Linkage of extracellular and intracellular control of cytosolic Ca2+ in rat osteoclasts in the presence of thapsigargin

Involvement of the Ca²⁺ signaling pathway in osteoprotegerin inhibition of osteoclast differentiation and maturation

The purpose of this study was to determine whether the Ca²⁺ signaling pathway is involved in the ability of osteoprotegerin (OPG) to inhibit osteoclast differentiation and maturation. RAW264.7 cells were incubated with macrophage colony-stimulating factor (M-CSF) + receptor activator of nuclear factor-κB ligand (RANKL) to stimulate osteoclastogenesis and then treated with different concentrations of OPG, an inhibitor of osteoclast differentiation. The intracellular Ca²⁺ concentration [Ca²⁺]_i and phosphorylation of Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) in the different treatment groups were measured by flow cytometry and Western blotting, respectively. The results confirmed that M-CSF + RANKL significantly increased [Ca²⁺]_i and CaMKII phosphorylation in osteoclasts (p < 0.01), and that these effects were subsequently decreased by OPG treatment. Exposure to specific inhibitors of the Ca²⁺ signaling pathway revealed that these changes varied between the different OPG treatment groups. Findings from the present study indicated that the Ca²⁺ signaling pathway is involved in both the regulation of osteoclastogenesis as well as inhibition of osteoclast differentiation and activation by OPG.

Osteoblasts detect pericellular calcium concentration increase via neomycin-sensitive voltage gated calcium channels

The mechanisms underlying the detection of critically loaded or micro-damaged regions of bone by bone cells are still a matter of debate. Our previous studies showed that calcium efflux originates from pre-failure regions of bone matrix and MC3T3-E1 osteoblasts respond to such efflux by an increase in the intracellular calcium concentration. The mechanisms by which the intracellular calcium concentration increases in response to an increase in the pericellular calcium concentration are unknown. Elevation of the intracellular calcium may occur via release from the internal calcium stores of the cell and/or via the membrane bound channels. The current study applied a wide range of pharmaceutical inhibitors to identify the calcium entry pathways involved in the process: internal calcium release from endoplasmic reticulum (ER, inhibited by thapsigargin and TMB-8), calcium receptor (CaSR, inhibited by calhex), stretch-activated calcium channel (SACC, inhibited by gadolinium), voltage-gated calcium channels (VGCC, inhibited by nifedipine, verapamil, neomycin, and ω-conotoxin), and calcium-induced-calcium-release channel (CICRC, inhibited by ryanodine and dantrolene). These inhibitors were screened for their effectiveness to block intracellular calcium increase by using a concentration gradient induced calcium efflux model which mimics calcium diffusion from the basal aspect of cells. The inhibitor(s) which reduced the intracellular calcium response was further tested on osteoblasts seeded on mechanically loaded notched cortical bone wafers undergoing damage. The results showed that only neomycin reduced the intracellular calcium response in osteoblasts, by 27%, upon extracellular calcium stimulus induced by concentration gradient. The inhibitory effect of neomycin was more pronounced (75% reduction in maximum fluorescence) for osteoblasts seeded on notched cortical bone wafers loaded mechanically to damaging load levels. These results imply that the increase in intracellular calcium occurs by the entry of extracellular calcium ions through VGCCs which are sensitive to neomycin. N-type and P-type VGCCs are potential candidates because they are observed in osteoblasts and they are sensitive to neomycin. The calcium channels identified in this study provide new insight into mechanisms underlying the targeted repair process which is essential to bone adaptation.

Calcium signaling in osteoclasts

It has long been known that many bone diseases, including osteoporosis, involve abnormalities in osteoclastic bone resorption. As a result, there has been intense study of the mechanisms that regulate both the differentiation and bone resorbing function of osteoclast cells. Calcium (Ca(2+)) signaling appears to play a critical role in the differentiation and functions of osteoclasts. Cytoplasmic Ca(2+) oscillations occur during RANKL-mediated osteoclastogenesis. Ca(2+) oscillations provide a digital Ca(2+) signal that induces osteoclasts to up-regulate and autoamplify nuclear factor of activated T cells c1 (NFATc1), a Ca(2+)/calcineurin-dependent master regulator of osteoclastogenesis. Here we review previous studies on Ca(2+) signaling in osteoclasts as well as recent breakthroughs in understanding the basis of RANKL-induced Ca(2+) oscillations, and we discuss possible molecular players in this specialized Ca(2+) response that appears pivotal for normal bone function. This article is part of a Special Issue entitled: 11th European Symposium on Calcium.

