GDP-L-fucose pyrophosphorylase. Purification, cDNA cloning, and properties of the enzyme

The enzyme that catalyzes the formation of GDP-L-fucose from GTP and beta-L-fucose-1-phosphate (i.e. GDP-beta-L-fucose pyrophosphorylase, GFPP) was purified about 560-fold from the cytosolic fraction of pig kidney. At this stage, there were still a number of protein bands on SDS gels, but only the 61-kDa band became specifically labeled with the photoaffinity substrate, azido-GDP-L-[32P]fucose. Several peptides from this 61-kDa band were sequenced and these sequences were used for cloning the gene. The cDNA clone yielded high levels of GFPP activity when expressed in myeloma cells and in a baculovirus system, demonstrating that the 61-kDa band is the authentic GFPP. The porcine tissue with highest specific activity for GFPP was kidney, with lung, liver, and pancreas being somewhat lower. GFPP was also found in Chinese hamster ovary, but not Madin-Darby canine kidney cells. Northern analysis showed the mRNA in human spleen, prostate, testis, ovary, small intestine, and colon. GFPP was stable at 4 (o)C in buffer containing 50 mM sucrose, with little loss of activity over a 9-day period. GTP was the best nucleoside triphosphate substrate but significant activity was also observed with ITP and to a lesser extent with ATP. The enzyme was reasonably specific for beta-L-fucose-1-P, but could also utilize alpha-D-arabinose-1-P to produce GDP-alpha-D-arabinose. The product of the reaction with GTP and alpha-L-fucose-1-P was characterized as GDP-beta-L-fucose by a variety of chemical and chromatographic methods.     

Novel inhibitors of fungal protein synthesis produced by a strain of Graphium putredinis. Isolation, characterisation and biological properties

The isolation and structure determination of 6 analogues of the fungal protein synthesis inhibitor GR135402, from Graphium putredinis, is described. The relative potencies of the compounds as protein synthesis inhibitors and as in vitro antifungal agents provide interesting insights into the structure-activity relationships in this series.     

Production of IgM hexamers by normal and autoimmune B cells: implications for the physiologic role of hexameric IgM

Secreted IgM is predominantly found as pentameric molecules, but IgM can also be secreted as hexamers by B cell lines. Murine hexamers activate the complement cascade more efficiently than pentamers, but the physiologic significance of hexameric IgM remains unknown. Here, we report that IgM hexamers and pentamers are cleared from the circulation with similar kinetics, suggesting that the predominance of pentameric IgM in vivo reflects the regulation of polymer assembly and secretion in responding B cells. Normal IgM-secreting B cells, particularly those from the peritoneal cavity, are capable of secreting abundant hexameric IgM in vitro. The disparity between the ability of B cells to secrete IgM hexamers in vitro and the paucity of this polymer in vivo suggest that IgM hexamers might be deleterious. In support of this, we demonstrate that the autoantibodies from a number of patients with cold agglutinin (CA) disease include both IgM hexamers and pentamers. The CA IgM hexamers lyse human erythrocytes in the presence of human complement more efficiently than CA IgM pentamers, suggesting a potential role for hexameric IgM in the pathogenesis of this autoimmune syndrome.     

Key role of the Cdx2 homeobox gene in extracellular matrix-mediated intestinal cell differentiation

