AQP4ex is a recently discovered isoform of AQP4 generated by a translational readthrough mechanism. It is strongly expressed at the astrocyte perivascular endfeet as a component of the supramolecular membrane complex, commonly called orthogonal array of particles (OAP), together with the canonical isoforms M1 and M23 of AQP4. Previous site-directed mutagenesis experiments suggested the potential role of serine³³¹ and serine³³⁵, located in the extended peptide of AQP4ex, in water channel activity by phosphorylation. In the present study we evaluated the effective phosphorylation of human AQP4ex. A small scale bioinformatic analysis indicated that only Ser³³⁵ is conserved in human, mouse and rat AQP4ex. The phosphorylation site of Ser³³⁵ was assessed through generation of phospho-specific antibodies in rabbits. Antibody specificity was first evaluated in binding phosphorylated peptide versus its unphosphorylated analog by ELISA, which was further confirmed by site-directed mutagenesis experiments. Western blot and immunofluorescence experiments revealed strong expression of phosphorylated AQP4ex (p-AQP4ex) in human brain and localization at the perivascular astrocyte endfeet in supramolecular assemblies identified by BN/PAGE experiments. All together, these data reveal, for the first time, the existence of a phosphorylated form of AQP4, at Ser³³⁵ in the extended sequence exclusive of AQP4ex. Therefore, we anticipate an important physiological role of p-AQP4ex in human brain water homeostasis.

Water deprivation triggers coordinated physiological responses to preserve body fluid balance, yet the molecular mechanisms that regulate aquaporin-mediated water transport under dehydration remain incompletely understood. Aquaporin-4 (AQP4), the main water channel in the brain and a basolateral water pathway in the kidney collecting duct, exists in multiple isoforms, including the translational readthrough variant AQP4ex, whose regulatory role is only beginning to be defined. Here, we investigated the effects of acute water deprivation (6–12 h) on AQP4 isoform expression and phosphorylation in a mouse kidney and brain. While total AQP4 and AQP4ex protein levels remained largely unchanged in both tissues, dehydration induced a marked and divergent regulation of the phosphorylated form of AQP4ex. Levels increased in the kidney medulla, consistent with enhanced antidiuretic water transport, but decreased in the cerebral cortex, suggesting a protective reduction in perivascular water permeability. No changes were detected in the cerebellum. These findings identify phosphorylation of AQP4ex as a rapid, tissue-specific regulatory mechanism that adjusts water flux according to the physiological needs of each organ, revealing an additional layer of control in systemic water homeostasis and highlighting AQP4ex as a potential target in dehydration-related and osmotic disorders. Future studies could explore the signaling pathways regulating AQP4ex phosphorylation and investigate its potential involvement in pathological conditions, such as diabetes insipidus or cerebral edema.