Basic fibroblast growth factor: an extracellular mechanotransducer in articular cartilage?

Prochondrogenic effect of decellularized extracellular matrix secreted from human induced pluripotent stem cell-derived chondrocytes

Cartilage is mainly composed of chondrocytes and the extracellular matrix (ECM), which exchange important biochemical and biomechanical signals necessary for differentiation and homeostasis. Human articular cartilage has a low ability for regeneration because it lacks blood vessels, nerves, and lymphatic vessels. Currently, cell therapeutics, including stem cells, provide a promising strategy for cartilage regeneration and treatment; however, there are various hurdles to overcome, such as immune rejection and teratoma formation. In this study, we assessed the applicability of the stem cell-derived chondrocyte ECM for cartilage regeneration. Human induced pluripotent stem cell (hiPSC)-derived chondrocytes (iChondrocytes) were differentiated, and decellularized ECM (dECM) was successfully isolated from cultured chondrocytes. Isolated dECM enhanced in vitro chondrogenesis of iPSCs when recellularized. Implanted dECM also restored osteochondral defects in a rat osteoarthritis model. A possible association with the glycogen synthase kinase-3 beta (GSK3β) pathway demonstrated the fate-determining importance of dECM in regulating cell differentiation. Collectively, we suggested the prochondrogenic effect of hiPSC-derived cartilage-like dECM and offered a promising approach as a non-cellular therapeutic for articular cartilage reconstruction without cell transplantation. STATEMENT OF SIGNIFICANCE: Human articular cartilage has low ability for regeneration and cell culture-based therapeutics could aid cartilage regeneration. Yet, the applicability of human induced pluripotent stem cell-derived chondrocyte (iChondrocyte) extracellular matrix (ECM) has not been elucidated. Therefore, we first differentiated iChondrocytes and isolated the secreted ECM by decellularization. Recellularization was performed to confirm the pro-chondrogenic effect of the decellularized ECM (dECM). In addition, we confirmed the possibility of cartilage repair by transplanting the dECM into the cartilage defect in osteochondral defect rat knee joint. We believe that our proof-of-concept study will serve as a basis for investigating the potential of dECM obtained from iPSC-derived differentiated cells as a non-cellular resource for tissue regeneration and other future applications.

Perlecan in Pericellular Mechanosensory Cell-Matrix Communication, Extracellular Matrix Stabilisation and Mechanoregulation of Load-Bearing Connective Tissues

In this study, we review mechanoregulatory roles for perlecan in load-bearing connective tissues. Perlecan facilitates the co-acervation of tropoelastin and assembly of elastic microfibrils in translamellar cross-bridges which, together with fibrillin and elastin stabilise the extracellular matrix of the intervertebral disc annulus fibrosus. Pericellular perlecan interacts with collagen VI and XI to define and stabilize this matrix compartment which has a strategic position facilitating two-way cell-matrix communication between the cell and its wider extracellular matrix. Cues from the extracellular matrix are fed through this pericellular matrix back to the chondrocyte, allowing it to perceive and respond to subtle microenvironmental changes to regulate tissue homeostasis. Thus perlecan plays a key regulatory role in chondrocyte metabolism, and in chondrocyte differentiation. Perlecan acts as a transport proteoglycan carrying poorly soluble, lipid-modified proteins such as the Wnt or Hedgehog families facilitating the establishment of morphogen gradients that drive tissue morphogenesis. Cell surface perlecan on endothelial cells or osteocytes acts as a flow sensor in blood and the lacunar canalicular fluid providing feedback cues to smooth muscle cells regulating vascular tone and blood pressure, and the regulation of bone metabolism by osteocytes highlighting perlecan's multifaceted roles in load-bearing connective tissues.

Posttraumatic osteoarthritis: what have we learned to advance osteoarthritis?

PURPOSE OF REVIEW

Current thinking in the study of posttraumatic osteoarthritis (PTOA) is overviewed: the osteoarthritis which follows acute joint injury. The review particularly highlights important publications in the last 18 months, also reflecting on key older literature, in terms of what have we have we learned and have yet to learn from PTOA, which can advance the osteoarthritis field as a whole.

RECENT FINDINGS

PTOA is a mechanically driven disease, giving insight into mechanical drivers for osteoarthritis. A mechanosensitive molecular tissue injury response (which includes activation of pain, degradative and also repair pathways) is triggered by acute joint injury and seen in osteoarthritis. Imaging features of PTOA are highly similar to osteoarthritis, arguing against it being a different phenotype. The inflammatory pathways activated by injury contribute to early joint symptoms. However, later structural changes appear to be dissociated from traditional measures of synovial inflammation.

SUMMARY

PTOA remains an important niche in which to understand processes underlying osteoarthritis and seek interventional targets. Whether PTOA has true molecular or clinical differences to osteoarthritis as a whole remains to be understood. This knowledge is important for a field where animal modelling of the disease relies heavily on the link between injury and osteoarthritis.

