jaw structure into a lower jaw-like structure. These results indicate that Gq/11 is necessary and sufficient to mediate the lower jaw patterning mechanism for Ednra in zebrafish.

Assessing candidate DLX-regulated genes in the first pharyngeal arch of chick embryos

BACKGROUND

Insights into the development and evolution of asymmetrical jaws will require an understanding of the gene regulatory networks that underpin the differential morphogenesis of the maxillary and mandibular domains of the first pharyngeal arch in a variety of gnathostomes. While a robust relationship has been demonstrated between jaw patterning and the Endothelin-Dlx gene axis, much less is known of the next level of genes in the jaw patterning hierarchy.

RESULTS

Several genes, whose expression depends on Dlx5 and/or Dlx6, have been identified in mice. Here, we examined the expression patterns of the chick orthologues of some of those genes, namely GSC, PITX1, HAND2, and GBX2, and tested their dependence on endothelin signaling to assess whether there is a conserved regulatory relationship between those genes in the chick embryo. To further validate these genes as direct DLX targets, we identified conserved non-coding sequences containing candidate DLX binding motifs and demonstrated DLX-responsiveness in vitro.

CONCLUSIONS

The evidence presented in this study combines to support the hypothesis that these four genes are direct targets of DLX transcription factors in the lower jaw-forming tissue.

The Gq/11 family of Gα subunits is necessary and sufficient for lower jaw development

Vertebrate jaw development is coordinated by highly conserved ligand-receptor systems such as the peptide ligand Endothelin 1 (Edn1) and Endothelin receptor type A (Ednra), which are required for patterning of lower jaw structures. The Edn1/Ednra signaling pathway establishes the identity of lower jaw progenitor cells by regulating expression of numerous patterning genes, but the intracellular signaling mechanisms linking receptor activation to gene regulation remain poorly understood. As a first step towards elucidating this mechanism, we examined the function of the Gq/11 family of Gα subunits in zebrafish using pharmacological inhibition and genetic ablation of Gq/11 activity and transgenic induction of a constitutively active Gq protein in edn1 -/- embryos. Genetic loss of Gq/11 activity fully recapitulated the edn1 -/- phenotype, with genes encoding G11 being most essential. Furthermore, inducing Gq activity in edn1 -/- embryos not only restored Edn1/Ednra-dependent jaw structures and gene expression signatures but also caused homeosis of the upper jaw structure into a lower jaw-like structure. These results indicate that Gq/11 is necessary and sufficient to mediate the lower jaw patterning mechanism for Ednra in zebrafish.

Characterization of runs of homozygosity islands in American mink using whole-genome sequencing data

The genome-wide analysis of runs of homozygosity (ROH) islands can be an effective strategy for identifying shared variants within a population and uncovering important genomic regions related to complex traits. The current study performed ROH analysis to characterize the genome-wide patterns of homozygosity, identify ROH islands and annotated genes within these candidate regions using whole-genome sequencing data from 100 American mink (Neogale vison). After sequence processing, variants were called using GATK and Samtools pipelines. Subsequent to quality control, 8,373,854 bi-allelic variants identified by both pipelines remained for further analysis. A total of 34,652 ROH segments were identified in all individuals, among which shorter segments (0.3-1 Mb) were abundant throughout the genome, approximately accounting for 84.39% of all ROH. Within these segments, we identified 63 ROH islands housing 156 annotated genes. The genes located in ROH islands were associated with fur quality (EDNRA, FGF2, FOXA2 and SLC24A4), body size/weight (MYLK4, PRIM2, FABP2, EYS and PHF3), immune capacity (IL2, IL21, PTP4A1, SEMA4C, JAK2, CCNA2 and TNIP3) and reproduction (ADAD1, KHDRBS2, INSL6, PGRMC2 and HSPA4L). Furthermore, Gene Ontology and KEGG pathway enrichment analyses revealed 56 and 9 significant terms (FDR-corrected p-value < 0.05), respectively, among which cGMP-PKG signalling pathway, regulation of actin cytoskeleton, and calcium signalling pathway were highlighted due to their functional roles in growth and fur characteristics. This is the first study to present ROH islands in American mink. The candidate genes from ROH islands and functional enrichment analysis suggest possible signatures of selection in response to the mink breeding targets, such as increased body length, reproductive performance and fur quality. These findings contribute to our understanding of genetic characteristics, and provide complementary information to assist with implementation of breeding strategies for genetic improvement in American mink.

Preparation of the new peptide drug ACTY116-loaded in situ forming implants and evaluation of its efficacy in pulmonary arterial hypertension and right ventricular hypertrophy induced by SU5416/hypoxia in mice

There is a lack of effective therapeutic drugs for pulmonary arterial hypertension. Previous studies have demonstrated the positive cardiovascular system protective effects of the new peptide ACTY116. However, its stability in ordinary aqueous solution injections is poor and its half-life in the body is short, which has hindered the development of preparations. This study aimed to prepare in situ forming implants (ISFIs) of the peptide ACTY116 and investigate its impact on pulmonary arterial hypertension. We prepared ISFIs using NMP/TA as a solvent and PLGA as a polymer. These ISFIs exhibited low viscosity, low toxicity and sustained release properties. In a mouse model of pulmonary hypertension induced by SU5416/hypoxia, both ISFIs and ACTY116 peptides effectively reduced pulmonary hypertension, cardiac hypertrophy and pulmonary blood vessel wall thickness. In conclusion, this study highlights the potential of ACTY116 as a treatment for pulmonary arterial hypertension and suggests that incorporating it into an in-situ gel implant could be a promising option.

DLX gene expression in the developing chick pharyngeal arches and relationship to endothelin signaling and avian jaw patterning

BACKGROUND

A hinged jaw that articulates with the skull base is a striking feature of the vertebrate head and has been greatly modified between, and within, vertebrate classes. Genes belonging to the DLX homeobox family are conserved mediators of local signaling pathways that distinguish the dorsal and ventral aspects of the first pharyngeal arch. Specifically, a subset of DLX genes are expressed in the cranial neural crest-derived mandibular ectomesenchyme in response to ventral endothelin signaling, an important step that confers the first arch with maxillary and mandibular identities. Downstream targets of DLX genes then execute the morphogenetic processes that lead to functional jaws. Identifying lineage-specific variations in DLX gene expression and the regulatory networks downstream of DLX action is necessary to understand how different kinds of jaws evolved.

