Loss of ephrin-A5 function disrupts lens fiber cell packing and leads to cataract

Abstract

Cell-cell interactions organize lens fiber cells into highly ordered structures to maintain transparency. However, signals regulating such interactions have not been well characterized. We report here that ephrin-A5, a ligand of the Eph receptor tyrosine kinases, plays a key role in lens fiber cell shape and cell-cell interactions. Lens fiber cells in mice lacking ephrin-A5 function appear rounded and irregular in cross-section, in contrast to their normal hexagonal appearance in WT lenses. Cataracts eventually develop in 87% of ephrin-A5 KO mice. We further demonstrate that ephrin-A5 interacts with the EphA2 receptor to regulate the adherens junction complex by enhancing recruitment of ␤-catenin to N-cadherin. These results indicate that the Eph receptors and their ligands are critical regulators of lens development and maintenance.␤-catenin ͉ Eph receptor ͉ N-cadherin C ataract, or the opacification of the lens, is the leading cause of visual impairment and blindness worldwide (1). The molecular events underlying lens development and the processes by which the lens maintains transparency over a lifetime are unclear (2). In addition, the cellular and biochemical mechanisms underlying the pathological changes leading to cataract remain poorly understood.The lens is composed of a single layer of epithelial cells on the anterior surface, which, over a lifetime, divide and differentiate into the underlying lens fiber cells that comprise the bulk of the lens (3, 4). Initially during lens development, primary lens fiber cells differentiate and elongate from the posterior pole. In later embryogenesis and throughout life, secondary lens fiber cells differentiate from lens epithelial cells located at the equator. In cross section, the lens fiber cells resemble flattened hexagons with two broad and four short sides (3). These cells are organized in a highly ordered and closely packed manner, and interact through extensive intercellular adhesion complexes including gap and adherens junctions (5). Fiber cell gap junctions are composed of connexins (Cx) 46 and 50 (6), inactivation of which leads to the degeneration of the inner fiber cells and the development of cataract in mice (7,8). Mutations in human Cx genes have also been associated with cataractogenesis (9, 10). As the lens is completely enclosed by an acellular, avascular capsule, it is believed that these cell-cell junctions are critical for providing nutrient transport, removal of metabolic wastes, and maintenance of homeostasis (11,12). In addition to gap junctions, widespread adherens junctions containing N-cadherin and its associated protein ␤-catenin exist between lens fiber cells (13-16), and may play important roles in lens development and function.Although cell-cell interaction is critical for maintaining lens transparency, little is known about the molecular mechanisms underlying these interactions. We have identified an unexpected regulator of lens fiber cell-cell interaction, the axon guidance molecule ephrin-A5 (17)(18)(19), and have shown that the loss...

Mutations of theEPHA2receptor tyrosine kinase gene cause autosomal dominant congenital cataract

“…These EPHA2 mutations either replace a functionally important amino acid in the SAM domain (Stapleton et al, 1999;Marchler-Bauer et al, 2007) or result in a strong interaction between the EPHA2 receptor and the LMW-PTP negative regulator. Moreover, a recent study on the ephrin-A5 knockout mice has demonstrated a critical role of the ephrin-A5 ligand and its EphA2 receptor in lens development and maintenance (Cooper et al, 2008). In an American family with autosomal dominant posterior polar CC, a group led by Alan Shiels, who performed the first genetic mapping of the CTPP1 locus in the British family described in our present studies (Ionides et al, 1997), reported independently an EPHA2 missense mutation affecting the SAM domain (Shiels et al, 2008).…”

Clinical and experimental advances in congenital and paediatric cataracts

“…It is also vital for regular lens development and differentiation, since knockout mutants of Efna5 develop cataract in approximately 87 per cent of the mutants. The lens fibre cells appear rounded and irregular in cross section [117]. Furthermore, ephrin-A5 is known to interact with the ephrin-A2 receptor (gene symbol EPHA2/Epha2) to regulate the adherens junction complex by recruitment of b-catenin to N-cadherin.…”

EphA2 and Src regulate equatorial cell morphogenesis during lens development

“…Recent studies, including our work, have reported that EphA2 or ephrin A5 mutations cause cataracts with variable severity or incomplete penetrance in humans and mice (Cheng and Gong, 2011;Cooper et al, 2008;Jun et al, 2009;Kaul et al, 2010;Masoodi et al, 2012;Park et al, 2012;Shi et al, 2012;Shiels et al, 2008;Sundaresan et al, 2012;Tan et al, 2011;Zhang et al, 2009). Bidirectional signals mediated by membrane-anchored ephrins and Eph receptor tyrosine kinases play important roles in a broad range of cell-cell recognition events, including axon pathfinding, early segmentation and organ morphogenesis, by modulating cell repulsive or adhesive signals through multiple downstream proteins, such as Ras/Rho, MAP kinase, Akt or FAK, that are crucial for intracellular signal transduction and cytoskeletal dynamics (Arvanitis and Davy, 2008;Himanen et al, 2007;Kullander and Klein, 2002).…”

“…Epha2 −/− knockout lenses have significant changes in refractive index (Shi et al, 2012). Efna5 −/− mice develop severe age-related nuclear cataracts (Cooper et al, 2008) or anterior cataracts (Cheng and Gong, 2011) depending on the strain background, and these differ from Epha2 −/− lens phenotypes. It is unclear how EphA2 or ephrin A5 mutations lead to these pathological changes or why variable lens phenotypes occur in Epha2 −/− or Efna5 −/− mice.…”