The mechanism of aluminum-independent G-protein activation by fluoride and magnesium. 31P NMR spectroscopy and fluorescence kinetic studies

Common Patterns of Hydrolysis Initiation in P-loop Fold Nucleoside Triphosphatases

The P-loop fold nucleoside triphosphate (NTP) hydrolases (also known as Walker NTPases) function as ATPases, GTPases, and ATP synthases, are often of medical importance, and represent one of the largest and evolutionarily oldest families of enzymes. There is still no consensus on their catalytic mechanism. To clarify this, we performed the first comparative structural analysis of more than 3100 structures of P-loop NTPases that contain bound substrate Mg-NTPs or their analogues. We proceeded on the assumption that structural features common to these P-loop NTPases may be essential for catalysis. Our results are presented in two articles. Here, in the first, we consider the structural elements that stimulate hydrolysis. Upon interaction of P-loop NTPases with their cognate activating partners (RNA/DNA/protein domains), specific stimulatory moieties, usually Arg or Lys residues, are inserted into the catalytic site and initiate the cleavage of gamma phosphate. By analyzing a plethora of structures, we found that the only shared feature was the mechanistic interaction of stimulators with the oxygen atoms of gamma-phosphate group, capable of causing its rotation. One of the oxygen atoms of gamma phosphate coordinates the cofactor Mg ion. The rotation must pull this oxygen atom away from the Mg ion. This rearrangement should affect the properties of the other Mg ligands and may initiate hydrolysis according to the mechanism elaborated in the second article.

Transcribing Genes the Hard Way: In Vitro Reconstitution of Nanoarchaeal RNA Polymerase Reveals Unusual Active Site Properties

Nanoarchaea represent a highly diverged archaeal phylum that displays many unusual biological features. The Nanoarchaeum equitans genome encodes a complete set of RNA polymerase (RNAP) subunits and basal factors. Several of the standard motifs in the active center contain radical substitutions that are normally expected to render the polymerase catalytically inactive. Here we show that, despite these unusual features, a RNAP reconstituted from recombinant Nanoarchaeum subunits is transcriptionally active. Using a sparse-matrix high-throughput screening method we identified an atypical stringent requirement for fluoride ions to maximize its activity under in vitro transcription conditions.

Chemical Aspects of Human and Environmental Overload with Fluorine

Over the last 100-120 years, due to the ever-increasing importance of fluorine-containing compounds in modern technology and daily life, the explosive development of the fluorochemical industry led to an enormous increase of emission of fluoride ions into the biosphere. This made it more and more important to understand the biological activities, metabolism, degradation, and possible environmental hazards of such substances. This comprehensive and critical review focuses on the effects of fluoride ions and organofluorine compounds (mainly pharmaceuticals and agrochemicals) on human health and the environment. To give a better overview, various connected topics are also discussed: reasons and trends of the advance of fluorine-containing pharmaceuticals and agrochemicals, metabolism of fluorinated drugs, withdrawn fluorinated drugs, natural sources of organic and inorganic fluorine compounds in the environment (including the biosphere), sources of fluoride intake, and finally biomarkers of fluoride exposure.

Mechanisms of Fluoride Toxicity: From Enzymes to Underlying Integrative Networks

Fluoride has been employed in laboratory investigations since the early 20th century. These studies opened the understanding of fluoride interventions to fundamental biological processes. Millions of people living in endemic fluorosis areas suffer from various pathological disturbances. The practice of community water fluoridation used prophylactically against dental caries increased concern of adverse fluoride effects. We assessed the publications on fluoride toxicity until June 2020. We present evidence that fluoride is an enzymatic poison, inducing oxidative stress, hormonal disruptions, and neurotoxicity. Fluoride in synergy with aluminum acts as a false signal in G protein cascades of hormonal and neuronal regulations in much lower concentrations than fluoride acting alone. Our review shows the impact of fluoride on human health. We suggest focusing the research on fluoride toxicity to the underlying integrative networks. Ignorance of the pluripotent toxic effects of fluoride might contribute to unexpected epidemics in the future.

Principles of fluoride toxicity and the cellular response: a review

Fluoride is ubiquitously present throughout the world. It is released from minerals, magmatic gas, and industrial processing, and travels in the atmosphere and water. Exposure to low concentrations of fluoride increases overall oral health. Consequently, many countries add fluoride to their public water supply at 0.7-1.5 ppm. Exposure to high concentrations of fluoride, such as in a laboratory setting often exceeding 100 ppm, results in a wide array of toxicity phenotypes. This includes oxidative stress, organelle damage, and apoptosis in single cells, and skeletal and soft tissue damage in multicellular organisms. The mechanism of fluoride toxicity can be broadly attributed to four mechanisms: inhibition of proteins, organelle disruption, altered pH, and electrolyte imbalance. Recently, there has been renewed concern in the public sector as to whether fluoride is safe at the current exposure levels. In this review, we will focus on the impact of fluoride at the chemical, cellular, and multisystem level, as well as how organisms defend against fluoride. We also address public concerns about fluoride toxicity, including whether fluoride has a significant effect on neurodegeneration, diabetes, and the endocrine system.

Invited review: Activation of G proteins by GTP and the mechanism of Gα-catalyzed GTP hydrolysis

This review addresses the regulatory consequences of the binding of GTP to the alpha subunits (Gα) of heterotrimeric G proteins, the reaction mechanism of GTP hydrolysis catalyzed by Gα and the means by which GTPase activating proteins (GAPs) stimulate the GTPase activity of Gα. The high energy of GTP binding is used to restrain and stabilize the conformation of the Gα switch segments, particularly switch II, to afford stable complementary to the surfaces of Gα effectors, while excluding interaction with Gβγ, the regulatory binding partner of GDP-bound Gα. Upon GTP hydrolysis, the energy of these conformational restraints is dissipated and the two switch segments, particularly switch II, become flexible and are able to adopt a conformation suitable for tight binding to Gβγ. Catalytic site pre-organization presents a significant activation energy barrier to Gα GTPase activity. The glutamine residue near the N-terminus of switch II (Glncat ) must adopt a conformation in which it orients and stabilizes the γ phosphate and the water nucleophile for an in-line attack. The transition state is probably loose with dissociative character; phosphoryl transfer may be concerted. The catalytic arginine in switch I (Argcat ), together with amide hydrogen bonds from the phosphate binding loop, stabilize charge at the β-γ bridge oxygen of the leaving group. GAPs that harbor "regulator of protein signaling" (RGS) domains, or structurally unrelated domains within G protein effectors that function as GAPs, accelerate catalysis by stabilizing the pre-transition state for Gα-catalyzed GTP hydrolysis, primarily by restraining Argcat and Glncat to their catalytic conformations. © 2016 Wiley Periodicals, Inc. Biopolymers 105: 449-462, 2016.

