Reduced COX-2 expression in aged mice is associated with impaired fracture healing

Osteocyte connexin hemichannels and prostaglandin E2 release dictate bone marrow mesenchymal stromal cell commitment

Bone is a dynamic organ constantly undergoing remodeling with both bone formation and resorption. Bone formation is mediated by osteoblasts originating from the differentiation of bone marrow (BM) mesenchymal stem and progenitor cells (BM-MSPCs). However, how bone cells communicate with BM-MSPCs to coordinate bone formation remains largely elusive. Here, we unveil a key role of osteocyte connexin 43 (Cx43) hemichannels in regulating the lineage commitment of BM-MSPCs. Two transgenic mouse models expressing dominant negative Cx43 mutants in osteocytes were used: R76W (inhibiting gap junctions) and Δ130 to 136 (inhibiting both hemichannels and gap junctions). BM-MSPCs from Δ130 to 136 mice showed enhanced adipogenic differentiation and reduced osteogenic potential, leading to increased BM adipocytes. Flow cytometry and single-cell RNA sequencing revealed shifts in BM-MSPC subsets, less osteogenic-biased MSPCs, and more adipogenic-biased MSPCs in Δ130 to 136 mice. Conversely, R76W, with more functional hemichannels, exhibited effects similar to WT mice or even greater opposite effects than Δ130 to 136 mice. Prostaglandin E2 (PGE2), released from active Cx43 hemichannels, inhibited adipogenesis and promoted osteogenesis via the PGE2 receptor EP4 and ERK1/2 signaling. Inhibition of Cx43 hemichannels or EP4 led to increased adipogenic-biased MSPCs. Moreover, administration of a Cx43(M1) antibody, which inhibits hemichannels, substantially increased BM adipocytes, accompanied by increased adipogenic-biased MSPCs, and decreased osteogenic-biased MSPCs. Our study highlights the pivotal role of osteocyte Cx43 hemichannels in BM-MSPC fate decision through PGE2 release, providing insights into the precise and highly regulated communication between matrix-bound bone cells and BM-MSPCs, which dictates bone formation and remodeling.

Targeting EP2 Receptor Improves Muscle and Bone Health in Dystrophin-/-/Utrophin-/- Double-Knockout Mice

Duchenne muscular dystrophy (DMD) is a severe genetic muscle disease occurring due to mutations of the dystrophin gene. There is no cure for DMD. Using a dystrophin-/-utrophin-/- (DKO-Hom) mouse model, we investigated the PGE2/EP2 pathway in the pathogenesis of dystrophic muscle and its potential as a therapeutic target. We found that Ep2, Ep4, Cox-2, 15-Pgdh mRNA, and PGE2 were significantly increased in DKO-Hom mice compared to wild-type (WT) mice. The EP2 and EP4 receptors were mainly expressed in CD68+ macrophages and were significantly increased in the muscle tissues of both dystrophin-/- (mdx) and DKO-Hom mice compared to WT mice. Osteogenic and osteoclastogenic gene expression in skeletal muscle also increased in DKO-Hom mice, which correlates with severe muscle heterotopic ossification (HO). Treatment of DKO-Hom mice with the EP2 antagonist PF04418948 for 2 weeks increased body weight and reduced HO and muscle pathology by decreasing both total macrophages (CD68+) and senescent macrophages (CD68+P21+), while increasing endothelial cells (CD31+). PF04418948 also increased bone volume/total volume (BV/TV), the trabecular thickness (Tb.Th) of the tibia trabecular bone, and the cortical bone thickness of both the femur and tibia without affecting spine trabecular bone microarchitecture. In summary, our results indicate that targeting EP2 improves muscle pathology and improves bone mass in DKO mice.

Cause and chondroprotective effects of prostaglandin E2 secretion during mesenchymal stromal cell chondrogenesis

Mesenchymal stromal cells (MSCs) that are promising for cartilage tissue engineering secrete high amounts of prostaglandin E2 (PGE2), an immunoactive mediator involved in endochondral bone development. This study aimed to identify drivers of PGE2 and its role in the inadvertent MSC misdifferentiation into hypertrophic chondrocytes. PGE2 release, which rose in the first three weeks of MSC chondrogenesis, was jointly stimulated by endogenous BMP, WNT, and hedgehog activity that supported the exogenous stimulation by TGF-β1 and insulin to overcome the PGE2 inhibition by dexamethasone. Experiments with PGE2 treatment or the inhibitor celecoxib or specific receptor antagonists demonstrated that PGE2, although driven by prohypertrophic signals, exerted broad autocrine antihypertrophic effects. This chondroprotective effect makes PGE2 not only a promising option for future combinatorial approaches to direct MSC tissue engineering approaches into chondral instead of endochondral development but could potentially have implications for the use of COX-2-selective inhibitors in osteoarthritis pain management.

Systemic inflammatory and gut microbiota responses to fracture in young and middle-aged mice

Age is a patient-specific factor that can significantly delay fracture healing and exacerbate systemic sequelae during convalescence. The basis for this difference in healing rates is not well-understood, but heightened inflammation has been suggested to be a significant contributor. In this study, we investigated the systemic cytokine and intestinal microbiome response to closed femur fracture in 3-month-old (young adult) and 15-month-old (middle-aged) female wild-type mice. Middle-aged mice had a serum cytokine profile that was distinct from young mice at days 10, 14, and 18 post-fracture. This was characterized by increased concentrations of IL-17a, IL-10, IL-6, MCP-1, EPO, and TNFα. We also observed changes in the community structure of the gut microbiota in both young and middle-aged mice that was evident as early as day 3 post-fracture. This included an Enterobacteriaceae bloom at day 3 post-fracture in middle-aged mice and an increase in the relative abundance of the Muribaculum genus. Moreover, we observed an increase in the relative abundance of the health-promoting Bifidobacterium genus in young mice after fracture that did not occur in middle-aged mice. There were significant correlations between serum cytokines and specific genera, including a negative correlation between Bifidobacterium and the highly induced cytokine IL-17a. Our study demonstrates that aging exacerbates the inflammatory response to fracture leading to high levels of pro-inflammatory cytokines and disruption of the intestinal microbiota.

Does Construct Type Matter? A Retrospective Review Comparing Outcomes of Distal Radius Fractures Treated with Standard Volar Plating versus Fragment-Specific Fixation

Background  Treatment of intra-articular distal radius fractures (DRFs) rests on anatomic internal fixation. Fragment-specific fixation (FSF) is applied when fracture pattern is too complex for standard volar plating (SVP), oftentimes with potential increased risk of complications. We hypothesized that patients undergoing FSF would achieve less wrist range of motion (ROM) with higher risk of complications compared with SVP. Methods  We conducted a retrospective review of 159 consecutive patients undergoing DRF fixation from 2017 to 2020. Patients < 18 years old, < 8 weeks' follow-up, open fractures, ipsilateral trauma, and fractures requiring dorsal spanning plate were excluded. Patient demographics, specific construct type, AO fracture classification, ROM, and complications were assessed. ROM was calculated using average flexion, extension, supination, and pronation. t -Tests were used to determine differences in ROM among construct types. Results  Ninety-two patients met all inclusion criteria: 59 underwent SVP and 33 underwent FSF. Average wrist ROM for patients undergoing SVP was 57 degrees/50 degrees flexion-extension and 87 degrees/88 degrees supination-pronation; average ROM for patients undergoing FSF was 55 degrees/49 degrees flexion-extension and 88 degrees/89 degrees supination-pronation. No significant differences were identified when comparing final wrist flexion ( p  = 0.08), extension ( p  = 0.33), supination ( p  = 0.35), or pronation ( p  = 0.21). Overall reoperation rate was 5% and higher for FSF (12%) versus SVP (2%). Highest reoperation rate was observed in the double volar hook cohort (80%; N  = 4). Conclusion  Construct type does not appear to affect final ROM if stable internal fixation is achieved. SVP and FSF had similar complication rates; however, double volar hook constructs resulted in increased reoperations likely from fixation failure and plate prominence. Level of Evidence  Level IV, retrospective review.

