Segregation of the receptor EphA7 from its tyrosine kinase-negative isoform on neurons in adult mouse brain

Loss of EphA7 Expression in Basal Cell Carcinoma by Hypermethylation of CpG Islands in the Promoter Region

Basal cell carcinoma (BCC) is the most common malignancy worldwide, with increasing incidence. BCCs present low mortality but high morbidity, and its pathogenesis remains unclear. Eph receptors have been implicated in tumorigenesis. EphA7 plays a role as a tumor suppressor in certain cancers. We checked EphA7 expression levels and methylation status in a set of BCCs, benign skin diseases, and compound nevus tissue samples using immunohistochemistry. EphA7 protein was positively expressed in normal basal cells, benign skin diseases, and compound nevus cells, but lost in areas of BCC tissues. We detected hypermethylation in BCC tissue samples with reduced expression of EphA7. There is a significant relationship between the expression level of EphA7 receptor protein and the methylation status of CpG islands in the EphA7 promoter region (P < 0.001). To our knowledge, this is the first study to report the EphA7 expression profile and hypermethylation of EphA7 in BCC. The role of the EphA7 gene and the status of hypermethylation in tumorigenesis and treatment of BCC warrant further investigation.

EphA7 isoforms differentially regulate cortical dendrite development

The shape of a neuron facilitates its functionality within neural circuits. Dendrites integrate incoming signals from axons, receiving excitatory input onto small protrusions called dendritic spines. Therefore, understanding dendritic growth and development is fundamental for discerning neural function. We previously demonstrated that EphA7 receptor signaling during cortical development impacts dendrites in two ways: EphA7 restricts dendritic growth early and promotes dendritic spine formation later. Here, the molecular basis for this shift in EphA7 function is defined. Expression analyses reveal that EphA7 full-length (EphA7-FL) and truncated (EphA7-T1; lacking kinase domain) isoforms are dynamically expressed in the developing cortex. Peak expression of EphA7-FL overlaps with dendritic elaboration around birth, while highest expression of EphA7-T1 coincides with dendritic spine formation in early postnatal life. Overexpression studies in cultured neurons demonstrate that EphA7-FL inhibits both dendritic growth and spine formation, while EphA7-T1 increases spine density. Furthermore, signaling downstream of EphA7 shifts during development, such that in vivo inhibition of mTOR by rapamycin in EphA7-mutant neurons ameliorates dendritic branching, but not dendritic spine phenotypes. Finally, direct interaction between EphA7-FL and EphA7-T1 is demonstrated in cultured cells, which results in reduction of EphA7-FL phosphorylation. In cortex, both isoforms are colocalized to synaptic fractions and both transcripts are expressed together within individual neurons, supporting a model where EphA7-T1 modulates EphA7-FL repulsive signaling during development. Thus, the divergent functions of EphA7 during cortical dendrite development are explained by the presence of two variants of the receptor.

The Evolutionary History of Ephs and Ephrins: Toward Multicellular Organisms

Eph receptor (Eph) and ephrin signaling regulate fundamental developmental processes through both forward and reverse signaling triggered upon cell–cell contact. In vertebrates, they are both classified into classes A and B, and some representatives have been identified in many metazoan groups, where their expression and functions have been well studied. We have extended previous phylogenetic analyses and examined the presence of Eph and ephrins in the tree of life to determine their origin and evolution. We have found that 1) premetazoan choanoflagellates may already have rudimental Eph/ephrin signaling as they have an Eph-/ephrin-like pair and homologs of downstream-signaling genes; 2) both forward- and reverse-downstream signaling might already occur in Porifera since sponges have most genes involved in these types of signaling; 3) the nonvertebrate metazoan Eph is a type-B receptor that can bind ephrins regardless of their membrane-anchoring structure, glycosylphosphatidylinositol, or transmembrane; 4) Eph/ephrin cross-class binding is specific to Gnathostomata; and 5) kinase-dead Eph receptors can be traced back to Gnathostomata. We conclude that Eph/ephrin signaling is of older origin than previously believed. We also examined the presence of protein domains associated with functional characteristics and the appearance and conservation of downstream-signaling pathways to understand the original and derived functions of Ephs and ephrins. We find that the evolutionary history of these gene families points to an ancestral function in cell–cell interactions that could contribute to the emergence of multicellularity and, in particular, to the required segregation of cell populations.

