The molecular organization of nerve membranes

Summary The lipid content and composition from an axolemma-rich preparation isolated from squid retinal axons was analyzed.The lipids, which accounted for 45.5% of the dry weight of this membrane, were composed of 22% cholesterol, 66.7% phospholipids and 5.2% free fatty acids. The negatively charged species phosphatidyl ethanolamine (37%), phosphatidyl serine (10%) and lysophosphatidyl ethanolamine (4%) made up 51% of the phospholipids. The amphoteric phosphatidyl choline and sphingomyelin accounted for 39% and 4%, respectively.The relative distribution of fatty acids in each of the isolated phospholipids was studied. The most remarkable feature of these phospholipids was the large proportion of long-chain polyunsaturated fatty acids. The 226 acyl chain accounted for 37% in phosphatidyl ethanolamine, 21.7% in phosphatidyl choline, 17.5% on phosphatidyl serine and 20.3% in sphingomyelin (all expressed as area %).The molar fraction of unsaturated fatty acids reached 65% in phosphatidyl ethanolamine and 42.0 and 44.8% in phosphatidyl choline and phosphatidyl serine, respectively. The double bond index in these species varied between 1.0 and 2.6.The lipid composition of the axolemma-rich preparation isolated from squid retinal axons appears to be similar to other excitable plasma membranes in two important features: (a) a low cholesterol/phospholipid molar ratio of 0.61; and (b) the polyunsaturated nature of the fatty acid of their phospholipids.This particular chemical composition may contribute a great deal to the molecular unstability of excitable membranes.The preceding papers of this series were published inArchives of Biochemistry and Biophysics.     

The action of α-amanitin in vivo on the synthesis and maturation of mouse liver ribonucleic acids

α-Amanitin acts in vitro and in vivo as a selective inhibitor of nucleoplasmic RNA polymerases. Treatment of mice with low doses of α-amanitin causes the following changes in the synthesis, maturation and nucleocytoplasmic transfer of liver RNA species. 1. The synthesis of the nuclear precursor of mRNA is strongly inhibited and all electrophoretic components are randomly affected. The labelling of cytoplasmic mRNA is blocked. These effects may be correlated with the rapid and lasting inhibition of nucleoplasmic RNA polymerase. 2. The synthesis and maturation of the nuclear precursor of rRNA is inhibited within 30min. (a) The initial effect is a strong (about 80%) inhibition of the early steps of 45S precursor rRNA maturation. (b) The synthesis of 45S precursor rRNA is also inhibited and the effect increases from about 30% at 30min to more than 70% at 150min. (c) The labelling of nuclear and cytoplasmic 28S and 18S rRNA is almost completely blocked. The labelling of nuclear 5S rRNA is inhibited by about 50%, but that of cytoplasmic 5S rRNA is blocked. (d) The action of α-amanitin on the synthesis of precursor rRNA cannot be correlated with the slight gradual decrease of nucleolar RNA polymerase activity (only 10–20% inhibition at 150min). (e) The inhibition of precursor rRNA maturation and synthesis precedes the ultrastructural lesions of the nucleolus detected by standard electron microscopy. 3. The synthesis of nuclear 4.6S precursor of tRNA is not affected by α-amanitin. However, the labelling of nuclear and cytoplasmic tRNA is decreased by about 50%, which indicates an inhibition of precursor tRNA maturation. The results of this study suggest that the synthesis and maturation of the precursor of rRNA and the maturation of the precursor of tRNA are under the control of nucleoplasmic gene products. The regulator molecules may be either RNA or proteins with exceedingly fast turnover.     

Informationstheoretisches lernmodell

Summary The formation of the interconnection stimulus-response in a learning system is analysed. The system, a technical or a biological one establishes this correspondence by processing the information fed back from the medium during the learning process. This information has two aspects: a quantitative one related to the probability of the events, and a qualitative one related to the utility of the events in view of a goal. Both aspects are taken into consideration; by successive experiences the system eliminates the double uncertainty concerning the probabilistic dependence: stimulu — sresponse — outcome and its utility. Learning implies then a system for evaluating the utilities of different outcomes in view of a goal, a memory to record them and a decision system for selecting the corresponding responses upon a given criterion.

