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

Diversity of Intercellular Communication Modes: A Cancer Biology Perspective

From the moment a cell is on the path to malignant transformation, its interaction with other cells from the microenvironment becomes altered. The flow of molecular information is at the heart of the cellular and systemic fate in tumors, and various processes participate in conveying key molecular information from or to certain cancer cells. For instance, the loss of tight junction molecules is part of the signal sent to cancer cells so that they are no longer bound to the primary tumors and are thus free to travel and metastasize. Upon the targeting of a single cell by a therapeutic drug, gap junctions are able to communicate death information to by-standing cells. The discovery of the importance of novel modes of cell-cell communication such as different types of extracellular vesicles or tunneling nanotubes is changing the way scientists look at these processes. However, are they all actively involved in different contexts at the same time or are they recruited to fulfill specific tasks? What does the multiplicity of modes mean for the overall progression of the disease? Here, we extend an open invitation to think about the overall significance of these questions, rather than engage in an elusive attempt at a systematic repertory of the mechanisms at play.

Low-intensity pulsed ultrasound regulates osteoblast-osteoclast crosstalk via EphrinB2/EphB4 signaling for orthodontic alveolar bone remodeling

Background: The limited regenerative potential of periodontal tissue remains a challenge in orthodontic treatment, especially with respect to alveolar bone remodeling. The dynamic balance between the bone formation of osteoblasts and the bone resorption of osteoclasts controls bone homeostasis. The osteogenic effect of low-intensity pulsed ultrasound (LIPUS) is widely accepted, so LIPUS is expected to be a promising method for alveolar bone regeneration. Osteogenesis is regulated by the acoustic mechanical effect of LIPUS, while the cellular perception, transduction mode and response regulation mechanism of LIPUS stimuli are still unclear. This study aimed to explore the effects of LIPUS on osteogenesis by osteoblast-osteoclast crosstalk and the underlying regulation mechanism. Methods: The effects of LIPUS on orthodontic tooth movement (OTM) and alveolar bone remodeling were investigated via rat model by histomorphological analysis. Mouse bone marrow mesenchymal stem cells (BMSCs) and bone marrow monocytes (BMMs) were purified and used as BMSC-derived osteoblasts and BMM-derived osteoclasts, respectively. The osteoblast-osteoclast co-culture system was used to evaluate the effect of LIPUS on cell differentiation and intercellular crosstalk by Alkaline phosphatase (ALP), Alizarin Red S (ARS), tartrate-resistant acid phosphatase (TRAP) staining, real-time quantitative PCR, western blotting and immunofluorescence. Results: LIPUS was found to improve OTM and alveolar bone remodeling in vivo, promote differentiation and EphB4 expression in BMSC-derived osteoblasts in vitro, particularly when cells were directly co-cultured with BMM-derived osteoclasts. LIPUS enhanced EphrinB2/EphB4 interaction between osteoblasts and osteoclasts in alveolar bone, activated the EphB4 receptor on osteoblasts membrane, transduced LIPUS-related mechanical signals to the intracellular cytoskeleton, and gave rise to the nuclear translocation of YAP in Hippo signaling pathway, thus regulating cell migration and osteogenic differentiation. Conclusions: This study shows that LIPUS modulates bone homeostasis by osteoblast-osteoclast crosstalk via EphrinB2/EphB4 signaling, which benefits the balance between OTM and alveolar bone remodeling.

Heterogeneity in the size of the apical surface of cortical progenitors

BACKGROUND

The apical surface (AS) of epithelial cells is highly specialized; it is important for morphogenetic processes that are essential to shape organs and tissues and it plays a role in morphogen and growth factor signaling. Apical progenitors in the mammalian neocortex are pseudoepithelial cells whose apical surface lines the ventricle. Whether changes in their apical surface sizes are important for cortical morphogenesis and/or other aspects of neocortex development has not been thoroughly addressed.

