Adenophostin A can stimulate Ca2+ influx without depleting the inositol 1,4,5-trisphosphate-sensitive Ca2+ stores in the Xenopus oocyte

Pharmacological Regulation of Endoplasmic Reticulum Structure and Calcium Dynamics: Importance for Neurodegenerative Diseases

The endoplasmic reticulum (ER) is the largest organelle of the cell, composed of a continuous network of sheets and tubules, and is involved in protein, calcium (Ca2+), and lipid homeostasis. In neurons, the ER extends throughout the cell, both somal and axodendritic compartments, and is highly important for neuronal functions. A third of the proteome of a cell, secreted and membrane-bound proteins, are processed within the ER lumen and most of these proteins are vital for neuronal activity. The brain itself is high in lipid content, and many structural lipids are produced, in part, by the ER. Cholesterol and steroid synthesis are strictly regulated in the ER of the blood-brain barrier protected brain cells. The high Ca2+ level in the ER lumen and low cytosolic concentration is needed for Ca2+-based intracellular signaling, for synaptic signaling and Ca2+ waves, and for preparing proteins for correct folding in the presence of high Ca2+ concentrations to cope with the high concentrations of extracellular milieu. Particularly, ER Ca2+ is controlled in axodendritic areas for proper neurito- and synaptogenesis and synaptic plasticity and remodeling. In this review, we cover the physiologic functions of the neuronal ER and discuss it in context of common neurodegenerative diseases, focusing on pharmacological regulation of ER Ca2+ Furthermore, we postulate that heterogeneity of the ER, its protein folding capacity, and ensuring Ca2+ regulation are crucial factors for the aging and selective vulnerability of neurons in various neurodegenerative diseases. SIGNIFICANCE STATEMENT: Endoplasmic reticulum (ER) Ca2+ regulators are promising therapeutic targets for degenerative diseases for which efficacious drug therapies do not exist. The use of pharmacological probes targeting maintenance and restoration of ER Ca2+ can provide restoration of protein homeostasis (e.g., folding of complex plasma membrane signaling receptors) and slow down the degeneration process of neurons.

Stimulation of inositol 1,4,5-trisphosphate (IP3) receptor subtypes by adenophostin A and its analogues

Inositol 1,4,5-trisphosphate receptors (IP3R) are intracellular Ca(2+) channels. Most animal cells express mixtures of the three IP3R subtypes encoded by vertebrate genomes. Adenophostin A (AdA) is the most potent naturally occurring agonist of IP3R and it shares with IP3 the essential features of all IP3R agonists, namely structures equivalent to the 4,5-bisphosphate and 6-hydroxyl of IP3. The two essential phosphate groups contribute to closure of the clam-like IP3-binding core (IBC), and thereby IP3R activation, by binding to each of its sides (the α- and β-domains). Regulation of the three subtypes of IP3R by AdA and its analogues has not been examined in cells expressing defined homogenous populations of IP3R. We measured Ca(2+) release evoked by synthetic adenophostin A (AdA) and its analogues in permeabilized DT40 cells devoid of native IP3R and stably expressing single subtypes of mammalian IP3R. The determinants of high-affinity binding of AdA and its analogues were indistinguishable for each IP3R subtype. The results are consistent with a cation-π interaction between the adenine of AdA and a conserved arginine within the IBC α-domain contributing to closure of the IBC. The two complementary contacts between AdA and the α-domain (cation-π interaction and 3″-phosphate) allow activation of IP3R by an analogue of AdA (3″-dephospho-AdA) that lacks a phosphate group equivalent to the essential 5-phosphate of IP3. These data provide the first structure-activity analyses of key AdA analogues using homogenous populations of all mammalian IP3R subtypes. They demonstrate that differences in the Ca(2+) signals evoked by AdA analogues are unlikely to be due to selective regulation of IP3R subtypes.

