Region-specific constitutive gene expression in the adult porcine meniscus

Current advances in engineering meniscal tissues: insights into 3D printing, injectable hydrogels and physical stimulation based strategies

The knee meniscus is the cushioning fibro-cartilage tissue present in between the femoral condyles and tibial plateau of the knee joint. It is largely avascular in nature and suffers from a wide range of tears and injuries caused by accidents, trauma, active lifestyle of the populace and old age of individuals. Healing of the meniscus is especially difficult due to its avascularity and hence requires invasive arthroscopic approaches such as surgical resection, suturing or implantation. Though various tissue engineering approaches are proposed for the treatment of meniscus tears, three-dimensional (3D) printing/bioprinting, injectable hydrogels and physical stimulation involving modalities are gaining forefront in the past decade. A plethora of new printing approaches such as direct light photopolymerization and volumetric printing, injectable biomaterials loaded with growth factors and physical stimulation such as low-intensity ultrasound approaches are being added to the treatment portfolio along with the contemporary tear mitigation measures. This review discusses on the necessary design considerations, approaches for 3D modeling and design practices for meniscal tear treatments within the scope of tissue engineering and regeneration. Also, the suitable materials, cell sources, growth factors, fixation and lubrication strategies, mechanical stimulation approaches, 3D printing strategies and injectable hydrogels for meniscal tear management have been elaborated. We have also summarized potential technologies and the potential framework that could be the herald of the future of meniscus tissue engineering and repair approaches.

"Slow walk" mimetic tensile loading maintains human meniscus tissue resident progenitor cells homeostasis in photocrosslinked gelatin hydrogel

Meniscus, the cushion in knee joint, is a load-bearing tissue that transfers mechanical forces to extracellular matrix (ECM) and tissue resident cells. The mechanoresponse of human tissue resident stem/progenitor cells in meniscus (hMeSPCs) is significant to tissue homeostasis and regeneration but is not well understood. This study reports that a mild cyclic tensile loading regimen of ∼1800 loads/day on hMeSPCs seeded in 3-dimensional (3D) photocrosslinked gelatin methacryloyl (GelMA) hydrogel is critical in maintaining cellular homeostasis. Experimentally, a "slow walk" biomimetic cyclic loading regimen (10% tensile strain, 0.5 Hz, 1 h/day, up to 15 days) is applied to hMeSPCs encapsulated in GelMA hydrogel with a magnetic force-controlled loading actuator. The loading significantly increases cell differentiation and fibrocartilage-like ECM deposition without affecting cell viability. Transcriptomic analysis reveals 332 mechanoresponsive genes, clustered into cell senescence, mechanical sensitivity, and ECM dynamics, associated with interleukins, integrins, and collagens/matrix metalloproteinase pathways. The cell-GelMA constructs show active ECM remodeling, traced using a green fluorescence tagged (GFT)-GelMA hydrogel. Loading enhances nascent pericellular matrix production by the encapsulated hMeSPCs, which gradually compensates for the hydrogel loss in the cultures. These findings demonstrate the strong tissue-forming ability of hMeSPCs, and the importance of mechanical factors in maintaining meniscus homeostasis.

The transplantation of particulated juvenile allograft cartilage and synovium for the repair of meniscal defect in a lapine model

Background

Synovium has been confirmed to be the primary contributor to meniscal repair. Particulated Juvenile Allograft Cartilage (PJAC) has demonstrated promising clinical effect on repairing cartilage. The synergistic effect of synovium and PJAC transplant on meniscal fibrocartilaginous repair is unclear. We hypothesize that the transplantation of synovium and PJAC synergistically facilitates meniscal regeneration and the donor cells within graft tissues still survive in the regenerated tissue at the last follow up (16 weeks postoperatively).

Methods

The study included 24 mature female rabbits, which were randomly divided into experimental and control groups. A cylindrical full-thickness defect measuring 2.0 ​mm was prepared in the avascular portion of the anterior horn of medial meniscus in both knees. The synovium and PJAC transplant were harvested from juvenile male rabbits (2 months after birth). The experimental group received synovium and PJAC transplant encapsulated with fibrin gel. The control groups received synovium transplant encapsulated with fibrin gel, pure fibrin gel and nothing. The macroscopic, imageological and histological evaluations of repaired tissue were performed at 8 weeks and 16 weeks postoperatively. The in situ hybridization (ISH) of male-specific sex-determining region Y-linked (SRY) gene was performed to detect the transplanted cells.

Results

The regenerated tissue in experimental group showed superior structural integrity, superficial smoothness, and marginal integration compared to control groups at 8 weeks or 16 weeks postoperatively. More meniscus-like fibrochondrocytes filled the repaired tissue in the experimental group, and the matrix surrounding these cell clusters demonstrated strongly positive safranin O and type 2 collagen immunohistochemistry staining. By SRY gene ISH, the positive SRY signal of experimental group could be detected at 8 weeks (75.72%, median) and 16 weeks (48.69%, median). The expression of SOX9 in experimental group was the most robust, with median positive rates of 65.52% at 8 weeks and 67.55% at 16 weeks.

Conclusion

The transplantation of synovium and PJAC synergistically facilitates meniscal regeneration. The donor cells survive for at least 16 weeks in the recipient.

The translational potential of this article

This study highlighted the positive effect of PJAC and synovium transplant on meniscal repair. We also clarified the potential repair mechanisms reflected by the survival of donor cells and upregulated expression of meniscal fibrochondrocytes related genes. Thus, based on our study, further clinical experiments are needed to investigate synovium and PJAC transplant as a possible treatment to meniscal defects.

A Tale of Two Loads: Modulation of IL-1 Induced Inflammatory Responses of Meniscal Cells in Two Models of Dynamic Physiologic Loading

Meniscus injuries are highly prevalent, and both meniscus injury and subsequent surgery are linked to the development of post-traumatic osteoarthritis (PTOA). Although the pathogenesis of PTOA remains poorly understood, the inflammatory cytokine IL-1 is elevated in synovial fluid following acute knee injuries and causes degradation of meniscus tissue and inhibits meniscus repair. Dynamic mechanical compression of meniscus tissue improves integrative meniscus repair in the presence of IL-1 and dynamic tensile strain modulates the response of meniscus cells to IL-1. Despite the promising observed effects of physiologic mechanical loading on suppressing inflammatory responses of meniscus cells, there is a lack of knowledge on the global effects of loading on meniscus transcriptomic profiles. In this study, we compared two established models of physiologic mechanical stimulation, dynamic compression of tissue explants and cyclic tensile stretch of isolated meniscus cells, to identify conserved responses to mechanical loading. RNA sequencing was performed on loaded and unloaded meniscus tissue or isolated cells from inner and outer zones, with and without IL-1. Overall, results from both models showed significant modulation of inflammation-related pathways with mechanical stimulation. Anti-inflammatory effects of loading were well-conserved between the tissue compression and cell stretch models for inner zone; however, the cell stretch model resulted in a larger number of differentially regulated genes. Our findings on the global transcriptomic profiles of two models of mechanical stimulation lay the groundwork for future mechanistic studies of meniscus mechanotransduction, which may lead to the discovery of novel therapeutic targets for the treatment of meniscus injuries.

Regional-specific meniscal extracellular matrix hydrogels and their effects on cell-matrix interactions of fibrochondrocytes

Decellularized meniscal extracellular matrix (ECM) material holds great potential for meniscus repair and regeneration. Particularly, injectable ECM hydrogel is highly desirable for the minimally invasive treatment of irregularly shaped defects. Although regional-specific variations of the meniscus are well documented, no ECM hydrogel has been reported to simulate zonally specific microenvironments of the native meniscus. To fill the gap, different (outer, middle, and inner) zones of porcine menisci were separately decellularized. Then the regionally decellularized meniscal ECMs were solubilized by pepsin digestion, neutralized, and then form injectable hydrogels. The hydrogels were characterized in gelation behaviors and mechanical properties and seeded with bovine fibrochondrocytes to evaluate the regionally biochemical effects on the cell-matrix interactions. Our results showed that the decellularized inner meniscal ECM (IM) contained the greatest glycosaminoglycan (GAG) content and the least collagen content compared with the decellularized outer meniscal ECM (OM) and middle meniscal ECM (MM). The IM hydrogel showed lower compressive strength than the OM hydrogel. When encapsulated with fibrochondrocytes, the IM hydrogel accumulated more GAG, contracted to a greater extent and reached higher compressive strength than that of the OM hydrogel at 28 days. Our findings demonstrate that the regionally specific meniscal ECMs present biochemical variation and show various effects on the cell behaviors, thus providing information on how meniscal ECM hydrogels may be utilized to reconstruct the microenvironments of the native meniscus.

Meniscus cell regional phenotypes: Dedifferentiation and reversal by biomaterial embedding

Meniscus injuries are common and a major cause of long-term joint degeneration and disability. Current treatment options are limited, so novel regenerative therapies or tissue engineering strategies are urgently needed. The development of new therapies is hindered by a lack of knowledge regarding the cellular biology of the meniscus and a lack of well-established methods for studying meniscus cells in vitro. The goals of this study were to (1) establish baseline expression profiles and dedifferentiation patterns of inner and outer zone primary meniscus cells, and (2) evaluate the utility of poly(ethylene glycol) diacrylate (PEGDA) and gelatin methacrylate (GelMA) polymer hydrogels to reverse dedifferentiation trends for long-term meniscus cell culture. Using reverse transcription-quantitative polymerase chain reaction, we measured expression levels of putative meniscus phenotype marker genes in freshly isolated meniscus tissue, tissue explant culture, and monolayer culture of inner and outer zone meniscus cells from porcine knees to establish baseline dedifferentiation characteristics, and then compared these expression levels to PEGDA/GelMA embedded passaged meniscus cells. COL1A1 showed robust upregulation, while CHAD, CILP, and COMP showed downregulation with monolayer culture. Expression levels of COL2A1, ACAN, and SOX9 were surprisingly similar between inner and outer zone tissue and were found to be less sensitive as markers of dedifferentiation. When embedded in PEGDA/GelMA hydrogels, expression levels of meniscus cell phenotype genes were significantly modulated by varying the ratio of polymer components, allowing these materials to be tuned for phenotype restoration, meniscus cell culture, and tissue engineering applications.

