Semaphorin 7A Accelerates the Inflammatory Osteolysis of Periapical Lesions

Semaphorin 7A (SEMA7A), the only class VII semaphorin member, has been considered as a potent immunomodulatory regulator whose function in periapical lesions remains unclear. In our previous study, we found that SEMA7A was up-regulated in human periapical periodontitis and might be involved in the immune response and tissue destruction of periapical lesions. In this research, we aimed to further explore the specifical regulatory role of SEMA7A as well as its regulatory mechanisms in the inflammatory progression of periapical lesions.

Human periodontal ligament cells (hPDLCs) were collected from intact, caries-free, and healthy third molars and stimulated with recombinant human/mouse SEMA7A (rSEMA7A). Real-time quantitative polymerase chain reaction (RT-qPCR), Western blot analysis, and enzyme-linked immunosorbent assay were used to detect the messenger RNA and protein levels of inflammatory cytokines and matrix metalloproteinases (MMPs) in hPDLCs. Twenty C57BL/6 mice were randomly divided into 4 groups: the healthy control group, pulp exposure group, pulp exposure and saline treatment group, and pulp exposure and rSEMA7A treatment group. Twenty microliters of sterile saline or 4 μg rSEMA7A were injected respectively into the buccal mucosa around the root apex at day 0, 7, and 14. Mandibular tissues were collected at day 21. Micro-computed tomographic and immunohistochemical staining were used to identify the bone destruction and inflammatory infiltration in periapical areas. Finally, an AKT inhibitor (LY294002) was used to pretreat hPDLCs before rSEMA7A stimulation to determine the role of AKT signaling activation in this process.

After treatment with rSEMA7A, the messenger RNA and protein levels of interleukin (IL)-1β, IL-18, cyclooxygenase-2, MMP-1, and MMP-3 were remarkably up-regulated in hPDLCs. For in vivo experiments, compared with the other 3 groups, the treatment of rSEMA7A aggravated the osteolysis of alveolar bone and promoted the infiltration of immune cells into the apex area, along with increased expression levels of IL-1β, IL-18, MMP-1, and MMP-3. Furthermore, we found that the proinflammatory role of SEMA7A could be inhibited by the application of the AKT inhibitor (LY294002).

SEMA7A likely aggravates the inflammatory reaction and bone destruction of existing periapical lesions. The proinflammatory role of SEMA7A in hPDLCs could partially be mediated through the AKT signaling transduction pathway.