Studies On The Effect Of Aquaporin Deletion In Mice On Intraocular Pressure And Aqueous Fluid Production

The transport of water across some cell plasma membranes is facilitated by aquaporin (AQP) water channels, which are small transmembrane proteins that function as passive conduits for osmotically or hydrostatically-driven water transport. 1992 Agre and associates serendipitously identified a member of the water channel family (CHIP28, subsequently renamed AQP1). There are at present 10 members of the mammalian aquaporin family (AQPO-AQP9). Structural and mutagenesis studies indicate that aquaporin monomers assemble in membrane as tetramers in which each monomer contains an independent water pore. Analysis of tissue distribution indicates aquaporin expression in many epithelia and endothelia that are involved in fluid transport.Aquaporin (AQP) water channels are expressed in cell types hi eye that are involved in aqueous fluid production and outflow. To date the evidence for a role of aquaporins in eye physiology has been indirect, consisting of tissue localization and cell culture experiments. There has until recently been a paucity of information about the functional role of aquaporins in mammalian physiology because of the lack of suitable inhibitors and animal models. Measurements of the static and dynamic properties of the aqueous fluid in the mouse eye were quite challenging.The purpose of this study was to test the hypothesis that aquaporins play a role in aqueous fluid dynamics and thus intraocular pressure regulation. Novel methods for measurement of intraocular pressure/volume and aqueous fluid production/outflow were applied in comparative studies of wildtype mice vs. mice lacking AQP1 and AQP4 (individually and together).Methods: Aqueous fluid production was measured by in vivo confbcal microscopy after transcomeal iontophoretic introduction of fluorescein into the aqueous fluid. Aqueous fluid volume and [C1~J were assayed in samples withdrawn by micropipettes. Intraocular pressure (TOP), pressure-dependent outflow, and anterior chamber compliance were determined from pressure measurements in response to pulsed and continuous fluid infusions into the anterior chamber using micropipettes.Result: In wildtype mice (GDI genetic background, age 4-6 weeks), IOP was 16.0 ?0.4 mmHg, aqueous fluid volume was 7.2 ?0.3 ul, aqueous fluid production was 3.6 ?0.2 ul/hr, aqueous fluid outflow was 0.36 ?0.06 ul/hr/mmHg, and anterior chamber compliance was 0.036 ?0.006 ul/mmHg (mean ?SE, 8-10 eyes). IOP was significantly decreased by up to 1.8 mmHg (PO.002) in age/litter matched mice lacking AQP1 and/or AQP4 in outbred GDI and inbred C57/W6 genetic backgrounds, and aqueous fluid production was decreased by up to 0.9 pl/hr (P<0.05 ). However aquaporin deletion did not significantly affect aqueous fluid outflow, volume or [Cl'J, or anterior chamber compliance. Conclusion: These results provide evidence for the involvement of aquaporins in intraocular pressure regulation by facilitating aqueous fluid secretion across the ciliary epithelium. Aquaporin inhibition may thus provide a novel approach for the treatment of glaucoma.