Crystallin mRNA concentrations and distribution in lens of normal and galactosemic rats. Implications in development of sugar cataracts.

It is well established that high concentrations of sugar in the lens of the eye eventually lead to fiber cell destruction and cataracts. In these studies the decrease in crystallin mRNAs was quantified as a result of influx of high concentrations of galactose into the lens of rats. The alpha A-, alpha B1-, and gamma-crystallin mRNA concentrations were assessed in normal lens and in lens undergoing development of sugar cataracts by northern blot and in situ hybridization methods. In a normal, 28-day-old lens, alpha A-crystallin mRNA accumulated to high levels throughout the fiberplasm, and alpha B-crystallin mRNA was present at low levels in epithelial cells, with increased expression in elongating epithelial and fiber cells. The beta B1-crystallin mRNA was distributed to about the same grain density throughout the fiberplasm but at significantly lower levels than alpha A-crystallin mRNA. The gamma-crystallin mRNA first emerged in the terminally differentiated fiber cell, with insignificant amounts detected in the elongating epithelial and fiber cells at the bow. Measurements of hybridization levels on the same RNA population isolated from a single lens showed that in the controls, alpha A-crystallin mRNA comprised about ten times the level of alpha B-crystallin mRNA and twice the level of beta B1- and gamma-crystallin mRNAs. In the cataractous lens the rate of decrease in the concentrations of alpha A-, alpha B- and beta B1-crystallin mRNAs was the same; the decrease in gamma-crystallin mRNA was far more severe. By 20 days of feeding of galactose, at the age of 48 days, gamma-crystallin mRNA diminished to about 9% of the control levels, alpha A-crystallin mRNA to 49%, alpha B-crystallin mRNA to 55%, and beta B1-crystallin mRNA to 65%. In the normal lens, at 48 days of age, the levels of alpha A-, alpha B-, and beta B1-crystallin mRNAs showed no significant changes; the gamma-crystallin mRNA level decreased significantly, to about 70% of the day-28 level, the time at which galactose feeding began. Overall, these data suggest that the loss in crystallin mRNAs in response to the development of galactose cataracts follows this order of decline: gamma greater than alpha B greater than alpha A greater than beta B1.