Homeostasis established by coordination of subcellular compartment plasticity improves spike encoding

Homeostasis in cells maintains their survival and functions. The plasticity at neurons and synapses may destabilize their signal encoding. The rapid recovery of cellular homeostasis is needed to secure the precise and reliable encoding of neural signals necessary for well-organized behaviors. We report a homeostatic process that is rapidly established through Ca(2+)-induced coordination of functional plasticity among subcellular compartments. An elevation of cytoplasmic Ca(2+) levels raises the threshold potentials and refractory periods of somatic spikes, and strengthens the signal transmission at glutamatergic and GABAergic synapses, in which synaptic potentiation shortens refractory periods and lowers threshold potentials. Ca(2+) signals also induce an inverse change of membrane excitability at the soma versus the axon. The integrative effect of Ca(2+)-induced plasticity among the subcellular compartments is homeostatic in nature, because it stabilizes neuronal activities and improves spike timing precision. Our study of neuronal homeostasis that is fulfilled by rapidly coordinating subcellular compartments to improve neuronal encoding sheds light on exploring homeostatic mechanisms in other cell types.

Inositol trisphosphate receptor Ca2+ release channels

The inositol 1,4,5-trisphosphate (InsP3) receptors (InsP3Rs) are a family of Ca2+ release channels localized predominately in the endoplasmic reticulum of all cell types. They function to release Ca2+ into the cytoplasm in response to InsP3 produced by diverse stimuli, generating complex local and global Ca2+ signals that regulate numerous cell physiological processes ranging from gene transcription to secretion to learning and memory. The InsP3R is a calcium-selective cation channel whose gating is regulated not only by InsP3, but by other ligands as well, in particular cytoplasmic Ca2+. Over the last decade, detailed quantitative studies of InsP3R channel function and its regulation by ligands and interacting proteins have provided new insights into a remarkable richness of channel regulation and of the structural aspects that underlie signal transduction and permeation. Here, we focus on these developments and review and synthesize the literature regarding the structure and single-channel properties of the InsP3R.

The role of nuclear envelope calcium in modifying nuclear pore complex structureThis paper is one of a selection of papers published in this Special Issue, entitled The Nucleus: A Cell Within A Cell

Some of the most important trafficking processes in cells involve transport across the nuclear envelope. Whether it is the import of transcription factors or the export of RNA, the only known portal across the double lipid bilayer that forms the nuclear envelope are the macromolecular pores known as nuclear pore complexes (NPCs). Understanding how signals influence the conformation of the NPC is important for testing models of, and perhaps modifying, transport across the nuclear envelope. Here we summarize high-resolution atomic force microscopy studies of NPC structure following manipulation of nuclear envelope calcium stores of nuclei from Xenopus laevis oocytes. The results show that the release of calcium from these stores through the specific activation of inositol 1,4,5-trisphosphate receptors leads to changes in NPC structure observable from both sides of the nuclear envelope. The diameter of the NPC is also sensitive to these calcium stores and increases upon calcium release. Western blot analysis reveals the presence of ryanodine receptors in the nuclear envelope of X. laevis oocytes, although in low abundance. Activation of these calcium channels also leads to the displacement of the central mass and changes in NPC diameter. This change in structure may involve a displacement of the cytoplasmic and nuclear rings of the NPC towards each other, leading to the apparent emergence of the central mass from both sides of the NPC. The changes in conformation and diameter of the NPC may alter cargo access and binding to phenylalanine-glycine repeats lining the pore, thus altering transport.

Nuclear side conformational changes in the nuclear pore complex following calcium release from the nuclear membrane