RESULTS

Here we show that apical progenitors are heterogeneous with respect to their apical surface area. In Efnb1 mutants, the size of the apical surface is modified and this correlates with discrete alterations of tissue organization without impacting apical progenitors proliferation.

CONCLUSIONS

Altogether, our data reveal heterogeneity in apical progenitors AS area in the developing neocortex and shows a role for Ephrin B1 in controlling AS size. Our study also indicates that changes in AS size do not have strong repercussion on apical progenitor behavior.

Cell sorting in vitro and in vivo: How are cadherins involved?

Animal tissues are composed of heterogenous cells, and their sorting into different compartments of the tissue is a pivotal process for organogenesis. Cells accomplish sorting by themselves-it is well known that singly dispersed cells can self-organize into tissue-like structures in vitro. Cell sorting is regulated by both biochemical and physical mechanisms. Adhesive proteins connect cells together, selecting particular partners through their specific binding properties, while physical forces, such as cell-cortical tension, control the cohesiveness between cells and in turn cell assembly patterns in mechanical ways. These processes cooperate in determining the overall cell sorting behavior. This article focuses on the 'cadherin' family of adhesion molecules as a biochemical component of cell-cell interactions, addressing how they regulate cell sorting by themselves or by cooperating with other factors. New ideas beyond the classical models of cell sorting are also discussed.

Cell segregation via differential collision modes between heterotypic cell populations

In tissue development and regeneration, the establishment of sharp boundaries between heterotypic cells is essential for the differentiation of tissue functions. During the dynamic rearrangements of constituent cells that result from cell division and collective migration, the segregation boundary encounters various challenges. Several studies have suggested that cortical actomyosin structures play a crucial role in the maintenance of the boundary interface of segregated cell populations, implicating actin-mediated stresses. Examining physical cellular properties such as motility, traction, and intercellular stress, we investigated the formation and maintenance of the stable segregation between epithelial and mesenchymal cell populations devoid of heterotypic adhesions. At the contact boundary, the homotypic adhesion-mediated epithelial aggregates exerted collision-mediated compression against the surrounding mesenchymal cells. Our results demonstrated that heterotypic cell populations established a robust interfacial boundary by accumulating stress from active collisions and repulsions between two dissimilar cell types. Furthermore, the moment of the heterotypic collisions was identified by the existence of a sharp rise in maximum shear stress within the cell cluster.

Autocrine/Paracrine Slit–Robo Signaling Controls Optic Lobe Development in Drosophila melanogaster

Cell segregation mechanisms play essential roles during the development of the central nervous system (CNS) to support its organization into distinct compartments. The Slit protein is a secreted signal, classically considered a paracrine repellent for axonal growth through Robo receptors. However, its function in the compartmentalization of CNS is less explored. In this work, we show that Slit and Robo3 are expressed in the same neuronal population of the Drosophila optic lobe, where they are required for the correct compartmentalization of optic lobe neuropils by the action of an autocrine/paracrine mechanism. We characterize the endocytic route followed by the Slit/Robo3 complex and detected genetic interactions with genes involved in endocytosis and actin dynamics. Thus, we report that the Slit-Robo3 pathway regulates the morphogenesis of the optic lobe through an atypical autocrine/paracrine mechanism in addition to its role in axon guidance, and in association with proteins of the endocytic pathway and small GTPases.

Eph/ephrin signaling controls cell contacts and formation of a structurally asymmetrical tissue boundary in the Xenopus gastrula

In the primitive vertebrate gastrula, the boundary between ectoderm and mesoderm is formed by Brachet's cleft. Here we examine Brachet's cleft and its control by Eph/ephrin signaling in Xenopus at the ultrastructural level and by visualizing cortical F-actin. We infer cortical tension ratios at tissue surfaces and their interface in normal gastrulae and after depletion of receptors EphB4 and EphA4 and ligands ephrinB2 and ephrinB3. We find that cortical tension downregulation at cell contacts, a normal process in adhesion, is asymmetrically blocked in the ectoderm by Eph/ephrin signals from the mesoderm. This generates high interfacial tension that can prevent cell mixing across the boundary. Moreover, it determines an asymmetric boundary structure that is suited for the respective roles of ectoderm and mesoderm, as substratum and as migratory layers. The Eph and ephrin isoforms also control different cell-cell contact types in ectoderm and mesoderm. Respective changes of adhesion upon isoform depletion affect adhesion at the boundary to different degrees but usually do not prohibit cleft formation. In an extreme case, a new type of cleft-like boundary is even generated where cortical tension is symmetrically increased on both sides of the boundary.

