Reduced osteoblastic population and defective mineralization in osteopetrotic (op/op) mice

Embryonic inhibition of colony-stimulating factor 1 receptor induces enlarged cartilaginous zone of the midpalatal suture in postnatal mice

OBJECTIVES

The midpalatal suture acts as the growth centre of the maxilla. Colony-stimulating factor 1 receptor (CSF1R) is essential for osteoclastogenesis. Deletion of CSF1R, and its ligand, results in significant craniofacial phenotypes but has not been studied in detail in the midpalatal suture.

MATERIALS AND METHODS

Pregnant ICR mice were treated with the CSF1R inhibitor PLX5622 at embryo Day 14.5 (E14.5) to E17.5. Pups at E18.5, postnatal Day 3 (P3) and P7 were collected for skeletal and histological staining. Osteoclasts were labelled using TRAP staining. PHH3 and TUNEL were employed to detect cell proliferation and apoptosis. Sox9, Ihh, and Col10a1 and Runx2, Col1a1, and DMP1 were used to detect chondrogenic differentiation and osteogenic differentiation, respectively. CD31, MMP9 and CTSK were utilized to assess vascular invasion and osteoclast secretion enzymes, respectively.

RESULTS

Embryonic inhibition of CSF1R resulted in a depletion of TRAP-positive cells and an enlarged cartilage zone of the midpalatal suture of postnatal mice. Compared to those in the control group, Sox9, Ihh, Col10a1, Runx2 and Col1a1 were upregulated, whereas TUNEL and DMP1 were decreased in this zone. In the trabecular region, Col10a1 was upregulated, while TUNEL, Col1a1 and DMP1 were downregulated. Moreover, the expression of MMP9, CTSK and CD31 was decreased, and invasion into the cartilage zone was delayed.

CONCLUSIONS

Embryonic inhibition of CSF1R led to an abnormally enlarged cartilaginous zone in the midpalatal suture, potentially due to delayed endochondral ossification caused by the depletion of osteoclasts. Additionally, we established a novel model of midpalatal suture dysplasia, offering prospects for future research.

CSF1R as a Therapeutic Target in Bone Diseases: Obvious but Not so Simple

PURPOSE OF REVIEW

The purpose of the review is to summarize the expression and function of CSF1R and its ligands in bone homeostasis and constraints on therapeutic targeting of this axis.

RECENT FINDINGS

Bone development and homeostasis depends upon interactions between mesenchymal cells and cells of the mononuclear phagocyte lineage (MPS), macrophages, and osteoclasts (OCL). The homeostatic interaction is mediated in part by the systemic and local production of growth factors, macrophage colony-stimulating factor (CSF1), and interleukin 34 (IL34) that interact with a receptor (CSF1R) expressed exclusively by MPS cells and their progenitors. Loss-of-function mutations in CSF1 or CSF1R lead to loss of OCL and macrophages and dysregulation of postnatal bone development. MPS cells continuously degrade CSF1R ligands via receptor-mediated endocytosis. As a consequence, any local or systemic increase or decrease in macrophage or OCL abundance is rapidly reversible. In principle, both CSF1R agonists and antagonists have potential in bone regenerative medicine but their evaluation in disease models and therapeutic application needs to carefully consider the intrinsic feedback control of MPS biology.

Novel insights into the coupling of osteoclasts and resorption to bone formation

Bone remodeling consists of resorption by osteoclasts (OCs) and formation by osteoblasts (OBs). Precise coordination of these activities is required for the resorbed bone to be replaced with an equal amount of new bone in order to maintain skeletal mass throughout the lifespan. This coordination of remodeling processes is referred to as the "coupling" of resorption to bone formation. In this review, we discuss the essential role for OCs in coupling resorption to bone formation, mechanisms for this coupling, and how coupling becomes less efficient or disrupted in conditions of bone loss. Lastly, we provide perspectives on targeting coupling to treat human bone disease.

Eldecalcitol effects on osteoblastic differentiation and function in the presence or absence of osteoclastic bone resorption

Eldecalcitol (ELD) is an active vitamin D3 analog, possesses anti-resorption properties and is an approved therapeutic drug for the treatment of osteoporosis in Japan. However, the effect of ELD on osteoblasts in a distinct cell microenvironment, including in the presence or absence of osteoclastic bone resorption, is undetermined. In the current study, the effect of bone resorption supernatant on the ELD-mediated regulation of viability, differentiation and receptor activator of ΝF-κB ligand/osteoprotegerin (RANKL/OPG) expression was assessed in MC3T3-E1 pre-osteoblast cells. The murine macrophage-like cell line RAW 264.7 was induced to differentiate into functional osteoblasts. Bone resorption supernatant was prepared by culturing osteoclast with a bovine cortical bone specimen. Mouse MC3T3-E1 cells were subsequently treated with ELD combined with differentiated osteoclast cell culture (OCS) or osteoclast bone resorption model supernatants. Cell counting kit-8, alkaline phosphatase (ALP) activity, reverse transcription-quantitative (RT-q) PCR and western blot analysis were used to assess cell viability, osteogenic activity and RANKL and OPG expression in MC3T3-E1 cells. The OCS and OCS + ELD treatment exhibited significantly increased MC3T3-E1 cell viability when compared with the control group. However, ELD, bone resorption culture supernatant (BRS) and ELD + BRS treatments significantly decreased MC3T3-E1 cell viability. The results of ALP activity analysis, RT-qPCR and western blot analysis demonstrated that ELD treatment and OCS decreased the osteogenic markers (ALP and RUNX2), however, BRS increased them. All treatments enhanced the expression of RANKL and RANKL/OPG ratio. The results of the current study revealed that ELD inhibits osteoblastic differentiation in vitro. However, in the presence of BRS, which mimics the local bone microenvironment in vivo, the net effect on osteogenesis was positive. Furthermore, osteoclasts and bone matrix-derived factors increased the RANKL/OPG ratio, thereby potentiating osteoclastic activity.

