Changes in geometry and cortical porosity in adult, ovary-intact rabbits after 5 months treatment with LY333334 (hPTH 1-34)

Evaluation of cortical bone remodeling in canines treated with daily and weekly administrations of teriparatide by establishing AI-driven morphometric analyses and GIS-based spatial mapping

Larger animal models with a well-developed Haversian system, as observed in humans, are ideal to analyze cortical bone remodeling in pharmacological studies of anti-osteoporosis drugs, although they have some limitations in controlling individual variability in size, weight, age, and number. This study aimed to morphometrically analyze cortical bone remodeling focusing on Haversian canals in dogs using four regimens of TPTD with daily and weekly administrations at lower and higher weekly doses (4.9 μg/kg/week and 19.8 μg/kg/week, respectively) for 9 months. A micro-computed tomography-based analysis showed no significant differences among regimen groups. By establishing artificial intelligence (AI)-driven morphometric analyses and geographical information system (GIS)-based spatial mapping of Haversian canals that does not require confocal microscopy but is possible with more commonly used wide field microscopes, we successfully observed significant morphometric distinctions among regimens applied even in dogs. Our analytical results suggested that the daily higher regimen specifically increased the number of eroded pores creating spaces between existing canals, thus stimulating cortical bone remodeling.

Daily administration of parathyroid hormone slows the progression of basic multicellular units in the cortical bone of the rabbit distal tibia

Basic Multicellular Units (BMUs) conduct bone remodeling, a critical process of tissue turnover which, if imbalanced, can lead to disease, including osteoporosis. Parathyroid hormone (PTH 1-34; Teriparatide) is an osteoanabolic treatment for osteoporosis; however, it elevates the rate of intra-cortical remodeling (activation frequency) leading, at least transiently, to increased porosity. The purpose of this study was to test the hypothesis that PTH not only increases the rate at which cortical BMUs are initiated but also increases their progression (Longitudinal Erosion Rate; LER). Two groups (n = 7 each) of six-month old female New Zealand white rabbits were both administered 30 μg/kg of PTH once daily for a period of two weeks to induce remodeling. Their distal right tibiae were then imaged in vivo by in-line phase contrast micro-CT at the Canadian Light Source synchrotron. Over the following two weeks the first group (PTH) received continued daily PTH while the second withdrawal group (PTHW) was administrated 0.9 % saline. At four weeks all animals were euthanized, their distal tibiae were imaged by conventional micro-CT ex vivo and histomorphometry was performed. Matching micro-CT datasets (in vivo and ex vivo) were co-registered in 3D and LER was measured from 612 BMUs. Counter to our hypothesis, mean LER was lower (p < 0.001) in the PTH group (30.19 ± 3.01 μm/day) versus the PTHW group (37.20 ± 2.77 μm/day). Despite the difference in LER, osteonal mineral apposition rate (On.MAR) did not differ between groups indicating the anabolic effect of PTH was sustained after withdrawal. The slowing of BMU progression by PTH warrants further investigation; slowed resorption combined with elevated bone formation rate, may play an important role in how PTH enhances coupling between resorption and formation within the BMU. Finally, the prolonged anabolic response following withdrawal may have utility in terms of optimizing clinical dosing regimens.

Direct Assessment of Rabbit Cortical Bone Basic Multicellular Unit Longitudinal Erosion Rate: A 4D Synchrotron-Based Approach

Cortical bone remodeling is carried out by basic multicellular units (BMUs), which couple resorption to formation. Although fluorochrome labeling has facilitated study of BMU formative parameters since the 1960s, some resorptive parameters, including the longitudinal erosion rate (LER), have remained beyond reach of direct measurement. Indeed, our only insights into this spatiotemporal parameter of BMU behavior come from classical studies that indirectly inferred LER. Here, we demonstrate a 4D in vivo method to directly measure LER through in-line phase contrast synchrotron imaging. The tibias of rabbits (n = 15) dosed daily with parathyroid hormone were first imaged in vivo (synchrotron micro-CT; day 15) and then ex vivo 14 days later (conventional micro-CT; day 29). Mean LER assessed by landmarking the co-registered scans was 23.69 ± 1.73 μm/d. This novel approach holds great promise for the direct study of the spatiotemporal coordination of bone remodeling, its role in diseases such as osteoporosis, as well as related treatments. © 2022 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Expansion of the osteocytic lacunar-canalicular system involved in pharmacological action of PTH revealed by AI-driven fluorescence morphometry in female rabbits

Osteoporosis is an age-related disorder that is characterized by reduced bone mass. Its prevention and treatment are important healthcare issues for maintaining social activity in aged societies. Although bone fractures mostly occur at sites of weakened cortical bone, pathophysiological and pharmacological evaluations of bone mass have tended to be predominantly assessed in trabecular bone. To statistically characterize cortical bone remodeling, we originally established multimode fluorescence imaging and artificial intelligence (AI)-driven morphometric analyses in six-month-old female rabbits with well-defined cortical remodeling, similar to that in humans. We evaluated three distinct administration frequencies of teriparatide [TPTD; human parathyroid hormone, hPTH (1–34)]: once (1/w), twice (2/w), and seven times (7/w) a week, with the same total dose (140 μg/kg/week). Our analyses revealed significant expansions of the osteocytic lacunar-canalicular system and Haversian canals accompanied by the development of cortical porosity and endosteal naïve bone formation induced by a frequent administration regimen (7/w) of TPTD; however, once-weekly (1/w) and twice-weekly (2/w) administration of TPTD showed little effect. These findings demonstrate a clear contrast between the effects of frequent and infrequent administration of TPTD on cortical bone metabolism and suggest that osteocytic bone remodeling is involved in the pharmacological action of PTH.

Lagomorpha as a Model Morphological System

Due to their global distribution, invasive history, and unique characteristics, European rabbits are recognizable almost anywhere on our planet. Although they are members of a much larger group of living and extinct mammals [Mammalia, Lagomorpha (rabbits, hares, and pikas)], the group is often characterized by several well-known genera (e.g., Oryctolagus, Sylvilagus, Lepus, and Ochotona). This representation does not capture the extraordinary diversity of behavior and form found throughout the order. Model organisms are commonly used as exemplars for biological research, but there are a limited number of model clades or lineages that have been used to study evolutionary morphology in a more explicitly comparative way. We present this review paper to show that lagomorphs are a strong system in which to study macro- and micro-scale patterns of morphological change within a clade that offers underappreciated levels of diversity. To this end, we offer a summary of the status of relevant aspects of lagomorph biology.

Cortical Bone Porosity in Rabbit Models of Osteoporosis

Cortical bone porosity is intimately linked with remodeling, is of growing clinical interest, and is increasingly accessible by imaging. Thus, the potential of animal models of osteoporosis (OP) to provide a platform for studying how porosity develops and responds to interventions is tremendous. To date, rabbit models of OP have largely focused on trabecular microarchitecture or bone density; some such as ovariectomy (OVX) have uncertain efficacy and cortical porosity has not been extensively reported. Our primary objective was to characterize tibial cortical porosity in rabbit-based models of OP, including OVX, glucocorticoids (GC), and OVX + GC relative to controls (SHAM). We sought to: (i) test the hypothesis that intracortical remodeling is elevated in these models; (ii) contrast cortical remodeling and porosity in these models with that induced by parathyroid hormone (1-34; PTH); and (iii) contrast trabecular morphology in the proximal tibia across all groups. Evidence that an increase in cortical porosity occurred in all groups was observed, although this was the least robust for GC. Histomorphometric measures supported the hypothesis that remodeling rate was elevated in all groups and also revealed evidence of uncoupling of bone resorption and formation in the GC and OVX + GC groups. For trabecular bone, a pattern of loss was observed for OVX, GC, and OVX + GC groups, whereas the opposite was observed for PTH. Change in trabecular number best explained these patterns. Taken together, the findings indicated rabbit models provide a viable and varied platform for the study of OP and associated changes in cortical remodeling and porosity. Intriguingly, the evidence revealed differing effects on the cortical and trabecular envelopes for the PTH model. © 2020 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR)..

Internal architecture of the mandibular condyle of rabbits is related to dietary resistance during growth

Although there is considerable evidence that bone responds to the loading environment in which it develops, few analyses have examined phenotypic plasticity or bone functional adaptation in the masticatory apparatus. Prior work suggests that masticatory morphology is sensitive to differences in food mechanical properties during development; however, the importance of the timing/duration of loading and variation in naturalistic diets is less clear. Here, we examined microstructural and macrostructural differences in the mandibular condyle in four groups of white rabbits (Oryctolagus cuniculus) raised for a year on diets that varied in mechanical properties and timing of the introduction of mechanically challenging foods, simulating seasonal variation in diet. We employed sliding semilandmarks to locate multiple volumes of interest deep to the mandibular condyle articular surface, and compared bone volume fraction, trabecular thickness and spacing, and condylar size/shape among experimental groups. The results reveal a shared pattern of bony architecture across the articular surface of all treatment groups, while also demonstrating significant among-group differences. Rabbits raised on mechanically challenging diets have significantly increased bone volume fraction relative to controls fed a less challenging diet. The post-weaning timing of the introduction of mechanically challenging foods also influences architectural properties, suggesting that bone plasticity can extend well into adulthood and that bony responses to changes in loading may be rapid. These findings demonstrate that bony architecture of the mandibular condyle in rabbits responds to variation in mechanical loading during an organism's lifetime and has the potential to track dietary variation within and among species.

