Cell attachment, collagen binding, and receptor analysis on bovine articular chondrocytes

Anisotropic Shape-Memory Alginate Scaffolds Functionalized with Either Type I or Type II Collagen for Cartilage Tissue Engineering

Regenerating articular cartilage and fibrocartilaginous tissue such as the meniscus is still a challenge in orthopedic medicine. While a range of different scaffolds have been developed for joint repair, none have facilitated the development of a tissue that mimics the complexity of soft tissues such as articular cartilage. Furthermore, many of these scaffolds are not designed to function in mechanically challenging joint environments. The overall goal of this study was to develop a porous, biomimetic, shape-memory alginate scaffold for directing cartilage regeneration. To this end, a scaffold was designed with architectural cues to guide cellular and neo-tissue alignment, which was additionally functionalized with a range of extracellular matrix cues to direct stem cell differentiation toward the chondrogenic lineage. Shape-memory properties were introduced by covalent cross-linking alginate using carbodiimide chemistry, while the architecture of the scaffold was modified using a directional freezing technique. Introducing such an aligned pore structure was found to improve the mechanical properties of the scaffold, and promoted higher levels of sulfated glycosaminoglycans (sGAG) and collagen deposition compared to an isotropic (nonaligned) pore geometry when seeded with adult human stem cells. Functionalization with collagen improved stem cell recruitment into the scaffold and facilitated more homogenous cartilage tissue deposition throughout the construct. Incorporating type II collagen into the scaffolds led to greater cell proliferation, higher sGAG and collagen accumulation, and the development of a stiffer tissue compared to scaffolds functionalized with type I collagen. The results of this study demonstrate how both scaffold architecture and composition can be tailored in a shape-memory alginate scaffold to direct stem cell differentiation and support the development of complex cartilaginous tissues.

A novel method to detect articular chondrocyte death during early stages of osteoarthritis using a non-invasive ApoPep-1 probe

Introduction

Current methods for early diagnosis of osteoarthritis (OA) are limited. We assessed whether in vivo detection of chondrocyte death by ApoPep-1 (CQRPPR), a peptide that binds to histone H1 of apoptotic and necrotic cells, could be used to detect the initiation of OA.

Methods

Apoptosis-induced ATDC5 cells were labeled with Annexin V and ApoPep-1. Surgical destabilization of the medial meniscus (DMM) was performed on both knees of 12-week-old male mice and severity of OA was determined by histological analysis according to the Osteoarthritis Research Society International (OARSI) guidelines. At 1, 2, 4, and 8 weeks post-surgery, mice were intravenously injected with fluorescence-labeled ApoPep-1 or control peptide and in vivo imaging was performed within 30 minutes of injection by near-infrared fluorescence (NIRF). Binding of ApoPep-1 to OA joints was demonstrated by ex vivo imaging and immunofluorescent staining using TUNEL and histone H1 and type II collagen antibodies.

Results

Strong signals of ApoPep-1 were observed on the apoptotic ATDC5 cells. Knees corresponded to grade II, III, and V OA at 2, 4, and 8 weeks after DMM, respectively. Between 2 and 8 weeks after surgery, the in vivo NIRF signal at OA-ApoPep1-injected joints was consistently stronger than sham-operated or OA-control peptide-injected joints. ApoPep-1, TUNEL, and histone H1 signals were stronger in grade II OA cartilage than sham-operated cartilage when detected by immunofluorescent staining. Type II collagen expression was similar between grade II OA and sham group.

Conclusion

ApoPep-1 can be used to detect OA in vivo by binding to apoptotic chondrocytes. This is a novel, sensitive, and rapid method which can detect apoptotic cells in OA rodent models soon after its onset.

Fabrication and characterization of nanoscale-roughened PCL/collagen micro/nanofibers treated with plasma for tissue regeneration

A nanoscale roughened poly(ε-caprolactone)/collagen fibrous mat was fabricated by the selective plasma-treatment method using a template with 800 nm holes.

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.

Surface modification of polycaprolactone scaffolds fabricated via selective laser sintering for cartilage tissue engineering

Surface modified porous polycaprolactone scaffolds fabricated via rapid prototyping techniques were evaluated for cartilage tissue engineering purposes. Polycaprolactone scaffolds manufactured by selective laser sintering (SLS) were surface modified through immersion coating with either gelatin or collagen. Three groups of scaffolds were created and compared for both mechanical and biological properties. Surface modification with collagen or gelatin improved the hydrophilicity, water uptake and mechanical strength of the pristine scaffold. From microscopic observations and biochemical analysis, collagen-modified scaffold was the best for cartilage tissue engineering in terms of cell proliferation and extracellular matrix production. Chondrocytes/collagen-modified scaffold constructs were implanted subdermally in the dorsal spaces of female nude mice. Histological and immunohistochemical staining of the retrieved implants after 8 weeks revealed enhanced cartilage tissue formation. We conclude that collagen surface modification through immersion coating on SLS-manufactured scaffolds is a feasible scaffold for cartilage tissue engineering in craniofacial reconstruction.

