Mutual interaction of neurons and astrocytes derived from iPSCs with APP V717L mutation developed the astrocytic phenotypes of Alzheimer's disease

BACKGROUND

The development of induced pluripotent stem cells (iPSCs) technology has enabled human cellular disease modeling for inaccessible cell types, such as neural cells in the brain. However, many of the iPSC-derived disease models established to date typically involve only a single cell type. These monoculture models are inadequate for accurately simulating the brain environment, where multiple cell types interact. The limited cell type diversity in monoculture models hinders the accurate recapitulation of disease phenotypes resulting from interactions between different cell types. Therefore, our goal was to create cell models that include multiple interacting cell types to better recapitulate disease phenotypes.

METHODS

To establish a co-culture model of neurons and astrocytes, we individually induced neurons and astrocytes from the same iPSCs using our novel differentiation methods, and then co-cultured them. We evaluated the effects of co-culture on neurons and astrocytes using immunocytochemistry, immuno-electron microscopy, and Ca2+ imaging. We also developed a co-culture model using iPSCs from a patient with familial Alzheimer's disease (AD) patient (APP V717L mutation) to investigate whether this model would manifest disease phenotypes not seen in the monoculture models.

RESULTS

The co-culture of the neurons and astrocytes increased the branching of astrocyte processes, the number of GFAP-positive cells, neuronal activities, the number of synapses, and the density of presynaptic vesicles. In addition, immuno-electron microscopy confirmed the formation of a tripartite synaptic structure in the co-culture model, and inhibition of glutamate transporters increased neuronal activity. Compared to the co-culture model of the control iPSCs, the co-culture model of familial AD developed astrogliosis-like phenotype, which was not observed in the monoculture model of astrocytes.

CONCLUSIONS

Co-culture of iPSC-derived neurons and astrocytes enhanced the morphological changes mimicking the in vivo condition of both cell types. The formation of the functional tripartite synaptic structures in the co-culture model suggested the mutual interaction between the cells. Furthermore, the co-culture model with the APP V717L mutation expressed in neurons exhibited an astrocytic phenotype reminiscent of AD brain pathology. These results suggest that our co-culture model is a valuable tool for disease modeling of neurodegenerative diseases.

P2X7 receptors and pannexin1 hemichannels shape presynaptic transmission

Over the last decades, since the discovery of ATP as a transmitter, accumulating evidence has been reported about the role of this nucleotide and purinergic receptors, in particular P2X7 receptors, in the modulation of synaptic strength and plasticity. Purinergic signaling has emerged as a crucial player in orchestrating the molecular interaction between the components of the tripartite synapse, and much progress has been made in how this neuron-glia interaction impacts neuronal physiology under basal and pathological conditions. On the other hand, pannexin1 hemichannels, which are functionally linked to P2X7 receptors, have appeared more recently as important modulators of excitatory synaptic function and plasticity under diverse contexts. In this review, we will discuss the contribution of ATP, P2X7 receptors, and pannexin hemichannels to the modulation of presynaptic strength and its impact on motor function, sensory processing, synaptic plasticity, and neuroglial communication, with special focus on the P2X7 receptor/pannexin hemichannel interplay. We also address major hypotheses about the role of this interaction in physiological and pathological circumstances.

Receptor-receptor interactions and microvesicle exchange as mechanisms modulating signaling between neurons and astrocytes

It is well known that astrocytes play a significant metabolic role in the nervous tissue, maintaining the homeostasis of the extracellular space and of the blood-brain barrier, and providing trophic support to neurons. In addition, however, evidence exists indicating astrocytes as important elements for brain activity through signaling exchange with neurons. Astrocytes, indeed, can sense synaptic activity and their molecular machinery responds to neurotransmitters released by neurons with cytoplasmic Ca²⁺ elevations that, in turn, stimulate the release of neuroactive substances (gliotransmitters) influencing nearby neurons. In both cell types the recognition and transduction of this complex pattern of signals is mediated by specific receptors that are also involved in mechanisms tuning the intercellular cross-talk between astrocytes and neurons. Two of these mechanisms are the focus of the present discussion. The first concerns direct receptor-receptor interactions leading to the formation at the cell membrane of multimeric receptor complexes. The cooperativity that emerges in the actions of orthosteric and allosteric ligands of the monomers forming the assembly provides the cell decoding apparatus with sophisticated and flexible dynamics in terms of recognition and signal transduction pathways. A further mechanism of plasticity involving receptors is based on the transfer of elements of the cellular signaling apparatus via extracellular microvesicles acting as protective containers, which can lead to transient changes in the transmitting/decoding capabilities of the target cell.

