Control of hippocampal dendritic spine morphology through ephrin-A3/EphA4 signaling

Inhibition of EphA4 signaling after ischemia-reperfusion reduces apoptosis of CA1 pyramidal neurons

Hippocampal CA1 pyramidal neurons are sensitive to ischemic damage. However, the cellular and molecular mechanisms underlying neuronal cell death caused by ischemia-reperfusion (I/R) are not completely clear. Here, we report that the ephrinA/EphA cell-cell interaction signaling pathway plays an important role in the apoptosis of hippocampal CA1 pyramidal neurons induced by I/R. We found that the expression of ephrinA3 and EphA4 is increased in the CA1 region following transient forebrain ischemia. Blocking ephrinA3/EphA4 interaction by EphA4-Fc, an inhibitor of EphA4, attenuated apoptotic neuronal cell death, likely through the inhibition of caspase-3 activation. These results reveal a novel function of ephrin/Eph signaling in the regulation of apoptosis in CA1 pyramidal neurons after I/R.

Involvement of the γ-secretase-mediated EphA4 signaling pathway in synaptic pathogenesis of Alzheimer's disease

Loss of synapses is associated with cognitive impairment in Alzheimer's disease (AD). However, the molecular mechanism underlying this synaptic impairment is not well understood. EphA4 is a substrate of γ-secretase, and the γ-secretase-cleaved EphA4 intracellular domain (EICD) is known to enhance the formation of dendritic spines via activation of the Rac signaling pathway. Here, we show that the amount of Rac1 is significantly reduced, and correlated with the level of EICD in the frontal lobes of AD patients. Biochemical analyses revealed that the amount of membrane-associated EICD was decreased and strongly correlated with the level of membrane-associated Rac1, which is considered to be active Rac1. The synaptic scaffolding protein, postsynaptic density (PSD)-95, was specifically decreased in AD, and the amount of PSD-95 correlated with the level of Rac1. Moreover, the amounts of Rac1 and PSD-95 were negatively correlated with the extent of tau phosphorylation, which is crucial for neurofibrillary tangle formation. These results suggest that attenuation of the EICD-mediated Rac signaling pathway is involved in the synaptic pathogenesis of AD.

Glial glutamate transport modulates dendritic spine head protrusions in the hippocampus

Accumulating evidence supports the idea that synapses are tripartite, whereby perisynaptic astrocytes modulate both pre- and postsynaptic function. Although some of these features have been uncovered by using electrophysiological methods, less is known about the structural interplay between synapses and glial processes. Here, we investigated how astrocytes govern the plasticity of individual hippocampal dendritic spines. Recently, we uncovered that a subgroup of innervated dendritic spines is able to undergo remodeling by extending spine head protrusions (SHPs) toward neighboring functional presynaptic boutons, resulting in new synapses. Although glutamate serves as a trigger, how this behavior is regulated is unknown. As astrocytes control extracellular glutamate levels through their high-affinity uptake transporters, together with their privileged access to synapses, we investigated a role for astrocytes in SHP formation. Using time-lapse confocal microscopy, we found that the volume overlap between spines and astrocytic processes decreased during the formation of SHPs. Focal application of glutamate also reduced spine-astrocyte overlap and induced SHPs. Importantly, SHP formation was prevented by blocking glial glutamate transporters, suggesting that glial control of extracellular glutamate is important for SHP-mediated plasticity of spines. Hence, the dynamic changes of both spines and astrocytes can rapidly modify synaptic connectivity.

Synaptic cell adhesion

Chemical synapses are asymmetric intercellular junctions that mediate synaptic transmission. Synaptic junctions are organized by trans-synaptic cell adhesion molecules bridging the synaptic cleft. Synaptic cell adhesion molecules not only connect pre- and postsynaptic compartments, but also mediate trans-synaptic recognition and signaling processes that are essential for the establishment, specification, and plasticity of synapses. A growing number of synaptic cell adhesion molecules that include neurexins and neuroligins, Ig-domain proteins such as SynCAMs, receptor phosphotyrosine kinases and phosphatases, and several leucine-rich repeat proteins have been identified. These synaptic cell adhesion molecules use characteristic extracellular domains to perform complementary roles in organizing synaptic junctions that are only now being revealed. The importance of synaptic cell adhesion molecules for brain function is highlighted by recent findings implicating several such molecules, notably neurexins and neuroligins, in schizophrenia and autism.

Ephrin regulation of synapse formation, function and plasticity

Synapses enable the transmission of information within neural circuits and allow the brain to change in response to experience. During the last decade numerous proteins that can induce synapse formation have been identified. Many of these synaptic inducers rely on trans-synaptic cell-cell interactions to generate functional contacts. Moreover, evidence now suggests that the same proteins that function early in development to regulate synapse formation may help to maintain and/or regulate the function and plasticity of mature synapses. One set of receptors and ligands that appear to impact both the development and the mature function of synapses are Eph receptors (erythropoietin-producing human hepatocellular carcinoma cell line) and their surface associated ligands, ephrins (Eph family receptor interacting proteins). Ephs can initiate new synaptic contacts, recruit and stabilize glutamate receptors at nascent synapses and regulate dendritic spine morphology. Recent evidence demonstrates that ephrin ligands also play major roles at synapses. Activation of ephrins by Eph receptors can induce synapse formation and spine morphogenesis, whereas in the mature nervous system ephrin signaling modulates synaptic function and long-term changes in synaptic strength. In this review we will summarize the recent progress in understanding the role of ephrins in presynaptic and postsynaptic differentiation, and synapse development, function and plasticity.

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.