Cortical bone vessel identification and quantification on contrast-enhanced MR images

Background

Cortical bone porosity is a major determinant of bone strength. Despite the biomechanical importance of cortical bone porosity, the biological drivers of cortical porosity are unknown. The content of cortical pore space can indicate pore expansion mechanisms; both of the primary components of pore space, vessels and adipocytes, have been implicated in pore expansion. Dynamic contrast-enhanced MRI (DCE-MRI) is widely used in vessel detection in cardiovascular studies, but has not been applied to visualize vessels within cortical bone. In this study, we have developed a multimodal DCE-MRI and high resolution peripheral QCT (HR-pQCT) acquisition and image processing pipeline to detect vessel-filled cortical bone pores.

Methods

For this in vivo human study, 19 volunteers (10 males and 9 females; mean age =63±5) were recruited. Both distal and ultra-distal regions of the non-dominant tibia were imaged by HR-pQCT (82 µm nominal resolution) for bone structure segmentation and by 3T DCE-MRI (Gadavist; 9 min scan time; temporal resolution =30 sec; voxel size 230×230×500 µm³) for vessel visualization. The DCE-MRI was registered to the HR-pQCT volume and the voxels within the MRI cortical bone region were extracted. Features of the DCE data were calculated and voxels were categorized by a 2-stage hierarchical kmeans clustering algorithm to determine which voxels represent vessels. Vessel volume fraction (volume ratio of vessels to cortical bone), vessel density (average vessel count per cortical bone volume), and average vessel volume (mean volume of vessels) were calculated to quantify the status of vessel-filled pores in cortical bone. To examine spatial resolution and perform validation, a virtual phantom with 5 channel sizes and an applied pseudo enhancement curve was processed through the proposed image processing pipeline. Overlap volume ratio and Dice coefficient was calculated to measure the similarity between the detected vessel map and ground truth.

Results

In the human study, mean vessel volume fraction was 2.2%±1.0%, mean vessel density was 0.68±0.27 vessel/mm³, and mean average vessel volume was 0.032±0.012 mm³/vessel. Signal intensity for detected vessel voxels increased during the scan, while signal for non-vessel voxels within pores did not enhance. In the validation phantom, channels with diameter 250 µm or greater were detected successfully, with volume ratio equal to 1 and Dice coefficient above 0.6. Both statistics decreased dramatically for channel sizes less than 250 µm.

Conclusions

We have a developed a multi-modal image acquisition and processing pipeline that successfully detects vessels within cortical bone pores. The performance of this technique degrades for vessel diameters below the in-plane spatial resolution of the DCE-MRI acquisition. This approach can be applied to investigate the biological systems associated with cortical pore expansion.

Abaloparatide increases bone mineral density and bone strength in ovariectomized rabbits with glucocorticoid-induced osteopenia

Glucorticoid (GC) therapy is the commonest cause of secondary osteoporosis. Ovariectomized rabbits receiving the GC methylprednisolone for 6 weeks exhibited relatively lower vertebral and femoral bone mass. Treatment with the PTH receptor agonist abaloparatide for 12 weeks during ongoing methylprednisolone administration increased cortical and trabecular bone mass and femur bending strength.

INTRODUCTION

Abaloparatide, an osteoanabolic PTHrP analog, increases bone mineral density (BMD) and reduces fracture risk in women with postmenopausal osteoporosis. This study assessed abaloparatide effects on BMD and bone strength in ovariectomized (OVX) rabbits with glucocorticoid (GC)-induced osteopenia.

METHODS

Thirty-two rabbits underwent OVX and 8 underwent sham surgery. One day later, 24 OVX rabbits began daily s.c. GC injections (methylprednisolone, 1 mg/kg/day) for 6 weeks, while 8 OVX and 8 sham controls received no GC. GC-challenged rabbits (8/group) then received GC (0.5 mg/kg/day) along with daily s.c. vehicle (GC-OVX), abaloparatide 5 μg/kg/day (ABL5), or 25 μg/kg/day (ABL25) for 12 weeks, and the no-GC OVX and sham controls received daily vehicle.

RESULTS

GC-OVX rabbits showed significant deficits in vertebral and proximal femur areal BMD, lower cortical area, thickness and volumetric BMD of the femur diaphysis, and reduced trabecular bone volume and volumetric BMD in the vertebra and distal femur versus sham controls. These deficits were significantly reversed in the ABL25 group, which also showed enhanced trabecular micro-architecture versus GC-OVX controls. Destructive bending tests showed significantly lower femur diaphysis ultimate load and bending rigidity of the femoral diaphysis in the GC-OVX group versus sham controls, whereas these parameters were similar in the ABL25 group vs sham controls.

CONCLUSIONS

Abaloparatide 25 μg/kg/day mitigated the adverse effects of GC administration on cortical and trabecular bone and improved femoral strength in OVX rabbits. These results suggest potential promise for abaloparatide as an investigational therapy for glucocorticoid-induced osteoporosis.

Effect of intermittent administration of teriparatide on the mechanical and histological changes in bone grafted with β-tricalcium phosphate using a rabbit bone defect model

Grafting β-tricalcium phosphate (TCP) is a well-established method for restoring bone defects; however, there is concern that the mechanical stability of the grafted β-TCP is not maintained during bone translation. Teriparatide has an anabolic effect, stimulating bone formation and increasing bone mineral density for the treatment of osteoporosis. The aim of the present study was to evaluate the effect of intermittent teriparatide treatment on changes in bone grafted with β-TCP using a rabbit bone defect model. Bone defects (5×15 mm) were created in the distal femoral condyle of Japanese white rabbits, and β-TCP granules of two different total porosities were manually grafted. Teriparatide (40 µg/kg) or 0.2% rabbit serum albumin solution as a vehicle control was subcutaneously injected three times per week following the surgery. At 4 or 8 weeks post-surgery, serum samples were obtained and the levels of γ-carboxylated osteocalcin (Gla-OC) were quantified using ELISA. Histomorphometry was also performed using sections of graft sites following staining for tartrate resistant acid phosphatase. Activity and mechanical strength (maximum shear strength, maximum shear stiffness and total energy absorption) were evaluated using an axial push-out load to failure test. Teriparatide treatment significantly increased (P<0.05) the serum levels of Gla-OC, a specific marker for bone formation, suggesting that teriparatide enhances bone formation in β-TCP-grafted rabbits. Furthermore teriparatide increased the degradation of β-TCP by bone remodeling (P<0.05) and promoted the formation of new bone following application of the graft compared with the control group (P<0.01). Furthermore, teriparatide suppressed the reduction in mechanical strength (P<0.05) during bone translation in bone defects grafted with β-TCP. The results of the present study demonstrate that teriparatide is effective in maintaining the mechanical stability of grafted β-TCP, possibly by promoting new bone formation.

Dietary variation and mechanical properties of articular cartilage in the temporomandibular joint: implications for the role of plasticity in mechanobiology and pathobiology

Due to their nature as tissue composites, skeletal joints pose an additional challenge in terms of evaluating the functional significance of morphological variation in their bony and cartilaginous components in response to altered loading conditions. Arguably, this complexity requires more direct means of investigating joint plasticity and performance than typically employed to analyze macro- and micro-anatomical phenomena. To address a significant gap in our understanding of the plasticity of the mammalian temporomandibular joint (TMJ), we investigated the histology and mechanical properties of condylar articular cartilage in rabbits subjected to long-term variation in diet-induced masticatory stresses, specifically cyclical loading. Three cohorts of male weanlings were raised for six months on different diets until adulthood. Following euthanasia, the TMJ condyles on one side were dissected away, fixed, decalcified, dehydrated, embedded and sectioned. Safranin O staining was employed to identify variation in proteoglycan content, which in turn was used to predict patterns of articular cartilage stiffness in contralateral condylar specimens for each treatment group. Hematoxylin and eosin staining was used to quantify diet-induced changes in chondrocyte hypertrophy and cellularity. Mechanical tests document significant decreases in articular cartilage stiffness corresponding to patterns of extracellular matrix relative protein abundance in rabbits subjected to greater cyclical loading. This indicates that TMJs routinely subjected to higher masticatory stresses due to a challenging diet eventually develop postnatal decreases in the ability to counter compressive loads during postcanine biting and chewing. These findings provide novel information regarding TMJ performance, with broader implications about the costs and benefits of phenotypic plasticity as well as implications for how such biological processes affect connective tissue mechanobiology and pathobiology.

