Retinoic acid signaling is essential for pancreas development and promotes endocrine at the expense of exocrine cell differentiation in Xenopus

How and when the vertebrate endoderm is first subdivided into discrete progenitor cell populations that will give rise to the different major organs, including pancreas and liver, are only poorly understood. We have used Xenopus laevis as a model system to characterize these events, since it is particularly suited to study the early embryonic patterning in vertebrates. Our experimental results support the notion that retinoic acid (RA) functions as an essential endodermal patterning signal in Xenopus and that it acts as early as during gastrulation. As a result of RA treatment, the expression of Sonic Hedgehog (Shh), a known inhibitor of pancreas development in other vertebrate systems, is negatively regulated in the dorsal prepancreatic endoderm. Furthermore, RA is found to promote endocrine at the expense of exocrine differentiation in the dorsal pancreas, correlating with a specific inhibition of Notch signaling activities in this territory. Conversely, RA enhances exocrine marker gene expression in the ventral pancreas.     

Experimental indication for the existence of multiple Trp rotamers in von Willebrand Factor A3 domain

The first step in both normal haemostasis and arterial thrombosis is the interaction between collagen, von Willebrand factor (vWF), and glycoprotein Ib. The A3 domain of vWF forms the principal binding site for collagen type I and type III. Inhibition of the vWF-collagen interaction by an anti-human vWF monoclonal antibody (MoAb) 82D6A3 can be a potential way to prevent arterial thrombosis. Identification of the epitope of MoAb 82D6A3 showed recently that the consensus sequence SPWR obtained by phage display could adopt the conformation of the discontinuous epitope. Modelling showed that Trp982 in the vWF had to obtain a more solvent accessible conformation. We performed a detailed fluorescence study of Trp982 in the vWF A3. Using the method described by Hellings et al. (Biophys J 2003;85:1894-1902), we were able to identify two different low-energy Trp982 rotamers and to link them with their experimentally derived fluorescence lifetimes. Fluorescence anisotropy showed no interconversion in the nanosecond timescale between the two different rotameric states. With these experiments, we gather strong indications for the existence of an exposed rotamer conformation and a rotamer that corresponds to the one observed in the X-ray structure. These results strongly support the modeling work (Vanhoorelbeke et al., J Biol Chem 2003;278:37815-37821).     

Paracrine actions of the stomach-derived leptin

Leptin, a 16 kilodalton protein-encoded by the ob gene, is involved in the regulation of food intake, body composition, and energy expenditure through a central feedback mechanism. Initially thought to be adipocyte-specific, the ob gene, as well as the leptin receptor, has been found in a variety of other tissues. Relevant to this review, the leptin gene and its receptor have been identified in the stomach, intestine, liver, and pancreas. Recent data also suggest that gut leptin may act locally within the gastrointestinal tract to influence intestinal functions such as nutrient absorption and may have a physiopathological implication. This review emphasises the concept that leptin may be a new gastrointestinal hormone.     

A human immunodeficiency virus type 1-infected individual with low viral load harbors a virus variant that exhibits an in vitro RNA dimerization defect

We investigated the in vitro RNA dimerization properties of the untranslated leader RNA derived from human immunodeficiency virus type 1 variants circulating in an individual with a low viral load and slow disease progression. The leader sequences of these viruses contain highly unusual polymorphisms within the dimerization initiation site (DIS): an insert that abolishes dimerization and a compensatory substitution. The dimerization of leader RNA from late stages of infection is further improved by additional mutations outside the DIS motif that facilitate a secondary structure switch from a dimerization-incompetent to a dimerization-competent RNA conformation.     

Flow cytometric and immunoblot assays for cell surface ADP-ribosylation using a monoclonal antibody specific for ethenoadenosine

NAD-dependent ADP-ribosylation is one of the posttranslational protein modifications. On mammalian cells, glycosylphosphatidylinositol-anchored cell surface ADP-ribosyltransferases (ARTs) ADP-ribosylate other cell surface proteins and thereby affect important cellular functions. Here we describe convenient flow-cytometric and immunoblot assays for monitoring ADP-ribosylation of cell surface proteins on living cells by exploiting the capacity of ARTs to utilize etheno-NAD as substrate. Etheno-ADP-ribosylation of cell surface proteins can be detected by flow cytometry with 1G4, a monoclonal antibody specific for ethenoadenosine. Labeling of cells with 1G4 is dependent on the expression of cell surface ARTs and occurs only after incubation of ART-expressing cells with etheno-NAD and not with etheno-ADP-ribose. Dose-response analyses show efficient 1G4 staining of ART-expressing cells at micromolar etheno-NAD concentrations. Half-maximal staining is obtained with 1-2 micro M etheno-NAD, saturation is reached at 5-20 micro M etheno-NAD. Immunoblot analyses confirm that ART-expressing cells incorporate ethenoadenosine covalently (i.e., SDS resistant) into several cell surface proteins. The flow-cytometric 1G4 staining assay can be used to identify subpopulations of cells expressing cell surface ART activity and to select ART(hi) cell variants. The immunoblot 1G4 staining assay can also be used to identify etheno-ADP-ribosylated target proteins. These new assays hold promise for many interesting applications in biochemistry and cell biology.     

