Redifferentiation of in vitro expanded adult articular chondrocytes by combining the hanging-drop cultivation method with hypoxic environment

The iPSC secretome is beneficial for in vitro propagation of primary osteoarthritic chondrocytes cell lines

BACKGROUND

One of the obstacles to autologous chondrocyte implantation (ACI) is obtaining a large quantity of chondrocytes without depletion of their properties. The conditioned medium (CM) from different subpopulations of stem cells (mesenchymal stromal cells (MSC) or induced pluripotent stem cells (iPSC)) could be a gamechanger. MSCs' potential is related to the donor's health and age, which could be omitted when, as a source, iPSCs are used. There is a lack of data regarding their use in the chondrocyte culture expansion. Thus, we wanted to verify whether iPSC-CM could be beneficial for the cell culture of primary chondrocyte cells.

METHODS

We added the iPSC-CMs from GPCCi001-A and ND 41658*H cells to the culture of primary chondrocyte cell lines isolated from OA patients (n = 6) for other two passages. The composition of the CM was evaluated using Luminex technology. Then, we analysed the senescence, proliferation rate and using flow cytometry: viability, distribution of cell cycle phases, production of reactive oxygen species (ROS) and double-strand breaks. The cartilage-related markers were evaluated using Western blot and immunofluorescence. Additionally, a three-dimensional cell culture was used to determine the potential to form cartilage particles.

RESULTS

iPSC-CM increased proliferation and diminished cell ROS production and senescence. CM influenced the cartilage-related protein expression and promoted the growth of cartilage particles. The cell exposed to CM did not lose the ECM proteins, suggesting the chondroprotective effect for prolonged culture time.

CONCLUSION

Our preliminary results suggest a beneficial effect on maintaining chondrocyte biology during in vitro expansion.

Banked Primary Progenitor Cells for Allogeneic Intervertebral Disc (IVD) Therapy: Preclinical Qualification and Functional Optimization within a Cell Spheroid Formulation Process

Background/Objectives: Biological products are emerging as therapeutic management options for intervertebral disc (IVD) degenerative affections and lower back pain. Autologous and allogeneic cell therapy protocols have been clinically implemented for IVD repair. Therein, several manufacturing process design considerations were shown to significantly influence clinical outcomes. The primary objective of this study was to preclinically qualify (chondrogenic potential, safety, resistance to hypoxic and inflammatory stimuli) cryopreserved primary progenitor cells (clinical grade FE002-Disc cells) as a potential cell source in IVD repair/regeneration. The secondary objective of this study was to assess the cell source's delivery potential as cell spheroids (optimization of culture conditions, potential storage solutions). Methods/Results: Safety (soft agar transformation, β-galactosidase, telomerase activity) and functionality-related assays (hypoxic and inflammatory challenge) confirmed that the investigated cellular active substance was highly sustainable in defined cell banking workflows, despite possessing a finite in vitro lifespan. Functionality-related assays confirmed that the retained manufacturing process yielded strong collagen II and glycosaminoglycan (GAG) synthesis in the spheroids in 3-week chondrogenic induction. Then, the impacts of various process parameters (induction medium composition, hypoxic incubation, terminal spheroid lyophilization) were studied to gain insights on their criticality. Finally, an optimal set of technical specifications (use of 10 nM dexamethasone for chondrogenic induction, 2% O2 incubation of spheroids) was set forth, based on specific fine tuning of finished product critical functional attributes. Conclusions: Generally, this study qualified the considered FE002-Disc progenitor cell source for further preclinical investigation based on safety, quality, and functionality datasets. The novelty and significance of this study resided in the establishment of defined processes for preparing fresh, off-the-freezer, or off-the-shelf IVD spheroids using a preclinically qualified allogeneic human cell source. Overall, this study underscored the importance of using robust product components and optimal manufacturing process variants for maximization of finished cell-based formulation quality attributes.

Advancements in Chondrocyte 3-Dimensional Embedded Culture: Implications for Tissue Engineering and Regenerative Medicine

Cartilage repair necessitates regenerative medicine because of the unreliable healing mechanism of cartilage. To yield a sufficient number of cells for transplantation, chondrocytes must be expanded in culture. However, in 2D culture, chondrocytes tend to lose their distinctive phenotypes and functionalities after serial passage, thereby limiting their efficacy for tissue engineering purposes. The mechanism of dedifferentiation in 2D culture can be attributed to various factors, including abnormal nuclear strength, stress-induced mitochondrial impairment, chromatin remodeling, ERK-1/2 and the p38/mitogen-activated protein kinase (MAPK) signaling pathway. These mechanisms collectively contribute to the loss of chondrocyte phenotype and reduced production of cartilage-specific extracellular matrix (ECM) components. Chondrocyte 3D culture methods have emerged as promising solutions to prevent dedifferentiation. Techniques, such as scaffold-based culture and scaffold-free approaches, provide chondrocytes with a more physiologically relevant environment, promoting their differentiation and matrix synthesis. These methods have been used in cartilage tissue engineering to create engineered cartilage constructs for transplantation and joint repair. However, chondrocyte 3D culture still has limitations, such as low viability and proliferation rate, and also difficulties in passage under 3D condition. These indicate challenges of obtaining a sufficient number of chondrocytes for large-scale tissue production. To address these issues, ongoing studies of many research groups have been focusing on refining culture conditions, optimizing scaffold materials, and exploring novel cell sources such as stem cells to enhance the quality and quantity of engineered cartilage tissues. Although obstacles remain, continuous endeavors to enhance culture techniques and overcome limitations offer a promising outlook for the advancement of more efficient strategies for cartilage regeneration.

