Bone structure and metabolism in a rodent model of male senile osteoporosis.

Osteoporosis is a common and severe condition in elderly men, which is poorly characterized. In order to identify the hallmarks of age-related bone loss in the male mammalian skeleton, we studied several aspects of bone structure and metabolism in 23-month-old male Sprague-Dawley rats and compared them to 5-month-old animals. Cancellous bone mineral density, bone volume and trabecular number were markedly reduced in the proximal tibia of aged rats when compared to the young rats. An increase in bone matrix material density indicating a reduced deposition of new bone matrix was seen. Also, serum levels of osteocalcin, a marker of bone formation, were reduced in old males. The decreased bone formation could in part be linked to the decreased serum insulin-like growth factor 1 (IGF-1) levels which were observed in these animals. Serum levels of RatLaps (c-terminal telopeptide of type I collagen) were increased. Interestingly, an ex vivo osteoclast generation assay revealed that bone marrow from aged rats formed fewer osteoclasts than that from young rats. Consistent with this observation, serum levels of soluble RANKL, a critical osteoblast derived factor for osteoclastogenesis, were decreased in aged rats and RANKL mRNA expression was slightly reduced in bone marrow cells. Elevated leptin and adiponectin levels present in these animals could have contributed further to impaired osteoclastogenesis. We conclude that aged male rodents are characterized by a severely diminished cancellous bone network and a bone turnover situation in which bone formation is decreased to such an extent that it is outweighed by bone resorption, despite a blunted osteoclast generation potential of the bone marrow.

Increased bone resorption and impaired bone microarchitecture in short-term and extended high-fat diet-induced obesity.

Although obesity traditionally has been considered a condition of low risk for osteoporosis, this classic view has recently been questioned. The aim of this study was to assess bone microarchitecture and turnover in a mouse model of high-fat diet-induced obesity. Seven-week-old male C57BL/6J mice (n = 18) were randomized into 3 diet groups. One third (n = 6) received a low-fat diet for 24 weeks, one third was kept on an extended high-fat diet (eHF), and the remaining was switched from low-fat to high-fat chow 3 weeks before sacrifice (sHF). Serum levels of insulin, leptin, adiponectin, osteocalcin, and cross-linked telopeptides of type I collagen (CTX) were measured. In addition, bone microarchitecture was analyzed by micro-computed tomography; and lumbar spine bone density was assessed by dual-energy x-ray absorptiometry. The CTX, body weight, insulin, and leptin were significantly elevated in obese animals (sHF: +48%, +24%, +265%, and +102%; eHF: +43%, +52%, +761%, and +292%). The CTX, body weight, insulin, and leptin showed a negative correlation with bone density and bone volume. Interestingly, short-term high-fat chow caused similar bone loss as extended high-fat feeding. Bone volume was decreased by 12% in sHF and 19% in eHF. Bone mineral density was 25% (sHF) and 27% (eHF) lower when compared with control mice on low-fat diet. As assessed by the structure model index, bone microarchitecture changed from plate- to rod-like appearance upon high-fat challenge. Trabecular and cortical thickness remained unaffected. Short-term and extended high-fat diet-induced obesity caused significant bone loss in male C57BL/6J mice mainly because of resorptive changes in trabecular architecture.

The HLA-B27 transgenic rat, a model of spondyloarthritis, has decreased bone mineral density and increased RANKL to osteoprotegerin mRNA ratio.

OBJECTIVE: Bone metabolism in spondyloarthritis (SpA) is not well elucidated. We investigated alterations in bone in the HLA-B27 transgenic rat, a model of SpA. METHODS: Femur, tibia, and lumbar vertebral bodies of disease-prone HLA-B27 transgenic, healthy HLA-B7 transgenic, and nontransgenic control rats were used for bone histomorphometric and dual energy x-ray absorptiometry (DEXA) analysis. Serum levels of type I collagen C-telopeptides (CTX), N-terminal propeptide of type I procollagen (P1NP), and osteocalcin, as well as receptor activator of nuclear factor-kappa B ligand (RANKL) and osteoprotegerin (OPG), were measured. RNA was isolated from the bone tissue of the femura to analyze gene expression of RANKL, OPG, and osteocalcin. RESULTS: Histomorphometric analysis indicated a significant decrease in bone volume as well as trabecular number and thickness in the HLA-B27 rats. Trabecular separation was increased. Numbers of osteoblasts, osteoclasts, and osteoid volume were not altered significantly. The decrease in bone mineral density was confirmed using DEXA. Levels of RANKL mRNA were significantly increased in the bone tissue of HLA-B27 transgenic rats, resulting in an increased RANKL to OPG ratio. Osteocalcin mRNA expression was also significantly elevated in bone of HLA-B27 rats. Serum levels of CTX, RANKL, OPG, P1NP, and osteocalcin did not differ significantly. CONCLUSION: Our data indicate that, similarly to SpA in humans, HLA-B27 transgenic rats show a reduced bone mass, and suggest an involvement of the RANKL/OPG system in the mechanism of bone loss in this disease. This model may be adequate to study osteoporosis in SpA.

Intracellular and surface RANKL are differentially regulated in patients with ankylosing spondylitis.

Ankylosing spondylitis (AS) is characterized by ankylosis of axial joints but osteoporosis is also a well-reported feature. T cells have been implicated as a source of receptor activator of NFkappaB ligand (RANKL) in inflammatory bone diseases. Hence, we assessed whether T cells in patients with AS act as a source of RANKL too. Therefore, we investigated the expression of RANKL on T cells from 21 patients with AS by flow cytometry. Bone mineral density (BMD) was evaluated by quantitative computer tomography (QCT) and dual X-ray absorptiometry (DXA) and correlated with serum levels of osteoprotegerin (OPG) and RANKL. BMD was decreased in 45% of all patients when measured with DXA (48% with QCT) and correlated negatively with OPG. Expression of intracellular RANKL was increased on CD4+ (84 vs. 70%) and CD8+ (85.2 vs. 65.3%, P < 0.05) T cells in patients with AS, whereas expression of membrane-bound RANKL was significantly lower (CD4+: 2.2 vs. 8.5% and CD8+: 0.7 vs. 3.2%, P < 0.01). Our results indicate that surface and intracellular RANKL production is differentially regulated on T cells of patients with AS.