Transient exposure to TGF-β3 improves the functional chondrogenesis of MSC-laden hyaluronic acid hydrogels

Injectable MSC Spheroid and Microgel Granular Composites for Engineering Tissue

Many cell types require direct cell-cell interactions for differentiation and function; yet, this can be challenging to incorporate into 3-dimensional (3D) structures for the engineering of tissues. Here, a new approach is introduced that combines aggregates of cells (spheroids) with similarly-sized hydrogel particles (microgels) to form granular composites that are injectable, undergo interparticle crosslinking via light for initial stabilization, permit cell-cell contacts for cell signaling, and allow spheroid fusion and growth. One area where this is important is in cartilage tissue engineering, as cell-cell contacts are crucial to chondrogenesis and are missing in many tissue engineering approaches. To address this, granular composites are developed from adult porcine mesenchymal stromal cell (MSC) spheroids and hyaluronic acid microgels and simulations and experimental analyses are used to establish the importance of initial MSC spheroid to microgel volume ratios to balance mechanical support with tissue growth. Long-term chondrogenic cultures of granular composites produce engineered cartilage tissue with extensive matrix deposition and mechanical properties within the range of cartilage, as well as integration with native tissue. Altogether, a new strategy of injectable granular composites is developed that leverages the benefits of cell-cell interactions through spheroids with the mechanical stabilization afforded with engineered hydrogels.

A Systems Approach to Biomechanics, Mechanobiology, and Biotransport

The human body represents a collection of interacting systems that range in scale from nanometers to meters. Investigations from a systems perspective focus on how the parts work together to enact changes across spatial scales, and further our understanding of how systems function and fail. Here, we highlight systems approaches presented at the 2022 Summer Biomechanics, Bio-engineering, and Biotransport Conference in the areas of solid mechanics; fluid mechanics; tissue and cellular engineering; biotransport; and design, dynamics, and rehabilitation; and biomechanics education. Systems approaches are yielding new insights into human biology by leveraging state-of-the-art tools, which could ultimately lead to more informed design of therapies and medical devices for preventing and treating disease as well as rehabilitating patients using strategies that are uniquely optimized for each patient. Educational approaches can also be designed to foster a foundation of systems-level thinking.

Developmental morphogens direct human induced pluripotent stem cells toward an annulus fibrosus-like cell phenotype

Introduction

Therapeutic interventions for intervertebral disc herniation remain scarce due to the inability of endogenous annulus fibrosus (AF) cells to respond to injury and drive tissue regeneration. Unlike other orthopedic tissues, such as cartilage, delivery of exogenous cells to the site of annular injury remains underdeveloped, largely due to a lack of an ideal cell source and the invasive nature of cell isolation. Human induced pluripotent stem cells (iPSCs) can be differentiated to specific cell fates using biochemical factors and are, therefore, an invaluable tool for cell therapy approaches. While differentiation protocols have been developed for cartilage and fibrous connective tissues (e.g., tendon), the signals that regulate the induction and differentiation of human iPSCs toward the AF fate remain unknown.

Methods

iPSC-derived sclerotome cells were treated with various combinations of developmental signals including transforming growth factor beta 3 (TGF-β3), connective tissue growth factor (CTGF), platelet derived growth factor BB (PDGF-BB), insulin-like growth factor 1 (IGF-1), or the Hedgehog pathway activator, Purmorphamine, and gene expression changes in major AF-associated ECM genes were assessed. The top performing combination treatments were further validated by using three distinct iPSC lines and by assessing the production of upregulated ECM proteins of interest. To conduct a broader analysis of the transcriptomic shifts elicited by each factor combination, and to compare genetic profiles of treated cells to mature human AF cells, a 96.96 Fluidigm gene expression array was applied, and principal component analysis was employed to identify the transcriptional signatures of each cell population and treatment group in comparison to native AF cells.

Results

TGF-β3, in combination with PDGF-BB, CTGF, or IGF-1, induced an upregulation of key AF ECM genes in iPSC-derived sclerotome cells. In particular, treatment with a combination of TGF-β3 with PDGF-BB for 14 days significantly increased gene expression of collagen II and aggrecan and increased protein deposition of collagen I and elastin compared to other treatment groups. Assessment of genes uniquely highly expressed by AF cells or SCL cells, respectively, revealed a shift toward the genetic profile of AF cells with the addition of TGF-β3 and PDGF-BB for 14 days.

Discussion

These findings represent an initial approach to guide human induced pluripotent stem cells toward an AF-like fate for cellular delivery strategies.

Physiologic Doses of TGF-β Improve the Composition of Engineered Articular Cartilage

For cartilage regeneration applications, transforming growth factor beta (TGF-β) is conventionally administered at highly supraphysiologic doses (10-10,000 ng/mL) in an attempt to cue cells to fabricate neocartilage that matches the composition, structure, and functional properties of native hyaline cartilage. While supraphysiologic doses enhance ECM biosynthesis, they are also associated with inducing detrimental tissue features, such as fibrocartilage matrix deposition, pathologic-like chondrocyte clustering, and tissue swelling. Here we investigate the hypothesis that moderated TGF-β doses (0.1-1 ng/mL), akin to those present during physiological cartilage development, can improve neocartilage composition. Variable doses of media-supplemented TGF-β were administered to a model system of reduced-size cylindrical constructs (Ø2-Ø3 mm), which mitigate the TGF-β spatial gradients observed in conventional-size constructs (Ø4-Ø6 mm), allowing for a novel assessment of the intrinsic effect of TGF-β doses on macroscale neocartilage properties and composition. The administration of physiologic TGF-β to reduced-size constructs yields neocartilage with native-matched sGAG content and mechanical properties while providing a more hyaline cartilage-like composition, marked by: 1) reduced fibrocartilage-associated type I collagen, 2) 77% reduction in the fraction of cells present in a clustered morphology, and 3) 45% reduction in the degree of tissue swelling. Physiologic TGF-β appears to achieve an important balance of promoting requisite ECM biosynthesis, while mitigating hyaline cartilage compositional deficits. These results can guide the development of novel physiologic TGF-β-delivering scaffolds to improve the regeneration clinical-sized neocartilage tissues.

Comparison of multiple synthetic chondroinductive factors in pellet culture against a TGF-β positive control

Despite the advances in surgical and cell therapy regenerative techniques for cartilage repair, the challenge is to overcome an inferior fibrocartilage repair tissue. In vitro, TGF-β1 and TGF-β3 are the primary growth factors employed to induce chondrogenic differentiation. However, the clinical application of native proteins may present challenges regarding stability, cost, or reproducibility. Therefore, there remains an unmet clinical need for the identification of small chondroinductive synthetic molecules. From the literature, two peptides-CM10 and CK2.1-appear to be promising candidates; however, they have not been directly compared to TGF-β with human bone marrow-derived stem cells (hBMSCs). Similarly, two promising compounds-kartogenin and SM04690-have been reported in the literature to exhibit chondroinductive potential in vivo and in vitro; however, kartogenin was not directly compared against TGF-β. In the current study, we evaluated the chondroinductive potential of CM10, CK2.1, kartogenin, and SM04690, and directly compared them to each other and to a TGF-β3 positive control. Following 21 days of culture, none of the evaluated chondrogenic factors, either individually or even in combinations of two, resulted in a higher gene expression of chondrogenic markers as compared to TGF-β3. Additionally, no collagen II gene expression was detected except in the TGF-β3 positive control group. Given that the evaluated factors have confirmed efficacy in the literature, but not in the current study with a positive control, there may be value in the future identification of new chondroinductive factors that are less situation-dependent, with rigorous evaluations of their effect on chondrogenesis using positive controls.

