Chondrogenic Differentiation of Adipose-Derived Stromal Cells in Combinatorial Hydrogels Containing Cartilage Matrix Proteins with Decoupled Mechanical Stiffness

Characterizing properties of scaffolds 3D printed with peptide-polymer conjugates

Three-dimensional (3D) printing is a popular biomaterials fabrication technique because it enables scaffold composition and architecture to be tuned for different applications. Modifying these properties can also alter mechanical properties, making it challenging to decouple biochemical and physical properties. In this study, inks containing peptide-poly(caprolactone) (PCL) conjugates were solvent-cast 3D printed to create peptide-functionalized scaffolds. We characterized how different concentrations of hyaluronic acid-binding (HAbind-PCL) or mineralizing (E3-PCL) conjugates influenced properties of the resulting 3D-printed constructs. The peptide sequences CGGGRYPISRPRKR (HAbind-PCL; positively charged) and CGGGAAAEEE (E3-PCL; negatively charged) enabled us to evaluate how conjugate chemistry, charge, and concentration affected 3D-printed architecture, conjugate location, and mechanical properties. For both HAbind-PCL and E3-PCL, conjugate addition did not affect ink viscosity, filament diameter, scaffold architecture, or scaffold compressive modulus. Increasing conjugate concentration in the ink prior to printing correlated with an increase in peptide concentration on the scaffold surface. Interestingly, conjugate type affected final conjugate location within the 3D-printed filament cross-section. HAbind-PCL conjugates remained within the filament bulk while E3-PCL conjugates were located closer to the filament surface. E3-PCL at all concentrations did not affect mechanical properties, but an intermediate HAbind-PCL concentration resulted in a moderate decrease in filament tensile modulus. These data suggest final conjugate location within the filament bulk may influence mechanical properties. However, no significant differences were observed between PCL filaments printed without conjugates and filaments printed with higher HAbind-PCL concentrations. These results demonstrate that this 3D printing platform can be used to functionalize the surface without significant changes to the physical properties of the scaffold. The downstream potential of this strategy will enable decoupling of biochemical and physical properties to fine-tune cellular responses and support functional tissue regeneration.

Prospects of magnetically based approaches addressing inflammation in tendon tissues

Tendon afflictions constitute a significant share of musculoskeletal diseases and represent a primary cause of incapacity worldwide. Unresolved/chronic inflammatory states have been associated with the onset and progression of tendon disorders, contributing to undesirable immune stimulation and detrimental tissue effects. Thus, targeting persistent inflammatory events could assist important developments to solve pathophysiological processes and innovative therapeutics to address impaired healing and accomplish complete tendon regeneration. This review overviews the impact of inflammation and inflammatory mediators in tendon niches, unveiling the importance of tendon cell populations and their signature features, and the influence of microenvironmental factors on inflamed and injured tendons. The demand for non-invasive instructive strategies to manage persistent inflammatory mediators, guide inflammatory pathways, and modulate cellular responses will also be approached by exploring the role of pulsed electromagnetic field (PEMF). PEMF alone or combined with more sophisticated systems triggered by magnetic fields will be considered in the design of successful therapies to control inflammation in tendinopathic conditions.

Effect of the uronic acid composition of alginate in alginate/collagen hybrid hydrogel on chondrocyte behavior

Introduction: Developing a culture system that can effectively maintain chondrocyte phenotype and functionalization is a promising strategy for cartilage repair. Methods: An alginate/collagen (ALG/COL) hybrid hydrogel using different guluronate/mannuronate acid ratio (G/M ratio) of alginates (a G/M ratio of 64/36 and a G/M ratio of 34/66) with collagen was developed. The effects of G/M ratios on the properties of hydrogels and their effects on the chondrocytes behaviors were evaluated. Results: The results showed that the mechanical stiffness of the hydrogel was significantly affected by the G/M ratios of alginate. Chondrocytes cultured on Mid-G/M hydrogels exhibited better viability and phenotype preservation. Moreover, RT-qPCR analysis showed that the expression of cartilage-specific genes, including SOX9, COL2, and aggrecan was increased while the expression of RAC and ROCK1 was decreased in chondrocytes cultured on Mid-G/M hydrogels. Conclusion: These findings demonstrated that Mid-G/M hydrogels provided suitable matrix conditions for cultivating chondrocytes and may be useful in cartilage tissue engineering. More importantly, the results indicated the importance of taking alginate G/M ratios into account when designing alginate-based composite materials for cartilage tissue engineering.

The Role of Extracellular Matrix and Hydrogels in Mesenchymal Stem Cell Chondrogenesis and Cartilage Regeneration

Diseases associated with articular cartilage disintegration or loss are still therapeutically challenging. The traditional treatment approaches only alleviate the symptoms while potentially causing serious side effects. The limited self-renewal potential of articular cartilage provides opportunities for advanced therapies involving mesenchymal stem cells (MSCs) that are characterized by a remarkable regenerative capacity. The chondrogenic potential of MSCs is known to be regulated by the local environment, including soluble factors and the less discussed extracellular matrix (ECM) components. This review summarizes the process of chondrogenesis, and also the biological properties of the ECM mediated by mechanotransduction as well as canonical and non-canonical signaling. Our focus is also on the influence of the ECM's physical parameters, molecular composition, and chondrogenic factor affinity on the adhesion, survival, and chondrogenic differentiation of MSCs. These basic biological insights are crucial for a more precise fabrication of ECM-mimicking hydrogels to improve cartilage tissue reconstruction. Lastly, we provide an overview of hydrogel classification and characterization. We also include the results from preclinical models combining MSCs with hydrogels for the treatment of cartilage defects, to support clinical application of this construct. Overall, it is believed that the proper combination of MSCs, hydrogels, and chondrogenic factors can lead to complex cartilage regeneration.

Functionalized Fluoropolymer-Compatibilized Elastomeric Bilayer Composites for Osteochondral Repair: Unraveling the Role of Substrate Stiffness and Functionalities

The osteochondral lesions and osteoarthritis-related complications continue to be clinically relevant challenges to be addressed by the biomaterials community. Hydrogel-based scaffolds have been widely investigated to enhance osteochondral regeneration, but the inferior mechanical properties together with poor functional stability are the major constraints in their clinical translation. The development of osteochondral implants with natural tissue-mimicking mechanical properties remains largely unexplored. In this perspective, the present study demonstrates a strategy to develop a bilayer osteochondral implant with an elastically stiff composite (poly(vinylidene difluoride)-reinforced BaTiO₃, PVDF/BT) and elastically compliant composite (maleic anhydride-functionalized PVDF/thermoplastic polyurethane/BaTiO₃, m-PVDF/TPU/BT). The compositional variation in polymer composites allowed the elastic modulus of the hybrid bilayer construct to vary from ∼2 GPa to ∼90 MPa, which enabled a better understanding of the substrate-stiffness-dependent cellular behavior and maturation of preosteoblasts and chondrocytes. The cellular functionalities on PVDF-based polymer matrices have been benchmarked against ultrahigh-molecular-weight polyethylene (UHMWPE), which is clinically used for a wide spectrum of orthopedic applications. The increased alkaline phosphatase (ALP) activity, collagen synthesis, and matrix mineralization confirmed the early differentiation of preosteoblasts on the PVDF/BT matrix with subchondral bone-like mechanical properties. On the contrary, the upregulated chondrogenic functionalities were recorded on m-PVDF/TPU/BT with an elevated level of collagen content, glycosaminoglycans, and proteoglycans. Emphasis has been laid on probing the regulation of the osteochondral behavior using tailored substrate stiffness and functionalities using compatibilized fluoropolymer-based elastomeric composites. Taken together, the results of this work conclusively establish the efficacy of the hybrid bilayer composite with natural tissue-mimicking mechanical properties for the functional repair of osteochondral defects.

