Página 5 dos resultados de 60878 itens digitais encontrados em 0.050 segundos
Resultados filtrados por Publicador: National Academy of Sciences

‣ Interactions of climate change with biological invasions and land use in the Hawaiian Islands: Modeling the fate of endemic birds using a geographic information system

Benning, Tracy L.; LaPointe, Dennis; Atkinson, Carter T.; Vitousek, Peter M.
Fonte: National Academy of Sciences Publicador: National Academy of Sciences
Tipo: Artigo de Revista Científica
Português
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The Hawaiian honeycreepers (Drepanidae) represent a superb illustration of evolutionary radiation, with a single colonization event giving rise to 19 extant and at least 10 extinct species [Curnutt, J. & Pimm, S. (2001) Stud. Avian Biol. 22, 15–30]. They also represent a dramatic example of anthropogenic extinction. Crop and pasture land has replaced their forest habitat, and human introductions of predators and diseases, particularly of mosquitoes and avian malaria, has eliminated them from the remaining low- and mid-elevation forests. Landscape analyses of three high-elevation forest refuges show that anthropogenic climate change is likely to combine with past land-use changes and biological invasions to drive several of the remaining species to extinction, especially on the islands of Kauai and Hawaii.

‣ On the relation between fluctuation and response in biological systems

Sato, Katsuhiko; Ito, Yoichiro; Yomo, Tetsuya; Kaneko, Kunihiko
Fonte: National Academy of Sciences Publicador: National Academy of Sciences
Tipo: Artigo de Revista Científica
Português
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A general relationship between fluctuation and response in a biological system is presented. The fluctuation is given by the variance of some quantity, whereas the response is given as the average change of that quantity for a given parameter change. We propose a relationship where the two are proportional, in a similar way to the fluctuation–dissipation theorem in physics. By studying an evolution experiment where fluorescence of protein in bacteria increases, we confirm our relation by observing a positive correlation between the speed of fluorescence evolution and the phenotypic fluctuation of the fluorescence over clone bacteria. The generality of the relationship as well as its relevance to evolution is discussed.

‣ Detection of multistability, bifurcations, and hysteresis in a large class of biological positive-feedback systems

Angeli, David; Ferrell, James E.; Sontag, Eduardo D.
Fonte: National Academy of Sciences Publicador: National Academy of Sciences
Tipo: Artigo de Revista Científica
Português
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It is becoming increasingly clear that bistability (or, more generally, multistability) is an important recurring theme in cell signaling. Bistability may be of particular relevance to biological systems that switch between discrete states, generate oscillatory responses, or “remember” transitory stimuli. Standard mathematical methods allow the detection of bistability in some very simple feedback systems (systems with one or two proteins or genes that either activate each other or inhibit each other), but realistic depictions of signal transduction networks are invariably much more complex. Here, we show that for a class of feedback systems of arbitrary order the stability properties of the system can be deduced mathematically from how the system behaves when feedback is blocked. Provided that this open-loop, feedback-blocked system is monotone and possesses a sigmoidal characteristic, the system is guaranteed to be bistable for some range of feedback strengths. We present a simple graphical method for deducing the stability behavior and bifurcation diagrams for such systems and illustrate the method with two examples taken from recent experimental studies of bistable systems: a two-variable Cdc2/Wee1 system and a more complicated five-variable mitogen-activated protein kinase cascade.

‣ Unique CD40-mediated biological program in B cell activation requires both type 1 and type 2 NF-κB activation pathways

