Mouse models of telomere dysfunction phenocopy skeletal changes found in human age-related osteoporosis

Authors

Tracy A. Brennan, Kevin P. Egan, Carter M. Lindborg, Qijun Chen, Mariya T. Sweetwyne, Kurt D. Hankenson, Sharon X. Xie, Frederick B. Johnson and Robert J. Pignolo

Abstract

A major challenge in the elderly is osteoporosis and the high risk of fracture. Telomere dysfunction is a cause of cellular senescence and telomere shortening which occurs with age in cells from most human tissues, including bone. Telomere defects contribute to the pathogenesis of two progeroid disorders characterized by premature osteoporosis, Werner syndrome and dyskeratosis congenital. It is hypothesized that telomere shortening contributes to bone aging. Using mice with disrupted telomere maintenance mechanisms, including mutants in Werner helicase (Wrn-/-), telomerase (Terc-/-) and Wrn-/- Terc-/- double mutants, we evaluated their skeletal phenotypes as models for human bone aging. Compared to young wild-type (WT) mice, micro-computerized tomography analysis revealed that young Terc-/- and Wrn-/-Terc-/- mice have decreased trabecular bone volume, trabecular number and trabecular thickness, as well as increased trabecular spacing. In cortical bone, young Terc-/- and Wrn-/-Terc-/- mice have increased cortical thinning, and increased porosity relative to age-matched WT mice. These trabecular and cortical changes were accelerated with age in Terc-/- and Wrn-/-Terc-/- mice compared to older WT mice. Histological quantification of osteoblasts in aged mice showed a similar number of osteoblasts in all genotypes; however, significant decreases in osteoid, mineralization surface, mineral apposition rate and bone formation rate in older Terc-/- and Wrn-/-Terc-/- bone suggest that osteoblast dysfunction is a prominent feature of precocious aging in these mice. Except in the Wrn-/- single mutant, osteoclast number did not increase in any genotype. Significant alterations in mechanical parameters (structure model index, degree of anistrophy, and moment of inertia) of the Terc-/- and Wrn-/-Terc-/- femurs compared to WT mice were also observed. Young Wrn-/-Terc-/- mice had a statistically significant increase in bone marrow fat content compared to young WT mice, which remained elevated in aged double mutants. Taken together, our results suggest that Terc-/- and Wrn-/-Terc-/- mutants recapitulate the human bone aging phenotype and are useful models for studying age-related osteoporosis.

Link To Article

http://dx.doi.org/10.1242/dmm.014928

Pediatric solid organ transplantation and osteoporosis: a descriptive study on bone histomorphometric findings

Authors

Inari S. Tamminen, Helena Valta, Hannu Jalanko, Sari Salminen, Mervi K. Mäyränpää, Hanna Isaksson, Heikki Kröger, Outi Mäkitie

Abstract

Background

Organ transplantation may lead to secondary osteoporosis in children. This study characterized bone histomorphometric findings in pediatric solid organ transplant recipients who were assessed for suspected secondary osteoporosis.

Methods

Iliac crest biopsies were obtained from 19 children (7.6–18.8 years, 11 male) who had undergone kidney (n = 6), liver (n = 9), or heart (n = 4) transplantation a median 4.6 years (range 0.6–16.3 years) earlier. All patients had received oral glucocorticoids at the time of the biopsy.

Results

Of the 19 patients, 21 % had sustained peripheral fractures and 58 % vertebral compression fractures. Nine children (47 %) had a lumbar spine BMD Z-score below −2.0. Histomorphometric analyses showed low trabecular bone volume (< −1.0 SD) in 6 children (32 %) and decreased trabecular thickness in 14 children (74 %). Seven children (37 %) had high bone turnover at biopsy, and low turnover was found in 6 children (32 %), 1 of whom had adynamic bone disease.

Conclusions

There was a great heterogeneity in the histological findings in different transplant groups, and the results were unpredictable using non-invasive methods. The observed changes in bone quality (i.e. abnormal turnover rate, thin trabeculae) rather than the actual loss of trabecular bone, might explain the increased fracture risk in pediatric solid organ transplant recipients.

