The role of microtubules in the regulation of proteoglycan synthesis in chondrocytes under hydrostatic pressure

Regulation of Oxygen Tension as a Strategy to Control Chondrocytic Phenotype for Cartilage Tissue Engineering and Regeneration

Cartilage defects and osteoarthritis are health problems which are major burdens on health care systems globally, especially in aging populations. Cartilage is a vulnerable tissue, which generally faces a progressive degenerative process when injured. This makes it the 11th most common cause of global disability. Conservative methods are used to treat the initial phases of the illness, while orthopedic management is the method used for more progressed phases. These include, for instance, arthroscopic shaving, microfracturing and mosaicplasty, and joint replacement as the final treatment. Cell-based implantation methods have also been developed. Despite reports of successful treatments, they often suffer from the non-optimal nature of chondrocyte phenotype in the repair tissue. Thus, improved strategies to control the phenotype of the regenerating cells are needed. Avascular tissue cartilage relies on diffusion for nutrients acquisition and the removal of metabolic waste products. A low oxygen content is also present in cartilage, and the chondrocytes are, in fact, well adapted to it. Therefore, this raises an idea that the regulation of oxygen tension could be a strategy to control the chondrocyte phenotype expression, important in cartilage tissue for regenerative purposes. This narrative review discusses the aspects related to oxygen tension in the metabolism and regulation of articular and growth plate chondrocytes and progenitor cell phenotypes, and the role of some microenvironmental factors as regulators of chondrocytes.

Regulation of chondrocyte biosynthetic activity by dynamic hydrostatic pressure: the role of TRP channels

INTRODUCTION

Chondrocytes perceive and respond to mechanical loading as signals that regulate their metabolism. Joint loading exposes chondrocytes to multiple modes of mechanical stress, including hydrostatic pressure; however, the mechanisms by which chondrocytes sense physiologically relevant levels of hydrostatic pressure are not well understood. We hypothesized that hydrostatic pressure is transduced to an intracellular signal through mechanosensitive membrane ion channels of chondrocytes. The goals of this study were to examine the effect of hydrostatic loading on the development of engineered cartilage tissue and the contribution of mechanosensitive ion channels on these hydrostatic loading effects.

METHODS

Using a 3D model of porcine chondrocytes in agarose, we applied specific chemical inhibitors to determine the role of transient receptor potential (TRP) ion channels TRPV1, TRPV4, TRPC3, and TRPC1 in transducing hydrostatic pressure.

RESULTS

Hydrostatic loading caused a frequency and magnitude-dependent decrease in sulfated glycosaminoglycans (S-GAG), without changes in DNA content. Inhibiting TRPC3 and TRPV4 decreased S-GAG content; however, only the inhibition of TRPV1 partially attenuated the hydrostatic loading-induced reduction in S-GAG content.

CONCLUSIONS

Our findings indicate that TRPV1 may serve as a transducer of hydrostatic pressure in chondrocytes, and provide further support for the role of TRPV4 in regulating chondrocyte anabolism, as well as initial evidence implicating TRPC3 in chondrogenesis. These findings add to our further understanding of the chondrocyte "channelome" and suggest that a range of ion channels mediate the transduction of different biophysical stimuli such as hydrostatic pressure, membrane stretch, or osmotic stress.

Effect of intermittent hydrostatic pressure on aging human chondrocyte cells

In this study, a hydrostatic pressure chamber (HPC) is designed and developed to host chondrocyte cell culture. External stimuli such as forces, pressure, vibration etc. are found to be significant factors on upregulating the relevant proteins for constructing the extracellular matrix (ECM) during culturing. The aim of this paper is to design a system which provides external stimuli during chondrocyte cell culture as well as to discover the relevant gene which can generate the repair and regeneration of aging cells. The system consists of a controllable HPC that provides intermittent hydrostatic pressure (HP) on the cultured cells. The chamber is capable of applying intermittent HP in the range: 0 to1 MPa, at a frequency of 0.5-1Hz. An investigation was undertaken to determine the improvement of human chondrocyte cells viz. of 3 sub-jects whose ages are 60 and above. The effect of HP on the aging cells is observed through the extracted ribonucleic acid (RNA) after the cell is treated with HP for two hours, each day, over four days. The experiments were conducted to observe the effect of HP on the level of collagen type I, collagen type II, and aggrecan. Results show that HP did little to help in upregulating the aggrecan and collagen type II in aged-chondrocyte cells. Further, it was found that the application of HP depended on the number of days applied. The results presented the possibility of ap-plying HP in regeneration of damaged cartilage in elderly.

Effects of cyclic tensile strain and microgravity on the distribution of actin fiber and Fat1 cadherin in murine articular chondrocytes

Chondrocytes as mechano-sensitive cells can sense and respond to mechanical stress throughout life. In chondrocytes, changes of structure and morphology in the cytoskeleton have been potentially involved in various mechano-transductions such as stretch-activated ion channels, integrins, and intracellular organelles. However, the mechanism of cytoskeleton rearrangement in response to mechanical loading and unloading remains unclear. In this study, we exposed chondrocytes to a physiological range of cyclic tensile strain as mechanical loading or to simulated microgravity by 3D-clinostat that produces an unloading environment. Based on microarray profiling, we focused on Fat1 that implicated in the formation and rearrangement of actin fibers. Next, we examined the relationship between the distribution of Fat1 proteins and actin fibers after cyclic tensile strain and microgravity. As a result, Fat1 proteins did not colocalize with actin stress fibers after cyclic tensile strain, but accumulated near the cell membrane and colocalized with cortical actin fibers after microgravity. Our findings indicate that Fat1 may mediate the rearrangement of cortical actin fibers induced by mechanical unloading.

Mechanism of Abnormal Chondrocyte Proliferation Induced by Piezo1-siRNA Exposed to Mechanical Stretch

Objective

To investigate the effect of small interfering RNA targeting mechanosensitive ion channel protein Piezo1 (Piezo1-siRNA) on abnormal chondrocyte proliferation exposed to mechanical stretch.

Methods

Construct and screen effective Piezo1-siRNA sequences and explore an appropriate method to transfect lentiviral vector into chondrocytes exposed to mechanical stretch. Western blot and RT-PCR were used to detect the mRNA and protein expression of Piezo1, Kif18A, and β-tubulin, respectively. Flow cytometry was used to measure the changes in the chondrocyte cycle. The proliferation of chondrocyte was evaluated by cell counting kit-8.