Chondrogenic Potential of Human Dental Pulp Stem Cells Cultured as Microtissues

Several tissue engineering stem cell-based procedures improve hyaline cartilage repair. In this work, the chondrogenic potential of dental pulp stem cell (DPSC) organoids or microtissues was studied. After several weeks of culture in proliferation or chondrogenic differentiation media, synthesis of aggrecan and type II and I collagen was immunodetected, and SOX9, ACAN, COL2A1, and COL1A1 gene expression was analysed by real-time RT-PCR. Whereas microtissues cultured in proliferation medium showed the synthesis of aggrecan and type II and I collagen at the 6^th week of culture, samples cultured in chondrogenic differentiation medium showed an earlier and important increase in the synthesis of these macromolecules after 4 weeks. Gene expression analysis showed a significant increase of COL2A1 after 3 days of culture in chondrogenic differentiation medium, while COL1A1 was highly expressed after 14 days. Cell-cell proximity promotes the chondrogenic differentiation of DPSCs and important synthesis of hyaline chondral macromolecules.

Human Adipose-Derived Stromal/Stem Cell Culture and Analysis Methods for Adipose Tissue Modeling In Vitro: A Systematic Review

Human adipose-derived stromal/stem cells (hASC) are widely used for in vitro modeling of physiologically relevant human adipose tissue. These models are useful for the development of tissue constructs for soft tissue regeneration and 3-dimensional (3D) microphysiological systems (MPS) for drug discovery. In this systematic review, we report on the current state of hASC culture and assessment methods for adipose tissue engineering using 3D MPS. Our search efforts resulted in the identification of 184 independent records, of which 27 were determined to be most relevant to the goals of the present review. Our results demonstrate a lack of consensus on methods for hASC culture and assessment for the production of physiologically relevant in vitro models of human adipose tissue. Few studies have assessed the impact of different 3D culture conditions on hASC adipogenesis. Additionally, there has been a limited use of assays for characterizing the functionality of adipose tissue in vitro. Results from this study suggest the need for more standardized culture methods and further analysis on in vitro tissue functionality. These will be necessary to validate the utility of 3D MPS as an in vitro model to reduce, refine, and replace in vivo experiments in the drug discovery regulatory process.

Gradient scaffold with spatial growth factor profile for osteochondral interface engineering

Osteochondral (OC) matrix design poses a significant engineering challenge due to the complexity involved with bone-cartilage interfaces. To better facilitate the regeneration of OC tissue, we developed and evaluated a biodegradable matrix with uniquely arranged bone and cartilage supporting phases: a poly(lactic-co-glycolic) acid (PLGA) template structure with a porosity gradient along its longitudinal axis uniquely integrated with hyaluronic acid hydrogel. Micro-CT scanning and imaging confirmed the formation of an inverse gradient matrix. Hydroxyapatite was added to the PLGA template which was then plasma-treated to increase hydrophilicity and growth factor affinity. An osteogenic growth factor (bone morphogenetic protein 2; BMP-2) was loaded onto the template scaffold via adsorption, while a chondrogenic growth factor (transforming growth factor beta 1; TGF-β1) was incorporated into the hydrogel phase. Confocal microscopy of the growth factor loaded matrix confirmed the spatial distribution of the two growth factors, with chondrogenic factor confined to the cartilaginous portion and osteogenic factor present throughout the scaffold. We observed spatial differentiation of human mesenchymal stem cells (hMSCs) into cartilage and bone cells in the scaffolds in vitro: cartilaginous regions were marked by increased glycosaminoglycan production, and osteogenesis was seen throughout the graft by alizarin red staining. In a dose-dependent study of BMP-2, hMSC pellet cultures with TGF-β1 and BMP-2 showed synergistic effects on chondrogenesis. These results indicate that development of an inverse gradient matrix can spatially distribute two different growth factors to facilitate chondrogenesis and osteogenesis along different portions of a scaffold, which are key steps needed for formation of an osteochondral interface.

Metabolic Labeling to Probe the Spatiotemporal Accumulation of Matrix at the Chondrocyte-Hydrogel Interface

Hydrogels are engineered with biochemical and biophysical signals to recreate aspects of the native microenvironment and to control cellular functions such as differentiation and matrix deposition. This deposited matrix accumulates within the pericellular space and likely affects the interactions between encapsulated cells and the engineered hydrogel; however, there has been little work to study the spatiotemporal evolution of matrix at this interface. To address this, metabolic labeling is employed to visualize the temporal and spatial positioning of nascent proteins and proteoglycans deposited by chondrocytes. Within covalently crosslinked hyaluronic acid hydrogels, chondrocytes deposit nascent proteins and proteoglycans in the pericellular space within 1 d after encapsulation. The accumulation of this matrix, as measured by an increase in matrix thickness during culture, depends on the initial hydrogel crosslink density with decreased thicknesses for more crosslinked hydrogels. Encapsulated fluorescent beads are used to monitor the hydrogel location and indicate that the emerging nascent matrix physically displaces the hydrogel from the cell membrane with extended culture. These findings suggest that secreted matrix increasingly masks the presentation of engineered hydrogel cues and may have implications for the design of hydrogels in tissue engineering and regenerative medicine.

Gradient Hydrogels for Optimizing Niche Cues to Enhance Cell-Based Cartilage Regeneration

Hydrogels have been widely used for cell delivery to enhance cell-based therapies for cartilage tissue regeneration. To better support cartilage deposition, it is imperative to determine hydrogel formulation with physical and biochemical cues that are optimized for different cell populations. Previous attempts to identify optimized hydrogels rely mostly on testing hydrogel formulations with discrete properties, which are time-consuming and require large amounts of cells and materials. Gradient hydrogels encompass a range of continuous changes in niche properties, therefore offering a promising solution for screening a wide range of cell-niche interactions using less materials and time. However, harnessing gradient hydrogels to assess how matrix stiffness modulates cartilage formation by different cell types in vivo have never been investigated before. The goal of this study is to fabricate gradient hydrogels for screening the effects of varying hydrogel stiffness on cartilage formation by mesenchymal stem cells (MSCs) and chondrocytes, respectively, the two most commonly used cell populations for cartilage regeneration. We fabricated stiffness gradient hydrogels with tunable dimensions that support homogeneous cell encapsulation. Using gradient hydrogels with tunable stiffness range, we found MSCs and chondrocytes exhibit opposite trend in cartilage deposition in response to stiffness changes in vitro. Specifically, MSCs require soft hydrogels with Young's modulus less than 5 kPa to support faster cartilage deposition, as shown by type II collagen and sulfated glycosaminoglycan staining. In contrast, chondrocytes produce cartilage more effectively in stiffer matrix (>20 kPa). We chose optimal ranges of stiffness for each cell population for further testing in vivo using a mouse subcutaneous model. Our results further validated that soft matrix (Young's modulus <5 kPa) is better in supporting MSC-based cartilage deposition in three-dimensional, whereas stiffer matrix (Young's modulus >20 kPa) is more desirable for supporting chondrocyte-based cartilage deposition. Our results show the importance of optimizing niche cues in a cell-type-specific manner and validate the potential of using gradient hydrogels for optimizing niche cues to support cartilage regeneration in vitro and in vivo. Impact statement The present study validates the utility of gradient hydrogels for determining optimal hydrogel stiffness for supporting cartilage regeneration using both chondrocytes and stem cells. We demonstrate that such gradient hydrogels can be used for fast optimizing matrix stiffness for specific cell type to support optimal cartilage regeneration. To our knowledge, this is the first demonstration of applying gradient hydrogels for assessing optimal niche cues that support tissue regeneration in vivo and may be used for assessing optimal niche cues for different cell types to regeneration of different tissues.