Changes in nuclear pore complex (NPC) structure are studied following treatments modifying the cisternal calcium levels located between the two lipid bilayers that together form the nuclear envelope. Since the NPC forms the only known passageway across the nuclear envelope, it plays a central role in nucleocytoplasmic transport. Understanding the origin of conformational changes that may affect this trafficking or modify cargo interactions with the NPC is, therefore, necessary to completely understand the function of these complex molecules. In previous studies on the cytoplasmic side of the nuclear envelope, a central mass was observed in the pore of the NPC and its location was shown to be sensitive to the cisternal calcium levels. Here we report atomic force microscopy (AFM) measurements on the nuclear side of the envelope, which also reveal a cisternal calcium dependence in the conformational state of the NPC. These measurements, made at the single nuclear pore level, reveal a displacement of the central mass towards the nuclear side of the membrane following treatments with adenophostin A, a specific agonist of calcium channels (inositol 1,4,5-trisphosphate (IP(3)) receptors) located in the nuclear envelope. We further demonstrate that these conformational changes are observed in nuclear pores lacking the basket structure while samples prepared in the presence of protease inhibitors retain baskets and block AFM measurements of the channel. While these measurements are unable to distinguish whether the central mass is cargo or an integral component of the NPC, its dose-dependent displacement with cisternal calcium levels does suggest links to transport or to changes in cargo interactions with the NPC. Taken together with previous measurements done on the cytoplasmic side of the nuclear envelope, these studies argue against a piston-like displacement of the central mass and instead suggest a more complicated mechanism. One possibility involves a concerted collapse of the NPC rings towards one another following cisternal calcium release, thus leading to the apparent emergence of the central mass from each side of the NPC.

Activation of Postsynaptic Ca2+Stores Modulates Glutamate Receptor Cycling in Hippocampal Neurons

We show that activation of postsynaptic inositol 1,4,5-tris-phosphate receptors (IP3Rs) with the IP3R agonist adenophostin A (AdA) produces large increases in AMPA receptor (AMPAR) excitatory postsynaptic current (EPSC) amplitudes at hippocampal CA1 synapses. Co-perfusion of the Ca2+chelator bis-( o-aminophenoxy)- N,N,N′,N′-tetraacetic acid strongly inhibited AdA-enhanced increases in EPSC amplitudes. We examined the role of AMPAR insertion/anchoring in basal synaptic transmission. Perfusion of an inhibitor of synaptotagmin-soluble n-ethylmaleimide-sensitive factor attachment protein (SNAP) receptor SNARE-mediated exocytosis depressed basal EPSC amplitudes, whereas a peptide that inhibits GluR2/3 interactions with postsynaptic density-95 (PDZ) domain proteins glutamate receptor interacting protein (GRIP)/protein interacting with C-kinase-1 (PICK1) enhanced basal synaptic transmission. These results suggest that constitutive trafficking and anchoring of AMPARs help maintain basal synaptic transmission. The regulation of postsynaptic AMPAR trafficking involves synaptotagmin-SNARE-mediated vesicle exocytosis and interactions between AMPARs and the PDZ domains in GRIP/PICK1. We show that inhibitors of synaptotagmin-SNARE-mediated exocytosis, or interactions between AMPARs and GRIP/PICK1, attenuated AdA-enhanced increases in EPSC amplitudes. These results suggest that IP3R-mediated Ca2+release can enhance AMPAR EPSC amplitudes through mechanisms that involve AMPAR-PDZ interactions and/or synaptotagmin-SNARE-mediated receptor trafficking.

Ca2+–calmodulin signalling pathway up-regulates GABA synaptic transmission through cytoskeleton-mediated mechanisms

We investigated the role of calcium (Ca(2+))/calmodulin (CaM) signaling pathways in modulating GABA synaptic transmission at CA1 pyramidal neurons in hippocampal slices. Whole-cell pipettes were used to record type A GABA receptor (GABA(A)R)-gated inhibitory postsynaptic currents (IPSCs) and to perfuse intracellularly modulators in the presence of glutamate receptor antagonists. GABA(A)R-gated IPSCs were enhanced by the postsynaptic infusions of adenophostin (1 microM), a potent agonist of inositol-1,4,5-triphosphate receptor (IP(3)R) that induces Ca(2+) release. The enhancement was blocked by co-infusing either 1,2-bis(2-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid (10 mM) or CaM-binding peptide (100 microM). Moreover, the postsynaptic infusion of Ca(2+)-CaM (40/10 microM) enhanced both evoked and spontaneous GABA(A)R-gated IPSCs. The enhancement was attenuated by co-infusing 100 microM CaM-KII(281-301), an autoinhibitory peptide of CaM-dependent protein kinases. These results indicate that postsynaptic Ca(2+)-CaM signaling pathways essentially enhance GABAergic synaptic transmission. In the investigation of synaptic targets for the enhancement, we found that IP(3)R agonist-enhanced GABA(A)R-gated IPSCs were attenuated by co-infusing colchicine (30 microM), vincristine (3 microM) or cytochalasin D (1 microM) that inhibits tubulin or actin polymerization, implying that actin filament and microtubules are involved. We conclude that postsynaptic Ca(2+)-CaM signaling pathways strengthen the function of GABAergic synapses via a cytoskeleton-mediated mechanism, probably the recruitment of receptors in the postsynaptic membrane.