Receptor tyrosine kinase EphA7 is required for interneuron connectivity at specific subcellular compartments of granule cells

Neuronal transmission is regulated by the local circuitry which is composed of principal neurons targeted at different subcellular compartments by a variety of interneurons. However, mechanisms that contribute to the subcellular localisation and maintenance of GABAergic interneuron terminals are poorly understood. Stabilization of GABAergic synapses depends on clustering of the postsynaptic scaffolding protein gephyrin and its interaction with the guanine nucleotide exchange factor collybistin. Lentiviral knockdown experiments in adult rats indicated that the receptor tyrosine kinase EphA7 is required for the stabilisation of basket cell terminals on proximal dendritic and somatic compartments of granular cells of the dentate gyrus. EphA7 deficiency and concomitant destabilisation of GABAergic synapses correlated with impaired long-term potentiation and reduced hippocampal learning. Reduced GABAergic innervation may be explained by an impact of EphA7 on gephyrin clustering. Overexpression or ephrin stimulation of EphA7 induced gephyrin clustering dependent on the mechanistic target of rapamycin (mTOR) which is an interaction partner of gephyrin. Gephyrin interactions with mTOR become released after mTOR activation while enhanced interaction with the guanine nucleotide exchange factor collybistin was observed in parallel. In conclusion, EphA7 regulates gephyrin clustering and the maintenance of inhibitory synaptic connectivity via mTOR signalling.

Eph Receptors, Ephrins, and Synaptic Function

Compelling new findings have revealed that receptor tyrosine kinases of the Eph family, along with their ephrin ligands, play an essential role in regulating the properties of developing mature excitatory synapses in the central nervous system. The cell surface localization of both the Eph receptors and the ephrins enables these proteins to signal bidirectionally at sites of cell-to-cell contact, such as synapses. Eph receptors and ephrins have indeed been implicated in multiple aspects of synaptic function, including clustering and modulating N-methyl-D-aspartate receptors, modifying the geometry of postsynaptic terminals, and influencing long-term synaptic plasticity and memory. In this review, we discuss how Eph receptors and ephrins are integrated into the molecular machinery that supports synaptic function.

Regional expression and ultrastructural localization of EphA7 in the hippocampus and cerebellum of adult rat

EphA7 is expressed in the adult central nervous system (CNS), where its roles are yet poorly defined. We mapped its distribution using in situ hybridization (ISH) and immunohistochemistry (IHC) combined with light (LM) and electron microscopy (EM) in adult rat and mouse brain. The strongest ISH signal was in the hippocampal pyramidal and granule cell layers. Moderate levels were detected in habenula, striatum, amygdala, the cingulate, piriform and entorhinal cortex, and in cerebellum, notably the Purkinje cell layer. The IHC signal distribution was consistent with ISH results, with transport of the protein to processes, as exemplified in the hippocampal neuropil layers and weakly stained pyramidal cell layers. In contrast, in the cerebellum, the Purkinje cell bodies were the most strongly immunolabeled elements. EM localized the cell surface-expression of EphA7 essentially in postsynaptic densities (PSDs) of dendritic spines and shafts, and on some astrocytic leaflets, in both hippocampus and cerebellum. Perikaryal and dendritic labeling was mostly intracellular, associated with the synthetic and trafficking machineries. Immunopositive vesicles were also observed in axons and axon terminals. Quantitative analysis in EM showed significant differences in the frequency of labeled elements between regions. Notably, labeled dendrites were ∼3-5 times less frequent in cerebellum than in hippocampus, but they were individually endowed with ∼10-40 times higher frequencies of PSDs, on their shafts and spines. The cell surface localization of EphA7, being preferentially in PSDs, and in perisynaptic astrocytic leaflets, provides morphologic evidence that EphA7 plays key roles in adult CNS synaptic maintenance, plasticity, or function. J. Comp. Neurol. 524:2462-2478, 2016. © 2016 Wiley Periodicals, Inc.