Anti-inflammatory effect of platelet-rich plasma on nucleus pulposus cells with response of TNF-α and IL-1

The purpose of this study was to investigate the anti-inflammatory effect of platelet-rich plasma (PRP) with collagen matrix on human nucleus pulposus (NP) cell in response to pro-inflammatory cytokines such as tumor necrosis factor-alpha (TNF-α) and interleukin-1 (IL-1). NP cells from human disks were cultured in a monolayer and maintained in the collagen matrix prior to the addition of recombinant human IL-1 and TNF-α. After applying IL-1 and TNF-α, PRP prepared by using a commercially available platelet concentration system was added. The response was investigated using real-time PCR for mRNA expression of type II collagen, aggrecan, matrix metalloproteinase-3 (MMP-3), and cyclooxygenase-2 (COX-2). The combination of IL-1β and TNF-α led to decrease of matrix synthesis gene expression such as collagen type II and aggrecan and increase of the degradation gene expression of COX-2 and MMP-3, compared to the control. Consecutive PRP exposure significantly recovered the down-regulated gene expression of collagen type II and aggrecan and significantly reduced the increased MMP-3 and COX-2 gene expression, compared to that of control groups with pro-inflammatory cytokines. The administration of PRP with collagen matrix markedly suppressed cytokine-induced pro-inflammatory degrading enzymes and mediators in the NP cell. It also rescued gene expression concerning matrix synthesis, thereby stabilizing NP cell differentiation.

Regulatory mechanisms for the development of growth plate cartilage

In vertebrates, most of the skeleton is formed through endochondral ossification. Endochondral bone formation is a complex process involving the mesenchymal condensation of undifferentiated cells, the proliferation of chondrocytes and their differentiation into hypertrophic chondrocytes, and mineralization. This process is tightly regulated by various factors including transcription factors, soluble mediators, extracellular matrices, and cell-cell and cell-matrix interactions. Defects of these factors often lead to skeletal dysplasias and short stature. Moreover, there is growing evidence that epigenetic and microRNA-mediated mechanisms also play critical roles in chondrogenesis. This review provides an overview of our current understanding of the regulators for the development of growth plate cartilage and their molecular mechanisms of action. A knowledge of the regulatory mechanisms underlying the proliferation and differentiation of chondrocytes will provide insights into future therapeutic options for skeletal disorders.

Cryopreservation-Induced Stress on Long-Term Preserved Articular Cartilage

Tissue engineered cartilage constructs have potential clinical applications in human healthcare. Their effective utilization is, however, hampered by the lack of an optimal cryopreservation procedure that ensures their availability as and when required at the patient’s bedside. Cryopreservation-induced stress represents a major barrier towards the cryopreservation of such tissue constructs, and they remain a scientific challenge despite the significant progress in the long-term storage and banking of isolated chondrocytes and thin cartilage tissue slices. These stresses are caused by intra- and extracellular ice crystallization, cryoprotectant (CPA) toxicity, suboptimal rates of cooling and warming, osmotic imbalance, and altered intracellular pH that might cause cellular death and/or a disruption of extracellular matrix (ECM). This paper reviews the cryopreservation-induced stresses on tissue engineered cartilages and discusses how they influence the integrity of the tissue during its long-term preservation. We have also reported how various antioxidants, vitamins, and plant extracts have been used to inhibit and overcome the stress during cryopreservation and provide promising results. Based on the reviewed information, the paper has also proposed some novel ways which might help in increasing the postthawing cell viability of cryopreserved cartilage.

Annexin-phospholipid interactions. Functional implications

Annexins constitute an evolutionary conserved multigene protein superfamily characterized by their ability to interact with biological membranes in a calcium dependent manner. They are expressed by all living organisms with the exception of certain unicellular organisms. The vertebrate annexin core is composed of four (eight in annexin A6) homologous domains of around 70 amino acids, with the overall shape of a slightly bent ring surrounding a central hydrophilic pore. Calcium- and phospholipid-binding sites are located on the convex side while the N-terminus links domains I and IV on the concave side. The N-terminus region shows great variability in length and amino acid sequence and it greatly influences protein stability and specific functions of annexins. These proteins interact mainly with acidic phospholipids, such as phosphatidylserine, but differences are found regarding their affinity for lipids and calcium requirements for the interaction. Annexins are involved in a wide range of intra- and extracellular biological processes in vitro, most of them directly related with the conserved ability to bind to phospholipid bilayers: membrane trafficking, membrane-cytoskeleton anchorage, ion channel activity and regulation, as well as antiinflammatory and anticoagulant activities. However, the in vivo physiological functions of annexins are just beginning to be established.

