Syndecan-2 selectively regulates VEGF-induced vascular permeability.

Vascular endothelial growth factor (VEGF)- driven increase in vascular permeability is a key feature of many disease states associated with inflammation and ischemic injury, contributing significantly to morbidity and mortality in these settings. Despite its importance, no specific regulators that preferentially control VEGF-dependent increase in permeability versus its other biological activities, have been identified. Here we report that a proteoglycan Syndecan-2 (Sdc2) regulates the interaction between a transmembrane phosphatase DEP1 and VEGFR2 by controlling cell surface levels of DEP1. In the absence of Sdc2 or the presence of an antibody that blocks Sdc2-DEP1 interaction, increased plasma membrane DEP1 levels promote selective dephosphorylation of the VEGFR2 Y951 site that is involved in permeability control. Either an endothelial-specific Sdc2 deletion or a treatment with an anti-Sdc2 antibody result in a highly significant reduction in stroke size due to a decrease in intracerebral edema.

Lighting up the cell surface with evanescent wave microscopy.

Evanescent wave microscopy, also termed total internal reflection fluorescence microscopy (TIR-FM), has shed new light on important cellular processes taking place near the plasma membrane. For example, this technique can enable the direct observation of membrane fusion of synaptic vesicles and the movement of single molecules during signal transduction. There has been a recent surge in the popularity of this technique with the advent of green-fluorescent protein (GFP) as a fluorescent marker and new technical developments. These technical developments and some of the latest applications of TIR-FM are the subject of this review.

Multicolour imaging of post-Golgi sorting and trafficking in live cells.

The biogenesis and maintenance of asymmetry is crucial to many cellular functions including absorption and secretion, signalling, development and morphogenesis. Here we have directly visualized the segregation and trafficking of apical (glycosyl phosphatidyl inositol-anchored) and basolateral (vesicular stomatitis virus glycoprotein) cargo in living cells using multicolour imaging of green fluorescent protein variants. Apical and basolateral cargo segregate progressively into large domains in Golgi/trans-Golgi network structures, exclude resident proteins, and exit in separate transport containers. These remain distinct and do not merge with endocytic structures suggesting that lateral segregation in the trans-Golgi network is the primary sorting event. Fusion with the plasma membrane was detected by total internal reflection microscopy and reveals differences between apical and basolateral carriers as well as new 'hot spots' for exocytosis.

A novel anionic modification of N-glycans on mammalian endothelial cells is recognized by activated neutrophils and modulates acute inflammatory responses.

We previously reported an unusual carboxylated modification on N:-glycans isolated from whole bovine lung. We have now raised IgG mAbs against the modification by immunization with biotinylated aminopyridine-derivatized glycans enriched for the anionic species and screening for Abs whose reactivities were abrogated by carboxylate neutralization of bovine lung glycopeptides. One such Ab (mAb GB3.1) was inhibited by carboxylated bovine lung glycopeptides and other multicarboxylated molecules, but not by glycopeptides in which the carboxylate groups were modified. The Ab recognized an epitope constitutively expressed on bovine, human, and other mammalian endothelial cells. Stimulated, but not resting, neutrophils bound to immobilized bovine lung glycopeptides in a carboxylate-dependent manner. The binding of activated neutrophils to immobilized bovine lung glycopeptides was inhibited both by mAb GB3.1 and by soluble glycopeptides in a carboxylate-dependent manner. The Ab also inhibited extravasation of neutrophils and monocytes in a murine model of peritoneal inflammation. This inhibition of cell trafficking correlated with the increased sequestration but reduced transmigration of leukocytes that were found to be adherent to the endothelium of the mesenteric microvasculature. Taken together, these results indicate that these novel carboxylated N:-glycans are constitutively expressed on vascular endothelium and participate in acute inflammatory responses by interaction with activated neutrophils.

Fusion of constitutive membrane traffic with the cell surface observed by evanescent wave microscopy.

