Eph/ephrin molecules--a hub for signaling and endocytosis

Human endothelial progenitor cells

Human endothelial progenitor cells (EPCs) have been generally defined as circulating cells that express a variety of cell surface markers similar to those expressed by vascular endothelial cells, adhere to endothelium at sites of hypoxia/ischemia, and participate in new vessel formation. Although no specific marker for an EPC has been identified, a panel of markers has been consistently used as a surrogate marker for cells displaying the vascular regenerative properties of the putative EPC. However, it is now clear that a host of hematopoietic and vascular endothelial subsets display the same panel of antigens and can only be discriminated by an extensive gene expression analysis or use of a variety of functional assays that are not often applied. This article reviews our current understanding of the many cell subsets that constitute the term EPC and provides a concluding perspective as to the various roles played by these circulating or resident cells in vessel repair and regeneration in human subjects.

EphrinA2 regulates clathrin mediated KSHV endocytosis in fibroblast cells by coordinating integrin-associated signaling and c-Cbl directed polyubiquitination

Kaposi's sarcoma-associated herpesvirus (KSHV) interacts with human dermal endothelial cell surface tyrosine kinase EphrinA2 (EphA2) and integrins (α3β1 and αVβ3) in the lipid raft (LR) region, and EphA2 regulates macropinocytic virus entry by coordinating integrin-c-Cbl associated signaling. In contrast, KSHV enters human foreskin fibroblast (HFF) cells by LR-independent clathrin mediated endocytosis. The present studies conducted to identify the key molecules regulating KSHV entry in HFF cells showed that KSHV induces association with integrins (αVβ5, αVβ3 and α3β1) and EphA2 in non-LR regions early during infection and activates EphA2, which in turn associates with phosphorylated c-Cbl, myosin IIA, FAK, Src, and PI3-K, as well as clathrin and its adaptor AP2 and effector Epsin-15 proteins. EphA2 knockdown significantly reduced these signal inductions, virus internalization and gene expression. c-Cbl knockdown ablated the c-Cbl mediated K63 type polyubiquitination of EphA2 and clathrin association with EphA2 and KSHV. Mutations in EphA2's tyrosine kinase domain (TKD) or sterile alpha motif (SAM) abolished its interaction with c-Cbl. Mutations in tyrosine kinase binding (TKB) or RING finger (RF) domains of c-Cbl resulted in very poor association of c-Cbl with EphA2 and decreased EphA2 polyubiquitination. These studies demonstrated the contributions of these domains in EphA2 and c-Cbl association, EphA2 polyubiquitination and virus-EphA2 internalization. Collectively, these results revealed for the first time that EphA2 influences the tyrosine phosphorylation of clathrin, the role of EphA2 in clathrin mediated endocytosis of a virus, and c-Cbl mediated EphA2 polyubiquitination directing KSHV entry in HFF cells via coordinated signal induction and progression of endocytic events, all of which suggest that targeting EphA2 and c-Cbl could block KSHV entry and infection.

Structurally encoded intraclass differences in EphA clusters drive distinct cell responses

The functional outcomes of ephrin binding to Eph receptors (Ephs) range from cell repulsion to adhesion. Here we used cell collapse and stripe assays to show contrasting effects of human ephrinA5 binding to EphA2 and EphA4. Despite equivalent ligand-binding affinities EphA4 triggered greater cell collapse, while EphA2-expressing cells adhered better to ephrinA5-coated surfaces. Chimeric receptors showed the ectodomain is a major determinant of cell response. We report crystal structures of EphA4 ectodomain alone and in complexes with ephrinB3 and ephrinA5. These revealed closed clusters with a dimeric or circular arrangement in the crystal lattice, contrasting with extended arrays previously observed for EphA2 ectodomain. Localization microscopy-based analyses showed ligand-stimulated EphA4 induces smaller clusters than EphA2. Mutant Ephs link these characteristics to interactions observed in the crystal lattices, suggesting a mechanism by which distinctive ectodomain surfaces determine clustering, and thereby signalling, properties.

Pharmacological inhibition of Eph receptors enhances glucose-stimulated insulin secretion from mouse and human pancreatic islets

AIMS/HYPOTHESIS

Type 2 diabetes is characterised by impaired glucose-stimulated insulin secretion (GSIS) from pancreatic islets. Since erythropoietin-producing hepatoma (Eph)-ephrin bidirectional signalling fine-tunes GSIS from pancreatic beta cells, we investigated Eph receptor tyrosine kinases (RTK) as potential drug targets for selectively increasing GSIS.

METHODS

Insulin secretion assays were carried out using mouse and human pancreatic islets as well as mouse insulinoma (MIN6) cells in the presence or absence of two Eph RTK inhibitors. Furthermore, the most potent inhibitor was injected into mice to evaluate its effects on glucose tolerance and plasma insulin levels.

RESULTS

We showed that the Eph RTK inhibitors selectively increased GSIS from MIN6 cells as well as mouse and human islets. Our results also showed that the insulin secretory effects of these compounds required Eph-ephrin signalling. Finally, pharmacological inhibition of Eph receptor signalling improved glucose tolerance in mice.

CONCLUSIONS/INTERPRETATION

We showed for the first time that Eph RTKs represent targets for small molecules to selectively increase GSIS and improve glucose tolerance.

Impaired EphA4 signaling leads to congenital hydronephrosis, renal injury, and hypertension

Experimental hydronephrosis induced by partial ureteral obstruction at 3 wk of age causes hypertension and renal impairment in adult rats and mice. Signaling by Ephrin receptors (Eph) and their ligands (ephrins) importantly regulates embryonic development. Genetically modified mice, where the cytoplasmic domain of the EphA4 receptor has been substituted by enhanced green fluorescent protein (EphA4gf/gf), develop spontaneous hydronephrosis and provide a model for further studies of the disorder. The present study aimed to determine if animals with congenital hydronephrosis develop hypertension and renal injuries, similar to that of experimental hydronephrosis. Ultrasound and Doppler techniques were used to visualize renal impairment in the adult mice. Telemetric blood pressure measurements were performed in EphA4gf/gf mice and littermate controls (EphA4+/+) during normal (0.7% NaCl)- and high (4% NaCl)-sodium conditions. Renal excretion, renal plasma flow, and glomerular filtration were studied, and histology and morphology of the kidneys and ureters were performed. EphA4gf/gf mice developed variable degrees of hydronephrosis that correlated with their blood pressure level. In contrast to EphA4+/+, the EphA4gf/gf mice displayed salt-sensitive hypertension, reduced urine concentrating ability, reduced renal plasma flow, and lower glomerular filtration rate. Kidneys from EphA4gf/gf mice showed increased renal injuries, as evidenced by fibrosis, inflammation, and glomerular and tubular changes. In conclusion, congenital hydronephrosis causes hypertension and renal damage, similar to that observed in experimentally induced hydronephrosis. This study further reinforces the supposed causal link between hydronephrosis and later development of hypertension in humans.

