Regulation of hippocampal synaptic plasticity by the tyrosine kinase receptor, REK7/EphA5, and its ligand, AL-1/Ephrin-A5

Genetic characterization of outbred Sprague Dawley rats and utility for genome-wide association studies

Sprague Dawley (SD) rats are among the most widely used outbred laboratory rat populations. Despite this, the genetic characteristics of SD rats have not been clearly described, and SD rats are rarely used for experiments aimed at exploring genotype-phenotype relationships. In order to use SD rats to perform a genome-wide association study (GWAS), we collected behavioral data from 4,625 SD rats that were predominantly obtained from two commercial vendors, Charles River Laboratories and Harlan Sprague Dawley Inc. Using double-digest genotyping-by-sequencing (ddGBS), we obtained dense, high-quality genotypes at 291,438 SNPs across 4,061 rats. This genetic data allowed us to characterize the variation present in Charles River vs. Harlan SD rats. We found that the two populations are highly diverged (F_ST > 0.4). Furthermore, even for rats obtained from the same vendor, there was strong population structure across breeding facilities and even between rooms at the same facility. We performed multiple separate GWAS by fitting a linear mixed model that accounted for population structure and using meta-analysis to jointly analyze all cohorts. Our study examined Pavlovian conditioned approach (PavCA) behavior, which assesses the propensity for rats to attribute incentive salience to reward-associated cues. We identified 46 significant associations for the various metrics used to define PavCA. The surprising degree of population structure among SD rats from different sources has important implications for their use in both genetic and non-genetic studies.

Outbred Sprague Dawley rats are among the most commonly used rats for neuroscience, physiology and pharmacological research; in the year 2020, 4,188 publications contained the keyword “Sprague Dawley”. Rats identified as “Sprague Dawley” are sold by several commercial vendors, including Charles River Laboratories and Harlan Sprague Dawley Inc. (now Envigo). Despite their widespread use, little is known about the genetic diversity of SD. We genotyped more than 4,000 SD rats, which we used for a genome-wide association study (GWAS) and to characterize genetic differences between SD rats from Charles River Laboratories and Harlan. Our analysis revealed extensive population structure both between and within vendors. The GWAS for Pavlovian conditioned approach (PavCA) identified a number of genome-wide significant loci for that complex behavioral trait. Our results demonstrate that, despite sharing an identical name, SD rats that are obtained from different vendors are very different. Future studies should carefully define the exact source of SD rats being used and may exploit their genetic diversity for genetic studies of complex traits.

Endothelial-Specific EphA4 Negatively Regulates Native Pial Collateral Formation and Re-Perfusion following Hindlimb Ischemia

Leptomeningeal anastomoses play a critical role in regulating vascular re-perfusion following obstruction, however, the mechanisms regulating their development remains under investingation. Our current findings indicate that EphA4 receptor is a novel negative regulator of collaterogenesis. We demonstrate that EphA4 is highly expressed on pial arteriole collaterals at post-natal day (P) 1 and 7, then significantly reduced by P21. Endothelial cell (EC)-specific loss of EphA4, EphA4f/f/Tie2::Cre (KO), resulted in an increase in the density but not diameter of pial collaterals compared to WT mice. ECs isolated from KO mice displayed a 3-fold increase in proliferation, enhanced migration, tube formation and elevated levels of phospho(p)-Akt compared to WT ECs. Attenuating p-Akt, using LY294002, reduced the proliferative and migration effects in the KO ECs. RNAseq analysis also revealed altered expression patterns for genes that regulate cell proliferation, vascular development, extracellular matrix and immune-mediate responses, namely MCP-1, MMP2 and angiopoietin-1. Lastly, we show that induction of hindlimb ischemia resulted in accelerated re-perfusion, collateral remodeling and reduced tissue necrosis in the absence of EC-specific EphA4 compared to WT mice. These findings demonstrate a novel role for EphA4 in the early development of the pial collateral network and suggests a role in regulating vascular remodeling after obstruction.

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.

Repetitive grooming and sensorimotor abnormalities in an ephrin-A knockout model for Autism Spectrum Disorders

EphA receptors and ephrin-A ligands play important roles in neural development and synaptic plasticity in brain regions where expression persists into adulthood. Recently, EPHA3 and EPHA7 gene mutations were linked with Autism Spectrum Disorders (ASDs) and developmental neurological delays, respectively. Furthermore, deletions of ephrin-A2 or ephrin-A3, which exhibit high binding affinity for EphA3 and EphA7 receptors, are associated with subtle deficits in learning and memory behavior and abnormalities in dendritic spine morphology in the cortex and hippocampus in mice. To better characterize a potential role for these ligands in ASDs, we performed a comprehensive behavioral characterization of anxiety-like, sensorimotor, learning, and social behaviors in ephrin-A2/-A3 double knockout (DKO) mice. The predominant phenotype in DKO mice was repetitive and self-injurious grooming behaviors such as have been associated with corticostriatal circuit abnormalities in other rodent models of neuropsychiatric disorders. Consistent with ASDs specifically, DKO mice exhibited decreased preference for social interaction in the social approach assay, decreased locomotor activity in the open field, increased prepulse inhibition of acoustic startle, and a shift towards self-directed activity (e.g., grooming) in novel environments, such as marble burying. Although there were no gross deficits in cognitive assays, subtle differences in performance on fear conditioning and in the Morris water maze resembled traits observed in other rodent models of ASD. We therefore conclude that ephrin-A2/-A3 DKO mice have utility as a novel ASD model with an emphasis on sensory abnormalities and restricted, repetitive behavioral symptoms.

Eph receptors: new players in Alzheimer's disease pathogenesis

Alzheimer's disease (AD) is devastating and leads to permanent losses of memory and other cognitive functions. Although recent genetic evidences strongly argue for a causative role of Aβ in AD onset and progression (Jonsson et al., 2012), its role in AD etiology remains a matter of debate. However, even if not the sole culprit or pathological trigger, genetic and anatomical evidences in conjunction with numerous pharmacological studies, suggest that Aβ peptides, at least contribute to the disease. How Aβ contributes to memory loss remains largely unknown. Soluble Aβ species referred to as Aβ oligomers have been shown to be neurotoxic and induce network failure and cognitive deficits in animal models of the disease. In recent years, several proteins were described as potential Aβ oligomers receptors, amongst which are the receptor tyrosine kinases of Eph family. These receptors together with their natural ligands referred to as ephrins have been involved in a plethora of physiological and pathological processes, including embryonic neurogenesis, learning and memory, diabetes, cancers and anxiety. Here we review recent discoveries on Eph receptors-mediated protection against Aβ oligomers neurotoxicity as well as their potential as therapeutic targets in AD pathogenesis.