Necl2/3-mediated mechanism for tripartite synapse formation

Ramified, polarized protoplasmic astrocytes interact with synapses via perisynaptic astrocyte processes (PAPs) to form tripartite synapses. These astrocyte-synapse interactions mutually regulate their structures and functions. However, molecular mechanisms for tripartite synapse formation remain elusive. We developed an in vitro co-culture system for mouse astrocytes and neurons that induced astrocyte ramifications and PAP formation. Co-cultured neurons were required for astrocyte ramifications in a neuronal activity-dependent manner, and synaptically-released glutamate and activation of astrocytic mGluR5 metabotropic glutamate receptor were likely involved in astrocyte ramifications. Astrocytic Necl2 trans-interacted with axonal Necl3, inducing astrocyte-synapse interactions and astrocyte functional polarization by recruiting EAAT1/2 glutamate transporters and Kir4.1 K+ channel to the PAPs, without affecting astrocyte ramifications. This Necl2/3 trans-interaction increased functional synapse number. Thus, astrocytic Necl2, synaptically-released glutamate and axonal Necl3 cooperatively formed tripartite glutamatergic synapses in vitro. Studies on hippocampal mossy fiber synapses in Necl3 knockout and Necl2/3 double knockout mice confirmed these previously unreported mechanisms for astrocyte-synapse interactions and astrocyte functional polarization in vivo.

Neuronal and astrocytic protein connections and associated adhesion molecules

Astrocytes are morphologically complex, with a myriad of processes which allow contact with other astrocytes, blood vessels, and neurons. Adhesion molecules expressed by these cells regulate this connectivity. Adhesion molecules are required to form and maintain functional neural circuits, but their importance and mechanisms of action, particularly in astrocyte-neuron contact, remain unresolved. Several studies of neuron-astrocyte connections have demonstrated the vital functions of adhesion molecules, including neuron-glia cell adhesion molecules, astrotactins, and protocadherins. In this review, we provide an overview and perspective of astrocyte-neuron contacts mediated by adhesion molecules in developing neural circuits and synapse formation, especially in the cerebellum. We also outline a novel mechanism of interaction between neurons and astrocytes in the tripartite synapses that has been recently found by our group.

Three's Company: Neuroimmune activation, sex, and memory at the tripartite synapse

The neuroimmune system is required for normal cognitive functions such as learning and memory in addition to its critical role in detecting and responding to invading pathogens and injury. Understanding the functional convergence of neurons, astrocytes, and microglia at the synapse, particularly in the hippocampus, is key to understanding the nuances of such diverse roles. In the healthy brain, communication between all three cells is important for regulating neuronal activation and synaptic plasticity mechanisms, and during neuroinflammation, the activity and functions of all three cells can produce and be modulated by inflammatory cytokines. An important remaining component to this system is the conclusive evidence of sex differences in hippocampal plasticity mechanisms, hormone modulation of synaptic plasticity, functional properties of hippocampal neurons, and in neuroimmune activation. Sex as a biological variable here is necessary to consider given sex differences in the prevalence of memory-related disorders such as Alzheimer's disease and Post-Traumatic Stress disorder, both of which present with neuroimmune dysregulation. To make meaningful progress towards a deeper understanding of sex biases in memory-related disease prevalence, I propose that the next chapter of psychoneuroimmune research must focus on the signal integration and transduction at the synapse between experience-dependent plasticity mechanisms, neuroimmune activation, and the influence of biological sex.

