[Show abstract][Hide abstract] ABSTRACT: During endochondral ossification, both the production of a cartilage template and the subsequent vascularisation of that template are essential precursors to bone tissue formation. Recent studies have found the application of both chondrogenic and vascular priming of mesenchymal stem cells (MSCs) enhanced the mineralisation potential of MSCs in vitro whilst also allowing for immature vessel formation. However, the in vivo viability, vascularisation and mineralisation potential of MSC aggregates that have been pre-conditioned in vitro by a combination of chondrogenic and vascular priming, has yet to be established. In this study, we test the hypothesis that a tissue regeneration approach that incorporates both chondrogenic priming of MSCs, to first form a cartilage template, and subsequent pre-vascularisation of the cartilage constructs, by co-culture with human umbilical vein endothelial cells (HUVECs) in vitro, will improve vessel infiltration and thus mineral formation once implanted in vivo.
Human MSCs were chondrogenically primed for 21 days, after which they were co-cultured with MSCs and HUVECs and cultured in endothelial growth medium for another 21 days. These aggregates were then implanted subcutaneously in nude rats for 4 weeks. We used a combination of bioluminescent imaging, microcomputed tomography, histology (Masson’s trichrome and Alizarin Red) and immunohistochemistry (CD31, CD146, and α-smooth actin) to assess the vascularisation and mineralisation potential of these MSC aggregates in vivo.
Pre-vascularised cartilaginous aggregates were found to have mature endogenous vessels (indicated by α-smooth muscle actin walls and erythrocytes) after 4 weeks subcutaneous implantation, and also viable human MSCs (detected by bioluminescent imaging) 21 days after subcutaneous implantation. In contrast, aggregates that were not pre-vascularised had no vessels within the aggregate interior and human MSCs did not remain viable beyond 14 days. Interestingly, the pre-vascularised cartilaginous aggregates were also the only group to have mineralised nodules within the cellular aggregates, whereas mineralisation occurred in the alginate surrounding the aggregates for all other groups.
Taken together these results indicate that a combined chondrogenic priming and pre-vascularisation approach for in vitro culture of MSC aggregates shows enhanced vessel formation and increased mineralisation within the cellular aggregate when implanted subcutaneously in vivo.
[Show abstract][Hide abstract] ABSTRACT: The use of soft, synthetic materials for the replacement of soft, load-bearing tissues has been largely unsuccessful due to a lack of materials with sufficient fatigue and wear properties, as well as a lack of fundamental understanding on the relationship between material structure and behavior under cyclic loads. In this study, we investigated the response of several soft, biomedical polymers to cyclic compressive stresses under aqueous conditions and utilized dynamic mechanical analysis and differential scanning calorimetry to evaluate the role of thermo-mechanical transitions on such behavior. Studied materials include: polycarbonate urethane, polydimethylsiloxane, four acrylate copolymers with systematically varied thermo-mechanical transitions, as well as bovine meniscal tissue for comparison. Materials showed compressive moduli between 2.3 and 1900MPa, with polycarbonate urethane (27.3MPa) matching closest to meniscal tissue (37.0MPa), and also demonstrated a variety of thermo-mechanical transition behaviors. Cyclic testing resulted in distinct fatigue-life curves, with failure defined as either classic fatigue fracture or a defined increased in maximum strain due to ratcheting. Our study found that polymers with sufficient dissipation mechanisms at the testing temperature, as evidenced by tan delta values, were generally tougher than those with less dissipation and exhibited ratcheting rather than fatigue fracture much like meniscal tissue. Strain recovery tests indicated that, for some toughened polymers, the residual strain following our cyclic loading protocol could be fully recovered. The similarity in ratcheting behavior, and lack of fatigue fracture, between the meniscal tissue and toughened polymers indicates that such polymers may have potential as artificial soft tissue.
