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Immobilization, as a result of motor-complete spinal cord injury (SCI), is associated with severe osteoporosis. Whether parathyroid hormone (PTH) administration would reduce bone loss after SCI remains unclear. Thus, female mice underwent sham or surgery to produce complete spinal cord transection. PTH (80 g/kg) or vehicle was injected subcutaneously (SC) daily starting on the day of surgery and continued for 35 days. Isolated tibias and femurs were examined by microcomputed tomography scanning (micro-CT) and histology and serum markers of bone turnover were measured. Micro-CT analysis of tibial metaphysis revealed that the SCI-vehicle animals exhibited 49% reduction in fractional trabecular bone volume and 18% in trabecular thickness compared to sham-vehicle controls. SCI-vehicle animals also had 15% lower femoral cortical thickness and 16% higher cortical porosity than sham-vehicle counterparts. Interestingly, PTH administration to SCI animals restored 78% of bone volume, increased connectivity to 366%, and lowered structure model index by 10% compared to sham-vehicle animals. PTH further favorably attenuated femoral cortical bone loss to 5% and prevented the SCI-associated cortical porosity. Histomorphometry evaluation of femurs of SCI-vehicle animals demonstrated a marked 49% and 38% decline in osteoblast and osteoclast number, respectively, and 35% reduction in bone formation rate. In contrast, SCI-PTH animals showed preserved osteoblast and osteoclast numbers and enhanced bone formation rate. Furthermore, SCI-PTH animals had higher levels of bone formation and resorption markers than either SCI- or sham-vehicle groups. Collectively, these findings suggest that intermittent PTH receptor activation is an effective therapeutic strategy to preserve bone integrity after severe immobilization.
© 2017 The Authors. Physiological Reports published by Wiley Periodicals, Inc. on behalf of The Physiological Society and the American Physiological Society.
Ceramic/polymer composite bone grafts offer the potential advantage of combining the osteoconductivity of ceramic component with the ductility of polymeric component, resulting in a graft that meets many of the desired properties for bone void fillers (BVF). However, the relative contributions of the polymer and ceramic components to bone healing are not well understood. In this study, we compared remodeling of low-viscosity (LV) ceramic/poly(ester urethane) composites to a ceramic BVF control in a sheep femoral condyle plug defect model. LV composites incorporating either ceramic (LV/CM) or allograft bone (LV/A) particles were evaluated. We hypothesized that LV/CM composites which have the advantageous handling properties of injectability, flowability, and settability would heal comparably to the CM control, which was evaluated for up to 2 years to study its long-term degradation properties. Remodeling of LV/CM was comparable to that observed for the CM control, as evidenced by new bone formation on the surface of the ceramic particles. At early time points (4 months), LV/CM composites healed similar to the ceramic clinical control, while LV/A components showed more variable healing due to osteoclast-mediated resorption of the allograft particles. At longer time points (12-15 months), healing of LV/CM composites was more variable due to the nonhomogeneous distribution and lower concentration of the ceramic particles compared to the ceramic clinical control. Resorption of the ceramic particles was almost complete at 2 years. This study highlights the importance of optimizing the loading and distribution of ceramic particles in polymer/ceramic composites to maximize bone healing. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 105B: 2333-2343, 2017.
© 2016 Wiley Periodicals, Inc.
Patients with chronic kidney disease (CKD) have an increased risk of fracture. Raloxifene is a mild antiresorptive agent that reduces fracture risk in the general population. Here we assessed the impact of raloxifene on the skeletal properties of animals with progressive CKD. Male Cy/+ rats that develop autosomal dominant cystic kidney disease were treated with either vehicle or raloxifene for five weeks. They were assessed for changes in mineral metabolism and skeletal parameters (microCT, histology, whole-bone mechanics, and material properties). Their normal littermates served as controls. Animals with CKD had significantly higher parathyroid hormone levels compared with normal controls, as well as inferior structural and mechanical skeletal properties. Raloxifene treatment resulted in lower bone remodeling rates and higher cancellous bone volume in the rats with CKD. Although it had little effect on cortical bone geometry, it resulted in higher energy to fracture and modulus of toughness values than vehicle-treated rats with CKD, achieving levels equivalent to normal controls. Animals treated with raloxifene had superior tissue-level mechanical properties as assessed by nanoindentation, and higher collagen D-periodic spacing as assessed by atomic force microscopy. Thus, raloxifene can positively impact whole-bone mechanical properties in CKD through its impact on skeletal material properties.