Parathyroid hormone stimulates bone regeneration in an atrophic non-union model in aged mice

BACKGROUND

Non-union formation still represents a major burden in trauma and orthopedic surgery. Moreover, aged patients are at an increased risk for bone healing failure. Parathyroid hormone (PTH) has been shown to accelerate fracture healing in young adult animals. However, there is no information whether PTH also stimulates bone regeneration in atrophic non-unions in the aged. Therefore, the aim of the present study was to analyze the effect of PTH on bone regeneration in an atrophic non-union model in aged CD-1 mice.

METHODS

After creation of a 1.8 mm segmental defect, mice femora were stabilized by pin-clip fixation. The animals were treated daily with either 200 µg/kg body weight PTH 1-34 (n = 17) or saline (control; n = 17) subcutaneously. Bone regeneration was analyzed by means of X-ray, biomechanics, micro-computed tomography (µCT) imaging as well as histological, immunohistochemical and Western blot analyses.

RESULTS

In PTH-treated animals bone formation was markedly improved when compared to controls. This was associated with an increased bending stiffness as well as a higher number of tartrate-resistant acid phosphatase (TRAP)-positive osteoclasts and CD31-positive microvessels within the callus tissue. Furthermore, PTH-treated aged animals showed a decreased inflammatory response, characterized by a lower number of MPO-positive granulocytes and CD68-positive macrophages within the bone defects when compared to controls. Additional Western blot analyses demonstrated a significantly higher expression of cyclooxygenase (COX)-2 and phosphoinositide 3-kinase (PI3K) in PTH-treated mice.

CONCLUSION

Taken together, these findings indicate that PTH is an effective pharmacological compound for the treatment of non-union formation in aged animals.

Gene-Activated Materials in Regenerative Dentistry: Narrative Review of Technology and Study Results

Treatment of a wide variety of defects in the oral and maxillofacial regions requires the use of innovative approaches to achieve best outcomes. One of the promising directions is the use of gene-activated materials (GAMs) that represent a combination of tissue engineering and gene therapy. This approach implies that biocompatible materials will be enriched with gene-carrying vectors and implanted into the defect site resulting in transfection of the recipient's cells and secretion of encoded therapeutic protein in situ. GAMs may be presented in various designs depending on the type of material, encoded protein, vector, and way of connecting the vector and the material. Thus, it is possible to choose the most suitable GAM design for the treatment of a particular pathology. The use of plasmids for delivery of therapeutic genes is of particular interest. In the present review, we aimed to delineate the principle of work and various designs of plasmid-based GAMs and to highlight results of experimental and clinical studies devoted to the treatment of periodontitis, jaw bone defects, teeth avulsion, and other pathologies in the oral and maxillofacial regions.

An in vivo study to investigate an original intramedullary bone graft harvesting technology

BACKGROUND

Harvesting bone graft (BG) from the intramedullary canal to treat bone defects is largely conducted using the Reamer-Irrigator-Aspirator (RIA) system. The RIA system uses irrigation fluid during harvesting, which may result in washout of osteoinductive factors. Here, we propose a new harvesting technology dedicated to improving BG collection without the potential washout effect of osteoinductive factors associated with irrigation fluid. This novel technology involves the conceptual approach of first aspirating the bone marrow (BM) with a novel aspirator prototype, followed by reaming with standard reamers and collecting the bone chips with the aspirator (reaming-aspiration method, R-A method). The aim of this study was to assess the harvesting efficacy and osteoinductive profile of the BG harvested with RIA 2 system (RIA 2 group) compared to the novel harvesting concept (aspirator + R-A method, ARA group).

METHODS

Pre-planning computed tomography (CT) imaging was conducted on 16 sheep to determine the femoral isthmus canal diameter. In this non-recovery study, sheep were divided into two groups: RIA 2 group (n = 8) and ARA group (n = 8). We measured BG weight collected from left femur and determined femoral cortical bone volume reduction in postoperative CT imaging. Growth factor and inflammatory cytokine amounts of the BGs were quantified using enzyme-linked immunosorbent assay (ELISA) methods.

RESULTS

The use of the stand-alone novel aspirator in BM collection, and in harvesting BG when the aspirator is used in conjunction with sequential reaming (R-A method) was proven feasible. ELISA results showed that the collected BG contained relevant amounts of growth factors and inflammatory cytokines in both the RIA 2 and the ARA group.

CONCLUSIONS

Here, we present the first results of an innovative concept for harvesting intramedullary BG. It is a prototype of a novel aspirator technology that enables the stepwise harvesting of first BM and subsequent bone chips from the intramedullary canal of long bones. Both the BG collected with the RIA 2 system and the aspirator prototype had the capacity to preserve the BG's osteoinductive microenvironment. Future in vivo studies are required to confirm the bone regenerative capacity of BG harvested with the innovative harvesting technology.

The Role of COX-2 and PGE2 in the Regulation of Immunomodulation and Other Functions of Mesenchymal Stromal Cells

The ability of MSCs to modulate the inflammatory environment is well recognized, but understanding the molecular mechanisms responsible for these properties is still far from complete. Prostaglandin E2 (PGE2), a product of the cyclooxygenase 2 (COX-2) pathway, is indicated as one of the key mediators in the immunomodulatory effect of MSCs. Due to the pleiotropic effect of this molecule, determining its role in particular intercellular interactions and aspects of cell functioning is very difficult. In this article, the authors attempt to summarize the previous observations regarding the role of PGE2 and COX-2 in the immunomodulatory properties and other vital functions of MSCs. So far, the most consistent results relate to the inhibitory effect of MSC-derived PGE2 on the early maturation of dendritic cells, suppressive effect on the proliferation of activated lymphocytes, and stimulatory effect on the differentiation of macrophages into M2 phenotype. Additionally, COX-2/PGE2 plays an important role in maintaining the basic life functions of MSCs, such as the ability to proliferate, migrate and differentiate, and it also positively affects the formation of niches that are conducive to both hematopoiesis and carcinogenesis.

Bifidobacterium longum supplementation improves age-related delays in fracture repair

Age-related delays in bone repair remains an important clinical issue that can prolong pain and suffering. It is now well established that inflammation increases with aging and that this exacerbated inflammatory response can influence skeletal regeneration. Recently, simple dietary supplementation with beneficial probiotic bacteria has been shown to influence fracture repair in young mice. However, the contribution of the gut microbiota to age-related impairments in fracture healing remains unknown. Here, we sought to determine whether supplementation with a single beneficial probiotic species, Bifidobacterium longum (B. longum), would promote fracture repair in aged (18-month-old) female mice. We found that B. longum supplementation accelerated bony callus formation which improved mechanical properties of the fractured limb. We attribute these pro-regenerative effects of B. longum to preservation of intestinal barrier, dampened systemic inflammation, and maintenance of the microbiota community structure. Moreover, B. longum attenuated many of the fracture-induced systemic pathologies. Our study provides evidence that targeting the gut microbiota using simple dietary approaches can improve fracture healing outcomes and minimize systemic pathologies in the context of aging.

Fracture Healing in the Setting of Endocrine Diseases, Aging, and Cellular Senescence

More than 2.1 million age-related fractures occur in the United States annually, resulting in an immense socioeconomic burden. Importantly, the age-related deterioration of bone structure is associated with impaired bone healing. Fracture healing is a dynamic process which can be divided into four stages. While the initial hematoma generates an inflammatory environment in which mesenchymal stem cells and macrophages orchestrate the framework for repair, angiogenesis and cartilage formation mark the second healing period. In the central region, endochondral ossification favors soft callus development while next to the fractured bony ends, intramembranous ossification directly forms woven bone. The third stage is characterized by removal and calcification of the endochondral cartilage. Finally, the chronic remodeling phase concludes the healing process. Impaired fracture healing due to aging is related to detrimental changes at the cellular level. Macrophages, osteocytes, and chondrocytes express markers of senescence, leading to reduced self-renewal and proliferative capacity. A prolonged phase of "inflammaging" results in an extended remodeling phase, characterized by a senescent microenvironment and deteriorating healing capacity. Although there is evidence that in the setting of injury, at least in some tissues, senescent cells may play a beneficial role in facilitating tissue repair, recent data demonstrate that clearing senescent cells enhances fracture repair. In this review, we summarize the physiological as well as pathological processes during fracture healing in endocrine disease and aging in order to establish a broad understanding of the biomechanical as well as molecular mechanisms involved in bone repair.

