Plant community composition and species richness in the High Arctic tundra: From the present to the future

Arctic plant communities are altered by climate changes. The magnitude of these alterations depends on whether species distributions are determined by macroclimatic conditions, by factors related to local topography, or by biotic interactions. Our current understanding of the relative importance of these conditions is limited due to the scarcity of studies, especially in the High Arctic. We investigated variations in vascular plant community composition and species richness based on 288 plots distributed on three sites along a coast‐inland gradient in Northeast Greenland using a stratified random design. We used an information theoretic approach to determine whether variations in species richness were best explained by macroclimate, by factors related to local topography (including soil water) or by plant‐plant interactions. Latent variable models were used to explain patterns in plant community composition. Species richness was mainly determined by variations in soil water content, which explained 35% of the variation, and to a minor degree by other variables related to topography. Species richness was not directly related to macroclimate. Latent variable models showed that 23.0% of the variation in community composition was explained by variables related to topography, while distance to the inland ice explained an additional 6.4 %. This indicates that some species are associated with environmental conditions found in only some parts of the coast–inland gradient. Inclusion of macroclimatic variation increased the model's explanatory power by 4.2%. Our results suggest that the main impact of climate changes in the High Arctic will be mediated by their influence on local soil water conditions. Increasing temperatures are likely to cause higher evaporation rates and alter the distribution of late‐melting snow patches. This will have little impact on landscape‐scale diversity if plants are able to redistribute locally to remain in areas with sufficient soil water.     

Fragment-based discovery of hepatitis C virus NS5b RNA polymerase inhibitors

Non-nucleoside inhibitors of HCV NS5b RNA polymerase were discovered by a fragment-based lead discovery approach, beginning with crystallographic fragment screening. The NS5b binding affinity and biochemical activity of fragment hits and inhibitors was determined by surface plasmon resonance (Biacore) and an enzyme inhibition assay, respectively. Crystallographic fragment screening hits with 1–10 mM binding affinity (K_D) were iteratively optimized to give leads with 200 nM biochemical activity and low μM cellular activity in a Replicon assay.     

Helix α4 of the Bacillus thuringiensis Cry1Aa Toxin Plays a Critical Role in the Postbinding Steps of Pore Formation

Helix α4 of Bacillus thuringiensis Cry toxins is thought to play a critical role in the toxins'' mode of action. Accordingly, single-site substitutions of many Cry1Aa helix α4 amino acid residues have previously been shown to cause substantial reductions in the protein''s pore-forming activity. Changes in protein structure and formation of intermolecular disulfide bonds were investigated as possible factors responsible for the inactivity of these mutants. Incubation of each mutant with trypsin and chymotrypsin for 12 h did not reveal overt structural differences with Cry1Aa, although circular dichroism was slightly decreased in the 190- to 210-nm region for the I132C, S139C, and V150C mutants. The addition of dithiothreitol stimulated pore formation by the E128C, I132C, S139C, T142C, I145C, P146C, and V150C mutants. However, in the presence of these mutants, the membrane permeability never reached that measured for Cry1Aa, indicating that the formation of disulfide bridges could only partially explain their loss of activity. The ability of a number of inactive mutants to compete with wild-type Cry1Aa for pore formation in brush border membrane vesicles isolated from Manduca sexta was also investigated with an osmotic swelling assay. With the exception of the L147C mutant, all mutants tested could inhibit the formation of pores by Cry1Aa, indicating that they retained receptor binding ability. These results strongly suggest that helix α4 is involved mainly in the postbinding steps of pore formation.During the last few decades, the insecticidal toxins produced by Bacillus thuringiensis have been used increasingly in the forms of formulated sprays and transgenic plants for the highly focused biological control of insect pests (29). At the same time, the mechanism by which these proteins form pores in the apical membrane of midgut epithelial cells of targeted insects has been studied extensively (7, 29). In the case of the three-domain Cry toxins, specificity is mostly attributable to their capacity to bind to certain proteins located on the surface of the intestinal membrane through specific segments of domains II and III, composed mainly of β sheets (16, 27). On the other hand, membrane insertion and pore formation are thought to occur through elements of domain I, composed of a bundle of six amphipathic α-helices surrounding the highly hydrophobic helix α5 (17, 20).Several lines of evidence indicate that helices α4 and α5 play a particularly important role in these processes (3). Spectroscopic studies with synthetic peptides corresponding to domain I helices revealed that α4 and α5 have the greatest propensity for insertion into artificial membranes, although insertion and pore formation were most efficient when α4 and α5 were connected by a segment corresponding to the α4-α5 loop of the toxin (13, 14). A particularly large number of single-site mutations with altered amino acids from these helices, which lead to a strong reduction in the toxicity and pore-forming ability of the toxin, have been characterized (2, 9, 10, 15, 18, 23, 25, 30, 31, 33). Finally, a site-directed chemical modification study has provided strong evidence that α4 lines the lumens of the pores formed by the toxin (23).Recent studies have established that toxin activity is especially sensitive to modifications not only in the charged residues of α4 (31) but in most of its hydrophilic residues (15). Furthermore, the loss of activity of most of these mutants did not result from an altered selectivity or size of the pores but from a reduced pore-forming capacity of the toxin (15, 31). In order to better understand the role of α4 in the mechanism of pore formation, the present study was carried out with a series of previously characterized Cry1Aa mutants in which most of the residues from this helix were replaced by cysteines (15). By subjecting these mutants to circular dichroism (CD), protease sensitivity, pore formation inhibition, and electrophoretic mobility analyses, our data suggest that the mutations in α4 which alter the pore-forming ability of Cry1Aa do so mainly by preventing the proper oligomerization or membrane insertion of the toxin.     