Auditory brainstem responses of ephrin-A2, ephrin-A5(-/-) and ephrin-A2A5(-/-) mice

Eph receptors and ephrin ligands are large families of cell surface proteins which have established roles in axonal growth and guidance. These are well characterized in the visual and somatosensory systems but are less well documented in the auditory pathway. We examined the possible functional role of two ephrin genes (ephrin-A2 and ephrin-A5) in the auditory system by measuring auditory brainstem responses (ABR) to tone bursts from 6 to 30 kHz in ephrin-A2(-/-), ephrin-A5(-/-) and ephrin-A2A5(-/-) (knockout) mice. At high frequencies, the ephrin-A2A5(-/-) mice exhibited thresholds that were significantly lower than in wild-type mice by approximately 20 dB, suggesting ephrin-A2 and ephrin-A5 may have frequency-specific effects on the auditory system. There were also alterations in ABR wave peak amplitudes that were specific to each mouse strain which suggested both peripheral and central involvement of EphA-ephrin-A signalling in auditory function.

Activation of EphA receptors mediates the recruitment of the adaptor protein Slap, contributing to the downregulation of N-methyl-D-aspartate receptors

Regulation of the activity of N-methyl-d-aspartate receptors (NMDARs) at glutamatergic synapses is essential for certain forms of synaptic plasticity underlying learning and memory and is also associated with neurotoxicity and neurodegenerative diseases. In this report, we investigate the role of Src-like adaptor protein (Slap) in NMDA receptor signaling. We present data showing that in dissociated neuronal cultures, activation of ephrin (Eph) receptors by chimeric preclustered eph-Fc ligands leads to recruitment of Slap and NMDA receptors at the sites of Eph receptor activation. Interestingly, our data suggest that prolonged activation of EphA receptors is as efficient in recruiting Slap and NMDA receptors as prolonged activation of EphB receptors. Using established heterologous systems, we examined whether Slap is an integral part of NMDA receptor signaling. Our results showed that Slap does not alter baseline activity of NMDA receptors and does not affect Src-dependent potentiation of NMDA receptor currents in Xenopus oocytes. We also demonstrate that Slap reduces excitotoxic cell death triggered by activation of NMDARs in HEK293 cells. Finally, we present evidence showing reduced levels of NMDA receptors in the presence of Slap occurring in an activity-dependent manner, suggesting that Slap is part of a mechanism that homeostatically modulates the levels of NMDA receptors.

Ephrin-A5 deficiency alters sensorimotor and monoaminergic development

The Eph receptors and their ligands, the ephrins, play an important role during neural development. In particular, ephrin-A5 is highly expressed in the developing nervous system in several brain regions including the olfactory bulb, frontal cortex, striatum and hypothalamus. Although a number of studies have characterized the expression of ephrin-A5 in these regions, very little is known about the functional consequences that might follow alterations in the expression of this ligand. Previously, we demonstrated that ephrin-A5 acts as a guidance molecule regulating the trajectory of the ascending midbrain dopaminergic pathways. In light of this finding and the critical role of dopamine in modulating a number of behaviors, we sought to determine whether loss of ephrin-A5 altered neurobehavioral development. Our results indicate that ephrin-A5-null mice exhibit delays in reaching developmental milestones and in the maturation of motor skills. In addition, they exhibit increased locomotor activity and reduced levels of brain monoamines. Therefore, we conclude that ephrin-A5 expression appears to be critical for proper development of central monoaminergic pathways and that its loss results in a number of neurodevelopmental abnormalities. Because alterations in monoamine function are associated with a variety of neurodevelopmental disorders, these data suggest that further study on the potential role of ephrin-A5 in such disorders is warranted.

Contextual learning increases dendrite complexity and EphrinB2 levels in hippocampal mouse neurons

Although the role of hippocampus in memory processing is well assessed, an association of experience-dependent behavioural modifications with hippocampal neuron morphological and biochemical changes deserves further characterisation. Here, we present evidence of dendritic alterations together with rapid accumulation of EphrinB2, a factor known to influence cell plasticity, in pyramidal neurons of the CA1 area of mouse hippocampus, during the formation of recent contextual fear memory. Male C57BL/6N mice exhibited a robust fear response 24h after contextual and cued fear conditioning. At this time and in the absence of the memory test, conditioned mice showed morphological alterations in hippocampal and lateral amygdala neurons. Western blot analysis of extracts from conditioned but not pseudoconditioned or naive mice showed a specific increase in the amount of EphrinB2 in the hippocampus but not the cortex. However, levels of EphA4 receptor, known to interact trans-synaptically with EphrinB2, did not change upon conditioning in extracts from the same structures. Finally, immunohistochemical analysis of the hippocampus and amygdala of conditioned mice showed increased levels of EphrinB2 in pyramidal neurons of the CA1 area, when compared to pseudoconditioned and control mice. Such increase was not observed in other hippocampal areas or the amygdala. These results suggest that rapid accumulation of EphrinB2 in hippocampal CA1 neurons is involved in the behavioural and cellular modifications induced by contextual fear conditioning. A similar mechanism does not appear to occur in lateral amygdala neurons, in spite of the robust behavioural and cellular modifications induced in such structure by cued fear conditioning.

EphA activation overrides the presynaptic actions of BDNF

The adult pattern of neural connectivity is shaped by repulsive and attractive factors, many of which are modulated by activity. Although much is known about the actions of these factors when studied in isolation, little is known about how they interact. To address this question, we examined the effects of sequential or coapplication of brain-derived neurotrophic factor (BDNF) and Fc-conjugated ephrin-A5 or EphA5 in cultured embryonic hippocampal neurons. BDNF promotes neurite outgrowth and synapse formation, and when applied acutely, it elicits an increase in ongoing synaptic activity. Members of the ephrin family of ligands and receptors can be repulsive and prevent formation of synaptic contacts. Acute exposure to either ephrin-A5-Fc or EphA5-Fc transiently enhanced synaptic activity when applied alone, but when applied prior to BDNF, they dramatically reduced the electrophysiological effects of the neurotrophin. Conversely, BDNF had no effect on subsequently applied ephrin-A5-Fc or EphA5-Fc. Consistent with this, ephrin-A5-Fc also prevented BDNF-induced activation of p42/44 MAPK. The effect of ephrin-A5-Fc appears to be presynaptic, as it prevented the BDNF-induced increase in spontaneous miniature postsynaptic current frequency, whereas EphA5-Fc did not. These results suggest that these factors can be categorized differently, with the contact-mediated activation of EphA receptors by ephrin-A5 overriding the diffusion-mediated effect of BDNF.

