Researchers use spectroscopic methods to look for signs of Alzheimer’s disease on the ­retina of mouse models. This opens up opportunities to detect the disease at an early stage in the future

Detecting neurodegenerative diseases in the eyes long before they break out: A European research team with the participation of Leibniz IPHT has come one step closer to this aim. Using a laser-based method, the researchers have succeeded in distinguishing the retina of an Alzheimer’s mouse model from a healthy one on the basis of its spectral fingerprint.

Unlike previous studies, the researchers do not rely on the controversial detection of certain biomarkers – such as protein deposits typical of Alzheimer’s disease – but use spectroscopic means to decipher the biochemical composition of the retina of mouse models. This enables them to detect minimal changes even before they are reflected in the retina. This would allow them to detect signs of emerging ­Alzheimer’s disease much earlier than it is possible with current methods.

“The literature describes that subtle biochemical modifications probably take place before they become visible on the retina,” explain Clara Stiebing and Izabella Jahn, first authors of the study. While morphological changes in the retinal layers can be diagnosed in vivo using standard optical coherence tomography (OCT), deviations in biochemical composition cannot be detected in this way. “This is where Raman spectroscopic investigation, as an adjunct to optical coherence tomography and fluorescence imaging, could make a crucial contribution to improving the accuracy of ocular disease diagnosis,” said Jürgen Popp, scientific director of Leibniz IPHT.

Together with the Medical University of Vienna and partners from Germany and the Netherlands, the Leibniz IPHT team is working on a novel diagnostic platform for age-related eye diseases and Alzheimer’s disease. It combines Raman spectroscopy with OCT.

Diagnostic features for drug development

Whether specific biomarkers for Alzheimer’s disease can be detected noninvasively in the retina is controversial in research. Some studies have identified the protein deposits characteristic of the disease – amyloid plaques and tau fibrils – in both human retinas and mouse models. Others, however, report the absence of these clues and question the diagnostic value of the approach.

Medical ­approval process to be ­completed shortly

By using spectroscopic methods to distinguish healthy and diseased samples for the first time, the researchers are now demonstrating a new way to detect Alzheimer’s disease in the retina. “We biochemically characterized two defined mouse models,” reports Clara Stiebing. In doing so, the researchers succeeded in biochemically identifying the individual layers of the retina on the basis of cross-sections via their different contents of nucleic acids, rhodopsin, lipids, and proteins. Using frontal images – which are closer to the intended in vivo application – they were able to distinguish healthy and diseased mouse retinas with an accuracy of 86 percent. Distinct accumulations of amyloid plaques could not be found in either the cross-sections or the frontal images.

The Jena research team had already shown in an earlier study that spectroscopic examinations of the retina – an eye scan using laser light – are theoretically possible and can provide valuable clues to disease (see also Annual Report 2019). “The question that needed to be answered after that was: can Raman spectroscopy detect differences between healthy and diseased retinas?” explains Izabella Jahn. “We have now shown that this is possible using the retina of mouse models. This was important to test the hypothesis that there are also differences in biochemical information in the retina in neurodegenerative diseases.”

The researchers performed the experiments on ex vivo samples from mice under ideal experimental conditions, i.e., without adhering to laser safety limits for ocular exposure. The question remains whether it is possible to obtain high-quality Raman spectra directly from living humans, Clara Stiebing said, “and whether these data can be used to generate robust statistical methods.” Of course, one must be careful about generalizing results from measurements in mice to humans as well, says Rainer Leitgeb from the Medical University of Vienna, who is coordinating the project. “However, the new results reinforce our specific goal of also being able to detect neurodegenerative diseases such as Alzheimer’s by a simple eye scan. Even without the previously assumed deposits of amyloid plaques, there are changes in the retina that could potentially be detected by Raman spectroscopy. This would provide an objective and comparable diagnostic feature, which would also be of great importance for drug development. The researchers show this in a study with mouse models. How specific these changes really are must ultimately be shown in the targeted human studies.”

The project partners are now building a device at the Medical University of Vienna that combines Raman spectroscopy with optical coherence tomography (OCT). Medical approval should be completed in the first half of 2021, Rainer Leitgeb reports. Then the device can be tested on the first patients.