Increases in cytosolic calcium, but not fluid flow, affect aggrecan mRNA levels in articular chondrocytes

Acid- and Volume-Sensitive Chloride Currents in Human Chondrocytes

Chondrocytes face extreme alterations of extracellular osmolarity and pH, which force them to appropriately regulate their cell volume (CV) and cellular pH. Perturbations of these mechanisms lead to chondrocyte death and ultimately to osteoarthritis (OA), the most common chronic joint diseases worldwide. OA hallmarks are altered cartilage hydration and severe fluid acidification. Impaired CV regulation and acidotoxicity contribute to disease progression and volume-sensitive anion channels are upregulated in OA. This study assessed the effect of hypotonicity and extracellular acidification on chondrocyte Cl^– conductances and CV regulation. Cl^– currents and membrane potentials were measured in human C28/I2 cells and primary human chondrocytes using the patch clamp technique. Intracellular pH was assessed by BCECF fluorescence, CV measurements were performed using the Coulter method, and cell viability/cell death by a resazurin assay. Hypotonic cell swelling caused activation of a volume-sensitive outwardly rectifying (VSOR) Cl^– current followed by a regulatory volume decrease (RVD), which was attenuated by the Cl^– channel blocker DCPIB. Extracellular, but not intracellular acidification to pH ≤ 5.0 elicited an acid-sensitive outwardly rectifying (ASOR) Cl^– conductance. Activation of either current depolarized the cell membrane potential. Under simultaneous hypotonic and acidic stimulation, VSOR and ASOR currents transiently coactivated, giving rise to a mixed current phenotype. Over time the VSOR current gradually vanished and the residual conductance showed a pure ASOR current phenotype. Extracellular acidification caused an isotonic CV gain and a complete suppression of RVD under hypotonic conditions. The results suggest that deactivation of the VSOR current under acidic conditions impairs CV regulation in chondrocytes, which is likely to compromise chondrocyte viability.

Glycosaminoglycan synthesis in the nucleus pulposus: Dysregulation and the pathogenesis of disc degeneration

Few human tissues have functions as closely linked to the composition of their extracellular matrices as the intervertebral disc. In fact, the hallmark of intervertebral disc degeneration, commonly accompanying low back and neck pain, is the progressive loss of extracellular matrix molecules - specifically the GAG-substituted proteoglycans. While this loss is often associated with increased extracellular catabolism via metalloproteinases and pro-inflammatory cytokines, there is strong evidence that disc degeneration is related to dysregulation of the enzymes involved in GAG biosynthesis. In this review, we discuss those environmental factors, unique to the disc, that control expression and function of XT-1, GlcAT-I, and ChSy/ChPF in the healthy and degenerative state. Additionally, we address the pathophysiology of aberrant GAG biosynthesis and highlight therapeutic strategies designed to augment the loss of extracellular matrix molecules that afflict the degenerative state.

Cell-based tissue engineering strategies used in the clinical repair of articular cartilage

One of the most important issues facing cartilage tissue engineering is the inability to move technologies into the clinic. Despite the multitude of current research in the field, it is known that 90% of new drugs that advance past animal studies fail clinical trials. The objective of this review is to provide readers with an understanding of the scientific details of tissue engineered cartilage products that have demonstrated a certain level of efficacy in humans, so that newer technologies may be developed upon this foundation. Compared to existing treatments, such as microfracture or autologous chondrocyte implantation, a tissue engineered product can potentially provide more consistent clinical results in forming hyaline repair tissue and in filling the entirety of the defect. The various tissue engineering strategies (e.g., cell expansion, scaffold material, media formulations, biomimetic stimuli, etc.) used in forming these products, as collected from published literature, company websites, and relevant patents, are critically discussed. The authors note that many details about these products remain proprietary, not all information is made public, and that advancements to the products are continuously made. Nevertheless, by understanding the design and production processes of these emerging technologies, one can gain tremendous insight into how to best use them and also how to design the next generation of tissue engineered cartilage products.

Cell-based tissue engineering strategies used in the clinical repair of articular cartilage

One of the most important issues facing cartilage tissue engineering is the inability to move technologies into the clinic. Despite the multitude of current research in the field, it is known that 90% of new drugs that advance past animal studies fail clinical trials. The objective of this review is to provide readers with an understanding of the scientific details of tissue engineered cartilage products that have demonstrated a certain level of efficacy in humans, so that newer technologies may be developed upon this foundation. Compared to existing treatments, such as microfracture or autologous chondrocyte implantation, a tissue engineered product can potentially provide more consistent clinical results in forming hyaline repair tissue and in filling the entirety of the defect. The various tissue engineering strategies (e.g., cell expansion, scaffold material, media formulations, biomimetic stimuli, etc.) used in forming these products, as collected from published literature, company websites, and relevant patents, are critically discussed. The authors note that many details about these products remain proprietary, not all information is made public, and that advancements to the products are continuously made. Nevertheless, by understanding the design and production processes of these emerging technologies, one can gain tremendous insight into how to best use them and also how to design the next generation of tissue engineered cartilage products.

