Isolated chondrons: a viable alternative for studies of chondrocyte metabolism in vitro

Cartilage-derived cells display heterogeneous pericellular matrix synthesis in agarose microgels

The pericellular matrix (PCM) surrounding chondrocytes is essential for articular cartilage tissue engineering. As the current isolation methods to obtain chondrocytes with their PCM (chondrons) result in a heterogeneous mixture of chondrocytes and chondrons, regenerating the PCM using a tissue engineering approach could prove beneficial. In this study, we aimed to discern the behavior of articular chondrocytes (ACs) in regenerating the PCM in such an approach and whether this would also be true for articular cartilage-derived progenitor cells (ACPCs), as an alternative cell source. Bovine ACs and ACPCs were encapsulated in agarose microgels using droplet-based microfluidics. ACs were stimulated with TGF-β1 and dexamethasone and ACPCs were sequentially stimulated with BMP-9 followed by TGF-β1 and dexamethasone. After 0, 3, 5, and 10 days of culture, PCM components, type-VI collagen and perlecan, and ECM component, type-II collagen, were assessed using flow cytometry and fluorescence microscopy. Both ACs and ACPCs synthesized the PCM before the ECM. It was seen for the first time that synthesis of type-VI collagen always preceded perlecan. While the PCM synthesized by ACs resembled native chondrons after only 5 days of culture, ACPCs often made less well-structured PCMs. Both cell types showed variations between individual cells and donors. On one hand, this was more prominent in ACPCs, but also a subset of ACPCs showed superior PCM and ECM regeneration, suggesting that isolating these cells may potentially improve cartilage repair strategies.

Cartilaginous endplates: A comprehensive review on a neglected structure in intervertebral disc research

The cartilaginous endplates (CEP) are key components of the intervertebral disc (IVD) necessary for sustaining the nutrition of the disc while distributing mechanical loads and preventing the disc from bulging into the adjacent vertebral body. The size, shape, and composition of the CEP are essential in maintaining its function, and degeneration of the CEP is considered a contributor to early IVD degeneration. In addition, the CEP is implicated in Modic changes, which are often associated with low back pain. This review aims to tackle the current knowledge of the CEP regarding its structure, composition, permeability, and mechanical role in a healthy disc, how they change with degeneration, and how they connect to IVD degeneration and low back pain. Additionally, the authors suggest a standardized naming convention regarding the CEP and bony endplate and suggest avoiding the term vertebral endplate. Currently, there is limited data on the CEP itself as reported data is often a combination of CEP and bony endplate, or the CEP is considered as articular cartilage. However, it is clear the CEP is a unique tissue type that differs from articular cartilage, bony endplate, and other IVD tissues. Thus, future research should investigate the CEP separately to fully understand its role in healthy and degenerated IVDs. Further, most IVD regeneration therapies in development failed to address, or even considered the CEP, despite its key role in nutrition and mechanical stability within the IVD. Thus, the CEP should be considered and potentially targeted for future sustainable treatments.

Harmonization and standardization of nucleus pulposus cell extraction and culture methods

Background

In vitro studies using nucleus pulposus (NP) cells are commonly used to investigate disc cell biology and pathogenesis, or to aid in the development of new therapies. However, lab-to-lab variability jeopardizes the much-needed progress in the field. Here, an international group of spine scientists collaborated to standardize extraction and expansion techniques for NP cells to reduce variability, improve comparability between labs and improve utilization of funding and resources.

Methods

The most commonly applied methods for NP cell extraction, expansion, and re-differentiation were identified using a questionnaire to research groups worldwide. NP cell extraction methods from rat, rabbit, pig, dog, cow, and human NP tissue were experimentally assessed. Expansion and re-differentiation media and techniques were also investigated.

Results

Recommended protocols are provided for extraction, expansion, and re-differentiation of NP cells from common species utilized for NP cell culture.

Conclusions

This international, multilab and multispecies study identified cell extraction methods for greater cell yield and fewer gene expression changes by applying species-specific pronase usage, 60-100 U/ml collagenase for shorter durations. Recommendations for NP cell expansion, passage number, and many factors driving successful cell culture in different species are also addressed to support harmonization, rigor, and cross-lab comparisons on NP cells worldwide.

Isolation of Chondrons from Hyaline Cartilage

In native healthy hyaline cartilage, the chondrocytes are surrounded by a pericellular matrix that has a distinct composition and function compared to the hyaline cartilage extracellular matrix. The chondrocyte together with its pericellular matrix is called a chondron. The type VI collagen, which is the main component of the pericellular matrix, is resistant to enzymatic digestion by pure collagenase and dispase that do digest the extracellular matrix. Therefore, this combination of enzymes can be used to enzymatically isolate chondrons from hyaline cartilage. Chondrons have a high potential for cartilage tissue engineering. This chapter describes in detail how chondrons can be isolated from hyaline cartilage for further use.

Enzymatic Isolation of Articular Chondrons: Is It Much Different Than That of Chondrocytes?

In native articular cartilage, chondrocytes (Chy) are completely capsulated by a pericellular matrix (PCM), together called the chondron (Chn). Due to its unique properties (w.r.t. territorial matrix) and importance in mechanotransduction, the PCM and Chn may be important in regenerative strategies. The current gold standard for the isolation of Chns from cartilage dates from 1997. Although previous research already showed the low cell yield and the heterogeneity of the isolated populations, their compositions and properties have never been thoroughly characterized. This study aimed to compare enzymatic isolation methods for Chy and Chns and characterizes the isolation efficiency and quality of the PCM. Bovine articular cartilage was digested according to the 5-h (5H) gold standard Chn isolation method (0.3% dispase +0.2% collagenase II), an overnight (ON) Chn isolation (0.15% dispase +0.1% collagenase II), and an ON Chy isolation (0.15% collagenase II +0.01% hyaluronidase). Type VI collagen staining, fluorescence-activated cell sorting (FACS) analysis, specific cell sorting, and immunohistochemistry were performed using a type VI collagen staining, to study their isolation efficiency and quality of the PCM. These analyses showed a heterogeneous mixture of Chy and Chns for all three methods. Although the 5H Chn isolation resulted in the highest percentage of Chns, the cell yield was significantly lower compared to the other isolation methods. FACS, based on the type VI collagen staining, successfully sorted the three identified cell populations. To maximize Chn yield and homogeneity, the ON Chn enzymatic digestion method should be combined with type VI collagen staining and specific cell sorting. Impact statement Since chondrocytes are highly dependent on their microenvironment for maintaining phenotypic stability, it is hypothesized that using chondrons results in superior outcomes in cartilage tissue engineering. This study reveals the constitution of cell populations obtained after enzymatic digestion of articular cartilage tissue and presents an alternative method to obtain a homogeneous population of chondrons. These data can improve the impact of studies investigating the effect of the pericellular matrix on neocartilage formation.

Pericellular heparan sulfate proteoglycans: Role in regulating the biosynthetic response of nucleus pulposus cells to osmotic loading

Background

Daily physiologic loading causes fluctuations in hydration of the intervertebral disc (IVD); thus, the embedded cells experience cyclic alterations to their osmotic environment. These osmotic fluctuations have been described as a mechanism linking mechanics and biology, and have previously been shown to promote biosynthesis in chondrocytes. However, this phenomenon has yet to be fully interrogated in the IVD. Additionally, the specialized extracellular matrix surrounding the cells, the pericellular matrix (PCM), transduces the biophysical signals that cells ultimately experience. While it is known that the PCM is altered in disc degeneration, whether it disrupts normal osmotic mechanotransduction has yet to be determined. Thus, our objectives were to assess: (1) whether dynamic osmotic conditions stimulate biosynthesis in nucleus pulposus cells, and (2) whether pericellular heparan sulfate proteoglycans (HSPGs) modulate the biosynthetic response to osmotic loading.

Methods

Bovine nucleus pulposus cells isolated with retained PCM were encapsulated in 1.5% alginate beads and treated with or without heparinase III, an enzyme that degrades the pericellular HSPGs. Beads were subjected to 1 h of daily iso-osmotic, hyper-osmotic, or hypo-osmotic loading for 1, 2, or 4 weeks. At each timepoint the total amount of extracellular and pericellular sGAG/DNA were quantified. Additionally, whether osmotic loading triggered alterations to HSPG sulfation was assessed via immunohistochemistry for the heparan sulfate 6-O-sulfertransferase 1 (HS6ST1) enzyme.

Results

Osmotic loading significantly influenced sGAG/DNA accumulation with a hyper-osmotic change promoting the greatest sGAG/DNA accumulation in the pericellular region compared with iso-osmotic conditions. Heparanase-III treatment significantly reduced extracellular sGAG/DNA but pericellular sGAG was not affected. HS6ST1 expression was not affected by osmotic loading.

