Ein Modell zur örtlich-zeitlichen Signalübertragung im visuellen System des Menschen auf der Basis der linearen Systemtheorie kontinuierlicher Medien

Zusammenfassung Als Modell für die Übertragung örtlich-zeitlich variabler Helligkeitsmuster durch das visuelle System des Menschen wird ein dreidimensionales, homogenes neuronales Geflecht betrachtet. Die Ausbreitung der neuronalen Erregung erfolgt anisotrop in afferenter und lateraler Richtung und wird durch eine modifizierte Wärmeleitungsgleichung beschrieben, deren Hauptmerkmal ein regenrativer Signalanteil ist. Die zeitabhängige Lösung beschreibt u.a. die zeitliche Übertragungscharakteristik für flimmernde Gitter im gesamten Frequenzbereich. Die stationäre Lösung beschreibt die örtliche Übertragungscharakteristik für Gitter, wobei auch die Struktur rezeptiver Felder mit inhibitorischem Umfeld deutlich erkennbar ist. Die Struktur der Lösung läßt verstehen, wie sich mit zunehmender Tiefe im neuronalen Geflecht der Übergang von der Ortsselektivität zur Ortsfrequenzselektivität vollzieht.

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A model for the spatial-temporal signal transmission properties of the human visual system based on the linear system theory of continuous media

In this paper, a three dimensional homogeneous neural web serves as a model for the transmission of spatial-temporal luminous pattern by the human visual system. The propagation of neuronal excitation in afferent and lateral direction is assumed to be anisotropic. It is described by a modified heat conduction equation a main feature of which is a regenerative signal component. The time-dependent solution describes f.e. the temporal transmission characteristics for flickering grids in the whole frequency range. The stationary solution contains the spatial transmission characteristics for grids as well as the structure of receptive fields with inhibitory surrounds. The structure of the solution makes it under-standable how the transition from spatial selectivity to spatial-frequency-selectivity develops with growing depth of the neuronal web.

     

Derivative chromosomal structures from a ring chromosome 4

Summary This report describes the results from cultured lymphocytes studied at metaphase, anaphase, and interphase from an individual with a ring chromosome 4. A ring was present in 90.1% of metaphases. Special attention was directed towards the occurrence of derivative chromosomal structures, such as partially duplicated and triplicated rings, tricentric rings, chains of 3 interlocked rings, rod-shaped chromosomes, pulverized rings, and others.The clinical features of the individual (small stature and impaired mental development, hypoplastic thumbs, ptosis palpebrae hypoplastic external male genitalia, abnormal dermatoglyphic pattern) did not conform to a specific phenotype.

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Zusammenfassung Die Lymphocyten eines Patienten mit Ringchromosom 4 wurden in der Meta-, Ana-und Interphase beobachtet. 90,1% der Metaphasen wiesen einen Ring auf. Besondere Aufmerksamkeit wurde abnormen, aus dem Ring entstandenen Chromosomen zugewandt. Es fanden sich u. a.: partielle Doppel-und Dreifachringe, Ringe mit 3 Zentromeren, Ringe aus 3 miteinander verketteten Ringen, stabförmige Chromosomen und pulverisierte Ringe.Die Symptome des Patienten (Minderwuchs, Schwachsinn, Hypoplasie der Daumen, Ptosis palpebrae, Hypoplasie des äußeren genitale, abnorme Dermatoglyphen) waren zum größten Teil unspezifish.

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This work was supported in part by grants from the Deutsche Forschungsgemeinschaft and is part of a thesis by R.N. for a medical doctorate.     

Developmental changes of nerve growth factor and its mRNA in the rat hippocampus: Comparison with choline acetyltransferase

Previous experiments have demonstrated that in the septo-hippocampal system choline acetyltransferase (ChAT) is induced by nerve growth factor (NGF) (Gnahn et al. (1983) Dev. Brain Res. 9, 45-52) and that hippocampal NGF and mRNANGF levels are correlated with the density of cholinergic innervation (Korsching et al. (1985) EMBO J. 4, 1389-1393). In the present investigation we have compared the developmental changes of ChAT, NGF, and mRNANGF levels in this system. During the postnatal development of the hippocampus the time courses of NGF and ChAT were well correlated including the most rapid increase between P12 and P14. This increase in hippocampal NGF was preceded by a corresponding increase in mRNANGF. The developmental changes in hippocampal NGF levels were also closely reflected by corresponding changes in the septum. This, together with previous observations (Korsching et al., 1985) that the adult septum, in spite of relatively high NGF levels, does not contain measurable quantities of mRNANGF, suggests that the NGF levels in the septum are determined by the quantity of NGF transported retrogradely from the field of innervation rather than by local synthesis. During the prenatal period hippocampal NGF levels were relatively high, whereas the mRNANGF was below the level of detection. Since the ingrowth of septal fibers, and with that also the removal of NGF by retrograde transport, begins around birth, the relatively high prenatal NGF levels probably result from an accumulation produced by a small copy number of mRNANGF prior to the removal of NGF by retrograde axonal transport. It is concluded that the correlation of the developmental changes in NGF and mRNANGF with the ChAT activity in the hippocampus further supports the concept of a physiological role of NGF in the central nervous system.     

