Objectives: 

Recent failures of clinical trials for Alzheimer's disease (AD) using anti-amyloid beta-peptide (Abeta) approaches suggest that it may be necessary to attack downstream targets in the Abeta cascade. To elucidate such targets, we studied the effect of oligomerized Abeta on astrocytes. Astrocytes are known to modulate neuronal excitability and synaptic transmission.

Here we evaluate the effects of Abeta on intracellular Ca2+ and glutamate levels in astrocytes in vitro, as well as in wild type mice (WT) and mice expressing the human amyloid protein precursor (hAPPtg). First, we used the glutamate sensing fluorescent reporter (SuperGluSnFR) developed by Roger Tsien to perform sensitive optical measurements of glutamate release in response to Abeta. Second, we performed microdialysis measurements of glutamate levels in the hippocampues of WT and hAPPtg mice.

Materials: 

Glutamate release from astrocytes was measured in vitro by FRET microscopy using SuperGluSnFR, for optical measurement of glutamate. In vivo levels of extracellular glutamate were analyzed using a combination of intracerebral microdialysis and liquid chromatography–mass spectrometry (LC-MS).

Results: 

Abeta oligomers induced intracellular Ca2+ elevation and glutamate release from astrocytes. In vivo, extracellular levels of glutamate were higher in hAPPtg mice than in WT mice.

Conclusions: 

Nanomolar concentrations of oligomerized (but not non-oligomerized) Abeta induced release of several tens of micromolar glutamate from cultured astrocytes. These levels of glutamate can cause excitotoxic damage and synaptic loss in neurons. In vivo measurements of the glutamate levels in APP brains confirmed these results. Taken together with prior studies, our work suggests that the neurotoxic effects of Abeta may be mediated at least in part by local release of glutamate from astrocytes. Our finding that oligomeric Abeta induces changes in intracellular Ca2+ levels and extracellular release of glutamate points to a role of glutamatergic signaling in the pathogenesis of AD and the importance of addressing excitotoxicity synaptic damage in the future treatment of AD.