The influence of matrix elasticity on chondrocyte behavior in 3D

Stable-Dynamic Hydrogels Mimicking the Pericellular Matrix for Articular Cartilage Repair

Cartilage regeneration requires a specialized biomechanical environment. Macroscopically, cartilage repair requires a protracted, stable mechanical environment, whereas microscopically, it involves dynamic interactions between cells and the extracellular matrix. Therefore, this study aims to design a hydrogel that meets the complex biomechanical requirements for cartilage repair. Dynamic hybrid hydrogels with temporal stability at the macroscale and dynamic properties at the microscale are successfully synthesized. The dynamic hybrid hydrogel simulates the stress relaxation and viscoelasticity of the pericellular matrix, facilitating effective interactions between the extracellular matrix and cells. The in vitro and in vivo experiments demonstrated that the hybrid hydrogel significantly promoted cartilage repair. The dynamic hybrid hydrogel alleviates abnormal actin polymerization, reduces intracellular stress, and increases the volume of individual cells. By modulating the cytoskeleton, the hybrid hydrogel inhibits Notch signal transduction in both the receptor and ligand cells, resulting in an improved cartilage phenotype. This study introduces an effective hybrid hydrogel scaffold that modulates the chondrocyte cytoskeleton and Notch signaling pathways by establishing an appropriate biomechanical environment, thus offering a promising material for cartilage repair.

Advancements in hydrogel design for articular cartilage regeneration: A comprehensive review

This review paper explores the cutting-edge advancements in hydrogel design for articular cartilage regeneration (CR). Articular cartilage (AC) defects are a common occurrence worldwide that can lead to joint breakdown at a later stage of the disease, necessitating immediate intervention to prevent progressive degeneration of cartilage. Decades of research into the biomedical applications of hydrogels have revealed their tremendous potential, particularly in soft tissue engineering, including CR. Hydrogels are highly tunable and can be designed to meet the key criteria needed for a template in CR. This paper aims to identify those criteria, including the hydrogel components, mechanical properties, biodegradability, structural design, and integration capability with the adjacent native tissue and delves into the benefits that CR can obtain through appropriate design. Stratified-structural hydrogels that emulate the native cartilage structure, as well as the impact of environmental stimuli on the regeneration outcome, have also been discussed. By examining recent advances and emerging techniques, this paper offers valuable insights into developing effective hydrogel-based therapies for AC repair.

A personalized osteoarthritic joint-on-a-chip as a screening platform for biological treatments

Osteoarthritis (OA) is a highly disabling pathology, characterized by synovial inflammation and cartilage degeneration. Orthobiologics have shown promising results in OA treatment thanks to their ability to influence articular cells and modulate the inflammatory OA environment. Considering their complex mechanism of action, the development of reliable and relevant joint models appears as crucial to select the best orthobiologics for each patient. The aim of this study was to establish a microfluidic OA model to test therapies in a personalized human setting. The joint-on-a-chip model included cartilage and synovial compartments, containing hydrogel-embedded chondrocytes and synovial fibroblasts, separated by a channel for synovial fluid. For the cartilage compartment, a Hyaluronic Acid-based matrix was selected to preserve chondrocyte phenotype. Adding OA synovial fluid induced the production of inflammatory cytokines and degradative enzymes, generating an OA microenvironment. Personalized models were generated using patient-matched cells and synovial fluid to test the efficacy of mesenchymal stem cells on OA signatures. The patient-specific models allowed monitoring changes induced by cell injection, highlighting different individual responses to the treatment. Altogether, these results support the use of this joint-on-a-chip model as a prognostic tool to screen the patient-specific efficacy of orthobiologics.

Dual-network DNA-silk fibroin hydrogels with controllable surface rigidity for regulating chondrogenic differentiation

Osteoarthritis (OA) is a common joint disease known for cartilage degeneration, leading to a substantial burden on individuals and society due to its high disability rate. However, current clinical treatments for cartilage defects remain unsatisfactory due to the unclear mechanisms underlying cartilage regeneration. Tissue engineering hydrogels have emerged as an attractive approach in cartilage repair. Recent research studies have indicated that stem cells can sense the mechanical strength of hydrogels, thereby regulating their differentiation fate. In this study, we present the groundbreaking construction of dual-network DNA-silk fibroin (SF) hydrogels with controllable surface rigidity. The supramolecular networks, formed through DNA base-pairing, induce the development of β-sheet structures by constraining and aggregating SF molecules. Subsequently, SF was cross-linked via horseradish peroxidase (HRP)-mediated enzyme reactions to form the second network. Experimental results demonstrated a positive correlation between the surface rigidity of dual-network DNA-SF hydrogels and the DNA content. Interestingly, it was observed that dual-network DNA-SF hydrogels with moderate surface rigidity exhibited the highest effectiveness in facilitating the migration of bone marrow mesenchymal stem cells (BMSCs) and their chondrogenic differentiation. Transcriptome sequencing further confirmed that dual-network DNA-SF hydrogels primarily enhanced chondrogenic differentiation of BMSCs by upregulating the Wnt and TGF-β signaling pathways while accelerating collagen II synthesis. Furthermore, in vivo studies revealed that dual-network DNA-SF hydrogels with moderate surface rigidity significantly accelerated cartilage regeneration. In summary, the dual-network DNA-SF hydrogels represent a promising and novel therapeutic strategy for cartilage regeneration.

From cells to organs: progress and potential in cartilaginous organoids research

While cartilage tissue engineering has significantly improved the speed and quality of cartilage regeneration, the underlying metabolic mechanisms are complex, making research in this area lengthy and challenging. In the past decade, organoids have evolved rapidly as valuable research tools. Methods to create these advanced human cell models range from simple tissue culture techniques to complex bioengineering approaches. Cartilaginous organoids in part mimic the microphysiology of human cartilage and fill a gap in high-fidelity cartilage disease models to a certain extent. They hold great promise to elucidate the pathogenic mechanism of a diversity of cartilage diseases and prove crucial in the development of new drugs. This review will focus on the research progress of cartilaginous organoids and propose strategies for cartilaginous organoid construction, study directions, and future perspectives.

