The misfolding and aggregation of amyloid-β peptides (Aβ) are considered early drivers of Alzheimer’s disease (AD), the most common neurodegenerative disease. The aggregation and accumulation of the Aβ peptides lead to loss of function and cell death of the neurons in the brain. It is estimated that there are currently 45–50 million people living with this progressive and incurable disease, and with a growing and aging world population, the number of diagnosed patients is predicted to quickly increase even further.
Aβ42 aggregation triggers cell death program
Aβ42 is one of the two major isoforms of Aβ found in the Alzheimer's patients’ brains and is shown to be more toxic and prone to form oligomers than the peptide Aβ40.
Increased Aβ42 production and aggregation is believed to trigger strong endoplasmic reticulum (ER) stress in the neurons. When the stress levels surpass the buffering capacity of cell, the cell death program is activated to remove irreversibly damaged cells, and parts of the brain die.
The aggregation of Aβ42 peptides is also involved in aberrant mitochondrial structures and functionality, which can increase the oxidative stress in the neurons. Brain cells are more susceptible to oxidative stress than other cells due to the higher metabolic activity and lower antioxidative activity. Oxidative stress may exacerbate progression of Alzheimer's disease through oxidative damage to cellular structures, proteins, lipids and DNA.
FMN supplementation increases resistance
The study, recently published in Nature Communications, shows that in yeast expressing toxic amyloid-β 42 peptide, FMN supplementation reduces the cellular levels of misfolded proteins and increases the cells’ resistance to oxidative stress.
“This study was made with the aim to find underlying mechanisms of modulating Aβ aggregation in vivo, in this case yeast cells. There is no known cure for the disease currently, therefor, researchers are looking for potential targets and drugs for early treatment. The faster we find a treatment that can act early on AD onset, the better chances the patients would have. The next step would be to prove that FMN supplementation can also increase the viability of other model organisms, such as C. elegans, mammalian cell lines etc. In the ideal case, a clinical trial would eventually be tested on patients,” says Xin Chen, MD PhD, at the Division of Systems and Synthetic Biology, and first author of the study.
FMN1 deletion affects viability of the cells
To identify genes involved in decreasing the toxicity of the Aβ42 peptide the researchers have performed a genome-wide synthetic genetic interaction array (SGA), using baker’s yeast Saccharomyces cerevisiae, as the model organism.
In collaboration with Professor Thomas Nyström and his team at the University of Gothenburg, the team of Dina Petranovic at Chalmers used a yeast deletion mutant library consisting of ~ 5500 strains with single gene deletions, which cover more than 80 per cent of the yeast genome. They created a new library which combined the Aβ42 expression with each deletion strain.
Based on the screen results, around 400 gene deletions were shown to significantly increase the toxicity of Aβ42, and the FMN1 gene was selected for further investigation. FMN1 encodes a riboflavin kinase, an essential enzyme responsible for catalysing the phosphorylation of riboflavin (Vitamin B2) into one of its active forms, flavin mononucleotide (FMN).
"Riboflavin proposed to have potential as a neuroprotective agent"
“One of the reasons we focused our main efforts into the riboflavin metabolism is because the FMN1 gene has a human ortholog which was found to be relevant in AD patients. Additionally, riboflavin was proposed in other models to have potential as a neuroprotective agent. If this is eventually shown to be relevant in clinical trials, maybe there could be a treatment based on a small molecule, which could be easier, cheaper and more convenient than many other options.” says Xin Chen.
The researchers also showed that the transcription levels of the human ortholog, RFK, are significantly decreased in Alzheimer's patients’ brain tissues, suggesting a conserved evolutionary function of riboflavin kinase in underlying processes that govern proteostasis management in cells.
FMN supplementation improved oxidative stress tolerance
A wide set up of experiments on yeast Aβ42 strains showed that supplement of FMN to culture medium reduced the Aβ42 induced cellular toxicity with increased viability. Cells with FMN supplementation showed reduced misfolded protein load, altered cellular metabolism and improved cell capacity to resist oxidative stress.
Also, FMN supplementation caused a global transcription response in the cells and significantly changed metabolic pathways related to the increased ratios between reduced and oxidized forms of redox cofactors. The improved redox homeostasis can be beneficial for oxidative stress tolerance and contribute to alleviated Aβ42 toxicity.
The next step in this line of research is to test the beneficial effects of FMN supplementation in other AD model organisms (such as the C. elegans, Drosophila, mice or mammalian cell lines) and further investigate the effects of FMN supplementation in other neurodegenerative disease models, such as Huntington’s and Parkinson’s disease.
Text: Susanne Nilsson Lindh
Photo: Johan Bodell and Martina Butorac
Read the study in Nature Communications