Scaffolds containing GAG-mimetic cellulose sulfate promote TGF-β interaction and MSC Chondrogenesis over native GAGs

Cartilage tissue engineering strategies seek to repair damaged tissue using approaches that include scaffolds containing components of the native extracellular matrix (ECM). Articular cartilage consists of glycosaminoglycans (GAGs) which are known to sequester growth factors. In order to more closely mimic the native ECM, this study evaluated the chondrogenic differentiation of mesenchymal stem cells (MSCs), a promising cell source for cartilage regeneration, on fibrous scaffolds that contained the GAG-mimetic cellulose sulfate. The degree of sulfation was evaluated, examining partially sulfated cellulose (pSC) and fully sulfated cellulose (NaCS). Comparisons were made with scaffolds containing native GAGs (chondroitin sulfate A, chondroitin sulfate C and heparin). Transforming growth factor-beta3 (TGF-β3) sequestration, as measured by rate of association, was higher for sulfated cellulose-containing scaffolds as compared to native GAGs. In addition, TGF-β3 sequestration and retention over time was highest for NaCS-containing scaffolds. Sulfated cellulose-containing scaffolds loaded with TGF-β3 showed enhanced chondrogenesis as indicated by a higher Collagen Type II:I ratio over native GAGs. NaCS-containing scaffolds loaded with TGF-β3 had the highest expression of chondrogenic markers and a reduction of hypertrophic markers in dynamic loading conditions, which more closely mimic in vivo conditions. Studies also demonstrated that TGF-β3 mediated its effect through the Smad2/3 signaling pathway where the specificity of TGF-β receptor (TGF- βRI)-phosphorylated SMAD2/3 was verified with a receptor inhibitor. Therefore, studies demonstrate that scaffolds containing cellulose sulfate enhance TGF-β3-induced MSC chondrogenic differentiation and show promise for promoting cartilage tissue regeneration.

Evaluation of surgical fixation methods for the implantation of melt electrowriting-reinforced hyaluronic acid hydrogel composites in porcine cartilage defects

The surgical repair of articular cartilage remains an ongoing challenge in orthopedics. Tissue engineering is a promising approach to treat cartilage defects; however, scaffolds must (i) possess the requisite material properties to support neocartilage formation, (ii) exhibit sufficient mechanical integrity for handling during implantation, and (iii) be reliably fixed within cartilage defects during surgery. In this study, we demonstrate the reinforcement of soft norbornene-modified hyaluronic acid (NorHA) hydrogels via the melt electrowriting (MEW) of polycaprolactone to fabricate composite scaffolds that support encapsulated porcine mesenchymal stromal cell (pMSC, three donors) chondrogenesis and cartilage formation and exhibit mechanical properties suitable for handling during implantation. Thereafter, acellular MEW-NorHA composites or MEW-NorHA composites with encapsulated pMSCs and precultured for 28 days were implanted in full-thickness cartilage defects in porcine knees using either bioresorbable pins or fibrin glue to assess surgical fixation methods. Fixation of composites with either biodegradable pins or fibrin glue ensured implant retention in most cases (80%); however, defects treated with pinned composites exhibited more subchondral bone remodeling and inferior cartilage repair, as evidenced by micro-computed tomography (micro-CT) and safranin O/fast green staining, respectively, when compared to defects treated with glued composites. Interestingly, no differences in repair tissue were observed between acellular and cellularized implants. Additional work is required to assess the full potential of these scaffolds for cartilage repair. However, these results suggest that future approaches for cartilage repair with MEW-reinforced hydrogels should be carefully evaluated with regard to their fixation approach for construct retention and surrounding cartilage tissue damage.

Stimulation of Chondrocyte and Bone Marrow Mesenchymal Stem Cell Chondrogenic Response by Polypyrrole and Polypyrrole/Gold Nanoparticles

Bone marrow mesenchymal stem cells (BMMSCs) possess a strong ability to differentiate into the chondrogenic lineage, which is important for cartilage regeneration. External stimuli, such as electrical stimulation (ES), are frequently studied for chondrogenic differentiation of BMMSCs; however, the application of conductive polymers such as polypyrrole (Ppy), has never been used for stimulating BMMSCs chondrogenesis in vitro before. Thus, the aim of this study was to evaluate the chondrogenic potential of human BMMSCs after stimulation with Ppy nanoparticles (Ppy NPs) and compare them to cartilage-derived chondrocytes. In this study, we tested Ppy NPs without and with 13 nm gold NPs (Ppy/Au) for BMMSCs and chondrocyte proliferation, viability, and chondrogenic differentiation for 21 days, without the use of ES. The results demonstrated significantly higher amounts of cartilage oligomeric matrix protein (COMP) in BMMSCs stimulated with Ppy and Ppy/Au NPs, as compared to the control. The expression of chondrogenic genes (SOX9, ACAN, COL2A1) in BMMSCs and chondrocytes were upregulated by Ppy and Ppy/Au NPs, as compared to controls. Histological staining with safranin-O indicated higher extracellular matrix production in Ppy and Ppy/Au NPs stimulated samples, as compared to controls. In conclusion, Ppy and Ppy/Au NPs stimulate BMMSC chondrogenic differentiation; however, BMMSCs were more responsive to Ppy, while chondrocytes possessed a stronger chondrogenic response to Ppy/Au NPs.

Electrically Conductive Hydrogels for Articular Cartilage Tissue Engineering

Articular cartilage is a highly specialized tissue found in diarthrodial joints, which is crucial for healthy articular motion. Despite its importance, articular cartilage has limited regenerative capacities, and the degeneration of this tissue is a leading cause of disability worldwide, with hundreds of millions of people affected. As current treatment options for cartilage degeneration remain ineffective, tissue engineering has emerged as an exciting approach to create cartilage substitutes. In particular, hydrogels seem to be suitable candidates for this purpose due to their biocompatibility and high customizability, being able to be tailored to fit the biophysical properties of native cartilage. Furthermore, these hydrogel matrices can be combined with conductive materials in order to simulate the natural electrochemical properties of articular cartilage. In this review, we highlight the most common conductive materials combined with hydrogels and their diverse applications, and then present the current state of research on the development of electrically conductive hydrogels for cartilage tissue engineering. Finally, the main challenges and future perspectives for the application of electrically conductive hydrogels on articular cartilage repair strategies are also discussed.

Functional tissue engineering of articular cartilage for biological joint resurfacing-The 2021 Elizabeth Winston Lanier Kappa Delta Award

Biological resurfacing of entire articular surfaces represents a challenging strategy for the treatment of cartilage degeneration that occurs in osteoarthritis. Not only does this approach require anatomically sized and functional engineered cartilage, but the inflammatory environment within an arthritic joint may also inhibit chondrogenesis and induce degradation of native and engineered cartilage. Here, we present the culmination of multiple avenues of interdisciplinary research leading to the development and testing of bioartificial cartilage for tissue-engineered resurfacing of the hip joint. The work is based on a novel three-dimensional weaving technology that is infiltrated with specific bioinductive materials and/or genetically-engineered stem cells. A variety of design approaches have been tested in vitro, showing biomimetic cartilage-like properties as well as the capability for long-term tunable and inducible drug delivery. Importantly, these cartilage constructs have the potential to provide mechanical functionality immediately upon implantation, as they will need to replace a majority, if not the entire joint surface to restore function. To date, these approaches have shown excellent preclinical success in a variety of animal studies, including the resurfacing of a large osteochondral defect in the canine hip, and are now well-poised for clinical translation.

