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Biomedical subjects

M Kühl

Publications and source records attributed to M Kühl.

At least 37 records · Page 2Linked to original sources
PubMed2000

Expression of Xenopus homologs of the beta-catenin binding protein pontin52.

Identification of pontin52 as an interaction partner of the Wnt/Wg signal transducer beta-catenin implicated a role for this protein in Wnt signaling. Here we describe the isolation of two Xenopus homologs of pontin52, Xpontin and Xreptin, and report the first expression pattern of vertebrate pontin52 homologs. Whole-mount in situ hybridization studies reveal a strong expression of Xpontin in neural crest cells and in later stages in different gastrointestinal organs. Xreptin is also expressed in neural crest cells, in particular in a subpopulation that give raise to the adrenal medulla.

ATPases Associated with Diverse Cellular Activitie↗
PubMed2000

Does a 100-km walking affect indicators of vitamin status?

The status of thiamin (B1), riboflavin (B2), ascorbic acid (AA), and tocopherol was determined in 60 leisure athletes (age 46 +/- 10 y, BMI 23.7 +/- 2.0 kg.m-2, VO2max 39.4 +/- 6.5 ml.min-1.kg-1), who completed a 100-km walking race. Vitamin plasma levels and activities of erythrocyte transketolase (ETK) and glutathione reductase (EGR) were measured before start, immediately after finishing and 6 hours later. The participators finished the entire distance in 14.25 h (average speed 7 km.h-1). Before start, all participators showed an excellent vitamin status (prevalences of low vitamin status ranged between 1.7 and 1.8%). Plasma tocopherol concentrations correlated significantly with increased age (r = 0.35, p = 0.008). Compared to the values before start, plasma concentrations of B1 and B2 as well as ETK and/or EGR were increased significantly after finishing. The raised levels persisted 6 hours after finishing, whereas AA remained unaltered. The univariate analysis of variance revealed that the change in vitamin status after finish and 6 hours later was in part highly dependent on age, BMI and the level of physical fitness. Despite the long distance, the extensive character of the 100-km walking with its low intensity did not deteriorate the measured indicators of vitamin status.

Adult↗
PubMed2000

The responses of photosynthesis and oxygen consumption to short-term changes in temperature and irradiance in a cyanobacterial mat (Ebro Delta, Spain).

We have evaluated the effects of short-term changes in incident irradiance and temperature on oxygenic photosynthesis and oxygen consumption in a hypersaline cyanobacterial mat from the Ebro Delta, Spain, in which Microcoleus chthonoplastes was the dominant phototrophic organism. The mat was incubated in the laboratory at 15, 20, 25 and 30 degrees C at incident irradiances ranging from 0 to 1,000 micromol photons m(-2) s(-1). Oxygen microsensors were used to measure steady-state oxygen profiles and the rates of gross photosynthesis, which allowed the calculation of areal gross photosynthesis, areal net oxygen production, and oxygen consumption in the aphotic layer of the mat. The lowest surface irradiance that resulted in detectable rates of gross photosynthesis increased with increasing temperature from 50 micromol photons m(-2) s(-1) at 15 degrees C to 500 micromol photons m(-2) s(-1) at 30 degrees C. These threshold irradiances were also apparent from the areal rates of net oxygen production and point to the shift of M. chthonoplastes from anoxygenic to oxygenic photosynthesis and stimulation of sulphide production and oxidation rates at elevated temperatures. The rate of net oxygen production per unit area of mat at maximum irradiance, J0, did not change with temperature, whereas, JZphot, the flux of oxygen across the lower boundary of the euphotic zone increased linearly with temperature. The rate of oxygen consumption per volume of aphotic mat increased with temperature. This increase occurred in darkness, but was strongly enhanced at high irradiances, probably as a consequence of increased rates of photosynthate exudation, stimulating respiratory processes in the mat. The compensation irradiance (Ec) marking the change of the mat from a heterotrophic to an autotrophic community, increased exponentially in this range of temperatures.

Cyanobacteria↗
PubMed2000

A new system for three-dimensional tracking of motile microorganisms.

A new three-dimensional (3D)-tracking system with optimized dark-field illumination is presented. It allows simultaneous 3D tracking of several free-swimming microorganisms with diameters of >10 microm. Resolution limits and illumination efficiencies for different size classes of microorganisms are treated analytically. First applications for 3D tracking of protists are demonstrated.

