ATP-induced ATP release from astrocytes

Propagation of interastrocyte Ca2+ waves is mediated by diffusion of extracellular adenosine triphosphate (ATP), and may require regenerative release of ATP. The ability of ATP to initiate release of intracellular ATP was assessed by labeling adenine nucleotide pools in astrocyte cultures with 14C-adenine. The 14C-purines released during exposure to ATP were then identified by thin-layer chromatography. ATP treatment caused a five-fold increase in release of 14C-ATP but not 14C-ADP or 14C-AMP, indicating selectivity for release of ATP. Other P2 receptor agonists also caused significant 14C-ATP release, and the P2 receptor antagonists suramin, reactive blue-2 and pyridoxalphosphate-6-azo(benzene-2,4-disulfonic acid) (PPADS) inhibited ATP-induced 14C-ATP release to varying degrees, suggesting the involvement of a P2 receptor. ATP-induced 14C-ATP release was not affected by chelation of intracellular Ca2+ with BAPTA-AM, or by blockers of Ca2+ release from intracellular stores or of extracellular Ca2+ influx, suggesting a Ca2+-independent response. ATP-induced 14C-ATP release was significantly inhibited by non-selective anion channel blockers but not by blockers of ATP-binding cassette proteins, gap junction hemichannels, or vesicular exocytosis. Release of adenine nucleotides induced by 0 Ca2+ was, in contrast, not selective for ATP, and was susceptible to inhibition by gap junction blockers. These findings indicate that astrocytes are capable of ATP-induced ATP release and support a role for regenerative ATP release in glial Ca2+ wave propagation.     

Intercellular Ca2+ waves in mechanically stimulated articular chondrocytes

Articular cartilage is a tissue designed to withstand compression during joint movement and, in vivo, is subjected to a wide range of mechanical loading forces. Mechanosensitivity has been demonstrated to influence chondrocyte metabolism and cartilage homeostasis, but the mechanisms underlying mechanotransduction in these cells are poorly understood. In many cell types mechanical stimulation induces increases of the cytosolic Ca2+ concentration that propagates from cell to cell as an intercellular Ca2+ wave. Cell-to-cell communication through gap junctions underlies tissue co-ordination of metabolism and sensitivity to extracellular stimuli: gap junctional permeability to intracellular second messengers allows signal transduction pathways to be shared among several cells, ultimately resulting in co-ordinated tissue responses. Mechanically-induced Ca2+ signalling was investigated with digital fluorescence video imaging in primary cultures of rabbit articular chondrocytes. Mechanical stimulation of a single cell, obtained by briefly distorting the plasmamembrane with a micropipette, induced a wave of increased Ca2+ that was communicated to surrounding cells. Intercellular Ca2+ spreading was inhibited by 18 alpha-glycyrrhetinic acid, suggesting the involvement of gap junctions in signal propagation. The functional expression of gap junctions was assessed, in confluent chondrocyte cultures, by the intercellular transfer of Lucifer yellow dye in microinjection experiments while the expression of connexin 43 could be detected in Western blots. A series of pharmacological tools known to interfere with the cell calcium handling capacity were employed to investigate the mechanism of mechanically-induced Ca2+ signalling. In the absence of extracellular Ca2+ mechanical stimulation induced communicated Ca2+ waves similar to controls. Mechanical stress induced Ca2+ influx both in the stimulated chondrocyte but not in the adjacent cells, as assessed by the Mn2+ quenching technique. Cells treatment with thapsigargin and with the phospholipase C inhibitor U73122 blocked mechanically-induced signal propagation. These results provide evidence that in chondrocytes mechanical stimulation activates phospholipase C, thus leading to an increase of intracellular inositol 1,4,5-trisphosphate. The second messenger, by permeating gap junctions, stimulates intracellular Ca2+ release in neighbouring cells. Intercellular Ca2+ waves may provide a mechanism to co-ordinate tissue responses in cartilage physiology.     

Coordinated intercellular calcium waves induced by noradrenaline in rat hepatocytes: dual control by gap junction permeability and agonist.

