Alpha-crystallin is an aggregate of two polypeptides, alphaA and alphaB, which are small heat shock proteins and act as molecular chaperones in vitro preventing stress-induced protein aggregation. Loss of chaperone activity of alpha-crystallin can lead to cataract formation. Targeted disruption of a mouse alphaA-crystallin gene induces cataract at an early age, implying a critical role of this protein in maintaining fiber cell transparency. Lens epithelial cells express relatively low levels of alphaA and alphaB. Previously it was thought that alphaA acted simply as a sink for unfolding proteins in lens fiber cells. However, we recently showed that lens epithelial cultures of alphaA knockout mice have 50 percent slower growth and an altered cell cycle distribution. Cells synchronized in mitosis had a higher level of alphaA, and alphaA was localized in the nucleus of mitotic lens epithelial cells. This suggests that alphaA expression may change the regulation of the cell cycle. These new findings may explain in part why the lenses of alphaA knockout mice are smaller than wild type, and may suggest a role of alpha-crystallin in deregulation of growth in secondary cataracts. We also found that lens epithelial cells derived from alphaB knockout mice demonstrate hyperproliferation and tetraploidy at an increased frequency. We propose a series of experiments to further examine the role of alpha-crystallin in cell growth. Wild type and alphaA knockout DNA synthesizing cells will be labeled in vivo to gain insight into the role of alphaA in the cell cycle. Cell cycle transit times will be determined in wild type and alphaA knockout cultured lens epithelial cells. Expression of alphaA and alphaB will be monitored at different phases in the cell cycle. Experiments will be designed to examine the interaction of alphaA and alphaB with cell cycle proteins to gain further insight into the molecular mechanism by which alpha-crystallin changes the regulation of the cell cycle. These mechanistic studies will be helpful in determining how the loss of alpha-crystallin function can lead to deregulation of normal cell growth and to cataract formation. Since secondary cataracts are mainly a growth problem, these studies may help gain insight into the mechanism of secondary cataract formation.
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