La perte de mémoire est un des premiers symptômes caractéristiques de la maladie d’Alzheimer (MA). Dans les cerveaux des patients, on retrouve des dépôts extracellulaires de plaques amyloïdes ainsi que des agrégats intracellulaires de la protéine tau. Les patients présentent également des déficits du métabolisme cérébral du glucose, une quinzaine d’années avant les premiers défauts cognitifs, suggérant que le métabolisme pourrait contribuer à la physiopathologie de la MA. Pour mieux comprendre les mécanismes qui relient le métabolisme énergétique et l’activité synaptique, nous nous sommes intéressés à la production de L-serine, une molécule dont la synthèse de novo dérive d’un intermédiaire de la glycolyse. La L-serine est le précurseur de la D-sérine, un acide aminé en conformation D présent en grande quantité dans le cerveau. La D-sérine est un co-agoniste des récepteurs au N-méthyl-D-aspartate (NMDA-R) nécessaire à la potentialisation à long terme (LTP) de l’activité synaptique dans l’hippocampe. La voie de biosynthèse de la L-serine est ainsi à l’interface entre métabolisme énergétique et activité synaptique. Afin d’étudier le rôle de cette voie, nous avons mis au point un modèle de délétion conditionnelle de la Phgdh, la première enzyme de la voie de biosynthèse de la L-serine. Nous avons injecté par stéréotaxie des vecteurs adéno-associés permettant l’expression de la Cre recombinase dans l’hippocampe de souris Phgdh(flox/flox), une lignée de souris qui possède des sites LoxP autour des exons 4 et 5 du gène de la Phgdh. Nous avons validé ce modèle en montrant que l’expression de Phgdh ainsi que les taux de D-serine diminuent d’environ 60% dans l’hippocampe des souris injectées. Nous avons ensuite réalisé des enregistrements électrophysiologiques sur tranches et nous avons mis en évidence une diminution de la LTP dans l’hippocampe des souris injectées avec la Cre recombinase. Ces souris présentent également un déficit de mémoire à long terme mis en évidence avec le test de la piscine de Morris. Ces déficits sont restaurés lorsque les souris reçoivent chroniquement un régime enrichi en L-serine. Ces résultats montrent que la biosynthèse de sérine est nécessaire et suffisante pour la plasticité synaptique et la mémoire à long terme.Afin d’étudier le rôle de cette voie dans la MA, nous avons mesuré l’expression de différentes enzymes dans des extraits d’hippocampes de patients atteints de MA et nous avons mis en évidence des changements significatifs dès les stades intermédiaires. Finalement, nous avons étudié un modèle murin de MA, les souris 3xTg, qui présentent des déficits métaboliques, synaptiques et comportementaux. Les déficits de LTP sont restaurés en ajoutant de la L- ou de la D-sérine de façon aigue sur les tranches d’hippocampe. Nous montrons que le déficit de mémoire spatiale à long terme peut être restauré par une supplémentation chronique en D-sérine, suggérant l’importance de cette voie dans le contexte de la MA. / Memory loss is among the first symptoms reported by patients suffering from Alzheimer’s disease (AD). AD is characterized by extracellular amyloid plaques and intracellular aggregations of tau. A decrease of brain glucose metabolism has also been described in the brain of AD patients. Since this decrease appears decades before memory loss, we hypothesize that metabolic deficits could directly contribute to AD physiopathology. To understand the mechanisms linking brain metabolism and synaptic activity, we proposed to study the production of L-serine, a signaling molecule whose de novo synthesis diverts part of the glycolytic flux. L-serine is the precursor of D-serine, a co-agonist of N-methyl-D-aspartate receptors (NMDA-R) that is required to maintain long term potentiation (LTP) of synaptic activity in the hippocampus. Since both L- and D-serine are formed through the activity of the Phosphorylated Pathway that diverts part of the glycolytic flux, any metabolic deficits may impact synaptic activity.We developed a model of conditional Phgdh deletion, the first enzyme of the phosphorylated pathway, by stereotaxically injecting Adeno-Associated Vectors allowing the expression of Cre recombinase in the hippocampus of Phgdh(flox/flox) mice, a mice strain with loxP sites flanking exons 4 and 5 of Phgdh gene. We validated this model showing that Phgdh expression and D-serine level are decreased by 60% in the hippocampus of injected mice. We performed electrophysiological recordings and showed that LTP is significantly reduced in mice injected with Cre recombinase. Those mice also show long term memory deficits in the Morris Water Maze test. Those deficits are restored by chronically feeding Cre injected mice with a diet enriched in L-serine indicating that serine biosynthesis is necessary and sufficient for synaptic plasticity and long term memory.To assess whether this pathway may be involved in AD pathogenesis, we quantified the expression of several enzymes of the serine biosynthesis pathway in human brain samples and found major changes in AD patients even at intermediate stages. To further investigate this hypothesis, we used 3xTg-AD mice, a mouse model for AD showing deficits in brain metabolism, synaptic activity and cognition. LTP deficits in 3xTg mice are restored by acute supplementation of L- or D-serine on hippocampal slices. We show that chronic administration of D-serine restores long term spatial memory. It suggests that serine biosynthesis is an important pathway in AD.