Autologous and Allogeneic Cytotherapies for Large Knee (Osteo)Chondral Defects: Manufacturing Process Benchmarking and Parallel Functional Qualification

Cytotherapies are often necessary for the management of symptomatic large knee (osteo)-chondral defects. While autologous chondrocyte implantation (ACI) has been clinically used for 30 years, allogeneic cells (clinical-grade FE002 primary chondroprogenitors) have been investigated in translational settings (Swiss progenitor cell transplantation program). The aim of this study was to comparatively assess autologous and allogeneic approaches (quality, safety, functional attributes) to cell-based knee chondrotherapies developed for clinical use. Protocol benchmarking from a manufacturing process and control viewpoint enabled us to highlight the respective advantages and risks. Safety data (telomerase and soft agarose colony formation assays, high passage cell senescence) and risk analyses were reported for the allogeneic FE002 cellular active substance in preparation for an autologous to allogeneic clinical protocol transposition. Validation results on autologous bioengineered grafts (autologous chondrocyte-bearing Chondro-Gide scaffolds) confirmed significant chondrogenic induction (COL2 and ACAN upregulation, extracellular matrix synthesis) after 2 weeks of co-culture. Allogeneic grafts (bearing FE002 primary chondroprogenitors) displayed comparable endpoint quality and functionality attributes. Parameters of translational relevance (transport medium, finished product suturability) were validated for the allogeneic protocol. Notably, the process-based benchmarking of both approaches highlighted the key advantages of allogeneic FE002 cell-bearing grafts (reduced cellular variability, enhanced process standardization, rationalized logistical and clinical pathways). Overall, this study built on our robust knowledge and local experience with ACI (long-term safety and efficacy), setting an appropriate standard for further clinical investigations into allogeneic progenitor cell-based orthopedic protocols.

Could hypoxia rehabilitate the osteochondral diseased interface? Lessons from the interplay of hypoxia and purinergic signals elsewhere

The osteochondral unit comprises the articular cartilage (90%), subchondral bone (5%) and calcified cartilage (5%). All cells present at the osteochondral unit that is ultimately responsible for matrix production and osteochondral homeostasis, such as chondrocytes, osteoblasts, osteoclasts and osteocytes, can release adenine and/or uracil nucleotides to the local microenvironment. Nucleotides are released by these cells either constitutively or upon plasma membrane damage, mechanical stress or hypoxia conditions. Once in the extracellular space, endogenously released nucleotides can activate membrane-bound purinoceptors. Activation of these receptors is fine-tuning regulated by nucleotides' breakdown by enzymes of the ecto-nucleotidase cascade. Depending on the pathophysiological conditions, both the avascular cartilage and the subchondral bone subsist to significant changes in oxygen tension, which has a tremendous impact on tissue homeostasis. Cell stress due to hypoxic conditions directly influences the expression and activity of several purinergic signalling players, namely nucleotide release channels (e.g. Cx43), NTPDase enzymes and purinoceptors. This review gathers experimental evidence concerning the interplay between hypoxia and the purinergic signalling cascade contributing to osteochondral unit homeostasis. Reporting deviations to this relationship resulting from pathological alterations of articular joints may ultimately unravel novel therapeutic targets for osteochondral rehabilitation. At this point, one can only hypothesize how hypoxia mimetic conditions can be beneficial to the ex vivo expansion and differentiation of osteo- and chondro-progenitors for auto-transplantation and tissue regenerative purposes.

Chondrocytes In Vitro Systems Allowing Study of OA

Osteoarthritis (OA) is an extremely complex disease, as it combines both biological-chemical and mechanical aspects, and it also involves the entire joint consisting of various types of tissues, including cartilage and bone. This paper describes the methods of conducting cell cultures aimed at searching for the mechanical causes of OA development, therapeutic solutions, and methods of preventing the disease. It presents the systems for the cultivation of cartilage cells depending on the level of their structural complexity, and taking into account the most common solutions aimed at recreating the most important factors contributing to the development of OA, that is mechanical loads. In-vitro systems used in tissue engineering to investigate the phenomena associated with OA were specified depending on the complexity and purposefulness of conducting cell cultures.

Spheroid-Based Tissue Engineering Strategies for Regeneration of the Intervertebral Disc

Degenerative disc disease, a painful pathology of the intervertebral disc (IVD), often causes disability and reduces quality of life. Although regenerative cell-based strategies have shown promise in clinical trials, none have been widely adopted clinically. Recent developments demonstrated that spheroid-based approaches might help overcome challenges associated with cell-based IVD therapies. Spheroids are three-dimensional multicellular aggregates with architecture that enables the cells to differentiate and synthesize endogenous ECM, promotes cell-ECM interactions, enhances adhesion, and protects cells from harsh conditions. Spheroids could be applied in the IVD both in scaffold-free and scaffold-based configurations, possibly providing advantages over cell suspensions. This review highlights areas of future research in spheroid-based regeneration of nucleus pulposus (NP) and annulus fibrosus (AF). We also discuss cell sources and methods for spheroid fabrication and characterization, mechanisms related to spheroid fusion, as well as enhancement of spheroid performance in the context of the IVD microenvironment.

Anisotropic Chitosan Scaffolds Generated by Electrostatic Flocking Combined with Alginate Hydrogel Support Chondrogenic Differentiation

The replacement of damaged or degenerated articular cartilage tissue remains a challenge, as this non-vascularized tissue has a very limited self-healing capacity. Therefore, tissue engineering (TE) of cartilage is a promising treatment option. Although significant progress has been made in recent years, there is still a lack of scaffolds that ensure the formation of functional cartilage tissue while meeting the mechanical requirements for chondrogenic TE. In this article, we report the application of flock technology, a common process in the modern textile industry, to produce flock scaffolds made of chitosan (a biodegradable and biocompatible biopolymer) for chondrogenic TE. By combining an alginate hydrogel with a chitosan flock scaffold (CFS+ALG), a fiber-reinforced hydrogel with anisotropic properties was developed to support chondrogenic differentiation of embedded human chondrocytes. Pure alginate hydrogels (ALG) and pure chitosan flock scaffolds (CFS) were studied as controls. Morphology of primary human chondrocytes analyzed by cLSM and SEM showed a round, chondrogenic phenotype in CFS+ALG and ALG after 21 days of differentiation, whereas chondrocytes on CFS formed spheroids. The compressive strength of CFS+ALG was higher than the compressive strength of ALG and CFS alone. Chondrocytes embedded in CFS+ALG showed gene expression of chondrogenic markers (COL II, COMP, ACAN), the highest collagen II/I ratio, and production of the typical extracellular matrix such as sGAG and collagen II. The combination of alginate hydrogel with chitosan flock scaffolds resulted in a scaffold with anisotropic structure, good mechanical properties, elasticity, and porosity that supported chondrogenic differentiation of inserted human chondrocytes and expression of chondrogenic markers and typical extracellular matrix.

