Identification and function of type-2 and type-3 ryanodine receptors in gut epithelial cells

Characterization of ryanodine receptors in rat colonic epithelium

AIM

Functional evidence suggests the presence of two types of intracellular Ca(2+) channels responsible for the release of Ca(2+) from Ca(2+)-stores, i.e. inositol-1,4,5-trisphosphate (IP(3)R) and ryanodine receptors (RyR), in rat colonic epithelium. Generally, three ryanodine receptor isoforms (RyR1-RyR3) are known; however, the type of RyR at this epithelium is unknown and was the focus of the present study.

METHODS

RyRs were characterized by molecular biological and immunohistochemical methods in the rat colon.

RESULTS

A transcript of RyR1 was found in mRNA from colonic crypts. In contrast, RyR2 and RyR3 were found in their corresponding reference tissues, but not in the cDNA from colonic crypts suggesting a predominant expression of the RyR1 isoform in this epithelium. In order to characterize the subcellular localization of RyR1, immunohistochemical experiments were performed. They showed that RyR1 is present in the lamina epithelialis mucosae and smooth muscle cells and is distributed equally along the whole crypt axis with no difference between surface and crypt cells. A double staining with IP(3)R3, the dominant cytoplasmic isoform of IP3Rs in this epithelium, revealed that there is only little colocalization of the two receptor subtypes within the epithelial cells. Furthermore, the epithelium is equipped with the enzyme CD38 responsible for the production of cyclic adenosine diphosphate ribose, the physiological agonist of RyR. RyRs are known to be activated by changes in the redox state. The oxidant, monochloramine evoked a ruthenium red-sensitive Ca(2+) release all over the crypt axis. This release was unaffected by prior stimulation of IP(3) receptors with ATP (and vice versa).

CONCLUSION

The present data suggest a functional separation of IP(3)- and ryanodine receptor-carrying Ca(2+) stores in the colonic epithelium.

17beta-estradiol rapidly mobilizes intracellular calcium from ryanodine-receptor-gated stores via a PKC-PKA-Erk-dependent pathway in the human eccrine sweat gland cell line NCL-SG3

We describe a novel rapid non-genomic effect of 17beta-estradiol (E2) on intracellular Ca2+ ([Ca2+]i) signalling in the eccrine sweat gland epithelial cell line NCL-SG3. E2 had no observable effect on basal [Ca2+]i, however exposure of cells to E2 in the presence of the microsomal Ca2+ ATPase pump inhibitor, thapsigargin, produced a secondary, sustained increase in [Ca2+]i compared to thapsigargin treatment alone, where cells responded with a transient single spike-like increase in [Ca2+]i. The E2-induced increase in [Ca2+]i was not dependent on the presence of extracellular calcium and was completely abolished by ryanodine (100 microM). The estrogen receptor antagonist ICI 182,780 (1 microM) prevented the E2-induced effects suggesting a role for the estrogen receptor in the release of [Ca2+]i from ryanodine-receptor-gated stores. The E2-induced effect on [Ca2+]i could also be prevented by the protein kinase C delta (PKCdelta)-specific inhibitor rottlerin (10 microM), the protein kinase A (PKA) inhibitor Rp-adenosine 3',5'-cyclic monophosphorothioate (200 microM) and the MEK inhibitor PD98059 (10 microM). We established E2 rapidly activates the novel PKC isoform PKCepsilon, PKA and Erk 1/2 MAPK in a PKCdelta and estrogen-receptor-dependent manner. The E2-induced effect was specific to 17beta-estradiol, as other steroids had no effect on [Ca2+]i. We have demonstrated a novel mechanism by which E2 rapidly modulates [Ca2+]i release from ryanodine-receptor-gated intracellular Ca2+ stores. The signal transduction pathway involves the estrogen receptor coupled to a PKC-PKA-Erk 1/2 signalling pathway.

