Liver X receptors regulate de novo lipogenesis in a tissue-specific manner in C57BL/6 female mice

The liver X receptors (LXRs) play a key role in cholesterol and bile acid metabolism but are also important regulators of glucose metabolism. Recently, LXRs have been proposed as a glucose sensor affecting LXR-dependent gene expression. We challenged wild-type (WT) and LXRαβ(-/-) mice with a normal diet (ND) or a high-carbohydrate diet (HCD). Magnetic resonance imaging showed different fat distribution between WT and LXRαβ(-/-) mice. Surprisingly, gonadal (GL) adipocyte volume decreased on HCD compared with ND in WT mice, whereas it slightly increased in LXRαβ(-/-) mice. Interestingly, insulin-stimulated lipogenesis of isolated GL fat cells was reduced on HCD compared with ND in LXRαβ(-/-) mice, whereas no changes were observed in WT mice. Net de novo lipogenesis (DNL) calculated from Vo(2) and Vco(2) was significantly higher in LXRαβ(-/-) than in WT mice on HCD. Histology of HCD-fed livers showed hepatic steatosis in WT mice but not in LXRαβ(-/-) mice. Glucose tolerance was not different between groups, but insulin sensitivity was decreased by the HCD in WT but not in LXRαβ(-/-) mice. Finally, gene expression analysis of adipose tissue showed induced expression of genes involved in DNL in LXRαβ(-/-) mice compared with WT animals as opposed to the liver, where expression of DNL genes was repressed in LXRαβ(-/-) mice. We thus conclude that absence of LXRs stimulates DNL in adipose tissue, but suppresses DNL in the liver, demonstrating opposite roles of LXR in DNL regulation in these two tissues. These results show tissue-specific regulation of LXR activity, a crucial finding for drug development.     

Relation between bacterial activity in the surface microlayer and estuarine hydrodynamics

Bacterial communities of the surface microlayer (SML) of the estuary Ria de Aveiro (Portugal) were characterized in terms of abundance and activity during a 2-year survey at two sites with distinct hydrodynamic properties (marine and brackish water zones). The hydrodynamic conditions were simulated using a bidimensional numerical model and related to the microbiological observations. The pattern of variation of bacterial biomass productivity (BBP) was distinct between the two sampling sites. At the outer site, BBP was significantly lower at the SML, whereas at the inner site, it was significantly enhanced at the SML. Although the total bacterial abundance was similar in the SML and underlying water (UW), the fraction of cells attached to particles was significantly higher at the SML (two to three times). The integration of microbiological results with environmental and hydrological variables shows that strong currents in the marine zone promote the vertical mixing, inhibiting the establishment of an SML bacterial community distinct from that of UW. In contrast, in the brackish water zone, lower current velocities provide conditions for enhancing the bacterial activity in the enriched SML. Estuarine dynamics influence the distribution and activity of microorganisms at the SML and in the water column, with anticipated impacts for the carbon cycle in the estuarine environment.     

Characterization of the effect of TIMAP phosphorylation on its interaction with protein phosphatase 1

TIMAP, TGF-β inhibited, membrane-associated protein, is highly abundant in endothelial cells (EC). We have shown earlier the involvement of TIMAP in PKA-mediated ERM (ezrin-radixin-moesin) dephosphorylation as part of EC barrier protection by TIMAP (Csortos et al., 2008). Emerging data demonstrate the regulatory role of TIMAP on protein phosphatase 1 (PP1) activity. We provide here evidence for specific interaction (K_a = 1.80 × 10⁶ M⁻¹) between non-phosphorylated TIMAP and the catalytic subunit of PP1 (PP1c) by surface plasmon resonance based binding studies. Thiophosphorylation of TIMAP by PKA, or sequential thiophosphorylation by PKA and GSK3β slightly modifies the association constant for the interaction of TIMAP with PP1c and decreases the rate of dissociation. However, dephosphorylation of phospho-moesin substrate by PP1cβ is inhibited to different extent in the presence of non- (∼60% inhibition), mono- (∼50% inhibition) or double-thiophosphorylated (<10% inhibition) form of TIMAP. Our data suggest that double-thiophosphorylation of TIMAP has minor effect on its binding ability to PP1c, but considerably attenuates its inhibitory effect on the activity of PP1c. PKA activation by forskolin treatment of EC prevented thrombin evoked barrier dysfunction and ERM phosphorylation at the cell membrane (Csortos et al., 2008). With the employment of specific GSK3β inhibitor it is shown here that PKA activation is followed by GSK3β activation in bovine pulmonary EC and both of these activations are required for the rescuing effect of forskolin in thrombin treated EC. Our results suggest that the forskolin induced PKA/GSK3β activation protects the EC barrier via TIMAP-mediated decreasing of the ERM phosphorylation level.     