Oxytocin is an anabolic bone hormone

We report that oxytocin (OT), a primitive neurohypophyseal hormone, hitherto thought solely to modulate lactation and social bonding, is a direct regulator of bone mass. Deletion of OT or the OT receptor (Oxtr) in male or female mice causes osteoporosis resulting from reduced bone formation. Consistent with low bone formation, OT stimulates the differentiation of osteoblasts to a mineralizing phenotype by causing the up-regulation of BMP-2, which in turn controls Schnurri-2 and 3, Osterix, and ATF-4 expression. In contrast, OT has dual effects on the osteoclast. It stimulates osteoclast formation both directly, by activating NF-kappaB and MAP kinase signaling, and indirectly through the up-regulation of RANK-L. On the other hand, OT inhibits bone resorption by mature osteoclasts by triggering cytosolic Ca(2+) release and NO synthesis. Together, the complementary genetic and pharmacologic approaches reveal OT as a novel anabolic regulator of bone mass, with potential implications for osteoporosis therapy.

Calcium/calmodulin-dependent kinase activity is required for efficient induction of osteoclast differentiation and bone resorption by receptor activator of nuclear factor kappa B ligand (RANKL)

Calcium/calmodulin-dependent protein kinase (CaMK) is a major down stream mediator of Ca(2+) signaling in a wide range of cellular functions, including ion channel and cell cycle regulation and neurotransmitter synthesis and release. Here we have investigated the role of the CaMK signaling pathway in osteoclast differentiation and bone resorption. We observed that the CaMKI, CaMKII gamma isoforms were present in both bone-marrow derived macrophages and RAW264.7 murine macrophage cell line, and that expression persisted during osteoclast differentiation in the presence of receptor activator of nuclear factor kappa B (NF-kappaB) ligand (RANKL). RANKL-induced differentiation was accompanied by increased cyclic AMP response element transcriptional activity, and ERK phosphorylation, which are both downstream targets of CaMK. Two selective inhibitors of CaMKs, KN-93 and KN-62, inhibited osteoclastogenesis in a time and concentration-dependent manner. This was accompanied by suppression of cathepsin K expression and osteoclastic bone resorption, which are markers for differentiated osteoclast function. KN-93 and KN-62 both inhibited RANKL-induced ERK phosphorylation and CREB transcriptional activity. These findings imply a role for CaMK in osteoclast differentiation and bone resorption.

Thapsigargin modulates osteoclastogenesis through the regulation of RANKL-induced signaling pathways and reactive oxygen species production

The mechanism by which TG modulates osteoclast formation and apoptosis is not clear. In this study, we showed a biphasic effect of TG on osteoclast formation and apoptosis through the regulation of ROS production, caspase-3 activity, cytosolic Ca2+, and RANKL-induced activation of NF-kappaB and AP-1 activities.

INTRODUCTION

Apoptosis and differentiation are among the consequences of changes in intracellular Ca2+ levels. In this study, we investigated the effects of the endoplasmic reticular Ca2+-ATPase inhibitor, thapsigargin (TG), on osteoclast apoptosis and differentiation.

MATERIALS AND METHODS

Both RAW264.7 cells and primary spleen cells were used to examine the effect of TG on RANKL-induced osteoclastogenesis. To determine the action of TG on signaling pathways, we used reporter gene assays for NF-kappaB and activator protein-1 (AP-1) activity, Western blotting for phospho-extracellular signal-related kinase (ERK), and fluorescent probes to measure changes in levels of intracellular calcium and reactive oxygen species (ROS). To assess rates of apoptosis, we measured changes in annexin staining, caspase-3 activity, and chromatin and F-actin microfilament structure.

