Anabolic actions of PTH in the skeletons of animals

A brief historical perspective reviews studies that tested the hypotheses that PTH induces an anabolic effect in bone, and that the gain in trabecular bone was not at the expense of cortical bone. As PTH reduces the risk of fracture in humans with osteoporosis, the myths that postulated cortical bone porosity and increased bone turnover might increase fracture risk, are examined in the light of data from animals with osteonal bone. These show that PTH "braces" the bone by immediately stimulating bone formation at modeling and remodeling sites. Increased porosity is a late event, occurring close to the neutral axis of bone where detrimental effects on biomechanical strength are unlikely. PTH increases bone mass by stimulating modeling in favor of bone formation, and restructures bone geometry via more extensive remodeling. Cell and genetic events induced in bone by PTH have been studied in rats and are time- and regimen-dependent. In addition to the stimulation of gene expression for matrix proteins, early genes upregulated by once daily PTH are those associated with matrix degradation and induction of osteoclastic resorption, indicative of possible mechanisms by which PTH may increase bone turnover. Boneforming surfaces are increased due to increased numbers of newly differentiated osteoblasts and retention of older osteoblasts by inhibition of apoptosis. After stopping treatment, the number of osteoblasts is quickly reduced and bone turnover returns to that of controls, slowing both bone formation and resorption. The increased proportion of bone undergoing PTH-induced remodeling requires maturation and completion of mineralization. These responses may explain the delay in reversal of gains in bone mass and biomechanical properties for at least two turnover cycles following withdrawal in large animal models. Thus, the skeletal benefits of PTH extend beyond the active treatment phase.     

Mineralized Tissue in Aging1

Mineralized tissues undergo continuous remodeling through an appropriate balance in activity of osteoblasts, which regulate bone formation, and osteoclasts, which regulate bone resorption. Identification of the factors which control the osteogenic process has not yet been accomplished. Such identification is vital however, if we are to understand any aberrations in remodeling, that may occur with disease and/or aging. It is known that aging is accompanied by a decrease in absolute bone volume, which may be attributed to alterations in bone specific factors that regulate osteoblast and osteoclast function. In order to better understand age associated bone changes, the author and colleagues have compared extracts of human fetal bone with adult bone using a chemotactic assay, SDS-polyacrylamide gel electrophoresis and molecular sieve chromatography. The results show that the extract of adult bone, when compared with fetal bone, had a relative increase in organic components of lower molecular weight. Second, that chemoattractant activity was present in the extract of fetal bone, but not of adult bone. These differences may in part be related to the clinical condition of increased fracture rate and decreased fracture healing and bone remodeling in older individuals when compared with younger populations.     

Effect of alendronate administration on bone mineral density and bone strength in castrated rats.

Castration of male rats leads to increased bone turnover and osteopenia. This study was conducted to examine the effects of the aminobisphosphonate alendronate on castration-induced bone changes. Bisphosphonates are drugs that inhibit bone turnover by decreasing the resorption. Since they suppress bone remodeling, they may also prevent the repair of microdamage and decrease bone strength. Although the mechanical properties of bones are directly related to the determination of fracture risk, bisphosphonate effects on the related variables have scarcely been investigated. Twenty-four male Wistar rats at two months of age were castrated or sham-operated to evaluate the effects of long-term administration (six months) of sodium alendronate at a dose of 1 mg/kg/day. The bones were tested mechanically by a three-point bending test in a Mini Bionix (MTS) testing system. High bone remodeling seen in castrated rats expressed by increased TrACP and B-ALP was suppressed by alendronate administration. Bone from castrated rats was characterized by a reduction in bone density as well as ash, calcium and phosphate content. Castration significantly altered mechanical properties of bone and femoral cortical thickness. When castrated rats were treated with high dose of alendronate, the changes in bone density resulting from castration were entirely prevented, and mechanical analysis revealed preserved mechanical strength of femur and cortical thickness. We conclude that castration induces cortical bone loss associated with high bone turnover in the male rat, and this bone loss can be prevented by alendronate through the inhibition of osteoclastic activity, while preserving the mechanical properties of bone. These results document the efficacy of alendronate, even at high doses, in preventing bone loss, loss of bone mechanical strength, and the rise in biochemical bone turnover indicators due to castration in rats, and raises the possibility that a alendronate could be equally effective in humans.     