Impaired hippocampal neurogenesis and vascular formation in ephrin-A5-deficient mice

Neurogenesis occurs throughout the life in the mammalian brain. The hippocampal dentate gyrus (DG) is one of the major regions of the adult neurogenesis, where neural stem/progenitor cells continuously generate new granule neurons, although molecular mechanisms underlying generation and maintenance of newly born neurons are still elusive. Here we show that ephrin-A5, a ligand for Eph receptor tyrosine kinases, plays multiple roles in both neurogenesis and vascular formation in the adult hippocampus. In mice lacking ephrin-A5 function, cell proliferation and survival of newborn neurons were severely reduced in the hippocampus DG. Furthermore, ephrin-A5-deficient mice exhibited altered distribution of EphA4 receptor in the vascular endothelial cells and increased narrower capillaries in the hippocampus DG. EphA/ephrin-A signaling thus plays crucial roles in the establishment and/or maintenance of the brain vascular system, as an essential constituent of the adult neurogenic niche.

Guidance molecules in synapse formation and plasticity

A major goal of modern neuroscience research is to understand the cellular and molecular processes that control the formation, function, and remodeling of chemical synapses. In this article, we discuss the numerous studies that implicate molecules initially discovered for their functions in axon guidance as critical regulators of synapse formation and plasticity. Insights from these studies have helped elucidate basic principles of synaptogenesis, dendritic spine formation, and structural and functional synapse plasticity. In addition, they have revealed interesting dual roles for proteins and cellular mechanisms involved in both axon guidance and synaptogenesis. Much like the dual involvement of morphogens in early cell fate induction and axon guidance, many guidance-related molecules continue to play active roles in controlling the location, number, shape, and strength of neuronal synapses during development and throughout the lifetime of the organism. This article summarizes key findings that link axon guidance molecules to specific aspects of synapse formation and plasticity and discusses the emerging relationship between the molecular and cellular mechanisms that control both axon guidance and synaptogenesis.

Bidirectional signaling of ErbB and Eph receptors at synapses

Synapse development and remodeling are regulated by a plethora of molecules such as receptor tyrosine kinases (RTKs), a family of cell surface receptors that play critical roles in neural development. Two families of RTKs implicated in synaptic functions, ErbBs and Ephs, share similar characteristics in terms of exhibiting forward and reverse signaling. In this review, we will discuss the latest advances in the functions of ErbBs and Ephs at the synapse, including dendritic spine morphogenesis, synapse formation and maturation, and synaptic transmission and plasticity. In addition to signaling at interneuronal synapses, communication between neuron and glia is increasingly implicated in the control of synaptic functions. Studies on RTKs and their cognate ligands in glial cells enhance our understanding on the nature of 'tripartite synapse'. Implications of these signaling events in human diseases will be discussed.

A TrkB/EphrinA interaction controls retinal axon branching and synaptogenesis

Toward understanding topographically specific branching of retinal axons in their target area, we have studied the interaction between neurotrophin receptors and members of the Eph family. TrkB and its ligand BDNF are uniformly expressed in the retina and tectum, respectively, and exert a branch-promoting activity, whereas EphAs and ephrinAs are expressed in gradients in retina and tectum and can mediate a suppression of axonal branching. We have identified a novel cis interaction between ephrinA5 and TrkB on retinal ganglion cell axons. TrkB interacts with ephrinA5 via its second cysteine-rich domain (CC2), which is necessary and sufficient for binding to ephrinA5. Their functional interaction is twofold: ephrinA5 augments BDNF-promoted retinal axon branching in the absence of its activator EphA7-Fc, whereas EphA7-Fc application abolishes branching in a local and concentration-dependent manner. The importance of TrkB in this process is shown by the fact that overexpression of an isolated TrkB-CC2 domain interfering with the ephrinA/TrkB interaction abolishes this regulatory interplay, whereas knockdown of TrkB via RNA interference diminishes the ephrinA5-evoked increase in branching. The ephrinA/Trk interaction is neurotrophin induced and specifically augments the PI-3 kinase/Akt pathway generally known to be involved in the promotion of branching. In addition, ephrinAs/TrkB modulate axon branching and also synapse formation of hippocampal neurons. Our findings uncover molecular mechanisms of how spatially restricted axon branching can be achieved by linking globally expressed branch-promoting with differentially expressed branch-suppressing activities. In addition, our data suggest that growth factors and the EphA-ephrinA system interact in a way that affects axon branching and synapse development.

Changes in attack behavior and activity in EphA5 knockout mice

During development, Eph tyrosine kinase receptors and their ephrin ligands function as axon guidance molecules while, in adults, these molecules appear to be involved in the regulation of neural plasticity and emotion. The absence of EphA5 receptor mediated forward signaling may cause alterations in connectivity of neural networks and boundary formation during development, including central monoaminergic systems. In the present studies, we demonstrated altered aggressive responses by animals lacking functional EphA5 receptors. These behavioral changes were accompanied by altered concentrations of serotonin (5-HT) and the metabolite, 5-HIAA, in the hypothalamus. The changes of serotonin activity in hypothalamus also result in increase of body weight in EphA5 knockout mice. Furthermore, EphA5 knockout mice exhibited a significant decrease in activity levels following exposure to naïve intruders in their home cages. We conclude that the EphA5 receptor may be involved in mediation of aggressive behavior regulated, in part, by hypothalamic serotonin.

