Biomedical subjects

J Michiels

Publications and source records attributed to J Michiels.

At least 37 records · Page 2Linked to original sources

PubMed1998

Differential regulation of Rhizobium etli rpoN2 gene expression during symbiosis and free-living growth.

The Rhizobium etli rpoN1 gene, encoding the alternative sigma factor sigma54 (RpoN), was recently characterized and shown to be involved in the assimilation of several nitrogen and carbon sources during free-living aerobic growth (J. Michiels, T. Van Soom, I. D'hooghe, B. Dombrecht, T. Benhassine, P. de Wilde, and J. Vanderleyden, J. Bacteriol. 180:1729-1740, 1998). We identified a second rpoN gene copy in R. etli, rpoN2, encoding a 54.0-kDa protein which displays 59% amino acid identity with the R. etli RpoN1 protein. The rpoN2 gene is cotranscribed with a short open reading frame, orf180, which codes for a protein with a size of 20.1 kDa that is homologous to several prokaryotic and eukaryotic proteins of similar size. In contrast to the R. etli rpoN1 mutant strain, inactivation of the rpoN2 gene did not produce any phenotypic defects during free-living growth. However, symbiotic nitrogen fixation was reduced by approximately 90% in the rpoN2 mutant, whereas wild-type levels of nitrogen fixation were observed in the rpoN1 mutant strain. Nitrogen fixation was completely abolished in the rpoN1 rpoN2 double mutant. Expression of rpoN1 was negatively autoregulated during aerobic growth and was reduced during microaerobiosis and symbiosis. In contrast, rpoN2-gusA and orf180-gusA fusions were not expressed aerobically but were strongly induced at low oxygen tensions or in bacteroids. Expression of rpoN2 and orf180 was abolished in R. etli rpoN1 rpoN2 and nifA mutants under all conditions tested. Under free-living microaerobic conditions, transcription of rpoN2 and orf180 required the RpoN1 protein. In symbiosis, expression of rpoN2 and orf180 occurred independently of the rpoN1 gene, suggesting the existence of an alternative symbiosis-specific mechanism of transcription activation.

Amino Acid Sequence↗

PubMed1998

luxI- and luxR-homologous genes of Rhizobium etli CNPAF512 contribute to synthesis of autoinducer molecules and nodulation of Phaseolus vulgaris.

Autoinduction plays an important role in intercellular communication among symbiotic and pathogenic gram-negative bacteria. We report here that a nitrogen-fixing symbiont of Phaseolus vulgaris, Rhizobium etli CNPAF512, produces at least seven different autoinducer molecules. One of them exhibits a growth-inhibitory effect like that of the bacteriocin small [N-(3R-hydroxy-7-cis-tetradecanoyl)-L-homoserine lactone]. At least two of the other autoinducers are synthesized by a LuxI-homologous autoinducer synthase. The corresponding luxI homologous gene (raiI) and a luxR homolog (raiR) have been identified and characterized. Enhanced expression of raiI is dependent on cell density and on the presence of one or more autoinducer molecules synthesized by R. etli CNPAF512. A raiI mutant was shown to release only three different autoinducer molecules; a raiR mutant releases four different autoinducer molecules. Examination of different mutants for nodulation of beans showed that raiI is involved in the restriction of nodule number, whereas nitrogen-fixing activity in terms of acetylene reduction per nodule was not affected.

Amino Acid Sequence↗

PubMed1998

The Rhizobium etli rpoN locus: DNA sequence analysis and phenotypical characterization of rpoN, ptsN, and ptsA mutants.

