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

Eph-ephrin signaling couples endothelial cell sorting and arterial specification

Cell segregation allows the compartmentalization of cells with similar fates during morphogenesis, which can be enhanced by cell fate plasticity in response to local molecular and biomechanical cues. Endothelial tip cells in the growing retina, which lead vessel sprouts, give rise to arterial endothelial cells and thereby mediate arterial growth. Here, we have combined cell type-specific and inducible mouse genetics, flow experiments in vitro, single-cell RNA sequencing and biochemistry to show that the balance between ephrin-B2 and its receptor EphB4 is critical for arterial specification, cell sorting and arteriovenous patterning. At the molecular level, elevated ephrin-B2 function after loss of EphB4 enhances signaling responses by the Notch pathway, VEGF and the transcription factor Dach1, which is influenced by endothelial shear stress. Our findings reveal how Eph-ephrin interactions integrate cell segregation and arteriovenous specification in the vasculature, which has potential relevance for human vascular malformations caused by EPHB4 mutations.

Ubiquitin ligase and signalling hub MYCBP2 is required for efficient EPHB2 tyrosine kinase receptor function

Eph receptor tyrosine kinases participate in a variety of normal and pathogenic processes during development and throughout adulthood. This versatility is likely facilitated by the ability of Eph receptors to signal through diverse cellular signalling pathways: primarily by controlling cytoskeletal dynamics, but also by regulating cellular growth, proliferation, and survival. Despite many proteins linked to these signalling pathways interacting with Eph receptors, the specific mechanisms behind such links and their coordination remain to be elucidated. In a proteomics screen for novel EPHB2 multi-effector proteins, we identified human MYC binding protein 2 (MYCBP2 or PAM or Phr1). MYCBP2 is a large signalling hub involved in diverse processes such as neuronal connectivity, synaptic growth, cell division, neuronal survival, and protein ubiquitination. Our biochemical experiments demonstrate that the formation of a complex containing EPHB2 and MYCBP2 is facilitated by FBXO45, a protein known to select substrates for MYCBP2 ubiquitin ligase activity. Formation of the MYCBP2-EPHB2 complex does not require EPHB2 tyrosine kinase activity and is destabilised by binding of ephrin-B ligands, suggesting that the MYCBP2-EPHB2 association is a prelude to EPHB2 signalling. Paradoxically, the loss of MYCBP2 results in increased ubiquitination of EPHB2 and a decrease of its protein levels suggesting that MYCBP2 stabilises EPHB2. Commensurate with this effect, our cellular experiments reveal that MYCBP2 is essential for efficient EPHB2 signalling responses in cell lines and primary neurons. Finally, our genetic studies in Caenorhabditis elegans provide in vivo evidence that the ephrin receptor VAB-1 displays genetic interactions with known MYCBP2 binding proteins. Together, our results align with the similarity of neurodevelopmental phenotypes caused by MYCBP2 and EPHB2 loss of function, and couple EPHB2 to a signalling effector that controls diverse cellular functions.

Ubiquitin ligase and signalling hub MYCBP2 is required for efficient EPHB2 tyrosine kinase receptor function

Eph receptor tyrosine kinases participate in a variety of normal and pathogenic processes during development and throughout adulthood. This versatility is likely facilitated by the ability of Eph receptors to signal through diverse cellular signalling pathways: primarily by controlling cytoskeletal dynamics, but also by regulating cellular growth, proliferation, and survival. Despite many proteins linked to these signalling pathways interacting with Eph receptors, the specific mechanisms behind such links and their coordination remain to be elucidated. In a proteomics screen for novel EPHB2 multi-effector proteins, we identified human MYC binding protein 2 (MYCBP2 or PAM or Phr1). MYCBP2 is a large signalling hub involved in diverse processes such as neuronal connectivity, synaptic growth, cell division, neuronal survival, and protein ubiquitination. Our biochemical experiments demonstrate that the formation of a complex containing EPHB2 and MYCBP2 is facilitated by FBXO45, a protein known to select substrates for MYCBP2 ubiquitin ligase activity. Formation of the MYCBP2-EPHB2 complex does not require EPHB2 tyrosine kinase activity and is destabilised by binding of ephrin-B ligands, suggesting that the MYCBP2-EPHB2 association is a prelude to EPHB2 signalling. Paradoxically, the loss of MYCBP2 results in increased ubiquitination of EPHB2 and a decrease of its protein levels suggesting that MYCBP2 stabilises EPHB2. Commensurate with this effect, our cellular experiments reveal that MYCBP2 is essential for efficient EPHB2 signalling responses in cell lines and primary neurons. Finally, our genetic studies in C. elegans provide in vivo evidence that the ephrin receptor VAB-1 displays genetic interactions with known MYCBP2 binding proteins. Together, our results align with the similarity of neurodevelopmental phenotypes caused by MYCBP2 and EPHB2 loss of function, and couple EPHB2 to a signaling effector that controls diverse cellular functions.

EPH/Ephrin Signaling in Normal Hematopoiesis and Hematologic Malignancies: Deciphering Their Intricate Role and Unraveling Possible New Therapeutic Targets

Erythropoietin-producing hepatocellular carcinoma receptors (EPHs) represent the largest family of receptor tyrosine kinases (RTKs). EPH interaction with ephrins, their membrane-bound ligands, holds a pivotal role in embryonic development, while, though less active, it is also implicated in various physiological functions during adult life. In normal hematopoiesis, different patterns of EPH/ephrin expression have been correlated with hematopoietic stem cell (HSC) maintenance and lineage-committed hematopoietic progenitor cell (HPC) differentiation, as well as with the functional properties of their mature offspring. Research in the field of hematologic malignancies has unveiled a rather complex involvement of the EPH/ephrinsignaling pathway in the pathophysiology of these neoplasms. Aberrations in genetic, epigenetic, and protein levels have been identified as possible players implicated both in tumor progression and suppression, while correlations have also been highlighted regarding prognosis and response to treatment. Initial efforts to therapeutically target the EPH/ephrin axis have been undertaken in the setting of hematologic neoplasia but are mainly confined to the preclinical level. To this end, deciphering the complexity of this signaling pathway both in normal and malignant hematopoiesis is necessary.

