Body size determines soil community assembly in a tropical forest

Tropical forests shelter an unparalleled biological diversity. The relative influence of environmental selection (i.e., abiotic conditions, biotic interactions) and stochastic–distance‐dependent neutral processes (i.e., demography, dispersal) in shaping communities has been extensively studied for various organisms, but has rarely been explored across a large range of body sizes, in particular in soil environments. We built a detailed census of the whole soil biota in a 12‐ha tropical forest plot using soil DNA metabarcoding. We show that the distribution of 19 taxonomic groups (ranging from microbes to mesofauna) is primarily stochastic, suggesting that neutral processes are prominent drivers of the assembly of these communities at this scale. We also identify aluminium, topography and plant species identity as weak, yet significant drivers of soil richness and community composition of bacteria, protists and to a lesser extent fungi. Finally, we show that body size, which determines the scale at which an organism perceives its environment, predicted the community assembly across taxonomic groups, with soil mesofauna assemblages being more stochastic than microbial ones. These results suggest that the relative contribution of neutral processes and environmental selection to community assembly directly depends on body size. Body size is hence an important determinant of community assembly rules at the scale of the ecological community in tropical soils and should be accounted for in spatial models of tropical soil food webs.     

In vitro and in vivo neurotoxicity of prion protein oligomers

The mechanisms underlying prion-linked neurodegeneration remain to be elucidated, despite several recent advances in this field. Herein, we show that soluble, low molecular weight oligomers of the full-length prion protein (PrP), which possess characteristics of PrP to PrPsc conversion intermediates such as partial protease resistance, are neurotoxic in vitro on primary cultures of neurons and in vivo after subcortical stereotaxic injection. Monomeric PrP was not toxic. Insoluble, fibrillar forms of PrP exhibited no toxicity in vitro and were less toxic than their oligomeric counterparts in vivo. The toxicity was independent of PrP expression in the neurons both in vitro and in vivo for the PrP oligomers and in vivo for the PrP fibrils. Rescue experiments with antibodies showed that the exposure of the hydrophobic stretch of PrP at the oligomeric surface was necessary for toxicity. This study identifies toxic PrP species in vivo. It shows that PrP-induced neurodegeneration shares common mechanisms with other brain amyloidoses like Alzheimer disease and opens new avenues for neuroprotective intervention strategies of prion diseases targeting PrP oligomers.     

An epidemiological study of reproductive function biomarkers in male welders

In a cross-sectional study, the serum concentrations of inhibin B and prolactin of 96 male current welders were compared with the concentrations measured in 96 age-matched referents. Also, 23 patients who were all former welders diagnosed as having welding-related manganism were studied. The current welders' geometric mean (GM) airborne exposure to manganese (Mn) was 121 µg m^-3 (range 7-2320). The serum concentrations of prolactin adjusted for age and smoking habits (GM 193 mIU l^-1 vs. 166 mIU l^-1; p=0.047) and inhibin B adjusted for alcohol consumption (arithmetic mean (AM) 151 ng l^-1 vs. 123 ng l^-1; p=0.001) were higher in the welders compared with the referents. The whole blood Mn concentration was associated with the serum prolactin concentrations. Tobacco smoking resulted in lower serum prolactin concentrations. The GM serum prolactin concentrations of the patients did not significantly differ from that of the referents, but their AM serum inhibin B concentration was statistically significantly lower. The results may suggest an effect of Mn on the pituitary that is reversible upon cessation of exposure. Lower inhibin B concentrations in the patients could point to a functional impairment of the testicular Sertoli cells, that may be caused by a welding fume component or other factors in their work environment.     

Nuclear and chloroplast DNA phylogeography reveals vicariance among European populations of the model species for the study of metal tolerance, Arabidopsis halleri (Brassicaceae)

Arabidopsis halleri is a pseudometallophyte involved in numerous molecular studies of the adaptation to anthropogenic metal stress. In order to test the representativeness of genetic accessions commonly used in these studies, we investigated the A. halleri population genetic structure in Europe. Microsatellite and nucleotide polymorphisms from the nuclear and chloroplast genomes, respectively, were used to genotype 65 populations scattered over Europe. The large-scale population structure was characterized by a significant phylogeographic signal between two major genetic units. The localization of the phylogeographic break was assumed to result from vicariance between large populations isolated in southern and central Europe, on either side of ice sheets covering the Alps during the Quaternary ice ages. Genetic isolation was shown to be maintained in western Europe by the high summits of the Alps, whereas admixture was detected in the Carpathians. Considering the phylogeographic literature, our results suggest a distinct phylogeographic pattern for European species occurring in both mountain and lowland habitats. Considering the evolution of metal adaptation in A. halleri, it appears that recent adaptations to anthropogenic metal stress that have occurred within either phylogeographic unit should be regarded as independent events that potentially have involved the evolution of a variety of genetic mechanisms.     

