Mechanism of mechanically induced intercellular calcium waves in rabbit articular chondrocytes and in HIG-82 synovial cells

Convergence of Calcium Channel Regulation and Mechanotransduction in Skeletal Regenerative Biomaterial Design

Cells are known to perceive their microenvironment through extracellular and intracellular mechanical signals. Upon sensing mechanical stimuli, cells can initiate various downstream signaling pathways that are vital to regulating proliferation, growth, and homeostasis. One such physiologic activity modulated by mechanical stimuli is osteogenic differentiation. The process of osteogenic mechanotransduction is regulated by numerous calcium ion channels-including channels coupled to cilia, mechanosensitive and voltage-sensitive channels, and channels associated with the endoplasmic reticulum. Evidence suggests these channels are implicated in osteogenic pathways such as the YAP/TAZ and canonical Wnt pathways. This review aims to describe the involvement of calcium channels in regulating osteogenic differentiation in response to mechanical loading and characterize the fashion in which those channels directly or indirectly mediate this process. The mechanotransduction pathway is a promising target for the development of regenerative materials for clinical applications due to its independence from exogenous growth factor supplementation. As such, also described are examples of osteogenic biomaterial strategies that involve the discussed calcium ion channels, calcium-dependent cellular structures, or calcium ion-regulating cellular features. Understanding the distinct ways calcium channels and signaling regulate these processes may uncover potential targets for advancing biomaterials with regenerative osteogenic capabilities.

Intersections Between Mitochondrial Metabolism and Redox Biology Mediate Posttraumatic Osteoarthritis

PURPOSE OF REVIEW

This review will cover foundational studies and recent findings that established key concepts for understanding the importance of redox biology to chondrocyte mitochondrial function and osteoarthritis pathophysiology after injury.

RECENT FINDINGS

Articular chondrocyte mitochondria can be protected with a wide variety of antioxidants that will be discussed within a framework suggested by classic studies. These agents not only underscore the importance of thiol metabolism and associated redox function for chondrocyte mitochondria but also suggest complex interactions with signal transduction pathways and other molecular features of osteoarthritis that require more thorough investigation. Emerging evidence also indicates that reductive stress could occur alongside oxidative stress. Recent studies have shed new light on historic paradoxes in chondrocyte redox and mitochondrial physiology, leading to the development of promising disease-modifying therapies for posttraumatic osteoarthritis.

Spontaneous calcium signaling of cartilage cells: from spatiotemporal features to biophysical modeling

Intracellular calcium ([Ca²⁺]_i) oscillation is a fundamental signaling response of cartilage cells under mechanical loading or osmotic stress. Chondrocytes are usually considered as nonexcitable cells with no spontaneous [Ca²⁺]_i signaling. This study proved that chondrocytes can exhibit robust spontaneous [Ca²⁺]_i signaling without explicit external stimuli. The intensity of [Ca²⁺]_i peaks from individual chondrocytes maintain a consistent spatiotemporal pattern, acting as a unique "fingerprint" for each cell. Statistical analysis revealed lognormal distributions of the temporal parameters of [Ca²⁺]_i peaks, as well as strong linear correlations between their means and sds. Based on these statistical findings, we hypothesized that the spontaneous [Ca²⁺]_i peaks may result from an autocatalytic process and that [Ca²⁺]_i oscillation is controlled by a threshold-regulating mechanism. To test these 2 mechanisms, we established a multistage biophysical model by assuming the spontaneous [Ca²⁺]_i signaling of chondrocytes as a combination of deterministic and stochastic processes. The theoretical model successfully explained the lognormal distribution of the temporal parameters and the fingerprint feature of [Ca²⁺]_i peaks. In addition, by using antagonists for 10 pathways, we revealed that the initiation of spontaneous [Ca²⁺]_i peaks in chondrocytes requires the presence of extracellular Ca²⁺, and that the PLC-inositol 1,4,5-trisphosphate pathway, which controls the release of calcium from the endoplasmic reticulum, can affect the initiation of spontaneous [Ca²⁺]_i peaks in chondrocytes. The purinoceptors and transient receptor potential vanilloid 4 channels on the plasma membrane also play key roles in the spontaneous [Ca²⁺]_i signaling of chondrocytes. In contrast, blocking the T-type or L-type voltage-gated calcium channel promoted the spontaneous calcium signaling. This study represents a systematic effort to understand the features and initiation mechanisms of spontaneous [Ca²⁺]_i signaling in chondrocytes, which are critical for chondrocyte mechanobiology.-Zhou, Y., Lv, M., Li, T., Zhang, T., Duncan, R., Wang, L., Lu, X. L. Spontaneous calcium signaling of cartilage cells: from spatiotemporal features to biophysical modeling.

Low-intensity pulsed ultrasound (LIPUS) treatment of cultured chondrocytes stimulates production of CCN family protein 2 (CCN2), a protein involved in the regeneration of articular cartilage: mechanism underlying this stimulation

OBJECTIVE

CCN family protein 2/connective tissue growth factor (CCN2/CTGF) promotes cartilage regeneration in experimental osteoarthritis (OA) models. However, CCN2 production is very low in articular cartilage. The aim of this study was to investigate whether or not CCN2 was promoted by cultured chondrocytes treated with low-intensity pulsed ultrasound (LIPUS) and to clarify its mechanism.

METHODS

Human chondrocytic cell line (HCS)-2/8, rat primary epiphyseal and articular cartilage cells, and Ccn2-deficient chondrocytes that impaired chondrocyte differentiation, were treated with LIPUS for 20 min at 3.0 MHz frequency and 60 mW/cm² power. Expressions of chondrocyte differentiation marker mRNAs were examined by real-time PCR (RT-PCR) analysis from HCS-2/8 cells and Ccn2-deficient chondrocytes at 30 min and 1 h after LIPUS treatment, respectively. CCN2 production was examined by Western blotting after 5 h of LIPUS treatment. Moreover, Ca²⁺ influx was measured by using a Fluo-4 probe.

RESULTS

The gene expression of chondrocyte differentiation markers and CCN2 production were increased in cultured chondrocytes treated with LIPUS. In addition, Ca²⁺ influx and phosphorylation of p38 mitogen-activated protein kinase (MAPK) and extracellular signal-regulated kinase (ERK)1/2 were increased by LIPUS treatment, and the stability of TRPV4 and BK_ca channel mRNAs was decreased by siRNA against CCN2. Consistent with those findings, the LIPUS-induced the gene expressions of type II collagen (COL2a1) and Aggrecan (ACAN) observed in wild-type cells were not observed in the Ccn2-deficient chondrocytes.

CONCLUSION

These data indicate that chondrocyte differentiation represented by CCN2 production was mediated via MAPK pathways activated by LIPUS-stimulated Ca²⁺ influx, which in turn was supported by the induced CCN2 molecules in articular chondrocytes.