Macrophage-lineage TRAP+ cells recruit periosteum-derived cells for periosteal osteogenesis and regeneration

The periosteum, a thin tissue that covers almost the entire bone surface, accounts for more than 80% of human bone mass and is essential for bone regeneration. Its osteogenic and bone regenerative abilities are well studied, but much is unknown about the periosteum. In this study, we found that macrophage-lineage cells recruit periosteum-derived cells (PDCs) for cortical bone formation. Knockout of colony stimulating factor-1 eliminated macrophage-lineage cells and resulted in loss of PDCs with impaired periosteal bone formation. Moreover, macrophage-lineage TRAP+ cells induced transcriptional expression of periostin and recruitment of PDCs to the periosteal surface through secretion of platelet-derived growth factor-BB (PDGF-BB), where the recruited PDCs underwent osteoblast differentiation coupled with type H vessel formation. We also found that subsets of Nestin+ and LepR+ PDCs possess multipotent and self-renewal abilities and contribute to cortical bone formation. Nestin+ PDCs are found primarily during bone development, whereas LepR+ PDCs are essential for bone homeostasis in adult mice. Importantly, conditional knockout of Pdgfrβ (platelet-derived growth factor receptor beta) in LepR+ cells impaired periosteal bone formation and regeneration. These findings uncover the essential role of periosteal macrophage-lineage cells in regulating periosteum homeostasis and regeneration.

RANKL, Ephrin-Eph and Wnt10b are key intercellular communication molecules regulating bone remodeling in autologous transplanted goldfish scales

This study aimed to investigate the precise data of gene expression, functions, and chronological relationships amongst communication molecules involved in the bone remodeling process with an in vivo model using autologous transplanted scales of goldfish. Autotransplantation of methanol-fixed cell-free scales triggers scale resorption and regeneration, as well as helps elucidate the process of bone remodeling. We investigated osteoclastic markers, osteoblastic markers, and gene expressions of communicating molecules (RANKL, ephrinB2, EphB4, EphA4, Wnt10b) by qPCR, in situ hybridization for Wnt10b, and immunohistochemistry for EphrinB2 and EphA4 proteins to elucidate the bone remodeling process. Furthermore, functional inhibition experiments for the signaling of ephrinB2/Eph, ephrin/EphA4, and Wnt10b using specific antibodies, revealed that these proteins are involved in key signaling pathways promoting normal bone remodeling. Our data suggests that the remodeling process comprises of two successive phases. In the first absorption phase, differentiation of osteoclast progenitors by RANKL is followed by the bone absorption by mature, active osteoclasts, with the simultaneous induction of osteoblast progenitors by multinucleated osteoclast-derived Wnt10b, and proliferation of osteoblast precursors by ehprinB2/EphB4 signaling. Subsequently, during the second formation phase, termination of bone resorption by synergistic cooperation occurs, with downregulation of RANKL expression in activated osteoblasts and Ephrin/EphA4-mediated mutual inhibition between neighboring multinucleated osteoclasts, along with simultaneous activation of osteoblasts via forward and reverse EphrinB2/EphB4 signaling between neighboring osteoblasts. In addition, the present study shows that autologous transplantation of methanol-fixed cell-free scale is an ideal in vivo model to study bone remodeling.

Cathepsin K Inhibitors for Osteoporosis: Biology, Potential Clinical Utility, and Lessons Learned

Cathepsin K is a cysteine protease member of the cathepsin lysosomal protease family. Although cathepsin K is highly expressed in osteoclasts, lower levels of cathepsin K are also found in a variety of other tissues. Secretion of cathepsin K from the osteoclast into the sealed osteoclast-bone cell interface results in efficient degradation of type I collagen. The absence of cathepsin K activity in humans results in pycnodysostosis, characterized by increased bone mineral density and fractures. Pharmacologic cathepsin K inhibition leads to continuous increases in bone mineral density for ≤5 years of treatment and improves bone strength at the spine and hip. Compared with other antiresorptive agents, cathepsin K inhibition is nearly equally efficacious for reducing biochemical markers of bone resorption but comparatively less active for reducing bone formation markers. Despite multiple efforts to develop cathepsin K inhibitors, potential concerns related to off-target effects of the inhibitors against other cathepsins and cathepsin K inhibition at nonbone sites, including skin and perhaps cardiovascular and cerebrovascular sites, prolonged the regulatory approval process. A large multinational randomized, double-blind phase III study of odanacatib in postmenopausal women with osteoporosis was recently completed. Although that study demonstrated clinically relevant reductions in fractures at multiple sites, odanacatib was ultimately withdrawn from the regulatory approval process after it was found to be associated with an increased risk of cerebrovascular accidents. Nonetheless, the underlying biology and clinical effects of cathepsin K inhibition remain of considerable interest and could guide future therapeutic approaches for osteoporosis.

Localization of Minodronate in Mouse Femora Through Isotope Microscopy

Minodronate is highlighted for its marked and sustained effects on osteoporotic bones. To determine the duration of minodronate's effects, we have assessed the localization of the drug in mouse bones through isotope microscopy, after labeling it with a stable nitrogen isotope ([(15)N]-minodronate). In addition, minodronate-treated bones were assessed by histochemistry and transmission electron microscopy (TEM). Eight-week-old male ICR mice received [(15)N]-minodronate (1 mg/kg) intravenously and were sacrificed after 3 hr, 24 hr, 1 week, and 1 month. Isotope microscopy showed that [(15)N]-minodronate was present mainly beneath osteoblasts rather than nearby osteoclasts. At 3 hr after minodronate administration, histochemistry and TEM showed osteoclasts with well-developed ruffled borders. However, osteoclasts were roughly attached to the bone surfaces and did not feature ruffled borders at 24 hr after minodronate administration. The numbers of tartrate-resistant acid phosphatase-positive osteoclasts and alkaline phosphatase-reactive osteoblastic area were not reduced suddenly, and apoptotic osteoclasts appeared in 1 week and 1 month after the injections. Von Kossa staining demonstrated that osteoclasts treated with minodronate did not incorporate mineralized bone matrix. Taken together, minodronate accumulates in bone underneath osteoblasts rather than under bone-resorbing osteoclasts; therefore, it is likely that the minodronate-coated bone matrix is resistant to osteoclastic resorption, which results in a long-lasting and bone-preserving effect.

