EphB2 and EphB3 forward signalling are required for palate development

The lack of EphB3 receptor prevents bone loss in mouse models of osteoporosis

Bone homeostasis is a complex process in which some Eph kinase receptors and their ephrin ligands appear to be involved. In the present study, we address this issue by examining, both in vitro and in vivo, the role of EphB2 and EphB3 in mesenchymal stromal/stem cell (MSC) differentiation into bone tissue. This was first evaluated by quantitative reverse transcription PCR (RT-qPCR) and histological staining in MSCs cultured in specific mediums revealing that although EphB2-/- MSCs mainly expressed pro-adipogenic transcription factors, EphB3-/- MSCs showed abundant osteogenic transcripts, such as Runx2, Msx2, and Sp7. To clarify the underlying molecular mechanisms, we found that the lack of EphB3 signaling alters the genetic profile of differentiating MSCs, reducing the expression of many inhibitory molecules and antagonists of the BMP signaling pathway, and increasing Bmp7 expression, a robust bone inductor. Then, to confirm the osteogenic role of EphB3 in vivo, we studied the condition of 2 mouse models of induced bone loss (ovariectomy or long-term glucocorticoid treatment). Interestingly, in both models, both WT and EphB2-/- mice equally developed the disease but EphB3-/- mice did not exhibit the typical bone loss, nor an increase in urine Ca2+ or blood serum CTX-1. This phenotype in EphB3-KO mice could be due to their significantly higher proportions of osteoprogenitor cells and preosteoblasts, and their lower number of osteoclasts, as compared with WT and EphB2-KO mice. Thus, we conclude that EphB3 acts as a negative regulator of the osteogenic differentiation, and its absence prevents bone loss in mice subjected to ovariectomy or dexamethasone treatment.

Genome-wide association analysis reveals 6 copy number variations associated with the number of cervical vertebrae in Pekin ducks

As a critical developmental stage in vertebrates, the vertebral column formation process is under strict control; however, we observed variations in the number of cervical vertebrae in duck populations in our previous study. Here, we further explored the variations in the number of vertebrae in two duck populations: 421 Pekin duck × mallard F2 ducks and 850 Pekin ducks. Using resequencing data of 125 Pekin ducks with different numbers of cervical vertebrae and 352 Pekin duck × mallard F2 ducks with different numbers of thoracic vertebrae, we detected whole-genome copy number variations (CNVs) and implemented a genome-wide association study (GWAS) to identify the genetic variants related to the traits. The findings verified the existence of variations in the number of cervical vertebrae in duck populations. The number of cervical vertebrae in most ducks was 15, while that in a small number of the ducks was 14 or 16. The number of cervical vertebrae had a positive influence on the neck production, and one cervical vertebra addition could increase 11 g or 2 cm of duck neck. Genome-wide CNV association analysis identified six CNVs associated with the number of cervical vertebrae, and the associated CNV regions covered 15 genes which included WNT10A and WNT6. These findings improve our understanding of the variations in the number of vertebrae in ducks and lay a foundation for future duck breeding.

Whole-genome sequencing reveals de-novo mutations associated with nonsyndromic cleft lip/palate

The majority (85%) of nonsyndromic cleft lip with or without cleft palate (nsCL/P) cases occur sporadically, suggesting a role for de novo mutations (DNMs) in the etiology of nsCL/P. To identify high impact protein-altering DNMs that contribute to the risk of nsCL/P, we conducted whole-genome sequencing (WGS) analyses in 130 African case-parent trios (affected probands and unaffected parents). We identified 162 high confidence protein-altering DNMs some of which are based on available evidence, contribute to the risk of nsCL/P. These include novel protein-truncating DNMs in the ACTL6A, ARHGAP10, MINK1, TMEM5 and TTN genes; as well as missense variants in ACAN, DHRS3, DLX6, EPHB2, FKBP10, KMT2D, RECQL4, SEMA3C, SEMA4D, SHH, TP63, and TULP4. Many of these protein-altering DNMs were predicted to be pathogenic. Analysis using mouse transcriptomics data showed that some of these genes are expressed during the development of primary and secondary palate. Gene-set enrichment analysis of the protein-altering DNMs identified palatal development and neural crest migration among the few processes that were significantly enriched. These processes are directly involved in the etiopathogenesis of clefting. The analysis of the coding sequence in the WGS data provides more evidence of the opportunity for novel findings in the African genome.

Investigating the effects of compound paralogous EPHB receptor mutations on mouse facial development

BACKGROUND

Variation in facial shape may arise from the combinatorial or overlapping actions of paralogous genes. Given its many members, and overlapping expression and functions, the EPH receptor family is a compelling candidate source of craniofacial morphological variation. We performed a detailed morphometric analysis of an allelic series of E14.5 Ephb1-3 receptor mutants to determine the effect of each paralogous receptor gene on craniofacial morphology.

RESULTS

We found that Ephb1, Ephb2, and Ephb3 genotypes significantly influenced facial shape, but Ephb1 effects were weaker than Ephb2 and Ephb3 effects. Ephb2^-/- and Ephb3^-/- mutations affected similar aspects of facial morphology, but Ephb3^-/- mutants had additional facial shape effects. Craniofacial differences across the allelic series were largely consistent with predicted additive genetic effects. However, we identified a potentially important nonadditive effect where Ephb1 mutants displayed different morphologies depending on the combination of other Ephb paralogs present, where Ephb1^+/- , Ephb1^-/- , and Ephb1^-/- ; Ephb3^-/- mutants exhibited a consistent deviation from their predicted facial shapes.

CONCLUSIONS

This study provides a detailed assessment of the effects of Ephb receptor gene paralogs on E14.5 mouse facial morphology and demonstrates how the loss of specific receptors contributes to facial dysmorphology.

Cellular and molecular mechanisms of EPH/EPHRIN signaling in evolution and development

The EPH receptor tyrosine kinases and their signaling partners, the EPHRINS, comprise a large class of cell signaling molecules that plays diverse roles in development. As cell membrane-anchored signaling molecules, they regulate cellular organization by modulating the strength of cellular contacts, usually by impacting the actin cytoskeleton or cell adhesion programs. Through these cellular functions, EPH/EPHRIN signaling often regulates tissue shape. Indeed, recent evidence indicates that this signaling family is ancient and associated with the origin of multicellularity. Though extensively studied, our understanding of the signaling mechanisms employed by this large family of signaling proteins remains patchwork, and a truly "canonical" EPH/EPHRIN signal transduction pathway is not known and may not exist. Instead, several foundational evolutionarily conserved mechanisms are overlaid by a myriad of tissue -specific functions, though common themes emerge from these as well. Here, I review recent advances and the related contexts that have provided new understanding of the conserved and varied molecular and cellular mechanisms employed by EPH/EPHRIN signaling during development.