Nuclear envelope wrinkling predicts mesenchymal progenitor cell mechano-response in 2D and 3D microenvironments

Exogenous mechanical cues are transmitted from the extracellular matrix to the nuclear envelope (NE), where mechanical stress on the NE mediates shuttling of transcription factors and other signaling cascades that dictate downstream cellular behavior and fate decisions. To systematically study how nuclear morphology can change across various physiologic microenvironmental contexts, we cultured mesenchymal progenitor cells (MSCs) in engineered 2D and 3D hyaluronic acid hydrogel systems. Across multiple contexts we observed highly 'wrinkled' nuclear envelopes, and subsequently developed a quantitative single-cell imaging metric to better evaluate how wrinkles in the nuclear envelope relate to progenitor cell mechanotransduction. We determined that in soft 2D environments the NE is predominately wrinkled, and that increases in cellular mechanosensing (indicated by cellular spreading, adhesion complex growth, and nuclear localization of YAP/TAZ) occurred only in absence of nuclear envelope wrinkling. Conversely, in 3D hydrogel and tissue contexts, we found NE wrinkling occurred along with increased YAP/TAZ nuclear localization. We further determined that these NE wrinkles in 3D were largely generated by actin impingement, and compared to other nuclear morphometrics, the degree of nuclear wrinkling showed the greatest correlation with nuclear YAP/TAZ localization. These findings suggest that the degree of nuclear envelope wrinkling can predict mechanotransduction state in mesenchymal progenitor cells and highlights the differential mechanisms of NE stress generation operative in 2D and 3D microenvironmental contexts.

Characterization and Comparison of Postnatal Rat Meniscus Stem Cells at Different Developmental Stages

Meniscus‐derived stem cells (MeSCs) are a potential cell source for meniscus tissue engineering. The stark morphological and structural changes of meniscus tissue during development indicate the complexity of MeSCs at different tissue regions and stages of development. In this study, we characterized and compared postnatal rat meniscus tissue and MeSCs at different tissue regions and stages of development. We observed that the rat meniscus tissue exhibited marked changes in tissue morphology during development, with day 7 being the most representative time point of different developmental stages. All rat MeSCs displayed typical stem cell characteristics. Rat MeSCs derived from day 7 inner meniscus tissue exhibited the highest self‐renewal capacity, cell proliferation, differentiation potential toward various mesenchymal lineage and the highest expression levels of chondrogenic genes and proteins. Transplantation of rat MeSCs derived from day 7 inner meniscus tissue promoted neo‐tissue formation and effectively protected joint surface cartilage in vivo. Our results demonstrated for the first time that rat MeSCs are not necessarily better at earlier developmental stages, and that rat MeSCs derived from day 7 inner meniscus tissue may be a superior cell source for effective meniscus regeneration and articular cartilage protection. This information could make a significant contribution to human meniscus tissue engineering in the future. stem cells translational medicine2019;8:1318&1329

(A): Meniscus tissue at different tissue regions and stages of development. (B): MeSCs at different tissue regions and stages of development. (C): Intra‐articular injection of MeSCs for meniscus regeneration and OA suppression. *Significant difference between two groups at p < .05. **Significant difference between two groups at p < .01. ***Significant difference between two groups at p < .001. ****Significant difference between two groups at p < .0001. N.S., No significant difference between two groups at p ≥ .05.

Metabolic responses of meniscal tissue to focal collagenase degeneration

Purpose: The objective of this study was to determine the responses of normal meniscus to collagenase activity. It was hypothesized that meniscal explants exposed to collagenase would significantly increase release of pro-inflammatory cytokines and degradative enzymes, in a dose-dependent manner, compared to control.Methods: Menisci were harvested from adult dogs (n = 6) euthanized for reasons unrelated to this study. Meniscal explants were created from the central portion of lateral and medial meniscus. Explants were injected with 100 µl collagenase at a concentration of 50 µg/ml, 5 µg/ml, or 0 µg/ml of collagenase. Explants were cultured for 12 days, and media were changed and collected every 3 days for biomarker analyses. Differences among collagenase concentrations were determined by a three factor ANOVA with adjustment for multiple comparisons, with pre-adjustment statistical significance set at p < 0.05.Results: When data from all explants were compared, the 50 µg group released significantly higher IL-6 and PGE2, and the 5 µg group released significantly higher levels of MMP-3 and CTX-II compared to the 0 µg group. Explants from the medial meniscus released significantly more MMP-1, MMP-2, MMP-3, and MMP-13 in response to stimulation with 5 µg/ml of collagenase compared to explants from the lateral meniscus.Discussion: The data from this study indicate that in response to localized degradative enzyme activity, the meniscus increases the release of pro-inflammatory and degradative biomarkers in a dose-dependent manner. Further, these data indicate potential differences in metabolic responses of lateral versus medial menisci to collagenase insult.

Postnatal deletion of Alk5 gene in meniscal cartilage accelerates age-dependent meniscal degeneration in mice

Activation of transforming growth factor-β (TGF-β) signaling has been used to enhance healing of meniscal degeneration in several models. However, the exact role and molecular mechanism of TGF-β signaling in meniscus maintenance and degeneration are still not understood due to the absence of in vivo evidence. In this study, we found that the expression of activin receptor-like kinases 5 (ALK5) in the meniscus was decreased with the progression of age and/or osteoarthritis induced meniscal degeneration. Col2α1 positive cells were found to be specifically distributed in the superficial and inner zones of the anterior horn, as well as the inner zone of the posterior horn in mice, indicating that Col2α1-CreER^T2 mice can be a used for studying gene function in menisci. Furthermore, we deleted Alk5 in Col2α1 positive cells in meniscus by administering tamoxifen. Alterations in the menisci structure were evaluated histologically. The expression levels of genes and proteins associated with meniscus homeostasis and TGF-β signaling were analyzed by quantitative real-time PCR analysis (qRT-PCR) and immunohistochemistry (IHC). Our results revealed severe and progressive meniscal degeneration phenotype in 3- and 6-month-old Alk5 cKO mice compared with Cre-negative control, including aberrantly increased hypertrophic meniscal cells, severe fibrillation, and structure disruption of meniscus. qRT-PCR and IHC results showed that disruption of anabolic and catabolic homeostasis of chondrocytes may contribute to the meniscal degeneration phenotype observed in Alk5 cKO mice. Thus, TGF-β/ALK5 signaling plays a chondro-protective role in menisci homeostasis, in part, by inhibiting matrix degradation and maintaining extracellular matrix proteins levels in meniscal tissues.

Arthroprotective Effects of Cf-02 Sharing Structural Similarity with Quercetin

In this study, we synthesized hundreds of analogues based on the structure of small-molecule inhibitors (SMIs) that were previously identified in our laboratory with the aim of identifying potent yet safe compounds for arthritis therapeutics. One of the analogues was shown to share structural similarity with quercetin, a potent anti-inflammatory flavonoid present in many different fruits and vegetables. We investigated the immunomodulatory effects of this compound, namely 6-(2,4-difluorophenyl)-3-(3-(trifluoromethyl)phenyl)-2H-benzo[e][1,3]oxazine-2,4(3H)-dione (Cf-02), in a side-by-side comparison with quercetin. Chondrocytes were isolated from pig joints or the joints of patients with osteoarthritis that had undergone total knee replacement surgery. Several measures were used to assess the immunomodulatory potency of these compounds in tumor necrosis factor (TNF-α)-stimulated chondrocytes. Characterization included the protein and mRNA levels of molecules associated with arthritis pathogenesis as well as the inducible nitric oxide synthase (iNOS)–nitric oxide (NO) system and matrix metalloproteinases (MMPs) in cultured chondrocytes and proteoglycan, and aggrecan degradation in cartilage explants. We also examined the activation of several important transcription factors, including nuclear factor-kappaB (NF-κB), interferon regulatory factor-1 (IRF-1), signal transducer and activator of transcription-3 (STAT-3), and activator protein-1 (AP-1). Our overall results indicate that the immunomodulatory potency of Cf-02 is fifty-fold more efficient than that of quercetin without any indication of cytotoxicity. When tested in vivo using the induced edema method, Cf-02 was shown to suppress inflammation and cartilage damage. The proposed method shows considerable promise for the identification of candidate disease-modifying immunomodulatory drugs and leads compounds for arthritis therapeutics.

Chondroprotective Effects and Mechanisms of Dextromethorphan: Repurposing Antitussive Medication for Osteoarthritis Treatment

Osteoarthritis (OA) is the most common joint disorder and primarily affects older people. The ideal anti-OA drug should have a modest anti-inflammatory effect and only limited or no toxicity for long-term use. Because the antitussive medication dextromethorphan (DXM) is protective in atherosclerosis and neurological diseases, two common disorders in aged people, we examined whether DXM can be protective in pro-inflammatory cytokine-stimulated chondrocytes and in a collagen-induced arthritis (CIA) animal model in this study. Chondrocytes were prepared from cartilage specimens taken from pigs or OA patients. Western blotting, quantitative PCR, and immunohistochemistry were adopted to measure the expression of collagen II (Col II) and matrix metalloproteinases (MMP). DXM significantly restored tumor necrosis factor-alpha (TNF-α)-mediated reduction of collagen II and decreased TNF-α-induced MMP-13 production. To inhibit the synthesis of MMP-13, DXM blocked TNF-α downstream signaling, including I kappa B kinase (IKK)α/β-IκBα-nuclear factor-kappaB (NF-κB) and c-Jun N-terminal kinase (JNK)-activator protein-1 (AP-1) activation. Besides this, DXM protected the CIA mice from severe inflammation and cartilage destruction. DXM seemed to protect cartilage from inflammation-mediated matrix degradation, which is an irreversible status in the disease progression of osteoarthritis. The results suggested that testing DXM as an osteoarthritis therapeutic should be a focus in further research.

