Sphingosine kinase 1 overexpression induces MFN2 fragmentation and alters mitochondrial matrix Ca2+ handling in HeLa cells.

Sphingosine kinase 1 (SK1) converts sphingosine to the bioactive lipid sphingosine 1-phosphate (S1P). S1P binds to G-protein-coupled receptors (S1PR1-5) to regulate cellular events, including Ca2+ signaling. The SK1/S1P axis and Ca2+ signaling both play important roles in health and disease. In this respect, Ca2+ microdomains at the mitochondria-associated endoplasmic reticulum (ER) membranes (MAMs) are of importance in oncogenesis. Mitofusin 2 (MFN2) modulates ER-mitochondria contacts, and dysregulation of MFN2 is associated with malignancies. We show that overexpression of SK1 augments agonist-induced Ca2+ release from the ER resulting in increased mitochondrial matrix Ca2+. Also, overexpression of SK1 induces MFN2 fragmentation, likely through increased calpain activity. Further, expressing putative calpain-cleaved MFN2 N- and C-terminal fragments increases mitochondrial matrix Ca2+ during agonist stimulation, mimicking the SK1 overexpression in cells. Moreover, SK1 overexpression enhances cellular respiration and cell migration. Thus, SK1 regulates MFN2 fragmentation resulting in increased mitochondrial Ca2+ and downstream cellular effects.

Bcl-2 proteins and calcium signaling: complexity beneath the surface.

Antiapoptotic Bcl-2-family members are well known for their 'mitochondrial' functions as critical neutralizers of proapoptotic Bcl-2-family members, including the executioner multidomain proteins Bax and Bak and the BH3-only proteins. It has been clear for more than 20 years that Bcl-2 proteins can impact intracellular Ca(2+) homeostasis and dynamics. Moreover, altered Ca(2+) signaling is increasingly linked to oncogenic behavior. Specifically targeting the Ca(2+)-signaling machinery may thus prove to be a valuable strategy for cancer treatment. Over 10 years ago a major controversy was recognized concerning whether or not Bcl-2 proteins exerted their antiapoptotic functions via Ca(2+) signaling through lowering the filling state of the endoplasmic reticulum (ER) Ca(2+) stores or by suppressing Ca(2+) release from the ER without affecting the filling state of this Ca(2+) store. Further research from different laboratories indicated a wide variety of mechanisms by which Bcl-2-family members can impact Ca(2+) signaling. In this review, we propose that antiapoptotic Bcl-2-family members are multimodal regulators of intracellular Ca(2+)-signaling events in cell survival and cell death. We will discuss how different Bcl-2-family members impact cell survival and cell death by regulating Ca(2+) transport systems at the ER, mitochondria and plasma membrane and by impacting the organization of organelles and how these insights can be exploited for causing cell death in cancer cells. Finally, we propose that the existing controversy reflects the diversity of links between Bcl-2 proteins and Ca(2+) signaling, as certainly not all targets or mechanisms will be operative in every cell type and every condition.

BAX inhibitor-1 is a Ca(2+) channel critically important for immune cell function and survival.

The endoplasmic reticulum (ER) serves as the major intracellular Ca(2+) store and has a role in the synthesis and folding of proteins. BAX (BCL2-associated X protein) inhibitor-1 (BI-1) is a Ca(2+) leak channel also implicated in the response against protein misfolding, thereby connecting the Ca(2+) store and protein-folding functions of the ER. We found that BI-1-deficient mice suffer from leukopenia and erythrocytosis, have an increased number of splenic marginal zone B cells and higher abundance and nuclear translocation of NF-κB (nuclear factor-κ light-chain enhancer of activated B cells) proteins, correlating with increased cytosolic and ER Ca(2+) levels. When put into culture, purified knockout T cells and even more so B cells die spontaneously. This is preceded by increased activity of the mitochondrial initiator caspase-9 and correlated with a significant surge in mitochondrial Ca(2+) levels, suggesting an exhausted mitochondrial Ca(2+) buffer capacity as the underlying cause for cell death in vitro. In vivo, T-cell-dependent experimental autoimmune encephalomyelitis and B-cell-dependent antibody production are attenuated, corroborating the ex vivo results. These results suggest that BI-1 has a major role in the functioning of the adaptive immune system by regulating intracellular Ca(2+) homeostasis in lymphocytes.

