Aquaporin-4 expression levels and mis-localization are inversely linked to peritumoral edema in gliomas of varying aggressiveness

Aquaporin-4 (AQP4) and its extended isoform, AQP4ex, are crucial for regulating brain water homeostasis. Mis-localization of these isoforms is implicated in various brain tumors, including glioblastoma multiforme (GBM). This study explores AQP4 isoform expression and localization in Pilocytic Astrocytomas (PA), a circumscribed astrocytic low-grade glioma, compared to GBM, an adult-type diffuse high grade glioma. We found significant upregulation of AQP4 and AQP4ex in PA, with notable mis-localization deviating from the typical perivascular localization seen in healthy tissue. This mis-localization mirrors the phenotype observed in AQP4ex knockout models, where impaired AQP4 localization is linked to disrupted water homeostasis and reduced waste clearance, despite overall increased AQP4 levels. Interestingly, PA shows minimal peritumoral edema and a relatively intact blood-brain barrier (BBB), with elevated phosphorylated AQP4ex (pAQP4ex) suggesting a role in stabilizing AQP4 function. In contrast, GBM exhibits reduced AQP4/AQP4ex expression, significant peritumoral edema, and BBB disruption. GFAP isoforms, GFAPκ and GFAPδ, are upregulated in PA, associated with Rosenthal fibers, indicating a stabilizing astrocytic response. GBM, however, shows generalized GFAP increase, reflecting aggressive gliosis and disrupted water homeostasis. In conclusion, both AQP4 expression levels and mis-localization are important factors influencing peritumoral edema and tumor aggressiveness in gliomas. This study positions AQP4 as a potential biomarker for glioma progression, offering insights into astrocytic function and paving the way for targeted therapies.