Conclusion

Results suggest that hyper-osmotic loading promotes matrix synthesis and that modifications to HSPGs directly influence the metabolic responses of cells to osmotic fluctuations. Collectively, results suggest degeneration-associated modifications to pericellular HSPGs may contribute to the altered mechanobiology observed in disease.

From Chondrocytes to Chondrons, Maintenance of Phenotype and Matrices Production in a Composite 3D Hydrogel Scaffold

Osteoarthritis (OA) is a degenerative disease characterized by articular cartilage (AC) degradation that affects more than 30 million people in the USA OA is managed with symptom-alleviating medications. Matrix-assisted autologous chondrocyte transplantation (MACT) is a tissue-engineered option, but current products are expensive and lack mechanical tunability or processability to match defects’ mechanical properties and anatomical shapes. Here, we explore the efficacy of a biocompatible hydrogel-based scaffold composed of sodium alginate, gelatin, and gum Arabic—referred to by SA–GEL–GA—to support bovine articular chondrocytes’ (bAChs) proliferation, pericellular matrix (PCM), and extracellular matrix (ECM) production. bAChs were grown for 45 days in SA–GEL–GA. Their viability; their live/dead status; histological staining; biochemical assays for glycosaminoglycans (GAGs) and collagen; atomic force microscopy (AFM) imaging; and immunofluorescence staining of collagen I, collagen II, aggrecan, and CD44 were tested. We found that SA–GEL–GA was not cytotoxic, induced cellular proliferation by 6.1-fold while maintaining a round morphology, and supported ECM deposition by producing 3.9-fold more GAG compared to day 0. bAChs transformed into chondrons and produced a PCM enriched with collagen II (3.4-fold), aggrecan (1.7-fold), and CD44 (1.3-fold) compared to day 0. In summary, SA–GEL–GA supported the proliferation, ECM production, and PCM production of bAChs in vitro.

Combination of chondrocytes and chondrons improves extracellular matrix production to promote the repairs of defective knee cartilage in rabbits

Background

Chondrons are composed of chondrocytes and the surrounding pericellular matrix (PCM) and function to enhance chondrocyte-mediated cartilage tissue engineering. This study aimed at investigating the potential effect of combined chondrocytes with chondrons on the production of proteoglycan and collagen-II (Col-2) and the repair of defective knee cartilage in rabbits.

Methods

Chondrocytes and chondrons were isolated from the knee cartilage of rabbits, and cultured alone or co-cultured for varying periods in vitro. Their morphology was characterized by histology. The levels of aggrecan (AGG), Col-2 and glycosaminoglycan (GAG) expression were quantified by qRT-PCR, Alcian blue-based precipitation and ELISA. The effect of combined chondrocytes with chondrons in alginate spheres on the repair of defective knee cartilage was examined in rabbits.

Results

The isolated chondrocytes and chondrons displayed unique morphology and began to proliferate on day 3 and 6 post culture, respectively, accompanied by completely degenerated PCM on day 6 post culture. Evidently, chondrocytes had stronger proliferation capacity than chondrons. Longitudinal analyses indicated that culture of chondrons, but not chondrocytes, increased AGG mRNA transcripts and GAG levels with time and Col-2 mRNA transcripts only on day 3 post culture. Compared with chondrocytes or chondrons alone, co-culture of chondrocytes and chondrons significantly up-regulated AGG and Col-2 expression and GAG production, particularly at a ratio of 1:1. Implantation with chondrocytes and chondrons at 1:1 significantly promoted the repair of defective knee cartilage in rabbits, accompanied by reduced the Wakiteni scores with time.

Conclusion

Combined chondrons with chondrocytes promoted the production of extracellular matrix and the repair of defective knee cartilage in rabbits.

The translational potential of this article

This study explores that the combination of chondrons and chondrocytes may be new therapeutic strategy for cartilage tissue engineering and repair of defective cartilage.

Physiological Effects of the Electrogenic Current Generated by the Na+/K+ Pump in Mammalian Articular Chondrocytes

Background: Although the chondrocyte is a nonexcitable cell, there is strong interest in gaining detailed knowledge of its ion pumps, channels, exchangers, and transporters. In combination, these transport mechanisms set the resting potential, regulate cell volume, and strongly modulate responses of the chondrocyte to endocrine agents and physicochemical alterations in the surrounding extracellular microenvironment. Materials and Methods: Mathematical modeling was used to assess the functional roles of energy-requiring active transport, the Na⁺/K⁺ pump, in chondrocytes. Results: Our findings illustrate plausible physiological roles for the Na⁺/K⁺ pump in regulating the resting membrane potential and suggest ways in which specific molecular components of pump can respond to the unique electrochemical environment of the chondrocyte. Conclusion: This analysis provides a basis for linking chondrocyte electrophysiology to metabolism and yields insights into novel ways of manipulating or regulating responsiveness to external stimuli both under baseline conditions and in chronic diseases such as osteoarthritis.

K+ and Ca2+ Channels Regulate Ca2+ Signaling in Chondrocytes: An Illustrated Review

An improved understanding of fundamental physiological principles and progressive pathophysiological processes in human articular joints (e.g., shoulders, knees, elbows) requires detailed investigations of two principal cell types: synovial fibroblasts and chondrocytes. Our studies, done in the past 8–10 years, have used electrophysiological, Ca²⁺ imaging, single molecule monitoring, immunocytochemical, and molecular methods to investigate regulation of the resting membrane potential (E_R) and intracellular Ca²⁺ levels in human chondrocytes maintained in 2-D culture. Insights from these published papers are as follows: (1) Chondrocyte preparations express a number of different ion channels that can regulate their E_R. (2) Understanding the basis for E_R requires knowledge of (a) the presence or absence of ligand (ATP/histamine) stimulation and (b) the extraordinary ionic composition and ionic strength of synovial fluid. (3) In our chondrocyte preparations, at least two types of Ca²⁺-activated K⁺ channels are expressed and can significantly hyperpolarize E_R. (4) Accounting for changes in E_R can provide insights into the functional roles of the ligand-dependent Ca²⁺ influx through store-operated Ca²⁺ channels. Some of the findings are illustrated in this review. Our summary diagram suggests that, in chondrocytes, the K⁺ and Ca²⁺ channels are linked in a positive feedback loop that can augment Ca²⁺ influx and therefore regulate lubricant and cytokine secretion and gene transcription.

IL-1β Damages Fibrocartilage and Upregulates MMP-13 Expression in Fibrochondrocytes in the Condyle of the Temporomandibular Joint

The temporomandibular joint (TMJ), which differs anatomically and biochemically from hyaline cartilage-covered joints, is an under-recognized joint in arthritic disease, even though TMJ damage can have deleterious effects on physical appearance, pain and function. Here, we analyzed the effect of IL-1β, a cytokine highly expressed in arthritic joints, on TMJ fibrocartilage-derived cells, and we investigated the modulatory effect of mechanical loading on IL-1β-induced expression of catabolic enzymes. TMJ cartilage degradation was analyzed in 8–11-week-old mice deficient for IL-1 receptor antagonist (IL-1RA^−/−) and wild-type controls. Cells were isolated from the juvenile porcine condyle, fossa, and disc, grown in agarose gels, and subjected to IL-1β (0.1–10 ng/mL) for 6 or 24 h. Expression of catabolic enzymes (ADAMTS and MMPs) was quantified by RT-qPCR and immunohistochemistry. Porcine condylar cells were stimulated with IL-1β for 12 h with IL-1β, followed by 8 h of 6% dynamic mechanical (tensile) strain, and gene expression of MMPs was quantified. Early signs of condylar cartilage damage were apparent in IL-1RA^−/− mice. In porcine cells, IL-1β strongly increased expression of the aggrecanases ADAMTS4 and ADAMTS5 by fibrochondrocytes from the fossa (13-fold and 7-fold) and enhanced the number of MMP-13 protein-expressing condylar cells (8-fold). Mechanical loading significantly lowered (3-fold) IL-1β-induced MMP-13 gene expression by condylar fibrochondrocytes. IL-1β induces TMJ condylar cartilage damage, possibly by enhancing MMP-13 production. Mechanical loading reduces IL-1β-induced MMP-13 gene expression, suggesting that mechanical stimuli may prevent cartilage damage of the TMJ in arthritic patients.