RESULTS

Here, we describe and compare the expression of all six DLX genes in the chick pharyngeal arches, focusing on the period of active patterning in the first arch. Disruption of endothelin signaling results in the down-regulation of ventral-specific DLX genes and confirms their functional role in avian jaw patterning.

CONCLUSIONS

This expression resource will be important for comparative embryology and for identifying synexpression groups of DLX-regulated genes in the chick.

Endothelin signaling in development

Since the discovery of endothelin 1 (EDN1) in 1988, the role of endothelin ligands and their receptors in the regulation of blood pressure in normal and disease states has been extensively studied. However, endothelin signaling also plays crucial roles in the development of neural crest cell-derived tissues. Mechanisms of endothelin action during neural crest cell maturation have been deciphered using a variety of in vivo and in vitro approaches, with these studies elucidating the basis of human syndromes involving developmental differences resulting from altered endothelin signaling. In this Review, we describe the endothelin pathway and its functions during the development of neural crest-derived tissues. We also summarize how dysregulated endothelin signaling causes developmental differences and how this knowledge may lead to potential treatments for individuals with gene variants in the endothelin pathway.

Mandibulofacial dysostosis with alopecia results from ETAR gain-of-function mutations via allosteric effects on ligand binding

Mutations of G protein-coupled receptors (GPCRs) cause various human diseases, but the mechanistic details are limited. Here, we establish p.E303K in the gene encoding the endothelin receptor type A (ETAR/EDNRA) as a recurrent mutation causing mandibulofacial dysostosis with alopecia (MFDA), with craniofacial changes similar to those caused by p.Y129F. Mouse models carrying either of these missense mutations exhibited a partial maxillary-to-mandibular transformation, which was rescued by deleting the ligand endothelin 3 (ET3/EDN3). Pharmacological experiments confirmed the causative ETAR mutations as gain of function, dependent on ET3. To elucidate how an amino acid substitution far from the ligand binding site can increase ligand affinity, we used molecular dynamics (MD) simulations. E303 is located at the intracellular end of transmembrane domain 6, and its replacement by a lysine increased flexibility of this portion of the helix, thus favoring G protein binding and leading to G protein-mediated enhancement of agonist affinity. The Y129F mutation located under the ligand binding pocket reduced the sodium-water network, thereby affecting the extracellular portion of helices in favor of ET3 binding. These findings provide insight into the pathogenesis of MFDA and into allosteric mechanisms regulating GPCR function, which may provide the basis for drug design targeting GPCRs.

Dissect Relationships Between Gene Co-expression and Functional Connectivity in Human Brain

Although recent evidence indicates an association between gene co-expression and functional connectivity in human brain, specific association patterns remain largely unknown. Here, using neuroimaging-based functional connectivity data of living brains and brain-wide gene expression data of postmortem brains, we performed comprehensive analyses to dissect relationships between gene co-expression and functional connectivity. We identified 125 connectivity-related genes (20 novel genes) enriched for dendrite extension, signaling pathway and schizophrenia, and 179 gene-related functional connections mainly connecting intra-network regions, especially homologous cortical regions. In addition, 51 genes were associated with connectivity in all brain functional networks and enriched for action potential and schizophrenia; in contrast, 51 genes showed network-specific modulatory effects and enriched for ion transportation. These results indicate that functional connectivity is unequally affected by gene expression, and connectivity-related genes with different biological functions are involved in connectivity modulation of different networks.

GATA3 is essential for separating patterning domains during facial morphogenesis

Neural crest cells (NCCs) within the mandibular and maxillary prominences of the first pharyngeal arch are initially competent to respond to signals from either region. However, mechanisms that are only partially understood establish developmental tissue boundaries to ensure spatially correct patterning. In the 'hinge and caps' model of facial development, signals from both ventral prominences (the caps) pattern the adjacent tissues whereas the intervening region, referred to as the maxillomandibular junction (the hinge), maintains separation of the mandibular and maxillary domains. One cap signal is GATA3, a member of the GATA family of zinc-finger transcription factors with a distinct expression pattern in the ventral-most part of the mandibular and maxillary portions of the first arch. Here, we show that disruption of Gata3 in mouse embryos leads to craniofacial microsomia and syngnathia (bony fusion of the upper and lower jaws) that results from changes in BMP4 and FGF8 gene regulatory networks within NCCs near the maxillomandibular junction. GATA3 is thus a crucial component in establishing the network of factors that functionally separate the upper and lower jaws during development.

Diversity of copy number variation in the worldwide goat population

Goats (Capra hircus) are an important farm animal species. Copy number variation (CNV) represents a major source of genomic structural variation. We investigated the diversity of CNV distribution in goats using CaprineSNP50 genotyping data generated by the ADAPTmap Project. We identified 6286 putative CNVs in 1023 samples from 50 goat breeds using PennCNV. These CNVs were merged into 978 CNV regions, spanning ~262 Mb of total length and corresponding to ~8.96% of the goat genome. We then divided the samples into six subgroups per geographic distribution and constructed a comparative CNV map. Our results revealed a population differentiation in CNV across different geographical areas, including Western Asia, Eastern Mediterranean, Alpine & Northern Europe, Madagascar, Northwestern Africa, and Southeastern Africa groups. The results of a cluster heatmap analysis based on the CNV count per individual across different groups was generally consistent with the one generated from the SNP data, likely reflecting the population history of different goat breeds. We sought to determine the gene content of these CNV events and found several important CNV-overlapping genes (e.g. EDNRA, ADAMTS20, ASIP, KDM5B, ADAM8, DGAT1, CHRNB1, CLCN7, and EXOSC4), which are involved in local adaptations such as coat color, muscle development, metabolic processes, osteopetrosis, and embryonic development. Therefore, this research generated an extensive CNV map in the worldwide population of goat, which offers novel insight into the goat genome and its functional annotation.

