The effect of age on gene expression in adult and juvenile rats following femoral fracture

Effect of age on biomaterial-mediated in situ bone tissue regeneration

Emerging studies show the potential application of synthetic biomaterials that are intrinsically osteoconductive and osteoinductive as bone grafts to treat critical bone defects. Here, the biomaterial not only assists recruitment of endogenous cells, but also supports cellular activities relevant to bone tissue formation and function. While such biomaterial-mediated in situ tissue engineering is highly attractive, success of such an approach relies largely on the regenerative potential of the recruited cells, which is anticipated to vary with age. In this study, we investigated the effect of the age of the host on mineralized biomaterial-mediated bone tissue repair using critical-sized cranial defects as a model system. Mice of varying ages, 1-month-old (juvenile), 2-month-old (young-adult), 6-month-old (middle-aged), and 14-month-old (elderly), were used as recipients. Our results show that the bio-mineralized scaffolds support bone tissue formation by recruiting endogenous cells for all groups albeit with differences in an age-related manner. Analyses of bone tissue formation after 2 and 8 weeks post-treatment show low mineral deposition and reduced number of osteocalcin and tartrate-resistant acid phosphatase (TRAP)-expressing cells in elderly mice.

STATEMENT OF SIGNIFICANCE

Tissue engineering strategies that promote tissue repair through recruitment of endogenous cells will have a significant impact in regenerative medicine. Previous studies from our group have shown that biomineralized materials containing calcium phosphate minerals can contribute to neo-bone tissue through recruitment and activation of endogenous cells. In this study, we investigated the effect of age of the recipient on biomaterial-mediated bone tissue repair. Our results show that the age of the recipient mouse had a significant impact on the quality and quantity of the engineered neo-bone tissues, in which delayed/compromised bone tissue formation was observed in older mice. These findings are in agreement with the clinical observations that age of patients is a key factor in bone repair.

The Effects of Age and Dose on Gene Expression and Segmental Bone Defect Repair After BMP-2 Delivery

Age is a well‐known influential factor in bone healing, with younger patients generally healing bone fractures more rapidly and suffering fewer complications compared with older patients. Yet the impact age has on the response to current bone healing treatments, such as delivery of bone morphogenetic protein 2 (BMP‐2), remains poorly characterized. It remains unclear how or if therapeutic dosing of BMP‐2 should be modified to account for age‐related differences in order to minimize potential adverse effects and consequently improve patient bone‐healing outcomes. For this study, we sought to address this issue by using a preclinical critically sized segmental bone defect model in rats to investigate age‐related differences in bone repair after delivery of BMP‐2 in a collagen sponge, the current clinical standard. Femoral defects were created in young (7‐week‐old) and adult (8‐month‐old) rats, and healing was assessed using gene expression analyses, longitudinal radiography, ex vivo micro‐computed tomography (µCT), as well as torsional testing. We found that young rats demonstrated elevated expression of genes related to osteogenesis, chondrogenesis, and matrix remodeling at the early 1‐week time point compared with adult rats. These early gene expression differences may have impacted long‐term healing as the regenerated bones of young rats exhibited higher bone mineral densities compared with those of adult rats after 12 weeks. Furthermore, the young rats demonstrated significantly more bone formation and increased mechanical strength when BMP‐2 dose was increased from 1 µg to 10 µg, a finding not observed in adult rats. Overall, these results indicate there are age‐related differences in BMP‐2‐mediated bone regeneration, including relative dose sensitivity, suggesting that age is an important consideration when implementing a BMP‐2 treatment strategy. © 2018 The Authors JBMR Plus published by Wiley Periodicals, Inc. on behalf of American Society for Bone and Mineral Research.

Pigs are useful for the molecular study of bone inflammation and regeneration in humans

Pigs are used with increased frequency to model different kinds of orthopedic surgical conditions. In order to show the full potential of porcine models in orthopedic research, it is therefore required to examine the expression of bone regulatory genes in pigs affected by orthopedic surgery and compare it to the expression in humans and mice as mice, are one of the most applied animal species in orthopedics today. In the present study, the local molecular response to drilling of a tibial implant cavity, and the subsequent insertion of a steel implant was examined in a porcine model. Pigs were euthanized five days after drilling of the bone. The molecular response of 73 different genes was analyzed using a high-throughput quantitative polymerase chain reaction platform and compared to histopathology. Histologically, it was found that bone remodeling was initiated on day 5 after surgery and was associated with upregulation of several genes involved in bone degradation and formation ( CTSK, ACP5, IBSP, RANK, RANKL and COL1A1). Interleukin-6 and several acute-phase proteins (C3, SAA and ITIH4) were significantly upregulated, indicating their importance in the initial process of healing and osseointegration. All tested bone morphogenic proteins (BMP2, -4 and -7) including their inhibitor noggin were also significantly upregulated. Surprisingly, vascular endothelial growth factor A was not found to be regulated five days after surgery while several other vascular growth factors (ANGPT1, ANGPT2 and PTN) were upregulated. The pig was found to be a useful model for elucidation of bone regulatory genes in humans.

Effects of Aging on Fracture Healing

PURPOSE OF REVIEW

This review summarizes research on the physiological changes that occur with aging and the resulting effects on fracture healing.

