Are osteoclasts needed for the bone anabolic response to parathyroid hormone? A study of intermittent parathyroid hormone with denosumab or alendronate in knock-in mice expressing humanized RANKL

Anti-RANKL Antibody For Active Charcot Foot Neuro-Osteoarthropathy in Patients with Diabetes and Chronic Kidney Disease

BACKGROUND

Charcot neuroosteoarthropathy (CNO) is characterized with increased osteoclastic activity that can be curbed with antiresorptive agents. Chronic kidney disease (CKD) precludes bisphosphonates but anti-receptor activator of nuclear factor-B ligand (anti-RANKL) antibody, denosumab, can be contemplated in CKD. We investigated denosumab for active CNO of foot in CKD for CNO remission.

METHODS

During the study period, 446 persons of diabetes with unilateral, active CNO of foot and CKD were identified and 78 were finally enrolled. Patients received either 60 mg denosumab (single-dose, subcutaneous) along with standard of care (SoC) as total contact cast (TCC) (group A; n = 26) or SoC (group B; n = 52) only. Patients were followed every 4 weeks until CNO remission and subsequently every 8 weeks until 48 weeks following remission. Remission was defined as temperature difference <2 °C between 2 feet confirmed twice (4 weeks apart) with clinical resolution of signs of inflammation. The primary outcome studied was proportion of patients achieving remission within 48 weeks and the time to remission.

RESULTS

Median age was 56.5 (48.8-65) and 57 (48.5-61.2) years, P = .57; duration of diabetes 16 (10-25.3) and 14.9 (10-19) years, P = .151; and estimated glomerular filtration rate 44.8 (21.1-65.6) and 45.7 (32.9-55.7) mL/min/1.73 m2, P = .771, in group A and B, respectively. Median temperature difference at presentation between the affected and opposite foot was 3.4 °C (2.7-6.9) and 3.2 °C (2.2-4.0), P = .119, respectively. All patients achieved remission in group A (100%) compared with 42 (80.8%) in group B (P = .006) (hazard ratio 0.52, 95% CI: 0.32-0.87; P = .012). The median time to remission was similar in the 2 groups (15 [11-25] and 17.5 [14-31.5] weeks, P = .229, respectively). 25-Hydroxyvitamin D3 >14 ng/mL was significantly associated (OR 9.5, 95% CI 1.04-87.5, P = .045) with remission.

CONCLUSION

Anti-RANKL antibody added to SoC (TCC) induces remission of active foot CNO in greater proportions of patients with diabetes and CKD.

Advances of long non-coding RNAs in osteoclast differentiation and osteoporosis

INTRODUCTION

Osteoclasts, which are responsible for bone resorption, are specialized multinucleated cells generated from monocyte/macrophage progenitor cells or hematopoietic stem cells (HSCs). Physiological bone remodeling can become pathological, such as osteoporosis, when osteoclastogenesis is out of balance. Thousands of long noncoding RNAs (lncRNAs) influence important molecular and biological processes. Recent research has revealed gene expression regulation function that numerous lncRNAs regulate nuclear domain organization, genome stability. Furthermore, the research of lncRNAs has substantial clinical implications for the treatment of existing and new diseases.

AREAS COVERED

In this review, we gather the most recent research on lncRNAs and their potential for basic research and clinical applications in osteoclast and osteoporosis. We also discuss the findings here in order to fully understand the role of lncRNAs in osteoclast differentiation and osteoporosis, as well as to provide a solid basis for future research exploring associated mechanisms and treatments.

EXPERT OPINION

LncRNA has been considered as an important role in the regulation of osteoclast differentiation and osteoporosis. It is exciting to investigate pathophysiological processes in osteoporosis and the therapeutic potential of lncRNAs. We hope that this review will offer promising prospects for the development of precision and individualized approaches to treatment.

Marantodes pumilum (Kacip Fatimah) Aqueous Extract Enhances Osteoblast and Suppresses Osteoclast Activities in Cancellous Bone of a Rat Model of Postmenopause

Evidence pointed towards the benefits of Marantodes pumilum in treating osteoporosis after menopause; however, the detailed mechanisms still have not been explored. Therefore, this study aims to identify the molecular mechanisms underlying M. pumilum's bone-protective effect via the involvement of RANK/RANKL/OPG and Wnt/β-catenin signaling pathways. Ovariectomized adult female rats were given M. pumilum leaf aqueous extract (MPLA) (50 and 100 mg/kg/day) and estrogen (positive control) orally for twenty-eight consecutive days. Following the treatment, rats were sacrificed, and femur bones were harvested. Blood was withdrawn for analysis of serum Ca2+, PO43-, and bone alkaline phosphatase (BALP) levels. The bone microarchitectural changes were observed by H&E and PAS staining and distribution and expression of RANK/RANKL/OPG and Wnt3a/β-catenin and its downstream proteins were determined by immunohistochemistry, immunofluorescence, Western blot, and real-time PCR. MPLA treatment increased serum Ca2+ and PO43- levels and reduced serum BALP levels (p < 0.05). Besides, deterioration in cancellous bone microarchitecture and the loss of bone glycogen and collagen content were mitigated by MPLA treatment. Levels of RANKL, Traf6, and NF-kB but not RANK in bone were decreased; however, levels of OPG, Wnt3a, LRP-5, Frizzled, Dvl, β-catenin, RUNX, and Bmp-2 in bone were increased following treatment with MPLA. In conclusion, MPLA helps to protect against bone deterioration in estrogen deficiency state and thus, this herb could potentially be used to ameliorate osteoporosis in women after menopause.

Hungry bone syndrome like presentation following single-dose denosumab for hypercalcaemia secondary to sarcoidosis with IgA nephropathy

A woman in her mid-50s with IgA nephropathy, sarcoidosis and steroid-induced diabetes mellitus presented with generalised paraesthesia and spontaneous tetany. She had received denosumab 60 mg subcutaneously 8 weeks previously for parathyroid hormone independent hypercalcaemia.At admission, she had severe hypocalcaemia (5 mg/dL), hypophosphataemia (1.9 mg/dL), hypomagnesaemia (1.4 mg/dL) and elevated serum creatinine (1.48 mg/dL) with prolonged QTc (corrected QT interval) on electrocardiograph. She initially received intravenous calcium and magnesium followed by oral calcium carbonate and calcitriol. Her prednisolone dose was tapered to 5 mg/day. Evaluation showed secondary hyperparathyroidism (1474 pg/mL) and elevated 1,25-dihydroxy vitamin D (195 pg/mL). After 1 week of oral calcium carbonate (3000 mg/day) and calcitriol (1.5 µg/day), she achieved normocalcaemia (8.1 mg/dL).To conclude, denosumab for hypercalcaemia with renal insufficiency causes prolonged severe symptomatic hypocalcaemia and hypophosphataemia mimicking hungry bone syndrome. It is important to periodically monitor for hypocalcaemia after denosumab.

Actions of Parathyroid Hormone Ligand Analogues in Humanized PTH1R Knockin Mice

Rodent models are commonly used to evaluate parathyroid hormone (PTH) and PTH-related protein (PTHrP) ligands and analogues for their pharmacologic activities and potential therapeutic utility toward diseases of bone and mineral ion metabolism. Divergence, however, in the amino acid sequences of rodent and human PTH receptors (rat and mouse PTH1Rs are 91% identical to the human PTH1R) can lead to differences in receptor-binding and signaling potencies for such ligands when assessed on rodent vs human PTH1Rs, as shown by cell-based assays in vitro. This introduces an element of uncertainty in the accuracy of rodent models for performing such preclinical evaluations. To overcome this potential uncertainty, we used a homologous recombination-based knockin (KI) approach to generate a mouse (in-host strain C57Bl/6N) in which complementary DNA encoding the human PTH1R replaces a segment (exon 4) of the murine PTH1R gene so that the human and not the mouse PTH1R protein is expressed. Expression is directed by the endogenous mouse promoter and hence occurs in all biologically relevant cells and tissues and at appropriate levels. The resulting homozygous hPTH1R-KI (humanized) mice were healthy over at least 10 generations and showed functional responses to injected PTH analog peptides that are consistent with a fully functional human PTH1R in target bone and kidney cells. The initial evaluation of these mice and their potential utility for predicting behavior of PTH analogues in humans is reported here.