EPH receptor tyrosine kinases phosphorylate the PAR-3 scaffold protein to modulate downstream signaling networks

EPH receptors (EPHRs) constitute the largest family among receptor tyrosine kinases in humans. They are mainly involved in short-range cell-cell communication events that regulate cell adhesion, migration, and boundary formation. However, the molecular mechanisms by which EPHRs control these processes are less understood. To address this, we unravel EPHR-associated complexes under native conditions using mass-spectrometry-based BioID proximity labeling. We obtain a composite proximity network from EPHA4, -B2, -B3, and -B4 that comprises 395 proteins, most of which were not previously linked to EPHRs. We examine the contribution of several BioID-identified candidates via loss-of-function in an EPHR-dependent cell-segregation assay. We find that the signaling scaffold PAR-3 is required for cell sorting and that EPHRs directly phosphorylate PAR-3. We also delineate a signaling complex involving the C-terminal SRC kinase (CSK), whose recruitment to PAR-3 is dependent on EPHR signals. Our work describes signaling networks by which EPHRs regulate cellular phenotypes.

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.

Interplay of Eph-Ephrin Signalling and Cadherin Function in Cell Segregation and Boundary Formation

The segregation of distinct cell populations to form sharp boundaries is crucial for stabilising tissue organisation, for example during hindbrain segmentation in craniofacial development. Two types of mechanisms have been found to underlie cell segregation: differential adhesion mediated by cadherins, and Eph receptor and ephrin signalling at the heterotypic interface which regulates cell adhesion, cortical tension and repulsion. An interplay occurs between these mechanisms since cadherins have been found to contribute to Eph-ephrin-mediated cell segregation. This may reflect that Eph receptor activation acts through multiple pathways to decrease cadherin-mediated adhesion which can drive cell segregation. However, Eph receptors mainly drive cell segregation through increased heterotypic tension or repulsion. Cadherins contribute to cell segregation by antagonising homotypic tension within each cell population. This suppression of homotypic tension increases the difference with heterotypic tension triggered by Eph receptor activation, and it is this differential tension that drives cell segregation and border sharpening.

Nuclear-capture of endosomes depletes nuclear G-actin to promote SRF/MRTF activation and cancer cell invasion

Signals are relayed from receptor tyrosine kinases (RTKs) at the cell surface to effector systems in the cytoplasm and nucleus, and coordination of this process is important for the execution of migratory phenotypes, such as cell scattering and invasion. The endosomal system influences how RTK signalling is coded, but the ways in which it transmits these signals to the nucleus to influence gene expression are not yet clear. Here we show that hepatocyte growth factor, an activator of MET (an RTK), promotes Rab17- and clathrin-dependent endocytosis of EphA2, another RTK, followed by centripetal transport of EphA2-positive endosomes. EphA2 then mediates physical capture of endosomes on the outer surface of the nucleus; a process involving interaction between the nuclear import machinery and a nuclear localisation sequence in EphA2's cytodomain. Nuclear capture of EphA2 promotes RhoG-dependent phosphorylation of the actin-binding protein, cofilin to oppose nuclear import of G-actin. The resulting depletion of nuclear G-actin drives transcription of Myocardin-related transcription factor (MRTF)/serum-response factor (SRF)-target genes to implement cell scattering and the invasive behaviour of cancer cells.

