Extracellular tau oligomers affect extracellular glutamate handling by astrocytes through downregulation of GLT‐1 expression and impairment of NKA1A2 function

Abstract

Aims: Several studies reported that astrocytes support neuronal communication by the release of gliotransmitters, including ATP and glutamate. Astrocytes also play a fundamental role in buffering extracellular glutamate in the synaptic cleft, thus limiting the risk of excitotoxicity in neurons. We previously demonstrated that extracellular tau oligomers (ex-oTau), by specifically targeting astrocytes, affect glutamate-dependent synaptic transmission via a reduction in gliotransmitter release. The aim of this work was to determine if ex-oTau also impair the ability of astrocytes to uptake extracellular glutamate, thus further contributing to ex-oTau-dependent neuronal dysfunction.Methods: Primary cultures of astrocytes and organotypic brain slices were exposed to ex-oTau (200 nM) for 1 h. Extracellular glutamate buffering by astrocytes was studied by: Na + imaging; electrophysiological recordings; high-performance liquid chromatography; Western blot and immunofluorescence. Experimental paradigms avoiding ex-oTau internalisation (i.e. heparin pre-treatment and amyloid precursor protein knockout astrocytes) were used to dissect intracellular vs extracellular effects of oTau.Results: Ex-oTau uploading in astrocytes significantly affected glutamate-transporter-1 expression and function, thus impinging on glutamate buffering activity. Ex-oTau also reduced Na-K-ATPase activity because of pump mislocalisation on the plasma membrane, with no significant changes in expression. This effect was independent of oTau internalisation and it caused Na + overload and membrane depolarisation in ex-oTautargeted astrocytes.Conclusions: Ex-oTau exerted a complex action on astrocytes, at both intracellular and extracellular levels. The net effect was dysregulated glutamate signalling in terms of both release and uptake that relied on reduced expression of glutamate-transporter-1, altered function and localisation of NKA1A1, and NKA1A2. Consequently, Na + gradients and all Na + -dependent transports were affected. Domenica Donatella Li Puma and Cristian Ripoli equally contributed to this work.

A Medicinal Chemistry Perspective on Excitatory Amino Acid Transporter 2 Dysfunction in Neurodegenerative Diseases

“…To a lesser extent, EAAT2 loss was observed in the occipital and frontal cortices . Complimentarily, in a very recent work it was shown that tau oligomers, following internalization into the astrocytic intracellular space, inhibited EAAT2 expression and decreased the capacity of astrocytes to uptake glutamate . These findings suggest that, perhaps, EAAT2 dysfunction and reactive astrogliosis are not only associated with Aβ pathology in AD but also with pathological tau in the later stages of the AD continuum.…”

Cytoplasmic HDAC4 recovers synaptic function in the 3×Tg mouse model of Alzheimer's disease

“…We found that the level of p‐Thr286‐CaMKII, used as a readout of its activation, was diminished in AD tissue compared with WT one (fold change AD vs WT: 0.39 ± 0.049, p < 0.001, t test; n = 3; Figure S1A ). Furthermore, treatment of hippocampal neurons with synaptotoxic concentrations of human synthetic Aβ (200 μM) or recombinant human tau 2N/4R (100 nM) [ 30 , 31 ] for 24 h also reduced p‐CaMKII levels indicating the contribution of the two AD hallmarks to CaMKII inhibition (p‐CaMKII fold change vs control: Aβ, 0.65 ± 0.10, p < 0.05; tau, 0.44 ± 0.06, p < 0.01, ANOVA with Bonferroni's test; n = 3; Figure S1B ).…”

“…Recombinant human tau 4R/2N was prepared as described in Li Puma et al [ 30 ]. Oligomerisation was achieved via the introduction of disulphide bonds through incubation with 1 mM H 2 O 2 at RT for 20 h and assessed by Western blot (WB).…”

“…We found that the level of p-Thr286-CaMKII, used as a readout of its activation, was diminished in AD tissue compared with WT one (fold change AD vs WT: 0.39 AE 0.049, p < 0.001, t test; n = 3; Figure S1A). Furthermore, treatment of hippocampal neurons with synaptotoxic concentrations of human synthetic Aβ (200 μM) or recombinant human tau 2N/4R (100 nM) [30,31] for 24 h also Other important players in the cytoplasmic retention of HDAC4 are the 14-3-3 proteins, a family that acts as adaptor or anchoring proteins. 14-3-3 β and ε have been reported as HDAC4 regulators.…”

ApoE4 expression disrupts tau uptake, trafficking, and clearance in astrocytes

“…Recent evidence indicates that astrocytes can process low levels of oligomeric tau, but as they are exposed to increasing pathogenic tau levels during disease progression, the astrocytes become detrimental (Sidoryk‐Wegrzynowicz et al, 2017) to surrounding neurons (Wang & Ye, 2021b). The astrocytic shift to a pathogenic tau‐induced neurotoxic phenotype (Ezerskiy et al, 2022; Gaikwad et al, 2021) is characterized by altered gliotransmitter release (Piacentini et al, 2017), impaired synaptic function (Li Puma et al, 2022), inflammatory cytokine release (Wang & Ye, 2021b), and altered mitochondrial dynamics (Richetin et al, 2020). Ultimately, this results in diminished neuronal support and neurodegeneration (Ezerskiy et al, 2022), though the influence of tau on specific astrocytic processes in relation to apoE genotype is unknown.…”