Modulation of allograft incorporation by continuous infusion of growth factors over a prolonged duration in vivo

Targeted activation of androgen receptor signaling in the periosteum improves bone fracture repair

Low testosterone level is an independent predictor of osteoporotic fracture in elderly men as well as increased fracture risk in men undergoing androgen deprivation. Androgens and androgen receptor (AR) actions are essential for bone development and homeostasis but their linkage to fracture repair remains unclear. Here we found that AR is highly expressed in the periosteum cells and is co-localized with a mesenchymal progenitor cell marker, paired-related homeobox protein 1 (Prrx1), during bone fracture repair. Mice lacking the AR gene in the periosteum expressing Prrx1-cre (AR^-/Y;Prrx1::Cre) but not in the chondrocytes (AR^-/Y;Col-2::Cre) exhibits reduced callus size and new bone volume. Gene expression data analysis revealed that the expression of several collagens, integrins and cell adhesion molecules were downregulated in periosteum-derived progenitor cells (PDCs) from AR^-/Y;Prrx1::Cre mice. Mechanistically, androgens-AR signaling activates the AR/ARA55/FAK complex and induces the collagen-integrin α2β1 gene expression that is required for promoting the AR-mediated PDCs migration. Using mouse cortical-defect and femoral graft transplantation models, we proved that elimination of AR in periosteum of host mice impairs fracture healing, regardless of AR existence of transplanted donor graft. While testosterone implanted scaffolds failed to complete callus bridging across the fracture gap in AR^-/Y;Prrx1::Cre mice, cell-based transplantation using DPCs re-expressing AR could lead to rescue bone repair. In conclusion, targeting androgen/AR axis in the periosteum may provide a novel therapy approach to improve fracture healing.

TGF-beta receptor mediated telomerase inhibition, telomere shortening and breast cancer cell senescence

Human telomerase reverse transcriptase (hTERT) plays a central role in telomere lengthening for continuous cell proliferation, but it remains unclear how extracellular cues regulate telomerase lengthening of telomeres. Here we report that the cytokine bone morphogenetic protein-7 (BMP7) induces the hTERT gene repression in a BMPRII receptor- and Smad3-dependent manner in human breast cancer cells. Chonic exposure of human breast cancer cells to BMP7 results in short telomeres, cell senescence and apoptosis. Mutation of the BMPRII receptor, but not TGFbRII, ACTRIIA or ACTRIIB receptor, inhibits BMP7-induced repression of the hTERT gene promoter activity, leading to increased telomerase activity, lengthened telomeres and continued cell proliferation. Expression of hTERT prevents BMP7-induced breast cancer cell senescence and apoptosis. Thus, our data suggest that BMP7 induces breast cancer cell aging by a mechanism involving BMPRII receptor- and Smad3-mediated repression of the hTERT gene.

[Spinal arthrodesis. basic science]

Spinal arthrodesis consists of a combination of a system of mechanical stabilisation of one or more vertebral segments with a biological substance that promotes osteoneogenesis, with aim of achieving the permanent fusion between areas more or less the same size of these segments. In spinal arthrodesis, the biological support par excellence is the autograft. However, obtaining this involves a high incidence of morbidity and, in cases of arthrodesis of more than one intervertebral space, the quantity available is usually insufficient. The extraction and implantation time prolongs the surgery, increasing the exposure to and risk of bleeding and infection. For these reasons, there is a search for substances that possess the properties of the autograft, avoiding the morbidity and added surgical time required to extract the autograft. The biomechanical-biological interaction in vertebral arthrodesis has been studied in this article.

What bone graft substitutes should we use in post-traumatic spinal fusion?