Endocytosis of receptor tyrosine kinases

Endocytosis is the major regulator of signaling from receptor tyrosine kinases (RTKs). The canonical model of RTK endocytosis involves rapid internalization of an RTK activated by ligand binding at the cell surface and subsequent sorting of internalized ligand-RTK complexes to lysosomes for degradation. Activation of the intrinsic tyrosine kinase activity of RTKs results in autophosphorylation, which is mechanistically coupled to the recruitment of adaptor proteins and conjugation of ubiquitin to RTKs. Ubiquitination serves to mediate interactions of RTKs with sorting machineries both at the cell surface and on endosomes. The pathways and kinetics of RTK endocytic trafficking, molecular mechanisms underlying sorting processes, and examples of deviations from the standard trafficking itinerary in the RTK family are discussed in this work.

Netrin-1-induced activation of Notch signaling mediates glioblastoma cell invasion

Glioblastoma multiforme is an aggressively invasive human brain cancer, which lacks effective treatment. The axonal guidance protein, netrin-1, is overexpressed in glioblastoma tumor biopsies. In Matrigel invasion assays we observed that experimental overexpression of netrin-1 increased cell invasiveness and its downregulation decreased invasiveness. Using tandem affinity purification and mass spectrometry protein identification we found that netrin-1 forms a complex with both Notch2 and Jagged1. Recombinant netrin-1 colocalized with Jagged1 and Notch2 at the cell surface and was also present in the intracellular vesicles with Jagged1, but not with Notch2. Netrin-1 activated Notch signaling and subsequent glioblastoma cell invasion. Interestingly, the recombinant central domain of netrin-1 counteracted the effects of the full-length netrin-1: it inhibited glioblastoma cell invasion and Notch activation by retaining the Notch signaling complex at the cell surface. This finding may give rise to therapeutic applications. These results reveal a new mechanism leading to glioblastoma cell invasion, in which netrin-1 activates Notch signaling.

Eph receptors and their ligands: promising molecular biomarkers and therapeutic targets in prostate cancer

Although at present, there is a high incidence of prostate cancer, particularly in the Western world, mortality from this disease is declining and occurs primarily only from clinically significant late stage tumors with a poor prognosis. A major current focus of this field is the identification of new biomarkers which can detect earlier, and more effectively, clinically significant tumors from those deemed "low risk", as well as predict the prognostic course of a particular cancer. This strategy can in turn offer novel avenues for targeted therapies. The large family of Receptor Tyrosine Kinases, the Ephs, and their binding partners, the ephrins, has been implicated in many cancers of epithelial origin through stimulation of oncogenic transformation, tumor angiogenesis, and promotion of increased cell survival, invasion and migration. They also show promise as both biomarkers of diagnostic and prognostic value and as targeted therapies in cancer. This review will briefly discuss the complex roles and biological mechanisms of action of these receptors and ligands and, with regard to prostate cancer, highlight their potential as biomarkers for both diagnosis and prognosis, their application as imaging agents, and current approaches to assessing them as therapeutic targets. This review demonstrates the need for future studies into those particular family members that will prove helpful in understanding the biology and potential as targets for treatment of prostate cancer.

Presenilin mediates neuroprotective functions of ephrinB and brain-derived neurotrophic factor and regulates ligand-induced internalization and metabolism of EphB2 and TrkB receptors

Activation of EphB receptors by ephrinB (efnB) ligands on neuronal cell surface regulates important functions, including neurite outgrowth, axonal guidance, and synaptic plasticity. Here, we show that efnB rescues primary cortical neuronal cultures from necrotic cell death induced by glutamate excitotoxicity and that this function depends on EphB receptors. Importantly, the neuroprotective function of the efnB/EphB system depends on presenilin 1 (PS1), a protein that plays crucial roles in Alzheimer's disease (AD) neurodegeneration. Furthermore, absence of one PS1 allele results in significantly decreased neuroprotection, indicating that both PS1 alleles are necessary for full expression of the neuroprotective activity of the efnB/EphB system. We also show that the ability of brain-derived neurotrophic factor (BDNF) to protect neuronal cultures from glutamate-induced cell death depends on PS1. Neuroprotective functions of both efnB and BDNF, however, were independent of γ-secretase activity. Absence of PS1 decreases cell surface expression of neuronal TrkB and EphB2 without affecting total cellular levels of the receptors. Furthermore, PS1-knockout neurons show defective ligand-dependent internalization and decreased ligand-induced degradation of TrkB and Eph receptors. Our data show that PS1 mediates the neuroprotective activities of efnB and BDNF against excitotoxicity and regulates surface expression and ligand-induced metabolism of their cognate receptors. Together, our observations indicate that PS1 promotes neuronal survival by regulating neuroprotective functions of ligand-receptor systems.

Henipavirus receptor usage and tropism

Nipah (NiV) and Hendra (HeV) viruses are the deadliest human pathogens within the Paramyxoviridae family, which include human and animal pathogens of global biomedical importance. NiV and HeV infections cause respiratory and encephalitic illness with high mortality rates in humans. Henipaviruses (HNV) are the only Paramyxoviruses classified as biosafety level 4 (BSL4) pathogens due to their extreme pathogenicity, potential for bioterrorism, and lack of licensed vaccines and therapeutics. HNV use ephrin-B2 and ephrin-B3, highly conserved proteins, as viral entry receptors. This likely accounts for their unusually broad species tropism, and also provides opportunities to study how receptor usage, cellular tropism, and end-organ pathology relates to the pathobiology of HNV infections. The clinical and pathologic manifestations of NiV and HeV virus infections are reviewed in the chapters by Wong et al. and Geisbert et al. in this issue. Here, we will review the biology of the HNV receptors, and how receptor usage relates to HNV cell tropism in vitro and in vivo.

