Involvement of alpha5beta1 integrin in matrix interactions and proliferation of chondrocytes

Collagen type I-based recombinant peptide promotes bone regeneration in rat critical-size calvarial defects by enhancing osteoclast activity at late stages of healing

Introduction

We recently demonstrated the bone-forming potential of medium-cross-linked recombinant collagen peptide (mRCP) in animal models of bone defects. However, these studies were limited to a 4-week observation period; therefore, in the present study, we aimed to further evaluate mRCP as a suitable bone graft material for the alveolar cleft by analyzing its bone-forming potential, osteogenic-inducing ability, and biodegradation over an extended period of 12 weeks, using a rat critical-size calvarial defect model.

Methods

Using Sprague-Dawley rats, we created critical-size calvarial defects through a surgical procedure. The defects were then filled with 3 mg of mRCP (mRCP group) or 18 mg of Cytrans® (CA) granules, which has a carbonate apatite-based composition resembling natural bone, was used as a reference material (CA group). For negative control, the defects were left untreated. Bone volume, total bone volume (bone volume including CA granules), and bone mineral density (BMD) in the defect were assessed using micro-computed tomography (μ-CT) at 0, 4, 8, and 12 weeks after implantation. Using histomorphometric analyses of hematoxylin and eosin (H&E)-stained sections, we measured the amount of newly formed bone and total newly formed bone (new bone including CA granules) in the entire defect site, as well as the amount of newly formed bone in the central side, two peripheral sides (left and right), periosteal (top) side, and dura mater (bottom) side. In addition, we measured the amount of residual bone graft material in the defect. Osteoclasts and osteoblasts in the newly formed bone were detected using tartrate-resistant acid phosphatase (TRAP) and alkaline phosphatase (ALP) staining, respectively.

Results

Bone volume in the mRCP group increased over time and was significantly larger at 8 and 12 weeks after surgery than at 4 weeks. The bone volume in the mRCP group was greater than that of the CA and control groups at 4, 8, and 12 weeks after implantation, and while the total bone volume was greater in the CA group after 4 and 8 weeks, the mRCP group had comparable levels of total bone volume to that of the CA group at 12 weeks after implantation. The BMD of the mRCP group reached similar levels to native calvaria bone at the same time point. H&E-stained sections revealed a larger amount of newly formed bone 12 weeks after implantation in the mRCP group compared to that of the CA and control groups. The total newly formed bone at 12 weeks after implantation was on par with that in the CA group. Furthermore, at the defect site, the area of newly formed bone was larger on the peripheral and dura mater sides. Notably, the number of osteoclasts in the mRCP group was higher than in the CA and control groups and peaked 8 weeks after implantation, which coincided with the timing of the greatest resorption of mRCP. Although the ALP-positive area was greater in the mRCP group compared to other groups, we did not detect any significant changes in the number of osteoblasts over time.

Conclusion

This study demonstrated the bone-forming potential of mRCP over an extended period of 12 weeks, suggesting that mRCP sufficiently resists resorption to promote bone formation through induction of osteoclast activation in the late stages of the healing period.

RGD-incorporated biomimetic cryogels for hyaline cartilage regeneration

Maintaining the integrity of articular cartilage is paramount to joint health and function. Under constant mechanical stress, articular cartilage is prone to injury that often extends to the underlying subchondral bone. In this study, we incorporated arginine-aspartate-glycine (RGD) peptide into chondroitin sulfate-based cryogel for hyaline cartilage regeneration. Known to promote cell adhesion and proliferation, RGD peptide is a double-edged sword for cartilage regeneration. Depending on the peptide availability in the microenvironment, RGD may aid in redifferentiation of dedifferentiated chondrocytes by mimicking physiological cell-matrix interaction or inhibit chondrogenic phenotype via excessive cell spreading. Here, we observed an increase in chondrogenic phenotype with RGD concentration. The group containing the highest RGD concentration (3 mM; RGD group) experienced a 24-fold increase inCOL2expression in the 1st week ofin vitroculture and formed native cartilage-resembling ectopic tissuein vivo. No sign of dedifferentiation (COL1) was observed in all groups. Within the concentration range tested (0-3 mM RGD), RGD promotes chondrocyte redifferentiation after monolayer expansion and thus, formation of hyaline cartilage tissue.

Application of Alginate Hydrogels for Next-Generation Articular Cartilage Regeneration

The articular cartilage has insufficient intrinsic healing abilities, and articular cartilage injuries often progress to osteoarthritis. Alginate-based scaffolds are attractive biomaterials for cartilage repair and regeneration, allowing for the delivery of cells and therapeutic drugs and gene sequences. In light of the heterogeneity of findings reporting the benefits of using alginate for cartilage regeneration, a better understanding of alginate-based systems is needed in order to improve the approaches aiming to enhance cartilage regeneration with this compound. This review provides an in-depth evaluation of the literature, focusing on the manipulation of alginate as a tool to support the processes involved in cartilage healing in order to demonstrate how such a material, used as a direct compound or combined with cell and gene therapy and with scaffold-guided gene transfer procedures, may assist cartilage regeneration in an optimal manner for future applications in patients.

TNIIIA2, The Peptide of Tenascin-C, as a Candidate for Preventing Articular Cartilage Degeneration

OBJECTIVE

TNIIIA2 is a peptide of the extracellular matrix glycoprotein tenascin-C. We evaluated whether intra-articular injection of TNIIIA2 could prevent articular cartilage degeneration without inducing synovitis in an osteoarthritis mice model.

DESIGN

Ten micrograms per milliliter of TNIIIA2 were injected into the knee joint of mice (group II) to evaluate the induction of synovitis. The control group received an injection of phosphate buffered saline (group I). Synovitis was evaluated using synovitis score 2 and 4 weeks after injection. The ligaments of knee joints of mice were transected to make the osteoarthritis model. After transection, 10 µg/mL of TNIIIA2 was injected into the knee joint (group IV). The control group received an injection of phosphate buffered saline after transection (group III). Histologic examinations were made using hematoxylin and eosin and safranin-O staining at 2, 4, 8, and 12 weeks postoperatively. An in vitro study was also performed to determine the mechanism by which TNIIIA2 prevents cartilage degeneration. Human chondrocytes were isolated, cultured, and treated with TNIIIA2. The expressions of various mRNAs, including inflammatory cytokines, and anabolic and catabolic factors for cartilage were compared using real-time polymerase chain reaction.

RESULTS

There were no differences between groups in the study of intra-articular injection of mice (group I vs. group II). In the osteoarthritis model, we found development of osteoarthritis was suppressed in group IV at 4 and 8 weeks. TNIIIA2 upregulated the expressions of tumor necrosis factor-α, matrix metalloproteinase 3, and basic fibroblast growth factor.

CONCLUSION

We demonstrated that TNIIIA2 could prevent cartilage degeneration without synovitis.

Bone formation potential of collagen type I-based recombinant peptide particles in rat calvaria defects

Introduction

This study aimed to examine the bone-forming ability of medium-cross-linked recombinant collagen peptide (mRCP) particles developedbased on human collagen type I, contains an arginyl-glycyl-aspartic acid-rich motif, fabricated as bone filling material, compared to that of the autologous bone graft.

Methods

Calvarial bone defects were created in immunodeficient rats though a surgical procedure. The rats were divided into 2 groups: mRCP graft and tibia bone graft (bone graft). The bone formation potential of mRCP was evaluated by micro-computed tomography and hematoxylin-eosin staining at 1, 2, 3, and 4 weeks after surgery, and the data were analyzed and compared to those of the bone graft.

