Allosteric modulation in spontaneously active mutant γ-aminobutyric acidA receptors

Tryptophan substitutions were made in the second transmembrane domain of the gamma-aminobutyric acid(A) (GABAA) receptor alpha and beta subunits and the resulting mutant receptors, containing alpha2(S270W) and/or beta1 (S265W), were expressed in Xenopus oocytes. Mutation of either or both subunits resulted in receptors that exhibited enhanced sensitivity to agonist and were spontaneously active in the absence of GABA. The spontaneous activity was blocked by picrotoxin or bicuculline. The enhancement of GABA-induced currents by pentobarbital, by the neurosteroid 5alpha-pregnan-3alpha-ol-20-one, and by the benzodiazepine flunitrazepam was dramatically reduced in the mutant receptors. These results are consistent with the idea that a mutation that promotes gating behavior in a ligand-gated ion channel will also show reduced effects of all positive allosteric modulators in a generalized manner, even when these modulators act at distinct sites on the receptor.

The crystal structure of yeast thiamin pyrophosphokinase

BACKGROUND

Thiamin pyrophosphokinase (TPK) catalyzes the transfer of a pyrophosphate group from ATP to vitamin B1 (thiamin) to form the coenzyme thiamin pyrophosphate (TPP). Thus, TPK is important for the formation of a coenzyme required for central metabolic functions. TPK has no sequence homologs in the PDB and functions by an unknown mechanism. The TPK structure has been determined as a significant step toward elucidating its catalytic action.

RESULTS

The crystal structure of Saccharomyces cerevisiae TPK complexed with thiamin has been determined at 1.8 A resolution. TPK is a homodimer, and each subunit consists of two domains. One domain resembles a Rossman fold with four alpha helices on each side of a 6 strand parallel beta sheet. The other domain has one 4 strand and one 6 strand antiparallel beta sheet, which form a flattened sandwich structure containing a jelly-roll topology. The active site is located in a cleft at the dimer interface and is formed from residues from domains of both subunits. The TPK dimer contains two compound active sites at the subunit interface.

CONCLUSIONS

The structure of TPK with one substrate bound identifies the location of the thiamin binding site and probable catalytic residues. The structure also suggests a likely binding site for ATP. These findings are further supported by TPK sequence homologies. Although possessing no significant sequence homology with other pyrophospokinases, thiamin pyrophosphokinase may operate by a mechanism of pyrophosphoryl transfer similar to those described for pyrophosphokinases functioning in nucleotide biosynthesis.

Earthquake triggering by seismic waves following the Landers and Hector Mine earthquakes

The proximity and similarity of the 1992, magnitude 7.3 Landers and 1999, magnitude 7.1 Hector Mine earthquakes in California permit testing of earthquake triggering hypotheses not previously possible. The Hector Mine earthquake confirmed inferences that transient, oscillatory 'dynamic' deformations radiated as seismic waves can trigger seismicity rate increases, as proposed for the Landers earthquake. Here we quantify the spatial and temporal patterns of the seismicity rate changes. The seismicity rate increase was to the north for the Landers earthquake and primarily to the south for the Hector Mine earthquake. We suggest that rupture directivity results in elevated dynamic deformations north and south of the Landers and Hector Mine faults, respectively, as evident in the asymmetry of the recorded seismic velocity fields. Both dynamic and static stress changes seem important for triggering in the near field with dynamic stress changes dominating at greater distances. Peak seismic velocities recorded for each earthquake suggest the existence of, and place bounds on, dynamic triggering thresholds. These thresholds vary from a few tenths to a few MPa in most places, depend on local conditions, and exceed inferred static thresholds by more than an order of magnitude. At some sites, the onset of triggering was delayed until after the dynamic deformations subsided. Physical mechanisms consistent with all these observations may be similar to those that give rise to liquefaction or cyclic fatigue.