The effect of chemically defined medium on spontaneous calcium signaling of in situ chondrocytes during long-term culture

Chemically defined serum-free medium has been shown to better maintain the mechanical integrity of articular cartilage explants than serum-supplemented medium during long-term in vitro culture, but little is known about its effect on cellular mechanisms. We hypothesized that the chemically defined culture medium could regulate the spontaneous calcium signaling of in situ chondrocytes, which may modulate the cellular metabolic activities. Bovine cartilage explants were cultured in chemically defined serum-free or serum-supplemented medium for four weeks. The spontaneous intracellular calcium ([Ca(2+)]i) signaling of in situ chondrocytes was longitudinally measured together along with the biomechanical properties of the explants. The spontaneous [Ca(2+)]i oscillations in chondrocytes were enhanced at the initial exposure of serum-supplemented medium, but were significantly dampened afterwards. In contrast, cartilage explants in chemically defined medium preserved the level of calcium signaling, and showed more responsive cells with higher and more frequent [Ca(2+)]i peaks throughout the four week culture in comparison to those in serum medium. Regardless of the culture medium that the explants were exposed, a positive correlation was detected between the [Ca(2+)]i responsive rate and the stiffness of cartilage (Spearman's rank correlation coefficient=0.762). A stable pattern of [Ca(2+)]i peaks was revealed for each chondrocyte, i.e., the spatiotemporal features of [Ca(2+)]i peaks from a cell were highly consistent during the observation period (15 min). This study showed that the beneficial effect of chemically defined culture of cartilage explants is associated with the spontaneous [Ca(2+)]i signaling of chondrocytes in cartilage.

Connexin43 enhances the expression of osteoarthritis-associated genes in synovial fibroblasts in culture

Background

Recent work has shown that the gap junction protein connexin43 (Cx43) is upregulated in cells of the joint during osteoarthritis (OA). Here we examined if the OA-associated increase in Cx43 expression impacts the function of synovial fibroblasts by contributing to the production of catabolic and inflammatory factors that exacerbate joint destruction in arthritic disease.

Methods

Using rabbit and human synovial fibroblast cell lines, we examined the effects of Cx43 overexpression and Cx43 siRNA-mediated knockdown on the gene expression of OA-associated matrix metalloproteinases (MMP1 and MMP13), aggrecanases (ADAMTS4 and ADAMTS5), and inflammatory factors (IL1, IL6 and PTGS2) by quantitative real time RT-PCR. We examined collagenase activity in conditioned media of cultured synovial cells following Cx43 overexpression. Lastly, we assessed the interplay between Cx43 and the NFκB cascade by western blotting and gene expression studies.

Results

Increasing Cx43 expression enhanced the gene expression of MMP1, MMP13, ADAMTS4, ADAMTS5, IL1, IL6 and PTGS2 and increased the secretion of collagenases into conditioned media of cultured synovial fibroblasts. Conversely, knockdown of Cx43 decreased expression of many of these catabolic and inflammatory genes. Modulation of Cx43 expression altered the phosphorylation of the NFκB subunit, p65, and inhibition of NFκB with chemical inhibitors blocked the effects of increased Cx43 expression on the mRNA levels of a subset of these catabolic and inflammatory genes.

Conclusions

Increasing or decreasing Cx43 expression alone was sufficient to alter the levels of catabolic and inflammatory genes expressed by synovial cells. The NFκB cascade mediated the effect of Cx43 on the expression of a subset of these OA-associated genes. As such, Cx43 may be involved in joint pathology during OA, and targeting Cx43 expression or function may be a viable therapeutic strategy to attenuate the catabolic and inflammatory environment of the joint during OA.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2474-15-425) contains supplementary material, which is available to authorized users.

Effects of cyclic equibiaxial mechanical stretch on α-BK and TRPV4 expression in equine chondrocytes

Background

Chondrocytes are regularly exposed to load-induced stimuli and have the capability to sense and respond to applied mechanical stress. However, the mechanisms involved in chondrocyte mechanotransduction are not clearly understood. The purpose of this study was to explore the effects of cyclic equibiaxial mechanical stretch on the expression of α-BK and TRPV4 channels.

Findings

Freshly isolated equine articular chondrocytes were subjected to mechanical stress (8% elongation at frequency of 0.5 Hz for 8 h). Western blotting was used to investigate the expression of BK_Ca and TRPV4 channel proteins. Mechanical stretch increased the expression of BK_Ca channels by 1.8 fold but TRPV4 expression was not affected.

Conclusions

Upregulation of BK_Ca channel may be the result of direct membrane stretch or elevated intracellular Ca²⁺.

Mechanical stimulation by ultrasound enhances chondrogenic differentiation of mesenchymal stem cells in a fibrin-hyaluronic acid hydrogel

Chondrogenic differentiation and cartilage tissue formation derived from stem cells are highly dependent on both biological and mechanical factors. This study investigated whether or not fibrin-hyaluronic acid (HA) coupled with low-intensity ultrasound (LIUS), a mechanical stimulation, produces an additive or synergistic effect on the chondrogenesis of rabbit mesenchymal stem cells (MSCs) derived from bone marrow. For the purpose of comparison, rabbit MSCs were first cultured in fibrin-HA or alginate hydrogels, and then subjected to chondrogenic differentiation in chondrogenic-defined medium for 4 weeks in the presence of either transforming growth factor-beta3 (TGF-β3) (10 ng/mL) or LIUS treatment (1.0 MHz and 200 mW/cm(2) ). The resulting samples were evaluated at 1 and 4 weeks by histological observation, chemical assays, and mechanical analysis. The fibrin-HA hydrogel was found to be more efficient than alginate in promoting chondrogenesis of the MSCs by producing a larger amount of sulfated glycosaminoglycans (GAGs) and collagen, and engineered constructs made with the hydrogel demonstrated higher mechanical strength. At 4 weeks of tissue culture, the chondrogenesis of the MSCs in fibrin-HA were shown to be further enhanced by treatment with LIUS, as observed by analyses for the amounts of GAGs and collagen, and mechanical strength testing. In contrast, TGF-β3, a well-known chondrogenic inducer, showed a marginal additive effect in the amount of collagen only. These results revealed that LIUS further enhanced chondrogenesis of the MSCs cultured in fibrin-HA, in vitro, and suggested that the combination of fibrin-HA and LIUS is a useful tool in constructing high-quality cartilage tissues from MSCs.