Results

The axial volume-rendered images demonstrated considerable bony bridging with the mRCP graft, but there was no significant difference in the bone volume and bone mineral density between the mRCP graft and bone graft at 4 weeks. The peripheral new bone density was significantly higher than the central new bone density and the bottom side score was significantly higher than the top side score at early stage in the regenerated bone within the bone defects.

Conclusion

These results indicate that mRCP has a high potential of recruiting osteogenic cells, comparable to that of autologous bone chips.

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Bone formation potential of mRCP were comparable to that of autogenous bone.

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mRCP particles exhibit high new bone formation potential in the calvaria defect.

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Bone bridging was observed over the entire defect in mRCP graft at 4 weeks.

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mRCP has a high potential of recruiting osteogenic cells comparable to bone graft.

Petaloid recombinant peptide enhances in vitro cartilage formation by synovial mesenchymal stem cells

In vitro chondrogenesis of mesenchymal stem cells (MSCs) mimics in vivo chondrogenesis of MSCs. However, the size of the cartilage pellets that can be attained in vitro is limited by current methods; therefore, some modifications are required to obtain larger pellets. Petaloid pieces of recombinant peptide (petaloid RCP) have the advantage of creating spaces between cells in culture. The RCP used here is based on the alpha-1 sequence of human collagen type I and contains 12 Arg-Gly-Asp motifs. We examined the effect and mechanisms of adding petaloid RCP on the in vitro chondrogenesis of human synovial MSCs by culturing 125k cells with or without 0.125 mg petaloid RCP in chondrogenic medium for 21 days. The cartilage pellets were sequentially analyzed by weight, sulfated glycosaminoglycan content, DNA retention, and histology. Petaloid RCP significantly increased the weight of the cartilage pellets: The petaloid RCP group weighed 7.7 ± 1.2 mg (n = 108), whereas the control group weighed 5.3 ± 1.6 mg. Sulfated glycosaminoglycan and DNA contents were significantly higher in the petaloid RCP group than in the control group. Light and transmission electron microscopy images showed that the petaloid RCP formed the framework of the pellet at day 1, the framework was broken by production of cartilage matrix by the synovial MSCs at day 7, and the cartilage pellet grew larger, with diffuse petaloid RCP remaining, at day 21. Therefore, petaloid RCP formed a framework for the pellet, maintained a higher cell number, and promoted in vitro cartilage formation of synovial MSCs. © 2018 The Authors. Journal of Orthopaedic Research® Published by Wiley Periodicals, Inc. J Orthop Res 37:1350-1357, 2019.

ECM signaling in cartilage development and endochondral ossification

During cartilage development chondrocytes undergo a multi-step process characterized by consecutive changes in cell morphology and gene expression. Cell proliferation, polarity, differentiation, and migration are influenced by chemical and mechanical signaling between the extracellular matrix (ECM) and the cell. Several structurally diverse transmembrane receptors such as integrins, discoidin domain receptor 2 (DDR 2), and CD44 mediate the crosstalk between cells and their ECM. However, the contribution of cell-matrix interactions during early chondrogenesis and further cartilage development through cell receptors and their signal transduction pathways is still not fully understood. Determination of receptor signaling pathways and the function of downstream targets will aid in a better understanding of musculoskeletal pathologies such as chondrodysplasia, and the development of new approaches for the treatment of cartilage disorders. We will summarize recent findings, linking cell receptors and their potential signaling pathways to the control of chondrocyte behavior during early chondrogenesis and endochondral ossification.

Maintenance of chondrocyte phenotype during expansion on PLLA microtopographies

Articular chondrocytes are difficult to grow, as they lose their characteristic phenotype following expansion on standard tissue culture plates. Here, we show that culturing them on surfaces of poly(L-lactic acid) of well-defined microtopography allows expansion and maintenance of characteristic chondrogenic markers. We investigated the dynamics of human chondrocyte dedifferentiation on the different poly(L-lactic acid) microtopographies by the expression of collagen type I, collagen type II and aggrecan at different culture times. When seeded on poly(L-lactic acid), chondrocytes maintained their characteristic hyaline phenotype up to 7 days, which allowed to expand the initial cell population approximately six times without cell dedifferentiation. Maintenance of cell phenotype was afterwards correlated to cell adhesion on the different substrates. Chondrocytes adhesion occurs via the α₅β₁ integrin on poly(L-lactic acid), suggesting cell–fibronectin interactions. However, α₂β₁ integrin is mainly expressed on the control substrate after 1 day of culture, and the characteristic chondrocytic markers are lost (collagen type II expression is overcome by the synthesis of collagen type I). Expanding chondrocytes on poly(L-lactic acid) might be an effective solution to prevent dedifferentiation and improving the number of cells needed for autologous chondrocyte transplantation.

Development of chondrocyte-seeded electrosprayed nanoparticles for repair of articular cartilage defects in rabbits

Due to limited self-healing capacity in cartilages, there is a rising demand for an innovative therapy that promotes chondrocyte proliferation while maintaining its biofunctionality for transplantation. Chondrocyte transplantation has received notable attention; however, the tendencies of cell de-differentiation and de-activation of biofunctionality have been major hurdles in its development, delaying this therapy from reaching the clinic. We believe it is due to the non-stimulative environment in the injured cartilage, which is unable to provide sustainable physical and biological supports to the newly grafted chondrocytes. Therefore, we evaluated whether providing an appropriate matrix to the transplanted chondrocytes could manipulate cell fate and recovery outcomes. Here, we proposed the development of electrosprayed nanoparticles composed of cartilage specific proteins, namely collagen type II and hyaluronic acid, for implantation with pre-seeded chondrocytes into articular cartilage defects. The fabricated nanoparticles were pre-cultured with chondrocytes before implantation into injured articular cartilage. The study revealed a significant potential for nanoparticles to support pre-seeded chondrocytes in cartilage repair, serving as a protein delivery system while improving the survival and biofunctionality of transplanted chondrocytes for prolonged period of time.

Introduction to Cartilage

Cartilage is a supporting connective tissue that, together with the bone, forms the framework supporting the body as a whole. There are many distinct types of cartilage, which exhibit numerous similarities as well as differences. Among them, articular cartilage is the best known and the most studied type. Articular cartilage is the thin layer of connective tissue that covers the articulating ends of bones in synovial (diarthrodial) joints. It provides a smooth surface for joint movement and acts as a load-bearing medium that protects the bone and distributes stress. The intense interest in articular cartilage is motivated by the critical role its degradation plays in arthritis and related joint diseases, which are the number one cause of disability in humans. This chapter discusses the physical, chemical and cellular properties of cartilage that give the tissue its extraordinary load-bearing characteristics.