An Investigation on the Regenerative Effects of Intra Articular Injection of Co-Cultured Adipose Derived Stem Cells with Chondron for Treatment of Induced Osteoarthritis

Purpose: Adipose tissue derived stem cells (ASCs) and chondrocytes are best cells for articular cartilage regeneration. Chondrocyte with peri-cellular matrix (PCM) is called chondron provides ideal microenviroment than chondrocytes. We aimed to evaluate the regenerative effects of intra-articular injection of ASCs co-cultures with chondron in induced osteoarthritis (OA).

Methods: ASC, from the peri-renal fat of male rat and chondron from primary newborn rat hyaline cartilage were isolated. ASCs were cultured for at least three passages in vitro. Six weeks after OA induction, rats were randomly distributed in five groups of control, osteoarthritic, ASC, chondron and co-cultured. ASCs (10⁷), chondrons (10⁷) and combination of chondrons and ASCs (10⁷) were injected into intra-articular space of the rat knee. The effect of treatments was evaluated by macroscopic and microscopic examinations. The expression levels of collagen type ΙΙ was studied by immunohistochemistry.

Results: Macroscopic appearance of the co-cultured group, showed much enhanced articular cartilage regeneration compared to ASC and chondron groups. H&E showed evidence of repair site of articular surface without erosion and fibrillation versus OA group which showed thin layer of hyaline cartilage over tidemark and spontaneous fibrocartilage formation. Metachromatic regions stained with toluidine blue were larger in treatment groups versus OA group. Strong intensity of type ΙΙ collagen staining was observed in co-culture group compared to other groups.

Conclusion: Co-culture of chondrons and ASCs increased articular hyaline cartilage formation and provides a useful tool to improve limitations of each of applied cells in this model.

Age-Correlated Phenotypic Alterations in Cells Isolated From Human Degenerated Intervertebral Discs With Contained Hernias

STUDY DESIGN

Human intervertebral disc (hIVD) cells were isolated from 41 surgically excised samples and assessed for their phenotypic alterations with age.

OBJECTIVE

Toward the design of novel anti-aging strategies to overcome degenerative disc disease (DDD), we investigated age-correlated phenotypic alterations that occur on primary hIVD cells.

SUMMARY OF BACKGROUND DATA

Although regenerative medicine holds great hope, much is still to be unveiled on IVD cell biology and its intrinsic signaling pathways, which can lead the way to successful therapies for IDD. A greater focus on age-related phenotypic changes at the cell level would contribute to establish more effective anti-aging/degeneration targets.

METHODS

The study was subdivided in four main steps: i) optimization of primary cells isolation technique; ii) high-throughput cell morphology analysis, by imaging flow cytometry (FC) and subsequent validation by histological analysis; iii) analysis of progenitor cell surface markers expression, by conventional FC; and iv) statistical analysis and correlation of cells morphology and phenotype with donor age.

RESULTS

Three subsets of cells were identified on the basis of their diameter: small cell (SC), large cell (LC), and super LC (SLC). The frequency of SCs decreased nearly 50% with age, whereas that of LCs increased nearly 30%. Interestingly, the increased cells size was due to an enlargement of the pericellular matrix (PCM). Moreover, the expression pattern for CD90 and CD73 was a reflexion of age, where older individuals show reduced frequencies of positive cells for those markers. Nevertheless, the elevated percentages of primary positive cells for the mesenchymal stem cells (MSCs) marker CD146 found, even in some older donors, refreshed hope for the hypothetical activation of the self-renewal potential of the IVD.

CONCLUSION

These findings highlight the remarkable morphological alterations that occur on hIVD cells with aging and degeneration, while reinforcing previous reports on the gradual disappearance of an endogenous progenitor cell population.

LEVEL OF EVIDENCE

N/A.

Human osteoarthritic chondrons outnumber patient- and joint-matched chondrocytes in hydrogel culture-Future application in autologous cell-based OA cartilage repair?

Autologous chondrocyte implantation (ACI) is used in 34-60% for osteoarthritic (OA) cartilage defects, although ACI is neither recommended nor designed for OA. Envisioning a hydrogel-based ACI for OA that uses chondrons instead of classically used chondrocytes, we hypothesized that human OA chondrons may outperform OA chondrocytes. We compared patient- and joint surface-matched human OA chondrons with OA chondrocytes cultured for the first time in a hydrogel, using a self-assembling peptide system. We determined yield, viability, cell numbers, mRNA expression, GAPDH mRNA enzyme activity, Collagen II synthesis (CPII) and degradation (C2C), and sulfated glycosaminoglycan. Ex vivo, mRNA expression was comparable. Over time, significant differences in survival led to 3.4-fold higher OA chondron numbers in hydrogels after 2 weeks (p = .002). Significantly, more enzymatically active GAPDH protein indicated higher metabolic activity. The number of cultures that expressed mRNA for Collagen Types I and VI, COMP, aggrecan, VEGF, TGF-β1, and FGF-2 (but not Collagen Types II and X) was different, resulting in a 3.5-fold higher number of expression-positive OA chondron cultures (p < .05). Measuring CPII and C2C per hydrogel, OA chondron hydrogels synthesized more than they degraded Collagen Type II, the opposite was true for OA chondrocytes. Per cell, OA chondrons but not OA chondrocytes displayed more synthesis than degradation. Thus, OA chondrons displayed superior biosynthesis and mRNA expression of tissue engineering and phenotype-relevant genes. Moreover, human OA chondrons displayed a significant survival advantage in hydrogel culture, whose presence, drastic extent, and timescale was novel and is clinically significant. Collectively, these data highlight the high potential of human OA chondrons for OA ACI, as they would outnumber and, thus, surpass OA chondrocytes.

Co-culture of chondrons and mesenchymal stromal cells reduces the loss of collagen VI and improves extracellular matrix production

Adult articular chondrocytes are surrounded by a pericellular matrix (PCM) to form a chondron. The PCM is rich in hyaluronan, proteoglycans, and collagen II, and it is the exclusive location of collagen VI in articular cartilage. Collagen VI anchors the chondrocyte to the PCM. It has been suggested that co-culture of chondrons with mesenchymal stromal cells (MSCs) might enhance extracellular matrix (ECM) production. This co-culture study investigates whether MSCs help to preserve the PCM and increase ECM production. Primary bovine chondrons or chondrocytes or rat MSCs were cultured alone to establish a baseline level for ECM production. A xenogeneic co-culture monolayer model using rat MSCs (20, 50, and 80%) was established. PCM maintenance and ECM production were assessed by biochemical assays, immunofluorescence, and histological staining. Co-culture of MSCs with chondrons enhanced ECM matrix production, as compared to chondrocyte or chondron only cultures. The ratio 50:50 co-culture of MSCs and chondrons resulted in the highest increase in GAG production (18.5 ± 0.54 pg/cell at day 1 and 11 ± 0.38 pg/cell at day 7 in 50:50 co-culture versus 16.8 ± 0.61 pg/cell at day 1 and 10 ± 0.45 pg/cell at day 7 in chondron monoculture). The co-culture of MSCs with chondrons appeared to decelerate the loss of the PCM as determined by collagen VI expression, whilst the expression of high-temperature requirement serine protease A1 (HtrA1) demonstrated an inverse relationship to that of the collagen VI. Together, this implies that MSCs directly or indirectly inhibited HtrA1 activity and the co-culture of MSCs with chondrons enhanced ECM synthesis and the preservation of the PCM.

Rapid Chondrocyte Isolation for Tissue Engineering Applications: The Effect of Enzyme Concentration and Temporal Exposure on the Matrix Forming Capacity of Nasal Derived Chondrocytes

Laboratory based processing and expansion to yield adequate cell numbers had been the standard in Autologous Disc Chondrocyte Transplantation (ADCT), Allogeneic Juvenile Chondrocyte Implantation (NuQu®), and Matrix-Induced Autologous Chondrocyte Implantation (MACI). Optimizing cell isolation is a key challenge in terms of obtaining adequate cell numbers while maintaining a vibrant cell population capable of subsequent proliferation and matrix elaboration. However, typical cell yields from a cartilage digest are highly variable between donors and based on user competency. The overall objective of this study was to optimize chondrocyte isolation from cartilaginous nasal tissue through modulation of enzyme concentration exposure (750 and 3000 U/ml) and incubation time (1 and 12 h), combined with physical agitation cycles, and to assess subsequent cell viability and matrix forming capacity. Overall, increasing enzyme exposure time was found to be more detrimental than collagenase concentration for subsequent viability, proliferation, and matrix forming capacity (sGAG and collagen) of these cells resulting in nonuniform cartilaginous matrix deposition. Taken together, consolidating a 3000 U/ml collagenase digest of 1 h at a ratio of 10 ml/g of cartilage tissue with physical agitation cycles can improve efficiency of chondrocyte isolation, yielding robust, more uniform matrix formation.