Probing the origin of matching functional jaws: roles of Dlx5/6 in cranial neural crest cells

Gnathostome jaws derive from the first pharyngeal arch (PA1), a complex structure constituted by Neural Crest Cells (NCCs), mesodermal, ectodermal and endodermal cells. Here, to determine the regionalized morphogenetic impact of Dlx5/6 expression, we specifically target their inactivation or overexpression to NCCs. NCC-specific Dlx5/6 inactivation (NCC^∆Dlx5/6) generates severely hypomorphic lower jaws that present typical maxillary traits. Therefore, differently from Dlx5/6 null-embryos, the upper and the lower jaws of NCC^∆Dlx5/6 mice present a different size. Reciprocally, forced Dlx5 expression in maxillary NCCs provokes the appearance of distinct mandibular characters in the upper jaw. We conclude that: (1) Dlx5/6 activation in NCCs invariably determines lower jaw identity; (2) the morphogenetic processes that generate functional matching jaws depend on the harmonization of Dlx5/6 expression in NCCs and in distinct ectodermal territories. The co-evolution of synergistic opposing jaws requires the coordination of distinct regulatory pathways involving the same transcription factors in distant embryonic territories.

Differing contributions of the first and second pharyngeal arches to tympanic membrane formation in the mouse and chick

We have proposed that independent origins of the tympanic membrane (TM), consisting of the external auditory meatus (EAM) and first pharyngeal pouch, are linked with distinctive middle ear structures in terms of dorsal-ventral patterning of the pharyngeal arches during amniote evolution. However, previous studies have suggested that the first pharyngeal arch (PA1) is crucial for TM formation in both mouse and chick. In this study, we compare TM formation along the anterior-posterior axis in these animals using Hoxa2 expression as a marker of the second pharyngeal arch (PA2). In chick, the EAM begins to invaginate at the surface ectoderm of PA2, not at the first pharyngeal cleft, and the entire TM forms in PA2. Chick-quail chimera that have lost PA2 and duplicated PA1 suggest that TM formation is achieved by developmental interaction between a portion of the EAM and the columella auris in PA2, and that PA1 also contributes to formation of the remaining part of the EAM. By contrast, in mouse, TM formation is highly associated with an interdependent relationship between the EAM and tympanic ring in PA1.

Negative regulation of endothelin signaling by SIX1 is required for proper maxillary development

Jaw morphogenesis is a complex event mediated by inductive signals that establish and maintain the distinct developmental domains required for formation of hinged jaws, the defining feature of gnathostomes. The mandibular portion of pharyngeal arch 1 is patterned dorsally by Jagged-Notch signaling and ventrally by endothelin receptor A (EDNRA) signaling. Loss of EDNRA signaling disrupts normal ventral gene expression, the result of which is homeotic transformation of the mandible into a maxilla-like structure. However, loss of Jagged-Notch signaling does not result in significant changes in maxillary development. Here we show in mouse that the transcription factor SIX1 regulates dorsal arch development not only by inducing dorsal Jag1 expression but also by inhibiting endothelin 1 (Edn1) expression in the pharyngeal endoderm of the dorsal arch, thus preventing dorsal EDNRA signaling. In the absence of SIX1, but not JAG1, aberrant EDNRA signaling in the dorsal domain results in partial duplication of the mandible. Together, our results illustrate that SIX1 is the central mediator of dorsal mandibular arch identity, thus ensuring separation of bone development between the upper and lower jaws.

Embryonic expression of endothelins and their receptors in lamprey and frog reveals stem vertebrate origins of complex Endothelin signaling

Neural crest cells (NCCs) are highly patterned embryonic cells that migrate along stereotyped routes to give rise to a diverse array of adult tissues and cell types. Modern NCCs are thought to have evolved from migratory neural precursors with limited developmental potential and patterning. How this occurred is poorly understood. Endothelin signaling regulates several aspects of NCC development, including their migration, differentiation, and patterning. In jawed vertebrates, Endothelin signaling involves multiple functionally distinct ligands (Edns) and receptors (Ednrs) expressed in various NCC subpopulations. To test the potential role of endothelin signaling diversification in the evolution of modern, highly patterned NCC, we analyzed the expression of the complete set of endothelin ligands and receptors in the jawless vertebrate, the sea lamprey (Petromyzon marinus). To better understand ancestral features of gnathostome edn and ednr expression, we also analyzed all known Endothelin signaling components in the African clawed frog (Xenopus laevis). We found that the sea lamprey has a gnathsotome-like complement of edn and ednr duplicates, and these genes are expressed in patterns highly reminiscent of their gnathostome counterparts. Our results suggest that the duplication and specialization of vertebrate Endothelin signaling coincided with the appearance of highly patterned and multipotent NCCs in stem vertebrates.

Viable Ednra Y129F mice feature human mandibulofacial dysostosis with alopecia (MFDA) syndrome due to the homologue mutation

Animal models resembling human mutations are valuable tools to research the features of complex human craniofacial syndromes. This is the first report on a viable dominant mouse model carrying a non-synonymous sequence variation within the endothelin receptor type A gene (Ednra c.386A>T, p.Tyr129Phe) derived by an ENU mutagenesis program. The identical amino acid substitution was reported recently as disease causing in three individuals with the mandibulofacial dysostosis with alopecia (MFDA, OMIM 616367) syndrome. We performed standardized phenotyping of wild-type, heterozygous, and homozygous Ednra^Y129F mice within the German Mouse Clinic. Mutant mice mimic the craniofacial phenotypes of jaw dysplasia, micrognathia, dysplastic temporomandibular joints, auricular dysmorphism, and missing of the squamosal zygomatic process as described for MFDA-affected individuals. As observed in MFDA-affected individuals, mutant Ednra^Y129F mice exhibit hearing impairment in line with strong abnormalities of the ossicles and further, reduction of some lung volumetric parameters. In general, heterozygous and homozygous mice demonstrated inter-individual diversity of expression of the craniofacial phenotypes as observed in MFDA patients but without showing any cleft palates, eyelid defects, or alopecia. Mutant Ednra^Y129F mice represent a valuable viable model for complex human syndromes of the first and second pharyngeal arches and for further studies and analysis of impaired endothelin 1 (EDN1)–endothelin receptor type A (EDNRA) signaling. Above all, Ednra^Y129F mice model the recently published human MFDA syndrome and may be helpful for further disease understanding and development of therapeutic interventions.

Genomic amplification of the caprine EDNRA locus might lead to a dose dependent loss of pigmentation

The South African Boer goat displays a characteristic white spotting phenotype, in which the pigment is limited to the head. Exploiting the existing phenotype variation within the breed, we mapped the locus causing this white spotting phenotype to chromosome 17 by genome wide association. Subsequent whole genome sequencing identified a 1 Mb copy number variant (CNV) harboring 5 genes including EDNRA. The analysis of 358 Boer goats revealed 3 alleles with one, two, and three copies of this CNV. The copy number is correlated with the degree of white spotting in goats. We propose a hypothesis that ectopic overexpression of a mutant EDNRA scavenges EDN3 required for EDNRB signaling and normal melanocyte development and thus likely lead to an absence of melanocytes in the non-pigmented body areas of Boer goats. Our findings demonstrate the value of domestic animals as reservoir of unique mutants and for identifying a precisely defined functional CNV.