Alginate-hyaluronic acid-collagen composite hydrogel favorable for the culture of chondrocytes and their phenotype maintenance

Articular cartilage has limited regeneration capacity, thus significant challenge has been made to restore the functions. The development of hydrogels that can encapsulate and multiply cells, and then effectively maintain the chondrocyte phenotype is a meaningful strategy to this cartilage repair. In this study, we prepared alginate-hyaluronic acid based hydrogel with type I collagen being incorporated, namely Alg-HA-Col composite hydrogel. The incorporation of Col enhanced the chemical interaction of molecules, and the thermal stability and dynamic mechanical properties of the resultant hydrogels. The primary chondrocytes isolated from rat cartilage were cultured within the composite hydrogel and the cell viability recorded revealed active proliferation over a period of 21 days. The mRNA levels of chondrocyte phenotypes, including SOX9, collagen type II, and aggrecan, were significantly up-regulated when the cells were cultured within the Alg-HA-Col gel than those cultured within the Alg-HA. Furthermore, the secretion of sulphated glycosaminoglycan, a cartilage-specific matrix molecule, was recorded higher in the collagen-added composite hydrogel. Although more in-depth studies are required such as the in vivo functions, the currently-prepared Alg-HA-Col composite hydrogel is considered to provide favorable 3-dimensional matrix conditions for the cultivation of chondrocytes. Moreover, the cell-cultured constructs may be useful for the cartilage repair and tissue engineering.

Spheroidal Organoids Reproduce Characteristics of Longitudinal Depth Zones in Bovine Articular Cartilage

Articular cartilage has multiple histologically distinct longitudinal depth zones. Development and pathogenesis occur throughout these zones. Cartilage explants, monolayer cell culture and reconstituted 3-dimensional cell constructs have been used for investigating mechanisms of pathophysiology in articular cartilage. Such models have been insufficient to reproduce zone-dependent cellular characteristics and extracellular matrix (ECM) upon investigation into cartilage development and pathogenesis. Therefore, we defined a chondrocyte spheroid model consistently formed with isolated chondrocytes from longitudinal depth zones without extrinsic materials. This spheroid showed zone-dependent characteristics of size, cartilage-specific ECM (collagen types I and II, aggrecan and keratan sulfate) and gene expressions of anabolic and catabolic molecules (matrix molecules and matrix metalloproteinase-13). In addition, the spheroid model is small enough to maintain the viability of cells and point symmetry to analyze the gradient of diffusive molecules. This spheroid organoid model will be useful to elucidate the mechanism of histogenesis and pathogenesis in articular cartilage.

Physically entrapped gelatin in polyethylene glycol scaffolds for three-dimensional chondrocyte culture

Developing tissue-engineered constructs for clinical use must satisfy the fundamental biologic parameters of biocompatibility, cell adhesiveness, and biodegradability. Physical entrapment of bioactive agents into synthetic polymers, as three-dimensional scaffolds, holds great promise for cell culture applications. Here, in an attempt to elucidate the effects of physical interlocking of natural and synthetic gel networks on cell responses within three-dimensional microenvironments, gelatin (of different concentrations) was physically incorporated into macroporous polyethylene glycol (PEG) hydrogels to fabricate PEG-GEL1 (10:1, PEG:gelatin) and PEG-GEL5 (10:5, PEG:gelatin). The effect of the physically entrapped gelatin on primary chondrocytes was investigated in relation to cell distribution, morphology and viability, proliferation, gene expression, and extracellular matrix accumulation in vitro. Our findings have shown successful incorporation of two different concentrations of gelatin into polyethylene glycol macroporous hydrogels through physical mixing. These physical blends not only enhanced chondrocyte adhesion and proliferation but also boosted gene expression of collagen II and aggrecan after 14 days in culture. Although results demonstrated that gelatin levels dropped sharply in PEG-GEL1 and PEG-GEL5 in the first 7 days, however evidently, after days 14 and 21 gelatin levels in both groups remained substantially unchanged and in turn enhanced glycosaminoglycan formation in vitro. Thus, the modification of polyethylene-glycol-based scaffolds with physically entrapped gelatin may be sufficient for dictating three-dimensional microenvironments for chondrocyte cultures.

Alpha 5 Integrin Mediates Osteoarthritic Changes in Mouse Knee Joints

Osteoarthritis (OA) is one of most common skeletal disorders and can affect synovial joints such as knee and ankle joints. α5 integrin, a major fibronectin receptor, is expressed in articular cartilage and has been demonstrated to play roles in synovial joint development and in the regulation of chondrocyte survival and matrix degradation in articular cartilage. We hypothesized that α5 integrin signaling is involved in pathogenesis of OA. To test this, we generated compound mice that conditionally ablate α5 integrin in the synovial joints using the Gdf5Cre system. The compound mice were born normally and had an overall appearance similar to the control mice. However, when the mutant mice received the OA surgery, they showed stronger resistance to osteoarthritic changes than the control. Specifically the mutant knee joints presented lower levels of cartilage matrix and structure loss and synovial changes and showed stronger biomechanical properties than the control knee joints. These findings indicate that α5 integrin may not be essential for synovial joint development but play a causative role in induction of osteoarthritic changes.

Regulation of α5 and αV Integrin Expression by GDF-5 and BMP-7 in Chondrocyte Differentiation and Osteoarthritis

The Integrin β1 family is the major receptors of the Extracellular matrix (ECM), and the synthesis and degradation balance of ECM is seriously disrupted during Osteoarthritis (OA). In this scenario, integrins modify their pattern expression and regulate chondrocyte differentiation in the articular cartilage. Members of the Transforming growth factor beta (Tgf-β) Superfamily, such as Growth differentiation factor 5 (Gdf-5) and Bone morphogenetic protein 7 (Bmp-7), play a key role in joint formation and could regulate the integrin expression during chondrocyte differentiation and osteoarthritis progression in an experimental OA rat model. Decrease of α5 integrin expression in articular cartilage was related with chondrocyte dedifferentiation during OA progression, while increase of α1, α2, and α3 integrin expression was related with fibrous areas in articular cartilage during OA. Hypertrophic chondrocytes expressed αV integrin and was increased in the articular cartilage of rats with OA. Integrin expression during chondrocyte differentiation was also analyzed in a micromass culture system of mouse embryo mesenchymal cells, micromass cultures was treated with Gdf-5 or Bmp-7 for 4 and 6 days, respectively. Gdf-5 induced the expression of the α5 sub-unit, while Bmp-7 induced the expression of the αV sub-unit. This suggests a switch in signaling for prehypertrophic chondrocyte differentiation towards hypertrophy, where Gdf-5 could maintain the articular chondrocyte phenotype and Bmp-7 would induce hypertrophy. Decrease of Ihh expression during late stages of OA in rat model suggest that the ossification in OA rat knees and endochondral ossification could be activated by Bmp-7 and αV integrin in absence of Ihh. Thus, chondrocyte phenotype in articular cartilage is similar to prehypetrophic chondrocyte in growth plate, and is preserved due to the presence of Indian hedgehog (Ihh), Gdf-5 and α5 integrin to maintain articular cartilage and prevent hypertrophy.

Evaluating the effects of charged oligopeptide motifs coupled with RGD on osteogenic differentiation of mesenchymal stem cells

Mesenchymal stem cells, due to their multilineage differentiation potential, have emerged as a promising cell candidate for cell-based therapy. In recent years, biomaterials were artificially synthesized to control the differentiation of mesenchymal stem cells. In this study, a series of charged or neutral oligopeptide motifs coupled with RGD were synthesized and used for surface modification using quartz substrates as model. Cell behaviors on the modified surfaces with different charged oligopeptide motifs were studied. It was found that these different charged oligopeptide motifs coupled with RGD were biocompatible for cell proliferation and adhesion. Moreover, it was demonstrated that the positively charged oligopeptide motif could inhibit osteogenic differentiation, while the negatively charged and neutral oligopeptide motifs could enhance osteogenic differentiation in the presence of RGD. This work may bring us enlightenment that different charged oligopeptide motifs coupled with RGD may be used for biomaterial surface modification for different stem cell-based therapies.

