Gene expression of single articular chondrocytes

Membrane stretch as the mechanism of activation of PIEZO1 ion channels in chondrocytes

Osteoarthritis is a chronic disease that can be initiated by altered joint loading or injury of the cartilage. The mechanically sensitive PIEZO ion channels have been shown to transduce injurious levels of biomechanical strain in articular chondrocytes and mediate cell death. However, the mechanisms of channel gating in response to high cellular deformation and the strain thresholds for activating PIEZO channels remain unclear. We coupled studies of single-cell compression using atomic force microscopy (AFM) with finite element modeling (FEM) to identify the biophysical mechanisms of PIEZO-mediated calcium (Ca2+) signaling in chondrocytes. We showed that PIEZO1 and PIEZO2 are needed for initiating Ca2+ signaling at moderately high levels of cellular deformation, but at the highest strains, PIEZO1 functions independently of PIEZO2. Biophysical factors that increase apparent chondrocyte membrane tension, including hypoosmotic prestrain, high compression magnitudes, and low deformation rates, also increased PIEZO1-driven Ca2+ signaling. Combined AFM/FEM studies showed that 50% of chondrocytes exhibit Ca2+ signaling at 80 to 85% nominal cell compression, corresponding to a threshold of apparent membrane finite principal strain of E = 1.31, which represents a membrane stretch ratio (λ) of 1.9. Both intracellular and extracellular Ca2+ are necessary for the PIEZO1-mediated Ca2+ signaling response to compression. Our results suggest that PIEZO1-induced signaling drives chondrocyte mechanical injury due to high membrane tension, and this threshold can be altered by factors that influence membrane prestress, such as cartilage hypoosmolarity, secondary to proteoglycan loss. These findings suggest that modulating PIEZO1 activation or downstream signaling may offer avenues for the prevention or treatment of osteoarthritis.

Human Salivary Histatin-1-Functionalized Gelatin Methacrylate Hydrogels Promote the Regeneration of Cartilage and Subchondral Bone in Temporomandibular Joints

The avascular structure and lack of regenerative cells make the repair of osteochondral defects in the temporomandibular joint (TMJ) highly challenging in the clinic. To provide a viable treatment option, we developed a methacrylated gelatin (Gel-MA) hydrogel functionalized with human salivary histatin-1 (Hst1). Gel-MA is highly biocompatible, biodegradable, and cost-effective. Hst1 is capable of activating a series of cell activities, such as adhesion, migration, differentiation, and angiogenesis. To evaluate the efficacy of Hst1/Gel-MA, critical-size osteochondral defects (3 mm in diameter and 3 mm in depth) of TMJ in New Zealand white rabbits were surgically created and randomly assigned to one of the three treatment groups: (1) control (no filling material); (2) Gel-MA hydrogel; (3) Hst1/Gel-MA hydrogel. Samples were retrieved 1, 2, and 4 weeks post-surgery and subjected to gross examination and a series of histomorphometric and immunological analyses. In comparison with the control and Gel-MA alone groups, Hst1/Gel-MA hydrogel was associated with significantly higher International Cartilage Repair Society score, modified O’Driscoll score, area percentages of newly formed bone, cartilage, collagen fiber, and glycosaminoglycan, and expression of collagen II and aggrecan. In conclusion, Hst1/Gel-MA hydrogels significantly enhance bone and cartilage regeneration, thus bearing promising application potential for repairing osteochondral defects.

Biotribology of Synovial Cartilage: A New Method for Visualization of Lubricating Film and Simultaneous Measurement of the Friction Coefficient

A healthy natural synovial joint is very important for painless active movement of the natural musculoskeletal system. The right functioning of natural synovial joints ensures well lubricated contact surfaces with a very low friction coefficient and wear of cartilage tissue. The present paper deals with a new method for visualization of lubricating film with simultaneous measurements of the friction coefficient. This can contribute to better understanding of lubricating film formation in a natural synovial joint. A newly developed device, a reciprocating tribometer, is used to allow for simultaneous measurement of friction forces with contact visualization by fluorescence microscopy. The software allowing for snaps processing and subsequent evaluation of fluorescence records is developed. The evaluation software and the follow-up evaluation procedure are also described. The experiments with cartilage samples and model synovial fluid are carried out, and the new software is applied to provide their evaluation. The primary results explaining a connection between lubrication and friction are presented. The results show a more significant impact of albumin proteins on the lubrication process, whereas its clusters create a more stable lubrication layer. A decreasing trend of protein cluster count, which corresponds to a decrease in the thickness of the lubrication film, is found in all experiments. The results highlight a deeper connection between the cartilage friction and the lubrication film formation, which allows for better understanding of the cartilage lubrication mechanism.