Alcohol actions on GABA(A) receptors: from protein structure to mouse behavior

This article represents the proceedings of a symposium at the 2000 ISBRA Meeting in Yokohama, Japan. The chairs were R. Adron Harris and Susumu Ueno. The presentations were (1) Protein kinase Cepsilon-regulated sensitivity of gamma-aminobutyric acid type A (GABAA) receptors to allosteric agonists, by Robert O. Messing, A. M. Sanchez-Perez, C. W. Hodge, T. McMahon, D. Wang, K. K. Mehmert, S. P. Kelley, A. Haywood, and M. F. Olive; (2) Genetic and functional analysis of a GABAA receptor gamma2 subunit variant: A candidate for quantitative trait loci involved in alcohol sensitivity and withdrawal, by Kari J. Buck and Heather M. Hood; (3) Tryptophan-scanning mutagenesis in GABAA receptor subunits: Channel gating and alcohol actions, by Susumu Ueno; and (4) Can a single binding site account for actions of alcohols on GABAA and glycine receptors? by R. Adron Harris, Yuri Blednov, Geoffrey Findlay, and Maria Paola Mascia.

Correlation between molecular volume and effects of n-alcohols on human neuronal nicotinic acetylcholine receptors expressed in Xenopus oocytes

Nicotinic acetylcholine receptors (nAChRs) are neurotransmitter-gated ion channels and like most such channels, ethanol and longer chain alcohols modulate their activity. In the present studies, the effects of alcohols were characterized on defined combinations of human neuronal nAChR subunits heterologously expressed in Xenopus oocytes. Short-chain alcohols, such as ethanol, propanol, and butanol potentiated ACh-induced currents in both alpha(2)beta(4) and alpha(4)beta(4) nAChRs. Longer chain alcohols, however, inhibited these receptor subtypes. Small increases in alcohol chain length were sufficient to produce a "crossover" from potentiation to inhibition. For the alpha(2)beta(4) receptor subunit combination, butanol clearly potentiated while pentanol inhibited ACh-induced current, whereas for alpha(4)beta(4) nAChR, propanol potentiated, butanol had no discernable effect, and pentanol inhibited receptor function. Fluorinated analogs of ethanol, propanol, and butanol were used to determine whether the effects of the alcohols were dependent upon chain length or whether another related attribute, such as molecular volume, was the defining characteristic. The experimental results support the hypothesis that for both alpha(2)beta(4) and alpha(4)beta(4) receptor subtypes, molecular volume appears to be the most important determinant of both the potency as well as the direction of modulation of nAChR function by n-alcohols and related compounds. Although it has been suggested that the inhibitory and facilitatory effects of alcohols are mediated by actions at different sites on the receptor molecule, the present data suggest the possibility that there may be a single site of alcohol action and that the nature of this action is dependent upon the physical properties of the molecule.

Regulation of branched-chain alpha-keto acid dehydrogenase kinase expression in rat liver

Branched-chain amino acids are toxic in excess but have to be conserved for protein synthesis. This is accomplished in large part by control of the activity of the branched-chain alpha-keto acid dehydrogenase complex by phosphorylation/dephosphorylation. Regulation of the activity of the hepatic enzyme appears particularly important, at least in rats, since an exceptional high activity of the complex in this tissue makes the liver the primary clearing house for excess branched-chain alpha-keto acids released by other tissues. The degree to which the branched-chain alpha-keto acid dehydrogenase complex is inactivated by phosphorylation is determined by the activity of the branched-chain alpha-keto acid dehydrogenase kinase, which is itself regulated by allosteric effectors as well as factors that affect its level of expression. Well established among these are the alpha-keto acid produced by leucine transamination, which is a potent inhibitor of the kinase, and starvation for dietary protein, which causes increased expression of the branched-chain alpha-keto acid dehydrogenase kinase. The latter finding resulted in the working hypothesis that nutrients and hormones regulate expression of the branched-chain alpha-keto acid dehydrogenase kinase. Evidence has been obtained for the involvement of thyroid hormone, glucocorticoids and ligands for peroxisome proliferator-activated receptor alpha. Thyroid hormone induces, whereas glucocorticoids and peroxisome proliferator-activated receptor alpha ligands repress, expression of the kinase. Increased blood levels of thyroid hormone are proposed to be responsible for increased expression of branched-chain alpha-keto acid dehydrogenase kinase in animals starved for protein.

