Distribution and Function of Macrophage Galactose-type C-type Lectin 2 (MGL2/CD301b): EFFICIENT UPTAKE AND PRESENTATION OF GLYCOSYLATED ANTIGENS BY DENDRITIC CELLS*

Dendritic cells (DCs) express cell surface lectins that are potentially involved in the recognition, uptake, and presentation of glycosylated foreign substances. A unique calcium-type (C-type) lectin, the macrophage galactose (Gal)-type C-type lectin (MGL/CD301) expressed on DCs, is thought to participate in the recognition of molecules from both altered self and pathogens due to its monosaccharide specificity for Gal and N-acetylgalactosamine (GalNAc). Although mice have two MGL genes, Mgl1 and Mgl2, their distinct roles have not been previously explored. The present report characterizes the properties of MGL2 by examining its distribution and its role in antigen presentation by DCs. We generated an MGL2-specific monoclonal antibody and examined MGL2 expression in tissues by immunohistochemistry and in isolated cells by flow cytometry. The cells reactive with this antibody were shown to be a portion of MGL1-expressing cells, mostly conventional DCs. Internalization of soluble polyacrylamide polymers (PAA) with α-GalNAc residues (GalNAc-PAA) by bone marrow-derived DCs (BM-DCs) was mediated by MGL2, as revealed by a comparison of Mgl1^−/− and Mgl2^−/− BM-DCs with wild-type BM-DCs. Biotinylated GalNAc-PAA conjugated to streptavidin (SAv) was more efficiently presented to SAv-primed T cells by BM-DCs than β-N-acetylglucosamine-PAA conjugated to SAv or SAv alone as shown by thymidine uptake and cytokine production. This is the first report that demonstrates the involvement of GalNAc residues in antigen uptake and presentation by DCs that lead to CD4⁺ T cell activation.     

Characterization of missense mutations and large deletions in the ALPL gene by sequencing and quantitative multiplex PCR of short fragments

Hypophosphatasia is a rare inherited bone disorder characterized by defective bone and dental mineralization and deficiency of serum and liver/bone/kidney alkaline phosphatase activity. The disease is due to mutations in the alkaline phosphatase liver-type (ALPL) gene. Gross deletions or insertions have not previously been reported in this gene. We report here the characterization of nine novel ALPL gene mutations in a series of 8 patients affected by various forms of hypophosphatasia. The newly discovered mutations included five missense mutations (c.368C --> A, c.814C--> T, c.1196C--> T, c.1199C--> T, c.1283G--> C), two small deletions (c.797_802del, c.1044_1055del), and two large deletions. The large deletions were detected by quantitative multiplex polymerase chain reaction (PCR) of short fluorescent fragments (QMPSF). We conclude that QMPSF slightly reduces the proportion of undetected mutations in hypophosphatasia and improves genetic counselling in the affected families.     

The PLB2 gene of Saccharomyces cerevisiae confers resistance to lysophosphatidylcholine and encodes a phospholipase B/lysophospholipase

The PLB1 gene of Saccharomyces cerevisiae encodes a protein that demonstrates phospholipase B, lysophospholipase, and transacylase activities. Several genes with significant homology to PLB1 exist in the S. cerevisiae genome, raising the possibility that other proteins may contribute to the total phospholipase B/lysophospholipase/transacylase activities of the cell. We report the isolation of a previously uncharacterized gene that is highly homologous to PLB1 and that, when overexpressed, confers resistance to 1-palmitoyllysophosphatidylcholine. This gene, which is located adjacent to the PLB1 gene on the left arm of chromosome XIII and which we refer to as PLB2, encodes a phospholipase B/lysophospholipase. Unlike PLB1, this gene product does not contain significant transacylase activity. The PLB2 gene product shows lysophospholipase activity toward lysophosphatidylcholine, lysophosphatidylserine, and lysophosphatidylethanolamine. Whereas deletion of either PLB1 or PLB2 resulted in the loss of 80% of cellular lysophospholipase activity, a plb1/plb2 double deletion mutant is completely devoid of lysophospholipase activity toward the preferred substrate lysophosphatidylcholine. Overexpression of PLB2 was associated with an increase in total cellular phospholipase B/lysophospholipase activity, as well as the appearance of significant lysophospholipase activity in the medium. Moreover, overexpression of PLB2 was associated with saturation at a higher cell density, and an increase in total cellular phospholipid content, but no change in phospholipid composition or fatty acid incorporation into cellular lipids. Deletion of PLB2 was not lethal and did not result in alteration of membrane phospholipid composition or content. PLB2 gene expression was found to be maximal during exponential growth conditions and was decreased in late phase, in a manner similar to other genes involved in phospholipid metabolism.     

