Glutamate Induces the Elongation of Early Dendritic Protrusions via mGluRs in Wild Type Mice, but Not in Fragile X Mice

Fragile X syndrome (FXS), the most common inherited from of autism and mental impairment, is caused by transcriptional silencing of the Fmr1 gene, resulting in the loss of the RNA-binding protein FMRP. Dendritic spines of cortical pyramidal neurons in affected individuals are abnormally immature and in Fmr1 knockout (KO) mice they are also abnormally unstable. This could result in defects in synaptogenesis, because spine dynamics are critical for synapse formation. We have previously shown that the earliest dendritic protrusions, which are highly dynamic and might serve an exploratory role to reach out for axons, elongate in response to glutamate. Here, we tested the hypothesis that this process is mediated by metabotropic glutamate receptors (mGluRs) and that it is defective in Fmr1 KO mice. Using time-lapse imaging with two-photon microscopy in acute brain slices from early postnatal mice, we find that early dendritic protrusions in layer 2/3 neurons become longer in response to application of glutamate or DHPG, a Group 1 mGluR agonist. Blockade of mGluR5 signaling, which reverses some adult phenotypes of KO mice, prevented the glutamate-mediated elongation of early protrusions. In contrast, dendritic protrusions from KO mice failed to respond to glutamate. Thus, absence of FMRP may impair the ability of cortical pyramidal neurons to respond to glutamate released from nearby pre-synaptic terminals, which may be a critical step to initiate synaptogenesis and stabilize spines

Astrocytic Ca2+ Waves Guide CNS Growth Cones to Remote Regions of Neuronal Activity

Activity plays a critical role in network formation during developmental, experience-dependent, and injury related remodeling. Here we report a mechanism by which axon trajectory can be altered in response to remote neuronal activity. Using photoconductive stimulation to trigger high frequency action potentials in rat hippocampal neurons in vitro, we find that activity functions as an attractive cue for growth cones in the local environment. The underlying guidance mechanism involves astrocyte Ca2+ waves, as the connexin-43 antagonist carbenoxolone abolishes the attraction when activity is initiated at a distance greater than 120 µm. The asymmetric growth cone filopodia extension that precedes turning can be blocked with CNQX (10 µM), but not with the ATP and adenosine receptor antagonists suramin (100 µM) and alloxazine (4 µM), suggesting non-NMDA glutamate receptors on the growth cone mediate the interaction with astrocytes. These results define a potential long-range signalling pathway for activity-dependent axon guidance in which growth cones turn towards directional, temporally coordinated astrocyte Ca2+ waves that are triggered by neuronal activity. To assess the viability of the guidance effect in an injury paradigm, we performed the assay in the presence of conditioned media from lipopolysaccharide (LPS) activated purified microglial cultures, as well as directly activating the glia present in our co-cultures. Growth cone attraction was not inhibited under these conditions, suggesting this mechanism could be used to guide regeneration following axonal injury

Corneal Epithelium Expresses a Variant of P2X7 Receptor in Health and Disease

Improper wound repair of the corneal epithelium can alter refraction of light resulting in impaired vision. We have shown that ATP is released after injury, activates purinergic receptor signaling pathways and plays a major role in wound closure. In many cells or tissues, ATP activates P2X7 receptors leading to cation fluxes and cytotoxicity. The corneal epithelium is an excellent model to study the expression of both the full-length P2X7 form (defined as the canonical receptor) and its truncated forms. When Ca2+ mobilization is induced by BzATP, a P2X7 agonist, it is attenuated in the presence of extracellular Mg2+ or Zn2+, negligible in the absence of extracellular Ca2+, and inhibited by the competitive P2X7 receptor inhibitor, A438079. BzATP enhanced phosphorylation of ERK. Together these responses indicate the presence of a canonical or full-length P2X7 receptor. In addition BzATP enhanced epithelial cell migration, and transfection with siRNA to the P2X7 receptor reduced cell migration. Furthermore, sustained activation did not induce dye uptake indicating the presence of truncated or variant forms that lack the ability to form large pores. Reverse transcription-polymerase chain reaction and Northern blot analysis revealed a P2X7 splice variant. Western blots identified a full-length and truncated form, and the expression pattern changed as cultures progressed from monolayer to stratified. Cross-linking gels demonstrated the presence of homo- and heterotrimers. We examined epithelium from age matched diabetic and non-diabetic corneas patients and detected a 4-fold increase in P2X7 mRNA from diabetic corneal epithelium compared to non-diabetic controls and an increased trend in expression of P2X7variant mRNA. Taken together, these data indicate that corneal epithelial cells express full-length and truncated forms of P2X7, which ultimately allows P2X7 to function as a multifaceted receptor that can mediate cell proliferation and migration or cell death

Astrocytes Optimize the Synaptic Transmission of Information

Chemical synapses transmit information via the release of neurotransmitter-filled vesicles from the presynaptic terminal. Using computational modeling, we predict that the limited availability of neurotransmitter resources in combination with the spontaneous release of vesicles limits the maximum degree of enhancement of synaptic transmission. This gives rise to an optimal tuning that depends on the number of active zones. There is strong experimental evidence that astrocytes that enwrap synapses can modulate the probabilities of vesicle release through bidirectional signaling and hence regulate synaptic transmission. For low-fidelity hippocampal synapses, which typically have only one or two active zones, the predicted optimal values lie close to those determined by experimentally measured astrocytic feedback, suggesting that astrocytes optimize synaptic transmission of information

P2Y receptors and pain transmission

It is widely accepted that the most important ATP receptors involved in pain transmission belong to the P2X3 and P2X2/3 subtypes, selectively expressed in small diameter dorsal root ganglion (DRG) neurons. However, several types of the metabotropic ATP (P2Y) receptors have also been found in primary afferent neurons; P2Y1 and P2Y2 receptors are typically expressed in small, nociceptive cells. Here we review the results available on the involvement of P2Y receptors in the modulation of pain transmission

