Book of Abstracts: Albany 2011
June 14-18 2011
©Adenine Press (2010)
Homologous and Heterologous Crystallin Interactions in Cataract
Age-related cataract is the most common cause of blindness worldwide. Nearly fifty percent of Americans above the age of 75 are diagnosed with this disease, and surgical intervention is the sole method of treatment at present. In the developing world, even this treatment is not readily available. These are compelling reasons to search for better treatments to delay, prevent or arrest cataract formation. Recent evidence suggests that age-related cataracts also have a genetic component (1). Therefore, determining the mechanisms underlying genetic cataracts with a known association to a protein-mutation is one important strategy towards understanding the molecular basis for cataract formation. This approach has the added advantage of addressing mechanisms of congenital and childhood cataracts which are difficult to treat because surgical intervention frequently leads to serious consequences (2). For these reasons, we have been determining the molecular mechanisms underlying a number of genetic cataracts by studying the mutant proteins associated with them.
The most common human cataract-associated mutations occur in γD-crystallin (HGD). In the singly-substituted HGD mutants we have studied so far, we find that the protein structure and stability do not change, despite a significant lowering in protein solubility in most cases, which leads to the formation of a condensed phase and consequent light scattering and opacity. One example is the Pro23 to Thr (P23T) mutation in HGD in which the mutant protein shows a dramatically lowered solubility, which is about 1/100th of that of the normal protein. Moreover, the solubility profile of P23T is "retrograde" ⎯ i.e. it increases as the temperature decreases. For this mutant we have shown that hydrophobic surface patches emerge as a result of the mutation, which are largely responsible for its retrograde solubility and aggregation (3). Recently, we have identified the residues, Tyr16, His22, Asp21, and Tyr50, in the P23T mutant that give rise to novel hydrophobic surface, as well as several residues where backbone fluctuations in different time-scales are restricted, providing a comprehensive understanding of how lens opacity could result from this mutation. For P23T and several other mutants, we have shown that changes in the homologous protein-protein interactions (or self-association) are responsible for the formation of the distinct condensed phase, which in turn is likely to lead to light scattering in the cataractous lens.
In an interesting recent development, we found that changes in homologous interactions leading to protein condensation and light scattering may not be the only mechanism leading to cataract (6). In our study of the Glu107 to Ala (E107A) mutant, we found that not only is the mutant protein very similar in structure and stability to the normal protein ⎯ as in all the previous cases we studied ⎯ but it is also as soluble as the normal protein (5). In fact, the E107A mutant does not form a condensed protein phase that could account for light scattering and cataract. However, mixtures of the mutant with another lens crystallin, namely α−crystallin, show increased light scattering compared to normal α−γ−crystallin mixtures. There is also a striking difference in the liquid-liquid phase separation behaviors: The two coexisting phases in the E107A−α mixtures differ much more in protein density than those that occur in HGD−α mixtures. In HGD−α mixtures, the de-mixing of phases occurs primarily by protein type while in E107A−α mixtures it is increasingly governed by protein density. Therefore, here it is clearly the heterologous attractive interactions between two different crystallins in the lens that lead to increased light scattering. It has been known for some time that the attractive interactions between various crystallins are optimized for stability, and any change ⎯ attractive or repulsive ⎯ is likely to lead to instability (5). Our data (5) support these theoretical predictions and emphasize the importance of examining detailed molecular mechanisms to build a comprehensive understanding of a complex disease. A brief commentary on our work is presented in (4).