A cell is a house with four rooms: mitochondria, nucleus, cytosol, and plasma membrane. Each room has its own life, and hydrogen peroxide, a small signaling molecule, works differently in each one. Before, we could only see that a fire had been lit somewhere in the house, but we didn't know which room. Now a glowing sensor has been placed in every room. This sensor is a protein called HyPer7, which changes its glow when peroxide appears nearby. The sensor is so sensitive that it notices even a single molecule of peroxide.
Scientists made four lines of flies, each with the sensor in its own room. Then they watched where and when the fire lit up. In the ovaries, mitochondria flared and went dark within six minutes after peroxide was added. In old flies, the cytosolic sensor in gut cells burned brighter than in young ones, meaning oxidative stress builds up there with age. In wing primordia, the fire appeared in different places: lit in one room, extinguished in another. In embryos, the membrane sensor glowed brightly in yolk granules and at the leading edge, when the embryo's edges come together like a zipper closing. Now it's clear that hydrogen peroxide doesn't just spill across the cell; it works differently in each room. Four windows are open, and the fire in each room can be seen separately.
The HyPer7 sensor is so sensitive that it can register oxidation from a single molecule of hydrogen peroxide that has diffused into the reaction zone. At the same time, it is reduced by the thioredoxin system, so it actually reports not just the presence of peroxide but the balance between oxidation and reduction.
The idea of seeing invisible signaling molecules inside a living cell echoes the concept of 'inner vision' from Stanisław Lem's novel Solaris, where an ocean-mind communicates through subtle chemical signals. Here, scientists gain the ability to literally watch the chemical language of cells in action.
Not published on arXiv. Here are its page, the PDF and the formalization: you can check it yourself, not only read our account.