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Alzheimer’s disease (AD) is a neurodegenerative disorder characterized by two major pathological hallmarks, amyloid plaques and neurofibrillary tangles. The accumulation of amyloid-β protein (Aβ) is an early event associated with synaptic and mitochondrial damage in AD. Therefore, molecular pathways underlying the neurotoxicity and generation of Aβ represent promising therapeutic targets for AD. Recent studies have shown that actin severing protein, Cofilin plays an important role in synaptic remodeling, mitochondrial dysfunction, and AD pathogenesis. However, whether Cofilin is an essential component of AD pathogenesis and how Aβ induced neurotoxicity impinges its signals to Cofilin are unclear. In my dissertation studies, we found Aβ oligomers bind with intermediate activation conformers of β1-integrin to induce the loss of surface β1-integrin and activation of Cofilin via Slingshot homology-1 (SSH1) activation. Specifically, conditional loss of β1-integrin prevented Aβ induced Cofilin activation, and allosteric modulation or activation of β1-integrin significantly reduced Aβ binding to neurons and mitigated Aβ42-induced reactive oxygen species (ROS) generation, mitochondrial dysfunction, synaptic proteins depletion, and apoptosis. Furthermore, we found that SSH1 reduction, which mitigated Cofilin activation, prevented Aβ-induced mitochondrial Cofilin translocation and apoptosis, while AD brain mitochondria contained significantly increased activated/oxidized Cofilin. In mechanistic support in vivo, we demonstrated that APP transgenic mice brains contain decreased SSH/Cofilin and SSH1/14-3-3 complexes which indicates that SSH-Cofilin activation occurred by releasing of SSH from 14-3-3. We also showed that genetic reduction in Cofilin rescues APP/Aβ-induced synaptic protein loss and gliosis, as well as impairments in synaptic plasticity and contextual memory in vivo. Our lab previously found that overexpression of the scaffolding protein RanBP9 increases Aβ production in cell lines and in transgenic mice, while promoting Cofilin activation and mitochondrial dysfunction. However, how endogenous RanBP9 activates cofilin and whether endogenous RanBP9 accelerates Aβ-induced deficits in synaptic plasticity, cofilin-dependent pathology, and cognitive impairments were unknown. In my dissertation studies, we found that endogenous RanBP9 positively regulates SSH1 levels and mediates A-induced translocation of Cofilin to mitochondria. Moreover, we demonstrated that endogenous RanBP9 mediates A-induced formation of Cofilin-actin rods in primary neurons. Endogenous level of RanBP9 was also required for Aβ-induced collapse of growth cones in immature neurons and depletion of synaptic proteins in mature neurons. In vivo, we also found APP transgenic mice exhibit significantly increased endogenous RanBP9 levels and that genetic reduction in RanBP9 rescued APP/Aβ-induced synaptic protein loss, gliosis, synaptic plasticity impairments, and contextual memory deficits. These findings indicated that endogenous RanBP9 not only promotes Aβ production but also meditate Aβ induced neurotoxicity via positively regulating SSH1. Taken together, these novel findings implicate essential involvement of β1-integrin–SSH1/RanBP9–Cofilin pathway in mitochondrial and synaptic dysfunction in AD pathogenesis.

Alzheimer’s disease is a debilitating, progressive neurodegenerative disorder in the elderly, characterized by severe loss of memory and higher cognitive functions. In the hundred years since its discovery, Alzheimer’s disease (AD) has traversed from the status of a ‘rare neurological oddity’ to one of the greatest challenges faced by healthcare and medicine in this millennium. A reported 44 million people currently suffer from AD but only 1 in 4 people have been diagnosed. Although AD has been an area of intense research for almost 50 years now, most studies have focused on the end stage disease. Years of study on the pathological cause underlying AD; have conclusively shown that the accumulation of the sticky peptide, Aβ, is one of the major triggers of AD pathogenesis. However, after the initial Aβ trigger, multiple processes contribute to disease progression, so that by the time a patient is diagnosed on the basis of overt behavioral phenotypes, it is difficult to understand and differentiate between the causative mechanisms and the consequential effects of the disease. It is, perhaps, because of this, that we are still struggling to find therapies for AD which will stop or at the very least slow the course of the disease. In the 2015 report on AD, issued by the Alzheimer’s association, much emphasis has been placed on the early diagnosis of AD and the revision of the diagnostic criteria for AD. According to the new guidelines proposed in 2011, AD has been divided into three stages where the first stage occurs before the appearance of overt behavioral symptoms such as memory loss, whereas by the 1984 guidelines, cognitive disabilities must have already occurred for diagnoses of AD. This proposed preclinical stage of AD has been defined, reflecting the current belief that AD pathogenesis begins almost 20 years before the occurrence of behavioral dysfunction. However, no diagnostic criteria are currently available to establish this stage. Hence, there is a need to understand the early pathogenic mechanisms of AD, which will yield early therapeutic targets as well as early diagnostic markers of