Osteocytes (OCY) are the end stage differentiation cells of the osteoblast lineage, and are incorporated in the bone matrix during bone formation. In doing so, OCY control the mineralisation of osteoid. OCY form a dense inter-connected network of cell bodies and cell processes throughout the mineralised matrix of bone. OCY viability depends on interstitial fluid flow along the OCY canaliculi, driven by pulsatile blood flow and loading of the skeleton. Maintenance of the density and viability of OCY are essential for bone health because OCY perform many important functions in bone. Firstly, OCY appear to initiate bone repair of bone microdamage. Secondly, OCY are almost certainly the cells, which initiate new bone formation in response to increased loading of bone. Thirdly, OCY are able to regulate the amount of new bone formation in bone remodelling cycles, at least in part by the production of a molecule called sclerostin (SCL). Mutations in the SCL gene, or deletion of the SCL gene in transgenic mice, are associated with particularly dense, fracture resistant bones. This information has led to development of anti-SCL antibodies as a potential anabolic therapy for bones. Bone loss in ovariectomised aged rats was shown recently to be reversed by treatment with neutralising SCL antibodies. There is also some data to suggest that these antibodies may promote fracture healing. Reduced OCY viability and/or density have been reported in association with osteoporotic fracture. OCY viability seems to be dependent on skeletal loading, adequate skeletal blood flow and estrogen in females. OCY viability is adversely affected by hypoxia, unloading of the skeleton and pharmacobiology, such as chronic exposure to glucocorticoids. Both micro and macro-fractures result in disruption of the OCY network, as do procedures such as drilling and cutting of bone. Because of the important roles of OCY in bone, new approaches to bone health may require the identification of agents to protect these cells from harmful influences in disease and ageing

Aim

Osteomyelitis is a difficult-to-treat disease with high chronification rates. The surgical amputation of the afflicted limb sometimes remains as the patients’ last resort. Several studies suggest an increase in mitochondrial fission as a possible contributor to the accumulation of intracellular reactive oxygen species and thereby to cell death of infectious bone cells. The aim of this study is to analyze the ultrastructural impact of bacterial infection and its accompanying microenvironmental tissue hypoxia on osteocytic and osteoblastic mitochondria.

Age-related fragility fractures are highly correlated with the loss of bone integrity and deteriorated morphology of the osteocytes. Previous studies have reported low-magnitude high-frequency vibration(LMHFV) promotes osteoporotic diaphyseal fracture healing to a greater extent than in age-matched normal fracture healing, yet how osteoporotic fractured bone responds to the mechanical signal has not been explored. As osteocytes are prominent for mechanosensing and initiating bone repair, we hypothesized that LMHFV could enhance fracture healing in ovariectomized metaphyseal fracture through morphological changes and mineralisation in the osteocyte Lacuno-canalicular Network(LCN). As most osteoporotic fractures occur primarily at the metaphysis, an osteoporotic metaphyseal fracture model was established.

A total of 72 six-month old female Sprague-Dawley rats (n=72) were obtained(animal ethical approval ref: 16–037-MIS). Half of the rats underwent bilateral ovariectomy(OVX) and kept for 3 months for osteoporosis induction. Metaphyseal fracture on left distal femur was created by osteotomy and fixed by a plate. Rats were then randomized to (1) OVX+LMHFV(20 mins/day and 5 days/week, 35Hz, 0.3g), (2) OVX control, (3) SHAM+LMHFV, (4) SHAM control. Assessments of morphological structural changes, functional markers of the LCN(Scanning Electron Microscopy, FITC-Imaris, immunohistochemistry), mineralization status(EDX, dynamic histomorphometry) and healing outcomes(X-ray, microCT, mechanical testing) were performed at week 1, 2 and 6 post-fracture. One‐way ANOVA with post-hoc test was performed. Statistical significance was set at p < 0.05.

