Impaired bone formation and increased osteoclastogenesis in mice lacking chemokine (C-C motif) ligand 5 (Ccl5)

Chemokines play crucial roles in the recruitment of specific hematopoietic cell types, and some of them have been suggested to be involved in the regulation of bone remodeling. Because we have previously observed that chemokine (C-C motif) ligand 2 (Ccl2) and Ccl5 are direct target genes of noncanonical Wnt signaling in osteoblasts, we analyzed the skeletal phenotypes of Ccl2-deficient and Ccl5-deficient mice. In line with previous studies, Ccl2-deficient mice display a moderate reduction of osteoclastogenesis at the age of 6 months. In contrast, 6-month-old Ccl5-deficient mice display osteopenia associated with decreased bone formation and increased osteoclastogenesis. Moreover, unlike in wild-type and Ccl2-deficient mice, large areas of their trabecular and endocortical bone surfaces are not covered by osteoblasts or bone-lining cells, and this is associated with a severe reduction of endosteal bone formation. Although this phenotype diminishes with age, it is important that we could further identify a reduced number of osteal macrophages in 6-month-old Ccl5-deficient mice, because this cell type has previously been reported to promote endosteal bone formation. Because Ccl5-deficient mice also display increased osteoclastogenesis, we finally addressed the question of whether osteal macrophages could differentiate into osteoclasts and/or secrete inhibitors of osteoclastogenesis. For that purpose we isolated these cells by CD11b affinity purification from calvarial cultures and characterized them ex vivo. Here we found that they are unable to differentiate into osteoblasts or osteoclasts, but that their conditioned medium mediates an antiosteoclastogenic effect, possibly caused by interleukin-18 (IL-18), an inhibitor of osteoclastogenesis expressed by osteal macrophages. Taken together, our data provide in vivo evidence supporting the previously suggested role of Ccl5 in bone remodeling. Moreover, to the best of our knowledge, Ccl5-deficient mice represent the first model with a spontaneous partial deficiency of osteal macrophages, a recently identified cell type, whose impact on bone remodeling is just beginning to be understood.

Osteoblast-specific expression of Fra-2/AP-1 controls adiponectin and osteocalcin expression and affects metabolism

Recent studies have established that the skeleton functions as an endocrine organ affecting metabolism through the osteoblast-derived hormone osteocalcin (Ocn). However, it is not fully understood how many transcription factors expressed in osteoblasts regulate the endocrine function. Here, we show that mice with osteoblast-specific deletion of Fra-2 (Fosl2) have low bone mass but increased body weight. In contrast, transgenic expression of Fra-2 in osteoblasts leads to increased bone mass and decreased body weight accompanied by reduced serum glucose and insulin levels, improved glucose tolerance and insulin sensitivity. In addition, mice lacking Fra-2 have reduced levels of circulating Ocn, but high adiponectin (Adipoq), whereas Fra-2 transgenic mice exhibit high Ocn and low Adipoq levels. Moreover, we found that Adipoq was expressed in osteoblasts and that this expression was transcriptionally repressed by Fra-2. These results demonstrate that Fra-2 expression in osteoblasts represents a novel paradigm for a transcription factor controlling the endocrine function of the skeleton.

Increased osteoblast and osteoclast indices in individuals with systemic mastocytosis

Indolent systemic mastocytosis (ISM) can trigger bone loss. However, the clinical relevance of different mast cell infiltration patterns for bone remains to be clarified. Here, we report increased bone turnover in individuals with ISM, and its extent is rather related to the type of mast cell distribution within the bone marrow than to the presence or absence of cutaneous manifestations.

INTRODUCTION

It is well established that ISM can trigger osteopenia or osteoporosis. However, neither the clinical relevance of the infiltration pattern of mast cells within the bone marrow nor the impact of the presence or absence of cutaneous mast cell infiltration has been elucidated.

METHODS

We retrospectively analysed 300 cases with histologically proven ISM of the bone marrow and performed quantitative histomorphometry for a subgroup of 159 patients that did not receive any treatment before the biopsies were taken. Most importantly, since 66 % of the patients displayed ISM without the characteristic skin lesions, we were able to compare ISM with or without cutaneous manifestation.

