Creation of an inositol 1,4,5-trisphosphate-sensitive Ca2+ store in secretory granules of insulin-producing cells

IP3R at ER-Mitochondrial Contact Sites: Beyond the IP3R-GRP75-VDAC1 Ca2+ Funnel

Membrane contact sites (MCS) circumvent the topological constraints of functional coupling between different membrane-bound organelles by providing a means of communication and exchange of materials. One of the most characterised contact sites in the cell is that between the endoplasmic reticulum and the mitochondrial (ERMCS) whose function is to couple cellular Ca2+ homeostasis and mitochondrial function. Inositol 1,4,5-trisphosphate receptors (IP3Rs) on the ER, glucose-regulated protein 75 (GRP 75) and voltage-dependent anion channel 1 (VDAC1) on the outer mitochondrial membrane are the canonical component of the Ca2+ transfer unit at ERMCS. These are often reported to form a Ca2+ funnel that fuels the mitochondrial low-affinity Ca2+ uptake system. We assess the available evidence on the IP3R subtype selectivity at the ERMCS and consider if IP3Rs have other roles at the ERMCS beyond providing Ca2+. Growing evidence suggests that all three IP3R subtypes can localise and regulate Ca2+ signalling at ERMCS. Furthermore, IP3Rs may be structurally important for assembly of the ERMCS in addition to their role in providing Ca2+ at these sites. Evidence that various binding partners regulate the assembly and Ca2+ transfer at ERMCS populated by IP3R-GRP75-VDAC1, suggesting that cells have evolved mechanisms that stabilise these junctions forming a Ca2+ microdomain that is required to fuel mitochondrial Ca2+ uptake.

Treatment of Acquired Hypothalamic Obesity: Now and the Future

The hypothalamus is the centre of neuroendocrine regulation of energy homeostasis and appetite. Maldevelopment of, or damage to, the key hypothalamic nuclei disrupts the coordinated balance between energy intake and expenditure leading, to rapid and excessive weight gain. Hypothalamic obesity is compounded by a disruption of the hypothalamic-pituitary axis, sleep disruption, visual compromise, and neurological and vascular sequalae. Amongst suprasellar tumors, craniopharyngioma is the most common cause of acquired hypothalamic obesity, either directly or following surgical or radiotherapeutic intervention. At present, therapy is limited to strategies to manage obesity but with a modest and variable impact. Current approaches include optimizing pituitary hormone replacement, calorie restriction, increased energy expenditure through physical activity, behavioral interventions, pharmacotherapy and bariatric surgery. Current pharmacotherapeutic approaches include stimulants that increase energy consumption, anti-diabetic agents, hypothalamic-pituitary substitution therapy, octreotide, and methionine aminopeptidase 2 (MetAP2) inhibitors. Some pharmacological studies of hypothalamic obesity report weight loss or stabilization but reported intervention periods are short, and others report no effect. The impact of bariatric surgery on weight loss in hypothalamic obesity again is variable. Novel or combined approaches to manage hypothalamic obesity are thus required to achieve credible and sustained weight loss. Identifying etiological factors contributing hypothalamic obesity may lead to multi-faceted interventions targeting hyperphagia, insulin resistance, decreased energy expenditure, sleep disturbance, hypopituitarism and psychosocial morbidity. Placebo-controlled trials using current single, or combination therapies are required to determine the impact of therapeutic agents. A well-defined approach to defining the location of hypothalamic damage may support the use of future targeted therapies. Intranasal oxytocin is currently being investigated as an anorexogenic agent. Novel agents including those targeting pro-opimelanocortin-C and AgRP/NPY expressing neurons and the MC4 receptor may result in better outcomes. This article discusses the current challenges in the management of hypothalamic obesity in children and young people and future therapeutic approaches to increasing weight loss and quality of life in these patients.

