Stimulation of excitatory amino acid release from adult mouse brain glia subcellular particles by high mobility group box 1 protein

The multifunctional protein high mobility group box 1 (HMGB1) is expressed in hippocampus and cerebellum of adult mouse brain. Our aim was to determine whether HMGB1 affects glutamatergic transmission by monitoring neurotransmitter release from glial (gliosomes) and neuronal (synaptosomes) re-sealed subcellular particles isolated from cerebellum and hippocampus. HMGB1 induced release of the glutamate analogue [³H]d-aspartate form gliosomes in a concentration-dependent manner, whereas nerve terminals were insensitive to the protein. The HMGB1-evoked release of [³H]d-aspartate was independent of modifications of cytosolic Ca²⁺, but it was blocked by dl-threo-β-benzyloxyaspartate (dl-TBOA), an inhibitor of glutamate transporters. HMGB1 also stimulated the release of endogenous glutamate in a Ca²⁺-independent and dl-TBOA-sensitive manner. These findings suggest the involvement of carrier-mediated release. Moreover, dihydrokainic acid, a selective inhibitor of glutamate transporter 1 (GLT1), does not block the effect of HMGB1, indicating a role for the glial glutamate-aspartate transporter (GLAST) subtype in this response. We also demonstrate that HMGB1/glial particles association is promoted by Ca²⁺. Furthermore, although HMGB1 can physically interact with GLAST and the receptor for advanced glycation end products (RAGE), only its binding with RAGE is promoted by Ca²⁺. These results suggest that the HMGB1 cytokine could act as a modulator of glutamate homeostasis in adult mammal brain.