Betwixt and Between: Intracranial Perspective on Zygomatic Arch Plasticity and Function in Mammals

The zygomatic arch is morphologically complex, providing a key interface between the viscerocranium and neurocranium. It also serves as an attachment site for masticatory muscles, thereby linking it to the feeding apparatus. Though morphological variation related to differential loading is well known for many craniomandibular elements, the adaptive osteogenic response of the zygomatic arch remains to be investigated. Here, experimental data are presented that address the naturalistic influence of masticatory loading on the postweaning development of the zygoma and other cranial elements. Given the similarity of bone-strain levels among the zygoma and maxillomandibular elements, a rabbit and pig model were used to test the hypothesis that variation in cortical bone formation and biomineralization along the zygomatic arch and masticatory structures are linked to increased stresses. It was also hypothesized that neurocranial structures would be minimally affected by varying loads. Rabbits and pigs were raised for 48 weeks and 8 weeks, respectively. In both experimental models, CT analyses indicated that elevated masticatory loading did not induce differences in cortical bone thickness of the zygomatic arch, though biomineralization was positively affected. Hypotheses were supported regarding bone formation for maxillomandibular and neurocranial elements. Varying osteogenic responses in the arch suggests that skeletal adaptation, and corresponding variation in performance, may reside differentially at one level of bony architecture. Thus, it is possible that phenotypic diversity in the mammalian zygoma is due more singularly to natural selection (vs. plasticity). These findings underscore the complexity of the zygomatic arch and, more generally, determinants of skull form. Anat Rec, 299:1646-1660, 2016. © 2016 Wiley Periodicals, Inc.

Administration frequency as well as dosage of PTH are associated with development of cortical porosity in ovariectomized rats

To investigate whether the administration frequency of parathyroid hormone (PTH) is associated with the development of cortical porosity, this study established 15 dosage regimens of teriparatide [human PTH(1–34), TPTD] with four distinct concentrations and four distinct administration frequencies of TPTD to 16-week-old ovariectomized rats. Our analyses demonstrated that the bone mineral density, mechanical properties, and bone turnover were associated with the total amount of TPTD administered. Our observations further revealed that the cortical porosity was markedly developed as a result of an increased administration frequency with a lower concentration of total TPTD administration in our setting, although the highest concentration also induced cortical porosity. Deconvolution fluorescence tiling imaging on calcein-labeled undecalcified bone sections also demonstrated the development of cortical porosity to be closely associated with the bone site where periosteal bone formation took place. This site-specific cortical porosity involved intracortical bone resorption and an increased number and proximity of osteocytic lacunae, occasionally causing fused lacunae. Taken together, these findings suggested the involvement of local distinctions in the rate of bone growth that may be related to the site-specific mechanical properties in the development of cortical porosity induced by frequent and/or high doses of TPTD.

Acute development of cortical porosity and endosteal naïve bone formation from the daily but not weekly short-term administration of PTH in rabbit

Teriparatide [human parathyroid hormone (1–34)], which exerts an anabolic effect on bone, is used for the treatment of osteoporosis in patients who are at a high risk for fracture. That the once-daily administration of teriparatide causes an increase in cortical porosity in animal models and clinical studies has been a matter of concern. However, it is not well documented that the frequency of administration and/or the total dose of teriparatide affect the cortical porosity. The present study developed 4 teriparatide regimens [20 μg/kg/day (D20), 40 μg/kg/day (D40), 140 μg/kg/week (W140) and 280 μg/kg/week (W280)] in the rabbit as a model animal with a well-developed Haversian system and osteons. The total weekly doses were equivalent in the low-dose groups (D20 and W140) and in the high-dose groups (D40 and W280). After the short-term (1 month) administration of TPDT, micro-CT, histomorphometry and three-dimensional second harmonic generation (3D-SHG) imaging to visualize the bone collagen demonstrated that daily regimens but not weekly regimens were associated with the significant development of cortical porosity and endosteal naïve bone formation by marrow fibrosis. We concomitantly monitored the pharmacokinetics of the plasma teriparatide levels as well as the temporal changes in markers of bone formation and resorption. The analyses in the present study suggested that the daily repeated administration of teriparatide causes more deleterious changes in the cortical microarchitecture than the less frequent administration of higher doses. The findings of the present study may have some implications for use of teriparatide in clinical treatment.

Effects of abaloparatide-SC (BA058) on bone histology and histomorphometry: The ACTIVE phase 3 trial

There are a number of effective treatments for osteoporosis but most are in the antiresorptive class of compounds. Abaloparatide-SC is a new osteoanabolic agent, which increased bone mineral density and lowered the risk of osteoporosis-related fractures in the phase 3 ACTIVE trial. The objective of this report is to describe the effects of abaloparatide-SC 80μg on bone histology and histomorphometry in iliac crest bone biopsies from this trial in which participants were randomized to receive blinded daily subcutaneous injections of placebo or abaloparatide-SC 80μg/d or open-label teriparatide 20μg/d for 18months. Iliac crest bone biopsies were obtained between 12 and 18months. Qualitative histological analysis of biopsies from abaloparatide-SC-treated patients revealed normal bone microarchitecture without evidence of adverse effects on mineralization or on the formation of normal lamellar bone. There were no bone marrow abnormalities, marrow fibrosis nor was there presence of excess osteoid or woven bone. There were few significant differences among the three treatment groups in a standard panel of static and dynamic histomorphometric indices. The mineral apposition rate was higher in the teriparatide-treated group than in the placebo-treated group. The eroded surface was lower in the abaloparatide-SC-treated group than in the placebo-treated group. Cortical porosity was higher in both the abaloparatide-SC- and the teriparatide-treated groups than in the placebo-treated group. We conclude that histological and histomorphometric analysis of iliac crest bone biopsies from subjects who were treated for up to 18months with abaloparatide-SC showed no evidence of concern for bone safety.

TRIAL REGISTRATION

ClinicalTrials.gov number NCT01343004.

Intracranial and hierarchical perspective on dietary plasticity in mammals

The effect of dietary properties on craniofacial form has been the focus of numerous functional studies, with increasingly more work dedicated to the importance of phenotypic plasticity. As bone is a dynamic tissue, morphological variation related to differential loading is well established for many masticatory structures. However, the adaptive osteogenic response of several cranial sites across multiple levels of bony organization remains to be investigated. Here, rabbits were obtained at weaning and raised for 48 weeks until adulthood in order to address the naturalistic influence of altered loading on the long-term development of masticatory and non-masticatory elements. Longitudinal data from micro-computed tomography (μCT) scans were used to test the hypothesis that variation in cortical bone formation and biomineralization in masticatory structures is linked to increased stresses during oral processing of mechanically challenging foods. It was also hypothesized that similar parameters for neurocranial structures would be minimally affected by varying loads as this area is characterized by low strains during mastication and reduced hard-tissue mechanosensitivity. Hypotheses were supported regarding bone formation for maxillomandibular and neurocranial elements, though biomineralization trends of masticatory structures did not mirror macroscale findings. Varying osteogenic responses in masticatory elements suggest that physiological adaptation, and corresponding variation in skeletal performance, may reside differentially at one level of bony architecture, potentially affecting the accuracy of behavioral and in silico reconstructions. Together, these findings underscore the complexity of bone adaptation and highlight functional and developmental variation in determinants of skull form.

Chewed out: an experimental link between food material properties and repetitive loading of the masticatory apparatus in mammals

Using a model organism (rabbits) that resembles a number of mammalian herbivores in key aspects of its chewing behaviors, we examined how variation in dietary mechanical properties affects food breakdown during mastication. Such data have implications for understanding phenotypic variation in the mammalian feeding apparatus, particularly with respect to linking jaw form to diet-induced repetitive loading. Results indicate that chewing frequency (chews/s) is independent of food properties, whereas chewing investment (chews/g) and chewing duration(s), which are proportional to repetitive loading of the jaws, are positively related to food stiffness and toughness. In comparisons of displacement-limited and stress-limited fragmentation indices, which respectively characterize the intraoral breakdown of tough and stiff foods, increases in chewing investment and duration are linked solely to stiffness. This suggests that stiffer foods engender higher peak loads and increased cyclical loading. Our findings challenge conventional wisdom by demonstrating that toughness does not, by itself, underlie increases in cyclical loading and loading duration. Instead, tough foods may be associated with such jaw-loading patterns because they must be processed in greater volumes owing to their lower nutritive quality and for longer periods of time to increase oral exposure to salivary chemicals.