Les Tauopathies sont un ensemble de maladies neurodégénératives caractérisées par l’agrégation de la protéine Tau dans les neurones, astrocytes et autres types cellulaires. Toutefois, les mécanismes sous-tendant la présence de Tau dans les astrocytes et les conséquences de Tau sur les astrocytes restent peu connus. L’objectif de ce projet a été d’étudier l’interaction entre les neurones porteurs de Tau soluble et/ou agrégée et les astrocytes avoisinants. Nous avons généré trois modèles de Tauopathie par transfert de gènes et avons caractérisé la pathologie Tau neuronale et astrocytaire par histologie et biologie moléculaire. Dans l’hippocampe, la surexpression de l’isoforme mutante hTAUP301L ou d’une forme pro-agrégeante hTAUProAggr, mais pas celle de la protéine sauvage hTAUWT, a entraîné la formation progressive d’agrégats aussi bien dans les neurones que dans les astrocytes. Nous avons montré que les inclusions astrocytaires de Tau étaient secondaires à une pathologie neuronale par différents schémas expérimentaux. En utilisant des vecteurs spécifiques de types cellulaires, nous avons également démontré le transfert bi-directionnel d’espèces de Tau entre neurones et astrocytes. Curieusement, nous avons observé une perte astrocytaire uniquement dans le subiculum du groupe hTAUWT qui ne présente pas d’inclusions astrocytaires de Tau. Nos données ont montré que, dans nos modèles, la tauopathie astrocytaire est secondaire à la présence de dégénérescences neurofibrillaires et ne résulte pas d’une agrégation liée à une surexpression réactionnelle de Tau endogène dans l’astrocyte. De plus, des amorces de Tau neuronales peuvent promouvoir l’agrégation de Tau astrocytaire et la Tau astrocytaire peut être transférée aux neurones. Par ailleurs, si les agrégats de Tau paraissent plutôt bénins pour les astrocytes, les conséquences fonctionnelles d’une telle astrogliopathie restent encore à évaluer. / Tauopathies are neurodegenerative diseases characterized by the aggregation of Tau protein in neurons, astrocytes and other cell types. However, the mechanisms leading to the presence of Tau aggregates in astrocytes and the consequences of Tau on astrocytes are poorly understood. The aim of this project was to study the relationship between neurons bearing soluble and/or aggregated Tau species and their neighboring astrocytes.We thus generated three gene transfer-based Tauopathy models and used immunohistological and molecular biology methods to characterize Tau pathology in neurons and astrocytes.In the hippocampus, overexpression of mutant hTAUP301L or that of a pro-aggregating variant hTAUProAggr, but not that of wild-type Tau hTAUWT, led to a gradual increase in the formation of aggregates not only in neurons but also in astrocytes. Using different experimental paradigms, we showed that astrocytic Tau was secondary to neuronal pathology. Using cell type-specific AAV- Tau vectors, we further demonstrated the bi-directional transfer of Tau species between neurons and astrocytes. Interestingly, we observed astrocyte loss in the subiculum only in the hTauWT group in the absence of any astrocytic Tau inclusions.Our data show that astroglial tauopathy is secondary to the presence of neurofibrillary tangles in our models and does not result from aggregation of overexpressed endogenous Tau in astrocytes. In addition, neuronal Tau seeds can promote the aggregation of astrocytic Tau and astrocytic Tau can be transferred to neurons. Furthermore, if Tau aggregates appear fairly innocuous for astrocytes, the functional consequences of such astroglial tauopathy still remain to be further assessed.