EphA signaling promotes actin-based dendritic spine remodeling through slingshot phosphatase

Actin cytoskeletal remodeling plays a critical role in transforming the morphology of subcellular structures across various cell types. In the brain, restructuring of dendritic spines through actin cytoskeleletal reorganization is implicated in the regulation of synaptic efficacy and the storage of information in neural circuits. However, the upstream pathways that provoke actin-based spine changes remain only partly understood. Here we show that EphA receptor signaling remodels spines by triggering a sequence of events involving actin filament rearrangement and synapse/spine reorganization. Rapid EphA signaling over minutes activates the actin filament depolymerizing/severing factor cofilin, alters F-actin distribution in spines, and causes transient spine elongation through the phosphatases slingshot 1 (SSH1) and calcineurin/protein phosphatase 2B (PP2B). This early phase of spine extension is followed by synaptic reorganization events that take place over minutes to hours and involve the relocation of pre/postsynaptic components and ultimately spine retraction. Thus, EphA receptors utilize discrete cellular and molecular pathways to promote actin-based structural plasticity of excitatory synapses.

Eph/ephrin signaling in cell-cell and cell-substrate adhesion

Cell-cell and cell-matrix adhesion are critical processes for the formation and maintenance of tissue patterns during development, as well as control of invasion and metastasis of cancer cells. Although great strides have been made regarding our understanding of the processes that play a role in cell adhesion and cell movement, the precise mechanisms by which diverse signaling events regulate cell and tissue architecture are poorly understood. One group of cell surface molecules, Eph receptor tyrosine kinases, and their membrane-bound ligands, ephrins, are key regulators in these processes. It is the ability of Eph/ephrin signaling pathways to regulate cell-cell adhesion and motility that establishes this family as a formidable system for regulating tissue separation and morphogenesis. Moreover, the de-regulation of this signaling system is linked to the promotion of more aggressive and metastatic tumors in humans.

Probing astrocyte function in fragile X syndrome

Astrocytes have been recognized as a class of cells that fill the space between neurons for more than a century. From their humble beginnings in the literature as merely space filling cells, an ever expanding list of functions in the CNS now exceeds the list of functions performed by neurons. In virtually all developmental and pathological conditions in the brain, astrocytes are involved in some capacity that directly affects neuronal function. Today we recognize that astrocytes are involved in the development and function of synaptic communication. Increasing evidence suggests that abnormal synaptic function may be a prominent contributing factor to the learning disability phenotype. With the discovery of FMRP in astrocytes, coupled with a role of astrocytes in synaptic function, research directed to glial neurobiology has never been more important. This chapter highlights the current knowledge of astrocyte function with a focus on their involvement in Fragile X syndrome.

Eph receptors at synapses: implications in neurodegenerative diseases

Precise regulation of synapse formation, maintenance and plasticity is crucial for normal cognitive function, and synaptic failure has been suggested as one of the hallmarks of neurodegenerative diseases. In this review, we describe the recent progress in our understanding of how the receptor tyrosine kinase Ephs and their ligands ephrins regulate dendritic spine morphogenesis, synapse formation and maturation, as well as synaptic plasticity. In particular, we discuss the emerging evidence implicating that deregulation of Eph/ephrin signaling contributes to the aberrant synaptic functions associated with cognitive impairment in Alzheimer's disease. Understanding how Eph/ephrin regulates synaptic function may therefore provide new insights into the development of therapeutic agents against neurodegenerative diseases.

Looking forward to EphB signaling in synapses

Eph receptors and their ligands ephrins comprise a complex signaling system with diverse functions that span a wide range of tissues and developmental stages. The variety of Eph receptor functions stems from their ability to mediate bidirectional signaling through trans-cellular Eph/ephrin interactions. Initially thought to act by directing repulsion between cells, Ephs have also been demonstrated to induce and maintain cell adhesive responses at excitatory synapses in the central nervous system. EphB receptors are essential to the development and maintenance of dendritic spines, which accommodate the postsynaptic sites of most glutamatergic excitatory synapses in the brain. Functions of EphB receptors are not limited to control of the actin cytoskeleton in dendritic spines, as EphB receptors are also involved in the formation of functional synaptic specializations through the regulation of glutamate receptor trafficking and functions. In addition, EphB receptors have recently been linked to the pathophysiology of Alzheimer's disease and neuropathic pain, thus becoming promising targets for therapeutic interventions. In this review, we discuss recent findings on EphB receptor functions in synapses, as well as the mechanisms of bidirectional trans-synaptic ephrin-B/EphB receptor signaling that shape dendritic spines and influence post-synaptic differentiation.

The immune molecule CD3zeta and its downstream effectors ZAP-70/Syk mediate ephrin signaling in neurons to regulate early neuritogenesis

Recent studies have highlighted the key role of the immune protein CD3ζ in the maturation of neuronal circuits in the CNS. Yet, the upstream signals that might recruit and activate CD3ζ in neurons are still unknown. In this study, we show that CD3ζ functions early in neuronal development and we identify ephrinA1-dependent EphA4 receptor activation as an upstream regulator of CD3ζ. When newly born neurons are still spherical, before neurite extension, we found a transient CD3ζ aggregation at the cell periphery matching the initiation site of the future neurite. This accumulation of CD3ζ correlated with a stimulatory effect on filopodia extension via a Rho-GEF Vav2 pathway and a repression of neurite outgrowth. Conversely, cultured neurons lacking CD3ζ isolated from CD3ζ(-/-) mice showed a decreased number of filopodia and an enhanced neurite number. Stimulation with ephrinA1 induces the translocation of both CD3ζ and its activated effector molecules, ZAP-70/Syk tyrosine kinases, to EphA4 receptor clusters. EphrinA1-induced growth cone collapse was abrogated in CD3ζ(-/-) neurons and was markedly reduced by ZAP-70/Syk inhibition. Moreover, ephrinA1-induced ZAP-70/Syk activation was inhibited in CD3ζ(-/-) neurons. Altogether, our data suggest that CD3ζ mediates the ZAP-70/Syk kinase activation triggered by ephrinA-activated pathway to regulate early neuronal morphogenesis.