The rpoN region of Rhizobium etli was isolated by using the Bradyrhizobium japonicum rpoN1 gene as a probe. Nucleotide sequence analysis of a 5,600-bp DNA fragment of this region revealed the presence of four complete open reading frames (ORFs), ORF258, rpoN, ORF191, and ptsN, coding for proteins of 258, 520, 191, and 154 amino acids, respectively. The gene product of ORF258 is homologous to members of the ATP-binding cassette-type permeases. ORF191 and ptsN are homologous to conserved ORFs found downstream from rpoN genes in other bacterial species. Unlike in most other microorganisms, rpoN and ORF191 are separated by approximately 1.6 kb. The R. etli rpoN gene was shown to control in free-living conditions the production of melanin, the activation of nifH, and the metabolism of C4-dicarboxylic acids and several nitrogen sources (ammonium, nitrate, alanine, and serine). Expression of the rpoN gene was negatively autoregulated and occurred independently of the nitrogen source. Inactivation of the ptsN gene resulted in a decrease of melanin synthesis and nifH expression. In a search for additional genes controlling the synthesis of melanin, an R. etli mutant carrying a Tn5 insertion in ptsA, a gene homologous to the Escherichia coli gene coding for enzyme I of the phosphoenolpyruvate:sugar phosphotransferase system, was obtained. The R. etli ptsA mutant also displayed reduced expression of nifH. The ptsN and ptsA mutants also displayed increased sensitivity to the toxic effects of malate and succinate. Growth of both mutants was inhibited by these C4-dicarboxylates at 20 mM at pH 7.0, while wild-type cells grow normally under these conditions. The effect of malate occurred independently of the nitrogen source used. Growth inhibition was decreased by lowering the pH of the growth medium. These results suggest that ptsN and ptsA are part of the same regulatory cascade, the inactivation of which renders the cells sensitive to toxic effects of elevated concentrations of malate or succinate.

Amino Acid Sequence↗

PubMed1998

Human brain regions involved in direction discrimination.

To obtain further evidence for the functional specialization and task-dependent processing in the human visual system, we used positron emission tomography to compare regional cerebral blood flow in two direction discrimination tasks and four control tasks. The stimulus configuration, which was identical in all tasks, included the motion of a random dot pattern, dimming of a fixation point, and a tone burst. The discrimination tasks comprised the identification of motion direction and successive direction discrimination. The control tasks were motion detection, dimming detection, tone detection, and passive viewing. There was little difference in the activation patterns evoked by the three detection tasks except for decreased activity in the parietal cortex during the detection of a tone. Thus attention to a nonvisual stimulus modulated different visual cortical regions nonuniformly. Comparison of successive discrimination with motion detection yielded significant activation in the right fusiform gyrus, right lingual gyrus, right frontal operculum, left inferior frontal gyrus, and right thalamus. The fusiform and opercular activation sites persisted even after subtracting direction identification from successive discrimination, indicating their involvement in temporal comparison. Functional magnetic resonance imaging (fMRI) experiments confirmed the weak nature of the activation of human MT/V5 by successive direction discrimination but also indicated the involvement of an inferior satellite of human MT/V5. The fMRI experiments moreover confirmed the involvement of human V3A, lingual, and parietal regions in successive discrimination. Our results provide further evidence for the functional specialization of the human visual system because the cortical regions involved in direction discrimination partially differ from those involved in orientation discrimination. They also support the principle of task-dependent visual processing and indicate that the right fusiform gyrus participates in temporal comparison, irrespective of the stimulus attribute.

Acoustic Stimulation↗

PubMed1998

Sequence analysis of the Rhizobium etli ribose kinase gene rbsK and its phylogenetic position.

DNA sequence analysis of a 1878-bp DNA fragment located downstream from the Rhizobium etli ptsN gene revealed the presence of an open reading frame coding for a protein of 300 amino acids. This protein is homologous to members of the PfkB subfamily of carbohydrate and carbohydrate phosphate kinases. Since the highest homology is observed with the ribokinases of Escherichia coli, Haemophilus influenzae and Bacillus subtilis, the isolated gene was named the R. etli rbsK gene. The eubacterial ribokinases form a cluster distinct from the cluster of ribokinase proteins of the archaebacteria Methanobacterium thermoautotrophicum, Methanococcus jannaschii and Sulfolobus solfoataricus, which form a more divergent group of proteins. R. etli RbsK has a molecular mass of 30.6 kDa and a calculated isoelectric point of 4.5. No homologues of Escherichia coli ORF284 and ORF90 were found downstream from R. etli ptsN.

Amino Acid Sequence↗

PubMed1997

Positron-emission tomography imaging of long-term shape recognition challenges.

Long-term visual memory performance was impaired by two types of challenges: a diazepam challenge on acquisition and a sensory challenge on recognition. Using positron-emission tomography regional cerebral blood flow imaging, we studied the effect of these challenges on regional brain activation during the delayed recognition of abstract visual shapes as compared with a baseline fixation task. Both challenges induced a significant decrease in differential activation in the left fusiform gyrus, suggesting that this region is involved in the automatic or volitional comparison of incoming and stored stimuli. In contrast, thalamic differential activation increased in response to memory challenges. This increase might reflect enhanced retrieval attempts as a compensatory mechanism for restoring recognition performance.

Adult↗

PubMed1997

Attention to one or two features in left or right visual field: a positron emission tomography study.

