Rapamycin affects early fracture healing in mice

DDIT3 deficiency accelerates bone remodeling during bone healing by enhancing osteoblast and osteoclast differentiation through ULK1-mediated autophagy

The coordination of osteoblasts and osteoclasts is essential for bone remodeling. DNA damage inducible script 3 (DDIT3) is an important regulator of bone and participates in cell differentiation, proliferation, autophagy, and apoptosis. However, its role in bone remodeling remains unexplored. Here, we found that Ddit3 knockout (Ddit3-KO) enhanced both bone formation and resorption. The increased new bone formation and woven bone resorption, i.e., enhanced bone remodeling capacity, was found to accelerate bone defect healing in Ddit3-KO mice. In vitro experiments showed that DDIT3 inhibited both osteoblast differentiation and Raw264.7 cell differentiation by regulating autophagy. Cell coculture assay showed that Ddit3-KO decreased the ratio of receptor activator of nuclear factor-κβ ligand (RANKL) to osteoprotegerin (OPG) in osteoblasts, and Ddit3-KO osteoblasts inhibited osteoclast differentiation. Meanwhile, DDIT3 knockdown (DDIT3-sh) increased receptor activator of nuclear factor-κβ (RANK) expression in Raw264.7 cells, and DDIT3-sh Raw264.7 cells promoted osteoblast differentiation, whereas, DDIT3 overexpression had the opposite effect. Mechanistically, DDIT3 promoted autophagy partly by increasing ULK1 phosphorylation at serine555 (pULK1-S555) and decreasing ULK1 phosphorylation at serine757 (pULK1-S757) in osteoblasts, thereby inhibiting osteoblast differentiation. DDIT3 inhibited autophagy partly by decreasing pULK1-S555 in Raw264.7 cells, thereby suppressing osteoclastic differentiation. Taken together, our data indicate that DDIT3 is one of the elements regulating bone remodeling and bone healing, which may become a potential target in bone defect treatment.

Recent advancement in vascularized tissue-engineered bone based on materials design and modification

Bone is one of the most vascular network-rich tissues in the body and the vascular system is essential for the development, homeostasis, and regeneration of bone. When segmental irreversible damage occurs to the bone, restoring its vascular system by means other than autogenous bone grafts with vascular pedicles is a therapeutic challenge. By pre-generating the vascular network of the scaffold in vivo or in vitro, the pre-vascularization technique enables an abundant blood supply in the scaffold after implantation. However, pre-vascularization techniques are time-consuming, and in vivo pre-vascularization techniques can be damaging to the body. Critical bone deficiencies may be filled quickly with immediate implantation of a supporting bone tissue engineered scaffold. However, bone tissue engineered scaffolds generally lack vascularization, which requires modification of the scaffold to aid in enhancing internal vascularization. In this review, we summarize the relationship between the vascular system and osteogenesis and use it as a basis to further discuss surgical and cytotechnology-based pre-vascularization strategies and to describe the preparation of vascularized bone tissue engineered scaffolds that can be implanted immediately. We anticipate that this study will serve as inspiration for future vascularized bone tissue engineered scaffold construction and will aid in the achievement of clinical vascularized bone.

Callus γδ T cells and microbe-induced intestinal Th17 cells improve fracture healing in mice

IL-17A (IL-17), a driver of the inflammatory phase of fracture repair, is produced locally by several cell lineages including γδ T cells and Th17 cells. However, the origin of these T cells and their relevance for fracture repair are unknown. Here, we show that fractures rapidly expanded callus γδ T cells, which led to increased gut permeability by promoting systemic inflammation. When the microbiota contained the Th17 cell-inducing taxon segmented filamentous bacteria (SFB), activation of γδ T cells was followed by expansion of intestinal Th17 cells, their migration to the callus, and improved fracture repair. Mechanistically, fractures increased the S1P receptor 1-mediated (S1PR1-mediated) egress of Th17 cells from the intestine and enhanced their homing to the callus through a CCL20-mediated mechanism. Fracture repair was impaired by deletion of γδ T cells, depletion of the microbiome by antibiotics (Abx), blockade of Th17 cell egress from the gut, or Ab neutralization of Th17 cell influx into the callus. These findings demonstrate the relevance of the microbiome and T cell trafficking for fracture repair. Modifications of microbiome composition via Th17 cell-inducing bacteriotherapy and avoidance of broad-spectrum Abx may represent novel therapeutic strategies to optimize fracture healing.

Interactions of B-lymphocytes and bone cells in health and disease

Bone remodeling occurs through the interactions of three major cell lineages, osteoblasts, which mediate bone formation, osteocytes, which derive from osteoblasts, sense mechanical force and direct bone turnover, and osteoclasts, which mediate bone resorption. However, multiple additional cell types within the bone marrow, including macrophages, T lymphocytes and B lymphocytes influence the process. The bone marrow microenvironment, which is supported, in part, by bone cells, forms a nurturing network for B lymphopoiesis. In turn, developing B lymphocytes influence bone cells. Bone health during homeostasis depends on the normal interactions of bone cells with other lineages in the bone marrow. In disease state these interactions become pathologic and can cause abnormal function of bone cells and inadequate repair of bone after a fracture. This review summarizes what is known about the development of B lymphocytes and the interactions of B lymphocytes with bone cells in both health and disease.

Motivating role of type H vessels in bone regeneration

Coupling between angiogenesis and osteogenesis has an important role in both normal bone injury repair and successful application of tissue‐engineered bone for bone defect repair. Type H blood vessels are specialized microvascular components that are closely related to the speed of bone healing. Interactions between type H endothelial cells and osteoblasts, and high expression of CD31 and EMCN render the environment surrounding these blood vessels rich in factors conducive to osteogenesis and promote the coupling of angiogenesis and osteogenesis. Type H vessels are mainly distributed in the metaphysis of bone and densely surrounded by Runx2⁺ and Osterix⁺ osteoprogenitors. Several other factors, including hypoxia‐inducible factor‐1α, Notch, platelet‐derived growth factor type BB, and slit guidance ligand 3 are involved in the coupling of type H vessel formation and osteogenesis. In this review, we summarize the identification and distribution of type H vessels and describe the mechanism for type H vessel‐mediated modulation of osteogenesis. Type H vessels provide new insights for detection of the molecular and cellular mechanisms that underlie the crosstalk between angiogenesis and osteogenesis. As a result, more feasible therapeutic approaches for treatment of bone defects by targeting type H vessels may be applied in the future.