RECENT FINDINGS

Aging affects the inflammatory response during fracture healing through senescence of the immune response and increased systemic pro-inflammatory status. Important cells of the inflammatory response, macrophages, T cells, mesenchymal stem cells, have demonstrated intrinsic age-related changes that could impact fracture healing. Additionally, vascularization and angiogenesis are impaired in fracture healing of the elderly. Finally, osteochondral cells and their progenitors demonstrate decreased activity and quantity within the callus. Age-related changes affect many of the biologic processes involved in fracture healing. However, the contributions of such changes do not fully explain the poorer healing outcomes and increased morbidity reported in elderly patients. Future research should address this gap in understanding in order to provide improved and more directed treatment options for the elderly population.

Bioinformatics analysis of time-series genes profiling to explore key genes affected by age in fracture healing

The present study was aimed to explore possible key genes and bioprocess affected by age during fracture healing. GSE589, GSE592 and GSE1371 were downloaded from gene expression omnibus database. The time-series genes of three age levels rats were firstly identified with hclust function in R. Then functional and pathway enrichment analysis for selected time-series genes were performed. Finally, the VennDiagram package of R language was used to screen overlapping n time-series genes. The expression changes of time-series genes in the rats of three age levels were classified into two types: one was higher expressed at 0 day, decreased at 3 day to 2 week, and increased from 4 to 6 week; the other was the opposite. Functional and pathways enrichment analysis showed that 12 time-series genes of adult and old rats were significantly involved in ECM-receptor interaction pathway. The expression changes of 11 genes were consistent with time axis, 10 genes were up-regulated at 3 days after fracture, and increased slowly in 6 week, while Itga2b was down-regulated. The functions of 106 overlapping genes were all associated with growth and development of bone after fracture. The key genes in ECM-receptor interaction pathway including Spp1, Ibsp, Tnn and Col3a1 have been reported to be related to fracture in literatures. The difference during fracture healing in three age levels rats is mainly related to age. The Spp1, Ibsp, Tnn and Col3a1 are possible potential age-related genes and ECM-receptor interaction pathway is the potential age-related process during fracture healing.

Current insights on the regenerative potential of the periosteum: molecular, cellular, and endogenous engineering approaches

While century old clinical reports document the periosteum's remarkable regenerative capacity, only in the past decade have scientists undertaken mechanistic investigations of its regenerative potential. At a Workshop at the 2012 Annual Meeting of Orthopaedic Research Society, we reviewed the molecular, cellular, and tissue scale approaches to elucidate the mechanisms underlying the periosteum's regenerative potential as well as translational therapies engineering solutions inspired by its remarkable regenerative capacity. The entire population of osteoblasts within periosteum, and at endosteal and trabecular bone surfaces within the bone marrow, derives from the embryonic perichondrium. Periosteal cells contribute more to cartilage and bone formation within the callus during fracture healing than do cells of the bone marrow or endosteum, which do not migrate out of the marrow compartment. Furthermore, a current healing paradigm regards the activation, expansion, and differentiation of periosteal stem/progenitor cells as an essential step in building a template for subsequent neovascularization, bone formation, and remodeling. The periosteum comprises a complex, composite structure, providing a niche for pluripotent cells and a repository for molecular factors that modulate cell behavior. The periosteum's advanced, "smart" material properties change depending on the mechanical, chemical, and biological state of the tissue. Understanding periosteum development, progenitor cell-driven initiation of periosteum's endogenous tissue building capacity, and the complex structure-function relationships of periosteum as an advanced material are important for harnessing and engineering ersatz materials to mimic the periosteum's remarkable regenerative capacity.

Alcohol induced epigenetic perturbations during the inflammatory stage of fracture healing

It is well recognized by orthopedic surgeons that fractures of alcoholics are more difficult to heal successfully and have a higher incidence of non-union, but the mechanism of alcohol's effect on fracture healing is unknown. In order to give direction for the study of the effects of alcohol on fracture healing, we propose to identify gene expression and microRNA changes during the early stages of fracture healing that might be attributable to alcohol consumption. As the inflammatory stage appears to be the most critical for successful fracture healing, this paper focuses on the events at day three following fracture or the stage of inflammation. Sprague-Dawley rats were placed on an ethanol-containing or pair-fed Lieber and DeCarli diet for four weeks prior to surgical fracture. Following insertion of a medullary pin, a closed mid-diaphyseal fracture was induced using a Bonnarens and Einhorn fracture device. At three days' post-fracture, the region of the fracture calluses was harvested from the right hind-limb. RNA was extracted and microarray analysis was conducted against the entire rat genome. There were 35 genes that demonstrated significant increased expression due to alcohol consumption and 20 that decreased due to alcohol. In addition, the expression of 20 microRNAs was increased and six decreased. In summary, while it is recognized that mRNA levels may or may not represent protein levels successfully produced by the cell, these studies reveal changes in gene expression that support the hypothesis that alcohol consumption affects events involved with inflammation. MicroRNAs are known to modulate mRNA and these findings were consistent with much of what was seen with mRNA microarray analysis, especially the involvement of smad4 which was demonstrated by mRNA microarray, microRNA and polymerase chain reaction.

