The ability of the lens in the human eye to change focus relies on a dense formation of proteins that can result in clumps that cloud the lens and lead to cataracts – except for special protective proteins that prevent this. Now a team in Munich, Germany has discovered an activation mechanism that can switch on one of these protective proteins to keep the lens clear.
The team, from the Technische Universitaet Muenchen (TUM), write about their findings in a recent online issue of the Proceedings of the National Academy of Sciences (PNAS).
They suggest the discovery may lead to alternative treatments for cataracts that do not require surgery.
Lens cells perform a remarkable feat. They produce a dense mix of proteins that give the lens its refractive power – its ability to change focus so we can see distant and close objects – yet at the same time keep the lens clear.
To overcome the problem of cloudiness, the lens cells produce and eliminate proteins in a completely different way to other cells – they make them once in the embryonic stage and preserve them for life. Unlike the proteins in the rest of our body, those in our lenses are as old as we are.
But to make the proteins last a lifetime, the lens cells have to keep them in a dissolved state, or they clump together and produce the cloudiness that is characteristic of cataracts.
And herein lies the clue to the German team’s discovery – they have found one of the mechanisms the cell uses to keep the proteins in a dissolved state for so long.
Scientists already knew that two related “heat shock” proteins, αA-crystallin and αB-crystallin, were involved. Heat shock proteins are present in all human cells and help stop other proteins clumping when the cell experiences strong heat or stress.
But until this study, little was known about the structure and behavior of the two crystallins, despite intensive research, as study author Johannes Buchner, professor for biotechnology at TUM, explains:
“The great challenge in the analysis of these two crystallin types lies in their inordinate variety. These proteins exist as a mixture of very different forms, each comprising a variable number of subunits. This makes it very difficult to distinguish the individual structures from one another.”
A few years ago, scientists at TUM solved the mystery of one of the crystallin proteins – they decoded the molecular structure of one of the most important forms of αB-crystallin. The protein is made of 24 subunits.
Under normal conditions, when a lens cell is not stressed, the protein exists in the form that the scientists decoded. But they realized this is just a resting form, and not the form that helps prevent other proteins clumping. So they reasoned there must a switching mechanism that triggers the formation of active forms of the protein.
In the study they describe how they found the trigger – when the cell is exposed to stress, such as heat, phosphate groups attach to the crystallin protein causing it to break up into its subunits. The protein subunits each bind to other proteins and stop them clumping. This is the active form of the crystallin.
The main challenge the team faced was resolving the structure of the protein, as co-author Sevil Weinkauf, professor for electron microscopy at TUM, explains:
“Imagine you only have a few pictures of a coffee cup’s shadow cast and want to infer the shape of the cup from that. Now, if you think that sounds difficult, try to imagine you have not just a single cup, but a cupboard full of china that you want to deduce from the shadow casts. It is precisely this daunting challenge that we met for αB-crystallin.”
The team believes their discovery of how the crystallin behaves could lead to new treatments for cataracts that do not require surgery. It may be possible to develop a drug that activates the αB-crystallin mechanism to clear up clouded lenses.
There could also be other applications, because the protein also plays a role in other cells. For instance it is too active in cancer cells and can stop them committing suicide. In that example, a drug could be developed to deactivate the protein.
Funds from the German Research Foundation helped finance the study.
In 2012, researchers at the Missouri University of Science and Technology in the US, found that eyedrops containing an antioxidant can prevent or heal cataracts and other degenerative eye disorders.