A threshold of mechanical strain intensity for the direct activation of osteoblast function exists in a murine maxilla loading model

Mature primary human osteocytes in mini organotypic cultures secrete FGF23 and PTH1-34-regulated sclerostin

Introduction

For decades, functional primary human osteocyte cultures have been crucially needed for understanding their role in bone anabolic processes and in endocrine phosphate regulation via the bone-kidney axis. Mature osteocyte proteins (sclerostin, DMP1, Phex and FGF23) play a key role in various systemic diseases and are targeted by successful bone anabolic drugs (anti-sclerostin antibody and teriparatide (PTH1-34)). However, cell lines available to study osteocytes produce very little sclerostin and low levels of mature osteocyte markers. We have developed a primary human 3D organotypic culture system that replicates the formation of mature osteocytes in bone.

Methods

Primary human osteoblasts were seeded in a fibrinogen / thrombin gel around 3D-printed hanging posts. Following contraction of the gel around the posts, cells were cultured in osteogenic media and conditioned media was collected for analysis of secreted markers of osteocyte formation.

Results

The organoids were viable for at least 6 months, allowing co-culture with different cell types and testing of bone anabolic drugs. Bulk RNAseq data displayed the developing marker trajectory of ossification and human primary osteocyte formation in vitro over an initial 8- week period. Vitamin D3 supplementation increased mineralization and sclerostin secretion, while hypoxia and PTH1-34 modulated sclerostin. Our culture system also secreted FGF23, enabling the future development of a bone-kidney-parathyroid-vascular multi-organoid or organ-on-a-chip system to study disease processes and drug effects using purely human cells.

Discussion

This 3D organotypic culture system provides a stable, long-lived, and regulated population of mature human primary osteocytes for a variety of research applications.

Effect of tension and compression on dynamic alveolar histomorphometry

Here, we tested the hypothesis that tensile and compressive stresses generated in the alveolar bone proper regulate site-specific cellular and functional changes in osteoclasts and osteoblasts. Thirty-two 13-week-old male mice were randomly divided into four groups: two experimental groups with vertical loading obliquely from the palatal side to the buccal side of the maxillary molar (0.9 N) 30 min per day for 8 or 15 days and unloaded controls (n = 8). Calcein and alizarin were administered 8 and 2 days before euthanization, respectively, to detect the time of bone formation. Undecalcified sections parallel to the occlusal plane were prepared on the palatal root and the surrounding alveolar bone in the middle of the root length. The alveolar perimeter was divided into 12 equal regions for site analysis, and the bone histomorphometric parameters were obtained for each region. Data from in vivo microfocus computed tomography were used to construct animal-specific finite element models. 2D stress distribution images were overlain on histology images obtained from the same location. Significant differences in the total perimeter between groups and between loading and unloading in each region were statistically analyzed (α = 0.05). Osteoclast counts and the alizarin label ratio were significantly higher in the loaded group than in the unloaded group in regions where the maximum von Mises and principal tensile stresses were the highest along the perimeter. The label ratio of calcein was significantly lower in the 8-day loaded group than in the unloaded group, indicating that the calcein-labeled surface was resorbed by osteoclasts that appeared during the loading period. The effect of loading was mitigated by an increase in the maximum principal compressive stress. We conclude that bone resorption and bone formation are functions of site-specific tension and compression in the alveolar bone proper, confirming our hypothesis. This finding is critical for the advancement of diagnosis and treatment planning in clinical dentistry.

Changes in the intra- and peri-cellular sclerostin distribution in lacuno-canalicular system induced by mechanical unloading

INTRODUCTION

Mechanical stimuli regulate Sclerostin (Scl), a negative regulator of bone formation, expression in osteocytes. However, the detailed Scl distribution in osteocytes in response to mechanical unloading remains unclear.

MATERIALS AND METHODS

Twelve-week-old male rats were used. The sciatic and femoral nerves on the right side were excised as mechanical unloading treatment. A sham operation was performed on the left side. One week after neurotrauma, the bone density of the femora was evaluated by peripheral quantitative computed tomography, and immunofluorescence was performed in coronal sections of the femoral diaphysis. The mean fluorescence intensity and fluorescent profile of Scl from the marrow to the periosteal side were analyzed to estimate the Scl expression and determine to which side (marrow or periosteal) the Scl prefers to distribute in response to mechanical unloading. The most sensitive region indicated by the immunofluorescence results was further investigated by transmission electron microscopy (TEM) with immunogold staining to show the Scl expression changes in different subcellular structures.

RESULTS

In femur distal metaphysis, neurotrauma-induced mechanical unloading significantly decreased the bone density, made the distribution of Scl closer to the marrow on the anterior and medial side, and increased the Scl expression only on the lateral side. TEM findings showed that only the expression of Scl in canaliculi was increased by mechanical unloading.

CONCLUSIONS

Our results showed that even short-term mechanical unloading is enough to decrease bone density, and mechanical unloading not only regulated the Scl expression but also changed the Scl distribution in both the osteocyte network and subcellular structures.