Rapid Chondrocyte Isolation for Tissue Engineering Applications: The Effect of Enzyme Concentration and Temporal Exposure on the Matrix Forming Capacity of Nasal Derived Chondrocytes

Laboratory based processing and expansion to yield adequate cell numbers had been the standard in Autologous Disc Chondrocyte Transplantation (ADCT), Allogeneic Juvenile Chondrocyte Implantation (NuQu®), and Matrix-Induced Autologous Chondrocyte Implantation (MACI). Optimizing cell isolation is a key challenge in terms of obtaining adequate cell numbers while maintaining a vibrant cell population capable of subsequent proliferation and matrix elaboration. However, typical cell yields from a cartilage digest are highly variable between donors and based on user competency. The overall objective of this study was to optimize chondrocyte isolation from cartilaginous nasal tissue through modulation of enzyme concentration exposure (750 and 3000 U/ml) and incubation time (1 and 12 h), combined with physical agitation cycles, and to assess subsequent cell viability and matrix forming capacity. Overall, increasing enzyme exposure time was found to be more detrimental than collagenase concentration for subsequent viability, proliferation, and matrix forming capacity (sGAG and collagen) of these cells resulting in nonuniform cartilaginous matrix deposition. Taken together, consolidating a 3000 U/ml collagenase digest of 1 h at a ratio of 10 ml/g of cartilage tissue with physical agitation cycles can improve efficiency of chondrocyte isolation, yielding robust, more uniform matrix formation.

Cyclic Tensile Strain Reduces TNF-α Induced Expression of MMP-13 by Condylar Temporomandibular Joint Cells

To investigate whether the disproportionate degradation of mandibular condyle cartilage in arthritic juvenile temporomandibular joint (TMJ) is related to distinctive responses of TMJ-derived cells to tumor necrosis factor-α (TNF-α), and whether mechanical loading affects this response. The effect of TNF-α (0.1-10 ng/ml) was tested on juvenile porcine TMJ cells isolated from the condyle, fossa, and disc, grown in 3D agarose gels. Expression of anabolic and catabolic factors was quantified by RT-qPCR and/or immunohistochemistry. Condylar cells were stimulated for 12 h with TNF-α (10 ng/ml), followed by 8 h of 6% cyclic tensile strain, and gene expression of MMPs was quantified. TNF-α (10 ng/ml) reduced the expression of the matrix proteins collagen types I and II after 6 h of incubation. Aggrecan gene expression was increased in the presence of 0.1 ng/ml TNF-α. The fossa and disc cells responded to TNF-α with an increased expression of the aggrecanase ADAMTS4. TNF-α enhanced MMP-13 gene and protein expression only by condylar cells. Mechanical loading reduced this effect. Cells isolated from the different cartilaginous structures reacted differently to TNF-α. Since the disc and fossa contain a very low level of proteoglycans in comparison to the condyle, the role played by ADAMTS4 in degradation of the fossa and disc might be limited. TNF-α induced MMP-13 expression by condylar cells might be involved in the degradation of the juvenile condyle. Since this expression was reduced by mechanical loading, functional loading with oral physiotherapy or orthodontic activators may help to reduce the catabolic effect of TNF-α. J. Cell. Physiol. 232: 1287-1294, 2017. © 2016 Wiley Periodicals, Inc.

Systematic study of cell isolation from bovine nucleus pulposus: Improving cell yield and experiment reliability

Differences in matrix compositions in human nucleus pulposus (NP) clinical samples demand different cell isolation protocols for optimal results but there is no clear guide about this to date. Sub-optimal protocols may result in low cell yield, limited reliability of results or even failure of experiments. Cell yield, viability and attachment of cells isolated from bovine NP tissue with different protocols were estimated by cell counting, Trypan blue staining and cell culturing respectively. RNA was extracted from isolated cells and quantified by Nanodrop spectrometry and RT-qPCR. Higher collagenase concentration, longer digestion duration and pronase pre-treatment increased the cell yield. Cell viability remained high (<5% dead cells) even after 0.2% collagenase treatment for overnight. NP cells remained to have high ACAN, COL2A1, CDH2, KRT18, and KRT19 expression compared to muscle cells for different cell isolation conditions tested. Digestion by collagenase alone without the use of pronase could isolate cells from human degenerated NP tissue but clusters of cells were observed. We suggest the use of the disappearance of tissue as an indirect measure of cells released. This study provides a guide for researchers to decide the parameters involved in NP cell isolation for optimal outcome.

Chondrons and the pericellular matrix of chondrocytes

In cartilage, chondrocytes are embedded within an abundant extracellular matrix (ECM). A typical chondron consists of a chondrocyte and the immediate surrounding pericellular matrix (PCM). The PCM has a patent structure, defined molecular composition, and unique physical properties that support the chondrocyte. Given this spatial position, the PCM is pivotal in mediating communication between chondrocytes and the ECM and, thus, plays a critical role in cartilage homeostasis. The biological function and mechanical properties of the PCM have been extensively studied, mostly in the form of chondrons. This review intends to summarize recent progress in chondron and chondrocyte PCM research, with emphasis on the re-establishment of the PCM by isolated chondrocytes or mesenchymal stem cells during chondrogenic differentiation, and the effects of the PCM on cartilage tissue formation.

Enhanced cell-induced articular cartilage regeneration by chondrons; the influence of joint damage and harvest site

OBJECTIVE

Interactions between chondrocytes and their native pericellular matrix provide optimal circumstances for regeneration of cartilage. However, cartilage diseases such as osteoarthritis change the pericellular matrix, causing doubt to them as a cell source for autologous cell therapy.

METHODS

Chondrons and chondrocytes were isolated from stifle joints of goats in which cartilage damage was surgically induced in the right knee. After 4 weeks of regeneration culture, DNA content and proteoglycan and collagen content and release were determined.

RESULTS

The cartilage regenerated by chondrons isolated from the damaged joint contained less proteoglycans and collagen compared to chondrons from the same harvest site in the nonoperated knee (P < 0.01). Besides, chondrons still reflected whether they were isolated from a damaged joint, even if they where isolated from the opposing or adjacent condyle. Although chondrocytes did not reflect this diseased status of the joint, chondrons always outperformed chondrocytes, even when isolated from the damaged joints (P < 0.0001). Besides increased cartilage production, the chondrons showed less collagenase activity compared to the chondrocytes.

CONCLUSION

Chondrons still outperform chondrocytes when they were isolated from a damaged joint and they might be a superior cell source for articular cartilage repair and cell-induced cartilage formation.

The structure and function of the pericellular matrix of articular cartilage

Chondrocytes in articular cartilage are surrounded by a narrow pericellular matrix (PCM) that is both biochemically and biomechanically distinct from the extracellular matrix (ECM) of the tissue. While the PCM was first observed nearly a century ago, its role is still under investigation. In support of early hypotheses regarding its function, increasing evidence indicates that the PCM serves as a transducer of biochemical and biomechanical signals to the chondrocyte. Work over the past two decades has established that the PCM in adult tissue is defined biochemically by several molecular components, including type VI collagen and perlecan. On the other hand, the biomechanical properties of this structure have only recently been measured. Techniques such as micropipette aspiration, in situ imaging, computational modeling, and atomic force microscopy have determined that the PCM exhibits distinct mechanical properties as compared to the ECM, and that these properties are influenced by specific PCM components as well as disease state. Importantly, the unique relationships among the mechanical properties of the chondrocyte, PCM, and ECM in different zones of cartilage suggest that this region significantly influences the stress-strain environment of the chondrocyte. In this review, we discuss recent advances in the measurement of PCM mechanical properties and structure that further increase our understanding of PCM function. Taken together, these studies suggest that the PCM plays a critical role in controlling the mechanical environment and mechanobiology of cells in cartilage and other cartilaginous tissues, such as the meniscus or intervertebral disc.

Clinical rehabilitation guidelines for matrix-induced autologous chondrocyte implantation on the tibiofemoral joint

Autologous chondrocyte implantation (ACI) has become an established technique for the repair of full-thickness chondral defects in the knee. Matrix-induced ACI (MACI) is the third and current generation of this surgical technique, and, while postoperative rehabilitation following MACI aims to restore normal function in each patient as quickly as possible by facilitating a healing response without overloading the repair site, current published guidelines appear conservative, varied, potentially outdated, and often based on earlier ACI surgical techniques. This article reviews the existing evidence-based literature pertaining to cell loading and postoperative rehabilitation following generations of ACI. Based on this information, in combination with the technical benefits provided by third-generation MACI in comparison to its surgical predecessors, we present a rehabilitation protocol for patients undergoing MACI in the tibiofemoral joint that has now been implemented for several years by our institution in patients with MACI, with good clinical outcomes.