Effects of cryo-processing on the mechanical and biological properties of poly(vinyl alcohol)-gelatin theta-gels

Poly(vinyl alcohol) (PVA), a synthetic, nontoxic polymer, is widely studied for use as a biomedical hydrogel due to its structural and physicomechanical properties. Depending on the synthesis method, PVA hydrogels can exhibit a range of selected characteristics-strength, creep resistance, energy dissipation, degree of crystallinity, and porosity. While the structural integrity and behavior of the hydrogel can be fine-tuned, common processing techniques result in a brittle, linear elastic material. In addition, PVA lacks functionality to engage and participate in cell adhesion, which can be a limitation for integrating PVA materials with tissue in situ. Thus, there is a need to further engineer PVA hydrogels to optimize its physicomechanical properties while enhancing cell adhesion and bioactivity. While the inclusion of gelatin into PVA hydrogels has been shown to impart cell-adhesive properties, the optimization of the mechanical properties of PVA-gelatin blends has not been studied in the context of traditional PVA hydrogel processing techniques. The incorporation of poly(ethylene glycol) with PVA prior to solidification forms an organized, cell instructive hydrogel with improved stiffness. The effect of cryo-processing, i.e., freeze-thaw (FT) cycling was elucidated by comparing 1 FT and 8 FT theta-cryo-gels and cryo-gels. To confirm the viability of the gels, human mesenchymal stem cell (hMSC) protein and sulfated glycosaminoglycan assays were performed to verify the nontoxicity and influence on hMSC differentiation. We have devised an elastic PVA-gelatin hydrogel utilizing the theta-gel and cryo-gel processing techniques, resulting in a stronger, more elastic material with greater potential as a scaffold for complex tissues.

Dual delivery of stem cells and insulin-like growth factor-1 in coacervate-embedded composite hydrogels for enhanced cartilage regeneration in osteochondral defects

Exogenous dual delivery of progenitor cell population and therapeutic growth factors (GFs) is one of alternative tissue engineering strategies for osteochondral tissue regeneration. In the present study, an implantable dual delivery platform was developed using coacervates (Coa) (i.e., a tertiary complex of poly(ethylene argininylaspartate diglyceride) (PEAD) polycation, heparin, and cargo insulin-like growth factor-1 (IGF-1), in thiolated gelatin (gelatin-SH)/ poly(ethylene glycol) diacrylate (PEGDA) interpenetrating network (IPN) hydrogels. Since Coa is able to protect cargo GF and maintain its long-term bioactivity, it is speculated that Coa-mediated delivery of chondrogenic factor IGF-1 with the aid of adipose-derived stem cells (ADSCs) would synergistically facilitate osteochondral tissue repair during physiological regeneration process. Our results indicate that gelatin-SH/PEGDA IPN hydrogels demonstrated biocompatibility and mechanical properties for a possible long-term transplantation, and PEAD-base Coa exhibited a sustained release of bioactive IGF-1 over 3 weeks. Subsequently, released IGF-1 from Coa could effectively induce chondrogenic differentiation of embedded ADSCs in the hydrogel, by showing enhanced glycosaminoglycan deposition and expression of chondrogenesis-associated genes. More importantly, at 12 weeks post-implantation in a rabbit full thickness osteochondral defect model, the quality of regenerative tissues in both chondral and subchondral layers was significantly improved in dual delivery of ADSC and IGF-1 in Coa encapsulated in gelatin-SH/PEGDA IPN hydrogels, as compared with a single delivery of ADSC only and a dual delivery without Coa. Therefore, we conclude that our Coa-embedded composite hydrogel platform could effectively augment osteochondral tissue regeneration holds promise for a feasible osteoarthritis therapeutic application.

Regulation of the Viscoelastic Properties of Hyaluronate-Alginate Hybrid Hydrogel as an Injectable for Chondrocyte Delivery

Modulation of the viscoelastic properties of hydrogels is critical in tissue engineering applications. In the present study, a hyaluronate-alginate hybrid (HAH) was synthesized by introducing alginate to the hyaluronate backbone with varying molecular weights (700-2500 kDa), and HAH hydrogels were prepared in the presence of calcium ions at the same cross-linking density. The storage shear moduli of the HAH hydrogels increased with the concomitant increase in the molecular weight of hyaluronate in the HAH polymer. The HAH hydrogels were also modified with arginine-glycine-aspartic acid (RGD) and histidine-alanine-valine (HAV) peptides to enhance cell-matrix and cell-cell interactions, respectively. The chondrogenic differentiation of ATDC5 cells encapsulated within the HAH hydrogels was enhanced with the increase in the storage shear moduli of the gels in vitro as well as in vivo. This approach of regulating the viscoelastic properties of hydrogels using polymers of varying molecular weights at the same cross-linking density may prove to be useful in various tissue engineering applications including cartilage regeneration.

Mixed Composition Microribbon Hydrogels Induce Rapid and Synergistic Cartilage Regeneration by Mesenchymal Stem Cells in 3D via Paracrine Signaling Exchange

Hydrogels are widely used matrices for mesenchymal stem cell (MSC)-based cartilage regeneration but often result in slow cartilage deposition with inferior mechanical strength. We recently reported a gelatin-based microribbon (μRB) scaffold, which contains macroporosity and substantially enhances the speed of cartilage formation by MSCs in 3D. However, our previous method cannot be used to fabricate different polymers into μRBs, and the effects of varying μRB compositions on MSC cartilage regeneration in 3D remain unknown. Here, we report a method that allows fabricating different polymers [gelatin, chondroitin sulfate, hyaluronic acid, and polyethylene glycol (PEG)] into μRB structures, which can be mixed in any ratio and cross-linked into 3D scaffolds in a modular manner. Mixing glycosaminoglycan μRBs with gelatin or PEG μRBs induced great synergy, resulting in fast cartilage deposition. After only 3 weeks of culture, leading mixed μRB composition reached high compressive strength on par with native cartilage. Such synergy can be recapitulated via exchange of soluble factors secreted by MSCs seeded in different μRB compositions in a dose-dependent manner. Tuning the ratio of mixed μRB compositions allowed further optimization of the quantity and speed of cartilage regeneration by MSCs. Together, our results validate mixed μRB compositions as a novel biomaterial tool for inducing synergy and accelerating MSC-based cartilage regeneration with biomimetic mechanical properties through paracrine signal exchange.

Osteochondral Regeneration Using Adipose Tissue-Derived Mesenchymal Stem Cells

Osteoarthritis (OA) is a major joint disease that promotes locomotor deficiency during the middle- to old-age, with the associated disability potentially decreasing quality of life. Recently, surgical strategies to reconstruct both articular cartilage and subchondral bone for OA have been diligently investigated for restoring joint structure and function. Adipose tissue-derived mesenchymal stem cells (AT-MSCs), which maintain pluripotency and self-proliferation ability, have recently received attention as a useful tool to regenerate osteocartilage for OA. In this review, several studies were described related to AT-MSC spheroids, with scaffold and scaffold-free three-dimensional (3D) constructs produced using “mold” or “Kenzan” methods for osteochondral regeneration. First, several examples of articular cartilage regeneration using AT-MSCs were introduced. Second, studies of osteochondral regeneration (not only cartilage but also subchondral bone) using AT-MSCs were described. Third, examples were presented wherein spheroids were produced using AT-MSCs for cartilage regeneration. Fourth, osteochondral regeneration following autologous implantation of AT-MSC scaffold-free 3D constructs, fabricated using the “mold” or “Kenzan” method, was considered. Finally, prospects of osteochondral regeneration by scaffold-free 3D constructs using AT-MSC spheroids were discussed.

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.

