In a house where hundreds of different cells live, old age does not arrive everywhere at once. Some rooms crumble fast, others hold out to the end. Rapamycin works like an electrician walking through the house flipping switches, but only the circuits where a shared fire of aging is already smoldering respond.
Reproductive tissue cells changed the most: 7 of the 15 most responsive cell types. The fat body, gut cells, oenocytes and the muscles that keep a fly airborne reacted strongly. Neurons and their glial helpers barely noticed the drug. It looks as if the electrician cannot reach the attic: the rapamycin molecule is too large to get into the brain.
In females the fire was stronger, and rapamycin put it out more actively: in nervous and muscle tissue the cellular clocks wound back, the number of muscle nuclei was preserved, and climbing improved. In males at 30 days of age, many cells responded neutrally or even harmfully, and protection appeared later. So the drug does not strike individual cells but a shared state that flares up in different parts of the house.
Previously it was unclear which cells were responsible for the lifespan extension and why the effect was stronger in females. Now it is clear: rapamycin shuts down CAT, and females accumulate more of this state. An atlas like this will help find cellular targets for anti-aging drugs and design treatment regimens that account for sex. The electrician does not fix every bulb separately: he throws the main breaker that feeds the fire.
Rapamycin was discovered in a soil sample from Easter Island and named after the island's local name, Rapa Nui. Its molecule weighs 914.2 daltons, which is quite a lot for a drug, and this may be what keeps it from getting into the brain.
The idea of a shared aging state that can be suppressed by a drug echoes Aldous Huxley's novel Brave New World, where people take soma to maintain youth and stability, though in reality rapamycin does not act that totally.
Not published on arXiv. Here are its page, the PDF and the formalization: you can check it yourself, not only read our account.