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The aquaporin (AQP) transmembrane proteins facilitate the movement of water across the plasma membrane. In the lens, AQP0 is expressed in fiber cells and AQP1 in the epithelium. Recently, two individuals were identified with congenital polymorphic autosomal dominant cataract, due to a single nucleotide base deletion mutation in the lens AQP0. The deletion modified the reading frame resulting in the addition of a premature stop codon. In the present study, we examined the water permeability properties, trafficking and dominant negative effects as well as cytotoxicity due to the mutant AQP0 (Δ213-AQP0) protein. The membrane water permeability (P_w) of Δ213-AQP0 expressing oocytes (14±1μm/s) was significantly lower than those expressing WT-AQP0 (25±3μm/s). P_w of water injected control oocytes was 13±2μm/s. Co-expression of WT-AQP0 with Δ213-AQP0 significantly lowered the P_w (18±3μm/s …