Integrin-β1 regulates chondrocyte proliferation and apoptosis through the upregulation of GIT1 expression

Chondrocytes play a critical role in the repair process of osteoarthritis, which is also known as degenerative arthritis. Integrins, as the key family of cell surface receptors, are responsible for the regulation of chondrocyte proliferation, differentiation, survival and apoptosis through the recruitment and activation of downstream adaptor proteins. Moreover, G-protein-coupled receptor kinase interacting protein-1 (GIT1) exerts its effects on cell proliferation and migration through interaction with various cytokines. It has been previously suggested that GIT1 acts as a vital protein downstream of the integrin-mediated pathway. In the present study, we investigated the effects of integrin-β1 on cell proliferation and apoptosis, as well as the underlying mechanisms in chondrocytes in vitro. Following transfection with a vector expressing integrin-β1, our results revealed that the overexpression of integrin-β1 enhanced GIT1 expression, whereas the knockdown of integrin-β1 by siRNA suppressed GIT1 expression. However, no significant effect was observed on integrin-β1 expression following the enforced overexpression of GIT1, which suggests that GIT1 is localized downstream of integrin-β1. In other words, integrin-β1 regulates the expression of GIT1. Furthermore, this study demonstrated that integrin-β1 and GIT1 increased the expression levels of aggrecan and type II collagen, thus promoting chondrocyte proliferation; however, they inhibited chondrocyte apoptosis. Taken together, our data demonstrate that integrin-β1 plays a vital role in chondrocyte proliferation, differentiation and apoptosis. GIT1 exerts effects similar to those of integrin-β1 and is a downstream target of integrin-β1.

The application of multiple biophysical cues to engineer functional neocartilage for treatment of osteoarthritis. Part II: signal transduction

The unique mechanoelectrochemical environment of cartilage has motivated researchers to investigate the effect of multiple biophysical cues, including mechanical, magnetic, and electrical stimulation, on chondrocyte biology. It is well established that biophysical stimuli promote chondrocyte proliferation, differentiation, and maturation within "biological windows" of defined dose parameters, including mode, frequency, magnitude, and duration of stimuli (see companion review Part I: Cellular Response). However, the underlying molecular mechanisms and signal transduction pathways activated in response to multiple biophysical stimuli remain to be elucidated. Understanding the mechanisms of biophysical signal transduction will deepen knowledge of tissue organogenesis, remodeling, and regeneration and aiding in the treatment of pathologies such as osteoarthritis. Further, this knowledge will provide the tissue engineer with a potent toolset to manipulate and control cell fate and subsequently develop functional replacement cartilage. The aim of this article is to review chondrocyte signal transduction pathways in response to mechanical, magnetic, and electrical cues. Signal transduction does not occur along a single pathway; rather a number of parallel pathways appear to be activated, with calcium signaling apparently common to all three types of stimuli, though there are different modes of activation. Current tissue engineering strategies, such as the development of "smart" functionalized biomaterials that enable the delivery of growth factors or integration of conjugated nanoparticles, may further benefit from targeting known signal transduction pathways in combination with external biophysical cues.

Role of integrins and their ligands in osteoarthritic cartilage

Osteoarthritis (OA) is a degenerative disease, which is characterized by articular cartilage destruction, and mainly affects the older people. The extracellular matrix (ECM) provides a vital cellular environment, and interactions between the cell and ECM are important in regulating many biological processes, including cell growth, differentiation, and survival. However, the pathogenesis of this disease is not fully elucidated, and it cannot be cured totally. Integrins are one of the major receptors in chondrocytes. A number of studies confirmed that the chondrocytes express several integrins including α5β1, αVβ3, αVβ5, α6β1, α1β1, α2β1, α10β1, and α3β1, and some integrins ligands might act as the OA progression biomarkers. This review focuses on the functional role of integrins and their extracellular ligands in OA progression, especially OA cartilage. Clear understanding of the role of integrins and their ligands in OA cartilage may have impact on future development of successful therapeutic approaches to OA.

In vivo analysis of Arg-Gly-Asp sequence/integrin α5β1-mediated signal involvement in embryonic enchondral ossification by exo utero development system

Enchondral ossification is a fundamental mechanism for longitudinal bone growth during vertebrate development. In vitro studies suggested that functional blockade with RGD peptides or with an antibody that interferes with integrin α5β1-ligand interactions inhibited pre-hypertrophic chondrocyte differentiation. The purpose of this study is to elucidate in vivo the roles of the integrin α5β1-mediated signal through the Arg-Gly-Asp (RGD) sequence in the cell-extracellular matrix (ECM) interaction in embryonic enchondral ossification by an exo utero development system. We injected Arg-Gly-Asp-Ser (RGDS) peptides and anti-integrin α5β1 antibody (α5β1 ab) in the upper limbs of mouse embryos at embryonic day (E) 15.5 (RGDS-injected limbs, α5β1 ab-injected limbs), and compared the effects on enchondral ossification with those found in the control limbs (Arg-Gly-Glu-Ser peptide-, mouse IgG-, or vehicle-injected, and no surgery) at E16.5. In the RGDS-injected limbs, the humeri were shorter and there were fewer BrdU-positive cells than in the control limbs. The ratios of cartilage length and area to those of the humerus were higher in the RGDS-injected limbs. The ratios of type X collagen to type 2 collagen mRNA and protein (Coll X/Coll 2) were significantly lower in the RGDS-injected limbs. In those limbs, TUNEL-positive cells were hardly observed, and the ratios of fractin to the Coll X/Coll 2 ratio were lower than in the control limbs. Furthermore, the α5β1 ab-injected limbs showed results similar to those of RGDS-injected limbs. The present in vivo study by exo utero development system showed that RGDS and α5β1 ab injection decreased chondrocyte proliferation, differentiation, and apoptosis in enchondral ossification, and suggested that the integrin α5β1-mediated ECM signal through the RGD sequence is involved in embryonic enchondral ossification.

Role of α2β1 integrins in mediating cell shape on microtextured titanium surfaces

Surface microroughness plays an important role in determining osteoblast behavior on titanium. Previous studies have shown that osteoblast differentiation on microtextured titanium substrates is dependent on alpha-2 beta-1 (α2β1) integrin signaling. This study used focused ion beam milling and scanning electron microscopy, combined with three-dimensional image reconstruction, to investigate early interactions of individual cells with their substrate and the role of integrin α2β1 in determining cell shape. MG63 osteoblast-like cells on sand blasted/acid etched (SLA) Ti surfaces after 3 days of culturing indicated decreased cell number, increased cell differentiation, and increased expression of mRNA levels for α1, α2, αV, and β1 integrin subunits compared to cells on smooth Ti (PT) surfaces. α2 or β1 silenced cells exhibited increased cell number and decreased differentiation on SLA compared to wild-type cells. Wild-type cells on SLA possessed an elongated morphology with reduced cell area, increased cell thickness, and more apparent contact points. Cells on PT exhibited greater spreading and were relatively flat. Silenced cells possessed a morphology and phenotype similar to wild-type cells grown on PT. These observations indicate that surface microroughness affects cell response via α2β1 integrin signaling, resulting in a cell shape that promotes osteoblastic differentiation.

