Stable expression of truncated inositol 1,4,5-trisphosphate receptor subunits in 3T3 fibroblasts. Coordinate signaling changes and differential suppression of cell growth and transformation

Calpain-cleaved type 1 inositol 1,4,5-trisphosphate receptor (InsP(3)R1) has InsP(3)-independent gating and disrupts intracellular Ca(2+) homeostasis

The type 1 inositol 1,4,5-trisphosphate receptor (InsP(3)R1) is a ubiquitous intracellular Ca(2+) release channel that is vital to intracellular Ca(2+) signaling. InsP(3)R1 is a proteolytic target of calpain, which cleaves the channel to form a 95-kDa carboxyl-terminal fragment that includes the transmembrane domains, which contain the ion pore. However, the functional consequences of calpain proteolysis on channel behavior and Ca(2+) homeostasis are unknown. In the present study we have identified a unique calpain cleavage site in InsP(3)R1 and utilized a recombinant truncated form of the channel (capn-InsP(3)R1) corresponding to the stable, carboxyl-terminal fragment to examine the functional consequences of channel proteolysis. Single-channel recordings of capn-InsP(3)R1 revealed InsP(3)-independent gating and high open probability (P(o)) under optimal cytoplasmic Ca(2+) concentration ([Ca(2+)](i)) conditions. However, some [Ca(2+)](i) regulation of the cleaved channel remained, with a lower P(o) in suboptimal and inhibitory [Ca(2+)](i). Expression of capn-InsP(3)R1 in N2a cells reduced the Ca(2+) content of ionomycin-releasable intracellular stores and decreased endoplasmic reticulum Ca(2+) loading compared with control cells expressing full-length InsP(3)R1. Using a cleavage-specific antibody, we identified calpain-cleaved InsP(3)R1 in selectively vulnerable cerebellar Purkinje neurons after in vivo cardiac arrest. These findings indicate that calpain proteolysis of InsP(3)R1 generates a dysregulated channel that disrupts cellular Ca(2+) homeostasis. Furthermore, our results demonstrate that calpain cleaves InsP(3)R1 in a clinically relevant injury model, suggesting that Ca(2+) leak through the proteolyzed channel may act as a feed-forward mechanism to enhance cell death.

c-Myb-dependent inositol 1,4,5-trisphosphate receptor type-1 expression in vascular smooth muscle cells

OBJECTIVE

The IP3 receptor-1 (IP3R1) mediates Ca2+ signals critical to vascular smooth muscle cell (VSMC) proliferation. The cell cycle-associated transcription factor c-Myb increases Ca2+ at the G1/S transition. Here we show the mechanism through which c-Myb regulates expression of IP3R1.

METHODS & RESULTS

Ribonuclease protection confirmed transcriptional start (TS), and qRT-PCR revealed a 6-fold increase in IP3R1 mRNA as immortalized VSMC progress from G0 to G1/S. A c-Myb neutralizing antibody decreased IP3R1 mRNA expression 3-fold, and abolished the 3.4-fold increase in IP3R1 protein observed at G1/S. Primary aortic VSMCs in culture and proliferating carotid VSMCs in vivo showed similar regulation of IP3R1 mRNA and protein. Sequence analysis of a 3.1-Kb mouse IP3R1 promoter revealed 17 putative c-Myb binding sites. Reporter assays demonstrated a 2-fold increase in promoter activity in G1/S- versus G0-synchronized VSMCs, which was abolished by functional c-Myb knockdown or deletion of promoter sequences upstream and downstream of TS. Point mutations in Myb sites-13 or -15 significantly blunted G1/S-specific promoter induction in both immortalized and primary VSMCs. Gel shift and ChIP confirmed binding of c-Myb to sites-13 and -15 in G1/S stage VSMCs.

CONCLUSION

c-Myb regulates cell cycle-associated IP3R1 transcription in VSMCs via specific highly conserved Myb-binding sites in the IP3R1 promoter.