The promising roles of macrophages in geriatric hip fracture

As aging becomes a global burden, the incidence of hip fracture (HF), which is the most common fracture in the elderly population and can be fatal, is rapidly increasing, and its extremely high fatality rate places significant medical and financial burdens on patients. Fractures trigger a complex set of immune responses, and recent studies have shown that with aging, the immune system shows decreased activity or malfunctions in a process known as immune senescence, leading to disease and death. These phenomena are the reasons why elderly individuals typically exhibit chronically low levels of inflammation and increased rates of infection and chronic disease. Macrophages, which are key players in the inflammatory response, are critical in initiating the inflammatory response, clearing pathogens, controlling the innate and adaptive immune responses and repairing damaged tissues. Tissue-resident macrophages (TRMs) are widely present in tissues and perform immune sentinel and homeostatic functions. TRMs are combinations of macrophages with different functions and phenotypes that can be directly influenced by neighboring cells and the microenvironment. They form a critical component of the first line of defense in all tissues of the body. Immune system disorders caused by aging could affect the biology of macrophages and thus the cascaded immune response after fracture in various ways. In this review, we outline recent studies and discuss the potential link between monocytes and macrophages and their potential roles in HF in elderly individuals.

DXAGE 2.0 - adult age at death estimation using bone loss in the proximal femur and the second metacarpal

The accurate age at death assessment of unidentified adult skeletal individuals is a critical research task in forensic anthropology, being a key feature for the determination of biological profiles of individual skeletal remains. We have previously shown that the age-related decrease of bone mineral density (BMD) in the proximal femur could be used to assess age at death in women (Navega et al., J Forensic Sci 63:497-503, 2018). The present study aims to generate models for age estimation in both sexes through bone densitometry of the femur and radiogrammetry of the second metacarpal. The training sample comprised 224 adults (120 females, 104 males) from the "Coimbra Identified Skeletal Collection," and different models were generated through least squares regression and general regression neural networks (GRNN). The models were operationalized in a user-friendly online interface at https://osteomics.com/DXAGE2/ . The mean absolute difference between the known and estimated age at death ranges from 9.39 to 13.18 years among women and from 10.33 to 15.76 among men with the least squares regression models. For the GRNN models, the mean absolute difference between documented and projected age ranges from 8.44 to 12.58 years in women and from 10.56 to 16.18 years in men. DXAGE 2.0 enables age estimation in incomplete and/or fragmentary skeletal remains, using alternative skeletal regions, with reliable results.

Low-Intensity Pulsed Ultrasound Stimulation for Bone Fractures Healing: A Review

Low-intensity pulsed ultrasound (LIPUS) is a developing technology, which has been proven to improve fracture healing process with minimal thermal effects. This noninvasive treatment accelerates bone formation through various molecular, biological, and biomechanical interactions with tissues and cells. Although LIPUS treatment has shown beneficial effects on different bone fracture locations, only very few studies have examined its effects on deeper bones. This study provides an overview on therapeutic ultrasound for fractured bones, possible mechanisms of action, clinical evidences, current limitations, and its future prospects.

Proteasome inhibition-enhanced fracture repair is associated with increased mesenchymal progenitor cells in mice

The ubiquitin/proteasome system controls the stability of Runx2 and JunB, proteins essential for differentiation of mesenchymal progenitor/stem cells (MPCs) to osteoblasts. Local administration of proteasome inhibitor enhances bone fracture healing by accelerating endochondral ossification. However, if a short-term administration of proteasome inhibitor enhances fracture repair and potential mechanisms involved have yet to be exploited. We hypothesize that injury activates the ubiquitin/proteasome system in callus, leading to elevated protein ubiquitination and degradation, decreased MPCs, and impaired fracture healing, which can be prevented by a short-term of proteasome inhibition. We used a tibial fracture model in Nestin-GFP reporter mice, in which a subgroup of MPCs are labeled by Nestin-GFP, to test our hypothesis. We found increased expression of ubiquitin E3 ligases and ubiquitinated proteins in callus tissues at the early phase of fracture repair. Proteasome inhibitor Bortezomib, given soon after fracture, enhanced fracture repair, which is accompanied by increased callus Nestin-GFP+ cells and their proliferation, and the expression of osteoblast-associated genes and Runx2 and JunB proteins. Thus, early treatment of fractures with Bortezomib could enhance the fracture repair by increasing the number and proliferation of MPCs.

Development of a murine model of ischemic osteonecrosis to study the effects of aging on bone repair

Age at onset is one of the most important predictors of outcome following ischemic osteonecrosis (ON). Currently, there is no well-established animal model to study the effects of age on the repair process following ischemic ON. The purpose of this study was to further advance a murine model of ischemic ON using four age groups of mice to determine the effects of aging on revascularization and bone repair following ischemic ON. Ischemia was surgically induced in the distal femoral epiphysis of four age groups of skeletally immature and mature mice; juvenile (5 weeks), adolescent (12 weeks), adult (22 weeks), and middle age (52 weeks). Mice were euthanized at 2 days or 4 weeks post-ischemia surgery to evaluate the extent of ON, revascularization, and bone repair. Terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling staining showed extensive cell death in the epiphysis of all four age groups at 2 days post-ischemia surgery. At 4 weeks, the juvenile mice followed by the adolescent mice had significantly greater revascularization and repair of the necrotic marrow space, increased osteoblast and osteoclast numbers, and increased bone formation rates compared to the adult and middle-age mice. Faster revascularization and bone healing were observed in the skeletally immature mice compared to the skeletally mature mice following ischemic ON. The findings resemble the clinical observation of aging on bone repair following ischemic ON. The mouse model may serve as a useful tool to investigate the mechanisms underlying the age-related impairment of bone repair in adolescent and adult ON and to develop novel therapeutic strategies.

Expression of nerve growth factor in the callus during fracture healing in a fracture model in aged mice

BACKGROUND

Impaired fracture healing results in extensive and prolonged disability and long-term pain. Previous studies reported that nerve growth factor (NGF) was expressed during fracture healing and that anti-NGF antibody improves physical activity associate with facture pain. However, NGF expression levels in delayed or non-union are not fully understood.

OBJECTIVE

We compared chronological changes in NGF in the callus of young mice after femur fracture with those in aged mice after femur fracture as a model of bone fracture in the elderly.

METHODS

We used young (age 8 weeks) and aged (age 10 months) male C57BL/6J mice. A fracture was generated in the femur. At 5, 7, 10, 14, 17, and 21 days after creation of a fracture, mRNA expression levels of Col2a1, Col10a1, NGF were evaluated using quantitative PCR. We examined NGF protein expression levels and localization in the callus at day 14 using ELISA and immunohistochemistry, respectively.

RESULTS

Expression of NGF in the callus after femur fracture in aged mice was significantly greater than that in young mice at days 5, 7, 10, 17, and 21 days. NGF protein levels in the callus of aged mice were also significantly higher than that in young mice. Immunohistochemical staining showed that NGF was heavily expressed in hypertrophic chondrocytes in the callus in aged mice.

CONCLUSIONS

It is suggested that delayed Col2a1 and Col10a1 expression reflects delayed chondrocyte formation and delayed chondrocyte maturation in aged mice and that higher NGF expression in aged mice at day 14 may be associated with the presence of remaining hypertrophic chondrocytes in callus with delaying endochondral ossification.

Targeting angiogenesis for fracture nonunion treatment in inflammatory disease

Atrophic fracture nonunion poses a significant clinical problem with limited therapeutic interventions. In this study, we developed a unique nonunion model with high clinical relevance using serum transfer-induced rheumatoid arthritis (RA). Arthritic mice displayed fracture nonunion with the absence of fracture callus, diminished angiogenesis and fibrotic scar tissue formation leading to the failure of biomechanical properties, representing the major manifestations of atrophic nonunion in the clinic. Mechanistically, we demonstrated that the angiogenesis defect observed in RA mice was due to the downregulation of SPP1 and CXCL12 in chondrocytes, as evidenced by the restoration of angiogenesis upon SPP1 and CXCL12 treatment in vitro. In this regard, we developed a biodegradable scaffold loaded with SPP1 and CXCL12, which displayed a beneficial effect on angiogenesis and fracture repair in mice despite the presence of inflammation. Hence, these findings strongly suggest that the sustained release of SPP1 and CXCL12 represents an effective therapeutic approach to treat impaired angiogenesis and fracture nonunion under inflammatory conditions.

