Haloperidol Affects Plasticity of Differentiated NG-108 Cells Through σ1R/IP<Subscript>3</Subscript>R1 Complex

Haloperidol is an antipsychotic agent that primarily acts as an antagonist of D2 dopamine receptors. Besides other receptor systems, it targets sigma 1 receptors (σ1Rs) and inositol 1,4,5-trisphosphate receptors (IP₃Rs). Aim of this work was to investigate possible changes in IP₃Rs and σ1Rs resulting from haloperidol treatment and to propose physiological consequences in differentiated NG-108 cells, i.e., effect on cellular plasticity. Haloperidol treatment resulted in up-regulation of both type 1 IP₃Rs (IP₃R1s) and σ1Rs at mRNA and protein levels. Haloperidol treatment did not alter expression of other types of IP₃Rs. Calcium release from endoplasmic reticulum (ER) mediated by increased amount of IP₃R1s elevated cytosolic calcium and generated ER stress. IP₃R1s were bound to σ1Rs, and translocation of this complex from ER to nucleus occurred in the group of cells treated with haloperidol, which was followed by increased nuclear calcium levels. Haloperidol-induced changes in cytosolic, reticular, and nuclear calcium levels were similar when specific σ1 blocker -BD 1047- was used. Changes in calcium levels in nucleus, ER, and cytoplasm might be responsible for alterations in cellular plasticity, because length of neurites increased and number of neurites decreased in haloperidol-treated differentiated NG-108 cells.     

Mitochondrial Calcium Signaling Driven by the IP3 Receptor

Many agonists bring about their effects on cellular functions through a rise incytosolic [Ca²⁺]([Ca²⁺]_c) mediated by the second messenger inositol 1,4,5-trisphosphate (IP₃). Imaging studiesof single cells have demonstrated that [Ca²⁺]_c signals display cell specific spatiotemporalorganization that is established by coordinated activation of IP₃ receptor Ca²⁺ channels.Evidence emerges that cytosolic calcium signals elicited by activation of the IP₃ receptors areefficiently transmitted to the mitochondria. An important function of mitochondrial calciumsignals is to activate the Ca²⁺-sensitive mitochondrial dehydrogenases, and thereby to meetdemands for increased energy in stimulated cells. Activation of the permeability transitionpore (PTP) by mitochondrial calcium signals may also be involved in the control of cell death.Furthermore, mitochondrial Ca²⁺ transport appears to modulate the spatiotemporal organizationof [Ca²⁺]_c responses evoked by IP₃ and so mitochondria may be important in cytosolic calciumsignaling as well. This paper summarizes recent research to elucidate the mechanisms andsignificance of IP₃-dependent mitochondrial calcium signaling.     

1α,25-dihydroxyvitamin D3 rapidly alters phospholipid metabolism in the nuclea envelope of osteoblasts

1α,25-Dihydroxyvitamin D₃ (1α, 25-(OH)₂D₃) has been shown to increase cytosolic calcium and inositol trophosphate levels in rat osteosarcoma cells (ROS 17/2.8) and to increase nuclear calcium in these cells. To determine the mechanism(s) of 1α, (OH)₂D₃-induced changes in the calcium, the effect of the hormone on phospholipid metabolism in isolated osteoblast nuclei wa assessed. 1α,25 (OH)₂D₃, 20 nM, increased inositol triphosphate levels in the nuclei after 5 min of treatment. The biologically inactive epimer, 1β,25-(OH)₂D₃, had no significant effect on inositol triphosphate levels. ATP, 1 mM, also increased inositol triphosphate levels in the isolated nuclei after 5 min. 1α,25-(OH)₂D₃, 20 nM, increased calcium in the isolated nuclei in the presence but not in the absence of extranuclear calcium with 5 min. Nuclear calcium was also increased within 5 min by ATP, 1 mM, and inositol triphosphate, 1 mM. The effects of ATP on nuclear calcium was not additive with 1α, 25-(OH)₂D₃, suggesting that these two agents increase nuclear calcium in these osteoblast-like cells by similar mechanisms. In summary, 1α,25-(OH)₂D₃ amd ATP rapidly increase inositol triphosphate levels in isolated from ROS 17/2.8 cells. The hormone, the nucleotide, and the inositol phospholipid nuclear calcium. Thus, the 1α,25-(OH)₂D₃ and ATP effects of nuclear calcium may be mediated by changes in phospholipid metabolism in the nuclei of these osteoblastlike cells. © Wiley-Liss, Inc.     