The EphA4 receptor regulates neuronal morphology through SPAR-mediated inactivation of Rap GTPases

Eph receptors play critical roles in the establishment and remodeling of neuronal connections, but the signaling pathways involved are not fully understood. We have identified a novel interaction between the C terminus of the EphA4 receptor and the PDZ domain of the GTPase-activating protein spine-associated RapGAP (SPAR). In neuronal cells, this binding mediates EphA4-dependent inactivation of the closely related GTPases Rap1 and Rap2, which have recently been implicated in the regulation of dendritic spine morphology and synaptic plasticity. We show that SPAR-mediated inactivation of Rap1, but not Rap2, is critical for ephrin-A-dependent growth cone collapse in hippocampal neurons and decreased integrin-mediated adhesion in neuronal cells. Distinctive effects of constitutively active Rap1 and Rap2 on the morphology of growth cones and dendritic spines support the idea that these two GTPases have different functions in neurons. Together, our data implicate SPAR as an important signaling intermediate that links the EphA4 receptor with Rap GTPase function in the regulation of neuronal morphology.

Altered EphA5 mRNA expression in rat brain with a single methamphetamine treatment

Methamphetamine is a potent and indirect dopaminergic agonist which can cause chronic brain dysfunctions including drug abuse, drug dependence and drug-induced psychosis. Methamphetamine is known to trigger molecular mechanisms involved in associative learning and memory, and thereby alter patterns of synaptic connectivity. The persistent risk of relapse in methamphetamine abuse, dependence and psychosis may be caused by such alterations in synaptic connectivity. EphA5 receptors constitute large families of tyrosine kinase receptor and are expressed almost exclusively in the nervous system, especially in the limbic structures. Recent studies suggest EphA5 to be important in the topographic projection, development, and plasticity of limbic structures, and to be involved in dopaminergic neurotransmission. We used in situ hybridization to examine whether methamphetamine alters EphA5 mRNA expression in the brains of adult male Wister rats. EphA5 mRNA was widely distributed in the medial frontal cortex, cingulate cortex, piriform cortex, hippocampus, habenular nucleus and amygdala. Compared to baseline expression at 0h, EphA5 mRNA was significantly decreased (by 20%) in the medial frontal cortex at 24h, significantly increased (by 30%) in the amygdala at 9 and 24h, significantly but transiently decreased (by 30%) in the habenular nucleus at 1h after a single injection of methamphetamine. Methamphetamine did not change EphA5 mRNA expression in the cingulate cortex, piriform cortex or hippocampus. Our results that methamphetamine altered EphA5 mRNA expression in rat brain suggest methamphetamine could affect patterns of synaptic connectivity, which might be responsible for methamphetamine-induced chronic brain dysfunctions.

Eph receptor and ephrin signaling in developing and adult brain of the honeybee (Apis mellifera)

Roles for Eph receptor tyrosine kinase and ephrin signaling in vertebrate brain development are well established. Their involvement in the modulation of mammalian synaptic structure and physiology is also emerging. However, less is known of their effects on brain development and their function in adult invertebrate nervous systems. Here, we report on the characterization of Eph receptor and ephrin orthologs in the honeybee, Apis mellifera (Am), and their role in learning and memory. In situ hybridization for mRNA expression showed a uniform distribution of expression of both genes across the developing pupal and adult brain. However, in situ labeling with Fc fusion proteins indicated that the AmEphR and Amephrin proteins were differentially localized to cell body regions in the mushroom bodies and the developing neuropiles of the antennal and optic lobes. In adults, AmEphR protein was localized to regions of synaptic contacts in optic lobes, in the glomeruli of antennal lobes, and in the medial lobe of the mushroom body. The latter two regions are involved in olfactory learning and memory in the honeybee. Injections of EphR-Fc and ephrin-Fc proteins into the brains of adult bees, 1 h before olfactory conditioning of the proboscis extension reflex, significantly reduced memory 24 h later. Experimental amnesia in the group injected with ephrin-Fc was apparent 1 h post-training. Experimental amnesia was also induced by post-training injections with ephrin-Fc suggesting a role in recall. This is the first demonstration that Eph molecules function to regulate the formation of memory in insects.

Bidirectional ephrin/Eph signaling in synaptic functions

Eph receptors, the largest family of receptor tyrosine kinases, and their membrane bound ligands, the ephrins, are involved in multiple developmental and adult processes within and outside of the nervous system. Bi-directional signaling from both the receptor and the ligand is initiated by ephrin-Eph binding upon cell-cell contact, and involves interactions with distinct subsets of downstream signaling molecules related to specific functions. In the CNS, Ephs and ephrins act as attractive/repulsive, migratory and cell adhesive cues during development and participate in synaptic functions in adult animals. In this review, we will focus on recent findings highlighting the functions of ephrin/Eph signaling in dendritic spine morphogenesis, synapse formation and synaptic plasticity.

EphB3 receptor and ligand expression in the adult rat brain

Eph receptors and ligands are two families of proteins that control axonal guidance during development. Their expression was originally thought to be developmentally regulated but recent work has shown that several EphA receptors are expressed postnatally. The EphB3 receptors are expressed during embryonic development in multiple regions of the central nervous system but their potential expression and functional role in the adult brain is unknown. We used in situ hybridization, immunohistochemistry, and receptor affinity probe in situ staining to investigate EphB3 receptors mRNA, protein, and ligand (ephrin-B) expression, respectively, in the adult rat brain. Our results indicate that EphB3 receptor mRNA and protein are constitutively expressed in discrete regions of the adult rat brain including the cerebellum, raphe pallidus, hippocampus, entorhinal cortex, and both motor and sensory cortices. The spatial profile of EphB3 receptors was co-localized to regions of the brain that had a high level of EphB3 receptor binding ligands. Its expression pattern suggests that EphB3 may play a role in the maintenance of mature neuronal connections or re-arrangement of synaptic connections during late stages of development.

Interaction between ephrins/Eph receptors and excitatory amino acid receptors: possible relevance in the regulation of synaptic plasticity and in the pathophysiology of neuronal degeneration

There is increasing evidence that Eph receptors and their transmembrane ligands, named ephrins, interact with glutamate receptors in both developing and adult neurons. EphB receptors interact with proteins that regulate the membrane trafficking of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptor subunits, and both ephrins and EphB receptors have been found to co-localize with N-methyl-d-aspartate (NMDA) receptors and to positively modulate NMDA receptor function. Moreover, pharmacologic activation of ephrin-Bs amplifies group-I metabotropic glutamate receptor signaling through mechanisms that involve NMDA receptors. The interaction with ionotropic or metabotropic glutamate receptors provides a substrate for the emerging role of ephrins and Eph receptors in the regulation of activity-dependent forms of synaptic plasticity, such as long-term potentiation and long-term depression, which are established electrophysiologic models of associative learning. In addition, these interactions explain the involvement of ephrins/Eph receptors in the regulation of pain threshold and epileptogenesis, as well as their potential implication in processes of neuronal degeneration. This may stimulate the search for new drugs that might modulate excitatory synaptic transmission by interacting with the ephrin/Eph receptor system.