[Show abstract][Hide abstract] ABSTRACT: Investigating biophysical cellular interactions in the circulation currently requires choosing between in vivo models, which are difficult to interpret due in part to the hemodynamic and geometric complexities of the vasculature; or in vitro systems, which suffer from non-physiologic assumptions and/or require specialized microfabrication facilities and expertise. To bridge that gap, we developed an in vitro "do-it-yourself" perfusable vasculature model that recapitulates in vivo geometries, such as aneurysms, stenoses, and bifurcations, and supports endothelial cell culture. These inexpensive, disposable devices can be created rapidly (<2 hours) with high precision and repeatability, using standard off-the-shelf laboratory supplies. Using these "endothelialized" systems, we demonstrate that spatial variation in vascular cell adhesion molecule (VCAM-1) expression correlates with the wall shear stress patterns of vascular geometries. We further observe that the presence of endothelial cells in stenoses reduces platelet adhesion but increases sickle cell disease (SCD) red blood cell (RBC) adhesion in bifurcations. Overall, our method enables researchers from all disciplines to study cellular interactions in physiologically relevant, yet simple-to-make, in vitro vasculature models.
[Show abstract][Hide abstract] ABSTRACT: Autologous bone grafting remains the gold standard in the treatment of large bone defects but is limited by tissue availability and donor site morbidity. Recombinant human bone morphogenetic protein-2 (rhBMP-2), delivered with a collagen sponge, is clinically used to treat large bone defects and complications such as delayed healing or nonunion. For the same dose of rhBMP-2, we have shown that a hybrid nanofiber mesh-alginate (NMA-rhBMP-2) delivery system provides longer-term release and increases functional bone regeneration in critically sized rat femoral bone defects compared with a collagen sponge. However, no comparisons of healing efficiencies have been made thus far between this hybrid delivery system and the gold standard of using autograft.
We compared the efficacy of the NMA-rhBMP-2 hybrid delivery system to morselized autograft and hypothesized that the functional regeneration of large bone defects observed with sustained BMP delivery would be at least comparable to autograft treatment as measured by total bone volume and ex vivo mechanical properties.
Bilateral critically sized femoral bone defects in rats were treated with either live autograft or with the NMA-rhBMP-2 hybrid delivery system such that each animal received one treatment per leg. Healing was monitored by radiography and histology at 2, 4, 8, and 12 weeks. Defects were evaluated for bone formation by longitudinal micro-CT scans over 12 weeks (n = 14 per group). The bone volume, bone density, and the total new bone formed beyond 2 weeks within the defect were calculated from micro-CT reconstructions and values compared for the 2-, 4-, 8-, and 12-week scans within and across the two treatment groups. Two animals were used for bone labeling with subcutaneously injected dyes at 4, 8, and 12 weeks followed by histology at 12 weeks to identify incremental new bone formation. Functional recovery was measured by ex vivo biomechanical testing (n = 9 per group). Maximum torque and torsional stiffness calculated from torsion testing of the femurs at 12 weeks were compared between the two groups.
The NMA-rhBMP-2 hybrid delivery system resulted in greater bone formation and improved biomechanical properties compared with autograft at 12 weeks. Comparing new bone volume within each group, the NMA-rhBMP-2-treated group had higher volume (p < 0.001) at 12 weeks (72.59 ± 18.34 mm(3)) compared with 8 weeks (54.90 ± 16.14) and 4 weeks (14.22 ± 9.59). The new bone volume was also higher at 8 weeks compared with 4 weeks (p < 0.001). The autograft group showed higher (p < 0.05) new bone volume at 8 weeks (11.19 ± 8.59 mm(3)) and 12 weeks (14.64 ± 10.36) compared with 4 weeks (5.15 ± 4.90). Between groups, the NMA-rhBMP-2-treated group had higher (p < 0.001) new bone volume than the autograft group at both 8 and 12 weeks. Local mineralized matrix density in the NMA-rhBMP-2-treated group was lower than that of the autograft group at all time points (p < 0.001). Presence of nuclei within the lacunae of the autograft and early appositional bone formation seen in representative histology sections suggested that the bone grafts remained viable and were functionally engrafted within the defect. The bone label distribution from representative sections also revealed more diffuse mineralization in the defect in the NMA-rhBMP-2-treated group, whereas more localized distribution of new mineral was seen at the edges of the graft pieces in the autograft group. The NMA-rhBMP-2-treated group also revealed higher torsional stiffness (0.042 ± 0.019 versus 0.020 ± 0.022 N-m/°; p = 0.037) and higher maximum torque (0.270 ± 0.108 versus 0.125 ± 0.137 N-m; p = 0.024) compared with autograft.
