Plasticity of neuron-glial interactions mediated by astrocytic EphARs

Astrocytes regulate neuronal network activity by mediating synapse remodeling

Based on experience during our life, neuronal connectivity continuously changes through structural remodeling of synapses. Recent studies have shown that the complex interaction between astrocytes and synapses regulates structural synapse remodeling by inducing the formation and elimination of synapses, as well as their functional maturation. Defects in this astrocyte-mediated synapse remodeling cause problems in not only neuronal network activities but also animal behaviors. Moreover, in various neurological disorders, astrocytes have been shown to play central roles in the initiation and progression of synaptic pathophysiology through impaired interactions with synapses. In this review, we will discuss recent studies identifying the novel roles of astrocytes in neuronal circuit remodeling, focusing on synapse formation and elimination. We will also discuss the potential implication of defective astrocytic function in evoking various brain disorders.

The Normal and Brain Tumor Vasculature: Morphological and Functional Characteristics and Therapeutic Targeting

Glioblastoma is among the most common tumor of the central nervous system in adults. Overall survival has not significantly improved over the last decade, even with optimizing standard therapeutic care including extent of resection and radio- and chemotherapy. In this article, we review features of the brain vasculature found in healthy cerebral tissue and in glioblastoma. Brain vessels are of various sizes and composed of several vascular cell types. Non-vascular cells such as astrocytes or microglia also interact with the vasculature and play important roles. We also discuss in vitro engineered artificial blood vessels which may represent useful models for better understanding the tumor-vessel interaction. Finally, we summarize results from clinical trials with anti-angiogenic therapy alone or in combination, and discuss the value of these approaches for targeting glioblastoma.

Role of astrocytes in synapse formation and maturation

Astrocytes are the most abundant glial cells in the mammalian brain and directly participate in the proper functioning of the nervous system by regulating ion homeostasis, controlling glutamate reuptake, and maintaining the blood-brain barrier. In the last two decades, a growing body of work also identified critical roles for astrocytes in regulating synaptic connectivity. Stemming from the observation that functional and morphological development of astrocytes occur concurrently with synapse formation and maturation, these studies revealed that both developmental processes are directly linked. In fact, astrocytes both physically contact numerous synaptic structures and actively instruct many aspects of synaptic development and function via a plethora of secreted and adhesion-based molecular signals. The complex astrocyte-to-neuron signaling modalities control different stages of synaptic development such as regulating the initial formation of structural synapses as well as their functional maturation. Furthermore, the synapse-modulating functions of astrocytes are evolutionarily conserved and contribute to the development and plasticity of diverse classes of synapses and circuits throughout the central nervous system. Importantly, because impaired synapse formation and function is a hallmark of many neurodevelopmental disorders, deficits in astrocytes are likely to be major contributors to disease pathogenesis. In this chapter, we review our current understanding of the cellular and molecular mechanisms by which astrocytes contribute to synapse development and discuss the bidirectional secretion-based and contact-mediated mechanisms responsible for these essential developmental processes.

Role of Astrocytes in the Neurogenic Niches

In the mammalian brain, highly specialized astrocytes serve as neural stem cells (NSCs) that divide and give rise to new neurons, in a process called neurogenesis. During embryonic development NSCs generate almost all neurons of the brain. Soon after birth the neurogenic potential of NSCs is highly reduced, and neurogenesis occurs only in two specialized brain regions called the neurogenic niches. Niche cells are essential to stem cells as they provide structural and nutritional support, and control fundamental stem cell decisions. Astrocytes, major components of the adult neurogenic niches, are evolving as important regulators of neurogenesis, by controlling NSC proliferation, fate choice, and differentiation of the progeny. Therefore, astrocytes contribute to neurogenesis in two ways: as NSCs and as niche cells. This review highlights the role of astrocyte-like NSCs during development and adulthood, and summarizes how niche astrocytes control the process of adult neurogenesis.

The Astrocyte-Neuron Interface: An Overview on Molecular and Cellular Dynamics Controlling Formation and Maintenance of the Tripartite Synapse

Astrocytes are known to provide trophic support to neurons and were originally thought to be passive space-filling cells in the brain. However, recent advances in astrocyte development and functions have highlighted their active roles in controlling brain functions by modulating synaptic transmission. A bidirectional cross talk between astrocytic processes and neuronal synapses define the concept of tripartite synapse. Any change in astrocytic structure/function influences neuronal activity which could lead to neurodevelopmental and neurodegenerative disorders. In this chapter, we briefly overview the methodologies used in deciphering the mechanisms of dynamic interplay between astrocytes and neurons.

