Digestion of Streptococcus pneumoniae cell walls with its major peptidoglycan hydrolase releases branched stem peptides carrying proinflammatory activity.

The peptidoglycan of Gram-positive bacteria is known to trigger cytokine release from peripheral blood mononuclear cells (PBMCs). However, it requires 100-1000 times more Gram-positive peptidoglycan than Gram-negative lipopolysaccharide to release the same amounts of cytokines from target cells. Thus, either peptidoglycan is poorly active or only part of it is required for PBMC activation. To test this hypothesis, purified Streptococcus pneumoniae walls were digested with their major autolysin N-acetylmuramoyl-L-alanine amidase, and/or muramidase. Solubilized walls were separated by reverse phase high pressure chromatography. Individual fractions were tested for their PBMC-stimulating activity, and their composition was determined. Soluble components had a Mr between 600 and 1500. These primarily comprised stem peptides cross-linked to various extents. Simple stem peptides (Mr <750) were 10-fold less active than undigested peptidoglycan. In contrast, tripeptides (Mr >1000) were >/=100-fold more potent than the native material. One dipeptide (inactive) and two tripeptides (active) were confirmed by post-source decay analysis. Complex branched peptides represented </=2% of the total material, but their activity (w/w) was almost equal to that of LPS. This is the first observation suggesting that peptidoglycan stem peptides carry high tumor necrosis factor-stimulating activity. These types of structures are conserved among Gram-positive bacteria and will provide new material to help elucidate the mechanism of peptidoglycan-induced inflammation.

Association of BAFF/BLyS overexpression and altered B cell differentiation with Sjögren's syndrome.

BAFF (BLyS, TALL-1, THANK, zTNF4) is a member of the TNF superfamily that specifically regulates B lymphocyte proliferation and survival. Mice transgenic (Tg) for BAFF develop an autoimmune condition similar to systemic lupus erythematosus. We now demonstrate that BAFF Tg mice, as they age, develop a secondary pathology reminiscent of Sjögren's syndrome (SS), which is manifested by severe sialadenitis, decreased saliva production, and destruction of submaxillary glands. In humans, SS also correlates with elevated levels of circulating BAFF, as well as a dramatic upregulation of BAFF expression in inflamed salivary glands. A likely explanation for disease in BAFF Tg mice is excessive survival signals to autoreactive B cells, possibly as they pass through a critical tolerance checkpoint while maturing in the spleen. The marginal zone (MZ) B cell compartment, one of the enlarged B cell subsets in the spleen of BAFF Tg mice, is a potential reservoir of autoreactive B cells. Interestingly, B cells with an MZ-like phenotype infiltrate the salivary glands of BAFF Tg mice, suggesting that cells of this compartment potentially participate in tissue damage in SS and possibly other autoimmune diseases. We conclude that altered B cell differentiation and tolerance induced by excess BAFF may be central to SS pathogenesis.

BAFF production by antigen-presenting cells provides T cell co-stimulation.

The B cell-activating factor from the tumor necrosis factor family (BAFF) is an important regulator of B cell immunity. Recently, we demonstrated that recombinant BAFF also provides a co-stimulatory signal to T cells. Here, we studied expression of BAFF in peripheral blood leukocytes and correlated this expression with BAFF T cell co-stimulatory function. BAFF is produced by antigen-presenting cells (APC). Blood dendritic cells (DC) as well as DC differentiated in vitro from monocytes or CD34+ stem cells express BAFF mRNA. Exposure to bacterial products further up-regulates BAFF production in these cells. A low level of BAFF transcription, up-regulated upon TCR stimulation, was also detected in T cells. Functionally, blockade of endogenous BAFF produced by APC and, to a lesser extent, by T cells inhibits T cell activation. Altogether, this indicates that BAFF may regulate T cell immunity during APC-T cell interactions and as an autocrine factor once T cells have detached from the APC.

