Ephrins negatively regulate cell proliferation in the epidermis and hair follicle

A Pharmacological Investigation of Eph-Ephrin Antagonism in Prostate Cancer: UniPR1331 Efficacy Evidence

The Eph kinases are the largest receptor tyrosine kinases (RTKs) family in humans. PC3 human prostate adenocarcinoma cells are a well-established model for studying Eph-ephrin pharmacology as they naturally express a high level of EphA2, a promising target for new cancer therapies. A pharmacological approach with agonists did not show significant efficacy on tumor growth in prostate orthotopic murine models, but reduced distal metastasis formation. In order to improve the comprehension of the pharmacological targeting of Eph receptors in prostate cancer, in the present work, we investigated the efficacy of Eph antagonism both in vitro and in vivo, using UniPR1331, a small orally bioavailable Eph-ephrin interaction inhibitor. UniPR1331 was able to inhibit PC3 cells' growth in vitro in a dose-dependent manner, affecting the cell cycle and inducing apoptosis. Moreover, UniPR1331 promoted the PC3 epithelial phenotype, downregulating epithelial mesenchymal transition (EMT) markers. As a consequence, UniPR1331 reduced in vitro PC3 migration, invasion, and vasculomimicry capabilities. The antitumor activity of UniPR1331 was confirmed in vivo when administered alone or in combination with cytotoxic drugs in PC3-xenograft mice. Our results demonstrated that Eph antagonism is a promising strategy for inhibiting prostate cancer growth, especially in combination with cytotoxic drugs.

Protein-Protein Interaction Inhibitors Targeting the Eph-Ephrin System with a Focus on Amino Acid Conjugates of Bile Acids

The role of the Eph-ephrin system in the etiology of pathological conditions has been consolidated throughout the years. In this context, approaches directed against this signaling system, intended to modulate its activity, can be strategic therapeutic opportunities. Currently, the most promising class of compounds able to interfere with the Eph receptor-ephrin protein interaction is composed of synthetic derivatives of bile acids. In the present review, we summarize the progresses achieved, in terms of chemical expansions and structure-activity relationships, both in the steroidal core and the terminal carboxylic acid group, along with the pharmacological characterization for the most promising Eph-ephrin antagonists in in vivo settings.

Expression and distribution of EPHA4 and Ephrin A3 in Aohan fine-wool sheep skin

The objective of this study was to identify the expression and distribution of EPHA4 and Ephrin A3 genes in the development and morphogenesis of hair follicles in fine-wool sheep. The results could lay a theoretical basis for understanding the molecular mechanism that regulates hair follicle development. The skin of Aohan fine-wool sheep at different developmental stages (embryonic day 90, E90d, and 120, E120d, and postnatal day 1, B1d, and 30, B30d) were selected. Real-time quantitative polymerase chain reaction (RT-qPCR) and immunohistochemistry were used to study the levels of mRNA and proteins, respectively. The RT-qPCR results showed that the mRNA expression level of EPHA4 at B1d was significantly lower than at E120d (p<0.01). The expression of Ephrin A3 at E120d was significantly higher than that at E90d and B1d (p<0.01). Immunohistochemical detection results showed that the level and localisation of EPHA4 and Ephrin A3 proteins had spatial and temporal specificity. EPHA4 expression in dermal papilla cells might be important for inducing Aohan fine-hair follicle regeneration and for controlling the properties of the hair. Ephrin A3 might play an important role in the redifferentiation of secondary hair follicles and might also be involved in the inhibition of apoptosis-related gene expression in hair follicles. The Ephrin A3 signalling pathway might accelerate the growth of fine-hair follicles and increase the density of hair follicles.

Drug Conjugates for Targeting Eph Receptors in Glioblastoma

Glioblastoma (GBM) is a complex and heterogeneous tumor that warrants a comprehensive therapeutic approach for treatment. Tumor-associated antigens offer an opportunity to selectively target various components of the GBM microenvironment while sparing the normal cells within the central nervous system. In this study, we conjugated a multivalent vector protein, QUAD 3.0, that can target four receptors: EphA3, EphA2, EphB2, and also IL-13RA2, spanning virtually 100% of the GBM microenvironment, to doxorubicin derivatives. The conjugates effectively bound to all four receptors, although to varying degrees, and delivered cytotoxic loads to both established and patient-derived GBM cell lines, with IC50 values in the low nM range. The conjugates were also non-toxic to animals. We anticipate that the QUAD 3.0 Dox conjugates will be further used in preclinical models and possibly clinics in the foreseeable future.