Phospholipases of mineralization competent cells and matrix vesicles: roles in physiological and pathological mineralizations

The present review aims to systematically and critically analyze the current knowledge on phospholipases and their role in physiological and pathological mineralization undertaken by mineralization competent cells. Cellular lipid metabolism plays an important role in biological mineralization. The physiological mechanisms of mineralization are likely to take place in tissues other than in bones and teeth under specific pathological conditions. For instance, vascular calcification in arteries of patients with renal failure, diabetes mellitus or atherosclerosis recapitulates the mechanisms of bone formation. Osteoporosis—a bone resorbing disease—and rheumatoid arthritis originating from the inflammation in the synovium are also affected by cellular lipid metabolism. The focus is on the lipid metabolism due to the effects of dietary lipids on bone health. These and other phenomena indicate that phospholipases may participate in bone remodelling as evidenced by their expression in smooth muscle cells, in bone forming osteoblasts, chondrocytes and in bone resorbing osteoclasts. Among various enzymes involved, phospholipases A₁ or A₂, phospholipase C, phospholipase D, autotaxin and sphingomyelinase are engaged in membrane lipid remodelling during early stages of mineralization and cell maturation in mineralization-competent cells. Numerous experimental evidences suggested that phospholipases exert their action at various stages of mineralization by affecting intracellular signaling and cell differentiation. The lipid metabolites—such as arachidonic acid, lysophospholipids, and sphingosine-1-phosphate are involved in cell signaling and inflammation reactions. Phospholipases are also important members of the cellular machinery engaged in matrix vesicle (MV) biogenesis and exocytosis. They may favour mineral formation inside MVs, may catalyse MV membrane breakdown necessary for the release of mineral deposits into extracellular matrix (ECM), or participate in hydrolysis of ECM. The biological functions of phospholipases are discussed from the perspective of animal and cellular knockout models, as well as disease implications, development of potent inhibitors and therapeutic interventions.

Intercellular Ca(2+) waves: mechanisms and function

Intercellular calcium (Ca(2+)) waves (ICWs) represent the propagation of increases in intracellular Ca(2+) through a syncytium of cells and appear to be a fundamental mechanism for coordinating multicellular responses. ICWs occur in a wide diversity of cells and have been extensively studied in vitro. More recent studies focus on ICWs in vivo. ICWs are triggered by a variety of stimuli and involve the release of Ca(2+) from internal stores. The propagation of ICWs predominately involves cell communication with internal messengers moving via gap junctions or extracellular messengers mediating paracrine signaling. ICWs appear to be important in both normal physiology as well as pathophysiological processes in a variety of organs and tissues including brain, liver, retina, cochlea, and vascular tissue. We review here the mechanisms of initiation and propagation of ICWs, the key intra- and extracellular messengers (inositol 1,4,5-trisphosphate and ATP) mediating ICWs, and the proposed physiological functions of ICWs.

Large, but not small sustained tensile strains stimulate adipogenesis in culture

Understanding the mechanoresponsiveness of adipocytes and the characteristics of the mechanical stimuli that regulate adipogenesis is critically important in establishing knowledge in regard to the long-term effects of a sedentary lifestyle (or immobility in extreme medical conditions) as well as concerning obesity and related diseases. In this study we subjected 3T3-L1 preadipocytes cultured on elastic substrata to different levels of static equiaxial tensile strains within the physiological range, up to substrate tensile strain (STS) of 12%, while inducing differentiation in the cultures. Based on prior work which revealed that adipogenesis is accelerated in cultures subjected to STS of 12% by activating the mitogen-activated protein kinase kinase signaling pathway, we were specifically interested in identifying the STS levels which trigger this process. We hence monitored the production and accumulation of lipid droplets (LDs) using a non-destructive, image-processing-based method that we have previously developed, for a period of 4 weeks. The experimental data demonstrated accelerated adipogenesis in the cultures subjected to STS levels of 6%, 9%, and 12% with respect to cultures subjected to STS of 3% and (non-stretched) control cultures. This accelerated adipogenic response to the large sustained STS manifested in significantly larger numbers and greater sizes of LDs in the cultures that were stretched to large STS levels (p < 0.05), starting at approximately day 14 following induction of differentiation. Hence, indeed, there appears to be a certain tensile strain threshold, or domain-which is found within the physiological range-above which the responsiveness of adipocytes to sustained static stretching increases and is manifested in accelerated adipogenesis.

Cell volume regulation in chondrocytes

Chondrocytes are the cells within cartilage which produce and maintain the extracellular matrix. Volume regulation in these cells is vital to their function and occurs in several different physiological and pathological contexts. Firstly, chondrocytes exist within an environment of changing osmolarity and compressive loads. Secondly, in osteoarthritic joint failure, cartilage water content changes and there is a notable increase in chondrocyte apoptosis. Thirdly, endochondral ossification requires chondrocyte swelling in association with hypertrophy. Regulatory volume decrease (RVD) and regulatory volume increase (RVI) have both been observed in articular chondrocytes and this review focuses on the mechanisms identified to account for these. There has been evidence so far to suggest TRPV4 is central to RVD; however other elements of the pathway have not yet been identified. Unlike RVD, RVI appears less robust in articular chondrocytes and there have been fewer mechanistic studies; the primary focus being on the Na(+)-K(+)-2Cl(-) co-transporter. The clinical significance of chondrocyte volume regulation remains unproven. Importantly however, transcript abundances of several ion channels implicated in volume control are changed in chondrocytes from osteoarthritic cartilage. A critical question is whether disturbances of volume regulation mechanisms lead to, result from or are simply coincidental to cartilage damage.

Static mechanical stretching accelerates lipid production in 3T3-L1 adipocytes by activating the MEK signaling pathway

Understanding mechanotransduction in adipocytes is important for research of obesity and related diseases. We cultured 3T3-L1 preadipocytes on elastic substrata and applied static tensile strains of 12% to the substrata while inducing differentiation. Using an image processing method, we monitored lipid production for a period of 3-4 wk. The ratio of %-lipid area per field of view (FOV) in the stretched over nonstretched cultures was significantly greater than unity (P < 0.05), reaching ∼1.8 on average starting from experimental day ∼10. The superior coverage of the FOV by lipids in the stretched cultures was due to significantly greater sizes of lipid droplets (LDs) with respect to nonstretched cultures, starting from experimental day ∼10 (P < 0.05), and due to significantly more LDs per cell between days ∼10 and ∼17 (P < 0.05). The statically stretched cells also differentiated significantly faster than the nonstretched cells within the first ∼10 days (P < 0.05). Adding peroxisome proliferator-activated receptor-γ (PPARγ) antagonist did not change these trends, as the %-lipid area per FOV in the stretched cultures that received this treatment was still significantly greater than in the nonstretched cultures without the PPARγ antagonist (14.44 ± 1.96% vs. 10.21 ± 3%; P < 0.05). Hence, the accelerated adipogenesis in the stretched cultures was not mediated through PPARγ. Nonetheless, inhibiting the MEK/MAPK signaling pathway reduced the extent of adipogenesis in the stretched cultures (13.53 ± 5.63%), bringing it to the baseline level of the nonstretched cultures without the MEK inhibitor (10.21 ± 3.07%). Our results hence demonstrate that differentiation of adipocytes can be enhanced by sustained stretching, which activates the MEK signaling pathway.

