Hypoxia-induced collagen crosslinking as a mechanism for enhancing mechanical properties of engineered articular cartilage

Evaluating stiffness of gastric wall using laser resonance frequency analysis for gastric cancer

Tumor stiffness is drawing attention as a novel axis that is orthogonal to existing parameters such as pathological examination. We developed a new diagnostic method that focuses on differences in stiffness between tumor and normal tissue. This study comprised a clinical application of laser resonance frequency analysis (L-RFA) for diagnosing gastric cancer. L-RFA allows for precise and contactless evaluation of tissue stiffness. We used a laser to create vibrations on a sample's surface that were then measured using a vibrometer. The data were averaged and analyzed to enhance accuracy. In the agarose phantom experiments, a clear linear correlation was observed between the Young's modulus of the phantoms (0.34-0.71 MPa) and the summation of normalized vibration peaks (Score) in the 1950-4050 Hz range (R2 = 0.93145). Higher Young's moduli also resulted in lower vibration peaks at the excitation frequency, signal-to-noise (S/N) ratios, and harmonic peaks. We also conducted L-RFA measurements on gastric cancer specimens from two patients who underwent robot-assisted distal gastrectomy. Our results revealed significant waveform differences between tumor and normal regions, similar to the findings in agarose phantoms, with tumor regions exhibiting lower vibration peaks at the excitation frequency, S/N ratios, and harmonic peaks. Statistical analysis confirmed significant differences in the score between normal and tumor regions (p = 0.00354). L-RFA was able to assess tumor stiffness and holds great promise as a noninvasive diagnostic tool for gastric cancer.

Inflammation-Associated Cytotoxic Agents in Tumorigenesis

Chronic inflammatory processes are related to all stages of tumorigenesis. As inflammation is closely associated with the activation and release of different cytotoxic agents, the interplay between cytotoxic agents and antagonizing principles is highlighted in this review to address the question of how tumor cells overcome the enhanced values of cytotoxic agents in tumors. In tumor cells, the enhanced formation of mitochondrial-derived reactive species and elevated values of iron ions and free heme are antagonized by an overexpression of enzymes and proteins, contributing to the antioxidative defense and maintenance of redox homeostasis. Through these mechanisms, tumor cells can even survive additional stress caused by radio- and chemotherapy. Through the secretion of active agents from tumor cells, immune cells are suppressed in the tumor microenvironment and an enhanced formation of extracellular matrix components is induced. Different oxidant- and protease-based cytotoxic agents are involved in tumor-mediated immunosuppression, tumor growth, tumor cell invasion, and metastasis. Considering the special metabolic conditions in tumors, the main focus here was directed on the disturbed balance between the cytotoxic agents and protective mechanisms in late-stage tumors. This knowledge is mandatory for the implementation of novel anti-cancerous therapeutic approaches.

Low-oxygen tension augments chondrocyte sensitivity to biomimetic thermomechanical cues in cartilage-engineered constructs

Chondrocytes respond to various biophysical cues, including oxygen tension, transient thermal signals, and mechanical stimuli. However, understanding how these factors interact to establish a unique regulatory microenvironment for chondrocyte function remains unclear. Herein, we explore these interactions using a joint-simulating bioreactor that independently controls the culture's oxygen concentration, evolution of temperature, and mechanical loading. Our analysis revealed significant coupling between these signals, resulting in a remarkable ∼14-fold increase in collagen type II (COL2a) and aggrecan (ACAN) mRNA expression. Furthermore, dynamic thermomechanical stimulation enhanced glycosaminoglycan and COL2a protein synthesis, with the magnitude of the biosynthetic changes being oxygen dependent. Additionally, our mechanistic study highlighted the crucial role of SRY-box transcription factor 9 (SOX9) as a major regulator of chondrogenic response, specifically expressed in response to combined biophysical signals. These findings illuminate the integration of various mechanobiological cues by chondrocytes and provide valuable insights for improving the extracellular matrix content in cartilage-engineered constructs.

3D Bioprinting of Hyaline Articular Cartilage: Biopolymers, Hydrogels, and Bioinks

The musculoskeletal system, consisting of bones and cartilage of various types, muscles, ligaments, and tendons, is the basis of the human body. However, many pathological conditions caused by aging, lifestyle, disease, or trauma can damage its elements and lead to severe disfunction and significant worsening in the quality of life. Due to its structure and function, articular (hyaline) cartilage is the most susceptible to damage. Articular cartilage is a non-vascular tissue with constrained self-regeneration capabilities. Additionally, treatment methods, which have proven efficacy in stopping its degradation and promoting regeneration, still do not exist. Conservative treatment and physical therapy only relieve the symptoms associated with cartilage destruction, and traditional surgical interventions to repair defects or endoprosthetics are not without serious drawbacks. Thus, articular cartilage damage remains an urgent and actual problem requiring the development of new treatment approaches. The emergence of biofabrication technologies, including three-dimensional (3D) bioprinting, at the end of the 20th century, allowed reconstructive interventions to get a second wind. Three-dimensional bioprinting creates volume constraints that mimic the structure and function of natural tissue due to the combinations of biomaterials, living cells, and signal molecules to create. In our case-hyaline cartilage. Several approaches to articular cartilage biofabrication have been developed to date, including the promising technology of 3D bioprinting. This review represents the main achievements of such research direction and describes the technological processes and the necessary biomaterials, cell cultures, and signal molecules. Special attention is given to the basic materials for 3D bioprinting-hydrogels and bioinks, as well as the biopolymers underlying the indicated products.

Micronutrient optimization for tissue engineered articular cartilage production of type II collagen

Tissue Engineering of cartilage has been hampered by the inability of engineered tissue to express native levels of type II collagen in vitro. Inadequate levels of type II collagen are, in part, due to a failure to recapitulate the physiological environment in culture. In this study, we engineered primary rabbit chondrocytes to express a secreted reporter, Gaussia Luciferase, driven by the type II collagen promoter, and applied a Design of Experiments approach to assess chondrogenic differentiation in micronutrient-supplemented medium. Using a Response Surface Model, 240 combinations of micronutrients absent in standard chondrogenic differentiation medium, were screened and assessed for type II collagen promoter-driven Gaussia luciferase expression. While the target of this study was to establish a combination of all micronutrients, alpha-linolenic acid, copper, cobalt, chromium, manganese, molybdenum, vitamins A, E, D and B7 were all found to have a significant effect on type II collagen promoter activity. Five conditions containing all micronutrients predicted to produce the greatest luciferase expression were selected for further study. Validation of these conditions in 3D aggregates identified an optimal condition for type II collagen promoter activity. Engineered cartilage grown in this condition, showed a 170% increase in type II collagen expression (Day 22 Luminescence) and in Young's tensile modulus compared to engineered cartilage in basal media alone.Collagen cross-linking analysis confirmed formation of type II-type II collagen and type II-type IX collagen cross-linked heteropolymeric fibrils, characteristic of mature native cartilage. Combining a Design of Experiments approach and secreted reporter cells in 3D aggregate culture enabled a high-throughput platform that can be used to identify more optimal physiological culture parameters for chondrogenesis.

Extracellular matrix remodeling in tumor progression and immune escape: from mechanisms to treatments

The malignant tumor is a multi-etiological, systemic and complex disease characterized by uncontrolled cell proliferation and distant metastasis. Anticancer treatments including adjuvant therapies and targeted therapies are effective in eliminating cancer cells but in a limited number of patients. Increasing evidence suggests that the extracellular matrix (ECM) plays an important role in tumor development through changes in macromolecule components, degradation enzymes and stiffness. These variations are under the control of cellular components in tumor tissue via the aberrant activation of signaling pathways, the interaction of the ECM components to multiple surface receptors, and mechanical impact. Additionally, the ECM shaped by cancer regulates immune cells which results in an immune suppressive microenvironment and hinders the efficacy of immunotherapies. Thus, the ECM acts as a barrier to protect cancer from treatments and supports tumor progression. Nevertheless, the profound regulatory network of the ECM remodeling hampers the design of individualized antitumor treatment. Here, we elaborate on the composition of the malignant ECM, and discuss the specific mechanisms of the ECM remodeling. Precisely, we highlight the impact of the ECM remodeling on tumor development, including proliferation, anoikis, metastasis, angiogenesis, lymphangiogenesis, and immune escape. Finally, we emphasize ECM "normalization" as a potential strategy for anti-malignant treatment.