Case report: Craniofrontonasal syndrome caused by a novel variant in the EFNB1 gene in a Colombian woman

Craniofrontonasal Syndrome is a very rare dominant X-linked genetic disorder characterized by symptoms such as hypertelorism, craniosynostosis, eye alterations, bifid nose tip, and longitudinal ridging and splitting of nails. Heterozygous females are usually the patients severely affected. To date, clinical or genetic data have not been published for these patients in Colombia. Here we report a female proband with coronal craniosynostosis, hypertelorism, strabismus, rotational nystagmus, high-arched palate, dental crowding, scoliosis, severe pectus excavatum, unilateral breast hypoplasia, and brachydactyly; diagnosed with Craniofrontonasal Syndrome with the novel heterozygous variant c.374A>C (p.Glu125Ala) in the EFNB1 gene. So far, she has been treated with physical therapy and surgical correction of the bifid nose and an umbilical hernia. To the best of our knowledge, this is the first report of a patient with this rare genetic disorder in Colombia, expanding its mutational spectrum and highlighting the importance of genetic evaluation of patients with craniosynostosis and facial dysmorphism.

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.

Cardiofacioneurodevelopmental syndrome: Report of a novel patient and expansion of the phenotype

The cardiofacioneurodevelopmental syndrome (CFNDS) is characterized by craniofacial anomalies including bilateral cleft lip and palate, cardiac, skeletal, and neurodevelopmental features and additional variable manifestations. Whole-exome sequencing revealed homozygous loss-of-function variants in CCDC32 (alternative name: C15orf57) in both previously described patients. ccdc32 deletion in zebrafish suggests a ciliary contribution to the pathomechanism. We report a 9-year-old female patient with CFNDS caused by a homozygous 32,583-bp deletion affecting CCDC32. Independent of the affected CCDC32 transcript variant this deletion likely leads to loss of the encoded protein. The patient had intellectual disability, marked hypertelorism, bilateral cleft lip and palate, and short stature. She had bilateral conductive hearing loss, small hands and feet, and finger abnormalities. Brain imaging disclosed hypoplastic corpus callosum. We describe a core phenotype comprising developmental delay and bilateral cleft lip and palate in the three individuals with CFNDS. Variable abnormalities of the face, brain, heart, fingers, and toes and postnatal growth retardation or microcephaly can be present. Possible involvement of the uncharacterized CCDC32 protein in the adapter protein 2 (AP2) complex regulating clathrin-mediated endocytosis has been reported. Cleft palate and cardiac defects observed in mice deficient of different AP2 subunits support a CCDC32 function in the AP2 complex.

Tracheal separation is driven by NKX2-1-mediated repression of Efnb2 and regulation of endodermal cell sorting

The mechanisms coupling fate specification of distinct tissues to their physical separation remain to be understood. The trachea and esophagus differentiate from a single tube of definitive endoderm, requiring the transcription factors SOX2 and NKX2-1, but how the dorsoventral site of tissue separation is defined to allocate tracheal and esophageal cell types is unknown. Here, we show that the EPH/EPHRIN signaling gene Efnb2 regulates tracheoesophageal separation by controlling the dorsoventral allocation of tracheal-fated cells. Ventral loss of NKX2-1 results in disruption of separation and expansion of Efnb2 expression in the trachea independent of SOX2. Through chromatin immunoprecipitation and reporter assays, we find that NKX2-1 likely represses Efnb2 directly. Lineage tracing shows that loss of NKX2-1 results in misallocation of ventral foregut cells into the esophagus, while mosaicism for NKX2-1 generates ectopic NKX2-1/EPHRIN-B2 boundaries that organize ectopic tracheal separation. Together, these data demonstrate that NKX2-1 coordinates tracheal specification with tissue separation through the regulation of EPHRIN-B2 and tracheoesophageal cell sorting.

Lewis et al. show that, in the development of the mammalian trachea and esophagus, cell fate specification is coupled with morphogenesis by NKX2-1-mediated repression of Efnb2. This establishes an EPH/EPHRIN boundary that drives cell allocation and physical separation of the trachea and esophagus.

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.

Long Non-Coding RNAs MALAT1 and NEAT1 in Non-syndromic Orofacial Clefts

Long non-coding RNAs (lncRNAs) are thought to play important roles in non-syndromic orofacial clefts (NSOFC). Clinical diagnosis was categorized as either non-syndromic cleft lip with or without cleft palate (NSCL/P), or non-syndromic cleft palate-only (NSCPO). Tissues excised from the trimmed wound edge were reserved as experimental samples; adjacent normal control was used as a positive control, and tissue from healthy individuals was used as a blank control. Target lncRNAs in the collected tissues were identified using microarrays and quantitative reverse transcription PCR (RT-qPCR). Immunohistochemical (IHC) staining and RT-qPCR were used to verify the target mRNAs. Pathway, gene ontology (GO) enrichment, and TargetScan predictions were employed to construct competing endogenous RNA networks (ceRNA networks) and explore their potential functions. RNA-Seq revealed 24 upregulated and 43 downregulated lncRNAs; MALAT1 and NEAT1 were screened and validated using RT-qPCR. Common NSOFC risk factors were positively correlated with MALAT1 and NEAT1 expression. Bioinformatics predicted four ceRNA networks; GO enrichment focused on their potential functions. RT-qPCR and IHC data were consistent with respect to expression levels of proteins and the mRNAs that encode them. As MALAT1 and NEAT1 are associated with the severity of NSOFC, they represent potential therapeutic targets and prognostic biomarkers.

Essential role of Msx1 in regulating anterior-posterior patterning of the secondary palate in mice

Development of the secondary palate displays molecular heterogeneity along the anterior-posterior axis; however, the underlying molecular mechanism remains largely unknown. MSX1 is an anteriorly expressed transcription repressor required for palate development. Here, we investigate the role of Msx1 in regional patterning of the secondary palate. The Wnt1-Cre-mediated expression of Msx1 (RosaMsx1^Wnt1-Cre) throughout the palatal mesenchyme leads to cleft palate in mice, associated with aberrant cell proliferation and cell death. Osteogenic patterning of the hard palate in RosaMsx1^Wnt1-Cre mice is severely impaired, as revealed by a marked reduction in palatine bone formation and decreased expression of the osteogenic regulator Sp7. Overexpression and knockout of Msx1 in mice show that the transcription repressor promotes the expression of the anterior palate-specific Alx1 but represses the expression of the medial-posterior palate genes Barx1, Meox2, and Tbx22. Furthermore, Tbx22 constitutes a direct Msx1 target gene in the secondary palate, suggesting that Msx1 can directly repress the expression of medial-posterior specific genes. Finally, we determine that Sp7 is downstream of Tbx22 in palatal mesenchymal cells, suggesting that a Msx1/Tbx22/Sp7 axis participates in the regulation of palate development. Our findings unveil a novel role for Msx1 in regulating the anterior-posterior growth and patterning of the secondary palate.