Histochemical assessment for osteoblastic activity coupled with dysfunctional osteoclasts in c-src deficient mice

Since osteoblastic activities are believed to be coupled with osteoclasts, we have attempted to histologically verify which of the distinct cellular circumstances, the presence of osteoclasts themselves or bone resorption by osteoclasts, is essential for coupled osteoblastic activity, by examining c-fos^-/- or c-src^-/- mice. Osteopetrotic c-fos deficient (c-fos^-/-) mice have no osteoclasts, while c-src deficient (c-src^-/-) mice, another osteopetrotic model, develop dysfunctional osteoclasts due to a lack of ruffled borders. c-fos^-/- mice possessed no tartrate-resistant acid phosphatase (TRAPase)-reactive osteoclasts, and showed very weak tissue nonspecific alkaline phosphatase (TNALPase)-reactive mature osteoblasts. In contrast, c-src^-/- mice had many TNALPase-positive osteoblasts and TRAPase-reactive osteoclasts. Interestingly, the parallel layers of TRAPase-reactive/osteopontin-positive cement lines were observed in the superficial region of c-src^-/- bone matrix. This indicates the possibility that in c-src^-/- mice, osteoblasts were activated to deposit new bone matrices on the surfaces that osteoclasts previously passed along, even without bone resorption. Transmission electron microscopy demonstrated cell-to-cell contacts between mature osteoblasts and neighboring ruffled border-less osteoclasts, and osteoid including many mineralized nodules in c-src^-/- mice. Thus, it seems likely that osteoblastic activities would be maintained in the presence of osteoclasts, even if they are dysfunctional.

Severe compromise of preosteoblasts in a surgical mouse model of bisphosphonate-associated osteonecrosis of the jaw

OBJECTIVES

The effect of amino-bisphosphonates on osteoblastic lineage and its potential contribution to the pathogenesis of bisphosphonate-associated osteonecrosis of the jaw (BONJ) remain controversial. We assessed the effects of zoledronic acid (ZOL) on bone and vascular cells of the alveolar socket using a mouse model of BONJ.

MATERIAL AND METHODS

Thirty-two mice were treated twice a week with either 100 μg/kg of ZOL or saline for 12 weeks. The first left maxillary molar was extracted at the third week. Alveolar sockets were assessed at both 3 weeks (intermediate) and 9 weeks (long-term) after molar extraction by semi-quantitative histomorphometry for empty lacunae, preosteoblasts (Osterix), osteoclasts (TRAP), and pericyte-like cells (CD146). Also, the bone microarchitecture was assessed by micro-CT.

RESULTS

Osteonecrotic-like lesions were observed in 21% of mice. Moreover, a decreased number of preosteoblasts contrasted with the increased number of osteoclasts at both time points. In addition, osteoclasts display multinucleation and detachment from the endosteal surface. Furthermore, the number of pericyte-like cells increased at the intermediate time point. The alveolar bone mass increased exclusively with long-term ZOL treatment.

CONCLUSION

The severe imbalance between bone-forming cells and bone-resorbing cells shown in this study could contribute to the pathogenesis of BONJ.

Effects of drug discontinuation after short-term daily alendronate administration on osteoblasts and osteocytes in mice

In order to determine whether osteoclastic bone resorption is restarted after withdrawn of bisphosphonates, we conducted histological examinations on murine osteoclasts, osteoblasts and osteocytes after discontinuation of a daily regimen of alendronate (ALN) with a dosage of 1 mg/kg/day for 10 days. After drug discontinuation, metaphyseal trabecular number and bone volume remained unaltered for the first 4 days. Osteoclast number did not increase, while the number of apoptotic osteoclasts was elevated. On the other hand, tissue non-specific alkaline phosphatase-immunoreactive area was markedly reduced after ALN discontinuation. In addition, osteocytes showed an atrophic profile with empty lacunar areas during and after ALN treatment. Interestingly, as early as 36 h after a single ALN injection, osteocytes show signs of atrophy despite the presence of active osteoblasts. Structured illumination microscopy system showed shortening of osteocytic cytoplasmic processes after drug cessation, suggesting a possible morphological and functional disconnection between osteocytes and osteoblasts. Taken together, it appears that osteoclastic bone resorption is not resumed after ALN discontinuation; also, osteoblasts and osteocytes hardly seem to recover once they are inactivated and atrophied by ALN. In summary, it seems that one must pay more attention to the responses of osteoblasts and osteocytes, rather focusing on the resuming of osteoclastic bone resorption after the ALN discontinuation.

Osteoclast TGF-β Receptor Signaling Induces Wnt1 Secretion and Couples Bone Resorption to Bone Formation

Osteoblast-mediated bone formation is coupled to osteoclast-mediated bone resorption. These processes become uncoupled with age, leading to increased risk for debilitating fractures. Therefore, understanding how osteoblasts are recruited to sites of resorption is vital to treating age-related bone loss. Osteoclasts release and activate TGF-β from the bone matrix. Here we show that osteoclast-specific inhibition of TGF-β receptor signaling in mice results in osteopenia due to reduced osteoblast numbers with no significant impact on osteoclast numbers or activity. TGF-β induced osteoclast expression of Wnt1, a protein crucial to normal bone formation, and this response was blocked by impaired TGF-β receptor signaling. Osteoclasts in aged murine bones had lower TGF-β signaling and Wnt1 expression in vivo. Ex vivo stimulation of osteoclasts derived from young or old mouse bone marrow macrophages showed no difference in TGF-β-induced Wnt1 expression. However, young osteoclasts expressed reduced Wnt1 when cultured on aged mouse bone chips compared to young mouse bone chips, consistent with decreased skeletal TGF-β availability with age. Therefore, osteoclast responses to TGF-β are essential for coupling bone resorption to bone formation, and modulating this pathway may provide opportunities to treat age-related bone loss.

