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K. Varadaraj, S. S. Kumari, R. T. Mathias; Unique and Analogous Functions of AQP0 for Lens Fiber Cell Architecture and Transparency. Invest. Ophthalmol. Vis. Sci. 2010;51(13):685.
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© ARVO (1962-2015); The Authors (2016-present)
To investigate whether aquaporin 0 (AQP0) performs only water channel function or has additional function/s to contribute towards lens transparency. Mutations as well as knockout of AQP0 result in dominant lens cataract. Among the multiple functions speculated for AQP0, only water channel function has been authentically proven. Lens switches protein expression from AQP1 in the equatorial epithelial cells to 40 times less efficient AQP0 in the differentiating secondary fiber cells. If AQP0 has only water permeability function, can the more efficient water channel, AQP1, transgenically expressed in the fiber cells compensate and restore lens transparency in the AQP0 knockout (AQP0 -/-) mouse lens? If not, it can be inferred that AQP0 performs distinctive role/s in the lens besides the water channel function.
We first generated a transgenic wild type mouse line (TgAQP1+/+) expressing human AQP1 in the lens fiber cells using the fiber cell-specific alphaA-crystallin promoter. TgAQP1+/+ mice were crossed with AQP0 knockout (AQP0-/-) mice to generate AQP0-/- mice expressing AQP1 in the lens fiber cells (TgAQP1+/+/AQP0-/-) to see whether lens transparency can be restored. RT-PCR, immunostaining, western blotting, confocal microscopy, electron microscopy, membrane water permeability assay and lens transparency evaluation techniques were used in this study.
Transgenic mice (TgAQP1+/+) showed increase in membrane water permeability without causing defects in lens morphology, ultrastructure or transparency compared to the wild type. AQP0-/- compromised homeostasis of the lenses as demonstrated by the development of cataract. Water permeability of TgAQP1+/+/AQP0-/- mice was 2.6 times more than that of the wild type. Transgene AQP1 reduced the severity of lens cataract and lessened the pace of acceleration of cataractogenesis; however, lens transparency was not completely restored. Lens fiber cells showed less-compact arrangement.
Partial restoration of lens transparency in AQP0 null mice expressing transgenic AQP1 in the fiber cells suggests that AQP0 and AQP1 have at least one analogous function, of being water channels. Using an animal model, this study also reveals that AQP1 cannot fully replace AQP0 functionally, implying AQP0 may have unique function/s. Absence of AQP0 could be the cause for the less-compact architecture of the fiber cells, indicating a possible cell-to-cell adhesion structural role for AQP0.
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