The anabolic action of intermittent PTH in combination with cathepsin K inhibitor or alendronate differs depending on the remodeling status in bone in ovariectomized mice

Preclinical Rodent Models for Human Bone Disease, Including a Focus on Cortical Bone

Preclinical models (typically ovariectomized rats and genetically altered mice) have underpinned much of what we know about skeletal biology. They have been pivotal for developing therapies for osteoporosis and monogenic skeletal conditions, including osteogenesis imperfecta, achondroplasia, hypophosphatasia, and craniodysplasias. Further therapeutic advances, particularly to improve cortical strength, require improved understanding and more rigorous use and reporting. We describe here how trabecular and cortical bone structure develop, are maintained, and degenerate with aging in mice, rats, and humans, and how cortical bone structure is changed in some preclinical models of endocrine conditions (eg, postmenopausal osteoporosis, chronic kidney disease, hyperparathyroidism, diabetes). We provide examples of preclinical models used to identify and test current therapies for osteoporosis, and discuss common concerns raised when comparing rodent preclinical models to the human skeleton. We focus especially on cortical bone, because it differs between small and larger mammals in its organizational structure. We discuss mechanisms common to mouse and human controlling cortical bone strength and structure, including recent examples revealing genetic contributors to cortical porosity and osteocyte network configurations during growth, maturity, and aging. We conclude with guidelines for clear reporting on mouse models with a goal for better consistency in the use and interpretation of these models.

Mechanical loading and parathyroid hormone effects and synergism in bone vary by site and modeling/remodeling regime

Intermittent injections of parathyroid hormone (PTH) and mechanical loading are both known to effect a net increase in bone mass. Fundamentally, bone metabolism can be divided into modeling (uncoupled formation or resorption) and remodeling (subsequent formation biologically coupled to resorption in space and time). Methods to delineate the bone response between these regimes are scant but have garnered recent attention and acceptance, and will be critical tools to properly assess short- and long-term efficacy of osteoporosis treatments. To this end, we employ a time-lapse micro-computed tomography strategy to quantify and localize modeling and remodeling volumes over 4 weeks of concurrent PTH treatment and mechanical loading. Modeled and remodeled volumes are probed for differences with respect to treatment, loading, and interactions thereof in trabecular and cortical bone compartments, which were further separated by plate/rod microarchitecture and periosteal/endosteal surfaces, respectively. Loading effects are further considered independently with regard to localized strain environments. Our findings indicate that in trabecular bone, PTH and loading stimulate anabolic modeling additively, and remodeling synergistically. PTH tends to lead to bone accumulation indiscriminate of trabecular microarchitecture, whereas loading tends to more strongly affect plates than rods. The cortical surfaces responded uniquely to PTH and loading, with synergistic effects on the periosteal surface for anabolic modeling, and on the endosteal surface for catabolic modeling. The increase in catabolic modeling due to loading, which is enhanced by PTH, is concentrated to areas of the endosteal surface under low strain and to our knowledge has not previously been reported. Taken together, the effects of PTH, loading, and their interactions, are shown to be dependent on the specific bone compartment and metabolic regime; this may explain some discrepancies in previously-reported findings.

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.

Disruption of the aldehyde dehydrogenase 2 gene increases the bone anabolic response to intermittent PTH treatment in an ovariectomized mouse model

Aldehyde dehydrogenase 2 (ALDH2) is the enzyme that oxidizes the acetaldehyde produced by alcohol metabolism. This variant not only affects the response to alcohol but is also associated with several diseases, such as esophageal cancer, myocardial infarction, and particularly osteoporosis. In our previous study, we reported that compared to wild-type (WT) mice, Aldh2 knockout (KO) mice naturally have a strong bone formation ability, and high expression of parathyroid hormone receptor (PTHR1) in osteocytes. The effect of the Aldh2 gene on bone metabolism in response to intermittent PTH treatment is unknown. The purpose of this study was to clarify the effect of the Aldh2 gene on the bone anabolic response to intermittent PTH treatment in ovariectomized mice. Female KO and WT mice were ovariectomized at 8 weeks of age. At 14 weeks of age, the KO and WT mice were divided into vehicle-treated (Veh) and PTH-treated (PTH) groups (i.e., the WT-Veh, WT-PTH, KO-Veh and KO-PTH groups). PTH (1-34) and vehicle were subcutaneously administered to each group at a dose of 40 μg/kg body weight (BW) five times per week for 4 weeks. Micro-CT showed that the bone volume (BV), trabecular number (Tb.N), connectivity density (Conn.D), and cortical thickness (Ct.Th) values in the KO-PTH mice were significantly higher than those in the KO-Veh mice. Histomorphometric analysis showed that the BV, Tb.N, and mineral apposition rate (MAR) values in the KO-PTH group were significantly higher than those in the KO-Veh group. The mRNA expression level of PTHR1 in the KO-PTH group was significantly increased and that of p21 in the KO-PTH group was significantly decreased compared with the levels in the KO-Veh group. The expression of PTHR in osteocytes from the KO-PTH group was also significantly increased compared with that in osteocytes from the KO-Veh group. Furthermore, cell cultures revealed that the ALP⁺CFU-f/total CFU-f percentage was significantly higher in the KO-PTH group than in the KO-Veh group. We concluded that in ovariectomized Aldh2 KO mice, the bone anabolic response to intermittent PTH treatment was significantly enhanced compared to that in WT mice, which may be mediated by the high expression level of PTHR1.