Physiological concentrations of soluble uric acid are chondroprotective and anti-inflammatory

High uric acid levels are a risk factor for cardiovascular disorders and gout; however, the role of physiological concentrations of soluble uric acid (sUA) is poorly understood. This study aimed to clarify the effects of sUA in joint inflammation. Both cell cultures of primary porcine chondrocytes and mice with collagen-induced arthritis (CIA) were examined. We showed that sUA inhibited TNF-α- and interleukin (IL)-1β–induced inducible nitric oxide synthase, cyclooxygenase-2 and matrix metalloproteinase (MMP)-13 expression. Examination of the mRNA expression of several MMPs and aggrecanases confirmed that sUA exerts chondroprotective effects by inhibiting the activity of many chondro-destructive enzymes. These effects attenuated collagen II loss in chondrocytes and reduced proteoglycan degradation in cartilage explants. These results were reproduced in chondrocytes cultured in three-dimensional (3-D) alginate beads. Molecular studies revealed that sUA inhibited the ERK/AP-1 signalling pathway, but not the IκBα-NF-κB signalling pathway. Increases in plasma uric acid levels facilitated by the provision of oxonic acid, a uricase inhibitor, to CIA mice exerted both anti-inflammatory and arthroprotective effects in these animals, as demonstrated by their arthritis severity scores and immunohistochemical analysis results. Our study demonstrated that physiological concentrations of sUA displayed anti-inflammatory and chondroprotective effects both in vitro and in vivo.

Meniscus, articular cartilage and nucleus pulposus: a comparative review of cartilage-like tissues in anatomy, development and function

The degradation of cartilage in the human body is impacted by aging, disease, genetic predisposition and continued insults resulting from daily activity. The burden of cartilage defects (osteoarthritis, rheumatoid arthritis, intervertebral disc damage, knee replacement surgeries, etc.) is daunting in light of substantial economic and social stresses. This review strives to broaden the scope of regenerative medicine and tissue engineering approaches used for cartilage repair by comparing and contrasting the anatomical and functional nature of the meniscus, articular cartilage (AC) and nucleus pulposus (NP). Many review papers have provided detailed evaluations of these cartilages and cartilage-like tissues individually but none have comprehensively examined the parallels and inconsistencies in signaling, genetic expression and extracellular matrix composition between tissues. For the first time, this review outlines the importance of understanding these three tissues as unique entities, providing a comparative analysis of anatomy, ultrastructure, biochemistry and function for each tissue. This novel approach highlights the similarities and differences between tissues, progressing research toward an understanding of what defines each tissue as distinctive. The goal of this paper is to provide researchers with the fundamental knowledge to correctly engineer the meniscus, AC and NP without inadvertently developing the wrong tissue function or biochemistry.

Anatomical region-dependent enhancement of 3-dimensional chondrogenic differentiation of human mesenchymal stem cells by soluble meniscus extracellular matrix

Extracellular matrix (ECM) derived from decellularized tissues has been found to promote tissue neogenesis, most likely mediated by specific biochemical and physical signaling motifs that promote tissue-specific differentiation of progenitor cells. Decellularized ECM has been suggested to be efficacious for the repair of tissue injuries. However, decellularized meniscus contains a dense collagenous structure, which impedes cell seeding and infiltration and is not readily applicable for meniscus repair. In addition, the meniscus consists of two distinct anatomical regions that differ in vascularity and cellular phenotype. The purpose of this study was to explore the region-specific bioactivity of solubilized ECM derived from the inner and outer meniscal regions as determined in 2D and 3D cultures of adult mesenchymal stem cells (MSCs). When added as a medium supplement to 2D cultures of MSCs, urea-extracted fractions of the inner (imECM) and outer meniscal ECM (omECM) enhanced cell proliferation while imECM most strongly upregulated fibrochondrogenic differentiation on the basis of gene expression profiles. When added to 3D cultures of MSCs seeded in photocrosslinked methacrylated gelatin (GelMA) hydrogels, both ECM fractions upregulated chondrogenic differentiation as determined by gene expression and protein analyses, as well as elevated sulfated glycosaminoglycan sGAG content, compared to ECM-free controls. The chondrogenic effect at day 21 was most pronounced with imECM supplementation, but equivalent between ECM groups by day 42. Despite increased cartilage matrix, imECM and omECM constructs possessed compressive moduli similar to controls. In conclusion, soluble meniscal ECM may be considered for use as a tissue-specific reagent to enhance chondrogenesis for MSC-based 3D cartilage tissue engineering.

STATEMENT OF SIGNIFICANCE

The inner region of the knee meniscus is frequently injured and possesses a poor intrinsic healing capacity. Solubilized extracellular matrix (ECM) derived from decellularized meniscus tissue may promote homologous differentiation of progenitor cells, thereby enhancing fibrocartilage formation within a meniscal lesion. However, the meniscus possesses regional variation in ultrastructure, biochemical composition, and cell phenotype, which may affect the bioactivity of soluble ECM derived from different regions of decellularized menisci. In this study, we demonstrate that urea-extracted fractions of ECM derived from the inner and outer regions of menisci enhance chondrogenesis in mesenchymal stem cells seeded in 3-dimensional photocrosslinkable hydrogels and that this effect is more strongly mediated by inner meniscal ECM. These findings suggest region-specific bioactivity of decellularized meniscal ECM.

Rapidly dissociated autologous meniscus tissue enhances meniscus healing: An in vitro study

PURPOSE

Treatment of meniscus tears is a persistent challenge in orthopedics. Although cell therapies have shown promise in promoting fibrocartilage formation in in vitro and preclinical studies, clinical application has been limited by the paucity of autologous tissue and the need for ex vivo cell expansion. Rapid dissociation of the free edges of the anterior and posterior meniscus with subsequent implantation in a meniscus lesion may overcome these limitations. The purpose of this study was to explore the effect of rapidly dissociated meniscus tissue in enhancing neotissue formation in a radial meniscus tear, as simulated in an in vitro explant model.

MATERIALS AND METHODS

All experiments in this study, performed at minimum with biological triplicates, utilized meniscal tissues from hind limbs of young cows. The effect of varying collagenase concentration (0.1%, 0.2% and 0.5% w/v) and treatment duration (overnight and 30 minutes) on meniscus cell viability, organization of the extracellular matrix (ECM), and gene expression was assessed through a cell metabolism assay, microscopic examination, and quantitative real-time reverse transcription polymerase chain reaction analysis, respectively. Thereafter, an explant model of a radial meniscus tear was used to evaluate the effect of a fibrin gel seeded with one of the following: (1) fibrin alone, (2) isolated and passaged (P2) meniscus cells, (3) overnight digested tissue, and (4) rapidly dissociated tissue. The quality of in vitro healing was determined through histological analysis and derivation of an adhesion index.

RESULTS

Rapid dissociation in 0.2% collagenase yielded cells with higher levels of metabolism than either 0.1% or 0.5% collagenase. When seeded in a three-dimensional fibrin hydrogel, both overnight digested and rapidly dissociated cells expressed greater levels of collagens type I and II than P2 meniscal cells at 1 week. At 4 and 8 weeks, collagen type II expression remained elevated only in the rapid dissociation group. Histological examination revealed enhanced healing in all cell-seeded treatment groups over cell-free fibrin controls at weeks 1, 4, and 8, but there were no significant differences across the treatment groups.

CONCLUSIONS

Rapid dissociation of meniscus tissue may provide a single-step approach to augment regenerative healing of meniscus repairs.

Platelet-Rich Plasma Increases the Levels of Catabolic Molecules and Cellular Dedifferentiation in the Meniscus of a Rabbit Model

Despite the susceptibility to frequent intrinsic and extrinsic injuries, especially in the inner zone, the meniscus does not heal spontaneously owing to its poor vascularity. In this study, the effect of platelet-rich plasma (PRP), containing various growth factors, on meniscal mechanisms was examined under normal and post-traumatic inflammatory conditions. Isolated primary meniscal cells of New Zealand white (NZW) rabbits were incubated for 3, 10, 14 and 21 days with PRP(−), 10% PRP (PRP(+)), IL(+) or IL(+)PRP(+). The meniscal cells were collected and examined using reverse-transcription polymerase chain reaction (RT-PCR). Culture media were examined by immunoblot analyses for matrix metalloproteinases (MMP) catabolic molecules. PRP containing growth factors improved the cellular viability of meniscal cells in a concentration-dependent manner at Days 1, 4 and 7. However, based on RT-PCR, meniscal cells demonstrated dedifferentiation, along with an increase in type I collagen in the PRP(+) and in IL(+)PRP(+). In PRP(+), the aggrecan expression levels were lower than in the PRP(−) until Day 21. The protein levels of MMP-1 and MMP-3 were higher in each PRP group, i.e., PRP(+) and IL(+)PRP(+), at each culture time. A reproducible 2-mm circular defect on the meniscus of NZW rabbit was used to implant fibrin glue (control) or PRP in vivo. After eight weeks, the lesions in the control and PRP groups were occupied with fibrous tissue, but not with meniscal cells. This study shows that PRP treatment of the meniscus results in an increase of catabolic molecules, especially those related to IL-1α-induced inflammation, and that PRP treatment for an in vivo meniscus injury accelerates fibrosis, instead of meniscal cartilage.

