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1.  Age-Related Skeletal Dynamics and Decrease in Bone Strength in DNA Repair Deficient Male Trichothiodystrophy Mice 
PLoS ONE  2012;7(4):e35246.
Accumulation of DNA damage caused by oxidative stress is thought to be one of the main contributors of human tissue aging. Trichothiodystrophy (TTD) mice have a mutation in the Ercc2 DNA repair gene, resulting in accumulation of DNA damage and several features of segmental accelerated aging. We used male TTD mice to study the impact of DNA repair on bone metabolism with age. Analysis of bone parameters, measured by micro-computed tomography, displayed an earlier decrease in trabecular and cortical bone as well as a loss of periosteal apposition and a reduction in bone strength in TTD mice with age compared to wild type mice. Ex vivo analysis of bone marrow differentiation potential showed an accelerated reduction in the number of osteogenic and osteoprogenitor cells with unaltered differentiation capacity. Adipocyte differentiation was normal. Early in life, osteoclast number tended to be increased while at 78 weeks it was significantly lower in TTD mice. Our findings reveal the importance of genome stability and proper DNA repair for skeletal homeostasis with age and support the idea that accumulation of damage interferes with normal skeletal maintenance, causing reduction in the number of osteoblast precursors that are required for normal bone remodeling leading to a loss of bone structure and strength.
PMCID: PMC3323647  PMID: 22506075
2.  Bone fragility and decline in stem cells in prematurely aging DNA repair deficient trichothiodystrophy mice 
Age  2011;34(4):845-861.
Trichothiodystrophy (TTD) is a rare, autosomal recessive nucleotide excision repair (NER) disorder caused by mutations in components of the dual functional NER/basal transcription factor TFIIH. TTD mice, carrying a patient-based point mutation in the Xpd gene, strikingly resemble many features of the human syndrome and exhibit signs of premature aging. To examine to which extent TTD mice resemble the normal process of aging, we thoroughly investigated the bone phenotype. Here, we show that female TTD mice exhibit accelerated bone aging from 39 weeks onwards as well as lack of periosteal apposition leading to reduced bone strength. Before 39 weeks have passed, bones of wild-type and TTD mice are identical excluding a developmental defect. Albeit that bone formation is decreased, osteoblasts in TTD mice retain bone-forming capacity as in vivo PTH treatment leads to increased cortical thickness. In vitro bone marrow cell cultures showed that TTD osteoprogenitors retain the capacity to differentiate into osteoblasts. However, after 13 weeks of age TTD females show decreased bone nodule formation. No increase in bone resorption or the number of osteoclasts was detected. In conclusion, TTD mice show premature bone aging, which is preceded by a decrease in mesenchymal stem cells/osteoprogenitors and a change in systemic factors, identifying DNA damage and repair as key determinants for bone fragility by influencing osteogenesis and bone metabolism.
PMCID: PMC3682057  PMID: 21814739
DNA repair syndromes; Mouse models; Aging; Bone strength; Bone fragility; Bone marrow stem cells
3.  Identification of receptor-type protein tyrosine phosphatase μ as a new marker for osteocytes 
Osteocytes are the predominant cells in bone, where they form a cellular network and display important functions in bone homeostasis, phosphate metabolism and mechanical transduction. Several proteins strongly expressed by osteocytes are involved in these processes, e.g., sclerostin, DMP-1, PHEX, FGF23 and MEPE, while others are upregulated during differentiation of osteoblasts into osteocytes, e.g., osteocalcin and E11. The receptor-type protein tyrosine phosphatase µ (RPTPμ) has been described to be expressed in cells which display a cellular network, e.g., endothelial and neuronal cells, and is implied in mechanotransduction. In a capillary outgrowth assay using metatarsals derived from RPTPμ-knock-out/LacZ knock-in mice, we observed that the capillary structures grown out of the metatarsals were stained blue, as expected. Surprisingly, cells within the metatarsal bone tissue were positive for LacZ activity as well, indicating that RPTPμ is also expressed by osteocytes. Subsequent histochemical analysis showed that within bone, RPTPμ is expressed exclusively in early-stage osteocytes. Analysis of bone marrow cell cultures revealed that osteocytes are present in the nodules and an enzymatic assay enabled the quantification of the amount of osteocytes. No apparent bone phenotype was observed when tibiae of RPTPμ-knock-out/LacZ knock-in mice were analyzed by μCT at several time points during aging, although a significant reduction in cortical bone was observed in RPTPμ-knock-out/LacZ knock-in mice at 20 weeks. Changes in trabecular bone were more subtle. Our data show that RPTPμ is a new marker for osteocytes.