Myelination during fracture healing in vivo in myelin protein zero (p0) transgenic medaka line

During the fracture healing process, osteoblasts and osteoclasts, as well as the nervous system are known to play important roles for signaling in the body. Glia cells contribute to the healing process by myelination, which can increase the speed of signals transmitted between neurons. However, the behavior of myelinating cells at a fracture site remains unclear. We developed a myelin protein zero (mpz)-EGFP transgenic medaka line for tracing myelinating cells. Mpz-enhanced green fluorescence protein (EGFP)-positive (mpz⁺) cells are driven by the 2.9-kb promoter of the medaka mpz gene, which is distributed throughout the nervous system, such as the brain, spinal cord, lateral line, and peripheral nerves. In the caudal fin region, mpz⁺ cells were found localized parallel with the fin ray (bone) in the adult stage. mpz⁺ cells were not distributed with fli-DsRed positive (fli⁺) blood vessels, but with some nerve fibers, and were dyed with the anti-acetylated tubulin antibody. We then fractured one side of the caudal lepidotrichia in a caudal fin of mpz-EGFP medaka and found a unique phenomenon, in that mpz⁺ cells were accumulated at 1 bone away from the fracture site. This mpz⁺ cell accumulation phenomenon started from 4 days after fracture of the proximal bone. Thereafter, mpz⁺ cells became elongated from the proximal bone to the distal bone and finally showed a crosslink connection crossing the fracture site to the distal bone at 28 days after fracture. Finally, the effects of rapamycin, known as a mTOR inhibitor, on myelination was examined. Rapamycin treatment of mpz-EGFP/osterix-DsRed double transgenic medaka inhibited not only the crosslink connection of mpz⁺ cells but also osterix⁺ osteoblast accumulation at the fracture site, accompanied with a fracture healing defect. These findings indicated that mTOR signaling plays important roles in bone formation and neural networking during fracture healing. Taken together, the present results are the first to show the dynamics of myelinating cells in vivo.

Local delivery of bone morphogenetic protein-2 from near infrared-responsive hydrogels for bone tissue regeneration

Achievement of spatiotemporal control of growth factors production remains a main goal in tissue engineering. In the present work, we combined inducible transgene expression and near infrared (NIR)-responsive hydrogels technologies to develop a therapeutic platform for bone regeneration. A heat-activated and dimerizer-dependent transgene expression system was incorporated into mesenchymal stem cells to conditionally control the production of bone morphogenetic protein 2 (BMP-2). Genetically engineered cells were entrapped in hydrogels based on fibrin and plasmonic gold nanoparticles that transduced incident energy of an NIR laser into heat. In the presence of dimerizer, photoinduced mild hyperthermia induced the release of bioactive BMP-2 from NIR-responsive cell constructs. A critical size bone defect, created in calvaria of immunocompetent mice, was filled with NIR-responsive hydrogels entrapping cells that expressed BMP-2 under the control of the heat-activated and dimerizer-dependent gene circuit. In animals that were treated with dimerizer, NIR irradiation of implants induced BMP-2 production in the bone lesion. Induction of NIR-responsive cell constructs conditionally expressing BMP-2 in bone defects resulted in the formation of new mineralized tissue, thus indicating the therapeutic potential of the technological platform.

Does mammalian target of rapamycin or sestrin 1 protein signaling have a role in bone fracture healing?

Background/aim

Fracture healing is a complex physiological process that involves a well-orchestrated series of biological events. The mammalian target of rapamycin (mTOR) and sestrin 1 (SESN 1) play a central role in cell metabolism, proliferation, and survival. The aim of our study is to present serum mTOR and SESN 1 levels by comparing patients with or without bone fractures. It is also a guide for further research on the roles of these proteins in fracture healing.

Materials and methods

A total of 34 patients (10 females, 24 males) with bone fractures and 32 controls (10 females, 22 males) participated in this study. After collecting serum venous blood samples, the quantitative sandwich ELISA technique was used for the determination of serum mTOR and SESN 1 levels.

Results

The mean serum mTOR level was significantly higher in the fracture group compared to the control group (P = 0.001). However, SESN 1 levels did not significantly differ between groups (P = 0.913).

Conclusion

We found that serum mTOR levels increased on the first day after fracture compared to the control group. However, we obtained no significant difference between groups in terms of SESN 1 levels. This study may guide further clinical studies investigating the potential role of mTOR signaling in the bone healing process.

VEGFA From Early Osteoblast Lineage Cells (Osterix+) Is Required in Mice for Fracture Healing

Bone formation via intramembranous and endochondral ossification is necessary for successful healing after a wide range of bone injuries. The pleiotropic cytokine, vascular endothelial growth factor A (VEGFA) has been shown, via nonspecific pharmacologic inhibition, to be indispensable for angiogenesis and ossification following bone fracture and cortical defect repair. However, the importance of VEGFA expression by different cell types during bone healing is not well understood. We sought to determine the role of VEGFA from different osteoblast cell subsets following clinically relevant models of bone fracture and cortical defect. Ubiquitin C (UBC), Osterix (Osx), or Dentin matrix protein 1 (Dmp1) Cre-ERT2 mice (male and female) containing floxed VEGFA alleles (VEGFA^fl/fl ) were either given a femur full fracture, ulna stress fracture, or tibia cortical defect at 12 weeks of age. All mice received tamoxifen continuously starting 2 weeks before bone injury and throughout healing. UBC Cre-ERT2 VEGFA^fl/fl (UBC cKO) mice, which were used to mimic nonspecific inhibition, had minimal bone formation and impaired angiogenesis across all bone injury models. UBC cKO mice also exhibited impaired periosteal cell proliferation during full fracture, but not stress fracture repair. Osx Cre-ERT2 VEGFA^fl/fl (Osx cKO) mice, but not Dmp1 Cre-ERT2 VEGFA^fl/fl (Dmp1 cKO) mice, showed impaired periosteal bone formation and angiogenesis in models of full fracture and stress fracture. Neither Osx cKO nor Dmp1 cKO mice demonstrated significant impairments in intramedullary bone formation and angiogenesis following cortical defect. These data suggest that VEGFA from early osteolineage cells (Osx+), but not mature osteoblasts/osteocytes (Dmp1+), is critical at the time of bone injury for rapid periosteal angiogenesis and woven bone formation during fracture repair. Whereas VEGFA from another cell source, not from the osteoblast cell lineage, is necessary at the time of injury for maximum cortical defect intramedullary angiogenesis and osteogenesis. © 2019 American Society for Bone and Mineral Research.

Vascular bundle transplantation combined with porous bone substituted scaffold for the treatment of early-stage avascular necrosis of femoral head

Various reasons leading to disruption of blood supply will result in avascular femoral head necrosis. Decompression of lesion area, structural support to subchondral bone, and rebuilding of blood supply system are the keys for a successful hip preserve surgery. Reconstruction of local vascular network is always a huge challenge in clinical. Based on tantalum rod implantation and free vascularized fibular grafting, we propose the combined application of vascular bundle transplantation and porous bone substitute scaffold implantation as a potential novel treatment method. It may simultaneously achieve decompression, support and blood supply reconstruction. The hypothesis provides some new ideals and possibilities for solving this clinical problem of femoral head necrosis.