The Blood-Brain Barrier and the EphR/Ephrin System: Perspectives on a Link Between Neurovascular and Neuropsychiatric Disorders

Interactions among endothelial cells (EC) forming blood vessels and their surrounding cell types are essential to establish the blood-brain barrier (BBB), an integral part of the neurovascular unit (NVU). Research on the NVU has recently seen a renaissance to especially understand the neurobiology of vascular and brain pathologies and their frequently occurring comorbidities. Diverse signaling molecules activated in the near proximity of blood vessels trigger paracellular pathways which regulate the formation and stabilization of tight junctions (TJ) between EC and thereby influence BBB permeability. Among regulatory molecules, the erythropoietin-producing-hepatocellular carcinoma receptors (EphR) and their Eph receptor-interacting signals (ephrins) play a pivotal role in EC differentiation, angiogenesis and BBB integrity. Multiple EphR-ligand interactions between EC and other cell types influence different aspects of angiogenesis and BBB formation. Such interactions additionally control BBB sealing properties and thus the penetration of substances into the brain parenchyma. Thus, they play critical roles in the healthy brain and during the pathogenesis of brain disorders. In this mini-review article, we aim at integrating the constantly growing literature about the functional roles of the EphR/ephrin system for the development of the vascular system and the BBB and in the pathogenesis of neurovascular and neuropsychiatric disorders. We suggest the hypothesis that a disrupted EphR/ephrin signaling at the BBB might represent an underappreciated molecular hub of disease comorbidity. Finally, we propose the possibility that the EphR/ephrin system bears the potential of becoming a novel target for the development of alternative therapeutic treatments, focusing on such comorbidities.

Physiology of Astroglia

Astrocytes are neural cells of ectodermal, neuroepithelial origin that provide for homeostasis and defense of the central nervous system (CNS). Astrocytes are highly heterogeneous in morphological appearance; they express a multitude of receptors, channels, and membrane transporters. This complement underlies their remarkable adaptive plasticity that defines the functional maintenance of the CNS in development and aging. Astrocytes are tightly integrated into neural networks and act within the context of neural tissue; astrocytes control homeostasis of the CNS at all levels of organization from molecular to the whole organ.

Physiology of Astroglia

Astrocytes are neural cells of ectodermal, neuroepithelial origin that provide for homeostasis and defense of the central nervous system (CNS). Astrocytes are highly heterogeneous in morphological appearance; they express a multitude of receptors, channels, and membrane transporters. This complement underlies their remarkable adaptive plasticity that defines the functional maintenance of the CNS in development and aging. Astrocytes are tightly integrated into neural networks and act within the context of neural tissue; astrocytes control homeostasis of the CNS at all levels of organization from molecular to the whole organ.

Developing HiPSC Derived Serum Free Embryoid Bodies for the Interrogation of 3-D Stem Cell Cultures Using Physiologically Relevant Assays

Although a number of in vitro disease models have been developed using hiPSCs, one limitation is that these two-dimensional (2-D) systems may not represent the underlying cytoarchitectural and functional complexity of the affected individuals carrying suspected disease variants. Conventional 2-D models remain incomplete representations of in vivo-like structures and do not adequately capture the complexity of the brain. Thus, there is an emerging need for more 3-D hiPSC-based models that can better recapitulate the cellular interactions and functions seen in an in vivo system. Here we report a protocol to develop a 3-D system from undifferentiated hiPSCs based on the serum free embryoid body (SFEB). This 3-D model mirrors aspects of a developing ventralized neocortex and allows for studies into functions integral to living neural cells and intact tissue such as migration, connectivity, communication, and maturation. Specifically, we demonstrate that the SFEBs using our protocol can be interrogated using physiologically relevant and high-content cell based assays such as calcium imaging, and multi-electrode array (MEA) recordings without cryosectioning. In the case of MEA recordings, we demonstrate that SFEBs increase both spike activity and network-level bursting activity during long-term culturing. This SFEB protocol provides a robust and scalable system for the study of developing network formation in a 3-D model that captures aspects of early cortical development.

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

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

Morphological plasticity of astroglia: Understanding synaptic microenvironment

Memory formation in the brain is thought to rely on the remodeling of synaptic connections which eventually results in neural network rewiring. This remodeling is likely to involve ultrathin astroglial protrusions which often occur in the immediate vicinity of excitatory synapses. The phenomenology, cellular mechanisms, and causal relationships of such astroglial restructuring remain, however, poorly understood. This is in large part because monitoring and probing of the underpinning molecular machinery on the scale of nanoscopic astroglial compartments remains a challenge. Here we briefly summarize the current knowledge regarding the cellular organisation of astroglia in the synaptic microenvironment and discuss molecular mechanisms potentially involved in use-dependent astroglial morphogenesis. We also discuss recent observations concerning morphological astroglial plasticity, the respective monitoring methods, and some of the newly emerging techniques that might help with conceptual advances in the area.

The inhibitory neurotransmitter GABA evokes long-lasting Ca(2+) oscillations in cortical astrocytes