Fibroblast growth factor 2 drives changes in gene expression following injury to murine cartilage in vitro and in vivo

OBJECTIVE

The articular cartilage is known to be highly mechanosensitive, and a number of mechanosensing mechanisms have been proposed as mediators of the cellular responses to altered mechanical load. These pathways are likely to be important in tissue homeostasis as well as in the pathogenesis of osteoarthritis. One important injury-activated pathway involves the release of pericellular fibroblast growth factor 2 (FGF-2) from the articular cartilage. Using a novel model of murine cartilage injury and surgically destabilized joints in mice, we examined the extent to which FGF-2 contributes to the cellular gene response to injury.

METHODS

Femoral epiphyses from 5-week-old wild-type mice were avulsed and cultured in serum-free medium. Explant lysates were Western blotted for phospho-JNK, phospho-p38, and phospho-ERK or were fixed for immunohistochemical analysis of the nuclear translocation of p65 (indicative of NF-κB activation). RNA was extracted from injured explants, rested explants that had been stimulated with recombinant FGF-2 or FGF-18, or whole joints from either wild-type mice or FGF-2(-/-) mice. Reverse transcription-polymerase chain reaction was performed to examine a number of inflammatory response genes that had previously been identified in a microarray analysis.

RESULTS

Murine cartilage avulsion injury resulted in rapid activation of the 3 MAP kinase pathways as well as NF-κB. Almost all genes identified in murine joints following surgical destabilization were also regulated in cartilage explants upon injury. Many of these genes, including those for activin A (Inhba), tumor necrosis factor-stimulated gene 6 (Tnfaip6), matrix metalloproteinase 19 (Mmp19), tissue inhibitor of metalloproteinases 1 (Timp1), and podoplanin (Pdpn), were significantly FGF-2 dependent following injury to cartilage in vitro and to joint tissues in vivo.

CONCLUSION

FGF-2-dependent gene expression occurs in vitro and in vivo in response to cartilage/joint injury in mice.

Characterizing the role of mechanical signals in gene regulatory networks using Long SAGE

A systems understanding of mechanical regulation is critical for determining how cells proliferate and differentiate. To better understand the biological process in which mechanical signals regulate cells, we globally investigated the gene expression profiling via long serial analysis of gene expression (Long SAGE) in osteoblasts after exposure to mechanical stretching. The analysis showed that the differentially expressed genes were related with many physiological processes, including signal transduction, cell proliferation and apoptosis. Several genes that were seldom or never studied in osteoblasts have been found in this study. We further analyzed the signal pathways and provided gene regulatory networks activated by mechanical signals. Many changed genes in our data were contributed to ECM-integrin-FAK mediated pathway and mainly influenced actin-cytoskeleton dynamic remodeling, cell proliferation and differentiation. We also provided evidence supporting the hypothesis that endoplasmic reticulum and mitochondrion were combined to dedicate to calcium regulation. Taken together, our experiments provided a systemic view on biological processes and mechanotransduction network in osteoblasts, suggesting that mechanical signals regulate osteoblast through a greater diversity of interactions and pathways than previously appreciated.

Effect of aging on cellular mechanotransduction

Aging is becoming a critical heath care issue and a burgeoning economic burden on society. Mechanotransduction is the ability of the cell to sense, process, and respond to mechanical stimuli and is an important regulator of physiologic function that has been found to play a role in regulating gene expression, protein synthesis, cell differentiation, tissue growth, and most recently, the pathophysiology of disease. Here we will review some of the recent findings of this field and attempt, where possible, to present changes in mechanotransduction that are associated with the aging process in several selected physiological systems, including musculoskeletal, cardiovascular, neuronal, respiratory systems and skin.

Comparative immunolocalisation of perlecan with collagen II and aggrecan in human foetal, newborn and adult ovine joint tissues demonstrates perlecan as an early developmental chondrogenic marker