Muscarinic receptor M3 contributes to intestinal stem cell maintenance via EphB/ephrin-B signaling

Acetylcholine (ACh) signaling through activation of nicotinic and muscarinic ACh receptors regulates expression of specific genes that mediate and sustain proliferation, differentiation, and homeostasis in the intestinal crypts. This signaling plays a pivotal role in the regulation of intestinal stem cell function, but the details have not been clarified. Here, we performed experiments using type 3 muscarinic acetylcholine receptor (M3) knockout mice and their intestinal organoids and report that endogenous ACh affects the size of the intestinal stem niche via M3 signaling. RNA sequencing of crypts identified up-regulation of the EphB/ephrin-B signaling pathway. Furthermore, using an MEK inhibitor (U0126), we found that mitogen-activated protein kinase/extracellular signal-regulated kinase (MAPK/ERK) signaling, which is downstream of EphB/ephrin-B signaling, is activated in M3-deficient crypts. Collectively, M3, EphB/ephrin-B, and the MAPK/ERK signaling cascade work together to maintain the homeostasis of intestinal epithelial cell growth and differentiation following modifications of the cholinergic intestinal niche.

EPH/EPHRIN regulates cellular organization by actomyosin contractility effects on cell contacts

EPH/EPHRIN signaling is essential to many aspects of tissue self-organization and morphogenesis, but little is known about how EPH/EPHRIN signaling regulates cell mechanics during these processes. Here, we use a series of approaches to examine how EPH/EPHRIN signaling drives cellular self-organization. Contact angle measurements reveal that EPH/EPHRIN signaling decreases the stability of heterotypic cell:cell contacts through increased cortical actomyosin contractility. We find that EPH/EPHRIN-driven cell segregation depends on actomyosin contractility but occurs independently of directed cell migration and without changes in cell adhesion. Atomic force microscopy and live cell imaging of myosin localization support that EPH/EPHRIN signaling results in increased cortical tension. Interestingly, actomyosin contractility also nonautonomously drives increased EPHB2:EPHB2 homotypic contacts. Finally, we demonstrate that changes in tissue organization are driven by minimization of heterotypic contacts through actomyosin contractility in cell aggregates and by mouse genetics experiments. These data elucidate the biomechanical mechanisms driving EPH/EPHRIN-based cell segregation wherein differences in interfacial tension, regulated by actomyosin contractility, govern cellular self-organization.

miR-31 Displays Subtype Specificity in Lung Cancer

miRNA rarely possess pan-oncogenic or tumor-suppressive properties. Most miRNAs function under tissue-specific contexts, acting as either tumor suppressors in one tissue, promoting oncogenesis in another, or having no apparent role in the regulation of processes associated with the hallmarks of cancer. What has been less clear is the role of miRNAs within cell types of the same tissue and the ability within each cell type to contribute to oncogenesis. In this study, we characterize the role of one such tissue-specific miRNA, miR-31, recently identified as the most oncogenic miRNA in lung adenocarcinoma, across the histologic spectrum of human lung cancer. Compared with normal lung tissue, miR-31 was overexpressed in patient lung adenocarcinoma, squamous cell carcinoma, and large-cell neuroendocrine carcinoma, but not small-cell carcinoma or carcinoids. miR-31 promoted tumor growth in mice of xenografted human adenocarcinoma and squamous cell carcinoma cell lines, but not in large- or small-cell carcinoma lines. While miR-31 did not promote primary tumor growth of large- and small-cell carcinoma, it did promote spontaneous metastasis. Mechanistically, miR-31 altered distinct cellular signaling programs within each histologic subtype, resulting in distinct phenotypic differences. This is the first report distinguishing diverse functional roles for this miRNA across the spectrum of lung cancers and suggests that miR-31 has broad clinical value in human lung malignancy. SIGNIFICANCE: These findings demonstrate the oncogenic properties of miR-31 in specific subtypes of lung cancer and highlight it as a potential therapeutic target in these subtypes. GRAPHICAL ABSTRACT: http://cancerres.aacrjournals.org/content/canres/81/8/1942/F1.large.jpg.

MicroRNA function in craniofacial bone formation, regeneration and repair

Bone formation in the craniofacial complex is regulated by cranial neural crest (CNC) and mesoderm-derived cells. Different elements of the developing skull, face, mandible, maxilla (jaws) and nasal bones are regulated by an array of transcription factors, signaling molecules and microRNAs (miRs). miRs are molecular modulators of these factors and act to restrict their expression in a temporal-spatial mechanism. miRs control the different genetic pathways that form the craniofacial complex. By understanding how miRs function in vivo during development they can be adapted to regenerate and repair craniofacial genetic anomalies as well as bone diseases and defects due to traumatic injuries. This review will highlight some of the new miR technologies and functions that form new bone or inhibit bone regeneration.

Isolation and Time-Lapse Imaging of Primary Mouse Embryonic Palatal Mesenchyme Cells to Analyze Collective Movement Attributes

Development of the palate is a dynamic process, which involves vertical growth of bilateral palatal shelves next to the tongue followed by elevation and fusion above the tongue. Defects in this process lead to cleft palate, a common birth defect. Recent studies have shown that palatal shelf elevation involves a remodeling process that transforms the orientation of the shelf from a vertical to a horizontal one. The role of the palatal shelf mesenchymal cells in this dynamic remodeling has been difficult to study. Time-lapse-imaging-based quantitative analysis has been recently used to show that primary mouse embryonic palatal mesenchymal (MEPM) cells can self-organize into a collective movement. Quantitative analyses could identify differences in mutant MEPM cells from a mouse model with palate elevation defects. This paper describes methods to isolate and culture MEPM cells from E13.5 embryos-specifically for time-lapse imaging-and to determine various cellular attributes of collective movement, including measures for stream formation, shape alignment, and persistence of direction. It posits that MEPM cells can serve as a proxy model for studying the role of palatal shelf mesenchyme during the dynamic process of elevation. These quantitative methods will allow investigators in the craniofacial field to assess and compare collective movement attributes in control and mutant cells, which will augment the understanding of mesenchymal remodeling during palatal shelf elevation. Furthermore, MEPM cells provide a rare mesenchymal cell model for investigation of collective cell movement in general.

