Midkine-deficiency increases the anabolic response of cortical bone to mechanical loading

The adaptive response of bone to load is dependent on molecular factors, including growth factor signaling, which is involved in the regulation of proliferation, differentiation and function of osteoblasts and osteoclasts. Based on a recent study, which has shown that the deficiency of growth factor midkine (Mdk) in mice at 12 and 18 months of age resulted in increased trabecular bone formation, we hypothesized that mechanically-induced bone remodeling may, at least in part, be dependent on Mdk expression. To investigate this, we loaded the ulnae of Mdk-deficient mice and appropriate wild-type mice at the age of 12 months using the in vivo ulna loading model. Histomorphometric quantification of the periosteal bone demonstrated an increased mineralizing surface, mineral apposition rate, and bone formation rate in ulnae of Mdk-deficient mice compared to wild-type mice in response to loading. Because Mdk has been shown to bind to a complex of receptor-type protein tyrosine phosphatase zeta (Ptprz) and low density lipoprotein receptor-related protein-6 (Lrp-6) together with the α4β1- and α6β1-integrins, we performed in vitro studies using osteoblastic cells, transiently over-expressing Mdk, Wnt-3a, and Ptprz to evaluate whether Mdk has a role in regulating bone formation by modulating Wnt signaling. We observed a negative effect of Mdk on Wnt signaling, the extent of which appeared to be dependent on Ptprz expression. Moreover, we performed in vitro loading studies with osteoblasts treated with recombinant Mdk and observed a negative effect on the expression of Wnt target genes, which play a critical role in osteoblast proliferation. In summary, our data demonstrate that Mdk-deficiency in mice has an anabolic effect on mechanically induced cortical bone formation. This could be due to an improved osteoblast function based on an enhancement of β-catenin-dependent Wnt signaling by both Mdk-deficiency and mechanical loading.

Control of bone formation by the serpentine receptor Frizzled-9

Although Wnt signaling in osteoblasts is of critical importance for the regulation of bone remodeling, it is not yet known which specific Wnt receptors of the Frizzled family are functionally relevant in this process. In this paper, we show that Fzd9 is induced upon osteoblast differentiation and that Fzd9(-/-) mice display low bone mass caused by impaired bone formation. Our analysis of Fzd9(-/-) primary osteoblasts demonstrated defects in matrix mineralization in spite of normal expression of established differentiation markers. In contrast, we observed a reduced expression of chemokines and interferon-regulated genes in Fzd9(-/-) osteoblasts. We also identified the ubiquitin-like modifier Isg15 as one potential downstream mediator of Fzd9 in these cells. Importantly, our molecular analysis further revealed that canonical Wnt signaling is not impaired in the absence of Fzd9, thus explaining the absence of a bone resorption phenotype. Collectively, our results reveal a previously unknown function of Fzd9 in osteoblasts, a finding that may have therapeutic implications for bone loss disorders.

Identification of molecular markers for articular cartilage

OBJECTIVE

The aim of the current study was to identify molecular markers for articular cartilage (AC) that can be used as tools for the quality control of tissue engineered (TE) cartilage.

DESIGN

A genome-wide expression analysis was performed using RNA isolated from articular and growth plate (GP) cartilage, both extracted from the knee joints of 6 weeks old minipigs. After confirming the specific expression for selected genes by RT-PCR, these were used as molecular markers for the quality control of TE cartilage.

RESULTS

Albeit several known chondrocyte markers were expressed to a similar extent in articular and GP cartilage, our genome-wide expression analysis led us to identify genes being selectively expressed in either GP or articular chondrocytes. These findings led us to perform a RT-PCR expression analysis for the corresponding genes to demonstrate the absence of GP-specific markers in TE cartilage, while common or AC markers were expressed.

CONCLUSIONS

Taken together, these results provide important novel insights into chondrocyte biology in general and AC in particular. In addition, it is reasonable to speculate, that some of the identified genes play distinct roles in the regulation of articular chondrocyte differentiation and/or function, thereby raising the possibility that they may serve as targets for non-operative therapies of osteoarthritis (OA).