EphA7 signaling guides cortical dendritic development and spine maturation

The process by which excitatory neurons are generated and mature during the development of the cerebral cortex occurs in a stereotyped manner; coordinated neuronal birth, migration, and differentiation during embryonic and early postnatal life are prerequisites for selective synaptic connections that mediate meaningful neurotransmission in maturity. Normal cortical function depends upon the proper elaboration of neurons, including the initial extension of cellular processes that lead to the formation of axons and dendrites and the subsequent maturation of synapses. Here, we examine the role of cell-based signaling via the receptor tyrosine kinase EphA7 in guiding the extension and maturation of cortical dendrites. EphA7, localized to dendritic shafts and spines of pyramidal cells, is uniquely expressed during cortical neuronal development. On patterned substrates, EphA7 signaling restricts dendritic extent, with Src and Tsc1 serving as downstream mediators. Perturbation of EphA7 signaling in vitro and in vivo alters dendritic elaboration: Dendrites are longer and more complex when EphA7 is absent and are shorter and simpler when EphA7 is ectopically expressed. Later in neuronal maturation, EphA7 influences protrusions from dendritic shafts and the assembling of synaptic components. Indeed, synaptic function relies on EphA7; the electrophysiological maturation of pyramidal neurons is delayed in cultures lacking EphA7, indicating that EphA7 enhances synaptic function. These results provide evidence of roles for Eph signaling, first in limiting the elaboration of cortical neuronal dendrites and then in coordinating the maturation and function of synapses.

Profiling Eph receptor expression in cells and tissues: a targeted mass spectrometry approach

The Eph receptor tyrosine kinase family includes many members, which are often expressed together in various combinations and can promiscuously interact with multiple ephrin ligands, generating intricate networks of intracellular signals that control physiological and pathological processes. Knowing the entire repertoire of Eph receptors and ephrins expressed in a biological sample is important when studying their biological roles. Moreover, given the correlation between Eph receptor/ephrin expression and cancer pathogenesis, their expression patterns could serve important diagnostic and prognostic purposes. However, profiling Eph receptor and ephrin expression has been challenging. Here we describe a novel and straightforward approach to catalog the Eph receptors present in cultured cells and tissues. By measuring the binding of ephrin Fc fusion proteins to Eph receptors in ELISA and pull-down assays, we determined that a mixture of four ephrins is suitable for isolating both EphA and EphB receptors in a single pull-down. We then used mass spectrometry to identify the Eph receptors present in the pull-downs and estimate their relative levels. This approach was validated in cultured human cancer cell lines, human tumor xenograft tissue grown in mice, and mouse brain tissue. The new mass spectrometry approach we have developed represents a useful tool for the identification of the spectrum of Eph receptors present in a biological sample and could also be extended to profiling ephrin expression.

Eph receptors are involved in the activity-dependent synaptic wiring in the mouse cerebellar cortex

Eph receptor tyrosine kinases are involved in many cellular processes. In the developing brain, they act as migratory and cell adhesive cues while in the adult brain they regulate dendritic spine plasticity. Here we show a new role for Eph receptor signalling in the cerebellar cortex. Cerebellar Purkinje cells are innervated by two different excitatory inputs. The climbing fibres contact the proximal dendritic domain of Purkinje cells, where synapse and spine density is low; the parallel fibres contact the distal dendritic domain, where synapse and spine density is high. Interestingly, Purkinje cells have the intrinsic ability to generate a high number of spines over their entire dendritic arborisations, which can be innervated by the parallel fibres. However, the climbing fibre input continuously exerts an activity-dependent repression on parallel fibre synapses, thus confining them to the distal Purkinje cell dendritic domain. Such repression persists after Eph receptor activation, but is overridden by Eph receptor inhibition with EphA4/Fc in neonatal cultured cerebellar slices as well as mature acute cerebellar slices, following in vivo infusion of the EphA4/Fc inhibitor and in EphB receptor-deficient mice. When electrical activity is blocked in vivo by tetrodotoxin leading to a high spine density in Purkinje cell proximal dendrites, stimulation of Eph receptor activation recapitulates the spine repressive effects of climbing fibres. These results suggest that Eph receptor signalling mediates the repression of spine proliferation induced by climbing fibre activity in Purkinje cell proximal dendrites. Such repression is necessary to maintain the correct architecture of the cerebellar cortex.