Quantitative tracking of passage and 3D culture effects on chondrocyte and fibrochondrocyte gene expression

Success in cartilage and fibrocartilage tissue engineering relies heavily on using an appropriate cell source. Many different cell sources have been identified, including primary and stem cells, along with experimental strategies to obtain the required number of cells or to induce chondrogenesis. However, no definitive method exists to quantitatively evaluate the similarity of the resulting cell phenotypes to those of the native cells between candidate strategies. In this study, we develop an integrative approach to enable such evaluations by deriving, from gene expression profiles, two quantitative metrics representing the nearest location within the range of native cell phenotypes and the deviation from it. As an example application to evaluating potential cell sources for cartilage or meniscus tissue engineering, we examine phenotypic changes of juvenile and adult articular chondrocytes and fibrochondrocytes across multiple passages and subsequent 3D culture. A substantial change was observed in cell phenotype due to the isolation process itself, followed by a clear progression toward the outer meniscal cell phenotype with passage. The new metrics also indicated that 3D culture moderately reduced the passage-induced deviation from the native meniscal phenotypes for juvenile chondrocytes and adult fibrochondrocytes, which was not obvious through examination of individual gene expressions. However, brief 3D culture alone did not move any of the cells towards an inner meniscal phenotype, the most relevant target for meniscal tissue engineering. This integrative approach of examining and combining multiple gene expressions can be used to evaluate various other tissue-engineering strategies to direct cells toward the desired phenotype. Copyright © 2015 John Wiley & Sons, Ltd.

Regional comparisons of porcine menisci

The purpose of this study was to analyze histologic, biochemical, and biomechanical differences between zonal, regional, and anatomic locations of porcine menisci. We evaluated six menisci removed from pigs. Medial and lateral menisci were divided into three regions: anterior, middle, and posterior. In each portion, the central zone (CZ) and peripheral zone (PZ) were examined histologically (hematoxylin & eosin, safranin O/Fast green, and picrosiriusred staining), using scanning electron microscopy, biochemically (hydroxyproline assay for collagen content and dimethylmethylene blue assay for glycosaminoglycan [GAG] content), and biomechanically (compression testing). Collagen content in the CZ was lower than that in the PZ. GAG content in the CZ was higher than that in the PZ. GAG content in the PZ of the posterior portion was significantly higher than that in the anterior and middle portions. Compression strength in the CZ was higher than that in the PZ. The differences in cellular phenotype, vascular penetration, and ECM not only between CZ and PZ but also among the anterior, middle, and posterior portions were clarified in the immature porcine meniscus. This result helps further our understanding of the biological characteristic of the meniscus. © 2014 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 32:1602-1611, 2014.

Relationship of gene expression in the injured human meniscus to body mass index: a biologic connection between obesity and osteoarthritis

OBJECTIVE

Higher body mass index (BMI) increases the risk of meniscus injury and knee osteoarthritis (OA). However, it is unknown if and how obesity affects meniscus biology. We analyzed transcriptome-wide gene expression profiles of injured human menisci to test the hypothesis that meniscal gene expression signatures relate to patient BMI.

METHODS

Meniscus samples were obtained from patients undergoing arthroscopic partial meniscectomy. Transcriptome-wide analysis of gene expression followed by validation of selected transcripts by QuantiGene Plex assay was performed. Correlations of gene expression with BMI and relative fold changes (≥1.5-fold) in 3 BMI categories (lean [BMI 18.5-24.9 kg/m(2) ], overweight [BMI 25.0-29.9 kg/m(2) ], and obese [BMI >30.0 kg/m(2) ]) were analyzed, and integrated functional classifications were probed computationally.

RESULTS

The obese versus overweight comparison resulted in the largest set of differences (565 transcripts) followed by obese versus lean (280 transcripts) and overweight versus lean (125 transcripts). Biologic reproducibility was confirmed by cluster analysis of expressed transcripts. Differentially regulated transcripts represented important functional classifications. Transcripts associated with oxygen transport, calcium ion binding, and cell homeostasis were elevated with BMI, while those related to extracellular matrix deposition, cell migration, and glucosamine metabolic processes were repressed. While these functional classifications may play key roles in cartilage/meniscus homeostasis, failure of extracellular matrix deposition and increase in calcium ion binding likely contribute to OA development following meniscal injury.

CONCLUSION

Our results indicate greater differences in gene expression between obese and overweight groups than between overweight and lean groups. This may indicate that there is a weight threshold at which injured meniscus responds severely to increased BMI. BMI-related changes in gene expression present a plausible explanation for the role of meniscal injury in OA development among obese patients.

Meniscus maturation in the swine model: changes occurring along with anterior to posterior and medial to lateral aspect during growth

The meniscus plays important roles in knee function and mechanics and is characterized by a heterogeneous matrix composition. The changes in meniscus vascularization observed during growth suggest that the tissue-specific composition may be the result of a maturation process. This study has the aim to characterize the structural and biochemical variations that occur in the swine meniscus with age. To this purpose, menisci were collected from young and adult pigs and divided into different zones. In study 1, both lateral and medial menisci were divided into the anterior horn, the body and the posterior horn for the evaluation of glycosaminoglycans (GAGs), collagen 1 and 2 content. In study 2, the menisci were sectioned into the inner, the intermediate and the outer zones to determine the variations in the cell phenotype along with the inner–outer direction, through gene expression analysis. According to the results, the swine meniscus is characterized by an increasing enrichment in the cartilaginous component with age, with an increasing deposition in the anterior horn (GAGs and collagen 2; P < 0.01 both); moreover, this cartilaginous matrix strongly increases in the inner avascular and intermediate zone, as a consequence of a specific differentiation of meniscal cells towards a cartilaginous phenotype (collagen 2, P < 0.01). The obtained data add new information on the changes that accompany meniscus maturation, suggesting a specific response of meniscal cells to the regional mechanical stimuli in the knee joint.

Effect of open wedge high tibial osteotomy on the lateral compartment in sheep. Part I: Analysis of the lateral meniscus

PURPOSE

To evaluate whether medial open wedge high tibial osteotomy (HTO) results in structural and biochemical changes in the lateral meniscus in adult sheep.

METHODS

Three experimental groups with biplanar osteotomies of the right proximal tibiae were tested: (a) closing wedge HTO resulting in 4.5° of tibial varus, (b) open wedge HTO resulting in 4.5° of tibial valgus (standard correction) and (c) open wedge HTO resulting in 9.5° of valgus (overcorrection), each of which was compared to the contralateral knees with normal limb axes. After 6 months, the lateral menisci were macroscopically and microscopically evaluated. The proteoglycan and DNA contents of the red-red and white-white zones of the anterior, middle and posterior third were determined.

RESULTS

Semiquantitative macroscopic and microscopic grading revealed no structural differences between groups. The red-red zone of the middle third of the lateral menisci of animals that underwent overcorrection exhibited a significant 0.7-fold decrease in mean DNA contents compared with the control knee without HTO (P = 0.012). Comparative estimation of the DNA and proteoglycan contents and proteoglycan/DNA ratios of all other parts and zones of the lateral menisci did not reveal significant differences between groups.

CONCLUSION

Open wedge HTO does not lead to significant macroscopic and microscopic structural changes in the lateral meniscus after 6 months in vivo. Overcorrection significantly decreases the proliferative activity of the cells in the red-red zone of the middle third in the sheep model.

Host serum is not indispensable in collagen performance in viable meniscal transplantation at 4-week incubation

PURPOSE

Viable meniscal transplantation has been criticized as an expensive and logistically demanding technique. The purpose was to compare the standard culture medium with another culture medium that is more widely available and easier to work with and to assess the collagen net ultrastructure architecture and the capacity of the preserved cells to produce proteins.

METHODS

Ten fresh lateral menisci were harvested. Each meniscus was divided into three parts; control group, fetal-bovinum-serum group and Insulin-Transferrin-Selenium group during 4 weeks. Cell metabolism was assessed with the gene expression of type I collagen, type II collagen and aggrecan. Collagen ultrastructure was assessed with transmission electron microscopy. The Collagen Meniscal Architecture scoring system was used to evaluate the degree of meniscal disarray.

RESULTS

Type I collagen was expressed more in the fetal-bovinum-serum group than in the ITS group (P = 0.036). No differences were found between cultured samples and control groups. Type II collagen showed decreased expression in both cultured groups compared with the control group. No differences were observed in the gene expression of aggrecan in either group. No differences were observed when the Collagen Meniscal Architecture scoring system was applied.

CONCLUSIONS

Insulin-Transferrin-Selenium-supplemented medium is at least as effective as the fetal-bovinum-serum-supplemented medium to preserve the net architecture of the meniscal tissue. Gene expression of the studied proteins was similar in the Insulin-Transferrin-Selenium group to that observed in the control group at 4 weeks. Insulin-Transferrin-Selenium might be a better alternative and might be used instead of fetal-bovinum-serum or an autologous host serum in order to preserve meniscal tissue, which precludes the necessity of obtaining host serum previously. Thus, viable meniscal transplantation would logistically be less complicated to perform.

Decreased hypertrophic differentiation accompanies enhanced matrix formation in co-cultures of outer meniscus cells with bone marrow mesenchymal stromal cells

Introduction

The main objective of this study was to determine whether meniscus cells from the outer (MCO) and inner (MCI) regions of the meniscus interact similarly to or differently with mesenchymal stromal stem cells (MSCs). Previous study had shown that co-culture of meniscus cells with bone marrow-derived MSCs result in enhanced matrix formation relative to mono-cultures of meniscus cells and MSCs. However, the study did not examine if cells from the different regions of the meniscus interacted similarly to or differently with MSCs.

Methods

Human menisci were harvested from four patients undergoing total knee replacements. Tissue from the outer and inner regions represented pieces taken from one third and two thirds of the radial distance of the meniscus, respectively. Meniscus cells were released from the menisci after collagenase treatment. Bone marrow MSCs were obtained from the iliac crest of two patients after plastic adherence and in vitro culture until passage 2. Primary meniscus cells from the outer (MCO) or inner (MCI) regions of the meniscus were co-cultured with MSCs in three-dimensional (3D) pellet cultures at 1:3 ratio, respectively, for 3 weeks in the presence of serum-free chondrogenic medium containing TGF-β1. Mono-cultures of MCO, MCI and MSCs served as experimental control groups. The tissue formed after 3 weeks was assessed biochemically, histochemically and by quantitative RT-PCR.

