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VENERDI’ 26 OTTOBRE 2018, ORE 15  – Aula Convegni

Dr. Roberto Piacentini
Institute of Human Physiology, Università Cattolica del Sacro Cuore

Increasing evidence demonstrates the pivotal role of tau in Alzheimer’s disease (AD) pathogenesis [1]. Hyperphosphorylated and misfolded tau accumulating inside neurons destroys axons, thus contributing to neuronal loss in AD. However, tau has been also found in extracellular medium, due to activity-dependent release from neurons, mainly by exosomes. We and other groups demonstrated that extracellular oligomeric tau (ex-oTau) exerts a strong synaptotoxic action [1-3]. In fact, 20-min application ex-oTau to hippocampal brain slices from WT mice impairs long-term potentiation at the CA3-CA1 synapse, and intra-hippocampal application of oTau affects memory [2,3]. However, oTau does not exert adverse effects in mice lacking APP (APP-KO mice) [3,4]. We demonstrated that, contrarily to WT cells, APP KO ones express low levels of Heparan Sulfate Proteoglycans (HSPGs) that are known to mediate Tau internalization in cells [5], thus supporting the hypothesis that ex-oTau synaptotoxicity depends on its intracellular accumulation. In Tauopathies, tau has been found accumulated in cells other than neurons, such as astrocytes and microglia [6]. However, the role of astrocytes in oTau synaptotoxicity has been poorly investigated. As part of the “tripartite synapse” astrocytes regulate synaptic transmission and plasticity by Ca2+-dependent release of gliotransmitters such as glutamate, ATP and D-serine [7,8]. They also participate in regulating the amount of extracellular glutamate by removing excess of this neurotransmitter through specific Na+dependent transporters (Excitatory Amino Acid Transporter, EAAT2). To this regard, it was also reported that pathogen tau (P301L) overexpression selectively in astrocytes leads to reduced expression of EAAT2 affectingneuromuscolar function [9]. Here we report that astrocytes mediate ex-oTau synaptotoxic effects by an altered management of neurotransmitters such as glutamate and ATP. In particular, we found that ex-oTau applied for 1h or less determines: i) synaptotoxicity, although it accumulates more rapidly and abundantly in astrocytes than in neurons; ii) alteration of ATP-induced Ca2+-dependent gliotransmitter release, in particular ATP and glutamate; iii) reduction of the amplitude of glutamate-induced EAAT2-mediated intracellular Na+ transients in astrocytes, reflecting glutamate internalization, that was accompanied by a significant reduction of EAAT2 expression; iv) depression of basal synaptic transmission and impairment of longterm potentiation at the hippocampal CA3-CA1 synapse, similar to that observed in oTau-untreated brain slices after metabolic inhibition of astrocytes by fluorocitrate. These effects were not observed when oTau was pre-treated with heparin, which impedes its interaction with HSPGs mediating tau internalization, or when ex-oTau was applied to APP-KO cells that cannot internalize oTau.
We also found that ex-oTau alters Na+/K+-ATPase (NKA) pump function by altering its cellular distribution, but not its expression. This alteration leads to increased Na+ basal levels and cell depolarization. Surprisingly, this effect was independent on oTau internalization and was observed on wild-type as well as APP KO cells including neurons. In conclusion, we found that ex-oTau exerts a dual action: i) intracellular, on astrocytes that are permissive to oTau, by affecting Ca2+ signaling and EAAT2 expression, that results in an altered gliotransmitter release and glutamate management; ii) extracellular, by affecting NKA distribution and function, thus inducing Na+ homeostasis dysregulation and cell depolarization. Together, these effects determined alteration of synaptic functions underlying memory.

References:
[1] Guerrero-Muñoz et al. 2015. Front Cell Neurosci. 9:464; [2] Fá et al. 2016. Sci Rep. 6:19393; [3] Puzzo et al. eLife 6:e26991; [4] Piacentini et al. 2017. Glia. 65:1302-16; [5] Holmes et al. 2013. PNAS. 110:E3138-47; ù [6] Kahlson&Colodner 2015. J Exp Neurosci. 9:43–50; [7] Newman 2003. Trends Neurosci. 26:536-42; [8] Bonasco et al. 2014. J Neurosci. 33:1483-92; [9] Dabir et al. 2006. J Neurosci. 26:644-54.

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