EAAT2 Expression in the Hippocampus, Subiculum, Entorhinal Cortex and Superior Temporal Gyrus in Alzheimer’s Disease

Abstract

Alzheimer’s disease (AD) is a neuropathological disorder characterized by the presence and accumulation of amyloid-beta plaques and neurofibrillary tangles. Glutamate dysregulation and the concept of glutamatergic excitotoxicity have been frequently described in the pathogenesis of a variety of neurodegenerative disorders and are postulated to play a major role in the progression of AD. In particular, alterations in homeostatic mechanisms, such as glutamate uptake, have been implicated in AD. An association with excitatory amino acid transporter 2 (EAAT2), the main glutamate uptake transporter, dysfunction has also been described. Several animal and few human studies examined EAAT2 expression in multiple brain regions in AD but studies of the hippocampus, the most severely affected brain region, are scarce. Therefore, this study aims to assess alterations in the expression of EAAT2 qualitatively and quantitatively through DAB immunohistochemistry (IHC) and immunofluorescence within the hippocampus, subiculum, entorhinal cortex, and superior temporal gyrus (STG) regions, between human AD and control cases. Although no significant EAAT2 density changes were observed between control and AD cases, there appeared to be increased transporter expression most likely localized to fine astrocytic branches in the neuropil as seen on both DAB IHC and immunofluorescence. Therefore, individual astrocytes are not outlined by EAAT2 staining and are not easily recognizable in the CA1–3 and dentate gyrus regions of AD cases, but the altered expression patterns observed between AD and control hippocampal cases could indicate alterations in glutamate recycling and potentially disturbed glutamatergic homeostasis. In conclusion, no significant EAAT2 density changes were found between control and AD cases, but the observed spatial differences in transporter expression and their functional significance will have to be further explored.

EAAT2 as a therapeutic research target in Alzheimer's disease: A systematic review

“…However, our previous study reported no significant quantitative differences in EAAT2 staining in hippocampal regions. Despite this, we did note higher EAAT2 staining in the neuropil that appeared to show less co-localization with GFAP stained astrocytic main branches, also perhaps indicative of EAAT2 astrocytic loss (Yeung et al, 2021 ). Interestingly, this EAAT2 expression pattern in AD cases (higher staining in the neuropil) appears similar to immunohistochemical images from the hippocampus published by Li et al ( 1997 ).…”

“…In addition, amyloid-beta administration has been shown to result in the mislocalization of EAAT2 (Manisha et al, 2020 ). However, it is uncertain if this is the reason behind EAAT2 alterations (both expressional and qualitative) previously documented in AD postmortem tissue (Li et al, 1997 ; Abdul et al, 2009 ; Hoshi et al, 2018 ; Yeung et al, 2021 ). After Aβ treatment, HNE and GLT-1 conjugation and impairments in glutamate uptake were reported in rat cortical synaptosomes (Keller et al, 1997 ).…”

“…More work is needed to establish the exact cellular mechanisms behind Aβ induced dysfunction of EAAT2. Interestingly, certain studies within the same brain region have shown contradictory results regarding EAAT2 protein expression (Abdul et al, 2009 ; Yeung et al, 2021 ). A possible explanation could be the methodology used.…”

“…Some research has concluded EAAT2 protein expression is reduced in AD postmortem hippocampal (Abdul et al, 2009 ) and temporal gyrus tissue (Hoshi et al, 2018 ). However, a recently published study by our laboratory group revealed no quantitative EAAT2 alterations in the hippocampus, subiculum, entorhinal cortex, or superior temporal gyrus (Yeung et al, 2021 ). Despite this, changes to the expression pattern of EAAT2 were evident, with higher immunoreactivity noticeable in the neuropil (Yeung et al, 2021 ).…”

“…However, a recently published study by our laboratory group revealed no quantitative EAAT2 alterations in the hippocampus, subiculum, entorhinal cortex, or superior temporal gyrus (Yeung et al, 2021 ). Despite this, changes to the expression pattern of EAAT2 were evident, with higher immunoreactivity noticeable in the neuropil (Yeung et al, 2021 ). The notion that EAAT2 expression level may not be altered in AD is further supported by earlier evidence from Beckstrøm et al ( 1999 ), who found varying EAAT2 levels in the inferior temporal gyrus for both control and AD cases, but no clear association with AD.…”