Bioprinting of Stem Cell Spheroids Followed by Post-Printing Chondrogenic Differentiation for Cartilage Tissue Engineering

Cartilage tissue presents low self-repair capability and lesions often undergo irreversible progression. Structures obtained by tissue engineering, such as those based in extrusion bioprinting of constructs loaded with stem cell spheroids may offer valuable alternatives for research and therapeutic purposes. Human mesenchymal stromal cell (hMSC) spheroids can be chondrogenically differentiated faster and more efficiently than single cells. This approach allows obtaining larger tissues in a rapid, controlled and reproducible way. However, it is challenging to control tissue architecture, construct stability, and cell viability during maturation. Herein, this work reports a reproducible bioprinting process followed by a successful post-bioprinting chondrogenic differentiation procedure using large quantities of hMSC spheroids encapsulated in a xanthan gum-alginate hydrogel. Multi-layered constructs are bioprinted, ionically crosslinked, and post chondrogenically differentiated for 28 days. The expression of glycosaminoglycan, collagen II and IV are observed. After 56 days in culture, the bioprinted constructs are still stable and show satisfactory cell metabolic activity with profuse extracellular matrix production. These results show a promising procedure to obtain 3D models for cartilage research and ultimately, an in vitro proof-of-concept of their potential use as stable chondral tissue implants.

The effect of allyl isothiocyanate on chondrocyte phenotype is matrix stiffness-dependent: Possible involvement of TRPA1 activation

Osteoarthritis (OA) is a chronic joint disease with increasing prevalence. Chondrocytes (CHs) are highly differentiated end-stage cells with a secretory phenotype that keeps the extracellular matrix (ECM) balanced and the cartilage environment stable. Osteoarthritis dedifferentiation causes cartilage matrix breakdown, accounting for one of the key pathogenesis of osteoarthritis. Recently, the activation of transient receptor potential ankyrin 1 (TRPA1) was claimed to be a risk factor in osteoarthritis by causing inflammation and extracellular matrix degradation. However, the underlying mechanism is still unknown. Due to its mechanosensitive property, we speculated that the role of TRPA1 activation during osteoarthritis is matrix stiffness-dependent. In this study, we cultured the chondrocytes from patients with osteoarthritis on stiff vs. soft substrates, treated them with allyl isothiocyanate (AITC), a transient receptor potential ankyrin 1 agonist, and compared the chondrogenic phenotype, containing cell shape, F-actin cytoskeleton, vinculin, synthesized collagen profiles and their transcriptional regulatory factor, and inflammation-related interleukins. The data suggest that allyl isothiocyanate treatment activates transient receptor potential ankyrin 1 and results in both positive and harmful effects on chondrocytes. In addition, a softer matrix could help enhance the positive effects and alleviate the harmful ones. Thus, the effect of allyl isothiocyanate on chondrocytes is conditionally controllable, which could be associated with transient receptor potential ankyrin 1 activation, and is a promising strategy for osteoarthritis treatment.

In Vitro and In Vivo Evaluation of 3D Constructs Engineered with Human iPSC-Derived Chondrocytes in Gelatin-Methacryloyl Hydrogel

Articular cartilage defects have limited healing potential and, when left untreated, can lead to osteoarthritis. Tissue engineering focuses on regenerating the damaged joint surface, preferably in an early stage. Here we investigate the regenerative potential of 3D constructs consisting of human iPSC-derived chondrocytes in gelatin-methacryloyl (GelMA) hydrogel for stable hyaline cartilage production. iPSC-derived chondrocytes are encapsulated in GelMA hydrogel at low (1x10⁷ mL^-1 ) and high (2x10⁷ mL^-1 ) density. In conventional medium, GelMA hydrogel supports the chondrocyte phenotype, as opposed to cells cultured in 3D in absence of hydrogel. Moreover, encapsulated iPSC-derived chondrocytes preserve their in vivo matrix formation capacity after 21 days in vitro. In differentiation medium, hyaline cartilage-like tissue forms after 21 days, demonstrated by highly sulfated glycosaminoglycans and collagen type II. Matrix deposition is delayed at low encapsulation density, corroborating with lower transcript levels of COL2A1. An ectopic assay in nude mice demonstrates further maturation of the matrix deposited in vitro. Direct ectopic implantation of iPSC-derived chondrocyte-laden GelMA, without in vitro priming, also generates hyaline cartilage-like tissue, albeit less mature. Since it is unclear what maturity upon implantation is desired for joint surface regeneration, this is an attractive technology to generate immature and more mature hyaline cartilage-like tissue. This article is protected by copyright. All rights reserved.

Bioinspired Injectable Hydrogels Dynamically Stiffen and Contract to Promote Mechanosensing-Mediated Chondrogenic Commitment of Stem Cells

Developing stiff and resilient injectable hydrogels that can mechanically support load-bearing joints while enabling chondrogenic differentiation of stem cells is a major challenge in the field of cartilage tissue engineering. In the present work, a triple-network injectable hydrogel system was engineered using Bombyx mori silk fibroin, carboxymethyl cellulose (CMC), and gelatin. The developed hydrogel demonstrated a simultaneous increase in both stiffness and contraction over time, thereby imparting a four-dimensional (4D) evolving niche to the cells. While resilience was provided by CMC, the dynamic alterations in the hydrogel matrix were attributed to the formation of β-sheets in silk. The engineered contraction facilitated condensation of cells that mimicked an important step during cartilage development. Subsequently, this led to downregulation of YAP signaling and enhanced chondrogenic commitment of stem cells. More importantly, the in vivo study showed that the ectopically regenerated cartilage was mature and closely resembled native articular cartilage. Overall, this strategy of engineering mechanotransduction that promotes chondrogenesis by contraction-mediated condensation is a promising and translatable approach for cartilage repair.