Systemic osteoprotegerin does not improve peri-implant bone volume or osseointegration in rabbits

Anti-RANKL (receptor activator of nuclear factor kappa-B ligand) agents function by blocking the differentiation of osteoclasts, thereby proving useful in the clinical management of postmenopausal osteoporosis. The effects of such agents on osseointegration is less well understood. The purpose of the current study was to investigate whether osteoprotegerin (OPG), an osteoclast inhibitor, enhances the known anabolic effects of mechanical loading (VEH) and intermittent PTH (iPTH) using a well-established rabbit model of osseointegration. In the first set of experiments, OPG was administered either alone or combined with iPTH to study its effects on measured bone mass. The second set of experiments was conducted using a higher dosage of OPG (10 mg/kg) to explore its early impact at the cellular and molecular levels. All subjects had mechanical load applied to the implant on one extremity, and no load applied on the contralateral side. In the first set of experiments, OPG alone decreased peri-implant bone mass compared to the mechanical loading group, whereas OPG + iPTH increased peri-implant bone mass compared to the OPG group. In the second set of experiments, high-dose OPG significantly decreased osteoclast number (-74.3%) at 1 week. However, this effect was not sustained as osteoclast number returned to baseline by 2 weeks. These results suggest that systemic administration of OPG does not enhance osseointegration, but rather has a detrimental effect.

Bisphosphonates impair the onset of bone formation at remodeling sites

Bisphosphonates are widely used anti-osteoporotic drugs targeting osteoclasts. They strongly inhibit bone resorption, but also strongly reduce bone formation. This reduced formation is commonly ascribed to the mechanism maintaining the resorption/formation balance during remodeling. The present study provides evidence for an additional mechanism where bisphosphonates actually impair the onset of bone formation after resorption. The evidence is based on morphometric parameters recently developed to assess the activities reversing resorption to formation. Herein, we compare these parameters in cancellous bone of alendronate- and placebo-treated postmenopausal osteoporotic patients. Alendronate increases the prevalence of eroded surfaces characterized by reversal cells/osteoprogenitors at low cell density and remote from active bone surfaces. This indicates deficient cell expansion on eroded surfaces - an event that is indispensable to start formation. Furthermore, alendronate decreases the coverage of these eroded surfaces by remodeling compartment canopies, a putative source of reversal cells/osteoprogenitors. Finally, alendronate strongly decreases the activation frequency of bone formation, and decreases more the formative compared to the eroded surfaces. All these parameters correlate with each other. These observations lead to a model where bisphosphonates hamper the osteoprogenitor recruitment required to initiate bone formation. This effect results in a larger eroded surface, thereby explaining the well-known paradox that bisphosphonates strongly inhibit bone resorption without strongly decreasing eroded surfaces. The possible mechanism for hampered osteoprogenitor recruitment is discussed: bisphosphonates may decrease the release of osteogenic factors by the osteoclasts, and/or bisphosphonates released by osteoclasts may act directly on neighboring osteoprogenitor cells as reported in preclinical studies.

Combination Therapy of PTH and Antiresorptive Drugs on Osteoporosis: A Review of Treatment Alternatives

Antiresorptive drugs have been widely used for osteoporosis. Intermittent parathyroid hormone (PTH), an anabolic agent, increases osteoblast production rate and inhibits apoptosis of osteoblasts, thus increasing skeletal mass besides improving bone microarchitecture and strength. Combination therapy for osteoporosis produced great interests and controversies. Therefore, we performed a systematic literature search from PubMed, EMBASE, Scopus, Web of Science, CINDHL, and the Cochrane Database of Systematic Reviews using the search terms PTH or teriparatide combined with bisphosphonate, alendronate, ibandronate, risedronate, raloxifene, denosumab, and zoledronic acid with the limit osteoporosis. At last, 36 related articles were included for further analysis. Findings from previous studies revealed that combination therapy in different conditions of naive or previous bisphosphonate treatment might have different outcomes. The use of combination therapy, however, may be an alternative option among osteoporotic patients with a history of bisphosphonate use. Combined teriparatide with denosumab appear to show the most substantial and clinically relevant skeletal benefits to osteoporotic patients. Additional research is necessary to define optimal methods of developing sequential and/or cyclical combinations of PTH and antiresorptive agents.

Re-thinking the bone remodeling cycle mechanism and the origin of bone loss

Proper bone remodeling necessarily requires that osteoblasts reconstruct the bone that osteoclasts have resorbed. However, the cellular events connecting resorption to reconstruction have remained poorly known. The consequence is a fragmentary understanding of the remodeling cycle where only the resorption and formation steps are taken into account. New tools have recently made possible to elucidate how resorption shifts to formation, thereby allowing to comprehend the remodeling cycle as a whole. This new knowledge is reviewed herein. It shows how teams of osteoclasts and osteoblast lineage cells are progressively established and how they are subjected therein to reciprocal interactions. Contrary to the common view, osteoclasts and osteoprogenitors are intermingled on the eroded surfaces. The analysis of the resorption and cell population dynamics shows that osteoprogenitor cell expansion and resorption proceed as an integrated mechanism; that a threshold cell density of osteoprogenitors on the eroded surface is mandatory for onset of bone formation; that the cell initiating osteoprogenitor cell expansion is the osteoclast; and that the osteoclast therefore triggers putative osteoprogenitor reservoirs positioned at proximity of the eroded bone surface (bone lining cells, canopy cells, pericytes). The interplay between magnitude of resorption and rate of cell expansion governs how soon bone reconstruction is initiated and may determine uncoupling and permanent bone loss if a threshold cell density is not reached. The clinical perspectives opened by these findings are discussed.

Effect of parathyroid hormone on the structural, densitometric and failure behaviors of mouse tibia in the spatiotemporal space

Parathyroid hormone (PTH) is an anabolic bone drug approved by the US Food and Drug Administration (FDA) to treat osteoporosis. However, previous studies using cross-sectional designs have reported variable and sometimes contradictory results. The aim of the present study was to quantify the localized effect of PTH on the structural and densitometric behaviors of mouse tibia and their links with the global mechanical behavior of bone using a novel spatiotemporal image analysis approach and a finite element analysis technique. Twelve female C57BL/6J mice were divided into two groups: the control and PTH treated groups. The entire right tibiae were imaged using an in vivo micro-computed tomography (μCT) system eight consecutive times. Next, the in vivo longitudinal tibial μCT images were rigidly registered and divided into 10 compartments across the entire tibial space. The bone volume (BV), bone mineral content (BMC), bone tissue mineral density (TMD), and tibial endosteal and periosteal areas (TEA and TPA) were quantified in each compartment. Additionally, finite element models of all the tibiae were generated to analyze the failure behavior of the tibia. It was found that both the BMC and BV started to increase in the proximal tibial region, and then the increases extended to the entire tibial region after two weeks of treatment (p < 0.05). PTH intervention significantly reduced the TEA in most tibial compartments after two weeks of treatment, and the TPA increased in most tibial regions after four weeks of treatment (p < 0.05). Tibial failure loads significantly increased after three weeks of PTH treatment (p < 0.01). The present study provided the first evidence of the localized effect of PTH on bone structural and densitometric properties, as well as their links with the global mechanical behaviors of bone, which are important pieces of information for unveiling the mechanism of PTH intervention.

Molecular understanding of pharmacological treatment of osteoporosis

Osteoporosis is a serious health concern, particularly in aged societies. The burden of osteoporosis with its associated morbidity and mortality due to fracture has become a critical socioeconomic problem.

Skeletal integrity is maintained through a balance of bone resorption and bone formation. The bone turnover process, called bone remodelling.

Recently, a number of anti-osteoporosis drugs with excellent anti-osteoporosis and fracture effects have been developed. They are mainly classified into two groups according to their effects on bone remodelling: anti-resorptive agents and anabolic agents.

Cite this article: EFORT Open Rev 2019;4:158-164. DOI: 10.1302/2058-5241.4.180018

Denosumab-Associated Severe Hypocalcemia in a Patient With Chronic Kidney Disease

Denosumab is a monoclonal antibody directed against the receptor activator of nuclear factor kappa B ligand (RANKL). Denosumab has been shown to reduce the risk of skeletal-related events, including spinal cord compression, pathologic fracture and hypercalcemia of malignancy in patients with bone metastases. Hypocalcemia is a known side effect of denosumab, occurring in an estimated 8-14% of the patients. Here, we present an asymptomatic patient with stage-5 chronic kidney disease and severe hypocalcemia who had received denosumab 1 month earlier.

