Cell autonomous TGFß signaling is essential for stem/progenitor cell recruitment into degenerative tendons.

Understanding cell recruitment in damaged tendons is critical for improvements in regenerative therapy. We recently reported that targeted disruption of transforming growth factor beta (TGFß) type II receptor in the tendon cell lineage (Tgfbr2ScxCre) resulted in resident tenocyte dedifferentiation and tendon deterioration in postnatal stages. Here we extend the analysis and identify direct recruitment of stem/progenitor cells into the degenerative mutant tendons. Cre-mediated lineage tracing indicates that these cells are not derived from tendon-ensheathing tissues or from a Scleraxis-expressing lineage, and they turned on tendon markers only upon entering the mutant tendons. Through immunohistochemistry and inducible gene deletion, we further find that the recruited cells originated from a Sox9-expressing lineage and their recruitment was dependent on cell autonomous TGFß signaling. The cells identified in this study thus differ from previous reports of cell recruitment into injured tendons and suggest a critical role for TGFß signaling in cell recruitment, providing insights that may support improvements in tendon repair.

Tgfß signaling is critical for maintenance of the tendon cell fate.

Studies of cell fate focus on specification, but little is known about maintenance of the differentiated state. In this study, we find that the mouse tendon cell fate requires continuous maintenance in vivo and identify an essential role for TGFß signaling in maintenance of the tendon cell fate. To examine the role of TGFß signaling in tenocyte function the TGFß type II receptor (Tgfbr2) was targeted in the Scleraxis-expressing cell lineage using the ScxCre deletor. Tendon development was not disrupted in mutant embryos, but shortly after birth tenocytes lost differentiation markers and reverted to a more stem/progenitor state. Viral reintroduction of Tgfbr2 to mutants prevented and even rescued tenocyte dedifferentiation suggesting a continuous and cell autonomous role for TGFß signaling in cell fate maintenance. These results uncover the critical importance of molecular pathways that maintain the differentiated cell fate and a key role for TGFß signaling in these processes.

Chondroitin-6-sulfate attenuates inflammatory responses in murine macrophages via suppression of NF-κB nuclear translocation.

Inflammation is a host protective response to noxious stimuli, and excessive production of pro-inflammatory mediators by macrophages (mφ) can lead to numerous pathological conditions. In this study, immunomodulatory effects of immobilized and soluble glycosaminoglycans (GAGs) on mouse-bone-marrow-derived mφ were compared by measuring nitric oxide (NO). We demonstrate here that all GAGs studied except for heparin were able to modulate interferon-γ/lipopolysaccharide (IFN-γ/LPS)-induced NO release by mφ to varying extents after 24h of incubation. In particular, the modulatory activities of soluble chondroitin-6-sulfate (C6S), hyaluronic acid and heparan sulfate altered markedly after covalent immobilization. Of these, soluble C6S exhibited the strongest NO inhibitory activity, and the inhibition was dose- and time-dependent. Moreover, C6S significantly reduced pro-inflammatory cytokines interleukin (IL)-6 and tumor necrosis factor (TNF)-α production by IFN-γ/LPS- or LPS-activated mφ. Specifically, the C6S-mediated suppression of mφ pro-inflammatory phenotype was accompanied by an increase in the IL-10 level, suggesting a possible switch towards anti-inflammatory/wound healing M2 state. In addition, the highest magnitude of inhibitory effects was obtained when cells were pre-treated with C6S prior to IFN-γ/LPS or LPS challenge, suggesting an additional role for C6S in protection against microbial infection. Further investigations reveal that the anti-inflammatory effects of C6S on activated mφ may be ascribed at least in part to suppression of NF-κB nuclear translocation.

Effects of biomimetic surfaces and oxygen tension on redifferentiation of passaged human fibrochondrocytes in 2D and 3D cultures.

Due to its limited healing potential within the inner avascular region, functional repair of the meniscus remains a significant challenge in orthopaedic surgery. Tissue engineering of a meniscus implant using meniscal cells offers the promise of enhancing the reparative process and achieving functional meniscal repair. In this work, using quantitative real-time reverse transcriptase polymerase chain reaction (RT-qPCR) analysis, we show that human fibrochondrocytes rapidly dedifferentiate during monolayer expansion on standard tissue culture flasks, representing a significant limit to clinical use of this cell population for meniscal repair. Previously, we have characterized and described the feasibility of a tailored biomimetic surface (C6S surface) for reversing dedifferentiation of monolayer-expanded rat meniscal cells. The surface is comprised of major meniscal extracellular matrix (ECM) components in the inner region, namely collagen I/II (at a 2:3 ratio) and chondroitin-6-sulfate. We thus have further evaluated the effects of the C6S surface, alongside a number of other tailored surfaces, on cell adhesion, proliferation, matrix synthesis and relevant marker gene expression (collagen I, -II, aggrecan and Sox-9 etc) of passaged human fibrochondrocytes in 2D (coated glass coverslips) and 3D (surface-modified polymeric scaffolds) environments. We show that the C6S surface is permissive for cell adhesion, proliferation and ECM synthesis, as demonstrated using DNA quantification, 1,9-dimethylmethylene blue (DMMB) assay, histology and immunohistochemistry. More importantly, RT-qPCR analyses corroborate the feasibility of the C6S surface for reversing phenotypic changes, especially the downregulation of collagen II, of dedifferentiated human fibrochondrocytes. Furthermore, human fibrochondrocyte redifferentiation was enhanced by hypoxia in the 3D cultures, independent of hypoxia inducible factor (HIF) transcriptional activity and was shown to potentially involve the transcriptional activation of Sox-9.

