Aberrant expression of the astrocyte glutamate transporter EAAT2 occurs in early-stage Alzheimer's disease and precedes hippocampal synaptic loss

Synaptically released glutamate is cleared mostly by astrocytic glutamate transporters. This function of astrocytes is crucial for regulating spatial and temporal synaptic specificity. In the hippocampus, greater than 80% of synaptically-released glutamate is taken up by the astrocytic glutamate transporter EAAT2, also known as GLT-1. To examine the role of A β in astrocyte dysfunction we investigated the impact of A β 1–42 exposure on the morphological integrity of EAAT2-positive astrocytes in acute hippocampal slices. To address the potential significance of astrocytic dysfunction in AD we examined EAAT2 expression in the hippocampus of mild cognitively impaired (MCI) and late-stage AD patients. Confocal microscopy was performed on hippocampi from normal subjects, subjects with a clinical dementia rating (CDR) = 0.5 (intended to approximate mild cognitive impairment), and AD subjects. Hippocampal slices were prepared from C57BL6 mouse brains (P14–21) using standard methods and were incubated for 30 minutes with 500nM synthetic human Aβ 4. Aβ 1–42 exposure induced rapid (approx. 30 min) re-sculpturing of astrocytic morphology in hippocampal slices that was evidenced by reduced process length (p<0.001) and reduced branching (p<0.05). This increase in astrocyte dysmorphology produced micro-domains where PSD95-positive synaptic puncta appeared separated from EAAT2-positive astrocytic processes. EAAT2 immunostaining of normal, CDR=0.5, and AD hippocampus revealed markedly similar astrocytic dysmorphology in both MCI and AD cases that was evidenced by loss of fine EAAT2-positive distal astrocyte processes, aberrant accumulation of EAAT2 in astrocyte cell bodies, and distinct micro-domains of EAAT2 loss in fields that were immuno-positive for both PSD95 and GFAP. In hilus significant loss of EAAT2 immunostaining in CDR=0.5 patients (p<0.034) and AD patients (p<0.019) preceded loss of PSD95 in CDR=0.05 patients (non-significant) and AD patients (p<0.001) compared to controls. Aβ 1–42 -induces astrocyte dysmorphology and mislocalization of EAAT2. Markedly similar astrocytic dysmorphology was observed in early and late-stage AD suggesting that AD pathogenesis may involve a form of early occurring synaptic neglect arising from disorganized glial/synapse morphologic associations. This may leave synapses increasingly isolated from efficient access to critical clearance functions and other forms of metabolic and trophic support that is provided by astrocytes.