RESULTS

At concentrations that caused a rapid rise in intracellular Ca2+, TG increased caspase-3 activity and promoted apoptosis in osteoclast-like cells (OLCs). Low concentrations of TG, which were insufficient to measurably alter intracellular Ca2+, unexpectedly suppressed caspase-3 activity and enhanced RANKL-induced osteoclastogenesis. At these lower concentrations, TG potentiated ROS production and RANKL-induced NF-kappaB activity, but suppressed RANKL-induced AP-1 activity and had little effect on ERK phosphorylation.

CONCLUSION

Our novel findings of a biphasic effect of TG are incompletely explained by our current understanding of TG action, but raise the possibility that low intensity or local changes in subcellular Ca2+ levels may regulate intracellular differentiation signaling. The extent of cross-talk between Ca2+ and RANKL-mediated intracellular signaling pathways might be important in determining whether cells undergo apoptosis or differentiate into OLCs.

Calcium sensing and cell signaling processes in the local regulation of osteoclastic bone resorption

The skeletal matrix in terrestrial vertebrates undergoes continual cycles of removal and replacement in the processes of bone growth, repair and remodeling. The osteoclast is uniquely important in bone resorption and thus is implicated in the pathogenesis of clinically important bone and joint diseases. Activated osteoclasts form a resorptive hemivacuole with the bone surface into which they release both acid and osteoclastic lysosomal hydrolases. This article reviews cell physiological studies of the local mechanisms that regulate the resorptive process. These used in vitro methods for the isolation, culture and direct study of the properties of neonatal rat osteoclasts. They demonstrated that both local microvascular agents and products of the bone resorptive process such as ambient Ca2+ could complement longer-range systemic regulatory mechanisms such as those that might be exerted through calcitonin (CT). Thus elevated extracellular [Ca2+], or applications of surrogate divalent cation agonists for Ca2+, inhibited bone resorptive activity and produced parallel increases in cytosolic [Ca2+], cell retraction and longer-term inhibition of enzyme release in isolated rat osteoclasts. These changes showed specificity, inactivation, and voltage-dependent properties that implicated a cell surface Ca2+ receptor (CaR) sensitive to millimolar extracellular [Ca2+]. Pharmacological, biophysical and immunochemical evidence implicated a ryanodine-receptor (RyR) type II isoform in this process and localized it to a unique, surface membrane site, with an outward-facing channel-forming domain. Such a surface RyR might function either directly or indirectly in the process of extracellular [Ca2+] sensing and in turn be modulated by cyclic adenosine diphosphate ribose (cADPr) produced by the ADP-ribosyl cyclase, CD38. The review finishes by speculating about possible detailed models for these transduction events and their possible interactions with other systemic mechanisms involved in Ca2+ homeostasis as well as the possible role of the RyR-based signaling mechanisms in longer-term cell regulatory processes.

Involvement of capacitive calcium entry and calcium store refilling in osteoclastic survival and bone resorption process

Bone resorption is closely dependent on osteoclastic survival and osteoclast apoptotic cell death could represent a key step at the end of this process. In order to precise the possible role of calcium movement in osteoclastic cell death, we investigated whether intracellular calcium store replenishment and capacitive calcium entry (CCE) are involved in osteoclastic survival and bone resorption. We demonstrate that (i). thapsigargin, a sarco-endoplasmic reticulum calcium ATPase pump (SERCA) blocker, decreases both osteoclastic survival and bone resorption process, (ii). 2-aminoethoxydiphenyl borate (2-APB) and SKF-96365, two store-operated channel (SOC) blockers, dramatically decrease osteoclastic survival and bone resorption and (iii). culture in calcium-free medium and thapsigargin exposure synergically inhibit osteoclastic survival which falls dramatically to a value close to 0% (P<0.001). Inversely, osteoclastic survival increases significantly when thapsigargin-treated cells are cultured in the presence of 20mM calcium, suggesting that increasing extracellular calcium concentration stimulates osteoclasts survival when the filling of intracellular stores is prevented. Taken together, our data strongly suggest that in osteoclasts, calcium movements between cellular compartments involved in the regulation of calcium signalling, such as calcium stores refilling and CCE, are closely associated to the regulation of osteoclast survival and bone resorption.