Effect of M-phase kinase phosphorylations on type 1 inositol 1,4,5-trisphosphate receptor-mediated Ca2+ responses in mouse eggs

The type 1 inositol 1,4,5-trisphosphate receptor (IP3R1) mediates increases in the intracellular concentration of Ca(2+) ([Ca(2+)]i) during fertilization in mammalian eggs. The activity of IP3R1 is enhanced during oocyte maturation, and phosphorylations by M-phase kinases are thought to positively regulate the activity of IP3R1. Accordingly, we and others have found that IP3R1 is phosphorylated at S(421), T(799) (by Cdk1) and at S(436) (by ERK). Nevertheless, the effects of these phosphorylations on the function of the receptor and their impact on [Ca(2+)]i oscillations in eggs have not been clearly examined. To address this, we expressed in mouse oocytes an IP3R1 variant with the three indicated phosphorylation sites replaced by acidic residues, IIIE-IP3R1, such that it would act like a constitutively phosphorylated IP3R1, and examined [Ca(2+)]i parameters in response to stimuli. We found that overexpression of wild type (wt-IP3R1) or IIIE-IP3R1 in oocytes containing endogenous receptors caused dominant negative-like effects on Ca(2+) release and oscillations. Therefore, we first selectively removed the endogenous IP3R1, and subsequently expressed the exogenous receptors. We found that in response to injection of PLCζ cRNA, eggs without endogenous IP3R1 failed to mount persistent Ca(2+) oscillations, although expression of wt-IP3R1 restored their [Ca(2+)]i oscillatory activity. We also observed that the Ca(2+) oscillatory ability and the sensitivity to IP3 in eggs expressing IIIE-IP3R1 were greater than in those expressing wt-IP3R1. Lastly, we found that exogenous IP3R1s are resistant to downregulation and support longer oscillations and of higher amplitude. Altogether, our results show that phosphorylations by Cdk1 and MAPK enhance the activity of IP3R1, which is consistent with its maximal activity observed at the time of fertilization and the role of Ca(2+) release in egg activation.

Distribution Profile of Inositol 1,4,5-Trisphosphate Receptor/Ca2+ Channels in α and β Cells of Pancreas: Dominant Localization in Secretory Granules and Common Error in Identification of Secretory Granule Membranes

OBJECTIVES

The α and β cells of pancreatic islet release important hormones in response to intracellular Ca increases that result from Ca releases through the inositol 1,4,5-trisphoshate receptor (IP3R)/Ca channels. Yet no systematic studies on distribution of IP3R/Ca channels have been done, prompting us to investigate the distribution of all 3 IP3R isoforms.

METHODS

Immunogold electron microscopy was performed to determine the presence and the relative concentrations of all 3 IP3R isoforms in 2 major organelles secretory granules (SGs) and the endoplasmic reticulum of α and β cells of rat pancreas.

RESULTS

All 3 IP3R isoforms were present in SG membranes of both cells, and the IP3R concentrations in SGs were ∼2-fold higher than those in the endoplasmic reticulum. Moreover, large halos shown in the electron microscope images of insulin-containing SGs of β cells were gap spaces that resulted from separation of granule membranes from the surrounding cytoplasm.

CONCLUSIONS

These results strongly suggest the important roles of SGs in IP3-induced, Ca-dependent regulatory secretory pathway in pancreas. Moreover, the accurate location of SG membranes of β cells was further confirmed by the location of another integral membrane protein synaptotagmin V and of membrane phospholipid PI(4,5)P2.

An IP3R3- and NPY-expressing microvillous cell mediates tissue homeostasis and regeneration in the mouse olfactory epithelium

Calcium-dependent release of neurotrophic factors plays an important role in the maintenance of neurons, yet the release mechanisms are understudied. The inositol triphosphate (IP3) receptor is a calcium release channel that has a physiological role in cell growth, development, sensory perception, neuronal signaling and secretion. In the olfactory system, the IP3 receptor subtype 3 (IP3R3) is expressed exclusively in a microvillous cell subtype that is the predominant cell expressing neurotrophic factor neuropeptide Y (NPY). We hypothesized that IP3R3-expressing microvillous cells secrete sufficient NPY needed for both the continual maintenance of the neuronal population and for neuroregeneration following injury. We addressed this question by assessing the release of NPY and the regenerative capabilities of wild type, IP3R3(+/-), and IP3R3(-/-) mice. Injury, simulated using extracellular ATP, induced IP3 receptor-mediated NPY release in wild-type mice. ATP-evoked NPY release was impaired in IP3R3(-/-) mice, suggesting that IP3R3 contributes to NPY release following injury. Under normal physiological conditions, both IP3R3(-/-) mice and explants from these mice had fewer progenitor cells that proliferate and differentiate into immature neurons. Although the number of mature neurons and the in vivo rate of proliferation were not altered, the proliferative response to the olfactotoxicant satratoxin G and olfactory bulb ablation injury was compromised in the olfactory epithelium of IP3R3(-/-) mice. The reductions in both NPY release and number of progenitor cells in IP3R3(-/-) mice point to a role of the IP3R3 in tissue homeostasis and neuroregeneration. Collectively, these data suggest that IP3R3 expressing microvillous cells are actively responsive to injury and promote recovery.