Evaluation of Mandibular Bone After Dental Extraction in Rats Treated With Antiresorptive Drugs

PURPOSE

Zoledronic acid (ZOL) and denosumab (Dmab) are commonly used to treat bone pathologies. Because these drugs suppress bone metabolism, this study sought to compare their effect on bone repair after tooth extraction.

MATERIALS AND METHODS

Four-week-old male Wistar rats were randomly assigned to 1 of 3 groups: ZOL 0.125 mg/kg, Dmab 0.25 mg/kg, or saline solution 10 mL/kg (control). After 1 week of treatment, the first left molar was extracted; the rats were euthanized at 28 days. The jaws were removed and photographed for macroscopic analysis of wound healing and then subjected to tomographic and histologic analyses. Immunohistochemistry was carried out against the receptor activator of nuclear factor-κB ligand (RANKL) and osteoprotegerin (OPG).

RESULTS

No difference in wound healing, presence of inflammatory infiltrate and bone sequestration, or osteocyte expression of RANKL and OPG was found among groups. Tomographic analysis showed that the ZOL group had less alveolar resorption and more complete alveolar repair compared with the other groups. There was a statistically significant difference in the OPG marker in the control (P = .008) and ZOL (P = .05) groups when comparing the extracted and non-extracted sides.

CONCLUSION

Systemic use of ZOL can improve alveolar bone healing; however, the potential risk for the development of osteonecrosis should be considered. Higher expression of OPG seems to be associated with the control of osteoclastogenesis during bone repair.

The skeletal impact of the chemotherapeutic agent etoposide

Effects of the chemotherapeutic agent etoposide on the skeleton were determined in mice. Numbers of bone marrow cells were reduced and myeloid cells were increased. Bone volume was significantly decreased with signs of inhibition of bone formation. Etoposide after pre-treatment with zoledronic acid still reduced bone but overall bone volume was higher than with etoposide alone.

INTRODUCTION

Chemotherapeutics target rapidly dividing tumor cells yet also impact hematopoietic and immune cells in an off target manner. A wide array of therapies have negative side effects on the skeleton rendering patients osteopenic and prone to fracture. This study focused on the pro-apoptotic chemotherapeutic agent etoposide and its short- and long-term treatment effects in the bone marrow and skeleton.

METHODS

Six- to 16-week-old mice were treated with etoposide (20-25 mg/kg) or vehicle control in short-term (daily for 5-9 days) or long-term (3×/week for 17 days or 6 weeks) regimens. Bone marrow cell populations and their phagocytic/efferocytic functions were analyzed by flow cytometry. Blood cell populations were assessed by CBC analysis. Bone volume and area compartments and osteoclast numbers were measured by microCT, histomorphometry, and TRAP staining. Biomarkers of bone formation (P1NP) and resorption (TRAcP5b) were assayed from serum. Gene expression in bone marrow was assessed using qPCR.

RESULTS

Flow cytometric analysis of the bone marrow revealed short-term etoposide reduced overall cell numbers and B220+ cells, with increased marrow apoptotic (AnnexinV+PI-) cells, mesenchymal stem-like cells, and CD68+, CD45+, and CD11b+ monocyte/myeloid cells (as a percent of the total marrow). After 6 weeks, the CD68+, Gr1+, CD11b+, and CD45+ cell populations were still relatively increased in etoposide-treated bone marrow. Skeletal phenotyping revealed etoposide decreased bone volume, trabecular thickness, and cortical bone volume. Gene expression in the marrow for the leptin receptor and CXCL12 were reduced with short-term etoposide, and an increased ratio of RANKL/OPG mRNA was observed. In whole bone, Runx2 and osteocalcin gene expressions were reduced, and in serum, P1NP was significantly reduced with etoposide. Treatment with the antiresorptive agent zoledronic acid prior to etoposide increased bone volume and improved the etoposide-induced decrease in skeletal parameters.

CONCLUSIONS

These data suggest that etoposide induces apoptosis in the bone marrow and significantly reduces parameters of bone formation with rapid reduction in bone volume. Pre-treatment with an antiresorptive agent results in a preservation of bone mass. Preventive approaches to preserving the skeleton should be considered in human clinical studies.

Longitudinal effects of Parathyroid Hormone treatment on morphological, densitometric and mechanical properties of mouse tibia

The use of Parathyroid Hormone (PTH) as bone anabolic is limited due to cost-benefit assessments. Preclinical studies evaluating the effects of PTH on bone have reported variable and often contradictory results. Here, we have applied a new approach using a combination of in-vivo longitudinal µCT, image processing techniques and finite element models to monitor early local changes in the whole tibia (divided in 40 compartments) and mechanical properties of female C57BL/6J mice treated with PTH 1-34, compared to controls. Compared with standard 3D bone morphometric analysis, our new approach allowed detection of much smaller and localised changes in bone mineral content (BMC) at very early time points (1 week vs 3 weeks with standard methods) and showed that changes do not occur uniformly over time and across the anatomical space. Indeed, in the PTH treated mice, significant changes in BMC were observed in the medial and posterior sectors of the proximal tibia, a week after treatment, and in the medial sector of the tibia midshaft region a week later (p < 0.05). By the third week, two thirds of the regions showed significantly higher values of BMC (p < 0.05). The effect of PTH on bone regional volume is similar to that on BMC, but there is almost no effect of PTH on bone tissue mineral density. The differences in estimated mechanical properties became significant after three weeks of treatment (p < 0.05). These results provide the first evidence of an early and localised PTH effect on murine bone, and show that our novel partitioning approach, compared to the standard evaluation protocol, allows a more precise quantification of bone changes following treatment, which would facilitate preclinical testing of novel mono- and/or combination therapies throughout the bone.

Estrogen Regulates Bone Turnover by Targeting RANKL Expression in Bone Lining Cells

Estrogen is critical for skeletal homeostasis and regulates bone remodeling, in part, by modulating the expression of receptor activator of NF-κB ligand (RANKL), an essential cytokine for bone resorption by osteoclasts. RANKL can be produced by a variety of hematopoietic (e.g. T and B-cell) and mesenchymal (osteoblast lineage, chondrocyte) cell types. The cellular mechanisms by which estrogen acts on bone are still a matter of controversy. By using murine reconstitution models that allow for selective deletion of estrogen receptor-alpha (ERα) or selective inhibition of RANKL in hematopoietic vs. mesenchymal cells, in conjunction with in situ expression profiling in bone cells, we identified bone lining cells as important gatekeepers of estrogen-controlled bone resorption. Our data indicate that the increase in bone resorption observed in states of estrogen deficiency in mice is mainly caused by lack of ERα-mediated suppression of RANKL expression in bone lining cells.

Pharmacologically Inactive Bisphosphonates as an Alternative Strategy for Targeting Osteoclasts: In Vivo Assessment of 5-Fluorodeoxyuridine-Alendronate in a Preclinical Model of Breast Cancer Bone Metastases

Bisphosphonates have effects that are antiresorptive, antitumor, and antiapoptotic to osteoblasts and osteocytes, but an effective means of eliciting these multiple activities in the treatment of bone metastases has not been identified. Antimetabolite-bisphosphonate conjugates have potential for improved performance as a class of bone-specific antineoplastic drugs. The primary objective of the study was to determine whether an antimetabolite-bisphosphonate conjugate will preserve bone formation concomitant with antiresorptive and antitumor activity. 5-FdU-ale, a highly stable conjugate between the antimetabolite 5-fluoro-2'-deoxyuridine and the bisphosphonate alendronate, was tested for its therapeutic efficacy in a mouse model of MDA-MB231 breast cancer bone metastases. In vitro testing revealed osteoclasts to be highly sensitive to 5-FdU-ale. In contrast, osteoblasts had significantly reduced sensitivity. Tumor cells were resistant in vitro but in vivo tumor burden was nevertheless significantly reduced compared with untreated mice. Sensitivity to 5-FdU-ale was not mediated through inhibition of farnesyl diphosphate synthase activity, but cell cycle arrest was observed. Although serum tartrate-resistant acid phosphatase (TRAP) levels were greatly reduced by both drugs, there was no significant decrease in the serum bone formation marker osteocalcin with 5-FdU-ale treatment. In contrast, there was more than a fivefold decrease in serum osteocalcin levels with alendronate treatment (p < 0.001). This finding is supported by time-lapse micro-computed tomography analyses, which revealed bone formation volume to be on average 1.6-fold higher with 5-FdU-ale treatment compared with alendronate (p < 0.001). We conclude that 5-FdU-ale, which is a poor prenylation inhibitor but maintains potent antiresorptive activity, does not reduce bone formation and has cytostatic antitumor efficacy. These results document that conjugation of an antimetabolite with bisphosphonates offers flexibility in creating potent bone-targeting drugs with cytostatic, bone protection properties that show limited nephrotoxicity. This unique class of drugs may offer distinct advantages in the setting of targeted adjuvant therapy and chemoprevention of bone diseases. © 2016 American Society for Bone and Mineral Research.