Genome-wide association study identifies multiple susceptibility loci for craniofacial microsomia

Craniofacial microsomia (CFM) is a rare congenital anomaly that involves immature derivatives from the first and second pharyngeal arches. The genetic pathogenesis of CFM is still unclear. Here we interrogate 0.9 million genetic variants in 939 CFM cases and 2,012 controls from China. After genotyping of an additional 443 cases and 1,669 controls, we identify 8 significantly associated loci with the most significant SNP rs13089920 (logistic regression P=2.15 × 10(-120)) and 5 suggestive loci. The above 13 associated loci, harboured by candidates of ROBO1, GATA3, GBX2, FGF3, NRP2, EDNRB, SHROOM3, SEMA7A, PLCD3, KLF12 and EPAS1, are found to be enriched for genes involved in neural crest cell (NCC) development and vasculogenesis. We then perform whole-genome sequencing on 21 samples from the case cohort, and identify several novel loss-of-function mutations within the associated loci. Our results provide new insights into genetic background of craniofacial microsomia.

Mutations in the endothelin receptor type A cause mandibulofacial dysostosis with alopecia

The endothelin receptor type A (EDNRA) signaling pathway is essential for the establishment of mandibular identity during development of the first pharyngeal arch. We report four unrelated individuals with the syndrome mandibulofacial dysostosis with alopecia (MFDA) who have de novo missense variants in EDNRA. Three of the four individuals have the same substitution, p.Tyr129Phe. Tyr129 is known to determine the selective affinity of EDNRA for endothelin 1 (EDN1), its major physiological ligand, and the p.Tyr129Phe variant increases the affinity of the receptor for EDN3, its non-preferred ligand, by two orders of magnitude. The fourth individual has a somatic mosaic substitution, p.Glu303Lys, and was previously described as having Johnson-McMillin syndrome. The zygomatic arch of individuals with MFDA resembles that of mice in which EDNRA is ectopically activated in the maxillary prominence, resulting in a maxillary to mandibular transformation, suggesting that the p.Tyr129Phe variant causes an EDNRA gain of function in the developing upper jaw. Our in vitro and in vivo assays suggested complex, context-dependent effects of the EDNRA variants on downstream signaling. Our findings highlight the importance of finely tuned regulation of EDNRA signaling during human craniofacial development and suggest that modification of endothelin receptor-ligand specificity was a key step in the evolution of vertebrate jaws.

Structure, Function, Pharmacology, and Therapeutic Potential of the G Protein, Gα/q,11

G protein coupled receptors (GPCRs) are one of the major classes of cell surface receptors and are associated with a group of G proteins consisting of three subunits termed alpha, beta, and gamma. G proteins are classified into four families according to their α subunit; Gα_i, Gα_s, Gα_12/13, and Gα_q. There are several downstream pathways of Gα_q of which the best known is upon activation via guanosine triphosphate (GTP), Gα_q activates phospholipase Cβ, hydrolyzing phosphatidylinositol 4,5-biphosphate into diacylglycerol and inositol triphosphate and activating protein kinase C and increasing calcium efflux from the endoplasmic reticulum. Although G proteins, in particular, the Gα_q/11 are central elements in GPCR signaling, their actual roles have not yet been thoroughly investigated. The lack of research of the role on Gα_q/11 in cell biology is partially due to the obscure nature of the available pharmacological agents. YM-254890 is the most useful Gα_q-selective inhibitor with antiplatelet, antithrombotic, and thrombolytic effects. YM-254890 inhibits Gα_q signaling pathways by preventing the exchange of guanosine diphosphate for GTP. UBO-QIC is a structurally similar compound to YM-254890, which can inhibit platelet aggregation and cause vasorelaxation in rats. Many agents are available for the study of signaling downstream of Gα_q/11. The role of G proteins could potentially represent a novel therapeutic target. This review will explore the range of pharmacological and molecular tools available for the study of the role of Gα_q/11 in GPCR signaling.

Cre recombinase-regulated Endothelin1 transgenic mouse lines: novel tools for analysis of embryonic and adult disorders

Endothelin-1 (EDN1) influences both craniofacial and cardiovascular development and a number of adult physiological conditions by binding to one or both of the known endothelin receptors, thus initiating multiple signaling cascades. Animal models containing both conventional and conditional loss of the Edn1 gene have been used to dissect EDN1 function in both embryos and adults. However, while transgenic Edn1 over-expression or targeted genomic insertion of Edn1 has been performed to understand how elevated levels of Edn1 result in or exacerbate disease states, an animal model in which Edn1 over-expression can be achieved in a spatiotemporal-specific manner has not been reported. Here we describe the creation of Edn1 conditional over-expression transgenic mouse lines in which the chicken β-actin promoter and an Edn1 cDNA are separated by a strong stop sequence flanked by loxP sites. In the presence of Cre, the stop cassette is removed, leading to Edn1 expression. Using the Wnt1-Cre strain, in which Cre expression is targeted to the Wnt1-expressing domain of the central nervous system (CNS) from which neural crest cells (NCCs) arise, we show that stable chicken β-actin-Edn1 (CBA-Edn1) transgenic lines with varying EDN1 protein levels develop defects in NCC-derived tissues of the face, though the severity differs between lines. We also show that Edn1 expression can be achieved in other embryonic tissues utilizing other Cre strains, with this expression also resulting in developmental defects. CBA-Edn1 transgenic mice will be useful in investigating diverse aspects of EDN1-mediated-development and disease, including understanding how NCCs achieve and maintain a positional and functional identity and how aberrant EDN1 levels can lead to multiple physiological changes and diseases.

