Effects of combination therapy with PTH and 17beta-estradiol on long bones of female mice

PTH(1-34) treatment and/or mechanical loading have different osteogenic effects on the trabecular and cortical bone in the ovariectomized C57BL/6 mouse

In preclinical mouse models, a synergistic anabolic response to PTH(1–34) and tibia loading was shown. Whether combined treatment improves bone properties with oestrogen deficiency, a cardinal feature of osteoporosis, remains unknown. This study quantified the individual and combined longitudinal effects of PTH(1–34) and loading on the bone morphometric and densitometric properties in ovariectomised mice. C57BL/6 mice were ovariectomised at 14-weeks-old and treated either with injections of PTH(1–34); compressive loading of the right tibia; both interventions concurrently; or both interventions on alternating weeks. Right tibiae were microCT-scanned from 14 until 24-weeks-old. Trabecular metaphyseal and cortical midshaft morphometric properties, and bone mineral content (BMC) in 40 different regions of the tibia were measured. Mice treated only with loading showed the highest trabecular bone volume fraction at week 22. Cortical thickness was higher with co-treatment than in the mice treated with PTH alone. In the mid-diaphysis, increases in BMC were significantly higher with loading than PTH. In ovariectomised mice, the osteogenic benefits of co-treatment on the trabecular bone were lower than loading alone. However, combined interventions had increased, albeit regionally-dependent, benefits to cortical bone. Increased benefits were largest in the mid-diaphysis and postero-laterally, regions subjected to higher strains under compressive loads.

Influence of long-term postmenopausal hormone-replacement therapy on estimated structural bone strength: a study in discordant monozygotic twins

Although postmenopausal hormone-replacement therapy (HRT) is known to prevent fractures, knowledge on the influence of long-term HRT on bone strength and its determinants other than areal bone mineral density is scarce. This study used a genetically controlled design with 24 monozygotic female twin pairs aged 54 to 72 years in which one cotwin was using HRT (mean duration 8 years) and the other had never used HRT. Estimated bone strength, cross-sectional area, volumetric bone mineral density, bone mineral mass, and cross-sectional density and mass distributions were assessed in the tibial shaft, distal tibia, and distal radius with peripheral computed tomography (pQCT). In the tibial shaft, HRT users had 9% [95% confidence interval (CI) 3%-15%] higher estimated bending strength than their nonusing cotwins. Larger cortical area and higher cortical bone mineral density accounted for this difference. The cortex was larger in the HRT users in the endocortical region. In the distal tibia, estimated compressive strength was 24% (95% CI 9%-40%) higher and in the distal radius 26% (95% CI 11%-41%) higher in the HRT users than in their nonusing cotwins owing to higher volumetric bone mineral density. No difference between users and nonusers was observed in total bone cross-sectional area in any measured bone site. The added mineral mass in the HRT users was distributed evenly within and between bone sites. In postmenopausal women, long-term HRT preserves estimated bone strength systemically by preventing bone mineral loss similarly in body weight-loaded and non-weight-loaded bone.

Short-term intermittent PTH 1-34 administration enhances bone formation in SCID/Beige mice

The anabolic effect of intermittent PTH on bone is variable depending on the species studied, duration/mode of administration, and location of skeletal response investigated. We tested the hypothesis low dose, short term, intermittent PTH 1-34 administration is sufficient to enhance bone formation without altering bone resorption. To test our hypothesis, mice were treated intermittently with one of three concentrations of PTH 1-34 (1 microg/kg; low, 10 microg/kg, or 20 microg/kg; high) for three weeks. The skeletal response was identified by quantifying: serum markers of bone turnover, cancellous bone parameters in distal femur, proximal tibia, and lumbar vertebrae by microCT, and number of osteoblasts and osteoclasts in distal femur. Mice receiving 20 microg/kg of PTH 1-34 demonstrated a 30% increase in serum osteocalcin, but no differences in serum calcium, type I collagen teleopeptides, or TRACP 5b. For all bones, microCT analysis suggested mice receiving 20 microg/kg of PTH 1-34 had increased cancellous bone mineral density, trabecular thickness and spacing, but decreased trabecular number. A 60% increase in the number of alkaline phosphatase positive osteoblasts in the distal femur was also observed in tissue sections; however, the number of TRAP positive osteoclasts was not different between test and control groups. While animals administered 10 microg/kg demonstrated similar trends for all bone turnover indices, such alterations were not observed in animals administered PTH 1-34 at 1 microg/kg per day. Thus, PTH 1-34, administered intermittently for three weeks at 20 microg/kg is sufficient to enhance bone formation without enhancing resorption.