Functionalized cell-free scaffolds for bone defect repair inspired by self-healing of bone fractures: A review and new perspectives

Studies have demonstrated that scaffolds, a component of bone tissue engineering, play an indispensable role in bone repair. However, these scaffolds involving ex-vivo cultivated cells seeded have disadvantages in clinical practice, such as limited autologous cells, time-consuming cell expansion procedures, low survival rate and immune-rejection issues. To overcome these disadvantages, recent focus has been placed on the design of functionalized cell-free scaffolds, instead of cell-seeded scaffolds, that can reduplicate the natural self-healing events of bone fractures, such as inflammation, cell recruitment, vascularization, and osteogenic differentiation. New approaches and applications in tissue engineering and regenerative medicine continue to drive the development of functionalized cell-free scaffolds for bone repair. In this review, the self-healing processes were highlighted, and approaches for the functionalization were summarized. Also, ongoing efforts and breakthroughs in the field of functionalization for bone defect repair were discussed. Finally, a brief summery and new perspectives for functionalization strategies were presented to provide guidelines for further efforts in the design of bioinspired cell-free scaffolds.

Association of HSP22 with mTOR in osteoblasts: regulation of TNF-α-stimulated IL-6 synthesis

Heat shock protein 22 (HSP22) is ubiquitously expressed in various types of cells including in osteoblasts. We previously reported that tumor necrosis factor (TNF)-α stimulates interleukin (IL)-6 synthesis via p44/p42 MAPK in osteoblast-like MC3T3-E1 cells and that mTOR/p70 S6 kinase (p70 S6K) negatively regulates the IL-6 synthesis. In this study, we investigated the involvement of HSP22 in TNF-α-stimulated-IL-6 synthesis and the underlying mechanism in MC3T3-E1 cells. HSP22 knockdown reduces TNF-α-stimulated release of IL-6. In addition, HSP22 knockdown strengthens TNF-α-induced phosphorylation of p70 S6K but suppresses that of p44/p42 MAPK. HSP22 coimmunoprecipitates with mTOR. HSP22 knockdown increases the basal levels of phosphorylated mTOR. These results strongly suggest that HSP22 interacts with mTOR and regulates TNF-α-induced IL-6 synthesis in osteoblasts.

Loss of Dnmt3b in Chondrocytes Leads to Delayed Endochondral Ossification and Fracture Repair

Despite advanced understanding of signaling mediated by local and systemic factors, the role of epigenetic factors in the regulation of bone regeneration remains vague. The DNA methyltransferases (Dnmts) Dnmt3a and Dnmt3b have tissue specific expression patterns and create unique methylation signatures to regulate gene expression. Using a stabilized murine tibia fracture model we find that Dnmt3b is induced early in fracture healing, peaks at 10 days post fracture (dpf), and declines to nearly undetectable levels by 28 dpf. Dnmt3b expression was cell-specific and stage-specific. High levels were observed in chondrogenic lineage cells within the fracture callus. To determine the role of Dnmt3b in fracture healing, Agc1CreERT2 ;Dnmt3bf/f (Dnmt3bAgc1ER ) mice were generated to delete Dnmt3b in chondrogenic cells. Dnmt3bAgc1ER fracture displayed chondrogenesis and chondrocyte maturation defect, and a delay in the later events of angiogenesis, ossification, and bone remodeling. Biomechanical studies demonstrated markedly reduced strength in Dnmt3bAgc1ER fractures and confirmed the delay in repair. The angiogenic response was reduced in both vessel number and volume at 10 and 14 dpf in Dnmt3bAgc1ER mice. Immunohistochemistry showed decreased CD31 expression, consistent with the reduced angiogenesis. Finally, in vitro angiogenesis assays with human umbilical vein endothelial cells (HUVECs) revealed that loss of Dnmt3b in chondrocytes significantly reduced tube formation and endothelial migration. To identify specific angiogenic factors involved in the decreased callus vascularization, a protein array was performed using conditioned media isolated from control and Dnmt3b loss-of-function chondrocytes. Several angiogenic factors, including CXCL12 and osteopontin (OPN) were reduced in chondrocytes following loss of Dnmt3b. DNA methylation analysis further identified hypomethylation in Cxcl12 promoter region. Importantly, the defects in tube formation and cell migration could be rescued by administration of CXCL12 and/or OPN. Altogether, our findings establish that Dnmt3b positively regulates chondrocyte maturation process, and its genetic ablation leads to delayed angiogenesis and fracture repair. © 2017 American Society for Bone and Mineral Research.

Mammalian target of rapamycin as a therapeutic target in osteoporosis

The mechanistic target of rapamycin (mTOR) plays a key role in sensing and integrating large amounts of environmental cues to regulate organismal growth, homeostasis, and many major cellular processes. Recently, mounting evidences highlight its roles in regulating bone homeostasis, which sheds light on the pathogenesis of osteoporosis. The activation/inhibition of mTOR signaling is reported to positively/negatively regulate bone marrow mesenchymal stem cells (BMSCs)/osteoblasts-mediated bone formation, adipogenic differentiation, osteocytes homeostasis, and osteoclasts-mediated bone resorption, which result in the changes of bone homeostasis, thereby resulting in or protect against osteoporosis. Given the likely importance of mTOR signaling in the pathogenesis of osteoporosis, here we discuss the detailed mechanisms in mTOR machinery and its association with osteoporosis therapy.

mTORC1 Inhibits NF-κB/NFATc1 Signaling and Prevents Osteoclast Precursor Differentiation, In Vitro and In Mice

The mechanistic target of rapamycin complex 1 (mTORC1) is a critical sensor for bone homeostasis and bone formation; however, the role of mTORC1 in osteoclast development and the underlying mechanisms have not yet been fully established. Here, we found that mTORC1 activity declined during osteoclast precursors differentiation in vitro and in vivo. We further targeted deletion of Raptor (mTORC1 key component) or Tsc1 (mTORC1 negative regulator) to constitutively inhibit or activate mTORC1 in osteoclast precursors (monocytes/macrophages), using LyzM-cre mice. Osteoclastic formation was drastically increased in cultures of Raptor deficient bone marrow monocytes/macrophages (BMMs), and Raptor-deficient mice displayed osteopenia with enhanced osteoclastogenesis. Conversely, BMMs lacking Tsc1 exhibited a severe defect in osteoclast-like differentiation and absorptive function, both of which were restored following rapamycin treatment. Importantly, expression of nuclear factor κ-light-chain-enhancer of activated B cells (NF-κB) and nuclear factor of activated T cells, cytoplasmic 1 (NFATc1), transcription factors that are essential for osteoclast differentiation was negatively regulated by mTORC1 in osteoclast lineages. These results provide evidence that mTORC1 plays as a critical role as an osteoclastic differentiation-limiting signal and suggest a potential drawback in treating bone loss-related diseases with mTOR inhibitors clinically. © 2017 American Society for Bone and Mineral Research.