Biomechanical properties and mechanobiology of the articular chondrocyte

To withstand physiological loading over a lifetime, human synovial joints are covered and protected by articular cartilage, a layer of low-friction, load-bearing tissue. The unique mechanical function of articular cartilage largely depends on the composition and structural integrity of the cartilage matrix. The matrix is produced by highly specialized resident cells called chondrocytes. Under physiological loading, chondrocytes maintain the balance between degradation and synthesis of matrix macromolecules. Under excessive loading or injury, however, degradation exceeds synthesis, causing joint degeneration and, eventually, osteoarthritis (OA). Hence, the mechanoresponses of chondrocytes play an important role in the development of OA. Despite its clear importance, the mechanobiology of articular chondrocytes is not well understood. To summarize our current understanding, here we review studies of the effect of mechanical forces on mechanical and biological properties of articular chondrocytes. First, we present the viscoelastic properties of the cell nucleus, chondrocyte, pericellular matrix, and chondron. Then we discuss how these properties change in OA. Finally, we discuss the responses of normal and osteoarthritic chondrocytes to a variety of mechanical stimuli. Studies reviewed here may provide novel insights into the pathogenesis of OA and may help in development of effective biophysical treatment.

α5β1 integrin induces the expression of noncartilaginous procollagen gene expression in articular chondrocytes cultured in monolayers

Introduction

Articular chondrocytes undergo an obvious phenotypic change when cultured in monolayers. During this change, or dedifferentiation, the expression of type I and type III procollagen is induced where normal chondrocytes express little type I and type III procollagen. In this study, we attempted to determine the mechanism(s) for the induction of such procollagen expression in dedifferentiating chondrocytes.

Methods

All experiments were performed using primary-cultured human articular chondrocytes under approval of institutional review boards. Integrin(s) responsible for the induction of type I and type III procollagen expression were specified by RNAi experiments. The signal pathway(s) involved in the induction were determined by specific inhibitors and RNAi experiments. Adenovirus-mediated experiments were performed to identify a small GTPase regulating the activity of integrins in dedifferentiating chondrocytes. The effect of inhibition of integrins on dedifferentiation was investigated by experiments using echistatin, a potent disintegrin. The effect of echistatin was investigated first with monolayer-cultured chondrocytes, and then with pellet-cultured chondrocytes.

Results

In dedifferentiating chondrocytes, α5β1 integrin was found to be involved in the induction of type I and type III procollagen expression. The induction was known to be mediated by v-akt murine thymoma viral oncogene homolog (AKT) signaling. Among the three AKT isoforms, AKT1 seemed to be most involved in the signaling. Elated RAS viral (r-ras) oncogene homolog (RRAS) was considered to regulate the progression of dedifferentiation by modulating the affinity and avidity of α5β1 integrin to ligands. Echistatin inhibited dedifferentiation of monolayer-cultured chondrocytes. Furthermore, the matrix formed by pellet-cultured chondrocytes more closely resembled that of normal cartilage compared with the controls.

Conclusions

The result of this study has shown, for the first time, that α5β1 integrin may be responsible for the induction of non-cartilaginous collagen expression in chondrocytes undergoing dedifferentiation. Again, this study has shown that the inhibition of ligand ligation to integrins may be an effective strategy to inhibit phenotypic change of cultured chondrocytes, and to improve the quality of matrix synthesized by primary cultured chondrocytes.

Effect of chondroitinase ABC on adhesion and behavior of synovial membrane-derived mesenchymal stem cells in rabbit partial-thickness chondral defects

Transplanted cells may have difficulty attaching to the surface of partial-thickness chondral lesions because of the anti-adhesive properties of the proteoglycan rich matrix. Therefore, the current study attempts to evaluate the effect of chondroitinase ABC (chABC) on the adhesion and behavior of transplanted synovial membrane-derived mesenchymal stem cells (SDSCs) in rabbit partial-thickness chondral defects. In ex vivo adhesion experiments, chABC treatment (0.1 U/ml) was increased in SDSC attachment to the cartilage explants, and significantly diminished by pretreatment with neutralizing antibody against fibronectin. In the in vivo experiments, 1 day and 4 weeks after the chABC treatment (0.1 and 1 U/ml), the immunoreactivity (IR) against CS-56 (intact chondroitin sulfate antibody) was markedly decreased; however, the IR of 2B6 (stub of the chondroitin 4-sulfate chain), 3B3 (stub of the chondroitin 6-sulfate chain), and fibronectin was increased. At 12 weeks, this IR returned to normal except in the high-dose chABC-treated group (1 U/ml). Furthermore, the attachment of SDSCs to the chondral defects after chABC treatment was increased at 7 days compared with that in the chondral defects pretreated with saline. However, the tissue repaired by SDSCs was negatively stained for type II collagen at 12 weeks. In conclusion, these results showed that the exposure to fibronectin by chABC treatment enhances the attachment of SDSCs to partial-thickness chondral defects. However, the tissue regenerated by SDSCs showed lack of hyaline cartilage regeneration. Thus, to understand the fate of transplanted MSCs in cartilage defect is very important for successful cell therapies.

Scientific evidence and rationale for the development of curcumin and resveratrol as nutraceutricals for joint health

Interleukin 1β (IL-1β) and tumor necrosis factor α (TNF-α) are key cytokines that drive the production of inflammatory mediators and matrix-degrading enzymes in osteoarthritis (OA). These proinflammatory cytokines bind to their respective cell surface receptors and activate inflammatory signaling pathways culminating with the activation of nuclear factor κB (NF-κB), a transcription factor that can be triggered by a host of stress-related stimuli including, excessive mechanical stress and ECM degradation products. Once activated, NF-κB regulates the expression of many cytokines, chemokines, adhesion molecules, inflammatory mediators, and several matrix-degrading enzymes. Therefore, proinflammatory cytokines, their cell surface receptors, NF-κB and downstream signaling pathways are therapeutic targets in OA. This paper critically reviews the recent literature and outlines the potential prophylactic properties of plant-derived phytochemicals such as curcumin and resveratrol for targeting NF-κB signaling and inflammation in OA to determine whether these phytochemicals can be used as functional foods.

New insights into cartilage repair - the role of migratory progenitor cells in osteoarthritis

Osteoarthritis is one of the most common musculo-skeletal diseases with a complex patholoy and a strong impact on cell biology, differentiation and migration behavior of mesenchymal stem cell-derived progenitor cells. In this review, we elucidate the influence of the pathologically altered extracellular matrix on progenitor cell behavior. Moreover, we discuss the modulation of progenitor cells especially of previously characterized chondrogenic progenitor cells (Koelling et al., 2009) in situ to enhance their regeneration potential. These options comprise the application of growth factors like fibroblast growth factor-2, a Runx-2 knock down and a contemporary anti-inflammatory therapy. This supports endogenous regeneration on behalf of the diseased osteoarthritic cartilage, which otherwise results mainly in an insufficient fibro-cartilaginous repair tissue. Furthermore, new results indicate a role of pericytes in osteoarthritis for these repair attempts. We discuss the biological mechanisms potentially leading to new therapeutic options in osteoarthritis to enhance regeneration in situ.