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

BACKGROUND

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

METHODOLOGY/PRINCIPAL FINDINGS

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

CONCLUSIONS

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

Production of VEGF receptor 1 and 2 mRNA and protein during endochondral bone repair is differential and healing phase specific

Physiological disturbances, including temporary hypoxia, are expected to drive angiogenesis during bone repair. Evidence suggests that the angiogenic ligand vascular endothelial growth factor (VEGF)-A plays an important role in this process. We characterized the expression of two receptors that are essential for mediating VEGF signaling, VEGFR1/Flt-1 and VEGFR2/Flk-1/KDR, in a mouse rib fracture model. Their mRNA and protein levels were assessed in four healing phases, which were characterized histologically as hemorrhage formation on postfracture day (PFD) 1, inflammatory response on PFD 3, initiation of callus development on PFD 7, and the presence of a mature callus on PFD 14. Transcript was detected for VEGFR1 and VEGFR2, as well as VEGF. While mRNA expression of VEGFR1 was monophasic throughout all healing phases, VEGFR2 showed a biphasic profile with significantly increased mRNA expression during callus formation and maturation. Expression of VEGF mRNA was characterized by a more gradual increase during callus formation. The protein level for VEGFR1 was below detection sensitivity during the initial healing phase. It was then restored to a stable level, detectable through the subsequent healing phases. Hence, the VEGFR1 protein levels partially mirrored the transcript expression profile. In comparison, the protein level of VEGFR2 increased gradually during the healing phases and peaked at callus maturation. This correlated well with the transcriptional expression of VEGFR2. Intact bone from age-matched male mice had considerable protein levels of VEGFR1 and VEGF, but no detectable VEGFR2. Together, these findings uncovered expression signatures of the VEGF-VEGFR axis in endochondral bone repair.

Rejuvenation of the inflammatory system stimulates fracture repair in aged mice

Age significantly reduces the regenerative capacity of the skeleton, but the underlying causes are unknown. Here, we tested whether the functional status of inflammatory cells contributes to delayed healing in aged animals. We created chimeric mice by bone marrow transplantation after lethal irradiation. In this model, chondrocytes and osteoblasts in the regenerate are derived exclusively from host cells while inflammatory cells are derived from the donor. Using this model, the inflammatory system of middle-aged mice (12 month old) was replaced by transplanted bone marrow from juvenile mice (4 weeks old), or age-matched controls. We found that the middle-aged mice receiving juvenile bone marrow had larger calluses and more bone formation during early stages and faster callus remodeling at late stages of fracture healing, indicating that inflammatory cells derived from the juvenile bone marrow accelerated bone repair in the middle-aged animals. In contrast, transplanting bone marrow from middle-aged mice to juvenile mice did not alter the process of fracture healing in juvenile mice. Thus, the roles of inflammatory cells in fracture healing may be age-related, suggesting the possibility of enhancing fracture healing in aged animals by manipulating the inflammatory system.

Transcriptional analysis of fracture healing and the induction of embryonic stem cell-related genes

Fractures are among the most common human traumas. Fracture healing represents a unique temporarily definable post-natal process in which to study the complex interactions of multiple molecular events that regulate endochondral skeletal tissue formation. Because of the regenerative nature of fracture healing, it is hypothesized that large numbers of post-natal stem cells are recruited and contribute to formation of the multiple cell lineages that contribute to this process. Bayesian modeling was used to generate the temporal profiles of the transcriptome during fracture healing. The temporal relationships between ontologies that are associated with various biologic, metabolic, and regulatory pathways were identified and related to developmental processes associated with skeletogenesis, vasculogenesis, and neurogenesis. The complement of all the expressed BMPs, Wnts, FGFs, and their receptors were related to the subsets of transcription factors that were concurrently expressed during fracture healing. We further defined during fracture healing the temporal patterns of expression for 174 of the 193 genes known to be associated with human genetic skeletal disorders. In order to identify the common regulatory features that might be present in stem cells that are recruited during fracture healing to other types of stem cells, we queried the transcriptome of fracture healing against that seen in embryonic stem cells (ESCs) and mesenchymal stem cells (MSCs). Approximately 300 known genes that are preferentially expressed in ESCs and approximately 350 of the known genes that are preferentially expressed in MSCs showed induction during fracture healing. Nanog, one of the central epigenetic regulators associated with ESC stem cell maintenance, was shown to be associated in multiple forms or bone repair as well as MSC differentiation. In summary, these data present the first temporal analysis of the transcriptome of an endochondral bone formation process that takes place during fracture healing. They show that neurogenesis as well as vasculogenesis are predominant components of skeletal tissue formation and suggest common pathways are shared between post-natal stem cells and those seen in ESCs.

Remodeling of fracture callus in mice is consistent with mechanical loading and bone remodeling theory

During the remodeling phase of fracture healing in mice, the callus gradually transforms into a double cortex, which thereafter merges into one cortex. In large animals, a double cortex normally does not form. We investigated whether these patterns of remodeling of the fracture callus in mice can be explained by mechanical loading. Morphologies of fractures after 21, 28, and 42 days of healing were determined from an in vivo mid-diaphyseal femoral osteotomy healing experiment in mice. Bone density distributions from microCT at 21 days were converted into adaptive finite element models. To assess the effect of loading mode on bone remodeling, a well-established remodeling algorithm was used to examine the effect of axial force or bending moment on bone structure. All simulations predicted that under axial loading, the callus remodeled to form a single cortex. When a bending moment was applied, dual concentric cortices developed in all simulations, corresponding well to the progression of remodeling observed experimentally and resulting in quantitatively comparable callus areas of woven and lamellar bone. Effects of biological differences between species or other reasons cannot be excluded, but this study demonstrates how a difference in loading mode could explain the differences between the remodeling phase in small rodents and larger mammals.