Substrate mechanics dictate cell-cell communication by gap junctions in stem cells from human apical papilla

It is recognized that the interaction between cells and their physical microenvironment plays a fundamental role in controlling cell behaviors and even in determining cell fate. Any change in the physical properties of the extracellular matrix (ECM), such as its topography, geometry, and stiffness, controls this interaction. In the current study, we revealed a potent interconnection between the cell-matrix interaction and cell-cell communication that is mediated by interface stiffness, and elucidated this process in stem cells from human apical papilla (hSCAPs) in terms of mechanosensing, mechanotransduction, and gap junction-mediated cell-cell communication. We first fabricated polydimethylsiloxane (PDMS) substrates with the same topography and geometry but different stiffnesses and found that the cell morphology of the hSCAPs actively changed to adapt to the difference in substrate stiffness. We also found that the hSCAPs secreted more fibronectin in response to the stiff substrate. The focal adhesion plaques were changed by altering the expression of focal adhesion kinase (FAK) and paxillin. The FAK and paxillin bound to connexin 43 and, as a result, altered the gap junction formation. By performing a Lucifer yellow transfer assay, we further confirmed that the interface stiffness mediated cell-cell communication in living hSCAPs through changes in gap junction tunnels. The intrinsic mechanism that mediated cell-cell communication by extracellular stiffness show the great influence of the interaction between cells and their external physical microenvironment and stress the importance of microenvironmental mechanics in organ development and diseases. STATEMENT OF SIGNIFICANCE: Biochemical factors could direct cell behaviors such as cell proliferation, migration, differentiation, cell cycling and apoptosis. Likewise, biophysical factors could also determine cell behaviors in all biological processes. In the current study, we revealed a potent interconnection between the cell-matrix interaction and cell-cell communication by elucidating the whole process from cell mechanosensing, mechanotransduction to gap junction-mediated cell-cell communication. This process occurs in a collective of cells but not in that of a single cell. Biophysical properties of ECM induced cell-to-cell communication indicates the importance of microenvironmental mechanics in organ development and diseases. These findings should be of great interest in all biological fields, especially in biomaterials - cell/molecular biology involved in the interactions between the cell and its matrix.

Bioengineered microenvironment to culture early embryos

The abnormalities of early post-implantation embryos can lead to early pregnancy loss and many other syndromes. However, it is hard to study embryos after implantation due to the limited accessibility. The success of embryo culture in vitro can avoid the challenges of embryonic development in vivo and provide a powerful research platform for research in developmental biology. The biophysical and chemical cues of the microenvironments impart significant spatiotemporal effects on embryonic development. Here, we summarize the main strategies which enable researchers to grow embryos outside of the body while overcoming the implantation barrier, highlight the roles of engineered microenvironments in regulating early embryonic development, and finally discuss the future challenges and new insights of early embryo culture.

Hybrid and Composite Scaffolds Based on Extracellular Matrices for Cartilage Tissue Engineering

IMPACT STATEMENT

Scaffolds fabricated from extracellular matrix (ECM) derivatives are composed of conducive structures for cell attachment, proliferation, and differentiation, but generally do not have proper mechanical properties and load-bearing capacity. In contrast, scaffolds based on synthetic biomaterials demonstrate appropriate mechanical strength, but the absence of desirable biological properties is one of their main disadvantages. To integrate mechanical strength and biological cues, these ECM derivatives can be conjugated with synthetic biomaterials. Hence, hybrid scaffolds comprising both advantages of synthetic polymers and ECM derivatives can be considered a robust vehicle for tissue engineering applications.

Microribbon-hydrogel composite scaffold accelerates cartilage regeneration in vivo with enhanced mechanical properties using mixed stem cells and chondrocytes

Juvenile chondrocytes are robust in regenerating articular cartilage, but their clinical application is hindered by donor scarcity. Stem cells offer an abundant autologous cell source but are limited by slow cartilage deposition with poor mechanical properties. Using 3D co-culture models, mixing stem cells and chondrocytes can induce synergistic cartilage regeneration. However, the resulting cartilage tissue still suffers from poor mechanical properties after prolonged culture. Here we report a microribbon/hydrogel composite scaffold that supports synergistic interactions using co-culture of adipose-derived stem cells (ADSCs) and neonatal chondrocytes (NChons). The composite scaffold is comprised of a macroporous, gelatin microribbon (μRB) scaffolds filled with degradable nanoporous chondroitin sulfate (CS) hydrogel. We identified an optimal CS concentration (6%) that best supported co-culture synergy in vitro. Furthermore, 7 days of TGF-β3 exposure was sufficient to induce catalyzed cartilage formation. When implanted in vivo, μRB/CS composite scaffold supported over a 40-fold increase in compressive moduli of cartilage produced by mixed ADSCs/NChons to ~330 kPa, which surpassed even the quality of cartilage produced by 100% NChons. Together, these results validate μRB/CS composite as a promising scaffold for cartilage regeneration using mixed populations of stem cells and chondrocytes.

Gelatin-Based Microribbon Hydrogels Accelerate Cartilage Formation by Mesenchymal Stem Cells in Three Dimensions

Hydrogels (HGs) are attractive matrices for cell-based cartilage tissue regeneration given their injectability and ability to fill defects with irregular shapes. However, most HGs developed to date often lack cell scale macroporosity, which restrains the encapsulated cells, leading to delayed new extracellular matrix deposition restricted to pericellular regions. Furthermore, tissue-engineered cartilage using conventional HGs generally suffers from poor mechanical property and fails to restore the load-bearing property of articular cartilage. The goal of this study was to evaluate the potential of macroporous gelatin-based microribbon (μRB) HGs as novel 3D matrices for accelerating chondrogenesis and new cartilage formation by human mesenchymal stem cells (MSCs) in 3D with improved mechanical properties. Unlike conventional HGs, these μRB HGs are inherently macroporous and exhibit cartilage-mimicking shock-absorbing mechanical property. After 21 days of culture, MSC-seeded μRB scaffolds exhibit a 20-fold increase in compressive modulus to 225 kPa, a range that is approaching the level of native cartilage. In contrast, HGs only resulted in a modest increase in compressive modulus of 65 kPa. Compared with conventional HGs, macroporous μRB scaffolds significantly increased the total amount of neocartilage produced by MSCs in 3D, with improved interconnectivity and mechanical strength. Altogether, these results validate gelatin-based μRBs as promising scaffolds for enhancing and accelerating MSC-based cartilage regeneration and may be used to enhance cartilage regeneration using other cell types as well.

Comparing Single Cell Versus Pellet Encapsulation of Mesenchymal Stem Cells in Three-Dimensional Hydrogels for Cartilage Regeneration

While the gold standard for inducing mesenchymal stem cell (MSC) chondrogenesis utilizes pellet culture, most tissue engineering strategies for cartilage regeneration encapsulate MSCs as single cells, partially due to the technical challenge to homogeneously encapsulate cell pellets in three-dimensional (3D) hydrogels. It remains unclear whether encapsulating MSCs as single cell suspension or cell aggregates in 3D hydrogels would enhance MSC-based cartilage formation. In this study, we determined that the optimal size of MSC micropellets (μPellets) that can be homogeneously encapsulated in hydrogels with high cell viability is 100 cells/pellet. Using optimized μPellet size, MSCs were encapsulated either as single cell suspension or μPellets in four soft hydrogel formulations with stiffness ranging 3-6 kPa. Regardless of hydrogel formulations, single cell encapsulation resulted in more neocartilage deposition with improved mechanical functions over μPellet encapsulation. For single cell encapsulation, polyethylene glycol (PEG) hydrogels containing chondroitin sulfate led to the most cartilage matrix deposition, with compressive modulus reaching 211 kPa after only 21 days, a range approaching the stiffness of native cartilage. The findings from this study offer valuable insights on guiding optimal method design for MSCs and hydrogel-based cartilage regeneration. The optimized μPellet encapsulation method may be broadly applicable to encapsulate other stem cell types or cancer cells as aggregates in hydrogels. Impact Statement While the gold standard for inducing mesenchymal stem cell (MSC) chondrogenesis utilizes pellet culture, it remains unclear whether encapsulating MSCs as cell pellets in three-dimensional hydrogels would enhance MSC-based cartilage formation. In this study, we determined the optimal size of MSC micropellet (μPellet) that can be homogeneously encapsulated in hydrogels with high cell viability. Unexpectedly, single cell encapsulation resulted in more robust new cartilage formation than μPellet encapsulation. Furthermore, tuning hydrogel formulation led to rapid cartilage regeneration with stiffness approaching that of native cartilage. The findings from this study would facilitate clinical translation of MSCs and hydrogel-based therapies for cartilage regeneration with optimized parameters.