Regenerative potentials of platelet-rich plasma enhanced by collagen in retrieving pro-inflammatory cytokine-inhibited chondrogenesis

This study was undertaken to evaluate the role of collagen matrix to enhance platelet-rich plasma (PRP) effects on pro-inflammatory cytokine-induced arthritic model. We have previously demonstrated the highly regenerative roles of PRP to restore disc degeneration and osteoporosis. In this study, PRP modulated by collagen matrix was used as a regenerative and anti-inflammatory mediator to rescue the chondrocyte degeneration induced by pro-inflammatory cytokines IL-1β (10 ng/ml)+TNF-α (20 ng/ml). First, the MTT result indicated that 1 ng/ml TGF-β1 in PRP showed an optimal dosage for chondrocytes proliferation. The chondrogenic-specific gene expressions were rescued by PRP from the inhibition of IL-1β+TNF-α, especially under the modulation of collagen matrix. The inflammatory molecules activated by IL-1β+TNF-α were also significantly diminished by PRP with collagen matrix. The membrane receptors integrin α1β1 and CD44 were strongly inhibited by IL-1β+TNF-α, while this inhibition was then recovered by PRP in collagen coating condition. In a 3D model encapsulated with collagen, PRP-induced chondrogenesis were highly enhanced, such as strong restoration of type II collagen and proteoglycan from the inhibition of IL-1β+TNF-α. The result indicated that collagen matrix enhances the effect of PRP on chondrogenesis in response to pro-inflammatory cytokines. The combination of PRP and collagen matrix might facilitate a physiological microenvironment beneficial for maintaining chondrocyte homeostasis and represents an advanced osteoarthritis therapy for clinical applications.

The effect of type II collagen coating of chitosan fibrous scaffolds on mesenchymal stem cell adhesion and chondrogenesis

The biocompatibility of chitosan and its similarity to glycosaminoglycans (GAG) make it attractive for cartilage tissue engineering. We have previously reported improved chondrogenesis but limited cell adhesion on chitosan scaffolds. Our objectives were to produce chitosan scaffolds coated with different densities of type II collagen and to evaluate the effect of this coating on mesenchymal stem cell (MSC) adhesion and chondrogenesis. Chitosan fibrous scaffolds were obtained by a wet spinning method and coated with type II collagen at two different densities. A polyglycolic acid mesh served as a reference group. The scaffolds were characterized by Fourier-transform infrared spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and type II collagen content. Constructs were analyzed after MSCs seeding via live/dead assay, weight and DNA evaluations, SEM, and TEM. Constructs were cultured in chondrogenic medium for 21 days prior to quantitative analysis (weight, DNA, and GAG), SEM, TEM, histology, immunohistochemistry, and quantitative real time polymerase chain reaction. The cell attachment and distribution after seeding correlated with the density of type II collagen. The cell number, the matrix production, and the expression of genes specific for chondrogenesis were improved after culture in collagen coated chitosan constructs. These findings encourage the use of type II collagen for coating chitosan scaffolds to improve MSCs adhesion and chondrogenesis, and confirm the importance of biomimetic scaffolds for tissue engineering.

Effect of collagen II coating on mesenchymal stem cell adhesion on chitosan and on reacetylated chitosan fibrous scaffolds

The biocompatibility and biomimetic properties of chitosan make it attractive for tissue engineering but its use is limited by its cell adhesion properties. Our objectives were to produce and characterize chitosan and reacetylated-chitosan fibrous scaffolds coated with type II collagen and to evaluate the effect of these chemical modifications on mesenchymal stem cell (MSC) adhesion. Chitosan and reacetylated-chitosan scaffolds obtained by a wet spinning method were coated with type II collagen. Scaffolds were characterized prior to seeding with MSCs. The constructs were analyzed for cell binding kinetics, numbers, distribution and viability. Cell attachment and distribution were improved on chitosan coated with type II collagen. MSCs adhered less to reacetylated-chitosan and collagen coating did not improve MSCs attachment on those scaffolds. These findings are promising and encourage the evaluation of the differentiation of MSCs in collagen-coated chitosan scaffolds. However, the decreased cell adhesion on reacetylated chitosan scaffold seems difficult to overcome and will limit its use for tissue engineering.