Intracellular signalling of epinephrine in rat hepatocytes during fetal development and hepatic regeneration

The changes in intracellular calcium concentration and IP₃ production after the addition of epinephrine were analysed in adult, fetal (20th–22nd day of intrauterine life), and regenerating rat hepatocytes (4 h–24 h after partial hepatectomy) to determine whether the signal transduction is the same in quiescent proliferating and differentiating cells.The epinephrine treatment causes a significative cytosolic calcium transient in hepatocytes isolated in the last day of fetal life (22-day old) and in the early stage of regeneration (4 h). This effect is not significant in the previous stage of fetal life (20-day old) and at the onset of M phase of cell cycle after partial hepatectomy (24 h).[³H]myo inositol incorporation into IP₃ and IP₄ is higher in 20 day fetal and regenerating hepatocytes with respect to the control. In these cells the epinephrine does not affect basal level of IP₃ and IP₄, while it causes a substantial increase of these inositol phosphates in adult hepatocytes.[³H]myo inositol incorporation into PIP₂ is very low at the 20th day of fetal life. Epinephrine has no effect on this parameter in fetal and regenerating hepatocytes.Our results show that the epinephrine signal is mediated differently in proliferating and in quiescent hepatocytes.     

Is there crosstalk between extracellular and intracellular calcium mobilization in jasmonic acid signaling

The changes in cytosolic Ca²⁺ levels play an important role in the jasmonic acid (JA) signal transduction pathway. We demonstrate that an increase in cytosolic free Ca²⁺ concentration ([Ca²⁺]_cyt) of Arabidopsis leaf cells was affected by pretreatment with heparin and 3,4,5-trimethoxybenzoic acid 8-(diethylamino)octyl ester (TMB-8). With pretreatment of heparin, an antagonist of inositol 1,4,5-trisphosphate (IP₃) sensitive channels, the basal and JA induced fluorescence of [Ca²⁺]_cyt were both decreased. Furthermore, heparin and TMB-8, another antagonist of IP₃ sensitive channels, enhanced the JA-induced gene expression of JR1. These data suggest that there may be a fine tune control system between extracellular and intracellular Ca²⁺ mobilization in JA signaling pathway.     

1α,25-Dihydroxyvitamin D3 rapidly increases nuclear calcium levels in rat osteosarcoma cells

1α,25-Dihydroxyvitamin D₃ increases intracellular calcium in rat osteoblast-like cells that possess the classic receptor (ROS 17/2.8) as well as those that lack the classic receptor (ROS 24/1), indicating that a separate signalling system mediates this rapid nongenomic action. To determine the intracellular sites of this calcium increase, cytosolic and nuclear fluorescence (340 nm/380 nm ratio) were measured in Fura 2AM loaded ROS 17/2.8 cells using digital microscopy. Within 5 min, cytosolic fluorescence increased by 29% (P < 0.05) and nuclear fluorescence by 30% (P < 0.01) after exposure to 1α,25-dihydroxyvitamin D₃ (20 nM). This effect was blocked by the inactive epimer 1β,25-dihydroxyvitamin D₃. In an individual cell, cytosolic and nuclear fluorescence increased gradually after 1, 3, and 5 min exposure to vitamin D. Nuclei were then isolated from ROS 17/2.8 cells to directly measure the hormone's effect on nuclear calcium. The calcium content of Fura 2AM loaded nuclei was not affected by increasing the calcium concentration in the incubation buffer from 50 nM to 200 nM. After 5 min, 1α,25-dihydroxyvitamin D₃, 20 nM, increased the calcium of isolated nuclei in medium containing 50 nM calcium and 200 nM calcium. 1β,25-dihydroxyvitamin D₃, 20 nM, had no effect on nuclear calcium but blocked the 1α,25-dihydroxyvitamin D₃ induced rise in the isolated nuclei. The results indicate that the nuclear membrane of the ROS 17/2.8 cells contain calcium permeability barriers and transport systems that are sensitive to and specific for 1α,25-dihydroxyvitamin D₃. 1α,25-Dihydroxyvitamin D₃ rapidly increases nuclear calcium levels in both intact cells and isolated nuclei suggesting that rapid nongenomic activation of nuclear calcium may play a functional role in osteoblastic activity.     

