Aggregation state determines the localization and function of M1-and M23-aquaporin-4 in astrocytes

The astrocyte water channel aquaporin-4 (AQP4) is expressed as heterotetramers of M1 and M23 isoforms in which the presence of M23-AQP4 promotes formation of large macromolecular aggregates termed orthogonal arrays. Here, we demonstrate that the AQP4 aggregation state determines its subcellular localization and cellular functions. Individually expressed M1 AQP4 was freely mobile in the plasma membrane and could diffuse into rapidly extending lamellipodial regions to support cell migration. In contrast, M23 AQP4 formed large arrays that did not diffuse rapidly enough to enter lamellipodia and instead stably bound adhesion complexes and polarized to astrocyte end-feet in vivo. Co-expressed M1- and M23 AQP4 formed aggregates of variable size that segregated due to diffusional sieving of small, mobile M1-AQP4-enriched arrays into lamellipodia and preferential interaction of large, M23 AQR4-enriched arrays with the extracellular matrix. Our results therefore demonstrate an aggregation state dependent mechanism for segregation of plasma membrane protein complexes that confers specific functional roles to M1- and M23-AQP4.