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Velia Fowler, Roberta Nowak, Junyuan Gao, Richard Mathias, Sondip Biswas, Woo-Kuen Lo; The Spectrin-Actin Network and Beaded Filaments Cooperate to Assemble Large Gap Junction Plaques, Facilitating Normal Ion Homeostasis in Mouse Lens Fiber Cells. Invest. Ophthalmol. Vis. Sci. 2013;54(15):5735.
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© ARVO (1962-2015); The Authors (2016-present)
Lens transparency depends critically on gap junction connexins (Cx46, Cx50) which assemble into large plaques on fiber cell membranes, controlling ionic homeostasis in the avascular lens. We investigated the contributions of the membrane-associated spectrin-actin network and lens-specific beaded filaments to gap junction plaque assembly and physiology in mouse lenses.
Mouse lenses with no beaded filaments (CP49-null), a disrupted spectrin-actin network with no Tropomodulin1 (Tmod1-null), and CP49-null/Tmod1-null lenses were studied. Lens intracellular hydrostatic pressure, Na+ levels and impedance as a function of distance from the lens center were measured. Cx46 and Cx50 levels were tested by western blotting. Assembly of Cx46 and Cx50 plaques was examined by confocal immunofluorescence microscopy of lens cryosections and by freeze fracture TEM in fiber cells of different ages.
In wild-type mouse lenses, Tmod1 and the spectrin-actin filament network localize to fiber cell membrane broad sides, but with gaps occupied by large CX46 and Cx50 plaques. Cx46 and Cx50 plaques are unchanged in Tmod1-null or CP49-null lenses, but are dispersed into smaller, punctate structures on membranes in CP49-null/Tmod1-null lenses which have a grossly disrupted spectrin-actin network. Freeze-fracture TEM also reveals disruption of large gap junction plaques as fiber cells mature in the double-knockout lenses. Structural changes in gap junction plaques are paralleled by functional changes in gap junctions. Differentiating and mature fiber cells of double knockout CP49-null/Tmod1-null lenses had significantly elevated intracellular hydrostatic pressure, Na+ levels and lower gap junction coupling conductance relative to wild-type lenses, or to lenses missing only CP49 or Tmod1, which were normal. Western blotting of Cx46 and Cx50 showed that protein levels and sizes were unchanged in CP49-null/Tmod1-null lenses.
CP49-null/Tmod1-null lenses have impaired gap junction communication, causing increased radial gradients for hydrostatic pressure, and Na+. We propose that a spectrin-actin network stabilized by Tmod1 and interactions with beaded filaments forms a ‘cage’ around gap junction plaques, corralling the individual channels and facilitating their coalescence into large, optimally functioning gap junction plaques.
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