We undertook a comparative immunolocalisation study on type II collagen, aggrecan and perlecan in a number of 12- to 14-week-old human foetal and postnatal (7-19 months) ovine joints including finger, toe, knee, elbow, hip and shoulder. This demonstrated that perlecan followed a virtually identical immunolocalisation pattern to that of type II collagen in the foetal tissues, but a slightly divergent localisation pattern in adult tissues. Aggrecan was also localised in the cartilaginous joint tissues, which were clearly delineated by toluidine blue staining and the type II collagen immunolocalisations. It was also present in the capsular joint tissues and in ligaments and tendons in the joint, which stained poorly or not at all with toluidine blue. In higher power microscopic views, antibodies to perlecan also stained small blood vessels in the synovial lining tissues of the joint capsule; however, this was not discernable in low power macroscopic views where the immunolocalisation of perlecan to pericellular regions of cells within the cartilaginous rudiments was a predominant feature. Perlecan was also evident in small blood vessels in stromal connective tissues associated with the cartilage rudiments and with occasional nerves in the vicinity of the joint tissues. Perlecan was expressed by rounded cells in the enthesis attachment points of tendons to bone and in rounded cells in the inner third of the meniscus, which stained prominently with type II collagen and aggrecan identifying the chondrogenic background of these cells and local compressive loads. Flattened cells within the tendon and in the surface laminas of articular cartilages and the meniscus did not express perlecan. Collected evidence presented herein, therefore, indicates that besides being a basement membrane component, perlecan is also a marker of chondrogenic cells in prenatal cartilages. In postnatal cartilages, perlecan displayed a pericellular localisation pattern rather than the territorial or interterritorial localisation it displayed in foetal cartilages. This may reflect processing of extracellular perlecan presumably as a consequence of intrinsic biomechanical loading on these tissues or to divergent functions for perlecan and type II collagen in adult compared to prenatal tissues.

Fibroblast growth factor 2 is an intrinsic chondroprotective agent that suppresses ADAMTS-5 and delays cartilage degradation in murine osteoarthritis

OBJECTIVE

We have previously identified in articular cartilage an abundant pool of the heparin-binding growth factor, fibroblast growth factor 2 (FGF-2), which is bound to the pericellular matrix heparan sulfate proteoglycan, perlecan. This pool of FGF-2 activates chondrocytes upon tissue loading and is released following mechanical injury. In vitro, FGF-2 suppresses interleukin-1-driven aggrecanase activity in human cartilage explants, suggesting a chondroprotective role in vivo. We undertook this study to investigate the in vivo role of FGF-2 in murine cartilage.

METHODS

Basal characteristics of the articular cartilage of Fgf2(-/-) and Fgf2(+/+) mice were determined by histomorphometry, nanoindentation, and quantitative reverse transcriptase-polymerase chain reaction. The articular cartilage was graded histologically in aged mice as well as in mice in which osteoarthritis (OA) had been induced by surgical destabilization of the medial meniscus. RNA was extracted from the joints of Fgf2(-/-) and Fgf2(+/+) mice following surgery and quantitatively assessed for key regulatory molecules. The effect of subcutaneous administration of recombinant FGF-2 on OA progression was assessed in Fgf2(-/-) mice.

RESULTS

Fgf2(-/-) mice were morphologically indistinguishable from wild-type (WT) animals up to age 12 weeks; the cartilage thickness and proteoglycan staining were equivalent, as was the mechanical integrity of the matrix. However, Fgf2(-/-) mice exhibited accelerated spontaneous and surgically induced OA. Surgically induced OA in Fgf2(-/-) mice was suppressed to levels in WT mice by subcutaneous administration of recombinant FGF-2. Increased disease in Fgf2(-/-) mice was associated with increased expression of messenger RNA of Adamts5, the key murine aggrecanase.

CONCLUSION

These data identify FGF-2 as a novel endogenous chondroprotective agent in articular cartilage.

Recent advances in annular pathobiology provide insights into rim-lesion mediated intervertebral disc degeneration and potential new approaches to annular repair strategies

The objective of this study was to assess the impact of a landmark annular lesion model on our understanding of the etiopathogenesis of IVD degeneration and to appraise current IVD repairative strategies. A number of studies have utilised the Osti sheep model since its development in 1990. The experimental questions posed at that time are covered in this review, as are significant recent advances in annular repair strategies. The ovine model has provided important spatial and temporal insights into the longitudinal development of annular lesions and how they impact on other discal and paradiscal components such as the NP, cartilaginous end plates, zygapophyseal joints and vertebral bone and blood vessels. Important recent advances have been made in biomatrix design for IVD repair and in the oriented and dynamic culture of annular fibrochondrocytes into planar, spatially relevant, annular type structures. The development of hyaluronan hydrogels capable of rapid in situ gelation offer the possibility of supplementation of matrices with cells and other biomimetics and represent a significant advance in biopolymer design. New generation biological glues and self-curing acrylic formulations which may be augmented with slow delivery biomimetics in microcarriers may also find application in the non-surgical repair of annular defects. Despite major advances, significant technical challenges still have to be overcome before the biological repair of this intractable connective tissue becomes a realistic alternative to conventional surgical intervention for the treatment of chronic degenerate IVDs.

