Nuclear factor erythroid 2–related factor 2 (Nrf2)/antioxidant response element (ARE) pathway is key in maintaining redox homeostasis and the pathogenesis of osteoarthritis (OA) involves oxidative distress. We thus investigated whether Nrf2/ARE signaling may control expression of key chondrogenic differentiation and hyaline cartilage maintenance factor SOX9.

In human C-28/I2 chondrocytes SOX9 expression was measured by RT–qPCR after shRNA-mediated knockdown of Nrf2 or its antagonist the Kelch-like erythroid cell-derived protein with cap “n” collar homology-associated protein 1 (Keap1). Putative ARE-binding sites in the proximal SOX9 promoter region were inactivated, cloned into pGL3, and co-transfected with phRL–TK for dual-luciferase assays to verify whether Nrf2 transcriptionally regulates SOX9. SOX9 promoter activity without and with Nrf2-inducer methysticin were analyzed. Sox9 expression in articular chondrocytes was correlated to cartilage thickness and degeneration in wild-type (WT) and Nrf2-knockout mice. Data were analyzed by one-way ANOVA, a Student's t-test, or Wilcoxon rank-sum test, according to the normal distribution. Statistical significance was set to p < 0.05.

While Keap1-specific RNAi increased SOX9 expression, Nrf2-specific RNAi significantly decreased it. Putative ARE sites (ARE₁, ARE₂) were identified in the SOX9 promoter region. ARE₂ mutagenesis significantly reduced SOX9 promoter activity, while truncation of ARE₁ did not. A functional ARE₂ site was thus essential for methysticin-mediated induction of SOX9 promoter activity. Knee cartilage of young Nrf2-knockout mice further revealed significantly fewer Sox9-positive chondrocytes as compared to old Nrf2-knockout animals, which further showed thinner cartilage and more severe cartilage erosion.

Our data suggest that SOX9 expression in articular cartilage is directly Nrf2-dependent and that pharmacological Nrf2 activation may hold potential to diminish age-dependent osteoarthritic changes in knee cartilage through improving protective SOX9 expression.

Many age-related diseases affect our skeletal system, but bone health-targeting drug development strategies still largely rely on 2D in vitro screenings. We aimed at developing a scaffold-free progenitor cell-based 3D biomineralization model for more physiological high-throughput screenings.

MC3T3-E1 pre-osteoblast spheroids were cultured in V-shaped plates for 28 days in alpha-MEM (10% FCS, 1% L-Gln, 1X NEAA) with 1% pen/strep, changed every two days, and differentiation was induced by 10mM b-glycerophosphate and 50µg/ml ascorbic-acid. Osteogenic cell differentiation was assessed through profiling mRNA expression of selected osteogenic markers by efficiency corrected normalized 2^DDCq RT-qPCR. Biomineralization in spheroids was evaluated by histochemistry (Alizarin Red/von Kossa staining), Alkaline phosphatase (Alp) activity, Fourier transform infrared spectroscopy (FTIR) analyses, micro-CT analyses, and scanning electron microscopy on critical point-dried samples. GraphPad Prism 9 analyses comprised Shapiro-Wilk and Brown-Forsythe tests as well as 2-way ANOVA with Tukey post-hoc and non-parametric Kruskal-Wallis with Dunn post-hoc tests.

During mineralization, as opposed to non-mineralizing conditions, characteristic mRNA expression profiles of selected early and late osteoblast differentiation markers (e.g., RunX, Alp, Col1a1, Bglap) were observed between day 0 and 28 of culture; Alp was strongly upregulated (p<0.001) from day 7 on, followed by its enzymatic activity (p<0.001). Bglap and Col1a1 expression peaked on (p<0.001) and from day 14 on (p<0.05), respectively. IHC revealed osteocalcin staining in the spheroid core regions at day 14, while type I collagen staining of the cores was most prominent from day 21 on. Alizarin Red and Von Kossa confirmed central and radially outwards expanding mineralization patterns between day 14 and day 28, which was accompanied by a steady increase in extracellular calcium deposition over time (p<0.001). Micro-CT analyses allowed quantitative appreciation of the overall increase in mineral density over time (day21, p<0.05; d28, p<0.001), while SEM-EDX and FTIR ultimately confirmed a bone-like hydroxyapatite mineral deposition in 3D.

