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排序方式: 共有588条查询结果,搜索用时 15 毫秒
1.
Mechanosensitive channel proteins are important safety valves against osmotic shock in bacteria, and are involved in sensing touch and sound waves in higher organisms. The mechanosensitive channel of small conductance (MscS) has been extensively studied. Pulsed electron-electron double resonance (PELDOR or DEER) of detergent-solubilized protein confirms that as seen in the crystal structure, the outer ring of transmembrane helices do not pack against the pore-forming helices, creating an apparent void. The relevance of this void to the functional form of MscS in the bilayer is the subject of debate. Here, we report PELDOR measurements of MscS reconstituted into two lipid bilayer systems: nanodiscs and bicelles. The distance measurements from multiple mutants derived from the PELDOR data are consistent with the detergent-solution arrangement of the protein. We conclude, therefore, that the relative positioning of the transmembrane helices is preserved in mimics of the cell bilayer, and that the apparent voids are not an artifact of detergent solution but a property of the protein that will have to be accounted for in any molecular mechanism of gating.  相似文献   
2.

Aims

We recently published that the positive inotropic response (PIR) to levosimendan can be fully accounted for by phosphodiesterase (PDE) inhibition in both failing human heart and normal rat heart. To determine if the PIR of the active metabolite OR-1896, an important mediator of the long-term clinical effects of levosimendan, also results from PDE3 inhibition, we compared the effects of OR-1896, a representative Ca2+ sensitizer EMD57033 (EMD), levosimendan and other PDE inhibitors.

Methods

Contractile force was measured in rat ventricular strips. PDE assay was conducted on rat ventricular homogenate. cAMP was measured using RII_epac FRET-based sensors.

Results

OR-1896 evoked a maximum PIR of 33±10% above basal at 1 μM. This response was amplified in the presence of the PDE4 inhibitor rolipram (89±14%) and absent in the presence of the PDE3 inhibitors cilostamide (0.5±5.3%) or milrinone (3.2±4.4%). The PIR was accompanied by a lusitropic response, and both were reversed by muscarinic receptor stimulation with carbachol and absent in the presence of β-AR blockade with timolol. OR-1896 inhibited PDE activity and increased cAMP levels at concentrations giving PIRs. OR-1896 did not sensitize the concentration-response relationship to extracellular Ca2+. Levosimendan, OR-1896 and EMD all increased the sensitivity to β-AR stimulation. The combination of either EMD and levosimendan or EMD and OR-1896 further sensitized the response, indicating at least two different mechanisms responsible for the sensitization. Only EMD sensitized the α1-AR response.

Conclusion

The observed PIR to OR-1896 in rat ventricular strips is mediated through PDE3 inhibition, enhancing cAMP-mediated effects. These results further reinforce our previous finding that Ca2+ sensitization does not play a significant role in the inotropic (and lusitropic) effect of levosimendan, nor of its main metabolite OR-1896.  相似文献   
3.
The bacterium Azotobacter vinelandii produces a family of seven secreted and calcium-dependent mannuronan C-5 epimerases (AlgE1–7). These epimerases are responsible for the epimerization of β-d-mannuronic acid (M) to α-l-guluronic acid (G) in alginate polymers. The epimerases display a modular structure composed of one or two catalytic A-modules and from one to seven R-modules having an activating effect on the A-module. In this study, we have determined the NMR structure of the three individual R-modules from AlgE6 (AR1R2R3) and the overall structure of both AlgE4 (AR) and AlgE6 using small angle x-ray scattering. Furthermore, the alginate binding ability of the R-modules of AlgE4 and AlgE6 has been studied with NMR and isothermal titration calorimetry. The AlgE6 R-modules fold into an elongated parallel β-roll with a shallow, positively charged groove across the module. Small angle x-ray scattering analyses of AlgE4 and AlgE6 show an overall elongated shape with some degree of flexibility between the modules for both enzymes. Titration of the R-modules with defined alginate oligomers shows strong interaction between AlgE4R and both oligo-M and MG, whereas no interaction was detected between these oligomers and the individual R-modules from AlgE6. A combination of all three R-modules from AlgE6 shows weak interaction with long M-oligomers. Exchanging the R-modules between AlgE4 and AlgE6 resulted in a novel epimerase called AlgE64 with increased G-block forming ability compared with AlgE6.  相似文献   
4.
For an ideal polysaccharide gel with a known total polymer chain contour length, crosslinks all of the same functionality and elastic chains all with the same contour length and stiffness, the gel crosslink density can readily be determined from measurements of the maximum volume of the swollen gel (Moe et al., (1991) Food Hydrocolloids, 5, (1/2), 119–123. In the case of randomly crosslinked polysaccharide gels, where the chain contour length between two adjacent crosslinks may vary greatly, it is often much more difficult to determine the crosslink density. This paper reports on an attempt to extend the use of maximum gel volume measurements to estimate crosslink density for the latter type of gel. This is done by calculating the maximum swelling volume for polymer networks with four-functional crosslinks, known elastic chain mean contour length and standard deviation. The numerical analysis involves the calculation of the equilibrium force at each crosslink as the network expands. This allows a detailed study of how the distribution of individual polymer chain contour lengths affects the maximum swelling volume. The computer simulation results are compared with the results from experimental measurements of the maximum volume of swollen covalently crosslinked sodium alginate gels.  相似文献   
5.