Maintenance of cartilaginous gene expression on extracellular matrix derived from serially passaged chondrocytes during in vitro chondrocyte expansion

The loss of cartilaginous phenotype during in vitro expansion culture of chondrocytes is a major barrier for the application of cartilage tissue engineering. The use of matrices mimicking the in vivo extracellular matrix (ECM) microenvironment is anticipated to be an efficient method to suppress chondrocyte phenotype loss. In this study, we developed several types of ECM derived from serially passaged chondrocytes for use as cell-culture substrata and compared their effects on chondrocyte functions. Primary bovine chondrocytes and serially passaged chondrocytes (at passages 2 and 6) were cultured on tissue-culture polystyrene. After culture, the cellular components were selectively removed from the ECM deposited by the cells. The remaining ECM proteins were used as cell-culture substrata. The composition of the deposited ECM depended on the culture stage of the serially passaged chondrocytes used for the ECM production. The deposited ECM supported the adhesion and proliferation of chondrocytes. The effects of the ECM on the chondrocyte dedifferentiation during in vitro passage culture differed dramatically depending on the phenotype of the chondrocytes used to produce the ECM. The primary chondrocyte-derived ECM delayed the chondrocyte dedifferentiation during in vitro passage culture and is a good candidate for chondrocyte subculture for tissue engineering.

Presence of cartilage stem/progenitor cells in adult mice auricular perichondrium

Background

Based on evidence from several other tissues, cartilage stem/progenitor cells in the auricular cartilage presumably contribute to tissue development or homeostasis of the auricle. However, no definitive studies have identified or characterized a stem/progenitor population in mice auricle.

Methodology/Principal Findings

The 5-bromo-2′-deoxyuridine (BrdU) label-retaining technique was used to label dividing cells in fetal mice. Observations one year following the labeling revealed that label-retaining cells (LRCs) were present specifically in auricular perichondrium at a rate of 0.08±0.06%, but LRCs were not present in chondrium. Furthermore, LRCs were successfully isolated and cultivated from auricular cartilage. Immunocytochemical analyses showed that LRCs express CD44 and integrin-α₅. These LRCs, putative stem/progenitor cells, possess clonogenicity and chondrogenic capability in vitro.

Conclusions/Significance

We have identified a population of putative cartilage stem/progenitor cells in the auricular perichondrium of mice. Further characterization and utilization of the cell population should improve our understanding of basic cartilage biology and lead to advances in cartilage tissue engineering and novel therapeutic strategies for patients with craniofacial defects, including long-term tissue restoration.

Viral arthritides

Viral infections may manifest as acute or chronic arthritis. Joint involvement arises from either direct infection of the joint, through an immunological response directed towards the virus or autoimmunity. Epidemiological clues to the diagnosis include geographic location and exposure to vector-borne, blood-borne or sexually transmitted viruses. Although not always possible, it is important to diagnose the pathogenic virus, usually by serology, nucleic acid tests or rarely, viral culture. In general, viral arthritides are self-limiting and treatment is targeted at symptomatic relief. This article focuses on the causes, clinical features, diagnosis and treatment of viral arthritides.

αvβ5 integrin promotes dedifferentiation of monolayer-cultured articular chondrocytes

OBJECTIVE

When cultured in monolayers, articular chondrocytes undergo an obvious phenotypic change. Although the involvement of integrins has been suggested, the exact mechanisms of the change have not been determined. This study was undertaken to clarify the mechanisms underlying the loss of chondrocyte phenotype early after plating.

METHODS

Primary cultured human articular chondrocytes were used for the experiments. Involvement of respective integrins in the phenotypic change was investigated in RNA interference (RNAi) experiments. A signaling pathway involved in the change was identified in experiments using specific inhibitors and adenoviruses encoding mutated genes involved in the pathway. Adenoviruses carrying mutated GTPases were used to determine the involvement of small GTPases in the process.

RESULTS

In monolayer-cultured chondrocytes, suppression of αv or β5 integrin expression by RNAi inhibited morphologic changes in the cells and increased (or prevented a reduction in) the expression of various cartilage matrix genes. Consistent results were obtained in experiments using a blocking antibody and a synthetic inhibitor of αvβ5 integrin. The decrease in cartilage matrix gene expression in chondrocytes after plating was mediated by ERK signaling, which was promoted primarily by αvβ5 integrin. In articular chondrocytes, the affinity of αvβ5 integrin for ligands was regulated by the small GTPase R-Ras. R-Ras was gradually activated in monolayer-cultured chondrocytes after plating, which caused a gradual decline in cartilage matrix gene expression through enhanced αvβ5 integrin activation and the subsequent increase in ERK signaling.

CONCLUSION

Our findings indicate that αvβ5 integrin may be involved in the change that occurs in monolayer-cultured chondrocytes after plating.

Effect of fiber diameter on the spreading, proliferation and differentiation of chondrocytes on electrospun chitosan matrices

Tissue-engineered neocartilage with appropriate biomechanical properties holds promise not only for graft applications but also as a model system for controlled studies of chondrogenesis. Our objective in the present research study is to better understand the impact of fiber diameter on the cellular activity of chondrocytes cultured on nanofibrous matrices. By using the electrospinning process, fibrous scaffolds with fiber diameters ranging from 300 nm to 1 μm were prepared and the physicomechanical properties of the scaffolds were characterized. Bovine articular chondrocytes were then seeded and maintained on the scaffolds for 7 and 14 days in culture. An upregulation in the gene expression of collagen II was noted with decreasing fiber diameters. For cells that were cultured on scaffolds with a mean fiber diameter of 300 nm, a 2-fold higher ratio of collagen II/collagen I was noted when compared to cells cultured on sponge-like scaffolds prepared by freeze drying and lyophilization. Integrin (α(5), αv, β(1)) gene expression was also observed to be influenced by matrix morphology. Our combined results suggest that matrix geometry can regulate and promote the retention of the chondrocyte genotype.

Effects of extracellular matrices derived from different cell sources on chondrocyte functions

Cell-derived extracellular matrices (ECMs) are a key factor in regulating cell functions in tissue engineering and regenerative medicine. The fact that cells are surrounded by their specific ECM in vivo elicits the need to elucidate the effects of ECM derived from different cell sources on cell functions. Here, three types of ECM were prepared by decellularizing cultured chondrocytes, fibroblasts, and mesenchymal stem cells (MSC) and used for chondrocyte culture to compare their effects on chondrocyte adhesion, proliferation, and differentiation. Chondrocyte adhesion to the chondrocyte-derived ECM was greater than those to the fibroblast- and MSC-derived ECM. Chondrocyte proliferation on the chondrocyte-derived ECM was lower than those on the fibroblast- and MSC-derived ECM. The ECM showed no evident effect on chondrocyte differentiation. The effects of ECM on cell functions depended on the cell source used to prepare the ECM.

Cell-matrix interactions and dynamic mechanical loading influence chondrocyte gene expression and bioactivity in PEG-RGD hydrogels

The pericellular matrix (PCM) surrounding chondrocytes is thought to play an important role in transmitting biochemical and biomechanical signals to the cells, which regulates many cellular functions including tissue homeostasis. To better understand chondrocytes interactions with their PCM, three-dimensional poly(ethylene glycol) (PEG) hydrogels containing Arg-Gly-Asp (RGD), the cell-adhesion sequence found in fibronectin and which is present in the PCM of cartilage, were employed. RGD was incorporated into PEG hydrogels via tethers at 0.1, 0.4 and 0.8 mM concentrations. Bovine chondrocytes were encapsulated in the hydrogels and subjected to dynamic compressive strains (0.3 Hz, 18% amplitude strain) for 48h, and their response assessed by cell morphology, ECM gene expression, cell proliferation and matrix synthesis. Incorporation of RGD did not influence cell morphology under free swelling conditions. However, the level of cell deformation upon an applied strain was greater in the presence of RGD. In the absence of dynamic loading, RGD appears to have a negative effect on chondrocyte phenotype, as seen by a 4.7-fold decrease in collagen II/collagen I expressions in 0.8mM RGD constructs. However, RGD had little effect on early responses of chondrocytes (i.e. cell proliferation and matrix synthesis/deposition). When isolating RGD as a biomechanical cue, cellular response was very different. Chondrocyte phenotype (collagen II/collagen I ratio) and proteoglycan synthesis were enhanced with higher concentrations of RGD. Overall, our findings demonstrate that RGD ligands enhance cartilage-specific gene expression and matrix synthesis, but only when mechanically stimulated, suggesting that cell-matrix interactions mediate chondrocyte response to mechanical stimulation.