A network analysis of angiogenesis/osteogenesis-related growth factors in bone tissue engineering based on in-vitro and in-vivo data: A systems biology approach

The principal purpose of tissue engineering is to stimulate the injured or unhealthy tissues to revive their primary function through the simultaneous use of chemical agents, cells, and biocompatible materials. Still, choosing the appropriate protein as a growth factor (GF) for tissue engineering is vital to fabricate artificial tissues and accelerate the regeneration procedure. In this study, the angiogenesis and osteogenesis-related proteins' interactions are studied using their related network. Three major biological processes, including osteogenesis, angiogenesis, and angiogenesis regulation, were investigated by creating a protein-protein interaction (PPI) network (45 nodes and 237 edges) of bone regeneration efficient proteins. Furthermore, a gene ontology and a centrality analysis were performed to identify essential proteins within a network. The higher degree in this network leads to higher interactions between proteins and causes a considerable effect. The most highly connected proteins in the PPI network are the most remarkable for their employment. The results of this study showed that three significant proteins including prostaglandin endoperoxide synthase 2 (PTGS2), TEK receptor tyrosine kinase (TEK), and fibroblast growth factor 18 (FGF18) were involved simultaneously in osteogenesis, angiogenesis, and their positive regulatory. Regarding the available literature, the results of this study confirmed that PTGS2 and FGF18 could be used as a GF in bone tissue engineering (BTE) applications to promote angiogenesis and osteogenesis. Nevertheless, TEK was not used in BTE applications until now and should be considered in future works to be examined in-vitro and in-vivo.

Psoralen, a natural phytoestrogen, improves diaphyseal fracture healing in ovariectomized mice: A preliminary study

Psoralen is an effective active component extracted from Psoraleacorylifolia, which can promote bone formation in osteoporotic animals. However, to the best of our knowledge, its effect on fracture healing has not yet been examined. In the present study, open femur fractures were created in ovariectomy (OVX)-induced osteoporotic mice. OVX mice were treated with psoralen (psoralen+OVX group) or physiological saline (OVX group) by oral gavage. Radiographic and histological results demonstrated progressed callus consolidation in the psoralen+OVX group compared with the OVX group after 10 and 21 days of treatment. Qualitative histological analysis showed that the number of osteoclasts was significantly reduced in the psoralen+OVX group after treatment. Moreover, reverse transcription-quantitative PCR analysis of callus samples showed increased expression of bone morphogenetic protein-2 (BMP-2) and osteoprotegerin (OPG), and decreased expression of receptor activator of nuclear factor-κB ligand (RANKL) at 10 and 21 days post injury in the psoralen+OVX group compared with the OVX group. Furthermore, western blot analysis showed that psoralen significantly increased the expression of estrogen receptor (ER)-α, but had no effect on ER-β expression; these results were further confirmed by immunohistochemistry. To conclude, these results indicated that psoralen may promote callus formation and inhibit osteoclast genesis by increasing BMP-2 and ER-α levels, and OPG/RANKL ratio. Consequently, psoralen could be a possible treatment for osteoporotic fracture-related complications.

High resolution imaging in bone tissue research-review

This review article focuses on imaging of bone tissue to understand skeletal health with regards to bone quality. Skeletal fragility fractures are due to bone diseases such as osteoporosis which result in low bone mass and bone mineral density (BMD) leading to high risk of fragility fractures. Recent advances in imaging and analysis technologies have highly benefitted the field of biological sciences. In particular, their application in skeletal health has been of significant importance in understanding bone mechanical behavior (structure and properties) at the tissue level. While synchrotron based microCT technique has remained the gold standard for non-destructive evaluation of structure in material and biological sciences, several lab based microCT systems have been developed to provide high resolution imaging of specimens with greater access, and ease of use in laboratory settings. Lab based microCT scanners are widely used in the bone field as a standard tool to evaluate three-dimensional (3D) morphologies of bone structure at image resolutions appropriate for bone samples from small animals to bone biopsy specimens from humans. Both synchrotron and standard lab based microCT systems provide high resolution imaging ex vivo for a small sized specimen. A few X-ray based systems are also commercially available for in vivo scanning at relatively low image resolutions. Synchrotron-based CT microscopy is being used for various ultra-high-resolution image analyses using complex 3D software. However, the synchrotron-based CT technology is in high demand, allows only limited numbers of specimens, expensive, requires complex additional instrumentation, and is not easily available to researchers as it requires access to a synchrotron source which is always limited. Therefore, desktop laboratory scanners (microXCT, Zeiss/Xradia, Scanco, SkyScan. etc.), mimicking the synchrotron based CT technology or image resolution, have been developed to solve the accessibility issues. These lab based scanners have helped both material science, and the bone field to investigate bone tissue morphologies at submicron mage resolutions. Considerable progress has been made in both in vivo and ex vivo imaging towards providing high resolution images of bone tissue. Both clinical and research imaging technologies will continue to improve and help understand osteoporosis and other related skeletal issues in order to develop targeted treatments for bone fragility. This review summarizes the high resolution imaging work in bone research.

Bifidobacterium adolescentis supplementation attenuates fracture-induced systemic sequelae

The gut microbiota is an important contributor to both health and disease. While previous studies have reported on the beneficial influences of the gut microbiota and probiotic supplementation on bone health, their role in recovery from skeletal injury and resultant systemic sequelae remains unexplored. This study aimed to determine the extent to which probiotics could modulate bone repair by dampening fracture-induced systemic inflammation. Our findings demonstrate that femur fracture induced an increase in gut permeability lasting up to 7 days after trauma before returning to basal levels. Strikingly, dietary supplementation with Bifidobacterium adolescentis augmented the tightening of the intestinal barrier, dampened the systemic inflammatory response to fracture, accelerated fracture callus cartilage remodeling, and elicited enhanced protection of the intact skeleton following fracture. Together, these data outline a mechanism whereby dietary supplementation with beneficial bacteria can be therapeutically targeted to prevent the systemic pathologies induced by femur fracture.

Can low intensity pulsed ultrasound (LIPUS) be used as an alternative to revision surgery for patients with non-unions following fracture fixation?

BACKGROUND

Non-union is a significant complication of fracture fixation surgery, and can negatively impact a patient's quality of life. Low intensity pulsed ultrasound (LIPUS) has been used to treat delayed or non-unions previously in the literature. The aim of this study was to determine the success rate of LIPUS treatment in patients with chronic fracture non-unions, and to establish the effect of systemic or local factors on its success.

METHODS

This was a retrospective, observational study which included all patients undergoing LIPUS treatment in a single institution. Patients deemed suitable for LIPUS underwent treatment for a period of 6 months from initiation. They were followed up with sequential radiographs to assess union at intervals of 6 weeks, 3 months, 6 months and 1 year. LIPUS treatment was considered to be successful when patients achieved clinical and radiological union, without the need for revision surgery.

RESULTS

A total of 46 patients were included in the study; 8 were lost to follow - up, leaving 38 patients for the final analysis. The mean age of patients was 47.03 ± 19.7 with a male to female ratio of 1.2:1. Union was achieved in 57.89%; the rest underwent revision surgery. There was no significant association between outcomes after LIPUS treatment and patients' age, gender, smoking status or type of non-union. Patients with a small inter-fragment bone gap were more likely to have a successful outcome after LIPUS (p = 0.041). Time to treatment did not have a statistically significant impact on outcomes after LIPUS. Interestingly, all 6 patients with diabetes in the study managed to achieve union after LIPUS.

CONCLUSIONS

This study demonstrates that LIPUS is not successful in a large proportion of patients with established fracture non-unions. However, it does represent a low risk treatment modality as an alternative to revision surgery, especially for patients with diabetes who have a small inter - fragment bone gap. More research in the form of large randomised controlled trials needs to be carried out to further assess the role of LIPUS in the treatment of non-unions.

The Life of a Fracture: Biologic Progression, Healing Gone Awry, and Evaluation of Union

New knowledge about the molecular biology of fracture-healing provides opportunities for intervention and reduction of risk for specific phases that are affected by disease and medications. Modifiable and nonmodifiable risk factors can prolong healing, and the informed clinician should optimize each patient to provide the best chance for union. Techniques to monitor progression of fracture-healing have not changed substantially over time; new objective modalities are needed.