Effects of teriparatide on cortical histomorphometric variables in postmenopausal women with or without prior alendronate treatment

Cortical bone, the dominant component of the human skeleton by volume, plays a key role in protecting bones from fracture. We analyzed the cortical bone effects of teriparatide treatment in postmenopausal women with osteoporosis who had previously received long-term alendronate (ALN) therapy or were treatment naïve (TN). Tetracycline-labeled paired iliac crest biopsies obtained from 29 ALN-pretreated and 16 TN women were evaluated for dynamic histomorphometric parameters of bone formation at the periosteal, endocortical and intracortical bone compartments, before and after 24months of teriparatide treatment. At baseline, the frequency of specimens without any endocortical and periosteal tetracycline labeling, and the percentage of quiescent osteons, was higher in the ALN than the TN group. Endocortical and periosteal mineralizing surface (MS/BS%), periosteal bone formation rate (BFR/BS), mineral apposition rate (MAR) and the number of intracortical forming osteons were significantly lower in the ALN-pretreated patients than in the TN group. Following teriparatide treatment, the frequency of endocortical and periosteal unlabeled biopsies decreased; in the ALN-pretreated group the percentage of quiescent osteons decreased and, in contrast, forming and resorbing osteons were increased. Teriparatide treatment resulted in significant increases of MAR in the endocortical, and MS/BS% in the periosteal compartment in the ALN-pretreated group. Most indices of bone formation remained lower in the ALN-pretreated group compared with the TN group at study end. Endocortical wall width was increased in both ALN-pretreated and TN groups. Cortical porosity and cortical thickness were significantly increased in the ALN-pretreated group after teriparatide treatment. Our results suggest that 24months of teriparatide treatment increases cortical bone formation and cortical turnover in patients who were either TN or had previous ALN therapy.

Experimental perspective on fallback foods and dietary adaptations in early hominins

The robust jaws and large, thick-enameled molars of the Plio-Pleistocene hominins Australopithecus and Paranthropus have long been interpreted as adaptations for hard-object feeding. Recent studies of dental microwear indicate that only Paranthropus robustus regularly ate hard items, suggesting that the dentognathic anatomy of other australopiths reflects rare, seasonal exploitation of hard fallback foods. Here, we show that hard-object feeding cannot explain the extreme morphology of Paranthropus boisei. Rather, analysis of long-term dietary plasticity in an animal model suggests year-round reliance on tough foods requiring prolonged postcanine processing in P. boisei. Increased consumption of such items may have marked the earlier transition from Ardipithecus to Australopithecus, with routine hard-object feeding in P. robustus representing a novel behaviour.

Teaching an old jaw new tricks: Diet-induced plasticity in a model organism, from weaning to adulthood

Many organisms exhibit a decrease in the ability to modify their phenotypes in response to shifts in environmental conditions as they mature. Such age-dependent plasticity has important implications in a variety of evolutionary and ecological contexts, particularly with respect to understanding adaptive responses to heterogeneous environments. In this study we used experimental diet manipulation to examine the life-history trajectory of plasticity in the feeding complex of a model organism, the white rabbit (Oryctolagus cuniculus). We demonstrate that, contrary to expectations derived from previous cross-sectional studies of skeletal plasticity, the jaws of weanlings and young adults exhibit similar increases in relative bone cross-sectional areas in response to the introduction of mechanically challenging foods into their diets. Furthermore, we present evidence that sensitivity to loading patterns persists well into adulthood in some regions of the masticatory apparatus in rabbits, indicating that there is an extended window of opportunity to respond to changes in dietary properties during an animal's life span. We conclude that certain aspects of the facial skeleton of rabbits, and perhaps mammals in general, are sensitive to environmental stimuli long after skeletal maturity is achieved, highlighting the importance of plasticity as a source of adaptive variation at later life-history stages.

Differing effects of PTH 1-34, PTH 1-84, and zoledronic acid on bone microarchitecture and estimated strength in postmenopausal women with osteoporosis: an 18-month open-labeled observational study using HR-pQCT

Whereas the beneficial effects of intermittent treatment with parathyroid hormone (PTH) (intact PTH 1-84 or fragment PTH 1-34, teriparatide) on vertebral strength is well documented, treatment may not be equally effective in the peripheral skeleton. We used high-resolution peripheral quantitative computed tomography (HR-pQCT) to detail effects on compartmental geometry, density, and microarchitecture as well as finite element (FE) estimated integral strength at the distal radius and tibia in postmenopausal osteoporotic women treated with PTH 1-34 (20 µg sc daily, n = 18) or PTH 1-84 (100 µg sc daily, n = 20) for 18 months in an open-label, nonrandomized study. A group of postmenopausal osteoporotic women receiving zoledronic acid (5 mg infusion once yearly, n = 33) was also included. Anabolic therapy increased cortical porosity in radius (PTH 1-34 32 ± 37%, PTH 1-84 39 ± 32%, both p < 0.001) and tibia (PTH 1-34 13 ± 27%, PTH 1-84 15 ± 22%, both p < 0.001) with corresponding declines in cortical density. With PTH 1-34, increases in cortical thickness in radius (2.0 ± 3.8%, p < 0.05) and tibia (3.8 ± 10.4%, p < 0.01) were found. Trabecular number increased in tibia with both PTH 1-34 (4.2 ± 7.1%, p < 0.05) and PTH 1-84 (5.3 ± 8.3%, p < 0.01). Zoledronic acid did not impact cortical porosity at either site but increased cortical thickness (3.0 ± 3.5%, p < 0.01), total (2.7 ± 2.5%, p < 0.001) and cortical density (1.5 ± 2.0%, p < 0.01) in tibia as well as trabecular volume fraction in radius (2.5 ± 5.1%, p < 0.05) and tibia (2.2 ± 2.2%, p < 0.01). FE estimated bone strength was preserved, but not increased, with PTH 1-34 and zoledronic acid at both sites, whereas it decreased with PTH 1-84 in radius (-2.8 ± 5.8%, p < 0.05) and tibia (-3.9 ± 4.8%, p < 0.001). Conclusively, divergent treatment-specific effects in cortical and trabecular bone were observed with anabolic and zoledronic acid therapy. The finding of decreased estimated strength with PTH 1-84 treatment was surprising and warrants confirmation.

PTH receptor signaling in osteocytes governs periosteal bone formation and intracortical remodeling

The periosteal and endocortical surfaces of cortical bone dictate the geometry and overall mechanical properties of bone. Yet the cellular and molecular mechanisms that regulate activity on these surfaces are far from being understood. Parathyroid hormone (PTH) has profound effects in cortical bone, stimulating periosteal expansion and at the same time accelerating intracortical bone remodeling. We report herein that transgenic mice expressing a constitutive active PTH receptor in osteocytes (DMP1-caPTHR1 mice) exhibit increased cortical bone area and an elevated rate of periosteal and endocortical bone formation. In addition, DMP1-caPTHR1 mice display a marked increase in intracortical remodeling and cortical porosity. Crossing DMP1-caPTHR1 mice with mice lacking the Wnt coreceptor, LDL-related receptor 5 (LRP5), or with mice overexpressing the Wnt antagonist Sost in osteocytes (DMP1-Sost mice) reduced or abolished, respectively, the increased cortical bone area, periosteal bone formation rate, and expression of osteoblast markers and Wnt target genes exhibited by the DMP1-caPTHR1 mice. In addition, DMP1-caPTHR1 lacking LRP5 or double transgenic DMP1-caPTHR1;DMP1-Sost mice exhibit exacerbated intracortical remodeling and increased osteoclast numbers, and markedly decreased expression of the RANK decoy receptor osteoprotegerin. Thus, whereas Sost downregulation and the consequent Wnt activation is required for the stimulatory effect of PTH receptor signaling on periosteal bone formation, the Wnt-independent increase in osteoclastogenesis induced by PTH receptor activation in osteocytes overrides the effect on Sost. These findings demonstrate that PTH receptor signaling influences cortical bone through actions on osteocytes and defines the role of Wnt signaling in PTH receptor action.

Comparative effects of teriparatide and strontium ranelate in the periosteum of iliac crest biopsies in postmenopausal women with osteoporosis

The periosteum contains osteogenic cells that regulate the outer shape of bone and contribute to determine its cortical thickness, size and position. We assessed the effects of subcutaneous injections of teriparatide (TPTD, 20μg/day) or oral strontium ranelate (SrR, 2g/day) in postmenopausal women with osteoporosis on new bone formation activity at the periosteal and endosteal bone surfaces using dynamic histomorphometric measurements. Evaluable tetracycline-labeled transiliac crest bone biopsies were analyzed from 27 patients in the TPTD group, and 22 in the SrR group after six months of treatment. Measurements were conducted on the thicker and thinner cortices separately, and comparisons between the thicker, thinner and combined cortices were carried out. At the combined periosteal cortex, the mineralization surface as a percent of bone surface (MS/BS%) was greater for TPTD (mean±SE: 8.08±1.22%) than SrR (3.22±1.05%) (p<0.005). The difference in mineral apposition rate (MAR) between TPTD (0.35±0.06μm/day) and SrR (0.14±0.06μm/day) was also significant (p<0.05), while that of bone formation rate per bone surface (BFR/BS) between TPTD (0.014±0.004 mm(3)/mm(2)/year) and SrR (0.004±0.003 mm(3)/mm(2)/year) was not (p=0.057). Statistically significant differences between the two treatments were also observed for MS/BS%, BFR/BS, MAR and the double-labeled perimeter in the periosteum of the thicker, but not thinner, iliac crest cortices. The comparison between the thicker and thinner cortices of both periosteal and endosteal surfaces showed statistically significant differences for MAR and the double-labeled perimeter for TPTD treated women. There were no statistically significant differences in any bone formation dynamic measurements between the two cortices in the SrR group. In conclusion, most of the bone formation and mineralization variables were significantly higher for TPTD- than SrR-treated women at both the periosteal and endosteal combined cortices. The response to TPTD for dynamic bone formation measurements in the periosteal surface was greater for the thicker than thinner cortex, but this difference was not significant in SrR treated patients. This may reflect a greater ability of TPTD to enhance responsiveness of bone to the mechanical loading environment. These effects on bone formation may underlie the improvement in bone quality in patients with osteoporosis treated with TPTD.