Multiple reprobing of Western blots after inactivation of peroxidase activity by its substrate, hydrogen peroxide

Sequential detections of different proteins on Western blot save time and precious samples. The main problem concerning reprobing is that stripping buffers can unbind both the antibody and the tested antigen. An original reprobing method has been set up based on horseradish peroxidase (HRP) inhibition after enhanced chemiluminescence detection. Instead of removing previously fixed antibodies as common stripping buffers do, the HRP activity linked to the secondary antibody is irreversibly inhibited by excess of hydrogen peroxide. A 15-min incubation allows one to perform at least five different sequential detections without losing significant amounts of blotted proteins.     

Importance of abiotic stress as a range-limit determinant for European plants: insights from species responses to climatic gradients

Aim We examined whether species occurrences are primarily limited by physiological tolerance in the abiotically more stressful end of climatic gradients (the asymmetric abiotic stress limitation (AASL) hypothesis) and the geographical predictions of this hypothesis: abiotic stress mainly determines upper‐latitudinal and upper‐altitudinal species range limits, and the importance of abiotic stress for these range limits increases the further northwards and upwards a species occurs. Location Europe and the Swiss Alps. Methods The AASL hypothesis predicts that species have skewed responses to climatic gradients, with a steep decline towards the more stressful conditions. Based on presence–absence data we examined the shape of plant species responses (measured as probability of occurrence) along three climatic gradients across latitudes in Europe (1577 species) and altitudes in the Swiss Alps (284 species) using Huisman–Olff–Fresco, generalized linear and generalized additive models. Results We found that almost half of the species from Europe and one‐third from the Swiss Alps showed responses consistent with the predictions of the AASL hypothesis. Cold temperatures and a short growing season seemed to determine the upper‐latitudinal and upper‐altitudinal range limits of up to one‐third of the species, while drought provided an important constraint at lower‐latitudinal range limits for up to one‐fifth of the species. We found a biome‐dependent influence of abiotic stress and no clear support for abiotic stress as a stronger upper range‐limit determinant for species with higher latitudinal and altitudinal distributions. However, the overall influence of climate as a range‐limit determinant increased with latitude. Main conclusions Our results support the AASL hypothesis for almost half of the studied species, and suggest that temperature‐related stress controls the upper‐latitudinal and upper‐altitudinal range limits of a large proportion of these species, while other factors including drought stress may be important at the lower range limits.     

Discrete and Effortful Imagined Movements Do Not Specifically Activate the Autonomic Nervous System

Background

The autonomic nervous system (ANS) is activated in parallel with the motor system during cyclical and effortful imagined actions. However, it is not clear whether the ANS is activated during motor imagery of discrete movements and whether this activation is specific to the movement being imagined. Here, we explored these topics by studying the baroreflex control of the cardiovascular system.

Methodology/Principal Findings

Arterial pressure and heart rate were recorded in ten subjects who executed or imagined trunk or leg movements against gravity. Trunk and leg movements result in different physiological reactions (orthostatic hypotension phenomenon) when they are executed. Interestingly, ANS activation significantly, but similarly, increased during imagined trunk and leg movements. Furthermore, we did not observe any physiological modulation during a control mental-arithmetic task or during motor imagery of effortless movements (horizontal wrist displacements).

Conclusions/Significance

We concluded that ANS activation during motor imagery is general and not specific and physiologically prepares the organism for the upcoming effortful action.     

Dimethylthiourea protects against chlorine induced changes in airway function in a murine model of irritant induced asthma

Background

Exposure to chlorine (Cl₂) causes airway injury, characterized by oxidative damage, an influx of inflammatory cells and airway hyperresponsiveness. We hypothesized that Cl₂-induced airway injury may be attenuated by antioxidant treatment, even after the initial injury.

Methods

Balb/C mice were exposed to Cl₂ gas (100 ppm) for 5 mins, an exposure that was established to alter airway function with minimal histological disruption of the epithelium. Twenty-four hours after exposure to Cl₂, airway responsiveness to aerosolized methacholine (MCh) was measured. Bronchoalveolar lavage (BAL) was performed to determine inflammatory cell profiles, total protein, and glutathione levels. Dimethylthiourea (DMTU;100 mg/kg) was administered one hour before or one hour following Cl₂ exposure.

Results

Mice exposed to Cl₂ had airway hyperresponsiveness to MCh compared to control animals pre-treated and post-treated with DMTU. Total cell counts in BAL fluid were elevated by Cl₂ exposure and were not affected by DMTU treatment. However, DMTU-treated mice had lower protein levels in the BAL than the Cl₂-only treated animals. 4-Hydroxynonenal analysis showed that DMTU given pre- or post-Cl₂ prevented lipid peroxidation in the lung. Following Cl₂ exposure glutathione (GSH) was elevated immediately following exposure both in BAL cells and in fluid and this change was prevented by DMTU. GSSG was depleted in Cl₂ exposed mice at later time points. However, the GSH/GSSG ratio remained high in chlorine exposed mice, an effect attenuated by DMTU.

Conclusion

Our data show that the anti-oxidant DMTU is effective in attenuating Cl₂ induced increase in airway responsiveness, inflammation and biomarkers of oxidative stress.