Tuning the Phenotype of Cartilage Tissue Mimics by Varying Spheroid Maturation and Methacrylamide-Modified Gelatin Hydrogel Characteristics

In hybrid bioprinting of cartilage tissue constructs, spheroids are used as cellular building blocks and combined with biomaterials for dispensing. However, biomaterial intrinsic cues can deeply affect cell fate and to date, the influence of hydrogel encapsulation on spheroid viability and phenotype has received limited attention. This study assesses this need and unravels 1) how the phenotype of spheroid-laden constructs can be tuned through adjusting the hydrogel physico-chemical properties and 2) if the spheroid maturation stage prior to encapsulation is a determining factor for the construct phenotype. Articular chondrocyte spheroids with a cartilage specific extracellular matrix (ECM) are generated and different maturation stages, early-, mid-, and late-stage (3, 7, and 14 days, respectively), are harvested and encapsulated in 10, 15, or 20 w/v% methacrylamide-modified gelatin (gelMA) for 14 days. The encapsulation of immature spheroids do not lead to a cartilage-like ECM production but when more mature mid- or late-stage spheroids are combined with a certain concentration of gelMA, a fibrocartilage-like as well as a hyaline cartilage-like phenotype can be induced. As a proof of concept, late-stage spheroids are bioprinted using a 10 w/v% gelMA-Irgacure 2959 solution with the aim to test the processing potential of the spheroid-laden bioink.

Feasibility of Human Platelet Lysate as an Alternative to Foetal Bovine Serum for In Vitro Expansion of Chondrocytes

Osteoarthritis (OA) is a degenerative joint disease that affects a lot of people worldwide. Current treatment for OA mainly focuses on halting or slowing down the disease progress and to improve the patient’s quality of life and functionality. Autologous chondrocyte implantation (ACI) is a new treatment modality with the potential to promote regeneration of worn cartilage. Traditionally, foetal bovine serum (FBS) is used to expand the chondrocytes. However, the use of FBS is not ideal for the expansion of cells mean for clinical applications as it possesses the risk of animal pathogen transmission and animal protein transfer to host. Human platelet lysate (HPL) appears to be a suitable alternative to FBS as it is rich in biological factors that enhance cell proliferation. Thus far, HPL has been found to be superior in promoting chondrocyte proliferation compared to FBS. However, both HPL and FBS cannot prevent chondrocyte dedifferentiation. Discrepant results have been reported for the maintenance of chondrocyte redifferentiation potential by HPL. These differences are likely due to the diversity in the HPL preparation methods. In the future, more studies on HPL need to be performed to develop a standardized technique which is capable of producing HPL that can maintain the chondrocyte redifferentiation potential reproducibly. This review discusses the in vitro expansion of chondrocytes with FBS and HPL, focusing on its capability to promote the proliferation and maintain the chondrogenic characteristics of chondrocytes.

3D composite engineered using supercritical CO2 decellularized porcine cartilage scaffold, chondrocytes, and PRP: Role in articular cartilage regeneration

At present, no definitive treatment for articular cartilage defects has been perfected. Most of the previous treatments involved multiple drilling and microfracture over defect sites with repair-related substances, which poses a limited therapeutic effect. End-stage therapy includes artificial knee joint replacement. In this study, we prepared a novel decellularized natural cartilage scaffold from porcine articular cartilage by supercritical CO₂ extraction technology and three-dimensional (3D) composites made using decellularized porcine cartilage graft (dPCG) as scaffolds, platelet-rich plasma (PRP), thrombin as signals and chondrocytes as cells for the treatment of articular cartilage defects. In this study, in vitro and in vivo cartilage regeneration and the expression of chondrogenic markers were examined. Decellularized cartilage graft (dPCG) was evaluated for the extent of cell and DNA removal. Residual cartilage ECM structure was confirmed to be type II collagen by SDS PAGE and immunostaining. The new 3D composite with dPCG (100 mg and 2 × 10⁶ chondrocytes) scaffold promotes chondrogenic marker expression in vitro. We found that the in vivo 3D composite implanted cartilage defect showed significant regeneration relative to the blank and control implant. Immunohistochemical staining showed increase of expression including Collagen type II and aggrecan in 3D composite both in vitro and in vivo studies. In this study, the bioengineered 3D composite by combining dPCG scaffold, chondrocytes, and PRP facilitated the chondrogenic marker expression in both in vitro and in vivo models with accelerated cartilage regeneration. This might serve the purpose of clinical treatment of large focal articular cartilage defects in humans in the near future.

Biological perspectives and current biofabrication strategies in osteochondral tissue engineering

Articular cartilage and the underlying subchondral bone are crucial in human movement and when damaged through disease or trauma impacts severely on quality of life. Cartilage has a limited regenerative capacity due to its avascular composition and current therapeutic interventions have limited efficacy. With a rapidly ageing population globally, the numbers of patients requiring therapy for osteochondral disorders is rising, leading to increasing pressures on healthcare systems. Research into novel therapies using tissue engineering has become a priority. However, rational design of biomimetic and clinically effective tissue constructs requires basic understanding of osteochondral biological composition, structure, and mechanical properties. Furthermore, consideration of material design, scaffold architecture, and biofabrication strategies, is needed to assist in the development of tissue engineering therapies enabling successful translation into the clinical arena. This review provides a starting point for any researcher investigating tissue engineering for osteochondral applications. An overview of biological properties of osteochondral tissue, current clinical practices, the role of tissue engineering and biofabrication, and key challenges associated with new treatments is provided. Developing precisely engineered tissue constructs with mechanical and phenotypic stability is the goal. Future work should focus on multi-stimulatory environments, long-term studies to determine phenotypic alterations and tissue formation, and the development of novel bioreactor systems that can more accurately resemble the in vivo environment.