Comparison of skeletal effects of ovariectomy versus chemically induced ovarian failure in mice

Bone loss associated with menopause leads to an increase in skeletal fragility and fracture risk. Relevant animal models can be useful for evaluating the impact of ovarian failure on bone loss. A chemically induced model of menopause in which mice gradually undergo ovarian failure yet retain residual ovarian tissue has been developed using the chemical 4-vinylcyclohexene diepoxide (VCD). This study was designed to compare skeletal effects of VCD-induced ovarian failure to those associated with ovariectomy (OVX). Young (28 day) C57Bl/6Hsd female mice were dosed daily with vehicle or VCD (160 mg/kg/d, IP) for 15 days (n = 6-7/group) and monitored by vaginal cytology for ovarian failure. At the mean age of VCD-induced ovarian failure (approximately 6 wk after onset of dosing), a different group of mice was ovariectomized (OVX, n = 8). Spine BMD (SpBMD) was measured by DXA for 3 mo after ovarian failure and OVX. Mice were killed approximately 5 mo after ovarian failure or OVX, and bone architecture was evaluated by microCT ex vivo. In OVX mice, SpBMD was lower than controls 1 mo after OVX, whereas in VCD-treated mice, SpBMD was not lower than controls until 2.9 mo after ovarian failure (p < 0.05). Both VCD-induced ovarian failure and OVX led to pronounced deterioration of trabecular bone architecture, with slightly greater effects in OVX mice. At the femoral diaphysis, cortical bone area and thickness did not differ between VCD mice and controls but were decreased in OVX compared with both groups (p < 0.05). Circulating androstenedione levels were preserved in VCD-treated mice but reduced in OVX mice relative to controls (p < 0.001). These findings support that (1) VCD-induced ovarian failure leads to trabecular bone deterioration, (2) bone loss is attenuated by residual ovarian tissue, particularly in diaphyseal cortical bone, and (3) the VCD mouse model can be a relevant model for natural menopause in the study of associated bone disorders.

Voluntary run training but not estradiol deficiency alters the tibial bone-soleus muscle functional relationship in mice

The study's objective was to investigate how estrogen deficiency and run training affect the tibial bone-soleus muscle functional relationship in mice. Female mice were assigned into one of two surgical conditions, ovariectomy (OVX) or sham ovariectomy (sham), and one of two activity conditions, voluntary wheel running (Run) or sedentary (Sed). To determine whether differences observed between OVX and sham conditions could be attributed to estradiol (E2), additional OVX mice were supplemented with E2. Tibial bones were analyzed for their functional capacities, ultimate load, and stiffness. Soleus muscles were analyzed for their functional capacities, maximal isometric tetanic force (Po), and peak eccentric force. The ratios of bone functional capacities to those of muscle were calculated. The bone functional capacities were affected by both surgical condition and activity but more strongly by surgical condition. Ultimate load and stiffness for the sham group were 7–12% greater than those for OVX animals ( P = 0.002), whereas only stiffness was greater for Run than for Sed animals (9%; P = 0.015). The muscle functional capacities were affected by both surgical condition and activity; however, in contrast to the bone, the muscle was more affected by activity. Po and peak eccentric force were 10–21% greater for Run than for Sed animals ( P ≤ 0.016), whereas only Po was greater in sham than in OVX animals (9%; P = 0.011). The bone-to-muscle ratios of functional capacities were affected by activity but not by surgical condition or E2 supplementation. Thus a mismatch of bone-muscle function occurred in mice that voluntarily ran on wheels, irrespective of estrogen status.