Protein 4.1N does not interact with the inositol 1,4,5-trisphosphate receptor in an epithelial cell line

Cytosolic Ca2+ regulates a variety of cell functions, and the spatial patterns of Ca2+ signals are responsible in part for the versatility of this second messenger. The subcellular distribution of the inositol 1,4,5-trisphosphate receptor (IP3R) is thought to regulate Ca2+-signaling patterns but little is known about how the distribution of the IP3R itself is regulated. Here we examined the relationship between the IP3R and the cytoskeletal linker protein 4.1N in the polarized WIF-B cell line because protein 4.1N regulates targeting of the type I IP3R in neurons, but WIF-B cells do not express this cytoskeletal protein. WIF-B cells expressed all three isoforms of the IP3R, and each isoform was distributed throughout the cell. These cells did not express the ryanodine receptor. Photorelease of microinjected, caged IP3 induced a rapid rise in cytosolic Ca2+, but the increase began uniformly throughout the cell rather than at a specific initiation site. Expression of protein 4.1N was not associated with redistribution of the IP3R or changes in Ca2+-signaling patterns. These findings are consistent with the hypothesis that the subcellular distribution of IP3R isoforms regulates the formation of Ca2+ waves, and the finding that interactions between protein 4.1N and the IP3R vary among cell types may provide an additional, tissue-specific mechanism to shape the pattern of Ca2+ waves.

Crosstalk between cAMP and Ca2+ signaling in non-excitable cells

An impressive array of cytosolic calcium ([Ca2+](i)) signals exert control over a broad range of physiological processes. The specificity and fidelity of these [Ca2+](i) signals is encoded by the frequency, amplitude, and sub-cellular localization of the response. It is believed that the distinct characteristics of [Ca2+](i) signals underlies the differential activation of effectors and ultimately cellular events. This "shaping" of [Ca2+](i) signals can be achieved by the influence of additional signaling pathways modulating the molecular machinery responsible for generating [Ca2+](i) signals. There is a particularly rich source of potential sites of crosstalk between the cAMP and the [Ca2+](i) signaling pathways. This review will focus on the predominant molecular loci at which these classical signaling systems interact to impact the spatio-temporal pattern of [Ca2+](i) signaling in non-excitable cells.

Regulation of Ca(2+) signaling in rat bile duct epithelia by inositol 1,4,5-trisphosphate receptor isoforms

Cytosolic Ca(2+) (Ca(i)(2+)) regulates secretion of bicarbonate and other ions in the cholangiocyte. In other cell types, this second messenger acts through Ca(2+) waves, Ca(2+) oscillations, and other subcellular Ca(2+) signaling patterns, but little is known about the subcellular organization of Ca(2+) signaling in cholangiocytes. Therefore, we examined Ca(2+) signaling and the subcellular distribution of Ca(2+) release channels in cholangiocytes and in a model cholangiocyte cell line. The expression and subcellular distribution of inositol 1,4,5-trisphosphate (InsP(3)) receptor (InsP(3)R) isoforms and the ryanodine receptor (RyR) were determined in cholangiocytes from normal rat liver and in the normal rat cholangiocyte (NRC) polarized bile duct cell line. Subcellular Ca(2+) signaling in cholangiocytes was examined by confocal microscopy. All 3 InsP(3)R isoforms were expressed in cholangiocytes, whereas RyR was not expressed. The type III InsP(3)R was the most heavily expressed isoform at the protein level and was concentrated apically, whereas the type I and type II isoforms were expressed more uniformly. The type III InsP(3)R was expressed even more heavily in NRC cells but was concentrated apically in these cells as well. Adenosine triphosphate (ATP), which increases Ca(2+) via InsP(3) in cholangiocytes, induced Ca(2+) oscillations in both cholangiocytes and NRC cells. Acetylcholine (ACh) induced apical-to-basal Ca(2+) waves. In conclusion, Ca(2+) signaling in cholangiocytes occurs as polarized Ca(2+) waves that begin in the region of the type III InsP(3)R. Differential subcellular localization of InsP(3)R isoforms may be an important molecular mechanism for the formation of Ca(2+) waves and oscillations in cholangiocytes. Because Ca(i)(2+) is in part responsible for regulating ductular secretion, these findings also may have implications for the molecular basis of cholestatic disorders.