Our results showed LMHFV could significantly enhance the morphology of the LCN. There was a 65.3% increase in dendritic branch points(p=0.03) and 93% increase in canalicular length(p=0.019) in the OVX-LMHFV group at week 2 post-fracture. Besides, a similar trend was also observed in the SHAM+LMHFV group, with a 43.4% increase in branch points and 53% increase in canaliculi length at week 2. A significant increase of E11 and DMP1 was observed in the LMHFV groups, indicating the reconstruction of the LCN. The decreasing sclerostin and increasing FGF23 at week 1 represented the active bone formation phase while the gradual increase at week 6 signified the remodelling phase. Furthermore, Ca/P ratio, mineral apposition rate and bone formation rate were all significantly enhanced in the OVX+LMHFV group. The overall bone mineral density in BV was significantly raised in the OVX+LMHFV group at week 2(p=0.043) and SHAM+LMHFV at week 6(p=0.04). Quantitative analysis of microCT showed BV/TV was significantly increased at week 2 in OVX+LMHFV group(p=0.008) and week 6(p=0.001) in both vibration groups. In addition, biomechanical testing revealed that the OVX+LMHFV group had a significantly higher ultimate load(p=0.03) and stiffness(p=0.02) at week 2.

To our best knowledge, this is the first report to illustrate LMHFV could enhance osteocytes' morphology, mineralisation status and healing outcome in a new osteoporotic metaphyseal fracture animal model. Our cumulative data supports that the mechanosensitivity of bone would not impair due to osteoporosis. The revitalized osteocyte LCN and upregulated osteocytic protein markers implied a better connectivity and transduction of signals between osteocytes, which may foster the osteoporotic fracture healing process through an enhanced mineralisation process. This could stimulate further mechanistic investigations with potential translation of LMHFV to our fragility fracture patients.

Bone metastases are common and severe complications of cancers. It is estimated to occur in 65–75% of breast and prostate cancer patients and cause 80% of breast cancer-related deaths. Metastasised cancer cells have devastating impacts on bone due to their ability to alter bone remodeling by interacting with osteoblasts and osteoclasts. Exercise, often used as an intervention for cancer patients, regulates bone remodeling via osteocytes. Therefore, we hypothesise that bone mechanical loading may regulate bone metastases via osteocytes. This provides novel insights into the impact of exercises on bone metastases. It will assist in designing cancer intervention programs that lowers the risk for bone metastases. Investigating the mechanisms for the observed effects may also identify potential drug targets.

MLO-Y4 osteocyte-like cells (gift of Dr. Bonewald, University of Missouri-Kansas City) on glass slides were placed in flow chambers and subjected to oscillatory fluid flow (1Pa; 1Hz; 2 hours). Media were extracted (conditioned media; CM) post-flow. RAW264.7 osteoclast precursors were conditioned in MLO-Y4 CM for 7 days. Migration of MDA-MB-231 breast cancer cells and PC3 prostate cancer cells towards CM was assayed using Transwell. Viability, apoptosis, and proliferation of the cancer cells in the CM were measured with Fixable Viability Dye eFluor 450, APOPercentage, and BrDu, respectively. P-values were calculated using Student's t-test.

Significantly more MDA-MB-231 and PC3 cells migrated towards the CM from MLO-Y4 cells with exposure to flow in comparison to CM from MLO-Y4 cells not exposed to flow. The preferential migration is abolished with anti-VEGF antibodies. MDA-MB-231 cells apoptosis rate was slightly lower in CM from MLO-Y4 cells exposed to flow, while proliferation rate was slightly higher. The current data showed no difference in cancer cells viability and adhesion to collagen between any two groups. On the other hand, it was observed that less MDA-MB-231 cells migrated towards CM from RAW264.7 cells conditioned in CM from MLO-Y4 cells stimulated with flow in comparison to those conditioned in CM from MLO-Y4 cells not stimulated with flow. TRAP staining results confirmed that there were less differentiated osteoclasts when RAW264.7 cells were cultured in CM from MLO-Y4 cells exposed to flow.

Overall, this study suggests that when only osteocytes and cancer cells are involved, osteocytes subjected to mechanical loading can promote metastases due to the increased secretion of VEGF. However, with the incorporation of osteoclasts, mechanical loading on osteocytes seems to reduce MDA-MB-231 cell migration. This is likely because osteocytes reduce osteoclastogenesis in response to mechanical stimulation, and osteoclasts have been shown to support cancer cells. Animal studies will also be conducted to verify the pro- or anti-metastatic effect of mechanical loading that is observed in the in vitro part of this study.

