Effects of osteoprotegerin administration on osteoclast differentiation and trabecular bone structure in osteoprotegerin-deficient mice

Rhizomelia and Impaired Linear Growth in a Girl with Juvenile Paget Disease: The Natural History of the Condition

In ultra-rare bone diseases, information on growth during childhood is sparse. Juvenile Paget disease (JPD) is an ultra-rare disease, characterized by loss of function of osteoprotegerin (OPG). OPG inhibits osteoclast activation via the receptor activator of nuclear factor-κB (RANK) pathway. In JPD, overactive osteoclasts result in inflammatory-like bone disease due to grossly elevated bone resorption. Knowledge on the natural history of JPD, including final height and growth, is limited. Most affected children receive long-term antiresorptive treatment, mostly with bisphosphonates, to contain bone resorption, which may affect growth. In this study, we report the follow-up of height, growth velocity, and skeletal maturation in a 16-year-old female patient with JPD. The patient was treated with cyclic doses of pamidronate starting at 2.5 years of age and with 2 doses of denosumab at the age of 8 years, when pamidronate was paused. In the following years, a sustainable decline in a height z-score and a stunted pubertal growth spurt; despite appropriate maturation of the epiphyseal plates of the left hand, the proximal right humerus and both femora were observed. Whether this reflects the growth pattern in JPD or might be associated to the antiresorptive treatments is unclear, since there is very limited information available on the effect of bisphosphonates and denosumab on growth and the growth plate in pediatric patients. Studies are needed to understand the natural history of an ultra-rare bone disease and to assess the effects of antiresorptive treatment on the growing skeleton.

Simulation of bone remodeling around a femoral prosthesis using a model that accounts for biological and mechanical interactions

The present study focuses on a model for three-dimensional bone remodeling of the human femur that considers cellular dynamics to determine the volume fraction of new bone. The model considers the interaction among responsive osteoblasts, active osteoblasts, and osteoclasts, as well as signaling molecules and parathyroid hormone (PTH). The stimulus of the model has a systemic origin due to the PTH effect, and a local origin due to the action of cytokines, growth factors, and mechanical stimuli near the site of the bone cells. The present work considers that the mechanical stimulus that activates cellular activity is obtained from stresses acting on the bone tissue and the number of daily loading cycles. In addition to simulating the bone modeling process in an intact femur, the numerical model is used to simulate bone adaptation in relation to the stress shielding phenomenon after the implantation of a femoral prosthesis. The results showed that the simulations provide a distribution of bone density that is similar to a radiograph and, in addition, allows the visualization of osteoblast and osteoclast dynamics in bone adaptation response after prosthesis implantation.

Generalized Degenerative Joint Disease in Osteoprotegerin (Opg) Null Mutant Mice

Bone structure is modulated by the interaction between receptor activator of nuclear factor-κB (RANK) and RANK ligand (RANKL). Osteoprotegerin (OPG), a decoy receptor for RANKL, modifies osteoclast-mediated bone resorption directly and spares articular cartilage indirectly in rodents with immune-mediated arthritis by preventing subchondral bone destruction. The OPG/RANKL balance also seems to be critical in maintaining joint integrity in osteoarthritis, a condition featuring articular bone and cartilage damage in the absence of profound inflammation. The current study explored the role of OPG in sparing articular cartilage by evaluating joint lesions in adult C57BL/6J mice lacking osteoprotegerin (Opg (-) (/-)). At 3, 5, 7, 9, and 12 months of age, both sexes of Opg (-) (/-) mice developed severe degenerative joint disease (DJD) characterized by progressive loss of cartilage matrix and eventually articular cartilage. Lesions developed earlier and more severely in Opg (-) (/-) mice relative to age-matched, wild-type (Opg (+) (/+)), or heterozygous (Opg (+) (/-)) littermates (P ≤ .05). The femorotibial joint was affected bilaterally at 3 months, while other key weight-bearing diarthrodial joints (eg, coxofemoral, scapulohumeral, humeroradioulnar) were affected later and unilaterally. Cortical bone in subchondral plates and long bone diaphyses of Opg (-) (/-) mice but not Opg (+/+) or Opg (+) (/-) animals was osteoporotic by 3 months of age (P ≤ .05); the extent of porosity was less than the degree of DJD. Closure of the physes in long bones (P ≤ .05) and cartilage retention in the femoral primary spongiosa (P ≤ .05) affected chiefly Opg (-) (/-) mice. These data suggest that OPG plays an essential direct role in maintaining cartilage integrity in the articular surfaces and physes.