RESULTS

We found that both forms of ISM were not only characterized by a decreased trabecular bone mass but also by an increased number of osteoclasts and osteoblasts. Interestingly, when we analysed these data in relation to mast cell distribution, we found that the bone cell numbers in cases with mast cell granulomas were significantly increased compared to cases with diffuse mast cell distribution. Moreover, evidence of increased bone turnover was also found in 16 patients displaying osteosclerosis.

CONCLUSION

Based on the largest cohort of bone biopsies from patients with ISM analysed so far, we could demonstrate high bone turnover, more specifically increased osteoblast and osteoclast numbers and surface indices, as a cause of the skeletal changes. Moreover, the severity of the bone disease is presumably rather dependent on the amount of mast cells and their distribution within the bone marrow irrespective of the presence or absence of cutaneous involvement.

Mutations in WNT1 cause different forms of bone fragility

We report that hypofunctional alleles of WNT1 cause autosomal-recessive osteogenesis imperfecta, a congenital disorder characterized by reduced bone mass and recurrent fractures. In consanguineous families, we identified five homozygous mutations in WNT1: one frameshift mutation, two missense mutations, one splice-site mutation, and one nonsense mutation. In addition, in a family affected by dominantly inherited early-onset osteoporosis, a heterozygous WNT1 missense mutation was identified in affected individuals. Initial functional analysis revealed that altered WNT1 proteins fail to activate canonical LRP5-mediated WNT-regulated β-catenin signaling. Furthermore, osteoblasts cultured in vitro showed enhanced Wnt1 expression with advancing differentiation, indicating a role of WNT1 in osteoblast function and bone development. Our finding that homozygous and heterozygous variants in WNT1 predispose to low-bone-mass phenotypes might advance the development of more effective therapeutic strategies for congenital forms of bone fragility, as well as for common forms of age-related osteoporosis.

Retinol deprivation partially rescues the skeletal mineralization defects of Phex-deficient Hyp mice

X-linked hypophosphatemic rickets (XLH) is a genetic disorder caused by mutational inactivation of the PHEX gene, encoding a transmembrane endopeptidase expressed in osteoblasts. Since several experiments involving Phex-deficient Hyp mice have demonstrated that an increased expression of Fgf23 in osteoblasts is causative for the renal phosphate loss characteristic of XLH, we performed genome-wide expression analysis to compare differentiated osteoblasts from wildtype and Hyp mice. Here we did not only observe the expected increase of Fgf23 expression in the latter ones, but also a differential expression of genes that are either induced by or involved in retinoic acid signaling, which led us to analyze whether dietary retinol deprivation would influence the phenotype of Hyp mice. Unexpectedly, feeding a retinol-free diet resulted in a partial rescue of the growth plate and bone mineralization defects in 6 weeks old Hyp mice. When we fed the same diet for 24 weeks the amount of non-mineralized bone matrix (osteoid) was reduced by more than 70%, although phosphate homeostasis was unaffected. In contrast, a dietary normalization of serum phosphate levels in Hyp mice reduced the osteoid amount by less than 30%, thereby demonstrating a previously unknown impact of retinol on the cell-autonomous mineralization defect of Phex-deficient osteoblasts.

Osteopetrosis, osteopetrorickets and hypophosphatemic rickets differentially affect dentin and enamel mineralization

Osteopetrosis (OP) is an inherited disorder of defective bone resorption, which can be accompanied by impaired skeletal mineralization, a phenotype termed osteopetrorickets (OPR). Since individuals with dysfunctional osteoclasts often develop osteomyelitis of the jaw, we have analyzed, if dentin and enamel mineralization are differentially affected in OP and OPR. Therefore, we have applied non-decalcified histology and quantitative backscattered electron imaging (qBEI) to compare the dental phenotypes of Src(-/-), oc/oc and Hyp(-/0) mice, which serve as models for OP, OPR and hypophosphatemic rickets, respectively. While both, Src(-/-) and oc/oc mice, were characterized by defects of molar root formation, only oc/oc mice displayed a severe defect of dentin mineralization, similar to Hyp(-/0) mice. Most importantly, while enamel thickness was not affected in either mouse model, the calcium content within the enamel phase was significantly reduced in oc/oc, but not in Src(-/-) or Hyp(-/0) mice. Taken together, these data demonstrate that dentin and enamel mineralization are differentially affected in Src(-/-) and oc/oc mice. Moreover, since defects of dental mineralization may trigger premature tooth decay and thereby osteomyelitis of the jaw, they further underscore the importance of discriminating between OP and OPR in the respective individuals.