Osteopetrosis in obese female rats is site-specifically inhibited by physical training

NEW FINDINGS

What is the central question of this study? Clinical studies suggest that obesity 'protects' against osteoporosis. However, these studies used only bone densitometry and assessed only one bone site, which is insufficient to enable conclusions to be drawn about the response of the whole skeleton. Furthermore, the effects of exercise on bone responses in obesity have not been explored previously. What is the main finding and what is its importance? We show that obesity causes osteopetrosis. Therefore, the classical perspective of 'protective effects of obesity' needs to be reviewed, and exercise is an important tool to avoid these alterations and to maintain the homeostasis of bone. A sedentary lifestyle and obesity induce systemic inflammatory responses. Although the effects of physical inactivity on osseous tissue have been well established, the effects of obesity on bone tissue remain controversial. Furthermore, the effects of physical training on bone tissue responses in the presence of diet-induced obesity are unknown. Our aim was to investigate the effects of obesity and physical training at multiple bone sites in rats. Female Wistar rats were divided into the following four groups: (i) control diet, non-trained (C-NT); (ii) high-refined carbohydrate-containing diet, non-trained (HC-NT); (iii) control diet, trained (C-T); and (iv) high-refined carbohydrate-containing diet, trained (HC-T). At 5 months of age, the rats were submitted to daily exercise for 30 min day(-1). After 13 weeks, blood samples, adipose and skeletal tissues were harvested. Two-way ANOVA was applied to detect differences (significance accepted when P ≤ 0.05). The HC-NT group exhibited increased body mass, adiposity, serum leptin, serum insulin, insulin resistance index and concentrations of tumour necrosis factor-α and interleukin-6. Obese rats (HC-NT) exhibited thickening of nasal bones, trabecular bones in the lumbar vertebrae and long bones in a site-dependent manner. The HC-T group exhibited similar adiposity and inflammatory results. Morphological analysis of the lumbar vertebrae in rats fed the HC diet revealed characteristics of osteopetrosis that were inhibited by exercise. In conclusion, the HC diet induced obesity and inflammatory/hormonal alterations and increased the trabecular bone in a site-dependent manner. However, obesity caused osteopetrosis in the lumbar vertebrae, which could be inhibited by physical training. Although exercise inhibited the development of bone alterations, physical training did not inhibit the HC diet-induced obesity responses.

PDGF-BB secreted by preosteoclasts induces angiogenesis during coupling with osteogenesis

Osteogenesis during bone modeling and remodeling is coupled with angiogenesis. A recent study shows that the specific vessel subtype, strongly positive for CD31 and Endomucin (CD31^hiEmcn^hi), couples angiogenesis and osteogenesis. We found that preosteoclasts secrete platelet derived growth factor-BB (PDGF-BB), inducing CD31^hiEmcn^hi vessels during bone modeling and remodeling. Mice with depletion of PDGF-BB in tartrate-resistant acid phosphatase positive (TRAP⁺) cell lineage (Pdgfb^–/–) show significantly lower trabecular and cortical bone mass, serum and bone marrow PDGF-BB concentrations, and CD31^hiEmcn^hi vessels compared to wild-type mice. In the ovariectomized (OVX) osteoporotic mouse model, concentrations of serum and bone marrow PDGF-BB and CD31^hiEmcn^hi vessels are significantly decreased. Inhibition of cathepsin K (CTSK) increases preosteoclast numbers, resulting in higher levels of PDGF-BB to stimulate CD31^hiEmcn^hi vessels and bone formation in OVX mice. Thus, pharmacotherapies that increase PDGF-BB secretion from preosteoclasts offer a novel therapeutic target for osteoporosis to promote angiogenesis for bone formation.

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.

Altered distribution of bone matrix proteins and defective bone mineralization in klotho-deficient mice

In an attempt to identify the histological properties of the klotho-deficient (kl/kl) bone matrix, bone mineralization and the localization of Ca(2+)-binding bone matrix proteins - osteocalcin, dentin matrix protein-1 (DMP-1) and matrix Gla protein (MGP) - were examined in kl/kl tibiae. While a widespread osteocalcin staining could be verified in the wild-type bone matrix, localization of the same protein in the kl/kl tibiae seemed rather restricted to osteocytes with only a faint staining of the whole bone matrix. In wild-type mice, MGP immunoreactivity was present at the junction between the epiphyseal bone and cartilage, and at the insertion of the cruciate ligaments. In kl/kl mice, however, MGP was seen around the cartilaginous cores of the metaphyseal trabeculae and in the periphery of some cells of the bone surface. DMP-1 was identified in the osteocytic canalicular system of wild-type tibiae, but in the kl/kl tibiae this protein was mostly found in the osteocytic lacunae and in the periphery of some cells of the bone surface. Mineralization of the kl/kl bone seemed somewhat defective, with broad unmineralized areas within its matrix. In these areas, mineralized osteocytes along with their lacunae and osteocytic cytoplasmic processes were found to have intense osteocalcin and DMP-1 staining. Taken together, it might be that the excessive production of Ca(2+)-binding molecules such as osteocalcin and DMP-1 by osteocytes concentrates mineralization around such cells, disturbing the completeness of mineralization in the kl/kl bone matrix.