Calcium-mobilizing agonists induce intracellular Ca2+ concentration ([Ca2+]i) changes thought to trigger cellular responses. In connected cells, rises in [Ca2+]i can propagate from cell to cell as intercellular Ca2+ waves, the mechanisms of which are not elucidated. Using fura2-loaded rat hepatocytes, we studied the mechanisms controlling coordination and intercellular propagation of noradrenaline-induced Ca2+ signals. Gap junction blockade with 18 alpha-glycyrrhetinic acid resulted in a loss of coordination between connected cells. We found that second messengers and [Ca2+]i rises in one hepatocyte cannot trigger Ca2+ responses in connected cells, suggesting that diffusion across gap junctions, while required for coordination, is not sufficient by itself for the propagation of intercellular Ca2+ waves. In addition, our experiments revealed functional differences between noradrenaline-induced Ca2+ signals in connected hepatocytes. These results demonstrate that intercellular Ca2+ signals in multicellular systems of rat hepatocytes are propagated and highly organized through complex mechanisms involving at least three factors. First, gap junction coupling ensures coordination of [Ca2+]i oscillations between the different cells; second, the presence of hormone at each hepatocyte is required for cell-cell Ca2+ signal propagation; and third, functional differences between adjacent connected hepatocytes could allow a 'pacemaker-like' intercellular spread of Ca2+ waves.     

Intercellular Ca2+ signaling in alveolar epithelial cells through gap junctions and by extracellular ATP

Inter- and extracellular-mediated changes in intracellular Ca2+ concentration ([Ca2+]i) can ensure coordinated tissue function in the lung. Cultured rat alveolar epithelial cells (AECs) have been shown to respond to secretagogues with increases in [Ca2+]i and have been shown to be gap junctionally coupled. However, communication of [Ca2+]i changes in AECs is not well defined. Monolayers of AECs were mechanically perturbed and monitored for [Ca2+]i changes. Perturbation of AECs was administered by a glass probe to either mechanically stimulate or mechanically wound individual cells. Both approaches induced a change in [Ca2+]i in the stimulated cell that was propagated to neighboring cells (Ca2+ waves). A connexin mimetic peptide shown to uncouple gap junctions eliminated Ca2+ waves in mechanically stimulated cells but had no effect on mechanically wounded cells. In contrast, apyrase, an enzyme that effectively removes ATP from the extracellular milieu, had no effect on mechanically stimulated cells but severely restricted mechanically wounded Ca2+ wave propagation. We conclude that AECs have the ability to communicate coordinated Ca2+ changes using both gap junctions and extracellular ATP.     

A quantitative model of purinergic junctional transmission of calcium waves in astrocyte networks

A principal means of transmitting intracellular calcium (Ca2+) waves at junctions between astrocytes involves the release of the chemical transmitter adenosine triphosphate (ATP). A model of this process is presented in which activation of purinergic P2Y receptors by ATP triggers the release of ATP, in an autocrine manner, as well as concomitantly increasing intracellular Ca2+. The dependence of the temporal characteristics of the Ca2+ wave are shown to critically depend on the dissociation constant (K(R)) for ATP binding to the P2Y receptor type. Incorporating this model astrocyte into networks of these cells successfully accounts for many of the properties of propagating Ca2+ waves, such as the dependence of velocity on the type of P2Y receptor and the time-lag of the Ca2+ wave behind the ATP wave. In addition, the conditions under which Ca2+ waves may jump from one set of astrocytes across an astrocyte-free lane to another set of astrocytes are quantitatively accounted for by the model. The properties of purinergic transmission at astrocyte junctions may determine many of the characteristics of Ca2+ propagation in networks of these cells.     

Intracellular Ca2+ stores are essential for injury induced Ca2+ signaling and re-endothelialization

