Synaptic ultrastructure in nerve terminals of Drosophila larvae overexpressing the learning gene dunce

We investigated synaptic ultrastructure of individual nerve ending varicosities at the Drosophila larval neuromuscular junction in transgenic larvae overexpressing the learning gene dunce (dnc) in the nervous system. It was previously shown that cAMP is reduced to one-third normal in these larvae and that they have fewer nerve terminal varicosities and smaller junction potentials, although transmitter release from individual nerve ending varicosities is not significantly altered. We tested the hypothesis that synaptic ultrastructure is modified to compensate for possible reduced efficacy of synaptic transmission resulting from lower than normal cAMP. Synaptic size and number of presynaptic dense bodies (active zone structures) per synapse are modestly enhanced in transgenic larvae overexpressing the dnc gene product and in rutabaga (rut(1)) mutant larvae, which have reduced adenylyl cyclase activity and reduced neural cAMP. The incidence of complex synapses (possessing 2 or more presynaptic dense bodies) was not consistently different in experimental larvae compared to controls. The observations suggest that chronic reduction of cAMP levels in the nervous system of Drosophila larvae, although leading to a modest compensatory change in synaptic structure, does not markedly alter several synaptic ultrastructural parameters which are thought to influence the strength of transmitter release; thus, homeostatic mechanisms do not act to maintain normal-sized junction potentials by altering synaptic structure.     

Lowering S-adenosylmethionine levels in Escherichia coli modulates C-to-T transition mutations

Deoxycytosine methylase (Dcm) enzyme activity causes mutagenesis in vitro either directly by enzyme-induced deamination of cytosine to uracil in the absence of the methyl donor, S-adenosylmethionine (SAM), or indirectly through spontaneous deamination of [5-methyl]cytosine to thymine. Using a Lac reversion assay, we investigated the contribution of the first mechanism to Dcm mutagenesis in vivo by lowering the levels of SAM. Escherichia coli SAM levels were lowered by reducing SAM synthetase activity via the introduction of a metK84 allele or by hydrolyzing SAM using the bacteriophage T3 SAM hydrolase. The metK84 strains exhibited increased C-to-T mutagenesis. Expression of the T3 SAM hydrolase gene, under the control of the arabinose-inducible P(BAD) promoter, effectively reduced Dcm-mediated genomic DNA methylation. However, increased mutagenesis was not observed until extremely high arabinose concentrations were used, and genome methylation at Dcm sites was negligible.     

Copper catalyzed oxidation of Alzheimer Abeta.

Abeta derived from amyloid plaques of Alzheimer's disease-affected brain contain several oxidative posttranslational modifications. In this study we have characterized the amino acid content of human amyloid-derived Abeta and compared it with that of human synthetic Abeta subjected to metal-catalyzed oxidation. Human amyloid derived Abeta has an increased content of arginine (46%) and glutamate/glutamine residues (28%), but a decreased content of histidine residues (-32%) as compared to the expected amino acid content. Incubation of synthetic human Abeta with Cu(II), but not Fe(III), in the presence of H2O2 similarly induced a decrease in histidine residues (-79%), but also a decrease in tyrosine residues (-28%). Our results suggest that histidine and tyrosine are most vulnerable to metal mediated oxidative attack, consistent with our earlier findings that Cu coordinated via histidine residues is redox competent. Our results suggest that the loss of histidine residues in human amyloid-derived Abeta may be a result of Cu oxidation, and that unidentified post-translational mechanisms operate to modify other amino acids of Abeta in vivo.     

Magnification in Drosophila: evidence for an inducible rDNA-specific recombination system

An experiment is described that provides evidence for an exchange mechanism to explain the increase in ribosomal gene number that occurs during bobbed magnification. We show that bobbed and bobbed-lethal alleles do not magnify in closed X chromosomes, but that a spontaneous ring opening restores normal magnification. The results provide strong evidence that the elementary magnifying event is unequal sister chromatid exchange, and can be interpreted in the framework of an inducible rDNA-specific recombination system as the basis of ribosomal gene magnification.     