Effect of age on vascularization during fracture repair

Age affects fracture repair; however, the underlying mechanisms are not well understood. The goal of this study was to assess the effects that age has on vascularization during fracture healing. Tibial fractures were created in juvenile (4-week-old), middle-aged (6-month-old), and elderly (18-month-old) mice. The length density and surface density of blood vessels within fracture calluses were analyzed using stereology at 7 days after fracture. The expression of molecules that regulate vascular invasion of the fracture callus was also compared among the three age groups by immunohistochemistry and in situ hybridization. At 7 days after fracture, juvenile mice had a higher surface density of blood vessels compared to the middle-aged and elderly. Hypoxia-inducible factor-1 alpha protein and transcripts of vascular endothelial growth factor were detected at 3 days postinjury in juvenile but not middle-aged and elderly mice. Stronger Mmp-9 and -13 expression was detected in fracture calluses at day 7 in the juvenile compared to the middle-aged and elderly mice. At 21 days postfracture, expression of both Mmps was more robust in the elderly than juvenile and middle-aged animals. These data indicate that age affects vascularization during fracture repair, and the changes we observed are directly correlated with altered expression of biochemical factors that regulate the process of angiogenesis. However, whether the increased vascularization is the cause or result of accelerated bone repair in juvenile animals remains unknown. Nonetheless, our results indicate that enhancing vascularization during fracture repair in the elderly may provide unique therapeutic opportunities.

Changes in mRNA gene expression during growth in the femoral head of the young rat

The rate of physeal growth slows as an animal matures with changes in mRNA gene expression due to the altered cellular activity. To measure the change in gene expression during the juvenile growth period, the femoral head, enclosing the proximal femoral physis, primary spongiosa, and articular cartilage, was collected from both femora of 16 female Sprague-Dawley rats between 4 and 10 weeks of age. One femur of each rat had had a mid-diaphyseal femoral fracture at 4 weeks of age. RNA was extracted and hybridized to 16 Affymetrix Rat Genomic 230 2.0 GeneChip microarrays with probe sets for 31,000 genes of which 18,200 were expressed. Of these, 8002 genes had a significant change in gene expression during growth, about half increasing and half decreasing. These changes included up-regulation with time of genes related to cartilage, blood vessels, osteoprotegerin, osteomodulin, and most ribosomal proteins. There was down-regulation with maturity of genes related to bone, growth-promoting cytokines, G proteins, GTPase-mediated signal transduction factors, cytokine receptors, mitosis, integrin-linked kinase, and the cytoskeleton. In summary, the slowing of growth with maturity was associated with changes in mRNA gene expression in the femoral head for a large number of genes. These changes in gene expression between young and mature rats suggest factors which are important for the support of the rapid linear growth during early life.

Genes with greater up-regulation in the fracture callus of older rats with delayed healing

The rate of bone formation to bridge a fracture gap slows with age. To explore potential pathogenic mechanisms and possible negative-feedback responses by the skeleton to this reduced rate of healing, mRNA transcripts up-regulated more and/or longer were studied in older rats with delayed healing. Female rats at 6 (young), 26 (adult), and 52 (old) weeks of age received unilateral diaphyseal femoral fractures with intramedullary rod stabilization. At 0, 0.4, 1, 2, 4, and 6 weeks after fracture, the fracture site was harvested. Total RNA was extracted, cRNA was prepared, and the cRNA was hybridized to 54 Affymetrix U34A microarrays (three arrays/age/time point). Transcripts for 180 genes were identified as up-regulated more and/or longer in old rats with delayed fracture healing. Of these, 60 were selected for more intense review. Significantly more and/or longer expression was seen in genes related to myofibroblasts, cell proliferation, calcification inhibition, TGF-beta activity, lipid metabolism, cell adhesion, and the cytoskeleton. Further study is needed to determine if these up-regulated transcripts are related to the pathological processes which slow healing or are related to attempts by the fracture tissue to stimulate bone to bridge the fracture gap.

Repair of segmental bone defects in rabbit tibiae using a complex of beta-tricalcium phosphate, type I collagen, and fibroblast growth factor-2

The objective of this study was to evaluate the effects of a complex of beta-tricalcium phosphate (beta-TCP) granules, collagen, and fibroblast growth factor-2 (FGF-2) on cortical bone repair in rabbits. Segmental bone defects of 5 mm in length were created in the middle of the tibial shaft. The defect was stabilized with a plate and screws, and was filled with 0.3 ml of a complex of beta-TCP granules and 5% collagen, with or without 200 microg of recombinant human fibroblast growth factor-2 (rhFGF-2). Bone regeneration and beta-TCP resorption were assessed by X-ray and micro-CT scanner. A three-point bending test was also performed. The results showed that the segmental bone defect was not only radiologically, but also mechanically healed with cortical bone 12 weeks after implantation of the complex with rhFGF-2. In contrast, after implantation of the complex without rhFGF-2, most of the defect was filled with beta-TCP and only a small amount of bone formation was found. These results suggest that resorption of beta-TCP is important for bone formation and may be promoted by FGF-2 in the beta-TCP implantation site. In addition, the complex of beta-TCP granules and collagen combined with rhFGF-2 provides a paste-like material that is easy to handle. This material may be of considerable use in the treatment of cortical bone defects.