IP3R2 levels dictate the apoptotic sensitivity of diffuse large B-cell lymphoma cells to an IP3R-derived peptide targeting the BH4 domain of Bcl-2.

Disrupting inositol 1,4,5-trisphosphate (IP3) receptor (IP3R)/B-cell lymphoma 2 (Bcl-2) complexes using a cell-permeable peptide (stabilized TAT-fused IP3R-derived peptide (TAT-IDP(S))) that selectively targets the BH4 domain of Bcl-2 but not that of B-cell lymphoma 2-extra large (Bcl-Xl) potentiated pro-apoptotic Ca(2+) signaling in chronic lymphocytic leukemia cells. However, the molecular mechanisms rendering cancer cells but not normal cells particularly sensitive to disrupting IP3R/Bcl-2 complexes are poorly understood. Therefore, we studied the effect of TAT-IDP(S) in a more heterogeneous Bcl-2-dependent cancer model using a set of 'primed to death' diffuse large B-cell lymphoma (DL-BCL) cell lines containing elevated Bcl-2 levels. We discovered a large heterogeneity in the apoptotic responses of these cells to TAT-IDP(S) with SU-DHL-4 being most sensitive and OCI-LY-1 being most resistant. This sensitivity strongly correlated with the ability of TAT-IDP(S) to promote IP3R-mediated Ca(2+) release. Although total IP3R-expression levels were very similar among SU-DHL-4 and OCI-LY-1, we discovered that the IP3R2-protein level was the highest for SU-DHL-4 and the lowest for OCI-LY-1. Strikingly, TAT-IDP(S)-induced Ca(2+) rise and apoptosis in the different DL-BCL cell lines strongly correlated with their IP3R2-protein level, but not with IP3R1-, IP3R3- or total IP3R-expression levels. Inhibiting or knocking down IP3R2 activity in SU-DHL-4-reduced TAT-IDP(S)-induced apoptosis, which is compatible with its ability to dissociate Bcl-2 from IP3R2 and to promote IP3-induced pro-apoptotic Ca(2+) signaling. Thus, certain chronically activated B-cell lymphoma cells are addicted to high Bcl-2 levels for their survival not only to neutralize pro-apoptotic Bcl-2-family members but also to suppress IP3R hyperactivity. In particular, cancer cells expressing high levels of IP3R2 are addicted to IP3R/Bcl-2 complex formation and disruption of these complexes using peptide tools results in pro-apoptotic Ca(2+) signaling and cell death.

Polycystin-1 and polycystin-2 are both required to amplify inositol-trisphosphate-induced Ca2+ release.

Autosomal dominant polycystic kidney disease is caused by loss-of-function mutations in the PKD1 or PKD2 genes encoding respectively polycystin-1 and polycystin-2. Polycystin-2 stimulates the inositol trisphosphate (IP(3)) receptor (IP(3)R), a Ca(2+)-release channel in the endoplasmic reticulum (ER). The effect of ER-located polycystin-1 is less clear. Polycystin-1 has been reported both to stimulate and to inhibit the IP(3)R. We now studied the effect of polycystin-1 and of polycystin-2 on the IP(3)R activity under conditions where the cytosolic Ca(2+) concentration was kept constant and the reuptake of released Ca(2+) was prevented. We also studied the interdependence of the interaction of polycystin-1 and polycystin-2 with the IP(3)R. The experiments were done in conditionally immortalized human proximal-tubule epithelial cells in which one or both polycystins were knocked down using lentiviral vectors containing miRNA-based short hairpins. The Ca(2+) release was induced in plasma membrane-permeabilized cells by various IP(3) concentrations at a fixed Ca(2+) concentration under unidirectional (45)Ca(2+)-efflux conditions. We now report that knock down of polycystin-1 or of polycystin-2 inhibited the IP(3)-induced Ca(2+) release. The simultaneous presence of the two polycystins was required to fully amplify the IP(3)-induced Ca(2+) release, since the presence of polycystin-1 alone or of polycystin-2 alone did not result in an increased Ca(2+) release. These novel findings indicate that ER-located polycystin-1 and polycystin-2 operate as a functional complex. They are compatible with the view that loss-of-function mutations in PKD1 and in PKD2 both cause autosomal dominant polycystic kidney disease.