Radial glia: progenitor, pathway, and partner

Radial glia (RG) are a glial cell type that can be found from the earliest stages of CNS development. They are clearly identifiable by their unique morphology, having a periventricular cell soma and a long process extending all the way to the opposite pial surface. Due to this striking morphology, RG have long been thought of as a transient substrate for neuron migration in the developing brain. In fact, RG cells, far from exclusively serving as a passive scaffold for cell migration, have a remarkably diverse range of critical functions in CNS development and function. These include serving as progenitors of neurons and glia both during development as well as in response to injury, helping to direct axonal and dendritic process outgrowth, and regulating synaptic development and function. RG also engage in extensive bidirectional signaling both with neurons and one another. This review describes the diversity of RG cell types in the CNS and discusses their many important activities.

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.

Long-term changes of spine dynamics and microglia after transient peripheral immune response triggered by LPS in vivo

Background

An episode of peripheral immune response may create long-lasting alterations in the neural network. Recent studies indicate a glial involvement in synaptic remodeling. Therefore it is postulated that both synaptic and glial changes could occur under the peripheral inflammation.

Results

We tested this possibility by in vivo two-photon microscopy of dendritic spines after induction of a peripheral immune response by lipopolysaccharide (LPS) treatment of mice.

We observed that the spines were less stable in LPS-treated mice. The accumulation of spine changes gradually progressed and remained low over a week after LPS treatment but became significantly larger at four weeks. Over eight weeks after LPS treatment, the fraction of eliminated spines amounted to 20% of the initial population and this persistent destabilization resulted in a reduction of the total spine density.

We next evaluated glial activation by LPS administration. Activation of microglia was confirmed by a persistent increase of Iba1 immunoreactivity. Morphological changes in microglia were observed two days after LPS administration and were partially recovered within one week but sustained over a long time period.

Conclusions

These results indicate long-lasting aggravating effects of a single transient peripheral immune response on both spines and microglia. The parallel persistent alterations of both spine turnover and the state of microglia in vivo suggest the presence of a pathological mechanism that sustains the enhanced remodeling of neural networks weeks after peripheral immune responses. This pathological mechanism may also underlie long-lasting cognitive dysfunctions after septic encephalopathy in human patients.

Which Comes First in Fragile X Syndrome, Dendritic Spine Dysgenesis or Defects in Circuit Plasticity?

The salient neuropathological defect in fragile X syndrome is the overabundance of immature dendritic spines in cortical pyramidal neurons. This review examines this anatomical synaptic defect in the context of other alterations in synaptic and circuit plasticity in fragile X mice. In theory, abnormal spines could lead to dysfunctional circuits and vice versa, so it is still not clear which problem comes first. Because of the tight structure-function relationships at the synapse, and given the significant overlap between signaling pathways that regulate spine shape/dynamics and long-term synaptic plasticity (both of which involve proteins regulated by fragile X mental retardation protein [FMRP]), it is argued that the two defects cannot be separated. It will be critical to determine whether neurons that lack FMRP and demonstrate alterations in long-term potentiation/depression also fail to undergo the expected enlargement/shrinkage of dendritic spines associated with those forms of synaptic plasticity or to establish clear links from FMRP signaling to either spine instability or defective synaptic plasticity, especially during critical periods of brain development. The resulting data will be vital in guiding translational research that can identify novel molecular targets for therapy in this devastating disorder.

EphA4 expression promotes network activity and spine maturation in cortical neuronal cultures

Background

Neurons form specific connections with targets via synapses and patterns of synaptic connectivity dictate neural function. During development, intrinsic neuronal specification and environmental factors guide both initial formation of synapses and strength of resulting connections. Once synapses form, non-evoked, spontaneous activity serves to modulate connections, strengthening some and eliminating others. Molecules that mediate intercellular communication are particularly important in synaptic refinement. Here, we characterize the influences of EphA4, a transmembrane signaling molecule, on neural connectivity.

Results

Using multi-electrode array analysis on in vitro cultures, we confirmed that cortical neurons mature and generate spontaneous circuit activity as cells differentiate, with activity growing both stronger and more patterned over time. When EphA4 was over-expressed in a subset of neurons in these cultures, network activity was enhanced: bursts were longer and were composed of more spikes than in control-transfected cultures. To characterize the cellular basis of this effect, dendritic spines, the major excitatory input site on neurons, were examined on transfected neurons in vitro. Strikingly, while spine number and density were similar between conditions, cortical neurons with elevated levels of EphA4 had significantly more mature spines, fewer immature spines, and elevated colocalization with a mature synaptic marker.

Conclusions

These results demonstrate that experimental elevation of EphA4 promotes network activity in vitro, supporting spine maturation, producing more functional synaptic pairings, and promoting more active circuitry.

Neuron-glia signaling: Implications for astrocyte differentiation and synapse formation

Glial cells are currently viewed as active partners of neurons in synapse formation. The close proximity of astrocytes to the synaptic cleft implicates that they strongly influence synapse function as well as suggests that these cells might be potential targets for neuronal-released molecules. In this review, we discuss the signaling pathways of astrocyte generation and the role of astrocyte-derived molecules in synapse formation in the central nervous system. Further, we discuss the role of the excitatory neurotransmitter, glutamate and transforming growth factor beta 1 (TGF-β1) pathway in astrocyte generation and differentiation. We provide evidence that astrocytes surrounding synapses are target of neuronal activity and shed light into the role of astroglial cells into neurological disorders associated with glutamate neurotoxicity.