Inositol 1,4,5-trisphosphate receptor phosphorylation in breast cancer

The aim of this study was to establish the type(s) of inositol 1,4,5-trisphosphate receptors (IP3Rs) in T47D breast cancer cells that regulate intracellular calcium (Ca2+) and whether they interact with cyclin (Cy), an important regulator of cyclin-dependent kinases (cdk), during cell cycle progression. Immunoblotting, immunoprecipitation, and pull-down assays were used to identify IP3R expression and interaction with Cy. The relative IP3R3 expression, as compared to IP3R1, was higher in these cells. Pull-down analysis showed that IP3R3 interacted with both CyA and CyB. The interaction with Cys and the phosphorylation of IP3Rs by Cy/cdk complexes provide a novel mechanism of regulating intracellular Ca2+ release and Ca2+-dependent signaling events in breast cancer.

Inositol 1,4,5-trisphosphate receptor (type 1) phosphorylation and modulation by Cdc2

Calcium (Ca2+) release from the endoplasmic reticulum (ER) controls numerous cellular functions including proliferation, and is regulated in part by inositol 1,4,5-trisphosphate receptors (IP3Rs). IP3Rs are ubiquitously expressed intracellular Ca2+-release channels found in many cell types. Although IP3R-mediated Ca2+ release has been implicated in cellular proliferation, the biochemical pathways that modulate intracellular Ca2+ release during cell cycle progression are not known. Sequence analysis of IP3R1 reveals the presence of two putative phosphorylation sites for cyclin-dependent kinases (cdks). In the present study, we show that cdc2/CyB, a critical regulator of eukaryotic cell cycle progression, phosphorylates IP3R1 in vitro and in vivo at both Ser(421) and Thr(799) and that this phosphorylation increases IP3 binding. Taken together, these results indicate that IP3R1 may be a specific target for cdc2/CyB during cell cycle progression.

Carboxyl-terminal sequences critical for inositol 1,4,5-trisphosphate receptor subunit assembly

The inositol 1,4,5-trisphosphate receptor (InsP(3)R) is a tetrameric assembly of conserved subunits that each contains six transmembrane regions (TMRs) localized near the carboxyl terminus. Receptor subunit assembly into a tetramer appears to be a multideterminant process involving an additive contribution of membrane spanning helices and the short cytosolic carboxyl terminus (residues 2590-2749). Previous studies have shown that of the six membrane-spanning regions in each subunit, the 5th and 6th transmembrane regions, and the carboxyl terminus are strong determinants for assembly. The fifth and sixth TMRs contain numerous beta-branched amino acids that may participate in coiled/coil formation via putative leucine zipper motifs. InsP(3)R truncation mutants were expressed in COS-1 cells and analyzed by sucrose density gradient sedimentation and gel filtration for their ability to assemble. Chemical cross-linking with the homobifunctional reagents sDST or DMS of mammalian and bacterially expressed carboxyl-terminal containing receptor fragments reveals that sequences within the carboxyl terminus confer the formation of subunit dimers. A series of InsP(3) receptor carboxyl-terminal fragments and glutathione S-transferase (GST)/InsP(3)R chimeras were expressed in Escherichia coli and used in an in vitro assay to elucidate the minimal sequence responsible for association of the carboxyl termini into dimers. The results presented here indicate that this minimal sequence is approximately 30 residues in length and is localized between residues 2629 and 2654. These residues are highly conserved between the three InsP(3)R isoforms ( approximately 80% identity) as well as the ryanodine receptor ( approximately 40% identity) and suggest that a conserved assembly motif may exist between the two intracellular receptor families. We propose that assembly of the InsP(3) receptor to a tetramer involves intersubunit interactions mediated through both the membrane-spanning regions and residues 2629-2654 of the carboxyl terminus possibly through the formation of a dimer of dimers.

Inositol 1,4,5-trisphosphate receptor down-regulation is activated directly by inositol 1,4,5-trisphosphate binding. Studies with binding-defective mutant receptors