Selective regulation of IP3-receptor-mediated Ca2+ signaling and apoptosis by the BH4 domain of Bcl-2 versus Bcl-Xl.

Antiapoptotic B-cell lymphoma 2 (Bcl-2) targets the inositol 1,4,5-trisphosphate receptor (IP(3)R) via its BH4 domain, thereby suppressing IP(3)R Ca(2+)-flux properties and protecting against Ca(2+)-dependent apoptosis. Here, we directly compared IP(3)R inhibition by BH4-Bcl-2 and BH4-Bcl-Xl. In contrast to BH4-Bcl-2, BH4-Bcl-Xl neither bound the modulatory domain of IP(3)R nor inhibited IP(3)-induced Ca(2+) release (IICR) in permeabilized and intact cells. We identified a critical residue in BH4-Bcl-2 (Lys17) not conserved in BH4-Bcl-Xl (Asp11). Changing Lys17 into Asp in BH4-Bcl-2 completely abolished its IP(3)R-binding and -inhibitory properties, whereas changing Asp11 into Lys in BH4-Bcl-Xl induced IP(3)R binding and inhibition. This difference in IP(3)R regulation between BH4-Bcl-2 and BH4-Bcl-Xl controls their antiapoptotic action. Although both BH4-Bcl-2 and BH4-Bcl-Xl had antiapoptotic activity, BH4-Bcl-2 was more potent than BH4-Bcl-Xl. The effect of BH4-Bcl-2, but not of BH4-Bcl-Xl, depended on its binding to IP(3)Rs. In agreement with the IP(3)R-binding properties, the antiapoptotic activity of BH4-Bcl-2 and BH4-Bcl-Xl was modulated by the Lys/Asp substitutions. Changing Lys17 into Asp in full-length Bcl-2 significantly decreased its binding to the IP(3)R, its ability to inhibit IICR and its protection against apoptotic stimuli. A single amino-acid difference between BH4-Bcl-2 and BH4-Bcl-Xl therefore underlies differential regulation of IP(3)Rs and Ca(2+)-driven apoptosis by these functional domains. Mutating this residue affects the function of Bcl-2 in Ca(2+) signaling and apoptosis.

Cytosolic Ca2+ signals depending on the functional state of the Golgi in HeLa cells.

The Golgi apparatus is, like the endoplasmic reticulum, an inositol-1,4,5-trisphosphate-sensitive Ca2+ store, but its role in setting up Ca2+ signals is not well understood. We have now measured histamine-induced Ca2+ signals in HeLa cells pretreated with brefeldin A, a fungal metabolite that leads to the fragmentation and subsequent disappearance of the Golgi apparatus by its reabsorption within the endoplasmic reticulum. Ca2+ responses in which the free cytoplasmic Ca2+ concentration returned to resting levels during the histamine stimulation (mainly baseline Ca2+ oscillations or a single Ca2+ peak) occurred more often in brefeldin A pretreated cells, resulting in a lower Ca2+ plateau in population measurements. The latencies before the onset of the Ca2+ signals were longer after brefeldin A pretreatment. These results suggest that the integrity of the Golgi apparatus contributes to the shaping of intracellular Ca2+ signals.

Receptor-operated Ca2+ entry mediated by TRPC3/TRPC6 proteins in rat prostate smooth muscle (PS1) cell line.

