The interaction of leucocidin with the cell membrane of the polymorphonuclear leucocyte

1. When leucocidin is incubated with leucocytes it is inactivated in solution and only a little adsorption takes place. This reaction has been used to purify the cell membrane. 2. The interaction of the membrane with leucocidin is very complex and at least three phenomena occur: (a) An inactivation of leucocidin in solution by large amounts of membrane which is synergistic between the two components of leucocidin, is thermolabile and is not inhibited by electrolyte. (b) An adsorption of leucocidin which is synergistic between the two components of leucocidin, does not proceed to the same extent as the inactivation in solution and is a function of the phospholipid components. Phospholipids isolated from the membrane adsorb leucocidin but the adsorption requires the presence of several molecular species. (c) Polymerization of leucocidin induced by tenfold smaller amounts of membrane than are required to bring about the first two interactions. The polymerization is reversed by adjustment of the ionic strength. It is due to the presence of the lipid components of the membrane. Different lipids are equally effective in inducing the polymerization. 3. Each component of leucocidin will polymerize in the absence of membranes and lose biological activity at low ionic strength. This is reversed by electrolyte and it does not proceed to the same extent as in the presence of membranes. 4. The nature of the interaction of leucocidin with cells, membranes and lipids and the spontaneous polymerization indicate that each component of leucocidin can adopt different isomeric forms. 5. The relationship of the interaction with the membrane to the cytotoxic effect of leucocidin is discussed.     

Natural occurrence of microbial sulphur oxidation by long-range electron transport in the seafloor

Recently, a novel mode of sulphur oxidation was described in marine sediments, in which sulphide oxidation in deeper anoxic layers was electrically coupled to oxygen reduction at the sediment surface. Subsequent experimental evidence identified that long filamentous bacteria belonging to the family Desulfobulbaceae likely mediated the electron transport across the centimetre-scale distances. Such long-range electron transfer challenges some long-held views in microbial ecology and could have profound implications for sulphur cycling in marine sediments. But, so far, this process of electrogenic sulphur oxidation has been documented only in laboratory experiments and so its imprint on the seafloor remains unknown. Here we show that the geochemical signature of electrogenic sulphur oxidation occurs in a variety of coastal sediment environments, including a salt marsh, a seasonally hypoxic basin, and a subtidal coastal mud plain. In all cases, electrogenic sulphur oxidation was detected together with an abundance of Desulfobulbaceae filaments. Complementary laboratory experiments in intertidal sands demonstrated that mechanical disturbance by bioturbating fauna destroys the electrogenic sulphur oxidation signal. A survey of published geochemical data and 16S rRNA gene sequences identified that electrogenic sulphide oxidation is likely present in a variety of marine sediments with high sulphide generation and restricted bioturbation, such as mangrove swamps, aquaculture areas, seasonally hypoxic basins, cold sulphide seeps and possibly hydrothermal vent environments. This study shows for the first time that electrogenic sulphur oxidation occurs in a wide range of marine sediments and that bioturbation may exert a dominant control on its natural distribution.     

Combination of herbivore removal and nitrogen deposition increases upland carbon storage

Ecosystem carbon (C) accrual and storage can be enhanced by removing large herbivores as well as by the fertilizing effect of atmospheric nitrogen (N) deposition. These drivers are unlikely to operate independently, yet their combined effect on aboveground and belowground C storage remains largely unexplored. We sampled inside and outside 19 upland grazing exclosures, established for up to 80 years, across an N deposition gradient (5–24 kg N ha^?1 yr^?1) and found that herbivore removal increased aboveground plant C stocks, particularly in moss, shrubs and litter. Soil C storage increased with atmospheric N deposition, and this was moderated by the presence or absence of herbivores. In exclosures receiving above 11 kg N ha^?1 year^?1, herbivore removal resulted in increased soil C stocks. This effect was typically greater for exclosures dominated by dwarf shrubs (Calluna vulgaris) than by grasses (Molinia caerulea). The same pattern was observed for ecosystem C storage. We used our data to predict C storage for a scenario of removing all large herbivores from UK heathlands. Predictions were made considering herbivore removal only (ignoring N deposition) and the combined effects of herbivore removal and current N deposition rates. Predictions including N deposition resulted in a smaller increase in UK heathland C storage than predictions using herbivore removal only. This finding was driven by the fact that the majority of UK heathlands receive low N deposition rates at which herbivore removal has little effect on C storage. Our findings demonstrate the crucial link between herbivory by large mammals and atmospheric N deposition, and this interaction needs to be considered in models of biogeochemical cycling.     