Arachidonic acid mobilizes Ca2+ from the endoplasmic reticulum and an acidic store in rat pancreatic β cells

In rat pancreatic β cells, arachidonic acid (AA) triggered intracellular Ca(2+) release. This effect could be mimicked by eicosatetraynoic acid, indicating that AA metabolism is not required. The AA-mediated Ca(2+) signal was not affected by inhibition of ryanodine receptors or emptying of ryanodine-sensitive store but was reduced by ∼70% following the disruption of acidic stores (treatment with bafilomycin A1 or glycyl-phenylalanyl-β-naphthylamide (GPN)). The action of AA did not involve TRPM2 channels or NAADP receptors because intracellular dialysis of adenosine diphosphoribose (ADPR; an activator of TRPM2 channels) or NAADP did not affect the AA response. In contrast, stimulation of IP(3) receptors via intracellular dialysis of adenophostin A, or exogenous application of ATP largely abolished the AA-mediated Ca(2+) signal. Intracellular dialysis of heparin abolished the ATP-mediated Ca(2+) signal but not the AA response, suggesting that the action of AA did not involve the IP(3)-binding site. Treatment with the SERCA pump inhibitor, thapsigargin, reduced the amplitude of the AA-mediated Ca(2+) signal by ∼70%. Overall, our finding suggests that AA mobilizes Ca(2+) from the endoplasmic reticulum as well as an acidic store and both stores could be depleted by IP(3) receptor agonist. The possibility of secretory granules as targets of AA is discussed.

Functional ryanodine receptors in the plasma membrane of RINm5F pancreatic beta-cells

Ryanodine receptors (RyR) are Ca²⁺ channels that mediate Ca²⁺ release from intracellular stores in response to diverse intracellular signals. In RINm5F insulinoma cells, caffeine, and 4-chloro-m-cresol (4CmC), agonists of RyR, stimulated Ca²⁺ entry that was independent of store-operated Ca²⁺ entry, and blocked by prior incubation with a concentration of ryanodine that inactivates RyR. Patch-clamp recording identified small numbers of large-conductance (γ_K = 169 pS) cation channels that were activated by caffeine, 4CmC or low concentrations of ryanodine. Similar channels were detected in rat pancreatic β-cells. In RINm5F cells, the channels were blocked by cytosolic, but not extracellular, ruthenium red. Subcellular fractionation showed that type 3 IP₃ receptors (IP₃R3) were expressed predominantly in endoplasmic reticulum, whereas RyR2 were present also in plasma membrane fractions. Using RNAi selectively to reduce expression of RyR1, RyR2, or IP₃R3, we showed that RyR2 mediates both the Ca²⁺ entry and the plasma membrane currents evoked by agonists of RyR. We conclude that small numbers of RyR2 are selectively expressed in the plasma membrane of RINm5F pancreatic β-cells, where they mediate Ca²⁺ entry.

Induction of autophagy promotes fusion of multivesicular bodies with autophagic vacuoles in k562 cells

Morphological and biochemical studies have shown that autophagosomes fuse with endosomes forming the so-called amphisomes, a prelysosomal hybrid organelle. In the present report, we have analyzed this process in K562 cells, an erythroleukemic cell line that generates multivesicular bodies (MVBs) and releases the internal vesicles known as exosomes into the extracellular medium. We have previously shown that in K562 cells, Rab11 decorates MVBs. Therefore, to study at the molecular level the interaction of MVBs with the autophagic pathway, we have examined by confocal microscopy the fate of MVBs in cells overexpressing green fluorescent protein (GFP)-Rab11 and the autophagosomal protein red fluorescent protein-light chain 3 (LC3). Autophagy inducers such as starvation or rapamycin caused an enlargement of the vacuoles decorated with GFP-Rab11 and a remarkable colocalization with LC3. This convergence was abrogated by a Rab11 dominant negative mutant, indicating that a functional Rab11 is involved in the interaction between MVBs and the autophagic pathway. Interestingly, we presented evidence that autophagy induction caused calcium accumulation in autophagic compartments. Furthermore, the convergence between the endosomal and the autophagic pathways was attenuated by the Ca2+ chelator acetoxymethyl ester (AM) of the calcium chelator 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA), indicating that fusion of MVBs with the autophagosome compartment is a calcium-dependent event. In addition, autophagy induction or overexpression of LC3 inhibited exosome release, suggesting that under conditions that stimulates autophagy, MVBs are directed to the autophagic pathway with consequent inhibition in exosome release.

