Laser fluorescence method makes new discovery about aging process

A team of researchers at the Leibniz-Institute for Molecular Pharmacology (FMP; Berlin, Germany) used fluorescence sensors to show that a certain area of the cell—the endoplasmic reticulum—loses its oxidative power in advanced age. If lost, many proteins can no longer mature properly and, at the same time, oxidative damage accumulates in the cytosol (the interior of the cell). The work opens up a new understanding of aging, but also of neurodegenerative diseases such as Alzheimer's or Parkinson's.

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In the endoplasmic reticulum (ER), proteins that are secreted—such as insulin or antibodies of the immune system—mature in an oxidative environment. A type of quality control, redox homoeostasis, ensures that the oxidative milieu is maintained and disulphide bridges can form. Disulphide bridges form and stabilize the 3D protein structure and are thus essential for a correct function of the secretory proteins, such as those migrating into the blood.

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Using genetically encoded sensors, the study shows that the opposing redox state in ER lumen and cytosol is altered during aging and upon disruption of proteostasis. The resulting redox imbalance can spread across tissues. (Image credit: FMP)

Because the ER loses its oxidative power in advanced age, which shifts the reducing/oxidizing equilibrium in this compartment, this leads to a decline in the capacity to form the disulphide bridges that are so important for correct protein folding. As a consequence, many proteins can no longer mature properly and become unstable.

"Up to now, it has been completely unclear what happens in the endoplasmic reticulum during the aging process. We have now succeeded in answering this question," says Dr. Janine Kirstein, first author of the study. At the same time, the scientists were able to show that there is a strong correlation between protein homoeostasis and redox equilibrium. "This is absolutely new and helps us to understand why secretory proteins become unstable and lose their function in advanced age and after stress. This may explain why the immune response declines as we get older," she explains.

The researchers also demonstrated the decline of the oxidative milieu of the ER after stress. When they synthesized amyloid protein fibrils in the cell, which cause diseases such as Alzheimer's, Parkinson's, or Huntington's disease, they set the same cascade in motion. Apart from this, they were able to show that amyloids that are synthesized in a certain tissue also have negative effects on the redox equilibrium in another tissue within the same organism. "Protein stress leads to the same effects as aging," Kirstein explains. "Our findings are thus not only interesting with regards to aging, but also concerning neurodegenerative diseases such as Alzheimer's."

For their experiments, the team of researchers used nematodes—an established model system for investigating aging processes on a molecular level. Since the nematode is transparent, the researchers were able to use fluorescence-based sensors to measure oxidation in the individual cell compartments. So, it was possible to track precisely in the living nematode how the redox condition changes with advancing age. In addition, the influence of protein aggregation on the redox homeostasis was investigated in cultivated cells of human origin. The data were fully consistent with those from the nematode.

The image depicts a single nematode muscle cell, which synthesizes the fluorescence sensor redox-GFP in the endoplasmic reticulum. The sensor is excited with a laser of two different wavelengths. Green areas reflect reducing conditions and blue areas oxidizing conditions. Scale bar: 10 µm. (Photo: FMP/Janine Kirstein)

"We gained a lot of insight, but have also learned that aging is much more complex than previously assumed," Kirstein says. For example, the mechanism of the signal transduction of protein folding stress to the redox equilibrium—both within the cell from one compartment to another and also between two different tissues—remains completely unclear.

The redox equilibrium may serve as a basis for new biomarkers for diagnosing both aging and neurodegenerative processes in the future. "The approach is less likely to be useful for therapeutic purposes at present, but the development of diagnostic tools is certainly conceivable," Kirstein says.

Full details of the work appear in The EMBO Journal; for more information, please visit http://dx.doi.org/10.15252/embj.201591711.

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