Monitoring the fusion of constitutive traffic with the plasma membrane has remained largely elusive. Ideally, fusion would be monitored with high spatial and temporal resolution. Recently, total internal reflection (TIR) microscopy was used to study regulated exocytosis of fluorescently labeled chromaffin granules. In this technique, only the bottom cellular surface is illuminated by an exponentially decaying evanescent wave of light. We have used a prism type TIR setup with a penetration depth of approximately 50 nm to monitor constitutive fusion of vesicular stomatitis virus glycoprotein tagged with the yellow fluorescent protein. Fusion of single transport containers (TCs) was clearly observed and gave a distinct analytical signature. TCs approached the membrane, appeared to dock, and later rapidly fuse, releasing a bright fluorescent cloud into the membrane. Observation and analysis provided insight about their dynamics, kinetics, and position before and during fusion. Combining TIR and wide-field microscopy allowed us to follow constitutive cargo from the Golgi complex to the cell surface. Our observations include the following: (1) local restrained movement of TCs near the membrane before fusion; (2) apparent anchoring near the cell surface; (3) heterogeneously sized TCs fused either completely; or (4) occasionally larger tubular-vesicular TCs partially fused at their tips.

Lipid rafts and signal transduction.

Signal transduction is initiated by complex protein-protein interactions between ligands, receptors and kinases, to name only a few. It is now becoming clear that lipid micro-environments on the cell surface -- known as lipid rafts -- also take part in this process. Lipid rafts containing a given set of proteins can change their size and composition in response to intra- or extracellular stimuli. This favours specific protein-protein interactions, resulting in the activation of signalling cascades.

Dual-color visualization of trans-Golgi network to plasma membrane traffic along microtubules in living cells.

The mechanisms and carriers responsible for exocytic protein trafficking between the trans-Golgi network (TGN) and the plasma membrane remain unclear. To investigate the dynamics of TGN-to-plasma membrane traffic and role of the cytoskeleton in these processes we transfected cells with a GFP-fusion protein, vesicular stomatitis virus G protein tagged with GFP (VSVG3-GFP). After using temperature shifts to block VSVG3-GFP in the endoplasmic reticulum and subsequently accumulate it in the TGN, dynamics of TGN-to-plasma membrane transport were visualized in real time by confocal and video microscopy. Both small vesicles (<250 nm) and larger vesicular-tubular structures (>1.5 microm long) are used as transport containers (TCs). These TCs rapidly moved out of the Golgi along curvilinear paths with average speeds of approximately 0.7 micrometer/second. Automatic computer tracking objectively determined the dynamics of different carriers. Fission and fusion of TCs were observed, suggesting that these late exocytic processes are highly interactive. To directly determine the role of microtubules in post-Golgi traffic, rhodamine-tubulin was microinjected and both labeled cargo and microtubules were simultaneously visualized in living cells. These studies demonstrated that exocytic cargo moves along microtubule tracks and reveals that carriers are capable of switching between tracks.

Unusual anionic N-linked oligosaccharides from bovine lung.

