Bone biomechanical properties in LRP5 mutant mice

The mutation responsible for the high bone mass (HBM) phenotype has been postulated to act through the adaptive response of bone to mechanical load resulting in denser and stronger skeletons in humans and animals. The bone phenotype of members of a HBM family is characterized by normally shaped bones that are exceptionally dense, particularly at load bearing sites [Cancer Res. 59 (1999) 1572]. The high bone mass (HBM) mutation was identified as a glycine to valine substitution at amino acid residue 171 in the gene coding for low-density lipoprotein receptor-related protein 5 (LRP5) [Bone Miner. Res. 16(4) (2001) 758]. Thus, efforts have focused on the examination of the role of LRP5 and the G171V mutation in bone mechanotransduction responses [J. Bone Miner. Res 18 (2002) 960]. Transgenic mice expressing the human G171V mutation have been shown to have skeletal phenotypes remarkably similar to those seen in affected individuals. In this study, we have identified differences in biomechanical (structural and apparent material) properties, bone mass/ash, and bone stiffness of cortical and cancellous bone driven by the G171V mutation in LRP5. As in humans, the LRP5 G171V plays an important role in regulating bone structural phenotypes in mice. These bone phenotypes include greater structural and apparent material properties in HBM HET as compared to non-transgenic littermates (NTG) mice. Body size and weight in HBM HET were similar to that in NTG control mice. However, the LRP5 G171V mutation in HET mice results in a skeleton that has greater structural (femoral shaft, femoral neck, tibiae, vertebral body) and apparent material (vertebral body) strength, percent bone ash weight (ulnae), and tibial stiffness. Despite similar body weight to NTG mice, the denser and stiffer bones in G171V mice may represent greater bone formation sensitivity to normal mechanical stimuli resulting in an overadaptation of skeleton to weight-related forces.

Differences in vertebral structure and strength of inbred female mouse strains

This study assessed mouse strain-related differences in vertebral biomechanics and histomorphometry in inbred mice strains shown to differ in bone mineral content (BMC) and areal density (BMD) (as measured by pDEXA). Lumbar vertebrae L3 to L5 were collected from three mice strains (C3H/HeJ[C3], C57BL/6J[B6], and DBA/2J[D2], n=12/strain, 4-month-old female, 22.2 +/- 0.3g). BMC and BMD were measured in L3 and L4 using peripheral dual energy x-ray absorptiometry. The L4 vertebral body was then mechanically tested in compression to determine structural properties (ultimate/yield load, stiffness) from load-displacement curves and derive apparent material properties (ultimate/yield stress, and modulus of elasticity). L5 was processed for histomorphometric evaluation. Vertebral BMC and BMD were greater in C3 than in B6 and D2 mice. Vertebral trabecular/cancellous bone volume was smaller in C3 than in D2 and B6 mice. Trabecular bone formation rates were greater in D2 than in B6 and C3 mice. Osteoid surface was smaller in C3 mice than in B6 and D2 mice. Differences in osteoclast and mineralizing surfaces were not detected among the three mouse strains. In addition, there were no significant differences in biomechanical properties between the three strains. Despite the greatest BMC and areal BMD in C3 mice, the lack of strain-related differences in vertebral body strength data suggests that the biomechanical properties may be affected by the bone distribution and/or complex combination of cortical and cancellous bone at this site.

Effects of nicotine on bone mass and strength in aged female rats

This study investigated the effects of nicotine on bone mass and biomechanical properties in aged, estrogen-replete (sham-operated) and estrogen-deplete (ovariectomized) female rats. Eight month old, retired breeder, sham-operated and ovariectomized Sprague-Dawley rats were left untreated for 12 weeks to establish cancellous osteopenia in the ovariectomized group. The animals were then administered saline, low dose nicotine (6.0 mg/kg/day) or high dose nicotine (9.0 mg/kg/day) via osmotic minipumps for 12 weeks. Vertebrae and femora were collected at necropsy for determination of bone mass and strength. As expected, ovariectomy had a negative effect on most endpoints evaluated. Vertebral body bone mineral content (BMC) and density (BMD) and the structural (ultimate load and yield load) and material (ultimate stress, yield stress, and flexural modulus of elasticity) strength properties were lower in the OVX rats than in the sham-operated rats. Femoral diaphysis BMC, BMD, ultimate load, and flexural modulus were also lower in the OVX rats than in the sham-operated rats. The nicotine doses administered resulted in serum nicotine levels that averaged 1.5-4.5-fold greater than those observed in heavy smokers. Despite the high doses used, nicotine had no effect on vertebral BMC, BMD, or any of the structural and material strength properties in either the OVX or the Sham rats. In addition, nicotine had no effect on femoral diaphysis BMC, BMD, ultimate load, stiffness, ultimate stress, or flexural modulus. Femoral yield load and stress were lower in low dose nicotine-treated rats than in vehicle-treated rats. However, differences were not detected between the high dose nicotine- and vehicle-treated rats for either femoral yield load or stress. The results suggest that tobacco agents other than nicotine are responsible for the decreased bone density and increased fracture risk as observed in smokers.