Fluoride Induces a Volume Reduction in CA1 Hippocampal Slices Via MAP Kinase Pathway Through Volume Regulated Anion Channels

Regulation of cell volume is an important aspect of cellular homeostasis during neural activity. This volume regulation is thought to be mediated by activation of specific transporters, aquaporin, and volume regulated anion channels (VRAC). In cultured astrocytes, it was reported that swelling-induced mitogen-activated protein (MAP) kinase activation is required to open VRAC, which are thought to be important in regulatory volume decrease and in the response of CNS to trauma and excitotoxicity. It has been also described that sodium fluoride (NaF), a recognized G-protein activator and protein phosphatase inhibitor, leads to a significant MAP kinase activation in endothelial cells. However, NaF's effect in volume regulation in the brain is not known yet. Here, we investigated the mechanism of NaF-induced volume change in rat and mouse hippocampal slices using intrinsic optical signal (IOS) recording, in which we measured relative changes in intracellular and extracellular volume as changes in light transmittance through brain slices. We found that NaF (1~5 mM) application induced a reduction in light transmittance (decreased volume) in CA1 hippocampus, which was completely reversed by MAP kinase inhibitor U0126 (10 µM). We also observed that NaF-induced volume reduction was blocked by anion channel blockers, suggesting that NaF-induced volume reduction could be mediated by VRAC. Overall, our results propose a novel molecular mechanism of NaF-induced volume reduction via MAP kinase signaling pathway by activation of VRAC.

3',4'-Dihydroxyflavonol reduces vascular contraction through Ca²⁺ desensitization in permeabilized rat mesenteric artery

3',4'-Dihydroxyflavonol (DiOHF) exerts endothelium-independent relaxation in rat aortic rings. In this study, we hypothesized that DiOHF reduces vascular contraction through Ca²⁺ desensitization in permeabilized third-order branches of rat mesenteric arteries. The third-order branches of rat mesenteric arteries were permeabilized with β-escin and subjected to tension measurement. Cumulative addition of phenylephrine (0.3-30 μM) produced concentration-dependent vascular contraction of endothelium-intact and endothelium-denuded arterial rings, which were inhibited by pretreatment with DiOHF (10, 30, or 100 μM). In addition, DiOHF dose-dependently decreased vascular contractions induced by 3.0 μM phenylephrine. β-Escin-permeabilized third-order branches of mesenteric arteries were contracted with Ca²⁺, NaF, or guanosine-5'-(γ-thio)triphosphate (GTPγS) 30 min after pretreatment with DiOHF or vehicle. Pretreatment with DiOHF for 30 min inhibited vascular contraction induced by cumulative additions of Ca²⁺ (pCa 9.0-6.0) or NaF (4.0-16.0 mM) in permeabilized arterial rings. Cumulative addition of DiOHF also reduced vascular contraction induced by Ca²⁺-controlled solution of pCa 6.0, 16.0 mM NaF, or 100 μM GTPγS in permeabilized arterial rings. DiOHF inhibited the increase in vascular tension provoked by calyculin A, even though it did not affect vascular tension already produced by calyculin A. DiOHF accelerated the relaxation induced by rapidly lowering Ca²⁺. DiOHF reduced vascular contraction through Ca²⁺ desensitization in permeabilized third-order branches of rat mesenteric arteries. These results suggest that DiOHF may have a therapeutic potential in the treatment of cardiovascular diseases.

Honokiol attenuates vascular contraction through the inhibition of the RhoA/Rho-kinase signalling pathway in rat aortic rings

Objectives

Honokiol is a small-molecule polyphenol isolated from the species Magnolia obovata. We hypothesized that honokiol attenuated vascular contractions through the inhibition of the RhoA/Rho-kinase signalling pathway.

Methods

Rat aortic rings were denuded of endothelium, mounted in organ baths, and subjected to contraction or relaxation. Phosphorylation of 20 kDa myosin light chains (MLC20), myosin phosphatase targeting subunit 1 (MYPT1) and protein kinase C (PKC)-potentiated inhibitory protein for heterotrimeric myosin light chain phosphatase (MLCP) of 17 kDa (CPI17) were examined by immunoblot. We also measured the amount of guanosine triphosphate RhoA as a marker for RhoA activation.

Key findings

Pretreatment with honokiol dose-dependently inhibited the concentration–response curves in response to sodium fluoride (NaF) or thromboxane A2 agonist U46619. Honokiol decreased the phosphorylation levels of MLC20, MYPT1Thr855 and CPI17Thr38 as well as the activation of RhoA induced by 8.0 mm NaF or 30 nm U46619.

Conclusions

These results demonstrated that honokiol attenuated vascular contraction through the inhibition of the RhoA/Rho-kinase signalling pathway.

Fluoride induces vascular contraction through activation of RhoA/Rho kinase pathway in isolated rat aortas

We hypothesized that fluoride induces vascular contraction through activation of the RhoA/Rho kinase pathway in isolated rat aortas. Rat aortic rings were mounted in organ baths and contracted with sodium fluoride (NaF). We measured the amount of GTP-RhoA as well as vascular tension. We also determined the level of phosphorylation of the myosin light chain (MLC(20)), myosin phosphatase targeting subunit 1 (MYPT1) and PKC-potentiated inhibitory protein for heterotrimeric MLCP of 17kDa (CPI17). In both physiological salt solution and Ca(2+)-free solution, NaF increased vascular tension and MLC(20) phosphorylation in dose-dependent manners. NaF increased not only phosphorylation level of MYPT1(Thr855) and CPI17(Thr38), but also the amount of GTP-RhoA. Both H1152 and Y27632, inhibitors of Rho kinase, but not Ro31-8220, an inhibitor of PKC, attenuated NaF-induced contraction and phosphorylation level of MLC(20), MYPT1(Thr855) and CPI17(Thr38). In conclusion, fluoride induces vascular contraction through activation of the RhoA/Rho kinase pathway.