Tripartite synaptomics: Cell-surface proximity labeling in vivo

The astrocyte is a central glial cell and plays a critical role in the architecture and activity of neuronal circuits and brain functions through forming a tripartite synapse with neurons. Emerging evidence suggests that dysfunction of tripartite synaptic connections contributes to a variety of psychiatric and neurodevelopmental disorders. Furthermore, recent advancements with transcriptome profiling, cell biological and physiological approaches have provided new insights into the molecular mechanisms into how astrocytes control synaptogenesis in the brain. In addition to these findings, we have recently developed in vivo cell-surface proximity-dependent biotinylation (BioID) approaches, TurboID-surface and Split-TurboID, to comprehensively understand the molecular composition between astrocytes and neuronal synapses. These proteomic approaches have discovered a novel molecular framework for understanding the tripartite synaptic cleft that arbitrates neuronal circuit formation and function. Here, this short review highlights novel in vivo cell-surface BioID approaches and recent advances in this rapidly evolving field, emphasizing how astrocytes regulate excitatory and inhibitory synapse formation in vitro and in vivo.

Sensing and Regulating Synaptic Activity by Astrocytes at Tripartite Synapse

Astrocytes are recognized as more important cells than historically thought in synaptic function through the reciprocal exchange of signaling with the neuronal synaptic elements. The idea that astrocytes are active elements in synaptic physiology is conceptualized in the Tripartite Synapse concept. This review article presents and discusses recent representative examples that highlight the heterogeneity of signaling in tripartite synapse function and its consequences on neural network function and animal behavior.

Preparation of Synaptoneurosomes to Study the Synapse in the Murine Cerebral Cortex

The synapse is a complex structure where the transmission of information takes place. Synaptic dysfunction is one of the earliest pathophysiological events in several diseases, such as traumatic brain injury, cerebral ischemia, and neurodegenerative diseases. Thus, a methodology to study synaptic structure and function is crucial for the development of potential strategies for the treatment of many neurological diseases. Synaptoneurosomes (SNs) are structures assembled by the sealed presynaptic bouton and the attached post-synaptic density. Despite the fact that for a long time it has been recognized that SNs are a powerful tool to study synaptic function, composition, and structure, its use has been limited by the requirement of relatively large amounts of material to successfully isolate them. Here we describe a three-step centrifugation procedure performed under hypotonic conditions to isolate SNs from small volumes of the cerebral cortex. Graphic abstract: Schematic flowchart for the preparation of synaptoneurosomes.

Ultrastructural and functional changes at the tripartite synapse during epileptogenesis in a model of temporal lobe epilepsy

The persistent unresponsiveness of many of the acquired epilepsies to traditional antiseizure medications has motivated the search for prophylactic drug therapies that could reduce the incidence of epilepsy in this at risk population. These studies are based on the idea of a period of epileptogenesis that can follow a wide variety of brain injuries. Epileptogenesis is hypothesized to involve changes in the brain not initially associated with seizures, but which result finally in seizure prone networks. Understanding these changes will provide crucial clues for the development of prophylactic drugs. Using the repeated low-dose kainate rat model of epilepsy, we have studied the period of epileptogenesis following status epilepticus, verifying the latent period with continuous EEG monitoring. Focusing on ultrastructural properties of the tripartite synapse in the CA1 region of hippocampus we found increased astrocyte ensheathment around both the presynaptic and postsynaptic elements, reduced synaptic AMPA receptor subunit and perisynaptic astrocyte GLT-1 expression, and increased number of docked vesicles at the presynaptic terminal. These findings were associated with an increase in frequency of the mEPSCs observed in patch clamp recordings of CA1 pyramidal cells. The results suggest a complex set of changes, some of which have been associated with increasingly excitable networks such as increased vesicles and mEPSC frequency, and some associated with compensatory mechanisms, such as increased astrocyte ensheathment. The diversity of ultrastructural and electrophysiological changes observed during epileptogeneiss suggests that potential drug targets for this period should be broadened to include all components of the tripartite synapse.

Diverse Actions of Astrocytes in GABAergic Signaling

An imbalance of excitatory and inhibitory neurotransmission leading to over excitation plays a crucial role in generating seizures, while enhancing GABAergic mechanisms are critical in terminating seizures. In recent years, it has been reported in many studies that astrocytes are deeply involved in synaptic transmission. Astrocytes form a critical component of the “tripartite” synapses by wrapping around the pre- and post-synaptic elements. From this location, astrocytes are known to greatly influence the dynamics of ions and transmitters in the synaptic cleft. Despite recent extensive research on excitatory tripartite synapses, inhibitory tripartite synapses have received less attention, even though they influence inhibitory synaptic transmission by affecting chloride and GABA concentration dynamics. In this review, we will discuss the diverse actions of astrocytic chloride and GABA homeostasis at GABAergic tripartite synapses. We will then consider the pathophysiological impacts of disturbed GABA homeostasis at the tripartite synapse.