Monitoring multiple angiogenesis-related molecules in the blood of cancer patients shows a correlation between VEGF-A and MMP-9 levels before treatment and divergent changes after surgical vs. conservative therapy.

Anti-angiogenic therapies are currently in cancer clinical trials, but to date there are no established tests for evaluating the angiogenic status of a patient. We measured 11 circulating angiogenesis-associated molecules in cancer patients before and after local treatment. The purpose of our study was to screen for possible relationships among the different molecules and between individual molecules and tumor burden. We measured VEGF-A, PlGF, SCF, MMP-9, EDB+ -fibronectin, sVEGFR-2, sVEGFR-1, salphaVbeta3, sTie-2, IL-8 and CRP in the blood of 22 healthy volunteers, 17 early breast, 17 early colorectal, and 8 advanced sarcoma/melanoma cancer patients. Breast cancer patients had elevated levels of VEGF-A and sTie-2, colorectal cancer patients of VEGF-A, MMP-9, sTie-2, IL-8 and CRP, and melanoma/sarcoma patients of sVEGFR-1. salphaVbeta3 was decreased in colorectal cancer patients. A correlation between VEGF-A and MMP-9 was found. After tumor removal, MMP-9 and salphaVbeta3 significantly decreased in breast and CRP in colorectal cancer, whereas sVEGFR-1 increased in colorectal cancer patients. In sarcoma/melanoma patients treated regionally with TNF and chemotherapy we observed a rise in VEGF-A, SCF, VEGFR-2, MMP-9, Tie-2 and CRP, a correlation between CRP and IL-8, and a decreased in sVEGFR-1 levels. In conclusion, among all factors measured, only VEGF-A and MMP-9 consistently correlated to each other, elevated CRP levels were associated with tumor burden, whereas sVEGF-R1 increased after tumor removal in colorectal cancer. Treatment with chemotherapy and TNF induced changes consistent with an angiogenic switch. These results warrant a prospective study to compare the effect of surgical tumor removal vs. chemotherapy on some of these markers and to evaluate their prognostic/predictive value.

MyD88, an adapter protein involved in interleukin-1 signaling.

MyD88 has a modular organization, an N-terminal death domain (DD) related to the cytoplasmic signaling domains found in many members of the tumor necrosis factor receptor (TNF-R) superfamily, and a C-terminal Toll domain similar to that found in the expanding family of Toll/interleukin-1-like receptors (IL-1R). This dual domain structure, together with the following observations, supports a role for MyD88 as an adapter in IL-1 signal transduction; MyD88 forms homodimers in vivo through DD-DD and Toll-Toll interactions. Overexpression of MyD88 induces activation of the c-Jun N-terminal kinase (JNK) and the transcription factor NF-kappaB through its DD. A point mutation in MyD88, MyD88-lpr (F56N), which prevents dimerization of the DD, also blocks induction of these activities. MyD88-induced NF-kappaB activation is inhibited by the dominant negative versions of TRAF6 and IRAK, which also inhibit IL-1-induced NF-kappaB activation. Overexpression of MyD88-lpr or MyD88-Toll (expressing only the Toll domain) acted to inhibit IL-1-induced NF-kappaB and JNK activation in a 293 cell line overexpressing the IL-1RI. MyD88 coimmunoprecipitates with the IL-1R signaling complex in an IL-1-dependent manner.

Apoptosis induced by death receptors.