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Der Beitrag stellt einen Teil der Untersuchungen dar, die zur Ausarbeitung der Doktor-Dissertation am Polytechnischen Institut Gheorghe Gheorghiu-Dej in Bukarest unternommen wurden.     

Modeling competition between Laurencia obtusa (Ceramiales,Rhodophyta) and Hypnea spinella (Gigartinales,Rhodophyta) at Cabo Frio Island,Rio de Janeiro,Brazil

The Lotka-Volterra competition model was used to represent the interaction between Laurencia obtusa and Hypnea spinella. A new model that considers effects of competition on algal carrying capacity is suggested. To test the models, data from field experiments conducted in an intertidal region at Cabo Frio Island, Rio de Janeiro, Brazil, were used. Both models showed that Hypnea was a stronger competitor than Laurencia. The model of interaction through the carrying capacity showed a stable coexistence between the algal populations and better represented the experimental data.     

Inositol 1,4,5-trisphosphate formation in leaves of winter oilseed rape plants in response to freezing, tissue water potential aed abscisic acid

Time courses of formation of inositol 1,4,5-trisphosphate (IP₃) were followed in the leaves of non-acclimated and cold (2°C)-acclimated winter oilseed rape ( Brassica napus L. var. oleifera ) plants, subjected to different freezing temperatures or to polyethylene glycol 8000 (PEG) and abscisic acid (ABA) treatments. Changes in water potential (Ψ_w) and in ABA level in the frost- and PEG-treated tissues were also determined. Results obtained indicate that temperatures sligthly higher than LT₅₀ induced a transient and substantial increase in IP₃ level, both in non-acclimated and cold-acclimated tissues. At comparable freezing temperature (–5°C) the response of cold-acclimated leaves was lower than that of non-acclimated ones. The PEG-depedent decrease in Ψ_w to –0.9 MPa or ABA (0.1 m M ) treatment gave rise to a transient increase in IP₃ content in non-acclimated tissues only. Collectively, the data indicate that cold acclimation of plants may lead to lower cell responsiveness to the factors studied in terms of induction of IP₃ formation. Changes in the IP₃ content, observed in the present experiments, support our previous suggestion that non-killing freezing temperatures may induce the phosphoinositide pathway, both in non-acclimated and cold-acclimated tissues. Lowering of tissue water potential to some threshold value or a high exogenous ABA supply may mimic the freezing-dependent reaction in the non-acclimated leaves.     

PGE2 involvement in experimental infection with Trypanosoma cruzi subpopulations

PGE₂ involvement in experimental Trypanosoma cruzi infection depends on the lethal capacity of the parasite subpopulation used. Mice acutely infected with non-lethal K98 displayed an enhancement in PGE₂ serum levels during the acute period, while those infected with lethal T. cruzi subpopulations (RA or K98-2) showed levels not different from normal mice. The enhancement detected in K98 group could be related both to an increased number of CD8+ T cell number and to enhanced PGE₂ release per cell by CD8+; values of PGE₂ release by adherent cells were not altered in this group. Treatment with cyclooxygenase inhibitors enhanced mortality rates of mice infected with K98, and administration of 16,16-dimethyl PGE₂ (dPGE) reversed this effect. However, mice infected with RA did not reduce their mortality rates by administration of diverse doses of dPGE. These findings suggest that PGE₂ could play a role in resistance in mice infected with K98.     