Effects of Adipokines and Insulin on Intracellular pH, Calcium Concentration, and Responses to Hypo-Osmolarity in Human Articular Chondrocytes from Healthy and Osteoarthritic Cartilage

OBJECTIVE

To evaluate the effects of adipokines and insulin on intracellular calcium concentration ([Ca(2+)]i) and pH (pHi) in human articular chondrocytes from healthy (CHC) and osteoarthritic cartilage (COC).

DESIGN

pHi and [Ca(2+)]i were measured using BCECF and Fura-2 fluorometric probes in CHC and COC under control conditions and following a hypotonic shock. The effects of interleukin-1β (IL1β), tumor necrosis factor-α (TNFα), insulin, leptin, resistin, and adiponectin were assessed.

RESULTS

pHi was lower in COC than in CHC. Only IL1β β decreased pHi in both cell types; all the agents enhanced pHi recovery following an ammonium prepulse in CHC, effect that was attenuated by Na(+)-H(+) exchanger inhibitors, but they had no effect in COC. Hypotonic shock (HTS) caused a pHi increase, which was significantly smaller in COC. All the hormones attenuated this response and the effect of IL1β was greater. The basal [Ca(2+)]i was similar in COC and CHC; IL1β, TNFα, and insulin increased the [Ca(2+)]i, but leptin, resistin, and adiponectin did not. These effects were greater in COC. This [Ca(2+)]i increase was dependent on extracellular Ca(2+) and attenuated by Na(+)-Ca(2+) exchanger inhibitors. HTS caused a [Ca(2+)]i increase, which was inhibited by transient receptor potential vanilloid blockers and attenuated by all the hormones tested with the exception of adiponectin.

CONCLUSIONS

These findings may help explain the association between obesity and osteoarthritis, in which these hormones are altered. The responses of CHC and COC are different, which suggests that a modification of pH and Ca(2+) homeostasis is part of the osteoarthritis pathophysiology.

Megakaryocytes are mechanically responsive and influence osteoblast proliferation and differentiation

Maintenance of bone mass and geometry is influenced by mechanical stimuli. Paradigms suggest that osteocytes embedded within the mineralized matrix and osteoblasts on the bone surfaces are the primary responders to physical forces. However, other cells within the bone marrow cavity, such as megakaryocytes (MKs), are also subject to mechanical forces. Recent studies have highlighted the potent effects of MKs on osteoblast proliferation as well as bone formation in vivo. We hypothesize that MKs are capable of responding to physical forces and that the interactions between these cells and osteoblasts can be influenced by mechanical stimulation. In this study, we demonstrate that two MK cell lines respond to fluid shear stress in culture. Furthermore, using laser capture microdissection, we isolated MKs from histologic sections of murine tibiae that were exposed to compressive loads in vivo. C-fos, a transcription factor shown to be upregulated in response to load in various tissue types, was increased in MKs from loaded relative to non-loaded limbs at a level comparable to that of osteocytes from the same limbs. We also developed a co-culture system to address whether mechanical stimulation of MKs in culture would impact osteoblast proliferation and differentiation. The presence of MKs in co-culture, but not conditioned media, had dramatic effects on proliferation of preosteoblast MC3T3-E1 cells in culture. Our data suggests a minimal decrease in proliferation as well as an increase in mineralization capacity of osteoblasts co-cultured with MKs exposed to shear compared to co-cultures with unstimulated MKs.

Calcium regulates cyclic compression-induced early changes in chondrocytes during in vitro cartilage tissue formation

A single application of cyclic compression (1kPa, 1Hz, 30min) to bioengineered cartilage results in improved tissue formation through sequential catabolic and anabolic changes mediated via cell shape changes that are regulated by α5β1 integrin and membrane-type metalloprotease (MT1-MMP). To determine if calcium was involved in this process, the role of calcium in regulating cell shape changes, MT1-MMP expression and integrin activity in response to mechanical stimulation was examined. Stimulation-induced changes in cell shape and MT1-MMP expression were abolished by chelation of extracellular calcium, and this effect was reversed by re-introduction of calcium. Spreading was inhibited by blocking stretch-activated channels (with gadolinium), while retraction was prevented by blocking the L-Type voltage-gated channel (with nifedipine); both compounds inhibited MT1-MMP upregulation. Calcium A23187 ionophore restored cellular response further supporting a role for these channels. Calcium regulated the integrin-mediated signalling pathway, which was facilitated through Src kinase. Both calcium- and integrin-mediated pathways converged on ERK-MAPK in response to stimulation. While both integrins and calcium signalling mediate chondrocyte mechanotransduction, calcium appears to play the major regulatory role. Understanding the underlying molecular mechanisms involved in chondrocyte mechanotransduction may lead to the development of improved bioengineered cartilage.