Results

According to the mRNA and protein expression of Piezo1, the effective siRNA sequence was successfully screened. Compared with the 0 h group, mechanical stretch upregulated the expression of Piezo1, Kif18A, and β-tubulin, resulting in chondrocyte cycle arrest and eventually inhibiting chondrocyte proliferation. Moreover, Piezo1-siRNA transfection effectively blocks this process and promotes the proliferation of chondrocyte.

Conclusion

Piezo1-siRNA can reduce the inhibition of chondrocyte proliferation induced by mechanical stretch via downregulating the expression of Kif18A and inhibiting the depolymerization of microtubules. Piezo1-siRNA plays a protective role in chondrocytes, which provides a potential method for the treatment of OA under abnormal mechanical stimulation.

Mechanical regulation of nucleocytoplasmic translocation in mesenchymal stem cells: characterization and methods for investigation

Mesenchymal stem cells (MSCs) have immune-modulatory and tissue-regenerative properties that make them a suitable and promising tool for cell-based therapy application. Since the bio-chemo-mechanical environment influences MSC fate and behavior, the understanding of the mechanosensors involved in the transduction of mechanical inputs into chemical signals could be pivotal. In this context, the nuclear pore complex is a molecular machinery that is believed to have a key role in force transmission and in nucleocytoplasmic shuttling regulation. To fully understand the nuclear pore complex role and the nucleocytoplasmic transport dynamics, recent advancements in fluorescence microscopy provided the possibility to study passive and facilitated nuclear transports also in mechanically stimulated cell culture conditions. Here, we review the current available methods for the investigation of nucleocytoplasmic shuttling, including photo-perturbation-based approaches, fluorescence correlation spectroscopy, and single-particle tracking techniques. For each method, we analyze the advantages, disadvantages, and technical limitations. Finally, we summarize the recent knowledge on mechanical regulation of nucleocytoplasmic translocation in MSC, the relevant progresses made so far, and the future perspectives in the field.

Effects of Rolling-Sliding Mechanical Stimulation on Cartilage Preserved In Vitro

Introduction

Mechanical stimulation is important for maintaining cartilage function. We used a loading device to exert rolling-sliding mechanical stimulation on cartilage preserved in vitro to investigate cartilage viability and the involved mechanisms.

Methods

Osteochondral grafts from pig knees were randomly classified into loading and control groups. The loading group cartilage was subjected to cycles of mechanical stimulation with specified frequency/time/pressure combinations every 3 days; Then the DMEM was refreshed, and the cartilage was preserved in vitro. The control group cartilage was preserved in DMEM throughout the process and was changed every 3 days. On days 14 and 28, the chondrocyte survival rate, histology, and Young's modulus of the cartilage were measured. Western blots were performed after 2 h of loading to evaluate the protein expression.

Results

The loading group showed a significantly higher chondrocyte survival rate, proteoglycan and type II collagen content, and Young's modulus than did the control group on day 14, but no statistically significant differences were found on day 28. After two hours of the loading, the phosphorylation levels of MEK and ERK1/2 increased, and the expression of caspase-3, cleaved caspase-3 and bax decreased.

Conclusion

These results suggest that periodic rolling-sliding mechanical stimulation can increase cartilage vitality in 2 weeks; a possible mechanism is that mechanical stimulation activates the MEK/ERK signalling pathway, thus inhibiting apoptotic protein expression. This loading preservation scheme could be used by cartilage tissue banks to improve cartilage preservation in vitro and enhance the quality of cartilage repair.

Cartilage biomechanics: A key factor for osteoarthritis regenerative medicine

Osteoarthritis (OA) is a joint disorder that is highly extended in the global population. Several researches and therapeutic strategies have been probed on OA but without satisfactory long-term results in joint replacement. Recent evidences show how the cartilage biomechanics plays a crucial role in tissue development. This review describes how physics alters cartilage and its extracellular matrix (ECM); and its role in OA development. The ECM of the articular cartilage (AC) is widely involved in cartilage turnover processes being crucial in regeneration and joint diseases. We also review the importance of physicochemical pathways following the external forces in AC. Moreover, new techniques probed in cartilage tissue engineering for biomechanical stimulation are reviewed. The final objective of these novel approaches is to create a cellular implant that maintains all the biochemical and biomechanical properties of the original tissue for long-term replacements in patients with OA.

Interplay between cytoskeletal polymerization and the chondrogenic phenotype in chondrocytes passaged in monolayer culture

Tubulin and actin exist as monomeric units that polymerize to form either microtubules or filamentous actin. As the polymerization status (monomeric/polymeric ratio) of tubulin and/or actin have been shown to be important in regulating gene expression and phenotype in non-chondrocyte cells, the objective of this study was to examine the role of cytoskeletal polymerization on the chondrocyte phenotype. We hypothesized that actin and/or tubulin polymerization status modulates the chondrocyte phenotype during monolayer culture as well as in 3D culture during redifferentiation. To test this hypothesis, articular chondrocytes were grown and passaged in 2D monolayer culture. Cell phenotype was investigated by assessing cell morphology (area and circularity), actin/tubulin content, organization and polymerization status, as well as by determination of proliferation, fibroblast and cartilage matrix gene expression with passage number. Bovine chondrocytes became larger, more elongated, and had significantly (P < 0.05) increased gene expression of proliferation-associated molecules (cyclin D1 and ki67), as well as significantly (P < 0.05) decreased cartilage matrix (type II collagen and aggrecan) and increased fibroblast-like matrix, type I collagen (COL1), gene expression by passage 2 (P2). Although tubulin polymerization status was not significantly (P > 0.05) modulated, actin polymerization was increased in bovine P2 cells. Actin depolymerization, but not tubulin depolymerization, promoted the chondrocyte phenotype by inducing cell rounding, increasing aggrecan and reducing COL1 expression. Knockdown of actin depolymerization factor, cofilin, in these cells induced further P2 cell actin polymerization and increased COL1 gene expression. To confirm that actin status regulated COL1 gene expression in human P2 chondrocytes, human P2 chondrocytes were exposed to cytochalasin D. Cytochalasin D decreased COL1 gene expression in human passaged chondrocytes. Furthermore, culture of bovine P2 chondrocytes in 3D culture on porous bone substitute resulted in actin depolymerization, which correlated with decreased expression of COL1 and proliferation molecules. In 3D cultures, aggrecan gene expression was increased by cytochalasin D treatment and COL1 was further decreased. These results reveal that actin polymerization status regulates chondrocyte dedifferentiation. Reorganization of the cytoskeleton by actin depolymerization appears to be an active regulatory mechanism for redifferentiation of passaged chondrocytes.