Activation of inositol 1,4,5-trisphosphate receptor is essential for the opening of mouse TRP5 channels

We studied the opening mechanism of Ca(2+)-permeable channels formed with mouse transient receptor potential type 5 (mTRP5) using Xenopus oocytes. After stimulation of coexpressed muscarinic M(1) receptors with acetylcholine (ACh) in a Ca(2+)-free solution, switching to 2 mM Ca(2+)-containing solution evoked a large Cl(-) current, which reflects the opening of endogenous Ca(2+)-dependent Cl(-) channels following Ca(2+) entry through the expressed channels. The ACh-evoked response was not affected by a depletion of Ca(2+) store with thapsigargin but was inhibited by preinjection of antisense oligodeoxynucleotides (ODNs) to G(q), G(11), or both. The mTRP5 channel response was also induced by a direct activation of G proteins with injection of guanosine 5'-3-O-(thio)triphosphate (GTP gamma S). The ACh- and GTP gamma S-evoked responses were inhibited by either pretreatment with a phospholipase C inhibitor, U73122, or an inositol-1,4,5-trisphosphate (IP(3)) receptor inhibitor, xestospongin C (XeC). An activation of IP(3) receptors with injection of adenophostin A (AdA) evoked the mTRP5 channel response in a dose-dependent manner. The AdA-evoked response was not suppressed by preinjection of antisense ODNs to G(q/11) or U73122 but was suppressed by either preinjection of XeC or a peptide mimicking the IP(3) binding domain of Xenopus IP(3) receptor. These findings suggest that the activation of IP(3) receptor is essential for the opening of mTRP5 channels, and that neither G proteins, phosphoinositide metabolism, nor depletion of the Ca(2+) store directly modifies the IP(3) receptor-linked opening of mTRP5 channels.

Calcium-calmodulin signalling pathway up-regulates glutamatergic synaptic function in non-pyramidal, fast spiking rat hippocampal CA1 neurons

1. The role of Ca(2+)-calmodulin (CaM) signalling cascades in modulating glutamatergic synaptic transmission on CA1 non-pyramidal fast-spiking neurons was investigated using whole-cell recording and perfusion in rat hippocampal slices. 2. Paired stimuli (PS), consisting of postsynaptic depolarization to 0 mV and presynaptic stimulation at 1 Hz for 30 s, enhanced excitatory postsynaptic currents (EPSCs) on non-pyramidal neurons in the stratum pyramidale (SP). The potentiation was reduced by the extracellular application of D-amino-5-phosphonovaleric acid (DAP-5, 40 microM), and blocked by the postsynaptic perfusion of 1,2-bis(2-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid (BAPTA, 10 mM), a CaM-binding peptide (100 microM) or CaMKII (281-301) (an autoinhibitory peptide of CaM-dependent protein kinases, 100 microM). 3. The application of adenophostin, an agonist of inositol trisphosphate receptors (IP(3)Rs) that evokes Ca(2+) release, into SP non-pyramidal neurons via the patch pipette (1 microM) enhanced EPSCs and occluded PS-induced synaptic potentiation. The co-application of BAPTA (10 mM) with adenophostin blocked synaptic potentiation. In addition, Ca(2+)-CaM (40:10 microM) induced synaptic potentiation, which occluded PS-induced potentiation and was attenuated by introducing CaMKII (281-301) (100 microM). EPSCs were sensitive to an antagonist of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptor (AMPAR). 4. Application of Ca(2+)-CaM into SP non-pyramidal neurons induced the emergence of AMPAR-mediated EPSCs that were not evoked by low stimulus intensity before perfusion. Ca(2+)-CaM also increased the amplitude and frequency of spontaneous EPSCs. A scavenger of nitric oxide, carboxy-PTIO (30 microM in slice-perfusion solution), did not affect these increases in sEPSCs. 5. The magnitude of PS-, adenophostin- or Ca(2+)-CaM-induced synaptic potentiation in SP non-pyramidal neurons increased during postnatal development. 6. These results indicate that Ca(2+)-CaM signalling pathways in CA1 SP non-pyramidal neurons up-regulate glutamatergic synaptic transmission probably through the conversion of inactive-to-active synapses.

