Why old bodies stop clearing out “zombie cells”: a failing recycling system

Two things go wrong in almost every aging tissue. Cells get worse at recycling their own worn-out proteins, and senescent cells pile up: cells that have stopped dividing, refuse to die, and send out inflammatory signals. In a young body, senescent cells are removed by the immune system, mainly by macrophages, the “big eaters” that swallow dead and damaged cells. In old bodies they linger. A study published in Nature Aging suggests the two problems are connected.

A recycling system with a doorman

Cells have several ways of breaking down proteins. This study is about one called chaperone-mediated autophagy (CMA). “Autophagy” means “self-eating”. In CMA, a helper protein (a chaperone) recognises proteins that carry a short tag, and delivers them to the cell’s recycling centre, the lysosome. A receptor on the lysosome surface, called LAMP2A, works like a doorman: it lets the tagged proteins in to be digested. CMA is known to decline with age in most tissues.

What the researchers found

Old cells can’t switch CMA on when they become senescent. In connective-tissue cells (fibroblasts) taken from the ears of young mice (4 months), CMA activity went up when the cells were pushed into senescence with a drug. Cells from old mice (23 months) started with less CMA and could not raise it.

Without CMA, senescent cells look “old” and send worse signals. When the researchers blocked CMA in young cells, about half of the protein changes typical of old cells appeared. These cells were not fully senescent, but when they did become senescent, the mix of molecules they released was different. That mix pushed neighbouring healthy cells towards senescence too.

Those signals disable the clean-up crew. The secretions of CMA-deficient senescent cells lowered CMA inside macrophages, and macrophages without CMA were worse at swallowing cells. The researchers traced this to a “don’t eat me” signalling system: cells display a protein that tells macrophages to leave them alone, and macrophages read it through a receptor called SIRPα. Normally CMA helps remove that receptor from the macrophage surface. Without CMA, the receptor stays, the brake on eating stays on, and senescent cells survive.

In living mice, the effect was visible. Old male mice whose macrophages lacked CMA had more senescent cells in fat tissue, liver and lung; in females the picture was less clear. Wounds in middle-aged mice with CMA-deficient macrophages healed more slowly, with more senescent cells left around the wound.

Boosting CMA helped. A small experimental molecule that raises CMA, given by mouth to mice from 18 months of age for 5 months, prevented the usual age-related rise in senescent cells in fat, liver and lung, in both sexes, and reduced scar tissue. Macrophages taken from old mice and treated with it for two days swallowed cells about as well as macrophages from young mice. In a mouse model of lung scarring, starting the drug early reduced scarring and inflammation; starting it a week later helped less.

How much does this mean for people?

Almost everything here was done in mice and cultured cells. The human evidence is indirect: in lung tissue samples, gene activity linked to CMA was lower in older people and in people with idiopathic pulmonary fibrosis (a scarring lung disease), and lysosomes from those patients’ lungs had less of the LAMP2A receptor. The authors note these human data are correlations that need to be confirmed. The CMA-boosting compound is a research tool, not an approved drug.

Why it matters

Most efforts against senescent cells try to kill them with drugs (senolytics). This work points to another route: help the body’s own immune system do the job again. It also shows how one hallmark of aging (failing protein recycling) can drive another (senescent cell build-up), which may explain why these problems tend to arrive together.