Cartilage repair represents an unmet medical challenge and cell-based approaches to engineer human articular cartilage are a promising solution. Here, we describe three-dimensional (3D) biomimetic hydrogels as an ideal tool for the expansion and maturation of human articular chondrocytes.
Human articular cartilage is highly susceptible to damage and has limited self-repair and regeneration potential. Cell-based strategies to engineer cartilage tissue offer a promising solution to repair articular cartilage. To select the optimal cell source for tissue repair, it is important to develop an appropriate culture platform to systematically examine the biological and biomechanical differences in the tissue-engineered cartilage by different cell sources. Here we applied a three-dimensional (3D) biomimetic hydrogel culture platform to systematically examine cartilage regeneration potential of juvenile, adult, and osteoarthritic (OA) chondrocytes. The 3D biomimetic hydrogel consisted of synthetic component poly(ethylene glycol) and bioactive component chondroitin sulfate, which provides a physiologically relevant microenvironment for in vitro culture of chondrocytes. In addition, the scaffold may be potentially used for cell delivery for cartilage repair in vivo. Cartilage tissue engineered in the scaffold can be evaluated using quantitative gene expression, immunofluorescence staining, biochemical assays, and mechanical testing. Utilizing these outcomes, we were able to characterize the differential regenerative potential of chondrocytes of varying age, both at the gene expression level and in the biochemical and biomechanical properties of the engineered cartilage tissue. The 3D culture model could be applied to investigate the molecular and functional differences among chondrocytes and progenitor cells from different stages of normal or aberrant development.
With its limited self-repair potential, human articular cartilage undergoes frequent irreversible damages. Extensive efforts are currently focused on the development of efficient cell-based approaches for treatment of articular cartilage injuries. The success of these cell-based therapies is highly dependent on the selection of an optimal cell source and the maintenance of its regenerative potential. Chondrocytes are a common cell source for cartilage repair, but they are limited in supply and can de-differentiate during in vitro expansion in 2D monolayer culture thereby limiting their generation of hyaline cartilage 1.
The aim of this protocol is to establish a 3-dimensional hydrogel platform for an in vitro comparative study of human chondrocytes from different ages and disease state. Unlike conventional two-dimensional (2D) culture, three-dimensional (3D) hydrogels allow chondrocytes to maintain their morphology and phenotype and provides a physiologically relevant environment enabling chondrocytes to produce cartilage tissue 2,3. In addition to providing a 3D physical structure for chondrocyte culture, hydrogels mimic the function of native cartilage extracellular matrix (ECM). Specifically, the inclusion of chondroitin sulfate methacrylate provides a potential reservoir for secreted paracrine factors 4 and enables cell-mediated degradation and matrix turnover 5. Although many 3D hydrogel culture systems have been utilized widely in various studies including agarose and alginate gels, we have used a biomimetic 3D culture system that has some distinct advantages for chondrocyte culture. Chondroitin sulfate (CS) is an abundant component in articular cartilage and the PEG-CS hydrogels have been shown to maintain and even enhance chondrogenic phenotype and facilitate cell-mediated matrix degradation and turnover 2,5. In addition, the mechanical properties of the hydrogel scaffold can be easily modulated by changing concentration of PEG and hence can be utilized to further enhance the regeneration potential of chondrocytes or a related cell type 6,7. PEG/CSMA is also biocompatible and hence has the potential for a direct clinical application in cartilage defects for example. The limitation for this system is its complexity and the use of photopolymerization that can potentially affect cell viability as compared to simpler systems like agarose, however the advantages for the chondrocyte culture outweigh the potential limitations.
The 3D hydrogel culture is compatible with conventional assay for evaluation of cell phenotype (gene expression, protein immunostaining) and functional outcome (quantification of cartilage matrix production, mechanical testing). This favorable 3D environment was tested to compare the tissue regeneration potential of human chondrocytes from three different aged populations in long-term 3D cultures.
The outcomes were evaluated via both phenotypic and functional assays. Juvenile, adult and OA chondrocytes showed differential responses in the 3D biomimetic hydrogel culture. After 3 and 6 weeks, chondrogenic gene expression was upregulated in juvenile and adult chondrocytes but was downregulated in OA chondrocytes. Deposition of cartilage tissue components including aggrecan, type II collagen, and glycosaminoglycan (GAG) was high for juvenile and adult chondrocytes but not for OA chondrocytes. The compressive moduli of the resulting cartilage constructs also exhibited similar trends. In conclusion, both juvenile and adult chondrocytes exhibited chondrogenic and cartilage matrix disposition up to 6 weeks of 3D culture in hydrogels. In contrast, osteoarthritic chondrocytes revealed a loss of cartilage phenotype and minimal ability to generate robust cartilage.
Comme indiqué dans ce protocole, les hydrogels 3D sont capables de maintenir chondrocytes phénotype dans la culture, en évitant le processus de dédifférenciation cellulaire dans les cellules fibrocartilagineuses habituellement rencontrés avec les cultures monocouches 15. En outre, des cultures à long terme de la construction chondrocytes- hydrogel ont révélé un environnement favorable qui maintient les caractéristiques cellulaires intrinsèques associés à l'âge et la maladie.
