Study Reveals Potential for Cartilage Regeneration
Cartilage is notoriously stubborn. Once it wears away, it tends not to come back. This issue lies at the heart of osteoarthritis, a challenging condition that stiffens joints, causes chronic pain, and can eventually compel millions into surgery for joint replacements.
A recent study in Science has found that researchers were able to prompt aging and injured joints in mice to regenerate healthy cartilage. They achieved this through an unconventional method: blocking a single enzyme associated with aging. Remarkably, this technique also spurred early signs of regeneration in human cartilage samples taken from knee replacement surgeries.
This research implies that what has long been seen as irreversible cartilage loss might someday be addressable at its root.
The Aging Enzyme
As we grow older, the smooth cartilage that cushions our bones gradually diminishes. Unlike skin or blood, cartilage lacks an automatic rejuvenation mechanism.
The study attributes much of this decline to an enzyme known as 15-PGDH, which increases with age in various tissues, earning it the nickname “gerozyme.”
Comparing the knee cartilage of younger and older mice, the researchers discovered that levels of 15-PGDH roughly doubled with age. These levels also rose following joint injuries akin to torn anterior cruciate ligaments, a common trigger for osteoarthritis in humans.
This trend piqued the researchers’ interest because 15-PGDH is known to break down molecules that aid tissue repair. Previous studies had shown that inhibiting the enzyme helped older mice rebuild muscle and increase strength. So, they wondered: could cartilage respond similarly?
To explore this, they treated older mice with a small-molecule drug that inhibited 15-PGDH. Some mice received the drug systemically, while others had direct injections into their knees. In both scenarios, the thin, damaged cartilage showed signs of thickening across the joint surface.
Further analysis revealed that the new tissue was hyaline cartilage—the smooth, low-friction type ideal for healthy joints—not the tougher fibrocartilage that often occurs during unsuccessful repair attempts.
“We were quite surprised by the extent of cartilage regeneration in aging mice,” said Nidhi Bhutani, an orthopaedic scientist and senior author of the study. “The results were remarkable.”
Reprogramming, Not Replacing
The treatment also assisted in healing joints after injuries. In mice with knee injuries resembling a torn ACL, researchers administered injections of the enzyme-blocking drug twice weekly for a month. Treated mice showed a significantly lower risk of developing osteoarthritis compared to untreated ones, whose joints quickly declined, showcasing elevated levels of the gerozyme.
Treated mice walked more normally, bearing more weight on their injured legs. The improvement in their joints, inferred from gait and weight distribution, indicated reduced discomfort.
What astonished the researchers was the mechanism behind this recovery. For years, scientists had sought stem cells in cartilage to aid in joint repair, but those cells were elusive.
In this case, stem cells were not necessary at all.
Rebuilding Cartilage
Instead, the researchers closely examined the cartilage cells already present in the joint. Utilizing a technique that traces gene activity in individual cells, they found that aging and injury pushed many cartilage cells into a damaging state, producing 15-PGDH and other molecules that degrade cartilage. Meanwhile, cells that typically maintain healthy cartilage became less frequent.
Inhibiting 15-PGDH reversed this trend. Cells that contributed to cartilage breakdown diminished, as did those linked to the formation of inferior cartilage. Simultaneously, the number of cells dedicated to creating and sustaining smooth, healthy cartilage nearly doubled.
This regeneration didn’t involve new cell creation; rather, the existing cells altered their behavior.
“This represents a novel method of regenerating adult tissue and holds significant clinical potential for treating arthritis related to aging or injury,” said Helen Blau, a stem cell biologist and senior author of the study. “We were searching for stem cells, yet they seem to play no role. It’s genuinely exciting.”
These findings suggest that cartilage retains an ability to repair itself well into adulthood. Although this capacity appears to diminish with age, it isn’t permanently lost.
Early Signs in Human Tissue
While mouse studies often spark hope, results can be less inspiring in humans. To see if human cartilage would respond similarly, researchers used tissue from knee replacement surgeries.
They treated samples from osteoarthritis patients with the 15-PGDH inhibitor for a week. The cartilage displayed reduced enzyme levels, lower gene expression tied to degradation and inflammation, and early indications of rebuilding the extracellular matrix essential for cartilage function.
“The mechanism is quite impressive and really reshapes our thinking about how tissue regeneration can occur,” Bhutani noted. “It’s evident that a significant pool of existing cells in cartilage is changing their gene expression patterns.”
This research aligns with a broader movement to develop treatments that modify osteoarthritis itself rather than simply alleviating pain. Approximately one in five adults in the United States suffers from this condition, leading to tens of billions in annual healthcare costs. Current medications often do little to slow cartilage loss, meaning surgery tends to be the last resort.
In contrast, the enzyme-blocking strategy targets a primary driver of degeneration and builds on existing studies. An oral form of a 15-PGDH inhibitor is already undergoing early clinical trials for age-related muscle weakness, showing a favorable safety profile in healthy participants.
This prior experience could expedite testing the drug for those with joint diseases. Nevertheless, many questions linger. Mouse joints differ from human joints, and regenerating cartilage in a living knee over years of use will be a far greater challenge than coaxing cells in a lab.
Still, this research challenges long-standing beliefs that adult cartilage is beyond repair. Instead, it presents the possibility that aging joints are not simply malfunctioning machines, but rather systems stuck in an inappropriate setting.