Death receptors belong to the TNF receptor family and are characterised by an intracellular death domain that serves to recruit adapter proteins such as TRADD and FADD and cysteine proteases such as Caspase-8. Activation of Caspase-8 on the aggregated receptor leads to apoptosis. Triggering of death receptors is mediated through the binding of specific ligands of the TNF family, which are homotrimeric type-2 membrane proteins displaying three receptor binding sites. There are various means of modulating the activation of death receptors. The status of the ligand (membrane-bound vs. soluble) is critical in the activation of Fas and of TRAIL receptors. Cleavage of membrane-bound FasL to a soluble form (sFasL) does not affect its ability to bind to Fas but drastically decreases its cytotoxic activity. Conversely, cross-linking epitope-tagged sFasL with anti-tag antibodies to mimic membrane-bound ligand results in a 1000-fold increase in cytotoxicity. This suggests that more than three Fas molecules need to be aggregated to efficiently signal apoptosis. Death receptors can also be regulated by decoy receptors. The cytotoxic ligand TRAIL interacts with five receptors, only two of which (TRAIL-R1 and -R2) have a death domain. TRAIL-R3 is anchored to the membrane by a glycolipid and acts as a dominant negative inhibitor of TRAIL-mediated apoptosis when overexpressed on TRAIL-sensitive cells. Intracellular proteins interacting with the apoptotic pathway are potential modulators of death receptors. FLIP resembles Caspase-8 in structure but lacks protease activity. It interacts with both FADD and Caspase-8 to inhibits the apoptotic signal of death receptors and, at the same time, can activate other signalling pathways such as that leading to NF-kappa B activation.

Maladies inflammatoires de l'intestin [Main therapeutic advances in IBD in 2007].

In 2008 three biological agents against TNFalpha will be available. The combination of infliximab with azathioprine is no longer recommended, as hepatosplenic lymphomas with a particularly bad prognosis have been associated with this combined therapy. Regular maintenance therapy with infliximab is as effective in preventing the development of anti-infliximab antibodies as co-administration of this anti-TNFalpha agent with an immunomodulator. The benefit of regular maintenance therapy is probably linked to the presence of residual trough levels of infliximab between perfusions.

Fas triggers an alternative, caspase-8-independent cell death pathway using the kinase RIP as effector molecule.

Cell death is achieved by two fundamentally different mechanisms: apoptosis and necrosis. Apoptosis is dependent on caspase activation, whereas the caspase-independent necrotic signaling pathway remains largely uncharacterized. We show here that Fas kills activated primary T cells efficiently in the absence of active caspases, which results in necrotic morphological changes and late mitochondrial damage but no cytochrome c release. This Fas ligand-induced caspase-independent death is absent in T cells that are deficient in either Fas-associated death domain (FADD) or receptor-interacting protein (RIP). RIP is also required for necrotic death induced by tumor necrosis factor (TNF) and TNF-related apoptosis-inducing ligand (TRAIL). In contrast to its role in nuclear factor kappa B activation, RIP requires its own kinase activity for death signaling. Thus, Fas, TRAIL and TNF receptors can initiate cell death by two alternative pathways, one relying on caspase-8 and the other dependent on the kinase RIP.

Immunosuppresseurs: usages systémiques et neurologiques [Systemic and neurological uses of immunosuppressive agents].

Involvement of the central or peripheral nervous system, frequently present in systemic inflammatory immune disorders, has to be considered a severe threat and requires aggressive immunosuppressive treatment to achieve rapid remission. This is usually obtained with high-dose systemic corticosteroids combined with cyclophosphamide. Once remission is obtained, immunosuppressive agents with a more favorable safety profile are needed to exert a corticosteroid-sparing effect and minimize adverse events. New therapeutic approaches are currently developed to treat autoimmune diseases, mostly linked to the definition of new indications for biological agents such as TNF-alpha antagonists and rituximab.

APRIL is critical for plasmablast survival in the bone marrow and poorly expressed by early-life bone marrow stromal cells.