Studies over the last decade provided evidence that in a dynamic interaction with neurons glial cell astrocytes contribut to fundamental phenomena in the brain. Most of the knowledge on this derives, however, from studies monitoring the astrocyte Ca²⁺ response to glutamate. Whether astrocytes can similarly respond to other neurotransmitters, including the inhibitory neurotransmitter GABA, is relatively unexplored. By using confocal and two photon laser‐scanning microscopy the astrocyte response to GABA in the mouse somatosensory and temporal cortex was studied. In slices from developing (P15‐20) and adult (P30‐60) mice, it was found that in a subpopulation of astrocytes GABA evoked somatic Ca²⁺ oscillations. This response was mediated by GABA_B receptors and involved both G_i/o protein and inositol 1,4,5‐trisphosphate (IP₃) signalling pathways. In vivo experiments from young adult mice, revealed that also cortical astrocytes in the living brain exibit GABA_B receptor‐mediated Ca²⁺ elevations. At all astrocytic processes tested, local GABA or Baclofen brief applications induced long‐lasting Ca²⁺ oscillations, suggesting that all astrocytes have the potential to respond to GABA. Finally, in patch‐clamp recordings it was found that Ca²⁺ oscillations induced by Baclofen evoked astrocytic glutamate release and slow inward currents (SICs) in pyramidal cells from wild type but not IP₃R2^−/− mice, in which astrocytic GABA_B receptor‐mediated Ca²⁺ elevations are impaired. These data suggest that cortical astrocytes in the mouse brain can sense the activity of GABAergic interneurons and through their specific recruitment contribut to the distinct role played on the cortical network by the different subsets of GABAergic interneurons. GLIA 2016;64:363–373

DNA Methylation of the EphA5 Promoter Is Associated with Rat Congenital Hypothyroidism

Thyroid hormones (THs) are essential for normal development of the mammalian central nervous system through regulation of TH-responsive genes. EphA5, an important TH-responsive gene encoding the tyrosine kinase receptor EphA5, regulates synaptogenesis initiation and synaptic remodeling during brain development. Abnormal EphA5 expression is involved in the development of congenital hypothyroidism (CH). To show the regulatory mechanism of EphA5 expression in CH rats, we analyzed the correlation between methylation of the EphA5 promoter and its expression in the hypothyroid hippocampus and hippocampal neurons. Demethylation treatment using 5'-azadeoxycytidine upregulated EphA5 expression and rescued the effects of hypermethylation, suggesting a novel regulatory mechanism of EphA5 expression in CH rats. Our results suggest a potentially new approach for the development of drugs to restore neurocognitive impairments associated with CH.

Connectivity and circuitry in a dish versus in a brain

In order to understand and find therapeutic strategies for neurological disorders, disease models that recapitulate the connectivity and circuitry of patients’ brain are needed. Owing to many limitations of animal disease models, in vitro neuronal models using patient-derived stem cells are currently being developed. However, prior to employing neurons as a model in a dish, they need to be evaluated for their electrophysiological properties, including both passive and active membrane properties, dynamics of neurotransmitter release, and capacity to undergo synaptic plasticity. In this review, we survey recent attempts to study these issues in human induced pluripotent stem cell-derived neurons. Although progress has been made, there are still many hurdles to overcome before human induced pluripotent stem cell-derived neurons can fully recapitulate all of the above physiological properties of adult mature neurons. Moreover, proper integration of neurons into pre-existing circuitry still needs to be achieved. Nevertheless, in vitro neuronal stem cell-derived models hold great promise for clinical application in neurological diseases in the future.

A time course analysis of the electrophysiological properties of neurons differentiated from human induced pluripotent stem cells (iPSCs)

Many protocols have been designed to differentiate human embryonic stem cells (ESCs) and human induced pluripotent stem cells (iPSCs) into neurons. Despite the relevance of electrophysiological properties for proper neuronal function, little is known about the evolution over time of important neuronal electrophysiological parameters in iPSC-derived neurons. Yet, understanding the development of basic electrophysiological characteristics of iPSC-derived neurons is critical for evaluating their usefulness in basic and translational research. Therefore, we analyzed the basic electrophysiological parameters of forebrain neurons differentiated from human iPSCs, from day 31 to day 55 after the initiation of neuronal differentiation. We assayed the developmental progression of various properties, including resting membrane potential, action potential, sodium and potassium channel currents, somatic calcium transients and synaptic activity. During the maturation of iPSC-derived neurons, the resting membrane potential became more negative, the expression of voltage-gated sodium channels increased, the membrane became capable of generating action potentials following adequate depolarization and, at day 48–55, 50% of the cells were capable of firing action potentials in response to a prolonged depolarizing current step, of which 30% produced multiple action potentials. The percentage of cells exhibiting miniature excitatory post-synaptic currents increased over time with a significant increase in their frequency and amplitude. These changes were associated with an increase of Ca²⁺ transient frequency. Co-culturing iPSC-derived neurons with mouse glial cells enhanced the development of electrophysiological parameters as compared to pure iPSC-derived neuronal cultures. This study demonstrates the importance of properly evaluating the electrophysiological status of the newly generated neurons when using stem cell technology, as electrophysiological properties of iPSC-derived neurons mature over time.

Mechanisms of hippocampal astrocytes mediation of spatial memory and theta rhythm by gliotransmitters and growth factors

Our knowledge about encoding and maintenance of spatial memory emphasizes the integrated functional role of the grid cells and the place cells of the hippocampus in the generation of theta rhythm in spatial memory formation. However, the role of astrocytes in these processes is often underestimated in their contribution to the required structural and functional characteristics of hippocampal neural network operative in spatial memory. We show that hippocampal astrocytes, by the secretion of gliotransmitters, such as glutamate, d-serine, and ATP and growth factors such as BDNF and by the expression of receptors and channels such as those of TNFα and aquaporin, have several diverse fuctions in spatial memory. We specifically focus on the role of astrocytes on five phases of spatial memory: (1) theta rhythm generation, (2) theta phase precession, (3) formation of spatial memory by mapping data of entorhinal grid cells into the place cells, (4) storage of spatial information, and (5) maintenance of spatial memory. Finally, by reviewing the literature, we propose specific mechanisms mentioned in the form of a hypothesis suggesting that astrocytes are important in spatial memory formation.