Ectopic Telomerase Expression Fails to Maintain Chondrogenic Capacity in Three-Dimensional Cultures of Clinically Relevant Cell Types

The poor healing capacity of cartilage and lack of effective treatment for associated disease and trauma makes it a strong candidate for a regenerative medicine approach. Potential therapies tested to date, although effective, have met with a number of intrinsic difficulties possibly related to limited autologous chondrocyte cell yield and quality of cartilage produced. A potential mechanism to bypass limited cell yields and improve quality of differentiation is to immortalize relevant cell types through the ectopic expression of telomerase. Pellet cultures of human chondrocytes (OK3), bone marrow mesenchymal stem cells (BMA13), and embryonic stem cell (H1 line)-derived cells (1C6) and their human telomerase reverse transcriptase (hTERT) transduced counterparts were maintained for 20 days in standard maintenance medium (MM) or transforming growth factor-β3-supplemented prochondrogenic medium (PChM). Pellets were assessed for volume and density by microcomputed tomography. Quantitative gene expression (COL1A2, COL2A1, COL3A1, COL6A3, COL10A1, ACAN, COMP, SOX9); sulfated glycosaminoglycans (sGAGs), and DNA quantification were performed. Histology and immunohistochemistry were used to determine matrix constituent distribution. Pellet culture in PChM resulted in significantly larger pellets with an overall increased density when compared with MM culture. Gene expression analysis revealed similarities in expression patterns between telomerase-transduced and parental cells in both MM and PChM. Of the three parental cell types examined OK3 and BMA13 produced similar amounts of pellet-associated sGAG in PChM (4.62 ± 1.20 and 4.91 ± 1.37 μg, respectively) with lower amounts in 1C6 (2.89 ± 0.52 μg), corresponding to 3.1, 2.3, and 1.6-fold increases from day 0. In comparison, telomerase-transduced cells all had much lower sGAG with OK3H at 2.74 ± 0.11 μg, BMA13H 1.29 ± 0.34 μg, and 1C6H 0.52 ± 0.01 μg corresponding to 1.2, 0.87, and 0.34-fold changes compared with day 0. Histology of day 20 pellets displayed reduced staining overall for collagens and sGAG in telomerase-transduced cells, most notably with alterations in aggrecan and collagen VI; all cells stained positively for collagen II. We conclude that while telomerase transduction may be an effective technique to extend cellular proliferative capacity, it is not sufficient in isolation to sustain a naive chondrogenic phenotype across multiple cell types.

Optimisation of high-quality total ribonucleic acid isolation from cartilaginous tissues for real-time polymerase chain reaction analysis

Objectives

Studies which consider the molecular mechanisms of degeneration and regeneration of cartilaginous tissues are seriously hampered by problematic ribonucleic acid (RNA) isolations due to low cell density and the dense, proteoglycan-rich extracellular matrix of cartilage. Proteoglycans tend to co-purify with RNA, they can absorb the full spectrum of UV light and they are potent inhibitors of polymerase chain reaction (PCR). Therefore, the objective of the present study is to compare and optimise different homogenisation methods and RNA isolation kits for an array of cartilaginous tissues.

Materials and Methods

Tissue samples such as the nucleus pulposus (NP), annulus fibrosus (AF), articular cartilage (AC) and meniscus, were collected from goats and homogenised by either the MagNA Lyser or Freezer Mill. RNA of duplicate samples was subsequently isolated by either TRIzol (benchmark), or the RNeasy Lipid Tissue, RNeasy Fibrous Tissue, or Aurum Total RNA Fatty and Fibrous Tissue kits. RNA yield, purity, and integrity were determined and gene expression levels of type II collagen and aggrecan were measured by real-time PCR.

Results

No differences between the two homogenisation methods were found. RNA isolation using the RNeasy Fibrous and Lipid kits resulted in the purest RNA (A260/A280 ratio), whereas TRIzol isolations resulted in RNA that is not as pure, and show a larger difference in gene expression of duplicate samples compared with both RNeasy kits. The Aurum kit showed low reproducibility.

Conclusion

For the extraction of high-quality RNA from cartilaginous structures, we suggest homogenisation of the samples by the MagNA Lyser. For AC, NP and AF we recommend the RNeasy Fibrous kit, whereas for the meniscus the RNeasy Lipid kit is advised.

Cite this article: M. Peeters, C. L. Huang, L. A. Vonk, Z. F. Lu, R. A. Bank, M. N. Helder, B. Zandieh Doulabi. Optimisation of high-quality total ribonucleic acid isolation from cartilaginous tissues for real-time polymerase chain reaction analysis. Bone Joint Res 2016;5:560–568. DOI: 10.1302/2046-3758.511.BJR-2016-0033.R3.

Biomimetic molecules lower catabolic expression and prevent chondroitin sulfate degradation in an osteoarthritic ex vivo model