Receptor regulation of gene expression of axon guidance molecules: implications for adaptation

Axon guidance molecules, critical for neurodevelopment, are also implicated in morphological and other neurodaptative changes mediated by physiological or pharmacological events in adult brain. As an example, the psychostimulant cocaine markedly alters axon guidance molecules in adult brain of cocaine-treated rats. To decipher a potential link between drug-induced activation of G-protein-coupled receptors (GPCRs) and modulation of axon guidance molecules, we investigated whether GPCR activity in a SK-N-MC human neuroepithelioma cell line (which expresses low levels of D1 dopamine receptors) affects gene expression of axon guidance molecules (semaphorins, ephrins, netrins, and their receptors). Using real-time polymerase chain reaction, we identified 17 of 26 axon guidance molecules in these cells, with varying levels of expression. Forskolin, which raised intracellular cAMP levels 340%, increased EphA5, EphB2, and Neuropilin1 expression, paralleling reported changes in the rat hippocampus after cocaine treatment. The dopamine receptor agonist dihydrexidine, which raised cAMP levels 22%, promoted regulatory changes in EphrinA1, EphrinA5, EphB1, DCC, and Semaphorin3C, whereas (+/-)-6-chloro-7,8-dihydroxy-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrobromide (SKF81297) altered EphA5, EphrinA1, EphrinA5, and neuropilin1. cAMP and other signal transduction pathways may regulate gene expression of axon guidance molecules, potentially linking monoamine receptor activation to signal transduction cascades, transcriptional regulation of axon guidance molecules, and alterations in neural networks.

Disruption of ephrin-A/EphA binding alters synaptogenesis and neural connectivity in the hippocampus

Ephrins are guidance cues that modulate axonal growth and the subsequent axonal topographic maps in many regions of the CNS. Here we studied the functional roles of ephrin-A/EphA interactions in the layer-specific pattern of axonal projections in the hippocampus by disrupting the ephrin-A signaling by over-expression of a soluble EphA receptor. Tracing experiments in EphA5-Fc over-expressing mice revealed that reduction of ephrin-A/EphA interactions did not affect the proper distribution of the main hippocampal afferents, i.e. entorhinal and commissural projections. However, further ultrastructural analyses showed a reduction in the density of synaptic terminals in the entorhinal and commissural termination layers in these mice. In addition, using anti-calbindin antibodies, we analyzed the dentate mossy fiber projections following disruption of ephrin-A/EphA interactions throughout developing hippocampus. While the main mossy fiber bundle appeared normal, the infrapyramidal bundle formed longer projections that established ectopic contacts in these transgenic mice. Later, the expected specific pruning of the infrapyramidal bundle was not observed at adult stages. Ultrastructural analyses confirmed a higher number of mossy fiber terminals in the infrapyramidal bundle in adult EphA5-Fc transgenic mice and showed that these terminals were larger and established a greater number of contacts than in controls. Our results demonstrate that ephrin-A/EphA interactions regulate the synaptogenesis of hippocampal afferents and the proper development and refinement of granule cell projections.

Imaging LTP of presynaptic release of FM1-43 from the rapidly recycling vesicle pool of Schaffer collateral-CA1 synapses in rat hippocampal slices

Recent studies using the styryl dye FM1-43 and two-photon microscopy to directly visualize transmitter release at CA3-CA1 excitatory synapses in the hippocampus have demonstrated that activity-dependent long-term potentiation (LTP) and long-term depression are associated with alterations in vesicular release. It is not known whether particular vesicle pools preferentially express these alterations or what second messenger cascades are involved. To address these questions, we selectively loaded FM1-43 into the rapidly recycling pool (RRP) of vesicles by use of a brief hypertonic shock to release and load the RRP. We demonstrate here that the induction of LTP can lead to a selective long-lasting enhancement in presynaptic release from the RRP, while reserve pool kinetics remain unchanged. LTP of RRP release was N-methyl-d-aspartate receptor-dependent and also required production of the intercellular messenger NO and activation of receptor tyrosine kinase. Measurement of FM1-43 stimulus-evoked uptake rates following induction of LTP confirmed that LTP produces more rapid recycling of vesicles released by electrical stimulation, consistent with an enhanced release probability from the RRP.

Retinal remodelling

Retinal degenerative diseases that progress through loss of photoreceptors initiate a sequence of events that culminates in negative remodelling of the retina. Initially, photoreceptor loss ablates glutamatergic signalling to the neural retina and eliminates coordinate Ca++-coupled homeostatic signalling. Retinal neurons react to this loss of glutamatergic input through retinal rewiring and migration of neurons throughout the axis of the retina. All diseases that kill photoreceptors trigger retinal remodelling as the final common pathway and cell death is a common feature. Retinal remodelling resembles CNS pathologic remodelling and constitutes a major challenge to all rescue strategies.

Retinal remodeling during retinal degeneration

Retinal degenerations, regardless of the initiating event or gene defect, often result in a loss of photoreceptors. This formal deafferentation of the neural retina eliminates the intrinsic glutamatergic drive of the sensory retina and, perhaps more importantly, removes coordinated Ca++-coupled signaling to the neural retina. As in other central nervous system degenerations, deafferentation activates remodeling. Neuronal remodeling is the common fate of all photoreceptor degenerations.