Osteoimmunology in Bone Fracture Healing

PURPOSE OF REVIEW

In the process of bone fracture healing, inflammation is thought to be an essential process that precedes bone formation and remodeling. We review recent studies on bone fracture healing from an osteoimmunological point of view.

RECENT FINDINGS

Based on previous observations that many types of immune cells infiltrate into the bone injury site and release a variety of molecules, recent studies have addressed the roles of specific immune cell subsets. Macrophages and interleukin (IL)-17-producing γδ T cells enhance bone healing, whereas CD8⁺ T cells impair bone repair. Additionally, IL-10-producing B cells may contribute to bone healing by suppressing excessive and/or prolonged inflammation. Although the involvement of other cells and molecules has been suggested, the precise underlying mechanisms remain elusive. Accumulating evidence has begun to reveal the deeper picture of bone fracture healing. Further studies are required for the development of novel therapeutic strategies for bone fracture.

Leptin accelerates the pathogenesis of heterotopic ossification in rat tendon tissues via mTORC1 signaling

Leptin, an adipocyte-derived cytokine associated with bone metabolism, is believed to play a critical role in the pathogenesis of heterotopic ossification (HO). The effect and underlying action mechanism of leptin were investigated on osteogenic differentiation of tendon-derived stem cells (TDSCs) in vitro and the HO formation in rat tendons. Isolated rat TDSCs were treated with various concentrations of leptin in the presence or absence of mTORC1 signaling specific inhibitor rapamycin in vitro. A rat model with Achilles tenotomy was employed to evaluate the effect of leptin on HO formation together with or without rapamycin treatment. In vitro studies with TDSCs showed that leptin increased the expression of osteogenic biomarkers (alkaline phosphatase, runt-related transcription factor 2, osterix, osteocalcin) and enhanced mineralization of TDSCs via activating the mTORC1 signal pathway (as indicated by phosphorylation of p70 ribosomal S6 kinase 1 and p70 ribosomal S6). However, mTORC1 signaling blockade with rapamycin treatment suppressed leptin-induced osteogenic differentiation and mineralization. In vivo studies showed that leptin promoted HO formation in the Achilles tendon after tenotomy, and rapamycin treatment blocked leptin-induced HO formation. In conclusion, leptin can promote TDSC osteogenic differentiation and heterotopic bone formation via mTORC1 signaling in both vitro and vivo model, which provides a new potential therapeutic target for HO prevention.

Pharmacological modulation of autophagy: therapeutic potential and persisting obstacles

Autophagy is central to the maintenance of organismal homeostasis in both physiological and pathological situations. Accordingly, alterations in autophagy have been linked to clinically relevant conditions as diverse as cancer, neurodegeneration and cardiac disorders. Throughout the past decade, autophagy has attracted considerable attention as a target for the development of novel therapeutics. However, such efforts have not yet generated clinically viable interventions. In this Review, we discuss the therapeutic potential of autophagy modulators, analyse the obstacles that have limited their development and propose strategies that may unlock the full therapeutic potential of autophagy modulation in the clinic.

Sildenafil Transiently Delays Early Alveolar Healing of Tooth Extraction Sockets

Bone is a unique tissue that has the ability to repair itself and return to full function. Bone regeneration is a well synchronized biological process that recapitulates embryonic bone development. The establishment of a functional vascular supply has been shown to be essential for proper ossification of newly deposited bone, and impaired angiogenesis as in advanced age, diabetes, and anti-cancer treatments affect bone repair. Endothelial Guanosine, 3', 5'-Cyclic Monophophate(cGMP) is known to support angiogenesis, and sildenafil, a Phosphodiesterase 5 (PDE5) antagonist, prevents cGMP hydrolysis and thereby, promotes the formation of new blood vessels. Since the development of functional vascular networks is critical to bone repair, we investigated the effects of sildenafil on early alveolar bone regeneration following exodontia. Our results demonstrate that per-oral administration of sildenafil (10 mg/kg/day) in rats delays the dissolution and replacement of the sanguine clot with granulation tissue. As a result, the number of replicating cells, a hallmark of regenerating tissue, observed on day 4 was remarkably lower in sildenafil-treated animals than their control counterparts (mean±SD; control: 47.35±9.21; sildenafil: 11.47±5.14). Similarly, cells expressing transcription factor Cbfa-1/Runx2 and osteopontin, markers of differentiating osteoblasts, were fewer in treated animals (mean±SD; control: 83.18 ± 4.60; sildenafil: 13.77 ± 4.63). Treatment with hydrolysis-resistant cyclic GMP (cGMP) showed findings similar to sildenafil-treated animals suggesting a negative impact of cGMP on early inflammatory phase of bone healing. However, histological differences were not significant between the 2 groups on day 8. Based on these findings, we conclude that sildenafil temporarily retards early events in alveolar bone healing.

VEGF-loaded biomimetic scaffolds: a promising approach to improve angiogenesis and osteogenesis in an ischemic environment

This study provides a promising approach to improve angiogenesis and osteogenesis in an ischemic environment.

Implications of the Interaction Between miRNAs and Autophagy in Osteoporosis

Imbalances between bone formation and resorption are the primary cause of osteoporosis. However, currently, a detailed molecular mechanism of osteoporosis is not available. Autophagy is the conserved process characterized by degrading and recycling aggregated proteins, intracellular pathogens, and damaged organelles. MicroRNAs (miRNAs) are novel regulatory factors that play important roles in numerous cellular processes, including autophagy, through the posttranscriptional regulation of gene expression. Conversely, autophagy plays a role in the regulation of miRNA homeostasis. Recent advances have revealed that both autophagy and miRNAs are involved in the maintenance of bone homoeostasis, whereas the role of the interaction of miRNAs with autophagy in osteoporosis remains unclear. In this paper, we review previous reports on autophagy, miRNAs, and their interaction in osteoporosis.

Energetic interventions for healthspan and resiliency with aging

Several behavioral and pharmacological strategies improve longevity, which is indicative of delayed organismal aging, with the most effective interventions extending both life- and healthspan. In free living creatures, maintaining health and function into old age requires resilience against a multitude of stressors. Conversely, in experimental settings, conventional housing of rodents limits exposure to such challenges, thereby obscuring an accurate assessment of resilience. Caloric restriction (CR) and exercise, as well as pharmacologic strategies (resveratrol, rapamycin, metformin, senolytics), are well established to improve indices of health and aging, but some paradoxical effects have been observed on resilience. For instance, CR potently retards the onset of age-related diseases, and improves lifespan to a greater extent than exercise in a variety of models. However, exercise has proven more consistently beneficial to organismal resilience against a broad array of stressors, including infections, surgery, wound healing and frailty. CR can improve cellular stress defenses and protect from frailty, but also impairs the response to infections, bed rest and healing. How an intervention will impact not only longevity, health and function, but also resiliency, is critical to better understanding translational implications. Thus, organismal robustness represents a critical, albeit understudied aspect of aging, which needs more careful attention in order to better inform on how putative age-delaying strategies will impact preservation of health and function in response to stressors with aging in humans.

