A predominant form of C-terminally end-cleaved AQP0 functions as an open water channel and an adhesion protein in AQP0^ΔC/ΔC mouse lens.

The purpose of this investigation was to find out whether C-terminally end-cleaved aquaporin 0 (AQP0), that is present predominantly in the lens mature fiber cells of the WT, functions as a water channel and a cell-to-cell adhesion (CTCA) protein in a knockin (KI) mouse model (AQP0^ΔC/ΔC) that does not express intact AQP0. A genetically engineered KI mouse model, AQP0^ΔC/ΔC, expressing only end-cleaved AQP0 was developed. This model expresses 1-246 amino acids of AQP0, instead of the full length 1-263 amino acids. Lens transparency of postnatal day 10 (P10) was analyzed qualitatively by dark field imaging. WT, AQP0^+/- and AQP0^+/ΔC lenses were transparent; AQP0^-/- and AQP0^ΔC/ΔC mouse lenses displayed loss of transparency. Lens fiber cell membrane vesicles (FCMVs) were prepared from wild type (WT), AQP0 heterozygous (AQP0^+/-), AQP0 knockout (AQP0^-/-), AQP0^+/ΔC and AQP0^ΔC/ΔC; water permeability (P_f) was measured using the osmotic shrinking method. CTCA assay was performed using adhesion-deficient L-cells and FCMVs prepared from the abovementioned genotypes. FCMVs of AQP0^+/- and AQP0^-/- showed a statistically significant reduction (P < 0.001) in P_f and CTCA compared to those of WT. AQP0^+/ΔC and AQP0^ΔC/ΔC FCMVs exhibited no statistically significant alteration (P > 0.05) in P_f compared to those of WT. However, CTCA of AQP0^+/ΔC AQP0^ΔC/ΔC FCMVs was significantly higher (P < 0.001) than that of WT FCMVs. Our experiments clearly show that C-terminally end-cleaved AQP0 can function both as a water channel and a CTCA molecule in the lens fiber cell membranes. Also, end-truncation plays an important role in increasing the CTCA between fiber cells.

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