Gap junctions and hemichannels in signal transmission, function and development of bone

Gap junctional intercellular communication (GJIC) mediated by connexins, in particular connexin 43 (Cx43), plays important roles in regulating signal transmission among different bone cells and thereby regulates development, differentiation, modeling and remodeling of the bone. GJIC regulates osteoblast formation, differentiation, survival and apoptosis. Osteoclast formation and resorptive ability are also reported to be modulated by GJIC. Furthermore, osteocytes utilize GJIC to coordinate bone remodeling in response to anabolic factors and mechanical loading. Apart from gap junctions, connexins also form hemichannels, which are localized on the cell surface and function independently of the gap junction channels. Both these channels mediate the transfer of molecules smaller than 1.2kDa including small ions, metabolites, ATP, prostaglandin and IP(3). The biological importance of the communication mediated by connexin-forming channels in bone development is revealed by the low bone mass and osteoblast dysfunction in the Cx43-null mice and the skeletal malformations observed in occulodentodigital dysplasia (ODDD) caused by mutations in the Cx43 gene. The current review summarizes the role of gap junctions and hemichannels in regulating signaling, function and development of bone cells. This article is part of a Special Issue entitled: The Communicating junctions, composition, structure and characteristics.

The emerging chondrocyte channelome

Chondrocytes are the resident cells of articular cartilage and are responsible for synthesizing a range of collagenous and non-collagenous extracellular matrix macromolecules. Whilst chondrocytes exist at low densities in the tissue (1-10% of the total tissue volume in mature cartilage) they are extremely active cells and are capable of responding to a range of mechanical and biochemical stimuli. These responses are necessary for the maintenance of viable cartilage and may be compromised in inflammatory diseases such as arthritis. Although chondrocytes are non-excitable cells their plasma membrane contains a rich complement of ion channels. This diverse channelome appears to be as complex as one might expect to find in excitable cells although, in the case of chondrocytes, their functions are far less well understood. The ion channels so far identified in chondrocytes include potassium channels (K(ATP), BK, K(v), and SK), sodium channels (epithelial sodium channels, voltage activated sodium channels), transient receptor potential calcium or non-selective cation channels and chloride channels. In this review we describe this emerging channelome and discuss the possible functions of a range of chondrocyte ion channels.

Characterization of a stretch-activated potassium channel in chondrocytes

Chondrocytes possess the capacity to transduce load-induced mechanical stimuli into electrochemical signals. The aim of this study was to functionally characterize an ion channel activated in response to membrane stretch in isolated primary equine chondrocytes. We used patch-clamp electrophysiology to functionally characterize this channel and immunohistochemistry to examine its distribution in articular cartilage. In cell-attached patch experiments, the application of negative pressures to the patch pipette (in the range of 20-200 mmHg) activated ion channel currents in six of seven patches. The mean activated current was 45.9 +/- 1.1 pA (n = 4) at a membrane potential of 33 mV (cell surface area approximately 240 microm(2)). The mean slope conductance of the principal single channels resolved within the total stretch-activated current was 118 +/- 19 pS (n = 6), and reversed near the theoretical potassium equilibrium potential, E(K+), suggesting it was a high-conductance potassium channel. Activation of these high-conductance potassium channels was inhibited by extracellular TEA (K(d) approx. 900 microM) and iberiotoxin (K(d) approx. 40 nM). This suggests that the current was largely carried by BK-like potassium (MaxiK) channels. To further characterize these BK-like channels, we used inside-out patches of chondrocyte membrane: we found these channels to be activated by elevation in bath calcium concentration. Immunohistochemical staining of equine cartilage samples with polyclonal antibodies to the alpha1- and beta1-subunits of the BK channel revealed positive immunoreactivity for both subunits in superficial zone chondrocytes. These experiments support the hypothesis that functional BK channels are present in chondrocytes and may be involved in mechanotransduction and chemotransduction.

Mechanotransduction pathways of low-intensity ultrasound in C-28/I2 human chondrocyte cell line

Low-intensity ultrasound (LIUS) has recently been considered to be an effective method to induce cartilage repair and/or regeneration after injury. Nevertheless, there is no study to provide a cellular mechanism or signal pathways of LIUS stimulation. The current study is designed to investigate the effects of LIUS on the mechanotransduction pathways in C-28/I2, an immortalized human chondrocyte cell line. C-28/I2 cells were treated with LIUS at an intensity of 200 mW/cm2 using Noblelife™ from Duplogen. The role of stretch-activated channels (SAC) and integrins that are most well-known mechanoreceptors on the chondrocyte cell surface was first examined in mediating the LIUS effects on the expression of type II collagen and aggrecan. When analysed by reverse transcriptase polymerase chain reaction (RT-PCR) and immunohistochemistry, gadolinium (a specific inhibitor of SACs) or GRGDSP (a peptide inhibitor of integrins) specifically reduced the LIUS-induced elevation of type II collagen and aggrecan expressions depending on the incubation time. In addition, the LIUS treatment of C-28/I2 cells induced the phosphorylation of c-Jun N-terminal kinase (JNK) and extracellular signal-regulated kinase (ERK) but not p38 kinase among the members of the mitogen-activated protein kinases (MAPKs). The phosphorylation of ERK by LIUS was repressed by a specific inhibitor of the ERK pathway and integrin function. These results suggest that the LIUS signal might be mediated via canonical mechanoreceptors of SACs and integrins and subsequently through JNK and ERK pathways. The present study provides the first evidence for the activation of the mechanotransduction pathways by LIUS in human chondrocytes.

Spontaneous oscillation and mechanically induced calcium waves in chondrocytes

The characteristics of spontaneous calcium (Ca(2+)) oscillation and mechanically induced Ca(2+) waves in articular chondrocytes were studied. In some, but not all, chondrocytes in sliced cartilage and primary cultures, we observed spontaneous oscillation of intracellular Ca(2+) that never spread to adjacent cells. In contrast, a mechanical stimulus to a single cell by touching with a glass rod induced an increase of intracellular Ca(2+) that spread to neighboring cells in a wave-like manner, even though there was no physical contact between the cells. This indicated the release of some paracrine factor from the mechanically stimulated cells. Application of ultrasonic vibration also induced an oscillation of intracellular Ca(2+). The application of a uridine 5'-triphosphate (UTP), UTP, induced a transient increase in intracellular Ca(2+) and the release of adenosine 5'-triphosphate (ATP) in cultured chondrocytes. A P2 receptor antagonist (suramin) and blockers of Cl(-) channels, niflumic acid and 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS), reduced the UTP-induced ATP release. The results indicated that Cl(-) channels were involved in the extracellular release of ATP following mechanical or P2Y receptor stimulation. Thus, ATP stimulation of P2Y receptors elicits an increase in intracellular Ca(2+), triggering further release of ATP from adjacent cells, thereby expanding the Ca(2+) wave in chondrocytes.

