A transgenic animal model of osmotic cataract. Part 1: over-expression of bovine Na+/myo-inositol cotransporter in lens fibers.

PURPOSE Intracellular osmotic stress is believed to be linked to the advancement of diabetic cataract. Although the accumulation of organic osmolytes (myo-inositol, sorbitol, taurine) is thought to protect the lens by maintaining osmotic homeostasis, the physiologic implication of osmotic imbalance (i.e., hyperosmotic stress caused by intracellular over-accumulation of organic osmolytes) on diabetic cataract formation is not clearly understood. Studies from this laboratory have identified several osmotic compensatory mechanisms thought to afford the lens epithelium, but not the lens fibers, protection from water stress during intervals of osmotic crisis. This model is founded on the supposition that the fibers of the lens are comparatively more susceptible to damage by osmotic insult than is the lens epithelium. To test this premise, several transgenic mouse lines were developed that over-express the bovine sodium/myo-inositol cotransporter (bSMIT) gene in lens fiber cells. METHODS Of the several transgenic mouse lines generated, two, MLR14 and MLR21, were analyzed in detail. Transgenic mRNA expression was analyzed in adult and embryonic transgenic mice by a coupled reverse transcriptase-polymerase chain reaction (RT-PCR) and in situ hybridization on embryonic tissue sections, respectively. Intralenticular myo-inositol content from individual mouse lenses was quantified by anion exchange chromatography and pulsed electrochemical detection. Ocular histology of embryonic day 15.5 (E15.5) embryos from both transgenic (TG) families was analyzed and compared to their respective nontransgenic (NTG) littermates. RESULTS Both RT-PCR and in situ hybridization determined that transgene expression was higher in line MLR21 than in line MLR14. Consistent with this, intralenticular myo-inositol from MLR21 TG mice was markedly higher compared with NTG littermates or MLR14 TG mice. Histologic analysis of E15.5 MLR21 TG embryos disclosed a marked swelling in the differentiating fibers of the bow region and subcapsular fibers of the central zone, whereas the lens epithelium appeared morphologically normal. The lenticular changes, initiated early during lens development in TG MLR21 embryos, result in severe bilateral nuclear cataracts readily observable in neonates under normal rearing and dietary conditions. In contrast, TG MLR14 pups reared under standard conditions produced no lens opacity. CONCLUSIONS Lens fiber swelling and related cataractous outgrowth positively correlated to the degree of lens bSMIT gene expression and intralenticular myo-inositol content. The affected (i.e., swollen) lens fibers appeared to be unable to cope with the water stress generated by the transgene-induced over-accumulation of myo-inositol and, as a result of this inability to osmoregulate, suffered osmotic damage due to water influx.