Parathyroid hormone exposure frequency regulates spatiotemporal coupling during bone remodeling in a functional bone model

Abstract

Abstract 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.