Results

Co-culture of inner (MCI) or outer (MCO) meniscus cells with MSCs resulted in neo-tissue with increased (up to 2.2-fold) proteoglycan (GAG) matrix content relative to tissues formed from mono-cultures of MSCs, MCI and MCO. Co-cultures of MCI or MCO with MSCs produced the same amount of matrix in the tissue formed. However, the expression level of aggrecan was highest in mono-cultures of MSCs but similar in the other four groups. The DNA content of the tissues from co-cultured cells was not statistically different from tissues formed from mono-cultures of MSCs, MCI and MCO. The expression of collagen I (COL1A2) mRNA increased in co-cultured cells relative to mono-cultures of MCO and MCI but not compared to MSC mono-cultures. Collagen II (COL2A1) mRNA expression increased significantly in co-cultures of both MCO and MCI with MSCs compared to their own controls (mono-cultures of MCO and MCI respectively) but only the co-cultures of MCO:MSCs were significantly increased compared to MSC control mono-cultures. Increased collagen II protein expression was visible by collagen II immuno-histochemistry. The mRNA expression level of Sox9 was similar in all pellet cultures. The expression of collagen × (COL10A1) mRNA was 2-fold higher in co-cultures of MCI:MSCs relative to co-cultures of MCO:MSCs. Additionally, other hypertrophic genes, MMP-13 and Indian Hedgehog (IHh), were highly expressed by 4-fold and 18-fold, respectively, in co-cultures of MCI:MSCs relative to co-cultures of MCO:MSCs.

Conclusions

Co-culture of primary MCI or MCO with MSCs resulted in enhanced matrix formation. MCI and MCO increased matrix formation similarly after co-culture with MSCs. However, MCO was more potent than MCI in suppressing hypertrophic differentiation of MSCs. These findings suggest that meniscus cells from the outer-vascular regions of the meniscus can be supplemented with MSCs in order to engineer functional grafts to reconstruct inner-avascular meniscus.

Zonal differences in meniscus matrix turnover and cytokine response

OBJECTIVE

To determine the mechanisms of meniscal degeneration and whether this varied zonally and from articular cartilage.

DESIGN

Normal ovine menisci were dissected into inner and outer zones and along with cartilage cultured ±IL-1α and TNFα. Glycosaminoglycan (GAG) and collagen release, and gene expression were quantified. Aggrecan proteolysis was analysed by Western blotting with neoepitope-specific antibodies. Matrix metalloproteinase (MMP)2, MMP9 and MMP13 activity was evaluated by gelatin zymography or fluorogenic assay.

RESULTS

Inner meniscus was more cartilaginous containing more GAG and expressing more ACAN and COL2A1 than outer zones. Higher expression of VCAN and ADAMTS4 in medial outer and both zones of the lateral meniscus reflected their embryologic origin from cells outside the cartilage anlagen. All meniscal regions released a greater % GAG in response to cytokines; only outer zones had cytokine-stimulated collagenolysis. Cytokine-induced aggrecanolysis was primarily due to increased ADAMTS cleavage in cartilage and inner menisci but MMPs in the outer menisci. Outer menisci always released more active MMP2 than other tissues and more active MMP13 in basal and TNF-stimulated cultures. Expression of ACAN, COL1A1 and COL2A1 was decreased by both cytokines in all tissues, while VCAN was increased by IL-1α in cartilage and inner menisci. Metalloproteinase expression was differentially regulated by IL-1α and TNFα: ADAMTS4, MMP1, MMP3 were upregulated more by IL-1α in inner zones whereas ADAMTS5, MMP13 and MMP9 were more upregulated by TNFα in outer zones.

CONCLUSIONS

Meniscal degeneration mechanisms are zonally-dependent, and may contribute to the enzymatic burden in the joint.

Regional effects of enzymatic digestion on knee meniscus cell yield and phenotype for tissue engineering

An abundant cell source is the cornerstone of most tissue engineering strategies, but extracting cells from the knee meniscus is hindered by its dense fibrocartilaginous matrix. Identifying a method to efficiently isolate meniscus cells is important, as it can reduce the cost and effort required to perform meniscus engineering research. In this study, six enzymatic digestion regimens used for cartilaginous cell isolation were used to isolate cells from the outer, middle, and inner regions of the bovine knee meniscus. Each regimen in each region was assessed in terms of cell yield, impact on cell phenotype, and cytotoxicity. All digestion regimens caused an overall upregulation of cartilage-specific genes Sox9, collagen type I (Col 1), collagen type II (Col 2), cartilage oligomeric matrix protein, and aggrecan (AGC) in cells from all meniscus regions, but was highest for cells isolated using 1075 U/mL of collagenase for 3 h (high collagenase). In response to isolation, outer meniscus cells showed highest upregulation of Sox9 and Col 1 genes, whereas greatest upregulation for middle meniscus cells was seen in Col 1 expression, and Col 2 expression for inner cells. Cell yield was highest in all regions when subjected to 45 min of 61 U/mL pronase followed by 3 h of 1075 U/mL collagenase (pronase/collagenase [P/C]) digestion regimen (outer: 6.57±0.37, middle: 12.77±1.41, inner: 22.17±1.47×10(6) cells/g tissue). The second highest cell yield was achieved using the low collagenase (LC) digestion regimen that applied 433 U/mL of collagenase for 18 h (outer: 1.95±0.54, middle: 3.3±4.4, inner: 6.06±2.44×10(6) cells/g tissue). Cytotoxicity analysis showed higher cell death in the LC group compared with the P/C group. Self-assembled constructs formed from LC-isolated cells were less dense than constructs formed from P/C-isolated cells, and P/C constructs showed higher glycosaminoglycan content and compressive moduli than LC constructs. All isolation methods tested resulted in similar phenotypic changes in meniscus cells from each region. These results indicate that, compared with other common isolation protocols, the P/C isolation method is able to more efficiently isolate meniscus cells from all regions that can produce tissue engineered constructs.

Interleukin-1, tumor necrosis factor-alpha, and transforming growth factor-beta 1 and integrative meniscal repair: influences on meniscal cell proliferation and migration

Introduction

Interleukin-1 (IL-1) and tumor necrosis factor-α (TNF-α) are up-regulated in injured and osteoarthritic knee joints. IL-1 and TNF-α inhibit integrative meniscal repair; however, the mechanisms by which this inhibition occurs are not fully understood. Transforming growth factor-β1 (TGF-β1) increases meniscal cell proliferation and accumulation, and enhances integrative meniscal repair. An improved understanding of the mechanisms modulating meniscal cell proliferation and migration will help to improve approaches for enhancing intrinsic or tissue-engineered repair of the meniscus. The goal of this study was to examine the hypothesis that IL-1 and TNF-α suppress, while TGF-β1 enhances, cellular proliferation and migration in cell and tissue models of meniscal repair.

Methods

A micro-wound assay was used to assess meniscal cell migration and proliferation in response to the following treatments for 0, 24, or 48 hours: 0 to 10 ng/mL IL-1, TNF-α, or TGF-β1, in the presence or absence of 10% serum. Proliferated and total cells were fluorescently labeled and imaged using confocal laser scanning microscopy and the number of proliferated, migrated, and total cells was determined in the micro-wound and edges of each image. Meniscal cell proliferation was also assessed throughout meniscal repair model explants treated with 0 or 10 ng/mL IL-1, TNF-α, or TGF-β1 for 14 days. At the end of the culture period, biomechanical testing and histological analyses were also performed. Statistical differences were assessed using an ANOVA and Newman-Keuls post hoc test.

Results

IL-1 and TNF-α decreased cell proliferation in both cell and tissue models of meniscal repair. In the presence of serum, TGF-β1 increased outer zone cell proliferation in the micro-wound and in the cross section of meniscal repair model explants. Both IL-1 and TNF-α decreased the integrative shear strength of repair and extracellular matrix deposition in the meniscal repair model system, while TGF-β1 had no effect on either measure.

Conclusions

Meniscal cell proliferation in vivo may be diminished following joint injury due to the up-regulation of inflammatory cytokines, thereby limiting native cellular repair of meniscal lesions. Therefore, therapies that can promote meniscal cell proliferation have promise to enhance meniscal repair and improve tissue engineering strategies.

A benzamide-linked small molecule HS-Cf inhibits TNF-α-induced interferon regulatory factor-1 in porcine chondrocytes: a potential disease-modifying drug for osteoarthritis therapeutics

BACKGROUND

Using tumor necrosis factor-alpha (TNF-α)-activated porcine chondrocytes as a screening tool, we aim to synthesize and identify small-molecule inhibitors preserving immunomodulatory effects as therapeutics for osteoarthritis (OA).

METHODS

Chondrocytes were isolated from pig joints. A minilibrary of 300 benzamide-linked small molecules was established. The levels of inducible nitric oxide synthase (iNOS) and nitric oxide (NO) were measured by Western blot and Griess reaction, respectively. Proteoglycan degradation in cartilage explants was determined by histochemistry analysis. The activation of transcription factors and protein kinases was determined by electrophoretic mobility shift assays or Western blots. Zymography and real-time reverse transcriptase-polymerase chain reaction were used to determine enzyme activity and expression of matrix metalloproteinases (MMPs) and aggrecanases, respectively.

RESULTS

Bioassay screening of benzamide-linked small molecules revealed that 2-hydroxy-N-[3-(trifluoromethyl)phenyl]benzamide (HS-Cf) was a potent inhibitor of NO production and iNOS expression in TNF-α-stimulated porcine chondrocytes. HS-Cf suppressed TNF-α-induced activity of MMP-13 and expressions of several aggrecanases and prevented TNF-α-mediated reduction of collagen II. Histochemistry analysis confirmed that HS-Cf could prevent TNF-α-induced degradation and release of proteoglycan/aggrecan in cartilage explants. Such effects by HS-Cf were likely through suppressing TNF-α-induced interferon regulatory factor-1 (IRF-1) but not nuclear factor-kappaB signaling. The significance of IRF-1 was further confirmed by short hairpin knockdown studies.

CONCLUSIONS

In a minilibrary containing 300 small molecules, we identified a benzamide-linked small molecule, HS-Cf, that through down-regulating TNF-α-induced IRF-1 activity suppressed chondrocyte activation and prevented cartilage destruction. HS-Cf might be a potential disease-modifying drug for OA therapeutics.