Assessment and Comparison of the Efficacy of Methotrexate, Prednisolone, Adalimumab, and Tocilizumab on Multipotency of Mesenchymal Stem Cells

Mesenchymal stem cell (MSC)-based articular regeneration might be beneficial for both protecting and rebuilding cartilaginous tissues in the management of rheumatoid arthritis. However, it is unclear how current immunosuppressive strategies influence the multipotency of MSCs. The present study was undertaken to profile the direct effectiveness of major antirheumatic drugs including methotrexate, prednisolone, adalimumab, and tocilizumab on the multipotency of MSCs, with a special focus on chondrogenesis. The inhibitory effects of methotrexate on adipogenesis, osteogenesis, and chondrogenesis were observed to occur in a dose-dependent manner in an in vitro differentiation system. Prednisolone enhanced adipogenesis, but reduced alkaline phosphatase activity in osteoprogenitors and suppressed the formation of chondrospheroids. Adalimumab suppressed alkaline phosphatase activity, while tocilizumab diminished osteogenesis and chondrogenesis of MSCs in vitro. Chondrogenesis of antirheumatic drug-treated MSCs was also evaluated in vivo using a scaffolded spheroid-engrafted murine model. The biologics examined appeared to be relatively safe for cartilaginous formation, but methotrexate and prednisolone exhibited opposing influences on chondrogenesis. Taken together, these results reveal the direct efficacy of major antirheumatic agents on the multipotency of MSCs. Therefore, our findings suggest that optimization of medication protocols is further required for therapeutic approaches involving cartilaginous tissue engineering.

Chondrogenic differentiation of mesenchymal stem/stromal cells on 3D porous poly (ε-caprolactone) scaffolds: Effects of material alkaline treatment and chondroitin sulfate supplementation

Cartilage defects resultant from trauma or degenerative diseases (e.g., osteoarthritis) can potentially be repaired using tissue engineering (TE) strategies combining progenitor cells, biomaterial scaffolds and bio-physical/chemical cues. This work examines promoting chondrogenic differentiation of human bone marrow mesenchymal stem/stromal cells (BM-MSCs) by combining the effects of modified poly (ε-caprolactone) (PCL) scaffolds hydrophilicity and chondroitin sulfate (CS) supplementation in a hypoxic 5% oxygen atmosphere. 3D-extruded PCL scaffolds, characterized by μCT, featured a 21 mm^-1 surface area to volume ratio, 390 μm pore size and approximately 100% pore interconnectivity. Scaffold immersion in sodium hydroxide solutions for different periods of time had major effects in scaffold surface morphology, wettability and mechanical properties, but without improvements on cell adhesion. In-situ chondrogenic differentiation of BM-MSC seeded in 3D-extruded PCL scaffolds resulted in higher cell populations and ECM deposition along all scaffold structure, when chondrogenesis was preceded by an expansion phase. Additionally, CS supplementation during BM-MSC expansion was crucial to enhance aggrecan gene expression, known as a hallmark of chondrogenesis. Overall, this study presents an approach to tailor the wettability and mechanical properties of PCL scaffolds and supports the use of CS-supplementation as a biochemical cue in integrated TE strategies for cartilage regeneration.

Adipose-Derived Mesenchymal Stem Cell Chondrospheroids Cultured in Hypoxia and a 3D Porous Chitosan/Chitin Nanocrystal Scaffold as a Platform for Cartilage Tissue Engineering

Articular cartilage degeneration is one of the most common causes of pain and disability in middle-aged and older people. Tissue engineering (TE) has shown great therapeutic promise for this condition. The design of cartilage regeneration constructs must take into account the specific characteristics of the cartilaginous matrix, as well as the avascular nature of cartilage and its cells' peculiar arrangement in isogenic groups. Keeping these factors in mind, we have designed a 3D porous scaffold based on genipin-crosslinked chitosan/chitin nanocrystals for spheroid chondral differentiation of human adipose tissue-derived mesenchymal stem cells (hASCs) induced in hypoxic conditions. First, we demonstrated that, under low oxygen conditions, the chondrospheroids obtained express cartilage-specific markers including collagen type II (COL2A1) and aggrecan, lacking expression of osteogenic differentiation marker collagen type I (COL1A2). These results were associated with an increased expression of hypoxia-inducible factor 1α, which positively directs COL2A1 and aggrecan expression. Finally, we determined the most suitable chondrogenic differentiation pattern when hASC spheroids were seeded in the 3D porous scaffold under hypoxia and obtained a chondral extracellular matrix with a high sulphated glycosaminoglycan content, which is characteristic of articular cartilage. These findings highlight the potential use of such templates in cartilage tissue engineering.

Generating Chondromimetic Mesenchymal Stem Cell Spheroids by Regulating Media Composition and Surface Coating

INTRODUCTION

Spheroids of mesenchymal stem cells (MSCs) in cartilage tissue engineering have been shown to enhance regenerative potential owing to their 3D structure. In this study, we explored the possibility of priming spheroids under different media to replace the use of inductive surface coatings for chondrogenic differentiation.

METHODS

Rat bone marrow-derived MSCs were organized into cell spheroids by the hanging drop technique and subsequently cultured on hyaluronic acid (HA) coated or non-coated well plates under different cell media conditions. Endpoint analysis included cell viability, DNA and Glycosaminoglycan (GAG) and collagen content, gene expression and immunohistochemistry.

RESULTS

For chondrogenic applications, MSC spheroids derived on non-coated surfaces outperformed the spheroids derived from HA-coated surfaces in matrix synthesis and collagen II gene expression. Spheroids on non-coated surfaces gave rise to the highest collagen and GAG when primed with medium containing insulin-like growth factor (IGF) for 1 week during spheroid formation. Spheroids that were grown in chondroinductive raw material-inclusive media such as aggrecan or chondroitin sulfate exhibited the highest Collagen II gene expression in the non-coated surface at 1 week.

CONCLUSION

Media priming by growth factors and raw materials might be a more predictive influencer of chondrogenesis compared to inductive-surfaces. Such tailored bioactivity of the stem cell spheroids in the stage of the spheroid formation may give rise to a platform technology that may eventually produce spheroids capable of chondrogenesis achieved by mere media manipulation, skipping the need for additional culture on a modified surface, that paves the way for cost-effective technologies.

Co-expression of 1α-hydroxylase and vitamin D receptor in human articular chondrocytes

BACKGROUND

The aim was to investigate whether resident chondrocytes in human articular cartilage and in subculture express vitamin D receptor (VDR) and the enzyme that hydroxylates the prohormone 25(OH)D₃ to the active hormone 1α,25(OH)₂D₃, namely 1α-hydroxylase (CYP27B1). Any putative effects of vitamin D on chondrocytes were also explored.