Chondrogenesis of adipose-derived adult stem cells in a poly-lactide-co-glycolide scaffold

Adult adipose-derived stem cells (ASCs) are considered to be an alternative cell source for cell-based cartilage repair because of their multiple differentiation potentials. This article addresses the chondrogenic differentiation of ASCs seeded into poly-lactide-co-glycolide (PLGA) scaffolds after implantation in a subcutaneous pocket of nude mice. Human ASCs were seeded into PLGA (polylactic acid:polyglycolic acid = 90:10) scaffolds and cultured in transforming growth factor beta 1 (TGF-beta1)-containing medium for 3 weeks in vitro. Then specimens were implanted into a subcutaneous pocket of severe combined immunodeficiency mice and harvested after 8 weeks. Chondrospecific messenger RNA (mRNA) expression was analyzed using reverse transcriptase polymerase chain reaction. Corresponding extracellular matrix (ECM) synthesis was demonstrated using immunohistochemical staining. Chondrospecific marker molecules such as collagen type II and type X, cartilage oligomeric matrix protein, and aggrecan subsequently increased during the 3 weeks period in vitro. After a further 8 weeks, in vivo samples pretreated with TGF-beta1 continued expressing collagen type II and aggrecan mRNA, and collagen type II was found within the ECM using immunohistochemistry. Chondrospecific mRNA was not detected in control samples. ASC-seeded PLGA scaffolds express a stable chondrogenic phenotype in a heterotopic model of cartilage transplantation and represent a suitable tool for tissue engineering of cartilage.

Signalling cascades in mechanotransduction: cell-matrix interactions and mechanical loading

Mechanical loading of articular cartilage stimulates the metabolism of resident chondrocytes and induces the synthesis of molecules to maintain the integrity of the cartilage. Mechanical signals modulate biochemical activity and changes in cell behavior through mechanotransduction. Compression of cartilage results in complex changes within the tissue including matrix and cell deformation, hydrostatic and osmotic pressure, fluid flow, altered matrix water content, ion concentration and fixed charge density. These changes are detected by mechanoreceptors on the cell surface, which include mechanosensitive ion channels and integrins that on activation initiate intracellular signalling cascades leading to tissue remodelling. Excessive mechanical loading also influences chondrocyte metabolism but unlike physiological stimulation leads to a quantitative imbalance between anabolic and catabolic activity resulting in depletion of matrix components. In this article we focus on the role of mechanical signalling in the maintenance of articular cartilage, and discuss how alterations in normal signalling can lead to pathology.

Rho-ROCK signaling differentially regulates chondrocyte spreading on fibronectin and bone sialoprotein

The mammalian growth plate is a dynamic structure rich in extracellular matrix (ECM). Interactions of growth plate chondrocytes with ECM proteins regulate cell behavior. In this study, we compared chondrocyte adhesion and spreading dynamics on fibronectin (FN) and bone sialoprotein (BSP). Chondrocyte adhesion and spreading were also compared with fibroblasts to analyze potential cell-type-specific effects. Chondrocyte adhesion to BSP is independent of posttranslational modifications but is dependent on the RGD sequence in BSP. Whereas chondrocytes and fibroblasts adhered at similar levels on FN and BSP, cells displayed more actin-dependent spread on FN despite a 16x molar excess of BSP adsorbed to plastic. To identify intracellular mediators responsible for this difference in spreading, we investigated focal adhesion kinase (FAK)-Src and Rho-Rho kinase (ROCK) signaling. Although activated FAK localized to the vertices of adhered chondrocytes, levels of FAK activation did not correlate with the extent of spreading. Furthermore, Src inhibition reduced chondrocyte spreading on both FN and BSP, suggesting that FAK-Src signaling is not responsible for less cell spreading on BSP. In contrast, inhibition of Rho and ROCK in chondrocytes increased cell spreading on BSP and membrane protrusiveness on FN but did not affect cell adhesion. In fibroblasts, Rho inhibition increased fibroblast spreading on BSP while ROCK inhibition changed membrane protrusiveness of FN and BSP. In summary, we identify a novel role for Rho-ROCK signaling in regulating chondrocyte spreading and demonstrate both cell- and matrix molecule-specific mechanisms controlling cell spreading.

Regulation of chondrocyte differentiation by the actin cytoskeleton and adhesive interactions

Chondrocyte differentiation is a multi-step process characterized by successive changes in cell morphology and gene expression. In addition to tight regulation by numerous soluble factors, these processes are controlled by adhesive events. During the early phase of the chondrocyte life cycle, cell-cell adhesion through molecules such as N-cadherin and neural cell adhesion molecule (N-CAM) is required for differentiation of mesenchymal precursor cells to chondrocytes. At later stages, for example in growth plate chondrocytes, adhesion signaling from extracellular matrix (ECM) proteins through integrins and other ECM receptors such as the discoidin domain receptor (DDR) 2 (a collagen receptor) and Annexin V is necessary for normal chondrocyte proliferation and hypertrophy. Cell-matrix interactions are also important for chondrogenesis, for example through the activity of CD44, a receptor for Hyaluronan and collagens. The roles of several signaling molecules involved in adhesive signaling, such as integrin-linked kinase (ILK) and Rho GTPases, during chondrocyte differentiation are beginning to be understood, and the actin cytoskeleton has been identified as a common target of these adhesive pathways. Complete elucidation of the pathways connecting adhesion receptors to downstream effectors and the mechanisms integrating adhesion signaling with growth factor- and hormone-induced pathways is required for a better understanding of physiological and pathological skeletal development.

Bone growth during daily or intermittent calcitriol treatment during renal failure with advanced secondary hyperparathyroidism

Calcitriol is a standard therapy for secondary hyperparathyroidism in chronic renal failure. We evaluated whether the effect of daily or intermittent calcitriol administration is more efficient in enhancing bone growth in renal failure with advanced secondary hyperparathyroidism in weanling 5/6 nephrectomized rats loaded with phosphorus to induce severe secondary hyperparathyroidism. The animals were treated daily or three times weekly with calcitriol for 4 weeks but the total weekly dose of calcitriol was the same. Although calcitriol increased the serum calcium, it did not lower parathyroid hormone (PTH) or improve tibia and body length. Animals with renal failure and advanced secondary hyperparathyroidism had decreased PTH/PTHrP, which was accompanied by an increase in the cyclin kinase inhibitor p57(Kip2). Calcitriol treatment upregulated the PTH/PTHrP receptor but also increased inhibitors of cell proliferation such as p21(Waf1/Cip1), IGFBP3, and FGFR3. Calcitriol also enhanced markers of chondrocyte differentiation, such as IGF1, Vitamin D receptor, FGF23, and bone morphogenetic protein-7. Receptor activator of nuclear factor-kappabeta ligand levels improved with calcitriol treatment but without changes in osteoprotegerin suggesting an enhancement of osteo/chondroclastogenesis and mineralization. Overall, both daily and intermittent calcitriol had similar effects on endochondral bone growth in phosphorus-loaded rats with renal failure.