Efficient siRNA delivery into tumor cells by p19-YSA fusion protein

For the efficient cytoplasmic delivery of siRNA in a receptor-specific fashion, we designed a p19-YSA fusion protein composed of p19 RNA binding protein and ephrin mimetic peptide (YSA peptide). The resulting recombinant protein had the high affinity for EphA2 receptor overexpressed on cancer cells as well as the complexing ability with siRNA, thus leading to tumor-targeted delivery of siRNA. The buried structure of siRNA within p19-YSA/siRNA complexes allowed the bound siRNAs to be protected from the external RNases, resulting in the enhanced stability of siRNA in serum conditions. The p19-YSA carriers could complex with siRNA in a size-dependent and sequence-independent manner and showed the pH-dependent complexing/dissocation behaviors with siRNA. In contrast to electrostatic interaction-mediated siRNA delivery systems such as cationic polymers/siRNA or cationic polypeptides/siRNA complexes, the bound siRNA within p19-YSA/siRNA complexes showed enhanced stability against large polyanions found outside cells, due to the nanomolar levels of affinity. Here, we demonstrated the superior efficiency of p19-YSA/siRNA complexes in RFP gene silencing, compared to untreated cells. These results provide an alternative approach to enhance the stability of siRNA as well as to achieve the targeted siRNA delivery.

Eph/ephrin signalling during development

Eph receptors and their membrane-tethered ligands have important functions in development. Trans interactions of Eph receptors with ephrins at cell-cell interfaces promote a variety of cellular responses, including repulsion, attraction and migration. Eph-ephrin signalling can be bi-directional and controls actin cytoskeleton dynamics, thereby leading to changes in cellular shape. This article provides an overview of the general structures and signalling mechanisms, and of typical developmental functions along with cell biological principles.

Eph receptor signaling in C. elegans

Eph receptor protein-tyrosine kinases are among the oldest known animal receptors and have greatly expanded in number during vertebrate evolution. Their complex transduction mechanisms are capable of bidirectional and bimodal (multi-response) signaling. Eph receptors are expressed in almost every cell type in the human body, yet their roles in development, physiology, and disease are incompletely understood. Studies in C. elegans have helped identify biological functions of these receptors, as well as transduction mechanisms. Here we review advances in our understanding of Eph receptor signaling made using the C. elegans model system.

Discrete ephrin-B1 expression by specific layers of the primate retinogeniculostriate system continues throughout postnatal and adult life

The molecular guidance cue ephrin-B1 has traditionally been associated with the early development of the visual system, encompassing retinocollicular mapping as well as development and maturation of synapses. Although little is known about its role in the visual system during the postnatal period and in adulthood, recent studies have demonstrated the expression of ephrin-B1 in the adult mouse brain, indicating a sustained role beyond early development. Therefore, we explored the spatiotemporal expression of ephrin-B1 in the postnatal and adult nonhuman primate visual system and demonstrated that a modulated expression continued following birth into adulthood in the lateral geniculate nucleus (LGN) and primary visual cortex (V1, striate cortex). This occurred in the layers involved in bidirectional geniculostriate communication: layers 3Bβ, 4, and 6 of V1 and the parvocellular (P) and magnocellular (M) layers of the LGN. Furthermore, discrete gradients between the ipsi- and contralateral inputs of the P and M layers of the LGN evolved between 1 month following birth and the start of the critical period (3 months), and continued into adulthood. We also detected the postsynaptic expression of ephrin-B1 by excitatory cells in adult LGN and V1 and a subset of interneurons in adult V1, suggestive of a more global rather than subtype-specific role. Together these results suggest a possible role for ephrin-B1 in the maturation of the primate retinogeniculostriate pathway throughout postnatal life, extending into adulthood.

Synapse dysfunction in autism: a molecular medicine approach to drug discovery in neurodevelopmental disorders

Autism and autism spectrum disorders (ASDs) affect millions of individuals worldwide. Despite increased autism diagnoses over the past 30 years, therapeutic intervention is often 'trial and error'. This approach has identified some beneficial agents, but complex heterogeneous disorders require a more personalized treatment regimen. Many ASD risk factors are genetic, implicating impaired synaptic development and function. Monogenetic disorders (e.g., fragile X syndrome, Rett syndrome, and neurofibromatosis) that have phenotypic overlap with autism provide insights into ASD pathology through the identification novel drug targets (e.g., glutamatergic receptors). Encouragingly, some of these novel drug targets provide symptomatic improvement, even in patients who have lived with ASDs for protracted periods of time. Consequently, a targeted drug discovery approach is expected to deliver improved agents for the treatment and management of ASDs. Here, we review the opportunities and challenges in drug development for autism and provide insight into the neurobiology of ASDs.

Eph/Ephrin signaling in injury and inflammation

The Eph/ephrin receptor-ligand system plays an important role in embryogenesis and adult life, principally by influencing cell behavior through signaling pathways, resulting in modification of the cell cytoskeleton and cell adhesion. There are 10 EphA receptors, and six EphB receptors, distinguished on sequence difference and binding preferences, that interact with the six glycosylphosphatidylinositol-linked ephrin-A ligands and the three transmembrane ephrin-B ligands, respectively. The Eph/ephrin proteins, originally described as developmental regulators that are expressed at low levels postembryonically, are re-expressed after injury to the optic nerve, spinal cord, and brain in fish, amphibians, rodents, and humans. In rodent spinal cord injury, the up-regulation of EphA4 prevents recovery by inhibiting axons from crossing the injury site. Eph/ephrin proteins may be partly responsible for the phenotypic changes to the vascular endothelium in inflammation, which allows fluid and inflammatory cells to pass from the vascular space into the interstitial tissues. Specifically, EphA2/ephrin-A1 signaling in the lung may be responsible for pulmonary inflammation in acute lung injury. A role in T-cell maturation and chronic inflammation (heart failure, inflammatory bowel disease, and rheumatoid arthritis) is also reported. Although there remains much to learn about Eph/ephrin signaling in human disease, and specifically in injury and inflammation, this area of research raises the exciting prospect that novel therapies will be developed that precisely target these pathways.