Aggrecan, the major proteoglycan in cartilage, serves to protect cartilage tissue from damage and degradation during the progression of osteoarthritis (OA). In cartilage extracellular matrix (ECM) aggrecan exists in an aggregate composed of several aggrecan molecules that bind to a single filament of hyaluronan. Each molecule of aggrecan is composed of a protein core and glycosaminoglycan sides chains, the latter of which provides cartilage with the ability to retain water and resist compressive loads. During the progression of OA, loss of aggrecan is considered to occur first, after which other cartilage matrix components become extremely susceptible to degradation. Proteolytic cleavage of the protein core of aggrecan by enzymes such as aggrecanases, prevent its binding to HA and lower cartilage mechanical strength. Here we present the use of HA-binding or collagen type II-binding molecules that functionally mimic aggrecan but lack known cleavage sites, protecting the molecule from proteolytic degradation. These molecules synthesized with chondroitin sulfate backbones conjugated to hyaluronan- or collagen type II- binding peptides, are capable of diffusing through a cartilage explant and adhering to the ECM of this tissue. The objective of this study was to test the functional efficacy of these molecules in an ex vivo osteoarthritic model to discern the optimal molecule for further studies. Different variations of chondroitin sulfate conjugated to the binding peptides were diffused through aggrecan depleted explants and assessed for their ability to enhance compressive stiffness, prevent CS degradation, and modulate catabolic (MMP-13 and ADAMTS-5) and anabolic (aggrecan and collagen type II) gene expression. A pilot in vivo study assessed the ability to retain the molecule within the joint space of an osteoarthritic guinea pig model. The results indicate chondroitin sulfate conjugated to hyaluronan-binding peptides is able to significantly restore equilibrium modulus and prevent CS degradation. All molecules demonstrated the ability to lower catabolic gene expression in aggrecan depleted explants. In order to enhance biosynthesis and regeneration, the molecules need to be coupled with an external stimulant such as a growth factor. The chondroitin sulfate molecule synthesized with HA-binding peptides demonstrated adherence to cartilage tissue and retention up to 6 hours in an ambulatory joint. Further studies will monitor the in vivo residence time and ability of the molecules to act as a disease-modifying agent.

A developmentally inspired combined mechanical and biochemical signaling approach on zonal lineage commitment of mesenchymal stem cells in articular cartilage regeneration

Articular cartilage is organized into multiple zones including superficial, middle and calcified zones with distinct cellular and extracellular components to impart lubrication, compressive strength, and rigidity for load transmission to bone, respectively. During native cartilage tissue development, changes in biochemical, mechanical, and cellular factors direct the formation of stratified structure of articular cartilage. The objective of this work was to investigate the effect of combined gradients in cell density, matrix stiffness, and zone-specific growth factors on the zonal organization of articular cartilage. Human mesenchymal stem cells (hMSCs) were encapsulated in acrylate-functionalized lactide-chain-extended polyethylene glycol (SPELA) gels simulating cell density and stiffness of the superficial, middle and calcified zones. The cell-encapsulated gels were cultivated in a medium supplemented with growth factors specific to each zone and the expression of zone-specific markers was measured with incubation time. Encapsulation of 60 × 10(6) cells per mL hMSCs in a soft gel (80 kPa modulus) and cultivation with a combination of TGF-β1 (3 ng mL(-1)) and BMP-7 (100 ng mL(-1)) led to the expression of markers for the superficial zone. Conversely, encapsulation of 15 × 10(6) cells per mL hMSCs in a stiff gel (320 MPa modulus) and cultivation with a combination of TGF-β1 (30 ng mL(-1)) and hydroxyapatite (3%) led to the expression of markers for the calcified zone. Further, encapsulation of 20 × 10(6) cells per mL hMSCs in a gel with 2.1 MPa modulus and cultivation with a combination of TGF-β1 (30 ng mL(-1)) and IGF-1 (100 ng mL(-1)) led to up-regulation of the middle zone markers. Results demonstrate that a developmental approach with gradients in cell density, matrix stiffness, and zone-specific growth factors can potentially regenerate zonal structure of the articular cartilage.

Controlled chondrogenesis from adipose-derived stem cells by recombinant transforming growth factor-β3 fusion protein in peptide scaffolds

Adipose-derived stem cells (ADSCs) are promising for cartilage repair due to their easy accessibility and chondrogenic potential. Although chondrogenesis of transforming growth factor-β (TGF-β) mediated mesenchymal stem cells (MSCs) is well established in vitro, clinical tissue engineering requires effective and controlled delivery of TGF-β in vivo. In this work, a self-assembled peptide scaffold was employed to construct cartilages in vivo through the chondrogenesis from ADSCs controlled by recombinant fusion protein LAP-MMP-mTGF-β3 that was transfected by lentiviral vectors. During this course, the addition of matrix metalloproteinases (MMPs) can trigger the release of mTGF-β3 from the recombinant fusion protein of LAP-MMP-mTGF-β3 in the combined scaffolds, thus stimulating the differentiation of ADSCs into chondrogenesis. The specific expression of cartilage genes was analyzed by real-time polymerase chain reaction and Western blot. The expression of chondrocytic markers was obviously upregulated to a higher level compared to the one by commonly used TGF-β3 alone. After 3 weeks of in vitro culturing, the hybrids with differentiated chondrogenesis were then injected subcutaneously into nude mice and retrieved after 4 weeks of culturing in vivo. Histological analysis also confirmed that the recombinant fusion protein was more effective for the formation of cartilage matrix than the cases either with TGF-β3 alone or without LAP-MMP-mTGF-β3 (P<0.05). This study demonstrates that controlled local delivery of the LAP-MMP-mTGF-β3 constructs can accelerate differentiation of ADSCs into the cartilage in vivo, which indicates the great potential of this hybrid in rapid therapy of osteoarthritis.