Surgical treatment of spinal fractures consists of postural reduction and segmental arthrodesis, together with an eventual performance of spinal canal decompression. Spinal arthrodesis consists of the combination of a hardware system for mechanical stabilisation together with a biological substance for enhancement of bone formation. To date, autologous graft is the only biological substance demonstrated to possess osteogenic properties. Cancellous bone graft has greater cellular activity than cortical graft, whereas cortical graft is stronger. Consequently, according to biological and biomechanical properties of autograft, spinal posterior arthrodesis is better enhanced by cancellous autograft, whereas anterior interbody tricortical bone is more suitable for anterior fusion. Allograft does not cause harvesting complications as autograft does, and also its amount is theoretically unlimited; nevertheless the rate of bone fusion facilitated by allograft is far from that enhanced by autograft given that allograft has no osteoprogenitor cells. There is little evidence on the efficacy of demineralised bone matrix for spinal fusion. Bone morphogenetic proteins (BMPs) are in use in spinal surgery, but their exact role with respect to type, dose, and carrier, together with their cost-effectiveness, need further clinical delineation. Calcium phosphate compounds appear to be good as carriers; however, they have no osteoinductive or osteogenic properties. Current clinical literature seem to indicate their usefulness for bony fusion in spinal surgery, when combined with bone marrow aspirate or used as an extender for autologous bone graft. Age, length of fusion, location, and concurrent diseases should be definitive for fusion outcome; papers on spinal arthrodesis should neatly stratify these variables. Unfortunately, since that is not the rule, conclusions drawn from current literature are very unreliable. Autograft remains the gold standard, and cancellous bone is advisable in posterolateral approaches, whereas tricortical iliac crest autograft appears appropriate for interbody support. In longer segments, its expansion with BMPs looks safe at least. Basic knowledge has been achieved from animal experiments, and clinical application of the findings to humans should be done very cautiously; in any case, both anterior and posterior arthrodesis must be protected with instrumentation used according to appropriate biomechanical principles. A combination of failure of the correct graft together with proper instrumentation will result in poorer outcome, even if the right graft is used.

Antibacterial activity of bone allografts: comparison of a new vancomycin-tethered allograft with allograft loaded with adsorbed vancomycin

Bacterial contamination of bone allograft is a significant complication of orthopedic surgery. To address this issue, we have engineered a method for covalently modifying bone allograft tissue with the antibiotic vancomycin. The goal of this investigation was to compare the biocidal properties of this new allograft material with those of vancomycin physisorbed onto graft material. The duration of antibiotic release from the vancomycin-modified allograft matrix was determined, and no elution was observed. In contrast, the adsorbed antibiotic showed a peak elution at 24h that then decreased over several days. We next used an Staphylococcus aureus disk diffusion assay to measure the activity of the eluted vancomycin. Again we found that no active antibiotic was eluted from the covalently modified allograft. Similarly, when the vancomycin-modified allograft morsel was used in the assay, no measurable elution was observed; amounts of antibiotic released from the adsorbed samples inhibited S. aureus growth for 4-7 days. Probably the most telling property of the allograft was that after 2 weeks, the tethered allograft was able to resist bacterial colonization. Unlike the elution system in which vancomycin was depleted over the course of days-weeks, the antibiotic on the allograft was stably bound even after 300 days, while its biocidal activity remained undiminished for 60 days. This finding was in stark contrast to the antibiotic impregnated allograft, which was readily colonized by bacteria. Finally we chose to evaluate three indicators of cell function: expression of a key transcription factor, expression of selected transcripts, and assessment of cell morphology. Since the tethered antibiotic appeared to have little or no effect on any of these activities, it was concluded that the stable, tethered antibiotic prevented bacterial infection while not modifying bone cell function.

Characterization of tunable FGF-2 releasing polyelectrolyte multilayers

Fibroblast growth factor 2 (FGF-2) is a potent mediator of stem cell differentiation and proliferation. Although FGF-2 has a well-established role in promoting bone tissue formation, flaws in its delivery have limited its clinical utility. Polyelectrolyte multilayer films represent a novel system for FGF-2 delivery that has promise for local, precisely controlled, and sustained release of FGF-2 from surfaces of interest, including medical implants and tissue engineering scaffolds. In this work, the loading and release of FGF-2 from synthetic hydrolytically degradable multilayer thin films of various architectures is explored; drug loading was tunable using at least three parameters (number of nanolayers, counterpolyanion, and type of degradable polycation) and yielded values of 7-45 ng/cm(2) of FGF-2. Release time varied between 24 h and approximately five days. FGF-2 released from these films retained in vitro activity, promoting the proliferation of MC3T3 preosteoblast cells. The use of biologically derived counterpolyanions heparin sulfate and chondroitin sulfate in the multilayer structures enhanced FGF-2 activity. The control over drug loading and release kinetics inform future in vivo bone and tissue regeneration models for the exploration of clinical relevance of LbL growth factor delivery films.

Bacterial colonization of bone allografts: establishment and effects of antibiotics

BACKGROUND

Bone grafts are frequently used to supplement bone stock and to establish structural stability. However, graft-associated infection represents a challenging complication leading to increased patient morbidity and healthcare costs.

QUESTIONS/PURPOSES

We therefore designed this study to (1) determine if increasing initial S. aureus inoculation of bone allograft results in a proportionate increase in colonization; (2) assess if antibiotics decrease colonization and if antibiotic tethering to allograft alters its ability to prevent bacterial colonization; and (3) determine if covalent modification alters the allograft topography or its biological properties.