Osteocytes: central conductors of bone biology in normal and pathological conditions

Osteocytes are the most abundant and longest-living cells in the adult skeleton. For a long time, osteocytes were considered static and inactive cells, but in recent years, it has been suggested that they represent the key responder to various stimuli that regulate bone formation and remodelling as well as one of the key endocrine regulators of bone metabolism. Osteocytes respond to mechanical stimuli by producing and secreting several signalling molecules, such as nitric oxide and prostaglandin E(2) , that initiate local bone remodelling. Moreover, they can control bone formation by modulating the WNT signalling pathway, an essential regulator of cell fate and commitment, as they represent the main source of sclerostin, a negative regulator of bone formation. Osteocytes can also act as an endocrine organ by releasing fibroblast growth factor 23 and several other proteins (DMP-1, MEPE, PHEX) that regulate phosphate metabolism. It has been demonstrated that various bone diseases are associated with osteocyte abnormalities, although it is not clear if these changes are the direct cause of the pathology or if they are secondary to the pathological changes in the bone microenvironment. Thus, a better understanding of these cells could offer exciting opportunities for new advances in the prevention and management of different bone diseases.

Meox2Cre-mediated disruption of CSF-1 leads to osteopetrosis and osteocyte defects

CSF-1, a key regulator of mononuclear phagocyte production, is highly expressed in the skeleton by osteoblasts/osteocytes and in a number of nonskeletal tissues such as uterus, kidney and brain. The spontaneous mutant op/op mouse has been the conventional model of CSF-1 deficiency and exhibits a pleiotropic phenotype characterized by osteopetrosis, and defects in hematopoiesis, fertility and neural function. Studies to further delineate the biologic effect of CSF-1 within various tissues have been hampered by the lack of suitable models. To address this issue, we generated CSF-1 floxed/floxed mice and demonstrate that Cre-mediated recombination using Meox2Cre, a Cre line expressed in epiblast during early embryogenesis, results in mice with ubiquitous CSF-1 deficiency (CSF-1KO). Homozygous CSF-1KO mice lacked CSF-1 in all tissues and displayed, in part, a similar phenotype to op/op mice that included: failure of tooth eruption, osteopetrosis, reduced macrophage densities in reproductive and other organs and altered hematopoiesis with decreased marrow cellularity, circulating monocytes and B cell lymphopoiesis. In contrast to op/op mice, CSF-1KO mice showed elevated circulating and splenic T cells. A striking feature in CSF-1KO mice was defective osteocyte maturation, bone mineralization and osteocyte-lacunar system that was associated with reduced dentin matrix protein 1 (DMP1) expression in osteocytes. CSF-1KO mice also showed a dramatic reduction in osteomacs along the endosteal surface that may have contributed to the hematopoietic and cortical bone defects. Thus, our findings show that ubiquitous CSF-1 gene deletion using a Cre-based system recapitulates the expected osteopetrotic phenotype. Moreover, results point to a novel link between CSF-1 and osteocyte survival/function that is essential for maintaining bone mass and strength during skeletal development.

Eldecalcitol, a second-generation vitamin D analog, drives bone minimodeling and reduces osteoclastic number in trabecular bone of ovariectomized rats

To elucidate the histological events that follow administration of eldecalcitol, a second-generation of vitamin D analog currently awaiting approval as a drug for treatment of osteoporosis, we employed the ovariectomy (OVX) rat model. OVX rats received vehicle or 30ng/kg of eldecalcitol, and sham-operated animals received vehicle only. Rats were sacrificed after 12weeks and had their femora and tibiae removed and processed for histochemical and histomorphometrical analyses. When compared with OVX group, osteoclastic number and bone resorption parameters were significantly reduced in eldecalcitol-treated rats, accompanied by decreased bone formation parameters. The preosteoblastic layer, with which osteoclastic precursors interact for mutual differentiation, was poorly developed in the eldecalcitol group, indicating less cell-to-cell contact between preosteoblasts and osteoclast precursors. Interestingly, eldecalcitol did promote a type of focal bone formation that is independent of bone resorption, a process known as bone minimodeling. While the number of ED-1-positive macrophages was higher in the bone marrow of treated rats, though osteoclastic number was deceased. Taken together, our findings suggest that eldecalcitol stimulates preosteoblastic differentiation rather than their proliferation, which in turn may prevent or diminish cell-to-cell contact between preosteoblasts and osteoclastic precursors, and therefore, lead to lower osteoclast numbers and decreased bone resorption.

Ultrastructural observation on cells meeting the histological criteria for preosteoblasts--a study in the mouse tibial metaphysis

Preosteoblasts are currently defined as the precursors of mature osteoblasts. These cells are morphologically diverse and may represent a continuum during osteoblast differentiation. We have attempted to categorize the different preosteoblastic phenotypes in vivo by examining bone cells expressing the runt-related transcription factor 2, alkaline phosphatase and BrdU incorporation - histological traits of a preosteoblast - under transmission electron microscopy (TEM). TEM observations demonstrated, at least, in part two preosteoblastic subtypes: (i) a cell rich in cisterns of rough endoplasmic reticulum (rER) with vesicles and vacuoles and (ii) a subtype featuring extended cytoplasmic processes that connect with distant cells, with a small amount of scattered cisterns of rER and with many vesicles and vacuoles. ER-rich cells, whose cellular machinery is similar to that of an osteoblast, were often seen adjacent to mature osteoblasts, and therefore, may be ready for terminal differentiation. In contrast, ER-poor and vesicle-rich cells extended their cytoplasmic processes to mature osteoblasts and, frequently, to bone-resorbing osteoclasts. The abundant vesicles and vacuoles identified in this cell type indicate that this cell is involved in vesicular transport rather than matrix synthesis activity. In summary, our study verified the morphological diversity and the ultrastructural properties of osteoblastic cells in vivo.