Biomimetic aggrecan reduces cartilage extracellular matrix from degradation and lowers catabolic activity in ex vivo and in vivo models

Aggrecan, a major macromolecule in cartilage, protects the extracellular matrix (ECM) from degradation during the progression of osteoarthritis (OA). However, aggrecan itself is also susceptible to proteolytic cleavage. Here, the use of a biomimetic proteoglycan (mAGC) is presented, which functionally mimics aggrecan but lacks the known cleavage sites, protecting the molecule from proteolytic degradation. The objective of this study is to test the efficacy of this molecule in ex vivo (human OA synovial fluid) and in vivo (Sprague-Dawley rats) osteoarthritic models. These results indicate that mAGC's may protect articular cartilage against the loss of key ECM components, and lower catabolic protein and gene expression in both models. This suppression of matrix degradation has the potential to provide a healthy environment for tissue repair.

Distinct patterns of gene expression in the superficial, middle and deep zones of bovine articular cartilage

Hyaline articular cartilage will not heal spontaneously, and lesions in hyaline articular cartilage often result in degenerative joint disease. Considerable progress has been made with respect to the responsive stem cells, inductive signals and extracellular scaffolding required for the optimal regeneration of cartilage. However, many challenges remain, such as topographic differences in the functional zones of articular cartilage. We hypothesized that a distinct set of differentially expressed genes define the surface, middle and deep zones of hyaline articular cartilage. Microarray analysis of bovine articular cartilage from the superficial and middle zones revealed 52 genes differentially expressed ≥ 10-fold and 114 additional genes differentially expressed ≥ five-fold. However, no genes were identified with a ≥ five-fold difference in expression when comparing articular cartilage from the middle and deep zones. There are distinct, differential gene expression patterns in the superficial and middle zones of hyaline articular cartilage that highlight the functional differences between these zones. This investigation has implications for the tissue engineering and regeneration of hyaline articular cartilage.

Regulation of articular chondrocyte aggrecan and collagen gene expression by multiple growth factor gene transfer

Gene transfer is a promising approach to the delivery of chondrotrophic growth factors to promote cartilage repair. It is unlikely that a single growth factor transgene will optimally regulate these cells. The aim of this study was to identify those growth factor transgene combinations that optimally regulate aggrecan, collagen type II and collagen type I gene expression by articular chondrocytes. We delivered combinations of the transgenes encoding fibroblast growth factor-2, insulin-like growth factor I, transforming growth factor beta1, bone morphogenetic protein-2, and/or bone morphogenetic protein-7 and assessed chondrocyte responses by measuring changes in the expression of aggrecan, type II collagen and type I collagen genes. These growth factor transgenes differentially regulated the magnitude and time course of all three-matrix protein genes. In concert, the transgenes regulated matrix gene expression in an interactive fashion that ranged from synergistic to inhibitory. Maximum stimulation of aggrecan (16-fold) and type II collagen (35-fold) expression was with the combination of IGF-I, BMP-2, and BMP-7 transgenes. The results indicate that the optimal choice of growth factor genes for cell-based cartilage repair cannot be predicted from observations of individual transgenes. Rather, such gene therapy will require an empirically based selection of growth factor gene combinations.

Engineering lubrication in articular cartilage

Despite continuous progress toward tissue engineering of functional articular cartilage, significant challenges still remain. Advances in morphogens, stem cells, and scaffolds have resulted in enhancement of the bulk mechanical properties of engineered constructs, but little attention has been paid to the surface mechanical properties. In the near future, engineered tissues will be able to withstand and support the physiological compressive and tensile forces in weight-bearing synovial joints such as the knee. However, there is an increasing realization that these tissue-engineered cartilage constructs will fail without the optimal frictional and wear properties present in native articular cartilage. These characteristics are critical to smooth, pain-free joint articulation and a long-lasting, durable cartilage surface. To achieve optimal tribological properties, engineered cartilage therapies will need to incorporate approaches and methods for functional lubrication. Steady progress in cartilage lubrication in native tissues has pushed the pendulum and warranted a shift in the articular cartilage tissue-engineering paradigm. Engineered tissues should be designed and developed to possess both tribological and mechanical properties mirroring natural cartilage. In this article, an overview of the biology and engineering of articular cartilage structure and cartilage lubrication will be presented. Salient progress in lubrication treatments such as tribosupplementation, pharmacological, and cell-based therapies will be covered. Finally, frictional assays such as the pin-on-disk tribometer will be addressed. Knowledge related to the elements of cartilage lubrication has progressed and, thus, an opportune moment is provided to leverage these advances at a critical step in the development of mechanically and tribologically robust, biomimetic tissue-engineered cartilage. This article is intended to serve as the first stepping stone toward future studies in functional tissue engineering of articular cartilage that begins to explore and incorporate methods of lubrication.