Ca(2+) influx through the osteoclastic plasma membrane ryanodine receptor

We predict that the type 2 ryanodine receptor isoform (RyR-2) located in the osteoclastic membrane functions as a Ca(2+) influx channel and as a divalent cation (Ca(2+)) sensor. Cytosolic Ca(2+) measurements revealed Ca(2+) influx in osteoclasts at depolarized membrane potentials. The cytosolic Ca(2+) change was, as expected, not seen in Ca(2+)-free medium and was blocked by the RyR modulator ryanodine. In contrast, at basal membrane potentials (approximately 25 mV) ryanodine triggered extracellular Ca(2+) influx that was blocked by Ni(2+). In parallel, single-channel recordings obtained from inside-out excised patches revealed a divalent cation-selective approximately 60-pS conductance in symmetric solutions of Ba-aspartate [Ba-Asp; reversal potential (E(rev)) approximately 0 mV]. In the presence of a Ba(2+) gradient, i.e., with Ba-Asp in the pipette and Na-Asp in the bath, channel conductance increased to approximately 120 pS and E(rev) shifted to 21 mV. The conductance was tentatively classified as a RyR-gated Ca(2+) channel as it displayed characteristic metastable states and was sensitive to ruthenium red and a specific anti-RyR antibody, Ab(34). To demonstrate that extracellular Ca(2+) sensing occurred at the osteoclastic surface rather than intracellularly, we performed protease protection assays using pronase. Preincubation with pronase resulted in markedly attenuated cytosolic Ca(2+) signals triggered by either Ni(2+) (5 mM) or Cd(2+) (50 microM). Finally, intracellular application of antiserum Ab(34) potently inhibited divalent cation sensing. Together, these results strongly suggest the existence of 1) a membrane-resident Ca(2+) influx channel sensitive to RyR modulators; 2) an extracellular, as opposed to intracellular, divalent cation activation site; and 3) a cytosolic CaM-binding regulatory site for RyR. It is likely therefore that the surface RyR-2 not only gates Ca(2+) influx but also functions as a sensor for extracellular divalent cations.

Receptor-operated osteoclast calcium sensing

Osteoclasts "sense" elevated extracellular calcium, which leads to cytoskeletal changes that may be linked to phospholipase C (PLC) activation and the associated rise in intracellular calcium ([Ca(2+)](i)). Since PLC is linked to transient receptor potential channels (trp), we hypothesized that receptor activated calcium influx due to this channel type would be activated by osteoclasts sensing [Ca(2+)](e). We found that high [Ca(2+)](e) induced similar intracellular Ca(2+) rises in chicken osteoclasts with or without intracellular Ca(2+) store depletion by either TPEN or thapsigargin, thus defining store-insensitive Ca(2+) influx. This store-insensitive calcium sensing component was blocked by the PLC antagonist U73122. Also, the calcium channel inhibitor SKF 96365, a blocker of store-independent trp-like channels, was effective in inhibiting calcium sensing in the presence of thapsigargin. Thus, a store-independent component of calcium sensing was associated with ion channels linked to PLC. Since receptor activated transient receptor potential (trp) family cation channels open in a PLC-dependent and store-independent manner, we suggest that receptor operated channels are activated in osteoclasts stimulated by high extracellular Ca(2+).

Direct microsensor measurement of nitric oxide production by the osteoclast

Nitric oxide (NO) triggers marked osteoclast retraction which closely resembles that due to Ca2+. The effect of Ca2+ has been attributed to a stimulated release of NO. Here, we show for the first time, by direct measurement with a microsensor, that osteoclasts do indeed produce NO and that this production is enhanced by a high Ca2+. We also show that the Ca2+ ionophore, A23187, mimics the latter. Furthermore, osteoclasts on dentine produce more NO than osteoclasts on glass and NO release from dentine-plated osteoclasts is much less sensitive to stimulation by Ca2+. Finally, the microsomal Ca2+ store-depleting agent, thapsigargin, attenuates NO release only from osteoclasts on glass, suggesting that stored Ca2+ has the dominant effect in modulating NO release from non-resorbing cells. NO is a powerful inhibitor of bone resorption: a direct demonstration of its production is therefore strong evidence for a role in modulating osteoclast function.