Activation of certain phosphoinositidase C-linked cell surface receptors is known to cause an acceleration of the proteolysis of inositol 1,4,5-trisphosphate (InsP3) receptors and, thus, lead to InsP3 receptor down-regulation. To gain insight into this process, we examined whether or not InsP3 receptor degradation is a direct consequence of InsP3 binding by analyzing the down-regulation of exogenous wild-type and binding-defective mutant InsP3 receptors expressed in SH-SY5Y human neuroblastoma cells. Stimulation of these cells with carbachol showed that wild-type exogenous receptors could be down-regulated but that the binding-defective mutant exogenous receptors were not. Thus, InsP3 binding appears to mediate down-regulation. To validate this conclusion, a comprehensive analysis of the effects of the exogenous receptors was undertaken. This showed that exogenous receptors (i) are localized appropriately within the cell, (ii) enhance InsP3-induced Ca2+ release in permeabilized cells, presumably by increasing the number of InsP3-sensitive Ca2+ channels, (iii) have minimal effects on Ca2+ mobilization and InsP3 formation in intact cells, (iv) form heteromers with endogenous receptors, and (v) do not alter the down-regulation of endogenous receptors. In total, these data show that the introduction of exogenous receptors into SH-SY5Y cells does not compromise intracellular signaling or the down-regulatory process. We can thus conclude that InsP3 binding directly activates InsP3 receptor degradation. Because InsP3 binding induces a conformational change in the InsP3 receptor, these data suggest that this change provides the signal for accelerated proteolysis.

A minimal model for calcium signal generated by tyrosine kinase and G protein linked receptors; a stochastic computer simulation with CALSIM

A software was designed to simulate the calcium signal following hormone or growth factor stimulation in epithelial cells. The software written in C runs on a PC under Windows environment. It is based on a Markov process where the dynamic of the system is characterised by phenomenological transition probabilities. Moreover a minimal model is proposed to analyse the role of plasma channels and IP3 receptors, together with the opposite action of the CaATPase pumps, in the cytosolic and endoplasmic reticulum (ER) calcium signal control. The simulation is applied on the calcium response following stimulation by carbacol (protein G coupled receptors) or epidermal growth factor (tyrosine kinase type receptors) in A431 epithelial cells. The experimental calcium signals can be grouped in three classes; a spike and a return to the basal level (signal A), a spike and a decrease to a plateau level (signal B) or a slow increase to a plateau (signal C). Epidermal growth factor induces signal A and B while carbacol gives signal B and C. When a 'pseudo' steady state is reached oscillations occur. Computer simulations show that signal A can result from the activation of IP3 receptors while signal C would result from the activation of the plasma channels; signal B appears as the additive contribution of both channels, while oscillations are compatible with a calcium induced calcium release mechanism. Simulations suggest that the calcium dynamic in the ER is a mirror of cytosolic calcium but that a simple way to produce similar calcium elevation in these two compartments is to activate plasma channels. Implications of such a mechanism is discussed.

InsP3 receptor is essential for growth and differentiation but not for vision in Drosophila

Phospholipase C (PLC) is the focal point for two major signal transduction pathways: one initiated by G protein-coupled receptors and the other by tyrosine kinase receptors. Active PLC hydrolyzes phosphatidylinositol bisphosphate (PIP2) into the two second messengers inositol 1,4,5-trisphosphate (InsP3) and diacyl glycerol (DAG). DAG activates protein kinase C, and InsP3 mobilizes calcium from intracellular stores via the InsP3 receptor. Changes in [Ca2+]i regulate the function of a wide range of target proteins, including ion channels, kinases, phosphatases, proteases, and transcription factors (Berridge, 1993). In the mouse, there are three InsP3R genes, and type 1 InsP3R mutants display ataxia and epileptic seizures (Matsumoto et al., 1996). In Drosophila, only one InsP3 receptor (InsP3R) gene is known, and it is expressed ubiquitously throughout development (Hasan and Rosbash, 1992; Yoshikawa et al., 1992; Raghu and Hasan, 1995). Here, we characterize Drosophila InsP3R mutants and demonstrate that the InsP3R is essential for embryonic and larval development. Interestingly, maternal InsP3R mRNA is sufficient for progression through the embryonic stages, but larval organs show asynchronous and defective cell divisions, and imaginal discs arrest early and fail to differentiate. We also generated adult mosaic animals and demonstrate that phototransduction, a model PLC pathway thought to require InsP3R, does not require InsP3R for signaling.