Canonical Wnt signaling inhibits osteoclastogenesis independent of osteoprotegerin

Canonical Wnt signaling through Fzd8 and β-catenin negatively regulates osteoclast differentiation and reduces bone resorption independent of osteoprotegerin.

Although Wnt signaling is considered a key regulatory pathway for bone formation, inactivation of β-catenin in osteoblasts does not affect their activity but rather causes increased osteoclastogenesis due to insufficient production of osteoprotegerin (Opg). By monitoring the expression pattern of all known genes encoding Wnt receptors in mouse tissues and bone cells we identified Frizzled 8 (Fzd8) as a candidate regulator of bone remodeling. Fzd8-deficient mice displayed osteopenia with normal bone formation and increased osteoclastogenesis, but this phenotype was not associated with impaired Wnt signaling or Opg production by osteoblasts. The deduced direct negative influence of canonical Wnt signaling on osteoclastogenesis was confirmed in vitro and through the generation of mice lacking β-catenin in the osteoclast lineage. Here, we observed increased bone resorption despite normal Opg production and a resistance to the anti-osteoclastogenic effect of Wnt3a. These results demonstrate that Fzd8 and β-catenin negatively regulate osteoclast differentiation independent of osteoblasts and that canonical Wnt signaling controls bone resorption by two different mechanisms.

Impaired bone mineralization accompanied by low vitamin D and secondary hyperparathyroidism in patients with femoral neck fracture

SUMMARY

Although it is well established that a decrease in bone mass increases the risk of osteoporotic fractures, the proportion of fractures attributable to areal bone mineral density (BMD) is rather low. Here, we have identified bone mineralization defects together with low serum 25-hydroxyvitamin D (25-(OH) D) levels as additional factors associated with femoral neck fractures.

INTRODUCTION

Osteoporotic fractures of the femoral neck are associated with increased morbidity and mortality. Although it is well established that a decrease in bone mass increases the risk of osteoporotic fractures, the proportion of fractures attributable to areal BMD is rather low. To identify possible additional factors influencing femur neck fragility, we analyzed patients with femoral neck fracture.

METHODS

We performed a detailed clinical and histomorphometrical evaluation on 103 patients with femoral neck fracture including dual-energy X-ray absorptiometry, laboratory parameters, and histomorphometric and bone mineral density distribution (BMDD) analyses of undecalcified processed biopsies of the femoral head and set them in direct comparison to skeletal healthy control individuals.

RESULTS

Patients with femoral neck fracture displayed significantly lower serum 25-(OH) D levels and increased serum parathyroid hormone (PTH) compared to controls. Histomorphometric analysis revealed not only a decreased bone volume and trabecular thickness in the biopsies of the patients, but also a significant increase of osteoid indices. BMDD analysis showed increased heterogeneity of mineralization in patients with femoral neck fracture. Moreover, patients with femoral neck fracture and serum 25-(OH) D levels below 12 μg/l displayed significantly thinner trabecular bone.

CONCLUSION

Taken together, our data suggest that impaired bone mineralization accompanied by low serum 25-(OH) D levels is of major importance in the etiology of femoral neck fractures. Therefore, balancing serum 25-(OH) D levels and thereby normalizing PTH serum levels may counteract pronounced mineralization defects and might decrease the incidence of femoral neck fractures.

Osteocalcin is associated with testosterone in the general population and selected patients with bone disorders

Research in the last decade has revealed that bone is not only a target tissue for numerous circulating hormones but functions as an endocrine organ itself. As a recent study demonstrated a stimulatory effect of the osteoblast-derived hormone osteocalcin (OCN) on testosterone production in mice, we investigated whether such an association can be replicated in humans. We used data from 1338 men (25-86 years) in the population-based epidemiological Study of Health in Pomerania and from 110 male outpatients with bone disorders (18-85 years) for the study. We analysed cross-sectional associations between OCN and total testosterone serum concentrations (TT), as well as associations between further markers of bone turnover [bone-specific alkaline phosphatase (BAP), serum C-terminal telopeptides of Type I collagen (CTX), urinary deoxypyridinoline] and TT using ordinary least square (OLS) regression models. Multivariable OLS models revealed a positive association between OCN and TT in the population-based (β coefficients for a one standard deviation increase, 0.590; standard error (SE), 0.175; p-value, <0.01) and patient-based (β coefficient, 0.575; SE, 0.132; p-value, <0.01) samples even after adjustment for age and body mass index (both samples), and time of blood sampling (population-based sample only). Furthermore, we observed positive associations between BAP and TT (β coefficient, 0.403; SE, 0.170; p-value, 0.02) as well as between CTX and TT (β coefficient, 0.733; SE, 0.172; p-value, <0.01) in men from the general population. The present investigation shows that OCN is associated with TT in the general population and in patients with bone disorders, and may thus indicate general male health status. Additional longitudinal observational studies are warranted to confirm our findings and future experimental research is necessary to elucidate potential mechanisms underlying the observed associations.

