Hypoxia Suppresses Glutamate Transport in Astrocytes

Abstract

Glutamate uptake by astrocytes is fundamentally important in the regulation of CNS function. Disruption of uptake can lead to excitotoxicity and is implicated in various neurodegenerative processes as well as a consequence of hypoxic/ischemic events. Here, we investigate the effect of hypoxia on activity and expression of the key glutamate transporters excitatory amino acid transporter 1 (EAAT1) [GLAST (glutamate-aspartate transporter)] and EAAT2 [GLT-1 (glutamate transporter 1)]. Electrogenic, Na ϩ -dependent glutamate uptake was monitored via whole-cell patch-clamp recordings from cortical astrocytes. Under hypoxic conditions (2.5 and 1% O 2 exposure for 24 h), glutamate uptake was significantly reduced, and pharmacological separation of uptake transporter subtypes suggested that the EAAT2 subtype was preferentially reduced relative to the EAAT1. This suppression was confirmed at the level of EAAT protein expression (via Western blots) and mRNA levels (via real-time PCR). These effects of hypoxia to inhibit glutamate uptake current and EAAT protein levels were not replicated by desferrioxamine, cobalt, FG0041, or FG4496, agents known to mimic effects of hypoxia mediated via the transcriptional regulator, hypoxia-inducible factor (HIF). Furthermore, the effects of hypoxia were not prevented by topotecan, which prevents HIF accumulation. In stark contrast, inhibition of nuclear factor-B (NF-B) with SN50 fully prevented the effects of hypoxia on glutamate uptake and EAAT expression. Our results indicate that prolonged hypoxia can suppress glutamate uptake in astrocytes and that this effect requires activation of NF-B but not of HIF. Suppression of glutamate uptake via this mechanism may be an important contributory factor in hypoxic/ischemic triggered glutamate excitotoxicity.

Bioinorganic Chemistry of Alzheimer’s Disease

“…503 Hypoxia also reduces the uptake and transport of glutamate in astrocytes, which could further facilitate excitotoxicity. 504 Much more direct evidence for oxidative stress being an underlying cause of the amyloid cascade comes from recent findings of a positive feedback loop between γ-and β-secretase activity triggered by oxidative stress. 88 β-secretase expression is increased by oxidative stress, 505,506 which could explain why hypoxia up-regulates β-secretase, since hypoxia leads to local oxidative stress from metabolic inefficiency, and mitochondrial inhibition has been shown to also up-regulate β-secretase in rats.…”

Astrocytic transporters in Alzheimer's disease

“…Likewise, α3 subunit protein levels and Na + .K + -ATPase activity are reduced in a double transgenic (APP and PS-1) mouse model of AD (205). With reference to the glial isoforms, exogenous application of Aβ [25][26][27][28][29][30][31][32][33][34][35] resulted in a decrease in α1 subunit protein levels (206). Most recently, the α3 neuronal specific Na + .K + -ATPase isoform is a target for Aβ assemblies, presenting a potential novel therapeutic strategy (207).…”

Sequential Generation of Two Distinct Synapse-Driven Network Patterns in Developing Neocortex

“…This property most likely derives from the strong sensitivity of cENOs to extracellular glutamate levels. Glutamate uptake by astrocytes was shown to be disrupted by hypoxia (Dallas et al, 2007). The astrocytic network might indeed be implicated in the control of cENOs even in normoxic conditions (Aguado et al, 2002).…”