Collagen type II-hyaluronan interactions - the effect of proline hydroxylation: a molecular dynamics study

Hyaluronan-collagen composites have been employed in numerous biomedical applications. Understanding the interactions between hyaluronan and collagen is particularly important in the context of joint cartilage function and the treatment of joint diseases. Many factors affect the affinity of collagen for hyaluronan. One of the important factors is the ratio of 3- or 4-hydroxy proline to proline residues. This article presents the results from molecular dynamics calculations of HA-collagen type II interactions with hyaluronan. The applied protocol employed docking and geometry optimization of complexes built using collagen structures with different numbers of hydroxyl groups attached to proline moieties. It was established that the hydroxyproline/proline ratio affects both structural and energetic features of the collagen-hyaluronan complex. Proline hydroxylation was found to significantly influence the number of all identified types of molecular forces, hydrophobic interactions, water bridges and hydrogen bonds, which can be formed between collagen and hyaluronan. Importantly, an increase in the hydroxyproline/proline ratio in the collagen chain increases the binding affinity for hyaluronan. This is illustrated by the linear correlation between the binding free energy and the hydroxylation degree. A comparison of the results obtained for 3 and 4 hydroxylation of proline indicates that the hydroxyl group attachment position plays a minor role in complex stabilization. However, a slightly stronger affinity was observed for 4 hydroxylation. In order to evaluate the effect of the aqueous environment on the collagen-hyaluronan complex stability, the enthalpic and entropic contributions to the free energy of solvation were analyzed.

Ion modulatory treatments toward functional self-assembled neocartilage

Signals that recapitulate in vitro the conditions found in vivo, such as hypoxia or mechanical forces, contribute to the generation of tissue-engineered hyaline-like tissues. The cell regulatory processes behind hypoxic and mechanical stimuli rely on ion concentration; iron is required to degrade the hypoxia inducible factor 1a (HIF1α) under normoxia, whereas the initiation of mechanotransduction requires the cytoplasmic increase of calcium concentration. In this work, we propose that ion modulation can be used to improve the biomechanical properties of self-assembled neocartilage constructs derived from rejuvenated expanded minipig rib chondrocytes. The objectives of this work were 1) to determine the effects of iron sequestration on self-assembled neocartilage constructs using two doses of the iron chelator deferoxamine (DFO), and 2) to evaluate the performance of the combined treatment of DFO and ionomycin, a calcium ionophore that triggers cytoplasmic calcium accumulation. This study employed a two-phase approach. In Phase I, constructs treated with a high dose of DFO (100 µM) exhibited an 87% increase in pyridinoline crosslinks, a 57% increase in the Young's modulus, and a 112% increase in the ultimate tensile strength (UTS) of the neotissue. In Phase II, the combined use of both ion modulators resulted in 150% and 176% significant increases in the Young's modulus and UTS of neocartilage constructs, respectively; for the first time, neocartilage constructs achieved a Young's modulus of 11.76±3.29 MPa and UTS of 4.20±1.24 MPa. The results of this work provide evidence that ion modulation can be employed to improve the biomechanical properties in engineered neotissues. STATEMENT OF SIGNIFICANCE: The translation of tissue-engineered products requires the development of strategies capable of producing biomimetic neotissues in a replicable, controllable, and cost-effective manner. Among other functions, Fe2+ and Ca2+ are involved in the control of the hypoxic response and mechanotransduction, respectively. Both stimuli, hypoxia and mechanical forces, are known to favor chondrogenesis. This study utilized ion modulators to improve the mechanical properties self-assembled neocartilage constructs derived from expanded and rejuvenated costal chondrocytes via Fe2+ sequestration and Ca2+ influx, alone or in combination. The results indicate that ion modulation induced tissue maturation and a significant improvement of the mechanical properties, and holds potential as a tool to mitigate the need for bioreactors and engineer hyaline-like tissues.

Remodeling Osteoarthritic Articular Cartilage under Hypoxic Conditions

Osteoarthritis (OA) is one of the leading joint diseases induced by abnormalities or inflammation in the synovial membrane and articular cartilage, causing severe pain and disability. Along with the cartilage malfunction, imbalanced oxygen uptake occurs, changing chondrocytes into type I collagen- and type X collagen-producing dedifferentiated cells, contributing to OA progression. However, mounting evidence suggests treating OA by inducing a hypoxic environment in the articular cartilage, targeting the inhibition of several OA-related pathways to bring chondrocytes into a normal state. This review discusses the implications of OA-diseased articular cartilage on chondrocyte phenotypes and turnover and debates the hypoxic mechanism of action. Furthermore, this review highlights the new understanding of OA, provided by tissue engineering and a regenerative medicine experimental design, modeling the disease into diverse 2D and 3D structures and investigating hypoxia and hypoxia-inducing biomolecules and potential cell therapies. This review also reports the mechanism of hypoxic regulation and highlights the importance of activating and stabilizing the hypoxia-inducible factor and related molecules to protect chondrocytes from mitochondrial dysfunction and apoptosis occurring under the influence of OA.

In vitro maturation and in vivo stability of bioprinted human nasal cartilage

The removal of skin cancer lesions on the nose often results in the loss of nasal cartilage. The cartilage loss is either surgically replaced with autologous cartilage or synthetic grafts. However, these replacement options come with donor-site morbidity and resorption issues. 3-dimensional (3D) bioprinting technology offers the opportunity to engineer anatomical-shaped autologous nasal cartilage grafts. The 3D bioprinted cartilage grafts need to embody a mechanically competent extracellular matrix (ECM) to allow for surgical suturing and resistance to contraction during scar tissue formation. We investigated the effect of culture period on ECM formation and mechanical properties of 3D bioprinted constructs of human nasal chondrocytes (hNC)-laden type I collagen hydrogel in vitro and in vivo. Tissue-engineered nasal cartilage constructs developed from hNC culture in clinically approved collagen type I and type III semi-permeable membrane scaffold served as control. The resulting 3D bioprinted engineered nasal cartilage constructs were comparable or better than the controls both in vitro and in vivo. This study demonstrates that 3D bioprinted constructs of engineered nasal cartilage are feasible options in nasal cartilage reconstructive surgeries.

Time course of 3D fibrocartilage formation by expanded human meniscus fibrochondrocytes in hypoxia

Adult human meniscus fibrocartilage is avascular and nonhealing after injury. Meniscus tissue engineering aims to replace injured meniscus with lab-grown fibrocartilage. Dynamic culture systems may be necessary to generate fibrocartilage of sufficient mechanical properties for implantation; however, the optimal static preculture conditions before initiation of dynamic culture are unknown. This study thus investigated the time course of fibrocartilage formation by human meniscus fibrochondrocytes on a three-dimensional biomaterial scaffold under various static conditions. Human meniscus fibrochondrocytes from partial meniscectomy were expanded to passage 1 (P1) or P2 (3.0 ± 0.4 and 6.5 ± 0.6 population doublings), seeded onto type I collagen scaffolds, and grown in hypoxia (HYP, 3% O₂ ) or normoxia (NRX, 20% O₂ ) for 3, 6, and 9 weeks. Mechanical properties were not different between P1 and P2 cell-based constructs. Mechanical properties were lower in HYP, increased continually in NRX only, and were positively correlated with glycosaminoglycan content and accumulation of hyaline cartilage-like matrix components. The most mechanically competent tissues (NRX/9 weeks) reached 1/5 of the native meniscus instantaneous compression modulus but had an increasingly hypertrophic matrix-forming phenotype. HYP consistently suppressed the hypertrophic phenotype. The results provide baselines of engineered meniscus fibrocartilage properties under static conditions, which can be used to select a preculture strategy for dynamic culture depending on the desired combination of mechanical properties, hyaline cartilage-like matrix abundance, and hypertrophic phenotype.

INTRACELLULAR CALCIUM AND SODIUM MODULATION OF SELF-ASSEMBLED NEOCARTILAGE USING COSTAL CHONDROCYTES

Ion signaling via Ca²⁺ and Na⁺ plays a key role in mechanotransduction and encourages a chondrogenic phenotype and tissue maturation. Here, we propose that the pleiotropic effects of Ca²⁺ and Na⁺ modulation can be used to induce maturation and improvement of neocartilage derived from re-differentiated expanded chondrocytes from minipig rib cartilage. Three ion modulators were employed: 1) 4α-phorbol-12,13-didecanoate (4-αPDD), an agonist of the Ca²⁺-permeable transient receptor potential vanilloid 4 (TRPV4), 2) ouabain, an inhibitor of the Na⁺/K⁺ pump, and 3) ionomycin, a Ca²⁺ ionophore. These ion modulators were used individually or in combination. While no beneficial effects were observed when using combinations of the ion modulators, single treatment of constructs with the three ion modulators resulted in multiple effects in structure-function relationships. The most significant findings were related to ionomycin. Treatment of neocartilage with ionomycin produced 61% and 115% increases in glycosaminoglycan and pyridinoline crosslink content, respectively, compared to the control. Moreover, treatment with this Ca²⁺ ionophore resulted in a 45% increase of the aggregate modulus, and a 63% increase in the tensile Young's modulus, resulting in aggregate and Young's moduli of 567 kPa and 8.43 MPa, respectively. These results support the use of ion modulation to develop biomimetic neocartilage using expanded re-differentiated costal chondrocytes.