EphA/ephrin-A signaling is critically involved in region-specific apoptosis during early brain development

EphAs and ephrin-As have been implicated in the morphogenesis of the developing brain. We found that EphA7 and ephrin-A5 are coexpressed in the dorsal midline (DM) of the diencephalon and anterior mesencephalon. Interestingly, programmed cell death (PCD) of the neural epithelial cells normally found in this region was reduced in ephrin-A5/ephrin-A2 dual-deficient embryos. In contrast, in vivo expression of ephrin-A5-Fc or full-length ephrin-A5 strongly induced apoptosis in neural epithelial cells and was accompanied by severe brain malformation during embryonic development. Expression of ephrinA5-Fc correlated with apoptosis of EphA7-expressing cells, whereas null mutation of ephrin-A5 resulted in the converse phenotype. Importantly, null mutation of caspase-3 or endogenous ephrin-A5 attenuated the PCD induced by ectopically overexpressed ephrin-A5. Together, our results suggest that brain region-specific PCD may occur in a region where EphAs cluster with neighboring ephrin-As through cell-cell contact.

Essential roles of EphB receptors and EphrinB ligands in endothelial cell function and angiogenesis

Eph receptor tyrosine kinases and their Ephrin ligands represent an important signaling system with widespread roles in cell physiology and disease. Receptors and ligands in this family are anchored to the cell surface; thus Eph/Ephrin interactions mainly occur at sites of cell-to-cell contact. EphB4 and EphrinB2 are the Eph/Ephrin molecules that play essential roles in vascular development and postnatal angiogenesis. Analysis of expression patterns and function has linked EphB4/EphrinB2 to endothelial cell growth, survival, migration, assembly, and angiogenesis. Signaling from these molecules is complex, with the potential for being bidirectional, emanating both from the Eph receptors (forward signaling) and from the Ephrin ligands (reverse signaling). In this review, we describe recent advances on the roles of EphB/EphrinB protein family in endothelial cell function and outline potential approaches to inhibit pathological angiogenesis based on this understanding.

Biophysics of substrate interaction: influence on neural motility, differentiation, and repair

The identity and behavior of a cell is shaped by the molecular and mechanical composition of its surroundings. Molecular cues have firmly established roles in guiding both neuronal fate decisions and the migration of cells and axons. However, there is growing evidence that topographical and rigidity cues in the extracellular environment act synergistically with these molecular cues. Like chemical cues, physical factors do not elicit a fixed response, but rather one that depends on the sensory makeup of the cell. Moreover, from developmental studies and the plasticity of neural tissue, it is evident that there is dynamic feedback between physical and chemical factors to produce the final morphology. Here, we focus on our current understanding of how these physical cues shape cellular differentiation and migration, and discuss their relevance to repairing the injured nervous system.

'Til Eph do us part': intercellular signaling via Eph receptors and ephrin ligands guides cerebral cortical development from birth through maturation

Eph receptors, the largest family of surface-bound receptor tyrosine kinases and their ligands, the ephrins, mediate a wide variety of cellular interactions in most organ systems throughout both development and maturity. In the forming cerebral cortex, Eph family members are broadly and dynamically expressed in particular sets of cortical cells at discrete times. Here, we review the known functions of Eph-mediated intercellular signaling in the generation of progenitors, the migration of maturing cells, the differentiation of neurons, the formation of functional connections, and the choice between life and death during corticogenesis. In synthesizing these results, we posit a signaling paradigm in which cortical cells maintain a life history of Eph-mediated intercellular interactions that guides subsequent cellular decision-making.

Cellular decisions in cardiac outflow tract and coronary development: an act by VEGF and NOTCH

Congenital cardiac abnormalities are, due to their relatively high frequency and severe impact on quality of life, an important focus in cardiovascular research. Recently, various human studies have revealed a high coincidence of VEGF and NOTCH polymorphisms with cardiovascular outflow tract anomalies, such as bicuspid aortic valves and Tetralogy of Fallot, next to predisposition for cardiovascular pathologies, including atherosclerosis and aortic valve calcification. This genetic association between VEGF/NOTCH mutations and congenital cardiovascular defects in humans has been supported by substantial proof from animal models, revealing interaction of both pathways in cellular processes that are crucial for cardiac development. This review focuses on the role of VEGF and NOTCH signaling and their interplay in cardiogenesis with special interest to coronary and outflow tract development. An overview of the association between congenital malformations and VEGF/NOTCH polymorphisms in humans will be discussed along with their potential mechanisms and processes as revealed by transgenic mouse models. The molecular and cellular interaction of VEGF and subsequent Notch-signaling in these processes will be highlighted.

The role of endocytosis in activating and regulating signal transduction

Endocytosis is increasingly understood to play crucial roles in most signaling pathways, from determining which signaling components are activated, to how the signal is subsequently transduced and/or terminated. Whether a receptor-ligand complex is internalized via a clathrin-dependent or clathrin-independent endocytic route, and the complexes' subsequent trafficking through specific endocytic compartments, to then be recycled or degraded, has profound effects on signaling output. This review discusses the roles of endocytosis in three markedly different signaling pathways: the Wnt, Notch, and Eph/Ephrin pathways. These offer fundamentally different signaling systems: (1) diffusible ligands inducing signaling in one cell, (2) membrane-tethered ligands inducing signaling in a contacting receptor cell, and (3) bi-directional receptor-ligand signaling in two contacting cells. In each of these systems, endocytosis controls signaling in fascinating ways, and comparison of their similarities and dissimilarities will help to expand our understanding of endocytic control of signal transduction across multiple signaling pathways.

Eph-dependent cell-cell adhesion and segregation in development and cancer

Numerous studies attest to essential roles for Eph receptors and their ephrin ligands in controlling cell positioning and tissue patterning during normal and oncogenic development. These studies suggest multiple, sometimes contradictory, functions of Eph-ephrin signalling, which under different conditions can promote either spreading and cell-cell adhesion or cytoskeletal collapse, cell rounding, de-adhesion and cell-cell segregation. A principle determinant of the balance between these two opposing responses is the degree of receptor/ligand clustering and activation. This equilibrium is likely altered in cancers and modulated by somatic mutations of key Eph family members that have emerged as candidate cancer markers in recent profiling studies. In addition, cross-talk amongst Ephs and with other signalling pathways significantly modulates cell-cell adhesion, both between and within Eph- and ephrin-expressing cell populations. This review summarises our current understanding of how Eph receptors control cell adhesion and morphology, and presents examples demonstrating the importance of these events in normal development and cancer.