The mineral dissolution function of osteoclasts is dispensable for hypertrophic cartilage degradation during long bone development and growth

During long bone development and post-natal growth, the cartilaginous model of the skeleton is progressively replaced by bone, a process known as endochondral ossification. In the primary spongiosa, osteoclasts degrade the mineralized cartilage produced by hypertrophic chondrocytes to generate cartilage trabeculae that osteoblasts embed in bone matrix. This leads to the formation of the trabecular bone network of the secondary spongiosa that will undergo continuous remodeling. Osteoclasts are specialized in mineralized tissue degradation, with the combined ability to solubilize hydroxyapatite and to degrade extracellular matrix proteins. We reported previously that osteoclasts lacking Dock5 could not degrade bone due to abnormal podosome organization and absence of sealing zone formation. Consequently, adult Dock5(-/-) mice have increased trabecular bone mass. We used Dock5(-/-) mice to further investigate the different functions of osteoclast during endochondral bone formation. We show that long bones are overall morphologically normal in developing and growing Dock5(-/-) mice. We demonstrate that Dock5(-/-) mice also have normal hypertrophic cartilage and cartilage trabecular network. Conversely, trabecular bone volume increased progressively in the secondary spongiosa of Dock5(-/-) growing mice as compared to Dock5(+/+) animals, even though their osteoclast numbers were the same. In vitro, we show that Dock5(-/-) osteoclasts do present acidic compartments at the ventral plasma membrane and produce normal amounts of active MMP9, TRAP and CtsK for matrix protein degradation but they are unable to solubilize minerals. These observations reveal that contrarily to bone resorption, the ability of osteoclasts to dissolve minerals is dispensable for the degradation of mineralized hypertrophic cartilage during endochondral bone formation.

α-Actinin-3 deficiency is associated with reduced bone mass in human and mouse

Bone mineral density (BMD) is a complex trait that is the single best predictor of the risk of osteoporotic fractures. Candidate gene and genome-wide association studies have identified genetic variations in approximately 30 genetic loci associated with BMD variation in humans. α-Actinin-3 (ACTN3) is highly expressed in fast skeletal muscle fibres. There is a common null-polymorphism R577X in human ACTN3 that results in complete deficiency of the α-actinin-3 protein in approximately 20% of Eurasians. Absence of α-actinin-3 does not cause any disease phenotypes in muscle because of compensation by α-actinin-2. However, α-actinin-3 deficiency has been shown to be detrimental to athletic sprint/power performance. In this report we reveal additional functions for α-actinin-3 in bone. α-Actinin-3 but not α-actinin-2 is expressed in osteoblasts. The Actn3(-/-) mouse displays significantly reduced bone mass, with reduced cortical bone volume (-14%) and trabecular number (-61%) seen by microCT. Dynamic histomorphometry indicated this was due to a reduction in bone formation. In a cohort of postmenopausal Australian women, ACTN3 577XX genotype was associated with lower BMD in an additive genetic model, with the R577X genotype contributing 1.1% of the variance in BMD. Microarray analysis of cultured osteoprogenitors from Actn3(-/-) mice showed alterations in expression of several genes regulating bone mass and osteoblast/osteoclast activity, including Enpp1, Opg and Wnt7b. Our studies suggest that ACTN3 likely contributes to the regulation of bone mass through alterations in bone turnover. Given the high frequency of R577X in the general population, the potential role of ACTN3 R577X as a factor influencing variations in BMD in elderly humans warrants further study.