A novel and thoroughly characterized versatile bone-like 3D biomineralization in vitro model was established, which allows for studying effects of pharmacological interventions on bone mineralization ex vivo under physiomimetic conditions. Ongoing studies currently aim at elucidating in how far it specifically recapitulates intramembranous ossification.

Nuclear factor erythroid 2–related factor 2 (Nrf2) is a crucial transcription factor to maintain cellular redox homeostasis, but is also affecting bone metabolism. As the association between Nrf2 and osteoporosis in elderly females is not fully elucidated, our aim was to shed light on the potential contribution of Nrf2 to the development of age-dependent osteoporosis using a mouse model.

Female wild-type (WT, n=18) and Nrf2-knockout (KO, n=12) mice were sacrificed at different ages (12 weeks=young mature adult, and 90 weeks=old), morphological cortical and trabecular properties of femoral bone analyzed by micro-computed tomography (µCT), and compared to histochemistry. Mechanical properties were derived from quasi-static compression tests and digital image correlation (DIC) used to analyze full-field strain distribution. Bone resorbing cells and aromatase expression by osteocytes were evaluated immunohistochemically and empty osteocyte lacunae counted in cortical bone. Wilcoxon rank sum test was used for data comparison and differences considered statistically significant at p<0.05.

When compared to old WT mice, old Nrf2-KO mice revealed a significantly reduced trabecular bone mineral density (BMD), cortical thickness (Ct.Th), cortical area (Ct.Ar), and cortical bone fraction (Ct.Ar/Tt.Ar). Surprisingly, these parameters were not different in skeletally mature young adult mice. Metaphyseal trabeculae were thin but present in all old WT mice, while no trabecular bone was detectable in 60% of old KO mice. Occurrence of empty osteocyte lacunae did not differ between both groups, but a significantly higher number of osteoclast-like cells and fewer aromatase-positive osteocytes were found in old KO mice. Furthermore, female Nrf2-KO mice showed an age-dependently reduced fracture resilience when compared to age-matched WT mice.

Our results confirmed lower bone quantity and quality as well as an increased number of bone resorbing cells in old female Nrf2-KO mice. Additionally, aromatase expression in osteocytes of old Nrf2-KO mice was compromised, which may indicate a chronic lack of estrogen in bones of old Nrf2-deficient mice. Thus, chronic Nrf2 loss seems to contribute to age-dependent progression of female osteoporosis.

Bioabsorbable metals hold a lot of potential as orthopaedic implant materials. Three metal families are currently being investigated: iron (Fe), magnesium (Mg) and zinc (Zn). Currently, however, biodegradation of such implants is poorly predictable. We thus used Direct Metal Printing to additively manufacture porous implants of a standardized bone-mimetic design and evaluated their mechanical properties and degradation behaviour, respectively, under in vivo-like conditions.

Atomized powder was manufactured to porous implants of repetitive diamond unit cells, using a ProX DMP 320 (Layerwise, Belgium) or a custom-modified ReaLizer SLM50 metal printer. Degradation behaviour was characterized under static and dynamic conditions in a custom-built bioreactor system (37ºC, 5% CO₂ and 20% O₂) for up of 28 days. Implants were characterized by micro-CT before and after in vivo-like degradation. Mechanical characterization (according to ISO 13314: 2011) was performed on an Instron machine (10kN load cell) at different immersion times in simulated body fluid (r-SBF). Morphology and composition of degradation products were analysed (SEM, JSM-IT100, JEOL). Topographically identical titanium (Ti-6Al-4V, Ti64) specimen served as reference.

