A Machine Learning-Based Image Segmentation Method to Quantify In Vitro Osteoclast Culture Endpoints.

Quantification of in vitro osteoclast cultures (e.g. cell number) often relies on manual counting methods. These approaches are labour intensive, time consuming and result in substantial inter- and intra-user variability. This study aimed to develop and validate an automated workflow to robustly quantify in vitro osteoclast cultures. Using ilastik, a machine learning-based image analysis software, images of tartrate resistant acid phosphatase-stained mouse osteoclasts cultured on dentine discs were used to train the ilastik-based algorithm. Assessment of algorithm training showed that osteoclast numbers strongly correlated between manual- and automatically quantified values (r = 0.87). Osteoclasts were consistently faithfully segmented by the model when visually compared to the original reflective light images. The ability of this method to detect changes in osteoclast number in response to different treatments was validated using zoledronate, ticagrelor, and co-culture with MCF7 breast cancer cells. Manual and automated counting methods detected a 70% reduction (p < 0.05) in osteoclast number, when cultured with 10 nM zoledronate and a dose-dependent decrease with 1-10 µM ticagrelor (p < 0.05). Co-culture with MCF7 cells increased osteoclast number by ≥ 50% irrespective of quantification method. Overall, an automated image segmentation and analysis workflow, which consistently and sensitively identified in vitro osteoclasts, was developed. Advantages of this workflow are (1) significantly reduction in user variability of endpoint measurements (93%) and analysis time (80%); (2) detection of osteoclasts cultured on different substrates from different species; and (3) easy to use and freely available to use along with tutorial resources.

Modulation of osteoblast differentiation and function by the P2X4 receptor.

Bone cells are known to express multiple P2 receptor subtypes, and the functional effects of receptor activation have been described for many of these. One exception is the P2X4 receptor, which despite strong expression in osteoblasts and osteoclasts, has no defined functional activity. This study used the selective P2X4 receptor antagonists, 5-BDBD and PSB-12062, to investigate the role of this receptor in bone. Both antagonists (≥ 0.1 µM) dose-dependently decreased bone formation by 60-100%. This was accompanied by a ≤ 70% decrease in alkaline phosphatase activity, a ≤ 40% reduction in cell number, and a ≤ 80% increase in the number of adipocytes present in the culture. The analysis of gene expression showed that levels of osteoblast marker genes (e.g. Alpl, Bglap) were decreased in 5-BDBD treated cells. Conversely, expression of the adipogenic transcription factor PPARG was increased 10-fold. In osteoclasts, high doses of both antagonists were associated with a reduction in osteoclast formation and resorptive activity by ≤ 95% and ≤ 90%, respectively. Taken together, these data suggest that the P2X4 receptor plays a role in modulating bone cell function. In particular, it appears to influence osteoblast differentiation favouring the osteogenic lineage over the adipogenic lineage.

The proton-activated ovarian cancer G protein-coupled receptor 1 (OGR1) is responsible for renal calcium loss during acidosis.

Hypercalciuria is a common feature during metabolic acidosis and associates to nephrolithiasis and nephrocalcinosis. The mechanisms sensing acidosis and inducing increased urinary calcium excretion are still unknown. Here we tested whether mice deficient for proton-activated Ovarian cancer G-protein coupled receptor 1 (OGR1 or Gpr68) have reduced urinary excretion of calcium during chronic metabolic acidosis. In the kidney, OGR1 mRNA was found in cells of the glomerulus, proximal tubule, and interstitium including endothelial cells. Wild type (OGR1+/+) and OGR1 knockout (OGR1-/-) mice were given standard chow without (control) or loaded with ammonium chloride for one or seven days to induce acute or chronic metabolic acidosis, respectively. No differences in responding to the acid load were observed in the knockout mice, except for higher plasma bicarbonate after one day. Bone mineral density, resorption activity of osteoclasts, and urinary deoxypyridinoline were similar between genotypes. During metabolic acidosis the expression levels of key proteins involved in calcium reabsorption, i.e. the sodium/proton exchanger (NHE3), the epithelial calcium-selective channel TRPV5, and the vitamin D-dependent calcium binding protein calbindin-D28k were all higher in the knockout mice compared to wild type mice. This is consistent with the previous demonstration that OGR1 reduces NHE3 activity in proximal tubules of mice. Wild-type mice displayed a non-linear positive association between urinary proton and calcium excretion which was lost in the knockout mice. Thus, OGR1 is a pH sensor involved in the hypercalciuria of metabolic acidosis by controlling NHE3 activity in the proximal tubule. Hence, novel drugs modulating OGR1 activity may improve renal calcium handling.

