The complement-derived anaphylatoxin C5a increases microglial GLT-1 expression and glutamate uptake in a TNF-α-independent manner

Microglia can express Na⁺-dependent high-affinity glutamate transporters during pathological conditions in the CNS. The transporter expression seems to be activation dependent, and we therefore sought to identify factors that could induce it, in addition to the well-known effect of lipopolysaccharide (LPS) that is mediated by tumour necrosis factor-α (TNF-α). The complement-derived anaphylatoxins C3a and C5a are of potential interest as the complement system is activated in nearly all insults to the nervous system, and both C3a and C5a have been shown to protect against excitotoxicity. We have found that C5a, but not C3a, increased the expression of the microglial glutamate transporter GLT-1 in a dose-dependent manner without eliciting or modulating the release of TNF-α. However, the increase was not as prominent as the one induced by LPS, indicating that the microglia are in different activity states. The increase in microglial GLT-1 expression led to an increased functional uptake of glutamate without affecting the release. This suggests that C5a-stimulated microglia can be self- and neuroprotective by removing extracellular glutamate.