Intracellular Na(+) and Ca(2+) modulation increases the tensile properties of developing engineered articular cartilage

OBJECTIVE

Significant collagen content and tensile properties are difficult to achieve in tissue-engineered articular cartilage. The aim of this study was to investigate whether treating developing tissue-engineered cartilage constructs with modulators of intracellular Na(+) or Ca(2+) could increase collagen concentration and construct tensile properties.

METHODS

Inhibitors of Na(+) ion transporters and stimulators of intracellular Ca(2+) were investigated for their ability to affect articular cartilage development in a scaffoldless, 3-dimensional chondrocyte culture. Using a systematic approach, we applied ouabain (Na(+)/K(+)-ATPase inhibitor), bumetanide (Na(+)/K(+)/2Cl(-) tritransporter inhibitor), histamine (cAMP activator), and ionomycin (a Ca(2+) ionophore) to tissue-engineered constructs for 1 hour daily on days 10-14 of culture and examined the constructs at 2 weeks or 4 weeks. The gross morphology, biochemical content, and compressive and tensile mechanical properties of the constructs were assayed.

RESULTS

The results of these experiments showed that 20 microM ouabain, 0.3 microM ionomycin, or their combination increased the tensile modulus by 40-95% compared with untreated controls and resulted in an increased amount of collagen normalized to construct wet weight. In constructs exposed to ouabain, the increased percentage of collagen per construct wet weight was secondary to decreased glycosaminoglycan production on a per-cell basis. Treatment with 20 microM ouabain also increased the ultimate tensile strength of neo-tissue by 56-86% at 4 weeks. Other construct properties, such as construct growth and type I collagen production, were affected differently by Na(+) modulation with ouabain versus Ca(2+) modulation with ionomycin.

CONCLUSION

These data are the first to show that treatments known to alter intracellular ion concentrations are a viable method for increasing the mechanical properties of engineered articular cartilage and identifying potentially important relationships to hydrostatic pressure mechanotransduction. Ouabain and ionomycin may be useful pharmacologic agents for increasing tensile integrity and directing construct maturation.

Activation of TonEBP by calcium controls {beta}1,3-glucuronosyltransferase-I expression, a key regulator of glycosaminoglycan synthesis in cells of the intervertebral disc

The goal of this investigation was to study the expression and regulation of beta1,3-Glucuronosyltransferase-I (GlcAT-I), a key enzyme regulating GAG synthesis in cells of the intervertebral disc. There was a robust expression of GlcAT-I in the nucleus pulposus in vivo. Treatment with the calcium ionophore ionomycin resulted in increased GlcAT-I expression, whereas GlcAT-I promoter constructs lacking TonE site or a mutant TonE were unresponsive to the ionophore. Experiments using TonEBP and DN-TonEBP constructs showed that TonEBP positively regulated GlcAT-I promoter activity. ChIP analysis confirmed binding of TonEBP to the promoter. We further validated the role of TonEBP in controlling GlcAT-I expression using mouse embryo fibroblasts from TonEBP null mice. GlcAT-I promoter activity in null cells was significantly lower than the wild type cells. In contrast to wild type cells, treatment with ionomycin failed to increase GlcAT-I promoter activity in null cells. We then investigated if calcineurin (Cn)-NFAT signaling played a regulatory role in GlcAT-I expression. Inhibition of Cn following ionomycin treatment did not block GlcAT-I and tauT, a TonEBP-responsive reporter activity. GlcAT-I promoter activity was suppressed by co-expression of Cn, NFAT2, NFAT3, and NFAT4. Moreover, following ionomycin treatment, fibroblasts from CnAalpha and CnAbeta null mice exhibited robust induction in GlcAT-I promoter activity compared with wild type cells. Results of these studies demonstrate that calcium regulates GlcAT-I expression in cells of the nucleus pulposus through a signaling network comprising both activator and suppressor molecules. The results suggest that by controlling both GAG and aggrecan synthesis, disc cells can autoregulate their osmotic environment and accommodate mechanical loading.

Chondrocyte intracellular calcium, cytoskeletal organization, and gene expression responses to dynamic osmotic loading

While chondrocytes in articular cartilage experience dynamic stimuli from joint loading activities, few studies have examined the effects of dynamic osmotic loading on their signaling and biosynthetic activities. We hypothesize that dynamic osmotic loading modulates chondrocyte signaling and gene expression differently than static osmotic loading. With the use of a novel microfluidic device developed in our laboratory, dynamic hypotonic loading (−200 mosM) was applied up to 0.1 Hz and chondrocyte calcium signaling, cytoskeleton organization, and gene expression responses were examined. Chondrocytes exhibited decreasing volume and calcium responses with increasing loading frequency. Phalloidin staining showed osmotic loading-induced changes to the actin cytoskeleton in chondrocytes. Real-time PCR analysis revealed a stimulatory effect of dynamic osmotic loading compared with static osmotic loading. These studies illustrate the utility of the microfluidic device in cell signaling investigations, and their potential role in helping to elucidate mechanisms that mediate chondrocyte mechanotransduction to dynamic stimuli.