Parathyroid Hormone (1-34) Might Not Improve Early Bone Healing after Sinus Augmentation in Healthy Rabbits

Purpose. This study evaluated the effect of administering intermittent parathyroid hormone [PTH (1-34), henceforth PTH] on the early-stage bone healing of maxillary sinus augmentation in healthy rabbits. Materials and Methods. Bovine bone mineral was grafted on the sinuses of 20 female New Zealand white rabbits. The animals were randomly divided into two groups, PTH (n = 10) or saline (n = 10), in which either PTH or saline was injected subcutaneously 5 days a week for 2 weeks. Half of the animals in each group were killed at 2 weeks postoperatively and the other half were killed at 4 weeks postoperatively. The dosage of PTH was 10 μg/kg/day. Radiographic and histomorphometric analyses were performed. Result. The new bone area (NBA) did not differ significantly between the PTH and saline groups. The NBA in the PTH group in the total augmented area and in the demarcated window, center, and Schneiderian membrane regions increased significantly from 2 to 4 weeks. The number of osteoclasts decreased significantly from 2 to 4 weeks in both groups, with no difference between the two groups. Conclusion. Intermittent PTH might not stimulate new bone formation in healthy rabbits during the first 4 weeks of healing.

One Year of Abaloparatide, a Selective Activator of the PTH1 Receptor, Increased Bone Formation and Bone Mass in Osteopenic Ovariectomized Rats Without Increasing Bone Resorption

Abaloparatide is a novel 34-amino acid peptide selected to be a potent and selective activator of the parathyroid hormone receptor (PTH1R) signaling pathway with 41% homology to PTH(1-34) and 76% homology to PTHrP(1-34). A 12-month treatment study was conducted in osteopenic ovariectomized (OVX) rats to characterize the mechanisms by which abaloparatide increases bone mass. Sprague-Dawley (SD) rats were subjected to OVX or sham surgery at age 6 months and left untreated for 3 months to allow OVX-induced bone loss. Ten OVX rats were euthanized after this bone depletion period, and the remaining OVX rats received daily subcutaneous injections of vehicle (n = 18) or abaloparatide at 1, 5, or 25 μg/kg/d (n = 18/dose level) for 12 months. Sham controls (n = 18) received vehicle daily. Bone densitometry and biochemical markers of bone formation and resorption were assessed longitudinally, and L₃ vertebra and tibia were collected at necropsy for histomorphometry. Abaloparatide increased biochemical bone formation markers without increasing bone resorption markers or causing hypercalcemia. Abaloparatide increased histomorphometric indices of bone formation on trabecular, endocortical, and periosteal surfaces without increasing osteoclasts or eroded surfaces. Abaloparatide induced substantial increases in trabecular bone volume and density and improvements in trabecular microarchitecture. Abaloparatide stimulated periosteal expansion and endocortical bone apposition at the tibial diaphysis, leading to marked increases in cortical bone volume and density. Whole-body bone mineral density (BMD) remained stable in OVX-Vehicle controls while increasing 25% after 12 months of abaloparatide (25 μg/kg). Histomorphometry and biomarker data suggest that gains in cortical and trabecular bone mass were attributable to selective anabolic effects of abaloparatide, without evidence for stimulated bone resorption. © 2016 American Society for Bone and Mineral Research.

A Novel, Direct NO Donor Regulates Osteoblast and Osteoclast Functions and Increases Bone Mass in Ovariectomized Mice

Most US Food and Drug Administration (FDA)-approved treatments for osteoporosis target osteoclastic bone resorption. Only PTH derivatives improve bone formation, but they have drawbacks, and novel bone-anabolic agents are needed. Nitrates, which generate NO, improved BMD in estrogen-deficient rats and may improve bone formation markers and BMD in postmenopausal women. However, nitrates are limited by induction of oxidative stress and development of tolerance, and may increase cardiovascular mortality after long-term use. Here we studied nitrosyl-cobinamide (NO-Cbi), a novel, direct NO-releasing agent, in a mouse model of estrogen deficiency-induced osteoporosis. In murine primary osteoblasts, NO-Cbi increased intracellular cGMP, Wnt/β-catenin signaling, proliferation, and osteoblastic gene expression, and protected cells from apoptosis. Correspondingly, in intact and ovariectomized (OVX) female C57Bl/6 mice, NO-Cbi increased serum cGMP concentrations, bone formation, and osteoblastic gene expression, and in OVX mice, it prevented osteocyte apoptosis. NO-Cbi reduced osteoclasts in intact mice and prevented the known increase in osteoclasts in OVX mice, partially through a reduction in the RANKL/osteoprotegerin gene expression ratio, which regulates osteoclast differentiation, and partially through direct inhibition of osteoclast differentiation, observed in vitro in the presence of excess RANKL. The positive NO effects in osteoblasts were mediated by cGMP/protein kinase G (PKG), but some of the osteoclast-inhibitory effects appeared to be cGMP-independent. NO-Cbi increased trabecular bone mass in both intact and OVX mice, consistent with its in vitro effects on osteoblasts and osteoclasts. NO-Cbi is a novel direct NO-releasing agent that, in contrast to nitrates, does not generate oxygen radicals, and combines anabolic and antiresorptive effects in bone, making it an excellent candidate for treating osteoporosis. © 2016 American Society for Bone and Mineral Research.

Longitudinal imaging of the ageing mouse

Several non-invasive imaging techniques are used to investigate the effect of pathologies and treatments over time in mouse models. Each preclinical in vivo technique provides longitudinal and quantitative measurements of changes in tissues and organs, which are fundamental for the evaluation of alterations in phenotype due to pathologies, interventions and treatments. However, it is still unclear how these imaging modalities can be used to study ageing with mice models. Almost all age related pathologies in mice such as osteoporosis, arthritis, diabetes, cancer, thrombi, dementia, to name a few, can be imaged in vivo by at least one longitudinal imaging modality. These measurements are the basis for quantification of treatment effects in the development phase of a novel treatment prior to its clinical testing. Furthermore, the non-invasive nature of such investigations allows the assessment of different tissue and organ phenotypes in the same animal and over time, providing the opportunity to study the dysfunction of multiple tissues associated with the ageing process. This review paper aims to provide an overview of the applications of the most commonly used in vivo imaging modalities used in mouse studies: micro-computed-tomography, preclinical magnetic-resonance-imaging, preclinical positron-emission-tomography, preclinical single photon emission computed tomography, ultrasound, intravital microscopy, and whole body optical imaging.

Overexpression of Gα11 in Osteoblast Lineage Cells Suppresses the Osteoanabolic Response to Intermittent PTH and Exercise

Intermittent parathyroid hormone (iPTH) treatment and mechanical loading are osteoanabolic stimuli that are partially mediated through actions on G protein-coupled receptors (GPCRs). GPCR signaling can be altered by heterotrimeric G protein Gα subunits levels, which can therefore lead to altered responses to such stimuli. Previous studies have suggested that enhanced signaling through the Gαq/11 pathway inhibits the osteoanabolic actions of PTH. The influence of Gαq/11 signaling on mechanotransduction, however, has not been reported in vivo. Using transgenic mice that specifically overexpress Gα11 in osteoblast lineage cells (G11-Tg mice), we investigated the skeletal effects of elevated Gα11 levels on iPTH and mechanical loading by treadmill exercise. Both regimens increased trabecular and cortical bone in Wild-Type (WT) mice as a result of increased bone formation. In G11-Tg mice, there was no change in trabecular or cortical bone and no increase in bone formation in response to iPTH or exercise. While exercise reduced osteoclast parameters in WT mice, these changes were diminished in G11-Tg mice as expression of M-csf and Trap remained increased. Collectively, our results suggest that osteoblastic upregulation of Gα11 is inhibitory to osteoanabolic actions of both PTH and exercise, and that its suppression may be a promising target for treating bone loss.

