[Reading Science] "Biology Books Need Rewriting"... New Cellular Organelle Discovered

US Rockefeller University Research Team
Confirms Role of Phosphate Storage in Drosophila Gut Cells

Cells of plants and animals are composed of organelles such as the nucleus, cell membrane, cytoplasm, and mitochondria. This is the content found in current biology textbooks. However, a new intracellular organelle that has never been identified before has been discovered, attracting attention.

Image showing the structure of a cell. The red color indicates mitochondria. Photo by Weizmann Institute of Science, Israel website.

A research team at Rockefeller University in the United States recently discovered a new intracellular organelle in fruit fly cells that functions as a phosphate storage site, and published their paper in the international journal Nature on the 4th.

Phosphate is a substance that plays an important role in cell growth, energy production, and signal transduction in plants, bacteria, yeast, and others. It performs essential functions in animal tissues and cells. However, its specific functions have not yet been studied in detail. The research team was initially studying the role of phosphate in tissue regeneration in the gut of fruit flies to understand how various diseases affect human intestinal cells, which led to this discovery.

The research team fed fruit flies with phosphonoformic acid (PFA), which limits phosphate absorption within cells. Upon observing the intestinal epithelial cells of the fruit flies, they confirmed that the number of cells rapidly increased due to phosphate deficiency. This rapid cell proliferation also occurred when the flies were fed food containing 10% less phosphate than the normal level, suggesting that phosphate plays an important role in regulating cell numbers.

Subsequently, the team investigated how phosphate exerts this effect by examining how phosphate deficiency influences gene expression. As a result, they focused on the PXo gene. This gene is similar to phosphate-sensing proteins found in mammalian genes, and the team discovered that when phosphate is deficient in cells, PXo expression is also reduced. They confirmed that the decrease in PXo gene expression triggers excessive cell division, and when PXo was overexpressed, cell division slowed down again.

The researchers particularly found that the PXo protein is associated with a unique oval-shaped structure that is unlike any previously known intracellular organelle. This new organelle is composed of multiple cell membranes, and the PXo protein passes through these membranes to transport phosphate. Additionally, the phosphate stored within this organelle is converted into phospholipids, which are major components of cell membranes.

Observations showed that when phosphate becomes deficient in fruit fly cells, this new organelle breaks down and releases the stored phospholipids to each cell. In other words, it functions as a phosphate storage site within the cell. Notably, this also acts as a signal to increase the production of new cells.

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The research team explained, "As the number of cells increases, more nutrients can be absorbed, which may help the intestinal epithelium maintain stable phosphate levels. It is beneficial for the tissue to regenerate more healthy cells." They added, "This study lays the foundation for investigating whether similar phosphate storage organelles exist in other animals, including humans."