Homologous and heterologous enhancers modulate spatial expression but not cell-type specificity of the murine γF-crystallin promoter

Previous studies have shown that mouse γF-crystallin sequences –759 to +45, which include the core promoter and two upstream enhancer elements, contain sufficient information for directing gene expression to terminally differentiated fiber cells of the ocular lens. To investigate the role that proximal sequences of the mouse γF-crystallin promoter play in the developmental regulation of gene expression, we generated transgenic mice containing the lacZ gene driven by either mouse γF-crystallin sequences –171 to +45, which lack functional enhancers, or a hybrid hamster αA-/mouse γF-crystallin promoter, which contains the hamster αA-crystallin enhancer instead of operational γF-crystallin enhancers. In situ analysis of lacZ expression in these mice revealed that the mouse γF-crystallin promoter segment –171 to +45, which shows low activity in vitro, is able to direct gene expression to the fiber cells in the nucleus of the lens. However, animals expressing γ171–lacZ show both a lower level of expression of the lacZ gene and a narrower pattern of staining in the lens nucleus than mice expressing γ759–lacZ, which contains the two enhancer elements located between –392 and –278 and –226 to –123. Moreover, animals expressing the lacZ gene driven by the hybrid αA-/γF-crystallin promoter show a pattern of staining in the lens nucleus similar to that of mice expressing γ759–lacZ, indicating that although the αA-crystallin enhancer is unable to alter the cell-type specificity conferred by the mouse γF-crystallin proximal promoter segment –171 to +45, it is able to expand the spatial domain of gene expression in a manner similar to the native γF-crystallin enhancers. The significance of these results, in terms of the mechanisms governing spatial regulation of gene expression, is discussed.