Showing papers in "Biochemical Society Transactions in 2005"

Rho GTPases and the control of cell behaviour.

Abstract: Rho, Rac and Cdc42, three members of the Rho family of small GTPases, each control a signal transduction pathway linking membrane receptors to the assembly and disassembly of the actin cytoskeleton and of associated integrin adhesion complexes. Rho regulates stress fibre and focal adhesion assembly, Rac regulates the formation of lamellipodia protrusions and membrane ruffles, and Cdc42 triggers filopodial extensions at the cell periphery. These observations have led to the suggestion that wherever filamentous actin is used to drive a cellular process, Rho GTPases are likely to play an important regulatory role. Rho GTPases have also been reported to control other cellular activities, such as the JNK (c-Jun N-terminal kinase) and p38 MAPK (mitogen-activated protein kinase) cascades, an NADPH oxidase enzyme complex, the transcription factors NF-κB (nuclear factor κB) and SRF (serum-response factor), and progression through G 1 of the cell cycle. Thus Rho, Rac and Cdc42 can regulate the actin cytoskeleton and gene transcription to promote co-ordinated changes in cell behaviour. We have been analysing the biochemical contributions of Rho GTPases in cell movement and have found that Rac controls cell protrusion, while Cdc42 controls cell polarity.

Signalling role of adipose tissue: adipokines and inflammation in obesity.

Abstract: White adipose tissue (WAT) is a major endocrine and secretory organ, which releases a wide range of protein signals and factors termed adipokines. A number of adipokines, including leptin, adiponectin, tumour necrosis factor alpha, IL-1beta (interleukin 1beta), IL-6, monocyte chemotactic protein-1, macrophage migration inhibitory factor, nerve growth factor, vascular endothelial growth factor, plasminogen activator inhibitor 1 and haptoglobin, are linked to inflammation and the inflammatory response. Obesity is characterized by a state of chronic mild inflammation, with raised circulating levels of inflammatory markers and the expression and release of inflammation-related adipokines generally rises as adipose tissue expands (adiponectin, which has anti-inflammatory action is an exception). The elevated production of inflammation-related adipokines is increasingly considered to be important in the development of diseases linked to obesity, particularly Type II diabetes and the metabolic syndrome. WAT is involved in extensive cross-talk with other organs and multiple metabolic systems through the various adipokines.

Protein import into mitochondria.

Abstract: Mitochondria comprise approx. 1000–3000 different proteins, almost all of which must be imported from the cytosol into the organelle. So far, six complex molecular machines, protein translocases, were identified that mediate this process. The TIM23 complex is a major translocase in the inner mitochondrial membrane. It uses two energy sources, namely membrane potential and ATP, to facilitate preprotein translocation across the inner membrane and insertion into the inner membrane. Recent research has led to the discovery of a number of new constituents of the TIM23 complex and to the unravelling of the mechanisms of preprotein translocation.

Thiol redox control via thioredoxin and glutaredoxin systems.

Abstract: The Trx (thioredoxin) and Grx (glutaredoxin) systems control cellular redox potential, keeping a reducing thiol-rich intracellular state, which on generation of reactive oxygen species signals through thiol redox control mechanisms. Here, we give a brief overview of the human Trx and Grx systems. The main part focuses on our current knowledge about mitochondrial Grx2, which facilitates mitochondrial redox homoeostasis during oxidative stress-induced apoptosis.

Expression of sweet taste receptors of the T1R family in the intestinal tract and enteroendocrine cells.

Abstract: The composition of the intestinal luminal content varies considerably with diet. It is important therefore that the intestinal epithelium senses and responds to these significant changes and regulates its functions accordingly. Although it is becoming evident that the gut epithelium senses and responds to luminal nutrients, little is known about the nature of the nutrient sensing molecule and the downstream cellular events. A prototype example is the modulation in the capacity of the gut to absorb monosaccharides via the intestinal luminal membrane Na + /glucose cotransporter, SGLT1. The experimental evidence suggests that luminal sugar is sensed by a glucose sensor residing on the luminal membrane of the gut epithelium and linked to a G-protein-coupled receptor, cAMP/PKA (protein kinase A) pathway, resulting ultimately in modulation of intestinal monosaccharide absorption. Here we report the expression, at mRNA and protein levels, of members of the T1R sweet taste receptors, and the α-subunit of the G-protein gustducin, in the small intestine and the enteroendocrine cell line, STC-1. In the small intestine, there is a highly coordinated expression of sweet taste receptors and gustducin, a G-protein implicated in intracellular taste signal transduction, throughout the gut. The potential involvement of these receptors in sugar sensing in the intestine will facilitate our understanding of intestinal nutrient sensing, with implications for better nutrition and health maintenance.