Fracture healing in mice under controlled rigid and flexible conditions using an adjustable external fixator

Mice are increasingly used to investigate mechanobiology in fracture healing. The need exists for standardized models allowing for adjustment of the mechanical conditions in the fracture gap. We introduced such a model using rigid and flexible external fixators with considerably different stiffness (axial stiffnesses of 18.1 and 0.82 N/mm, respectively). Both fixators were used to stabilize a 0.5 mm osteotomy gap in the femur of C57BL/6 mice (each n = 8). Three-point bending tests, µCT, and histomorphometry demonstrated a different healing pattern after 21 days. Both fixations induced callus formation with a mixture of intramembranous and enchondral ossification. Under flexible conditions, the bending stiffness of the callus was significantly reduced, and a larger but qualitatively inferior callus with a significantly lower fraction of bone but a higher fraction of cartilage and soft tissue was formed. Monitoring of the animal movement and the ground reaction forces demonstrated physiological loading with no significant differences between the groups, suggesting that the differences in healing were not based on a different loading behavior. In summary, flexible external fracture fixation of the mouse femur led to delayed fracture healing in comparison to a more rigid situation.

Apolipoprotein E-dependent inverse regulation of vertebral bone and adipose tissue mass in C57Bl/6 mice: modulation by diet-induced obesity

The long prevailing view that obesity is generally associated with beneficial effects on the skeleton has recently been challenged. Apolipoprotein E (apoE) is known to influence both adipose tissue and bone. The goal of the current study was to examine the impact of apoE on the development of fat mass and bone mass in mice under conditions of diet-induced obesity (DIO). Four week-old male C57BL/6 (WT) and apoE-deficient (apoE(-/-)) mice received a control or a diabetogenic high-fat diet (HFD) for 16 weeks. The control-fed apoE(-/-) animals displayed less total fat mass and higher lumbar trabecular bone volume (BV/TV) than WT controls. When stressed with HFD to induce obesity, apoE(-/-) mice had a lower body weight, lower serum glucose, insulin and leptin levels and accumulated less white adipose tissue mass at all sites including bone marrow. While WT animals showed no significant change in BV/TV and bone formation rate (BFR), apoE deficiency led to a decrease of BV/TV and BFR when stressed with HFD. Bone resorption parameters were not affected by HFD in either genotype. Taken together, under normal dietary conditions, apoE-deficient mice acquire less fat mass and more bone mass than WT littermates. When stressed with HFD to develop DIO, the difference of total body fat mass becomes larger and the difference of bone mass smaller between the genotypes. We conclude that apoE is involved in an inverse regulation of bone mass and fat mass in growing mice and that this effect is modulated by diet-induced obesity.

Fra-2/AP-1 controls bone formation by regulating osteoblast differentiation and collagen production

The activator protein-1 (AP-1) transcription factor complex, in particular the Fos proteins, is an important regulator of bone homeostasis. Fra-2 (Fosl2), a Fos-related protein of the AP-1 family, is expressed in bone cells, and newborn mice lacking Fra-2 exhibit defects in chondrocytes and osteoclasts. Here we show that Fra-2-deficient osteoblasts display a differentiation defect both in vivo and in vitro. Moreover, Fra-2-overexpressing mice are osteosclerotic because of increased differentiation of osteoblasts, which appears to be cell autonomous. Importantly, the osteoblast-specific osteocalcin (Oc) gene and collagen1α2 (col1α2) are transcriptional targets of Fra-2 in both murine and human bone cells. In addition, Fra-2, Oc, and col1 are expressed in stromal cells of human chondroblastic and osteoblastic osteosarcomas (Os's) as well as during osteoblast differentiation of human Os cell lines. These findings reveal a novel function of Fra-2/AP-1 as a positive regulator of bone and matrix formation in mice and humans.