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

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

Dermis isolated adult stem cells for cartilage tissue engineering

Adult stem cells from the dermal layer of skin are an attractive alternative to primary cells for meniscus engineering, as they may be easily obtained and used autologously. Recently, chondroinducible dermis cells from caprine skin have shown promising characteristics for cartilage tissue engineering. In this study, their multilineage differentiation capacity is determined, and methods of expanding and tissue engineering these cells are investigated. It was found that these cells could differentiate along adipogenic, osteogenic, and chondrogenic lineages, allowing them to be termed dermis isolated adult stem cells (DIAS cells). Focusing on cartilage tissue engineering, it was found that passaging these cells in chondrogenic medium and forming them into self-assembled tissue engineered constructs caused upregulation of collagen type II and COMP gene expression. Further investigation showed that applying transforming growth factor β1 (TGF-β1) or bone morphogenetic protein 2 (BMP-2) to DIAS constructs caused increased sulfated glycosaminoglycan content. Additionally, TGF-β1 treatment caused significant increases in compressive properties and construct contraction. In contrast, BMP-2 treatment resulted in the largest constructs, but did not increase compressive properties. These results show that DIAS cells can be easily manipulated for cartilage tissue engineering strategies, and may also be a useful cell source for other mesenchymal tissues.

Gene expression of alginate-embedded chondrocyte subpopulations and their response to exogenous IGF-1 delivery

The delivery of growth factors to aid in cartilage engineering has received considerable attention. However, phenotypical differences between chondrocyte cell populations and their distinct responses to growth factors are not fully understood. To address this issue, we have investigated the gene expression of chondrocytes isolated from the superficial, middle, and deep zones of bovine articular cartilage. A three-dimensional (3D) alginate bead model was used to encapsulate zonal chondrocytes and culture with or without exogenous insulin-like growth factor-1(IFG-1) delivery. Following culture, mRNA expression of type I collagen, type II collagen, aggregan, IGF-1 and IGF-1 binding protein (IGF-BP3) were analysed at 1, 4 and 8 days. To the best of our knowledge, this is the first study to investigate gene expression of IGF-1 and IGF-BP3 by zone, and among the first studies to investigate growth factor delivery to chondrocytes in a 3D culture environment. Histological images and cell count data confirm the isolation of chondrocyte subpopulations, and gene expression data show distinct profiles for each zone, both with and without IGF-1 delivery. The data also show similar gene expression for the middle and deep zone cells, while the superficial zone group displays unique activity. Deep zone cells appear the most robust in their phenotype retention and most responsive to IGF-1 delivery. The results highlight differences in metabolic activity and varying responses to delivered growth factors between zonal chondrocyte populations.

Phenotypic variations in chondrocyte subpopulations and their response to in vitro culture and external stimuli

Articular cartilage defects have limited capacity to self-repair, and cost society up to 60 billion dollars annually in both medical treatments and loss of working days. Recent developments in cartilage tissue engineering have resulted in many new products coming to market or entering clinical trials. However, there is a distinct lack of treatments which aim to recreate the complex zonal organization of articular cartilage. Cartilage tissue withstands repetitive strains throughout an individual's lifetime and provides frictionless movement between joints. The structure and composition of its intricately organized extracellular matrix varies with tissue depth to provide optimal resistance to loading, ensure ease of movement, and integrate with the subchondral bone. Each tissue zone is specially designed to resist the load it experiences, and maximize the tissue properties needed for its location. It is unlikely that a homogenous solution to tissue repair will be able to optimally restore the function of such a heterogeneous tissue. For zonal engineering of articular cartilage to become practical, maintenance of phenotypically stable zonal cell populations must be achieved. The chondrocyte phenotype varies considerably by zone, and it is the activity of these cells that help achieve the structural organization of the tissue. This review provides an examination of literature which has studied variations in cellular phenotype between cartilage zones. By doing so, we have identified critical differences between cell populations and highlighted areas of research which show potential in the field. Current research has made the morphological and metabolic variations between these cell populations clear, but an ideal way of maintaining these differences in vitro culture is yet to be established. Combinations of delivered growth factors, mechanical loading, and layered three-dimensional culture systems all show potential for achieving this goal. Furthermore, differentiation of progenitor cell populations into chondrocyte subpopulations may also hold promise for achieving large numbers of zonal chondrocytes. Success of the field lies in establishing methods of retaining phenotypically stable cell populations for in vitro culture.

Hyaluronan reversed proteoglycan synthesis inhibited by mechanical stress: possible involvement of antioxidant effect

INTRODUCTION

Abnormal mechanical stress loaded on the cartilage leads to the osteoarthritis (OA). Although intraarticular hyaluronan (HA) injection is an effective treatment for OA, the underlying mechanism has not been made clear.

METHODS

Mechanical compression was loaded on the bovine cartilage using the Biopress system. Proteoglycan (PG) and reactive oxygen species (ROS) synthesis were measured with [(35)S] incorporation and fluorescent dye, respectively. Accumulation of peroxynitrite was determined with western blotting using nitrotyrosine antibody.

RESULTS

Mechanical compression inhibited PG synthesis and enhanced ROS. Externally added HA reversed stress-inhibited PG synthesis and attenuated ROS synthesis. HA also significantly decreased the generation of nitrotyrosine.

CONCLUSIONS

HA neutralized stress-enhanced ROS synthesis and resulted in the reversing of PG synthesis. These data suggest that HA plays an anabolic effect as an antioxidant.