Tart Cherry Prevents Bone Loss through Inhibition of RANKL in TNF-Overexpressing Mice

Current drugs for the treatment of rheumatoid arthritis-associated bone loss come with concerns about their continued use. Thus, it is necessary to identify natural products with similar effects, but with fewer or no side effects. We determined whether tart cherry (TC) could be used as a supplement to prevent inflammation-mediated bone loss in tumor necrosis factor (TNF)-overexpressing transgenic (TG) mice. TG mice were assigned to a 0%, 5%, or 10% TC diet, with a group receiving infliximab as a positive control. Age-matched wild-type (WT) littermates fed a 0% TC diet were used as a normal control. Mice were monitored by measurement of body weight. Bone health was evaluated via serum biomarkers, microcomputed tomography (µCT), molecular assessments, and mechanical testing. TC prevented TNF-mediated weight loss, while it did not suppress elevated levels of interleukin (IL)-1β and IL-6. TC also protected bone structure from inflammation-induced bone loss with a reduced ratio of receptor activator of nuclear factor kappa-B ligand (RANKL)/osteoprotegerin (OPG) to a degree comparable to infliximab. Furthermore, unlike with infliximab, TC exhibited a moderate improvement in TNF-mediated decline in bone stiffness. Thus, TC could be used as a prophylactic regimen against future fragility fractures in the context of highly chronic inflammation.

A novel role for cathepsin K in periosteal osteoclast precursors during fracture repair

Osteoporosis management is currently centered around bisphosphonates, which inhibit osteoclast (OC) bone resorption but do not affect bone formation. This reduces fracture risk, but fails to restore healthy bone remodeling. Studies in animal models showed that cathepsin K (CatK) inhibition by genetic deletion or chemical inhibitors maintained bone formation while abrogating resorption during bone remodeling and stimulated periosteal bone modeling. Recently, periosteal mononuclear tartrate-resistant acid phosphatase-positive (TRAP⁺ ) osteoclast precursors (OCPs) were shown to augment angiogenesis-coupled osteogenesis. CatK gene deletion increased osteoblast differentiation via enhanced OCP and OC secretion of platelet-derived growth factor (PDGF)-BB and sphingosine 1 phosphate. The effects of periosteum-derived OCPs on bone remodeling are unknown, particularly with regard to fracture repair. We hypothesized that periosteal OCPs derived from CatK-null (Ctsk^-/- ) mice may enhance periosteal bone formation during fracture repair. We found fewer periosteal OCPs in Ctsk^-/- mice under homeostatic conditions; however, after fracture, this population increased in number relative to that seen in wild-type (WT) mice. Enhanced TRAP staining and greater expression of PDGF-BB were observed in fractured Ctsk^-/- femurs relative to WT femurs. This early pattern of augmented PDGF-BB expression in Ctsk^-/- mice may contribute to improved fracture healing by enhancing callus mineralization in Ctsk^-/- mice.

Alendronate does not prevent long bone fragility in an inactive rat model

The lack of estrogen and inactivity are both important in the pathogenesis of osteoporosis in elderly women, and there have been no appropriate rodent studies to examine the effects of common bisphosphonates on these two components separately. We compared the efficacy of alendronate (ALN) on the long bones of aged female rats, which were sedentary, estrogen deficient, or both. The rats were either forced to remain in a sitting position or allowed to walk in standard cages with or without ALN administration. The 8-week experimental period began 5 weeks after ovariectomy or sham surgery. Parameters of the hindlimb bones were determined by a three-point bending test, peripheral quantitative computed tomography, microfocus computed tomography, confocal laser Raman microspectroscopy, and dynamic histomorphometry. Regardless of ovariectomy, ALN was ineffective against the deterioration of breaking stress caused by sitting even though the trabecular bone mineral density was significantly higher in the sitting-ALN groups. Toughness was significantly deficient in the ovariectomy sitting-ALN group. This was in agreement with the bone geometry with a greater marrow space. Sitting also increased the mineral-to-matrix ratio and the carbonate-to-phosphate ratio, both indicative of aged bone. A greater loss of proteinaceous amide intensity compared with mineral intensity resulted in an increased mineral-to-matrix ratio in the presence of ALN. Sitting resulted in deficits in the quality and the geometry of cortical bone, resulting in fragility. The use of bisphosphonates, such as ALN, may provide a therapy best suited for osteoporotic individuals whose daily activity is not limited.

