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B. Danysh, K. Czymmek, M. K. Duncan; Anionic Domains of the Lens Capsule Control Size and Charge Selective Diffusion. Invest. Ophthalmol. Vis. Sci. 2007;48(13):2025.
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The lens capsule is a relatively thick extracellular matrix protecting the lens from viral and bacterial invasion while allowing smaller molecules necessary for growth and development to pass between the lens and ocular environment. Heparan sulfate proteoglycans (HSPG) and other molecules found in the capsule contain anionic domains that may contribute to this selectivity. Here we test the hypothesis that these anionic domains contribute to selective diffusion within the lens capsule.
Fluorescence Recovery After Photobleaching (FRAP) was used to quantitate diffusion of charged and uncharged 10kDa and 70kDa FITC labeled dextran molecules within the capsules of intact mouse lenses. Anionic domains in the capsule were masked by preincubation with the polycationic molecules polybrene and rhodamine.
There was no significant difference in the diffusion rates of uncharged 10kDa (2.3nm Stokes radius) dextrans in polycation treated versus untreated lens capsules. However, 70kDa (6.0nm Stokes radius) uncharged dextrans diffused almost 50% faster in the polycation treated lens capsules. Anionic 10kDa dextrans diffused much slower within the capsule than neutral or cationic molecules of the same size while 70kDa anionic dextran was unable to enter the capsule efficiently. Polycation pretreatment of the capsule to neutralize its anionic charge abolished the ability of the capsule to exclude anionic dextrans.ConclusionMatrices such as the lens capsule have been proposed to possess 5-7nm pores lined with anionic heparan sulfate attached to HSPGs as well as carboxyl groups from various ECM proteins. Here we demonstrate that the anionic charge of the lens capsule results in the exclusion of highly anionic molecules of similar size to these pores from the capsule and retards the movement of anions whose size would otherwise allow them to transit the capsule easily. Further, our data indicate that the anionically charged HSPG side chains extend into these pores reducing the pore radius and contribute to size selectivity of the capsule for neutral molecules of Stoke’s radii between 5-7 nm.
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