Disinhibition Mediates a Form of Hippocampal Long-Term Potentiation in Area CA1

The hippocampus plays a central role in memory formation in the mammalian brain. Its ability to encode information is thought to depend on the plasticity of synaptic connections between neurons. In the pyramidal neurons constituting the primary hippocampal output to the cortex, located in area CA1, firing of presynaptic CA3 pyramidal neurons produces monosynaptic excitatory postsynaptic potentials (EPSPs) followed rapidly by feedforward (disynaptic) inhibitory postsynaptic potentials (IPSPs). Long-term potentiation (LTP) of the monosynaptic glutamatergic inputs has become the leading model of synaptic plasticity, in part due to its dependence on NMDA receptors (NMDARs), required for spatial and temporal learning in intact animals. Using whole-cell recording in hippocampal slices from adult rats, we find that the efficacy of synaptic transmission from CA3 to CA1 can be enhanced without the induction of classic LTP at the glutamatergic inputs. Taking care not to directly stimulate inhibitory fibers, we show that the induction of GABAergic plasticity at feedforward inhibitory inputs results in the reduced shunting of excitatory currents, producing a long-term increase in the amplitude of Schaffer collateral-mediated postsynaptic potentials. Like classic LTP, disinhibition-mediated LTP requires NMDAR activation, suggesting a role in types of learning and memory attributed primarily to the former and raising the possibility of a previously unrecognized target for therapeutic intervention in disorders linked to memory deficits, as well as a potentially overlooked site of LTP expression in other areas of the brain.     

Predicting Habitat Utilization and Extent of Ecosystem Disturbance by an Increasing Herbivore Population

Herbivory can lead to shifts in ecosystem state or changes in ecosystem functioning, and recovery from herbivory is particularly slow in disturbance-sensitive ecosystems such as arctic tundra. Herbivore impacts on ecosystems are variable in space and time due to population fluctuations and selective utilization of habitats; thus there is a need to accurately predict herbivore impacts at the landscape scale. The habitat utilization and extent of disturbance caused by increasing populations of pink-footed geese (Anser brachyrhynchus) foraging in the high arctic tundra of Svalbard were assessed using a predictive model of the population’s habitat use. Pink-footed geese arrive in Svalbard in early spring when they forage for belowground plant parts; this foraging (called grubbing) can cause vegetation loss and soil disturbance. Surveys of the extent and intensity of grubbing were carried out to develop predictive models that were subsequently tested against data collected during the following year from different areas. Both habitat type at a particular point and the amount of preferred fen habitat in the surrounding area were powerful predictors of grubbing likelihood and the developed model correctly classified over 69% of validation observations with an AUC of 0.75. Pink-footed geese showed a strong preference for wetter habitats within low-lying landscapes. Extrapolation of the predictive model across the archipelago showed that a maximum potential area of 2300 km² (3.8% of the archipelago) could be disturbed by grubbing. Thus, increasing populations of geese may cause large-scale vegetation loss and soil disturbance in arctic ecosystems.     