METHODS

Cartilage from human osteoarthritic knee joints, cultured chondrocytes and cells grown in 3D spheroids were examined for the expression of VDR and 1α-hydroxylase by PCR, Western blots and immunolabelling. Receptor engagement was judged by visualizing nuclear translocation. The effects of 25(OH)D₃ and 1α,25(OH)₂D₃ on chondrocyte functions were assessed in proliferation-, chondrogenesis- and cartilage signature-gene expression assays. The capability of chondrocytes to hydroxylate 25(OH)D₃ was determined by measuring the concentration of metabolites. Finally, a putative regulation of receptor and enzyme expression by 1α,25(OH)₂D₃ or interleukin (IL)-1β, was investigated by Western blot.

RESULTS

Gene expression was positive for VDR in freshly isolated cells from native cartilage, cells subcultured in monolayers and in spheroids, whereas protein expression, otherwise judged low, was apparent in monolayers. Nuclear translocation of VDR occurred upon 1α,25(OH)₂D₃ treatment. Transcripts for 1α-hydroxylase were detected in freshly isolated cells, cultured cells and spheroids. Western blots and immunolabelling detected 1α-hydroxylase protein in all materials, while staining of tissue appeared confined to cells at the superficial layer. A dose-dependent 1α,25(OH)₂D₃ production was measured when the enzyme substrate was supplied to cell cultures. Western blots revealed that the VDR, but not 1α-hydroxylase, was induced by IL-1β treatment in adherent cells. Proliferation in monolayers was enhanced by both 25(OH)D₃ and 1α,25(OH)₂D₃, and both compounds had negative effects on chondrogenesis and cartilage-matrix genes.

CONCLUSIONS

VDR expression in resident cartilage chondrocytes, generally considered differentiated cells, is elusive. A similar pattern applies for redifferentiated chondrocytes in spheroid cultures, whereas dedifferentiated cells, established in monolayers, stably express VDR. Both 25(OH)D₃ and 1α,25(OH)₂D₃ are able to potentiate cell proliferation but have a negative impact in proteoglycan synthesis. Chondrocytes express 1α-hydroxylase and may contribute to the production of 1α,25(OH)₂D₃ into the joint environment. Effects of vitamin D could be unfavourable in the context of cartilage matrix synthesis.

Colony, hanging drop, and methylcellulose three dimensional hypoxic growth optimization of renal cell carcinoma cell lines

Renal cell carcinoma (RCC) is the most lethal of the common urologic malignancies, comprising 3% of all human neoplasms, and the incidence of kidney cancer is rising annually. We need new approaches to target tumor cells that are resistant to current therapies and that give rise to recurrence and treatment failure. In this study, we focused on low oxygen tension and three-dimensional (3D) cell culture incorporation to develop a new RCC growth model. We used the hanging drop and colony formation methods, which are common in 3D culture, as well as a unique methylcellulose (MC) method. For the experiments, we used human primary RCC cell lines, metastatic RCC cell lines, human kidney cancer stem cells, and human healthy epithelial cells. In the hanging drop assay, we verified the potential of various cell lines to create solid aggregates in hypoxic and normoxic conditions. With the semi-soft agar method, we also determined the ability of various cell lines to create colonies under different oxygen conditions. Different cell behavior observed in the MC method versus the hanging drop and colony formation assays suggests that these three assays may be useful to test various cell properties. However, MC seems to be a particularly valuable alternative for 3D cell culture, as its higher efficiency of aggregate formation and serum independency are of interest in different areas of cancer biology.

Interplay between cytoskeletal polymerization and the chondrogenic phenotype in chondrocytes passaged in monolayer culture

Tubulin and actin exist as monomeric units that polymerize to form either microtubules or filamentous actin. As the polymerization status (monomeric/polymeric ratio) of tubulin and/or actin have been shown to be important in regulating gene expression and phenotype in non-chondrocyte cells, the objective of this study was to examine the role of cytoskeletal polymerization on the chondrocyte phenotype. We hypothesized that actin and/or tubulin polymerization status modulates the chondrocyte phenotype during monolayer culture as well as in 3D culture during redifferentiation. To test this hypothesis, articular chondrocytes were grown and passaged in 2D monolayer culture. Cell phenotype was investigated by assessing cell morphology (area and circularity), actin/tubulin content, organization and polymerization status, as well as by determination of proliferation, fibroblast and cartilage matrix gene expression with passage number. Bovine chondrocytes became larger, more elongated, and had significantly (P < 0.05) increased gene expression of proliferation-associated molecules (cyclin D1 and ki67), as well as significantly (P < 0.05) decreased cartilage matrix (type II collagen and aggrecan) and increased fibroblast-like matrix, type I collagen (COL1), gene expression by passage 2 (P2). Although tubulin polymerization status was not significantly (P > 0.05) modulated, actin polymerization was increased in bovine P2 cells. Actin depolymerization, but not tubulin depolymerization, promoted the chondrocyte phenotype by inducing cell rounding, increasing aggrecan and reducing COL1 expression. Knockdown of actin depolymerization factor, cofilin, in these cells induced further P2 cell actin polymerization and increased COL1 gene expression. To confirm that actin status regulated COL1 gene expression in human P2 chondrocytes, human P2 chondrocytes were exposed to cytochalasin D. Cytochalasin D decreased COL1 gene expression in human passaged chondrocytes. Furthermore, culture of bovine P2 chondrocytes in 3D culture on porous bone substitute resulted in actin depolymerization, which correlated with decreased expression of COL1 and proliferation molecules. In 3D cultures, aggrecan gene expression was increased by cytochalasin D treatment and COL1 was further decreased. These results reveal that actin polymerization status regulates chondrocyte dedifferentiation. Reorganization of the cytoskeleton by actin depolymerization appears to be an active regulatory mechanism for redifferentiation of passaged chondrocytes.