Methylmercury Alters Eph and Ephrin Expression During Neuronal Differentiation of P19 Embryonal Carcinoma Cells

Developmental exposure to methylmercury (MeHg) induces a spectrum of neurological impairment characterized by cognitive disturbance, sensory/motor deficit, and diffuse structural abnormalities of the brain. These alterations may arise from neural path-finding errors during brain development, resulting from disturbances in the function of morphoregulatory guidance molecules. The Eph family of tyrosine kinase receptors and their ligands, the ephrins, guide neuronal migration and neurite pathfinding mainly via repulsive intercellular interactions. The present study examined the effects of MeHg on mRNA and protein expression profiles of Ephs and ephrins in the P19 embryonal carcinoma (EC) cell line and its neuronal derivatives. Undifferentiated control P19 cells displayed low- to undetectable levels of mRNA for ephrins or Ephs, with the sole exception of EphA2 which was highly expressed. Upon differentiation into neurons, the ephrin expression increased progressively through day 10. Similarly, expression of the Ephs, including EphsA3, -A4, -A8, -B2, -B3, -B4, and -B6, increased significantly. In contrast, EphA2 expression decreased in day 2, 6 and 10 control neurons. Treatment with MeHg did not affect the expression of mRNA for ephrins or Ephs in undifferentiated P19 cells. However, treatment of differentiating neurons with MeHg for 24 h caused consistent increases in ligand mRNA expression, particularly ephrin-A5, -A6, -B1, and -B2. Similarly, MeHg induced variable increases in mRNA expression of receptors EphA2, -A3, -B3, and -B6. A trend toward a concentration-response relationship was observed for the alterations in Eph receptor mRNA expression although increases at the low and mid concentrations did not reach statistical significance. Immunoblots for ligand and receptor proteins mirrored the increases in the mRNA levels at the 0.5 and 1.5 microM MeHg concentrations but showed decreased protein levels compared to controls at the 3.0 microM concentration. Alterations in the Eph/ephrin family of repulsion molecules may represent an important mechanism in developmental MeHg neurotoxicity.

Nova regulates brain-specific splicing to shape the synapse

Alternative RNA splicing greatly increases proteome diversity and may thereby contribute to tissue-specific functions. We carried out genome-wide quantitative analysis of alternative splicing using a custom Affymetrix microarray to assess the role of the neuronal splicing factor Nova in the brain. We used a stringent algorithm to identify 591 exons that were differentially spliced in the brain relative to immune tissues, and 6.6% of these showed major splicing defects in the neocortex of Nova2-/- mice. We tested 49 exons with the largest predicted Nova-dependent splicing changes and validated all 49 by RT-PCR. We analyzed the encoded proteins and found that all those with defined brain functions acted in the synapse (34 of 40, including neurotransmitter receptors, cation channels, adhesion and scaffold proteins) or in axon guidance (8 of 40). Moreover, of the 35 proteins with known interaction partners, 74% (26) interact with each other. Validating a large set of Nova RNA targets has led us to identify a multi-tiered network in which Nova regulates the exon content of RNAs encoding proteins that interact in the synapse.

Induction of ephrin-B1 and EphB receptors during denervation-induced plasticity in the adult mouse hippocampus

Abstract It has been widely demonstrated that Eph receptors and their ephrin ligands play multiple pivotal roles in the development of the nervous system. However, less is known about their roles in the adult brain. Here we reported the expression of ephrin-B1 and its cognate EphB receptors in the adult mouse hippocampus at 3, 7, 15, 30 and 60 days after transections of the entorhinal afferents. In situ hybridization and immunohistochemistry showed the time-dependent up-regulation of ephrin-B1 in the denervated areas of the hippocampus, which initiated at 3 days postlesion (dpl), reached maximal levels at 7-15 dpl, remained slightly elevated at 30 dpl and recovered to normal levels by 60 dpl. Double labeling of ephrin-B1 and glial fibrillary acidic protein revealed that ephrin-B1-expressing cells in the denervated areas were reactive astrocytes. Furthermore, a ligand-binding assay using ephrin-B1/Fc chimera protein also displayed the up-regulation of EphB receptors in the denervated areas of the hippocampus in a similar manner to that of ephrin-B1. Within the first week postlesion, the EphB receptors were expressed by reactive astrocytes. After 7 dpl, however, EphB receptors were expressed not only by reactive astrocytes but also first by sprouting axons and later by regrowing dendrites. These results suggest that the ephrin-B1/EphB system may participate in the lesion-induced plasticity processes in the adult mouse hippocampus.

Denervation-induced spatiotemporal upregulation of ephrin-A2 in the mouse hippocampus after transections of the perforant path

Transections of the entorhinal afferent fibers to hippocampus, perforant path (PP), result in the denervation in specific hippocampal subregions, which is followed by a series of plastic events including axon sprouting and reactive synaptogenesis. Many growth-associated molecules are thought to participate in these events. In the present study, we proved the upregulation of ephrin-A2 in the denervated areas of the ipsilateral hippocampus following PP transections. Interestingly, when the elevation of ephrin-A2 reached the maximum axon sprouting in the denervated areas almost finished, implying the possible inhibitory effect of ephrin-A2 on sprouting. In addition, ephrin-A2 expression was observed in synapses during reactive synaptogenesis, suggesting that this molecule might also be implicated in the formation and maturation of synapses in the denervated areas.

Functions of ephrin/Eph interactions in the development of the nervous system: emphasis on the hippocampal system

Ephrins and their Eph receptors are membrane-anchored proteins that have key roles in the development of the Central Nervous System. The main characteristics of ephrin/Eph interactions are that their effect is mediated by cell-to-cell contacts and that they can propagate bidirectional signals downstream of the ligand-receptor complex. These characteristics make ephrins and Eph receptors critical cues in the regulation of migrating cells or axons, and in the establishment of tissue patterns and topographic maps in distinct regions of the developing brain. In addition, ephrins and Eph receptors regulate synapse formation and plasticity. These roles would be promoted by complementary gradual expression of receptors and ligands in the neurons involved. Although, historically, ephrins and Eph receptors have been considered as repulsion signals through barriers or gradients, new evidence indicates that they may be both inhibitory and permissive/active cues depending on expression levels. The expression of distinct ligands and receptors in the developing and mature hippocampus suggests that these proteins are involved in distinct processes during the development and maturation of the hippocampal region. In fact, recent studies have shown that ephrin/Eph signaling participates in the formation of the layer-specific patterns of hippocampal afferents, in synaptogenesis and in plasticity. Therefore, ephrin/Eph interactions should be considered a crucial system in the development and maturation of the brain regions, including the hippocampus.