The endoplasmic reticulum in PC12 cells. Evidence for a mosaic of domains differently specialized in Ca2+ handling

Velocity and isopycnic gradient centrifugation were employed to fractionate post-nuclear supernatants rapidly prepared from PC12 cells in order to characterize areas of the endoplasmic reticulum involved in various aspects of intracellular Ca2+ homeostasis. The endoplasmic reticulum Ca2+ pumping activity, defined by three properties studied in parallel in the isolated fractions; thapsigargin-sensitive uptake of 45Ca2+, Ca2+-dependent, thapsigargin-sensitive protein phosphorylation and Western blotting of sarcoplasmic reticulum calcium ATPase (SERCA) 2b and putative SERCA3 ATPases, was concentrated primarily in a few fractions located at the top and toward the bottom of velocity and isopycnic gradients, respectively. The endoplasmic reticulum Ca2+ release channel, the inositol 1,4,5-trisphosphate receptor, was concentrated in the same fractions as the Ca2+ pumps, and additionally in a few fractions distinctly poor in SERCAs. In contrast, two lumenal markers (protein disulfide isomerase and calreticulin, the major Ca2+ storage protein of non-muscle endoplasmic reticulum) were enriched in the middle fractions of the velocity gradients while calnexin, a Ca2+-binding membrane protein, was more widely distributed throughout the gradients. These results document a considerable degree of functional and compositional heterogeneity in the endoplasmic reticulum of neurosecretory PC12 cells. Even in the limited areas that appear specialized for rapid Ca2+ uptake and release the ratio between pumps and channels varies considerably. Within the rest of the system, insulated from short-term fluctuations of Ca2+ concentration, Ca2+-binding proteins appear to be extensively distributed, in agreement with the idea that the Ca2+ content of the endoplasmic reticulum serves multiple functions.

Pharmacological and functional properties of voltage-independent Ca2+ channels

During the last few years, considerable progress has taken place in our knowledge of the molecular and functional properties of the various voltage-independent Ca2+ channels. In addition to the ionotropic receptor-channels (ROCs), that are not discussed in the present review, these channels include the SMOCs, activated via second messengers or other transducing processes directly triggered by receptor activation; and the SOCCs, activated as a consequence of depletion of the rapidly exchanging Ca2+ stores in the cytoplasm. In parallel, a pharmacological approach to the study of these channels has been developed, based primarily on heterogeneous drugs already known for different biological effects, and subsequently recognized as voltage-independent Ca(2+)-channel blockers. From the systematic analysis of the effects of these drugs new information has emerged about SMOCs and SOCCs function. In addition, pharmacological blockade of these channels appears to have beneficial therapeutic effects in pathological conditions such as tumoral cell growth, inflammation and immunity. At the moment the field is rapidly evolving, with major developments expected in the years ahead.

Nitric oxide action on growth factor-elicited signals. Phosphoinositide hydrolysis and [Ca2+]i responses are negatively modulated via a cGMP-dependent protein kinase I pathway

The role of nitric oxide (NO) in the phosphatidylinositol 4,5-bisphosphate (PIP2) hydrolysis and intracellular Ca2+ release responses induced by epidermal, platelet-derived, and fibroblast growth factors was investigated in three cell lines, a clone of NIH-3T3 fibroblasts overexpressing epidermal growth factor receptors and the tumoral epithelial cells A431 and KB. In all three cell types, pretreatment with NO donors decreased growth factor-induced PIP2 and Ca2+ responses, whereas pretreatment with NO synthase inhibitors increased them. The Ca2(+)-dependent PIP2 hydroysis induced by micromolar concentrations of the Ca2+ ionophore, ionomycin, was also modulated negatively and positively by NO donors and synthase inhibitors, respectively. In contrast, the Ca2+ content of the intracellular stores was unaffected by the various pretreatments employed. NO donors and synthase inhibitors induced an increase and decrease, respectively, of the intracellular cGMP formation in all three cell lines investigated. All of the effects of the NO donors were mimicked by 8-bromo-cGMP administration and abolished by pretreatment with the specific blocker of the cGMP-dependent protein kinase I, KT5823, which by itself mimicked the effects of the synthase inhibitors. Together with previous observations on G protein-coupled receptors, the present results demonstrate that PIP2 hydrolysis and Ca2+ release occur under the feedback control of NO, independently of the phospholipase C (beta, gamma, or delta type) involved and of the mechanism of activation. Such a control, which appears to be effected by the cGMP-dependent protein kinase I acting at the level of the phospholipases C themselves, might ultimately contribute to the inhibitory role of NO on growth previously observed with various cell types.