The efficiency and plasticity of mitochondrial energy transduction

Abstract: Since it was first realized that biological energy transduction involves oxygen and ATP, opinions about the amount of ATP made per oxygen consumed have continually evolved. The coupling efficiency is crucial because it constrains mechanistic models of the electron-transport chain and ATP synthase, and underpins the physiology and ecology of how organisms prosper in a thermodynamically hostile environment. Mechanistically, we have a good model of proton pumping by complex III of the electron-transport chain and a reasonable understanding of complex IV and the ATP synthase, but remain ignorant about complex I. Energy transduction is plastic: coupling efficiency can vary. Whether this occurs physiologically by molecular slipping in the proton pumps remains controversial. However, the membrane clearly leaks protons, decreasing the energy funnelled into ATP synthesis. Up to 20% of the basal metabolic rate may be used to drive this basal leak. In addition, UCP1 (uncoupling protein 1) is used in specialized tissues to uncouple oxidative phosphorylation, causing adaptive thermogenesis. Other UCPs can also uncouple, but are tightly regulated; they may function to decrease coupling efficiency and so attenuate mitochondrial radical production. UCPs may also integrate inputs from different fuels in pancreatic β-cells and modulate insulin secretion. They are exciting potential targets for treatment of obesity, cachexia, aging and diabetes. Abbreviations: ANT, adenine nucleotide translocase; H+/O ratio, H+/2e ratio, H+/ATP ratio, number of protons translocated across the mitochondrial inner membrane for each oxygen atom reduced to water, pair of electrons transferred from donor to acceptor, or ATP synthesized; P/O ratio, number of ATP molecules made by mitochondria from ADP for each oxygen atom consumed during substrate oxidation and oxidative phosphorylation; ROS, reactive oxygen species; UCP, uncoupling protein

The role of cell-derived oligomers of Aβ in Alzheimer's disease and avenues for therapeutic intervention

Abstract: Burgeoning evidence suggests that soluble oligomers of Aβ (amyloid β-protein) are the earliest effectors of synaptic compromise in Alzheimer9s disease. Whereas most other investigators have employed synthetic Aβ peptides, we have taken advantage of a β-amyloid precursor protein-overexpressing cell line (referred to as 7PA2) that secretes sub-nanomolar levels of low- n oligomers of Aβ. These are composed of heterogeneous Aβ peptides that migrate on SDS/PAGE as dimers, trimers and tetramers. When injected into the lateral ventricle of rats in vivo , these soluble oligomers inhibit hippocampal long-term potentiation and alter the memory of a complex learned behaviour. Biochemical manipulation of 7PA2 medium including immunodepletion with Aβ-specific antibodies and fractionation by size-exclusion chromatography allowed us to unambiguously attribute these effects to low- n oligomers. Using this paradigm we have tested compounds directed at three prominent amyloid-based therapeutic targets: inhibition of the secretases responsible for Aβ production, inhibition of Aβ aggregation and immunization against Aβ. In each case, compounds capable of reducing oligomer production or antibodies that avidly bind Aβ oligomers also ameliorate the synaptotoxic effects of these natural, cell-derived oligomers.

Increased risk of Alzheimer's disease in Type II diabetes: insulin resistance of the brain or insulin-induced amyloid pathology?