Redifferentiation of High-Throughput Generated Fibrochondrocyte Micro-Aggregates: Impact of Low Oxygen Tension

In meniscus tissue engineering strategies, enhancing the matrix quality of the neomeniscal tissue is important. When the differentiated phenotype of fibrochondrocytes is lost, the quality of the matrix becomes compromised. The objective of this study was to produce uniform fibrochondrocyte micro-aggregates with desirable phenotype and tissue homogeneity in large quantities using a simple and reproducible method. Furthermore, we investigated if hypoxia could enhance the matrix quality. Porcine fibrochondrocytes were expanded at 21% oxygen until passage 3 (P3) and a gene expression profile was determined. P3 fibrochondrocytes were cultivated in chondrogenic medium at 5 and 21% oxygen in high-throughput agarose chips containing 2,865 microwells 200 µm in diameter. Evaluation included live/dead staining, histological examination, immunohistochemistry, dimethylmethylene blue assay and real-time reverse transcriptase quantitative polymerase chain reaction of the micro-aggregates. Gene expression analysis showed a drastic decline in collagen II and high expression of collagen I during monolayer culture. After 4 days, uniform and stable micro-aggregates could be produced. The redifferentiation and matrix quality of the hypoxic cultured micro-aggregates were enhanced relative to the normoxic cultures. Sulfated glycosaminoglycan synthesis was significantly higher, and collagen II expression and the collagen II/collagen I ratio were significantly upregulated in the hypoxic cultures. High-throughput production of uniform microtissues holds promise for the generation of larger-scale tissue engineering constructs or optimization of redifferentiation mechanisms for clinical applications.

Human articular chondrocytes express functional leukotriene B4 receptors

Leukotriene B4 (LTB4) is a potent chemoattractant associated with the development of osteoarthritis (OA), while its receptors BLT1 and BLT2 have been found in synovium and subchondral bone. In this study, we have investigated whether these receptors are also expressed by human cartilage cells and their potential effects on cartilage cells. The expression of LTB4 receptors in native tissue and cultured cells was assessed by immunohistochemistry, immunocytochemistry, polymerase chain reaction (PCR) and electron microscopy. The functional significance of the LTB4 receptor expression was studied by Western blotting, using phospho-specific antibodies in the presence or absence of receptor antagonists. In further studies, the secretion of pro-inflammatory cytokines, growth factors and metalloproteinases by LTB4-stimulated chondrocytes was measured by multiplex protein assays. The effects of LTB4 in cartilage signature gene expression in cultured cells were assessed by quantitative PCR, whereas the LTB4-promoted matrix synthesis was determined using 3D pellet cultures. Both receptors were present in cultured chondrocytes, as was confirmed by immunolabelling and PCR. The relative quantification by PCR demonstrated a higher expression of the receptors in cells from healthy joints compared with OA cases. The stimulation of cultured chondrocytes with LTB4 resulted in a phosphorylation of downstream transcription factor Erk 1/2, which was reduced after blocking BLT1 signalling. No alteration in the secretion of cytokine and metalloproteinases was recorded after challenging cultured cells with LTB4; likewise, cartilage matrix gene expression and 3D tissue synthesis were unaffected. Chondrocytes express BLT1 and BLT2 receptors, and LTB4 activates the downstream Erk 1/2 pathway by engaging the high-affinity receptor BLT1. However, any putative role in cartilage biology could not be revealed, and remains to be clarified.

Hypoxia combined with spheroid culture improves cartilage specific function in chondrocytes

Controlling the chondrocyte phenotype and function in a physiologically relevant microenvironment remains a major challenge for cartilage repair in tissue engineering applications. This work presents a straightforward strategy to create a high throughput concave microwell array used for generating multicellular spheroids of chondrocytes and facilitating the maintenance of the articular chondrocyte phenotype and function by combining 3D spheroid culture with hypoxia. The polydimethylsiloxane (PDMS) concave microwells were simply produced from a concave SU-8 template fabricated using a soft-lithography approach and easily adopted for size-controlled spheroid culture. 3D spheroid culture was observed to facilitate the cartilage-specific phenotype and function maintenance as compared to 2D monolayer culture. Combining hypoxia with spheroid culture markedly increased the expressions of cartilage-specific collagen II and aggrecan at protein and mRNA levels. The hypoxia-inducible factor (HIF) signaling pathway was found to get involved in phenotype maintenance, metabolism and differentiation of chondrocytes by regulating HIF-1α and HIF-2α, respectively. The established approach provides a useful platform for a wide range of applications in the field of cartilage biology, stem cell research and high throughput 3D drug testing in cancer.

Repair and tissue engineering techniques for articular cartilage

Chondral and osteochondral lesions due to injury or other pathology commonly result in the development of osteoarthritis, eventually leading to progressive total joint destruction. Although current progress suggests that biologic agents can delay the advancement of deterioration, such drugs are incapable of promoting tissue restoration. The limited ability of articular cartilage to regenerate renders joint arthroplasty an unavoidable surgical intervention. This Review describes current, widely used clinical repair techniques for resurfacing articular cartilage defects; short-term and long-term clinical outcomes of these techniques are discussed. Also reviewed is a developmental pipeline of acellular and cellular regenerative products and techniques that could revolutionize joint care over the next decade by promoting the development of functional articular cartilage. Acellular products typically consist of collagen or hyaluronic-acid-based materials, whereas cellular techniques use either primary cells or stem cells, with or without scaffolds. Central to these efforts is the prominent role that tissue engineering has in translating biological technology into clinical products; therefore, concomitant regulatory processes are also discussed.

Cartilage Tissue Formation from Dedifferentiated Chondrocytes by Codelivery of BMP-2 and SOX-9 Genes Encoding Bicistronic Vector

Articular cartilage, when damaged by degenerative disease or trauma, has limited ability for self-repair. Recently, many trials have demonstrated that gene therapy combined with tissue engineering techniques would be a promising approach for cartilage regeneration. Bone morphogenetic protein 2 (BMP-2) is an important signal for upregulation of osteogenesis and chondrogenesis of stem cells. Sex-determining region Y box gene 9 (SOX-9) has also been reported as one of the key transcription factors for chondrogenesis. We hypothesized that codelivery of BMP-2 and SOX-9 genes would result in improved efficiency of recovery of normal chondrogenic properties in dedifferentiated chondrocytes. To this aim, we constructed a bicistronic vector encoding the BMP-2 and SOX-9 genes linked to the “self-cleaving” 2A peptide sequence. After gene delivery to dedifferentiated chondrocytes using a microporator transfection system, we confirmed over 65% delivery efficiency of the BMP-2 and SOX-9 genes. According to RT-PCR analysis and Alcian blue staining, simultaneous delivery of BMP-2/SOX-9 resulted in significantly increased expression of chondrogenesis-related markers (type II collagen and aggrecan) and GAG matrix formation compared with individual delivery of the BMP-2 or SOX-9 gene. Six weeks after in vivo transplantation, BMP-2/SOX-9 genes also showed a significant increase in cartilage formation compared with the BMP-2 or SOX-9 gene. These results demonstrate that codelivery of two chondrogenic lineage-determining genes can enhance normal chondrogenic properties of dedifferentiated chondrocytes followed by improved cartilage formation.

