High fat diet-induced modifications in membrane lipid and mitochondrial-membrane protein signatures precede the development of hepatic insulin resistance in mice.

OBJECTIVE: Excess lipid intake has been implicated in the pathophysiology of hepatosteatosis and hepatic insulin resistance. Lipids constitute approximately 50% of the cell membrane mass, define membrane properties, and create microenvironments for membrane-proteins. In this study we aimed to resolve temporal alterations in membrane metabolite and protein signatures during high-fat diet (HF)-mediated development of hepatic insulin resistance. METHODS: We induced hepatosteatosis by feeding C3HeB/FeJ male mice an HF enriched with long-chain polyunsaturated C18:2n6 fatty acids for 7, 14, or 21 days. Longitudinal changes in hepatic insulin sensitivity were assessed via the euglycemic-hyperinsulinemic clamp, in membrane lipids via t-metabolomics- and membrane proteins via quantitative proteomics-analyses, and in hepatocyte morphology via electron microscopy. Data were compared to those of age- and litter-matched controls maintained on a low-fat diet. RESULTS: Excess long-chain polyunsaturated C18:2n6 intake for 7 days did not compromise hepatic insulin sensitivity, however, induced hepatosteatosis and modified major membrane lipid constituent signatures in liver, e.g. increased total unsaturated, long-chain fatty acid-containing acyl-carnitine or membrane-associated diacylglycerol moieties and decreased total short-chain acyl-carnitines, glycerophosphocholines, lysophosphatidylcholines, or sphingolipids. Hepatic insulin sensitivity tended to decrease within 14 days HF-exposure. Overt hepatic insulin resistance developed until day 21 of HF-intervention and was accompanied by morphological mitochondrial abnormalities and indications for oxidative stress in liver. HF-feeding progressively decreased the abundance of protein-components of all mitochondrial respiratory chain complexes, inner and outer mitochondrial membrane substrate transporters independent from the hepatocellular mitochondrial volume in liver. CONCLUSIONS: We assume HF-induced modifications in membrane lipid- and protein-signatures prior to and during changes in hepatic insulin action in liver alter membrane properties - in particular those of mitochondria which are highly abundant in hepatocytes. In turn, a progressive decrease in the abundance of mitochondrial membrane proteins throughout HF-exposure likely impacts on mitochondrial energy metabolism, substrate exchange across mitochondrial membranes, contributes to oxidative stress, mitochondrial damage, and the development of insulin resistance in liver.

Feasibility and quality development of biomaterials in the pretest studies of the German National Cohort.

BACKGROUND: The German National Cohort (GNC) is designed to address research questions concerning a wide range of possible causes of major chronic diseases (e.g. cancer, diabetes, infectious, allergic, neurologic and cardiovascular diseases) as well as to identify risk factors and prognostic biomarkers for early diagnosis and prevention of these diseases. The collection of biomaterials in combination with extensive information from questionnaires and medical examinations represents one of the central study components. OBJECTIVES: In two pretest studies of the German National Cohort conducted between 2011 and 2013, a range of biomaterials from a defined number of participants was collected. Ten study centres were involved in pretest 1 and 18 study centres were involved in pretest 2. Standard operation procedures (SOP) were developed and evaluated to minimize pre-analytical artefacts during biosample collection. Within the pretest studies different aspects concerning feasibility of sample collection/preparation [pretest 1 (a)] and quality control of biomarkers and proteome analyses were investigated [pretest 1 (b), (c)]. Additionally, recruitment of study participants for specific projects and examination procedures of all study centres in a defined time period according to common standards as well as transportation and decentralized storage of biological samples were tested (pretest 2). These analyses will serve as the basis for the biomaterial collection in the main study of the GNC starting in 2014. MATERIALS AND METHODS: Participants, randomly chosen from the population (n = 1000 subjects recruited at ten study sites in pretest 1) were asked to donate blood, urine, saliva and stool samples. Additionally, nasal and oropharyngeal swabs were collected at the study sites and nasal swabs were collected by the participants at home. SOPs for sample collection, preparation, storage and transportation were developed and adopted for pretest 2. In pretest 2, 18 study sites (n = 599 subjects) collected biomaterials mostly identical to pretest 1. Biomarker analyses to test the quality of the biomaterials were performed. RESULTS: In pretest 1 and 2, it was feasible to collect all biomaterials from nearly all invited participants without major problems. The mean response rate of the subjects was 95 %. As one important result we found for example that after blood draw the cellular fraction should be separated from the plasma and serum fractions during the first hour with no significant variation for up to 6 h at 4 ℃ for all analysed biomarkers. Moreover, quality control of samples using a proteomics approach showed no significant clustering of proteins according to different storage conditions. All developed SOPs were validated for use in the main study after some adaptation and modification. Additionally, electronic and paper documentation sheets were developed and tested to record time stamps, volumes, freezing times, and aliquot numbers of the collected biomaterials. DISCUSSION: The collection of the biomaterials was feasible without major problems at all participating study sites. However, the processing times were in some cases too long. To avoid pre-analytical artefacts in sample collection, appropriate standardisation among the study sites is necessary. To achieve this, blood and urine collection will have to be adapted to specific conditions of usage of liquid handling robots, which will be available at all participating study centres in the main study of the GNC. Strict compliance with the SOPs, thorough training of the staff and accurate documentation are mandatory to obtain high sample quality for later analyses. The so obtained biomaterials represent a valuable resource for research on infectious and other common complex diseases in the GNC.