Muscle fiber type comparison of PDH kinase activity and isoform expression in fed and fasted rats

Fiber type specificity for expression of all three rat skeletal muscle pyruvate dehydrogenase kinase (PDK) isoforms (PDK1, 2, and 4) was determined in fed and 24-h fasted rats. PDK activity and isoform protein and mRNA contents were determined in white gastrocnemius (WG; fast-twitch glycolytic), red gastrocnemius (RG; fast-twitch oxidative), and soleus (Sol; slow-twitch oxidative) muscles. PDK activity was lower in WG compared with oxidative muscles (RG, Sol) in both fed and fasted rats. PDK activities from fed muscles were 0.12 +/- 0.04, 0.30 +/- 0.01, and 0.36 +/- 0.08 min(-1) in WG, Sol, and RG, respectively, and increased in fasted muscles (0.36 +/- 0.09, 0.68 +/- 0.18, and 0.80 +/- 0.14 min(-1)). This correlated with increased PDK4 protein and to a lesser extent with PDK4 mRNA. PDK2 protein was not different between fiber types in fed or fasted rats, but PDK2 mRNA content was twofold greater in RG from fasted rats compared with fed rats. PDK1 was unaltered by fasting in all muscle types at both the protein and mRNA level, but in both fed and fasted rats had much greater protein and mRNA content in the oxidative vs. glycolytic muscles. In conclusion, PDK activity and PDK1 and 4 protein and mRNA were lower in glycolytic vs. oxidative muscles from fed and fasted rats. Fasting for 24 h induced a two- to threefold increase in PDK activity that was mainly due to increases in PDK4 protein and mRNA. PDK1 and 2 protein and mRNA were generally unaltered by fasting in all fiber types, except for increased PDK2 mRNA in the fast oxidative fibers. Because the PDK isoforms vary greatly in their kinetic properties, their relative proportions in the three fiber types at any given time during fasting could significantly alter the acute regulation of the pyruvate dehydrogenase complex.

Effect of fibrin bandage fibrinogen concentration on blood loss after grade V liver injury in swine

OBJECTIVE

To determine the effect of fibrinogen concentration of dry fibrin bandages on blood loss after grade V liver injury.

METHODS

Twenty-four pigs were used. Grade V liver injuries were induced and treated with dry fibrin bandages containing 0, 4, 8, or 15 mg fibrinogen/cm2. Animals were monitored for 60 minutes. Blood loss, fluid use, hematological data, and hemostasis were assessed.

RESULTS

Post-treatment blood losses (mean and 95% confidence interval [CI]) were 1,560 mL (356-6,844), 372 mL (65-2,134), 225 mL (51-992), and 127 mL (22-732) in the 0-, 4-, 8-, and 15-mg groups, respectively. Only the 15-mg group had results significantly lower than the 0-mg group (p < 0.05). Blood loss was negatively related to fibrinogen concentration (p < 0.05).

CONCLUSION

Fibrinogen concentration was inversely related to blood loss after grade V liver injury. The 15-mg formulation was the only one that significantly reduced blood loss.

Regulation of the activity of branched-chain 2-oxo acid dehydrogenase (BCODH) complex by binding BCODH kinase

Branched-chain 2-oxo acid dehydrogenase (BCODH) kinase is responsible for inactivation of BCODH complex by phosphorylation of the complex. Activity of the kinase towards its substrate, the E1 component of the BCODH complex, is known dependent upon binding of the kinase to the E2 component. The possible existence as well as importance of unbound mitochondrial BCODH kinase has been largely ignored in previous studies. Evidence is presented here for the existence of free and bound BCODH kinase in the matrix space of rat liver mitochondria. Furthermore, in female rats, in which diurnal variations in liver BCODH complex and kinase activities occur, the amount of the kinase bound to the complex changes between morning and evening without a change in total kinase protein. Activity of the kinase correlates with the amount of bound rather than total kinase protein, suggesting only the bound form is active. Changes in amount of kinase bound and therefore active appear responsible for diurnal variation in BCODH complex activity in the female rat. We propose that change in the amount of bound BCODH kinase is a key feature of a novel regulatory mechanism for determining the activity state of the BCODH complex.