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.

Adrenocorticotropin evokes transient elevations in intracellular free calcium ([Ca2+]i) and increases basal [Ca2+]i in resting chondrocytes through a phospholipase C-dependent mechanism

Both clinical and in vitro evidence points to the involvement of the melanocortin peptide, ACTH, in the terminal differentiation of chondrocytes. Terminal differentiation along the endochondral pathway is responsible for linear growth, but also plays a role in osteoarthritic cartilage degeneration. Chondrocyte terminal differentiation is associated with an incremental increase in chondrocyte basal intracellular free calcium ([Ca(2+)](i)), and ACTH agonism of melanocortin receptors is known to mobilize [Ca(2+)](i.) Using differentiated resting chondrocytes highly expressing type II collagen and aggrecan, we examined the influence of both ACTH and dexamethasone treatment on matrix gene transcription and [Ca(2+)](i). Resting chondrocytes treated concurrently with dexamethasone and ACTH expressed matrix gene transcripts in a pattern consistent with that of rapid terminal differentiation. Using the fluorescent Ca(2+) indicator, fura-2, we determined that ACTH evokes transient increases in [Ca(2+)](i) and elevates basal Ca(2+) levels in resting chondrocytes. The transient increases were initiated intracellularly, were abrogated by the phospholipase C-specific inhibitor, U73122, and were partly attenuated by myo-inositol 1,4,5-triphosphate receptor inhibition via 10 mm caffeine. The initial intracellular release also resulted in store-operated calcium entry, presumably through store-operated channels. Dexamethasone priming increased both the initial ACTH-evoked [Ca(2+)](i) release and the subsequent store-operated calcium entry. These data demonstrate roles for ACTH and glucocorticoid in the regulation of chondrocyte terminal differentiation. Because the actions of ACTH are mediated through known G protein-coupled receptors, the melanocortin receptors, these data may provide a new therapeutic target in the treatment of growth deficiencies and cartilage degeneration.

ACTH enhances chondrogenesis in multipotential progenitor cells and matrix production in chondrocytes

The association of melanocortin peptide overproduction with enhanced linear growth prompted the current investigation of adrenocorticotropin hormone (ACTH) effects on multipotential chondroprogenitor populations and committed chondrocytes in culture. Two multipotential progenitor populations, rat bone marrow stromal cells (BMSC) and the clonal multipotential cell line RCJ3.1, and two committed chondrocyte populations, resting chondrocytes (RC) isolated from the rib of young rats and the chondrocyte restricted cell line RCJ3.1C5.18 (C5.18), were cultured in differentiation medium plus or minus ACTH. Alcian blue stain was used to quantitate proteoglycan matrix production in all populations treated with a range of ACTH concentrations. Changes in proliferation due to ACTH treatment of all cell types were measured using 3H-thymidine incorporation. Differences in matrix production of ACTH-treated and -untreated RC and C5.18 cells were determined using 3H-proline incorporation. Relative transcript expression of the chondrocyte matrix proteins collagen type II (COLL II) and aggrecan (AGR) in treated and untreated cells was analyzed by Northern blot. Collagen type X (COLL X), a marker of hypertrophic differentiation, was measured in committed chondrocytic populations. Western analysis was used to detect the melanocortin-3 receptor (MC3-R), which was a suspected mediator of the ACTH signal. Matrix deposition was dose-dependently increased by ACTH in all cell populations as measured by alcian blue stain. ACTH treatment increased proliferation in multipotential progenitor populations (BMSC and RCJ3.1) while proliferation was decreased in committed chondrocyte populations (RC and C5.18). Total protein and total cell-associated collagen production were significantly increased by ACTH treatment in committed populations. Relative COLL II and AGR transcript expressions were significantly increased in both the RC- and C5.18-committed population and very significantly increased in the progenitor populations. Additionally, collagen type X expression was detected earlier and in greater abundance in ACTH-treated committed chondrocyte populations. Finally, the melanocortin-3 receptor was detected in all examined cell types by Western blot. These data show that ACTH promotes the development of the chondrocyte phenotype from multipotential mesenchymal progenitor populations and increases matrix production and differentiation of committed chondrocytes. These findings, together with the detection of the MC3-R in all of these cell types, indicate a role for the melanocortin system in chondrogenesis.