Nerve growth factor alone triggers osteoarthritis-like joint changes in healthy mice

When researchers at a leading arthritis research center injected a specialized protein called nerve growth factor directly into the knee joints of healthy mice, something unexpected happened. The joints became swollen and increasingly sensitive to pain. Over just four weeks, the mice's knees began to look and behave as if they had osteoarthritis — even though the protective cartilage showed no visible damage.

The study, published in Arthritis & Rheumatology, reveals that nerve growth factor, or NGF, does far more than signal pain. It can fundamentally reshape a joint. "NGF is much more than a pain messenger — it can actually change the structure of a joint," the researchers noted.

This discovery solves a long-standing puzzle in osteoarthritis treatment. Antibodies designed to block NGF were remarkably effective at reducing pain in human trials, offering hope to the 528 million people worldwide living with this degenerative joint disease. But the FDA halted further trials when some patients developed rapidly progressive osteoarthritis after treatment. No one could explain why blocking a pain signal would cause joints to break down faster.

To find answers, researchers injected NGF once weekly into the knee joints of healthy male mice for four weeks, while a control group received neutral injections. They tracked joint swelling, pain sensitivity, structural changes, and bone condition, while also analyzing genetic changes in joint cells and nerves.

The results were striking. Beyond visible swelling, the joint lining became inflamed, thickened, and developed scar tissue. NGF activated genes associated with nerve growth, scarring, and bone formation — with synovial lining fibroblasts, the cells that maintain joint health, being most affected. Pain-sensing nerve fibers sprouted new branches deep into the joint lining and into emerging bony growths called pre-osteophytes.

The bone beneath the cartilage also grew denser. Together, these changes created a joint that looked and functioned like one affected by osteoarthritis.

The findings offer a path forward. Understanding exactly where NGF and its receptors are located in joint tissues — and how these distributions shift with age and disease — could enable researchers to design treatments that block harmful structural changes while preserving NGF's protective functions. For the millions of people above age 50 managing aching joints, this research brings the possibility of safer, more effective therapies closer to reality.