Pdgfra and Pdgfrb Genetically Interact in the Murine Neural Crest Cell Lineage to Regulate Migration and Proliferation

Cranial neural crest cells (cNCCs) are migratory, multipotent cells that originate from the forebrain to the hindbrain and eventually give rise to the cartilage and bone of the frontonasal skeleton, among other derivatives. Signaling through the two members of the platelet-derived growth factor receptor (PDGFR) family of receptor tyrosine kinases, alpha and beta, plays critical roles in the cNCC lineage to regulate craniofacial development during murine embryogenesis. Further, the PDGFRs have been shown to genetically interact during murine craniofacial development at mid-to-late gestation. Here, we examined the effect of ablating both Pdgfra and Pdgfrb in the murine NCC lineage on earlier craniofacial development and determined the cellular mechanisms by which the observed phenotypes arose. Our results confirm a genetic interaction between the two receptors in this lineage, as phenotypes observed in an allelic series of mutant embryos often worsened with the addition of conditional alleles. The defects observed here appear to stem from aberrant cNCC migration, as well as decreased proliferation of the facial mesenchyme upon combined decreases in PDGFRα and PDGFRβ signaling. Importantly, we found that PDGFRα plays a predominant role in cNCC migration whereas PDGFRβ primarily contributes to proliferation of the facial mesenchyme past mid-gestation. Our findings provide insight into the distinct mechanisms by which PDGFRα and PDGFRβ signaling regulate cNCC activity and subsequent craniofacial development in the mouse embryo.

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.

Extracranial midline defects in a patient with craniofrontonasal syndrome with a novel EFNB1 mutation

We report a female patient with craniofrontonasal syndrome (CFNS) who in addition showed other cranial and extracranial midline defects including partial corpus callosum agenesis, ocular melanocytosis, pigmentary glaucoma, duplex collecting system, uterus didelphys, and septate vagina. She was found to have a novel pathogenic variant in exon 5 of EFNB1, c.646G>T (p.Glu216*) predicted to cause premature protein truncation. From our review, we found at least 39 published CFNS patients with extracranial midline defects, comprising congenital diaphragmatic hernia, congenital heart defects, umbilical hernia, hypospadias, and less frequently, sacrococcygeal teratomas, and internal genital anomalies in females. These findings support that the EFNB1 mutations have systemic consequences disrupting morphogenetic events at the extracranial midline. Though these are not rigorously included as midline defects, we found at least 10 CFNS patients with congenital anomalies of the kidney and urinary tract, all females. Additionally, uterus didelphys and ocular melanocytosis observed in our patient are proposed also as a previously unreported EFNB1-related midline defects. In addition, this case may be useful for considering the intentional search for genitourinary anomalies in future patients with CFNS, which will be helpful to define their frequency in this entity.

EphrinB2 overexpression enhances osteogenic differentiation of dental pulp stem cells partially through ephrinB2-mediated reverse signaling

Background

Alveolar bone loss is a frequent occurrence. Dental pulp stem cells (DPSCs) which have invasive accessibility and high osteogenic potential is a promising source for cell-based bone regeneration. EphrinB2 is involved in bone homeostasis and osteogenesis. The aim of this study was to investigate the effect and mechanism of ephrinB2 overexpression on osteogenic differentiation of DPSCs and bone defect repair.

Methods

EphrinB2 expression was analyzed during osteogenic induction of human DPSCs (hDPSCs). Endogenous ephrinB2 expression in hDPSCs was then upregulated using EfnB2 lentiviral vectors. The effect of ephrinB2 overexpression on osteogenic differentiation capacity of hDPSCs was investigated in vitro, and activation of ephrinB2-EphB4 bidirectional signaling in ephrinB2-overexpressing hDPSCs was detected. In vivo, a canine alveolar bone defect model was established and canine DPSCs (cDPSCs) were cultured, characterized, EfnB2-tranfected, and combined with a PuraMatrix scaffold. Micro-CT analysis was performed to evaluate the therapeutic effect of ephrinB2-overexpressing cDPSCs on bone defect repair.

Results

EphrinB2 was upregulated after osteogenic induction of hDPSCs. EphrinB2 overexpression enhanced osteogenic differentiation capacity of hDPSCs in vitro. Moreover, p-ephrinB2 instead of p-EphB4 was upregulated by ephrinB2 overexpression, and activation of ephrinB2-mediated reverse signaling promoted osteogenic differentiation of hDPSCs. In a canine bone defect model, ephrinB2 overexpression in cDPSCs significantly improved trabecular bone volume per tissue volume (BV/TV) and trabecular thickness, as demonstrated by radiographic analysis.

Conclusions

EphrinB2 overexpression enhanced osteogenic potential of DPSCs partially via upregulation of ephrinB2-mediated reverse signaling and effectively promoted alveolar bone defect repair.

Electronic supplementary material

The online version of this article (10.1186/s13287-019-1540-2) contains supplementary material, which is available to authorized users.

Craniofacial, orofacial and dental disorders: the role of the RAS/ERK pathway

Deviations from the precisely coordinated programme of human head development can lead to craniofacial and orofacial malformations often including a variety of dental abnormalities too. Although the aetiology is still unknown in many cases, during the last decades different intracellular signalling pathways have been genetically linked to specific disorders. Among these pathways, the RAS/extracellular signal-regulated kinase (ERK) signalling cascade is the focus of this review since it encompasses a large group of genes that when mutated cause some of the most common and severe developmental anomalies in humans. We present the components of the RAS/ERK pathway implicated in craniofacial and orodental disorders through a series of human and animal studies. We attempt to unravel the specific molecular targets downstream of ERK that act on particular cell types and regulate key steps in the associated developmental processes. Finally we point to ambiguities in our current knowledge that need to be clarified before RAS/ERK-targeting therapeutic approaches can be implemented.

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.

MAPK and PI3K signaling: At the crossroads of neural crest development

Receptor tyrosine kinase-mediated growth factor signaling is essential for proper formation and development of the neural crest. The many ligands and receptors implicated in these processes signal through relatively few downstream pathways, frequently converging on the MAPK and PI3K pathways. Despite decades of study, there is still considerable uncertainty about where and when these signaling pathways are required and how they elicit particular responses. This review summarizes our current understanding of growth factor-induced MAPK and PI3K signaling in the neural crest.

