The sticking point: How integrins bind to their ligands

The integrin adhesion receptors are alpha beta heterodimers that exist in different ligand-binding states. Because of their large size and conformational lability, it has been difficult to determine how they interact with their ligands. Ligand-binding sites have been identified in the beta subunit, and now more recently in the 'I' domain and EF-hand-like domains V and VI of the alpha subunit. We speculate here about how these various sites might operate together to bind ligand in a stable manner.     

Apoptosis induction by Bid requires unconventional ubiquitination and degradation of its N-terminal fragment

Bcl-2 family member Bid is subject to autoinhibition; in the absence of stimuli, its N-terminal region sequesters the proapoptotic Bcl-2 homology 3 (BH3) domain. Upon proteolytic cleavage in its unstructured loop, Bid is activated, although structural data reveal no apparent resulting conformational change. We found that, upon Bid cleavage, the N-terminal fragment (tBid-N) is ubiquitinated and degraded, thus freeing the BH3 domain in the C-terminal fragment (tBid-C). Ubiquitination of tBid-N is unconventional because acceptor sites are neither lysines nor the N terminus. Chemical approaches implicated thioester and hydroxyester linkage of ubiquitin and mutagenesis implicated serine and possibly threonine as acceptor residues in addition to cysteine. Acceptor sites reside predominantly but not exclusively in helix 1, which is required for ubiquitination and degradation of tBid-N. Rescue of tBid-N from degradation blocked Bid's ability to induce mitochondrial outer membrane permeability but not mitochondrial translocation of the cleaved complex. We conclude that unconventional ubiquitination and proteasome-dependent degradation of tBid-N is required to unleash the proapoptotic activity of tBid-C.     

Impact of resistant starch on body fat patterning and central appetite regulation

Background

Adipose tissue patterning has a major influence on the risk of developing chronic disease. Environmental influences on both body fat patterning and appetite regulation are not fully understood. This study was performed to investigate the impact of resistant starch (RS) on adipose tissue deposition and central regulation of appetite in mice.

Methodology and Principle Findings

Forty mice were randomised to a diet supplemented with either the high resistant starch (HRS), or the readily digestible starch (LRS). Using ¹H magnetic resonance (MR) methods, whole body adiposity, intrahepatocellular lipids (IHCL) and intramyocellular lipids (IMCL) were measured. Manganese-enhanced MRI (MEMRI) was used to investigate neuronal activity in hypothalamic regions involved in appetite control when fed ad libitum. At the end of the interventional period, adipocytes were isolated from epididymal adipose tissue and fasting plasma collected for hormonal and adipokine measurement. Mice on the HRS and LRS diet had similar body weights although total body adiposity, subcutaneous and visceral fat, IHCL, plasma leptin, plasma adiponectin plasma insulin/glucose ratios was significantly greater in the latter group. Adipocytes isolated from the LRS group were significantly larger and had lower insulin-stimulated glucose uptake. MEMRI data obtained from the ventromedial and paraventricular hypothalamic nuclei suggests a satiating effect of the HRS diet despite a lower energy intake.

Conclusion and Significance

Dietary RS significantly impacts on adipose tissue patterning, adipocyte morphology and metabolism, glucose and insulin metabolism, as well as affecting appetite regulation, supported by changes in neuronal activity in hypothalamic appetite regulation centres which are suggestive of satiation.     

Ferrichrome utilization in a mesorhizobial population: microevolution of a three-locus system

The ability to utilize the siderophore ferrichrome as an iron source was found to be a variable trait in a field population of mesorhizobia. To investigate the genetic basis of this variation, genes required for ferrichrome utilization (fhu genes) were characterized in Mesorhizobium strain R88B, an Fhu(+) member of the population. Functional fhu genes were present at three loci. Two genes of the ferrichrome ABC transporter, fhuBD, were identified at an fhu1 locus downstream of the symbiosis island that was integrated at the phe-tRNA gene. The fhuA gene encoding the ferrichrome outer membrane receptor was located in the fhu2 locus together with non-functional fhuDB genes, while the fhuC gene encoding the ATPase required for ferrichrome transport was part of the fhu3 locus that included genes required to form a functional TonB complex. None of the fhu genes were present in the sequenced Mesorhizobium loti strain MAFF303099. Comparisons with MAFF303099 suggested that the fhu2 and fhu3 loci evolved through small-scale (< 5 kb) acquisitions and deletions. Despite their independent origins, the three fhu loci were coordinately regulated in response to iron availability. Within the mesorhizobial population, the ability to utilize ferrichrome was most strongly correlated with the presence of the fhuA gene. We hypothesize that the ferrichrome transport system evolved through cycles of gene acquisition and deletion, with the positive selection pressure of an iron-poor or siderophore-rich environment being offset by the negative pressure of the outer membrane receptor being a target for phage.     