Cartilage tissue engineering on fibrous chitosan scaffolds produced by a replica molding technique

The biocompatibility of chitosan and its similarity with glycosaminoglycans make it attractive as a scaffold for cartilage engineering. Fibrous scaffolds may simulate cartilage extracellular matrix structure and promote chondrocyte functions. Our objectives were to produce chitosan fibers of different size and evaluate their potential for chondrogenesis. A novel replica molding technique was developed to produce chitosan nonwoven scaffolds made of fiber measuring 4, 13, or 22 mum in width. A polyglycolic acid mesh (PGA) served as a reference group. Controls were analyzed 48 h after seeding porcine chondrocytes via scanning electron microscopy (SEM), DNA, and glycosaminoglycan (GAG) quantifications. Constructs were cultured for 21 days prior to confocal microscopy, SEM, histology, and quantitative analysis (weight, water, DNA, GAG and collagen II). Chondrocytes maintained their phenotypic appearance and a viability above 85% on the chitosan scaffolds. Chondrocytes attach preferentially to PGA, resulting in a greater cellularity of these constructs. However, based on the GAG/DNA and Collagen II/DNA ratios, matrix production per chondrocyte was improved in chitosan constructs, especially on smaller fibers. The differences between PGA and chitosan are more likely to result from the chemical composition rather than their structural characteristics. Although chitosan appears to promote matrix formation, further studies should be aimed at improving its cell adhesion properties.

Chondrocyte aggregation in suspension culture is GFOGER-GPP- and beta1 integrin-dependent

Isolated chondrocytes form aggregates in suspension culture that maintain chondrocyte phenotype in a physiological pericellular environment. The molecular mechanisms involved in chondrocyte aggregation have not been previously identified. Using this novel suspension culture system, we performed mRNA and protein expression analysis along with immunohistochemistry for potential cell adhesion molecules and extracellular matrix integrin ligands. Inhibition of aggregation assays were performed using specific blocking agents. We found that: (i) direct cell-cell interactions were not involved in chondrocyte aggregation, (ii) chondrocytes in aggregates were surrounded by a matrix rich in collagen II and cartilage oligomeric protein (COMP), (iii) aggregation depends on a beta1-integrin, which binds a triple helical GFOGER sequence found in collagens, (iv) integrin alpha10-subunit is the most highly expressed alpha-subunit among those tested, including alpha5, in aggregating chondrocytes. Taken together, this body of evidence suggests that the main molecular interaction involved in aggregation of phenotypically stable chondrocytes is the alpha10beta1-collagen II interaction.

Collagen/annexin V interactions regulate chondrocyte mineralization

Physiological mineralization in growth plate cartilage is highly regulated and restricted to terminally differentiated chondrocytes. Because mineralization occurs in the extracellular matrix, we asked whether major extracellular matrix components (collagens) of growth plate cartilage are directly involved in regulating the mineralization process. Our findings show that types II and X collagen interacted with cell surface-expressed annexin V. These interactions led to a stimulation of annexin V-mediated Ca(2+) influx resulting in an increased intracellular Ca(2+) concentration, [Ca(2+)](i), and ultimately increased alkaline phosphatase activity and mineralization of growth plate chondrocytes. Consequently, stimulation of these interactions (ascorbate to stimulate collagen synthesis, culturing cells on type II collagen-coated dishes, or overexpression of full-length annexin V) resulted in increase of [Ca(2+)](i), alkaline phosphatase activity, and mineralization of growth plate chondrocytes, whereas inhibition of these interactions (3,4-dehydro-l-proline to inhibit collagen secretion, K-201, a specific annexin channel blocker, overexpression of N terminus-deleted mutant annexin V that does not bind to type II collagen and shows reduced Ca(2+) channel activities) decreased [Ca(2+)](i), alkaline phosphatase activity, and mineralization. In conclusion, the interactions between collagen and annexin V regulate mineralization of growth plate cartilage. Because annexin V is up-regulated during pathological mineralization events of articular cartilage, it is possible that these interactions also regulate pathological mineralization.

The effect of chondrocyte cryopreservation on cartilage engineering

Chondrocytes were collected from the stifle joints of four pigs to study the effect of cryopreservation on the chondrogenic potential of chondrocytes. Half of the cells were cryopreserved for 3months. Polyglycolic acid scaffolds were cultured with fresh or cryopreserved chondrocytes for 4weeks. Cell morphology and the quality of engineered tissue were evaluated by scanning electron microscopy, histopathology and biochemical methods. More cells attached to scaffolds at 48h when fresh chondrocytes were seeded. At 4weeks, the numbers of cells, DNA and collagen II were greater in constructs engineered by fresh cells. However, the collagen II/DNA ratio did not differ between the two groups. More matrix was identified on a scanning electron microscope and by histopathology in the fresh group. Cartilage engineered with cryopreserved chondrocytes may contain less matrix and fewer cells. These findings most likely resulted from a lack of cell attachment on the matrix secondary to cryopreservation. Future studies are needed to further evaluate the mechanism by which cryopreservation may affect chondrocyte attachment.