Thy-1-Mediated phosphatidylinositol turnover in cultured rat glomerular mesangial cell

Thy-1 glycoprotein is expressed in rat glomerular mesangial cells, and anti-Thy-1 nephritis induced by anti-Thy-1 antibodies is a model of human renal diseases. In this study, we examined Thy-1-mediated biological reactions in cultured rat glomerular mesangial cells utilizing two anti-Thy-1 monoclonal antibodies (mAbs), 1-22-3 and OX-7. Incubation of the cells with these mAbs resulted in increased inositol trisphosphate (IP₃) levels. The rise in IP₃ produced by mAb 1-22-3 was greater than that produced by mAb OX-7 at the same dose. Incubation of mesangial cells with these mAbs resulted in an increase in the intracellular free calcium concentration ([Ca²⁺]i). mAb 1-22-3 induced a sustained increase in [Ca²⁺]i, while that induced by mAb OX-7 lasted 1-2 min, then decreased to the basal level. An transient increase in [Ca²⁺]i was also observed in Ca²⁺-free medium, indicating that these [Ca²⁺]i increases are due to release of Ca²⁺ from internal stores by IP₃ without calcium flux across cell membrane. When cells were pretreated with protein tyrosine kinase (PTK) inhibitors (herbimycin A or genistein), Thy-1-mediated increases in [Ca²⁺]i were inhibited. These data suggest that Thy-1 induces the production of IP₃ (including inositol 1,4,5-triphosphate, an intracellular Ca²⁺-releasing factor) and that PTKs may contribute to the Thy-1-mediated elevation of [Ca²⁺]i which presumably results from phospholipase C activation following Thy-1-mediated signaling in rat mesangial cells. © 1996 Wiley-Liss, Inc.     

A novel fluorescent method employing the FRET-based biosensor “LIBRA” for the identification of ligands of the inositol 1,4,5-trisphosphate receptors

LIBRA is a fluorescent biosensor of inositol 1,4,5-trisphosphate (IP₃) and is composed of the ligand-binding domain of the rat type 3 IP₃ receptor and cyan and yellow fluorescent proteins. We examined the responses of LIBRA and its IP₃-insensitive mutant LIBRA-N to compounds known to inhibit IP₃-induced Ca²⁺ release. Heparin, a competitive antagonist of IP₃ receptors, increased the emission ratio of LIBRA but not that of LIBRA-N. In contrast, 2-aminoethoxydiphenyl borate, a known non-competitive inhibitor of IP₃ receptor, decreased the emission ratios of both LIBRA and LIBRA-N. Thus, the concurrent use of LIBRA-N with LIBRA identifies nonspecific responses. These results indicate that LIBRA and its mutant control can be used to detect specific agonists and antagonists of IP₃ receptors. We also demonstrate the utility of LIBRA and LIBRA-N in discriminating between specific and nonspecific responses in intact cells.     