Calcium sensitization induced by sodium fluoride in permeabilized rat mesenteric arteries

It was hypothesized that NaF induces calcium sensitization in Ca(2+)-controlled solution in permeabilized rat mesenteric arteries. Rat mesenteric arteries were permeabilized with beta-escin and subjected to tension measurement. NaF potentiated the concentration-response curves to Ca(2+) (decreased EC(50) and increased E(max)). Cumulative addition of NaF (4.0, 8.0 and 16 mM) also increased vascular tension in Ca(2+)-controlled solution at pCa 7.0 or pCa 6.5, but not at pCa 8.0. NaF-induced vasocontraction and GTPgammaS-induced vasocontraction were not additive. NaF-induced vasocontraction at pCa 7.0 was inhibited by pretreatment with Rho kinase inhibitors H1152 or Y27632 but not with a MLCK inhibitor ML-7 or a PKC inhibitor Ro31-8220. NaF induces calcium sensitization in a Ca(2+)-dependent manner in beta-escin-permeabilized rat mesenteric arteries. These results suggest that NaF is an activator of the Rho kinase signaling pathway during vascular contraction.

NaF Activates MAPKs and Induces Apoptosis in Odontoblast-like Cells

The cytotoxic effects of fluoride on odontoblasts are not clear. In this study, we examined whether NaF induces apoptosis in MDPC-23 odontoblast-like cells and the involvement of mitogen-activated protein kinase (MAPK) signaling pathways in NaF-induced apoptosis. MDPC-23 cells incubated with 5 mM NaF for 24 hrs exhibited caspase-3 activation, cleavage of poly(ADP-ribose) polymerase, DNA fragmentation, and an increase in cytoplasmic nucleosomes. Prior to the induction of apoptosis, all MAPKs examined were phosphorylated, but in a different manner. In contrast to the sustained phosphorylation of c-Jun NH2-terminal kinase (JNK) and p38, NaF exposure induced a biphasic phosphorylation of extracellular signal-regulated protein kinase (ERK). NaF-induced apoptosis was markedly suppressed by treatment with the JNK inhibitor, SP600125, and mildly suppressed by the MAPK/ERK kinase inhibitor, U0126. Inhibition of p38 activity did not protect cells from apoptosis. Thus, exposure to NaF induces apoptosis in odontoblast-like cells, depending on JNK and, less significantly, ERK pathways.

Kinetic analysis of beta-phosphoglucomutase and its inhibition by magnesium fluoride

The isomerization of beta-glucose-1-phosphate (betaG1P) to beta-glucose-6-phosphate (G6P) catalyzed by beta-phosphoglucomutase (betaPGM) has been examined using steady- and presteady-state kinetic analysis. In the presence of low concentrations of beta-glucose-1,6-bisphosphate (betaG16BP), the reaction proceeds through a Ping Pong Bi Bi mechanism with substrate inhibition (kcat = 65 s(-1), K(betaG1P) = 15 microM, K(betaG16BP) = 0.7 microM, Ki = 122 microM). If alphaG16BP is used as a cofactor, more complex kinetic behavior is observed, but the nonlinear progress curves can be fit to reveal further catalytic parameters (kcat = 74 s(-1), K(betaG1P) = 15 microM, K(betaG16BP) = 0.8 microM, Ki = 122 microM, K(alphaG16BP) = 91 microM for productive binding, K(alphaG16BP) = 21 microM for unproductive binding). These data reveal that variations in the substrate structure affect transition-state affinity (approximately 140,000-fold in terms of rate acceleration) substantially more than ground-state binding (110-fold in terms of binding affinity). When fluoride and magnesium ions are present, time-dependent inhibition of the betaPGM is observed. The concentration dependence of the parameters obtained from fitting these progress curves shows that a betaG1P x MgF3(-) x betaPGM inhibitory complex is formed under the reaction conditions. The overall stability constant for this complex is approximately 2 x 10(-16) M(5) and suggests an affinity of the MgF3(-) moiety to this transition-state analogue (TSA) of < or = 70 nM. The detailed kinetic analysis shows how a special type of TSA that does not exist in solution is assembled in the active site of an enzyme. Further experiments show that under the conditions of previous structural studies, phosphorylated glucose only persists when bound to the enzyme as the TSA. The preference for TSA formation when fluoride is present, and the hydrolysis of substrates when it is not, rules out the formation of a stable pentavalent phosphorane intermediate in the active site of betaPGM.

Evidence for two distinct Mg2+ binding sites in G(s alpha) and G(i alpha1) proteins

The function of guanine nucleotide binding (G) proteins is Mg(2+) dependent with guanine nucleotide exchange requiring higher metal ion concentration than guanosine 5'-triphosphate hydrolysis. It is unclear whether two Mg(2+) binding sites are present or if one Mg(2+) binding site exhibits different affinities for the inactive GDP-bound or the active GTP-bound conformations. We used furaptra, a Mg(2+)-specific fluorophore, to investigate Mg(2+) binding to alpha subunits in both conformations of the stimulatory (G(s alpha)) and inhibitory (G(i alpha1)) regulators of adenylyl cyclase. Regardless of the conformation or alpha protein studied, we found that two distinct Mg(2+) sites were present with dissimilar affinities. With the exception of G(s alpha) in the active conformation, cooperativity between the two Mg(2+) sites was also observed. Whereas the high affinity Mg(2+) site corresponds to that observed in published X-ray structures of G proteins, the low affinity Mg(2+) site may involve coordination to the terminal phosphate of the nucleotide.

Cyclic AMP-independent CGRP8-37-sensitive receptors mediate adrenomedullin-induced decrease of CaCl2-contraction in pregnant rat mesenteric artery

OBJECTIVES

We tested the hypothesis that adrenomedullin reduces calcium influx independent of potassium channels in depolarized endothelium-denuded mesenteric artery from pregnant rats.