The Effect of Scaffold Modulus on the Morphology and Remodeling of Fetal Mesenchymal Stem Cells

Hydrogel materials have been successfully used as matrices to explore the role of biophysical and biochemical stimuli in directing stem cell behavior. Here, we present our findings on the role of modulus in guiding bone marrow fetal mesenchymal stem cell (BMfMSC) fate determination using semi-synthetic hydrogels made from PEG-fibrinogen (PF). The BMfMSCs were cultivated in the PF for up to 2 weeks to study the influence of matrix modulus (i.e., cross-linking density of the PF) on BMfMSC survival, morphology and integrin expression. Both two-dimensional (2D) and three-dimensional (3D) culture conditions were employed to examine the BMfMSCs as single cells or as cell spheroids. The hydrogel modulus affected the rate of BMfMSC metabolic activity, the integrin expression levels and the cell morphology, both as single cells and as spheroids. The cell seeding density was also found to be an important parameter of the system in that high densities were favorable in facilitating more cell-to-cell contacts that favored higher metabolic activity. Our findings provide important insight about design of a hydrogel scaffold that can be used to optimize the biological response of BMfMSCs for various tissue engineering applications.

Chasing Chimeras - The elusive stable chondrogenic phenotype

The choice of the best-suited cell population for the regeneration of damaged or diseased cartilage depends on the effectiveness of culture conditions (e.g. media supplements, three-dimensional scaffolds, mechanical stimulation, oxygen tension, co-culture systems) to induce stable chondrogenic phenotype. Herein, advances and shortfalls in in vitro, preclinical and clinical setting of various in vitro microenvironment modulators on maintaining chondrocyte phenotype or directing stem cells towards chondrogenic lineage are critically discussed. Chondrocytes possess low isolation efficiency, limited proliferative potential and rapid phenotypic drift in culture. Mesenchymal stem cells are relatively readily available, possess high proliferation potential, exhibit great chondrogenic differentiation capacity, but they tend to acquire a hypertrophic phenotype when exposed to chondrogenic stimuli. Embryonic and induced pluripotent stem cells, despite their promising in vitro and preclinical data, are still under-investigated. Although a stable chondrogenic phenotype remains elusive, recent advances in in vitro microenvironment modulators are likely to develop clinically- and commercially-relevant therapies in the years to come.

Biochemical and Mechanical Gradients Synergize To Enhance Cartilage Zonal Organization in 3D

Articular cartilage is characterized by zonal organizations containing dual gradients of biochemical cues and mechanical cues. However, how biochemical gradient interacts with the mechanical gradient to drive the cartilage zonal development remains largely unknown. Here, we report the development of a dual-gradient hydrogel platform as a 3D niche to elucidate the relative contributions of biochemical and mechanical niche gradients in modulating zonal-specific chondrocyte responses and cartilage zonal organization. Chondroitin sulfate (CS), a major constituent of cartilage extracellular matrix, was chosen as the biochemical cue. Poly(ethylene glycol), a bioinert polymer, was used to create the stiffness gradient. Dual-gradient hydrogels upregulated cartilage marker expressions and increased chondrocyte proliferation and collagen deposition in a zonal-dependent manner. Hydrogels with CS gradient alone exhibited poor mechanical strength and degraded prematurely after 1 week of culture. While CS gradient alone did not support long-term culture, adding CS gradient to mechanical-gradient hydrogels substantially enhanced cell proliferation, glycosaminoglycan production, and collagen deposition compared to mechanical-gradient hydrogels alone. These results suggest that biochemical and mechanical gradient cues synergize to enhance cartilage zonal organization by chondrocytes in 3D. Together, our results validate the potential of dual-gradient hydrogels as a 3D cell niche for cartilage regeneration with zonal organization and may be used to recreate other tissue interfaces.

Mechanically cartilage-mimicking poly(PCL-PTHF urethane)/collagen nanofibers induce chondrogenesis by blocking NF-kappa B signaling pathway

Cartilage cannot self-repair and thus regeneration is a promising approach to its repair. Here we developed new electrospun nanofibers, made of poly (ε-caprolactone)/polytetrahydrofuran (PCL-PTHF urethane) and collagen I from calf skin (termed PC), to trigger the chondrogenic differentiation of mesenchymal stem cells (MSCs) and the cartilage regeneration in vivo. We found that the PC nanofibers had a modulus (4.3 Mpa) lower than the PCL-PTHF urethane nanofibers without collagen I from calf skin (termed P) (6.8 Mpa) although both values are within the range of the modulus of natural cartilage (1-10 MPa). Both P and PC nanofibers did not show obvious difference in the morphology and size. Surprisingly, in the absence of the additional chondrogenesis inducers, the softer PC nanofibers could induce the chondrogenic differentiation in vitro and cartilage regeneration in vivo more efficiently than the stiffer P nanofibers. Using mRNA-sequence analysis, we found that the PC nanofibers outperformed P nanofibers in inducing chondrogenesis by specifically blocking the NF-kappa B signaling pathway to suppress inflammation. Our work shows that the PC nanofibers can serve as building blocks of new scaffolds for cartilage regeneration and provides new insights on the effect of the mechanical properties of the nanofibers on the cartilage regeneration.

Matrix stiffness regulates the differentiation of tendon-derived stem cells through FAK-ERK1/2 activation

Tendon derived stem cells (TDSCs) were vital in tendon homeostasis. Nevertheless, the regulation of TDSCs differentiation in tendinopathy is unclear. Matrix stiffness modulated stem cells differentiation, and matrix stiffness of tendinopathic tissues decreased significantly. In order to clarify the role of matrix stiffness in TDSCs differentiation, they were cultured on the gelatin hydrogels with the stiffness from 2.34 ± 1.48 kPa to 24.09 ± 14.03 kPa. The effect of matrix stiffness on TDSCs proliferation and differentiation were investigated with CCK8 assay, immunofluorescences, real time PCR and western blot. It was found the proliferation of TDSCs increased and more stress fibers formed with increasing matrix stiffness. The differentiation of TDSCs into tenogenic, chondrogenic, and osteogenic lineages were inhibited on stiff hydrogel evidenced by reduced expression of tenocyte markers THBS4, TNMD, SCX, chondrocyte marker COL2, and osteocyte markers Runx2, Osterix, and ALP. Furthermore, the phosphorylation of FAK and ERK1/2 were enhanced when TDSCs grew on stiff hydrogel. After FAK or ERK1/2 was inhibited, the effect of matrix stiffness on differentiation of TDSCs was inhibited as well. The above results indicated matrix stiffness modulated the proliferation and differentiation of TDSCs, and the regulation effect could correlate to the activation of FAK or ERK1/2.

Functionality of decellularized matrix in cartilage regeneration: A comparison of tissue versus cell sources

Increasing evidence indicates that decellularized extracellular matrices (dECMs) derived from cartilage tissues (T-dECMs) or chondrocytes/stem cells (C-dECMs) can support proliferation and chondrogenic differentiation of cartilage-forming cells. However, few review papers compare the differences between these dECMs when they serve as substrates for cartilage regeneration. In this review, after an introduction of cartilage immunogenicity and decellularization methods to prepare T-dECMs and C-dECMs, a comprehensive comparison focuses on the effects of T-dECMs and C-dECMs on proliferation and chondrogenic differentiation of chondrocytes/stem cells in vitro and in vivo. Key factors within dECMs, consisting of microarchitecture characteristics and micromechanical properties as well as retained insoluble and soluble matrix components, are discussed in-depth for potential mechanisms underlying the functionality of these dECMs in regulating chondrogenesis. With this information, we hope to benefit dECM based cartilage engineering and tissue regeneration for future clinical application.