A function for tyrosine phosphorylation of type 1 inositol 1,4,5-trisphosphate receptor in lymphocyte activation

Sustained elevation of intracellular calcium by Ca²⁺ release–activated Ca²⁺ channels is required for lymphocyte activation. Sustained Ca²⁺ entry requires endoplasmic reticulum (ER) Ca²⁺ depletion and prolonged activation of inositol 1,4,5-trisphosphate receptor (IP₃R)/Ca²⁺ release channels. However, a major isoform in lymphocyte ER, IP3R1, is inhibited by elevated levels of cytosolic Ca²⁺, and the mechanism that enables the prolonged activation of IP₃R1 required for lymphocyte activation is unclear. We show that IP₃R1 binds to the scaffolding protein linker of activated T cells and colocalizes with the T cell receptor during activation, resulting in persistent phosphorylation of IP₃R1 at Tyr353. This phosphorylation increases the sensitivity of the channel to activation by IP₃ and renders the channel less sensitive to Ca²⁺-induced inactivation. Expression of a mutant IP3R1-Y353F channel in lymphocytes causes defective Ca²⁺ signaling and decreased nuclear factor of activated T cells activation. Thus, tyrosine phosphorylation of IP3R1-Y353 may have an important function in maintaining elevated cytosolic Ca²⁺ levels during lymphocyte activation.     

Differential expression of type 2 and type 3 inositol 1,4,5-trisphosphate receptor mRNAs in various mouse tissues: in situ hybridization study

The inositol 1,4,5-trisphosphate receptor (IP₃R) is an intracellular Ca²⁺ release channel responsible for mobilizing stored Ca²⁺. Three different receptor types have been molecularly cloned, and their genes have been classified into a family. The gene for the type 1 receptor (IP₃R1) is predominantly expressed in cerebellar Purkinje neurons, but its gene product is localized widely in a variety of tissues; however, there is little information on what types of cells express the other two receptor types, type 2 and type 3 (IP₃R2 and IP₃R3, respectively). We studied the expression of the IP₃R gene family in various mouse tissues by in situ hybridization histochemistry. Compared with IP₃R1, the levels of expression of IP₃R2 and IP₃R3 mRNAs were low in all of the tissues tested. IP₃R2 mRNA was localized in the intralobular duct cells of the submandibular gland, the urinary tubule cells of the kidney, the epithelial cells of epididymal ducts and the follicular granulosa cells of the ovary, while the IP₃R3 mRNA was distributed in gastric cells, salivary and pancreatic acinar cells and the epithelium of the small intestine. All of these cells which express either IP₃R2 or IP₃R3 mRNA are known to have a secretory function in which IP₃/Ca²⁺ signalling has been shown to be involved, and thus either IP₃R2 or IP₃R3 may be a prerequisite to secretion in these cells.     

Cyanide-stimulated inositol 1,4,5-trisphosphate formation: An intracellular neurotoxic signaling cascade

Cyanide-induced neurotoxicity is associated with altered cellular Ca²⁺ homeostasis resulting in sustained elevation of cytosolic Ca²⁺. In order to characterize the effect of cyanide on intracellular signaling mechanisms, the interaction of KCN with the inositol 1,4,5-triphosphate Ca²⁺ signaling system was determined in the PC12 cell line. KCN in the concentration range of 1.0–100 μM produced a rapid rise in intracellular IP₃ levels (peak level occurred within 60 sec); 10 μM KCN elevated intracellular levels of IP₃ to 148% of control levels. This response was mediated by phospholipase C (PLC) since U73122, a specific PLC inhibitor, blocked the response. Removal of Ca²⁺ from the incubation medium and chelation of intracellular Ca²⁺ with BAPTA partially attenuate the cyanide-stimulated IP₃ generation, showing that the response is partially Ca²⁺ dependent. Also, treatment of cells with nifedipine or LaCl₃, Ca²⁺ channel blockers, partially blocked the generation of IP₃. This study shows that cyanide in concentrations as low as 1 μM stimulates IP₃ generation that may be mediated by receptor and nonreceptor IP₃ production since they have differential dependence on Ca²⁺. It is proposed that this response is an early intracellular signaling action that can contribute to altered Ca²⁺ homeostasis characteristic of cyanide neurotoxicity. © 1997 John Wiley & Sons, Inc.     

