An inducible explant model for dissecting osteoclast-osteoblast coordination in health and disease

Metabolic bone diseases are a collection of disorders resulting in diminished skeletal integrity and changes in bone mass due to perturbations in the life-long process of bone remodeling. Perturbations in the number, size and nuclear multiplicity of osteoclasts underpin the development of diverse metabolic bone diseases that impact >13% of adults over age 50 world-wide. Each metabolic bone disease (e.g., osteoporosis, Paget’s disease, fibrous dysplasia (FD), osteopetrosis) presents with unique phenotypes, rises from distinct etiologies and progresses with disparate severities, but all are underpinned by a breakdown in osteoclast formation/function. These perturbations of osteoclast formation/function either stem from or cause dysfunctional osteoclast-osteoblast coordination. Unfortunately, a mechanistic understanding of osteoclast-osteoblast coordination and communication is lacking and represents a major barrier to understanding the biology underpinning bone remodeling and the development of effective treatments targeting this process. Here we have developed an inducible ex vivo culture model that models osteoclast-osteoblast coordination in the bone remodeling compartment. Doxycycline addition to cultures activates GαsR201C expression and RANKL release from osteoprogenitors, which elicits the differentiation and fusion of neighboring preosteoclasts. In turn, multinucleated osteoclast formation promotes the proliferation of osteoprogenitors, accompanied by the robust release of RANK+ extracellular vesicles, all within ∼4 days. This system recapitulates many aspects of the complex osteoclast-osteoblast coordination required for the function of the bone remodeling compartment in both health and diseases underpinned by excessive osteoclast formation. Moreover, based on the ease of isolation, culture, reproducibility and the general adaptability of these cultures to a variety of assays, we expect that this new model will expedite the investigation of osteoclast-osteoblast coordination and osteoclast fusion in bone remodeling and offer a powerful tool for evaluating signaling cascades and novel therapeutic interventions in osteoclast-linked skeletal disease. One-Sentence Summary Conditional, inducible, ex vivo marrow explants offer a novel tool for studying osteoclast formation and osteoclast-osteoblast coordination in a rapid convenient culture model. ### Competing Interest Statement The authors have declared no competing interest.