Prostate smooth muscle cells predominantly express alpha1-adrenoceptors (alpha1-AR). alpha1-AR antagonists induce prostate smooth muscle relaxation and therefore they are useful therapeutic compounds for the treatment of benign prostatic hyperplasia symptoms. However, the Ca(2+) entry pathways associated with the activation of alpha1-AR in the prostate have yet to be elucidated. In many cell types, mammalian homologues of transient receptor potential (TRP) genes, first identified in Drosophila, encode TRPC (canonical TRP) proteins. They function as receptor-operated channels (ROCs) which are involved in various physiological processes such as contraction, proliferation, apoptosis, and differentiation. To date, the expression and function of TRPC channels have not been studied in prostate smooth muscle. In fura-2 loaded PS1 (a prostate smooth muscle cell line) which express endogenous alpha1A-ARs, alpha-agonists epinephrine (EPI), and phenylephrine (PHE) induced Ca(2+) influx which depended on the extracellular Ca(2+) and PLC activation but was independent of PKC activation. Thus, we have tested two membrane-permeable analogues of diacylglycerol (DAG), oleoyl-acyl-sn-glycerol (OAG) and 1,2-dioctanoyl-sn-glycerol (DOG). They initiated Ca(2+) influx whose properties were similar to those induced by the alpha-agonists. Sensitivity to 2-aminoethyl diphenylborate (2-APB), SKF-96365 and flufenamate implies that Ca(2+)-permeable channels mediated both alpha-agonist- and OAG-evoked Ca(2+) influx. Following the sarcoplasmic reticulum (SR) Ca(2+) store depletion by thapsigargin (Tg), a SERCA inhibitor, OAG and PHE were both still able to activate Ca(2+) influx. However, OAG failed to enhance Ca(2+) influx when added in the presence of an alpha-agonist. RT-PCR and Western blotting performed on PS1 cells revealed the presence of mRNAs and the corresponding TRPC3 and TRPC6 proteins. Experiments using an antisense strategy showed that both alpha-agonist- and OAG-induced Ca(2+) influx required TRPC3 and TRPC6, whereas the Tg-activated ("capacitative") Ca(2+) entry involved only TRPC3 encoded protein. It may be thus concluded that PS1 cells express TRPC3 and TRPC6 proteins which function as receptor- and store-operated Ca(2+) entry pathways.

[Intracellular calcium-releasing channels as cellular targets for immunophilins: a molecular, functional and structural analysis]. / Intracellulaire calcium-vrijzettingskanalen als cellulaire doelwitten voor immunofilines: een moleculaire, functionele en structurele analyse.

In this study, the FKBP12-binding properties of IP3Rs and RyRs were compared. Although the primary sequence of IP3Rs en RyRs contained a putative FKBP12-binding site, the functional, molecular and structural properties of these sites appeared to be completely different. For RyRs, FKBPs appear to function as associated proteins that are important for the functional regulation of the channel, thereby stabilizing the RyR complex. For IP3Rs, FKBPs might be involved in the de novo protein synthesis of the IP3Rs and the folding of the peptide chain to a functional IP3R protein, thereby functioning as helper enzymes. Hence, it is very unlikely that they function as associated regulatory proteins of the IP3R. In addition, we provided evidence that FKBP 12 is also an important regulating protein of the Ca(2+)-flux properties of the RyR3. FKBP12 clearly modulated both RyR3-mediated global and local Ca(2+)-responses.

Calcium release from the Golgi apparatus and the endoplasmic reticulum in HeLa cells stably expressing targeted aequorin to these compartments.