Abstract: Type II diabetes mellitus (DM2) is associated with an increased risk of cognitive dysfunction and dementia. The increased risk of dementia concerns both Alzheimer's disease and vascular dementia. Although some uncertainty remains into the exact pathogenesis, several mechanisms through which DM2 may affect the brain have now been identified. First, factors related to the 'metabolic syndrome', a cluster of metabolic and vascular risk factors (e.g. dyslipidaemia and hypertension) that is closely linked to DM2, may be involved. A number of these risk factors are predictors of cerebrovascular disease, accelerated cognitive decline and dementia. Secondly, hyperglycaemia may be involved, through adverse effects of potentially 'toxic' glucose metabolites on the brain and its vasculature. Thirdly, insulin itself may be involved. Insulin can directly modulate synaptic plasticity and learning and memory, and disturbances in insulin signalling pathways in the periphery and in the brain have recently been implicated in Alzheimer's disease and brain aging. Insulin also regulates the metabolism of beta-amyloid and tau, the building blocks of amyloid plaques and neurofibrillary tangles, the neuropathological hallmarks of Alzheimer's disease. In this paper, the evidence for the association between DM2 and dementia and for each of these underlying mechanisms will be reviewed, with emphasis on the role of insulin itself.

Polyunsaturated fatty acids and inflammation.

Abstract: The n-6 polyunsaturated fatty acid, arachidonic acid, is a precursor of prostaglandins, leukotrienes and related compounds that have important roles as mediators and regulators of inflammation. Consuming increased amounts of long chain n-3 polyunsaturated fatty acids (found in oily fish and fish oils) results in a partial replacement of the arachidonic acid in cell membranes by eicosapentaenoic and docosahexaenoic acids. This leads to decreased production of arachidonic acid-derived mediators. This alone is a potentially beneficial anti-inflammatory effect of n-3 fatty acids. However, n-3 fatty acids have a number of other effects that might occur downstream of altered eicosanoid production or are independent of this. For example, they result in suppressed production of pro-inflammatory cytokines and can modulate adhesion molecule expression. These effects occur at the level of altered gene expression.

Nitrogen assimilation and nitrogen control in cyanobacteria

Abstract: Nitrogen sources commonly used by cyanobacteria include ammonium, nitrate, nitrite, urea and atmospheric N2, and some cyanobacteria can also assimilate arginine or glutamine. ABC (ATP-binding cassette)-type permeases are involved in the uptake of nitrate/nitrite, urea and most amino acids, whereas secondary transporters take up ammonium and, in some strains, nitrate/nitrite. In cyanobacteria, nitrate and nitrite reductases are ferredoxin-dependent enzymes, arginine is catabolized by a combination of the urea cycle and arginase pathway, and urea is degraded by a Ni2+-dependent urease. These pathways provide ammonium that is incorporated into carbon skeletons through the glutamine synthetase–glutamate synthase cycle, in which 2-oxoglutarate is the final nitrogen acceptor. The expression of many nitrogen assimilation genes is subjected to regulation being activated by the nitrogen-control transcription factor NtcA, which is autoregulatory and whose activity appears to be influenced by 2-oxoglutarate and the signal transduction protein PII. In some filamentous cyanobacteria, N2 fixation takes place in specialized cells called heterocysts that differentiate from vegetative cells in a process strictly controlled by NtcA. Abbreviations: ABC, ATP-binding cassette; CAP, catabolite gene activator protein

Nitric oxide and nitrosative stress tolerance in bacteria.