The persistence of serum IgG antibodies elicited in human infants is much shorter than when such responses are elicited later in life. The reasons for this rapid waning of antigen-specific antibodies elicited in infancy are yet unknown. We have recently shown that adoptively transferred tetanus toxoid (TT)-specific plasmablasts (PBs) efficiently reach the bone marrow (BM) of infant mice. However, TT-specific PBs fail to persist in the early-life BM, suggesting that they fail to receive the molecular signals that support their survival/differentiation. Using a proliferation-inducing ligand (APRIL)- and B-cell activating factor (BAFF) B-lymphocyte stimulator (BLyS)-deficient mice, we demonstrate here that APRIL is a critical factor for the establishment of the adult BM reservoir of anti-TT IgG-secreting cells. Through in vitro analyses of PB/plasma cell (PC) survival/differentiation, we show that APRIL induces the expression of Bcl-X(L) by a preferential binding to heparan sulfate proteoglycans at the surface of CD138(+) cells. Last, we identify BM-resident macrophages as the main cells that provide survival signals to PBs and show that this function is slowly acquired in early life, in parallel to a progressive acquisition of APRIL expression. Altogether, this identifies APRIL as a critical signal for PB survival that is poorly expressed in the early-life BM compartment.

Implication of TNF family members in the immune response to the infection with the intracellular protozoan parasite "Leishmania major"

Suite à une infection avec le protozoaire Leishmania major (L. major), les souris sensibles de souche BALB/c développent des lésions progressives associées à une maturation des cellules CD4+ TH2 sécrétant de l'IL-4. A l'inverse, les souris résistantes de souche C57BL/6 guérissent à terme, sous l'influence de l'expansion des cellules CD4+ TH1 produisant de l'IFNy qui a un effet synergique avec le TNF ("tumor necrosis factor") sur l'activation des macrophages et leur fonction leishmanicide. Lors de notre étude nous avons montré que des souris C57BL/6 doublement déficientes en TNF et FasL ("Fas ligand") infectées par L. major ne guérissaient ni leur lésions ni ne contrôlaient la réplication de parasites malgré une réponse de type TH1. Bien que l'activité de synthétase inductible de l'oxyde nitrique ("iNOs") soit comparable chez les souris doublement ou simplement déficientes, seules celles déficientes en FasL ont démontré une incapacité à contrôler la réplication parasitaire. De surcroît il est apparu que le FasL a un effet synergique avec l'IFNy. L'adjonction de FasL à une culture cellulaire de macrophages stimulés par l'IFNy conduit à une activation de ces cellules. Celle-ci est démontrée par l'augmentation de la production de TNF et de NO par les macrophages ainsi que par l'élimination des parasites intracellulaires par ces mêmes cellules. Alors que le FasL et l'IFNy semblent essentiels au contrôle de la réplication des pathogènes intracellulaires, la contribution de TNF s'oriente davantage vers le contrôle de l'inflammation. L'activation macrophagique via Fas précède la mort cellulaire qui survient quelques jours plus tard. Cette mort cellulaire programmée était indépendante de la cascade enzymatique des caspases, au vu de l'absence d'effet de l'inhibiteur non-spécifique ZVAD-fmk des caspases. Ces résultats suggèrent que l'interaction Fas-FasL agit comme une costimulation nécessaire à une activation efficace des macrophages, la mort cellulaire survenant consécutivement à l'activation des macrophages.¦-¦Upon infection with the protozoan parasite Leishmania major (L. major), susceptible BALB/c mice develop non healing lesions associated with the maturation of CD4+ TH2 cells secreting IL-4. In contrast, resistant C57BL/6 mice are able to heal their lesions, because of CD4+ TH1 cell expansion and production of high levels of IFNy, which synergizes with tumour necrosis factor (TNF) in activating macrophages to their microbicidal state. In our study we showed that C57BL/6 mice lacking both TNF and Fas ligand (FasL) infected with L. major neither resolved their lesions nor controlled L. major replication despite a strong TH1 response. Although comparable inducible nitric oxide synthase (iNOs) was measured in single or double deficient mice, only mice deficient in FasL failed to control the parasite replication. Moreover FasL synergized with IFNy for the induction of leishmanicidal activity within macrophages infected with L. major in vitro. Addition of FasL to IFNy stimulated macrophages led to their activation, as reflected by the secretion of tumour necrosis factor and nitrite oxide, as well as the induction of their microbicidal activity, resulting in the killing of intracellular L. major. While FasL along with IFNy and iNOs appeared to be essential for the complete control of intracellular pathogen replication, the contribution of TNF appeared more important in controlling the inflammation on the site of infection. Macrophage activation via Fas pathway preceded cell death, which occurred a few days after Fas mediated activation. This program cell death was independent of caspase enzymatic activities as revealed by the lack of effect of ZVAD-fmk, a pan-caspase inhibitor. These results suggested that the Fas-FasL pathway, as part of the classical activation pathway of the macrophages, is essential in the stimulation of macrophage leading to a microbicidal state and to AICD, and may thus contribute to the pathogenesis of L. major infection.