Cell interactions in bone tissue engineering

Bone fractures, where the innate regenerative bone response is compromised, represent between 4 and 8 hundred thousands of the total fracture cases, just in the United States. Bone tissue engineering (TE) brought the notion that, in cases such as those, it was preferable to boost the healing process of bone tissue instead of just adding artificial parts that could never properly replace the native tissue. However, despite the hype, bone TE so far could not live up to its promises and new bottom-up approaches are needed. The study of the cellular interactions between the cells relevant for bone biology can be of essential importance to that. In living bone, cells are in a context where communication with adjacent cells is almost permanent. Many fundamental works have been addressing these communications nonetheless, in a bone TE approach, the 3D perspective, being part of the microenvironment of a bone cell, is as crucial. Works combining the study of cell-to-cell interactions in a 3D environment are not as many as expected. Therefore, the bone TE field should not only gain knowledge from the field of fundamental Biology but also contribute for further understanding the biology of bone. In this review, a summary of the main works in the field of bone TE, aiming at studying cellular interactions in a 3D environment, and how they contributed towards the development of a functional engineered bone tissue, is presented.

Increased connective tissue extracellular matrix in the op/op model of osteopetrosis

Mice that are homozygous for the recessive osteopetrosis spontaneous mutation (op/op) develop severe osteopetrosis due to a defect in the production of macrophage colony-stimulating factor (M-CSF) and a deficiency in monocyte-derived osteoclasts. Our study describes a novel soft tissue finding in an osteopetrosis (B6C3Fe a/a-Csf1(op)/J) mouse model. Tissues were obtained from B6C3Fe a/a-Csf1(op)/J mice and age-matched wild-type mice, processed for hematoxylin and eosin sections, and comprehensive light microscopic tissue evaluation was performed. Mutant mice had characteristic traits of op/op deficiency including missing incisors and domed skulls. Histologically, the bone marrow cavity was effaced by interweaving thick bony trabeculae consistent with osteopetrosis. An increase in a finely granular, basophilic interstitial extracellular matrix (ECM) was observed in the subcutaneous connective tissue of the op/op mice when compared with controls. Histochemically, the ECM was negative with periodic acid Schiff and stained dark blue with alcian blue at a pH of 2.5, indicating that it is composed primarily of nonsulfated glycosaminoglycans (GAGs). This work suggests an increased ECM that is composed mainly of GAGs located in the subcutaneous tissue in op/op mice. This increase in ECM may be related to altered matrix production or turnover because of changes in M-CSF production.

Local communication on and within bone controls bone remodeling

Bone remodeling is required for healthy calcium homeostasis and for repair of damage occurring with stress and age. Osteoclasts resorb bone and osteoblasts form bone. These processes normally occur in a tightly regulated sequence of events, where the amount of formed bone equals the amount of resorbed bone, thereby restoring the removed bone completely. Osteocytes are the third cell type playing an essential role in bone turnover. They appear to regulate activation of bone remodeling, and they exert both positive and negative regulation on both osteoclasts and osteoblasts. In this review, we consider the intricate communication between these bone cells in relation to bone remodeling, reviewing novel data from patients with mutations rendering different cell populations inactive, which have shown that these interactions are more complex than originally thought. We highlight the high probability that a detailed understanding of these processes will aid in the development of novel treatments for bone metabolic disorders, i.e. we discuss the possibility that bone resorption can be attenuated pharmacologically without a secondary reduction in bone formation.

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

We hypothesized that the anabolic action of parathyroid hormone (PTH) with the anti-catabolic agents cathepsin K inhibitor and alendronate differs depending on the remodeling status in the bone. C57/BL/6J mice, 8 weeks of age, were subjected to ovariectomized (OVX) or sham surgery. At 6 weeks after surgery, the mice were treated with cathepsin K inhibitor, alendronate, or a vehicle (daily, for 8 weeks), with or without PTH (1-34) (5 times/week, for the last 4 weeks). We assessed the bone chemical markers of the serum and urine, bone mineral density (BMD), histomorphomery in the primary and secondary spongiosa of the proximal tibia after fluorescence labeling, primary cell culture, and mRNA expressions in bone marrow cells. Cathepsin K inhibitor and alendronate significantly increased the BMD and the bone volume of the primary and secondary spongiosa, with a reduction of the urinary C-telopeptide of type I collagen that was increased by OVX, respectively. Cathepsin K inhibitor augmented the anabolic action of PTH on the BMD and bone volume at both the primary and secondary spongiosa, while alendronate had the same effect on the BMD and bone volume only at the primary spongiosa. Cathepsin K inhibitor did not decrease serum osteocalcin with or without PTH, while alendronate did decrease it. Cathepsin K inhibitor did not decrease the values of osteoclast number or bone formation rate with or without PTH, while alendronate decreased those values and increased osteoclast apoptosis. The combination of PTH and cathepsin K inhibitor increased alkaline phosphatase-positive CFU-f formation and c-fos, osterix, and osteocalcin mRNA expressions of bone marrow cells as well as PTH alone, while the combination of PTH and alendronate decreased those values. This study demonstrated that alendronate enhances the anabolic action of PTH at the primary spongiosa, but blunts it in the remodeling trabecular bone, while cathepsin K inhibitor enhances the action at both sites in OVX mice. In conclusion, the anabolic action of intermittent PTH in combination with cathepsin K inhibitor or alendronate differs depending on the remodeling status of bone in OVX mice.