RESULTS

Adrenomedullin reduced the CaCl(2)-induced contraction, while the receptor antagonist calcitonin gene-related peptide (CGRP)(8-37), but not adrenomedullin(22-52), reversed these effects. Adenylate cyclase inhibition by SQ22536 did not prevent adrenomedullin effects on CaCl(2)-induced contraction. Adrenomedullin did not inhibit depolarization-induced calcium entry to isolated vascular smooth muscle. Inhibition of myosin light-chain (MLC) phosphatase by calyculin A reversed the effects of adrenomedullin on contraction caused by submillimolar concentrations of CaCl(2), while adrenomedullin still inhibited contraction caused by higher concentrations of CaCl(2). However, the ratio of phosphorylated to total myosin phosphatase target 1, the regulatory subunit of MLC phosphatase, did not change with adrenomedullin, indicating a lack of MLC phosphatase activation. Interestingly, sodium fluoride, a nonspecific protein phosphatase inhibitor, completely blocked the effect of adrenomedullin on CaCl(2)-induced contraction. Adrenomedullin inhibited calcium mobilization from intracellular stores induced by thapsigargin.

CONCLUSION

Adrenomedullin inhibits CaCl(2)-induced contraction, without affecting calcium influx, through a CGRP(8-37)-sensitive receptor, but not using the cyclic adenosine monophosphate pathway, probably through activation of protein phosphatases. Inhibition of intracellular calcium release is an additional role played by adrenomedullin in calcium homeostasis in vascular smooth muscle.

Insight into catalysis of a unique GTPase reaction by a combined biochemical and FTIR approach

Rap1 and Rap2 are the only small guanine nucleotide-binding proteins of the Ras superfamily that do not use glutamine for GTP hydrolysis. Moreover, Rap1GAP, which stimulates the GTPase reaction of Rap1 10(5)-fold, does not have the classical "arginine finger" like RasGAP but presumably, introduces an asparagine residue into the active site. Here, we address the requirements of this unique reaction in detail by combining various biochemical methods, such as fluorescence spectroscopy, stopped-flow and time-resolved Fourier transform infrared spectroscopy (FTIR). The fluorescence spectroscopic assay monitors primarily protein-protein interaction steps, while FTIR resolves simultaneously the elementary steps of functional groups labor-free, but it is less sensitive and needs higher concentrations. Combining both methods allows us to distinguish weather mechanistic defects caused by mutation are due to affinity or due to functionality. We show that several mutations of Asn290 block catalysis. Some of the mutants, however, still form a complex with Rap1*GDP in the presence of BeF(x) but not AlF(x), supporting the notion that fluoride complexes are indicators of the ground versus transition state. Mutational analysis also shows that Thr61 is not required for catalysis. While replacement of Thr61 of Rap1 by Leu eliminates GTPase activation by Rap1GAP, the T61A and T61Q mutants have only a minor effect on catalysis, but change the relative rates of cleavage and (P(i)(-)) release. While Rap1GAP(N290A) is completely inactive on wild-type Rap1, it can act on Rap1(T61Q), arguing that Asn290 in trans has a role in catalysis similar to that of the intrinsic Gln in Ras and Rho. Finally, since FTIR works at high, and thus mostly saturating, concentrations, it can clearly separate effects on affinity from purely catalytic modifications, showing that Arg388, conserved between RapGAPs and mutated in the homologous RheBGAP Tuberin, affects binding affinity severely but has no effect on the cleavage reaction itself.

Is competition between Li+ and Mg2+ the underlying theme in the proposed mechanisms for the pharmacological action of lithium salts in bipolar disorder?

Lithium salts have been in use for the treatment of bipolar disorder for more than 50 years, but their pharmacological mode of action remains a matter of conjecture. Li(+) and Mg(2+) share many physicochemical properties. Not surprisingly, many reported cellular targets for Li(+) action involve Mg(2+)-activated enzymes, which are inhibited by Li(+). In this Account, we describe results from our and other laboratories that suggest that a competition mechanism between Li(+) and Mg(2+) ions for Mg(2+)-binding sites in cellular components is the underlying theme in putative mechanisms of Li(+) action.

Mechanism of fluoride-induced MAP kinase activation in pulmonary artery endothelial cells

In this study, we demonstrate that challenge of endothelial cells (EC) with NaF, a recognized G protein activator and protein phosphatase inhibitor, leads to a significant Erk activation, with increased phosphorylation of the well-known Erk substrate caldesmon. Inhibition of the Erk MAPK, MEK, by U0126 produces a marked decrease in NaF-induced caldesmon phosphorylation. NaF transiently increases the activity of the MEK kinase known as Raf-1 (approximately 3- to 4-fold increase over basal level), followed by a sustained Raf-1 inhibition (approximately 3- to 4-fold decrease). Selective Raf-1 inhibitors (ZM-336372 and Raf-1 inhibitor 1) significantly attenuate NaF-induced Erk and caldesmon phosphorylation. Because we have previously shown that Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) participates in Erk activation in thrombin-challenged cells, we next explored if CaMKII is involved in NaF-induced EC responses. We found that in NaF-treated EC, CaMKII activity increases in a time-dependent manner with maximal activity at 10 min (approximately 4-fold increase over a basal level). Pretreatment with KN93, a specific CaMKII inhibitor, attenuates NaF-induced barrier dysfunction and Erk phosphorylation. The Rho inhibitor C3 exotoxin completely abolishes NaF-induced CaMKII activation. Collectively, these data suggest that sequential activation of Raf-1, MEK, and Erk is modulated by Rho-dependent CaMKII activation and represents important NaF-induced signaling response. Caldesmon phosphorylation occurring by an Erk-dependent mechanism in NaF-treated pulmonary EC may represent a link between NaF stimulation and contractile responses of endothelium.

Inhibition of the ATPase activity of Escherichia coli ATP synthase by magnesium fluoride

Inhibition of ATPase activity of Escherichia coli ATP synthase by magnesium fluoride (MgFx) was studied. Wild-type F(1)-ATPase was inhibited potently, albeit slowly, when incubated with MgCl(2), NaF, and NaADP. The combination of all three components was required. Reactivation of ATPase activity, after removal of unbound ligands, occurred with half-time of approximately 14 h at 22 degrees C and was quasi-irreversible at 4 degrees C. Mutant F(1)-ATPases, in which catalytic site residues involved in transition state formation were modified, were found to be resistant to inhibition by MgFx. The data demonstrate that MgFx in combination with MgADP behaves as a tight-binding transition state analog in E. coli ATP synthase.