Application of DNA microarrays to study human alcoholism

An emerging idea is that long-term alcohol abuse results in changes in gene expression in the brain and that these changes are responsible at least partly for alcohol tolerance, dependence and neurotoxicity. The overall goal of our research is to identify genes which are differentially expressed in the brains of well-characterized human alcoholics as compared with non-alcoholics. This should identify as-yet-unknown alcohol-responsive genes, and may well confirm changes in the expression of genes which have been delineated in animal models of alcohol abuse. Cases were carefully selected and samples pooled on the basis of relevant criteria; differential expression was monitored by microarray hybridization. The inherent diversity of human alcoholics can be exploited to identify genes associated with specific pathological processes, as well as to assess the effects of concomitant disease, severity of brain damage, drinking behavior, and factors such as gender and smoking history. Initial results show selective changes in gene expression in alcoholics; of particular importance is a coordinated reduction in genes coding for myelin components.

Different hypotensive responses to intravenous bovine and human thrombin preparations in swine

BACKGROUND

Accidental intravenous introduction of commercial bovine thrombin (BT) during use of fibrin glue may result in profound hypotension. Commercial human thrombin (HT) is now available. This study compared the effects of intravenous BT versus HT in swine.

METHODS

Swine received 30 U/kg BT, 60 U/kg BT, 30 U/kg HT, or 60 U/kg HT intravenously. Mean arterial pressure (MAP) and survival were monitored for 30 minutes. Thrombin purities and in vitro activities were examined.

RESULTS

MAP nadir was lower (p < 0.05) after BT, 27.7 +/- 3.3% (mean +/- SEM) of pretreatment MAP, compared with 41.1 +/- 3.7% after HT. Five of six animals died after 60 U/kg BT, whereas all others survived (p < 0.05). Histology suggested more severe disseminated intravascular coagulation after BT. HT was purer than BT. In vitro activities were similar.

CONCLUSION

Both BT and HT produced hypotension. HT appeared safer, because of higher purity. Regardless of source and purity, thrombin must be used with caution.

A transmembrane site determines sensitivity of neuronal nicotinic acetylcholine receptors to general anesthetics

Neuronal nicotinic acetylcholine receptors (nAChRs) are potential targets for a wide variety of general anesthetics. We recently showed that alpha(4)beta(2) nAChRs are more sensitive than alpha(4)beta(4) receptors to the gaseous anesthetics nitrous oxide and xenon. The present study examines chimeric and point mutant rat nAChRs expressed in Xenopus oocytes and identifies a single amino acid residue (beta(2)-Val(253) or beta(4)-Phe(255)) near the middle of the second transmembrane segment (TM2) that determines gaseous anesthetic sensitivity. Mutations of this residue in beta subunits and the homologous residue of alpha(4) subunits (alpha(4)-Val(254)) showed that this position also determines sensitivities of nAChRs to acetylcholine, isoflurane, pentobarbital, and hexanol. In contrast, these mutations did not affect actions of ketamine. The positively charged sulfhydryl-specific reagent methanethiosulfonate ethylammonium reacted with a cysteine introduced at alpha(4)-Val(254) or beta(2)-Val(253), and irreversibly reduced anesthetic sensitivities of nAChRs. Propyl methanethiosulfonate is an anesthetic analog that covalently binds to a TM2 site of gamma-aminobutyric acid(A) and glycine receptors and irreversibly enhances receptor function. However, propyl methanethiosulfonate reversibly inhibited cysteine-substitution mutants at alpha(4)-Val(254) or beta(2)-Val(253) of nAChRs, and did not affect anesthetic sensitivity. Thus, residues alpha(4)-Val(254) and beta(2)-Val(253) alter channel gating and determine anesthetic sensitivity of nAChRs, but are not likely to be anesthetic-binding sites.