Biomechanical properties and mechanobiology of the articular chondrocyte

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

CCN2/CTGF is required for matrix organization and to protect growth plate chondrocytes from cellular stress

CCN2 (connective tissue growth factor (CTGF/CCN2)) is a matricellular protein that utilizes integrins to regulate cell proliferation, migration and survival. The loss of CCN2 leads to perinatal lethality resulting from a severe chondrodysplasia. Upon closer inspection of Ccn2 mutant mice, we observed defects in extracellular matrix (ECM) organization and hypothesized that the severe chondrodysplasia caused by loss of CCN2 might be associated with defective chondrocyte survival. Ccn2 mutant growth plate chondrocytes exhibited enlarged endoplasmic reticula (ER), suggesting cellular stress. Immunofluorescence analysis confirmed elevated stress in Ccn2 mutants, with reduced stress observed in Ccn2 overexpressing transgenic mice. In vitro studies revealed that Ccn2 is a stress responsive gene in chondrocytes. The elevated stress observed in Ccn2−/− chondrocytes is direct and mediated in part through integrin α5. The expression of the survival marker NFκB and components of the autophagy pathway were decreased in Ccn2 mutant growth plates, suggesting that CCN2 may be involved in mediating chondrocyte survival. These data demonstrate that absence of a matricellular protein can result in increased cellular stress and highlight a novel protective role for CCN2 in chondrocyte survival. The severe chondrodysplasia caused by the loss of CCN2 may be due to increased chondrocyte stress and defective activation of autophagy pathways, leading to decreased cellular survival. These effects may be mediated through nuclear factor κB (NFκB) as part of a CCN2/integrin/NFκB signaling cascade.

Response of Chondrocytes to Local Mechanical Injury in an Ex Vivo Model

BACKGROUND

Our goal was to set up an ex vivo culture system to assess whether cartilage wounding (partial-thickness defects) can induce morphological changes in neighboring chondrocytes and whether these cells can translocate to the surface of the defect.

METHODS

Two-millimeter partial-depth defects were created in human osteochondral explants followed by culture for up to 4 weeks. Frozen sections of defects and defect-free regions were labeled using immunofluorescence for a plasma membrane protein, CD44, and actin with TRITC-phalloidin. Viable nuclei were detected with Hoechst 33342. Differential interference contrast (DIC), confocal, and transmission electron microscopy (TEM) were used to examine process extension.

RESULTS

Significant changes in cell morphology occurred in response to wounding in the superficial and deep cartilage zones. These included cell flattening, polarization of the actin cytoskeleton, extension of pseudopods projecting towards the edge of the defect, and interactions of these filopodia with collagen fibers. Cell density decreased progressively in the 300-µm zone adjacent to the defect to an average of approximately 25% to 35% after 3 weeks. Concomitant increases in cell density in the defect margin were observed. By contrast, minimal changes were seen in the middle cartilage zone.

CONCLUSIONS

These novel observations strongly suggest active cartilage cell responses and movements in response to wounding. It is proposed that cartilage cells use contact guidance on fibrillated collagen to move into and populate defect areas in the superficial and deep zones.

Effects of Hydrostatic Loading on a Self-Aggregating, Suspension Culture-Derived Cartilage Tissue Analog

OBJECTIVE

Many approaches are being taken to generate cartilage replacement materials. The goal of this study was to use a self-aggregating suspension culture model of chondrocytes with mechanical preconditioning.

DESIGN

Our model differs from others in that it is based on a scaffold-less, self-aggregating culture model that produces a cartilage tissue analog that has been shown to share many similarities with the natural cartilage phenotype. Owing to the known loaded environment under which chondrocytes function in vivo, we hypothesized that applying force to the suspension culture-derived chondrocyte biomass would improve its cartilage-like characteristics and provide a new model for engineering cartilage tissue analogs.

RESULTS

In this study, we used a specialized hydrostatic pressure bioreactor system to apply mechanical forces during the growth phase to improve biochemical and biophysical properties of the biomaterial formed. We demonstrated that using this high-density suspension culture, a biomaterial more consistent with the hyaline cartilage phenotype was produced without any foreign material added. Unpassaged chondrocytes responded to a physiologically relevant hydrostatic load by significantly increasing gene expression of critical cartilage molecule collagen and aggrecan along with other cartilage relevant genes, CD44, perlecan, decorin, COMP, and iNOS.

CONCLUSIONS

This study describes a self-aggregating bioreactor model without foreign material or scaffold in which chondrocytes form a cartilage tissue analog with many features similar to native cartilage. This study represents a promising scaffold-less, methodological advancement in cartilage tissue engineering with potential translational applications to cartilage repair.

A proteomic approach for identification and localization of the pericellular components of chondrocytes

Although the pericellular matrix (PCM) plays a central role in the communication between chondrocytes and extracellular matrix, its composition is largely unknown. In this study, the PCM was investigated with a proteomic approach using chondrons, which are enzymatically isolated constructs including the chondrocyte and its surrounding PCM. Chondrons and chondrocytes alone were isolated from human articular cartilage. Proteins extracted from chondrons and chondrocytes were used for two-dimensional electrophoresis. Protein spots were quantitatively compared between chondron and chondrocyte gels. Cellular proteins, which had similar density between chondron and chondrocyte gels, did not proceed for analysis. Since chondrons only differ from chondrocytes in association of the PCM, protein spots in the chondron gels that had higher quantity than that in the chondrocyte gels were selected as candidates of the PCM components and processed for mass spectrometry. Among 15 identified peptides, several were fragments of the three type VI collagen chains (α-1, α-2, and α-3). Other identified PCM proteins included triosephosphate isomerase, transforming growth factor-β induced protein, peroxiredoxin-4, ADAM (A disintegrin and metalloproteinases) 28, and latent-transforming growth factor beta-binding protein-2. These PCM components were verified with immunohisto(cyto)chemistry for localization in the PCM region of articular cartilage. The abundance of type VI collagen in the PCM emphasizes its importance to the microenvironment of chondrocytes. Several proteins were localized in the PCM of chondrocytes for the first time and that warrants further investigation for their functions in cartilage biology.

The protective role of the pericellular matrix in chondrocyte apoptosis

INTRODUCTION

This study was designed to quantify the role of the pericellular matrix (PCM) in chondrocyte apoptosis using chondrons, which are a cartilage functional unit including a chondrocyte and its associated PCM.

METHODS

Chondrocytes and chondrons were enzymatically isolated from human articular cartilage and exposed to monosodium iodoacetate (MIA) and staurosporine for apoptosis induction. Chondrons were defined by the presence of type VI collagen, a basic component of the PCM. Apoptosis of chondrocytes and chondrons was measured with annexin V binding by flow cytometry and verified with terminal dUTP nick end-labeling staining. In a separate experiment, isolated chondrocytes were treated with soluble type VI collagen, before or after apoptosis induction with MIA, and cell death was measured by the activity of LDH and terminal dUTP nick end-labeling staining.

RESULTS

Chondrocytes treated with MIA incurred 27% cell death, compared with 12% in chondrons. On treating with MIA, 9% of chondrocytes underwent apoptosis, compared with only 1.6% of chondrons. Similarly, staurosporine induced 13% apoptosis in chondrocytes, whereas it was 3% in chondrons. Preincubation of type VI collagen effectively prevented chondrocytes from MIA-induced cell death. After apoptosis was induced with MIA, however, treatment with type VI collagen failed to rescue chondrocytes from death.

CONCLUSION

The PCM, a native microenvironment of chondrocytes, protects chondrocytes from apoptosis. Type VI collagen is a functional component of the PCM that contributes to the survival of chondrocytes.

Collagen-induced expression of collagenase-3 by primary chondrocytes is mediated by integrin &alpha;1 and discoidin domain receptor 2: a protein kinase C-dependent pathway

OBJECTIVES

To investigate whether maintaining the chondrocyte's native pericellular matrix prevents collagen-induced up-regulation of collagenase-3 (MMP-13) and whether integrin α1 (ITGα1) and/or discoidin domain receptor 2 (DDR2) modulate MMP-13 expression and which signalling pathway plays a role in collagen-stimulated MMP-13 expression.

METHODS

Goat articular chondrocytes and chondrons were cultured on collagen coatings. Small interfering RNA (siRNA) oligonucleotides targeted against ITGα1 and DDR2 were transfected into primary chondrocytes. Chemical inhibitors for mitogen-activated protein kinase kinase (MEK1) (PD98059), focal adhesion kinase (FAK) (FAK inhibitor 14), mitogen-activated protein kinase 8 (JNK) (SP600125) and protein kinase C (PKC) (PKC412), and a calcium chelator (BAPTA-AM) were used in cell cultures. Real-time PCR was performed to examine gene expression levels of MMP-13, ITGα1 and DDR2 and collagenolytic activity was determined by measuring the amount of hydroxyproline released in the culture medium.

