Low-dose ghrelin infusion--evidence against a hormonal role in food intake

Ghrelin is the only peripheral orexigenic peptide of gastrointestinal origin. Its preprandial increase is supposed to initiate food intake. This assumption is based on studies with intravenously infused ghrelin in rather high doses and the correlation between ghrelin levels and hunger sensations. As yet it is unclear whether or not low dose ghrelin resulting in physiological and moderately supraphysiological plasma levels has an effect on hunger sensations, the wish for food intake and / or the quantity of the meal consumed. We examined 20 normal-weight males (age 25±1.7 years, BMI 24±0.5 kg/m(2)) in a prospective double-blind randomized fashion. On two different days they obtained a ghrelin infusion 1 ng/kg/min or intravenous saline starting one hour after a standardized meal. Hunger and satiety ratings were documented by visual analogue scales. A second meal was served on demand and consumed until feeling satiated. Time point of the second meal as well as ingested calories were registered. Prior to the start of i.v. ghrelin the postprandial decrease of active plasma ghrelin by 30 pg/ml was comparable. In the controls the postprandial reduction was significant until 210 min compared to basal. With i.v. ghrelin basal levels were reached within 10 min. The maximal rise was twice basal. No effect was observed on hunger and satiety ratings. The time period between the meals and the food quantity of the second meal were similar. During ghrelin infusion glucose and growth hormone but not insulin and cortisol levels were significantly higher after the second meal compared to saline. The present data demonstrate for the first time the effect of a low dose ghrelin infusion on food intake. Neither physiological nor moderably supraphysiological ghrelin levels were associated with any change of the various food intake parameters determined. These data do not favour a hormonal role of peripheral ghrelin in the regulation of food intake.     

<Emphasis Type="Italic">Listeria</Emphasis> bacteriophage peptidoglycan hydrolases feature high thermoresistance and reveal increased activity after divalent metal cation substitution

The ability of the bacteriophage-encoded peptidoglycan hydrolases (endolysins) to destroy Gram-positive bacteria from without makes these enzymes promising antimicrobials. Recombinant endolysins from Listeria monocytogenes phages have been shown to rapidly lyse and kill the pathogen in all environments. To determine optimum conditions regarding application of recombinant Listeria phage endolysins in food or production equipments, properties of different Listeria endolysins were studied. Optimum NaCl concentration for the amidase HPL511 was 200 nM and 300 mM for the peptidases HPL118, HPL500, and HPLP35. Unlike most other peptidoglycan hydrolases, all four enzymes exhibited highest activity at elevated pH values at around pH 8–9. Lytic activity was abolished by EDTA and could be restored by supplementation with various divalent metal cations, indicating their role in catalytic function. While substitution of the native Zn²⁺ by Ca²⁺ or Mn²⁺ was most effective in case of HPL118, HPL500, and HPLP35, supplementation with Co²⁺ and Mn²⁺ resulted in an approximately 5-fold increase in HPL511 activity. Interestingly, the glutamate peptidases feature a conserved SxHxxGxAxD zinc-binding motif, which is not present in the amidases, although they also require centrally located divalent metals for activity. The endolysins HPL118, HPL511, and HPLP35 revealed a surprisingly high thermostability, with up to 35% activity remaining after 30 min incubation at 90°C. The available data suggest that denaturation at elevated temperatures is reversible and may be followed by rapid refolding into a functional state.     

Chloroplast ultrastructure and thylakoid polypeptide composition are affected by different salt concentrations in the halophytic plant Arthrocnemum macrostachyum

The effect of different external salt concentrations, from 0 mM to 1030 mM NaCl, on photosynthetic complexes and chloroplast ultrastructure in the halophyte Arthrocnemum macrostachyum was studied. Photosystem II, but not Photosystem I or cytochrome b6/f, was affected by salt treatment. We found that the PsbQ protein was never expressed, whereas the amounts of PsbP and PsbO were influenced by salt in a complex way. Analyses of Photosystem II intrinsic proteins showed an uneven degradation of subunits with a loss of about 50% of centres in the 0 mM NaCl treated sample. Also the shape of chloroplasts, as well as the organization of thylakoid membranes were affected by NaCl concentration, with many grana containing few thylakoids at 1030 mM NaCl and thicker grana and numerous swollen thylakoids at 0 mM NaCl. The PsbQ protein was found to be depleted also in thylakoids from other halophytes.     

Blockade of the activin receptor IIb activates functional brown adipogenesis and thermogenesis by inducing mitochondrial oxidative metabolism

Brown adipose tissue (BAT) is a key tissue for energy expenditure via fat and glucose oxidation for thermogenesis. In this study, we demonstrate that the myostatin/activin receptor IIB (ActRIIB) pathway, which serves as an important negative regulator of muscle growth, is also a negative regulator of brown adipocyte differentiation. In parallel to the anticipated hypertrophy of skeletal muscle, the pharmacological inhibition of ActRIIB in mice, using a neutralizing antibody, increases the amount of BAT without directly affecting white adipose tissue. Mechanistically, inhibition of ActRIIB inhibits Smad3 signaling and activates the expression of myoglobin and PGC-1 coregulators in brown adipocytes. Consequently, ActRIIB blockade in brown adipose tissue enhances mitochondrial function and uncoupled respiration, translating into beneficial functional consequences, including enhanced cold tolerance and increased energy expenditure. Importantly, ActRIIB inhibition enhanced energy expenditure only at ambient temperature or in the cold and not at thermoneutrality, where nonshivering thermogenesis is minimal, strongly suggesting that brown fat activation plays a prominent role in the metabolic actions of ActRIIB inhibition.     