Copyright © 2015 International Society of Nephrology. Published by Elsevier Inc. All rights reserved.
The bone marrow is a hypoxic microenvironment that is rich in growth factors and blood vessels and is readily colonized by tumor cells disseminated from numerous cancers including tumors of the breast, prostate, lung, and skin. The origin of metastatic growth promoting factors for tumor cells disseminated to the bone marrow is derived from multiple sources: the bone matrix, which is a reservoir for growth factors, and cells residing in the marrow and along bone surfaces, such as osteoblasts, osteoclasts, macrophages, and T cells, which secrete cytokines and chemokines. Low oxygen levels within the bone marrow induce hypoxia signaling pathways such as hypoxia inducible factor (HIF), which is regulated by oxygen requiring prolyl hydroxylases (PHDs) and von Hippel-Lindau (VHL) tumor suppressor. These hypoxia signaling pathways have profound effects on bone development and homeostasis. Likewise, hypoxic conditions observed in local breast and prostate tumors point to a role for hypoxia-inducible genes in metastasis to and colonization of the bone marrow. This review will explore the role of hypoxia-regulated factors in bone development and remodeling, and how these elements may contribute to solid tumor metastasis to the bone.
Copyright © 2015 Elsevier Inc. All rights reserved.
Established clinical approaches to treat bone voids include the implantation of autograft or allograft bone, ceramics, and other bone void fillers (BVFs). Composites prepared from lysine-derived polyurethanes and allograft bone can be injected as a reactive liquid and set to yield BVFs with mechanical strength comparable to trabecular bone. In this study, we investigated the effects of porosity, allograft particle size, and matrix mineralization on remodeling of injectable and settable allograft/polymer composites in a rabbit femoral condyle plug defect model. Both low viscosity and high viscosity grafts incorporating small (<105 μm) particles only partially healed at 12 weeks, and the addition of 10% demineralized bone matrix did not enhance healing. In contrast, composite grafts with large (105-500 μm) allograft particles healed at 12 weeks postimplantation, as evidenced by radial μCT and histomorphometric analysis. This study highlights particle size and surface connectivity as influential parameters regulating the remodeling of composite bone scaffolds.
© 2015 Wiley Periodicals, Inc.
The inner ear vestibular system has numerous projections on central brain centers that regulate sympathetic outflow, and skeletal sympathetic projections affect bone remodeling by inhibiting bone formation by osteoblasts and promoting bone resorption by osteoclasts. In this study, we show that bilateral vestibular lesions in mice cause a low bone mass phenotype associated with decreased bone formation and increased bone resorption. This reduction in bone mass is most pronounced in lower limbs, is not associated with reduced locomotor activity or chronic inflammation, and could be prevented by the administration of the β-blocker propranolol and by genetic deletion of the β2-adrenergic receptor, globally or specifically in osteoblasts. These results provide novel experimental evidence supporting a functional autonomic link between central proprioceptive vestibular structures and the skeleton. Because vestibular dysfunction often affects the elderly, these results also suggest that age-related bone loss might have a vestibular component and that patients with inner ear pathologies might be at risk for fracture. Lastly, these data might have relevance to the bone loss observed in microgravity, as vestibular function is altered in this condition as well. © 2015 American Society for Bone and Mineral Research.
© 2015 American Society for Bone and Mineral Research.
