Relationships between carbon and hydrogen isotope ratios and nitrogen levels in leaves ofClusia species and two other Clusiaceae genera at various sites and different altitudes in Venezuela

Samples of the Clusiaceae generaClusia, Oedematopus andDystovomita were collected at various sites and different altitudes in northern and south-western Venezuela. Analyses of stable isotopes of carbon and hydrogen and of leaf-nitrogen levels were performed on the dried samples. Correlations among these variables, i.e. carbon isotope discrimination (), hydrogen isotope ratio (D) and N-levels, and with altitude were assessed. In the samples, where values of above 15 indicate predominant performance of C₃ photosynthesis, there were slight tendencies of increasing , D and N-levels with increasing altitude and of increasing with increasing N. Although these correlations taken separately were not statistically significant, they support each other and indicate increasing transpiration and increased leaf-nutrient supply at increasing altitude. Performance of crassulacean acid metabolism (CAM) in species ofClusia appears to be restricted to altitudes below 1500 m a.s.l. There was a significant negative correlation of with altitude in the samples, where values of below 10 indicated predominant performance of CAM. This suggests that phases II and IV of CAM are progressively suppressed towards the upper altitudinal limit of CAM inClusia in northern Venezuela. It is concluded that among the large number of environmental factors and combinations thereof, which determine the expression of CAM inClusia and trigger C₃-CAM transitions in C₃/CAM intermediate species, low availability of water is the most important.     

Membrane topology of the chromate transporter ChrA of Pseudomonas aeruginosa

The membrane topology of the plasmid-encoded Pseudomonas aeruginosa ChrA protein, which effluxes chromate ions, was determined by the analysis of translational fusions with reporter enzymes alkaline phosphatase and beta-galactosidase. A novel 13-TMS (transmembrane segments) topology, with the N-terminus located in the cytoplasm and the C-terminus in the periplasmic space, was consistent with the enzyme activities determined in both Escherichia coli and P. aeruginosa. Alignment of the two halves of ChrA showed significant sequence homology, with TMS I, II, III, IV, V and VI displaying similarity to TMS VIII, IX, X, XI, XII and XIII, respectively, although with opposite membrane orientations. This suggests that ChrA arose from the duplication of a gene encoding a 6-TMS ancestral protein, followed by the insertion of extra TMS VII. These data also suggest that the two halves of ChrA may carry out distinct functions for the transport of chromate.     

The disruption of a gene encoding a putative arylesterase impairs pyruvate dehydrogenase complex activity and nitrogen fixation in Sinorhizobium meliloti

Nitrogen-fixing Sinorhizobium meliloti cells depend upon dicarboxylic acids as carbon and energy sources. The metabolism of these intermediate compounds of the trichloroacetic acid cycle is dependent upon the availability of acetyl-coenzyme A (CoA). In bacteroids, the combined activities of malic enzymes and pyruvate dehydrogenase (PDH) have been proposed to be responsible for the anaplerotic synthesis of acetyl-CoA. We obtained a S. meliloti mutant strain, PD3, in which a Tn5 insertion led to a significant decrease in the overall PDH activity. The genetic characterization of this mutant revealed that the transposon is located at the 3' end of a gene (ada) encoding a putative arylesterase. The mutant PD3 is deficient in nitrogen fixation, which strengthens the physiological importance of PDH activity in the symbiosis of S. meliloti with alfalfa plants.     

Wnt signaling involvement in beta-amyloid-dependent neurodegeneration

Alzheimer's disease (AD) is a progressive dementia paralleled by selective neuronal death, which is probably caused by the cytotoxic effects of the amyloid-beta peptide (Abeta). We have observed that Abeta-dependent neurotoxicity induces a loss of function of Wnt signaling components and that activation of this signaling cascade prevent such cytotoxic effects. Therefore we propose that compounds which mimic this signaling cascade may be candidates for therapeutic intervention in Alzheimer's patients.     

Decidual lymphocytes of human spontaneous abortions induce apoptosis but not necrosis in JEG-3 extravillous trophoblast cells

The human decidua contains an unusually high proportion of lymphocytes, mainly NK and T cells, which are potentially cytotoxic to the trophoblast when they are stimulated with certain cytokines. Given the high incidence of spontaneous abortion in humans and other species, our working hypothesis is that decidual lymphocytes are involved in immunological mechanisms that attack the trophoblast and induce abortion when any gestational problem arises. To test this hypothesis, flow cytometry was used to compare decidual lymphocyte populations in first-trimester spontaneous abortions and elective terminations of first-trimester pregnancy. We found significantly higher proportions of decidual lymphocytes that expressed activation markers, and of T cells (mainly T helper cells) in spontaneous abortions than in elective terminations of pregnancy. Decidual lymphocytes from spontaneous abortion, like decidual lymphocytes from elective termination of pregnancy and peripheral blood lymphocytes, were however, unable to lyse the JEG-3 extravillous cytotrophoblast cell line in a (51)Cr-release assay. Nevertheless, decidual lymphocytes from spontaneous abortion, unlike decidual lymphocytes from elective termination of pregnancy and peripheral blood lymphocytes, induced apoptosis in JEG-3 cells as determined by DNA fragment-release assay. Hematoxylin and eosin staining showed a significantly higher proportion of apoptotic JEG-3 cells when these cells were treated with decidual lymphocytes from spontaneous abortion than when JEG-3 cells were cultured with decidual lymphocytes from elective termination of pregnancy. The ultrastructural signs of apoptosis were confirmed by electron microscopy. These data support the hypothesis that activated decidual lymphocytes participate in human spontaneous abortion by inducing apoptosis but not necrosis of the trophoblast.     