Pulsed low-dose RANKL as a potential therapeutic for postmenopausal osteoporosis

A number of studies in model animal systems and in the clinic have established that RANKL promotes bone resorption. Paradoxically, we found that pulsing ovariectomized mice with low-dose RANKL suppressed bone resorption, decreased the levels of proinflammatory effector T cells and led to increased bone mass. This effect of RANKL is mediated through the induction of FoxP3⁺CD25⁺ regulatory CD8⁺ T cells (Tc_REG) by osteoclasts. Here, we show that pulses of low-dose RANKL are needed to induce Tc_REG, as continuous infusion of identical doses of RANKL by pump did not induce Tc_REG. We also show that low-dose RANKL can induce Tc_REG at 2, 3, 6, and 10 weeks after ovariectomy. Our results show that low-dose RANKL treatment in ovariectomized mice is optimal at once-per-month doses to maintain the bone mass. Finally, we found that treatment of ovariectomized mice with the Cathepsin K inhibitor odanacatib also blocked Tc_REG induction by low-dose RANKL. We interpret this result to indicate that antigens presented to CD8⁺ T cells by osteoclasts are derived from the bone protein matrix because Cathepsin K degrades collagen in the bone. Taken together, our studies provide a basis for using low-dose RANKL as a potential therapeutic for postmenopausal osteoporosis.

Effect of Concurrent Use of Whole-Body Vibration and Parathyroid Hormone on Bone Structure and Material Properties of Ovariectomized Mice

This study was designed to determine the effectiveness of whole-body vibration (WBV) and intermittent parathyroid hormone (iPTH) in combination against estrogen deficiency-induced osteoporosis. Female C57BL/6J mice were bilaterally ovariectomized (OVX, n = 40) or sham-operated (sham-OVX, n = 8) at 9 weeks of age. Two weeks later, the OVX mice were randomly divided into four groups (n = 10 each): the control group (c-OVX) and groups treated with iPTH (p-OVX), WBV (w-OVX) and both (pw-OVX). The p-OVX and pw-OVX groups were given human PTH (1-34) at a dose of 30 µg/kg/day. The w-OVX and pw-OVX groups were exposed to WBV at an acceleration of 0.3 g and 45 Hz for 20 min/day. All mice were euthanized after the 18-day treatment, and the left tibiae were harvested. The proximal metaphyseal region was µCT-scanned, and its cortical bone cross-section was analyzed by Fourier transform infrared microspectroscopy and nanoindentation testing. A single application of iPTH or WBV to OVX mice had no effect on bone structure or material properties of cortical bone, which were compromised in comparison to those in sham-OVX mice. The combination of iPTH and WBV improved trabecular bone volume, thickness, and connectivity in OVX mice. Although the combined treatment failed to improve cortical bone structure, its mineral maturity and hardness were restored to the levels observed in sham-OVX mice. There was no evidence of interaction between the two treatments, and the combined effects seemed to be additive. These results suggest combining WBV with iPTH has great potential for treating postmenopausal osteoporosis.

Combination therapy with ONO-KK1-300-01, a cathepsin K inhibitor, and parathyroid hormone results in additive beneficial effect on bone mineral density in ovariectomized rats

This study examined the effects of a novel cathepsin K inhibitor, ONO-KK1-300-01 (KK1-300), used concurrently with parathyroid hormone (PTH) in ovariectomized (OVX) rats. KK1-300 (3 mg/kg, twice daily), alendronate (1 mg/kg, once daily) or vehicle were orally administered to OVX rats for 56 days, starting the day after ovariectomy, followed by combination treatment with or without PTH (3 μg/kg, subcutaneously three times a week) for another 28 days. OVX control animals exhibited a significant increase in both bone resorption (urinary deoxypyridinoline; DPD) and formation markers (serum osteocalcin) as well as microstructural changes associated with decreased bone mineral density (BMD). Combination treatment with KK1-300 and PTH significantly decreased urinary DPD and increased serum osteocalcin, indicating a sustained beneficial effect compared to the effect of each mono-therapy. On the other hand, combination therapy with alendronate and PTH weakened the PTH-induced increase in osteocalcin. In proximal tibia, combination treatment with KK1-300 and PTH increased BMD to a level significantly higher than that achieved following single treatment with KK1-300 or PTH alone. On the other hand, combination treatment with alendronate and PTH failed to produce any significant additive effect on BMD following single treatment with alendronate or PTH alone. Microstructural analysis revealed that the PTH-induced increase in bone formation (MS/BS and BFR/BS) was fully maintained following combination treatment with KK1-300 and PTH, but not following combination treatment with alendronate and PTH. These findings indicate that KK1-300, unlike alendronate, has an additive effect on the preventive action of PTH on bone loss in OVX rats.