Mutations in endothelin 1 cause recessive auriculocondylar syndrome and dominant isolated question-mark ears

Auriculocondylar syndrome (ACS) is a rare craniofacial disorder with mandibular hypoplasia and question-mark ears (QMEs) as major features. QMEs, consisting of a specific defect at the lobe-helix junction, can also occur as an isolated anomaly. Studies in animal models have indicated the essential role of endothelin 1 (EDN1) signaling through the endothelin receptor type A (EDNRA) in patterning the mandibular portion of the first pharyngeal arch. Mutations in the genes coding for phospholipase C, beta 4 (PLCB4) and guanine nucleotide binding protein (G protein), alpha inhibiting activity polypeptide 3 (GNAI3), predicted to function as signal transducers downstream of EDNRA, have recently been reported in ACS. By whole-exome sequencing (WES), we identified a homozygous substitution in a furin cleavage site of the EDN1 proprotein in ACS-affected siblings born to consanguineous parents. WES of two cases with vertical transmission of isolated QMEs revealed a stop mutation in EDN1 in one family and a missense substitution of a highly conserved residue in the mature EDN1 peptide in the other. Targeted sequencing of EDN1 in an ACS individual with related parents identified a fourth, homozygous mutation falling close to the site of cleavage by endothelin-converting enzyme. The different modes of inheritance suggest that the degree of residual EDN1 activity differs depending on the mutation. These findings provide further support for the hypothesis that ACS and QMEs are uniquely caused by disruption of the EDN1-EDNRA signaling pathway.

Endothelin regulates neural crest deployment and fate to form great vessels through Dlx5/Dlx6-independent mechanisms

Endothelin-1 (Edn1), originally identified as a vasoconstrictor peptide, is involved in the development of cranial/cardiac neural crest-derived tissues and organs. In craniofacial development, Edn1 binds to Endothelin type-A receptor (Ednra) to induce homeobox genes Dlx5/Dlx6 and determines the mandibular identity in the first pharyngeal arch. However, it remains unsolved whether this pathway is also critical for pharyngeal arch artery development to form thoracic arteries. Here, we show that the Edn1/Ednra signaling is involved in pharyngeal artery development by controlling the fate of neural crest cells through a Dlx5/Dlx6-independent mechanism. Edn1 and Ednra knock-out mice demonstrate abnormalities in pharyngeal arch artery patterning, which include persistent first and second pharyngeal arteries, resulting in additional branches from common carotid arteries. Neural crest cell labeling with Wnt1-Cre transgene and immunostaining for smooth muscle cell markers revealed that neural crest cells abnormally differentiate into smooth muscle cells at the first and second pharyngeal arteries of Ednra knock-out embryos. By contrast, Dlx5/Dlx6 knockout little affect the development of pharyngeal arch arteries and coronary arteries, the latter of which is also contributed by neural crest cells through an Edn-dependent mechanism. These findings indicate that the Edn1/Ednra signaling regulates neural crest differentiation to ensure the proper patterning of pharyngeal arch arteries, which is independent of the regional identification of the pharyngeal arches along the dorsoventral axis mediated by Dlx5/Dlx6.

Preotic neural crest cells contribute to coronary artery smooth muscle involving endothelin signalling

Neural crest cells constitute a multipotent cell population that gives rise to diverse cell lineages. The neural crest arising from the postotic hindbrain is known as the 'cardiac' neural crest, and contributes to the great vessels and outflow tract endocardial cushions, but the neural crest contribution to structures within the heart remains largely controversial. Here we demonstrate that neural crest cells from the preotic region migrate into the heart and differentiate into coronary artery smooth muscle cells. Preotic neural crest cells preferentially distribute to the conotruncal region and interventricular septum. Ablation of the preotic neural crest causes abnormalities in coronary septal branch and orifice formation. Mice and chicks lacking endothelin signalling show similar abnormalities in the coronary artery, indicating its involvement in neural crest-dependent coronary artery formation. This is the first report that reveals the preotic neural crest contribution to heart development and smooth muscle heterogeneity within a coronary artery.

Arhgef15 promotes retinal angiogenesis by mediating VEGF-induced Cdc42 activation and potentiating RhoJ inactivation in endothelial cells

BACKGROUND

Drugs inhibiting vascular endothelial growth factor (VEGF) signaling are globally administered to suppress deregulated angiogenesis in a variety of eye diseases. However, anti-VEGF therapy potentially affects the normal functions of retinal neurons and glias which constitutively express VEGF receptor 2. Thus, it is desirable to identify novel drug targets which are exclusively expressed in endothelial cells (ECs). Here we attempted to identify an EC-specific Rho guanine nucleotide exchange factor (GEF) and evaluate its role in retinal angiogenesis.

METHODOLOGY/PRINCIPAL FINDINGS

By exploiting fluorescence-activated cell sorting and microarray analyses in conjunction with in silico bioinformatics analyses, we comprehensively identified endothelial genes in angiogenic retinal vessels of postnatal mice. Of 9 RhoGEFs which were highly expressed in retinal ECs, we show that Arhgef15 acted as an EC-specific GEF to mediate VEGF-induced Cdc42 activation and potentiated RhoJ inactivation, thereby promoting actin polymerization and cell motility. Disruption of the Arhgef15 gene led to delayed extension of vascular networks and subsequent reduction of total vessel areas in postnatal mouse retinas.

CONCLUSIONS/SIGNIFICANCE

Our study provides information useful to the development of new means of selectively manipulating angiogenesis without affecting homeostasis in un-targeted tissues; not only in eyes but also in various disease settings such as cancer.

Cell proliferation and apoptosis in the fusion of human primary and secondary palates

The markers of cell proliferation (Ki-67) and apoptosis [caspase-3, TdT-mediated biotin-dUTP nick-end labelling (TUNEL)] and the expression of syndecan-1 and heat shock protein 70 (Hsp70) were analyzed immunohistochemically in 11 developing human palates, from developmental weeks 6 to 10. During fusion of the primary palate, the proportion of proliferating cells decreased from 42 to 32% and the proportion of apoptotic cells decreased from 11 to 7% in the medial-edge epithelium. At later stages, the proportions of both types of cells decreased in the ectomesenchyme, except for proliferating cells in its non-condensing part. At developmental weeks 9-10, the epithelial seam in the secondary palate comprised 28% proliferative cells and 5% apoptotic cells. While condensing ectomesenchyme contained more apoptotic cells than proliferating cells, the opposite was observed for the non-condensing ectomesenchyme. Co-expression of syndecan-1 and Hsp70 was detected in cells budding from the epithelial seam. Our study indicates similar principles for human primary palate and secondary palate fusion, and parallel persistence of proliferation and apoptotic activity. While proliferation enables growth and fusion of different palatal primordia, apoptosis results in the removal of of large numbers epithelial cells at the fusion point. The disintegration of seam remnants seems to be executed through the processes of change in protein content and cell migration, probably leading to cell death as their final outcome.

