Nested bird and micro-habitat assemblages in a peatland archipelago

Biotic assemblages of insular habitats are nested when poor assemblages are subsets of richer ones. Nestedness of species assemblages is frequent and may result from selective extinction or frequent colonization in insular habitats. It may also be created by a nested distribution of habitats among islands or by sampling bias. We sampled 67 isolated peatlands (7–843 ha) in southern Quebec, Canada, to measure nestedness of bird species assemblages among peatlands and assess the habitat nestedness hypothesis. Species and microhabitat assemblages were both strongly nested among peatlands. Whether sites were ranked by species richness, microhabitat richness or peatland area had no effect on nestedness. However, microhabitat nestedness was significantly reduced when sites were sorted by area rather than by microhabitat richness. As expected, if bird-microhabitat associations are responsible for the nested pattern of distribution, we found a positive correlation between the contributions of bird species and microhabitats to individual site nestedness. Nevertheless, microhabitat assemblages were significantly less nested than bird species assemblages, possibly because of frequent recolonization by birds or uneven sampling among sites. Received: 12 June 1998 / Accepted: 20 September 1998     

The crystal structure of RhoA in complex with the DH/PH fragment of PDZRhoGEF, an activator of the Ca(2+) sensitization pathway in smooth muscle

Calcium sensitization in smooth muscle is mediated by the RhoA GTPase, activated by hitherto unspecified nucleotide exchange factors (GEFs) acting downstream of Galphaq/Galpha(12/13) trimeric G proteins. Here, we show that at least one potential GEF, the PDZRhoGEF, is present in smooth muscle, and its isolated DH/PH fragment induces calcium sensitization in the absence of agonist-mediated signaling. In vitro, the fragment shows high selectivity for the RhoA GTPase. Full-length fragment is required for the nucleotide exchange, as the isolated DH domain enhances it only marginally. We crystallized the DH/PH fragment of PDZRhoGEF in complex with nonprenylated human RhoA and determined the structure at 2.5 A resolution. The refined molecular model reveals that the mutual disposition of the DH and PH domains is significantly different from other previously described complexes involving DH/PH tandems, and that the PH domain interacts with RhoA in a unique mode. The DH domain makes several specific interactions with RhoA residues not conserved among other Rho family members, suggesting the molecular basis for the observed specificity.     

Biophysical characterization of synthetic rhamnolipids

Synthetic rhamnolipids, derived from a natural diacylated glycolipid, RL-2,2(14), produced by Burkholderia (Pseudomonas) plantarii, were analyzed biophysically. Changes in the chemical structures comprised variations in the length, the stereochemistry and numbers of the lipid chains, numbers of rhamnoses, and the occurrence of charged or neutral groups. As relevant biophysical parameters, the gel (beta) to liquid crystalline (alpha) phase behavior of the acyl chains of the rhamnoses, their three-dimensional supramolecular aggregate structure, and the ability of the compounds to intercalate into phospholipid liposomes in the absence and presence of lipopolysaccharide-binding protein were monitored. Their biological activities were examined as the ability to induce cytokines in human mononuclear cells and to induce chemiluminescence in monocytes. Depending on the particular chemical structures, the physicochemical parameters as well as the biological test systems show large variations. This relates to the acyl chain fluidity, aggregate structure, and intercalation ability, as well as the bioactivity. Most importantly, the data extend our conformational concept of endotoxicity, based on the intercalation of naturally originating amphiphilic virulence factors into membranes from immune cells. This 'endotoxin conformation', produced by amphiphilic molecules with a hydrophilic charged backbone and apolar hydrophobic moiety, and adopting inverted cubic aggregate structures, causes high mechanical stress in target immune cells on integral proteins, eventually leading to cell activation. Furthermore, biologically inactive rhamnolipids with lamellar aggregate structures antagonize the endotoxin-induced activity in a way similar to lipid A-derived antagonists.     

Aminoguanidine inhibits caspase-3 and calpain activation without affecting microglial activation following neonatal transient cerebral ischemia

Microglial cells, the resident macrophages of the CNS, can be both beneficial and detrimental to the brain. These cells play a central role as mediators of neuroinflammation associated with many neurodegenerative states, including cerebral ischemia. Because microglial cells are both a major source of inducible nitric oxide synthase (iNOS)/nitric oxide (NO) production locally in the injured brain and are activated by NO-mediated injury, we tested whether iNOS inhibition reduces microglial activation and ischemic injury in a neonatal focal ischemia-reperfusion model. Post-natal day 7 rats were subjected to a 2 h transient middle cerebral artery (MCA) occlusion. Pups with confirmed injury on diffusion-weighted magnetic resonance imaging (MRI) during occlusion were administered 300 mg/kg/dose aminoguanidine (AG) or vehicle at 0, 4 and 18 h after reperfusion, and animals were killed at 24 or 72 h post-reperfusion. The effect of AG on microglial activation as judged by the acquisition of ED1 immunoreactivity and proliferation of ED1-positive cells, on activation of cell death pathways and on injury volume, was determined. The study shows that while AG attenuates caspase 3 and calpain activation in the injured tissue, treatment does not affect the rapidly occurring activation and proliferation of microglia following transient MCA occlusion in the immature rat, or reduce injury size.     

Foraging Response of Turkish Honey Bee Subspecies to Flower Color Choices and Reward Consistency

Foraging behavior of Apis mellifera caucasica, A.m. carnica and A.m. syriaca in Turkey was studied for intrinsic subspecies-based differences. Models of forager flower-color fidelity, risk sensitive behavior and maximizing net gain were tested. Foragers were presented artificial flower patches containing blue, white and yellow flowers. Some bees of each subspecies showed high fidelity to yellow flowers, while others favored blue and white flowers. The degree of fidelity, however, differed among subspecies and was dependent upon which color was favored. Bees of all subspecies demonstrated risk indifferent behavior regardless of whether they favored yellow flowers or blue and white flowers. Flower handling time differed among subspecies and increased with reward quantity, and when a reward was present. Flight time between consecutive flowers also differed among honey bee subspecies. Foragers of all subspecies had a higher net gain when visiting flowers with consistent rewards.     

Autocrine galectin-1 promotes collective cell migration of squamous cell carcinoma cells through up-regulation of distinct integrins

We found that high galectin-1 (Gal-1) mRNA levels were associated with invasive squamous cell carcinoma (SCC) cells that expressed Snail, an epithelial-to-mesenchymal transition (EMT) regulator. Both Gal-1 overexpression and soluble Gal-1 treatment accelerated invasion and collective cell migration, along with activation of cdc42 and Rac. Soluble Gal-1 activated c-Jun N-terminal kinase to increase expression levels of integrins α2 and β5, which were essential for Gal-1 dependent collective cell migration and invasiveness. Soluble Gal-1 also increased the incidence of EMT in Snail-expressing SCC cells; these were a minor population with an EMT phenotype under growing conditions. Our findings indicate that soluble Gal-1 promotes invasiveness through enhancing collective cell migration and increasing the incidence of EMT.