The skeleton in primary hyperparathyroidism: a review focusing on bone remodeling, structure, mass, and fracture.

The mechanisms behind the influence of PHPT on the skeleton are closely connected with bone turnover. Throughout life, the skeleton is continuously renewed by bone remodeling, a process which serves the purpose of repairing damaged bone and adapting the skeleton to changes in physical load. In this process, old bone is removed by osteoclastic resorption and new bone is laid down by osteoblastic formation. Bone mass increases with growth in the first decades of life, and around the age of 30 years the peak bone mass is reached. Thereafter, as a result of mechanisms involving bone remodeling, a net bone loss is seen: 1) A reversible bone loss because of increase in the remodeling space, i.e., the amount of bone resorped but not yet reformed during the remodeling cycle. This mechanism leads to decrease in average trabecular thickness and cortical width, and to increase in cortical porosity. 2) An irreversible bone loss caused by negative bone balance, where the amount of bone formed by the osteoblasts is exceeded by the amount of bone resorbed by the osteoclasts at the same remodeling site. Consequently, progressive thinning of trabecular elements, reduced cortical width and increased cortical porosity is seen. 3) Finally, perforation of trabecular plates by deep resorption lacunae leads to complete irreversible removal of structural bone components. Parathyroid hormone, together with vitamin D, are the principal modulators in calcium homeostasis. The main actions of PTH are executed in bone and kidneys. In the kidneys, PTH increases the tubular re-absorption of calcium, thereby tending to increase serum calcium. PTH also induces increased conversion of 25(OH)-D to 1,25(OH)2-D. This last action, enhances intestinal calcium absorption and increased skeletal calcium mobilization, which further adds to the circulating calcium pool. In bone, the "acute" regulatory actions of PTH on serum calcium are probably accompliced via activation of osteocytes and lining cells. A second mechanism of PTH in bone is the regulation of bone remodeling. The action seems to be an increased recruitment from osteoblastic precursor cells and activation of mature osteoclasts. It is supposed that these responses are predominantly mediated indirectly through actions on osteoblast-like or nonosteoblast-like stromal cells, as osteoclasts themselves to not have PTH receptors. Bone metabolism and bone mass are studied by biochemical bone markers, bone histomorphometry, and densitometry. As bone markers and bone histomorphometry give information on bone metabolism from different points of view, these methods are preferably combined. Histomorphometry gives detailed information about bone turnover on cellular level, the whole remodeling sequence is described, and the bone balance can be calculated. However, they focus on a small volume, and may, therefore, not be representative for the whole skeleton. On the other hand, studies of bone markers supply general information about turnover in the whole skeleton, but they do not give facts on the bone turnover on the cellular or tissue level and bone balance. Bone densitometry is the principal method in studying bone mass, but valuable information concerning bone structure also comes from histomorphometry. Bone remodeling is considerably increased in PHPT. Studies of bone markers show increase in both resorptive and formative markers, and the increases seem to be of equivalent size. This is in agreement with histomorphometric findings and shows that the coupling between resorption and formation is preserved. By histomorphometry on iliac crest biopsies, trabecular bone remodeling is found increased by 50%, judged by the increase in activation frequency; a measure of how often new remodeling is initiated on the trabecular bone surface. In PHPT, such remodeling activity is repeated about once every year. Reconstruction of the whole remodeling sequence does not show major deviations in lengths of the resorptive and formative periods compared to normal. Furthermore, the amount of bone removed by the osteoclasts during the resorptive phase is matched by the amount of new bone formed by the osteoblasts leading to a bone balance very close to zero. Compared with trabecular bone, the turnover rate in cortical bone is considerably lower, around 10%. Remodeling of the cortical bone takes place at the endocortical, the pericortical, and the Haversian surfaces. Endocortical bone remodeling activities are very similar to trabecular remodeling activities with good correlation between individual parameters. Periosteal remodeling activity is negligible in PHPT, as it is in the normal state. Cortical porosity, which reflects the remodeling activity on the Haversian surface, is increased by 30-65% in PHPT. (ABSTRACT TRUNCATED)     

