Thesis (M. Med. Sc.)--University of Hong Kong, 2005.

During aging, a decline in metabolism appears to be associated with altered hippocampal function. Thus, 1-13C-glucose and 2-13C-acetate were employed to assess neural metabolism in the dentate gyrus and CA1 of F344 rats with respect to aging and cognitive status. The results obtained when 1-13C-glucose was used as the metabolic substrate suggest that glucose metabolism may not be altered in neural tissue itself with respect to aging or a decline in hippocampal function. In contrast, the use of 2-13C-acetate, a substrate that is preferentially metabolized by astrocytes, revealed significantly increased astroglial metabolism associated with age and preserved hippocampal function. Specifically, greater 2-13C-acetate incorporation into glutamine and glutamate was observed in the dentate gyrus and CA1 of aged rats that performed similar to young rats in the Morris water maze task. Since glutamate is the primary excitatory neurotransmitter of the hippocampus and plays a central role in synaptic plasticity, the mechanism proposed to underlie learning and memory, these finding were taken to represent an adaptive metabolic response by astroglia in the aged hippocampus. Subsequently, astroglia metabolism could potentially be a future target of therapeutic strategies for age-related cognitive decline.

Epilepsy is a neurological disorder that presents a significant public health burden, with an estimated five million people being newly diagnosed each year. However, current therapeutics designed to modify neuronal processes, provide no relief to 1-in-3 epileptic patients. Additionally, no disease modifying therapies currently exist to treat the underlying pathological processes involved in epileptogenesis. The overarching goal of this project is to further characterize the role astrocytes play in epileptogenesis, in hopes of revealing novel therapeutic targets to benefit patients who otherwise have no effective treatment options. System xc- (SXC), a cystine/glutamate antiporter expressed in astrocytes, is one such target that has been shown to play a critical role in establishing ambient extracellular glutamate levels in both health and disease. SXC has been shown to play a major role in setting ambient glutamatergic tone in the central nervous system (CNS) as pharmacological inhibition of SXC, using (S)-4-carboxyphenylglycine (S-4-CPG) or antisense xCT, resulted in a 60% reduction in extrasynaptic glutamate in the nucleus accumbens. Additionally, investigations in tumor-associated epilepsy revealed that overexpression of SXC seen in glioblastomas lead to higher levels of peritumoral glutamate, neuronal excitotoxicity, and ultimately seizures. These studies also found that SXC inhibition with sulfasalazine (SAS), an FDA approved drug and potent inhibitor of SXC, can ameliorate seizure burden in a glioblastoma mouse model. Therefore, the principal objective of this study is to further investigate the role of astrocytic SXC activity in epileptogenesis and seizure generation. In doing so, we also evaluated the efficacy of SAS in reducing seizure burden in vivo using an astrogliosis-mediated epilepsy mouse model. In this dissertation we show that (1) SXC inhibition, using SAS, is able to decrease induced epileptiform activity in multiple models of chemically induced hyperexcitability (2) this is due to a preferential decrease of NMDAR-mediated currents and (3) SXC inhibition, via SAS, decreases seizure burden in vivo in an astrogliosis-mediated epilepsy model. / Doctor of Philosophy / Epilepsy, characterized by unpredictable seizures, affects approximately 2.2 million Americans, with 150,000 new cases being diagnosed each year. Seizures typically occur when there is an imbalance between the excitatory and inhibitory processes in the brain. Because neurons are the primary cell in the brain that carry out these processes, clinically used anti-epileptic drugs (AEDs) work by either decreasing neuronal excitation or increasing neuronal inhibition. Despite success with managing seizures, up to 1-in-3 patients with epilepsy do not find any relief with existing AEDs. A statistic that has not changed in over 50 years of drug development. With this in mind, the overarching goal of this dissertation is to explore the efficacy of targeting non-neuronal processes to treat epilepsy and broaden the search for new AED targets by further characterizing a unique mouse model of epilepsy. One such target studied in our lab is system xc- (SXC), a glutamate/cystine antiporter present on astrocytes, a non-neuronal cell that provides maintenance, support and protection for neurons. Investigations in tumor-associated epilepsy from our lab revealed that hyperactivity of SXC in tumor cells was directly related to the development of tumor-associated epilepsy. These studies also revealed that SXC inhibition using sulfasalazine (SAS), an FDA approved drug, can decrease seizure burden in a tumor mouse model. Therefore, the principal objective of this study is to further investigate the role of astrocytic SXC activity in the development of epilepsy and seizure generation. In this dissertation we show that SXC inhibition, using SAS, is able to decrease neuronal hyperactivity and decreases seizure burden in an astrogliosis-mediated epilepsy model.

Thesis (Ph.D.)--University of Alabama at Birmingham, 2009. / Title from PDF title page (viewed on July 19, 2010). Includes bibliographical references.

