These studies test several novel hypotheses concerning the control of terminal differentiation in chick lens fibers with respect to growth factors, inhibition of mitosis and cell-cell interactions. Since multiple growth factors are known to influence diverse aspects of lens cell behavior, a profile of relevant growth factor receptors and their associated signal transduction mechanisms will initially be compiled for both post-mitotic epithelial cells and fiber cells. These observations will test the hypothesis that distinct growth factor related mechanisms characterize recognizable stages of lens fiber development. In addition, these observations can be utilized as new criteria for assessing the effectiveness of all subsequent cell culture manipulations on the accumulation of fiber-like traits. The next major hypothesis to be tested is that inhibition of mitosis during initial stages of terminal differentiation is actively regulated by cell surface receptors. Three in vitro models will examine the role of intracellular protein kinases, extracellular matrix interactions and cell adhesion glycoproteins in maintaining mitotic withdrawal as cell-cell interactions increase during aggregated cell growth. These models will also be utilized to test the hypothesis that, in mitotically inhibited cells, sequential or combined growth factor stimulation elicits a more complete profile of differentiated traits. The final major hypothesis to be tested is that lens fiber development is capable of being regulated by the endogenous production of growth factors or their receptors. The monitoring of differentiation-specific events in culture, aside from the previously mentioned growth factor mechanisms, will include more traditional and additional original criteria. Well established criteria include an increased protein to DNA ratio, nuclear pyknosis and DNA fragmentation, the persistent synthesis of specific crystallins, and the accumulation to high levels of gap junction proteins. A new gauge of lens fiber differentiation entails the accumulation, phosphorylation, and insolubilization of a 49 kD cytoskeletal protein. Not only is this marker affected by receptor-mediated mechanisms, but it serves as a reliable endpoint for fiber development. An additional marker of lens fiber terminal differentiation to be examined is the relative abundance of NCAM in cell membranes. These studies will more fully define how differentiation in the lens is controlled and may lend fresh insight into additional, more fundamental causes of cataract development.