Multiplexed calcium imaging of single-synapse activity and astroglial responses in the intact brain

All-optical registration of neuronal and astrocytic activities within the intact mammalian brain has improved significantly with recent advances in optical sensors and biophotonics. However, relating single-synapse release events and local astroglial responses to sensory stimuli in an intact animal has not hitherto been feasible. Here, we present a multiplexed multiphoton excitation imaging approach for assessing the relationship between presynaptic Ca2+ entry at thalamocortical axonal boutons and perisynaptic astrocytic Ca2+ elevations, induced by whisker stimulation in the barrel cortex of C57BL/6 mice. We find that, unexpectedly, Ca2+ elevations in the perisynaptic astrocytic regions consistently precede local presynaptic Ca2+ signals during spontaneous brain activity associated with anaesthesia. The methods described here can be adapted to a variety of optical sensors and are compatible with experimental designs that might necessitate repeated sampling of single synapses over a longitudinal behavioural paradigm.

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.

Synaptic neuron-astrocyte communication is supported by an order of magnitude analysis of inositol tris-phosphate diffusion at the nanoscale in a model of peri-synaptic astrocyte projection

Background

Astrocytes were conceived for decades only as supporting cells of the brain. However, the observation of Ca2+ waves in astrocyte synctitia, their neurotransmitter receptor expression and gliotransmitter secretion suggested a role in information handling, conception that has some controversies. Synaptic Neuron-Astrocyte metabotropic communication mediated by Inositol tris-phosphate (SN-AmcIP3) is supported by different reports. However, some models contradict this idea and Ca2+ stores are 1000 ± 325 nm apart from the Postsynaptic Density in the Perisynaptic Astrocyte Projections (PAP’s), suggesting that SN-AmcIP3 is extrasynaptic. However, this assumption does not consider IP3 Diffusion Coefficient (Dab), that activates IP3 Receptor (IP3R) releasing Ca2+ from intracellular stores.

Results

In this work we idealized a model of a PAP (PAPm) to perform an order of magnitude analysis of IP3 diffusion using a transient mass diffusion model. This model shows that IP3 forms a concentration gradient along the PAPm that reaches the steady state in milliseconds, three orders of magnitude before IP3 degradation. The model predicts that IP3 concentration near the Ca2+ stores may activate IP3R, depending upon Phospholipase C (PLC) number and activity. Moreover, the PAPm supports that IP3 and extracellular Ca2+ entry synergize to promote global Ca2+ transients.

Conclusion

The model presented here indicates that Ca2+ stores position in PAP’s does not limit SN-AmcIP3.

Electronic supplementary material

The online version of this article (10.1186/s13628-018-0043-3) contains supplementary material, which is available to authorized users.

Cell adhesion and matricellular support by astrocytes of the tripartite synapse

Astrocytes contribute to the formation, function, and plasticity of synapses. Their processes enwrap the neuronal components of the tripartite synapse, and due to this close interaction they are perfectly positioned to modulate neuronal communication. The interaction between astrocytes and synapses is facilitated by cell adhesion molecules and matricellular proteins, which have been implicated in the formation and functioning of tripartite synapses. The importance of such neuron-astrocyte integration at the synapse is underscored by the emerging role of astrocyte dysfunction in synaptic pathologies such as autism and schizophrenia. Here we review astrocyte-expressed cell adhesion molecules and matricellular molecules that play a role in integration of neurons and astrocytes within the tripartite synapse.

Local energy on demand: Are 'spontaneous' astrocytic Ca2+-microdomains the regulatory unit for astrocyte-neuron metabolic cooperation?