De novo neo-hyaline-cartilage from bovine organoids in viscoelastic hydrogels

Regenerative therapies for articular cartilage are currently clinically available. However, they are associated with several drawbacks that require resolution. Optimizing chondrocyte expansion and their assembly, can reduce the time and costs of these therapies and more importantly increase their clinical success. In this study, cartilage organoids were quickly mass produced from bovine chondrocytes with a new suspension expansion protocol. This new approach led to massive cell proliferation, high viability and the self-assembly of organoids. These organoids were composed of collagen type II, type VI, glycosaminoglycans, with Sox9 positive cells, embedded in a pericellular and interterritorial matrix similarly to hyaline cartilage. With the goal of producing large scale tissues, we then encapsulated these organoids into alginate hydrogels with different viscoelastic properties. Elastic hydrogels constrained the growth and fusion of the organoids inhibiting the formation of a tissue. In contrast, viscoelastic hydrogels allowed the growth and fusion of the organoids into a homogenous tissue that was rich in collagen type II and glycosaminoglycans. The encapsulation of organoids to produce in vitro neocartilage also proved to be superior to the conventional method of encapsulating 2D expanded chondrocytes. This study describes a multimodal approach that involves chondrocyte expansion, organoid formation and their assembly into neohyaline-cartilage which proved to be superior to the current standard approaches used in cartilage tissue engineering. STATEMENT OF SIGNIFICANCE: In this manuscript, we describe a new and simple methodology to quickly mass produce self-assembling cartilage organoids. Due to their matrix content and structure similarities with native cartilage, these organoids on their own have the potential to revolutionize cartilage research and the manner in which we study signaling pathways, disease progression, tissue engineering, drug development, etc. Furthermore, these organoids and their fast mass production were combined with a key relatively ignored hydrogel characteristic, viscoelasticity, to demonstrate their fusion into a neo-tissue. This has the potential to open the door for large scale cartilage regeneration such as for entire joint surfaces.

Evaluation of Marine Agarose Biomaterials for Tissue Engineering Applications

Five agarose types (D1LE, D2LE, LM, MS8 and D5) were evaluated in tissue engineering and compared for the first time using an array of analysis methods. Acellular and cellular constructs were generated from 0.3–3%, and their biomechanical properties, in vivo biocompatibility (as determined by LIVE/DEAD, WST-1 and DNA release, with n = 6 per sample) and in vivo biocompatibility (by hematological and biochemical analyses and histology, with n = 4 animals per agarose type) were analyzed. Results revealed that the biomechanical properties of each hydrogel were related to the agarose concentration (p < 0.001). Regarding the agarose type, the highest (p < 0.001) Young modulus, stress at fracture and break load were D1LE, D2LE and D5, whereas the strain at fracture was higher in D5 and MS8 at 3% (p < 0.05). All agaroses showed high biocompatibility on human skin cells, especially in indirect contact, with a correlation with agarose concentration (p = 0.0074 for LIVE/DEAD and p = 0.0014 for WST-1) and type, although cell function tended to decrease in direct contact with highly concentrated agaroses. All agaroses were safe in vivo, with no systemic effects as determined by hematological and biochemical analysis and histology of major organs. Locally, implants were partially encapsulated and a pro-regenerative response with abundant M2-type macrophages was found. In summary, we may state that all these agarose types can be safely used in tissue engineering and that the biomechanical properties and biocompatibility were strongly associated to the agarose concentration in the hydrogel and partially associated to the agarose type. These results open the door to the generation of specific agarose-based hydrogels for definite clinical applications such as the human skin, cornea or oral mucosa.

One-Step Photoactivation of a Dual-Functionalized Bioink as Cell Carrier and Cartilage-Binding Glue for Chondral Regeneration

Cartilage defects can result in pain, disability, and osteoarthritis. Hydrogels providing a chondroregeneration-permissive environment are often mechanically weak and display poor lateral integration into the surrounding cartilage. This study develops a visible-light responsive gelatin ink with enhanced interactions with the native tissue, and potential for intraoperative bioprinting. A dual-functionalized tyramine and methacryloyl gelatin (GelMA-Tyr) is synthesized. Photo-crosslinking of both groups is triggered in a single photoexposure by cell-compatible visible light in presence of tris(2,2'-bipyridyl)dichlororuthenium(II) and sodium persulfate as initiators. Neo-cartilage formation from embedded chondroprogenitor cells is demonstrated in vitro, and the hydrogel is successfully applied as bioink for extrusion-printing. Visible light in situ crosslinking in cartilage defects results in no damage to the surrounding tissue, in contrast to the native chondrocyte death caused by UV light (365-400 nm range), commonly used in biofabrication. Tyramine-binding to proteins in native cartilage leads to a 15-fold increment in the adhesive strength of the bioglue compared to pristine GelMA. Enhanced adhesion is observed also when the ink is extruded as printable filaments into the defect. Visible-light reactive GelMA-Tyr bioinks can act as orthobiologic carriers for in situ cartilage repair, providing a permissive environment for chondrogenesis, and establishing safe lateral integration into chondral defects.