Differing calcification processes in cultured vascular smooth muscle cells and osteoblasts.

Arterial medial calcification (AMC) is the deposition of calcium phosphate mineral, often as hydroxyapatite, in the medial layer of the arteries. AMC shares some similarities to skeletal mineralisation and has been associated with the transdifferentiation of vascular smooth muscle cells (VSMCs) towards an osteoblast-like phenotype. This study used primary mouse VSMCs and calvarial osteoblasts to directly compare the established and widely used in vitro models of AMC and bone formation. Significant differences were identified between osteoblasts and calcifying VSMCs. First, osteoblasts formed large mineralised bone nodules that were associated with widespread deposition of an extracellular collagenous matrix. In contrast, VSMCs formed small discrete regions of calcification that were not associated with collagen deposition and did not resemble bone. Second, calcifying VSMCs displayed a progressive reduction in cell viability over time (≤7-fold), with a 50% increase in apoptosis, whereas osteoblast and control VSMCs viability remained unchanged. Third, osteoblasts expressed high levels of alkaline phosphatase (TNAP) activity and TNAP inhibition reduced bone formation by to 90%. TNAP activity in calcifying VSMCs was ∼100-fold lower than that of bone-forming osteoblasts and cultures treated with ß-glycerophosphate, a TNAP substrate, did not calcify. Furthermore, TNAP inhibition had no effect on VSMC calcification. Although, VSMC calcification was associated with increased mRNA expression of osteoblast-related genes (e.g. Runx2, osterix, osteocalcin, osteopontin), the relative expression of these genes was up to 40-fold lower in calcifying VSMCs versus bone-forming osteoblasts. In summary, calcifying VSMCs in vitro display some limited osteoblast-like characteristics but also differ in several key respects: 1) their inability to form collagen-containing bone; 2) their lack of reliance on TNAP to promote mineral deposition; and, 3) the deleterious effect of calcification on their viability.

Inhibition of vascular smooth muscle cell calcification by ATP analogues.

Arterial medial calcification (AMC) has been associated with phenotypic changes in vascular smooth muscle cells (VSMCs) that reportedly makes them more osteoblast-like. Previous work has shown that ATP/UTP can inhibit AMC directly via P2 receptors and indirectly by NPP1-mediated hydrolysis to produce the mineralisation inhibitor, pyrophosphate (PPi). This study investigated the role of P2X receptors in the inhibitory effects of extracellular nucleotides on VSMC calcification. We found that Bz-ATP, α,ß-meATP and ß,γ-meATP inhibited calcification by up to 100%. Culture in a high-phosphate medium (2 mM) was associated with increased VSMC death and apoptosis; treatment with Bz-ATP, α,ß-meATP and ß,γ-meATP reduced apoptosis to levels seen in non-calcifying cells. Calcification was also associated with alterations in the protein levels of VSMC (e.g. SM22α and SMA) and osteoblast-associated (e.g. Runx2 and osteopontin) markers; Bz-ATP, α,ß-meATP and ß,γ-meATP attenuated these changes in protein expression. Long-term culture with Bz-ATP, α,ß-meATP and ß,γ-meATP resulted in lower extracellular ATP levels and an increased rate of ATP breakdown. P2X receptor antagonists failed to prevent the inhibitory effects of these analogues suggesting that they act via P2X receptor-independent mechanisms. In agreement, the breakdown products of α,ß-meATP and ß,γ-meATP (α,ß-meADP and methylene diphosphonate, respectively) also dose-dependently inhibited VSMC calcification. Furthermore, the actions of Bz-ATP, α,ß-meATP and ß,γ-meATP were unchanged in VSMCs isolated from NPP1-knockout mice, suggesting that the functional effects of these compounds do not involve NPP1-mediated generation of PPi. Together, these results indicate that the inhibitory effects of ATP analogues on VSMC calcification and apoptosis in vitro may be mediated, at least in part, by mechanisms that are independent of purinergic signalling and PPi.