RESULTS

Maintaining the chondrocyte's native pericellular matrix prevented MMP-13 up-regulation and collagenolytic activity when the cells were cultured on a collagen coating. Silencing of ITGα1 and DDR2 reduced MMP-13 gene expression and collagenolytic activity by primary chondrocytes cultured on collagen. Incubation with the PKC inhibitor strongly reduced MMP-13 gene expression levels. Gene expression levels of MMP-13 were also decreased by chondrocytes incubated with the MEK, FAK or JNK inhibitor.

CONCLUSION

Maintaining the native pericellular matrix of chondrocytes prevents collagen-induced up-regulation of MMP-13. Both ITGα1 and DDR2 modulate MMP-13 expression after direct contact between chondrocytes and collagen. PKC, FAK, MEK and JNK are involved in collagen-stimulated expression of MMP-13.

Characterization of ex vivo-generated bovine and human cartilage by immunohistochemical, biochemical, and magnetic resonance imaging analyses

Osteoarthritis (OA) is a prevalent age-associated disease involving altered chondrocyte homeostasis and cartilage degeneration. The avascular nature of cartilage and the altered chondrocyte phenotype characteristic of OA severely limit the capacity for in vivo tissue regeneration. Cell- and tissue-based repair has the potential to revolutionize treatment of OA, but those approaches have exhibited limited clinical success to date. In this study, we test the hypothesis that bovine and human chondrocytes in a collagen type I scaffold will form hyaline cartilage ex vivo with immunohistochemical, biochemical, and magnetic resonance (MR) endpoints similar to the original native cartilage. Chondrocytes were isolated from 1- to 3-week-old calf knee cartilage or from cartilage obtained from human total knee arthroplasties, suspended in 2.7 mg/mL collagen I, and plated as 300 microL spot cultures with 5 x 10(6) each. Medium formulations were varied, including the amount of serum, the presence or absence of ascorbate, and treatments with cytokines. Bovine chondrocytes generated metachromatic territorial and interstitial matrix and accumulated type II collagen over time. Type VI collagen was confined primarily to the pericellular region. The ex vivo-formed bovine cartilage contained more chondroitin sulfate per dry weight than native cartilage. Human chondrocytes remained viable and generated metachromatic territorial matrix, but were unable to support interstitial matrix accumulation. MR analysis of ex vivo-formed bovine cartilage revealed evidence of progressively maturing matrix, but MR-derived indices of tissue quality did not reach those of native cartilage. We conclude that the collagen-spot culture model supports formation and maturation of three-dimensional hyaline cartilage from active bovine chondrocytes. Future studies will focus on determining the capacity of human chondrocytes to show comparable tissue formation.

Biodegradable and injectable cure-on-demand polyurethane scaffolds for regeneration of articular cartilage

This paper describes the synthesis and characterization of an injectable methacrylate functionalized urethane-based photopolymerizable prepolymer to form biodegradable hydrogels. The tetramethacrylate prepolymer was based on the reaction between two synthesized compounds, diisocyanato poly(ethylene glycol) and monohydroxy dimethacrylate poly(epsilon-caprolactone) triol. The final prepolymer was hydrated with phosphate-buffered saline (pH 7.4) to yield a biocompatible hydrogel containing up to 86% water. The methacrylate functionalized prepolymer was polymerized using blue light (450 nm) with an initiator, camphorquinone and a photosensitizer, N,N-dimethylaminoethyl methacrylate. The polymer was stable in vitro in culture media over the 28 days tested (1.9% mass loss); in the presence of lipase, around 56% mass loss occurred over the 28 days in vitro. Very little degradation occurred in vivo in rats over the same time period. The polymer was well tolerated with very little capsule formation and a moderate host tissue response. Human chondrocytes, seeded onto Cultispher-S beads, were viable in the tetramethacrylate prepolymer and remained viable during and after polymerization. Chondrocyte-bead-polymer constructs were maintained in static and spinner culture for 8 weeks. During this time, cells remained viable, proliferated and migrated from the beads through the polymer towards the edge of the polymer. New extracellular matrix (ECM) was visualized with Masson's trichrome (collagen) and Alcian blue (glycosaminoglycan) staining. Further, the composition of the ECM was typical for articular cartilage with prominent collagen type II and type VI and moderate keratin sulphate, particularly for tissue constructs cultured under dynamic conditions.

Preservation of the chondrocyte's pericellular matrix improves cell-induced cartilage formation

The extracellular matrix surrounding chondrocytes within a chondron is likely to affect the metabolic activity of these cells. In this study we investigated this by analyzing protein synthesis by intact chondrons obtained from different types of cartilage and compared this with chondrocytes. Chondrons and chondrocytes from goats from different cartilage sources (articular cartilage, nucleus pulposus, and annulus fibrosus) were cultured for 0, 7, 18, and 25 days in alginate beads. Real-time polymerase chain reaction analyses indicated that the gene expression of Col2a1 was consistently higher by the chondrons compared with the chondrocytes and the Col1a1 gene expression was consistently lower. Western blotting revealed that Type II collagen extracted from the chondrons was cross-linked. No Type I collagen could be extracted. The amount of proteoglycans was higher for the chondrons from articular cartilage and nucleus pulposus compared with the chondrocytes, but no differences were found between chondrons and chondrocytes from annulus fibrosus. The expression of both Mmp2 and Mmp9 was higher by the chondrocytes from articular cartilage and nucleus pulposus compared with the chondrons, whereas no differences were found with the annulus fibrosus cells. Gene expression of Mmp13 increased strongly by the chondrocytes (>50-fold), but not by the chondrons. Taken together, our data suggest that preserving the pericellular matrix has a positive effect on cell-induced cartilage production.

Biomechanical properties of single chondrocytes and chondrons determined by micromanipulation and finite-element modelling

A chondrocyte and its surrounding pericellular matrix (PCM) are defined as a chondron. Single chondrocytes and chondrons isolated from bovine articular cartilage were compressed by micromanipulation between two parallel surfaces in order to investigate their biomechanical properties and to discover the mechanical significance of the PCM. The force imposed on the cells was measured directly during compression to various deformations and then holding. When the nominal strain at the end of compression was 50 per cent, force relaxation showed that the cells were viscoelastic, but this viscoelasticity was generally insignificant when the nominal strain was 30 per cent or lower. The viscoelastic behaviour might be due to the mechanical response of the cell cytoskeleton and/or nucleus at higher deformations. A finite-element analysis was applied to simulate the experimental force-displacement/time data and to obtain mechanical property parameters of the chondrocytes and chondrons. Because of the large strains in the cells, a nonlinear elastic model was used for simulations of compression to 30 per cent nominal strain and a nonlinear viscoelastic model for 50 per cent. The elastic model yielded a Young's modulus of 14 ± 1 kPa (mean ± s.e.) for chondrocytes and 19 ± 2 kPa for chondrons, respectively. The viscoelastic model generated an instantaneous elastic modulus of 21 ± 3 and 27 ± 4 kPa, a long-term modulus of 9.3 ± 0.8 and 12 ± 1 kPa and an apparent viscosity of 2.8 ± 0.5 and 3.4 ± 0.6 kPa s for chondrocytes and chondrons, respectively. It was concluded that chondrons were generally stiffer and showed less viscoelastic behaviour than chondrocytes, and that the PCM significantly influenced the mechanical properties of the cells.

Genome-wide analyses of gene expression during mouse endochondral ossification

Background

Endochondral ossification is a complex process involving a series of events that are initiated by the establishment of a chondrogenic template and culminate in its replacement through the coordinated activity of osteoblasts, osteoclasts and endothelial cells. Comprehensive analyses of in vivo gene expression profiles during these processes are essential to obtain a complete understanding of the regulatory mechanisms involved.

Methodology/Principal Findings

To address these issues, we completed a microarray screen of three zones derived from manually segmented embryonic mouse tibiae. Classification of genes differentially expressed between each respective zone, functional categorization as well as characterization of gene expression patterns, cytogenetic loci, signaling pathways and functional motifs both confirmed reported data and provided novel insights into endochondral ossification. Parallel comparisons of the microdissected tibiae data set with our previously completed micromass culture screen further corroborated the suitability of micromass cultures for modeling gene expression in chondrocyte development. The micromass culture system demonstrated striking similarities to the in vivo microdissected tibiae screen; however, the micromass system was unable to accurately distinguish gene expression differences in the hypertrophic and mineralized zones of the tibia.