TEMPO-Oxidized Cellulose Nanofiber-Alginate Hydrogel as a Bioink for Human Meniscus Tissue Engineering

Objective: The avascular inner regions of the knee menisci cannot self-heal. As a prospective treatment, functional replacements can be generated by cell-based 3D bioprinting with an appropriate cell source and biomaterial. To that end, human meniscus fibrochondrocytes (hMFC) from surgical castoffs of partial meniscectomies as well as cellulose nanofiber-alginate based hydrogels have emerged as a promising cell source and biomaterial combination. The objectives of the study were to first find the optimal formulations of TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl)-oxidized cellulose nanofiber/alginate (TCNF/ALG) precursors for bioprinting, and then to use them to investigate redifferentiation and synthesis of functional inner meniscus-like extracellular matrix (ECM) components by expanded hMFCs. Methods: The rheological properties including shear viscosity, thixotropic behavior recovery, and loss tangent of selected TCNF/ALG precursors were measured to find the optimum formulations for 3D bioprinting. hMFCs were mixed with TCNF/ALG precursors with suitable formulations and 3D bioprinted into cylindrical disc constructs and crosslinked with CaCl2 after printing. The bioprinted constructs then underwent 6 weeks of in vitro chondrogenesis in hypoxia prior to analysis with biomechanical, biochemical, molecular, and histological assays. hMFCs mixed with a collagen I gel were used as a control. Results: The TCNF/ALG and collagen-based constructs had similar compression moduli. The expression of COL2A1 was significantly higher in TCNF/ALG. The TCNF/ALG constructs showed more of an inner meniscus-like phenotype while the collagen I-based construct was consistent with a more outer meniscus-like phenotype. The expression of COL10A1 and MMP13 were lower in the TCNF/ALG constructs. In addition, the immunofluorescence of human type I and II collagens were evident in the TCNF/ALG, while the bovine type I collagen constructs lacked type II collagen deposition but did contain newly synthesized human type I collagen.

The effect of redox signaling on extracellular matrix changes in diabetic wounds leading to amputation

INTRODUCTION

& Objectives: Redox signaling is a critical regulator in the process of wound healing. This signaling pathway can be effective in the development or healing of diabetic ulcers through the ECM.In this study, the structure of extracellular matrix investigated in relation to redox signaling in the tissue of patients with diabetic ulcers that lead to organ amputation.

MATERIALS AND METHODS

The case-control design on diabetic patients ulcers as case group and non-diabetic limb ischemia as control were used.Hematoxylin-eosin, trichrome, and elastin staining methods were used for pathological evaluations of ECM. MDA, total thiol, and SOD levels were measured using ELISA kits to assess the oxidative stress level. Also, NO level was measured by using ELISA kits in both groups. Expression levels of genes MMP2, MMP9, and HIF were detected using real-time PCR with SYBR-green assay.

RESULTS

The pathological results showed an increase in the thickness of collagen and elastin fibers. Lipids atrophy was visible in the tissue isolated from the diabetic wound group. The amount of MAD to evaluate the level of lipid oxidation in patients with diabetic Ulcer was significantly higher than the control group(p < 0.01). Thiol level was significantly lower in the diabetic ulcer group than in the control group(p < 0.0001). The expression of metalloproteinases 2 and 9 genes in the tissues isolated from diabetic ulcers was lower than the control group(p < 0.0001). While the expression of the HIF gene in this group was higher than the control group(p < 0.0001).

CONCLUTION

In the diabetic wound, the HIF secretion due to hypoxic conditions is beneficial for matrix deposition and prevents protease activity, but if the hypoxia persists, it can lead to ECM deposition subsequently increases the tissue pressure, increases of the collagen I-to-collagen III ratio in collagen accumulation that due to more hypoxia , lipidsAtrophy and eventually amputation.

Engineered human meniscus' matrix-forming phenotype is unaffected by low strain dynamic compression under hypoxic conditions

Low oxygen and mechanical loading may play roles in regulating the fibrocartilaginous phenotype of the human inner meniscus, but their combination in engineered tissues remains unstudied. Here, we investigated how continuous low oxygen (“hypoxia”) combined with dynamic compression would affect the fibrocartilaginous “inner meniscus-like” matrix-forming phenotype of human meniscus fibrochondrocytes (MFCs) in a porous type I collagen scaffold. Freshly-seeded MFC scaffolds were cultured for 4 weeks in either 3 or 20% O₂ or pre-cultured for 2 weeks in 3% O₂ and then dynamically compressed for 2 weeks (10% strain, 1 Hz, 1 h/day, 5 days/week), all with or without TGF-β3 supplementation. TGF-β3 supplementation was found necessary to induce matrix formation by MFCs in the collagen scaffold regardless of oxygen tension and application of the dynamic compression loading regime. Neither hypoxia under static culture nor hypoxia combined with dynamic compression had significant effects on expression of specific protein and mRNA markers for the fibrocartilaginous matrix-forming phenotype. Mechanical properties significantly increased over the two-week loading period but were not different between static and dynamic-loaded tissues after the loading period. These findings indicate that 3% O₂ applied immediately after scaffold seeding and dynamic compression to 10% strain do not affect the fibrocartilaginous matrix-forming phenotype of human MFCs in this type I collagen scaffold. It is possible that a delayed hypoxia treatment and an optimized pre-culture period and loading regime combination would have led to different outcomes.

Physioxia Stimulates Extracellular Matrix Deposition and Increases Mechanical Properties of Human Chondrocyte-Derived Tissue-Engineered Cartilage

Cartilage tissue has been recalcitrant to tissue engineering approaches. In this study, human chondrocytes were formed into self-assembled cartilage sheets, cultured in physiologic (5%) and atmospheric (20%) oxygen conditions and underwent biochemical, histological and biomechanical analysis at 1- and 2-months. The results indicated that sheets formed at physiological oxygen tension were thicker, contained greater amounts of glycosaminoglycans (GAGs) and type II collagen, and had greater compressive and tensile properties than those cultured in atmospheric oxygen. In all cases, cartilage sheets stained throughout for extracellular matrix components. Type II-IX-XI collagen heteropolymer formed in the neo-cartilage and fibrils were stabilized by trivalent pyridinoline cross-links. Collagen cross-links were not significantly affected by oxygen tension but increased with time in culture. Physiological oxygen tension and longer culture periods both served to increase extracellular matrix components. The foremost correlation was found between compressive stiffness and the GAG to collagen ratio.

Collagen: quantification, biomechanics, and role of minor subtypes in cartilage

Collagen is a ubiquitous biomaterial in vertebrate animals. Although each of its 28 subtypes contributes to the functions of many different tissues in the body, most studies on collagen or collagenous tissues have focussed on only one or two subtypes. With recent developments in analytical chemistry, especially mass spectrometry, significant advances have been made toward quantifying the different collagen subtypes in various tissues; however, high-throughput and low-cost methods for collagen subtype quantification do not yet exist. In this Review, we introduce the roles of collagen subtypes and crosslinks, and describe modern assays that enable a deep understanding of tissue physiology and disease states. Using cartilage as a model tissue, we describe the roles of major and minor collagen subtypes in detail; discuss known and unknown structure-function relationships; and show how tissue engineers may harness the functional characteristics of collagen to engineer robust neotissues.