We previously described a diverse family of sulfated anionic N-linked oligosaccharides released by peptide: N-glycosidase F (PNGaseF) from calf pulmonary artery endothelial (CPAE) cells (Roux, L., Holoyda, S., Sundblad, G., Freeze, H.H., and Varki, A. (1988) J. Biol. Chem. 263, 8879-8889). Since a major fraction of the intact lung consists of endothelial cells, we reasoned that bovine lung might be a rich source of similar molecules. Total N-linked oligosaccharides from bovine lung acetone powder were released by PNGaseF, labeled by [3H]NaBH4 reduction, and the anionic fractions were studied with a variety of techniques. The sugar chains with lesser negative charge (designated Class I) share several properties of conventional multiantennary complex-type chains. However, unlike the case with CPAE cells, sialic acids account only for a minority of the anionic properties and only a small proportion carry sulfate esters. A variety of different treatments indicate that most of the unexplained negative charge is due to multiple carboxylic acid groups. Resistance to beta-glucuronidase and alpha-iduronidase suggests that these may be previously undescribed modifications of mammalian oligosaccharides. The most highly charged N-linked chains (designated Class II) are more similar in general structure to the corresponding ones from CPAE cells, although relatively more abundant. Their high charge is primarily due to chondroitin sulfate, heparin/heparan sulfate, or keratan sulfate glycosaminoglycan chains. Sequential digestion studies suggest that a significant proportion of these molecules have more than one type of glycosaminoglycan chain associated with them. Compositional analysis indicates the presence of xylose residues in Class II, but not Class I molecules. However, unlike the case with conventional glycosaminoglycans, these residues are not at the reducing terminus. Most previously reported structures of complex-type N-linked oligosaccharides are derived from the glycoproteins of blood cells, plasma, or the secretions of cultured mammalian cells. This library of N-linked oligosaccharides from an intact mammalian organ (lung) contains a high proportion of novel anionic sugar chains whose structures are different from conventional complex-type sialylated chains and only partially related to those from CPAE cells. Further exploration of the N-linked chains of intact mammalian tissues seems warranted.

Advances in the use of biotinylated diaminopyridine (BAP) as a versatile fluorescent tag for oligosaccharides.

We recently described a novel fluorescent compound, 2-amino,6-amidobiotinyl-pyridine (BAP), that allows the tagging of oligosaccharides, their fractionation by reversed-phase HPLC with picomole scale detection, and the formation of functional neoglycoprotein equivalents with (strept) avidin for the detection of receptors and the generation of monospecific antibodies (Rothenberg et al., Proc. Natl Acad. Sci. USA, 90, 11939-11943, 1993). Here, we describe the enhancement of this approach by the following. (i) A simple one-step purification of BAP from its synthetic precursors and other reactants. (ii) Development of HPLC sizing column methods to quickly purify BAP-coupled oligosaccharides away from free BAP and other reactants. (iii) Development of anion-exchange and amine-adsorption HPLC procedures for the fractionation of BAP-oligosaccharide adducts by charge and size, respectively. (iv) Investigation of the affinity of BAP-oligosaccharides for (strept)avidin, confirming the formation of stable complexes. (v) The use of BAP for sensitive monosaccharide compositional analysis of glycoproteins. (vi) Formation of stable BAP adducts without reduction and its implications for the mechanism of adduct formation. These advances make available a multitude of techniques for the fractionation of BAP-coupled oligosaccharides based on several different physical parameters. Distinct species of BAP-coupled oligosaccharides can be isolated and subjected to detailed structural analysis. Such defined molecules form stable complexes with streptavidin that are effectively neoglycoproteins, which can be used in a variety of biological applications. Notably, all of these approaches require relatively inexpensive materials and conventional equipment available in most laboratories.

Biotinylated diaminopyridine: an approach to tagging oligosaccharides and exploring their biology.

Fluorescent tagging of free oligosaccharides by reductive amination permits sensitive detection and fractionation of these molecules. To expand the scope of this approach, we have synthesized a fluorescent reagent, 2-amino-(6-amidobiotinyl)pyridine. This reagent can tag oligosaccharides under nondegradative conditions with high efficiency. The resulting adducts show excellent fractionation by reverse-phase HPLC with sensitive detection in the low picomole range. When combined with sequential exoglycosidase digestion, stepwise sequencing of the sugar chains is possible. The biotinyl group can also be used to recover the sugar chain from reaction mixtures. The high-affinity interaction of the biotinyl group with multivalent avidin or streptavidin can be used to create the functional equivalent of neoglycoproteins carrying multiple copies of oligosaccharides of defined structure. These complexes allow the production of IgG antibodies directed against the oligosaccharide chain. They can also harness the power of (strept)avidin-biotin technology for the detection and isolation of oligosaccharide-specific receptors from native sources of recombinant libraries.