Efficient cell surface labelling of live zebrafish embryos: wash-free fluorescence imaging for cellular dynamics tracking and nanotoxicity evaluation

We developed a red-fluorescent stain capable of achieving wash-free plasma membrane imaging in vitro and in vivo.

MUNC13 REGULATES SECRETION AT THE GOLGI IN A LOCALIZATION-DEPENDENT MANNER

Background: Constitutive secretion is critical for the maintenance of eukaryotic cell structure and function. Our lab has shown that Rab34 is required for secretion at the Golgi^1, and that the C1 domain-containing protein, Munc13, is an effector of Rab34^2. Current studies seek to elucidate potential roles for Munc13in secretion at the Golgi. Methods: Using a temperature-sensitive mutant of the Vesicular Stomatitis G-protein fused to GFP (VSVG-GFP) to monitor secretion, we examinedthe role of Munc13 in secretion in HeLa cells. Cells transfected with VSVG-GFP were treated with Munc13, amutant lacking the C1 domain (C1-less), and the phorbol esters TPA andPDBu. The rate of VSVG-GFP secretion was monitored using surface labelling of plasmalemmal VSVG-GFP and spinning disc confocal microscopy. Results: TPA treatment resulted in an increase in the rate of VSVG-GFP appearance at the plasma membrane. Co-transfection of either Munc13 or C1-less alone also resulted in an increased rate of VSVG-GFP transport. Transfection of Munc13 plus TPA treatment resulted in amarked decrease in the rate of VSVG-GFP transport. Since TPA treatment relocalizes Munc13 to the plasma membrane, this result suggests that the availability of Munc13 in the cytosol is required for its effect on VSVG-GFP secretion. Conclusions: Munc13 over-expression increases the rate of VSVG-GFP secretion to the plasma membrane. Sequestration of Munc13 at the plasma membrane with TPA abrogates thiseffect, and reduces the rate of VSVG-GFP secretion. We propose that Munc13 effects VSVG-GFP secretion via its interaction with Rab34 at the Golgi. References: 1. Goldenberg, NM, S. Grinstein, M. Silverman. Golgi-bound Rab34 is a Novel Member ofthe Secretory Pathway. Mol BiolCell. 18(12):4762-4771 (2007). 2. Speight, P, M. Silverman.Diacylglycerol-Activated Hmunc13 Serves as an Effector of the GTPaseRab34. Traffic.6(10):858-865 (2005).

Characterization of RagA and RagB in Porphyromonas gingivalis: study using gene-deletion mutants

The major outer-membrane proteins RagA and RagB of Porphyromonas gingivalis are considered to form a receptor complex functionally linked to TonB. In this study, P. gingivalis mutants with ragA, ragB or both deleted were constructed from strain W83 as the parent to examine the physiological and pathological functions of RagA and RagB. The double-deletion mutant completely lacked both RagA and RagB, whereas the ΔragA mutant reduced RagB expression considerably and the ΔragB mutant produced degraded RagA. Growth of the three mutants in a nutrient-rich medium and synthetic media containing digested protein as a unique nutrient source was similar to that of the parental strain; however, both the ΔragA and ΔragAB mutants exhibited very slow growth in a synthetic medium containing undigested, native protein, and the two mutants tended to lose their viability during experiments, although gingipain (protease) activities were unchanged in the mutants. A mouse model showed that the ΔragB mutant had reduced virulence. Cell-surface labelling with biotin and dextran revealed that both RagA and RagB localized on the outermost cell surface. A cross-linking experiment using wild-type P. gingivalis showed that RagA and RagB were closely associated with each other. Furthermore, co-immunoprecipitation confirmed that RagA and RagB formed a protein–protein complex. These results suggest that physically associated RagA and RagB may stabilize themselves on the cell surface and function as active transporters of large degradation products of protein and in part as a virulence factor.