Role of the lysyl oxidase family in organ development (Review)

Lysyl oxidase proteins (LOXs) are amine oxidases, which are mainly located in smooth muscle cells and fibroblasts and serve an important role in the formation of the extracellular matrix (ECM) in a copper-dependent manner. Owing to the ability of LOX proteins to modulate crosslinking between collagens and to promote the deposition of other fibers, they serve crucially in organogenesis and the subsequent organ development, as well as disease initiation and progression. In addition, ECM formation significantly influences organ morphological formation in both cancer- and non-tumor-related diseases, in addition to cellular epigenetic transformation and migration, under the influence of LOXs. A number of different signaling pathways regulate the LOXs expression and their enzymatic activation. The tissue remodeling and transformation process shares some resemblance between oncogenesis and embryogenesis. Additionally the roles that LOXs serve appeared to be stressed during oncogenesis and tumor metastasis. It has also been indicated LOXs have a noteworthy role in non-tumor diseases. Nonetheless, the role of LOXs in systemic or local organ development and disease control remains unknown. In the present study, the essential roles that LOXs play in embryogenesis were unveiled partially, whereas the role of LOXs in organ or systematic development requires further investigations. The present review aimed to discuss the roles of members of the LOX family in the context of the remodeling of organogenesis and organ development. In addition, the consequences of the malfunction of these proteins related to the development of abnormalities and resulting diseases is discussed.

Site-Dependent Lineage Preference of Adipose Stem Cells

Adult stem cells have unique properties in both proliferation and differentiation preference. In this study, we hypothesized that adipose stem cells have a depot-dependent lineage preference. Four rabbits were used to provide donor-matched adipose stem cells from either subcutaneous adipose tissue (ScAT) or infrapatellar fat pad (IPFP). Proliferation and multi-lineage differentiation were evaluated in adipose stem cells from donor-matched ScAT and IPFP. RNA sequencing (RNA-seq) and proteomics were conducted to uncover potential molecular discrepancy in adipose stem cells and their corresponding matrix microenvironments. We found that stem cells from ScAT exhibited significantly higher proliferation and adipogenic capacity compared to those from donor-matched IPFP while stem cells from IPFP displayed significantly higher chondrogenic potential compared to those from donor-matched ScAT. Our findings are strongly endorsed by supportive data from transcriptome and proteomics analyses, indicating a site-dependent lineage preference of adipose stem cells.

Hypoxia Inducible Factor-1α in Osteochondral Tissue Engineering

Damage to osteochondral (OC) tissues can lead to pain, loss of motility, and progress to osteoarthritis. Tissue engineering approaches offer the possibility of replacing damaged tissues and restoring joint function; however, replicating the spatial and functional heterogeneity of native OC tissue remains a pressing challenge. Chondrocytes in healthy cartilage exist in relatively low-oxygen conditions, while osteoblasts in the underlying bone experience higher oxygen pressures. Such oxygen gradients also exist in the limb bud, where they influence OC tissue development. The cellular response to these spatial variations in oxygen pressure, which is mediated by the hypoxia inducible factor (HIF) pathway, plays a central role in regulating osteo- and chondrogenesis by directing progenitor cell differentiation and promoting and maintaining appropriate extracellular matrix production. Understanding the role of the HIF pathway in OC tissue development may enable new approaches to engineer OC tissue. In this review, we discuss strategies to spatially and temporarily regulate the HIF pathway in progenitor cells to create functional OC tissue for regenerative therapies. Impact statement Strategies to engineer osteochondral (OC) tissue are limited by the complex and varying microenvironmental conditions in native bone and cartilage. Indeed, native cartilage experiences low-oxygen conditions, while the underlying bone is relatively normoxic. The cellular response to these low-oxygen conditions, which is mediated through the hypoxia inducible factor (HIF) pathway, is known to promote and maintain the chondrocyte phenotype. By using tissue engineering scaffolds to spatially and temporally harness the HIF pathway, it may be possible to improve OC tissue engineering strategies for the regeneration of damaged cartilage and its underlying subchondral bone.

High-Throughput, Temporal and Dose Dependent, Effect of Vitamins and Minerals on Chondrogenesis

Tissue engineered hyaline cartilage is plagued by poor mechanical properties largely due to inadequate type II collagen expression. Of note, commonly used defined chondrogenic media lack 14 vitamins and minerals, some of which are implicated in chondrogenesis. Type II collagen promoter-driven Gaussia luciferase was transfected into ATDC5 cells to create a chondrogenic cell with a secreted-reporter. The reporter cells were used in an aggregate-based chondrogenic culture model to develop a high-throughput analytic platform. This high-throughput platform was used to assess the effect of vitamins and minerals, alone and in combination with TGFβ1, on COL2A1 promoter-driven expression. Significant combinatorial effects between vitamins, minerals, and TGFβ1 in terms of COL2A1 promoter-driven expression and metabolism were discovered. An “optimal” continual supplement of copper and vitamin K in the presence of TGFβ1 gave a 2.5-fold increase in COL2A1 promoter-driven expression over TGFβ1 supplemented media alone in ATDC5 cells.

Surgical and tissue engineering strategies for articular cartilage and meniscus repair

Injuries to articular cartilage and menisci can lead to cartilage degeneration that ultimately results in arthritis. Different forms of arthritis affect ~50 million people in the USA alone, and it is therefore crucial to identify methods that will halt or slow the progression to arthritis, starting with the initiating events of cartilage and meniscus defects. The surgical approaches in current use have a limited capacity for tissue regeneration and yield only short-term relief of symptoms. Tissue engineering approaches are emerging as alternatives to current surgical methods for cartilage and meniscus repair. Several cell-based and tissue-engineered products are currently in clinical trials for cartilage lesions and meniscal tears, opening new avenues for cartilage and meniscus regeneration. This Review provides a summary of surgical techniques, including tissue-engineered products, that are currently in clinical use, as well as a discussion of state-of-the-art tissue engineering strategies and technologies that are being developed for use in articular cartilage and meniscus repair and regeneration. The obstacles to clinical translation of these strategies are also included to inform the development of innovative tissue engineering approaches.

The Role of Hypoxia in Corneal Extracellular Matrix Deposition and Cell Motility

The cornea is an excellent model tissue to study how cells adapt to periods of hypoxia as it is naturally exposed to diurnal fluxes in oxygen. It is avascular, transparent, and highly innervated. In certain pathologies, such as diabetes, limbal stem cell deficiency, or trauma, the cornea may be exposed to hypoxia for variable lengths of time. Due to its avascularity, the cornea requires atmospheric oxygen, and a reduction in oxygen availability can impair its physiology and function. We hypothesize that hypoxia alters membrane stiffness and the deposition of matrix proteins, leading to changes in cell migration, focal adhesion formation, and wound repair. Two systems-a 3D corneal organ culture model and polyacrylamide substrates of varying stiffness-were used to examine the response of corneal epithelium to normoxic and hypoxic environments. Exposure to hypoxia alters the deposition of the matrix proteins such as laminin and Type IV collagen. In addition, previous studies had shown a change in fibronectin after injury. Studies performed on matrix-coated acrylamide substrates ranging from 0.2 to 50 kPa revealed stiffness-dependent changes in cell morphology. The localization, number, and length of paxillin pY118- and vinculin pY1065-containing focal adhesions were different in wounded corneas and in human corneal epithelial cells incubated in hypoxic environments. Overall, these results demonstrate that low-oxygenated environments modify the composition of the extracellular matrix, basal lamina stiffness, and focal adhesion dynamics, leading to alterations in the function of the cornea. Anat Rec, 2019. © 2019 Wiley Periodicals, Inc.

Structure-function relationships of fetal ovine articular cartilage

It is crucial that the properties of engineered neocartilage match healthy native cartilage to promote the functional restoration of damaged cartilage. To accurately assess the quality of neocartilage and the degree of biomimicry achieved, its properties must be evaluated against native cartilage and tissue from which the cells for neocartilage formation were sourced. Fetal ovine cartilage is a promising and translationally relevant cell source with which to engineer neocartilage, yet, it is largely non-characterized. The influence of biomechanics during cartilage development, as well as their potential impact on structure-function relationships in utero motivates additional study of fetal cartilage. Toward providing tissue engineering design criteria and elucidating structure-function relationships, 11 locations across four regions of the fetal ovine stifle were characterized. Locational and regional differences were found to exist. Although differences in GAG content were observed, compressive stiffness did not vary or correlate with any biochemical component. Patellar cartilage tensile stiffness and strength were significantly greater than those of the medial condyle. Tensile modulus and UTS significantly correlated with pyridinoline content. More advanced zonal organization, more intense collagen II staining, and greater collagen and pyridinoline contents in the trochlear groove and patella suggest these regions exhibit a more advanced maturational state than others. Regional differences in functional properties and their correlations suggest that structure-function relationships emerge in utero. These data address the dearth of information of the fetal ovine stifle, may serve as a repository of information for cartilage engineering strategies, and may help elucidate functional adaptation in fetal articular cartilage. STATEMENT OF SIGNIFICANCE: Engineered neocartilage must be evaluated against healthy native cartilage and cell source tissue to determine its quality and degree of biomimicry. While fetal ovine cartilage has emerged as a promising and translationally relevant cell source with which to engineer neocartilage, it is largely non-characterized. Therefore, 11 locations across four regions (medial condyle, lateral condyle, trochlear groove, and patella) of the fetal ovine stifle were characterized. Importantly, locational and regional differences in functional properties were observed, and significant correlations of tensile properties to collagen and crosslink contents were detected, suggesting that functional adaptation begins in utero. This study provides a repository of quantitative information, clarifies the developmental order of cartilage functional properties, and informs future cartilage engineering efforts.