Emerging insights into the role of calcium ions in osteoclast regulation

Osteoclasts are exposed to unusually high, millimolar, Ca2+ concentrations and can "sense" changes in their ambient Ca2+ concentration during resorption. This results in a sharp cystolic Ca2+ increase through both Ca2+ release and Ca2+ influx. The rise in cystolic Ca2+ is transduced finally into an inhibition of bone resorption. We have shown that a type 2 ryanodine receptor isoform, expressed uniquely in the osteoblast plasma membrane, functions as a Ca2+ influx channel, and possibly as a Ca2+ sensor. Ryanodine receptors are ordinarily microsomal membrane Ca2+ release channels. They have only recently been shown to be expressed a other sites, including nuclear membranes. At the latter site, ryanodine receptors gate nucleoplasmic Ca2+ influx. Nucleoplasmic Ca2+, in turn, regulates key nuclear processes, including gene expression and apoptosis. Here, we review potential mechanisms underlying the recognition, movement, and actions of Ca2+ in the osteoclast.

A novel calcium sensor stimulating inositol phosphate formation and [Ca2+]i signaling expressed by GCT23 osteoclast-like cells

Osteoclast activity is inhibited by elevated [Ca2+]o; however, the underlying molecular mechanism is unknown. We used the human osteoclast-like cells GCT23 to elucidate their cation-sensing properties. Cells responded to elevated [Ca2+]o with rapid concentration-dependent [Ca2+]i transients (EC50 = 7.8 mm, time to peak 44 +/- 4 sec) that were due to release from intracellular stores, followed by Ca2+ influx across the plasma membrane. Ca2+ store depletion by thapsigargin, endothelin-1, or bradykinin activated calcium entry pathways. Cells responded similarly to Ni2+ and Cd2+ with albeit slower kinetics (EC50 <10 microm and <100 microm, times to peak 140 +/- 25 sec and 150 +/- 24 sec, respectively). The three cations stimulated inositol phosphate production (two-fold, p <.02) similar to bradykinin (2.5-fold, p <. 002), which activates a phospholipase C (PLC)-coupled receptor in GCT23 cells. The cells did not respond to 0.1-1 mM Gd3+ or neomycin B, indicating that the parathyroid calcium receptor (PCaR) is not functionally expressed. In confirmation, PCaR could not be detected by reverse transcriptase polymerase chain reaction in GCT23 cells and in mouse osteoclasts, and the calcimimetic compound NPS R-568 failed to produce the left shift of the concentration-response curve characteristic for PCaR. Our data demonstrate for the first time that cation sensing by osteoclast-like GCT23 cells is mediated by a PLC-coupled receptor that is not identical to PCaR.

The effect of extracellularly applied divalent cations on cytosolic Ca2+ in murine leydig cells: evidence for a Ca2+-sensing receptor