Astrocytes are a neural cell type critically involved in maintaining brain energy homeostasis as well as signaling. Like neurons, astrocytes are a heterogeneous cell population. Cortical astrocytes show a complex morphology with a highly branched aborization and numerous fine processes ensheathing the synapses of neighboring neurons, and typically extend one process connecting to blood vessels. Recent studies employing genetically encoded fluorescent calcium (Ca²⁺) indicators have described 'spontaneous' localized Ca²⁺-transients in the astrocyte periphery that occur asynchronously, independently of signals in other parts of the cells, and that do not involve somatic Ca²⁺ transients; however, neither it is known whether these Ca²⁺-microdomains occur at or near neuronal synapses nor have their molecular basis nor downstream effector(s) been identified. In addition to Ca²⁺ microdomains, sodium (Na⁺) transients occur in astrocyte subdomains, too, most likely as a consequence of Na⁺ co-transport with the neurotransmitter glutamate, which also regulates mitochondrial movements locally - as do cytoplasmic Ca²⁺ levels. In this review, we cover various aspects of these local signaling events and discuss how structural and biophysical properties of astrocytes might foster such compartmentation. Astrocytes metabolically interact with neurons by providing energy substrates to active neurons. As a single astrocyte branch covers hundreds to thousands of synapses, it is tempting to speculate that these metabolic interactions could occur localized to specific subdomains of astrocytes, perhaps even at the level of small groups of synapses. We discuss how astrocytic metabolism might be regulated at this scale and which signals might contribute to its regulation. We speculate that the astrocytic structures that light up transiently as Ca²⁺-microdomains might be the functional units of astrocytes linking signaling and metabolic processes to adapt astrocytic function to local energy demands. The understanding of these local regulatory and metabolic interactions will be fundamental to fully appreciate the complexity of brain energy homeostasis as well as its failure in disease and may shed new light on the controversy about neuron-glia bi-directional signaling at the tripartite synapse.

From in silico astrocyte cell models to neuron-astrocyte network models: A review

The idea that astrocytes may be active partners in synaptic information processing has recently emerged from abundant experimental reports. Because of their spatial proximity to neurons and their bidirectional communication with them, astrocytes are now considered as an important third element of the synapse. Astrocytes integrate and process synaptic information and by doing so generate cytosolic calcium signals that are believed to reflect neuronal transmitter release. Moreover, they regulate neuronal information transmission by releasing gliotransmitters into the synaptic cleft affecting both pre- and postsynaptic receptors. Concurrent with the first experimental reports of the astrocytic impact on neural network dynamics, computational models describing astrocytic functions have been developed. In this review, we give an overview over the published computational models of astrocytic functions, from single-cell dynamics to the tripartite synapse level and network models of astrocytes and neurons.

Molecular and cellular physiology of sodium-dependent glutamate transporters

Glutamate is the major excitatory transmitter in the vertebrate brain. After its release from presynaptic nerve terminals, it is rapidly taken up by high-affinity sodium-dependent plasma membrane transporters. While both neurons and glial cells express these excitatory amino acid transporters (EAATs), the majority of glutamate uptake is accomplished by astrocytes, which convert synaptically-released glutamate to glutamine or feed it into their own metabolism. Glutamate uptake by astrocytes not only shapes synaptic transmission by regulating the availability of glutamate to postsynaptic neuronal receptors, but also protects neurons from hyper-excitability and subsequent excitotoxic damage. In the present review, we provide an overview of the molecular and cellular characteristics of sodium-dependent glutamate transporters and their associated anion permeation pathways, with a focus on astrocytic glutamate transport. We summarize their functional properties and roles within tripartite synapses under physiological and pathophysiological conditions, exemplifying the intricate interactions and interrelationships between neurons and glial cells in the brain.

Behavioral sequelae of astrocyte dysfunction: focus on animal models of schizophrenia

Astrocytes regulate multiple processes in the brain ranging from trophic support of developing neurons to modulation of synaptic neurotransmission and neuroinflammation in adulthood. It is, therefore, understandable that pathogenesis and pathophysiology of major psychiatric disorders involve astrocyte dysfunctions. Until recently, there has been the paucity of experimental approaches to studying the roles of astrocytes in behavioral disease. A new generation of in vivo models allows us to advance our understanding of the roles of astrocytes in psychiatric disorders. This review will evaluate the recent studies that focus on the contribution of astrocyte dysfunction to behavioral alterations pertinent to schizophrenia and will propose the possible solutions of the limitations of the existing approaches.