Influence of pH on osteoclasts treated with zoledronate and alendronate.

OBJECTIVES: The objectives of this study were to analyze the effect of pH on the growth and activity of osteoclasts treated with different doses of two nitrogen-containing BPs, zoledronate and alendronate. MATERIALS AND METHODS: Murine osteoclasts cultured on dentine disks were treated with zoledronate (50 or 500 nM) or alendronate (500 or 5 µM) at two different pH values (7.4 or 7.0). Osteoclasts were counted with transmitted light microscopy, apoptosis/necrosis was studied with flow cytometry and confocal microscopy, and resorption pit number and depth were calculated using reflected light and scanning electron microscopy. RESULTS: The osteoclast count on dentine disks was significantly (p < 0.001) reduced by zoledronate or alendronate treatment at pH 7.0 in comparison to treatment with the same doses at pH 7.4 and untreated disks (controls). The percentage of apoptotic cells was significantly increased by treatment with 500 nM zoledronate or 5 µM alendronate at pH 7.0 in comparison to the same doses at pH 7.4. The number and depth of resorption pits were significantly lower in disks treated at each BP dose studied than in untreated controls at pH 7.0. CONCLUSIONS: Zoledronate and alendronate at therapeutic doses have an adverse effect on the viability and resorptive activity of osteoclasts when the local medium pH is reduced. CLINICAL RELEVANCE: These findings suggest that periodontal or peri-implant oral cavity infection may be a key trigger of the cascade of events that lead to BRONJ.

Inhibition of arterial medial calcification and bone mineralization by extracellular nucleotides: The same functional effect mediated by different cellular mechanisms.

Arterial medial calcification (AMC) is thought to share some outward similarities to skeletal mineralization and has been associated with the transdifferentiation of vascular smooth muscle cells (VSMCs) to an osteoblast-like phenotype. ATP and UTP have previously been shown to inhibit bone mineralization. This investigation compared the effects of extracellular nucleotides on calcification in VSMCs with those seen in osteoblasts. ATP, UTP and the ubiquitous mineralization inhibitor, pyrophosphate (PPi ), dose dependently inhibited VSMC calcification by ≤85%. Culture of VSMCs in calcifying conditions was associated with an increase in apoptosis; treatment with ATP, UTP, and PPi reduced apoptosis to levels seen in non-calcifying cells. Extracellular nucleotides had no effect on osteoblast viability. Basal alkaline phosphatase (TNAP) activity was over 100-fold higher in osteoblasts than VSMCs. ATP and UTP reduced osteoblast TNAP activity (≤50%) but stimulated VSMC TNAP activity (≤88%). The effects of extracellular nucleotides on VSMC calcification, cell viability and TNAP activity were unchanged by deletion or inhibition of the P2Y2 receptor. Conversely, the actions of ATP/UTP on bone mineralization and TNAP activity were attenuated in osteoblasts lacking the P2Y2 receptor. Ecto-nucleotide pyrophosphatase/phosphodiesterase 1 (NPP1) hydrolyses ATP and UTP to produce PPi . In both VSMCs and osteoblasts, deletion of NPP1 blunted the inhibitory effects of extracellular nucleotides suggesting involvement of P2 receptor independent pathways. Our results show that although the overall functional effect of extracellular nucleotides on AMC and bone mineralization is similar there are clear differences in the cellular mechanisms mediating these actions.

Allopurinol and oxypurinol promote osteoblast differentiation and increase bone formation.