Articular cartilage tissue engineering: the role of signaling molecules

Effective early disease modifying options for osteoarthritis remain lacking. Tissue engineering approach to generate cartilage in vitro has emerged as a promising option for articular cartilage repair and regeneration. Signaling molecules and matrix modifying agents, derived from knowledge of cartilage development and homeostasis, have been used as biochemical stimuli toward cartilage tissue engineering and have led to improvements in the functionality of engineered cartilage. Clinical translation of neocartilage faces challenges, such as phenotypic instability of the engineered cartilage, poor integration, inflammation, and catabolic factors in the arthritic environment; these can all contribute to failure of implanted neocartilage. A comprehensive understanding of signaling molecules involved in osteoarthritis pathogenesis and their actions on engineered cartilage will be crucial. Thus, while it is important to continue deriving inspiration from cartilage development and homeostasis, it has become increasingly necessary to incorporate knowledge from osteoarthritis pathogenesis into cartilage tissue engineering.

Changes in Ultrastructure and Cytoskeletal Aspects of Human Normal and Osteoarthritic Chondrocytes Exposed to Interleukin-1β and Cyclical Hydrostatic Pressure

The aim of this study was to examine the ultrastructure and cytoskeletal organization in human normal and Osteoarhritic (OA) chondrocytes, exposed to interleukin-1β (IL-1β) and cyclic hydrostatic pressure (HP). Morphological examination by transmission electron microscopy (TEM) and scanning electron microscopy (SEM) revealed differences between normal and OA chondrocytes at the nuclear and cytoplasmic level. IL-1β (5 ng/mL) induced a decrease of the number of mitochondria and Golgi bodies and a significant increase on the percentage of cells rich in vacuolization and in marginated chromatin. Cyclical HP (1–5 MPa, 0.25 Hz, for 3 h) did not change the morphology of normal chondrocytes, but had a beneficial effect on OA chondrocytes increasing the number of organelles. Normal and OA cells subjected to IL-1β and HP recovered cytoplasmic ultrastructure. Immunofluorescence (IF) examination of normal chondrocytes showed an actin signal polarized on the apical sides of the cytoplasm, tubulin and vimentin uniformly distributed throughout cytoplasm and vinculin revealed a punctuated pattern under the plasma membrane. In OA chondrocytes, these proteins partially lost their organization. Stimulation with IL-1β caused, in both type of cells, modification in the cytoskeletal organization; HP counteracted the negative effects of IL-1β. Our results showed structural differences at nuclear, cytoplasmic and cytoskeletal level between normal and OA chondrocytes. IL-1β induced ultrastructural and cytoskeletal modifications, counteracted by a cyclical low HP.

What quantitative mechanical loading stimulates in vitro cultivation best?

Articular cartilage has limited regeneration capacities. One of the factors that appear to affect the in vitro cultivation of articular cartilage is mechanical stimulation. So far, no combination of parameters has been identified that offers the best results. The goal is to review the literature in search of the best available set of quantitative mechanical stimuli that lead to optimal in vitro cultivation.The databases Scopus and PubMed were used to survey the literature, and strict in- and exclusion criteria were applied regarding the presence of quantitative data. The review was performed by studying the type of loading (hydrostatic compression or direct compression), the loading magnitude, the frequency and the loading regime (duration of the loading) in comparison to quantitative evidence of cartilage quality response (cellular, signaling and mechanical).Thirty-three studies met all criteria of which 8 studied human, 20 bovine, 2 equine, 1 ovine, 1 porcine and 1 canine cells using four different types of cultivated constructs. Six studies investigated loading magnitude within the same setup, three studies the frequency, and seven the loading regime. Nine studies presented mechanical tissue response. The studies suggest that a certain threshold exits for enhanced cartilage in vitro cultivation of explants (>20 % strain and 0.5 Hz), and that chondrocyte-seeded cultivated constructs show best results when loaded with physiological mechanical stimuli. That is a loading pressure between 5-10 MPa and a loading frequency of 1 Hz exerted at intermittent intervals for a period of a week or longer. Critical aspects remain to be answered for translation into in vivo therapies.

Mechanical regulation of mesenchymal stem cell differentiation

Biophysical cues play a key role in directing the lineage commitment of mesenchymal stem cells or multipotent stromal cells (MSCs), but the mechanotransductive mechanisms at play are still not fully understood. This review article first describes the roles of both substrate mechanics (e.g. stiffness and topography) and extrinsic mechanical cues (e.g. fluid flow, compression, hydrostatic pressure, tension) on the differentiation of MSCs. A specific focus is placed on the role of such factors in regulating the osteogenic, chondrogenic, myogenic and adipogenic differentiation of MSCs. Next, the article focuses on the cellular components, specifically integrins, ion channels, focal adhesions and the cytoskeleton, hypothesized to be involved in MSC mechanotransduction. This review aims to illustrate the strides that have been made in elucidating how MSCs sense and respond to their mechanical environment, and also to identify areas where further research is needed.

Effects of the mycotoxin nivalenol on bovine articular chondrocyte metabolism in vitro

Objective

Kashin-Beck Disease (KBD) is an endemic, age-related degenerative osteoarthropathy and its cause is hypothesised to involve Fusarium mycotoxins. This study investigated the Fusarium mycotoxin Nivalenol (NIV) on the metabolism of bovine articular chondrocytes in vitro.

Design

The effect 0.0–0.5 µg/ml NIV on transcript levels of types I and II collagen, aggrecan, matrix metalloproteinases (MMPs), a disintegrin and metalloproteinase with thrombospondin motif (ADAMTS) and the tissue inhibitors of MMPs (TIMPs) was investigated using quantitative PCR. Amounts of sulphated glycosaminoglycans, MMPs and TIMPs were assessed using the Dimethylmethylene Blue assay, gelatin zymography and reverse gelatin zymography respectively. Cytoskeletal organisation was analysed using confocal microscopy and cytoskeletal gene and protein levels were measured by quantitative PCR and Western blot analysis, respectively.

Results

NIV caused a dose-dependent increase in aggrecan transcription with a concomitant retention of sGAG in the cell lysate. Furthermore, NIV significantly increased MMPs-2, -3 & -9, ADAMTS-4 and -5, and TIMP-2 and -3 transcript levels but inhibited type I collagen, MMP 1 and TIMP 1 mRNA levels. NIV promoted extensive cytoskeletal network remodelling, particularly with vimentin where a dose-dependent peri-nuclear aggregation occurred.