Combinatorial roles for BMPs and Endothelin 1 in patterning the dorsal-ventral axis of the craniofacial skeleton

Bone morphogenetic proteins (BMPs) play crucial roles in craniofacial development but little is known about their interactions with other signals, such as Endothelin 1 (Edn1) and Jagged/Notch, which pattern the dorsal-ventral (DV) axis of the pharyngeal arches. Here, we use transgenic zebrafish to monitor and perturb BMP signaling during arch formation. With a BMP-responsive transgene, Tg(Bre:GFP), we show active BMP signaling in neural crest (NC)-derived skeletal precursors of the ventral arches, and in surrounding epithelia. Loss-of-function studies using a heat shock-inducible, dominant-negative BMP receptor 1a [Tg(hs70I:dnBmpr1a-GFP)] to bypass early roles show that BMP signaling is required for ventral arch development just after NC migration, the same stages at which we detect Tg(Bre:GFP). Inhibition of BMP signaling at these stages reduces expression of the ventral signal Edn1, as well as ventral-specific genes such as hand2 and dlx6a in the arches, and expands expression of the dorsal signal jag1b. This results in a loss or reduction of ventral and intermediate skeletal elements and a mis-shapen dorsal arch skeleton. Conversely, ectopic BMP causes dorsal expansion of ventral-specific gene expression and corresponding reductions/transformations of dorsal cartilages. Soon after NC migration, BMP is required to induce Edn1 and overexpression of either signal partially rescues ventral skeletal defects in embryos deficient for the other. However, once arch primordia are established the effects of BMPs become restricted to more ventral and anterior (palate) domains, which do not depend on Edn1. This suggests that BMPs act upstream and in parallel to Edn1 to promote ventral fates in the arches during early DV patterning, but later acquire distinct roles that further subdivide the identities of NC cells to pattern the craniofacial skeleton.

Developmental patterns of Ki-67, bcl-2 and caspase-3 proteins expression in the human upper jaw

The distribution of the Ki-67, bcl-2 and caspase-3 proteins was immunohistochemically analyzed in the developing human upper jaw (5th-10th gestational weeks). During this period, proliferative activity gradually decreased from higher levels at the earliest stages (50-52%) to lower levels, both in the jaw ectomesenchyme and in the epithelium. The highest expression of bcl-2 protein was found in the epithelium and ectomesenchyme of areas displaying lower rates of cell proliferation. High levels of caspase-3 protein were detected during the earliest stages of jaw development, indicating an important role for apoptosis in morphogenesis of early derivatives of the maxillary prominences. The number of Ki-67, bcl-2 and caspase-3 positive cells changed in a temporally and spatially restricted manner, coincidently with upper jaw differentiation. While apoptosis might control cell number, bcl-2 could act in suppression of apoptosis and enhancement of cell differentiation. A fine balance between cell proliferation (Ki-67), death (caspase-3) and cell survival (bcl-2) characterized early human upper jaw development. A rise in the number of apoptotic cells always temporally coincided with the decrease in number of surviving bcl-2 positive cells within the palatal region. Therefore, the upper jaw development seems to be controlled by the precisely defined expression of genes for proliferation, apoptosis and cell survival.

Establishment of mice expressing EGFP in the placode-derived inner ear sensory cell lineage and FACS-array analysis focused on the regional specificity of the otocyst

In this study, we established a novel enhanced green fluorescent protein (EGFP) reporter mouse line that enables the visualization of the placode-derived inner ear sensory cell lineage. EGFP was initially expressed in the otic placode and throughout its differentiation process into the inner ear sensory patches. At embryonic day 10.5 (E10.5), EGFP was expressed in the ventral and dorsomedial region of the otocyst. These regions could mainly give rise to the cochlea, including the organ of Corti, and the saccule, including the macula and the endolymphatic duct. The region could also give rise to cells that will develop as either prosensory cells or statoacoustic ganglion neuroblasts. By using this line and fluorescence-activated cell sorting (FACS)-array technology, we developed a new gene expression profile of the regional specificity of the otocyst. EGFP-positive regions include the Otx1-positive region, which could be clearly distinguished from EGFP-negative regions. The signal log ratio of microarray data showed high efficiency in predicting the genes expressed mainly in the ventral and/or dorsomedial otocyst and the data could be mined to uncover many novel genes involved in inner ear morphogenesis and cell fate regulation. Additionally, these data suggest that some novel genes enriched in EGFP-positive regions may be potentially involved in human congenital sensorineural hearing loss. This reporter line could play important roles in the use of animal models for detailed analysis of the differentiation process into the sensory patches and the identification of regional-specific gene networks and novel gene functions in the developing inner ear.

Identification and developmental analysis of endothelin receptor type-A expressing cells in the mouse kidney

The endothelin (Edn) system plays pleiotropic roles in renal function and various disease processes through two distinct G protein-coupled receptors, Edn receptors type-A (Ednra) and type-B (Ednrb). However, difficulties in the accurate identification of receptor-expressing cells in situ have made it difficult to dissect their diverse action in renal (patho)physiology. We have recently established mouse lines in which lacZ and EGFP are 'knocked-in' to the Ednra locus to faithfully mark Ednra-expressing cells. Here we analyzed these mice for their expression in the kidney to characterize Ednra-expressing cells. Ednra expression was first observed in undifferentiated mesenchymal cells around the ureteric bud at E12.5. Thereafter, Ednra expression was widely observed in vascular smooth muscle cells, JG cells and mesenchymal cells in the interstitium. After growth, the expression became confined to vascular smooth muscle cells, pericytes and renin-producing JG cells. By contrast, most cells in the nephron and vascular endothelial cells did not express Ednra. These results indicate that Ednra expression may be linked with non-epithelial fate determination and differentiation of metanephric mesenchyme. Ednra-lacZ/EGFP knock-in mice may serve as a useful tool in studies on renal function and pathophysiology of various renal diseases.