Regulation of bone formation by osteoclasts involves Wnt/BMP signaling and the chemokine sphingosine-1-phosphate

Under most conditions, resorbed bone is nearly precisely replaced in location and amount by new bone. Thus, it has long been recognized that bone loss through osteoclast-mediated bone resorption and bone replacement through osteoblast-mediated bone formation are tightly coupled processes. Abundant data conclusively demonstrate that osteoblasts direct osteoclast differentiation. Key questions remain, however, as to how osteoblasts are recruited to the resorption site and how the amount of bone produced is so precisely controlled. We hypothesized that osteoclasts play a crucial role in the promotion of bone formation. We found that osteoclast conditioned medium stimulates human mesenchymal stem (hMS) cell migration and differentiation toward the osteoblast lineage as measured by mineralized nodule formation in vitro. We identified candidate osteoclast-derived coupling factors using the Affymetrix microarray. We observed significant induction of sphingosine kinase 1 (SPHK1), which catalyzes the phosphorylation of sphingosine to form sphingosine 1-phosphate (S1P), in mature multinucleated osteoclasts as compared with preosteoclasts. S1P induces osteoblast precursor recruitment and promotes mature cell survival. Wnt10b and BMP6 also were significantly increased in mature osteoclasts, whereas sclerostin levels decreased during differentiation. Stimulation of hMS cell nodule formation by osteoclast conditioned media was attenuated by the Wnt antagonist Dkk1, a BMP6-neutralizing antibody, and by a S1P antagonist. BMP6 antibodies and the S1P antagonist, but not Dkk1, reduced osteoclast conditioned media-induced hMS chemokinesis. In summary, our findings indicate that osteoclasts may recruit osteoprogenitors to the site of bone remodeling through SIP and BMP6 and stimulate bone formation through increased activation of Wnt/BMP pathways.

Building bone to reverse osteoporosis and repair fractures

An important, unfilled clinical need is the development of new approaches to improve fracture healing and to treat osteoporosis by increasing bone mass. Recombinant forms of bone morphogenetic protein 2 (BMP2) and BMP7 are FDA approved to promote spinal fusion and fracture healing, respectively, and the first FDA-approved anabolic drug for osteoporosis, parathyroid hormone, increases bone mass when administered intermittently but can only be given to patients in the US for two years. As we discuss here, the tremendous explosion over the last two decades in our fundamental understanding of the mechanisms of bone remodeling has led to the prospect of mechanism-based anabolic therapies for bone disorders.

Histological and elemental analyses of impaired bone mineralization in klotho-deficient mice

The klotho gene-deficient mouse is known as an animal model for an accelerated gerontic state, mimicking osteoporosis, skin atrophy, ectopic calcification, and gonadal dysplasia. To elucidate the influence of klotho deficiency on bone mineralization, we examined the ultrastructures of osteoblasts and bone matrices in addition to performing the elemental mapping of calcium, phosphorus, and magnesium in the bone. Under anesthesia, 4- and 5-week-old klotho-deficient mice (klotho(-/-)mice) and their wild-type littermates were perfused with either 4% paraformaldehyde for light microscopic observation or 4% paraformaldehyde and 0.0125% glutaraldehyde for electron microscopic observation. The femurs and tibiae were processed for both observations. Paraffin sections were subject to alkaline phosphatase and tartrate resistant acid phosphatase histochemistry. Semithin and ultrathin sections obtained from epoxy resin-embedded specimens were used for detecting mineralization - according to von Kossa's staining method - and for elemental mapping by electron probe micro-analyzer, respectively. Alkaline phosphatase-positive plump osteoblasts adjacent to the growth plate normally developed cell organelles in the klotho(-/-)metaphyses. This, however, contrasted with the flattened osteoblasts covering the metaphyseal trabeculae and accompanied by small tartrate resistant acid phosphatase-positive osteoclasts. The wild-type mice displayed the mineralized matrix at the zone of hypertrophic chondrocyte of the growth plate and well-mineralized metaphyseal trabeculae parallel to the longitudinal axis of the bone. Alternatively, the klotho(-/-)mice demonstrated a thick mineralized matrix from the proliferative zone of the growth plate as well as the large non-mineralized area in the metaphyseal trabeculae. Consistently, electron probe micro-analysis verified sporadic distributions of higher or lower concentrations of calcium and phosphorus in each trabecule of the klotho(-/-)mice. The distribution of magnesium, however, was almost uniform. Under transmission electron microscopy, osteoblasts on the metaphyseal trabeculae displayed less-developed cell organelles in the klotho(-/-)mice. Thus, the klotho deficiency appears not only to reduce osteoblastic population, but also to disturb bone mineralization.

Alendronate treatment promotes bone formation with a less anisotropic microstructure during intramembranous ossification in rats

There are safety concerns regarding administration of bisphosphonates to children. Little is known about the effects of bisphosphonates on bone matrix organization during bone modeling. The present study examined the effects of alendronate (ALN) on bone matrices formed by intramembranous ossification in the appendicular growing skeleton. ALN was administered to 1-week-old Sprague-Dawley rats at a dose of 0, 35, or 350 microg/kg/week for 4 or 8 weeks. The position of femoral diaphysis formed exclusively by intramembranous ossification was identified, and cross sections of cortical bone at this position were analyzed. Bone mineral density (BMD) and geometric parameters were evaluated using peripheral quantitative computed tomography. The preferential orientation degree of biological apatite (BAp) crystals in the bone longitudinal direction, which shows the degree of bone matrix anisotropy, was evaluated using microbeam X-ray diffraction analysis. We analyzed bone histomorphometrical parameters and performed bone nanomechanical tests to examine the material properties of newly developing cortical bone. The preferential orientation degree of BAp crystals significantly decreased in 35 microg/kg/week ALN-treated groups compared with vehicle-treated groups, although there were no significant differences in BMD between the two groups. The periosteal mineral apposition rate significantly increased in the 35 microg/kg/week ALN-treated group. We found a high negative correlation between bone matrix anisotropy and the regional periosteal mineral apposition rate (r = -0.862, P < 0.001). Nanomechanical tests revealed that 35 microg/kg/week ALN administration caused deterioration of the material properties of the bone microstructure. These new findings suggest that alendronate affects bone matrix organization and promotes bone formation with a less anisotropic microstructure during intramembranous ossification.

Are nonresorbing osteoclasts sources of bone anabolic activity?