Molecular biology of insect olfaction: recent progress and conceptual models

Insects have an enormous impact on global public health as disease vectors and as agricultural enablers as well as pests and olfaction is an important sensory input to their behavior. As such it is of great value to understand the interplay of the molecular components of the olfactory system which, in addition to fostering a better understanding of insect neurobiology, may ultimately aid in devising novel intervention strategies to reduce disease transmission or crop damage. Since the first discovery of odorant receptors in vertebrates over a decade ago, much of our view on how the insect olfactory system might work has been derived from observations made in vertebrates and other invertebrates, such as lobsters or nematodes. Together with the advantages of a wide range of genetic tools, the identification of the first insect odorant receptors in Drosophila melanogaster in 1999 paved the way for rapid progress in unraveling the question of how olfactory signal transduction and processing occurs in the fruitfly. This review intends to summarize much of this progress and to point out some areas where advances can be expected in the near future.

"Low" concentrations of sodium fluoride inhibit neurotransmitter release from the guinea-pig superior cervical ganglion

The role of G proteins and related second messenger system on the modulation of acetylcholine release from [3H]choline-preloaded guinea-pig superior cervical ganglion was investigated using the potent general activator NaF. The electrically evoked (1 Hz, 5 min) [3H] release was inhibited by "low" F- concentrations (1-2.5 mM), by the adenylyl cyclase blocker MDL 12330A (10 microM), alone and in combination with 1 mM NaF, and increased by 0.5 mM 8Br-cAMP, 100 microM forskolin and 0.5 mM 3-isobutyl-1-methylxantine. No effect of 1 mM F- was observed on spontaneous release. Fluoride-induced inhibition was counteracted by the G protein blocker sulmazole (1 mM), forskolin and alteration of calcium influx by increasing [Ca2+]out from 2.2 to 6 mM, raising the rate of stimulation (10 Hz, 30 s), or broadening the presynaptic action potential with 10 microM 4-aminopyridine and 50 microM tetraethylammonium chloride. Thus a NaF-sensitive G protein, linked to cAMP synthesis, is determinant for the inhibition of neurosecretion in this cholinergic synapse, involving Ca2+-dependent mechanisms.

Effect of Li+ upon the Mg2+-dependent activation of recombinant Gialpha1

Although lithium salts have been used in the treatment and prophylaxis of manic-depressive or bipolar patients for 50 years, the mechanism of the pharmacologic action of Li+ is unknown. Based on activity studies of inhibitory and stimulatory guanine-binding (G) proteins in rat cortical membranes, it was proposed that Li+ inhibition of G-proteins may account for its pharmacologic action. We used the purified alpha subunit of the recombinant inhibitory G-protein, rGialpha1, and found that Li+ at therapeutic levels significantly inhibited the formation of the GDP.AlF4-.rGialpha1 complex. Because our studies were conducted with a purified, metal-reconstituted G-protein rather than with cell membrane suspensions, our Li+ inhibition results lend additional support to the G-protein hypothesis for Li+ action.

Lipid-dependent targeting of G proteins into rafts

Domains rich in sphingolipids and cholesterol, or rafts, may organize signal transduction complexes at the plasma membrane. Raft lipids are believed to exist in a state similar to the liquid-ordered phase. It has been proposed that proteins with a high affinity for an ordered lipid environment will preferentially partition into rafts (Melkonian, K. A., Ostermeyer, A. G., Chen, J. Z., Roth, M. G., and Brown, D. A. (1999) J. Biol. Chem. 274, 3910-3917). We investigated the possibility that lipid-lipid interactions between lipid-modified proteins and raft lipids mediate targeting of proteins to these domains. G protein monomers or trimers were reconstituted in liposomes, engineered to mimic raft domains. Assay for partitioning of G proteins into rafts was based on Triton X-100 insolubility. Myristoylation and palmitoylation of Galpha(i) were necessary and sufficient for association with liposomes and partitioning into rafts. Strikingly, the amount of fatty-acylated Galpha(i) in rafts was significantly reduced when myristoylated Galpha(i) was thioacylated with cis-unsaturated fatty acids instead of saturated fatty acids such as palmitate. Prenylated betagamma subunits were excluded from rafts, whether reconstituted alone or with fatty-acylated alpha subunits. These results suggest that the structural difference between lipids that modify proteins is one basis for the selectivity of protein targeting to rafts.

Inhibition of myosin ATPase by metal fluoride complexes

Magnesium (Mg2+) is the physiological divalent cation stabilizing nucleotide or nucleotide analog in the active site of myosin subfragment 1 (S1). In the presence of fluoride, Mg2+ and MgADP form a complex that traps the active site of S1 and inhibits myosin ATPase. The ATPase inactivation rate of the magnesium trapped S1 is comparable but smaller than the other known gamma-phosphate analogs at 1.2 M-1 s-1 with 1 mM MgCl2. The observed molar ratio of Mg/S1 in this complex of 1.58 suggests that magnesium occupies the gamma-phosphate position in the ATP binding site of S1 (S1-MgADP-MgFx). The stability of S1-MgADP-MgFx at 4 degrees C was studied by EDTA chase experiments but decomposition was not observed. However, removal of excess fluoride causes full recovery of the K+-EDTA ATPase activity indicating that free fluoride is necessary for maintaining a stable trap and suggesting that the magnesium fluoride complex is bonded to the bridging oxygen of beta-phosphate more loosely than the other known phosphate analogs. The structure of S1 in S1-MgADP-MgFx was studied with near ultraviolet circular dichroism, total tryptophan fluorescence, and tryptophan residue 510 quenching measurements. These data suggest that S1-MgADP-MgFx resembles the M**.ADP.Pi steady-state intermediate of myosin ATPase. Gallium fluoride was found to compete with MgFx for the gamma-phosphate site in S1-MgADP-MgFx. The ionic radius and coordination geometry of magnesium, gallium and other known gamma-phosphate analogs were compared and identified as important in determining which myosin ATPase intermediate the analog mimics.

Determination of the depth of penetration of the alpha subunit of retinal G protein in membranes: a spectroscopic study

This paper reports the fluorescence quenching of the alpha subunit of retinal rod outer segment G protein (Gtalpha) by vesicles of brominated phospholipids. Two different brominated phospholipids with the bromine quencher groups attached at the 6-7 and 9-10 positions in one of the fatty acyl chains have been used to estimate the depth of penetration of the Gtalpha protein in the lipid vesicles using steady-state fluorescence quenching techniques. Our studies provide evidence of the interaction between Gtalpha protein, in its active conformation, with the lipid vesicles mimicking natural membranes. This study demonstrates that in vitro the distance between fluorescent tryptophan site of Gtalpha and the membrane surface is approximately 6.5 A.