ATP-induced calcium increase as a potential first signal in mechanical tissue trauma. A laser scanning microscopic study on cultured mouse skeletal myocytes

Although it is known that after major tissue trauma, local incidents in the mechanically destroyed muscle tissue form the basis of subsequently occurring severe inflammatory reactions, the very first events taking place immediately after myocyte destruction have not been studied on the single cell level thus far. Therefore, in this study, the reaction of cultured C2C12 mouse skeletal myocytes to lethal injury was examined using laser scanning microscopy. Mechanical rupture of one single myocyte induced an immediate accumulation of calcium in its cytosol and nuclei, as detected by an increase in the fluorescence intensity of the intracellular calcium-sensitive dye Fluo-3. The intracellular calcium elevation propagated further to the adjacent, noninjured myocytes in a wave-like fashion within seconds. The calcium increase detected in these neighboring cells was higher and up to 1000 times more extended than the physiological calcium spike that induces C2C12 myocyte contraction. Wave propagation did not depend on gap junctional communication but occurred via liberation of nucleotides, mainly ATP, but presumably also UTP and others, from the destroyed cell and subsequent calcium release from the sarcoplasmic reticulum via a purinoceptor-mediated mechanism in the adjacent cells. These findings suggest a decisive role of ATP and related nucleotides in the pathogenesis of tissue trauma because they appear to initiate the signaling mechanism from injured myocytes to the surrounding tissue and potentially to the whole body.

P2Y receptors play a critical role in epithelial cell communication and migration

Cellular injury induces a complex series of events that involves Ca2+ signaling, cell communication, and migration. One of the first responses following mechanical injury is the propagation of a Ca2+ wave (Klepeis et al. [2001] J Cell Sci 114(Pt 23):4185-4195). The wave is generated by the extracellular release of ATP, which also induces phosphorylation of ERK (Yang et al. [2004] J Cell Biochem 91(5):938-950). ATP and other nucleotides, which bind to and activate specific purinergic receptors were used to mimic injury. Our goal was to determine which of the P2Y purinergic receptors are expressed and stimulated in corneal epithelial cells and which signaling pathways are activated leading to changes in cell migration, an event critical for wound closure. In this study, we demonstrated that the P2Y1, P2Y2, P2Y4, P2Y6, and P2Y11 receptors were present in corneal epithelial cells. A potency profile was determined by Ca2+ imaging for nucleotide agonists as follows: ATP > or = UTP > ADP > or = UDP. In contrast, negligible responses were seen for beta,gamma-meATP, a general P2X receptor agonist and adenosine, a P1 receptor agonist. Homologous desensitization of the Ca2+ response was observed for the four nucleotides. However, P2Y receptor internalization and degradation was not detected following stimulation with ATP, which is in contrast to EGFR internalization observed in response to EGF. ATP induced cell migration was comparable to that of EGF and was maximal at 1 microM. Cells exposed to ATP, UTP, ADP, and UDP demonstrated a rapid twofold increase in phosphorylation of paxillin at Y31 and Y118, however, there was no activation elicited by beta,gamma-meATP or adenosine. Additional studies demonstrated that wound closure was inhibited by reactive blue 2. These results indicate that P2Y receptors play a critical role in the injury repair process.

A novel method for assessing effects of hydrostatic fluid pressure on intracellular calcium: a study with bovine articular chondrocytes

Chondrocytes in articular cartilage are exposed to hydrostatic pressure and distortional stress during weight bearing and joint loading. Because these stresses occur simultaneously in articular cartilage, the mechanism of mechanosignal transduction due to hydrostatic pressure alone in chondrocytes is not clear. In this study, we attempted to characterize the change in intracellular calcium concentration ([Ca2+]i) in response to the application of hydrostatic fluid pressure (HFP) to cultured bovine articular chondrocytes isolated from defined surface (SZ) and middle zones (MZ) by using a fluorescent indicator (X-rhod-1 AM), a novel custom-made pressure-proof optical chamber, and laser confocal microscopy. Critical methodology implemented in this experiment involved application of high levels of HFP to the cells and the use of a novel imaging apparatus to measure the peak [Ca2+]i in individual cells. The peak [Ca2+]i in MZ cells cultured for 5 days showed a significant twofold increase after the application of HFP at constant 0.5 MPa for 5 min. The peak [Ca2+]i in SZ cells was lower (43%) than that of MZ cells. The peak was suppressed with an inhibitor of dantrolene, gadolinium, or a calcium ion-free buffer, but not with verapamil. This study indicated that the increase in [Ca2+]i in chondrocytes to HFP is dependent on the zonal origin. HFP stimulates calcium mobilization and stretch-activated channels.

Do changes in the mechanical properties of articular cartilage promote catabolic destruction of cartilage and osteoarthritis?

Osteoarthritis (OA) is a joint disease characterized by cartilage degeneration, a thickening of subchondral bone, and formation of marginal osteophytes. Previous mechanical characterization of cartilage in our laboratory suggests that energy storage and dissipation is reduced in osteoarthritis as the extent of fibrillation and fissure formation increases. It is not clear whether the loss of energy storage and dissipation characteristics is a result of biochemical and/or biophysical changes that occur to hyaline cartilage in joints. The purpose of this study is to present data, on the strain rate dependence of the elastic and viscous behaviors of cartilage, in order to further characterize changes that occur in the mechanical properties that are associated with OA. We have previously hypothesized that the changes seen in the mechanical properties of cartilage may be due to altered mechanochemical transduction by chondrocytes. Results of incremental tensile stress-strain tests at strain rates between 100%/min and 10,000%/min conducted on OA cartilage indicate that the slope of the elastic stress-strain curve increases with increasing strain rate, unlike the reported behavior of skin and self-assembled collagen fibers. It is suggested that the strain-rate dependence of the elastic stress-strain curve is due to the presence of large quantities of proteoglycans (PGs), which protect articular cartilage by increasing the apparent stiffness. The increased apparent stiffness of articular cartilage at high strain rates may limit the stresses borne and prolong the onset of OA. It is further hypothesized that increased compressive loading of chondrocytes in the intermediate zone of articular cartilage occurs as a result of normal wear to the superficial zone or from excessive impact loading. Once the superficial zone of articular cartilage is worn away, the tension is decreased throughout all cartilage zones leading to increased chondrocyte compressive loading and up-regulation of mechanochemical transduction processes that elaborate catabolic enzymes.