Mini-dystrophin expression down-regulates overactivation of G protein-mediated IP3 signaling pathway in dystrophin-deficient muscle cells

We present here evidence for the enhancement of an inositol 1,4,5-trisphosphate (IP3) mediated calcium signaling pathway in myotubes from dystrophin-deficient cell lines (SolC1(−)) as compared to a cell line from the same origin but transfected with mini-dystrophin (SolD(+)). With confocal microscopy, we demonstrated that calcium rise, induced by the perifusion of a solution containing a high potassium concentration, was higher in SolC1(−) than in SolD(+) myotubes. The analysis of amplitude and kinetics of the calcium increase in SolC1(−) and in SolD(+) myotubes during the exposure with SR Ca²⁺ channel inhibitors (ryanodine and 2-APB) suggested the presence of two mechanisms of SR calcium release: (1) a fast SR calcium release that depended on ryanodine receptors and (2) a slow SR calcium release mediated by IP3 receptors. Detection analyses of mRNAs (reverse transcriptase [RT]-PCR) and proteins (Western blot and immunolocalization) demonstrated the presence of the three known isoforms of IP3 receptors in both SolC1(−) and SolD(+) myotubes. Furthermore, analysis of the kinetics of the rise in calcium revealed that the slow IP3-dependent release may be increased in the SolC1(−) as compared to the SolD(+), suggesting an inhibitory effect of mini-dystrophin in this signaling pathway. Upon incubation with pertussis toxin (PTX), an inhibitory effect similar to that of the IP3R inhibitor (2-APB) was observed on K⁺-evoked calcium release. This result suggests the involvement of a Gi protein upstream of the IP3 pathway in these stimulation conditions. A hypothetical model is depicted in which both Gi protein and IP3 production could be involved in K⁺-evoked calcium release as well as a possible interaction with mini-dystrophin. Our findings demonstrate the existence of a potential relationship between mini-dystrophin and SR calcium release as well as a regulatory role of mini-dystrophin on intracellular signaling.

Organelles Containing Inositol Trisphosphate Receptor Type 2 in Adrenal Medullary Cells

To identify which organelles contained inositol trisphosphate (InsP(3)) receptor type 2 (InsP(3)R2) in adrenal medullary (AM) cells, immunocytochemical and biochemical studies were performed on AM cells of several species. InsP(3)R2-like immunoreactive materials produced by two different anti-InsP(3)R2 antibodies (Abs) (Chemicon and Sigma) were distributed in rat AM cells in agreement with BODIPY-FL-InsP(3) binding sites. For two other Abs (KM1083 and Santa Cruz), some of the anti-InsP(3)R2 immunoreactive materials were stained with an anti-dopamine-beta-hydroxylase Ab, but not by BODIPY-FL-InsP(3). BODIPY-FL-thapsigargin binding sites were consistent with a distribution of the endoplasmic reticulum (ER) identified by an anti-calnexin Ab, and a prior application of thapsigargin significantly eliminated BODIPY-FL-thapsigargin bindings, suggesting that BODIPY-FL-thapsigargin bindings were mediated by thapsigargin, but not the fluorescence molecule. The anti-InsP(3)R2 Ab that produced stainings consistent with BODIPY-FL-InsP(3) bindings recognized a protein with about 250 kDa. A fractional analysis of bovine adrenal medullae revealed that the 250 kDa InsP(3)R2 was detected in a crude membrane fraction, but not in a secretory granule fraction. The results suggest that the InsP(3)R2 was present in the ER, but not in secretory granules in AM cells.