Mechanotransduction and Stiffness-Sensing: Mechanisms and Opportunities to Control Multiple Molecular Aspects of Cell Phenotype as a Design Cornerstone of Cell-Instructive Biomaterials for Articular Cartilage Repair

Since material stiffness controls many cell functions, we reviewed the currently available knowledge on stiffness sensing and elucidated what is known in the context of clinical and experimental articular cartilage (AC) repair. Remarkably, no stiffness information on the various biomaterials for clinical AC repair was accessible. Using mRNA expression profiles and morphology as surrogate markers of stiffness-related effects, we deduced that the various clinically available biomaterials control chondrocyte (CH) phenotype well, but not to equal extents, and only in non-degenerative settings. Ample evidence demonstrates that multiple molecular aspects of CH and mesenchymal stromal cell (MSC) phenotype are susceptible to material stiffness, because proliferation, migration, lineage determination, shape, cytoskeletal properties, expression profiles, cell surface receptor composition, integrin subunit expression, and nuclear shape and composition of CHs and/or MSCs are stiffness-regulated. Moreover, material stiffness modulates MSC immuno-modulatory and angiogenic properties, transforming growth factor beta 1 (TGF-β1)-induced lineage determination, and CH re-differentiation/de-differentiation, collagen type II fragment production, and TGF-β1- and interleukin 1 beta (IL-1β)-induced changes in cell stiffness and traction force. We then integrated the available molecular signaling data into a stiffness-regulated CH phenotype model. Overall, we recommend using material stiffness for controlling cell phenotype, as this would be a promising design cornerstone for novel future-oriented, cell-instructive biomaterials for clinical high-quality AC repair tissue.

Stiffness Matters: Fine-Tuned Hydrogel Elasticity Alters Chondrogenic Redifferentiation

Biomechanical cues such as shear stress, stretching, compression, and matrix elasticity are vital in the establishment of next generation physiological in vitro tissue models. Matrix elasticity, for instance, is known to guide stem cell differentiation, influence healing processes and modulate extracellular matrix (ECM) deposition needed for tissue development and maintenance. To better understand the biomechanical effect of matrix elasticity on the formation of articular cartilage analogs in vitro, this study aims at assessing the redifferentiation capacity of primary human chondrocytes in three different hydrogel matrices of predefined matrix elasticities. The hydrogel elasticities were chosen to represent a broad spectrum of tissue stiffness ranging from very soft tissues with a Young’s modulus of 1 kPa up to elasticities of 30 kPa, representative of the perichondral-space. In addition, the interplay of matrix elasticity and transforming growth factor beta-3 (TGF-β3) on the redifferentiation of primary human articular chondrocytes was studied by analyzing both qualitative (viability, morphology, histology) and quantitative (RT-qPCR, sGAG, DNA) parameters, crucial to the chondrotypic phenotype. Results show that fibrin hydrogels of 30 kPa Young’s modulus best guide chondrocyte redifferentiation resulting in a native-like morphology as well as induces the synthesis of physiologic ECM constituents such as glycosaminoglycans (sGAG) and collagen type II. This comprehensive study sheds light onto the mechanobiological impact of matrix elasticity on formation and maintenance of articular cartilage and thus represents a major step toward meeting the need for advanced in vitro tissue models to study both re- and degeneration of articular cartilage.

Experimental study on the mechanical properties of biological hydrogels of different concentrations

BACKGROUND

Biological hydrogels provide a conducive three-dimensional extracellular matrix environment for encapsulating and cultivating living cells. Microenvironmental modulus of hydrogels dictates several characteristics of cell functions such as proliferation, adhesion, self-renewal, differentiation, migration, cell morphology and fate. Precise measurement of the mechanical properties of gels is necessary for investigating cellular mechanobiology in a variety of applications in tissue engineering. Elastic properties of gels are strongly influenced by the amount of crosslinking density.

OBJECTIVE

The main purpose of the present study was to determine the elastic modulus of two types of well-known biological hydrogels: Agarose and Gelatin Methacryloyl.

METHODS

Mechanical properties such as Young's modulus, fracture stress and failure strain of the prescribed gels with a wide range of concentrations were determined using tension and compression tests.

RESULTS

The elastic modulus, failure stress and strain were found to be strongly influenced when the amount of concentration in the hydrogels was changed. The elastic modulus for a lower level of concentration, not considered in this study, was also predicted using statistical analysis.

CONCLUSIONS

Closed matching of the mechanical properties of the gels revealed that the bulk tension and compression tests could be confidently used for assessing mechanical properties of delicate biological hydrogels.

Microniche geometry modulates the mechanical properties and calcium signaling of chondrocytes

In articular cartilage, the function of chondrocytes is strongly related to their zone-specific microniche geometry defined by pericellular matrix. Microniche geometry is critical for regulating the phenotype and function of the chondrocyte in native cartilage and tissue engineering constructs. However the role of microniche geometry in the mechanical properties and calcium signaling of chondrocytes remains unknown. To recapitulate microniche geometry at single-cell level, we engineered three basic physiological-related polydimethylsiloxane (PDMS) microniches geometries fabricated using soft lithography. We cultured chondrocytes in these microniche geometries and quantified cell mechanical properties using atomic force microscopy (AFM). Fluorescent calcium indicator was used to record and quantify cytosolic Ca²⁺ oscillation of chondrocytes in different geometries. Our work showed that microniche geometry modulated the mechanical behavior and calcium signaling of chondrocytes. The ellipsoidal microniches significantly enhanced the mechanical properties of chondrocytes compared to spheroidal microniche. Additionally, ellipsoidal microniches can markedly improved the amplitude but weakened the frequency of cytosolic Ca²⁺ oscillation in chondrocytes than spheroidal microniche. Our work might reveal a novel understanding of chondrocyte mechanotransduction and therefore be useful for designing cell-instructive scaffolds for functional cartilage tissue engineering.

Effects of Electrical Stimulation on Stem Cells

Recent interest in developing new regenerative medicine- and tissue engineering-based treatments has motivated researchers to develop strategies for manipulating stem cells to optimize outcomes in these potentially, game-changing treatments. Cells communicate with each other, and with their surrounding tissues and organs via electrochemical signals. These signals originate from ions passing back and forth through cell membranes and play a key role in regulating cell function during embryonic development, healing, and regeneration. To study the effects of electrical signals on cell function, investigators have exposed cells to exogenous electrical stimulation and have been able to increase, decrease and entirely block cell proliferation, differentiation, migration, alignment, and adherence to scaffold materials. In this review, we discuss research focused on the use of electrical stimulation to manipulate stem cell function with a focus on its incorporation in tissue engineering-based treatments.

