Sixteen hours of fasting differentially affects hepatic and muscle insulin sensitivity in mice

Fasting readily induces hepatic steatosis. Hepatic steatosis is associated with hepatic insulin resistance. The purpose of the present study was to document the effects of 16 h of fasting in wild-type mice on insulin sensitivity in liver and skeletal muscle in relation to 1) tissue accumulation of triglycerides (TGs) and 2) changes in mRNA expression of metabolically relevant genes. Sixteen hours of fasting did not show an effect on hepatic insulin sensitivity in terms of glucose production in the presence of increased hepatic TG content. In muscle, however, fasting resulted in increased insulin sensitivity, with increased muscle glucose uptake without changes in muscle TG content. In liver, fasting resulted in increased mRNA expression of genes promoting gluconeogenesis and TG synthesis but in decreased mRNA expression of genes involved in glycogenolysis and fatty acid synthesis. In muscle, increased mRNA expression of genes promoting glucose uptake, as well as lipogenesis and beta-oxidation, was found. In conclusion, 16 h of fasting does not induce hepatic insulin resistance, although it causes liver steatosis, whereas muscle insulin sensitivity increases without changes in muscle TG content. Therefore, fasting induces differential changes in tissue-specific insulin sensitivity, and liver and muscle TG contents are unlikely to be involved in these changes.     

Acute inhibition of hepatic beta-oxidation in APOE*3Leiden mice does not affect hepatic VLDL secretion or insulin sensitivity

Hepatic VLDL and glucose production is enhanced in type 2 diabetes and associated with hepatic steatosis. Whether the derangements in hepatic metabolism are attributable to steatosis or to the increased availability of FA metabolites is not known. We used methyl palmoxirate (MP), an inhibitor of carnitine palmitoyl transferase I, to acutely inhibit hepatic FA oxidation and investigated whether the FAs were rerouted into VLDL secretion and whether this would affect hepatic glucose production. After an overnight fast, male APOE3*Leiden transgenic mice received an oral dose of 10 mg/kg MP. Administration of MP led to an 83% reduction in plasma beta-hydroxybutyrate (ketone body) levels compared with vehicle-treated mice (0.47 +/- 0.07 vs. 2.81 +/- 0.16 mmol/l, respectively; P < 0.01), indicative of impaired ketogenesis. Plasma FFA levels were increased by 32% and cholesterol and insulin levels were decreased by 17% and 50%, respectively, in MP-treated mice compared with controls. MP treatment led to a 30% increase in liver triglyceride (TG) content. Surprisingly, no effect on hepatic VLDL-TG production was observed between the groups at 8 h after MP administration. In addition, the capacity of insulin to suppress endogenous glucose production was unaffected in MP-treated mice compared with controls. In conclusion, acute inhibition of FA oxidation increases hepatic lipid content but does not stimulate hepatic VLDL secretion or reduce insulin sensitivity.     

Mapping the collagen-binding site in the von Willebrand factor-A3 domain

The multimeric glycoprotein von Willebrand factor (VWF) mediates platelet adhesion to collagen at sites of vascular damage. The binding site for collagen types I and III is located in the VWF-A3 domain. Recently, we showed that His(1023), located near the edge between the "front" and "bottom" faces of A3, is critical for collagen binding (Romijn, R. A., Bouma, B., Wuyster, W., Gros, P., Kroon, J., Sixma, J. J., and Huizinga, E. G. (2001) J. Biol. Chem. 276, 9985-9991). To map the binding site in detail, we introduced 22 point mutations in the front and bottom faces of A3. The mutants were expressed as multimeric VWF, and binding to collagen type III was evaluated in a solid-state binding assay and by surface plasmon resonance. Mutation of residues Asp(979), Ser(1020), and His(1023) nearly abolished collagen binding, whereas mutation of residues Ile(975), Thr(977), Val(997), and Glu(1001) reduced binding affinity about 10-fold. Together, these residues define a flat and rather hydrophobic collagen-binding site located at the front face of the A3 domain. The collagen-binding site of VWF-A3 is distinctly different from that of the homologous integrin alpha(2) I domain, which has a hydrophilic binding site located at the top face of the domain. Based on the surface characteristics of the collagen-binding site of A3, we propose that it interacts with collagen sequences containing positively charged and hydrophobic residues. Docking of a collagen triple helix on the binding site suggests a range of possible engagements and predicts that at most eight consecutive residues in a collagen triple helix interact with A3.     