EphB3 as a Potential Mediator of Developmental and Reparative Osteogenesis

The ephrin-B family of membrane-bound ligands is involved in skeletal patterning, osteogenesis, and bone homeostasis. Yet, despite the increasing collection of data affirming their importance in bone, the Eph tyrosine kinases that serve as the receptors for these ephrins in osteoblast stem cell niches remain unidentified. Here we report the expression of EphB3 at sites of bone growth in the embryo, especially at the calvaria suture fronts, periosteum, chondrocytes, and trabeculae of developing long bones. Strong EphB3 expression persisted in the adult calvarial sutures and in the proliferative chondrocytes of long bones, both of which are documented niches for osteoblastic stem cells. We observed EphB3-positive cells in the tissue filling a created calvarial injury, further implying EphB3 involvement in bone healing. Genetic knockout of EphB3 caused an increase in the bone tissue volume as a fraction of total volume in 6-week-old calvaria and in femoral trabecular density, compared to wild type controls. This difference resolved by 12 weeks of age, when we instead observed an increase in the bone volume of femoral trabeculae and in trabecular thickness. Our data identify EphB3 as a candidate regulator of osteogenesis either alone or in combination with other bone-expressed Ephs, and indicate that it appears to function as a limiter of bone growth.

Loss of coordinated expression between ribosomal and mitochondrial genes revealed by comprehensive characterization of a large family with a rare Mendelian disorder

Non-canonical intronic variants are a poorly characterized yet highly prevalent class of alterations associated with Mendelian disorders. Here, we report the first RNA expression and splicing analysis from a family whose members carry a non-canonical splice variant in an intron of RPL11 (c.396 +3A>G). This mutation is causative for Diamond Blackfan Anemia (DBA) in this family despite incomplete penetrance and variable expressivity. Our analyses revealed a complex pattern of disruptions with many novel junctions of RPL11. These include an RPL11 transcript that is translated with a late stop codon in the 3' untranslated region (3'UTR) of the main isoform. We observed that RPL11 transcript abundance is comparable among carriers regardless of symptom severity. Interestingly, both the small and large ribosomal subunit transcripts were significantly overexpressed in individuals with a history of anemia in addition to congenital abnormalities. Finally, we discovered that coordinated expression between mitochondrial components and RPL11 was lost in all carriers, which may lead to variable expressivity. Overall, this study highlights the importance of RNA splicing and expression analyses in families for molecular characterization of Mendelian diseases.

PIP2 promotes conformation-specific dimerization of the EphA2 membrane region

The impact of the EphA2 receptor on cancer malignancy hinges on the two different ways it can be activated. EphA2 induces antioncogenic signaling after ligand binding, but ligand-independent activation of EphA2 is pro-oncogenic. It is believed that the transmembrane (TM) domain of EphA2 adopts two alternate conformations in the ligand-dependent and the ligand-independent states. However, it is poorly understood how the difference in TM helical crossing angles found in the two conformations impacts the activity and regulation of EphA2. We devised a method that uses hydrophobic matching to stabilize two conformations of a peptide comprising the EphA2 TM domain and a portion of the intracellular juxtamembrane (JM) segment. The two conformations exhibit different TM crossing angles, resembling the ligand-dependent and ligand-independent states. We developed a single-molecule technique using styrene maleic acid lipid particles to measure dimerization in membranes. We observed that the signaling lipid PIP2 promotes TM dimerization, but only in the small crossing angle state, which we propose corresponds to the ligand-independent conformation. In this state the two TMs are almost parallel, and the positively charged JM segments are expected to be close to each other, causing electrostatic repulsion. The mechanism PIP2 uses to promote dimerization might involve alleviating this repulsion due to its high density of negative charges. Our data reveal a conformational coupling between the TM and JM regions and suggest that PIP2 might directly exert a regulatory effect on EphA2 activation in cells that is specific to the ligand-independent conformation of the receptor.

Aberrant cell segregation in the craniofacial primordium and the emergence of facial dysmorphology in craniofrontonasal syndrome

Craniofrontonasal syndrome (CFNS) is a rare X-linked disorder characterized by craniofacial, skeletal, and neurological anomalies and is caused by mutations in EFNB1. Heterozygous females are more severely affected by CFNS than hemizygous males, a phenomenon called cellular interference that results from EPHRIN-B1 mosaicism. In Efnb1 heterozygous mice, mosaicism for EPHRIN-B1 results in cell sorting and more severe phenotypes than Efnb1 hemizygous males, but how craniofacial dysmorphology arises from cell segregation is unknown and CFNS etiology therefore remains poorly understood. Here, we couple geometric morphometric techniques with temporal and spatial interrogation of embryonic cell segregation in mouse mutant models to elucidate mechanisms underlying CFNS pathogenesis. By generating EPHRIN-B1 mosaicism at different developmental timepoints and in specific cell populations, we find that EPHRIN-B1 regulates cell segregation independently in early neural development and later in craniofacial development, correlating with the emergence of quantitative differences in face shape. Whereas specific craniofacial shape changes are qualitatively similar in Efnb1 heterozygous and hemizygous mutant embryos, heterozygous embryos are quantitatively more severely affected, indicating that Efnb1 mosaicism exacerbates loss of function phenotypes rather than having a neomorphic effect. Notably, neural tissue-specific disruption of Efnb1 does not appear to contribute to CFNS craniofacial dysmorphology, but its disruption within neural crest cell-derived mesenchyme results in phenotypes very similar to widespread loss. EPHRIN-B1 can bind and signal with EPHB1, EPHB2, and EPHB3 receptor tyrosine kinases, but the signaling partner(s) relevant to CFNS are unknown. Geometric morphometric analysis of an allelic series of Ephb1; Ephb2; Ephb3 mutant embryos indicates that EPHB2 and EPHB3 are key receptors mediating Efnb1 hemizygous-like phenotypes, but the complete loss of EPHB1-3 does not fully recapitulate the severity of CFNS-like Efnb1 heterozygosity. Finally, by generating Efnb1+/Δ; Ephb1; Ephb2; Ephb3 quadruple knockout mice, we determine how modulating cumulative receptor activity influences cell segregation in craniofacial development and find that while EPHB2 and EPHB3 play an important role in craniofacial cell segregation, EPHB1 is more important for cell segregation in the brain; surprisingly, complete loss of EPHB1-EPHB3 does not completely abrogate cell segregation. Together, these data advance our understanding of the etiology and signaling interactions underlying CFNS dysmorphology.