Dysfunction of NMDA receptors in Alzheimer's disease

N-methyl-d-aspartate receptors (NMDARs) play a pivotal role in the synaptic transmission and synaptic plasticity thought to underlie learning and memory. NMDARs activation has been recently implicated in Alzheimer’s disease (AD) related to synaptic dysfunction. Synaptic NMDARs are neuroprotective, whereas overactivation of NMDARs located outside of the synapse cause loss of mitochondrial membrane potential and cell death. NMDARs dysfunction in the glutamatergic tripartite synapse, comprising presynaptic and postsynaptic neurons and glial cells, is directly involved in AD. This review discusses that both beta-amyloid (Aβ) and tau perturb synaptic functioning of the tripartite synapse, including alterations in glutamate release, astrocytic uptake, and receptor signaling. Particular emphasis is given to the role of NMDARs as a possible convergence point for Aβ and tau toxicity and possible reversible stages of the AD through preventive and/or disease-modifying therapeutic strategies.

Targeted Gene Resequencing (Astrochip) to Explore the Tripartite Synapse in Autism-Epilepsy Phenotype with Macrocephaly

The frequent co-occurrence of autism spectrum disorders (ASD) and epilepsy, or paroxysmal EEG abnormalities, defines a condition termed autism-epilepsy phenotype (AEP). This condition results, in some cases , from dysfunctions of glial inwardly rectifying potassium channels (Kir), which are mainly expressed in astrocytes where they mediate neuron-glia communication. Macrocephaly is also often comorbid with autism-epilepsy (autism-epilepsy phenotype with macrocephaly, MAEP), and it is tempting to hypothesize that shared pathogenic mechanisms might explain concurrence of these conditions. In the present study, we assessed whether protein pathways involved, along with Kir channels, in astrocyte-neuron interaction at the tripartite synapse play a role in the etiopathogenesis of MAEP. Using a targeted resequencing methodology, we investigated the coding regions of 35 genes in 61 patients and correlated genetic results with clinical features. Variants were subdivided into 12 classes and clustered into four groups. We detected rare or previously unknown predicted deleterious missense changes in GJA1, SLC12A2, SNTA1, EFNA3, CNTNAP2, EPHA4, and STXBP1 in seven patients and two high-frequency variants in DLG1 in six individuals. We also found that a group of variants (predicted deleterious and non-coding), segregating with the comorbid MAEP/AEP subgroups, belong to proteins specifically involved in glutamate transport and metabolism (namely, SLC17A6, GRM8, and GLUL), as well as in potassium conductance (KCNN3). This "endophenotype-oriented" study, performed using a targeted strategy, helped to further delineate part of the complex genetic background of ASD, particularly in the presence of coexisting macrocephaly and/or epilepsy/paroxysmal EEG, and suggests that use of stringent clinical clustering might be an approach worth adopting in order to unravel the complex genomic data in neurodevelopmental disorders.

A digital neurmorphic circuit for a simplified model of astrocyte dynamics

Recent neurophysiologic findings have shown that astrocytes (the most abundant type of glial cells) are active partners in neural information processing and regulate the synaptic transmission dynamically. Motivated by these findings, in the present research, a digital neuromorphic circuit to implement the astrocyte dynamics is developed. To model the dynamics of the intracellular Ca(2+) waves produced by astrocytes, we utilize a simplified model which considers the main physiological pathways of neuron-astrocyte interactions. Next, a digital circuit for the astrocyte dynamic is proposed which is simulated using ModelSim and finally, it is implemented in hardware on the ZedBoard. The results of hardware synthesis, FPGA implementations are in agreement with MATLAB and ModelSim simulations and confirm that the proposed digital astrocyte is suitable for applications in reconfigurable neuromorphic devices which implement biologically brain circuits.

Astrocyte-neuron interplay in maladaptive plasticity

The complexity of neuronal networks cannot only be explained by neuronal activity so neurobiological research in the last decade has focused on different components of the central nervous system: the glia. Glial cells are fundamental elements for development and maintenance of physiological brain work. New data confirm that glia significantly influences neuronal communication through specific molecules, named "gliotransmitters", and their related receptors. This new approach to the traditional model of the way synapses work is also supported by changes occurring in pathological conditions, such as neurodegenerative diseases or toxic/traumatic injury to nervous system. Experimental models have revealed that glial cells are the starting point of damage progression that subsequently involves neurons. The "bedside to bench" approach has demonstrated that clinical phenotypes are strictly related to neuronal death, however it is conceivable that the disease begins earlier, years before clinical onset. This temporal gap is necessary to determine complex changes in the neuro-glial network organization and produce a "maladaptive plasticity". We review the function of glial cells in health and disease, pointing the putative mechanisms of maladaptive plasticity, suggesting that glial cells may represent a fascinating therapeutic target to prevent irreversible neuronal cell death.