Bone material properties and mineral matrix contributions to fracture risk or age in women and men

The strength of bone is related to its mass and geometry, but also to the physical properties of the tissue itself. Bone tissue is composed primarily of collagen and mineral, each of which changes with age, and each of which can be affected by pharmaceutical treatments designed to prevent or reverse the loss of bone. With age, there is a decrease in collagen content, which is associated with an increased mean tissue mineralization, but there is no difference in cross-link levels compared to younger adult bone. In osteoporosis, however, there is a decrease in the reducible collagen cross-links without an alteration in collagen concentration; this would tend to increase bone fragility. In older people, the mean tissue age (MTA) increases, causing the tissue to become more highly mineralized. The increased bone turnover following menopause may reduce global MTA, and would reduce overall tissue mineralization. Bone strength and toughness are positively correlated to bone mineral content, but when bone tissue becomes too highly mineralized, it tends to become brittle. This reduces its toughness, and makes it more prone to fracture from repeated loads and accumulated microcracking. Most approved pharmaceutical treatments for osteoporosis suppress bone turnover, increasing MTA and mineralization of the tissue. This might have either or both of two effects. It could increase bone volume from refilling of the remodeling space, reducing the risk for fracture. Alternatively, the increased MTA could increase the propensity to develop microcracks, and reduce the toughness of bone, making it more likely to fracture. There may also be changes in the morphology of the mineral crystals that could affect the homogeneity of the tissue and impact mechanical properties. These changes might have large positive or negative effects on fracture incidence, and could contribute to the paradox that both large and small increases in density have about the same effect on fracture risk. Bone mineral density measured by DXA does not discriminate between density differences caused by volume changes, and those caused by changes in mineralization. As such, it does not entirely reflect material property changes in aging or osteoporotic bone that contribute to bone's risk for fracture.     

Mechanisms of action of thyroid hormones in the skeleton

Background

Thyroid hormones regulate skeletal development, acquisition of peak bone mass and adult bone maintenance. Abnormal thyroid status during childhood disrupts bone maturation and linear growth, while in adulthood it results in altered bone remodeling and an increased risk of fracture

Scope of Review

This review considers the cellular effects and molecular mechanisms of thyroid hormone action in the skeleton. Human clinical and population data are discussed in relation to the skeletal phenotypes of a series of genetically modified mouse models of disrupted thyroid hormone signaling.

Major Conclusions

Euthyroid status is essential for normal bone development and maintenance. Major thyroid hormone actions in skeletal cells are mediated by thyroid hormone receptor α (TRα) and result in anabolic responses during growth and development but catabolic effects in adulthood. These homeostatic responses to thyroid hormone are locally regulated in individual skeletal cell types by the relative activities of the type 2 and 3 iodothyronine deiodinases, which control the supply of the active thyroid hormone 3,5,3’-L-triiodothyronine (T3) to its receptor.

General Significance

Population studies indicate that both thyroid hormone deficiency and excess are associated with an increased risk of fracture. Understanding the cellular and molecular basis of T3 action in skeletal cells will lead to the identification of new targets to regulate bone turnover and mineralization in the prevention and treatment of osteoporosis. This article is part of a Special Issue entitled Thyroid hormone signaling.     