Cerebral ischemia is a highly debilitating condition where shortage of oxygen and glucose leads to profuse cell death. Insufficient blood supply to the brain leads to cerebral ischemia and increase in extracellular lactate concentrations. Rise in lactate concentration and the leak potassium channel TREK1 have been independently associated with cerebral ischemia. Lactate is a neuroprotective metabolite whose concentrations increase to 15-30 mM during ischemia and TREK1 is a neuroprotective potassium channel which is upregulated during ischemia. Recent literature suggests lactate to be neuroprotective and TREK1 knockout mice show an increased sensitivity to brain and spinal cord ischemia, however the connecting link between the two is missing. We hypothesized that lactate might interact with TREK1 channels and mediate neuroprotection. The aim of this study was to investigate the effect of lactate on activity and expression of TREK1 channels and evaluate the role of lactate-TREK1 interaction in conferring neuroprotection in the ischemia-prone hippocampus Ischemic concentrations (15-30 mM) of lactate at pH 7.4 increased whole cell TREK1 current in CA1 stratum radiator astrocytes and caused membrane hyperpolarization. We confirmed the intracellular action of lactate on TREK1 in hippocampal slices using mono carboxylate transporter blockers. The intracellular effect of lactate on TREK1 channels is specific since other mono carboxylates such as pyruvate at pH 7.4 failed to increase TREK1 current. We used immunostaining, western blot and electrophysiology to show that 15-30 mM of lactate increased functional TREK1 protein expression by 1.5-3 fold in hippocampal astrocytes. Next, we performed quantitative PCR to investigate if the increase in TREK1 protein expression was due to increased transcription and found that lactate stimulated TREK1 mRNA transcription to increase TREK1 protein. Lactate mediated increase in TREK1 expression was dependent on protein kinase A as inhibitors of protein kinase A abolished the increase in TREK1 mRNA and protein. The role of lactate-TREK1 interaction in neuroprotection was subsequently investigated using an in vitro oxygen glucose deprivation model of ischemia. Addition of 30 mM lactate to oxygen glucose deprived slices reduced neuronal death in the hippocampal CA1 pyramidal layer. However, 30 mM lactate failed to reduce cell death in rat hippocampal slices treated with TREK1 channel blockers signifying the requirement of active TREK1 channels for lactate mediated neuroprotection. However, lactate in the presence of protein kinase inhibitor failed to reduce cell death. This might be related to the role of protein kinase A in upregulation of TREK1 channels. We also estimated CA1 pyramidal neuronal TREK1 channel expression and found both lactate and oxygen glucose deprivation to decrease TREK1 channel expression that was surprisingly opposite to the effects on astrocytes. As TREK1 channel activation and upregulation decreases neuronal excitability, a decrease in neuronal TREK1 channel expression in response to lactate is expected to cause higher neuronal death and fails to explain lactate mediated neuroprotection. Since, lactate upregulated TREK1 channel expression and functional activity in CA1 stratum radiate astrocytes, we reasoned that the lactate mediated neuroprotection might be via astrocytic TREK1 channels requiring viable functional astrocytes. This was tested by disrupting astrocyte function using gliotoxin, and estimating cell death in oxygen glucose deprived hippocampal slices. Lactate failed to reduce cell death in presence of gliotoxin signifying the importance of viable astrocytes for lactate mediated neuroprotection. The above effects were specific to lactate as pyruvate failed to increase TREK1 expression and reduce cell death. TREK1 channels contribute to neuroprotection by enhancing potassium buffering and glutamate clearance capacity of astrocytes. We propose that lactate promotes neuronal survival in hippocampus by increasing TREK1 channel expression and activity in astrocytes during ischemia. This pathway serves as an alternate mechanism of neuroprotection.