Utilisation d'un hydrogel biomimétique 3D présente plusieurs avantages. Tout d'abord, l'inclusion de sulfate de chondroïtine (CS), une composante majeure trouvé dans le cartilage articulaire, permettent aux cellules de dégrader la matrice d'hydrogel en sécrétant chondroitinase et fixer cartilage nouvellement synthétisé extra-cellulaire matrice 5, 16. En outre, CS a été démontré pour avoir des propriétés anti-inflammatoires dans l'articulation arthritique. L'hydrogel biomimétique peut également être utilisé en tant que matériau d'échafaudage pour la distribution des cellules dans la réparation du cartilage, et peut être modifié chimiquementpour faciliter une meilleure intégration de 17,18 tissu biomatériau.
L'utilisation des hydrogels de PEG-CS permet cultures à long terme des chondrocytes et l'évaluation des propriétés biochimiques et mécaniques. Ici, nous montrons comment cette plate-forme peut être utile pour les analyses comparatives des diverses sources de chondrocytes différenciés afin de définir le type de cellule optimale pour l'ingénierie de cartilage. Fait intéressant, les chondrocytes encapsulés dans des hydrogels restent viables et prolifèrent en fonction de leurs capacités intrinsèques. Les supports de la composition d'hydrogel, en fait, la croissance de sains jeunes et adultes chondrocytes comme représenté sur la Figure 2. La composition et la structure des hydrogels décrits favorise également la formation de tissu cartilagineux, comme indiqué par le dépôt d'une matrice extracellulaire fonctionnelle évaluée par des glycosaminoglycanes (GAG ) quantification.
Un avantage supplémentaire est que les constructions de chondrocytes hydrogelpeuvent être évalués pour les propriétés mécaniques du tissu cartilagineux nouvellement formé. Notez que le test de compression simple doit être effectué sur l'hydrogel acellulaire pour la comparaison. Les hydrogels, en fait, ont une rigidité intrinsèque en raison de la rigidité des groupements CS. Souche de compression simple de 5-20% (à un taux de déformation de 1% / s) peut être appliqué pour le contrôle mécanique du tissu cartilagineux 11,12 depuis la souche physiologique connu par le tissu du cartilage sous condition de chargement a été signalé à être 10-20 13,14%. La réponse des deux cellules chargées et acellulaire hydrogels à contrôle mécanique a été évaluée à la culture point final. Dans l'exemple décrit ci-dessus, nous avons observé une rigidité comparable des constructions contenant des chondrocytes juvéniles et adultes par opposition à la raideur inférieure des constructions contenant des chondrocytes arthrosiques. De telles propriétés mécaniques de la construction de cellules d'hydrogel permettent l'évaluation des propriétés fonctionnelles de latissu formé donnant une analyse en profondeur de la capacité de la maturation des cellules.
En conclusion, l'utilisation des hydrogels biomimétiques 3D pour étudier le potentiel différent de population de chondrocytes pour générer tissu cartilagineux peut être largement appliquée. Outre les études in vitro décrits ici, la transplantation in vivo des constructions cellulaires chargées peut être envisagé d'étudier la maturation des cellules et le potentiel de régénération dans le contexte physiologique. D'autres modifications de la plate-forme d'hydrogel avec des facteurs biomimétiques supplémentaires peuvent également être envisagées pour optimiser la prolifération des chondrocytes et de la maturation.
The authors have nothing to disclose.
The authors would like to acknowledge Stanford Department of Orthopaedic Surgery and Stanford Coulter Translational Seed Grant for funding. J.H.L. would like to thank National Science Foundation Graduate Fellowship and DARE Doctoral Fellowship for support.
juvenile chondrocytes (Clonetics™ Normal Human Chondrocyte Cell System ) | Lonza | CC-2550 | |
adult chondrocytes (Clonetics™ Normal Human Chondrocyte Cell System) | Lonza | CC-2550 | |
poly(ethylene glycol diacrylate) | Laysan Bio | ACRL-PEG-ACRL-1000-1g | |
2-morpholinoethanesulfonic acid | Sigma | M5287 | |
photoinitiator | Irgacure | 2959 | |
sodium chloride | Sigma | S9888 | |
chondroitin sulfate sodium salt | Sigma | C9819 | |
N-hydroxysuccinimide | Sigma | 130672 | |
1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide hydrochloride | Sigma | E1769 | |
2-aminoethyl methacrylate | Sigma | 516155 | |
dialysis tubing | Spectrum Laboratories | 132700 | |
Collagenase 2 | Worthington Biochemical | LS004177 | |
Collagenase 4 | Worthington Biochemical | LS004189 | |
DMEM/F12 media | HyClone, Thermo Scientific | SH3002301 | |
live/dead assay | Life Technologies | L3224 | |
Tri reagent | Life Technologies | AM9738 | |
Quant-iT™ PicoGreen® dsDNA Assay Kit | Invitrogen | P11496 | |
Sodium phosphate dibasic | Sigma | S3264 | |
Ethylenediaminetetraacetic acid disodium salt | Sigma | E5134 | |
L-Cysteine | Sigma | C1276 | |
1,9-dimethylmethylene blue | Sigma | 341088 | |
Instruments | |||
UV light equipment – XX-15LW Bench Lamp, 365nm | UVP | 95-0042-07 | |
Instron 5944 testing system | Instron Corporation | E5940 |