Nondestructive assessment of collagen hydrogel cross-linking using time-resolved autofluorescence imaging

We investigate the use of a fiber-based, multispectral fluorescence lifetime imaging (FLIm) system to nondestructively monitor changes in mechanical properties of collagen hydrogels caused by controlled application of widely used cross-linking agents, glutaraldehyde (GTA) and ribose. Postcross-linking, fluorescence lifetime images are acquired prior to the hydrogels being processed by rheological or tensile testing to directly probe gel mechanical properties. To preserve the sterility of the ribose-treated gels, FLIm is performed inside a biosafety cabinet (BSC). A pairwise correlation analysis is used to quantify the relationship between mean hydrogel fluorescence lifetimes and the storage or Young's moduli of the gels. In the GTA study, we observe strong and specific correlations between fluorescence lifetime and the storage and Young's moduli. Similar correlations are not observed in the ribose study and we postulate a reason for this. Finally, we demonstrate the ability of FLIm to longitudinally monitor dynamic cross-link formation. The strength of the GTA correlations and deployment of our fiber-based FLIm system inside the aseptic environment of a BSC suggests that this technique may be a valuable tool for the tissue engineering community where longitudinal assessment of tissue construct maturation in vitro is highly desirable.

Application of Extrusion-Based Hydrogel Bioprinting for Cartilage Tissue Engineering

Extrusion-based bioprinting (EBB) is a rapidly developing technique that has made substantial progress in the fabrication of constructs for cartilage tissue engineering (CTE) over the past decade. With this technique, cell-laden hydrogels or bio-inks have been extruded onto printing stages, layer-by-layer, to form three-dimensional (3D) constructs with varying sizes, shapes, and resolutions. This paper reviews the cell sources and hydrogels that can be used for bio-ink formulations in CTE application. Additionally, this paper discusses the important properties of bio-inks to be applied in the EBB technique, including biocompatibility, printability, as well as mechanical properties. The printability of a bio-ink is associated with the formation of first layer, ink rheological properties, and crosslinking mechanisms. Further, this paper discusses two bioprinting approaches to build up cartilage constructs, i.e., self-supporting hydrogel bioprinting and hybrid bioprinting, along with their applications in fabricating chondral, osteochondral, and zonally organized cartilage regenerative constructs. Lastly, current limitations and future opportunities of EBB in printing cartilage regenerative constructs are reviewed.

Physioxia Promotes the Articular Chondrocyte-Like Phenotype in Human Chondroprogenitor-Derived Self-Organized Tissue

INTRODUCTION

Biomaterial-based tissue engineering has not successfully reproduced the structural architecture or functional mechanical properties of native articular cartilage. In scaffold-free tissue engineering systems, cells secrete and organize the entire extracellular matrix over time in response to environmental signals such as oxygen level. In this study, we investigated the effect of oxygen on the formation of neocartilage from human-derived chondrogenic cells.

MATERIALS AND METHODS

Articular chondrocytes (ACs) and articular cartilage progenitor cells (ACPs) derived from healthy human adults were guided toward cell condensation by centrifugation onto plate inserts that were uncoated or coated with either agarose or fibronectin. Neocartilage discs were cultured at hyperoxic (20%) or physioxic (5%) oxygen levels, and biochemical, biomechanical, and molecular analyses were used to compare the cartilage produced by ACs versus ACPs.

RESULTS

Fibronectin-coated inserts proved optimal for growing cartilaginous discs from both cell types. In comparison with culture in hyperoxia, AC neocartilage cultured at physioxia exhibited a significant increase in chondrogenic gene expression, proteoglycan production, and mechanical properties with a concomitant decrease in collagen content. At both oxygen levels, ACP-derived neocartilage produced tissue with significantly enhanced mechanical properties and collagen content relative to AC-derived neocartilage. Both ACs and ACPs produced substantial collagen II and reduced levels of collagens I and X in physioxia relative to hyperoxia. Neocartilage from ACPs exhibited anisotropic organization characteristic of native cartilage with respect to collagen VI of the pericellular matrix when compared with AC-derived neocartilage; however, only ACs produced abundant surface-localized lubricin.

DISCUSSION AND CONCLUSIONS

Guiding human-derived cells toward condensation and subsequent culture in physioxia promoted the articular cartilage tissue phenotype for ACs and ACPs. Unlike ACs, ACPs are clonable and highly expandable while retaining chondrogenicity. The ability to generate large tissues utilizing a scaffold-free approach from a single autologous progenitor cell may represent a promising source of neocartilage destined for cartilage repair.

Applications of Tissue Engineering in Joint Arthroplasty: Current Concepts Update

Research in tissue engineering has undoubtedly achieved significant milestones in recent years. Although it is being applied in several disciplines, tissue engineering's application is particularly advanced in orthopedic surgery and in degenerative joint diseases. The literature is full of remarkable findings and trials using tissue engineering in articular cartilage disease. With the vast and expanding knowledge, and with the variety of techniques available at hand, the authors aimed to review the current concepts and advances in the use of cell sources in articular cartilage tissue engineering.

Influence of oxygen levels on chondrogenesis of porcine mesenchymal stem cells cultured in polycaprolactone scaffolds

Chondrogenesis of mesenchymal stem cells (MSCs) is known to be regulated by a number of environmental factors, including local oxygen levels. The hypothesis of this study is that the response of MSCs to hypoxia is dependent on the physical and chemical characteristics of the substrate used. The objective of this study was to explore how different modifications to polycaprolactone (PCL) scaffolds influenced the response of MSCs to hypoxia. PCL, PCL-hyaluronic acid (HA), and PCL-Bioglass^® (BG) scaffolds were seeded with MSCs derived from bone marrow and cultured for 35 days under normoxic or low oxygen conditions, and the resulting biochemical properties of the MSC laden construct were assessed. Low oxygen tension has a positive effect over cell proliferation and macromolecules biosynthesis. Furthermore, hypoxia enhanced the distribution of collagen and glycosaminoglycans (GAGs) deposition through the scaffold. On the other hand, MSCs displayed certain material dependent responses to hypoxia. Low oxygen tension had a positive effect on cell proliferation in BG and HA scaffolds, but only a positive effect on GAGs synthesis in PCL and HA scaffolds. In conclusion, hypoxia increased cell viability and expression of chondrogenic markers but the cell response was modulated by the type of scaffold used. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 105A: 1684-1691, 2017.

The Role of the Lysyl Oxidases in Tissue Repair and Remodeling: A Concise Review

Tissue injury provokes a series of events containing inflammation, new tissue formation and tissue remodeling which are regulated by the spatially and temporally coordinated organization. It is an evolutionarily conserved, multi-cellular, multi-molecular process via complex signalling network. Tissue injury disorders present grievous clinical problems and are likely to increase since they are generally associated with the prevailing diseases such as diabetes, hypertension and obesity. Although these dynamic responses vary not only for the different types of trauma but also for the different organs, a balancing act between the tissue degradation and tissue synthesis is the same. In this process, the degradation of old extracellular matrix (ECM) elements and new ones' synthesis and deposition play an essential role, especially collagens. Lysyl oxidase (LOX) and four lysyl oxidase-like proteins are a group of enzymes capable of catalyzing cross-linking reaction of collagen and elastin, thus initiating the formation of covalent cross-links that insolubilize ECM proteins. In this way, LOX facilitates ECM stabilization through ECM formation, development, maturation and remodeling. This ability determines its potential role in tissue repair and regeneration. In this review, based on the current in vitro, animal and human in vivo studies which have shown the significant role of the LOXs in tissue repair, e.g., tendon regeneration, ligament healing, cutaneous wound healing, and cartilage remodeling, we focused on the role of the LOXs in inflammation phase, proliferation phase, and tissue remodeling phase of the repair process. By summarizing its healing role, we hope to shed light on the understanding of its potential in tissue repair and provide up to date therapeutic strategies towards related injuries.