EphA7 promotes myogenic differentiation via cell-cell contact

The conversion of proliferating skeletal muscle precursors (myoblasts) to terminally-differentiated myocytes is a critical step in skeletal muscle development and repair. We show that EphA7, a juxtacrine signaling receptor, is expressed on myocytes during embryonic and fetal myogenesis and on nascent myofibers during muscle regeneration in vivo. In EphA7^-/- mice, hindlimb muscles possess fewer myofibers at birth, and those myofibers are reduced in size and have fewer myonuclei and reduced overall numbers of precursor cells throughout postnatal life. Adult EphA7^-/- mice have reduced numbers of satellite cells and exhibit delayed and protracted muscle regeneration, and satellite cell-derived myogenic cells from EphA7^-/- mice are delayed in their expression of differentiation markers in vitro. Exogenous EphA7 extracellular domain will rescue the null phenotype in vitro, and will also enhance commitment to differentiation in WT cells. We propose a model in which EphA7 expression on differentiated myocytes promotes commitment of adjacent myoblasts to terminal differentiation.

Eph/ephrin family anchored on exosome facilitate communications between cells

Interactions of Ephrins and Eph receptors at cell membranes play crucial role in boundary formation and axon guidance. Extracellular vesicles (EVs), such as exosomes, are formed by cells communicating with each other in paracrine or endocrine manner. Until now, it is thought that direct cell-cell contact is necessary for ephrin and Eph receptor signal transduction. In this review, we discuss recent data that indicate the existence of a novel Eph-ephrin family anchored exosome signaling pathway in long-range intercellular communication and provide evidence that this type of signaling elicits cellular responses in cancer cells, independent of juxtacrine interactions. We emphasize that exosome-anchored Eph/ephrin involves a variety of biological processes and transduction signals, which may serve as a potential diagnostic biomarker.

The EphA2 receptor is activated through induction of distinct, ligand-dependent oligomeric structures

The EphA2 receptor tyrosine kinase is capable of activating multiple diverse signaling pathways with roles in processes such as tissue homeostasis and cancer. EphA2 is known to form activated oligomers in the presence of ephrin-A ligands. Here, we characterize the lateral interactions between full-length EphA2 molecules in the plasma membrane in the presence of three types of ligands (dimeric ephrinA1-Fc, monomeric ephrinA1, and an engineered peptide ligand) as well as in the absence of ligand, using a quantitative FRET technique. The data show that EphA2 forms higher-order oligomers and two different types of dimers that all lead to increased EphA2 tyrosine phosphorylation, which is indicative of increased kinase-dependent signaling. We find that different ligands stabilize conformationally distinct oligomers that are assembled through two different interfaces. Our results suggest that these different oligomeric assemblies could have distinct signaling properties, contributing to the diverse activities of the EphA2 receptor.

Deo Singh et al. use Fully Quantified Spectral Imaging-FRET to show that the EphA2 receptor forms dimers or higher order oligomers depending on the type of ligand, and that different ligands stabilize EphA2 dimers through distinct interfaces. These findings may explain how EphA2 activates diverse signaling pathways.

Pharmacological evaluation of new bioavailable small molecules targeting Eph/ephrin interaction

Eph/ephrin system is an emerging target for cancer therapy but the lack of potent, stable and orally bioavailable compounds is impairing the development of the field. Since 2009 our research group has been devoted to the discovery and development of small molecules targeting Eph/ephrin system and our research culminated with the synthesis of UniPR129, a potent but problematic Eph/ephrin antagonist. Herein, we describe the in vitro pharmacological properties of two derivatives (UniPR139 and UniPR502) stemmed from structure of UniPR129. These two compounds acted as competitive and reversible antagonists of all Eph receptors reducing both ephrin-A1 and -B1 binding to EphAs and EphBs receptors in the low micromolar range. The compounds acted as antagonists inhibiting ephrin-A1-dependent EphA2 activation and UniPR139 exerted an anti-angiogenic effect, inhibiting HUVEC tube formation in vitro and VEGF-induced vessel formation in the chick chorioallantoic membrane assay. Finally, the oral bioavailability of UniPR139 represents a step forward in the search of molecules targeting the Eph/ephrin system and offers a new pharmacological tool useful for future in vivo studies.