PIXImus bone densitometer and associated technical measurement issues of skeletal growth in the young rat

The PIXImus dual-energy X-ray absorptiometer (DXA) is designed to measure body composition, bone mineral content (BMC), area (BA), and density (BMD) in mice and rats. The aims of this study were to longitudinally measure BMC, BA, and BMD in growing rats and to identify potential technical problems associated with the PIXImus. Total femur and lumbar DXA measurements, body weight, and length of initially 3-week-old rats (n = 10) were taken at weeks 5, 9, and 14. BMC and BMD of femoral metaphyseal and diaphyseal regions rich in trabecular and cortical bone, respectively, were obtained. Results showed significant increases in body weight, total femur BMC and BMD, lumbar area, length, BMC, and BMD at each time point. There was a significant positive correlation between body weight and total femur BMD (r = 0.97, P < 0.001) as well as lumbar BMD (r = 0.99, P < 0.001). BMD values for the femoral metaphyseal region and the lumbar spine were also positively correlated (r = 0.96, P < 0.01). Several technical issues (e.g., positioning of animals), difficulties (e.g., in analysis of images), and limitations (e.g., inability to detect underdeveloped calcified bone in growing animals and bone edge detection) of the software pertinent to the PIXImus were evident. In conclusion, despite limitations in the software, the PIXImus is a valuable tool for studying skeletal development of growing rats.

Lrp5-independent activation of Wnt signaling by lithium chloride increases bone formation and bone mass in mice

One of the well characterized cell biologic actions of lithium is the inhibition of glycogen synthase kinase-3beta and the consequent activation of canonical Wnt signaling. Because deficient Wnt signaling has been implicated in disorders of reduced bone mass, we tested whether lithium could improve bone mass in mice. We gavage-fed lithium chloride to 8-week-old mice from three different strains (Lrp5(-/-), SAMP6, and C57BL/6) and assessed the effect on bone metabolism after 4 weeks of therapy. Lrp5(-/-) mice lack the Wnt coreceptor low-density lipoprotein receptor-related protein 5 and have markedly reduced bone mass. Lithium, which is predicted to act downstream of this receptor, restored bone metabolism and bone mass to near wild-type levels in these mice. SAMP6 mice have accelerated osteoporosis due to inadequate osteoblast renewal. Lithium significantly improved bone mass in these mice and in wild-type C57BL/6 mice. We found that lithium activated canonical Wnt signaling in cultured calvarial osteoblasts from Lrp5(-/-) mice ex vivo and that lithium-treated mice had increased expression of Wnt-responsive genes in their bone marrow cells in vivo. These data lead us to conclude that lithium enhances bone formation and improves bone mass in mice and that it may do so via activation of the canonical Wnt pathway. Lithium has been used safely and effectively for over half a century in the treatment of bipolar illness. Prospective studies in patients receiving lithium should determine whether it also improves bone mass in humans.

Dual energy X ray absorptiometry of ex vivo HcB/Dem mouse long bones: left are denser than right

Dual energy X ray absorptiometry (DXA) has become a popular analytical technique in mice and other small animals. Comparative study of bone properties at different anatomical sites is an active area of study in model organisms. Such investigations require that site-specific data be generated and interpreted. There are no published data addressing the degree to which contralateral mouse bones resemble each other in the absence of an experimental intervention, nor are there data addressing the correlation of bone densitometry measurements between anatomically distant sites. To address these gaps in our knowledge, we used DXA to compare excised mouse femora and humeri. At the population level, left bones were slightly but significantly denser than right bones, with an overall adjusted bone mineral density (BMD) difference of 0.7 +/- 0.3 and 0.5 +/- 0.2 mg/cm2 at the femur and humerus, respectively. At the level of bone pairs from a single animal, absolute adjusted BMD disparities between the right and left sides were 2.3 +/- 1.9 mg/cm2 at the femur and 1.7 +/- 1.4 mg/cm2 at the humerus. Correlation coefficients between left and right sides were 0.78 for adjusted BMD at both sites. The correlation coefficient between side-averaged femoral and humeral BMD was 0.81, but ranged between 0.70 and 0.75 when limited to ipsilateral or contralateral femur-humerus pairs. Our findings suggest the desirability of randomizing limbs for treatment in studies using contralateral limb controls. These observations may represent the densitometric manifestation of behavioral and neuroanatomical lateralization in laboratory mice.

