Protein–protein interactions and lens transparency

doi:10.1016/j.exer.2008.08.018

Experimental Eye Research

Volume 87, Issue 6, 10 December 2008, Pages 496-501

https://doi.org/10.1016/j.exer.2008.08.018 Get rights and content

Abstract

Past studies have identified posttranslational modifications of human lens proteins occurring during cataract formation, and have also demonstrated that protein–protein interactions exist between different lens crystallins. Based upon current theories of lens transparency, these posttranslational modifications and their possible effects upon crystallin interactions may be the key to understanding why the lens is able to transmit light, and why transmission is decreased during cataractogenesis. This review will summarize current knowledge of posttranslational modifications during human cataractogenesis, and will propose their possible role in protein–protein interactions that are thought to be necessary for lens transparency. Based upon this premise, model systems will be described that will test the validity of the theory.

Introduction

The lens is comprised of a very high concentration of protein (approximately 300 mg/ml) that is necessary for the refractile properties of the tissue. In spite of this, and because of this very high protein concentration, the newborn human lens is completely transparent. During aging, the lens slowly loses some of its transparency, then looses more transparency during development of the senile cataract. In a small percentage of patients with congenital cataract, opacification occurs at a much younger age, due to genetic mutation.

In order to fully understand the molecular mechanisms involved in lens opacification, it is first necessary to understand why a tissue of such high protein concentration is transparent. The protein composition of mature lens fiber cells is comprised of three general classes of proteins; alpha crystallins, beta crystallins, and gamma crystallins (Bloemendal, 1977). The alpha crystallins are comprised of two highly related sequences, the beta crystallins are mainly comprised of six highly related sequences, and the gamma crystallins are mainly comprised of three highly related sequences (Lampi et al., 1997). In the human lens, the alpha crystallins form oligomers of large size, the beta crystallins form oligomers of smaller size, and the gamma crystallins exist as monomers (Bloemendal, 1977).

Why is the lens with such a high concentration of protein transparent? Trokel (1962) suggested that a “paracrystalline state” of these proteins minimized light scattering, and resulted in the lens transparency. However, Benedek (1971) predicted that a paracrystalline array was not necessary, and that a limited degree of short-range order due to repulsive, nearest-neighbor interactions at high protein density was sufficient for transparency. In a classic study, Delaye and Tardieu (1983) used small angle X-ray scattering to demonstrate the absence of protein spatial order in concentrated solutions of lens proteins, to show that liquid-like, short-range order of proteins and the consequent lack of large scale fluctuations were sufficient to explain the transparency of the lens.

Section snippets

Short-range order involves interactions of crystallins

Tardieu et al. (1992) used X-ray scattering and osmotic pressure studies to demonstrate interactions amongst gamma crystallins, a finding that was confirmed by Stevens et al. (1995) using NMR. Based upon light scattering studies, Bettelheim and Chen (1998) suggested that there also existed interactions between different classes of crystallins, with alpha–gamma interactions being the strongest. Additional studies using NMR (Carver et al., 1994), light scattering (Thurston et al., 1999),

Weak interactions between lens crystallins change during aging/cataractogenesis

If these interactions are important in lens transparency, is it possible they change during lens aging and/or cataractogenesis? Surface plasmon resonance demonstrated an increase interaction of alpha crystallins with a covalently modified form of gammaB crystallin found in aged bovine lens (Peterson et al., 2005), while microequilibrium dialysis showed that some interactions of alpha and gamma crystallin actually decreased during aging (Takemoto and Ponce, 2006, Takemoto et al., 2008). Using a

Covalent changes during human cataractogenesis

The noncovalent, weak interactions of lens proteins discussed in Section 3 could be perturbed by covalent changes of the lens proteins themselves. Such covalent changes may play an initiating role in the development of opacification. A summary of known covalent changes and their possible physiochemical consequences would help clarify the exact biochemical mechanism(s) involved in cataractogenesis. In human congenital cataracts occurring at an early age, the chemical change resulting in loss of