Zarnegar, Brian; He, Jeannie Q.; Oganesyan, Gagik; Hoffmann, Alexander; Baltimore, David; Cheng, Genhong
Fonte: National Academy of Sciences Publicador: National Academy of Sciences
Tipo: Artigo de Revista Científica
Português
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B lymphocytes can be activated by many different stimuli. However, the mechanisms responsible for the signaling and functional specificities of individual stimuli remain to be elucidated. Here, we have compared the contribution of the type 1 (p50-dependent) and type 2 (p52-dependent) NF-κB activation pathways to cell survival, proliferation, homotypic aggregation, and specific gene regulation of murine primary B lymphocytes. Whereas lipopolysaccharide (LPS) and B cell activation factor (BAFF) mainly activate the type 1 or type 2 pathways, respectively, CD40 ligand (CD40L) strongly activates both. Rescue of spontaneous apoptosis is diminished in p52–/– B cells after BAFF stimulation and in p50–/–c-Rel–/– B cells after LPS stimulation. Interestingly, significant CD40-induced B cell survival is still observed even in p50–/–c-Rel–/–p65–/+ B cells, which is correlated with the ability of CD40L to up-regulate Bcl-xL expression in these cells. CD40L- and LPS-induced B cell proliferation, as well as up-regulation of proliferation-related genes, however, are greatly reduced in c-Rel–/– and p50–/–c-Rel–/– B cells but are normal in p52–/– B cells. We have further demonstrated that both c-Rel and p52 are required for CD40-mediated B cell homotypic aggregation...

‣ Revisiting quorum sensing: Discovery of additional chemical and biological functions for 3-oxo-N-acylhomoserine lactones

Kaufmann, Gunnar F.; Sartorio, Rafaella; Lee, Sang-Hyeup; Rogers, Claude J.; Meijler, Michael M.; Moss, Jason A.; Clapham, Bruce; Brogan, Andrew P.; Dickerson, Tobin J.; Janda, Kim D.
Fonte: National Academy of Sciences Publicador: National Academy of Sciences
Tipo: Artigo de Revista Científica
Português
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Bacteria use small diffusible molecules to exchange information in a process called quorum sensing. An important class of autoinducers used by Gram-negative bacteria is the family of N-acylhomoserine lactones. Here, we report the discovery of a previously undescribed nonenzymatically formed product from N-(3-oxododecanoyl)-L-homoserine lactone; both the N-acylhomoserine and its novel tetramic acid degradation product, 3-(1-hydroxydecylidene)-5-(2-hydroxyethyl)pyrrolidine-2,4-dione, are potent antibacterial agents. Bactericidal activity was observed against all tested Gram-positive bacterial strains, whereas no toxicity was seen against Gram-negative bacteria. We propose that Pseudomonas aeruginosa utilizes this tetramic acid as an interference strategy to preclude encroachment by competing bacteria. Additionally, we have discovered that this tetramic acid binds iron with comparable affinity to known bacterial siderophores, possibly providing an unrecognized mechanism for iron solubilization. These findings merit new attention such that other previously identified autoinducers be reevaluated for additional biological functions.

‣ Recent biological invasion may hasten invasional meltdown by accelerating historical introductions

Grosholz, Edwin D.
Fonte: National Academy of Sciences Publicador: National Academy of Sciences
Tipo: Artigo de Revista Científica
Português
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Biological invasions are rapidly producing planet-wide changes in biodiversity and ecosystem function. In coastal waters of the U.S., >500 invaders have become established, and new introductions continue at an increasing rate. Although most species have little impact on native communities, some initially benign introductions may occasionally turn into damaging invasions, although such introductions are rarely documented. Here, I demonstrate that a recently introduced crab has resulted in the rapid spread and increase of an introduced bivalve that had been rare in the system for nearly 50 yr. This increase has occurred through the positive indirect effects of predation by the introduced crab on native bivalves. I used field and laboratory experiments to show that the mechanism is size-specific predation interacting with the different reproductive life histories of the native (protandrous hermaphrodite) and the introduced (dioecious) bivalves. These results suggest that positive interactions among the hundreds of introduced species that are accumulating in coastal systems could result in the rapid transformation of previously benign introductions into aggressively expanding invasions. Even if future management efforts reduce the number of new introductions...

‣ Growth and shape stability of a biological membrane adhesion complex in the diffusion-mediated regime

Shenoy, V. B.; Freund, L. B.
Fonte: National Academy of Sciences Publicador: National Academy of Sciences
Tipo: Artigo de Revista Científica
Português
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We examine the process of expansion of a focal adhesion complex by which a biological membrane containing mobile binders adheres to a substrate with complementary binders. Attention is focused on the situation, common among living cells, in which the mean mobile binder density is insufficient to overcome generic resistance to close approach of the membrane to its substrate. For the membrane to adhere, binders must be recruited from adjacent regions to join an adhesion patch of density adequate for adhesion, thereby expanding the size of the patch. The specific configuration examined is the expansion of a circular adhesion zone for which diffusive binder transport driven by a chemical potential gradient is the mechanism of binder recruitment. An aspect of the process of particular interest is the stability of the circular shape of the expanding front. It is found that the adhesion front radius increases as √t, where t is the time elapsed since nucleation, and that the circular shape becomes unstable under sinusoidal perturbations for radii large compared with the nucleation size, as observed in recent experiments.