Mouse embryonic palatal mesenchymal cells maintain stemness through the PTEN-Akt-mTOR autophagic pathway

BACKGROUND

Both genetic and environmental factors are implicated in the pathogenesis of cleft palate. However, the molecular and cellular mechanisms that regulate the development of palatal shelves, which are composed of mesenchymal cells, have not yet been fully elucidated. This study aimed to determine the stemness and multilineage differentiation potential of mouse embryonic palatal mesenchyme (MEPM) cells in palatal shelves and to explore the underlying regulatory mechanism associated with cleft palate formation.

METHODS

Palatal shelves excised from mice models were cultured in vitro to ascertain whether MEPM are stem cells through immunofluorescence and flow cytometry. The osteogenic, adipogenic, and chondrogenic differentiation potential of MEPM cells were also determined to characterize MEPM stemness. In addition, the role of the PTEN-Akt-mTOR autophagic pathway was investigated using quantitative RT-PCR, Western blotting, and transmission electron microscopy.

RESULTS

MEPM cells in culture exhibited cell surface marker expression profiles similar to that of mouse bone marrow stem cells and exhibited positive staining for vimentin (mesodermal marker), nestin (ectodermal marker), PDGFRα, Efnb1, Osr2, and Meox2 (MEPM cells markers). In addition, exposure to PDGFA stimulated chemotaxis of MEPM cells. MEPM cells exhibited stronger potential for osteogenic differentiation as compared to that for adipogenic and chondrogenic differentiation. Undifferentiated MEPM cells displayed a high concentration of autophagosomes, which disappeared after differentiation (at passage four), indicating the involvement of PTEN-Akt-mTOR signaling.

CONCLUSIONS

Our findings suggest that MEPM cells are ectomesenchymal stem cells with a strong osteogenic differentiation potential and that maintenance of their stemness via PTEN/AKT/mTOR autophagic signaling prevents cleft palate development.

Getting direction(s): The Eph/ephrin signaling system in cell positioning

In vertebrates, the Eph/ephrin family of signaling molecules is a large group of membrane-bound proteins that signal through a myriad of mechanisms and effectors to play diverse roles in almost every tissue and organ system. Though Eph/ephrin signaling has functions in diverse biological processes, one core developmental function is in the regulation of cell position and tissue morphology by regulating cell migration and guidance, cell segregation, and boundary formation. Often, the role of Eph/ephrin signaling is to translate patterning information into physical movement of cells and changes in morphology that define tissue and organ systems. In this review, we focus on recent advances in the regulation of these processes, and our evolving understanding of the in vivo signaling mechanisms utilized in distinct developmental contexts.

Cellular organization and boundary formation in craniofacial development

Craniofacial morphogenesis is a highly dynamic process that requires changes in the behaviors and physical properties of cells in order to achieve the proper organization of different craniofacial structures. Boundary formation is a critical process in cellular organization, patterning, and ultimately tissue separation. There are several recurring cellular mechanisms through which boundary formation and cellular organization occur including, transcriptional patterning, cell segregation, cell adhesion and migratory guidance. Disruption of normal boundary formation has dramatic morphological consequences, and can result in human craniofacial congenital anomalies. In this review we discuss boundary formation during craniofacial development, specifically focusing on the cellular behaviors and mechanisms underlying the self-organizing properties that are critical for craniofacial morphogenesis.

Role of IGF1 and EFN-EPH signaling in skeletal metabolism

Insulin-like growth factor 1(IGF1) and ephrin ligand (EFN)-receptor (EPH) signaling are both crucial for bone cell function and skeletal development and maintenance. IGF1 signaling is the major mediator of growth hormone-induced bone growth, but a host of different signals and factors regulate IGF1 signaling at the systemic and local levels. Disruption of the Igf1 gene results in reduced peak bone mass in both experimental animal models and humans. Additionally, EFN-EPH signaling is a complex system which, particularly through cell-cell interactions, contributes to the development and differentiation of many bone cell types. Recent evidence has demonstrated several ways in which the IGF1 and EFN-EPH signaling pathways interact with and depend upon each other to regulate bone cell function. While much remains to be elucidated, the interaction between these two signaling pathways opens a vast array of new opportunities for investigation into the mechanisms of and potential therapies for skeletal conditions such as osteoporosis and fracture repair.

Closing the Gap: Mouse Models to Study Adhesion in Secondary Palatogenesis

Secondary palatogenesis occurs when the bilateral palatal shelves (PS), arising from maxillary prominences, fuse at the midline, forming the hard and soft palate. This embryonic phenomenon involves a complex array of morphogenetic events that require coordinated proliferation, apoptosis, migration, and adhesion in the PS epithelia and underlying mesenchyme. When the delicate process of craniofacial morphogenesis is disrupted, the result is orofacial clefting, including cleft lip and cleft palate (CL/P). Through human genetic and animal studies, there are now hundreds of known genetic alternations associated with orofacial clefts; so, it is not surprising that CL/P is among the most common of all birth defects. In recent years, in vitro cell-based assays, ex vivo palate cultures, and genetically engineered animal models have advanced our understanding of the developmental and cell biological pathways that contribute to palate closure. This is particularly true for the areas of PS patterning and growth as well as medial epithelial seam dissolution during palatal fusion. Here, we focus on epithelial cell-cell adhesion, a critical but understudied process in secondary palatogenesis, and provide a review of the available tools and mouse models to better understand this phenomenon.