Behavior of Human Articular Chondrocytes During In Vivo Culture in Closed, Permeable Chambers

The exact contribution of transplanted chondrocytes for cartilage tissue repair prior expansion in monolayer culures remains undetermined. At our laboratory, we have created a new permeable chamber to study the chondrogenesis of dedifferentiated cells implanted ectopically in a closed and controlled environment. The behavior of chondrocytes has been studied in settings frequently used in clinical approaches during transplantation, namely injection of autologous chondrocyte cells in suspension (ACI), cells soaked in collagen membranes (MACI), and cells applied in a polymer gel (fibrin). As controls, we have tested the redifferentiation of chondrocytes in cell aggregates, and we have checked the proper functionality of chambers both in vitro and in vivo. After retrieval, firmed tissue-like shapes were recovered only from chambers containing cells seeded in membranes. Histomorphological, immunohistochemical, and ultrastructural analyses revealed synthesis of fibrous-like tissue, characterized by low-density collagen fibers, low collagen type II, abundant collagen type I, and low amounts of proteoglycans. Additionally, neither the collagen membranes nor the fibrin gel was reabsorbed by cells. In summary, our results show that the newly developed permeable chambers function correctly, allowing proper cell feeding and preventing cell leakage or host cell invasion. Additionally, our results suggest that, under these circumstances, chondrocytes are not able to orchestrate formation of hyaline cartilage and have little capacity to degrade artificial membranes or carrier gels such as fibrin. These are interesting observations that should be considered for understanding what role the transplanted chondrocytes play during restoration of articular cartilage after implantation.

Comparing scaffold-free and fibrin-based adipose-derived stromal cell constructs for adipose tissue engineering: an in vitro and in vivo study

Success of adipose tissue engineering for soft tissue repair has been limited by insufficient adipogenic differentiation, an unfavorable host response, and insufficient vascularization. In this study, we examined how scaffold-free spheroid and fibrin-based environments impact these parameters in human adipose-derived stromal cell (ASC)-based adipose constructs. ASCs were differentiated in spheroids or fibrin-based constructs. After 7 days, conditioned medium was collected and spheroids/fibrin-based constructs were either harvested or implanted subcutaneously in athymic mice. Following 7 days of implantation, the number of blood vessels in fibrin-based constructs was significantly higher than in spheroids (93±45 vs. 23±11 vessels/mm(2)), and the inflammatory response to fibrin-based constructs was less severe. The reasons for these results were investigated further in vitro. We found that ASCs in fibrin-based constructs secreted significantly higher levels of the angiogenic factors VEGF and HGF and lower levels of the inflammatory cytokine IL-8. Furthermore, ASCs in fibrin-based constructs secreted significantly higher levels of leptin and showed a 2.5-fold upregulation of the adipogenic transcription factor PPARG and a fourfold to fivefold upregulation of the adipocyte-specific markers FABP4, perilipin, and leptin. These results indicate that fibrin-based ASC constructs are potentially more suitable for ASC-based adipose tissue reconstruction than scaffold-free spheroids.

Scaffold-free 3D cellulose acetate membrane-based cultures form large cartilaginous constructs

Scaffold-free three-dimensional (3D) cultures provide clinical potential in cartilage regeneration. The purpose of this study was to characterize a scaffold-free 3D membrane-based culture system, in which human articular chondrocytes were cultivated on a cellulose acetate membrane filter, and compare it to pellet and monolayer cultures. Chondrocytes were expanded in monolayer culture for up to 5 passages, transferred to membrane-based or pellet cultures and harvested after 7 or 21 days. The chondrogenic potential was assessed by histology (toluidine blue, safranin-O), immunohistochemistry for collagen type II and quantitative analysis of collagen type II α(1) (COL2A1). Membrane-based cultures (P1) formed flexible disc-like constructs (diameter 4000 µm, thickness 150 µm) with a large smooth surface after 7 days. Positive safranin-O and collagen type II staining was found in membrane-based and pellet cultures at P1-3. Expression of COL2A1 after 7 days was increased in both culture systems compared to monolayer culture up to P3, whereas cells from monolayer > P3 did not redifferentiate. The best results for COL2A1 expression were obtained from membrane-based cultures at P1. After 21 days the membrane-based cultures did not express COL2A1. We concluded that membrane-based and pellet cultures showed the ability to promote redifferentiation of chondrocytes expanded in monolayer culture. The number of cell passages had an impact on the chondrogenic potential of cells. Membrane-based cultures provided the highest COL2A1 expression and a large, smooth and cartilage-like surface. As these are appropriate features for clinical applications, we assume that membrane-based cultures might be of use in cartilage regeneration if they displayed similar results in vivo.

Comparative Analyses of the Secretome from Dedifferentiated and Redifferentiated Adult Articular Chondrocytes

OBJECTIVE

The main goal of this study was to compare the secretion products derived from human articular chondrocytes established in either long-term monolayer cultures or in scaffold-free 3-dimensional (3-D) cultures.

METHODS

Stable isotope labeling of amino acids in cell culture (SILAC) was applied to investigate quantitatively the differences between proteins secreted from dedifferentiated and redifferentiated chondrocytes. Proteins in cell supernatants were resolved by 1-D gel electrophoresis and analyzed by mass spectrometry. The results from the proteomic analyses were validated by immunoblotting. Additionally, antibody arrays were used to screen culture supernatants for 79 different morphogens.

RESULTS

Quantitative SILAC showed that some relevant growth factors such as CTGF or GAS6 were elevated in monolayers, along with proteins characteristic of a dedifferentiated phenotype such as collagen type I and tenascin. In spheroids, data showed overexpression of some cartilage-specific proteins such as aggrecan, together with important matrix regulators such as chitinase-3-like protein and stromelysin-1. Antibody arrays revealed that chondrocytes in monolayer secrete higher levels of leukocyte-activating agents such as MCP-1 and GRO, whereas the spheroid configuration favors the production of cell morphogens such as MCSF and VEGF.