Bone growth retardation in mouse embryos expressing human collagenase 1

Cellular growth and differentiation are readouts of multiple signaling pathways from the intercellular and/or extracellular milieu. The extracellular matrix through the activation of cellular receptors transmits these signals. Therefore, extracellular matrix proteolysis could affect cell fate in a variety of biological events. However, the biological consequence of inadequate extracellular matrix degradation in vivo is not clear. We developed a mouse model expressing human collagenase (matrix metalloproteinase-1, MMP-1) under the control of Col2a1 promoter. The mice showed significant growth retardation during embryogenesis and a loss of the demarcation of zonal structure and columnar array of the cartilage. Immunological examination revealed increased degradation of type II collagen and upregulation of fibronectin and alpha(5)-integrin subunit in the transgenic cartilage. The resting zone and proliferating zone of the growth plate cartilage exhibited a simultaneous increase in bromodeoxyuridine (BrdU)-incorporated proliferating cells and terminal deoxynucleotidyl transferase-mediated X-dUTP nick-end labeling-positive apoptotic cells, respectively. Chondrocyte differentiation was not disturbed in the transgenic mice as evidenced by normal expression of the Ihh and type X collagen expression. These data demonstrate that type II collagen proteolysis is an important determinant for the skeletal outgrowth through modulation of chondrocyte survival and cartilagenous growth.

Self-assembly of fibrochondrocytes and chondrocytes for tissue engineering of the knee meniscus

Chondrocyte self-assembly in high-density scaffoldless culture has shown success in producing articular cartilage constructs, and a similar process could be applied to fibrocartilage tissue engineering. Three cell combinations were compared in self-assembly culture-100% chondrocytes, 100% meniscal fibrochondrocytes, and 50:50 co-cultures of fibrochondrocytes and chondrocytes with the goal of creating a proteoglycan, collagen I, and collagen II matrix similar to native meniscus. Two culture surfaces were also compared for self-assembly: agarose-coated wells and tissue culture plastic. After 4 weeks, the resulting self-assembled chondrocyte constructs were 10.24+/-0.63 mm in diameter and 0.96+/-0.14 mm thick, weighing 84.5+/-7.2 mg. Co-culture constructs were smaller and weighed 22.5+/-1.0 mg. In contrast, the fibrochondrocyte constructs contracted into spheres weighing 1.3+/-0.3 mg. Immunostaining showed collagen II in the chondrocyte constructs, both collagen I and collagen II in the co-cultures, and only collagen I in the fibrochondrocyte constructs. Collagen densities for chondrocyte, co-culture, and fibrochondrocyte constructs were 41+/-3, 38+/-3, and 20+/-2 microg/mg dry weight, and glycosaminoglycan densities were 230+/-2, 80+/-6, and 10+/-1 microg/mg dry weight, respectively. Self-assembled co-cultures, with their mixed collagen I and II matrix and robust gross characteristics, appear promising for tissue engineering of the knee meniscus.

In vitro culture of enzymatically isolated chondrons: a possible model for the initiation of osteoarthritis

The aim of this study was to assess whether enzymatically isolated chondrons from normal adult articular cartilage could be used as a model for the onset of osteoarthritis, by comparison with mechanically extracted chondrons from osteoarthritic cartilage. Enzymatically isolated chondrons (EC) were cultured for 4 weeks in alginate beads and agarose gel constructs. Samples were collected at days 1 and 2, and weekly thereafter. Samples were immunolabelled for types II and VI collagen, keratan sulphate and fibronectin and imaged using confocal microscopy. Mechanically extracted chondrons (MC) were isolated, immunohistochemically stained for type VI collagen and examined by confocal microscopy. In culture, EC showed the following characteristics: swelling of the chondron capsule, cell division within the capsule and remodelling of the pericellular microenvironment. This was followed by chondrocyte migration through gaps in the chondron capsule. Four types of cell clusters formed over time in both alginate beads and agarose constructs. Cells within clusters exhibited quite distinct morphologies and also differed in their patterns of matrix deposition. These differences in behaviour may be due to the origin of the chondrocytes in the intact tissue. The behaviour of EC in culture paralleled the range of morphologies observed in MC, which presented as single and double chondrons and large chondron clusters. This preliminary study indicates that EC in culture share similar structural characteristics with MC isolated from osteoarthritic cartilage, confirming that some processes that occur in osteoarthritis, such as pericellular remodelling, take place in EC cultures. The study of EC in culture may therefore provide an additional tool to investigate the mechanisms operating during the initial stages of osteoarthritis. Further investigation of specific osteoarthritic phenotype markers will, however, be required in order to validate the value of this model.

CCN2 (Connective Tissue Growth Factor) is essential for extracellular matrix production and integrin signaling in chondrocytes

The matricellular protein CCN2 (Connective Tissue Growth Factor; CTGF) is an essential mediator of ECM composition, as revealed through analysis of Ccn2 deficient mice. These die at birth due to complications arising from impaired endochondral ossification. However, the mechanism(s) by which CCN2 mediates its effects in cartilage are unclear. We investigated these mechanisms using Ccn2 ( -/- ) chondrocytes. Expression of type II collagen and aggrecan were decreased in Ccn2 (-/-) chondrocytes, confirming a defect in ECM production. Ccn2 ( -/- ) chondrocytes also exhibited impaired DNA synthesis and reduced adhesion to fibronectin. This latter defect is associated with decreased expression of alpha5 integrin. Moreover, CCN2 can bind to integrin alpha5beta1 in chondrocytes and can stimulate increased expression of integrin alpha5. Consistent with an essential role for CCN2 as a ligand for integrins, immunofluorescence and Western blot analysis revealed that levels of focal adhesion kinase (FAK) and extracellular signal-regulated kinase (ERK)1/2 phosphorylation were reduced in Ccn2 ( -/- ) chondrocytes. These findings argue that CCN2 exerts major effects in chondrocytes through its ability to (1) regulate ECM production and integrin alpha5 expression, (2) engage integrins and (3) activate integrin-mediated signaling pathways.

Time-dependent changes in adhesive force between chondrocytes and silk fibroin substrate

In tissue engineering for cartilage repair using scaffold, initial chondrocyte-material interactions are significantly important for the following cell behaviors such as phenotypic expression and matrix synthesis. Silk fibroin scaffold is considered to be one of the useful materials in/on which chondrocytes can proliferate without dedifferentiating into fibroblast-like cells and can organize a hyaline-like tissue. For the purpose of seeking some useful aspects for designing scaffold, initial adhesive force of chondrocytes to the surface of fibroin substrate was measured by using a lab-made apparatus applying the cantilever beam method. It was found that the adhesive force per unit spreading area of chondrocytes on fibroin substrate had a clear peak between 6 and 12h after seeding. From the results of immunofluorescence staining for actin and vinculin during this period, it could be thought that an immature formation of actin fibers which was uniquely observed at the periphery of cells attaching to fibroin substrate did not contribute to the increase of adhesive force. Results in this study suggested that surface of the fibroin substrate was gradually covered with some substances which inhibit the adhesion during this period. These cell-material interactions have a possibility to be useful information for designing the adhesive performance of scaffold surface in cartilage regeneration.