‣ Remote analysis of biological invasion and biogeochemical change

Asner, Gregory P.; Vitousek, Peter M.
Fonte: National Academy of Sciences Publicador: National Academy of Sciences
Tipo: Artigo de Revista Científica
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We used airborne imaging spectroscopy and photon transport modeling to determine how biological invasion altered the chemistry of forest canopies across a Hawaiian montane rain forest landscape. The nitrogen-fixing tree Myrica faya doubled canopy nitrogen concentrations and water content as it replaced native forest, whereas the understory herb Hedychium gardnerianum reduced nitrogen concentrations in the forest overstory and substantially increased aboveground water content. This remote sensing approach indicates the geographic extent, intensity, and biogeochemical impacts of two distinct invaders; its wider application could enhance the role of remote sensing in ecosystem analysis and management.

‣ Comparison of the protein–protein interfaces in the p53–DNA crystal structures: Towards elucidation of the biological interface

Ma, Buyong; Pan, Yongping; Gunasekaran, K.; Venkataraghavan, R. Babu; Levine, Arnold J.; Nussinov, Ruth
Fonte: National Academy of Sciences Publicador: National Academy of Sciences
Tipo: Artigo de Revista Científica
Português
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p53, the tumor suppressor protein, functions as a dimer of dimers. However, how the tetramer binds to the DNA is still an open question. In the crystal structure, three copies of the p53 monomers (containing chains A, B, and C) were crystallized with the DNA-consensus element. Although the structure provides crucial data on the p53–DNA contacts, the active oligomeric state is unclear because the two dimeric (A–B and B–C) interfaces present in the crystal cannot both exist in the tetramer. Here, we address the question of which of these two dimeric interfaces may be more biologically relevant. We analyze the sequence and structural properties of the p53–p53 dimeric interfaces and carry out extensive molecular dynamics simulations of the crystal structures of the human and mouse p53 dimers. We find that the A–B interface residues are more conserved than those of the B–C. Molecular dynamics simulations show that the A–B interface can provide a stable DNA-binding motif in the dimeric state, unlike B–C. Our results indicate that the interface between chains A–B in the p53–DNA complex constitutes a better candidate for a stable biological interface, whereas the B–C interface is more likely to be due to crystal packing. Thus...

‣ Biological effects in unirradiated human tissue induced by radiation damage up to 1 mm away

Belyakov, Oleg V.; Mitchell, Stephen A.; Parikh, Deep; Randers-Pehrson, Gerhard; Marino, Stephen A.; Amundson, Sally A.; Geard, Charles R.; Brenner, David J.
Fonte: National Academy of Sciences Publicador: National Academy of Sciences
Tipo: Artigo de Revista Científica
Português
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A central tenet in understanding the biological effects of ionizing radiation has been that the initially affected cells were directly damaged by the radiation. By contrast, evidence has emerged concerning “bystander” responses involving damage to nearby cells that were not themselves directly traversed by the radiation. These long-range effects are of interest both mechanistically and for assessing risks from low-dose exposures, where only a small proportion of cells are directly hit. Bystander effects have been observed largely by using single-cell in vitro systems that do not have realistic multicellular morphology; no studies have as yet been reported in three-dimensional, normal human tissue. Given that the bystander phenomenon must involve cell-to-cell interactions, the relevance of such single-cell in vitro studies is questionable, and thus the significance of bystander responses for human health has remained unclear. Here, we describe bystander responses in a three-dimensional, normal human-tissue system. Endpoints were induction of micronucleated and apoptotic cells. A charged-particle microbeam was used, allowing irradiation of cells in defined locations in the tissue yet guaranteeing that no cells located more than a few micrometers away receive any radiation exposure. Unirradiated cells up to 1 mm distant from irradiated cells showed a significant enhancement in effect over background...