Aging organs aren't worn out. In mice, 'zombie' cells piled up.
Our organs seem to age all together. We picture that as parts wearing out. But in aged mice, one change to the immune system moved the brain, heart, muscle and fat back toward youth at once. This looks less like wear and tear, and more like a cleanup crew that quit.
One change to the cleanup crew. Five organs moved toward youth.
Here's what they did. In aged male mice, they turned down one receptor, called EP2, on the immune system's cleanup cells. Those cells are tissue-resident macrophages. They did it two ways. They deleted EP2 in those cells, and they blocked it with an experimental compound. Either way, the mice's memory, frailty, grip strength, body fat and heart all moved toward youthful states.
Aging you can't square with wear and tear.
Think about how we picture aging. Parts wear out. You'd fix each failing organ. But they fixed no organ. They took one brake off one kind of immune cell. And the brain, heart, muscle and fat all moved back toward youth together. Wear and tear can't do that. It points to a cleanup that stopped.
Every organ keeps a cleanup crew. Aging jams its brake.
Every organ keeps its own cleanup crew. These cells are called tissue-resident macrophages. Their job never stops. They eat dying and worn-out cells before those cells cause trouble. Their busiest target is the neutrophil, the body's most common white blood cell. When one grows old and no one clears it, it turns into a kind of zombie. It leaks enzymes and sticky nets onto its neighbors. With age, the authors think, a molecule called PGE2 settles on a receptor on the crew, called EP2. EP2 acts like a brake. It dials down the crew's energy and its appetite. So the zombie cells pile up, and they poison the cells around them.
Release the brake two ways. The cleanup comes back.
So is the brake really the cause? Or would anything that woke up the immune system do it? They released the brake two separate ways. In one group of aged mice, they deleted EP2 only in the cleanup crew. In another, they gave the experimental compound that blocks it. Both times, the crew went back to clearing the zombie neutrophils. And here's the telling part. The aged crew wasn't bad at eating every dying cell. It was worst at the zombies. Underneath, EP2 had been jamming the grab that locks a zombie onto the macrophage, and the swallow that follows. Take the brake off, and both come back.
The clearest proof: the crew fails at the zombies, then recovers.
You can watch this in a dish. Put aged cleanup cells next to senescent neutrophils, and they mostly leave them alone. Release the brake, and they clear them again. That points one way, from failing to fixed. But look at how they released that brake. In one group it was genetic engineering in a mouse. In the other, an experimental compound. Neither is a drug a person could take. And no one measured how long these mice lived. Every result here is about clearing cells and organ function, not living longer.
The same brake is already jammed in aging human organs.
Here's the part that makes you lean in. The same brake is already jammed in aging people. In aged human liver and heart, EP2 sits high on the cleanup cells, the zombie neutrophils pile up, and the crew barely touches them. The same pattern as the mice, only seen, never released. But no one has released that brake in a person to find out if the cleanup comes back. There's no medicine you could take for it, and that experiment hasn't been run.
In these aged mice, some of what looked like organs wearing out was a cleanup crew held back by a single brake — release it, and the crew went back to work.
So in these aged mice, some of what looked like organs wearing out was really a cleanup crew held back by one brake. Release it, and the crew went back to work, and several organs moved back toward youth. In these mice, some of aging was a cleanup that stopped.