Conclusion

NIV exposure to chondrocytes decreased matrix deposition, whilst enhancing selective catabolic enzyme production, suggesting its potential for induction of cellular catabolism. This NIV-induced extracellular matrix remodelling may be due to extensive remodelling/disassembly of the cytoskeletal elements. Collectively, these findings support the hypothesis that trichothecene mycotoxins, and in particular NIV, have the potential to induce matrix catabolism and propagate the pathogenesis of KBD.

Tissue-engineered cartilage: the crossroads of biomaterials, cells and stimulating factors

Damage to cartilage represents one of the most challenging tasks of musculoskeletal therapeutics due to its limited propensity for healing and regenerative capabilities. Lack of current treatments to restore cartilage tissue function has prompted research in this rapidly emerging field of tissue regeneration of functional cartilage tissue substitutes. The development of cartilaginous tissue largely depends on the combination of appropriate biomaterials, cell source, and stimulating factors. Over the years, various biomaterials have been utilized for cartilage repair, but outcomes are far from achieving native cartilage architecture and function. This highlights the need for exploration of suitable biomaterials and stimulating factors for cartilage regeneration. With these perspectives, we aim to present an overview of cartilage tissue engineering with recent progress, development, and major steps taken toward the generation of functional cartilage tissue. In this review, we have discussed the advances and problems in tissue engineering of cartilage with strong emphasis on the utilization of natural polymeric biomaterials, various cell sources, and stimulating factors such as biophysical stimuli, mechanical stimuli, dynamic culture, and growth factors used so far in cartilage regeneration. Finally, we have focused on clinical trials, recent innovations, and future prospects related to cartilage engineering.

Identification of mechanosensitive genes during skeletal development: alteration of genes associated with cytoskeletal rearrangement and cell signalling pathways

Background

Mechanical stimulation is necessary for regulating correct formation of the skeleton. Here we test the hypothesis that mechanical stimulation of the embryonic skeletal system impacts expression levels of genes implicated in developmentally important signalling pathways in a genome wide approach. We use a mutant mouse model with altered mechanical stimulation due to the absence of limb skeletal muscle (Splotch-delayed) where muscle-less embryos show specific defects in skeletal elements including delayed ossification, changes in the size and shape of cartilage rudiments and joint fusion. We used Microarray and RNA sequencing analysis tools to identify differentially expressed genes between muscle-less and control embryonic (TS23) humerus tissue.

Results

We found that 680 independent genes were down-regulated and 452 genes up-regulated in humeri from muscle-less Spd embryos compared to littermate controls (at least 2-fold; corrected p-value ≤0.05). We analysed the resulting differentially expressed gene sets using Gene Ontology annotations to identify significant enrichment of genes associated with particular biological processes, showing that removal of mechanical stimuli from muscle contractions affected genes associated with development and differentiation, cytoskeletal architecture and cell signalling. Among cell signalling pathways, the most strongly disturbed was Wnt signalling, with 34 genes including 19 pathway target genes affected. Spatial gene expression analysis showed that both a Wnt ligand encoding gene (Wnt4) and a pathway antagonist (Sfrp2) are up-regulated specifically in the developing joint line, while the expression of a Wnt target gene, Cd44, is no longer detectable in muscle-less embryos. The identification of 84 genes associated with the cytoskeleton that are down-regulated in the absence of muscle indicates a number of candidate genes that are both mechanoresponsive and potentially involved in mechanotransduction, converting a mechanical stimulus into a transcriptional response.

Conclusions

This work identifies key developmental regulatory genes impacted by altered mechanical stimulation, sheds light on the molecular mechanisms that interpret mechanical stimulation during skeletal development and provides valuable resources for further investigation of the mechanistic basis of mechanoregulation. In particular it highlights the Wnt signalling pathway as a potential point of integration of mechanical and molecular signalling and cytoskeletal components as mediators of the response.

Cyclic hydrostatic pressure promotes a stable cartilage phenotype and enhances the functional development of cartilaginous grafts engineered using multipotent stromal cells isolated from bone marrow and infrapatellar fat pad

The objective of this study was to investigate how joint specific biomechanical loading influences the functional development and phenotypic stability of cartilage grafts engineered in vitro using stem/progenitor cells isolated from different source tissues. Porcine bone marrow derived multipotent stromal cells (BMSCs) and infrapatellar fat pad derived multipotent stromal cells (FPSCs) were seeded in agarose hydrogels and cultured in chondrogenic medium, while simultaneously subjected to 10MPa of cyclic hydrostatic pressure (HP). To mimic the endochondral phenotype observed in vivo with cartilaginous tissues engineered using BMSCs, the culture media was additionally supplemented with hypertrophic factors, while the loss of phenotype observed in vivo with FPSCs was induced by withdrawing transforming growth factor (TGF)-β3 from the media. The application of HP was found to enhance the functional development of cartilaginous tissues engineered using both BMSCs and FPSCs. In addition, HP was found to suppress calcification of tissues engineered using BMSCs cultured in chondrogenic conditions and acted to maintain a chondrogenic phenotype in cartilaginous grafts engineered using FPSCs. The results of this study point to the importance of in vivo specific mechanical cues for determining the terminal phenotype of chondrogenically primed multipotent stromal cells. Furthermore, demonstrating that stem or progenitor cells will appropriately differentiate in response to such biophysical cues might also be considered as an additional functional assay for evaluating their therapeutic potential.

A feasibility study for evaluation of mechanical properties of articular cartilage with a two-electrode electrical impedance method

BACKGROUND

Since articular cartilage has important mechanical properties such as load-bearing, shock absorption and lubrication for activities in daily life, it is important to evaluate the mechanical properties of repaired cartilage in terms of whether those properties are the same as those of natural cartilage. The purpose of this study was to investigate the effectiveness of an electrical impedance method for quantitatively measuring the mechanical properties of cartilage.

METHODS

Cartilage specimens were harvested from porcine knee joint, and two kinds of enzyme-treated cartilages were prepared to investigate the correlation between mechanical and electrical properties resulting from changes in the structure of the extracellular matrix. Collagenase solution and hyaluronidase solution were used to digest the collagen fibril and proteoglycan, respectively. Electrical impedance measurement, indentation test and biochemical analysis were carried out for the enzyme-treated cartilages.