RANKL blockade prevents and treats aggressive osteosarcomas

Osteosarcoma (OS) is the most common primary bone cancer, which occurs primarily in children and adolescents, severely affecting survivors' quality of life. Despite its chemosensitivity and treatment advances, long-term survival rates for OS patients have stagnated over the last 20 years. Thus, it is necessary to develop new molecularly targeted therapies for this metastatic bone cancer. Mutations in TP53 and RB are linked to OS predisposition and to the evolution of spontaneous OS. We established receptor activator of nuclear factor κB ligand (RANKL) as a therapeutic target for suppression and prevention of OS. Combined conditional osteoblast-specific deletions of Rb, p53, and the protein kinase A (PKA) regulatory subunit Prkar1α genes in genetically engineered mouse models (GEMMs) generate aggressive osteosarcomas, characterized by PKA, RANKL, and osteoclast hyperactivity. Whole-body Rankl deletion completely abrogates tumorigenesis. Although osteoblastic Rank deletion has little effect, osteoclastic Rank deletion delays tumorigenesis and prolongs life span. The latter is associated with inactivation of osteoclastogenesis and up-regulation of the tumor suppressor phosphatase and tensin homolog (PTEN). Further, we use these GEMMs as preclinical platforms to show that RANKL blockade with RANK-Fc arrests tumor progression and improves survival and also inhibits lung metastasis. Moreover, preemptive administration of RANK-Fc completely prevents tumorigenesis in mice highly predisposed to this aggressive cancer. Denosumab, a fully human monoclonal antibody against RANKL, is currently used to treat patients with osteoporosis or bone metastases. Our studies provide a strong rationale to consider RANKL blockade for the treatment and prevention of aggressive RANKL-overexpressing OS in humans.

Response to Therapy With Teriparatide, Denosumab, or Both in Postmenopausal Women in the DATA (Denosumab and Teriparatide Administration) Study Randomized Controlled Trial

Both antiresorptive and anabolic osteoporosis medications increase bone mineral density (BMD), but no single agent can restore normal bone strength in most osteoporotic patients. Moreover, the magnitude and consistency of the patient response to each individual agent vary depending on the anatomic site. In the DATA study, we reported that in postmenopausal osteoporotic women, 2 years of combined denosumab and teriparatide increase mean BMD at the hip and spine more than either drug alone. In the current analysis, we wished to determine if the individual rates of BMD response were also greater among women treated with both drugs. In DATA, 94 postmenopausal osteoporotic women (ages 51-91) were randomized to receive teriparatide (20 mcg subcutaneously daily), denosumab (60 mg subcutaneously every 6 mo), or both medications for 24 mo. The BMD of the total hip (TH), femoral neck (FN), and lumbar spine (LS) were assessed by dual-energy X-ray absorptiometry. The 82 subjects who completed all 2-yr treatments were analyzed. Responders were defined as experiencing BMD increases of >3%. An "excellent response" was defined as an increase of >6%. Over 24 mo, TH BMD increased by >3% in 36%, 53%, and 92% of women in the teriparatide, denosumab, and combination groups, respectively, and by >6% in 11%, 17%, and 50% in the teriparatide, denosumab, and combination groups, respectively (p < 0.01 for all comparisons vs combination). FN response rates were similar to TH. In the LS, BMD increased by >3% in 85%, 93%, and 100% of women in the teriparatide, denosumab, and combination groups, respectively (p = nonsignificant for all comparisons) and by >6% in 63%, 78%, and 100% of women in the teriparatide, denosumab, and combination groups, respectively (combination vs teriparatide, p = 0.001; combination vs denosumab, p = 0.016). In summary, more women treated with 24 mo of combined denosumab and teriparatide achieved a significant response at the TH and FN than those treated with either drug alone. All women treated with both agents together experienced an excellent response at the LS. These results support the continued investigation of combined denosumab and teriparatide therapy in postmenopausal osteoporotic women utilizing clinical endpoints such as fracture reduction.

Suppression of p38α MAPK Signaling in Osteoblast Lineage Cells Impairs Bone Anabolic Action of Parathyroid Hormone

Intermittent parathyroid hormone administration (iPTH) increases bone mass and strength by stimulating osteoblast number and activity. PTH exerts its anabolic effects through cAMP/protein kinase A (PKA) signaling pathway in mature osteoblasts and osteocytes. Here, we show that inactivation of the p38α MAPK-encoding gene with the use of an osteocalcin-cre transgene prevents iPTH bone anabolic action. Indeed, iPTH fails to increase insulin-like growth factor 1 expression, osteoblast number and activity, and bone formation in mice lacking p38α in osteoblasts and osteocytes. Moreover, iPTH-induced expression of receptor activator of NF-κB ligand (RANKL) and subsequent increased bone resorption are suppressed in those mice. Finally, we found that PTH activates p38α MAPK downstream of cAMP/PKA signaling pathway in mature osteoblasts. Our findings identify p38α MAPK as a key component of PTH signaling in osteoblast lineage cells and highlight its requirement in iPTH osteoanabolic activity. © 2015 American Society for Bone and Mineral Research.

Denosumab-associated osteonecrosis of the jaw affects osteoclast formation and differentiation: Pathological features of two cases

Medication-related osteonecrosis of the jaw (ONJ) is caused by antiresorptive (bisphosphonates and denosumab) and antiangiogenic agents, with the first report of denosumab-related ONJ emerging in 2010. To date, although certain case reports on denosumab-related ONJ have been published, those of ONJ caused by a single application of the drug are scarce. In addition, only one report described the histopathological features of this condition, although not completely; only the sequestrum resected by conservative surgery was evaluated. Although conservative treatment is recommended, the effectiveness of extensive surgery in the early stages of bisphosphonate-related ONJ has been described in recent years. Here we report the clinical and histopathological features of denosumab-related ONJ caused by single application of the drug, which was treated by extensive surgery in two patients. Histopathological analysis revealed a decreased number of osteoclasts in viable bone around the sequestrum, and these appeared morphologically immature, as indicated by the presence of very few nuclei. These findings are different from those for bisphosphonate-related ONJ and may assist in elucidating the mechanism underlying denosumab-related ONJ. Furthermore, extensive surgery may be effective for the management of this condition.

The inhibitory effects of a RANKL-binding peptide on articular and periarticular bone loss in a murine model of collagen-induced arthritis: a bone histomorphometric study

INTRODUCTION

We designed OP3-4 (YCEIEFCYLIR), a cyclic peptide, to mimic the soluble osteoprotegerin (OPG), and was proven to bind to RANKL (receptor activator of NF-κB ligand), thereby inhibiting osteoclastogenesis. We recently found that another RANKL binding peptide, W9, could accelerate bone formation by affecting RANKL signaling in osteoblasts. We herein demonstrate the effects of OP3-4 on bone formation and bone loss in a murine model of rheumatoid arthritis.

METHODS

Twenty-four seven-week-old male DBA/1J mice were used to generate a murine model of collagen-induced arthritis (CIA). Then, vehicle or OP3-4 (9 mg/kg/day or 18 mg/kg/day) was subcutaneously infused using infusion pumps for three weeks beginning seven days after the second immunization. The arthritis score was assessed, and the mice were sacrificed on day 49. Thereafter, radiographic, histological and biochemical analyses were performed.

RESULTS

The OP3-4 treatment did not significantly inhibit the CIA-induced arthritis, but limited bone loss. Micro-CT images and quantitative measurements of the bone mineral density revealed that 18 mg/kg/day OP3-4 prevented the CIA-induced bone loss at both articular and periarticular sites of tibiae. As expected, OP3-4 significantly reduced the CIA-induced serum CTX levels, a marker of bone resorption. Interestingly, the bone histomorphometric analyses using undecalcified sections showed that OP3-4 prevented the CIA-induced reduction of bone formation-related parameters at the periarticular sites.

CONCLUSION

The peptide that mimicked OPG prevented inflammatory bone loss by inhibiting bone resorption and stimulating bone formation. It could therefore be a useful template for the development of small molecule drugs for inflammatory bone loss.