The NMA-rhBMP-2 hybrid delivery system improved bone formation and restoration of biomechanical function of rat segmental bone defects compared with autograft treatment.
Delivery systems that allow prolonged availability of BMP may provide an effective clinical alternative to autograft treatment for repair of segmental bone defects. Future studies in a large animal model comparing mixed cortical-trabecular autograft and the NMA-rhBMP-2 hybrid delivery system are the next step toward clinical translation of this approach.
Clinical Orthopaedics and Related Research 04/2015; 473(9). DOI:10.1007/s11999-015-4312-z · 2.77 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Characterization of articular cartilage morphology and composition using microcomputed tomography (microCT) techniques requires the use of contrast agents to enhance X-ray attenuation of the tissue. This chapter describes the use of an anionic iodinated contrast agent at equilibrium with articular cartilage. In this technique, negatively charged contrast agent molecules distribute themselves inversely with respect to the negatively charged proteoglycans (PGs) within the cartilage tissue (Palmer et al. Proc Natl Acad Sci U S A 103:19255–19260, 2006). This relationship allows for assessment of cartilage degradation, as areas of high X-ray attenuation have been shown to correspond to areas of depleted PGs (Palmer et al. Proc Natl Acad Sci U S A 103:19255–19260, 2006; Xie et al. Osteoarthritis Cartilage 18:65–72, 2010).
[Show abstract][Hide abstract] ABSTRACT: Pioglitazone, the peroxisome proliferator-activated receptor-gamma (PPAR-γ) agonist is an effective therapy for type 2 diabetes, but has been associated with increased risk for bone fracture. Preclinical studies suggest that PPAR-α agonists (e.g., fenofibrate) increase bone mineral density/content, although clinical data on bone effects of fibrates are lacking. We investigated the effects of pioglitazone (10 mg/kg/day) and fenofibrate (25 mg/kg/day) on bone strength and bone histomorphometric parameters in osteopenic ovariectomized (OVX) rats. An additional group of rats received a combination of pioglitazone + fenofibrate to mimic the effects of a dual PPAR-α/γ agonist. The study consisted of a 13-week treatment phase followed by a 6-week treatment-free recovery period. Pioglitazone significantly reduced biomechanical strength at the lumbar spine and femoral neck compared with rats administered fenofibrate. Co-treatment with pioglitazone + fenofibrate had no significant effect on bone strength in comparison with OVX vehicle controls. Histomorphometric analysis of the proximal tibia revealed that pioglitazone suppressed bone formation and increased bone resorption at both cancellous and cortical bone sites relative to OVX vehicle controls. In contrast, fenofibrate did not affect bone resorption and only slightly suppressed bone formation. Discontinuation of pioglitazone treatment, both in the monotherapy and in the combination therapy arms, resulted in restoration of bone formation and resorption rates, demonstrating reversibility of effects. The above data support the concept that dual activation of PPAR-γ and PPAR-α attenuates the negative effects of PPAR-γ agonism on bone strength.