RESULTS

The water content increased with enzyme treatment time, and the permeability of the treated cartilages increased with decreasing glycosaminoglycan content for both types of enzyme-treated cartilages. The aggregate modulus and the electrical resistivity for both types of enzyme-treated cartilages decreased with increasing permeability after 12-h treatment. The aggregate modulus and the electrical resistivity for both types of treated cartilages decreased with increasing water content and permeability after 24-h treatment. The electrical resistivity and the aggregate modulus of articular cartilage depended not only on the water content, but also on the permeability, and the electrical resistivity for both types of enzyme-treated cartilages was found to be significantly linearly correlated with the aggregate modulus.

CONCLUSIONS

These results showed that the aggregate modulus of articular cartilage can be estimated by measuring its electrical impedance.

Biomechanical influence of cartilage homeostasis in health and disease

There is an urgent demand for long term solutions to improve osteoarthritis treatments in the ageing population. There are drugs that control the pain but none that stop the progression of the disease in a safe and efficient way. Increased intervention efforts, augmented by early diagnosis and integrated biophysical therapies are therefore needed. Unfortunately, progress has been hampered due to the wide variety of experimental models which examine the effect of mechanical stimuli and inflammatory mediators on signal transduction pathways. Our understanding of the early mechanopathophysiology is poor, particularly the way in which mechanical stimuli influences cell function and regulates matrix synthesis. This makes it difficult to identify reliable targets and design new therapies. In addition, the effect of mechanical loading on matrix turnover is dependent on the nature of the mechanical stimulus. Accumulating evidence suggests that moderate mechanical loading helps to maintain cartilage integrity with a low turnover of matrix constituents. In contrast, nonphysiological mechanical signals are associated with increased cartilage damage and degenerative changes. This review will discuss the pathways regulated by compressive loading regimes and inflammatory signals in animal and in vitro 3D models. Identification of the chondroprotective pathways will reveal novel targets for osteoarthritis treatments.

Involvement of the cytoskeletal elements in articular cartilage homeostasis and pathology

The cytoskeleton of all cells is a three-dimensional network comprising actin microfilaments, tubulin microtubules and intermediate filaments. Studies in many cell types have indicated roles for these cytoskeletal proteins in many diverse cellular processes including alteration of cell shape, movement of organelles, migration, endocytosis, secretion, cell division and extracellular matrix assembly. The cytoskeletal networks are highly organized in structure enabling them to fulfil their biological functions. This review will primarily focus on the organization and function of the three major cytoskeletal networks in articular cartilage chondrocytes. Articular cartilage is a major load-bearing tissue of the synovial joint; it is well known that the cytoskeleton acts as a physical interface between the chondrocytes and the extracellular matrix in 'sensing' mechanical stimuli. The effect of mechanical load on cytoskeletal element expression and organization will also be reviewed. Abnormal mechanical load is widely believed to be a risk factor for the development of osteoarthritis. Several studies have intimated that the major cytoskeletal networks are disorganized or often absent in osteoarthritic cartilage chondrocytes. The implications and possible reasoning for this are more widely discussed and placed into context with their potential relevance to disease and therapeutic strategies.

Hydrostatic pressure in articular cartilage tissue engineering: from chondrocytes to tissue regeneration

Cartilage has a poor intrinsic healing response, and neither the innate healing response nor current clinical treatments can restore its function. Therefore, articular cartilage tissue engineering is a promising approach for the regeneration of damaged tissue. Because cartilage is exposed to mechanical forces during joint loading, many tissue engineering strategies use exogenous stimuli to enhance the biochemical or biomechanical properties of the engineered tissue. Hydrostatic pressure (HP) is emerging as arguably one of the most important mechanical stimuli for cartilage, although no optimal treatment has been established across all culture systems. Therefore, this review evaluates prior studies on articular cartilage involving the use of HP, with a particular emphasis on the treatments that appear promising for use in future studies. Additionally, this review addresses HP bioreactor design, chondroprotective effects of HP, the use of HP for chondrogenic differentiation, the effects of high pressures, and HP mechanotransduction.

Effect of Cytoskeletal Disruption on Mechanotransduction of Hydrostatic Pressure by C3H10T1/2 Murine Fibroblasts

Cyclic hydrostatic pressure of physiological magnitude (< 10 MPa) stimulates chondrogenic differentiation of mesenchymal stem cells, but mechanotransduction mechanisms are not well understood. It was hypothesized that an intact cytoskeleton would be required for uninhibited mechanotransduction of hydrostatic pressure. Therefore we examined the effects of drugs which selectively interfere with actin and tubulin polymerization on pressure-induced upregulation of aggrecan and col2a1 (type II collagen) mRNA expression. C3H10T1/2 cells were cultured as pellets in either 4µM cytochalasin D or 4µM nocodazole and subjected to 3 days of cyclic hydrostatic compression (1 Hz, 5 MPa, 2 h per day). Phalloidin staining and indirect immunostaining with anti α-tubulin antibody confirmed disruption of microfilament and microtubule assemblies, respectively. Real time RT-PCR revealed that both drugs substantially lowered the basal level of aggrecan and col2a1 mRNA, but that neither drug prevented a pressure-stimulated increase in gene expression relative to the altered basal state. Thus upregulation of macromolecular gene expression by cyclic hydrostatic pressure did not require a completely intact cytoskeleton.

Chondrocyte response to high regimens of cyclic hydrostatic pressure in 3-dimensional engineered constructs

PURPOSE

Despite widespread use of 3-dimensional (3D) micro-porous scaffolds to promote their potential application in cartilage tissue engineering, only a few studies have examined the response to hydrostatic pressure of engineered constructs. A high cyclic pressurization, currently believed to be the predominant mechanical signal perceived by cells in articular cartilage, was used here to stimulate bovine articular chondrocytes cultured in a synthetic 3D porous scaffold (DegraPol).

METHODS

Construct cultivation lasted 3 days with applied pressurization cycles of amplitude 10 MPa, frequency 0.33 Hz, and stimulation sessions of 4 hours/day.

RESULTS

At 3 days of culture, with respect to pre-culture conditions, the viability of the pressurized constructs did not vary, whereas it underwent a 16% drop in the unpressurized controls. Synthesis of alfa-actin was 34% lower in all cultured constructs. Synthesis of collagen II/collagen I did not vary in pressurized constructs, was 76% lower in unpressurized controls, and was around 230% higher in pressurized constructs with respect to unpressurized controls. Chondrocytes showed a phenotypic spherical morphology at time zero and at 3 days of pressurized culture.