STATEMENT OF SIGNIFICANCE

The use of decellularized extracellular matrix (dECM) is becoming a promising approach for tissue engineering and regeneration. Compared to dECM derived from cartilage tissue, recently reported dECM from cell sources exhibits a distinct role in cell based cartilage regeneration. In this review paper, for the first time, tissue and cell based dECMs are comprehensively compared for their functionality in cartilage regeneration. This information is expected to provide an update for dECM based cartilage regeneration.

Engineering Hydrogel Microenvironments to Recapitulate the Stem Cell Niche

Stem cells are a powerful resource for many applications including regenerative medicine, patient-specific disease modeling, and toxicology screening. However, eliciting the desired behavior from stem cells, such as expansion in a naïve state or differentiation into a particular mature lineage, remains challenging. Drawing inspiration from the native stem cell niche, hydrogel platforms have been developed to regulate stem cell fate by controlling microenvironmental parameters including matrix mechanics, degradability, cell-adhesive ligand presentation, local microstructure, and cell-cell interactions. We survey techniques for modulating hydrogel properties and review the effects of microenvironmental parameters on maintaining stemness and controlling differentiation for a variety of stem cell types. Looking forward, we envision future hydrogel designs spanning a spectrum of complexity, ranging from simple, fully defined materials for industrial expansion of stem cells to complex, biomimetic systems for organotypic cell culture models.

A comparative study of chondroitin sulfate and heparan sulfate for directing three-dimensional chondrogenesis of mesenchymal stem cells

Background

Mesenchymal stem cells (MSCs) hold great promise for cartilage repair given their relative abundance, ease of isolation, and chondrogenic potential. To enhance MSC chondrogenesis, extracellular matrix components can be incorporated into three-dimensional (3D) scaffolds as an artificial cell niche. Chondroitin sulfate (CS)-containing hydrogels have been shown to support 3D chondrogenesis, but the effects of varying CS concentration and hydrogel stiffness on 3D MSC chondrogenesis remains elusive. Heparan sulfate (HS) is commonly used as a growth factor reservoir due to its ability to sequester growth factors; however, how it compares to CS in supporting 3D MSC chondrogenesis remains unknown.

Methods

We fabricated photocrosslinkable hydrogels containing physiologically relevant concentrations (0–10%) of CS or HS with two stiffnesses (~7.5 kPa and ~ 36 kPa) as a 3D niche for MSC chondrogenesis.

Results

CS is a more potent factor in enhancing MSC chondrogenesis, especially in soft hydrogels (~ 7.5 kPa). A moderate dosage of CS (5%) led to the highest amount of neocartilage deposition. Stiff hydrogels (~ 36 kPa) generally inhibited neocartilage formation regardless of the biochemical cues.

Conclusions

Taken together, the results from this study demonstrated that CS-containing hydrogels at low mechanical stiffness can provide a promising scaffold for enhancing MSC-based cartilage tissue regeneration.

Electronic supplementary material

The online version of this article (doi:10.1186/s13287-017-0728-6) contains supplementary material, which is available to authorized users.

Hyaluronic Acid Accelerates Tendon-to-Bone Healing After Rotator Cuff Repair

BACKGROUND

There is growing evidence that the subacromial injection of hyaluronic acid (HA) is effective for pain relief in rotator cuff tears; however, its effect on tendon-to-bone healing remains unknown.

PURPOSE

To examine the effect of HA on the chondrogenesis of mesenchymal stem cells (MSCs) in vitro and on tendon-to-bone healing in a rotator cuff repair model.

STUDY DESIGN

Controlled laboratory study.

METHODS

Bilateral complete tears of the infraspinatus tendon were made in rabbits and subsequently repaired. Before closure, 1 mL HA was applied to the repaired site, and phosphate-buffered saline was used in the opposite side as a control. Biomechanical, histological, and immunohistochemical analyses were performed at 4, 8, and 12 weeks after surgery. After euthanizing each animal, the bone marrow was isolated from the femoral bone in the same rabbits. Then, MSCs were cultured in media for chondrogenic differentiation, and the chondral pellet production and cartilage-related gene expression levels in the cells were examined at various concentrations of HA.

RESULTS

At 4 and 8 weeks after surgery, ultimate load-to-failure was significantly greater in the HA group than in the control group (45.61 ± 9.0 N vs 32.42 ± 9.4 N at 4 weeks, 90.7 ± 16.0 N vs 66.97 ± 10.0 N at 8 weeks; both P < .05) but not at 12 weeks after surgery (109.6 ± 40.2 N vs 108.1 ± 42.6 N, P > .05). Linear stiffness was not significant throughout the time point evaluation. The chondroid formation area at the tendon-bone interface stained by safranin O (control vs HA group) was 0.33% ± 0.7% versus 13.5% ± 12.3% at 4 weeks after surgery ( P < .05) and 3.0% ± 5.9% versus 12.9% ± 12.9% at 8 weeks after surgery ( P < .05), but there was no significant difference at 12 weeks after surgery. Maturity of collagen at the repaired site stained by PicroSirius Red (control vs HA group) was 16.2 ± 10.6 versus 43.5 ± 21.3 at 4 weeks after surgery ( P < .05), but there were no significant differences at 8 and 12 weeks after surgery. MSCs were cultured in media for chondrogenic differentiation, and the chondral pellet production and cartilage-related gene expression levels in the cells were examined at various concentrations of HA. The number of CD44-positive cells (control vs HA group) was 8.3% ± 1.4% versus 26.2% ± 5.2% at 3 days after surgery ( P < .05), 1.8% ± 1.1% versus 26.6% ± 11.6% at 4 weeks after surgery ( P < .05), 0.6% ± 0.9% versus 0.5% ± 0.6% at 8 weeks after surgery ( P > .05), and 1.8% ± 4.0% versus 5.4% ± 4.2% at 12 weeks after surgery ( P > .05). Compared with the control group, HA significantly increased the volume of cartilaginous pellet produced by MSCs (0.0016 ± 0.0015 mm³ at 0 mg/mL of HA, 0.0041 ± 0.0023 mm³ at 1.0 mg/mL, and 0.0041 ± 0.0018 mm³ at 4.0 mg/mL), with increased mRNA expression (relative ratio to control) of type 2 collagen (1.34 ± 0.38), SOX9 (1.58 ± 0.31), and aggrecan (1.30 ± 0.22) genes in the pellet ( P < .01).

CONCLUSION

HA accelerated tendon-to-bone healing in the rotator cuff repair model, enhancing the biomechanical strength and increasing chondroid formation and tendon maturity at the tendon-bone interface. Based on the data of in vitro experiments, HA-activated MSCs may play a crucial role in the acceleration of tendon-to-bone healing.

CLINICAL RELEVANCE

The data suggest the relevance of clinical application of HA to accelerate tendon-to-bone healing. It may decrease the number of retears after surgery.

Effect of alginate concentration on chondrogenesis of co-cultured human adipose-derived stem cells and nasal chondrocytes: a biological study

Background

The three-dimensional (3D) system is one of the important factors to engineer a biocompatible and functional scaffold for the applications of cell-based therapies for cartilage repair. The 3D alginate hydrogels system has previously been shown to potentially promote chondrogenesis. The chondrocytic differentiation of co-cultured adipose-derived stem cells (ADSCs) and nasal chondrocytes (NCs) within alginate constructs are hypothesized to be influenced by concentration of alginate hydrogel. In this study, we evaluated the effects of alginate concentration on chondrogenic differentiation of ADSCs and NCs co-cultured in a biological approach.