PMCID: PMC4468792  PMID: 25850409
Osteocytes; RPTPμ; Bone; Micro-CT; Animal model
4.  Increased physical activity severely induces osteoarthritic changes in knee joints with papain induced sulfate-glycosaminoglycan depleted cartilage 
Articular cartilage needs sulfated-glycosaminoglycans (sGAGs) to withstand high pressures while mechanically loaded. Chondrocyte sGAG synthesis is regulated by exposure to compressive forces. Moderate physical exercise is known to improve cartilage sGAG content and might protect against osteoarthritis (OA). This study investigated whether rat knee joints with sGAG depleted articular cartilage through papain injections might benefit from moderate exercise, or whether this increases the susceptibility for cartilage degeneration.
sGAGs were depleted from cartilage through intraarticular papain injections in the left knee joints of 40 Wistar rats; their contralateral joints served as healthy controls. Of the 40 rats included in the study, 20 rats remained sedentary, and the other 20 were subjected to a moderately intense running protocol. Animals were longitudinally monitored for 12 weeks with in vivo micro-computed tomography (μCT) to measure subchondral bone changes and single-photon emission computed tomography (SPECT)/CT to determine synovial macrophage activation. Articular cartilage was analyzed at 6 and 12 weeks with ex vivo contrast-enhanced μCT and histology to measure sGAG content and cartilage thickness.
All outcome measures were unaffected by moderate exercise in healthy control joints of running animals compared with healthy control joints of sedentary animals. Papain injections in sedentary animals resulted in severe sGAG-depleted cartilage, slight loss of subchondral cortical bone, increased macrophage activation, and osteophyte formation. In running animals, papain-induced sGAG-depleted cartilage showed increased cartilage matrix degradation, sclerotic bone formation, increased macrophage activation, and more osteophyte formation.
Moderate exercise enhanced OA progression in papain-injected joints and did not protect against development of the disease. This was not restricted to more-extensive cartilage damage, but also resulted in pronounced subchondral sclerosis, synovial macrophage activation, and osteophyte formation.
PMCID: PMC3978821  PMID: 24472689
5.  Systemic treatment with pulsed electromagnetic fields do not affect bone microarchitecture in osteoporotic rats 
International Orthopaedics  2012;36(7):1501-1506.
Pulsed electromagnetic fields (PEMF) are currently used in the treatment of spinal fusions and non-unions. There are indications that PEMF might also be effective in the treatment of osteoporosis. In this study we examined whether whole-body PEMF treatment affects the bone microarchitecture in an osteoporotic rat model.
Twenty-week-old female rats were ovariectomised (n = 20). Four different PEMF treatment protocols based on previous experimental studies and based on clinically used PEMF signals were examined (2 h/day, 5 days/week). A control group did not receive PEMF. At zero, three and six weeks cancellous and cortical bone architectural changes at the proximal tibia were evaluated using in vivo microCT scanning.
PEMF treatment did not induce any changes in cancellous or cortical bone compared to untreated controls.
Although previous studies have shown strong effects of PEMF in osteoporosis we were unable to demonstrate this in any of the treatment protocols. Using in vivo microCT scanning we were able to identify small bone changes in time. Subtle differences in the experimental set-up might explain the differences in study outcomes in the literature. Since PEMF treatment is safe, future experimental studies on the effect of PEMF on bone can better be performed directly on humans, eliminating the potential translation issues between animals and humans. In this study we found no support for the use of PEMF in the treatment of osteoporosis.