Increased expression of transthyretin in leptin-deficient ob/ob mice is not causative for their major phenotypic abnormalities

The hormone leptin is a critical regulator of adipogenesis and energy metabolism. Similarly, leptin-deficient ob/ob mice display various metabolic abnormalities, including not only obesity and insulin resistance, but also hypogonadism and high bone mass. By genome-wide expression analysis using hypothalamus RNA from wild-type and ob/ob mice, we observed the increased expression of the gene for transthyretin (Ttr) in the latter, as confirmed by quantitative real-time-polymerase chain reaction. Because Ttr encodes a carrier protein for retinol transport, and because we further found increased retinol levels in the serum of ob/ob mice, we investigated whether the additional absence of Ttr would influence the ob/ob phenotype. It was found that Ttr-deficient ob/ob mice were indistinguishable from ob/ob littermates in terms of body weight, as well as serum glucose, insulin and cholesterol levels. Although all of these parameters were identical to wild-type controls in Ttr-deficient mice, we found that the sole deletion of Ttr caused a significant increase of trabecular bone mass, bone marrow adiposity and mean adipocyte area in white adipose tissue. Interestingly, all these latter parameters were highest in Ttr-deficient ob/ob mice, and only in these mice did we observe a full penetrance of liver steatosis at 24 weeks of age. Taken together, our data demonstrate that the increased expression of Ttr in ob/ob mice does not cause (but rather attenuates) their phenotypic abnormalities.

Pharmacological estrogen administration causes a FSH-independent osteo-anabolic effect requiring ER alpha in osteoblasts

Postmenopausal osteoporosis is characterized by declining estrogen levels, and estrogen replacement therapy has been proven beneficial for preventing bone loss in affected women. While the physiological functions of estrogen in bone, primarily the inhibition of bone resorption, have been studied extensively, the effects of pharmacological estrogen administration are still poorly characterized. Since elevated levels of follicle-stimulating hormone (FSH) have been suggested to be involved in postmenopausal bone loss, we investigated whether the skeletal response to pharmacological estrogen administration is mediated in a FSH-dependent manner. Therefore, we treated wildtype and FSHβ-deficicent (Fshb^−/−) mice with estrogen for 4 weeks and subsequently analyzed their skeletal phenotype. Here we observed that estrogen treatment resulted in a significant increase of trabecular and cortical bone mass in both, wildtype and Fshb^−/− mice. Unexpectedly, this FSH-independent pharmacological effect of estrogen was not caused by influencing bone resorption, but primarily by increasing bone formation. To understand the cellular and molecular nature of this osteo-anabolic effect we next administered estrogen to mouse models carrying cell specific mutant alleles of the estrogen receptor alpha (ERα). Here we found that the response to pharmacological estrogen administration was not affected by ERα inactivation in osteoclasts, while it was blunted in mice lacking the ERα in osteoblasts or in mice carrying a mutant ERα incapable of DNA binding. Taken together, our findings reveal a previously unknown osteo-anabolic effect of pharmacological estrogen administration, which is independent of FSH and requires DNA-binding of ERα in osteoblasts.

Accelerated growth plate mineralization and foreshortened proximal limb bones in fetuin-A knockout mice