Ca2+ waves require sequential activation of inositol trisphosphate receptors and ryanodine receptors in pancreatic acini

BACKGROUND & AIMS

The inositol 1,4,5-trisphosphate (InsP3) receptor (InsP3R) and the ryanodine receptor (RyR) are the principal Ca2+-release channels in cells and are believed to serve distinct roles in cytosolic Ca2+ (Ca(i)2+) signaling. This study investigated whether these receptors instead can release Ca2+ in a coordinated fashion.

METHODS

Apical and basolateral Ca(i)2+ signals were monitored in rat pancreatic acinar cells by time-lapse confocal microscopy. Caged forms of second messengers were microinjected into individual cells and then photoreleased in a controlled fashion by either UV or 2-photon flash photolysis.

RESULTS

InsP3 increased Ca(i)2+ primarily in the apical region of pancreatic acinar cells, whereas the RyR agonist cyclic adenosine diphosphate ribose (cADPR) increased Ca(i)2+ primarily in the basolateral region. Apical-to-basal Ca(i)2+ waves were induced by acetylcholine and initiation of these waves was blocked by the InsP3R inhibitor heparin, whereas propagation into the basolateral region was inhibited by the cADPR inhibitor 8-amino-cADPR. To examine integration of apical and basolateral Ca(i)2+ signals, Ca2+ was selectively released either apically or basolaterally using 2-photon flash photolysis. Ca(i)2+ increases were transient and localized in unstimulated cells. More complex Ca(i)2+ signaling patterns, including polarized Ca(i)2+ waves, were observed when Ca2+ was photoreleased in cells stimulated with subthreshold concentrations of acetylcholine.

CONCLUSIONS

Polarized Ca(i)2+ waves are induced in acinar cells by serial activation of apical InsP3Rs and then basolateral RyRs, and subcellular release of Ca2+ coordinates the actions of these 2 types of Ca2+ channels. This subcellular integration of Ca2+-release channels shows a new level of complexity in the formation of Ca(i)2+ waves.

Phosphorylation of inositol 1,4,5-trisphosphate receptors in parotid acinar cells. A mechanism for the synergistic effects of cAMP on Ca2+ signaling

Acetylcholine-evoked secretion from the parotid gland is substantially potentiated by cAMP-raising agonists. A potential locus for the action of cAMP is the intracellular signaling pathway resulting in elevated cytosolic calcium levels ([Ca(2+)](i)). This hypothesis was tested in mouse parotid acinar cells. Forskolin dramatically potentiated the carbachol-evoked increase in [Ca(2+)](i), converted oscillatory [Ca(2+)](i) changes into a sustained [Ca(2+)](i) increase, and caused subthreshold concentrations of carbachol to increase [Ca(2+)](i) measurably. This potentiation was found to be independent of Ca(2+) entry and inositol 1,4,5-trisphosphate (InsP(3)) production, suggesting that cAMP-mediated effects on Ca(2+) release was the major underlying mechanism. Consistent with this hypothesis, dibutyryl cAMP dramatically potentiated InsP(3)-evoked Ca(2+) release from streptolysin-O-permeabilized cells. Furthermore, type II InsP(3) receptors (InsP(3)R) were shown to be directly phosphorylated by a protein kinase A (PKA)-mediated mechanism after treatment with forskolin. In contrast, no evidence was obtained to support direct PKA-mediated activation of ryanodine receptors (RyRs). However, inhibition of RyRs in intact cells, demonstrated a role for RyRs in propagating Ca(2+) oscillations and amplifying potentiated Ca(2+) release from InsP(3)Rs. These data indicate that potentiation of Ca(2+) release is primarily the result of PKA-mediated phosphorylation of InsP(3)Rs, and may largely explain the synergistic relationship between cAMP-raising agonists and acetylcholine-evoked secretion in the parotid. In addition, this report supports the emerging consensus that phosphorylation at the level of the Ca(2+) release machinery is a broadly important mechanism by which cells can regulate Ca(2+)-mediated processes.