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.

Control of synapse development and plasticity by Rho GTPase regulatory proteins

Synapses are specialized cell-cell contacts that mediate communication between neurons. Most excitatory synapses in the brain are housed on dendritic spines, small actin-rich protrusions extending from dendrites. During development and in response to environmental stimuli, spines undergo marked changes in shape and number thought to underlie processes like learning and memory. Improper spine development, in contrast, likely impedes information processing in the brain, since spine abnormalities are associated with numerous brain disorders. Elucidating the mechanisms that regulate the formation and plasticity of spines and their resident synapses is therefore crucial to our understanding of cognition and disease. Rho-family GTPases, key regulators of the actin cytoskeleton, play essential roles in orchestrating the development and remodeling of spines and synapses. Precise spatio-temporal regulation of Rho GTPase activity is critical for their function, since aberrant Rho GTPase signaling can cause spine and synapse defects as well as cognitive impairments. Rho GTPases are activated by guanine nucleotide exchange factors (GEFs) and inhibited by GTPase-activating proteins (GAPs). We propose that Rho-family GEFs and GAPs provide the spatiotemporal regulation and signaling specificity necessary for proper Rho GTPase function based on the following features they possess: (i) existence of multiple GEFs and GAPs per Rho GTPase, (ii) developmentally regulated expression, (iii) discrete localization, (iv) ability to bind to and organize specific signaling networks, and (v) tightly regulated activity, perhaps involving GEF/GAP interactions. Recent studies describe several Rho-family GEFs and GAPs that uniquely contribute to spinogenesis and synaptogenesis. Here, we highlight several of these proteins and discuss how they occupy distinct biochemical niches critical for synaptic development.

Imaging of Ca2+ and related signaling molecules and investigation of their functions in the brain

Intracellular Ca(2+) signaling, and related mechanisms involving inositol 1,4,5-trisphosphate (IP(3)), nitric oxide, and the excitatory neurotransmitter glutamate, play a major role in the regulation of cellular function in the brain. Due to the complex morphology of central neurons, the correct spatiotemporal distribution of signaling molecules is essential. Thus, imaging studies have been particularly useful in elucidating the functions of these signaling molecules. The advancement of imaging methods, together with the development of a new method for the specific inhibition of intracellular IP(3) signaling, have made it possible to identify pathways that are regulated by Ca(2+) signals in the brain, including Ca(2+)-dependent synaptic maintenance and glial cell-dependent neurite growth. Further investigation of Ca(2+)-related signaling is expected to increase our understanding of brain function in the future.

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.

Preferential control of basal dendritic protrusions by EphB2

The flow of information between neurons in many neural circuits is controlled by a highly specialized site of cell-cell contact known as a synapse. A number of molecules have been identified that are involved in central nervous system synapse development, but knowledge is limited regarding whether these cues direct organization of specific synapse types or on particular regions of individual neurons. Glutamate is the primary excitatory neurotransmitter in the brain, and the majority of glutamatergic synapses occur on mushroom-shaped protrusions called dendritic spines. Changes in the morphology of these structures are associated with long-lasting modulation of synaptic strength thought to underlie learning and memory, and can be abnormal in neuropsychiatric disease. Here, we use rat cortical slice cultures to examine how a previously-described synaptogenic molecule, the EphB2 receptor tyrosine kinase, regulates dendritic protrusion morphology in specific regions of the dendritic arbor in cortical pyramidal neurons. We find that alterations in EphB2 signaling can bidirectionally control protrusion length, and knockdown of EphB2 expression levels reduces the number of dendritic spines and filopodia. Expression of wild-type or dominant negative EphB2 reveals that EphB2 preferentially regulates dendritic protrusion structure in basal dendrites. Our findings suggest that EphB2 may act to specify synapse formation in a particular subcellular region of cortical pyramidal neurons.

Glia delimit shape changes of sensory neuron receptive endings in C. elegans

Neuronal receptive endings, such as dendritic spines and sensory protrusions, are structurally remodeled by experience. How receptive endings acquire their remodeled shapes is not well understood. In response to environmental stressors, the nematode Caenorhabditis elegans enters a diapause state, termed dauer, which is accompanied by remodeling of sensory neuron receptive endings. Here, we demonstrate that sensory receptive endings of the AWC neurons in dauers remodel in the confines of a compartment defined by the amphid sheath (AMsh) glial cell that envelops these endings. AMsh glia remodel concomitantly with and independently of AWC receptive endings to delimit AWC receptive ending growth. Remodeling of AMsh glia requires the OTD/OTX transcription factor TTX-1, the fusogen AFF-1 and probably the vascular endothelial growth factor (VEGFR)-related protein VER-1, all acting within the glial cell. ver-1 expression requires direct binding of TTX-1 to ver-1 regulatory sequences, and is induced in dauers and at high temperatures. Our results demonstrate that stimulus-induced changes in glial compartment size provide spatial constraints on neuronal receptive ending growth.

Mechanisms of synapse and dendrite maintenance and their disruption in psychiatric and neurodegenerative disorders

Emerging evidence indicates that once established, synapses and dendrites can be maintained for long periods, if not for the organism's entire lifetime. In contrast to the wealth of knowledge regarding axon, dendrite, and synapse development, we understand comparatively little about the cellular and molecular mechanisms that enable long-term synapse and dendrite maintenance. Here, we review how the actin cytoskeleton and its regulators, adhesion receptors, and scaffolding proteins mediate synapse and dendrite maintenance. We examine how these mechanisms are reinforced by trophic signals passed between the pre- and postsynaptic compartments. We also discuss how synapse and dendrite maintenance mechanisms are compromised in psychiatric and neurodegenerative disorders.