Atrial natriuretic peptide inhibits the phosphoinositide hydrolysis in murine Leydig tumor cells

The ability of ANP to inhibit the hydrolysis of phosphoinositides was examined in [³H] myoinositol-labeled intact murine Leydig tumor (MA-10) cells. Arginine vasopressin (AVP) stimulated the formation of inositol monophosphate (IP₁), inositol bisphosphate (IP₂), and inositol trisphosphate (IP₃) both in a time- and dose- dependent manner in MA-10 cells. ANP inhibited the AVP-induced formation of IP₁, IP₂, and IP₃ in these cells. The inhibitory effect of ANP on the AVP-stimulated formation of IP₁, IP₂, and IP₃ accounted for 30%, 38% and 42%, respectively, which was observed at the varying concentrations of AVP. ANP caused a dose-dependent attenuation in AVP-stimulated production of IP₁, IP₂ and IP₃ with maximum inhibition at 100 nM concentration of ANP. The production of inositol phosphates was inhibited in the presence of 8- bromo cGMP in a dose-dependent manner, whereas dibutyryl-cAMP had no effect on the generation of these metabolites. The LY 83583, an inhibitor of guanylyl cyclase and cGMP production, abolished the inhibitory effect of ANP on the AVP-stimulated production of inositol phosphates. Furthermore, 10 M LY 83583 also inhibited the ANP-stimulated guanylyl cyclase activity and the intracellular accumulation of cGMP by more than 65–70%. The inhibition of eGMP-dependent protein kinase by H-8, significantly restored the levels of AVP-stimulated inositol phosphates in the presence of either ANP or exogenous 8-bromo cGMP. The results of this study suggest that ANP exerts an inhibitory effect on the production of inositol phosphates in murine Leydig tumor (MA-10) cells by mechanisms involving cGMP and cGMP-dependent protein kinase.Established Investigator of the American Heart Association     

IP3-mediated cytosolic and nuclear calcium elevation in NRK-52E cells using ‘caged’ GPIP2

Alterations in calcium homeostasis play a pivotal role in the cellular response to injury. Increases in the concentration of cytosolic free calcium ([Ca²⁺]i) result in a variety of calcium mediated toxic responses such as cytoskeletal alterations, mitochondrial damage, and over-expression of gene products. Inositol trisphosphate is a second messenger that links external cell surface signals to [Ca²⁺]i elevation. The present study explored the use of caged glycerophosphoryl-myo-inositol-1,4,5-bisphosphate (GPIP₂) to mediate a rapid and prolonged increase in [Ca²⁺]i in a normal rat kidney epithelial cell line (NRK-52E). In intact NRK-52E cells, UV photolysis of microinjected GPIP₂ resulted in a 3–4-fold sustained increase in [Ca²⁺]_i. Graded photolytic release of GPIP2 also resulted in calcium-mediated morphologic alterations, as shown by confocal microscopy, with cellular blebs apparent within 30 min. There was no apparent increase in [Ca²⁺]i or morphologic alterations in control cells microinjected with calcium indicator and equally exposed to UV light. Subsequent application of thapsigargin or ionomycin (1.0 μM) produced a rapid and transient increase in [Ca²⁺]i. In addition, we show that activation of IN stores results in increased concentration of ionized nuclear calcium, ([Ca²⁺]n) which persists longer than the increase in [Ca²⁺]i. These findings indicate that GPIP₂ mediates a rapid and sustained elevation in [Ca²⁺]n and [Ca²⁺]i and this IP₃-mediated calcium elevation is translated to the nucleus in rat kidney epithelial cells.     