PMCID: PMC3385882  PMID: 22249842
6.  Vitamin K supplementation increases vitamin K tissue levels but fails to counteract ectopic calcification in a mouse model for pseudoxanthoma elasticum 
Pseudoxanthoma elasticum (PXE) is an autosomal recessive disorder in which calcification of connective tissue leads to pathology in skin, eye and blood vessels. PXE is caused by mutations in ABCC6. High expression of this transporter in the basolateral hepatocyte membrane suggests that it secretes an as-yet elusive factor into the circulation which prevents ectopic calcification. Utilizing our Abcc6−/− mouse model for PXE, we tested the hypothesis that this factor is vitamin K (precursor) (Borst et al. 2008, Cell Cycle). For 3 months, Abcc6−/− and wild-type mice were put on diets containing either the minimum dose of vitamin K required for normal blood coagulation or a dose that was 100 times higher. Vitamin K was supplied as menaquinone-7 (MK-7). Ectopic calcification was monitored in vivo by monthly micro-CT scans of the snout, as the PXE mouse model develops a characteristic connective tissue mineralization at the base of the whiskers. In addition, calcification of kidney arteries was measured by histology. Results show that supplemental MK-7 had no effect on ectopic calcification in Abcc6−/− mice. MK-7 supplementation increased vitamin K levels (in skin, heart and brain) in wild-type and in Abcc6−/− mice. Vitamin K tissue levels did not depend on Abcc6 genotype. In conclusion, dietary MK-7 supplementation increased vitamin K tissue levels in the PXE mouse model but failed to counteract ectopic calcification. Hence, we obtained no support for the hypothesis that Abcc6 transports vitamin K and that PXE can be cured by increasing tissue levels of vitamin K.
PMCID: PMC3195265  PMID: 21725681
Pseudoxanthoma elasticum; ABC transporter; Vitamin K; Ectopic calcification; Vascular calcification; Connective tissue; Cardiovascular; Vitamins; Calcium metabolism; Mouse models
7.  The peroxisome proliferator-activated receptor (PPAR) alpha agonist fenofibrate maintains bone mass, while the PPAR gamma agonist pioglitazone exaggerates bone loss, in ovariectomized rats 
Activation of peroxisome proliferator-activated receptor (PPAR)gamma is associated with bone loss and increased fracture risk, while PPARalpha activation seems to have positive skeletal effects. To further explore these effects we have examined the effect of the PPARalpha agonists fenofibrate and Wyeth 14643, and the PPARgamma agonist pioglitazone, on bone mineral density (BMD), bone architecture and biomechanical strength in ovariectomized rats.
Fifty-five female Sprague-Dawley rats were assigned to five groups. One group was sham-operated and given vehicle (methylcellulose), the other groups were ovariectomized and given vehicle, fenofibrate, Wyeth 14643 and pioglitazone, respectively, daily for four months. Whole body and femoral BMD were measured by dual X-ray absorptiometry (DXA), and biomechanical testing of femurs, and micro-computed tomography (microCT) of the femoral shaft and head, were performed.
Whole body and femoral BMD were significantly higher in sham controls and ovariectomized animals given fenofibrate, compared to ovariectomized controls. Ovariectomized rats given Wyeth 14643, maintained whole body BMD at sham levels, while rats on pioglitazone had lower whole body and femoral BMD, impaired bone quality and less mechanical strength compared to sham and ovariectomized controls. In contrast, cortical volume, trabecular bone volume and thickness, and endocortical volume were maintained at sham levels in rats given fenofibrate.
The PPARalpha agonist fenofibrate, and to a lesser extent the PPARaplha agonist Wyeth 14643, maintained BMD and bone architecture at sham levels, while the PPARgamma agonist pioglitazone exaggerated bone loss and negatively affected bone architecture, in ovariectomized rats.