myo-Inositol 1,4,5-trisphosphate and Ca(2+)/calmodulin-dependent factors mediate transduction of compression-induced signals in bovine articular chondrocytes

Although the effects of mechanical loading on chondrocyte metabolic activities have been extensively characterized, the sequence of events through which extracellular mechanical signals are transduced into chondrocytes and ultimately modulate cell activities is not well understood. Here, studies were performed to map out the sequential intracellular signalling pathways through which compression-induced signals modulate aggrecan mRNA levels in bovine articular chondrocytes. Bovine articular cartilage explants were subjected to a compressive stress of 0.1 MPa for 1 h in the presence or absence of inhibitors or antagonists of the phosphoinositol and Ca(2+)/calmodulin signalling pathways in order to determine the roles of second messengers and effector molecules of these pathways in transducing the compression-induced signals. In the absence of the inhibitors, aggrecan mRNA levels were stimulated by compression 2-4-fold relative to levels in tare-loaded (see below) explants. Treatment of the explants with graded levels of the protein kinase C inhibitor chelerythrine or bisindolylmaleimide I, followed by 1 h compressive loading, did not significantly alter the load-induced elevation of aggrecan mRNA levels. In contrast, thapsigargin, which depletes the Ins(1,4,5)P3-sensitive intracellular Ca(2+) stores, completely blocked the load response without significantly altering aggrecan mRNA levels in tare-loaded explants. Similarly, antagonists of the Ca(2+)/calmodulin signalling pathway dose-dependently or completely blocked the load-response. The results obtained demonstrate that transduction of the compression-induced aggrecan mRNA-regulating signals requires Ins(1,4,5)P3- and Ca(2+)/calmodulin-dependent signalling processes in bovine articular chondrocytes.

[Cartilage: from biomechanics to physical therapy]

OBJECTIVES

To review the current knowledge about the relationship between physical activities, cartilage biology, osteoarthritis and rehabilitation.

METHOD

PubMed, Ovid, Cochrane Data base were interrogated for the period 1966-2000. Key words were: chondrocyte, cartilage, osteoarthritis, mechanical stimulation, exercises, physical therapy, rehabilitation. Were reviewed: the mechanical biology of the chondrocytes and the cartilage, the mechanisms of transduction, the metabolic response of the chondrocytes to mechanical stresses; the effects of physical activity and immobilization on the cartilage in animal models, the main studies on the epidemiology of limbs osteoarthritis and clinical trials on rehabilitation.

RESULTS

In vitro studies have demonstrated that some molecules are involved in the transduction of mechanical stress into intracellular biological event. Chondrocytes and cartilage are sensitive to mechanical stress and cartilage extracellular matrix synthesis and degradation can be modulated by mechanical events. Applications of cyclic loads usually lead to an enhanced matrix synthesis while static loads usually decrease matrix production. In animal models, intensive physical activity or immobilization lead to cartilage alteration mimicking osteoarthritis. In human, intensive and prolonged physical activities are probably associated with hip and knee osteoarthritis. However, there is evidence that exercise therapy and continuous passive motion have beneficial effects on patients with knee or hip osteoarthritis. Fundamental and clinical studies are still needed to determine if exercise programs could have an effect on chondromodulation. Continuous passive motion could help, in the future, to better understand the relationship between mechanical stimulation and cartilage homeostasis.

CONCLUSION

Rehabilitation could be beneficial in the therapeutic management of limbs osteoarthritis. The protocols of rehabilitation should however be more evaluated in controlled trials.

Growth factors but not gap junctions play a role in injury-induced Ca2+ waves in epithelial cells

This paper characterizes the early responses of epithelial cells to injury. Ca2+ is an important early messenger that transiently increases in the cytoplasm of cells in response to external stimuli. Its elevation leads to the regulation of signaling pathways responsible for the downstream events important for wound repair, such as cell migration and proliferation. Live cell imaging in combination with confocal laser scanning microscopy of fluo-3 AM loaded cells was performed. We found that mechanical injury in a confluent region of cells creates an elevation in Ca2+ that is immediately initiated at the wound edge and travels as a wave to neighboring cells, with [Ca2+]i returning to background levels within two minutes. Addition of epidermal growth factor (EGF), but not platelet-derived growth factor-BB, resulted in increased [Ca2+]i, and EGF specifically enhanced the amplitude and duration of the injury-induced Ca2+ wave. Propagation of the Ca2+ wave was dependent on intracellular Ca2+ stores, as was demonstrated using both thapsigargin and Ca2+ chelators (EGTA and BAPTA/AM). Injury-induced Ca2+ waves were not mediated via gap junctions, as the gap-junction inhibitors 1-heptanol and 18α-glycyrrhetinic acid did not alter wave propagation, nor did the cells recover in photobleaching experiments. Additional studies also demonstrated that the wave could propagate across an acellular region. The propagation of the injury-induced Ca2+ wave occurs via diffusion of an extracellular mediator, most probably via a nucleotide such as ATP or UTP, that is released upon cell damage.

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Dual mechanism of intercellular communication in HOBIT osteoblastic cells: a role for gap-junctional hemichannels

Intercellular communication allows tissue coordination of cell metabolism and sensitivity to extracellular stimuli. Paracrine stimulation and cell-to-cell coupling through gap junctions induce the formation of complex cellular networks, which favors the intercellular exchange of nutrients and second messengers. Intercellular Ca2+ signaling was investigated in human osteoblast-like initial transfectant (HOBIT) cells, a human osteoblastic cell line in which cells retain most of the osteoblastic differentiation markers. HOBIT cells express connexin43 (Cx43) clustered at the cell-to-cell boundary and display functional intercellular coupling as assessed by the intercellular transfer of Lucifer yellow. Mechanical stimulation of a single cell induced a wave of increased Ca2+ that was radially propagated to surrounding cells. Treatment of cells with thapsigargin blocked mechanically induced signal propagation. Intercellular Ca2+ spreading and dye transfer were inhibited by 18alpha-glycyrrhetinic acid (18-GA), showing the involvement of gap junctions in signal propagation. Pretreatment of cells with suramin or with apyrase decreased the extent of wave propagation, suggesting that ATP-mediated paracrine stimulation contribute to cell-to-cell signaling. The functional expression of gap-junctional hemichannels was evidenced in experiments of Mn2+ quenching, extracellular dye uptake, and intracellular Ca2+ release, activated by uptake of inositol 1,4,5-trisphosphate (InsP3) from the external medium. Gap-junctional hemichannels were activated by low extracellular Ca2+ concentrations and inhibited by 18-GA. A role for Cx hemichannels in adenosine triphosphate (ATP) release and paracrine stimulation is suggested.

