Osteoclasts actively remodel both the mineral and proteinaceous components of bone during normal growth and development as well as pathologic states ranging from osteoporosis to bone metastasis. The cysteine proteinase cathepsin K confers osteoclasts with potent type I collagenolytic activity; however, cathepsin K–null mice, as well as cathepsin K–mutant humans, continue to remodel bone and degrade collagen by as-yet-undefined effectors. Here, we identify a cathepsin K–independent collagenolytic system in osteoclasts that is composed of a functionally redundant network of the secreted matrix metalloproteinase MMP9 and the membrane-anchored matrix metalloproteinase MMP14. Unexpectedly, whereas deleting either of the proteinases individually leaves bone resorption intact, dual targeting of Mmp9 and Mmp14 inhibited the resorptive activity of mouse osteoclasts in vitro and in vivo and human osteoclasts in vitro. In vivo, Mmp9/Mmp14 conditional double-knockout mice exhibited marked increases in bone density and displayed a highly protected status against either parathyroid hormone– or ovariectomy-induced pathologic bone loss. Together, these studies characterize a collagenolytic system operative in mouse and human osteoclasts and identify the MMP9/MMP14 axis as a potential target for therapeutic interventions for bone-wasting disease states.

Regenerative endodontics aims to replace inflamed/necrotic pulp tissues with regenerated pulp-like tissues to revitalize teeth and improve life quality. Pulp revascularization case reports, which showed successful clinical and radiographic outcomes, indicated the possible clinical application of pulp regeneration via cell homing strategy. From a clinical point of view, functional pulp-like tissues should be regenerated with the characterization of vascularization, re-innervation, and dentin deposition with a regulated rate similar to that of normal pulp. Efficient root canal disinfection and proper size of the apical foramen are the two requisite preconditions for pulp regeneration. Progress has been made on pulp regeneration via cell homing strategies. This review focused on the requisite preconditions and cell homing strategies for pulp regeneration. In addition to the traditionally used mechanical preparation and irrigation, antibiotics, irrigation assisted with EndoVac apical negative-pressure system, and ultrasonic and laser irradiation are now being used in root canal disinfection. In addition, pulp-like tissues could be formed with the apical foramen less than 1 mm, although more studies are needed to determine the appropriate size. Moreover, signaling molecules including stromal cell derived factor (SDF-1α), basic Fibroblast Growth Factor (bFGF), Platelet Derived Growth Factor (PDGF), stem cell factor (SCF), and Granulocyte Colony-Stimulating Factor (G-CSF) were used to achieve pulp-like tissue formation via a cell homing strategy. Studies on the cell sources of pulp regeneration might give some indications on the signaling molecular selection. The active recruitment of endogenous cells into root canals to regenerate pulp-like tissues is a novel concept that may offer an unprecedented opportunity for the near-term clinical translation of current biology-based therapies for dental pulp regeneration.

The clinical translation of regenerative endodontics demands further development of suitable scaffolds. Here, we assessed the possibility of using silk fibroin scaffold for pulp regeneration with dental pulp stem cells (DPSCs) and basic fibroblast growth factor (bFGF) in ectopic root canal transplantation model. Porous silk fibroin scaffolds were fabricated using freeze-drying technique (with or without bFGF incorporation), and characterized by scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy. DPSCs were isolated, characterized, seeded onto scaffolds, and inserted into the tooth root fragments. Cell viability and morphology were tested in the 3D model in vitro using CCK8 assay and SEM. Furthermore, the ectopic transplantation model was used to verify the generation of pulp-like tissue in DPSCs seeded silk fibroin scaffold with bFGF, as examined by histological analysis. DPSCs seeded in silk fibroin scaffold survived, exhibited cytoplasmic elongation in scaffolds at least 4 weeks in culture. bFGF promoted DPSCs viability in tooth fragments/scaffolds (TSS) between 7 and 28 days. Pulp-like tissue was generated in the bFGF-incorporated TSS with DPSCs. Histologically, the generated tissue was shown to be with well vascularity, have new matrix deposition and dentin-like tissue formation, and consist of both the transplanted and host-derived cells. Collectively, these data support the use of bFGF-incorporated silk fibroin scaffold as a highly promising scaffold candidate for future treatment concepts in regenerative endodontics to save teeth.