Evidence of functional ryanodine receptor involved in apoptosis of prostate cancer (LNCaP) cells

BACKGROUND

Very little is known about the functional expression and the physiological role of ryanodine receptors in nonexcitable cells, and in prostate cancer cells in particular. Nonetheless, different studies have demonstrated that calcium is a major factor involved in apoptosis. Therefore, the calcium-regulatory mechanisms, such as ryanodine-mediated calcium release, may play a substantial role in the regulation of apoptosis.

METHODS

We assessed the presence of such functional receptors in LNCaP prostate cancer cells, using fluorimetric measurements of intracellular calcium and expression assays of mRNA encoding ryanodine receptors.

RESULTS

We show here that LNCaP cells responded to caffeine, a ryanodine receptor agonist, by mobilizing calcium. Another ryanodine receptor agonist, 4-chloro-m-cresol, had a similar effect and promoted calcium release. These effects were inhibited by pretreatment with ryanodine or thapsigargin. In addition to a calcium release, caffeine was able to produce a calcium entry blocked by nickel. We used a reverse transcription-polymerase chain reaction assay to investigate the expression of ryanodine receptors in LNCaP cells. Two types of ryanodine receptor mRNAs were expressed in LNCaP cells: RyR1 and RyR2 mRNAs. Finally, we show that ryanodine receptor activation by caffeine slightly stimulates apoptosis of prostate cancer cells, and that the inhibition of these receptors by ryanodine protects the cells against apoptosis.

CONCLUSIONS

The combination of results showed that LNCaP cells, derived from a human prostate cancer, express functional RyRs able to mobilize Ca(2+) from intracellular stores and which might control apoptosis.

Expression and subcellular localization of the ryanodine receptor in rat pancreatic acinar cells

The ryanodine receptor (RyR) is the principal Ca2+-release channel in excitable cells, whereas the inositol 1,4,5-trisphosphate (InsP3) receptor (InsP3R) is primarily responsible for Ca2+ release in non-excitable cells, including epithelia. RyR also is expressed in a number of non-excitable cell types, but is thought to serve as an auxiliary or alternative Ca2+-release pathway in those cells. Here we use reverse transcription PCR to show that a polarized epithelium, the pancreatic acinar cell, expresses the type 2, but not the type 1 or 3, isoform of RyR. We furthermore use immunochemistry to demonstrate that the type 2 RyR is distributed throughout the basolateral and, to a lesser extent, the apical region of the acinar cell, but is excluded from the trigger zone, where cytosolic Ca2+ signals originate in this cell type. Since propagation of Ca2+ waves in acinar cells is sensitive to ryanodine, caffeine and Ca2+, these findings suggest that Ca2+ waves in this cell type result from the co-ordinated release of Ca2+, first from InsP3Rs in the trigger zone, then from RyRs elsewhere in the cell. RyR may play a fundamental role in Ca2+ signalling in polarized epithelia, including for Ca2+ signals initiated by InsP3.

Expression and subcellular localization of the ryanodine receptor in rat pancreatic acinar cells

The ryanodine receptor (RyR) is the principal Ca2+-release channel in excitable cells, whereas the inositol 1,4,5-trisphosphate (InsP3) receptor (InsP3R) is primarily responsible for Ca2+ release in non-excitable cells, including epithelia. RyR also is expressed in a number of non-excitable cell types, but is thought to serve as an auxiliary or alternative Ca2+-release pathway in those cells. Here we use reverse transcription PCR to show that a polarized epithelium, the pancreatic acinar cell, expresses the type 2, but not the type 1 or 3, isoform of RyR. We furthermore use immunochemistry to demonstrate that the type 2 RyR is distributed throughout the basolateral and, to a lesser extent, the apical region of the acinar cell, but is excluded from the trigger zone, where cytosolic Ca2+ signals originate in this cell type. Since propagation of Ca2+ waves in acinar cells is sensitive to ryanodine, caffeine and Ca2+, these findings suggest that Ca2+ waves in this cell type result from the co-ordinated release of Ca2+, first from InsP3Rs in the trigger zone, then from RyRs elsewhere in the cell. RyR may play a fundamental role in Ca2+ signalling in polarized epithelia, including for Ca2+ signals initiated by InsP3.

