Role of Astrocyte EAAT2/GLT1 Failure in Alzheimer's Disease Pathogenesis

Astrocytes are ideally positioned to support neuronal/synaptic needs for trophic factors, metabolic homeostasis, and protection from toxicity. While reactive astrogliosis is a prominent feature of AD, this offers very limited insight about how astrocytes influence the disease process or how they may be harmed. One of the most important functions of astrocytes is to clear extracellular glutamate to prevent excitotoxicity. Glutamate taken up by astrocytes is also used as a metabolic substrate for biosynthesis of other neuro- transmitters like GABA. Thus, there are at least two major ways astrocytic glutamate clearance protects the brain. In cortex and hippocampus, the glutamate transporter Slc1a2 (also called GLT1 or EAAT2) plays the most important role in glutamate clearance. Most, but not all Slc1a2 is in astrocytes. Our research team has shown that: (i) Slc1a2 is disturbed in AD; (ii) Slc1a2 loss in an AD mouse model accelerates onset of cognitive impairment; (iii) A42 slows synaptically-released glutamate uptake in hippo- campal slices; and (iv) mice with reduced astrocytic Slc1a2 display significant transcriptomic overlaps with AD. These data complement strong work from other groups and collectively argue that Slc1a2 dysfunction may play an important role in AD. However, additional critical questions need to be answered to better understand how astrocytic Slc1a2 may interact with A42 and tau pathology. Specifically, is there pathogenic synergy among these processes? In AD more needs to be uncovered about the relationship between neurons and the fine (often GFAP-negative) astrocytic processes expressing nearly all Slc1a2 in the brain—an anatomical relationship that is crucial to their function. In addition, there is insufficient data supporting the hypothesis that astrocytic Slc1a2 can play a contributing or causal role in exacerbating A42 and tau pathology. The goal of this project is to fill these knowledge gaps. First, we will use novel mice with reduced Slc1a2 specifically in astrocytes; and with adenoviral vectors (AAVs) expressing A42 and tauP301L, dissect the in vivo molecular interactions between these pathogenic pathways. We will address whether astrocytes are lost in response to A42 and/or tau. We will use a novel lentivirus system expressing Slc1a2, which infects astrocytes, to test whether specifically rescuing astrocytic Slc1a2 ameliorates neuropathology, as well as Slc1a2 function. Second, using state-of-the art patch clamp methods that directly measure astrocyte glutamate clearance, dissect how Slc1a2 loss, A42, and tau expression interact to affect astrocytic glutamate clearance. We will address how these pathogenic processes influence astrocytic Slc1a2 that regulate synaptic network excitability by supporting GABAergic transmission. Third, using well-characterized postmortem brains from control, prodromal, and AD patients we will test the potential translational significance of the glutamate transporter and astrocyte neuropathology we have reported and is suggested by our new preliminary data. Together, these data hold promise of advancing our knowledge of Slc1a2 as a potential molecular target for intervention in AD.