Gliotransmission by prostaglandin e(2): a prerequisite for GnRH neuronal function?

Over the past four decades it has become clear that prostaglandin E(2) (PGE(2)), a phospholipid-derived signaling molecule, plays a fundamental role in modulating the gonadotropin-releasing hormone (GnRH) neuroendocrine system and in shaping the hypothalamus. In this review, after a brief historical overview, we highlight studies revealing that PGE(2) released by glial cells such as astrocytes and tanycytes is intimately involved in the active control of GnRH neuronal activity and neurosecretion. Recent evidence suggests that hypothalamic astrocytes surrounding GnRH neuronal cell bodies may respond to neuronal activity with an activation of the erbB receptor tyrosine kinase signaling, triggering the release of PGE(2) as a chemical transmitter from the glia themselves, and, in turn, leading to the feedback regulation of GnRH neuronal activity. At the GnRH neurohemal junction, in the median eminence of the hypothalamus, PGE(2) is released by tanycytes in response to cell-cell signaling initiated by glial cells and vascular endothelial cells. Upon its release, PGE(2) causes the retraction of the tanycyte end-feet enwrapping the GnRH nerve terminals, enabling them to approach the adjacent pericapillary space and thus likely facilitating neurohormone diffusion from these nerve terminals into the pituitary portal blood. In view of these new insights, we suggest that synaptically associated astrocytes and perijunctional tanycytes are integral modulatory elements of GnRH neuronal function at the cell soma/dendrite and nerve terminal levels, respectively.

APC(Cdh1) mediates EphA4-dependent downregulation of AMPA receptors in homeostatic plasticity

Homeostatic plasticity is crucial for maintaining neuronal output by counteracting unrestrained changes in synaptic strength. Chronic elevation of synaptic activity by bicuculline reduces the amplitude of miniature excitatory postsynaptic currents (mEPSCs), but the underlying mechanisms of this effect remain unclear. We found that activation of EphA4 resulted in a decrease in synaptic and surface GluR1 and attenuated mEPSC amplitude through a degradation pathway that requires the ubiquitin proteasome system (UPS). Elevated synaptic activity resulted in increased tyrosine phosphorylation of EphA4, which associated with the ubiquitin ligase anaphase-promoting complex (APC) and its activator Cdh1 in neurons in a ligand-dependent manner. APC(Cdh1) interacted with and targeted GluR1 for proteasomal degradation in vitro, whereas depletion of Cdh1 in neurons abolished the EphA4-dependent downregulation of GluR1. Knockdown of EphA4 or Cdh1 prevented the reduction in mEPSC amplitude in neurons that was a result of chronic elevated activity. Our results define a mechanism by which EphA4 regulates homeostatic plasticity through an APC(Cdh1)-dependent degradation pathway.

Regulation of synaptic connectivity by glia

The human brain contains more than 100 trillion (10(14)) synaptic connections, which form all of its neural circuits. Neuroscientists have long been interested in how this complex synaptic web is weaved during development and remodelled during learning and disease. Recent studies have uncovered that glial cells are important regulators of synaptic connectivity. These cells are far more active than was previously thought and are powerful controllers of synapse formation, function, plasticity and elimination, both in health and disease. Understanding how signalling between glia and neurons regulates synaptic development will offer new insight into how the nervous system works and provide new targets for the treatment of neurological diseases.

Morphine induces AMPA receptor internalization in primary hippocampal neurons via calcineurin-dependent dephosphorylation of GluR1 subunits

Chronic morphine treatment resulting in the alteration of postsynaptic levels of AMPA receptors, thereby modulating synaptic strength, has been reported. However, the mechanism underlying such drug-induced synaptic modification has not been resolved. By monitoring the GluR1 trafficking in primary hippocampal neurons using the pHluorin-GluR1 imaging and biotinylation studies, we observed that prolonged morphine exposure significantly induced loss of synaptic and extrasynaptic GluR1 by internalization. The morphine-induced GluR1 endocytosis was independent of neural network activities or NMDA receptor activities, as neither blocking the sodium channels with tetrodotoxin nor NMDA receptors with dl-APV altered the effects of morphine. Instead, morphine-induced GluR1 endocytosis is attributed to a change in the phosphorylation state of the GluR1 at Ser(845) as morphine significantly decreased the dephosphorylation of GluR1 at this site. Such changes in Ser(845) phosphorylation required morphine-induced activation of calcineurin, based on the observations that a calcineurin inhibitor, FK506, completely abrogated the dephosphorylation, and morphine treatment led to an increase in calcineurin enzymatic activity, even in the presence of dl-APV. Importantly, pretreatment with FK506 and overexpression of the GluR1 mutants, S845D (phospho-mimic) or S845A (phospho-blocking) attenuated the morphine-induced GluR1 endocytosis. Therefore, the calcineurin-mediated GluR1-S845 dephosphorylation is critical for the morphine-induced changes in the postsynaptic AMPA receptor level. Together, these findings reveal a novel molecular mechanism for opioid-induced neuronal adaptation and/or synaptic impairment.