Endothelialization repairs the lining of damaged vasculature and is a key process in preventing thrombosis and restenosis. It has been demonstrated that extracellular calcium ([Ca2+](o)) influx is important for subsequent endothelialization. The role of intracellular Ca2+ stores in mechanical denudation induced intracellular calcium ([Ca2+](i)) rise and endothelialization remains to be demonstrated. Using monolayer culture of a human endothelial cell line (human umbilical vein endothelial cell, HUVEC), we investigated [Ca2+](i) wave propagation and re-endothelialization following mechanical denudation. Consistent with previous reports for other types of cells, mechanical denudation induces calcium influx, which is essential for [Ca2+](i) rise and endothelialization. Moreover, we found that intracellular Ca(2+) stores are also essential for denudation induced [Ca2+](i) wave initiation and propagation, and the subsequent endothelialization. Thapsigargin which depletes intracellular Ca2+ stores completely abolished [Ca2+](i) wave generation and endothelialization. Xestospongin C (XeC), which prevents Ca2+ release from intracellular Ca2+ stores by inhibition of inositol 1,4,5-trisphosphate (IP(3)) receptor, inhibited intercellular Ca2+ wave generation and endothelialization following denudation. Purinergic signaling through a suramin sensitive mechanism and gap junction communication also contribute to in intercellular Ca(2+) wave propagation and re-endothelialization. We conclude that intracellular Ca2+ stores, in addition to extracellular Ca2+, are essential for intracellular Ca2+ signaling and subsequent endothelialization following mechanical denudation.     

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.     

Mechanosensitivity and intercellular communication in HOBIT osteoblastic cells: a possible role for gap junction hemichannels

Mechanically induced intercellular Ca2+ signalling was investigated in differentiated HOBIT osteoblastic cells. HOBIT cells express connexin43 clustered at the cell-to-cell boundary and display functional intercellular coupling assessed by intercellular transfer of Lucifer yellow. Mechanical stimulation of single cells, besides leading to an intracellular Ca2+ rise, 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 was inhibited by 18alpha-glycyrrhetinic acid, demonstrating the involvement of gap junctions in signal propagation. Suramin and apyrase decreased the extent of wave propagation, suggesting that ATP-mediated paracrine stimulation contribute to cell-to-cell signalling. 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 from the external medium. Gap-junctional hemichannels were activated by low extracellular Ca2+ concentrations and inhibited by 18alpha-glycyrrhetinic acid.     

Intercellular calcium waves in primary cultured rat mesenteric smooth muscle cells are mediated by connexin43

Intercellular Ca(2+) wave propagation between vascular smooth muscle cells (SMCs) is associated with the propagation of contraction along the vessel. Here, we characterize the involvement of gap junctions (GJs) in Ca(2+) wave propagation between SMCs at the cellular level. Gap junctional communication was assessed by the propagation of intercellular Ca(2+) waves and the transfer of Lucifer Yellow in A7r5 cells, primary rat mesenteric SMCs (pSMCs), and 6B5N cells, a clone of A7r5 cells expressing higher connexin43 (Cx43) to Cx40 ratio. Mechanical stimulation induced an intracellular Ca(2+) wave in pSMC and 6B5N cells that propagated to neighboring cells, whereas Ca(2+) waves in A7r5 cells failed to progress to neighboring cells. We demonstrate that Cx43 forms the functional GJs that are involved in mediating intercellular Ca(2+) waves and that co-expression of Cx40 with Cx43, depending on their expression ratio, may interfere with Cx43 GJ formation, thus altering junctional communication.     

The gap junction cellular internet: connexin hemichannels enter the signalling limelight

Cxs (connexins), the protein subunits forming gap junction intercellular communication channels, are transported to the plasma membrane after oligomerizing into hexameric assemblies called connexin hemichannels (CxHcs) or connexons, which dock head-to-head with partner hexameric channels positioned on neighbouring cells. The double membrane channel or gap junction generated directly couples the cytoplasms of interacting cells and underpins the integration and co-ordination of cellular metabolism, signalling and functions, such as secretion or contraction in cell assemblies. In contrast, CxHcs prior to forming gap junctions provide a pathway for the release from cells of ATP, glutamate, NAD+ and prostaglandin E2, which act as paracrine messengers. ATP activates purinergic receptors on neighbouring cells and forms the basis of intercellular Ca2+ signal propagation, complementing that occuring more directly via gap junctions. CxHcs open in response to various types of external changes, including mechanical, shear, ionic and ischaemic stress. In addition, CxHcs are influenced by intracellular signals, such as membrane potential, phosphorylation and redox status, which translate external stresses to CxHc responses. Also, recent studies demonstrate that cytoplasmic Ca2+ changes in the physiological range act to trigger CxHc opening, indicating their involvement under normal non-pathological conditions. CxHcs not only respond to cytoplasmic Ca2+, but also determine cytoplasmic Ca2+, as they are large conductance channels, suggesting a prominent role in cellular Ca2+ homoeostasis and signalling. The functions of gap-junction channels and CxHcs have been difficult to separate, but synthetic peptides that mimic short sequences in the Cx subunit are emerging as promising tools to determine the role of CxHcs in physiology and pathology.     

