A colloidal approach to eye lens protein mixtures: relevance for cataract formation

A broad and constantly growing class of diseases, such as Alzheimer's disease, sickle-cell or cataract, involves protein association phenomena as an essential aspect. The common denominator of all the members of this class of molecular condensation diseases is an attractive energy of interaction between specific biologic molecules which produces condensation into dense, frequently insoluble mesoscopic phases. Understanding interprotein interactions is essential since it is the subtle interplay between interprotein attraction, repulsion and solution entropy that leads to the condensed protein phases. Among these diseases, cataract is the world's leading cause of blindness and effective prevention or non-surgical cure are still lacking. The loss of transparency of the eye lens found in cataract originates from the alteration of the spatial distribution of the lens crystallin proteins. Biological studies of the chaperone properties of crystallins have given valuable information about their aggregation in diluted environment. However, since the crystallins are present at high concentration in the eye lens cells, it is crucial to complement these studies with investigations at physiological concentrations where emergent mixture properties, like phase transitions, are expected. The eye lens cytoplasm contains a solution of mainly three classes of water soluble proteins, called α-, β- and γ-crystallin. In this thesis we develop a coarse-grained model for binary mixtures of α- and γ-crystallin proteins. By analyzing our numerical results in conjunction with experimental neutron scattering data, the interactions between the proteins are modeled. We demonstrate that transparency of the eye lens is greatly enhanced by a weak, short-range attraction between α- and γ-crystallin. Provided it is not too strong, such mutual attraction considerably decreases the critical temperature and the corresponding opacity due to light scattering, and it is consequently essential for eye lens transparency. The phase diagram of the binary α-γ model mixture is then investigated via thermodynamic perturbation theory. The instability boundary of the crystallin mixtures is found to depend on the α-γ attraction in a manner that is both extremely sensitive and non-monotonic, in excellent agreement with the experimental and numerical results. Moreover, the composition of the coexisting phases depends strongly on the strength of the α-γ interaction. In light of the tie lines determination it appears that a decrease of the binding affinity of α and γ-crystallins with ageing involves small energy changes in the attraction between the crystallins and can produce sufficient inhomogeneities to lead to the opacification of the lens. The colloidal approach developed in this thesis could be applied in the future to study other mixtures of crystallin proteins and provide new insights into their interactions and thermodynamic stability when combined with scattering experiments and cloud point measurements.

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