CEACAM1 isoforms with different cytoplasmic domains show different localization, organization and adhesive properties in polarized epithelial cells

CEACAM1 is a signaling cell adhesion molecule expressed in epithelia,vessel endothelia and leukocytes. It is expressed as two major isoforms with different cytoplasmic domains. CEACAM1 occurs both in cell-cell contact areas and on apical surfaces of polarized epithelial cells, but it is not known how the different isoforms are distributed in polarized cells or what the functions of CEACAM1 are in the apical surfaces. We investigated the localization and organization of the two CEACAM1 isoforms in transfected,polarized MDCK cells by confocal microscopy and differential surface labelling. CEACAM1-L was found on both the apical and the lateral surfaces,whereas CEACAM1-S appeared exclusively on the apical surfaces. Maintenance of the lateral localization of CEACAM1-L required homophilic binding between CEACAM1-L molecules on adjacent cells. Double-labelling with anti-CEACAM1 antibodies directed against different epitopes indicated that apical CEACAM1-L occurred either in a homophilic adhesive state or in a free non-adhesive state. CEACAM1-S appeared almost exclusively in the homophilic adhesive state. These findings suggest that CEACAM1 mediates adhesive bonds between adjacent microvilli on the apical surfaces.

Identification of nucleolin as a new L-selectin ligand

Apart from leucocyte–endothelial interactions, the adhesion molecule L-selectin mediates the homotypic adhesion of leucocytes during recruitment at sites of acute inflammation, as well as intercellular adhesion of haematopoietic progenitor cells during haematopoiesis. There is evidence that, in addition to P-selectin glycoprotein ligand-1, other as-yet-unidentified proteins function as L-selectin ligands on human leucocytes and haematopoietic progenitor cells. In the present study, we show: (i) by affinity chromatography on L-selectin–agarose; (ii) by protein identification using MS; and (iii) by covalent cell-surface labelling with sulphosuccinimidyl-2-(biotinamido)ethyl-1,3-dithiopropionate that the multifunctional nuclear protein nucleolin is partly exposed on the cell surface, and is a ligand of L-selectin in human leucocytes and haematopoietic progenitor cells.

GLUT4 vesicle trafficking in rat adipocytes after ethanol feeding: regulation by heterotrimeric G-proteins

Long-term ethanol consumption decreases insulin-stimulated glucose uptake in isolated rat adipocytes. Here we investigate the mechanisms for this decrease. Male Wistar rats were fed for 4 weeks with a liquid diet containing 35% of the calories from ethanol and compared with pair-fed controls. Stimulation of 3-O-methylglucose transport in isolated adipocytes by insulin was decreased by 70% after ethanol feeding. However, stimulation by insulin of the tyrosine phosphorylation of the p85 subunit of phosphoinositide 3-kinase and the phosphorylation of Akt were not affected by ethanol feeding. GLUT4 was mobilized from intracellular light microsomes in response to insulin in both pair-fed and ethanol-fed rats, resulting in 4.3-fold and 3.3-fold increases in GLUT4 associated with plasma membrane in pair-fed and ethanol-fed rats respectively. Surface-accessible GLUT4, assessed by a trypsin cleavage assay or cell-surface labelling with bis-mannose photolabel, was increased 2.3-fold and 1.6-fold respectively, in pair-fed rats after treatment with insulin. In contrast, insulin did not increase surface-accessible GLUT4 in ethanol-fed rats. Treatment of adipocytes with R-phenylisopropyladenosine, an adenosine A1 receptor agonist, increased the transport of 3-O-methylglucose and trypsin-accessible GLUT4, in adipocytes from both pair-fed and ethanol-fed rats. These results demonstrate that whereas the insulin-mediated signalling and translocation of GLUT4 to the plasma membrane is maintained after ethanol feeding, the final fusion of GLUT4 vesicles to the plasma membrane is disrupted, preventing the stimulation of glucose uptake by insulin. Fusion of GLUT4 with the plasma membrane can be stimulated by the activation of adenosine A1 receptors.