Identification of the collagen-binding site of the von Willebrand factor A3-domain

Von Willebrand factor (vWF) is a multimeric glycoprotein that mediates platelet adhesion and thrombus formation at sites of vascular injury. vWF functions as a molecular bridge between collagen and platelet receptor glycoprotein Ib. The major collagen-binding site of vWF is contained within the A3 domain, but its precise location is unknown. To localize the collagen-binding site, we determined the crystal structure of A3 in complex with an Fab fragment of antibody RU5 that inhibits collagen binding. The structure shows that RU5 recognizes a nonlinear epitope consisting of residues 962-966, 981-997, and 1022-1026. Alanine mutants were constructed of residues Arg(963), Glu(987), His(990), Arg(1016), and His(1023), located in or close to the epitope. Mutants were expressed as fully processed multimeric vWF. Mutation of His(1023) abolished collagen binding, whereas mutation of Arg(963) and Arg(1016) reduced collagen binding by 25-35%. These residues are part of loops alpha3beta4 and alpha1beta2 and alpha-helix 3, respectively, and lie near the bottom face of the domain. His(1023) and flanking residues display multiple conformations in available A3-crystal structures, suggesting that binding of A3 to collagen involves an induced-fit mechanism. The collagen-binding site of A3 is located distant from the top face of the domain where collagen-binding sites are found in homologous integrin I domains.     

Molecular Architecture of the Centriole Proteome: The Conserved WD40 Domain Protein POC1 Is Required for Centriole Duplication and Length Control

Centrioles are intriguing cylindrical organelles composed of triplet microtubules. Proteomic data suggest that a large number of proteins besides tubulin are necessary for the formation and maintenance of a centriole''s complex structure. Expansion of the preexisting centriole proteome from the green alga Chlamydomonas reinhardtii revealed additional human disease genes, emphasizing the significance of centrioles in normal human tissue homeostasis. We found that two classes of ciliary disease genes were highly represented among the basal body proteome: cystic kidney disease (especially nephronophthisis) syndromes, including Meckel/Joubert-like and oral-facial-digital syndrome, caused by mutations in CEP290, MKS1, OFD1, and AHI1/Jouberin proteins and cone-rod dystrophy syndrome genes, including UNC-119/HRG4, NPHP4, and RPGR1. We further characterized proteome of the centriole (POC) 1, a highly abundant WD40 domain-containing centriole protein. We found that POC1 is recruited to nascent procentrioles and localizes in a highly asymmetrical pattern in mature centrioles corresponding to sites of basal-body fiber attachment. Knockdown of POC1 in human cells caused a reduction in centriole duplication, whereas overexpression caused the appearance of elongated centriole-like structures. Together, these data suggest that POC1 is involved in early steps of centriole duplication as well as in the later steps of centriole length control.     

Photochemical activities of reaction centers from Rhodopseudomonas sphaeroides at low temperature and in the presence of chaotropic agents.

Light-induced absorbance changes were measured at low temperatures in reaction center preparations from Rhodopseudomonas sphaeroides. Absorbance difference spectra measured at 100 degrees K show that ubiquinone is photoreduced at this temperature, both by continuous light and by a short actinic flash. The reduction occurred with relatively high efficiency. These results give support to the idea that ubiquinone is involved in the primary photochemical reaction in Rhodopseudomonas sphaeroides. Reduction of ubiquinone was accompanied by a shift of the infrared absorption band of bacteriopheophytin. The rate of decay of the primary photoproducts (P+870 and ubisemiquinone) appeared to be approximately independent of temperature below 180 degrees K and above 270 degrees K; in the region between 180 and 270 degrees K it increased with decreasing temperature. The rate of decay was not affected by 0-phenanthroline. Secondary reactions were inhibited by lowering the temperature. The light-induced absorbance changes were inhibited by chaotropic agents, like thiocyanate and perchlorate. It was concluded that these agents lower the efficiency of the primary photoconversion. The kinetics indicated that the degree of inhibition was not the same for all reaction centers. The absorption spectrum of the photoconverted reaction centers appeared to be somewhat modified by thiocyanate.     

Scope and limitations of rotating-frame nuclear Overhauser enhancement spectroscopy applied to oligosaccharides

Two-dimensional rotating-frame nuclear Overhauser enhancement spectroscopy has been applied to milk oligosaccharides containing the blood-group determinants Lea, Led, or Leb. The results indicate a large number of dipolar interactions that are consistent with the preferred conformations described for these blood-group determinants. Despite careful experimental conditions, coherent transfer processes are not suppressed completely, allowing only a qualitative evaluation of the nuclear Overhauser data. The 1H- and 13C-n.m.r. spectra are assigned by comparison with literature data and by application of two-dimensional 13C-1H shift-correlation spectroscopy.     

Electron microscopy of the rabbit pineal organ in vitro

The results presented in this study show that the rabbit pineal organ cultured in vitro retained its in vivo fine structure for at least eight days. However, the Golgi complex in the light pinealocytes stopped forming dense core vesicles while vesicle-crowned ribbons increased in number. After addition of norepinephrine to the culture medium, the Golgi complex once more began the production of dense core vesicles. Terminals of light pinealocytic processes then often contained Golgi dense core vesicles in close contact with the cell membrane, suggesting the release of the vesicular content into the intercellular and perivascular spaces. A close topographical relationship between Golgi dense core vesicles and vesicle-crowned ribbons was observed.