T cell costimulation by the TNF ligand BAFF.

The TNF ligand family member BAFF (B cell activating factor belonging to the TNF family, also called Blys, TALL-1, zTNF-4, or THANK) is an important survival factor for B cells [corrected]. In this study, we show that BAFF is able to regulate T cell activation. rBAFF induced responses (thymidine incorporation and cytokine secretion) of T cells, suboptimally stimulated through their TCR. BAFF activity was observed on naive, as well as on effector/memory T cells (both CD4+ and CD8+ subsets), indicating that BAFF has a wide function on T cell responses. Analysis of the signal transduced by BAFF into T cells shows that BAFF has no obvious effect on T cell survival upon activation, but is able to deliver a complete costimulation signal into T cells. Indeed, BAFF is sufficient to induce IL-2 secretion and T cell division, when added to an anti-TCR stimulation. This highlights some differences in the BAFF signaling pathway in T and B cells. In conclusion, our results indicate that BAFF may play a role in the development of T cell responses, in addition to its role in B cell homeostasis.

The HVEM network: new directions in targeting novel costimulatory/co-inhibitory molecules for cancer therapy.

The regulation of the immune system is controlled by many cell surface receptors. A prominent representative is the 'molecular switch' HVEM (herpes virus entry mediator) that can activate either proinflammatory or inhibitory signaling pathways. HVEM ligands belong to two distinct families: the TNF-related cytokines LIGHT and lymphotoxin-α, and the Ig-related membrane proteins BTLA and CD160. HVEM and its ligands have been involved in the pathogenesis of various autoimmune and inflammatory diseases, but recent reports indicate that this network may also be involved in tumor progression and resistance to immune response. Here we summarize the recent advances made regarding the knowledge on HVEM and its ligands in cancer cells, and their potential roles in tumor progression and escape to immune responses. Blockade or enhancement of these pathways may help improving cancer therapy.

Gliotransmission in the hippocampus, mechanisms and interaction with synaptic functions