Morphine modulation of thrombospondin levels in astrocytes and its implications for neurite outgrowth and synapse formation

Opioid receptor signaling via EGF receptor (EGFR) transactivation and ERK/MAPK phosphorylation initiates diverse cellular responses that are cell type-dependent. In astrocytes, multiple μ opioid receptor-mediated mechanisms of ERK activation exist that are temporally distinctive and feature different outcomes. Upon discovering that chronic opiate treatment of rats down-regulates thrombospondin 1 (TSP1) expression in the nucleus accumbens and cortex, we investigated the mechanism of action of this modulation in astrocytes. TSP1 is synthesized in astrocytes and is released into the extracellular matrix where it is known to play a role in synapse formation and neurite outgrowth. Acute morphine (hours) reduced TSP1 levels in astrocytes. Chronic (days) opioids repressed TSP1 gene expression and reduced its protein levels by μ opioid receptor and ERK-dependent mechanisms in astrocytes. Morphine also depleted TSP1 levels stimulated by TGFβ1 and abolished ERK activation induced by this factor. Chronic morphine treatment of astrocyte-neuron co-cultures reduced neurite outgrowth and synapse formation. Therefore, inhibitory actions of morphine were detected after both acute and chronic treatments. An acute mechanism of morphine signaling to ERK that entails depletion of TSP1 levels was suggested by inhibition of morphine activation of ERK by a function-blocking TSP1 antibody. This raises the novel possibility that acute morphine uses TSP1 as a source of EGF-like ligands to activate EGFR. Chronic morphine inhibition of TSP1 is reminiscent of the negative effect of μ opioids on EGFR-induced astrocyte proliferation via a phospho-ERK feedback inhibition mechanism. Both of these variations of classical EGFR transactivation may enable opiates to diminish neurite outgrowth and synapse formation.

A role for thrombospondin-1 deficits in astrocyte-mediated spine and synaptic pathology in Down's syndrome

BACKGROUND

Down's syndrome (DS) is the most common genetic cause of mental retardation. Reduced number and aberrant architecture of dendritic spines are common features of DS neuropathology. However, the mechanisms involved in DS spine alterations are not known. In addition to a relevant role in synapse formation and maintenance, astrocytes can regulate spine dynamics by releasing soluble factors or by physical contact with neurons. We have previously shown impaired mitochondrial function in DS astrocytes leading to metabolic alterations in protein processing and secretion. In this study, we investigated whether deficits in astrocyte function contribute to DS spine pathology.

METHODOLOGY/PRINCIPAL FINDINGS

Using a human astrocyte/rat hippocampal neuron coculture, we found that DS astrocytes are directly involved in the development of spine malformations and reduced synaptic density. We also show that thrombospondin 1 (TSP-1), an astrocyte-secreted protein, possesses a potent modulatory effect on spine number and morphology, and that both DS brains and DS astrocytes exhibit marked deficits in TSP-1 protein expression. Depletion of TSP-1 from normal astrocytes resulted in dramatic changes in spine morphology, while restoration of TSP-1 levels prevented DS astrocyte-mediated spine and synaptic alterations. Astrocyte cultures derived from TSP-1 KO mice exhibited similar deficits to support spine formation and structure than DS astrocytes.

CONCLUSIONS/SIGNIFICANCE

These results indicate that human astrocytes promote spine and synapse formation, identify astrocyte dysfunction as a significant factor of spine and synaptic pathology in the DS brain, and provide a mechanistic rationale for the exploration of TSP-1-based therapies to treat spine and synaptic pathology in DS and other neurological conditions.

EphB-mediated degradation of the RhoA GEF Ephexin5 relieves a developmental brake on excitatory synapse formation

The mechanisms that promote excitatory synapse formation and maturation have been extensively studied. However, the molecular events that limit excitatory synapse development so that synapses form at the right time and place and in the correct numbers are less well understood. We have identified a RhoA guanine nucleotide exchange factor, Ephexin5, which negatively regulates excitatory synapse development until EphrinB binding to the EphB receptor tyrosine kinase triggers Ephexin5 phosphorylation, ubiquitination, and degradation. The degradation of Ephexin5 promotes EphB-dependent excitatory synapse development and is mediated by Ube3A, a ubiquitin ligase that is mutated in the human cognitive disorder Angelman syndrome and duplicated in some forms of Autism Spectrum Disorders (ASDs). These findings suggest that aberrant EphB/Ephexin5 signaling during the development of synapses may contribute to the abnormal cognitive function that occurs in Angelman syndrome and, possibly, ASDs.

Formation and maturation of the calyx of Held

Sound localization requires precise and specialized neural circuitry. A prominent and well-studied specialization is found in the mammalian auditory brainstem. Globular bushy cells of the ventral cochlear nucleus (VCN) project contralaterally to neurons of the medial nucleus of the trapezoid body (MNTB), where their large axons terminate on cell bodies of MNTB principal neurons, forming the calyces of Held. The VCN-MNTB pathway is necessary for the accurate computation of interaural intensity and time differences; MNTB neurons provide inhibitory input to the lateral superior olive, which compares levels of excitation from the ipsilateral ear to levels of tonotopically matched inhibition from the contralateral ear, and to the medial superior olive, where precise inhibition from MNTB neurons tunes the delays of binaural excitation. Here we review the morphological and physiological aspects of the development of the VCN-MNTB pathway and its calyceal termination, along with potential mechanisms that give rise to its precision. During embryonic development, VCN axons grow towards the midline, cross the midline into the region of the presumptive MNTB and then form collateral branches that will terminate in calyces of Held. In rodents, immature calyces of Held appear in MNTB during the first few days of postnatal life. These calyces mature morphologically and physiologically over the next three postnatal weeks, enabling fast, high fidelity transmission in the VCN-MNTB pathway.