Allopurinol and its active metabolite, oxypurinol are widely used in the treatment of gout and hyperuricemia. They inhibit xanthine oxidase (XO) an enzyme in the purine degradation pathway that converts xanthine to uric acid. This investigation examined the effect of allopurinol and oxypurinol on bone formation, cell number and viability, gene expression and enzyme activity in differentiating and mature, bone-forming osteoblasts. Although mRNA expression remained relatively constant, XO activity decreased over time with mature osteoblasts displaying reduced levels of uric acid (20% decrease). Treatment with allopurinol and oxypurinol (0.1-1 µM) reduced XO activity by up to 30%. At these concentrations, allopurinol and oxypurinol increased bone formation by osteoblasts ~4-fold and ~3-fold, respectively. Cell number and viability were unaffected. Both drugs increased tissue non-specific alkaline phosphatase (TNAP) activity up to 65%. Osteocalcin and TNAP mRNA expression was increased, 5-fold and 2-fold, respectively. Expression of NPP1, the enzyme responsible for generating the mineralisation inhibitor, pyrophosphate, was decreased 5-fold. Col1α1 mRNA expression and soluble collagen levels were unchanged. Osteoclast formation and resorptive activity were not affected by treatment with allopurinol or oxypurinol. Our data suggest that inhibition of XO activity promotes osteoblast differentiation, leading to increased bone formation in vitro.

Lack of effect of adenosine on the function of rodent osteoblasts and osteoclasts in vitro.

Extracellular ATP, signalling through P2 receptors, exerts well-documented effects on bone cells, inhibiting mineral deposition by osteoblasts and stimulating the formation and resorptive activity of osteoclasts. The aims of this study were to determine the potential osteotropic effects of adenosine, the hydrolysis product of ATP, on primary bone cells in vitro. We determined the effect of exogenous adenosine on (1) the growth, alkaline phosphatase (TNAP) activity and bone-forming ability of osteoblasts derived from the calvariae of neonatal rats and mice and the marrow of juvenile rats and (2) the formation and resorptive activity of osteoclasts from juvenile mouse marrow. Reverse transcription polymerase chain reaction (RT-PCR) analysis showed marked differences in the expression of P1 receptors in osteoblasts from different sources. Whilst mRNA for the A1 and A2B receptors was expressed by all primary osteoblasts, A2A receptor expression was limited to rat bone marrow and mouse calvarial osteoblasts and the A3 receptor to rat bone marrow osteoblasts. We found that adenosine had no detectable effects on cell growth, TNAP activity or bone formation by rodent osteoblasts in vitro. The analogue 2-chloroadenosine, which is hydrolysed more slowly than adenosine, had no effects on rat or mouse calvarial osteoblasts but increased TNAP activity and bone formation by rat bone marrow osteoblasts by 30-50 % at a concentration of 1 µM. Osteoclasts were found to express the A2A, A2B and A3 receptors; however, neither adenosine (≤100 µM) nor 2-chloroadenosine (≤10 µM) had any effect on the formation or resorptive activity of mouse osteoclasts in vitro. These results suggest that adenosine, unlike ATP, is not a major signalling molecule in the bone.

Acidosis is a key regulator of osteoblast ecto-nucleotidase pyrophosphatase/phosphodiesterase 1 (NPP1) expression and activity.

