STUDIES ON LENS DEVELOPMENT
 
The studies on crystallin transcriptional regulation have focused on the bB1-crystallin gene.  This lens-specific gene is expressed at high levels in elongating lens fiber cells in both chickens and rodents, and the bB1-crystallin protein accumulates to 1-8% of the water soluble protein of the lens.  I have studied the spatial and temporal regulation of the chicken bB1-crystallin promoter in transgenic mice and examined the cis-elements and trans-factors responsible for its lens specificity.  Although, the 5' flanking sequences of the chicken and mouse bB1-crystallin promoters have little sequence similarity, the chicken promoter from -434 to +30 is sufficient to drive lens specific expression at or near the expression level of endogenous mouse bB1-crystallin in the embryonic mouse lens.  Cellular localization of the reporter gene has confirmed that the spatial distribution of expression mimics that of endogenous mouse bB1-crystallin.  Deletions and mutations of the chicken bB1-crystallin promoter have led to the identification of some of the elements responsible for this high level of expression and has demonstrated that -101 to +30 is sufficient for lens specificity.  In addition, cis-element analysis is being conducted on the mouse bB1-crystallin promoter in order to explain the conservation of promoter function between mouse and chicken in spite of the lack of obvious sequence similarity.  Studies on the identification of transfactors responsible for the lens fiber cell specific expression of bB1-crystallin has been progressing rapidly and has shown that the homeobox protein Pax-6, a gene essential for the early specification of the lens, represses the expression of bB1-crystallin in lens epithelial cells while another homeobox protein, Prox 1, which is expressed later in lens development, activates expression of chicken bB1-crystallin.  Thus, it seems possible that competition between these two factors is at least partially responsible for the lens fiber cell specificity of bB1-crystallin. These studies will hopefully give insight into how the terminal differentiation of an organ is controlled by differential gene regulation and how the perturbation of these pathways can lead to disease.