Animals↗
PubMed2000

Induction of atrial fibrillation with rapid high voltage ventricular pacing for ventricular fibrillation conversion testing. The Ventak AV II DR Study.

OBJECTIVE: To assess whether rapid high voltage ventricular pacing can also induce atrial fibrillation, and whether the induction of atrial fibrillation during ventricular fibrillation conversion testing is related to the patient's heart disease. DESIGN: Prospective study of 50 patients who received the dual chamber implantable cardioverter-defibrillator (ICD) Ventak AV II DR (Guidant) as a first implant. This device can record atrial activity even during a ventricular fibrillation episode and can induce atrial fibrillation by rapid atrial bursts. MAIN OUTCOME MEASURES: Frequency of atrial fibrillation after induction of ventricular fibrillation; clinical characteristics of patients with and without induced atrial fibrillation; frequency of atrial fibrillation induced by rapid atrial bursts during predischarge testing. RESULTS: Atrial fibrillation was observed in 40 of the 217 ventricular fibrillation episodes (18%) that could be detected immediately after delivery of high voltage pacing. The biphasic ICD shock for termination of ventricular fibrillation also terminated the atrial fibrillation in all cases. The 40 episodes of simultaneous atrial and ventricular fibrillation occurred in 18 patients (36%). The distribution of the clinical characteristics of the patients and the inducibility of atrial fibrillation during predischarge testing were similar in those with and without induced atrial fibrillation. CONCLUSIONS: Rapid high voltage ventricular pacing frequently induces atrial fibrillation, which was terminated by the subsequent biphasic ICD shock. The induction of atrial fibrillation seems to be a non-specific phenomenon, unrelated to the clinical status of the patient.

Adult↗
PubMed1999

A Microsensor Study of the Interaction between Purple Sulfur and Green Sulfur Bacteria in Experimental Benthic Gradients.

> Abstract The interaction between the purple sulfur bacterium Thiocapsa roseopersicina and the green sulfur bacterium Prosthecochloris aestuarii was studied in a gradient chamber under a 16-hours light-8-hours dark regime. The effects of interaction were inferred by comparing the final outcome of a mixed culture experiment with those of the respective axenic cultures using the same inoculation densities and experimental conditions. Densities of bacteria were deduced from radiance microprofiles, and the chemical microenvironment was investigated with O2, H2S, and pH microelectrodes. P. aestuarii always formed a biofilm below the maximal oxygen penetration depth and its metabolism was strictly phototrophic. In contrast, T. roseopersicina formed a bilayer in both the mixed and the axenic culture. The top layer formed by the latter organism was exposed to oxygen, and chemotrophic sulfide oxidation took place during the dark periods, while the bottom layer grew phototrophically during the light periods only. In the mixed culture, the relative density of P. aestuarii was lower than in the axenic culture, which reflects the effects of the competition for sulfide. However, the relative density of T. roseopersicina was actually higher in the mixed culture than in the corresponding axenic culture, indicating a higher growth yield on sulfide in the mixed culture experiment. Several hypotheses are proposed to explain the effects of the interaction.

Journal Article↗
PubMed1999

Dynamics of anoxygenic photosynthesis in an experimental green sulphur bacteria biofilm.

The dynamics of sulphide oxidation in an experimental biofilm of the green sulphur bacterium, Prosthecochloris aestuarii, were studied using a newly developed light-dark cycling procedure. The biofilm was grown for 6 weeks in a benthic gradient chamber, in which gradients of light, sulphide and oxygen were imposed experimentally. The H2S concentrations and pH were measured with microsensors as a function of depth in the biofilm and of time after a change in illumination status. The sulphide oxidation rates were calculated as a function of time and depth in the biofilm using a numerical procedure to solve the non-stationary general diffusion equation. A close agreement was found between the areal rates of anoxygenic photosynthesis during the cycling procedure and the steady state before the cycling experiment. For the different layers of the biofilm, the maximum activity was observed after 10-12min of light exposure. After this maximum, sulphide oxidation decreased concomitantly with sulphide concentration, indicating sulphide limitation of anoxygenic photosynthesis. This lag time limits the application of the standard dark-light shift method with a brief light exposure of a few seconds and, therefore, the numerical procedure described in this study enables the depth distribution of anoxygenic photosynthesis rates in microbial mats to be determined more accurately.