Micro-CT analyses confirmed average strut sizes (420 ± 4 μm), and porosity (64%), to be close to design values. After 28 days of in vivo-like degradation, scaffolds were macroscopically covered by degradation products in an alloy-specific manner. Weight loss after cleaning also varied alloy-specifically, as did the change in pH value of the r-SBF. Corrosion time-dependent changes in Young's moduli from 1200 to 800 MPa for Mg, 1000 to 700 MPa for Zn and 48-8 MPa for iron were statistically significant.

In summary, DMP allows to accurately control interconnectivity and topology of implants from all three families and micro-structured design holds potential to optimize their degradation speed. This first systematic report sheds light into how design influences degradation behaviour under in vivo-like conditions to help developing new standards for future medical device evaluation.

Biodegradable metals as orthopaedic implant materials receive substantial scientific and clinical interest. Marketed cardiovascular products confirm good biocompatibility of iron. Solid iron biodegrades slowly in vivo and has got supra-physiological mechanical properties as compared to bone and porous implants can be optimized for specific orthopaedic applications. We used Direct Metal Printing (DMP)3 to additively manufacture (AM) scaffolds of pure iron with fine-tuned bone-mimetic mechanical properties and improved degradation behavior to characterize their biocompatibility under static and dynamic 3D culture conditions using a spectrum of different cell types.

Atomized iron powder was used to manufacture scaffolds with a repetitive diamond unit cell design on a ProX DMP 320 (Layerwise/3D Systems, Belgium). Mechanical characterization (Instron machine with a 10kN load cell, ISO 13314: 2011), degradation behavior under static and dynamic conditions (37ºC, 5% CO2 and 20% O2) for up of 28 days, with μCT as well as SEM/energy-dispersive X-ray spectroscopy (EDS) (SEM, JSM-IT100, JEOL) monitoring under in vivo-like conditions. Biocompatibility was comprehensively evaluated using a broader spectrum of human cells according to ISO 10993 guidelines, with topographically identical titanium (Ti-6Al-4V, Ti64) specimen as reference. Cytotoxicity was analyzed by two-way ANOVA and post-hoc Tukey's multiple comparisons test (α = 0.05).

By μCT, as-built strut size (420 ± 4 μm) and porosity of 64% ± 0.2% were compared to design values (400 μm and 67%, respectively). After 28 days of biodegradation scaffolds showed a 3.1% weight reduction after cleaning, while pH-values of simulated body fluids (r-SBF) increased from 7.4 to 7.8. Mechanical properties of scaffolds (E = 1600–1800 MPa) were still within the range for trabecular bone, then. At all tested time points, close to 100% biocompatibility was shown with identically designed titanium (Ti64) controls (level 0 cytotoxicity). Iron scaffolds revealed a similar cytotoxicity with L929 cells throughout the study, but MG-63 or HUVEC cells revealed a reduced viability of 75% and 60%, respectively, already after 24h and a further decreased survival rate of 50% and 35% after 72h. Static and dynamic cultures revealed different and cell type-specific cytotoxicity profiles. Quantitative assays were confirmed by semi-quantitative cell staining in direct contact to iron and morphological differences were evident in comparison to Ti64 controls.

This first report confirms that DMP allows accurate control of interconnectivity and topology of iron scaffold structures. While microstructure and chemical composition influence degradation behavior - so does topology and environmental in vitro conditions during degradation. While porous magnesium corrodes too fast to keep pace with bone remodeling rates, our porous and micro-structured design just holds tremendous potential to optimize the degradation speed of iron for application-specific orthopaedic implants. Surprisingly, the biological evaluation of pure iron scaffolds appears to largely depend on the culture model and cell type. Pure iron may not yet be an ideal surface for osteoblast- or endothelial-like cells in static cultures. We are currently studying appropriate coatings and in vivo-like dynamic culture systems to better predict in vivo biocompatibility.

