A groundbreaking study has uncovered the crucial role of two proteins in the abnormal bone growth that can occur after serious injuries. This phenomenon, known as heterotopic ossification (HO), can lead to long-term disability and pain for patients, yet its underlying biological processes have remained a mystery.
Led by Dr. Benjamin Levi from the Center for Organogenesis at the University of Texas Southwestern, the research team has shed light on how these proteins, thrombospondin 1 (TSP1) and thrombospondin 2 (TSP2), contribute to the unexpected bone formation in soft tissues. The study, published in Bone Research, offers a new understanding of how injured tissue can be 'reprogrammed' to support bone growth and suggests potential avenues for preventing this debilitating complication.
Dr. Levi emphasizes the significance of these findings: "Our research reveals the central role these proteins play in shaping the healing environment. When their activity is reduced, the abnormal bone growth is dramatically decreased."
Previous studies hinted at the influence of changes in the extracellular matrix (ECM) on tissue healing, but the specific molecular signals involved were unclear. This study aimed to identify the key factors that guide this healing process.
Using a mouse model involving burn and tendon injuries, known triggers of HO, the researchers tracked cellular and tissue changes over time with advanced genetic and imaging tools. They employed single-cell RNA sequencing, spatial transcriptomics, and high-resolution imaging to analyze collagen fibers and bone formation.
The analyses showed that TSP1 is primarily produced by immune cells called macrophages at the injury's center, with lower levels in mesenchymal progenitor cells (MPCs). In contrast, TSP2 is mainly produced by MPCs around the damaged area's edges.
The researchers also discovered that these proteins influence the arrangement of collagen fibers. In normal healing, collagen is flexible and loosely organized. However, in injured tissue with active thrombospondin signaling, the fibers become tightly aligned, creating a structure that promotes bone growth.
To test the proteins' essentiality, the team studied mice lacking both TSP1 and TSP2. In these mice, collagen fibers were disorganized, and abnormal bone growth was significantly reduced. Dr. Levi explains, "When we removed both proteins, the tissue lost the supportive framework needed for ectopic bone development, resulting in much less harmful bone formation."
Scans confirmed that these mice had smaller bone deposits in tendons and surrounding tissues, with their normal skeleton remaining unaffected. This suggests that targeting these proteins could reduce abnormal bone growth without interfering with healthy bone development.
The study also identified a regulatory protein, FUBP1, which controls TSP2 production. When FUBP1 levels were reduced, TSP2 levels dropped, weakening the signals that promote tissue remodeling.
While these findings are based primarily on animal models, further research is needed to confirm their applicability to humans and the safety of targeting these mechanisms. Nevertheless, the study provides valuable insights into the role of thrombospondin signaling in HO after injury.
Dr. Levi concludes, "HO can have a profound impact on patients' lives. By understanding the roles of TSP1 and TSP2 in HO formation, we aim to develop therapies that target these proteins and prevent HO before it causes permanent damage."
This research opens up new possibilities for treating and preventing HO, offering hope to those affected by this condition.