Nitrogen transfer between herbivores and their forage species

Herbivores may increase the productivity of forage plants; however, this depends on the return of nutrients from faeces to the forage plants. The aim of this study was to test if nitrogen (N) from faeces is available to forage plants and whether the return of nutrients differs between plant species using ¹⁵N natural abundance in faeces and plant tissue. To investigate the effect of grazing on N transfer, we carried out a grazing experiment in wet and mesic tundra on high Arctic Spitsbergen using barnacle geese (Branta leucopsis) as the model herbivore. N inputs (from faeces) increased with grazing pressure at both the wet and mesic sites, with the greatest N input from faeces at the wet site. The δ¹⁵N ratio in plant tissue from grazed plots was enriched in mosses and the dwarf shrub species, reflecting the δ¹⁵N signature of faeces-derived N, but no such pattern was observed in the dominant grasses. This study demonstrates that the δ¹⁵N signature of faeces and forage species is a useful tool to explore how grazing impacts on N acquisition. Our findings suggest that plant species which acquire their N close to the soil surface (e.g. mosses) access more of the N from faeces than species with deeper root systems (e.g. grasses) suggesting a transfer of N from the preferred forage species to the mosses and dwarf shrubs, which are less preferred by the geese. In conclusion, the moss layer appears to disrupt the nitrogen return from herbivores to their forage species.     

Catalytic and immunochemical properties of hepatic cytochrome P450 1A in three avian species treated with β-naphthoflavone or isosafrole

Induction of cytochrome P450 1A (CYP1A) can be used as a biomarker of exposure to planar halogenated aromatic hydrocarbons (PHAHs). Our objective was to characterize the induction of CYP1A activity and protein in three avian species following in vivo treatment with β-naphthoflavone (BNF) and/or isosafrole. Alkoxyresorufin-O-dealkylase (alk-ROD) activities of hepatic microsomes from Herring Gulls (Larus argentatus) (HGs), Double-crested Cormorants (Phalacrocorax auritus) (DCCs) and chickens (Gallus domesticus) were measured using ethoxy-, methoxy-, pentoxy- and benzyloxy-resorufin, in the presence and absence of the inhibitors ellipticine or furafylline. Immunoreactivity of microsomal proteins with antibodies to several CYP1A proteins was investigated. CYP1A protein and alk-ROD activities of HGs and DCCs, but not chickens, were induced by isosafrole. Ellipticine was a potent and non-selective inhibitor of alk-ROD activity in all three species, while furafylline inhibition of alk-ROD activities varied among species and treatments. In all three species, BNF induced a protein immunoreactive with monoclonal antibody to CYP1A1 from the marine fish Stenotomus chrysops (scup), but a CYP1A2-like protein was not detected in avian microsomes probed with polyclonal antibodies to mouse CYP1A2. Variations in responses among avian species indicate that CYP1A proteins and substrate specificities should be characterized for each species used in PHAH biomonitoring programs.     

The effect of triisopropyl phosphate on the mobility of surface concanavalin a receptors and on the locomotion of polymorphonuclear leucocytes

The non-phosphorylating organophosphorus compound triisopropyl phosphate, which is known to inhibit rabbit leucocyte locomotion, can stimulate the locomotion of guinea pig leucocytes under certain conditions. Different methods of preparing guinea pig leucocyte monolayers can give preparations with different proportions of motile cells. With preparations that contain relatively slowly moving cells triisopropyl phosphate increases the number of stationary cells without significantly affecting the speed of the cells that remain motile. Most rabbit leucocytes labelled with fluorescein-labelled concanavalin A form caps within 5–10 min at 37 °C. In contrast the rate of cap formation in guinea pig leucocytes is much slower and after 20 min many cells have only random patches. Triisopropyl phosphate accelerates cap formation in guinea pig leucocytes but not in rabbit leucocytes. The local anaesthetic nupercaine inhibits cap and patch formation in rabbit and guinea pig leucocytes. Inhibition of rabbit leucocyte locomotion is induced by concanavalin A at 1 μg/ml. These results are briefly related to the known effects of triisopropyl phosphate on the isolated leucocyte plasma membrane.