Matrix stiffness-regulated cellular functions under different dimensionalities

The microenvironments that cells encounter with in vitro.

Stiffness Sensing by Cells

Physical stimuli are essential for the function of eukaryotic cells, and changes in physical signals are important elements in normal tissue development as well as in disease initiation and progression. The complexity of physical stimuli and the cellular signals they initiate are as complex as those triggered by chemical signals. One of the most important, and the focus of this review, is the effect of substrate mechanical properties on cell structure and function. The past decade has produced a nearly exponentially increasing number of mechanobiological studies to define how substrate stiffness alters cell biology using both purified systems and intact tissues. Here we attempt to identify common features of mechanosensing in different systems while also highlighting the numerous informative exceptions to what in early studies appeared to be simple rules by which cells respond to mechanical stresses.

Impact of Hydrogel Stiffness on Differentiation of Human Adipose-Derived Stem Cell Microspheroids

Hydrogels represent an attractive material platform for realization of three-dimensional (3D) tissue-engineered constructs, as they have tunable mechanical properties, are compatible with different types of cells, and resemble elements found in natural extracellular matrices. So far, numerous hydrogel-cartilage/bone tissue engineering (TE)-related studies were performed by utilizing a single cell encapsulation approach. Although multicellular spheroid cultures exhibit advantageous properties for cartilage or bone TE, the chondrogenic or osteogenic differentiation potential of stem cell microspheroids within hydrogels has not been investigated much. This study explores, for the first time, how stiffness of gelatin-based hydrogels (having a storage modulus of 538, 3584, or 7263 Pa) affects proliferation and differentiation of microspheroids formed from telomerase-immortalized human adipose-derived stem cells (hASC/hTERT). Confocal microscopy indicates that all tested hydrogels supported cell viability during their 3- to 5-week culture period in the control, chondrogenic, or osteogenic medium. Although in the softer hydrogels cells from neighboring microspheroids started outgrowing and interconnecting within a few days, their protrusion was slower or limited in stiffer hydrogels or those cultured in chondrogenic medium, respectively. High expressions of chondrogenic markers (SOX9, ACAN, COL2A1), detected in all tested hydrogels, proved that the chondrogenic differentiation of hASC/hTERT microspheroids was very successful, especially in the two softer hydrogels, where superior cartilage-specific properties were confirmed by Alcian blue staining. These chondrogenically induced samples also expressed COL10A1, a marker of chondrocyte hypertrophy. Interestingly, the hydrogel itself (with no differentiation medium) showed a slight chondrogenic induction. Regardless of the hydrogel stiffness, in the samples stimulated with osteogenic medium, the expression of selected markers RUNX2, BGLAP, ALPL, and COL1A1 was not conclusive. Nevertheless, the von Kossa staining confirmed the presence of calcium deposits in osteogenically stimulated samples in the two softer hydrogels, suggesting that these also favor osteogenesis. This observation was also confirmed by Alizarin red quantification assay, with which higher amounts of calcium were detected in the osteogenically induced hydrogels than in their controls. The presented data indicate that the encapsulation of adipose-derived stem cell microspheroids in gelatin-based hydrogels show promising potential for future applications in cartilage or bone TE. Impact Statement Osteochondral defects represent one of the leading causes of disability in the world. Although numerous tissue engineering (TE) approaches have shown success in cartilage and bone tissue regeneration, achieving native-like characteristics of these tissues remains challenging. This study demonstrates that in the presence of a corresponding differentiation medium, gelatin-based hydrogels support moderate osteogenic and excellent chondrogenic differentiation of photo-encapsulated human adipose-derived stem cell microspheroids, the extent of which depends on hydrogel stiffness. Because photosensitive hydrogels are a convenient material platform for creating stiffness gradients in three dimensions, the presented microspheroid-hydrogel encapsulation strategy holds promise for future strategies of cartilage or bone TE.

Hydrogels of agarose, and methacrylated gelatin and hyaluronic acid are more supportive for in vitro meniscus regeneration than three dimensional printed polycaprolactone scaffolds

In this study, porcine fibrochondrocyte-seeded agarose, methacrylated gelatin (GelMA), methacrylated hyaluronic acid (MeHA) and GelMA-MeHA blend hydrogels, and 3D printed PCL scaffolds were tested under dynamic compression for potential meniscal regeneration in vitro. Cell-carrying hydrogels produced higher levels of extracellular matrix (ECM) components after a 35-day incubation than the 3D printed PCL. Cells on GelMA exhibited strong cell adhesion (evidenced with intense paxillin staining) and dendritic cell morphology, and produced an order of magnitude higher level of collagen (p < 0.05) than other materials. On the other hand, cells in agarose exhibited low cell adhesion and round cell morphology, and produced higher levels of glycosaminoglycans (GAGs) (p < 0.05) than other materials. A low level of ECM production and a high level of cell proliferation were observed on the 3D printed PCL. Dynamic compression at 10% strain enhanced GAG production in agarose (p < 0.05), and collagen production in GelMA. These results show that hydrogels have a higher potential for meniscal regeneration than the 3D printed PCL, and depending on the material used, fibrochondrocytes could be directed to proliferate or produce cartilaginous or fibrocartilaginous ECM. Agarose and MeHA could be used for the regeneration of the inner region of meniscus, while GelMA for the outer region.