Evidence for distinct mechanisms of transition state stabilization of GTPases by fluoride

GTPase-activating proteins (GAPs) function by stabilizing the GTPase transition state. This has been most clearly demonstrated by the formation of a high-affinity complex between various GAPs and GDP-bound GTPases in the presence of aluminum tetrafluoride, which can mimic the gamma-phosphate of GTP. Herein, we report that p190 RhoGAP forms a high-affinity complex with Rho GTPases in the presence of fluoride ions, suggesting that p190 also functions to stabilize the GTPase transition state. However, this Rho-p190 complex does not require aluminum ions or even guanine nucleotide, indicating a distinct role for fluoride that is not consistent with the gamma-phosphate-mimicking hypothesis. These results indicate that it is necessary to reconsider the assumed role of fluoride in stabilizing a variety of other GTPase-GAP interactions where the requirement for aluminum or guanine nucleotide has not yet been addressed.

Fluoride stimulates cystic fibrosis transmembrane conductance regulator Cl- channel activity

While studying the regulation of the cystic fibrosis transmembrane conductance regulator (CFTR), we found that addition of F- to the cytosolic surface of excised, inside-out membrane patches reversibly increased Cl- current in a dose-dependent manner. Stimulation required prior phosphorylation and the presence of ATP. F- increased current even in the presence of deferoxamine, which chelates Al3+, suggesting that stimulation was not due to AlF4-. F- also stimulated current in a CFTR variant that lacked a large part of the R domain, suggesting that the effect was not mediated via this domain. Studies of single channels showed that F- increased the open-state probability by slowing channel closure from bursts of activity; the mean closed time between bursts and single-channel conductance was not altered. These results suggested that F- influenced regulation by the cytosolic domains, most likely the nucleotide-binding domains (NBDs). Consistent with this, we found that mutation of a conserved Walker lysine in NBD2 changed the relative stimulatory effect of F- compared with wild-type CFTR, whereas mutation of the Walker lysine in NBD1 had no effect. Based on these and previous data, we speculate that F- interacts with CFTR, possibly via NBD2, and slows the rate of channel closure.

Macroscopic and single-channel chloride currents in neuropile glial cells of the leech central nervous system

In patch-clamp experiments we characterized four Cl- channels (42 pS, 70 pS, 80 pS and 229 pS) underlying the large Cl- conductance of leech neuropile glial cells. They differed with respect to their gating, their rectification and their activity in the cell-attached configuration, showed the selectivity sequence I->Cl->/=Br->F- and were impermeable to SO42-. The four channels were blocked by NPPB, DPC, niflumic acid and DIDS and exhibited either three or four sublevel states. The outward rectifying 42 pS, 70 pS and 80 pS Cl- channels were classified as intermediate conductance Cl- channels and they could contribute to the high Cl- conductance of the glial membrane, which stabilizes the glial membrane potential. The inward rectifying 229 pS Cl- channel is very similar to vertebrate high conductance Cl- channels, which are assumed to be part of an emergency system that is activated under pathophysiological conditions. In voltage-clamp experiments we calculated that the Cl- conductance amounts to one-third of the total membrane conductance. Reduction of this Cl- conductance by Cl- channel inhibitors markedly depolarized the glial cell membrane. These prominent depolarizations depended on Na+ influx and in most cases the glial cells failed to regulate their membrane potential following wash-out of the inhibitors.

An evaluation of strategies available for the identification of GTP-binding proteins required in intracellular signalling pathways

Strategies which can be used to elucidate the nature of a GTP-binding regulatory protein (G-protein) involved in an intracellular pathway of interest in the complex environment of the cell are described and evaluated. A desirable strategy is considered to be one in which the first stage indicates a requirement for one or more G-proteins, provides information on whether a monomeric, trimeric or other type of G-protein is involved, and gives some idea of the G-protein sub-class. In the second stage the specific G-protein involved is identified. Approaches available for investigations in the first stage include the use of analogues of GTP and GDP, AlF4-, inhibitors of G-protein isoprenylation, bacterial toxins which covalently modify G-proteins, and the introduction of a purified GDP dissociation inhibitor, GDP exchange and/or GTP-ase activating protein. Identification of the specific G-protein in the second stage can be achieved using anti G-protein antibodies, G-protein-or receptor-derived peptides, antisense G-protein RNA and over-expressed, constitutively-active or dominant-negative G-protein mutants. The correct interpretation of results obtained with GTP and GDP analogues and AlF4- in the first stage is complex and often difficult, and requires a thorough understanding of the functions and mechanisms of activation of G-proteins. Nevertheless, it is important to reach the correct conclusion at this stage since considerable time and expense are usually required for investigations in the second stage.

Cyclosporin A potentiates receptor-activated [Ca2+]c increase

The use of the immunosuppressant cyclosporin A (CsA) is frequently associated with hypertension. Drug-induced local vasoconstriction appears to be responsible for this effect. Using fura-2 and 45Ca2+ efflux techniques, we have examined variations in the cytosolic calcium concentration ([Ca2+]c) in rat aortic smooth muscle cells and have shown that increases in [Ca2+]c after [Arg8]vasopressin, serotonin, endothelin-1 or angiotensin II stimulation were potentiated after preincubation of cells with CsA. This effect was independent of cyclophilin or calcineurin inhibition by CsA. Measurements of inositol phosphates (InsPn) after agonist stimulation showed that CsA also potentiated their formation. As for 45Ca2+ efflux this effect was not related to cyclophilin or calcineurin inhibition. Direct stimulation of G proteins with aluminium tetrafluoride induced an increase in InsPn formation and 45Ca2+ efflux. Neither of these responses was potentiated by CsA. These results indicate that CsA acts on a target upstream of G protein activation, possibly at the receptor level, resulting in a potentiation of InsPn formation and subsequent calcium increase.