Combined Effects of Soy Isoflavones and β-Carotene on Osteoblast Differentiation

Soy isoflavones, genistein, daidzein and its metabolite equol, as well as β-carotene have been reported to be effective for maintaining bone health. However, it remains to be elucidated whether combining soy isoflavones with β-carotene is beneficial to bone formation. This study investigated the combined effect of soy isoflavones and β-carotene on the differentiation of MC3T3-E1 preosteoblastic cells. Daidzein and genistein alone did not affect cell growth but increased alkaline phosphatase (ALP) activity. Beta-carotene alone inhibited cell growth and markedly enhanced ALP activity. Soy isoflavones combined with β-carotene resulted in higher ALP activity than treatment with isoflavones or β-carotene alone. We observed significant main effects of β-carotene on the enhanced expression of Runx2, ALP, and ostepontin mRNA, whereas there was a significant main effect of soy isoflavones on the expression of osterix mRNA. To investigate how β-carotene affected osteoblast differentiation, MC3T3-E1 cells were treated with retinoic acid receptor (RAR) pan-antagonist combined with β-carotene. Osteopontin and ALP mRNA expression levels, which were increased following treatment with β-carotene, were significantly suppressed by the RAR pan-antagonist. This suggests treatment with β-carotene enhanced early osteoblastic differentiation, at least in part via RAR signaling. These results indicate that a combination of isoflavones and β-carotene may be useful for maintaining a positive balance of bone turnover by inducing osteoblast differentiation.

An NMR metabolomic study on the effect of alendronate in ovariectomized mice

Alendronate sodium (Fosamax) is most widely used for the prevention and treatment of osteoporosis. It is a type of anti-resorptive agent that reduces the risk of fractures by changing bone turnover and bone mineral density. We investigated the effect of Fosamax on a mouse model of osteoporosis. Twenty-seven female C57BL/6JNarl mice were divided into three groups: sham, ovariectomized (OVX) and OVX + Fosamax (Fosamax). After 23 weeks, bone density of femurs was analyzed using microcomputed tomography (micro-CT), and serum was analyzed for osteoblast and osteoclast activity, as well as metabolites using nuclear magnetic resonance (NMR) spectroscopy. Fosamax increased bone mineral density and cortical bone thickness, and decreased osteoblast activity slightly. Fosamax did not significantly change osteoclast activity. Serum metabolomics revealed that Fosamax had profound effects on overall metabolism, as significantly higher concentrations of metabolites associated with energy metabolism (including TCA-cycle intermediates and glucose), 3-hydroxybutyrate, taurine, allantoin, acetate, and ethanol, as well as lower concentrations of aspartate were observed in the Fosamax-treated mice compared with the OVX mice. These results suggest that alendronate may work by increasing bone density through altered metabolic activity.

Increased fracture callus mineralization and strength in cathepsin K knockout mice

Cathepsin K (CatK) is a cysteine protease, expressed predominantly in osteoclasts (OC) which degrades demineralized bone matrix. Novel selective inhibitors of CatK are currently being developed for the treatment of postmenopausal osteoporosis. Pharmacological inhibition of CatK reduces OC resorption activity while preserving bone formation in preclinical models. Disruption of the CatK gene in mice also results in high bone mass due to impaired bone resorption and elevated formation. Here, we assessed mid-shaft femoral fracture healing in 8-10week old CatK knock-out (KO) versus wild type (WT) mice. Fracture healing and callus formation were determined in vivo weekly via X-ray, and ex vivo at days 14, 18, 28 and 42 post-fracture by radiographic scoring, micro-computed tomography (μCT), histomorphometry and terminal mechanical four point bend strength testing. Radiological evaluation indicated accelerated bone healing and remodeling for CatK KO animals based on increased total radiographic scores that included callus opacity and bridging at days 28 and 42 post-fracture. Micro-CT based total callus volume was similar in CatK KO and WT mice at day 14. Callus size in CatK KO mice was 25% smaller than that in WT mice at day 18, statistically significant by day 28 and exhibited significantly higher mineralized tissue volume and volumetric BMD as compared to WT by day 18 onward. Osteoclast surface and osteoid surface trended higher in CatK KO calluses at all time-points and osteoblast number was also significantly increased at day 28. Increased CatK KO callus mineral density was reflected in significant increases in peak load and stiffness over WT at day 42 post-fracture. Regression analysis indicated a positive correlation (r=0.8671; p<0.001) between callus BMC and peak load indicating normal mineral properties in CatK KO calluses. Taken together, gene deletion of cathepsin K in mice accelerated callus size resolution, significantly increased callus mineralized mass, and improved mechanical strength as compared to wild type mice.