Inner meniscus cells maintain higher chondrogenic phenotype compared with outer meniscus cells

Meniscus cells have several distinct properties in cellular morphology and extracellular matrix production. Inner meniscus cells are considered to have more chondrocytic phenotype compared with outer meniscus cells. However, the chondrogenic property of each meniscus cell has not been elucidated in detail. In this study, we investigated the difference between human inner and outer meniscus-derived cells in extracellular matrix deposition and chondrogenic potential. Monolayer-cultured inner meniscus cells showed small and ovoid shapes though slender and fibroblastic cells were obtained from outer half of human meniscus. The syntheses of type II collagen and safranin O-stained proteoglycans were increased in chondrogenic pellets derived from inner meniscus cells, rather than in outer meniscus cell-derived pellets. On the other hand, adipogenic lipid vacuoles were equally accumulated in both inner and outer meniscus cells after adipogenic treatment. Chondrogenic treatments also enhanced the expression of chondrogenic marker genes, such as Sry-type HMG box (SOX) 9, Scleraxis, and alpha1(II) collagen, in inner meniscus cells. However, SOX9 expression was not increased in outer meniscus cells even after chondrogenic treatment. This study demonstrated that inner meniscus cells maintained higher chondrogenic potential compared with outer meniscus cells. Our results suggest that the difference between inner and outer meniscus cells in chondrogenic property might have an essential role in preserving a zone-specific meniscal feature.

Advanced glycation end products cause collagen II reduction by activating Janus kinase/signal transducer and activator of transcription 3 pathway in porcine chondrocytes

OBJECTIVES

The major risk factor for OA is ageing; however, the mechanisms remain largely unclear. We investigated the effects and mechanisms of advanced glycation end products (AGEs) that accumulate in aged joints in chondrocytes.

METHODS

Porcine chondrocytes or cartilage fragments were prepared. Gene expression of MMPs and a disintegrin and metalloproteinase with thrombospondin motifs (ADAMTS) was assessed by real-time RT-PCR. Gelatin zymography was used to determine MMP-13 enzyme activity. Histochemistry or immunoblotting analysis was applied to determine the expression of collagen II, proteoglycan and aggrecan. Electrophoretic mobility shift assay and immunoblotting were used to study the activation of signal transducer and activator of transcription 3 (STAT3). Genetic manipulations with short hairpin RNA (shRNA) or dominant negative constructs were applied.

RESULTS

AGE enhanced expression and enzyme activity of MMP and ADAMTS genes and resulted in reduction of collagen II. Both janus kinase 2 (JAK2) and JAK3 inhibitors suppressed AGE-induced MMP-13, ADAMTS-4 and ADAMTS-5 expression and enzyme activity. Inhibition of JAK2 or JAK3 prevented AGE-mediated decrease of collagen II in chondrocytes and proteoglycan (aggrecan) degradation in cartilage fragments. In addition, interference of STAT3 expression inhibited AGE-induced MMP-13 and ADAMTS enzyme activities and mRNA levels. Furthermore, expression of the dominant negative receptor of AGE (DN-RAGE) blocked AGE-induced STAT3 phosphorylation.

CONCLUSION

Blocking JAK/STAT3 signalling pathway inhibited AGE-induced activation of MMP-13 and ADAMTS and prevented AGE-mediated decrease of collagen II and proteoglycan (aggrecan). The results indicated that JAK/STAT3 pathway may be a potential target for designing disease-modifying drugs for the treatment of OA.

Chondroprotective effects and mechanisms of resveratrol in advanced glycation end products-stimulated chondrocytes

Introduction

Accumulation of advanced glycation end products (AGEs) in joints contributes to the pathogenesis of cartilage damage in osteoarthritis (OA). We aim to explore the potential chondroprotective effects of resveratrol on AGEs-stimulated porcine chondrocytes and cartilage explants.

Methods

Chondrocytes were isolated from pig joints. Activation of the IκB kinase (IKK)-IκBα-nuclear factor-kappaB (NF-κB) and c-Jun N-terminal kinase (JNK)/extracellular signal-regulated kinase (ERK)-activator protein-1 (AP-1) pathways was assessed by electrophoretic mobility shift assay (EMSA), Western blot and transfection assay. The levels of inducible nitric oxide synthase (iNOS)-NO and cyclooxygenase-2 (COX-2)-prostaglandin E₂ (PGE₂) were measured by Western blot, Griess reaction or ELISA. The expression and enzyme activity of matrix metalloproteinase-13 (MMP-13) were determined by real time RT/PCR and gelatin zymography, respectively.

Results

We show that AGEs-induced expression of iNOS and COX-2 and production of NO and PGE₂ were suppressed by resveratrol. Such effects of resveratrol were likely mediated through inhibiting IKK-IκBα-NF-κB and JNK/ERK-AP-1 signaling pathways induced by AGEs. By targeting these critical signaling pathways, resveratrol decreased AGEs-stimulated expression and activity of MMP-13 and prevented AGEs-mediated destruction of collagen II. Histochemistry analysis further confirms that resveratrol could prevent AGEs-induced degradation of proteoglycan and aggrecan in cartilage explants.

Conclusions

The present study reveals not only the effects and mechanisms regarding how resveratrol may protect cartilage from AGEs-mediated damage but also the potential therapeutic benefit of resveratrol in the treatment of OA.

The influence of an aligned nanofibrous topography on human mesenchymal stem cell fibrochondrogenesis

Fibrocartilaginous tissues serve critical load-bearing functions in numerous joints throughout the body. As these structures are often injured, there exists great demand for engineered tissue for repair or replacement. This study assessed the ability of human marrow-derived mesenchymal stem cells (MSCs) to elaborate a mechanically functional, fibrocartilaginous matrix in a nanofibrous microenvironment. Nanofibrous scaffolds, composed of ultra-fine biodegradable polymer fibers, replicate the structural and mechanical anisotropy of native fibrous tissues and serve as a 3D micro-pattern for directing cell orientation and ordered matrix formation. MSCs were isolated from four osteoarthritic (OA) patients, along with meniscal fibrochondrocytes (FC) which have proven to be a potent cell source for engineering fibrocartilage. Cell-seeded nanofibrous scaffolds were cultured in a chemically-defined medium formulation and mechanical, biochemical, and histological features were evaluated over 9 weeks. Surprisingly, and contrary to previous studies with juvenile bovine cells, matrix assembly by adult human MSCs was dramatically hindered compared to donor-matched FCs cultured similarly. Unlike FCs, MSCs did not proliferate, resulting in sparsely colonized constructs. Increases in matrix content, and therefore changes in tensile properties, were modest in MSC-seeded constructs compared to FC counterparts, even when normalized to the lower cell number in these constructs. To rule out the influence of OA sourcing on MSC functional potential, constructs from healthy young donors were generated; these constructs matured no differently than those formed with OA MSCs. Importantly, there was no difference in matrix production of MSCs and FCs when cultured in pellet form, highlighting the sensitivity of human MSCs to their 3D microenvironment.

CD14-negative isolation enhances chondrogenesis in synovial fibroblasts

Synovial membrane has been shown to contain mesenchymal stem cells. We hypothesized that an enriched population of synovial fibroblasts would undergo chondrogenic differentiation and secrete cartilage extracellular matrix to a greater extent than would a mixed synovial cell population (MSCP). The optimum doses of transforming growth factor beta 1 (TGF-beta1) and insulin-like growth factor 1 (IGF-1) for chondrogenesis were investigated. CD14-negative isolation was used to obtain a porcine cell population enriched in type-B synovial fibroblasts (SFB) from an MSCP. The positive cell surface markers in SFB were CD90, CD44, and cadherin-11. SFB and MSCP were cultured in the presence of 20 ng/mL TGF-beta1 for 7 days, and SFB were demonstrated to have higher chondrogenic potential. Further dose-response studies were carried out using the SFB cells and several doses of TGF-beta1 (2, 10, 20, and 40 ng/mL) and/or IGF-1 (1, 10, 100, and 500 ng/mL) for 14 days. TGF-beta1 supplementation was essential for chondrogenesis and prevention of cell death, whereas IGF-1 did not have a significant effect on the SFB cell number or glycosaminoglycan production. This study demonstrates that the CD14-negative isolation yields an enhanced cell population SFB that is more potent than MSCP as a cell source for cartilage tissue engineering.

Aggrecanolysis and in vitro matrix degradation in the immature bovine meniscus: mechanisms and functional implications

Introduction

Little is known about endogenous or cytokine-stimulated aggrecan catabolism in the meniscal fibrocartilage of the knee. The objectives of this study were to characterize the structure, distribution, and processing of aggrecan in menisci from immature bovines, and to identify mechanisms of extracellular matrix degradation that lead to changes in the mechanical properties of meniscal fibrocartilage.

Methods

Aggrecanase activity in the native immature bovine meniscus was examined by immunolocalization of the aggrecan NITEGE neoepitope. To investigate mechanisms of cytokine-induced aggrecan catabolism in this tissue, explants were treated with interleukin-1α (IL-1) in the absence or presence of selective or broad spectrum metalloproteinase inhibitors. The sulfated glycosaminoglycan (sGAG) and collagen contents of explants and culture media were quantified by biochemical methods, and aggrecan catabolism was examined by Western analysis of aggrecan fragments. The mechanical properties of explants were determined by dynamic compression and shear tests.

Results

The aggrecanase-generated NITEGE neoepitope was preferentially localized in the middle and outer regions of freshly isolated immature bovine menisci, where sGAG density was lowest and blood vessels were present. In vitro treatment of explants with IL-1 triggered the accumulation of NITEGE in the inner and middle regions. Middle region explants stimulated with IL-1 exhibited substantial decreases in sGAG content, collagen content, and mechanical properties. A broad spectrum metalloproteinase inhibitor significantly reduced sGAG loss, abrogated collagen degradation, and preserved tissue mechanical properties. In contrast, an inhibitor selective for ADAMTS-4 and ADAMTS-5 was least effective at blocking IL-1-induced matrix catabolism and loss of mechanical properties.