Conclusions/Significance

These studies allow us to better understand gene expression patterns in the growth plate and endochondral bones and provide an important technical resource for comparison of gene expression in diseased or experimentally-manipulated cartilages. Ultimately, this work will help to define the genomic context in which genes are expressed in long bones and to understand physiological and pathological ossification.

Glycosaminoglycans in the pericellular matrix of chondrons and chondrocytes

This is the first study to quantitate and profile the glycosaminoglycan (GAG) composition of the pericellular matrix (PCM) of chondrons and chondrocytes using the highly sensitive technique; fluorophore-assisted carbohydrate electrophoresis (FACE). Bovine articular chondrocytes and chondrons were isolated enzymatically. High cell yield and viability were obtained for both preparations. Chondrons had strong immunofluorescent labeling for keratan sulphate and chondroitin-6 sulphate but no labeling for hyaluronan. We compared the immunofluorescent data with FACE. The quantities of total keratan sulphate were determined to be 0.013 +/- 0.002 pg cell(-1) and 0.032 +/- 0.003 pg cell(-1) in the chondrocyte and chondron preparations, respectively. Four internal keratan sulphate sugars were detected (gal beta 1,4glcNAc6S, gal6S beta 1,4glcNAc6S, glcNAc beta 1,3gal and glcNAc6S beta 1,3gal) for both preparations but they were present at significantly higher concentrations in chondron preparations (P < 0.01). Total chondroitin sulphate (CS) was determined to be 0.054 +/- 0.004 pg cell(-1) and 0.077 +/- 0.005 pg cell(-1) for chondrocyte and chondron preparations, respectively. Unsulphated CS disaccharide levels were similar but chondrons had significantly more chondroitin-4 sulphated disaccharides and chondroitin-6 sulphated disaccharides (P < 0.05). Hyaluronan acid was present at low concentrations (0.010 +/- 0.001 pg cell(-1)) in both chondrocytes and chondrons. In this study, enzyme digestion coupled with FACE separation revealed new information about the differences in GAGs from isolated chondrocyte and chondron preparations. Further investigation of the differences in GAGs from chondrocytes and chondrons from different zones of articular cartilage may be useful for tissue engineering approaches.

Low-serum media and dynamic deformational loading in tissue engineering of articular cartilage

High-serum media have been shown to produce significant improvement in the properties of tissue-engineered articular cartilage when applied in combination with dynamic deformational loading. To mitigate concerns regarding the culture variability introduced by serum, we examined the interplay between low-serum/ITS-supplemented media and dynamic deformational loading. Our results show that low serum/ITS supplementation does not support the same level of tissue formation as compared to high serum controls. In free-swelling culture, using a combination of ITS with concentrations of FBS above 2% negated the beneficial effects of ITS. Although there were beneficial effects with loading and 0.2%FBS + ITS, these constructs significantly underperformed relative to 20%FBS constructs. At 2%FBS + ITS, the free-swelling construct stiffness and composition approached or exceeded that of 20%FBS constructs. With dynamic loading, the properties of 2%FBS + ITS constructs were significantly lower than free-swelling controls and 20%FBS constructs by day 42. By priming the chondrocytes in 20%FBS prior to exposure to low-serum/ITS media, we observed that low-serum/ITS media produced significant enhancement in tissue properties compared to constructs grown continuously in 20%FBS.

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

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

Structure of pericellular matrix around agarose-embedded chondrocytes

OBJECTIVE

Determine whether the structure of the type VI collagen component of the chondrocyte pericellular matrix (PCM) generated by agarose-embedded chondrocytes in culture is similar to that found in native articular cartilage.

METHODS

Confocal microscopy, quick-freeze deep-etch electron microscopy, and real-time polymerase chain reaction (PCR) were used to investigate temporal and spatial patterns of type VI collagen protein deposition and gene expression by bovine chondrocytes during 4 weeks of culture within a 2% agarose hydrogel. Similar analyses were performed on chondrocytes within samples of intact cartilage obtained from the same joint surfaces as those used for cell isolation for comparison.

RESULTS

Type VI collagen accumulated uniformly around cells embedded in agarose, with the rate of deposition slowing after the second week. After 1 week, PCM fibrils were observed to be oriented perpendicular to the cell surface, in contrast with the primarily tangential fibrillar arrangement observed in native articular cartilage. Expression of col6 in agarose-embedded cells was initially much higher ( approximately 400%) than that in chondrocytes within cartilage. Expression of col6 in the cultured chondrocytes declined by approximately 60% after 1 week, and remained stable thereafter.

CONCLUSIONS

PCM structure and composition around cells in a hydrogel scaffold may be different than that in native cartilage, with potential implications for mass transport, mechanotransduction, and ultimately, the success of tissue engineering approaches.

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

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

Zonal changes in the three-dimensional morphology of the chondron under compression: the relationship among cellular, pericellular, and extracellular deformation in articular cartilage

The pericellular matrix (PCM) is a narrow region of tissue that completely surrounds chondrocytes in articular cartilage. Previous theoretical models of the "chondron" (the PCM with enclosed cells) suggest that the structure and properties of the PCM may significantly influence the mechanical environment of the chondrocyte. The objective of this study was to quantify changes in the three-dimensional (3D) morphology of the chondron in situ at different magnitudes of compression applied to the cartilage extracellular matrix. Fluorescence immunolabeling for type-VI collagen was used to identify the boundaries of the cell and PCM, and confocal microscopy was used to form 3D images of chondrons from superficial, middle, and deep zone cartilage in explants compressed to 0%, 10%, 30%, and 50% surface-to-surface strain. Lagrangian tissue strain, determined locally using texture correlation, was highly inhomogeneous and revealed depth-dependent compressive stiffness and Poisson's ratio of the extracellular matrix. Compression significantly decreased cell and chondron height and volume, depending on the zone and magnitude of compression. In the superficial zone, cellular-level strains were always lower than tissue-level strains. In the middle and deep zones, however, tissue strains below 25% were amplified at the cellular level, while tissue strains above 25% were decreased at the cellular level. These findings are consistent with previous theoretical models of the chondron, suggesting that the PCM can serve as either a protective layer for the chondrocyte or a transducer that amplifies strain, such that cellular-level strains are more homogenous throughout the tissue depth despite large inhomogeneities in local ECM strains.

Pericellular matrilins regulate activation of chondrocytes by cyclic load-induced matrix deformation

Pericellular matrix is at the ideal location to be involved in transmitting mechanical signals from the microenvironment to a cell. We found that changes of the content of matrilins that link various pericellular molecules surrounding chondrocytes affect mechanical stimulation of chondrocyte proliferation and gene expression. Thus, pericellular matrilins may play a role in chondrocyte mechanotransduction.

INTRODUCTION

Chondrocytes reside in a capsule of pericellular matrix (chondron), which has been hypothesized to play a critical role in transducing mechanical signals to the cell. In this study, we test the hypothesis that the levels of matrilin (MATN)-1 and -3, major components of the chondrocyte pericellular matrix network, regulate activation of chondrocyte proliferation and differentiation by cyclic load-induced matrix deformation.

MATERIALS AND METHODS

Functional matrilins were decreased by expressing a dominant negative mini-MATN in primary chondrocytes or by using MATN1-null chondrocytes. The abundance of matrilins was also increased by expressing a wildtype MATN1 or MATN3 in chondrocytes. Chondrocytes were cultured in a 3D sponge subjected to cyclic deformation at 1 Hz. Chondrocyte gene expression was quantified by real-time RT-PCR and by Western blot analysis. Matrilin pericellular matrix assembly was examined by immunocytochemistry.

RESULTS

Elimination of functional matrilins from pericellular matrix abrogated mechanical activation of Indian hedgehog signaling and abolished mechanical stimulation of chondrocyte proliferation and differentiation. Excessive or reduced matrilin content decreased mechanical response of chondrocytes.

CONCLUSIONS

Normal content of matrilins is essential to optimal activation of chondrocytes by mechanical signals. Our data suggest that the sensitivity of chondrocytes to the changes in the microenvironment can be adjusted by altering the content of matrilins in pericellular matrix. This finding supports a critical role of pericellular matrix in chondrocyte mechano-transduction and has important implications in cartilage tissue engineering and mechanical adaptation.