Subcellular localization of collapsin response mediator proteins to lipid rafts

Collapsin response mediator proteins (CRMPs) are involved in signal transduction after exposure of neural cells to the axon guidance molecule Semaphorin 3A/collapsin. All five known CRMPs are expressed in the developing cerebral cortex and neocortical neurons are responsive to Semaphorin 3A. Here, we examine the expression and subcellular localization of CRMPs in neocortical neurons and in neonatal rat brain. In neocortical neurons CRMP-4 was detected in the perikaryon with a diffuse cytosolic distribution. In neurites and at growth cones punctate staining patterns were observed. Extraction of neuron cultures with methyl-beta-cyclodextrin to deplete cholesterol caused rapid redistribution of the punctate CRMP-4 staining into larger patches and abundant growth cone collapse. Western blotting of brain extracts demonstrated for all CRMPs the existence of soluble, detergent-extractable, and Triton X-100-resistant forms. Furthermore, sucrose density gradient centrifugation after solubilization of brain membranes with Triton X-100 revealed that CRMP-1, -3, -5, and to a lower extent CRMP-4 are associated with a detergent-resistant fraction with low buoyant density, but CRMP-2 was not detectable in this fraction. Thus, we propose that lipid rafts form sites for the compartmentalization of signaling events involving specific CRMPs and that the integrity of these membrane microdomains is essential for the maintenance of growth cones.     

Sulfenic acid formation in human serum albumin by hydrogen peroxide and peroxynitrite

Human serum albumin (HSA), the most abundant protein in plasma, has been proposed to have an antioxidant role. The main feature responsible for this property is its only thiol, Cys34, which comprises approximately 80% of the total free thiols in plasma and reacts preferentially with reactive oxygen and nitrogen species. Herein, we show that the thiol in HSA reacted with hydrogen peroxide with a second-order rate constant of 2.26 M(-1) s(-1) at pH 7.4 and 37 degrees C and a 1:1 stoichiometry. The formation of intermolecular disulfide dimers was not observed, suggesting that the thiol was being oxidized beyond the disulfide. With the reagent 7-chloro-4-nitrobenzo-2-oxa-1,3-diazol (NBD-Cl), we were able to detect the formation of sulfenic acid (HSA-SOH) from the UV-vis spectra of its adduct. The formation of sulfenic acid in Cys34 was confirmed by mass spectrometry using 5,5-dimethyl-1,3-cyclohexanedione (dimedone). Sulfenic acid was also formed from exposure of HSA to peroxynitrite, the product of the reaction between nitric oxide and superoxide radicals, in the absence or in the presence of carbon dioxide. The latter suggests that sulfenic acid can also be formed through free radical pathways since following reaction with carbon dioxide, peroxynitrite yields carbonate radical anion and nitrogen dioxide. Sulfenic acid in HSA was remarkably stable, with approximately 15% decaying after 2 h at 37 degrees C under aerobic conditions. The formation of glutathione disulfide and mixed HSA-glutathione disulfide was determined upon reaction of hydrogen peroxide-treated HSA with glutathione. Thus, HSA-SOH is proposed to serve as an intermediate in the formation of low molecular weight disulfides, which are the predominant plasma form of low molecular weight thiols, and in the formation of mixed HSA disulfides, which are present in approximately 25% of circulating HSA.     

In situ identification of protein structural changes in prion-infected tissue

Transmissible spongiform encephalopathies (TSE) are fatal neurodegenerative disorders characterized by the conversion of the normal prion protein (PrP(C)) into aggregates of its pathological conformer (PrP(Sc)). The mechanism behind this structural conversion is unclear. We report the identification of disease-related protein structural differences directly within the tissue environment. Utilizing a synchrotron infrared (IR) light source, IR images of protein structure were obtained at a subcellular resolution, revealing regions of decreased alpha-helical content and elevated beta-sheet structure in and around infected neurons in the 263 K scrapie hamster model. PrP(Sc) immunostaining of the same tissue demonstrated that the elevated beta-sheet regions correspond to regions where the misfolded structure of PrP(Sc) is located. No evidence of these structural changes was observed in normal neurons.     

Effects of perinatal maternal food restriction on pituitary-gonadal axis and plasma leptin level in rat pup at birth and weaning and on timing of puberty

The effects of maternal 50% food restriction (FR) during the last week of gestation and/or lactation on pituitary-gonadal axis (at birth and weaning), on circulating levels of leptin (at weaning), and on the onset of puberty have been determined in rats at birth and at weaning. Maternal FR during pregnancy has no effect at term on the litter size, on the basal level of testosterone in male pups, and on the drastic surge of circulating testosterone that occurs 2 h after birth. At weaning, similar retardation of body growth is observed in male and female pups from mothers exposed to FR. This undernutrition induces the most drastic effects when it is performed during both gestation and lactation or during lactation alone. Drastic retardation of testicle growth with reduction of cross-sectional area and intratubular lumen of the seminiferous tubules is observed in male pups from mothers exposed to undernutrition during both gestation and lactation or during lactation alone. Maternal FR during the perinatal period reduces circulating levels of FSH in male pups without affecting LH and testosterone concentrations. Maternal FR does not affect circulating levels of LH, estradiol, and progesterone in female pups. Female pups from mothers exposed to FR during both gestation and lactation show a significant increase of plasma FSH as well as a drastic retardation of ovarian growth. The follicular population was also altered. The number of antral follicles of small size (vesicular follicles) was increased, although the number of antral follicles of large size (graafian follicles) was reduced. Maternal FR occurring during both late gestation and lactation (male and female pups), during lactation alone (male and female pups), or during late gestation (female pups) induces a drastic reduction of plasma leptin and fat mass in pups at weaning. The onset of puberty is delayed in pups of both sexes from mothers exposed to FR during lactation and during both gestation and lactation. In conclusion, these data demonstrate that a perinatal growth retardation induced by maternal FR has long-term consequences on both size and histology of the genitals, on plasma gonadotropins and leptin levels, on fat stores at weaning, and on the onset of puberty.