SUMMARYAstrocytes represent the largest cell population in the human brain. In addition to a well established role as metabolic support for neuronal activity, in the last years these cells have been found to accomplish other important and, sometimes, unexpected functions. The tight enwrapping of synapses by astrocytic processes and the predominant expression of glutamate uptake carriers in the astrocytic rather than neuronal plasma membranes brought to the definition of a critical involvement of astrocytes in the clearance of glutamate from synaptic junctions. Moreover, several publications showed that astrocytes are able to release chemical transmitters (gliotransmitters) suggesting their active implication in the control of synaptic functions. Among gliotransmitters, the best characterized is glutamate, which has been proposed to be released from astrocytes in a Ca2+ dependent manner via exocytosis of synaptic-like microvesicles.In my thesis I present results leading to substantial advancement of the understanding of the mechanisms by which astrocytes modulate synaptic activity in the hippocampus, notably at excitatory synapses on dentate granule cells. I show that tumor necrosis factor- alpha (TNFa), a molecule that is generally involved in immune system functions, critically controls astrocyte-to-synapse communication (gliotransmission) in the brain. With constitutive levels of TNFa present, activation of purinergic G protein-coupled receptors in astrocytes, called P2Y1 receptors, induces localized intracellular calcium ([Ca2+]j) elevation in astrocytic processes (measured by two-photon microscopy) followed by glutamate release and activation of pre-synaptic NMDA receptors resulting in synaptic potentiation. In preparations lacking TNFa, astrocytes respond with identical [Ca2+]i elevations but fail to induce neuromodulation. I find that TNFa specifically controls the glutamate release step of gliotransmission. Addition of very low (picomolar) TNFa concentrations to preparations lacking the cytokine, promptly reconstitutes both normal exocytosis in cultured astrocytes and gliotransmission in hippocampal slices. These data provide the first demonstration that gliotransmission and its synaptic effects are controlled not only by astrocyte [Ca2+]i elevations but also by permissive/homeostatic factors like TNFa.In addition, I find that higher and presumably pathological TNFa concentrations do not act just permissively but instead become direct and potent triggers of glutamate release from astrocytes, leading to a strong enhancement of excitatory synaptic activity. The TNFa action, like the one observed upon P2Y1R activation, is mediated by pre-synaptic NMDA receptors, but in this case the effect is long-lasting, and not reversible. Moreover, I report that a necessary molecular target for this action of TNFa is TNFR1, one of the two specific receptors for the cytokine, as I found that TNFa was unable to induce synaptic potentiation when applied in slices from TNFR1 knock-out (Tnfrlv") mice. I then created a double transgenic mouse model where TNFR1 is knocked out in all cells but can be re-expressed selectively in astrocytes and I report that activation of the receptors in these cells is sufficient to reestablish TNFa-dependent long-lasting potentiation of synaptic activity in the TNFR1 knock-out mice.I therefore discovered that TNFa is a primary molecule displaying both permissive and instructive roles on gliotransmission controlling synaptic functions. These reports might have profound implications for the understanding of both physiological and pathological processes associated to TNFa production, including inflammatory processes in the brain.RÉSUMÉLes astrocytes sont les cellules les plus abondantes du cerveau humain. Outre leur rôle bien établi dans le support métabolique de l'activité neuronale, d'autres fonctions importantes, et parfois inattendues de ces cellules ont été mises en lumière au cours de ces dernières années. Les astrocytes entourent étroitement les synapses de leurs fins processus qui expriment fortement les transporteurs du glutamate et permettent ainsi aux astrocytes de jouer un rôle critique dans l'élimination du glutamate de la fente synaptique. Néanmoins, les astrocytes semblent être capables de jouer un rôle plus intégratif en modulant l'activité synaptique, notamment par la libération de transmetteurs (gliotransmetteurs). Le gliotransmetteur le plus étudié est le glutamate qui est libéré par l'exocytose régulée de petites vésicules ressemblant aux vésicules synaptiques (SLMVs) via un mécanisme dépendant du calcium.Les résultats présentés dans cette thèse permettent une avancée significative dans la compréhension du mode de communication de ces cellules et de leur implication dans la transmission de l'information synaptique dans l'hippocampe, notamment des synapses excitatrices des cellules granulaires du gyrus dentelé. J'ai pu montrer que le « facteur de nécrose tumorale