Increased trabecular bone formation in mice lacking the growth factor midkine

Midkine (Mdk) and pleiotrophin (Ptn) comprise a family of heparin-binding growth factors known primarily for their effects on neuronal cells. Since transgenic mice overexpressing Ptn have been reported to display increased bone density, we have previously analyzed Ptn-deficient mice but failed to detect any abnormality of skeletal development and remodeling. Together with the finding that Mdk expression increases in the course of primary osteoblast differentiation, we reasoned that Mdk, rather than Ptn, could play a physiologic role in bone formation. Here, we show that Mdk-deficient mice display an increased trabecular bone volume at 12 and 18 months of age, accompanied by cortical porosity. Histomorphometric quantification demonstrated an increased bone-formation rate compared with wild-type littermates, whereas bone resorption was differentially affected in trabecular and cortical bone of Mdk-deficient mice. To understand the effect of Mdk on bone formation at the molecular level, we performed a genome-wide expression analysis of primary osteoblasts and identified Ank and Enpp1 as Mdk-induced genes whose decreased expression in Mdk-deficient osteoblasts may explain, at least in part, the observed skeletal phenotype. Finally, we performed ovariectomy and observed bone loss only in wild-type but not in Mdk-deficient animals. Taken together, our data demonstrate that Mdk deficiency, at least in mice, results in an increased trabecular bone formation, thereby raising the possibility that Mdk-specific antagonists might prove beneficial in osteoporosis therapy.

The clock genes Period 2 and Cryptochrome 2 differentially balance bone formation

Background

Clock genes and their protein products regulate circadian rhythms in mammals but have also been implicated in various physiological processes, including bone formation. Osteoblasts build new mineralized bone whereas osteoclasts degrade it thereby balancing bone formation. To evaluate the contribution of clock components in this process, we investigated mice mutant in clock genes for a bone volume phenotype.

Methodology/Principal Findings

We found that Per2^Brdm1 mutant mice as well as mice lacking Cry2^−/− displayed significantly increased bone volume at 12 weeks of age, when bone turnover is high. Per2^Brdm1 mutant mice showed alterations in parameters specific for osteoblasts whereas mice lacking Cry2^−/− displayed changes in osteoclast specific parameters. Interestingly, inactivation of both Per2 and Cry2 genes leads to normal bone volume as observed in wild type animals. Importantly, osteoclast parameters affected due to the lack of Cry2, remained at the level seen in the Cry2^−/− mutants despite the simultaneous inactivation of Per2.

Conclusions/Significance

This indicates that Cry2 and Per2 affect distinct pathways in the regulation of bone volume with Cry2 influencing mostly the osteoclastic cellular component of bone and Per2 acting on osteoblast parameters.

Glucocorticoids suppress bone formation by attenuating osteoblast differentiation via the monomeric glucocorticoid receptor

Development of osteoporosis severely complicates long-term glucocorticoid (GC) therapy. Using a Cre-transgenic mouse line, we now demonstrate that GCs are unable to repress bone formation in the absence of glucocorticoid receptor (GR) expression in osteoblasts as they become refractory to hormone-induced apoptosis, inhibition of proliferation, and differentiation. In contrast, GC treatment still reduces bone formation in mice carrying a mutation that only disrupts GR dimerization, resulting in bone loss in vivo, enhanced apoptosis, and suppressed differentiation in vitro. The inhibitory GC effects on osteoblasts can be explained by a mechanism involving suppression of cytokines, such as interleukin 11, via interaction of the monomeric GR with AP-1, but not NF-kappaB. Thus, GCs inhibit cytokines independent of GR dimerization and thereby attenuate osteoblast differentiation, which accounts, in part, for bone loss during GC therapy.

BMP-7-induced ectopic bone formation and fracture healing is impaired by systemic NSAID application in C57BL/6-mice

