With the support of federal research funding, Johns Hopkins Medicine investigators, led by pathologist Aaron W. James, M.D., Ph.D., discovered that sensory neurons in skeletal tissue are constantly sensing their environment and are quick to respond to injuries. Now, through new research published in Science Translational Medicine, they describe how this response profoundly affects growth plates, the soft cartilage at the end of bones in children that support normal growth and development, and how it responds to targeted treatment in mice.
Following a growth plate injury, a rapid invasion of nerve fibers and blood vessels triggers the formation of an unwanted "bony bar," a rigid bonelike structure that can ultimately impede recovery and lead to stunted or altered bone growth. Through this new research, the team found that inhibiting TrkA+ sensory neurons with long-acting bupivacaine, an FDA-approved nerve-numbing anesthetic, reduced this abnormal growth by up to 60%."These findings suggest that the somatosensory nervous system does not just sense pain after growth plate injury, but actively responds in order to orchestrate a healing response," says James, the principal investigator of the James Laboratory. "This proof-of-concept study will bring us closer to see if therapeutically targeting these neurons can provide clinical benefits for patients."
Up to 30% of childhood skeletal injuries affect growth plates. Major falls, car accidents and sports injuries are leading causes. Fortunately, most children recover. However, in some cases, a growth plate injury can lead to long-term deformities, such as one leg being longer than another and require surgical care.
Ongoing research efforts related to this pathology are dedicated to understanding which molecular cues are transmitted between nerves and skeletal cells. In this context, the James Laboratory investigators found that sensory neurons send signals to launch this abnormal bone growth via pleiotrophin (PTN) signaling. To study the effects of inhibiting this pathway, they gave the long-acting nerve-blocker bupivacaine to mice with bone fractures and compared the response to controls that did not receive the treatment. The anesthetic was given twice a week, and the investigators assessed outcomes after one week and six weeks.
Through various analyses, the investigators found that inhibiting TrkA+ sensory neurons with the nerve blocker reduced the sudden bone growth in cartilage by up to 60% in mice with bone fractures after six weeks compared to controls. They also observed parallel mechanisms in human tissue samples, which were provided from people who received surgery for growth plate injuries.
Through additional investigation with mice, which included assessing underlying mechanisms from single-cell RNA sequencing analysis, the investigators observed more than 300 genes that were upregulated or downregulated as a response to nerve changes, implicating that the PTN pathway, which had not yet been studied, is involved in nerve-bone interactions. Overall, they found that the nerve blocker helped prevent or reduce the expression of genes that prompt the body to regenerate or build nerves, blood vessels and bones.
"More and more, our group and others have suggested — through using experimental models and clinical correlates — that sensory nerves play an outsized role in regulating skeletal pathobiology," says James. "Our hopes are to identify the mechanistic underpinnings of these interactions, such as in the growth plate, and leverage targeted therapies to both improve pain management and to prevent abnormal growth plate repair."
James and the investigators add that the study also provides new directions for research. For example, growth plate cartilage (at baseline) lacks nerve fibers and blood vessels. The underlying mechanisms surrounding this structure, and with the growth plate repelling neurons, is understudied. In addition, most skeletal pathologies are painful. While this study focuses on orthopaedic trauma, common infectious and neoplastic pathologies of the skeleton are also heralded by bone pain. Future steps for research can also focus on understanding if these pathologies can be regulated upstream by the sensory nervous system.
Additional authors, all of whom are from the pathology department at the Johns Hopkins University School of Medicine, include Xin Xing, Beicheng Du, Myles Zhou, Chunbao Rao, Qizhi Qin, Austin Chen, Sowmya Ramesh, Mary Archer, Manyu Zhu, Neelima Thottapplillil, JiHye Yea and Ziyi Wang, as well as Erin Honcharuk, a pediatric orthopaedic surgeon and an assistant professor of orthopaedic surgery at Johns Hopkins Medicine.
This research was supported by grants from the National Institutes of Health (P01 AG066603, R01 AR079171, R01 AR079171-07S1, R21AR078919, R21AR083544, R01 DE031488, R01 DE031028), Alex's Lemonade Stand Foundation (22-26743), the American Cancer Society (DBG-23-1155131-01-IBCD), the Maryland Stem Cell Research Foundation (2021-MSCRFD-5641) and the Department of Defense (USAMRAA HT9425-24-1-0051).
James is a scientific advisory board chairman, a consultant for Novadip LLC, and on the editorial board of the journals Bone Research, Stem Cells and the American Journal of Pathology.
To learn about growth plate injuries, visit
hopkinsmedicine.org/health/conditions-and-diseases/growth-plate-fractures.
To learn about the James Lab, visit labs.pathology.jhu.edu/james.