Aberrant methylation of EphA7 in human prostate cancer and its relation to clinicopathologic features

EphA7 is a member of Eph/ephrins family and play diverse roles in carcinogenesis. The aim of our study was to investigate functional and structural alterations of EphA7 in prostate cancer and determine if those findings correlate with the clinicopathologic features of prostate cancer. Forty-eight prostate carcinomas, 31 benign prostate hyperplasias, 5 normal prostate tissues and 3 prostate cell lines (LNCaP, DU145 and PC-3) were examined with quantitative RT-PCR, methylation-specific PCR and immunohistochemistry. Downregulation or loss of EphA7 mRNA expression was detected in 23 of 48 (47.9%) prostate carcinomas, and 2 of 31 (6.5%) hyperplasias. Methylation of the EphA7 promoter region was present in 20 of 48 (41.7%) of carcinomas and 6 of 31 (19.3%) hyperplasias, respectively. Immunostaining analysis showed EphA7 protein was absent in 10 of 30 (33.3%) carcinoma samples available and 8 of them (80.0%) exhibited hypermethylation. The frequency of EphA7 methylation was higher in cancer patients with higher Gleason score. Treatment of DU145 cells harboring methylation with 5-aza-2'-deoxycytidine reactivated expression of EphA7. Ectopic expression of EphA7 in DU145 cells did not suppress cell growth but inhibited colony formation. Our study provides evidence that epigenetic inactivation of EphA7 may be involved in prostate carcinogenesis.

Increased expression of EphA7 correlates with adverse outcome in primary and recurrent glioblastoma multiforme patients

Background

Malignant gliomas are lethal cancers, highly dependent on angiogenesis and treatment options and prognosis still remain poor for patients with recurrent glioblastoma multiforme (GBM). Ephs and ephrins have many well-defined functions during embryonic development of central nervous system such as axon mapping, neural crest cell migration, hindbrain segmentation and synapse formation as well as physiological and abnormal angiogenesis. Accumulating evidence indicates that Eph and ephrins are frequently overexpressed in different tumor types including GBM. However, their role in tumorigenesis remains controversial, as both tumor growth promoter and suppressor potential have been ascribed to Eph and ephrins while the function of EphA7 in GBM pathogenesis remains largely unknown.

Methods

In this study, we investigated the immunohistochemical expression of EphA7 in a series of 32 primary and recurrent GBM and correlated it with clinical pathological parameters and patient outcome. In addition, intratumor microvascular density (MVD) was quantified by immunostaining for endothelial cell marker von Willebrand factor (vWF).

Results

Overexpression of EphA7 protein was predictive of the adverse outcome in GBM patients, independent of MVD expression (p = 0.02). Moreover, high density of MVD as well as higher EphA7 expression predicted the disease outcome more accurately than EphA7 variable alone (p = 0.01). There was no correlation between MVD and overall survival or recurrence-free survival (p > 0.05). However, a statistically significant correlation between lower MVD and tumor recurrence was observed (p = 0.003).

Conclusion

The immunohistochemical assessment of tissue EphA7 provides important prognostic information in GBM and would justify its use as surrogate marker to screen patients for tyrosine kinase inhibitor therapy.

