The Gerozyme in Your Joints: What Stanford's Cartilage Discovery Tells Us About Aging, and About Removal

There is a word coming out of Stanford that deserves a place in how we think about aging: gerozyme.

It names a class of enzymes that grow more abundant as we age and, in doing so, actively push tissue toward decline. Not wear and tear, not bad luck: a specific, measurable molecule that rises with age and tells cells to behave old. In a series of studies that has now reached cartilage, Stanford laboratories have shown that if you block the gerozyme, aged tissue begins to repair itself.

The gerozyme story is, at its heart, a removal story. And removal, not addition, is the idea at the center of therapeutic plasma exchange.

What Stanford found

The enzyme's full name is 15-hydroxyprostaglandin dehydrogenase, mercifully shortened to 15-PGDH. Its job is to break down prostaglandin E2 (PGE2), a signal the body uses to switch on repair: PGE2 wakes the stem cells that rebuild injured muscle and supports regeneration in nerve, bone, blood, and gut. 15-PGDH is the off-switch for that signal, and it climbs with age.

Helen Blau's group first showed this in muscle, reporting in 2021 that the enzyme roughly doubles in aged muscle and that blocking it in old mice restored mass, strength, and endurance. In 2023 the team coined the term gerozyme and extended the finding to nerve. The newest chapter, published in Science with Nidhi Bhutani, asked whether the same enzyme was at work in the joints. It was. In the knee cartilage of aged mice, 15-PGDH was about twice as abundant as in young animals. When the team gave a small-molecule inhibitor, either systemically or injected into the joint, cartilage that had thinned and frayed with age thickened back across the joint surface. It was true hyaline cartilage, the smooth load-bearing surface, not the inferior fibrocartilage that usually fills in after damage. Mice with injuries resembling ACL tears were far less likely to develop osteoarthritis.

Two findings make this more than another mouse study. First, it held up in human tissue: cartilage taken from patients undergoing knee replacement showed fewer aging, cartilage-degrading cells and the early signature of regeneration after a week of treatment. Second, and more surprising, the regeneration did not come from stem cells. Cartilage has famously few of them, which is the textbook reason joints do not heal. Instead, the chondrocytes already living in the tissue reset their gene expression and returned to a younger, matrix-building state. As Bhutani put it, "a large pool of already existing cells in cartilage are changing their gene expression patterns." The cells did not multiply. They changed their minds.

This matters because osteoarthritis affects roughly one in five American adults, and no approved drug slows or reverses it; the toolkit is pain control and, eventually, joint replacement. An oral 15-PGDH inhibitor has already cleared Phase 1 safety trials for muscle weakness, and the Stanford group hopes a cartilage trial will follow.

Aging as an accumulation problem

The gerozyme concept is bigger than any one tissue. It belongs to a way of thinking about aging that has gathered force over the past decade: that a meaningful part of growing old is driven not only by damage inside cells but by signals, an accumulating chemical environment that instructs healthy cells to behave like old ones. Change the signal and you change the behavior. The cells were never broken; they were being told to stand down.

That framework comes out of the parabiosis experiments, in which a young mouse and an old mouse are surgically joined to share one bloodstream. The old animal's tissues grow measurably younger, the young animal's grow older, and aging, it turns out, travels in the circulation. The field named the traffic: anti-geronic factors that keep tissue youthful, and pro-geronic factors that drive it toward decline. With age the balance tips, the pro-aging signals rise, and cells find themselves marinating in a chemistry that tells them to age. A gerozyme like 15-PGDH is a pro-aging actor of exactly this kind, just operating inside the tissue rather than circulating through it.

A removal story, not an addition story

For years the longevity field chased addition: find the youthful ingredient in young blood, bottle it, and you would have a rejuvenation drug. The most famous candidate, a protein called GDF11, made magazine covers as the fountain-of-youth molecule, then fell apart under replication. The lesson the field absorbed was humility about magic ingredients.

The sturdier finding runs the other way. When researchers at Berkeley, working with Dr. Dobri Kiprov, replaced about half an old mouse's plasma with nothing but saline and albumin, no young blood and no added factors, they expected nothing to happen. Instead the animals' muscle repair, liver health, and brain-cell birth all improved, matching what young blood had done. They had not added youth; they had removed the accumulated old signal and let the body settle toward a younger balance. Lowering the burden was enough.

That is the same shape as the gerozyme result. Stanford did not give the joint a youthful ingredient; they took away the effect of an aging one, and the existing cells did the rest. Whether you lower a pro-aging signal by inhibiting an enzyme in the tissue or by removing accumulated pro-aging cargo from the blood, you are working the same lever: take away what has built up, and let the body heal itself.

This is the logic of therapeutic plasma exchange. TPE does not transfuse youth in; it removes a large fraction of the plasma outright, along with everything dissolved in it, and replaces that volume with clean albumin solution. When the Berkeley group profiled older people across repeated exchanges, the proteome did not drop uniformly, as simple dilution would predict. It shifted toward a younger pattern: pro-aging factors fell while beneficial regulators of immune function and tissue repair were restored upward, and markers of cellular senescence and DNA damage declined. That is the signature of a reset, not a thinning.

One important distinction

15-PGDH is a protein, and a small, measurable amount of it does circulate in the blood, so an apheresis session would clear some of that fraction along with everything else. But that is not where the enzyme does its damage. 15-PGDH is a cytosolic enzyme: it degrades the body's pro-repair prostaglandins from inside the cell, within the chondrocytes of the cartilage, the fibers of aging muscle, the immune cells of the joint. The trace in plasma is spillover, not the active pool. Plasma exchange cannot reach 15-PGDH where it works, which is why clearing the blood would not be expected to reproduce what Stanford achieved by delivering an inhibitor into the tissue itself. That is a different tool for a different compartment.

What plasma exchange physically removes is the circulating layer of the same problem: the pro-inflammatory cytokines of "inflammaging," the toxic secretions of senescent cells, the large protein-bound and antibody species the kidney and liver were never built to clear. So the gerozyme discovery is not proof that TPE removes this particular enzyme. It is independent corroboration of the principle TPE rests on: that aging is, in significant measure, an accumulation of pro-aging signals; that lowering that burden can reverse the aging phenotype; and that the body's existing cells, given a quieter environment, will often reset themselves without any need to add stem cells or magic ingredients.

Two laboratories, two tools, one idea: that with age the body fills up with something that drives decline, and that taking it away can let the body heal.

Dr. Allen P. Green is a Board-Certified Clinical Pathologist specializing in therapeutic plasma exchange for longevity, Alzheimer's disease, autoimmune conditions, and environmental detoxification. Trained at UT Southwestern Medical Center under Dr. Ravi Sarode, Dr. Green has supervised over 500 TPE procedures and has published multiple peer-reviewed papers in apheresis and transfusion medicine. He serves as Associate Medical Director at Global Apheresis in Mill Valley, California.

This article describes preclinical and early-stage research. A 15-PGDH inhibitor has not been approved for cartilage regeneration or any age-related indication, and therapeutic plasma exchange is not an approved treatment for aging. Nothing here is medical advice. If you are considering whether plasma exchange fits your situation, schedule a discovery call.