La maladie d'Alzheimer est une maladie neurodégénérative associée à une perte progressive des fonctions mnésiques et cognitives, qui font suite à des dysfonctions synaptiques. Ces dysfonctions sont en partie dues à la présence d’oligomères solubles du peptide amyloïde β (Aβo). Chez l’homme ces oligomères sont déjà présents durant la phase asymptomatique de la maladie et semblent expliquer la perte synaptique et les dysfonctions caractéristiques de la maladie. Cependant, les mécanismes clés qui initient ce dysfonctionnement et cette perte synaptique restent inconnus. Il est actuellement admis que la synapse compte un 3ème partenaire qui joue un rôle majeur dans l'intégrité structurale et fonctionnelle de la synapse, l’astrocyte. Grâce à leurs prolongements, les astrocytes peuvent enrober la plupart des synapses et contribuer activement aux changements morphologiques et fonctionnels observés durant l'activité synaptique. Cependant leur implication a été peu étudiée dans le contexte de la maladie d'Alzheimer.Le but de ces travaux de thèse, était d’étudier de quelle manière Aβo modifie l’activité calcique astrocytaire et quelles peuvent en être les répercussions sur l’activité synaptique. Ainsi, nous avons mis en évidence sur tranche aigue d’hippocampes de souris, une hyperactivité calcique généralisée, au sein du réseau astrocytaire et au niveau des prolongements fins astrocytaires. Cette hyperactivité s’est révélée être dépendante d’un canal membranaire récemment identifié au niveau astrocytaire, le canal TRPA1. Cette hyperactivité calcique dépendante de TRPA1, se manifeste également très précocement, avant l’apparition de plaques amyloïdes, dans un modèle de souris transgénique pour la maladie d’Alzheimer. Au niveau neuronal, nous avons observé une augmentation de la transmission synaptique basale en présence d’Aβo. De manière intéressante, l’inhibition pharmacologique de TRPA1 permet de bloquer l’hyperactivité calcique induite par Aβo et de rétablir l’activité synaptique précédemment perturbée par l’Aβo. Dans leur ensemble, nos résultats suggèrent que les astrocytes soient une cible privilégiée d’Aβo lors des phases asymptomatiques de la maladie d’Alzheimer, et que le blocage de l’hyperactivité calcique astrocytaire puisse garantir l’intégrité synaptique en présence d’Aβo. / Alzheimer’s disease is a neurodegenerative disorder associated with a progressive loss of cognitive functions following synaptic dysfunctions. These synaptic alterations are mainly due to oligomeric forms of amyloid β peptide (Aβo). In humans, these oligomers are already present during the silent phase of the disease and seem to explain synaptic loss and synaptic dysfunctions. However, key mechanisms that initiate synaptic loss and synaptic dysfunctions remain unknown. It is now well established that there is a third component of the synapse, playing major role in morphological and functional synaptic integrity, the astrocyte. Thanks to their processes, astrocytes can enwrap most of synapses and actively participate to morphological and functional changes observed during synaptic activity. Still, their involvement in Alzheimer’s disease is under-investigated.The aim of these thesis works, was to evaluate how Aβo modify astrocytic calcium activity and what could be the repercussions on synaptic activity. We have observed on mice hippocampal acute brain slices a global calcic hyperactivity, within the astrocytic network and inside fine processes. This hyperactivity was dependent on a recently identified astrocytic channel, the TRPA1 channel. This TRPA1-dependent calcic hyperactivity shows up also very early, before amyloid plaques formation, in a transgenic mouse model of Alzheimer’s disease. On the neuronal side, glutamatergic synaptic transmission was increased by Aβo. Interestingly, pharmacological inhibition of TRPA1 could block astrocytic calcium hyperactivity and restore glutamatergic synaptic activity previously disturbed by Aβo. Overall, our data suggests that the astrocyte is a frontline target of Aβo during the prodromal phase of Alzheimer’s disease, and that blockade of astrocytic calcium hyperactivity could preserve synaptic integrity even when Aβo is applied.

Les astrocytes sont les cellules les plus abondantes dans le cerveau où ils sontimpliquées dans une myriade de fonctions telles que la neurogénèse, l’homéostasieionique, le soutien métabolique, l’élimination des substances toxiques et dans laréponse aux lésions cérébrales. Des altérations fonctionnelles des astrocytes ont étéassociées avec des pathologies telles que l’épilepsie, la depression et laschizophrénie. A ce titre, l’étude de la contribution astrocytaire aux fonctionssynaptiques revêt un intérêt clinique et sociétal assez conséquent. Dans cette thèse,j’ai évalué le potentiel de plusieurs préparations permettant l’analyse des protéinesastrocytaires impliquées dans la plasticité synaptique et j’ai utilisé celle qui se prêtaitle mieux à la quantification des niveaux de protéines astrocytaires qui se trouventêtre régulées dans différent modèles de plasticité synaptique. J’ai caractériséplusieurs préparations qui peuvent être utilisées pour évaluer la contribution desastrocytes à la plasticité synaptique et j’ai identifié de nombreuses protéinesastrocytaires régulées par la plasticité synaptique et qui sont susceptibles d’êtreciblées lors de prochaines études destinées à identifier les mécanismes d’action desastrocytes dans un contexte physiologique et pathologique. / Astrocytes are the most abundant cell type in the brain and mediate a myriad offunctions, including neurogenesis, ion homeostasis, metabolic support, clearance oftoxic substances and responses to brain injuries. Alterations in astrocyte functionhave been linked with neurological disorders such as epilepsy, depression, dementiaand schizophrenia, and thus the continued study of astrocytic contributions tosynaptic function are of clinical and societal relevance. In this thesis I have evaluatedthe potential utility of several preparations for the assessment of astrocyte proteinsinvolved in the regulation of synaptic plasticity, and employed the most suitable ofthese preparations to measure regulation in astrocyte protein levels in models ofsynaptic plasticity. I have characterized several preparations that can be used toevaluate astrocyte contributions to synaptic plasticity and identified numerousastrocyte-enriched proteins regulated by synaptic plasticity that can be targeted infuture studies to elaborate upon the mechanisms of action of astrocytes in bothphysiological and pathological contexts.

Ph.D.

published_or_final_version / Medical Sciences / Master / Master of Medical Sciences

published_or_final_version / Molecular Biology / Master / Master of Philosophy

Diss. (sammanfattning) Göteborg : Göteborgs universitet, 2008. / Härtill 4 uppsatser.