μCT-based, in vivo dynamic bone histomorphometry allows 3D evaluation of the early responses of bone resorption and formation to PTH and alendronate combination therapy

Current osteoporosis treatments improve bone mass by increasing net bone formation: anti-resorptive drugs such as bisphosphonates block osteoclast activity, while anabolic agents such as parathyroid hormone (PTH) increase bone remodeling, with a greater effect on formation. Although these drugs are widely used, their role in modulating formation and resorption is not fully understood, due in part to technical limitations in the ability to longitudinally assess bone remodeling. Importantly, it is not known whether or not PTH-induced bone formation is independent of resorption, resulting in controversy over the effectiveness of combination therapies that use both PTH and an anti-resorptive. In this study, we developed a μCT-based, in vivo dynamic bone histomorphometry technique for rat tibiae, and applied this method to longitudinally track changes in bone resorption and formation as a result of treatment with alendronate (ALN), PTH, or combination therapy of both PTH and ALN (PTH+ALN). Correlations between our μCT-based measures of bone formation and measures of bone formation based on calcein-labeled histology (r=0.72-0.83) confirm the accuracy of this method. Bone remodeling parameters measured through μCT-based in vivo dynamic bone histomorphometry indicate an increased rate of bone formation in rats treated with PTH and PTH+ALN, together with a decrease in bone resorption measures in rats treated with ALN and PTH+ALN. These results were further supported by traditional histology-based measurements, suggesting that PTH was able to induce bone formation while bone resorption was suppressed.

Pharmacological inhibition of Dock5 prevents osteolysis by affecting osteoclast podosome organization while preserving bone formation

Osteoporosis is caused by excessive activity of bone-degrading osteoclasts over bone-forming osteoblast. Standard antiosteolytic treatments inhibit bone resorption by inducing osteoclast loss, with the adverse effect of hindering also bone formation. Formation of the osteoclast sealing zone requires Dock5, a guanine nucleotide exchange factor for the small GTPase Rac, and C21, a chemical inhibitor of Dock5, decreases bone resorption by cultured osteoclasts. Here we show that C21 directly inhibits the exchange activity of Dock5 and disrupts osteoclast podosome organization. Remarkably, C21 administration protects mice against bone degradation in models recapitulating major osteolytic diseases: menopause, rheumatoid arthritis and bone metastasis. Furthermore, C21 administration does not affect bone formation and is not toxic. Our results validate the pharmacological inhibition of Dock5 as a novel therapeutic route for fighting osteolytic diseases while preserving bone formation.

Osteoporosis therapy: a novel insight from natural homeostatic system in the skeleton

The skeleton normally responds to mechanical environment to maintain the resulting elastic deformation (strain) of bone, while increased bone strength by an osteoporosis drug results in decreased bone strain. Thus, it can be hypothesized that the effect of osteoporosis therapy is limited by natural homeostatic system in the skeleton. This logic is consistent with the fact that there exists a powerful effect that returns bone mass to its pre-treatment level after the withdrawal of treatment with osteoporosis agents. The present hypothesis provides a new significant insight into the mechanisms by which osteoporosis drugs improve bone fragility. Here we briefly discuss the effects of teriparatide, romosozumab, and odanacatib on bones in animals and humans.

Intermittent PTH administration and mechanical loading are anabolic for periprosthetic cancellous bone

The purpose of this study was to determine the individual and combined effects on periprosthetic cancellous bone of intermittent parathyroid hormone administration (iPTH) and mechanical loading at the cellular, molecular, and tissue levels. Porous titanium implants were inserted bilaterally on the cancellous bone of adult rabbits beneath a loading device attached to the distal lateral femur. The left femur received a sham loading device. The right femur was loaded daily, and half of the rabbits received daily PTH. Periprosthetic bone was evaluated up to 28 days for gene expression, histology, and µCT analysis. Loading and iPTH increased bone mass by a combination of two mechanisms: (1) Altering cell populations in a pro-osteoblastic/anti-adipocytic direction, and (2) controlling bone turnover by modulating the RANKL-OPG ratio. At the tissue level, BV/TV increased with both loading (+53%, p < 0.05) and iPTH (+54%, p < 0.05). Combined treatment showed only small additional effects at the cellular and molecular levels that corresponded to a small additive effect on bone volume (+13% compared to iPTH alone, p > 0.05). This study suggests that iPTH and loading are potential therapies for enhancing periprosthetic bone formation. The elucidation of the cellular and molecular response may help further enhance the combined therapy and related targeted treatment strategies.

Individual and combining effects of anti-RANKL monoclonal antibody and teriparatide in ovariectomized mice

We examined the individual and combined effects of teriparatide and anti-RANKL (receptor activator of nuclear factor κB ligand) monoclonal antibody in ovariectomized mice. Three-month-old female C57BL/6 mice were ovariectomized (OVX) or sham operated. Four weeks after OVX, they were assigned to 3 different groups to receive anti-RANKL monoclonal antibody (Ab) alone (5 mg/kg single injection at 4 weeks after OVX, Ab group), teriparatide alone (80 μg/kg daily injection for 4 weeks from 4 weeks after OVX, PTH group), or mAb plus teriparatide (Ab + PTH group). Mice were sacrificed 8 weeks after OVX. Bone mineral density (BMD) was measured at the femur and lumbar spine. Hind limbs were subjected to histological and histomorphometric analysis. Serum osteocalcin and CTX-I levels were measured to investigate the bone turnover. Compared with Ab group, Ab + PTH group showed a significant increase in BMD at distal femur and femoral shaft. Cortical bone volume was significantly increased in PTH and Ab + PTH groups compared with Ab group. Bone turnover in Ab + PTH group was suppressed to the same degree as in Ab group. The number of TRAP-positive multinucleated cells was markedly reduced in Ab and Ab + PTH groups. These results suggest that combined treatment of teriparatide with anti-RANKL antibody has additive effects on BMD in OVX mice compared with individual treatment.

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The effects of anti-RANKL antibody and teriparitide in ovariectomized mice was studied.

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Anti-RANKL antibody and teriparatide increased bone mass at the distinct regions.

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Combination of anti-RANKL antibody and teriparatide had additive effects.

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Teriparitide, but not anti-RANKL antibody, increased periosteal bone formation.

Enhanced individual trabecular repair and its mechanical implications in parathyroid hormone and alendronate treated rat tibial bone

Combined parathyroid hormone (PTH) and bisphosphonate (alendronate-ALN) therapy has recently been shown to increase bone volume fraction and plate-like trabecular structure beyond either monotherapy. To identify the mechanism through which plate-like structure was enhanced, we used in vivo microcomputed tomography (μCT) of the proximal tibia metaphysis and individual trabecular dynamics (ITD) analysis to quantify connectivity repair (incidences of rod connection and plate perforation filling) and deterioration (incidences of rod disconnection and plate perforation). Three-month-old female, intact rats were scanned before and after a 12 day treatment period of vehicle (Veh, n = 5), ALN (n = 6), PTH (n = 6), and combined (PTH+ALN, n = 6) therapy. Additionally, we used computational simulation and finite element (FE) analysis to delineate the contributions of connectivity repair or trabecular thickening to trabecular bone stiffness. Our results showed that the combined therapy group had greater connectivity repair (5.8 ± 0.5% connected rods and 2.0 ± 0.3% filled plates) beyond that of the Veh group, resulting in the greatest net gain in connectivity. For all treatment groups, increases in bone volume due to thickening (5-31%) were far greater than those due to connectivity repair (2-3%). Newly formed bone contributing only to trabecular thickening caused a 10%, 41%, and 69% increase in stiffness in the ALN, PTH, and PTH+ALN groups, respectively. Moreover, newly formed bone that led to connectivity repair resulted in an additional improvement in stiffness, with the highest in PTH+ALN (by an additional 12%), which was significantly greater than either PTH (5.6%) or ALN (4.5%). An efficiency ratio was calculated as the mean percent increase in stiffness divided by mean percent increase in BV for either thickening or connectivity repair in each treatment. For all treatments, the efficiency ratio of connectivity repair (ALN: 2.9; PTH: 3.4; PTH+ALN: 4.4) was higher than that due to thickening (ALN: 2.0; PTH: 1.7; PTH+ALN: 2.2), suggesting connectivity repair required less new bone formation to induce larger gains in stiffness. We conclude that through rod connection and plate perforation filling PTH+ALN combination therapy improved bone stiffness in a more efficient and effective manner than either monotherapy.