Fabrication of MSC-laden composites of hyaluronic acid hydrogels reinforced with MEW scaffolds for cartilage repair

Hydrogels are of interest in cartilage tissue engineering due to their ability to support the encapsulation and chondrogenesis of mesenchymal stromal cells (MSCs). However, features such as hydrogel crosslink density, which can influence nutrient transport, nascent matrix distribution, and the stability of constructs during and after implantation must be considered in hydrogel design. Here, we first demonstrate that more loosely crosslinked (i.e. softer, ∼2 kPa) norbornene-modified hyaluronic acid (NorHA) hydrogels support enhanced cartilage formation and maturation when compared to more densely crosslinked (i.e. stiffer, ∼6-60 kPa) hydrogels, with a >100-fold increase in compressive modulus after 56 d of culture. While soft NorHA hydrogels mature into neocartilage suitable for the repair of articular cartilage, their initial moduli are too low for handling and they do not exhibit the requisite stability needed to withstand the loading environments of articulating joints. To address this, we reinforced NorHA hydrogels with polycaprolactone (PCL) microfibers produced via melt-electrowriting (MEW). Importantly, composites fabricated with MEW meshes of 400µm spacing increased the moduli of soft NorHA hydrogels by ∼50-fold while preserving the chondrogenic potential of the hydrogels. There were minimal differences in chondrogenic gene expression and biochemical content (e.g. DNA, GAG, collagen) between hydrogels alone and composites, whereas the composites increased in compressive modulus to ∼350 kPa after 56 d of culture. Lastly, integration of composites with native tissue was assessedex vivo; MSC-laden composites implanted after 28 d of pre-culture exhibited increased integration strengths and contact areas compared to acellular composites. This approach has great potential towards the design of cell-laden implants that possess both initial mechanical integrity and the ability to support neocartilage formation and integration for cartilage repair.

Mechanotransducive Biomimetic Systems for Chondrogenic Differentiation In Vitro

Osteoarthritis (OA) is a long-term chronic joint disease characterized by the deterioration of bones and cartilage, which results in rubbing of bones which causes joint stiffness, pain, and restriction of movement. Tissue engineering strategies for repairing damaged and diseased cartilage tissue have been widely studied with various types of stem cells, chondrocytes, and extracellular matrices being on the lead of new discoveries. The application of natural or synthetic compound-based scaffolds for the improvement of chondrogenic differentiation efficiency and cartilage tissue engineering is of great interest in regenerative medicine. However, the properties of such constructs under conditions of mechanical load, which is one of the most important factors for the successful cartilage regeneration and functioning in vivo is poorly understood. In this review, we have primarily focused on natural compounds, particularly extracellular matrix macromolecule-based scaffolds and their combinations for the chondrogenic differentiation of stem cells and chondrocytes. We also discuss different mechanical forces and compression models that are used for In Vitro studies to improve chondrogenic differentiation. Summary of provided mechanical stimulation models In Vitro reviews the current state of the cartilage tissue regeneration technologies and to the potential for more efficient application of cell- and scaffold-based technologies for osteoarthritis or other cartilage disorders.

Nanofibrous hyaluronic acid scaffolds delivering TGF-β3 and SDF-1α for articular cartilage repair in a large animal model

Focal cartilage injuries have poor intrinsic healing potential and often progress to osteoarthritis, a costly disease affecting almost a third of adults in the United States. To treat these patients, cartilage repair therapies often use cell-seeded scaffolds, which are limited by donor site morbidity, high costs, and poor efficacy. To address these limitations, we developed an electrospun cell-free fibrous hyaluronic acid (HA) scaffold that delivers factors specifically designed to enhance cartilage repair: Stromal Cell-Derived Factor-1α (SDF-1α; SDF) to increase the recruitment and infiltration of mesenchymal stem cells (MSCs) and Transforming Growth Factor-β3 (TGF-β3; TGF) to enhance cartilage tissue formation. Scaffolds were characterized in vitro and then deployed in a large animal model of full-thickness cartilage defect repair. The bioactivity of both factors was verified in vitro, with both SDF and TGF increasing cell migration, and TGF increasing matrix formation by MSCs. In vivo, however, scaffolds releasing SDF resulted in an inferior cartilage healing response (lower mechanics, lower ICRS II histology score) compared to scaffolds releasing TGF alone. These results highlight the importance of translation into large animal models to appropriately screen scaffolds and therapies, and will guide investigators towards alternative growth factor combinations. STATEMENT OF SIGNIFICANCE: This study addresses an area of orthopaedic medicine in which treatment options are limited and new biomaterials stand to improve patient outcomes. Those suffering from articular cartilage injuries are often destined to have early onset osteoarthritis. We have created a cell-free nanofibrous hyaluronic acid (HA) scaffold that delivers factors specifically designed to enhance cartilage repair: Stromal Cell-Derived Factor-1α (SDF-1α; SDF) to increase the recruitment and infiltration of mesenchymal stem cells (MSCs) and Transforming Growth Factor-β3 (TGF-β3; TGF) to enhance cartilage tissue formation. To our knowledge, this study is the first to evaluate such a bioactive scaffold in a large animal model and demonstrates the capacity for dual growth factor release.

Applications of Vibrational Spectroscopy for Analysis of Connective Tissues

Advances in vibrational spectroscopy have propelled new insights into the molecular composition and structure of biological tissues. In this review, we discuss common modalities and techniques of vibrational spectroscopy, and present key examples to illustrate how they have been applied to enrich the assessment of connective tissues. In particular, we focus on applications of Fourier transform infrared (FTIR), near infrared (NIR) and Raman spectroscopy to assess cartilage and bone properties. We present strengths and limitations of each approach and discuss how the combination of spectrometers with microscopes (hyperspectral imaging) and fiber optic probes have greatly advanced their biomedical applications. We show how these modalities may be used to evaluate virtually any type of sample (ex vivo, in situ or in vivo) and how "spectral fingerprints" can be interpreted to quantify outcomes related to tissue composition and quality. We highlight the unparalleled advantage of vibrational spectroscopy as a label-free and often nondestructive approach to assess properties of the extracellular matrix (ECM) associated with normal, developing, aging, pathological and treated tissues. We believe this review will assist readers not only in better understanding applications of FTIR, NIR and Raman spectroscopy, but also in implementing these approaches for their own research projects.

Optimized Media Volumes Enable Homogeneous Growth of Mesenchymal Stem Cell-Based Engineered Cartilage Constructs

Despite marked advances in the field of cartilage tissue engineering, it remains a challenge to engineer cartilage constructs with homogeneous properties. Moreover, for engineered cartilage to make it to the clinic, this homogeneous growth must occur in a time-efficient manner. In this study we investigated the potential of increased media volume to expedite the homogeneous maturation of mesenchymal stem cell (MSC) laden engineered constructs over time in vitro. We assessed the MSC-laden constructs after 4 and 8 weeks of chondrogenic culture using bulk mechanical, histological, and biochemical measures. These assays were performed on both the intact total constructs and the construct cores to elucidate region-dependent differences. In addition, local strain transfer was assessed to quantify depth-dependent mechanical properties throughout the constructs. Our findings suggest that increased media volume enhances matrix deposition early in culture and ameliorates unwanted regional heterogeneities at later time points. Taken together, these data support the use of higher media volumes during in vitro culture to hasten tissue maturation and increase the core strength of tissue constructs. These findings will forward the field of cartilage tissue engineering and the translation of tissue engineered constructs.

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.

Enzymatic Synthesis of Glycans and Glycoconjugates

Glycoconjugates have great potential to improve human health in a multitude of different ways and fields. Prominent examples are human milk oligosaccharides and glycosaminoglycans. The typical choice for the production of homogeneous glycoconjugates is enzymatic synthesis. Through the availability of expression and purification protocols, recombinant Leloir glycosyltransferases are widely applied as catalysts for the synthesis of a wide range of glycoconjugates. Extensive utilization of these enzymes also depends on the availability of activated sugars as building blocks. Multi-enzyme cascades have proven a versatile technique to synthesize and in situ regenerate nucleotide sugar.In this chapter, the functions and mechanisms of Leloir glycosyltransferases are revisited, and the advantage of prokaryotic sources and production systems is discussed. Moreover, in vivo and in vitro pathways for the synthesis of nucleotide sugar are reviewed. In the second part, recent and prominent examples of the application of Leloir glycosyltransferase are given, i.e., the synthesis of glycosaminoglycans, glycoconjugate vaccines, and human milk oligosaccharides as well as the re-glycosylation of biopharmaceuticals, and the status of automated glycan assembly is revisited.