Some osteopetrotic mutations lead to low resorption, increased numbers of osteoclasts, and increased bone formation, whereas other osteopetrotic mutations lead to low resorption, low numbers of osteoclasts, and decreased bone formation. Elaborating on these findings, we discuss the possibility that osteoclasts are the source of anabolic signals for osteoblasts. In normal healthy individuals, bone formation is coupled to bone resorption in a tight equilibrium. When this delicate balance is disturbed, the net result is pathological situations, such as osteopetrosis or osteoporosis. Human osteopetrosis, caused by mutations in proteins involved in the acidification of the resorption lacuna (ClC-7 or the a3-V-ATPase), is characterized by decreased resorption in face of normal or even increased bone formation. Mouse mutations leading to ablation of osteoclasts (e.g., loss of macrophage-colony stimulating factor [M-CSF] or c-fos) lead to secondary negative effects on bone formation, in contrast to mutations where bone resorption is abrogated with sustained osteoclast numbers, such as the c-src mice. These data indicate a central role for osteoclasts, and not necessarily their resorptive activity, in the control of bone formation. In this review, we consider the balance between bone resorption and bone formation, reviewing novel data that have shown that this principle is more complex than originally thought. We highlight the distinct possibility that osteoclast function can be divided into two more or less separate functions, namely bone resorption and stimulation of bone formation. Finally, we describe the likely possibility that bone resorption can be attenuated pharmacologically without the undesirable reduction in bone formation.

Optimising antiresorptive therapies in postmenopausal women: why do we need to give due consideration to the degree of suppression?

Accelerated bone turnover with bone resorption exceeding bone formation is a major mechanism underlying postmenopausal bone loss and hence the development of osteoporosis. Accordingly, inhibition of bone resorption is a rational approach for the prevention of osteoporosis. In this context, the most logical option, hormone replacement therapy, reverses the rate of bone turnover to premenopausal levels, whereas the magnitude of inhibition by amino-bisphosphonates and the recently introduced anti-receptor activator of NFkappaB ligand (RANKL) antibody often exceeds this. As bone turnover has crucial implications for the continuous renewal of bone tissue, the over-suppression of bone turnover has potential consequences for bone quality and strength. Long-term treatment with potent bisphosphonates has recently been associated with osteonecrosis of the jaw and dose-dependent increases in micro-crack accumulation in animals. Although these observations are the subject of ongoing discussions, it is timely to discuss whether the over-suppression of bone turnover below premenopausal levels is really our ultimate goal when defining the success criteria for antiresorptive agents. In this review, the implications of high and excessively low bone turnover of endogenous origin for bone quality, fracture risk and integrity of the jaw are discussed. In addition, animal and clinical research revealing initial findings regarding the potential adverse effects of drug-induced suppression of bone remodeling are summarised. The inhibition of bone resorption, which is either transient between doses (e.g. with calcitonin) or does not exceed premenopausal levels (with hormone replacement therapy or selective estrogen receptor modulators), is preferable because it not only provides similar antifracture efficacy but can also assist in the maintenance of the dynamic repair of micro-cracks/micro-fractures.

Histological evidence of the altered distribution of osteocytes and bone matrix synthesis in klotho-deficient mice

Mice homozygous for klotho gene deletion are well established aging models as they mimic certain aspects of human senescence e.g. osteoporosis. Induced senescence may affect cellular functions and alter the histological properties of the extracellular matrices. The present study examined the histological and ultrastructural features of osteocytes and the surrounding bone matrix in klotho-deficient mice. As expected, osteoblasts showed a flattened shape with a weak immunoreactivity for alkaline phosphatase, and the bone matrix contained many empty osteocytic lacunae. The walls of both normal and empty lacunae were intensely immunopositive for osteopontin and dentin matrix protein-1, but featured an inconsistent immunoreactivity for osteocalcin and type I collagen. Not surprisingly, TUNEL-positivity, indicative of apoptosis, was found in many osteoblasts, osteocytes, and bone marrow cells of the klotho-deficient mice. In transmission electron microscopy, an amorphous matrix containing non-collagenous organic materials was recognizable around osteoblasts and in the osteocytic lacunae. Some osteoblasts on the bone surface featured these amorphous materials in vacuoles associated with their trans-Golgi network, indicating that, under klotho-deficient conditions, they synthesize and secrete the non-collagenous structures. Some osteocytes displayed pyknosis or degenerative traits. Thus, our findings provide histological evidence that klotho gene deletion influences the spatial distribution of osteocytes and the synthesis of bone matrix proteins in addition to the accelerated aging of bone cells.

An essential requirement for osteoclasts in refined bone-like tissue reconstruction in vitro

One crucial but often disregarded problem in tissue-engineered bone is that osteoblasts in culture significantly differ in function and behavior from their counterparts in vivo. This difference is represented as an inability of cultured osteoblasts to form lamellar bone-like structures and deposit the characteristic mineral of native bone, as well as their loss of polarity of matrix secretion, marked by tissue growth bridging the pore interconnections of scaffolds. These abnormalities of in vitro bone formation are a major cause of the current failure to yield functional and mechanically competent bone. Recent studies with osteoclast deficient animal models have definitely revealed that the regulatory effect of osteoclasts is essential for normal bone formation. The abnormalities of bone formation present in osteoclast deficient animals are largely reflected in current tissue-engineered bone. Reasonably, the abnormalities of in vitro bone formation most likely result from the absence of osteoclasts in current bone formation strategies. Therefore, it is hypothesized that osteoclast introduction will provide a necessary solution to the critical problems in bone tissue engineering. The way of osteoclast introduction is the first challenge in exploring its roles in bone tissue engineering. In vivo studies have shown that maturation of preosteoclasts, the cells just before fusion to form multinucleated cells, requires formation and accumulation of bone matrix despite the independence of preosteoclast appearance and bone formation. Thus, the application of preosteoclasts to biomineralized matrix for osteoclastogenesis is a physiologically relevant strategy for osteoclast introduction. In conclusion, osteoclast introduction holds the promise of structural and functional improvement of tissue-engineered bone.