Spatial regulation of receptor tyrosine kinases in development and cancer

During development and tissue homeostasis, patterns of cellular organization, proliferation and movement are highly choreographed. Receptor tyrosine kinases (RTKs) have a crucial role in establishing these patterns. Individual cells and tissues exhibit tight spatial control of the RTKs that they express, enabling tissue morphogenesis and function, while preventing unwarranted cell division and migration that can contribute to tumorigenesis. Indeed, RTKs are deregulated in most human cancers and are a major focus of targeted therapeutics. A growing appreciation of the essential role of spatial RTK regulation during development prompts the realization that spatial deregulation of RTKs is likely to contribute broadly to cancer development and may affect the sensitivity and resistance of cancer to pharmacological RTK inhibitors.

Regulation of VEGF signaling by membrane traffic

Recent findings have drawn attention to the role of membrane traffic in the signaling of vascular endothelial growth factor (VEGF). The significance of this development stems from the pivotal function of VEGF in vasculogenesis and angiogenesis. The outline of the regulation of VEGF receptor (VEGFR) signaling by membrane traffic is similar to that of the epidermal growth factor receptor (EGFR), a prototype of the intertwining between membrane traffic and signaling. There are, however, unique features in VEGFR signaling that are conferred in part by the involvement of the co-receptor neuropilin (Nrp). Nrp1 and VEGFR2 are integrated into membrane traffic through the adaptor protein synectin, which recruits myosin VI, a molecular motor that drives inward trafficking [17,21,64]. The recent detection of only mild vascular defects in a knockin mouse model that expresses Nrp1 lacking a cytoplasmic domain [104], questions the co-receptor's role in VEGF signaling and membrane traffic. The regulation of endocytosis by ephrin-B2 is another feature unique to VEGR2/3 [18,19], but it awaits a mechanistic explanation. Current models do not fully explain how membrane traffic bridges between VEGFR and the downstream effectors that produce its functional outcome, such as cell migration. VEGF-A appears to accomplish this task in part by recruiting endocytic vesicles carrying RhoA to internalized active VEGFR2 [58].

Initiation of synapse formation by Wnt-induced MuSK endocytosis

In zebrafish, the MuSK receptor initiates neuromuscular synapse formation by restricting presynaptic growth cones and postsynaptic acetylcholine receptors (AChRs) to the center of skeletal muscle cells. Increasing evidence suggests a role for Wnts in this process, yet how muscle cells respond to Wnt signals is unclear. Here, we show that in vivo, wnt11r and wnt4a initiate MuSK translocation from muscle membranes to recycling endosomes and that this transition is crucial for AChR accumulation at future synaptic sites. Moreover, we demonstrate that components of the planar cell polarity pathway colocalize to recycling endosomes and that this localization is MuSK dependent. Knockdown of several core components disrupts MuSK translocation to endosomes, AChR localization and axonal guidance. We propose that Wnt-induced trafficking of the MuSK receptor to endosomes initiates a signaling cascade to align pre- with postsynaptic elements. Collectively, these findings suggest a general mechanism by which Wnt signals shape synaptic connectivity through localized receptor endocytosis.

Optimal cellular mobility for synchronization arising from the gradual recovery of intercellular interactions

Cell movement and intercellular signaling occur simultaneously during the development of tissues, but little is known about how movement affects signaling. Previous theoretical studies have shown that faster moving cells favor synchronization across a population of locally coupled genetic oscillators. An important assumption in these studies is that cells can immediately interact with their new neighbors after arriving at a new location. However, intercellular interactions in cellular systems may need some time to become fully established. How movement affects synchronization in this situation has not been examined. Here, we develop a coupled phase oscillator model in which we consider cell movement and the gradual recovery of intercellular coupling experienced by a cell after movement, characterized by a moving rate and a coupling recovery rate, respectively. We find (1) an optimal moving rate for synchronization and (2) a critical moving rate above which achieving synchronization is not possible. These results indicate that the extent to which movement enhances synchrony is limited by a gradual recovery of coupling. These findings suggest that the ratio of time scales of movement and signaling recovery is critical for information transfer between moving cells.

The GATA2 transcriptional network is requisite for RAS oncogene-driven non-small cell lung cancer

Non-small cell lung cancer (NSCLC) is the most frequent cause of cancer deaths worldwide; nearly half contain mutations in the receptor tyrosine kinase/RAS pathway. Here we show that RAS-pathway mutant NSCLC cells depend on the transcription factor GATA2. Loss of GATA2 reduced the viability of NSCLC cells with RAS-pathway mutations, whereas wild-type cells were unaffected. Integrated gene expression and genome occupancy analyses revealed GATA2 regulation of the proteasome, and IL-1-signaling, and Rho-signaling pathways. These pathways were functionally significant, as reactivation rescued viability after GATA2 depletion. In a Kras-driven NSCLC mouse model, Gata2 loss dramatically reduced tumor development. Furthermore, Gata2 deletion in established Kras mutant tumors induced striking regression. Although GATA2 itself is likely undruggable, combined suppression of GATA2-regulated pathways with clinically approved inhibitors caused marked tumor clearance. Discovery of the nononcogene addiction of KRAS mutant lung cancers to GATA2 presents a network of druggable pathways for therapeutic exploitation.

EphBs: an integral link between synaptic function and synaptopathies

The assembly and function of neuronal circuits rely on selective cell-cell interactions to control axon targeting, generate pre- and postsynaptic specialization and recruit neurotransmitter receptors. In neurons, EphB receptor tyrosine kinases mediate excitatory synaptogenesis early during development, and then later coordinate synaptic function by controlling synaptic glutamate receptor localization and function. EphBs direct synapse formation and function to regulate cellular morphology through downstream signaling mechanisms and by interacting with glutamate receptors. In humans, defective EphB-dependent regulation of NMDA receptor (NMDAR) localization and function is associated with neurological disorders, including neuropathic pain, anxiety disorders and Alzheimer's disease (AD). Here, we propose that EphBs act as a central organizer of excitatory synapse formation and function, and as a key regulator of diseases linked to NMDAR dysfunction.

