Research reportAstrocytic glutamatergic transporters are involved in Aβ-induced synaptic dysfunction
Introduction
Alzheimer’s disease (AD) is the most common form of dementia in individuals over 65 years of age and is characterized by the accumulation of beta-amyloid (Aβ) and tau. Although the precise molecular mechanism of AD remains unknown, extensive research indicates that the Aβ protein plays a major role in the pathogenesis. There is a strong correlation between the levels of soluble Aβ and the severity of dementia in humans (Hefti et al., 2013, Wang et al., 2016). Experimentally, soluble Aβ oligomers were found to selectively block long-term potentiation (LTP), a cellular basis of memory (Jo et al., 2011, Kimura et al., 2012, Nomura et al., 2012, Walsh et al., 2002). Furthermore, the impairment of synaptic plasticity can be detected before the Aβ deposits in the mouse models of AD (Fowler et al., 2014).
The best correlation of memory deficits in AD patients is synapse loss (Robinson et al., 2014). A synaptic dysfunction is observed prior to neuron loss in mouse models of AD and coincides with the onset of memory deficits (Mucke et al., 2000). Emerging evidence suggests early cognitive decline in AD may result from a dysregulation of excitatory glutamatergic neurotransmission by soluble Aβ, leading to synaptic alterations and tau phosphorylation (Tackenberg et al., 2013). Extracellular glutamate concentrations are tightly controlled by a family of membrane transporters predominantly expressed by perisynaptic astrocytes (Danbolt, 2001). Excitatory amino acid transporters (EAATs) play an important role in the uptake of glutamate in the synaptic cleft. Five mammalian EAATs have been characterized: GLAST (glutamate/aspartate transporter, also called EAAT1), GLT-1 (glutamate transporter-1, also called EAAT2), EAAC1 (excitatory amino acid carrier-1, also called EAAT3), EAAT4 and EAAT5 (Arriza et al., 1997, Bunch et al., 2009, Divito and Underhill, 2014). Studies have shown that glutamate transporter expression was significantly decreased at both gene and protein levels in brains of sporadic AD patients (Jacob et al., 2007). Synthetic Aβ has been shown to inhibit glutamate uptake and decrease glutamatergic transporters expression in various preparations (Fernandez-Tome et al., 2004, Li et al., 2009, Matos et al., 2008, Tong et al., 2017, Zoia et al., 2011). Recently, several groups also confirmed that there is correlation between AD and glutamate transporters (Meeker et al., 2015, Takahashi et al., 2015, Zumkehr et al., 2015). As a potentially primary or early pathogenic factor, Aβ oligomers were shown by us to inhibit glutamate uptake in mouse hippocampal synaptosomes, similar to the effect of a glutamate uptake blocker, TBOA (Lei et al., 2016, Li et al., 2009). Because 80% of extracellular glutamate is taken up by GLT-1 and GLAST on astrocytes, the rest being taken back up into neurons (Lopez-Bayghen and Ortega, 2011), we want to test whether astrocytic glutamate transporters have any effect on the synaptic plasticity by Aβ. Although there are some reports demonstrating that Aβ could lead to the down-regulation of astrocytic glutamate uptake capacity (Fernandez-Tome et al., 2004, Harkany et al., 2000, Matos et al., 2008), there is no direct evidence to show that the Aβ-induced synaptic dysfunction is related to astrocytic glutamate uptake. We hypothesize that the Aβ oligomers interrupt the astrocytic glutamate uptake, increase the extracellular glutamate level, further activate the extrasynaptic GluN2B-containing NMDA receptors, and thus impair LTP and facilitate Ltd. Here we use different sources of soluble Aβ oligomers to detect the synaptic plasticity changes by the administration of selective astrocytic glutamate transporters inhibitors.
Section snippets
Aβ inhibits hippocampal LTP via interrupt glutamate transporter function
We previously demonstrated that soluble Aβ oligomers enhance LTD by decreasing glutamate uptake (Li et al., 2009). In order to further explore whether the Aβ impaired hippocampal LTP is connected to changes of glutamate transporters, we first measured the magnitudes of LTP inhibited by different doses of cell-derivate Aβ oligomers from 7PA2 CM (conditioned medium) (Walsh et al., 2002, Walsh et al., 2005), a well-studied cell-secreted human Aβ. The 7PA2 CM in 1x contains around 1.25–6.37 ng Aβ (
Discussion
Here we showed that soluble Aβ oligomers impair hippocampal LTP and promote LTD analogous to the effects of TBOA, a glutamate uptake inhibitor, and can be blocked by TBOA. More importantly, the astrocytic glutamate transporters are involved in the dysfunction of synapses induced by Aβ oligomers as Aβ oligomers impaired hippocampal LTP could be occluded by astrocytic glutamate transporters inhibitor, TFB-TBOA, and Aβ oligomers reduce the astrocytic glutamate transporters, EAAT1 and EAAT2,
Preparations of Aβ
Aβ peptide was prepared as describe previously (Lei et al., 2016, Li et al., 2009). Briefly, we used the conditioned medium (CM) of a CHO cell line (7PA2) that stably expresses human APP751 containing the V717F AD mutation (Podlisny et al., 1995, Welzel et al., 2014) to obtained secreted human Aβ peptide. Cells were grown in Dulbecco’s modified Eagle’s medium (DMEM) and were washed and cultured overnight (∼15 h) in serum-free medium after reaching ∼95% confluency. To wipe off debris and dead
Acknowledgments
This work was supported by research grants from National Key R&D Program of China (No. 2016YFC1306600), the National Natural Science Foundation of China (No. 81271428, No. 81471292, No. U1503222, No. 81430021 and No. U1603281), the key point Science Foundation of Guangdong of China (No. 2015A030311021), a grant supported by technology project of Guangzhou (No. 201504281820463), the collaborative innovation foundation of Guangzhou science and technology bureau (No. 2018-1202-SF-0019), the
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Authors contribute equally to this work.