Functional characterization of TRPV4 as an osmotically sensitive ion channel in porcine articular chondrocytes

OBJECTIVE

Transient receptor potential vanilloid 4 (TRPV4) is a Ca(2+)-permeable channel that can be gated by tonicity (osmolarity) and mechanical stimuli. Chondrocytes, the cells in cartilage, respond to their osmotic and mechanical environments; however, the molecular basis of this signal transduction is not fully understood. This study was undertaken to demonstrate the presence and functionality of TRPV4 in chondrocytes.

METHODS

TRPV4 protein expression was measured by immunolabeling and Western blotting. In response to TRPV4 agonist/antagonists, osmotic stress, and interleukin-1 (IL-1), changes in Ca(2+) signaling, cell volume, and prostaglandin E(2) (PGE(2)) production were measured in porcine chondrocytes using fluorescence microscopy, light microscopy, or immunoassay, respectively.

RESULTS

TRPV4 was expressed abundantly at the RNA and protein levels. Exposure to 4alpha-phorbol 12,13-didecanoate (4alphaPDD), a TRPV4 activator, caused Ca(2+) signaling in chondrocytes, which was blocked by the selective TRPV4 antagonist, GSK205. Blocking TRPV4 diminished the chondrocytes' response to hypo-osmotic stress, reducing the fraction of Ca(2+) responsive cells, the regulatory volume decrease, and PGE(2) production. Ca(2+) signaling was inhibited by removal of extracellular Ca(2+) or depletion of intracellular stores. Specific activation of TRPV4 restored the defective regulatory volume decrease caused by IL-1. Chemical disruption of the primary cilium eliminated Ca(2+) signaling in response to either 4alphaPDD or hypo-osmotic stress.

CONCLUSION

Our findings indicate that TRPV4 is present in articular chondrocytes, and chondrocyte response to hypo-osmotic stress is mediated by this channel, which involves both an extracellular Ca(2+) and intracellular Ca(2+) release. TRPV4 may also be involved in modulating the production or influence of proinflammatory molecules in response to osmotic stress.

IGF-1 does not moderate the time-dependent transcriptional patterns of key homeostatic genes induced by sustained compression of bovine cartilage

OBJECTIVE

To determine changes in chondrocyte transcription of a range of anabolic, catabolic and signaling genes following simultaneous treatment of cartilage with Insulin-like growth factor-1 (IGF-1) and ramp-and-hold mechanical compression, and compare with effects on biosynthesis.

METHODS

Explant disks of bovine calf cartilage were slowly compressed (unconfined) over 3-min to their 1mm cut-thickness (0%-compression) or to 50%-compression with or without 300 ng/ml IGF-1. Expression of 24 genes involved in cartilage homeostasis was measured using qPCR at 2, 8, 24, 32, 48 h after compression +/-IGF-1. Clustering analysis was used to identify groups of co-expressed genes to further elucidate mechanistic pathways.

RESULTS

IGF-1 alone stimulated gene expression of aggrecan and collagen II, but simultaneous 24h compression suppressed this effect. Compression alone up-regulated expression of matrix metalloproteinase (MMP)-3, MMP-13, a disintegrin and metalloproteinase with thrombospondin motif (ADAMTS)-5 and transforming growth factor (TGF)-beta, an effect not reversed by simultaneous IGF-1 treatment. Temporal changes in expression following IGF-1 treatment were generally slower than that following compression. Clustering analysis revealed five distinct groups within which the pairings, tissue inhibitor of metalloproteinase (TIMP)-3 and ADAMTS-5, MMP-1 and IGF-2, and IGF-1 and Collagen II, were all robustly co-expressed, suggesting inherent regulation and feedback in chondrocyte gene expression. While aggrecan synthesis was transcriptionally regulated by IGF-1, inhibition of aggrecan synthesis by sustained compression appeared post-transcriptionally regulated.

CONCLUSION

Sustained compression markedly altered the effects of IGF-1 on expression of genes involved in cartilage homeostasis, while IGF-1 was largely unable to moderate the transcriptional effects of compression alone. The demonstrated co-expressed gene pairings suggest a balance of anabolic and catabolic activity following simultaneous mechanical and growth factor stimuli.

Cartilage homeostasis in health and rheumatic diseases

As the cellular component of articular cartilage, chondrocytes are responsible for maintaining in a low-turnover state the unique composition and organization of the matrix that was determined during embryonic and postnatal development. In joint diseases, cartilage homeostasis is disrupted by mechanisms that are driven by combinations of biological mediators that vary according to the disease process, including contributions from other joint tissues. In osteoarthritis (OA), biomechanical stimuli predominate with up-regulation of both catabolic and anabolic cytokines and recapitulation of developmental phenotypes, whereas in rheumatoid arthritis (RA), inflammation and catabolism drive cartilage loss. In vitro studies in chondrocytes have elucidated signaling pathways and transcription factors that orchestrate specific functions that promote cartilage damage in both OA and RA. Thus, understanding how the adult articular chondrocyte functions within its unique environment will aid in the development of rational strategies to protect cartilage from damage resulting from joint disease. This review will cover current knowledge about the specific cellular and biochemical mechanisms that regulate cartilage homeostasis and pathology.