Maintenance of chondrocyte phenotype during expansion on PLLA microtopographies

Articular chondrocytes are difficult to grow, as they lose their characteristic phenotype following expansion on standard tissue culture plates. Here, we show that culturing them on surfaces of poly(L-lactic acid) of well-defined microtopography allows expansion and maintenance of characteristic chondrogenic markers. We investigated the dynamics of human chondrocyte dedifferentiation on the different poly(L-lactic acid) microtopographies by the expression of collagen type I, collagen type II and aggrecan at different culture times. When seeded on poly(L-lactic acid), chondrocytes maintained their characteristic hyaline phenotype up to 7 days, which allowed to expand the initial cell population approximately six times without cell dedifferentiation. Maintenance of cell phenotype was afterwards correlated to cell adhesion on the different substrates. Chondrocytes adhesion occurs via the α₅β₁ integrin on poly(L-lactic acid), suggesting cell–fibronectin interactions. However, α₂β₁ integrin is mainly expressed on the control substrate after 1 day of culture, and the characteristic chondrocytic markers are lost (collagen type II expression is overcome by the synthesis of collagen type I). Expanding chondrocytes on poly(L-lactic acid) might be an effective solution to prevent dedifferentiation and improving the number of cells needed for autologous chondrocyte transplantation.

Mechanical confinement regulates cartilage matrix formation by chondrocytes

Cartilage tissue equivalents formed from hydrogels containing chondrocytes could provide a solution for replacing damaged cartilage. Previous approaches have often utilized elastic hydrogels. However, elastic stresses may restrict cartilage matrix formation and alter the chondrocyte phenotype. Here we investigated the use of viscoelastic hydrogels, in which stresses are relaxed over time and which exhibit creep, for three-dimensional (3D) culture of chondrocytes. We found that faster relaxation promoted a striking increase in the volume of interconnected cartilage matrix formed by chondrocytes. In slower relaxing gels, restriction of cell volume expansion by elastic stresses led to increased secretion of IL-1β, which in turn drove strong up-regulation of genes associated with cartilage degradation and cell death. As no cell-adhesion ligands are presented by the hydrogels, these results reveal cell sensing of cell volume confinement as an adhesion-independent mechanism of mechanotransduction in 3D culture, and highlight stress relaxation as a key design parameter for cartilage tissue engineering.

Impact of expansion and redifferentiation under hypothermia on chondrogenic capacity of cultured human septal chondrocytes

A critical limitation in the cultivation of cartilage for tissue engineering is the dedifferentiation in chondrocytes, mainly during in vitro amplification. Despite many previous studies investigating the influence of various conditions, no data exist concerning the effects of hypothermia. Our aim has been to influence chondrocyte dedifferentiation in vitro by hypothermic conditions. Chondrocytes were isolated from cartilage biopsies and seeded in monolayer and in three-dimensional pellet-cultures. Each cell culture was either performed at 32.2°C or 37°C during amplification. Additionally, the influence of the redifferentiation of chondrocytes in three-dimensional cell culture was examined at 32.2°C and 37°C after amplification at 32.2°C or 37°C. An 3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium (MTS) assay was used to measure cell proliferation in monolayer, whereas the polymerase chain reaction and immunohistochemical and histological staining were used in three-dimensional pellet-cultures. Real-time polymerase chain reaction was employed to measure the relative expression of the target genes collagen II, collagen I, aggrecan and versican. Ratios were estimated between collagen II/collagen I and aggrecan/versican to evaluate differentiation. A higher value of these ratios indicated an advantageous status of differentiation. In monolayer, hypothermia at 32.2°C slowed down the proliferation rate of chondrocytes significantly, being up to two times lower at 32.2°C compared with culture at 37°C. Simultaneously, hypothermia in monolayer decelerated dedifferentiation. The ratio of aggrecan/versican was significantly higher at 32.2°C compared with that at 37°C. In three-dimensional pellet-culture, the chondrocytes redifferentiated at 32.2°C and at 37°C, and this process is more distinct at 37°C than at 32.2°C. Similar results were obtained for the ratios of collagen II/collagen I and aggrecan/versican and were supported by immunochemical and histological staining. Thus, hypothermic conditions for chondrocytes are mainly advantageous in monolayer culture. In three-dimensional pellet-culture, redifferentiation predominates at 37°C compared with at 32.2°C. In particular, the results from the monolayer cultures show potential in the avoidance of dedifferentiation.

Subcellular domain-dependent molecular hierarchy of SFK and FAK in mechanotransduction and cytokine signaling

Focal adhesion kinase (FAK) and Src family kinases (SFK) are known to play critical roles in mechanotransduction and other crucial cell functions. Recent reports indicate that they reside in different microdomains of the plasma membrane. However, little is known about their subcellular domain-dependent roles and responses to extracellular stimuli. Here, we employed fluorescence resonance energy transfer (FRET)-based biosensors in conjunction with collagen-coupled agarose gels to detect subcellular activities of SFK and FAK in three-dimensional (3D) settings. We observed that SFK and FAK in the lipid rafts and nonrafts are differently regulated by fluid flow and pro-inflammatory cytokines. Inhibition of FAK in the lipid rafts blocked SFK response to fluid flow, while inhibition of SFK in the non-rafts blocked FAK activation by the cytokines. Ex-vivo FRET imaging of mouse cartilage explants showed that intermediate level of interstitial fluid flow selectively decreased cytokine-induced SFK/FAK activation. These findings suggest that SFK and FAK exert distinctive molecular hierarchy depending on their subcellular location and extracellular stimuli.

Effect of matrix stiffness on osteoblast functionalization

OBJECTIVES

Stiffness of bone tissue differs response to its physiological or pathological status, such as osteoporosis or osteosclerosis. Consequently, the function of cells residing in bone tissue including osteoblasts (OBs), osteoclasts and osteocytes will be affected. However, to the best of our knowledge, the detailed mechanism of how extracellular matrix stiffness affects OB function remains unclear.

MATERIALS AND METHODS

We conducted a study that exposed rat primary OBs to polydimethylsiloxane substrates with varied stiffness to investigate the alterations of cell morphology, osteoblastic differentiation and its potential mechanism in mechanotransduction.