Accelerated cartilage resorption by chondroclasts during bone fracture healing in osteoprotegerin-deficient mice

Receptor activator of nuclear factor-kappaB ligand (RANKL) and osteoprotegerin (OPG), a decoy receptor of RANKL, maintain bone mass by regulating the differentiation of osteoclasts, which are bone-resorbing cells. Endochondral bone ossification and bone fracture healing involve cartilage resorption, a less well-understood process that is needed for replacement of cartilage by bone. Here we describe the role of OPG produced by chondrocytes in chondroclastogenesis. Fracture healing in OPG(-/-) mice showed faster union of the fractured bone, faster resorption of the cartilaginous callus, and an increased number of chondroclasts at the chondroosseous junctions compared with that in wild-type littermates. When a cultured pellet of OPG(-/-) chondrocytes was transplanted beneath the kidney capsule, the pellet recruited many chondroclasts. The pellet showed the ability to induce tartrate-resistant acid phosphatase-positive multinucleated cells from RAW 264.7 cells in vitro. Finally, OPG(-/-) chondrocytes (but not wild-type chondrocytes) cultured with spleen cells induced many tartrate-resistant acid phosphatase-positive multinucleated cells. The expression of RANKL and OPG in chondrocytes was regulated by several osteotropic factors including 1,25-dihydroxyvitamin D(3), PTHrP, IL-1alpha, and TNF-alpha. Thus, local OPG produced by chondrocytes probably controls cartilage resorption as a negative regulator for chondrocyte-dependent chondroclastogenesis.

Theoretical investigation of the role of the RANK-RANKL-OPG system in bone remodeling

The RANK-RANKL-OPG system is an essential signaling pathway involved in bone cell-cell communication, with ample evidence that modification of the RANK-RANKL-OPG signaling pathway has major effects on bone remodeling. The first focus of this paper is to demonstrate that a theoretical model of bone cell-cell interactions is capable of qualitatively reproducing changes in bone associated with RANK-RANKL-OPG signaling. To do this we consider either biological experiments or bone diseases related to receptor and/or ligand deficiencies, including RANKL over-expression, ablation of OPG production and/or RANK receptor modifications. The second focus is to investigate a wide range of possible therapeutic strategies for re-establishing bone homeostasis for various pathologies of the RANK-RANKL-OPG pathway. These simulations indicate that bone diseases associated with the RANK-RANKL-OPG pathway are very effective in triggering bone resorption compared to bone formation. These results align with Hofbauer's "convergence hypothesis", which states that catabolic bone diseases most effectively act through the RANK-RANKL-OPG system. Additionally, we demonstrate that severity of catabolic bone diseases strongly depends on how many components of this pathway are affected. Using optimization algorithms and the theoretical model, we identify a variety of successful "virtual therapies" for different disease states using both single and dual therapies.

Osteoclast polarization is not required for degradation of bone matrix in rachitic FGF23 transgenic mice

Hypophosphatemic transgenic (tg) mice overexpressing FGF23 in osteoblasts display disorganized growth plates and reduced bone mineral density characteristic of rickets/osteomalacia. These FGF23 tg mice were used as an in vivo model to examine the relation between osteoclast polarization, secretion of proteolytic enzymes and resorptive activity. Tg mice had increased mRNA expression levels of the osteoblast differentiation marker Runx2 and mineralization-promoting proteins alkaline phosphatase and bone sialoprotein in the long bones compared to wild type (wt) mice. In contrast, expression of alpha1(I) collagen, osteocalcin, dentin matrix protein 1 and osteopontin was unchanged, indicating selective activation of osteoblasts promoting mineralization. The number of osteoclasts was unchanged in tg compared to wt mice, as determined by histomorphometry, serum levels of TRAP 5b activity as well as mRNA expression levels of TRAP and cathepsin K. However, tg mice displayed elevated serum concentrations of C-terminal telopeptide of collagen I (CTX) indicative of increased bone matrix degradation. The majority of osteoclasts in FGF23 tg mice lacked ultrastructural morphological signs of proper polarization. However, they secreted both cathepsin K and MMP-9 at levels comparable to osteoclasts with ruffled borders. Mineralization of bone matrix thus appears essential for inducing osteoclast polarization but not for secretion of osteoclast proteases. Finally, release of CTX by freshly isolated osteoclasts was increased on demineralized compared to mineralized bovine bone slices, indicating that the mineral component limits collagen degradation. We conclude that ruffled borders are implicated in acidification and subsequent demineralization of the bone matrix, however not required for matrix degradation. The data collectively provide evidence that osteoclasts, despite absence of ruffled borders, effectively participate in the degradation of hypomineralized bone matrix in rachitic FGF23 tg mice.