A recent study published in Science by a team led by Johns Hopkins Medicine researchers has identified a key role for sensory neurons in bone fracture healing. The research, conducted in mice, demonstrates that nerves responsible for sensing pain also actively guide the repair of skeletal injuries.
"For the first time, we have mapped the circuitry of this neural network, defined which specific sensory neurons innervate [supply nerves to] bone, determined how these neurons change after an injury and identified which signals they produce that are necessary to promote bone formation and repair," said Zhao Li, M.D., Ph.D., senior research specialist at the James Laboratory in the Johns Hopkins University School of Medicine Department of Pathology.
The team used retrograde tracing with a laboratory-engineered adeno-associated virus to identify dorsal root ganglion (DRG) neurons involved in bone healing. "The technique, known as retrograde tracing, is akin to following a single electrical wire from a light bulb back through the walls to find where the circuit breaker lies," explained Aaron James, M.D., Ph.D., professor of pathology at Johns Hopkins and leader of the James Laboratory.
James added: "We combined retrograde tracing with a second technique, single-cell RNA sequencing isolation, to study individual innervating DRG nerve cells in mice - before and after bone fractures - and then isolated them to determine the proteins each one produces. Putting the data for all of the profiled cells together, we created the first comprehensive single-cell atlas of bone-innervating sensory neurons, a map of the neural network and signals necessary for bone repair."
Previous work from 2019 had shown that two proteins—tropomyosin receptor kinase-A (TrkA) and nerve growth factor (NGF)—are involved in promoting new bone formation after fractures. "When we blocked the response of TrkA+ neurons, either genetically or chemically, we saw a dramatic reduction in not only innervation but also in the three follow-up activities critical to successful recovery from a fracture: blood vessel formation, production of bone-synthesizing cells and mineralization of new bone," said James. "This finding indicated that fracture repair is truly dependent on the neural signaling directed by TrkA-expressing nerve fibers, yet the downstream molecular underpinnings were still unknown."
The new study further explored how DRG neurons shift roles during healing. According to Li: "What we discovered is that there are dynamic changes associated with sensory neuron response to bone injury and that these changes reflect how bone repair is done in phases. Just after injury, DRG neurons are nociceptors... but then at later timepoints, they enter a different phase... where they produce and release proteins that promote...bone and cartilage."
Researchers identified three nerve-derived morphogens—transforming growth factor beta 1 (TGFB1), fibroblast growth factor 9 (FGF9), and sonic hedgehog (SHH)—as being expressed during early phases of repair. By removing or blocking these neuron types in mice with fractures—a process called denervation—they observed impaired cell proliferation and poor overall healing. Li stated: "We saw in the denervated mice that there was defective skeletal cell proliferation...and overall poor bone repair." Further analysis highlighted FGF9 as essential for communication between nerve cells and those responsible for rebuilding bones.
James concluded: "By bridging neuroscience, skeletal biology and regenerative medicine, we now know that nociceptive DRG nerve cells—specialized for transmitting severe pain after bone injury—simultaneously drive bone regeneration." He added: "Resolving the paradox of how nociceptors play this dual role...gives us a potential target for drugs that might one day enhance bone healing..."
The research team included members from Johns Hopkins Medicine as well as collaborators from University of Maryland School of Medicine; University of South Florida; University of Texas at Dallas; and University of Texas Southwestern Medical Center.
Funding came from federal sources such as grants from several National Institutes of Health agencies—including National Institute of Arthritis and Musculoskeletal and Skin Diseases—and support from organizations like Alex's Lemonade Stand Foundation and American Cancer Society.