In vitro activation of the NADPH oxidase by fluoride. Possible involvement of a factor activating GTP hydrolysis on Rac (Rac-GAP)

The possible mechanism of activation of the NADPH oxidase by fluoride was investigated in the cell-free system. It is shown that the stimulatory effect of fluoride is inhibited by guanosine 5'-O-(2-thiodiphosphate) (GDP[S]) and potentiated by GTP. The effect of fluoride is not additive with GTP[S]. Fluoride activation requires the presence of Mg2+ in millimolar concentration but is independent of Al3+. The activating effect of fluoride is preserved in solubilized membrane extract after removal of the majority of heterotrimeric GTP-binding proteins by immunoadsorption. Fluoride has no direct action either on the nucleotide exchange of GTP hydrolysis of the isolated Rac protein. In contrast, fluoride effectively inhibits Rac-GTPase activity enhanced by a membrane component. In this way, fluoride could prolong the prevalence of Rac in the GTP-bound state and, as a consequence, activate NADPH oxidase. The possibility of the involvement of a membrane-bound Rac GTPase-activating protein activity in the physiological regulation of the enzyme is raised.

Enhanced contractile responses of arteries from streptozotocin diabetic rats to sodium fluoride

1. Previous studies from this laboratory have demonstrated that alpha 1-adrenoceptor-mediated increases in tension and phosphoinositide metabolism are enhanced in the aorta and mesenteric arteries from diabetic rats. The purpose of the present investigation was to determine whether contractile responses to sodium fluoride (NaF), which directly stimulates GTP-binding proteins (G-proteins), are also enhanced in diabetic arteries. 2. NaF (1-20 mM) in the presence of 10 microM aluminium chloride produced slowly developing, concentration-dependent contractions in mesenteric arteries from three month streptozotocin-diabetic (60 mg kg-1, i.v.) male Wistar rats and age-matched control rats. The maximum contractile response but not the sensitivity to NaF was significantly greater in mesenteric arteries from diabetic than from control rats, as was the response to noradrenaline (NA). Maximum contractile responses of aorta and caudal artery from diabetic rats to NaF were also significantly enhanced. 3. Removal of the endothelium and denervation with 6-hydroxydopamine did not significantly alter the maximum contractile response of mesenteric arteries from either control or diabetic rats to NaF. Similarly, NaF had no effect on cyclic AMP levels in aorta, and no difference in cyclic AMP levels, either basally or in the presence of NaF, was detected between control and diabetic rat aorta. 4. Contractile responses of mesenteric arteries from both control and diabetic rats to NaF were diminished in calcium-free Krebs solution, but the NaF response remained significantly elevated in mesenteric arteries from diabetic rats compared to control. 5. Ryanodine (30 microM) which depletes intracellular calcium stores, nifedipine (3 microM) which blocks dihydropyridine-sensitive calcium channels and calphostin C (0.5 microM) which selectively inhibits protein kinase C, all significantly inhibited maximum contractile responses of mesenteric arteries from control and diabetic rats to NaF. There were no significant differences between control and diabetic arteries in the relative magnitude of the inhibition produce by the three antagonist. 6. These data suggest that there may be increased activation of the same signalling processes that mediate NA-stimulated vasoconstriction, perhaps contraction-associated G-proteins or the effectors coupled to these G-proteins, in response to NaF in mesenteric arteries from diabetic rats. This may also be responsible for the enhanced contractile responses of these arteries to alpha 1-adrenoceptor stimulation.

Analysis of effects of corticotropin, forskolin and fluoride on activity of adenylate cyclase of bovine adrenal cortex

A mathematical model relating the activity of adenylate cyclase (AC) with concentrations of stimulators, equilibrium dissociation constants, specific activity and efficacies of AC depending on the states of its binding sites has been developed and used for analysis of the data on activation of AC of bovine adrenal cortex plasma membranes presented in (De Foresta et al. (1987) FEBS Lett. 216, 107-112). Equilibrium dissociation constants. chi h and chi l, corresponding to high- and low-affinity forskolin-binding sites were estimated to be 0.37 and 17 microM: these constants characterize forskolin's potency more adequately than does ED50, the concentration eliciting half-asymptotic activity of AC. Corticotropin does not affect the affinity of AC for forskolin whereas fluoride increases this affinity, thus augmenting forskolin's potency. Hormone receptor of adenylate cyclase of bovine adrenal cortex has been suggested to have two or more binding sites for corticotropin. Some unidentified factor(s) may be responsible for the differences found in adenylate cyclase activity in different experiments carried out under similar conditions. The model applied for the analysis may be thought to be the best means for the moment to relate dose-response dependencies with what is known or can be hypothesized about the mechanisms underlying activation of adenylate cyclase.

Interactive effects of fluoride and aluminum uptake and accumulation in bones of rabbits administered both agents in their drinking water

Fluoride (F) and aluminum (Al), which are known to form a strong complex, are both present in finished drinking water. The effect of F and AI on one another's tissue accumulation was determined using adult male New Zealand white rabbits. Thirty-six rabbits (three per group) were given Purina Rabbit Chow and drinking water containing no F or AI, F alone (1, 4, or 50 ppm F as NaF), Al alone, (100 or 500 ppm Al as AlCl3), or a combination of F and Al, ad libitum for 10 wk. None of these treatments altered food intake or weight gain in these rabbits. However, rabbits treated with 1 ppm F and 500 ppm Al consumed significantly less water than control rabbits. The F accumulation in plasma, urine, incisors, and tibia was increased as the F addition to the drinking water increased within groups receiving a single concentration of Al. In contrast, F accumulation in plasma, urine, incisors, and tibia decreased as the Al concentration increased within groups receiving a single F concentration, indicative of decreased intestinal absorption. Importantly, Al levels in tibia were significantly increased by the addition of F to the drinking water, even in animals receiving no Al in their drinking water. The effect of F on Al accumulation in bone was confirmed by our evaluating Al levels in sterna harvested from rats treated with 0 or 79 ppm F (as NaF in the drinking water) in a study conducted by the National Toxicology Program (Bucher et al., 1991). Therefore, some of the osteotoxicity seemingly associated with high F levels in bone may be due to the accumulation of Al or an Al-F complex.