CONCLUSIONS

Although the passage from 2D expansion to 3D geometry was effective to guide cell differentiation, only mechanical conditioning enabled the maintenance and further cell differentiation toward a mature chondrocytic phenotype.

Hydrostatic pressure sensation in cells: integration into the tensegrity model

Hydrostatic pressure (HP) is a mechanical stimulus that has received relatively little attention in the field of the cell biology of mechanotransduction. Generalized models, such as the tensegrity model, do not provide a detailed explanation of how HP might be detected. This is significant, because HP is an important mechanical stimulus, directing cell behaviour in a variety of tissues, including cartilage, bone, airways, and the vasculature. HP sensitivity may also be an important factor in certain clinical situations, as well as under unique environmental conditions such as microgravity. While downstream cellular effects have been well characterized, the initial HP sensation mechanism remains unclear. In vitro evidence shows that HP affects cytoskeletal polymerization, an effect that may be crucial in triggering the cellular response. The balance between free monomers and cytoskeletal polymers is shifted by alterations in HP, which could initiate a cellular response by releasing and (or) activating cytoskeleton-associated proteins. This new model fits well with the basic tenets of the existing tensegrity model, including mechanisms in which cellular HP sensitivity could be tuned to accommodate variable levels of stress.

Differential expression of type X collagen in a mechanically active 3-D chondrocyte culture system: a quantitative study

Objective

Mechanical loading of cartilage influences chondrocyte metabolism and gene expression. The gene encoding type X collagen is expressed specifically by hypertrophic chondrocytes and up regulated during osteoarthritis. In this study we tested the hypothesis that the mechanical microenvironment resulting from higher levels of local strain in a three dimensional cell culture construct would lead to an increase in the expression of type X collagen mRNA by chondrocytes in those areas.

Methods

Hypertrophic chondrocytes were isolated from embryonic chick sterna and seeded onto rectangular Gelfoam sponges. Seeded sponges were subjected to various levels of cyclic uniaxial tensile strains at 1 Hz with the computer-controlled Bio-Stretch system. Strain distribution across the sponge was quantified by digital image analysis. After mechanical loading, sponges were cut and the end and center regions were separated according to construct strain distribution. Total RNA was extracted from the cells harvested from these regions, and real-time quantitative RT-PCR was performed to quantify mRNA levels for type X collagen and a housing-keeping gene 18S RNA.

Results

Chondrocytes distributed in high (9%) local strain areas produced more than two times type X collagen mRNA compared to the those under no load conditions, while chondrocytes located in low (2.5%) local strain areas had no appreciable difference in type X collagen mRNA production in comparison to non-loaded samples. Increasing local strains above 2.5%, either in the center or end regions of the sponge, resulted in increased expression of Col X mRNA by chondrocytes in that region.

Conclusion

These findings suggest that the threshold of chondrocyte sensitivity to inducing type X collagen mRNA production is more than 2.5% local strain, and that increased local strains above the threshold results in an increase of Col X mRNA expression. Such quantitative analysis has important implications for our understanding of mechanosensitivity of cartilage and mechanical regulation of chondrocyte gene expression.

Shear and Compression Differentially Regulate Clusters of Functionally Related Temporal Transcription Patterns in Cartilage Tissue

Chondrocytes are subjected to a variety of biophysical forces and flows during physiological joint loading, including mechanical deformation, fluid flow, hydrostatic pressure, and streaming potentials; however, the role of these physical stimuli in regulating chondrocyte behavior is still being elucidated. To isolate the effects of these forces, we subjected intact cartilage explants to 1-24 h of continuous dynamic compression or dynamic shear loading at 0.1 Hz. We then measured the transcription levels of 25 genes known to be involved in cartilage homeostasis using real-time PCR and compared the gene expression profiles obtained from dynamic compression, dynamic shear, and our recent results on static compression amplitude and duration. Using clustering analysis, we determined that transcripts for proteins with similar function had correlated responses to loading. However, the temporal expression patterns were strongly dependent on the type of loading applied. Most matrix proteins were up-regulated by 24 h of dynamic compression or dynamic shear, but down-regulated by 24 h of 50% static compression, suggesting that cyclic matrix deformation is a key stimulator of matrix protein expression. Most matrix proteases were up-regulated by 24 h under all loading types. Transcription factors c-Fos and c-Jun maximally responded within 1 h to all loading types. Pre-incubating cartilage explants with either a chelator of intracellular calcium or an inhibitor of the cyclic AMP pathway demonstrated the involvement of both pathways in transcription induced by dynamic loading.

Chondrocyte mechanotransduction: effects of compression on deformation of intracellular organelles and relevance to cellular biosynthesis

OBJECTIVE

The effects of mechanical deformation of intact cartilage tissue on chondrocyte biosynthesis in situ have been well documented, but the mechanotransduction pathways that regulate such phenomena have not been elucidated completely. The goal of this study was to examine the effects of tissue deformation on the morphology of a range of intracellular organelles which play a major role in cell biosynthesis and metabolism.

DESIGN

Using chemical fixation, high pressure freezing, and electron microscopy, we imaged chondrocytes within mechanically compressed cartilage explants at high magnification and quantitatively and qualitatively assessed changes in organelle volume and shape caused by graded levels of loading.

RESULTS

Compression of the tissue caused a concomitant reduction in the volume of the extracellular matrix (ECM), chondrocyte, nucleus, rough endoplasmic reticulum, and mitochondria. Interestingly, however, the Golgi apparatus was able to resist loss of intraorganelle water and retain a portion of its volume relative to the remainder of the cell. These combined results suggest that a balance between intracellular mechanical and osmotic gradients govern the changes in shape and volume of the organelles as the tissue is compressed.

CONCLUSIONS

Our results lead to the interpretive hypothesis that organelle volume changes appear to be driven mainly by osmotic interactions while shape changes are mediated by structural factors, such as cytoskeletal interactions that may be linked to extracellular matrix deformations. The observed volume and shape changes of the chondrocyte organelles and the differential behavior between organelles during tissue compression provide evidence for an important mechanotransduction pathway linking translational and post-translational events (e.g., elongation and sulfation of glycosaminoglycans (GAGs) in the Golgi) to cell deformation.