EphB3 protein is associated with histological grade and FIGO stage in ovarian serous carcinomas

Eph (Erythropoietin-producing human hepatocellular carcinoma cell) is the largest subfamily of receptor tyrosine kinases. Eph receptors and their ephrin ligands are involved in embryonic development and physiological processes. Aberrant expression of Eph/ephrin may contribute to a variety of diseases including cancer. EphB3 is a member of Eph receptors and has been found to play roles in carcinogenesis of some types of human cancer. But, its expression and clinical significance in ovarian serous carcinoma have not been well investigated and are unknown. In this study, a set of ovarian tissues including normal fallopian tube, serous borderline tumor, and serous carcinoma were subjected to immunohistochemistry using a specific polyclonal antibody for EphB3. The relationship between EphB3 expression and clinicopathological parameters was statistically analyzed. EphB3 was strongly expressed in all fallopian tube specimens (19/19, 100%). EphB3 was negatively or weekly expressed in 1 of 17 (5.8%) in borderline tumors and 26 of 50 (52.0%) in serous carcinomas, moderately expressed in 7 of 17 (41.2%) in borderline tumors and 14 of 50 (28%) in serous carcinomas, and strongly expressed in 9 17 (52.9%) in borderline tumors and 10 of 50 (20%) in serous carcinomas. EphB3 expression is significantly reduced in serous carcinomas compared with normal fallopian tubes and borderline tumors (p < 0.001). EphB3 expression is negatively associated with histological grade (p < 0.001, r_s = -0.613) and FIGO stage (p = 0.001, r_s = -0.464) of serous carcinomas. Our results show EphB3 protein lost in ovarian serous carcinoma and is associated with tumor grade and FIGO stage, which indicate EphB3 protein may play a role in carcinogenesis of ovarian serous carcinoma and may be used as a molecular marker for prognosis.

Pigeonetics takes flight: Evolution, development, and genetics of intraspecific variation

Intensive artificial selection over thousands of years has produced hundreds of varieties of domestic pigeon. As Charles Darwin observed, the morphological differences among breeds can rise to the magnitude of variation typically observed among different species. Nevertheless, different pigeon varieties are interfertile, thereby enabling forward genetic and genomic approaches to identify genes that underlie derived traits. Building on classical genetic studies of pigeon variation, recent molecular investigations find a spectrum of coding and regulatory alleles controlling derived traits, including plumage color, feather growth polarity, and limb identity. Developmental and genetic analyses of pigeons are revealing the molecular basis of variation in a classic example of extreme intraspecific diversity, and have the potential to nominate genes that control variation among other birds and vertebrates in general.

Unidirectional Eph/ephrin signaling creates a cortical actomyosin differential to drive cell segregation

Cell segregation is the process by which cells self-organize to establish developmental boundaries, an essential step in tissue formation. Cell segregation is a common outcome of Eph/ephrin signaling, but the mechanisms remain unclear. In craniofrontonasal syndrome, X-linked mosaicism for ephrin-B1 expression has been hypothesized to lead to aberrant Eph/ephrin-mediated cell segregation. Here, we use mouse genetics to exploit mosaicism to study cell segregation in the mammalian embryo and integrate live-cell imaging to examine the underlying cellular and molecular mechanisms. Our data demonstrate that dramatic ephrin-B1-mediated cell segregation occurs in the early neuroepithelium. In contrast to the paradigm that repulsive bidirectional signaling drives cell segregation, unidirectional EphB kinase signaling leads to cell sorting by the Rho kinase-dependent generation of a cortical actin differential between ephrin-B1- and EphB-expressing cells. These results define mechanisms of Eph/ephrin-mediated cell segregation, implicating unidirectional regulation of cortical actomyosin contractility as a key effector of this fundamental process.

Bidirectional ephrin signaling in bone

The interaction between ephrin ligands (efn) and their receptors (Eph) is capable of inducing forward signaling, from ligand to receptor, as well as reverse signaling, from receptor to ligand. The ephrins are widely expressed in many tissues, where they mediate cell migration and adherence, properties that make the efn-Eph signaling critically important in establishing and maintaining tissue boundaries. The efn-Eph system has also received considerable attention in skeletal tissues, as ligand and receptor combinations are predicted to mediate interactions between the different types of cells that regulate bone development and homeostasis. This review summarizes our current understanding of efn-Eph signaling with a particular focus on the expression and functions of ephrins and their receptors in bone.

Regulation of cell protrusions by small GTPases during fusion of the neural folds

Epithelial fusion is a crucial process in embryonic development, and its failure underlies several clinically important birth defects. For example, failure of neural fold fusion during neurulation leads to open neural tube defects including spina bifida. Using mouse embryos, we show that cell protrusions emanating from the apposed neural fold tips, at the interface between the neuroepithelium and the surface ectoderm, are required for completion of neural tube closure. By genetically ablating the cytoskeletal regulators Rac1 or Cdc42 in the dorsal neuroepithelium, or in the surface ectoderm, we show that these protrusions originate from surface ectodermal cells and that Rac1 is necessary for the formation of membrane ruffles which typify late closure stages, whereas Cdc42 is required for the predominance of filopodia in early neurulation. This study provides evidence for the essential role and molecular regulation of membrane protrusions prior to fusion of a key organ primordium in mammalian development.

DOI:http://dx.doi.org/10.7554/eLife.13273.001

The neural tube is an embryonic structure that gives rise to the brain and spinal cord. It originates from a flat sheet of cells – the neural plate – that rolls up and fuses to form a tube during development. If this closure fails, it leads to birth defects such as spina bifida, a condition that causes severe disability because babies are born with an exposed and damaged spinal cord.

As the edges of the neural plate meet, they need to fuse together to produce a closed tube. It was known that cells at these edges extend protrusions. However, it was unclear how these protrusions are regulated, whether they arise from neural or non-neural cells and whether or not they are required for the neural tube to close fully.

By studying mutant mouse embryos, Rolo et al. found that cellular protrusions are indeed required for the neural tube to close completely. These protrusions proved to be regulated by proteins called Rac1 and Cdc42, which control the filaments inside the cell that are responsible for cell shape and movement. Rolo et al. also found that the cells that give rise to the protrusions are not part of the neural plate itself. Instead, these cells are neighboring cells from the layer that later forms the epidermis of the skin (the surface ectoderm).

Future studies will need to investigate which signals instruct those precise cells to make protrusions and to discover what happens to the protrusions after contact is made with cells on the opposite side. It will also be important to determine whether spina bifida may arise in humans if the protrusions are defective or absent.

DOI:http://dx.doi.org/10.7554/eLife.13273.002

Ephrin Ligands and Eph Receptors Show Regionally Restricted Expression in the Developing Palate and Tongue

The Eph family receptor-interacting (ephrin) ligands and erythropoietin-producing hepatocellular carcinoma (Eph) receptors constitute the largest known family of receptor tyrosine kinases. Ephrin ligands and their receptors form an important cell communication system with widespread roles in normal physiology and disease pathogenesis. In order to investigate potential roles of the ephrin-Eph system during palatogenesis and tongue development, we have characterized the cellular mRNA expression of family members EphrinA1-A3, EphA1–A8, and EphrinB2, EphB1, EphB4 during murine embryogenesis between embryonic day 13.5–16.5 using radioactive in situ hybridization. With the exception of EphA6 and ephrinA3, all genes were regionally expressed during the process of palatogenesis, with restricted and often overlapping domains. Transcripts were identified in the palate epithelium, localized at the tip of the palatal shelves, in the mesenchyme and also confined to the medial epithelium seam. Numerous Eph transcripts were also identified during tongue development. In particular, EphA1 and EphA2 demonstrated a highly restricted and specific expression in the tongue epithelium at all stages examined, whereas EphA3 was strongly expressed in the lateral tongue mesenchyme. These results suggest regulatory roles for ephrin-EphA signaling in development of the murine palate and tongue.