Extracellular agonists mobilize Ca2+ from SERCA-comprising intracellular Ca2+ stores located in both the Golgi apparatus and the endoplasmic reticulum. Ca2+ release from both these compartments was studied in HeLa cells stably expressing the luminescent Ca2+ indicator aequorin specifically targeted to these compartments. Changes in lumenal [Ca2+] as detected by the aequorin measurements were correlated with parallel changes in total Ca2+ content of the stores. The latencies and initial rates of Ca2+ release from the Golgi apparatus and the endoplasmic reticulum were quite similar. However, maximal Ca2+ release measured with Golgi-targeted aequorin terminated faster than that from the endoplasmic reticulum. The rate and extent of Ca2+ depletion from both compartments correlated well with the peak amplitude of the cytosolic [Ca2+] rise. Time-course experiments further revealed that the peak of the cytosolic Ca2+ response occurred before the lumenal [Ca2+] reached its lowest level. We conclude that both the Golgi apparatus and the endoplasmic reticulum contribute to the rise in cytosolic [Ca2+] upon agonist stimulation, but the kinetics of the Ca2+ release are different.

Inositol trisphosphate producing agonists do not mobilize the thapsigargin-insensitive part of the endoplasmic-reticulum and Golgi Ca2+ store.

Non-mitochondrial intracellular Ca2+ stores contain both thapsigargin-sensitive sarco(endo)plasmic-reticulum Ca2+-ATPases (SERCA) and thapsigargin-insensitive secretory-pathway Ca2+-ATPases (SPCA1). We now have studied the Ca2+-release properties of the compartments associated with these pumps in intact, i.e. non-permeabilized, cells of different origin (HeLa, keratinocytes, 16HBE14o-, COS-1, A7r5) and with different approaches (45Ca2+ fluxes, Ca2+ imaging and measurements of the free luminal [Ca2+] in the endoplasmic-reticulum and the Golgi apparatus using targeted aequorin). Application of an extracellular agonist in the absence of thapsigargin induced in all cells a Ca2+ release from both the endoplasmic-reticulum and the Golgi apparatus. The agonists were not able to release Ca2+ in the presence of 10 microM thapsigargin, except in COS-1 cells overexpressing SPCA1, where this pump not only appeared in the Golgi compartment but also overflowed into the agonist-sensitive part of the endoplasmic-reticulum. We conclude that the subcompartments of the endoplasmic-reticulum and of the Golgi complex that endogenously express SPCA1 are insensitive to agonist stimulation.

Calcineurin and intracellular Ca2+-release channels: regulation or association?

The Ca(2+)- and calmodulin-dependent phosphatase calcineurin was reported to interact with the inositol 1,4,5-trisphosphate receptor (IP(3)R) and the ryanodine receptor (RyR) and to modulate their phosphorylation status and activity. However, controversial data on the molecular mechanisms involved and on the functional relevance of calcineurin for these channel-complexes have been described. Hence, we will focus on the functional importance of calcineurin for IP(3)R and RyR function and on the different mechanisms by which Ca(2+)-dependent dephosphorylation can affect the gating of those intracellular Ca(2+)-release channels. Since many studies made use of immunosuppressive drugs that are inhibiting calcineurin activity, we will also have to take the different side effects of these drugs into account for the proper interpretation of the effects of calcineurin on intracellular Ca(2+)-release channels. In addition, it became recently known that various other phosphatases and kinases can associate with these channels, thereby forming macromolecular complexes. The relevance of these enzymes for IP(3)R and RyR functioning will be reviewed since in some cases they could interfere with the effects ascribed to calcineurin. Finally, we will discuss the downstream effects of calcineurin on the regulation of the expression levels of intracellular Ca(2+)-release channels as well as the relation between IP(3)R- and RyR-mediated Ca(2+) release and calcineurin-dependent gene expression.

Similar Ca(2+)-signaling properties in keratinocytes and in COS-1 cells overexpressing the secretory-pathway Ca(2+)-ATPase SPCA1.