Method

The co-cultured cells of 2:1 ADSCs-to-NCs ratio were encapsulated in alginate constructs in one of three concentrations (1.0%, 1.2% and 1.5%) and cultured under serum free conditions for 7 days. Cell viability, cell proliferation, immunohistochemical, gycosaminogylycans (GAG) synthesis, and gene expression were examined.

Results

Overall, the 1.2% alginate concentration group was relatively effective in chondrocytic differentiation in comparable to other groups. The cell morphology, cell viability, and cell proliferation revealed initial chondrogenic differentiation by the formation of cell clusters as well as the high permeability for exchange of solutes. The formation of newly synthesis cartilage-specific extracellular matrix in 1.2% group was demonstrated by positive immunohistochemical staining of collagen type II. The co-cultured cells in 1.2% group highly expressed COL II, ACP and SOX-9, compared to 1.0% and 1.5% groups, denote the retention of cartilaginous-specific phenotype by suppressing the undifferentiation stem cell markers of SOX-2 and OCT-4. The study showed 1.2% group was less likely to differentiate towards osteogenesis by downregulating hyperthrophy chondrocytic gene of COL X and osseous marker genes of OSC and OSP.

Conclusion

This study suggests that variations in the alginate concentration of co-cultured ADSCs and NCs influenced the chondrogenesis. The remarkable biological performance on chondrogenic differentiation in regulating the concentration of alginate 3D culture provides new insights into the cell cross-talk and demonstrates the effectiveness in regenerative therapies of cartilage defects in tissue engineering.

Modeling Physiological Events in 2D vs. 3D Cell Culture

Cell culture has become an indispensable tool to help uncover fundamental biophysical and biomolecular mechanisms by which cells assemble into tissues and organs, how these tissues function, and how that function becomes disrupted in disease. Cell culture is now widely used in biomedical research, tissue engineering, regenerative medicine, and industrial practices. Although flat, two-dimensional (2D) cell culture has predominated, recent research has shifted toward culture using three-dimensional (3D) structures, and more realistic biochemical and biomechanical microenvironments. Nevertheless, in 3D cell culture, many challenges remain, including the tissue-tissue interface, the mechanical microenvironment, and the spatiotemporal distributions of oxygen, nutrients, and metabolic wastes. Here, we review 2D and 3D cell culture methods, discuss advantages and limitations of these techniques in modeling physiologically and pathologically relevant processes, and suggest directions for future research.

Tissue-specific bioactivity of soluble tendon-derived and cartilage-derived extracellular matrices on adult mesenchymal stem cells

BACKGROUND

Biological scaffolds composed of tissue-derived extracellular matrix (ECM) can promote homologous (i.e., tissue-specific) cell differentiation through preservation of biophysical and biochemical motifs found in native tissues. Solubilized ECMs derived from decellularized tendon and cartilage have recently been promoted as tissue-specific biomaterials, but whether tissue-specific bioactivity is preserved following solubilization is unknown. This study explored the tissue-specific bioactivity of soluble decellularized tendon and cartilage ECMs on human bone marrow-derived mesenchymal stem cells (MSCs) presented across different culture microenvironments, including two-dimensional (2D) tissue culture plastic, aligned electrospun nanofibers, cell pellets, and cell-seeded photocrosslinkable hydrogels.

METHODS

Tendon and cartilage ECMs were decellularized using established methods and solubilized either via pepsin digestion or urea extraction. The effect of soluble ECMs on cell proliferation and differentiation was initially explored by supplementing basal medium of human MSCs cultured on 2D tissue culture plastic. In subsequent experiments, MSCs were cultured on aligned electrospun nanofibers, ascell pellets, or encapsulated within photocrosslinkable methacrylated gelatin (GelMA) hydrogels. Urea-extracted tendon and cartilage ECMs were added as supplements.

RESULTS

Pepsin-digested ECMs did not promote homologous differentiation in human MSCs, whether provided as a medium supplement or three-dimensional (3D) hydrogels. In contrast, urea-extracted ECMs tended to promote tissue-specific differentiation of MSCs cultured in 2D and 3D microenvironments. The application of the small molecule TGF-β signaling inhibitor SB-431542 largely negated the tissue-specific gene expression patterns mediated by tendon and cartilage ECMs. This suggests that the action of endogenous TGF-β was required, but was not sufficient, to impart tissue-specific bioactivity of urea-extracted ECMs. When urea-extracted cartilage ECM was incorporated within a photocurable GelMA hydrogel it independently enhanced chondrogenesis in encapsulated MSCs, and showed additive prochondrogenesis upon TGF-β supplementation in the medium.

CONCLUSIONS

Urea-extracted ECM fractions of decellularized tendon and cartilage are soluble supplements capable of enhancing tissue-specific differentiation of adult stem cells.

Effect of hydrogel material composition on hBMSC differentiation into zone-specific neo-cartilage: engineering human articular cartilage-like tissue with spatially varying properties

Material composition alone can be used to direct human bone marrow stromal cells into distinct, zone-specific cell phenotypes and spatially-varying, multi-layered material scaffolds can generate complex, patterned tissue structures.

Effects of Hydrogel Stiffness and Extracellular Compositions on Modulating Cartilage Regeneration by Mixed Populations of Stem Cells and Chondrocytes In Vivo

Cell-based therapies offer great promise for repairing cartilage. Previous strategies often involved using a single cell population such as stem cells or chondrocytes. A mixed cell population may offer an alternative strategy for cartilage regeneration while overcoming donor scarcity. We have recently reported that adipose-derived stem cells (ADSCs) can catalyze neocartilage formation by neonatal chondrocytes (NChons) when mixed co-cultured in 3D hydrogels in vitro. However, it remains unknown how the biochemical and mechanical cues of hydrogels modulate cartilage formation by mixed cell populations in vivo. The present study seeks to answer this question by co-encapsulating ADSCs and NChons in 3D hydrogels with tunable stiffness (∼1-33 kPa) and biochemical cues, and evaluating cartilage formation in vivo using a mouse subcutaneous model. Three extracellular matrix molecules were examined, including chondroitin sulfate (CS), hyaluronic acid (HA), and heparan sulfate (HS). Our results showed that the type of biochemical cue played a dominant role in modulating neocartilage deposition. CS and HA enhanced type II collagen deposition, a desirable phenotype for articular cartilage. In contrast, HS promoted fibrocartilage phenotype with the upregulation of type I collagen and failed to retain newly deposited matrix. Hydrogels with stiffnesses of ∼7-33 kPa led to a comparable degree of neocartilage formation, and a minimal initial stiffness was required to retain hydrogel integrity over time. Results from this study highlight the important role of matrix cues in directing neocartilage formation, and they offer valuable insights in guiding optimal scaffold design for cartilage regeneration by using mixed cell populations.

The Synergistic Effects of Matrix Stiffness and Composition on the Response of Chondroprogenitor Cells in a 3D Precondensation Microenvironment

Improve functional quality of cartilage tissue engineered from stem cells requires a better understanding of the functional evolution of native cartilage tissue. Therefore, a biosynthetic hydrogel was developed containing RGD, hyaluronic acid and/or type-I collagen conjugated to poly(ethylene glycol) acrylate to recapitulate the precondensation microenvironment of the developing limb. Conjugation of any combination of the three ligands did not alter the shear moduli or diffusion properties of the PEG hydrogels; thus, the influence of ligand composition on chondrogenesis could be investigated in the context of varying matrix stiffness. Gene expression of ligand receptors (CD44 and the b1-integrin) as well as markers of condensation (cell clustering and N-cadherin gene expression) and chondrogenesis (Col2a1 gene expression and sGAG production) by chondroprogenitor cells in this system were modulated by both matrix stiffness and ligand composition, with the highest gene expression occurring in softer hydrogels containing all three ligands. Cell proliferation in these 3D matrices for 7 d prior to chondrogenic induction increased the rate of sGAG production in a stiffness-dependent manner. This biosynthetic hydrogel supports the features of early limb-bud condensation and chondrogenesis and is a novel platform in which the influence of the matrix physicochemical properties on these processes can be elucidated.