Regulation of cell differentiation by Eph receptor and ephrin signaling

There is increasing evidence that in addition to having major roles in morphogenesis, in some tissues Eph receptor and ephrin signaling regulates the differentiation of cells. In one mode of deployment, cell contact dependent Eph-ephrin activation induces a distinct fate of cells at the interface of their expression domains, for example in early ascidian embryos and in the vertebrate hindbrain. In another mode, overlapping Eph receptor and ephrin expression underlies activation within a cell population, which promotes or inhibits cell differentiation in bone remodelling, neural progenitors and keratinocytes. Eph-ephrin activation also contributes to formation of the appropriate number of progenitor cells by increasing or decreasing cell proliferation. These multiple roles of Eph receptor and ephrin signaling may enable a coupling between morphogenesis and the differentiation and proliferation of cells.

Convergent Evolution of Head Crests in Two Domesticated Columbids Is Associated with Different Missense Mutations in EphB2

Head crests are important display structures in wild bird species and are also common in domesticated lineages. Many breeds of domestic rock pigeon (Columba livia) have crests of reversed occipital feathers, and this recessive trait is associated with a nonsynonymous coding mutation in the intracellular kinase domain of EphB2 (Ephrin receptor B2). The domestic ringneck dove (Streptopelia risoria) also has a recessive crested morph with reversed occipital feathers, and interspecific crosses between crested doves and pigeons produce crested offspring, suggesting a similar genetic basis for this trait in both species. We therefore investigated EphB2 as a candidate for the head crest phenotype of ringneck doves and identified a nonsynonymous coding mutation in the intracellular kinase domain that is significantly associated with the crested morph. This mutation is over 100 amino acid positions away from the crest mutation found in rock pigeons, yet both mutations are predicted to negatively affect the function of ATP-binding pocket. Furthermore, bacterial toxicity assays suggest that "crest" mutations in both species severely impact kinase activity. We conclude that head crests are associated with different mutations in the same functional domain of the same gene in two different columbid species, thereby representing striking evolutionary convergence in morphology and molecules.

TGFβ3 regulates periderm removal through ΔNp63 in the developing palate

The periderm is a flat layer of epithelium created during embryonic development. During palatogenesis, the periderm forms a protective layer against premature adhesion of the oral epithelia, including the palate. However, the periderm must be removed in order for the medial edge epithelia (MEE) to properly adhere and form a palatal seam. Improper periderm removal results in a cleft palate. Although the timing of transforming growth factor β3 (TGFβ3) expression in the MEE coincides with periderm degeneration, its role in periderm desquamation is not known. Interestingly, murine models of knockout (-/-) TGFβ3, interferon regulatory factor 6 (IRF6) (-/-), and truncated p63 (ΔNp63) (-/-) are born with palatal clefts because of failure of the palatal shelves to adhere, suggesting that these genes regulate palatal epithelial differentiation. However, despite having similar phenotypes in null mouse models, no studies have analyzed the possible association between the TGFβ3 signaling cascade and the IRF6/ΔNp63 genes during palate development. Recent studies indicate that regulation of ΔNp63, which depends on IRF6, facilitates epithelial differentiation. We performed biochemical analysis, gene activity and protein expression assays with palatal sections of TGFβ3 (-/-), ΔNp63 (-/-), and wild-type (WT) embryos, and primary MEE cells from WT palates to analyze the association between TGFβ3 and IRF6/ΔNp63. Our results suggest that periderm degeneration depends on functional TGFβ3 signaling to repress ΔNp63, thereby coordinating periderm desquamation. Cleft palate occurs in TGFβ3 (-/-) because of inadequate periderm removal that impedes palatal seam formation, while cleft palate occurs in ΔNp63 (-/-) palates because of premature fusion.

Investigation of the interactions between the EphB2 receptor and SNEW peptide variants

EphB2 interacts with cell surface-bound ephrin ligands to relay bidirectional signals. Overexpression of the EphB2 receptor protein has been linked to different types of cancer. The SNEW (SNEWIQPRLPQH) peptide binds with high selectivity and moderate affinity to EphB2, inhibiting Eph-ephrin interactions by competing with ephrin ligands for the EphB2 high-affinity pocket. We used rigorous free energy perturbation (FEP) calculations to re-evaluate the binding interactions of SNEW peptide with the EphB2 receptor, followed by experimental testing of the computational results. Our results provide insight into dynamic interactions of EphB2 with SNEW peptide. While the first four residues of the SNEW peptide are already highly optimized, change of the C-terminal end of the peptide has the potential to improve SNEW-binding affinity. We identified a PXSPY motif that can be similarly aligned with several other EphB2-binding peptides.

Embryonic expression of EphA receptor genes in mice supports their candidacy for involvement in cleft lip and palate

BACKGROUND

Eph receptors, comprising the A- and B-subfamilies, are the largest family of receptor tyrosine kinases in the mammalian genome, and their function is critical for morphogenesis in a variety of contexts. Whereas signaling through B-type Ephs has been demonstrated to play a role in cleft lip and palate (CL/P), the involvement of A-type Ephs has not been examined in this context notwithstanding a recent genome-wide association study that identified the EPHA3 locus as a candidate for non-syndromic CL/P.

RESULTS

Here, we present a systematic analysis of the gene expression patterns for the nine EphA receptors at progressive stages of mouse development and find that EphA3, EphA4, and EphA7 exhibit restricted overlapping patterns of expression during palate development. We find that homozygous mutation of EphA3 or compound homozygous mutation of EphA3 and EphA4 in mice does not result in defective midfacial development, supporting the possibility of redundant function with EphA7. We also document previously undescribed expression patterns in other tissues of the craniofacial complex including the lacrimal duct and salivary glands.

CONCLUSIONS

Together, these results are consistent with the hypothesis that mutations in EPHA family genes may cause CL/P and also suggest that functional redundancy between family members may be at play.