In-vivo imaging of the fracture healing in medaka revealed two types of osteoclasts before and after the callus formation by osteoblasts

The fracture healing research, which has been performed in mammalian models not only for clinical application but also for bone metabolism, revealed that generally osteoblasts are induced to enter the fracture site before the induction of osteoclasts for bone remodeling. However, it remains unknown how and where osteoclasts and osteoblasts are induced, because it is difficult to observe osteoclasts and osteoblasts in a living animal. To answer these questions, we developed a new fracture healing model by using medaka. We fractured one side of lepidotrichia in a caudal fin ray without injuring the other soft tissues including blood vessels. Using the transgenic medaka in which osteoclasts and osteoblasts were visualized by GFP and DsRed, respectively, we found that two different types of functional osteoclasts were induced before and after osteoblast callus formation. The early-induced osteoclasts resorbed the bone fragments and the late-induced osteoclasts remodeled the callus. Both types of osteoclasts were induced near the surface on the blood vessels, while osteoblasts migrated from adjacent fin ray. Transmission electron microscopy revealed that no significant ruffled border and clear zone were observed in early-induced osteoclasts, whereas the late-induced osteoclasts had clear zones but did not have the typical ruffled border. In the remodeling of the callus, the expression of cox2 mRNA was up-regulated at the fracture site around vessels, and the inhibition of Cox2 impaired the induction of the late-induced osteoclasts, resulting in abnormal fracture healing. Finally, our developed medaka fracture healing model brings a new insight into the molecular mechanism for controlling cellular behaviors during the fracture healing.     

Predicting changes in mechanical properties of trabecular bone by adaptive remodeling

Because changes in the mechanical properties of bone are closely related to trabecular bone remodeling, methods that consider the temporal morphological changes induced by adaptive remodeling of trabecular bone are needed to estimate long-term fracture risk and bone quality in osteoporosis. We simulated bone remodeling using simplified and pig trabecular bone models and estimated the morphology of healthy and osteoporotic cases. We then displayed the fracture risk of the remodeled models based on a cumulative histogram from high stress. The histogram showed more elements had higher stresses in the osteoporosis model, indicating that the osteoporosis model had a greater risk.     

Bisphosphonate mechanism of action

Nitrogen-containing bisphosphonates (N-BPs) are potent inhibitors of bone resorption widely used in the treatment of osteoporosis and other bone degrading disorders. At the tissue level, N-BPs reduce bone turnover, increase bone mass and mineralization, measured clinically as a rise in bone mineral density, increase bone strength and reduce fracture risk. At the cellular level, N-BPs, localize preferentially at sites of bone resorption, where mineral is exposed, are taken up by ostoclasts and inhibit osteoclast activity. The bone formation that follows incroporates the N-BP in the matrix, where it becomes pharmacologically inactive until released at a future time during bone remodeling. At the molecular level, N-BPs inhibit an enzyme in the cholesterol synthesis pathway, farnesyl diphosphate synthase. As a result, there is a reduction in the lipid geranylgeranyl diphosphate, which prenylates GTPases required for cytoskeletal organization and vesicular traffic in the osteoclast, leading to osteoclast inactivation.     

A computational assessment of the independent contribution of changes in canine trabecular bone volume fraction and microarchitecture to increased bone strength with suppression of bone turnover

This study addressed the effects of changes in trabecular microarchitecture induced by suppressed bone turnover-including changes to the remodeling space-on the trabecular bone strength-volume fraction characteristics independent of changes in tissue material properties. Twenty female beagle dogs, aged 1-2 years, were treated daily with either oral saline (n=10 control) or high doses of oral risedronate (0.5mg/kg/day, n=10 suppressed) for a period of 1 year, the latter designed (and confirmed) to substantially suppress bone turnover. High-resolution micro-CT-based finite element models (18-mum voxel size) of canine trabecular bone cores (n=2 per vertebral body) extracted from the T-10 vertebrae were analyzed in both compressive and torsional loading cases. The same tissue-level material properties were used in all models, thus providing measures of tissue-normalized strength due only to changes in the microarchitecture. Suppressed bone turnover resulted in more plate-like architecture with a thicker and more dense trabecular structure, but the relationship between the microarchitectural parameters and volume fraction was unaltered (p>0.05). Though the suppressed group had a greater tissue-normalized strength as compared to the control group (p<0.001) for both compressive and torsional loading, the relationship between tissue-normalized strength and volume fraction was not significantly altered for compression (p>0.13) or torsion (p>0.09). In this high-density, non-osteoporotic animal model, the increases in tissue-normalized strength seen with suppression of bone turnover were entirely commensurate with increases in bone volume fraction and thus, no evidence of microarchitecture-related or "stress-riser" effects which may disproportionately affect strength were found.     