Gliotransmitters travel in time and space

The identification of the presence of active signaling between astrocytes and neurons in a process termed gliotransmission has caused a paradigm shift in our thinking about brain function. However, we are still in the early days of the conceptualization of how astrocytes influence synapses, neurons, networks, and ultimately behavior. In this Perspective, our goal is to identify emerging principles governing gliotransmission and consider the specific properties of this process that endow the astrocyte with unique functions in brain signal integration. We develop and present hypotheses aimed at reconciling confounding reports and define open questions to provide a conceptual framework for future studies. We propose that astrocytes mainly signal through high-affinity slowly desensitizing receptors to modulate neurons and perform integration in spatiotemporal domains complementary to those of neurons.

Role of synaptic plasticity and EphA5-ephrinA5 interaction within the ventromedial hypothalamus in response to recurrent hypoglycemia

Hypoglycemia stimulates counterregulatory hormone release to restore euglycemia. This protective response is diminished by recurrent hypoglycemia, limiting the benefits of intensive insulin treatment in patients with diabetes. We previously reported that EphA5 receptor-ephrinA5 interactions within the ventromedial hypothalamus (VMH) influence counterregulatory hormone responses during acute hypoglycemia in nondiabetic rats. In this study, we examined whether recurrent hypoglycemia alters the capacity of the ephrinA5 ligand to activate VMH EphA5 receptors, and if so, whether these changes could contribute to pathogenesis of defective glucose counterregulation in response to a standard hypoglycemic stimulus. The expression of ephrinA5, but not EphA5 receptors within the VMH, was reduced by antecedent recurrent hypoglycemia. In addition, the number of synaptic connections was increased and astroglial synaptic coverage was reduced. Activation of VMH EphA5 receptors via targeted microinjection of ephrinA5-Fc before a hyperinsulinemic hypoglycemic clamp study caused a reduction in the glucose infusion rate in nondiabetic rats exposed to recurrent hypoglycemia. The increase in the counterregulatory response to insulin-induced hypoglycemia was associated with a 150% increase in glucagon release (P < 0.001). These data suggest that changes in ephrinA5/EphA5 interactions and synaptic plasticity within the VMH, a key glucose-sensing region in the brain, may contribute to the impairment in glucagon secretion and counterregulatory responses caused by recurrent hypoglycemia.

Astrocyte-synapse structural plasticity

The function and efficacy of synaptic transmission are determined not only by the composition and activity of pre- and postsynaptic components but also by the environment in which a synapse is embedded. Glial cells constitute an important part of this environment and participate in several aspects of synaptic functions. Among the glial cell family, the roles played by astrocytes at the synaptic level are particularly important, ranging from the trophic support to the fine-tuning of transmission. Astrocytic structures are frequently observed in close association with glutamatergic synapses, providing a morphological entity for bidirectional interactions with synapses. Experimental evidence indicates that astrocytes sense neuronal activity by elevating their intracellular calcium in response to neurotransmitters and may communicate with neurons. The precise role of astrocytes in regulating synaptic properties, function, and plasticity remains however a subject of intense debate and many aspects of their interactions with neurons remain to be investigated. A particularly intriguing aspect is their ability to rapidly restructure their processes and modify their coverage of the synaptic elements. The present review summarizes some of these findings with a particular focus on the mechanisms driving this form of structural plasticity and its possible impact on synaptic structure and function.

GluN3A: an NMDA receptor subunit with exquisite properties and functions

N-methyl-D-aspartate receptors (NMDAR) are pivotal for synaptic plasticity and memory formation. Conventional NMDAR consist of heterotetrameric structures composed of GluN1 and GluN2 subunits. A third subunit, GluN3, can also assemble with NMDAR subunits giving a remarkable modification of their heteromeric structure, forming a "nonconventional" NMDAR. As a consequence, the stoichiometry and kinetic properties of the receptors are dramatically changed. Among the GluN3 family, the GluN3A subunit has been the focus of a large amount of studies during recent years. These studies reveal that GluN3A is transiently expressed during development and could play a role in the fine tuning of neuronal networks as well as associated diseases. Moreover, GluN3A distribution outside the postsynaptic densities, including perisynaptic astrocytes, places it at a strategic position to play an important role in the interactions between neurons and glial cells. This review highlights GluN3A properties and addresses its role in neurophysiology and associated pathologies.