De novo 2q36.1q36.3 interstitial deletion involving the PAX3 and EPHA4 genes in a fetus with spina bifida and cleft palate

BACKGROUND

Interstitial 2q36 deletion is a rare event. Only two previously published cases of 2q36 deletions were characterized using array-CGH. This is the first case diagnosed prenatally.

METHODS

We report on the prenatal diagnosis of a 2q36.1q36.3 interstitial deletion in a fetus with facial dysmorphism, spina bifida, and cleft palate.

RESULTS

Array-CGH analysis revealed a 5.6 Mb interstitial deletion of the long arm of chromosome 2q36.1q36.3, including the PAX3 and EPHA4 genes.

CONCLUSION

The present study reinforces the hypothesis that PAX3 haploinsufficiency may be associated with neural tube defects in humans and suggests that the EPHA4 gene might be implicated during palate development. This report also illustrates the added value of array-CGH to detect cryptic chromosomal imbalances in malformed fetuses and to improve genetic counseling prenatally.

A chemical genetic approach reveals distinct EphB signaling mechanisms during brain development

EphB receptor tyrosine kinases control multiple steps in nervous system development. However, it remains unclear whether EphBs regulate these different developmental processes directly or indirectly. In addition, as EphBs signal through multiple mechanisms, it has been challenging to define which signaling functions of EphBs regulate particular developmental events. To address these issues, we engineered triple knockin mice in which the kinase activity of three neuronally expressed EphBs can be rapidly, reversibly, and specifically blocked. Using these mice we demonstrate that the tyrosine kinase activity of EphBs is required for axon guidance in vivo. By contrast, EphB-mediated synaptogenesis occurs normally when the kinase activity of EphBs is inhibited suggesting that EphBs mediate synapse development by an EphB tyrosine kinase-independent mechanism. Taken together, these experiments reveal that EphBs control axon guidance and synaptogenesis by distinct mechanisms, and provide a new mouse model for dissecting EphB function in development and disease.

Ephrin-B stimulation of calvarial bone formation

INTRODUCTION

Ephrin-B2 on osteoclasts was reported to promote bone formation as part of homeostasis by activating the EphB4 tyrosine kinase receptor on osteoblasts. Little is known about the role of ephrin-B signaling to EphBs in developmental bone formation.

RESULTS

We observed expression of an ephrin-B2 LacZ chimeric allele in the periosteum, sutural bone fronts, and dura mater of embryonic and neonatal mice. Expression in the adult skull was confined to sutures, but was heavily upregulated at sites of bone injury. Culture of embryonic calvariae with soluble recombinant ephrin-B2/Fc doubled their bone content without altering suture width or overall skull morphology. Ephrin-B2/Fc also stimulated osteoblast marker gene expression in cultured MC3T3 preosteoblastic cells without the need for type 1 collagen-induced differentiation. EphB4 was absent in embryonic and adult skulls. However, EphB1 and EphB2, both physiological receptors for ephrin-Bs, were expressed at sites of osteogenesis, and EphB1 knockout mice displayed a reduction in calvarial bone content compared to controls.

CONCLUSIONS

These data support a role for ephrin-B2 in the development and healing of bone through activation of osteoblast-specific gene expression. EphB1 and EphB2 are likely candidates receptors for the ephrin-B2 in bone.

Epithelial fusion during neural tube morphogenesis

Adhesion and fusion of epithelial sheets marks the completion of many morphogenetic events during embryogenesis. Neural tube closure involves an epithelial fusion sequence in which the apposing neural folds adhere initially via cellular protrusions, proceed to a more stable union, and subsequently undergo remodeling of the epithelial structures to yield a separate neural tube roof plate and overlying nonneural ectoderm. Cellular protrusions comprise lamellipodia and filopodia, and studies in several different systems emphasize the critical role of RhoGTPases in their regulation. How epithelia establish initial adhesion is poorly understood but, in neurulation, may involve interactions between EphA receptors and their ephrinA ligands. Epithelial remodeling is spatially and temporally correlated with apoptosis in the dorsal neural tube midline, but experimental inhibition of this cell death does not prevent fusion and remodeling. A variety of molecular signaling systems have been implicated in the late events of morphogenesis, but genetic redundancy, for example among the integrins and laminins, makes identification of the critical players challenging. An improved understanding of epithelial fusion can provide insights into normal developmental processes and may also indicate the mode of origin of clinically important birth defects.

Ephrin regulation of palate development

Studies of palate development are motivated by the all too common incidence of cleft palate, a birth defect that imposes a tremendous health burden and can leave lasting disfigurement. Although, mechanistic studies of palate growth and fusion have focused on growth factors such as Transforming Growth Factor ß-3 (Tgfß3), recent studies have revealed that the ephrin family of membrane bound ligands and their receptors, the Ephs, play central roles in palatal morphogenesis, growth, and fusion. In this mini-review, we will discuss the recent findings by our group and others on the functions of ephrins in palatal development.

Mechanisms of tissue fusion during development

Tissue fusion events during embryonic development are crucial for the correct formation and function of many organs and tissues, including the heart, neural tube, eyes, face and body wall. During tissue fusion, two opposing tissue components approach one another and integrate to form a continuous tissue; disruption of this process leads to a variety of human birth defects. Genetic studies, together with recent advances in the ability to culture developing tissues, have greatly enriched our knowledge of the mechanisms involved in tissue fusion. This review aims to bring together what is currently known about tissue fusion in several developing mammalian organs and highlights some of the questions that remain to be addressed.

Bone cell interactions through Eph/ephrin: bone modeling, remodeling and associated diseases

Bones cannot properly form or be maintained without cell-cell interactions through ephrin ligands and Eph receptors. Cell culture analysis and evaluation of genetic mouse models and human diseases reveal various ephrins and Eph functions in the skeletal system. Migration, attachment and spreading of mesenchymal stem cells are regulated by ephrinB ligands and EphB receptors. ephrinB1 loss-of-function is associated with craniofrontonasal syndrome (CFNS) in humans and mice. In bone remodeling, ephrinB2 is postulated to act as a “coupling stimulator.” In that case, bidirectional signaling between osteoclastic ephrinB2 and osteoblastic EphB4 suppresses osteoclastic bone resorption and enhances osteoblastic bone formation, facilitating the transition between these two states. Parathyroid hormone (PTH) induces ephrinB2 in osteoblasts and enhances osteoblastic bone formation. In contrast to ephrinB2, ephrinA2 acts as a “coupling inhibitor,” since ephrinA2 reverse signaling into osteoclasts enhances osteoclastogenesis and EphA2 forward signaling into osteoblasts suppresses osteoblastic bone formation and mineralization. Furthermore, ephrins and Ephs likely modulate pathological conditions such as osteoarthritis, rheumatoid arthritis, multiple myeloma and osteosarcoma. This review focuses on ephrin/Eph-mediated cell-cell interactions in bone biology.

