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Les récepteurs du glutamate de type AMPA (rAMPA) sont les récepteurs ionotroniques responsables de la majeure partie des courants excitateurs rapides lors de la transmission synaptique dans le système nerveux central. Le nombre de rAMPA stabilisés à la synapse est responsable en partie de l’intensité de la transmission synaptique et de nombreux phénomènes de plasticité synaptique. Les rAMPA se répartissent en trois populations en équilibre dynamique: les récepteurs intracellulaires, les récepteurs extra-synaptiques, et les récepteurs synaptiques stabilisés au niveau de la densité post-synaptique. L’implication des protéines transmembranaires régulatrices des rAMPA (TARP) dans la stabilisation des rAMPA est établie, et repose au moins en partie sur la liaison de la protéine TARP γ-2 avec la protéine d’échafaudage PSD-95. Dans l’hippocampe, siège de nombreux phénomènes de plasticité, l’isoforme γ-8 est particulièrement enrichie. La TARP γ-8 a pour particularité de posséder un domaine C-terminal plus long que son homologue γ-2 et de s’exprimer au niveau synaptique et extra-synaptique. Mon travail de thèse à consisté à étudier les mécanismes moléculaires mis en jeu dans la régulation de la liaison des protéines TARP γ-2 et γ-8 avec la protéine PSD-95, ainsi que l’implication respective des deux isoformes dans la régulation de la mobilité latérale des rAMPA. Les résultats majeurs de cette étude sont : a) l’interaction entre γ-2 et PSD-95 est régulée par la longueur apparent du domaine C-terminal de γ-2 modulée par la phosphorylation; b) γ-8 lie PSD-95 dans les compartiments synaptiques et extra-synaptiques, toutefois cette interaction n’est pas corrélée avec une immobilisation des rAMPA. Ces résultats suggèrent que γ-2 et γ-8 jouent des rôles bien distincts dans l’adressage des rAMPA à la synapse. / AMPA type glutamate receptors (AMPARs) are ionotropic receptors responsible for most excitatory transmission in the central nervous system. The number of stabilized AMPARs in front of glutamate release sites determines in large part the strength of synaptic transmission and variation in this number is thought to underlie numerous forms of synaptic plasticity. AMPARs are present in three main subcellular pools between which they are in a dynamic equilibrium by processes of trafficking: intracellular receptors, extrasynaptic receptors, and synaptic receptors stabilized at the postsynaptic density (PSD). Transmembrane AMPAR regulatory proteins (TARPs) are known to be implicated in AMPAR stabilization at the synapse through the interaction of TARP γ-2/8 with the scaffolding protein PSD-95. In the hippocampus, a structure exhibiting various synaptic plasticity patterns, γ-8 is the most abundant TARP. This isoform is characterized by a longer C-terminal fragment than γ-2 and a synaptic and extrasynaptic localization. During my Ph.D, I studied the molecular mechanisms involved in the regulation of TARP γ-2 and γ-8 binding to PSD-95 and their respective roles in regulating AMPAR lateral mobility. The main results are: a) γ-2 interaction with PSD-95 is regulated by the apparent length of its C-terminus domain that is modulated by phosphorylation; b) γ-8 binds PSD-95 in synaptic and extrasynaptic compartment however this interaction is not correlated with AMPAR immobilization. Altogether, those results suggest that those two TARP isoforms have independent functional roles.

Mammalian neurons rapidly undergo excitotoxic cell death during anoxia, while neurons from the anoxia-tolerant painted turtle can survive without oxygen for hours without apparent damage. An anoxia-mediated decrease in AMPA receptor currents are an important part of the turtle’s natural defence however the mechanism underlying it is unknown. Here I investigate a mechanism that involves activation of a mitochondrial KATP channel that subsequently signals a decrease in AMPAR currents. Whole-cell AMPAR currents were stable during normoxia, but anoxia or pharmacological activation of mKATP channels resulted in a 50% decrease in AMPAR currents. Conversely, mKATP antagonists blocked the anoxia-mediated decrease. Mitochondrial KCa channel modulators responded similarly. Blocking the Ca2+-uniporter also reduced normoxic AMPAR currents by 40%, and including BAPTA in the recording abolished the anoxia or agonist-mediated decrease. Therefore, the mKATP channel is involved in the anoxia-mediated down-regulation of AMPAR activity and is a common mechanism to reduce glutamatergic excitability.