In situ mechanical properties of the chondrocyte cytoplasm and nucleus

The way in which the nucleus experiences mechanical forces has important implications for understanding mechanotransduction. Knowledge of nuclear material properties and, specifically, their relationship to the properties of the bulk cell can help determine if the nucleus directly experiences mechanical loads, or if it is a signal transduction mechanism secondary to cell membrane deformation that leads to altered gene expression. Prior work measuring nuclear material properties using micropipette aspiration suggests that the nucleus is substantially stiffer than the bulk cell [Guilak, F., Tedrow, J.R., Burgkart, R., 2000. Viscoelastic properties of the cell nucleus. Biochem. Biophys. Res. Commun. 269, 781-786], whereas recent work with unconfined compression of single chondrocytes showed a nearly one-to-one correlation between cellular and nuclear strains [Leipzig, N.D., Athanasiou, K.A., 2008. Static compression of single chondrocytes catabolically modifies single-cell gene expression. Biophys. J. 94, 2412-2422]. In this study, a linearly elastic finite element model of the cell with a nuclear inclusion was used to simulate the unconfined compression data. Cytoplasmic and nuclear stiffnesses were varied from 1 to 7 kPa for several combinations of cytoplasmic and nuclear Poisson's ratios. It was found that the experimental data were best fit when the ratio of cytoplasmic to nuclear stiffness was 1.4, and both cytoplasm and nucleus were modeled as incompressible. The cytoplasmic to nuclear stiffness ratio is significantly lower than prior reports for isolated nuclei. These results suggest that the nucleus may behave mechanically different in situ than when isolated.

Gene expression profiling of chondrocytes from a porcine impact injury model

OBJECTIVE

To identify differentially expressed genes between axially impacted and control articular cartilage taken from porcine patellae maintained in organ culture for 14 days.

METHODS

Porcine patellae were impacted perpendicular to the articular surface to create an impact injury. Intact patellae (control and impacted) were maintained in culture for 14 days. Total RNA was then extracted from the articular cartilage beneath the impaction and used to prepare two Serial Analysis of Gene Expression (SAGE) libraries. Approximately 42,500 SAGE long tags were sequenced from the libraries. The expression of select genes was confirmed by quantitative real-time PCR analysis.

RESULTS

Thirty-nine SAGE tags were significantly differentially expressed in the impacted and control libraries, representing 30 different annotated pig genes. These genes represented gene products associated with matrix molecules, iron and phosphate transport, protein biosynthesis, skeletal development, cell proliferation, lipid metabolism and the inflammatory response. Twenty-three of the 30 genes were down-regulated in the impacted library and five were up-regulated in the impacted library. Quantitative real-time PCR follow-up of four genes supported the results found with SAGE.

CONCLUSION

We have identified 30 putative genes differentially expressed in a porcine impact injury model and validated these findings for four of these genes using real-time PCR. Results using this impact injury model have contributed further evidence that damaged chondrocytes may de-differentiate into fibroblast-like cells and proliferate in an attempt to repair themselves. Additional work is underway to study these genes in further detail at earlier time points to provide a more complete story about the fate of chondrocytes in articular cartilage following an injury.

Autologous versus pooled human serum for articular chondrocyte growth

PURPOSE

To evaluate the effect of autologous human serum (AHS) versus pooled human serum (PHS) versus foetal bovine serum (FBS) for growth of articular chondrocytes and formation of chondrocytefibrin constructs.

METHODS

Experiments with monolayer culture expansion of human articular chondrocytes were performed using basic culture media supplemented with 10% AHS, PHS, or FBS. Growth kinetics and specific phenotypic expression of the serially expanded chondrocytes were evaluated. Large-scale culture expansion was used to obtain about 30 million cells to form chondrocyte-fibrin constructs. All constructs were implanted subcutaneously at the dorsum part of athymic nude mice for 8 weeks. The in vivo constructs were evaluated using histological and gene expression studies.

RESULTS

The morphology of primary cultured chondrocytes (P0) was polygonal and became more elongated and larger after serial passages (P1, P2, and P3). This was comparable for AHS, PHS, and FBS. Total cell yields accumulated for AHS (28 million) and PHS (41 million) were significantly higher than those for FBS (4 million). After 8 weeks of implantation, in vivo chondrocyte-fibrin constructs demonstrated a glistening white and firm texture, comparable to normal hyaline cartilage. All constructs exhibited histo-architectural characteristics of well-distributed cartilage-isolated cells embedded within basophilic ground substance. Presence of accumulated proteoglycans cartilage-rich matrix was indicated by positive orange-red Safranin O staining. During monolayer culture expansion, collagen type II gene expression was down-regulated, while collagen type I gene expression was up-regulated. Collagen type II--the specific chondrogenesis marker--was re-expressed in the in vivo chondrocyte-fibrin construct.

CONCLUSION

AHS and PHS are better than FBS for in vitro cultivation of human articular chondrocytes.

