Intact AQP0 performs cell-to-cell adhesion.

Aquaporins (AQPs) constitute a major conduit for movement of water across plasma membranes. AQP0 is expressed in the fiber cells and is critical for lens transparency and homeostasis as mutations and knockout have resulted in dominant lens cataract. Several functions have been attributed for AQP0. In vitro and ex vivo experiments from several laboratories have confirmed the water permeability function of AQP0. However, this function seems paradoxical when the lens switches protein expression from AQP1 in the equatorial epithelial cells to 40 times less efficient AQP0 in the differentiating fiber cells. A possible explanation is AQP0 may perform unique function/s besides being a water pore. Indirect evidences including those from structural studies indicate a cell-to-cell adhesion role for AQP0. However, there is a lack of experimental evidence directly demonstrating the cell-to-cell adhesion capability of AQP0. We studied the adhesion property of human intact AQP0 by expressing it in adhesion-deficient mouse fibroblast L-cells using a newly devised method as well as a traditional assay. Our results reveal that AQP0 indeed can perform cell-to-cell adhesion. AQP1, two alternate splice variants of AQP4 (AQP4-M1and AQP4-M23) and E-cadherin were also tested to validate the results. Cell-to-cell adhesion and cell aggregation properties of AQP0 expressing L-cells were less than those of the positive control L-cells expressing mouse E-cadherin and greater than those of AQP4-M23. AQP1 or AQP4-M1 expressing cells did not show cell-to-cell adhesion or cell aggregation. To our knowledge, this is the first report validating the possible structural role of intact AQP0 as a cell-to-cell adhesion protein, using an in vitro expression system.