Impact of expansion and redifferentiation conditions on chondrogenic capacity of cultured chondrocytes

Cartilage regeneration based on isolated and culture-expanded chondrocytes is studied in a variety of in vitro models, but with varying morphological quality of tissue synthesized. The goal of the present study was to investigate the extent of the influence of expansion and redifferentiation conditions on final tissue morphology by comparing 2 expansion and redifferentiation methods. Chondrocytes from 9 human donors were expanded in medium without growth factor supplementation (basic expansion condition [BEC]) or in medium with basic fibroblast growth factor (bFGF) supplementation (growth factor supplemented expansion condition [GFSEC]). After expansion, cells were either redifferentiated in pellet culture or seeded on collagen type II-coated filters. Post-expansion mRNA levels of collagen type I and II and Sox-5, -6, and 9, measured by semiquantitative real-time polymerase chain reaction (PCR), suggested that expansion in GFSEC results in increased dedifferentiation compared to BEC. However, after 28 days of redifferentiation culture, morphology of tissue synthesized by GFSEC-expanded chondrocytes scored significantly higher on the Bern scale compared to BEC (6.4 +/- 0.3 points vs. 4.5 +/- 0.3 points in pellet culture and 6.0 +/- 0.4 points vs. 4.5 +/- 0.3 points on collagen-coated filters; p < 0.05). Expansion in GFSEC compared to BEC increased proteoglycan (PG) synthesis rate at day 9 (4.0-fold in pellet culture and 1.9-fold on collagen-coated filters; p < 0.01), PG release (6.7-fold in pellet culture and 3.2-fold on collagen-coated filters; p < 0.001), and final PG content at day 28 (1.6-fold in pellet culture and 1.5-fold on collagen-coated filters; p < 0.05). Redifferentiation on collagen-coated filters compared to pellet culture increased PG synthesis rate at day 9 (5.2-fold in BEC-expanded chondrocytes and 2.6-fold in GFSEC-expanded chondrocytes; p < 0.01), PG release (4.2-fold in BEC-expanded chondrocytes and 3.1-fold in GFSECexpanded chondrocytes; p < 0.01), and final PG content (1.3-fold in BEC-expanded chondrocytes and 1.9- fold in GFSEC-expanded chondrocytes; p < 0.01). Moreover, as visualized via electron microscopy, chondrocytes and organization of extracellular matrix cultured on filters was more similar to those found for hyaline cartilage. In conclusion, chondrocyte expansion in GFSEC and redifferentiation on collagen-coated filters resulted in most optimal chondrogenesis.

Maturational differences in superficial and deep zone articular chondrocytes

To examine whether differences in chondrocytes from skeletally immature versus adult individuals are important in cartilage healing, repair, or tissue engineering, superficial zone chondrocytes (SZC, from within 100 microm of the articular surface) and deep zone chondrocytes (DZC, from 30%-45% of the deepest un-mineralized part of articular cartilage) were harvested from immature (1-4 months) and young adult (18-36 months) steers and compared. Cell size, matrix gene expression and protein levels, integrin levels, and chemotactic ability were measured in cells maintained in micromass culture for up to 7 days. Regardless of age, SZC were smaller, had a lower type II to type I collagen gene expression ratio, and higher gene expression of SZ proteins than their DZC counterparts. Regardless of zone, chondrocytes from immature steers had higher levels of Sox 9 and type II collagen gene expression. Over 7 days in culture, the SZC of immature steers had the highest rate of proliferation. Phenotypically, the SZC of immature and adult steers were more stable than their respective DZC. Cell surface alpha5 and alpha2 integrin subunit levels were higher in the SZC of immature than of adult steers, whereas beta1 integrin subunit levels were similar. Both immature and adult SZC were capable of chemotaxis in response to fetal bovine serum or basic fibroblast growth factor. Our data indicate that articular chondrocytes vary in the different zones of cartilage and with the age of the donor. These differences may be important for cartilage growth, tissue engineering, and/or repair.

Effect of extracellular matrix protein on the rate of proteoglycan synthesis in rabbit intervertebral disc cells

OBJECTIVE

The extracellular matrix (ECM) is very important for fundamental cellular processes. However, the effects of ECM proteins on intervertebral disc (IVD) cell proliferation and metabolism have not been clarified. To verify the effects of ECM proteins on DNA and proteoglycan (PG) synthesis of IVD cells, PG synthesis rate was measured in IVD cells cultured in monolayer with or without ECM protein.

METHODS

Nucleus pulposus (NP) cells and anulus fibrosus (AF) cells isolated from adolescent rabbits were cultured in monolayer with or without ECM protein and at different concentrations of ECM protein for 4-6 days. [S]Sulfate incorporation into PG in the cell-associated matrix (CM) formed around cells and the further-removed matrix (FRM) in labeling medium was measured and standardized to DNA content.