Static compression of single chondrocytes catabolically modifies single-cell gene expression

Previous work has established that mechanical forces can lead to quantifiable alterations in cell function. However, how forces change gene expression in a single cell and the mechanisms of force transmission to the nucleus are poorly understood. Here we demonstrate that the gene expression of proteins related to the extracellular matrix in single articular chondrocytes is modified by compressive forces in a dosage-dependent manner. Increasing force exposure catabolically shifts single-cell mRNA levels of aggrecan, collagen IIa, and tissue inhibitor of metalloproteinase-1. Cytohistochemistry reveals that the majority of strain experienced by the cell is also experienced by the nucleus, resulting in considerable changes in nuclear volume and structure. Transforming growth factor-beta1 and insulin-like growth factor-I offer mechanoprotection and recovery of gene expression of aggrecan and metalloproteinase-1. These results suggest that forces directly influence gene transcription and may do so by changing chromatin conformation.

Epigenetic regulation of leptin affects MMP-13 expression in osteoarthritic chondrocytes: possible molecular target for osteoarthritis therapeutic intervention

OBJECTIVE

To investigate whether epigenetic mechanisms can regulate leptin's expression and affect its downstream targets as metalloproteinases 3,9,13 in osteoarthritic chondrocytes.

METHODS

DNA methylation in leptin promoter was measured by DNA bisulfite sequencing, and mRNA expression levels were measured by real-time quantitative PCR in osteoarthritic as well as in normal cartilage. Osteoarthritic articular cartilage samples were obtained from two distinct locations of the knee (n = 15); from the main defective area of maximum load (advanced osteoarthritis (OA)) and from adjacent macroscopically intact regions (minimal OA). Using small interference RNA, we tested if leptin downregulation would affect matrix metalloproteinase (MMP)-13 activity. We also evaluated the effect of the demethylating agent, 5'-Aza-2-deoxycytidine (AZA) and of the histone deacetylase inhibitor trichostatin A (TSA) on leptin expression in chondrocyte cultures. Furthermore, we performed chromatin immunoprecipitation in leptin's promoter area.

RESULTS

We found a correlation between leptin expression and DNA methylation and also that leptin controls MMP-13 activity in chondrocytes. Leptin's downregulation with small interference RNA inhibited MMP-13 expression dramatically. After 5-AZA application in normal chondrocytes, leptin's methylation was decreased, while its expression was upregulated, and MMP-13 was activated. Furthermore, TSA application in normal chondrocyte cultures increased leptin's expression. Also, chromatin immunoprecipitation in leptin's promoter after TSA treatment revealed that histone H3 lysines 9 and 14 were acetylated.

CONCLUSION

We found that epigenetic mechanisms regulate leptin's expression in chondrocytes affecting its downstream target MMP-13. Small interference RNA against leptin deactivated directly MMP-13, which was upregulated after leptin's epigenetic reactivation, raising the issue of leptin's therapeutic potential for osteoarthritis.

Hyperosmotic stress-induced apoptotic signaling pathways in chondrocytes

Articular chondrocytes have a well-developed osmoregulatory system that enables cells to survive in a constantly changing osmotic environment. However, osmotic loading exceeding that occurring under physiological conditions severely compromises chondrocyte function and leads to degenerative changes. The aim of the present study was to investigate the form of cell death and changes in apoptotic signaling pathways under hyperosmotic stress using a primary chondrocyte culture. Cell viability and apoptosis assays performed with annexin V and propidium iodide staining showed that a highly hyperosmotic medium (600 mOsm) severely reduced chondrocyte viability and led mainly to apoptotic cell death, while elevating osmotic pressure within the physiological range caused no changes compared to isosmotic conditions. Western blot analysis revealed that a 600 mOsm hyperosmotic environment induced the activation of proapoptotic members of the mitogen-activated protein kinase family such as c-Jun N-terminal kinase (JNK) and p38, and led to an increased level of extracellular signal regulated kinase (ERK1/2). Hyperosmotic stress also induced the activation of caspase-3. In summary, our results show that hyperosmotic stress leads to mainly apoptotic cell death via the involvement of proapoptotic signaling pathways in a primary chondrocyte culture.

Differences in matrix accumulation and hypertrophy in superficial and deep zone chondrocytes are controlled by bone morphogenetic protein

Despite the knowledge that superficial zone chondrocytes (SZC, located within 100 mum of the articular surface) and deep zone chondrocytes (DZC, located near the calcified zone) have distinct phenotypes, previous studies on bone morphogenetic proteins (BMPs) have not differentiated its effects on SZC versus DZC. Using a pellet culture model we have compared phenotype, morphology and matrix accumulation in SZC and DZC with or without adenovirus-mediated overexpression of BMP2 or -7 or the BMP antagonist Noggin. Greater accumulation of proteoglycan (PG)-rich matrix in the untreated DZC was associated with a hypertrophic phenotype with large cell diameters and high gene expression levels of runt-related transcription factor-2 (Runx2) as well as higher endogenous BMP activity. Noggin overexpression decreased matrix accumulation and cell diameters in SZC and DZC, confirming a role for endogenous BMP in both processes. In DZC, overexpression of either BMP2 or -7 increased cell diameter without increasing PG-rich matrix accumulation. In contrast, in SZC, BMP overexpression increased matrix accumulation and type II collagen gene expression without increasing cell diameter. These data indicate that differences in endogenous BMP activity level and responsiveness to BMPs define, in part, the differences between the SZC and DZC phenotype. They also suggest that SZC may be a more appropriate target for BMP therapy than DZC.