Osteoblast connexin43 modulates skeletal architecture by regulating both arms of bone remodeling

Cx43 in osteogenic cells controls both arms of the bone-remodeling cycle via direct actions on osteoblast differentiation and function and indirect modulation of osteoclastogenesis. These result in changes remindful of those that occur in skeletal disuse or aging and disclose a far broader function of Cx43 in skeletal biology.

Connexin43 (Cx43) has an important role in skeletal homeostasis, and Cx43 gene (Gja1) mutations have been linked to oculodentodigital dysplasia (ODDD), a human disorder characterized by prominent skeletal abnormalities. To determine the function of Cx43 at early steps of osteogenesis and its role in the ODDD skeletal phenotype, we have used the Dermo1 promoter to drive Gja1 ablation or induce an ODDD mutation in the chondro-osteogenic linage. Both Gja1 null and ODDD mutant mice develop age-related osteopenia, primarily due to a progressive enlargement of the medullary cavity and cortical thinning. This phenotype is the consequence of a high bone turnover state, with increased endocortical osteoclast-mediated bone resorption and increased periosteal bone apposition. Increased bone resorption is a noncell autonomous defect, caused by exuberant stimulation of osteoclastogenesis by Cx43-deficient bone marrow stromal cells, via decreased Opg production. The latter is part of a broad defect in osteoblast differentiation and function, which also results in abnormal structural and material properties of bone leading to decreased resistance to mechanical load. Thus Cx43 in osteogenic cells is a critical regulator of both arms of the bone remodeling cycle, its absence causing structural changes remindful of aged or disused bone.

Masticatory loading, function, and plasticity: a microanatomical analysis of mammalian circumorbital soft-tissue structures

In contrast to experimental evidence regarding the postorbital bar, postorbital septum, and browridge, there is exceedingly little evidence regarding the load-bearing nature of soft-tissue structures of the mammalian circumorbital region. This hinders our understanding of pronounced transformations during primate origins, in which euprimates evolved a postorbital bar from an ancestor with the primitive mammalian condition where only soft tissues spanned the lateral orbital margin between frontal bone and zygomatic arch. To address this significant gap, we investigated the postorbital microanatomy of rabbits subjected to long-term variation in diet-induced masticatory stresses. Rabbits exhibit a masticatory complex and feeding behaviors similar to primates, yet retain a more primitive mammalian circumorbital region. Three cohorts were obtained as weanlings and raised on different diets until adult. Following euthanasia, postorbital soft tissues were dissected away, fixed, and decalcified. These soft tissues were divided into inferior, intermediate, and superior units and then dehydrated, embedded, and sectioned. H&E staining was used to characterize overall architecture. Collagen orientation and complexity were evaluated via picrosirius-red staining. Safranin-O identified proteoglycan content with additional immunostaining performed to assess Type-II collagen expression. Surprisingly, the ligament along the lateral orbital wall was composed of elastic fibrocartilage. A more degraded organization of collagen fibers in this postorbital fibrocartilage is correlated with increased masticatory forces due to a more fracture-resistant diet. Furthermore, the lack of marked changes in the extracellular composition of the lateral orbital wall related to tissue viscoelasticity suggests it is unlikely that long-term exposure to elevated masticatory stresses underlies the development of a bony postorbital bar.

Effect of intermittent treatment with human Parathyroid Hormone 1-34 in SAMP6 senescence-accelerated mice

We examined trabecular and cortical bone in the senescence-accelerated mouse prone 6 (SAMP6) murine model of senile osteoporosis after treatment with human PTH 1-34. Sixteen-week-old female SAMP6 mice were assigned to control and PTH groups. PTH (20 microg/kg) was administered sc 3 times a week for 12 weeks. The control mouse strain, senescence-accelerated mouse resistant 1 (SAMR1), was used for comparison. The femoral metaphysis and diaphysis were used to measure bone mineral density (BMD), analyze the trabecular and the cortical structure by micro-computed tomography, and for conducting the bone strength test. PTH significantly attenuated the loss of BMD, improved the trabecular bone microstructure, and increased the bone strength in the femoral metaphysis. We did not find any differences in the bone strength of the femoral diaphysis after PTH treatment, although the cortical bone volume and cortical thickness were improved. Although the cortical thickness increased, the cortical bone density decreased, likely because of the increase of cortical porosity in the distal metaphysis after administration of PTH.

Effect of teriparatide [rhPTH(1,34)] and calcitonin on intertransverse process fusion in a rabbit model

STUDY DESIGN

Randomized, double-blinded, placebo controlled animal study.

OBJECTIVE

To evaluate the effect of teriparatide and calcitonin after an intertransverse process spinal fusion in a rabbit model.

SUMMARY OF BACKGROUND DATA

It is widely recognized that some osteoporosis medications, including bisphosphonates, can interfere with bone healing. Although prescribed frequently in the treatment of osteoporosis, the effect of teriparatide and calcitonin on spinal fusion has not been fully elucidated. We hypothesized that teriparatide, being the only anabolic medication for osteoporosis treatment, would have a beneficial effect on spine fusion.

METHODS

Fifty-one New Zealand white rabbits underwent a posterolateral L5-L6 intertransverse process arthrodesis using autogenous iliac crest bone graft. The rabbits were randomly divided into 3 groups. All animals received daily subcutaneous injections of group I (n = 17) 1 mL of saline placebo; group II (n = 17) 10 microg/kg/day of teriparatide; group III (n = 17) 14 IU/animal of calcitonin during the 8-week postoperative period. Postmortem analyses included manual palpation, radiographic, biomechanical, and histologic assessment. Three random 10x fields were examined/graded within the cephalad, middle, and caudal regions of each section (810 fields). Fusion quality was graded using the Emery histologic scale (0-7 based on fibrous/bone content of the fusion mass).

RESULTS

Histologic fusion rates for teriparatide averaged 86.7% and was significantly greater than the autograft control group (50%) (P = 0.033). Radiographically, there was a strong trend towards teriparatide being superior to the calcitonin group (85.7% vs. 56.3%, respectively; P = 0.07). The average Emery grading score was 5.99 +/- 1.46 SD for the autologous group and 6.26 +/- 0.93 SD for the teriparatide group (P = 0.031). Although not significant, the teriparatide group showed less motion in flexion/extension, lateral bending, and axial rotation.

CONCLUSION

Our results suggest that teriparatide enhances spinal fusion while calcitonin has a neutral effect. The teriparatide group had the best histologic fusion rate and Emery scores, while the calcitonin group was similar to the saline controls. Although not significant, the teriparatide group had a strong trend towards superior radiographic fusion over the calcitonin group.

Phenotypic plasticity and function of the hard palate in growing rabbits

Morphological variation related to differential loading is well known for many craniomandibular elements. Yet, the function of the hard palate, and in particular the manner in which cortical and trabecular bone of the palate respond to masticatory loads, remains more ambiguous. Here, experimental data are presented that address the naturalistic influence of biomechanical loading on the postweaning development and structure of the hard palate. A rabbit model was used to test the hypothesis that variation in the morphology of the hard palate is linked to variation in masticatory stresses. Rabbit siblings were divided as weanlings into soft and hard/tough dietary treatment groups of 10 subjects each and were raised for 15 weeks until subadulthood. MicroCT analyses indicate that rabbits subjected to elevated masticatory loading developed hard palates with significantly greater bone area, greater cortical bone thickness along the oral lamina, and thicker anterior palates. Such diet-induced levels of palatal plasticity are comparable to those for other masticatory elements, which likely reflect osteogenic responses for maintaining the functional integrity of the palate vis-à-vis elevated stresses during unilateral mastication. These data support a role for mechanical loading in the determination of palatal morphology, especially its internal structure, in living and fossil mammals such as the hominin Paranthropus. Furthermore, these findings have potential implications for the evolution of the mammalian secondary hard palate as well as for clinical considerations of human oral pathologies.