Journal of Bone and Mineral Metabolism 12/2014; 33(6). DOI:10.1007/s00774-014-0632-4 · 2.46 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Tissue engineering strategies have utilized a wide spectrum of synthetic and naturally-derived scaffold materials. Synthetic scaffolds are better defined and offer the ability to precisely and reproducibly control their properties, while naturally-derived scaffolds typically have inherent biological and structural properties that may facilitate tissue growth and remodeling. More recently, efforts to design optimized biomaterial scaffolds have blurred the line between these two approaches. Naturally-derived scaffolds can be engineered through the manipulation of intrinsic properties of the pre-existing backbone (e.g. structural properties), as well as the addition of controllable functional components (e.g. biological properties). Chemical and physical processing techniques used to modify structural properties of synthetic scaffolds have been tailored and applied to naturally-derived materials. Such strategies include manipulation of mechanical properties, degradation, and porosity. Furthermore, biofunctional augmentation of natural scaffolds via incorporation of exogenous cells, proteins, peptides, or genes has been shown to enhance functional regeneration over endogenous response to the material itself. Moving forward, the regenerative mode of action of naturally-derived materials requires additional investigation. Elucidating such mechanisms will allow for the determination of critical design parameters to further enhance efficacy and capitalize on the full potential of naturally-derived scaffolds.
[Show abstract][Hide abstract] ABSTRACT: Localized intra-articular delivery of anti-inflammatory proteins can reduce inflammation in osteoarthritis but poses a challenge because of rapid clearance within few hours of injection. On page 1562, A. J. García and co-workers report a new class of polymer that forms self-assembled nanoparticles with therapeutic proteins for prolonged retention in intra-articular joint spaces compared to bolus protein doses.
[Show abstract][Hide abstract] ABSTRACT: Localized intra-articular delivery of anti-inflammatory proteins can reduce inflammation in osteoarthritis but poses a challenge because of raid clearance within few hours of injection. A new class of polymer is developed that forms self-assembled nanoparticles ranging from 300 to 900 nm and demonstrates particle size dependent prolonged retention in intra-articular joint spaces compared to bolus protein over a period of 14 d.
[Show abstract][Hide abstract] ABSTRACT: Autograft treatment of large bone defects and fracture non-unions is complicated by limited tissue availability and donor site morbidity. Polymeric biomaterials such as alginate hydrogels provide an attractive tissue engineering alternative due to their biocompatibility, injectability, and tunable degradation rates. Irradiated RGD-alginate hydrogels have been used to deliver proteins such as bone morphogenetic protein-2 (BMP-2), to promote bone regeneration and restoration of function in a critically sized rat femoral defect model. However, slow degradation of irradiated alginate hydrogels may impede integration and remodeling of the regenerated bone to its native architecture. Oxidation of alginate has been used to promote degradation of alginate matrices. The objective of this study was to evaluate the effects of alginate oxidation on BMP-2 release and bone regeneration. We hypothesized that oxidized-irradiated alginate hydrogels would elicit an accelerated release of BMP-2, but degrade faster in vivo, facilitating the formation of higher quality, more mature bone compared to irradiated alginate. Indeed, oxidation of irradiated alginate did accelerate in vitro BMP-2 release. Notably, the BMP-2 retained within both constructs was bioactive at 26 days, as observed by induction of alkaline phosphatase activity and positive Alizarin Red S staining of MC3T3-E1 cells. From the in vivo study, robust bone regeneration was observed in both groups through 12 weeks by radiography, micro-computed tomography analyses, and biomechanical testing. Bone mineral density was significantly greater for the oxidized-irradiated alginate group at 8 weeks. Histological analyses of bone defects revealed enhanced degradation of oxidized-irradiated alginate and suggested the presence of more mature bone after 12 weeks of healing.