Extracellular invertase is an essential component of cytokinin-mediated delay of senescence

Leaf senescence is the final stage of leaf development in which the nutrients invested in the leaf are remobilized to other parts of the plant. Whereas senescence is accompanied by a decline in leaf cytokinin content, exogenous application of cytokinins or an increase of the endogenous concentration delays senescence and causes nutrient mobilization. The finding that extracellular invertase and hexose transporters, as the functionally linked enzymes of an apolasmic phloem unloading pathway, are coinduced by cytokinins suggested that delay of senescence is mediated via an effect on source-sink relations. This hypothesis was further substantiated in this study by the finding that delay of senescence in transgenic tobacco (Nicotiana tabacum) plants with autoregulated cytokinin production correlates with an elevated extracellular invertase activity. The finding that the expression of an extracellular invertase under control of the senescence-induced SAG12 promoter results in a delay of senescence demonstrates that effect of cytokinins may be substituted by these metabolic enzymes. The observation that an increase in extracellular invertase is sufficient to delay leaf senescence was further verified by a complementing functional approach. Localized induction of an extracellular invertase under control of a chemically inducible promoter resulted in ectopic delay of senescence, resembling the naturally occurring green islands in autumn leaves. To establish a causal relationship between cytokinins and extracellular invertase for the delay of senescence, transgenic plants were generated that allowed inhibition of extracellular invertase in the presence of cytokinins. For this purpose, an invertase inhibitor was expressed under control of a cytokinin-inducible promoter. It has been shown that senescence is not any more delayed by cytokinin when the expression of the invertase inhibitor is elevated. This finding demonstrates that extracellular invertase is required for the delay of senescence by cytokinins and that it is a key element of the underlying molecular mechanism.     

Population dynamics of an introduced bacterium degrading chlorinated benzenes in a soil column and in sewage sludge

The capacity of the -Proteobacterium Pseudomonas sp. strain P51, which degrades chlorinated benzenes, to metabolize 1,2,4-trichlorobenzene (TCB) under environmental conditions was tested by its release into two experimental systems. The first system consisted of laboratory scale microcosms which were operated with and without the addition of TCB and which were inoculated with sludge from a wastewater treatment plant. The second system consisted of a non sterile, water saturated soil column. We determined survival of strain P51 after its introduction and its ability to degrade TCB. The population dynamics was followed by selective plating and applying the polymerase chain reaction (PCR) to detect strain P51 and the chlorobenzene ( tcb) genes on catabolic plasmid pP51. The results showed a completely different behaviour of strain P51 in the two habitats under the applied conditions. In the soil column the P51 bacteria inoculated the entire area and their population reached 2 × 10⁶ cells/g soil. The population remained active since TCB was degraded to concentrations below the detection limit of 30 g/l. In the sludge microcosms, the number of strain P51 cells immediately decreased from 4 × 10⁷ cells/ml to 10⁵ cells/ml over a period of 2 days after inoculation, and then the strain disappeared to levels below our detection limit (10³–10⁴ cells/ml). In the reactor without TCB the population of P51 maintained a stable value of 10⁵ cells/ml during 8 days but then also decreased to levels below the detection limit. In addition, no significant TCB degradation was found in the sludge reactors. The influence of presence of TCB on maintenance of strain P51 in the two habitats is discussed. This work demonstrates the possibility to successfully apply preselected strains to degrade otherwise poorly degradable substances in complex mixed microbial communities. However, survival and activity may depend strongly on the type of system into which the strain is introduced.     

A comparative categorization of gene flux in diverse microbial species

Microbial genomes harbor genomic islands (GIs), genes presumably acquired via horizontal gene transfer (HGT). We compared GIs of hyperthermophilic, thermophilic, mesophilic, and pathogenic/nonpathogenic species and of small and large genomes. The COG database was used to characterize gene-encoded functions. Putative donors were determined to quantify gene flux between superkingdoms. In hyperthermophiles, more than 10% of the genes were on average acquired across the superkingdom border. For thermophiles and particularly mesophiles, we identified a nearly unidirectional export from bacteria to archaea. Additionally, we analyzed GI composition for Escherichia, and pairs of Listeria, Rhizobiales, Methanosarcinaceae, and Thermus thermophilus/Deinococcus radiodurans. For Escherichia and Listeria, the composition of GIs in pathogenic and nonpathogenic species did not differ significantly with respect to encoded COG classes. The analysis of related genomes showed that the composition of GIs cannot be explained with trends of gene content known to depend on genome size.     