Lacker's model: control of follicular growth and ovulation in domestic species

Lacker (1981) and Lacker & Akin (1988) developed a mathematical model of follicular maturation and ovulation; this model of only four parameters accounts for a large number of results obtained over the past decade or more on the control of follicular growth and ovulation in mammals. It establishes a single law of maturation for each follicle which describes the interactions between growing follicles. The function put forward is sufficient to explain the constancy of the number of ovulations or large follicles in a female as well as the variability of this number among strains or species and for either induced or spontaneous ovulators. According to the model, the number of ovulations or large follicles lies between two limits that are themselves simple functions of two parameters of the model. Moreover, Lacker's model exhibits interesting characteristics in agreement with results obtained by physiologists: in particular, it predicts that the number of ovulating or large follicles is independent of:

1. the total number of maturing follicles,
2. the process of recruitment of newly maturing follicles towards the terminal maturation (Poisson or other),
3. the form of the LH or FSH secretion curves as functions of the systemic level of oestradiol. The model further predicts that
4. selection and dominance of follicles result from the feedback between the ovary and the hypophysis through the interactions between follicles; these interactions are expressed by the maturation function of the model.
5. recovery from atresia is possible for a follicle: from decreasing, the rate of secretion of oestradiol may increase.
6. the revised model suggests a renewal of follicles during the sexual cycle, as “waves of follicular growth”.

Lacker's model is a model of strict dominance; it maintains a hierarchy of the follicles as soon as they start their final maturation to the ovulations as that is observed in bird or reptile ovary. Such a strict hierarchy is possible but it is probably not a general situation in all mammals. We therefore modified the maturing function of the follicle in order to make it compatible with the observations of physiologists: follicles always interact as in the initial model but they individually become old, the hierarchy of follicles can be modified with time and the largest follicles do not indefinitely grow as in the initial model.     

Probability of mast cells to mediate anaphylaxis in skeletal muscle

Are there enough mast cells in denervated skeletal muscle to account for autopharmacological mediation of the antigen potentials (APs) elicited by microtaps? Through rough qualitative estimations, some authors have suggested a positive answer to this question. However, in view of measurements performed in this investigation of both the density of mast cells and the diffusion coefficient of antigens, the probability of such mediated effects was found to be relatively low:P=0.016 for egg albumin andP=0.004 for ferritin. Therefore, most APs induced by microtaps should be attributed to the direct effect of antigen over the sensitized muscle fibers. Yet, both the density of mast cells found in this work and the known amount of histamine they are capable of releasing when challenged with antigen, support the hypothesis regarding the involvement of these cells when antigen is massively superfused so as to induce Schultz-Dale reactions in muscle strips. Under this circumstance, the direct and mediated mechanisms may coexist.     

Intrinsic pKas of ionizable residues in proteins: An explicit solvent calculation for lysozyme

Molecular dynamics simulations of triclinic hen egg white lysozyme in aqueous solution were performed to calculate the intrinsic pK_as of 14 ionizable residues. An all-atom model was used for both solvent and solute, and a single 180 ps simulation in conjunction with a Gaussian fluctuation analysis method was used. An advantage of the Gaussian fluctuation method is that it only requires a single simulation of the system in a reference state to calculate all the pK_as in the protein, in contrast to multiple simulations for the free energy perturbation method. pK_int shifts with respect to reference titratable residues were evaluated and compared to results obtained using a finite difference Poisson-Boltzmann (FDPB) method with a continuum solvent model; overall agreement with the direction of the shifts was generally observed, though the magnitude of the shifts was typically larger with the explicit solvent model. The contribution of the first solvation shell to the total charging free energies of the titratable groups was explicitly evaluated and found to be significant. Dielectric shielding between pairs of titratable groups was examined and found to be smaller than expected. The effect of the approximations used to treat the long-range interactions on the pK_int shifts is discussed. © 1994 Wiley-Liss, Inc.     

Replication of the promiscuous plasmid pLSI: a region encompassing the minus origin of replication is associated with stable plasmid inheritance

Deletion of a region of the promiscuous plasmid pLS1 encompassing the initiation signals for the synthesis of the plasmid lagging strand led to plasmid instability in Streptococcus pneumoniae and Bacillus subtilis. This defect could not be alleviated by increasing the number of copies (measured as double-stranded plasmid DNA) to levels similar to those of the wild-type plasmid pLS1. Our results indicate that in the vicinity of, or associated with the single-stranded origin region of pLS1 there is a plasmid component involved in its stable inheritance. Homology was found between the DNA gyrase binding site within the par region of plasmid pSC101 and the pLS1 specific recombination site RS_R.