Increased intracortical bone remodeling during lactation in beagle dogs

There are substantial changes in skeletal and mineral metabolism during pregnancy and lactation. The purpose of this study was to determine the changes in intracortical bone remodeling and turnover during lactation in beagle dogs. A femur and rib were obtained from dogs near the end of lactation or soon after weaning and compared with nonlactating controls. Rib cortical bone had much higher bone turnover rates than did femoral diaphyseal cortical bone. The number of single-labeled osteons and the number of resorption spaces were significantly greater during lactation in both the rib and the femur. Additionally, the mineral apposition rate, basic multicellular unit activation frequency, and bone turnover rates were greater in the femoral cortical bone from the lactating dogs than from the controls. These data demonstrate that during lactation, intracortical bone remodeling increases, and this may provide a mechanism for the skeleton to be responsive to the calcium requirements of the mother. In addition, these data may help explain the transient decreases in cortical bone mineral density that are reported to occur during human lactation.     

Effects of 17 β-estradiol on the expression of interstitial collagenases-8 and −13 (MMP-8 and MMP-13) and tissue inhibitor of metalloproteinase-1 (TIMP-1) in ovariectomized rat osteoblastic cells

Estrogen plays an important role in maintaining normal bone metabolism via the direct or indirect regulation of bone cells. Osteoblastic cells, as the target cells of estrogen, can secrete multiple matrix metalloproteinases (MMPs) that participate in bone remodeling. It has been demonstrated that bone loss induced by estrogen deficiency is closely related to the abnormal expression of multiple MMPs in osteoblastic cells. However, the regulating action of estrogen on the expression of interstitial collagenases MMP-8 and MMP-13 in osteoblastic cells in vivo remains unclear. We used an ovariectomized osteoporotic rat model to analyze the changes in the histomorphometric parameters of bone after and without treatment with 17-estradiol (E₂); We also used immunohistochemistry and in situ hybridization to observe changes in the expression of mRNA and the proteins MMP-8, MMP-13 and TIMP-1 in osteoblastic cells in rat proximal tibia. In this study, we found that in the ovariectomized rat the expression of MMP-13 mRNA and protein increased markedly, whereas the expression of MMP-8 and TIMP-1 mRNA and protein did not change significantly. Our analysis showed that the expression of MMP-13 protein was correlated positively to bone trabecular separation, osteoid surface area, and negatively to trabecular numbers and the percentage of trabecula bone volume/total tissue volume. Our results suggest that MMP-13 plays an important role in estrogen deficiency-induced bone loss, while estrogen can inhibit bone resorption and reduce bone turnover rate by down-regulating the expression of MMP-13 in osteoblastic cells.     

Subchondral bone response to injected adipose-derived stromal cells for treating osteoarthritis using an experimental rabbit model

Although articular cartilage is the target of osteoarthritis (OA), its deterioration is not always clearly associated with patient symptoms. Because a functional interaction between cartilage and bone is crucial, the pathophysiology of OA and its treatment strategy must focus also on subchondral bone. We investigated whether adipose-derived stromal cells (ASCs) injected into a joint at two different concentrations could prevent subchondral bone damage after the onset of mild OA in a rabbit model. We measured both volumetric and densitometric aspects of bone remodeling. Although OA can stimulate bone remodeling either catabolically or anabolically over time, the accelerated turnover does not allow complete mineralization of new bone and therefore gradually reduces its density. We measured changes in morphometric and densitometric bone parameters using micro-CT analysis and correlated them with the corresponding parameters in cartilage and meniscus. We found that ASCs promoted cartilage repair and helped counteract the accelerated bone turnover that occurs with OA.     