Effect of Systemic Intermittent Administration of Human Parathyroid Hormone (RHPTH[1–34]) on the Resistance to Reverse Torque in Rabbit Tibiae

The aim of this study was to evaluate the effect of intermittent administration of human parathyroid hormone [rhPTH (1–34)] on the removal torque of implants placed in rabbit tibiae. Twenty male New Zealand rabbits were submitted to implant surgery. Each animal received one machined screw-type implant (3.75 mm diameter × 8 mm length) in the proximal metaphysis of the right tibia. The rabbits were then divided into 2 groups: the test group (n = 10) received 6 μg/kg of rhPTH (1–34) subcutaneously in the dorsal region 3 days a week, and the control group (n = 10) received placebo. Removal torque was performed at 28 and 56 days after implant placement for both groups. The mean removal torque values at 28 days were 37.0 ± 4.36 Ncm and 47.4 ± 6.77 Ncm for control and test groups respectively (P &lt; .05). At 56 days the reverse torque was 45.8 ± 3.96 Ncm for the control group and 55.8 ± 2.86 Ncm for the test group, indicating that the removal torque was significantly higher in the test groups (P &lt; .05). These results demonstrated that intermittent treatment with rhPTH (1–34) enhanced the removal torque of implants in rabbit tibiae.

Differentiation and proliferation of periosteal osteoblast progenitors are differentially regulated by estrogens and intermittent parathyroid hormone administration

The periosteum is now widely recognized as a homeostatic and therapeutic target for actions of sex steroids and intermittent PTH administration. The mechanisms by which estrogens suppress but PTH promotes periosteal expansion are not known. In this report, we show that intermittent PTH(1-34) promotes differentiation of periosteal osteoblast precursors as evidenced by the stimulation of the expression or activity of alkaline phosphatase as well as of targets of the bone morphogenetic protein 2 (BMP-2) and Wnt pathways. In contrast, 17beta-estradiol (E2) had no effect by itself. However, it attenuated PTH- or BMP-2-induced differentiation of primary periosteal osteoblast progenitors. Administration of intermittent PTH to ovariectomized mice induced rapid phosphorylation of the BMP-2 target Smad1/5/8 in the periosteum. A replacement dose of E2 had no effect by itself but suppressed PTH-induced phosphorylation of Smad1/5/8. In contrast to its effects to stimulate periosteal osteoblast differentiation, PTH promoted and subsequently suppressed proliferation of periosteal osteoblast progenitors in vitro and in vivo. E2 promoted proliferation and attenuated the antiproliferative effect of PTH. Both hormones protected periosteal osteoblasts from apoptosis induced by various proapoptotic agents. These observations suggest that the different effects of PTH and estrogens on the periosteum result from opposing actions on the recruitment of early periosteal osteoblast progenitors. Intermittent PTH promotes osteoblast differentiation from periosteum-derived mesenchymal progenitors through ERK-, BMP-, and Wnt-dependent signaling pathways. Estrogens promote proliferation of early osteoblast progenitors but inhibit their differentiation by osteogenic agents such as PTH or BMP-2.

Effects of a one-month treatment with PTH(1-34) on bone formation on cancellous, endocortical, and periosteal surfaces of the human ilium

Using bone histomorphometry, we found that a 1-month treatment with PTH(1-34) [hPTH(1-34)] stimulated new bone formation on cancellous, endocortical, and periosteal bone surfaces. Enhanced bone formation was associated with an increase in osteoblast apoptosis.

INTRODUCTION

The precise mechanisms by which hPTH(1-34) increases bone mass and improves bone structure are unclear. Using bone histomorphometry, we studied the early effects of treating postmenopausal women with osteoporosis with hPTH(1-34).

MATERIALS AND METHODS

Tetracycline-labeled iliac crest bone biopsies were obtained from 27 postmenopausal women with osteoporosis who were treated for 1 month with hPTH(1-34), 50 microg daily subcutaneously. The results were compared with tetracycline-labeled biopsies from a representative control group of 13 postmenopausal women with osteoporosis.

RESULTS

The bone formation rate on the cancellous and endocortical surfaces was higher in hPTH(1-34)-treated women than in control women by factors of 4.5 and 5.0, respectively. We also showed a 4-fold increase in bone formation rate on the periosteal surface, suggesting that hPTH(1-34) has the potential to increase bone diameter in humans. On the cancellous and endocortical surfaces, the increased bone formation rate was primarily caused by stimulation of formation in ongoing remodeling units, with a modest amount of increased formation on previously quiescent surfaces. hPTH(1-34)-stimulated bone formation was associated with an increase in osteoblast apoptosis, which may reflect enhanced turnover of the osteoblast population and may contribute to the anabolic action of hPTH(1-34).

CONCLUSIONS

These findings provide new insight into the cellular basis by which hPTH(1-34) improves cancellous and cortical bone architecture and geometry in patients with osteoporosis.

Bone fragility: failure of periosteal apposition to compensate for increased endocortical resorption in postmenopausal women

The increase in bone fragility after menopause results from reduced periosteal bone formation and increased endocortical resorption. Women with highest remodeling had greatest loss of bone mass and estimated bone strength, whereas those with low remodeling lost less bone and maintained estimated bone strength.

INTRODUCTION

Bone loss from the inner (endocortical) surface contributes to bone fragility, whereas deposition of bone on the outer (periosteal) surface is believed to be an adaptive response to maintain resistance to bending.

MATERIALS AND METHODS

To test this hypothesis, changes in bone mass and estimated indices of bone geometry and strength of the one-third distal radius, bone turnover markers, and fracture incidence were measured annually in 821 women 30-89 years of age for 7.1 +/- 2.5 years. The analyses were made in 151 premenopausal women, 33 perimenopausal women, 279 postmenopausal women, and 72 postmenopausal women receiving hormone replacement therapy (HRT).

RESULTS

In premenopausal women, periosteal apposition increased the radius width, partly offsetting endocortical resorption; therefore, the estimated cortical thickness decreased. Outward displacement of the thinner cortex maintained bone mass and cortical area and increased estimated bending strength. Estimated endocortical resorption accelerated during perimenopause, whereas periosteal apposition decreased. Further cortical thinning occurred, but estimated bending strength was maintained by modest outward cortical displacement. Endocortical resorption accelerated further during the postmenopausal years, whereas periosteal apposition declined further; cortices thinned, but because outward displacement was minimal, estimated cortical area and bending strength now decreased. Women with highest remodeling had the greatest loss of bone mass and strength. Women with low remodeling lost less bone and maintained estimated bone strength. In HRT-treated women, loss of bone strength was partly prevented. These structural indices predicted incident fractures; a 1 SD lower section modulus doubled fracture risk.

CONCLUSIONS

Periosteal apposition does not increase after menopause to compensate for bone loss; it decreases. Bone fragility of osteoporosis is a consequence of reduced periosteal bone formation and increased endocortical resorption. Understanding the mechanisms of the age-related decline in periosteal apposition will identify new therapeutic targets. On the basis of our results, it may be speculated that the stimulation of periosteal apposition will increase bone width and improve skeletal strength.

Parathyroid hormone and teriparatide for the treatment of osteoporosis: a review of the evidence and suggested guidelines for its use

All therapies currently recommended for the management of osteoporosis act mainly to inhibit bone resorption and reduce bone remodeling. PTH and its analog, teriparatide [recombinant human PTH(1-34)], represent a new class of anabolic therapies for the treatment of severe osteoporosis, having the potential to improve skeletal microarchitecture. Significant reductions in both vertebral and appendicular fracture rates have been demonstrated in the phase III trial of teriparatide, involving elderly women with at least one prevalent vertebral fracture before the onset of therapy. However, there is as yet no evidence that the antifracture efficacy of PTH will be superior to the bisphosphonates, whereas cost-utility estimates suggest that teriparatide is significantly more expensive. Teriparatide should be considered as treatment for postmenopausal women and men with severe osteoporosis, as well as for patients with established glucocorticoid-induced osteoporosis who require long-term steroid treatment. Teriparatide should also be considered for the management of individuals at particularly high risk for fractures, including subjects who are younger than age 65 and who have particularly low bone mineral density measurements (T scores < or = 3.5). Teriparatide therapy is not recommended for more than 2 yr, based, in part, on the induction of osteosarcoma in a rat model of carcinogenicity. Total daily calcium intake from both supplements and dietary sources should be limited to 1500 mg together with adequate vitamin D intake (< or =1000 U/d). Monitoring of serum calcium may be safely limited to measurement after 1 month of treatment; mild hypercalcemia may be treated by withdrawing dietary calcium supplements, reducing the dosing frequency of PTH, or both. At present, concurrent therapy with antiresorptive therapy, particularly bisphosphonates, should be avoided, although sequential therapy with such agents may consolidate the beneficial effects upon the skeleton after PTH is discontinued.

beta-Arrestin2 regulates the differential response of cortical and trabecular bone to intermittent PTH in female mice

Cytoplasmic arrestins regulate PTH signaling in vitro. We show that female beta-arrestin2(-/-) mice have decreased bone mass and altered bone architecture. The effects of intermittent PTH administration on bone microarchitecture differed in beta-arrestin2(-/-) and wildtype mice. These data indicate that arrestin-mediated regulation of intracellular signaling contributes to the differential effects of PTH at endosteal and periosteal bone surfaces.