FoxO6 regulates Hippo signaling and growth of the craniofacial complex

The mechanisms that regulate post-natal growth of the craniofacial complex and that ultimately determine the size and shape of our faces are not well understood. Hippo signaling is a general mechanism to control tissue growth and organ size, and although it is known that Hippo signaling functions in neural crest specification and patterning during embryogenesis and before birth, its specific role in postnatal craniofacial growth remains elusive. We have identified the transcription factor FoxO6 as an activator of Hippo signaling regulating neonatal growth of the face. During late stages of mouse development, FoxO6 is expressed specifically in craniofacial tissues and FoxO6-/- mice undergo expansion of the face, frontal cortex, olfactory component and skull. Enlargement of the mandible and maxilla and lengthening of the incisors in FoxO6-/- mice are associated with increases in cell proliferation. In vitro and in vivo studies demonstrated that FoxO6 activates Lats1 expression, thereby increasing Yap phosphorylation and activation of Hippo signaling. FoxO6-/- mice have significantly reduced Hippo Signaling caused by a decrease in Lats1 expression and decreases in Shh and Runx2 expression, suggesting that Shh and Runx2 are also linked to Hippo signaling. In vitro, FoxO6 activates Hippo reporter constructs and regulates cell proliferation. Furthermore PITX2, a regulator of Hippo signaling is associated with Axenfeld-Rieger Syndrome causing a flattened midface and we show that PITX2 activates FoxO6 expression. Craniofacial specific expression of FoxO6 postnatally regulates Hippo signaling and cell proliferation. Together, these results identify a FoxO6-Hippo regulatory pathway that controls skull growth, odontogenesis and face morphology.

Association of EPHA3 Gene Polymorphisms with Nonsyndromic Cleft Lip With or Without Cleft Palate

AIMS

Nonsyndromic cleft lip with or without palate (NSCL/P) represents a complex condition caused by genetic and environmental factors. The aim of this study was to investigate the relationship between the EPHA3 polymorphisms and NSCL/P.

MATERIALS AND METHODS

To investigate the relationship between five EPHA3 single nucleotide polymorphisms (SNPs) and NSCL/P, we selected 180 affected patients and 167 normal controls from the Chinese Han Population. EPHA3 SNPs (rs7650466, rs1398197, rs17801309, rs1054750, and rs7632427) were genotyped using the SNaPshot technique; bioinformatic analyses were performed to determine if any of them were potentially functional SNPs.

RESULTS

The rs7650466 T allele was associated with the incidence of NSCL/P (OR, 0.211; 95% CI, 0.131-0.338; adjusted p = 4.881 × 10^-10) and cleft lip with or without palate (CL/P) (OR, 0.176; 95% CI, 0.104-0.297; adjusted p = 3.617 × 10^-10), as well as with protective and dominant effects in both conditions. The rs7650466 T allele could be associated with reduced risk of the malformation. In a bioinformatics analysis, we found potential matching sites (miR-1255a, miR-125a-3p, miR-143, and miR-552) for rs7650466 and preliminarily analyzed its potential function.

CONCLUSIONS

Collectively, our data suggest that the EPHA3 rs7650466 polymorphism confers genetic risk for NSCL/P in the Chinese Han Population. Furthermore, rs7650466 is associated with CL/P incidence in stratified analyses, but not with cleft palate only.

Modulation of cellular polarization and migration by ephrin/Eph signal-mediated boundary formation

Compartment boundaries are essential for ensuring proper cell organization during embryo development and in adult tissues, yet the mechanisms underlying boundary establishment are not completely understood. A number of mechanisms, including (i) differential adhesion, (ii) differential tension, and (iii) cell signaling-mediated cell repulsion, are known to contribute and likely a context-dependent balance of each of these dictates boundary implementation. The ephrin/Eph signaling pathway is known to impact boundary formation in higher animals. In different contexts, ephrin/Eph signaling is known to modulate adhesive properties and migratory behavior of cells. Furthermore it has been proposed that ephrin/Eph signaling may modulate cellular tensile properties, leading to boundary implementation. It remains unclear however, whether, in different contexts, ephrin/Eph act through distinct dominant action modes (e.g. differential adhesion vs. cell repulsion), or whether ephrin/Eph signaling elicits multiple cellular changes simultaneously. Here, using micropatterning of cells over-expressing either EphB3 or ephrinB1, we assess the contribution of each these factors in one model. We show that in this system ephrinB1/EphB3-mediated boundaries are accompanied by modulation of tissue-level architecture and polarization of cell migration. These changes are associated with changes in cell shape and cytoskeletal organization also suggestive of altered cellular tension.

High-throughput analysis reveals novel maternal germline RNAs crucial for primordial germ cell preservation and proper migration

During oogenesis, hundreds of maternal RNAs are selectively localized to the animal or vegetal pole, including determinants of somatic and germline fates. Although microarray analysis has identified localized determinants, it is not comprehensive and is limited to known transcripts. Here, we utilized high-throughput RNA-sequencing analysis to comprehensively interrogate animal and vegetal pole RNAs in the fully grown Xenopus laevis oocyte. We identified 411 (198 annotated) and 27 (15 annotated) enriched mRNAs at the vegetal and animal pole, respectively. Ninety were novel mRNAs over 4-fold enriched at the vegetal pole and six were over 10-fold enriched at the animal pole. Unlike mRNAs, microRNAs were not asymmetrically distributed. Whole-mount in situ hybridization confirmed that all 17 selected mRNAs were localized. Biological function and network analysis of vegetally enriched transcripts identified protein-modifying enzymes, receptors, ligands, RNA-binding proteins, transcription factors and co-factors with five defining hubs linking 47 genes in a network. Initial functional studies of maternal vegetally localized mRNAs show that sox7 plays a novel and important role in primordial germ cell (PGC) development and that ephrinB1 (efnb1) is required for proper PGC migration. We propose potential pathways operating at the vegetal pole that highlight where future investigations might be most fruitful.

Asymmetry at cell-cell interfaces direct cell sorting, boundary formation, and tissue morphogenesis

During development, cells of seemingly homogenous character sort themselves out into distinct compartments in order to generate cell types with specialized features that support tissue morphogenesis and function. This process is often driven by receptors at the cell membrane that probe the extracellular microenvironment for specific ligands and alter downstream signaling pathways impacting transcription, cytoskeletal organization, and cell adhesion to regulate cell sorting and subsequent boundary formation. This review will focus on two of these receptor families, Eph and Notch, both of which are intrinsically non-adhesive and are activated by a unique set of ligands that are asymmetrically distributed from their receptor on neighboring cells. Understanding the requirement of asymmetric ligand-receptor signaling at the membrane under homeostatic conditions gives insight into how misregulation of these pathways contributes to boundary disruption in diseases like cancer.