ATP signalling in epilepsy

This paper focuses on a role for ATP neurotransmission and gliotransmission in the pathophysiology of epileptic seizures. ATP along with gap junctions propagates the glial calcium wave, which is an extraneuronal signalling pathway in the central nervous system. Recently astrocyte intercellular calcium waves have been shown to underlie seizures, and conventional antiepileptic drugs have been shown to attenuate these calcium waves. Blocking ATP-mediated gliotransmission, therefore, represents a potential target for antiepileptic drugs. Furthermore, while knowledge of an antiepileptic role for adenosine is not new, a recent study showed that adenosine accumulates from the hydrolysis of accumulated ATP released by astrocytes and is believed to inhibit distant synapses by acting on adenosine receptors. Such a mechanism is consistent with a surround-inhibitory mechanism whose failure would predispose to seizures. Other potential roles for ATP signalling in the initiation and spread of epileptiform discharges may involve synaptic plasticity and coordination of synaptic networks. We conclude by making speculations about future developments

Regulation of cell-to-cell communication mediated by astrocytic ATP in the CNS

It has become apparent that glial cells, especially astrocytes, not merely supportive but are integrative, being able to receive inputs, assimilate information and send instructive chemical signals to other neighboring cells including neurons. At first, the excitatory neurotransmitter glutamate was found to be a major extracellular messenger that mediates these communications because it can be released from astrocytes in a Ca2+-dependent manner, diffused, and can stimulate extra-synaptic glutamate receptors in adjacent neurons, leading to a dynamic modification of synaptic transmission. However, recently extracellular ATP has come into the limelight as an important extracellular messenger for these communications. Astrocytes express various neurotransmitter receptors including P2 receptors, release ATP in response to various stimuli and respond to extracellular ATP to cause various physiological responses. The intercellular communication “Ca2+ wave” in astrocytes was found to be mainly mediated by the release of ATP and the activation of P2 receptors, suggesting that ATP is a dominant “gliotransmitter” between astrocytes. Because neurons also express various P2 receptors and synapses are surrounded by astrocytes, astrocytic ATP could affect neuronal activities and even dynamically regulate synaptic transmission in adjacent neurons as if forming a “tripartite synapse” In this review, we summarize the role of astrocytic ATP, as compared with glutamate, in gliotransmission and synaptic transmission in neighboring cells, mainly focusing on the hippocampus. Dynamic communication between astrocytes and neurons mediated by ATP would be a key event in the processing or integration of information in the CNS

Glutamate regulation of calcium and IP3 oscillating and pulsating dynamics in astrocytes

Recent years have witnessed an increasing interest in neuron-glia communication. This interest stems from the realization that glia participates in cognitive functions and information processing and is involved in many brain disorders and neurodegenerative diseases. An important process in neuron-glia communications is astrocyte encoding of synaptic information transfer: the modulation of intracellular calcium dynamics in astrocytes in response to synaptic activity. Here, we derive and investigate a concise mathematical model for glutamate-induced astrocytic intracellular Ca2+ dynamics that captures the essential biochemical features of the regulatory pathway of inositol 1,4,5-trisphosphate (IP3). Starting from the well-known two-state Li-Rinzel model for calcium-induced-calcium release, we incorporate the regulation of the IP3 production and phosphorylation. Doing so we extended it to a three-state model (referred as the G-ChI model), that could account for Ca2+ oscillations triggered by endogenous IP3 metabolism as well as by IP3 production by external glutamate signals. Compared to previous similar models, our three-state models include a more realistic description of the IP3 production and degradation pathways, lumping together their essential nonlinearities within a concise formulation. Using bifurcation analysis and time simulations, we demonstrate the existence of new putative dynamical features. The cross-couplings between IP3 and Ca2+ pathways endows the system with self-consistent oscillator properties and favor mixed frequency-amplitude encoding modes over pure amplitude modulation ones. These and additional results of our model are in general agreement with available experimental data and may have important implications on the role of astrocytes in the synaptic transfer of information.Comment: 42 pages, 16 figures, 1 table. Figure filenames mirror figure order in the paper. Ending "S" in figure filenames stands for "Supplementary Figure". This article was selected by the Faculty of 1000 Biology: "Genevieve Dupont: Faculty of 1000 Biology, 4 Sep 2009" at http://www.f1000biology.com/article/id/1163674/evaluatio

Bidirectional Coupling between Astrocytes and Neurons Mediates Learning and Dynamic Coordination in the Brain: A Multiple Modeling Approach

In recent years research suggests that astrocyte networks, in addition to nutrient and waste processing functions, regulate both structural and synaptic plasticity. To understand the biological mechanisms that underpin such plasticity requires the development of cell level models that capture the mutual interaction between astrocytes and neurons. This paper presents a detailed model of bidirectional signaling between astrocytes and neurons (the astrocyte-neuron model or AN model) which yields new insights into the computational role of astrocyte-neuronal coupling. From a set of modeling studies we demonstrate two significant findings. Firstly, that spatial signaling via astrocytes can relay a “learning signal” to remote synaptic sites. Results show that slow inward currents cause synchronized postsynaptic activity in remote neurons and subsequently allow Spike-Timing-Dependent Plasticity based learning to occur at the associated synapses. Secondly, that bidirectional communication between neurons and astrocytes underpins dynamic coordination between neuron clusters. Although our composite AN model is presently applied to simplified neural structures and limited to coordination between localized neurons, the principle (which embodies structural, functional and dynamic complexity), and the modeling strategy may be extended to coordination among remote neuron clusters