RESULTS

Distinctive differences of cell shapes and vinculin expression in rat osteoblasts were detected on different PDMS substrates. As representatives for OB function, expression of alkaline phosphatase, Runx2 and osteocalcin were identified and showed a decrease trend as substrates become soft, which is associated with the Rho/ROCK signalling pathway.

CONCLUSIONS

Our results indicated substrate elasticity as a potent regulator in OBs functionalization, which may pave a way for further understanding of bone diseases as well as a potential therapeutic alternative in tissue regeneration.

The effect of matrix stiffness on biomechanical properties of chondrocytes

The behavior of chondrocytes is regulated by multiple mechanical microenvironmental cues. During development and degenerative disease of articular cartilage, as an external signal, the extracellular matrix stiffness of chondrocytes changes significantly, but whether and how this biophysical cue affects biomechanical properties of chondrocytes remain elusive. In the present study, we designed supporting-biomaterials as  mimics of native pericellular matrix to study the effect of matrix stiffness on chondrocyte morphology and F-actin distribution. Furthermore, the active mechanical behavior of chondrocytes during sensing and responding to different matrix stiffness was quantitatively investigated using atom force microscope technique and theoretical model. Our results indicated that stiffer matrix tends to increase the cell spreading area, the percentage of irregular cell shape distribution and mechanical parameters including elastic modulus (E_elastic), instantaneous modulus (E₀), relaxed modulus (E_R) and apparent viscosity (μ) of chondrocytes. Knowledge of matrix stiffness-dependent biomechanical behaviors of chondrocytes has important implications for optimizing matrix material and advancing chondrocyte-based applications for functional tissue engineering.

Yield stress determines bioprintability of hydrogels based on gelatin-methacryloyl and gellan gum for cartilage bioprinting

Bioprinting of chondrocyte-laden hydrogels facilitates the fabrication of constructs with controlled organization and shape e.g. for articular cartilage implants. Gelatin-methacryloyl (gelMA) supplemented with gellan gum is a promising bio-ink. However, the rheological properties governing the printing process, and the influence of gellan gum on the mechanical properties and chondrogenesis of the blend, are still unknown. Here, we investigated the suitability of gelMA/gellan for cartilage bioprinting. Multiple concentrations, ranging from 3% to 20% gelMA with 0%-1.5% gellan gum, were evaluated for their printability, defined as the ability to form filaments and to incorporate cells at 15 °C-37 °C. To support the printability assessment, yield stress and viscosity of the hydrogels were measured. Stiffness of UV-cured constructs, as well as cartilage-like tissue formation by embedded chondrocytes, were determined in vitro. A large range of gelMA/gellan concentrations were printable with inclusion of cells and formed the bioprinting window. The addition of gellan gum improved filament deposition by inducing yielding behavior, increased construct stiffness and supported chondrogenesis. High gellan gum concentrations, however, did compromise cartilage matrix production and distribution, and even higher concentrations resulted in too high yield stresses to allow cell encapsulation. This study demonstrates the high potential of gelMA/gellan blends for cartilage bioprinting and identifies yield stress as a dominant factor for bioprintability.

Poly(trimethylene carbonate-co-ε-caprolactone) promotes axonal growth

Mammalian central nervous system (CNS) neurons do not regenerate after injury due to the inhibitory environment formed by the glial scar, largely constituted by myelin debris. The use of biomaterials to bridge the lesion area and the creation of an environment favoring axonal regeneration is an appealing approach, currently under investigation. This work aimed at assessing the suitability of three candidate polymers – poly(ε-caprolactone), poly(trimethylene carbonate-co-ε-caprolactone) (P(TMC-CL)) (11∶89 mol%) and poly(trimethylene carbonate) - with the final goal of using these materials in the development of conduits to promote spinal cord regeneration. Poly(L-lysine) (PLL) coated polymeric films were tested for neuronal cell adhesion and neurite outgrowth. At similar PLL film area coverage conditions, neuronal polarization and axonal elongation was significantly higher on P(TMC-CL) films. Furthermore, cortical neurons cultured on P(TMC-CL) were able to extend neurites even when seeded onto myelin. This effect was found to be mediated by the glycogen synthase kinase 3β (GSK3β) signaling pathway with impact on the collapsin response mediator protein 4 (CRMP4), suggesting that besides surface topography, nanomechanical properties were implicated in this process. The obtained results indicate P(TMC-CL) as a promising material for CNS regenerative applications as it promotes axonal growth, overcoming myelin inhibition.

Neocartilage formation from mesenchymal stem cells grown in type II collagen-hyaluronan composite scaffolds

Three-dimensional (3D) collagen type II-hyaluronan (HA) composite scaffolds (CII-HA) which mimics the extracellular environment of natural cartilage were fabricated in this study. Rheological measurements demonstrated that the incorporation of HA increased the compression modulus of the scaffolds. An initial in vitro evaluation showed that scaffolds seeded with porcine chondrocytes formed cartilaginous-like tissue after 8 weeks, and HA functioned to promote the growth of chondrocytes into scaffolds. Placenta-derived multipotent cells (PDMC) and gingival fibroblasts (GF) were seeded on tissue culture polystyrene (TCPS), CII-HA films, and small intestinal submucosa (SIS) sheets for comparing their chondrogenesis differentiation potentials with those of adipose-derived adult stem cells (ADAS) and bone marrow-derived mesenchymal stem cells (BMSC). Among different cells, PDMC showed the greatest chondrogenic differentiation potential on both CII-HA films and SIS sheets upon TGF-β3 induction, followed by GF. This was evidenced by the up-regulation of chondrogenic genes (Sox9, aggrecan, and collagen type II), which was not observed for cells grown on TCPS. This finding suggested the essential role of substrate materials in the chondrogenic differentiation of PDMC and GF. Neocartilage formation was more obvious in both PDMC and GF cells plated on CII-HA composite scaffolds vs. 8-layer SIS at 28 days in vitro. Finally, implantation of PDMC/CII-HA constructs into NOD-SCID mice confirmed the formation of tissue-engineered cartilage in vivo.