Approaches for In Situ Monitoring of Matrix Development in Hydrogel-Based Engineered Cartilage

Near infrared (NIR) spectroscopy using a fiber optic probe shows great promise for the nondestructive in situ monitoring of tissue engineered construct development; however, the NIR evaluation of matrix components in samples with high water content is challenging, as water absorbances overwhelm the spectra. In this study, we established approaches by which NIR spectroscopy can be used to select optimal individual engineered hydrogel constructs based on matrix content and mechanical properties. NIR spectroscopy of dry standard compounds allowed identification of several absorbances related to collagen and/or proteoglycan (PG), of which only two could be identified in spectra obtained from hydrated constructs, at ∼5940 and 5800 cm-1. In dry sample mixtures, the ratio of these peaks correlated positively to collagen and negatively to PG. In NIR spectra from engineered cartilage hydrogels, these peaks reflected higher collagen and PG content and dynamic modulus values, permitting the differentiation of constructs with poor and good matrix development. Similarly, the increasing baseline offset in raw NIR spectra also reflected matrix development in hydrated constructs. However, weekly monitoring of NIR spectra and the peaks at ∼5940 and 5800 cm-1 was not adequate to differentiate individual constructs based on matrix composition. Interestingly, changes in the baseline offset of raw spectra could be used to evaluate the growth trajectory of individual constructs. These results demonstrate an optimal approach for the use of fiber optic NIR spectroscopy for in situ monitoring of the development of engineered cartilage, which will aid in identifying individual constructs for implantation. Impact statement A current demand in tissue engineering is the establishment of nondestructive approaches to evaluate construct development during growth in vitro. In this article, we demonstrate original nondestructive approaches by which fiber optic NIR spectroscopy can be used to assess matrix (PG and collagen) formation and mechanical properties in hydrogel-based constructs. Our data provide a cohesive molecular-based approach for in situ longitudinal evaluation of construct development during growth in vitro. The establishment of these approaches is a valuable step toward the real-time identification and selection of constructs with optimal properties, which may lead to successful tissue integration upon in vivo implantation.

Mesenchymal stem cells for cartilage regeneration

Cartilage injuries are typically caused by trauma, chronic overload, and autoimmune diseases. Owing to the avascular structure and low metabolic activities of chondrocytes, cartilage generally does not self-repair following an injury. Currently, clinical interventions for cartilage injuries include chondrocyte implantation, microfracture, and osteochondral transplantation. However, rather than restoring cartilage integrity, these methods only postpone further cartilage deterioration. Stem cell therapies, especially mesenchymal stem cell (MSCs) therapies, were found to be a feasible strategy in the treatment of cartilage injuries. MSCs can easily be isolated from mesenchymal tissue and be differentiated into chondrocytes with the support of chondrogenic factors or scaffolds to repair damaged cartilage tissue. In this review, we highlighted the full success of cartilage repair using MSCs, or MSCs in combination with chondrogenic factors and scaffolds, and predicted their pros and cons for prospective translation to clinical practice.

3D bioprinting via an in situ crosslinking technique towards engineering cartilage tissue

3D bioprinting is a promising approach for the repair of cartilage tissue after damage due to injury or disease; however, the design of 3D printed scaffolds has been limited by the availability of bioinks with requisite printability, cytocompatibility, and bioactivity. To address this, we developed an approach termed in situ crosslinking that permits the printing of non-viscous, photocrosslinkable bioinks via the direct-curing of the bioink with light through a photopermeable capillary prior to deposition. Using a norbornene-modified hyaluronic acid (NorHA) macromer as a representative bioink and our understanding of thiol-ene curing kinetics with visible light, we varied the printing parameters (e.g., capillary length, flow rate, light intensity) to identify printing conditions that were optimal for the ink. The printing process was cytocompatible, with high cell viability and homogenous distribution of mesenchymal stromal cells (MSCs) observed throughout printed constructs. Over 56 days of culture in chondrogenic media, printed constructs increased in compressive moduli, biochemical content (i.e., sulfated glycosaminoglycans, collagen), and histological staining of matrix associated with cartilage tissue. This generalizable printing approach may be used towards the repair of focal defects in articular cartilage or broadly towards widespread biomedical applications across a range of photocrosslinkable bioinks that can now be printed.

Fibrochondrogenic differentiation potential of tendon-derived stem/progenitor cells from human patellar tendon

Background

Bone–tendon junction (BTJ) is a unique structure connecting tendon and bone through a fibrocartilage zone. Owing to its unique structure, the regeneration of BTJ remains a challenge. Here, we study the fibrochondrogenic differentiation of human tendon-derived stem/progenitor cells (TSPCs) both in vitro and in vivo.

Methods

TSPCs were isolated from human patellar tendon tissues and investigated for their multidifferentiation potential. TSPCs were cultured in chondrogenic medium with transforming growth factor beta 3 (TGF-β3) and BMP-2 in vitro ​and examined for the expression of fibrochondrogenic marker genes by quantitative real-time reverse transcription polymerase chain reaction, enzyme-linked immunosorbent assay, and immunofluorescence. TSPCs pretreated were also seeded in collage II sponge and then transplanted in immunocompromised nude mice to examine if the fibrochondrogenic characteristics were conserved in vivo.

Results

We found that TSPCs were differentiated towards fibrochondrogenic lineage, accompanied by the expression of collagen I, collagen II, SRY-box transcription factor 9 (Sox 9), and tenascin C. Furthermore, after TSPCs were seeded in collagen II sponge and transplanted in immunocompromised nude mice, they expressed fibrochondrogenic genes, including proteoglycan, collagen I, and collagen II.

Conclusion

Taken together, this study showed that TSPCs are capable of differentiating towards fibrocartilage-like cells, and the fibrochondrogenic characteristics were conserved even in vivo, and thus might have the potential application for fibrocartilage regeneration in BTJ repair.

The translational potential of this article

TSPCs are able to differentiate into fibrocartilage-like cells and thus might well be one potential cell source for fibrocartilage regeneration in a damaged BTJ repair.

Decorin Regulates the Aggrecan Network Integrity and Biomechanical Functions of Cartilage Extracellular Matrix

Joint biomechanical functions rely on the integrity of cartilage extracellular matrix. Understanding the molecular activities that govern cartilage matrix assembly is critical for developing effective cartilage regeneration strategies. This study elucidated the role of decorin, a small leucine-rich proteoglycan, in the structure and biomechanical functions of cartilage. In decorin-null cartilage, we discovered a substantial reduction of aggrecan content, the major proteoglycan of cartilage matrix, and mild changes in collagen fibril nanostructure. This loss of aggrecan resulted in significantly impaired biomechanical properties of cartilage, including decreased modulus, elevated hydraulic permeability, and reduced energy dissipation capabilities. At the cellular level, we found that decorin functions to increase the retention of aggrecan in the neo-matrix of chondrocytes, rather than to directly influence the biosynthesis of aggrecan. At the molecular level, we demonstrated that decorin significantly increases the adhesion between aggrecan and aggrecan molecules and between aggrecan molecules and collagen II fibrils. We hypothesize that decorin plays a crucial structural role in mediating the matrix integrity and biomechanical functions of cartilage by providing physical linkages to increase the adhesion and assembly of aggrecan molecules at the nanoscale.

