New insight into mechanisms of transcellular propagation of tau and α-synuclein in neurodegenerative diseases

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Title: New insight into mechanisms of transcellular propagation of tau and α-synuclein in neurodegenerative diseases
Author: Yan, Xu
Contributor: University of Helsinki, Faculty of Biological and Environmental Sciences, Department of Biosciences
Publisher: Helsingin yliopisto
Date: 2017-02-10
Language: en
Thesis level: Doctoral dissertation (article-based)
Abstract: Progressive development of pathology in neuroanatomically connected brain regions is a common feature of many neurodegenerative diseases. The spread of disease pathology is suggested to be dependent on the transmissibility of disease-associated proteins, particularly soluble aggregates of misfolded proteins. Emerging evidence suggests that many disease-associated proteins such as α-synuclein (aSyn) and tau, in certain misfolded and aggregated states convert from physiologically normal proteins into forms that lead to progression of disease pathology in a template-dependent manner, which is also known as seeding . The propagation and the proteinopathy have been suggested to occur via cell-to-cell transmission. The exact mechanisms involved in the seeding and spreading process are incompletely understood. In this thesis work, three critical steps of the seeding pathway (a process involves multiple steps), the intracellular aggregation, cellular release and uptake of aSyn and tau, were carefully studied primarily via a newly developed platform based on protein-fragment complementation assay. The main findings of this thesis are: a)Prolyl oligopeptidase (PREP) is a serine peptidase that was previously known to accelerate the process of aSyn aggregation and suppress autophagy clearance in cells and transgenic aSyn mice. The results of this thesis show that PREP directly interacts with aSyn in neuro2A cells and cell-free environment, and enhances aSyn dimerization, which is an early event in aSyn aggregation pathway. In addition, the PREP-mediated aSyn dimerization can be antagonized by KYP-2047, a small-molecule PREP inhibitor. b) Late-onset Alzheimer s disease (LOAD) susceptibility genes affect the individual risk of developing Alzheimer s disease, which is one of the common tauopathies. In this work, the functional connection between selected LOAD susceptibility genes and cell-to-cell transmission of tau was studied in vitro. We observed that RNAi knockdown of CD2AP and FRMD4A reduced tau secretion, and knockdown of APOE reduced tau uptake in HEK293T cells. Further mechanistic studies revealed that FRMD4A modulates tau secretion via the FRMD4A-cytohesin-Arf6 signalling pathway and the Par6/aPKC polarity signalling complex. This data, for the first time, demonstrates a functional connection between LOAD risk genes and cell-to-cell propagation of tau. c) Following internalization, extracellular, hyperphosphorylated tau was found to be recruited to stress granules, transient non-membraneous cytosolic structures composed of RNA and self-aggregating RNA-binding proteins. Tau recruitment was dependent on TIA-1, an RNA-binding stress granule protein. Importantly, the stress granules induced by and containing internalized tau were resistant to normal clearance and associated with increased sensitivity of cells to other stresses. This data describe a previously unrecognized mechanism and pathological consequence of cell-to-cell propagation of tau-mediated by stress granules, which have previously been associated with the pathophysiology of various neurodegenerative diseases. Overall, the work described in this thesis provides several novel findings that improve our understanding of cellular mechanisms underlying the development and spreading of aSyn and tau-related neurodegenerative pathologies. These pieces of knowledge may be potential avenues towards the development of crucial therapeutics against aSyn and tau-related neurodegenerative diseases.Asteittainen patologian kehittyminen neuroanatomisesti kytketyillä aivoalueilla on yleinen piirre monissa hermostoa rappeuttavissa sairauksissa. Patologian leviämisen uskotaan olevan riippuvainen tautiin liittyvien proteiinien, erityisesti väärin laskostuneiden proteiinien liukoisten aggregaattien, leviämisestä. Kasvava todistusaineisto osoittaa, että monet sairauteen liittyvät proteiinit, kuten α-synukleiini (Asyn) ja tau muuttuvat tietyissä väärin laskostuneissa ja aggregoituneissa muodoissa fysiologisesti normaaleista proteiinista muotoihin, jotka johtavat taudin patologian etenemiseen templaatista riippuvaisella tavalla, jota kutsutaan kylvöksi. Etenemisen ja proteinopatian on ehdotettu tapahtuvan solu-solu -transmissiolla. Kylvöön ja leviämiseen liittyvät tarkat mekanismit tunnetaan vielä puutteellisesti. Tässä tutkielmassa kolmea kriittistä vaihetta kylvöreitissä (prosessiin liittyy useita vaiheita), solunsisäistä aggregoitumista sekä aSyn ja taun vapauttamista ja sisäänottoa soluissa, tutkittiin huolelliseti pääasiassa äskettäin kehitetyllä proteiini-fragmentti -komplementaatioanalyysiin pohjautuvalla alustalla.
Subject: neuroscience
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