Melatonin stimulates inositol-1,4,5-trisphosphate and Ca2+ release from INS1 insulinoma cells

The effects of melatonin in mammalian cells are exerted via specific receptors or are related to its free radical scavenging activity. It has previously been reported that melatonin inhibits insulin secretion in the pancreatic islets of the rat and in rat insulinoma INS1 cells via Gi-protein-coupled MT1 receptors and the cyclic adenosine 3',5'-monophosphate pathway. However, the inositol-1,4,5-trisphosphate (IP3) pathway is involved in the insulin secretory response as well, and the melatonin signal may play a part in its regulation. This paper addresses the involvement of the second messengers IP3 and intracellular Ca2+ ([Ca2+]i) in the signalling cascade of melatonin in the rat insulinoma INS1 cell, a model for the pancreatic beta-cell. For this purpose melatonin at concentrations ranging from 1 to 100 nmol/L, carbachol and the nonselective melatonin receptor antagonist luzindole were used to stimulate INS1 cell batches, followed by an IP3-mass assay and Ca2+ imaging. Molecular biological studies relating to the mRNA of IP3 receptor (IP3R) subtypes and their relative abundance in INS1 cells showed expression of IP3R-1, IP3R-2 and IP3R-3 mRNA. In conclusion, we found that in rat insulinoma INS1 cells there is a dose-dependent stimulation of IP3 release by melatonin, which is accompanied by a likewise transient increase in [Ca2+]i concentrations. The melatonin effect observed mimics carbachol action. It can be abolished by 30 micromol/L luzindole and is sustained in Ca2+-free medium, suggesting a mechanism that includes the depletion of Ca2+ from intracellular stores.

Mapping protein expression in mouse pancreatic islets by immunolabeling and electron energy loss spectrum-imaging

A combination of immuno-electron microscopy and electron energy-loss spectrum-imaging was used to map the distributions of endocrine polypeptide hormones and proteins in mouse pancreatic islet of Langerhans. Tissue was analyzed from control animals and from mice that were heterozygous for the Anx7 gene, which defines a Ca2+/GTP-dependent membrane fusion and ion channel protein. The heterozygous Anx7 (+/-) mouse displays defects in IP3 receptor mediated Ca2+ signaling and insulin secretion. Therefore, information was obtained about the distributions of the hormones insulin and glucagon, as well as the proteins ANX7 and the IP3 receptor. Insulin secretion appears to be defective in the mutants. It was found from immunolabeling experiments that expression of the IP3 receptor is reduced in mutant islets compared to control islets. Subcellular distributions of sulfur and nitrogen obtained by electron energy-loss spectrum-imaging showed that the insulin concentrations of beta granules were essentially the same in control and mutant islets. By contrast, immunogold labeling of mutant islets shows more insulin immunoreactivity in the beta granules. It follows that insulin may be packaged differently in mutant islets, making antigenic determinants more available to the labeling antibody. The increased rate of insulin secretion in the hyperplastic mutant islets can be explained by compensatory increases in islet size, rather than by an increased insulin concentration in the beta cells. The results indicate that reduced ANX7 expression leads to defects in the IP3 receptor expression in the endocrine cells of the mutant mouse. Increased size of the islet or of adrenal medulla may be a compensatory mechanism for secretion defect by individual endocrine cells. Defects in IP3 receptor expression, and documented consequences of a Ca2+ signaling defect, lead to other changes in organelles such as the mitochondrial number in islet beta-cells. The effects and consequences of reduced ANX7 expression on mitochondria are evident in ultrastructural observations.

Use of somatostatin receptor ligands in obesity and diabetic complications

Somatostatin (SMS) is a potent inhibitory molecule. It inhibits both exocrine and endocrine secretory functions of the pancreas, suppresses growth hormone secretion and reduces the level of insulin-like growth factor-1. Long-acting somatostatin analogues were currently investigated for potential clinical benefits in two settings: (a) control of hyperinsulinaemia in obesity and (b) control of an excess of pro-angiogenic factors in diabetes-associated retinal complications. In two randomized, controlled trials the long-acting somatostatin analogue octreotide retarded progression of the microvascular complications in pre-proliferative and advanced stages of diabetic retinopathy. Inhibition of the early phase of insulin secretion by use of octreotide in patients with hypothalamic obesity resulted in weight loss and improved quality of life. Efficacy of octreotide correlated to residual beta-cell activity prior to the treatment. Obesity and diabetes mellitus are the most common chronic metabolic disorders in the world. The use of somatostatin analogues addressing the various hormonal imbalances of these disorders may provide a novel concept for their pharmacological treatment.