Bioactive factors for cartilage repair and regeneration: Improving delivery, retention, and activity

Articular cartilage is a remarkable tissue whose sophisticated composition and architecture allow it to withstand complex stresses within the joint. Once injured, cartilage lacks the capacity to self-repair, and injuries often progress to joint wide osteoarthritis (OA) resulting in debilitating pain and loss of mobility. Current palliative and surgical management provides short-term symptom relief, but almost always progresses to further deterioration in the long term. A number of bioactive factors, including drugs, corticosteroids, and growth factors, have been utilized in the clinic, in clinical trials, or in emerging research studies to alleviate the inflamed joint environment or to promote new cartilage tissue formation. However, these therapies remain limited in their duration and effectiveness. For this reason, current efforts are focused on improving the localization, retention, and activity of these bioactive factors. The purpose of this review is to highlight recent advances in drug delivery for the treatment of damaged or degenerated cartilage. First, we summarize material and modification techniques to improve the delivery of these factors to damaged tissue and enhance their retention and action within the joint environment. Second, we discuss recent studies using novel methods to promote new cartilage formation via biofactor delivery, that have potential for improving future long-term clinical outcomes. Lastly, we review the emerging field of orthobiologics, using delivered and endogenous cells as drug-delivering "factories" to preserve and restore joint health. Enhancing drug delivery systems can improve both restorative and regenerative treatments for damaged cartilage. STATEMENT OF SIGNIFICANCE: Articular cartilage is a remarkable and sophisticated tissue that tolerates complex stresses within the joint. When injured, cartilage cannot self-repair, and these injuries often progress to joint-wide osteoarthritis, causing patients debilitating pain and loss of mobility. Current palliative and surgical treatments only provide short-term symptomatic relief and are limited with regards to efficiency and efficacy. Bioactive factors, such as drugs and growth factors, can improve outcomes to either stabilize the degenerated environment or regenerate replacement tissue. This review highlights recent advances and novel techniques to enhance the delivery, localization, retention, and activity of these factors, providing an overview of the cartilage drug delivery field that can guide future research in restorative and regenerative treatments for damaged cartilage.

Chondrocyte and mesenchymal stem cell derived engineered cartilage exhibits differential sensitivity to pro-inflammatory cytokines

Tissue engineering is a promising approach for the repair of articular cartilage defects, with engineered constructs emerging that match native tissue properties. However, the inflammatory environment of the damaged joint might compromise outcomes, and this may be impacted by the choice of cell source in terms of their ability to operate anabolically in an inflamed environment. Here, we compared the response of engineered cartilage derived from native chondrocytes and mesenchymal stem cells (MSCs) to challenge by TNFα and IL-1β in order to determine if either cell type possessed an inherent advantage. Compositional (extracellular matrix) and functional (mechanical) characteristics, as well as the release of catabolic mediators (matrix metalloproteinases [MMPs], nitric oxide [NO]) were assessed to determine cell- and tissue-level changes following exposure to IL-1β or TNF-α. Results demonstrated that MSC-derived constructs were more sensitive to inflammatory mediators than chondrocyte-derived constructs, exhibiting a greater loss of proteoglycans and functional properties at lower cytokine concentrations. While MSCs and chondrocytes both have the capacity to form functional engineered cartilage in vitro, this study suggests that the presence of an inflammatory environment is more likely to impair the in vivo success of MSC-derived cartilage repair. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:2901-2910, 2018.

Role of dexamethasone in the long-term functional maturation of MSC-laden hyaluronic acid hydrogels for cartilage tissue engineering

The purpose of study was to investigate the maturation of mesenchymal stem cells (MSC) laden in HA constructs with various combinations of chemically defined medium (CM) components and determine the impact of dexamethasone and serum on construct properties. Constructs were cultured in CM with the addition or withdrawal of media components or were transferred to serum containing media that partially represents an in vivo-like condition where pro-inflammatory signals are present. Constructs cultured in CM+ (CM with TGF-β3) and DEX- (CM+ without dexamethasone) conditions produced robust matrix, while those in ITS/BSA/LA- (CM+ without ITS/BSA/LA) and Serum+ (10% FBS with TGF-β3) produced little matrix. While construct properties in DEX- were greater than those in CM+ at 4 weeks, properties in CM+ and DEX- reversed by 8 weeks. While construct properties in DEX- were greater than those in CM+ at 4 weeks, the continued absence or removal of dexamethasone resulted in marked GAG loss by 8 weeks. Conversely, the continued presence or new addition of dexamethasone at 4 weeks further improved or maintained construct properties through 8 weeks. Finally, when constructs were converted to Serum (in the continued presence of TGF-β3 with or without dexamethasone) after pre-culture in CM+ for 4 weeks, GAG loss was attenuated with addition of dexamethasone. Interestingly, however, collagen content and type was not impacted. In conclusion, dexamethasone influences the functional maturation of MSC-laden HA constructs, and may help to maintain properties during long-term culture or with in vivo translation by repressing pro-inflammatory signals. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:1717-1727, 2018.

Dose and Timing of N-Cadherin Mimetic Peptides Regulate MSC Chondrogenesis within Hydrogels

The transmembrane glycoprotein N-cadherin (NCad) mediates cell-cell interactions found during mesenchymal condensation and chondrogenesis. Here, NCad-derived peptides (i.e., HAV) are incorporated into hyaluronic acid (HA) hydrogels with encapsulated mesenchymal stem cells (MSCs). Since the dose and timing of NCad signaling are dynamic, HAV peptide presentation is tuned via alterations in peptide concentration and incorporation of an ADAM10-cleavable domain between the hydrogel and the HAV motif, respectively. HA hydrogels functionalized with HAV result in dose-dependent increases in early chondrogenesis of encapsulated MSCs and resultant cartilage matrix production. For example, type II collagen and glycosaminoglycan production increase ≈9- and 2-fold with the highest dose of HAV (i.e., 2 × 10^-3 m), respectively, when compared to unmodified hydrogels, while incorporation of an efficient ADAM10-cleavable domain between the HAV peptide and hydrogel abolishes increases in chondrogenesis and matrix production. Treatment with a small-molecule ADAM10 inhibitor restores the functional effect of the HAV peptide, indicating that timing and duration of HAV peptide presentation is crucial for robust chondrogenesis. This study demonstrates a nuanced approach to the biofunctionalization of hydrogels to better emulate the complex cell microenvironment during embryogenesis toward stem-cell-based cartilage production.

Biphasic Scaffolds from Marine Collagens for Regeneration of Osteochondral Defects

Background: Collagens of marine origin are applied increasingly as alternatives to mammalian collagens in tissue engineering. The aim of the present study was to develop a biphasic scaffold from exclusively marine collagens supporting both osteogenic and chondrogenic differentiation and to find a suitable setup for in vitro chondrogenic and osteogenic differentiation of human mesenchymal stroma cells (hMSC). Methods: Biphasic scaffolds from biomimetically mineralized salmon collagen and fibrillized jellyfish collagen were fabricated by joint freeze-drying and crosslinking. Different experiments were performed to analyze the influence of cell density and TGF-β on osteogenic differentiation of the cells in the scaffolds. Gene expression analysis and analysis of cartilage extracellular matrix components were performed and activity of alkaline phosphatase was determined. Furthermore, histological sections of differentiated cells in the biphasic scaffolds were analyzed. Results: Stable biphasic scaffolds from two different marine collagens were prepared. An in vitro setup for osteochondral differentiation was developed involving (1) different seeding densities in the phases; (2) additional application of alginate hydrogel in the chondral part; (3) pre-differentiation and sequential seeding of the scaffolds and (4) osteochondral medium. Spatially separated osteogenic and chondrogenic differentiation of hMSC was achieved in this setup, while osteochondral medium in combination with the biphasic scaffolds alone was not sufficient to reach this ambition. Conclusions: Biphasic, but monolithic scaffolds from exclusively marine collagens are suitable for the development of osteochondral constructs.