Previous work has shown that acidosis prevents bone nodule formation by osteoblasts in vitro by inhibiting mineralisation of the collagenous matrix. The ratio of phosphate (Pi ) to pyrophosphate (PPi ) in the bone microenvironment is a fundamental regulator of bone mineralisation. Both Pi and PPi , a potent inhibitor of mineralisation, are generated from extracellular nucleotides by the actions of ecto-nucleotidases. This study investigated the expression and activity of ecto-nucleotidases by osteoblasts under normal and acid conditions. We found that osteoblasts express mRNA for a number of ecto-nucleotidases including NTPdase 1-6 (ecto-nucleoside triphosphate diphosphohydrolase) and NPP1-3 (ecto-nucleotide pyrophosphatase/phosphodiesterase). The rank order of mRNA expression in differentiating rat osteoblasts (day 7) was Enpp1 > NTPdase 4 > NTPdase 6 > NTPdase 5 > alkaline phosphatase > ecto-5-nucleotidase > Enpp3 > NTPdase 1 > NTPdase 3 > Enpp2 > NTPdase 2. Acidosis (pH 6.9) upregulated NPP1 mRNA (2.8-fold) and protein expression at all stages of osteoblast differentiation compared to physiological pH (pH 7.4); expression of other ecto-nucleotidases was unaffected. Furthermore, total NPP activity was increased up to 53% in osteoblasts cultured in acid conditions (P < 0.001). Release of ATP, one of the key substrates for NPP1, from osteoblasts, was unaffected by acidosis. Further studies showed that mineralised bone formation by osteoblasts cultured from NPP1 knockout mice was increased compared with wildtypes (2.5-fold, P < 0.001) and was partially resistant to the inhibitory effect of acidosis. These results indicate that increased NPP1 expression and activity might contribute to the decreased mineralisation observed when osteoblasts are exposed to acid conditions.

Role of the P2Y13 receptor in the differentiation of bone marrow stromal cells into osteoblasts and adipocytes.

Accumulating evidence indicates that extracellular nucleotides, signaling through purinergic receptors, play a significant role in bone remodeling. Mesenchymal stem cells (MSCs) express functional P2Y receptors whose expression level is regulated during osteoblast or adipocyte differentiation. P2Y13 -deficient mice were previously shown to exhibit a decreased bone turnover associated with a reduction in the number of both osteoblasts and osteoclasts on the bone surfaces. We therefore examined whether P2Y13 R activation was involved in the osteogenic differentiation of MSC. Our study demonstrated that ADP stimulation of P2Y13 R(+/+) (but not P2Y13 R(-/-) ) adherent bone marrow stromal cells (BMSCs) increased significantly the formation of alkaline phosphatase-colony-forming units (CFU-ALP) as well as the expression of osteoblastic markers (osterix, alkaline phosphatase, and collagen I) involved in the maturation of preosteoblasts into osteoblasts. The number of CFU-ALP obtained from P2Y13 R(-/-) BMSC and the level of osteoblastic gene expression after osteogenic stimulation were strongly reduced compared to those obtained in wild-type cell cultures. In contrast, when P2Y13 R(-/-) BMSCs were incubated in an adipogenic medium, the number of adipocytes generated and the level of adipogenic gene expression (PPARγ2 and Adipsin) were higher than those obtained in P2Y13 R(+/+) MSC. Interestingly, we observed a significant increase of the number of bone marrow adipocytes in tibia of P2Y13 R(-/-) mice. In conclusion, our findings indicate that the P2Y13 R plays an important role in the balance of osteoblast and adipocyte terminal differentiation of bone marrow progenitors. Therefore, the P2Y13 receptor can be considered as a new pharmacological target for the treatment of bone diseases like osteoporosis. STEM Cells 2013;31:2747-2758.

PPAR agonists stimulate adipogenesis at the expense of osteoblast differentiation while inhibiting osteoclast formation and activity.

Drugs used in the treatment of type 2 diabetes and cardiovascular disease, specifically peroxisome proliferator-activated receptor (PPAR) agonists, have been reported to affect bone cell function and fracture risk. In this study, we assessed the direct effects of PPAR-γ agonists (rosiglitazone and troglitazone), used in the treatment of diabetes, and a PPAR-α agonist (fenofibrate), used to treat hyperlipidaemia, on the function of primary osteoblasts and osteoclasts. Formation of 'trabecular' bone structures by rat calvarial osteoblasts was reduced by up to 85% in cultures treated with rosiglitazone and by 45% in troglitazone-treated or fenofibrate-treated cultures; at the same time, lipid droplet formation was increased by 40-70%. The expression of key osteogenic markers was similarly downregulated in cultures treated with PPAR agonists, whereas adipogenesis markers were upregulated. Formation of osteoclasts in cultures derived from mouse marrow diminished with fenofibrate treatment, whereas both glitazones reduced resorptive activity without affecting osteoclast number. Metformin, although not a PPAR agonist, is also commonly used in the treatment of type 2 diabetes. Here, metformin was found to have no effect on bone cell function. Taken together, these data suggest that PPAR-γ agonists may enhance bone loss via increased adipogenesis at the expense of osteoblast formation. In contrast, PPAR-α agonists may prevent bone loss. Given that the prevalence of diabetes and cardiovascular disease is expected to rise significantly, greater attention may need to be paid to the effects of PPAR agonists on bone homeostasis.