Abstract: Nitric oxide is not only an obligatory intermediate in denitrification, but also a signalling and defence molecule of major importance. However, the basis of resistance to NO and RNS (reactive nitrogen species) is poorly understood in many microbes. The cellular targets of NO and RNS [e.g. metalloproteins, thiols in proteins, glutathione and Hcy (homocysteine)] may themselves serve as signal transducers, sensing NO and RNS, and resulting in altered gene expression and synthesis of protective enzymes. The properties of a number of such protective mechanisms are outlined here, including globins, flavorubredoxin, diverse enzymes with NO- or S -nitrosothiol-reducing properties and other redox proteins with poorly defined roles in protection from nitrosative stresses. However, the most fully understood mechanism for NO detoxification involves the enterobacterial flavohaemoglobin (Hmp). Aerobically, Hmp detoxifies NO by acting as an NO denitrosylase or ‘oxygenase’ and thus affords inducible protection of growth and respiration, and aids survival in macrophages. The flavohaemoglobin-encoding gene of Escherichia coli , hmp , responds to the presence of NO and RNS in an SoxRS-independent manner. Nitrosating agents, such as S -nitrosoglutathione, deplete cellular Hcy and consequently modulate activity of the MetR regulator that binds the hmp promoter. Regulation of Hmp synthesis under anoxic conditions involves nitrosylation of 4Fe-4S clusters in the global transcriptional regulator, FNR. The foodborne microaerophilic pathogen, Campylobacter jejuni , also expresses a haemoglobin, Cgb, but it does not possess the reductase domain of Hmp. A Cgb-deficient mutant of C. jejuni is hypersensitive to RNS, whereas cgb expression and holoprotein synthesis are specifically increased on exposure to RNS, resulting in NO-insensitive respiration. A ‘systems biology’ approach, integrating the methodologies of bacterial molecular genetics and physiology with post-genomic technologies, promises considerable advances in our understanding of bacterial NO tolerance mechanisms in pathogenesis.

Insulin signal transduction in human skeletal muscle: identifying the defects in Type II diabetes.

Abstract: Type II diabetes is characterized by defects in insulin action on peripheral tissues, such as skeletal muscle, adipose tissue and liver and pancreatic beta-cell defects. Since the skeletal muscle accounts for approx. 75% of whole body insulin-stimulated glucose uptake, defects in this tissue play a major role in the impaired glucose homoeostasis in Type II diabetic patients. Thus identifying defective steps in this process may reveal attractive targets for drug development to combat insulin resistance and Type II diabetes. This review will describe the effects of insulin on glucose transport and other metabolic events in skeletal muscle that are mediated by intracellular signalling cascades. Evidence for impaired activation of the insulin receptor signalling cascade and defective glucose transporter 4 translocation in the skeletal muscle from Type II diabetic patients will be presented. Through the identification of the intracellular defects in insulin action that control glucose homoeostasis, a better understanding of the disease pathogenesis can be gained and strategies for intervention may be developed.

Formate and its role in hydrogen production in Escherichia coli.

Abstract: The production of dihydrogen by Escherichia coli and other members of the Enterobacteriaceae is one of the classic features of mixed-acid fermentation. Synthesis of the multicomponent, membrane-associated FHL (formate hydrogenlyase) enzyme complex, which disproportionates formate into CO(2) and H(2), has an absolute requirement for formate. Formate, therefore, represents a signature molecule in the fermenting E. coli cell and factors that determine formate metabolism control FHL synthesis and consequently dihydrogen evolution.

Embryonic stem (ES) cells and embryonal carcinoma (EC) cells: opposite sides of the same coin.

Abstract: Embryonal carcinoma (EC) cells are the stem cells of teratocarcinomas, and the malignant counterparts of embryonic stem (ES) cells derived from the inner cell mass of blastocyst-stage embryos, whether human or mouse. On prolonged culture in vitro , human ES cells acquire karyotypic changes that are also seen in human EC cells. They also ‘adapt’, proliferating faster and becoming easier to maintain with time in culture. Furthermore, when cells from such an ‘adapted’ culture were inoculated into a SCID (severe combined immunodeficient) mouse, we obtained a teratocarcinoma containing histologically recognizable stem cells, which grew out when the tumour was explanted into culture and exhibited properties of the starting ES cells. In these features, the ‘adapted’ ES cells resembled malignant EC cells. The results suggest that ES cells may develop in culture in ways that mimic changes occurring in EC cells during tumour progression.