The role of attention in figure-ground segregation in areas V1 and V4 of the visual cortex

Our visual system segments images into objects and background. Figure-ground segregation relies on the detection of feature discontinuities that signal boundaries between the figures and the background and on a complementary region-filling process that groups together image regions with similar features. The neuronal mechanisms for these processes are not well understood and it is unknown how they depend on visual attention. We measured neuronal activity in V1 and V4 in a task where monkeys either made an eye movement to texture-defined figures or ignored them. V1 activity predicted the timing and the direction of the saccade if the figures were task relevant. We found that boundary detection is an early process that depends little on attention, whereas region filling occurs later and is facilitated by visual attention, which acts in an object-based manner. Our findings are explained by a model with local, bottom-up computations for boundary detection and feedback processing for region filling.     

Modeling the influence of optic flow on grid cell firing in the absence of other cues1

Information from the vestibular, sensorimotor, or visual systems can affect the firing of grid cells recorded in entorhinal cortex of rats. Optic flow provides information about the rat’s linear and rotational velocity and, thus, could influence the firing pattern of grid cells. To investigate this possible link, we model parts of the rat’s visual system and analyze their capability in estimating linear and rotational velocity. In our model a rat is simulated to move along trajectories recorded from rat’s foraging on a circular ground platform. Thus, we preserve the intrinsic statistics of real rats’ movements. Visual image motion is analytically computed for a spherical camera model and superimposed with noise in order to model the optic flow that would be available to the rat. This optic flow is fed into a template model to estimate the rat’s linear and rotational velocities, which in turn are fed into an oscillatory interference model of grid cell firing. Grid scores are reported while altering the flow noise, tilt angle of the optical axis with respect to the ground, the number of flow templates, and the frequency used in the oscillatory interference model. Activity patterns are compatible with those of grid cells, suggesting that optic flow can contribute to their firing.     

Climate change in size-structured ecosystems

One important aspect of climate change is the increase in average temperature, which will not only have direct physiological effects on all species but also indirectly modifies abundances, interaction strengths, food-web topologies, community stability and functioning. In this theme issue, we highlight a novel pathway through which warming indirectly affects ecological communities: by changing their size structure (i.e. the body-size distributions). Warming can shift these distributions towards dominance of small- over large-bodied species. The conceptual, theoretical and empirical research described in this issue, in sum, suggests that effects of temperature may be dominated by changes in size structure, with relatively weak direct effects. For example, temperature effects via size structure have implications for top-down and bottom-up control in ecosystems and may ultimately yield novel communities. Moreover, scaling up effects of temperature and body size from physiology to the levels of populations, communities and ecosystems may provide a crucially important mechanistic approach for forecasting future consequences of global warming.     

Effect of thymoquinone on cytosolic pH and Na+/H+ exchanger activity in mouse dendritic cells

The anti-inflammatory Nigella sativa component thymoquinone compromises the function of dendritic cells (DCs), key players in the regulation of innate and adaptive immunity. DC function is regulated by the Na(+)/H(+) exchanger (NHE), which is stimulated by lipopolysaccharides (LPS) and required for LPS-induced cell swelling, reactive oxygen species (ROS) production, TNF-α release and migration. Here we explored, whether thymoquinone influences NHE activity in DCs. To this end, bone marrow derived mouse DCs were treated with LPS in the absence and presence of thymoquinone (10 μM). Cytosolic pH (pH(i)) was determined from 2',7'-bis-(2-carboxyethyl)-5-(and-6)-carboxyfluorescein (BCECF) fluorescence, NHE activity from the Na(+)-dependent realkalinization following an ammonium pulse, cell volume from forward scatter in FACS analysis, ROS production from 2',7'-dichlorodihydrofluorescein diacetate (DCFDA) fluorescence, TNF-α production utilizing ELISA and DC migration with transwell migration assays. As a result, exposure of DCs to LPS (1 μg/ml) led within 4 hours to transient increase of NHE activity. Thymoquinone did not significantly modify cytosolic pH or cellular NHE activity in the absence of LPS, but abrogated the effect of LPS on NHE activity. Accordingly, in the presence of thymoquinone LPS-treatment resulted in cytosolic acidification. LPS further increased forward scatter and ROS formation, effects similarly abrogated by thymoquinone. Again, in the absence of LPS, thymoquinone did not significantly modify ROS formation and cell volume. LPS further triggered TNF-α release and migration, effects again blunted in the presence of thymoquinone. NHE1 inhibitor cariporide (10 μM) blunted LPS induced TNF-α release and migration. The effects of thymoquinone on NHE activity and migration were reversed upon treatment of the cells with t-butyl hydroperoxide (TBOOH, 5 μM). In conclusion, thymoquinone blunts LPS induced NHE activity, cell swelling, oxidative burst, cytokine release and migration of bone marrow derived murine dendritic cells. NHE inhibition may thus contribute to the antiinflammatory action of thymoquinone.     

Computational analysis of plasmepsin IV bound to an allophenylnorstatine inhibitor

The plasmepsin proteases from the malaria parasite Plasmodium falciparum are attracting attention as putative drug targets. A recently published crystal structure of Plasmodium malariae plasmepsin IV bound to an allophenylnorstatine inhibitor [Clemente, J.C. et al. (2006) Acta Crystallogr. D 62, 246-252] provides the first structural insights regarding interactions of this family of inhibitors with plasmepsins. The compounds in this class are potent inhibitors of HIV-1 protease, but also show nM binding affinities towards plasmepsin IV. Here, we utilize automated docking, molecular dynamics and binding free energy calculations with the linear interaction energy LIE method to investigate the binding of allophenylnorstatine inhibitors to plasmepsin IV from two different species. The calculations yield excellent agreement with experimental binding data and provide new information regarding protonation states of active site residues as well as conformational properties of the inhibitor complexes.