Mutations in the ubiquitously expressed secretory-pathway Ca(2+)-ATPase (SPCA1) Ca(2+) pump result in Hailey-Hailey disease, which almost exclusively affects the epidermal part of the skin. We have studied Ca(2+) signaling in human keratinocytes by measuring the free Ca(2+) concentration in the cytoplasm and in the lumen of both the Golgi apparatus and the endoplasmic reticulum. These signals were compared with those recorded in SPCA1-overexpressing and control COS-1 cells. Both the sarco(endo)plasmic-reticulum Ca(2+)-ATPase (SERCA) and SPCA1 can mediate Ca(2+) uptake into the Golgi stacks. Our results indicate that keratinocytes mainly used the SPCA1 Ca(2+) pump to load the Golgi complex with Ca(2+) whereas the SERCA Ca(2+) pump was mainly used in control COS-1 cells. Cytosolic Ca(2+) signals in keratinocytes induced by extracellular ATP or capacitative Ca(2+) entry were characterized by an unusually long latency reflecting extra Ca(2+) buffering by an SPCA1-containing Ca(2+) store, similarly as in SPCA1-overexpressing COS-1 cells. Removal of extracellular Ca(2+) elicited spontaneous cytosolic Ca(2+) transients in keratinocytes, similarly as in SPCA1-overexpressing COS-1 cells. With respect to Ca(2+) signaling keratinocytes and SPCA1-overexpressing COS-1 cells therefore behaved similarly but differed from control COS-1 cells. The relatively large contribution of the SPCA1 pumps for loading the Golgi stores with Ca(2+) in keratinocytes may, at least partially, explain why mutations in the SPCA1 gene preferentially affect the skin in Hailey-Hailey patients.

Mechanisms of ATP-induced calcium signaling and growth arrest in human prostate cancer cells.

This study investigates the calcium mechanisms involved in growth arrest induced by extracellular ATP in DU-145 androgen-independent human prostate cancer cells. Exposure of DU-145 cells to 100 microM ATP produced an increase in cytoplasmic calcium concentration ([Ca(2+)](i)), due to a mobilization of calcium from the endoplasmic reticulum stores and to subsequent capacitative calcium entry (CCE). We have shown that this [Ca(2+)](i) increase occurs after stimulation by ATP of the phospholipase C (PLC) pathway. For the first time, we have identified the inositol 1,4,5-trisphosphate receptor (IP(3)R) isoforms expressed in this cell line and have demonstrated a participation of protein kinase C in CCE. Using fluorescence imaging, we have shown that a long-term treatment with ATP leads to a decrease in the intraluminal endoplasmic reticulum calcium concentration as well as in the amount of releasable Ca(2+). Modulating extracellular free calcium concentrations indicated that variations in [Ca(2+)](i) did not affect the ATP-induced growth arrest of DU-145 cells. However, treating cells with 1 nM thapsigargin (TG) to deplete intracellular calcium pools prevented the growth arrest induced by ATP. Altogether, these results indicate that growth arrest induced in DU-145 cells by extracellular ATP is not correlated with an increase in [Ca(2+)](i) but rather with a decrease in intracellular calcium pool content.

Pharmacology of inositol trisphosphate receptors.

In almost all cells, cytosolic Ca(2+) is a crucial intracellular messenger, regulating many cellular processes. In non-excitable as well as in some excitable cells, Ca(2+) release from the intracellular stores into the cytoplasm is primarily initiated by the second messenger inositol 1,4,5-trisphosphate (IP(3)), which interacts with the IP(3) receptor (IP(3)R), a tetrameric intracellular Ca(2+)-release channel. This review focuses on the pharmacological modulation of the various functionally important sub-domains of the IP(3)R, including the IP(3)-binding domain, calmodulin-binding sites, adenine nucleotide-binding sites and the sites for interaction for FK506-binding proteins and other regulators. We will particularly focus on the pharmacological tools that interfere with these domains and discuss their relative specificity for the IP(3)R, thereby indicating their potential usefulness for unraveling the complex functional regulation of the IP(3)R.

Homer proteins and InsP(3) receptors co-localise in the longitudinal sarcoplasmic reticulum of skeletal muscle fibres.