Novel techniques in the development of osteoporosis drug therapy: the osteoclast ruffled-border vacuolar H(+)-ATPase as an emerging target

INTRODUCTION

Bone loss occurs in many diseases, including osteoporosis, rheumatoid arthritis and periodontal disease. For osteoporosis alone, it is estimated that 75 million people are afflicted worldwide, with high risks of fractures and increased morbidity and mortality. The demand for treatment consumes an ever-increasing share of healthcare resources. Successive generations of antiresorptive bisphosphonate drugs have reduced side effects, minimized frequency of dosing, and increased efficacy in halting osteoporotic bone loss, but their shortcomings have remained significant to the extent that a monoclonal antibody antiresorptive has recently taken a significant market share. Yet this latter, paradigm-shifting approach has its own drawbacks.

AREAS COVERED

This review summarizes recent literature on bone-remodeling cell and molecular biology and the background for existing approaches and emerging therapeutics and targets for treating osteoporosis. The authors discuss vacuolar H(+)-ATPase (V-ATPase) molecular biology and the recent advances in targeting the osteoclast ruffled-border V-ATPase (ORV) for the development of novel antiresorptive drugs. They also cover examples from the V-ATPase-targeted drug discovery literature, including conventional molecular biology methods, in silico drug discovery, and gene therapy in more detail as proofs of concept.

EXPERT OPINION

Existing therapeutic options for osteoporosis have limitations and inherent drawbacks. Thus, the search for novel approaches to osteoporosis drug discovery remains relevant. Targeting the ORV may be one of the more selective means of regulating bone resorption. Furthermore, this approach may be effective without removing active osteoclasts from the finely balanced osteoclast-osteoblast coupling required for normal bone remodeling.

Osteocyte control of bone remodeling: is sclerostin a key molecular coordinator of the balanced bone resorption-formation cycles?

Osteocytes, entrapped within a newly mineralized bone matrix, possess a unique cellular identity due to a specialized morphology and a molecular signature. These features endow them to serve as a bone response mechanism for mechanical stress in their microenvironment. Sclerostin, a primarily osteocyte product, is widely considered as a mechanotranduction key molecule whose expression is suppressed by mechanical loading, or it is induced by unloading. This review presents a model suggesting that sclerostin is major mediator for integrating mechanical, local, and hormonal signals, sensed by the osteocytes, in controlling the remodeling apparatus. This central role is achieved through interplay between two opposing mechanisms: (1) unloading-induced high sclerostin levels, which antagonize Wnt-canonical-β-catenin signaling in osteocytes and osteoblasts, permitting simultaneously Wnt-noncanonical and/or other pathways in osteocytes and osteoclasts, directed at bone resorption; (2) mechanical loading results in low sclerostin levels, activation of Wnt-canonical signaling, and bone formation. Therefore, adaptive bone remodeling occurring at a distinct bone compartment is orchestrated by altered sclerostin levels, which regulate the expression of the other osteocyte-specific proteins, such as RANKL, OPG, and proteins encoded by "mineralization-related genes" (DMP1, PHEX, and probably FGF23). For example, under specific terms, sclerostin regulates differential RANKL and OPG production, and creates a dynamic RANKL/OPG ratio, leading either to bone formation or resorption. It also controls the expression of PHEX, DMP1, and most likely FGF23, leading to either bone matrix mineralization or its inhibition. Such opposing up- or down-regulation of remodeling phases allows osteocytes to function as an "external unit", ensuring transition from bone resorption to bone formation.Mini Abstract: The osteocyte network plays a central role in directing bone response either to mechanical loading, or to unloading, leading correspondingly to bone formation or resorption. This review shows a key role of the osteocyte-produced sclerostin as a major mediator of the molecular mechanisms involved in the process of adaptive bone remodeling.

Future directions for new medical entities in osteoporosis

Odanacatib, a selective cathepsin K inhibitor, decreases bone resorption, whereas osteoclast number increases and bone formation is maintained, perhaps even increased on some cortical surfaces. In a phase 2 clinical trial, post-menopausal women receiving odanacatib presented a sustained reduction of bone resorption markers, whereas procollagen type 1 N-terminal propeptide returned to normal. In turn areal bone mineral density increased continuously at both spine and hip for up to 5 years. Blosozumab and romosozumab are sclerostin neutralizing antibodies that exert potent anabolic effects on both trabecular and cortical compartments. A phase 2 clinical trial has reported areal bone mineral density gains at spine and hip that were greater with romosozumab compared with placebo, but also with teriparatide. It also showed that antagonizing sclerostin results in a transient stimulation of bone formation but progressive inhibition of bone resorption. Other new medical entities that are promising for the treatment of osteoporosis include abaloparatide, a parathyroid hormone-related analogue with improved bone formation-resorption ratio.

Soluble guanylate cyclase as a novel treatment target for osteoporosis

Osteoporosis is a major health problem leading to fractures that cause substantial morbidity and mortality. Current osteoporosis therapies have significant drawbacks, creating a need for novel bone-anabolic agents. We previously showed that the nitric oxide/cyclic GMP (cGMP)/protein kinase G pathway mediates some of the anabolic effects of estrogens and mechanical stimulation in osteoblasts and osteocytes, leading us to hypothesize that cGMP-elevating agents may have bone-protective effects. We tested cinaciguat, a prototype of a novel class of soluble guanylate cyclase activators, in a mouse model of estrogen deficiency-induced osteoporosis. Compared with sham-operated mice, ovariectomized mice had lower serum cGMP concentrations, which were largely restored to normal by treatment with cinaciguat or low-dose 17β-estradiol. Microcomputed tomography of tibiae showed that cinaciguat significantly improved trabecular bone microarchitecture in ovariectomized animals, with effect sizes similar to those obtained with estrogen replacement therapy. Cinaciguat reversed ovariectomy-induced osteocyte apoptosis as efficiently as estradiol and enhanced bone formation parameters in vivo, consistent with in vitro effects on osteoblast proliferation, differentiation, and survival. Compared with 17β-estradiol, which completely reversed the ovariectomy-induced increase in osteoclast number, cinaciguat had little effect on osteoclasts. Direct guanylate cyclase stimulators have been extremely well tolerated in clinical trials of cardiovascular diseases, and our findings provide proof-of-concept for this new class of drugs as a novel, anabolic treatment strategy for postmenopausal osteoporosis, confirming an important role of nitric oxide/cGMP/protein kinase G signaling in bone.

Osteoclasts: more than 'bone eaters'

As the only cells definitively shown to degrade bone, osteoclasts are key mediators of skeletal diseases including osteoporosis. Bone-forming osteoblasts, and hematopoietic and immune system cells, each influence osteoclast formation and function, but the reciprocal impact of osteoclasts on these cells is less well appreciated. We highlight here the functions that osteoclasts perform beyond bone resorption. First, we consider how osteoclast signals may contribute to bone formation by osteoblasts and to the pathology of bone lesions such as fibrous dysplasia and giant cell tumors. Second, we review the interaction of osteoclasts with the hematopoietic system, including the stem cell niche and adaptive immune cells. Connections between osteoclasts and other cells in the bone microenvironment are discussed within a clinically relevant framework.

A closer look at the immediate trabecula response to combined parathyroid hormone and alendronate treatment

Daily injections of parathyroid hormone (PTH) are the only FDA-approved anabolic treatment for osteoporosis; however PTH is only clinically approved for treatment periods of up to 24months. To enhance its anabolic effect, combining PTH with anti-resorptive therapy was proposed and expected to maximize the effectiveness of PTH. The current study aimed to elucidate structural mechanisms through which combination therapy can further improve bone strength over a limited treatment window of 12days, to more closely examine the early phase of the anabolic window. We examined 30 female rats treated with either vehicle (Veh), alendronate (ALN), PTH, or both PTH and ALN (PTH+ALN). Standard and individual trabecula segmentation (ITS)-based microstructural analyses were performed using in vivo micro-computed tomography. We found an increase in BV/TV in all treatments with the highest in the PTH+ALN group. Tb.Th* increased in both PTH and PTH+ALN groups well beyond that of the Veh or ALN group. SMI decreased in all treatments with PTH+ALN having the greatest tendency toward plate-like structures. ITS confirmed the trend toward more plate-like structures with increased plate Tb.N* and increased plate-to-rod ratio that was most pronounced in the PTH+ALN group. Using image-based finite element analysis, we demonstrated that stiffness increased in all treatment groups, again with the largest increase in the PTH+ALN group, indicating the resulting structural implications of increased plate-like structure. Static and dynamic bone histomorphometry and a serum resorption marker confirmed that PTH+ALN significantly increased bone formation activities and suppressed bone resorption activities. Overall the results indicate that PTH+ALN treatment has an additive effect due to a preferential increase in plate-like structures.