Rapamycin-induced autophagy activity promotes bone fracture healing in rats

Autophagy is a crucial mediating process for normal bone cell function and metabolism in physiology or pathology. Rapamycin has been demonstrated to induce the autophagy pathway by inhibiting the mammalian target of rapamycin (mTOR) pathway. However, the contribution of autophagy in orthopedic diseases is rarely reported. The aim of the present study was to evaluate the capacity of pharmacologically induced autophagy to modify disease function in a rat model of bone fracture. A femur fracture model was established via surgery in adult male Sprague-Dawley rats. Rapamycin (n=63 rats) or dimethyl sulfoxide (DMSO) vehicle control (n=63 rats) was administered intraperitoneally for 2, 4 and 6 weeks, and 21 randomly selected rats were sacrificed in each group at each time point. X-ray micro-computed tomography and hematoxylin and eosin staining were used to evaluate the extent of fracture healing in each group. The effects of rapamycin on autophagy, mTOR signaling and the expression levels of vascular endothelial growth factor (VEGF) and proliferating cell nuclear antigen (PCNA) were analyzed using immunohistochemistry, immunofluorescence staining and western blot analysis. Rapamycin affected the mTOR signaling pathway in rats following fracture, as indicated by the inhibition of the phosphorylation of ribosomal protein S6, a target of mTOR, and activation of microtubule-associated protein 2 light chain 3, a key marker of autophagy. Histomorphometry and image examination indicated that the number of osteoblasts in each section was significantly (P<0.01) increased in the rapamycin group compared with the control group, and this was associated with a significant (P<0.05) increase in mineralized callus fraction. Furthermore, rapamycin treatment increased the expression levels of VEGF and PCNA in the rat callus tissue. These results suggest that rapamycin may serve a beneficial function in fracture healing, and that the underlying mechanism may involve the activation of autophagy.

Longitudinal analysis of osteogenic and angiogenic signaling factors in healing models mimicking atrophic and hypertrophic non-unions in rats

Impaired bone healing can have devastating consequences for the patient. Clinically relevant animal models are necessary to understand the pathology of impaired bone healing. In this study, two impaired healing models, a hypertrophic and an atrophic non-union, were compared to physiological bone healing in rats. The aim was to provide detailed information about differences in gene expression, vascularization and histology during the healing process. The change from a closed fracture (healing control group) to an open osteotomy (hypertrophy group) led to prolonged healing with reduced mineralized bridging after 42 days. RT-PCR data revealed higher gene expression of most tested osteogenic and angiogenic factors in the hypertrophy group at day 14. After 42 days a significant reduction of gene expression was seen for Bmp4 and Bambi in this group. The inhibition of angiogenesis by Fumagillin (atrophy group) decreased the formation of new blood vessels and led to a non-healing situation with diminished chondrogenesis. RT-PCR results showed an attempt towards overcoming the early perturbance by significant up regulation of the angiogenic regulators Vegfa, Angiopoietin 2 and Fgf1 at day 7 and a further continuous increase of Fgf1, -2 and Angiopoietin 2 over time. However µCT angiograms showed incomplete recovery after 42 days. Furthermore, lower expression values were detected for the Bmps at day 14 and 21. The Bmp antagonists Dan and Twsg1 tended to be higher expressed in the atrophy group at day 42. In conclusion, the investigated animal models are suitable models to mimic human fracture healing complications and can be used for longitudinal studies. Analyzing osteogenic and angiogenic signaling patterns, clear changes in expression were identified between these three healing models, revealing the importance of a coordinated interplay of different factors to allow successful bone healing.

The multifaceted role of the vasculature in endochondral fracture repair

Fracture healing is critically dependent upon an adequate vascular supply. The normal rate for fracture delayed or non-union is estimated to be between 10 and 15%, and annual fracture numbers are approximately 15 million cases per year. However, when there is decreased vascular perfusion to the fracture, incidence of impaired healing rises dramatically to 46%. Reduction in the blood supply to the fracture can be the result of traumatic injuries that physically disrupt the vasculature and damage supportive soft tissue, the result of anatomical location (i.e., distal tibia), or attributed to physiological conditions such as age, diabetes, or smoking. The role of the vasculature during repair is multifaceted and changes during the course of healing. In this article, we review recent insights into the role of the vasculature during fracture repair. Taken together these data highlight the need for an updated model for endochondral repair to facilitate improved therapeutic approaches to promote bone healing.

Role of angiogenesis in bone repair

Bone vasculature plays a vital role in bone development, remodeling and homeostasis. New blood vessel formation is crucial during both primary bone development as well as fracture repair in adults. Both bone repair and bone remodeling involve the activation and complex interaction between angiogenic and osteogenic pathways. Interestingly studies have demonstrated that angiogenesis precedes the onset of osteogenesis. Indeed reduced or inadequate blood flow has been linked to impaired fracture healing and old age related low bone mass disorders such as osteoporosis. Similarly the slow penetration of host blood vessels in large engineered bone tissue grafts has been cited as one of the major hurdle still impeding current bone construction engineering strategies. This article reviews the current knowledge elaborating the importance of vascularization during bone healing and remodeling, and the current therapeutic strategies being adapted to promote and improve angiogenesis.

Mechanical activation of mammalian target of rapamycin pathway is required for cartilage development

Mechanical stress regulates development by modulating cell signaling and gene expression. However, the cytoplasmic components mediating mechanotransduction remain unclear. In this study, elimination of muscle contraction during chicken embryonic development resulted in a reduction in the activity of mammalian target of rapamycin (mTOR) in the cartilaginous growth plate. Inhibition of mTOR activity led to significant inhibition of chondrocyte proliferation, cartilage tissue growth, and expression of chondrogenic genes, including Indian hedgehog (Ihh), a critical mediator of mechanotransduction. Conversely, cyclic loading (1 Hz, 5% matrix deformation) of embryonic chicken growth plate chondrocytes in 3-dimensional (3D) collagen scaffolding induced sustained activation of mTOR. Mechanical activation of mTOR occurred in serum-free medium, indicating that it is independent of growth factor or nutrients. Treatment of chondrocytes with Rapa abolished mechanical activation of cell proliferation and Ihh gene expression. Cyclic loading of chondroprogenitor cells deficient in SH2-containing protein tyrosine phosphatase 2 (Shp2) further enhanced mechanical activation of mTOR, cell proliferation, and chondrogenic gene expression. This result suggests that Shp2 is an antagonist of mechanotransduction through inhibition of mTOR activity. Our data demonstrate that mechanical activation of mTOR is necessary for cell proliferation, chondrogenesis, and cartilage growth during bone development, and that mTOR is an essential mechanotransduction component modulated by Shp2 in the cytoplasm.