Purinergic signalling in the musculoskeletal system.

It is now widely recognised that extracellular nucleotides, signalling via purinergic receptors, participate in numerous biological processes in most tissues. It has become evident that extracellular nucleotides have significant regulatory effects in the musculoskeletal system. In early development, ATP released from motor nerves along with acetylcholine acts as a cotransmitter in neuromuscular transmission; in mature animals, ATP functions as a neuromodulator. Purinergic receptors expressed by skeletal muscle and satellite cells play important pathophysiological roles in their development or repair. In many cell types, expression of purinergic receptors is often dependent on differentiation. For example, sequential expression of P2X5, P2Y1 and P2X2 receptors occurs during muscle regeneration in the mdx model of muscular dystrophy. In bone and cartilage cells, the functional effects of purinergic signalling appear to be largely negative. ATP stimulates the formation and activation of osteoclasts, the bone-destroying cells. Another role appears to be as a potent local inhibitor of mineralisation. In osteoblasts, the bone-forming cells, ATP acts via P2 receptors to limit bone mineralisation by inhibiting alkaline phosphatase expression and activity. Extracellular ATP additionally exerts significant effects on mineralisation via its hydrolysis product, pyrophosphate. Evidence now suggests that purinergic signalling is potentially important in several bone and joint disorders including osteoporosis, rheumatoid arthritis and cancers. Strategies for future musculoskeletal therapies might involve modulation of purinergic receptor function or of the ecto-nucleotidases responsible for ATP breakdown or ATP transport inhibitors.

Hypothermia inhibits osteoblast differentiation and bone formation but stimulates osteoclastogenesis.

It has long been known that core body temperature declines with age, with temperatures of 35.5°C or below common in the elderly. However, the effects of temperature reduction on bone cell function and skeletal homeostasis have been little studied. We investigated the effects of mild hypothermia (35.5°C) and severe hypothermia (34°C) on bone-forming osteoblasts, and bone-resorbing osteoclasts. Formation of 'trabecular' bone structures by rat calvarial osteoblasts was reduced by 75% at 35.5°C and by 95% at 34°C after 14-16 days culture, compared to 37°C. In addition to reductions in osteoblast cell number, expression of mRNAs for Runx2, alkaline phosphatase, osteocalcin and type I collagen were also down-regulated in hypothermia. In contrast, formation of osteoclasts in mononuclear cell cultures derived from mouse marrow, showed a 1.5 to 2-fold stimulation in hypothermia; resorption pit formation was similarly increased. Taken together, these data show that hypothermia exerts reciprocal effects on bone cell function by retarding osteoblast differentiation and bone formation, whilst increasing osteoclastogenesis and thus resorption. These results suggest the possibility that hypothermia in the elderly could potentially have a direct, negative impact on bone metabolism.

Evidence that pyrophosphate acts as an extracellular signalling molecule to exert direct functional effects in primary cultures of osteoblasts and osteoclasts.