Can deterministic mechanical size effects contribute to fracture and microdamage accumulation in trabecular bone?

Failure of bone under monotonic and cyclic loading is related to the bone mineral density, the quality of the bone matrix, and the evolution of microcracks. The theory of linear elastic fracture mechanics has commonly been applied to describe fracture in bone. Evidence is presented that bone failure can be described through a non-linear theory of fracture. Thereby, deterministic size effects are introduced. Concepts of a non-linear theory are applied to discern how the interaction among bone matrix constituents (collagen and mineral), microcrack characteristics, and trabecular architecture can create distinctively differences in the fracture resistance at the bone tissue level. The non-linear model is applied to interpret pre-clinical data concerning the effects of anti-osteoporotic agents on bone properties. The results show that bisphosphonate (BP) treatments that suppress bone remodeling will change trabecular bone in ways such that the size of the failure process zone relative to the trabecular thickness is reduced. Selective estrogen receptor modulators (SERMs) that suppress bone remodeling will change trabecular bone in ways such that the size of the failure process zone relative to the trabecular thickness is increased. The consequences of these changes are reflected in bone mechanical response and predictions are consistent with experimental observations in the animal model which show that BP treatment is associated with more brittle fracture and microcracks without altering the average length of the cracks, whereas SERM treatments lead to a more ductile fracture and mainly increase crack length with a smaller increase in microcrack density. The model suggests that BPs may be more effective in cases in which bone mass is very low, whereas SERMS may be more effective when milder osteoporotic symptoms are present.     

Biomarkers for osteoporosis management: utility in diagnosis, fracture risk prediction and therapy monitoring

Osteoporosis is a systemic disease characterized by low bone mass and microarchitectural deterioration of bone tissue, resulting in an increased risk of fracture. While the level of bone mass can be estimated by measuring bone mineral density (BMD) using dual X-ray absorptiometry (DXA), its measurement does not capture all the risk factors for fracture. Quantitative changes in skeletal turnover can be assessed easily and non-invasively by the measurement of serum and urinary biochemical markers; the most sensitive markers include serum osteocalcin, bone specific alkaline phosphatase, the N-terminal propeptide of type I collagen for bone formation, and the crosslinked C- (CTX) and N- (NTX) telopeptides of type I collagen for bone resorption. Advances in our knowledge of bone matrix biochemistry, most notably of post-translational modifications in type I collagen, are likely to lead to the development of new biochemical markers that reflect changes in the material property of bone, an important determinant of bone strength. Among those, the measurement of the urinary ratio of native (alpha) to isomerized (beta) CTX - an index of bone matrix maturation - has been shown to be predictive of fracture risk independently of BMD and bone turnover.In postmenopausal osteoporosis, levels of bone resorption markers above the upper limit of the premenopausal range are associated with an increased risk of hip, vertebral, and nonvertebral fracture, independent of BMD. Therefore, the combined use of BMD measurement and biochemical markers is helpful in risk assessment, especially in those women who are not identified as at risk by BMD measurement alone. Levels of bone markers decrease rapidly with antiresorptive therapies, and the levels reached after 3-6 months of therapy have been shown to be more strongly associated with fracture outcome than changes in BMD. Preliminary studies indicate that monitoring changes of bone formation markers could also be useful to monitor anabolic therapies, including intermittent parathyroid hormone administration and, possibly, to improve adherence to treatment. Thus, repeated measurements of bone markers during therapy may help improve the management of osteoporosis in patients.     