To explore the role of homeobox genes in the intestine, the human colon adenocarcinoma cell line Caco2-TC7 has been stably transfected with plasmids synthesizing Cdx1 and Cdx2 sense and antisense RNAs. Cdx1 overexpression or inhibition by antisense RNA does not markedly modify the cell differentiation markers analyzed in this study. In contrast, Cdx2 overexpression stimulates two typical markers of enterocytic differentiation: sucrase-isomaltase and lactase. Cells in which the endogenous expression of Cdx2 is reduced by antisense RNA attach poorly to the substratum. Conversely, Cdx2 overexpression modifies the expression of molecules involved in cell-cell and cell-substratum interactions and in transduction process: indeed, E-cadherin, integrin-beta4 subunit, laminin-gamma2 chain, hemidesmosomal protein, APC, and alpha-actinin are upregulated. Interestingly, most of these molecules are preferentially expressed in vivo in the differentiated villi enterocytes rather than in crypt cells. Cdx2 overexpression also results in the stimulation of HoxA-9 mRNA expression, an homeobox gene selectively expressed in the colon. In contrast, Cdx2-overexpressing cells display a decline of Cdx1 mRNA, which is mostly found in vivo in crypt cells. When implanted in nude mice, Cdx2-overexpressing cells produce larger tumors than control cells, and form glandular and villus-like structures. Laminin-1 is known to stimulate intestinal cell differentiation in vitro. In the present study, we demonstrate that the differentiating effect of laminin-1 coatings on Caco2-TC7 cells is accompanied by an upregulation of Cdx2. To further document this observation, we analyzed a series of Caco2 clones in which the production of laminin-alpha1 chain is differentially inhibited by antisense RNA. We found a positive correlation between the level of Cdx2 expression, that of endogenous laminin-alpha1 chain mRNA and that of sucrase-isomaltase expression in these cell lines. Taken together, these results suggest (a) that Cdx1 and Cdx2 homeobox genes play distinct roles in the intestinal epithelium, (b) that Cdx2 provokes pleiotropic effects triggering cells towards the phenotype of differentiated villus enterocytes, and (c) that Cdx2 expression is modulated by basement membrane components. Hence, we conclude that Cdx2 plays a key role in the extracellular matrix-mediated intestinal cell differentiation.     

Enteropathogenic and enterohaemorrhagic Escherichia coli: more subversive elements

Enteropathogenic (EPEC) and enterohaemorrhagic Escherichia coli (EHEC) constitute a significant risk to human health worldwide. Both pathogens colonize the intestinal mucosa and, by subverting intestinal epithelial cell function, produce a characteristic histopathological feature known as the 'attaching and effacing' (A/E) lesion. Although EPEC was the first E. coli to be associated with human disease in the 1940s and 1950s, it was not until the late 1980s and early 1990s that the mechanisms and bacterial gene products used to induce this complex brush border membrane lesion and diarrhoeal disease started to be unravelled. During the past few months, there has been a burst of new data that have revolutionized some basic concepts of the molecular basis of bacterial pathogenesis in general and EPEC pathogenesis in particular. Major breakthroughs and developments in the genetic basis of A/E lesion formation, signal transduction, protein translocation, host cell receptors and intestinal colonization are highlighted in this review.     

Design and synthesis of potent, selective inhibitors of endothelin-converting enzyme

Endothelin-1 is the most potent peptidic vasoconstrictor discovered to date. The final step of posttranslational processing of this peptide is the conversion of its precursor by endothelin-converting enzyme-1 (ECE-1), a metalloprotease which displays high amino acid sequence identity with neutral endopeptidase 24.11 (NEP) especially at the catalytic center. A series of potent and selective arylacetylene-containing ECE-1 inhibitors have been prepared. (S, S)-3-Cyclohexyl-2-[[5-(2, 4-difluorophenyl)-2-[(phosphonomethyl)amino]pent-4-ynoyl]amino] propio nic acid (47), an arylacetylene amino phosphonate dipeptide, was found to inhibit ECE-1 and NEP with IC50 values of 14 nM and 2 microM, respectively. Similarly, (S)-[[1-[(2-biphenyl-4-ylethyl)carbamoyl]-4-(2-fluorophenyl)but-3- yny l]amino]methyl]phosphonic acid (56), an arylacetylene amino phosphonate amide, had IC50's of 33 nM and 6.5 microM for ECE-1 and NEP, respectively. Slight modification of the aryl moiety was found to have dramatic effects on ECE-1/NEP selectivity. The 2-fluoro dipeptide analogue, (S, S)-2-[[5-(2-fluorophenyl)-2-[(phosphonomethyl)amino]pent-4-ynoyl]+ ++amin o]-4-methylpentanoic acid (40), showed a 72-fold selectivity for ECE-1 over NEP, while the 3-fluoro dipeptide analogue, (S, S)-2-[[5-(3-fluorophenyl)-2-[(phosphonomethyl)amino]pent-4-ynoyl]+ ++amin o]-4-methylpentanoic acid (22), was equipotent for ECE-1 and NEP. Several of these inhibitors were shown to be potent in blocking ET-1 production in vivo as demonstrated by the big ET-1-induced pressor response in rats. These potent inhibitors are the most selective for ECE-1 reported to date and are envisaged to have a variety of therapeutic applications.