Additive effect of PTH (1-34) and zoledronate in the prevention of disuse osteopenia in rats

Immobilization is known to cause a rapid bone loss due to increased osteoclastic bone resorption and decreased osteoblastic bone formation. Zoledronate (Zln) is a potent anti-resorptive pharmaceutical, while intermittent PTH is a potent bone anabolic agent. The aim of the present study was to investigate whether PTH or Zln alone or in combination could prevent immobilization-induced osteopenia. Immobilization was achieved by injecting 4IU Botox (BTX) into the right hind limb musculature. Seventy-two 16-week-old female Wistar rats were randomized into 6 groups; baseline (Base), control (Ctrl), BTX, BTX+PTH, BTX+Zln, and BTX+PTH+Zln. PTH (1-34) (80μg/kg) was given 5days/week and Zln (100μg/kg) was given once at study start. The animals were killed after 4weeks of treatment. The bone properties were evaluated using DEXA, μCT, dynamic bone histomorphometry, and mechanical testing. BTX resulted in lower femoral trabecular bone volume fraction (BV/TV) (-25%, p<0.05), lower tibial trabecular bone formation rate (BFR/BS) (-29%, p<0.05), and lower bone strength (Fmax) at the distal femur (-19%, p<0.001) compared with Ctrl. BTX+PTH resulted in higher femoral BV/TV (+31%, p<0.05), higher tibial trabecular BFR/BS (+297%, p<0.05), and higher Fmax at the distal femur (+11%, p<0.05) compared with BTX. BTX+Zln resulted in higher femoral BV/TV (+36%, p<0.05), lower tibial trabecular BFR/BS (-93%, p<0.05), and higher Fmax at the distal femur (+10%, p<0.05) compared with BTX. BTX+PTH+Zln resulted in higher femoral BV/TV (+70%, p<0.001), higher tibial trabecular BFR/BS (+59%, p<0.05), and higher Fmax at the distal femur (+32%, p<0.001) compared with BTX. In conclusion, BTX-induced immobilization led to lower BV/TV, BFR/BS, and Fmax. In general, PTH or Zln alone prevented the BTX-induced osteopenia, whereas PTH and Zln given in combination not only prevented, but also increased BV/TV and BFR/BS, and maintained Fmax at the distal femoral metaphysis compared with Ctrl.

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.

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.

Effects of pharmacological inhibition of cathepsin K on fracture repair in mice

Cathepsin K inhibitors (CatK-I) have been developed and established to restore bone mass in both animal models of bone loss and postmenopausal osteoporotic patients. We investigated the effects of a CatK-I L-006235 on bone repair and compared to alendronate (ALN) for its known effects on fracture healing in preclinical models. Femoral fractures were performed on wild type mice that were given vehicle (CON), CatK-I or ALN from day 0 post-fracture until euthanasia. Radiologic and micro-CT analyses demonstrated that CatK-I enhanced mineralization within the calluses at day 21 post-fracture, but to a lesser degree than ALN. Histological analyses showed residual unmineralized and mineralized cartilage in the calluses of CatK-I and ALN treated groups at day 21 post-fracture compared to that in CON. CatK-I enhanced the number of tartrate-resistant acid phosphatase positive (TRAP+) osteoclasts in the fracture calluses compared to ALN and CON treated groups. However, relative levels of serum C-terminal telopeptides of type I collagen (CTX) normalized to the number of TRAP+ osteoclasts within the calluses were significantly decreased in both CatK-I and ALN groups compared to CON. Additionally, the percentages of osteoblast surface over mineralized calluses and levels of the bone formation marker serum N-terminal propeptide of type I procollagen (P1NP) were comparable between CatK-I versus CON groups, while these bone formation parameters were decreased by ALN. Taken together, these results indicate that unlike ALN, CatK-I inhibits osteoclastic activity without changing bone formation, and the inhibition of CatK delayed but did not abrogate callus remodeling during bone repair.

An activator of the cAMP/PKA/CREB pathway promotes osteogenesis from human mesenchymal stem cells

Mesenchymal stem cells (MSCs) are multipotent adult stem cells capable of differentiating along the osteoblast, adipocyte, and chondrocyte lineages. Regulation of MSCs differentiation may be a useful tool for regenerative medicine and cell-based therapy. The discovery of small molecule that activates the osteogenic differentiation of MSCs could aid in the development of a new anabolic drug for osteoporosis treatment. We identified CW008, a derivative of pyrazole-pyridine, that stimulates osteoblast differentiation of human MSCs and increases bone formation in ovariectomized mice. CW008 promotes osteogenesis by activating cAMP/PKA/CREB signaling pathway and inhibiting leptin secretion. These results suggest that CW008 is an agonist of cAMP/PKA/CREB pathway in osteogenic differentiation and that application of CW008 may be useful for the treatment of bone-related diseases and for the study of bone biology.