Upregulation of inflammatory genes and downregulation of sclerostin gene expression are key elements in the early phase of fragility fracture healing

Background

Fracture healing is orchestrated by a specific set of events that culminates in the repair of bone and reachievement of its biomechanical properties. The aim of our work was to study the sequence of gene expression events involved in inflammation and bone remodeling occurring in the early phases of callus formation in osteoporotic patients.

Methodology/Principal Findings

Fifty-six patients submitted to hip replacement surgery after a low-energy hip fracture were enrolled in this study. The patients were grouped according to the time interval between fracture and surgery: bone collected within 3 days after fracture (n = 13); between the 4^th and 7^th day (n = 33); and after one week from the fracture (n = 10). Inflammation- and bone metabolism-related genes were assessed at the fracture site. The expression of pro-inflammatory cytokines was increased in the first days after fracture. The genes responsible for bone formation and resorption were upregulated one week after fracture. The increase in RANKL expression occurred just before that, between the 4^th–7^th days after fracture. Sclerostin expression diminished during the first days after fracture.

Conclusions

The expression of inflammation-related genes, especially IL-6, is highest at the very first days after fracture but from day 4 onwards there is a shift towards bone remodeling genes, suggesting that the inflammatory phase triggers bone healing. We propose that an initial inflammatory stimulus and a decrease in sclerostin-related effects are the key components in fracture healing. In osteoporotic patients, cellular machinery seems to adequately react to the inflammatory stimulus, therefore local promotion of these events might constitute a promising medical intervention to accelerate fracture healing.     

A semi-empirical cell dynamics model for bone turnover under external stimulus

The normal periodic turnover of bone is referred to as remodeling. In remodeling, old or damaged bone is removed during a 'resorption' phase and new bone is formed in its place during a 'formation' phase in a sequence of events known as coupling. Resorption is preceded by an 'activation' phase in which the signal to remodel is initiated and transmitted. Remodeling is known to involve the interaction of external stimuli, bone cells, calcium and phosphate ions, and several proteins, hormones, molecules, and factors. In this study, a semi-empirical cell dynamics model for bone remodeling under external stimulus that accounts for the interaction between bone mass, bone fluid calcium, bone calcium, and all three major bone cell types, is presented. The model is formulated to mimic biological coupling by solving separately and sequentially systems of ODEs for the activation, resorption, and formation phases of bone remodeling. The charateristic time for resorption (20 days) and the amount of resorption (~0.5%) are fixed for all simulations, but the formation time at turnover is an output of the model. The model was used to investigate the effects of different types of strain stimuli on bone turnover under bone fluid calcium balance and imbalance conditions. For bone fluid calcium balance, the model predicts complete turnover after 130 days of formation under constant 1000 microstrain stimulus; after 47 days of formation under constant 2000 microstrain stimulus; after 173 days of formation under strain-free conditions, and after 80 days of formation under monotonic increasing strain stimulus from 1000 to 2000 microstrain. For bone fluid calcium imbalance, the model predicts that complete turnover occurs after 261 days of formation under constant 1000 microstrain stimulus and that turnover never occurs under strain-free conditions. These predictions were not impacted by mean dynamic input strain stimuli of 1000 and 2000 microstrain at 1 Hz and 1000 microstrain amplitude. The formation phase of remodeling lasts longer than the resorption phase, increased strain stimulus accelerates bone turnover, while absence of strain significantly delays or prevents it, and formation time for turnover under monotonic increasing strain conditions is intermediate to those for constant strain stimuli at the minimum and maximum monotonic strain levels. These results are consistent with the biology, and with Frost's mechanostat theory.     