Altered expression of mitochondrial genes in response to fracture in old rats

BACKGROUND

Old rats require more time for bone to bridge a fracture gap than young rats. To explore possible mitochondrial dysfunction in this delay, we measured levels of mRNA derived from mitochondrial genes in healing fractures of young, adult, and old rats.

METHODS

Diaphyseal femoral fractures were induced in female rats at 6, 26, and 52 weeks of age (young, adult, and old rats, respectively). At baseline, at 3 days, and 1, 2, 4, and 6 weeks after fracture, the fracture site was harvested. Total RNA was extracted, and cRNA was prepared and hybridized to 54 Affymetrix U34A microarrays (2 rats/array and 3 arrays/age/time point).

RESULTS

Radiographic union occurred progressively later with age. Of the 107 mitochondria-related genes reviewed, all 8 located within the mitochondrial DNA and some nuclear genes (of the electron transport chain and tricarboxylic acid cycle) showed a prolonged reduction in gene expression after fracture in the oldest rats, to about half the level of expression detected in young rats at 6 weeks after fracture. The transcript levels of apoptotic genes increased after fracture in rats of all three ages, while the transcript levels of 23 mitochondriarelated genes were largely unaltered after fracture.

INTERPRETATION

Delayed fracture healing in old rats is associated with reduced mRNA expression of genes forming the mitochondrial energy pathways.

Fracture healing in the elderly patient

Clinical experience gives rise to the impression that there are differences in fracture healing in different age groups. It is evident that fractures heal more efficiently in children than in adults. However, minimal objective knowledge exists to evaluate this assumption. Temporal, spatial, and cellular quantitative and qualitative interrelationships, as well as signaling molecules and extracellular matrix have not been comprehensively and adequately elucidated for fracture healing in the geriatric skeleton. The biological basis of fracture healing will provide a context for revealing the pathophysiology of delayed or even impaired bone regeneration in the elderly. We will summarize experimental studies on age-related changes at the cellular and molecular level that will add to the pathophysiological understanding of the compromised bone regeneration capacity believed to exist in the elderly patient. We will suggest why this understanding would be useful for therapeutics focused on bone regeneration, in particular fracture healing at an advanced age.

Young, adult, and old rats have similar changes in mRNA expression of many skeletal genes after fracture despite delayed healing with age

Genes active in fracture healing are not well understood. Because age slows skeletal repair, the change in gene expression between animals of differing ages may illuminate novel pathways important to this healing response. To explore this, 6-, 26-, and 52-week-old female Sprague-Dawley rats were subjected to mid-diaphyseal femoral fracture with intramedullary fixation. The fracture callus was collected at 0, 0.4 (3 days), 1, 2, 4, or 6 weeks after fracture. RNA was extracted and pooled between two animals for each sample. Three samples were done for each time point for each age for a total of 54 Affymetrix U34A GeneChip microarrays. Of the 8700 genes on each array, 3300 were scored as present. Almost all of these genes were affected by femoral fracture with either upregulation or downregulation in the 6 weeks after fracture. Upregulated genes included markers for matrix genes for both cartilage and bone, osteoblasts, osteocytes, osteoclasts, fibroblasts, and mast cells. Downregulated genes included genes related to blood cell synthesis. Nearly all genes presently associated with bone metabolism showed the same response to fracture healing regardless of the age of the animal. In conclusion, skeletal fracture led to similar changes in RNA expression for most skeletal genes despite the delay in the formation of bone to bridge the fracture gap in old rats. Defects in the healing of skeletal trauma in older rats may lie in systems not normally studied by skeletal biologists.

Superior Effect of MD05, Beta-Tricalcium Phosphate Coated With Recombinant Human Growth/Differentiation Factor-5, Compared to Conventional Bone Substitutes in the Rat Calvarial Defect Model

BACKGROUND

MD05 consists of beta-tricalcium phosphate (beta-TCP) coated with recombinant human growth/differentiation factor-5 (rhGDF-5) and is under evaluation as an osteoinductive and osteoconductive bone graft material for use in dental and maxillofacial applications. The objective of this study was to compare the bone regenerative properties of MD05 with those of conventional commercially available bone substitutes.

METHODS

Full-thickness, 6-mm diameter, calvarial critical-size defects (two per animal) were created in adult Sprague-Dawley rats. Groups of rats were implanted with the following: 1) MD05; 2) bovine bone mineral; 3) bovine bone mineral with collagen; 4) bovine bone mineral with synthetic peptide, 5) beta-TCP (from two different manufacturers); or 6) no filling material (sham controls). Blinded macroscopic analysis, histopathologic analysis, and histomorphometric analysis were carried out 6 weeks after implantation.