alpha » (TNFa), une cytokine communément associée au système immunitaire, est aussi fondamentale pour la communication entre astrocyte et synapse. Lorsqu'un niveau constitutif très bas de TNFa est présent, l'activation des récepteurs purinergiques P2Y1 (des récepteurs couplés à protéine G) produit une augmentation locale de calcium (mesurée en microscopie bi-photonique) dans l'astrocyte. Cette dernière déclenche ensuite une libération de glutamate par les astrocytes conduisant à l'activation de récepteurs NMDA présynaptiques et à une augmentation de l'activité synaptique. En revanche, dans la souris TNFa knock-out cette modulation de l'activité synaptique par les astrocytes n'est pas bien qu'ils présentent toujours une excitabilité calcique normale. Nous avons démontré que le TNFa contrôle spécifiquement l'exocytose régulée des SLMVs astrocytaires en permettant la fusion synchrone de ces vésicules et la libération de glutamate à destination des récepteurs neuronaux. Ainsi, nous avons, pour la première fois, prouvé que la modulation de l'activité synaptique par l'astrocyte nécessite, pour fonctionner correctement, des facteurs « permissifs » comme le TNFa, agissant sur le mode de sécrétion du glutamate astrocytaire.J'ai pu, en outre, démontrer que le TNFa, à des concentrations plus élevées (celles que l'on peut observer lors de conditions pathologiques) provoque une très forte augmentation de l'activité synaptique, agissant non plus comme simple facteur permissif mais bien comme déclencheur de la gliotransmission. Le TNFa provoque 1'activation des récepteurs NMD A pré-synaptiques (comme dans le cas des P2Y1R) mais son effet est à long terme et irréversible. J'ai découvert que le TNFa active le récepteur TNFR1, un des deux récepteurs spécifiques pour le TNFa. Ainsi, l'application de cette cytokine sur une tranche de cerveau de souris TNFR1 knock-out ne produit aucune modification de l'activité synaptique. Pour vérifier l'implication des astrocytes dans ce processus, j'ai ensuite mis au point un modèle animal doublement transgénique qui exprime le TNFR1 uniquement dans les astrocytes. Ce dernier m'a permis de prouver que l'activation des récepteurs TNFR1 astrocytaires est suffisante pour induire une augmentation de l'activité synaptique de manière durable.Nous avons donc découvert que le TNFa possède un double rôle, à la fois un rôle permissif et actif, dans le contrôle de la gliotransmission et, par conséquent, dans la modulation de l'activité synaptique. Cette découverte peut potentiellement être d'une extrême importance pour la compréhension des mécanismes physiologiques et pathologiques associés à la production du TNFa, en particulier lors de conditions inflammatoires.RÉSUMÉ GRAND PUBLICLes astrocytes représentent la population la plus nombreuse de cellules dans le cerveau humain. On sait, néanmoins, très peu de choses sur leurs fonctions. Pendant très longtemps, les astrocytes ont uniquement été considérés comme la colle du cerveau, un substrat inerte permettant seulement de lier les cellules neuronales entre elles. Il n'y a que depuis peu que l'on a découvert de nouvelles implications de ces cellules dans le fonctionnement cérébral, comme, entre autres, une fonction de support métabolique de l'activité neuronale et un rôle dans la modulation de la neurotransmission. C'est ce dernier aspect qui fait l'objet de mon projet de thèse.Nous avons découvert que l'activité des synapses (régions qui permettent la communication d'un neurone à un autre) qui peut être potentialisée par la libération du glutamate par les astrocytes, ne peut l'être que dans des conditions astrocytaires très particulières. Nous avons, en particulier, identifié une molécule, le facteur de nécrose tumorale alpha (TNFa) qui joue un rôle critique dans cette libération de glutamate astrocytaire.Le TNFa est surtout connu pour son rôle dans le système immunitaire et le fait qu'il est massivement libéré lors de processus inflammatoires. Nous avons découvert qu'en concentration minime, correspondant à sa concentration basale, le TNFa peut néanmoins exercer un rôle indispensable en permettant la communication entre l'astrocyte et le neurone. Ce mode de fonctionnement est assez probablement représentatif d'un processus physiologique qui permet d'intégrer la communication astrocyte/neurone au fonctionnement général du cerveau. Par ailleurs, nous avons également démontré qu'en quantité plus importante, le TNFa change son mode de fonctionnement et agit comme un stimulateur direct de la libération de glutamate par l'astrocyte et induit une activation persistante de l'activité synaptique. Ce mode de fonctionnement est assez probablement représentatif d'un processus pathologique.Nous sommes également arrivés à ces conclusions grâce à la mise en place d'une nouvelle souche de souris doublement transgéniques dans lesquelles seuls les astrocytes (etnon les neurones ou les autres cellules cérébrales) sont capables d'être activés par le TNFa.