Mechanisms and physiological significance of the cholinergic control of pancreatic beta-cell function

Acetylcholine (ACh), the major parasympathetic neurotransmitter, is released by intrapancreatic nerve endings during the preabsorptive and absorptive phases of feeding. In beta-cells, ACh binds to muscarinic M(3) receptors and exerts complex effects, which culminate in an increase of glucose (nutrient)-induced insulin secretion. Activation of PLC generates diacylglycerol. Activation of PLA(2) produces arachidonic acid and lysophosphatidylcholine. These phospholipid-derived messengers, particularly diacylglycerol, activate PKC, thereby increasing the efficiency of free cytosolic Ca(2+) concentration ([Ca(2+)](c)) on exocytosis of insulin granules. IP3, also produced by PLC, causes a rapid elevation of [Ca(2+)](c) by mobilizing Ca(2+) from the endoplasmic reticulum; the resulting fall in Ca(2+) in the organelle produces a small capacitative Ca(2+) entry. ACh also depolarizes the plasma membrane of beta-cells by a Na(+)- dependent mechanism. When the plasma membrane is already depolarized by secretagogues such as glucose, this additional depolarization induces a sustained increase in [Ca(2+)](c). Surprisingly, ACh can also inhibit voltage-dependent Ca(2+) channels and stimulate Ca(2+) efflux when [Ca(2+)](c) is elevated. However, under physiological conditions, the net effect of ACh on [Ca(2+)](c) is always positive. The insulinotropic effect of ACh results from two mechanisms: one involves a rise in [Ca(2+)](c) and the other involves a marked, PKC-mediated increase in the efficiency of Ca(2+) on exocytosis. The paper also discusses the mechanisms explaining the glucose dependence of the effects of ACh on insulin release.

Differential modulation of inositol 1,4,5-trisphosphate receptor type 1 and type 3 by ATP

Binding of ATP to the inositol 1,4,5-trisphosphate receptor (IP(3)R) results in a more pronounced Ca(2+)release in the presence of inositol 1,4,5-trisphosphate (IP(3)). Two recently published studies demonstrated a different ATP sensitivity of IP(3)-induced Ca(2+)release in cell types expressing different IP(3)R isoforms. Cell types expressing mainly IP(3)R3 were less sensitive to ATP than cell types expressing mainly IP(3)R1 (Missiaen L, Parys JB, Sienaert I et al. Functional properties of the type 3 InsP(3)receptor in 16HBE14o- bronchial mucosal cells. J Biol Chem 1998;273: 8983-8986; Miyakawa T, Maeda A, Yamazawa T et al. Encoding of Ca(2+)signals by differential expression of IP(3)receptor subtypes. EMBO J 1999;18: 1303-1308). In order to investigate the difference in ATP sensitivity between IP(3)R isoforms at the molecular level, microsomes of Sf9 insect cells expressing full-size IP(3)R1 or IP(3)R3 were covalently labeled with ATP by using the photoaffinity label 8-azido[alpha-(32)P]ATP. ATP labeling of the IP(3)R was measured after immunoprecipitation of IP(3)Rs with isoform-specific antibodies, SDS-PAGE and Phosphorimaging. Unlabeled ATP inhibited covalent linking of 8-azido[alpha-(32)P]ATP to the recombinant IP(3)R1 and IP(3)R3 with an IC(50)of 1.6 microM and 177 microM, respectively. MgATP was as effective as ATP in displacing 8-azido[alpha-(32)P]ATP from the ATP-binding sites on IP(3)R1 and IP(3)R3, and in stimulating IP(3)-induced Ca(2+)release from permeabilized A7r5 and 16HBE14o- cells. The interaction of ATP with the ATP-binding sites on IP(3)R1 and IP(3)R3 was different from its interaction with the IP(3)-binding domains, since ATP inhibited IP(3)binding to the N-terminal 581 amino acids of IP(3)R1 and IP(3)R3 with an IC(50)of 353 microM and 4.0 mM, respectively. The ATP-binding sites of IP(3)R1 bound much better ATP than ADP, AMP and particularly GTP, while IP(3)R3 displayed a much broader nucleotide specificity. These results therefore provide molecular evidence for a differential regulation of IP(3)R1 and IP(3)R3 by ATP.

