Impaired plasticity of intrinsic excitability in the dentate gyrus alters spike transfer in a mouse model of Alzheimer's disease.

Alzheimer's disease (AD) is a progressive neurodegenerative disease characterized by cognitive decline related to deficits in synaptic transmission and plasticity. We report in APP/PS1 mice, a double transgenic mouse model of AD, that females displayed an early burden of Aß plaques load in the stratum moleculare of the dentate gyrus (DG) together with prominent neuroinflammatory activation of astrocytes and microglia. Robust deficits in hippocampus-dependent memory tasks were observed in APP/PS1 female mice as early as 3 months of age. We then studied the functional properties of the lateral perforant path (LPP) to DG granule cells. Remarkably DG granule cells displayed higher intrinsic excitability in APP/PS1 female mice. We showed that the long term potentiation of population spike amplitude induced by high frequency stimulation (HFS) at LPP-DG granule cells synapse is impaired in APP/PS1 female mice. HFS induced plasticity of intrinsic excitability in DG granule cells without inducing noticeable modification of synaptic strength. Furthermore, the enhanced intrinsic excitability was potentiated to a greater extent in APP/PS1 as compared to control mice following HFS. Our study shows that changes in the intrinsic excitability of DG granule cells in AD contribute to the dysfunctional transfer of information from the entorhinal cortex to the hippocampus.

PGE2-EP3 signaling pathway impairs hippocampal presynaptic long-term plasticity in a mouse model of Alzheimer's disease.

Alzheimer's disease (AD) is a progressive neurodegenerative disease characterized by early cognitive deficits linked to synaptic dysfunction and loss. Considerable evidence suggests that neuroinflammation contributes to AD. Prostaglandin E2 (PGE2), a key neuroinflammatory molecule, modulates hippocampal synaptic transmission and plasticity. We investigated the effect of PGE2 on synaptic transmission and presynaptic plasticity at synapses between mossy fibers from the dentate gyrus and CA3 pyramidal cells (Mf-CA3 synapse). These synapses are involved in mnemonic processes and consequently may be of relevance for AD. We provide evidence that although PGE2 had no effect both on either basal transmission or short-term plasticity, it strongly impaired presynaptic Mf-CA3 long-term potentiation (LTP) by acting on PGE2 receptor 3 (EP3) receptors. During aging, hippocampal levels of PGE2 markedly increased in the APP/PS1 mouse model of AD and impaired specifically presynaptic LTP via a PGE2-EP3 signaling pathway. In summary, the building up of PGE2 during the progression of AD leads to specific impairment of hippocampal presynaptic plasticity and highlights EP3 receptors as a potential target to alleviate cognitive deficits in AD.

Synaptic plasticity and spatial working memory are impaired in the CD mouse model of Williams-Beuren syndrome.

Mice heterozygous for a complete deletion (CD) equivalent to the most common deletion found in individuals with Williams-Beuren syndrome (WBS) recapitulate relevant features of the neurocognitive phenotype, such as hypersociability, along with some neuroanatomical alterations in specific brain areas. However, the pathophysiological mechanisms underlying these phenotypes still remain largely unknown. We have studied the synaptic function and cognition in CD mice using hippocampal slices and a behavioral test sensitive to hippocampal function. We have found that long-term potentiation (LTP) elicited by theta burst stimulation (TBS) was significantly impaired in hippocampal field CA1 of CD animals. This deficit might be associated with the observed alterations in spatial working memory. However, we did not detect changes in presynaptic function, LTP induction mechanisms or AMPA and NMDA receptor function. Reduced levels of Brain-derived neurotrophic factor (BDNF) were present in the CA1-CA3 hippocampal region of CD mice, which could account for LTP deficits in these mice. Taken together, these results suggest a defect of CA1 synapses in CD mice to sustain synaptic strength after stimulation. These data represent the first description of synaptic functional deficits in CD mice and further highlights the utility of the CD model to study the mechanisms underlying the WBS neurocognitive profile.

Microglia-derived purines modulate mossy fibre synaptic transmission and plasticity through P2X4 and A1 receptors.