Inhibition of the mitochondrial permeability transition improves bone fracture repair

Bone fracture is accompanied by trauma, mechanical stresses, and inflammation - conditions known to induce the mitochondrial permeability transition. This phenomenon occurs due to opening of the mitochondrial permeability transition pore (MPTP) promoted by cyclophilin D (CypD). MPTP opening leads to more inflammation, cell death and potentially to disruption of fracture repair. Here we performed a proof-of-concept study and tested a hypothesis that protecting mitochondria from MPTP opening via inhibition of CypD improves fracture repair. First, our in vitro experiments indicated pro-osteogenic and anti-inflammatory effects in osteoprogenitors upon CypD knock-out or pharmacological inhibition. Using a bone fracture model in mice, we observed that bone formation and biomechanical properties of repaired bones were significantly increased in CypD knock-out mice or wild type mice treated with a CypD inhibitor, NIM811, when compared to controls. These effects were evident in young male but not female mice, however in older (13 month-old) female mice bone formation was also increased during fracture repair. In contrast to global CypD knock-out, mesenchymal lineage-specific (Prx1-Cre driven) CypD deletion did not result in improved fracture repair. Our findings implicate MPTP in bone fracture and suggest systemic CypD inhibition as a modality to promote fracture repair.

Comparative transcriptomic analysis identifies distinct molecular signatures and regulatory networks of chondroclasts and osteoclasts

BACKGROUND

Chondroclasts and osteoclasts have been previously identified as the cells capable of resorbing mineralized cartilage and bone matrices, respectively. While both cell types appear morphologically similar, contain comparable ultrastructural features, and express tartrate-resistant acid phosphatase (TRAP), however, no information is available about the genomic similarities and differences between osteoclasts and chondroclasts.

METHODS

To address this question, we laser captured homogeneous populations of TRAP-positive cells that interact with bone (osteoclasts) and TRAP-positive cells that interact with mineralized cartilage (chondroclasts) on the same plane from murine femoral fracture callus sections. We then performed a global transcriptome profiling of chondroclasts and osteoclasts by utilizing a mouse genome Agilent GE 4X44K V2 microarray platform. Multiple computational approaches and interaction networks were used to analyze the transcriptomic landscape of osteoclasts and chondroclasts.

RESULTS

Our systematic and comprehensive analyses using hierarchical clustering and principal component analysis (PCA) demonstrate that chondroclasts and osteoclasts are transcriptionally distinct cell populations and exhibit discrete transcriptomic signatures as revealed by multivariate analysis involving scatter plot, volcano plot, and heatmap analysis. TaqMan qPCR was used to validate the microarray results. Intriguingly, the functional enrichment and integrated network analyses revealed distinct Gene Ontology terms and molecular pathways specific to chondroclasts and osteoclasts and further suggest that subsets of metabolic genes were specific to chondroclasts. Protein-protein interaction (PPI) network analysis showed an abundance of structured networks of metabolic pathways, ATP synthesis, and proteasome pathways in chondroclasts. The regulatory network analysis using transcription factor-target gene network predicted a pool of genes including ETV6, SIRT1, and ATF1 as chondroclast-specific gene signature.

CONCLUSIONS

Our study provides an important genetic resource for further exploration of chondroclast function in vivo. To our knowledge, this is the first demonstration of genetic landscape of osteoclasts from chondroclasts identifying unique molecular signatures, functional clustering, and interaction network.

Gender differences in tibial fractures healing in normal and muscular dystrophic mice

Duchenne muscular dystrophy (DMD) patients have a high fracture risk and poor fracture healing. The dystrophin^-/- (mdx) mouse is a murine model of DMD and exhibits delayed bone fracture healing. Since our research team has shown that adult stem cells, such as muscle-derived stem cells, display a gender difference in their osteogenic potential with the male cells being more osteogenic, we hypothesize that a potential gender differences may exist during bone healing in normal and mdx mice. To test this hypothesis, wild-type (WT) and mdx mice underwent tibial fracture surgery and microCT live scanning biweekly. The mice were sacrificed at 6 weeks post-surgery and the calluses were collected for histological analysis. To further investigate the mechanism, another two sets of mice were sacrificed at 10 days after fracture for RNA extraction and gene expression analysis and histology. MicroCT results showed, at 6 weeks post- surgery, the calluses were larger but showed less remodeling in both normal and mdx male mice when compared to females, at the same time point. However, females had higher callus bone volume density and an increase in osteoclast (OCs) number. At 10 days after fracture surgery, male mice had formed larger calluses, whereas females formed well-remodeled calluses with more osteoblasts and a greater bone area for both WT and mdx mice. Higher IGF-1 expression was observed in male mdx mice when compared to their female counterparts, whereas female WT mice had higher BMP-9 expression when compared to WT males. In conclusion, male mice formed larger bone calluses than females during tibial fracture healing for both WT and mdx mice. This may be attributed to higher IGF-1 expression, activation of Wnt/β-catennin signaling pathway and greater OB numbers during callus formation. Female mice achieved better bone remodeling in the regenerated bone with higher bone quality due to increased OC numbers that promote faster remodeling of the fracture calluses, and higher BMP-9 expression levels. Therefore, gender is one of many factors that need to be considered for both animal and human bone research.

Fracture Healing in Collagen-Related Preclinical Models of Osteogenesis Imperfecta

Osteogenesis imperfecta (OI) is a genetic bone dysplasia characterized by bone deformities and fractures caused by low bone mass and impaired bone quality. OI is a genetically heterogeneous disorder that most commonly arises from dominant mutations in genes encoding type I collagen (COL1A1 and COL1A2). In addition, OI is recessively inherited with the majority of cases resulting from mutations in prolyl-3-hydroxylation complex members, which includes cartilage-associated protein (CRTAP). OI patients are at an increased risk of fracture throughout their lifetimes. However, non-union or delayed healing has been reported in 24% of fractures and 52% of osteotomies. Additionally, refractures typically go unreported, making the frequency of refractures in OI patients unknown. Thus, there is an unmet need to better understand the mechanisms by which OI affects fracture healing. Using an open tibial fracture model, our study demonstrates delayed healing in both Col1a2 ^G610c/+ and Crtap ^-/- OI mouse models (dominant and recessive OI, respectively) that is associated with reduced callus size and predicted strength. Callus cartilage distribution and chondrocyte maturation were altered in OI, suggesting accelerated cartilage differentiation. Importantly, we determined that healed fractured tibia in female OI mice are biomechanically weaker when compared with the contralateral unfractured bone, suggesting that abnormal OI fracture healing OI may prime future refracture at the same location. We have previously shown upregulated TGF-β signaling in OI and we confirm this in the context of fracture healing. Interestingly, treatment of Crtap ^-/- mice with the anti-TGF-β antibody 1D11 resulted in further reduced callus size and predicted strength, highlighting the importance of investigating dose response in treatment strategies. These data provide valuable insight into the effect of the extracellular matrix (ECM) on fracture healing, a poorly understood mechanism, and support the need for prevention of primary fractures to decrease incidence of refracture and deformity in OI patients. © 2020 American Society for Bone and Mineral Research.

Deletion of Runx1 in osteoclasts impairs murine fracture healing through progressive woven bone loss and delayed cartilage remodeling

Conditional deletion of the transcription factor Runt-related transcription factor 1 (Runx1) in myeloid osteoclast precursors promotes osteoclastogenesis and subsequent bone loss. This study posits whether Runx1 regulates clastic cell-mediated bone and cartilage resorption in the fracture callus. We first generated mice, in which Runx1 was conditionally abrogated in osteoclast precursors (LysM-Cre;Runx1^F/F ; Runx1 cKO). Runx1 cKO and control mice were then subjected to experimental mid-diaphyseal femoral fractures. Our study found differential resorption of bony and calcified cartilage callus matrix by osteoclasts and chondroclasts within Runx1 cKO calluses, with increased early bony callus resorption and delayed calcified cartilage resorption. There was an increased number of osteoclasts and chondroclasts in the chondro-osseous junction of Runx1 cKO calluses starting at day 11 post-fracture, with minimal woven bone occupying the callus at day 18 post-fracture. LysM-Cre;Runx1^F/F mutant mice had increased bone compliance at day 28, but their strength and work to failure were comparable with controls. Taken together, these results indicate that Runx1 is a critical transcription factor in controlling osteoclastogenesis that negatively regulates bone and cartilage resorption in the fracture callus. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 38:1007-1015, 2020.