[Show abstract][Hide abstract] ABSTRACT: Bone loss due to age and disuse contributes to osteoporosis and increases fracture risk. It has been hypothesized that such bone loss can be attenuated by modulation of the C-C chemokine receptor 2 (CCR2) and/or its ligands. The objectives of this study were to examine the effects of genetic elimination of CCR2 on cortical and trabecular bones in the mouse tibia and how bone loss was impacted following disuse and estrogen loss. Female CCR2 knockout (CCR2(-/-)) and wildtype mice underwent ovariectomy (OVX) or denervation of musculature adjacent to the tibia (DEN) to induce bone loss. Cortical and trabecular structural properties as well as mechanical properties (i.e., strength) of tibial bones were measured. Compared to wildtype mice, CCR2(-/-) mice had tibiae that were up to 9 % larger and stronger; these differences could be explained mainly by the 17 % greater body mass (P < 0.001) of CCR2(-/-) mice. The majority of the tibia's structural and functional responses to OVX and DEN were similar regardless of the lack or presence of CCR2, indicating that CCR2 is not protective against bone loss per se. These findings indicate that while CCR2(-/-) mice do have larger and stronger bones than do wildtype mice, there is minimal evidence that CCR2 elimination provides protection against bone loss during disuse and estrogen loss.
Calcified Tissue International 09/2014; 95(5). DOI:10.1007/s00223-014-9914-z · 3.27 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Craniosynostosis is the premature fusion of cranial sutures, which can result in progressive cranial deformations, increased intracranial pressure, and restricted brain growth. Most cases of craniosynostosis require surgical reconstruction of the cranial vault with the goal of increasing the intracranial volume and correcting the craniofacial deformities. However, patients often experience rapid post-operative bone regrowth, known as re-synostosis, which necessitates additional surgical intervention. Bone morphogenetic protein (BMP) inhibitors have tremendous potential to treat re-synostosis, but the realization of a clinically viable inhibitor-based therapeutic requires the development of a delivery vehicle that can localize the release to the site of administration. Here, we present an in situ rapidly crosslinking injectable hydrogel that has the properties necessary to encapsulate co-administered proteins and demonstrate that the delivery of rmGremlin1 via our hydrogel system delays bone regrowth in a weanling mouse model of re-synostosis. Our hydrogel is composed of two mutually reactive poly(ethylene glycol) macromolecules, which when mixed crosslink via a bio-orthogonal Cu free click reaction. Hydrogels containing Gremlin caused a dose dependent inhibition of bone regrowth. In addition to craniofacial applications, our injectable click hydrogel has the potential to provide customizable protein, small molecule, and cell delivery to any site accessible via needle or catheter.
[Show abstract][Hide abstract] ABSTRACT: Skeletal development and growth are complex processes regulated by multiple microenvironmental cues, including integrin-ECM interactions. The β1 sub-family of integrins is the largest integrin sub-family and constitutes the main integrin binding partners of collagen I, the major ECM component of bone. As complete β1 integrin knockout results in embryonic lethality, studies of β1 integrin function in vivo rely on tissue-specific gene deletions. While multiple in vitro studies indicate that β1 integrins are crucial regulators of osteogenesis and mineralization, in vivo osteoblast-specific perturbations of β1 integrins have resulted in mild and sometimes contradictory skeletal phenotypes. To further investigate the role of β1 integrins on skeletal phenotype, we used the Twist2-Cre, Osterix-Cre and osteocalcin-Cre lines to generate conditional β1 integrin deletions, where Cre is expressed primarily in mesenchymal condensation, pre-osteoblast, and mature osteoblast lineage cells respectively within these lines. Mice with Twist2-specific β1 integrin disruption were smaller, had impaired skeletal development, especially in the craniofacial and vertebral tissues at E19.5, and did not survive beyond birth. Osterix-specific β1 integrin deficiency resulted in viable mice which were normal at birth but displayed early defects in calvarial ossification, incisor eruption and growth as well as femoral bone mineral density, structure, and mechanical properties. Although these defects persisted into adulthood, they became milder with age. Finally, a lack of β1 integrins in mature osteoblasts and osteocytes resulted in minor alterations to femur structure but had no effect on mineral density, biomechanics or fracture healing. Taken together, our data indicate that β1 integrin expression in early mesenchymal condensations play an important role in skeletal ossification, while β1 integrin-ECM interactions in pre-osteoblast, odontoblast- and hypertrophic chondryocyte-lineage cells regulate incisor eruption and perinatal bone formation in both intramembranously and endochondrally formed bones in young, rapidly growing mice. In contrast, the osteocalcin-specific β1 integrin deletion had only minor effects on skeletal phenotype.