ATP induces Ca(2+) signaling in human chondrons cultured in three-dimensional agarose films

OBJECTIVE

In vivo, chondrocytes are surrounded by an extracellular matrix, preventing direct cell-to-cell contact. Consequently, intercellular communication through gap junctions is unlikely. However, signaling at a distance is possible through extracellular messengers such as nitric oxide (NO) and nucleotides and nucleosides, adenosine triphosphate (ATP), uridine triphosphate (UTP), or adenosine diphosphate (ADP). We hypothesized that chondrons, chondrocytes surrounded by their native pericellular matrix, increase their intracellular calcium concentration ([Ca(2+)]ic) in response to ATP and other signaling molecules and that the source of Ca(2+) is from intracellular stores. The objectives of this study were to determine if chondrons in a 3-D gel respond to ATP by increasing [Ca(2+)]ic through a purinoceptor mechanism and to test whether chondrons in whole tissue samples would respond to ATP in a similar fashion.

DESIGN

Human chondrons, cultured in a three-dimensional agarose gel or in whole cartilage loaded with Fura-2AM, a calcium sensitive dye, were stimulated with 1, 5 and 10 microM ATP. A ratio-imaging fluorescence technique was used to quantitate the [Ca(2+)]ic.

RESULTS

ATP-stimulated chondrons increased their [Ca(2+)]ic from a basal level of 60 nM to over 1000 nM. Chondrons incubated in calcium-free medium also increased their [Ca(2+)]ic in response to ATP, indicating the source of Ca(2+) was not extracellular. ATP-induced calcium signaling was inhibited in chondrons pre-treated with suramin, a generic purinoceptor blocker. In addition, UTP and adenosine 5'-O-(3-thiotriphosphate) (ATPgammas) induced a calcium response, but 2-methylthio-ATP (2-MeSATP), ADP, and adenosine did not induce a significant increase in [Ca(2+)]ic, substantiating that the P2Y2 purinoceptor was dominant. Chondrons in whole cartilage increased [Ca(2+)]ic in response to ATP.

CONCLUSIONS

We conclude that chondrons in 3-D culture respond to ATP by increasing [Ca(2+)]ic via P2Y2 receptor activation. Thus, ATP can pass through the agarose gel and the pericellular matrix, bind purinoceptors and increase intracellular Ca(2+) in a signaling response.

Do gap junctions couple interstitial cells of Cajal pacing and neurotransmission to gastrointestinal smooth muscle?

Interstitial cells of Cajal (ICC) pace gastrointestinal phasic activity and transmit nerve activity. Gap junctions may couple these cells to smooth muscle, but no functional evidence exists. The objective of this study was to use uncouplers of gap junctions, 18 alpha-glycyrrhetenic acid and its water-soluble analogue carbenoxolone, to evaluate if gap junctions function in pacing and neurotransmission. After inhibition of nerve function with tetrodotoxin (TTX) and N(G)-nitro-L-arginine (L-NOARG), ionomycin- or carbachol-initiated regular phasic activities of circular muscle strips from canine colon and ileum. In some cases, the primary ICC network responsible for pacing was removed. The effects of inhibitors of gap junction conductance (10(-5)-10(-4) mol L(-1)) on frequencies and amplitudes of contraction were compared to appropriate time controls. Lower oesophageal sphincter (LOS) relaxations to nerve stimulation were studied before and after inhibition of gap junction functions. No major changes in LOS relaxations or frequencies of colonic or ileal contractions occurred, but amplitudes of contractions decreased from these agents. Similar results were obtained when the myenteric plexus-ICC network of ileum was removed. Regular phasic activity was not obtained after removal of the colon submuscular plexus ICC. These findings suggest that mechanisms other than gap junctions couple gut pacemaking activity and nerve transmission.

Inhibition of gap-junctional communication induces the trans-differentiation of osteoblasts to an adipocytic phenotype in vitro

Osteoblasts and adipocytes are thought to differentiate from a common stromal progenitor cell. These two phenotypically mature cell types show a high degree of plasticity, which can be observed when cells are grown under specific culture conditions. Gap junctions are abundant among osteoblastic cells in vivo and in vitro, whereas they are down-regulated during adipogenesis. Gap junctional communication (GJC) modulates the expression of genes associated with the mature osteoblastic phenotype. Inhibition of GJC utilizing 18-alpha-glycyrrhetinic acid (AGRA) blocks the maturation of pre-osteoblastic cells in vitro. Moreover, cytoplasmic lipid droplets are detectable at the end of the culture period, suggesting that GJC inhibition may favor an adipocytic phenotype. We used several human osteoblastic cell lines, as well as bone-derived primary osteoblastic cells, to show that confluent cultures of human osteoblastic cells grown under osteogenic conditions developed an adipocytic phenotype after 3 days of complete inhibition of GJC using AGRA or oleamide, two dissimilar nontoxic reversible inhibitors. Development of an adipogenic phenotype was confirmed by the accumulation of triglyceride droplets and the increase in mRNA expression of the adipocytic markers peroxisome proliferator-activated receptor gamma2 and lipoprotein lipase. Glycyrrhizic acid, a noninhibitory AGRA analog, or alpha-bromopalmitate, a nondegradable fatty acid, had no effect. Modulation of skeletal GJC may represent a new pharmacological target by which inhibition of marrow adipogenesis can take place with the parallel enhancement of osteoblastogenesis, thus providing a novel therapeutic approach to the treatment of human age-related osteopenic diseases and postmenopausal osteoporosis.

Effects of cAMP on intercellular coupling and osteoblast differentiation

Bone-forming cells are organized in a multicellular network interconnected by gap junctions. Direct intercellular communication via gap junctions is an important component of bone homeostasis, coordinating cellular responses to external signals and promoting osteoblast differentiation. The cAMP pathway, a major intercellular signal transduction mechanism, regulates osteoblastic function and metabolism. We investigated the effects of this second messenger on junctional communication and on the expression of differentiation markers in human HOBIT osteoblastic cells. Increased levels of cAMP induce posttranslational modifications (i.e., phosphorylations) of connexin43 and enhancement of gap junction assembly, resulting in an increased junctional permeance to Lucifer yellow and to a positive modulation of intercellular Ca(2+) waves. Increased intercellular communication, however, was accompanied by a parallel decrease of alkaline phosphatase activity and by an increase of osteocalcin expression. cAMP-dependent stimulation of cell-to-cell coupling induces a complex modulation of bone differentiation markers.