Contrasting effects of mosaic structure on alpha and beta diversity of bird assemblages in a human‐modified landscape

Habitat loss and fragmentation are key processes causing biodiversity loss in human‐modified landscapes. Knowledge of these processes has largely been derived from measuring biodiversity at the scale of ‘within‐habitat’ fragments with the surrounding landscape considered as matrix. Yet, the loss of variation in species assemblages ‘among’ habitat fragments (landscape‐scale) may be as important a driver of biodiversity loss as the loss of diversity ‘within’ habitat fragments (local‐scale). We tested the hypothesis that heterogeneity in vegetation cover is important for maintaining alpha and beta diversity in human‐modified landscapes. We surveyed bird assemblages in eighty 300‐m‐long transects nested within twenty 1‐km² vegetation ‘mosaics’, with mosaics assigned to four categories defined by the cover extent and configuration of native eucalypt forest and exotic pine plantation. We examined bird assemblages at two spatial scales: 1) within and among transects, and 2) within and among mosaics. Alpha diversity was the mean species diversity within‐transects or within‐mosaics and beta diversity quantified the effective number of compositionally distinct transects or mosaics. We found that within‐transect alpha diversity was highest in vegetation mosaics defined by continuous eucalypt forest, lowest in mosaics of continuous pine plantation, and at intermediate levels in mosaics containing eucalypt patches in a pine matrix. We found that eucalypt mosaics had lower beta diversity than other mosaic types when ignoring relative abundances, but had similar or higher beta diversity when weighting with species abundances. Mosaics containing both pine and eucalypt forest differed in their bird compositional variation among transects, despite sharing a similar suite of species. This configuration effect at the mosaic scale reflected differences in vegetation composition among transects. Maintaining heterogeneity in vegetation cover could help to maintain variation among bird assemblages across landscapes, thus partially offsetting local‐scale diversity losses due to fragmentation. Critical to this is the retention of remnant native vegetation.     

Variation in Eastern cottonwood (Populus deltoides Bartr.) phloem sap content caused by leaf development may affect feeding site selection behavior of the aphid, Chaitophorous populicola Thomas (Homoptera: Aphididae)

Apterous populations of Chaitophorous populicola Thomas (Homoptera: Aphididae) appear to track Eastern cottonwood (Populus deltoides Bartr.) leaf development. Few aphids occur on mature leaves. Marked individual aphids on leaves of different developmental stages were observed through a period of new leaf initiation. Nymph and adult C. populicola frequently track leaf development by moving up to younger leaves. A comparison of phloem sap constituents and leaf toughness among leaf developmental stages revealed some differences that could be used by C. populicola to determine leaf age. Phloem sap exudates, collected from P. deltoides leaves of different developmental stages, were analyzed by high-performance liquid chromatography for free amino acids and the phenolic glycoside salicin. Sucrose concentration in exudates, indicative of phloem sap exudation rate, was uniform among leaf stages. Of 20 amino acids examined, only aspartic acid and gamma-amino-n-butyric acid (GABA) concentrations differed significantly between leaf stages. Forward stepwise discriminant function analysis showed that seven of the amino acids analyzed are useful for classifying leaf maturity groupings. Aphid-infested cottonwoods had lower cystine concentrations in phloem sap than aphid-free plants. Salicin concentration was significantly higher in new leaves. Leaf toughness was assessed by lignin density and distance measurements in petiole cross-sections. Rapidly expanding leaves had significantly less lignification and new leaves had shorter distances to the vascular bundles than senescent leaves. These physiological and phytochemical differences among P. deltoides leaf developmental stages may contribute to the leaf stage selection patterns exhibited by the aphid, C. populicola.     

Differential protein expression in the corpus callosum (splenium) of human alcoholics: a proteomics study

It is widely accepted that the chronic use of alcohol induces metabolic abnormalities and neuronal damage in the brain, which can lead to cognitive dysfunction. Neuroimaging studies reveal that alcohol-induced brain damage is region specific and prominent damage has been observed in both gray and white matter of the prefrontal cortex, and a wide range of white matter structures including the corpus callosum. Molecular mechanisms underlying these structural changes are largely unknown. Using proteomics we have analysed the changes in protein expression in the splenium of the corpus callosum in two different alcoholic groups. Protein extracts from splenium of 22 human brains (nine controls, seven uncomplicated alcoholics and six complicated alcoholics with hepatic cirrhosis-designated complicated) were separated using two-dimensional gel electrophorosis. Image analysis revealed that there were significant alterations in protein expression for 25 protein spots in the uncomplicated alcoholic group and 45 in the complicated group compared to control (P<0.05; ANOVA). In a total of 72 spots (identified as 36 proteins), 15 (identified as 14 proteins) spots overlapped between two alcoholic groups. Another 32 protein spots (26 different proteins) were identified only in the complicated alcoholics. It is therefore possible that these 26 proteins in the complicated group are likely to be the results of hepatic compromise. When compared with our previous data of white matter from the prefrontal cortex in alcoholics, large numbers of identified proteins in the splenium are different. This suggests that there may be different mechanisms causing alcohol-induced brain damage in different regions of the white matter. Our data also indicate the importance of other pathways including oxidative stress, lipid peroxidation and apoptosis as potential causes of alcohol-induced brain damage.