Kindling and status epilepticus models of epilepsy: rewiring the brain

This review focuses on the remodeling of brain circuitry associated with epilepsy, particularly in excitatory glutamate and inhibitory GABA systems, including alterations in synaptic efficacy, growth of new connections, and loss of existing connections. From recent studies on the kindling and status epilepticus models, which have been used most extensively to investigate temporal lobe epilepsy, it is now clear that the brain reorganizes itself in response to excess neural activation, such as seizure activity. The contributing factors to this reorganization include activation of glutamate receptors, second messengers, immediate early genes, transcription factors, neurotrophic factors, axon guidance molecules, protein synthesis, neurogenesis, and synaptogenesis. Some of the resulting changes may, in turn, contribute to the permanent alterations in seizure susceptibility. There is increasing evidence that neurogenesis and synaptogenesis can appear not only in the mossy fiber pathway in the hippocampus but also in other limbic structures. Neuronal loss, induced by prolonged seizure activity, may also contribute to circuit restructuring, particularly in the status epilepticus model. However, it is unlikely that any one structure, plastic system, neurotrophin, or downstream effector pathway is uniquely critical for epileptogenesis. The sensitivity of neural systems to the modulation of inhibition makes a disinhibition hypothesis compelling for both the triggering stage of the epileptic response and the long-term changes that promote the epileptic state. Loss of selective types of interneurons, alteration of GABA receptor configuration, and/or decrease in dendritic inhibition could contribute to the development of spontaneous seizures.

Disruption of EphA/ephrin-A signaling in the nigrostriatal system reduces dopaminergic innervation and dissociates behavioral responses to amphetamine and cocaine

We have investigated functional roles of EphA/ephrin-A signaling in the development and function of the nigrostriatal system by overexpressing a soluble, broad-range EphA receptor antagonist in the central nervous system of transgenic mice. Adult transgenic mice showed a 30-40% reduction in the total volume of the substantia nigra (SN) without detectable differences in the number of dopaminergic neurons. Using fluorogold retrograde tracing from the striatum, we detected a 40-50% reduction in the number of dopaminergic neurons that could be traced from this structure in transgenic mice, suggesting that, a lower proportion of these cells were able to reach the striatum after disruption of EphA/ephrin-A signaling. In spite of this, total dopamine content in the striatum of transgenic mice was comparable to wild type. Analysis of locomotor activity and its regulation by pharmacological treatments that stimulate dopaminergic transmission revealed an unexpected dissociation of the behavioral responses to amphetamine and cocaine. In particular, transgenic mice were relatively insensitive to amphetamine while retaining normal responsiveness to cocaine, which, to the best of our knowledge, represents the first report of a dissociation of the behavioral responses to these two psychostimulants. Together, these results reveal an unexpected role for EphA/ephrin-A signaling in the normal connectivity and function of midbrain dopaminergic neurons.

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.

EphA/ephrin-A interactions during optic nerve regeneration: restoration of topography and regulation of ephrin-A2 expression

During visual system development, interactions between Eph tyrosine kinase receptors and their ligands, the ephrins, guide retinal ganglion cell (RGC) axons to their topographic targets in the optic tectum. Here we show that Eph/ephrin interactions are also involved in restoring topography during RGC axon regeneration in goldfish. Following optic nerve crush, EphA/ephrin-A interactions were blocked by intracranial injections of recombinant Eph receptor (EphA3-AP) or phospho-inositol phospholipase-C. Topographic errors with multiple inputs to some tectal loci were detected electrophysiologically and increased projections to caudal tectum demonstrated by RT-97 immunohistochemistry. In EphA3-AP-injected fish, ephrin-A2-expressing cells in the retino-recipient tectal layers were reduced in number compared to controls and their distribution was no longer graded. The findings, supported by in vitro studies, implicate EphA/ephrin-A interactions in restoring precise topography and in regulating ephrin-A2 expression during regeneration.

Neurochemical and behavioral deficits consequent to expression of a dominant negative EphA5 receptor

The Eph family tyrosine kinase receptors and their ligands have been linked to axon guidance and topographic mapping of the developing central nervous system. More specifically, the EphA5 receptor has been shown to play a role in development of hippocamposeptal, retinotectal and thalamocortical projections. Recently, a line of transgenic mice was developed which expresses a truncated EphA5 receptor lacking a functional tyrosine kinase domain. In a previous study, axonal tracing revealed that medial hippocampal axons in this strain projected laterally and ventrally away from their normal target area. In the current study, both transgenic and wild-type controls were evaluated in unconditioned (rotorod and locomotor activity) and conditioned (water maze and active avoidance) behavior tasks which tested hippocampal and striatal functioning. Compared to controls, the transgenic strain did not show differences in rotorod motor activity but did show a transient deficit in spatial navigation ability and a consistent impairment in active avoidance. The dominant-negative mutant receptor also resulted in a decrease in striatal dopamine and serotonin concentrations with no change in hippocampal monoamines. Collectively, these data suggest that animals expressing a truncated EphA5 receptor show deficits related to striatal functioning.

EphA/ephrin-A interactions regulate epileptogenesis and activity-dependent axonal sprouting in adult rats

The Eph family of tyrosine kinase receptors and their ligands, ephrins, are distributed in gradients and serve as molecular guidance cues for axonal patterning during neuronal development. Most of these molecules are also expressed in mature brain. Thus, we examine here the potential roles of such molecules in plasticity and activity-dependent mossy fiber sprouting of adult CNS. We show that the ligand ephrin-A3 and the receptor EphA5 are expressed in complementary gradients in the adult rat mossy fiber system. Using the kindling model, we demonstrate that exogenous immunoadhesins that affect the interaction of endogenous EphA receptors and ephrin-A ligands modulate the development of kindling, one type of long-term plasticity, in mature rat brain. These immunoadhesins, combined with epileptogenic stimulations, alter both the extent and the pattern of collateral axonal sprouting in the mossy fiber pathway. Our results suggest that EphA receptors and ephrin-A ligands modify neuronal plasticity and may serve as spatial cues that modulate the development and pattern of activation-dependent axonal growth in adult CNS.

Genetic approaches to molecular and cellular cognition: a focus on LTP and learning and memory

Long-term potentiation (LTP) is the predominant experimental model for the synaptic plasticity mechanisms thought to underlie learning and memory. This review is focused on the contributions of genetics to the understanding of the role of LTP in learning and memory. These studies have used a combination of genetics, molecular biology, neurophysiology, and psychology to demonstrate that molecular mechanisms of synaptic plasticity are critical for learning and memory. Because of the large scope of this literature, we focus primarily on genetic studies of hippocampal-dependent learning. Altogether, these findings not only demonstrate a role for plasticity in learning, they also lay down the foundations for the new field of molecular and cellular cognition.