Conclusions

Aggrecanase-mediated aggrecanolysis, typical of degenerative articular cartilage, may play a physiologic role in the development of the immature bovine meniscus. IL-1-induced release of sGAG and loss of mechanical properties can be ascribed primarily to the activity of MMPs or aggrecanases other than ADAMTS-4 and ADAMTS-5. These results may have implications for the clinical management of osteoarthritis.

Chondrocytes and meniscal fibrochondrocytes differentially process aggrecan during de novo extracellular matrix assembly

Aggrecan is an extracellular matrix molecule that contributes to the mechanical properties of articular cartilage and meniscal fibrocartilage, but the abundance and processing of aggrecan in these tissues are different. The objective of this study was to compare patterns of aggrecan processing by chondrocytes and meniscal fibrochondrocytes in tissue explants and cell-agarose constructs. The effects of transforming growth factor-beta 1 (TGF-beta1) stimulation on aggrecan deposition and processing were examined, and construct mechanical properties were measured. Fibrochondrocytes synthesized and retained less proteoglycans than did chondrocytes in tissue explants and agarose constructs. In chondrocyte constructs, TGF-beta1 induced the accumulation of a 120-kDa aggrecan species previously detected in mature bovine cartilage. Fibrochondrocyte-seeded constructs contained high-molecular-weight aggrecan but lacked aggrecanase-generated fragments found in native, immature meniscus. In addition, reflecting the lesser matrix accumulation, fibrochondrocyte constructs had significantly lower compression moduli than did chondrocyte constructs. These cell type-specific differences in aggrecan synthesis, retention, and processing may have implications for the development of functional engineered tissue grafts.

Central and peripheral region tibial plateau chondrocytes respond differently to in vitro dynamic compression

OBJECTIVE

The objective of this study was to test the hypotheses that chondrocytes from distinct regions of the porcine tibial plateau: (1) display region-specific baseline gene expression, and (2) respond differently to in vitro mechanical loading.

METHODS

Articular cartilage explants were obtained from central (not covered by meniscus) and peripheral (covered by meniscus) regions of porcine tibial plateaus. For baseline gene expression analysis, samples were snap frozen. To determine the effect of mechanical loading, central and peripheral region explants were exposed to equivalent dynamic compression (0-100 kPa) and compared to site-matched free-swelling controls (FSCs). mRNA levels for type II collagen (CII), aggrecan (AGGR), matrix metalloproteinase 1 (MMP-1), MMP-3, MMP-13, A disintegrin and metalloproteinase with thrombospondin motifs 4 (ADAM-TS4), ADAM-TS5, tissue inhibitor of metalloproteinases 1 (TIMP-1), TIMP-2, and tumor necrosis factor alpha (TNFalpha) were quantified using real time polymerase chain reaction (RT-PCR).

RESULTS

At baseline, mRNA levels for the structural proteins CII and AGGR were approximately twofold greater in the central region compared with peripheral region explants. In vitro dynamic compression strongly affected expression levels for CII, AGGR, MMP-3, and TIMP-2 relative to FSCs. Response differed significantly by region, with greater upregulation of CII, AGGR, and MMP-3 in central region explants.

CONCLUSIONS

Chondrocytes from different regions of the porcine tibial plateau express mRNA for structural proteins at different levels and respond to equivalent in vitro mechanical loading with distinctive changes in gene expression. These regional biological variations appear to be related to the local mechanical environment in the normal joint, and thus may indicate a sensitivity of the joint to conditions that alter joint loading such as anterior cruciate ligament (ACL) injury, meniscectomy, or joint instability.

Inhibition of matrix metalloproteinases enhances in vitro repair of the meniscus

Damage or injury of the meniscus is associated with onset and progression of knee osteoarthritis (OA). The intrinsic repair capacity of the meniscus is inhibited by inflammatory cytokines, such as interleukin-1 (IL-1). Using an in vitro meniscal repair model system, we examined the hypothesis that inhibition of matrix metalloproteinases (MMPs) in the presence of IL-1 will enhance repair of meniscal lesions. Integrative repair of the meniscus was examined between two concentric explants cultured with IL-1 and various MMP inhibitors for 14 days. Throughout the culture period, we assessed total specific MMP activity in the media. At harvest, biomechanical testing to assess the strength of repair and histologic staining were performed. IL-1 decreased the shear strength of repair, as compared with control explants. In the presence of IL-1, the broad-spectrum MMP inhibitor GM 6001 decreased the MMP activity in the media, increased the shear strength of repair, and enhanced tissue repair in the interface. However, individual MMP inhibitors did not alter the shear strength of repair in either the presence or absence of IL-1. These findings suggest IL-1 may inhibit meniscal repair through upregulation of MMPs, but inhibition of multiple MMPs may be necessary to promote integrative meniscal repair.

Meniscal tissue explants response depends on level of dynamic compressive strain

OBJECTIVE

Following partial meniscectomy, the remaining meniscus is exposed to an altered loading environment. In vitro 20% dynamic compressive strains on meniscal tissue explants has been shown to lead to an increase in release of glycosaminoglycans from the tissue and increased expression of interleukin-1alpha (IL-1alpha). The goal of this study was to determine if compressive loading which induces endogenously expressed IL-1 results in downstream changes in gene expression of anabolic and catabolic molecules in meniscal tissue, such as MMP expression.

METHOD

Relative changes in gene expression of MMP-1, MMP-3, MMP-9, MMP-13, A Disintegrin and Metalloproteinase with ThromboSpondin 4 (ADAMTS4), ADAMTS5, TNFalpha, TGFbeta, COX-2, Type I collagen (COL-1) and aggrecan and subsequent changes in the concentration of prostaglandin E(2) released by meniscal tissue in response to varying levels of dynamic compression (0%, 10%, and 20%) were measured. Porcine meniscal explants were dynamically compressed for 2h at 1Hz.

RESULTS

20% dynamic compressive strains upregulated MMP-1, MMP-3, MMP-13 and ADAMTS4 compared to no dynamic loading. Aggrecan, COX-2, and ADAMTS5 gene expression were upregulated under 10% strain compared to no dynamic loading while COL-1, TIMP-1, and TGFbeta gene expression were not dependent on the magnitude of loading.

CONCLUSION

This data suggests that changes in mechanical loading of the knee joint meniscus from 10% to 20% dynamic strain can increase the catabolic activity of the meniscus.

Tissue engineering with meniscus cells derived from surgical debris

OBJECTIVE

Injuries to the avascular regions of the meniscus fail to heal and so are treated by resection of the damaged tissue. This alleviates symptoms but fails to restore normal load transmission in the knee. Tissue engineering functional meniscus constructs for re-implantation may improve tissue repair. While numerous studies have developed scaffolds for meniscus repair, the most appropriate autologous cell source remains to be determined. In this study, we hypothesized that the debris generated from common meniscectomy procedures would possess cells with potential for forming replacement tissue. We also hypothesized that donor age and the disease status would influence the ability of derived cells to generate functional, fibrocartilaginous matrix.

METHODS

Meniscus derived cells (MDCs) were isolated from waste tissue of 10 human donors (seven partial meniscectomies and three total knee arthroplasties) ranging in age from 18 to 84 years. MDCs were expanded in monolayer culture through passage 2 and seeded onto fiber-aligned biodegradable nanofibrous scaffolds and cultured in a chemically defined media. Mechanical properties, biochemical content, and histological features were evaluated over 10 weeks of culture.

RESULTS

Results demonstrated that cells from every donor contributed to increasing biochemical content and mechanical properties of engineered constructs. Significant variability was observed in outcome parameters (cell infiltration, proteoglycan and collagen content, and mechanical properties) amongst donors, but these variations did not correlate with patient age or disease condition. Strong correlations were observed between the amount of collagen deposition within the construct and the tensile properties achieved. In scaffolds seeded with particularly robust cells, construct tensile moduli approached maxima of approximately 40 MPa over the 10-week culture period.

CONCLUSIONS

This study demonstrates that cells derived from surgical debris are a potent cell source for engineered meniscus constructs. Results further show that robust growth is possible in MDCs from middle-aged and elderly patients, highlighting the potential for therapeutic intervention using autologous cells.

Col2a1 lineage tracing reveals that the meniscus of the knee joint has a complex cellular origin

The knee joint consists of multiple interacting tissues that are prone to injury- and disease-related degeneration. Although much is known about the structure and function of the knee's constituent tissues, relatively little is known about their cellular origin and the mechanisms governing their segregation. To investigate the origin and segregation of knee tissues in vivo we performed lineage tracing using a Col2a1-Cre/R26R mouse model system and compared the data obtained with actual Col2a1 expression. These studies demonstrated that at E13.5 the interzone at the presumptive joint site forms when cells within the Col2a1-expressing anlagen cease expression of Col2a1 and not through cellular invasion into the anlagen. Later in development these interzone cells form the cruciate ligament and inner medial meniscus of the knee. At E14.5, after interzone formation, cells that had never expressed Col2a1 appeared in the joint and formed the lateral meniscus. Furthermore, cells with a Col2a1-positive expression history combined with the negative cells to form the medial meniscus. The invading cells started to express Col2a1 1 week after birth, resulting in all cells within the meniscus synthesizing collagen II. These findings support a model of knee development in which cells present in the original anlagen combine with invading cells in the formation of this complex joint.