Kaposi's sarcoma-associated herpesvirus interacts with EphrinA2 receptor to amplify signaling essential for productive infection

Kaposi's sarcoma-associated herpesvirus (KSHV), etiologically associated with Kaposi's sarcoma, uses integrins (α3β1, αVβ3, and αVβ5) and associated signaling to enter human dermal microvascular endothelial cells (HMVEC-d), an in vivo target of infection. KSHV infection activated c-Cbl, which induced the selective translocation of KSHV into lipid rafts (LRs) along with the α3β1, αVβ3, and xCT receptors, but not αVβ5. LR-translocated receptors were monoubiquitinated, leading to productive macropinocytic entry, whereas non-LR-associated αVβ5 was polyubiquitinated, leading to clathrin-mediated entry that was targeted to lysosomes. Because the molecule(s) that integrate signal pathways and productive KSHV macropinocytosis were unknown, we immunoprecipitated KSHV-infected LR fractions with anti-α3β1 antibodies and analyzed them by mass spectrometry. The tyrosine kinase EphrinA2 (EphA2), implicated in many cancers, was identified in this analysis. EphA2 was activated by KSHV. EphA2 was also associated with KSHV and integrins (α3β1 and αVβ3) in LRs early during infection. Preincubation of virus with soluble EphA2, knockdown of EphA2 by shRNAs, or pretreatment of cells with anti-EphA2 monoclonal antibodies or tyrosine kinase inhibitor dasatinib significantly reduced KSHV entry and gene expression. EphA2 associates with c-Cbl-myosin IIA and augmented KSHV-induced Src and PI3-K signals in LRs, leading to bleb formation and macropinocytosis of KSHV. EphA2 shRNA ablated macropinocytosis-associated signaling events, virus internalization, and productive nuclear trafficking of KSHV DNA. Taken together, these studies demonstrate that the EphA2 receptor acts as a master assembly regulator of KSHV-induced signal molecules and KSHV entry in endothelial cells and suggest that the EphA2 receptor is an attractive target for controlling KSHV infection.

The notch ligand delta-like 4 regulates multiple stages of early hemato-vascular development

Background

In mouse embryos, homozygous or heterozygous deletions of the gene encoding the Notch ligand Dll4 result in early embryonic death due to major defects in endothelial remodeling in the yolk sac and embryo. Considering the close developmental relationship between endothelial and hematopoietic cell lineages, which share a common mesoderm-derived precursor, the hemangioblast, and many key regulatory molecules, we investigated whether Dll4 is also involved in the regulation of early embryonic hematopoiesis.

Methodology/Principal Findings

Using Embryoid Bodies (EBs) derived from embryonic stem cells harboring hetero- or homozygous Dll4 deletions, we observed that EBs from both genotypes exhibit an abnormal endothelial remodeling in the vascular sprouts that arise late during EB differentiation, indicating that this in vitro system recapitulates the angiogenic phenotype of Dll4 mutant embryos. However, analysis of EB development at early time points revealed that the absence of Dll4 delays the emergence of mesoderm and severely reduces the number of blast-colony forming cells (BL-CFCs), the in vitro counterpart of the hemangioblast, and of endothelial cells. Analysis of colony forming units (CFU) in EBs and yolk sacs from Dll4^+/− and Dll4^−/− embryos, showed that primitive erythropoiesis is specifically affected by Dll4 insufficiency. In Dll4 mutant EBs, smooth muscle cells (SMCs) were seemingly unaffected and cardiomyocyte differentiation was increased, indicating that SMC specification is Dll4-independent while a normal dose of this Notch ligand is essential for the quantitative regulation of cardiomyogenesis.

Conclusions/Significance

This study highlights a previously unnoticed role for Dll4 in the quantitative regulation of early hemato-vascular precursors, further indicating that it is also involved on the timely emergence of mesoderm in early embryogenesis.

Should I stay or should I go? Ephs and ephrins in neuronal migration

In neuroscience, Ephs and ephrins are perhaps best known for their role in axon guidance. It was first shown in the visual system that graded expression of these proteins is instrumental in providing molecular coordinates that define topographic maps, particularly in the visual system, but also in the auditory, vomeronasal and somatosensory systems as well as in the hippocampus, cerebellum and other structures. Perhaps unsurprisingly, the role of these proteins in regulating cell-cell interactions also has an impact on cell mobility, with evidence that Eph-ephrin interactions segregate cell populations based on contact-mediated attraction or repulsion. Consistent with these studies, evidence has accumulated that Ephs and ephrins play important roles in the migration of specific cell populations in the developing and adult brain. This review focusses on two examples of neuronal migration that require Eph/ephrin signalling - radial and tangential migration of neurons in cortical development and the migration of newly generated neurons along the rostral migratory stream to the olfactory bulb in the adult brain. We discuss the challenge involved in understanding how cells determine whether they respond to signals by migration or axon guidance.

Podocytes: gaining a foothold

In an attempt to understand the basis of glomerular disease, significant progress has been made in understanding the mechanisms that determine podocyte development and the maintenance of podocyte health. This review examines recent advances in this area focusing on the podocyte intercellular junction, actin cytoskeletal dynamics, and determinants of podocyte cell polarity, autophagy and mTOR biology.

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

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

VEGF signaling inside vascular endothelial cells and beyond

Vascular endothelial growth factor-A (VEGF-A) has long been recognized as the key regulator of vascular development and function in health and disease. VEGF is a secreted polypeptide that binds to transmembrane tyrosine kinase VEGF receptors on the plasma membrane, inducing their dimerization, activation and assembly of a membrane-proximal signaling complex. Recent studies have revealed that many key events of VEGFR signaling occur inside the endothelial cell and are regulated by endosomal receptor trafficking. Plasma membrane VEGFR interacting molecules, including vascular guidance receptors Neuropilins and Ephrins also regulate VEGFR endocytosis and trafficking. VEGF signaling is increasingly recognized for its roles outside of the vascular system, notably during neural development, and blood vessels regulate epithelial branching morphogenesis. We review here recent advances in our understanding of VEGF signaling and its biological roles.