Sequential in vivo imaging of osteogenic stem/progenitor cells during fracture repair

Bone turns over continuously and is highly regenerative following injury. Osteogenic stem/progenitor cells have long been hypothesized to exist, but in vivo demonstration of such cells has only recently been attained. Here, in vivo imaging techniques to investigate the role of endogenous osteogenic stem/progenitor cells (OSPCs) and their progeny in bone repair are provided. Using osteo-lineage cell tracing models and intravital imaging of induced microfractures in calvarial bone, OSPCs can be directly observed during the first few days after injury, in which critical events in the early repair process occur. Injury sites can be sequentially imaged revealing that OSPCs relocate to the injury, increase in number and differentiate into bone forming osteoblasts. These methods offer a means of investigating the role of stem cell-intrinsic and extrinsic molecular regulators for bone regeneration and repair.

What is the impact of immunosuppressive treatment on the post-transplant renal osteopathy?

Although glucocorticoid therapy is considered to be the main pathogenic factor, a consistent body of evidence suggests that other immunosuppressants might also play an important role in the development of the post-transplant renal osteopathy (PRO) through their pleiotropic pharmacological effects. Glucocorticoids seem to induce osteoclasts' activity suppressing the osteoblasts while data regarding other immunosuppressive drugs are still controversial. Mycophenolate mofetil and azathioprine appear to be neutral regarding the bone metabolism. However, the study analyzing any independent effect of antimetabolites on bone turnover has not been conducted yet. Calcineurin inhibitors (CNIs) induce trabecular bone loss in rodent, with contradictory results in renal transplant recipients. Suppression of vitamin D receptor is probably the underlying mechanism of renal calcium wasting in renal transplant recipients receiving CNI. In spite of an increased 1,25(OH)2 vitamin D level, the kidney is not able to reserve calcium, suggesting a role of vitamin D resistance that may be related to bone loss. More efforts should be invested to determine the role of CNI in PRO. In particular, data regarding the role of mammalian target of rapamycin inhibitors (mTORi), such as sirolimus and everolimus, in the PRO development are still controversial. Rapamycin markedly decreases bone longitudinal growth as well as callus formation in experimental models, but also lowers the rate of bone resorption markers and glomerular filtration in clinical studies. Everolimus potently inhibits primary mouse and human osteoclast activity as well as the osteoclast differentiation. It also prevents the ovariectomy-induced loss of cancellous bone by 60 %, an effect predominantly associated with a decreased osteoclast-mediated bone resorption, resulting in a partial preservation of the cancellous bone. At present, there is no clinical study analyzing the effect of everolimus on bone turnover in renal transplant recipients or comparing sirolimus versus everolimus impact on bone, so only general conclusions could be drawn. Hence, the use of mTORi might be useful in patients with PRO due to their possible potential to inhibit osteoclast activity which might lead to a decreased rate of bone resorption. In addition, it should be also emphasized that they might inhibit osteoblast activity which may lead to a decreased bone formation and adynamic bone disease. Further studies are urgently needed to solve these important clinical dilemmas.

Effects of local insulin delivery on subperiosteal angiogenesis and mineralized tissue formation during fracture healing

Local insulin delivery has been shown to improve osseous healing in diabetic animals. The purpose of this study was to quantify the effects of local intramedullary delivery of saline or Ultralente insulin (UL) on various fracture healing parameters using an in vivo non-diabetic BB Wistar rat model. Quantitation of local insulin levels showed a rapid release of insulin from the fractured femora, demonstrating complete release at 2 days. RT-PCR analysis revealed that the expression of early osteogenic markers (Col1α2, osteopontin) was significantly enhanced with UL treatment when compared with saline controls (p < 0.05). Significant differences in VEGF + cells and vascularity were evident between the treatment and control groups at day 7 (p < 0.05). At day 21, histomorphometric analysis demonstrated a significant increase in percent mineralized tissue in the UL-treated animals compared with controls (p < 0.05), particularly within the subperiosteal region of the fracture callus. Mechanical testing at 4 weeks showed significantly greater mechanical strength for UL-treated animals (p < 0.05), but healing in control animals caught up at 6 weeks post-fracture. These results suggest that the primary osteogenic effect of UL during the early stages of fracture healing (1-3 weeks) is through an increase in osteogenic gene expression, subperiosteal angiogenesis, and mineralized tissue formation.

Rapamycin increases neuroblastoma xenograft and host stromal derived osteoprotegerin inhibiting osteolytic bone disease in a bone metastasis model

PURPOSE

Osteoprotegerin (OPG) is a decoy receptor for the Receptor of NF-κB (RANK) ligand that can inhibit osteoclastogenesis. Previous studies have suggested that Mammalian Target of Rapamycin (mTOR) inhibition upregulates OPG production. We tested the hypothesis that the mTOR inhibitor rapamycin could inhibit neuroblastoma bone metastases through its action on OPG.

EXPERIMENTAL DESIGN

An orthotopic model of bone metastasis was established. Mice with established disease were subsequently treated with rapamycin (5mg/kg IP daily) or vehicle control (DMSO 1:1000). X-rays were obtained twice a week to detect pathologic fractures. Serum OPG levels were measured by ELISA after two weeks of treatment.

RESULTS

Mice with bone disease receiving rapamycin had increased serum levels of OPG in the CHLA-20 mice compared to controls (36.89 pg/mL ± 3.90 vs 18.4 pg/mL ± 1.67, p=0.004) and NB1691 tumor-bearing groups (46.03 ± 2.67 pg/mL vs 17.96 ± 1.84pg/mL, p=0.001), and a significantly longer median time to pathologic fractures with CHLA-20 (103 days vs 74.5 days, p=0.014) and NB1691 xenografts.

CONCLUSION

In a xenograft model, increased OPG expression correlated with a delay to pathologic fracture suggesting a potential role for mTOR inhibitors in the treatment of neuroblastoma bone metastases.