Polarized exocytosis in MDCK cells is regulated by phosphorylation

Protein phosphorylation and dephosphorylation systems modulate many cellular activities and have recently been implicated in the in vitro transport of newly synthesized proteins. Here we show that polarized transport from the Golgi to the plasma membrane in intact MDCK cells is regulated by phosphorylation-dephosphorylation. Transport is inhibited by the phosphatase inhibitor okadaic acid and is stimulated by the kinase inhibitor staurosporine. Stimulation of apical transport exceeds stimulation of basolateral transport by up to 5-fold. We also find that the G protein activator aluminum fluoride, which stimulates transport to the surface at low fluoride concentrations as previously reported, inhibits transport at higher concentrations. In the nonpolarized fibroblast cell line CV-1, neither staurosporine nor aluminum fluoride stimulates transport to the cell surface. Our results suggest that the phosphorylation-dephosphorylation system, like the G protein, may be involved in the specialized sorting process characteristic of polarized cells. We show some evidence that these two mechanisms of regulation may act through common intermediates.

A pleiotropic effect of fluoride on signal transduction in macrophages: is it mediated by GPT-binding proteins?

The activation of GTP-binding proteins (G-proteins) by sodium fluoride + aluminum (AlF4-) was shown in several cell free systems. In the intact cell, NaF +/- aluminum was shown to activate various signal transduction pathways and indirect evidence is in line with effector mechanisms involving regulation of G-protein activity. We have explored the effect of NaF on several components of signal transduction pathways in macrophages. NaF was shown to reduce intracellular ATP levels and to suppress agonist-induced protein tyrosine phosphorylation and reactive oxygen species formation. NaF led to in situ activation of nitrogen activated protein kinase, phospholipase A2 and PtdIns-phospholipase C. Addition of AlCl(3) or deferoxamine, a chelator of aluminum, had little or no effect on NaF mediated enzyme activation. The results suggest that at least some of the pleiotropic effects of NaF in intact cells may not be mediated by G-protein activation but rather by depletion of ATP which is essential for protein phosphorylation reactions.

The effect of GTP on the aluminum fluoride- and forskolin-activated adenylyl cyclase from human embryonic kidney 293 cells

GTP has been shown to inhibit AlF4(-)-stimulated, and to activate forskolin-stimulated adenylyl cyclase activity in the presence of Mg2+ in cell membranes from human embryonic kidney 293 cells. The maximal inhibitory response of AlF4(-)-stimulated adenylyl cyclase activity by GTP was not dependent on the concentration of Mg2+, but was so in the case of forskolin-activated activity at all forskolin concentrations assayed. Mn2+ ions stimulated AlF4(-)- or forskolin-activated adenylyl cyclase activity to a greater extent than Mg2+. The inhibition of AlF4(-)-stimulated cyclase by GTP was still observed with Mn2+, but the activation of forskolin-stimulated cyclase by GTP was not. When assayed together, Mn2+ and Mg2+ showed non-additive behaviours with respect to the amount of cyclic AMP formed after AlF4(-)-stimulation of adenylyl cyclase. The temperature dependence of the activation of adenylyl cyclase by forskolin, AlF4- or under basal conditions was observed to be somehow different in the presence of Mn2+ than in the presence of Mg2+ ions. Cholera toxin treatment produced a markedly increased cyclase activity, specially when assayed with AlF4-. In the case of forskolin-activated adenylyl cyclase, UTP and CTP were unable to reproduce the cyclase activation detected with GTP. However, in the case of AlF4(-)-stimulated adenylyl cyclase, UTP was as good as GTP at inhibiting cyclase activity, and CTP virtually eliminated the activation of the cyclase with AlF4-.

Selective inhibition of T-type Ca2+ channels by Ro 40-5967

The present study shows that the chemically novel nondihydropyridine Ca2+ antagonist, Ro 40-5967, blocks T-type divalent ion currents in vascular muscle cells. T-type Ca2+ channels were blocked selectively and completely by therapeutic concentrations of 1 to 10 mumol/L Ro 40-5967, at which there was only 25% to 70% block of L-type Ca2+ currents. Using the combination of Ro 40-5967 and nisoldipine, a dihydropyridine selective for L-type Ca2+ channels, we found that all Ca2+ current could be completely blocked; thus, Ro 40-5967 is the first Ca2+ channel blocker to eliminate dihydropyridine-insensitive voltage-dependent Ca2+ current at therapeutically useful concentrations. The stepwise sequential block of T- and L-type Ca2+ currents demonstrated in the present study fulfills the functional criterion for the separate identity of the two Ca2+ channel types, and introduces a pharmacological tool that promises to be important in the exploration of T-type Ca2+ channel function.

Nanomolar arachidonic acid influences the respiratory burst in eosinophils and neutrophils induced by GTP-binding protein. A comparative study of the respiratory burst in bovine eosinophils and neutrophils

To investigate a possible role of phospholipase A2 (PLA2) in the respiratory burst in bovine eosinophilic and neutrophilic leukocytes dependent on GTP-binding protein (G-protein), we permeabilized these cells with Staphylococcus aureus alpha-toxin and induced NADPH oxidase activity with the non-hydrolysable GTP analogue GTP[S] or the aluminium tetrafluoro complex AlF4-. Under same experimental conditions, cells responded with different onset times. The onset time for eosinophils was 50-200 s, for neutrophils it was only a few seconds. GTP[S] stimulated in neutrophils only 5% of the respiratory burst compared to eosinophils, whereas AlF4(-)-induced comparable responses (neutrophils 120% of eosinophils). GDP inhibited these responses with an IC50 value of 2.4 mM. Arachidonic acid showed, with the exception of AlF4- stimulated neutrophils, on both stimuli and cell types an enhancing effect (150%) that reached its maximum at 0.1-1 microM. The PLA2 inhibitor 4-bromophenacylbromide reduced the GTP[S]- and AlF4(-)-induced response almost completely (10 microM) and the inhibition was not significantly different for eosinophils and neutrophils (IC50 1-3 microM). If the respiratory burst was reduced with 4-bromophenacylbromide to 1-4% of the original value, 10% of the basal NADPH oxidase activity could be restored by addition of only 20-100 nM arachidonic acid. In addition, the PLA2 activator adriamycin enhanced the response in a dose-dependent manner and in the same order as arachidonic acid did. The results presented above suggest that the respiratory burst may be regulated by different low-molecular-mass and/or heterotrimeric G-proteins and an active role for arachidonic acid or its metabolites in the activation and the maintenance of the direct G-protein-stimulated respiratory burst in bovine eosinophils and neutrophils.