Plekhh1, a partner of myosin 1 and an effector of EphB2 controls the cortical actin network for cell repulsion

EphB2/ephrinB signalling that plays a major role in cell segregation during embryonic development and tissue homeostasis, induces an important reorganization of the cortical actin network. We have previously reported that myosin 1b contributes to the reorganisation of the cortical actin network upon EphB2 signalling. In this report we have identified Plekhh1, as a new partner of members of the myosin 1 family and EphB2 receptors. Plekhh1 interacts with myosin 1b via its N-terminus domain and with EphB2 via its C-terminus domain. Furthermore, Plekhh1 is tyrosine-phosphorylated, and this depends on EphB2 kinase activity. Such as the manipulation of the expression level of myosin 1b and myosin 1c, manipulation of Plekhh1 expression levels reveals that Plekhh1 controls the formation of filopodia, the length of focal adhesions and the formation of blebs. Furthermore, binding of Plekhh1 interacting domain to myosin 1b increases the motor activity of myosin 1b in vitro. Together our data show that Plekhh1 is an effector of EphB2 and suggest that Plekhh1 regulates the cortical actin network via the interaction of its N-terminus domain with myosin 1 upon EphB2/ephrinB signalling.

Segmentation and patterning of the vertebrate hindbrain

During early development, the hindbrain is sub-divided into rhombomeres that underlie the organisation of neurons and adjacent craniofacial tissues. A gene regulatory network of signals and transcription factors establish and pattern segments with a distinct anteroposterior identity. Initially, the borders of segmental gene expression are imprecise, but then become sharply defined, and specialised boundary cells form. In this Review, we summarise key aspects of the conserved regulatory cascade that underlies the formation of hindbrain segments. We describe how the pattern is sharpened and stabilised through the dynamic regulation of cell identity, acting in parallel with cell segregation. Finally, we discuss evidence that boundary cells have roles in local patterning, and act as a site of neurogenesis within the hindbrain.

Myosin cables control the timing of tissue internalization in the Drosophila embryo

Compartment boundaries prevent cell mixing during animal development. In the early Drosophila embryo, the mesectoderm is a group of glial precursors that separate ectoderm and mesoderm, forming the ventral midline. Mesectoderm cells undergo one round of oriented divisions during axis elongation and are eventually internalized 6 h later. Using spinning disk confocal microscopy and image analysis, we found that after dividing, mesectoderm cells reversed their planar polarity. The polarity factor Bazooka was redistributed to mesectoderm-mesectoderm cell interfaces, and the molecular motor non-muscle Myosin II and its upstream activator Rho-kinase (Rok) accumulated at mesectoderm-ectoderm (ME) interfaces, forming supracellular cables flanking the mesectoderm on either side of the tissue. Laser ablation revealed the presence of increased tension at ME cables, where Myosin was stabilized, as shown by fluorescence recovery after photobleaching. We used laser nanosurgery to reduce tension at the ME boundary, and we found that Myosin fluorescence decreased rapidly, suggesting a role for tension in ME boundary maintenance. Mathematical modelling predicted that increased tension at the ME boundary was necessary to prevent the premature establishment of contacts between the two ectodermal sheets on opposite sides of the mesectoderm, thus controlling the timing of mesectoderm internalization. We validated the model in vivo: Myosin inhibition disrupted the linearity of the ME boundary and resulted in early internalization of the mesectoderm. Our results suggest that the redistribution of Rok polarizes Myosin and Bazooka within the mesectoderm to establish tissue boundaries, and that ME boundaries control the timely internalization of the mesectoderm as embryos develop.

EphrinB2-EphB2 signaling for dendrite protection after neuronal ischemia in vivo and oxygen-glucose deprivation in vitro