Bone Canopies in Pediatric Renal Osteodystrophy

Pediatric renal osteodystrophy (ROD) is characterized by changes in bone turnover, mineralization, and volume that are brought about by alterations in bone resorption and formation. The resorptive and formative surfaces on the cancellous bone are separated from the marrow cavity by canopies consisting of a layer of flat osteoblastic cells. These canopies have been suggested to play a key role in the recruitment of osteoprogenitors during the process of bone remodeling. This study was performed to address the characteristics of the canopies above bone formation and resorption sites and their association with biochemical and bone histomorphometric parameters in 106 pediatric chronic kidney disease (CKD) patients (stage 2–5) across the spectrum of ROD. Canopies in CKD patients often appeared as thickened multilayered canopies, similar to previous reports in patients with primary hyperparathyroidism. This finding contrasts with the thin appearance reported in healthy individuals with normal kidney function. Furthermore, canopies in pediatric CKD patients showed immunoreactivity to the PTH receptor (PTHR1) as well as to the receptor activator of nuclear factor kappa-B ligand (RANKL). The number of surfaces with visible canopy coverage was associated with plasma parathyroid hormone (PTH) levels, bone formation rate, and the extent of remodeling surfaces. Collectively, these data support the conclusion that canopies respond to the elevated PTH levels in CKD and that they possess the molecular machinery necessary to respond to PTH signaling.     

Osteoimmunology and osteoporosis

The concept of osteoimmunology is based on growing insight into the links between the immune system and bone at the anatomical, vascular, cellular, and molecular levels. In both rheumatoid arthritis (RA) and ankylosing spondylitis (AS), bone is a target of inflammation. Activated immune cells at sites of inflammation produce a wide spectrum of cytokines in favor of increased bone resorption in RA and AS, resulting in bone erosions, osteitis, and peri-inflammatory and systemic bone loss. Peri-inflammatory bone formation is impaired in RA, resulting in non-healing of erosions, and this allows a local vicious circle of inflammation between synovitis, osteitis, and local bone loss. In contrast, peri-inflammatory bone formation is increased in AS, resulting in healing of erosions, ossifying enthesitis, and potential ankylosis of sacroiliac joints and intervertebral connections, and this changes the biomechanical competence of the spine. These changes in bone remodeling and structure contribute to the increased risk of vertebral fractures (in RA and AS) and non-vertebral fractures (in RA), and this risk is related to severity of disease and is independent of and superimposed on background fracture risk. Identifying patients who have RA and AS and are at high fracture risk and considering fracture prevention are, therefore, advocated in guidelines. Local peri-inflammatory bone loss and osteitis occur early and precede and predict erosive bone destruction in RA and AS and syndesmophytes in AS, which can occur despite clinically detectable inflammation (the so-called 'disconnection'). With the availability of new techniques to evaluate peri-inflammatory bone loss, osteitis, and erosions, peri-inflammatory bone changes are an exciting field for further research in the context of osteoimmunology.     

Role of Cbl-PI3K Interaction during Skeletal Remodeling in a Murine Model of Bone Repair

Mice in which Cbl is unable to bind PI3K (YF mice) display increased bone volume due to enhanced bone formation and repressed bone resorption during normal bone homeostasis. We investigated the effects of disrupted Cbl-PI3K interaction on fracture healing to determine whether this interaction has an effect on bone repair. Mid-diaphyseal femoral fractures induced in wild type (WT) and YF mice were temporally evaluated via micro-computed tomography scans, biomechanical testing, histological and histomorphometric analyses. Imaging analyses revealed no change in soft callus formation, increased bony callus formation, and delayed callus remodeling in YF mice compared to WT mice. Histomorphometric analyses showed significantly increased osteoblast surface per bone surface and osteoclast numbers in the calluses of YF fractured mice, as well as increased incorporation of dynamic bone labels. Furthermore, using laser capture micro-dissection of the fracture callus we found that cells lacking Cbl-PI3K interaction have higher expression of Osterix, TRAP, and Cathepsin K. We also found increased expression of genes involved in propagating PI3K signaling in cells isolated from the YF fracture callus, suggesting that the lack of Cbl-PI3K interaction perhaps results in enhanced PI3K signaling, leading to increased bone formation, but delayed remodeling in the healing femora.