Neuroprotection by VNUT-mediated microglial ATP exocytosis in the ischemic brain

[Speaker] Ha P. N Le:1
[Co-author] Yuri Hirayma:1,2, Schuichi Koizumi:1
1:Neuropharmacology, Department of Neuropharmacology, Interdisciplinary Graduate School of Medicine, University of Yamanashi, Japan, 2:Department of Pharmacy, University of Yamanashi Hospital, Japan

ATP is known to be increased in response to transient brain ischemia, but whether this ATP increase is beneficial or harmful to ischemic brain injury is controversial, and a mechanism underlying the ATP increase is unknown. Vesicular nucleotide transporter (VNUT), a transporter responsible for vesicular storage of ATP, is recently an emerging key factor for purinergic chemical transmission in various physiological and pathological phenomena; however, a contribution of VNUT and its physiological consequences to ischemia also remain unknown.

Middle cerebral artery occlusion (MCAO) with reperfusion was used to induce experimental stroke in adult male mice. In vivo microdialysis was used to measure extracellular ATP concentrations following ischemia in the brain of freely moving mice.

Transient brain ischemia increased expression of VNUT preferentially in microglia in the ischemic region. VNUT deficiency exacerbates brain infarction following transient MCAO, which indicates neuroprotective potential of microglial VNUT. Extracellular ATP level was increased during transient ischemic period and returned to the baseline immediately after reperfusion in both WT and VNUT-KO mice. Interestingly, after returning to the baseline, ATP levels had a tendency to increase again and persisted at high levels until 72 hours after MCAO in WT mice; however, ATP increase in this later phase was not observed in VNUT-KO mice. To investigate the mechanism underlying microglial VNUT upregulation following brain ischemia, cytokine array was performed. We focused on IL-6, the cytokine that was significantly increased at 1 day after transient MCAO, and we found that when added to cultured microglia, IL-6 was able to induce microglial VNUT mRNA expression in vitro.

These results suggest that VNUT being upregulated in response to ischemia via IL-6- mediated mechanisms, has a vital role for exocytosis of ATP from microglia in the ischemic brain. The microglial ATP exocytosis contributes to neuroprotection against transient ischemic brain injury.

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