Sustained hypoxia enhances chondrocyte matrix synthesis

Modulating endochondral ossification of multipotent stromal cells for bone regeneration

For years it has been recognized that engineering of large bone constructs will be feasible only if the hurdle of vascularization is overcome. Attempts to engineer bone tissue have predominantly focused on intramembranous (direct) bone formation. A relatively new and most likely more physiological approach in this line is endochondral bone formation, comprising an intermediate cartilaginous stage. Cartilage in nature is an avascular tissue and its cells are equipped to survive the poor oxygenation and nutritional conditions inherent to implanted tissues. Subsequent terminal differentiation (hypertrophy) of the chondrocytes initiates the formation of a mineralized matrix that will then be converted into bone. Through this mechanism, our long bones grow and most fractures heal through the process of secondary fracture healing. The feasibility of the attractive concept of endochondral bone tissue engineering has already been shown. Most emphasis has gone to the multipotent stromal cells because of their great potential for expansion and differentiation and immunoprivileged nature. This review will focus on the promises and current status of this new field. Further, potent modulators of endochondral bone tissue engineering, including oxygen tension and mechanical stimuli, will be discussed.

Combined 3D and hypoxic culture improves cartilage-specific gene expression in human chondrocytes

BACKGROUND AND PURPOSE

In vitro expansion of autologous chondrocytes is an essential part of many clinically used cartilage repair treatments. Native chondrocytes reside in a 3-dimensional (3D) network and are exposed to low levels of oxygen. We compared monolayer culture to combined 3D and hypoxic culture using quantitative gene expression analysis.

METHODS

Cartilage biopsies were collected from the intercondylar groove in the distal femur from 12 patients with healthy cartilage. Cells were used for either monolayer or scaffold culture. The scaffolds were clinically available MPEG-PLGA scaffolds (ASEED). After harvesting of cells for baseline investigation, the remainder was divided into 3 groups for incubation in conditions of normoxia (21% oxygen), hypoxia (5% oxygen), or severe hypoxia (1% oxygen). RNA extractions were performed 1, 2, and 6 days after the baseline time point, respectively. Quantitative RT-PCR was performed using assays for RNA encoding collagen types 1 and 2, aggrecan, sox9, ankyrin repeat domain-37, and glyceraldehyde-3-phosphate dehydrogenase relative to 2 hypoxia-stable housekeeping genes.

RESULTS

Sox9, aggrecan, and collagen type 2 RNA expression increased with reduced oxygen. On day 6, the expression of collagen type 2 and aggrecan RNA was higher in 3D culture than in monolayer culture.

INTERPRETATION

Our findings suggest that there was a combined positive effect of 3D culture and hypoxia on cartilage-specific gene expression. The positive effects of 3D culture alone were not detected until day 6, suggesting that seeding of chondrocytes onto a scaffold for matrix-assisted chondrocyte implantation should be performed earlier than 2 days before implantation.

In-vivo generation of bone via endochondral ossification by in-vitro chondrogenic priming of adult human and rat mesenchymal stem cells

BACKGROUND

Bone grafts are required to repair large bone defects after tumour resection or large trauma. The availability of patients' own bone tissue that can be used for these procedures is limited. Thus far bone tissue engineering has not lead to an implant which could be used as alternative in bone replacement surgery. This is mainly due to problems of vascularisation of the implanted tissues leading to core necrosis and implant failure. Recently it was discovered that embryonic stem cells can form bone via the endochondral pathway, thereby turning in-vitro created cartilage into bone in-vivo. In this study we investigated the potential of human adult mesenchymal stem cells to form bone via the endochondral pathway.

METHODS

MSCs were cultured for 28 days in chondrogenic, osteogenic or control medium prior to implantation. To further optimise this process we induced mineralisation in the chondrogenic constructs before implantation by changing to osteogenic medium during the last 7 days of culture.