Expression and regulation of pyruvate dehydrogenase kinase isoforms in the developing rat heart and in adulthood: role of thyroid hormone status and lipid supply

Activation of the pyruvate dehydrogenase (PDH) complex (PDHC) promotes glucose disposal, whereas inactivation conserves glucose. The PDH kinases (PDHKs) regulate glucose oxidation through inhibitory phosphorylation of PDHC. The adult rat heart contains three PDHK isoforms PDHK1, PDHK2 and PDHK4. Using Western-blot analysis, with specific antibodies raised against individual recombinant PDHK1, PDHK2 and PDHK4, the present study investigated PDHK isoform expression in the developing rat heart and adulthood. We identified clear differences in the patterns of protein expression of each of these PDHK isoforms during the first 3 weeks of post-natal development, with most marked up-regulation of isoforms PDHK1 and PDHK4. Distinctions between the three cardiac PDHK isoforms were also demonstrated with respect to post-neonatal maturational up-regulation; with greatest up-regulation of PDHK1 and least up-regulation of PDHK4 from the post-neonatal period until maturity. The study also examined the role of thyroid hormone status and lipid supply on PDHK isoform expression. We observed marked selective increases in the amount of PDHK4 protein present relative to total cardiac protein in both hyperthyroidism and high-fat feeding. Overall, our data identify PDHK isoform PDHK1 as being of more potential regulatory importance for glucose oxidation in the adult compared with the neonatal heart, and cardiac PDHK4 as a PDHK isoform whose expression is specifically responsive to changes in lipid supply, suggesting that its up-regulation during early post-natal life may be the perinatal switch to use fatty acids as the energy source. We also identify regulation of pyruvate sensitivity of cardiac PDHK as a physiological variable, a change in which requires factors in addition to a change in lipid supply.

Parasite-mediated down-regulation of collagen-induced arthritis (CIA) in DA rats

Microbial infection can impact on the course of autoimmune disease, both in disease-inducing and disease-protecting capacities. Here we investigated if infection with Trypanosoma brucei brucei (Tbb), the protozoan causative agent of African Sleeping Sickness, could ameliorate the course of CIA in the Dark Agouti rat, an experimental model which shares many features with human rheumatoid arthritis. Infection of animals with living, but not inoculation with dead Tbb resulted in complete or significant reduction of clinical arthritic symptoms. Infection prior to collagen immunization was more effective than a later treatment, and this effect was related to the level of parasitaemia. Using reverse transcriptase-polymerase chain reaction we detected an increase in interferon-gamma mRNA in the draining lymph nodes of Tbb-treated animals relative to controls at day 28 after disease induction. Transforming growth factor-beta could be detected in the lymph nodes in four out of six animals that had received Tbb. In the joints, immunohistochemistry revealed reduced production of tumour necrosis factor-alpha in Tbb-treated animals relative to controls. The most striking difference between Tbb-infected and control groups, as measured by ELISA, was the down-regulation of anti-collagen II IgG antibody responses in parasite-infected animals. We conclude that live parasites can exert an immunomodulatory and protective effect in CIA in which several mechanisms may work in parallel, although the almost complete down-regulation of the anti-collagen antibody response may alone explain the protective effect in CIA. The described model may be useful in further attempts to use the mechanisms involved in parasite immune defence to prevent and treat certain autoimmune conditions.

Gene expression in human alcoholism: microarray analysis of frontal cortex

BACKGROUND

Changes in brain gene expression are thought to be responsible for the tolerance, dependence, and neurotoxicity produced by chronic alcohol abuse, but there has been no large scale study of gene expression in human alcoholism.

METHODS

RNA was extracted from postmortem samples of superior frontal cortex of alcoholics and nonalcoholics. Relative levels of RNA were determined by array techniques. We used both cDNA and oligonucleotide microarrays to provide coverage of a large number of genes and to allow cross-validation for those genes represented on both types of arrays.