Palatogenesis: morphogenetic and molecular mechanisms of secondary palate development

Mammalian palatogenesis is a highly regulated morphogenetic process during which the embryonic primary and secondary palatal shelves develop as outgrowths from the medial nasal and maxillary prominences, respectively, remodel and fuse to form the intact roof of the oral cavity. The complexity of control of palatogenesis is reflected by the common occurrence of cleft palate in humans. Although the embryology of the palate has long been studied, the past decade has brought substantial new knowledge of the genetic control of secondary palate development. Here, we review major advances in the understanding of the morphogenetic and molecular mechanisms controlling palatal shelf growth, elevation, adhesion and fusion, and palatal bone formation.

Eph/ephrin signaling: genetic, phosphoproteomic, and transcriptomic approaches

The Eph receptor tyrosine kinases and their ephrin partners compose a large and complex family of signaling molecules involved in a wide variety of processes in development, homeostasis, and disease. The complexity inherent to Eph/ephrin signaling derives from several characteristics of the family. First, the large size and functional redundancy/compensation by family members presents a challenge in defining their in vivo roles. Second, the capacity for bidirectional signaling doubles the potential complexity, since every member has the ability to act both as a ligand and a receptor. Third, Ephs and ephrins can utilize a wide array of signal transduction pathways with a tremendous diversity of cell biological effect. The daunting complexity of Eph/ephrin signaling has increasingly prompted investigators to resort to multiple technological approaches to gain mechanistic insight. Here we review recent progress in the use of advanced mouse genetics in combination with proteomic and transcriptomic approaches to gain a more complete understanding of signaling mechanism in vivo. Integrating insights from such disparate approaches provides advantages in continuing to advance our understanding of how this multifarious group of signaling molecules functions in a diverse array of biological contexts.

Effects of phenytoin and Echinacea purpurea extract on proliferation and apoptosis of mouse embryonic palatal mesenchymal cells

Cleft palate is one of the most common birth defects. Several environment factors are involved in the disorder, such as smoking, vitamin deficiency and teratogens. We investigated the teratogenic agent phenytoin and extract of the immunostimulant Echinacea purpurea in the etiology of cleft palate associated with the proliferation and apoptosis of mouse embryonic palatal mesenchymal (MEPM) cells. We measured the effects of phenytoin, E. purpurea extract, and the mixture of phenytoin and E. purpurea extract on the cell viability of MEPM cells by CCK-8 assay and on the proliferation and apoptosis of MEPM cells by BrdU labeling assay, flow cytometry, and TUNEL assay. Exposure to phenytoin for 24 h inhibited cell proliferation and increased cell apoptosis of MEPM cells, and E. purpurea extract had the reverse effect. Importantly, treatment with the mixture of phenytoin and E. purpurea extract increased the proliferation and decreased the apoptosis of MEPM cells as compared with treatment with phenytoin alone. The teratogenic effect of phenytoin on cleft palate is associated with the proliferation and apoptosis of MEPM cells, and E. purpurea extract may have a protective effect.

Ephrin reverse signaling controls palate fusion via a PI3 kinase-dependent mechanism

Secondary palate fusion requires adhesion and epithelial-to-mesenchymal transition (EMT) of the epithelial layers on opposing palatal shelves. This EMT requires transforming growth factor β3 (TGFβ3), and its failure results in cleft palate. Ephrins, and their receptors, the Ephs, are responsible for migration, adhesion, and midline closure events throughout development. Ephrins can also act as signal-transducing receptors in these processes, with the Ephs serving as ligands (termed "reverse" signaling). We found that activation of ephrin reverse signaling in chicken palates induced fusion in the absence of TGFβ3, and that PI3K inhibition abrogated this effect. Further, blockage of reverse signaling inhibited TGFβ3-induced fusion in the chicken and natural fusion in the mouse. Thus, ephrin reverse signaling is necessary and sufficient to induce palate fusion independent of TGFβ3. These data describe both a novel role for ephrins in palate morphogenesis, and a previously unknown mechanism of ephrin signaling.

Ephrin-B reverse signaling controls septation events at the embryonic midline through separate tyrosine phosphorylation-independent signaling avenues

We report that the disruption of bidirectional signaling between ephrin-B2 and EphB receptors impairs morphogenetic cell-cell septation and closure events during development of the embryonic midline. A novel role for reverse signaling is identified in tracheoesophageal foregut septation, as animals lacking the cytoplasmic domain of ephrin-B2 present with laryngotracheoesophageal cleft (LTEC), while both EphB2/EphB3 forward signaling and ephrin-B2 reverse signaling are shown to be required for midline fusion of the palate. In a third midline event, EphB2/EphB3 are shown to mediate ventral abdominal wall closure by acting principally as ligands to stimulate ephrin-B reverse signaling. Analysis of new ephrin-B2(6YFΔV) and ephrin-B2(ΔV) mutants that specifically ablate ephrin-B2 tyrosine phosphorylation- and/or PDZ domain-mediated signaling indicates there are at least two distinct phosphorylation-independent components of reverse signaling. These involve both PDZ domain interactions and a non-canonical SH2/PDZ-independent form of reverse signaling that may utilize associations with claudin family tetraspan molecules, as EphB2 and activated ephrin-B2 molecules are specifically co-localized with claudins in epithelia at the point of septation. Finally, the developmental phenotypes described here mirror common human midline birth defects found with the VACTERL association, suggesting a molecular link to bidirectional signaling through B-subclass Ephs and ephrins.