Matrix metalloproteinases regulate the formation of dendritic spine head protrusions during chemically induced long-term potentiation

Dendritic spines are are small membranous protrusions that extend from neuronal dendrites and harbor the majority of excitatory synapses. Increasing evidence has shown that matrix metalloproteinases (MMPs), a family of extracellularly acting and Zn²⁺-dependent endopeptidases, are able to rapidly modulate dendritic spine morphology. Spine head protrusions (SHPs) are filopodia-like processes that extend from the dendritic spine head, representing a form of postsynaptic structural remodeling in response to altered neuronal activity. Herein, we show that chemically induced long-term potentiation (cLTP) in dissociated hippocampal cultures upregulates MMP-9 activity that controls the formation of SHPs. Blocking of MMPs activity or microtubule dynamics abolishes the emergence of SHPs. In addition, autoactive recombinant MMP-9, promotes the formation of SHPs in organotypic hippocampal slices. Furthermore, spines with SHPs gained postsynaptic α-amino-3-hydroxyl-5-methyl-4-isoxazole propionic acid (AMPA) receptors upon cLTP and the synaptic delivery of AMPA receptors was controlled by MMPs. The present results strongly imply that MMP-9 is functionally involved in the formation of SHPs and the control of postsynaptic receptor distribution upon cLTP.

EphA5-EphrinA5 interactions within the ventromedial hypothalamus influence counterregulatory hormone release and local glutamine/glutamate balance during hypoglycemia

Activation of β-cell EphA5 receptors by its ligand ephrinA5 from adjacent β-cells has been reported to decrease insulin secretion during hypoglycemia. Given the similarities between islet and ventromedial hypothalamus (VMH) glucose sensing, we tested the hypothesis that the EphA5/ephrinA5 system might function within the VMH during hypoglycemia to stimulate counterregulatory hormone release as well. Counterregulatory responses and glutamine/glutamate concentrations in the VMH were assessed during a hyperinsulinemic-hypoglycemic glucose clamp study in chronically catheterized awake male Sprague-Dawley rats that received an acute VMH microinjection of ephrinA5-Fc, chronic VMH knockdown, or overexpression of ephrinA5 using an adenoassociated viral construct. Local stimulation of VMH EphA5 receptors by ephrinA5-Fc or ephrinA5 overexpression increased, whereas knockdown of VMH ephrinA5 reduced counterregulatory responses during hypoglycemia. Overexpression of VMH ephrinA5 transiently increased local glutamate concentrations, whereas ephrinA5 knockdown produced profound suppression of VMH interstitial fluid glutamine concentrations in the basal state and during hypoglycemia. Changes in ephrinA5/EphA5 interactions within the VMH, a key brain glucose-sensing region, act in concert with islets to restore glucose homeostasis during acute hypoglycemia, and its effect on counterregulation may be mediated by changes in glutamate/glutamine cycling.

Morphological plasticity of rodent astroglia

In the past two decades, there has been an explosion of research on the role of neuroglial interactions in the control of brain homeostasis in both physiological and pathological conditions. Astrocytes, a subtype of glia in the central nervous system, are dynamic signaling elements that regulate neurogenesis and development of brain circuits, displaying intimate dynamic relationships with neurons, especially at synaptic sites where they functionally integrate the tripartite synapse. When astrocytes are isolated from the brain and maintained in culture, they exhibit a polygonal shape unlike their precursors in vivo. However, cultured astrocytes can be induced to undergo morphological plasticity leading to process formation, either by interaction with neurons or by the influence of pharmacological agents. This review highlights studies on the molecular mechanisms underlying morphological plasticity in astrocyte cultures and intact brain tissue, both in situ and in vivo.

Developing neurons form transient nanotubes facilitating electrical coupling and calcium signaling with distant astrocytes

Despite the well-documented cooperation between neurons and astrocytes little is known as to how these interactions are initiated. We show here by differential interference contrast microscopy that immature hippocampal neurons generated short protrusions towards astrocytes resulting in tunneling nanotube (TNT) formation with an average lifetime of 15 minutes. Fluorescence microscopy revealed that all TNTs between the two cell types contained microtubules but 35% of them were F-actin negative. Immunolabeling against connexin 43 showed that this gap junction marker localized at the contact site of TNTs with astrocytes. Using optical membrane-potential measurements combined with mechanical stimulation, we observed that ~35% of immature neurons were electrically coupled with distant astrocytes via TNTs up to 5 hours after co-culture but not after 24 hours. Connexin 43 was expressed by most neurons at 5 hours of co-culture but was not detected in neurons after 24 hours. We show that TNTs mediated the propagation of both depolarization and transient calcium signals from distant astrocytes to neurons. Our findings suggest that within a limited maturation period developing neurons establish electrical coupling and exchange of calcium signals with astrocytes via TNTs, which correlates with a high neuronal expression level of connexin 43. This novel cell-cell communication pathway between cells of the central nervous system provides new concepts in our understanding of neuronal migration and differentiation.

Improving FRET dynamic range with bright green and red fluorescent proteins

A variety of genetically encoded reporters use changes in fluorescence (or Förster) resonance energy transfer (FRET) to report on biochemical processes in living cells. The standard genetically encoded FRET pair consists of CFPs and YFPs, but many CFP-YFP reporters suffer from low FRET dynamic range, phototoxicity from the CFP excitation light and complex photokinetic events such as reversible photobleaching and photoconversion. We engineered two fluorescent proteins, Clover and mRuby2, which are the brightest green and red fluorescent proteins to date and have the highest Förster radius of any ratiometric FRET pair yet described. Replacement of CFP and YFP with these two proteins in reporters of kinase activity, small GTPase activity and transmembrane voltage significantly improves photostability, FRET dynamic range and emission ratio changes. These improvements enhance detection of transient biochemical events such as neuronal action-potential firing and RhoA activation in growth cones.