Specification and morphogenesis of astrocytes

Astrocytes are the most abundant cell type in the mammalian brain. Interest in astrocyte function has increased dramatically in recent years because of their newly discovered roles in synapse formation, maturation, efficacy, and plasticity. However, our understanding of astrocyte development has lagged behind that of other brain cell types. We do not know the molecular mechanism by which astrocytes are specified, how they grow to assume their complex morphologies, and how they interact with and sculpt developing neuronal circuits. Recent work has provided a basic understanding of how intrinsic and extrinsic mechanisms govern the production of astrocytes from precursor cells and the generation of astrocyte diversity. Moreover, new studies of astrocyte morphology have revealed that mature astrocytes are extraordinarily complex, interact with many thousands of synapses, and tile with other astrocytes to occupy unique spatial domains in the brain. A major challenge for the field is to understand how astrocytes talk to each other, and to neurons, during development to establish appropriate astrocytic and neuronal network architectures.

The glia of Caenorhabditis elegans

Glia have been, in many ways, the proverbial elephant in the room. Although glia are as numerous as neurons in vertebrate nervous systems, technical and other concerns had left research on these cells languishing, whereas research on neurons marched on. Importantly, model systems to study glia had lagged considerably behind. A concerted effort in recent years to develop the canonical invertebrate model animals, Drosophila melanogaster and Caenorhabditis elegans, as settings to understand glial roles in nervous system development and function has begun to bear fruit. In this review, we summarize our current understanding of glia and their roles in the nervous system of the nematode C. elegans. The recent studies we describe highlight the similarities and differences between C. elegans and vertebrate glia, and focus on novel insights that are likely to have general relevance to all nervous systems.

Bergmann glial ensheathment of dendritic spines regulates synapse number without affecting spine motility

In the cerebellum, lamellar Bergmann glial (BG) appendages wrap tightly around almost every Purkinje cell dendritic spine. The function of this glial ensheathment of spines is not entirely understood. The development of ensheathment begins near the onset of synaptogenesis, when motility of both BG processes and dendritic spines are high. By the end of the synaptogenic period, ensheathment is complete and motility of the BG processes decreases, correlating with the decreased motility of dendritic spines. We therefore have hypothesized that ensheathment is intimately involved in capping synaptogenesis, possibly by stabilizing synapses. To test this hypothesis, we misexpressed GluR2 in an adenoviral vector in BG towards the end of the synaptogenic period, rendering the BG α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs) Ca2+-impermeable and causing glial sheath retraction. We then measured the resulting spine motility, spine density and synapse number. Although we found that decreasing ensheathment at this time does not alter spine motility, we did find a significant increase in both synaptic pucta and dendritic spine density. These results indicate that consistent spine coverage by BG in the cerebellum is not necessary for stabilization of spine dynamics, but is very important in the regulation of synapse number.

Ephrin-A5 and EphA5 interaction induces synaptogenesis during early hippocampal development

BACKGROUND

Synaptogenesis is a fundamental step in neuronal development. For spiny glutamatergic synapses in hippocampus and cortex, synaptogenesis involves adhesion of pre and postsynaptic membranes, delivery and anchorage of pre and postsynaptic structures including scaffolds such as PSD-95 and NMDA and AMPA receptors, which are glutamate-gated ion channels, as well as the morphological maturation of spines. Although electrical activity-dependent mechanisms are established regulators of these processes, the mechanisms that function during early development, prior to the onset of electrical activity, are unclear. The Eph receptors and ephrins provide cell contact-dependent pathways that regulate axonal and dendritic development. Members of the ephrin-A family are glycosyl-phosphatidylinositol-anchored to the cell surface and activate EphA receptors, which are receptor tyrosine kinases.

METHODOLOGY/PRINCIPAL FINDINGS

Here we show that ephrin-A5 interaction with the EphA5 receptor following neuron-neuron contact during early development of hippocampus induces a complex program of synaptogenic events, including expression of functional synaptic NMDA receptor-PSD-95 complexes plus morphological spine maturation and the emergence of electrical activity. The program depends upon voltage-sensitive calcium channel Ca2+ fluxes that activate PKA, CaMKII and PI3 kinase, leading to CREB phosphorylation and a synaptogenic program of gene expression. AMPA receptor subunits, their scaffolds and electrical activity are not induced. Strikingly, in contrast to wild type, stimulation of hippocampal slices from P6 EphA5 receptor functional knockout mice yielded no NMDA receptor currents.

CONCLUSIONS/SIGNIFICANCE

These studies suggest that ephrin-A5 and EphA5 signals play a necessary, activity-independent role in the initiation of the early phases of synaptogenesis. The coordinated expression of the NMDAR and PSD-95 induced by eprhin-A5 interaction with EphA5 receptors may be the developmental switch that induces expression of AMPAR and their interacting proteins and the transition to activity-dependent synaptic regulation.

Assisted morphogenesis: glial control of dendrite shapes

Neurons display a myriad of dendritic architectures, reflecting their diverse roles in information processing and transduction in the nervous system. Recent findings suggest that neuronal signals may not account for all aspects of dendrite morphogenesis. Observations from C. elegans and other organisms suggest that glial cells can affect dendrite length and guidance, as well as localization and shapes of dendritic receptive structures, such as dendritic spines and sensory cilia. Thus, besides direct roles in controlling neuronal activity, glia contribute to neuron function by ensuring that neurons attain their proper shapes.

RapGAPs in brain: multipurpose players in neuronal Rap signalling

Small Rap guanosine-tri-phosphate (GTP)ases are crucially involved in many cellular processes, including cell proliferation, differentiation, survival, adhesion and movement. In line, it has been shown that Rap signalling is involved in various aspects of neuronal differentiation, like the establishment of neuronal polarity or axonal growth cone movement. Rap GTPases can be activated by a wide variety of external stimuli, and this is mediated by specific guanine nucleotide exchange factors (RapGEFs). Inactivation of RapGTP can be achieved with the aid of specific GTPase-activating proteins (RapGAPs). In the brain, the most prominent RapGAPs are Rap1GAP and those of the spine-associated RapGAP (SPAR) family. This latter family consists of three members (SPAR1-3), from which two of them, namely SPAR1 and 2, have been investigated in more detail. As such, the localization of RapGAPs is crucially important in regulating Rap signalling at various sites in the cell and, for both SPAR1 and 2, enrichment at synaptic sites has been demonstrated. In recent years particularly the role of SPAR1 in shaping dendritic spine morphology has attracted considerable interest. In this review we will summarize the described actions of different RapGAPs expressed in the brain, and we will focus in particular on the SPAR family members.