Anaerobiosis↗
PubMed1999

Aerotaxis in Desulfovibrio.

Aerotaxis of two sulphate-reducing bacteria, the freshwater strain Desulfovibrio desulfuricans CSN (DSM 9104) and the marine strain Desulfovibrio oxyclinae N13 (DSM 11498), was studied using capillary microslides, microscopy and oxygen microsensors. The bacteria formed ring-shaped bands in oxygen diffusion gradients surrounding O2 bubbles, which were placed into anoxic sulphate-free cell suspensions in capillary microslides. The radial expansion of the oxic volume by diffusion was stopped by aerobic respiration. Bands were formed by cells avoiding high O2 levels near the O2 bubble, as well as by cells entering from the surrounding anoxic zone. At the inner edge of the bands, O2 levels of up to 20% air saturation (50 microM O2) were found, while the outer edge always coincided with the oxic-anoxic interface. Ring diameters and O2 concentrations at the inner edge of the band depended on the cell density and the strain used in the suspension. Band formation did not occur in the absence of an electron donor (5mM lactate) or when N2 gas bubbles were used. Both strains were highly motile with velocities of approximately equals 32 microm s(-1) during forward runs, and 7 microm s(-1) during backward runs respectively. Within the bands, cells moved in circles of about 20 microm diameter, while cells outside the band exhibited straighter or only slightly bent traces. It is concluded that the capacity of respiration at high rates and the positive and negative aerotactical responses of Desulfovibrio provide an efficient strategy for removing O2 from the habitat in situations where sufficient electron donors and high cell densities are present.

Chemotaxis↗
PubMed1999

A randomized controlled clinical trial comparing ventricular fibrillation detection time between two transvenous defibrillator models. The Acute Ventak AV investigator group.

The Ventak AV is an implantable cardioverter defibrillator with dual chamber pacing capability. Features include detection and treatment of ventricular arrhythmias, detection of atrial arrhythmias, as well as dual chamber pacing. The objective of the investigation was to verify the efficacy of the Ventak AV in detecting ventricular fibrillation in the presence of dual chamber pacing. Thirty-three patients, who were to receive an implantable defibrillator were randomized (1:1) in a paired comparison study to the Ventak AV (study device) and the Ventak Mini (control) during defibrillation threshold testing. In order to create a "worst case scenario" for sensing of ventricular fibrillation, pacing was performed at high lower rate limit values (Ventak AV DDD pacing at 150/min, Ventak Mini at VVI 100/min). Ventricularfibrillation was induced and the randomized device was allowed to detect and treat the arrhythmia. This test was repeated for each patient using the alternate device in a randomized order, such that all patients were tested with both devices. The mean ventricular fibrillation detection time for the Ventak AV was 2.0+/-0.11 seconds and for the control device the detection time was 1.8+/-0.11 seconds (P = 0.26). Appropriate tachyarrhythmia therapy decision was documented in all episodes for both devices. The study patient population demonstrated equivalent ventricular fibrillation detection time between the Ventak AV and the Ventak Mini. The Ventak AV demonstrated effectiveness in detecting ventricular fibrillation in the presence of high rate dual chamber pacing.

Adult↗
PubMed1999

Quantifying microbial diversity: morphotypes, 16S rRNA genes, and carotenoids of oxygenic phototrophs in microbial mats.

We quantified the diversity of oxygenic phototrophic microorganisms present in eight hypersaline microbial mats on the basis of three cultivation-independent approaches. Morphological diversity was studied by microscopy. The diversity of carotenoids was examined by extraction from mat samples and high-pressure liquid chromatography analysis. The diversity of 16S rRNA genes from oxygenic phototrophic microorganisms was investigated by extraction of total DNA from mat samples, amplification of 16S rRNA gene segments from cyanobacteria and plastids of eukaryotic algae by phylum-specific PCR, and sequence-dependent separation of amplification products by denaturing-gradient gel electrophoresis. A numerical approach was introduced to correct for crowding the results of chromatographic and electrophoretic analyses. Diversity estimates typically varied up to twofold among mats. The congruence of richness estimates and Shannon-Weaver indices based on numbers and proportional abundances of unique morphotypes, 16S rRNA genes, and carotenoids unveiled the underlying diversity of oxygenic phototrophic microorganisms in the eight mat communities studied.