Alternative mechanisms of initiating translation of mammalian mRNAs

Abstract: Of all the steps in mRNA translation, initiation is the one that differs most radically between prokaryotes and eukaryotes. Not only is there no equivalent of the prokaryotic Shine-Dalgarno rRNA-mRNA interaction, but also what requires only three initiation factor proteins (aggregate size approximately 125 kDa) in eubacteria needs at least 28 different polypeptides (aggregate >1600 kDa) in mammalian cells, which is actually larger than the size of the 40 S ribosomal subunit. Translation of the overwhelming majority of mammalian mRNAs occurs by a scanning mechanism, in which the 40 S ribosomal subunit, primed for initiation by the binding of several initiation factors including the eIF2 (eukaryotic initiation factor 2)-GTP-MettRNA(i) complex, is loaded on the mRNA immediately downstream of the 5'-cap, and then scans the RNA in the 5'-->3' direction. On recognition of (usually) the first AUG triplet via base-pairing with the Met-tRNA(i) anticodon, scanning ceases, triggering GTP hydrolysis and release of eIF2-GDP. Finally, ribosomal subunit joining and the release of the other initiation factors completes the initiation process. This sketchy outline conceals the fact that the exact mechanism of scanning and the precise roles of the initiation factors remain enigmatic. However, the factor requirements for initiation site selection on some viral IRESs (internal ribosome entry sites/segments) are simpler, and investigations into these IRES-dependent mechanisms (particularly picornavirus, hepatitis C virus and insect dicistrovirus IRESs) have significantly enhanced our understanding of the standard scanning mechanism. This article surveys the various alternative mechanisms of initiation site selection on mammalian (and other eukaryotic) cellular and viral mRNAs, starting from the simplest (in terms of initiation factor requirements) and working towards the most complex, which paradoxically happens to be the reverse order of their discovery.

Before and beyond ABA: upstream sensing and internal signals that determine ABA accumulation and response under abiotic stress.

Abstract: Sensing and signalling events that detect abiotic stress-induced changes in plant water status and initiate downstream stress responses such as ABA (abscisic acid) accumulation and osmoregulation remain uncharacterized in plants. Although conclusive results are lacking, recent results from plants, and analogies to signalling in other organisms, suggest possible mechanisms for sensing altered water status and initial transduction of that signal. Internal signals that act downstream of ABA and modulate stress responses to reflect the type and severity of the stress and the metabolic status of the plant are also not well understood. Two specific types of signalling, sugar sensing and reactive oxygen signalling, are likely to be modulators of ABA response under stress. For both upstream sensing and signalling of plant water status as well as downstream modulation of ABA response, present results suggest several genetic strategies with high potential to increase our understanding of the molecular basis by which plants sense and respond to altered water status.

1994–2004: 10 years of research on the anaerobic oxidation of ammonium

Abstract: The obligately anaerobic ammonium oxidation (anammox) reaction with nitrite as primary electron acceptor is catalysed by the planctomycete-like bacteria Brocadia anammoxidans , Kuenenia stuttgartiensis and Scalindua sorokinii . The anammox bacteria use a complex reaction mechanism involving hydrazine as an intermediate. They have a unique prokaryotic organelle, the anammoxosome, surrounded by ladderane lipids, which exclusively contains the hydrazine oxidoreductase as the major protein to combine nitrite and ammonia in a one-to-one fashion. In addition to the peculiar microbiology, anammox was shown to be very important in the oceanic nitrogen cycle, and proved to be a very good alternative for treatment of high-strength nitrogenous waste streams. With the assembly of the K. stuttgartiensis genome at Genoscope, Evry, France, the anammox reaction has entered the genomic and proteomic era, enabling the elucidation of many intriguing aspects of this fascinating microbial process.

Aβ-degrading enzymes: modulators of Alzheimer's disease pathogenesis and targets for therapeutic intervention

Abstract: The accumulation of Aβ (amyloid β-protein) peptides in the brain is a pathological hallmark of all forms of AD (Alzheimer9s disease) and reducing Aβ levels can prevent or reverse cognitive deficits in mouse models of the disease. Aβ is produced continuously and its concentration is determined in part by the activities ofseveral degradative enzymes, including NEP (neprilysin), IDE (insulin-degrading enzyme), ECE-1 (endothelinconverting enzyme 1) and ECE-2, and probably plasmin. Decreased activity of any of these enzymes due to genetic mutation, or age- or disease-related alterations in gene expression or proteolytic activity, may increase the risk for AD. Conversely, increased expression of these enzymes may confer a protective effect. Increasing Aβ degradation through gene therapy, transcriptional activation or even pharmacological activation of the Aβ-degrading enzymes represents a novel therapeutic strategy for the treatment of AD that is currently being evaluated in cell-culture and animal models. In this paper, we will review the roles of NEP, IDE, ECE and plasmin in determining endogenous Aβ concentration, highlighting recent results concerning the regulation of these enzymes and their potential as therapeutic targets.