RESULTS

After 8 weeks of subcutaneous implantation in mice, bone and bone marrow formation was observed in 8 of 9 constructs cultured in chondrogenic medium. No bone was observed in any samples cultured in osteogenic medium. Switch to osteogenic medium for 7 days prevented formation of bone in-vivo. Addition of β-glycerophosphate to chondrogenic medium during the last 7 days in culture induced mineralisation of the matrix and still enabled formation of bone and marrow in both human and rat MSC cultures. To determine whether bone was formed by the host or by the implanted tissue we used an immunocompetent transgenic rat model. Thereby we found that osteoblasts in the bone were almost entirely of host origin but the osteocytes are of both host and donor origin.

CONCLUSIONS

The preliminary data presented in this manuscript demonstrates that chondrogenic priming of MSCs leads to bone formation in vivo using both human and rat cells. Furthermore, addition of β-glycerophosphate to the chondrogenic medium did not hamper this process. Using transgenic animals we also demonstrated that both host and donor cells played a role in bone formation. In conclusion these data indicate that in-vitro chondrogenic differentiation of human MSCs could lead to an alternative and superior approach for bone tissue engineering.

Oxygen tension differentially regulates the functional properties of cartilaginous tissues engineered from infrapatellar fat pad derived MSCs and articular chondrocytes

BACKGROUND

For current tissue engineering or regenerative medicine strategies, chondrocyte (CC)- or mesenchymal stem cell (MSC)-seeded constructs are typically cultured in normoxic conditions (20% oxygen). However, within the knee joint capsule a lower oxygen tension exists.

OBJECTIVE

The objective of this study was to investigate how CCs and infrapatellar fad pad derived MSCs will respond to a low oxygen (5%) environment in 3D agarose culture. Our hypothesis was that culture in a low oxygen environment (5%) will enhance the functional properties of cartilaginous tissues engineered using both cell sources.

EXPERIMENTAL DESIGN

Cell-encapsulated agarose hydrogel constructs (seeded with CCs or infrapatellar fat pad (IFP) derived MSCs) were prepared and cultured in a chemically defined serum-free medium in the presence (CCs and MSCs) or absence (CCs only) of transforming growth factor-beta3 (TGF-β3) in normoxic (20%) or low oxygen (5%) conditions for 42 days. Constructs were assessed at days 0, 21 and 42 in terms of mechanical properties, biochemical content and histologically.

RESULTS

Low oxygen tension (5%) was observed to promote extracellular matrix (ECM) production by CCs cultured in the absence of TGF-β3, but was inhibitory in the presence of TGF-β3. In contrast, a low oxygen tension enhanced chondrogenesis of IFP constructs in the presence of TGF-β3, leading to superior mechanical functionality compared to CCs cultured in identical conditions.

CONCLUSIONS

Extrapolating the results of this study to the in vivo setting, it would appear that joint fat pad derived MSCs may possess a superior potential to generate a functional repair tissue in low oxygen tensions. However, in the context of in vitro cartilage tissue engineering, CCs maintained in normoxic conditions in the presence of TGF-β3 generate the most mechanically functional tissue.

Advances in Tissue Engineering Techniques for Articular Cartilage Repair

The limited repair potential of human articular cartilage contributes to development of debilitating osteoarthritis and remains a great clinical challenge. This has led to evolution of cartilage treatment strategies from palliative to either reconstructive or reparative methods in an attempt to delay or "bridge the gap" to joint replacement. Further development of tissue engineering-based cartilage repair methods have been pursued to provide a more functional biological tissue. Currently, tissue engineering of articular cartilage has three cornerstones; a cell population capable of proliferation and differentiation into mature chondrocytes, a scaffold that can host these cells, provide a suitable environment for cellular functioning and serve as a sustained-release delivery vehicle of chondrogenic growth factors and thirdly, signaling molecules and growth factors that stimulate the cellular response and the production of a hyaline extracellular matrix (ECM). The aim of this review is to summarize advances in each of these three fields of tissue engineering with specific relevance to surgical techniques and technical notes.