The effects of local vanadium treatment on angiogenesis and chondrogenesis during fracture healing

This study quantified the effects of local intramedullary delivery of an organic vanadium salt, which may act as an insulin-mimetic on fracture healing. Using a BB Wistar rat femoral fracture model, local vanadyl acetylacetonate (VAC) was delivered to the fracture site and histomorphometry, mechanical testing, and immunohistochemistry were performed. Callus percent cartilage was 200% higher at day 7 (p < 0.05) and 88% higher at day 10 (p < 0.05) in the animals treated with 1.5 mg/kg of VAC. Callus percent mineralized tissue was 37% higher at day 14 (p < 0.05) and 31% higher at day 21 (p < 0.05) in the animals treated with 1.5 mg/kg of VAC. Maximum torque to failure was 104% and 154% higher at 4 weeks post-fracture (p < 0.05) for the healing femurs from the VAC-treated (1.5 and 3.0 mg/kg) animals. Animals treated with other VAC doses demonstrated increased mechanical parameters at 4 weeks (p < 0.05). Immunohistochemistry detected 62% more proliferating cells at days 7 (p < 0.05) and 94% more at day 10 (p < 0.05) in the animals treated with 1.5 mg/kg VAC. Results showed 100% more vascular endothelial growth factor-C (VEGF-C) positive cells and 80% more blood vessels at day 7 (p < 0.05) within the callus subperiosteal region of VAC-treated animals (1.5 mg/kg) compared to controls. The results suggest that local VAC treatment affects chondrogenesis and angiogenesis within the first 7-10 days post-fracture, which leads to enhanced mineralized tissue formation and accelerated fracture repair as early as 3-4 weeks post-fracture.

Potential Therapeutic Roles for Inhibition of the PI3K/Akt/mTOR Pathway in the Pathophysiology of Diabetic Retinopathy

Novel therapeutics such as inhibitors of PI3K/Akt/mTOR pathway presents a unique opportunity for the management of diabetic retinopathy (DR). Second generation mTOR inhibitors have the prospect to be efficacious in managing various stages of disease progression in DR. During early stages, the mTOR inhibitors suppress HIF-1α, VEGF, leakage, and breakdown of the blood-retinal barrier. These mTOR inhibitors impart a pronounced inhibitory effect on inflammation, an early component with diverse ramifications influencing the progression of DR. These inhibitors suppress IKK and NF-κB along with downstream inflammatory cytokines, chemokines, and adhesion molecules. In proliferative DR, mTOR inhibitors suppress several growth factors that play pivotal roles in the induction of pathological angiogenesis. Lead mTOR inhibitors in clinical trials for ocular indications present an attractive treatment option for chronic use in DR with favorable safety profile and sustained ocular pharmacokinetics following single dose. Thereby, reducing dosing frequency and risk associated with chronic drug administration.

An isolated cryptic peptide influences osteogenesis and bone remodeling in an adult mammalian model of digit amputation

Biologic scaffolds composed of extracellular matrix (ECM) have been used successfully in preclinical models and humans for constructive remodeling of functional, site-appropriate tissue after injury. The mechanisms underlying ECM-mediated constructive remodeling are not completely understood, but scaffold degradation and site-directed recruitment of progenitor cells are thought to play critical roles. Previous studies have identified a cryptic peptide derived from the C-terminal telopeptide of collagen IIIα that has chemotactic activity for progenitor cells. The present study characterized the osteogenic activity of the same peptide in vitro and in vivo in an adult murine model of digit amputation. The present study showed that the cryptic peptide increased calcium deposition, alkaline phosphatase activity, and osteogenic gene expression in human perivascular stem cells in vitro. Treatment with the cryptic peptide in a murine model of mid-second phalanx digit amputation led to the formation of a bone nodule at the site of amputation. In addition to potential therapeutic implications for the treatment of bone injuries and facilitation of reconstructive surgical procedures, cryptic peptides with the ability to alter stem cell recruitment and differentiation at a site of injury may serve as powerful new tools for influencing stem cell fate in the local injury microenvironment.

Intravital microscopic studies of angiogenesis during bone defect healing in mice calvaria

PURPOSE

Due to the great availability of specific antibodies, gene-targeted animals and knockout strains, mouse models came into the focus of musculoskeletal research. Herein, we introduce a calvarian defect model in mice that allows the repetitive analysis of blood vessel formation during bone repair by intravital microscopy.

METHODS

The right parietal calvaria of 20 adult CD-1 mice were exposed by skin excision. Under continuous irrigation, a circular defect (Ø0.75 mm) was drilled into the calvarium without penetrating the inner cortical shell. A circular glass (Ø12 mm; thickness 0.15 mm) was fixed by two microscrews (M1; length 2mm) to cover the bone defect. Angiogenesis was analysed by intravital microscopy at days 0, 3, 6, 9, 12, 15, 18 and 21. In addition, bone repair was evaluated by histomorphometry at days 3, 6, 9 and 15. Immunohistochemical stainings for the angiogenic growth factor vascular endothelial growth factor (VEGF) and the cell proliferation marker proliferating cell nuclear antigen (PCNA) were performed to assess angiogenic and proliferative activity during healing of the calvarian defect.

RESULTS

Histomorphometry showed a typical pattern of intramembranous bone repair. Osseous bridging of the defect was observed at day 9. This was associated with the formation of a neo-periosteum, which covered the new woven bone and contained a dense network of newly formed blood vessels. At day 9, particularly cells of the neo-periosteum showed intense staining for VEGF, whilst PCNA-positive staining was found mainly in osteoblasts. At day 15, the major fraction of fibrous tissue was replaced by bone undergoing extensive remodelling. Intravital microscopy revealed an increase of vascular density between days 3 and 15. Blood vessel diameters showed an increase between days 3 and 9 and a subsequent decrease between days 9 and 21.

CONCLUSIONS

The present calvarian defect model provides a powerful tool to evaluate the process of angiogenesis during intramembranous bone repair in mice.

Fracture healing is accelerated in the absence of the adaptive immune system

Fracture healing is a unique biologic process starting with an initial inflammatory response. As in other regenerative processes, bone and the immune system interact closely during fracture healing. This project was aimed at further elucidating how the host immune system participates in fracture healing. A standard closed femoral fracture was created in wild-type (WT) and recombination activating gene 1 knockout (RAG1(-/-)) mice lacking the adaptive immune system. Healing was investigated using micro-computed tomography (µCT), biomechanical testing, and histologic and mRNA expression analyses. Biomechanical testing demonstrated a significantly higher torsional moment on days 14 and 21 in the RAG1(-/-) mice compared to the WT group. µCT evaluation of RAG1(-/-) specimens showed earlier mineralization and remodeling. Histologically, endochondral ossification and remodeling were accelerated in the RAG1(-/-) compared with the WT mice. Histomorphometric analysis on day 7 showed a significantly higher fraction of bone and a significantly lower fraction of cartilage in the callus of the RAG1(-/-) mice than in the WT mice. Endochondral ossification was accelerated in the RAG1(-/-) mice. Lymphocytes were present during the physiologic repair process, with high numbers in the hematoma on day 3 and during formation of the hard callus on day 14 in the WT mice. Expression of inflammatory cytokines was reduced in the RAG1(-/-) mice. In contrast, expression of anti-inflammatory interleukin 10 (IL-10) was strongly upregulated in RAG1(-/-) mice, indicating protective effects. This study revealed an unexpected phenotype of enhanced fracture healing in RAG1(-/-) mice, suggesting detrimental functions of lymphocytes on fracture healing. The shift from proinflammatory to anti-inflammatory cytokines suggests that immunomodulatory intervention strategies that maximise the regenerative and minimize the destructive effects of inflammation may lead to enhanced fracture repair.