The plasma protein fetuin-A/alpha2-HS-glycoprotein (genetic symbol Ahsg) is a systemic inhibitor of extraskeletal mineralization, which is best underscored by the excessive mineral deposition found in various tissues of fetuin-A deficient mice on the calcification-prone genetic background DBA/2. Fetuin-A is known to accumulate in the bone matrix thus an effect of fetuin-A on skeletal mineralization is expected. We examined the bones of fetuin-A deficient mice maintained on a C57BL/6 genetic background to avoid bone disease secondary to renal calcification. Here, we show that fetuin-A deficient mice display normal trabecular bone mass in the spine, but increased cortical thickness in the femur. Bone material properties, as well as mineral and collagen characteristics of cortical bone were unaffected by the absence of fetuin-A. In contrast, the long bones especially proximal limb bones were severely stunted in fetuin-A deficient mice compared to wildtype littermates, resulting in increased biomechanical stability of fetuin-A deficient femora in three-point-bending tests. Elevated backscattered electron signal intensities reflected an increased mineral content in the growth plates of fetuin-A deficient long bones, corroborating its physiological role as an inhibitor of excessive mineralization in the growth plate cartilage matrix - a site of vigorous physiological mineralization. We show that in the case of fetuin-A deficiency, active mineralization inhibition is a necessity for proper long bone growth.

Divergent resorbability and effects on osteoclast formation of commonly used bone substitutes in a human in vitro-assay

Bioactive bone substitute materials are a valuable alternative to autologous bone transplantations in the repair of skeletal defects. However, clinical studies have reported varying success rates for many commonly used biomaterials. While osteoblasts have traditionally been regarded as key players mediating osseointegration, increasing evidence suggests that bone-resorbing osteoclasts are of crucial importance for the longevity of applied biomaterials. As no standardized data on the resorbability of biomaterials exists, we applied an in vitro-assay to compare ten commonly used bone substitutes. Human peripheral blood mononuclear cells (PBMCs) were differentiated into osteoclasts in the co-presence of dentin chips and biomaterials or dentin alone (control) for a period of 28 days. Osteoclast maturation was monitored on day 0 and 14 by light microscopy, and material-dependent changes in extracellular pH were assessed twice weekly. Mature osteoclasts were quantified using TRAP stainings on day 28 and their resorptive activity was determined on dentin (toluidin blue staining) and biomaterials (scanning electron microscopy, SEM). The analyzed biomaterials caused specific changes in the pH, which were correlated with osteoclast multinuclearity (r = 0.942; p = 0.034) and activity on biomaterials (r = 0.594; p = 0.041). Perossal led to a significant reduction of pH, nuclei per osteoclast and dentin resorption, whereas Tutogen bovine and Tutobone human strikingly increased all three parameters. Furthermore, natural biomaterials were resorbed more rapidly than synthetic biomaterials leading to differential relative resorption coefficients, which indicate whether bone substitutes lead to a balanced resorption or preferential resorption of either the biomaterial or the surrounding bone. Taken together, this study for the first time compares the effects of widely used biomaterials on osteoclast formation and resorbability in an unbiased approach that may now aid in improving the preclinical evaluation of bone substitute materials.

The transcriptional profile of mesenchymal stem cell populations in primary osteoporosis is distinct and shows overexpression of osteogenic inhibitors

Primary osteoporosis is an age-related disease characterized by an imbalance in bone homeostasis. While the resorptive aspect of the disease has been studied intensely, less is known about the anabolic part of the syndrome or presumptive deficiencies in bone regeneration. Multipotent mesenchymal stem cells (MSC) are the primary source of osteogenic regeneration. In the present study we aimed to unravel whether MSC biology is directly involved in the pathophysiology of the disease and therefore performed microarray analyses of hMSC of elderly patients (79–94 years old) suffering from osteoporosis (hMSC-OP). In comparison to age-matched controls we detected profound changes in the transcriptome in hMSC-OP, e.g. enhanced mRNA expression of known osteoporosis-associated genes (LRP5, RUNX2, COL1A1) and of genes involved in osteoclastogenesis (CSF1, PTH1R), but most notably of genes coding for inhibitors of WNT and BMP signaling, such as Sclerostin and MAB21L2. These candidate genes indicate intrinsic deficiencies in self-renewal and differentiation potential in osteoporotic stem cells. We also compared both hMSC-OP and non-osteoporotic hMSC-old of elderly donors to hMSC of ∼30 years younger donors and found that the transcriptional changes acquired between the sixth and the ninth decade of life differed widely between osteoporotic and non-osteoporotic stem cells. In addition, we compared the osteoporotic transcriptome to long term-cultivated, senescent hMSC and detected some signs for pre-senescence in hMSC-OP.

Our results suggest that in primary osteoporosis the transcriptomes of hMSC populations show distinct signatures and little overlap with non-osteoporotic aging, although we detected some hints for senescence-associated changes. While there are remarkable inter-individual variations as expected for polygenetic diseases, we could identify many susceptibility genes for osteoporosis known from genetic studies. We also found new candidates, e.g. MAB21L2, a novel repressor of BMP-induced transcription. Such transcriptional changes may reflect epigenetic changes, which are part of a specific osteoporosis-associated aging process.