Mahogunin Ring Finger-1 (MGRN1) E3 Ubiquitin Ligase Inhibits Signaling from Melanocortin Receptor by Competition with G��s

Mahogunin ring finger-1 (MGRN1) is a RING domain-containing ubiquitin ligase mutated in mahoganoid, a mouse mutation causing coat color darkening, congenital heart defects, high embryonic lethality, and spongiform neurodegeneration. The melanocortin hormones regulate pigmentation, cortisol production, food intake, and body weight by signaling through five G protein-coupled receptors positively coupled to the cAMP pathway (MC1R–MC5R). Genetic analysis has shown that mouse Mgrn1 is an accessory protein for melanocortin signaling that may inhibit MC1R and MC4R by unknown mechanisms. These melanocortin receptors (MCRs) regulate pigmentation and body weight, respectively. We show that human melanoma cells express 4 MGRN1 isoforms differing in the C-terminal exon 17 and in usage of exon 12. This exon contains nuclear localization signals. MGRN1 isoforms decreased MC1R and MC4R signaling to cAMP, without effect on β₂-adrenergic receptor. Inhibition was independent on receptor plasma membrane expression, ubiquitylation, internalization, or stability and occurred upstream of Gα_s binding to/activation of adenylyl cyclase. MGRN1 co-immunoprecipitated with MCRs, suggesting a physical interaction of the proteins. Significantly, overexpression of Gα_s abolished the inhibitory effect of MGRN1 and decreased co-immunoprecipitation with MCRs, suggesting competition between MGRN1 and Gα_s for binding to MCRs. Although all MGRN1s were located in the cytosol in the absence of MCRs, exon 12-containing isoforms accumulated in the nuclei upon co-expression with the receptors. Therefore, MGRN1 inhibits MCR signaling by a new mechanism involving displacement of Gα_s, thus accounting for key features of the mahoganoid phenotype. Moreover, MGRN1 might provide a novel pathway for melanocortin signaling from the cell surface to the nucleus.     

Evidence of an American origin for symbiosis-related genes in Rhizobium lusitanum

Randomly amplified polymorphic DNA (RAPD) analysis was used to investigate the diversity of 179 bean isolates recovered from six field sites in the Arcos de Valdevez region of northwestern Portugal. The isolates were divided into 6 groups based on the fingerprint patterns that were obtained. Representatives for each group were selected for sequence analysis of 4 chromosomal DNA regions. Five of the groups were placed within Rhizobium lusitanum, and the other group was placed within R. tropici type IIA. Therefore, the collection of Portuguese bean isolates was shown to include the two species R. lusitanum and R. tropici. In plant tests, the strains P1-7, P1-1, P1-2, and P1-16 of R. lusitanum nodulated and formed nitrogen-fixing symbioses both with Phaseolus vulgaris and Leucaena leucocephala. A methyltransferase-encoding nodS gene identical with the R. tropici locus that confers wide host range was detected in the strain P1-7 as well as 24 others identified as R. lusitanum. A methyltransferase-encoding nodS gene also was detected in the remaining isolates of R. lusitanum, but in this case the locus was that identified with the narrow-host-range R. etli. Representatives of isolates with the nodS of R. etli formed effective nitrogen-fixing symbioses with P. vulgaris and did not nodulate L. leucocephala. From sequence data of nodS, the R. lusitanum genes for symbiosis were placed within those of either R. tropici or R. etli. These results would support the suggestion that R. lusitanum was the recipient of the genes for symbiosis with beans from both R. tropici and R. etli.     

Systematics and evolutionary significance of the small Abrocomidae from the early Miocene of southern South America

Octodontoidea is the most species-rich clade among hystricomorph rodents, and has a fossil record going back to at least the late Oligocene. Affinities of fossils previous to the late Miocene differentiation of the extant families Abrocomidae, Echimyidae and Octodontidae are controversial, essentially because these fossils may share few apomorphies with modern species. In fact, pre-late Miocene representatives of Abrocomidae had not been recognised until very recently. Here we revise the early Miocene genus Acarechimys, originally assigned to Echimyidae, and alternatively to stem Octodontoidea or to Octodontidae. A systematic and parsimony-based phylogenetic analysis of the species traditionally included in Acarechimys showed that this genus is part of stem Abrocomidae. These results are primarily supported by morphology of the mandible and lower molars. Acarechimys is here restricted to three species, A. minutus, A. pulchellus and Acarechimys pascuali sp. nov., while another species, A. constans, is here transferred to a new abrocomid genus. The remaining species were nested within Octodontidae. According to these results, Abrocomidae might have been as diverse as its sister clade Octodontidae-Echimyidae during the late Oligocene–early Miocene. Extinction of this diversity would have resulted in marked loss of evolutionary history, with extant abrocomids being currently restricted to late-diverged euhypsodont representatives.