The sulfation pattern of chondroitin sulfate from articular cartilage explants in response to mechanical loading

Chondrocytes within articular cartilage experience complete unloading between loading cycles thereby utilizing mechanical signals to regulate their own anabolic and catabolic activities. Structural alterations of proteoglycans (PGs) during aging and the development of osteoarthritis (OA) have been reported; whether these can be attributed to altered load or compression is largely unknown. We report here on experiments in which the effect of intermittent loading on the fine structure of newly synthesized chondroitin sulfate (CS) in bovine articular cartilage explants was examined. Tissues were subjected for 6 days to cyclic compressive pressure using a sinusoidal waveform of 0.1, 0.5 or 1.0 Hz frequency with a peak stress of 0.5 MPa for a period of 5, 10 or 20 s, followed by an unloading period lasting 10, 100 or 1000 s. During the final 18 h of the culture, cartilage explants were radiolabeled with 50 microCi/ml D-6-[3H]glucosamine, and newly synthesized as well as endogenous CS chains were isolated after proteinase solubilization of the tissue. CS chains were depolymerized with chondroitinase ABC and ACII, and the 3H-digestion products were quantified after fractionation by high-performance anion-exchange chromatography using a CarboPac PA1 column. Intermittently applied cyclic mechanical loading did not affect the proportion of 4- and 6-sulfated disaccharide repeats, but caused a significant decrease in the abundance of the 4,6-disulfated nonreducing terminal galNAc residues. In addition, loading induced elongation of CS chains. Taken together, these data provide evidence for the first time that long-term in vitro loading results in marked and reproducible changes in the fine structure of newly synthesized CS, and that accumulation of such chains may in turn modify the physicochemical and biological response of articular cartilage. Moreover, data presented here suggest that in vitro dynamic compression of cartilage tissue can induce some of the same alterations in CS sulfation that have previously been shown to occur during the development of degenerative joint diseases such as OA.

Articular cartilage functional histomorphology and mechanobiology: a research perspective

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

Mechanoregulation of human articular chondrocyte aggrecan and type II collagen expression by intermittent hydrostatic pressure in vitro

This study addressed the hypothesis that duration and magnitude of applied intermittent hydrostatic pressure (IHP) are critical parameters in regulation of normal human articular chondrocyte aggrecan and type II collagen expression. Articular chondrocytes were isolated from knee cartilage and maintained as primary, high-density monolayer cultures. IHP was applied at magnitudes of 1, 5 and 10 MPa at 1 Hz for durations of either 4 h per day for one day (4 x 1) or 4 h per day for four days (4 x 4). Total cellular RNA was isolated and analyzed for aggrecan and type II collagen mRNA signal levels using specific primers and reverse transcription polymerase chain reaction (RT-PCR) nested with beta-actin primers as internal controls. With a 4x1 loading regimen, aggrecan mRNA signal levels increased 1.3- and 1.5-fold at 5 and 10 MPa, respectively, relative to beta-actin mRNA when compared to unloaded cultures. Changing the duration of loading to a 4x4 regimen increased aggrecan mRNA signal levels by 1.4-, 1.8- and 1.9-fold at loads of 1, 5 and 10 MPa, respectively. In contrast to the effects of IHP on aggrecan, type II collagen mRNA signal levels were only upregulated at loads of 5 and 10 MPa with the 4x4 loading regimen. Analysis of cell-associated protein by western blotting confirmed that IHP increased aggrecan and type II collagen in chondrocyte extracts. These data demonstrate that duration and magnitude of applied IHP differentially alter chondrocyte matrix protein expression. The results show that IHP provides an important stimulus for increasing cartilage matrix anabolism and may contribute to repair and regeneration of damaged or diseased cartilage.

cDNA array reveals mechanosensitive genes in chondrocytic cells under hydrostatic pressure

Hydrostatic pressure (HP) has a profound effect on cartilage metabolism in normal and pathological conditions, especially in weight-bearing areas of the skeletal system. As an important component of overall load, HP has been shown to affect the synthetic capacity and well-being of chondrocytes, depending on the mode, duration and magnitude of pressure. In this study we examined the effect of continuous HP on the gene expression profile of a chondrocytic cell line (HCS-2/8) using a cDNA array containing 588 well-characterized human genes under tight transcriptional control. A total of 51 affected genes were identified, many of them not previously associated with mechanical stimuli. Among the significantly up-regulated genes were immediate-early genes, and genes involved in heat-shock response (hsp70, hsp40, hsp27), and in growth arrest (GADD45, GADD153, p21(Cip1/Waf1), tob). Markedly down-regulated genes included members of the Id family genes (dominant negative regulators of basic helix-loop-helix transcription factors), and cytoplasmic dynein light chain and apoptosis-related gene NIP3. These alterations in the expression profile induce a transient heat-shock gene response and activation of genes involved in growth arrest and cellular adaptation and/or differentiation.

Determination of cellular strains by combined atomic force microscopy and finite element modeling

Many organs adapt to their mechanical environment as a result of physiological change or disease. Cells are both the detectors and effectors of this process. Though many studies have been performed in vitro to investigate the mechanisms of detection and adaptation to mechanical strains, the cellular strains remain unknown and results from different stimulation techniques cannot be compared. By combining experimental determination of cell profiles and elasticities by atomic force microscopy with finite element modeling and computational fluid dynamics, we report the cellular strain distributions exerted by common whole-cell straining techniques and from micromanipulation techniques, hence enabling their comparison. Using data from our own analyses and experiments performed by others, we examine the threshold of activation for different signal transduction processes and the strain components that they may detect. We show that modulating cell elasticity, by increasing the F-actin content of the cytoskeleton, or cellular Poisson ratio are good strategies to resist fluid shear or hydrostatic pressure. We report that stray fluid flow in some substrate-stretch systems elicits significant cellular strains. In conclusion, this technique shows promise in furthering our understanding of the interplay among mechanical forces, strain detection, gene expression, and cellular adaptation in physiology and disease.

Glycosaminoglycans and beta-amyloid, prion and tau peptides in neurodegenerative diseases

Protein aggregation into dense filamentous inclusions is a characteristic feature of many etiologically diverse neurodegenerative disorders including Alzheimer's disease (AD), spongiform encephalopathies, and tauopathies. Thus, beta-amyloid peptide (Abeta) accumulates within senile amyloid plaques in AD, protease-resistant prion protein constitutes the amyloid deposits in spongiform encephalopathies and tau protein gives rise to neurofibrillary tangles (NFT) both in AD and in tauopathies. Curiously, these abnormal protein inclusions contain, in addition to their major peptide components, some associated sulfated glycosaminoglycans (sGAG). Here we discuss the proposal that the binding of sGAG to aggregate-forming peptides may modify the pathogenic process depending on their subcellular localization.