Nonsteroidal antiinflammatory drugs (NSAIDs) are known to potentially impair the fracture healing process. The aim of the present study was to determine if the impairment of bone healing by systemic NSAID application is, at least in part, due to an interaction of NSAIDs with the bone anabolic BMP-7 pathway. Therefore, we first analyzed fracture healing in control and diclofenac-treated mice, where we not only found a significant impairment of fracture healing due to diclofenac treatment as assessed by biomechanical testing and microCT imaging, but also found high coexpression of bone morphogenetic protein-7 (BMP-7) and cyclooxygenase-2 (COX-2) within the fracture callus of both groups. To experimentally address the possible interaction between BMP-7 and COX-2, we then induced ectopic bone formation in control (n = 10) and diclofenac-treated mice (n = 10) by application of BMP-7 (recombinant human OP-1, rhOP-1) into the hamstring muscles. After 20 days of treatment, each ectopic bone nodule was analyzed by contact-radiography, microCT, histology, and histomorphometry. Diclofenac application decreased the trabecular number and bone mass in the ectopic bone nodules significantly due to reduced osteoblast number and activity. These data demonstrate that the bone anabolic effect of BMP-7 and fracture healing is impaired by diclofenac application, and suggest that the potential negative impact of NSAIDs on fracture healing is, at least in part, due to interference with BMP-7 signaling.

Negative regulation of bone formation by the transmembrane Wnt antagonist Kremen-2

Wnt signalling is a key pathway controlling bone formation in mice and humans. One of the regulators of this pathway is Dkk1, which antagonizes Wnt signalling through the formation of a ternary complex with the transmembrane receptors Krm1/2 and Lrp5/6, thereby blocking the induction of Wnt signalling by the latter ones. Here we show that Kremen-2 (Krm2) is predominantly expressed in bone, and that its osteoblast-specific over-expression in transgenic mice (Col1a1-Krm2) results in severe osteoporosis. Histomorphometric analysis revealed that osteoblast maturation and bone formation are disturbed in Col1a1-Krm2 mice, whereas bone resorption is increased. In line with these findings, primary osteoblasts derived from Col1a1-Krm2 mice display a cell-autonomous differentiation defect, impaired canonical Wnt signalling and decreased production of the osteoclast inhibitory factor Opg. To determine whether the observed effects of Krm2 on bone remodeling are physiologically relevant, we analyzed the skeletal phenotype of 24 weeks old Krm2-deficient mice and observed high bone mass caused by a more than three-fold increase in bone formation. Taken together, these data identify Krm2 as a regulator of bone remodeling and raise the possibility that antagonizing KRM2 might prove beneficial in patients with bone loss disorders.

Reorganization of the femoral cortex due to age-, sex-, and endoprosthetic-related effects emphasized by osteonal dimensions and remodeling

Cortical remodeling is of particular importance to femoral strength. However, osteonal morphology and its quantitative changes are still poorly understood. Therefore, osteons as the cortexes basic structural units should serve for histomorphometrical assessment of reorganization mechanisms. Total hip replacement (THR) cases as well as relevant controls were investigated to gain insight into changes within stress shielded as well as age- and sex-associated bone with emphasis on the underlying osseous cell interactions. Control (n = 23) and THR (lifetime Ø14 months, n = 8) femora were prepared to grinding specimen. Osteon density (On.Dn.), osteon area (On.Ar.), haversian canal area (H.Ar.), osteon mean wall thickness (MWT), and cortical porosity (Ct.Po.) were evaluated periosteal and endosteal. Osteonal activity (OA) was determined by mineralization differentiation of active and quiescent osteons. Sex- and age-related differences in osteonal remodeling were evident in controls. Females showed age-dependent increases of H.Ar. during concomitant changes of On.Ar. and MWT resulting in elevated Ct.Po. at the entire cortex. Males showed this occurrence only periosteal. Increased OA and therefore higher remodeling rates were found in females above 70 years. THR led to decreased On.Dn. at the entire cortex. Periosteal increased OA represented increased periprosthetic remodeling resulting in elevated H.Ar., which enhanced Ct.Po.. Massive losses of endosteal circular osteonal structures following THR are reflected in a spongy reorganized cortex leading to increased Ct.Po.. Imbalances in osteoblast-osteoclast interactions whose intensity is related to the respective cortex regions occur in both stress shielded and aging bone and may thus contribute to implant failure and fracture risk.

