Astrocytes are the most abundant and heterogeneous type of glial cells in the brain, we tested the hypothesis that amyloid fibrils impair astrocyte glutamate transporters, thus leading to an abnormal accumulation of extrasynaptic glutamate, which in turn contributes to microglial pruning of glutamatergic synapses and memory deficiency in the rodent models of AD.

Aβ1-40 fibrils were microinjected bilaterally into hippocampal CA1 areas in rats. In another cohorts, DL-threo-beta-benzyloxyaspartate (DL-TBOA), a specific inhibitor of glutamate transporter 1 (GLT1), or ceftriaxone (used to enhance GLT1 function) were also microinjected into hippocampal CA1 areas. Immunostaining and immunoblotting were performed to detect the expression of GLT1 in astrocytes. Complement C1q production and microglial pruning of glutamatergic synapses were assessed by 3D immunofluorescence imaging. Morris water maze test was performed to evaluate the cognitive function.

A significant GLT1 reduction in the hippocampal astrocytes in rats injected with amyloid fibrils, indicating dysfunctional glutamate uptake. Microinjection of ceftriaxone significantly attenuated C1q expression and endocytosis of vGluT1 within microglia, and improved behavioral performance in the modeled rats. These results suggested that upregulation of GluT1 expression restored the synaptic microglial pruning and cognition impaired by amyloid fibrils. Meanwhile, in naïve rats, microinjection of DL-TBOA substantially increased C1q expression, increased localization of vGluT1 within microglia, and impaired performance in the Morris water maze. This suggests that dysfunctional GluT1, which likely leads to extracellular accumulation of glutamate, contributes to C1q-mediated microglial pruning of synapses and cognitive deficits induced by amyloid fibrils.

Reduction of astrocytic GLT1 contributes to C1q production and microglial pruning of glutamatergic synapses, hippocampal synaptic dysfunction and memory deficit in the AD model.