Focal sarcoplasmic reticulum calcium stores and diffuse inositol 1,4,5-trisphosphate and ryanodine receptors in human myometrium

The myometrial transcriptome changes in mares with endometrosis

Mares with endometrosis exhibit histological changes not only in the endometrium but also in the myometrium that suggest possible functional impairment. The molecular background of these changes is not well understood. We hypothesize that the transcriptomic profile of the mare myometrium varies depending on the degree of endometrosis in mares. Myometria were collected from mares in the mid-luteal phase of the estrous cycle with endometrium categories I, IIA, IIB, and III (∑n = 23), according to Kenney and Doig´s histopathological classification. Myometrial RNA was isolated and subjected to RNA-seq analysis to identify differentially expressed transcriptionally active regions (deTARs) and their contribution to signaling pathways (KEGG database) and biological processes (GO terms). In results, 665, 491 and 499 deTARs were found in the myometrium of mares with endometrium IIA vs I, IIB vs I and III vs I, respectively. 200 common deTARs in the myometrium across all stages of endometrosis (IIA, IIB, and III) vs I were identified. Evaluated deTARs enriched several KEGG pathways including calcium signaling, cAMP signaling, oxytocin signaling, ECM-receptor interaction, and focal adhesion, and were classified into various GO terms including adaptive immune response, tissue homeostasis, muscle contractions, muscle development, and other. In conclusion, transcriptomic alterations in the myometrium of mares with endometrosis may indicate an impaired function of the contractile machinery, mechanisms regulating calcium influx and handling, as well as changes in ECM composition, leading to a decreased contractile activity and structural changes in the myometrium of affected mares.

Sarcoplasmic reticulum function in smooth muscle

The sarcoplasmic reticulum (SR) of smooth muscles presents many intriguing facets and questions concerning its roles, especially as these change with development, disease, and modulation of physiological activity. The SR's function was originally perceived to be synthetic and then that of a Ca store for the contractile proteins, acting as a Ca amplification mechanism as it does in striated muscles. Gradually, as investigators have struggled to find a convincing role for Ca-induced Ca release in many smooth muscles, a role in controlling excitability has emerged. This is the Ca spark/spontaneous transient outward current coupling mechanism which reduces excitability and limits contraction. Release of SR Ca occurs in response to inositol 1,4,5-trisphosphate, Ca, and nicotinic acid adenine dinucleotide phosphate, and depletion of SR Ca can initiate Ca entry, the mechanism of which is being investigated but seems to involve Stim and Orai as found in nonexcitable cells. The contribution of the elemental Ca signals from the SR, sparks and puffs, to global Ca signals, i.e., Ca waves and oscillations, is becoming clearer but is far from established. The dynamics of SR Ca release and uptake mechanisms are reviewed along with the control of luminal Ca. We review the growing list of the SR's functions that still includes Ca storage, contraction, and relaxation but has been expanded to encompass Ca homeostasis, generating local and global Ca signals, and contributing to cellular microdomains and signaling in other organelles, including mitochondria, lysosomes, and the nucleus. For an integrated approach, a review of aspects of the SR in health and disease and during development and aging are also included. While the sheer versatility of smooth muscle makes it foolish to have a "one model fits all" approach to this subject, we have tried to synthesize conclusions wherever possible.

A review of recent insights into the role of the sarcoplasmic reticulum and Ca entry in uterine smooth muscle