published_or_final_version / Anatomy / Doctoral / Doctor of Philosophy

Oxidative stress is a feature of many chronic neurodegenerative diseases as well as a contributing factor in acute disorders including stroke. Fork head class of transcription factors (Foxos) play a key role in promoting oxidative stress-induced apoptosis in neurons through the upregulation of a number of pro-apoptotic genes. Here I demonstrate that synaptic NMDA receptor activity not only promotes Foxos nuclear exclusion but also suppresses the expression of Foxo1 in a PI3K-dependent fashion. I also found that Foxo1 is in fact, a Foxo target gene and that it is subject to a feed-forward inhibition by synaptic activity, which is thought to result in longerterm suppression of Foxo downstream gene expression than previously thought. The nuclear factor (erythroid 2-related) factor 2 (Nrf2) is another transcription factor involved in oxidative stress and the key regulator of many genes, whose products form important intrinsic antioxidant systems. In the CNS, artificial activation of Nrf2 in astrocytes has been shown to protect nearby neurons from oxidative insults. However, the extent to which Nrf2 in astrocytes could respond to endogenous signals such as mild oxidative stress is less clear. The data presented herein, demonstrate for the first time that endogenous Nrf2 could be activated by mild oxidative stress and that this activation is restricted to astrocytes. Contrary to the established dogma, I found that mild oxidative stress induces the astrocytic Nrf2 pathway in a manner distinct from the classical Keap1 antagonism employed by prototypical Nrf2 inducers. The mechanism was found to involve direct regulation of Nrf2's transactivation properties. Overall these results advance our knowledge of the molecular mechanism(s) associated with the control of endogenous antioxidant defences by physiological signals.

Stroke or cerebral vascular accident (CVA) is the 4th leading cause of mortality and the principle cause of long-term disability in the United States. Unfortunately, current reperfusion-based treatments (e.g. thrombolysis, tPA) cannot be administered to the majority of patients presenting with ischemic stroke. Accordingly, new treatments for ischemic stroke are desperately needed. Reactive astrocytes perform key roles in tissue repair and remodeling following stroke such as preservation and repair of the blood-brain barrier, modulation of immune cell invasion, glutamate uptake and neuroprotection, and glial scar formation. The proliferative subpopulation of reactive astrocytes found immediately adjacent to the infarct core after stroke (known as the peri-infarct area) is particularly important for protecting the brain parenchyma from ischemic damage and inflammation. Defining the signaling network that controls reactive astrocyte formation and function has potential to provide new treatment strategies for patients ineligible for reperfusion therapy. Notch1 signaling is required for the proliferation of peri-infarct reactive astrocytes after stroke. To identify downstream targets and potential functional effectors of Notch1 signaling in reactive astrocytes, we developed an ex vivo forward signaling screen. To generate large quantities of adult reactive astrocytes, we employed adult Reactive astrocyte-derived Neural Stem Cells (Rad-NSCs) isolated from the peri-infarct area of mice after stroke. Astrocytes re-differentiated from Rad-NSCs (AstroRad-NSC) were then exposed to immobilized Jagged-1, a Notch1 ligand. In response to Jagged-1, many genes involved in reactive astrocyte-mediated tissue protection, metabolic regulation, angiogenesis and glial scar formation were up-regulated. Of special interest, several genes for proteins that regulate with glutamate uptake and metabolism were increased by Jagged-1/Notch signaling, including the glial-specific GLutamate-ASpartate Transporter (GLAST). With loss-of-function experiments, we determined that deletion of Notch1 decreased GLAST transcript and protein levels in cultured AstroRad-NSC. Furthermore, we isolated reactive astrocytes directly from cerebral cortex after stroke and confirmed the effects of Notch1 on GLAST in vivo. Our results suggest that treatments designed to stimulate Notch1 signaling after stroke may promote glutamate uptake, thereby decreasing excitotoxicity and neuronal cell death. Binding of Endothelin peptides to the type B Endothelin receptor (ETBR) has been shown to alter cell proliferation. Investigating a possible relationship between Jagged-1/Notch1 and Endothelin signaling in reactive astrocytes, we determined that Notch1 signaling regulated ETBR indirectly, by activating STAT3, an unidentified transcriptional activator of ETBR. Using inducible transgenic astrocyte-specific conditional knockout (cKO) mice (GFAP-ETBR-cKO), we found that specific deletion of ETBR in reactive astrocytes phenocopied the defect in reactive astrocyte proliferation observed in our previous work with GFAP-Notch1-cKO mice. Notably, the Notch1-STAT3-ETBR axis we identified is likely to control reactive astrocyte proliferation in most, if not all, forms of CNS injury. The experimental results presented in this doctoral dissertation provide novel insight into signaling mechanisms that may someday be exploited to improve care for patients with stroke and other forms of CNS injury or disease.

Mémoire numérisé par la Division de la gestion de documents et des archives de l'Université de Montréal.