INTRODUCTION

The effects of PTH differ at endosteal and periosteal surfaces, suggesting that PTH activity in these compartments may depend on some yet unidentified mechanism(s) of regulation. The action of PTH in bone is mediated primarily by intracellular cAMP, and the cytoplasmic molecule beta-arrestin2 plays a central role in this signaling regulation. Thus, we hypothesized that arrestins would modulate the effects of PTH on bone in vivo.

MATERIALS AND METHODS

We used pDXA, muCT, histomorphometry, and serum markers of bone turnover to assess the skeletal response to intermittent PTH (0, 20, 40, or 80 mug/kg/day) in adult female mice null for beta-arrestin2 (beta-arr2(-/-)) and wildtype (WT) littermates (7-11/group).

RESULTS AND CONCLUSIONS

beta-arr2(-/-) mice had significantly lower total body BMD, trabecular bone volume fraction (BV/TV), and femoral cross-sectional area compared with WT. In WT females, PTH increased total body BMD, trabecular bone parameters, and cortical thickness, with a trend toward decreased midfemoral medullary area. In beta-arr2(-/-) mice, PTH not only improved total body BMD, trabecular bone architecture, and cortical thickness, but also dose-dependently increased femoral cross-sectional area and medullary area. Histomorphometry showed that PTH-stimulated periosteal bone formation was 2-fold higher in beta-arr2(-/-) compared with WT. Osteocalcin levels were significantly lower in beta-arr2(-/-) mice, but increased dose-dependently with PTH in both beta-arr2(-/-) and WT. In contrast, whereas the resorption marker TRACP5B increased dose-dependently in WT, 20-80 mug/kg/day of PTH was equipotent with regard to stimulation of TRACP5B in beta-arr2(-/-). In summary, beta-arrestin2 plays an important role in bone mass acquisition and remodeling. In estrogen-replete female mice, the ability of intermittent PTH to stimulate periosteal bone apposition and endosteal resorption is inhibited by arrestins. We therefore infer that arrestin-mediated regulation of intracellular signaling contributes to the differential effects of PTH on cancellous and cortical bone.

Does early PTH treatment compromise bone strength? The balance between remodeling, porosity, bone mineral, and bone size

The treatment of osteoporotic women with recombinant human parathyroid hormone (rhPTH) increases bone mineral density and reduces fracture risk. However, there has been concern that the initiation of therapy in women with low bone mass may cause an early and transient increased fracture risk because PTH stimulates bone remodeling, which in its first phase is associated with bone resorption. Animal and human studies suggest, however, that the stimulation of remodeling caused by rhPTH(1-34) does not lead to a deterioration of bone's mechanical properties or to an increased fracture risk even early in the treatment. There are several reasons for this. Bone biomarkers associated with formation rise earlier than those associated with resorption, suggesting that there is an initial period prior to the stimulation of remodeling during which bone formation occurs on surfaces without prior resorption. This initial period of formation may protect the patient from the later small and transient losses that occur through remodeling. Moreover, the increased remodeling occurs on cancellous surfaces or close to the endosteal surface of bone, where its mechanical effect is minimal. Additionally, these transient losses may be compensated by periosteal apposition that maintains the overall strength of the bone. Thus, the early stimulation of bone formation without prior resorption, and the redistribution of bone from cancellous and endocortical surfaces to the periosteal surface combine to prevent the mechanical deterioration that could otherwise occur with a transient acceleration of bone remodeling in a patient with low bone mass.

Response of induced bone defects in horses to collagen matrix containing the human parathyroid hormone gene

OBJECTIVE

To determine whether human parathyroid hormone (hPTH) gene in collagen matrix could safely promote bone formation in diaphyseal or subchondral bones of horses.

ANIMALS

8 clinically normal adult horses.

PROCEDURE

Amount, rate, and quality of bone healing for 13 weeks were determined by use of radiography, quantitative computed tomography, and histomorphometric analysis. Diaphyseal cortex and subchondral bone defects of metacarpi were filled with hPTH(1-34) gene-activated matrix (GAM) or remained untreated. Joints were assessed on the basis of circumference, synovial fluid analysis, pain on flexion, lameness, and gross and histologic examination.

RESULTS

Bone volume index was greater for cortical defects treated with hPTH(1-34) GAM, compared with untreated defects. Bone production in cortical defects treated with hPTH(1-34) GAM positively correlated with native bone formation in untreated defects. In contrast, less bone was detected in hPTH(1-34) GAM-treated subchondral bone defects, compared with untreated defects, and histology confirmed poorer healing and residual collagen sponge.

CONCLUSIONS AND CLINICAL RELEVANCE

Use of hPTH(1-34) GAM induced greater total bone, specifically periosteal bone, after 13 weeks of healing in cortical defects of horses. The hPTH(1-34) GAM impeded healing of subchondral bone but was biocompatible with joint tissues. Promotion of periosteal bone formation may be beneficial for healing of cortical fractures in horses, but the delay in onset of bone formation may negate benefits. The hPTH(1-34) GAM used in this study should not be placed in articular subchondral bone defects, but contact with articular surfaces is unlikely to cause short-term adverse effects.

Continuous parathyroid hormone induces cortical porosity in the rat: effects on bone turnover and mechanical properties

We examined the time course effects of continuous PTH on cortical bone and mechanical properties. PTH increased cortical bone turnover and induced intracortical porosity with no deleterious effect on bone strength. Withdrawal of PTH increased maximum torque to failure and stiffness with no change in energy absorbed.

INTRODUCTION

The skeletal response of cortical bone to parathyroid hormone (PTH) is complex and species dependent. Intermittent administration of PTH to rats increases periosteal and endocortical bone formation but has no known effects on intracortical bone turnover. The effects of continuous PTH on cortical bone are not clearly established.

MATERIALS AND METHODS

Eighty-four 6-month-old female Sprague-Dawley rats were divided into three control, six PTH, and two PTH withdrawal (WD) groups. They were subcutaneously implanted with osmotic pumps loaded with vehicle or 40 microg/kg BW/day human PTH(1-34) for 1, 3, 5, 7, 14, and 28 days. After 7 days, PTH was withdrawn from two groups of animals for 7 (7d-PTH/7d-WD) and 21 days (7d-PTH/21d-WD). Histomorphometry was performed on periosteal and endocortical surfaces of the tibial diaphysis in all groups. microCT of tibias and mechanical testing by torsion of femora were performed on 28d-PTH and 7d-PTH/21d-WD animals.

RESULTS AND CONCLUSIONS

Continuous PTH increased periosteal and endocortical bone formation, endocortical osteoclast perimeter, and cortical porosity in a time-dependent manner, but did not change the mechanical properties of the femur, possibly because of addition of new bone onto periosteal and endocortical surfaces. Additionally, withdrawal of PTH restored normal cortical porosity and increased maximum torque to failure and stiffness. We conclude that continuous administration of PTH increased cortical porosity in rats without having a detrimental effect on bone mechanical properties.

Teriparatide (Biosynthetic Human Parathyroid Hormone 1-34): A New Paradigm in the Treatment of Osteoporosis

The ideal treatment of osteoporosis should preferably prevent fractures through normalization of bone mass and bone micro-architecture. Biosynthetic human parathyroid hormone 1-34 (teriparatide) was recently approved in the EU and the USA as the first anabolic treatment of osteoporosis. The effects of teriparatide are mediated by the G-protein-dependent, parathyroid hormone receptor-1 in the cell membrane. The binding of the ligand to the receptor activates adenylate cyclase and a number of phospholipases (A, C, and D) and increases intracellular levels of cAMP and calcium. Intermittent teriparatide increases the number of osteoblasts and bone formation by activation of pre-existing osteoblasts, increased differentiation of lining cells, and reduced osteoblast apoptosis. Anabolic effects of teriparatide on bone have been demonstrated in several species. It increases bone mass, structural integrity, bone diameter, and bone strength. Clinical efficacy was demonstrated in a randomized study comprising 1637 post-menopausal women with osteoporosis showing a 65% and 35% reduction of the relative risk of vertebral and appendicular fractures, respectively, during 18 months of treatment. Moreover, bone mineral density in the lumbar spine and hip increased by 9.7% and 2.6%, respectively. Similar effects on bone mineral density have been reported in men with osteoporosis and in glucocorticoid-induced osteoporosis, however, fracture data are limited in these groups. Direct comparison with alendronate revealed that teriparatide has a more pronounced effect on bone mineral density. Teriparatide should be used in combination with calcium plus vitamin D, and may be combined with hormonal replacement therapy. In contrast, alendronate attenuates the effect of teriparatide. The efficacy of other combinations remains uncertain. After termination of teriparatide, bone mineral density of the lumbar spine is reduced by approximately 2-3% after 2 1/2 years. This decrease is prevented by treatment with bisphosphonates. The most frequent adverse effects with teriparatide are nausea, headache, dizziness, and leg cramps, however, only the latter two differed significantly between the groups receiving teriparatide 20 microg/day and placebo. In the pivotal clinical study, reduced dosage or termination of therapy due to hypercalcaemia was necessary in 3% and 0.2%, respectively. In a rat toxicology study, in which teriparatide was administered in high dosages for an extended period of time, osteosarcoma was seen in a significant number of animals. However, none of the approximately 2800 patients in clinical trials has developed osteosarcoma. Teriparatide constitutes a break-through in the treatment of severe osteoporosis, although a number of issues about the optimal use of teriparatide remains unsettled. The published data provide proof of concept on anabolic therapy which changes several paradigms of bone physiology. Other parathyroid hormone analogues are being investigated in clinical trials and the development of non-peptide, small molecules targeted at the parathyroid hormone receptor may be envisaged.