Vitamin B12: chemistry and biochemistry.

Abstract: Vitamin B12, the ‘antipernicious anaemia factor’, is required for human and animal metabolism. It was discovered in the late 1940s and its unique corrin ligand was revealed approx. 10 years later by X-ray crystallography. The B12-coenzymes are cofactors in various important enzymatic reactions and are particularly relevant in the metabolism of anaerobic microorganisms. Microorganisms are the only natural sources of the B12-derivatives, whereas most spheres of life (except for the higher plants) depend on these cobalt corrinoids. Abbreviations: B12r, cob(II)alamin; GM, glutamate mutase; MetH, methionine synthase; MMCM, methyl-malonyl CoA mutase; RNR-Ll, ribonucleotide reductase of Lactobacillus leichmanii

Channelrhodopsins: directly light-gated cation channels.

Abstract: Phototaxis and photophobic responses of green algae are mediated by rhodopsins with microbial type chromophores, i.e. all- trans -retinal in the ground state. The green alga Chlamydomonas reinhardtii was recently completely sequenced and the EST (expressed sequence tag) database was made public. We and others detected overlapping partial cDNA sequences that encode two proteins which we termed channelopsins (Chops). The N-terminal half of chop1 (∼300 of 712 amino acids) comprises hypothetical seven-transmembrane segments with sequence similarity to the proton pump bacteriorhodopsin and the chloride pump halorhodopsin. Even though the overall sequence homology is low, several amino acids are conserved that define the retinal-binding site and the H + -transporting network in BR (bacteriorhodopsin). Expression of Chop1, or only the hydrophobic core, in Xenopus laevis oocytes, enriched with retinal, produced a light-gated conductance (maximum at approx. 500 nm), which shows characteristics of a channel [ChR1 (channelrhodopsin-1)] that is selectively permeable for protons. Also ChR2 (737 amino acids) is an ion channel that is switched directly by light and also here the hydrophobic N-terminal half of the protein is sufficient to enable light-sensitive channel activity. The action spectrum is blue-shifted (maximum at approx. 460 nm) with respect to ChR1. In addition to protons, ChR2 is permeable to univalent and bivalent cations. We suggest that ChRs are involved in phototaxis of green algae. We show that heterologous expression of ChR2 is useful to manipulate intracellular p Ca or membrane potential of animal cells, simply by illumination.

Localization and function of Arf family GTPases.

Abstract: Arfs are a family of Ras-related GTP-binding proteins that function in the regulation of membrane trafficking and structure. The six mammalian Arf proteins are expressed ubiquitously and so it is anticipated that each will have a distinct localization and function within the cell. It has been assumed that much of this specificity will be defined by determining which regulators of Arfs, the GEFs (guanine nucleotide-exchange factors) and GAPs (GTPase-activating proteins) function with which Arf proteins. Although in vitro assays may indicate Arf preferences for the numerous Arf GEFs and GAPs that have been identified, in the cell the different Arfs, GEFs and GAPs are targeted to specific compartments where they carry out their functions. We have embarked on studies to define regions of the Arf1 and Arf6 proteins that determine their sites of action and specific activities at the Golgi and plasma membrane respectively. Chimaeras were made between Arf1 and Arf6 in order to identify regions of the protein that contributed to targeting and function. Whereas Arf6 is targeted to the plasma membrane through multiple regions along the protein, we have found a Golgi-targeting region in Arf1 that is sufficient to target Arf6 to the Golgi complex.

Multiple roles of arginine/serine-rich splicing factors in RNA processing

Abstract: SR proteins (serine- and arginine-rich proteins) are an evolutionarily conserved family consisting of essential pre-mRNA splicing factors. Since their discovery and initial characterization, roles of SR proteins in pre-mRNA splicing and in subsequent steps of post-transcriptional gene expression have expanded significantly. The current hypotheses suggest that SR proteins are multifunctional adaptor molecules that may couple distinct steps of RNA metabolism. In the present study, we will provide an overview of the roles of SR proteins in different steps of post-transcriptional gene expression.