CONCLUSION

Our results show that some classic dedifferentiation and redifferentiation markers are differentially expressed in 2-D or 3-D culture configurations. Other cell/matrix regulatory molecules are also found to be differentially expressed by chondrocytes in 2-D and 3-D conditions by SILAC and antibody arrays. Our data bring new information for understanding the biology of chondrocytes in general and the process of cartilage tissue reconstruction in particular.

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.

The secretory profiles of cultured human articular chondrocytes and mesenchymal stem cells: implications for autologous cell transplantation strategies

This study was undertaken to compare the phenotype of human articular chondrocytes (ACs) and bone marrow-derived mesenchymal stem cells (MSCs) after cell expansion by studying the spectrum of proteins secreted by cells into the culture medium. ACs and MSCs were expanded in monolayer cultures for some weeks, as done in standard cell transplantation procedures. Initially, the expression of cartilage signature genes was compared by real-time PCR. Metabolic labeling of proteins (SILAC) in combination with mass spectrometry (LC/MS-MS) was applied to investigate differences in released proteins. In addition, multiplex assays were carried out to quantify the amounts of several matrix metalloproteases (MMPs) and their natural inhibitors (TIMPs). Expanded chondrocytes showed a slightly higher expression of cartilage-specific genes than MSCs, whereas the overall spectra of released proteins were very similar for the two cell types. In qualitative terms MSCs seemed to secrete similar number of extracellular matrix proteins (43% vs. 45% of total proteins found) and catabolic agents (9% vs. 10%), and higher number of anabolic agents (12 % vs. 7%) compared to ACs. Some matrix-regulatory agents such as serpins, BMP-1, and galectins were detected only in MSC supernatants. Quantitative analyses of MMPs and TIMPs revealed significantly higher levels of MMP-1, MMP-2, MMP-3, and MMP-7 in the medium of ACs. Our data show that after the expansion phase, both ACs and MSCs express a dedifferentiated phenotype, resembling each other. ACs hold a phenotype closer to native cartilage at the gene expression level, whereas MSCs show a more anabolic profile by looking at the released proteins pattern. Our data together with the inherent capability of MSCs to maintain their differentiation potential for longer cultivation periods would favor the use of these cells for cartilage reconstruction.

Anabolic and catabolic responses of human articular chondrocytes to varying oxygen percentages

Introduction

Oxygen is a critical parameter proposed to modulate the functions of chondrocytes ex-vivo as well as in damaged joints. This article investigates the effect of low (more physiological) oxygen percentage on the biosynthetic and catabolic activity of human articular chondrocytes (HAC) at different phases of in vitro culture.

Methods

HAC expanded in monolayer were cultured in pellets for two weeks (Phase I) or up to an additional two weeks (Phase II). In each Phase, cells were exposed to 19% or 5% oxygen. Resulting tissues and culture media were assessed to determine amounts of produced/released proteoglycans and collagens, metalloproteinases (MMPs), collagen degradation products and collagen fibril organization using biochemical, (immuno)-histochemical, gene expression and scanning electron microscopy analyses. In specific experiments, the hypoxia-inducible factor-1α (HIF-1α) inhibitor cadmium chloride was supplemented in the culture medium to assess the involvement of this pathway.

Results

Independent from the oxygen percentage during expansion, HAC cultured at 5% O₂ (vs 19% O₂) during Phase I accumulated higher amounts of glycosaminoglycans and type II collagen and expressed reduced levels of MMP-1 and MMP-13 mRNA and protein. Switching to 19% oxygen during Phase II resulted in reduced synthesis of proteoglycan and collagen, increased release of MMPs, accumulation of type II collagen fragments and higher branching of collagen fibrils. In contrast, reducing O₂ during Phase II resulted in increased proteoglycan and type II collagen synthesis and reduced expression and release of MMP-13 mRNA and protein. Supplementation of cadmium chloride during differentiation culture at 5% O₂ drastically reduced the up-regulation of type II collagen and the down-regulation of MMP-1 mRNA.

Conclusions

The application of more physiologic oxygen percentage during specific phases of differentiation culture enhanced the biosynthetic activity and reduced the activity of catabolic enzymes implicated in cartilage breakdown. Modulation of the oxygen percentage during HAC culture may be used to study pathophysiological events occurring in osteoarthritis and to enhance properties of in vitro engineered cartilaginous tissues.

Enhanced chondrogenic differentiation of human bone marrow-derived mesenchymal stem cells in low oxygen environment micropellet cultures

Chondrogenesis of mesenchymal stem cells (MSCs) is typically induced when they are condensed into a single aggregate and exposed to transforming growth factor-beta (TGF-beta). Hypoxia, like aggregation and TGF-beta delivery, may be crucial for complete chondrogenesis. However, the pellet dimensions and associated self-induced oxygen gradients of current chondrogenic methods may limit the effectiveness of in vitro differentiation and subsequent therapeutic uses. Here we describe the use of embryoid body-forming technology to produce microscopic aggregates of human bone marrow MSCs (BM-MSCs) for chondrogenesis. The use of micropellets reduces the formation of gradients within the aggregates, resulting in a more homogeneous and controlled microenvironment. These micropellet cultures (approximately 170 cells/micropellet) as well as conventional pellet cultures (approximately 2 x 10(5) cells/pellet) were chondrogenically induced under 20% and 2% oxygen environments for 14 days. Compared to conventional pellets under both environments, micropellets differentiated under 2% O(2) showed significantly increased sulfated glycosaminoglycan (sGAG) production and more homogeneous distribution of proteoglycans and collagen II. Aggrecan and collagen II gene expressions were increased in pellet cultures differentiated under 2% O(2) relative to 20% O(2) pellets but 2% O(2) micropellets showed even greater increases in these genes, as well as increased SOX9. These results suggest a more advanced stage of chondrogenesis in the micropellets accompanied by more homogeneous differentiation. Thus, we present a new method for enhancing MSC chondrogenesis that reveals a unique relationship between oxygen tension and aggregate size. The inherent advantages of chondrogenic micropellets over a single macroscopic aggregate should allow for easy integration with a variety of cartilage engineering strategies.

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.