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A Human Lens Model of Cortical Cataract: Ca²⁺-Induced Protein Loss, Vimentin Cleavage and Opacification

From the School of Biological Sciences, University of East Anglia, Norwich, United Kingdom.

PURPOSE. Cortical cataract in humans is associated with Ca²⁺ overload and protein loss, and although animal models of cataract have implicated Ca²⁺-activated proteases in this process, it remains to be determined whether the human lens responds in this manner to conditions of Ca²⁺ overload. The purpose of these experiments was to investigate Ca²⁺-induced opacification and proteolysis in the organ-cultured human lens.

METHODS. Donor human lenses were cultured in Eagle’s minimum essential medium (EMEM) for up to 14 days. The Ca²⁺ ionophore ionomycin was used to induce a Ca²⁺ overload. Lenses were loaded with [³H]-amino acids for 48 hours. After a 24-hour control efflux period, lenses were cultured in control EMEM (Ca²⁺ 1.8 mM), EMEM + 5 µM ionomycin, or EMEM + 5 µM ionomycin + 5 mM EGTA (Ca²⁺ <1 µM). Efflux of proteins and transparency were monitored daily. Protein distribution and cytoskeletal proteolysis were analyzed at the end of the experiment. Cytoskeletal proteins were isolated and separated by sodium dodecyl sulfate–polyacrylamide gel electrophoresis (SDS-PAGE). Western blot analyses were probed with anti-vimentin antibody (clone V9) and detected by enhanced chemiluminescence.

RESULTS. Lenses cultured under control conditions remained transparent for 14 days in EMEM with no added supplements or serum. The lenses synthesized proteins and had a low rate of protein efflux throughout the experimental period. Ionomycin treatment resulted in cortical opacification, which was inhibited when external Ca²⁺ was chelated with EGTA. Exposure to ionomycin also led to an efflux of [³H]-labeled protein, amounting to 41% of the labeled protein over the 7-day experimental period, compared with 12% in ionomycin + EGTA–treated lenses. Efflux was accounted for by loss from the lens soluble protein (crystallin) fraction. Western blot analysis of the cytoskeletal protein vimentin (56 kDa) revealed a distinct breakdown product of 48 kDa in ionomycin-treated lenses that was not present when Ca²⁺ was chelated with EGTA. In addition, high-molecular-weight proteins (~115 kDa and 235 kDa) that cross-reacted with the vimentin antibody were observed in ionomycin-treated lenses. The Ca²⁺-induced changes were not age dependent.

CONCLUSIONS. Human lenses can be successfully maintained in vitro, remaining transparent for extended periods. Increased intracellular Ca²⁺ induces cortical opacification in the human lens. Ca²⁺-dependent cleavage and cross-linking of vimentin supports possible roles for calpain and transglutaminase in the opacification process. This human lens calcium-induced opacification (HLCO) model enables investigation of the molecular mechanisms of opacification, and the data help to explain the loss of protein observed in human cortical cataractous lenses in vivo.

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