GABA receptor rho1 subunit interacts with a novel splice variant of the glycine transporter, GLYT-1

Ionotropic gamma-aminobutyric acid (GABA(A) and GABA(C)) receptors mediate fast synaptic inhibition in the central nervous system. GABA(C) receptors are expressed predominantly in the retina on bipolar cell axon terminals, and are thought to mediate feedback inhibition from GABAergic amacrine cells. Utilizing the yeast two-hybrid system, we previously identified MAP1B as a binding partner of the GABA(C) receptor rho1 subunit. Here we describe the isolation of an additional rho1 interacting protein: a novel C-terminal variant of the glycine transporter GLYT-1. We show that GLYT-1 exists as four alternatively spliced mRNAs which encode proteins expressing one of two possible intracellullar N- and C-terminal domains. Variants containing the novel C terminus efficiently transport glycine when expressed in COS cells, but with unusual kinetics. We have confirmed the interaction between the novel C terminus and rho1 subunit and demonstrated binding in heterologous cells. This interaction may be crucial for the integration of GABAergic and glycinergic neurotransmission in the retina.

Defects in inositol 1,4,5-trisphosphate receptor expression, Ca(2+) signaling, and insulin secretion in the anx7(+/-) knockout mouse

The mammalian anx7 gene codes for a Ca(2+)-activated GTPase, which supports Ca(2+)/GTP-dependent secretion events and Ca(2+) channel activities in vitro and in vivo. To test whether anx7 might be involved in Ca(2+) signaling in secreting pancreatic beta cells, we knocked out the anx7 gene in the mouse and tested the insulin-secretory properties of the beta cells. The nullizygous anx7 (-/-) phenotype is lethal at embryonic day 10 because of cerebral hemorrhage. However, the heterozygous anx7 (+/-) mouse, although expressing only low levels of ANX7 protein, is viable and fertile. The anx7 (+/-) phenotype is associated with a substantial defect in insulin secretion, although the insulin content of the islets, is 8- to 10-fold higher in the mutants than in the normal littermate control. We infer from electrophysiological studies that both glucose-stimulated secretion and voltage-dependent Ca(2+) channel functions are normal. However, electrooptical recordings indicate that the (+/-) mutation has caused a change in the ability of inositol 1,4,5-trisphosphate (IP(3))-generating agonists to release intracellular calcium. The principle molecular consequence of lower anx7 expression is a profound reduction in IP(3) receptor expression and function in pancreatic islets. The profound increase in islets, beta cell number, and size may be a means of compensating for less efficient insulin secretion by individual defective pancreatic beta cells. This is a direct demonstration of a connection between glucose-activated insulin secretion and Ca(2+) signaling through IP(3)-sensitive Ca(2+) stores.

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.

Evidence for vacuolar-type proton pumps in nonmitochondrial and inositol 1,4,5-trisphosphate-sensitive calcium stores of insulin-secreting cells

This study examines whether acidic, vacuolar-type, proton-pump-carrying organelles of insulin-secreting cells (clonal endocrine pancreatic cell line INS-1) function as rapidly exchanging, inositol 1,4,5-trisphosphate-sensitive calcium stores. Calcium uptake into calcium stores will be modulated by the proton concentration within the stores, since calcium pumps in general appear to mediate a countertransport of calcium with protons. We therefore tested for sensitivity of calcium sequestration by nonmitochondrial stores (inhibition of mitochondrial calcium uptake by 2 microM ruthenium red) in saponin-permeabilized cells to proton-conducting ionophores and proton pump inhibition, using this as a marker for involvement of acidic organelles. Calcium sequestration was partially inhibited by the protonophores nigericin (10-50 microM) and carbonylcyanide m-chlorophenylhydrazone (CCCP; 20-50 microM), as well as by inclusion of 30 mM NH4Cl. Bafilomycin A1, a potent and selective inhibitor of vacuolar-type proton pumps, alone (1 - 500 nM) had no effect on calcium sequestration. however, it induced an inhibitory effect in the presence of nigericin or CCCP, even at low concentrations (5 microM) of these ionophores, lacking itself an inhibitory action on calcium sequestration. Bafilomycin A1 then was already maximally active at a concentration as low as 10 nM. Corres ponding to inhibition of total nonmitochondrial calcium sequestration, filling of inositol 1,4,5-trisphosphate-sensitive stores was decreased or even abolished by the protonophores alone or the protonophores combined with bafilomycin A1. We conclude that vacuolar-type proton pumps are present in at least a part of nonmitochondrial and inositol 1,4,5-trisphosphate-sensitive calcium stores in INS-1 cells. This assigns these stores to organelles such as secretory granules, the trans Golgi network, or endosomes. Luminal acidity of these stores will stimulate calcium sequestration by providing more protons for countertransport of calcium by calcium pumps. High concentrations of protonophores may be required for inhibitory effects because otherwise the proton pumps may be able to compensate sufficiently for ionophore-mediated proton loss. The lack of effect of bafilomycin A1 without protonophores may be due to a sufficient luminal buffering capacity or to preceding inhibition of the pump by an inside-positive transmembrane potential.