InsP(3)-induced Ca(2+) release in permeabilized invertebrate photoreceptors: a link between phototransduction and Ca(2+) stores

Using the low-affinity fluorescent Ca(2+) indicators, Mag-Fura-2 and Mag-Fura Red, we studied light- and InsP(3)-induced Ca(2+) release in permeabilized microvillar photoreceptors of the medicinal leech, Hirudo medicinalis. Two major components of the phosphoinositide signaling pathway, phospholipase-C and the InsP(3) receptor, were characterized immunologically and appropriately localized in photoreceptors. Whereas phospholipase-C was abudantly expressed in photoreceptive microvilli, InsP(3) receptors were found mostly in submicrovillar endoplasmic reticulum (SER). Permeabilization of the peripheral plasma membrane with saponin allowed direct measurements of luminal free Ca(2+) concentration (Ca(L)) changes. Confocal Ca(2+) imaging using Mag-Fura Red demonstrated that Ins(1,4,5)P(3) mobilizes Ca(2+) from SER. As detected with Mag-Fura-2, a brief 50ms light flash activated rapid Ca(2+) depletion of SER, followed by an effective refilling within 1min of dark adaptation after the light flash. Sensitivity to Ins(1,4,5)P(3) of the Ca(2+) release from SER in leech photoreceptors was accompanied by irreversible uncoupling of phototransduction from Ca(2+) release. Depletion of Ca(2+) stores was induced by Ins(1,4,5)P(3)(EC(50)= 4.75 microM) and the hyper-potent agonist adenophostin A (EC(50)/40nM) while the stereoisomer L-myo Ins(1,4,5)P(3) was totally inactive. Ins(1,4,5)P(3)- or adenophostin A-induced Ca(2+) release was inhibited by 0.1-1mg/ml heparin. The Ca(2+) pump inhibitors, cyclopiazonic acid and thapsigargin, in the presence of Ins(1,4,5)P(3), completely depleted Ca(2+) stores in leech photoreceptors.

Sperm factor induces intracellular free calcium oscillations by stimulating the phosphoinositide pathway

Injection of a porcine cytosolic sperm factor (SF) or of a porcine testicular extract into mammalian eggs triggers oscillations of intracellular free calcium ([Ca(2+)](i)) similar to those initiated by fertilization. To elucidate whether SF activates the phosphoinositide (PI) pathway, mouse eggs or SF were incubated with U73122, an inhibitor of events leading to phospholipase C (PLC) activation and/or of PLC itself. In both cases, U73122 blocked the ability of SF to induce [Ca(2+)](i) oscillations, although it did not inhibit Ca(2+) release caused by injection of inositol 1,4,5-triphosphate (IP(3)). The inactive analogue, U73343, had no effect on SF-induced Ca(2+) responses. To determine at the single cell level whether SF triggers IP(3) production concomitantly with a [Ca(2+)](i) rise, SF was injected into Xenopus oocytes and IP(3) concentration was determined using a biological detector cell combined with capillary electrophoresis. Injection of SF induced a significant increase in [Ca(2+)](i) and IP(3) production in these oocytes. Using ammonium sulfate precipitation, chromatographic fractionation, and Western blotting, we determined whether PLCgamma1, PLCgamma2, or PLCdelta4 and/or its splice variants, which are present in sperm and testis, are responsible for the Ca(2+) activity in the extracts. Our results revealed that active fractions do not contain PLCgamma1, PLCgamma2, or PLCdelta4 and/or its splice variants, which were present in inactive fractions. We also tested whether IP(3) could be the sensitizing stimulus of the Ca(2+)-induced Ca(2+) release mechanism, which is an important feature of fertilized and SF-injected eggs. Eggs injected with adenophostin A, an IP(3) receptor agonist, showed enhanced Ca(2+) responses to CaCl(2) injections. Thus, SF, and probably sperm, induces [Ca(2+)](i) rises by persistently stimulating IP(3) production, which in turn results in long-lasting sensitization of Ca(2+)-induced Ca(2+) release. Whether SF is itself a PLC or whether it acts upstream of the egg's PLCs remains to be elucidated.