Cellular subpopulations in the bone marrow play distinct and unexplored functions in skeletal homeostasis. This study delineated a unique role of osteal macrophages in bone and parathyroid hormone (PTH)-dependent bone anabolism using murine models of targeted myeloid-lineage cell ablation. Depletion of c-fms(+) myeloid lineage cells [via administration of AP20187 in the macrophage Fas-induced apoptosis (MAFIA) mouse model] reduced cortical and trabecular bone mass and attenuated PTH-induced trabecular bone anabolism, supporting the positive function of macrophages in bone homeostasis. Interestingly, using a clodronate liposome model with targeted depletion of mature phagocytic macrophages an opposite effect was found with increased trabecular bone mass and increased PTH-induced anabolism. Apoptotic cells were more numerous in MAFIA versus clodronate-treated mice and flow cytometric analyses of myeloid lineage cells in the bone marrow showed that MAFIA mice had reduced CD68(+) cells, whereas clodronate liposome-treated mice had increased CD68(+) and CD163(+) cells. Clodronate liposomes increased efferocytosis (clearance of apoptotic cells) and gene expression associated with alternatively activated M2 macrophages as well as expression of genes associated with bone formation including Wnt3a, Wnt10b, and Tgfb1. Taken together, depletion of early lineage macrophages resulted in osteopenia with blunted effects of PTH anabolic actions, whereas depletion of differentiated macrophages promoted apoptotic cell clearance and transformed the bone marrow to an osteogenic environment with enhanced PTH anabolism. These data highlight a unique function for osteal macrophages in skeletal homeostasis.
Changes in bone remodeling induced by pharmacological and genetic manipulation of β-adrenergic receptor (βAR) signaling in osteoblasts support a role of sympathetic nerves in the regulation of bone remodeling. However, the contribution of endogenous sympathetic outflow and nerve-derived norepinephrine (NE) to bone remodeling under pathophysiological conditions remains unclear. We show here that differentiated osteoblasts, like neurons, express the norepinephrine transporter (NET), exhibit specific NE uptake activity via NET and can catabolize, but not generate, NE. Pharmacological blockade of NE transport by reboxetine induced bone loss in WT mice. Similarly, lack of NE reuptake in norepinephrine transporter (Net)-deficient mice led to reduced bone formation and increased bone resorption, resulting in suboptimal peak bone mass and mechanical properties associated with low sympathetic outflow and high plasma NE levels. Last, daily sympathetic activation induced by mild chronic stress was unable to induce bone loss, unless NET activity was blocked. These findings indicate that the control of endogenous NE release and reuptake by presynaptic neurons and osteoblasts is an important component of the complex homeostatic machinery by which the sympathetic nervous system controls bone remodeling. These findings also suggest that drugs antagonizing NET activity, used for the treatment of hyperactivity disorders, may have deleterious effects on bone accrual.
Osteomyelitis is a common manifestation of invasive Staphylococcus aureus infection. Pathogen-induced bone destruction limits antimicrobial penetration to the infectious focus and compromises treatment of osteomyelitis. To investigate mechanisms of S. aureus-induced bone destruction, we developed a murine model of osteomyelitis. Microcomputed tomography of infected femurs revealed that S. aureus triggers profound alterations in bone turnover. The bacterial regulatory locus sae was found to be critical for osteomyelitis pathogenesis, as Sae-regulated factors promote pathologic bone remodeling and intraosseous bacterial survival. Exoproteome analyses revealed the Sae-regulated protease aureolysin as a major determinant of the S. aureus secretome and identified the phenol-soluble modulins as aureolysin-degraded, osteolytic peptides that trigger osteoblast cell death and bone destruction. These studies establish a murine model for pathogen-induced bone remodeling, define Sae as critical for osteomyelitis pathogenesis, and identify protease-dependent exoproteome remodeling as a major determinant of the staphylococcal virulence repertoire.
Copyright © 2013 Elsevier Inc. All rights reserved.
The skeleton is no longer seen as a static, isolated, and mostly structural organ. Over the last two decades, a more complete picture of the multiple functions of the skeleton has emerged, and its interactions with a growing number of apparently unrelated organs have become evident. The skeleton not only reacts to mechanical loading and inflammatory, hormonal, and mineral challenges, but also acts of its own accord by secreting factors controlling the function of other tissues, including the kidney and possibly the pancreas and gonads. It is thus becoming widely recognized that it is by nature an endocrine organ, in addition to a structural organ and site of mineral storage and hematopoiesis. Consequently and by definition, bone homeostasis must be tightly regulated and integrated with the biology of other organs to maintain whole body homeostasis, and data uncovering the involvement of the central nervous system (CNS) in the control of bone remodeling support this concept. The sympathetic nervous system (SNS) represents one of the main links between the CNS and the skeleton, based on a number of anatomic, pharmacologic, and genetic studies focused on β-adrenergic receptor (βAR) signaling in bone cells. The goal of this report was to review the data supporting the role of the SNS and βAR signaling in the regulation of skeletal homeostasis.