Preproenkephalin (Penk) is expressed in differentiated osteoblasts, and its deletion in Hyp mice partially rescues their bone mineralization defect

Although our understanding of the molecular mechanisms controlling osteoblast differentiation and function is steadily increasing, there are still many open questions, especially regarding the regulation of bone matrix mineralization. For instance, while there is hallmark evidence for the importance of the endopeptidase Phex, whose inactivation in Hyp mice or human patients causes X-linked hypophosphatemic rickets, it is still largely unknown how Phex controls bone mineralization since a physiological substrate for its endopeptidase activity has not been identified yet. Using a genome-wide expression analysis comparing primary calvarial osteoblasts, we have identified preproenkephalin (Penk) as a gene that is selectively expressed in mineralized cultures. Since a role of enkephalin in the regulation of bone remodeling has been suggested previously and since Leu-enkephalin is known to be cleaved by Phex, we analyzed whether Penk expression in osteoblasts is physiologically relevant. Through skeletal analysis of a Penk-deficient mouse model, we found that Penk expression is dispensable for bone development and remodeling since we could not detect any defect following nondecalcified bone histology and histomorphometry compared to wild-type littermates. When Penk was deleted in Phex-deficient Hyp mice, however, we observed a significant reduction of the osteoid enrichment at 24 weeks of age, whereas their disturbance of mineral homeostasis was not affected by the additional absence of the Penk gene. Taken together, our data provide the first in vivo analysis concerning the role of Penk in osteoblasts.

Osteolytic prostate cancer cells induce the expression of specific cytokines in bone-forming osteoblasts through a Stat3/5-dependent mechanism

Prostate cancer primarily metastasizes to bone, and the interaction of cancer cells with bone cells results in a local activation of bone formation and/or bone resorption. Since the cellular and molecular mechanisms underlying the development of these tumor-induced osteoblastic or osteolytic lesions are still poorly understood, we have compared the effects of two prostate cancer cell lines, osteoblastic MDA-PCa-2b cells and osteolytic PC-3 cells, on bone-forming osteoblasts. Using Affymetrix Gene Chip hybridization followed by qRT-PCR confirmation we were able to identify specific genes, including Smpd3 and Dmp1, whose expression is significantly reduced upon treatment with PC-3-conditioned medium. Moreover, we observed that PC-3-conditioned medium led to a marked induction of several cytokine genes, including Cxcl5, Cxcl12 and Tnfsf11, the latter one encoding for the osteoclast differentiation factor Rankl. Likewise, when we analyzed the effects of MDA-PCa-2b- and PC-3-conditioned medium on signal transduction in osteoblasts we did not only observe opposite effects on the canonical Wnt signalling pathway, but also a specific induction of Erk and Stat phosphorylation by PC-3-conditioned medium. Most importantly, the induction of Cxcl5, Cxcl12 and Tnfsf11 in osteoblasts by PC-3-conditioned medium was abrogated by the Stat3/5 inhibitor piceatannol, whereas the selective blockade of Stat1 and Erk activation had no effect. Together with the finding, that activated Stat3 in osteoblasts was detectable in bone biopsies from patients with osteolytic metastases, our data suggest that the Stat3/5-dependent activation of cytokine expression in osteoblasts may have a significant impact on cancer cell migration and proliferation, but also on osteoclast activation.

High bone turnover and accumulation of osteoid in patients with neurofibromatosis 1

Although it is known that neurofibromatosis 1 (NF1) patients suffer from vitamin D deficiency and display decreased bone mineral density (BMD), a systematic clinical and histomorphometrical analysis is absent. Our data demonstrate that NF1 patients display high bone turnover and accumulation of osteoid and that supplementation of vitamin D has a beneficial effect on their BMD.

INTRODUCTION

Neurofibromatosis 1 results in a wide range of clinical manifestations, including decreased BMD. Although it has been reported that NF1 patients have decreased vitamin D serum levels, the manifestation of the disease at the bone tissue level has rarely been analyzed.

METHODS

Thus, we performed a clinical evaluation of 14 NF1 patients in comparison to age- and sex-matched control individuals. The analysis included dual X-ray absorptiometry osteodensitometry, laboratory parameters, histomorphometric and quantitative backscattered electron imaging (qBEI) analyses of undecalcified bone biopsies.

