APP/AICD‐dependent expression of Glypican 4 mediates accumulation of tau oligomers in astrocytes and their synaptotoxic action

Abstract

AbstractBackgroundSeveral studies including ours reported the detrimental effects of extracellular tau oligomers (ex‐oTau) on glutamatergic synaptic transmission and plasticity. Astrocytes greatly internalize ex‐oTau thus contributing to their clearance. However, glial cell engulfment of oTau leads to alteration of neuro/gliotransmitter handling that negatively affects synaptic function. We previously reported that amyloid precursor protein (APP) is required for oTau internalization in astrocytes but the molecular mechanisms underlying this phenomenon has not been clearly identified yet. Here we investigated the role of glypican 4 (GPC4), a member of the large family of heparan sulfate proteoglycans that are membrane glycoproteins highly expressed in astrocytes playing a critical role in protein trafficking through the plasma membrane.MethodThe role of GPC4 in ex‐oTau accumulation in astrocytes and the resulting synaptotoxic action were studied in in vitro and ex‐vivo models obtained from wild‐type C57/bl6 and transgenic mice in which either APP was knocked‐out or it contained the non‐phosphorylatable amino acid alanine replacing threonine 688 (APPTA mice). In these experimental models we performed immunofluorescence analyses, Western blot and real‐time PCR experiments, confocal Ca2+ imaging, high‐pressure liquid chromatography, FM1‐43 imaging, electrophysiological recordings and Chromatin immunoprecipitation assays.ResultTreatment of cultured astrocytes with a specific anti‐GPC4 antibody significantly reduced oTau internalization in astrocytes and prevented oTau‐induced alterations of intracellular Ca2+ signaling and gliotransmitter release. As such, anti‐GPC4 treatment spared neurons from the astrocyte‐mediated synaptotoxic action of ex‐oTau. Specifically, after counteracting the interaction between ex‐oTau and GPC4 no significant decreases in synaptic vesicular release and synaptic protein expression were observed in neurons co‐cultured with astrocytes following exposure to ex‐oTau. The latter also failed to inhibit hippocampal LTP at CA3‐CA1 synapses. Of note, we found that the expression of GPC4 depended on APP and, in particular, on its C‐terminal domain, AICD, generated by γ‐secretase‐dependent cleavage of APP, that can bind Gpc4 promoter. Accordingly, GPC4 expression was significantly reduced in both APP‐KO ad APPTA mice, and ex‐oTau did not exert any significant detrimental action on synaptic function in APP‐KO mice.ConclusionCollectively, our data indicate that GPC4 expression is APP/AICD‐dependent, it mediates oTau accumulation in astrocytes and the resulting synaptotoxic effects.