Scientists find novel natural dye for screening cells

Researchers discover a natural food pigment which can distinguish between living and dead cells

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TOKYO—Working with in vitro cultures is a cost-effective and easily repeatable way of gaining insight into the interactions between cells or microorganisms and specific chemical compounds like drugs, nutrients and toxins. And to properly assess the toxicity of a compound, it is necessary to find a reliable and efficient way to distinguish living cells from cells killed due to toxicity.
Researchers have found several methods to tell living and dead cells apart, and one popular approach is the dye exclusion test (DET), which uses synthetic dyes. In conventional DET, a dye such as trypan blue or methylene blue selectively permeates and stains dead cells, distinguishing them from live cells. Unfortunately, synthetic dyes have been known to damage living cells in the culture as well. This renders them unusable for long-term studies with a single culture.
Now, a team of scientists from the Tokyo University of Science has recently discovered an alternative to DET with synthetic dyes: DET using a natural pigment made from Monascus purpureus (MP), a mold species traditionally used in Asia for the production of fermented foods.
The team was comprised of Assistant Professor Ryoma Tagawa, Professor Yoshikazu Higami, Professor Eiji Tokunaga and Assistant Professor Kyohei Yamashita, who is lead author of this and two other studies on MP.
In the MDPI Biology study, Dr. Yamashita and colleagues proved that MP can be used to ascertain the viability of breast cancer cells. They found that, unlike trypan blue, MP does not damage living cells and is robust against a typical chemotherapy drug cisplatin. MP took ten minutes to stain dead cells, and only costs a tenth of what trypan blue does.
“The proposed natural pigment enables the long-term monitoring of the life and death of cells, which may bring about improvements in the efficiency of biomass production, basic research on metabolic mechanisms, and applied research in fields such as breeding,” Yamashita remarked.
The discovery of MP as a tool for distinguishing dead cells was serendipitous. Yamashita and a colleague were working alongside Dr. Koji Yamada and Dr. Kengo Suzuki from Euglena Co., Ltd. to find effective ways of culturing Euglena gracilis, a type of single-cell algae, in foods. They stumbled upon the usefulness of MP, and another natural dye called anthocyanin pigment, for studying cell health over time.
The results of their study are published in PeerJ. It was the world’s first report on the application of natural food pigments in cell viability assays. Yamashita then went on to lead another study, published in Nature Research, demonstrating the applicability of MP in DET for another single-cell organism species with Paramecium — a vastly different structure.
In addition to its use as a reagent to monitor the life and death of cells, Yamashita noted that MP is nutritious to living cells, and has anti-oxidative characteristics. This makes the pigment useful for boosting culture efficiency and performing quality control in the food industry, where safe fermentation is critical. It is also safe to humans and the environment.
The applicability of MP to completely different kinds of cells — breast cancer, Euglena, and Paramecium — has made Yamashita very optimistic about its potential.
“Our natural pigment could be the tool that opens up new research fields involving the determination of the causes behind the death of cells,” Yamashita stated. “Moreover, natural pigments are highly likely to have useful properties that have not yet been found, and there is much room for exploration.”

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