On the limits of finite element models created from (micro)CT datasets and used in studies of bone-implant-related biomechanical problems

Patient-specific approach is gaining a wide popularity in computational simulations of biomechanical systems. Simulations (most often based on the finite element method) are to date routinely created using data from imaging devices such as computed tomography which makes the models seemingly very complex and sophisticated. However, using a computed tomography in finite element calculations does not necessarily enhance the quality or even credibility of the models as these depend on the quality of the input images. Low-resolution (medical-)CT datasets do not always offer detailed representation of trabecular bone in FE models and thus might lead to incorrect calculation of mechanical response to external loading. The effect of image resolution on mechanical simulations of bone-implant interaction has not been thoroughly studied yet. In this study, the effect of image resolution on the modeling procedure and resulting mechanical strains in bone was analyzed on the example of cranial implant. For this purpose, several finite element models of bone interacting with fixation-screws were generated using seven computed tomography datasets of a bone specimen but with different image resolutions (ranging from micro-CT resolution of 25 μm to medical-CT resolution of 1250 μm). The comparative analysis revealed that FE models created from images of low resolution (obtained from medical computed tomography) can produce biased results. There are two main reasons: 1. Medical computed tomography images do not allow generating models with complex trabecular architecture which leads to substituting of the intertrabecular pores with a fictitious mass; 2. Image gray value distribution can be distorted resulting in incorrect mechanical properties of the bone and thus in unrealistic or even completely fictitious mechanical strains. The biased results of calculated mechanical strains can lead to incorrect conclusion, especially when bone-implant interaction is investigated. The image resolution was observed not to significantly affect stresses in the fixation screw itself; however, selection of bone material representation might result in significantly different stresses in the screw.

Effect of load-induced local mechanical strain on peri-implant bone cell activity related to bone resorption and formation in mice: An analysis of histology and strain distributions

The purpose of this study was to investigate the effect of load-induced local mechanical strain on bone cell activity of peri-implant bone in mice. Titanium implants were placed in the maxillae of 13-week-old male C57BL/6J mice and subjected to intermittent 0.15 N, 0.3 N, or 0.6 N loads for 30 min/day for 6 days. The animals were sacrificed 2 days after the final loading. Unloaded mice were used as controls. An animal-specific three-dimensional finite element model was constructed based on morphological data retrieved from in vivo microfocus computed tomography for each mouse to calculate the mechanical strain distribution. Strain distribution images were overlaid on corresponding histological images of the same site in the same animal. The buccal cervical region of the peri-implant bone was predetermined as the region of interest (ROI). Each ROI was divided by four strain intensity levels: 0-20 με, 20-60 με, 60-100 με, and ≥100 με, and the bone histomorphometric parameters were analyzed by the total area of each strain range for all loaded samples. The distance between the calcified front and calcein labeling as a parameter representing the mineral apposition rate was significantly greater in the areas with strain intensity ≥100 με than in the area with strain intensity <100 με, suggesting that the bone formation activity of osteoblasts was locally enhanced by a higher mechanical strain. However, the shrunken osteocytes and the empty osteocyte lacunae were significantly lower in the highest strain area, suggesting that osteoclastogenesis was more retarded in higher strain areas than in lower strain areas. The histomorphometric parameters were not affected geometrically in the unloaded animals, suggesting that the load-induced mechanical strain caused differences in the histomorphometric parameters. Our findings support the hypothesis that bone cell activity related to bone resorption and formation is local strain-dependent on implant loading.

A novel inhibitor of NF-κB-inducing kinase prevents bone loss by inhibiting osteoclastic bone resorption in ovariectomized mice

Musculoskeletal diseases and disorders, including osteoporosis and rheumatoid arthritis are diseases that threaten a healthy life expectancy, and in order to extend the healthy life expectancy of elderly people, it is important to prevent bone and joint diseases and disorders. We previously reported that alymphoplasia (aly/aly) mice, which have a loss-of-function mutation in the Nik gene involved in the processing of p100 to p52 in the alternative NF-κB pathway, show mild osteopetrosis with a decrease in the osteoclast number, suggesting that the alternative NF-κB pathway is a potential drug target for ameliorating bone diseases. Recently, the novel NF-κB-inducing kinase (NIK)-specific inhibitor compound 33 (Cpd33) was developed, and we examined its effect on osteoclastic bone resorption in vitro and in vivo. Cpd33 inhibited the receptor activator of NF-κB ligand (RANKL)-induced osteoclastogenesis accompanied by a decrease in the expression of nfatc1, dc-stamp, and cathepsin K, markers of osteoclast differentiation, without affecting the cell viability, in a dose-dependent manner. Cdp33 specifically suppressed the RANKL-induced processing of p100 to p52 but not the phosphorylation of p65 or the degradation or resynthesis of IκBα in osteoclast precursors. Cpd33 also suppressed the bone-resorbing activity in mature osteoclasts. Furthermore, Cdp33 treatment prevented bone loss by suppressing the osteoclast formation without affecting the osteoblastic bone formation in ovariectomized mice. Taken together, NIK inhibitors may be a new option for patients with a reduced response to conventional pharmacotherapy or who have serious side effects.

Peptide drugs accelerate BMP-2-induced calvarial bone regeneration and stimulate osteoblast differentiation through mTORC1 signaling

Both W9 and OP3‐4 were known to bind the receptor activator of NF‐κB ligand (RANKL), inhibiting osteoclastogenesis. Recently, both peptides were shown to stimulate osteoblast differentiation; however, the mechanism underlying the activity of these peptides remains to be clarified. A primary osteoblast culture showed that rapamycin, an mTORC1 inhibitor, which was recently demonstrated to be an important serine/threonine kinase for bone formation, inhibited the peptide‐induced alkaline phosphatase activity. Furthermore, both peptides promoted the phosphorylation of Akt and S6K1, an upstream molecule of mTORC1 and the effector molecule of mTORC1, respectively. In the in vivo calvarial defect model, W9 and OP3‐4 accelerated BMP‐2‐induced bone formation to a similar extent, which was confirmed by histomorphometric analyses using fluorescence images of undecalcified sections. Our data suggest that these RANKL‐binding peptides could stimulate the mTORC1 activity, which might play a role in the acceleration of BMP‐2‐induced bone regeneration by the RANKL‐binding peptides.