Expression of typical osteoclast markers by PBMCs after PEG-induced fusion as a model for studying osteoclast differentiation

Differential regulation of skeletal stem/progenitor cells in distinct skeletal compartments

Skeletal stem/progenitor cells (SSPCs), characterized by self-renewal and multipotency, are essential for skeletal development, bone remodeling, and bone repair. These cells have traditionally been known to reside within the bone marrow, but recent studies have identified the presence of distinct SSPC populations in other skeletal compartments such as the growth plate, periosteum, and calvarial sutures. Differences in the cellular and matrix environment of distinct SSPC populations are believed to regulate their stemness and to direct their roles at different stages of development, homeostasis, and regeneration; differences in embryonic origin and adjacent tissue structures also affect SSPC regulation. As these SSPC niches are dynamic and highly specialized, changes under stress conditions and with aging can alter the cellular composition and molecular mechanisms in place, contributing to the dysregulation of local SSPCs and their activity in bone regeneration. Therefore, a better understanding of the different regulatory mechanisms for the distinct SSPCs in each skeletal compartment, and in different conditions, could provide answers to the existing knowledge gap and the impetus for realizing their potential in this biological and medical space. Here, we summarize the current scientific advances made in the study of the differential regulation pathways for distinct SSPCs in different bone compartments. We also discuss the physical, biological, and molecular factors that affect each skeletal compartment niche. Lastly, we look into how aging influences the regenerative capacity of SSPCs. Understanding these regulatory differences can open new avenues for the discovery of novel treatment approaches for calvarial or long bone repair.

RANKL interferes with osteoclastogenesis in PEG-fused U937 cells through LGR4

INTRODUCTION

RANKL plays an important role in the differentiation and maturation process of preosteoclast cells. The osteoclast is a multinucleated cell that can have various sizes and a variable number of nuclei. However, there are no models that allow us to understand how successive cell fusions have a limit, or how cell fusion is regulated.

METHODOLOGY

The present investigation was aimed to determine whether fusing U937 cells with PEG to generate osteoclast-like cells expresses LGR4 and whether applying RANKL to these cells modifies osteoclastic activity compared to non-PEG-fused and RANKL-treated cells.

RESULTS

By fusing U937 cells with PEG, it was found that the LGR4 receptor expression was promoted as early as 24 hours of culture. Applying RANKL before or after fusion inhibits osteoclastic activity. Interfering RANKL interaction with LGR4 in PEG-treated cells recovers and increases cell fusion and osteoclastic activity. PEG-fused U937 cells show osteoclast markers similar to those observed in the classical RANKL-stimulated cell model.

CONCLUSION

Our model allows us to understand that RANKL has fusogenic activity during the first days of culture and in fused cells modulates fusion, contributing to differentiate the role of RANKL before and after fusion through LGR4.

Characterization of different osteoclast phenotypes in the progression of bone invasion by oral squamous cell carcinoma

The present study aimed to characterize different phenotypes of osteoclasts in the progression of bone invasion by oral squamous cell carcinoma (OSCC). A local bone invasion model of OSCC was established by injecting SCC25 human OSCC cells into the center of calvariae in nude mice, and all mice were found to have a typical bone resorption area. Staining for tartrate-resistant acid phosphatase (TRAP) revealed various types of giant osteoclasts in the tumour-bone interface. Bone marrow cells (BMCs) were isolated from the nude mice for primary osteoclast culture, but only a few giant osteoclasts were generated. Additionally, special blood centrifuge tubes were utilized to obtain large numbers of peripheral blood mononuclear cells (PBMCs). Using magnetic activated cell sorting (MACS) and the cytokines colony-stimulating factor (CSF) and receptor activator of nuclear factor-κb ligand (RANKL), we differentiated human osteoclasts from CD14+ monocytes of PBMCs. Bone resorption was further confirmed by a bone resorption assay. Finally, Transwell inserts were used for indirect cell co-culture of SCC25 cells and CD14+ monocytes. Expression of specific osteoclast markers was detected by real-time PCR and western blotting. After co-culture for 3 and 6 days, conditioned medium (CM) of SCC25 cells stimulated the expression of osteoclast markers, and additional osteoclasts were detected through staining of TRAP and F-actin. In the present study distinct osteoclast phenotypes were observed in the established bone invasion animal model, and were confirmed using various primary osteoclast cultures. CM of OSCC cells may promote the expression of osteoclast markers and induce the differentiation of monocytes to mature osteoclasts, which can resorb adjacent bone tissue.