Cell type-specific localization of Ephs pairing with ephrin-B2 in the rat postnatal pituitary gland

Sox2-expressing stem/progenitor cells in the anterior lobe of the pituitary gland form two types of micro-environments (niches): the marginal cell layer and dense cell clusters in the parenchyma. In relation to the mechanism of regulation of niches, juxtacrine signaling via ephrin and its receptor Eph is known to play important roles in various niches. The ephrin and Eph families are divided into two subclasses to create ephrin/Eph signaling in co-operation with confined partners. Recently, we reported that ephrin-B2 localizes specifically to both pituitary niches. However, the Ephs interacting with ephrin-B2 in these pituitary niches have not yet been identified. Therefore, the present study aims to identify the Ephs interacting with ephrin-B2 and the cells that produce them in the rat pituitary gland. In situ hybridization and immunohistochemistry demonstrated cell type-specific localization of candidate interacting partners for ephrin-B2, including EphA4 in cells located in the posterior lobe, EphB1 in gonadotropes, EphB2 in corticotropes, EphB3 in stem/progenitor cells and EphB4 in endothelial cells in the adult pituitary gland. In particular, double-immunohistochemistry showed cis-interactions between EphB3 and ephrin-B2 in the apical cell membranes of stem/progenitor cell niches throughout life and trans-interactions between EphB2 produced by corticotropes and ephrin-B2 located in the basolateral cell membranes of stem/progenitor cells in the early postnatal pituitary gland. These data indicate that ephrin-B2 plays a role in pituitary stem/progenitor cell niches by selective interaction with EphB3 in cis and EphB2 in trans.

The role of Eph receptors and Ephrins in the skin

Eph receptors and Ephrin ligands are widely expressed in the skin. Various studies have been carried out to identify the effects of these molecules on many aspects of skin development. Here we summarize the literature that has identified roles for Eph receptors and Ephrins in the skin, focusing mainly on the epidermis, hair follicles, and cutaneous innervation. This review may help direct and focus further investigations into the role of Eph receptors and Ephrins in the development, maintenance, and repair processes in cutaneous biology.

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.

Eph receptor and ephrin function in breast, gut, and skin epithelia

Epithelial cells are tightly coupled together through specialized intercellular junctions, including adherens junctions, desmosomes, tight junctions, and gap junctions. A growing body of evidence suggests epithelial cells also directly exchange information at cell-cell contacts via the Eph family of receptor tyrosine kinases and their membrane-associated ephrin ligands. Ligand-dependent and -independent signaling via Eph receptors as well as reverse signaling through ephrins impact epithelial tissue homeostasis by organizing stem cell compartments and regulating cell proliferation, migration, adhesion, differentiation, and survival. This review focuses on breast, gut, and skin epithelia as representative examples for how Eph receptors and ephrins modulate diverse epithelial cell responses in a context-dependent manner. Abnormal Eph receptor and ephrin signaling is implicated in a variety of epithelial diseases raising the intriguing possibility that this cell-cell communication pathway can be therapeutically harnessed to normalize epithelial function in pathological settings like cancer or chronic inflammation.

Targeting EphA3 inhibits cancer growth by disrupting the tumor stromal microenvironment

Eph receptor tyrosine kinases are critical for cell-cell communication during normal and oncogenic tissue patterning and tumor growth. Somatic mutation profiles of several cancer genomes suggest EphA3 as a tumor suppressor, but its oncogenic expression pattern and role in tumorigenesis remain largely undefined. Here, we report unexpected EphA3 overexpression within the microenvironment of a range of human cancers and mouse tumor xenografts where its activation inhibits tumor growth. EphA3 is found on mouse bone marrow-derived cells with mesenchymal and myeloid phenotypes, and activation of EphA3(+)/CD90(+)/Sca1(+) mesenchymal/stromal cells with an EphA3 agonist leads to cell contraction, cell-cell segregation, and apoptosis. Treatment of mice with an agonistic α-EphA3 antibody inhibits tumor growth by severely disrupting the integrity and function of newly formed tumor stroma and microvasculature. Our data define EphA3 as a novel target for selective ablation of the tumor microenvironment and demonstrate the potential of EphA3 agonists for anticancer therapy.

Localization of juxtacrine factor ephrin-B2 in pituitary stem/progenitor cell niches throughout life

We have recently reported that Sox2-expressing pituitary stem/progenitor cells contact each other via a tight-junction protein CAR to form stem/progenitor cell niches in the marginal cell layer facing the lumen and in the clusters scattered in the parenchyma of the anterior lobe. However, the microenvironment of the niche for the maintenance of stem cell function in the pituitary remains obscure. In this study of pituitary stem/progenitor cell niches, we have attempted to identify the expression of juxtacrine factor ephrin and its receptor. We have found that ephrin-B2 is expressed in the pituitary throughout development but changes its localization pattern. Notably, in the adult pituitary, ephrin-B2 immuno-signals occur in SOX2-, E-cadherin-, and CAR-triple-positive stem/progenitor cells in the niches. Our data suggest that ephrin-B2 signaling has an important role in the formation of pituitary stem/progenitor cell niches and in pituitary organogenesis.

