| Summary: | More than 20 years have passed since the first demonstration of Aquaporin-9 (AQP9) in the brain. Yet its precise localization and function in brain tissue remain unresolved. In peripheral tissues, AQP9 is expressed in leukocytes where it is involved in systemic inflammation processes. In this study, we hypothesized that AQP9 plays a proinflammatory role in the brain, analogous to its role in the periphery. We also explored whether <i>Aqp9</i> is expressed in microglial cells, which would be supportive of this hypothesis. Our results show that targeted deletion of <i>Aqp9</i> significantly suppressed the inflammatory response to the parkinsonian toxin 1-methyl-4-phenylpyridinium (MPP<sup>+</sup>). This toxin induces a strong inflammatory response in brain. After intrastriatal injections of MPP<sup>+</sup>, the increase in transcript levels of proinflammatory genes was less pronounced in AQP9<sup>−/−</sup> mice compared with wild-type controls. Further, in isolated cell subsets, validated by flow cytometry we demonstrated that Aqp9 transcripts are expressed in microglial cells, albeit at lower concentrations than in astrocytes. The present analysis provides novel insight into the role of AQP9 in the brain and opens new avenues for research in the field of neuroinflammation and chronic neurodegenerative disease.
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