Expression of hypoxia-inducible factor-1α in synovial fluid and articular cartilage is associated with disease severity in knee osteoarthritis

The aim of the present study was to examine hypoxia-inducible factor 1α (HIF-1α) levels in the synovial fluid and articular cartilage of patients with primary knee osteoarthritis (OA) and to investigate their association with the severity of disease. A total of 36 patients with knee OA and ten healthy controls were enrolled. Anteroposterior knee radiographs and/or Mankin scores were assessed to determine the disease severity of the affected knee. Radiographic grading of OA in the knee was performed according to Kellgren-Lawrence criteria. HIF-1α levels in synovial fluid were measured using enzyme-linked immunosorbent assay, whereas HIF-1α levels in articular cartilage were assessed with immunohistochemical methods. Compared with healthy controls, OA patients exhibited an increased HIF-1α concentration in synovial fluid (218.17±25.12 vs. 156.66±7.74 pg/ml; P<0.001) and articular cartilage (P<0.05). Furthermore, synovial fluid HIF-1α levels demonstrated a positive correlation with articular cartilage HIF-1α levels (Pearson's P=0.815; P<0.001). Subsequent analysis showed that synovial fluid HIF-1α levels were significantly correlated with the severity of disease (Spearman's ρ=0.933; P<0.001). Furthermore, articular cartilage levels of HIF-1α also correlated with disease severity (Spearman's ρ=-0.967; P<0.001). The findings of the present study suggested that HIF-1α in synovial fluid and articular cartilage is associated with progressive joint damage and is likely to be a useful biomarker for determining disease severity and progression in knee OA.

Disparate response of articular- and auricular-derived chondrocytes to oxygen tension

PURPOSE/AIM

To determine the effect of reduced (5%) oxygen tension on chondrogenesis of auricular-derived chondrocytes. Currently, many cell and tissue culture experiments are performed at 20% oxygen with 5% carbon dioxide. Few cells in the body are subjected to this supra-physiological oxygen tension. Chondrocytes and their mesenchymal progenitors are widely reported to have greater chondrogenic expression when cultured at low, more physiological, oxygen tension (1-7%). Although generally accepted, there is still some controversy, and different culture methods, species, and outcome metrics cloud the field. These results are, however, articular chondrocyte biased and have not been reported for auricular-derived chondrocytes.

MATERIALS AND METHODS

Auricular and articular chondrocytes were isolated from skeletally mature New Zealand White rabbits, expanded in culture and differentiated in high density cultures with serum-free chondrogenic media. Cartilage tissue derived from aggregate cultures or from the tissue engineered sheets were assessed for biomechanical, glycosaminoglycan, collagen, collagen cross-links, and lysyl oxidase activity and expression.

RESULTS

Our studies show increased proliferation rates for both auricular and articular chondrocytes at low (5%) O2 versus standard (20%) O2. In our scaffold-free chondrogenic cultures, low O2 was found to increase articular chondrocyte accumulation of glycosaminoglycan, but not cross-linked type II collagen, or total collagen. Conversely, auricular chondrocytes accumulated less glycosaminoglycan, cross-linked type II collagen and total collagen under low oxygen tension.

CONCLUSIONS

This study highlights the dramatic difference in response to low O2 of chondrocytes isolated from different anatomical sites. Low O2 is beneficial for articular-derived chondrogenesis but detrimental for auricular-derived chondrogenesis.

Mesenchymal Stem Cell Seeding of Porcine Small Intestinal Submucosal Extracellular Matrix for Cardiovascular Applications

In this study, we investigate the translational potential of a novel combined construct using an FDA-approved decellularized porcine small intestinal submucosa extracellular matrix (SIS-ECM) seeded with human or porcine mesenchymal stem cells (MSCs) for cardiovascular indications. With the emerging success of individual component in various clinical applications, the combination of SIS-ECM with MSCs could provide additional therapeutic potential compared to individual components alone for cardiovascular repair. We tested the in vitro effects of MSC-seeding on SIS-ECM on resultant construct structure/function properties and MSC phenotypes. Additionally, we evaluated the ability of porcine MSCs to modulate recipient graft-specific response towards SIS-ECM in a porcine cardiac patch in vivo model. Specifically, we determined: 1) in vitro loading-capacity of human MSCs on SIS-ECM, 2) effect of cell seeding on SIS-ECM structure, compositions and mechanical properties, 3) effect of SIS-ECM seeding on human MSC phenotypes and differentiation potential, and 4) optimal orientation and dose of porcine MSCs seeded SIS-ECM for an in vivo cardiac application. In this study, histological structure, biochemical compositions and mechanical properties of the FDA-approved SIS-ECM biomaterial were retained following MSCs repopulation in vitro. Similarly, the cellular phenotypes and differentiation potential of MSCs were preserved following seeding on SIS-ECM. In a porcine in vivo patch study, the presence of porcine MSCs on SIS-ECM significantly reduced adaptive T cell response regardless of cell dose and orientation compared to SIS-ECM alone. These findings substantiate the clinical translational potential of combined SIS-ECM seeded with MSCs as a promising therapeutic candidate for cardiac applications.

Articular cartilage tissue engineering: the role of signaling molecules

Effective early disease modifying options for osteoarthritis remain lacking. Tissue engineering approach to generate cartilage in vitro has emerged as a promising option for articular cartilage repair and regeneration. Signaling molecules and matrix modifying agents, derived from knowledge of cartilage development and homeostasis, have been used as biochemical stimuli toward cartilage tissue engineering and have led to improvements in the functionality of engineered cartilage. Clinical translation of neocartilage faces challenges, such as phenotypic instability of the engineered cartilage, poor integration, inflammation, and catabolic factors in the arthritic environment; these can all contribute to failure of implanted neocartilage. A comprehensive understanding of signaling molecules involved in osteoarthritis pathogenesis and their actions on engineered cartilage will be crucial. Thus, while it is important to continue deriving inspiration from cartilage development and homeostasis, it has become increasingly necessary to incorporate knowledge from osteoarthritis pathogenesis into cartilage tissue engineering.

A Tissue-Engineered Chondrocyte Cell Sheet Induces Extracellular Matrix Modification to Enhance Ventricular Biomechanics and Attenuate Myocardial Stiffness in Ischemic Cardiomyopathy

There exists a substantial body of work describing cardiac support devices to mechanically support the left ventricle (LV); however, these devices lack biological effects. To remedy this, we implemented a cell sheet engineering approach utilizing chondrocytes, which in their natural environment produce a relatively elastic extracellular matrix (ECM) for a cushioning effect. Therefore, we hypothesized that a chondrocyte cell sheet applied to infarcted and borderzone myocardium will biologically enhance the ventricular ECM and increase elasticity to augment cardiac function in a model of ischemic cardiomyopathy (ICM). Primary articular cartilage chondrocytes of Wistar rats were isolated and cultured on temperature-responsive culture dishes to generate cell sheets. A rodent ICM model was created by ligating the left anterior descending coronary artery. Rats were divided into two groups: cell sheet transplantation (1.0 × 10(7) cells/dish) and no treatment. The cell sheet was placed onto the surface of the heart covering the infarct and borderzone areas. At 4 weeks following treatment, the decreased fibrotic extension and increased elastic microfiber networks in the infarct and borderzone areas correlated with this technology's potential to stimulate ECM formation. The enhanced ventricular elasticity was further confirmed by the axial stretch test, which revealed that the cell sheet tended to attenuate tensile modulus, a parameter of stiffness. This translated to increased wall thickness in the infarct area, decreased LV volume, wall stress, mass, and improvement of LV function. Thus, the chondrocyte cell sheet strengthens the ventricular biomechanical properties by inducing the formation of elastic microfiber networks in ICM, resulting in attenuated myocardial stiffness and improved myocardial function.