1. The effect of extracellularly applied divalent cations upon cytosolic Ca2+ levels ([Ca2+]) was investigated in fura-2-loaded mouse Leydig (TM3) cells. 2. The extracellular application of Ca2+ (2.5-15 mM) or Ni2+ (0.5-5 mM) elicited concentration-dependent elevations in cytosolic [Ca2+] that were followed by decays to baseline levels. Extracellular Mg2+ (0.8-15 mM) failed to influence cytosolic [Ca2+]. 3. Conditioning applications of Ca2+ (2.5-10 mM), Mg2+ (2.5-15 mM) or Ni2+ (0.5-5 mM) all attenuated the cytosolic Ca2+ response to a subsequent test application of 5 mM [Ni2+]. 4. The amplitude of Ni2+-induced cytosolic Ca2+ signals remained constant in low-Ca2+ solutions. Such findings suggest a participation of Ca2+ release from intracellular stores. In parallel, depletion of Ca2+ stores by either ionomycin (5 microM, in low-Ca2+ solutions) or thapsigargin (4 microM) abolished or attenuated Ni2+-induced Ca2+ transients. 5. Ionomycin (5 microM) elevated cytosolic [Ca2+] in Ca2+-free solutions even after prior Ni2+ application, indicating the presence of Ni2+-insensitive stores. 6. Caffeine (250 and 500 microM) elevated cytosolic [Ca2+] and attenuated Ni2+-induced Ca2+ release. Furthermore, TM3 cells stained intensely with a specific anti-ryanodine receptor antiserum, Ab34. These findings suggest that Ca2+ release is regulated by ryanodine receptors. 7. Both membrane depolarization and hyperpolarization, brought about by changes in extracellular [K+] ([K+]e) in the presence of valinomycin (5 microM), altered the waveform of the Ni2+-induced cytosolic Ca2+ signal. Hyperpolarization, in addition, diminished the response magnitude. Such voltage-induced response modulation localizes the regulatory events to the Leydig cell plasma membrane. 8. We propose the existence of a cell surface divalent cation (Ca2+) receptor in Leydig cells, the activation of which triggers Ca2+ fluxes through ryanodine receptors.

Mode of action of interleukin-6 on mature osteoclasts. Novel interactions with extracellular Ca2+ sensing in the regulation of osteoclastic bone resorption

We describe a physiologically significant mechanism through which interleukin-6 (IL-6) and a rising ambient Ca2+ interact to regulate osteoclastic bone resorption. VOXEL-based confocal microscopy of nonpermeabilized osteoclasts incubated with anti- IL-6 receptor antibodies revealed intense, strictly peripheral plasma membrane fluorescence. IL-6 receptor expression in single osteoclasts was confirmed by in situ reverse transcriptase PCR histochemistry. IL-6 (5 ng/l to 10 microg/l), but not IL-11 (10 and 100 microg/l), reversed the inhibition of osteoclastic bone resorption induced by high extracellular Ca2+ (15 mM). The IL-6 effect was abrogated by excess soluble IL-6 receptor (500 microg/l). Additionally, IL-6 (5 pg/l to 10 microg/l) inhibited cytosolic Ca2+ signals triggered by high Ca2+ or Ni2+. In separate experiments, osteoclasts incubated in 10 mM Ca2+ or on bone released more IL-6 than those in 1.25 mM Ca2+. Furthermore, IL-6 mRNA histostaining was more intense in osteoclasts in 10 or 20 mM Ca2+ than cells in 1.25 mM Ca2+. Similarly, IL-6 receptor mRNA histostaining was increased in osteoclasts incubated in 5 or 10 mM Ca2+. Thus, while high Ca2+ enhances IL-6 secretion, the released IL-6 attenuates Ca2+ sensing and reverses inhibition of resorption by Ca2+. Such an autocrine-paracrine loop may sustain osteoclastic activity in the face of an inhibitory Ca2+ level generated locally during resorption.

Localized calcium signaling in multinucleated osteoclasts

Localized intracellular Ca2+ ([Ca2+]i) pulses, fluctuations, and repetitive spikes were detected in multinucleated rabbit osteoclasts in the presence of serum and in response to calcitonin using the fluorescent calcium indicator fluo-3 and a laser scanning microscope. We observed that these [Ca2+], changes were often restricted within a region of the cell body or propagated from the initial region of occurrence to other parts of the cell body but not to all parts. These observations suggest the existence of significant barriers to Ca2+ transport between different cytoplasmic regions of the osteoclast. To further investigate this phenomenon, we mechanically perturbed different cellular regions by touching locally with a micropipette. This usually induced a local increase in cytosolic and nuclear free [Ca2+]i. In some cases there was propagation of the [Ca2+]i increase to other regions but with part of the cell body not affected. Those regions of the cell body to which the [Ca2+]i increase did not propagate had a [Ca2+]i response to a direct mechanical perturbation. Our data show that osteoclasts can have different [Ca2+]i activities in apparently equivalent cellular regions, no matter how generated. This suggests that there can be a number of spatially separate Ca2+ regulatory systems within an osteoclast cell body.