ATP-dependent adenophostin activation of inositol 1,4,5-trisphosphate receptor channel gating: kinetic implications for the durations of calcium puffs in cells

The inositol 1,4,5-trisphosphate (InsP(3)) receptor (InsP(3)R) is a ligand-gated intracellular Ca(2+) release channel that plays a central role in modulating cytoplasmic free Ca(2+) concentration ([Ca(2+)](i)). The fungal metabolite adenophostin A (AdA) is a potent agonist of the InsP(3)R that is structurally different from InsP(3) and elicits distinct calcium signals in cells. We have investigated the effects of AdA and its analogues on single-channel activities of the InsP(3)R in the outer membrane of isolated Xenopus laevis oocyte nuclei. InsP(3)R activated by either AdA or InsP(3) have identical channel conductance properties. Furthermore, AdA, like InsP(3), activates the channel by tuning Ca(2+) inhibition of gating. However, gating of the AdA-liganded InsP(3)R has a critical dependence on cytoplasmic ATP free acid concentration not observed for InsP(3)-liganded channels. Channel gating activated by AdA is indistinguishable from that elicited by InsP(3) in the presence of 0.5 mM ATP, although the functional affinity of the channel is 60-fold higher for AdA. However, in the absence of ATP, gating kinetics of AdA-liganded InsP(3)R were very different. Channel open time was reduced by 50%, resulting in substantially lower maximum open probability than channels activated by AdA in the presence of ATP, or by InsP(3) in the presence or absence of ATP. Also, the higher functional affinity of InsP(3)R for AdA than for InsP(3) is nearly abolished in the absence of ATP. Low affinity AdA analogues furanophostin and ribophostin activated InsP(3)R channels with gating properties similar to those of AdA. These results provide novel insights for interpretations of observed effects of AdA on calcium signaling, including the mechanisms that determine the durations of elementary Ca(2+) release events in cells. Comparisons of single-channel gating kinetics of the InsP(3)R activated by InsP(3), AdA, and its analogues also identify molecular elements in InsP(3)R ligands that contribute to binding and activation of channel gating.

Calreticulin modulates capacitative Ca2+ influx by controlling the extent of inositol 1,4,5-trisphosphate-induced Ca2+ store depletion

Calreticulin (CRT) is a highly conserved Ca(2+)-binding protein that resides in the lumen of the endoplasmic reticulum (ER). We overexpressed CRT in Xenopus oocytes to determine how it could modulate inositol 1,4,5-trisphosphate (InsP(3))-induced Ca(2+) influx. Under conditions where it did not affect the spatially complex elevations in free cytosolic Ca(2+) concentration ([Ca(2+)](i)) due to InsP(3)-induced Ca(2+) release, overexpressed CRT decreased by 46% the Ca(2+)-gated Cl(-) current due to Ca(2+) influx. Deletion mutants revealed that CRT requires its high capacity Ca(2+)-binding domain to reduce the elevations of [Ca(2+)](i) due to Ca(2+) influx. This functional domain was also required for CRT to attenuate the InsP(3)-induced decline in the free Ca(2+) concentration within the ER lumen ([Ca(2+)](ER)), as monitored with a "chameleon" indicator. Our data suggest that by buffering [Ca(2+)](ER) near resting levels, CRT may prevent InsP(3) from depleting the intracellular stores sufficiently to activate Ca(2+) influx.

