Atsuhiko Hikita

Bone homeostasis is maintained by coupling, in which osteoclast-mediated bone resorption is followed by osteoblast-mediated bone formation at the same site. Parathyroid hormone (PTH), including the PTH(1-34) analog, exerts distinct anabolic and catabolic effects on bone depending on its dosing pattern. However, how temporal PTH(1-34) exposure regulates the dynamics of bone remodeling-related cell populations and their spatial coupling remains unclear. Here, we used a functional bone model (FBM) generated from osteoblasts and osteoclast precursors to examine phase-specific responses to PTH(1-34). The FBM spans formation, resorption, and reformation phases, and PTH(1-34) was applied during the resorption and reformation phases. Longitudinal two-photon microscopy with second-harmonic generation imaging enabled non-destructive tracking of matrix resorption and reformation and quantified spatiotemporal changes in the volume and morphology of fluorescently labeled cells. Seven regimens were compared: vehicle, intermittent PTH once-weekly at 10 or 50 nM, intermittent PTH three-times-weekly at 10 or 50 nM, or continuous PTH at 10 or 50 nM. Once-weekly intermittent PTH enhanced osteoclastogenesis and SHG-defined resorption, yet preserved spatial coupling, osteoblast repopulation, and recovery of SHG volume. In contrast, three-times-weekly and continuous PTH resulted in more profound and more persistent matrix loss, weaker spatial coupling between resorption areas and refilling, and limited reformation. These findings identify exposure frequency, together with dose, as a key regulator of PTH-driven remodeling trajectories and demonstrate that PTH(1-34) exposure frequency tunes coupling in the FBM, establishing this platform for optimizing PTH-based regenerative strategies and for broader bone remodeling research.

Introduction: Osteoporosis is a global health issue. Bisphosphonates that are commonly used to treat osteoporosis suppress both bone resorption and subsequent bone formation. Inhibition of cathepsin K, a cysteine proteinase secreted by osteoclasts, was reported to suppress bone resorption while preserving or increasing bone formation. Analyses of the different effects of antiresorptive reagents such as bisphosphonates and cysteine proteinase inhibitors will contribute to the understanding of the mechanisms underlying bone remodeling. Materials and Methods: Our team has developed an in vitro system in which bone remodeling can be temporally observed at the cellular level by 2-photon microscopy. We used this system in the present study to examine the effects of the cysteine proteinase inhibitor E-64 and those of zoledronic acid on bone remodeling. Results: In the control group, the amount of the reduction and the increase in the matrix were correlated in each region of interest, indicating the topological and quantitative coordination of bone resorption and formation. Parameters for osteoblasts, osteoclasts, and matrix resorption/formation were also correlated. E-64 disrupted the correlation between resorption and formation by potentially inhibiting the emergence of spherical osteoblasts, which are speculated to be reversal cells in the resorption sites. Conclusion: These new findings help clarify coupling mechanisms and will contribute to the development of new drugs for osteoporosis.