The uterine sacroplasmic reticulum (SR) takes up and stores calcium [Ca], using an ATPase (SERCA) and the Ca-buffering proteins, calsequestrin and calreticulin. This stored Ca can be released via IP(3)-gated Ca channels. Decreases in luminal Ca concentration [Ca] have been directly measured following agonist stimulation. During spontaneous contractions however, there appears to be no involvement of the SR, as Ca entry and efflux across the plasma membrane account for these phasic contractions. After over-viewing current knowledge concerning SR structure and function, we highlight three areas of research which suggest new ways of looking at the role of the SR in the uterus, although they may be controversial or speculative at the moment. Firstly, we review the evidence for the function, if any, of Ca-induced SR Ca release channels, the ryanodine receptor (RyR) and the lack of Ca sparks (the elemental release events from RyRs), in the uterus. Secondly, we ask does regulation of SERCA by the accessory protein, phospholamban, occur in the uterus and what is the effect of knocking out phospholamban on uterine activity? Thirdly, we address the question of when and how store-operated Ca entry occurs in the myometrium. By analogy with other, usually less excitable tissues, is there a mechanism that links store Ca depletion to plasma membrane Ca entry in smooth muscle cells within intact uterus and is it physiologically relevant and regulated? Are the recently described proteins ORAI and STIM-1 involved in uterine store-operated Ca entry? We end the review by integrating these new insights with previous data to present a new working model of the SR in the uterus.

Full length ryanodine receptor subtype 3 encodes spontaneous calcium oscillations in native duodenal smooth muscle cells

Two isoforms of the ryanodine receptor subtype 3 (RYR3) have been described in smooth muscle. The RYR3 short isoform (RYR3S) negatively regulates the calcium-induced calcium release mechanism encoded by the RYR2, whereas the role of the full length isoform of RYR3 (RYR3L) was still unclear. Here, we describe RYR-dependent spontaneous Ca(2+) oscillations measured in 10% of native duodenum myocytes. We investigated the role of RYR3 isoforms in these spontaneous Ca(2+) signals. Inhibition of RYR3S expression by antisense oligonucleotides revealed that both RYR2 and RYR3L were able to propagate spontaneous Ca(2+) waves that were distinguishable by frequency analysis. When RYR3L expression was inhibited, the spontaneous Ca(2+) oscillations were never observed, indicating that RYR3S inhibited the function of RYR2. RYR2 expression inhibition led to Ca(2+) oscillations identical to those observed in control cells suggesting that RYR3S did not functionally interact with RYR3L. The presence and frequency of RYR3L-dependent Ca(2+) oscillations were dependent on sarcoplasmic reticulum Ca(2+) content as revealed by long-term changes of the extracellular Ca(2+) concentration. Our study shows that, in native duodenal myocytes, the spontaneous Ca(2+) waves are encoded by the RYR3L alone, which activity is regulated by sarcoplasmic reticulum Ca(2+) loading.

Role of the calcium store in uterine contractility

This article assesses the nature of the sarcoplasmic reticulum (SR) in uterine smooth muscle. Modern imagining techniques have revealed new information about the location and density of Ca storage and release. Release mechanisms, including IP(3) and Ca itself, via ryanodine receptors (RyR), as well as possible roles for cyclic ADP ribose, and the contribution of the SR to relaxation are detailed. The role of the SR Ca-ATPase in both decay of the Ca transient and maintaining Ca homeostasis is reviewed. Recent data on the role of local Ca signals from the SR in contributing to membrane excitability and contractility are discussed, along with interactions with ion channels in lipid microdomains.

Complex effects of ryanodine on the sarcoplasmic reticulum Ca2+ levels in smooth muscle cells

We have studied the effects of ryanodine and inhibition of the sarco/endoplasmic reticulum Ca(2+) ATPase (SERCA) with thapsigargin, on both [Ca(2+)](i) and the sarcoplasmic reticulum (SR) Ca(2+) level during caffeine-induced Ca(2+) release in single smooth muscle cells. Incubation with 10 microM ryanodine did not inhibit the first caffeine-induced [Ca(2+)](i) response, although it abolished the [Ca(2+)](i) response to a second application of caffeine. To assess whether ryanodine was inducing a permanent depletion of the internal Ca(2+) stores, we measured the SR Ca(2+) level with Mag-Fura-2. The magnitude of the caffeine-induced reduction in the SR Ca(2+) level was not augmented by incubating cells with 1 microM ryanodine. Moreover, on removal of caffeine, the SR Ca(2+) levels partially recovered in 61% of the cells due to the activity of thapsigargin-sensitive SERCA pumps. Unexpectedly, 10 microM ryanodine instead of inducing complete depletion of SR Ca(2+) stores markedly reduced the caffeine-induced SR Ca(2+) response. It was necessary to previously inhibit SERCA pumps with thapsigargin for ryanodine to be able to induce caffeine-triggered permanent depletion of SR Ca(2+) stores. These data suggest that the effect of ryanodine on smooth muscle SR Ca(2+) stores was markedly affected by the activity of SERCA pumps. Our data highlight the importance of directly measuring SR Ca(2+) levels to determine the effect of ryanodine on the internal Ca(2+) stores.