Mammalian neurons rapidly undergo excitotoxic cell death during anoxia, while neurons from the anoxia-tolerant painted turtle can survive without oxygen for hours without apparent damage. An anoxia-mediated decrease in AMPA receptor currents are an important part of the turtle’s natural defence however the mechanism underlying it is unknown. Here I investigate a mechanism that involves activation of a mitochondrial KATP channel that subsequently signals a decrease in AMPAR currents. Whole-cell AMPAR currents were stable during normoxia, but anoxia or pharmacological activation of mKATP channels resulted in a 50% decrease in AMPAR currents. Conversely, mKATP antagonists blocked the anoxia-mediated decrease. Mitochondrial KCa channel modulators responded similarly. Blocking the Ca2+-uniporter also reduced normoxic AMPAR currents by 40%, and including BAPTA in the recording abolished the anoxia or agonist-mediated decrease. Therefore, the mKATP channel is involved in the anoxia-mediated down-regulation of AMPAR activity and is a common mechanism to reduce glutamatergic excitability.

Disruptions in memory are a hallmark feature of several psychiatric diseases. These illnesses are often marred by an inability to recognize that a stimulus or event as been previously experienced, a phenomenon known as recognition memory. Previous study has demonstrated that cognitive disruptions reflect aberrant signaling, including disruptions in synaptic plasticity, in key regions of the brain, such as prefrontal cortex (PFC), hippocampus, and perirhinal cortex (PRh). However, in the case of recognition memory, how these disruptions arise and what specific plasticity mechanisms are involved is less clear. An understanding of the etiological factors underlying disruption and the synaptic processes involved in recognition will greatly advance the treatment and prevention of psychiatric disorders. As a result, the present thesis examined recognition memory in rodents in two experiments. In the first experiment, we blocked the endocytosis of AMPA receptors during the encoding, consolidation, or retrieval phase of object recognition memory using local PRh infusions of the cell membrane permeable Tat-GluA23Y interference peptide. Tat-GluA23Y infusion before the encoding and consolidation phases did not alter memory. In contrast, Tat-GluA23Y infusion prior to the retrieval phase significantly disrupted memory. These results indicate a distinct role for AMPA receptor endocytosis during a specific phase (retrieval) of visual recognition memory. In the second experiment, pregnant dams were treated with PolyI:C (4mg/kg, i.v.) on gestational day (GD) 15, and both the male and female offspring of these rats were tested as young adults in three different recognition memory tests: spontaneous novel object recognition, novel object location recognition, and object-in-place recognition. Male, but not female, rats were impaired in an object-in-place memory test that depends on processing between medial temporal lobe and PFC. However, neither male nor female rats were impaired on tests of simpler discriminations dependent on the medial temporal lobe. These findings support clinical studies demonstrating impaired object location binding in clinical populations and further demonstrate the plausibility of prenatal immune activation as an etiological factor in neurodevelopmental disease. Taken together, these results highlight the importance of a specific form of synaptic plasticity during the recognition of familiar stimuli and demonstrate that early life adversity can disrupt recognition memory processes.