CONCLUSIONS

NP cells in type I or type II collagen-coated plates significantly increased the rate of PG synthesis in both the CM and the FRM compared with those in uncoated plates and in fibronectin-coated plates; however, AF cells with ECM proteins did not increase the rate significantly. The rate of PG synthesis of nucleus cells was contra-dose dependent on both type I and type II collagen.

Chondrocyte phenotypes on different extracellular matrix monolayers

Chondrocytes undergo a process of dedifferentiation in monolayer culture that is characterized by a transition to a fibroblast-like phenotype. This behavioral change poses a challenge for tissue-engineered cartilage constructs, as approaches using autologous cells require expansion in vitro. Because chondrocytes express a variety of integrin receptors specific to different adhesive proteins, we hypothesized that chondrocytes expanded on various underlying protein monolayers would have different phenotypic responses. Bovine articular chondrocytes were cultured for up to 2 weeks on tissue culture plastic, fibronectin, collagen type I or collagen type II substrate in the presence or absence of ascorbate. Contrary to our hypothesis, the extracellular matrix protein substrates used in this study did not significantly alter the changes in chondrocyte morphology, gene expression, matrix formation, or cytoskeletal organization. Cells on all substrates assembled equivalent matrices, which may have subsequently regulated cell behavior. In cultures with ascorbate, populations of round and spread cells emerged after 1 week, with round cells expressing collagen type II and the differentiated phenotype and spread cells dedifferentiating. In cultures without ascorbate, chondrocytes rapidly adhered and spread onto organized fibronectin matrices via the alpha5beta1 integrin, which has been associated with survival and proliferation of chondrocytes in vitro. These findings indicate that expanding chondrocytes on protein monolayers may not be an effective solution to preventing dedifferentiation and improving autologous chondrocyte transplantation.

Effect of substrate mechanics on chondrocyte adhesion to modified alginate surfaces

This study characterized the attachment of chondrocytes to RGD-functionalized alginate by examining the effect of substrate stiffness on cell attachment and morphology. Bovine chondrocytes were added to wells coated with 2% alginate or RGD-alginate. The alginate was crosslinked with divalent cations ranging from 1.25 to 62.5 mmol/g alginate. Attachment to RGD-alginate was 10-20 times higher than attachment to unmodified alginate and was significantly inhibited by antibodies to integrin subunits alpha3l and beta1, cytochalasin-D, and soluble RGD peptide. The equilibrium level and rate of attachment increased with crosslink density and substrate stiffness. Substrate stiffness also regulated chondrocyte morphology, which changed from a rounded shape with nebulous actin on weaker substrates to a predominantly flat morphology with actin stress fibers on stiffer substrates. The dependence of attachment on integrins and substrate stiffness suggests that chondrocyte integrins may play a role in sensing the mechanical properties of the matrices to which they are attached.

Chondrocytes attach to hyaline or calcified cartilage and bone

OBJECTIVE

The initial attachment of transplanted chondrocytes to the surface of a cartilage defect is crucial for the success of chondrocyte transplantation. The purpose of this study was to investigate the early interaction of chondrocytes with the deep or calcified zones of cartilage or the subchondral bone, joint surfaces to which transplanted chondrocytes might have to attach in vivo.

DESIGN

Freshly isolated (primary) or passaged (P1) chondrocytes were seeded on the top of bone plugs having either a surface composed of mid-deep zone hyaline cartilage or calcified cartilage or bone only. The percent of cells that attached, the role of integrins in cell attachment, and gene expression after placement of the cells on the different surfaces were determined.

RESULTS

Both primary and passaged chondrocytes attached efficiently to all three surfaces (over 88% of seeded cells). The chondrocytes showed a punctate distribution of beta 1-integrin and vinculin, which in areas co-localized with actin, suggesting that the cells formed focal adhesions. The primary chondrocytes had a different shape, appearance of focal contacts, and actin distribution when compared to passaged cells and these did not appear to be influenced by the type of surface to which the cells attached. Blocking either beta 1-integrin or alpha v beta 5 integrin partially inhibited (between 27 to 48% and 26 to 37% respectively) attachment of both primary and passaged chondrocytes to all surfaces. Blocking alpha v beta 3 had no effect on adhesion. There was expression of type II collagen and aggrecan core protein mRNA by 2h. The different surfaces did not appear to affect the expression of these genes up to 24h although gene levels were lower in passaged cells.

CONCLUSIONS

Chondrocytes, either freshly isolated or passaged, have the potential to adhere to the different joint surfaces that could be exposed in a cartilage defect. Understanding how chondrocytes adhere and interact with damaged joint surfaces may help identify methods to enhance the retention of transplanted cells in the defect site and cartilage tissue formation.