Oxygen-glucose deprivation-induced enhancement of extracellular ATP-P2Y purinoceptors signaling for the propagation of astrocytic calcium waves

Waves of elevated intracellular free Ca²⁺ that propagate between neighboring astrocytes (Ca²⁺ waves) are important for the communication among astrocytes. We have previously revealed that focal photolysis of a caged calcium ionophore results in an increase in the concentration of intracellular Ca²⁺ in the target astrocytes, then the increase propagates to neighboring astrocytes through gap junctions. The extracellular ATP-purinoceptors signaling pathways are not primarily responsible for the propagation of the photolytic flash-induced Ca²⁺ waves. Here we examined whether and if so how the dynamics of Ca²⁺ waves changed after treatment with sublethal simulated ischemia; oxygen-glucose deprivation (OGD). OGD treatment increased the astrocytic expression of P2Y₁ and P2Y₂ receptors early during reperfusion, resulting in an increase in the propagating waves speed. In contrast, the expression of a gap junction protein was not changed significantly by the OGD suggesting that the extracellular ATP-P2Y receptors signaling pathways were preferentially enhanced after OGD. The present method to induce Ca²⁺ waves by focal photolysis of a caged calcium ionophore may provide a valuable tool with which to analyze glial Ca²⁺ waves under not only normal but also pathologic conditions.     

ATP- and Gap Junction–dependent Intercellular Calcium Signaling in Osteoblastic Cells

Many cells coordinate their activities by transmitting rises in intracellular calcium from cell to cell. In nonexcitable cells, there are currently two models for intercellular calcium wave propagation, both of which involve release of inositol trisphosphate (IP₃)- sensitive intracellular calcium stores. In one model, IP₃ traverses gap junctions and initiates the release of intracellular calcium stores in neighboring cells. Alternatively, calcium waves may be mediated not by gap junctional communication, but rather by autocrine activity of secreted ATP on P₂ purinergic receptors. We studied mechanically induced calcium waves in two rat osteosarcoma cell lines that differ in the gap junction proteins they express, in their ability to pass microinjected dye from cell to cell, and in their expression of P2Y₂ (P_2U) purinergic receptors. ROS 17/2.8 cells, which express the gap junction protein connexin43 (Cx43), are well dye coupled, and lack P_2U receptors, transmitted slow gap junction-dependent calcium waves that did not require release of intracellular calcium stores. UMR 106-01 cells predominantly express the gap junction protein connexin 45 (Cx45), are poorly dye coupled, and express P_2U receptors; they propagated fast calcium waves that required release of intracellular calcium stores and activation of P_2U purinergic receptors, but not gap junctional communication. ROS/P_2U transfectants and UMR/Cx43 transfectants expressed both types of calcium waves. Gap junction–independent, ATP-dependent intercellular calcium waves were also seen in hamster tracheal epithelia cells. These studies demonstrate that activation of P_2U purinergic receptors can propagate intercellular calcium, and describe a novel Cx43-dependent mechanism for calcium wave propagation that does not require release of intracellular calcium stores by IP₃. These studies suggest that gap junction communication mediated by either Cx43 or Cx45 does not allow passage of IP₃ well enough to elicit release of intracellular calcium stores in neighboring cells.     

A three-dimensional model of intercellular calcium signaling in epithelial cells

We have developed a fully three-dimensional (3D) model of calcium signaling in epithelial cells based on a set of reaction diffusion equations that are solved on a large-scale finite-element code in three dimensions. We have explicitly included the cellular compartments including the cell nucleus, cytoplasm, and gap junctions. The model allows for buffering of free Ca2+, calcium-induced calcium release, and the explicit inclusion of mobile buffers. To make quantitative comparisons to experimental results, we used fluorescence microscopy images of cells to generate an accurate mesh describing cell morphology. We found that Ca2+ wave propagation through the tissue is a function of both initial conditions used to start the wave and various geometrical parameters that affect propagation such as gap junction density and distribution, and the presence of nuclei. The exogenous dyes used in experimental imaging also affect wave propagation.     