The whoosh and trickle of calcium signalling

The importance of phospholipase C catalysed hydrolysis of phosphatidylinositol-(4,5)bisphosphate (PtdIns(4,5)P2) to inositol-(1,4,5)trisphosphate (Ins(1,4,5)P3) and sn-1,2-diacylglycerol in the signal transduction pathways of eukaryote cells, in response to extracellular stimuli, is now widely recognised. Although nearly 60 naturally occurring inositol phosphates have been identified in mammalian cells, mobilisation of Ca2+ from the intracellular stores has been most commonly attributed to the generation of Ins(1,4,5)P3 [1]. However, there is increasing evidence for the presence of ryanodine receptors (RyR) in non-excitable cells and for cADP-ribose (cADPr) as the signalling molecule responsible for Ca2+ release via the RyR. But what is the purpose for the co-existence of these two intracellular Ca2+ channels in non-excitable cells and why are they so heterogeneous in their distribution? These questions were explored at the recent International Symposium Calcium Signalling in Inflammatory Cells. Depletion of the intracellular Ca2+ pools is followed by entry of Ca2+ into the cell across the plasma membrane, but the mechanism(s) underlying this 'capacitative Ca2+ entry' is not well understood. Many potential signalling pathways which may account for capacitative Ca2+ entry have been proposed although none have been unanimously accepted. New developments in the elucidation of the mechanism responsible for capacitative Ca2+ entry and how Ca2+ entry is regulated, together with progress in the characterisation of plasma membrane Ca2+ entry channels were also discussed at this symposium.

Type I, II, and III inositol 1,4,5-trisphosphate receptors are unequally susceptible to down-regulation and are expressed in markedly different proportions in different cell types

The type I inositol 1,4,5-trisphosphate (InsP3) receptor can be rapidly depleted from cells during stimulation of phosphoinositide hydrolysis because its degradation is accelerated (Wojcikiewicz, R. J. H., Furuichi, T., Nakade, S., Mikoshiba, K., and Nahorski, S. R. (1994) J. Biol. Chem. 269, 7963-7969). The present study examines the regulatory properties of type II and III InsP3 receptors. Initially, the relative abundance of InsP3 receptors was defined in a range of cell types by quantitative immunoblotting. These studies showed that the proportions in which type I, II, and III InsP3 receptors are expressed differs greatly and that some cells (for example, AR4-2J rat pancreatoma cells) express all three receptors. Analysis of the effects of cholecystokinin and bombesin on AR4-2J cells showed that each of the InsP3 receptors could be down-regulated during activation of phosphoinositide hydrolysis, but that depletion of the type II receptor was limited. Such a discrepancy was also seen in rat cerebellar granule cells and was found to result from the type II receptor being relatively resistant to degradation. In conclusion, type I, II, and III receptors can all be down-regulated, but with different characteristics. As the relative abundance of InsP3 receptors is extremely variable, the extent to which activation of the down-regulatory process alters intracellular signaling will vary depending on which InsP3 receptors are expressed.

Shear stress-induced [Ca2+]i transients and oscillations in mouse fibroblasts are mediated by endogenously released ATP

The effects of ATP, U-73122, apyrase, and saline shear stress on [Ca2+]i homeostasis were studied in fura-2 loaded, mouse fibroblast cells (L929), both in suspension and plated on glass. Release of internal Ca2+ was induced by ATP, via a receptor identified pharmacologically as a P2U type. In single cells, low concentrations of ATP evoked [Ca2+]i oscillations. These events were blocked by the putative phospholipase C inhibitor, U-73122 (but not by the inactive analog U-73343) and by the ATP/ADPase, apyrase. In addition, both these agents reduced the [Ca2+]i of unstimulated cells, especially after stirring, and blocked spontaneously occurring [Ca2+]i oscillations, which suggested an already activated state of the ATP receptor, independent from exogenous stimulations. Moreover, it was found that stirring of the cells was correlated with a steady accumulation of inositol phosphates, also blockable by apyrase, and that [Ca2+]i mobilization could be induced by puffs of saline in single cells. The transition to a Ca(2+)-free environment also provoked [Ca2+]i oscillations, most likely via the increase in ATP4- concentration. This evidence suggests that endogenous ATP is released from L fibroblasts in response to fluid shear stress, and this results in an autocrine, tonic up-regulation of the phosphoinositide signaling system and an ensuing alteration in Ca2+ homeostasis. Up until now, such a response to shear stress was believed to be unique to endothelial cells.