RESULTS

NF1 patients display significantly lower 25-(OH)-cholecalciferol serum levels and decreased BMD compared to control individuals. Histomorphometric analysis did not only reveal a reduced trabecular bone volume in biopsies from NF1 patients, but also a significantly increased osteoid volume and increased numbers of osteoblasts and osteoclasts. Moreover, qBEI analysis revealed a significant decrease of the calcium content in biopsies from NF1 patients. To address the question whether a normalization of calcium homeostasis improves BMD in NF1 patients, we treated four patients with cholecalciferol for 1 year, which resulted in a significant increase of BMD.

CONCLUSION

Taken together, our data provide the first complete histomorphometric analysis from NF1 patients. Moreover, they suggest that low vitamin D levels significantly contribute to the skeletal defects associated with the disease.

Impaired gastric acidification negatively affects calcium homeostasis and bone mass

Activation of osteoclasts and their acidification-dependent resorption of bone is thought to maintain proper serum calcium levels. Here we show that osteoclast dysfunction alone does not generally affect calcium homeostasis. Indeed, mice deficient in Src, encoding a tyrosine kinase critical for osteoclast activity, show signs of osteopetrosis, but without hypocalcemia or defects in bone mineralization. Mice deficient in Cckbr, encoding a gastrin receptor that affects acid secretion by parietal cells, have the expected defects in gastric acidification but also secondary hyperparathyroidism and osteoporosis and modest hypocalcemia. These results suggest that alterations in calcium homeostasis can be driven by defects in gastric acidification, especially given that calcium gluconate supplementation fully rescues the phenotype of the Cckbr-mutant mice. Finally, mice deficient in Tcirg1, encoding a subunit of the vacuolar proton pump specifically expressed in both osteoclasts and parietal cells, show hypocalcemia and osteopetrorickets. Although neither Src- nor Cckbr-deficient mice have this latter phenotype, the combined deficiency of both genes results in osteopetrorickets. Thus, we find that osteopetrosis and osteopetrorickets are distinct phenotypes, depending on the site or sites of defective acidification.

Case report: multiple fractures in a patient with mutations of TWIST1 and TNSALP

Hypophosphatasia is a rare inherited disorder characterized by defective skeletal mineralization and low alkaline phosphatase activities in the serum. The genetic cause of hypophosphatasia is believed related to inactivating mutations in the TNSALP gene, encoding tissue-nonspecific alkaline phosphatase. Another rare inheritable disease, Saethre-Chotzen syndrome, leads to premature fusion of the cranial sutures caused by heterozygous mutations of the human TWIST1 gene. Because the two disorders apparently are not genetically related (only reported individually) yet both involve defective skeletal formation, we believe it is important to report our findings on a patient harboring mutations of TNSALP and TWIST1.

The protein tyrosine phosphatase Rptpzeta is expressed in differentiated osteoblasts and affects bone formation in mice

Tyrosine phosphorylation of intracellular substrates is one mechanism to regulate cellular proliferation and differentiation. Protein tyrosine phosphatases (PTPs) act by dephosphorylation of substrates and thereby counteract the activity of tyrosine kinases. Few PTPs have been suggested to play a role in bone remodeling, one of them being Rptpzeta, since it has been shown to be suppressed by pleiotrophin, a heparin-binding molecule affecting bone formation, when over-expressed in transgenic mice. In a genome-wide expression analysis approach we found that Ptprz1, the gene encoding Rptpzeta, is strongly induced upon terminal differentiation of murine primary calvarial osteoblasts. Using RT-PCR and Western Blotting we further demonstrated that differentiated osteoblasts, in contrast to neuronal cells, specifically express the short transmembrane isoform of Rptpzeta. To uncover a potential role of Rptpzeta in bone remodeling we next analyzed the skeletal phenotype of a Rptpzeta-deficient mouse model using non-decalcified histology and histomorphometry. Compared to wildtype littermates, the Rptpzeta-deficient mice display a decreased trabecular bone volume at the age of 50 weeks, caused by a reduced bone formation rate. Likewise, Rptpzeta-deficient calvarial osteoblasts analyzed ex vivo display decreased expression of osteoblast markers, indicating a cell-autonomous defect. This was confirmed by the finding that Rptpzeta-deficient osteoblasts had a diminished potential to form osteocyte-like cellular extensions on Matrigel-coated surfaces. Taken together, these data provide the first evidence for a physiological role of Rptpzeta in bone remodeling, and thus identify Rptpzeta as the first PTP regulating bone formation in vivo.