Rigorous conditions for food-web intervality in high-dimensional trophic niche spaces

Food webs represent trophic (feeding) interactions in ecosystems. Since the late 1970s, it has been recognized that food-webs have a surprisingly close relationship to interval graphs. One interpretation of food-web intervality is that trophic niche space is low-dimensional, meaning that the trophic character of a species can be expressed by a single or at most a few quantitative traits. In a companion paper we demonstrated, by simulating a minimal food-web model, that food webs are also expected to be interval when niche-space is high-dimensional. Here we characterize the fundamental mechanisms underlying this phenomenon by proving a set of rigorous conditions for food-web intervality in high-dimensional niche spaces. Our results apply to a large class of food-web models, including the special case previously studied numerically.     

Mitochondrial autophagy by Bnip3 involves Drp1-mediated mitochondrial fission and recruitment of Parkin in cardiac myocytes

The Bcl2/adenovirus E1B 19-kDa interacting protein 3 (Bnip3) is an atypical BH3-only protein that is associated with mitochondrial dysfunction and cell death. Bnip3 is also a potent inducer of mitochondrial autophagy, and in this study we have investigated the mechanisms by which Bnip3 induces autophagy in cardiac myocytes. We found that Bnip3 induced mitochondrial translocation of dynamin-related protein 1 (Drp1), a protein involved in mitochondrial fission in adult myocytes. Drp1-mediated mitochondrial fission correlated with increased autophagy, and inhibition of Drp1 reduced Bnip3-mediated autophagy. Overexpression of Drp1K38E, a dominant negative of Drp1, or mitofusin 1 prevented mitochondrial fission and autophagy by Bnip3. Also, inhibition of mitochondrial fission or autophagy resulted in increased death of myocytes overexpressing Bnip3. Moreover, Bnip3 promoted translocation of the E3 ubiquitin ligase Parkin to mitochondria, which was prevented in the presence of a Drp1 inhibitor. Interestingly, induction of autophagy by Bnip3 was reduced in Parkin-deficient myocytes. Thus our data suggest that induction of autophagy in response to Bnip3 is a protective response activated by the cell that involves Drp1-mediated mitochondrial fission and recruitment of Parkin.     

Genetic structure of pastoral and farmer populations in the African Sahel

Traditional pastoralists survive in few places in the world. They can still be encountered in the African Sahel, where annual alternations of dry and wet seasons force them to continual mobility. Little is known about the genetic structure of these populations. We present here the population distribution of 312 hypervariable segment I mitochondrial DNA (mtDNA) and 364 Y-short tandem repeat haplotypes in both farmer and pastoralist groups from the Lake Chad Basin and the West African Sahel. We show that the majority of pastoral populations (represented in the African Sahel by the Fulani nomads) fail to show significant departure from neutrality for mtDNA as evidenced by Fu's Fs statistics and exhibit lower levels of intrapopulation diversity measures for mtDNA when contrasted with farmers. These differences were not observed for the Y chromosome. Furthermore, analyses of molecular variance and population distributions of the mtDNA haplotypes show more heterogeneity in the sedentary groups than in the pastoralists. On the other hand, pastoralists retain a signature of a wide phylogenetic distance contributing to their male gene pool, whereas in at least some of the farmer populations, a founder effect and/or drift might have led to the presence of a single major lineage. Interestingly, these observations are in contrast with those recorded in Central Asia, where similar comparisons of farmer and pastoral groups have recently been carried out. We can conclude that in Africa, there have been no substantial mating exchanges between the Fulani pastoralists coming to the Lake Chad Basin from the West African Sahel and their farmer neighbors. At the same time, we suggest that the emergence of pastoralism might be an earlier and/or a demographically more important event than the introduction of sedentary agriculture, at least in this part of Africa.     