Inframolecular studies of the protonation of adenophostin A: comparison with 1-D-myo-inositol 1,4,5-trisphosphate

Adenophostin A is a glyconucleotide natural product with the highest known potency for the D-myo-inositol 1,4,5-trisphosphate receptor. Using synthetic adenophostin A we have investigated the macroscopic and microscopic protonation process of this compound by performing (31)P NMR, (1)H NMR, and potentiometric titration experiments. The logarithms of the first to the fourth stepwise protonation constants are, respectively, log K(1) = 8.48, log K(2) = 6.20, log K(3) = 4.96, and log K(4) = 3.80. The latter constant refers to the protonation equilibrium involving the N1 adenine nitrogen. From the microconstants the protonation fractions of each individual phosphate group can be calculated. Remarkably, the ionization state of the phosphates of adenophostin A at near physiological pH is very similar to those of inositol 1,4,5-trisphosphate, indicating that differences in phosphate charge cannot account for the high potency of this molecule. The analysis of the (1)H chemical shifts vs pH provided complementary conformational information. In particular, a slight "wrongway shift" of H1" can be related to the protonation of P2, thus indicating a short H1"-P2 distance. Our results are in line with a recently published model in which, however, a certain degree of constraint would keep the ribose 2'-phosphate moiety close to the glucose ring phosphates.

Molecular recognition of adenophostin, a very potent Ca2+ inducer, at the D-myo-inositol 1,4,5-trisphosphate receptor

The recognition mode of adenophostin A at the D-myo-inositol 1,4, 5-trisphosphate [Ins(1,4,5)P(3)] receptor was investigated. Comparison of conformations of Ins(1,4,5)P(3) and adenophostin A by using the combination of NMR and molecular mechanics (MM) calculations demonstrated that adenophostin A adopted a moderately extended conformation regarding the distance between the 2'-phosphoryl group and the 3' ',4' '-bisphosphate motif, as suggested previously [Wilcox, R. A. et al. (1995) Mol. Pharmacol. 47, 1204-1211]. Based on the nuclear Overhauser effect (NOE) observed between 3'-H and 1' '-H and on MM calculations, the molecular shape of adenophostin A proved to be an extended form at least in solution, in contrast to Wilcox's compactly folded, preliminary hairpin model. GlcdR(2,3',4')P(3), an adenophostin analogue without adenine moiety, was found to be less potent than adenophostin A and almost equipotent to Ins(1,4,5)P(3). We propose the possibility that (i) the optimal spatial arrangement of the three phosphoryl groups and/or (ii) the interaction of the adenine moiety of adenophostin A with the putative binding site, if it exists in the vicinity of the Ins(1,4,5)P(3)-binding site, might account for the exceptional potency of adenophostin A.

Adenophostin A induces spatially restricted calcium signaling in Xenopus laevis oocytes

The activation of intracellular calcium release and calcium entry across the plasmalemma in response to intracellular application of inositol 2,4,5-trisphosphate and adenophostin A, two metabolically stable agonists for inositol 1,4,5-trisphosphate receptors, was investigated using Xenopus laevis oocytes and confocal imaging. Intracellular injection of inositol 2,4,5-trisphosphate induced a rapidly spreading calcium signal associated with regenerative calcium waves; the calcium signal filled the peripheral regions of the cell in 1-5 min. Injection of high concentrations of adenophostin A (250 nM) similarly induced rapidly spreading calcium signals. Injection of low concentrations of adenophostin A resulted in calcium signals that spread slowly (>1 h). With extremely low concentrations of adenophostin A (approximately 10 pM), stable regions of Ca2+ release were observed that did not expand to peripheral regions. When the adenophostin A-induced calcium signal was restricted to central regions, compartmentalized calcium oscillations were sometimes observed. Restoration of extracellular calcium caused a rise in cytoplasmic calcium restricted to the region of adenophostin A-induced calcium mobilization. The limited diffusion of adenophostin A provides an opportunity to examine calcium signaling processes under spatially restricted conditions and provides insights into mechanisms of intracellular calcium oscillations and capacitative calcium entry.