RESULTS

Expression levels were determined for over 4000 genes and 163 of these were found to differ by 40% or more between alcoholics and nonalcoholics. Analysis of these changes revealed a selective reprogramming of gene expression in this brain region, particularly for myelin-related genes which were down-regulated in the alcoholic samples. In addition, cell cycle genes and several neuronal genes were changed in expression.

CONCLUSIONS

These gene expression changes suggest a mechanism for the loss of cerebral white matter in alcoholics as well as alterations that may lead to the neurotoxic actions of ethanol.

Induction of early atherosclerosis in CBA/J mice by combination of Trypanosoma cruzi infection and a high cholesterol diet

In addition to established factors such as hyperlipidemia, smoking and hypertension, inflammation and infection have recently been implicated as major risk factors for atherosclerotic disease. Proatherogenic effects induced by infection may be related to both systemic inflammation and to direct effects on the vascular wall. We report here that a high fat diet combined with a protozoal infection with known tropism to the heart induced early atherosclerosis and intimal inflammatory infiltrates (CD4+, CD8+ cells and macrophages) in aortas of all (n = 7) CBA/J mice investigated. These mice are normally quite resistant to atherosclerotic development and in the control group (n = 7) receiving only a fatty diet, only one mouse presented a lesion. This lesion was completely devoid of infiltrating CD8+ cells. Parasite-infected mice receiving a normal diet exhibited vasculitis, but no signs of atherosclerosis and control mice receiving normal diet, as expected, exhibited neither signs of vasculitis nor atherosclerosis. Secretion of IL-6, TNF-alpha, and IFN-gamma were demonstrated in all atherosclerotic lesions and IL-6 appeared to be the dominant cytokine, both in the lesions themselves as well as in the intimal-medial junction. There were no traces of parasites present in the artery wall, indicating that atherosclerosis was induced via an indirect route. We conclude that a high fat diet in conjunction with infection and systemic (or localized) inflammation may have a strong proatherogenic effect. Finally, we suggest that CBA/J mice infected with T. cruzi parasites and given a fatty diet could serve as a useful experimental model in the continued analysis of factors contributing to the induction of atherosclerosis.

Allosteric modulation in spontaneously active mutant gamma-aminobutyric acid(A) receptors [corrected]

Tryptophan substitutions were made in the second transmembrane domain of the gamma-aminobutyric acid(A) (GABA(A)) receptor alpha and beta subunits and the resulting mutant receptors, containing alpha(2)(S270W) and/or beta(1)(S265W), were expressed in Xenopus oocytes. Mutation of either or both subunits resulted in receptors that exhibited enhanced sensitivity to agonist and were spontaneously active in the absence of GABA. The spontaneous activity was blocked by picrotoxin or bicuculline. The enhancement of GABA-induced currents by pentobarbital, by the neurosteroid 5alpha-pregnan-3alpha-ol-20-one, and by the benzodiazepine flunitrazepam was dramatically reduced in the mutant receptors. These results are consistent with the idea that a mutation that promotes gating behavior in a ligand-gated ion channel will also show reduced effects of all positive allosteric modulators in a generalized manner, even when these modulators act at distinct sites on the receptor.

Screening of several H-2 congenic mouse strains identified H-2(q) mice as highly susceptible to MOG-induced EAE with minimal adjuvant requirement

We identified H-2(q) as a susceptible genotype for MOG-induced EAE by systematic screening of a series of H-2 congenic B10 mouse strains. A series of H-2(q)-bearing strains with divergent gene backgrounds were subsequently investigated. DBA/1 mice were highly susceptible to MOG(1-125)- and MOG(79-96)-induced EAE in the absence of pertussis toxin. Immunisation with MOG(1-125) and MOG(79-96) induced an autoreactive T-cell response in DBA/1 mice. Brain histopathology revealed T-cell and macrophage-infiltrated lesions with associated demyelination. The important features which make this an appropriate model of human disease are high sensitivity to MOG and dependence of an immunodominant peptide region homologous to that implicated in multiple sclerosis.