GABA release by hippocampal astrocytes

Astrocytes can directly influence neuronal activity through the release of various transmitters acting on membrane receptors expressed by neurons. However, in contrast to glutamate and ATP for instance, the release of GABA (γ-amino-butyric acid) by astrocytes is still poorly documented. Here, we used whole-cell recordings in rat acute brain slices and electron microscopy to test whether hippocampal astrocytes release the inhibitory transmitter GABA. We observed that slow transient inhibitory currents due to the activation of GABA_A receptors occur spontaneously in principal neurons of the three main hippocampal fields (CA1, CA3, and dentate gyrus). These currents share characteristics with the slow NMDA receptor-mediated currents previously shown to result from astrocytic glutamate release: they occur in the absence of synaptic transmission and have variable kinetics and amplitudes as well as low frequencies. Osmotic pressure reduction, known to enhance transmitter release from astrocytes, similarly increased the frequency of non-synaptic GABA and glutamate currents. Simultaneous occurrence of slow inhibitory and excitatory currents was extremely rare. Yet, electron microscopy examination of immunostained hippocampal sections shows that about 80% of hippocampal astrocytes [positive for glial fibrillary acidic protein (GFAP)] were immunostained for GABA. Our results provide quantitative characteristics of the astrocyte-to-neuron GABAergic signaling. They also suggest that all principal neurons of the hippocampal network are under a dual, excitatory and inhibitory, influence of astrocytes. The relevance of the astrocytic release of GABA, and glutamate, on the physiopathology of the hippocampus remains to be established.

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.

Attenuation of scratch-induced reactive astrogliosis by novel EphA4 kinase inhibitors

The EphA4 receptor and its ephrin ligands are involved in astrocytic gliosis following CNS injury. Therefore, a strategy aimed at the blockade of EphA4 signaling could have broad therapeutic interest in brain disorders. We have identified novel small molecule inhibitors of EphA4 kinase in specific enzymatic and cell-based assays. In addition, we have demonstrated in two in vitro models of scratch injury that EphA4 receptor kinase is activated through phosphorylation and is involved in the repopulation of the wound after the scratch. A potent EphA4 kinase inhibitor significantly inhibited wound closure and reduced the accumulation of the reactive astrocytes inside the scratch. We have also shown that after the transient focal cerebral ischemia in rats, a large glial scar is formed by the accumulation of astrocytes and chondroitin sulfate proteoglycan surrounding the infarcted tissue at 7 days and 14 days of reperfusion. EphA4 protein expression is highly up-regulated in the same areas at these time points, supporting its potential role in the glial scar formation and maintenance. Taken together, these results suggest that EphA4 kinase inhibitors might interfere with the astrogliosis reaction and thereby lead to improved neurological outcome after ischemic injury.

Expression profile and role of EphrinA1 ligand after spinal cord injury

Spinal cord injury (SCI) triggers the re-expression of inhibitory molecules present in early stages of development, contributing to prevention of axonal regeneration. Upregulation of EphA receptor tyrosine kinases after injury suggest their involvement in the nervous system's response to damage. However, the expression profile of their ephrinA ligands after SCI is unclear. In this study, we determined the expression of ephrinA ligands after contusive SCI. Adult Sprague-Dawley female rats were injured using the MASCIS impactor device at the T10 vertebrae, and levels of ephrinA mRNA and protein determined at different time points. Identification of the cell phenotype expressing the ephrin ligand and colocalization with Eph receptors was performed with immunohistochemistry and confocal microscopy. Behavioral studies were made, after blocking ephrinA1 expression with antisense (AS) oligonucleotides, to assess hindlimb locomotor activity. Real-time PCR demonstrated basal mRNA levels of ephrin (A1, A2, A3, and A5) in the adult spinal cord. Interestingly, ephrinA1 was the only ligand whose mRNA levels were significantly altered after SCI. Although ephrinA1 mRNA levels increased after 2 weeks and remain elevated, we did not observe this pattern at the protein level as revealed by western blot analysis. Immunohistochemical studies showed ephrinA1 expression in reactive astrocytes, axons, and neurons and also their colocalization with EphA4 and A7 receptors. Behavioral studies revealed worsening of locomotor activity when ephrinA1 expression was reduced. This study suggests that ephrinA1 ligands play a role in the pathophysiology of SCI.

Differential cycling rates of Kv4.2 channels in proximal and distal dendrites of hippocampal CA1 pyramidal neurons