Ciglitizone inhibits cell proliferation in human uterine leiomyoma via activation of store-operated Ca2+ channels

This study investigated the acute effects of a peroxisome proliferator-activated receptor (PPAR)-γ ligand, ciglitizone, on cell proliferation and intracellular Ca2+ signaling in human normal myometrium and uterine leiomyoma. Changes in intracellular Ca2+ concentration ([Ca2+]i) were measured with fura-2 AM, and cellular viabilities were determined by viable cell count and 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2H-tetrazolium bromide reduction assay. Ciglitizone (100 μM) induced greater inhibition of cell proliferation in uterine leiomyoma than in myometrium. Ciglitizone also dose-dependently increased [Ca2+]i in both myometrium and uterine leiomyoma; these [Ca2+]i increases were inhibited by PPAR-γ antagonists and raloxifene. Ciglitizone-induced [Ca2+]i increase showed only an initial peak in normal myometrial cells, whereas in uterine leiomyoma there was a second sustained [Ca2+]i increase as well. The initial [Ca2+]i increase in both myometrium and uterine leiomyoma resulted from the release of Ca2+ by the sarcoplasmic reticulum via activation of ryanodine receptors. The second [Ca2+]i increase was observed only in uterine leiomyoma because of a Ca2+ influx via an activation of store-operated Ca2+ channels (SOCCs). Cell proliferation was inhibited and secondary [Ca2+]i increase in uterine leiomyoma was attenuated by cotreatment of ciglitizone with a SOCC blocker, lanthanum. The results suggest that ciglitizone inhibits cell proliferation and increases [Ca2+]i through the activation of SOCCs, especially in human uterine leiomyoma.

Functional architecture of the SR calcium store in uterine smooth muscle

Sarcoplasmic reticulum (SR) is abundant in uterine smooth muscle cells. The functional role of this organelle in the regulation of uterine myocytes is not fully understood. The data available in the literature suggest that SR plays a dual role: as a source of calcium and as a calcium sink shaping calcium transients produced by membrane depolarisation and uterotonic agonists. Advances in digital imaging techniques including confocal microscopy of isolated living cells, and the development of methods for direct measurement of intraluminal calcium, has triggered a substantial increase in the number of publications elucidating the role of intracellular stores in calcium signalling. In this paper we review the literature and our own work on the SR calcium store in uterine smooth muscle cells.

The role of cyclic-ADP-ribose-signaling pathway in oxytocin-induced Ca2+ transients in human myometrium cells