Targeting Bortezomib to Bone Increases Its Bone Anabolic Activity and Reduces Systemic Adverse Effects in Mice

Bortezomib (Btz) is a proteasome inhibitor approved by the FDA to treat multiple myeloma. It also increases bone volume by promoting osteoblast differentiation and inhibiting osteoclastogenesis in mice. However, Btz has severe systemic adverse effects, which would limit its use as a bone anabolic agent. Here, we designed and synthesized a bone-targeted form of Btz by conjugating it to a bisphosphonate (BP) with no antiresorptive activity. We report that BP-Btz inhibited osteoclast formation and bone resorption and stimulated osteoblast differentiation in vitro similar to Btz. In vivo, BP-Btz increased bone volume more effectively than Btz in three mouse models: untreated wild-type mice, mice with ovariectomy, and aged mice with tibial factures. Importantly, BP-Btz had significantly less systemic side effects than Btz, including less thymic cell death, sympathetic nerve damage, and thrombocytopenia, and it improved survival rates in aged mice. Thus, BP-Btz represents a novel anabolic agent to treat conditions, such as postmenopausal and age-related bone loss. Bone targeting is an attractive approach to repurpose approved drugs to treat skeletal diseases. © 2019 American Society for Bone and Mineral Research. © 2019 American Society for Bone and Mineral Research.

Decreased pericellular matrix production and selection for enhanced cell membrane repair may impair osteocyte responses to mechanical loading in the aging skeleton

Transient plasma membrane disruptions (PMD) occur in osteocytes with in vitro and in vivo loading, initiating mechanotransduction. The goal here was to determine whether osteocyte PMD formation or repair is affected by aging. Osteocytes from old (24 months) mice developed fewer PMD (-76% females, -54% males) from fluid shear than young (3 months) mice, and old mice developed fewer osteocyte PMD (-51%) during treadmill running. This was due at least in part to decreased pericellular matrix production, as studies revealed that pericellular matrix is integral to formation of osteocyte PMD, and aged osteocytes produced less pericellular matrix (-55%). Surprisingly, osteocyte PMD repair rate was faster (+25% females, +26% males) in osteocytes from old mice, and calcium wave propagation to adjacent nonwounded osteocytes was blunted, consistent with impaired mechanotransduction downstream of PMD in osteocytes with fast PMD repair in previous studies. Inducing PMD via fluid flow in young osteocytes in the presence of oxidative stress decreased postwounding cell survival and promoted accelerated PMD repair in surviving cells, suggesting selective loss of slower-repairing osteocytes. Therefore, as oxidative stress increases during aging, slower-repairing osteocytes may be unable to successfully repair PMD, leading to slower-repairing osteocyte death in favor of faster-repairing osteocyte survival. Since PMD are an important initiator of mechanotransduction, age-related decreases in pericellular matrix and loss of slower-repairing osteocytes may impair the ability of bone to properly respond to mechanical loading with bone formation. These data suggest that PMD formation and repair mechanisms represent new targets for improving bone mechanosensitivity with aging.

Emerging role of stem cell-derived extracellular microRNAs in age-associated human diseases and in different therapies of longevity

Organismal aging involves the progressive decline in organ function and increased susceptibility to age-associated diseases. This has been associated with the aging of stem cell populations within the body that decreases the capacity of stem cells to self-renew, differentiate, and regenerate damaged tissues and organs. This review aims to explore how aging is associated with the dysregulation of stem cell-derived extracellular vesicles (SCEVs) and their corresponding miRNA cargo (SCEV-miRNAs), which are short non-coding RNAs involved in post-transcriptional regulation of target genes. Recent evidence has suggested that in aging stem cells, SCEV-miRNAs may play a vital role regulating various processes that contribute to aging: cellular senescence, stem cell exhaustion, telomere length, and circadian rhythm. Hence, further clarifying the age-dependent molecular mechanisms through which SCEV-miRNAs exert their downstream effects may inform a greater understanding of the biology of aging, elucidate their role in stem cell function, and identify important targets for future regenerative therapies. Additionally, current studies evaluating therapeutic role of SCEVs and SCEV-miRNAs in treating several age-associated diseases are also discussed.

PTH1-34 improves bone healing by promoting angiogenesis and facilitating MSCs migration and differentiation in a stabilized fracture mouse model

Objective

PTH_1-34 (parathyroid hormone 1–34) is the only clinical drug to promote osteogenesis. MSCs (mesenchymal stem cells) have multidirectional differentiation potential and are closely related to fracture healing. This study was to explore the effects of PTH_1-34 on proliferation and differentiation of endothelial cells and MSCs in vitro, and on angiogenesis, and MSCs migration during fracture healing in vivo.

Methods

Mice with stabilized fracture were assigned to 4 groups: CON, PTH (PTH_1-34 40 μg/kg/day), MSC (transplanted with 10⁵ MSCs), PTH+MSCs. Mice were sacrificed 14 days after fracture, and callus tissues were harvested for microCT scan and immunohistochemistry analysis. The effects of PTH_1-34 on angiogenesis, and MSCs differentiation and migration were assessed by wound healing, tube formation and immunofluorescence staining.

Results

Treatment with either PTH_1-34, or MSCs promoted bone healing and vascular formation in fracture callus. The callus bone mass, bone volume, and bone mineral density were all greater in PTH and/or MSC groups than they were in CON (p<0.05). PTH_1-34 increased small vessels formation (diameter ≤50μm), whereas MSCs increased the large ones (diameter >50μm). Expression of CD31 within calluses and trabecular bones were significantly higher in PTH_1-34 treated group than that of not (p<0.05). Expression of CD31, VEGFR, VEGFR2, and vWF was upregulated, and wound healing and tube formation were increased in MSCs treated with PTH_1-34 compared to that of control.

Conclusions

PTH_1-34 improved the proliferation and differentiation of endothelial cells and MSCs, enhancing migration of MSCs to bone callus to promote angiogenesis and osteogenesis, and facilitating fracture healing.

Individual Effector/Regulator T Cell Ratios Impact Bone Regeneration

There is increasing evidence that T lymphocytes play a key role in controlling endogenous regeneration. Regeneration appears to be impaired in case of local accumulation of CD8+ effector T cells (T_EFF), impairing endogenous regeneration by increasing a primary “useful” inflammation toward a damaging level. Thus, rescuing regeneration by regulating the heightened pro-inflammatory reaction employing regulatory CD4+ T (T_Reg) cells could represent an immunomodulatory option to enhance healing. Hypothesis was that CD4+ T_Reg might counteract undesired effects of CD8+ T_EFF. Using adoptive T_Reg transfer, bone healing was consistently improved in mice possessing an inexperienced immune system with low amounts of CD8+ T_EFF. In contrast, mice with an experienced immune system (high amounts of CD8+ T_EFF) showed heterogeneous bone repair with regeneration being dependent upon the individual T_EFF/T_Reg ratio. Thus, the healing outcome can only be improved by an adoptive T_Reg therapy, if an unfavorable T_EFF/T_Reg ratio can be reshaped; if the individual CD8+ T_EFF percentage, which is dependent on the individual immune experience can be changed toward a favorable ratio by the T_Reg transfer. Remarkably, also in patients with impaired fracture healing the T_EFF/T_Reg ratio was higher compared to uneventful healers, validating our finding in the mouse osteotomy model. Our data demonstrate for the first time the key-role of a balanced T_EFF/T_Reg response following injury needed to reach successful regeneration using bone as a model system. Considering this strategy, novel opportunities for immunotherapy in patients, which are at risk for impaired healing by targeting T_EFF cells and supporting T_Reg cells to enhance healing are possible.