Uncoupled angiogenesis and osteogenesis in nicotine-compromised bone healing

Nicotine is the main chemical component responsible for tobacco addiction. This study aimed to evaluate the influence of nicotine on angiogenesis and osteogenesis and the associated expression of angiogenic and osteogenic mediators during bone healing. Forty-eight adult New Zealand White rabbits were randomly assigned to a nicotine group and a control group. Nicotine pellets (1.5 g, 60-day time release) or placebo pellets were implanted in the neck subcutaneous tissue. The nicotine or placebo exposure time for all the animals was 7 weeks. Unilateral mandibular distraction osteogenesis was performed. Eight animals in each group were euthanized on day 5, day 11 of active distraction, and week 1 of consolidation, respectively. The mandibular samples were subjected to radiographic, histologic, immunohistochemical, and real-time reverse-transcriptase polymerase chain reaction examinations. Nicotine exposure upregulated the expression of hypoxia inducible factor 1alpha and vascular endothelial growth factor and enhanced angiogenesis but inhibited the expression of bone morphogenetic protein 2 and impaired bone healing. The results indicate that nicotine decouples angiogenesis and osteogenesis in this rabbit model of distraction osteogenesis, and the enhanced angiogenesis cannot compensate for the adverse effects of nicotine on bone healing.

Growth failure associated with sirolimus: case report

An 11-year-old girl, who was a renal transplant recipient, developed linear growth failure associated in time with sirolimus (SRL) treatment. After 5 years of functional graft [creatinine clearance (CCr) 90 ml/min per 1.73 m(2) body surface area], she developed acute renal failure due to calcineurin inhibitor-related hemolytic uremic syndrome, and cyclosporine A was replaced by SRL. Before the drug change, she had been growing normally (5.5 cm/year) and had reached the 33.9 percentile (P) of height (z-height -0.41), similar to her target. Two years later, her height had decreased to P 6th (z-height -1.54), as her growth velocity had diminished to 2.2 cm/year, despite optimal renal function (CCr 68 ml/min per 1.73 m(2)). Human recombinant growth hormone was needed to promote her catch-up growth and achieve the P 49th of height (z-height -0.03). SRL may have deleterious effects on growing children due its characteristic anti-proliferative and anti-angiogenic properties. Pediatric transplant recipients' linear growth should be cautiously monitored while they are being given SRL.

Fracture healing and drug therapies in osteoporosis

Fracture repair has not been fully optimised and there is opportunity to increase the healing rate and reduce the number of complications using pharmacological means. While most anti-osteoporosis drugs have been widely tested for their ability to decrease the risk of osteoporotic fractures, fragility fractures still occur in patients under medical intervention. The primary purpose of this systematic review is to understand these underlying mechanisms between bone and drug therapies in osteoporosis and the overall promotion of fracture healing and callus formation. Databases such as MEDLINE, Google Scholar, EMBASE and CINAHL were searched and nine articles met all inclusion criteria. We report that there is still large controversy and a need for clinical trials to address the deficiencies found in animal models. There is no clear evidence yet as to whether complications during the course of healing are attributable to implant anchorage problems in osteoporotic bone or to possibly delayed healing in the aged.

Advances in the establishment of defined mouse models for the study of fracture healing and bone regeneration

The availability of a broad spectrum of antibodies and gene-targeted animals caused an increasing interest in mouse models for the study of molecular mechanisms of fracture healing and bone regeneration. In most murine fracture models, the tibia or the femur is fractured using a 3-point bending device (closed models) or is osteotomized using an open surgical approach (open models). For fracture studies in mice, the tibia has to be considered less appropriate compared with the femur because the stabilization of the fracture is more difficult due to its triangular, distally declining caliber and its bowed longitudinal axis. Biomechanical factors critically influence the bone healing process. Thus, the use of stable osteosynthesis techniques is also of interest in murine fracture models. To achieve stable fixation, several biomechanically standardized implants have recently been introduced, including a locking nail and an intramedullary compression screw. Other implants, such as a pin-clip, an external fixator, and a locking plate, additionally allow the stabilization of fractures with distinct gap sizes. This enables the study of healing of critical size defects and nonunions. The use of these implants further allows a rigid fixation of fractures in bridle bones, which is essential for fracture studies in animals suffering from metabolic bone diseases like osteoporosis. In general, the analysis of bone healing in these models includes different imaging techniques and histologic, immunohistochemical, biomechanical, and molecular methods. To evaluate the impact of different osteosynthesis techniques on physical activity and rehabilitation, gait analysis may additionally be performed. By this, the gait of the animals can be visualized and quantitatively analyzed using modified running wheels and dynamic high-resolution radiography systems. Taken together, a variety of different murine femur fracture models have become available, providing defined biomechanical conditions for fracture research. The use of these mouse models may now allow studying the influence of fracture stabilization techniques on molecular mechanisms of bone healing.

Bone remodeling during fracture repair: The cellular picture

Fracture healing is a complex event that involves the coordination of a variety of different processes. Repair is typically characterized by four overlapping stages: the initial inflammatory response, soft callus formation, hard callus formation, initial bony union and bone remodeling. However, repair can also be seen to represent a juxtaposition of two distinct forces: anabolism or tissue formation, and catabolism or remodeling. These anabolic/catabolic concepts are useful for understanding bone repair without giving the false impression of temporally distinct stages that operate independently. They are also relevant when considering intervention. In normal bone development, bone remodeling conventionally refers to the removal of calcified bone tissue by osteoclasts. However, in the context of bone repair there are two phases of tissue catabolism: the removal of the initial cartilaginous soft callus, followed by the eventual remodeling of the bony hard callus. In this review, we have attempted to examine catabolism/remodeling in fractures in a systematic fashion. The first section briefly summarizes the traditional four-stage view of fracture repair in a physiological manner. The second section highlights some of the limitations of using a temporal rather than process-driven model and summarizes the anabolic/catabolic paradigm of fracture repair. The third section examines the cellular participants in soft callus remodeling and in particular the role of the osteoclast in endochondral ossification. Finally, the fourth section examines the effects of delaying osteoclast-dependent hard callus remodeling and also poses questions regarding the crosstalk between anabolism and catabolism in the latter stages of fracture repair.