Human myometrial contraction plays a fundamental role in labor. Dysfunction of uterine contraction is an important cause of labor progression failure. Although the mechanisms controlling uterine contraction are not completely understood, intracellular Ca2+ mobilization plays an important role during uterine contraction. Several mechanisms of intracellular Ca2+ mobilization are present in smooth muscle, but in the human uterus, only 1,4,5-trisphosphate-induced Ca2+ release has been studied extensively. Ryanodine receptor channels are present in myometrium. We determined the role of the cyclic ADP-ribose (cADPR)-signaling pathway in oxytocin-induced intracellular Ca2+ [(Ca2+)i] transients in human myometrial cells. We found that oxytocin-induced Ca2+ transient is dependent on several sources of Ca2+, including extracellular Ca2+ and intracellular Ca2+ stores. In addition, we found that both the 1,4,5-trisphosphate- and the cADPR-induced Ca2+ releasing systems are important for the induction of [Ca2+]i transients by oxytocin in human myometrial cells. Furthermore, we investigated TNFalpha regulation of oxytocin-induced [Ca2+]i transients, CD38 cyclase activity, and CD38 expression in human myometrial cells. We found that oxytocin-induced [Ca2+]i transients were significantly increased by 50 ng/ml TNF. Similarly, CD38 mRNA levels, CD38 expression, and cyclase activity were increased by TNFalpha, thus increasing cADPR levels. We propose that a complex interaction between multiple signaling pathways is important for the development of intracellular Ca2+ transients induced by oxytocin and that TNFalpha may contribute for the myometrium preparation for labor by regulating the cADPR-signaling pathway. The observation that the cADPR-signaling pathway is important for the development of intracellular Ca2+ transients in human myometrial cells raises the possibility that this signaling pathway could serve as a target for the development of new therapeutic strategies for abnormal myometrial contraction observed during pregnancy.

SERCA pump optimizes Ca2+ release by a mechanism independent of store filling in smooth muscle cells

Thapsigargin-sensitive sarco/endoplasmic reticulum Ca(2+) pumps (SERCAs) are involved in maintaining and replenishing agonist-sensitive internal stores. Although it has been assumed that release channels act independently of SERCA pumps, there are data suggesting the opposite. Our aim was to study the relationship between SERCA pumps and the release channels in smooth muscle cells. To this end, we have rapidly blocked SERCA pumps with thapsigargin, to avoid depletion of the internal Ca(2+) stores, and induced Ca(2+) release with either caffeine, to open ryanodine receptors, or acetylcholine, to open inositol 1,4,5-trisphosphate receptors. Blocking SERCA pumps produced smaller and slower agonist-induced [Ca(2+)](i) responses. We determined the Ca(2+) level of the internal stores both indirectly, measuring the frequency of spontaneous transient outward currents, and directly, using Mag-Fura-2, and demonstrated that the inhibition of SERCA pumps did not produce a reduction of the sarco/endoplasmic reticulum Ca(2+) levels to explain the decrease in the agonist-induced Ca(2+) responses. It appears that SERCA pumps are involved in sustaining agonist-induced Ca(2+) release by a mechanism that involves the modulation of Ca(2+) availability in the lumen of the internal stores.

The cyclic-ADP-ribose signaling pathway in human myometrium

Human myometrial contraction plays a fundamental role in labor. Dysfunction of uterine contraction is an important cause of failure in progression of labor. The mechanisms of control of uterine contractions are not completely understood. It appears that intracellular Ca(2+) mobilization may play an important role during uterine contraction. Several mechanisms of intracellular Ca(2+) mobilization have been described. However, in human uterus only the inositol 1,4,5-trisphosphate-induced Ca(2+) release has been extensively studied to date. In view of the identification of the presence of functional ryanodine channels in myometrium, we explored the role of the endogenous regulator of the ryanodine channel cyclic-ADP-ribose in human myometrial Ca(2+) regulation. Cyclic-ADP-ribose (cADPR) is a naturally occurring nucleotide implicated in the regulation of the gating properties of the ryanodine channel, in fact cADPR may be a second messenger that activates the ryanodine receptor. Here we explore the components of the cADPR system in human myometrium. We found that human myometrium contains all the components of the cADPR pathway including (1) cADPR-activated microsomal Ca(2+) release and (2) enzymes responsible for synthesis and degradation of cADPR and, furthermore, that intracellular levels of cADPR were detected in human myometrial tissue. These data indicate that the cADPR system is present and operational in human myometrial tissue. Further research is warranted to determine the role of this new signaling molecule in uterine contraction.