Disruptions in memory are a hallmark feature of several psychiatric diseases. These illnesses are often marred by an inability to recognize that a stimulus or event as been previously experienced, a phenomenon known as recognition memory. Previous study has demonstrated that cognitive disruptions reflect aberrant signaling, including disruptions in synaptic plasticity, in key regions of the brain, such as prefrontal cortex (PFC), hippocampus, and perirhinal cortex (PRh). However, in the case of recognition memory, how these disruptions arise and what specific plasticity mechanisms are involved is less clear. An understanding of the etiological factors underlying disruption and the synaptic processes involved in recognition will greatly advance the treatment and prevention of psychiatric disorders. As a result, the present thesis examined recognition memory in rodents in two experiments. In the first experiment, we blocked the endocytosis of AMPA receptors during the encoding, consolidation, or retrieval phase of object recognition memory using local PRh infusions of the cell membrane permeable Tat-GluA23Y interference peptide. Tat-GluA23Y infusion before the encoding and consolidation phases did not alter memory. In contrast, Tat-GluA23Y infusion prior to the retrieval phase significantly disrupted memory. These results indicate a distinct role for AMPA receptor endocytosis during a specific phase (retrieval) of visual recognition memory. In the second experiment, pregnant dams were treated with PolyI:C (4mg/kg, i.v.) on gestational day (GD) 15, and both the male and female offspring of these rats were tested as young adults in three different recognition memory tests: spontaneous novel object recognition, novel object location recognition, and object-in-place recognition. Male, but not female, rats were impaired in an object-in-place memory test that depends on processing between medial temporal lobe and PFC. However, neither male nor female rats were impaired on tests of simpler discriminations dependent on the medial temporal lobe. These findings support clinical studies demonstrating impaired object location binding in clinical populations and further demonstrate the plausibility of prenatal immune activation as an etiological factor in neurodevelopmental disease. Taken together, these results highlight the importance of a specific form of synaptic plasticity during the recognition of familiar stimuli and demonstrate that early life adversity can disrupt recognition memory processes.

The α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs) are ligand-gated glutamatergic ion channels that mediate most excitatory neurotransmission in the brain. Alterations in AMPAR synaptic accumulation mediate synaptic plasticity, including long-term potentiation, long-term depression and homeostatic synaptic plasticity. AMPAR abundance in neurons is determined by balanced processes of protein translation and degradation. Changes in AMPAR function and trafficking have direct impacts on synaptic transmission and cognitive functions. However, the molecular mechanisms regulating AMPAR expression and dynamics in neurons remain largely unknown. In this thesis, two molecular mechanisms that regulate AMPAR translation and protein stability through two different signaling pathways, 5' adenosine monophosphate-activated protein kinase (AMPK) and sirtuin 2 (SIRT2), are described. It is shown that SIRT2, a NAD+-dependent protein deacetylase, directly controls AMPAR stability by regulating AMPAR acetylation. For the first time, we discovered that AMPARs are subject to lysine acetylation, a novel form of post-translational modification for glutamate receptors. Under basal conditions, AMPARs are highly acetylated at their intracellular C termini, which protects against ubiquitination to antagonize AMPAR endocytosis and degradation, leading to prolonged receptor half-life. SIRT2 is also identified as the enzyme responsible for AMPAR deacetylation. Knockdown of SIRT2 led to elevated AMPAR acetylation and reduced ubiquitination, and consequently, increased AMPAR levels and synaptic transmission. SIRT2 knockout mice displayed weakened synaptic plasticity and impaired learning and memory. Resveratrol is a phytoalexin that has been shown to increase AMPAR expression and synaptic accumulation in neurons. The resveratrol effect on AMPAR expression is independent of sirtuin 1, the conventional target of resveratrol, but rather is mediated by AMPK and its downstream phosphoinositide 3-kinase (PI3K)/Akt pathway. Application of the AMPK activator, 5-aminoimidazole-4-carboxamide 1-β-D-ribofuranoside (AICAR), to neurons mimics the effects of resveratrol on both signaling and AMPAR expression. The resveratrol-induced increase in AMPAR expression results from elevated protein synthesis through the AMPK-PI3K pathway activation. These studies describe novel regulatory mechanisms responsible for the control of AMPAR protein amount and subcellular distribution in neurons, providing insights into our understanding of synaptic plasticity, brain function and neurological disorders. / 2017-12-06T00:00:00Z

No description available.

No description available.