A heat wave and dispersal cause dominance shift and decrease biomass in experimental metacommunities

In experimental metacommunities with marine benthic microalgae, we tested whether heat stress changes effects of connectivity and habitat heterogeneity on metacommunity structure and functioning, by manipulating a simulated heat wave, dispersal frequency and a light intensity gradient. We found that all measures of mean local and regional diversity and community biomass significantly declined after the heat wave and showed no sign of recovery. Additionally, dispersal decreased diversity and increased dominance in both the heat stressed and control communities. Together the heat wave and high dispersal frequency induced a dominance shift by spreading a temperature tolerant but low yielding species from its source patches with low light intensity across the metacommunity, an effect that increased with time. Although different species became dominant at high dispersal frequency with and without the heat wave, the shift towards a temperature tolerant species was not sufficient to maintain total community biomass. Thus, short‐term disturbance may cause longer‐term loss of ecosystem function due to dominance shifts in the composition of communities. This study illustrates the importance of employing multispecies approaches when attempting to predict responses of communities to environmental changes.     

The sushi domains of secreted GABA(B1) isoforms selectively impair GABA(B) heteroreceptor function

GABA(B) receptors are the G-protein-coupled receptors for gamma-aminobutyric acid (GABA), the main inhibitory neurotransmitter in the brain. GABA(B) receptors are promising drug targets for a wide spectrum of psychiatric and neurological disorders. Receptor subtypes exhibit no pharmacological differences and are based on the subunit isoforms GABA(B1a) and GABA(B1b). GABA(B1a) differs from GABA(B1b) in its ectodomain by the presence of a pair of conserved protein binding motifs, the sushi domains (SDs). Previous work showed that selectively GABA(B1a) contributes to heteroreceptors at glutamatergic terminals, whereas both GABA(B1a) and GABA(B1b) contribute to autoreceptors at GABAergic terminals or to postsynaptic receptors. Here, we describe GABA(B1j), a secreted GABA(B1) isoform comprising the two SDs. We show that the two SDs, when expressed as a soluble protein, bind to neuronal membranes with low nanomolar affinity. Soluble SD protein, when added at nanomolar concentrations to dissociated hippocampal neurons or to acute hippocampal slices, impairs the inhibitory effect of GABA(B) heteroreceptors on evoked and spontaneous glutamate release. In contrast, soluble SD protein neither impairs the activity of GABA(B) autoreceptors nor impairs the activity of postsynaptic GABA(B) receptors. We propose that soluble SD protein scavenges an extracellular binding partner that retains GABA(B1a)-containing heteroreceptors in proximity of the presynaptic release machinery. Soluble GABA(B1) isoforms like GABA(B1j) may therefore act as dominant-negative inhibitors of heteroreceptors and control the level of GABA(B)-mediated inhibition at glutamatergic terminals. Of importance for drug discovery, our data also demonstrate that it is possible to selectively impair GABA(B) heteroreceptors by targeting their SDs.     

On the Rho'd: the regulation of membrane protrusions by Rho-GTPases

Cell migration entails the formation of cellular protrusions such as lamellipodia or filopodia, the growth of which is powered by the polymerisation of actin filaments abutting the plasma membrane. Specific Rho-GTPase subfamilies are able to drive different types of protrusions. However, significant crosstalk between Rho-family members and the interplay of distinct Rho-effectors regulating or modulating actin reorganization in protrusions complicate the picture of how precisely they are initiated and maintained. Here, we briefly sketch our current knowledge on structure and dynamics of different protrusions as well as their regulation by Rho-GTPases. We also comment on topical, unresolved controversies in the field, with special emphasis on the interrelation of different protrusion types, and on the composition of the nanomachineries driving them.     

Requirement for Tyrosine Kinase-ERK1/2 Signaling in α1β1 Integrin-Mediated Collagen Matrix Remodeling by Rat Mesangial Cells

Abnormal mesangial extracellular matrix remodeling by mesangial cells (MCs) is the hallmark of progressive glomerulonephritis (GN). We recently showed, using a type I collagen gel contraction assay, that α1β1 integrin-dependent MC adhesion and migration are necessary cell behaviors for collagen matrix remodeling. To further determine the mechanism of α1β1 integrin-mediated collagen remodeling, we studied the signaling pathways of MCs that participate in the regulation of collagen gel contraction. Immunoprecipitation and phosphotyrosine detection revealed that gel contraction is associated with the enhanced activity and phosphorylation of ERK1/2 by MCs. The tyrosine kinase inhibitors herbimycin and genistein inhibited collagen gel contraction dose dependently. Furthermore, targeting ERK1/2 activity with a MEK inhibitor, PD98059, and antisense ERK1/2 hindered gel contraction in a dose-dependent manner. Similar inhibitory effects on gel contraction and ERK1/2 phosphorylation were observed when MC-mediated gel contraction was performed in the presence of function-blocking anti-α1 or anti-β1 integrin antibodies. However, cell adhesion and migration assays indicated that PD98059 and antisense ERK1/2 blocked α1β1 integrin-dependent MC migration, but did not interfere with collagen adhesion, although there was a marked decrease in ERK1/2 phosphorylation and ERK1/2 protein expression in cell adhesion on type I collagen. None of the above could affect membrane expression of α1β1 integrin. These results suggested that ERK1/2 activation is critical for the α1β1 integrin-dependent MC migration necessary for collagen matrix reorganization. We therefore conclude that ERK1/2 may serve as a possible target for pharmacological inhibition of pathological collagen matrix formation in GN.     