Teriparatide [PTH(1-34)] strengthens the proximal femur of ovariectomized nonhuman primates despite increasing porosity

OVX monkeys treated for 18 months with 1 or 5 microg/kg/d teriparatide [PTH (1-34)] had significantly stronger proximal femora relative to ovariectomized controls. Teriparatide enhancement of cortical area, cortical width, and trabecular bone volume seemed to more than compensate for the dose-dependent increase in cortical porosity. Beneficial effects of teriparatide treatment on the proximal femur persisted beyond the treatment period and may extend to the marrow.

INTRODUCTION

We conducted a detailed quantitative analysis of the effects of teriparatide on the proximal femur of ovariectomized monkeys. Teriparatide increased bone mass, enhanced structural architecture, and strengthened the hip, despite increasing cortical porosity.

MATERIALS AND METHODS

Monkeys were treated with vehicle (sham or OVX controls), 1 microg/kg/day teriparatide [parathyroid hormone (1-34); PTH1], or 5 microg/kg/day teriparatide (PTH5) for 18 months or for 12 months followed by 6 months of treatment withdrawal (PTH1W and PTH5W, respectively). Excised proximal femora were analyzed by microCT, conventional histomorphometry, and biomechanics.

RESULTS AND CONCLUSIONS

The femoral neck showed significant reduction in trabecular bone volume (BV/TV) for OVX compared with sham, whereas PTH1 BV/TV was restored to sham levels and PTH5 BV/TV was greater than sham and OVX. The withdrawal groups had BV/TVs intermediate between sham and OVX. PTH1 had trabecular number (Tb.N) greater than OVX, and PTH5 Tb.N was greater than sham and OVX. The withdrawal groups had Tb.Ns intermediate between sham and OVX. No differences between groups were observed for trabecular orientation or trabecular thickness. Teriparatide dose-dependently increased bone formation rate and activation frequency in the femoral neck. Cellular composition analyses suggested a tendency of ovariectomy to increase adiposity of marrow by 100%, whereas PTH tended to reduce adipocyte number and increase osteoblast number compared with OVX. Analyses of the cortex showed dose-dependent elevation of cortical porosity, which was consistent with enhanced bone turnover with treatment. Cortical porosity was reduced after withdrawal of teriparatide, because PTH1W cortical porosity was lower than OVX, whereas PTH5W cortical porosity was intermediate between sham and OVX. Increased cortical porosity did not weaken the proximal femora. Biomechanics showed that ovariectomy weakened proximal femora compared with sham, but PTH1, PTH5, and PTH1W were stronger than OVX and not different from sham. PTH5W strength was intermediate between sham and OVX. Therefore, teriparatide had beneficial effects on the proximal femur, despite increasing cortical porosity. Cortical porosity did not adversely affect the mechanical integrity of the proximal femora, because enhanced cortical area and trabecular bone volume more than compensated for the porosity. Much of the beneficial effects of teriparatide were retained after 6 months withdrawal from treatment. PTH effects on the femoral neck were not limited to bone but may include inhibition of OVX-stimulated adiposity of the marrow.

Trabecular bone response to mechanical and parathyroid hormone stimulation: the role of mechanical microenvironment

Bone response under combined mechanical and PTH stimuli is important in osteoporosis. A rat tail animal model with computer modeling was used to examine bone response to loading and PTH. PTH enhances and sustains increased bone formation rate, which directly correlates to mechanical microenvironment, suggesting beneficial effects of combined PTH treatment and exercise in preventing osteoporosis.

INTRODUCTION

Using an in vivo rat tail vertebra model combined with a specimen-specific, high-resolution microcomputed tomography (microCT)-based finite element analysis (FEA) technique, trabecular bone response to combined dynamic compressive loading and parathyroid hormone (PTH) stimulation was characterized.

MATERIALS AND METHODS

Two hundred twenty-four male Sprague-Dawley rats were randomly divided into seven treatment groups: (1) Control, (2) vehicle + 0N, (3) PTH + 0N, (4) vehicle + 50N, (5) PTH + 50N, (6) vehicle + 100N, and (7) PTH + 100N, with three treatment durations (1, 2, or 4 weeks). Rat PTH(1-34) was administered daily in the PTH-stimulated groups approximately 3 h before daily mechanical stimulation with 0, 50, or 100N dynamic compressive loading. microCT-based FEA was performed for each loaded vertebra after death. Bone histomorphometry was performed on trabecular bone with double fluorochrome labeling to assess bone formation.

RESULTS

Daily mechanical loading or PTH administration significantly increased bone formation rate (BFR) compared with control or V + 0N with significant increases in both mineral apposition rate (MAR) and labeled bone surface (LS/BS). PTH, when combined with mechanical loading, enhanced BFR mainly through a significant increase in MAR after the first week and through a significant increase in LS/BS after 2 and 4 weeks. Synergistic effects in BFR were present when PTH was combined with mechanical loading, especially after 2 and 4 weeks, where the increase in BFR was sustained. However, when either PTH or mechanical loading was the only stimulus, the bone formation response diminished to the level of Control animals after 4 weeks. Furthermore, significant correlations were observed between the bone formation indices and trabecular bone tissue mechanical microenvironments at 1 and 2 weeks, with PTH administration enhancing and sustaining these correlations into 4 weeks.

CONCLUSIONS

The synergistic effects of combined PTH and mechanical stimulation on trabecular bone formation rate suggest a potential benefit for combined PTH administration and exercise in the treatment of osteoporosis.

New bone formation with teriparatide [human parathyroid hormone-(1-34)] is not retarded by long-term pretreatment with alendronate, estrogen, or raloxifene in ovariectomized rats

With the ready availability of several osteoporosis therapies, teriparatide [human PTH-(1-34)] is likely to be prescribed to postmenopausal women with prior exposure to agents that prevent bone loss, such as bisphosphonates, estrogen, or selective estrogen receptor modulators. Therefore, we evaluated the ability of once daily teriparatide to induce bone formation in ovariectomized (Ovx) rats with extended prior exposure to various antiresorptive agents, such as alendronate (ABP), 17 alpha-ethinyl estradiol (EE), or raloxifene (Ral). Sprague Dawley rats were Ovx and treated with ABP (28 microg/kg, twice weekly), EE (0.1 mg/kg per d), or Ral (1 mg/kg per d) for 10 months before switching to teriparatide 30 microg/kg per d for another 2 months. Analysis of the proximal tibial metaphysis showed that all three antiresorptive agents prevented ovariectomy-induced bone loss after 10 months, but were mechanistically distinct, as shown by histomorphometry. Before teriparatide treatment, ABP strongly suppressed activation frequency and bone formation rate to below levels in other treatment groups, whereas these parameters were not different from sham values for EE or Ral. Trabecular area for ABP, EE, and Ral were greater than that in Ovx controls. However, the trabecular bone effects of ABP were attributed not only to effects on the secondary spongiosa, but also to the preservation of primary spongiosa, which was prevented from remodeling. After 2 months of teriparatide treatment, lumbar vertebra showed relative bone mineral density increases of 18%, 7%, 11%, and 10% for vehicle/teriparatide, ABP/teriparatide, EE/teriparatide, and Ral/teriparatide, respectively, compared with 10 month levels. Histomorphometry showed that trabecular area was increased by 105%, 113%, 36%, and 48% for vehicle/teriparatide, ABP/teriparatide, EE/teriparatide, and Ral/teriparatide, respectively, compared with 10 month levels. Teriparatide enhanced mineralizing surface, mineral apposition rate, and bone formation rate in all groups. Compression testing of vertebra showed that teriparatide improved strength (peak load) and toughness in all groups to a proportionately similar extent compared with 10 month levels. These data showed a surprising ability of the rat skeleton to respond to teriparatide despite extensive pretreatment with ABP, EE, or Ral. Therefore, the mature skeleton of Ovx rats remains highly responsive to the appositional effects of teriparatide regardless of pretreatment status in terms of cancellous bone area or rate of bone turnover.