The role of calcitonin and alpha-calcitonin gene-related peptide in bone formation

The Calca gene encodes two polypeptides, calcitonin (CT) and alpha-calcitonin gene-related peptide (alpha-CGRP), generated through alternative splicing. While CT, a hormone mainly produced by thyroidal C cells, has been described as a major regulator of bone resorption, alpha-CGRP, a neuropeptide expressed in the cells of the central and peripheral nervous system, is mostly known as a regulator of vascular tone. Surprisingly, the generation and skeletal analyses of two mouse deficiency models has recently uncovered a physiological function for both peptides in the regulation of bone formation. In the first model, where the replacement of exons 2-5 of the Calca gene resulted in the combined deficiency of CT and alpha-CGRP, an increased bone formation rate (BFR) was observed, whereas decreased BFR was found in the second model, where the introduction of a translational termination codon into exon 5 of the Calca gene resulted in the specific absence of alpha-CGRP.

Polyethylene particle-induced bone resorption in alpha-calcitonin gene-related peptide-deficient mice

This study investigates the impact of alpha-CGRP on bone metabolism after implantation of polyethylene particles. alpha-CGRP knockout mice showed less osteolysis compared with wildtype mice. The local neurogenic microenvironment might be a crucial factor in particle-induced osteolysis.

INTRODUCTION

Periprosthetic osteolysis is the major reason for aseptic loosening in joint arthroplasty. This study aimed to investigate the potential impact of alpha-calcitonin gene-related peptide (alpha-CGRP) deficiency on bone metabolism under conditions of polyethylene particle-induced osteolysis.

MATERIALS AND METHODS

We used the murine calvarial osteolysis model based on polyethylene particles in 14 C57BL 6 mice and 14 alpha-CGRP-deficient mice divided into four groups of 7 mice each. Groups 1 (C57BL/J 6) and 3 (alpha-CGRP knockout) received sham surgery, and groups 2 (C57BL/J 6) and 4 (alpha-CGRP knockout) were treated with polyethylene particles. Qualitative and quantitative 3D analyses were performed using microCT. In addition, bone resorption was measured within the midline suture by histological examination. The number of osteoclasts was determined by counting the TRACP(+) cells. Calvarial bone was tested for RANKL expression by RT-PCR and immunocytochemistry.

RESULTS

Bone resorption was significantly reduced in alpha-CGRP-deficient mice compared with their corresponding wildtype C57BL 6 mice as confirmed by histomorphometric data (p < 0.001) and microCT (p < 0.01). Osteoclast numbers were significantly reduced in group 3 and the particle subgroup compared with group 1 (p < 0.001). We observed a >3-fold increase of basal RANKL mRNA levels within group 1 compared with group 3. Additional low RANKL immunochemistry staining was noted in groups 3 and 4.

CONCLUSIONS

In conclusion, alpha-CGRP knockout mice did not show the expected extended osteolysis compared with wildtype mice expressing alpha-CGRP. One of the most reasonable explanations for the observed decrease in osteolysis could be linked to the osteoprotegerin (OPG)/RANK/RANKL system in alpha-CGRP-deficient animals. As a consequence, the fine tuning of osteoclasts mediating resorption in alpha-CGRP-null mice may be deregulated.

High bone mineral density in pycnodysostotic patients with a novel mutation in the propeptide of cathepsin K

INTRODUCTION

Pycnodysostosis is typically associated with short stature, multiple fractures without adequate trauma and high bone density on x-ray. The increased bone density is due to a genetic defect of cathepsin K, leading to dysfunctional osteoclastic bone resorption and bone remodeling. We wanted to know how this defect influences the trabecular and cortical volumetric bone mineral density of long bones as measured quantitatively by pQCT.

