Eph-ephrin Signaling Affects Eye Lens Fiber Cell Intracellular Voltage and Membrane Conductance

Abstract

The avascular eye lens generates its own microcirculation that is required for maintaining lifelong lens transparency. The microcirculation relies on sodium ion flux, an extensive network of gap junction (GJ) plaques between lens fiber cells and transmembrane water channels. Disruption of connexin proteins, the building blocks of GJs, or aquaporins, which make up water and adhesion channels, lead to lens opacification or cataracts. Recent studies have revealed that disruption of Eph-ephrin signaling, in particular the receptor EphA2 and the ligand ephrin-A5, in humans and mice lead to congenital and age-related cataracts. We investigated whether changes in lens transparency in EphA2 or ephrin-A5 knockout (–/–) mice is related to changes in GJ coupling and lens fluid and ion homeostasis. Immunostaining revealed changes in connexin 50 (Cx50) subcellular localization in EphA2–/– peripheral lens fibers and alteration in aquaporin 0 (Aqp0) staining patterns in ephrin-A5–/– and EphA2–/– inner mature fiber cells. Surprisingly, there was no obvious change in GJ coupling in knockout lenses. However, there were changes in fiber cell membrane conductance and intracellular voltage in knockout lenses from 3-month-old mice. These knockout lenses displayed decreased conductance of mature fiber membranes and were hyperpolarized compared to control lenses. This is the first demonstration that the membrane conductance of lens fibers can be regulated. Together these data suggest that EphA2 may be needed for normal Cx50 localization to the cell membrane and that conductance of lens fiber cells requires normal Eph-ephrin signaling and water channel localization.

Roles of Eph-Ephrin Signaling in the Eye Lens Cataractogenesis, Biomechanics, and Homeostasis

“…Surprisingly, these defects do not affect cell-cell coupling in the lens, but disrupt the normal intracellular voltage and membrane conductance of lens fibers. This is the first evidence that these properties of lens fibers could be modulated and that Eph-ephrin signaling is involved in maintaining the ion homeostasis of fiber cells ( Cheng et al, 2021 ).…”

“…Recently, disruption of Eph-ephrin signaling in human patients has been associated with congenital and age-related cataracts, defined as any opacity in the transparent eye lens ( Shiels et al, 2008 ; Jun et al, 2009 ; Zhang et al, 2009 ; Kaul et al, 2010 ; Tan et al, 2011 ; Sundaresan et al, 2012 ; Dave et al, 2013 ; Patel et al, 2017 ; Berry et al, 2018 ; Zhai et al, 2019 ). Mouse models are now being used to understand the mechanism of cataractogenesis ( Cooper et al, 2008 ; Jun et al, 2009 ; Cheng and Gong, 2011 ; Shi et al, 2012 ; Cheng et al, 2013 ; Son et al, 2013 ; Biswas et al, 2016 ; Cheng et al, 2017 ; Cheng et al, 2021 ; Zhou et al, 2021 ; Cheng et al, 2022 ). In this review, we highlight the different functions of Eph-ephrin signaling in the lens, describe how genetic background influences cataract phenotypes, as well as provide some insights into future directions and potential therapeutic strategies that can be tested to understand the pathogenesis of age-related cataracts.…”

“…These defects in cell-cell adhesion through adherens junctions likely lead to EMT, and the cluster of abnormal anterior epithelial cells invade the underlying fiber cell layer to cause anterior cataracts in the ephrin-A5 −/− mice ( Figures 3B,C ) ( Cheng and Gong, 2011 ). Interestingly, the hexagonal packing of fiber cells in ephrin-A5 −/− lenses in C57BL/6J background mice appears relatively normal ( Cheng and Gong, 2011 ; Cheng et al, 2017 ; Cheng et al, 2021 ). Based on these studies, genetic background strongly influences cataract phenotype and severity in ephrin-A5 −/− mice.…”

“…The avascular lens relies on a network of gap junction plaques, water channels, and active transport of sodium ions out of the lens to generate its own microcirculation current to bring in nutrients and remove waste ( Mathias et al, 1981 ; Mathias et al, 1997 ). The loss of EphA2 alters connexin 50 (Cx50) localization to lens fiber cell membranes, presumably compromising gap junction plaque formation and cell-cell communication ( Cheng et al, 2021 ). Loss of either ephrin-A5 or EphA2 also changes the localization of aquaporin 0, a protein that makes up water channels between cells ( Cheng et al, 2021 ).…”

“…The loss of EphA2 alters connexin 50 (Cx50) localization to lens fiber cell membranes, presumably compromising gap junction plaque formation and cell-cell communication ( Cheng et al, 2021 ). Loss of either ephrin-A5 or EphA2 also changes the localization of aquaporin 0, a protein that makes up water channels between cells ( Cheng et al, 2021 ). Surprisingly, these defects do not affect cell-cell coupling in the lens, but disrupt the normal intracellular voltage and membrane conductance of lens fibers.…”