In order to rescue neuronal function, neuroprotection should be required not only for the neuron soma but also the dendrites. Here, we propose the hypothesis that ephrin-B2-EphB2 signaling may be involved in dendritic degeneration after ischemic injury. A mouse model of focal cerebral ischemia with middle cerebral artery occlusion (MCAO) method was used for EphB2 signaling test in vivo. Primary cortical neuron culture and oxygen-glucose deprivation were used to assess EphB2 signaling in vitro. siRNA and soluble ephrin-B2 ectodomain were used to block ephrin-B2-Ephb2 signaling. In the mouse model of focal cerebral ischemia and in neurons subjected to oxygen-glucose deprivation, clustering of ephrin-B2 with its receptor EphB2 was detected. Phosphorylation of EphB2 suggested activation of this signaling pathway. RNA silencing of EphB2 prevented neuronal death and preserved dendritic length. To assess therapeutic potential, we compared the soluble EphB2 ectodomain with the NMDA antagonist MK801 in neurons after oxygen-glucose deprivation. Both agents equally reduced lactate dehydrogenase release as a general marker of neurotoxicity. However, only soluble EphB2 ectodomain protected the dendrites. These findings provide a proof of concept that ephrin-B2-EphB2 signaling may represent a novel therapeutic target to protect both the neuron soma as well as dendrites against ischemic injury.

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.

EPHecting cell contact by increasing cortical tension

EPH/EPHRIN signaling is crucial to the segregation of cell populations during the morphogenesis of many tissues. In this issue, Kindberg et al. (2021. J. Cell Biol.https://doi.org/10.1083/jcb.202005216) show that EPH activation can drive both heterotypic cell repulsion and homotypic aggregation by triggering increased cortical tension.

Forced to communicate: Integration of mechanical and biochemical signaling in morphogenesis

Morphogenesis is a physical process that requires the generation of mechanical forces to achieve dynamic changes in cell position, tissue shape, and size as well as biochemical signals to coordinate these events. Mechanical forces are also used by the embryo to transmit detailed information across space and detected by target cells, leading to downstream changes in cellular properties and behaviors. Indeed, forces provide signaling information of complementary quality that can both synergize and diversify the functional outputs of biochemical signaling. Here, we discuss recent findings that reveal how mechanical signaling and biochemical signaling are integrated during morphogenesis and the possible context-specific advantages conferred by the interactions between these signaling mechanisms.

Cell sorting and morphogenesis in early Drosophila embryos

The regionalisation of growing tissues into compartments that do not mix is thought to be a common motif of animal development. Compartments and compartmental boundaries were discovered by lineage studies in the model organism Drosophila. Since then, many compartment boundaries have been identified in developing tissues, from insects to vertebrates. These are important for animal development, because boundaries localize signalling centres that control tissue morphogenesis. Compartment boundaries are boundaries of lineage restriction, where specific mechanisms keep boundaries straight and cells segregated. Here, we review the mechanisms of cell sorting at boundaries found in early Drosophila embryos. The parasegmental boundaries, separating anterior from posterior compartments in the embryo, keep cells segregated by increasing actomyosin contractility at boundary cell-cell interfaces. Differential actomyosin contractility in turn promotes fold formation and orients cell division. Earlier in development, actomyosin differentials are also important for cell sorting during axis extension. Specific cell surface asymmetries and signalling pathways are required to initiate and maintain these actomyosin differentials.

Genetic Mouse Models and Induced Pluripotent Stem Cells for Studying Tracheal-Esophageal Separation and Esophageal Development

Esophagus and trachea arise from a common origin, the anterior foregut tube. The compartmentalization process of the foregut into the esophagus and trachea is still poorly understood. Esophageal atresia/tracheoesophageal fistula (EA/TEF) is one of the most common gastrointestinal congenital defects with an incidence rate of 1 in 2,500 births. EA/TEF is linked to the disruption of the compartmentalization process of the foregut tube. In EA/TEF patients, other organ anomalies and disorders have also been reported. Over the last two decades, animal models have shown the involvement of multiple signaling pathways and transcription factors in the development of the esophagus and trachea. Use of induced pluripotent stem cells (iPSCs) to understand organogenesis has been a valuable tool for mimicking gastrointestinal and respiratory organs. This review focuses on the signaling mechanisms involved in esophageal development and the use of iPSCs to model and understand it.