PMCID: PMC3127763  PMID: 21615901
8.  Dietary magnesium, not calcium, prevents vascular calcification in a mouse model for pseudoxanthoma elasticum 
Pseudoxanthoma elasticum (PXE) is a heritable disorder characterized by ectopic calcification of connective tissue in skin, Bruch’s membrane of the eye, and walls of blood vessels. PXE is caused by mutations in the ABCC6 gene, but the exact etiology is still unknown. While observations on patients suggest that high calcium intake worsens the clinical symptoms, the patient organization PXE International has published the dietary advice to increase calcium intake in combination with increased magnesium intake. To obtain more data on this controversial issue, we examined the effect of dietary calcium and magnesium in the Abcc6−/− mouse, a PXE mouse model which mimics the clinical features of PXE. Abcc6−/− mice were placed on specific diets for 3, 7, and 12 months. Disease severity was measured by quantifying calcification of blood vessels in the kidney. Raising the calcium content in the diet from 0.5% to 2% did not change disease severity. In contrast, simultaneous increase of both calcium (from 0.5% to 2.0%) and magnesium (from 0.05% to 0.2%) slowed down the calcification significantly. Our present findings that increase in dietary magnesium reduces vascular calcification in a mouse model for PXE should stimulate further studies to establish a dietary intervention for PXE.
PMCID: PMC2859158  PMID: 20177653
PXE; ABCC6; Calcium; Magnesium; Vascular calcification; ABC transporter; Histopathology; Calcium metabolism; Cardiovascular; Connective tissue; Diet
9.  Dual energy x-ray absorptiometry analysis contributes to the prediction of hip osteoarthritis progression 
Arthritis Research & Therapy  2009;11(6):R162.
To determine if structural bone parameters obtained from dual energy X-ray absorptiometry (DXA) contribute to the prediction of progression of hip osteoarthritis (OA) and to test if the difference between the most affected (OA) hip and the contralateral hip adds to this prediction.
The study group involves a prospective cohort of 189 patients that met the American College of Rheumatology (ARC) classification criteria for hip osteoarthritis. Progression was defined as 20% joint space narrowing or total hip replacement within a two years follow up. Software was developed to calculate geometrical aspects and bone mineral density (BMD) in different regions of interest of the proximal femur. Logistic regression was used to test if Kellgren and Lawrence (K-L) scores and DXA parameters can predict progression of OA. Models were compared using -2log likelihood tests, R2 Nagelkerke and areas under the Receiver Operator Characteristic curves, assessed using 10-fold cross validation.
The model that included the DXA variables was significantly better in predicting hip OA progression than the model with K-L score of the affected side alone (P < 0.01). The addition of the differences in DXA parameters between the most affected and contralateral hip in the superior part of the femoral head, trochanteric and intertrochanteric area further improved the prediction of progression (P < 0.05). K-L score of the affected side was still the most significant single variable in the models.
DXA parameters can significantly contribute to the prediction of progression in patients with hip osteoarthritis. The analysis of the DXA differences between the hips of the patient represents a small but significant contribution to this prediction. These analyses show the importance of bone density changes in the etiology of OA.
PMCID: PMC3003541  PMID: 19883507
10.  Renal Ca2+ wasting, hyperabsorption, and reduced bone thickness in mice lacking TRPV5 
Journal of Clinical Investigation  2003;112(12):1906-1914.
Ca2+ ions play a fundamental role in many cellular processes, and the extracellular concentration of Ca2+ is kept under strict control to allow the proper physiological functions to take place. The kidney, small intestine, and bone determine the Ca2+ flux to the extracellular Ca2+ pool in a concerted fashion. Transient receptor potential (TRP) cation channel subfamily V, members 5 and 6 (TRPV5 and TRPV6) have recently been postulated to be the molecular gatekeepers facilitating Ca2+ influx in these tissues and are members of the TRP family, which mediates diverse biological effects ranging from pain perception to male aggression. Genetic ablation of TRPV5 in the mouse allowed us to investigate the function of this novel Ca2+ channel in maintaining the Ca2+ balance. Here, we demonstrate that mice lacking TRPV5 display diminished active Ca2+ reabsorption despite enhanced vitamin D levels, causing severe hypercalciuria. In vivo micropuncture experiments demonstrated that Ca2+ reabsorption was malfunctioning within the early part of the distal convolution, exactly where TRPV5 is localized. In addition, compensatory hyperabsorption of dietary Ca2+ was measured in TRPV5 knockout mice. Furthermore, the knockout mice exhibited significant disturbances in bone structure, including reduced trabecular and cortical bone thickness. These data demonstrate the key function of TRPV5 in active Ca2+ reabsorption and its essential role in the Ca2+ homeostasis.
PMCID: PMC297001  PMID: 14679186

Results 1-10 (10)