Extracellular pyrophosphate (PPi) is well known for its fundamental role as a physiochemical mineralisation inhibitor. However, information about its direct actions on bone cells remains limited. This study shows that PPi decreased osteoclast formation and resorptive activity by ≤50 %. These inhibitory actions were associated with reduced expression of genes involved in osteoclastogenesis (Tnfrsf11a, Dcstamp) and bone resorption (Ctsk, Car2, Acp5). In osteoblasts, PPi present for the entire (0-21 days) or latter stages of culture (7-21/14-21 days) decreased bone mineralisation by ≤95 %. However, PPi present for the differentiation phase only (0-7/0-14 days) increased bone formation (≤70 %). Prolonged treatment with PPi resulted in earlier matrix deposition and increased soluble collagen levels (≤2.3-fold). Expression of osteoblast (RUNX2, Bglap) and early osteocyte (E11, Dmp1) genes along with mineralisation inhibitors (Spp1, Mgp) was increased by PPi (≤3-fold). PPi levels are regulated by tissue non-specific alkaline phosphatase (TNAP) and ecto-nucleotide pyrophosphatase/phosphodiesterase 1 (NPP1). PPi reduced NPP1 expression in both cell types whereas TNAP expression (≤2.5-fold) and activity (≤35 %) were increased in osteoblasts. Breakdown of extracellular ATP by NPP1 represents a key source of PPi. ATP release from osteoclasts and osteoblasts was decreased ≤60 % by PPi and by a selective TNAP inhibitor (CAS496014-12-2). Pertussis toxin, which prevents Gαi subunit activation, was used to investigate whether G-protein coupled receptor (GPCR) signalling mediates the effects of PPi. The actions of PPi on bone mineralisation, collagen production, ATP release, gene/protein expression and osteoclast formation were abolished or attenuated by pertussis toxin. Together these findings show that PPi, modulates differentiation, function and gene expression in osteoblasts and osteoclasts. The ability of PPi to alter ATP release and NPP1/TNAP expression and activity indicates that cells can detect PPi levels and respond accordingly. Our data also raise the possibility that some actions of PPi on bone cells could be mediated by a Gαi-linked GPCR.

Hypoxia mimetics restore bone biomineralisation in hyperglycaemic environments.

Diabetic patients have an increased risk of fracture and an increased occurrence of impaired fracture healing. Diabetic and hyperglycaemic conditions have been shown to impair the cellular response to hypoxia, via an inhibited hypoxia inducible factor (HIF)-1α pathway. We investigated, using an in vitro hyperglycaemia bone tissue engineering model (and a multidisciplinary bone characterisation approach), the differing effects of glucose levels, hypoxia and chemicals known to stabilise HIF-1α (CoCl2 and DMOG) on bone formation. Hypoxia (1% O2) inhibited bone nodule formation and resulted in discrete biomineralisation as opposed to the mineralised extracellular collagen fibres found in normoxia (20% O2). Unlike hypoxia, the use of hypoxia mimetics did not prevent nodule formation in normal glucose level. Hyperglycaemic conditions (25 mM and 50 mM glucose) inhibited biomineralisation. Interestingly, both hypoxia mimetics (CoCl2 and DMOG) partly restored hyperglycaemia inhibited bone nodule formation. These results highlight the difference in osteoblast responses between hypoxia mimetics and actual hypoxia and suggests a role of HIF-1α stabilisation in bone biomineralisation that extends that of promoting neovascularisation, or other system effects associated with hypoxia and bone regeneration in vivo. This study demonstrates that targeting the HIF pathway may represent a promising strategy for bone regeneration in diabetic patients.

Sheep condyle model evaluation of bone marrow cell concentrate combined with a scaffold for repair of large osteochondral defects.