The complete denitrification pathway of the symbiotic, nitrogen-fixing bacterium Bradyrhizobium japonicum.

Abstract: Denitrification is an alternative form of respiration in which bacteria sequentially reduce nitrate or nitrite to nitrogen gas by the intermediates nitric oxide and nitrous oxide when oxygen concentrations are limiting. In Bradyrhizobium japonicum , the N2-fixing microsymbiont of soya beans, denitrification depends on the napEDABC , nirK , norCBQD , and nosRZDFYLX gene clusters encoding nitrate-, nitrite-, nitric oxide- and nitrous oxide-reductase respectively. Mutational analysis of the B. japonicum nap genes has demonstrated that the periplasmic nitrate reductase is the only enzyme responsible for nitrate respiration in this bacterium. Regulatory studies using transcriptional lacZ fusions to the nirK , norCBQD and nosRZDFYLX promoter region indicated that microaerobic induction of these promoters is dependent on the fixLJ and fixK 2 genes whose products form the FixLJ–FixK2 regulatory cascade. Besides FixK2, another protein, nitrite and nitric oxide respiratory regulator, has been shown to be required for N-oxide regulation of the B. japonicum nirK and norCBQD genes. Thus nitrite and nitric oxide respiratory regulator adds to the FixLJ–FixK2 cascade an additional control level which integrates the N-oxide signal that is critical for maximal induction of the B. japonicum denitrification genes. However, the identity of the signalling molecule and the sensing mechanism remains unknown. Abbreviations: CRP, cAMP receptor protein; Fnr, fumarate and nitrate reductase; Nap, periplasmic nitrate reductase; Nar, membrane-bound nitrate reductase; Nir, nitrite reductase; NnrR, nitrite and nitric oxide respiratory regulator; Nor, nitric oxide reductase; Nos, nitrous oxide reductase; Tat, twin arginine translocon

Sugar sensing and signalling networks in plants

Abstract: Plant sugar signalling operates in a complex network with plant-specific hormone signalling pathways. Hexokinase was identified as an evolutionarily conserved glucose sensor that integrates light, hormone and nutrient signalling to control plant growth and development.

Serpentine plant MLO proteins as entry portals for powdery mildew fungi

Abstract: In the dicotyledonous plant species Arabidopsis and the monocot barley, presence of specific isoforms of the family of heptahelical plasma membrane-localized MLO proteins is required for successful host-cell invasion by ascomycete powdery mildew fungi. Absence of these MLO proteins, either caused by natural polymorphisms or induced lesions in the respective Mlo genes, results in failure of fungal sporelings to penetrate the plant cell wall. As a consequence, recessively inherited cell-autonomous mlo resistance is effective against all known isolates of powdery mildew fungi colonizing either barley or Arabidopsis. Barley MLO interacts constitutively with the cytoplasmic calcium sensor calmodulin, but the strength of this interaction increases transiently during fungal pathogenesis. In addition, MLO as well as ROR2, a plasma membrane-resident syntaxin also implicated in mlo penetration resistance, focally accumulate at sites of attempted fungal attack, thereby defining a novel pathogen-triggered micro-domain. In conclusion, powdery mildew fungi appear to specifically corrupt MLO to modulate vesicle-associated processes at the plant cell periphery for successful pathogenesis.

Making crossovers during meiosis.

Abstract: Homologous recombination (HR) is required to promote both correct chromosome segregation and genetic variation during meiosis. For this to be successful recombination intermediates must be resolved to generate reciprocal exchanges or 'crossovers' between the homologous chromosomes (homologues) during the first meiotic division. Crossover recombination promotes faithful chromosome segregation by establishing connections (chiasmata) between the homologues, which help guide their proper bipolar alignment on the meiotic spindle. Recent studies of meiotic recombination in both the budding and fission yeasts have established that there are at least two pathways for generating crossovers. One pathway involves the resolution of fully ligated four-way DNA junctions [HJs (Holliday junctions)] by an as yet unidentified endonuclease. The second pathway appears to involve the cleavage of the precursors of ligated HJs, namely displacement (D) loops and unligated/nicked HJs, by the Mus81-Eme1/Mms4 endonuclease.