Occurrence of cleft-palate and alteration of Tgf-β(3) expression and the mechanisms leading to palatal fusion in mice following dietary folic-acid deficiency

Folic acid (FA) is essential for numerous bodily functions. Its decrease during pregnancy has been associated with an increased risk of congenital malformations in the progeny. The relationship between FA deficiency and the appearance of cleft palate (CP) is controversial, and little information exists on a possible effect of FA on palate development. We investigated the effect of a 2-8 weeks' induced FA deficiency in female mice on the development of CP in their progeny as well as the mechanisms leading to palatal fusion, i.e. cell proliferation, cell death, and palatal-shelf adhesion and fusion. We showed that an 8 weeks' maternal FA deficiency caused complete CP in the fetuses although a 2 weeks' maternal FA deficiency was enough to alter all the mechanisms analyzed. Since transforming growth factor-β(3) (TGF-β(3)) is crucial for palatal fusion and since most of the mechanisms impaired by FA deficiency were also observed in the palates of Tgf-β(3)null mutant mice, we investigated the presence of TGF-β(3) mRNA, its protein and phospho-SMAD2 in FA-deficient (FAD) mouse palates. Our results evidenced a large reduction in Tgf-β(3) expression in palates of embryos of dams fed an FAD diet for 8 weeks; Tgf-β(3) expression was less reduced in palates of embryos of dams fed an FAD diet for 2 weeks. Addition of TGF-β(3) to palatal-shelf cultures of embryos of dams fed an FAD diet for 2 weeks normalized all the altered mechanisms. Thus, an insufficient folate status may be a risk factor for the development of CP in mice, and exogenous TGF-β(3) compensates this deficit in vitro.

Ephrin-B1 forward signaling regulates craniofacial morphogenesis by controlling cell proliferation across Eph-ephrin boundaries

Mutations in the X-linked human EPHRIN-B1 gene result in cleft palate and other craniofacial anomalies as part of craniofrontonasal syndrome (CFNS), but the molecular and developmental mechanisms by which ephrin-B1 controls the underlying developmental processes are not clear. Here we demonstrate that ephrin-B1 plays an intrinsic role in palatal shelf outgrowth in the mouse by regulating cell proliferation in the anterior palatal shelf mesenchyme. In ephrin-B1 heterozygous mutants, X inactivation generates ephrin-B1-expressing and -nonexpressing cells that sort out, resulting in mosaic ephrin-B1 expression. We now show that this process leads to mosaic disruption of cell proliferation and post-transcriptional up-regulation of EphB receptor expression through relief of endocytosis and degradation. The alteration in proliferation rates resulting from ectopic Eph-ephrin expression boundaries correlates with the more severe dysmorphogenesis of ephrin-B1(+/-) heterozygotes that is a hallmark of CFNS. Finally, by integrating phosphoproteomic and transcriptomic approaches, we show that ephrin-B1 controls proliferation in the palate by regulating the extracellular signal-regulated kinase/mitogen-activated protein kinase (ERK/MAPK) signal transduction pathway.

Identification of EphrinB1 expression in prostatic mesenchyme and a role for EphB-EphrinB signalling in prostate development

Paracrine signalling from mesenchyme to epithelium plays a key role in regulating prostate organogenesis and it is important to identify the mesenchymally expressed molecules that regulate organ growth, though currently few such molecules are known. Tyrosine kinase signalling via EphB receptors has been characterised in many developmental processes, and EphB3 mRNA expression was detected in prostate inductive mesenchyme in previous gene profiling studies. This led us to examine the expression and function of EphrinB signalling in prostate development, to determine if EphrinB ligands might function as mesenchymal paracrine regulators of prostate growth. Using PCR, wholemount in situ hybridisation, and immunohistochemistry we examined the expression of EphB receptors and EphrinB ligands in rat prostate during development to determine which showed mesenchymal expression. EphB3 and EphrinB1 transcripts and proteins were expressed in the mesenchyme of developing prostate and in female urogenital mesenchyme and smooth muscle. The function of EphrinB signalling was examined using in vitro organ culture assays of ventral prostate (VP), which were treated with EphB3-Fc and EphrinB1-Fc proteins to inhibit or augment Ephrin signalling. Addition of recombinant EphB3-Fc resulted in a significant decrease in VP organ size, while recombinant EphrinB1-Fc resulted in a significant increase in VP organ size and epithelial proliferation. Additionally, EphrinB1-Fc reduced the degree of epithelial branching in VP organs and increased ductal tip size, though without disrupting normal differentiation. We have identified expression of EphrinB1 in prostatic mesenchyme and suggest that the EphrinB signalling system acts as a regulator of prostate growth. EphrinB-EphB signalling may function as an autocrine regulator of mesenchyme and/or as a paracrine regulator of epithelia.

Expression and requirement of T-box transcription factors Tbx2 and Tbx3 during secondary palate development in the mouse

Formation of the mammalian secondary palate is a highly regulated and complex process. Impairment of the underlying cellular and molecular programs often results in cleft palate, a common birth defect in mammals. Here we report that Tbx2 and Tbx3, two closely related genes encoding T-box transcription factors, are expressed in the mesenchyme of the mouse palatal structures during development. Mice homozygous mutant for Tbx2 and mice double heterozygous for Tbx2 and Tbx3 exhibit a cleft palate phenotype arguing for an important contribution of Tbx2 and Tbx3 to palatogenesis. In Tbx2-deficient embryos, the bilateral primordial palatal shelves form but are smaller and retarded in the outgrowth process. They do not make contact but retain the potential to fuse. Development of other craniofacial structures appears normal, suggesting that impaired palate formation in Tbx2-mutant mice is caused by a primary defect in the palatal shelf mesenchyme. This is further supported by increased cell proliferation and apoptosis accompanied by increased expression of Bmp4 and CyclinD1 in Tbx2-deficient palatal shelves. Hence, Tbx2 and Tbx3 function overlappingly to control growth of the palatal shelf mesenchyme.

Ephrin-B1 regulates axon guidance by reverse signaling through a PDZ-dependent mechanism

Mutations in the ephrin-B1 gene result in craniofrontonasal syndrome (CFNS) in humans, a congenital disorder that includes a wide range of craniofacial, skeletal, and neurological malformations. In addition to the ability of ephrin-B1 to forward signal through its cognate EphB tyrosine kinase receptors, ephrin-B1 can also act as a receptor and transduce a reverse signal by either PDZ-dependent or phosphorylation-dependent mechanisms. To investigate how ephrin-B1 acts to influence development and congenital disease, we generated mice harboring a series of targeted point mutations in the ephrin-B1 gene that independently ablate specific reverse signaling pathways, while maintaining forward signaling capacity. We demonstrate that both PDZ and phosphorylation-dependent reverse signaling by ephrin-B1 are dispensable for craniofacial and skeletal development, whereas PDZ-dependent reverse signaling by ephrin-B1 is critical for the formation of a major commissural axon tract, the corpus callosum. Ephrin-B1 is strongly expressed within axons of the corpus callosum, and reverse signaling acts autonomously in cortical axons to mediate an avoidance response to its signaling partner EphB2. These results demonstrate the importance of PDZ-dependent reverse signaling for a subset of Ephrin-B1 developmental roles in vivo.