Cadherin cell-cell adhesion junctions establish the epithelial cell phenotype, prevent their epithelial to mesenchymal transition, define cell cytoarchitecture and signal morphogenesis. We seek to understand the function of these cell-cell junctions as guardians against the induction of cataract. Our studies show a unique structure/function relationship between cadherin junctions, their cytoskeletal linkages and lens differentiation state. Dynamic, highly detergent-soluble N-cadherin/2-catenin adherens junctions define the cytoarchitecture of lens epithelial cells while establishment of the highly ordered and elongated cytoarchitecture of differentiating lens fiber cells involves stabilization of cadherin junctions. We discovered the presence of a novel N-cadherin junctional complex in lens fiber cells, linked to the intermediate filament cytoskeleton through 3-catenin. We will investigate this novel cadherin complex for its role in providing the elongating lens fiber cells with the unique structural properties required for their differentiation and stability. In addition, we will study the differentiation-specific role of N-cadherin junctional complexes in the organization of the cytoskeleton and examine the role of the cytoskeleton in stabilizing cadherin differentiation-specific function. Interestingly, Src family tyrosine kinases (SFKs), regulators of cadherin junctions, have a dual role in lens differentiation. The SFK Src maintains lens epithelial cells in a proliferative, undifferentiated state and the SFK Fyn signals lens fiber cell differentiation. In this proposal we continue our investigation of the mechanisms of SFK regulation of N-cadherin junctional stability, and N-cadherin-linked SFK signaling of lens cell differentiation, in a pathway involving PI3K and Rac. Our previous studies show that the unregulated, constitutive activation of Src kinases disassembles lens cadherin junctions. We now find in a culture model for stress-induced cataract that lens opacities result from activation of a p38 stress kinase/Src kinase signaling pathway that targets the cadherin junctions. If Src kinase activation is blocked, both cadherin junctions and normal lens cytoarchitecture are maintained, and the development of lens opacities is prevented. We will investigate the mechanism of N-cadherin junction destabilization in cataract, focusing on the role protein tyrosine phosphatases (PTPs) as molecular regulators of both Src kinases and cadherin junctions. PTPs are inactivated by reactive oxygen species, a common cause of stress-induced cataract. It is hoped that in the long-term, understanding of this pathway will provide suitable pharmaceutical targets for the prevention of cataract. Will investigate our hypotheses in three specific aims: (1) Examine the hypothesis that N-cadherin regulates the cytoskeleton to establish and maintain lens structure and stability; (2) Examine the hypothesis that the Src kinases regulate lens cell differentiation state by their action as both upstream regulators and downstream signaling effectors of cadherin junctions; (3) Examine the hypothesis that misregulation of Src kinases abrogates the role of cadherin junctions and the cytoskeleton as protectors of lens structural stability and guardians against cataract.
Cataracts are opacities of the lens that interfere with its ability to focus an image on the retina. While many of the properties of cataract have been determined, the mechanism of cataract induction and potential methods of cataract prevention still remain elusive. The long term goals of this proposal are to understand the role of cadherin cell-cell junctions as guardians against this lens disease. ? ? ?
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