Nitrous oxide and methane fluxes of a pristine slope mire in the German National Park Harz Mountains

Pristine peatlands covered by Histosols (bogs and fens) with high water table and a restricted oxygen (O₂) availability are known to have low emissions of nitrous oxide (N₂O) but may be a significant source for atmospheric methane (CH₄) which are both important greenhouse gases. For the first time N₂O and CH₄ fluxes of a pristine slope mire in the German Harz Mountains have been monitored. Previously reported peatlands are characterised by anaerobic conditions due to high water table levels. Slope mires monitored here receive O₂ through slope water inflow. Gas fluxes have been monitored deploying closed chamber method on a central non-forested area and a forested area at the periphery of the slope mire. By means of groundwater piezometers water table levels, ammonium and nitrate contents as well as hydro-chemical variables like oxygen content and redox potential of the mire pore water have been concurrently measured with trace gas fluxes at both monitoring sites of the slope mire. The slope mire took up small amounts of atmospheric methane at a rate of −0.02 ± 0.01 kg C ha⁻¹ year⁻¹ revealing no significant difference between the forested and non-forested site. Higher uptake rates were observed during low water table level. In contrast to pristine peatlands influx of oxygen containing pore water into slope mire does limit reduction processes and resultant CH₄ emission. N₂O fluxes of the forested and non-forested sites of the slope mire did not differ and amounted to 0.25 ± 0.44 kg N ha⁻¹ year⁻¹. Higher emissions were observed at low water table levels and during thawing periods. In spite of favourable conditions N₂O fluxes of the slope mire have been comparable to those of pristine peatlands.     

Anaerobic Cometabolic Conversion of Benzothiophene by a Sulfate-Reducing Enrichment Culture and in a Tar-Oil-Contaminated Aquifer

Anaerobic cometabolic conversion of benzothiophene was studied with a sulfate-reducing enrichment culture growing with naphthalene as the sole source of carbon and energy. The sulfate-reducing bacteria were not able to grow with benzothiophene as the primary substrate. Metabolite analysis was performed with culture supernatants obtained by cometabolization experiments and revealed the formation of three isomeric carboxybenzothiophenes. Two isomers were identified as 2-carboxybenzothiophene and 5-carboxybenzothiophene. In some experiments, further reduced dihydrocarboxybenzothiophene was identified. No other products of benzothiophene degradation could be determined. In isotope-labeling experiments with a [¹³C]bicarbonate-buffered culture medium, carboxybenzothiophenes which were significantly enriched in the ¹³C content of the carboxyl group were formed, indicating the addition of a C₁ unit from bicarbonate to benzothiophene as the initial activation reaction. This finding was consistent with the results of earlier studies on anaerobic naphthalene degradation with the same culture, and we therefore propose that benzothiophene was cometabolically converted by the same enzyme system. Groundwater analyses of the tar-oil-contaminated aquifer from which the naphthalene-degrading enrichment culture was isolated exhibited the same carboxybenzothiophene isomers as the culture supernatants. In addition, the benzothiophene degradation products, in particular, dihydrocarboxybenzothiophene, were significantly enriched in the contaminated groundwater to concentrations almost the same as those of the parent compound, benzothiophene. The identification of identical metabolites of benzothiophene conversion in the sulfate-reducing enrichment culture and in the contaminated aquifer indicated that the same enzymatic reactions were responsible for the conversion of benzothiophene in situ.     

Neurite Fasciculation Mediated by Complexes of Axonin-1 and Ng Cell Adhesion Molecule

Neural cell adhesion molecules composed of immunoglobulin and fibronectin type III-like domains have been implicated in cell adhesion, neurite outgrowth, and fasciculation. Axonin-1 and Ng cell adhesion molecule (NgCAM), two molecules with predominantly axonal expression exhibit homophilic interactions across the extracellular space (axonin- 1/axonin-1 and NgCAM/NgCAM) and a heterophilic interaction (axonin-1–NgCAM) that occurs exclusively in the plane of the same membrane (cis-interaction). Using domain deletion mutants we localized the NgCAM homophilic binding in the Ig domains 1-4 whereas heterophilic binding to axonin-1 was localized in the Ig domains 2-4 and the third FnIII domain. The NgCAM–NgCAM interaction could be established simultaneously with the axonin-1–NgCAM interaction. In contrast, the axonin-1–NgCAM interaction excluded axonin-1/axonin-1 binding. These results and the examination of the coclustering of axonin-1 and NgCAM at cell contacts, suggest that intercellular contact is mediated by a symmetric axonin-1₂/NgCAM₂ tetramer, in which homophilic NgCAM binding across the extracellular space occurs simultaneously with a cis-heterophilic interaction of axonin-1 and NgCAM. The enhanced neurite fasciculation after overexpression of NgCAM by adenoviral vectors indicates that NgCAM is the limiting component for the formation of the axonin-1₂/NgCAM₂ complexes and, thus, neurite fasciculation in DRG neurons.