Eye contact: Proteins implicated in wet age-related macular degeneration

Macular degeneration

Age-related macular degeneration (AMD) is a common disease of the eye which is the leading cause of blindness in elderly people around the world. It affects the central part of the vision due to defects in the macula, a small part of the retina which is important for seeing things directly in front of you. As such, it affects activities like reading and watching television.

Of the two types of AMD, dry AMD is the more common but wet AMD is the more harmful introducing serious changes to vision more quickly than its dry counterpart. It accounts for about 90% of all cases of blindness. We know about the physical changes that occur in the eye, like the development of new blood vessels underneath the macula that cause scarring and damage sight, but little is known about how the disease develops.

In China, it has been estimated that up to 15.5% of the elderly are afflicted with AMD, so scientists associated with Nanjing Medical University have decided to try and learn more about the molecular basis of the condition. Qinghuai Liu and colleagues thought that a proteomics study to identify proteins involved in the progress of AMD might lead to new targets for drug therapy.

Pooled aqueous humour

The basis of the study was the investigation of aqueous humour, the watery liquid that fills the space between the cornea and the lens. Although it is separated from the macula and the retina at the rear of the eye, aqueous humour has been used to investigate several conditions of the eye. The protein content is generally quite low but it has been shown to be affected by wet AMD and other ocular diseases.

In this new study, the research group analysed the aqueous humour from six Chinese patients with wet AMD and compared it with that of six further patients of similar age who had cataracts but no signs of AMD. It would have been more appropriate to collect aqueous humour from healthy adults to compare with the AMD samples but this was not considered to be ethically acceptable.

The samples from each group were pooled to ensure that there was sufficient protein in the subsequent extracts for mass spectrometric analysis. The proteins were precipitated then separated from each other by two-dimensional gel electrophoresis. After staining and image analysis of the resulting protein spots on the gels, the proteins of interest were removed for identification by conventional procedures using matrix-assisted laser desorption/ionisation mass spectrometry on a tandem time-of-flight instrument.

Protein targets

The abundances of 78 proteins were more than two-fold different between the AMD samples and the controls, and 68 of these were identified by mass spectrometry. Some of these have been implicated in earlier studies of AMD and give some clues to the processes that are occurring during the onset and development of AMD.

For instance, several crystallins were in lower abundance in the AMD patients compared with the controls. The research team speculated that the α-crystallin A and B chains were associated with the formation of extracellular material, known as drusen, in deposits under the retina. Drusen consist of lipids and proteins and increase the risk of developing AMD, without actually causing it directly.

Based on earlier work, the team thought that α-crystallin B chain could also be involved in the production of new blood vessels (angiogenesis) in the eye and would make an effective target for new drugs for wet AMD. α-Crystallins are also known to prevent protein aggregation under stress and might protect the retina from oxidative stress. Their reduced abundance in AMD could indicate a breakdown in stress protection.

Proteins classified as chemokines were also implicated in wet AMD, along with complement factor I and an isoform of serum albumin. Overall, the functions of most of the altered proteins were found to be associated with angiogenesis, oxidative stress, apoptosis, cell division, tumorigenesis and immunomodulation.

Despite being a preliminary study, the identification of so many proteins of altered abundances in aqueous humour from wet AMD patients is an important step forward to understanding the disease. It is limited by the small sample set, although variations between individual patients are smoothed out by pooling the samples.

With further study, biomarkers of wet AMD could be identified and new drugs targeting specific proteins could be designed to reduce or halt the progression of the disease in patients, before their sight becomes too affected.