Additive and synergistic effects of bFGF and hypoxia on leporine meniscus cell-seeded PLLA scaffolds

Injuries to avascular regions of menisci do not heal and result in significant discomfort to patients. Current treatments, such as partial meniscectomy, alleviate these symptoms in the short term but lead to premature osteoarthritis as a result of compromised stability and changes in knee biomechanics. Thus, tissue engineering of the meniscus may provide an alternative treatment modality to overcome this problem. In this experiment, a scaffold-based tissue-engineering approach was utilized to regenerate the meniscus. Meniscus cells were cultured on poly-L-lactic acid scaffolds in normoxic (approximately 21% oxygen) or hypoxic (approximately 2% oxygen) conditions in the presence or absence of the growth factor, basic fibroblast growth factor (bFGF). At t = 4 weeks, histological sections of constructs showed presence of collagen and glycosaminoglycan (GAG) in all groups. Immunohistochemical staining showed the presence of collagen I in all groups and collagen II in groups cultured under hypoxic conditions. bFGF in the culture medium significantly increased cell number/construct by 25%, regardless of culture conditions. For GAG/construct, synergistic increases were observed in constructs cultured in hypoxic conditions and bFGF (two-fold) when compared to constructs cultured in normoxic conditions. Compressive tests showed synergistic increases in the relaxation modulus and coefficient of viscosity and additive increases in the instantaneous modulus for constructs cultured under hypoxic conditions and bFGF, when compared to constructs cultured under normoxic conditions. Overall, these results demonstrate that bFGF and hypoxia can significantly enhance the ability of meniscus cells to produce GAGs and improve the compressive properties of tissue-engineered meniscus constructs in vitro.

Validation of suitable house keeping genes for hypoxia-cultured human chondrocytes

Background

Hypoxic culturing of chondrocytes is gaining increasing interest in cartilage research. Culturing of chondrocytes under low oxygen tension has shown several advantages, among them increased synthesis of extracellular matrix and increased redifferentiation of dedifferentiated chondrocytes. Quantitative gene expression analyses such as quantitative real-time PCR (qRT-PCR) are powerful tools in the investigation of underlying mechanisms of cell behavior and are used routinely for differentiation and phenotype assays. However, the genes used for normalization in normoxic cell-cultures might not be suitable in the hypoxic environment. The objective of this study was to determine hypoxia-stable housekeeping genes (HKG) for quantitative real-time PCR (qRT-PCR) in human chondrocytes cultured in 21%, 5% and 1% oxygen by geNorm and NormFinder analyses.

Results

The chondrocytic response to the hypoxic challange was validated by a significant increase in expression of the hypoxia-inducible gene ankyrin repeat 37 as well as SOX9 in hypoxia. When cultured on the 3-dimentional (3D) scaffold TATA-binding protein (TBP) exhibited the highest expression stability with NormFinder while Ribosomal protein L13a (RPL13A) and beta2-microglobulin (B2M) were the most stable using geNorm analysis. In monolayer RPL13A were the most stable gene using NormFinder, while geNorm assessed RPL13A and human RNA polymerase II (RPII) as most stable. When examining the combination of (3D) culturing and monolayer RPL13A and B2M showed the highest expression stability from geNorm analysis while RPL13A also showed the highest expression stability using NormFinder. Often used HKG such as beta actin (ACTB) and glyceraldehyde-3-phosphate dehydrogenase (GAPDH) were the most unstable genes investigated in all comparisons. The pairwise variations for the two most stable HKG in each group were all below the cut-off value of 0.15, suggesting that the two most stable HKG from geNorm analysis would be sufficient for qRT-PCR.

Conclusion

All data combined we recommend RPL13A, B2M and RPII as the best choice for qRT-PCR analyses when comparing normoxic and hypoxic cultured human chondrocytes although other genes might also be suitable. However, the matching of HKG to target genes by means of a thorough investigation of the stability in each study would always be preferable.