RESULTS

New bone formation assessed histomorphometrically was about five times greater with MD05 than with the other bone substitutes tested, and bone repair was well advanced in MD05-filled defects after 6 weeks. The extent of fibrous tissue and residual implant were significantly lower in the MD05 group. In contrast to the other materials, the use of MD05 was associated with the complete osseous bridging of the defect and with the presence of normal bone marrow. The osteoinductive effect of rhGDF-5 was apparent from the more pronounced bone ingrowth observed with MD05 compared to the beta-TCP carrier alone. All implants showed good biocompatibility.

CONCLUSION

MD05 achieved superior bone regeneration compared to conventional materials and is a promising new bone substitute for dental and maxillofacial applications.

Bone marrow stromal cells of young and adult rats respond similarly to platelet-released supernatant and bone morphogenetic protein-6 in vitro

BACKGROUND

Age-related changes in periodontal bone regeneration, osseointegration of dental implants, and graft consolidation are increasingly considered in treatment planning. This study was intended to show whether aging is associated with a diminished responsiveness of osteoprogenitor cells to growth and differentiation factors.

METHODS

We compared the capacity of bone marrow stromal cells harvested from young and adult rats to proliferate, migrate, and differentiate into the osteogenic lineage following exposure to platelet-released supernatant (PRS) or bone morphogenetic protein-6 (BMP-6). Bone marrow stromal cells were isolated from 12 young rats aged 6 weeks and 12 adult rats aged 9 months. Proliferation was assessed by 3[H]thymidine incorporation, migration was evaluated with the Boyden chamber assay, and osteogenic differentiation was deduced from alkaline phosphatase activity.

RESULTS

Irrespective of the donor age, bone marrow stromal cells showed increased mitogenic activity and chemotactic motility when exposed to PRS. Adult bone marrow stromal cells had higher alkaline phosphatase activities at baseline and upon incubation with BMP-6 than cells obtained from young animals. There was no difference between the two groups in the slope of the alkaline phosphatase activity curve following stimulation with BMP-6.

CONCLUSIONS

The data demonstrate that, irrespective of their age, bone marrow stromal cells respond similarly to PRS and BMP-6 under in vitro conditions. These findings suggest that osteoprogenitor cells within the bone marrow of adult rats retain their juvenile potential to respond to growth and differentiation factors, which are released naturally or are applied therapeutically at sites of bone regeneration.

Analysis of fracture healing by large-scale transcriptional profile identified temporal relationships between metalloproteinase and ADAMTS mRNA expression

The aim of this study was to validate the use of transcriptional profiling as a means of characterizing the complex interactions of the thousands of genes that are expressed during fracture healing. Standard mid-diaphyseal tibia fractures were generated in C57/B6 murine tibiae and the transcriptional expression of approximately 13,000 genes was assessed. Three time points after fracture were assessed: day 3, representative of the inflammatory phase; day 10, representative of the peak of cartilage formation; and day 21, representative of the period of primary bone formation and coupled remodeling. A self-organizing mapping approach of the data revealed the temporal relationships between the expression of mRNAs for extracellular matrix proteins and the proteases that degrade the proteoglycan and collagenous matrices. A broad group of extracellular matrix protein mRNAs representative of basement membranes, blood vessels and cartilage all showed elevated expression over the first 21 days of fracture healing. The sorting of the data identified an orderly temporal expression of the metalloproteinases and ADAMTS during the progression of fracture healing with (MMP2/MMP14/TIMP2) and ADAMTS4 and 15 preceding the expression of (MMP9/MMP13). Based on their patterns of expression, relative to the known activities of the encoded proteolytic enzymes, our results suggest that the dissolution of cartilage protoeglycans proceeds before the underlying collagenous components of the matrix are removed. The exclusion of several mRNAs that are normally expressed by osteoclasts in the profiles of mRNAs from days 3 and 10 suggests that osteoclastic activity was largely absent during the early periods of cartilage tissue formation and that proteoglycan and specific collagenase activities, precedes or is prerequisite to later osteoclast infiltration into the remodeling tissues.

Recombinant human BMP-7 effectively prevents non-union in both young and old rats

The purpose of this study was to evaluate the influence of age on the effectiveness of rhBMP-7 treatment in a fracture with severe periosteal damage that is known to result in non-union formation. Closed stabilized femur fractures were produced in 3-month-old and 18-month-old rats. The fracture site was exposed and 2 mm of the periosteum cauterized circumferentially to impair normal fracture healing. The cauterized fracture site was immediately treated with either 100 microg rhBMP-7 (BMP-7 group), or with 25 microL of vehicle alone (control group). Fracture healing was evaluated with radiographs taken at 3 and 6 weeks. Animals were sacrificed at 3 and 6 weeks and specimens subjected to biomechanical and histological evaluation. In both age groups, none of the control animals healed throughout the 6 weeks experimental duration. All of the rhBMP-7-treated 3-month-old animals were radiographically healed at 3 weeks. In comparison, only 56% (9/16) of the rhBMP-7-treated 18-month-old animals were radiographically healed at 3 weeks. At 6 weeks, however, all of the 18-month-old rhBMP-7-treated animals had healed. Histology revealed slower healing in the 18-month-old animals. Treatment with rhBMP-7 significantly increased all of the biomechanical properties in both age groups. In the 3-month-old animals the mechanical strength approached that of the intact femur at 3 weeks, while in the 18-month-old animals this did not occur until 6 weeks. In conclusion, rhBMP-7 can effectively stimulate fracture repair in both young (3-month-old) and old (18-month-old) rats. However, the effect of rhBMP-7 on the rate of fracture healing is greater in young rats compared to old rats.