Articular cartilage functional histomorphology and mechanobiology: a research perspective

The histomorphogenesis of articular cartilage is regulated during skeletal development by the intermittent forces and motions imposed at diarthrodial joints. A key feature in this development is the formation of the superficial, transitional, radial, and calcified cartilage zones through the cartilage thickness. The histomorphological, biological, and mechanical characteristics of these zones can be correlated with the distributions of pressures, deformations, and pressure-induced fluid flow that are created in vivo. In a mature joint, cyclic loads produce cyclic hydrostatic fluid pressure through the entire cartilage thickness that is comparable in magnitude to the applied joint pressure. Prolonged physical activity can cause the total cartilage thickness to decrease about 5%, although the consolidation strains vary tremendously in the different zones. The superficial zone can experience significant fluid exudation and consolidation (compressive strains) in the range of 60% while the radial zone experiences relatively little fluid flow and consolidation. The topological variation in the histomorphologic appearance of articular cartilage is influenced by the local mechanical loading of chondrocytes in the different zones. Patterns of stress, strain, and fluid flow created in the joint result in spatial and temporal changes in the rates of synthesis and degradation of matrix proteins. When viewed over the course of a lifetime, even subtle difference in these cellular processes can affect the micro- and macro-morphology of articular cartilage. This hypothesis is supported by in vivo and ex vivo experiments where load-induced changes in matrix synthesis and catabolism, gene expression, and signal transduction pathways have been observed.

Role of pericellular matrix in development of a mechanically functional neocartilage

The role of the chondrocyte pericellular matrix (PCM) was examined in a three-dimensional chondrocyte culture system to determine whether retention of the native pericellular matrix could stimulate collagen and proteoglycan accumulation and also promote the formation of a mechanically functional hyaline-like neocartilage. Porcine chondrocytes and chondrons, consisting of the chondrocyte with its intact pericellular matrix, were maintained in pellet culture for up to 12 weeks. Sulfated glycosaminoclycans and type II collagen were measured biochemically. Immunocytochemistry was used to examine collagen localization as well as cell distribution within the pellets. In addition, the equilibrium compressive moduli of developing pellets were measured to determine whether matrix deposition contributed to the mechanical stiffness of the cartilage constructs. Pellets increased in size and weight over a 6-week period without apparent cell proliferation. Although chondrocytes quickly rebuilt a PCM rich in type VI collagen, chondron pellets accumulated significantly more proteoglycan and type II collagen than did chondrocyte pellets, indicating a greater positive effect of the native PCM. After 5 weeks in chondron pellets, matrix remodeling was evident by microscopy. Cells that had been uniformly distributed throughout the pellets began to cluster between large areas of interterritorial matrix rich in type II collagen. After 12 weeks, clusters were stacked in columns. A rapid increase in compressive strength was observed between 1 and 3 weeks in culture for both chondron and chondrocyte pellets and, by 6 weeks, both had achieved 25% of the equilibrium compressive stiffness of cartilage explants. Retention of the in vivo PCM during chondrocyte isolation promotes the formation of a mechanically functional neocartilage construct, suitable for modeling the responses of articular cartilage to chemical stimuli or mechanical compression.

Integrin-mediated adhesion of human articular chondrocytes to cartilage

OBJECTIVE

To determine 1) the kinetics and strength of adhesion of human articular chondrocytes to a cut cartilage surface, and 2) the role of specific integrins in mediating such adhesion, using an in vitro model.

METHODS

Human articular chondrocytes isolated from cadaveric donors (mean +/- SD age 38 +/- 13 years) were cultured in high-density or low-density monolayer. Following release from culture with trypsin and a 2-2.5-hour recovery period, chondrocytes were analyzed either for adhesion to cartilage or for integrin expression by flow cytometry.

RESULTS

Following culture in monolayer, adhesion of chondrocytes to cartilage increased with time, from 6-16% at 10 minutes to a maximum of 59-82% at 80-320 minutes. After 80 minutes of adhesion, the resistance of cells to flow-induced shear stress (50% detachment) was approximately 21 Pa. Chondrocyte adhesion to cartilage decreased with pretreatment of cells with monoclonal antibodies that bound to and blocked certain integrins. After an 80-minute incubation time, adhesion of chondrocytes cultured in high-density monolayer decreased from the value of IgG1-treated controls (55%) with blocking of the beta1 integrin subunit (to 23%) or with blocking of alpha 5 beta 1 (to 36%). Following expansion of chondrocytes in low-density monolayer, the mechanisms of adhesion to cartilage were generally similar. After an 80-minute incubation time, adhesion of chondrocytes cultured in low-density monolayer decreased from the value of IgG1-treated controls (62%) with blocking of the beta1 integrin subunit (to 30%) or with blocking of alpha 5 beta 1 (to 44%). Additionally, adhesion of these cells decreased to 46% by blocking of alpha v beta 5, with a similar trend in effect for chondrocytes cultured in high-density monolayer. Blocking of the alpha 1 or alpha 3 integrin subunits or alpha v beta 3 had no detectable effect on adhesion, even though these receptors were detected by flow cytometry.