In human vision, two features of the same object can be identified concurrently without loss of accuracy. Performance declines, however, when the features belong to different objects in opposite visual fields. We hypothesized that different positron emission tomography activation patterns would reflect these behavioral results. We first delineated an attention network for single discriminations in left or right visual field and then compared this with the activation pattern when subjects divided attention over two features of a single object or over two objects in opposite hemifields. When subjects attended to a single feature, parietal, premotor, and anterior cingulate cortex were activated. These effects were strongest in the right hemisphere and were, remarkably, unaffected by the direction of attention. In contrast, direction of attention affected occipital and frontal activity: right occipital and left lateral frontal activity were higher with attention to the left, whereas right lateral frontal activity was higher with attention to the right. When subjects identified two features of the same object, parietal, premotor, and anterior cingulate activity was enhanced further, predominantly this time in the left hemisphere. Again, there was no direction sensitivity. Direction-sensitive activation of lateral frontal cortex also was increased. Finally, when subjects divided their attention over opposite hemifields, activity in the direction-sensitive occipital and frontal regions fell to a level midway between those seen during exclusively leftward or rightward attention. Thus, the behavioral efficiency with which we attend to multiple features of a single peripheral object is paralleled by enhanced activity in structures generally active during peripheral selective attention as well as in structures that depend on the specific direction of attention, most notably lateral frontal cortex. In addition, in the direction-sensitive regions, dividing attention over hemifields causes a compromise pattern between the extreme levels obtained during unilateral attention.

Adolescent↗

PubMed1997

The arginine deiminase pathway in Rhizobium etli: DNA sequence analysis and functional study of the arcABC genes.

Sequence analysis upstream of the Rhizobium etli fixLJ homologous genes revealed the presence of three open reading frames homologous to the arcABC genes of Pseudomonas aeruginosa. The P. aeruginosa arcABC genes code for the enzymes of the arginine deiminase pathway: arginine deiminase, catabolic ornithine carbamoyltransferase (cOTCase), and carbamate kinase. OTCase activities were measured in free-living R. etli cells and in bacteroids isolated from bean nodules. OTCase activity in free-living cells was observed at a different pH optimum than OTCase activity in bacteroids, suggesting the presence of two enzymes with different characteristics and different expression patterns of the corresponding genes. The characteristics of the OTCase isolated from the bacteroids were studied in further detail and were shown to be similar to the properties of the cOTCase of P. aeruginosa. The enzyme has a pH optimum of 6.8 and a molecular mass of approximately 450 kDa, is characterized by a sigmoidal carbamoyl phosphate saturation curve, and exhibits a cooperativity for carbamoyl phosphate. R. etli arcA mutants, with polar effects on arcB and arcC, were constructed by insertion mutagenesis. Bean nodules induced by arcA mutants were still able to fix nitrogen but showed a significantly lower acetylene reduction activity than nodules induced by the wild type. No significant differences in nodule dry weight, plant dry weight, and number of nodules were found between the wild type and the mutants. Determination of the OTCase activity in extracts from bacteroids revealed a strong decrease in activity of this enzyme in the arcA mutant compared to the wild-type strain. Finally, we observed that expression of an R. etli arcA-gusA fusion was strongly induced under anaerobic conditions.

Acetylene↗

PubMed1997

The routine determination of the endogenous thrombin potential, first results in different forms of hyper- and hypocoagulability.

The area under the thrombin generation curve (the endogenous thrombin potential; ETP) has been proposed as a parameter for plasma-based hypercoagulability and to monitor anticoagulant treatment. We present an ETP assay for the routine laboratory using a centrifugal analyser. Throughput is 30 samples/h, within and between run imprecision is 4-5.6%. Suitable substrates were developed for the ranges of 10-500% and 2-100% of normal. Independent of tissue factor concentration (if > 4 pM), the normal value of the extrinsic ETP is 384.8 +/- 51.7 nM.min. The intrinsic ETP, triggered by ellagic acid, is 414 +/- 41 nM.min. The ETP is decreased to 15 and 35% of normal by oral anticoagulation (INR 2.5-4.0) and by heparin administration (APTT 1.5-2.5 x control). The ETP is increased in untreated subjects with congenital antithrombin deficiency and in women using oral contraceptives. In deep vein thrombosis (phlebographically confirmed), it is increased by 29.4% (extrinsic) and 53% (intrinsic). In (angiographically assessed) coronary artery disease the increase is by 10% and 17% respectively.

