Water in a sink swirls as it drains away. A cell caught in the whirlpool of aging is much the same: it cannot climb out, and every turn pulls it deeper. Scientists have now found what sets that whirlpool spinning: a protein called ApoD, which old cells release into their surroundings.
It all starts when old cells stop moving. Take the fluid they were grown in, add it to young cells, and the young ones lose their ability to move too. The culprit turned out to be the protein ApoD. It slips into a young cell and raises the level of another protein, SUN1. That protein is part of the bridge between the cell nucleus and its internal skeleton. When SUN1 builds up too much, the cell feels as if it is being shaken nonstop, and it ages. But here is the twist: SUN1 makes the cell produce even more ApoD. So you get a loop: ApoD raises SUN1, SUN1 raises ApoD. The whirlpool spins faster.
This loop also triggers other hallmarks of aging: the cell loses the protein that props up the nucleus from within, its energy drops, and harmful molecules that damage DNA pile up. The scientists tested this in mice: inject ApoD into an injured muscle, and it heals worse. The whirlpool of aging drags down not just individual cells but whole tissues. So why does aging speed up after 60? The ApoD-SUN1 loop offers an answer: once a tissue holds many such loops, they synchronize, and aging comes down like an avalanche, the way water first circles slowly and then roars down the drain. Break that loop, and you may be able to slow the whirlpool.
The protein ApoD (apolipoprotein D) was first discovered as a component of high-density lipoproteins, but its exact function long remained a mystery. It turned out to be produced in the brain in Alzheimer's disease and to protect neurons from oxidative stress. Now it turns out that ApoD is also an aging factor that old cells send to young ones to make them "grow old" too.
The idea that aging can be transmitted through blood brings to mind Aldous Huxley's novel Brave New World, where people age and die on a schedule. In reality, the mechanism turned out to be subtler: not just an "aging factor" in the blood, but a feedback loop that amplifies itself.
Not published on arXiv. Here are its page, the PDF and the formalization: you can check it yourself, not only read our account.