Winter diet and foraging range of gentoo penguins (<Emphasis Type="Italic">Pygoscelis papua</Emphasis>) from Kidney Cove,Falkland Islands

The winter diet and foraging range of gentoo penguins, Pygoscelis papua, were studied at Kidney Cove, Falkland Islands. The mean wet mass of the 56 stomach-content samples collected from May to October was 32.7ᇃ.4 g. The diet consisted generally of cephalopods, crustaceans and fish, as well as two other molluscs. Among the six cephalopod species identified, the commercially fished Patagonian squid, Loligo gahi, had the highest abundance and was also the main prey by reconstituted mass (53% of the total reconstituted mass). Lobster krill, Munida gregaria, one of five species of crustaceans, was the most abundant prey species by frequency of occurrence and by number (68% and 60%, respectively). Rock cod, Patagonotothen ramsayi, accounted for the majority of the fish diet with 34% of the total reconstituted mass. Most prey species identified in the winter diet were also abundant diet components during the breeding season. However, the known biology of the prey species and their rate of digestion indicated that, in winter, adults may forage further offshore than during the breeding season. This assumption was supported by the results obtained from two birds satellite-tracked during the study period. Both birds remained mainly in inshore waters and returned frequently ashore, but one penguin foraged up to 276 km from the coast. The differences in the foraging behaviour of the two birds were reflected in significant differences with regard to time spent underwater, distance travelled per day and calculated travelling speed. Furthermore, the progress of cohorts of L. gahi over the winter is consistent with results from life-cycle studies in this region and suggests that birds have been foraging in the feeding grounds of L. gahi.     

Differentially oriented populations of actin filaments generated in lamellipodia collaborate in pushing and pausing at the cell front

Eukaryotic cells advance in phases of protrusion, pause and withdrawal. Protrusion occurs in lamellipodia, which are composed of diagonal networks of actin filaments, and withdrawal terminates with the formation of actin bundles parallel to the cell edge. Using correlated live-cell imaging and electron microscopy, we have shown that actin filaments in protruding lamellipodia subtend angles from 15-90 degrees to the front, and that transitions from protrusion to pause are associated with a proportional increase in filaments oriented more parallel to the cell edge. Microspike bundles of actin filaments also showed a wide angular distribution and correspondingly variable bilateral polymerization rates along the cell front. We propose that the angular shift of filaments in lamellipodia serves in adapting to slower protrusion rates while maintaining the filament densities required for structural support; further, we suggest that single filaments and microspike bundles contribute to the construction of the lamella behind and to the formation of the cell edge when protrusion ceases. Our findings provide an explanation for the variable turnover dynamics of actin filaments in lamellipodia observed by fluorescence speckle microscopy and are inconsistent with a current model of lamellipodia structure that features actin filaments branching at 70 degrees in a dendritic array.     

Major Changes in the Ecology of the Wadden Sea: Human Impacts, Ecosystem Engineering and Sediment Dynamics

Shallow soft-sediment systems are mostly dominated by species that, by strongly affecting sediment dynamics, modify their local environment. Such ecosystem engineering species can have either sediment-stabilizing or sediment-destabilizing effects on tidal flats. They interplay with abiotic forcing conditions (wind, tide, nutrient inputs) in driving the community structure and generating spatial heterogeneity, determining the composition of different communities of associated species, and thereby affecting the channelling of energy through different compartments in the food web. This suggests that, depending on local species composition, tidal flats may have conspicuously different geomorphology and biological functions under similar external conditions. Here we use a historical reconstruction of benthic production in the Wadden Sea to construct a framework for the relationships between human impacts, ecosystem engineering and sediment dynamics. We propose that increased sediment disturbances by human exploitation interfere with biological controls of sediment dynamics, and thereby have shifted the dominant compartments of both primary and secondary production in the Wadden Sea, transforming the intertidal from an internally regulated and spatially heterogeneous, to an externally regulated and spatially homogenous system. This framework contributes to the general understanding of the interaction between biological and environmental control of ecosystem functioning, and suggests a general framework for predicting effects of human impacts on soft-bottom ecosystems.