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Intracellular Trafficking Mechanisms of Synaptic Dysfunction in Alzheimer's Disease.

ABSTRACT: Alzheimer's disease (AD) is the most common neurodegenerative disease characterized by progressive memory loss. Although AD neuropathological hallmarks are extracellular amyloid plaques and intracellular tau tangles, the best correlate of disease progression is synapse loss. What causes synapse loss has been the focus of several researchers in the AD field. Synapses become dysfunctional before plaques and tangles form. Studies based on early-onset familial AD (eFAD) models have supported that synaptic transmission is depressed by ?-amyloid (A?) triggered mechanisms. Since eFAD is rare, affecting only 1% of patients, research has shifted to the study of the most common late-onset AD (LOAD). Intracellular trafficking has emerged as one of the pathways of LOAD genes. Few studies have assessed the impact of trafficking LOAD genes on synapse dysfunction. Since endocytic traffic is essential for synaptic function, we reviewed A?-dependent and independent mechanisms of the earliest synaptic dysfunction in AD. We have focused on the role of intraneuronal and secreted A? oligomers, highlighting the dysfunction of endocytic trafficking as an A?-dependent mechanism of synapse dysfunction in AD. Here, we reviewed the LOAD trafficking genes APOE4, ABCA7, BIN1, CD2AP, PICALM, EPH1A, and SORL1, for which there is a synaptic link. We conclude that in eFAD and LOAD, the earliest synaptic dysfunctions are characterized by disruptions of the presynaptic vesicle exo- and endocytosis and of postsynaptic glutamate receptor endocytosis. While in eFAD synapse dysfunction seems to be triggered by A?, in LOAD, there might be a direct synaptic disruption by LOAD trafficking genes. To identify promising therapeutic targets and biomarkers of the earliest synaptic dysfunction in AD, it will be necessary to join efforts in further dissecting the mechanisms used by A? and by LOAD genes to disrupt synapses.

SUBMITTER: Perdigao C

PROVIDER: S-EPMC7180223 | biostudies-literature | 2020

REPOSITORIES: biostudies-literature

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Intracellular Trafficking Mechanisms of Synaptic Dysfunction in Alzheimer's Disease.

Perdigão Catarina C Barata Mariana A MA Araújo Margarida N MN Mirfakhar Farzaneh S FS Castanheira Jorge J Guimas Almeida Cláudia C

Frontiers in cellular neuroscience 20200417

Alzheimer's disease (AD) is the most common neurodegenerative disease characterized by progressive memory loss. Although AD neuropathological hallmarks are extracellular amyloid plaques and intracellular tau tangles, the best correlate of disease progression is synapse loss. What causes synapse loss has been the focus of several researchers in the AD field. Synapses become dysfunctional before plaques and tangles form. Studies based on early-onset familial AD (eFAD) models have supported that sy ...[more]

PMID: 32362813

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Project description:Despite 50+ years of clinical use as anxiolytics, anti-convulsants, and sedative/hypnotic agents, the mechanisms underlying benzodiazepine (BZD) tolerance are poorly understood. BZDs potentiate the actions of gamma-aminobutyric acid (GABA), the primary inhibitory neurotransmitter in the adult brain, through positive allosteric modulation of ?2 subunit containing GABA type A receptors (GABAARs). Here we define key molecular events impacting ?2 GABAAR and the inhibitory synapse gephyrin scaffold following initial sustained BZD exposure in vitro and in vivo. Using immunofluorescence and biochemical experiments, we found that cultured cortical neurons treated with the classical BZD, diazepam (DZP), presented no substantial change in surface or synaptic levels of ?2-GABAARs. In contrast, both ?2 and the postsynaptic scaffolding protein gephyrin showed diminished total protein levels following a single DZP treatment in vitro and in mouse cortical tissue. We further identified DZP treatment enhanced phosphorylation of gephyrin Ser270 and increased generation of gephyrin cleavage products. Selective immunoprecipitation of ?2 from cultured neurons revealed enhanced ubiquitination of this subunit following DZP exposure. To assess novel trafficking responses induced by DZP, we employed a ?2 subunit containing an N terminal fluorogen-activating peptide (FAP) and pH-sensitive green fluorescent protein (?2pHFAP). Live-imaging experiments using ?2pHFAP GABAAR expressing neurons identified enhanced lysosomal targeting of surface GABAARs and increased overall accumulation in vesicular compartments in response to DZP. Using fluorescence resonance energy transfer (FRET) measurements between ?2 and ?2 subunits within a GABAAR in neurons, we identified reductions in synaptic clusters of this subpopulation of surface BZD sensitive receptor. Additional time-series experiments revealed the gephyrin regulating kinase ERK was inactivated by DZP at multiple time points. Moreover, we found DZP simultaneously enhanced synaptic exchange of both ?2-GABAARs and gephyrin using fluorescence recovery after photobleaching (FRAP) techniques. Finally we provide the first proteomic analysis of the BZD sensitive GABAAR interactome in DZP vs. vehicle treated mice. Collectively, our results indicate DZP exposure elicits down-regulation of gephyrin scaffolding and BZD sensitive GABAAR synaptic availability via multiple dynamic trafficking processes.

Project description:Introduction: ?-Amyloid protein (A?) putatively plays a seminal role in synaptic loss in Alzheimer's disease (AD). While there is no consensus regarding the synaptic-relevant species of A?, it is known that A? oligomers (A?Os) are noticeably increased in the early stages of AD, localizing at or within the synapse. In cell and animal models, A?Os have been shown to attach to synapses and instigate synapse dysfunction and deterioration. To establish the pathological mechanism of synaptic loss in AD, it will be important to identify the synaptic targets to which A?Os attach. Methods: An unbiased approach using far western ligand blots has identified three synaptic proteins to which A?Os specifically attach. These proteins (p100, p140, and p260) were subsequently enriched by detergent extraction, ultracentrifugation, and CHT-HPLC column separation, and sequenced by LC-MS/MS. P100, p140, and p260 were identified. These levels of A?Os targets in human AD and aging frontal cortexes were analyzed by quantitative proteomics and western-blot. The polyclonal antibody to A?Os was developed and used to block the toxicity of A?Os. The data were analyzed with one-way analysis of variance. Results: A?Os binding proteins p100, p140, and p260 were identified as Na/K-ATPase, synGap, and Shank3, respectively. ?3-Na/K-ATPase, synGap, and Shank3 proteins showed loss in the postsynaptic density (PSD) of human AD frontal cortex. In short term experiments, oligomers of A? inhibited Na/K-ATPase at the synapse. Na/K-ATPase activity was restored by an antibody specific for soluble forms of A?. ?3-Na/K-ATPase protein and synaptic ?-amyloid peptides were pulled down from human AD synapses by co-immunoprecipitation. Results suggest synaptic dysfunction in early stages of AD may stem from inhibition of Na/K-ATPase activity by A? oligomers, while later stages could hypothetically result from disrupted synapse structure involving the PSD proteins synGap and Shank3. Conclusion: We identified three A?O binding proteins as ?3-Na/K-ATPase, synGap, and Shank3. Soluble A? oligomers appear capable of attacking neurons via specific extracellular as well as intracellular synaptic proteins. Impact on these proteins hypothetically could lead to synaptic dysfunction and loss, and could serve as novel therapeutic targets for AD treatment by antibodies or other agents.

Dataset Information

Intracellular Trafficking Mechanisms of Synaptic Dysfunction in Alzheimer's Disease.

Publications

Intracellular Trafficking Mechanisms of Synaptic Dysfunction in Alzheimer's Disease.

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