Developing stratified epithelia: lessons from the epidermis and thymus

Stratified squamous epithelial cells are found in a number of organs, including the skin epidermis and the thymus. The progenitor cells of the developing epidermis form a multi-layered epithelium and appendages, like the hair follicle, to generate an essential barrier to protect against water loss and invasion of foreign pathogens. In contrast, the thymic epithelium forms a three-dimensional mesh of keratinocytes that are essential for positive and negative selection of self-restricted T cells. While these distinct stratified epithelial tissues derive from distinct embryonic germ layers, both tissues instruct immunity, and the epithelial differentiation programs and molecular mechanisms that control their development are remarkably similar. In this review, we aim to highlight some of the similarities between the thymus and the skin epidermis and its appendages during developmental specification.

Eph receptors and ephrins: therapeutic opportunities

The erythropoietin-producing hepatocellular carcinoma (Eph) receptor tyrosine kinase family plays important roles in developmental processes, adult tissue homeostasis, and various diseases. Interaction with Eph receptor-interacting protein (ephrin) ligands on the surface of neighboring cells triggers Eph receptor kinase-dependent signaling. The ephrins can also transmit signals, leading to bidirectional cell contact-dependent communication. Moreover, Eph receptors and ephrins can function independently of each other through interplay with other signaling systems. Given their involvement in many pathological conditions ranging from neurological disorders to cancer and viral infections, Eph receptors and ephrins are increasingly recognized as attractive therapeutic targets, and various strategies are being explored to modulate their expression and function. Eph receptor/ephrin upregulation in cancer cells, the angiogenic vasculature, and injured or diseased tissues also offer opportunities for Eph/ephrin-based targeted drug delivery and imaging. Thus, despite the challenges presented by the complex biology of the Eph receptor/ephrin system, exciting possibilities exist for therapies exploiting these molecules.

Dynamic Wnt5a expression in murine hair follicle cycle and its inhibitory effects on follicular

OBJECTIVE

To analyze the dynamic expression of Wnt family member 5A (Wingless-type MMTV integration Wnt site family, member 5a) in murine hair cycle and its inhibitory effects on follicle in vivo.

METHODS

Situ hybridization in full-thickness skin was used to observe the change of mouse protein expression in different growth stages, and Ad-Wnt5a was injected after defeathering to observe the hair follicle growth in vivo.

RESULTS

The Wnt5a mRNA was expressed at birth, and was firstly increased then decreased along with the progress of the hair cycle. It reached the peak in advanced stage of growth cycle (P<0.05). Rhoa and β-catenin expression levels were significantly decreased in three groups. Rac2 expression was significantly up-regulated, and the expression level of Wnt5a, Shh and Frizzled2 was increased, but less significantly than group 2.

CONCLUSIONS

The expression of Wnt5a mRNA is consistent with change of murine follicle cycle, and has obvious inhibitory effects on the growth of hair follicle in vivo, indicating that it is antagonistic to Wnts pathway and interferes the growth of follicle together.

Skin as a living coloring book: how epithelial cells create patterns of pigmentation

The pigmentation of mammalian skin and hair develops through the interaction of two basic cell types - pigment donors and recipients. The pigment donors are melanocytes, which produce and distribute melanin through specialized structures. The pigment recipients are epithelial cells, which acquire melanin and put it to use, collectively yielding the pigmentation visible to the eye. This review will focus on the pigment recipients, the historically less understood cell type. These end-users of pigment are now known to exert a specialized control over the patterning of pigmentation, as they identify themselves as melanocyte targets, recruit pigment donors, and stimulate the transfer of melanin. As such, this review will discuss the evidence that the skin is like a coloring book: the pigment recipients create a 'picture,' a blueprint for pigmentation, which is colorless initially but outlines where pigment should be placed. Melanocytes then melanize the recipients and 'color in' the picture.

Heterogeneity and plasticity of epidermal stem cells

The epidermis is an integral part of our largest organ, the skin, and protects us against the hostile environment. It is a highly dynamic tissue that, during normal steady-state conditions, undergoes constant turnover. Multiple stem cell populations residing in autonomously maintained compartments facilitate this task. In this Review, we discuss stem cell behaviour during normal tissue homeostasis, regeneration and disease within the pilosebaceous unit, an integral structure of the epidermis that is responsible for hair growth and lubrication of the epithelium. We provide an up-to-date view of the pilosebaceous unit, encompassing the heterogeneity and plasticity of multiple discrete stem cell populations that are strongly influenced by external cues to maintain their identity and function.

Eph receptor tyrosine kinases in cancer stem cells

Eph receptor tyrosine kinases (RTKs) and their ligands, ephrins, play critical roles in development, tissue homeostasis, and cancer. Because Eph receptors are expressed in most adult stem cell niches and in many types of cancers, it has been long suspected that this family of RTKs may also regulate the function of cancer stem-like cells (CSCs). This review will focus on recent studies to elucidate the contribution of Eph/ephrin molecules in CSC self-renewal and tumorigenicity, as well as describe efforts to target these molecules in cancer. Because CSCs are often resistant to therapeutic intervention and have been shown to depend on Eph RTKs for self-renewal, targeting Eph receptors may hold promise for the treatment of drug-resistant cancers.

