Defects in lens differentiation or homeostasis can result in cataract, the major cause of visual impairment in humans. An understanding of the mechanisms that control lens development and maintain lens transparency is therefore an essential part of addressing a major international health issue. Gap junctions participate in joining lens epithelial and fiber cells into a metabolic and ionic syncytium essential for lens clarity. The highest level of gap junction-mediated intercellular coupling (GJIC) in the lens is at the equator, the region where epithelial cells differentiate into secondary fibers. The applicant has previously developed a serum-free system to culture primary embryonic chick lens cells from the peripheral epithelium, the cell type that in vitro most closely recapitulates the in vivo processes of epithelial-to-fiber differentiation and fiber-type gap junction formation. We have reported that in this system, FGF (either recombinant or from vitreous humor) upregulates the expression of the fiber differentiation markers examined and increases GJIC in parallel but at least partially independent processes. FGF activity was shown to diffuse out of intact vitreous bodies, the major in vivo reservoir of growth factors for the lens. These and additional studies led to a novel model of how FGF-mediated sustained activation of the ERK MAP kinase contributes to the asymmetry of GJIC believed to be essential for lens clarity (Le and Musil 2001 J Cell Biol 154:197-216). Preliminary results presented in this application provide the first evidence that BMP also upregulates fiber marker expression and (synergistically with FGF) GJIC in cultured lens cells, and that vitreous humor contains a diffusible BMP-like activity. Other studies support the novel hypothesis that epithelial-to-fiber differentiation may be modulated by a unique postranslational modification (polysialylation) of the cell adhesion molecule NCAM. The goals of the proposed studies are: (1) to investigate the signal transduction pathways by which FGF and/or BMP upregulate GJIC and fiber marker expression in cultured chick and rat lens cells, and elucidate the role of vitreous humor-derived BMP in these processes in vivo; (2) to study how FGF and/or BMP upregulate GJIC without increasing gap junction channel number; (3) to determine the role of NCAM polysialylation in epithelial-to-fiber differentiation; and (4) to utilize my expertise in gap junction formation and degradation to investigate how the turnover of lens gap junctions is decreased from a t1/2 less than or equal too 5 h to being stable for years upon differentiation of epithelial cells to mature fibers. These studies will provide new insights into how growth factor signaling and cell-cell interactions contribute to the unique structure and function of the lens.