The role of changes in extracellular matrix of cartilage in the presence of inflammation on the pathology of osteoarthritis

Osteoarthritis (OA) is a degenerative disease that affects various tissues surrounding joints such as articular cartilage, subchondral bone, synovial membrane, and ligaments. No therapy is currently available to completely prevent the initiation or progression of the disease partly due to poor understanding of the mechanisms of the disease pathology. Cartilage is the main tissue afflicted by OA, and chondrocytes, the sole cellular component in the tissue, actively participate in the degeneration process. Multiple factors affect the development and progression of OA including inflammation that is sustained during the progression of the disease and alteration in biomechanical conditions due to wear and tear or trauma in cartilage. During the progression of OA, extracellular matrix (ECM) of cartilage is actively remodeled by chondrocytes under inflammatory conditions. This alteration of ECM, in turn, changes the biomechanical environment of chondrocytes, which further drives the progression of the disease in the presence of inflammation. The changes in ECM composition and structure also prevent participation of mesenchymal stem cells in the repair process by inhibiting their chondrogenic differentiation. This review focuses on how inflammation-induced ECM remodeling disturbs cellular activities to prevent self-regeneration of cartilage in the pathology of OA.

Response of sheep chondrocytes to changes in substrate stiffness from 2 to 20 Pa: effect of cell passaging

AIM

The influence of culture substrate stiffness (in the kPa range) on chondrocyte behavior has been described. Here we describe the response to variations in substrate stiffness in a soft range (2-20 Pa), as it may play a role in understanding cartilage physiopathology.

METHODS

We developed a system for cell culture in substrates with different elastic moduli using collagen hydrogels and evaluated chondrocytes after 2, 4, and 7 days in monolayer and three-dimensional (3D) cultures. Experiments were performed in normoxia and hypoxia in order to describe the effect of a low oxygen environment on chondrocytes. Finally, we also evaluated if dedifferentiated cells preserve the capacity for mechanosensing.

RESULTS

Chondrocytes showed less proliferating activity when cultured in monolayer in the more compliant substrates. Expression of the cartilage markers Aggrecan (Acan), type II collagen (Col2a1), and Sox9 was upregulated in the less stiff gels (both in monolayer and in 3D culture). Stiffer gels induced an organization of the actin cytoskeleton that correlated with the loss of a chondrocyte phenotype. When cells were cultured in hypoxia, we observed changes in the cellular response that were mediated by HIF-1α. Results in 3D hypoxia cultures were opposite to those found in normoxia, but remained unchanged in monolayer hypoxic experiments. Similar results were found for dedifferentiated cells.

CONCLUSIONS

Chondrocytes respond differently according to the stiffness of the substrate. This response depends greatly on the oxygen environment and on whether the chondrocyte is embedded or grown onto the hydrogel, since mechanosensing capacity was not lost with cell expansion.

Processed xenogenic cartilage as innovative biomatrix for cartilage tissue engineering: effects on chondrocyte differentiation and function

One key point in the development of new bioimplant matrices for the reconstruction and replacement of cartilage defects is to provide an adequate microenvironment to ensure chondrocyte migration and de novo synthesis of cartilage-specific extracellular matrix (ECM). A recently developed decellularization and sterilization process maintains the three-dimensional (3D) collagen structure of native septal cartilage while increasing matrix porosity, which is considered to be crucial for cartilage tissue engineering. Human primary nasal septal chondrocytes were amplified in monolayer culture and 3D-cultured on processed porcine nasal septal cartilage scaffolds. The influence of chondrogenic growth factors on neosynthesis of ECM proteins was examined at the protein and gene expression levels. Seeding experiments demonstrated that processed xenogenic cartilage matrices provide excellent environmental properties for human nasal septal chondrocytes with respect to cell adhesion, migration into the matrix and neosynthesis of cartilage-specific ECM proteins, such as collagen type II and aggrecan. Matrix biomechanical stability indicated that the constructs retrieve full stability and function during 3D culture for up to 42 days, proportional to collagen type II and GAG production. Thus, processed xenogenic cartilage offers a suitable environment for human nasal chondrocytes and has promising potential for cartilage tissue engineering in the head and neck region.

Chondrocyte redifferentiation in 3D: the effect of adhesion site density and substrate elasticity

To obtain sufficient cell numbers for cartilage tissue engineering with autologous chondrocytes, cells are typically expanded in monolayer culture. As a result, they lose their chondrogenic phenotype in a process called dedifferentiation, which can be reversed upon transfer into a 3D environment. We hypothesize that the properties of this 3D environment, namely adhesion site density and substrate elasticity, would influence this redifferentiation process. To test this hypothesis, chondrocytes were expanded in monolayer and their phenotypical transition was monitored. Agarose hydrogels manipulated to give different RGD adhesion site densities and mechanical properties were produced, cells were incorporated into the gels to induce redifferentiation, and constructs were analyzed to determine cell number and extracellular matrix production after 2 weeks of 3D culture. The availability of adhesion sites within the gels inhibited cellular redifferentiation. Glycosaminoglycan production per cell was diminished by RGD in a dose-dependent manner and cells incorporated into gels with the highest RGD density, remained positive for collagen type I and produced the least collagen type II. Substrate stiffness, in contrast, did not influence cellular redifferentiation, but softer gels contained higher cell numbers and ECM amounts after 2 weeks of culture. Our results indicate that adhesion site density but not stiffness influences the redifferentiation process of chondrocytes in 3D. This knowledge might be used to optimize the redifferentiation process of chondrocytes and thus the formation of cartilage-like tissue.