Adenomatous polyposis coli (APC), beta-catenin, and cadherin are expressed in human bone and cartilage

AIMS

Members of the cadherin and catenin families are involved in chondrogenesis and catenin gene mutations have been detected in malignant tumours of bone. This study was undertaken to assess in detail expression of cadherin, beta-catenin and the associated tumour suppressor gene product APC in bone and cartilage at different stages of human skeletal maturity and in non-neoplastic and neoplastic osteoarticular disease.

METHODS AND RESULTS

Immunohistochemical staining of formalin-fixed paraffin-embedded normal and osteoarthritic adult articular cartilage, fetal growth plate and a series of tumours of bone and cartilage was undertaken with a panel of antibodies against APC, beta-catenin, and pan-cadherin. This study demonstrated expression of APC, beta-catenin and cadherin in normal and diseased bone and cartilage. APC was present both in osteoblasts and osteoclasts but not in osteocytes. Although only weak APC staining of occasional growth plate hypertrophic chondrocytes and normal articular chondrocytes was seen, APC staining was increased in osteoarthritic articular cartilage. beta-catenin and pan-cadherin staining was strongly positive in osteoclasts and osteoblasts, with expression being lost when bone cells differentiated into osteocytes. Expression of APC, beta-catenin and pan-cadherin in bone tumours was similar to that of non-neoplastic adult tissues.

CONCLUSIONS

These findings suggest previously unrecognized roles for APC in regulation of function of chondrocytes, osteoblasts and osteoclasts and support the view that catenin-cadherin interactions are important in regulation of bone cell activity. Abnormalities of expression or function of these molecules may be important in formation of bone tumours and their clinical behaviour.

The differential distribution of acetylated and detyrosinated alpha-tubulin in the microtubular cytoskeleton and primary cilia of hyaline cartilage chondrocytes

The primary cilium is a ubiquitous cytoplasmic organelle of unknown function. Ultrastructural evidence of primary cilia in chondrocytes, and their colocalisation with the Golgi apparatus, has led to speculation that these structures are functionally linked. To investigate the relationship between these organelles, we examined the molecular anatomy of the microtubular cytoskeleton in the chondrocytes of chick embryo sterna. Thick cryosections were immunolabelled with antibodies directed against acetylated alpha-tubulin (C3B9), detyrosinated alpha-tubulin (ID5) and total alpha-tubulin (TAT), and imaged at high magnification using confocal laser scanning microscopy. Transmission electron microscopy confirmed the ultrastructure of the chondrocyte primary cilium and its structural relationship to the Golgi apparatus. Detyrosinated and acetylated alpha-tubulins were concentrated in the centrioles, centrosome and microtubule organising centre adjacent to the nucleus, with total alpha-tubulin distributed throughout the cytoplasm. ID5 stained the primary cilium at an incidence of 1 per cell, its colocalisation with C3B9 identifying the primary cilium as one of the most stable features of the microtubular cytoskeleton. Primary cilia varied from 1 to 4 microm in length, and 3 patterns of projection into the extracellular matrix were identified; (1) full extension and matrix contact, with minor undulations along the length; (2) partial extension and matrix contact, with a range of bending deflections; (3) cilium reclined against the cell surface with minimal matrix contact. Ultrastructural studies identified direct connections between extracellular collagen fibres and the proteins which decorate ciliary microtubules, suggesting a matrix-cilium-Golgi continuum in hyaline chondrocytes. These results strengthen the hypothesis that the primary cilium acts as a 'cellular cybernetic probe' capable of transducing environmental information from the extracellular matrix, communicating this information to the centrosome. and regulating the exocytosis of Golgi-derived secretory vesicles.

The cells of the rabbit meniscus: their arrangement, interrelationship, morphological variations and cytoarchitecture

Four major morphologically distinct classes of cells were identified within the adult rabbit meniscus using antibodies to cytoskeletal proteins. Two classes of cell were present in the fibrocartilage region of the meniscus. These meniscal cells exhibited long cellular processes that extended from the cell body. A third cell type found in the inner hyaline-like region of the meniscus had a rounded form and lacked projections. A fourth cell type with a fusiform shape and no cytoplasmic projections was found along the superficial regions of the meniscus. Using a monoclonal antibody to connexin 43, numerous gap junctions were observed in the fibrocartilage region, whereas none were seen in cells either from the hyaline-like or the superficial zones of the meniscus. The majority of the cells within the meniscus exhibited other specific features such as primary cilia and 2 centrosomes. The placement of the meniscal cell subtypes as well as their morphology and architecture support the supposition that their specific characteristics underlie the ability of the meniscus to respond to different types of environmental mechanical loads.

Differential regulation of stress proteins by high hydrostatic pressure, heat shock, and unbalanced calcium homeostasis in chondrocytic cells

High hydrostatic pressure (HP) has recently been shown to increase cellular heat shock protein 70 (Hsp70) level in a specific way that does not involve transcriptional activation of the gene, but rather the stabilisation of the mRNA for Hsp70. In this study, we investigated whether there are other observable changes caused by HP stress, and compared them with those induced by certain other forms of stressors. A chondrocytic cell line T/C28a4 was exposed to 30 MPa continuous HP, heat shock at 43 degrees C, and increased cytosolic calcium concentration by the addition of sarco-endoplasmic reticulum Ca(2+) ATPase inhibitor thapsigargin (25 nM) or calcium ionophore A23187 (1 microM) in the cultures. The protein synthesis was studied by in vitro metabolic labelling followed by one- and two-dimensional polyacrylamide gel electrophoresis, and mass spectrometry was utilized to confirm the identity of the protein spots on two-dimensional gels. Continuous 30 MPa HP increased remarkably the relative labelling of Hsp70. Labelling of Hsp90 was also increased by 15-20%, although no clear change was evident at the protein level in Western blots. Elevated intracellular Ca(2+) concentration induced by thapsigargin and calcium ionophore A23187 increased mainly the synthesis of glucose-regulated protein 78 (Grp78/BiP), whereas Hsp70 and Hsp90 were decreased by the treatment. Heat shock was the strongest inducer of Hsp70 and Hsp90. This study further confirmed the induction of Hsp70 in chondrocytic cells exposed to high HP, but it also showed that calcium-mediated responses are unlikely to cause the stress response observed in the hydrostatically pressurized cells.