Early ischemic preconditioning against focal transient and permanent brain ischemia in rats: role of collateral circulation

We hypothesize that early ischemic preconditioning (IPC) can afford protection against focal brief and prolonged cerebral ischemia with subsequent reperfusion as well as permanent brain ischemia in rats by amelioration of regional cerebral blood flow. Adult male Wistar rats (n=97) were subjected to transient (30 and 60 minutes) and permanent middle cerebral artery (MCA) occlusion. IPC protocol consisted of two episodes of 5-min common carotid artery occlusion + 5-min reperfusion prior to test ischemia either followed by 48 hours of reperfusion or not. Triphenyltetrazolium chloride and Evans blue were used for delineation of infarct size and anatomical area at risk (comprises ischemic penumbra and ischemic core), respectively. Blood flow in the MCA vascular bed was measured with use of Doppler ultrasound. The IPC resulted in significant infarct size limitation in both transient and permanent MCA occlusion. Importantly, IPC caused significant reduction of area at risk after 30 min of focal ischemia as compared to controls [med(min-max) 11.4% (3.59-2 0.35%) vs. 2.47% (0.8-9.31%), p = 0.018] but it failed to influence area at risk after 5 min of ischemia [med(min-max) 7.61% (6.32-10.87%) vs. 8.2% (4.87-9.65%), p > 0.05]. No differences in blood flow were found between IPC and control groups using Doppler ultrasound. This is suggestive of the fact that IPC does not really influence blood flow in the large cerebral arteries such as MCA but it might have some effect on smaller arteries. It seems that, along with well established cytoprotective effects of IPC, IPC-mediated reduction of area at risk by means of improvement in local cerebral blood flow may contribute to infarct size limitation after focal transient and permanent brain ischemia in rats.     

Synthesis and in vitro activity of N-benzyl-1-(2,3-dichlorophenyl)-1H-tetrazol-5-amine P2X(7) antagonists

Synthesis and biological evaluation of a novel class of substituted N-benzyl-1-(2,3-dichlorophenyl)-1H-tetrazol-5-amine derivatives resulted in the identification of potent P2X₇ antagonists. These compounds were assayed for activity at both the human and rat P2X₇ receptors. On the benzyl moiety, a variety of functional groups were tolerated, including both electron-withdrawing and electron-donating substituents. Ortho-substitution on the benzyl group provided the greatest potency. The ortho-substituted analogs showed approximately 2.5-fold greater potency at human compared to rat P2X₇ receptors. Compounds 12 and 38 displayed hP2X₇pIC₅₀s >7.8 with less than 2-fold difference in potency at the rP2X₇.     

Impact of the N-terminal secretor domain on YopD translocator function in Yersinia pseudotuberculosis type III secretion

Type III secretion systems (T3SSs) secrete needle components, pore-forming translocators, and the translocated effectors. In part, effector recognition by a T3SS involves their N-terminal amino acids and their 5' mRNA. To investigate whether similar molecular constraints influence translocator secretion, we scrutinized this region within YopD from Yersinia pseudotuberculosis. Mutations in the 5' end of yopD that resulted in specific disruption of the mRNA sequence did not affect YopD secretion. On the other hand, a few mutations affecting the protein sequence reduced secretion. Translational reporter fusions identified the first five codons as a minimal N-terminal secretion signal and also indicated that the YopD N terminus might be important for yopD translation control. Hybrid proteins in which the N terminus of YopD was exchanged with the equivalent region of the YopE effector or the YopB translocator were also constructed. While the in vitro secretion profile was unaltered, these modified bacteria were all compromised with respect to T3SS activity in the presence of immune cells. Thus, the YopD N terminus does harbor a secretion signal that may also incorporate mechanisms of yopD translation control. This signal tolerates a high degree of variation while still maintaining secretion competence suggestive of inherent structural peculiarities that make it distinct from secretion signals of other T3SS substrates.