Recent data have provided evidence that microglia, the brain-resident macrophage-like cells, modulate neuronal activity in both physiological and pathophysiological conditions, and microglia are therefore now recognized as synaptic partners. Among different neuromodulators, purines, which are produced and released by microglia, have emerged as promising candidates to mediate interactions between microglia and synapses. The cellular effects of purines are mediated through a large family of receptors for adenosine and for ATP (P2 receptors). These receptors are present at brain synapses, but it is unknown whether they can respond to microglia-derived purines to modulate synaptic transmission and plasticity. Here, we used a simple model of adding immune-challenged microglia to mouse hippocampal slices to investigate their impact on synaptic transmission and plasticity at hippocampal mossy fibre (MF) synapses onto CA3 pyramidal neurons. MF-CA3 synapses show prominent forms of presynaptic plasticity that are involved in the encoding and retrieval of memory. We demonstrate that microglia-derived ATP differentially modulates synaptic transmission and short-term plasticity at MF-CA3 synapses by acting, respectively, on presynaptic P2X4 receptors and on adenosine A1 receptors after conversion of extracellular ATP to adenosine. We also report that P2X4 receptors are densely located in the mossy fibre tract in the dentate gyrus-CA3 circuitry. In conclusion, this study reveals an interplay between microglia-derived purines and MF-CA3 synapses, and highlights microglia as potent modulators of presynaptic plasticity.

Impaired interleukin-1beta and c-Fos expression in the hippocampus is associated with a spatial memory deficit in P2X(7) receptor-deficient mice.

Recent evidence suggests that interleukin-1beta (IL-1beta), which was originally identified as a proinflammatory cytokine, is also required in the brain for memory processes. We have previously shown that IL-1beta synthesis in the hippocampus is dependent on P2X(7) receptor (P2X(7)R), which is an ionotropic receptor of ATP. To substantiate the role of P2X(7)R in both brain IL-1beta expression and memory processes, we examined the induction of IL-1beta mRNA expression in the hippocampus of wild-type (WT) and homozygous P2X(7) receptor knockout mice (P2X(7)R(-/-)) following a spatial memory task. The spatial recognition task induced both IL-1beta mRNA expression and c-Fos protein activation in the hippocampus of WT but not of P2X(7)R(-/-) mice. Remarkably, P2X(7)R(-/-) mice displayed spatial memory impairment in a hippocampal-dependant task, while their performances in an object recognition task were unaltered. Taken together, our results show that P2X(7)R plays a critical role in spatial memory processes and the associated hippocampal IL-1beta mRNA synthesis and c-Fos activation.

Dual modulation of synaptic transmission in the nucleus tractus solitarius by prostaglandin E2 synthesized downstream of IL-1beta.

The activation of the innate immune system induces the production of blood-borne proinflammatory cytokines like interleukin-1beta (IL-1beta), which in turn triggers brain-mediated adaptative responses referred to as sickness behaviour. These responses involve the modulation of neural networks in key regions of the brain. The nucleus tractus solitarius (NTS) of the brainstem is a key nucleus for immune-to-brain signalling. It is the main site of termination of vagal afferents and is adjacent to the area postrema, a circumventricular organ allowing blood-borne action of circulating IL-1beta. Although it is well described that IL-1beta activates cerebral endothelial and glial cells, it is still unknown if and how IL-1beta or downstream-synthesized molecules impact NTS synaptic function. In this study we report that IL-1beta did not modulate NTS synaptic transmission per se, whereas prostaglandin E(2) (PGE(2)), which is produced downstream of IL-1beta, produced opposite effects on spontaneous and evoked release. On the one hand, PGE(2) facilitated glutamatergic transmission between local NTS neurons by enhancing the frequency of spontaneous excitatory postsynaptic currents through a presynaptic receptor different from the classical EP1-4 subtypes. On the other hand, PGE(2) also depressed evoked excitatory input from vagal afferent terminals through presynaptic EP3 receptors coupled to G-proteins linked to adenylyl cyclase and protein kinase A activity. Our data show that IL-1beta-induced PGE(2) can modulate evoked and spontaneous release in the NTS differentially through different mechanisms. These data unravel some molecular mechanisms by which innate immune stimuli could signal to, and be integrated within, the brainstem to produce central adaptative responses.

In vitro and in vivo evidence for a role of the P2X7 receptor in the release of IL-1 beta in the murine brain.