Bone 08/2014; 68. DOI:10.1016/j.bone.2014.08.008 · 3.97 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The objective of the study was to determine if low intensity, high frequency vibration training impacted the musculoskeletal system in a mouse model of Duchenne muscular dystrophy, relative to healthy mice. Three-week old wildtype (n = 26) and mdx mice (n = 22) were randomized to non-vibrated or vibrated (45 Hz and 0.6 g, 15 min/d, 5 d/wk) groups. In
vivo and ex
vivo contractile function of the anterior crural and extensor digitorum longus muscles, respectively, were assessed following 8 wks of vibration. Mdx mice were injected 5 and 1 days prior to sacrifice with Calcein and Xylenol, respectively. Muscles were prepared for histological and triglyceride analyses and subcutaneous and visceral fat pads were excised and weighed. Tibial bones were dissected and analyzed by micro-computed tomography for trabecular morphometry at the metaphysis, and cortical geometry and density at the mid-diaphysis. Three-point bending tests were used to assess cortical bone mechanical properties and a subset of tibiae was processed for dynamic histomorphometry. Vibration training for 8 wks did not alter trabecular morphometry, dynamic histomorphometry, cortical geometry, or mechanical properties (P≥0.34). Vibration did not alter any measure of muscle contractile function (P≥0.12); however the preservation of muscle function and morphology in mdx mice indicates vibration is not deleterious to muscle lacking dystrophin. Vibrated mice had smaller subcutaneous fat pads (P = 0.03) and higher intramuscular triglyceride concentrations (P = 0.03). These data suggest that vibration training at 45 Hz and 0.6 g did not significantly impact the tibial bone and the surrounding musculature, but may influence fat distribution in mice.
PLoS ONE 08/2014; 9(8):e104339. DOI:10.1371/journal.pone.0104339 · 3.23 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Despite progress in bone tissue engineering, the healing of critically sized diaphyseal defects remains a clinical challenge. A stem cell based approach is an attractive alternative to current treatment techniques. The objective of this study was to examine the ability of adult stem cells to enhance bone formation when co-delivered with the osteoinductive factor bone morphogenetic protein-2 (BMP-2) in a biologically functionalized hydrogel. First, adipose and bone marrow derived stem cells (ADSCs and BMMSCs) were screened for their potential to form bone when delivered in an RGD functionalized alginate hydrogel using a subcutaneous implant model. BMMSCs co-delivered with BMP-2 produced significantly more mineralized tissue compared to either ADSCs co-delivered with BMP-2 or acellular hydrogels containing BMP-2. Next, the ability of BMMSCs to heal a critically sized diaphyseal defect with a non-healing dose of BMP-2 was tested using the alginate hydrogel as an injectable cell carrier. The effect of timing of therapeutic delivery on bone regeneration was also tested in the diaphyseal model. A 7 day delayed injection of the hydrogel into the defect site resulted in less mineralized tissue formation than immediate delivery of the hydrogel. By 12 weeks, BMMSC loaded hydrogels produced significantly more bone than acellular constructs regardless of immediate or delayed treatment. For immediate delivery, bridging of defects treated with BMMSC loaded hydrogels occurred at a rate of 75% compared to a 33% bridging rate for acellular treated defects. No bridging was observed in any of the delayed delivery samples for any of the groups. Therefore, for this cell based bone tissue engineering approach, immediate delivery of constructs leads to an overall enhanced healing response compared to delayed delivery techniques. Further, these studies demonstrate that co-delivery of adult stem cells, specifically BMMSCs, with BMP-2 enhances bone regeneration in a critically sized femoral segmental defect compared to acellular hydrogels containing BMP-2.
Tissue Engineering Part A 07/2014; 21(1-2). DOI:10.1089/ten.TEA.2014.0057 · 4.70 Impact Factor