Journal Article↗
PubMed1999

The Wnt/Wg signal transducer beta-catenin controls fibronectin expression.

beta-Catenin stabilizes the cadherin cell adhesion complex but, as a component of the Wnt/Wg signaling pathway, also controls gene expression by forming a heterodimer with a transcription factor of the LEF-TCF family. We demonstrate that the substrate adhesion molecule fibronectin is a direct target of Wnt/Wg signaling. Nuclear depletion of beta-catenin following cadherin transfection in Xenopus fibroblasts resulted in downregulation of fibronectin expression which was restored by activating the Wnt/Wg signaling cascade via LiCl treatment or transfection of either Xwnt-8 or beta-catenin. We isolated the Xenopus fibronectin gene (FN) promoter and found four putative LEF-TCF binding sites. By comparing the activities of different fibronectin gene reporter constructs in fibroblasts and cadherin transfectants, the LEF-TCF site at position -368 was identified as a Wnt/Wg response element. LEF-1-related proteins were found in nuclei of the fibroblasts but were absent in a kidney epithelial cell line. Consistent with the lack of these transcription factors, the FN promoter was silent in the epithelial cells but was activated upon transfection of LEF-1. Wild-type Xenopus Tcf-3 (XTcf-3) was unable to activate FN promoter reporter constructs, while a mutant lacking the groucho binding region behaved like LEF-1. In contrast to XTcf-3, LEF-1 does not interact with groucho proteins, which turn TCFs into activators or repressors (J. Roose, M. Molenaar, J. Hurenkamp, J. Peterson, H. Brantjes, P. Moerer, M. van de Wetering, O. Destreé, and H. Clevers, Nature 395:608-612, 1998). Together these data provide evidence that expressing LEF-1 enables fibroblasts, in contrast to epithelial cells, to respond to the Wnt/Wg signal via beta-catenin in stimulating fibronectin gene transcription. Our findings further promote the idea that due to its dual function, beta-catenin regulates the balance between cell-cell and cell-substrate adhesion.

Animals↗
PubMed1998

Functional interaction of an axin homolog, conductin, with beta-catenin, APC, and GSK3beta.

Control of stability of beta-catenin is central in the wnt signaling pathway. Here, the protein conductin was found to form a complex with both beta-catenin and the tumor suppressor gene product adenomatous polyposis coli (APC). Conductin induced beta-catenin degradation, whereas mutants of conductin that were deficient in complex formation stabilized beta-catenin. Fragments of APC that contained a conductin-binding domain also blocked beta-catenin degradation. Thus, conductin is a component of the multiprotein complex that directs beta-catenin to degradation and is located downstream of APC. In Xenopus embryos, conductin interfered with wnt-induced axis formation.

Adenomatous Polyposis Coli Protein↗
PubMed1998

Expression of the Armadillo family member p120cas1B in Xenopus embryos affects head differentiation but not axis formation.

The Armadillo family is formed by proteins which possess an Arm domain comprising multiple copies of a 42-amino-acid motif, the Arm repeat, initially described for the Drosophila segment polarity gene product Armadillo. The Arm domain serves in protein-protein interactions which are required for the family members Armadillo, beta-catenin and plakoglobin to mediate cell-cell adhesion and Wnt/Wingless signalling. Similarily, p120cas, the Arm domain containing src substrate, also binds to cadherins and becomes tyrosine phosphorylated in response to a variety of stimuli. However, a putative function of p120cas in adhesion or signalling has not yet been demonstrated. It has also not been shown until now that an Arm domain is a common signal transduction motif. Using Xenopus embryos we show by expression of murine p120cas1B (mp120cas1B) in ventral blastomeres that this catenin cannot replace beta-catenin function in dorsal axis formation. Thus, the presence of an Arm domain per se is not sufficient to activate the Wnt/Wg pathway. Indeed, injection of mp120cas1B into dorsal blastomeres led instead to delayed blastopore closure and posteriorized phenotypes with malformed head structures indicative of disturbed gastrulation movements. Because neither convergent extension behaviour nor adhesion to fibronectin was altered in the injected embryos we assume that mp120cas1B influences motility or orientation of migrating mesodermal cells.