METHODS

Three siblings of a consanguineous family were admitted to our hospital because of multiple fractures. Pycnodysostosis was diagnosed based on the clinical presentation with the characteristic dense appearance of their bones on x-ray. The distal and proximal radius of the patients and of control subjects was scanned using a Stratec XCT-2000 pQCT scanner and data were processed using the software provided by the manufacturer. Genomic DNA was extracted from blood samples of all three patients and their parents. The coding exons of the cathepsin K gene (CTSK) were amplified and sequenced.

RESULTS

The patients displayed the typical features of pycnodysostosis: Short stature, delay of closure of the fontanelles, hypoplasia of the maxilla, spondylolysis of the lumbar spine, stubby hands and feet and a history of multiple fractures. Volumetric bone density was much higher in pycnodysostotic bone than in the control bones 686 +/- 28 mg/cm(3) in patients vs. 290 +/- 6 mg/cm(3) in controls; p = 0.001), especially in the trabecular compartment (733 +/- 26 mg/cm(3) in patients vs. 195 +/- 8 mg/cm(3) in controls; p < 0.001), but also in the cortical bone (1108 +/- 22 in patients vs. 1020 +/- 17 in controls; p < 0.01). In contrast to this finding, the patients displayed an elevation of alkaline phosphatase in the serum and free deoxypyridinoline-crosslinks (DPD) in the urine, suggesting osteomalacia. Sequencing of the cathepsin K gene revealed homozygosity for a novel missense mutation in all three patients predicting the amino acid exchange from arginine to tryptophan at position 46 (R46W).

CONCLUSION

We present here for the first time quantitative data on the mineral density of bones of pycnodysostotic patients with a novel mutation in the propeptide of cathepsin K. The elevated bone mineral density in the cortex and the changes in the serum markers suggest an effect of cathepsin K not only on bone volume, but also on bone mineralization. This might in part explain the increased susceptibility to fractures of patients with pycnodysostosis.

Calcitonin deficiency in mice progressively results in high bone turnover

Although the pharmacological action of calcitonin (CT) as an inhibitor of bone resorption is well established, there is still some controversy regarding its physiological function. Unexpectedly, Calca-deficient mice lacking CT and alpha-calcitonin gene-related peptide (alphaCGRP) were described to have a high bone mass phenotype caused by increased bone formation with normal bone resorption. Here we show that these mice develop a phenotype of high bone turnover with age, suggesting that CT is a physiological inhibitor of bone remodeling.

INTRODUCTION

The absence of significant changes in bone mineral density caused by decline or overproduction of CT in humans has raised the question, whether the pharmacological action of CT as an inhibitor of bone resorption is also of physiological relevance. To study the physiological role of mammalian CT, we have analyzed the age-dependent bone phenotype of two mouse models, one lacking CT and alphaCGRP (Calca-/-), the other one lacking only alphaCGRP (alphaCGRP-/-).

MATERIALS AND METHODS

Bones from wildtype, Calca-/- -mice and alphaCGRP-/- -mice were analyzed at the ages of 6, 12 and 18 months using undecalcified histology. Differences of bone remodeling were quantified by static and dynamic histomorphometry as well as by measuring the urinary collagen degradation products. To rule out secondary mechanisms underlying the observed phenotype, we determined serum concentrations of relevant hormones using commercially available antibody-based detection kits.

RESULTS

Whereas alphaCGRP-/- -mice display an osteopenia at all ages analyzed, the Calca-/- -mice develop a phenotype of high bone turnover with age. Histomorphometric analysis performed at the age of 12 months revealed significant increases of bone formation and bone resorption specifically in the Calca-/- -mice. This severe phenotype that can result in hyperostotic lesions, can not be explained by obvious endocrine abnormalities other than the absence of CT.

CONCLUSIONS

In addition to the previously described increase of bone formation in the Calca-deficient mice, we have observed that there is also an increase of bone resorption with age. This suggests that CT has a dual action as an inhibitor of bone remodeling, which may explain why alterations of CT serum levels in humans do not result in major changes of bone mineral density.