The ricinosomes of senescing plant tissue bud from the endoplasmic reticulum.

The ricinosome (synonym, precursor protease vesicle) is a novel organelle, found so far exclusively in plant cells. Electron microscopic studies suggest that it buds off from the endoplasmic reticulum in senescing tissues. Biochemical support for this unusual origin now comes from the composition of the purified organelle, which contains large amounts of a 45-kDa cysteine endoprotease precursor with a C-terminal KDEL motif and the endoplasmic reticulum lumen residents BiP (binding protein) and protein disulfide isomerase. Western blot analysis, peptide sequencing, and mass spectrometry demonstrate retention of KDEL in the protease proform. Acidification of isolated ricinosomes causes castor bean cysteine endopeptidase activation, with cleavage of the N-terminal propeptide and the C-terminal KDEL motif. We propose that ricinosomes accumulate during senescence by programmed cell death and are activated by release of protons from acidic vacuoles.

Deamidation as a widespread phenomenon in two-dimensional polyacrylamide gel electrophoresis of human blood plasma proteins.

The human plasma protein patterns obtained by two-dimensional polyacrylamide gel electrophoresis (2-D PAGE) is a good model system for post-translational modifications because of the existence of several "ladders" of protein spots [Anderson, N. L., Anderson, N. G., Electrophoresis 1991, 12, 883-906], so-called "trains" of spots. Our investigation of several proteins, amongst others beta2-microglobulin and the haptoglobin chains, found the differences in isoelectric points (p/) to be due to deamidation of asparagines. After enzymatic cleavage with endopeptidases in the 2-D polyacrylamide gel, the asparagine and deamidated asparagine containing peptides were separated and quantified by reversed-phase HPLC. In order to separate these peptides, a neutral pH system was established and, as a result, the differences in hydrophobicity of asparagine-containing and deamidated asparagine-containing peptides increased. But how do deamidated asparagines contribute to the observed spot pattern? One spot in the 2-D gel consists of a mixture of protein species with the same number of deamidated asparagines but on different sequence position sites. The difference between the spots in the "ladder" is a growing number of negative charges introduced in the protein by an increasing number of deamidated asparagines. As a consequence, the mass difference between two spots is exactly 1 Da, which is shown in this paper for intact protein masses and the corresponding deamidated peptides.

High-level expression in Escherichia coli of selenocysteine-containing rat thioredoxin reductase utilizing gene fusions with engineered bacterial-type SECIS elements and co-expression with the selA, selB and selC genes.

Mammalian thioredoxin reductase (TrxR) catalyzes reduction of thioredoxin and many other substrates, and is a central enzyme for cell proliferation and thiol redox control. The enzyme is a selenoprotein and can therefore, like all other mammalian selenoproteins, not be directly expressed in Escherichia coli, since selenocysteine-containing proteins are synthesized by a highly species-specific translation machinery. This machinery involves a secondary structure, SECIS element, in the selenoprotein-encoding mRNA, directing selenocysteine insertion at the position of an opal (UGA) codon, normally conferring termination of translation. It is species-specific structural features and positions in the selenoprotein mRNA of the SECIS elements that hitherto have hampered heterologous production of recombinant selenoproteins. We have discovered, however, that rat TrxR can be expressed in E. coli by fusing its open reading frame with the SECIS element of the bacterial selenoprotein formate dehydrogenase H. A variant of the SECIS element designed to encode the conserved carboxyterminal end of the enzyme (-Sec-Gly-COOH) and positioning parts of the SECIS element in the 3'-untranslated region was also functional. This finding revealed that the SECIS element in bacteria does not need to be translated for full function and it enabled expression of enzymatically active mammalian TrxR. The recombinant selenocysteine-containing TrxR was produced at dramatically higher levels than formate dehydrogenase O, the only endogenous selenoprotein expressed in E. coli under the conditions utilized, demonstrating a surprisingly high reserve capacity of the bacterial selenoprotein synthesis machinery under aerobic conditions. Co-expression with the selA, selB and selC genes (encoding selenocysteine synthase, SELB and tRNA(Sec), respectively) further increased the efficiency of the selenoprotein production and thereby also increased the specific activity of the recombinant TrxR to about 25 % of the native enzyme, with as much as 20 mg produced per liter of culture. These results show that with the strategy utilized here, the capacity of selenoprotein synthesis in E. coli is more than sufficient for making possible the use of the bacteria for production of recombinant selenoproteins.