Diverse roles of Eph/ephrin signaling in the mouse lens

Recent genetic studies show that the Eph/ephrin bidirectional signaling pathway is associated with both congenital and age-related cataracts in mice and humans. We have investigated the molecular mechanisms of cataractogenesis and the roles of ephrin-A5 and EphA2 in the lens. Ephrin-A5 knockout ⁻/⁻ mice often display anterior polar cataracts while EphA2⁻/⁻ lenses show very mild cortical or nuclear cataracts at weaning age. The anterior polar cataract of ephrin-A5⁻/⁻ lenses is correlated with multilayers of aberrant cells that express alpha smooth muscle actin, a marker for mesenchymal cells. Only select fiber cells are altered in ephrin-A5⁻/⁻ lenses. Moreover, the disruption of membrane-associated β-catenin and E-cadherin junctions is observed in ephrin-A5⁻/⁻ lens central epithelial cells. In contrast, EphA2⁻/⁻ lenses display normal monolayer epithelium while disorganization is apparent in all lens fiber cells. Immunostaining of ephrin-A5 proteins, highly expressed in lens epithelial cells, were not colocalized with EphA2 proteins, mainly expressed in lens fiber cells. Besides the previously reported function of ephrin-A5 in lens fiber cells, this work suggests that ephrin-A5 regulates β-catenin signaling and E-cadherin to prevent lens anterior epithelial cells from undergoing the epithelial-to-mesenchymal transition while EphA2 is essential for controlling the organization of lens fiber cells through an unknown mechanism. Ephrin-A5 and EphA2 likely interacting with other members of Eph/ephrin family to play diverse functions in lens epithelial cells and/or fiber cells.