How changes in interconnectivity affect the bulk properties of articular cartilage: a fibre network study

The remarkable compressive strength of articular cartilage arises from the mechanical interactions between the tension-resisting collagen fibrils and swelling proteoglycan proteins within the tissue. These interactions are facilitated by a significant level of interconnectivity between neighbouring collagen fibrils within the extracellular matrix. A reduction in interconnectivity is suspected to occur during the early stages of osteoarthritic degeneration. However, the relative contribution of these interconnections towards the bulk mechanical properties of articular cartilage has remained an open question. In this study, we present a simple 2D fibre network model which explicitly represents the microstructure of articular cartilage as collection of discrete nodes and linear springs. The transverse stiffness and swelling properties of this fibre network are studied, and a semi-analytic relationship which relates these two macroscopic properties via microscopic interconnectivity is derived. By comparing this derived expression to previously published experimental data, we show that although a reduction in network interconnectivity accounts for some of the observed changes in the mechanical properties of articular cartilage as degeneration occurs, a decrease in matrix interconnectivity alone do not provide a full account of this process.     

Bioconversion of Lithocholic Acid Under Anaerobic Conditions by Pseudomonas sp. Strain NCIB 10590

The biotransformation of lithocholic acid by Pseudomonas sp. strain NCIB 10590 under anaerobic conditions was studied. The major products were identified as androsta-1,4-diene-3,17-dione and 3-oxochol-4-ene-24-oic acid. The minor products included 17β-hydroxyandrost-4-ene-3-one, 17β-hydroxyandrosta-1,4-diene-3-one, 3-oxo-5β-cholan-24-oic acid, 3-oxochola-1,4-diene-24-oic acid, 3-oxopregn-4-ene-20-carboxylic acid, and 3-oxopregna-1,4-diene-20-carboxylic acid. Anaerobiosis increases the number of metabolites produced by Pseudomonas sp. NCIB 10590 from lithocholic acid.     

Differences in hydroxylation and binding of Notch and HIF-1α demonstrate substrate selectivity for factor inhibiting HIF-1 (FIH-1)

FIH-1, factor inhibiting hypoxia-inducible factor-1 (HIF-1), regulates oxygen sensing by hydroxylating an asparagine within HIF-α. It also hydroxylates asparagines in many proteins containing ankyrin repeats, including Notch1–3, p105 and IκBα. Relative binding affinity and hydroxylation rate are crucial determinants of substrate selection and modification. We determined the contributions of substrate sequence composition and length and of oxygen concentration to the FIH-1-binding and/or hydroxylation of Notch1–4 and compared them with those for HIF-1α. We also demonstrated hydroxylation of two asparagines in Notch2 and 3, corresponding to Sites 1 and 2 of Notch1, by mass spectrometry for the first time.Our data demonstrate that substrate length has a much greater influence on FIH-1-dependent hydroxylation of Notch than of HIF-1α, predominantly through binding affinity rather than maximal reaction velocity. The K_m value of FIH-1 for Notch1, <0.2 μM, is at least 250-fold lower than that of 50 μM for HIF-1α. Site 1 of Notch1–3 appeared the preferred site of FIH-1 hydroxylation in these substrates. Interestingly, binding of Notch4 to FIH-1 was observed with an affinity almost 10-fold lower than for Notch1–3, but no hydroxylation was detected. Importantly, we demonstrate that the K_m of FIH-1 for oxygen at the preferred Site 1 of Notch1–3, 10–19 μM, is an order of magnitude lower than that for Site 2 or HIF-1α. Hence, at least during in vitro hydroxylation, Notch is likely to become efficiently hydroxylated by FIH-1 even under relatively severe hypoxic conditions, where HIF-1α hydroxylation would be reduced.     