Therapeutic perspectives of Eph-ephrin system modulation

Eph receptors are the largest class of kinase receptors and, together with their ligands ephrins, they have a primary role in embryogenesis. Their expression has been found deregulated in several cancer tissues and, in many cases, abnormal levels of these proteins have been correlated to a poor prognosis. Recently, the Eph-ephrin system was found to be deregulated in other pathological processes, involving the nervous and cardiovascular systems. The increasing body of evidence supports the Eph-ephrin system as a target not only for the treatment of solid tumors, but also to face other critical diseases such as amyotrophic lateral sclerosis and diabetes driving current efforts toward the development of pharmacological tools potentially able to treat these pathologies.

Complementary expression and repulsive signaling suggest that EphB2 and ephrin-B1 are possibly involved in epithelial boundary formation at the squamocolumnar junction in the rodent stomach

Eph receptors and ephrin ligands are cell-cell communication molecules with well-defined roles in cell adhesion, migration, and tissue boundary formation. However, their expression levels in the squamocolumnar epithelial junction region at the distal esophagus are completely unknown. We examined EphB2 and ephrin-B1 localization in the squamocolumnar epithelial junction region between the proximal and distal stomach of the rodents. Immunostaining showed complimentary expression patterns along the proximal-to-distal axis of the gastric epithelia across the junction: EphB2 expression was maximal around the epithelial junction and sharply decreased in the stratified squamous epithelium at a short distance from the junction, whereas ephrin-B1 was strongly expressed in the stratified squamous epithelium at a distance from the junction and sharply decreased toward the junction. These expression patterns suggest that EphB2/ephrin-B1 signaling occurs preferentially in the epithelia across the junction, where the receptor and ligand expression highly overlap. We also show that (1) EphB2 preferentially binds ephrin-B1, and (2) cell repulsion/lateral migration was induced in primary cultured gastric keratinocytes on ephrin-B1-Fc- and EphB2-Fc-coated surfaces. On the basis of these findings, we propose that EphB2 and ephrin-B1 are possibly involved in epithelial boundary formation at the squamocolumnar junction.

Alteration of the EphA2/Ephrin-A signaling axis in psoriatic epidermis

EphA2 is a receptor tyrosine kinase (RTK) that triggers keratinocyte differentiation upon activation and subsequent downregulation by ephrin-A1 ligand. The objective of this study was to determine whether the EphA2/ephrin-A1 signaling axis was altered in psoriasis, an inflammatory skin condition in which keratinocyte differentiation is abnormal. Microarray analysis of skin biopsies from psoriasis patients revealed increased mRNA transcripts for several members of this RTK family in plaques, including the EphA1, EphA2, and EphA4 subtypes prominently expressed by keratinocytes. Of these, EphA2 showed the greatest upregulation, a finding that was confirmed by quantitative reverse-transcriptase-PCR, immunohistochemistry (IHC), and ELISA. In contrast, psoriatic lesions exhibited reduced ephrin-A ligand immunoreactivity. Exposure of primary keratinocytes induced to differentiate in high calcium or a three-dimensional (3D) raft culture of human epidermis to a combination of growth factors and cytokines elevated in psoriasis increased EphA2 mRNA and protein expression while inducing S100A7 and disrupting differentiation. Pharmacological delivery of a soluble ephrin-A1 peptidomimetic ligand led to a reduction in EphA2 expression and ameliorated proliferation and differentiation in raft cultures exposed to EGF and IL-1α. These findings suggest that ephrin-A1-mediated downregulation of EphA2 supports keratinocyte differentiation in the context of cytokine perturbation.

Microenvironment-dependent homeostasis and differentiation of epidermal basal undifferentiated keratinocytes and their clinical applications in skin repair

Skin homeostasis is maintained by controlling the balance between proliferation and differentiation of epidermal stem cells. The microenvironment, including extrinsic stresses, growth factors, soluble molecules, cell-ECM and cell-cell communications, plays an important role in cell fate determination in vivo and in vitro. In response to external signals, keratinocytes cooperate with other cell types to modulate and facilitate the wound microenvironment during wound healing; however, the aberrant signals or conjunctions in the environment will lead to pathologic abnormalities. In addition, despite some drawbacks, the epidermal stem-cellbased bioengineered skin substitutes have greatly improved the quality of cutaneous repair. Thus, exploring the characteristics and regulation mechanisms of microenvironment-dependent homeostasis and differentiation of epidermal basal undifferentiated keratinocytes is necessary to understand skin development and wound repair and to design novel therapeutic strategies for skin wound healing.