Metabolic adaptation to the chronic loss of Ca2+ signaling induced by KO of IP3 receptors or the mitochondrial Ca2+ uniporter

Calcium signaling is essential for regulating many biological processes. Endoplasmic reticulum inositol trisphosphate receptors (IP₃Rs) and the mitochondrial Ca²⁺ uniporter (MCU) are key proteins that regulate intracellular Ca²⁺ concentration. Mitochondrial Ca²⁺ accumulation activates Ca²⁺-sensitive dehydrogenases of the tricarboxylic acid (TCA) cycle that maintain the biosynthetic and bioenergetic needs of both normal and cancer cells. However, the interplay between calcium signaling and metabolism is not well understood. In this study, we used human cancer cell lines (HEK293 and HeLa) with stable KOs of all three IP₃R isoforms (triple KO [TKO]) or MCU to examine metabolic and bioenergetic responses to the chronic loss of cytosolic and/or mitochondrial Ca²⁺ signaling. Our results show that TKO cells (exhibiting total loss of Ca²⁺ signaling) are viable, displaying a lower proliferation and oxygen consumption rate, with no significant changes in ATP levels, even when made to rely solely on the TCA cycle for energy production. MCU KO cells also maintained normal ATP levels but showed increased proliferation, oxygen consumption, and metabolism of both glucose and glutamine. However, MCU KO cells were unable to maintain ATP levels and died when relying solely on the TCA cycle for energy. We conclude that constitutive Ca²⁺ signaling is dispensable for the bioenergetic needs of both IP₃R TKO and MCU KO human cancer cells, likely because of adequate basal glycolytic and TCA cycle flux. However, in MCU KO cells, the higher energy expenditure associated with increased proliferation and oxygen consumption makes these cells more prone to bioenergetic failure under conditions of metabolic stress.     

Exogenous nitric oxide-induced release of calcium from intracellular IP3 receptor-sensitive stores via S-nitrosylation in respiratory burst-dependent neutrophils

PMA-induced respiratory burst neutrophils were exposed to exogenous nitric oxide (NO) donor sodium nitroprusside (SNP) to study the effect of NO on calcium signaling. A sharp rise of cytosolic calcium concentration ([Ca²⁺]_c) was triggered by 1 mM SNP with and without external calcium. We found that GF 109203X, a specific inhibitor of protein kinase C, DPI, a putative inhibitor of the respiratory burst-generating NADPH oxidase, and 2-DG, a non-metabolizable analog of glucose, completely inhibited the SNP-induced rise of [Ca²⁺]_c in PMA-activated respiratory burst neutrophils. Meanwhile, 2-APB and TMB-8, two potent IP₃ receptor inhibitors, prevented calcium increase respectively. Furthermore, N-ethylmaleimide (NEM), a specific cysteine alkylating agent, evidently abolished the [Ca²⁺]_c elevation. In contrast, the sGC inhibitor NS2028 had little effect on the rise of [Ca²⁺]_c. Taken together, these results indicated that exogenous NO induced the release of calcium from intracellular IP₃ receptor-sensitive stores of neutrophils via S-nitrosylation in a respiratory burst-dependent manner.     

Characterization of Inositol Phosphates in Carrot (Daucus carota L.) Cells

We have shown previously that inositol-1,4,5-trisphosphate (IP₃) stimulates an efflux of ⁴⁵Ca²⁺ from fusogenic carrot protoplasts (M Rincón, WF Boss [1987] Plant Physiol 83: 395-398). In light of these results, we suggested that IP₃ might serve as a second messenger for the mobilization of intracellular Ca²⁺ in higher plant cells. To determine whether or not IP₃ and other inositol phosphates were present in the carrot cells, the cells were labeled with myo-[2-³H]inositol for 18 hours and extracted with ice-cold 10% trichloroacetic acid. The inositol metabolites were separated by anion exchange chromatography and by paper electrophoresis. We found that [³H]inositol metabolites coeluted with inositol bisphosphate (IP₂) and IP₃ when separated by anion exchange chromatography. However, we could not detect IP₂ or IP₃ when the inositol metabolites were analyzed by paper electrophoresis even though the polyphosphoinositides, which are the source of IP₂ and IP₃, were present in these cells. Thus, [³H] inositol metabolites other than IP₂ and IP₃ had coeluted on the anion exchange columns. The data indicate that either IP₃ is rapidly metabolized or that it is not present at a detectable level in the carrot cells.     