Link To Article

http://dx.doi.org/10.1007/s00467-014-2771-1

Japanese Medaka: A Non-Mammalian Vertebrate Model for Studying Sex and Age-Related Bone Metabolism In Vivo

Authors

Admane H. Shanthanagouda, Bao-Sheng Guo, Rui R. Ye, Liang Chao, Michael W. L. Chiang, Gopalakrishnan Singaram, Napo K. M. Cheung, Ge Zhang, Doris W. T. Au

Abstract

Background

In human, a reduction in estrogen has been proposed as one of the key contributing factors for postmenopausal osteoporosis. Rodents are conventional models for studying postmenopausal osteoporosis, but the major limitation is that ovariectomy is needed to mimic the estrogen decline after menopause. Interestingly, in medaka fish (Oryzias latipes), we observed a natural drop in plasma estrogen profile in females during aging and abnormal spinal curvature was apparent in old fish, which are similar to postmenopausal women. It is hypothesized that estrogen associated disorders in bone metabolism might be predicted and prevented by estrogen supplement in aging O. latipes, which could be corresponding to postmenopausal osteoporosis in women.

Principal findings

In O. latipes, plasma estrogen was peaked at 8 months old and significantly declined after 10, 11 and 22 months in females. Spinal bone mineral density (BMD) and micro-architecture by microCT measurement progressively decreased and deteriorated from 8 to 10, 12 and 14 months old, which was more apparent in females than the male counterparts. After 10 months old, O. latipes were supplemented with 17α-ethinylestradiol (EE2, a potent estrogen mimic) at 6 and 60 ng/mg fish weight/day for 4 weeks, both reduction in spinal BMD and deterioration in bone micro-architecture were significantly prevented. The estrogenic effect of EE2 in O. latipes was confirmed by significant up-regulation of four key estrogen responsive genes in the liver. In general, bone histomorphometric analyses indicated significantly lowered osteoblasts and osteoclasts numbers and surfaces on vertebrae of EE2-fed medaka.

Significance

We demonstrate osteoporosis development associated with natural drop in estrogen level during aging in female medaka, which could be attenuated by estrogen treatment. This small size fish is a unique alternative non-mammalian vertebrate model for studying estrogen-related molecular regulation in postmenopausal skeletal disorders in vivo without ovariectomy.

Link To Article

http://dx.doi.org/10.1371/journal.pone.0088165

The Rho-GEF Kalirin regulates bone mass and the function of osteoblasts and osteoclasts

Authors

Su Huang, Pierre P. Elensite, Kornchanok Wayakanon, Prashant Mandela, Betty A. Eipper, Richard E. Mains, Matthew R. Allen, Angela Bruzzaniti

Abstract

Bone homeostasis is maintained by the balance between bone resorption by osteoclasts and bone formation by osteoblasts. Dysregulation in the activity of the bone cells can lead to osteoporosis, a disease characterized by low bone mass and an increase in bone fragility and risk of fracture. Kalirin is a novel GTP-exchange factor protein that has been shown to play a role in cytoskeletal remodeling and dendritic spine formation in neurons. We examined Kalirin expression in skeletal tissue and found that it was expressed in osteoclasts and osteoblasts. Furthermore, micro-CT analyses of the distal femur of global Kalirin knockout (Kal-KO) mice revealed significantly reduced trabecular and cortical bone parameters in Kal-KO mice, compared to WT mice, with significantly reduced bone mass in 8, 14 and 36 week-old female Kal-KO mice. Male mice also exhibited a decrease in bone parameters but not the level seen in female mice. Histomorphometric analyses also revealed decreased bone formation rate in 14 week-old female Kal-KO mice, as well as decreased osteoblast number/bone surface and increased osteoclast surface/bone surface. Consistent with our in vivo findings, the bone resorbing activity and differentiation of Kal-KO osteoclasts was increased in vitro. Although alkaline phosphatase activity by Kal-KO osteoblasts was increased in vitro, Kal-KO osteoblasts showed decreased mineralizing activity, as well as decreased secretion of OPG, which was inversely correlated with ERK activity. Taken together, our findings suggest that deletion of Kalirin directly affects osteoclast and osteoblast activity, leading to decreased OPG secretion which is likely to alter the RANKL/OPG ratio, promoting osteoclastogenesis. Therefore, Kalirin may play a role in paracrine and/or endocrine signaling events that control skeletal bone remodeling and the maintenance of bone mass.

