Deficient astrocyte metabolism impairs glutamine synthesis and neurotransmitter homeostasis in a mouse model of Alzheimer's disease

Jens V Andersen,Sofie K Christensen,Emil W Westi,Marta Diaz-Delcastillo,Heikki Tanila,Arne Schousboe,Blanca I Aldana,Helle S Waagepetersen

doi:10.1016/j.nbd.2020.105198

Jens V Andersen, Sofie K Christensen + Show 6 more

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https://doi.org/10.1016/j.nbd.2020.105198

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Abstract

Alzheimer's disease (AD) leads to cerebral accumulation of insoluble amyloid-β plaques causing synaptic dysfunction and neuronal death. Neurons rely on astrocyte-derived glutamine for replenishment of the amino acid neurotransmitter pools. Perturbations of astrocyte glutamine synthesis have been described in AD, but whether this functionally affects neuronal neurotransmitter synthesis is not known. Since the synthesis and recycling of neurotransmitter glutamate and GABA are intimately coupled to cellular metabolism, the aim of this study was to provide a functional investigation of neuronal and astrocytic energy and neurotransmitter metabolism in AD. To achieve this, we incubated acutely isolated cerebral cortical and hippocampal slices from 8-month-old female 5xFAD mice, in the presence of 13C isotopically enriched substrates, with subsequent gas chromatography–mass spectrometry (GC–MS) analysis. A prominent neuronal hypometabolism of [U-13C]glucose was observed in the hippocampal slices of the 5xFAD mice. Investigating astrocyte metabolism, using [1,2-13C]acetate, revealed a marked reduction in glutamine synthesis, which directly hampered neuronal synthesis of GABA. This was supported by an increased metabolism of exogenously supplied [U-13C]glutamine, suggesting a neuronal metabolic compensation of the reduced astrocytic glutamine supply. In contrast, astrocytic metabolism of [U-13C]GABA was reduced, whereas [U-13C]glutamate metabolism was unaffected. Finally, astrocyte de novo synthesis of glutamate and glutamine was hampered, whereas the enzymatic capacity of glutamine synthetase for ammonia fixation was maintained. Collectively, we demonstrate that deficient astrocyte metabolism leads to reduced glutamine synthesis, directly impairing neuronal GABA synthesis in the 5xFAD brain. These findings suggest that astrocyte metabolic dysfunction may be fundamental for the imbalances of synaptic excitation and inhibition in the AD brain.

Highlights

Alzheimer’s disease (AD) is a progressive neurodegenerative disor der characterized by gradual cognitive decline and dementia (Masters et al, 2015; Querfurth and LaFerla, 2010)
13C labeling of glutamate M + 1 was significantly reduced in the cerebral cortical slices of the 5xFAD mice and the same was observed for gluta mine. These results demonstrate a maintained anaplerotic activity via pyruvate carboxylase (PC), whereas astrocyte de novo synthesis of glutamate and glutamine is reduced in the 5xFAD cerebral cortical slices
We demonstrate that deficient astrocyte gluta mine synthesis directly hampers neuronal synthesis of neurotransmitter

Summary

Introduction

Alzheimer’s disease (AD) is a progressive neurodegenerative disor der characterized by gradual cognitive decline and dementia (Masters et al, 2015; Querfurth and LaFerla, 2010). A central pathological feature of AD is the accumulation of insoluble amyloid-β (Aβ) aggre gates, predominantly manifesting in the cerebral cortex and hippo campus (Masters et al, 2015). Cerebral accumulation of Aβ leads to a complex cascade of cellular responses arising decades before clinical symptoms (De Strooper and Karran, 2016). The most abundant glial cell type, the astrocyte, reacts strongly to Aβ pathology. Since astrocytes are crucial for synaptic formation and signaling processes, it has been hypothesized that malfunctioning astrocytes may facilitate or accelerate synaptic dysfunction and neurodegeneration in AD (De Strooper and Karran, 2016; Oksanen et al, 2019; Carter et al, 2019; Walton and Dodd, 2007; Steele and Robinson, 2012)

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