Raman spectroscopic imaging for quantification of depth-dependent and local heterogeneities in native and engineered cartilage

Articular cartilage possesses a remarkable, mechanically-robust extracellular matrix (ECM) that is organized and distributed throughout the tissue to resist physiologic strains and provide low friction during articulation. The ability to characterize the make-up and distribution of the cartilage ECM is critical to both understand the process by which articular cartilage undergoes disease-related degeneration and to develop novel tissue repair strategies to restore tissue functionality. However, the ability to quantitatively measure the spatial distribution of cartilage ECM constituents throughout the tissue has remained a major challenge. In this experimental investigation, we assessed the analytical ability of Raman micro-spectroscopic imaging to semi-quantitatively measure the distribution of the major ECM constituents in cartilage tissues. Raman spectroscopic images were acquired of two distinct cartilage tissue types that possess large spatial ECM gradients throughout their depth: native articular cartilage explants and large engineered cartilage tissue constructs. Spectral acquisitions were processed via multivariate curve resolution to decompose the "fingerprint" range spectra (800-1800 cm-1) to the component spectra of GAG, collagen, and water, giving rise to the depth dependent concentration profile of each constituent throughout the tissues. These Raman spectroscopic acquired-profiles exhibited strong agreement with profiles independently acquired via direct biochemical assaying of spatial tissue sections. Further, we harness this spectroscopic technique to evaluate local heterogeneities through the depth of cartilage. This work represents a powerful analytical validation of the accuracy of Raman spectroscopic imaging measurements of the spatial distribution of biochemical components in a biological tissue and shows that it can be used as a valuable tool for quantitatively measuring the distribution and organization of ECM constituents in native and engineered cartilage tissue specimens.

Near-Infrared Spectroscopy Predicts Compositional and Mechanical Properties of Hyaluronic Acid-Based Engineered Cartilage Constructs

Hyaluronic acid (HA) has been widely used for cartilage tissue engineering applications. However, the optimal time point to harvest HA-based engineered constructs for cartilage repair is still under investigation. In this study, we investigated the ability of a nondestructive modality, near-infrared spectroscopic (NIR) analysis, to predict compositional and mechanical properties of HA-based engineered cartilage constructs. NIR spectral data were collected from control, unseeded constructs, and twice per week by fiber optic from constructs seeded with chondrocytes during their development over an 8-week period. Constructs were harvested at 2, 4, 6, and 8 weeks, collagen and sulfated glycosaminoglycan content measured using biochemical assays, and the mechanical properties of the constructs evaluated using unconfined compression tests. NIR absorbances associated with the scaffold material, water, and engineered cartilage matrix, were identified. The NIR-determined matrix absorbance plateaued after 4 weeks of culture, which was in agreement with the biochemical assay results. Similarly, the mechanical properties of the constructs also plateaued at 4 weeks. A multivariate partial least square model based on NIR spectral input was developed to predict the moduli of the constructs, which resulted in a prediction error of 10% and R value of 0.88 for predicted versus actual values of dynamic modulus. Furthermore, the maximum increase in moduli was calculated from the first derivative of the curve fit of NIR-predicted and actual moduli values over time, and both occurred at ∼2 weeks. Collectively, these data suggest that NIR spectral data analysis could be an alternative to destructive biochemical and mechanical methods for evaluation of HA-based engineered cartilage construct properties.

Vibrational spectroscopy and imaging: applications for tissue engineering

Tissue engineering (TE) approaches strive to regenerate or replace an organ or tissue. The successful development and subsequent integration of a TE construct is contingent on a series of in vitro and in vivo events that result in an optimal construct for implantation. Current widely used methods for evaluation of constructs are incapable of providing an accurate compositional assessment without destruction of the construct. In this review, we discuss the contributions of vibrational spectroscopic assessment for evaluation of tissue engineered construct composition, both during development and post-implantation. Fourier transform infrared (FTIR) spectroscopy in the mid and near-infrared range, as well as Raman spectroscopy, are intrinsically label free, can be non-destructive, and provide specific information on the chemical composition of tissues. Overall, we examine the contribution that vibrational spectroscopy via fiber optics and imaging have to tissue engineering approaches.

* Thermosensitive Poly(N-vinylcaprolactam) Injectable Hydrogels for Cartilage Tissue Engineering

Injectable hydrogels have gained prominence in the field of tissue engineering for minimally invasive delivery of cells for tissue repair and in the filling of irregular defects. However, many injectable hydrogels exhibit long gelation times or are not stable for long periods after injection. To address these concerns, we used thermosensitive poly(N-vinylcaprolactam) (PNVCL) hydrogels due to their cytocompatibility and fast response to temperature stimuli. Changes in the PNVCL molecular weight and concentration enabled the development of hydrogels with tunable mechanical properties and fast gelation times (<60 s when the temperature was raised from room temperature to physiologic temperature). Chondrocytes (CHs) and mesenchymal stem cells were encapsulated in PNVCL hydrogels and exhibited high viability (∼90%), as monitored by Live/Dead staining and Alamar Blue assays. Three-dimensional constructs of CH-laden PNVCL hydrogels supported cartilage-specific extracellular matrix production both in vitro and after subcutaneous injection in nude rats for up to 8 weeks. Moreover, biochemical analyses of constructs demonstrated a time-dependent increase in glycosaminoglycans (GAGs) and collagen, which were significantly augmented in the implants cultured in vivo. Histological analyses also demonstrated regular distribution of synthesized cartilage components, including abundant GAGs and type II collagen. The findings from this study demonstrate thermosensitive PNVCL as a candidate injectable biomaterial to deliver cells for cartilage tissue engineering.

Next Generation Tissue Engineering of Orthopedic Soft Tissue-to-Bone Interfaces

Soft tissue-to-bone interfaces are complex structures that consist of gradients of extracellular matrix materials, cell phenotypes, and biochemical signals. These interfaces, called entheses for ligaments, tendons, and the meniscus, are crucial to joint function, transferring mechanical loads and stabilizing orthopedic joints. When injuries occur to connected soft tissue, the enthesis must be re-established to restore function, but due to structural complexity, repair has proven challenging. Tissue engineering offers a promising solution for regenerating these tissues. This prospective review discusses methodologies for tissue engineering the enthesis, outlined in three key design inputs: materials processing methods, cellular contributions, and biochemical factors.

The Good the Bad and the Ugly of Glycosaminoglycans in Tissue Engineering Applications

High sulfation, low cost, and the status of heparin as an already FDA- and EMA- approved product, mean that its inclusion in tissue engineering (TE) strategies is becoming increasingly popular. However, the use of heparin may represent a naïve approach. This is because tissue formation is a highly orchestrated process, involving the temporal expression of numerous growth factors and complex signaling networks. While heparin may enhance the retention and activity of certain growth factors under particular conditions, its binding 'promiscuity' means that it may also inhibit other factors that, for example, play an important role in tissue maintenance and repair. Within this review we focus on articular cartilage, highlighting the complexities and highly regulated processes that are involved in its formation, and the challenges that exist in trying to effectively engineer this tissue. Here we discuss the opportunities that glycosaminoglycans (GAGs) may provide in advancing this important area of regenerative medicine, placing emphasis on the need to move away from the common use of heparin, and instead focus research towards the utility of specific GAG preparations that are able to modulate the activity of growth factors in a more controlled and defined manner, with less off-target effects.