Effects of nicotine on bone and calciotropic hormones in aged ovariectomized rats

The objective of this investigation was to assess the effects of chronic nicotine administration on bone status and serum calcium and calciotropic hormone levels in aged, estrogen-replete (intact, sham-operated) and estrogen-deplete (ovariectomized) female rats. Eight-month-old sham-operated (sham) and ovariectomized (ovx) retired breeder rats were maintained untreated for 3 months to allow for the development of osteopenia in the ovx group. The animals were then administered either saline, low dose nicotine (6.0 mg/kg/day), or high dose nicotine (9.0 mg/kg/day) via osmotic minipumps for 3 months. Blood was drawn at necropsy for determination of serum nicotine, cotinine, Ca, PTH, 25(OH)D, and 1,25(OH)(2)D. Right tibiae were collected and processed undecalcified for cancellous and cortical bone histomorphometry. Histomorphometric endpoints evaluated at the proximal tibial metaphysis included cancellous bone volume (BV/TV), osteoclast surface (Oc.S), osteoid surface (OS), mineralizing surface (MS), mineral apposition rate (MAR), and bone formation rate (BFR). Histomorphometric endpoints evaluated at the tibial diaphysis included cortical area (Ct.Ar), marrow area (Ma.Ar), and periosteal and endocortical MS, MAR, and BFR. Ovariectomy resulted in lower cancellous BV/TV and Ct.Ar and higher cancellous, endocortical, and periosteal MS and BFR. The presence of nicotine in serum confirmed successful delivery of the drug via osmotic minipumps. Administration of nicotine at the high dose resulted in lower serum 25(OH)D levels but differences in serum Ca or PTH were not detected with either nicotine treatment. Differences with nicotine treatment were also not detected for Oc.S at the proximal tibia. While treatment with nicotine at the high dose resulted in higher MS and BFR, in both sham and ovx rats, there were no differences due to nicotine treatment in cancellous BV/TV. Marrow area was greater in rats treated with nicotine than in rats treated with vehicle. However, differences with nicotine treatment were not detected in Ct.Ar in either intact or ovx rats. Overall, these findings indicate that steady state nicotine exposure does not alter bone mass in intact or ovx rats but may have detrimental effects on body storage of vitamin D.

Effects of nicotine on bone mass, turnover, and strength in adult female rats

This study investigated the effects of nicotine, the chemical responsible for tobacco addiction, on bone and on serum mineral and calcitropic hormone levels in adult, female rats to help resolve a current controversy regarding the impact of nicotine on bone health. Seven-month-old rats received either saline (n = 12), low-dose nicotine (4.5 mg/kg/day, n = 2), or high-dose nicotine (6.0 mg/kg/day, n = 12) administered subcutaneously via osmotic minipumps for 3 months. Blood, femora, tibiae, and lumbar vertebrae (3-5) were collected at necropsy for determination of serum mineral and hormonal concentrations, bone density (femora and vertebrae), bone turnover (tibiae), and bone strength (femora). The presence of nicotine in serum (111 +/- 7 and 137 +/- 10 ng/ml for the low- and high-dose nicotine groups, respectively) confirmed successful delivery of the drug via osmotic minipumps. Nicotine-induced treatment differences were not detected in serum calcium, 25-hydroxyvitamin D, and 1,25-dihydroxyvitamin D. However, serum phosphorus and parathyroid hormone (PTH) were higher in rats treated with high-dose nicotine, and serum calcitonin was lower in rats treated with both high- and low-dose nicotine than in control rats. Nicotine treatment had no effect on tibial cancellous or cortical bone turnover or femoral bone mineral content (BMC) and density (BMD). Femoral ultimate load and vertebral BMC were lower in rats treated with high-dose nicotine than in control rats. We conclude that nicotine at serum concentrations 2.5-fold greater than the average in smokers has limited detrimental effects on bone in normal, healthy female rats.