Evaluation criteria for musculoskeletal and craniofacial tissue engineering constructs: a conference report

Over the past 20 years, tissue engineering (TE) has evolved into a thriving research and commercial development field. However, applying TE strategies to musculoskeletal (MSK) and craniofacial tissues has been particularly challenging since these tissues must also transmit loads during activities of daily living. To address this need, organizers invited a small group of bioengineers, surgeons, biologists, and material scientists from academia, industry, and government to participate in a two and half-day conference to develop general and tissue-specific criteria for evaluating new concepts and tissue-engineered constructs, including threshold values of success. Participants were assigned to four breakout groups representing commonly injured tissues, including tendon and ligament, articular cartilage, meniscus and temporomandibular joint, and bone and intervertebral disc. Working in multidisciplinary teams, participants first carefully defined one or two important unmet clinical needs for each tissue type, including current standards of care and the potential impact of TE solutions. The groups then sought to identify important parameters for evaluating repair outcomes in preclinical studies and to specify minimally acceptable values for these parameters. The importance of in vitro TE studies was then discussed in the context of these preclinical studies. Where data were not currently available from clinical, preclinical, or culture studies, the groups sought to identify important areas of preclinical research needed to speed the development process. This report summarizes the findings of the conference.

Transfer of macroscale tissue strain to microscale cell regions in the deformed meniscus

Cells within fibrocartilaginous tissues, including chondrocytes and fibroblasts of the meniscus, ligament, and tendon, regulate cell biosynthesis in response to local mechanical stimuli. The processes by which an applied mechanical load is transferred through the extracellular matrix to the environment of a cell are not fully understood. To better understand the role of mechanics in controlling cell phenotype and biosynthetic activity, this study was conducted to measure strain at different length scales in tissue of the fibrocartilaginous meniscus of the knee joint, and to define a quantitative parameter that describes the strain transferred from the far-field tissue to a microenvironment surrounding a cell. Experiments were performed to apply a controlled uniaxial tensile deformation to explants of porcine meniscus containing live cells. Using texture correlation analyses of confocal microscopy images, two-dimensional Lagrangian and principal strains were measured at length scales representative of the tissue (macroscale) and microenvironment in the region of a cell (microscale) to yield a strain transfer ratio as a measure of median microscale to macroscale strain. The data demonstrate that principal strains at the microscale are coupled to and amplified from macroscale principal strains for a majority of cell microenvironments located across diverse microstructural regions, with average strain transfer ratios of 1.6 and 2.9 for the maximum and minimum principal strains, respectively. Lagrangian strain components calculated along the experimental axes of applied deformations exhibited considerable spatial heterogeneity and intersample variability, and suggest the existence of both strain amplification and attenuation. This feature is consistent with an in-plane rotation of the principal strain axes relative to the experimental axes at the microscale that may result from fiber sliding, fiber twisting, and fiber-matrix interactions that are believed to be important for regulating deformation in other fibrocartilaginous tissues. The findings for consistent amplification of macroscale to microscale principal strains suggest a coordinated pattern of strain transfer from applied deformation to the microscale environment of a cell that is largely independent of these microstructural features in the fibrocartilaginous meniscus.

Human meniscus cells express hypoxia inducible factor-1alpha and increased SOX9 in response to low oxygen tension in cell aggregate culture

In previous work we demonstrated that the matrix-forming phenotype of cultured human cells from whole meniscus was enhanced by hypoxia (5% oxygen). Because the meniscus contains an inner region that is devoid of vasculature and an outer vascular region, here we investigate, by gene expression analysis, the separate responses of cells isolated from the inner and outer meniscus to lowered oxygen, and compared it with the response of articular chondrocytes. In aggregate culture of outer meniscus cells, hypoxia (5% oxygen) increased the expression of type II collagen and SOX9 (Sry-related HMG box-9), and decreased the expression of type I collagen. In contrast, with inner meniscus cells, there was no increase in SOX9, but type II collagen and type I collagen increased. The articular chondrocytes exhibited little response to 5% oxygen in aggregate culture, with no significant differences in the expression of these matrix genes and SOX9. In both aggregate cultures of outer and inner meniscus cells, but not in chondrocytes, there was increased expression of collagen prolyl 4-hydroxylase (P4H)α(I) in response to 5% oxygen, and this hypoxia-induced expression of P4Hα(I) was blocked in monolayer cultures of meniscus cells by the hypoxia-inducible factor (HIF)-1α inhibitor (YC-1). In fresh tissue from the outer and inner meniscus, the levels of expression of the HIF-1α gene and downstream target genes (namely, those encoding P4Hα(I) and HIF prolyl 4-hydroxylase) were significantly higher in the inner meniscus than in the outer meniscus. Thus, this study revealed that inner meniscus cells were less responsive to 5% oxygen tension than were outer meniscus cells, and they were both more sensitive than articular chondrocytes from a similar joint. These results suggest that the vasculature and greater oxygen tension in the outer meniscus may help to suppress cartilage-like matrix formation.

Inhibition of integrative repair of the meniscus following acute exposure to interleukin-1 in vitro

Damage or loss of the meniscus is associated with progressive osteoarthritic degeneration of the knee joint. Injured and degenerative joints are characterized by elevated levels of the pro-inflammatory cytokine interleukin-1 (IL-1), which with prolonged exposure can induce catabolic and anti-anabolic activities that inhibit tissue repair. We used an in vitro model system to examine the hypotheses that acute exposure to IL-1 inhibits meniscal repair, and that an IL-1-mediated increase in matrix metalloproteinase (MMP) activity is associated with the inhibition of repair. Integrative tissue repair was studied between concentric explants of porcine medial menisci that were treated with IL-1alpha acutely (100 pg/mL for 1 or 3 days) or chronically (100 pg/mL for entire culture duration). After 14 and 28 days in culture, biomechanical testing, cell viability, and histology were performed to assess meniscal repair. Total specific MMP activity in the culture media was measured using a quenched fluorescent substrate. As little as 1 day of IL-1 exposure significantly reduced shear strength, cell accumulation, and tissue repair compared to controls. IL-1 exposure for 1 or 3 days significantly increased MMP activity that subsided by day 9. With chronic IL-1 exposure, MMP activity remained elevated for the duration of culture and was negatively correlated with repair strength. Our study shows that short-term exposure to physiologically relevant concentrations of IL-1 significantly reduces meniscal repair in vitro, and thus may potentially inhibit the intrinsic repair response in vivo. The suppression of IL-1 or MMP expression and/or activity warrant investigation as potential strategies for promoting meniscal repair.

Interleukin-1 and tumor necrosis factor alpha inhibit repair of the porcine meniscus in vitro

OBJECTIVE

Injury or removal of the knee meniscus leads to progressive joint degeneration, and current surgical therapies for meniscal tears seek to maximally preserve meniscal structure and function. However, the factors that influence intrinsic repair of the meniscus are not well understood. The goal of this study was to investigate the capacity of meniscus tissue to repair a simulated defect in vitro and to examine the effect of pro-inflammatory cytokines on this process.

METHODS

Cylindrical explants were harvested from the outer one-third of medial porcine menisci. To simulate a full-thickness defect, a central core was removed and reinserted immediately into the defect. Explants were cultured for 2, 4, or 6 weeks in serum-containing media in the presence or absence of interleukin-1 (IL-1) or tumor necrosis factor alpha (TNF-alpha), and meniscal repair was investigated using mechanical testing and fluorescence confocal microscopy.

RESULTS

Meniscal lesions in untreated samples showed a significant capacity for intrinsic repair in vitro, with increasing cell accumulation and repair strength over time in culture. In the presence of IL-1 or TNF-alpha, no repair was observed despite the presence of abundant viable cells.

CONCLUSIONS

This study demonstrates that the meniscus exhibits an intrinsic repair response in vitro. However, the presence of pro-inflammatory cytokines completely inhibited repair. These findings suggest that increased levels of pro-inflammatory cytokines post-injury or under arthritic conditions may inhibit meniscal repair. Therefore, inhibition of these cytokines may provide a means of accelerating repair of damaged or injured menisci in vivo.

Regional differences in prostaglandin E2 and nitric oxide production in the knee meniscus in response to dynamic compression

Injury or loss of the knee meniscus is associated with altered joint stresses that lead to progressive joint degeneration. The goal of this study was to determine if dynamic mechanical compression influences the production of inflammatory mediators by meniscal cells. Dynamic compression increased prostaglandin E2 (PGE(2)) and nitric oxide (NO) production over a range of stress magnitudes (0.0125-0.5 MPa) in a manner that depended on stress magnitude and zone of tissue origin. Inner zone explants showed greater increases in PGE(2) and NO production as compared to outer zone explants. Meniscal tissue expressed NOS2 and NOS3 protein, but not NOS1. Mechanically induced NO production was blocked by NOS inhibitors, and the non-selective NOS inhibitor L-NMMA augmented PGE(2) production in the outer zone only. These findings suggest that the meniscus may serve as an intra-articular source of pro-inflammatory mediators, and that alterations in the magnitude or distribution of joint loading could significantly influence the production of these mediators in vivo.

Repair response of the inner and outer regions of the porcine meniscus in vitro

BACKGROUND

The menisci are essential intra-articular structures that contribute to knee function, and meniscal injury or loss is associated with joint degeneration. Tears of the outer vascularized zone have a greater potential for repair than do tears in the inner avascular region.

OBJECTIVE AND HYPOTHESIS

Develop an in vitro explant model to examine the hypothesis that differences exist in the intrinsic repair response between the outer and inner region of the meniscus.

STUDY DESIGN

Controlled laboratory study.

METHODS

Cylindrical explants were harvested from the outer one third and inner two thirds of medial porcine menisci. To simulate a full-thickness defect, a central core was removed and reinserted immediately. Explants were cultured for 2, 4, or 6 weeks, and meniscal healing was investigated using mechanical testing, histologic analysis, and fluorescence confocal microscopy.

RESULTS

Over the 6-week culture period, meniscal explants exhibited migration of cells into the repair site, followed by increased tissue formation that bridged the interface. The repair strength increased significantly over time, with no differences between the 2 regions.

CONCLUSION

The findings show that explants from the avascular inner zone and vascular outer zone of the meniscus exhibit similar healing potential and repair strength in vitro.

CLINICAL RELEVANCE

These findings support the hypothesis that the regional differences in meniscal repair observed clinically are owed to the additional vascular supply of the outer meniscus rather than intrinsic differences between the extracellular matrix and cells from these 2 areas.