Single-cell analysis of osmoregulation reveals heterogeneity of aquaporin 4 functionality in human astrocytes

The water channel aquaporin 4 (AQP4) contributes to water flow and waste removal across the blood-brain barrier and its levels, organization and localization are perturbed in various neurological diseases, including Alzheimer's Disease. This renders AQP4 a potentially valuable therapeutic target. However, most functional assays aimed at identifying modulators of AQP4 function are performed with primary rodent cells and do not consider inter-cellular variations in AQP4 abundance and presentation. To address this, we have established and applied a robust live cell microscopy assay that captures the contribution of AQP4 in the osmotically driven (de-)quenching of the vital dye Calcein-AM with single-cell resolution. Using human astrocytoma cells, we found that performing measurements on cellular regions instead of whole fields of view yielded a more sensitive readout of the osmotic response, which correlated with AQP4 abundance. Stable co-expression of the two major AQP4 isoforms, but not of the individual isoforms, provoked a faster adaptation to osmotic changes, while siRNA-mediated knockdown of AQP4 had the opposite effect. Post-hoc correlation with the canonical membrane marker CD44 revealed that the speed of the osmotic response scaled with AQP4 membrane enrichment. Coating the substrate with laminin promoted AQP4 membrane enrichment, while cell confinement with fixed-size micropatterns further increased the speed of osmoregulation, underscoring the influence of extracellular factors. The osmotic response of primary fetal astrocytes and human iPSC-derived astrocyte models was comparable to AQP4-deficient astrocytoma cells, in line with their low AQP4 levels and indicative of their immature state. In conclusion, a correlative single-cell approach based on the quantification of Calcein-AM quenching capacity, AQP4 abundance and AQP4 membrane enrichment, allows resolving osmoregulation in a more sensitive manner and reveals heterogeneity between and within human astrocyte (-like) cultures, which could prove crucial for future screens aimed at identifying AQP4 modulators.