Seeing clearly: X-rays help

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  • Published: Aug 15, 2013
  • Author: David Bradley
  • Channels: X-ray Spectrometry
thumbnail image: Seeing clearly: X-rays help

Cataract insight

Allosteric mechanism of water-channel gating by   Ca2+–calmodulin Image credit: Nature/Gonen et al

The protein that maintains the clarity of the lens in the human eye has been analysed using X-ray crystallography and could provide new insights into how to treat potentially blinding cataracts and various other health conditions.

Aquaporin proteins act as water channels between cells and are present in lots of different tissues in different organisms. Aquaporin zero (AQP0) is not quite as ubiquitous, it is found only in the lens of the mammalian eye. The lens is known to comprise of unique cells - lens fibres - that themselves are mostly water and proteins known as crystallins. It is the tight packing of these fibres and the ordering effect of the crystallin proteins that creates the uniform medium within the eye that can transmit light as well as, if not more effectively, than glass.

However, the lens can sometimes be prone to abnormal development and for many people succumbs to years of exposure to ultraviolet light and age-related changes leading to the condition known as cataract - clouding of the lens that reduces its transparency and ultimately leads to compromised visual acuity and ultimately blindness. Indeed, cataracts are the most common cause of blindness globally; 1 in 6 people over the age of 40 develop cataracts and half of those over the age of 80 years. Surgery can remove a cataract lens and replace it with an artificial lens, restoring vision with appropriate prescription spectacles. However, there are factors other than age, such as diabetes, smoking and various genetic factors associated with mutation in the AQP0 gene that can increase the risk of a person developing cataracts. As such, this protein represents a novel target for treating or potentially preventing cataract without surgery through pharmaceutical intervention.

Visual plumbing

"The AQP0 channel is believed to play a vital role in maintaining the transparency of the lens and in regulating water volume in the lens fibres, so understanding the molecular details of how water flows through the channel could lead to a better understanding of cataract," explains Houmam Araj, who oversees programs on lens, cataract and oculomotor systems at NIH's National Eye Institute (NEI), which helped fund the research.

The closing of the AQP0 channel is regulated by another protein, calmodulin, which is a calcium-sensitive molecule, the functional mechanism of which is not entirely clear. At least one model of this protein suggests that calmodulin is nothing more than a molecular bath plug that fills the open channel to prevent water flow. However, the latest collaborative study between researchers at the University of California, Irvine, and the Janelia Farm Research Campus in Ashburn, Virginia, suggests this is not the case and that the action of the molecular bath plug is a lot subtler.

Plugging the knowledge gap

Writing in the journal Nature Structural and Molecular Biology James Hall and Douglas Tobias who lead the UC Irvine team and Tamir Gonen team leader Janelia Farm and their colleagues explain how calmodulin acts like a bath plug that grabs hold of the plughole in AQP0 and the open channel and pulls it closed. Previously, Gonen and colleagues had used X-ray crystallography to look at the protein involved. However, to get a dynamic, working, view of the protein, the team first used electron microscopy to view AQP0 and calmodulin bound together. Then they combined their microscopy and crystallography data to generate computerized models of how the two proteins interact and to identify the most critical amino acid residues within AQP0. They then knocked out the essential residues one by one in mutant forms of the protein to test the models.

The team explains that rather than being a simple molecular bath plug, calmodulin acts like a gate valve in a plumbing fixture. The AQP0 channel is made up of four identical barrel-shaped units, bundled together side by side. The researchers found that in the presence of calcium, calmodulin binds to one unit and then a second. This leads to a twisting of the channel pulling a sequence of amino acids within each unit into the channel's core impeding the flow of water molecules through the channel.

The new study not only has implications for one day finding anti-cataract medication but provides new understanding of the behaviour of calmodulin and its interactions with various other protein channels. It is, after all, also linked directly to the function of many other ion channels involved in the contractions of the heart and other muscles and in nerve signalling. This first structural model of calmodulin bound to a complete protein channel is an important step towards a better understanding of how those channels behave and what arises when they got awry.

Related Links

Nature Struct Mol Biol 2013, online: "Allosteric mechanism of water-channel gating by Ca2+ - calmodulin"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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