Mechanism of intracellular Ca(2+)-wave propagation elicited by mechanical stimulation in cultured endothelial CPAE cells

Intra- and intercellular Ca(2+)-signaling during mechanical stimulation in calf pulmonary artery endothelial cells (CPAE) was investigated with digital fluorescence microscopy. Mechanical stimulation of a CPAE cell in a Ca(2+)-containing solution revealed a rise of the free intracellular Ca(2+)-concentration ([Ca(2+)](i)) in the mechanically stimulated cell (MS) proceeding to the neighboring (NB) cells as an intercellular Ca(2+)-wave. Experiments in Ca(2+)-free solution, containing 2mM EGTA, demonstrated that a detectable [Ca(2+)](i)-transient in the MS cell is not always a requisite for intercellular communication (IC). The Ca(2+)-wave propagation was not affected by changes in membrane potential and was not mediated by voltage-dependent Ca(2+)-channels. Ca(2+)-influx through the Ni(2+)-sensitive Ca(2+)-pathway occurred in the MS as could be assessed by Mn(2+)-quenching experiments. The intra- and intercellular Ca(2+)-wave was triggered by the release of thapsigargin-sensitive intracellular Ca(2+)-stores. Phospholipase C (PLC) inhibition by U73122 reduced the Ca(2+)-amplitude of the MS cell and almost completely inhibited the IC, indicating that the Ca(2+)-release in the MS and NB cells is PLC/inositol 1,4,5-trisphosphate (IP(3)) mediated.

Interleukin-1beta increases the functional expression of connexin 43 in articular chondrocytes: evidence for a Ca2+-dependent mechanism

Cell-to-cell interactions and gap junctions-dependent communication are crucially involved in chondrogenic differentiation, whereas in adult articular cartilage direct intercellular communication occurs mainly among chondrocytes facing the outer cartilage layer. Chondrocytes extracted from adult articular cartilage and grown in primary culture express connexin 43 (Cx43) and form functional gap junctions capable of sustaining the propagation of intercellular Ca2+ waves. Degradation of articular cartilage is a characteristic feature of arthritic diseases and is associated to increased levels of Interleukin-1 (IL-1) in the synovial fluid. We have examined the effects of IL-1 on gap junctional communication in cultured rabbit articular chondrocytes. Incubation with IL-1 potentiated the transmission of intercellular Ca2+ waves and the intercellular transfer of Lucifer yellow. The stimulatory effect was accompanied by a dose-dependent increase in the expression of Cx43 and by an enhanced Cx43 immunostaining at sites of cell-to-cell contact. IL-1 stimulation induced a dose-dependent increase of cytosolic Ca2+ and activates protein tyrosine phosphorylation. IL-1-dependent up-regulation of Cx43 could be prevented by intracellular Ca2+ chelation but not by inhibitors of protein tyrosine kinases, suggesting a crucial role of cytosolic Ca2+ in regulating the expression of Cx43. IL-1 is one of the most potent cytokines that promotes cartilage catabolism; its modulation of intercellular communication represents a novel mechanism by which proinflammatory mediators regulate the activity of cartilage cells.

Intercellular calcium waves in HeLa cells expressing GFP-labeled connexin 43, 32, or 26

This study was undertaken to obtain direct evidence for the involvement of gap junctions in the propagation of intercellular Ca(2+) waves. Gap junction-deficient HeLa cells were transfected with plasmids encoding for green fluorescent protein (GFP) fused to the cytoplasmic carboxyl termini of connexin 43 (Cx43), 32 (Cx32), or 26 (Cx26). The subsequently expressed GFP-labeled gap junctions rendered the cells dye- and electrically coupled and were detected at the plasma membranes at points of contact between adjacent cells. To correlate the distribution of gap junctions with the changes in [Ca(2+)](i) associated with Ca(2+) waves and the distribution of the endoplasmic reticulum (ER), cells were loaded with fluorescent Ca(2+)-sensitive (fluo-3 and fura-2) and ER membrane (ER-Tracker) dyes. Digital high-speed microscopy was used to collect a series of image slices from which the three-dimensional distribution of the gap junctions and ER were reconstructed. Subsequently, intercellular Ca(2+) waves were induced in these cells by mechanical stimulation with or without extracellular apyrase, an ATP-degrading enzyme. In untransfected HeLa cells and in the absence of apyrase, cell-to-cell propagating [Ca(2+)](i) changes were characterized by initiating Ca(2+) puffs associated with the perinuclear ER. By contrast, in Cx-GFP-transfected cells and in the presence of apyrase, [Ca(2+)](i) changes were propagated without initiating perinuclear Ca(2+) puffs and were communicated between cells at the sites of the Cx-GFP gap junctions. The efficiency of Cx expression determined the extent of Ca(2+) wave propagation. These results demonstrate that intercellular Ca(2+) waves may be propagated simultaneously via an extracellular pathway and an intracellular pathway through gap junctions and that one form of communication may mask the other.

Mechanisms contributing to fluid-flow-induced Ca2+ mobilization in articular chondrocytes

We previously showed that fluid flow, which chondrocytes experience in vivo and which results in a variety of morphological and metabolic changes in cultured articular chondrocytes, can also stimulate a rise in intracellular calcium concentration ([Ca2+]i). However, the mechanism by which Ca2+ is mobilized in response to flow is unclear. In this study, we investigated the roles of intracellular Ca2+ stores, G-proteins, and extracellular ATP in the flow-induced Ca2+ response in bovine articular chondrocytes (BAC). Cells loaded with the Ca2+ sensitive dye Fura-2 were exposed to steady flow at 34 ml/min (37 dynes/cm2) in a parallel plate flow chamber. Whereas ryanodine and caffeine had no effect, both neomycin and thapsigargin significantly decreased the Ca2+(i) response to flow, suggesting a role for Ca2+ store release, possibly through an inositol 1,4,5-trisphosphate (IP3)-dependent mechanism. Twenty-four-hour treatment with pertussis toxin also significantly decreased the response, suggesting that the mechanism may be G-protein regulated. In addition, ATP release by chondrocytes does not appear to mediate the flow-induced Ca2+ response because suramin, a P2 purinergic blocker, had no effect. These results suggest that BAC respond rapidly to changes in their mechanical environment, such as increased fluid flow, by a mechanism that involves IP3 stimulated Ca2+(i) release and G-protein activation.

Assays using digital fluorescence: 1985-1998

Luminescence continues to provide comprehensive literature surveys which will be published in most issues. These are a continuation of the literature surveys begun in 1986 in the Journal of Bioluminescence and Chemiluminescence which, up until 1998, encompassed more than 6000 references cited by year or specialized topic. With this newly named journal these searches are expanding to reflect the journal's wider scope. In future we will cover all fundamental and applied aspects of biological and chemical luminescence and include not only bioluminescence and chemiluminescence but also fluorescence, time resolved fluorescence, electrochemiluminescence, phosphorescence, sonoluminescence, lyoluminescence and triboluminescence. The compilers would be pleased to receive any comments from the readership. Contact by e-mail: L.J. Kricka: larry_kricka@path1a.med.upenn.edu or P.E. Stanley: Stanley@LUMIWEB.COM Copyright 1999 John Wiley & Sons, Ltd.