Xanthones et C-glucosides flavoniques du genre Gentiana (sous-genre Gentianella)

Xanthones with 1,3,5,8 or 1,3,4,5,8-oxidation pattern, the C-glucosides mangiferin, isoorientin and swertisin, were isolated from the aerial parts of G. germanica and G. ramosa. The distribution of these compounds within the subgenus Gentianella is given. Phenolic patterns in Gentiana and in Swertia are compared.     

GPR54 regulates ERK1/2 activity and hypothalamic gene expression in a Gα(q/11) and β-arrestin-dependent manner

G protein-coupled receptor 54 (GPR54) is a G(q/11)-coupled 7 transmembrane-spanning receptor (7TMR). Activation of GPR54 by kisspeptin (Kp) stimulates PIP(2) hydrolysis, Ca(2+) mobilization and ERK1/2 MAPK phosphorylation. Kp and GPR54 are established regulators of the hypothalamic-pituitary-gonadal (HPG) axis and loss-of-function mutations in GPR54 are associated with an absence of puberty and hypogonadotropic hypogonadism, thus defining an important role of the Kp/GPR54 signaling system in reproductive function. Given the tremendous physiological and clinical importance of the Kp/GPR54 signaling system, we explored the contributions of the GPR54-coupled G(q/11) and β-arrestin pathways on the activation of a major downstream signaling molecule, ERK, using G(q/11) and β-arrestin knockout mouse embryonic fibroblasts. Our study revealed that GPR54 employs the G(q/11) and β-arrestin-2 pathways in a co-dependent and temporally overlapping manner to positively regulate ERK activity and pERK nuclear localization. We also show that while β-arrestin-2 potentiates GPR54 signaling to ERK, β-arrestin-1 inhibits it. Our data also revealed that diminished β-arrestin-1 and -2 expression in the GT1-7 GnRH hypothalamic neuronal cell line triggered distinct patterns of gene expression following Kp-10 treatment. Thus, β-arrestin-1 and -2 also regulate distinct downstream responses in gene expression. Finally, we showed that GPR54, when uncoupled from the G(q/11) pathway, as is the case for several naturally occurring GPR54 mutants associated with hypogonadotropic hypogonadism, continues to regulate gene expression in a G protein-independent manner. These new and exciting findings add significantly to our mechanistic understanding of how this important receptor signals intracellularly in response to kisspeptin stimulation.     

Surface nucleolin participates in both the binding and endocytosis of lactoferrin in target cells.

Lactoferrin (Lf), a multifunctional molecule present in mammalian secretions and blood, plays important roles in host defense and cancer. Indeed, Lf has been reported to inhibit the proliferation of cancerous mammary gland epithelial cells and manifest a potent antiviral activity against human immunodeficiency virus and human cytomegalovirus. The Lf-binding sites on the cell surface appear to be proteoglycans and other as yet undefined protein(s). Here, we isolated a Lf-binding 105 kDa molecular mass protein from cell extracts and identified it as human nucleolin. Medium-affinity interactions ( approximately 240 nm) between Lf and purified nucleolin were further illustrated by surface plasmon resonance assays. The interaction of Lf with the cell surface-expressed nucleolin was then demonstrated through competitive binding studies between Lf and the anti-human immunodeficiency virus pseudopeptide, HB-19, which binds specifically surface-expressed nucleolin independently of proteoglycans. Interestingly, binding competition studies between HB-19 and various Lf derivatives in proteoglycan-deficient hamster cells suggested that the nucleolin-binding site is located in both the N- and C-terminal lobes of Lf, whereas the basic N-terminal region is dispensable. On intact cells, Lf co-localizes with surface nucleolin and together they become internalized through vesicles of the recycling/degradation pathway by an active process. Morever, a small proportion of Lf appears to translocate in the nucleus of cells. Finally, the observations that endocytosis of Lf is inhibited by the HB-19 pseudopeptide, and the lack of Lf endocytosis in proteoglycan-deficient cells despite Lf binding, point out that both nucleolin and proteoglycans are implicated in the mechanism of Lf endocytosis.     

Phosphate availability affects the tonoplast localization of PLDzeta2, an Arabidopsis thaliana phospholipase D

Under phosphate deprivation, higher plants change their lipid composition and recycle phosphate from phospholipids. A phospholipase D, PLDzeta2, is involved in this recycling and in other cellular functions related to plant development. We investigated the localization of Arabidopsis PLDzeta2 by cell fractionation and in vivo GFP confocal imaging. AtPLDzeta2 localizes to the tonoplast and the Nter regulatory domain is sufficient for its sorting. Under phosphate deprivation, AtPLDzeta2 remains located in the tonoplast but its distribution is uneven. We observed PLDzeta2-enriched tonoplast domains preferentially positioned close to mitochondria and beside chloroplasts. In absence of PLDzeta2, membrane developments were visualized inside vacuoles.     

Loss of [3H]kainate and of NMDA-displaceable [3H]glutamate binding sites in brain in thiamine deficiency: Results of a quantitative autoradiographic study

Previous studies suggest that alterations of brain glutamate synthesis and release occur in experimental thiamine deficiency. In order to assess the integrity of post-synaptic glutamatergic receptors in thiamine deficiency, binding sites for [³H]glutamate (displaced by NMDA), [³H]-kainate, and [³H]quisqualate (AMPA sites) were evaluated using Quantitative Receptor Autoradiography in rat brain following 14 days of treatment with the central thiamine antagonist pyrithiamine. Compared to pair-fed controls, brains of symptomatic thiamine-deficient animals contained significantly fewer NMDA-displaceable binding sites in cerebral cortex, medial septum and hippocampus. It has been suggested that NMDA-receptor mediated glutamate excitotoxicity plays a role in the pathogenesis of neuronal loss in thiamine deficiency. If such is the case, the selective loss of NMDA binding sites in cerebral cortex and hippocampus offers a possible explanation for the relative nonvulnerability of these brain regions to pyrithiamine-induced thiamine deficiency. [³H]quisqualate (AMPA) binding sites were unchanged in all brain regions of pyrithiamine-treated rats whereas [³H]kainate sites were significantly reduced in density in medial and lateral thalamus. The decline in these binding sites may be due to neuronal loss in pyrithiamine-induced thiamine deficiency. Alterations of glutamatergic synaptic function involving both NMDA and kainate receptor subclasses could contribute to the pathogenesis of neurological dysfunction in Wernicke's Encephalopathy in humans.     

Analysis of the distribution of the kinetochore protein Ndc10p in <Emphasis Type="Italic"> Saccharomyces cerevisiae </Emphasis>using 3-D modeling of mitotic spindles

Ndc10p is one of the DNA-binding constituents of the kinetochore in Saccharomyces cerevisiae but light microscopy analysis suggests that Ndc10p is not limited to kinetochore regions. We examined the localization of Ndc10p using immunoelectron microscopy and showed that Ndc10p is associated with spindle microtubules from S-phase through anaphase. By serial section reconstruction of mitotic spindles combined with immunogold detection, we showed that Ndc10p interacts with microtubules laterally as well as terminally. About 50% of the gold label in serial section reconstructions of short mitotic spindles was associated with the walls of spindle microtubules. Interaction of kinetochore components with microtubule walls was also shown for kinetochore protein Ndc80p. Our data suggest that at least a subset of kinetochore-associated protein is dispersed throughout the mitotic spindle.