Ecdysone receptor and ultraspiracle proteins are tyrosine phosphorylated during adult development of silkmoths

20-hydroxy ecdysone (20E) is essential to promote adult development in diapausing silkmoth pupae. Increases in protein tyrosine/serine-phosphorylations observed soon after 20E administration supported the initial hypothesis that activation of receptor tyrosine kinase-ras-MAPK pathway could be responsible for the growth promoting effects of 20E. This report pertains to the high levels of protein tyrosine phosphorylations (PTP) that occurred later during the growth to differentiation transition because of its novelty and relevance to 20E dependence of adult development. Further analyses demonstrated that both ecdysone receptor (EcR) and ultraspiracle (USP), the two dimerizing partners of the functional ecdysone receptor, are tyrosine phosphorylated coincidental with high PTP. Enhanced PTP during growth to differentiation transition and concomitant tyrosine phosphorylation of EcR and USP was shown to occur in another silkmoth species pointing to the necessity of similar protein tyrosine phosphorylation pathways for adult development. Properly timed increases in tissue protein tyrosine kinase (PTK) activity could explain the enhancement of PTP in the wing epidermis of both the silkmoths. Thymidine incorporation measurements showed that cessation of DNA synthesis preceded the increase in PTK activity thus emphasizing a role for PTP in aspects of tissue physiology related to differentiative events rather than cell proliferation. Phosphatase and tyrosine kinase inhibitors (Tyrphostins) had minimal effects on adult wing development in vivo. However, the escape of the adult from the pupal case was blocked by a tyrphostin indicating the importance of PTKs in eclosion.

A role for epha2 in cell migration and refractive organization of the ocular lens

PURPOSE

The Epha2 receptor is a surprisingly abundant component of the membrane proteome of vertebrate lenses. In humans, genetic studies have linked mutations in EPHA2 to inherited and age-related forms of cataract, but the function of Epha2 in the lens is obscure. To gain insights into the role of Epha2, a comparative analysis of lenses from wild-type and Epha2(-/-) mice was performed.

METHODS

Epha2 distribution was examined using immunocytochemistry and Western blot analysis. Lens optical quality was assessed by laser refractometry. Confocal microscopy was used to analyze cellular phenotypes.

RESULTS

In wild-type lenses, Epha2 was expressed by lens epithelial cells and elongating fibers but was degraded during the later stages of fiber differentiation. Epha2-null lenses retained their transparency, but two key optical parameters, lens shape and internal composition, were compromised in Epha2(-/-) animals. Epha2-null lenses were smaller and more spherical than age-matched wild-type lenses, and laser refractometry revealed a significant decrease in refractive power of the outer cell layers of mutant lenses. In the absence of Epha2, fiber cells deviated from their normal course and terminated at sutures that were no longer centered on the optical axis. Patterning defects were also noted at the level of individual cells. Wild-type fiber cells had hexagonal cross-sectional profiles with membrane protrusions extending from the cell vertices. In contrast, Epha2(-/-) cells had irregular profiles, and protrusions extended from all membrane surfaces.

CONCLUSIONS

These studies indicate that Epha2 is not required for transparency but does play an indispensable role in the cytoarchitecture and refractive quality of the lens.

Notch4 normalization reduces blood vessel size in arteriovenous malformations

Abnormally enlarged blood vessels underlie many life-threatening disorders including arteriovenous (AV) malformations (AVMs). The core defect in AVMs is high-flow AV shunts, which connect arteries directly to veins, "stealing" blood from capillaries. Here, we studied mouse brain AV shunts caused by up-regulation of Notch signaling in endothelial cells (ECs) through transgenic expression of constitutively active Notch4 (Notch4*). Using four-dimensional two-photon imaging through a cranial window, we found that normalizing Notch signaling by repressing Notch4* expression converted large-caliber, high-flow AV shunts to capillary-like vessels. The structural regression of the high-flow AV shunts returned blood to capillaries, thus reversing tissue hypoxia. This regression was initiated by vessel narrowing without the loss of ECs and required restoration of EphB4 receptor expression by venous ECs. Normalization of Notch signaling resulting in regression of high-flow AV shunts, and a return to normal blood flow suggests that targeting the Notch pathway may be useful therapeutically for treating diseases such as AVMs.

Evidence for a dual function of EphB4 as tumor promoter and suppressor regulated by the absence or presence of the ephrin-B2 ligand

Overexpression of the receptor tyrosine kinase EphB4 is common in epithelial cancers and linked to tumor progression by promoting angiogenesis, increasing survival and facilitating invasion and migration. However, other studies have reported loss of EphB4 suggesting a tumor suppressor function in some cancers. These opposing roles may be regulated by (i) the presence of the primary ligand ephrin-B2 that regulates pathways involved in tumor suppression or (ii) the absence of ephrin-B2 that allows EphB4 signaling via ligand-independent pathways that contribute to tumor promotion. To explore this theory, EphB4 was overexpressed in the prostate cancer cell line 22Rv1 and the mammary epithelial cell line MCF-10A. Overexpressed EphB4 localized to lipid-rich regions of the plasma membrane and confirmed to be ligand-responsive as demonstrated by increased phosphorylation of ERK1/2 and internalization. EphB4 overexpressing cells demonstrated enhanced anchorage-independent growth, migration and invasion, all characteristics associated with an aggressive phenotype, and therefore supporting the hypothesis that overexpressed EphB4 facilitates tumor promotion. Importantly, these effects were reversed in the presence of ephrin-B2 which led to a reduction in EphB4 protein levels, demonstrating that ligand-dependent signaling is tumor suppressive. Furthermore, extended ligand stimulation caused a significant decrease in proliferation that correlated with a rise in caspase-3/7 and -8 activities. Together, these results demonstrate that overexpression of EphB4 confers a transformed phenotype in the case of MCF-10A cells and an increased metastatic phenotype in the case of 22Rv1 cancer cells and that both phenotypes can be restrained by stimulation with ephrin-B2, in part by reducing EphB4 levels.

The cis side of juxtacrine signaling: a new role in the development of the nervous system

Cell-cell communication by juxtacrine signaling plays a key role in the development of the nervous system, from cell fate determination through axonal guidance to synaptogenesis. Interestingly, several juxtacrine signaling systems exhibit an inhibitory interaction between receptors and ligands in the same cell, termed cis inhibition. These include the Notch, semaphorin and ephrin signaling systems. Here we review the role of cis inhibition in these signaling systems in the development of the nervous system. We compare and contrast cis inhibition mechanisms and discuss their potential cellular function as a threshold-generating mechanism. The prevalence of cis inhibition suggests that these interactions and their functional regulatory roles may serve as a general design principle for juxtacrine signaling-mediated processes during and beyond neurodevelopment.