Low turnover osteoporosis in sheep induced by hypothalamic-pituitary disconnection

The hypothalamus is of critical importance in regulating bone remodeling. This is underscored by the fact that intracerebroventricular-application of leptin in ewe leads to osteopenia. As a large animal model of osteoporosis, this approach has some limitations, such as high technical expenditure and running costs. Therefore we asked if a surgical ablation of the leptin signaling axis would have the same effects and would thereby be a more useful model. We analyzed the bone phenotype of ewe after surgical hypothalamo-pituitary disconnection (HPD + OVX) as compared to control ewe (OVX) after 3 and 12 months. Analyses included histomorphometric characterization, micro-CT and measurement of bone turnover parameters. Already 3 months after HPD we found osteopenic ewe with a significantly decreased bone formation (69%) and osteoclast activity (49%). After a period of 12 months the HPD group additionally developed an (preclinical) osteoporosis with significant reduction (33%) of femoral cortical thickness, as compared to controls (OVX). Taken together, HPD leads after 12 month to osteoporosis with a reduction in both trabecular and cortical bone caused by a low bone turnover situation, with reduced osteoblast and osteoclast activity, as compared to controls (OVX). The HPD-sheep is a suitable large animal model of osteoporosis. Furthermore our results indicate that an intact hypothalamo-pituitary axis is required for activation of bone turnover.

WNT5A is induced by inflammatory mediators in bone marrow stromal cells and regulates cytokine and chemokine production

WNT5A has recently been implicated in inflammatory processes, but its role as a bone marrow stromal cell (BMSC)-derived mediator of joint inflammation in arthritis is unclear. Here, we investigated whether inflammatory stimuli induce WNT5A in BMSC to control inflammatory responses. WNT5A levels were determined in human BMSC after stimulation with lipopolysaccharide (LPS) or tumor necrosis factor α (TNF-α,) and in synovial cells and tissue of patients with rheumatoid arthritis (RA) and human TNF-α transgenic (hTNFtg) mice. A microarray analysis of WNT5A-treated murine osteoblasts was performed using Affymetrix gene chips. The regulation of cytokine/chemokine expression was confirmed by qPCR, ELISA, and Luminex technology in BMSC after stimulation with WNT5A or WNT5A knockdown. Relevant signaling pathways were identified using specific inhibitors. Migration of MACS-purified T lymphocytes and monocytes was assessed using the FluoroBlok system. WNT5A expression was increased threefold in BMSC after stimulation with LPS or TNF-α. Synovial fibroblasts from patients with RA showed a twofold increase of WNT5A expression compared with control cells, and its expression was highly induced in the synovial tissue of patients with RA and hTNFtg mice. Microarray analysis of WNT5A-treated osteoblasts identified cytokines and chemokines as targets. The induction of IL-1β, IL-6, CCL2, CCL5, CXCL1, and CXCL5 by WNT5A was confirmed in BMSC and depended on the activation of the NF-κB, mitogen-activated protein (MAPK), and Akt pathways. Accordingly, knockdown of WNT5A markedly reduced the basal and LPS-induced cytokine/chemokine production. Finally, migration of monocytes and T cells toward the supernatant of WNT5A-treated BMSC was increased by 25% and 20%, respectively. This study underlines the critical role of BMSC-derived WNT5A in the regulation of inflammatory processes and suggests its participation in the pathogenesis of RA.

Estradiol increases hematopoietic stem and progenitor cells independent of its actions on bone

Hematopoietic stem and progenitor cells reside in vascular and endosteal niches in the bone marrow. Factors affecting bone remodeling were reported to influence numbers and mobilization of hematopoietic stem cells. We therefore analyzed the effects of estradiol acting anabolic on bone integrity. Here we observe that estradiol increases progenitor cell numbers in the vascular but not in the endosteal compartment independent of its estrogen receptor α-dependent anabolic bone effects. Hematopoietic progenitors capable of reconstituting lethally irradiated mice are increased by enhanced cell cycle entry, leading to a diminished long-term reconstitution potential after serial transplantation. We demonstrate that estradiol action on stromal cells potently favors hematopoietic progenitor/stem cell frequency accompanied by enhanced expression of cell adhesion molecules. Finally, estradiol treatment enhances retention of hematopoietic stem cells in the vascular niche of the bone marrow. We describe for the first time the mechanism of estrogen action on hematopoietic stem and progenitor cells.