Regulation of tumor initiation and metastatic progression by Eph receptor tyrosine kinases

In recent years, a growing body of evidence has indicated that signaling molecules previously implicated in axon guidance are important regulators of multistep tumorigenesis and progression. Eph receptors and ephrins belong to this special class of molecules that play important roles in both axon guidance and cancer. Tremendous progress has been made in the past few years in both understanding the role of Eph receptors and ephrins in cancer and designing therapeutic strategies for cancer therapy. This review will focus on new advances in elucidating the contribution of Eph/ephrin molecules to key processes in tumor initiation and metastatic progression, including cancer cell proliferation, invasion and metastasis, and tumor angiogenesis.

Eph-2B, acting as an extracellular ligand, induces differentiation markers in epidermal keratinocytes

In the bi-directional signaling system comprising ephrins (EFNs) and ephrin receptors (Ephs), both EFNs and Ephs simultaneously function both as ligands and as receptors. Importantly, the EFN/Eph system is deregulated in human cancers and has been implicated in the metastatic processes because of its effects on the adhesion and migration of epithelial cells. The idiosyncratic function of Ephs, membrane-bound receptor kinases, as extracellular signaling ligands, has not been extensively studied. This prompted us to explore the transcriptional targets regulated by Ephs acting solely as ligands. To define the ligand function of EphB2 in human epidermal keratinocytes, we treated these cells with EphB2 as Fc-conjugate dimmers, which thus act exclusively as extracellular ligands. We compared the EphB2 and EFNA4 effects during a 48 h time course, using transcriptional profiling. We found that EphB2, acting as a ligand, promotes epidermal differentiation. For example, EphB2 induces expression of markers of epidermal differentiation, including keratins KRT1 and KRT10, SPRRs, desmosomal proteins and cell cycle inhibitors, while suppressing basal layer markers, integrins and cell cycle proteins. The effects of EphB2 are delayed relative to those of EFNA4. Unlike EFNA4, EphB2 did not induce lipid metabolism proteins, this particular aspect of epidermal differentiation seems not to be regulated by EphB2. Our results define the transcriptional targets of the reverse signaling by EphB2 acting exclusively as a ligand and begin to characterize this intriguing function of Ephs.

Eph and ephrins in epithelial stem cell niches and cancer

The family of Eph tyrosine kinase receptors is an important part of signaling pathways involved in development, tissue homeostasis and tumorigenesis. Binding and activation of the receptors by their ligands, the ephrins, result in bidirectional signaling into both receptor and ligand expressing cells. Adult stem cell niches and tumors frequently express receptors and ligands, although their function is only beginning to be understood. Thus, Eph receptors and ephrins have become important molecules for understanding basic biological processes as well as tumorigenesis, and are promising targets for potential therapeutic intervention in human disease.

Eph/ephrin interactions modulate muscle satellite cell motility and patterning

During development and regeneration, directed migration of cells, including neural crest cells, endothelial cells, axonal growth cones and many types of adult stem cells, to specific areas distant from their origin is necessary for their function. We have recently shown that adult skeletal muscle stem cells (satellite cells), once activated by isolation or injury, are a highly motile population with the potential to respond to multiple guidance cues, based on their expression of classical guidance receptors. We show here that, in vivo, differentiated and regenerating myofibers dynamically express a subset of ephrin guidance ligands, as well as Eph receptors. This expression has previously only been examined in the context of muscle-nerve interactions; however, we propose that it might also play a role in satellite cell-mediated muscle repair. Therefore, we investigated whether Eph-ephrin signaling would produce changes in satellite cell directional motility. Using a classical ephrin 'stripe' assay, we found that satellite cells respond to a subset of ephrins with repulsive behavior in vitro; patterning of differentiating myotubes is also parallel to ephrin stripes. This behavior can be replicated in a heterologous in vivo system, the hindbrain of the developing quail, in which neural crest cells are directed in streams to the branchial arches and to the forelimb of the developing quail, where presumptive limb myoblasts emigrate from the somite. We hypothesize that guidance signaling might impact multiple steps in muscle regeneration, including escape from the niche, directed migration to sites of injury, cell-cell interactions among satellite cell progeny, and differentiation and patterning of regenerated muscle.

