Rise and fall of crystallin gene messenger levels during fibroblast growth factor induced terminal differentiation of lens cells

Abstract

Explanted rat lens epithelial cells differentiate synchronously in vitro to lens fiber cells in the presence of basic fibroblast growth factor (bFGF). We have monitored the expression of the three rat crystallin gene families, the α-, β-, and γ-crystallin genes, during this process. The expression of these gene families is sequentially activated, first the α-crystallin genes at Day 1, then the β-crystallin genes at Day 3, and finally the γ-crystallin genes at Day 8. The steady state levels of α- and β-crystallin mRNA are not affected by incubation with actinomycin D, suggesting that these mRNAs are stable. Nevertheless, all crystallin mRNAs disappear from the differentiated explants between Days 10 and 11, a process signaled by bFGF. At this time a novel abundant mRNA appears. Cloning and sequencing showed that this mRNA encoded aldose reductase. Our results suggest a novel model for the regulation of crystallin synthesis during lens cell differentiation: a gene pulse delivers a certain amount of stable mRNA, this mRNA is removed at a later stage of differentiation by a stage-specific breakdown mechanism. Each of these regulatory steps requires a signal from bFGF.