The structure, function, and cellular regulation of ryanodine-sensitive Ca2+ release channels

The fundamental biological process of Ca2+ signaling is known to be important in most eukaryotic cells, and inositol 1,2,5-trisphosphate and ryanodine receptors, intracellular Ca2+ release channels encoded by two distantly related gene families, are central to this phenomenon. Ryanodine receptors in the sarcoplasmic reticulum of skeletal and cardiac muscle have a predominant role in excitation-contraction coupling, but the channels are also present in the endoplasmic reticulum of noncontractile tissues including the central nervous system and the immune system. In all, three highly homologous ryanodine receptor isoforms have been identified, all very large proteins which assemble as (homo)tetramers of approximately 2 MDa. They contain large cytoplasmically disposed regulatory domains and are always associated with other structural or regulatory proteins, including calmodulin and immunophilins, which can have marked effects on channel function. The type 1 isoform in skeletal muscle is electromechanically coupled to surface membrane voltage sensors, whereas the remaining isoforms appear to be activated solely by endogenous cytoplasmic second messengers or other ligands, including Ca2+ itself ("Ca(2+)-induced Ca2+ release"). This review concentrates on ryanodine receptor structure-function relationships as probed by a variety of methods and on the molecular mechanisms of channel modulation at the cellular level (including evidence for the regulation of gene expression and transcription). It also touches on the relevance of ryanodine receptors to complex cellular functions and disease.

Purification and characterization of ryanodine receptor 3 from mammalian tissue

The ryanodine receptors are intracellular Ca2+ release channels that play a key role in cell signaling via Ca2+. There are three isoforms. Isoform 1 from skeletal muscle and isoform 2 from heart have been characterized. Isoform 3 is widely distributed in many mammalian tissues although in minuscule amounts. Its low abundance has hampered its study. We now describe methodology to isolate mammalian isoform 3 in amounts sufficient for biochemical and biophysical characterization. Bovine diaphragm sarcoplasmic reticulum fractions enriched in terminal cisternae containing both isoforms 1 (>95%) and 3 (<5% of the ryanodine binding) served as starting source. Isoform 3 was selectively immunoprecipitated from the 3-[(3-cholamidopropyl)-dimethylammonio]-1-propanesulfonic acid (CHAPS)-solubilized fraction and eluted with peptide epitope. Isoform 3 thus prepared is highly purified as characterized by SDS-polyacryamide gel electrophoresis, Coomassie Blue staining, and by high affinity ryanodine binding. The purified isoform 3 was incorporated into planar lipid bilayers, and its channel properties were studied. Channel characteristics in common with the other two isoforms are slope conductance, higher selectivity to Ca2+ versus K+ (PCa/K approximately 6), and response to drugs and ligands. In its response to Ca2+ and ATP, it more closely resembles isoform 2. The first two-dimensional structure of isoform 3 was obtained by cryoelectron microscopy and image enhancement techniques.

Spatial domains of Ca2+ signaling in secretory epithelial cells

Secretory epithelial cells are found in exocrine organs such as the pancreas and are also found in the lining of the lungs and gut. One important regulator of cell function in epithelial cells is the concentration of cytosolic Ca2+. The study of Ca2+ signaling in these cells has a long history and recent work has now identified, at the molecular level, key components in the Ca2+ signaling cascade. Furthermore, advances in fluorescent imaging techniques has enabled a detailed insight into the subcellular distribution of the agonist-evoked [Ca2+]i signal. A number of spatially different [Ca2+]i responses have been identified. Firstly, global [Ca2+]i signals are observed in response to high agonist concentrations. Secondly, at lower agonist concentrations trains of local [Ca2+]i spikes, restricted to the secretory pole region of pancreatic acinar cells, have been identified. Finally, these local [Ca2+]i spikes have now been further devolved into microdomains of [Ca2+]i elevation. The [Ca2+]i signal within a single microdomain has been shown to be the crucial trigger in the regulation of the ion channels important in fluid secretion.