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.

EphrinB1/EphB3b Coordinate Bidirectional Epithelial-Mesenchymal Interactions Controlling Liver Morphogenesis and Laterality

Positioning organs in the body often requires the movement of multiple tissues, yet the molecular and cellular mechanisms coordinating such movements are largely unknown. Here, we show that bidirectional signaling between EphrinB1 and EphB3b coordinates the movements of the hepatic endoderm and adjacent lateral plate mesoderm (LPM), resulting in asymmetric positioning of the zebrafish liver. EphrinB1 in hepatoblasts regulates directional migration and mediates interactions with the LPM, where EphB3b controls polarity and movement of the LPM. EphB3b in the LPM concomitantly repels hepatoblasts to move leftward into the liver bud. Cellular protrusions controlled by Eph/Ephrin signaling mediate hepatoblast motility and long-distance cell-cell contacts with the LPM beyond immediate tissue interfaces. Mechanistically, intracellular EphrinB1 domains mediate EphB3b-independent hepatoblast extension formation, while EpB3b interactions cause their destabilization. We propose that bidirectional short- and long-distance cell interactions between epithelial and mesenchyme-like tissues coordinate liver bud formation and laterality via cell repulsion.

Generation of an immortalized mouse embryonic palatal mesenchyme cell line

Palatogenesis is a complex morphogenetic process, disruptions in which result in highly prevalent birth defects in humans. In recent decades, the use of model systems such as genetically-modified mice, mouse palatal organ cultures and primary mouse embryonic palatal mesenchyme (MEPM) cultures has provided significant insight into the molecular and cellular defects underlying cleft palate. However, drawbacks in each of these systems have prevented high-throughput, large-scale studies of palatogenesis in vitro. Here, we report the generation of an immortalized MEPM cell line that maintains the morphology, migration ability, transcript expression and responsiveness to exogenous growth factors of primary MEPM cells, with increased proliferative potential over primary cultures. The immortalization method described in this study will facilitate the generation of palatal mesenchyme cells with an unlimited capacity for expansion from a single genetically-modified mouse embryo and enable mechanistic studies of palatogenesis that have not been possible using primary culture.

EPHRIN-B1 Mosaicism Drives Cell Segregation in Craniofrontonasal Syndrome hiPSC-Derived Neuroepithelial Cells

Although human induced pluripotent stem cells (hiPSCs) hold great potential for the study of human diseases affecting disparate cell types, they have been underutilized in seeking mechanistic insights into the pathogenesis of congenital craniofacial disorders. Craniofrontonasal syndrome (CFNS) is a rare X-linked disorder caused by mutations in EFNB1 and characterized by craniofacial, skeletal, and neurological anomalies. Heterozygous females are more severely affected than hemizygous males, a phenomenon termed cellular interference that involves mosaicism for EPHRIN-B1 function. Although the mechanistic basis for cellular interference in CFNS has been hypothesized to involve Eph/ephrin-mediated cell segregation, no direct evidence for this has been demonstrated. Here, by generating hiPSCs from CFNS patients, we demonstrate that mosaicism for EPHRIN-B1 expression induced by random X inactivation in heterozygous females results in robust cell segregation in human neuroepithelial cells, thus supplying experimental evidence that Eph/ephrin-mediated cell segregation is relevant to pathogenesis in human CFNS patients.

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A novel iPSC line can effectively model a craniofacial condition

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Eph/ephrin-mediated cell segregation underlies CFNS

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Cell segregation occurs in CFNS neuroepithelial progenitor cells

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Neuroepithelial progenitors are a possible cell of origin for CFNS dysmorphogenesis

Molecular Analysis of Ephrin A4 and Ephrin B1 in a Rabbit Model of Craniosynostosis: Likely Exclusion as the Loci of Origin

Craniosynostosis (CS) has a prevalence of approximately 1 in every 2000 live births and is characterized by the premature fusion of one or more cranial sutures. Failure to maintain the cell lineage boundary at the coronal suture is thought to be involved in the pathology of some forms of CS. The Ephrin family of receptor tyrosine kinases consists of membrane-bound receptors and ligands that control cell patterning and the formation of developmental boundaries. Mutations in the ephrin A4 (EFNA4) and ephrin B1 (EFNB1) ligands have been linked to nonsyndromic CS and craniofrontonasal syndrome, respectively, in patient samples. We have previously described a colony of rabbits with a heritable pattern of coronal suture synostosis, although the genetic basis for synostosis within this model remains unknown. The present study was performed to determine if EFNA4 or EFNB1 could be the loci of the causal mutation in this unique animal model. Sequencing of EFNA4 and EFNB1 was performed using templates obtained from wild-type (n = 4) and craniosynostotic (n = 4) rabbits. No structural coding errors were identified in either gene. A single-nucleotide transversion was identified in one wild-type rabbit within the third intron of EFNA4. These data indicate that the causal locus for heritable CS in this rabbit model is not located within the structural coding regions of either EFNA4 or EFNB1.

Computational Model of Secondary Palate Fusion and Disruption

Morphogenetic events are driven by cell-generated physical forces and complex cellular dynamics. To improve our capacity to predict developmental effects from chemical-induced cellular alterations, we built a multicellular agent-based model in CompuCell3D that recapitulates the cellular networks and collective cell behavior underlying growth and fusion of the mammalian secondary palate. The model incorporated multiple signaling pathways (TGFβ, BMP, FGF, EGF, and SHH) in a biological framework to recapitulate morphogenetic events from palatal outgrowth through midline fusion. It effectively simulated higher-level phenotypes (e.g., midline contact, medial edge seam (MES) breakdown, mesenchymal confluence, and fusion defects) in response to genetic or environmental perturbations. Perturbation analysis of various control features revealed model functionality with respect to cell signaling systems and feedback loops for growth and fusion, diverse individual cell behaviors and collective cellular behavior leading to physical contact and midline fusion, and quantitative analysis of the TGF/EGF switch that controls MES breakdown-a key event in morphogenetic fusion. The virtual palate model was then executed with theoretical chemical perturbation scenarios to simulate switch behavior leading to a disruption of fusion following chronic (e.g., dioxin) and acute (e.g., retinoic acid) chemical exposures. This computer model adds to similar systems models toward an integrative "virtual embryo" for simulation and quantitative prediction of adverse developmental outcomes following genetic perturbation and/or environmental disruption.