Gene expression during fracture healing in rats comparing intramedullary fixation to plate fixation by DNA microarray

OBJECTIVE

This study was designed to compare mRNA gene expression in healing diaphyseal femoral fractures between those injuries treated with intramedullary nails and those treated with internal plate fixation.

DESIGN

RNA gene expression was measured at 1 day, 3 days, and 1, 2, 4, and 6 weeks after surgery in the fracture callus of rats randomized to femoral shaft fracture with intramedullary nail fixation, rigid plate fixation, or sham fracture.

SETTING

AAALAC-accredited vivarium of an independent academic medical center.

ANIMALS

Fifty-seven, adult, female, Sprague-Dawley rats at 16 weeks of age.

INTERVENTION

Femoral fracture with intramedullary nail fixation, femoral fracture with plate and screw fixation, or sham surgery with no fracture.

MAIN OUTCOME MEASUREMENTS

RNA expression for 8700 genes was measured with 19 Affymetrix U34A microarrays. The fracture callus was significantly larger with intramedullary nail fixation than with plate fixation. Most genes responded to fracture with a change in mRNA expression. Most of the responding genes followed the same time course for both fixation methods. This included genes related to growth factors, bone matrix, mast cells, most nerve factors, and hematopoiesis. The intramedullary nail group had significantly greater up-regulation for transcripts related to cartilage, cell division, inflammation, and the acetylcholine receptor. There was significantly greater up-regulation in the plate group for genes related to macrophage activity.

CONCLUSIONS

There were differentially expressed genes present between the 2 surgical groups that may give insight into the control of fracture repair.

Comparison of BMP-2 and -4 for rat mandibular bone regeneration at various doses1

OBJECTIVE

To compare mandibular bone regeneration with bone morphogenetic proteins-2 and -4 (BMP-2 and -4) at varying doses.

STUDY DESIGN

Defects were created in the left hemi-mandibles of 82 Sprague-Dawley rats. The defects were filled with a hyaluronic acid polymer loaded with 0.01, 0.1, 1, or 10 microg of BMP-2 or -4. Control groups consisted of animals with unfilled defects, or with defects filled with the hyaluronic acid sponges loaded with growth factor dilution buffer. Animals were killed after 8 weeks, and the hemi-mandibles were analyzed histologically using stereologic techniques.

RESULTS

Mandibles implanted with carriers containing 10 microg of BMP-2 or -4 differed significantly from controls in terms of new bone area (p = 0.01 and p = 0.0001, respectively). Marrow space development occurred in a dose-dependent fashion (p < 0.0001 for both growth factors), and this effect was more pronounced for BMP-2 at larger doses (p < 0.0001 at 1 and 10 microg doses). New bone areas and volumes did not differ significantly between the growth factors. While defects implanted with BMP-4 tended to have thicker cortical bone and more trabecular bone, at least partial defect bridging was achieved in a greater number of defects implanted with BMP-2 (47%) than with BMP-4 (35%).

CONCLUSION

Although similar areas and volumes of new bone were induced with BMP-2 and -4, differences were noted in the quality of bone generated with each growth factor. The results indicate a threshold dose for acute administration between 1 and 10 mug BMP-2 for bony union in this model, and > or =10 microg for BMP-4.

SIGNIFICANCE

These findings suggest that differences in bone growth factor osteogenic potential deserve further study and may have an impact on the translation of osteoinductive protein therapy into clinical practice.

Cellular basis for age-related changes in fracture repair

The goal of this work was to define cellular and molecular changes that occur during fracture healing as animals age. We compared the molecular, cellular, and histological progression of skeletal repair in juvenile (4 weeks old), middle-aged (6 months old), and elderly (18 months old) mice at 3, 5, 7, 10, 14, 21, 28, and 35 days post-fracture, using a non-stabilized tibia fracture model. Our histological and molecular analyses demonstrated that there was a sharp decline in fracture healing potential between juvenile and middle-aged animals, while a more subtle decrease in healing potential was apparent between middle-aged and elderly mice. By three days after fracture, chondrocytes expressing Collagen type II, and osteoblasts expressing osteocalcin, were present in calluses of juvenile, but not middle-aged or elderly, mice. At day 5 immature chondrocytes and osteoblasts were observed in calluses of middle-aged and elderly mice. While at this time, chondrocytes in juvenile mice were expressing Collagen type X (ColX) indicating that chondrocyte maturation was already underway. At day 7, chondrocytes expressing ColX were abundant in middle-aged mice while a small domain of ColX-positive chondrocytes were observed in elderly mice. Further, in juvenile and middle-aged mice, but not elderly mice, vascular invasion of the cartilage was underway by day 7. Juvenile mice had replaced nearly all of the cartilage by day 14, while cartilage was still present in the callus of middle-aged mice at day 21 and in elderly mice at day 28. In addition to these delays, histomorphometry revealed that elderly and middle-aged mice formed less bone than juveniles (p<0.001), while cartilage production was unaffected (p>0.22). Collectively, these data suggest that enhancing cell differentiation, improving osteoblast function, and accelerating endochondral ossification may significantly benefit the elderly.