AD. One of the earliest documented events in AD pathogenesis is synaptic dysfunction, which is later manifested as loss of dendritic spines. Deficits in long term potentiation (LTP) has been demonstrated in Aβ exposed hippocampal slices as well as in mouse models of AD, much before the appearance of pathological hallmarks such as plaques and tangles as well as overt behavioral phenotypes. While these and other studies indicate clearly that elevated levels of soluble Aβ peptide leads to impairment of synaptic function, the underlying molecular mechanisms are yet to be elucidated. One of the purported mediators of Aβ induced dysfunction is oxidative stress. The Aβ peptide, especially the Aβ42, is a self aggregating peptide with a propensity to form peptidyl radicals. Interaction of the peptidyl radicals with biomolecules leads to the generation of more free radical species via cascading chain reactions. Additionally, Aβ peptide has also been demonstrated to have synaptotoxic effects via its effect on NMDA receptors and calcium influx leading to deregulated reactive oxygen species (ROS) production as well as excitotoxicity. Hence, with a view to understanding Aβ mediated early synaptic dysfunction in AD, we studied early signaling changes in the synaptosomes derived from the cortex of APP/PS1 mice model of AD at various ages. The APP/PS1 model contains a mouse/human chimeric APP gene bearing the KM670/671NL Swedish mutation and the human PS1 gene with an exon 9 deletion. These mice exhibit behavioral deficits from 7 months of age while plaque deposition and gliosis become apparent by 9 months of age. We chose to study both pre-symptomatic ages (1 and 3 months old) as well as post symptomatic (9 months old) mice. Post nuclear supernatant (PNS) as well as synaptosomes were isolated from the cortex of APP/PS1 and age matched control mice. We assayed the levels of reactive oxygen species (ROS) in the PNS and the synaptosomes of post symptomatic 9 months old APP/PS1 mice and age matched controls. In contrast to reports of enhanced oxidative stress markers in the brains of AD patients, we did not find any increase in the levels of ROS in the PNS of post symptomatic APP/PS1 mice compared to age matched controls. However, synaptosomes from the cortex of these animals exhibited a significant increase in ROS levels in APP/PS1 mice compared to controls. We further found that there was significant increase in the ROS levels in synaptosomes, but not PNS, of very young asymptomatic 1 and 3 months old APP/PS1 mice. This is a first demonstration of synapse specific increase in oxidative stress in AD mice, as young as 1 month of age, indicating that disease specific mechanisms operate at the synapse much before the appearance of any overt cellular or behavioral symptoms. The increase in synaptic ROS levels correlated with a small but significant increase in the levels of Aβ42 in the brains of APP/PS1 mice compared to controls. We also found a concurrent change in the redox status of the cytoskeletal protein, actin, at the synapse. As early as 1 month of age, there was a significant decrease in the protein level of reduced actin indicating that there is an increase in the level of oxidized actin at the synapse. This loss of reduced actin was specific to the fibrillar pool of actin while no significant change was observed in the redox status of the monomeric globular pool of actin. Oxidation of actin has been demonstrated to lead to its depolymerization. Concurrently, we found a significant loss of fibrillar actin in the synaptosomes of APP/PS1 mice. Actin is the major cytoskeletal protein at the synapse. Changes in the globular to fibrillar actin ratio at the synapse at early pre-symptomatic ages in APP/PS1 mice will likely lead to structural and consequent functional changes at the synapse. This could potentially be one of the triggers of synaptic dysfunction in AD. Furthermore, changes in the Akt-mTOR signaling pathway was also observed in the synaptosomes of 1 month old APP/PS1 mice, which is sustained at 9 months. There was a significant loss of the mTOR-pS6K-4EBP1 axis in the synaptosomes, but not PNS, of APP/PS1 mice. We found that loss of Akt signaling, as evinced by loss of Akt phosphorylation, Akt kinase activity as well as loss of phosphorylation of downstream effector GSK3β, potentially underlies the loss of mTOR signaling. Further, the loss of Akt signaling is mediated by synapse specific redox modification of Akt and consequent interaction with the protein phosphatase PP2a. Loss of the Akt-mTOR signaling at the synapse is indicative of deficits in local protein translation. Loss of this essential synaptic function, which plays critical roles in synapse maintenance as well as synaptic plasticity during learning and memory, at an early age, will have long ranging impact on synaptic function such as long term potentiation (LTP) in APP/PS1 mice. Our study is the first demonstration of oxidative stress and consequent signaling changes which occur specifically at the synapse of very young 1 month old APP/PS1 mice. These changes occur much before the appearance of overt phenotype such as plaque deposition and behavioral dysfunction but sustain till the appearance of classical pathological hallmarks. Hence, the study demonstrates that disease progression starts much before previously thought and provides us a critical time window during which therapeutic strategies designed to delay or stop these changes might change the course of AD.

