Lens-preferred activity of chicken δ1- and δ2-crystallin enhancers in transgenic mice and evidence for retinoic acid-responsive regulation of the δ1-crystallin gene

There are two tandemly linked δ-crystallin genes (5' δ-δ2 3') in the chicken, with the δ1-crystallin gene being expressed much more highly (50- 100-fold) in the embryonic lens than the δ2-crystallin gene. Previous transfection experiments have shown that a lens preferred enhancer exists in the third intron of each chicken δ-crystallin gene. In the present investigation we have used transgenic mice to establish that both the chicken δ1- and δ2-crystallin enhancers are preferentially active in the mouse lens in combination with their homologous promoter and the chloramphenicol acetyltransferase (CAT) reporter gene. The promoter/CAT constructs lacking the enhancers were inactive in the transgenic mice. In one case, a truncated δ2-crystallin promoter (-308/+24) in combination with the enhancer was also active in the Purkinje cells of the cerebellum of the transgenic mice, which could prove useful in future experiments. Finally, retinoic acid receptors (RARβ) activated the δ1-crystallin, but not the δ2-crystallin enhancer in the recombinant plasmids in cotransfected embryonic chicken lens epithelial cells treated with retinoic acid. This activation did not occur when using the core enhancer (fragment B4)lacking surrounding flanking sequences (fragment B3 and B5) of the enhancer. Together these experiments show that the chicken δ-crystallin enhancers show lens-preference in transgenic mice despite the absence of δ-crystallin in this species and add retinoic acid nuclear receptors to the growing list of transcription factors (including Pax-6, Sox-2, and δEF3) that directly or indirectly contribute to the high expression of the δ1-crystallin gene in the lens.