Cyclooxygenase 2 augments osteoblastic but suppresses chondrocytic differentiation of CD90+ skeletal stem cells in fracture sites

Cyclooxygenase 2 (COX-2) is essential for normal tissue repair. Although COX-2 is known to enhance the differentiation of mesenchymal stem cells (MSCs), how COX-2 regulates MSC differentiation into different tissue-specific progenitors to promote tissue repair remains unknown. Because it has been shown that COX-2 is critical for normal bone repair and local COX-2 overexpression in fracture sites accelerates fracture repair, this study aimed to determine the MSC subsets that are targeted by COX-2. We showed that CD90⁺ mouse skeletal stem cells (mSSCs; i.e., CD45^-Tie2^-AlphaV⁺ MSCs) were selectively recruited by macrophage/monocyte chemoattractant protein 1 into fracture sites following local COX-2 overexpression. In addition, local COX-2 overexpression augmented osteoblast differentiation and suppressed chondrocyte differentiation in CD90⁺ mSSCs, which depended on canonical WNT signaling. CD90 depletion data demonstrated that local COX-2 overexpression targeted CD90⁺ mSSCs to accelerate fracture repair. In conclusion, CD90⁺ mSSCs are promising targets for the acceleration of bone repair.

IGF-1 reverses the osteogenic inhibitory effect of dexamethasone on BMP9-induced osteogenic differentiation in mouse embryonic fibroblasts via PI3K/AKT/COX-2 pathway

Glucocorticoid-Induced Osteoporosis (GIOP) is a prevalent clinical complication caused by large dose administration of glucocorticoids, such as Dexamethasone (Dex) and Prednisone. GIOP may lead to fractures and even Osteonecrosis of the Femoral Head (ONFH). It has been reported that glucocorticoids inhibit osteogenesis via the suppression of osteogenic differentiation in Mesenchymal Stem Cells (MSCs), but the precise mechanism underlying this suppression awaits further investigation. Meanwhile, novel and efficacious therapies are recommended to cope with GIOP. In this study, we demonstrated that Dex had the inhibitory effect on Bone Morphogenetic Protein 9 (BMP9)-induced ALP activities and matrix mineralization in Mouse Embryonic Fibroblasts (MEFs). In addition, the study confirmed that Dex decreased the expression of osteogenic markers such as Runx2 and OPN. However, the inhibitory effect of Dex on these osteogenic markers can be reversed when combined with insulin-like growth factor 1 (IGF-1). Regarding the inhibitory mechanism, we found that the level of AKT and p-AKT can be decreased by Dex and that Ly294002, the PI3K inhibitor, can block the reversal effect of IGF-1. Moreover, the knockdown or inhibition of COX-2 produced similar results to those of Ly294002. Our findings indicated that IGF-1 may reverse the osteogenic inhibitory effect of Dex via PI3K/AKT pathway, which may be associated with the up-regulation of COX-2. This study may provide new clinical management strategy for GIOP cases.

Mesenchymal stem cell-macrophage crosstalk and bone healing

Recent research has brought about a clear understanding that successful fracture healing is based on carefully coordinated cross-talk between inflammatory and bone forming cells. In particular, the key role that macrophages play in the recruitment and regulation of the differentiation of mesenchymal stem cells (MSCs) during bone regeneration has been brought to focus. Indeed, animal studies have comprehensively demonstrated that fractures do not heal without the direct involvement of macrophages. Yet the exact mechanisms by which macrophages contribute to bone regeneration remain to be elucidated. Macrophage-derived paracrine signaling molecules such as Oncostatin M, Prostaglandin E2 (PGE2), and Bone Morphogenetic Protein-2 (BMP2) have been shown to play critical roles; however the relative importance of inflammatory (M1) and tissue regenerative (M2) macrophages in guiding MSC differentiation along the osteogenic pathway remains poorly understood. In this review, we summarize the current understanding of the interaction of macrophages and MSCs during bone regeneration, with the emphasis on the role of macrophages in regulating bone formation. The potential implications of aging to this cellular cross-talk are reviewed. Emerging treatment options to improve facture healing by utilizing or targeting MSC-macrophage crosstalk are also discussed.

Cellular biology of fracture healing

The biology of bone healing is a rapidly developing science. Advances in transgenic and gene-targeted mice have enabled tissue and cell-specific investigations of skeletal regeneration. As an example, only recently has it been recognized that chondrocytes convert to osteoblasts during healing bone, and only several years prior, seminal publications reported definitively that the primary tissues contributing bone forming cells during regeneration were the periosteum and endosteum. While genetically modified animals offer incredible insights into the temporal and spatial importance of various gene products, the complexity and rapidity of healing-coupled with the heterogeneity of animal models-renders studies of regenerative biology challenging. Herein, cells that play a key role in bone healing will be reviewed and extracellular mediators regulating their behavior discussed. We will focus on recent studies that explore novel roles of inflammation in bone healing, and the origins and fates of various cells in the fracture environment. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res.

Influences of donor and host age on human muscle-derived stem cell-mediated bone regeneration

Background

Human muscle-derived stem cells (hMDSCs) have been shown to regenerate bone efficiently when they were transduced with Lenti-viral bone morphogenetic protein 2 (LBMP2). However, whether the age of hMDSCs and the animal host affect the bone regeneration capacity of hMDSCs and mechanism are unknown which prompted the current study.

Methods

We isolated three gender-matched young and old populations of skeletal muscle stem cells, and tested the influence of cells’ age on in vitro osteogenic differentiation using pellet culture before and after Lenti-BMP2/green fluorescent protein (GFP) transduction. We further investigated effects of the age of hMDSCs and animal host on hMDSC-mediated bone regeneration in a critical-size calvarial bone defect model in vivo. Micro-computer tomography (CT), histology, and immunohistochemistry were used to evaluate osteogenic differentiation and mineralization in vitro and bone regeneration in vivo. Western blot, quantitative polymerase chain reaction (PCR), and oxidative stress assay were performed to detect the effects of age of hMDSCs on cell survival and osteogenic-related genes. Serum insulin-like growth factor 1 (IGF1) and receptor activator of nuclear factor-kappa B ligand (RANKL) were measured with an enzyme-linked immunosorbent assay (ELISA).

Results

We found LBMP2/GFP transduction significantly enhanced osteogenic differentiation of hMDSCs in vitro, regardless of donor age. We also found old were as efficient as young LBMP2/GFP-transduced hMDSCs for regenerating functional bone in young and old mice. These findings correlated with lower phosphorylated p38MAPK expression and similar expression levels of cell survival genes and osteogenic-related genes in old hMDSCs relative to young hMDSCs. Old cells exhibited equivalent resistance to oxidative stress. However, both young and old donor cells regenerated less bone in old than young hosts. Impaired bone regeneration in older hosts was associated with high bone remodeling due to higher serum levels of RANKL and lower level of IGF-1.

Conclusion

hMDSC-mediated bone regeneration was not impaired by donor age when hMDSCs were transduced with LBMP2/GFP, but the age of the host adversely affected hMDSC-mediated bone regeneration. Regardless of donor and host age, hMDSCs formed functional bone, suggesting a promising cell resource for bone regeneration.

Electronic supplementary material

The online version of this article (10.1186/s13287-018-1066-z) contains supplementary material, which is available to authorized users.

Changes in ephrin gene expression during bone healing identify a restricted repertoire of ephrins mediating fracture repair

To identify the repertoire of ephrin genes that might regulate endochondral bone fracture repair, we examined changes in ephrin ligand and receptor (Eph) gene expression in fracture callus tissues during bone fracture healing. Ephrin and Eph proteins were then localized in the fracture callus tissues present when changes in gene expression were observed. Ephrin gene expression was widespread in fracture tissues, but the repertoire of ephrin genes with significant changes in expression that might suggest a regulatory role in fracture callus development was restricted to the ephrin A family members Epha4, Epha5 and the ephrin B family member Efnb1. After 3 weeks of healing, Epha4 fracture expression was downregulated from 1.3- to 0.8-fold and Epha5 fracture expression was upregulated from 1.2- to 1.5-fold of intact contralateral femur expression, respectively. Efnb1 expression was downregulated from 1.5- to 1.2-fold after 2 weeks post-fracture. These ephrin proteins were localized to fracture callus prehypertrophic chondrocytes and osteoblasts, as well as to the periosteum and fibrous tissues. The observed positive correlation between mRNA levels of EfnB1 with Col10 and Epha5 with Bglap, together with colocalized expression with their respective proteins, suggest that EfnB1 is a positive mediator of prehypertrophic chondrocyte development and that Epha5 contributes to osteoblast-mediated mineralization of fracture callus. In contrast, mRNA levels of Epha4 and Efnb1 correlated negatively with Bglap, thus suggesting a negative role for these two ephrin family members in mature osteoblast functions. Given the number of family members and widespread expression of the ephrins, a characterization of changes in ephrin gene expression provides a basis for identifying ephrin family members that might regulate the molecular pathways of bone fracture repair. This approach suggests that a highly restricted repertoire of ephrins, EfnB1 and EphA5, are the major mediators of fracture callus cartilage hypertrophy and ossification, respectively, and proposes candidates for additional functional study and eventual therapeutic application.