Capacitative calcium entry

Images

A ryanodine receptor-like molecule expressed in the osteoclast plasma membrane functions in extracellular Ca2+ sensing

Ryanodine receptors (RyRs) reside in microsomal membranes where they gate Ca2+ release in response to changes in the cytosolic Ca2+ concentration. In the osteoclast, a divalent cation sensor, the Ca2+ receptor (CaR), located within the cell's plasma membrane, monitors changes in the extracellular Ca2+ concentration. Here we show that a RyR-like molecule is a functional component of this receptor. We have demonstrated that [3H] ryanodine specifically binds to freshly isolated rat osteoclasts. The binding was displaced by ryanodine itself, the CaR agonist Ni2+ and the RyR antagonist ruthenium red. The latter also inhibited cytosolic Ca2+ elevations induced by Ni2+. In contrast, the responses to Ni2+ were strongly potentiated by an antiserum Ab129 raised to an epitope located within the channel-forming domain of the type II RyR. The antiserum also stained the surface of intact, unfixed, trypan blue-negative osteoclasts. Serial confocal sections and immunogold scanning electron microscopy confirmed a plasma membrane localization of this staining. Antiserum Ab34 directed to a putatively intracellular RyR epitope expectedly did not stain live osteoclasts nor did it potentiate CaR activation. It did, however, stain fixed, permeabilized cells in a distinctive cytoplasmic pattern. We conclude that an RyR-like molecule resides within the osteoclast plasma membrane and plays in important role in extracellular Ca2+ sensing.

Calcium signal induced by mechanical perturbation of osteoclasts

Multinucleated osteoclasts from rabbit long bone, 1-6 days in culture, respond to mechanical perturbation with a transient increase of intracellular calcium concentration ([Ca2+]i), as measured with the fluorescent indicator fluo-3 on a confocal laser scanning microscope. In experiments with different extracellular calcium concentrations (from 11.8 mM to calcium-free), the incidence, the magnitude, and the duration of [Ca2+]i responses decreases with decreasing bathing [Ca2+]. Following mechanical perturbation, a thapsigargin-induced [Ca2+]i response has a lower magnitude than the thapsigargin-induced response without mechanical perturbation. In thapsigargin-pretreated osteoclasts the mechanical perturbation-induced rise in [Ca2+]i is larger and longer than in control cells. Ni2+ inhibits the incidence and decreases both the magnitude and the duration of the responses, while nifedipine, verapamil, and Gd3+ have no effect. These measurements show that rabbit osteoclasts transduce a mechanical perturbation of the cell membrane into a [Ca2+]i signal via both a calcium influx and an internal calcium release.

Calcium inflow in cells transfected with cloned rat and porcine calcitonin receptors

Ca2+ fluxes were examined in HEK 293 cells stably expressing the rat or porcine calcitonin receptors (CTRs). Calcitonin (CT) rapidly increased cytosolic Ca2+ ([Ca2+]i) concentrations in these cells in a manner which was sustained in the presence of extracellular Ca2+ ([Ca2+]e). In cells pretreated with CT, elevation of the [Ca2+]e concentration resulted in a further increase in [Ca2+]i which was concentration-dependent with respect to both the concentration of CT and the increment of [Ca2+]e. Untransfected cells, cells transfected with vector alone, and CTR-transfected cells not treated with CT, were unresponsive to [Ca2+]e. The microsomal Ca(2+)-ATPase inhibitor thapsigargin was able to mimic both the acute [Ca2+]i fluxes and responsiveness to [Ca2+]e mediated by CT in these cells. The CT-induced responsiveness to [Ca2+]e was neither mimicked by, nor affected by, activators of the cAMP or protein kinase C pathways. Treatment of cells with pertussis toxin influenced neither the primary Ca2+ fluxes in response to CT or thapsigargin nor the agonist-induced [Ca2+]e influx. Nifedipine failed to block responses to either CT or thapsigargin. These results lead to the important conclusion that the CTR participates in receptor-activated Ca2+ inflow, in which depletion of intracellular Ca2+ pools leads secondarily to influx of extracellular Ca2+.