Expression of inositol 1,4,5-trisphosphate receptors changes the Ca2+ signal of Xenopus oocytes

The receptors for the second messenger inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] form a family of closely related proteins that play an important role in regulating the free intracellular Ca2+ concentration. To test the hypothesis that changing the expression level of Ins(1,4,5)P3 receptors could alter the Ins(1,4,5)P3-mediated Ca2+ signal, we overexpressed Ins(1,4,5)P3 receptor type 1 (InsP3R-1) or type 3 (InsP3R-3) in Xenopus laevis oocytes. Expression of InsP3R-1 increased the velocity of the propagating waves of intracellular Ca2+ release but did not affect the Ins(1,4,5)P3-induced entry of extracellular Ca2+ across the plasma membrane. In contrast, expression of intracellular Ca2+ but markedly increased the magnitude and duration of Ca2+ influx. Immunolocalization studied revealed InsP3R-3 at the endoplasmic reticulum, with a relatively stronger signal at or near the plasma membrane. The results suggest that changing the expression level of an InsP3R can alter the Ins(1,4,5)P3-mediated Ca2+ signal and that InsP3R-1 and InsP3R-3 may have different biological functions.

Identification of three isoforms of the InsP3 receptor in human myometrial smooth muscle

The mechanisms responsible for the mobilisation of Ca2+ from intracellular stores sensitive to inositol trisphosphate (InsP3) were studied in saponin-permeabilised human myometrial cells in which the sarcoplasmic reticulum was pre-loaded with 45Ca2+. InsP3-induced 45Ca2+ release was measured over the InsP3 concentration range of 100 nM to 100 microM and showed a graded response. InsP3-induced 45Ca2+ release was inhibited by heparin (20-40 microg/ml) but not significantly affected by caffeine. The Ca2+ sensitivity of InsP3-induced Ca2+ release was measured under conditions which were designed to exclude interference with Ca2+ released by the ryanodine receptor/channel complex. The data showed a bell-shaped relationship with the InsP3 receptor (InsP3R) functional at 10 nM, becoming maximally activated at 300 nM but inhibited at 10 microM Ca2+. Messenger RNA encoding for three isoforms of InsP3R, type I, II and type III, was shown to be present. The relative expression levels of these messengers were obtained by ratio-PCR analysis and the levels of expression of the different isoforms were found to differ between individual patients.

The inositol triphosphate receptor family

Specific receptors on intracellular membranes mediate the Ca2+ mobilization induced by the second messenger molecule D-myo-inositol 1,4,5-triphosphate (IP3). Most cell types appear to contain multiple receptor isoforms. The review summarizes recent progress on IP3 receptor biology with a particular emphasis on distinctive structural and regulatory features of the individual isoforms.

Inositol 1,4,5-trisphosphate receptors, secretory granules and secretion in endocrine and neuroendocrine cells

Recent studies have revealed the presence of inositol 1,4,5-trisphosphate receptors in the secretory granules of endocrine and neuroendocrine cells. This distribution suggests that inositol 1,4,5-trisphosphate-regulated release of granule stores of Ca2+ might facilitate the secretory process. In addition, inositol 1,4,5-trisphosphate receptors might participate directly in the biogenesis of secretory granules. The presence of inositol 1,4,5-trisphosphate receptors in synaptic nerve terminals raises the possibility that they might also be involved in the control of neurotransmitter release.