Modulation of collagen II fiber formation in 3-D porous scaffold environments.

Collagen II, a major extracellular matrix component in cartilaginous tissues, undergoes fibrillogenesis under physiological conditions. The present study explored collagen II fiber formation in solution and in two- (coverslip) and three-dimensional (scaffold) environments under different incubation conditions. These conditions include variations in adsorption buffers, the presence of 1-ethyl-3-(3-dimenthylaminopropyl) carbodiimide/N-hydroxysuccinimide crosslinker and the nature of the material surfaces. We extend our observations of collagen II fiber formation in two dimensions to develop an approach for the formation of a fibrillar collagen II network throughout surface-modified polylactide-co-glycolide porous scaffolds. Morphologically, the collagen II network is similar to that present in native articular cartilage. Biological validation of the resultant optimized functional scaffold, using rat bone marrow-derived mesenchymal stem cells, shows appreciable cell infiltration throughout the scaffold with enhanced cell spreading at 24h post-seeding. This economic and versatile approach is thus believed to have significant potential in cartilage tissue engineering applications.

Interactions of meniscal cells with extracellular matrix molecules: towards the generation of tissue engineered menisci.

Menisci are one of the most commonly injured parts of the knee. Conventional surgical interventions are often associated with a long-term increased risk of osteoarthritis. Meniscal tissue engineering utilizes natural or synthetic matrices as a scaffold to guide tissue repair or regeneration in three dimensions. Studies have shown that a diverse cellular response can be triggered depending on the composition of the surrounding extracellular matrix (ECM) components. As such, attempts have been made to replace or repair meniscus defects using tissue grafts or reconstituted ECM components prepared from a multitude of tissues. This commentary summarizes the most recent data on the response of meniscal cells to ECM components, both in vivo and in vitro, and focuses on their potential roles in meniscal repair and regeneration. We also discuss our recent investigations into the interactions of meniscal cells and a self assembled biomimetic surface composed of meniscal ECM molecules. The biological effects conferred by the biomimetic surface, in terms of cell adhesion, proliferation, gene expression profiles and matrix synthesis, were evaluated. Finally, some suggested directions for future research in this field are outlined.

Interactions between meniscal cells and a self assembled biomimetic surface composed of hyaluronic acid, chitosan and meniscal extracellular matrix molecules.

Menisci are one of the most commonly injured parts of the knee with a limited healing potential. This study focuses on fabrication and characterization of biomimetic surfaces for meniscal tissue engineering. To achieve this, a combination of hyaluronic acid/chitosan (HA/CH) mutilayers with covalently immobilized major extracellular matrix (ECM) components of native meniscus, namely collagen I/II (COL.I/II) and chondroitin-6-sulfate (C6S) was employed. Adsorption of the biomolecules was monitored using a quartz crystal microbalance with dissipation (QCM-D) and fourier transform-surface plasmon resonance (FT-SPR). Immobilization of the biomolecules onto HA/CH mutilayers was achieved by sequential adsorption, with optimum binding at a molar ratio of 1.4:1 (COL.I/II: C6S). After adding COL.I/II, the layers became relatively more rigid and large aggregates were evident. The effects of the modified surfaces on cell proliferation, gene expression and proteoglycan production of rat meniscal cells were examined. Quantitative real-time reverse transcriptase polymerase chain reaction (RT-qPCR) analysis showed that primary meniscal cells dedifferentiated rapidly after one passage in monolayer culture. This process could be reversed by culturing the cells on C6S surfaces, as indicated by increases in both collagen II and aggrecan gene expression, as well as proteoglycan production. Cells with abundant lipid vacuoles were evident on all the surfaces over an extended culture period. The results demonstrate the feasibility of C6S surfaces to avoid the dedifferentiation that normally occurs during monolayer expansion of meniscal cells.

Enhanced chondrogenic differentiation of human bone marrow-derived mesenchymal stem cells in low oxygen environment micropellet cultures.

Chondrogenesis of mesenchymal stem cells (MSCs) is typically induced when they are condensed into a single aggregate and exposed to transforming growth factor-beta (TGF-beta). Hypoxia, like aggregation and TGF-beta delivery, may be crucial for complete chondrogenesis. However, the pellet dimensions and associated self-induced oxygen gradients of current chondrogenic methods may limit the effectiveness of in vitro differentiation and subsequent therapeutic uses. Here we describe the use of embryoid body-forming technology to produce microscopic aggregates of human bone marrow MSCs (BM-MSCs) for chondrogenesis. The use of micropellets reduces the formation of gradients within the aggregates, resulting in a more homogeneous and controlled microenvironment. These micropellet cultures (approximately 170 cells/micropellet) as well as conventional pellet cultures (approximately 2 x 10(5) cells/pellet) were chondrogenically induced under 20% and 2% oxygen environments for 14 days. Compared to conventional pellets under both environments, micropellets differentiated under 2% O(2) showed significantly increased sulfated glycosaminoglycan (sGAG) production and more homogeneous distribution of proteoglycans and collagen II. Aggrecan and collagen II gene expressions were increased in pellet cultures differentiated under 2% O(2) relative to 20% O(2) pellets but 2% O(2) micropellets showed even greater increases in these genes, as well as increased SOX9. These results suggest a more advanced stage of chondrogenesis in the micropellets accompanied by more homogeneous differentiation. Thus, we present a new method for enhancing MSC chondrogenesis that reveals a unique relationship between oxygen tension and aggregate size. The inherent advantages of chondrogenic micropellets over a single macroscopic aggregate should allow for easy integration with a variety of cartilage engineering strategies.