EphA7-ephrin-A5 signaling in mouse somatosensory cortex: developmental restriction of molecular domains and postnatal maintenance of functional compartments

Members of the Eph family of receptor tyrosine kinases and their ligands, the ephrins, are expressed in distinct patterns in the forming cortex. EphA7 is expressed early in cortical development, becoming concentrated in anterior and posterior domains, whereas ephrin-A5 is expressed later in corticogenesis, highest in the middle region that has low levels of EphA7. The EphA7 gene produces full-length and truncated isoforms, which are repulsive and adhesive, respectively. Analysis of cortical RNA expression demonstrates that proportions of these isoforms change with time, from a more repulsive mix during embryogenesis to a more permissive mix postnatally. To examine how EphA7 and ephrin-A5 influence the formation of cortical regions, EphA7-/- mice were analyzed. Within the cortex of EphA7-/- mice, the distribution of ephrin-A5 was more extensive, encompassing its usual medial domain but also extending more posteriorly toward the occipital pole. Moreover, relative levels of ephrin-A5 along the cortex's anatomical axes changed in EphA7-/- animals, creating less striking shifts in ligand abundance. Furthermore, in vivo functional studies revealed that EphA7 exerts a repulsive influence on ephrin-A5-expressing cells during corticogenesis. In contrast, EphA7 appears to mediate permissive interactions in the postnatal cortex: the area of somatosensory cortex was significantly reduced in EphA7-/- mice. A similar reduction was present in ephrin-A5-/- animals and a more pronounced decrease was observed in EphA7/ephrin-A5-/- cortex. Taken together, this study supports a role for EphA7 and ephrin-A5 in the establishment and maintenance of certain cortical domains and suggests that the nature of their interactions changes with cortical maturity.

Eph receptors in the adult brain

The Eph receptors are a large family of receptor tyrosine kinases with important roles in the establishment of neuronal and vascular networks during embryonic development. The functions of Eph receptors in the adult brain have only recently been investigated, and the results are forcing us to amend the conventional view that these molecules function predominantly in a developmental context. This review summarizes this rapidly expanding new area of research, which has shown that the Eph receptors regulate the structure and physiological function of excitatory synapses through multiple mechanisms, and might thus play a significant role in higher brain functions.

Corpus callosum deficiency in transgenic mice expressing a truncated ephrin-A receptor

The A-class of the erythropoietin-producing hepatocellular carcinoma cell-derived (EphA) tyrosine kinase receptors and their ligands, the A-ephrins, play critical roles in the specification of topographic axon projection maps during development. In this study, the role of the EphA subfamily in callosal projections was investigated using transgenic mice expressing a kinase deletion mutant of EphA5. In approximately half of these transgenic mice, cerebral cortical neurons in various cortical regions (primary and secondary somatosensory cortices and frontal as well as visual areas) failed to project to the contralateral cortex. When commissural axons were examined with DiI labeling, few callosal fibers were found to traverse the midline in both the adult and neonatal transgenic mice. This defect in callosal development correlates with the expression of the transgene, because neurons in the superficial layers of the motor cortex, where transgene expression is low, show normal contralateral projection through the corpus callosum. In addition, multiple EphA receptors are expressed in callosal neurons and ephrin-A5 stimulates neurite outgrowth of callosal neurons in vitro. The midline glia structures important for callosal axon midline crossing appear normal in the transgenic mice, suggesting that the defects are unrelated to defective guidance structures at the midline. These observations suggest critical functions for EphA receptor in establishing callosal connections during brain development.

EphA4 is not required for Purkinje cell compartmentation

The Purkinje cells of both the adult and the developing cerebellar cortex are organized into parasagittal stripes or 'segments' expressing a variety of biochemical markers. We show that in the developing mouse cerebellar cortex, members of the Eph receptor gene family are expressed in mediolaterally alternating Purkinje cell segments. Since members of the Eph receptors family have been shown to play a role in hindbrain segmentation and boundary formation (Philos. Trans. R. Soc. Lond. B: Biol. Sci. 355 (2000) 993), we analyzed the effect of a null mutation of the EphA4 gene on Purkinje cell compartmentation. Using well characterized markers of Purkinje cell compartmentation in both the developing and the adult cerebellum, we observed no significant alteration in the banding pattern of these markers between the EphA4 knockout mice and their wild type controls. The ribboned pattern of migrating granule cells in the developing cerebellum also appears unaltered. The expression of other members of this gene family, including ephrin-B2, EphA2, and ephrin-A1, in a compartmentalized pattern within the Purkinje cell layer suggests a possible redundancy and/or a compensation of EphA4 function in the segmental patterning of cerebellar Purkinje cells.

