Ismael Y. Karkache

Our understanding of the different developmental origins of osteoclast progenitors and their respective roles during homeostatic bone remodeling at different skeletal sites as well as their roles within bone regeneration and degenerative conditions is evolving. In this narrative review article, we summarize what is known about the developmental origins, anatomical sources, and markers of osteoclast progenitors. We touch on how osteoclast progenitors vary during different disease contexts, including periodontitis, rheumatoid arthritis, and osteoarthritis. In addition, we also characterize osteoclast progenitors that contribute to bone healing and define changes observed with advancing age. In this regard, we offer a critical review of gaps within our understanding and opportunities for future development within the field. Because of their diverse nature under different contexts, identifying and characterizing osteoclast progenitors may help to advance condition-specific therapies.

Background/Objectives: In contrast to endochondral bone healing, the process of intramembranous bone regeneration is poorly understood. This limits our ability to repair and regenerate the craniofacial skeleton to either correct deformity or optimally heal tissues following injury. While there are several preclinical models of intramembranous regeneration within the craniofacial skeleton, some are not load bearing and others are technically challenging. The goal of this pilot study is therefore to describe a simple method for induction of cortical defects within the mandible that does not involve compounding injury to the surrounding tissues. Methods: Single cortex defects were generated in the mandible body of 8-week-old male and female mice. The extent of bone regeneration within the defect was characterized at days 0, 3, 14, and 28 following defect generation via micro-computed tomography and histology. Conclusions: Observed healing was predictable and reproducible and resulted in intramembranous bone formation. This model will help aid the understanding of intramembranous bone healing in load bearing bones (e.g., mandible) within the craniofacial skeleton

Prior work demonstrated that Phlpp1 deficiency alters trabecular bone mass and enhances M-CSF responsiveness, but the cell types and requirement of Phlpp1 for this effect were unclear. To understand the function of Phlpp1 within myeloid lineage cells, we crossed Phlpp1 floxed mice with mice harboring LysM-Cre. Micro-computed tomography of the distal femur of 12-week-old mice revealed a 30% increase in bone volume per total volume of Phlpp1 female conditional knockouts, but we did not observe significant changes within male Phlpp1 cKO_LysM mice. Bone histomorphmetry of the proximal tibia further revealed that Phlpp1 cKO_LysM females exhibited elevated osteoclast numbers, but conversely had reduced levels of serum markers of bone resorption as compared to littermate controls. Osteoblast number and serum markers of bone formation were unchanged. In vitro assays confirmed that Phlpp1 ablation enhanced osteoclast number and area, but limited bone resorption. Additionally, reconstitution with exogenous Phlpp1 suppressed osteoclast numbers. Dose response assays demonstrated that Phlpp1^−/− cells are more responsive to M-CSF, but reconstitution with Phlpp1 abrogated this effect. Furthermore, small molecule-mediated Phlpp inhibition enhanced osteoclast numbers and size. Enhanced phosphorylation of Phlpp substrates—including Akt, ERK1/2, and PKCζ—accompanied these observations. In contrast, actin cytoskeleton disruption occurred within Phlpp inhibitor treated osteoclasts. Moreover, Phlpp inhibition reduced resorption of cells cultured on bovine bone slices in vitro. Our results demonstrate that Phlpp1 deficiency within myeloid lineage cells enhances bone mass by limiting bone resorption while leaving osteoclast numbers intact; moreover, we show that Phlpp1 represses osteoclastogenesis and controls responses to M-CSF.