Divergent roles of astrocytic versus neuronal EAAT2 deficiency on cognition and overlap with aging and Alzheimer's molecular signatures

The excitatory amino acid transporter 2 (EAAT2) is the major glutamate transporter in the brain expressed predominantly in astrocytes and at low levels in neurons and axonal terminals. EAAT2 expression is reduced in aging and sporadic Alzheimer's disease (AD) patients' brains. The role EAAT2 plays in cognitive aging and its associated mechanisms remains largely unknown. Here, we show that conditional deletion of astrocytic and neuronal EAAT2 results in age-related cognitive deficits. Astrocytic, but not neuronal EAAT2, deletion leads to early deficits in short-term memory and in spatial reference learning and long-term memory. Neuronal EAAT2 loss results in late-onset spatial reference long-term memory deficit. Neuronal EAAT2 deletion leads to dysregulation of the kynurenine pathway, and astrocytic EAAT2 deficiency results in dysfunction of innate and adaptive immune pathways, which correlate with cognitive decline. Astrocytic EAAT2 deficiency also shows transcriptomic overlaps with human aging and AD. Overall, the present study shows that in addition to the widely recognized astrocytic EAAT2, neuronal EAAT2 plays a role in hippocampus-dependent memory. Furthermore, the gene expression profiles associated with astrocytic and neuronal EAAT2 deletion are substantially different, with the former associated with inflammation and synaptic function similar to changes observed in human AD and gene expression changes associated with inflammation similar to the aging human brain. 6 mice across 4 groups (gfapCre+ EAAT2delta/+; gfap-Cre- EAAT2+/+; syn-Cre+ EAAT2-/-; syn-Cre- EAAT2+/+) were sequenced to study differential expression b/w conditions兴奋性氨基酸转运体2（excitatory amino acid transporter 2, EAAT2）是大脑内主要的谷氨酸转运体，主要表达于星形胶质细胞，在神经元及轴突末梢中呈低水平表达。在衰老及散发性阿尔茨海默病（sporadic Alzheimer's disease, AD）患者的脑组织中，EAAT2的表达水平会出现下调。目前学界对于EAAT2在认知衰老中发挥的作用及其相关调控机制仍不甚明晰。本研究证实，条件性敲除星形胶质细胞与神经元中的EAAT2会导致年龄相关性认知功能缺损。仅敲除星形胶质细胞而非神经元的EAAT2，会引发短期记忆、空间参考学习及长期记忆的早期缺损；而神经元EAAT2缺失则会导致迟发性空间参考长期记忆障碍。神经元EAAT2缺失会引发犬尿氨酸通路（kynurenine pathway）的失调，星形胶质细胞EAAT2缺乏则会导致固有免疫与适应性免疫通路功能异常，上述变化均与认知衰退密切相关。此外，星形胶质细胞EAAT2缺乏的转录组特征与人类衰老及AD存在显著重叠。总体而言，本研究表明，除了已被广泛认知的星形胶质细胞EAAT2外，神经元EAAT2同样在海马依赖型记忆中发挥重要作用。进一步而言，与星形胶质细胞和神经元EAAT2缺失相关的基因表达谱存在显著差异：前者的炎症与突触功能变化与人类AD中观察到的改变高度相似，且其炎症相关基因表达变化与人类衰老大脑的特征一致。本研究共对4组小鼠（每组6只，分别为gfap-Cre+ EAAT2delta/+；gfap-Cre- EAAT2+/+；syn-Cre+ EAAT2-/-；syn-Cre- EAAT2+/+）进行转录组测序，以探究不同实验条件间的差异表达情况。