Adolescent↗

PubMed1996

The influence of stimulus location on the brain activation pattern in detection and orientation discrimination. A PET study of visual attention.

This PET study concerns changes in regional cerebral blood flow (rCBF) associated with orientation discrimination tasks and with simple detection of the stimulus. The difference in rCBF between discrimination and detection represents the discrimination, or 'task', component of the response. We have examined how such differences are influenced by the position of the visual stimulus and also how they change when a peripheral distractor is added to a relevant central stimulus. We first determined the regions in which the task produced the main effect regardless of stimulus position or distraction with an additional stimulus. In these selected regions, we determined the interactions between task and stimulus position and the interactions between task and stimulus addition. Five regions were more active during orientation discrimination than during stimulus detection: the inferior occipital cortex, the right putamen, the superior parietal lobule, the anterior cingulate cortex and the left lower and the right upper premotor area. Stimulus addition interacted with task only in the lower premotor area. Interactions between the task and stimulus position occurred in the occipital and parietal cortex and in the putamen. When a central stimulus was presented instead of a peripheral one the difference between orientation discrimination and stimulus detection was significantly larger in the inferior occipital lobe and in the right putamen. Conversely, the difference between orientation discrimination and stimulus detection in the superior parietal lobule was significantly larger when a peripheral stimulus was presented instead of a central one.

Adult↗

PubMed1995

Structural and functional analysis of the fixLJ genes of Rhizobium leguminosarum biovar phaseoli CNPAF512.

The fixLJ genes of Rhizobium leguminosarum biovar phaseoli CNPAF512 were identified by DNA hybridization of a genomic library with an internal fragment of the Rhizobium meliloti fixJ gene. The nucleotide sequence was determined and the corresponding amino acid sequence was aligned with the amino acid sequences of the FixL proteins of R. meliloti, Bradyrhizobium japonicum and Azorhizobium caulinodans. While the FixJ protein and the carboxy-terminal part of the FixL protein are highly homologous to the other FixL and FixJ proteins, the homology in the central heme-binding, oxygen-sensing domain and in the amino-terminal domain of FixL is very low. The R. leguminosarum bv. phaseoli FixL protein does not contain the heme-binding motif defined for the previously described FixL proteins. R. leguminosarum bv. phaseoli fixLJ and fixJ mutants were constructed. These mutants can still fix nitrogen, albeit at a reduced level. Expression analysis of nifA-gusA and nifH-gusA fusions in the constructed mutants revealed that the R. leguminosarum bv. phaseoli fixLJ genes are involved in microaerobic nifH expression but not in nifA expression.

Amino Acid Sequence↗

PubMed1995

Cyclospora sp.: a coccidian that causes diarrhoea in travellers.

Cyclospora sp. is a coccidian (protozoan) that has been incriminated in cases of diarrhoea in particular among travellers. We report two cases of intestinal Cyclospora infection in immunocompetent Belgians, who travelled to Indonesia. Although the diarrhoea persisted for several days recovery seemed to occur spontaneously.

Adult↗

PubMed1995

The use of magnetic resonance angiography in stereotactic neurosurgery.

The authors discuss the advantages and disadvantages of the use of magnetic resonance (MR) angiography images in stereotactic neurosurgery. Current computer programs designed to assist the neurosurgeon in the planning of stereotactic neurosurgical interventions use intraarterial digital subtraction angiography images to visualize the blood vessels. Magnetic resonance angiography is a recent technique with a number of advantages over the digital subtraction method: it is less invasive and less prone to complications; it provides truly three-dimensional data sets that can be viewed from any direction; and it can visualize both stationary and flowing tissues with the same imaging device and localizer frame. Although digital subtraction images are still superior in contrast and vascular detail, state-of-the-art high-resolution MR angiography sequences provide sufficient vascular detail for planning surgery. Contrast-enhanced MR angiography images were acquired using adapted gradient-echo sequences to compensate for flow-induced distortions; postacquisition distortion correction was not necessary. Five methods to integrate and inspect a possible trajectory in the MR angiography data are discussed. Initial clinical experience with eight patients led to the conclusion that MR angiography is a valuable imaging modality that can be integrated reliably into a stereotactic neurosurgery planning procedure.

Adult↗

PubMed1995

[Jehan Yperman: a Belgian of the 3rd section is brought back to life after centuries of neglect].