CONCLUSION

Under the culture and seeding conditions studied, beta1, alpha 5 beta 1, and alpha v beta 5 integrins mediate human chondrocyte adhesion to cartilage. These chondrocyte integrins have a potential role in the initial adhesion and retention of chondrocytes at a cartilage defect site following clinical procedures of chondrocyte transplantation.

Modeling the dynamic composition of engineered cartilage

Mathematical models to describe extracellular matrix (ECM) deposition and scaffold degradation in cell-polymer constructs for the design of engineered cartilage were developed and validated. The ECM deposition model characterized a product-inhibition mechanism in the concentration of cartilage molecules, collagen and glycosaminoglycans (GAG). The scaffold degradation model used first-order kinetics to describe hydrolysis (not limited by diffusion) of biodegradable polyesters, polyglycolic acid and polylactic acid. Each model was fit to published accumulation and degradation data. As experimental validation, cell-polymer constructs (n=24) and unseeded scaffolds (n=24) were cultured in vitro. Biochemical assays for ECM content and measurements of scaffold mass were performed at 1, 2, 4, 6, 8, or 10 weeks (n=8 per time point). The models demonstrated a strong fit with published data and experimental results (R(2)=0.75 to 0.99) and predicted the temporal total construct mass with reasonable accuracy (30% RMS error). This approach can elucidate mechanisms governing accumulation/degradation and may be coupled with structure-function relationships to describe time-dependent changes in construct elastic properties.

Interaction of chondrocytes, extracellular matrix and growth factors: relevance for articular cartilage tissue engineering

The abundant extracellular matrix of articular cartilage has to be maintained by a limited number of chondrocytes. Vice versa, the extracellular matrix has an important role in the regulation of chondrocyte function.

OBJECTIVE

In this review we discuss the role of the extracellular matrix in the regulation of chondrocyte function and the relevance for cartilage tissue engineering. To reach this goal the international literature on this subject has been searched with a major focus on the last 5 years.

RESULTS

Structural matrix macromolecules (e.g. collagen, hyaluronate), but also growth factors (e.g. IGF-I, TGF beta) entrapped in the matrix and released under specific conditions affect chondrocyte behavior. These factors communicate with the chondrocyte via specific membrane receptors. In this way there is a close interaction between the extracellular and intracellular milieu. Articular cartilage has a limited capacity of intrinsic repair, which has resulted in the development of tissue engineering approaches to repair damaged cartilage. Successful application of scaffolds has to take into account the important role of both soluble and insoluble matrix-derived factors in cartilage homeostasis.

CONCLUSION

Functional tissue engineering will only be realized when the scaffolds used will provide cartilage cells with the correct extracellular signals.

Mechanisms of chondrocyte adhesion to cartilage: role of beta1-integrins, CD44, and annexin V

The initial adhesion of transplanted chondrocytes to surrounding host cartilage may be important in the repair of articular defects. Adhesion may position cells to secrete molecules that fill the defect and integrate repair tissue with host tissue. While chondrocytes are known to become increasingly adherent to cartilage with time, the molecular basis for this is unknown. The objective of this study was to investigate the role of beta1-integrin, CD44, and annexin V receptors in chondrocyte adhesion to cartilage. Chondrocytes were cultured in high density monolayer, released with trypsin, and allowed to recover in suspension for 2 h at 37 degrees C. Under these conditions, flow cytometry analysis showed that chondrocytes expressed beta1-integrins, CD44, and annexin V. In a rapid screening assay to assess chondrocyte adhesion to cartilage, cell detachment decreased from 79% at 10 min following transplantation to 10% at 320 min. Treatment of cells with a monoclonal antibody to block beta1-integrins significantly increased chondrocyte detachment from cartilage compared to untreated controls. Similarly, results from a parallel-plate shear flow adhesion assay showed that blocking beta1-integrins significantly increased chondrocyte detachment from cartilage compared to untreated controls at each level of applied shear (0-70 Pa). In both assays, treatment of cells with reagents that block CD44 (hyaluronan oligosaccharides or monoclonal Ab IM7) or annexin V (polyclonal Ab #8958) had no detectable effect on adhesion. With cartilage treated with chondroitinase ABC, blocking beta1-integrins also increased chondrocyte detachment, while blocking CD44 and annexin V also had no detectable effect. Under the conditions studied here, beta1-integrins appear to mediate chondrocyte adhesion to a cut cartilage surface. Delineation of the mechanisms of adhesion may have clinical implications by allowing cell manipulations or matrix treatments to enhance chondrocyte adhesion and retention at a defect site.