Roles of EphB3/ephrin-B1 in feather morphogenesis

The ephrin receptor (Eph) tyrosine kinases and their ephrin ligands are involved in morphogenesis during organ formation. We studied their role in feather morphogenesis, focusing on ephrin-B1 and its receptor EphB3. Early in feather development, ephrin-B1 mRNA and protein were found to be expressed in the dermal condensation, but not in the inter-bud mesenchyme. Later, in feather buds, expression was found in both the epithelium and mesenchyme. In the feather follicle, ephrin-B1 protein expression was found to be enriched in the feather filament epithelium and in the marginal plate which sets the boundary between the barb ridges. EphB3 mRNA was also expressed in epithelia. In the feather bud, its expression was restricted to the posterior bud. In the follicle, its expression formed a circle at the bud base which may set the boundary between bud and inter-bud domains. Perturbation with ephrin-B1/Fc altered feather primordia segregation and feather bud elongation. Analyses revealed that ephrin-B1/Fc caused three types of changes: blurred placode boundaries with loose dermal condensations, incomplete follicle invagination with less compact dermal papillae, and aberrant barb ridge patterning in feather filament morphogenesis. Thus, while ephrin-B1 suppression does not inhibit the initial emergence of a new epithelial domain, Eph/ephrin-B1 interaction is required for its proper completion. Consequently, we propose that interaction between ephrin-B1 and its receptor is involved in boundary stabilization during feather morphogenesis.

PI3K/AKT/mTOR Pathway in Angiogenesis

The phosphatidylinositol 3-kinase (PI3K)/AKT/mammalian target of rapamycin (mTOR) pathway is activated in the majority of human cancers. This pathway is known to play a key role in numerous cellular functions including proliferation, adhesion, migration, invasion, metabolism, and survival, but in the current review we focus on its role in angiogenesis. PI3K activation may occur via RAS mutation, loss of phosphatase and tensin homolog (PTEN), or by increased expression of growth factor receptors such as epidermal growth factor receptor. There is a connection between the PI3K pathway and angiogenesis. Hypoxia leads to HIF-1α stabilization and is a major stimulus for increased vascular endothelial growth factor (VEGF) production by tumor cells. However, activation of the PI3K/AKT pathway in tumor cells can also increase VEGF secretion, both by hypoxia-inducible factor 1 (HIF-1) dependent and independent mechanisms. The PI3K/AKT pathway also modulates the expression of other angiogenic factors such as nitric oxide and angiopoietins. Numerous inhibitors targeting the PI3K/AKT/mTOR pathway have been developed, and these agents have been shown to decrease VEGF secretion and angiogenesis. The effect of these inhibitors on tumor vasculature can be difficult to predict. The vasculature of tumors is aberrant, leading to sluggish bloodflow and elevated interstitial blood pressure, which can be perpetuated by the high levels of VEGF. Hence, decreasing VEGF expression can paradoxically lead to vascular normalization and improved bloodflow in some tumors. In addition to its importance in cancer, the PI3K pathway also plays an essential role in the formation of normal blood vessels during development. Embryos with kinase-dead p110α catalytic subunit of PI3K develop vascular defects. Stimulation of endothelial cells by VEGF leads to activation of the PI3K pathway within these cells, which is important for cell migration. Sustained endothelial activation of AKT1 has been shown to induce the formation of structurally abnormal blood vessels that recapitulate the aberrations of tumor vessels. Hence, the PI3K pathway plays an important role in regulating angiogenesis both in normal tissues and in cancers.

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.

Basic and therapeutic aspects of angiogenesis

Blood vessels form extensive networks that nurture all tissues in the body. Abnormal vessel growth and function are hallmarks of cancer and ischemic and inflammatory diseases, and they contribute to disease progression. Therapeutic approaches to block vascular supply have reached the clinic, but limited efficacy and resistance pose unresolved challenges. Recent insights establish how endothelial cells communicate with each other and with their environment to form a branched vascular network. The emerging principles of vascular growth provide exciting new perspectives, the translation of which might overcome the current limitations of pro- and antiangiogenic medicine.

Functional redundancy and compensation among members of gap junction protein families?

Gap junctions are intercellular conduits for small molecules made up by protein subunits called connexins. A large number of connexin genes were found in mouse and man, and most cell types express several connexins, lending support to the view that redundancy and compensation among family members exist. This review gives an overview of the current knowledge on redundancy and functional compensation - or lack thereof. It takes into account the different properties of connexin subunits which comprise gap junctional intercellular channels, but also the compatibility of connexins in gap junctions. Most insight has been gained by the investigation of mice deficient for one or more connexins and transgenic mice with functional replacement of one connexin gene by another. Most single deficient mice show phenotypical alterations limited to critical developmental time points or to specific organs and tissues, while mice doubly deficient for connexins expressed in the same cell type usually show more severe phenotypical alterations. Replacement of a connexin by another connexin in some cases gave rise to rescue of phenotypical alterations of connexin deficiencies, which were restricted to specific tissues. In many tissues, connexin substitution did not restore phenotypical alterations of connexin deficiencies, indicating that connexins are specialized in function. In some cases, fatal consequences arose from the replacement. The current consensus gained from such studies is that redundancy and compensation among connexins exists at least to a limited extent. This article is part of a Special Issue entitled: The Communicating junctions, composition, structure and characteristics.

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.

The mouse IAPE endogenous retrovirus can infect cells through any of the five GPI-anchored Ephrin A proteins

The IAPE (Intracisternal A-type Particles elements with an Envelope) family of murine endogenous retroelements is present at more than 200 copies in the mouse genome. We had previously identified a single copy that proved to be fully functional, i.e. which can generate viral particles budding out of the cell and infectious on a series of cells, including human cells. We also showed that IAPE are the progenitors of the highly reiterated IAP elements. The latter are now strictly intracellular retrotransposons, due to the loss of the envelope gene and re-localisation of the associated particles in the course of evolution. In the present study we searched for the cellular receptor of the IAPE elements, by using a lentiviral human cDNA library and a pseudotype assay on transduced cells. We identified Ephrin A4, a GPI-anchored molecule involved in several developmental processes, as a receptor for the IAPE pseudotypes. We also found that the other 4 members of the Ephrin A family -but not those of the closely related Ephrin B family- were also able to mediate IAPE cell entry, thus significantly increasing the amount of possible cell types susceptible to IAPE infection. We show that these include mouse germline cells, as illustrated by immunohistochemistry experiments, consistent with IAPE genomic amplification by successive re-infection. We propose that the uncovered properties of the identified receptors played a role in the accumulation of IAPE elements in the mouse genome, and in the survival of a functional copy.

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.