The ideal bone substituting biomaterials should possess bone-mimicking mechanical properties; have of porous interconnected structure, and adequate biodegradation behaviour to enable full recovery of bony defects. Direct metal printed porous scaffolds hold potential to satisfy all these requirements and were additively manufactured (AM) from atomized WE43 magnesium alloy powder with grain sizes between 20 and 60 μm. Their micro-structure, mechanical properties, degradation behavior and biocompatibility was then evaluated in vitro. Firstly, post-processing values nicely followed design parameters. Next, Young's moduli were similar to that of trabecular bone (i.e., E = 700–800 MPa) even after 28 days of simulated in vivo-like corrosion by in vitro immersion. Also, a relatively moderate hydrogen evolution, corresponding to a calculated 19.2% of scaffold mass loss, was in good agreement with 20.7% volume reduction as derived from reconstructed μCT images. Finally, only moderate cytotoxicity (i.e., level 0, <25%), even after extensive ISO 10993-conform testing for 72 h using MG-63 cells, was determined using WE43 extracts (2 way ANOVA, post-hoc Tukey's multiple comparisons test; α = 0.05). Cytotoxicity was further evaluated by direct live-dead staining assays, revealing a higher cell death in static culture. However, intimate cell-metal contact was observed by SEM. In summary, while pure WE43 may not yet be an ideal surface for cell adhesion, this novel AM process allows for adjusting biodegradation through topological design. Our approach holds tremendous potential to develop functional and biodegradable implants for orthopaedic applications.

Background

Large bone defects still challenge the orthopaedic surgeon. Local vascularity at the site of the fracture has an important influence on the healing procedure. Vascular endothelial growth factor (VEGF) and it's receptor (VEGFR2) are potent inducer of angiogenesis during the fracture healing. Aim of the present study was the investigation of critical size fracture (CSF) healing in VEGFR2-luc mice using tailored scaffolds.

Background

Transcription factor nuclear factor E2p45-related factor 2 (Nrf2) is crucial for controlling the antioxidant response and maintaining cellular redox homeostasis. Binding of Nrf2 to antioxidant response elements (ARE) promotes the expression of anti-oxidative stress enzymes. In osteoblasts, Nrf2 directly interacts with Runx2, a strong transcriptional activator of osteoblast-specific genes. Sox9, a key regulator of chondrocyte differentiation is dominant over Runx2 in mesenchymal chondrogenic precursors. We therefore aimed to elucidate the role of Nrf2, and its regulation of Sox9, in chondrocytes.

Adult chondrocytes experience a hypoxic environment in vivo. Culturing chondrocytes under oxygen tension that more closely resembles the in vivo situation, i.e. hypoxic conditions, has been shown to have positive effects on matrix synthesis. During redifferentiation of expanded chondrocytes, hypoxia increased collagen type II expression. However, the mechanism by which hypoxia enhances redifferentiation is still incompletely elucidated. We employed micro-bioreactor technology to elucidate the contribution of TGF-β superfamily ligands to the chondrocyte differentiation process under hypoxic conditions in vitro.

Dedifferentiated chondrocytes in alginate were cultured for 48 hours under hypoxic (1% pO2) or normoxic (20%) conditions, using specialized bioreactor technology. Gene expression of chondrocyte-specific markers (SOX9, COL2A1, COL1A1, AGC1 and MMP13) as well as established hypoxia-controlled genes (GDF1-, PHD3, HAS2, VEGF, COX2) and components of the TGF-β superfamily signaling pathways were analyzed by qPCR and protein expression after 48 hours in combination with TGF-β superfamily ligand-specific siRNA as well as selected TGF-β superfamily receptor inhibitors.

Hypoxic culture showed robust upregulation of the selected hypoxia-specific marker genes. In addition, well-established chondrocyte-specific markers like SOX9 and collagen type II were upregulated. TGF-β isoforms were selectively upregulated under hypoxia on both mRNA and protein level. In addition, both Activin receptor-like kinases, ALK1 and ALK5, were upregulated under hypoxia, while respective type II and III receptors were unresponsive. The hypoxia-induced COL2 expression was abrogated by TGF-β2 siRNA, as was ALK5 inhibition. Our data strongly indicates that TGF-β superfamily signaling pathways are involved in chondrocyte redifferentiation under low oxygen tension in vitro.