Mice over-expressing salmon calcitonin have strongly attenuated osteoarthritic histopathological changes after destabilization of the medial meniscus

OBJECTIVE

Calcitonin is well-known for its inhibitory actions on bone-resorbing osteoclasts and recently potential beneficial effects on cartilage were shown. We investigated effects of salmon calcitonin (sCT) on the articular cartilage and bone, after destabilization of the medial meniscus (DMM) in normal and sCT over-expressing mice.

DESIGN

Bone phenotype of transgenic (TG) C57Bl/6 mice over-expressing sCT at 6 months and 12 months was investigated by (1) serum osteocalcin and urinary deoxypyridinoline and (2) dynamic and normal histomorphometry of vertebrae bodies. In subsequent evaluation of cartilage and subchondral bone changes, 44 10-week old TG or wild-type (WT) mice were randomized into four groups and subjected to DMM or sham-operations. After 7 weeks animals were sacrificed, and knee joints were isolated for histological analysis.

RESULTS

Trabecular bone volume (BV/TV) increased 150% after 6 months and 300% after 12 months in sCT-expressing mice when compared to WT controls (P<0.05). Osteoblast number, bone formation rate and osteocalcin measurements were not affected in TG mice over-expressing sCT. In WT animals, a 5-fold increase in the quantitative erosion index was observed after DMM, and the semi-quantitative OARSI score showed over 400% (P<0.001) increase, compared to sham-operated WT mice. DMM-operated TG mice were protected against cartilage erosion and showed a 65% and 64% (P<0.001) reduction, respectively, for the two histopathological evaluation methods.

CONCLUSIONS

sCT over-expressing mice had higher bone volume, and were protected against cartilage erosion. These data suggest that increased levels of sCT may hamper the pathogenesis of osteoarthritis (OA). However more studies are necessary to confirm these preliminary results.

The role of apolipoprotein E in bone metabolism

Apolipoprotein E (apoE) is a major structural apolipoprotein of several lipoprotein classes. Over the last 13 years, numerous studies have focused on the question whether human apoE affects bone phenotypes and, more recently, whether apoE regulates bone metabolism in mice. Here, we first provide a brief background introduction into the structure, established physiological and pathophysiological functions of apoE, and will then discuss the new aspects of the emerging role of apoE in bone.

Transgenic over-expression of interleukin-33 in osteoblasts results in decreased osteoclastogenesis

Interleukin-33 (IL-33) is the most recently identified member of the IL-1 family of cytokines, which is primarily known for its proinflammatory functions. We have previously reported that IL-33 is expressed by bone-forming osteoblasts, and that administration of recombinant IL-33 to bone marrow cultures inhibits their differentiation into bone-resorbing osteoclasts. Likewise, while the inhibitory effect of IL-33 on osteoclast differentiation was fully abolished in cultures lacking the IL-33 receptor ST2, mice lacking ST2 displayed low bone mass caused by increased osteoclastogenesis. Although these data suggested a physiological role of IL-33 as an inhibitor of bone resorption, direct in vivo evidence supporting such a function was still missing. Here we describe the generation and bone histomorphometric analysis of a transgenic mouse model (Col1a1-Il33) over-expressing IL-33 specifically in osteoblasts. While we did not observe differences in osteoblast number and bone formation between wildtype and Col1a1-Il33 mice, the number of osteoclasts was significantly reduced compared to wildtype littermates in two independent transgenic lines. Since we did not observe quantitative differences in the populations of eosinophils, neutrophils, basophils or M2-macrophages from the bone marrow of wildtype and Col1a1-Il33 mice, our data demonstrate that an inhibition of osteoclastogenesis is one of the major physiological functions of IL-33, at least in mice.

p65-Dependent production of interleukin-1β by osteolytic prostate cancer cells causes an induction of chemokine expression in osteoblasts