Structural complexity and functional diversity of endoplasmic reticulum Ca(2+) stores

Considerable evidence, including recent direct observations, suggest that endoplasmic reticulum (ER) Ca(2+) stores in neurons, glia, and other cell types, consists of spatially-distinct compartments that can be individually loaded and unloaded. In addition, sub-plasmalemmal ('junctional') components of the ER (jER) are functionally coupled to the overlying plasmalemmal (PL) microdomains in PL-jER units named 'PLasmERosomes'. The PL microdomains and the jER contain clusters of specific transport proteins that regulate Na(+) and Ca(2+) concentrations in the tiny cytosolic space between the PL and jER. This organization helps the ER to produce the many types of complex local and global Ca(2+) signals that are responsible for the simultaneous control of numerous neuronal and glial functions.

Differing mechanisms of inhibition of calcium increases in human uterine myocytes by indomethacin and nimesulide

OBJECTIVE

Indomethacin, an inhibitor of cyclooxygenase types 1 and 2, and nimesulide, a cyclooxygenase 2 selective inhibitor, are both well-known inhibitors of prostaglandin production. It has been assumed that the tocolytic mechanism of nimesulide and indomethacin is only through decreased prostaglandin production. The purpose of this study was to test the hypothesis that either nimesulide or indomethacin, or both, has a mechanism of action on human myocytes other than inhibition of prostaglandin production.

STUDY DESIGN

Human uterine myometrium was obtained from consenting patients during cesarean deliveries. Myocytes were cultured, plated, and loaded with a calcium-dependent fluorescent dye, calcium green 1. The relative concentrations of intracellular free calcium were determined by measurement of time-dependent fluorescence changes by means of a video fluorimeter. In all experiments, cells were stimulated with prostaglandin F2alpha, 30 micromol/L. Experiments were performed without pretreatment (control) or with pretreatment consisting of indomethacin, 10 micromol/L, or nimesulide, 30 micromol/L. The percentages of cells demonstrating calcium increases were counted and compared by means of the Fisher exact test. A P value =.05 was considered significant.

RESULTS

After prostaglandin F2alpha exposure, 33% of cells showed an increase in intracellular free calcium under control conditions. When pretreated with nimesulide, 39% of cells responded to prostaglandin F2alpha (P =.59). When pretreated with indomethacin, only 16% of cells responded to prostaglandin F2alpha (P =.019).

CONCLUSIONS

Pretreatment with nimesulide failed to reduce the fraction of cells that responded to prostaglandin F2alpha. In contrast, pretreatment with indomethacin significantly reduced the fraction of responding cells. These data suggest that, in vitro, indomethacin exhibits a mechanism of tocolysis other than inhibition of prostaglandin synthesis.

The mechanism of propagation of intracellular calcium waves in cultured human uterine myocytes

OBJECTIVE

The primary goal of this work was to determine the relative importance of sarcoplasmic reticulum inositol 1,4,5-triphosphate receptors and ryanodine receptors in the mechanism of intracellular calcium wave propagation in human uterine myocytes. A secondary goal was to identify the rate-determining step of calcium wave propagation.

STUDY DESIGN

Pregnant human myometrium was obtained at the time of cesarean delivery, enzymatically dispersed, and cultured through several passages. Intracellular calcium wave velocities were measured with video fluorescence microscopy and the calcium-dependent fluorescent dye calcium green 1. Experimental conditions were modified by exposure of the cells to ruthenium red (blocked ryanodine receptor), ryanodine (locked open ryanodine receptor), oxytocin (increased inositol-1,4,5-triphosphate), sodium butyrate (intracellular acidification), ammonium chloride (intracellular alkalinization), and elevation of temperature (from 19 degrees C to 30 degrees C).