The receptor tyrosine kinase EPHB6 regulates catecholamine exocytosis in adrenal gland chromaffin cells

The erythropoietin-producing human hepatocellular receptor EPH receptor B6 (EPHB6) is a receptor tyrosine kinase that has been shown previously to control catecholamine synthesis in the adrenal gland chromaffin cells (AGCCs) in a testosterone-dependent fashion. EPHB6 also has a role in regulating blood pressure, but several facets of this regulation remain unclear. Using amperometry recordings, we now found that catecholamine secretion by AGCCs is compromised in the absence of EPHB6. AGCCs from male knockout (KO) mice displayed reduced cortical F-actin disassembly, accompanied by decreased catecholamine secretion through exocytosis. This phenotype was not observed in AGCCs from female KO mice, suggesting that testosterone, but not estrogen, contributes to this phenotype. Of note, reverse signaling from EPHB6 to ephrin B1 (EFNB1) and a 7-amino acid-long segment in the EFNB1 intracellular tail were essential for the regulation of catecholamine secretion. Further downstream, the Ras homolog family member A (RHOA) and FYN proto-oncogene Src family tyrosine kinase (FYN)-proto-oncogene c-ABL-microtubule-associated monooxygenase calponin and LIM domain containing 1 (MICAL-1) pathways mediated the signaling from EFNB1 to the defective F-actin disassembly. We discuss the implications of EPHB6's effect on catecholamine exocytosis and secretion for blood pressure regulation.

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.

Actomyosin regulation by Eph receptor signaling couples boundary cell formation to border sharpness

The segregation of cells with distinct regional identity underlies formation of a sharp border, which in some tissues serves to organise a boundary signaling centre. It is unclear whether or how border sharpness is coordinated with induction of boundary-specific gene expression. We show that forward signaling of EphA4 is required for border sharpening and induction of boundary cells in the zebrafish hindbrain, which we find both require kinase-dependent signaling, with a lesser input of PDZ domain-dependent signaling. We find that boundary-specific gene expression is regulated by myosin II phosphorylation, which increases actomyosin contraction downstream of EphA4 signaling. Myosin phosphorylation leads to nuclear translocation of Taz, which together with Tead1a is required for boundary marker expression. Since actomyosin contraction maintains sharp borders, there is direct coupling of border sharpness to boundary cell induction that ensures correct organisation of signaling centres.

Trans-endocytosis elicited by nectins transfers cytoplasmic cargo, including infectious material, between cells

Here, we show that cells expressing the adherens junction protein nectin-1 capture nectin-4-containing membranes from the surface of adjacent cells in a trans-endocytosis process. We find that internalized nectin-1-nectin-4 complexes follow the endocytic pathway. The nectin-1 cytoplasmic tail controls transfer: its deletion prevents trans-endocytosis, while its exchange with the nectin-4 tail reverses transfer direction. Nectin-1-expressing cells acquire dye-labeled cytoplasmic proteins synchronously with nectin-4, a process most active during cell adhesion. Some cytoplasmic cargo remains functional after transfer, as demonstrated with encapsidated genomes of measles virus (MeV). This virus uses nectin-4, but not nectin-1, as a receptor. Epithelial cells expressing nectin-4, but not those expressing another MeV receptor in its place, can transfer infection to nectin-1-expressing primary neurons. Thus, this newly discovered process can move cytoplasmic cargo, including infectious material, from epithelial cells to neurons. We name the process nectin-elicited cytoplasm transfer (NECT). NECT-related trans-endocytosis processes may be exploited by pathogens to extend tropism. This article has an associated First Person interview with the first author of the paper.