Technetium-99 conjugated with methylene diphosphonate ameliorates ovariectomy-induced osteoporotic phenotype without causing osteonecrosis in the jaw

Technetium-99 conjugated with methylene diphosphonate ((99)Tc-MDP) is a novel bisphosphonate derivative without radioactivity and has been successfully used to treat arthritis in China for years. Since bisphosphonate therapy has the potential to induce bisphosphonate-related osteonecrosis of the jaw (BRONJ), we examined whether (99)Tc-MDP represents a new class of bisphosphonate for antiresorptive therapy to ameliorate estrogen deficiency-induced bone resorption with less risk of causing BRONJ. We showed that (99)Tc-MDP-treated, ovariectomized (OVX) mice had significantly improved bone mineral density and trabecular bone volume in comparison to the untreated OVX group by inhibiting osteoclasts and enhancing osteogenic differentiation of bone marrow mesenchymal stem cells. To determine the potential of inducing BRONJ, (99)Tc-MDP/dexamethasone (Dex) or zoledronate/Dex was administered into C57BL/6J mice via the tail vein, followed by extraction of maxillary first molars. Interestingly, (99)Tc-MDP treatment showed less risk to induce osteonecrosis in the maxillary bones compared to zoledronate treatment group, partially because (99)Tc-MDP neither suppressed adaptive regulatory T cells nor activated the inflammatory T-helper-producing interleukin-17 cells. Taken together, our findings demonstrate that (99)Tc-MDP therapy may be a promising approach in the treatment of osteoporosis with less risk of causing BRONJ.

Mechanical loading-related changes in osteocyte sclerostin expression in mice are more closely associated with the subsequent osteogenic response than the peak strains engendered

Osteocyte sclerostin is regulated by loading and disuse in mouse tibiae but is more closely related to subsequent local osteogenesis than the peak strains engendered.

INTRODUCTION

The purpose of this study was to assess the relationship between loading-related change in osteocyte sclerostin expression, local strain magnitude, and local bone modeling/remodeling.

METHODS

The right tibiae of 19-week-old female C57BL/6 mice were subjected to non-invasive, dynamic axial loading and/or to sciatic neurectomy-induced disuse. The sclerostin status of osteocytes was evaluated immunohistochemically, changes in bone mass by micro-computed tomography, new bone formation by histomorphometry, and loading-induced strain by strain gauges and finite element analysis.

RESULTS

In cortical bone of the tibial shaft, loading engendered strains of similar peak magnitude proximally and distally. Proximally, sclerostin-positive osteocytes decreased and new bone formation increased. Distally, there was neither decrease in sclerostin-positive osteocytes nor increased osteogenesis. In trabecular bone of the proximal secondary spongiosa, loading decreased sclerostin-positive osteocytes and increased bone volume. Neither occurred in the primary spongiosa. Disuse increased sclerostin-positive osteocytes and decreased bone volume at all four sites. Loading reversed this sclerostin upregulation to a level below baseline in the proximal cortex and secondary spongiosa.

CONCLUSION

Loading-related sclerostin downregulation in osteocytes of the mouse tibia is associated preferentially with regions where new bone formation is stimulated rather than where high peak strains are engendered. The mechanisms involved remain unclear, but could relate to peak surface strains not accurately reflecting the strain-related osteogenic stimulus or that sclerostin regulation occurs after sufficient signal processing to distinguish between local osteogenic and non-osteogenic responses.

Odanacatib reduces bone turnover and increases bone mass in the lumbar spine of skeletally mature ovariectomized rhesus monkeys

Odanacatib (ODN) is a selective and reversible inhibitor of cathepsin K (CatK) currently being developed as a once-weekly treatment for osteoporosis. In this study, we evaluated the effects of ODN on bone turnover, bone mineral density (BMD), and bone strength in the lumbar spine of estrogen-deficient, skeletally mature rhesus monkeys. Ovariectomized (OVX) monkeys were treated in prevention mode for 21 months with either vehicle, ODN 6 mg/kg, or ODN 30 mg/kg (p.o., q.d.) and compared with intact animals. ODN treatment persistently suppressed the bone resorption markers (urinary NTx [75% to 90%] and serum CTx [40% to 55%]) and the serum formation markers (BSAP [30% to 35%] and P1NP [60% to 70%]) versus vehicle-treated OVX monkeys. Treatment with ODN also led to dose-dependent increases in serum 1-CTP and maintained estrogen deficiency-elevated Trap-5b levels, supporting the distinct mechanism of CatK inhibition in effectively suppressing bone resorption without reducing osteoclast numbers. ODN at both doses fully prevented bone loss in lumbar vertebrae (L₁ to L₄) BMD in OVX animals, maintaining a level comparable to intact animals. ODN dose-dependently increased L₁ to L₄ BMD by 7% in the 6 mg/kg group (p < 0.05 versus OVX-vehicle) and 15% in the 30 mg/kg group (p < 0.05 versus OVX-vehicle) from baseline. Treatment also trended to increase bone strength, associated with a positive and highly significant correlation (R = 0.838) between peak load and bone mineral content of the lumbar spine. Whereas ODN reduced bone turnover parameters in trabecular bone, the number of osteoclasts was either maintained or increased in the ODN-treated groups compared with the vehicle controls. Taken together, our findings demonstrated that the long-term treatment with ODN effectively suppressed bone turnover without reducing osteoclast number and maintained normal biomechanical properties of the spine of OVX nonhuman primates.