Adenophostin A and inositol 1,4,5-trisphosphate differentially activate Cl- currents in Xenopus oocytes because of disparate Ca2+ release kinetics

Depletion of endoplasmic reticulum Ca2+ stores induces Ca2+ entry from the extracellular space by a process termed "store-operated Ca2+ entry" (SOCE). It has been suggested that the novel fungal metabolite adenophostin-A may be able to stimulate Ca2+ entry without stimulating Ca2+ release from stores. To test this idea further, we compared Ca2+ release, SOCE, and the stimulation of Ca2+-activated Cl- currents in Xenopus oocytes in response to inositol 1,4,5-trisphosphate (IP3) and adenophostin-A injection. IP3 stimulated an outward Cl- current, ICl1-S, in response to Ca2+ release from stores followed by an inward current, ICl2, in response to SOCE. In contrast, low concentrations of adenophostins (AdAs) activated ICl2 without activating ICl1-S, consistent with the suggestion that AdA can activate Ca2+ entry without stimulating Ca2+ release. However, when Ca2+ entry has been stimulated by AdA, Ca2+ stores are largely depleted of Ca2+, as assessed by the inability of ionomycin to release additional Ca2+. The Ca2+ release stimulated by AdA, however, was 7 times slower than the release stimulated by IP3, which could explain the minimal activation of ICl1-S; when Ca2+ is released slowly, the threshold level required for ICl1-S activation is not attained.

Effect of adenophostin A on Ca2+ entry and calcium release-activated calcium current (Icrac) in rat basophilic leukemia cells

In most non-excitable cells, calcium influx is signaled by depletion of intracellular calcium stores, a process known as capacitative calcium entry. Adenophostin A, a potent activator of the inositol 1, 4,5-trisphosphate receptor, has been reported to activate Ca2+ entry in Xenopus oocytes to a greater extent than expected on the basis of its ability to release calcium stores. In this study, we compared the abilities of adenophostin A and inositol 2,4,5-trisphosphate ((2, 4,5)IP3) to release Ca2+ from intracellular stores, to activate Ca2+ entry, and to activate calcium release-activated calcium current (Icrac) in rat basophilic leukemia cells. Under conditions of low intracellular Ca2+ buffering (0.1 mM BAPTA), adenophostin A-induced Ca2+ release and activation of Icrac could be monitored simultaneously. However, other reagents that would be expected to deplete Ca2+ stores ((2,4,5)IP3, 3-fluoro-inositol 1,4, 5-trisphosphate, thapsigargin, and ionomycin) were unable to activate Icrac under this low Ca2+ buffering condition. Adenophostin A activated Icrac after a significant delay, longer than the delay for Ca2+ release. Thus, adenophostin A activates Icrac as a consequence of release of intracellular Ca2+, rather than directly acting on store-operated channels. The unique ability of adenophostin A to activate Icrac under conditions of low intracellular Ca2+ buffering suggests an additional site of action, perhaps in preventing or reducing rapid Ca2+-dependent inactivation of store-operated Ca2+ channels.

Kinetics of elementary Ca2+ puffs evoked in Xenopus oocytes by different Ins(1,4,5)P3 receptor agonists

Elementary Ca2+ puffs form the basic building blocks of global Ins(1, 4,5)P3-evoked Ca2+ signals. In Xenopus oocytes, Ca2+ puffs evoked by the high-affinity agonist adenophostin were shorter and smaller than puffs evoked by Ins(1,4,5)P3 and the lower affinity analogue Ins(2,4, 5)P3. Agonist-specific mechanisms, therefore, play a role in shaping local Ca2+ release events, but termination of Ca2+ flux is not delimited simply by agonist dissociation.

Double-stranded-RNA-activated protein kinase (PKR) regulates Ca2+ stores in Xenopus oocytes

Expression of the double-stranded-RNA-dependent protein kinase (PKR) in Xenopus oocytes attenuated Ca2+ entry-dependent membrane currents activated by depletion of Ca2+ stores, whereas expression of a dominant-negative PKR mutant had the opposite effect. These results appeared to be due to perturbation of releasable Ca2+ stores, and not actions of PKR on protein synthesis. PKR may thus have novel protein substrates and cellular functions in Ca2+ storage and signalling.