Eph receptor deficiencies lead to altered cochlear function

Ephrins and Eph receptors are a family of molecules that have been implicated in many developmental processes including neuronal network formation, guidance of cell migration, and axonal pathfinding. These molecules exhibit the ability to send bidirectional signals following ligand-receptor interactions resulting from cell-cell contacts. Gene-targeted knockout mice of B-class ephrins and Eph receptors have been shown to display phenotypic responses that correlate with anatomical defects. For example, disruption of the EphB2 receptor leads to defects of the vestibular system, including pathfinding abnormalities in efferent axons and reduced endolymph production. Such developmental distortions lead to deficiencies in ionic homeostasis and repetitive circling behaviors. The present study demonstrates that B-class ephrins and Eph receptors are expressed in cochlear tissues, suggesting that they may play some role in auditory function. To determine whether ephrins and Eph receptors have a functional role in the peripheral auditory system, distortion-product otoacoustic emission (DPOAE) levels, collected across a broad frequency range, were compared between groups of mice expressing different Eph receptor genotypes. In particular, EphB1 and EphB3 receptor knockout mice exhibited significantly diminished DPOAE levels as compared to wild-type littermates, indicating that these specific Eph receptors are necessary for normal cochlear function.

mRNA expression of ephrins and Eph receptor tyrosine kinases in the neonatal and adult mouse central nervous system

Ephrins and Eph receptors are a family of molecules that have been implicated in axonal pathfinding. A unique feature of B-class ephrins and Eph receptors is their ability to transmit bidirectional signals in both ephrin- and Eph receptor-expressing cells upon cell-cell contact. These signals can lead to cytoskeletal alterations that have been attributed to regulating neuronal growth responses. Examination of gene-target knockout mice has supported this hypothesis, revealing numerous developmental defects in the nervous systems of mice mutant for both B-class ephrins and Eph receptors. To examine the potential scope of action for these genes in the nervous system, we have used in situ hybridization to study the mRNA expression of ephrins (B1, B2, and B3) and Eph receptors (B1, B2, B3, A4) in neonatal and adult mice. We found ephrins and Eph receptors to be expressed throughout the CNS. Expression was observed in the epithelium and migratory regions of the neonate and adult tissues as well as in discrete regions of high plasticity, including the adult olfactory bulb, hippocampus, and cerebellum. These studies suggest additional potential roles for these molecules in the postnatal and adult CNS and will serve as a guide in the detailed evaluation of mutant mice.

Dephrin, a transmembrane ephrin with a unique structure, prevents interneuronal axons from exiting the Drosophila embryonic CNS

Ephrin/Eph signalling is crucial for axonal pathfinding in vertebrates and invertebrates. We identified the Drosophila ephrin orthologue, Dephrin, and describe for the first time the role of ephrin/Eph signalling in the embryonic central nervous system (CNS). Dephrin is a transmembrane ephrin with a unique N terminus and an ephrinB-like cytoplasmic tail. Dephrin binds and interacts with DEph, the Drosophila Eph-like receptor, and Dephrin and DEph are confined to different neuronal compartments. Loss of Dephrin or DEph causes the abberant exit of interneuronal axons from the CNS, whereas ectopic expression of Dephrin halts axonal growth. We propose that the longitudinal tracts in the Drosophila CNS are moulded by a repulsive outer border of Dephrin expression.

Kinase-independent requirement of EphB2 receptors in hippocampal synaptic plasticity

During development, Eph receptors mediate the repulsive axon guidance function of ephrins, a family of membrane attached ligands with their own receptor-like signaling potential. In cultured glutamatergic neurons, EphB2 receptors were recently shown to associate with NMDA receptors at synaptic sites and were suggested to play a role in synaptogenesis. Here we show that Eph receptor stimulation in cultured neurons modulates signaling pathways implicated in synaptic plasticity, suggesting cross-talk with NMDA receptor-activated pathways. Mice lacking EphB2 have normal hippocampal synapse morphology, but display defects in synaptic plasticity. In EphB2(-/-) hippocampal slices, protein synthesis-dependent long-term potentiation (LTP) was impaired, and two forms of synaptic depression were completely extinguished. Interestingly, targeted expression of a carboxy-terminally truncated form of EphB2 rescued the EphB2 null phenotype, indicating that EphB2 kinase signaling is not required for these EphB2-mediated functions.

Expression of Eph receptors in skeletal muscle and their localization at the neuromuscular junction

The participation of ephrins and Eph receptors in guiding motor axons during muscle innervation has been well documented, but little is known about their expression and functional significance in muscle at later developmental stages. Our present study investigates the expression and localization of Eph receptors and ephrins in skeletal muscle. Prominent expression of EphA4, EphA7, and ephrin-A ligands was detected in muscle during embryonic development. More importantly, both EphA4 and EphA7, as well as ephrin-A2, were localized at the neuromuscular junction (NMJ) of adult muscle. Despite their relative abundance, they were not localized at the synapses during embryonic stages. The concentration of EphA4, EphA7, and ephrin-A2 at the NMJ was observed at postnatal stages and the synaptic localization became prominent at later developmental stages. In addition, expression of Eph receptors was increased by neuregulin and after nerve injury. Furthermore, we demonstrated that overexpression of EphA4 led to tyrosine phosphorylation of the actin-binding protein cortactin and that EphA4 was coimmunoprecipitated with cortactin in muscle. Taken together, our findings indicate that EphA4 is associated with the actin cytoskeleton. Since actin cytoskeleton is critical to the formation and stability of NMJ, the present findings raise the intriguing possibility that Eph receptors may have a novel role in NMJ formation and/or maintenance.

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.

Eph receptors and neural plasticity

Eph receptor tyrosine kinases are largely known for their involvement in brain development but, as some of these receptor tyrosine kinases are also expressed in adults, their possible role in the mature nervous system has begun to be explored. Evidence for the involvement of Eph receptors in synaptic plasticity, learning and memory is only emerging and needs corroboration. However, it is likely that the actions of Eph kinases in the adult brain will attract significant attention and become a fertile research area, as occurred in the case of the neurotrophins.