Role of forward and reverse signaling in Eph receptor and ephrin mediated cell segregation

Eph receptor and ephrin signaling has a major role in segregating distinct cell populations to form sharp borders. Expression of interacting Ephs and ephrins typically occurs in complementary regions, such that polarised activation of both components occurs at the interface. Forward signaling through Eph receptors can drive cell segregation, but it is unclear whether reverse signaling through ephrins can also contribute. We have tested the role of reverse signaling, and of polarised versus non-polarised activation, in assays in which contact repulsion drives cell segregation and border sharpening. We find that polarised forward signaling drives stronger segregation than polarised reverse signaling. Nevertheless, reverse signaling contributes since bidirectional Eph and ephrin activation drives stronger segregation than unidirectional forward signaling alone. In contrast, non-polarised Eph activation drives little segregation. We propose that although polarised forward signaling is the principal driver of segregation, reverse signaling enables bidirectional repulsion which prevents mingling of each population into the other.

EPHB6 controls catecholamine biosynthesis by up-regulating tyrosine hydroxylase transcription in adrenal gland chromaffin cells

EPHB6 is a member of the erythropoietin-producing hepatocellular kinase (EPH) family and a receptor tyrosine kinase with a dead kinase domain. It is involved in blood pressure regulation and adrenal gland catecholamine (CAT) secretion, but several facets of EPHB6-mediated CAT regulation are unclear. In this study, using biochemical, quantitative RT-PCR, immunoblotting, and gene microarray assays, we found that EPHB6 up-regulates CAT biosynthesis in adrenal gland chromaffin cells (AGCCs). We observed that epinephrine content is reduced in the AGCCs from male Ephb6-KO mice, caused by decreased expression of tyrosine hydroxylase, the rate-limiting enzyme in CAT biosynthesis. We demonstrate that the signaling pathway from EPHB6 to tyrosine hydroxylase expression in AGCCs involves Rac family small GTPase 1 (RAC1), MAP kinase kinase 7 (MKK7), c-Jun N-terminal kinase (JNK), proto-oncogene c-Jun, activator protein 1 (AP1), and early growth response 1 (EGR1). On the other hand, signaling via extracellular signal-regulated kinase (ERK1/2), p38 mitogen-activated protein kinase, and ELK1, ETS transcription factor (ELK1) was not affected by EPHB6 deletion. We further report that EPHB6's effect on AGCCs was via reverse signaling through ephrin B1 and that EPHB6 acted in concert with the nongenomic effect of testosterone to control CAT biosynthesis. Our findings elucidate the mechanisms by which EPHB6 modulates CAT biosynthesis and identify potential therapeutic targets for diseases, such as hypertension, caused by dysfunctional CAT biosynthesis.

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.

Evolutionary emergence of the rac3b/rfng/sgca regulatory cluster refined mechanisms for hindbrain boundaries formation

Developmental programs often rely on parallel morphogenetic mechanisms that guarantee precise tissue architecture. While redundancy constitutes an obvious selective advantage, little is known on how novel morphogenetic mechanisms emerge during evolution. In zebrafish, rhombomeric boundaries behave as an elastic barrier, preventing cell intermingling between adjacent compartments. Here, we identify the fundamental role of the small-GTPase Rac3b in actomyosin cable assembly at hindbrain boundaries. We show that the novel rac3b/rfng/sgca regulatory cluster, which is specifically expressed at the boundaries, emerged in the Ostariophysi superorder by chromosomal rearrangement that generated new cis-regulatory interactions. By combining 4C-seq, ATAC-seq, transgenesis, and CRISPR-induced deletions, we characterized this regulatory domain, identifying hindbrain boundary-specific cis-regulatory elements. Our results suggest that the capacity of boundaries to act as an elastic mesh for segregating rhombomeric cells evolved by cooption of critical genes to a novel regulatory block, refining the mechanisms for hindbrain segmentation.