Patch occupancy of North American mammals: is patchiness in the eye of the beholder?

Aim Intraspecific variation in patch occupancy often is related to physical features of a landscape, such as the amount and distribution of habitat. However, communities occupying patchy environments typically exhibit non‐random distributions in which local assemblages of species‐poor patches are nested subsets of assemblages occupying more species‐rich patches. Nestedness of local communities implies interspecific differences in sensitivity to patchiness. Several hypotheses have been proposed to explain interspecific variation in responses to patchiness within a community, including differences in (1) colonization ability, (2) extinction proneness, (3) tolerance to disturbance, (4) sociality and (5) level of adaptation to prevailing environmental conditions. We used data on North American mammals to compare the performance of these ‘ecological’ hypotheses and the ‘physical landscape’ hypothesis. We then compared the best of these models against models that scaled landscape structure to ecologically relevant attributes of individual species. Location North America. Methods We analysed data on prevalence (i.e. proportion of patches occupied in a network of patches) and occupancy for 137 species of non‐volant mammals and twenty networks consisting of four to seventy‐five patches. Insular and terrestrial networks exhibited significantly different mean levels of prevalence and occupancy and thus were analysed separately. Indicator variables at ordinal and family levels were included in models to correct for effects caused by phylogeny. Akaike's information criterion was used in conjunction with ordinary least squares and logistic regression to compare hypotheses. Results A patch network's physical structure, indexed using patch area and isolation, received the greatest support among models predicting the prevalence of species on insular networks. Niche breadth (diet and habitat) received the greatest support for predicting prevalence of species occupying terrestrial networks. For both insular and terrestrial systems, physical features (patch area and isolation) received greater support than any of the ecological hypotheses for predicting species occupancy of individual patches. For terrestrial systems, scaling patch area by its suitability to a focal species and by individual area requirements of the species, and scaling patch isolation by species‐specific dispersal ability and niche breadth, resulted in models of patch occupancy that were superior to models relying solely on physical landscape features. For all selected models, unexplained levels of variation were high. Main conclusions Stochasticity dominated the systems we studied, indicating that random events are probably quite important in shaping local communities. With respect to deterministic factors, our results suggest that forces affecting species prevalence and occupancy may differ between insular and terrestrial systems. Physical features of insular systems appeared to swamp ecological differences among species in determining prevalence and occupancy, whereas species with broad niches were disproportionately represented in terrestrial networks. We hypothesize that differential extinction over long time periods in highly variable networks has driven nestedness of mammalian communities on islands, whereas differential colonization over shorter time‐scales in more homogeneous networks probably governed the local structure of terrestrial communities. Our results also demonstrate that integration of a species' ecological traits with physical features of a patch network is superior to reliance on either factor separately when attempting to predict the species' probability of patch occupancy in terrestrial systems.     

Postnatal development of catecholamine uptake and storage of the newborn rat heart.

The ability of atria and ventricular slices of heart removed from Sprague-Dawley rats at ages 1–21 days to take up and store catecholamines was studied to correlate this activity with the previously observed physiologic immaturity of the cardiac sympathetic nervous system in the neonatal mammal. Total uptake and subcellular distribution at 1, 4, 7, 14, and 21 days were determined at various time intervals between 5 and 30 min. In the artria there was a small amount of uptake observed during the first postnatal day, which increased only slightly by 4 days of age. The greatest change occurred between 4 and 7 days. Uptakes at 7, 14, and 21 days were similar to adult values. Reserpine inhibited the 30-min uptake 57% during the first day of life, increasing to 69% inhibition by 7 days of age. Uptake of norepinephrine into the ventricular slices, both with and without reserpine, was less at each age point until 14 days of age, at which time it equaled atrial uptake. The uptake and the effect of reserpine in the particulate fraction of homogenate from both atria and ventricular slices paralleled the developmental pattern seen in the slice. Application of Michaelis-Menten kinetics to uptake at 4 and 7 days revealed a common K_m but differing V_max. These studies suggest that the increases in the ability of the developing rat heart to accumulate norepinephrine is due to increases in the number or storage capacity of synaptic vesicles.