The role of Eph/ephrin molecules in stromal–hematopoietic interactions

Bone marrow mesenchymal stromal/stem cells(BMSC) are fundamental regulatory elements of the hematopoietic stem cell niche; however, the molecular signals that mediate BMSC support of hematopoiesis are poorly understood. Recent studies indicate that BMSC and hematopoietic stem/progenitors cells differentially express the Eph cell surface tyrosine kinase receptors, and their ephrinligands. Eph/ephrin interactions are thought to mediate cross-talk between BMSC and different hematopoietic cell populations to influence cell development, migration and function. This review summarizes Eph/ephrin interactions in the regulation of BMSC communication with hematopoietic stem/progenitor cells and discusses Eph/ephrintargeted therapeutic strategies that are currently being pursued or various hematotological malignancies.

PDGFRβ regulates craniofacial development through homodimers and functional heterodimers with PDGFRα

Craniofacial development is a complex morphogenetic process, disruptions in which result in highly prevalent human birth defects. While platelet-derived growth factor (PDGF) receptor α (PDGFRα) has well-documented functions in this process, the role of PDGFRβ in murine craniofacial development is not well established. We demonstrate that PDGFRα and PDGFRβ are coexpressed in the craniofacial mesenchyme of mid-gestation mouse embryos and that ablation of Pdgfrb in the neural crest lineage results in increased nasal septum width, delayed palatal shelf development, and subepidermal blebbing. Furthermore, we show that the two receptors genetically interact in this lineage, as double-homozygous mutant embryos exhibit an overt facial clefting phenotype more severe than that observed in either single-mutant embryo. We reveal a physical interaction between PDGFRα and PDGFRβ in the craniofacial mesenchyme and demonstrate that the receptors form functional heterodimers with distinct signaling properties. Our studies thus uncover a novel mode of signaling for the PDGF family during vertebrate development.

X-linked heterozygous mutations in ARR3 cause female-limited early onset high myopia

PURPOSE

To identify genetic mutations in three families with early onset high myopia (eoHM) limited to female members.

METHODS

Genomic DNA was collected from participating members of families XF1, XF2, and XF3. Genome-wide linkage scans were performed on the largest family (XF1). Whole exome sequencing was performed on seven samples, including five samples (four affected and one unaffected) from family XF1, as well as the two probands from family XF2 and XF3. Variants were analyzed with multistep bioinformatics analyses. Sanger-dideoxy sequencing was used to verify candidate variations in families and controls.

RESULTS

The genome-wide linkage scans performed on family XF1 detected a candidate locus on chromosome Xp11.1-Xq13.3 with a maximum logarithm of the odds (LOD) score of 2.48 and 3.01 for markers DXS991 and DXS986, respectively. Parallel whole exome sequencing identified a novel c.893C>A (p.Ala298Asp) mutation in ARR3 located on Xq13.1 in family XF1, which was shared by all four affected individuals but not the unaffected individual. Two other novel mutations in ARR3, c.298C>T (p.Arg100*) and c.239T>C (p.Leu80Pro), were detected in families XF2 and XF3, respectively. These mutations were predicted to be damaging and were not present in the normal controls and existing databases. All three mutations cosegregated with eoHM in each of the three families, in which all heterozygous female members are affected whereas all hemizygous male family members are not affected. Transmission of the mutations and eoHM in the three families demonstrates an unusual pattern of X-linked female-limited inheritance.

CONCLUSIONS

These data suggest that heterozygous mutations in ARR3 might be responsible for X-linked female-limited eoHM in the three families, a pattern contrary to the standard X-linked recessive trait. To our knowledge, eoHM is the first human disease associated with mutations in ARR3 and the second X-linked female-limited disease identified thus far. Identification of ARR3 associated with X-linked female-limited trait provides not only additional evidence of this unusual hereditary pattern but also an additional model for investigating the molecular mechanism responsible for female-limited phenotypes.

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

Eph/ephrins drive cell segregation and boundary formation. O’Neill et al. discover that segregation is driven by unidirectional kinase-dependent EphB signaling. Unidirectional signaling generates a cortical actin differential between ephrin-B1– and EphB2-expressing cells and requires ROCK activity for 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.

Understanding craniosynostosis as a growth disorder

Craniosynostosis is a condition of complex etiology that always involves the premature fusion of one or multiple cranial sutures and includes various anomalies of the soft and hard tissues of the head. Steady progress in the field has resulted in identifying gene mutations that recurrently cause craniosynostosis. There are now scores of mutations on many genes causally related to craniosynostosis syndromes, though the genetic basis for the majority of nonsyndromic cases is unknown. Identification of these genetic mutations has allowed significant progress in understanding the intrinsic properties of cranial sutures, including mechanisms responsible for normal suture patency and for pathogenesis of premature suture closure. An understanding of morphogenesis of cranial vault sutures is critical to understanding the pathophysiology of craniosynostosis conditions, but the field is now poised to recognize the repeated changes in additional skeletal and soft tissues of the head that typically accompany premature suture closure. We review the research that has brought an understanding of premature suture closure within our reach. We then enumerate the less well-studied, but equally challenging, nonsutural phenotypes of craniosynostosis conditions that are well characterized in available mouse models. We consider craniosynostosis as a complex growth disorder of multiple tissues of the developing head, whose growth is also targeted by identified mutations in ways that are poorly understood. Knowledge gained from studies of humans and mouse models for these conditions underscores the diverse, associated developmental anomalies of the head that contribute to the complex phenotypes of craniosynostosis conditions presenting novel challenges for future research. WIREs Dev Biol 2016, 5:429-459. doi: 10.1002/wdev.227 For further resources related to this article, please visit the WIREs website.

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.

Eph-ephrin signaling in nervous system development

Ephrins and Eph receptors enable contact-mediated interactions between cells at every stage of nervous system development. In spite of their broad binding affinities, Eph proteins facilitate specificity in neuronal migration and axon targeting. This review focuses on recent studies that demonstrate how these proteins interact with each other, and with other signaling pathways, to guide specificity in a diverse set of developmental processes.