Odanacatib treatment increases hip bone mass and cortical thickness by preserving endocortical bone formation and stimulating periosteal bone formation in the ovariectomized adult rhesus monkey

Odanacatib (ODN) is a selective and reversible inhibitor of cathepsin K (CatK). Previously, ODN was shown to increase bone mineral density (BMD) and maintained normal bone strength at the spine in ovariectomized (OVX) rhesus monkeys. Here, we further characterize the effects of ODN on BMD, bone strength, and dynamic histomorphometric analyses of the hip from the same monkeys. Animals were treated for 21 months with vehicle, 6 or 30 mg/kg ODN (p.o., q.d.). ODN increased femoral neck (FN) BMD by 11% and 15% (p < 0.07) and ultimate load by 25% (p < 0.05) and 30% (p < 0.01) versus vehicle. Treatment-related increases in ultimate load positively correlated with the increased FN BMD, bone mineral content (BMC), and cortical thickness. Histomorphometry of FN and proximal femur (PF) revealed that ODN reduced trabecular and intracortical bone formation rate (BFR) but did not affect long-term endocortical BFR. Moreover, ODN stimulated long-term FN and PF periosteal BFR by 3.5-fold and 6-fold with the 30 mg/kg dose versus vehicle, respectively. Osteoclast surfaces were either unaffected or trended higher (~twofold) in endocortical and trabecular surfaces in the ODN group. Lastly, ODN increased cortical thickness of FN by 21% (p = 0.08) and PF by 19% (p < 0.05) versus vehicle after 21 months of treatment. Together, both doses of ODN increased bone mass and improved bone strength at the hip. Unlike conventional antiresorptives, ODN displayed site-specific effects on trabecular versus cortical bone formation. The drug provided marked increases in periosteal bone formation and cortical thickness in OVX monkeys, suggesting that CatK inhibition may represent a novel therapeutic approach for the treatment of osteoporosis.

Involvement of PRIP, phospholipase C-related, but catalytically inactive protein, in bone formation

PRIP (phospholipase C-related, but catalytically inactive protein) is a novel protein isolated in this laboratory. PRIP-deficient mice showed increased serum gonadotropins, but decreased gonadal steroid hormones. This imbalance was similar to that for the cause of bone disease, such as osteoporosis. In the present study, therefore, we analyzed mutant mice with special reference to the bone property. We first performed three-dimensional analysis of the femur of female mice. The bone mineral density and trabecular bone volume were higher in mutant mice. We further performed histomorphometrical assay of bone formation parameters: bone formation rate, mineral apposition rate, osteoid thickness, and osteoblast number were up-regulated in the mutant, indicating that increased bone mass is caused by the enhancement of bone formation ability. We then cultured primary cells isolated from calvaria prepared from both genotypes. In mutant mice, osteoblast differentiation, as assessed by alkaline phosphatase activity and the expression of osteoblast differentiation marker genes, was enhanced. Moreover, we analyzed the phosphorylation of Smad1/5/8 in response to bone morphogenetic protein, with longer phosphorylation in the mutant. These results indicate that PRIP is implicated in the negative regulation of bone formation.

Cathepsin K: its skeletal actions and role as a therapeutic target in osteoporosis

Bone remodeling consists of two phases--bone resorption and bone formation--that are normally balanced. When bone resorption exceeds bone formation, pathologic processes, such as osteoporosis, can result. Cathepsin K is a member of the papain family of cysteine proteases that is highly expressed by activated osteoclasts. Cathepsin K readily degrades type I collagen, the major component of the organic bone matrix. With such a major role in the initial process of bone resorption, cathepsin K has become a therapeutic target in osteoporosis. The antiresorptive properties of cathepsin K inhibitors have been studied in phase I and phase II clinical trials. Phase III studies are currently underway for odanacatib, a selective cathepsin K inhibitor.

Inhibition of osteoclast function reduces hematopoietic stem cell numbers in vivo

Osteoblasts play a crucial role in the hematopoietic stem cell (HSC) niche; however, an overall increase in their number does not necessarily promote hematopoiesis. Because the activity of osteoblasts and osteoclasts is coordinately regulated, we hypothesized that active bone-resorbing osteoclasts would participate in HSC niche maintenance. Mice treated with bisphosphonates exhibited a decrease in proportion and absolute number of Lin(-)cKit(+)Sca1(+) Flk2(-) (LKS Flk2(-)) and long-term culture-initiating cells in bone marrow (BM). In competitive transplantation assays, the engraftment of treated BM cells was inferior to that of controls, confirming a decrease in HSC numbers. Accordingly, bisphosphonates abolished the HSC increment produced by parathyroid hormone. In contrast, the number of colony-forming-unit cells in BM was increased. Because a larger fraction of LKS in the BM of treated mice was found in the S/M phase of the cell cycle, osteoclast impairment makes a proportion of HSCs enter the cell cycle and differentiate. To prove that HSC impairment was a consequence of niche manipulation, a group of mice was treated with bisphosphonates and then subjected to BM transplantation from untreated donors. Treated recipient mice experienced a delayed hematopoietic recovery compared with untreated controls. Our findings demonstrate that osteoclast function is fundamental in the HSC niche.