Dysfunctional synapse in Alzheimer's disease - A focus on NMDA receptors

Alzheimer's disease (AD) is the most prevalent form of dementia in the elderly. Alterations capable of causing brain circuitry dysfunctions in AD may take several years to develop. Oligomeric amyloid-beta peptide (Aβ) plays a complex role in the molecular events that lead to progressive loss of function and eventually to neurodegeneration in this devastating disease. Moreover, N-methyl-D-aspartate (NMDA) receptors (NMDARs) activation has been recently implicated in AD-related synaptic dysfunction. Thus, in this review we focus on glutamatergic neurotransmission impairment and the changes in NMDAR regulation in AD, following the description on the role and location of NMDARs at pre- and post-synaptic sites under physiological conditions. In addition, considering that there is currently no effective ways to cure AD or stop its progression, we further discuss the relevance of NMDARs antagonists to prevent AD symptomatology. This review posits additional information on the role played by Aβ in AD and the importance of targeting the tripartite glutamatergic synapse in early asymptomatic and possible reversible stages of the disease through preventive and/or disease-modifying therapeutic strategies. This article is part of the Special Issue entitled 'The Synaptic Basis of Neurodegenerative Disorders'.

Acute isolation and transcriptome characterization of cortical astrocytes and microglia from young and aged mice

Astrocytes and microglia become reactive in many neurological disorders resulting in phenotypic and functional alterations. Both cell types might also display functional changes during normal aging. To identify gene signatures and changes in basal cellular functions of astrocytes and microglia in relation to aging, we isolated viable astrocytes and microglia from young adult and aged mouse cortices and determined their gene expression profile. Aged astrocytes, compared with young astrocytes, showed an increased inflammatory phenotype and increased 'zinc ion binding.' Young astrocytes showed higher expression of genes involved in 'neuronal differentiation' and hemoglobin synthesis. Astrocyte expression of genes involved in neuronal signaling remains high throughout age. Aged microglia had higher expression of genes involved in 'vesicle release,' 'zinc ion binding,' and genes within the tumor necrosis factor-ligand family and young microglia had increased transcript levels of C-C motif chemokines. These data provide a transcriptome database of cell-type enriched genes of astrocytes and microglia from adult mice and give insight into the differential gene signature of astrocytes and microglia in relation to normal aging.

A mathematical model of the tripartite synapse: astrocyte-induced synaptic plasticity

In this paper, we present a biologically detailed mathematical model of tripartite synapses, where astrocytes modulate short-term synaptic plasticity. The model consists of a pre-synaptic bouton, a post-synaptic dendritic spine-head, a synaptic cleft and a peri-synaptic astrocyte controlling Ca(2 + ) dynamics inside the synaptic bouton. This in turn controls glutamate release dynamics in the cleft. As a consequence of this, glutamate concentration in the cleft has been modeled, in which glutamate reuptake by astrocytes has also been incorporated. Finally, dendritic spine-head dynamics has been modeled. As an application, this model clearly shows synaptic potentiation in the hippocampal region, i.e., astrocyte Ca(2 + ) mediates synaptic plasticity, which is in conformity with the majority of the recent findings (Perea and Araque (Science 317, 1083-1086, 2007); Henneberger et al. (Nature 463, 232-236, 2010); Navarrete et al. (PLoS Biol. 10, e1001259, 2012)).

The impact of the glial spatial buffering on the K(+) Nernst potential

Astrocytes play a critical role in CNS metabolism, regulation of volume and ion homeostasis of the interstitial space. Of special relevance is their clearance of K(+) that is released by active neurons into the extracellular space. Mathematical analysis of a modified Nernst equation for the electrochemical equilibrium of neuronal plasma membranes, suggests that K(+) uptake by glial cells is not only relevant during neuronal activity but also has a non-neglectable impact on the basic electrical membrane properties, specifically the resting membrane potential, of neurons and might be clinically valuable as a factor in the genetics and epigenetics of the epilepsy and tuberous sclerosis complex.