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Sondip K. Biswas, Lawrence Brako, Woo-Kuen Lo; Abnormal Formation of Extensive Aquaporin Junctions Which Significantly Alters Fiber Cell Shape Is Not Associated with the Late-onset Lens Opacity in Cav1-KO Mice. Invest. Ophthalmol. Vis. Sci. 2012;53(14):1047.
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Aquaporin junctions (AJs) are believed to play adhesion function between lens fiber cells. Our freeze-fracture studies have shown that Aqp0 proteins are arranged into square arrays to form asymmetrical AJs between mature lens fiber cells. These junctions are specifically associated with undulating tongue-and-groove (ridge) membrane surfaces. Here we examined the causative effect between extensive AJ formation and the late-onset lens opacity in cav1-KO mice. Caveolin1 is thought to play a role in cholesterol transport in many cell types.
Lenses from cav1 wild-type and KO mice between 7 days and 22 months old were studied with light microscopy, scanning EM, freeze-fracture TEM, freeze-fracture immunogold labeling and filipin cytochemistry for cholesterol detection.
Light microscopy showed that freshly isolated lenses from cav1-KO mice did not exhibit any detectable lens opacity until 18 months old. The light opacity was first observed in the lens nucleus and later extended to the inner cortical area. Scanning EM first revealed the abnormal formation of abundant AJs in the outer cortical fibers (~200μm deep from the capsule surface) toward the lens core in cav1-KO mice at 30 days old; this abnormal formation pattern persisted throughout all the ages studied. In contrast, the normal distribution of abundant AJs was restricted to the inner mature fibers (~500μm deep) and the lens core in the same-age WT mice. The abnormally-formed AJs with extensive undulating ridge pattern covered the entire fiber membrane surface, which significantly altered the fiber cell configurations. Freeze-fracture TEM revealed that distinct arrangement of square arrays was distributed along the single undulating membranes of newly-formed AJs. FRIL studies confirmed the asymmetrical labeling of Aqp0 antibody in square-array junctions. By filipin cytochemistry, the filipin-cholesterol-complexes were found preferentially distributed in the square arrays-free area (cholesterol-rich), but not in the square arrays-rich region of the junctions.
This study indicates that although abnormal formation of extensive AJs dramatically alters the cortical fiber-cell shapes, it does not directly cause any detectable lens opacity in the young and mature cav1-KO mice. It is proposed that the resulting extensive AJs may provide extra adhesive support between fiber cells to maintain compactness and transparency despite their significant cell-shape changes. The cav1-KO mice would be an excellent animal model to critically examine the formation process and roles of AJs in maintaining the normal lens membrane architecture.
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