Accuracy and precision of PIXImus densitometry for ex vivo mouse long bones: comparison of technique and software version

Many densitometric studies in mice assess bone mineral density (BMD) at specified regions of interest, often using ex vivo specimens. In the present study, we sought to determine the precision and accuracy of ex vivo densitometry of mouse bones, comparing two software versions and two data acquisition techniques. The newer software allows manual adjustment of the threshold value for bone, improving the ability to analyze bone edges correctly. Root mean square standard deviations were 2-3 mg/cm2, with coefficients of variation ranging between 3% and 5% for femora and humeri and between 6% and 7% for radii. The regression coefficients for bone mineral content as a function of ash mass were near 1 for femora and humeri, but considerably lower for radii. Coefficients of determination were inversely related to bone size, with R2 values exceeding 0.9 at the femur, 0.8 at the humerus, and ranging between 0.3 and 0.6 at the radius. We found that our instrument has a position artifact, with BMD and bone mineral content dependent on the specimen's coordinates in the scanned field. Our findings establish the limitations of ex vivo densitometry with the PIXImus and support our recommendation that investigators seek position artifacts in their instruments.

Bone is a target for the antidiabetic compound rosiglitazone

Rosiglitazone is an FDA-approved oral antidiabetic agent for the treatment of type 2 diabetes. This compound improves insulin sensitivity through the activation of the nuclear receptor, peroxisome proliferator-activated receptor-gamma (PPAR-gamma). In addition to sensitizing cells to insulin, the PPAR-gamma2 isoform appears to be critical for the regulation of osteoblast and adipocyte differentiation from common mesenchymal bone marrow progenitors. We have demonstrated previously that PPAR-gamma2 activated with rosiglitazone acts as a dominant inhibitor of osteoblastogenesis in murine bone marrow in vitro. Here, we show that in vivo, rosiglitazone administration results in significant bone loss. When rosiglitazone (20 microg/g body weight/d) was given to 6-month-old, nondiabetic C57BL/6 mice for 7 wk, a significant decrease in total body bone mineral density was observed. Analysis of bone microarchitecture, using micro-computed tomography, demonstrated a decrease in bone volume, trabecular width, and trabecular number and an increase in trabecular spacing. Histomorphometric analysis showed a decrease in bone formation rate, with a simultaneous increase in fat content in the bone marrow. Changes in bone morphology and structure were accompanied by changes in the expression of osteoblast- and adipocyte-specific marker genes; the expression of the osteoblast-specific genes Runx2/Cbfa1, Dlx5, and alpha1(I)collagen were decreased, whereas the expression of the adipocyte-specific fatty acid binding protein aP2, was increased. These in vivo data suggest that rosiglitazone therapy may pose a significant risk of adverse skeletal effects in humans.

Anabolic action of parathyroid hormone on cortical and cancellous bone differs between axial and appendicular skeletal sites in mice

The mouse is being increasingly used to study the anabolic action of parathyroid hormone (PTH) on the skeleton. The efficacy of intermittent PTH treatment on bone varies widely among tested strains of mice with differences in peak bone mass and structure. We have therefore examined the responses of skeletal sites with high or low cancellous bone mass to PTH treatment in a single strain with genetically low bone mass. Mature C57BL/6 mice were ovariectomized (ovx) or sham operated and, after 4 weeks, treated with PTH(1-34) (40 microg/kg/day, 5 days/week sc) or vehicle for 3 or 7 weeks. Two doses of fluorescent labels were given to the animals 9 and 3 days before euthanasia. Histomorphometry was performed on sections of the proximal tibia, tibial diaphysis, and vertebral body. The results indicate that 4 to 11 weeks of ovx induced a approximately 44% loss of cancellous bone in the proximal tibia and a approximately 25% loss of cancellous bone in the vertebra with impaired trabecular architecture and high bone turnover. In the intact animals, PTH increased cancellous bone volume to a greater extent in the vertebral body than in the proximal tibia, a site with lower cancellous bone volume at the outset. In the ovx mice, PTH increased cancellous bone volume to a greater extent in the vertebral body, a site displaying moderate cancellous bone loss, than in the proximal tibia, a site with severe cancellous bone loss. Conversely, the treatment added a little cortical bone to the tibia, a highly loaded site, but did not significantly increase cortical width of the vertebral body, a less loaded site. We conclude that, for intermittent PTH treatment to be maximally effective, there must be an adequate number of trabeculae present at the beginning of treatment, regardless of estrogen status. Our results also support an interaction between PTH anabolic action and mechanical loading.