The essential role of the N-terminal domain of the orange carotenoid protein in cyanobacterial photoprotection: importance of a positive charge for phycobilisome binding

Most cyanobacteria, under high light conditions, decrease the amount of energy arriving at the reaction centers by increasing thermal energy dissipation at the level of the phycobilisome, the extramembranous antenna. This mechanism is induced by photoactivation of the Orange Carotenoid Protein (OCP). To identify how the activated OCP interacts with phycobilisomes (PBs), several OCP mutants were constructed, and the influence of mutations on photoactivity, stability, and binding to PBs was characterized. The disruption of the salt bridge between Arg155 and Glu244, which stabilizes the interaction between the N- and C-terminal domains, increased the rate of photoactivity and the stability of the photoactivated OCP, suggesting that the activated OCP has an open structure with decreased interdomain interaction. Changing Glu244 to leucine had no effect on OCP binding to PBs. By contrast, substitution of Arg155 with a neutral or a negatively charged amino acid largely decreased OCP binding to the PBs, whereas substitution with a lysine slightly perturbed the interaction. These results strongly suggest that the surface of the N-terminal domain, containing the Arg155, interacts with the PB and that the positive charge of Arg155 plays a key role in photoprotection.     

Intracellular hypertonicity is responsible for water flux associated with Na+/glucose cotransport

Detection of a significant transmembrane water flux immediately after cotransporter stimulation is the experimental basis for the controversial hypothesis of secondary active water transport involving a proposed stoichiometry for the human Na(+)/glucose cotransporter (SGLT1) of two Na(+), one glucose, and 264 water molecules. Volumetric measurements of Xenopus laevis oocytes coexpressing human SGLT1 and aquaporin can be used to detect osmotic gradients with high sensitivity. Adding 2 mM of the substrate alpha-methyl-glucose (alphaMG) created mild extracellular hypertonicity and generated a large cotransport current with minimal cell volume changes. After 20, 40, and 60 s of cotransport, the return to sugar-free, isotonic conditions was accompanied by measurable cell swelling averaging 0.051, 0.061, and 0.077 nl/s, respectively. These water fluxes are consistent with internal hypertonicities of 1.5, 1.7, and 2.2 mOsm for these cotransport periods. In the absence of aquaporin, the measured hypertonicites were 4.6, 5.0, and 5.3 mOsm for the same cotransport periods Cotransport-dependent water fluxes, previously assumed to be water cotransport, could be largely explained by hypertonicities of such amplitudes. Using intracellular Na(+) injection and Na(+)-selective electrode, the intracellular diffusion coefficient for Na(+) was estimated at 0.29 +/- 0.03 x 10(-5) cm(2) s(-1). Using the effect of intracellular alphaMG injection on the SGLT1-mediated outward current, the intracellular diffusion coefficient of alphaMG was estimated at 0.15 +/- 0.01 x 10(-5) cm(2) s(-1). Although these intracellular diffusion coefficients are much lower than in free aqueous solution, a diffusion model for a single solute in an oocyte would require a diffusion coefficient three times lower than estimated to explain the local osmolyte accumulation that was experimentally detected. This suggests that either the diffusion coefficients were overestimated, possibly due to the presence of convection, or the diffusion in cytosol of an oocyte is more complex than depicted by a simple model.     

Interactions between GABA-B1 receptors and Kir 3 inwardly rectifying potassium channels

γ-aminobutyric acid (GABA) is the principal inhibitory neurotransmitter in the mammalian brain. It acts via both ionotropic GABA-A and metabotropic GABA-B receptors. We evaluated the interaction of receptors with members of the inwardly rectifying potassium (Kir 3) channel family, which also play an important role in neuronal transmission and membrane excitability. These channels are functionally regulated by GABA-B receptors. Possible physical interactions between GABA-B receptor and Kir 3 channels expressed in HEK cells were evaluated using Bioluminescence Resonance Energy Transfer (BRET) experiments, co-immunoprecipitation and confocal microscopy. Our data indicate that Kir 3 channels and Gβγ subunits can interact with the GABA-B₁ subunits independently of the GABA-B₂ subunit or Kir 3.4 which are ultimately responsible for their targetting to the cell surface. Thus signalling complexes containing GABA-B receptors, G proteins and Kir channels are formed shortly after biosynthesis most likely in the endoplasmic reticulum.     

From analog to digital: exploring cell dynamics with single quantum dots

Semiconductor quantum dots (QDs) have emerged as new fluorescent probes for biology. When combined with ultrasensitive optical techniques, they allow motions of individual biomolecules to be tracked in live cells with high signal-to-noise and over unprecedented durations. Single QD imaging readily offers a powerful tool to investigate the organization in cell membranes. Altogether QDs will contribute to more advanced biological imaging and enable new studies on the dynamics of cellular processes.Robert Feulgen Lecture 2005 presented at the Joint Meeting of the Society for Histochemistry and The Histochemical Society in Noordwijkerhout, The Netherlands     

Functional organisation and gain of activity: the case of the antibacterial tetra-para-guanidinoethyl-calix[4]arene

The antibacterial activities of the para-guanidinoethylphenol and of its cyclic tetramer, the tetra-para-guanidinoethyl-calix[4]arene, have been evaluated on reference gram-positive and gram-negative bacteria. Antibiotic disk diffusion assays completed by micromethod technique were employed to determine if a synergistic effect could be expected from the spatial organisation of the monomer into its cyclic tetrameric analogue. Disk diffusion assays and microdilution experiments revealed better properties for the calixarene species, with a real and important gain of activity with regards to the monomer.