Coordination of Fusion and Trafficking of Pre-osteoclasts at the Marrow–Bone Interface
Fusion is the final osteoclast differentiation step leading to bone resorption. In healthy trabecular bone, osteoclast fusion is restricted to bone surfaces undergoing resorption, and necessarily requires site-specific recruitment of mononucleated pre-osteoclasts originating from the bone marrow. However, the spatiotemporal mechanism coordinating recruitment and fusion is poorly investigated. Herein we identify a collagen/vascular network as a likely structure supporting this mechanism. We therefore used multiplex immunohistochemistry and electron microscopy on human iliac crest bone samples, in combination with functional assays performed in vitro with osteoclasts generated from healthy blood donors. First, we found that putative pre-osteoclasts are in close vicinity of a network of collagen fibers associated with vessels and bone remodeling compartment canopies. Based on 3D-reconstructions of serial sections, we propose that this network may serve as roads leading pre-osteoclasts to resorption sites, as reported for cell migration in other tissues. Importantly, almost all these bone marrow pre-osteoclasts, but only some osteoclasts, express the collagen receptor OSCAR, which is reported to induce fusion competence. Furthermore, differentiating osteoclasts cultured on collagen compared to mineral show higher fusion rates, higher expression of fusogenic cytokines, and a CD47 plasma membrane distribution pattern reported to be typical of a pre-fusion state—thus collectively supporting collagen-induced fusion competence. Finally, these in vitro assays show that collagen induces high cell mobility. The present data lead to a model where collagen fibers/vasculature support the coordination between traffic and fusion of pre-osteoclasts, by serving as a physical road and inducing fusion competence as well as cell mobility.
KeywordsCollagen Osteoclasts Fusion Recruitment OSCAR
We wish to thank Jacob Bastholm Olesen, Birgit MacDonald, Kaja Søndergaard Laursen, Karin Trampedach, and Vibeke Nielsen for their excellent technical assistance. This study was financed through general funding by Lillebaelt Hospital, Aarhus University Hospital, and Odense University Hospital.
Designing research study: KS, TLA, and JMD; conducting experiments: KS and TLA; acquiring data: KS, TLA, MH, LR, and NM; analyzing data: KS, TLA, and JMD; writing the manuscript: KS, TLA, and JMD; editing and correcting the manuscript: KS, TLA, MH, LR, NM, and JMD; final approval of manuscript: KS, TLA, MH, LR, NM, and JMD.
Compliance with Ethical Standards
Conflict of interest
Kent Søe, Thomas Levin Andersen, Maja Hinge, Lars Rolighed, Niels Marcussen, and Jean-Marie Delaisse declare that they have no conflict of interests.
Human and Animal Rights and Informed Consent
All procedurse performed in studies involving human participants were in accordance with the ethical standards of the institutional and/or national research committee (The Scientific Eithical Committee for the Region of Southern Denmark S-20070121 and S20070019) and with the 1964 Helsinki declaration and its later amendments or comparable ethical standards.
- 2.Jaworski ZF, Duck B, Sekaly G (1981) Kinetics of osteoclasts and their nuclei in evolving secondary Haversian systems. J Anat 133:397–405Google Scholar
- 12.Baron R, Neff L, Tran VP et al (1986) Kinetic and cytochemical identification of osteoclast precursors and their differentiation into multinucleated osteoclasts. Am J Pathol 122:363–378Google Scholar
- 23.Gordon H, Sweets HH (1936) A simple method for the silver impregnation of reticulum. Am J Pathol 12(545–552):1Google Scholar
- 31.Nakamura I, Pilkington MF, Lakkakorpi PT et al (1999) Role of alpha(v)beta(3) integrin in osteoclast migration and formation of the sealing zone. J Cell Sci 112(Pt 2):3985–3993Google Scholar
- 38.Tezuka K, Tezuka Y, Maejima A et al (1994) Molecular cloning of a possible cysteine proteinase predominantly expressed in osteoclasts. J Biol Chem 269:1106–1109Google Scholar
- 47.Fuller K, Owens JM, Chambers TJ (1995) Macrophage inflammatory protein-1 alpha and IL-8 stimulate the motility but suppress the resorption of isolated rat osteoclasts. J Immunol 154:6065–6072Google Scholar