The P2X(7) receptor (P2X(7)R) is a purinoceptor expressed predominantly by cells of immune origin, including microglial cells. P2X(7)R has a role in the release of biologically active proinflammatory cytokines such as IL-1 beta, IL-6 and TNFalpha. Here we demonstrate that when incubated with lipopolysaccharide (LPS), glial cells cultured from brain of P2X(7)R(-/-) mice produce less IL-1 beta compared to glial cells from brains of wild-type mice. This is not the case for TNFalpha and IL-6. Our results indicate a selective effect of the P2X7R gene deletion on release of IL-1 beta release but not of IL-6 and TNFalpha. In addition, we confirm that only microglial cells produce IL-1beta, and this release is dependent on P2X(7)R and ABC1 transporter. Because IL-1 beta is a key regulator of the brain cytokine network and P2X(7)R is an absolute requirement for IL-1 beta release, we further investigated whether response of brain cytokines to LPS in vivo was altered in P2X(7)R(-/-) mice compared to wild-type mice. IL-1 beta and TNFalpha mRNAs were less elevated in the brain of P2X(7)R(-/-) than in the brain of wild-type mice in response to systemic LPS. These results show that P2X7R plays a key role in the brain cytokine response to immune stimuli, which certainly applies also to cytokine-dependent alterations in brain functions including sickness behavior.

NFkappaB activates in vivo the synthesis of inducible Cox-2 in the brain.

Interleukin-1beta (IL-1beta) induces cyclooxygenase-2 (Cox-2) expression in many of its cellular targets resulting in production and release of prostaglandins. Although IL-1beta-induced Cox-2 expression most likely requires activation of nuclear transcription factor kappa B (NFkappaB) pathway, this has never been formally demonstrated in vivo. We tested this using a specific inhibitor of NFkappaB activation, the NEMO binding domain (NBD) peptide, that has been shown previously to be effective in various in vivo models of acute inflammation. Incubation of rat glioma cells with the NBD peptide blocked IL-1beta-induced NFkappaB nuclear translocation. Furthermore, after injection of a biotinylated version of the NBD peptide into the lateral ventricle of the brain, we found that it readily diffused to its potential cellular targets in vivo. To test the effects of the peptide on NFkappaB activation and Cox-2 expression in the brain, we injected it intracerebroventricularly (36 microg/rat) into rats before intraperitoneal injection of IL-1beta (60 microg/kg). Treatment with NBD peptide completely abolished IL-1beta-induced NFkappaB activation and Cox-2 synthesis in microvasculature. In contrast, the peptide had no effect on constitutive neuronal Cox-2. These findings strongly support the hypothesis that IL-1beta-induced NFkappaB activation plays a major role in transmission of immune signals from the periphery to the brain.

ATP binding cassette transporter ABC1 is required for the release of interleukin-1beta by P2X7-stimulated and lipopolysaccharide-primed mouse Schwann cells.

Schwann cells are best known as myelinating glial cells of the peripheral nervous system, but they also participate actively in the sphere of immunity by producing pro-inflammatory cytokines, such as interleukin-1beta (IL-1beta). In a previous study, we demonstrated that posttranslational processing of IL-1beta by immune-challenged Schwann cells required the P2X7 receptor. Remarkably, the release of IL-1beta was not associated with cell death, indicating the involvement of an active mechanism. ATP binding cassette (ABC) transporters are known to transport leaderless secretory proteins, such as IL-1beta; therefore, we investigated whether such transporters were at work in Schwann cells. Mouse Schwann cells expressed ABC1 transporter mRNA and displayed the functional protein. Glybenclamide and diisothiocyanato-stilbene-disulfonic acid (DIDS), two blockers of chloride fluxes that drive the export activity of ABC1 transporters, inhibited IL-1beta release without altering its intracellular processing. Enhancing chloride efflux potentiated the release of IL-1beta, while decreasing it led to a strong reduction in its release. Because the stimulation of the P2X7 receptor also activates a chloride conductance, we investigated the possibility of a sole anionic pathway mobilized by the P2X7 receptor and ABC1. Glybenclamide and DIDS had no significant effects on the P2X7-activated chloride current suggesting therefore the existence of two different pathways. In summary, ABC1 transporters are required for the release of IL-1beta by mouse Schwann cells. Being associated together with chloride conductance, P2X7 receptors and ABC1 transporters delineate a subtle and complex regulation of IL-1beta production in mammalian Schwann cells. Furthermore, ABC1 transporters could be a target of therapeutic interest for regulating IL-1beta activity in neuroinflammation disorders.