Sterile inflammation is a key component of a variety of diseases including, gout, arthritis, type 1 diabetes, Alzheimer’s disease and multiple sclerosis (MS). Sterile inflammation induces the recruitment of immune cells via chemokines, such as CCL5 and CXCL10. Expression of these chemokines is dependent on IRF-1. Recently the FDA has approved the use of a pro-drug, FTY720 that after phosphorylation becomes a S1P mimetic for the treatment of MS. This report describes two novel and opposing mechanisms of S1P action in sterile inflammation. First, intracellular S1P acts as a cofactor of cIAP2 that inducesIL-1-dependent K63-polyubiquitination of IRF-1, which leads to the recruitment of immune cells to the site of inflammation. Conversely, extracellular S1P provides a feedback loop that inhibits CXCL10 and CCL5 expression through S1PR2 signaling. Accordingly, immune cell infiltration to sites of sterile inflammation is increased in S1PR2-/- production via calcium-dependent, but cAMP- and PKA-independent mechanisms that likely involve c-Fos expression and unconventional PKC activation. Elevated c-Fos could competitively inhibit CCL5 expression directly or indirectly via blocking IFN production. These two novel pathways highlight unexpected aspects of S1P signaling, and provide potential mechanisms that can be exploited for the improvement of therapeutics for the treatment of MS.

No description available.

Joie Jie Wang. / Thesis (M.Phil.)--Chinese University of Hong Kong, 1993. / Includes bibliographical references (leaves 121-132). / ABSTRACT --- p.i / ACKNOWLEDGEMENTS --- p.iii / LIST OF TABLE --- p.vii / LIST OF FIGURES --- p.viii / LIST OF ABBREVIATIONS --- p.xii / Chapter CHAPTER I. --- INTRODUCTION --- p.1 / Chapter I.1. --- Fibre tracts of the rat spinal cord --- p.1 / Chapter I.2. --- Histopathological responses to spinal cord injuries --- p.2 / Chapter I.3. --- Failure of CNS regeneration --- p.4 / Intrinsic inability of CNS neurons themselves to regenerate --- p.4 / Inappropriate synapse without normal functioning --- p.5 / Progressive necrosis and cystic cavities --- p.5 / Autoimmune explanation for the failure of regeneration --- p.6 / Glial scarring --- p.6 / Absence of Schwann cells in the CNS --- p.7 / Lack of requisite growth factors --- p.8 / Chapter I.4. --- The use of transplants --- p.9 / Transplants of fetal nerve tissues --- p.9 / Transplants of peripheral nerve tissues --- p.10 / Transplants of neuroglial cells --- p.11 / Transplants of central neurons --- p.12 / Chapter I.5. --- Objectives of the present study --- p.13 / Chapter CHAPTER II. --- METERIALS AND METHODS --- p.15 / Chapter II.1. --- Hemisection of rats --- p.15 / Chapter II.2. --- Preparation of purified cortical astrocytes --- p.15 / Chapter II.3. --- Scanning electron microscopy (SEM) --- p.18 / Chapter II.4. --- HistologýؤLight microscopy --- p.19 / Chapter II.5. --- Measurement of volume of scar tissue --- p.19 / Chapter II.6. --- Immunofluorescence staining --- p.20 / Chapter II.7. --- Transmission electron microscopy --- p.23 / Chapter II.8. --- Comparison of expression of various proteins in the spinal cord --- p.24 / Polyacrylainide gel electrophoresis --- p.24 / Western blotting --- p.26 / Chapter CHAPTER III. --- RESULTS --- p.28 / Chapter III.1. --- Survival of cultured astrocytes --- p.28 / Chapter III.2. --- Light microscopy --- p.28 / Hemotoxylin and Eosin staining --- p.28 / Toluidine Blue staining --- p.30 / PHAL labelled astrocytes --- p.31 / Immunofluorescence staining --- p.32 / N-CAM --- p.32 / GFAP --- p.33 / NF --- p.34 / Chapter III.3. --- Transmission electron microscopy (TEM) --- p.35 / Chapter III.4. --- Determination of the volume of scar tissue --- p.37 / Chapter III.5. --- Gel electrophoresis --- p.38 / Chapter III.6. --- Immunoblotting and densitometry --- p.38 / Chapter CHAPTER IV. --- DISCUSSION AND CONCLUSIONS --- p.115 / REFERENCES --- p.121