Irreversible effects of retinoic acid pulse on Xenopus jaw morphogenesis: new insight into cranial neural crest specification

Jaws are formed by cephalic neural crest (CNCCs) and mesodermal cells migrating to the first pharyngeal arch (PA1). A complex signaling network involving different PA1 components then establishes the jaw morphogenetic program. To gather insight on this developmental process, in this study, we analyze the teratogenic effects of brief (1-15 min) pulses of low doses of retinoic acid (RA: 0.25-2 µM) or RA agonists administered to early Xenopus laevis (X.l.) embryos. We show that these brief pulses of RA cause permanent craniofacial defects specifically when treatments are performed during a 6-hr window (developmental stages NF15-NF23) that covers the period of CNCCs maintenance, migration, and specification. Earlier or later treatments have no effect. Similar treatments performed at slightly different developmental stages within this temporal window give rise to different spectra of malformations. The RA-dependent teratogenic effects observed in Xenopus can be partially rescued by folinic acid. We provide evidence suggesting that in Xenopus, as in the mouse, RA causes craniofacial malformations by perturbing signaling to CNCCs. Differently from the mouse, where RA affects CNCCs only at the end of their migration, in Xenopus, RA has an effect on CNCCs during all the period ranging from their exit from the neural tube until their arrival in the PA1. Our findings provide a conceptual framework to understand the origin of individual facial features and the evolution of different craniofacial morphotypes.

Knock-in approaches

Molecular genetic strategies to study gene function in mice or to generate a mouse model for a human disease are continuously under development. The application and importance of knock-in approaches are increasing. This chapter elaborates on novel developments for the generation of knock-in mice. Special emphasis is given to recombinase-mediated cassette exchange, a new emerging knock-in strategy that enables easy generation of a series of different knock-in mutations within one gene.

Regulation of facial morphogenesis by endothelin signaling: insights from mice and fish

Craniofacial morphogenesis is accomplished through a complex set of developmental events, most of which are initiated in neural crest cells within the pharyngeal arches. Local patterning cues from the surrounding environment induce gene expression within neural crest cells, leading to formation of a diverse set of skeletal elements. Endothelin-1 (Edn1) is one of the primary signals that establishes the identity of neural crest cells within the mandibular portion of the first pharyngeal arch. Signaling through its cognate receptor, the endothelin-A receptor, is critical for patterning the ventral/distal portion of the arch (lower jaw) and also participates with Hox genes in patterning more posterior arches. Edn1/Ednra signaling is highly conserved between mouse and zebrafish, and genetic analyses in these two species have provided complementary insights into the patterning cues responsible for establishing the craniofacial complex as well as the genetic basis of facial birth defect syndromes.

Endothelin receptor type A expression defines a distinct cardiac subdomain within the heart field and is later implicated in chamber myocardium formation

The avian and mammalian heart originates from two distinct embryonic regions: an early differentiating first heart field and a dorsomedially located second heart field. It remains largely unknown when and how these subdivisions of the heart field divide into regions with different fates. Here, we identify in the mouse a subpopulation of the first (crescent-forming) field marked by endothelin receptor type A (Ednra) gene expression, which contributes to chamber myocardium through a unique type of cell behavior. Ednra-lacZ/EGFP-expressing cells arise in the ventrocaudal inflow region of the early linear heart tube, converge to the midline, move anteriorly along the outer curvature and give rise to chamber myocardium mainly of the left ventricle and both atria. This movement was confirmed by fluorescent dye-labeling and transplantation experiments. The Ednra-lacZ/EGFP-expressing subpopulation is characterized by the presence of Tbx5-expressing cells. Ednra-null embryonic hearts often demonstrate hypoplasia of the ventricular wall, low mitotic activity and decreased Tbx5 expression with reciprocal expansion of Tbx2 expression. Conversely, endothelin 1 stimulates ERK phosphorylation and Tbx5 expression in the early embryonic heart. These results indicate that early Ednra expression defines a subdomain of the first heart field contributing to chamber formation, in which endothelin 1/Ednra signaling is involved. The present finding provides an insight into how subpopulations within the crescent-forming (first) heart field contribute to the coordination of heart morphogenesis through spatiotemporally defined cell movements.

Molecular mechanisms of cranial neural crest cell migration and patterning in craniofacial development

During vertebrate craniofacial development, neural crest cells (NCCs) contribute much of the cartilage, bone and connective tissue that make up the developing head. Although the initial patterns of NCC segmentation and migration are conserved between species, the variety of vertebrate facial morphologies that exist indicates that a complex interplay occurs between intrinsic genetic NCC programs and extrinsic environmental signals during morphogenesis. Here, we review recent work that has begun to shed light on the molecular mechanisms that govern the spatiotemporal patterning of NCC-derived skeletal structures – advances that are central to understanding craniofacial development and its evolution.

Spatio-temporal dynamics of gene expression of the Edn1-Dlx5/6 pathway during development of the lower jaw

The morphogenesis of the vertebrate skull results from highly dynamic integrated processes involving the exchange of signals between the ectoderm, the endoderm, and cephalic neural crest cells (CNCCs). Before migration CNCCs are not committed to form any specific skull element, molecular signals exchanged in restricted regions of tissue interaction are crucial in providing positional identity to the CNCCs mesenchyme and activate the specific morphogenetic process of different skeletal components of the head. In particular, the endothelin-1 (Edn1)-dependent activation of Dlx5 and Dlx6 in CNCCs that colonize the first pharyngeal arch (PA1) is necessary and sufficient to specify maxillo-mandibular identity. Here, to better analyze the spatio-temporal dynamics of this process, we associate quantitative gene expression analysis with detailed examination of skeletal phenotypes resulting from combined allelic reduction of Edn1, Dlx5, and Dlx6. We show that Edn1-dependent and -independent regulatory pathways act at different developmental times in distinct regions of PA1. The Edn1-->Dlx5/6-->Hand2 pathway is already active at E9.5 during early stages of CNCCs colonization. At later stages (E10.5) the scenario is more complex: we propose a model in which PA1 is subdivided into four adjacent territories in which distinct regulations are taking place. This new developmental model may provide a conceptual framework to interpret the craniofacial malformations present in several mouse mutants and in human first arch syndromes. More in general, our findings emphasize the importance of quantitative gene expression in the fine control of morphogenetic events.