Implications of osteoblast-osteoclast interactions in the management of osteoporosis by antiresorptive agents denosumab and odanacatib

Antiresorptive agents, used in the treatment of osteoporosis, inhibit either osteoclast formation or function. However, with these approaches, osteoblast activity is also reduced because of the loss of osteoclast-derived coupling factors that serve to stimulate bone formation. This review discusses how osteoclast inhibition influences osteoblast function, comparing the actions of an inhibitor of osteoclast formation [anti-RANKL/Denosumab (DMAB)] with that of a specific inhibitor of osteoclastic cathepsin K activity [Odanacatib (ODN)]. Denosumab rapidly and profoundly, but reversibly, reduces bone formation. In contrast, preclinical studies and clinical trials of ODN showed that bone formation at some skeletal sites was preserved although resorption was reduced. This preservation of bone formation appears to be due to effects of coupling factors, secreted by osteoclasts and released from demineralized bone matrix. This indicates that bone resorptive activities of osteoclasts are separable from their coupling activities.

Three-dimensional image registration improves the long-term precision of in vivo micro-computed tomographic measurements in anabolic and catabolic mouse models

Micro-computed tomography (micro-CT) is a widely used technique to track bone structural and mineral changes in small animals in vivo. Precise definition of volumes of interest (VOIs) in follow-up scans is required to accurately quantify these changes. To improve precision, VOIs can be transferred from baseline images onto follow-ups using image registration. We studied the performance of a registration procedure applied to in vivo data sets of anabolic and osteoporotic bone changes in mice. Micro-CT image data from two separate CD1 mouse data sets were studied. The first included a group treated with parathyroid hormone (PTH) and control and the second, an ovariectomy (OVX) group and control. Micro-CT was performed once per week for 4 weeks at the proximal tibia starting at treatment onset (PTH data set) or after surgery (OVX data set). A series consisting entirely of user-defined VOIs and a registered series where VOIs defined at baseline were transferred to follow-ups were created. Standard bone structural and mineral measurements were calculated. Image registration resulted in a 13-56 % reduction in precision error. Significant effects of registration to detect PTH-induced changes in BV/TV and trabecular BMD were observed. When changes were very pronounced or small, the qualitative improvement observed for the registered data set did not reach statistical significance. This study documents an increase in long-term precision of micro-CT measurements with image registration. Sensitivity to detect changes was improved but not uniform for all parameters. Future study of this technique on images with a smaller voxel size (<19 μm) may capture the effect in greater detail, in particular for trabecular thickness, where changes may be too small to be observed with the voxel size used here. Our results document the value of registration and indicate that the magnitude of improvement depends on the model and treatment chosen.

Bone biology and anabolic therapies for bone: current status and future prospects

Bone is continuously remodelled at many sites asynchronously throughout the skeleton, with bone formation and resorption balanced at these sites to retain bone structure. Negative balance resulting in bone loss and osteoporosis, with consequent fractures, has mainly been prevented or treated by anti-resorptive drugs that inhibit osteoclast formation and/or activity, with new prospects now of anabolic treatments that restore bone that has been lost. The anabolic effectiveness of parathyroid hormone has been established, and an exciting new prospect is presented of neutralising antibody against the osteocyte protein, sclerostin. The cellular actions of these two anabolic treatments differ, and the mechanisms will need to be kept in mind in devising their best use. On present evidence it seems likely that treatment with either of these anabolic agents will need to be followed by anti-resorptive treatment in order to maintain bone that has been restored. No matter how effective anabolic therapies for the skeleton become, it seems highly likely that there will be a continuing need for safe, effective anti-resorptive drugs.

An experimental therapy to improve skeletal growth and prevent bone loss in a mouse model overexpressing IL-6

Premature osteoporosis and stunted growth are common complications of childhood chronic inflammatory disease. Presently, no treatment regimens are available for these defects in juvenile diseases. We identified the sequential Fc-OPG/hPTH treatment as an experimental therapy that improves the skeletal growth and prevents the bone loss in a mouse model overexpressing IL-6.

INTRODUCTION

Premature osteoporosis and stunted growth are common complications of childhood chronic inflammatory diseases and have a significant impact on patients' quality of life. Presently, no treatment regimens are available for these defects in juvenile diseases. To test a new therapeutic approach, we used growing mice overexpressing the pro-inflammatory cytokine IL-6 (TG), which show a generalized bone loss and stunted growth.

METHODS

Since TG mice present increased bone resorption and impaired bone formation, we tested a combined therapy with the antiresorptive modified osteoprotegerin, Fc-OPG, and the anabolic PTH. We injected TG mice with Fc-OPG once at the 4th day of life and with hPTH(1-34) everyday from the 16th to the 30th day of age.

RESULTS

A complete prevention of growth and bone defects was observed in treated mice due to normalization of osteoclast and osteoblast parameters. Re-establishment of normal bone turnover was confirmed by RT-PCR analysis and by in vitro experiments that revealed the full rescue of osteoclast and osteoblast functions. The phenotypic recovery of TG mice was due to the sequential treatment, because TG mice treated with Fc-OPG or hPTH alone showed an increase of body weight, tibia length, and bone volume to intermediate levels between those observed in vehicle-treated WT and TG mice.

CONCLUSIONS

Our results identified the sequential Fc-OPG/hPTH treatment as an experimental therapy that improves the skeletal growth and prevents the bone loss in IL-6 overexpressing mice, thus providing the proof of principle for a therapeutic approach to correct these defects in juvenile inflammatory diseases.

Mechanisms of osteoclast-dependent bone formation

Should we believe that osteoclasts are only involved in bone resorption? What about their contribution to bone formation? In this article I will review evidence that bone formation can be regulated by osteoclasts. Why is this? Likely because in the physiologic condition of bone remodeling, bone resorption and formation are balanced, and there is no better way to control this equilibrium than through a concerted action between the two cell types. Although the influence of osteoblasts on osteoclastic bone resorption is well documented and consolidated over time, what osteoclasts do to regulate osteoblast activity is still matter of intense investigation. The original hypothesis that all is in the osteoblast-seeking factors stored in the bone matrix, released and activated during bone resorption, is now being challenged by several studies, suggesting that osteoclasts are also capable of producing 'clastokines' that regulate osteoblast performance. Indeed, several of them have been demonstrated to orchestrate osteoclast-osteoblast activities. However, we are probably still at the dawn of a new era, and future work will tell us whether any of these clastokines can be exploited to stimulate bone formation and rebalance bone remodeling in skeletal diseases.

Release of bone markers in immediately loaded and nonloaded dental implants: a randomized clinical trial

The aim of this study was to compare the release of bone markers during osseointegration of immediately loaded and nonloaded implants. Forty patients who were indicated for rehabilitation with dental implants randomly received either implant and prosthesis placement within 72 hours (group IM) or implant insertion and no prosthesis placement (group NL). Peri-implant crevicular fluid was collected immediately after implant insertion and 7, 15, 30, 60, 90, and 120 days after surgery and levels of osteoprotegerin, transforming growth factors, osteocalcin, osteopontin, and parathyroid hormone were evaluated using Luminex assay. Bleeding index and peri-implantar sulcus depth were also evaluated. The data were compared using statistical tests (α = 5%). No statistical difference was found regarding demographic and clinical parameters (p > .05). Transforming growth factors, osteoprotegerin, osteopontin, and parathyroid hormone presented an earlier release peak in group IM than in NL group (p < .05). Osteocalcin achieved higher levels in group IM versus group NL between 7 and 30 days of evaluation (p < .05). It may be concluded that earlier loading positively modulates bone mediators release around immediately loaded implants when compared with nonloaded dental implants.