Differentiation of answer of glioma C6 cells to SERCA pump inhibitors by actin disorganization

Capacitative calcium entry, usually evoked by receptor-ligand binding, may be also studied in the model system of calcium release after SERCA pump inhibition. We have previously found that disorganization of actin cytoskeleton has no effect on calcium influx into glioma C6 cells after thapsigargin administration [Biochem. Biophys. Res. Commun. 296 (2002) 484]. In the present work we show that the effect of other SERCA pump inhibitors depends on the endoplasmic reticulum distribution in a cell. Changing this distribution leads to changes in calcium release from ER stores. Intensity of calcium influx in the capacitative phase of cell answer does not depend on actin cytoskeleton state; however, administration of cytochalasin D significantly slows down signal build-up. While cyclopiazonic acid acts very similarly to thapsigargin, cytoskeleton disorganization leads to rise of calcium signal after administration of 2,5-di-(t-butyl)-1,4-benzohydroquinone. This effect may be caused by specific binding of this inhibitor to SERCA3 isoform of pump protein only.     

Kinetic studies on the inhibition of GABA-T by gamma-vinyl GABA and taurine

Gamma-aminobutyric acid transaminase (GABA-T, EC 2.6.1.19) is a pyridoxal phosphate (PLP) dependent enzyme that catalyzes the degradation of gamma-aminobutyric acid. The kinetics of this reaction are studied in vitro, both in the absence, and in the presence of two inhibitors: gamma-vinyl GABA (4-aminohex-5-enoic acid), and a natural product, taurine (ethylamine-2-sulfonic acid). A kinetic model that describes the transamination process is proposed. GABA-T from Pseudomonas fluorescens is inhibited by gamma-vinyl GABA and taurine at concentrations of 51.0 and 78.5 mM. Both inhibitors show competitive inhibition behavior when GABA is the substrate and the inhibition constant (Ki) values for gamma-vinyl GABA and taurine were found to be 26 +/- 3 mM and 68 +/- 7 mM respectively. The transamination process of alpha-ketoglutarate was not affected by the presence of gamma-vinyl GABA, whereas, taurine was a noncompetitive inhibitor of GABA-T when alpha-ketoglutarate was the substrate. The inhibition dissociation constant (Kii) for this system was found to be 96 +/- 10 mM. The Michaelis-Menten constant (Km) in the absence of inhibition, was found to be 0.79 +/- 0.11 mM, and 0.47 +/- 0.10 mM for GABA and alpha-ketoglutarate respectively.     

Potent,extra-channel influence of several calcium-channel modulators on striatal binding of [3H]tyramine

A number of Ca²⁺-, K⁺-, and Na⁺-channel modulators has been tested with respect to their effects on [³H]tyramine (TY) binding, as a putative marker for the vesicular dopamine (DA) transporter in striatal membrane preparations containing vesicle ghosts. Among organic Ca²⁺-channel modulators, the diphenylalkylamines tested consistently inhibited TY binding: the order of potency was prenylamine>lidoflazine>flunarizine>cinnarizine, with K_i values of 0.1, 0.2, 0.5 and 1.2 M, respectively. Low (up to 100 nM) concentrations of prenylamine did competitively inhibit TY binding, and higher concentrations provoked a mixed-type inhibition. Furthermore, LIGAND-analysis of competition curves revealed a high- and a low-affinity binding site for prenylamine and flunarizine. The TY binding process was also sensitive to selected K⁺- and Na⁺-channel modulators. Since several Ca²⁺-antagonists are known to affect H⁺-ATPase and the bioenergetics of catecholamine storage vesicles in chromaffin granules, thus affecting monoamine storage, the energy requirements for the formation of the TY/carrier complex were here assessed, assuming similarity between chromaffin granules and synaptic vesicles. TY binding, though not reflecting endovesicle-sequestered TY, was indeed strongly sensitive (with K_i coefficients in the fM or low nM range) to the dissipation of the vesicular transmembrane proton concentration ( pH), electrical ( ), and proton electrochemical ( H⁺) gradients, provoked by a number of specifically targeted agents. It is concluded that Ca²⁺-channel agents of the diphenylalkylamine group may directly affect striatal TY binding due to an extrachannel-regulated competition with TY for the vesicular carrier of DA, as well indirectly, by disruption of the transmembrane energization of the reserpine-sensitive carrier.