Cell segregation and border sharpening by Eph receptor-ephrin-mediated heterotypic repulsion

Eph receptor and ephrin signalling has a major role in cell segregation and border formation, and may act through regulation of cell adhesion, repulsion or tension. To elucidate roles of cell repulsion and adhesion, we combined experiments in cell culture assays with quantitations of cell behaviour which are used in computer simulations. Cells expressing EphB2, or kinase-inactive EphB2 (kiEphB2), segregate and form a sharp border with ephrinB1-expressing cells, and this is disrupted by knockdown of N-cadherin. Measurements of contact inhibition of locomotion reveal that EphB2-, kiEphB2- and ephrinB1-expressing cells have strong heterotypic and weak homotypic repulsion. EphB2 cells have a transient increase in migration after heterotypic activation, which underlies a shift in the EphB2–ephrinB1 border but is not required for segregation or border sharpening. Simulations with the measured values of cell behaviour reveal that heterotypic repulsion can account for cell segregation and border sharpening, and is more efficient than decreased heterotypic adhesion. By suppressing homotypic repulsion, N-cadherin creates a sufficient difference between heterotypic and homotypic repulsion, and enables homotypic cohesion, both of which are required to sharpen borders.

Long-lived force patterns and deformation waves at repulsive epithelial boundaries

For an organism to develop and maintain homeostasis, cell types with distinct functions must often be separated by physical boundaries. The formation and maintenance of such boundaries are commonly attributed to mechanisms restricted to the cells lining the boundary. Here we show that, besides these local subcellular mechanisms, the formation and maintenance of tissue boundaries involves long-lived, long-ranged mechanical events. Following contact between two epithelial monolayers expressing, respectively, EphB2 and its ligand ephrinB1, both monolayers exhibit oscillatory patterns of traction forces and intercellular stresses that tend to pull cell-matrix adhesions away from the boundary. With time, monolayers jam, accompanied by the emergence of deformation waves that propagate away from the boundary. This phenomenon is not specific to EphB2/ephrinB1 repulsion but is also present during the formation of boundaries with an inert interface and during fusion of homotypic epithelial layers. Our findings thus unveil a global physical mechanism that sustains tissue separation independently of the biochemical and mechanical features of the local tissue boundary.

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.

EphA4 Regulates Neuroblast and Astrocyte Organization in a Neurogenic Niche

Significant migration cues are required to guide and contain newly generated rodent subventricular zone (SVZ) neuroblasts as they transit along the lateral ventricles and then through the anterior forebrain to their ultimate site of differentiation in the olfactory bulbs (OBs). These cues enforce strict neuroblast spatial boundaries within the dense astroglial meshwork of the SVZ and rostral migratory stream (RMS), yet are permissive to large-scale neuroblast migration. Therefore, the molecular mechanisms that define these cues and control dynamic interactions between migratory neuroblasts and surrounding astrocytes are of particular interest. We found that deletion of EphA4 and specifically ablation of EphA4 kinase activity resulted in misaligned neuroblasts and disorganized astrocytes in the RMS/SVZ, linking EphA4 forward signaling to SVZ and RMS spatial organization, orientation, and regulation. In addition, within a 3 week period, there was a significant reduction in the number of neuroblasts that reached the OB and integrated into the periglomerular layer, revealing a crucial role for EphA4 in facilitating efficient neuroblast migration to the OB. Single-cell analysis revealed that EPHA4 and its EFN binding partners are expressed by subpopulations of neuroblasts and astrocytes within the SVZ/RMS/OB system resulting in a cell-specific mosaic, suggesting complex EphA4 signaling involving both homotypic and heterotypic cell-cell interactions. Together, our studies reveal a novel molecular mechanism involving EphA4 signaling that functions in stem cell niche organization and ultimately neuroblast migration in the anterior forebrain.SIGNIFICANCE STATEMENT The subventricular zone neurogenic stem cell niche generates highly migratory neuroblasts that transit the anterior forebrain along a defined pathway to the olfactory bulb. Postnatal and adult brain organization dictates strict adherence to a narrow migration corridor. Subventricular zone neuroblasts are aligned in tightly bundled chains within a meshwork of astrocytes; however, the cell-cell cues that organize this unique, cell-dense migration pathway are largely unknown. Our studies show that forward signaling through the EphA4 tyrosine kinase receptor, mediated by ephrins expressed by subpopulations of neuroblasts and astrocytes, is required for compact, directional organization of neuroblasts and astrocytes within the pathway and efficient transit of neuroblasts through the anterior forebrain to the olfactory bulb.

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