Background

Meiotic recombination has traditionally been explained based on the structural requirement to stabilize homologous chromosome pairs to ensure their proper meiotic segregation. Competing hypotheses seek to explain the emerging findings of significant heterogeneity in recombination rates within and between genomes, but intraspecific comparisons of genome-wide recombination patterns are rare. The honey bee (Apis mellifera) exhibits the highest rate of genomic recombination among multicellular animals with about five cross-over events per chromatid.

Results

Here, we present a comparative analysis of recombination rates across eight genetic linkage maps of the honey bee genome to investigate which genomic sequence features are correlated with recombination rate and with its variation across the eight data sets, ranging in average marker spacing ranging from 1 Mbp to 120 kbp. Overall, we found that GC content explained best the variation in local recombination rate along chromosomes at the analyzed 100 kbp scale. In contrast, variation among the different maps was correlated to the abundance of microsatellites and several specific tri- and tetra-nucleotides.

Conclusions

The combined evidence from eight medium-scale recombination maps of the honey bee genome suggests that recombination rate variation in this highly recombining genome might be due to the DNA configuration instead of distinct sequence motifs. However, more fine-scale analyses are needed. The empirical basis of eight differing genetic maps allowed for robust conclusions about the correlates of the local recombination rates and enabled the study of the relation between DNA features and variability in local recombination rates, which is particularly relevant in the honey bee genome with its exceptionally high recombination rate.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1281-2) contains supplementary material, which is available to authorized users.  相似文献   
6.
In recent decades we have seen rapid and co‐occurring changes in landscape structure, species distributions and even climate as consequences of human activity. Such changes affect the dynamics of the interaction between major forest pest species, such as bark beetles (Coleoptera: Curculionidae, Scolytinae), and their host trees. Normally breeding mostly in broken or severely stressed spruce; at high population densities some bark beetle species can colonise and kill healthy trees on scales ranging from single trees in a stand to multi‐annual landscape‐wide outbreaks. In Eurasia, the largest outbreaks are caused by the spruce bark beetle, Ips typographus (Linnaeus), which is common and shares a wide distribution with its main host, Norway spruce (Picea abies Karst.). A large literature is now available, from which this review aims to synthesize research relevant for the population dynamics of I. typographus and co‐occurring species under changing conditions. We find that spruce bark beetle population dynamics tend to be metastable, but that mixed‐species and age‐heterogeneous forests with good site‐matching tend to be less susceptible to large‐scale outbreaks. While large accumulations of logs should be removed and/or debarked before the next swarming period, intensive removal of all coarse dead wood may be counterproductive, as it reduces the diversity of predators that in some areas may play a role in keeping I. typographus populations below the outbreak threshold, and sanitary logging frequently causes edge effects and root damage, reducing the resistance of remaining trees. It is very hard to predict the outcome of interspecific interactions due to invading beetle species or I. typographus establishing outside its current range, as they can be of varying sign and strength and may fluctuate depending on environmental factors and population phase. Most research indicates that beetle outbreaks will increase in frequency and magnitude as temperature, wind speed and precipitation variability increases, and that mitigating forestry practices should be adopted as soon as possible considering the time lags involved.  相似文献   
7.
Olav  Hogstad 《Ibis》1984,126(3):296-306
A Goldcrest Regulus regulus population living in a spruce-dominated forest near Oslo (c.60°N), in southern Norway, was studied during the winters (November-March) from 1966–67 to 1971–72. A significant reduction in population size occurred between November and March, the recorded values varying from 76 to 96%, with a mean reduction for all six winters studied of 86%. This winter decline in numbers was significantly correlated with both precipitation (i.e., snowfall) and ambient temperature, but not with bird density. Spiders were the predominant prey items of the Goldcrests, and it is suggested that the birds experience food shortages that affect their survival rate. Although not statistically significant, a mean change in the ratio of males:females from 2:1 in October-November to 10:1 in December—February was observed, suggesting that a greater mortality of females than males occurs during the winter.
Partly based on data for colour-banded birds, it is suggested that the Goldcrests form social flocks, which move around as individual units within territories of about 9 ha (7.5 12 ha, n = 5) in extent. The mean flock size decreased significantly from 4.57 ( n = 157) birds in November-December to 2.82 ( n = 74) birds in January-February, probably because of the reductions in the size of the local population during the winter. The mean flock size in November-December varied inversely with the ambient temperature ( r = -0.92).  相似文献   