Osteopontin regulates anabolic effect in human menopausal osteoporosis with intermittent parathyroid hormone treatment

In this pilot study, we demonstrated that women with osteopontin (OPN) over-expression show less resistance to postmenopausal osteoporosis than women with normal OPN levels. We hypothesized that the levels of plasma OPN could be used as a treatment indicator for intermittent parathyroid hormone (PTH)-treated menopausal osteoporosis. We demonstrated that plasma OPN levels could be used as a biomarker for early treatment response.

INTRODUCTION

Animal studies indicate that OPN-deficient mice are resistant to ovariectomy induced osteoporosis. Our pilot study also demonstrated women with OPN over expression may show less resistance to postmenopausal osteoporosis. The role of plasma OPN in PTH1-34-treated osteoporosis remains unclear.

METHODS

From September 2005 to September 2006, 31 menopausal women over 45 years of age with severe osteoporosis were enrolled in our study. Subjects were treated with PTH1-34 subcutaneously at a dose of 20 μg/day. Plasma OPN levels and BMD of the lumbar spine and hip were measured using ELISA and dual-energy X-ray absorptiometry at baseline, 3, 6, and 9 months. Response to the treatment was assessed by the sequential change in bone mineral density and OPN expression using a general linear mixed model.

RESULTS

The plasma OPN decreased sequentially and significantly throughout the 9-month treatment course from 20.75 ± 5.36 to 11.2 ± 4.37 ng/ml (p < 0.001). The sequential improvement in the T-score and Z-score was significant in the lumbar spine but not in the hip area. In the lumbar spine, when the plasma OPN decreased by 1 ng/ml the T-score increased by 0.0406 and the Z-score increased by 0.0572 of lumbar spine.

CONCLUSION

OPN levels are related to the anabolic effect of PTH in human postmenopausal osteoporosis. Plasma OPN levels could be used as a biomarker for early treatment response.

The biphenyl-carboxylate derivative ABD328 is a novel orally active antiresorptive agent

We previously described a novel series of biphenyl carboxylic acid derivatives which have potent antiresorptive effects in vitro and in vivo and do not affect osteoblast function. However, none of the previous compounds showed oral activity, probably because they were esters, which would be expected to be metabolized very rapidly. Here, we tested whether derivatives where the ester link was replaced by a ketone link were orally active. Compounds were tested in murine osteoclast and osteoblast cultures and in the mouse ovariectomy (OVX) model of osteoporosis. The ketones were at least as potent at inhibiting osteoclast formation and RANKL signaling in vitro as the esters and did not inhibit osteoblast differentiation or function. The basic ketone-linked compound ABD68 was only partially able to inhibit OVX-induced bone loss at an oral dose of 20 mg/kg daily. Substitutions on the phenyl rings increased the potency of the compounds in vitro and may prevent metabolism of the compounds in vivo. The most promising derivative, ABD328, completely prevented OVX-induced bone loss when administered by intraperitoneal injection at 3 mg/kg daily. Furthermore, ABD328 was also able to fully prevent OVX-induced bone loss when given orally at 10 mg/kg daily. The results indicate that biphenyl carboxylates like ABD328 are oral candidate drugs for the treatment of diseases characterized by increased bone resorption, such as postmenopausal osteoporosis.

Mechanical stimulation and intermittent parathyroid hormone treatment induce disproportional osteogenic, geometric, and biomechanical effects in growing mouse bone

Mechanical loading and intermittent parathyroid (iPTH) treatment are both osteoanabolic stimuli and are regulated by partially overlapping cellular signaling pathways. iPTH has been shown clinically to be effective in increasing bone mass and reducing fracture risk. Likewise, mechanical stimulation can significantly enhance bone apposition and prevent bone loss, but its clinical effects on fracture susceptibility are less certain. Many of the osteogenic effects of iPTH are localized to biomechanically suboptimal bone surfaces, whereas mechanical loading directs new bone formation to high-stress areas and not to strain-neutral areas. These differences in localization in new tissue, resulting from load-induced versus iPTH-induced bone accumulation, should affect the relation between bone mass and bone strength, or "tissue economy." We investigated the changes in bone mass and strength induced by 6 weeks of mechanical loading and compared them to changes induced by 6 weeks of iPTH treatment. Loading and iPTH both increased ulnar bone accrual, as measured by bone mineral density and content, and fluorochrome-derived bone formation. iPTH induced a significantly greater increase in bone mass than loading, but ulnar bone strength was increased approximately the same amount by both treatments. Mechanical loading during growth can spatially optimize new bone formation to improve structural integrity with a minimal increase in mass, thereby increasing tissue economy, i.e., the amount of strength returned per unit bone mass added. Furthermore, exercise studies in which only small changes in bone mass are detected might be more beneficial to bone health and fracture resistance than has commonly been presumed.