The heterogeneous expression of voltage-gated channels in dendrites suggests that neurons perform local microdomain computations at different regions. It has been shown that A-type K(+) channels have a nonuniform distribution along the primary apical dendrite in CA1 pyramidal neurons, increasing with distance from the soma. Kv4.2 channels, which are responsible for the somatodendritic A-type K(+) current in CA1 pyramidal neurons, shape local synaptic input, and regulate the back-propagation of APs into dendrites. Experiments were performed to test the hypothesis that Kv4.2 channels are differentially trafficked at different regions along the apical dendrite during basal activity and upon stimulation in CA1 neurons. Proximal (50-150 μm from the soma, primary and oblique) and distal (>200 μm) apical dendrites were selected. The fluorescence recovery after photobleaching (FRAP) technique was used to measure basal cycling rates of EGFP-tagged Kv4.2 (Kv4.2g). We found that the cycling rate of Kv4.2 channels was one order of magnitude slower at both primary and oblique dendrites between 50 and 150 μm from the soma. Kv4.2 channel cycling increased significantly at 200 to 250 μm from the soma. Expression of a Kv4.2 mutant lacking a phosphorylation site for protein kinase-A (Kv4.2gS552A) abolished this distance-dependent change in channel cycling; demonstrating that phosphorylation by PKA underlies the increased mobility in distal dendrites. Neuronal stimulation by α-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate (AMPA) treatment increased cycling of Kv4.2 channels significantly at distal sites only. This activity-dependent increase in Kv4.2 cycling at distal dendrites was blocked by expression of Kv4.2gS552A. These results indicate that distance-dependent Kv4.2 mobility is regulated by activity-dependent phosphorylation of Kv4.2 by PKA.

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.

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.

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.

Glia: the many ways to modulate synaptic plasticity

Synaptic plasticity consists in a change in synaptic strength that is believed to be the basis of learning and memory. Synaptic plasticity has been for a very long period of time a hallmark of neurons. Recent advances in physiology of glial cells indicate that astrocyte and microglia possess all the features to participate and modulate the various form of synaptic plasticity. Indeed beside their respective supportive and immune functions an increasing number of study demonstrate that astrocytes and microglia express receptors for most neurotransmitters and release neuroactive substances that have been shown to modulate neuronal activity and synaptic plasticity. Because glial cells are all around synapses and release a wide variety of neuroactive molecule during physiological and pathological conditions, glial cells have been reported to modulate synaptic plasticity in many different ways. From change in synaptic coverage, to release of chemokines and cytokines up to dedicated "glio" transmitters release, glia were reported to affect synaptic scaling, homeostatic plasticity, metaplasticity, long-term potentiation and long-term depression.

Regulation of radial glial motility by visual experience

Radial glia in the developing optic tectum express the key guidance molecules responsible for topographic targeting of retinal axons. However, the extent to which the radial glia are themselves influenced by retinal inputs and visual experience remains unknown. Using multiphoton live imaging of radial glia in the optic tectum of intact Xenopus laevis tadpoles in conjunction with manipulations of neural activity and sensory stimuli, radial glia were observed to exhibit spontaneous calcium transients that were modulated by visual stimulation. Structurally, radial glia extended and retracted many filopodial processes within the tectal neuropil over minutes. These processes interacted with retinotectal synapses and their motility was modulated by nitric oxide (NO) signaling downstream of neuronal NMDA receptor (NMDAR) activation and visual stimulation. These findings provide the first in vivo demonstration that radial glia actively respond both structurally and functionally to neural activity, via NMDAR-dependent NO release during the period of retinal axon ingrowth.

Neuron-glia communication via EphA4/ephrin-A3 modulates LTP through glial glutamate transport

Astrocytes are critical participants in synapse development and function, but their role in synaptic plasticity is unclear. Eph receptors and their ephrin ligands have been suggested to regulate neuron-glia interactions, and EphA4-mediated ephrin reverse signaling is required for synaptic plasticity in the hippocampus. Here we show that long-term potentiation (LTP) at the CA3-CA1 synapse is modulated by EphA4 in the postsynaptic CA1 cell and by ephrin-A3, a ligand of EphA4 that is found in astrocytes. Lack of EphA4 increased the abundance of glial glutamate transporters, and ephrin-A3 modulated transporter currents in astrocytes. Pharmacological inhibition of glial glutamate transporters rescued the LTP defects in EphA4 (Epha4) and ephrin-A3 (Efna3) mutant mice. Transgenic overexpression of ephrin-A3 in astrocytes reduces glutamate transporter levels and produces focal dendritic swellings possibly caused by glutamate excitotoxicity. These results suggest that EphA4/ephrin-A3 signaling is a critical mechanism for astrocytes to regulate synaptic function and plasticity.

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.

Synapse development and plasticity: roles of ephrin/Eph receptor signaling

The receptor tyrosine kinase Eph and its membrane-bound ligand ephrin are emerging key players in synapse formation and plasticity in the central nervous system. Understanding how ephrin/Eph regulate synapse formation and functions is often complicated by the fact that both ligands and receptors are expressed in the pre-synaptic and post-synaptic neurons and upon their interaction, bi-directional signaling cascades can be triggered. By elucidating the respective downstream targets and generating signaling-deficient mutants, the specific roles of forward (Eph receptor) and reverse (ephrin) signaling are beginning to be unraveled. In this review, we summarize recent advances in our understanding of how ephrin and Eph differentially participate in specific aspects of synapse formation in developing neurons, and activity-dependent plasticity in the adult brain.