Astrocytes and synaptic plasticity

Synaptic plasticity, the ability of neurons to change the number and strength of their synapses, has long been considered the sole province of the neuron. Yet neurons do not function in isolation; they are a part of elaborate glial networks where they are intimately associated with astrocytes. Astrocytes make extensive contacts with synaptic sites where they release soluble factors that can increase synapse number, provide synaptic insulation restricting the spread of neurotransmitter to neighboring synapses, and release neuroactive compounds, gliotransmitters, that can directly influence synaptic transmission. During periods of synaptogenesis, astrocyte processes are highly mobile and may contribute to the stabilization of new synapses. As our understanding of the extent of their influence at the synapse unfolds, it is clear that astrocytes are well poised to modulate multiple aspects of synaptic plasticity.

Generation of an EphA4 conditional allele in mice

Ephrins and Eph receptor tyrosine kinases are cell-surface molecules that serve a multitude of functions in cell-cell communication in development, physiology, and disease. EphA4 is a promiscuous member of the EphA subclass of Eph receptors and can bind to both EphrinAs and EphrinBs. In addition to its well-established roles in guiding the development of neuronal connectivity, EphA4 has been implicated for a role in synaptic plasticity, vascular formation, axon regeneration, and central nervous system repair following injury. However, the study of its role in the adult stage has been hampered by confounding developmental defects in EphA4 germline mutants. Here, we report the generation and molecular characterization of an EphA4 conditional allele along with a novel null allele with a knockin fluorescent reporter gene (mCFP). The conditional allele will be useful in ascertaining postdevelopmental and/or cell type-specific function of EphA4 in physiology, injury, and disease.

Three-dimensional relationships between perisynaptic astroglia and human hippocampal synapses

Perisynaptic astroglia are critical for normal synaptic development and function. Little is known, however, about perisynaptic astroglia in the human hippocampus. When mesial temporal lobe epilepsy (MTLE) is refractory to medication, surgical removal is required for seizure quiescence. To investigate perisynaptic astroglia in human hippocampus, we recovered slices for several hours in vitro from three surgical specimens and then quickly fixed them to achieve high-quality ultrastructure. Histological samples from each case were found to have mesial temporal sclerosis with Blumcke Type 1a (mild, moderate) or 1b (severe) pathology. Quantitative analysis through serial section transmission electron microscopy in CA1 stratum radiatum revealed more synapses in the mild (10/10 microm(3)) than the moderate (5/10 microm(3)) or severe (1/10 microm(3)) cases. Normal spines occurred in mild and moderate cases, but a few multisynaptic spines were all that remained in the severe case. Like adult rat hippocampus, perisynaptic astroglial processes were preferentially associated with larger synapses in the mild and moderate cases, but rarely penetrated the cluster of axonal boutons surrounding multisynaptic spines. Synapse perimeters were only partially surrounded by astroglial processes such that all synapses had some access to substances in the extracellular space, similar to adult rat hippocampus. Junctions between astroglial processes were observed more frequently in moderate than mild case, but were obscured by densely packed intermediate filaments in astroglial processes of the severe case. These findings suggest that perisynaptic astroglial processes associate with synapses in human hippocampus in a manner similar to model systems and are disrupted by severe MTLE pathology.

EphA4 signaling in juveniles establishes topographic specificity of structural plasticity in the hippocampus

The formation and loss of synapses is involved in learning and memory. Distinct subpopulations of permanent and plastic synapses coexist in the adult brain, but the principles and mechanisms underlying the establishment of these distinctions remain unclear. Here we show that in the hippocampus, terminal arborizations (TAs) with high plasticity properties are specified at juvenile stages, and account for most synapse turnover of adult mossy fibers. Out of 9-12 giant terminals along CA3, distinct subpopulations of granule neurons revealed by mouse reporter lines exhibit 0, 1, or >2 TAs. TA specification involves a topographic rule based on cell body position and EphA4 signaling. Upon disruption of EphA4 signaling or PSA-NCAM in juvenile circuits, single-TA mossy fibers establish >2 TAs, suggesting that intra-axonal competition influences plasticity site selection. Therefore, plastic synapse specification in juveniles defines sites of synaptic remodeling in the adult, and hippocampal circuit plasticity follows unexpected topographic principles.

Astrocytes prevent abnormal neuronal development in the fragile x mouse

Astrocytes are now distinguished as major regulators of neuronal growth and synaptic development. Recently, they have been identified as key players in the progression of a number of developmental disorders; however, in fragile X syndrome (FXS), the role of astrocytes is not known. Using a coculture design, we found that hippocampal neurons exhibited abnormal dendritic morphology and a decreased number of presynaptic and postsynaptic protein aggregates when they were grown on astrocytes from a fragile X mouse. Moreover, we found that normal astrocytes could prevent the development of abnormal dendrite morphology and preclude the reduction of presynaptic and postsynaptic protein clusters in neurons from a fragile X mouse. These experiments are the first to establish a role for astrocytes in the altered neurobiology of FXS. Our results support the notion that astrocytes contribute to abnormal dendrite morphology and the dysregulated synapse development in FXS.