Aim of the study was to evaluate if abrasion-arthroplasty (AAP) and abrasion-chondroplasty (ACP) leads to a release of mesenchymal stem cell (MSC) like cells from the bone marrow to the joint cavity where they probably differentiate into a chondrogenic phenotype.

Introduction: Osteomyelitis often causes functional impairment due to tissue destruction and the incidence of this condition appears to be increasing. Antimicrobial peptides (AP) are effectors of the innate defence system and play a key role in host protection at cellular surfaces. Human beta-defensins (HBD) represent a major subclass of antimicrobial peptides and act as a first line defence through their broad spectrum of potent antimicrobial activity (1). The aim of the present in vitro and in vivo investigations was to study the expression and regulation of HBD-2 and -3 in the case of bacterial bone infection and to analyze the effects of immunosuppressive drugs on bone-derived AP-expression.

Methods: Samples of healthy human bone, osteomyelitic bone and cultured osteoblasts (primary-, hFOB- and SAOS-2 cells) were assessed for the expression of HBD-2/-3 by RT-PCR, immunohistochemistry or ELISA. Regulation of HBD-2/-3 was studied after exposure to Staphylococcus aureus (SAS) or Pseudomonas aeruginosa (PAS), proinflammatory cytokines (IL-1, 10ng/ml) and immunosuppressive drugs (glucocorticoids, methotrexate) and was assayed by ELISA. An osteomyelitis mouse model was performed to demonstrate the regulation of the murine homologues of HBD-2/-3 by real time RT-PCR and immunohistochemistry.

Results: ELISA experiments demonstrated, that samples of infected bone produce higher levels of endogenous antibiotics such as HBD-2 when compared with samples of healthy bone. After exposure of osteoblasts to bacteria or proinflammatory cytokines a clear HBD-2/-3 induction was observed. Additional treatment with glucocorticoids or methotrexate prevented bacteria mediated HBD-2 induction in cultured osteoblasts. The osteomyelitis mouse model demonstrated transcriptional up-regulation of the murine HBD-homologues in bone after intra-osseous contamination of the tibia.

Discussion: Our study firstly demonstrate that osteoblasts are able to produce anti-inflammatory peptides such as HBD-2 in vitro and in an animal model of staphylococcal osteomyelitis. We provide evidence for a new role of osteoblasts during infection of bone tissues, namely, the ability to produce antimicrobial peptides and modulating immune responses in inflammatory bone diseases.

Immunosuppressive drugs such as glucocorticoids or methotrexate may increase the susceptibility to bone infection by decreasing AP-expression levels in case of microbial challenge. Novel approaches to management are required particularly in the era of multi-resistant bacterial strains. Current investigation will focus on the regulation of human β-Defensins in bone and may allow artificial amplification for prevention of bacterial bone infection in the future.

Our aim was to investigate vascular endothelial growth factor (VEGF) expression after lacerations of a meniscus in a rabbit model. Specimens of meniscus were examined using immunohistochemistry, enzyme-linked immunoassay and the reverse transcription polymerase chain reaction after one, two, five or ten weeks. In the periphery of the meniscus 90% of the lacerations had healed after five and ten weeks, but no healing was observed in the avascular area. Expression of VEGF protein and VEGF mRNA was found in the meniscus of both the operated and the contralateral sites but both were absent in control rabbits which had not undergone operation. The highest expression of VEGF was found in the avascular area after one week (p < 0.001). It then lessened at both the vascular and avascular areas, but still remained greater in comparison with the control meniscus (p < 0.05). Despite greater expression of VEGF, angiogenesis failed at the inner portion. These findings demonstrated the poor healing response in the avascular area which may not be caused by an intrinsic cellular insufficiency to stimulate angiogenesis.