Link To Article

http://dx.doi.org/10.1016/j.bone.2013.12.023

Sclerostin Inhibition Reverses Skeletal Fragility in an Lrp5-Deficient Mouse Model of OPPG Syndrome

Authors

Rajendra Kedlaya, Shreya Veera, Daniel J. Horan, Rachel E. Moss, Ugur M. Ayturk, Christina M. Jacobsen, Margot E. Bowen, Chris Paszty, Matthew L. Warman, and Alexander G. Robling

Abstract

Osteoporosis pseudoglioma syndrome (OPPG) is a rare genetic disease that produces debilitating effects in the skeleton. OPPG is caused by mutations in LRP5, a WNT co-receptor that mediates osteoblast activity. WNT signaling through LRP5, and also through the closely related receptor LRP6, is inhibited by the protein sclerostin (SOST). It is unclear whether OPPG patients might benefit from the anabolic action of sclerostin neutralization therapy (an approach currently being pursued in clinical trials for postmenopausal osteoporosis) in light of their LRP5 deficiency and consequent osteoblast impairment. To assess whether loss of sclerostin is anabolic in OPPG, we measured bone properties in a mouse model of OPPG (Lrp5−/−), a mouse model of sclerosteosis (Sost−/−), and in mice with both genes knocked out (Lrp5−/−;Sost−/−). Lrp5−/−;Sost−/− mice have larger, denser, and stronger bones than do Lrp5−/− mice, indicating that SOST deficiency can improve bone properties via pathways that do not require LRP5. Next, we determined whether the anabolic effects of sclerostin depletion in Lrp5−/− mice are retained in adult mice by treating 17-week-old Lrp5−/− mice with a sclerostin antibody for 3 weeks. Lrp5+/+ and Lrp5−/− mice each exhibited osteoanabolic responses to antibody therapy, as indicated by increased bone mineral density, content, and formation rates. Collectively, our data show that inhibiting sclerostin can improve bone mass whether LRP5 is present or not. In the absence of LRP5, the anabolic effects of SOST depletion can occur via other receptors (such as LRP4/6). Regardless of the mechanism, our results suggest that humans with OPPG might benefit from sclerostin neutralization therapies.

Link To Article

http://dx.doi.org/10.1126/scitranslmed.3006627

Bone quality and strength are greater in growing male rats fed fructose compared with glucose

Authors

Erica F. Bass, Clifton A. Bailee, Richard D. Lewis, Silvia Q. Giraudo

Abstract

Optimization of peak bone mass during adolescence is important for osteoporosis prevention. Studies in rodents and humans have demonstrated the harmful effects of sugar intake on bone health. With the high levels of sucrose in the diets of adolescents, it is necessary to understand the influence of glucose and fructose on growing bones. This study compared the effects of dietary glucose and fructose on bone formation, microarchitecture, and strength. Because of the different metabolic effects of glucose and fructose, we hypothesized that their individual effects on bone would be different. Eighteen male Sprague-Dawley rats (age, 60 days) were randomly assigned to high-fructose (n = 9; 40% fructose, 10% glucose) or high-glucose diet (n = 9; 50% glucose) for 12 weeks. Bone measurements included histology and histomorphometry of trabecular bone in the distal femur and a 3-point bending test of the whole tibia. Whole liver mass and postprandial serum glucose, insulin, and triglycerides were used to assess differences in energy metabolism between the diets. There were no differences in food intake, body weight, or visceral adiposity between groups, but fructose consumption led to heavier livers (P = .001) and elevated serum triglycerides (P = .00). The distal femurs of fructose-fed rats had greater bone volume (bone volume/total volume; P = .03), lower bone surface (bone surface/bone volume; P = .02), and thicker trabeculae (trabecular thickness; P = .01). The tibias of the fructose-fed rats also withstood a greater maximum flexure load (P = .032). These results indicate that consumption of the high-fructose diet resulted in stronger bones with enhanced microarchitecture than consumption of the high-glucose diet.

Link to Article

http://dx.doi.org/10.1016/j.nutres.2013.08.006