RESULTS

Wave velocities were found to be the same for spontaneously occurring (9.6 +/- 2.6 microm/s) and oxytocin-stimulated (10.3 +/- 3.4 microm/s) waves. Advance treatment of the cells with ryanodine or ruthenium red failed to change oxytocin-stimulated wave velocities from control values. The temperature dependence of calcium wave velocities was studied across the range 19 degrees C to 30 degrees C. Plots of wave velocities versus the inverse of the temperature yielded apparent activation energies that were the same for spontaneous (13.2 +/- 0.3 kcal/mol) and oxytocin-induced (14.3 +/- 1.6 kcal/mol) waves. After intracellular acidification by treatment with butyrate (20 mmol/L) wave velocities increased by 44%. Wave velocities decreased by 35% after treatment with ammonium chloride (20 mmol/L).

CONCLUSION

Propagation of intracellular calcium waves in cultured human uterine myocytes exhibited mechanisms of sarcoplasmic reticulum calcium release that could use either inositol 1,4,5-triphosphate receptors alone or ryanodine receptors alone, or both together. The rate-determining step for calcium wave propagation was diffusion of calcium though a highly buffered cytoplasm.

Simultaneous measurements of changes in sarcoplasmic reticulum and cytosolic

1. The role of the sarcoplasmic reticulum (SR) was investigated in spontaneous and agonist-induced uterine Ca2+ transients, by combining low- (mag-fluo-4) and high-affinity (fura-2) indicators to measure intraluminal SR ([Ca2+]L) and cytosolic ([Ca2+]i) calcium concentration, simultaneously, in single smooth muscle cells from pregnant rat uterus. 2. Carbachol or ATP, in the absence of extracellular Ca2+, decreased [Ca2+]L and increased [Ca2+]i. Although some replenishment (around 50 %) occurred in its absence, extracellular Ca2+ was required for full replenishment of the SR Ca2+. 3. In 4/15 cells, ATP evoked oscillations of [Ca2+]i. These were accompanied by successive release and re-uptake of SR Ca2+. Inhibition of the SR Ca2+-ATPase with thapsigargin abolished the oscillations and luminal changes. 4. Spontaneous [Ca2+]i transients produced no detectable changes in [Ca2+]L. The larger [Ca2+]i transients evoked by high-K+ depolarisation increased [Ca2+]L. Spontaneous activity was inhibited when [Ca2+]L was increased. 5. These data show that it is possible to simultaneously measure SR and cytosolic [Ca2+], and to investigate their response to agonist application and spontaneous activity.

Intracellular calcium gradients in cultured human uterine smooth muscle: a functionally important subplasmalemmal space

The plasma membrane contains the key elements for the control of coupling excitation to contraction in smooth muscle. The superficial calcium buffer barrier, initially proposed by van Breemen for vascular smooth muscle, may participate in the regulation of calcium entry in other smooth muscle types. To investigate the relationship between the sarcoplasmic reticulum (SR) and the plasma membrane in myometrial smooth muscle cells, we performed experiments using videofluorescence imaging and cell-attached electrophysiology. The cell-attached patch was used as a reporter for the free calcium in the subplasmalemmal space by monitoring openings of the Maxi-K channel. Calcium green-1 was used to simultaneously monitor changes of the deep cytosolic calcium concentrations. The cell with the patch attached was stimulated via an intercellular calcium wave from an adjacent cell. In this fashion, release of SR calcium was accomplished with minimal disturbance of the plasma membrane and the subplasmalemmal space of the cell studied. With physiological bathing solution, six of seven calcium waves activated Maxi-K channels. Surprisingly, the Maxi-K channels began opening 6.3 +/- 4.7s (range 2.6-15.0s) after the wave passed the pipette location. When plasma membrane calcium fluxes were inhibited with 100 microM lanthanum, no Maxi-K channel openings were observed in six of seven experiments. These results are best explained by a subplasmalemmal space in which the calcium concentration is largely controlled by store-operated channels. These results suggest the superficial buffer barrier as merely one aspect of subplasmalemmal regulation of calcium dynamics, and emphasize the importance of store-operated calcium channels during dynamic calcium changes.