| Ocular lens fiber cells have a well-developed cytoskeletal network and numerous specialized intercellular junctions. In addition to three major cytoskeletal elements (microfilaments, intermediate filaments, and microtubules) common to all eukaryotic cells, fiber cells contain a unique cytoskeletal element called the beaded filament (BF). Biochemical and immunocytochemical studies have shown two fiber-cell-specific proteins, called CP49 and filensin, to be the principal components of the beaded filament. The primary amino acid sequences for both proteins from multiple species established that both are related to the intermediate filament family (IF) of proteins. However, both proteins proved to be the most divergent IF proteins yet identified.; The observation that the two BF proteins were restricted in expression to the lens fiber cell, combined with their extreme degree of divergence from the remainder of the IF family, led to the suggestion that BF proteins must play a critical role in the biology of the lens fiber cell, a role which is unique to that cell.; In this study, I tested the hypothesis that BFs are required for achieving and/or maintaining structural specializations of fiber cells. Possible roles include, elongation and differentiation of lens epithelial cells to form fiber cells, transient redistribution of organelles, fiber cell architecture, long-range order of fiber cells and ultimately the optical clarity of lens.; I sought to determine a function for the beaded filament of the ocular lens, using a targeted genomic deletion approach. This method directly tested several hypotheses of CP49/BF function, but also had the potential to suggest new function through creation of a “loss-of-function” mutant. The results establish that the CP49 is not required for many hypothesized functions such as normal differentiation, elongation, and organelle positioning/processing, but that it is required for optimal clarity. |