Mechanisms of ephrin-Eph signalling in development, physiology and disease

Eph receptor Tyr kinases and their membrane-tethered ligands, the ephrins, elicit short-distance cell-cell signalling and thus regulate many developmental processes at the interface between pattern formation and morphogenesis, including cell sorting and positioning, and the formation of segmented structures and ordered neural maps. Their roles extend into adulthood, when ephrin-Eph signalling regulates neuronal plasticity, homeostatic events and disease processes. Recently, new insights have been gained into the mechanisms of ephrin-Eph signalling in different cell types, and into the physiological importance of ephrin-Eph in different organs and in disease, raising questions for future research directions.

Report of a family with craniofrontonasal syndrome

Craniofrontonasal syndrome (CFNS, OMIM 304110) paradoxically presents a severe phenotype in heterozygous females and a mild or a normal phenotype in hemizygous males. Hypertelorism is seen in almost all of the female CFNS patients; craniosynostosis, facial asymmetry, and bifid nose are the other major clinical features. Most of the males are mildly affected, frequently only with hypertelorism. Here, we report a family with a G151S mutation in the EFNB1 gene. The mutation was identified in two severely affected sisters and paradoxically in their clinically unaffected father. The father on whom we report is the first male patient genetically proved to carry a CFNS-causing mutation and not presenting any signs nor symptoms of CFNS.

From Bench to Bedside and Back: Improving Diagnosis and Treatment of Craniofacial Malformations Utilizing Animal Models

Craniofacial anomalies are among the most common birth defects and are associated with increased mortality and, in many cases, the need for lifelong treatment. Over the past few decades, dramatic advances in the surgical and medical care of these patients have led to marked improvements in patient outcomes. However, none of the treatments currently in clinical use address the underlying molecular causes of these disorders. Fortunately, the field of craniofacial developmental biology provides a strong foundation for improved diagnosis and for therapies that target the genetic causes of birth defects. In this chapter, we discuss recent advances in our understanding of the embryology of craniofacial conditions, and we focus on the use of animal models to guide rational therapies anchored in genetics and biochemistry.

Craniofrontonasal dysplasia: variability of the frontonasal suture and implications for treatment

Craniofrontonasal dysplasia's (CFND's) phenotypic range includes hypertelorism, coronal craniosynostosis, frontonasal dysplasia, and digital anomalies. The variable expression is paradoxical for an X-linked syndrome because hemizygous males are less affected than heterozygous females. We describe a case of CFND due to a c.30>T EFNB1 gene mutation. In place of the typical craniosynostosis found in CFND, she presented with a superiorly displaced nasion and an anomalously positioned frontonasal suture. This report reveals an unreported malformation in CFND and its surgical implications.

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.

Receptor tyrosine kinases modulate distinct transcriptional programs by differential usage of intracellular pathways

Receptor tyrosine kinases (RTKs) signal through shared intracellular pathways yet mediate distinct outcomes across many cell types. To investigate the mechanisms underlying RTK specificity in craniofacial development, we performed RNA-seq to delineate the transcriptional response to platelet-derived growth factor (PDGF) and fibroblast growth factor (FGF) signaling in mouse embryonic palatal mesenchyme cells. While the early gene expression profile induced by both growth factors is qualitatively similar, the late response is divergent. Comparing the effect of MEK (Mitogen/Extracellular signal-regulated kinase) and PI3K (phosphoinositide-3-kinase) inhibition, we find the FGF response is MEK dependent, while the PDGF response is PI3K dependent. Furthermore, FGF promotes proliferation but PDGF favors differentiation. Finally, we demonstrate overlapping domains of PDGF-PI3K signaling and osteoblast differentiation in the palate and increased osteogenesis in FGF mutants, indicating this differentiation circuit is conserved in vivo. Our results identify distinct responses to PDGF and FGF and provide insight into the mechanisms encoding RTK specificity.

DOI:http://dx.doi.org/10.7554/eLife.07186.001

Cells produce many different proteins that play a variety of important roles. For example, proteins called receptor tyrosine kinases can detect particular molecules and send signals to other parts of the cell to regulate the activity (or “expression”) of genes involved in cell division, movement, and other processes.

Humans have 58 receptor tyrosine kinases, and defects in these proteins have been linked to diseases such as cancer and diabetes. However, many different receptors regulate the activities of shared sets of genes, so it is not clear how an individual receptor can specifically control the genes involved in a particular process.

Two receptor tyrosine kinases called PDGFR and FGFR are crucial for the development of the face, palate, and head in humans and other animals. Vasudevan et al. used a technique called RNA-sequencing to find out which genes are regulated by these receptors in mouse palate cells. The experiments show that there is a common set of genes whose activities change quickly—within 1 hour—in response to the activation of either PDGFR or FGFR. However, several hours later, cells in which PDGFR is activated have different patterns of gene expression compared to those with active FGFR.

Vasudevan et al. also found that FGFR promotes cell division, while PDGFR promotes the changing of palate cells into different types with more specialized roles. These different outcomes arise because PDGFR and FGFR use different signaling pathways that involve distinct proteins. For example, a protein called PI3K is critical for changes in gene expression in response to PDGFR but not FGFR.

These results suggest that PGDRF and FGFR control different cellular processes in the palate by sending distinct signals into the cell. Understanding the receptor tyrosine kinases and the networks of genes they activate will help us to identify the signals that are important for other processes, such as the development of the face.

DOI:http://dx.doi.org/10.7554/eLife.07186.002

Receptor tyrosine kinase signaling: regulating neural crest development one phosphate at a time

Receptor tyrosine kinases (RTKs) bind to a subset of growth factors on the surface of cells and elicit responses with broad roles in developmental and postnatal cellular processes. Receptors in this subclass consist of an extracellular ligand-binding domain, a single transmembrane domain, and an intracellular domain harboring a catalytic tyrosine kinase and regulatory sequences that are phosphorylated either by the receptor itself or by various interacting proteins. Once activated, RTKs bind signaling molecules and recruit effector proteins to mediate downstream cellular responses through various intracellular signaling pathways. In this chapter, we highlight the role of a subset of RTK families in regulating the activity of neural crest cells (NCCs) and the development of their derivatives in mammalian systems. NCCs are migratory, multipotent cells that can be subdivided into four axial populations, cranial, cardiac, vagal, and trunk. These cells migrate throughout the vertebrate embryo along defined pathways and give rise to unique cell types and structures. Interestingly, individual RTK families often have specific functions in a subpopulation of NCCs that contribute to the diversity of these cells and their derivatives in the mammalian embryo. We additionally discuss current methods used to investigate RTK signaling, including genetic, biochemical, large-scale proteomic, and biosensor approaches, which can be applied to study intracellular signaling pathways active downstream of this receptor subclass during NCC development.