Efficient conditional and promoter-specific in vivo expression of cDNAs of choice by taking advantage of recombinase-mediated cassette exchange using FlEx gene traps

Recombinase-mediated cassette exchange (RMCE) exploits the possibility to unidirectionally exchange any genetic material flanked by heterotypic recombinase recognition sites (RRS) with target sites in the genome. Due to a limited number of available pre-fabricated target sites, RMCE in mouse embryonic stem (ES) cells has not been tapped to its full potential to date. Here, we introduce a universal system, which allows the targeted insertion of any given transcriptional unit into 85 742 previously annotated retroviral conditional gene trap insertions, representing 7013 independent genes in mouse ES cells, by RMCE. This system can be used to express any given cDNA under the control of endogenous trapped promoters in vivo, as well as for the generation of transposon ‘launch pads’ for chromosomal region-specific ‘Sleeping Beauty’ insertional mutagenesis. Moreover, transcription of the gene-of-interest is only activated upon Cre-recombinase activity, a feature that adds conditionality to this expression system, which is demonstrated in vivo. The use of the RMCE system presented in this work requires one single-cloning step followed by one overnight gateway clonase reaction and subsequent cassette exchange in ES cells with efficiencies of 40% in average.

Prx1 and Prx2 cooperatively regulate the morphogenesis of the medial region of the mandibular process

Mice lacking both Prx1 and Prx2 display severe abnormalities in the mandible. Our analysis showed that complete loss of Prx gene products leads to growth abnormalities in the mandibular processes evident as early as embryonic day (E) 10.5 associated with changes in the survival of the mesenchyme in the medial region. Changes in the gene expression in the medial and lateral regions were related to gradual loss of a subpopulation of mesenchyme in the medial region expressing eHand. Our analysis also showed that Prx gene products are required for the initiation and maintenance of chondrogenesis and terminal differentiation of the chondrocytes in the caudal and rostral ends of Meckel's cartilage. The fusion of the mandibular processes in the Prx1/Prx2 double mutants is caused by accelerated ossification. These observations together show that, during mandibular morphogenesis, Prx gene products play multiple roles including the cell survival, the region-specific terminal differentiation of Meckelian chondrocytes and osteogenesis.

Elucidating timing and function of endothelin-A receptor signaling during craniofacial development using neural crest cell-specific gene deletion and receptor antagonism

Cranial neural crest cells (NCCs) play an intimate role in craniofacial development. Multiple signaling cascades participate in patterning cranial NCCs, some of which are regulated by endothelin-A receptor (Ednra) signaling. Ednra(-/-) embryos die at birth from severe craniofacial defects resulting from disruption of neural crest cell patterning and differentiation. These defects include homeotic transformation of lower jaw structures into upper jaw-like structures, suggesting that some cephalic NCCs alter their "identity" in the absence of Ednra signaling. To elucidate the temporal necessity for Ednra signaling in vivo, we undertook two strategies. We first used a conditional knockout strategy in which mice containing a conditionally targeted Ednra allele (Ednra(fl)) were bred with mice from the Hand2-Cre and Wnt1-Cre transgenic mouse strains, two strains in which Cre expression occurs at different time periods within cranial NCCs. In our second approach, we used an Ednra-specific antagonist to treat wild type pregnant mice between embryonic days E8.0 and E10.0, a time frame encompassing the early migration and proliferation of cranial NCCs. The combined results suggest that Ednra function is crucial for NCC development between E8.25 and E9.0, a time period encompassing the arrival of NCCs in the arches and/or early post-migratory patterning. After this time period, Ednra signaling is dispensable. Interestingly, middle ear structures are enlarged and malformed in a majority of Ednra(fl/fl);Wnt1-Cre embryos, instead resembling structures found in extinct predecessors of mammals. These observations suggest that the advent of Ednra signaling in cranial NCCs may have been a crucial event in the evolution of the mammalian middle ear ossicles.

A high-speed congenic strategy using first-wave male germ cells

Background

In laboratory mice and rats, congenic breeding is essential for analyzing the genes of interest on specific genetic backgrounds and for analyzing quantitative trait loci. However, in theory it takes about 3–4 years to achieve a strain carrying about 99% of the recipient genome at the tenth backcrossing (N10). Even with marker-assisted selection, the so-called ‘speed congenic strategy’, it takes more than a year at N4 or N5.

Methodology/Principal Findings

Here we describe a new high-speed congenic system using round spermatids retrieved from immature males (22–25 days of age). We applied the technique to three genetically modified strains of mice: transgenic (TG), knockin (KI) and N-ethyl-N-nitrosourea (ENU)-induced mutants. The donor mice had mixed genetic backgrounds of C57BL/6 (B6)∶DBA/2 or B6∶129 strains. At each generation, males used for backcrossing were selected based on polymorphic marker analysis and their round spermatids were injected into B6 strain oocytes. Backcrossing was repeated until N4 or N5. For the TG and ENU-mutant strains, the N5 generation was achieved on days 188 and 190 and the proportion of B6-homozygous loci was 100% (74 markers) and 97.7% (172/176 markers), respectively. For the KI strain, N4 was achieved on day 151, all the 86 markers being B6-homozygous as early as on day 106 at N3. The carrier males at the final generation were all fertile and propagated the modified genes. Thus, three congenic strains were established through rapid generation turnover between 41 and 44 days.

Conclusions/Significance

This new high-speed breeding strategy enables us to produce congenic strains within about half a year. It should provide the fastest protocol for precise definition of the phenotypic effects of genes of interest on desired genetic backgrounds.

An endothelin-1 switch specifies maxillomandibular identity

Articulated jaws are highly conserved structures characteristic of gnathostome evolution. Epithelial-mesenchymal interactions within the first pharyngeal arch (PA1) instruct cephalic neural crest cells (CNCCs) to form the different skeletal elements of the jaws. The endothelin-1 (Edn1)/endothelin receptor type-A (Ednra)-->Dlx5/6-->Hand2 signaling pathway is necessary for lower jaw formation. Here, we show that the Edn1 signaling is sufficient for the conversion of the maxillary arch to mandibular identity. Constitutive activation of Ednra induced the transformation of upper jaw, maxillary, structures into lower jaw, mandibular, structures with duplicated Meckel's cartilage and dermatocranial jaws constituted by 4 dentary bones. Misexpression of Hand2 in the Ednra domain caused a similar transformation. Skeletal transformations are accompanied by neuromuscular remodeling. Ednra is expressed by most CNCCs, but its constitutive activation affects predominantly PA1. We conclude that after migration CNCCs are not all equivalent, suggesting that their specification occurs in sequential steps. Also, we show that, within PA1, CNCCs are competent to form both mandibular and maxillary structures and that an Edn1 switch is responsible for the choice of either morphogenetic program.