Connexin mimetic peptides reversibly inhibit Ca(2+) signaling through gap junctions in airway cells

The effect of peptides with sequences derived from connexins, the constituent proteins of gap junctions, on mechanically stimulated intercellular Ca(2+) signaling in tracheal airway epithelial cells was studied. Three peptides with sequences corresponding to connexin extracellular loop regions reversibly restricted propagation of Ca(2+) waves to neighboring cells. Recovery of communication began within 10 min of removal of the peptides, with inhibition totally reversed by 20-40 min. The peptides were shown to be more effective in inhibiting Ca(2+) waves than glycyrrhetinic acid or oleamide. Inhibition of intercellular Ca(2+) waves by connexin mimetic peptides did not affect the Ca(2+) response to extracellular ATP. Although the intracellular Ca(2+) response of tracheal epithelial cells to ATP was greatly reduced by either pretreatment with high doses of ATP or application of apyrase, mechanically stimulated intercellular Ca(2+) signaling was not affected by these agents. We conclude that connexin mimetic peptides are effective and reversible inhibitors of gap junctional communication of physiologically significant molecules that underlie Ca(2+) wave propagation in tracheal epithelial cells and propose a potential mechanism for the mode of action of mimetic peptides.     

Abnormalities in gap junctions and Ca2+ dynamics in cardiomyocytes at the border zone of myocardial infarcts

Abnormalities in gap junction function and Ca2+ dynamics are believed to be important factors in arrhythmogenesis after myocardial infarction. To elucidate the relationship between changes in Ca2+ dynamics and gap junctions, we analyzed by real-time in situ Ca2+ imaging of fluo-3 loaded whole hearts the spatiotemporal occurrence of Ca2+ waves and the localization of connexin43 (Cx43) at the border zone of myocardial infarcts induced in the rat by coronary ligation. At early time points (2-4 hours postligation), different regions of the left ventricle showed distinct changes in cytosolic free Ca2+ concentrations [Ca2+]i. While some cardiomyocytes of infarcted regions exhibited high levels of resting fluo-3 fluorescence, at border zones frequent Ca2+ waves were observed. Some of the waves were abolished by spontaneous Ca2+ transients and others were not. Intact myocardium apart from infarcted regions exhibited homogenous Ca2+ transients. Confocal imaging of Cx43 and actin filaments in the rat heart fixed 2 hours after coronary ligation revealed that Cx43 was markedly decreased in the area of myocyte necrosis with contraction bands and in the neighboring myocardium. These results suggest that abnormal expression and function of gap junctions could be associated with Ca2+ waves at the border zone of myocardial infarcts, possibly through Ca2+ overload.     

Calcium signalling in glial cells

Calcium signals are the universal way of glial responses to the various types of stimulation. Glial cells express numerous receptors and ion channels linked to the generation of complex cytoplasmic calcium responses. The glial calcium signals are able to propagate within glial cells and to create a spreading intercellular Ca2+ wave which allow information exchange within the glial networks. These propagating Ca2+ waves are primarily mediated by intracellular excitable media formed by intracellular calcium storage organelles. The glial calcium signals could be evoked by neuronal activity and vice versa they may initiate electrical and Ca2+ responses in adjacent neurones. Thus glial calcium signals could integrate glial and neuronal compartments being therefore involved in the information processing in the brain.     