Zonal variations in the three-dimensional morphology of the chondron measured in situ using confocal microscopy

OBJECTIVE

Chondrocytes in articular cartilage are surrounded by a narrow pericellular matrix (PCM), which together with the enclosed cell(s) are termed the "chondron". Although the precise function of this tissue region is unknown, previous studies provide indirect evidence that the PCM plays an important role in governing the local mechanical environment of chondrocytes. In particular, theoretical models of the chondron under mechanical loading suggest that the shape, size, and biomechanical properties of the PCM significantly influence the stress-strain and fluid flow environment of the cell. The goal of this study was to quantify the three-dimensional morphology of chondron in situ using en bloc immunolabeling of type VI collagen coupled with fluorescence confocal microscopy.

METHODS

Three-dimensional reconstructions of intact, fluorescently labeled chondrons were made from stacks of confocal images recorded in situ from the superficial, middle, and deep zones of porcine articular cartilage of the medial femoral condyle.

RESULTS

Significant variations in the shape, size, and orientation of chondrocytes and chondrons were observed with depth from the tissue surface, revealing flattened discoidal chondrons in the superficial zone, rounded chondrons in the middle zone, and elongated, multicellular chondrons in the deep zone.

CONCLUSIONS

The shape and orientation of the chondron appear to reflect the local collagen architecture of the interterritorial matrix, which varies significantly with depth. Quantitative measurements of morphology of the chondron and its variation with site, disease, or aging may provide new insights into the influence of this structure on physiology and the pathology of articular cartilage.

The pericellular matrix as a transducer of biomechanical and biochemical signals in articular cartilage

The pericellular matrix (PCM) is a narrow tissue region surrounding chondrocytes in articular cartilage, which together with the enclosed cell(s) has been termed the "chondron." While the function of this region is not fully understood, it is hypothesized to have important biological and biomechanical functions. In this article, we review a number of studies that have investigated the structure, composition, mechanical properties, and biomechanical role of the chondrocyte PCM. This region has been shown to be rich in proteoglycans (e.g., aggrecan, hyaluronan, and decorin), collagen (types II, VI, and IX), and fibronectin, but is defined primarily by the presence of type VI collagen as compared to the extracellular matrix (ECM). Direct measures of PCM properties via micropipette aspiration of isolated chondrons have shown that the PCM has distinct mechanical properties as compared to the cell or ECM. A number of theoretical and experimental studies suggest that the PCM plays an important role in regulating the microenvironment of the chondrocyte. Parametric studies of cell-matrix interactions suggest that the presence of the PCM significantly affects the micromechanical environment of the chondrocyte in a zone-dependent manner. These findings provide support for a potential biomechanical function of the chondrocyte PCM, and furthermore, suggest that changes in the PCM and ECM properties that occur with osteoarthritis may significantly alter the stress-strain and fluid environments of the chondrocytes. An improved understanding of the structure and function of the PCM may provide new insights into the mechanisms that regulate chondrocyte physiology in health and disease.

Comparison of gene expression profile between human chondrons and chondrocytes: a cDNA microarray study

OBJECTIVE

The chondron is a basic unit of articular cartilage that includes the chondrocyte and its pericellular matrix (PCM). This current study was designed to investigate the effects of the chondron PCM on the gene expression profile of chondrocytes.

DESIGN

Chondrons and chondrocytes were enzymatically isolated from human articular cartilage, and maintained in pellet culture. Pellets of chondrons or chondrocytes were collected at days 1, 3 and 5 for cDNA microarray analysis.

RESULTS

In comparison with chondrocytes alone, chondrons had 258 genes, in a broad range of functional categories, either up- or downregulated at the three time points tested. At day 1, 26 genes were significantly upregulated in chondrons and four downregulated in comparison to chondrocytes. At day 3, the number of upregulated chondron genes was 97 and the number downregulated was 43. By day 5, there were more downregulated genes (56) than upregulated genes (32) in chondrons. Upregulation of a group of heat shock proteins (HSPA1A, HSPA2 and HSPA8) in chondrons was validated by real time reverse transcription polymerase chain reaction (RT-PCR). Genes related to chondrocyte hypertrophy and dedifferentiation such as SSP1 and DCN were downregulated in chondrons as compared to the expression in chondrocytes.

CONCLUSION

The presence of the PCM in chondrons has a profound influence on chondrocyte gene expression. Upregulation of the heat shock protein 70 may contribute to the robustness and active matrix production of chondrons. The intact PCM may further stabilize the phenotype of chondrocytes within chondrons.

Localization of Type VI Collagen in Tissue-Engineered Cartilage on Polymer Scaffolds

Together, the chondrocyte and its pericellular matrix have been collectively termed the chondron. Current opinion is that the pericellular matrix has both protective and signalling functions between chondrocyte and extracellular matrix. Formation of a native chondrocyte pericellular matrix or chondron structure might therefore be advantageous when tissue engineering a functional hyaline cartilage construct. The presence of chondrons has not been previously described in cartilage engineered on a scaffold. In this paper, we describe a modified immunochemical method to detect collagen VI, a key molecular marker for the pericellular matrix, and an investigation of type VI collagen distribution in engineered hyaline cartilage constructs. Cartilage constructs were engineered from adult human or bovine hyaline chondrocytes cultured on sponge or nonwoven fiber based HYAFF 11 scaffolds. Type VI collagen was detected in all constructs, but a distinctive, high-density, chondron-like distribution of collagen VI was present only in constructs exhibiting additional features of hyaline cartilage engineered using nonwoven HYAFF 11. Chondron structures were localized in areas of the extracellular matrix displaying strong collagen II and GAG staining of constructs where type II collagen composed a high percentage (over 65%) of the total collagen.

Zonal uniformity in mechanical properties of the chondrocyte pericellular matrix: micropipette aspiration of canine chondrons isolated by cartilage homogenization

The pericellular matrix (PCM) is a region of tissue that surrounds chondrocytes in articular cartilage and together with the enclosed cells is termed the chondron. Previous studies suggest that the mechanical properties of the PCM, relative to those of the chondrocyte and the extracellular matrix (ECM), may significantly influence the stress-strain, physicochemical, and fluid-flow environments of the cell. The aim of this study was to measure the biomechanical properties of the PCM of mechanically isolated chondrons and to test the hypothesis that the Young's modulus of the PCM varies with zone of origin in articular cartilage (surface vs. middle/deep). Chondrons were extracted from articular cartilage of the canine knee using mechanical homogenization, and the elastic properties of the PCM were determined using micropipette aspiration in combination with theoretical models of the chondron as an elastic incompressible half-space, an elastic compressible bilayer, or an elastic compressible shell. The Young's modulus of the PCM was significantly higher than that reported for isolated chondrocytes but over an order of magnitude lower than that of the cartilage ECM. No significant differences were observed in the Young's modulus of the PCM between surface zone (24.0 +/- 8.9 kPa) and middle/deep zone cartilage (23.2 +/- 7.1 kPa). In combination with previous theoretical biomechanical models of the chondron, these findings suggest that the PCM significantly influences the mechanical environment of the chondrocyte in articular cartilage and therefore may play a role in modulating cellular responses to micromechanical factors.

Osteoarthritic changes in the biphasic mechanical properties of the chondrocyte pericellular matrix in articular cartilage

The pericellular matrix (PCM) is a narrow region of cartilaginous tissue that surrounds chondrocytes in articular cartilage. Previous modeling studies indicate that the mechanical properties of the PCM relative to those of the extracellular matrix (ECM) can significantly affect the stress-strain, fluid flow, and physicochemical environments of the chondrocyte, suggesting that the PCM plays a biomechanical role in articular cartilage. The goals of this study were to measure the mechanical properties of the PCM using micropipette aspiration coupled with a linear biphasic finite element model, and to determine the alterations in the mechanical properties of the PCM with osteoarthritis (OA). Using a recently developed isolation technique, chondrons (the chondrocyte and its PCM) were mechanically extracted from non-degenerate and osteoarthritic human cartilage. The transient mechanical behavior of the PCM was well-described by a biphasic model, suggesting that the viscoelastic response of the PCM is attributable to flow-dependent effects, similar to that of the ECM. With OA, the mean Young's modulus of the PCM was significantly decreased (38.7+/-16.2 kPa vs. 23.5+/-12.9 kPa, p < 0.001), and the permeability was significantly elevated (4.19+/-3.78 x10(-17) m(4)/Ns vs. 10.2+/-9.38 x 10(-17) m(4)/Ns, p < 0.01). The Poisson's ratio was similar for both non-degenerate and OA PCM (0.044+/-0.063 vs. 0.030+/-0.068, p > 0.6). These findings suggest that the PCM may undergo degenerative processes with OA, similar to those occurring in the ECM. In combination with previous theoretical models of cell-matrix interactions in cartilage, our findings suggest that changes in the properties of the PCM with OA may have an important influence on the biomechanical environment of the chondrocyte.