Animals↗
PubMed1998

In situ methods for assessment of microorganisms and their activities.

Recent technical developments in the field of molecular biology and microsensors are beginning to enable microbiologists to study the abundance, localization and activity of microorganisms in situ. The various new methods on their own bear high potential but it is the combination of studies on structure and function of microbial communities that will yield the most detailed insights in the way microorganisms operate in nature.

Bacteria↗
PubMed1997

Two novel human members of an emerging mammalian gene family related to mono-ADP-ribosylating bacterial toxins.

Mono-ADP-ribosylation is one of the posttranslational protein modifications regulating cellular metabolism, e.g., nitrogen fixation, in prokaryotes. Several bacterial toxins mono-ADP-ribosylate and inactivate specific proteins in their animal hosts. Recently, two mammalian GPI-anchored cell surface enzymes with similar activities were cloned (designated ART1 and ART2). We have now identified six related expressed sequence tags (ESTs) in the public database and cloned the two novel human genes from which these are derived (designated ART3 and ART4). The deduced amino acid sequences of the predicted gene products show 28% sequence identity to one another and 32-41% identity vs the muscle and T cell enzymes. They contain signal peptide sequences characteristic of GPI anchorage. Southern Zoo blot analyses suggest the presence of related genes in other mammalian species. By PCR screening of somatic cell hybrids and by in situ hybridization, we have mapped the two genes to human chromosomes 4p14-p15.1 and 12q13.2-q13.3. Northern blot analyses show that these genes are specifically expressed in testis and spleen, respectively. Comparison of genomic and cDNA sequences reveals a conserved exon/intron structure, with an unusually large exon encoding the predicted mature membrane proteins. Secondary structure prediction analyses indicate conserved motifs and amino acid residues consistent with a common ancestry of this emerging mammalian enzyme family and bacterial mono(ADP-ribosyl)transferases. It is possible that the four human gene family members identified so far represent the "tip of an iceberg," i.e., a larger family of enzymes that influences the function of target proteins via mono-ADP-ribosylation.

ADP Ribose Transferases↗
PubMed1997

Wnt signalling goes nuclear.

The Wnt signalling cascade is a highly conserved signalling pathway throughout the animal kingdom. In Xenopus, Wnt signalling functions in mesodermal dorsoventral patterning. Earlier work on deciphering the components of the wnt signalling cascade left a gap between cytosolic beta-catenin, the final member of the cascade, and the nuclear target genes. Several recent papers now reveal how the Wnt signal is transmitted into the nucleus. Surprisingly, beta-catenin directly interacts with the transcription factor LEF-1/XTCF-3, and thereby is not only translocated into the nucleus but also modulates the properties of LEF-1/XTCF-3 as a transcription factor.

Animals↗
PubMed1997

Fritz: a secreted frizzled-related protein that inhibits Wnt activity.

Signaling molecules of the Wnt gene family are involved in the regulation of dorso-ventral, segmental and tissue polarity in Xenopus and Drosophila embryos. Members of the frizzled gene family, such as Drosophila frizzled-2 and rat frizzled-1, have been shown encode Wnt binding activity and to engage intracellular signal transduction molecules known to be part of the Wnt signaling pathway. Here we describe the cloning and characterization of Fritz, a mouse (mfiz) and human (hfiz) gene which codes for a secreted protein that is structurally related to the extracellular portion of the frizzled genes from Drosophila and vertebrates. The Fritz protein antagonizes Wnt function when both proteins are ectopically expressed in Xenopus embryos. In early gastrulation, mouse fiz mRNA is expressed in all three germ layers. Later in embryogenesis fiz mRNA is found in the central and peripheral nervous systems, nephrogenic mesenchyme and several other tissues, all of which are sites where Wnt proteins have been implicated in tissue patterning. We propose a model in which Fritz can interfere with the activity of Wnt proteins via their cognate frizzled receptors and thereby modulate the biological responses to Wnt activity in a multitude of tissue sites.

Amino Acid Sequence↗