Bone Marrow-derived Endothelial Progenitor Cells Are Associated with Bone Mass and Strength

OBJECTIVE

Blood vessels of bone are thought to influence osteogenesis of bone. No clinical studies have determined whether angiogenesis is related to bone mass and gene expression of growth factors. We compared bone marrow endothelial progenitor cells (EPC), which control angiogenesis of bone in postmenopausal women incurring fragility fracture, with osteoporosis or traumatic fracture with normal bone mass (COM).

METHODS

Bone specimens were obtained from age-matched women with osteoporosis or COM. Mononuclear cells were isolated and EPC were detected by flow cytometry. The expression levels of specific genes were measured. Bone mineral density (BMD) was determined, and serum markers of bone turnover also were measured. Differences between OP and COM were assessed with Student t test or Mann-Whitney U test, and correlations were determined using Spearman's correlation.

RESULTS

Compared with COM, patients with OP had significantly lower levels of serum osteocalcin, procollagen type-1 N-terminal propeptide, and 25-hydroxy vitamin D, as well as decreased BMD of total hip and femoral neck and fewer bone marrow EPC. Expression levels of vascular endothelial growth factor, angiopoietin-1 (Ang-1), angiopoietin 2 (Ang-2), and the osteoblast-specific genes runt-related transcription factor 2 (RUNX2) and osterix in bone were significantly lower in OP than in COM. We determined that mature EPC were correlated positively with BMD of the femoral neck and total hip, gene expression of Ang-1, RUNX2, and CD31, and negatively with gene expression of receptor activator of nuclear factor-κB ligand and Ang-2.

CONCLUSION

Our results demonstrate correlations of bone marrow EPC with bone mass and gene expression of growth factors, which support a hypothesis of crosstalk between angiogenesis and osteogenesis in bone health.

Ageing: Is there a role for arachidonic acid and other bioactive lipids? A review

Ageing is inevitable. Recent studies suggest that it could be delayed. Low-grade systemic inflammation is seen in type 2 diabetes mellitus, hypertension and endothelial dysfunction that are common with increasing age. In all these conditions, an alteration in arachidonic acid (AA) metabolism is seen in the form of increased formation of pro-inflammatory eicosanoids and decreased production of anti-inflammatory lipoxins, resolvins, protectins and maresins and decreased activity of desaturases. Calorie restriction, exercise and parabiosis delay age-related changes that could be related to enhanced proliferation of stem cells, decrease in inflammation and transfer of GDF-11 (growth differentiation factor-11) and other related molecules from the young to the old, increase in the formation of lipoxin A4, resolvins, protectins and maresins, hydrogen sulfide (H2S) and nitric oxide (NO); inhibition of ageing-related hypothalamic or brain IKK-β and NF-kB activation, decreased gonadotropin-releasing hormone (GnRH) release resulting in increased neurogenesis and consequent decelerated ageing. This suggests that hypothalamus participates in ageing process. N-acylethanolamines (NAEs) and lipid-derived signalling molecules can be tuned favorably under dietary restriction to extend lifespan and/or prevent advanced age associated diseases in an mTOR dependent pathway manner. Sulfur amino acid (SAA) restriction increased hydrogen sulfide (H2S) production and protected tissues from hypoxia and tissue damage. Anti-inflammatory metabolites formed from AA such as LXA4, resolvins, protectins and maresins enhance production of NO, CO, H2S; suppress NF-kB expression and alter mTOR expression and thus, may aid in delaying ageing process. Dietary restriction and exercise enhance AA metabolism to form LXA4, resolvins, protectins and maresins that have anti-inflammatory actions. AA and their metabolites also influence stem cell biology, enhance neurogenesis to improve memory and augment autophagy to prolong life span. Thus, AA and other PUFAs and their anti-inflammatory metabolites inhibit inflammation, augment stem cell proliferation, restore to normal lipid-derived signaling molecules and NO and H2S production, enhance autophagy and prolong life span.

Teriparatide (human PTH1-34) compensates for impaired fracture healing in COX-2 deficient mice

Genetic ablation of cyclooxygenase-2 (COX-2) in mice is known to impair fracture healing. To determine if teriparatide (human PTH_1-34) can promote healing of Cox-2-deficient fractures, we performed detailed in vivo analyses using a murine stabilized tibia fracture model. Periosteal progenitor cell proliferation as well as bony callus formation was markedly reduced in Cox-2^-/- mice at day 10 post-fracture. Remarkably, intermittent PTH_1-34 administration increased proliferation of periosteal progenitor cells, restored callus formation on day 7, and enhanced bone formation on days 10, 14 and 21 in Cox-2-deficient mice. PTH_1-34 also increased biomechanical torsional properties at days 10 or 14 in all genotypes, consistent with enhanced bony callus formation by radiologic examinations. To determine the effects of intermittent PTH_1-34 for callus remodeling, TRAP staining was performed. Intermittent PTH_1-34 treatment increased the number of TRAP positive cells per total callus area on day 21 in Cox-2^-/- fractures. Taken together, the present findings indicate that intermittent PTH_1-34 treatment could compensate for COX-2 deficiency and improve impaired fracture healing in Cox-2-deficient mice.

Preclinical models for orthopedic research and bone tissue engineering

In this review, we broadly define and discuss the preclinical rodent models that are used for orthopedics and bone tissue engineering. These range from implantation models typically used for biocompatibility testing and high-throughput drug screening, through to fracture and critical defect models used to model bone healing and severe orthopedic injuries. As well as highlighting the key methods papers describing these techniques, we provide additional commentary based on our substantive practical experience with animal surgery and in vivo experimental design. This review also briefly touches upon the descriptive and functional outcome measures and power calculations that are necessary for an informative study. Obtaining informative and relevant research outcomes can be very dependent on the model used, and we hope this evaluation of common models will serve as a primer for new researchers looking to undertake preclinical bone studies. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:832-840, 2018.

Bone healing in an aged murine fracture model is characterized by sustained callus inflammation and decreased cell proliferation

Geriatric fractures take longer to heal and heal with more complications than those of younger patients; however, the mechanistic basis for this difference in healing is not well understood. To improve this understanding, we investigated cell and molecular differences in fracture healing between 5-month-old (young adult) and 25-month-old (geriatric) mice healing utilizing high-throughput analysis of gene expression. Mice underwent bilateral tibial fractures and fracture calluses were harvested at 5, 10, and 20 days post-fracture (DPF) for analysis. Global gene expression analysis was performed using Affymetrix MoGene 1.0 ST microarrays. After normalization, data were compared using ANOVA and evaluated using Principal Component Analysis (PCA), CTen, heatmap, and Incromaps analysis. PCA and cross-sectional heatmap analysis demonstrated that DPF followed by age had pronounced effects on changes in gene expression. Both un-fractured and 20 DPF aged mice showed increased expression of immune-associated genes (CXCL8, CCL8, and CCL5) and at 10 DPF, aged mice showed increased expression of matrix-associated genes, (Matn1, Ucma, Scube1, Col9a1, and Col9a3). Cten analysis suggested an enrichment of CD8+ cells and macrophages in old mice relative to young adult mice and, conversely, a greater prevalence of mast cells in young adult mice relative to old. Finally, consistent with the PCA data, the classic bone healing pathways of BMP, Indian Hedgehog, Notch and Wnt clustered according to the time post-fracture first and age second.

CLINICAL SIGNIFICANCE

Greater understanding of age-dependent molecular changes with healing will help form a mechanistic basis for therapies to improve patient outcomes. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:149-158, 2018.