Activation of HIF-1alpha in exponentially growing cells via hypoxic stimulation is independent of the Akt/mTOR pathway

Accumulation of HIF-1alpha during normoxic conditions at high cell density has previously been shown to occur and can be used to stabilize HIF-1alpha protein in the absence of a specific anaerobic chamber. However, the impact and origin of this pool of HIF-1alpha, obtained under normoxia, has been underestimated. In this study, we have systematically compared the related pools of HIF-1alpha stabilized in normoxia by high cell density to those obtained at low density in hypoxia. At first glance, these two stimuli appear to have similar outcomes: HIF-1alpha stabilization and induction of HIF-1-dependent genes. However, upon careful analysis, we observed that molecular mechanisms involved are different. We clearly demonstrate that density-dependant HIF-1alpha accumulation during normoxia is due to the cells high consumption of oxygen, as demonstrated by using a respiration inhibitor (oligomycin) and respiratory-defective mutant cells (GSK3). Finally and most importantly, our data indicate that a decrease in AKT activity followed by a total decrease in p70(S6K) phosphorylation reflecting a decrease in mTOR activity occurs during high oxygen consumption, resulting from high cell density. In contrast, hypoxia, even at severe low O(2) levels, only slightly impacts upon the mTOR pathway under low cell density conditions. Thus, activation of HIF-1alpha in exponentially growing cells via hypoxic stimulation is independent of the Akt/mTOR pathway whereas HIF-1alpha activation obtained in high confluency is totally dependent on mTOR pathway as rapamycin totally impaired (i) HIF-1alpha stabilization and (ii) mRNA levels of CA9 and BNIP3, two HIF-target genes.     

Population genetic structure and long-distance dispersal among seabird populations: implications for colony persistence

Dramatic local population decline brought about by anthropogenic-driven change is an increasingly common threat to biodiversity. Seabird life history traits make them particularly vulnerable to such change; therefore, understanding population connectivity and dispersal dynamics is vital for successful management. Our study used a 357-base pair mitochondrial control region locus sequenced for 103 individuals and 18 nuclear microsatellite loci genotyped for 245 individuals to investigate population structure in the Atlantic and Pacific populations of the pelagic seabird, Leach's storm-petrel Oceanodroma leucorhoa leucorhoa. This species is under intense predation pressure at one regionally important colony on St Kilda, Scotland, where a disparity between population decline and predation rates hints at immigration from other large colonies. AMOVA, F(ST), Φ(ST) and Bayesian cluster analyses revealed no genetic structure among Atlantic colonies (Global Φ(ST) = -0.02 P > 0.05, Global F(ST) = 0.003, P > 0.05, STRUCTURE K = 1), consistent with either contemporary gene flow or strong historical association within the ocean basin. The Pacific and Atlantic populations are genetically distinct (Global Φ(ST) = 0.32 P < 0.0001, Global F(ST) = 0.04, P < 0.0001, STRUCTURE K = 2), but evidence for interocean exchange was found with individual exclusion/assignment and population coalescent analyses. These findings highlight the importance of conserving multiple colonies at a number of different sites and suggest that management of this seabird may be best viewed at an oceanic scale. Moreover, our study provides an illustration of how long-distance movement may ameliorate the potentially deleterious impacts of localized environmental change, although direct measures of dispersal are still required to better understand this process.     

The comparative biology of diving in two genera of European Dytiscidae (Coleoptera)

Surfacing behaviour is fundamental in the ecology of aquatic air-breathing organisms; however, it is only in vertebrates that the evolutionary ecology of diving has been well characterized. Here, we explore the diving behaviour of dytiscid beetles, a key group of surface-exchanging freshwater invertebrates, by comparing the dive responses of 25 taxa (Deronectes and Ilybius spp.) acclimated at two temperatures. The allometric slopes of dive responses in these dytiscids appear similar to those of vertebrate ectotherms, supporting the notion that metabolic mode shapes the evolution of diving performance. In both genera, beetles spend more time submerged than on the surface, and surface time does not vary with the temperature of acclimation. However, presumably in order to meet increased oxygen demand at higher temperatures, Deronectes species increase surfacing frequency, whereas Ilybius species decrease dive time, an example of 'multiple solutions.' Finally, widespread northern species appear to possess higher diving performances than their geographically restricted southern relatives, something which may have contributed to their range expansion ability.     

Envelope structure of Synechococcus sp. WH8113, a nonflagellated swimming cyanobacterium

Background  

Many bacteria swim by rotating helical flagellar filaments [1]. Waterbury et al. [15] discovered an exception, strains of the cyanobacterium Synechococcus that swim without flagella or visible changes in shape. Other species of cyanobacteria glide on surfaces [2,7]. The hypothesis that Synechococcus might swim using traveling surface waves [6,13] prompted this investigation.