Striated muscle represents one of the best models for studies on Ca(2+) signalling. However, although much is known on the localisation and molecular interactions of the ryanodine receptors (RyRs), far less is known on the localisation and on the molecular interactions of the inositol trisphosphate receptors (InsP(3)Rs) in striated muscle cells. Recently, members of the Homer protein family have been shown to cluster type 1 metabotropic glutamate receptors (mGluR1) in the plasma membrane and to interact with InsP(3)R in the endoplasmic reticulum of neurons. Thus, these scaffolding proteins are good candidates for organising plasma membrane receptors and intracellular effector proteins in signalosomes involved in intracellular Ca(2+) signalling. Homer proteins are also expressed in skeletal muscle, and the type 1 ryanodine receptor (RyR1) contains a specific Homer-binding motif. We report here on the relative sub-cellular localisation of InsP(3)Rs and Homer proteins in skeletal muscle cells with respect to the localisation of RyRs. Immunofluorescence analysis showed that both Homer and InsP(3)R proteins present a staining pattern indicative of a localisation at the Z-line, clearly distinct from that of RyR1. Consistent herewith, in sub-cellular fractionation experiments, Homer proteins and InsP(3)R were both found in the fractions enriched in longitudinal sarcoplasmic reticulum (LSR) but not in fractions of terminal cisternae that are enriched in RyRs. Thus, in skeletal muscle, Homer proteins may play a role in the organisation of a second Ca(2+) signalling compartment containing the InsP(3)R, but are apparently not involved in the organisation of RyRs at triads.

IP(3)-mediated Ca(2+) signals in human neuroblastoma SH-SY5Y cells with exogenous overexpression of type 3 IP(3) receptor.

Human neuroblastoma SH-SY5Y cells, predominantly expressing type 1 inositol 1,4,5-trisphosphate (IP(3)) receptor (IP(3)R), were stably transfected with IP(3)R type 3 (IP(3)R3) cDNA. Immunocytochemistry experiments showed a homogeneous cytoplasmic distribution of type 3 IP(3)Rs in transfected and selected high expression cloned cells. Using confocal Ca(2+) imaging, carbachol (CCh)-induced Ca(2+) release signals were studied. Low CCh concentrations (< or = 750 nM) evoked baseline Ca(2+) oscillations. Transfected cells displayed a higher CCh responsiveness than control or cloned cells. Ca(2+) responses varied between fast, large Ca(2+) spikes and slow, small Ca(2+) humps, while in the clone only Ca(2+) humps were observed. Ca(2+) humps in the transfected cells were associated with a high expression level of IP(3)R3. At high CCh concentrations (10 microM) Ca(2+) transients in transfected and cloned cells were similar to those in control cells. In the clone exogenous IP(3)R3 lacked the C-terminal channel domain but IP(3)-binding capacity was preserved. Transfected cells mainly expressed intact type 3 IP(3)Rs but some protein degradation was also observed. We conclude that in transfected cells expression of functional type 3 IP(3)Rs causes an apparent higher affinity for IP(3). In the clone, the presence of degraded receptors leads to an efficient cellular IP(3) buffer and attenuated IP(3)-evoked Ca(2+) release.

Washing out of lipophilic compounds induces a transient increase in the passive Ca(2+) leak in permeabilized A7r5 cells.

We have investigated how the immunosuppressant drug FK506 affected the basal Ca(2+) leak in permeabilized A7r5 cells. Non-mitochondrial Ca(2+) stores loaded to steady state with Ca(2+) slowly lost their accumulated Ca(2+) during incubation in a Ca(2+)-free efflux medium. FK506 up to 100 microM had no effect on the basal Ca(2+) leak. In contrast, the rate of Ca(2+) release proceeded much faster immediately after washing out FK506. The increase in rate of Ca(2+) release after washing out of this compound depended on both its initial concentration and on the time of pre-incubation. A similar effect was also observed after removing another immunosuppressant drug (rapamycin) and after removing the inositol 1,4,5-trisphosphate receptor inhibitor xestospongin C. Since all these substances have a high octanol/H(2)O partition coefficient and accumulate in the endoplasmic reticulum membrane, we suggest that the transient increase in the basal Ca(2+) leak is due to the sudden removal of these lipophilic substances from the membrane.