Leptin: a potential cognitive enhancer?

Abstract: It is well documented that the hormone leptin signals information regarding the status of fat stores to hypothalamic nuclei, which in turn control feeding behaviour and body weight. However, leptin and its receptor are widely expressed in many extra-hypothalamic brain regions, including hippocampus, brain stem and cerebellum. Moreover, evidence is accumulating that leptin has other neuronal functions that are unrelated to its effects on energy homeostasis. Indeed a role for leptin in neuronal development has been suggested as leptin-deficient rodents display abnormal brain development and leptin actively participates in the development of the hypothalamus. In the hippocampus, leptin is a potential cognitive enhancer as genetically obese rodents with dysfunctional leptin receptors display impairments in hippocampal synaptic plasticity. Moreover, direct administration of leptin into the hippocampus can facilitate hippocampal LTP (long-term potentiation) in vivo and improve memory processing in mice. At the cellular level, we have also shown that leptin has the capacity to convert short-term potentiation into LTP. Here, we review the data that leptin influences hippocampal synaptic plasticity via enhancing NMDA (N-methyl-D-aspartate) receptor function. We also provide evidence that rapid trafficking of NMDA receptors to the plasma membrane may underlie the effects of leptin on excitatory synaptic strength.

A genomic view of the complexity of mammalian proteolytic systems.

Abstract: Proteolytic enzymes play an essential role in different physiological processes, including development, reproduction and host defence, as well as in numerous pathologies, like inflammatory diseases, neurological disorders or cancer. The completion of the human genome sequence allowed us to determine that more than 2% of all human genes are proteases or protease inhibitors, reflecting the importance of proteolysis in human biology. To understand better the complexity of proteases in human and other model organisms, we have used the available genome sequences of different mammalian organisms, including mouse, rat and chimpanzee, to identify and compare their degradomes, the complete set of protease genes in these species. Surprisingly, the rodent protease complement is more complex when compared with that of primates, mainly due to the expansion of protease families implicated in reproduction and host defence. Similarly, most differences between human and chimpanzee proteases are found in genes implicated in the immune system, which might explain some of the differences between both organisms. We have also found several genes implicated in reproduction, nutrition and the immune system, which are functional in rat, mouse or chimpanzee, but have been inactivated by mutations in the human lineage. These findings suggest that pseudogenization of specific protease genes has been a mechanism contributing to the evolution of the human genome. Finally, we found that proteases implicated in human hereditary diseases, and especially in neurodegenerative disorders, are highly conserved among mammals.

eIF2B, a mediator of general and gene-specific translational control

Abstract: eIF2B (eukaryotic initiation factor 2B) is a multisubunit protein that is required for protein synthesis initiation and its regulation in all eukaryotic cells. Mutations in eIF2B have also recently been found to cause a fatal human disease called CACH (childhood ataxia with central nervous system hypomyelination) or VWM (vanishing white matter disease). This review provides a general background to translation initiation and mechanisms known to control eIF2B function, before describing molecular genetic and biochemical analysis of eIF2B structure and function, integrating work from studies of the yeast and mammalian eIF2B proteins.

Role of the phosphoinositide 3-kinase pathway in mouse embryonic stem (ES) cells

Abstract: Mouse ES (embryonic stem) cells maintain pluripotency with robust proliferation in vitro. ES cells share some similarities with cancer cells, such as anchorage-independent growth, loss of contact inhibition and tumour formation. After differentiation, ES cells lose pluripotency and tumorigenicity. Recent studies showed that the PI3K (phosphoinositide 3-kinase) pathway is important for proliferation, survival and maintenance of pluripotency in ES cells. The PI3K pathway is activated by growth factors and cytokines including insulin and leukaemia inhibitory factor. In addition to these exogenous factors, the PI3K pathway is endogenously activated by the constitutively active Ras family protein ERas (ES cell-expressed Ras). The PI3K pathway utilizes multiple downstream effectors including mTOR (mammalian target of rapamycin), which we have shown to be essential for proliferation in mouse ES cells and early embryos.