Passive strain-induced matrix synthesis and organization in shape-specific, cartilaginous neotissues

Tissue-engineered musculoskeletal soft tissues typically lack the appropriate mechanical robustness of their native counterparts, hindering their clinical applicability. With structure and function being intimately linked, efforts to capture the anatomical shape and matrix organization of native tissues are imperative to engineer functionally robust and anisotropic tissues capable of withstanding the biomechanically complex in vivo joint environment. The present study sought to tailor the use of passive axial compressive loading to drive matrix synthesis and reorganization within self-assembled, shape-specific fibrocartilaginous constructs, with the goal of developing functionally anisotropic neotissues. Specifically, shape-specific fibrocartilaginous neotissues were subjected to 0, 0.01, 0.05, or 0.1 N axial loads early during tissue culture. Results found the 0.1-N load to significantly increase both collagen and glycosaminoglycan synthesis by 27% and 67%, respectively, and to concurrently reorganize the matrix by promoting greater matrix alignment, compaction, and collagen crosslinking compared with all other loading levels. These structural enhancements translated into improved functional properties, with the 0.1-N load significantly increasing both the relaxation modulus and Young's modulus by 96% and 255%, respectively, over controls. Finite element analysis further revealed the 0.1-N uniaxial load to induce multiaxial tensile and compressive strain gradients within the shape-specific neotissues, with maxima of 10.1%, 18.3%, and -21.8% in the XX-, YY-, and ZZ-directions, respectively. This indicates that strains created in different directions in response to a single axis load drove the observed anisotropic functional properties. Together, results of this study suggest that strain thresholds exist within each axis to promote matrix synthesis, alignment, and compaction within the shape-specific neotissues. Tailoring of passive axial loading, thus, presents as a simple, yet effective way to drive in vitro matrix development in shape-specific neotissues toward more closely achieving native structural and functional properties.

Culture temperature affects human chondrocyte messenger RNA expression in monolayer and pellet culture systems

Cell-based therapy has been explored for articular cartilage regeneration. Autologous chondrocyte implantation is a promising cell-based technique for repairing articular cartilage defects. However, there are several issues such as chondrocyte de-differentiation. While numerous studies have been designed to overcome some of these issues, only a few have focused on the thermal environment that can affect chondrocyte metabolism and phenotype. In this study, the effects of different culture temperatures on human chondrocyte metabolism- and phenotype-related gene expression were investigated in 2D and 3D environments. Human chondrocytes were cultured in a monolayer or in a pellet culture system at three different culture temperatures (32°C, 37°C, and 41°C) for 3 days. The results showed that the total RNA level, normalized to the threshold cycle value of internal reference genes, was higher at lower temperatures in both culture systems. Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and citrate synthase (CS), which are involved in glycolysis and the citric acid cycle, respectively, were expressed at similar levels at 32°C and 37°C in pellet cultures, but the levels were significantly lower at 41°C. Expression of the chondrogenic markers, collagen type IIA1 (COL2A1) and aggrecan (ACAN), was higher at 37°C than at 32°C and 41°C in both culture systems. However, this phenomenon did not coincide with SRY (sex-determining region Y)-box 9 (SOX9), which is a fundamental transcription factor for chondrogenesis, indicating that a SOX9-independent pathway might be involved in this phenomenon. In conclusion, the expression of chondrocyte metabolism-related genes at 32°C was maintained or enhanced compared to that at 37°C. However, chondrogenesis-related genes were further induced at 37°C in both culture systems. Therefore, manipulating the culture temperature may be an advantageous approach for regulating human chondrocyte metabolic activity and chondrogenesis.

Hypoxic culture of bone marrow-derived mesenchymal stromal stem cells differentially enhances in vitro chondrogenesis within cell-seeded collagen and hyaluronic acid porous scaffolds

Introduction

The quality of cartilaginous tissue derived from bone marrow mesenchymal stromal stem cell (BMSC) transplantation has been correlated with clinical outcome. Therefore, culture conditions capable of modulating tissue phenotype, such as oxygen tension and scaffold composition, are under investigation. The objective of this study was to assess the effect of hypoxia on in vitro BMSC chondrogenesis within clinically approved porous scaffolds composed of collagen and hyaluronic acid (HA). It was hypothesized that hypoxic isolation/expansion and differentiation would improve BMSC chondrogenesis in each construct.

Methods

Ovine BMSCs were isolated and expanded to passage 2 under hypoxia (3% oxygen) or normoxia (21% oxygen). Cell proliferation and colony-forming characteristics were assessed. BMSCs were seeded at 10 million cells per cubic centimeter on cylindrical scaffolds composed of either collagen I sponge or esterified HA non-woven mesh. Chondrogenic differentiation was performed in a defined medium under hypoxia or normoxia for 14 days. Cultured constructs were assessed for gene expression, proteoglycan staining, glycosaminoglycan (GAG) quantity, and diameter change.

Results

Isolation/expansion under hypoxia resulted in faster BMSC population doublings per day (P <0.05), whereas cell and colony counts were not significantly different (P = 0.60 and 0.30, respectively). Collagen and HA scaffolds seeded with BMSCs that were isolated, expanded, and differentiated under hypoxia exhibited superior aggrecan and collagen II mRNA expressions (P <0.05), GAG quantity (P <0.05), and proteoglycan staining in comparison with normoxia. GAG/DNA was augmented with hypoxic isolation/expansion in all constructs (P <0.01). Comparison by scaffold composition indicated increased mRNA expressions of hyaline cartilage-associated collagen II, aggrecan, and SOX9 in collagen scaffolds, although expression of collagen X, which is related to hypertrophic cartilage, was also elevated (P <0.05). Proteoglycan deposition was not significantly improved in collagen scaffolds unless culture involved normoxic isolation/expansion followed by hypoxic differentiation. During chondrogenesis, collagen-based constructs progressively contracted to 60.1% ± 8.9% of the initial diameter after 14 days, whereas HA-based construct size was maintained (109.7% ± 4.2%).

Conclusions

Hypoxic isolation/expansion and differentiation enhance in vitro BMSC chondrogenesis within porous scaffolds. Although both collagen I and HA scaffolds support the creation of hyaline-like cartilaginous tissue, variations in gene expression, extracellular matrix formation, and construct size occur during chondrogenesis.

Electronic supplementary material

The online version of this article (doi:10.1186/s13287-015-0075-4) contains supplementary material, which is available to authorized users.

Harnessing biomechanics to develop cartilage regeneration strategies

As this review was prepared specifically for the American Society of Mechanical Engineers H.R. Lissner Medal, it primarily discusses work toward cartilage regeneration performed in Dr. Kyriacos A. Athanasiou's laboratory over the past 25 years. The prevalence and severity of degeneration of articular cartilage, a tissue whose main function is largely biomechanical, have motivated the development of cartilage tissue engineering approaches informed by biomechanics. This article provides a review of important steps toward regeneration of articular cartilage with suitable biomechanical properties. As a first step, biomechanical and biochemical characterization studies at the tissue level were used to provide design criteria for engineering neotissues. Extending this work to the single cell and subcellular levels has helped to develop biochemical and mechanical stimuli for tissue engineering studies. This strong mechanobiological foundation guided studies on regenerating hyaline articular cartilage, the knee meniscus, and temporomandibular joint (TMJ) fibrocartilage. Initial tissue engineering efforts centered on developing biodegradable scaffolds for cartilage regeneration. After many years of studying scaffold-based cartilage engineering, scaffoldless approaches were developed to address deficiencies of scaffold-based systems, resulting in the self-assembling process. This process was further improved by employing exogenous stimuli, such as hydrostatic pressure, growth factors, and matrix-modifying and catabolic agents, both singly and in synergistic combination to enhance neocartilage functional properties. Due to the high cell needs for tissue engineering and the limited supply of native articular chondrocytes, costochondral cells are emerging as a suitable cell source. Looking forward, additional cell sources are investigated to render these technologies more translatable. For example, dermis isolated adult stem (DIAS) cells show potential as a source of chondrogenic cells. The challenging problem of enhanced integration of engineered cartilage with native cartilage is approached with both familiar and novel methods, such as lysyl oxidase (LOX). These diverse tissue engineering strategies all aim to build upon thorough biomechanical characterizations to produce functional neotissue that ultimately will help combat the pressing problem of cartilage degeneration. As our prior research is reviewed, we look to establish new pathways to comprehensively and effectively address the complex problems of musculoskeletal cartilage regeneration.

Repair and tissue engineering techniques for articular cartilage

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

Digoxin and adenosine triphosphate enhance the functional properties of tissue-engineered cartilage

Toward developing engineered cartilage for the treatment of cartilage defects, achieving relevant functional properties before implantation remains a significant challenge. Various chemical and mechanical stimuli have been used to enhance the functional properties of engineered musculoskeletal tissues. Recently, Ca(2+)-modulating agents have been used to enhance matrix synthesis and biomechanical properties of engineered cartilage. The objective of this study was to determine whether other known Ca(2+) modulators, digoxin and adenosine triphosphate (ATP), can be employed as novel stimuli to increase collagen synthesis and functional properties of engineered cartilage. Neocartilage constructs were formed by scaffold-free self-assembling of primary bovine articular chondrocytes. Digoxin, ATP, or both agents were added to the culture medium for 1 h/day on days 10-14. After 4 weeks of culture, neocartilage properties were assessed for gross morphology, biochemical composition, and biomechanical properties. Digoxin and ATP were found to increase neocartilage collagen content by 52-110% over untreated controls, while maintaining proteoglycan content near native tissue values. Furthermore, digoxin and ATP increased the tensile modulus by 280% and 180%, respectively, while the application of both agents increased the modulus by 380%. The trends in tensile properties were found to correlate with the amount of collagen cross-linking. Live Ca(2+) imaging experiments revealed that both digoxin and ATP were able to increase Ca(2+) oscillations in monolayer-cultured chondrocytes. This study provides a novel approach toward directing neocartilage maturation and enhancing its functional properties using novel Ca(2+) modulators.

