Osteonecrosis of the Jaw-a Bone Site-Specific Effect of Bisphosphonates.

A known complication that can occur in patients using bisphosphonates (BPs) is osteonecrosis of the jaw (ONJ). ONJ features bone exposure that may be associated with severe pain, swelling, local infection, and pathological fracture of the jaw. Current literature indicates that a complex combination of factors is necessary to induce ONJ. Several hypotheses about the pathophysiology of ONJ were previously reported. Here, we review these hypotheses and introduce new ideas and suggestions on this topic, focusing on bone site-specific cells, and the effect that BPs and other anti-resorptive drugs have on those cells. Gaining more insight into bone site-specific effects may help to better understand the pathogenesis ONJ, and contribute to the development of new bone site-specific anti-resorptive drugs.

Implications of high-dosage bisphosphonate treatment on bone tissue in the jaw and knee joint.

Bisphosphonates are bone antiresorptive agents traditionally used on a relatively large scale for treatment of bone metabolic diseases and on a smaller scale for bone metastasis treatment. A study on the effects of bisphosphonate treatment on healthy instead of diseased animals will give more insight into the basic mechanisms of bisphosphonates and their effects on different bone sites. We aimed to assess the effect of BP on the mouse knee and jaw joint. Three-month old female C57BL/6 mice were used (twenty-four and eighteen control and experimental group, respectively). At baseline and after treatment with zoledronic acid (ZA) for one, three or six months, we combined bone assessment via µCT and additional histology. Our results showed that, in the knee joint, ZA treatment increased TMD, bone volume, trabecular thickness but did not influence cortical thickness. In both control and ZA group, a higher trabecular TMD compared to cortical TMD was seen. Unseen in the knee joint, ZA treatment in the jaw joint resulted in bone-site specific changes in mineralization; a significant time-dependent higher TMD was evident in the subchondral bone compared to the most distal region of the condyle. MicroCT images revealed the presence of mineral in this region and histology showed that this region did not contain mature bone tissue but cartilage-like tissue. Our data indicate the possibility of site-specific negative side effects, i.e., disturbing normal mandibular development under the influence of bisphosphonate therapy.

Osteoclast fusion and fission.

Osteoclasts are specialized multinucleated cells with the unique capacity to resorb bone. Despite insight into the various steps of the interaction of osteoclast precursors leading to osteoclast formation, surprisingly little is known about what happens with the multinucleated cell itself after it has been formed. Is fusion limited to the short period of its formation, or do osteoclasts have the capacity to change their size and number of nuclei at a later stage? To visualize these processes we analyzed osteoclasts generated in vitro with M-CSF and RANKL from mouse bone marrow and native osteoclasts isolated from rabbit bones by live cell microscopy. We show that osteoclasts fuse not only with mononuclear cells but also with other multinucleated cells. The most intriguing finding was fission of the osteoclasts. Osteoclasts were shown to have the capacity to generate functional multinucleated compartments as well as compartments that contained apoptotic nuclei. These compartments were separated from each other, each giving rise to a novel functional osteoclast or to a compartment that contained apoptotic nuclei. Our findings suggest that osteoclasts have the capacity to regulate their own population in number and function, probably to adapt quickly to changing situations.

A lineage-tracing tool to map the fate of hypoxic tumour cells.

Intratumoural hypoxia is a common characteristic of malignant treatment-resistant cancers. However, hypoxia-modification strategies for the clinic remain elusive. To date, little is known on the behaviour of individual hypoxic tumour cells in their microenvironment. To explore this issue in a spatial and temporally controlled manner, we developed a genetically encoded sensor by fusing the O2-labile hypoxia-inducible factor 1α (HIF-1α) protein to eGFP and a tamoxifen-regulated Cre recombinase. Under normoxic conditions, HIF-1α is degraded but, under hypoxia, the HIF-1α-GFP-Cre-ERT2 fusion protein is stabilised and in the presence of tamoxifen activates a tdTomato reporter gene that is constitutively expressed in hypoxic progeny. We visualise the random distribution of hypoxic tumour cells from hypoxic or necrotic regions and vascularised areas using immunofluorescence and intravital microscopy. Once tdTomato expression is induced, it is stable for at least 4 weeks. Using this system, we could show in vivo that the post-hypoxic cells were more proliferative than non-labelled cells. Our results demonstrate that single-cell lineage tracing of hypoxic tumour cells can allow visualisation of their behaviour in living tumours using intravital microscopy. This tool should prove valuable for the study of dissemination and treatment response of post-hypoxic tumour cells in vivo at single-cell resolution.This article has an associated First Person interview with the joint first authors of the paper.

Jaw bone marrow-derived osteoclast precursors internalize more bisphosphonate than long-bone marrow precursors.

Bisphosphonates (BPs) are widely used in the treatment of several bone diseases, such as osteoporosis and cancers that have metastasized to bone, by virtue of their ability to inhibit osteoclastic bone resorption. Previously, it was shown that osteoclasts present at different bone sites have different characteristics. We hypothesized that BPs could have distinct effects on different populations of osteoclasts and their precursors, for example as a result of a different capacity to endocytose the drugs. To investigate this, bone marrow cells were isolated from jaw and long bone from mice and the cells were primed to differentiate into osteoclasts with the cytokines M-CSF and RANKL. Before fusion occurred, cells were incubated with fluorescein-risedronate (FAM-RIS) for 4 or 24h and uptake was determined by flow cytometry. We found that cultures obtained from the jaw internalized 1.7 to 2.5 times more FAM-RIS than long-bone cultures, both after 4 and 24h, and accordingly jaw osteoclasts were more susceptible to inhibition of prenylation of Rap1a after treatment with BPs for 24h. Surprisingly, differences in BP uptake did not differentially affect osteoclastogenesis. This suggests that jaw osteoclast precursors are less sensitive to bisphosphonates after internalization. This was supported by the finding that gene expression of the anti-apoptotic genes Bcl-2 and Bcl-xL was higher in jaw cells than long bone cells, suggesting that the jaw cells might be more resistant to BP-induced apoptosis. Our findings suggest that bisphosphonates have distinct effects on both populations of osteoclast precursors and support previous findings that osteoclasts and precursors are bone-site specific. This study may help to provide more insights into bone-site-specific responses to bisphosphonates.