Ectodomain structures of Eph receptors

Eph receptors, the largest subfamily of receptor tyrosine kinases (RTKs), and their ephrin ligands are important mediators of cell-cell communication that regulate axon guidance, long-term potentiation, and stem cell development, among others. By now, many Eph receptors and ephrins have also been found to play important roles in the progression of cancer. Since both the receptor and the ligand are membrane-bound, their interaction leads to the multimerization of both molecules to distinct clusters within their respective plasma membranes, resulting in the formation of discrete signaling centers. In addition, and unique to Eph receptors and ephrins, their interaction initiates bi-directional signaling cascades where information is transduced in the direction of both the receptor- and the ligand-bearing cells. The Ephs and the ephrins are divided into two subclasses, A and B, based on their affinities for each other and on sequence conservation. Crystal structures and other biophysical studies have indicated that isolated extracellular Eph and ephrin domains initially form high-affinity heterodimers around a hydrophobic loop of the ligand that is buried in a hydrophobic pocket on the surface of the receptor. The dimers can then further arrange by weaker interactions into higher-order Eph/ephrin clusters observed in vivo at the sites of cell-cell contact. Although the hetero-dimerization is a universal way to initiate signaling, other extracellular domains of Ephs are involved in the formation of higher-order clusters. The structures also show important differences defining the unique partner preferences of the two ligand and receptor subclasses, namely, how subclass specificity is determined both by individual interacting residues and by the precise architectural arrangement of ligands and receptors within the complexes.

Eph/ephrin signaling in epidermal differentiation and disease

Eph receptor tyrosine kinases mediate cell-cell communication by interacting with ephrin ligands residing on adjacent cell surfaces. In doing so, these juxtamembrane signaling complexes provide important contextual information about the cellular microenvironment that helps orchestrate tissue morphogenesis and maintain homeostasis. Eph/ephrin signaling has been implicated in various aspects of mammalian skin physiology, with several members of this large family of receptor tyrosine kinases and their ligands present in the epidermis, hair follicles, sebaceous glands, and underlying dermis. This review focuses on the emerging role of Eph receptors and ephrins in epidermal keratinocytes where they can modulate proliferation, migration, differentiation, and death. The activation of Eph receptors by ephrins at sites of cell-cell contact also appears to play a key role in the maturation of intercellular junctional complexes as keratinocytes move out of the basal layer and differentiate in the suprabasal layers of this stratified, squamous epithelium. Furthermore, alterations in the epidermal Eph/ephrin axis have been associated with cutaneous malignancy, wound healing defects and inflammatory skin conditions. These collective observations suggest that the Eph/ephrin cell-cell communication pathway may be amenable to therapeutic intervention for the purpose of restoring epidermal tissue homeostasis and integrity in dermatological disorders.

Role of Eph/ephrin tyrosine kinase in malignant glioma

Accumulating evidence has revealed that the tyrosine kinases play a major role in glioma proliferation and invasion. The largest family of tyrosine kinases, the Eph family, and its ligands, the ephrins, are frequently overexpressed in glioma, suggesting important roles for their bidirectional signals in glioma pathobiology. Ephs bind to cell surface-associated ephrin ligands on neighboring cells and have many biological functions during embryonic development of the central nervous system, including axon mapping, cell migration, and angiogenesis. Recent findings suggest that Eph/ephrin signaling affects glioma cell growth, migration, and invasion in vitro and in vivo. However, their roles in glioma seem complex, because both tumor growth promoter and suppressor potentials have been ascribed to Ephs and ephrins. Here, we review recent advances in research on the role of Eph/ephrin signaling in glioma and suggest that the Eph/ephrin system could be a potential target of glioma therapy.

EphB signaling controls lineage plasticity of adult neural stem cell niche cells

Stem cells remain in specialized niches over the lifespan of the organism in many organs to ensure tissue homeostasis and enable regeneration. How the niche is maintained is not understood, but is probably as important as intrinsic stem cell self-renewal capacity for tissue integrity. We here demonstrate a high degree of phenotypic plasticity of the two main niche cell types, ependymal cells and astrocytes, in the neurogenic lateral ventricle walls in the adult mouse brain. In response to a lesion, astrocytes give rise to ependymal cells and ependymal cells give rise to niche astrocytes. We identify EphB2 forward signaling as a key pathway regulating niche cell plasticity. EphB2 acts downstream of Notch and is required for the maintenance of ependymal cell characteristics, thereby inhibiting the transition from ependymal cell to astrocyte. Our results show that niche cell identity is actively maintained and that niche cells retain a high level of plasticity.

The ephrin A1-EphA2 system promotes cardiac stem cell migration after infarction

RATIONALE

Understanding the mechanisms that regulate trafficking of human cardiac stem cells (hCSCs) may lead to development of new therapeutic approaches for the failing heart.

OBJECTIVE

We tested whether the motility of hCSCs in immunosuppressed infarcted animals is controlled by the guidance system that involves the interaction of Eph receptors with ephrin ligands.