Taking cues from the extracellular matrix to design bone-mimetic regenerative scaffolds

There is an ongoing need for effective materials that can replace autologous bone grafts in the clinical treatment of bone injuries and deficiencies. In recent years, research efforts have shifted away from a focus on inert biomaterials to favor scaffolds that mimic the biochemistry and structure of the native bone extracellular matrix (ECM). The expectation is that such scaffolds will integrate with host tissue and actively promote osseous healing. To further enhance the osteoinductivity of bone graft substitutes, ECM-mimetic scaffolds are being engineered with a range of growth factors (GFs). The technologies used to generate GF-modified scaffolds are often inspired by natural processes that regulate the association between endogenous ECMs and GFs. The purpose of this review is to summarize research centered on the development of regenerative scaffolds that replicate the fundamental collagen-hydroxyapatite structure of native bone ECM, and the functionalization of these scaffolds with GFs that stimulate critical events in osteogenesis.

Modulating stem cell–chondrocyte interactions for cartilage repair using combinatorial extracellular matrix-containing hydrogels

Combinatorial ECM-containing hydrogels with decoupled mechanical and biochemical cues modulate cartilage formation by a stem cell and chondrocyte co-culture in 3D.

Physically crosslinked polyvinyl alcohol and gelatin interpenetrating polymer network theta-gels for cartilage regeneration

Theta-gels are hydrogels that form during the solidification and phase separation of two dislike polymers, in which a low molecular weight polymer behaves as a porogen and is removed through dialysis. For this study, interpenetrating polymer network (IPN) hydrogels were formed between polyvinyl alcohol (PVA) and gelatin using theta-gel fabrication techniques, i.e., in the presence of a porogen. The addition of gelatin to a PVA theta-gel, formed with a porogen, polyethylene glycol (PEG), created macro-porous hydrogels, and increased shear storage moduli and elastic moduli, compared to PVA-gelatin scaffold controls. A reduction in PVA crystallinity was verified by Fourier transform infrared (FTIR) spectroscopy in hydrogels fabricated using a porogen, i.e., PVA-PEG-gelatin, compared to PVA, PVA-PEG, or PVA-gelatin hydrogels alone. Van Geison staining confirmed the retention of gelatin after dialysis. A range of hydrogel moduli was achieved by optimizing PVA concentration, molecular weight, and gelatin concentration. PVA-gelatin hydrogels maintained primary human mesenchymal stem cell (MSC) viability. Soft (∼10 kPa) and stiff (∼100 kPa) PVA-gelatin hydrogels containing type II collagen significantly increased glycosaminoglycan (GAG) production compared to controls. PVA-gelatin hydrogels, formed using theta-gel techniques, warrant further investigation as articular cartilage tissue engineering scaffolds.

Developing Extracellular Matrix Technology to Treat Retinal or Optic Nerve Injury(1,2,3)

Adult mammalian CNS neurons often degenerate after injury, leading to lost neurologic functions. In the visual system, retinal or optic nerve injury often leads to retinal ganglion cell axon degeneration and irreversible vision loss. CNS axon degeneration is increasingly linked to the innate immune response to injury, which leads to tissue-destructive inflammation and scarring. Extracellular matrix (ECM) technology can reduce inflammation, while increasing functional tissue remodeling, over scarring, in various tissues and organs, including the peripheral nervous system. However, applying ECM technology to CNS injuries has been limited and virtually unstudied in the visual system. Here we discuss advances in deriving fetal CNS-specific ECMs, like fetal porcine brain, retina, and optic nerve, and fetal non-CNS-specific ECMs, like fetal urinary bladder, and the potential for using tissue-specific ECMs to treat retinal or optic nerve injuries in two platforms. The first platform is an ECM hydrogel that can be administered as a retrobulbar, periocular, or even intraocular injection. The second platform is an ECM hydrogel and polymer "biohybrid" sheet that can be readily shaped and wrapped around a nerve. Both platforms can be tuned mechanically and biochemically to deliver factors like neurotrophins, immunotherapeutics, or stem cells. Since clinical CNS therapies often use general anti-inflammatory agents, which can reduce tissue-destructive inflammation but also suppress tissue-reparative immune system functions, tissue-specific, ECM-based devices may fill an important need by providing naturally derived, biocompatible, and highly translatable platforms that can modulate the innate immune response to promote a positive functional outcome.

3D Hydrogel Scaffolds for Articular Chondrocyte Culture and Cartilage Generation

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.

In vitro and in vivo biocompatibility, bioavailability and tolerance of an injectable vehicle for adipose-derived stem/stromal cells for plastic surgery indications

Soft tissue reconstruction is a challenge in plastic surgery, when replacing lost materials and correcting contour defects. Many permanent and temporary fillers have been used to restore the volume of these lesions, but often with poor results and even complications. Adipose-derived stem/stromal cells (ASCs) and adipose tissue engineering have been suggested as valuable alternatives. In order to inject these cultured cells, it was essential to find a suitable vehicle. The purpose of this study was to evaluate Cytocare(®), an injectable medical device, composed of hyaluronic acid plus amino acids, vitamins and mineral salts. First, ASC viability and bioavailability in the 3 different available Cytocare(®) formulations using the MTT test were assessed; then an animal experiment, testing the tolerance after intradermal injections of both Cytocare(®) alone and with ASCs was carried out. Our in vitro results demonstrate a high biocompatibility of Cytocare(®) resulting in a better viability of ASCs when cultured in Cytocare(®) compared to culture medium (p < 0.05, Mann and Whitney). Cytocare(®) also permits their bioavailability and proliferation, making it a potential transfer vehicle that can retain the cells before their integration around the recipient site. Finally, our animal experiment shows that the ASC + Cytocare(®) combination is well tolerated. In conclusion, Cytocare(®) can be used as a biocompatible scaffold for cultured ASCs in therapeutic treatments, ensuring ASC bioavailability, as well as evidence of excellent tolerance in nude mice.

Mechanism of regulation of stem cell differentiation by matrix stiffness

Stem cell behaviors are regulated by multiple microenvironmental cues. As an external signal, mechanical stiffness of the extracellular matrix is capable of governing stem cell fate determination, but how this biophysical cue is translated into intracellular signaling remains elusive. Here, we elucidate mechanisms by which stem cells respond to microenvironmental stiffness through the dynamics of the cytoskeletal network, leading to changes in gene expression via biophysical transduction signaling pathways in two-dimensional culture. Furthermore, a putative rapid shift from original mechanosensing to de novo cell-derived matrix sensing in more physiologically relevant three-dimensional culture is pointed out. A comprehensive understanding of stem cell responses to this stimulus is essential for designing biomaterials that mimic the physiological environment and advancing stem cell-based clinical applications for tissue engineering.

Union is strength: matrix elasticity and microenvironmental factors codetermine stem cell differentiation fate

Stem cells are an attractive cellular source for regenerative medicine and tissue engineering applications due to their multipotency. Although the elasticity of the extracellular matrix (ECM) has been shown to have crucial impacts in directing stem cell differentiation, it is not the only contributing factor. Many researchers have recently attempted to design microenvironments that mimic the stem cell niche with combinations of ECM elasticity and other cues, such as ECM physical properties, soluble biochemical factors and cell-cell interactions, thereby driving cells towards their preferred lineages. Here, we briefly discuss the effect of matrix elasticity on stem cell lineage specification and then summarize recent advances in the study of the combined effects of ECM elasticity and other cues on the differentiation of stem cells, focusing on two aspects: biophysical and biochemical factors. In the future, biomedical scientists will continue investigating the union strength of matrix elasticity and microenvironmental cues for manipulating stem cell fates.