Periprosthetic joint infection (PJI) is a potentially devastating complication of joint replacement surgery. Osteocytes comprise 90–95% of all cells in hard bone tissue, are long-lived and are becoming increasingly recognised as a critical cell type in the regulation of bone and systemic physiology. The purpose of this study was to examine role of these cells in PJI pathophysiology and aetiology, with the rationale that their involvement could contribute to the difficulty in detecting and clearing PJI. This study examined the ability of human osteocytes to become infected by Staphylococcus aureus and the responses of both the host cell and pathogen in this scenario. Several S. aureus (MRSA) strains were tested for their ability to infect human primary osteocyte-like cells in vitro and human bone samples ex vivo. Bone biopsies were retrieved from patients undergoing revision total hip arthroplasty for either aseptic loosening associated with osteolysis, or for PJI. Retrieved bacterial colony number from cell lysates and colony morphology were determined. Gene expression was measured by microarray/bioinformatics analysis and/or real-time RT-PCR. Exposure to planktonic S. aureus (approx. 100 CFU/cell) resulted in intracellular infection of human osteocyte-like cells. We found no evidence of increased rates of osteocyte cell death in bacteria exposed cultures. Microarray analysis of osteocyte gene expression 24h following exposure revealed more than 1,500 differentially expressed genes (fold-change more than 2, false discovery rate p < 0.01). The gene expression patterns were consistent with a strong innate immune response and altered functionality of the osteocytes. Consistent patterns of host gene expression were observed between experimentally infected osteocyte-like cultures and human bone, and in PJI patient bone samples. Internalised bacteria switched to the quasi-dormant small colony variant (SCV) form over a period of 5d, and the ensuing infection appeared to reach a stable state. S. aureus infection of viable osteocytes was also identified in bone taken from PJI patients. We have demonstrated [1] that human osteocytes can become infected by S. aureus and respond robustly by producing immune mediators. The bony location of the infected osteocyte may render them refractory to clearance by immune cells, and osteocytes may therefore be an immune-privileged cell type. The phenotypic switch of S. aureus to SCV, a form less sensitive to most antibiotics and one associated with intracellular survival, suggests that infection of osteocytes may contribute to a chronic disease state. The osteocyte may therefore serve as a reservoir of bacteria for reinfection, perhaps explaining the high prevalence of infections that only become apparent after long periods of time or recur following surgical/medical treatment. Our findings also provide a biological rationale for the recognised need for aggressive bone debridement in the surgical management of PJI

Aim. This study describes the histologic changes seen with a gentamicin-eluting synthetic bone graft substitute (BGS)(1) in managing bone defects after resection of chronic osteomyelitis (cOM). Method. 154 patients with mean follow-up of 21.8 months (12–56) underwent treatment of cOM with an antibiotic-loaded BGS for defect filling. Nine patients had subsequent surgery, not related to infection recurrence, allowing biopsy of the implanted material. These biopsies were harvested between 19 days and two years after implantation, allowing a description of the material's remodelling over time. Samples were fixed in formalin and stained with haematoxylin-eosin. Immunohistochemistry, using an indirect immunoperoxidase technique, identified the osteocyte markers Dentine Matrix Protein-1 (DMP-1) and Podoplanin, the macrophage/osteoclast marker CD68, and the macrophage marker CD14. Results. The material was actively remodelled and was osteoconductive. There was evidence of osteoblast recruitment, leading to osteoid and intramembranous formation of woven and lamellar bone on the material's surface, seen most prominently in areas of well-vascularised fibrous tissue. Osteocytes in woven bone expressed the markers DMP-1 and Podoplanin. No cartilage or endochondral ossification was seen. There was a prominent (CD14+/ CD68+) macrophage response to the BSG and macrophages within reparative cellular and collagenous fibrous tissue. In biopsies taken between 4 and 5 months, there were bone trabeculae containing BGS of mainly woven but partly lamellar type. Giant cells on the surface of newly formed mineralised osteoid and woven bone expressed an osteoclast phenotype (CD68+/CD14-). In later biopsies (up to 2 years), larger bone trabeculae were seen more frequently within well-vascularised reparative fibrous tissue. The BGS was replaced with predominantly lamellar bone. One biopsy was taken from an extraosseous leak of BGS into the soft tissues, behind the distal tibia. The histology showed a heavy macrophage infiltrate, but notably no evidence of osteoid or bone formation in the material or surrounding soft tissues. Conclusion. There was clear evidence that this BGS is osteoconductive with first osteoid then woven and lamellar bone being formed. DMP-1 and podoplanin-expressing osteocytes present in woven and lamellar bone demonstrate osteoclastic bone remodelling. Increased lamellar bone was noted in later samples and bone formation was most prominent in well-vascularised areas. There was on-going remodelling of the material beyond one year. The BGS did not ossify in soft tissue. The hydroxyapatite scaffold in this material is probably responsible for its high osteoconductivity and potential to be transformed into bone