Involvement of protein–protein changes in lens opacification

Based upon studies described in this review, Fig. 2 shows the possible involvement of the magnitude of protein–protein interactions upon lens opacification. In the normal transparent lens [B], weak interactions between heterologous crystallins such as alpha and gamma crystallins ensure a uniform protein density across the lens, resulting in lens transparency. During aging of the normal lens [A], a decrease in these interactions, possibly to zero (Takemoto and Ponce, 2006, Takemoto et al., 2008

Other causes of lens opacification

Although this review hypothesizes that alterations in crystallin interactions play a major role in opacification, it is possible that other biochemical/biophysical mechanisms are also important in loss of lens transparency. Alterations in protein–protein interaction may not only involve crystallins, but may also involve non-crystallin components. The uniformity in lens protein density could be stabilized by the presence of cytoskeletal proteins, which have been shown to bind to crystallins, and

Test of hypothesis: future studies

Based upon the figures and tables of this review, we hypothesize that posttranslational modifications or genetic mutation result in change in protein–protein interactions between lens crystallins, which result in a decrease in lens transparency. With available methodology, it should be possible to rigorously test this hypothesis.

First, a more systemic characterization should be done of posttranslational modifications specific to the human cataractous lens. The continuing development of more

Acknowledgements

This publication was made possible by grants from the National Eye Institute and grant number P20 RR016475 from the National Center for Research Resources (NCRR), a component of the National Institutes of Health (NIH), to LT.

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ReviewProtein–protein interactions and lens transparency

Abstract

Introduction

Section snippets

Short-range order involves interactions of crystallins

Weak interactions between lens crystallins change during aging/cataractogenesis

Covalent changes during human cataractogenesis

Involvement of protein–protein changes in lens opacification

Other causes of lens opacification

Test of hypothesis: future studies

Acknowledgements

Am. J. Hum. Genet.

Int. J. Biol. Macromol.

J. Biol. Chem.

Exp. Eye Res.

Biochim. Biophys. Acta

J. Biol. Chem.

Exp. Eye Res.

Investig. Ophthalmol. Vis. Sci.

Am. J. Hum. Genet.

Exp. Eye Res.

J. Biol. Chem.

Biochim. Biophys. Acta

Biochem. Biophys. Res. Commun.

Exp. Eye Res.

Exp. Eye Res.

Exp. Eye Res.

Theory of transparency of the eye

Appl. Opt.

Freezable and non-freezable water content of cataractous human lenses

Investig. Ophthalmol. Vis. Sci.

Topographic correspondence between total and non-freezable water content and the appearance of cataract in human lens

Curr. Eye Res.

The verterbrate eye lens

Science

Morphological characterization of the alphaA- and alphaB- crystallin double knockout mouse lens

BMC Ophthalmol.

The ageing lens

Ophthalmologica

Differential binding of mutant (R116C) and wildtype alpha crystallin to actin

Curr. Eye Res.

Phase-separation inhibitors and prevention of selenite cataract

Proc. Natl. Acad. Sci. U.S.A.

Lens cytoskeleton and transparency: a model

Eye

Structural and functional changes in the alpha A crystallin R116C mutant in hereditary cataracts

Biochemistry

Short-range order of crystallin proteins accounts for eye lens transparency

Nature

Alteration of protein–protein interactions of congenital cataract crystallin mutants

Investig. Ophthalmol. Vis. Sci.

Selective oxidation of cysteine and methionine in normal and senile cataractous lenses

Proc. Natl. Acad. Sci. U.S.A.

Congenital cataract and macular hypoplasia in humans associated with a de novo mutation in CRYAA and compound heterozygous mutations in P

Graefes Arch. Clin. Exp. Ophthalmol.

Effect of deamidation of asparagine 146 on functional and structural properties of human lens alphaB-crystallin

Investig. Ophthalmol. Vis. Sci.

Review
Protein–protein interactions and lens transparency