8.
In this study we investigated hollow oaks (Quercus robur, Q. petrea) situated in open landscapes and in forests in Norway in northern Europe, and compared their importance for rare and threatened beetles (Coleoptera). Old, hollow oak trees, both in parks and in forests, were extremely rich in red-listed beetles, and hosted a high proportion of threatened species. The proportion of oak associated species and the mean number of red-listed beetle species per tree was similar in the two site types, but rarefaction showed that for a certain number of individuals, oaks in forests had more threatened and near-threatened species than oaks in parks. The species composition also differed between site types: Park oaks had a higher proportion of species associated with hollows and animal nests, whereas in forests, there was a higher proportion of species depending on dead oak wood in general. Four factors were significant in explaining the richness of red-listed beetles in our study: Tree circumference, cavity decay stage, proportion of oak in the surroundings, and coarse woody debris (CWD) in the surroundings. Forest oaks were smaller, but they still trapped a species richness comparable to that of the larger park oaks—probably a result of high amounts of CWD in the surroundings. We show that oaks in open landscapes and oaks in forest have only partly overlapping beetle assemblages and, thus, cannot be substituted in conservation. Planning for conservation of red-listed beetles associated with this key habitat demands a large scale perspective, both in space and time, as the surroundings have important effects on associated threatened and near threatened species.  相似文献   
9.
Aim. To identify and discuss validity aspects on so called negative and non-positive studies. Methods. Arguments and examples are drawn from experiences in occupational health epidemiology regarding the interpretation of more or less equivocal study results. Results and conclusions. A negative study may be defined as showing a result that goes against the investigated hypothesis of an increased (or prevented) risk. Traditionally, studies with a risk estimate (relative risk or odds ratio) above but close to unity are also referred to as negative, given a narrow confidence interval (CI) that includes unity. A risk estimate above unity with the CI including unity is non-positive, however, but an estimate below unity with upper CI bond exceeding unity might be seen as possibly negative or non-negative. A weaker “significance” than usually required should perhaps be accepted when evaluating serious hazards. In contrast to positive studies, the negative and non-positive studies tend to escape criticism in spite of questionable validity that may have obscured existing risks (or preventive effects). Even stronger arguments can be made in criticising negative and non-positive studies than positive studies, for example, regarding selection phenomena, and observational problems regarding exposure and outcome. Negative confounding should be considered although usually weak. In case-control studies, so-called over-matching may obscure an existing risk as could the “healthy worker effect” in cohort studies. Small scale non-positive studies should be made available for meta-analyses and when considering studies that do not convincingly show a risk; those who are exposed should be given the “benefit of the doubt.”  相似文献   
10.
Two of the signature genetic events that occur in human gliomas, EGFR amplification and IDH mutation, are poorly represented in experimental models in vitro. EGFR amplification, for example, occurs in 40 to 50% of GBM, and yet, EGFR amplification is rarely preserved in cell cultures derived from human tumors. To analyze the fate of EGFR amplified and IDH mutated cells in culture, we followed the development over time of cultures derived from human xenografts in nude rats enriched for tumor cells with EGFR amplification and of cultures derived from patient samples with IDH mutations, in serum monolayer and spheroid suspension culture, under serum and serum free conditions. We observed under serum monolayer conditions, that nestin positive or nestin and SMA double positive rat stromal cells outgrew EGFR amplified tumor cells, while serum spheroid cultures preserved tumor cells with EGFR amplification. Serum free suspension culture exhibited a more variable cell composition in that the resultant cell populations were either predominantly nestin/SOX2 co-expressing rat stromal cells or human tumor cells, or a mixture of both. The selection for nestin/SMA positive stromal cells under serum monolayer conditions was also consistently observed in human oligodendrogliomas and oligoastrocytomas with IDH mutations. Our results highlight for the first time that serum monolayer conditions can select for stromal cells instead of tumor cells in certain brain tumor subtypes. This result has an important impact on the establishment of new tumor cell cultures from brain tumors and raises the question of the proper conditions for the growth of the tumor cell populations of interest.  相似文献   
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