Nonlinear Gap Junctions Enable Long-Distance Propagation of Pulsating Calcium Waves in Astrocyte Networks

A new paradigm has recently emerged in brain science whereby communications between glial cells and neuron-glia interactions should be considered together with neurons and their networks to understand higher brain functions. In particular, astrocytes, the main type of glial cells in the cortex, have been shown to communicate with neurons and with each other. They are thought to form a gap-junction-coupled syncytium supporting cell-cell communication via propagating Ca²⁺ waves. An identified mode of propagation is based on cytoplasm-to-cytoplasm transport of inositol trisphosphate (IP₃) through gap junctions that locally trigger Ca²⁺ pulses via IP₃-dependent Ca²⁺-induced Ca²⁺ release. It is, however, currently unknown whether this intracellular route is able to support the propagation of long-distance regenerative Ca²⁺ waves or is restricted to short-distance signaling. Furthermore, the influence of the intracellular signaling dynamics on intercellular propagation remains to be understood. In this work, we propose a model of the gap-junctional route for intercellular Ca²⁺ wave propagation in astrocytes. Our model yields two major predictions. First, we show that long-distance regenerative signaling requires nonlinear coupling in the gap junctions. Second, we show that even with nonlinear gap junctions, long-distance regenerative signaling is favored when the internal Ca²⁺ dynamics implements frequency modulation-encoding oscillations with pulsating dynamics, while amplitude modulation-encoding dynamics tends to restrict the propagation range. As a result, spatially heterogeneous molecular properties and/or weak couplings are shown to give rise to rich spatiotemporal dynamics that support complex propagation behaviors. These results shed new light on the mechanisms implicated in the propagation of Ca²⁺ waves across astrocytes and the precise conditions under which glial cells may participate in information processing in the brain.     

The functional expression of connexin 43 in articular chondrocytes is increased by interleukin 1beta: evidence for a Ca2+-dependent mechanism

Cell-to-cell interactions and gap junctions-dependent communication are crucially involved in chondrogenic differentiation, while 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 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 connexin 43 and by an enhanced connexin 43 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 connexin 43 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 connexin 43. 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.     

Ca2+ waves in astrocytes.

The glial cell is the most numerous cell type in the central nervous system and is believed to play an important role in guiding brain development and in supporting adult brain function. One type of glial cell, the astrocyte also may be an integral computational element in the brain since it undergoes neurotransmitter-triggered signalling. Here we review the role of the astrocyte in the central nervous system, emphasizing receptor-mediated Ca2+ physiology. One focus is the recent discovery that the neurotransmitter glutamate induces a variety of intracellular Ca2+ changes in astrocytes. Simple Ca2+ spikes or intracellular Ca2+ oscillations often appear spatially uniform. However, in many instances, the Ca2+ rise has a significant spatial dimension, beginning in one part of the cell it spreads through the rest of the cell in the form of a wave. With high enough agonist concentration an astrocyte syncitium supports intercellular waves which propagate from cell to cell over relatively long distances. We present results of experiments using more specific pharmacological glutamate receptor agonists. In addition to describing the intercellular Ca2+ wave we present evidence for another form of intercellular signalling. Some possible functions of a long-range glial signalling system are also discussed.     

Propagation of fast and slow intercellular Ca(2+) waves in primary cultured arterial smooth muscle cells

Smooth muscle contraction is regulated by changes in cytosolic Ca²⁺ concentration ([Ca²⁺]_i). In response to stimulation, Ca²⁺ increase in a single cell can propagate to neighbouring cells through gap junctions, as intercellular Ca²⁺ waves. To investigate the mechanisms underlying Ca²⁺ wave propagation between smooth muscle cells, we used primary cultured rat mesenteric smooth muscle cells (pSMCs). Cells were aligned with the microcontact printing technique and a single pSMC was locally stimulated by mechanical stimulation or by microejection of KCl. Mechanical stimulation evoked two distinct Ca²⁺ waves: (1) a fast wave (2 mm/s) that propagated to all neighbouring cells, and (2) a slow wave (20 μm/s) that was spatially limited in propagation. KCl induced only fast Ca²⁺ waves of the same velocity as the mechanically induced fast waves. Inhibition of gap junctions, voltage-operated calcium channels, inositol 1,4,5-trisphosphate (IP₃) and ryanodine receptors, shows that the fast wave was due to gap junction mediated membrane depolarization and subsequent Ca²⁺ influx through voltage-operated Ca²⁺ channels, whereas, the slow wave was due to Ca²⁺ release primarily through IP₃ receptors. Altogether, these results indicate that temporally and spatially distinct mechanisms allow intercellular communication between SMCs. In intact arteries this may allow fine tuning of vessel tone.