Toward a high-resolution structure of IP3R channel

The ability of cells to maintain low levels of Ca²⁺ under resting conditions and to create rapid and transient increases in Ca²⁺ upon stimulation is a fundamental property of cellular Ca²⁺ signaling mechanism. An increase of cytosolic Ca²⁺ level in response to diverse stimuli is largely accounted for by the inositol 1,4,5-trisphosphate receptor (IP₃R) present in the endoplasmic reticulum membranes of virtually all eukaryotic cells. Extensive information is currently available on the function of IP₃Rs and their interaction with modulators. Very little, however, is known about their molecular architecture and therefore most critical issues surrounding gating of IP₃R channels are still ambiguous, including the central question of how opening of the IP₃R pore is initiated by IP₃ and Ca²⁺. Membrane proteins such as IP₃R channels have proven to be exceptionally difficult targets for structural analysis due to their large size, their location in the membrane environment, and their dynamic nature. To date, a 3D structure of complete IP₃R channel is determined by single-particle cryo-EM at intermediate resolution, and the best crystal structures of IP₃R are limited to a soluble portion of the cytoplasmic region representing ∼15% of the entire channel protein. Together these efforts provide the important structural information for this class of ion channels and serve as the basis for further studies aiming at understanding of the IP₃R function.     

The BRCA1 Tumor Suppressor Binds to Inositol 1,4,5-Trisphosphate Receptors to Stimulate Apoptotic Calcium Release

The inositol 1,4,5-trisphosphate receptor (IP₃R) is a ubiquitously expressed endoplasmic reticulum (ER)-resident calcium channel. Calcium release mediated by IP₃Rs influences many signaling pathways, including those regulating apoptosis. IP₃R activity is regulated by protein-protein interactions, including binding to proto-oncogenes and tumor suppressors to regulate cell death. Here we show that the IP₃R binds to the tumor suppressor BRCA1. BRCA1 binding directly sensitizes the IP₃R to its ligand, IP₃. BRCA1 is recruited to the ER during apoptosis in an IP₃R-dependent manner, and, in addition, a pool of BRCA1 protein is constitutively associated with the ER under non-apoptotic conditions. This is likely mediated by a novel lipid binding activity of the first BRCA1 C terminus domain of BRCA1. These findings provide a mechanistic explanation by which BRCA1 can act as a proapoptotic protein.     

Effects of both glucose and IP<Subscript>3</Subscript> concentrations on action potentials in pancreatic β-cells

Abstarct Considering the ATP-driven (SERCA) pump flux as function of glucose concentration and the calcium flux from the endoplasmic reticulum (ER) through the IP₃R channel, the calcium-based phantom bursting model (PBM) of β-cells (Bertram and Sherman in Bull Math Biol 66:1313, 2004) is theoretically extended to discuss the effects of glucose and inositol 1,4,5-trisphosphate (IP₃) concentration on the membrane potential activities. When IP₃ concentration is fixed, it is found that there is a critical glucose concentration at which electrical bursting oscillations transfer into spiking, and the critical concentration of glucose is increased with the increasing of IP₃ concentration. To get the bursting oscillations in β-cells, our theoretical results show that the stimulatory glucose concentration should be more than 10 mM, which is consistent with the normal physiological IP₃ level. When the stochastic opening and closing of IP₃R channels are considered, it is shown that the membrane potential oscillation transfers from spiking to bursting with the channel number decreasing, and the average cytosolic free Ca²⁺ concentration is increased with the increase of glucose concentration.