Multiple roles of EPH receptors and ephrins in neural development

The control of cell movement during development is essential for forming and stabilizing the spatial organization of tissues and cell types. During initial steps of tissue patterning, distinct regional domains or cell types arise at appropriate locations, and the movement of cells is constrained in order to maintain spatial relationships during growth. In other situations, the guidance of migrating cells or neuronal growth cones to specific destinations underlies the establishment or remodeling of a pattern. Eph receptor tyrosine kinases and their ephrin ligands are key players in controlling these cell movements in many tissues and at multiple stages of patterning.

Regulation of repulsion versus adhesion by different splice forms of an Eph receptor

Eph tyrosine kinase receptors and their membrane-bound ephrin ligands mediate cell interactions and participate in several developmental processes. Ligand binding to an Eph receptor results in tyrosine phosphorylation of the kinase domain, and repulsion of axonal growth cones and migrating cells. Here we report that a subpopulation of ephrin-A5 null mice display neural tube defects resembling anencephaly in man. This is caused by the failure of the neural folds to fuse in the dorsal midline, suggesting that ephrin-A5, in addition to its involvement in cell repulsion, can participate in cell adhesion. During neurulation, ephrin-A5 is co-expressed with its cognate receptor EphA7 in cells at the edges of the dorsal neural folds. Three different EphA7 splice variants, a full-length form and two truncated versions lacking kinase domains, are expressed in the neural folds. Co-expression of an endogenously expressed truncated form of EphA7 suppresses tyrosine phosphorylation of the full-length EphA7 receptor and shifts the cellular response from repulsion to adhesion in vitro. We conclude that alternative usage of different splice forms of a tyrosine kinase receptor can mediate cellular adhesion or repulsion during embryonic development.

Distribution of the receptor EphA7 and its ligands in development of the mouse nervous system

EphA7 is a receptor tyrosine kinase of the Eph family. We have mapped EphA7 immunoreactivity and ligand binding in mouse embryo heads and developing brain. Immunoreactivity for the full-length receptor is found in all the cell populations that express EphA7 mRNA. In particular, it is located on growing axons from EphA7-expressing neurons, both in the trigeminal nerve and in developing brain. In many cases it persists in terminal fields in adult brain. Ligand is detected in a largely complementary distribution in embryos, but is surprisingly weak or undetectable in the target regions of many EphA7-positive axons postnatally.

Eph receptors and ephrins demarcate cerebellar lobules before and during their formation

The formation of the ten cerebellar lobules is an unsolved problem in brain development. We report a screen for the four subfamilies of Eph receptors and their ligands (ephrins) in developing mouse cerebellum, using soluble receptor-immunoglobulin and ligand-immunoglobulin fusion proteins, and antibodies against EphA and ephrin-B proteins. Our results identify Eph receptors and ephrins as the first molecules known to demarcate individual lobules during development. Staining for ephrin-A ligands is in lobule VIII as it forms, across the whole width of the cerebellum. Staining for three EphA receptors approximately coincides with presumptive lobules VI and/or VII before and just after birth, whereas a fourth EphA receptor (EphA4, which binds ligands of both subfamilies) has more widespread expression. Staining for EphB receptors is in lobules VII, VIII, and IX. Staining for ephrin-B ligands is much weaker, becomes detectable only after birth, and does not appear to be lobule-specific. Staining for all subfamilies spreads to at least some adjacent lobules as maturation proceeds. The lobule-specific patterns appear before the lobules form, and initially extend across the width of the cerebellum, in spite of the lesser conservation of the lateral extensions of the lobules. These expression patterns define previously unknown developmental units and suggest that Eph family proteins may contribute to cerebellar morphogenesis.