METHODS AND RESULTS

Within the cardiac niches, cardiomyocytes expressed preferentially the ephrin A1 ligand, whereas hCSCs possessed the EphA2 receptor. Treatment of hCSCs with ephrin A1 resulted in the rapid internalization of the ephrin A1-EphA2 complex, posttranslational modifications of Src kinases, and morphological changes consistent with the acquisition of a motile cell phenotype. Ephrin A1 enhanced the motility of hCSCs in vitro, and their migration in vivo following acute myocardial infarction. At 2 weeks after infarction, the volume of the regenerated myocardium was 2-fold larger in animals injected with ephrin A1-activated hCSCs than in animals receiving control hCSCs; this difference was dictated by a greater number of newly formed cardiomyocytes and coronary vessels. The increased recovery in myocardial mass with ephrin A1-treated hCSCs was characterized by further restoration of cardiac function and by a reduction in arrhythmic events.

CONCLUSIONS

Ephrin A1 promotes the motility of EphA2-positive hCSCs, facilitates their migration to the area of damage, and enhances cardiac repair. Thus, in situ stimulation of resident hCSCs with ephrin A1 or their ex vivo activation before myocardial delivery improves cell targeting to sites of injury, possibly providing a novel strategy for the management of the diseased heart.

Specific and shared targets of ephrin A signaling in epidermal keratinocytes

Both ephrins (EFNs) and their receptors (Ephs) are membrane-bound, restricting their interactions to the sites of direct cell-to-cell interfaces. They are widely expressed, often co-expressed, and regulate developmental processes, cell adhesion, motility, survival, proliferation, and differentiation. Both tumor suppressor and oncogene activities are ascribed to EFNs and Ephs in various contexts. A major conundrum regarding the EFN/Eph system concerns their large number and functional redundancy given the promiscuous cross-activation of ligands and receptors and the overlapping intracellular signaling pathways. To address this issue, we treated human epidermal keratinocytes with five EFNAs individually and defined the transcriptional responses in the cells. We found that a large set of genes is coregulated by all EFNAs. However, although the responses to EFNA3, EFNA4, and EFNA5 are identical, the responses to EFNA1 and EFNA2 are characteristic and distinctive. All EFNAs induce epidermal differentiation markers and suppress cell adhesion genes, especially integrins. Ontological analysis showed that all EFNAs induce cornification and keratin genes while suppressing wound healing-associated, signaling, receptor, and extracellular matrix-associated genes. Transcriptional targets of AP1 are selectively suppressed by EFNAs. EFNA1 and EFNA2, but not the EFNA3, EFNA4, EFNA5 cluster, regulate the members of the ubiquitin-associated proteolysis genes. EFNA1 specifically induces collagen production. Our results demonstrate that the EFN-Eph interactions in the epidermis, although promiscuous, are not redundant but specific. This suggests that different members of the EFN/Eph system have specific, clearly demarcated functions.

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

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

Ligand targeting of EphA2 enhances keratinocyte adhesion and differentiation via desmoglein 1

Cell–cell adhesion and communication maintains epithelial tissue homeostasis. This study demonstrates that ephrin ligands target the EphA2 receptor to dampen Erk1/2 signaling without affecting cell proliferation and identifies a novel link with desmoglein 1, a desmosomal cadherin that enhances epidermal adhesion and differentiation.

EphA2 is a receptor tyrosine kinase that is engaged and activated by membrane-linked ephrin-A ligands residing on adjacent cell surfaces. Ligand targeting of EphA2 has been implicated in epithelial growth regulation by inhibiting the extracellular signal-regulated kinase 1/2 (Erk1/2)-mitogen activated protein kinase (MAPK) pathway. Although contact-dependent EphA2 activation was required for dampening Erk1/2-MAPK signaling after a calcium switch in primary human epidermal keratinocytes, the loss of this receptor did not prevent exit from the cell cycle. Incubating keratinocytes with a soluble ephrin-A1-Fc peptide mimetic to target EphA2 further increased receptor activation leading to its down-regulation. Moreover, soluble ligand targeting of EphA2 restricted the lateral expansion of epidermal cell colonies without limiting proliferation in these primary cultures. Rather, ephrin-A1-Fc peptide treatment promoted epidermal cell colony compaction and stratification in a manner that was associated with increased keratinocyte differentiation. The ligand-dependent increase in keratinocyte adhesion and differentiation relied largely upon the up-regulation of desmoglein 1, a desmosomal cadherin that maintains the integrity and differentiated state of suprabasal keratinocytes in the epidermis. These data suggest that keratinocytes expressing EphA2 in the basal layer may respond to ephrin-A1–based cues from their neighbors to facilitate entry into a terminal differentiation pathway.