Fernand Leduc de 1943 à 1985

The authors look at the body of Leduc's pictorial output and evaluate his research into light and colour. Includes the transcriptions of four of the artist's letters and some technical comments on the 111 works presented. Biographical notes. Bibl. 3 p

Increased flexibility and liposome-binding capacity of CD1e at endosomal pH

International audienceThe plasma membrane proteins CD1a, CD1b and CD1c are expressed by human dendritic cells, the professional antigen-presenting cells of the immune system, and present lipid antigens to T lymphocytes. CD1e belongs to the same family of molecules, but accumulates as a membrane-associated form in the Golgi compartments of immature dendritic cells and as a soluble cleaved form in the lysosomes of mature dendritic cells. In lysosomes, the N-terminal propeptide of CD1e is also cleaved, but the functional consequences of this step are unknown. Here, we investigated how the pH changes encountered during transport to lysosomes affect the structure of CD1e and its ligand-binding properties. Circular dichroism studies demonstrated that the secondary and tertiary structures of recombinant CD1e were barely altered by pH changes. Nevertheless, at acidic pH, guanidium chloride-induced unfolding of CD1e molecules required lower concentrations of denaturing agent. The nonfunctional L194P allelic variant was found to be structurally less stable at acidic pH than the functional forms, providing an explanation for the lack of its detection in lysosomes. The number of water-exposed hydrophobic patches that bind 8-anilinonaphthalene-1-sulfonate was higher in acidic conditions, especially for the L194P variant. CD1e molecules interacted with lipid surfaces enriched in anionic lipids, such as bis(monoacylglycero)phosphate, a late endosomal/lysosomal lipid, especially at acidic pH, or when the propeptide was present. Altogether, these data indicate that, in the late endosomes/lysosomes of DCs, the acid pH promotes the binding of lipid antigens to CD1e through increased hydrophobic and ionic interactions

Birbeck granule-like "organized smooth endoplasmic reticulum" resulting from the expression of a cytoplasmic YFP-tagged langerin.

Langerin is required for the biogenesis of Birbeck granules (BGs), the characteristic organelles of Langerhans cells. We previously used a Langerin-YFP fusion protein having a C-terminal luminal YFP tag to dynamically decipher the molecular and cellular processes which accompany the traffic of Langerin. In order to elucidate the interactions of Langerin with its trafficking effectors and their structural impact on the biogenesis of BGs, we generated a YFP-Langerin chimera with an N-terminal, cytosolic YFP tag. This latter fusion protein induced the formation of YFP-positive large puncta. Live cell imaging coupled to a fluorescence recovery after photobleaching approach showed that this coalescence of proteins in newly formed compartments was static. In contrast, the YFP-positive structures present in the pericentriolar region of cells expressing Langerin-YFP chimera, displayed fluorescent recovery characteristics compatible with active membrane exchanges. Using correlative light-electron microscopy we showed that the coalescent structures represented highly organized stacks of membranes with a pentalaminar architecture typical of BGs. Continuities between these organelles and the rough endoplasmic reticulum allowed us to identify the stacks of membranes as a form of "Organized Smooth Endoplasmic Reticulum" (OSER), with distinct molecular and physiological properties. The involvement of homotypic interactions between cytoplasmic YFP molecules was demonstrated using an A206K variant of YFP, which restored most of the Langerin traffic and BG characteristics observed in Langerhans cells. Mutation of the carbohydrate recognition domain also blocked the formation of OSER. Hence, a "double-lock" mechanism governs the behavior of YFP-Langerin, where asymmetric homodimerization of the YFP tag and homotypic interactions between the lectin domains of Langerin molecules participate in its retention and the subsequent formation of BG-like OSER. These observations confirm that BG-like structures appear wherever Langerin accumulates and confirm that membrane trafficking effectors dictate their physiology and, illustrate the importance of molecular interactions in the architecture of intracellular membranes

Correction: An essential role for α4A-tubulin in platelet biogenesis

International audienceNo abstract availabl

Continuity of the BG-like structures with the rough ER.

<p>Transfected M10 cells stably expressing YFP-Lang were processed for electron microscopy. M10 cells stably expressing YFP-Lang were processed for FIB/SEM. The surface of the block was ion-milled and serial images were acquired. A 3D reconstruction was then obtained from the image stack after manual segmentation. (A) A stack of BG-like membranes and sacs of the rough ER in the same plane with (B) manual segmentation of the different objects of interest. The BG-like structures appear in orange, yellow and red and the ER in green. (C and D) A 3D reconstruction demonstrating that continuity (blue segments) exists between the BG-like membranes and the ER (two different angles of view, see also <b><a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0060813#pone.0060813.s007" target="_blank">Video S1</a></b>). Scale bars: 500 nm.</p

FRAP analysis of the mobilities of Langerin/YFP chimeras.

<p>(A) Selected regions of M10 cells stably expressing either Lang-YFP (upper panels) or YFP-Lang (middle and lower panels) were bleached (arrows) and the fluorescence in single z-sections was recorded approximately every 1.6 s. (B) The fluorescence in the bleached area was quantified and plotted against time after correction for the change in total fluorescence. At least 10 cells were analyzed for each plot; error bars indicate the standard error of the mean. F_i and F_m designate the immobile and mobile fractions of the Langerin/YFP chimeras, respectively.</p

Schematic view of the proposed mechanism of formation of BG-like OSER.

<p>The ER lumen (L) is depicted in pale gray and the cytoplasm (C) in white. Homotypic interactions between the CRD domains of Langerin (C-terminal, intra-luminal position, dark gray stars) are responsible for the luminal zipping of Langerin-enriched ER membranes, while homotypic interactions between YFP molecules (N-terminal, cytoplasmic position, green circles) are involved in the stacking of BG-like membranes. This “two-lock mechanism” could plausibly explain the high rigidity and immobility of these structures.</p