Jehan Yperman (13th-14th century) is a great figure of medicine in Ypres. With a grant supported by the rich medieval city, he continued to perfect his knowledge and completed his studies under Lanfranc (Guido Lanfranchi) in Paris. The Master transmits to him the ideas of the well-known Universities of Salerne and Bologne. Yperman started his surgical-ophthalmological works in Ypres and wrote some noteworthy treatises on general and surgical medicine. Ophthalmology played an important part in these treatises. And so, how was he forgotten during five centuries? How was he revived? The many elogious publications during the last years, in several countries, are a guarantee of the value and survival.

Belgium↗

PubMed1994

Characterization of the Rhizobium leguminosarum biovar phaseoli nifA gene, a positive regulator of nif gene expression.

We report the isolation, mutational analysis and the nucleotide sequence of the Rhizobium leguminosarum bv. phaseoli nifA gene. Comparison of the deduced amino acid sequence with other NifA sequences indicated the presence of the conserved central activator and the C-terminal DNA-binding domains. Nodules elicited by a R. leguminosarum bv. phaseoli nifA mutant were symbiotically ineffective. The expression of a nifA-gusA fusion was shown to be independent on the oxygen status of the cell. We cloned the three nifH copies of R. leguminosarum bv. phaseoli and determined the nucleotide sequence of their promoter regions. The expression of nifH-gusA fusions is induced under microaerobic conditions and is dependent on the presence of NifA.

Amino Acid Sequence↗

PubMed1994

On the problem of geometric distortion in magnetic resonance images for stereotactic neurosurgery.

In this paper, we discuss the issue of geometric distortion in magnetic resonance (MR) images used to plan stereotactic neurosurgical interventions. We analyze the process for the case of Fourier transform imaging and demonstrate that spatial misregistrations are fundamentally due to two causes: deviations of the magnetic field from its ideal value and blood flow. This enables us to relate the causes of geometric distortion to the MR imaging system, the patient and the stereotactic localizer frame. Based on the general model, we propose model refinements and discuss methods for the quantification and correction of all causes. The results of our calculations and experiments indicate that, using the proposed corrections, MRI and MR angiography should be considered valuable and reliable acquisition modalities for the planning of stereotactic neurosurgical interventions.

Brain↗

PubMed1994

Effects of temperature stress on bean-nodulating Rhizobium strains.

High soil temperatures in tropical areas limit nodulation and dinitrogen fixation by strains of Rhizobium. Several heat-tolerant bean-nodulating Rhizobium strains have been isolated previously. However, the basis of their resistance to heat remains unknown. In this study, we compared the effects of heat on symbiotic nitrogen fixation, cell survival, amino acid uptake, and protein synthesis in a heat-tolerant (CIAT899) and a heat-sensitive (CNPAF512) bean-nodulating Rhizobium strain. Acetylene reduction activity of nodulated roots excised from unstressed plants was strongly diminished at 35 or 40 degrees C when plants were nodulated either by CIAT899 or by CNPAF512. When these strains were tested under free-living conditions, survival at 40 degrees C as well as the kinetics of l-[S]methionine uptake and protein synthesis at 35 and 40 degrees C indicated the higher tolerance of CIAT899 than of CNPAF512 to thermal stress. The synthesis of heat shock proteins was detected in both strains, although at different temperatures. Increased synthesis of 14 heat shock proteins in CNPAF512 and of 6 heat shock proteins in CIAT899 was observed at 40 and 45 degrees C, respectively. A heat shock protein of approximately 21 kDa, of which the synthesis was strongest in both Rhizobium strains upon a temperature shift up, was also conserved in several other bean-nodulating rhizobia. Acquired thermotolerance in CIAT899 was shown to depend on protein synthesis.

Journal Article↗

PubMed1994

How does the stereotactic workstation help the neurosurgeon?

At the KUL University of Leuven a workstation for the planning of neurosurgical stereotactic procedures has been developed. Its benefits are illustrated in three exemplary cases. The CT and/or MR images, acquired under stereotactic conditions, are transmitted via a PACS network (picture archiving and communication systems) directly to the stereotactic workstation in the operating theater. Target and entry point can be accurately defined on zoomed images. The trajectory can be checked and modified on all registered data sets and on resliced images along any plane. Maximum intensity projection of magnetic resonance angiography data sets along any arbitrary direction show the relative position of the blood vessels and the trajectory. During the preceding 32 months 29 patients were operated on using the stereotactic workstation. Postoperatively no new neurological deficit was observed in any of these patients. The workstation improves patient safety and increases the accuracy of neurosurgical stereotactic operations, because it helps the neurosurgeon to avoid blood vessels and/or important functional areas.

Adult↗