Cellular basis for age-related changes in fracture repair

The goal of this work was to define cellular and molecular changes that occur during fracture healing as animals age. We compared the molecular, cellular, and histological progression of skeletal repair in juvenile (4 weeks old), middle-aged (6 months old), and elderly (18 months old) mice at 3, 5, 7, 10, 14, 21, 28, and 35 days post-fracture, using a non-stabilized tibia fracture model. Our histological and molecular analyses demonstrated that there was a sharp decline in fracture healing potential between juvenile and middle-aged animals, while a more subtle decrease in healing potential was apparent between middle-aged and elderly mice. By three days after fracture, chondrocytes expressing Collagen type II, and osteoblasts expressing osteocalcin, were present in calluses of juvenile, but not middle-aged or elderly, mice. At day 5 immature chondrocytes and osteoblasts were observed in calluses of middle-aged and elderly mice. While at this time, chondrocytes in juvenile mice were expressing Collagen type X (ColX) indicating that chondrocyte maturation was already underway. At day 7, chondrocytes expressing ColX were abundant in middle-aged mice while a small domain of ColX-positive chondrocytes were observed in elderly mice. Further, in juvenile and middle-aged mice, but not elderly mice, vascular invasion of the cartilage was underway by day 7. Juvenile mice had replaced nearly all of the cartilage by day 14, while cartilage was still present in the callus of middle-aged mice at day 21 and in elderly mice at day 28. In addition to these delays, histomorphometry revealed that elderly and middle-aged mice formed less bone than juveniles (p<0.001), while cartilage production was unaffected (p>0.22). Collectively, these data suggest that enhancing cell differentiation, improving osteoblast function, and accelerating endochondral ossification may significantly benefit the elderly.

Early signals for fracture healing

Fracture healing requires the cooperation of multiple molecular signaling pathways. To better understand this cascade of transcriptional events, we compared the gene expression profiles between intact bone and fractured bone at days 1, 2, and 4 using a rat femur model of bone healing. Cluster analysis identified several groups of genes with dynamic temporal expression patterns and stage-specific functions. The immediate-response genes are highlighted by binding activity, transporter activity, and energy derivation. We consider these activities as critical signals for initiation of fracture healing. The continuously increased genes are characterized by those directly involved in bone repair, thus, representing bone specific forefront workers. The constantly upregulated genes tend to regulate general cell growth and are enriched with genes that are involved in tumorigenesis, suggesting common pathways between two processes. The constantly downregulated genes predominantly involve immune response, the significance of which remains for further investigation. Knowledge acquired through this analysis of transcriptional activities at the early stage of bone healing will contribute to our understanding of fracture repair and bone-related pathological conditions.

Comparison of mRNA gene expression by RT-PCR and DNA microarray

Few studies have compared the quantification of mRNA by DNA microarray to the results obtained by reverse transcription PCR (RT-PCR). In this study, mRNA was collected from the healing femoral fracture callus of adult and juvenile rats at various times after fracture. Ten samples were measured by both methods for 26 genes. For RT-PCR, mRNA was reverse transcribed, amplified, electrophoresed, blotted, and probed with 32P-labeled internal oligonucleotides, which were quantified. For DNA microarray, the mRNA was processed to biotin-labeled cRNA, hybridized to 10 Affymetrix Rat U34A microarrays, and quantified. Correlation coefficients (r) for each gene for the agreement between RT-PCR and microarray ranged from -0.48 to +0.93. This variation made the interpretation gene-specific. Genes with moderate expression levels gave the highest r values. Increased numbers of absent calls by the microarray software and increased separation between the location of the PCR primers and the microarray probes both led to reduced agreement. Microarray analysis suggested a floor effect in expression levels measured by RT-PCR for two genes. In conclusion, moderate mRNA expression levels with overlap in the location of PCR primers and microarray probes can yield good agreement between these two methods.

Altered mRNA expression of genes related to nerve cell activity in the fracture callus of older rats: A randomized, controlled, microarray study

Background

The time required for radiographic union following femoral fracture increases with age in both humans and rats for unknown reasons. Since abnormalities in fracture innervation will slow skeletal healing, we explored whether abnormal mRNA expression of genes related to nerve cell activity in the older rats was associated with the slowing of skeletal repair.

Methods

Simple, transverse, mid-shaft, femoral fractures with intramedullary rod fixation were induced in anaesthetized female Sprague-Dawley rats at 6, 26, and 52 weeks of age. At 0, 0.4, 1, 2, 4, and 6 weeks after fracture, a bony segment, one-third the length of the femur, centered on the fracture site, including the external callus, cortical bone, and marrow elements, was harvested. cRNA was prepared and hybridized to 54 Affymetrix U34A microarrays (3/age/time point).

Results

The mRNA levels of 62 genes related to neural function were affected by fracture. Of the total, 38 genes were altered by fracture to a similar extent at the three ages. In contrast, eight neural genes showed prolonged down-regulation in the older rats compared to the more rapid return to pre-fracture levels in younger rats. Seven genes were up-regulated by fracture more in the younger rats than in the older rats, while nine genes were up-regulated more in the older rats than in the younger.

Conclusions

mRNA of 24 nerve-related genes responded differently to fracture in older rats compared to young rats. This differential expression may reflect altered cell function at the fracture site that may be causally related to the slowing of fracture healing with age or may be an effect of the delayed healing.