Skeletal metastases are a frequent complication of prostate, breast and lung cancer, and the interactions of tumor cells with bone-forming osteoblasts and bone-resorbing osteoclasts have been suggested to play critical roles in disease progression. We have previously shown that treatment of primary murine osteoblasts with conditioned medium of the human osteolytic prostate cancer cell line PC-3 results in a rapid induction of chemokine expression, thereby providing further evidence for a molecular crosstalk between bone and tumor cells. The aim of our current study was to identify PC-3-derived molecules mediating this effect. Using Affymetrix Gene Chip hybridization followed by qRT-PCR we were able to confirm that the expression of chemokine-encoding genes is markedly induced in human primary osteoblasts following incubation with PC-3-conditioned medium. Since this induction was significantly affected upon alteration of p65-levels in PC-3 cells, we performed a second genome-wide expression analysis to identify p65-regulated cytokines, which were then tested for their ability to induce chemokine expression. Here we observed that interleukin-1β (IL-1B) did not only increase the expression of chemokines in osteoblasts, but also the phosphorylation of p65 and thereby its own expression. Since immunohistochemistry on bone biopsy sections from prostate cancer metastases demonstrated IL-1B expression in both, tumor cells and osteoblasts, our data suggest that IL-1B is one of the relevant cytokines involved in the skeletal complications of cancer metastases.

Skeletal mineralization defects in adult hypophosphatasia--a clinical and histological analysis

Histomorphometry and quantitative backscattered electron microscopy of iliac crest biopsies from patients with adult hypophosphatasia not only confirmed the expected enrichment of non-mineralized osteoid, but also demonstrated an altered trabecular microarchitecture, an increased number of osteoblasts, and an impaired calcium distribution within the mineralized bone matrix.

INTRODUCTION

Adult hypophosphatasia is an inherited disorder of bone metabolism caused by inactivating mutations of the ALPL gene, encoding tissue non-specific alkaline phosphatase. While it is commonly accepted that the increased fracture risk of the patients is the consequence of osteomalacia, there are only few studies describing a complete histomorphometric analysis of bone biopsies from affected individuals. Therefore, we analyzed iliac crest biopsies from eight patients and set them in direct comparison to biopsies from healthy donors or from individuals with other types of osteomalacia.

METHODS

Histomorphometric analysis was performed on non-decalcified sections stained either after von Kossa/van Gieson or with toluidine blue. Bone mineral density distribution was quantified by backscattered electron microscopy.

RESULTS

Besides the well-documented enrichment of non-mineralized bone matrix in individuals suffering from adult hypophosphatasia, our histomorphometric analysis revealed alterations of the trabecular microarchitecture and an increased number of osteoblasts compared to healthy controls or to individuals with other types of osteomalacia. Moreover, the analysis of the mineralized bone matrix revealed significantly decreased calcium content in patients with adult hypophosphatasia.

CONCLUSIONS

Taken together, our data show that adult hypophosphatasia does not solely result in an enrichment of osteoid, but also in a considerable degradation of bone quality, which might contribute to the increased fracture risk of the affected individuals.

Interleukin-33 is expressed in differentiated osteoblasts and blocks osteoclast formation from bone marrow precursor cells

Since the hematopoetic system is located within the bone marrow, it is not surprising that recent evidence has demonstrated the existence of molecular interactions between bone and immune cells. While interleukin 1 (IL-1) and IL-18, two cytokines of the IL-1 family, have been shown to regulate differentiation and activity of bone cells, the role of IL-33, another IL-1 family member, has not been addressed yet. Since we observed that the expression of IL-33 increases during osteoblast differentiation, we analyzed its possible influence on bone formation and observed that IL-33 did not affect matrix mineralization but enhanced the expression of Tnfsf11, the gene encoding RANKL. This finding led us to analyze the skeletal phenotype of Il1rl1-deficient mice, which lack the IL-33 receptor ST2. Unexpectedly, these mice displayed normal bone formation but increased bone resorption, thereby resulting in low trabecular bone mass. Since this finding suggested a negative influence of IL-33 on osteoclastogenesis, we next analyzed osteoclast differentiation from bone marrow precursor cells and observed that IL-33 completely abolished the generation of TRACP(+) multinucleated osteoclasts, even in the presence of RANKL and macrophage colony-stimulating factor (M-CSF). Although our molecular studies revealed that IL-33 treatment of bone marrow cells caused a shift toward other hematopoetic lineages, we further observed a direct negative influence of IL-33 on the osteoclastogenic differentiation of RAW264.7 macrophages, where IL-33 repressed the expression of Nfatc1, which encodes one of the key transciption factors of osteoclast differentiation. Taken together, these findings have uncovered a previously unknown function of IL-33 as an inhibitor of bone resorption.