METHODS

Allograft bone and vancomycin-modified bone (VAN-bone) was challenged with different doses of S. aureus for times out to 24 hours in the presence or absence of solution vancomycin. Bacterial colonization was assessed by fluorescence, scanning electron microscopy (SEM), and by direct colony counting. Cell density and distribution of osteoblast-like cells on control and modified allograft were then compared.

RESULTS

Bacterial attachment was apparent within 6 hours with colonization and biofilm formation increasing with time and dose. Solution vancomycin failed to prevent bacterial attachment whereas VAN-bone successfully resisted colonization. The allograft modification did not affect the attachment and distribution of osteoblast-like cells.

CONCLUSIONS

Allograft bone was readily colonized by S. aureus and covered by a biofilm with especially florid growth in natural topographic niches. Using a novel covalent modification, allograft bone was able to resist colonization by organisms while retaining the ability to allow adhesion of osteoblastic cells.

CLINICAL RELEVANCE

Generation of allograft bone that can resist infection in vivo would be important in addressing one of the most challenging problems associated with the use of allograft, namely infection.

BMP-2/PLGA delayed-release microspheres composite graft, selection of bone particulate diameters, and prevention of aseptic inflammation for bone tissue engineering

Autogenous bone grafts are widely used in the repair of bone defects. Growth factors such as bone morphogenetic protein 2 (BMP-2) can induce bone regeneration and enhance bone growth. The combination of an autogenous bone graft and BMP-2 may provide a better osteogenic effect than either treatment alone, but BMP-2 is easily inactivated in body fluid. The objective of this study was to develop a technique that can better preserve the in vivo activity of BMP-2 incorporated in bone grafts. In this study, we first prepared BMP-2/poly(lactic-co-glycolic acid) (PLGA) delayed-release microspheres, and then combined collagen, the delayed-release microspheres, and rat autologous bone particulates to form four groups of composite grafts with different combinations: collagen in group A; collagen combined with bone particulates in group B; collagen combined with BMP-2/PLGA delayed-release microspheres in group C; and collagen combined with both bone particulates and BMP-2/PLGA delayed-release microspheres in group D. The four groups of composite grafts were implanted into the gluteus maximus pockets in rats. The ectopic osteogenesis and ALP level in group D (experimental group) were compared with those in groups A, B, and C (control groups) to study whether it had higher osteogenic capability. Results showed that the composite graft design increased the utility of BMP-2 and reduced the required dose of BMP-2 and volume of autologous bone. The selection of bone particulate diameter had an impact on the osteogenetic potential of bone grafts. Collagen prevented the occurrence of aseptic inflammation and improved the osteoinductivity of BMP-2. These results showed that this composite graft design is effective and feasible for use in bone repair.

Bone morphogenetic protein-7 inhibits telomerase activity, telomere maintenance, and cervical tumor growth

Telomere maintenance is critical in tumor cell immortalization. Here, we report that the cytokine bone morphogenetic protein-7 (BMP7) inhibits telomerase activity that is required for telomere maintenance in cervical cancer cells. Application of human recombinant BMP7 triggers a repression of the human telomerase reverse transcriptase (hTERT) gene, shortening of telomeres, and hTERT repression-dependent cervical cancer cell death. Continuous treatment of mouse xenograft tumors with BMP7, or silencing the hTERT gene, results in sustained inhibition of telomerase activity, shortening of telomeres, and tumor growth arrest. Overexpression of hTERT lengthens telomeres and blocks BMP7-induced tumor growth arrest. Thus, BMP7 negatively regulates telomere maintenance, inducing cervical tumor growth arrest by a mechanism of inducing hTERT gene repression.

A perspective: engineering periosteum for structural bone graft healing

Autograft is superior to both allograft and synthetic bone graft in repair of large structural bone defect largely due to the presence of multipotent mesenchymal stem cells in periosteum. Recent studies have provided further evidence that activation, expansion and differentiation of the donor periosteal progenitor cells are essential for the initiation of osteogenesis and angiogenesis of donor bone graft healing. The formation of donor cell-derived periosteal callus enables efficient host-dependent graft repair and remodeling at the later stage of healing. Removal of periosteum from bone autograft markedly impairs healing whereas engraftment of multipotent mesenchymal stem cells on bone allograft improves healing and graft incorporation. These studies provide rationale for fabrication of a biomimetic periosteum substitute that could fit bone of any size and shape for enhanced allograft healing and repair. The success of such an approach will depend on further understanding of the molecular signals that control inflammation, cellular recruitment as well as mesenchymal stem cell differentiation and expansion during the early phase of the repair process. It will also depend on multidisciplinary collaborations between biologists, material scientists and bioengineers to address issues of material selection and modification, biological and biomechanical parameters for functional evaluation of bone allograft healing.