Nuclear factor kappaB nuclear translocation as a crucial marker of brain response to interleukin-1. A study in rat and interleukin-1 type I deficient mouse.

The signalling pathways that mediate early central effects of interleukin-1 (IL-1) during the acute phase reaction have been poorly elucidated. Interaction of IL-1beta to its specific receptor interleukin-1 receptor type I (IL-1RI) leads to nuclear factor kappa B (NuFkappaB) nuclear translocation and a robust transcriptional activation of inhibitor of kappa B alpha (IkappaBalpha) within the rat brain. Indeed, we demonstrated that IL-1RI expressed in blood brain barrier (BBB) cells and in circumventricular organs (CVOs) is crucial for p65-NFkappaB translocation induced by peripheral injection of IL-1beta. Moreover, it has been previously shown that monitoring IkappaBalpha mRNA synthesis is an effective tool to investigate the activity of the transcription factor NFkappaB into the CNS. However in the present study we observed time-related and cell-type differences between IkappaBalpha mRNA synthesis and p65-NFkappaB translocation. This indicates that the expression of IkappaBalpha mRNA does not strictly parallel p65-NFkappaB nuclear translocation, suggesting that these markers are not interchangeable to investigate NFkappaB activity but must be studied together. Thus, we hypothesize that IL-1beta reached the brain across the CVOs that lack a BBB and endothelial cells all over the brain and interacted with its receptors to induce NFkappaB translocation. The study of the consequences of the impairment of NFkappaB pathway activation in in vivo experimentation should bring important clues about the precise role of this transcription factor.

[The immune status of Schwann cells: what is the role of the P2X7 receptor?]. / Le statut immun de la cellule de Schwann: quels rôles pour le récepteur P2X7?

The peripheral nervous system (PNS) displays structural barriers and a lack of lymphatic drainage which strongly limit the access of molecules and cells from the immune system. In addition, the PNS has the ability to set up some specific mechanisms of immune protection to limit the pathogenicity of inflammation processes following insults by pathogens or inflammatory autoimmune diseases like the Guillain-Barré syndrome. Schwann cells are among the most prominent cells which can display immune capabilities in the PNS. Numerous in vitro studies have shown that Schwann cells were indeed able to display a large repertoire of properties, ranging from the participation to antigen presentation, to secretion of pro- and anti-inflammatory cytokines, chemokines and neurotrophic factors. In vivo studies have confirmed the immune capabilities of Schwann cells. The aim of this review is to present how Schwann cells can participate to the initiation, the regulation and the termination of the immune response in the light of the recent discovery of the Schwann cell expression of purinergic P2X7 receptors.

Maturation and release of interleukin-1beta by lipopolysaccharide-primed mouse Schwann cells require the stimulation of P2X7 receptors.

The P2X7 receptor, mainly expressed by immune cells, is a ionotropic receptor activated by high concentration of extracellular ATP. It is involved in several processes relevant to immunomodulation and inflammation. Among these processes, the production of extracellular interleukin-1beta (IL-1beta), a pro-inflammatory cytokine, plays a major role in the activation of the cytokine network. We have investigated the role of P2X7 receptor and of an associated calcium-activated potassium conductance (BK channels) in IL-1beta maturation and releasing processes by Schwann cells. Lipopolysaccharide-primed Schwann cells synthesized large amounts of pro-IL-1beta but did not release detectable amounts of pro or mature IL-1beta. ATP on its own had no effect on the synthesis of pro-IL-1beta, but a co-treatment with lipopolysaccharide and ATP led to the maturation and the release of IL-1beta by Schwann cells. Both mechanisms were blocked by oxidized ATP. IL-1beta-converting enzyme (ICE), the caspase responsible for the maturation of pro-IL-1beta in IL-1beta, was activated by P2X7 receptor stimulation. The specific inhibition of ICE by the caspase inhibitor Ac-Tyr-Val-Ala-Asp-aldehyde blocked the maturation of IL-1beta. In searching for a link between the P2X7 receptor and the activation of ICE, we found that enhancing potassium efflux from Schwann cells upregulated the production of IL-1beta, while strongly reducing potassium efflux led to opposite effects. Blocking BK channels actually modulated IL-1beta release. Taken together, these results show that P2X7 receptor stimulation and associated BK channels, through the activation of ICE, leads to the maturation and the release of IL-1beta by immune-challenged Schwann cells.