Sorting out astrocyte physiology from pharmacology

A number of exciting findings have been made in astrocytes during the past 15 years that have led many researchers to redefine how the brain works. Astrocytes are now widely regarded as cells that propagate Ca(2+) over long distances in response to stimulation, and, similar to neurons, release transmitters (called gliotransmitters) in a Ca(2+)-dependent manner to modulate a host of important brain functions. Although these discoveries have been very exciting, it is essential to place them in the proper context of the approaches used to obtain them to determine their relevance to brain physiology. This review revisits the key observations made in astrocytes that greatly impact how they are thought to regulate brain function, including the existence of widespread propagating intercellular Ca(2+) waves, data suggesting that astrocytes signal to neurons through Ca(2+)-dependent release of glutamate, and evidence for the presence of vesicular machinery for the regulated exocytosis of gliotransmitters.

Morphological changes in hippocampal astrocytes induced by environmental enrichment in mice

Environmental enrichment is known to induce plastic changes in the brain, including morphological changes in hippocampal neurons, with increases in synaptic and spine densities. In recent years, the evidence for a role of astrocytes in regulating synaptic transmission and plasticity has increased, and it is likely that morphological and functional changes in astrocytes play an important role in brain plasticity. Our study was designed to evaluate changes in astrocytes induced by environmental enrichment in the CA1 region of the hippocampus, focusing on astrocytic density and on morphological changes in astrocytic processes. After 8 weeks of environmental enrichment starting at weaning, male CF-1 mice presented no significant changes in astrocyte number or in the density of glial fibrillary acidic protein (GFAP) immunoreactivity in the stratum radiatum. However, they did present changes in astrocytic morphology in the same region, as expressed by a significant increase in the ramification of astrocytic processes measured by the Sholl concentric circles method, as well as by an increase in the number and length of primary processes extending in a parallel orientation to CA1 nerve fibers. This led astrocytes to acquire a more stellate morphology, a fact which could be related to the increase in hippocampal synaptic density observed in previous studies. These findings corroborate the idea that structural changes in astrocytic networks are an integral part of plasticity processes occurring in the brain.

Communication between neurons and astrocytes: relevance to the modulation of synaptic and network activity

Neuromodulation is a fundamental process in the brain that regulates synaptic transmission, neuronal network activity and behavior. Emerging evidence demonstrates that astrocytes, a major population of glial cells in the brain, play previously unrecognized functions in neuronal modulation. Astrocytes can detect the level of neuronal activity and release chemical transmitters to influence neuronal function. For example, recent findings show that astrocytes play crucial roles in the control of Hebbian plasticity, the regulation of neuronal excitability and the induction of homeostatic plasticity. This review discusses the importance of astrocyte-to-neuron signaling in different aspects of neuronal function from the activity of single synapses to that of neuronal networks.

Developmental course of EphA4 cellular and subcellular localization in the postnatal rat hippocampus

From embryonic development to adulthood, the EphA4 receptor and several of its ephrin-A or -B ligands are expressed in the hippocampus, where they presumably play distinct roles at different developmental stages. To help clarify these diverse roles in the assembly and function of the hippocampus, we examined the cellular and subcellular localization of EphA4 in postnatal rat hippocampus by light and electron microscopic immunocytochemistry. On postnatal day (P) 1, the EphA4 immunostaining was robust in most layers of CA1, CA3, and dentate gyrus and then decreased gradually, until P21, especially in the cell body layers. At the ultrastructural level, focal spots of EphA4 immunoreactivity were detected all over the plasma membrane of pyramidal and granule cells, between P1 and P14, from the perikarya to the dendritic and axonal extremities, including growth cones and filopodia. This cell surface immunoreactivity then became restricted to the synapse-associated dendritic spines and axon terminals by P21. In astrocytes, the EphA4 immunolabeling showed a similar cell surface redistribution, from the perikarya and large processes at P1-P7, to small perisynaptic processes at P14-P21. In both cell types, spots of EphA4 immunoreactivity were also detected, with an incidence decreasing with maturation, on the endoplasmic reticulum, Golgi apparatus, and vesicles, organelles involved in protein synthesis, posttranslational modifications, and transport. The cell surface evolution of EphA4 localization in neuronal and glial cells is consistent with successive involvements in the developmental movements of cell bodies first, followed by process outgrowth and guidance, synaptogenesis, and finally synaptic maintenance and plasticity.

Morphogenesis and regulation of Bergmann glial processes during Purkinje cell dendritic spine ensheathment and synaptogenesis

Astrocytes have an important role in synaptic formation and function but how astrocytic processes become associated with synaptic structures during development is not well understood. Here we analyzed the pattern of growth of the processes extending off the main Bergmann glial (BG) shafts during synaptogenesis in the cerebellum. We found that during this period, BG process outgrowth was correlated with increased ensheathment of dendritic spines. In addition, two-photon time-lapse imaging revealed that BG processes were highly dynamic, and processes became more stable as the period of spine ensheathment progressed. While process motility was dependent on actin polymerization, activity of cytoskeletal regulators Rac1 and RhoG did not play a role in glial process dynamics or density, but was critical for maintaining process length. We extended this finding to probe the relationship between process morphology and ensheathment, finding that shortened processes result in decreased coverage of the spine. Furthermore, we found that areas in which BG expressed dn-Rac1, and therefore had a lower level of synaptic ensheathment, showed an overall increase in synapse number. These analyses reveal how BG processes grow to surround synaptic structures, elucidate the importance of BG process structure for proper development of synaptic ensheathment, and reveal a role for ensheathment in synapse formation.