* Optimization of Preculture Conditions to Maximize the In Vivo Performance of Cell-Seeded Engineered Intervertebral Discs

The development of engineered tissues has progressed over the past 20 years from in vitro characterization to in vivo implementation. For musculoskeletal tissue engineering in particular, the emphasis of many of these studies was to select conditions that maximized functional and compositional gains in vitro. However, the transition from the favorable in vitro culture environment to a less favorable in vivo environment has proven difficult, and, in many cases, engineered tissues do not retain their preimplantation phenotype after even short periods in vivo. Our laboratory recently developed disc-like angle-ply structures (DAPS), an engineered intervertebral disc for total disc replacement. In this study, we tested six different preculture media formulations (three serum-containing and three chemically defined, with varying doses of transforming growth factor β3 [TGF-β3] and varying strategies to introduce serum) for their ability to preserve DAPS composition and metabolic activity during the transition from in vitro culture to in vivo implantation in a subcutaneous athymic rat model. We assayed implants before and after implantation to determine collagen content, glycosaminoglycan (GAG) content, metabolic activity, and magnetic resonance imaging (MRI) characteristics. A chemically defined media condition that incorporated TGF-β3 promoted the deposition of GAG and collagen in DAPS in vitro, the maintenance of accumulated matrix in vivo, and minimal changes in the metabolic activity of cells within the construct. Preculture in serum-containing media (with or without TGF-β3) was not compatible with DAPS maturation, particularly in the nucleus pulposus (NP) region. All groups showed increased collagen production after implantation. These findings define a favorable preculture strategy for the translation of engineered discs seeded with disc cells.

The use of mesenchymal stem cells for cartilage repair and regeneration: a systematic review

BACKGROUND

The management of articular cartilage defects presents many clinical challenges due to its avascular, aneural and alymphatic nature. Bone marrow stimulation techniques, such as microfracture, are the most frequently used method in clinical practice however the resulting mixed fibrocartilage tissue which is inferior to native hyaline cartilage. Other methods have shown promise but are far from perfect. There is an unmet need and growing interest in regenerative medicine and tissue engineering to improve the outcome for patients requiring cartilage repair. Many published reviews on cartilage repair only list human clinical trials, underestimating the wealth of basic sciences and animal studies that are precursors to future research. We therefore set out to perform a systematic review of the literature to assess the translation of stem cell therapy to explore what research had been carried out at each of the stages of translation from bench-top (in vitro), animal (pre-clinical) and human studies (clinical) and assemble an evidence-based cascade for the responsible introduction of stem cell therapy for cartilage defects. This review was conducted in accordance to PRISMA guidelines using CINHAL, MEDLINE, EMBASE, Scopus and Web of Knowledge databases from 1st January 1900 to 30th June 2015. In total, there were 2880 studies identified of which 252 studies were included for analysis (100 articles for in vitro studies, 111 studies for animal studies; and 31 studies for human studies). There was a huge variance in cell source in pre-clinical studies both of terms of animal used, location of harvest (fat, marrow, blood or synovium) and allogeneicity. The use of scaffolds, growth factors, number of cell passages and number of cells used was hugely heterogeneous.

SHORT CONCLUSIONS

This review offers a comprehensive assessment of the evidence behind the translation of basic science to the clinical practice of cartilage repair. It has revealed a lack of connectivity between the in vitro, pre-clinical and human data and a patchwork quilt of synergistic evidence. Drivers for progress in this space are largely driven by patient demand, surgeon inquisition and a regulatory framework that is learning at the same pace as new developments take place.

Mouse Mesenchymal Progenitor Cells Expressing Adipogenic and Osteogenic Transcription Factors Suppress the Macrophage Inflammatory Response

Mesenchymal progenitor cell characteristics that can identify progenitor populations with specific functions in immunity are actively being investigated. Progenitors from bone marrow and adipose tissue regulate the macrophage (MΦ) inflammatory response by promoting the switch from an inflammatory to an anti-inflammatory phenotype. Conversely, mesenchymal progenitors from the mouse aorta (mAo) support and contribute to the MΦ response under inflammatory conditions. We used cell lines with purported opposing immune-regulatory function, a bone marrow derived mesenchymal progenitor cell line (D1) and a mouse aorta derived mesenchymal progenitor cell line (mAo). Their interaction and regulation of the MΦ cell response to the inflammatory mediator, lipopolysaccharide (LPS), was examined by coculture. As expected, D1 cells suppressed NO, TNF-α, and IL-12p70 production but MΦ phagocytic activity remained unchanged. The mAo cells enhanced NO and TNF-α production in coculture and enhanced MΦ phagocytic activity. Using flow cytometry and PCR array, we then sought to identify sets of MSC-associated genes and markers that are expressed by these progenitor populations. We have determined that immune-supportive mesenchymal progenitors highly express chondrogenic and tenogenic transcription factors while immunosuppressive mesenchymal progenitors highly express adipogenic and osteogenic transcription factors. These data will be useful for the isolation, purification, and modification of mesenchymal progenitors to be used in the treatment of inflammatory diseases.

A Closed Chondromimetic Environment within Magnetic-Responsive Liquified Capsules Encapsulating Stem Cells and Collagen II/TGF-β3 Microparticles

TGF-β3 is enzymatically immobilized by transglutaminase-2 action to poly(l-lactic acid) microparticles coated with collagen II. Microparticles are then encapsulated with stem cells inside liquified spherical compartments enfolded with a permselective shell through layer-by-layer adsorption. Magnetic nanoparticles are electrostatically bound to the multilayered shell, conferring magnetic-response ability. The goal of this study is to engineer a closed environment inside which encapsulated stem cells would undergo a self-regulated chondrogenesis. To test this hypothesis, capsules are cultured in chondrogenic differentiation medium without TGF-β3. Their biological outcome is compared with capsules encapsulating microparticles without TGF-β3 immobilization and cultured in normal chondrogenic differentiation medium containing soluble TGF-β3. Glycosaminoglycans quantification demosntrates that similar chondrogenesis levels are achieved. Moreover, collagen fibrils resembling the native extracellular matrix of cartilage can be observed. Importantly, the genetic evaluation of characteristic cartilage markers confirms the successful chondrogenesis, while hypertrophic markers are downregulated. In summary, the engineered capsules are able to provide a suitable and stable chondrogenesis environment for stem cells without the need of TGF-β3 supplementation. This kind of self-regulated capsules with softness, robustness, and magnetic responsive characteristics is expected to provide injectability and in situ fixation, which is of great advantage for minimal invasive strategies to regenerate cartilage.

Effects of Mesenchymal Stem Cell and Growth Factor Delivery on Cartilage Repair in a Mini-Pig Model

OBJECTIVE

We have recently shown that mesenchymal stem cells (MSCs) embedded in a hyaluronic acid (HA) hydrogel and exposed to chondrogenic factors (transforming growth factor-β3 [TGF-β3]) produce a cartilage-like tissue in vitro. The current objective was to determine if these same factors could be combined immediately prior to implantation to induce a superior healing response in vivo relative to the hydrogel alone.

DESIGN

Trochlear chondral defects were created in Yucatan mini-pigs (6 months old). Treatment groups included an HA hydrogel alone and hydrogels containing allogeneic MSCs, TGF-β3, or both. Six weeks after surgery, micro-computed tomography was used to quantitatively assess defect fill and subchondral bone remodeling. The quality of cartilage repair was assessed using the ICRS-II histological scoring system and immunohistochemistry for type II collagen.

RESULTS

Treatment with TGF-β3 led to a marked increase in positive staining for collagen type II within defects (P < 0.05), while delivery of MSCs did not (P > 0.05). Neither condition had an impact on other histological semiquantitative scores (P > 0.05), and inclusion of MSCs led to significantly less defect fill (P < 0.05). For all measurements, no synergistic interaction was found between TGF-β3 and MSC treatment when they were delivered together (P > 0.05).

CONCLUSIONS

At this early healing time point, treatment with TGF-β3 promoted the formation of collagen type II within the defect, while allogeneic MSCs had little benefit. Combination of TGF-β3 and MSCs at the time of surgery did not produce a synergistic effect. An in vitro precultured construct made of these components may be required to enhance in vivo repair in this model system.