Alpha10 integrin expression is up-regulated on fibroblast growth factor-2-treated mesenchymal stem cells with improved chondrogenic differentiation potential

Mesenchymal stem cells (MSCs) are multipotent cells that have the capacity to differentiate into various different cell lineages and can generate bone, cartilage and adipose tissue. MSCs are presently characterized using a broad range of different cell-surface markers that are not exclusive to MSCs and not sensitive to culture conditions or differentiation capacity. We show that the integrin subunits alpha10 and alpha11 of the collagen binding integrins alpha10beta1 and alpha11beta1 are expressed by human MSCs in monolayer cultures. We also demonstrate that the expression of alpha10 increases, while alpha1 and alpha11 decrease, during aggregate culture of MSCs in chondrogenic medium. Alpha10beta1 is expressed by chondrocytes in cartilage, whereas alpha11beta1 integrins are predominantly expressed by subsets of the fibroblastic lineage. In extensive monolayer cultures of MSCs, alpha10 expression is down-regulated. We show that this down-regulation is reversed by fibroblast growth factor-2 (FGF-2) treatment. Addition of FGF-2 to MSCs not only results in increased alpha10 expression, but also in decreased alpha11 expression. FGF-2 treatment of MSCs has been shown to keep the cells more multipotent and also induces cell proliferation and Sox-9 up-regulation. We demonstrate improved chondrogenecity as well as increased collagen-dependant migratory potential of FGF-2-treated MSCs having a high alpha10 expression. We also demonstrate expression of alpha10 and alpha11 integrin subunits in the endosteum and periosteum of mice, but very low or not detectable expression levels in freshly aspired human or mouse BM. We show that MSCs with high chondrogenic differentiation potential are highly alpha10 positive and propose alpha10 as a potential marker to predict the differentiation state of MSCs.

The effects of local bFGF release and uniaxial strain on cellular adaptation and gene expression in a 3D environment: implications for ligament tissue engineering

The objectives of this investigation were (1) to characterize the growth factor release profile of a basic fibroblast growth factor (bFGF)-coated three-dimensional (3D) polymer scaffold under static and cyclically strained conditions, and (2) to delineate the individual and collective contributions of locally released bFGF and mechanical strain on cellular morphology and gene expression in this 3D system. Scaffolds were treated with I(125)-bFGF and subjected to mechanical strain or maintained in a static environment and the media sampled for factor release over a period of 6 days. Over the first 10 hours, a burst release of 25% of the incorporated growth factor into the surrounding media was noted. At 24 hours, approximately 40% of the bFGF was released into the media, after which steady state was achieved and minimal subsequent release was noted. Mechanical stimulation had no effect on growth factor release from the scaffold in this system. To test the concerted effects of bFGF and mechanical stimulation on bone marrow stromal cells (BMSCs), scaffolds were loaded with 0, 100, or 500 ng of bFGF, seeded with cells, and subjected to mechanical strain or maintained in a static environment. Scaffolds were harvested at 1, 7, and 21 days for RT-PCR and histomorphometry. All scaffolds subjected to growth factor and/or mechanical stimulation demonstrated cellular adherence and spreading at 21 days. Conversely, in the absence of both bFGF and mechanical stimulation, cells demonstrated minimal cytoplasmic spread. Moreover, at 21 days, cells subjected to both mechanical stimulation and bFGF (500 ng) demonstrated the highest upregulation of stress-resistive (collagen I, III) and stress-responsive proteins (tenascin-C). The effect of growth factor may be dose sensitive, however, as unstrained scaffolds treated with 100 ng of bFGF demonstrated upregulation of gene expression comparable to strained scaffolds treated with lower doses of bFGF (0 or 100 ng). In conclusion, results from this study suggest that the stimulatory effects of bFGF are dose sensitive and appear to be influenced by the addition of mechanical strain. The concurrent application of biochemical and mechanical stimuli may be important in promoting the adaptation of BMSCs and driving the transcription of genes essential for synthesis of a functional ligament replacement tissue.

Chondroitin sulfate sulfation motifs as putative biomarkers for isolation of articular cartilage progenitor cells

Osteoarthritis is a chronic, debilitating joint disease characterized by progressive destruction of articular cartilage. Recently, a number of studies have identified a chondroprogenitor cell population within articular cartilage with significant potential for repair/regeneration. As yet, there are few robust biomarkers of these cells. In this study, we show that monoclonal antibodies recognizing novel chondroitin sulfate sulfation motif epitopes in glycosaminoglycans on proteoglycans can be used to identify metabolically distinct subpopulations of cells specifically within the superficial zone of the tissue and that flow cytometric analysis can recognize these cell subpopulations. Fluorochrome co-localization analysis suggests that the chondroitin sulfate sulphation motifs are associated with a range of cell and extracellular matrix proteoglycans within the stem cell niche that include perlecan and aggrecan but not versican. The unique distributions of these sulphation motifs within the microenvironment of superficial zone chondrocytes, seems to designate early stages of stem/progenitor cell differentiation and is consistent with these molecules playing a functional role in regulating aspects of chondrogenesis. The isolation and further characterization of these cells will lead to an improved understanding of the role novel chondroitin sulfate sulfation plays in articular cartilage development and may contribute significantly to the field of articular cartilage repair.