Secreted Protein Del-1 Promotes Bone Regeneration During Resolution of Periodontitis

Objectives: Resolution of inflammation is an active process that restores functional homeostasis, although the mechanisms involved are incompletely understood. The secreted protein Del-1 was recently identified as an endogenous homeostatic factor that regulates neutrophil recruitment and inflammation in the periodontal tissue. The aim of this study was to explore the potential role of Del-1 in inflammation resolution and bone regeneration.
Methods: Del-1-deficient (Del-1^-/-) and -sufficient (Del-1^+/+) mice (n=5-6/group) were subjected to 10-day ligature-induced periodontitis followed by ligature removal for 5 days to facilitate resolution. Bone gain was quantified morphometrically and newly regenerated bone was confirmed by modified Masson’s-trichrome staining. Primary cultures of Del-1^-/- and Del-1^+/+ osteoblastic progenitors were treated with osteogenic medium and their calcification potential was determined by Alizarin-Red-S staining (n=4cultures/group). Del-1 and mutant versions thereof were tested in vivo and in vitro to identify critical component(s) of Del-1 involved in bone gain and osteogenic differentiation. The receptor involved was determined by pull-down assay followed by receptor-knockdown experiments in osteoblastic progenitors.
Results: After ligature removal, periodontal inflammation was resolved and bone was regenerated in Del-1^+/+ mice but not in Del-1^-/- mice (unpaired t-test,P<0.01). Consistently, Del-1^-/- osteoblastic progenitors were defective in calcified nodule formation. Local gingival injection of Del-1 in Del-1^-/- mice restored inflammation resolution and bone gain (unpaired t-test, P<0.01). Mechanistic analysis in vivo and in vitro using Del-1 and mutants thereof showed that bone gain and osteogenic differentiation were dependent on interactions between the RGD motif on the N-terminal one-third of Del-1 and the β3-integrin on osteoblastic progenitors. Whereas intact Del-1 was required for optimal resolution of inflammation, its RGD-containing N-terminal part was sufficient to promote osteogenic differentiation and bone regeneration.
Conclusions: Del-1 is shown for the first time to promote bone regeneration during periodontitis resolution, a function that might be exploited therapeutically to treat human periodontitis.