Derivation of epithelial-like cells from eyelid fat-derived stem cells in thermosensitive hydrogel

Injectable hydrogel is one of the great interests for tissue engineering and cell encapsulation. In the study, the thermosensitive chitosan/gelatin/β-glycerol phosphate (C/G/GP) disodium salt hydrogels were designed and investigated by different analyses. The eye fat-derived stem cells were used to evaluate the biocompatibility of hydrogels based on their phenotypic profile, viability, proliferation, and attachment ability. The results show that the sol/gel transition temperature of the C/G/GP hydrogel was in the range of 31.1-33.8 °C at neutral pH value, the gelation time was shortened, and the gel strength also improved at body temperature when compared with the C/GP hydrogel. In vitro cell culture experiments with eyelid fat-derived stem cells in hydrogel showed beneficial effects on the cell phenotypic morphology, proliferation, and differentiation. Microscopic figures showed that the eyelid fat stem cell were firmly anchored to the substrates and were able to retain a normal stem cell phenotype. Immunocytochemistry (ICC) and real-time-PCR results revealed change in the expression profile of eyelid fat stem cells grown with hydrogels when compared to those grown on control in epithelial induction condition. This study indicates that using chitosan/gelatin/β-glycerol phosphate hydrogel for cell culture is feasible and may apply in minimal invasive surgery in the future.

Heterogeneous engineered cartilage growth results from gradients of media-supplemented active TGF-β and is ameliorated by the alternative supplementation of latent TGF-β

Transforming growth factor beta (TGF-β) has become one of the most widely utilized mediators of engineered cartilage growth. It is typically exogenously supplemented in the culture medium in its active form, with the expectation that it will readily transport into tissue constructs through passive diffusion and influence cellular biosynthesis uniformly. The results of this investigation advance three novel concepts regarding the role of TGF-β in cartilage tissue engineering that have important implications for tissue development. First, through the experimental and computational analysis of TGF-β concentration distributions, we demonstrate that, contrary to conventional expectations, media-supplemented exogenous active TGF-β exhibits a pronounced concentration gradient in tissue constructs, resulting from a combination of high-affinity binding interactions and a high cellular internalization rate. These gradients are sustained throughout the entire culture duration, leading to highly heterogeneous tissue growth; biochemical and histological measurements support that while biochemical content is enhanced up to 4-fold at the construct periphery, enhancements are entirely absent beyond 1 mm from the construct surface. Second, construct-encapsulated chondrocytes continuously secrete large amounts of endogenous TGF-β in its latent form, a portion of which undergoes cell-mediated activation and enhances biosynthesis uniformly throughout the tissue. Finally, motivated by these prior insights, we demonstrate that the alternative supplementation of additional exogenous latent TGF-β enhances biosynthesis uniformly throughout tissue constructs, leading to enhanced but homogeneous tissue growth. This novel demonstration suggests that latent TGF-β supplementation may be utilized as an important tool for the translational engineering of large cartilage constructs that will be required to repair the large osteoarthritic defects observed clinically.

Pediatric laryngotracheal reconstruction with tissue-engineered cartilage in a rabbit model

OBJECTIVES/HYPOTHESIS

To develop an effective rabbit model of in vitro- and in vivo-derived tissue-engineered cartilage for laryngotracheal reconstruction (LTR).

STUDY DESIGN

1) Determination of the optimal scaffold 1% hyaluronic acid (HA), 2% HA, and polyglycolic acid (PGA) and in vitro culture time course using a pilot study of 4 by 4-mm in vitro-derived constructs analyzed on a static culture versus zero-gravity bioreactor for 4, 8, and 12 weeks, with determination of compressive modulus and histology as outcome measures. 2) Three-stage survival rabbit experiment utilizing autologous auricular chondrocytes seeded in scaffolds, either 1% HA or PGA. The constructs were cultured for the determined in vitro time period and then cultured in vivo for 12 weeks. Fifteen LTRs were performed using HA cartilage constructs, and one was performed with a PGA construct. All remaining specimens and the final reconstructed larynx underwent mechanical testing, histology, and glycosaminoglycan (GAG) content determination, and then were compared to cricoid control specimens (n = 13) and control LTR using autologous thyroid cartilage (n = 18).

METHODS

1) One rabbit underwent an auricular punch biopsy, and its chondrocytes were isolated and expanded and then encapsulated in eight 4 by 4-mm discs of 1% HA, 2% HA, PGA either in rotary bioreactor or static culture for 4, 8, and 12 weeks, respectively, with determination of compressive modulus, GAG content, and histology. 2) Sixteen rabbits underwent ear punch biopsy; chondrocytes were isolated and expanded. The cells were seeded in 13 by 5 by 2.25-mm UV photopolymerized 1% HA (w/w) or calcium alginate encapsulated synthetic PGA (13 × 5 × 2 mm); the constructs were then incubated in vitro for 12 weeks (the optimal time period determined above in paragraph 1) on a shaker. One HA and one PGA construct from each animal was tested mechanically and histologically, and the remaining eight (4 HA and 4 PGA) were implanted in the neck. After 12 weeks in vivo, the most optimal-appearing HA construct was used as a graft for LTR in 15 rabbits and PGA in one rabbit. The seven remaining specimens underwent hematoxylin and eosin, Safranin O, GAG content determination, and flexural modulus testing. At 12 weeks postoperative, the animals were euthanized and underwent endoscopy. The larynges underwent mechanical and histological testing. All animals that died underwent postmortem examination, including gross and microhistological analysis of the reconstructed airway.

RESULTS

Thirteen of the 15 rabbits that underwent LTR with HA in vitro- and in vivo-derived tissue-engineered cartilage constructs survived. The 1% HA specimens had the highest modulus and GAG after 12 weeks in vitro. The HA constructs became well integrated in the airway, supported respiration for the 12 weeks, and were histologically and mechanically similar to autologous cartilage.

CONCLUSIONS

The engineering of in vitro- and in vivo-derived cartilage with HA is a novel approach for laryngotracheal reconstruction. The data suggests that the in vitro- and in vivo-derived tissue-engineered approaches may offer a promising alternative to current strategies used in pediatric airway reconstruction, as well as other head and neck applications.

LEVEL OF EVIDENCE

NA. Laryngoscope, 126:S5-S21, 2016.

Integrating biologically inspired nanomaterials and table-top stereolithography for 3D printed biomimetic osteochondral scaffolds

The osteochondral interface of an arthritic joint is notoriously difficult to regenerate due to its extremely poor regenerative capacity and complex stratified architecture. Native osteochondral tissue extracellular matrix is composed of numerous nanoscale organic and inorganic constituents. Although various tissue engineering strategies exist in addressing osteochondral defects, limitations persist with regards to tissue scaffolding which exhibit biomimetic cues at the nano to micro scale. In an effort to address this, the current work focused on 3D printing biomimetic nanocomposite scaffolds for improved osteochondral tissue regeneration. For this purpose, two biologically-inspired nanomaterials have been synthesized consisting of (1) osteoconductive nanocrystalline hydroxyapatite (nHA) (primary inorganic component of bone) and (2) core-shell poly(lactic-co-glycolic) acid (PLGA) nanospheres encapsulated with chondrogenic transforming growth-factor β1 (TGF-β1) for sustained delivery. Then, a novel table-top stereolithography 3D printer and the nano-ink (i.e., nHA + nanosphere + hydrogel) were employed to fabricate a porous and highly interconnected osteochondral scaffold with hierarchical nano-to-micro structure and spatiotemporal bioactive factor gradients. Our results showed that human bone marrow-derived mesenchymal stem cell adhesion, proliferation, and osteochondral differentiation were greatly improved in the biomimetic graded 3D printed osteochondral construct in vitro. The current work served to illustrate the efficacy of the nano-ink and current 3D printing technology for efficient fabrication of a novel nanocomposite hydrogel scaffold. In addition, tissue-specific growth factors illustrated a synergistic effect leading to increased cell adhesion and directed stem cell differentiation.