Emergence of scaffold-free approaches for tissue engineering musculoskeletal cartilages

This review explores scaffold-free methods as an additional paradigm for tissue engineering. Musculoskeletal cartilages-for example articular cartilage, meniscus, temporomandibular joint disc, and intervertebral disc-are characterized by low vascularity and cellularity, and are amenable to scaffold-free tissue engineering approaches. Scaffold-free approaches, particularly the self-assembling process, mimic elements of developmental processes underlying these tissues. Discussed are various scaffold-free approaches for musculoskeletal cartilage tissue engineering, such as cell sheet engineering, aggregation, and the self-assembling process, as well as the availability and variety of cells used. Immunological considerations are of particular importance as engineered tissues are frequently of allogeneic, if not xenogeneic, origin. Factors that enhance the matrix production and mechanical properties of these engineered cartilages are also reviewed, as the fabrication of biomimetically suitable tissues is necessary to replicate function and ensure graft survival in vivo. The concept of combining scaffold-free and scaffold-based tissue engineering methods to address clinical needs is also discussed. Inasmuch as scaffold-based musculoskeletal tissue engineering approaches have been employed as a paradigm to generate engineered cartilages with appropriate functional properties, scaffold-free approaches are emerging as promising elements of a translational pathway not only for musculoskeletal cartilages but for other tissues as well.

Promoting increased mechanical properties of tissue engineered neocartilage via the application of hyperosmolarity and 4α-phorbol 12,13-didecanoate (4αPDD)

Osteoarthritis, a degenerative disease of the load-bearing joints, greatly reduces quality of life for millions of Americans and places a tremendous cost on the American healthcare system. Due to limitations of current treatments, tissue engineering of articular cartilage may provide a promising therapeutic option to treat cartilage defects. However, cartilage tissue engineering has yet to recapitulate the functional properties of native tissue. During normal joint loading, cartilage tissue experiences variations in osmolarity and subsequent changes in ionic concentrations. Motivated by these known variations in the cellular microenvironment, this study sought to improve the mechanical properties of neocartilage constructs via the application of hyperosmolarity and transient receptor potential vanilloid 4 (TRPV4) channel activator 4α-phorbol 12,13-didecanoate (4αPDD). It was shown that 4αPDD elicited significant increases in compressive properties. Importantly, when combined, 4αPDD positively interacted with hyperosmolarity to modulate its effects on tensile stiffness and collagen content. Thus, this study supports 4αPDD-activated channel TRPV4 as a purported mechanosensor and osmosensor that can facilitate the cell and tissue level responses to improve the mechanical properties of engineered cartilage. To our knowledge, this study is the first to systematically evaluate the roles of hyperosmolarity and 4αPDD on the functional (i.e., mechanical and biochemical) properties of self-assembled neotissue. Future work may combine 4αPDD-induced channel activation with other chemical and mechanical stimuli to create robust neocartilages suitable for treatment of articular cartilage defects.

Topographic variations in biomechanical and biochemical properties in the ankle joint: an in vitro bovine study evaluating native and engineered cartilage

PURPOSE

The purposes of this study were to identify differences in the biomechanical and biochemical properties among the articulating surfaces of the ankle joint and to evaluate the functional and biological properties of engineered neocartilage generated using chondrocytes from different locations in the ankle joint.

METHODS

The properties of the different topographies within the ankle joint (tibial plafond, talar dome, and distal fibula) were evaluated in 28 specimens using 7 bovine ankles; the femoral condyle was used as a control. Chondrocytes from the same locations were used to form 28 neocartilage constructs by tissue engineering using an additional 7 bovine ankles. The functional properties of neocartilage were compared with native tissue values.

RESULTS

Articular cartilage from the tibial plafond, distal fibula, talar dome, and femoral condyle exhibited Young modulus values of 4.8 ± 0.5 MPa, 3.9 ± 0.1 MPa, 1.7 ± 0.2 MPa, and 4.0 ± 0.5 MPa, respectively. The compressive properties of the corresponding tissues were 370 ± 22 kPa, 242 ± 18 kPa, 255 ± 26 kPa, and 274 ± 18 kPa, respectively. The tibial plafond exhibited 3-fold higher tensile properties and 2-fold higher compressive and shear moduli compared with its articulating talar dome; the same disparity was observed in neocartilage. Similar trends were detected in biochemical data for both native and engineered tissues.

CONCLUSIONS

The cartilage properties of the various topographic locations within the ankle are significantly different. In particular, the opposing articulating surfaces of the ankle have significantly different biomechanical and biochemical properties. The disparity between tibial plafond and talar dome cartilage and chondrocytes warrants further evaluation in clinical studies to evaluate their exact role in the pathogenesis of ankle lesions.

CLINICAL RELEVANCE

Therapeutic modalities for cartilage lesions need to consider the exact topographic source of the cells or cartilage grafts used. Furthermore, the capacity of generating neocartilage implants from location-specific chondrocytes of the ankle joint may be used in the future as a tool for the treatment of chondral lesions.

Cartilage tissue engineering using dermis isolated adult stem cells: the use of hypoxia during expansion versus chondrogenic differentiation

Dermis isolated adult stem (DIAS) cells, a subpopulation of dermis cells capable of chondrogenic differentiation in the presence of cartilage extracellular matrix, are a promising source of autologous cells for tissue engineering. Hypoxia, through known mechanisms, has profound effects on in vitro chondrogenesis of mesenchymal stem cells and could be used to improve the expansion and differentiation processes for DIAS cells. The objective of this study was to build upon the mechanistic knowledge of hypoxia and translate it to tissue engineering applications to enhance chondrogenic differentiation of DIAS cells through exposure to hypoxic conditions (5% O₂) during expansion and/or differentiation. DIAS cells were isolated and expanded in hypoxic (5% O₂) or normoxic (20% O₂) conditions, then differentiated for 2 weeks in micromass culture on chondroitin sulfate-coated surfaces in both environments. Monolayer cells were examined for proliferation rate and colony forming efficiency. Micromasses were assessed for cellular, biochemical, and histological properties. Differentiation in hypoxic conditions following normoxic expansion increased per cell production of collagen type II 2.3 fold and glycosaminoglycans 1.2 fold relative to continuous normoxic culture (p<0.0001). Groups expanded in hypoxia produced 51% more collagen and 23% more GAGs than those expanded in normoxia (p<0.0001). Hypoxia also limited cell proliferation in monolayer and in 3D culture. Collectively, these data show hypoxic differentiation following normoxic expansion significantly enhances chondrogenic differentiation of DIAS cells, improving the potential utility of these cells for cartilage engineering.

Combined use of chondroitinase-ABC, TGF-β1, and collagen crosslinking agent lysyl oxidase to engineer functional neotissues for fibrocartilage repair

Patients suffering from damaged or diseased fibrocartilages currently have no effective long-term treatment options. Despite their potential, engineered tissues suffer from inferior biomechanical integrity and an inability to integrate in vivo. The present study identifies a treatment regimen (including the biophysical agent chondroitinase-ABC, the biochemical agent TGF-β1, and the collagen crosslinking agent lysyl oxidase) to prime highly cellularized, scaffold-free neofibrocartilage implants, effecting continued improvement in vivo. We show these agents drive in vitro neofibrocartilage matrix maturation toward synergistically enhanced Young's modulus and ultimate tensile strength values, which were increased 245% and 186%, respectively, over controls. Furthermore, an in vitro fibrocartilage defect model found this treatment regimen to significantly increase the integration tensile properties between treated neofibrocartilage and native tissue. Through translating this technology to an in vivo fibrocartilage defect model, our results indicate, for the first time, that a pre-treatment can prime neofibrocartilage for significantly enhanced integration potential in vivo, with interfacial tensile stiffness and strength increasing by 730% and 745%, respectively, compared to integration values achieved in vitro. Our results suggest that specifically targeting collagen assembly and organization is a powerful means to augment overall neotissue mechanics and integration potential toward improved clinical feasibility.