AIMS: Minimally manipulated cells, such as autologous bone marrow concentrates (BMC), have been investigated in orthopaedics as both a primary therapeutic and augmentation to existing restoration procedures. However, the efficacy of BMC in combination with tissue engineering is still unclear. In this study, we aimed to determine whether the addition of BMC to an osteochondral scaffold is safe and can improve the repair of large osteochondral defects when compared to the scaffold alone. METHODS: The ovine femoral condyle model was used. Bone marrow was aspirated, concentrated, and used intraoperatively with a collagen/hydroxyapatite scaffold to fill the osteochondral defects (n = 6). Tissue regeneration was then assessed versus the scaffold-only group (n = 6). Histological staining of cartilage with alcian blue and safranin-O, changes in chondrogenic gene expression, microCT, peripheral quantitative CT (pQCT), and force-plate gait analyses were performed. Lymph nodes and blood were analyzed for safety. RESULTS: The results six months postoperatively showed that there were no significant differences in bone regrowth and mineral density between BMC-treated animals and controls. A significant upregulation of messenger RNA (mRNA) for types I and II collagens in the BMC group was observed, but there were no differences in the formation of hyaline-like cartilage between the groups. A trend towards reduced sulphated glycosaminoglycans (sGAG) breakdown was detected in the BMC group but this was not statistically significant. Functional weightbearing was not affected by the inclusion of BMC. CONCLUSION: Our results indicated that the addition of BMC to scaffold is safe and has some potentially beneficial effects on osteochondral-tissue regeneration, but not on the functional endpoint of orthopaedic interest. Cite this article: Bone Joint Res 2021;10(10):677-689.

Bioengineering the ameloblastoma tumour to study its effect on bone nodule formation.

Ameloblastoma is a benign, epithelial cancer of the jawbone, which causes bone resorption and disfigurement to patients affected. The interaction of ameloblastoma with its tumour stroma drives invasion and progression. We used stiff collagen matrices to engineer active bone forming stroma, to probe the interaction of ameloblastoma with its native tumour bone microenvironment. This bone-stroma was assessed by nano-CT, transmission electron microscopy (TEM), Raman spectroscopy and gene analysis. Furthermore, we investigated gene correlation between bone forming 3D bone stroma and ameloblastoma introduced 3D bone stroma. Ameloblastoma cells increased expression of MMP-2 and -9 and RANK temporally in 3D compared to 2D. Our 3D biomimetic model formed bone nodules of an average surface area of 0.1 mm2 and average height of 92.37 [Formula: see text] 7.96 µm over 21 days. We demonstrate a woven bone phenotype with distinct mineral and matrix components and increased expression of bone formation genes in our engineered bone. Introducing ameloblastoma to the bone stroma, completely inhibited bone formation, in a spatially specific manner. Multivariate gene analysis showed that ameloblastoma cells downregulate bone formation genes such as RUNX2. Through the development of a comprehensive bone stroma, we show that an ameloblastoma tumour mass prevents osteoblasts from forming new bone nodules and severely restricted the growth of existing bone nodules. We have identified potential pathways for this inhibition. More critically, we present novel findings on the interaction of stromal osteoblasts with ameloblastoma.

Acidosis, hypoxia and bone.

Bone homeostasis is profoundly affected by local pH and oxygen tension. It has long been recognised that the skeleton contains a large reserve of alkaline mineral (hydroxyapatite), which is ultimately available to neutralise metabolic H(+) if acid-base balance is not maintained within narrow limits. Bone cells are extremely sensitive to the direct effects of pH: acidosis inhibits mineral deposition by osteoblasts but it activates osteoclasts to resorb bone and other mineralised tissues. These reciprocal responses act to maximise the availability of OH(-) ions from hydroxyapatite in solution, where they can buffer excess H(+). The mechanisms by which bone cells sense small pH changes are likely to be complex, involving ion channels and receptors in the cell membrane, as well as direct intracellular effects. The importance of oxygen tension in the skeleton has also long been known. Recent work shows that hypoxia blocks the growth and differentiation of osteoblasts (and thus bone formation), whilst strongly stimulating osteoclast formation (and thus bone resorption). Surprisingly, the resorptive function of osteoclasts is unimpaired in hypoxia. In vivo, tissue hypoxia is usually accompanied by acidosis due to reduced vascular perfusion and increased glycolytic metabolism. Thus, disruption of the blood supply can engender a multiple negative impact on bone via the direct actions of reduced pO(2) and pH on bone cells. These observations may contribute to our understanding of the bone disturbances that occur in numerous settings, including ageing, inflammation, fractures, tumours, anaemias, kidney disease, diabetes, respiratory disease and smoking.