京都大学 / 0048 / 新制・課程博士 / 博士(医科学) / 甲第20811号 / 医科博第82号 / 新制||医科||6(附属図書館) / 京都大学大学院医学研究科医科学専攻 / (主査)教授 長船 健二, 教授 妻木 範行, 教授 村井 俊哉 / 学位規則第4条第1項該当 / Doctor of Medical Science / Kyoto University / DFAM

La maladie de Parkinson (PD) et la démence à corps de Lewy (DLB) sont les deuxième et troisième maladies neurodégénératives les plus communes et font partie d’une classe de maladies appelées synucléinopathies. Les synucléinopathies sont associées à une pathologie liée à l’α-synucléine (α-syn) laquelle se caractérise par une accumulation de cette protéine dans les neurones, formant ainsi les corps de Lewy. L’α-syn pathologique se retrouve aussi sous forme d’oligomères et de fibrilles, qui sont toxiques pour les neurones et leurs synapses. L’une des premières anomalies observables chez les patients atteints de synucléinopathies est la dysfonction synaptique, souvent combinée à une perte de synapses. Il a été rapporté que les oligomères d’α-syn retrouvés au niveau des synapses précèdent la formation de corps de Lewy dans les neurones et leur transmission semble être associée à la progression des symptômes. Pourtant, les mécanismes moléculaires sous-jacents la dysfonction synaptique causée par l’α-syn restent inconnus. D’autre part, le fonctionnement normal des synapses est fortement régulé par une famille de protéines appelées organisateurs synaptiques. Les organisateurs synaptiques, incluant la protéine neurexine, sont des molécules d’adhésion cellulaire qui régulent la synaptogenèse, la plasticité, la libération des neurotransmetteurs et les fonctions cognitives. De plus, nous avons préliminairement montré que l’α-syn interagit avec l’isoforme β des neurexines (NRXs) (β-NRXs). Mon projet avait donc pour but de caractériser l’interaction α-syn/β-NRX et d’évaluer comment celle-ci contribue à la pathologie liée à l’α-syn. Nous avons émis l’hypothèse que cette interaction affecte la fonction synaptogénique liée aux NRXs et son trafic. Dans un premier temps, pour tester notre hypothèse, l’interaction α-syn/β-NRX a été évaluée grâce à des analyses de liaison à la surface cellulaire. Il a été constaté que les oligomères d’α-syn se lient fortement à NRX1,2β de manière dépendante du domaine riche en histidine (HRD), caractéristique de l’isoforme β, et cela sans perturber sa liaison à ses ligands endogènes postsynaptiques, neuroligine 1 (NLG1) et « leucine rich repeat transmembrane neuronal 2 » (LRRTM2). De plus, à travers des essais d’internalisation, nous avons observé que les oligomères d’α-syn altèrent le trafic de NRX1β en augmentant son internalisation de façon dépendante au HRD et altèrent également la différenciation NRX-dépendante de la synapse en synapse inhibitrice. Par conséquent, nous suggérons que cette internalisation accrue pourrait affecter la fonction synaptogénique associée aux NRXs. Ce travail contribue à une meilleure compréhension sur la façon dont l’α-syn provoque un dysfonctionnement synaptique, fournissant de nouvelles perspectives moléculaires et pharmacologiques sur les synucléinopathies. / Parkinson’s disease (PD) and dementia with Lewy bodies (DLB) are the second and the third most common neurodegenerative disorders and are part of a class of diseases called synucleinopathies. Synucleinopathies are associated with an α-synuclein (α-syn) pathology which shows an accumulation of α-syn in neurons, forming Lewy bodies. This pathological α-syn can form oligomers and fibrils, which are toxic for neurons and their synapses. One of the first changes to occur in patients’ brain with synucleinopathies is synaptic dysfunction often combined with synapse loss. Synaptic α-syn oligomers were revealed to precede the formation of Lewy bodies, and their transmission to other neurons to correlate with the progression of the symptoms. Yet, the molecular mechanisms underlying how α-syn leads to synaptic dysfunction are unknown. Synaptic function is highly regulated by a protein family called synaptic organizers. Synaptic organizers are cell adhesion molecules that regulate synaptogenesis, plasticity, neurotransmitter release, synaptic plasticity and cognitive functions. Of this family, we have found that α-syn interacts with the β-isoforms of the neurexins (NRXs) family members (β-NRXs). My project aimed to characterize α-syn/β-NRX interaction and to evaluate how this interaction contributes to α-syn pathology. We hypothesized that this interaction affects NRX trafficking and its synaptic function. Firstly, to test our hypothesis, the α-syn/β-NRX interaction was characterized by performing cell surface binding assays. I found that α-syn oligomers strongly bind to NRX1,2β in a histidine rich domain (HRD)-dependent manner, without disrupting NRX binding to its postsynaptic binding partners, neuroligin 1 (NLG1) and leucine rich repeat transmembrane neuronal 2 (LRRTM2). Moreover, using internalization assays, we discovered that α-syn oligomers impair NRX trafficking by increasing NRX1β internalization in an HRD-dependent manner and impair NRX-dependent inhibitory presynaptic differentiation. Thereby, we suggest that this increased internalization affects the inhibitory synaptogenic function of NRX-based synaptic organizing complexes. This work contributes to a better understanding of how α-syn causes synaptic dysfunction, providing promising new molecular mechanisms and pharmacological insights into synucleinopathies.